78 10 13233 https://digitalcollections.lakeheadu.ca/files/original/18fd51ee65642289cfb33fc14030102e.pdf 71db64055c86846756ee627afe76f091 PDF Text Text 70th Annual Meeting Institute on Lake Superior Geology Houghton, Michigan May 15-18, 2024 Proceedings Volume 70 Part 1 - Program and Abstracts �70th Annual Meeting Institute on Lake Superior Geology Houghton, Michigan May 15-18, 2024 Sponsored by: A. E. Seaman Mineral Museum Great Lakes Research Center Department of Geological and Mining Engineering and Sciences Michigan Technological University Meeting Co-Chairs Theodore J. Bornhorst, Erika C. Vye, Patrice Cobin, and James DeGraff Proceedings Volume 70 Part 1: Program and Abstracts Edited by Theodore J. Bornhorst and Erika C. Vye Cover Photo: The only known color photograph of in situ colorless calcite crystals with inclusions of native copper. Vug is about 15 cm across and 30 cm deep; located at the top of the Knowlton basalt lava flow at the 4 th level, 850 ft stope, of the Caledonia Mine, Michigan. Photo taken in 1994 soon after the vug was blasted open. Native copper in the calcite crystals has not been visibly altered despite being about 1 billion years old. Photograph by Theodore J. Bornhorst i �70th Institute on Lake Superior Geology Volume 70 consists of: Part 1: Program and Abstracts Part 2: Field Trip Guidebook Trip 1: Mesoproterozoic Midcontinent Rift-filling Strata and Native Copper Deposits of the Keweenaw Peninsula, Michigan Trip 2: Mining History and Geology of the Quincy Mine, Keweenaw Peninsula Native Copper District, Michigan Trip 3: Geoheritage of Buffalo Reef: Industrial Impact on Land, Culture, and Fish Sovereignty Trip 4: Keweenaw Fault System Geometry and Kinematics: Clues to Its Nature and Origin Trip 5: Geology and History of a Native Copper Mine: Adventure Mine, Ontonagon County, Michigan Trip 6: Southern Complex Granitoids, Gneisses, and Migmatites: New Data, Discoveries, and Perspectives Trip 7: Landslides on the Ontonagon River at Military Hill Reference to material in Part 2 should follow the example below: Authors, 2024, Field Trip title, 70th Institute on Lake Superior Geology, Abstracts and Proceedings, v. 70, Part 2, Field Trip Guidebook, p. xx-xx. Proceedings Volume 70, Part 1: Program and Abstracts and Part 2: Field Trip Guidebook are published by the 70th Institute on Lake Superior Geology and distributed by the Institute Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca Some figures in this volume were submitted by authors in color but are printed black and white. Full color imagery will appear in the digital version of the volume when it is available on-line at: http://www.lakesuperiorgeology.org ISSN 1042-99 ii �Part 1: Program and Abstracts Table of Contents Institutes on Lake Superior Geology, 1955-2024 ............................................................. iv Sam Goldich and the Goldich Medal ............................................................................... vii Goldich Medal Guidelines ................................................................................................ ix Goldich Medalists ............................................................................................................. xi 2024 Goldich Medal Recipient ......................................................................................... xi Goldich Medal Committee ............................................................................................... xi Citation for 2024 Goldich Medal Recipient..................................................................... xii Honoring the Pioneers of Lake Superior Geology……………………………………….xiv Citation for 2024 Pioneer of Lake Superior Geology Recipient………………………...xv In Memoriam……………………………………………………………………………xix Eisenbrey Student Travel Awards ................................................................................... xx Joe Mancuso Student Research Award ........................................................................... xxi Doug Duskin Student Paper Awards and 2024 Student Paper Awards Committee ...... xxii Board of Directors and 2024 ILSG Meeting Volunteers .............................................. xxiii 2024 ILSG Meeting Volunteers and Session Chairs…………………………………..xxiv Field Trip Leaders and Guidebook Authors .................................................................. xxv Banquet Speaker Robert M Hazen ................................................................................ xxvi Report of the Chair of the 69th Annual Meeting ........................................................ xxvii Sponsors ........................................................................................................................ xxxi Technical Program ....................................................................................................... xxxii Abstracts ........................................................................................................................ xliii iii �Institutes on Lake Superior Geology, 1955-2024 # Date Place Chairs 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 Minneapolis, Minnesota Houghton, Michigan East Lansing, Michigan Duluth, Minnesota Minneapolis, Minnesota Madison, Wisconsin Port Arthur, Ontario Houghton, Michigan Duluth, Minnesota Ishpeming, Michigan St. Paul, Minnesota Sault Ste. Marie, Michigan East Lansing, Michigan Superior, Wisconsin Oshkosh, Wisconsin Thunder Bay, Ontario Duluth, Minnesota Houghton, Michigan Madison, Wisconsin Sault Ste. Marie, Ontario Marquette, Michigan St. Paul, Minnesota Thunder Bay, Ontario C.E. Dutton A.K. Snelgrove B.T. Sandefur R.W. Marsden G.M. Schwartz & C. Craddock E.N. Cameron E.G. Pye A.K. Snelgrove H. Lepp A.T. Broderick P.K. Sims & R.K. Hogberg R.W. White W.J. Hinze A.B. Dickas G.L. LaBerge M.W. Bartley & E. Mercy D.M. Davidson J. Kalliokoski M.E. Ostrom P.E. Giblin J.D. Hughes M. Walton M.M. Kehlenbeck iv �# 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 Date 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 Place Milwaukee, Wisconsin Duluth, Minnesota Eau Claire, Wisconsin East Lansing, Michigan International Falls, Minnesota Houghton, Michigan Wausau, Wisconsin Kenora, Ontario Wisconsin Rapids, Wisconsin Wawa, Ontario Marquette, Michigan Duluth, Minnesota Thunder Bay, Ontario Eau Claire, Wisconsin Hurley, Wisconsin Eveleth, Minnesota Houghton, Michigan Marathon, Ontario Cable, Wisconsin Sudbury, Ontario Minneapolis, Minnesota Marquette, Michigan Thunder Bay, Ontario Madison, Wisconsin Kenora, Ontario Iron Mountain, Michigan Duluth, Minnesota Nipigon, Ontario Sault Ste. Marie, Ontario Lutsen, Minnesota Marquette, Michigan Ely, Minnesota International Falls, Minnesota 57 58 59 60 61 62 63 64 65 66 67 68 69 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 Ashland, Wisconsin Thunder Bay, Ontario Houghton, Michigan Hibbing, Minnesota Dryden, Ontario Duluth, Minnesota Wawa, Ontario Iron Mountain, Michigan Terrace Bay, Ontario Meeting cancelled Virtual meeting Sudbury, Ontario Eau Claire, Wisconsin v Chairs G. Mursky D.M. Davidson P.E. Myers W.C. Cambray D.L. Southwick T.J. Bornhorst G.L. LaBerge C.E. Blackburn J.K. Greenberg E.D. Frey & R.P. Sage J. S. Klasner J.C. Green M.M. Kehlenbeck P.E. Myers A.B. Dickas D.L. Southwick T.J. Bornhorst M.C. Smyk L.G. Woodruff R.P. Sage & W. Meyer J.D. Miller & M.A. Jirsa T.J. Bornhorst & R.S. Regis S.A. Kissin & P. Fralick M.G. Mudrey & Jr., B.A. Brown P. Hinz & R.C. Beard L. Woodruff & W.F. Cannon S. Hauck & M. Severson M. Smyk & P. Hollings A. Wilson & R. Sage L. Woodruff & J. Miller T.J. Bornhorst & J. Klasner J. Miller, G. Hudak, D. Peterson M. Jirsa, P. Hollings & T. Boerboom, P. Hinz & M.Smyk T. Fitz P. Hollings T.J. Bornhorst & A. Blaske J. Miller & M. Jirsa R. Cundari & P. Hinz J. Miller, C. Schardt & D. Peterson A. Pace, A. Wilson & T.J. Bornhorst L. Woodruff, W. Cannon & E.K. Stewart P. Hollings & M.C. Smyk Cancelled by the COVID-19 pandemic M. Jirsa, M. Smyk & P. Hollings R.M. Easton & W. Bleeker R. Lodge, E.K. Stewart, & C. Ames �# 70 Date Place 2024 Houghton, Michigan Chairs T.J. Bornhorst, E. Vye, P. Cobin, & J. DeGraff vi �Sam Goldich and the Goldich Medal Sam Goldich received an A.B. from the University of Minnesota in 1929, a M.A. from Syracuse University in 1930, and a Ph.D. from the University of Minnesota in 1936. During World War II Sam worked for the U.S. Geological Survey in mineral exploration. In 1948, Sam returned to the University of Minnesota, and became Professor and Director of the Rock Analysis Laboratory the following year. He rejoined the U.S. Geological Survey in 1959 and was appointed as the first Branch Chief of the Branch of Isotope Geology. Sam returned to academia in 1964 when he went to Pennsylvania State University. He left PSU in 1965 and moved to the State University of New York at Stony Brook, where he stayed for 3 years. Restless yet again, he moved to Northern Illinois University in 1968 where he was a professor until his retirement in 1977. Sam’s final move was to Denver where he became an emeritus at the Colorado School of Mines. Sam died in 2000, less than a month before his 92nd birthday. In the late 1970s, Geological Society of America Special Paper 182, which included seminal geochronological studies by Sam Goldich and coworkers on the Archean rocks of the Minnesota River Valley, was nearing completion. At this time various ILSG regulars began discussing the possibility of recognizing Sam for his pioneering work on the resolution of age relationships and thus the geology of Precambrian rocks in the Lake Superior region. Three members, R.W. Ojakangas, J.O. Kalliokoski, and G.B. Morey, presented the idea to the ILSG Board of Directors in 1978. The Board approved the creation of an award, provided funding could be obtained. It was suggested that collecting one or two dollars at registration for a dedicated account would provide resources for striking the medal. A general request was made to the ILSG membership for donations and Sam himself offered a challenge grant to match the contributions. In total, $4,000 was collected and thus began the work of creating the Goldich Medal. The initial Goldich Award was presented to Sam by G.B. Morey in 1979 and consisted of a large paper proclamation. For the actual medal, G.B. Morey consulted with the foundry on production details, while Dick Ojakangas and Jorma Kalliokoski worked on the design of the award, suggesting that it be given for “outstanding contributions to the geology of the Lake Superior region.” Simultaneously, a committee of J.O. Kalliokoski, W.F. Cannon, M.M Kehlenbeck, G.B. Morey, and G. Mursky developed the Award Guidelines that were approved by the ILSG Board. By 1981 all the elements of the Goldich Award had come together, and the second recipient, Carl E. Dutton, Jr., received the Goldich Medal for 50 years of significant contributions to the understanding of the geology of the Lake Superior region. Since the beginning, the Awards Committee has consisted of individuals representing industry, government and academia, with each member of the Committee serving for three years. The medal is now awarded every year at the annual ILSG meeting. Reference: Morey, G.B. and Hanson, G.N. (editors). 1980. Selected studies of Archean gneisses and Lower Proterozoic rocks, southern Canadian Shield. Geological Society of America, Special Paper 182, 175 p. Prepared by various Goldich Medal Awardees, 2007 vii �INSTITUTE ON LAKE SUPERIOR GEOLOGY GOLDICH MEDAL viii �Goldich Medal Guidelines (Adopted by the Board of Directors, 1981; amended 1999) Preamble The Institute on Lake Superior Geology was born in 1955, as documented by the fact that the 27th annual meeting was held in 1981. The Institute’s continuing objectives are to deal with those aspects of geology that are related geographically to Lake Superior; to encourage the discussion of subjects and sponsoring field trips that will bring together geologists from academia, government surveys, and industry; and to maintain an informal but highly effective mode of operation. During the course of its existence, the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have made particularly noteworthy and meritorious contributions to the understanding of Lake Superior geology and mineral deposits. The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years. Subsequent medalists and this year’s recipient are listed in the table below. Award Guidelines 1) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution to, the geology of the Lake Superior region. 2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of three members, one to serve for three years, one for two years, and one for one year. The member with the briefest incumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In his/her third year this member shall be the chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSG membership. The out-going, senior member of the Board of Directors shall act as liaison between the Board and the Committee for a period of one year. 3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of the Board of Directors, who will then inform the Board of the nominee. 4) The Board of Directors normally will accept the nominee of the Committee, inform the medalist, and have one medal engraved appropriately for presentation at the next meeting of the Institute. 5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award. ix �Nominating Procedures 1) The deadline for nominations is November 1. Nominations shall be taken at any time by the Goldich Medal Committee. Committee members may themselves nominate candidates; however, Board members may not solicit for or support individual nominees. 2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation, lists of publications, curriculum vita’s, and evidence of contributions to Lake Superior geology and to the Institute. 3) Nominations are not restricted to Institute attendees but are open to anyone who has worked on and contributed to the understanding of Lake Superior geology. Selection Guidelines 1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including: a) importance of relevant publications; b) promotion of discovery and utilization of natural resources; c) contributions to understanding of the natural history and environment of the region; d) generation of new ideas and concepts; and e) contributions to the training and education of geoscientists and the public. 2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institute meetings, presentation of talks and posters, and service on Institute boards, committees, and field trips. 3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of the Committee members. 4) There are several points to be considered by the Goldich Medal Committee: a) An attempt should be made to maintain a balance of medal recipients from each of the three estates—industry, academia, and government. b) It must be noted that industry geoscientists are at a disadvantage in that much of their work in not published. 5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedly one of the Institute’s great strengths and should be nurtured by equitable recognition of excellence in both countries. x �Goldich Medalists 1979 Samuel S. Goldich 1998 Zell Peterman 2016 Mark A. Jirsa 1980 not awarded 1999 Tsu-Ming Han 2017 Philip Fralick 1981 Carl E. Dutton, Jr 2000 John C. Green 1982 Ralph W. Marsden 2001 John S. Klasner 2018 Val W. Chandler 2019 Mark Severson 1983 Burton Boyum 2002 Ernest K. Lehmann 2020 not awarded 1984 Richard W. Ojakangas 2003 Klaus J. Schulz 2021 Alan MacTavish 1985 Paul K. Sims 2004 Paul Weiblen 2022 Terrence J. Boerboom 1986 G.B. Morey 2005 Mark Smyk 2023 Peter Hollings 1987 Henry H. Halls 2006 Michael G. Mudrey 2024 Suzanne W. Nicholson 1988 Walter S. White 2007 Joseph Mancuso 1989 Jorma Kalliokoski 2008 Theodore J. Bornhorst 1990 Kenneth C. Card 2009 L. Gordon Medaris, Jr 1991 William Hinze 2010 William D. Addison & 1992 William F. Cannon Gregory R. Brumpton 1993 Donald W. Davis 2011 Dean M. Rossell 1994 Cedric Iverson 2012 James D. Miller 1995 Gene La Berge 2013 Tom Waggoner 1996 David L. Southwick 1997 Ronald P. Sage 2014 Laurel Woodruff 2015 Rodney J. Ikola 2024 GOLDICH MEDAL RECIPIENT Suzanne W. Nicholson Goldich Medal Committee Serving through the meeting year shown in parentheses. Dorothy Campbell (2019-2024) Ontario Geological Survey, Government Member (Committee Chair) Dean Peterson (2022-2025) Big Rock Exploration, Industry Member Marcia Bjornerud (2023-2026) Lakehead University, Academic Member xi �Citation for the 2024 Goldich Medal Recipient Suzanne W. Nicholson It is a pleasure and honor to present the 2024 Goldich Medal to our close friend and colleague, Suzanne Nicholson, recently retired from a long and fruitful career at the U.S. Geological Survey. Suzanne began working for the USGS as a student field assistant in 1978, and then, after completing a master’s degree at the University of Massachusetts, was hired as a full-time employee in 1981. Suzanne’s interest in the geology of the Lake Superior region began with her dissertation work with Paul Weiblen at the University of Minnesota on felsic magmatism in the Portage Lake Volcanics in Michigan, part of the Mesoproterozoic Midcontinent Rift System (MRS). This study included detailed mapping and sampling (typically big samples that had to be carried long distances) followed by major and trace element whole rock analysis and determination of a suite of radiogenic isotopes (Sr, Nd, Pb). Using modern petrologic methods, her research documented the presence of two distinct felsic magma types, one derived by partial melting of felsic basement and the other related to rift basalts through partial melting and/or fractional crystallization. Suzanne also was one of the first to provide comprehensive radiogenic isotope analyses of host basalts, documenting their distinctive isotopic character and that of their mantle sources. Her 1990 seminal publication (Nicholson, S.W., and Shirey, S.B., 1990, Midcontinent Rift volcanism in the Lake Superior region: Sr, Nd, and Pb isotopic evidence for a mantle plume origin: Journal of Geophysical Research, v. 95, p. 10,85110,868) described the unique geochemical character of rift magmatism around the Lake Superior region. Her on-going interest in MRS geochemistry culminated in the 1997 paper that established a rift-wide correlation of MRS basalts (Nicholson, S.W., Schulz, K.J., Shirey, S.B., and Green, J.B., 1997, Rift-wide correlation of 1.1 Ga Midcontinent Rift System basalts: multiple mantle sources during rift development: Canadian Journal of Earth Sciences, v. 34, p. 504-520), providing a foundation for future interpretations of MRS-related volcanic rock geochemistry. Suzanne was also a leader in advancing an understanding of the spatial-temporal evolution of MRS metallogeny (Nicholson, S.W., Cannon, W.F., and Schulz, K.J., 1992, Metallogeny of the Midcontinent Rift System of North America: Precambrian Research, v. 58, p. 355-386), further refined in a 2020 paper (Woodruff, L.G., Schulz, K.J., Nicholson, S.W., and Dicken, C.L., 2020, Mineral deposits of the Mesoproterozoic Midcontinent Rift system in the Lake Superior region A space and time classification: Ore Geology Reviews, 103716). Along with colleagues from the USGS, Suzanne helped produce a series of 1:100,000-scale geologic maps for the MRS and adjacent rocks from the Keweenaw Peninsula, extending through Michigan into northern Wisconsin to the Minnesota state line. These maps summarized legacy mapping and, along with new fieldwork, resulted in interpretations and correlations that xii �are the current standard for understanding the distribution and origin of the MRS volcanic and intrusive rocks of that area. Suzanne also initiated and continues to lead an on-going cooperative government/academia effort to compile and digitize existing MRS geology, geochemistry, isotope data, and age dates that will promote and direct future research of the region. Throughout her career, Suzanne was a careful and meticulous researcher who held her own results to a very high standard for accuracy, completeness, and thoroughly documented interpretations. Through the years, Suzanne has been a strong supporter of the Institute on Lake Superior Geology. She was a first or co-author on 17 abstracts presented at ILSG meetings from 1990 through 2019, a co-leader for two ILSG field trips, and co-editor for the 1996 Proceedings, Part 1- Program and Abstracts volume. Suzanne also was always willing and able to help with anything needed at ILSG meetings (a common trait among ILSG participants), such as acting as a session chair or serving on the student paper committee. In 2015, Suzanne moved into increasingly responsible managerial positions within the USGS, which curtailed her direct involvement with research in the Lake Superior region. In 2020, she received the U.S. Department of Interior's second highest honorary award—the Meritorious Service Award— in recognition of her scientific leadership and noteworthy contributions to the USGS Mineral Resources Program. Suzanne retired from her position as Associate Program Coordinator for the USGS Mineral Resources Program in 2021 but was retained for 2 years as an annuitant to keep the Program on budgetary track during a time of transition. Her qualities as a scientist transferred to her administrative duties, demonstrating the same dedication and skills she brought to her research. Now that Suzanne’s service to the Program has ended, we look forward to her return to MRSrelated research as a USGS Emeritus scientist. Throughout her managerial tenure, Suzanne never lost her attachment to the Lake Superior region and was able to promote and maintain funding for ongoing regional project work for her USGS colleagues. This support resulted in many new and exciting discoveries, such as tracing the extent and nature of the Sudbury ejecta layer across Michigan and Wisconsin, and tackling legacy seismic data to help understand the tectonicmagmatic evolution of the MRS. Through her thoughtful discussions, critical reviews, cheerful field assistance, and friendship for the past 40-some years, Suzanne helped enrich the lives and careers of many people, including those of her fellow USGS MRS aficionados. We remain a convivial group and all of us look back fondly on the times we spent together in the field. Who could forget death marches across Isle Royale, or raccoons swiping rhyolite samples in the Porcupine Mountains, or six long weeks at the Hurley Holiday Inn, among our many other adventures? So now, the three of us, all former recipients of the Goldich Medal, are joined by Suzanne in that honor. In recognition of her decades of accomplishments and dedication to the geology of the Lake Superior region and to the Institute on Lake Superior Geology, it is our pleasure to present the 2024 Goldich Medal to its second female recipient, Suzanne Nicholson. Citation by: Laurel G. Woodruff, USGS, Goldich Medal Winner, 2014 Klaus J. Schulz, USGS, retired, Goldich Medal Winner, 2003 William F. Cannon, USGS, Emeritus, Goldich Medal Winner, 1992 xiii �Honoring the Pioneers of Lake Superior Geology (Adopted by the Board of Directors, 2016) Preamble At the suggestion of Gene LaBerge, the 2016 executive board agreed to implement a program to recognize historic pioneers in the understanding of geology in the Lake Superior region. Beginning with the 2017 annual meeting, nominations will be accepted from the membership for geologists whose work was conducted primarily before the inception of the Institute in 1955. Biographical sketches of those pioneers will be presented at future annual meetings so that all may appreciate the value of their contributions. Selection of nominees will be decided in part by the organizing committee of each year's annual meeting, in consultation with the Board, to ensure equitable geographic representation in the selection process. Award Guidelines 1) Nominations from the membership will be submitted via the Institute web site and forwarded to the Chair of the next Annual Meeting. The nominations will be no more than half a page in length and will summarize the contribution of the nominee. 2) The Organizing Committee will select one or two individuals to be highlighted at the next Annual meeting and submit those names to the Board for approval. 3) The nominator will be requested to prepare a brief presentation to be given during the next annual meeting with a summary to be included in the Proceedings volume. 4) Unsuccessful nominations will be kept by the Secretary for two years and forwarded to the next meeting Chair; these nominations may be resubmitted at a later date. The Board will review this award every five years. Pioneers of Lake Superior Geology 2017 Douglass Houghton (1809-1845) 2018-20 not presented 2021 Newton Horace Winchell (1839-1914) 2022 Thomas Leslie Tanton (1890-1971) 2023 Thomas Benton Brooks (1836-1900) 2024 Roland Duer Irving (1847-1888) xiv �2024 Citation for the Roland Duer Irving Pioneer of Lake Superior Geology It is my great honor to nominate and promote Roland Duer Irving (1847-1888) as the 2024 Pioneer of Lake Superior Geology. I suspect that many ILSG members are unfamiliar with Dr. Irving and his many truly pioneering contributions to our understanding of various aspects of Lake Superior geology. Were it not for his premature death at the age of 41, I have no doubt that his continued work on the Precambrian geology of the Lake Superior region would have ranked him as one of the greatest Lake Superior geologists of his time. As it stands, his nearly two decades of mapping, petrography, and geochemical studies and mentoring of students at the University of Wisconsin provided a firm and rational foundation for our further understanding of Lake Superior geology. Roland Duer Irving Roland Duer Irving was born in New York City on April 29th, 1847 (1847 – 1888) as grand-nephew to the classic American novelist-essayist Washington Irving and the New York State Supreme Court Justice, John Duer. John Wesley Powell (2nd USGS Director 1881-1894) noted in his memoriam of Dr. Irving (Powell, 1891) that in his youth, “Roland was subject to frequent and alarming attacks of illness, also to a weakness of sight, which proved to be his greatest obstacle through life” and as such “his early education was at home, his sisters and his father being his instructor”. Ultimately, he enrolled at Columbia College School of Mines in 1863, and with the continued help of his sisters, he graduated in 1868 with a degree in mining engineering. During and after his time at Columbia, he worked for coal mines and smelters in Pennsylvania and New Jersey. In 1870, he was offered a mining and metallurgy chair position at the University of Wisconsin. Irving’s arrival at the University of Wisconsin in 1870 marked the emergence of the “Wisconsin School of Precambrian Geology” (Dott, 2001). He quickly gained prominence within the university as a faculty leader and outside the university as a research investigator (Curti and Carstenson, 1949). Soon after his arrival, the Wisconsin Geological Survey was established by the legislature in 1873 with Irving, T.C. Chamberlain, and Moses Strong serving assistant geologists. By 1876, Chamberlin took the reins as chief geologist of the survey, a position he would hold until its legislative termination in 1879. In 1880, Clarence King, first director of the US Geological Survey, recruited both Chamberlin and Irving to join the USGS in an effort to develop a geologic map of the entire United States. In 1881, Chamberlin was appointed director of the Glacial Division of the USGS. In 1882, Irving was appointed to head the USGS’s Lake Superior Precambrian Division, all the while continuing as head of the Department of Mineralogy and Geology at the University of Wisconsin. xv �During Roland Irving’s teaching and research time with the University of Wisconsin, the Wisconsin Geological Survey and the USGS, he came to mentor and collaborate with several notable geologists who would make their own mark on Lake Superior geology (Dott, 2001). Charles Van Hise arrived at UW as a geology student under Irving’s supervision in 1874. He completed his BS in 1879, his MS in 1882, and his PhD in 1892 (1st PhD at UW). With the passing of Dr. Irving in 1888, Van Hise became not only the principal geologist for the USGS’s Lake Superior Division, but also the head of Wisconsin’s geology department. Another notable student of Roland Irving’s at Wisconsin was Florence Bascom. She conducted a petrographic study of the Mellen Complex under the supervision of Irving and Van Hise and received the second ever MS degree in geology from UW in 1887. She later earned her doctorate degree from Johns Hopkins in 1893, the first woman in the US to be awarded a PhD in geology. Irving’s work with the Wisconsin Geological Survey (1873-1879) involved many aspects of Wisconsin geology. In Volume 1 (actually published last in 1883), which was intended to be a general summary of the geology, natural history and economic geology of the state for the general education of the public, Irving contributed chapters on the minerals, rock types, and iron ores of the state. In Volume 2 (1877), Irving reported on the Precambrian, Paleozoic, and Quaternary geology of central Wisconsin. His descriptions of the general structure and lithologic attributes of the Baraboo Quartzite and its unconformable relationship with adjacent “Silurian” (Paleozoic) rocks is particularly noteworthy. Volume 3 (1880), which focussed on the geology of Northern Wisconsin, included Irving’s summary report on the general geology of the Lake Superior region (Part 1) and a more detailed report on the geology of the eastern Lake Superior District (Part III). This work, which was based on field studies conducted between 1875 and 1878, formed the basis of his subsequent USGS work detailing the overall geology and structure of the Keweenawan System in the Lake Superior region. It is noteworthy that the renown petrographer, Raphel Pumpelly, contributed a chapter on the petrography of Keweenawan rocks (Part II) collected by Irving and others. Irving relied heavily on petrographic examination of field samples in his subsequent USGS work. In the final volume (#4, 1882) devoted to the geology, paleontology, natural history, and glacial geology of the southern half of the state Irving’s contribution focussed on the field and petrographic attributes of crystalline rocks of the Wisconsin River valley. He recruited his MS student, Charles Van Hise, to carry out most of the petrographic descriptions. Joining the US Geological Survey in 1880 as head of the Lake Superior Precambrian Division, Irving took advantage of being able to explore beyond the confines of Wisconsin and immediately embarked on his long-standing desire to produce a “resume of the results obtained in the Lake Superior country by other geologists up to the present” (Geology of Wisconsin, Volume III (1880) Part 1, p. 3). Building on his own studies of the Keweenawan System in northern Wisconsin, he reviewed and, where appropriate, integrated all former geologic studies in the Lake Superior dating back to the Michigan surveys of Douglass Houghton (1831-1844), the surveys of Upper Canada starting with Logan (1846), and the work of Joseph Norwood in northeastern Minnesota as part of the D.D. Owen US Survey (1847-1852). Between July 1880 and March 1882, Irving conducted reconnaissance mapping, along with a crew of five assistant geologists, in several poorly understood areas throughout the Lake Superior basin. xvi �In 1880, the Minnesota Geological Survey, headed by N.H. Winchell, was in its 9th year of existence, but had only just begun to map the Precambrian geology of the state. As such, Irving decided to spend much of his mapping efforts on the north shore of Lake Superior between Duluth and Nipigon Bay to ascertain how it correlated with the south shore. This occurred at a time when many frontier states were developing their own geologic surveys with the expressed purpose of excluding the federal survey. The USGS already had a strong foothold in Michigan and after the ending of the Wisconsin survey in 1879, developed a strong presence there as well. Suffice it to say that Irving’s work in Minnesota was not well received or valued by the Winchell’s Minnesota Survey. Notwithstanding Winchell objections, Irving’s publication of USGS Monograph 5 - The Copperbearing Rocks of Lake Superior (Irving, 1883) proved to be a remarkably complete and accurate picture of the geology and structure of Keweenawan System (Figure 1). The many important observations and interpretations about Keweenawan geology put forth by Irving include: • formalizing the lithostratigraphy of the Keweenawan System • • defining the synclinal structure of the lavas in the Lake Superior area recognizing that eruptive rocks consist of basic, intermediate, and felsic types • noting no obvious relation of volcanic type to stratigraphic position • interpreting that basic lavas were erupted subaerially from fissures, not ash-generating volcanoes • recognizing that amygdaloidal zones capping basalts are themselves volcanic (not sedimentary) • accurately estimating the thickness of the North Shore Volcanics to be about 18,000’ • interpreting gabbroic and granitic rocks to be intrusive into the volcanic rocks (thus younger) and likely formed in staging chambers that fed surface eruptions Following on the publication of Monograph 5, Irving continued to apply his geologic and petrographic expertise to studies of other Precambrian systems (greenstones, quartzites, and iron formations) in collaboration with students and USGS colleagues. When Roland Irving unexpectedly died (from “paralysis”, perhaps a stroke) on May 30, 1888, he was engaged with Van Hise on another USGS monograph (#19) on the Gogebic Iron Range, which was published posthumously (Irvine and Van Hise, 1892). This monograph launched Charles Van Hise on a career path to becoming an internationally recognized expert on Lake Superior iron formations. While Van Hise will ultimately be recognized as pioneer of Lake Superior geology, it is fitting that we first acknowledge the remarkable accomplishments of his advisor and mentor, Roland Duer Irving. One can only imagine the professional stature he would have attained were he not struck down at the peak of his creativity and expertise. xvii �Figure 1: Plate 1 of USGS Monograph 5 by R.D. Irving, 1883 References Curti, M., and Carstenson, V., 1949, The University of Wisconsin, A History, 1848-1925 (v. 1). Madison, Wisconsin, University of Wisconsin Press, 739 p. Dott, Robert H., Jr., 2001, The remarkable legacy of the Wisconsin School of Precambrian Geology. Geoscience Wisconsin, v. 18, p. 27-40. Irving, R.D., 1883, The Copper-bearing Rocks of Lake Superior. USGS Monograph 5, 464p. Irving, R.D., and Van Hise, C.R., 1892, Penokee Iron-Bearing Series of Michigan and Wisconsin. USGS Monograph 19, 534p. Powell, J.W., 1891, Roland Duer Irving. Eleventh Annual report of the Director of the United States Geological Survey, Part 1- Geology: 1889-1890 p. 38-42. Citation by: James Miller University of Minnesota-Duluth xviii �In Memoriam Louis Mattson Obituary 12/31/2023 Louis A. Mattson, 89, of Pengilly, MN. passed away December 31, 2023, in Grand Rapids, MN. He was a long-time member of the Institute on Lake Superior Geology. The son of commercial fishermen, Lou was the last surviving member of the Mattson Tobin Harbor Fishery on Michigan’s Isle Royale. The fishery on Isle Royale, and family homesteads settled in the 1890’s at Larsmont, and the French River on Minnesota’s North Shore were part of Lou’s DNA. If you knew Lou, you knew about the family legacy on Lake Superior. The landscape of northern Minnesota inspired Lou to pursue a BS in Geology from the University of Minnesota Duluth and an MS in Geology from the University of Minnesota and the Colorado School of Mines. This education would lead to a 30-year career highlighted by travel around the world while working for M.A. Hanna’s Minerals Research Laboratory in Nashwauk. Lou’s sharp mind did not rest in retirement. He extensively researched family genealogy culminating in connections with relatives in Larsmo, Finland, enhanced his boat collection at the home he and Peggy built on Swan Lake, supported the Isle Royale Friends and Family Association (IRFFA). xix �Eisenbrey Student Travel Awards The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name “Eisenbrey” was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. “Ned” Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader - he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The number of awards and value are determined by the annual Chair in consultation with the Secretary and Treasurer. Recipients will be announced at the end of the annual meeting. The following general criteria will be considered by the annual Chair, who is responsible for the selection: 1) The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. 2) Students who are the senior author on either an oral or poster paper will be given favored consideration. 3) It is desirable for two or more students to jointly request travel assistance. 4) In general, priority will be given to those in the Institute region who are farthest away from the meeting location. 5) Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. Successful applicants will receive their awards during the meeting. xx �Joe Mancuso Student Research Awards The 2005 Board of Directors established the ILSG Student Research Fund with $10,000 US from the Institute’s general fund to encourage student research on the geology of the Lake Superior region. A minimum of two awards of $500 US each for research expenses (but not travel expenses) will be made each year. Students are expected to present their research orally or during a poster session at an ILSG meeting. The award winners will also be automatically eligible for the Eisenbrey Travel Awards. To allow the fund to grow, the Fund will receive one-half of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. • The ILSG Board of Directors will be responsible for selecting a minimum of two awards each year. The ILSG Treasurer will issue the awards. • The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. • The applications are due to the ILSG Secretary by August 31st of each year. Awards will be made by October 1st of each year. • Names of the award recipients will be announced at the next annual meeting and posted on the ILSG website. • Details of the application process can be found on the ILSG web site. • The proposal will need to be signed by the researcher’s supervisor. The 2012 Board of Directors approved modification of the fund’s name, adding “Mancuso” to reflect the many contributions of Joseph Mancuso to the organization and sizeable donations made in his name. “Doc Joe,” as he was known by his students, taught geology for 36 years at Bowling Green State University, Ohio. He advised many graduate students in field-oriented research, and frequently brought them to Institute meetings. Joe was the 2007 Goldich Medalist. In fall 2023, the ILSG Board of Directors selected two students to be granted research funding of $500 each from the Joe Mancuso Student Research Fund. The awardees were: Adrian Perez Avila Lakehead University Braxton Murphy Michigan Technological University Department of Geological and Mining Engineering and Sciences TOPIC: Characterization of the host rocks to mineralization in the Shebandowan greenstone belt in the vicinity of the Moss Lake deposit, NW Ontario TOPIC: Determine the relative paleostress state and tectonic conditions that resulted in formation and movement of faults making up the Keweenaw fault system near Houghton, Michigan, USA. Zsuzsanna P. Allerton University of Minnesota- Twinn Cities TOPIC: Investigate the timing and genesis of massive and semi-massive hematite ore bodies located in the Neoarchean (~2.7 Ga) Lake Vermilion/Soudan Underground Mine State Park (SSP) Farhan Ahmed Bhuiyan University of Minnesota- Duluth TOPIC: Evaluating post-depositional mineral reactions in the 1.71 – 1.47 Ga Freedom Formation, Baraboo, WI xxi �Doug Duskin Student Paper Awards Each year, the Institute selects the best of student presentations and honors the presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting, and from generous donations to the fund in honor of Doug Duskin—an exploration geologist and longtime friend of the Institute. The 2012 ILSG Board of Directors approved adding Doug’s name to the award to acknowledge his contributions and distribute those donations in a manner that would have pleased him. The Duskin Student Paper Committee is appointed by the Meeting Chair. Criteria for best student paper—last modified by the Board in 2001—follow: 1) The contribution must be demonstrably the work of the student. 2) The student must present the contribution in-person. 3) The Student Paper Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. 4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. 5) The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). 6) The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required but is left to the discretion of the Committee. 7) The names of award recipients shall be included as part of the annual Chair’s report that appears in the next volume of the Institute. Student papers will be noted on the Program. 2024 Student Paper Awards Committee Stacy Saari – Minnesota Department of Natural Resources (Committee Chair) Paula Leier-Englehardt – HydroGeo Solutions LLC, Wisconsin Dan Hirvi – Consulting Geologist, Michigan Allison Severson – Minnesota Geological Survey xxii �Board of Directors Theodore J. Bornhorst, Chair (2024-2027) — Michigan Technological University Carysn Ames (2023-2026) — Wisconsin Geological and Natural History Survey Mike Easton (2022-2025) — Ontario Geological Survey Mark Smyk (2019-2024) — Lakehead University Peter Hollings Secretary (2019-2024) — Lakehead University Mark A. Jirsa Treasurer (2022-2025) — Minnesota Geological Survey Board member through the close of the meeting year shown in parentheses. xxiii �2024 ILSG Meeting Michigan Tech Volunteers Great Lakes Research Center Daniel J. Lizzadro-McPherson Student Volunteers: Affiliated with Michigan Tech Jhuleyssy Liesseth Sánchez Aguilar Gabriel Ahrendt Katherine Langfield Marie, Lansbery Braxton Murphy Abe Stone 2024 ILSG Meeting Session Chairs Allan Blaske, GEI Consultants Amy Radakovich Block, Minnesota Geological Survey Patty Cobin, A. E. Seaman Mineral Museum, Michigan Tech Mary Louise Hill, Lakehead University Allan MacTavish, AGC GeoConsulting Ashley Quigley, Michigan Geological Survey Bernie Saini-Eidukat, North Dakota State University Mark Smyk, Lakehead University xxiv �Field Trip Leaders and Guidebook Authors Field trips have been the mainstay of the ILSG since its inception 70 years ago. We give special thanks to the field trip leaders and guidebook authors who volunteered their time and talent in carrying that tradition forward. Trip 1: Mesoproterozoic Midcontinent Rift-filling Strata and Native Copper Deposits of the Keweenaw Peninsula, Michigan Ted Bornhorst (Michigan Tech) Trip 2: Mining History and Geology of the Quincy Mine, Keweenaw Peninsula Native Copper District, Michigan Tom Wright (Quincy Mine Hoist Association) Jim DeGraff and Ted Bornhorst (Michigan Tech) Trip 3: Geoheritage of Buffalo Reef: Industrial Impact on Land, Culture, and Fish Sovereignty Erika Vye, Charlie Kerfoot (Michigan Tech) Stephanie Swart (Michigan Department of Environmental Quality) Dione Price and Evelyn Ravindran (Keweenaw Bay Indian Community) Trip 4: Keweenaw Fault Geometry and Kinematics: Clues to Its Nature and Origin Jim DeGraff, Katherine Langfield, and Dan Lizzadro-McPherson (Michigan Tech) Trip 5: Adventure Mine, Ontonagon County, Michigan: Geology and History of a Native Copper Mine Matt Portfleet (Adventure Mining Company) Ted Bornhorst (Michigan Tech) Trip 6: Southern Complex Granitoids, Gneisses and Migmatites: New Data, Discoveries, and Perspectives Chad Deering (Michigan Tech) Trip 7: Landslides in the Glacial Lake Ontonagon Sediments Stan Vitton and Mohammad Sadeghi (Michigan Technological University) xxv �Mineral Informatics: A New Frontier in Understanding Earth Robert M. Hazen Banquet Speaker Senior Staff Scientist, Earth and Planets Laboratory Carnegie Institution for Science, Washington, DC 20015 Email: rhazen@carnegiescience.edu The story of Earth is a 4.5-billion-year saga of dramatic transformations, driven by physical, chemical, and biological processes. The co-evolution of life and rocks unfolded in an irreversible sequence of evolutionary stages. Each stage re-sculpted our planet’s surface, while introducing new planetary processes and phenomena. This grand and intertwined tale of Earth’s living and non-living spheres is coming into ever-sharper focus, thanks to advances in “mineral informatics” - a field that employs large and growing mineral data resources to tell the deep-time stories of our evolving planet. Minerals are remarkably information rich, holding dozens of trace and minor elements, scores of stable isotopes, solid and fluid inclusions, chemical zoning, twinning, exsolution, countless defects, and a host of optical, magnetic, electrical, and other properties. Every mineral specimen is a time capsule waiting to be opened—waiting to tell its story. This lecture will explore some of the advanced data analytical and visualization methods that are shining new light on the old field of mineralogy, while revealing in ever greater clarity the co-evolution of the geosphere and biosphere. A network diagram of all known minerals colored by the way the minerals form. For example, red indicates high-temperature igneous minerals, while green indicates minerals formed by life. xxvi �REPORT OF THE 69th ANNUAL MEETING OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY Robert Lodge (University of Wisconsin-Eau Claire), Esther Stewart, and Carsyn Ames (Wisconsin Geologic and Natural History Survey) hosted the 69th Annual Institute on Lake Superior Geology on April 23 – 26, 2023 at the Lismore Hotel and Conference Center in Eau Claire, Wisconsin. The meeting consisted of two days of technical sessions with pre- and posttechnical session field trips. First, we would like to thank the meeting sponsors for their generous support, either through direct funding or in-kind support, namely: Talon Metals, American Institute of Professional Geologists, Geological Society of Minnesota, Crystal Cave, and Visit Eau Claire. We also thank the Individual Contributors to the Student Travel Scholarship fund: Val Chandler, Jim DeGraff, Thomas Erickson, Tom Fitz, Dave Good, Bob Mahin, Gordon Medaris Jr., Jim Miller, Steven Pinta, Tod Roush, and Gerry White. The 2023 meeting was the first conference held in the US since the 2018 Iron Mountain meeting, the first in Wisconsin since the 2011 Ashland meeting, and the first meeting in Eau Claire since 1991. Total meeting registration was 126, including 19 students. Attendance from both Canadian and United States was excellent despite other conferences in the Lake Superior region in April and May (GAC-MAC, Sudbury; Northcentral GSA, Grand Rapids). The technical program was nevertheless excellent with a good array of topics from Archean and Paleoproterozoic geology, to Midcontinent Rift geology and mineralization, to Quaternary Geology, Geoscience Education and Geoheritage. In addition, a student-industry networking lunch was held at the Riverview Room in the Eau Claire Public Library and an evening social was held at Reboot Social. Proceedings Volume 69 was published in two parts. Part 1 – Program and Abstracts, compiled and edited by Carsyn Ames (WGNHS) contains 54 published abstracts for 34 oral and 19 poster presentations. Students presented 8 oral and 10 poster presentations. Part 2 – Field Trip Guidebooks, was compiled and edited by Robert Lodge (UWEC). It contains descriptions of two pre-meeting and two post-meeting field trips. Hard copies of the Abstract Volume and Field Trip Guidebooks for trip participants were printed by University Printing at the University of Wisconsin-Eau Claire. Both volumes are available for download from the Institute on Lake Superior Geology website. The 69th ILSG marked only the third time in the Institute’s long history that its annual meeting was held in Eau Claire, the last time being in 1991. Plans for another Wisconsin-based ILSG meeting had been discussed for a while. With recent work in the Paleoproterozoic geology and mineralization in the Penokean orogen in northwestern Wisconsin and the central location of Eau Claire, it seemed appropriate to host the meeting there. Eau Claire sits on the boundary between Precambrian Shield, Paleozoic Platform, and the terminus of the continental ice sheet and allowed organizers to host four field trips examining billions of years of geologic history. Two field trips focused on the Precambrian geology of the Penokean orogen exposed in the Chippewa and Eau Claire River Valleys. While the preconference field trip got to see historic flooding on the Chippewa River (there are not many days when a bunch of Precambrian geologists are xxvii �looking at the river rather than the rocks), waters receded for the post-conference field trip. Field stops on this trip were originally (in some cases, exclusively) described in previous ILSG meetings (Eau Claire, 1980; 1991) but benefitted from new research and analyses and new viewpoints on the tectonics and metallogeny of the region. One fieldtrip visited classic exposures Paleozoic stratigraphy around the Eau Claire and Menominee regions and enjoyed lunch in an ancient meteorite impact structure. One field trip visited Quaternary geology and fluvial geomorphology of the Chippewa River valley. All the field trips, and the meeting itself, were blessed with good weather. Total field trip participation was 116 (excluding leaders and volunteer drivers). A list of field trips is provided below: Pre-meeting field trips (and leaders) on Tuesday, April 23. 1) Precambrian Geology of the Chippewa River valley: A Transect through the Marshfield Terrane (Robert Lodge, Bob Hooper, UW-Eau Claire) 2) Wisconsin’s Paleozoic stratigraphy and tour of Crystal Cave (Carsyn Ames, Esther Stewart, Bill Batten, Eric Stewart, Ian Orland, WGNHS) Post-meeting field trips (and leaders) on Friday, April 26. 3) Precambrian Geology of the Eau Claire River valley: Re-discovering the Eau Claire Volcanic Complex (Robert Lodge, Evan Weber, Bob Hooper, UW-Eau Claire) 4) Quaternary Geology and Geomorphology of the Eau Claire Region (Doug Faulker, UW-Eau Claire; Elmo Rawling, WGNHS; Phil Larson, Minnesota State University, Mankato) Many registrants attended the welcoming reception on Tuesday evening. Furthermore, the vast majority of registrants and invited guests attended the annual ILSG banquet on Wednesday night. Although a Homer Award overview presentation was given, no “recipients” were identified during the 2023 annual meeting, or in the previous 4 years! As always, a highlight of the post-banquet activities was presentation of the 2023 Goldich Medal. This year’s very deserving recipient was Dr. Pete Hollings. The Goldich Medal citation was presented by Mark Smyk, his colleague for many years. Mark described Pete’s many contributions to the ILSG, to the greater understanding of Archean and Proterozoic geology of the Lake Superior region, and his commitment to students. Pete is indeed a worthy recipient of this prestigious award. The 69th ILSG continued the post-banquet guest speaker tradition. Curt Meine, a conservation biologist, historian, and writer from the Aldo Leopold Foundation and Center for Humans and Nature, gave a presentation entitled Imagining “Conservation Geology”: Lessons from the Driftless Area. His talk provided an insightful viewpoint of how geology and landscapes integrate with history and culture in the Driftless Area of central Wisconsin. In 2023, the student paper committee remarked on the high quality of student research across all participants and had a difficult time of selecting the best among the excellent oral and poster presentations. The committee awarded four prizes for the best oral and poster presentations by xxviii �both undergraduate and graduate students. The best graduate oral presentation was awarded to Justin Jonsson and his talk on “Petrogenesis of the mineralized horizons in the Offset and Creek zones, Lac des Iles Complex, N. Ontario”. The best undergraduate oral presentation was awarded to Blaize Briggs for his talk on “Quetico-Wabigoon Subprovince Boundary in the Superior Province north of Thunder Bay, Ontario, Canada”. The best graduate poster presentation was awarded to Fransisca Nunez Ferreira for her poster on “Morphometry and formation process of eskers developed under the Chippewa Lobe of the Laurentide Ice Sheet”. The best undergraduate poster presentation was awarded to Lillian Glodowski for her poster on “Characterizing volcanic host stratigraphy and syn-volcanic intrusions at the Lynne Zn-Pb-Cu deposit, Oneida Co., Wisconsin”. Eisenbrey Student Travel Grants were given to twelve students: Zsuzsanna Allerton, Ryan Barkley, Blaize Briggs, Tianna Groeneveld, Justin Jonsson, Daniel Lizzardo-McPherson, Francisca Nunez Ferreira, Jordan Peterzon, Sam Ghantous, Madeline Taylor, BJ Itai, and Katherine Langfield. The Institute’s Board of Directors met on Thursday, April 25, 2023, and a brief overview of the meeting notes is provided below: 1. Accepted report of the Chairs for the 68th ILSG, as published in the Proceedings volume, and minutes of last Board meeting, May, 2022 (Hollings) 2. Received, discussed, and accepted 2022-2023 ILSG Financial Summary (Jirsa). 3. Received, discussed, and accepted 2022-2023 report of the Secretary (Hollings). 4. Approved Carsyn Ames as on-going ILSG Board member 5. Discussed and approved Amy Radakovich Block as Assistant Treasurer in a non-voting role (end of term 2026). 6. Discussed and approved replacing Steve Kissin as the “member from academia” on Goldich Committee (end of term 2023) with Marcia Bjornerud 7. Approved Houghton as the site for the 70th annual ILSG meeting. The meeting will be hosted by Ted Bornhorst. 8. Discussed the role of the Michigan Tech archives as the host of hard copies of the publications of the Institute. A formal agreement has been signed with the Archives who will request financial support as needed rather than the previous model of providing a donation of $1 per attendee. 9. A number of future meeting locations were discussed. Peter Hinz has offered Kenora as a future site and Mark Jirsa is keen to host the Mountain Iron meeting that was cancelled due to the pandemic. Bernie Saini-Eidukat to be approached to see if he is still interested in organizing a meeting in St Cloud. 10. The cost of insurance was again discussed and it was agreed that the Board of Directors insurance should be maintained and that the costs would be included in the cost of each meeting. Given the high costs quoted for field trip insurance the Board to investigate field trip insurance options. Hollings to approach GAC. Amy to approach GSA and Carsyn to approach a risk advisor. The Board discussed embedding a Liability waiver in the registration process. 11. The Board discussed embedding a photo release in the meeting and/or field trip registration process such that anyone registering for the field trip is aware they are agreeing to be photographed and have their image used in ILSG publications/website etc. It was agreed that the Institute does not want to turn anyone away from the meeting/trips simply because xxix �they do not want to be photographed, and the result is that before any photos are uploaded to the website, someone on the Board (or a future social media position) will need to go through the photos and make sure that no one who has NOT signed a photo release is shown in a shot where they are identifiable. The 69th ILSG meeting was a great success, and we wish to thank all the people who contributed to that success, field trip leaders and drivers, UWEC student volunteers, and businesses and organizations in downtown Eau Claire that hosted and entertained visitors. Patty Cobin and Ted Bornhorst (A.E. Seaman Mineral Museum, Michigan Technological University) handled the premeeting registration and supplied the poster boards. Thanks also go to the staff at Lismore Hotel and Conference Center who helped the meeting and banquet run smoothly and providing lunches and snacks during the technical sessions. Thanks to Eau Claire Public Library for hosting our student-industry luncheon, Reboot Social for hosting our post-meeting evening social, Eau Claire Cheese and Deli for field trip lunches, and Eau Claire Student Transit for bus transportation for fieldtrips. Robert Lodge (UWEC), Carsyn Ames (WGNHS), and Esther Stewart (WGHNS) Co-Chairs, 69th Institute on Lake Superior Geology xxx �Donations to Support Student Participation at the Annual Meeting of the Institute on Lake Superior Geology A SPECIAL THANK YOU TO OUR INDIVIDUAL CONTRIBUTORS Roger Anderson Aaron Hirsch Wouter Bleeker Allan MacTavish Terry Boerboom Bob Mahin Ted Bornhorst Gordon Medaris Jr. Alex Brown Jim Miller Michael Carr Rick Sandri Val Chandler Isabel Serrano Kate Clover Mark Severson Abraham Drost Jim Small Thomas Erickson Gerry White Annia Fayon Graham Wilson Mary Louise Hill xxxi �TECHNICAL PROGRAM xxxii �Wednesday May 15, 2024 All field trips begin and end at the Michigan Tech Memorial Union Building Parking tickets are given between 7 am to 4 pm weekdays. Between ticketing hours all vehicles need a parking pass; these will be available from trip leaders. Pre-meeting Field Trips May 15, 2024 8:00 am - 5:00 pm PRE-MEETING FIELD TRIPS Trip 1: Mesoproterozoic Midcontinent Rift-filling Strata and Native Copper Deposits of the Keweenaw Peninsula, Michigan Ted Bornhorst (Michigan Tech University) Trip 2: Mining History and Geology of the Quincy Mine, Keweenaw Peninsula Native Copper District, Michigan Tom Wright (Quincy Mine Hoist Association), Jim DeGraff, Katherine Langfield, and Ted Bornhorst (Michigan Tech) Trip 3: Geoheritage of Buffalo Reef: Industrial Impact on Land, Culture, and Fish Sovereignty Erika Vye and Charlie Kerfoot (Michigan Tech) Stephanie Swart (Michigan Department of Environmental Quality) Dione Price and Evelyn Ravindran (Keweenaw Bay Indian Community) Wednesday evening May 15, 2024 4:00 pm - 8:00 pm Registration (2nd floor, Michigan Tech Memorial Union) 6:30 pm - 8:30 pm Poster Setup and Viewing (2nd floor, Michigan Tech Memorial Union) 6:30 pm - 8:30 pm Welcoming Reception (2nd floor, Michigan Tech Memorial Union) xxxiii �* Denotes a student eligible for Best Student Paper Award. To be eligible students must have graduated no more than one month before the ILSG meeting, be first author, and present the paper at the meeting + Denotes author that will present the paper if different than the first author. Thursday - May 16, 2024 All vehicles need a parking pass between 7am to 4pm weekdays; these will be available from registration 7:15 am - noon Registration (2nd floor, Michigan Tech Memorial Union) 8:15 am OPENING REMARKS (2nd floor, Michigan Tech Memorial Union) Ted Bornhorst, Erika Vye, Patty Cobin, and Jim DeGraff Co-Chairs, 2024 ILSG TECHNICAL SESSION I – ORAL PRESENTATIONS Session Chair: Mark Smyk 8:20 Jim MILLER Roland Duer Irving - Pioneer of Lake Superior geology 8:35 Graham WILSON, Charles BUTT, Robert GARRETT and Heather ROBINSON R.W. Boyle’s History of Geochemistry and Cosmochemistry 8:55 James DeGRAFF, Nolan GAMET, Katherine LANGFIELD, Daniel LIZZADROMcPHERSON, Sophie MUELLER, and Colin TYRRELL Transpressional Nature of the Keweenaw Fault System, Lake Superior Region, and Its Relationship to Grenville Orogenesis 9:15 Alex BROWN Re-interpretation of hydrothermal alteration, mineralization and host-rock oxidation to form the Keweenaw native copper lodes, northern Michigan 9:35 Esther STEWART, Michael TAPPA, Ann BAUER, Anthony PRAVE, and Latisha BRENGMAN Geochemical fingerprints from the late Mesoproterozoic epeiric seaway of the Nonesuch Formation, Wisconsin and Michigan, USA 9:55 END OF TECHNICAL SESSION I 9:55-10:10 COFFEE BREAK xxxiv �TECHNICAL SESSION II – ORAL PRESENTATIONS Session Chair: Ashley Quigley 10:10 Sarah GORDEE, Madison RIAN, Stacy SAARI, and Matthew CARTER Description and application of the Consolidated Minerals Database to support geological investigations: an example from the Cuyuna Range, central Minnesota 10:30 Stacy SAARI, Sarah GORDEE, Madison RIAN, and Matt CARTER Compiled historical drillhole and geochemical data from the Cuyuna Range, Minnesota, provides powerful new insights for geological and mineral potential investigations. 10:50 Bob MAHIN, Ashley QUIGLEY, John YELLICH, John ESCH, and Nolan GAMET Critical Mineral Systems in the Upper Peninsula of Michigan, A Cooperative Effort Between the USGS and the Michigan Geological Survey 11:10 Paul BEDROSIAN, Dana PETERSON, and Bennett HOOGENBOOM Geophysical imaging of the Paleoproterozoic Animikie basin in Minnesota 11:30 Dan HOLLIS Use of Ambient Noise Tomography for Mineral Exploration in the Lake Superior Region 11:50 END OF TECHNICAL SESSION II 11:50-1:10 LUNCH BREAK and ILSG BOARD OF DIRECTORS MEETING - lunches not provided to conference attendees- TECHNICAL SESSION III- ORAL PRESENTATIONS Session Chair: Mary Louise Hill 1:10 *Demily THIBODEAU-BELLO, Mary Louise HILL, Andrew CONLY, An evaluation of structural and mineralogical controls on gold mineralization on the GoldRich property in the Abbie Lake area, Wawa, Ontario 1:30 Dean PETERSON and Alex STEINER The geology and ore deposit model of the high-grade Emily Manganese Deposit, Cuyuna Range, Minnesota: Results from the 2023 drilling program 1:50 *Gabriel AHRENDT and Aleksey SMIRNOV Rock magnetic investigation of the Vulcan Iron Formation: Unveiling Paleoproterozoic Paleoenvironments 2:10 *Zsuzsanna ALLERTON, George HUDAK, Christian TEYSSIER, Annia FAYON, Martin DANIŠIK, Liam COURTNEY-DAVIES, and Phillip LARSON Geochronology and geochemistry of hematite ore in northeastern Minnesota xxxv �2:30 Joyashish THAKURTA and Beau HAAG Sulfur-isotope ratios in Paleoproterozoic Michigamme Formation at the Lake Superior Region: Implications on basin evolution and ambient seawater composition in the Greater Animikie Basin 2:50 *Jordan PETERZON, Noah PHILLIPS, Pete HOLLINGS, and Lionnel DJON Deformation conditions, micromechanics, and fault zone development in mafic protoliths at the Lac des Iles mine, northwestern Ontario 3:10 END OF TECHNICAL SESSION III 3:10-3:30 COFFEE BREAK TECHNICAL SESSION IV – POSTER PRESENTATIONS Session Chair: Allan Blaske and Patty Cobin 3:30-5:00 AUTHORS PRESENT AT THEIR POSTERS 5:00 END OF TECHNICAL SESSION IV Thursday evening May 15, 2024 6:00 pm RECEPTION AND CASH BAR (2nd floor, Michigan Tech Memorial Union) 7:00 pm ANNUAL BANQUET (2nd floor, Michigan Tech Memorial Union) 2024 Goldich Medal Recipient: Suzanne W. Nicholson Banquet Speaker: Robert M. Hazen, Carnegie Institution for Science “Mineral Informatics: A New Frontier in Understanding Earth” xxxvi �Friday - May 17, 2024 All vehicles need a parking pass available from co-chairs 8:15 INTRODUCTORY REMARKS AND UPDATES (2nd floor, Michigan Tech Memorial Union) Ted Bornhorst, Erika Vye, Patty Cobin, and Jim DeGraff Co-Chairs, 2024 ILSG TECHNICAL SESSION V – ORAL PRESENTATIONS Session Chair: Bernie Saini-Eidukat 8:20 *Farhan Ahmed BHUIYAN, Latisha BRENGMAN, and Esther STEWART Assessing depositional and post-depositional mineral associations in the <1.71 Ga Freedom Formation, Baraboo, WI, USA. 8:40 Jack MALONE, Ryan CLARK, Amira HARRIS-BOMMARITO, and David MALONE Baraboo Interval Quartzites in Iowa: Reassessing the Origin and Provenance of the Washington County Quartzite, SE Iowa 9:00 Gordon MEDARIS, Chloe BONAMICI, Phil BROWN, Laurel GOODWIN, Brian JICHA, Brad SINGER, Michael SPICUZZA and John VALLEY, The Evolution of Baraboo Interval Sedimentary Rocks: Deposition at 1.63 Ga and Metamorphism at 1.47 Ga 9:20 Amy Radakovich BLOCK, George HUDAK, and Kate SOUDERS Insights into the southwestern Superior Province: New igneous geochronology and geochemistry in northwestern Minnesota, USA 9:40 Ryan CLARK, David PEATE, Allison KUSICK, Kenny HORKLEY, and Chris MACFARLANE Baddeleyite age reveals timing of the Northeast Iowa Intrusive Complex (NEIIC) 10:00 END OF TECHNICAL SESSION V 10:00-10:20 COFFEE BREAK TECHNICAL SESSION VI – POSTER PRESENTATIONS Session Chair: Allan Blaske and Patty Cobin 10:20-11:40 AUTHORS PRESENT AT THEIR POSTERS 11:40 END OF TECHNICAL SESSION VI 11:40-1:00 LUNCH BREAK xxxvii �TECHNICAL SESSION VII – ORAL PRESENTATIONS Session Chair: Allan MacTavish 1:00 Jim MILLER and John GREEN Two decades of teaching the geologic heritage of Minnesota’s North Shore at the North House Folk School, Grand Marais 1:20 Erika VYE, Daniel LIZZADRO-MCPHERSON, and James JUIP The Keweenaw Geoheritage Summer Internship Experience 1:40 Eric NOWARIAK, S, Allison SEVERSON, and Amy Radakovich BLOCK Lithostratigraphy and Geochronology of the Lower Northeast Sequence of the North Shore Volcanic Group, Cook County, MN, USA 2:00 Pete HOLLINGS and Mark SMYK New Insights into the Geology and Geochemistry of the Osler Group and Related Rocks, Midcontinent Rift System, Northern Lake Superior, Ontario 2:20 David GOOD MCR Synthesis 1. Characterizing the MCR mantle plume 2:40 END OF TECHNICAL SESSION VII 2:40-3:00 COFFEE BREAK and TAKE DOWN POSTERS TECHNICAL SESSION VIII – ORAL PRESENTATIONS Session Chair: Amy Radakovich Block 3:00 Bill ROSE and James DeGRAFF Lidar Topography: Bright opportunity for reading Keweenaw Landscapes 3:20 Wouter BLEEKER, Natasha WODICKA, Sandra KAMO, Michael HAMILTON, Quinn EMON, and Jennifer SMITH The Lake Superior area “event layer”: Testing the connection with the Sudbury impact 3:40 Tien GRAUCH, S. HELLER, Laurel WOODRUFF, and Esther STEWART Revisiting geophysical interpretations of the Midcontinent Rift below Lake Superior— Insights from GLIMPCE seismic-reflection line C 4:00 Aaron HIRSCH Recent developments on the use of the Horizontal-to-Vertical Spectral Ratio (HVSR) passive seismic method to determine depth to bedrock in Minnesota 4:20 END OF TECHNICAL SESSION VIII xxxviii �4:20 Presentation of Student Awards Best Student Paper Awards – Stacy Saari Student Travel/Participation Awards – Ted Bornhorst 4:40 Concluding Remarks and Field Trips Ted Bornhorst, Erika Vye, Patty Cobin, and Jim DeGraff Co-Chairs, 2024 ILSG END OF TECHNICAL SESSIONS OF THE 70th ANNUAL MEETING Friday Evening May 17, 2024 7 pm ATDC Building across the parking lot from the A.E. Seaman Mineral Museum 2024 Edith D. and E. Wm. Heinrich Lecture “Mineral Evolution: A Case Study of a New Natural Law" by Robert M. Hazen Sponsored by the Edith D. and E. Wm. Heinrich Mineralogical Research Foundation and the A. E. Seaman Mineral Museum Saturday May 18, 2024 Field trips begin and end at the Michigan Tech Memorial Union Parking pass not needed on weekend. 8:00 am – 5:00 pm POST-MEETING FIELD TRIPS Trip 4: Keweenaw Fault Geometry and Kinematics: Clues to Its Nature and Origin Jim DeGraff, Katherine Langfield, and Dan Lizzadro-McPherson (Michigan Tech) Trip 5: Adventure Mine, Ontonagon County, Michigan: Geology and History of a Native Copper Mine Matt Portfleet (Adventure Mining Company), Ted Bornhorst (Michigan Tech) Trip 6: Southern Complex Granitoids, Gneisses and Migmatites: New Data, Discoveries, and Perspectives Chad Deering (Michigan Tech) Trip 7: Landslides in the Glacial Lake Ontonagon Sediments Stan Vitton (Michigan Tech) xxxix �POSTER PRESENTATIONS * Denotes a student eligible for Best Student Paper Award. To be eligible students must have graduated no more than one month before the ILSG meeting, be first author, and present the paper at the meeting + Denotes author that will present the paper if different than the first author. Numbered Posters and Abstracts are in sequential order Poster Number 1. withdrawn 2. Sheree HINZ GeologyOntario: a powerful search tool for Ontario explorationists Therese PETTIGREW, Robert CUNDARI, Rebecca PRICE, and Manuel DUGUET Identification of Fertile Parent Granitoid Units in the Superior Province of Ontario withdrawn 3. Mia MORSON and + Shannon ZUREVINSKI Quartz trace element chemistry: Exploring the link between a fertile parental granite and a mineralized pegmatite 4. *Kevin MEXIA, Pete HOLLINGS Geochemistry of Midcontinent Rift-related intrusive rocks of the Sunday Lake intrusion 5. Justin JONSSON, Paul MALEGUS, Sophie CHURCHLEY, and Rebecca PRICE Characterizing the geochemistry and nickel-copper-platinum group elements potential of mafic and ultramafic intrusions in northwestern Ontario withdrawn 6. *Andrea Paola CORREDOR BRAVO, Pete HOLLINGS, Matthew BRZOZOWSKI, and Geoff HEGGIE Magmatic and hydrothermal evolution of the Mesoproterozoic Current ultramafic PGECu-Ni deposit within the Thunder Bay North Intrusive Complex: insights from trace elements, Nd, Sr, O, and H isotopes 7. Max LAXER and + David GOOD Building a 3D model for Cu/Pd inflection points throughout the Marathon PGE-Cu Deposit 8. *Vlad SHESHNEV, Pete HOLLINGS, Noah PHILLIPS, Ryan WESTON, Matt DELLER, and Dana CAMPBELL Geochemistry and Petrology of the Eagle’s Nest Intrusion, McFaulds Lake Greenstone Belt, Northern Ontario xl �9. *Yiruo XU and Robert HOLDER Cooling of an Archean metamorphic terrane: garnet diffusion study of the Quetico Subprovince, Canada 10. *Clare BEAUDRY, Madelyn HESS, Cristian PEREIRA, and Bernhardt SAINI-EIDUKAT Petrology and geochemistry of Precambrian basement rocks in Walsh County, North Dakota 11. *Cristian PEREIRA, Timothy NESHEIM, Jeffrey D. VERVOORT, and Bernhardt SAINI-EIDUKAT Major element geochemistry and first zircon U-Pb age dates of Precambrian basement rocks in eastern North Dakota 12. Jamey JONES, Bill CANNON, Ben DRENTH, and Paul O’SULLIVAN Geologic and tectonic implications of detrital zircon U-Pb ages from the Dickinson Group in the western Upper Peninsula of Michigan, USA 13. Bill CANNON, A. SOUDERS, Ben DRENTH, and Robert AYUSO The Sacred Heart Orogeny in Michigan: Latest Archean Granites and the Great Lakes Tectonic Zone 14. A. SOUDERS, W. CANNON, B. DRENTH, R. SALERNO, J. THOMPSON, and P. SYLVESTER New LA-ICP-MS U-Pb geochronology of Archean rocks, central Upper Peninsula, Michigan, USA: a step toward refining the final assembly of the Superior craton 15. *Zsuzsanna ALLERTON, Annia FAYON, George HUDAK, Christian TEYSSIER, Liam COURTNEY-DAVIES, and Martin DANIŠIK Geochronology campaign in northeastern Minnesota 16. Ross SALERNO, Bill CANNON, Amanda SOUDERS, Jay THOMPSON Understanding the evolution of the upper Midwest Archean gneiss dome corridor using apatite, titanite, and monazite LA-ICP-MS U-Pb geochronology and microstructural analyses 17. *Trent EDIGER and Marcia BJØRNERUD Glimpses of a Paleoproterozoic landscape: Analysis of exhumed topography on Archean basement rocks northwest of Marquette, Michigan 18. Rebecca STOKES, Bill CANNON, and Ross SALERNO Characteristics of graphitization across a metamorphic gradient in the Michigamme Formation of the Marquette Trough and Baraga Basin, MI 19. Tom BUCHHOLZ, Alexander FALSTER, and Wm. SIMMONS Preliminary mineralogy of a pegmatite in the pyroxene syenites of the Stettin Complex, Wausau Complex, Marathon County, Wisconsin xli �20. *Katherine LANGFIELD, Nolan GAMET, James DeGRAFF Cross-sectional Geometry of the Keweenaw Fault System between Hancock and Mohawk, Upper Peninsula of Michigan 21. *Braxton MURPHY, Katherine LANGFIELD, and James DeGRAFF Geometry, Slip Kinematics, and Deformation along the Hancock Fault in the Quincy Mine Workings, Upper Peninsula of Michigan 22. *Kenz CARLTON, Basil TIKOFF, and Esther STEWART The Honey Creek Structure, Sauk County, Wisconsin: Asymmetric Faulting Associated with Seismic-Induced Fluid Escape 23. *Alex LAWRENCE, Adam VANDERKIN, and Robert LODGE Volcanic and Hydrothermal Reconstruction of the Paleoproterozoic Butler Zn-Cu occurrence, Clark County, Wisconsin 24. *Lyndsie VICKERS, and Robert LODGE Petrology and Geochemistry of Felsic Magmatism in the Paleoproterzoic Eau Claire Volcanic Complex, Northcentral Wisconsin 25. *Dan SHAKKED, Lucas ROBARGE, and Robert LODGE Analysis of deformation-related structures in the Eau Claire Volcanic Complex, Wisconsin 26. *Gwendolyn MARTIN and Marcia BJØRNERUD Investigating the origin of pervasive breccias in the Paleoproterozoic Saunders Formation in northern Wisconsin 27. Aaron HIRSCH, Emma SCHNEIDER Lithostratigraphic discrimination of Quaternary core in Minnesota using magnetic susceptibility 28. Bill ROSE and Erika VYE Michigan Coastal Path: A Social Commitment to Geoeducation 29. Bill ROSE and Erika VYE Jacobsville geoheritage is globally celebrated and locally loved 30. *Alice MARTIN, Zsuzsanna ALLERTON, Emma JOHNSON, Annia FAYON, Jim ESSIG, Sarah GUY-LEVAR,George HUDAK The Soudan Geology Trail Project: Let’s talk about rocks in northeastern Minnesota A tribute to Jean Peterman Kemp Zimmer and Jeanne Seaman Farnum by the A.E. Seaman Mineral Museum: Trailblazers for Women in Geology xlii �ABSTRACTS xliii ��Rock magnetic investigation of the Vulcan Iron Formation: Unveiling Paleoproterozoic Paleoenvironments AHRENDT, Gabriel1 and SMIRNOV, Aleksey1,2 1 Department of Geological Mining and Engineering Sciences, Michigan Technological University, 1400 Townsend Dr, Houghton, MI 49931 2 Department of Physics, Michigan Technological University, 1400 Townsend Dr, Houghton, MI 49931 The Paleoproterozoic (~1.88 Ga) Vulcan Iron Formation, located in the Southwestern Upper Peninsula of Michigan, is a significant Superior-type Banded Iron Formation, comprising four main members. The lower, Traders Member is characterized by banded ferruginous-siliciclastic layers with distinct alternating layers of ferric iron. The middle, Brier Member is a fissile slate with varying concentrations of magnetite from low to locally enriched. The upper, Curry Member, is an oolitic iron formation enriched with specular hematite and lacking noticeable banding. In some locations, the Curry Member is overlain by the ferric slate referred to as the Loretto Member. We conducted comprehensive rock magnetic investigations of three lower formation members, using thermal demagnetization of natural remanent magnetization, magnetic hysteresis and first-order reversal curve measurements, and thermomagnetic analyses. Our findings suggest that the members may be genetically distinct, reflecting shifts in depositional regimes that dramatically affected their texture and mineralogy. The Traders Member, characterized by abundant small paramagnetic grains, likely formed during a period of rapid subsidence and soluble transport of ferrous iron into a euxinic basin, followed by alternating periods of CO2 fixing and sulphide-oxidizing cyanobacteria. The subsequent transition to a shallow, foreshortened basin as the Pembine-Wasau terrane accreted led to the increased silica saturation and local concentration of superparamagnetic ferrous iron mud, forming the Brier slate. A further evolution due to the flooding of a shallow sea inducing high turbidity and increased oxygenation in the water column, resulted in the formation of the Curry Member, marked by a mix of magnetically hard minerals, including specular hematite. We speculate that, subsequently, a decrease in sea level, associated with the basin’s contraction, created conditions conducive to high silica input from continental margins and a change in the biotic regime which reduced the formation of granules and led to the creation of the Lorretto slate member. 1 �Geochronology campaign in northeastern Minnesota ALLERTON, Zsuzsanna1, FAYON, Annia1, HUDAK, George1, TEYSSIER, Christian1, COURTNEY-DAVIES, Liam2, and DANIŠIK, Martin3 1 Earth and Environmental Sciences, University of Minnesota, Minneapolis, MN 55455, USA Geological Sciences, University of Colorado, Thermochronology Research and Instrumentation Lab, Boulder, CO 80309-0399, USA 3 School of Earth and Planetary Sciences Department, John de Laeter Centre, Curtin University, Perth, WA 6845. Australia 2 Northeastern Minnesota is known from volcanic rocks along the north shore of Lake Superior and the intrusive suit of the Duluth Igneous Complex (DC) to its west with associated Ni-Cu-PGE mineralization (Leu, 2015; Miller, 2002). These lithologic units are part of the ~1100 Ma Midcontinent Rift System (MRS). During the rifting event, the hot (~1000°C) DC was emplaced into the “cold” Neoarchean (~2700 Ma) monzo-granitic Giants Range Batholith (GRB) footwall, resulting in contact metamorphism, sulfide mineralization (Benko et al., 2015), and hydrothermal activity. The focus of this project is to constrain the hydrothermal effect of DC emplacement and other regional events by tracking isotopic, chemical, and textural changes in accessory minerals zircon and apatite along a transect from the DC/GRB contact westward into the Archean basement. Samples in this study are collected from the GRB and the Purvis Lake tonalite. Six of the GRB samples were analyzed for U-Pb dating of in-situ apatite, and twelve GRB samples and one Purvis Lake tonalite sample were disaggregated to isolate individual zircon grains to acquire UPb radiometric dates (Figure 1). U-Pb data were obtained by Laser Ablation-Inductively Coupled Figure 1: Simplified geologic map of Minnesota's arrowhead region showing the study area (yellow inset). Samples are numbered within lithological units of the Giants Range Batholith (GRB) and Purvis Lake tonalite. Maps are modified from Griffin and Morey (1969) and Peterson and Jirsa (1999). 2 �Plasma Mass Spectrometry (LA-ICP-MS) at laboratories of University of Santa Barbara (in-situ apatite) and University of Colorado, Boulder (zircon separates). Results show bimodal dates signifying crystallization age and the timing of hydrothermal alteration. In-situ apatite U-Pb yields 2594.3 ± 30.8 Ma ages at ~ 2 km from the contact, and the rest of the apatite U-Pb dates suggest Pb-loss as a function of hydrothermal alteration from ~1067.24 ± 7.64 Ma to ~1084.89 ± 9.23 Ma within ~1 km of the contact. Zircons separated from twelve samples yield U-Pb crystallization ages ranging from ~2640 to 2700 Ma, and lower intercepts ranging from ~700 to 1200 Ma with uncertainties of ~4-200 Ma. The average lower intercept is ~1150 Ma, and we interpret these ages to record hydrothermal activity-induced Pbloss. Crystallization age and error increase, and the timing of hydrothermally driven Pb-loss becomes more elusive with distance from the contact. The tonalite ~20 km from the DC/GRB contact yields a crystallization age of 2708 ± 25 Ma and displays Pb-loss at 1140 ± 116 Ma. The large uncertainty associated with Pb-loss might be suggestive of multiple hydrothermal events. These data are consistent with hematite (U-Th)/He dates of the massive hematite ore bodies of Soudan Iron Mine, ~40 km (map distance) from the DC/GRB contact, that record hydrothermal alteration at ~ 1100 Ma (see Allerton at al., 2024 “Geochemistry and geochronology of hematite ore in northeastern Minnesota”, ILSG 2024). References Benko, Z., Mogessie, A., Molnar, F., Severson, M., Hauck, S., & Raic, S., 2015. Partial melting processes and Cu-Ni-PGE mineralization in the footwall of the South Kawishiwi Intrusion at the Spruce Road Deposit, Duluth Complex, Minnesota. Economic Geology and the Bulletin of the Society of Economic Geologists, 110(5), 1269-1293. Leu, A., 2016. Geology and Petrology of the Wilder Lake Intrusion, Duluth Complex, Northeastern Minnesota [thesis]. Miller, J., & Minnesota Geological Survey, 2002. Geology and mineral potential of the Duluth complex and related rocks of northeastern Minnesota. Report of investigations (Minnesota Geological Survey; 58). Saint Paul: University of Minnesota, Minnesota Geological Survey. Peterson, D. M., and Jirsa, M.A., 1999. Bedrock geologic map and mineral exploration data, western Vermilion district, St. Louis and Lake Counties, northeastern Minnesota: MGS Miscellaneous Map M-98, scale 1:48,000. Griffin, W. L. and Morey, G. B., 1969. Geology of the Isaac Lake Quadrangle, St. Louis County, Minnesota. Published in Cooperation with the Minnesota Department of Iron Range Resources and Rehabilitation. Minnesota Geological Survey 5 P-8 Special Publication Series. University of Minnesota. 3 �Geochronology and geochemistry of hematite ore in northeastern Minnesota ALLERTON, Zsuzsanna1, HUDAK, George1, TEYSSIER, Christian1, FAYON, Annia1, DANIŠIK, Martin2, COURTNEY-DAVIES, Liam3, and LARSON, Phillip4 1 Earth & Environmental Sciences, University of Minnesota, Minneapolis, MN 55455, USA School of Earth and Planetary Sciences, John de Laeter Centre, Curtin University, Perth, WA 6845, Australia 3 Geological Sciences, University of Colorado Boulder, Thermochronology Research and Instrumentation Lab, Boulder, CO 80309-0399, USA 4 Earth and Environmental Sciences, University of Minnesota, Duluth, MN 55812, USA 2 The Neoarchean Lake Vermilion-Soudan Underground Mine State Park is known for its Algoma-type banded iron formation (BIF). The BIF encloses lens-shape high-grade (63-65% Fe) iron ore locally. The well-established view is that massive to semi-massive hematite ore bodies are the product of hydrothermal alteration of BIF, during which process hydrothermal fluids leached silica, resulting in volume reduction (production of vug spaces) and Fe-replacement (Gruner 1930; Klinger, 1960; Thompson, 2015). Studies have postulated that ore mineralization was syn- or post-depositional with BIF (Gruner, 1926; Thompson, 2015), but the absolute timing of hematite ore had not been established until now. Figure 1: The diagram illustrates hematite mineralization consistent with the timing of Yavapai and Mazatzal orogenies based on U-Pb ages, and Midcontinent Rift System signatures overprint mineralization ages with (U-Th)/He analysis. Presented is a novel technique based on coupled U-Pb and (U-Th)/He hematite radiometric dating (Courtney-Davies et al., 2022) to determine the formation age and thermal history recorded by hematite. Initial electron probe microanalyses (EPMA) allowed for hematite characterization (microcrystalline and microplaty) that helped locating inclusion-free mineral surfaces for radiometric age dating. U-Pb Laser AblationInductively Coupled-Plasma Mass Spectrometry results suggest Paleoproterozoic mineralization at 1740.4±72.5 Ma and 1640.8±47.2 Ma and (UTh)/He ages clustered at 1093.1±16.4 Ma, the latter indicating a hydrothermal overprint of the original mineralization event (Figure 1). We propose a regional scale model that describes the hydrothermal alteration of Archean BIF at ~1700-1600 Ma with the formation of hematite ore including the growth of microcrystalline then microplaty textures, followed 4 �by a thermal overprint at ~1100 Ma associated with the development of the Midcontinent Rift System (Figure 2). Figure 2: Schematic diagrams display S-N cross-section starting with A) pre-D2, showing Gafvert Lake sequence (GL) unconformably above the Soudan Member (SM) that is stratigraphically above the Lower Member (LM) of Ely Formation. B) D2 regional transpression resulting in right lateral shear zones within SM, constrained to 2685-2674 Ma from dating of regional metamorphic fabrics (Lodge et al., 2013). C) Orogenic magmatism—depicted by a mafic dike (MD) at 1700-1600 Ma—generates hydrothermal fluids, and shear zones are utilized for fluid flow and facilitate a 2-stage hematite ore mineralization (microcrystalline and microplaty). D) The Midcontinent Rift System at ~1100 Ma results in hydrothermal overprint of original mineralization recorded in hematite (U-Th)/He ages. Whole-rock — including major, trace and rare earth elements—lithogeochemical analysis has been performed on four iron formation and ore samples, and results are currently being processed. Klinger (1969) proposed a volume-to-volume replacement mineralization, while Thomson (2015) calculated 39% volume loss by silica leaching and 9% Fe mass gain with Fe replacement. Additionally, isocon analysis (Grant, 2005) is underway to better understand the relationship between mobile and immobile elements during ore mineralization and the paragenesis of hematite. References Courtney-Davies, L., et al., 2022. Hematite geochronology reveals a tectonic trigger for iron ore mineralization during Nuna breakup: Geology, v. 50, p. 1318-1323, doi: 10.1130/G50374.1. Grant, J.A., 2005, Isocon analysis: A brief review of the method and applications: Physics and Chemistry of the Earth, Parts A/B/C, v. 30, p. 997–1004, doi: 10.1016/j.pce.2004.11.003. Gruner, J. W., 1926. Hydrothermal alteration of iron ores of the Lake Superior type—a modified theory: Economic Geology, v. 32, p.121-130. Gruner, J.W., 1930. Hydrothermal oxidation and leaching experiments; their bearing on the origin of Lake Superior hematite-iron ores: Economic Geology, v. 25, p. 697-719. Klinger, F.L., 1960. Geology and ore deposits of the Soudan mine, St. Louis County, Minnesota [thesis]. Lodge, R.W.D., et al., 2013. New U-Pb geochronology from Timiskaming-type assemblages in the Shebandowan and Vermilion greenstone belts, Wawa subprovince, Superior Craton: implications for the Neoarchean development of the southwestern Superior Province, Precambrian Research, v. 235, p. 264-277. Schulz, K. J., 1982. The magmatic evolution of the Vermilion greenstone belt of Minnesota: Tectonophysics, v. 190, p. 233-268. Thompson, A., 2015. A hydrothermal model for metasomatism of Neoarchean Algoma-Type banded iron formation to massive hematite ore at the Soudan Mine, NE Minnesota [thesis]. 5 �Petrology and geochemistry of Precambrian basement rocks in Walsh County, North Dakota BEAUDRY, Clare1, HESS, Madelyn1, PEREIRA, Cristian1, and SAINI-EIDUKAT, Bernhardt1,2 1 Department of Earth, Environmental and Geospatial Sciences, 2Department of Chemistry and Biochemistry, North Dakota State University, Fargo, ND 58102, USA In 1977, thirty-two cores were drilled in eastern North Dakota and western Minnesota along the Red River, for the purpose of evaluating uranium potential (Figure 1). The project was funded by the Department of Energy and overseen by Bendix Corporation. A technical report (Moore, 1978), a M.S. thesis that focused on the weathered horizon at the top of the Precambrian bedrock (Kelley, 1980), and several ILSG abstracts were published. For this study, three cores from Walsh County, North Dakota were sampled at the North Dakota Geological Survey Drill Core Library (Grand Forks, ND). Samples were taken from RRVD #17, RRVD #18, and RRVD #19A to focus the study to Walsh County, ND. Figure 2 shows the lithology of the three cores and outlines sample locations. Figure 2: Stratigraphic column of RRVD drill core Precambrian layers. Black Xs indicate sample locations. Numbers to the left correspond to the XRF analysis in Table 1. Data taken from Moore (1979) and optical observations. Figure 1: Location map of Eastern North Dakota and Western Minnesota. Era of Precambrian Bedrock is outlined. Red River Valley Drill Cores are outlined. Petrography and whole rock geochemical analyses (Table 1) were carried out on Precambrian layers. Precambrian sediments are buried under younger layers in Eastern North Dakota, the sampled areas are underlain by Archean gneiss, (Klasner and King, 1986). RRVD #17 was characterized as quartz monzonite with heavier alterations of biotite and feldspar farther up in the core. The alterations may be due to stronger weathering agents on the paleoweathered horizon. RRVD #18 was characterized as a granodiorite with uniform foliation and mineral percentages throughout the drill core. RRVD #19A was characterized as a gneissic granite with 6 �higher foliation as the sample increases in depth. The top of the cores is bleached, likely an effect of paleoweathering processes. Analyses were plotted on AFM and TAS diagrams (Figure 3). Table 1: RRVD #17-785, 2: RRVD #18-645.5, 3: RRVD #18-655.5, 4: RRVD #19A-1284.5, 5: #19A1291, 6: #19A-1296.5. Chemical data from NDSU XRF analysis. wt% 1 2 SiO2 59.1 69.2 TiO2 0.58 Al2O3 Fe2O3 MnO 3 4 5 6 71 68.2 73.5 73.6 0.39 0.32 0.31 0.21 0.22 23.6 14.7 13.8 20.4 13.5 13.1 6.77 0.07 3.55 0.05 3.06 0.04 3.53 0.03 2.54 0.03 2.57 0.03 MgO 2.25 1.19 1.04 0.87 0.44 0.44 CaO 3.44 3.83 3.44 N.D. 3.12 2.8 Na2O 5.32 5.23 5.51 N.D. 4.82 4.32 K2O 1.84 1.46 1.32 6.4 1.36 2.47 P2O5 0.20 0.15 0.13 0.07 0.09 0.07 Total 103.1 99.75 99.66 99.81 99.61 99.62 Figure 3: Classification diagrams for measured samples. REFERENCES: Kelley, L.I., 1980, Kaolinitic weathering zone on Precambrian basement rocks, Red River Valley, eastern North Dakota and northwestern Minnesota. M.S. Thesis, University of North Dakota. 85 pp. Klasner, J.S. and E. R. King. 1986. Precambrian basement geology of North and South Dakota. Canadian Journal of Earth Sciences. 23(8): 1083-1102. https://doi.org/10.1139/e86-109 Moore, W. L., 1978, A preliminary report on the geology of the Red River Valley Drilling Project, eastern North Dakota and northwestern Minnesota: Bendix Field Engineering Company Subcontract H77-059-E, 292p. https://www.osti.gov/biblio/6538603 doi:10.2172/6538603. 7 �Geophysical imaging of the Paleoproterozoic Animikie basin in Minnesota BEDROSIAN, Paul A., PETERSON, Dana E. and HOOGENBOOM, Bennett E. U.S. Geological Survey, Bldg 20, MS 964, Denver Federal Center, Denver, CO 80225 The 1.88-1.83 Ga Penokean orogen is preserved as a discontinuous fold belt stretching nearly 1500 km from central Minnesota to eastern Ontario. In Minnesota, the supracrustal sequence occupies a NW-facing salient broadly divided into a southern fold-and-thrust belt and a northern tectonic foredeep. The former consists of volcanic and sedimentary rocks in several structural panels while the latter - the ‘main bowl’ Animikie basin (AB) - consists of thick sedimentary sequences and is one of the least deformed remnants of this former continentalmargin. Metasedimentary rocks of the AB include chemical sedimentary rocks (e.g., iron formation of the Mesabi iron range) and turbidites of the Virginia and Thomson Formations. The latter are an important source of sulfur for Ni-Cu mineralization within the intruding Duluth Complex and satellite intrusions. The USGS has been collecting geophysical data in the AB and surrounding areas, including airborne electromagnetic, broadband and nodal seismic, and magnetotelluric data. These data and resulting models reveal the main bowl AB to be more complex than suggested from surface geological mapping. High-electrical conductivity is mapped throughout the basin, including along its northern edge where it is linked to the gently dipping bedded-pyrrhotite-unit and along the steeply dipping SW edge of the Duluth Complex, where it may reflect a hornfels zone formed during contact metamorphism. At the basin scale, a discontinuous bowl-shaped high conductivity zone extends to ~5 km depth. This intra-basin conductor shows some relation to deformation boundaries, such as a demarcation between rocks exhibiting folding and cleavage and those that do not. Some deep (>5 km) geophysical variations are likely related to structural variations within the Archean basement or within thrust panels inferred to project some distance beneath the basin. Where exposed, strong conductors within some of the adjacent thrust belts suggest a correlation with metamorphic grade. Elevated conductivity can, in most cases, be related to a combination of metallic sulfides and graphite. A 9-20 km deep, steeply dipping conductive band is also imaged internal to the Duluth Complex and adjacent to modeled high-density bodies interpreted as magmatic feeder zones. We interpret this conductor as a remnant of AB metasediments preserved within the complex and speculate that the AB played an important control on magma emplacement. 8 �Assessing depositional and post-depositional mineral associations in the <1.71 Ga Freedom Formation, Baraboo, WI, USA. BHUIYAN, Farhan Ahmed 1, BRENGMAN, Latisha 1, and STEWART, Esther 2 1 University of Minnesota Duluth, Earth & Environmental Sciences Department, University of Minnesota Duluth, 1114 Kirby Drive, Heller Hall 229, Duluth, MN 55812. 2 Wisconsin Geological and Natural History Survey, University of Wisconsin-Madison Division of Extension, 3817 Mineral Point Rd, Madison, WI 53705. Some geochemical proxy records suggest that the period following Earth’s initial rise in atmospheric oxygen ~2.4 billion years ago was marked by low, fluctuating oxygen levels (Lyons et al., 2014; 2021). Such conditions would likely have made the ocean’s photic zone inhospitable to multicellular life forms that require oxygen (e.g. Krause et al., 2022). Interpreting the trajectory of Earth’s oxygenation is complicated due to uncertainties in the diagenetic effects on redox proxy records and limited preservation, especially across the late Paleoproterozoic and early Mesoproterozoic (e.g. Slotznick et al., 2022). To fill in critical knowledge gaps in surface redox conditions and geochemical characteristics of Mid-Proterozoic depositional environments, we investigated the Freedom Formation, a <1.71 Ga and >1.47 Ga iron-rich chemical sedimentary unit preserved in historic drill cores near Baraboo, WI, USA (Stewart et al., 2021). Our goal is to decipher primary redox information that links back to the depositing fluid. To accomplish this goal, and separate primary depositional signatures from post-depositional overprinting, we integrate core observations, mineral, petrographic, and geochemical datasets. The Freedom Formation includes a lower unit composed of thin-bedded, interlaminated, fine-grained clastic and chemical sediments and an upper unit composed of dolomite. We document a coarsening upward sequence in the lower Freedom Formation, accompanied by mineralogical changes in three drill cores (H122, H22, and H23). Two of the cores (H122 and H22) show a transition in mineralogy from a base assembly of chamosite, quartz, and magnetite to a Mn-carbonate and hematite-dominant assembly towards the top of the sections. This mineralogical transition is accompanied by an increase in the proportion of sand-sized material, a decrease in mud-sized material, and a noticeable transition to carbonate. Veining and disrupted beds occur throughout all the cores. Whole rock geochemical samples targeting carbonate-rich beds across the lower Freedom Formation indicate a decline in clastic contamination up section, marked by falling Al2O3 concentrations and reduced Zr/Hf ratios. Positive shale normalized Eu/Eu*SN anomalies indicate a role for hydrothermal fluids in precipitation of the authigenic mineral phases. Throughout the lower units, anoxic conditions are dominant, indicated by positive shale-normalized cerium (Ce/Ce*SN) anomalies. Interpreting the observed mineralogical transition in drill cores and the geochemical dataset requires detailed, systematic petrographic observation to distinguish the relative order of events and develop a paragenetic sequence. We identify texturally early minerals based on criteria outlined in LaBerge (1964) and separate those from post-depositional phases to interpret the history of the unit. To classify as a texturally-early phase, minerals must meet the following criteria: 1) be very fine-grained (where no grain size reduction can be attributed to metamorphism); 2) form even and consistent grain size distributions throughout the sample; 3) form the main component of granules or mud-sized particles in fine-grained layers characterized by a granular or particulate textural pattern; and 4) be associated with sedimentary features like 9 �bedding. From this work, we identified quartz and chamosite as the texturally earliest phases in the lowermost Freedom Formation. Towards the top of the lower Freedom Formation, carbonate phases were most commonly identified as texturally earliest. Additionally, the following key observations were made: (1) if quartz is not present, chamosite is the texturally earliest phase; (2) if chamosite is not present, then carbonate is the texturally earliest phase; (3) nano-scale hematite exists at boundaries of chamosite, quartz, carbonate, and stilpnomelane crystals, and within veinlets; (4) euhedral magnetite cross-cuts all other phases, and is often associated directly with chamosite; and (5) large hematite sometimes crosscuts small magnetite, or forms oxidized rims on euhedral magnetite crystals; (6) multiple generations of quartz and oxides exist; and (7) at least two types of carbonate are present (Fe- and Mn-rich and poor). Combining core and mineral datasets, we note that because multiple generations of oxides are present Figure 1: Reflected light photomicrograph of slide no: H122 538 (Fig. 1), post-formational fluid FF (50X) documenting oxidized hematite rims on magnetite flow may directly connect to crystals. observed redox changes in oxide phases. The most critical of these post-depositional observations is the oxidation of magnetite rims (Fig. 1). Overall, across all the cores, independent of redox changes observed in oxide phases, we note a transition in texturally early phases from reduced fine-grained, Fe2+- containing minerals to Mn-Carbonate. This mineralogical transition is marked by anoxic geochemical signatures and possibly indicates minor variations in oxygen conditions during the formation of the mineral phases preserved in the Freedom Formation. References: Krause, A. J., W. Mills, B. J., Merdith, A. S., Lenton, T. M., & Poulton, S. W. (2022). Extreme variability in atmospheric oxygen levels in the late Precambrian. Science Advances. https://doi.org/abm8191 LaBerge, G. L. (1964). Development of magnetite in iron formations of the Lake Superior region. Economic Geology, 59(7), 1313–1342. Lyons, T. W., Diamond, C. W., Planavsky, N. J., Reinhard, C. T., & Li, C. (2021). Oxygenation, life, and the planetary system during Earth’s middle history: An overview. Astrobiology, 21(8), 906–923. Lyons, T. W., Reinhard, C. T., & Planavsky, N. J. (2014). The rise of oxygen in Earth’s early ocean and atmosphere. Nature, 506(7488), 307–315. Slotznick, S. P., Johnson, J. E., Rasmussen, B., Raub, T. D., Webb, S. M., Zi, J. W., Kirschvink, J. L., & Fischer, W. W. (2022). Reexamination of 2.5-Ga “whiff” of oxygen interval points to anoxic ocean before GOE. Science Advances, 8(1), eabj7190. Stewart, E. K., Brengman, L. A., & Stewart, E. D. (2021). Revised Provenance, Depositional Environment, and Maximum Depositional Age for the Baraboo (< ca. 1714 Ma) and Dake (< ca. 1630 Ma) Quartzites, Baraboo Hills, Wisconsin. The Journal of Geology, 129(1), 1–31. 10 �The Lake Superior area “event layer”: Testing the connection with the Sudbury impact BLEEKER, Wouter1, WODICKA, Natasha1, KAMO, Sandra2, HAMILTON, Michael2, EMON, Quinn1, and SMITH, Jennifer1 1 Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8; wouter.bleeker@canada.ca Jack Satterly Geochronology Lab., University of Toronto, 22 Ursula Franklin St., Toronto, ON M5S 3B1 2 Following the initial publication of Addison et al. (2005) [1], there has been growing recognition of a major “event layer” near the top of the Gunflint Formation in the Thunder Bay area [e.g. 2,3,4], and at equivalent stratigraphic levels in the Marquette Range Supergroup of Michigan and Wisconsin [5,6]. Despite some initial hesitation [7,8], a consensus quickly emerged that this event layer represents the local manifestations, in the Lake Superior area, of the 1850 Ma Sudbury impact event that, ~600 km to the east, formed a multiring impact crater centered on the Sudbury area (see [9,10] for a recent review). The deformed and partially preserved remnants of this crater are known as the “Sudbury Structure” and, with a reconstructed final crater diameter of ~300 km, it represents the largest terrestrial impact crater known in the geological record [9]. As such, it would have had far-reaching effects extending out to several crater diameters from “ground zero”, in addition to a global fall-out layer of impact material (cf. the global K-Pg ejecta layer). A curious question, raised in Bleeker & Kamo (2022) [10], is why then this event layer is not more widely recognized in Canada, in places where ca. 1850 Ma basinal stratigraphy is reasonably well preserved (e.g., Mistassini Basin, Fox River Belt, Belcher Islands, Labrador Trough etc.)? Some of these localities are not much farther away from a “ground zero” near Sudbury. This has prompted us to undertake further tests of the putative link between the Lake Superior area event layer and the Sudbury impact structure. Our first test is to more precisely date, by CA-ID-TIMS, felsic ash layers in the lowermost Rove Formation, i.e. the first well-defined and well-preserved tuff layers overlying the event layer. Currently our results suggest the oldest of these tuff layers is ca. 1842 Ma, thus tightening the permissible time interval for the event layer to 1856-1842 Ma [cf. 1,2]. Our second test (in progress) is to attempt a precise CA-ID-TIMS zircon date of the ca. 1850 Ma Peavy Pond Granodiorite (which currently has a SHRIMP age with ±11 Myr uncertainty, see [11]). This granodiorite is known to intrude the lower Michigamme Slates, Baraga Group, in Michigan (W. Cannon, pers. comm. 2023), thus constraining a minimum age for the event layer. Our third and potentially most definitive test is to identify “tracers” in the event layer of the Lake Superior area that can be uniquely tied to target rocks of the Sudbury area. One such tracer would be 2460 Ma zircons from the Copper Cliff Rhyolite and its subvolcanic intrusions (Creighton and Murray granites) that are unique to the area and represent the final felsic rift volcanism/magmatism of the lowermost Huronian Supergroup [12,9,10]. For this test we processed a large bulk sample (~7.5 kg) of the event layer, with its diagnostic grey accretionary lapilli, from the HWY 588 locality west of Thunder Bay. Zircons were separated and mounted for SHRIMP U-Pb analysis at the Geological Survey of Canada, Ottawa. Eighty three zircon grains were spot dated, of which 71 returned high-quality results (Figure 1). Figure 1: U-Pb concordia plot for spot dates by SHRIMP on 71 zircon grains from the Gunflint event layer, HWY 588 roadside outcrop. 11 �The age distribution shows several well-defined clusters with 2(3) of the dated zircons defining a small but discrete subpopulation at ca. 2460 Ma. One of these zircons shows possible shock features (PDFs, planar deformation features; see Figure 1 inset) identified during picking. Although this particular grain is likely ca. 2460 Ma in origin, its result shows considerable discordance and should thus be treated with caution. Nevertheless, we think the 2460 Ma subpopulation uniquely ties fall-out material in the event layer, including rare shocked quartz grains [e.g. 2], to the Sudbury crater and its target rocks. In addition to the conclusive result of the 2460 Ma zircons, the data also identify a distinct ca. 2310-2320 Ma subpopulation of zircon grains that are known to first show up (in a regional stratigraphic sense) in the Gordon Lake Formation of the upper Huronian Supergroup. These zircon grains could have been delivered to the Thunder Bay area either as 1) ejecta from the Sudbury impact event, or perhaps more likely 2) as reworked detrital zircons from widespread felsic ash material that was deposited across the wider Superior craton at 23102320 Ma. Finding shock features in these grains would favour the first scenario, whereas a total absence of shock features would favour the second scenario. To further constrain the impact event, we also subsampled the large sample from the HWY 588 event layer into 6 small slabs with varying abundances of 1–3 cm accretionary lapilli (i.e. from ~5 to ~95 vol% lapilli) and analyzed these for major and trace elements, and for low-level PGE abundances. Results show a negative correlation between siderophile elements such as Ir (also Ru, Ni, Cr etc.) and lapilli abundance, indicating that the lapilli consist largely of diluting material and are not the optimum target for identifying the nature of the impactor [e.g. 13]. The highest Ir content of 0.3–0.4 ppb, i.e. ~1–2 orders of magnitude above average crustal values, actually occurs in laminated, dark, fine sand- to silt-size sediments that overlie the lapilli-rich horizon fall-out material (~0.5–1.0 m above). Future work will entail more detailed sampling of this overlying stratigraphy to identify the Ir peak and define the detailed mineralogy and Ir deportment in this material. Incidentally, values of 0.3–0.5 ppb Ir are also the maximum recorded values in the upper Onaping Formation filling the Sudbury crater and overlying its melt sheet [14]. From our initial results it appears that a maximum of impactor material condensed relatively late and was least diluted in fine grained fall-out material near the top of the event layer, well above the accretionary lapilli. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005. Geology, vol. 33(3), p. 193–196. Addison, W.D., Brumpton, G.R., Davis, D.W., Fralick, P.W., and Kissin, S.A., 2010. GSA Special Paper 465, p. 245–268. Jirsa, M.A., Fralick, P.W., Weiblen, P.W., and Anderson, J.L.B., 2011. GSA Field Guide 24, p. 147–169. Huber, M.S., McDonald, I., and Koeberl, C., 2014. Meteoritics & Planetary Science, vol. 49(10), p. 1749–1768. Pufahl, P.K., Hiatt, E.E., Stanley, C.R., Morrow, J.R., Nelson, G.J., and Edwards, C.T., 2007. Geology, vol. 35(9), p. 827–830. Cannon, W.F., Schulz, K.J., Horton Jr, J.W., and Kring, D.A., 2010. GSA Bulletin, vol. 122(1–2), p. 50–75. Kissin, S.A., and Fralick, P.W., 1997. Journal of the Royal Astronomical Society of Canada, vol. 91(5), p. 216. Kissin, S.A., Okamoto, M., Addison, W.D., and Brumpton, G.R., 2000. 46th Annual Meeting of the Institute on Lake Superior Geology, vol. 46, part 1, p. 31–32. Bleeker, W., and Kamo, S., 2022a. 68th Annual Meeting of the Institute on Lake Superior Geology, vol. 68, part 1, p. 5–6. Bleeker, W., and Kamo, S., 2022b. 68th Annual Meeting of the Institute on Lake Superior Geology, vol. 68, part part 2, Field Trip Guidebook, p. 4–57. Ayuso, R.A., Schulz, K.J., Cannon, W.F., Woodruff, L.G., Vazquez, J.A., Foley, N.K., and Jackson, J., 2018. 64th Annual Meeting of Institute on Lake Superior Geology, vol. 64, part 1, p. 7–8. Bleeker, W., Kamo, S.L., Ames, D.E., and Davis, D., 2015. Geological Survey of Canada Open File 7856, p. 151–166. Mougel, B., Moynier, F., Göpel, C., and Koeberl, C., 2017. Earth and Planetary Science Letters, vol. 460, p. 105–111. Mungall, J.E., Ames, D.E., and Hanley, J.J., 2004. Nature, vol. 429(6991), p. 546–548. 12 �Insights into the southwestern Superior Province: New igneous geochronology and geochemistry in northwestern Minnesota, USA BLOCK, Amy Radakovich1, HUDAK, George J.2, SOUDERS, A. Kate3 1 Minnesota Geological Survey, 2609 Territorial Road, St. Paul, MN 55114 University of Minnesota –- Twin Cities, 116 Church Street SE, Minneapolis, MN 55455 3 U.S. Geological Survey, Denver, CO 80225 2 The U.S. Geological Survey Earth Mapping Resources Initiative (Earth MRI) program recently funded acquisition of new airborne geophysical (Allen Langhans and Drenth, 2023), geochronologic, and geochemical data in a part of the Superior Province in northwest Minnesota that is prospective for numerous Archean critical-mineral-producing systems. The study area comprises three subprovinces of the Archean Superior Province (Fig. 1). Previous work in the area (Jirsa et al., 2012; Jirsa et al., 1999) has been severely limited by an absence of outcrop, sparse drill hole data, and the existence of only one geochronologic age. These new ages and geochemical analyses, obtained through the Earth MRI program, represent the first highresolution geologic data in the Neoarchean subprovinces of the southwestern Superior Province. Seven new U-Pb zircon LA-ICP-MS magmatic ages (Souders, in review) establish the timing of intrusive activity in the southwestern extent of both the Wawa and Wabigoon subprovinces. A biotite-hornblende tonalite in the Red Lake Falls pluton (207Pb/206Pb weighted mean age of 2701 ± 4 Ma, 2s), a biotite tonalite in the Snake River batholith (207Pb/206Pb weighted mean age of 2738 ± 12 Ma, 2s), and a diorite in the Grygla pluton (207Pb/206Pb weighted mean age of 2771 Ma ± 8 Ma, 2s) define three distinct Neoarchean episodes of intermediate intrusive activity near the present-day southern margin of the Wabigoon subprovince. A preliminary magmatic age from a small hornblende monzodiorite stock in the Wabigoon indicates ca. 2727 intermediatemafic intrusive activity. In the Wawa subprovince, a 207Pb/206Pb weighted mean age of 2702 ± 6.5 Ma age (2s) from the Fertile pluton biotite granodiorite indicates Neoarchean intermediate intrusive activity at the northern margin of the Wawa subprovince coincident with similar activity in the Wabigoon. A small mafic body that intrudes a mafic volcanic sequence in the Wawa yields a preliminary age of ca. 2690 Ma. Finally, a combined 207Pb/206Pb weighted mean age from two closely spaced anorthosite samples confirms a Neoarchean (2737 ± 4.5 Ma, 2s) age for the Mentor Anorthosite Intrusive Complex (MAIC). High-precision CA-TIMS U-Pb zircon analyses provide age constraints on supracrustal rocks in both subprovinces. Two trachyandesite lapilli tuff samples from the Wabigoon subprovince yield 207 Pb/206Pb weighted mean ages of ca. 2730 Ma and ca. 2733 Ma (Block et al., in prep. b), >25 Ma older than the few other volcanic ages from the Wabigoon in Minnesota. Two feldspathic wackes in the Wawa subprovince are still being processed for ages. Newly dated intermediate intrusions are LREE enriched and have arc-like trace element patterns. Discrimination diagrams indicate that these intrusions are generally I-type, calc-alkaline, volcanic arc-granites. Samples from the MAIC exhibit complex REE patterns, and their interpretation is less straightforward. The newly dated intermediate volcanic samples from the Wabigoon are also calc-alkaline and exhibit arc-like signatures. In combination with ~130 additional geochemical analyses and detailed petrography, results presented here provide significant insight into the tectonic evolution of the southwestern Superior Province and invite comparison with well-studied rock packages in the Wabigoon and Abitibi provinces in Canada. 13 �Figure 1. Bedrock geology map of northwestern MN, USA. Units within the project area (Block et al., in prep. a) are shown in the legend. Units outside the map area are from Jirsa et al. (2012). Newly obtained LA-ICPMS U-Pb ages are shown as yellow stars, and newly obtained TIMS U-Pb ages are shown as green stars. References Allen Langhans, A.D., and Drenth, B.J., 2023, Airborne magnetic and radiometric survey, northwestern Minnesota, 2021: U.S. Geological Survey data release, https://doi.org/10.5066/P97D2JJE. Block, Amy Radakovich, Drenth, Benjamin J., Souders, A. Kate, Hudak III, George J, Hirsch, Aaron C., and Saari, Stacy M., in prep. A, Geologic map of the Mentor Igneous Complex Focus Area, Northwest Minnesota: Minnesota Geological Survey, Miscellaneous Map Series M-200, scale: 1:100,000. Block, Amy Radakovich, Hudak III, George J, Souders, A. Kate, Drenth, Benjamin J., Schmitz, M., Hirsch, Aaron C., and Saari, Stacy M., in prep. b, Preliminary investigation of the geologic history and critical mineral potential of the Mentor Igneous Complex Focus Area, Northwest Minnesota: Minnesota Geological Survey, Report of Investigations 74. Jirsa, M. A., Boerboom, T. J., Chandler, V. W., 2012, Geologic Map of Minnesota, Precambrian Geology: Minnesota Geological Survey, Map S-22, 1:500,000. Jirsa, M. A., Chandler, V. W., and Runkel, A. C., 1999, Bedrock geologic map of northwestern Minnesota: Minnesota Geological Survey, Miscellaneous Map Series M-92, 1:200,000. Souders A.K., in review. U-Pb geochronology of the Mentor Anorthosite Intrusive Complex (MAIC) and regional plutonic units: U.S. Geological Survey data release, https://doi.org/10.5066/P9WMD477. 14 �Magmatic and hydrothermal evolution of the Mesoproterozoic Current ultramafic PGE-CuNi deposit within the Thunder Bay North Intrusive Complex: insights from trace elements, Nd, Sr, O, and H isotopes CORREDOR BRAVO, Andrea Paola1, HOLLINGS, Pete1, BRZOZOWSKI, Matthew1, and HEGGIE, Geoff2 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada Clean Air Metals, 1004 Alloy Drive, Thunder Bay, ON P7B 6A5 Canada 2 The Mesoproterozoic PGE-Cu-Ni enriched Current intrusion, part of the Thunder Bay North Intrusive Complex, is located 50 km northeast of Thunder Bay, Ontario. The northwest-trending intrusion is a 3.4 km long conduit-type that intruded the rocks of the Quetico Basin during the early stages of the Midcontinent Rift System (MRS; Woodruff et al, 2020). The intrusion has four mineralized zones; the Current and Bridge Zone in the northwest are characterized by shallow and thin features; in the middle lies the BeaverCloud Zone, characterized by its substantial thickness, while the southeast is the deepest 437Southeast Anomaly (SEA) Zone (Kuntz et al., 2022). The intrusion exhibits a primitive mantle- Figure 1. Schematic model of the Current intrusion and the Quetico country rock. normalized pattern resembling ocean island Illustration compiled in Leapfrog using data basalt, characterized by LREE enrichment and provided by Clean Air Metals Inc. small positive anomalies in Nb, La, and Ce, consistent with minimal continental crust contamination. The La/Smn values in the Current intrusion samples, ranging from 1.8 to 2.6, align with previous studies, indicating a basaltic magma derived from an enriched mantle plume. The enriched nature of the magma in the Current intrusion is consistent with other mineralized and unmineralized intrusions associated with the MRS (Escape, Seagull, Lone Island intrusion, and Nipigon Embayment; Heggie, 2005; Hollings et al., 2007b; Caglioti, 2023; Yahia, 2023). The intrusion has slightly lower Sri (0.7021 to 0.7043) and εNd (-1.18 to -4.02) values compared to typical the mantle source at 1100 Ma. Therefore, it is suggested that the plume-derived magma interacted with an enriched subcontinental lithospheric mantle, which may have contributed to the slightly negative εNd values of the intrusion.The stable isotope analysis data from the Current intrusion indicates an interaction between magmatic mantle-derived fluids (δ2H from −40 to −80‰, δ18O from 5.5 to 7.0‰), meteoric fluids (δ2H <-80‰, δ18O <5.5‰), and devolatilization/ dehydration fluids of the Quetico country rocks (δ18O >7‰). Three distinct domains within the intrusion were identified based on alteration intensity and micro-textural observations and each showing varying secondary mineral assemblages Domain A consists of antigorite, tremolite, clinochlore, epidote, pyrite, cubanite, millerite, secondary pyrrhotite ± chamosite ± sericite, and ± secondary magnetite, Domain B consists of 15 �lizardite-chrysotile, tremolite, clinochlore, epidote, pyrite, cubanite, millerite ± sericite, and ± secondary magnetite Domain C consists of talc and carbonates. Domain A and B have characteristics of interaction with meteoric, mantle, and/or subcontinental lithospheric mantle -derived fluids, whereas Domain C is associated with fluids from devolatilization of the country rock and is overprinted on Domains A and B. The alteration processes in the different domains Figure 2. δ18O and δ2H values of bulk rock in the four involved two distinct fluid types at mineralized zones of the Current intrusion (Current, varying temperatures, Domain A likely Bridge, Beaver-Cloud, and 437-SEA) and the involved higher temperatures (>300°C) surrounding country rock of the Quetico basin. and fluids rich in H2O. In contrast, domain B was altered by fluids at lower temperatures (<300°C). Later CO2-bearing fluids of Domain C overprinted earlier alteration at temperatures below 50°C. The alteration of the intrusion also resulted in significant volume reduction of primary sulfides and oxides that have been replaced by secondary minerals, such as chalcopyrite and pyrrhotite were replaced by secondary magnetite and pyrite and primary magnetite was replaced by pyrite and chamosite. References Caglioti, C. (2023). PGE–Cu–Ni sulfide mineralization of the Mesoproterozoic Escape intrusion, northwestern Ontario (MSc). Lakehead University, Thunder Bay, Ontario. https://knowledgecommons.lakeheadu.ca/handle/2453/5195 Heggie, G. (2005). Whole rock geochemistry, mineral chemistry, petrology, and Pt, Pd mineralization of the Seagull Intrusion, Northwestern Ontario. Lakehead University, Thunder Bay, Ontario. https://knowledgecommons.lakeheadu.ca/handle/2453/689 Hollings, P., Richardson, A., Creaser, R. A., and Franklin, J. M. (2007b). Radiogenic isotope characteristics of the Mesoproterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario. Canadian Journal of Earth Sciences, 44(8), 1111-1129. https://doi.org/10.1139/e06-128 Kuntz, G., Wissent, B., Boyk, K., Harkonen, H., Jones, L., Muir, W., Buss, B., and Peacock, B. (2022). NI 43- 101 Technical report and preliminary economic assessment for the Thunder Bay North Project, Thunder Bay, Ontario Woodruff, L. G., Schulz, K. J., Nicholson, S. W., and Dicken, C. L. (2020). Mineral deposits of the Mesoproterozoic Midcontinent Rift system in the Lake Superior region–a space and time classification. Ore Geology Reviews, 126, 103716 Yahia, K. (2023). Geochemistry, petrography, geochronology, and radiogenic isotopes of the weakly mineralized intrusions in Thunder Bay North Igneous Complex (MSc). Lakehead University, Thunder Bay, Ontario. https://knowledgecommons.lakeheadu.ca/handle/2453/5283 16 �Re-interpretation of hydrothermal alteration, mineralization and host-rock oxidation to form the Keweenaw native copper lodes, northern Michigan BROWN, Alex C. 13250 rue Acadie, Pierrefonds, Quebec, Canada, H9A 1K9, acbrown@polymtl.ca The sandstone/conglomerate-hosted portions of the native copper ores of northern Michigan (e.g., the Calumet and Hecla Conglomerate ores) occur mostly in deeply reddish sediments. In the immediate vicinity of native copper ores, the reddish sediments appear to have been hydrothermally bleached to salmon-red colors (Butler and Burbank, 1929; Cornwall, 1956; White, 1968; Weege and Pollock, 1972). This communication notes that fine-grained salmoncolored clastic sediments may host fine-grained disseminations of native copper enclosed by salmon-red aureoles, not unlike grey reduction halos commonly found in red sandstones (Figs. 1, 2). If the interpreted origin and preservation of reduction halos in red sandstones is applied to the native copper-hosting aureoles in the Calumet and Hecla Conglomerate, the deep reddening of the conglomerates hosting native copper of the Keweenaw Peninsula may be interpreted as a post-ore event. Redbed sandstones commonly show centimeter-scale reduction spots and blotches, e.g., rift-hosted Carboniferous clastic sediments of eastern Canada (Poll and Sutherland, 1976), the Permian fluvial Abo Formation of New Mexico (Bensing et al., 2005), and the Jacobsville sandstones of northern Michigan. Petrographic and chemical analyses of reduction spots in the Abo Formation indicate that those reduction spots have never been reddened – ferrous clastic grains within the reduction spots are still ferrous while similar grains in the enclosing red sandstone are oxidized (Bensing et al., 2005). Interpretation: wood trash in the cores of reduction spots maintained elliptical reducing conditions in the immediate vicinity of wood trash (i.e., within the grey halos), while oxidizing post-sedimentary water reddened all other portions of the clastic sediments. Figure 1. Carboniferous redbeds of Dorchester Cape, New Brunswick, Canada, showing abundant centimetric-scale reduction spots with dark-greyish cores centered on fossilized organic matter. Figure 2. Close view of greyish reduction spots in redbeds of Figure 1 (tip of hammer for scale). Cores of reduction spots contain fossil wood debris and base-metal sulfides e.g., chalcocite, partially oxidized to malachite). 17 �Native copper in the Calumet and Hecla Conglomerate occurs as interstitial fillings in conglomerates and as fine-grained disseminations in finer sandy sediments. Curiously, very finegrained disseminations of native copper in fine-grained sediments are observed to be surrounded by elliptical salmon-red sediment (Fig. 3). A possible, chemically justified interpretation: native copper was deposited with salmon-red alteration, mostly within highly permeable conglomeratic portions of the sandstone-conglomerates and also as very fine-grains in associated sandy sediments. Subsequently, all sandstone-conglomerates were thoroughly oxidized to their classic deep-red color during post-ore circulations of oxygenated ground water, except in the finergrained sediments where local salmon-red alteration was preserved against reddening by reduction-inducing fine grains of native copper. Post-ore deep-red oxidation of copper in the fine-grained sediment was inhibited by the poor permeability of this fine-grained sediment to late deep-reddening groundwaters, but also by the reducing property of metallic copper. Figure 3. Cut and epoxy-ed sample of Calumet and Hecla Conglomerate native copper ore. Upper half: Deep red, coarse conglomeratic sediment with coarse-grained native copper. Lower half: Fine-grained clastic sediment containing fine-grained native copper enclosed by “bleached” salmon-red alteration halos. Bleached halos and core native copper are equated here to reduction spots with fossil wood debris common in redbed sandstones (see text for explanation). References Bensing, J.P., Mozley, P.S., and Dunbar, N.W., 2005. Importance of clay in iron transport and sediment reddening: evidence from reduction features of the Abo Formation, New Mexico, U.S.A. Sedimentary Research, 75: 562–571. Butler, B.S. and Burbank, W.S., 1929. The copper deposits of Michigan. US Geol. Surv. Prof. Paper 144, 238 p. Cornwall, H.R., 1956. A summary of ideas on the origin of native copper deposits. Economic Geology, 51: 615–631. Weege, R.J. and Pollock, J.P., 1972. The geology of two new mines in the native copper district of Michigan. Economic Geology, 67: 622–633. White, W.S., 1968. The native-copper deposits of northern Michigan, in Ridge, J.D., ed., Ore Deposits of the United States, 1933–1967 (Graton-Sales Volume 1), American Inst. Min. Metall. & Petrol. Eng., 303–326. 18 �Preliminary mineralogy of a pegmatite in the pyroxene syenites of the Stettin Complex, Wausau Complex, Marathon County, Wisconsin Buchholz, Thomas 1, Falster, Alexander 2, and Simmons, Wm 2 1 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494 MP Research Group, Maine Mineral and Gem Museum, PO Box 500, 99 Main Street, Bethel, Maine 04217, USA 2 2 The Stettin Complex is the oldest (1565 +3-5 Ma, Van Wyck 1994) and most alkalic of the four intrusions that comprise the Wausau Syenite Complex, and is composed of various syenite phases. Recently an opportunity arose to examine the mineralogy of a pegmatite located in the pyroxene syenites of the Stettin Complex. Upper portions of the roughly horizontal pegmatite are below tilled soils but in-situ, are weathered, and fragments are coated with Fe-oxides and clays. Excavations over the last several years has exposed somewhat fresher material at depth and allowed better study. The pegmatite is zoned, but numerous included syenite screens complicate evaluation. Pegmatite-host syenite contacts are often sharp with no notable contact zones, suggesting relatively minor temperature contrast between the two phases, but thin 2-3 cm reaction zones are also common, with minor coarsening of feldspar and arfvedsonite compared to host syenite, small miaroles, scattered patches of abundant, tiny pink zircons and yet unidentified minerals, and in the freshest material, fluorite. The upper weathered portions of the dike, probably corresponding to border and intermediate zones, consist of anorthoclase (often showing “moonstone” visual effects; the recovery of these feldspars is the objective of those working the dike), highly altered former pyroxenes(?) with-sparse remnant hedenbergite, arfvedsonite, quartz, abundant clear pink to orange zircons and other accessory phases, including small miarolitic cavities. Per Medaris & Koellner (2010), pyroxenes in the Stettin complex range from Fe-rich diopside, to hedenbergite and aegirine. In this pegmatite pyroxenes appear to have been affected by late-stage oxidizing fluids, altering Fe2+-rich pyroxenes to Fe3+ rich smectite-group clays ± Fe-oxyhydroxides with sparse remnants of hedenbergite, while aegirine is absent from the dike. Ca released by pyroxene alteration may have contributed to the formation of various late-stage Ca-rich species. Deeper interior zones are mostly anorthoclase with arfvedsonite and other accessory minerals, with contact zones (or lack thereof) repeated around included syenite fragments. Graphic quartz-anorthoclase intergrowths “graphic syenite” are locally common, as are small miarolitic cavities. Several small-volume pegmatite units are: rare irregular patches of granular albite +- larger anorthoclase crystals, with abundant pyrochlore(?) crystals and possibly other species; and enigmatic thin 2-5 cm thick irregular veins or pods, mainly quartz, albite and anorthoclase: these are confined to the pegmatite and do not enter the host syenite, and includes sparse arfvedsonite, abundant cassiterite grains (≈2-4 μm), fergusonite, metamict zircons (some showing Hf-enrichment), sparse microlite (Ta-dominant pyrochlore group), sparse tantalite-(Mn) (Ta-Mn dominant columbite-group species), tiny grains of barite, and other yet-unidentified species. These anomalous pods or veins may be derived from highly fractionated late-stage differentiates. Perhaps high F activity supported unusual enrichment of HFSE in latecrystallizing melt. . 19 �Other accessory mineralogy of the main dike includes: Aeschynite-(Ce): Elongated, dark greyblack crystals in anorthoclase. Synchysite/Parisite: Small red to pinkish hexagonal crystals. High Ca contents suggest they are either synchysite or parisite. Bavenite(?): Small white bladed crystals included in clear quartz crystals; tentative ID based on their morphology and presence of Ca, Si and O (EDS). Calcite: White crusts and masses in vugs from lower portions of the excavation. Chevkinite-group(?): Dark grains with white borders; often heavily altered to soft, chalky, fine-grained niobian Ti-oxides with minor Th, Ca, LREE ± Si and Al. Columbite-group: Sparse columbite-(Fe) noted as inclusions in a porous fergusonite-(Y) grain. Fayalite: Uncommon gray radiating acicular crystals and glassy brown grains associated with arfvedsonite. Fergusonite-(Y): Small yellowish to reddish-brown tapering crystals in anorthoclase, arfvedsonite and miaroles. Ferro-anthophyllite: Uncommon, patches of white acicular crystals in smectite-rich altered pyroxenes. Fluorapatite: Sparse crystals in vugs with arfvedsonite. Fluorite: Late in vugs and isolated grains, likely often removed by weathering. Graphite: Sparse Thin hexagonal platy crystals, typically showing thin hexagonal overgrowths. Ilmenite: Common; thin black metallic plates with a pyrophanite component in anorthoclase and miaroles. Kainosite-(Y)(?): One off-white crystal in pegmatite, appears to be a Ca-Y-LREE silicate, may be kainosite-(Y). Magnetite: Common as irregular masses, rarely as well-formed octahedral crystals. Molybdenite: Sparse as thin soft hexagonal plates. Monazite-(Ce): Uncommon, small brick-red crystals in feldspar and in vugs. Niocalite(?): Yellow to pale yellow-brown elongated crystals in anorthoclase. Very sparse. Some compositions strongly suggest niocalite, others are yet-unidentified species. Pyrochlore: Rare: yellow-brown octahedral crystals. Quartz: Common, generally as a later-stage mineral. Siderite: now absent, but goethite pseudomorphs after probable siderite are common in small vugs. Thorite: Rare; red to red-black grains associated with fergusonite-(Y). Titanite: Uncommon, as brown to redbrown grains. Zircon: Abundant in upper intermediate zone, less so in coarse interior zones. As work continues, it is likely that additional phases will be identified, as there are a number of unknowns awaiting further work, and much material awaits cleaning and study. Thanks are due to Austin Gausmann, Bill Schoenfuss, and Trent and Shana Rebeck for access to the pegmatite. REFERENCES: Medaris, L. Gordon Jr., Koellner, Susan E., 2010. Ferromagnesian minerals in the Stettin Syenite Complex, Marathon County, Wisconsin: compositions and contrasts with the Wolf River Batholith (abstract): Institute on Lake Superior Geology Proceedings, 56th Annual Meeting, International Falls, MN, v. 56, part 1, p. 42-43. Van Wyck, N. 1994. The Wolf River A-type magmatic event in Wisconsin: U/Pb and Sm/Nd constraints on timing and petrogenesis (abstract): Institute on Lake Superior Geology, 40th Annual Meeting, Part 1, Program and Abstracts, p. 81-82. 20 �The Sacred Heart Orogeny in Michigan: Latest Archean Granites and the Great Lakes Tectonic Zone CANNON, W. F.1, SOUDERS, A. K2, DRENTH, Benjamin J.2, AYUSO, Robert A.1 1 U.S. Geological Survey, Reston, VA 20192, 2U.S. Geological Survey, Denver, CO 80225 The Sacred Heart Orogeny, as defined in Minnesota (Schmitz, et al., 2018), is the terminal Archean contractional and magmatic event during which the Minnesota River Valley Subprovince (MRVS) was sutured to the southern edge of the Superior craton along the Great Lakes Tectonic Zone (GLTZ), accompanied by voluminous granitic magmatism. The ages of those granites are tightly clustered from 2.58-2.6 Ga. In Michigan’s Upper Peninsula voluminous granites of the Southern Complex, the eastern extension of the MRVS, have ages very similar to those in Minnesota and were emplaced near and within the GLTZ during suturing. The granitic phase of the Bell Creek Gneiss (Cannon and Simmons, 1973), a coarse-grained, K-spar megacrystic granite, is well dated (Petryk, 2019; Barth, 2023). The average of ten dates is 2.56 Ga. An additional age from SHRIMP analyses (Ayuso, presented here) is 2584+5.3/-2.79 Ma, fully consistent with previous determinations. We have recently recognized and dated a batholithic-scale intrusion of medium-grained, massive, K-rich granite, informally the New Swanzy granite, that borders the granitic phase of the Bell Creek Gneiss to the east and is adjacent to and interacted with the GLTZ during suturing. A regionally extensive negative gravity anomaly south and southeast of the exposed granite suggests that the batholith is substantially larger than its exposed portion (see map below). Both granites intruded an older gneiss complex with ages ranging from 2.6-2.9 Ga. The Bell Creek granite has an internal foliation defined by alignment of K-spar megacrysts broadly conformable with trends of adjacent gneisses, and contains no angular xenoliths. In contrast, the New Swanzy granite, although compositionally uniform as judged in outcrop, has numerous pegmatitic segregations and dikes. It has sharp cross-cutting contacts with older gneisses and contains many xenoliths. These characteristics and several instances of dikes of New Swanzy-like granites cutting the Bell Creek, suggests that the New Swanzy granite post-dates the Bell Creek Gneiss, at least slightly, and has an intrusive contact with it. The New Swanzy granite has a complex interaction with sheared rocks of the GLTZ. The GLTZ is a 2.5 km-wide, SW-dipping zone of dextral shear-thrusting where rocks of the MRVS were thrust northward over the Superior craton (Sims, 1991). The age of suturing was suggested to be about 2.69 Ga. Close to the GLTZ, the New Swanzy granite has a shear foliation parallel to the GLTZ. Within the GLTZ areas of granite are enveloped in intensely sheared rocks whose protolith is uncertain. Many dikes and stringers of granite cut mylonitic foliation. These show varying intensity of deformation, but all were emplaced after the most intense shearing. An undeformed granite dike was intruded across shear foliation at 2559±19.5 Ma (U-Pb apatite). We consider this to set a minimum age for deformation in the GLTZ in this region. Thus, the New Swanzy seems best interpreted as a syntectonic granite emplaced adjacent to the active suture late in development of the GLTZ The data summarized here indicate the latest Archean tectonic and intrusive events in northern Michigan and Minnesota are correlative. Thus, we deem it appropriate to extend the Minnesota-derived term “Sacred Heart Orogeny” to the culminating phase of development of the Southern Complex in Michigan. Because rocks of the Southern Complex are exposed in direct contact with the GLTZ, they present a unique opportunity to study the interaction of intrusion and suturing of the MRVS to the Wawa-Abitibi terrane. 21 �A-Massive New Swanzy granite cutting country rock gneiss. B- GLTZ mylonite cut by granitic stringers with varying degrees of deformation. C- undeformed 2.56 Ga granite dike cutting foliation of GLTZ. References Barth, E. G., 2023, Age and chemistry of the Bell Creek Batholith: Michigan Technological University, M.S. Thesis https://doi.org/10.37099/mtu.dc.etdr/1589 Cannon, W.F., and Simmons, G C., 1973, Geology of part of the Southern Complex, Marquette District, Michigan: Journal of Research of the U.S. Geological Survey, v.1, n.2, p. 165-173. Petryk, B. 2019, The origin of an Archean batholith in Michigan’s Upper Peninsula: Michigan Technological University, M.S. Thesis. https://doi.org/10.37099/mtu.dc.etdr/932 Sims, P.K, 1991, Great Lakes Tectonic Zone in Marquette Area, Michigan-implications for Archean tectonics in north-central United States: U.S. Geological Survey Bulletin 1904-E, 17 p. Schmitz, M.D., Southwick, D.L., Bickford, M.E., Mueller, P.A., and Samson, S.D., 2018, Neoarchean and Paleoproterozoic events in the Minnesota River Valley subprovince, with implications for southern Superior craton evolution and correlation: Precambrian Research, v.316, p. 206-226. 22 �The Honey Creek Structure, Sauk County, Wisconsin: Asymmetric Faulting Associated with Seismic-Induced Fluid Escape Kenz CARLTON1, Basil TIKOFF1, & Esther K. STEWART2 1 2 University of Wisconsin–Madison, Department of Geoscience, 1215 West Dayton Street, Madison, Wisconsin 53706, USA Wisconsin Geological and Natural History Survey, UW-Madison Division of Extension, Madison, Wisconsin 53705, USA The Honey Creek structure occurs in the Balfanz quarry in Sauk County, Wisconsin. It is south of the Baraboo syncline and in the vicinity of the Denzer syncline. The Honey Creek structure is dominantly a N dipping fault (striking ~250 and dipping ~30 using right hand rule) that deforms the Ordovician Oneota Formation and underlying Cambrian Jordan Formation. The hanging wall contains units that are stratigraphically younger than those immediately adjacent across the fault; this geometry requires that the Honey Creek structure contains a normal fault. The footwall side of the fault was deformed, such that the beds were rotated to a nearly vertical orientation and occasionally overturned next to the fault. The folding in the footwall is consistent with drag on a reverse fault, which is opposite to the direction of inferred stratigraphic offset. Soft sediment deformation, interpreted as sand injection emanating from the Jordan Formation, is prevalent alongside the fault on the footwall side, although a sand lens crosscuts the fault in one place. A smaller-scale structure is located less than 30 m S of the Honey Creek structure. This feature has a similar strike and dip, normal sense of offset, and folding of the footwall. It differs from the Honey Creek structure insofar as there is brecciation but no sand injection along the fault. Finally, there appears to be a recumbent fold located less than 40 m S from the main Honey Creek structure, with a vergence away from the Honey Creek structure. We interpret this recumbent fold to have formed in the same deformational event. We consider two possible interpretations for this structure, both of which invoke significant ground shaking. First, the deformation could result from intracratonic seismicity. The timing of deformation (Early Ordovician) is broadly consistent with that of the Taconic orogeny, although the orientation of the fault is at a high angle to the inferred regional shortening direction (EW). Its location, directly south of the Baraboo syncline, could be consistent with reactivation of a Proterozoic fault. Second, the deformation could result from a distant (<200 km) meteor impact. The timing of deformation is consistent with a swarm of meteorites that occurred at the same time in the upper Midwest, and resulted in a number of craters (e.g., Decorah, Elm Creek, etc.). This interpretation is consistent with regional soft-sediment deformation in the Oneota Formation. 23 �Baddeleyite age reveals timing of the Northeast Iowa Intrusive Complex (NEIIC) CLARK, Ryan1, PEATE, David2, KUSICK, Allison2,3, HORKLEY, Kenny4, and MACFARLANE, Chris5 1 Iowa Geological Survey, University of Iowa, 300 Trowbridge Hall, Iowa City, IA, 52242, USA University of Iowa, Department of Earth & Environmental Sciences, 115 Trowbridge Hall, Iowa City, IA, 52242, USA 3 University of Wisconsin-Milwaukee, Department of Geosciences, 3209 N. Maryland Avenue, Milwaukee, WI, 53211, USA 4 University of Iowa, Materials Analysis, Testing and Fabrication Facility, 205 N. Madison Street, Iowa City, IA, 52242, USA 5 University of New Brunswick, Department of Earth Sciences, 2 Bailey Drive, Fredericton, New Brunswick, Canada 2 Recent geophysical surveys over portions of the entirely concealed Northeast Iowa Intrusive Complex (NEIIC) have provided a clearer picture of the region’s Precambrian basement geology (Drenth et al., 2015 and 2020). High amplitude magnetic and gravity anomalies remain the focus of further research into their mineral resource potential, due in part to the likelihood that the NEIIC is related to the ~1,100 Ma Midcontinent Rift System (MRS). A core drilled into the northeast-trending Osborne Anomaly in Clayton County, Iowa provides the only samples in the vicinity of the NEIIC. The Osborne core encountered 722 feet (220 m) of mafic-ultramafic rocks that has been previously described as olivine-plagioclase cumulate. Recent screening using a portable X-ray fluorescence (pXRF) spectrometer revealed elevated concentrations of zirconium (Figure 1) as well as aluminum and potassium in several discrete zones of late stage melt (Clark et al., 2019). Datable minerals in the form of zirconolite and baddeleyite have been identified in samples from these zones. Obtaining a reliable age from the Osborne core has been paramount to making the argument that the NEIIC is Keweenawan and thus possibly related to other magmatic intrusive terranes in the Lake Superior Region. A recent study (Drenth et al, 2020) obtained an age of ~1,170 Ma from LA-ICP-MS analyses of apatite crystals from the Osborne core. However, accurate U-Pb ages on apatites are often limited by the need for a precise correction for the common Pb component. Here, we present new U-Pb ages on baddeleyite crystals from a depth of 2,416.3 feet (736.5 m) that were analyzed by LA-ICP-MS at the University of New Brunswick. The U-Pb crystallization age of 1,148 ± 14 Ma (weighted average of six concordant baddeleyite analyses) stands as the first reliable date to come from the NEIIC region. This age is comparable with other intrusions outboard of the MRS, such as the Corson Diabase in eastern South Dakota (1,149 ± 7 Ma), the Great Abitibi dike (1,141 ± 2 Ma), and the Inspiration diabase (1,159 ± 33 Ma) (McCormick et al., 2017 and references therein), and indicate a wider regional magmatic event that pre-dated initiation of the MRS by ~50 Ma. The general age of these intrusions has been interpreted as early stage magmatism related to the onset of the MRS. The latest geophysical survey over the majority of the southern portion of the NEIIC shows that the Osborne Anomaly is cut by NEIIC intrusions (Drenth et al., 2020), thus providing a maximum emplacement age of ~1,150 Ma. 24 �Figure 1. Graph of Zr concentration by depth from two separate rounds of pXRF analyses illustrates two distinct zones of Zr-enrichment. Inset backscattered electron image shows an elongated zirconolite crystal (gray) with inter-grown baddeyelite crystal (white) from a sample at 2,614.3 feet depth. References Clark, R.J., Anderson, R.R., and Peate, D.W., 2019. The northeast Iowa intrusive complex: a magmatic conundrum related to the Midcontinent Rift System. Geological Society of America Abstracts with Programs, v. 51, no. 2. Drenth, B.J., Anderson, R.R., Schulz, K.J., Feinberg, J.M., Chandler, V.M., and Cannon, W.F., 2015. What lies beneath: geophysical mapping of a concealed Precambrian intrusive complex along the Iowa-Minnesota border. Canadian Journal of Earth Science, v. 52: 279-293. Drenth, B.J., Souders, A.K., Schulz, K.J., Feinberg, J.M., Anderson, R.R., Chandler, V.M., Cannon, W.F., and Clark, R.J., 2020. Evidence for a concealed Midcontinent Rift-related northeast Iowa intrusive complex. Precambrian Research, v. 347. McCormick, K.A., Chamberlain, K.R., and Paterson, C.J., 2018. U-Pb baddeleyite crystallization age for a Corson diabase intrusion: possible Midcontinent Rift magmatism in eastern South Dakota. Canadian Journal of Earth Science, v. 55: 111-117. 25 �Transpressional Nature of the Keweenaw Fault System, Lake Superior Region, and Its Relationship to Grenville Orogenesis DeGRAFF, James 1, GAMET, Nolan 2, LANGFIELD, Katherine 1, LIZZADROMcPHERSON, Daniel 1, MUELLER, Sophie 3, and TYRRELL, Colin 4 1 Michigan Technological University, Houghton, MI 49931 Michigan Geological Survey, Marquette, MI 49855 3 Nevada Gold Mines, Elko, NV 89801 4 Self-empoyed, Mass City, MI 49948 2 The Keweenaw fault system (KFS) is a connected set of faults that extends along the southern margin of the Midcontinent Rift System from northwest Wisconsin to near Keweenaw Point in Michigan. A component of reverse slip has thrust Portage Lake Volcanics (PLV, 1.1 Ga) southeastward over younger, mostly flat-lying Jacobsville Sandstone (JS) on some faults in the system (Fig. 1). This motion enhanced a regional northwesterly tilt to PLV strata, produced counter-regional tilt near major fault segments, and locally tilted footwall JS strata to vertical and overturned attitudes (1, 2). Regionally, the KFS azimuth changes by 65° from 35° near Houghton to 100° at Big Bay, as does the strike of PLV layers. Locally, the Keweenaw fault on published maps changes azimuth by up to 85° at unusual bends, some of which have been attributed to offsets on transverse faults. These changes in fault azimuth are important clues to the geometry of faults making up the KFS and to their individual and collective slip behavior. If opposing rock masses across the KFS are relatively rigid, a reasonable assumption, the fault system cannot be pure dip slip everywhere along its curved path, which inference also applies to its component faults. Mapping along the KFS since 2017 reveals that the sinuous, mostly single fault trace on published maps oversimplifies important structural relationships. In any part of the system, three directional fault sets are recognized: (1) a dominant set that defines the KFS trend and locally separates steeper dipping PLV layers to the northwest from shallower dipping PLV layers to the southeast; (2) splay faults angled 15-30° clockwise from set 1; and (3) connector faults angled 3575° counterclockwise from set 1 that join footwall splays to the main fault trend (Fig. 1). The three fault sets maintain these angular relationships as the curved KFS changes direction from near Houghton to the tip of the peninsula. Interconnections between faults define fault-bounded blocks with long dimensions roughly parallel to the local KFS trend. The fault-bounded blocks and footwall splay faults defining their southeastern and southern edges are arrayed in a left-stepping pattern along the KFS, suggesting a component of right-lateral strike slip. Analysis of fault-slip data, i.e. slickenlines and slip-sense indicators, from the fault population along the KFS indicates that the system’s slip characteristics change between Houghton and the tip of the Keweenaw Peninsula. Near Houghton where the KFS trends northeasterly, the ratio of strike slip to dip slip is about 1:1 and is bimodal, whereas the ratio is more than 2:1 and is unimodal near Bête Grise Bay where the KFS trends easterly. Geologic relationships across some faults are consistent with their northwest and north sides sliding to the right and upward relative to opposing sides. Inversion of fault-slip data indicates that a strike-slip regime existed near Keweenaw Point with an azimuth of maximum shortening of about 100°, which favors a component of right-lateral slip on the KFS. Folding within and adjacent to the fault-bounded blocks exhibits two styles. Multiple folds subparallel to shorter northeast and east ends of fault-bounded blocks (i.e. NE- to N-trending axes) formed by shortening across such boundaries. In contrast, single folds subparallel to longer sides of the blocks in footwall JS strata (i.e. NE- to ESE-trending axes) formed by drape 26 �of JS strata over steeply dipping, mostly strike-slip faults with little to no shortening across them. Based on this evidence, we infer that oblique slip on the KFS becomes mostly right-lateral strike slip near Keweenaw Point and that crustal shortening is along a line roughly perpendicular to the Grenville front about 550 kilometers to the east-southeast (Fig. 1). Acknowledgements: Funding was provided by the USGS EDMAP program, matched by MTU’s Department of Geology and Mining Engineering and Sciences, and supplemented by grants from the Michigan Space Grant Consortium, Keweenaw Community Forest Company, and the ILSG. We thank the Michigan Geological Survey for its sponsorship and G. Hubbell, I. Gannon, G. Hemmila, G. Ahrendt, J. Hawes, B. Murphy, B. Heusdens, and D. Breen for fieldwork assistance. Figure 1: Keweenaw fault system (black lines) north of Portage Lake, Michigan. Five largest fault-bounded blocks numbered 1 – 5. Black arrows show inferred maximum shortening direction. Inset map modified from Northwestern University maps online (https://www.earth.northwestern.edu/spree/Maps.html). References 1. Cannon, W.F. and Nicholson, S.W., 2001, Geologic Map of the Keweenaw Peninsula and Adjacent Area, Michigan: United States Geological Survey, Map I-2696, Scale = 1:100,000. 2. DeGraff, J.M. and Carter, B.T., 2022, Detached structural model of the Keweenaw fault system, Lake Superior region, North America: Implications for its origin and relationship to the Midcontinent Rift System: Geological Society of America Bulletin, v. 135, no. 1/2, p. 449–466. 27 �Glimpses of a Paleoproterozoic landscape: Analysis of exhumed topography on Archean basement rocks northwest of Marquette, Michigan EDIGER, Trent and BJØRNERUD, Marcia Geosciences Department, Lawrence University, Appleton Wisconsin 54911 Background and purpose: Northwest of Marquette, MI, between the Yellow Dog River on the north, and Silver Lake and the Little Garlic River on the south, the modern land surface lies close to the nonconformity between an Archean granite-greenstone complex (the ~2.7 Ga Compeau Creek Gneiss and Mona Schist) and Paleoproterozoic metasedimentary rocks (the ~1.85 Ga Michigamme Formation). High areas are underlain by Archean rocks, while lower ones are underlain by the Michigamme Fm., suggesting that Michigamme sediments accumulated on an ancient land surface of hills and valleys with up to 70 m of relief. Although the Archean and Proterozoic rocks are locally in fault contact, good exposures of the nonconformity, together with systematic fining of grain size in the Michigamme with distance from Archean highs, support the interpretation that much of this area is an ancient exhumed landscape. Further evidence for a regional Paleoproterozoic landscape with significant relief comes from observations that the Sudbury ejecta layer in the region occurs at a wide range of stratigraphic heights above the base of the Michigamme Formation, and locally directly on Archean rocks (Cannon et al., 2010). The modern topography of this region is also qualitatively different - more rugged – than that seen on Archean rocks exposed only 25 km to the south, along the Black River east of Republic, even though the two areas share the same glacial history. In the study area, we suspect that the primary effect of glacial erosion was to remove the soft Michigamme Fm. from high spots, reexposing the sub-Michigamme surface. In the southern area, either the Michigamme Formation was never deposited or the pre-glacial landscape had been already been eroded to a level below the nonconformity. We believe, therefore, that the study area preserves a low-fidelity version of a Paleoproterozoic landscape and can provide insight into patterns of erosion and weathering at a time before land plants and modern atmospheric conditions. On an Earth with no vegetation and little to no soil, eroded sediments would have had a shorter residence time on landscapes and in river systems. Bedrock rivers would have been more common than they are today, and the primary mechanisms of landscape evolution would have been corrosion (chemical weathering) in a CO 2rich atmosphere, corrasion (abrasion of bedrock by entrained sediment), and cavitation (pitting of bedrock surfaces by bubble implosion in turbulent waters). Faults, joints, and other bedrock features would have been the primary influences on river channel location and potholes would have played an important role in channel development (Wohl, 1998). The goal of the study was to develop quantitative metrics to characterize the exhumed ancient landscape and contrast these with modern topography in areas with similar bedrock in order to gain a better understanding of geomorphologic processes in Paleoproterozoic time. Methods: The boundaries of the 165 km2 study area -- the extent of the exposed Archean nonconformity surface -- were drawn based on the provisional geologic map by Klasner et al. (1979) in combination with field observations and visual assessment of the topography. The rocks in the southern comparison area along the Black River are not a granite-greenstone complex like those in the study area, but they do include a mix of felsic and mafic lithologies (Archean gneisses and Paleoproterozoic dikes; Cannon, 1975) and thus serve as a reasonable analog. In order to understand the role of climate in generating relief in granite-greenstone complexes, we also analyzed the topography of two other granite-greenstone terranes: the Pilbara craton in the desert of northwestern Australia (ca. 3.5-3.2 Ga) and the Umburanas complex in the rainforest of Bahia 28 �Province, Brazil (ca. 3.4-3.1 Ga). In both areas, the bedrock lies close to the surface and the regions have been tectonically stable since at least Mesoproterozoic time. Digital Elevation Models (DEMs) for the study site and comparison site came from LiDAR data collected by the USGS 3D Elevation Program and were accessed via OpenTopography’s data map. DEMs for the Bahia province in Brazil and the Pilbara craton in Australia were generated by the Shuttle Radio Topography Mission and accessed through USGS EarthExplorer, and Geoscience Australia’s data map, respectively. When necessary, DEMs were merged into a single feature layer in Esri ArcGIS Pro 3.2.0 and clipped to the areas of interest. Roughness visualizations were calculated by determining the difference between the highest and lowest elevation cell in each 3x3 rectangular pixel neighborhood (Wilson et al. 2007). In ArcGIS Pro, focal statistics (statistical operations on each pixel based on specified neighboring pixels), were used to generate maximum and minimum elevation rasters. These intermediary rasters were then used to quantify roughness and create visualizations using the raster calculator tool. Results: By several measures, the topography of the study area is significantly more rugged than that of both the Black River comparison site and the Pilbara Craton. The roughest 90 m2 parcel in the study area has 73 m of relief compared with 27 m for the Black River and 46 m for the Pilbara. The study site also has a greater percent of land area in the highest roughness classes (>16 m of relief within 90 m2 parcels). The Black River area and Pilbara craton are surprisingly similar in the distribution of elevations, despite representing very different erosional conditions (glacial scouring and desert exposure, respectively). However, along the Black River, lithology seems to have little control on topography while in the Pilbara craton, contacts between Archean batholiths and volcanogenic sediments are the roughest areas. The Umbaranas site is much rougher than the other three, with up to 392 m of local relief where greenstones are exposed, possibly reflecting the wide range of volcanic and sedimentary lithologies in the Umburanas complex (Barbosa & Sabaté, 2002). In the study site NW of Marquette, topographic roughness is concentrated along linear zones – presumably erosion-enhanced faults or fractures in the Archean bedrock. These straight paleochannels differ from the meandering shapes typical of alluvial (sediment-dominated) river systems. Moreover, some of these channels have scalloped edges, a possible record of their evolution through linkage of bedrock potholes (Wohl, 1998). In summary, the topography of the study area differs not only from that of the nearby Black River site, which shared the same recent glacial history, but also from both the desert and rainforest sites. We suggest, therefore, that the region northwest of Marquette represents a Paleoproterozoic bedrock landscape that may have developed under warm, wet conditions in the absence of vegetation, a combination that does not occur on Earth today. References cited Barbosa, J., & Sabaté, P., 2002. Geological features and Paleoproterozoic collision of four crustal segments, Sao Francisco craton, Anais Da Academia Brasileira de Ciências, 74, 343–359. Cannon, W.F., Schulz, K., Horton, J., & Kring, D., 2010. The Sudbury ejecta layer in the Paleoproterozoic iron ranges of northern Michigan, USA. GSA Bulletin, 122, 50-75. Klasner, J., Cannon, W.F., & Brock, M., 1979. Bedrock geologic map of Baraga, Dead River and Clark Creek basins, Marquette County Michigan, USGS Open File Report 79-1305. Cannon, W.F., 1975. Bedrock geologic map of the Republic Quadrangle, Marquette County, Michigan. USGS Miscellaneous Investigations Series Map I-862. Wohl, E. 1998. Bedrock channel morphology. Rivers Over Rock. AGU Monograph 107, 133-149. Wilson, M., OConnell, B., Brown, C., Guinan, J., & Grehan, A, 2007. Multiscale terrain analysis of multibeam bathymetry data. Marine Geodesy, 30, 3–35. 29 �MCR Synthesis 1. Characterizing the MCR mantle plume GOOD, David Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street, London, ON N6A 5B7, Canada The Midcontinent Rift (Keweenawan Large Igneous Province) contains the most diverse assemblage of mafic rock types for any LIP on earth with 9 distinct basalt groups and more than 15 major Ni-Cu-PGE occurrences or deposits. The main objective of the MCR synthesis is to build a coherent model to explain the vast array of observations, geochemical data and interpretations presented by numerous researchers over the past four decades. The project is subdivided into 4 related objectives: 1) Recognition of the key geochemical features of global rift/LIP settings that we should see in the MCR; 2) Build a classification scheme for all basalts, gabbros and ultramafic rocks using high-precision incompatible trace elements; 3) Apply analytical tools and modelling to unravel petrogenesis of recognized groupings; and 4) Represent the results in a model for the MCR that highlights spatial and temporal relationships in the rift. This project was inspired by several key events over the past decade, each of which indicate the project is feasible at this time. Proof of concept tests for objectives 2, 3 and 4 were presented in 2023 indicating a high degree of confidence for the success of this 4-to-5-year project. A few researchers have identified various stratigraphic units in the MCR to have originated by partial melting in the mantle plume and used their inherent isotopic or trace element compositions to model melt-crustal interaction and the petrogenesis of various intrusions that host Cu-Ni-PGE deposits. These plume-related basalt units include the lower Siemens Creek and Kallander Creek basalt in Michigan and the lower Osler and Mamainse Point basalt in Ontario. But these units each present slightly different trace element characteristics, so the question arises as to what criteria are useful for distinguishing mantle plume magmas from those generated in the upper mantle or at different depths within the plume. The main criteria for identifying plume magmas are based on ocean island basalt-like trace element characteristics. In this study, two MCR plume basalt types are identified (Groups 1 and 5) using the combination of λ1-λ2 REE coefficients, TiO2/Yb, and Gd/Yb diagrams. The differences between groups 1 and 5 are best explained by partial melting at different depths, based on differences between majorite (>~300 km) and pyrope garnet fractionation, respectively. Group 1 includes basalts from the Lower Osler and lower Kallander creek groups and the highly fractionated Devon volcanic unit. Group 5 includes basalts from the lower series A and B units at Mamainse Point, Central Osler Volcanic Group and the lower Siemens Creek basalt located at the Skinny, Bluff and Bond Falls sites in Michigan. A well-understood and fundamental characteristic of highly incompatible trace element ratios is their use to correlate basalt and intrusive rocks. These typically unique trace element signatures can be used in a manner like finger printing. However, in all cases, care must be taken, particularly for TiO2, to evaluate clinopyroxene or spinel fractionation, as is the case for basalt and intrusions in Group 1. Based on these comparisons, the Bovine, Current Lake, Disraeli, Haystack, Hele, Kitto, Riverdale Sill, Seagull, Shillabeer, and Thunder Intrusions belong to Group 1, whereas the McIntyre, Jackfish, and Logan sills belong to Group 5. 30 �Description and application of the Consolidated Minerals Database to support geological investigations: an example from the Cuyuna Range, central Minnesota GORDEE, Sarah 1, RIAN, Madison 1, SAARI, Stacy 1, and CARTER, Matthew1 1 Minnesota Department of Natural Resources, Division of Lands and Minerals, 1525 3rd Ave E, Hibbing, MN 55746 Over the past ~50 years, the Minnesota Department of Natural Resources (DNR) Lands and Minerals Division has amassed numerous collections of mineral exploration-related documents, amounting to well over 10,000 hardcopy materials containing geoscience and related land data. Curating these collections has proven to be challenging given the sheer volume of documents from multitudinous sources, necessitating a concerted solution to manage these materials. The Consolidated Minerals Database (CMD) is under development by the DNR to support the initiative to bring the agency’s collections of historical and contemporary documents into digital format and to make them readily available for public use. It is a database of unique collections containing cross-referenced documents with linkages to other internal and external databases, including the Hibbing Drill Core Library (DCL) database, and a web map, where geospatially linked documents can be retrieved from specific localities or regions. The various collections comprising the CMD are designated by project, company, or other relevant shared interest(s). Documents are individually entered into a particular collection and assigned a unique numerical identifier, which is used to cross-reference to different databases. Metadata (e.g., title, date, source) are recorded in a series of entries, and attributes of the document (e.g., scope, subject, content, methods, materials, discipline) are classified in a series of dropdown menus, enabling users to search and find documents meeting specific criteria relating to documents’ contents and origin. In the current initiative, the DNR utilizes the CMD intake application to produce digital records of documents from the Cuyuna Range in central Minnesota, where exploration and mining for iron and manganese ores was active throughout much of the early-middle 20th century. The objective is to curate mineral exploration documents and compile geological data from these records in a large database. A synthesis of these data will help to better understand the geological architecture and extent of historical exploration in the region, and the compiled datasets will help to evaluate the potential for additional iron, manganese, and other resources. Historical documents from the Cuyuna mining district are stored in the Hibbing Lands and Minerals office. Dozens of different exploration companies drilled at least 12,000 boreholes and created thousands of documents spanning multiple decades of mineral exploration and mining in this district. Relevant documents in this collection range from 1905 to the 1970s, and include geological maps, surface maps with drillhole collars and associated metadata, mine maps (surface, subsurface, infrastructure), tables with geochemical data, drillhole profiles with tabular geochemical data, geological information and interpretations, geological cross-sections, field notes, and notes and correspondences regarding property ownership, exploration results and resource estimates. Because of the number of companies involved and diversity in the presentation of data it is necessary to address certain challenges before curation into CMD. Documents are first sorted by company and locality in the Public Land Survey System, which allows for the identification and removal of duplicate maps and other documents shared 31 �among and across different companies. Once sorted, all relevant documents with clear datasets and sufficient metadata to identify the source and locality are curated into the CMD. Following curation, plan-view maps are converted to picture format and brought into ArcGIS, where they are spatially located using georeferencing methods. This method helps to resolve problematic drillhole locations, and to identify less obvious duplicate documents and datasets, including drillholes that were renamed over time as operators changed hands. Drillhole collar locations can then be added to a database of known drillholes in the region, and integrated and compared to cores from the DCL database. Once correctly positioned, individual geospatial datasets, such as geological logs, and geochemical and geophysical data, are extracted from each document. Extracting and compiling geologic data such as geologic logs and assays into tabular format has been a challenging endeavor. These data were hand-written or typed using a typewriter, and utilizing optical character recognition technology to extract text is not straightforward. However, transcribing the data by hand is a cumbersome and protracted process, and potentially introduces errors that must be checked for quality assurance. With the advance of artificial intelligence (AI) and machine learning, it is now possible to train an AI model to extract data into tables. Training the AI data extraction model is an efficient process. First, pages (10 minimum) containing example data listed in an internally consistent format are imported; then table(s) are delineated using the associated headers, labels, and rows per each page. Once the model is trained, numerous documents or pages of the same format are uploaded and the data are auto-extracted, and the output reflects the model’s specified number of tables, columns, and rows. Before the outputted data are extracted, they are enhanced within the model workflow to account for spelling errors, incorrect symbols, etc., so that it is unnecessary to resolve errors individually by hand. Once a model is trained for a specific table format, thousands of pages of tabular data can be extracted into a tabular database en masse in a matter of minutes. Using MircoMine modeling software, the tabulated data are visualized in a 3D model, where any remaining tabulation errors are identified and corrected. This process is ongoing, as many still-uncurated documents remain in the Cuyuna collection. To date, nearly 2,000 documents totalling over 20,000 pages have been scanned, georeferenced and lodged in the CMD, and the drillhole database contains over 4,000 individual drillholes with assay data totaling over 60,000 lines. Incompleteness notwithstanding, the current database is a growing and ever-refining, data compilation from thousands of geological investigations. Together, the newly compiled data are sufficiently expansive to make new observations and interpretations pertaining to the geology and distribution and style of iron and manganese resources, as well as the potential for other base and precious metal resources, in the Cuyuna Range. 32 �Revisiting geophysical interpretations of the Midcontinent Rift below Lake Superior— Insights from GLIMPCE seismic-reflection line C GRAUCH, V.J.S.1, HELLER, S.J.2, WOODRUFF, Laurel G.3, and STEWART, Esther K.4 1 U.S. Geological Survey, MS 973, Federal Center, Denver, CO 80225 U.S. Geological Survey, MS 939, Federal Center, Denver, CO 80225 3 U.S. Geological Survey, 2280 Woodale Drive, St. Paul, MN 55112 4 Wisconsin Geological & Natural History Survey, 3817 Mineral Point Road, Madison, WI 53705 2 The 1.1 Ga Midcontinent Rift System (MRS) has been investigated in the Lake Superior region for more than a century, driven by mineral exploration, academic study, and, for a brief time, oil and gas exploration. Limited outcrops on land and the extent of MRS rocks under the lake have motivated many workers to use geophysical methods to investigate the nature and extent of the rift. The most influential geophysical data for modern paradigms has come from seismic-reflection profiles collected by the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE) in the late 1980s. Notably, many previous workers have used interpretations of GLIMPCE line C (Fig. 1, inset) to demonstrate the architecture of the MRS in western Lake Superior. A recurring theme from the previous work is that syn-extensional basalt flows accumulated in half-grabens bounded by normal growth faults, which then reactivated as reverse faults in response to later compression (e.g., Cannon et al., 1989; Hinze et al., 1992; Dickas and Mudrey, 1997; Stein et al., 2015). We are revisiting GLIMPCE line C by constructing a detailed velocity model for conversion of the seismic data measured in two-way travel time to a section plotted versus depth (Grauch et al., 2023). This approach allows for digital verification of the modeled velocities and more accurate depiction of thicknesses and dips of units to tie to geology onshore. We have constructed an analogous gravity model along line C that provides independent evaluation of our velocity model using velocity-density relations developed from analysis of region-wide rock property compilations (Grauch, 2023). Preliminary results from the velocity modeling, depth conversion, and ties to onshore geology have led to a significantly different view of Line C as primarily a sag basin rather than a half-graben, showing both syn- and post-magmatic subsidence (Fig. 1; Grauch et al., 2023). Narrow intervals of high velocities, which indicate a composition of gabbro (Grauch, 2023), emanate upwards along both sides of the sag basin from an inferred mantle bulge. The intervals are associated with strong linear reflections that truncate sub-horizontal layers in the sag basin and may obscure any minor faulting that occurred before or after intrusion. Cross-cutting mafic intrusions provide an alternate explanation for the termination of layers that was previously thought to indicate major faulting. This new view of line C implies that basin subsidence was the dominant process in the development of rift stage troughs rather than major half-graben structures. Other important interpretations include the following. • Portage Lake Volcanics show syn-magmatic basin subsidence • The Lower Oronto Group section shows post-magmatic basin subsidence • Onlap of Upper Oronto Group onto tilted Porcupine Volcanics suggest the deformation pre-dated deposition of Oronto Group sediments 33 �• Rocks of the lower northeast sequence of the North Shore Group may connect to rocks of similar age from the south shore that lie underneath the sag basin. Figure 1. Interpreted depth section for GLIMPCE Line C. No vertical exaggeration. NSVG, North Shore Volcanic Group. PLV, Portage Lake Volcanics. The new rendition of the Line C seismic data also raises several questions. • How do the sedimentary sections correlate from north to south? • What caused the truncation of volcanic layers at the volcanic-sedimentary contact in the middle of the seismic section and was reverse faulting involved? • What is the tectonic process that drove the syn-magmatic subsidence? • Where does the reverse Keweenaw fault extend into the section from the south shore and what was its influence? These and other questions can be addressed through the construction of velocity models and depth conversions of other seismic lines in the lake. Future insights will benefit from a three-dimensional view that these additional seismic lines will provide. Cannon, W.F., Green, A.C., Hutchinson, D.R., Lee, M.W., Milkereit, B., Behrendt, J.C., Halls, H.C., Green, J.C., Dickas, A.B., Morey, G.B., Sutcliffe, R.H., and Spencer, C., 1989, The North American Midcontinent rift beneath Lake Superior from GLIMPCE seismic reflection profiling: Tectonics, v. 8, p. 305-332., https://doi.org/10.1029/TC008i002p00305 . Dickas, A.B., and Mudrey, M.G., Jr., 1997, Segmented structure of the Middle Proterozoic Midcontinent System, North America, in R.J. Ojakangas, A.B. Dickas, and J.C. Green (eds.), Middle Proterozoic to Cambrian Rifting, Central North America: Geological Society of America Special Paper 312, 37-46., https://doi.org/10.1130/08137-2312-4.37 . Grauch, V.J.S., 2023, Compressional wave seismic velocity, bulk density, and their empirical relations for geophysical modeling of the Midcontinent Rift System in the Lake Superior region: U.S. Geological Survey Scientific Investigations Report 2023-5061, 60 p., https://doi.org/10.3133/sir20235061. Grauch, V.J.S., Heller, Sam J., Stewart, Esther K., and Woodruff, Laurel G. 2023. Exploring the geology of the Midcontinent Rift under western Lake Superior using a preliminary velocity model of seismic line GLIMPCE C, in Ames, C. (ed.), 69th Annual Institute on Lake Superior Geology Proceedings—Part 1, Program and Abstracts, p. 37-38. Hinze, W. J., Allen, D. J., Fox, A. J., Sunwood, D., Woelk, T., and Green, A. G., 1992, Geophysical investigations and crustal structure of the North American Midcontinent Rift system: Tectonophysics, v. 213, p. 17-32. Stein, C.A., Kley, J., Stein, S., Hindle, D., and Keller, G.R., 2015, North America’s Midcontinent Rift: When rift met LIP: Geosphere, v. 11, p. 1607-1616. 34 �GeologyOntario: a powerful search tool for Ontario explorationists HINZ, Sheree1 Ontario Geological Survey, 435 James Street South, Thunder Bay, ON, P7E 6S7 Canada W ith dr aw n In March of 2023, the Ministry of Mines released a new online platform to search and query Ontario Geological Survey data. The Ontario Geological Survey maintains and provides public access to a wealth of geological information including maps, publications, assessment files, mineral inventory points, miscellaneous data releases, geophysical data, abandoned mine site information, and more. Though this information has been available online for many years, the previous iteration of GeologyOntario had significant constraints, including a lack of spatial search abilities, and users experienced challenges in finding relevant information. The new GeologyOntario consists of separate text (Figure 1) and spatial search (Figures 2 and 3) tools which provides ample opportunities to discover information. Geological information is often dependent on spatial data, and the new spatial search tool runs on a powerful Esri-based system, allowing clients to build queries to focus on the types of data relevant to their interests, within the geographical areas they are working. The new GeologyOntario Search Hub is located at https://geology-ontario-en-mndm.hub.arcgis.com/. Figure 1. GeologyOntario text search page (https://www.geologyontario.mines.gov.on.ca/). 35 �aw n dr W ith Figure 2. GeologyOntario spatial search page showing regional geology, mineral inventory, and the results of a search for mineral inventory points listing lithium as a primary commodity in the area of the Separation Rapids pluton(https://mndm.maps.arcgis.com/apps/webappviewer/index.html?id=66ee0efe4d3c4816963737dbdb 890708). Figure 3. GeologyOntario spatial search page with regional geology, mineral inventory, assessment files, Resident Geologist Program (RGP) site visits, and exploration activity layers active. 36 �Recent developments on the use of the Horizontal-to-Vertical Spectral Ratio (HVSR) passive seismic method to determine depth to bedrock in Minnesota HIRSCH, Aaron C.1 1 Minnesota Geological Survey, University of Minnesota, 2609 Territorial Road, St. Paul MN 55114 Bedrock depth is an important dataset for water resource management, hydrological studies, mineral exploration, and general well planning. In Minnesota, bedrock depth is highly variable; thin to nonexistent in the northeast, up to 250m+ in areas to the west, and irregular elsewhere. In areas where bedrock depth is not known from existing water, exploration, or scientific drilling, various geophysical techniques can be used. One of these methods is the Horizontal-to-Vertical Spectral Ratio (HVSR) (Nogoshi and Igarashi, 1971; Nakamura, 1989) which utilizes horizontal ambient noise surface wave frequencies that are excited and amplified dependent on the depth to the basement bedrock below a less dense and seismically slower velocity upper layer (i.e. unconsolidated glacial sediments). The HVSR method has been utilized in Minnesota to estimate the depth to bedrock since the late 2000’s (Chandler and Lively, 2014) and has become a standard measurement in the MN County Geological Atlas program (e.g. Bauer et al., 2023; Mayer et al., 2023). The initial HVSR dataset used 1647 passive seismic measurements with 303 locations with a known bedrock depth, also known as control points, to develop parameters to accurately estimate the depth to bedrock across Minnesota (Chandler and Lively, 2016). The Minnesota Geological Survey has now collected a total of over 6000 HVSR measurements and 480 control points resulting in a new assessment from the larger and more geographically and geologically widespread dataset. Analyses has included a new quantitative data quality ranking using international HVSR guidelines (SESAME, 2004) and new control parameters have been investigated. The shape of the HVSR curve is now being captured in a passive seismic database due to its relationship with bedrock depth topography, bedrock weathering, and the underlying velocity structure. Ongoing evaluation of this database will help refine the HVSR depth to bedrock estimation and more accurately identify potential bedrock valleys while future work will include measuring densities and ultrasonic velocities of Quaternary cores to constrain the control point parameters more accurately. References Bauer, E. J., Cicha, J., Radakovich Block, A., Jirsa, M. A., Hirsch, A. C., Meyer, G. N., Scott, S. B., Lively, R. S.. (2023). C-55, Geologic Atlas of Otter Tail County, Minnesota. Minnesota Geological Survey. Retrieved from the University of Minnesota Digital Conservancy, https://hdl.handle.net/11299/256920. Chandler, V. W., Lively, R. S., 2016, Utility of the horizontal-to-vertical spectral ratio passive seismic method for estimating thickness of Quaternary sediments in Minnesota and adjacent parts of Wisconsin, Interpretation, Vol. 4, No. 3, p. SH71-SH90. http://dx.doi.org/10.1190/INT-20150212.1. Chandler, V.W., and Lively, R.S., 2014. OFR14-01, Evaluation of the horizontal-to-vertical spectral ratio (HVSR) passive seismic method for estimating the thickness of Quaternary deposits in Minnesota and adjacent parts of Wisconsin. Minnesota Geological Survey. Retrieved from the University of Minnesota Digital Conservancy, https://hdl.handle.net/11299/162792. Mayer, J. A., Bradley, M. C., Retzler, A. J., Severson, A. R., Jirsa, M. A., Chandler, V.W., Conrad, D. R., Gowan, A. S., Radakovich Block, A., and Hamilton, J. D., 2023. C-58, Geologic Atlas of Lincoln 37 �County, Minnesota. Minnesota Geological Survey. Retrieved from the University of Minnesota Digital Conservancy, https://hdl.handle.net/11299/260212. Nakamura, Y., 1989, A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface: Quarterly Report Railway Technical Research Institute, 25–30. Nogoshi, M., and Igarashi, T., 1971. On the amplitude characteristics of microtremor (part 2) (in Japanese with English abstract): Journal of the Seismological Society of Japan, 24, 26–40. SESAME, 2004. Guidelines for the implementation of the H/V spectral ratio technique on ambient vibrations. Measurements, processing, and interpretation: WP12 European commission — Research general directorate project no. EVG1-CT-2000-0026 SESAME, report D23.12, 62, http://www.gripweb.org/gripweb/sites/default/files/HV_User_Guidelines.pdf. 38 �Lithostratigraphic discrimination of Quaternary core in Minnesota using magnetic susceptibility HIRSCH, Aaron C.1, SCHNEIDER, Emma, L.1 1 Minnesota Geological Survey, University of Minnesota, 2609 Territorial Road, St. Paul MN 55114 Most of Minnesota is covered by Quaternary sediments deposited during multiple glaciation events. This Quaternary stratigraphy is highly complex due to multiple glacial events in which ice lobes emanated from differing locations north of Minnesota and deposited sediment (diamict, till) of variable thicknesses (up to 250m), provenance, and morphology across the state (Johnson et al., 2016). The Minnesota Geological Survey uses grain counts, color, sedimentary structures, and composition to establish Quaternary lithostratigraphic units that distinguish these deposits by lithology, stratigraphy, and geomorphology. Nine lithostratigraphic regions were identified using cuttings, outcrops, and rotary sonic core (Johnson et al., 2016). Magnetic susceptibility measurements were taken at 1-2 meter intervals from many of these rotary sonic cores during core analysis by applying a magnetic field to the core and recording the magnetic response. This study was conducted to determine if magnetic susceptibility measurements from cores can aid unit correlation across Minnesota regions as part of a USGS funded data preservation project. Over 11,000 measurements were recorded in a newly established Quaternary magnetic susceptibility database with over 7,000 measurements assigned to a lithostratigraphic formation and unit interpretation. Magnetic susceptibility logs were generated for each measured core and statistics calculated for each unit. Analysis of this database has identified lithostratigraphic units with distinctive magnetic susceptibility ranges as compared to nearby and similarly aged units. Due to these results, this newly established database functions as another tool for lithostratigraphic identification of Quaternary sediments and local and regional correlations. References Johnson, Mark D., Adams, Roberta S., Gowan, Angela S., Harris, Kenneth L., Hobbs, Howard C., Jennings, Carrie E., Knaeble, Alan R., Lusardi, Barbara A., and Meyer, Gary N., 2016. RI-68 Quaternary Lithostratigraphic Units of Minnesota. Minnesota Geological Survey. Retrieved from the University of Minnesota Digital Conservancy, https://hdl.handle.net/11299/177675 39 �New Insights into the Geology and Geochemistry of the Osler Group and Related Rocks, Midcontinent Rift System, Northern Lake Superior, Ontario HOLLINGS, Pete and SMYK, Mark Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada Ongoing geological reconnaissance and lithogeochemical sampling were undertaken on parts of the Black Bay Peninsula, St. Ignace Island and neighbouring islands in 2022 and 2023. New field and geochemical data have helped to both distinguish lithostratigraphic units and suggest common magmatic histories in developing a model for the Midcontinent Rift System (MRS)related Osler Volcanic Group and related intrusive rocks. The Osler Group (1108-1105 Ma), a ~3 km-thick succession of predominantly basaltic flows and clastic sedimentary rocks on the north shore of Lake Superior, represents some of the earliest MRS magmatism. Previous studies have largely focused on the paleomagnetism (e.g. SwansonHysell et al., 2019; Halls, 1974) and geochemistry (e.g. Hollings et al., 2007; Keays and Lightfoot, 2015) of the flood basalts in developing a stratigraphic sequence. However, only basic mapping and some initial studies had been conducted on felsic extrusive and intrusive rocks (e.g. St. Ignace Island Complex, Hollings et al., 2023, Smyk et al., 2006; geochronology, SwansonHysell et al., 2019 and references therein). Sampling efforts were most recently focused on these felsic rocks in order to determine their geochemical affinity and to suggest how they, and related mafic igneous rocks, may fit in with the provisional tectono-magmatic model for the Osler Group. Felsic rocks include the Agate Point rhyolite flows (1105.15 Ma); thin felsic fragmental units; aphanitic felsite; and massive, subvolcanic(?), quartz-feldspar-phyric rocks (aka “quartz-feldspar porphyry”/QFP). These red, brown or gray rocks occur predominantly on St. Ignace Island (including in the core of the St. Ignace Island Complex (SIC)) and on smaller islands, south to Agate Point. Rhyolites tend to be LREE-enriched (La/Smn= 5.20-5.47), have higher total REE than the QFPs and more pronounced negative Ti anomalies. The majority of QFPs, including those in the core of the SIC, tend to display a coherent, tightly grouped REE trend, characterized by moderate LREE enrichment (La/Smn= 2.34-4.71, averaging 3.50), relatively flat HREE patterns and pronounced negative Nb anomalies. This similarity in the REE distribution of both extrusive and subvolcanic felsic rocks suggests that they may share a common magmatic and fractionation history. In contrast, the basaltic flows into which felsic rocks have been emplaced have flatter REE distribution patterns (La/Smn= 1.61-3.51) than those of the felsic rocks, with less-pronounced negative Ti anomalies. Mafic and felsic flows situated above an unconformity/conglomerate at Bullers Bay, St. Ignace Island, display pronounced negative Nb anomalies whereas those below do not. This suggests that the lower flows are part of the more primitive Lower Formation of the Osler Group, while the flows above the unconformity resemble those of the more crustally contaminated Central Formation (cf. Keays and Lightfoot 2015; Hollings et al. 2007) as delineated on nearby Simpson Island. 40 �Gabbroic rocks occur at the margin of the SIC, in the Moss Lake Intrusion and as numerous diabase sills and dykes with various orientations which intrude the supracrustal rocks. SIC and Moss Lake gabbro samples display similar REE patterns, characterized by moderate LREE enrichment (La/Smn= 2.59-3.23), moderate negative Ti anomalies and pronounced negative Nb anomalies. By comparison, smaller, diabasic dykes and sills have relatively flat REE distribution and less-pronounced negative Ti anomalies. Prominent, regional-scale mafic dykes (i.e. McEachan, Shesheeb, Paps) display lower total REE and lack negative Sm anomalies. Hollings et al. (2023) suggested that the rocks of the SIC likely formed as the result of emplacement of a large mafic magma chamber at the base of the Osler volcanic pile that triggered partial melting to generate felsic end members which then ascended to shallower levels in the crust. The SIC QFPs are geochemically similar to both the massive, subvolcanic(?) QFPs elsewhere on St. Ignace Island and nearby islands, as well as to the rhyolites at Agate Point, suggesting a similar origin for all of these felsic rocks. REFERENCES Halls, H., 1974, A paleomagnetic reversal in the Osler Volcanic Group, northern Lake Superior: Canadian Journal of Earth Sciences, v. 11, p. 1200–1207, doi:10.1139/e74-113. Hollings, P., Fralick, P. and Cousens, B. 2007. Early history of the Midcontinent Rift inferred from geochemistry and sedimentology of the Mesoproterozoic Osler Group, northwestern Ontario. Canadian Journal of Earth Sciences, 44, 389–412, https://doi.org/10.1139/e06-084. Hollings, P., Hanley, J., Smyk, M., Heaman, L., Cousens, B., and Zajacz, Z. 2023. The ~ 1.1 Ga St. Ignace Island Complex, Northern Ontario, Canada: Evidence for Magma Mixing and Crustal Melting in the Generation of Midcontinent Rift-Related Bimodal Magmas and Implications for Regional Metallogeny, Journal of Petrology, Volume 64, Issue 6, June 2023, egad032, https://doi.org/10.1093/petrology/egad032. Keays, R.R. and Lightfoot, P.C. 2015. Geochemical stratigraphy of the Keweenawan Midcontinent Rift volcanic rocks with regional implications for the genesis of associated Ni, Cu, Co, and platinum group element sulfide mineralization. Economic Geology, 110, 1235– 1267, https://doi.org/10.2113/econgeo.110.5.1235. Smyk, M.C., Hollings, P.N. and Heaman, L. 2006. Preliminary investigations of the petrology, geochemistry and geochronology of the St. Ignace Island Complex, Midcontinent Rift, northern Lake Superior, Ontario; 52nd Institute on Lake Superior Geology, Annual Meeting, Sault Ste. Marie, Ontario, May, 2006, Proceedings Volume 52, Part 1, p.61-62. Swanson-Hysell, N.L., Ramezani, J., Fairchild, L.M., and Rose, I.R. 2019. Failed rifting and fast drifting: Midcontinent Rift development, Laurentia’s rapid motion and the driver of Grenvillian orogenesis; GSA Bulletin 131 (5-6), 913-940. 41 �Use of Ambient Noise Tomography for Mineral Exploration in the Lake Superior Region HOLLIS, Dan1 1 Sisprobe SAS, 831 Pacific Street, #1A, Morro Bay, California, 93442 USA Abstract Ambient noise tomography (ANT) is a relatively new passive seismic tool used in mineral exploration. The method has been successfully used in the Lake Superior region in mapping subsurface structure and rock properties. This presentation will provide a brief introduction to the ANT method and review recent ANT work done in the Lake Superior region. Introduction Exploration for mineral resources uses a variety of geophysical methods to detect and delineate mineral deposits and systems in order to optimize core drilling programs: gravity, magnetics, active-source reflection seismic and electromagnetics methods to name a few. Ambient noise tomography is a relatively new seismic geophysical tool that has seen increasing use in the past couple of years for mineral resource exploration. ANT uses natural earth vibrations and human-generated seismic vibrations to image subsurface structure and map physical properties of the subsurface. Four ANT surveys for mineral resources in the Lake Superior region have been completed: three surveys within the Coldwell Complex near the town of Marathon, Ontario, and one in the Duluth Complex in northeastern Minnesota. Ambient Noise Tomography Method The Earth is constantly vibrating. For the ANT exploration method, useful vibrations, sometimes referred to as “ambient seismic noise”, are generated by hydrosphere-lithosphere interaction such as the oceanic microseism caused by swell (a similar microseism is caused by swell in Lake Superior and the other Great Lakes), and anthropogenic sources such as vehicular traffic, industrial sites, railroads, and other human activity. An ANT survey uses continuous passive seismic data collected with an array of nodal seismometers (“nodes”) and uses surface waves to image the subsurface. The data recording duration is usually between 1 to 4 weeks. Surface waves are dispersive with different frequencies propagating at different velocities related to the seismic velocities of subsurface lithology. Frequency-velocity dispersion curves are picked for all receiver pairs. These dispersion curves serve as input for a tomographic process resulting in an array of frequency-velocity points for each cell in a grid. All cells within the grid are inverted to produce depth-velocity profiles and the result is a 3D shear wave velocity (Vs) cube where velocity is the seismic velocity of the lithology and subsurface structure interpreted from the velocity model. Case Studies Coldwell Complex, Marathon Area, Ontario The first Marathon ANT survey was collected in October 2017 in the Marathon area over a VMS target. This first survey was intended as a noise test using 31one-component (vertical) 10 Hz nodes. The purpose of the noise test was to characterize the spectral power, temporal variation and azimuthal distribution of the local ambient noise. The collected data was also used to cross-correlate all receiver pairs to assess the signal-to-noise ratio of surface waves in the virtual source gathers and processed to produce a crude 3D velocity model which showed agreement with available core hole data. The positive results of the noise test led to the go-ahead for an expanded survey over the target. 42 �An 8.6 km2 expanded ANT survey over the Marathon target was collected in November-December 2017 using 91 one-component (vertical) 10 Hz nodes. The resulting 3D velocity volume had usable imaging down to 1500 meters and imaged the gabbro intrusion slab target. Details about the noise test and expanded ANT survey and its interpretation can be found in Hollis et al. In July-August 2018, the Sally ANT survey was conducted over an exploration target several kilometers to the northwest of the Marathon surveys. The Sally survey was collected using 196 three-component 5 Hz nodes. This survey demonstrated the effectiveness of using three-component data to produce a more accurate velocity model. Details of the Sally survey can be found in Lavoué et al. With the good results of the expanded Marathon survey, funding was obtained through the European Union Horizon 2020 program to conduct larger, higher resolution survey again over the Marathon target in order to test the limits of the ANT method and to test other potential ANT analyses and passive seismic methods. This survey was acquired in September-October 2018 using 983 one-component (vertical) 10 Hz nodes. This third Marathon ANT survey generated several publications on its results some of which are listed in the Reference section. Duluth Complex, Northeast Minnesota In September 2023, a 32 km2 survey was collected over a helium exploration target in Lake County, Minnesota. This survey used 183 three-component seismic nodes. Logging of a post-survey confirmation well has helium shows between 533 – 671 meters which agrees with the interpreted reservoir depth range from the ANT 3D data (Pulsar Helium). Conclusion Past work in the Lake Superior region has shown the ambient noise tomography is an effective tool for mineral exploration in the area. References Dales, P., L. Pinzon-Ricon, F. Brenguier, P. Boué, N. Arndt, J. McBride, F. Lavoué, C. J. Bean, S. Beaupretre, R. Fayjaloun, et al. (2020). Virtual Sources of Body Waves from Noise Correlations in a Mineral Exploration Context, Seismological Research Letters XX, 1–9, doi: 10.1785/0220200023. Hollis D., McBride J., Good D., Arndt N., et al (2019). Ambient noise surface wave tomography at the Marathon PGM-Cu deposit, Ontario, Canada, CSEG Recorder, June 2019. Lavoué A., Nicholas Arndt, John McBride, Aurélien Mordret, Florent Brenguier, Pierre Boué, Roméo Courbis, Sophie Beauprêtre, Charles Beard, Dan Hollis, and Richard Lynch, (2020), Ambient noise Rayleigh and Love wave tomography beneath the Sally Palladium-Copper Deposit (Ontario, Canada), SEG Technical Program Expanded Abstracts : 2075-2079. Pulsar Helium, https://files.elfsightcdn.com/eafe4a4d-3436-495d-b748-5bdce62d911d/2f2bca29-47e64d88-851e-d97bbca643b5/Pulsar_corp_deck_20Mar24x_FINAL-compressed.pdf. Accessed 3/29/2024. Sharma H., Molnar S., Hollis D. and McBride J. (2018). Application of ambient-noise analysis and velocity modeling in mineral exploration. SEG Technical Program, Expanded Abstracts, 3072–3076. Teodor, Daniela & Beard, Charles & Pinzon-Rincon, Laura & Mordret, Aurelien & Lavoué, François & Beaupretre, Sophie & Boué, Pierre & Brenguier, Florent. (2021). High-frequency ambient noise surface wave tomography at the Marathon PGE-Cu deposit (Ontario, Canada). 10.5194/egusphere-egu21-13152. 43 �Geologic and tectonic implications of detrital zircon U-Pb ages from the Dickinson Group in the western Upper Peninsula of Michigan, USA JONES, James V.1, CANNON, William F.2, DRENTH, Benjamin J.3 and O’SULLIVAN, Paul4 1 U.S. Geological Survey, Anchorage, AK 99508, USA jvjones@usgs.gov U.S. Geological Survey, Reston, VA 20192, USA 3 U.S. Geological Survey, Denver, CO 80225, USA 4 GeoSep Services LLC, Moscow, ID 83843, USA 2 In the Lake Superior region of the northern United States and southern Canada, Paleoproterozoic metasedimentary successions record the breakup of southern Superia (in present coordinates) that began ca. 2.3 Ga and the eventual transition to long-lived accretionary orogenesis along the southern Laurentia margin ca. 1.90–1.85 Ga. These successions are difficult to correlate for reasons that include contrasts in thickness and facies at multiple scales, similarities in depositional environment through hundreds of millions of years of sedimentation, and polyphase tectonism that variably produced intense deformational and metamorphic overprints. Detrital zircon U-Pb geochronology is useful for correlating siliciclastic strata that are widespread throughout the successions and for identifying provenance patterns in space and time. We present new data for samples collected from ca. 2.3–1.8 Ga strata across the western Upper Peninsula of Michigan and northern Wisconsin that provide a baseline for regional geologic mapping and correlations with similar strata regionally to globally. Our findings provide new insights into stratigraphic relationships of the ca. 2.1 Ga Dickinson Group and require revision of the depositional history, tectonic evolution, and regional significance of the succession. The Dickinson Group is a distinctive succession of metasedimentary and metavolcanic rocks exposed only in the Felch trough area of the western Upper Peninsula. The strata are bounded by the Randville Dolomite of the Chocolay Group to the north and Archean banded gneiss to the south. These bounding contacts are mostly interpreted to be structural. The lowermost unit of the Dickinson Group is the East Branch Arkose, a coarse cobble to boulder conglomerate that contains rounded clasts of granite and quartzite in a matrix of feldspathic to lithic wacke. The conglomerate is moderately sorted and generally matrix-supported. At a few localities, the East Branch appears in unconformable contact with coarse-grained granite, interpreted as one of the ca. 2.6 Ga batholiths that are common in the southern Superior Province. At these basal localities, cobbles are strongly flattened and the entire unit contains a well-developed foliation defined by the flattened cobbles and aligned biotite in the sedimentary matrix. The East Branch Arkose is overlain by the Solberg Schist, the lower part of which contains fine-grained mafic schist and amphibolite together with discontinuous calc-silicate horizons up to 15 cm thick. Compositional layering in the Solberg is isoclinally folded with a consistent foliation defined by fine-grained chlorite and amphibole. The middle Solberg contains a ~100-ft-thick bed of iron-formation called the Skunk Creek Member that includes biotitehornblende schist and thinly bedded metachert with magnetite layers (James, 1958). The upper Solberg is made up of interlayered biotite quartzite, massive gray quartz-mica schist, and staurolite-biotite schist. The Solberg Schist is overlain by the Six-Mile Lake Amphibolite, which is made up of fine- to medium-grained amphibolite with a strong tectonic foliation defined by hornblende. As originally mapped, the Dickinson Group defines a subvertical, south-facing homocline and was previously thought to be Archean based on an inferred gradational contact between the Six-Mile Lake Amphibolite and the Archean banded gneiss (James, 1958; James et 44 �al., 1961). However, detrital zircon data published by Craddock et al. (2013) showed that the East Branch arkose was deposited ca. 2.1 Ga or later, thus implying a Paleoproterozoic age for the entire succession. Our detrital zircon U-Pb data from the East Branch Arkose match previously published data from Craddock et al. (2013) and are dominated by ca. 2.6 Ga grains interpreted to reflect local granitic sources that are also observed as cobbles. Rare, but statistically significant ca. 2.1 Ga populations, confirm the Paleoproterozoic maximum depositional age. Mafic Solberg Schist that overlies the arkose does not contain abundant zircon, although some small grains that were recovered show a mix of ages ranging from ca. 3.1 to 2.6 Ga and a small ca. 2.1 Ga population. Detrital zircon age spectra from upper Solberg exposures are distinctly different, though. Samples of biotite quartzite and staurolite schist both contain prominent ca. 1.86–1.84 Ga age populations together with more minor ca 2.5 and 2.3 Ga age populations. The upper Solberg age spectra closely match samples of the Michigamme Formation from throughout the surrounding region, suggesting that the upper siliciclastic component of the Solberg schist should, instead, be mapped as Baraga Group. This revised interpretation raises the possibility that the mafic volcanic rocks and iron formation of the lower and middle Solberg Schist could also correlate with the lower Baraga and(or) Menominee Groups, though additional data are required to test these possibilities. Furthermore, it raises questions about the age of the Six-Mile Lake Amphibolite, the uppermost unit of James’ (1958) Dickinson Group. We suggest that the Six-Mile Lake may be Archean as previously inferred by James (1958), in which case its concealed contact with the upper Solberg or Michigamme Formation would be tectonic rather than depositional. We are presently working to test this revised hypothesis through new geochronology and 40Ar/39Ar thermochronology across the contact. The actual depositional age of the East Branch Arkose remains uncertain, as it can be younger than the ca. 2.1 Ga detrital zircon age populations that it contains. This age population overlaps with the ca. 2.1 Ga porphyritic red granite that crops out among the western exposures of Dickinson Group strata, though cross-cutting relationships between the Dickinson and porphyritic red granite are not observed. A ca. 2.1 Ga depositional age for the arkose would require rapid unroofing of the coeval granite in a manner not presently observed elsewhere in the region. In summary, prior interpretations of a continuous ca. 2.1–2.0 Ga Dickinson Group succession in the western Upper Peninsula of Michigan are not consistent with new detrital zircon ages from siliciclastic strata previously mapped as the upper Solberg Schist. These units correlate with the Michigamme Formation instead and raise new questions about the age, setting, and tectonic evolution of multiple Archean and Paleoproterozoic units in the region. References cited Craddock, J.P., Rainbird, R.H., Davis, W.J., Davidson, C., Vervoort, J.D., Konstantinou, A., Boerboom, T., Vorhies, S., Kerber, L., and Lundquist, B., 2013, Detrital zircon geochronology and provenance of the Paleoproterozoic Huron (~2.4-2.2 Ga) and Animikie (~2.2-1.8 Ga) basins, southern Superior Province: Journal of Geology, v. 121, p. 623–644, https://doi.org/10.1086/673265. Drenth, B.J., Cannon, W.F., Schulz, K.J., and Ayuso, R.A., 2021, Geophysical insights into Paleoproterozoic tectonics of the Superior Province, central Upper Peninsula, Michigan, USA: Precambrian Research, v. 359, https://doi.org/10.1016/j.precamres.2021.106205. James, H.L., 1958, Stratigraphy of pre-Keweenawan rocks in parts of northern Michigan: U.S. Geological Survey Professional Paper 314-C, 44 p. James, H.L., Clark, L.D., Lamey, C.A., and Pettijohn, F.J., 1961, Geology of central Dickinson County, Michigan: U.S. Geological Survey Professional Paper 310, 176 p. 45 �Characterizing the geochemistry and nickel-copper-platinum group elements potential of mafic and ultramafic intrusions in northwestern Ontario JONSSON, Justin1, MALEGUS, Paul1, CHURCHLEY, Sophie1, PRICE, Rebecca1 1 aw n Resident Geologist Program, Ontario Geological Survey, Ministry of Mines, Suite B002, 435 James Street South, Thunder Bay, ON P7E 6S7 Canada Globally, magmatic sulphide deposits host significant resources of nickel, copper, cobalt and platinum group elements (PGE). These deposits occur as concentrations of sulphide minerals hosted within mafic to ultramafic intrusive rocks and are widespread across Ontario, occurring in every Precambrian geologic terrane. Ontario is home to 10 operating mines in magmatic sulphide deposits: 9 within the Paleoproterozoic Sudbury Igneous Complex and one within the Neoarchean Lac des Iles Complex. W ith dr In 1999, Operation Treasure Hunt was initiated by the Ontario Government to stimulate mineral exploration by acquiring new airborne geophysical data, surficial and bedrock geochemical data, and development of new methods. In 2003, following completion of the Operation Treasure Hunt project, the Ontario Geological Survey published a report (Vaillancourt et al. 2003) that assessed 109 mafic to ultramafic intrusions across Ontario. The purpose of this part of Operation Treasure Hunt was to characterize and publish data for intrusions known to be prospective for PGE-dominated magmatic sulphide mineralization. Many of the intrusions studied during Operation Treasure Hunt were host to significant known mineralization, including current and past-producing mines, and several of these intrusions are the focus of ongoing mineral exploration. Despite the work by Vaillancourt et al. (2003), there are hundreds of mafic to ultramafic intrusions in Ontario that have not been systematically assessed for magmatic sulphide mineralization potential. Many of these intrusions have favourable characteristics for potentially containing magmatic sulphide deposits, including geophysical anomalies (e.g., magnetic, conductivity), overburden geochemical anomalies and known sulphide mineralization. In 2023, the Resident Geologist Program of the Ontario Geological Survey initiated a project to systematically characterize geochemistry of a subset of mafic-ultramafic intrusions in northwestern Ontario that largely have not been subject to significant historical evaluation by academic researchers, government surveys, or mineral exploration companies. Evaluating the geochemistry of mafic to ultramafic intrusions can provide insight into the magma history, tectonic setting and potential for economic metal endowment. Factors that may influence metal endowment, that can be determined from the examination of geochemical data, include determining magma source characteristics, the timing of sulphur saturation and the degree of interaction of the magma(s) with their country rocks. Careful evaluation of physical characteristics and whole-rock geochemistry can inform future mineral exploration and/or the development of models for the emplacement of mafic to ultramafic intrusions and any hosted mineralization. 46 �W ith dr aw n Initial sample collection and analytical work took place during 2023. Areas of interest are shown in Figure 1, and include the Red Lake, Onaman–Tashota, and Heaven Lake greenstone belts. In this display, we provide examples of preliminary results and interpretations from areas targeted in the first year of field work, including the Trout Bay intrusion (Red Lake greenstone belt), Westwood intrusion (northeast of the Lumby Lake greenstone belt), and the Big Ghee Lake intrusion (south of the Shebandowan greenstone belt). Figure 38.1. Simplified bedrock geology map of a portion of northwestern Ontario, showing project target areas: Red Lake greenstone belt (outlined in blue); Heaven Lake greenstone belt (outlined in black); and Onaman–Tashota greenstone belt (outlined in white). Regional geology modified from Ontario Geological Survey (2011). References Ontario Geological Survey 2011. 1:250 000 scale bedrock geology of Ontario; Ontario Geological Survey, Miscellaneous Release—Data 126 – Revision 1. Vaillancourt, C., Sproule, R.A., MacDonald, C.A. and Lesher, C.M. 2003. Investigation of mafic-ultramafic intrusions in Ontario and implications for platinum group element mineralization: Operation Treasure Hunt; Ontario Geological Survey, Open File Report 6102, 335p. 47 �Cross-sectional Geometry of the Keweenaw Fault System between Hancock and Mohawk, Upper Peninsula of Michigan LANGFIELD, Katherine1, GAMET, Nolan2, DeGRAFF, James1 1 Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, MI, USA 2 Michigan Geological Survey, Marquette, MI, USA The Keweenaw fault system (KFS) is a major compressional feature along the Keweenaw Peninsula near the southern edge of the Midcontinent Rift System (MRS). The MRS formed in the Mesoproterozoic when a major extensional event split the ancient North American continent across the Upper Midwest, yielding large volumes of basaltic lava such as the Portage Lake Volcanics (1.1 Ga, PLV). The PLV strata were thrust southeastward over the Jacobsville Sandstone (JS) along the KFS during post-rift compression by the Grenville Orogeny (1), and some have postulated an earlier origin by normal faulting during rifting (2,3). A recent interpretation based partly on cross-section modeling is that faults making up the KFS are parts of a detached thrust system that formed during the Grenville Orogeny (4). Faults of the KFS have been interpreted to have dip slip – recent reverse slip and possibly earlier normal slip. Since 2017, bedrock mapping and analysis of fault-slip indicators have revealed a significant component of right-lateral strike slip on the KFS, which at its northeast end is inferred to have twice the magnitude of north-side-up reverse slip (5). The collective oblique motion across the KFS is accommodated on fault segments with three distinct orientations that overlap and intersect: (1) major segments parallel to the KFS trend, (2) splay faults striking clockwise to major segments by less than 35°, and (3) shorter connector faults striking counterclockwise to major segments and splays by up to 75° (Fig. 1). The ratio of dip slip to strike slip should vary among faults with such a range of orientations, as should the style of deformation in their hanging walls and footwalls. To help understand these relationships, cross-sections were constructed across various fault components of the KFS using recent mapping data, heritage data from published maps, and drill hole data. Cross-section work employed the dip-domain-bisector method and principles of detached thrust systems and conservation of volume. The new cross-sections attempt to model the subsurface geometry of the segmented KFS and to build on previous work in the area (4). Important unknowns are the JS thickness in the footwall and how JS strata deform adjacent to major faults. A minimum JS thickness of 800 meters was assumed, based on Mayflower drill hole #41 that crosses the Keweenaw fault at 476 meters below sea level (Fig. 2). Ductile deformation of a poorly indurated, mud-prone section near the base of JS was the method used to accommodate flexural slip in the overlying section, but other mechanisms remain to be investigated. A common feature of cross-sections transverse to the KFS trend is a thrust sheet with shallowly dipping PLV strata between a major fault segment and a splay fault. The cross-sections are adding to our understanding of deformation within the KFS and to the tectonic forces that created it. Acknowledgements This project was funded by the USGS EDMAP program (Award G21AC10681), Department of Geological and Mining Engineering and Science of Michigan Tech, ILSG Student Research Fund, Michigan Space Grant Consortium, and sponsored by the Michigan Geological Survey. We thank Tom 48 �Wright for access to Quincy Mine; Ian Gannon, Breeanne Heusdens, Jack Hawes, Braxton Murphy, and Dillon Breen for field assistance; and Dan Lizzadro-McPherson for ArcGIS assistance. References 1. Cannon, W.F., 1994, Closing of the Midcontinent rift ‒ A far-field effect of Grenvillian compression: Geology, v. 22, p.155-158. 2. Cannon, W.F., Green, A.G., Hutchinson, D.R. and nine others, 1989, The North American Midcontinent Rift beneath Lake Superior from GLIMPCE seismic reflection profiling: Tectonics, v. 8, p. 305-332. 3. Hinze, W.J., Braile, L.W., and Chandler, V.W., 1990, A geophysical profile of the southern margin of the Midcontinent Rift System in western Lake Superior: Tectonics, v. 9, no. 2, p. 303-310. 4. DeGraff, J.M. and Carter, B.T., 2022, Detached structural model of the Keweenaw fault system, Lake Superior region, North America: Implications for its origin and relationship to the Midcontinent Rift System: Geological Society of America Bulletin, v. 135, no. 1/2, p. 449–466. 5. Lizzadro-McPherson., D.J., 2023, Structural Analysis and Slip Kinematics of the Keweenaw Fault System between Bête Grise Bay and Gratiot Lake, Keweenaw County, Michigan: Michigan Technological University M.S. thesis, 140 p. Figure 1: Updated bedrock geologic map and legend of study area, with fault segments labelled: KF – Keweenaw Fault, HFHancock Fault, AGF – Allouez Gap Fault Figure 2: Crosssection showing Keweenaw (KF) and Hancock Faults (HF) at Douglass-Houghton Falls. Main units: JS – Jacobsville Sandstone, PLV - Portage Lake Volcanics 49 �Volcanic and Hydrothermal Reconstruction of the Paleoproterozoic Butler Zn-Cu occurrence, Clark County, Wisconsin LAWRENCE, Alex1, VANDERKIN, Adam1, and LODGE, Robert, W.D.1 1 Department of Geology, University of Wisconsin-Eau Claire, Eau Claire, WI 54701 USA The Butler Zn-Cu occurrence is located in western Clark County, northcentral Wisconsin, and is an example of a volcanogenic massive sulfide (VMS) deposit. These VMS deposits are mined globally for numerous metals including Zn, Cu, Pb, Ag, and Au, and are formed at or near the seafloor in extensional submarine volcanic environments through the discharge of hot, metalrich hydrothermal fluids (e.g. Franklin et al., 2005). The Butler occurrence is hosted in the Paleoproterozoic Eau Claire Volcanic Complex (1.8-1.9 Ga) within the Marshfield terrane of the Penokean Orogen (Shultz and Cannon, 2007; DeMatties, 2022). Historically, the interpreted setting for Penokean volcanism within the Marshfield terrane was a “continental’ setting with younger magmas emplaced within older Archean crust. New data from the Eau Claire Volcanic Complex suggests the absence of Archean crust during Penokean volcanism (Weber et al., 2023). The goal of this project is to interpret the volcanic and hydrothermal setting of the Butler Zn-Cu deposit and test whether the lithostratigraphic and petrochemical associations fit an oceanic or continental model. Extensional environments that form VMS deposits can exist in both oceanic and continental settings. This imparts unique lithostratigraphic (Franklin et al. 2005) and petrochemical (Piercey, 2011) characteristics on the host stratigraphy and alteration styles. The lithostratigraphy of continental-associated VMS tends to be more felsic in nature with a higher abundance of siliciclastic rocks relative to oceanic-settings. Mafic rocks are much more abundant in ocean environments whereas are rare and largely intrusive in continental settings. Chemically, felsic rocks in continental settings are HFSE- and REE-enriched while mafic rocks are typically alkalic- to MORB-affinities commonly found in continental rift settings. Approximately 2700 linear feet of drill core from the Butler occurrence were re-logged and sampled for petrographic and geochemical characterization of the host volcanic and hydrothermally-altered rocks. Petrography divided the host strata into three main units: 1) felsic volcanic rocks, 2) amphibolite, and 3) metapelite. The felsic volcanic rocks are fine-grained, foliated quartzofeldspathic schists (Figure 1A) that have layered volcaniclastic textures and local stretched and flattened lapilli fragments. The amphibolite units (Figure 1B) are fine- to mediumgrained, homogenous, and largely unaltered suggesting an intrusive origin that post-dates the main VMS-forming event. The metapelite units (Figure 1C) are made up of a micaceous matrix composed of muscovite, chlorite, and biotite. The metapelite units are characterized by large porphyroblasts of garnet, staurolite, and/or cordierite. Hydrothermally-altered rocks that host sulfide mineralization are metamorphosed to biotite±chlorite±talc schists and calc-silicate mineral assemblages. The sulfide mineralization is primarily pyrite with variable amounts of chalcopyrite, and sphalerite. Massive sulfides (Figure 1D) are weakly banded with chloritic gangue while semi-massive vein-type mineralization is found throughout altered rocks. The relative abundance of the felsic and amphibolite units coupled with an intrusive origin for amphibolite, suggests a bimodal-felsic type VMS, described in the paper Volcanogenic Massive Sulfide Deposits that is typical of continental magmatism (Franklin et al., 2005). Mafic units are interpreted to be island arc to MORB-type based on Ti vs. V discrimination plots. Felsic volcanic rocks have an FII-type affinity on Zr/Y vs. Y discrimination diagrams and have within plate affinities on Nb vs. Y discrimination diagrams. Geochemical abundances of the host rocks support a continental petrochemical association. 50 �Figure 1. Photographs of core samples from the Butler deposit featuring the host rocks of the VMS. White scale bar equals about 1 cm. (A) Foliated felsic volcanic rock that is the main host rock of the Butler formation. (B) Amphibolite unit, image displays how homogenous the matrix is. (C) Metapelite unit, tan staurolite porphyroblasts along with large purple to grey cordierite porphyroblasts found throughout the matrix. (D) Massive sulfide unit containing pyrite and chalcopyrite. References DeMatties, T.A., 2022. Exploration-resource assessment of productive felsic volcanic centers in the Paleoproterozoic Penokean volcanic belt of northern Wisconsin, Michigan and East-central Minnesota, USA: Ore Geology Reviews, v. 141: 104489. Franklin, J. M., Gibson, H. L., Jonasson, I. R., and Galley, A. G., 2005, Volcanogenic massive sulfide deposits, in Hedenquist, J. F. H., Goldfarb, R. J., and Richards, J. P., eds., Economic Geology, 100th Anniversary Volume, p. 523-560. Piercey, S. J., 2011, The setting, style, and role of magmatism in the formation of volcanogenic massive sulfide deposits: Mineralium Deposita, v. 46, p. 449-471. Schulz, K.J., and Cannon, W.F., 2007. The Penokean orogeny in the Lake Superior region: Precambrian Research, v. 157: 4-25. Weber, E.M., Lodge, R.W.D., Marsh, J.H. (2023). U/Pb geochronology and zircon petrochronology of Paleoproterozoic magmas from the Marshfield terrane, Penokean Orogen, Wisconsin. Institute on Lake Superior Geology Proceedings, 69th Annual Meeting, Eau Claire, Wisconsin, Part 1Program and Abstracts, p. 97-98. 51 �Building a 3D model for Cu/Pd inflection points throughout the Marathon PGE-Cu Deposit LAXER Max1, GOOD David1 1 Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street, London, ON N6A 5B7, Canada The Marathon PGE-Cu deposit is hosted in the North American Midcontinent rift system, a failed continental rift (Good et al., 2015). Magmatic activity in the area created an optimal environment for the formation of economically significant sulphides (Smith et al., 2022) bearing copper (Cu), and platinum group elements (PGE) at the Marathon deposit. The deposit lies in the Two Duck Lake Gabbro, a subophitic, coarse-grained intrusion located at the eastern margin of the Coldwell Complex. This study explores how Cu/Pd varies in 3D space at the deposit scale and aims to use it as a vectoring tool to guide exploration. The ratio of Cu/Pd is a useful marker of the enrichment of Pd relative to the mantle. Low Pd relative to Cu indicates previous Pd depletion due to the early formation of sulphides in the intruding magma that formed the deposit, whereas a relatively higher Pd concentration implies Pd enrichment (Barnes et al., 1993). The 3D model helps to visualize the positions of the abrupt shifts (inflection points) in Cu/Pd ratio throughout the Marathon deposit. Identifying and modelling Cu/Pd inflection points facilitated the search for trends in mineralization. To create the model, a data filtration process was employed to define wide mineralization intervals containing at least 80 ppm Cu and 0.15 ppm Pd. Zones of continuous mineralization of at least 16 m in length were identified. To identify inflection points the difference in Cu/Pd ratio was evaluated at 10 m intervals. The mineralized zones were searched for points that surpassed the thresholds found to constitute trend reversals in Cu/Pd (ΔCu/Pd >5000 or <-5000). Approximately 1150 inflection points have been identified in 404 drillholes from a dataset of 997 drillholes and 61960 assays. Figure 1. Graphs showing the trends of concentration Cu and Pd in ppm, the ratio of Cu/Pd, and the 10 m difference calculations used to identify inflection points, down drill hole M-20-541 (depth in m) at the Marathon PGE-Cu deposit. The dashed lines indicate filtration cut-offs for Cu (80 ppm) and Pd (0.15 ppm) and the inflection point thresholds in the Cu/Pd graph (at -5000 and 5000). 52 �Three zones of interest were identified within the model of the deposit with distinct trends in the occurrence Cu/Pd ratio inflection points (Fig. 2). Area 1 included zones of high grade Pd mineralization occurring independently of any high-grade copper or any inflection points. In Area 2 the arrangement of inflection points suggests a boundary which aligns with the paleosurface at the contact between the Footwall and the Main Zone. A fault runs through Area 3 (Good et al., 2015), along which there are no Cu/Pd inflection points, indicating that there may be a link between faulting and the consistency of Cu/Pd ratio. The most prominent pattern observed in the 3D model of the inflection points was that Cu/Pd correlates better with lithological changes than with shifts in Cu and Pd grade. Area 1 Area 2 Area 3 Figure 2. Views of the Leapfrog model of the Marathon deposit, including all Cu and Pd assays and all inflection points. Showing plan views of the whole deposit, Areas 1 and 3 and a cross-section of Area 2. References Barnes, S.-J., Couture, J., Sawyer, E., & Bouchaib, C., 1993. Nickel-copper occurrences in the BelleterreAngliers Belt of the Pontiac subprovince and the use of Cu-PD ratios in interpreting platinumgroup element distributions. Economic Geology, 88(6), 1402–1418. Good, D. J., Epstein, R., McLean, K., Linnen, R. L., & Samson, I. M., 2015. Evolution of the main zone at the marathon Cu-PGE sulfide deposit, Midcontinent Rift, Canada: Spatial relationships in a magma conduit setting. Economic Geology, 110(4), 983–1008. Smith, J. M., Ripley, E. M., Li, C., Shirey, S. B., & Benson, E. K. (2022). Magmatic origin for the massive sulfide ores in the sedimentary country rocks of mafic–ultramafic intrusions in the midcontinent rift system. Mineralium Deposita, 57(7), 1189–1210. 53 �Critical Mineral Systems in the Upper Peninsula of Michigan, A Cooperative Effort Between the USGS and the Michigan Geological Survey MAHIN, Robert2, QUIGLEY, Ashley2 YELLICH, John1, ESCH, John1, and GAMET, Nolan2, 1 Michigan Geological Survey, Western Michigan University, Kalamazoo MI 49008-5241 2 Michigan Geological Survey, Western Michigan University, Gwinn MI 49841 In 2018, the U.S. Geologic Survey (USGS) released a list of critical minerals defined as “non-fuel mineral or mineral material essential to the economic or national security of the U.S., and which has a supply chain vulnerable to disruption” and updated it in 2022 to a total of 50 critical minerals (Burton, 2022). Since 2021, President Biden has made the domestic supply of critical minerals a national priority. With federal funding, the USGS Earth Mapping Resource Initiative (EMRI) is collaborating with State geological surveys on geologic mapping and critical mineral assessments, as well as inventorying and characterizing mine wastes. The USGS has identified broad focus areas within the United States to target critical minerals (Hammarstrom and others, 2023). These focus areas are based on known mineral occurrences and favorable geologic settings. The Precambrian of the Upper Peninsula (UP) figures in 17 mineral systems. The Michigan Geological Survey (MGS) has narrowed the list to nine systems in the UP to focus our future work (Table 1). With the support of the USGS, forthcoming mapping and geochemical reconnaissance programs by the MGS over the next few years will assess these systems. Name of focus area Midcontinent Rift magmatic sulfide Ni-Cu-PGE Manganese (Mn) in ironformations Graphite in black shales Mineral system Deposit type(s) Critical minerals in Critical minerals the deposit types Identified Mafic magmatic Nickel-copper-PGE sulfide Co, Ni, PGE, Te Nickel, Co, PGE Marine chemocline Iron-manganese Co, Mn Manganese Metamorphic Graphite (carbonaceous sed) Humboldt Granite Porphyry Sn (granite-related) Porphyry/skarn Humboldt Granite Magmatic REE Southern Complex pegmatites Porphyry Sn (granite-related) Pegmatite LCT Mesoproterozoic Phosphate Marine chemocline Peavey Pond Complex IOA-IOCG Western Upper Peninsula, IOCG IOA-IOCG Peralkaline syenite/granite/rhyolite/ alaskite/pegmatites Graphite Be, Nb, Sc, Sn, Ta, W Be, Fl, Hf, Nb, REE, Ta, Te, V, Zr Be, Ce, Li, Nb, Sc, Ta, Sn Co, REE Phosphate Iron oxide apatite; Iron oxide copper Co, REE gold Iron oxide apatite; Iron oxide copper Co, REE gold Trace Trace Trace Phosphate Unknown Unknown Table 1: USGS-MGS Critical Mineral Focus Areas for the UP (modified from Hammarstrom and others, 2023) The existence of some critical minerals is well-established in the UP, such as magmatic sulfide Ni-Cu-Pt-Pd-Co. Others, such as graphite, manganese, and phosphate have been documented in small occurrences or as accessory minerals in larger deposits (Cannon and Klasner, 1976: Hwang and others, 1986; James and others, 1968; Peterman and others, 1987, 54 �Mancuso, 1975). Evidence for critical minerals such as rare-earth elements, beryllium, and fluorspar in pegmatites, granites, and iron-oxide-copper-gold/oxide apatite deposits (IOCG/IOA) is sparse. A limited number of studies of UP pegmatites and the Humboldt granite have identified trace REE, Be, and Ta minerals (e.g. Buchholz and others, 2014; Johnson and others, 2015; Moss, 1975, Schulz and others, 1988). Mineralization directly tied to IOCG/IOA has not been identified, although the tectonic history and metal endowment suggests the UP is a permissive, if not prospective region for them. The USGS has also mounted a mine waste characterization program intended to identify potentially recoverable critical minerals in historical mine stockpiles, waste piles and tailings and prioritized by size, potential mineral resources. As part of the effort, the MGS has identified over 80 mine sites in the UP within EMRI critical mineral focus areas that have published references to possible critical mineral content. Future assessments will involve representative sampling of mine waste features and geochemical evaluations. References Buchholz, T. W., Simmons, W. B., and Falster, A.U., 2014: Accessory mineralogy of the Black River Pegmatite and Humboldt granite, Marquette County, Michigan. In Fortieth Rochester Mineralogical Symposium: Contributed Papers in Specimen Mineralogy, Part 1, Rocks & Min., 89:4, 370-374. Burton, J., 2022. U.S. Geological Survey Releases 2022 List of Critical Minerals: https://www.usgs.gov/news/national-news-release/us-geological-survey-releases-2022-list-criticalminerals. Cannon, W.F. & Klasner, J.S., 1976. Phosphorite and other apatite-bearing sedimentary rocks in the Precambrian of Northern Michigan: US Geological Survey Circular, 746, 6 p. Hammarstrom, J.M., Woodruff, L.G., and Dicken, C.L., 2023, Critical mineral deposits of the United States: U.S. Geological Survey data release, https://doi.org/10.5066/P9K1HBNT Hwang, J. Y., Carlson, D. H., Johnson, A. M., and Van Alstine, J., 1986. Preliminary investigation of graphite resources in Michigan, in Process Mineralogy VI: Applications to Precious Mineral Deposits, Industrial Minerals, Coal, Liberation, Mineral Processing, Agglomeration, Metallurgical Products and Refractories, with Special Emphasis on Cathodoluminescence Microscopy, (Ed. by, Hagni, R. D.), p. 315- 327. James, H.L., Dutton, C.E., Pettijohn, F.J. and Wier, K.L., 1968. Geology and ore deposits of the Iron River-Crystal Falls District, Iron County, Michigan: U.S. Geological Survey Professional Paper 570, 127 p. Johnson, Christopher M. and Van Daalen, Christopher M., 2015. Mineralogy and geochemistry of Late Archean and Paleoproterozoic granites and pegmatites in the Northern Penokean terrane of Marquette and Dickinson Counties, Michigan. University of New Orleans Theses and Dissertations. 2088. Mancuso, J.J., 1975. Carbonate-apatite in Precambrian cherty iron formation, Baraga County, Michigan. Economic Geology, 70, p. 583-586. Moss, Michael J., 1975. Some pegmatites near Gwinn, Michigan. Master's Thesis 2451, Western Michigan University. Peterman, J.F., Johnson, A.M. and Van Alstine, J., 1987. Geological and geophysical investigation of graphite resources in Upper Michigan, in Institute on Lake Superior Geology, 33rd Annual Meeting, Proceedings and Abstracts, p. 53-54. Schulz, K.J., P.K. Sims, Z.E. Peterman. 1988. A post-tectonic rare-metal-rich granite in the southern complex, Upper Peninsula, Michigan. in Institute on Lake Superior Geology, 34th Annual Meeting, Proceedings and Abstracts, p. 34. 55 �Baraboo Interval Quartzites in Iowa: Reassessing the Origin and Provenance of the Washington County Quartzite, SE Iowa MALONE, Jack1, CLARK, Ryan1, HARRIS-BOMMARITO, Amira2, and MALONE, David2 1 Iowa Geological Survey, 300 Trowbridge Hall, University of Iowa, Iowa City, IA 52242 Geography-Geology, Felmley Hall of Science, Illinois State University, Normal, Illinois 61790 2 The Washington County Quartzite (WCQ) in southeastern Iowa is the southernmost occurrence of the Baraboo Interval quartzites in the midcontinent region (Figure 1). Two drill holes encountered poorly sorted quartzite and phyllite at a depth greater than 2,300 feet, likely deposited in a braided fluvial or deltaic environment near the Laurentian continental margin on the Columbia supercontinent. One hundred new LA-ICPMS detrital zircon ages from the WCQ show a prominent 1.78 Ga age peak, representing local Yavapai-aged basement, a secondary peak at 1.8-1.9 Ga representing a distal Penokean source, and minor <2.5 Ga peak derived from distal sources in the Superior Province. Multidimensional scaling of other Baraboo Interval quartzites and potential sources show that the WCQ is indistinguishable from the lower interval of the Baraboo quartzite (Figure 2). Cumulative and stacked probability plots also reflect principal source areas locally derived by erosion of underlying Yavapai-aged crust and distally derived Penokean and older sources from the Pembine-Wausau Terrane or southern Superior Province. The WCQ likely serves as the down slope equivalent during initial Baraboo deposition. Figure 1. Geological map of Precambrian basement rocks in the northern midcontinent (modified from Medaris et al., 2021). Baraboo Interval Inliers are B = Barron, F = Flambeau, M = McCaslin, T = Thunder Mountain, R = Rib Mountain, and N = Necedah. SLTZ = Spirit Lake Tectonic Zone, ECMP = East-Central Minnesota Batholith, GLTZ= Great Lakes Tectonic Zone, and NFZ = Niagara Fault Zone. 56 �Figure 2. Three-dimensional multi-dimensional scaling plot of Baraboo Interval strata in the northern midcontinent. Included data compiled from Malone et al. (2022), Van Wyck and Norman (2004), Stewart et al. (2018), Stewart et al. (2021), and Medaris et al. (2021). The red circle is the WCQ from this study. The yellow cluster includes samples from the lower members of the Baraboo Quartzite in the Baraboo Hills that are dominated by Yavapai-age zircons. The blue cluster includes the middle members of the Baraboo Quartzite in the Baraboo hills, other quartzites north of the SLTZ, and Necedah reflects a complex sedimentary provenance that includes proximal and distally derived zircons from the Penokean Province, Superior Province, and Trans-Hudson belt. The purple cluster includes the Waterloo Quartzite and the upper members of the Baraboo Quartzite in the Baraboo Hills that are dominated by southerly derived Mazatzal-age zircons. References Malone, D.H., Craddock, J.P., Holm, D., Krieger, A., and Baumann, S.J., 2022. Continent‐scale sediment dispersal for the Proterozoic Baraboo Interval quartzites in the Laurentian midcontinent. Terra Nova, 34(6): 503-511. Medaris, L.G., Jr., Singer, B.S., Jicha, B.R., Malone, D.H., Schwartz, J.J., Stewart, E.K., Van Lankvelt, A., Williams, M.L., and Reiners, P.W., 2021. Early Mesoproterozoic evolution of midcontinental Laurentia: Defining the geon 14 Baraboo orogeny. Geoscience Frontiers, 12: 101174. Stewart, E.K., Brengman, L.A., and Stewart, E.D., 2021. Revised Provenance, Depositional Environment, and Maximum Depositional Age for the Baraboo (<ca. 1714 Ma) and Dake (<ca. 1630 Ma) Quartzites, Baraboo Hills, Wisconsin. Journal of Geology, 129: 1-31 . Stewart, E.D., Stewart, E.K., Walker, A., and Zambito, J.J., IV., 2018. Revisiting the Paleoproterozoic Baraboo interval in southern Wisconsin: Evidence for syn-depositional tectonism along the south-central margin of Laurentia. Precambrian Research, 314: 221-239. Van Wyck, N., and Norman, M., 2004. Detrital zircon ages from early Proterozoic quartzites, Wisconsin, support rapid weathering and deposition of mature quartz arenites. Journal of Geology, 112: 305-315. 57 �The Soudan Geology Trail Project: Let’s talk about rocks in northeastern Minnesota MARTIN, Alice1, ALLERTON, Zsuzsanna1, JOHNSON, Emma1, FAYON, Annia1, ESSIG, Jim2, GUY-LEVAR, Sarah2, HUDAK, G. H. 1 1 Earth and Environmental Science Department, University of Minnesota, 150 John Tate Hall, 116 Church St. SE, Minneapolis, MN 55455, USA 2 Minnesota Department of Natural Resources, 1302 McKinley Park Rd, Soudan, MN 55782, USA As part of a larger research endeavor within the Archean terrane of northern MN, we are developing educational outreach content created to engage the public with portions of the important geology of the region. Rocks exposed in the Lake Vermilion-Soudan Underground Mine State Park, located near Tower, MN, record glimpses of environmental and tectonic conditions from 2.7 billion years ago to the present, including mysteries of the early Earth, complexities of modern history, and possibilities of the future. The planned content will be designed to follow outcrops located along a paved trail that runs through the park (Figure 1). Figure 1: This figure shows recently exposed units along the proposed trail (bold line in center of the figure). Starting at the banded iron formation (A), the trail leads north then east (clockwise) to the next units (B) consisting of pillow basalts and basaltic lava flows, followed by felsic tuff (C) and lastly a large, exposed outcrop along the paved road is chlorite schist with intertwined with banded iron formation (D). 58 �The chosen outcrops consist of well-preserved greenschist-facies metamorphosed igneous, sedimentary, and sheared rocks. Selected rock units along the trail include Neoarchean basaltic lava flows, pillow basalts, felsic tuff, gabbro, oxide-facies banded iron formation, and chloriteand sericite-dominant schists. These rock units represent an ancient submarine volcanic and hydrothermal environment that was subsequently regionally deformed (Hudak et al., 2016). The work being done will be included in educational materials designed to communicate the scientific content in digestible ways. Plain language writing, paired with visuals, modern analogues and analogies will present the information in a variety of ways with the intention of supporting a range of learning styles. The materials will be available in physical forms (on paper and/or trail signs) and with QR codes which will link to additional online content. Visual illustrations will be designed in collaboration with a Minnesota high school student. Combining educational material with the hands-on outdoor experience of visiting the trail and highlighted outcrops aims to facilitate cognitive development and understanding of the long history and importance of the regional geology. This initiative is a collaboration among park officials Jim Essig (Park Manager) and Sarah Guy-Levar (Interpretive Supervisor) from the MN Department of Natural Resources, the University of Minnesota Department of Earth and Environmental Sciences, and local and state educators. References Hudak, G.J., Peterson, D.M., Radakovich, A., Pignotta, G., Schwierske, K., and Students from the 2010-2013 Precambrian Research Center Geology Field Camp, 2016, Bedrock geologic map of Lake Vermilion/Soudan Underground Mine State Park – Report to the Minnesota Department of Natural Resources: Natural Resources Research Institute, University of Minnesota Duluth, Technical Report NRRI/TR-2016/20, 23 p. 59 �Investigating the origin of pervasive breccias in the Paleoproterozoic Saunders Formation in northern Wisconsin MARTIN, Gwendolyn and BJØRNERUD, Marcia Department of Geosciences, Lawrence University, 711 E Boldt Way, Appleton WI 54911 The Paleoproterozoic Saunders Formation is an enigmatic unit with limited exposure along the Brule River, which forms the border between northern Wisconsin and the Upper Peninsula of Michigan. The unit occurs just north of the Niagara Fault zone, the Penokean-age (ca. 1.88 Ga) tectonic suture between the Superior Craton and the Wisconsin Magmatic Terranes (Schulz & Cannon, 2007). Variously described as a “massive dolomite” (Allen 1910), a “silicified dolomite” (Sims, 1992), and a “silica rock” (Cannon, 1986), the Saunders Fm. is thought to be part of the lower Chocolay Group, correlative with the Randville, Bad River, and Kona Dolomites, and possibly also the quartzites underlying these units (Sturgeon River, Sunday, and Mesnard Fms.). Each of these carbonate formations is overlain by a major unconformity, at ca. 2.1 Ga., representing at least 100 million years of erosion. Every published description of the Saunders Fm. mentions that it tends to be brecciated, yet the nature of these breccias has not been explored in detail. Dutton & Linebaugh (1967) suggested that the Saunders Fm. represents a condensed section of basal Chocolay quartzite and dolomite, related to the formation of the regional unconformity. James et al. (1968) similarly hypothesized that “silcretes” within the Saunders had formed by Proterozoic weathering but also pointed out that none of the other Chocolay Group carbonate units displays evidence of such deep weathering. They speculated, therefore, that the Saunders breccias could have had a tectonic origin but did not pursue that hypothesis further. The purpose of this study was to characterize and interpret Saunders breccias in outcrops at Brule River Cliffs State Natural Area in Wisconsin. At this site, outcrops of the Saunders Formation are of two types: 1) beige to orange-colored dolostone with a ‘gritty’ but otherwise massive (unveined, unlaminated) texture; and 2) dramatically fragmented dolostone with extensive ‘stockwork” quartz veins that constitute most of the rock mass. Immediately southwest of the Natural Area boundary, large boulders of dolomite-matrix breccias with angular chert fragments are common. Although these are not in situ, we suspect they come from the Saunders Fm., and were transported ca. 12 km by glacial ice. If so, these chert breccias represent a third, distinct textural type within the Saunders. The gritty dolomite, which is typically unveined, has a distinctive diamictic, granular texture, with scattered mm-sized, rounded grains set in a much finer matrix. In thin section, the matrix also appears granular, unlike the crystalline texture typical of most carbonate rocks. Similar gritty/ granular dolomites have been observed along a major upper crustal fault zone in Namibia. Rowe et al. (2012) interpreted these unusual textures as records of decarbonation and fluidized granular flow caused by rapid frictional heating during seismic slip in rocks that had been at ambient crustal temperatures of around 200°C. (Carbonate rocks typically devolatilize, rather than melt, during seismic slip, so pseudotachylyte is rare along faults cutting through dolostone). The absence of talc or other calc-silicate metamorphic minerals in the Saunders Formation points to subgreenschist temperatures in an upper crustal setting comparable to the Namibian case. The veined breccias have an ‘exploded’ look, with quartz veins in multiple orientations that appear to have increased the volume of the rock mass more than 100%. The isolated fragments of host dolostone have narrow, slab-like shapes that suggest fragmentation occurred partly along bedding planes. In thin section, most of the veins have a coarse, blocky texture with no preferred orientation of crystals. Fluid inclusions arrays are common, particularly in the interiors of the crystals. Some of the vein quartz shows slight undulose extinction. The chert breccia boulders 60 �found southwest (in the down-ice direction) of the Saunders outcrops have the texture of cataclasites. The angular fragments of chert within these breccias appear to represent thin silicified stromatolitic layers that were fractured and dismembered. We interpret these three textural types of the Saunders Fm. as distinct areas within a major Penokean-age fault zone. The chert breccias may represent the outer part of the fault zone, dominated by non-seismic cataclasis. The gritty dolomite, bearing evidence of co-seismic heating, would have been closer to the fault core, together with the heavily veined dolostone, whose ‘exploded’ nature points to extreme dilational strain and forceful fluid influx with little cataclasis or grinding. The large amounts of vein material relative to the host rock, as well the blocky texture of the veins, are consistent with the introduction of large volumes of overpressured, silicasupersaturated fluids into the shallow crust during the propagation of a fault rupture upward from depth. Such fluid influx can happen when co-seismic slip breaks the barrier between deep crustal, low-permeability rocks in which fluids are at lithostatic pressures and overlying high-permeability rocks with fluids at hydrostatic pressure (Cox and Munroe, 2016). Silica-rich fluids traveling upwards from below this barrier would be far from chemical equilibrium in the shallow crust, and they would rapidly precipitate their dissolved silica, easily overcoming kinetic quartz growth limits that exist under equilibrium conditions (Williams and Fagereng, 2022). This interpretation of the Saunders breccias is supported by oxygen isotope analyses of seven vein quartz samples, all of which yielded 18O values between 15.94 to 17.46 VSMOW. Although brecciated textures described in previous studies of the Saunders formation may be related to deep weathering and the post-Saunders unconformity, the breccias exposed in the Brule River Cliffs Natural Area are clearly tectonic -- and probably coseismic -- in origin. The Saunders Formation thus provides further evidence for great earthquakes at various crustal depths along major fault zones during the Penokean orogeny (Larson & Bjørnerud, 2017; Taylor & Bjørnerud, 2023). References cited Allen, R., 1910. The Iron River iron-bearing district. Mich. Geol. Biol. Survey Pub. 3, Ser. 2, 151 p. Cannon, W., 1986. Bedrock geologic map of the Iron River 1º x 2º quadrangle. USGS Map I-1360-B. Cox, S. & Munroe, S., 2016. Breccia formation by particle fluidization in fault zones. Am. J. Science, 316, 241-278. Dutton, C. & Linebaugh, R., 1967. Map of Precambrian geology of Menominee district, USGS Map I-466. James, H., et al., 1968. Geology & ore deposits of Iron River-Crystal Falls District. USGS Prof. Paper 570. Larson, M. & Bjørnerud, M., 2017. Seismic slip, mylonitization and fluid flow along the Penokean TwelveFoot Falls shear zone, Marinette County, NE Wisconsin. Proc. Inst. Lake Superior Geol., 63, 56-57. Rowe, C., Fagereng, Å., Miller, J. & Mapani, B., 2012. Signature of coseismic decarbonation in dolomitic fault rocks of the Naukluft Thrust, Namibia. Earth & Planetary Science Letters, 333, 200-210. Schulz, K. & Cannon, W., 2007. Penokean orogeny in the Lake Superior Region. Precam. Res. 157, 4-25. Sims, P.K., 1992. Geologic map of Precambrian rocks, southern Lake Superior region, USGS Map I-2185 Taylor, M., and Bjørnerud, M., 2023. Deciphering the metamorphic and deformational history of the Hardwood Gneiss, Felch District, Michigan. Proc. Inst. Lake Superior Geol., 69, 89-90. Williams, R. and Fagereng, Å., 2022. The role of quartz in the seismic cycle. Rev. Geophysics, 60, 2021RG000768. 61 �The Evolution of Baraboo Interval Sedimentary Rocks: Deposition at 1.63 Ga and Metamorphism at 1.47 Ga MEDARIS, Gordon Jr., BONAMICI, Chloe, BROWN, Phil, GOODWIN, Laurel, JICHA, Brian, SINGER, Brad, SPICUZZA, Michael, VALLEY, John Department of Geoscience. University of Wisconsin–Madison, Madison, Wisconsin 53706 Supermature siliciclastic sedimentary rocks of the Baraboo Interval (Dott, 1983) were deposited in the southern Lake Superior region following the 1.65-1.63 Ga Mazatzal orogeny and subsequently experienced 1.47 Ga fluid-rock interactions related to the trans-Laurentian Pinware-Baraboo-Picuris orogeny (Daniel et al., 2022). Metamorphic fluid-rock interactions include dehydration and metasomatic varieties, the latter having been promoted by regional-scale advective flow of brines along permeable channels in the various Baraboo Interval occurrences. In the Baraboo Range, south-central Wisconsin, the supermature sedimentary rocks are composed of five oxides, viz. SiO2, Al2O3, Fe2O3, TiO2, and H2O, with CaO, Na2O, and K2O having been largely removed during weathering of the igneous basement. During metamorphism, the original sedimentary mineral assemblage of kaolinite + quartz + hematite + rutile was transformed to one of pyrophyllite + quartz + hematite + rutile through the dehydration reaction, Al2Si2O5(OH)4 (kln) + 2SiO2 (qtz) = Al2Si4O10(OH)2 (prl) + H2O (fluid) Note that in shale, which consisted mostly of kaolinite, the appearance of pyrophyllite is accompanied by diaspore, as expressed by the dehydration reaction, 2Al2Si2O5(OH)4 (kln) = Al2Si4O10(OH)2 (prl) + 2AlO(OH) (dsp) + 2H2O (fluid) Figure 1. Schematic cross-section of the Baraboo Range, indicating the various metasomatic mineral assemblages; the numbers specify 40Ar/39Ar plateau ages for muscovite. Folding and metamorphism in the Baraboo Range were accompanied by advective flow of brines and potassium metasomatism along the base of the quartzite and in the overlying slate (Fig. 1). At the base of the quartzite, kaolinite was replaced by muscovite in paleosol, kaolinite was replaced by pyrophyllite and accompanied by precipitation of muscovite in metapelite (pipestone), and thin diaspore hydrothermal veins (bordered by muscovite) intruded quartzite 62 �above the metapelite. Slate that overlies the quartzite consists of muscovite, chlorite, quartz, hematite, and rutile and contains 4.7% to 6.4% K2O, due to potassium metasomatism, compared to 3.5% K2O in average shale. The paleosol at the base of the quartzite in Baxter Hollow (Fig. 1), which is 796 cm thick, experienced a total flux of 0.46 mol cm-2 K2O during metasomatism. Five additional Proterozoic paleosols in the southern Lake Superior region, ranging in thickness from 300 cm to 950 cm, also experienced potassium metasomatism, with K2O fluxes between 0.22 and 0.73 mol cm-2, respectively; for all six paleosols taken together, K2Oflux = 0.00066  thicknesscm + 0.06, for which R2 = 0.82. SiO2 was mobilized high in the quartzite section at the base of the metasiltstone and metapelite horizon in the south limb of the syncline, where quartz was precipitated in several bedding-parallel slickenfiber layers, 3 mm to 8 mm thick (Fig. 1). Individual slickenfibers are cylindrical, having a:b:c fabric ratios of 8:1:1, in which the a-dimension is up to 4 mm in length. Slickenfibers plunge down-dip approximately perpendicular to the fold axis of the syncline, and slickenfiber steps record top-to-the-south shear. SiO2 was also mobilized above the metasiltstone/metapelite horizon, where quartz was precipitated in bedding-parallel quartzite breccia zones up to 100 m thick (Fig. 1). The breccia zones consist of angular red quartzite fragments cemented by a stockwork of white quartz veins that consist predominantly of coarse-grained quartz and small amounts of specular hematite and locally, coarse-grained muscovite. Euhedral quartz crystals occur in late-stage vugs, some of which are partly to completely filled by kaolinite. Values of 18O (-2‰ to +31‰ VSMOW, SIMS) in euhedral quartz correlate with complex patterns of growth zoning and healed fractures (SEM-CL) to reveal multiple fluid events, including high-T (~300 oC) and low-T (50-100 oC) exchange with hydrothermal and meteoric fluids (Schranz et al., 2017). The pyrophyllite + diaspore mineral assemblage in metapelite constrains the temperature of Baraboo recrystallization to between 315 oC and 360 oC at a pressure of 2.0 kbar. An isochor for fluid inclusions in quartz in a folded quartz vein in metapelite, combined with phase equilibrium considerations, yields T-P conditions between 320 oC, 2.7 kbar, and 385 oC, 4.0 kbar, corresponding to a thermal gradient of ~30 oC/km. Expressed another way, the thermal gradient for metamorphism of the Baraboo quartzite was ~1700 oC/GPa, which places Baraboo metamorphism in the high T/P type of metamorphism (775 oC/GPa < T/P < 2000 oC/GPa), as defined by Brown and Johnson (2019). 40 Ar/39Ar plateau ages for muscovite in paleosol (1467 ± 11 Ma), hydrothermal veins (1478 ± 12 Ma), quartzite breccia (1472 ± 3 Ma), and four samples of slate (between 1493 ± 3 and 1473 ± 3 Ma) demonstrate that recrystallization and K-metasomatism in the Baraboo Range were contemporaneous with emplacement of the 1476-1470 Ma Wolf River A-type ferroan granitic batholith in Wisconsin. Such metamorphism and magmatism in Wisconsin represent the local expression of the continental-scale geon 14 Pinware-Baraboo-Picuris orogeny, which is characterized by high T/P metamorphism and A-type ferroan granitic batholiths. References Brown, M. and Johnson, T., 2019. Metamorphism and the evolution of subduction on Earth. Am. Mineral., 104: 1065-1082; Dott, R.H. Jr., 1983. The Proterozoic red quartzite enigma in the north-central U.S. – resolved by plate collision? Geol. Soc. Am. Mem., 160: 129-141; Daniel, C.G., et al., 2023. Linking the Pinware, Baraboo, and Picuris orogens: Recognition of a trans-Laurentian ca. 1520-1340 Ma orogenic belt. Geol Soc. Am. Mem., 220: 175-190; Schranz, L., et al., 2017, Stable oxygen isotopes, fluid inclusions, and microstructures in Baraboo Quartzite breccia. Proc. ILSG, v. 63/1: 83-84. 63 �Geochemistry of Midcontinent Rift-related intrusive rocks of the Sunday Lake intrusion MEXIA, Kevin1 and HOLLINGS, Pete1 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, On P7B 1J4, Canada. The Sunday Lake intrusion is located 25 km north of Thunder Bay, Ontario and hosts Ni-CuPGE mineralization. It has an age of 1109.0±1.3 (Bleeker et al., 2020), and is related to the ~1115 to 1106 Ma magmatic event of the Midcontinent Rift System (MRS; Heaman et al. 2007). The intrusion is emplaced in Archean rocks of the Quetico Basin along the Crock Fault (Flank, 2017). It is not exposed at surface but was first identified from airborne magnetic surveys. In 2008, platinum and palladium mineralization was discovered by HTX Minerals Corp. In 2017 Impala Canada Ltd. Partnered on a joint venture with more than 30 holes drilled to date. The intrusion is funnel-shaped with a width of up to 1.5 kilometers and is 3 kilometers in length. It varies from 350 meters to 1000 meters in thickness. The intrusion consists of maficultramafic layers divided into three series: the Upper Gabbro Series, the Lower Gabbro Series, and the Ultramafic Series (Flank, 2017). Reef-style mineralization present in the lower zones of the intrusion consists of disseminated to blebby chalcopyrite-pyrrhotite-pyrite bearing olivine melagabbro (Fig. 1; Miller, 2020). One hole intersected the basal zones of the intrusion with more than 20 meters of mineralization at 2.11 g/t Pt, 0.95 g/t Pd, 0.16g/t Au, 0.26% Cu, and 0.11% Ni (Flank, 2017). The objectives of this project are to characterize the paragenesis of the Sunday Lake intrusion and the Ni-Cu-PGE mineralization, investigate the effects of crustal contamination on mineralization within the Sunday Lake Intrusion, and to place the Sunday Lake intrusion within the evolution of the MRS. This project utilizes two representative drill holes from which a total of 71 samples were collected. A total of thirty polished thin sections were generated for petrographic studies. Rocks were classified based on relative proportions of olivine, clinopyroxene, and plagioclase with modal rock names such as melagabbro, olivine melagabbro, and wehrlites. Fifty-five samples were analyzed for major and trace elements. Spider diagrams show different compositions within the layered intrusion, with primitive samples having trends consistent with a plume-like composition (Fig. 2A) while others suggest interaction with and contamination by host rocks (Fig. 2B). Variation in the behavior of trace elements suggest contamination, assimilation, and fractional crystallization processes were involved in the magmatic evolution of the intrusion. Sixteen samples have been sent for Sm-Nd and Rb-Sr isotope studies. The results of this study will be used to assess the source of mineralization and extent of contamination of the Sunday Lake Intrusion. 64 �A SL23KM41 Cpy Po B 5 mm Gangue Figure 1. Photomicrograph in reflected natural light (PPL) of a gabbroic sample containing pyrrhotite and chalcopyrite. Polished thin section scanned using a Zeiss microscope. References Figure 2. Primitive mantle normalized REE spider diagram of two samples. A: Sample showing a plume-like trend. B: Sample suggesting an interaction with the host rock. Normalising values from Sun and McDonough (1989). 9 Bleeker, W., et al. "The Midcontinent Rift and its mineral systems: Overview and temporal constraints of Ni-Cu-PGE mineralized intrusions." Targeted Geoscience Initiative 5 (2020): 7-35. Flank, S. (2017). The Petrography, Geochemistry and Stratigraphy of the Sunday Lake Intrusion, Jacques Township, Ontario. School of graduate studies. Heaman, L. M., Easton, R. M., Hart, T. R., MacDonald, C. A., Hollings, P., & Smyk, M. (2007). Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario. Canadian Journal of Earth Sciences, 44(8), 1055-1086. Miller, J.D., Green, J.C., and Severson, M.J. (2002). Terminology, nomenclature, and classification of Keweenawan igneous rocks of northeastern Minnesota. In Miller, J.D. Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.E., and Wahl, T.E., Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, p. 5-20. Miller, J.D. (2020). Report on the Petrography, Geochemistry, and Lithostratigraphy of DDH SL10-026 from the Southern Sunday Lake Intrusion. JDM GeoConsulting. Sun, S. S., & McDonough, W. F. (1989). Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. Geological Society, London, Special Publications, 42(1), 313-345. Wold, R.J., Hinze, W.J. (1982). Geology and tectonics of the Lake Superior basin. Geol. Soc. Am. Mem. 156, 280. 65 �TWO DECADES OF TEACHING THE GEOLOGIC HERITAGE OF MINNESOTA’S NORTH SHORE AT THE NORTH HOUSE FOLK SCHOOL, GRAND MARAIS MILLER, Jim1 and GREEN, John2 1 2 Department of Earth and Environmental Science, UMD – retired; current residence: Shuniah, ON Department of Earth and Environmental Science, UMD – retired; current residence: Duluth, MN Since 1997, the North House Folk School, located in Grand Marais, Minnesota, has been promoting lifelong learning in the traditional arts and crafts and in knowledge about our northern culture and environment - present and past. Starting with a dozen courses at its inception, North House currently offers over 350 classes per year to over 3,000 students. From 2004 to 2010, John Green lent his expertise to North House by offering weekend lectures and field courses each year on basic geology and the geology of the North Shore and the Gunflint Trail. As a bonus, he also compiled lists of the native plants seen on these field excursions. In 2013, Jim Miller revived the course and has come to offer 2-3 courses per year that explore the Midcontinent Rift geology of the North Shore (What’s This Rock series) and the diverse geology at the end of the Gunflint Trail (Geology up the Trail series). By 2022, three different North Shore classes are offered (two per year, rotating in May and August) that explore different segments of the shore: What’s this Rock? – Grand Portage to Grand Marais, What’s this Rock Too? - Grand Marais to Tettegouche State Park, and What’s this Rock 3? - Tettegouche to Two Harbors (Fig. 1). From 2014 to 2019, a mid-October weekend field trip at the head of the Gunflint Trail was run out of the Gunflint. We hope to reinstate this course this fall or next, but will run it out of Grand Marais. Due to the Covid pandemic, no in-person field courses were permitted during 2020 and most of 2021 (one WTR course was run in October 2021, but participants drove their own vehicles). North House opted to host on-line webinars during the winter of 2021-22. Jim presented three webinar series. In January 2021, three lectures were offered on North Shore geology which was virtually attended by 104 students. During March 2022, three lectures were presented on the geology of Minnesota State Parks and Waysides with 83 people logged in. Then, in January 2023, a two-lecture webinar on the geology of the Gunflint Trail was viewed by 50 people. With the lifting of all Covid restrictions in the winter of 2022-23, North House reverted to only offering in-person classes. As currently taught, the three weekend WTR courses start with an introductory meeting on Friday evening on the North House campus in Grand Marais to discuss trip logistics and provide a geologic overview. Saturday is devoted entirely to a field trip that visits various classic geological exposures along the North Shore. Travel has typically involved carpooling with personal vehicles, but starting in August, 2023, the field trips have used a mini-bus. This or a 15passenger van will be the preferred method of transport going forward. In the evening, participants have the option to gather for informal discussions or a special lecture on various topics, especially at wood-fired pizza party held either Friday or Saturday evenings at the North 66 �House campus. The weekend concludes with a half-day field trip on Sunday morning, after which the group gathers either on a cobbled beach on Lake Superior to practice their newfound rock identification skills. Each field course is limited to about 15 registrants. A total of over 400 students have attended the field courses we have taught at North House over the past 20 years. Participants have ranged in age from 10 to 80 and come from all over the US and Canada, though most are from Minnesota, especially the Twin Cities. Their backgrounds range from those who have never heard of plate tectonics, to those who have had a few geology courses in their past. The common denominator among all participants is that they can all be characterized as being “rock curious”. Figure 1: Geology of northeastern Minnesota showing the general locations of geology field courses currently taught at North House Folk School - three “What’s this Rock?” courses and a Gunflint Trail (GFT) course. 67 �Quartz trace element chemistry: Exploring the link between a fertile parental granite and a mineralized pegmatite MORSON, Mia, and ZUREVINSKI, Shannon Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada Recent studies have proposed the use of pegmatitic quartz trace element chemistry as an indicator to lithium mineralization of potential economic pegmatite deposits (Müller et al., 2021). Using Laser Ablation- Inductively Coupled Mass Spectrometry (LA-ICP MS), the trace elements are determined in situ within a single quartz grain. Trace elements such as Al, Ge, Ti, P, B, and Fe3+, can substitute for Si4+ at very low concentrations, and the elements H, Li, Na, K, Fe2+ can enter the quartz crystal lattice via interstitial lattice positions (Götze et al., 2004). In this study, quartz from the 2685 Ma fertile Ghost Lake granitic batholith (GLB) and related Mavis Lake pegmatites group (Dryden, Ontario) were used to compare trends in quartz trace element concentrations within a single system (Figure 1). The focus of this study was to test the applicability of using quartz to (1) help identify fertile parental granitoid plutons and (2) help decipher any internal fractionation trends in mineralized pegmatites. If trace element concentrations in quartz from the granitic parent show any relationship to the mineralized pegmatites, this would prove beneficial in other exploration programs where potentially enriched Li pegmatites have not yet been identified. It could allow for an early assessment of an S-type granitoid and show indication of a potential nearby mineralized pegmatite, essentially the technique could be deemed a ‘fertility indicator’. Furthermore, if the quartz trace element compositions in the pegmatite zones show enrichment trends similar to the proposed model of Černý (1991), the technique would prove powerful in defining potential areas for further investigation (i.e. following the trend of enrichment). Figure 1: Study sample location through the zones of fractionation and enrichment, from the Ghost Lake Batholith to the Mavis Pegmatites (modified from Breaks and Selway, 1991). The results show correlations with increasing Li, Al, and Ge, and decreasing Ti from the GLB fertile parent granite to mineralized Mavis Lake pegmatites (Figure 2). The quartz trace elements can be used to form a simple fractionation model, similar to the pegmatite fractionation model of Černý (1991) to show elemental enrichments in a fertile LCT pegmatite system upon evolution (Figure 3). In summary, the quartz chemistry indicates fractionation and enrichment 68 �trends can be identified across the pegmatite zones. It is still unclear whether or not the technique could be applied to granites in order to assess fertility, as more work is required to understand the key differences in the quartz chemistry between a barren granite and a fertile parent granite. Figure 2: Trends in quartz trace element concentration in the GLB granites, intermediate beryl columbite zone, and mineralized Mavis Lake pegmatites. (a) Al vs Li bivariate plot. (b) Ti vs Ge bivariate plot. Figure 3: Fractionation model showing quartz trace element trends within the GLB-Mavis Lake system (modified from Černý, 1991). Breaks F.W., Selway, J.B., Tindle, A.G., 2005. Fertile peraluminous granites and related rare-element pegmatites, Superior Province of Ontario. Rare-Eelement Geochemistry and Mineral Deposits: Geological Association of Canada (GAC) Short Coarse Notes 17: 87-125. Černý, P., 1991. Rare-element granitic pegmatites. Part 1: Anatomy and internal evolution of pegmatite deposits. Part 2: Regional to global environments and petrogenesis. Geoscience Canada 18:49– 81. Götze, J., Plötze, M., Graupner, T., Hallbauer, D.K., Bray, C. J., 2004. Trace element incorporation into quartz: A combined study by ICP-MS, electron spin resonance, cathodoluminescence, capillary ion analysis, and gas chromatography. Geochimica et Cosmochimica Acta, 68(18): 3741–3759. Müller A., Keyser W., Simmons W. B., Webber K., Wise M., Beurlen H., Garate-Olave I., Roda-Robles E., Galliski M. A., 2021. Quartz chemistry of granitic pegmatites; implications for classification, genesis and exploration. Chemical Geology, 584:1-17. 69 �Geometry, Slip Kinematics, and Deformation along the Hancock Fault in the Quincy Mine Workings, Upper Peninsula of Michigan MURPHY, Braxton, LANGFIELD, Katherine, DeGRAFF, James Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, MI, USA 49931 The Hancock fault is one of several major compressional features along the southern edge of the Midcontinent Rift System. It forms part of the Keweenaw fault system (KFS) whose connected segments follow the spine of Michigan’s Keweenaw Peninsula. The Hancock fault is a splay in the hanging wall of the KFS that intersects the main Keweenaw fault zone at an acute angle to define a thrust slice (Fig. 1). It extends along an azimuth of 55° for 17 kilometers from a point west of Houghton to its intersection with the main Keweenaw fault zone between Calumet and Lake Linden. Volcanic and sedimentary layers of the Portage Lake Volcanics (PLV, 1.1 Ga) are shown on bedrock geology maps with left-lateral offset across the Hancock fault (1, 2). Figure 1: Hancock (HF) and Keweenaw (KF) faults shown on USGS bedrock geology maps (1, 2). Major layers: pb-Bohemia conglomerate, psc-Scales Creek flow, pkKearsarge flow, pgGreenstone flow, chc-Copper Harbor conglomerate (base). Like other faults of the KFS, the Hancock fault has a major component of reverse slip that occurred during compression related to the Grenville Orogeny (3). Recent mapping and fault-slip measurements on the population of faults associated with the KFS reveal that the fault system has a right-lateral component of strike slip. This raises the question of whether the leftlateral offset of units across the Hancock fault in map view can be reconciled with net reverse and right-lateral slip along the KFS. The work reported here builds on previously reported work on the Hancock fault in the Quincy Mine workings to address this and other questions about its geometry, slip kinematics, and deformation (4). An adit in east Hancock provides access to the 7th level of the historic Quincy Mine, whose workings run along and across the Hancock fault at four locations (Figs. 1 and 2). The current phase of the project focused on acquiring data from the fault southwest of the adit, which data 70 �were combined with data previously collected from the adit toward the northeast. Fault-slip measurements were made on all accessible fault surfaces and consisted of fault attitude, slickenline rake, and slip sense where possible. The Hancock fault’s strike and dip were measured at well-exposed locations, and the thickness of its gouge and breccia were measured where possible. Orientations were measured using the FieldMove Clino app as well as a Brunton compass. Stereonet and FaultKin freeware were used to plot and analyze the orientation data. Figure 2: Geology along the Hancock fault in the Quincy adit and connected mine workings. The project is ongoing but some results have emerged. The Hancock fault cuts upward across stratigraphy in the direction of thrusting at angles between 4° and 18°, similar to cut-off angles for the Keweenaw fault (5). The low cut-off angles of both faults, which have significant reverse slip, are consistent with the properties of a detached thrust system. Both faults also have components of strike slip as indicated by the population of nearby faults, but right-lateral slip is slightly dominant over left-lateral slip. This implies that the significant left-lateral offset of PLV layers seen in map view is the result of mostly reverse slip on the Hancock fault as it cuts slightly clockwise to strike of the layers. Acknowledgements Funding for this work was provided by the ILSG Student Research Fund and is gratefully acknowledged. References 1. Cornwall, H.R. and Wright, J.C., 1956a, Geologic Map of the Hancock Quadrangle, Michigan: U.S. Geological Survey, Mineral Investigations Field Studies Map MF-46, scale 1:24,000. 2. Cornwall, H.R. and Wright, J.C., 1956b, Geologic Map of the Laurium Quadrangle, Michigan: U.S. Geological Survey, Mineral Investigations Field Studies Map MF-47, scale 1:24,000. 3. Cannon, W.F., 1994, Closing of the Midcontinent Rift: a far-field effect of Grenvillian compression: Geology, v. 22, pp. 155-158. 4. Langfield, K.M., DeGraff, J.M., and Gamet, N.G., 2023, Slip kinematics of the Keweenaw and Hancock faults within the Midcontinent Rift System, Upper Peninsula of Michigan: Inst. on Lake Superior Geology, 69th An. Meeting, Eau Claire, WI, Part 1 – Program and Abstracts, v. 69, p. 50-51. 5. DeGraff, J.M. and Carter, B.T., 2023, Detached structural model of the Keweenaw fault system, Lake Superior region, North America: Implications for its origin and relationship to the Midcontinent Rift System: Geological Society of America Bulletin, v. 135, no. 1/2, p. 449–466. 71 �Lithostratigraphy and Geochronology of the Lower Northeast Sequence of the North Shore Volcanic Group, Cook County, MN, USA NOWARIAK, Eric S., SEVERSON, Allison R., BLOCK, Amy Radakovich Minnesota Geological Survey, Department of Earth and Environmental Sciences, University of Minnesota-Twin Cities, MN, USA The Lower northeast sequence (LNE) of the North Shore Volcanic Group (NSVG) in northeastern Minnesota represents some of the earliest known sedimentary and volcanic rocks of the Midcontinent Rift System (MRS) including the Puckwunge Formation (PF), Grand Portage Lavas (GPL), Esther Lake Lavas (ELL), and the Hovland Lavas (HL) (Miller and others, 2002). New 1:24,000 field mapping in northern Cook County, paired with U/Pb TIMS zircon geochronology have established an updated lithostratigraphic sequence of the LNE and understanding of its relationships with surrounding intrusive rocks, providing new insights on magmatic evolution and spreading rates during the Plateau Stage of MRS magmatism. The LNE is a shallow to moderately south dipping bimodal volcanic sequence that youngs southward and is segregated from the overlying Upper northeast sequence by the cross-cutting Brule-Hovland Gabbro, a complex of texturally and mineralogically varied gabbroic and diabasic intrusions (Fig. 1). To the north, the LNE is bounded predominantly by cross-cutting Early Stage Duluth Complex granophyric intrusions and locally by underlying Paleoproterozoic metasedimentary sequences. The arenitic sandstones and conglomerates of the PF, which forms the base of the LNE, lie unconformably on top of Paleoproterozoic bedrock and forms the base of the LNE on to which the NSVG rocks were erupted. GPL volcanics were erupted disconformably atop the PF, and consist of geochemically primitive basalts, as compared to overlying LNE volcanics (Mattis, 1972). Basal units of the GPL are defined by pillowed and fragmental olivine basalts, transitioning to thick flows of massive to sparsely amygdaloidal basalts with rubbly tops. The boundary between the GPL and the overlying ELL is marked by a distinct change from olivine tholeiite basalts to thick, pilotaxitic flows of amygdaloidal, oxide-rich ferroandesites and andesitic basalts which exhibit steeper REE 72 Figure 1. Schematic Stratigraphic Section of the LNE showing locations of geochronologic samples. See text for details. �profiles than GPL basalts. U/Pb zircon geochronologic analysis of a thin rhyolite flow interlayered with the massive andesitic basalts near the base of the ELL returned an age of ca. 1105.4 Ma (Fig. 1). Bimodal volcanics of the HL sequence above the ELL are typified by feldspar-phyric to glomeroporphyritic flows of basalt, basaltic andesite, trachyandesite, and rhyolites. Plagioclase phenocrysts within the mafic and intermediate lithologies are fractured, locally resorbed, contain inclusions of olivine and clinopyroxene, and have compositions estimated to be more calcic than the groundmass feldspars. Mafic and intermediate lithologies are locally pillowed, though most flows do not show evidence of subaqueous eruption. Rhyolites constitute a large volume of the upper half of the HL sequence. These rhyolites exhibit abundant autobreccia textures, flow foliation, pumice fragments, and phenocrysts of feldspar and quartz. Microscopically, HL rhyolites are texturally diverse, containing microlites, perlitic structures, fiamme, and spherules interpreted to be a product of volatile-rich lavas and pyroclastic flows. U/Pb zircon analyses of upper HL rhyolites returned ages of ca. 1105.69 Ma and 1106.0 Ma (Fig. 1). Basalt flows intercalated with the rhyolite units are thick, structured flows with basal breccias, columnar jointed massive flow centers, and amygdaloidal flow tops. Subvolcanic, plagioclase ultraphyric diabase dikes and sills of Burnell’s (1976) Brule Lake Porphyry (BLP) are abundant throughout the HL. The BLP contains 20-70% plagioclase phenocrysts hosted within a compositionally varied matrix, and although they have not been dated, a synvolcanic interpretation has been applied to these intrusions based on their local amygdaloidal nature and textural similarities to the HL. New ages from rhyolites in the basal portion of the ELL and upper portion of the HL at ca. 1106 Ma suggest these lavas are volcanic expressions of the coeval Early Stage Duluth Complex Cucumber Lake and Misquah Hills granophyres exposed to the north of the LNE (Vervoort and others, 2007). The Early Stage granophyres are interpreted to be a product of assimilation of continental crust (Vervoort and others, 2007). Such a change in magma composition is reflected in the lithologic and geochemical character of the ELL and HL as compared to the underlying GPL. The overlap in ages presented here suggests rapid eruption rates at ca. 1106 Ma, which indicate the entire ELL and HL sequence erupted within ~1 Ma. Phenocryst-rich volcanic and hypabyssal rocks throughout the HL and BLP may have been derived from anorthositic cumulates that formed in the roof of a mid-crustal staging chamber during a period of slow rift spreading, and subsequently remobilized during magma recharge and venting events at 1106 Ma. References Burnell, J.R., Jr., 1976, Petrology and structural relations of the Brule Lake intrusions, Cook County, Minnesota: Minneapolis, University of Minnesota, M.S. thesis, 105 p., 1 pl. Mattis, A. F., 1972. The Petrology and Sedimentation of the Basal Keweenawan Sandstones of the North and South Shores of Lake Superior. University of Minnesota – Duluth, M.S. thesis. Miller, James D., Jr.; Green, J.C.; Severson, M.J.; Chandler, V.W.; Hauck, S.A.; Peterson, D.M.; Wahl, T.E., 2002. RI-58 Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey. Vervoort, J. D., Wirth, K., Kennedy, B., Sandland, T., & Harpp, K. S., 2007. The magmatic evolution of the Midcontinent rift: New geochronologic and geochemical evidence from felsic magmatism. Precambrian Research, 157(1-4), 235-268. 73 �Major element geochemistry and first zircon U-Pb age dates of Precambrian basement rocks in eastern North Dakota PEREIRA, Cristian1, NESHEIM, Timothy2, VERVOORT, Jeffrey D. 3, and SAINIEIDUKAT, Bernhardt1,4 1 Department of Earth, Environmental and Geospatial Sciences, North Dakota State University, Fargo, ND 58102, USA 2 North Dakota Geological Survey, 2835 Campus Rd., Grand Forks, ND 58202 USA School of the Environment: Earth Sciences, Washington State University, Pullman, WA 99164 USA 3 4 Dept of Chemistry and Biochemistry, North Dakota State University, Fargo, ND 58102, USA We are re-examining cores of the 1977 Red River Valley Drilling Project (Moore, 1978). Other previous work includes study of the paleoweathered horizon on the Precambrian bedrock (Kelley, 1980), Klasner and King (1986), Sims et al. (1991) and various ILSG abstracts. We sampled the Precambrian portions of three of these cores (RRVD #5, #8, #11) from eastern North Dakota, and a core cut by Kennecott Exploration Company in 2010 (10NDV001; Nesheim, 2013) (Fig. 1; Table 1). Samples were analyzed at Washington State University (WSU) for U-Pb zircon age dates. Kelley (1980) reported major element analyses and new analyses were carried out at WSU and North Dakota State University using XRF (Table 2). Figure 1. Precambrian geology map of North Dakota after Nesheim (2012) and Sims et al. (1991), with location map and core locations (stars) for this study Table 1. Summary of samples, lithology, and zircon age dates Core / depth Lat / Long lithology Zircon age (MSWD) RRVD 5 46.225514 Fine to medium grained 2715.0 +/- 18.1 Ma (3.4) 379 ft (115.5 m) -96.932504 quartz monzonite RRVD 8A 46.897502 Fine to medium grained 2782.7±9.3 Ma (0.72) 600 ft (182.9 m) -97.370662 chlorite gneiss RRVD 11 47.614926 Medium grained biotite 2 populations: younger 693 ft (211.2 m) -97.291738 granitoid 2671±23.2 Ma (3.0) 10NDV001 48.61706 Medium grained 2694.5±13.6 Ma (1.19) 837 ft (255.1 m) -97.316902 magnetite-rich granitic gneiss 74 �Table 2. Whole rock major element analyses wt.% 1 2 3 4 SiO2 74.00 68.91 64.30 65.80 TiO2 0.16 0.18 0.13 0.42 Al2O3 13.7 10.43 17.40 15.20 Fe2O3 1.36 2.04 4.67 FeO 1.65 MnO 0.01 0.31 0.02 0.10 MgO 0.04 0.44 0.71 0.00 CaO 1.49 4.60 0.91 1.65 Na2O 4.60 0.87 8.16 6.00 K2O 4.22 6.82 5.96 5.90 P2O5 0.09 0.03 0.06 0.05 SO3 0.04 LOI 5.44 sum 99.67 99.72 99.68 99.79 1: RRVD 5-383.5''; 2: RRVD 8A-602'; 3: RRVD 11-695'; 4: 10NDV001-836' Figure 2. U-Pb concordia diagrams for zircons from the Precambrian core samples with weighted mean 207 Pb/206Pb ages. Error ellipses represent 2SE uncertainties. Open ellipses with thick grey lines depict outlier U-Pb zircon analyses removed from final age determinations. The analyzed rocks contain 64-74 wt.% SiO2, with RRVD 11-695' showing high total alkalis (Na2O+K2O = 14.12 wt. %). All show Neoarchean zircon ages (2.7 –2.8 Ga) with the granitoids showing slightly younger ages than the gneisses (Table 1; Fig. 2). Sample RRVD 11-693 appears to be a 2 component rock with two zircon populations. These chemical results and measured ages are consistent with those measured in other areas of the Superior Craton (cf. Li et al., 2020). REFERENCES: Kelley, L.I., 1980, Kaolinitic weathering zone on Precambrian basement rocks, Red River Valley, eastern North Dakota and northwestern Minnesota. M.S. Thesis, University of North Dakota. 85 pp. Klasner, J.S. and E. R. King. 1986a. Precambrian basement geology of North and South Dakota. Canadian Journal of Earth Sciences. 23(8): 1083-1102. https://doi.org/10.1139/e86-109 Li, D., Hollings, P., Chen, H., Sun, X., Tan, C., and Zurevinski, S., 2020, Zircon U–Pb and Lu–Hf systematics of the major terranes of the Western Superior Craton, Canada: Mantle-crust interaction and mechanism(s) of craton formation, Gondwana Research, v. 78, p. 261-277. Moore, W. L., 1978, A preliminary report on the geology of the Red River Valley Drilling Project, eastern North Dakota and northwestern Minnesota: Bendix Field Engineering Company Subcontract H77-059-E, 292p. https://www.osti.gov/biblio/6538603 doi:10.2172/6538603 Nesheim, T., 2012, Review of Radiometric Ages from North Dakota’s Precambrian Basement. North Dakota Geological Survey Geologic Investigations No. 160. Nesheim, T., 2013, Recent Diamond Exploration in Eastern North Dakota. NDGS GeoNews, p. 5-7. Sims, P.K., Peterman, Z.E., Hildenbrand, T.G., and Mahan, S., 1991, Precambrian Basement Map of the Trans-Hudson Orogen and adjacent terranes, northern Great Plains, U.S.A.: USGS Miscellaneous Investigations Series Map, I-2214. 75 �The geology and ore deposit model of the high-grade Emily Manganese Deposit, Cuyuna Range, Minnesota: Results from the 2023 drilling program PETERSON, Dean1, and STEINER, Alex1 1 Big Rock Exploration, 2505 West Superior Street, Duluth, MN 55806. The Emily deposit is the highest-grade manganese resource in the USA. The deposit is located along the western margin of the Paleoproterozoic Animikie Basin (Southwick and Morey, 1991) and is hosted by the Emily Iron Formation, a shallow water Superior type iron formation. Recent work by Big Rock Exploration on a drilling program for Electric Metals has identified coherent zones of high-grade mineralization (30 to ≥40 wt.% Mn) over a 1.25-kilometer strike length. An ore deposit model has been developed that incorporates the deposition of primary thin-bedded manganese-iron carbonates (Fig. 1) and later conversion into massive manganese oxide through early folding (Fig. 2) and prolonged periods of weathering, oxidation, and erosion. Figure 1. Model of the primary depositional setting of the Paleoproterozoic Superior-type iron formations of the western Animikie basin. Figure 2. Schematic model for the formation of the Emily District thrust-front folds as related to the Penokean fold & thrust belt of the Cuyuna North and South iron ranges. 76 �Historic exploration and drilling in the 1940’s and 1950’s by Pickands Mather and US Steel identified iron and manganese-bearing mineralization within the Emily Iron Formation (Strond, 1959). US Steel developed but did not implement a preliminary mine plan for mining of the Emily Deposit. Following approximately 50 years of inactivity, Cooperative Mineral Resources (subsidiary of Crow Wing Power) pursued a pilot mining operation using pressurized water that ultimately proved unsuccessful. As a follow up investigation into the outcomes of the pilot mining, a small-scale drill program was completed in 2010-2012. An Emily deposit drilling program was designed and executed by Big Rock Exploration, LLC, in 2022-2023. A total of 29 drill holes were completed to extend mineralization and refine the previous resource estimates. A total of 13,107 feet of drilling was completed for this program. Data collected for this project includes lithological, structural, geotechnical, and geochemical data from drill cores as well as geophysical data from selected drill holes. Through interpretation of legacy, recent and new drilling data, Big Rock Exploration has redefined the stratigraphy of the Emily Iron Formation and developed an ore deposit model for the high-grade manganese oxides of the Emily deposit (Fig. 3). This ore deposit model and associated geological model have been used to support an updated and expanded mineral resource estimate for the Emily Deposit, to be completed by Forte Dynamics. Figure 3. Integrated stratigraphy, permeability, texture, and Mn-grade diagram for the Emily deposit. REFERENCES Southwick, D.L. and Morey, G.B., 1991, Tectonic imbrication and foredeep development in the Penokean orogen, east-central Minnesota; an interpretation based on regional geophysics and results of test drilling, U.S. Geological Survey Bulletin 1904-C, pp. C1–C17. Strong, R., 1959, Report on Geological Investigation of the Cuyuna District, Minnesota, 1949-1959, US Steel Internal Report, 318 pages. 77 �Deformation conditions, micromechanics, and fault zone development in mafic protoliths at the Lac des Iles mine, northwestern Ontario PETERZON, Jordan1, PHILLIPS, Noah1, HOLLINGS, Pete1, and DJON, Lionnel2 1 Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1, Canada Impala Canada, 69 Yonge Street, Suite 700 Toronto, ON M5E 1K3, Canada 2 Faults and their associated damage zones are important geologic structures that serve as permeable pathways through the upper crust; however, the effect of host lithology on fault core development and damage zone structure remains poorly constrained. The development of fault cores and damage zones is typically controlled by the strength and composition of the protolith, conditions of deformation, and fluid chemistry (Caine et al., 1996). Fault zones are characterized by a variably developed fault core composed of unconsolidated gouge or silicified breccias, outward into a highly fractured damage zone and then a relatively unaltered protolith. Trapped mineralization may be offset or remobilized by later faulting. Faults may act as conduits or barriers for fluid flow depending on the proportion of fault core to damage zone (i.e., the fault zone architecture; Caine et al., 1996; Faulkner et al., 2010). Permeability is typically enhanced in damage zones due to the high density of fractures and is diminished in fault cores due to the presence of clay-rich fault gouges. This study examines deformation conditions and fluid-rock interaction of fault zones within the Lac des Iles Complex. The Lac des Iles Complex is a series of mafic-ultramafic intrusive bodies occurring within the Marmion terrane of the Superior Province (Figure 1). The complex has been dated at 2689 ± 1.0 Ma and was emplaced into a ~3.01 to ~2.68 Ga granitegreenstone terrane (Djon et al., 2018). Extensive Ni-Cu-PGE mineralization has been offset by two late reverse faults in the high-grade zones (>4 g/t Pd) called the Camp Lake fault and the Offset fault. A depletion in Pd is observed within the damage zone of each fault, approximately 145 – 180 meters from the actual fault. This depletion is likely due to late fluid flow within the damage zones. Fault cores in tonalite mainly are composed of breccias with calcite to quartz-rich matrix, while fault cores in gabbro are composed of chlorite-rich gouges (Figure 2). Fracture densities in felsic protoliths have a higher fracture density than mafic protoliths suggesting that fluid flow would be more effective in felsic protoliths which may have contributed to depleted mineralization. This implies that host rock lithology strongly affects fault zone structure, including alteration assemblages, fracture densities, and permeabilities. We hypothesize that the development of a frictionally weak, chlorite-rich fault core impeded the development of a more fracture-dense damage zone in the gabbros. Electron microprobe analyses on chlorite grains reveal three generations of chlorite growth have occurred: pre-faulting at ~350°C, syn-faulting at ~150 – 200°C, and post-faulting at ~150°C (Figure 3). Elemental gains and losses from unaltered protolith to fault core were examined to understand the interactions between alteration and fault zones. Within fault cores and damage zones, there is an observable gain in Mg and Fe in mafic protoliths, due to the precipitation of new chlorite within the fault zone. In mafic protoliths, fluid-rock interactions play an important role in the development of fault core and damage zone structures. 78 �Figure 2 Variations in drill core with proximity to faulting. Figure 1 Local geology of the Lac des Iles Intrusion with faults of study. Modified from Djon et al., (2018). Figure 3 Results of chlorite thermometry from electron microprobe analyses. References Caine, J.S., Evans, J.P., and Forster, C. B., 1996. Fault zone architecture and permeability structure. Geology, 24 (11): 1025-1028. Djon, M.L., Peck, D.C., Olivo, G.R., Miller, J.D., and Joy, B., 2008. Contrasting Style of Pd-rich Magmatic Sulfide Mineralization in the Lac des Iles Intrusive Complex, Ontario, Canada. Economic Geology, 113 (3): 741-767. Faulkner, D.R., Jackson, C.A.L., Lunn, R.J., Schlische, R.W., Shipton, Z.K., Wibberley, C.A.J., and Withjack, M.O., 2010. A review of recent developments concerning the structure, and fluid flow properties of fault zones. Journal of Structural Geology, 32 (11): 1557-1575. 79 �Identification of Fertile Parent Granitoid Units in the Superior Province of Ontario PETTIGREW, Therese1, CUNDARI, Robert1, PRICE, Rebecca2, and DUGUET, Manuel3 1 Ontario Geological Survey, 435 James St. South, Thunder Bay, ON P7E 6S7 Ontario Geological Survey, 227 Howey St, Red Lake, ON P0V 2M0 3 Ontario Geological Survey, 933 Ramsey Lake Rd, Sudbury, ON P3E 6B5 Canada aw n 2 Peraluminous granites are widely distributed throughout the Superior Province of Ontario, most notably within and adjacent to the metasedimentary rocks of the English River and Quetico subprovinces from which they were derived by partial melting. There has been a significant amount of work that proposes a direct genetic relationship between peraluminous, S-type granitoids (i.e., fertile parent granites) and rare-element pegmatites of the lithium-cesiumtantalum (LCT) group across the world (see for instance Černý, 1989, 1991; Wise, Müller and Simmons, 2022, and references therein). W ith dr A fertile granite is the parental granite to rare-element pegmatite intrusions. Many granitic melts have the capability to generate fertile granite plutons that will, in turn, produce even more fractionated melts enriched in incompatible elements. In the case of the LCT group pegmatites, the residual melt may percolate into the surrounding host rock and crystallize rare-element pegmatites (Breaks, Selway and Tindle, 2003). Identifying fertile parent granites is an important step in the exploration for rare-element pegmatites as it greatly reduces the search area on a regional scale (Breaks and Tindle, 1997). A significant amount of work was performed by the Ontario Geological Survey (OGS) in the late 1990s and early 2000s to improve our understanding of rare-element pegmatites and their parent granitoid units in the Superior Province, with a focus on northwestern Ontario (e.g., Breaks, Selway and Tindle, 2003). In 2022, ten areas of the Superior Province in Ontario were identified for study as part of the fertile granite project (Figure 1). Locations for sampling were selected to complement the existing granitoid geochemical databases acquired by the OGS, as well as to provide coverage in areas previously not investigated for the presence of fertile granitoid rocks and associated LCT group pegmatites. The 2022 field season was intended as a preliminary investigation of the selected areas. A total of 100 samples were collected (Figure 1) and analyzed for major, trace element and rare earth element geochemistry at the Geoscience Laboratories (Sudbury) to identify potential fertile parent granite bodies. Due to several staffing changes during the spring and summer of 2023, focus on the project was delayed and did not resume until the winter of 2023-24. Further work in support of the fertile granite project will include preliminary evaluation of the geochemical data set generated during the summer of 2022, compilation of geochemical data from previous studies and planning for additional sampling during the 2024 field season. The primary deliverable of the project will be an MRD compiling previously released whole-rock geochemical data related to fertile granites. This compilation will be supplemented with the new whole-rock geochemical data acquired during this study. Additionally, several articles will be generated and released during the course of the project in both the Resident Geologist Program Recommendations for Exploration (released annually in January) and the Report of Activities (published annually in the spring). 80 �aw n dr W ith Figure 1. Locations of fertile granite project target areas (outlined in black) and sample locations (green dots) collected in 2022. Regional geology from Ontario Geological Survey (2011, see publication for a detailed geological legend). Subprovince boundaries are based on Stott (2011) and are outlined in blue (from Cundari, 2022). References Breaks, F.W., Selway, J.B. and Tindle, A.G., 2003. Fertile peraluminous granites and related rare-element mineralization in pegmatites, Superior Province, northwest and northeast Ontario: Operation Treasure Hunt; Ontario Geological Survey, Open File Report 6099, 179 p. Breaks, F.W. and Tindle, A.G., 1997. Rare-element exploration potential of the Separation Lake area: An emerging target for Bikita-type mineralization in the Superior Province of northwest Ontario; in Summary of Field Work and Other Activities, 1997, Ontario Geological Survey, Miscellaneous Paper 168, p. 72-88. Černý, P., 1989. Exploration strategy and methods for pegmatite deposits of tantalum; in Lanthanides, tantalum and niobium, Springer-Verlag, New York, p. 274-302. ——— 1991. Rare-element granitic pegmatites. Part 1: Anatomy and internal evolution of pegmatite deposits. Part 2: Regional to global environments and petrogenesis; Geoscience Canada, v.18, p. 4981. Cundari, R.M., 2022. Identification of Fertile Parent Granitoid Units in the Superior Province of Ontario: Project Description; in Summary of Field Work and Other Activities, 2022, Ontario Geological Survey, Open File Report 6390, p. 30-1 to 30-5. Ontario Geological Survey, 2011. 1:250 000 scale bedrock of Ontario; Ontario Geological Survey, Miscellaneous Release—Data 126 – Revision 1. Stott, G.M., 2011. A revised terrane subdivision of the Superior Province in Ontario; Ontario Geological Survey, Miscellaneous Release—Data 278. Wise, M.A., Müller, A. and Simmons, W.B., 2022. A proposed new mineralogical classification system for granitic pegmatites; The Canadian Mineralogist, v.60, p. 229-248. 81 �Lidar Topography: Bright opportunity for Reading Keweenaw Landscapes ROSE, Bill1 and DeGRAFF, James1 1Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931 USA A GeoAtlas for Keweenaw, Houghton, and Baraga counties will soon be publicly accessible as an exciting new tool for understanding landscapes through lidar surveys and convenient geospatial display tools. In this presentation, we discuss hypotheses that emerged from a “first look” at this remarkable data which reveals greater detail than standard topographic maps. The purpose of this discussion is to stimulate robust investigation to build new geological awareness of geomorphology. The work may be a useful element for geoeducation because of the improved resolution. 82 �Figure 1: Lidar topography may reveal in situ differentiation within thick lavas of the Portage Lake Volcanics. Here, lidar data from the Keweenaw GeoAtlas is used with field mapping data from Cornwall (1951) and Longo (1984). These thick lavas reveal cooling of about 1000 years where the interior texture of the basalt is pegmatoidal, contrasting with ophitic textures near the bottom and top of the flow where solidification was faster. Arrows show the same crossections on lidar and geologic maps. Figure 2: Geology at Traverse Island in Keweenaw Bay. Left -aerial image from Google Earth with offshore tracing of Jacobsville Sandstone strata (yellow) and fractures (red). Onshore features are from Denning (1949). Right – onshore tracing of sandstone strata from 2-m resolution Lidar data. White outline is a caprock of “quartzite” with a possible channel form at its western edge. References Cornwall, H.R., 1951, Differentiation in Magmas of the Keweenawan Series, J Geol, v. 59, pp. 151-172. Denning, R.M., 1949, The Petrology of the Jacobsville Sandstone, Lake Superior: Michigan College of Mining and Technology [MTU], M.S. thesis, 71 p. Longo, A.A., 1984, A correlation for a middle Keweenawan flood basalt: the Greenstone flow, Isle Royale and Keweenaw Peninsula, Michigan, M.S. thesis, Michigan Technological University, Houghton, MI, 198 pp. 83 �Michigan Coastal Path: A Social Commitment to Geoeducation ROSE, Bill1 and VYE, Erika2 1 Geological and Mining Engineering & Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931 USA 2 Great Lakes Research Center, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931 USA The geology of the Midcontinent Rift is beautifully exposed for researchers, teachers, students, and geotourists in the Keweenaw Peninsula and Isle Royale (http://carnegiekeweenaw.org/social-post/keweenaw-shorelines-bill-rose). Michigan holds title to the surrounding bottomlands of the Great Lakes under the Public Trust Doctrine in addition to a public trust interest in the shorelands up to the ordinary high water mark (OHWM). Michigan confronts the challenge of discerning the boundaries between public trust interests and private property rights at the shore (Norton et al, 2013). In 1968, the Michigan Legislature adopted an elevation-based approach for discerning the ordinary high water mark (OHWM). In 2005, the Michigan Supreme Court reaffirmed that Michigan's public trust interest extends up to the OHWM, but it left unresolved questions of exactly how the two methods of marking ordinary high water relate to one another, and precisely how far up the shore the state has authority to regulate private shoreline development extends. (Norton et al, 2011). Here we describe the definitions of high water lines for geologic discussion. How may both landowner and hiker amicably agree on the high water mark when we meet along the shore? Figure 1: Shoreline exposures reveal the rock details clearly - veins of calcite (near the Copper Harbor Light) and native copper (Washington Island). 84 �Figure 2: Shoreline of Copper Harbor Conglomerate on Manitou Island. The zonation and succession of shorelines may be seen and used to define the high water mark. References Norton, R. K., Meadows, G.A., and Meadows, L.A. (2013). The deceptively complicated “elevation ordinary high-water mark” and the problem with using it on a Laurentian Great Lakes shore. Journal of Great Lakes Research, Volume 39, Issue 4, pp 527-535. Norton, R.K. & Meadows, G.A. (2014). Land and water governance on the shores of the Laurentian Great Lakes. Water International 39:6, pages 901-920. 85 �Jacobsville Geoheritage is Globally Celebrated and Locally Loved ROSE, Bill1 and VYE, Erika2 1 Geological and Mining Engineering & Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931 USA 2 Great Lakes Research Center, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931 USA The Jacobsville Sandstone is a well-known red bed sandstone of Neoproterozoic age from Upper Michigan, USA (Cannon and Nicholson, 2001) and is part of the Keweenaw Supergroup related to the Midcontinent Rift System. The rift formed ~1100 Ma and is a ~3000 km long feature in North America, centered on the Lake Superior area. The Jacobsville is the youngest of the area’s Precambrian rocks and was deposited during the Rigolet Phase of the Grenvillian Orogeny (1010-980 Ma) (Hodgin et al 2022). Cliff exposures show crossbedding and channels interpreted as fluvial deposits. Jacobsville Sandstone was a fashionable building stone in much of Eastern North America. From 1885 to 1920, it was used in hundreds of prominent buildings including the famous Astoria Hotel in New York City (Eckert, 2000). It was mined from several quarry sites near Jacobsville, Michigan. The location is part of a significant geoheritage location where native copper has also been mined, valued, and utilized for thousands of years. The development of copper mining drove extensive immigration of Europeans to Upper Michigan. The Jacobsville quarries offered an alternative to underground employment in the booming mining industry of the Keweenaw. Since quarrying has ceased, Jacobsville quarries have been overgrown and are often overlooked. Highlighting the significance of these places and increasing access offers an opportunity to teach locals and visitors about Earth's history and natural/cultural resources. It connects people to a significant element of Keweenaw geoheritage often eclipsed by the history of copper mining. In recent years we have been building awareness of the geohistory and geoheritage of Jacobsville quarrying. This awareness is building educational outreach focused on this remarkable rock formation which features in many local towns. The International Union of Geological Sciences (IUGS) and UNESCO’s International Geoscience Programme (IGP) have announced that the Jacobsville Sandstone - a rock formation named for Jacobsville, Michigan - is now one of only 15 Global Heritage Stone Resources (GHSR) in the world and the first in the United States (Rose et al, 2017). Global Heritage Stone Resources (GHSR) are scientific designations created and managed by the Heritage Stone Subcommission – HSS (IUGS/IAEG) to enhance the geological knowledge, use, and conservation of natural stones of historical importance worldwide. Highlights of recent Jacobsville geoheritage efforts include boat tours that explore the rock exposures in spectacular cliff views and Michigan historic signage in Houghton and other towns. References WF Cannon and SW Nicholson, 2001, Geologic Map of the Keweenaw and Adjacent Area Michigan: U.S. Geological Survey Map I-2696, scale = 1:100,000. WI Rose, EC Vye, CA Stein, DH Malone, JP Craddock and S Stein (2017) Jacobsville Sandstone: A candidate for nomination for Global Heritage Stone Resource, Michigan, USA. Episodes 40 (3), 213-219 86 �K.B. Eckert, (2000). The Sandstone Architecture of the Lake Superior Region. Wayne State University Press, Detroit, USA, 344 p. EB Hodgin, NL Swanson-Hysell, JM DeGraff, ARC Kylander-Clark, MD Schmitz, AC Turner, Y Zhang, DA Stolper (2022). Final inversion of the Midcontinent Rift during the Rigolet Phase of the Grenvillian Orogeny. Geology 2022; 50 (5): 547–551 Fig 1: Red Jacket firehouse in Calumet (built in 1898–99) – National Register of Historic Places. Use through Creative Commons by Andrew Jameson. Fig 2: One of many cliff exposures of the Neoproterozoic Jacobsville Sandstone, here about 1 km N of the town of Jacobsville, 19 km SE of Houghton, in Michigan’s Keweenaw Peninsula. Cliff exposures are found in dozens of locations within Keweenaw Bay. Photo by Steve Brimm. Fig 3: Jacobsville Quarry near Portage Entry, in operation, about 1895 (MTU Neg 03965, Michigan Technological University Archives and Copper Country Historical Collections, Houghton, Michigan Michigan Technological University Archives). 87 �Compiled historical drillhole and geochemical data from the Cuyuna Range, Minnesota, provides powerful new insights for geological and mineral potential investigations. SAARI, Stacy1, GORDEE, Sarah1, RIAN, Madison1, and CARTER, Matt1 1 Minnesota Department of Natural Resources, Lands and Minerals, 1525 Third Ave. East, Hibbing, MN 55746 The Minnesota Department of Natural Resources (DNR) staff of Lands and Minerals (LAM) compiled a large tabular dataset of drill hole locations, geological logs, and geochemical data from the Cuyuna Iron Range of central Minnesota as part of the federally funded Earth Mapping Resources Initiative (Earth MRI). These data will help to determine where potential resources of iron and manganese, as well as other critical minerals, may exist in the underlying bedrock. Iron in the Cuyuna Range was discovered in 1903 by Cuyler Adams and was actively mined until 1984. Prior to the end of WW1, there were 37 active mines on the Cuyuna Range. The lack of outcrop hindered the geological understanding and definition of the resources, and over 12,000 exploration holes were drilled over this period (Morey et al., 1977). Some of the historical explorers and mining operators in this area included: Orelands Mining Company, Evergreen Mining, Pittsburg-Pacific, Pickands-Mather, Oliver Iron Mining (division of US Steel), Inland Steel, Rogers-Brown Company, and Zontelli Brothers (Sutherland, 2016). Since the end of active mining in this district, the DNR LAM office in Hibbing has accumulated thousands of mining and mineral exploration documents from various companies. Current estimates suggest that the Cuyuna Range is among the top three largest manganese occurrences in the United States, justifying continued interest in its resource potential (Strong, 1959; Beltrame et al., 1981; Kilgore and Thomas, 1982; Cannon et al., 2017). From the 1940s to 1960s, the US Bureau of Mines (USBM) assembled, coded, and entered location and geochemical data for about 40% of the 12,833 drill holes available from the USBM Minnesota Mineral Development Atlas. USBM used criteria such as depth of overburden, past mining activity, availability of manganese data, and the availability of the sample material to determine which data to prioritize. All data were entered from public land survey (PLS) sections if there were fewer than 80 drillholes. For PLS sections exceeding 80 drillholes, then only 5 drillholes were entered for each quarter-quarter. This compilation resulted in data for 5,045 drillholes across the entire Cuyuna Range (Morey et al., 1977). Further, in the early 1990s, the Minnesota Geological Survey (MGS) created several databases to compile the assays and geologic logs from the USBMs dataset. Their database contains around 12,000 holes which have limited assay data and generalized geologic logs. Many of these drillhole sites were also entered in the Minnesota Well Index. As part of the Earth MRI project the MGS transferred thousands of maps and drill logs to the DNR. Compiling and managing 10,000s of documents and drill hole locations is a complicated and enormous undertaking, however, the use of geospatial software (i.e., ArcGIS), artificial intelligence and machine learning, and MicroMine 3D modelling software has accelerated the process. Without these technological resources, it would be difficult and time consuming to track 88 �duplication among the various exploration documents as well as the rebranding of drillhole names by successive explorers. The DNR merged the USBM and MGS databases and used the MicroMine software to help identify and resolve missing intervals, overlapping intervals, missing or incorrect azimuth and inclination, drillholes without analytical data, missing total depth, values beyond the end of the drillhole, and coincident drillhole collars. It would be nearly impossible to uncover these errors by hand. DNR staff curated and compiled a collection of US Steel data from the 1950s that was not included in its entirety in either the MGS or USBM compilations. These and other historic exploration data added drill logs and assays for hundreds of holes to the DNR’s drillhole compilation, mainly from the Emily area. Intervals that were assayed from these holes were often missing geological information, likely because the alteration and mineralization made interpretation difficult for the earliest explorers. Any missing collar elevations were obtained from 2012 1-m Lidar, knowing that inaccuracies may exist from previously mined areas or existing stockpiles post-exploration. DNR staff consolidated lithology types by limiting modifiers related to alteration or mineralization and extracted all assay data to create a working 3D model of the Emily district. Not only will future users have access to this model, but they will also be able to easily search by key words, sort based on geochemical results, and view the geographical location of the data. The data compilation is the first of a three-phase project which will be followed by ground and airborne geophysics and later supported by petrographical, lithochemical and geochronological analyses. Subsequent geologic mapping, mineral potential evaluation, and a geological interpretation for the rest of the Cuyuna district will complete this project. The project will culminate in an Earth MRI data release and report in 2026. REFERENCES: Beltrame, R.J., Holtzman, R.C., and Wahl, T.E., 1981, Manganese resources of the Cuyuna Range, eastcentral Minnesota: Saint Paul, Minn., Minnesota Geological Survey Report of Investigations 24, 22 p. Cannon, W.F., Kimball, B.E., and Corathers, L.A., 2017, Manganese, in chap. L of Schulz, K.J., DeYoung, J.H., Jr., Seal, R.R., II, and Bradley, D.C., eds., Critical mineral resources of the United States—Economic and environmental geology and prospects for future supply: USGS Professional Paper 1802, p. L1–L28. Jirsa, M. A., Boerboom, T. J., Chandler, V. W., 2012, Geologic Map of Minnesota, Precambrian Geology, Minnesota Geological Survey Map S-22, 1:500,000. Kilgore, C.C., and Thomas, P.R., 1982, Manganese availability - Domestic: U.S. Bureau of Mines Information Circular 8889, 14 p. Morey, G.B., Broberg, J., Beltrame, R.J., and Holtzman, R.C., 1977, Manganese-Bearing Ores of the Cuyuna Iron Range, East-Central Minnesota, MGS Report of Investigation for Grant U.S.D.I., Bureau of Mines G0264002, 185 p. Strong, R., 1959, Report on Geological Investigation of the Cuyuna District, Minnesota, 1949-1959, U.S. Steel - Oliver Iron Mining Division, 301 p., 6 plates. Sutherland, Frederick E., 2016, The Cuyuna Range: Legacy of a 20th Century Industrial Community. Ph.D. thesis, Michigan Technological University, 271 p. 89 �Understanding the evolution of the upper Midwest Archean gneiss dome corridor using apatite, titanite, and monazite LA-ICP-MS U-Pb geochronology and microstructural analyses SALERNO, Ross1, CANNON, William1, SOUDERS, Amanda2, and THOMPSON, Jay2 1 U.S. Geological Survey, Reston, VA 20192, 2U.S. Geological Survey, Denver, CO 80225 The origin of the Archean gneiss dome corridor stretching across Minnesota, Wisconsin, and northern Michigan is an important question for understanding the Paleoproterozoic tectonic evolution of the upper Midwest. The formation of these gneiss domes was originally attributed to orogenic collapse during the Penokean orogeny (1.86-1.83 Ga) (Schneider et al., 2004). More but more recent work, however, indicates their exhumation may be more closely linked to a suite of younger structures and metamorphism which are broadly concurrent with the Yavapai event between 1.78-1.75 Ga (e.g., Tinkham and Marshak, 2004; Schulz and Cannon, 2007). In this study, we leverage new LA-ICP-MS U-Pb geochronology and microstructural observations using EBSD (electron backscatter diffraction) to shed light on the timing of metamorphism and deformation related to the exhumation of these domes to understand the broader tectonic framework in which they formed. We investigate a suite of metamorphosed and deformed rocks collected from both inside and adjacent to these gneiss domes in northern Michigan (Figure 1). We show that these rocks have metamorphic U-Pb ages ranging from Neoarchean to Mesoproterozoic, reflecting the prolonged tectonic history of the southern margin of Laurentia (Figure 2). In the Paleoarchean Watersmeet gneiss, titanite grains have U-Pb intercept ages of 2550±46 (2σ, n=36) Ma, concurrent with the Sacred Heart orogeny. The U-Pb concordia ages of apatite in the Watersmeet gneiss at 1869±32 (2σ, n=27) Ma, and monazite U-Pb ages in the Hardwood gneiss at 1826±21 (2σ, n=36) Ma, reflect metamorphism of these rocks during the Penokean orogeny. Several samples have apatite U-Pb concorida ages that indicate heating continued for tens of millions of years after the end of the Penokean orogeny at about 1830 Ma: at 1815±32 Ma (2σ, n=17) in the Republic trough, at 1803±29 Ma (2σ, n=49) in the Hardwood gneiss, and at 1796±29 (2σ, n=51)Ma in the Michigamme Formation directly adjacent to the Watersmeet dome. Our dataset documents the influence of post-Penokean orogenic events on the rocks of the gneiss dome corridor in northern Michigan. In the Neoarchean Carney Lake gneiss, migmatitic rocks have titanite U-Pb ages of 1752±71 Ma (2σ, n=53), indicating reactivation during the Yavapai orogeny. In the Solberg schist, in the Felch trough, titanite grains have recrystallized into aggregates of subgrains, likely formed during deformation. These titanite grains have U-Pb ages of 1713±32 Ma (2σ, n=82), which we interpret to reflect the timing of deformation-induced recrystallization. The U-Pb ages of apatite in the Solberg schist are markedly younger at 1588±28 Ma (2σ, n=46) and align with the timing of the Mazatzal orogeny. Together, these new U-Pb data add to a growing body of evidence that the present architecture of the gneiss dome corridor in the upper Midwest is at least in part due to post-Penokean orogenic events. 90 �Figure 1. Generalized geologic map of the study area (modified from Tinkham and Marshack, 2004) showing the locations of sample sites with yellow stars. Black dots show the position of towns W: Watersmeet, R: Republic, H: Hardwood, and M: Marquette. Figure 2. The LA-ICP-MS U-Pb ages of apatite, titanite, and monazite for the samples in this study. The vertical bars represent the timing of major orogenic events along the southern margin of the Superior craton and Laurentia. References Schneider. S., Holm. D., O’Boyle. C., Hamilton. M., Jercinovic. M., 2004, Paleoproterozoic development of a gneiss dome corridor in the southern Lake Superior region, USA: GSA Special Paper 380, 339357. Schulz. K., Cannon. W., 2007, The Penokean orogeny in the Lake Superior Region: Precambrian Research, 157, 4-5. Tinkham. D., Marshak. S., 2004, Precambrian dome and keel structure in the Penokean orogenic belt of northern Michigan, USA: GSA Special Paper 380, 321-338. 91 �Analysis of deformation-related structures in the Eau Claire Volcanic Complex, Wisconsin SHAKKED, Daniel, L.1, ROBARGE, Lucas, C.1 and LODGE, Robert W.D. 1 1 Department of Geology and Environmental Sciences, University of Wisconsin-Eau Claire, Eau Claire, WI 54701, USA This study is focused on the Penokean-aged (1.8-1.9 Ga) deformation fabrics and microstructures in the Big Falls region of the Eau Claire Volcanic Complex (ECVC), Northwestern Wisconsin. Varying intensities of deformation and metamorphism within the Penokean Orogeny are extensive and well documented, particularly in the External Domain in the northern parts of the orogen. However, the southernmost regions are more poorly studied because outcrops are present as rare erosional outliers in river channels. Structural interpretation, and by association terrane boundaries, have largely been inferred from geophysical data. This project focused on describing the structural and metamorphic fabric development at two field areas: one studying the origin of the gneissic banding within the amphibolitic banded gneiss (Figure 1A), while the other being a study on the genesis of the migmatitic fabrics (Figure 1B). The goal of this research is to petrographically and geochemically interpret the deformation mechanisms of the ECVC and improve its tectonic context to the rest of the Penokean orogeny. There are two main volcanic terranes within the Penokean Orogeny and are sutured together by the Eau Pleine Shear Zone. The Pembine-Wausau terrane is characterized as a juvenile arc-system formed through subduction and was accreted onto the southern edge of the Superior Craton. This was followed by the accretion of the Marshfield Terrane; an Archean microcontinent overprinted by Paleoproterozoic magmatism. Historical interpretations of the ECVC suggested these rocks were deposited on the Marshfield Terrane, but recent geochemical and petrochronological studies show a mantle-derived, oceanic affinity (Lodge et al, 2023; Weber et al., 2023). Therefore, revisiting and reinterpreting the tectonic context of the ECVC to the Marshfield Terrane is warranted. The previous interpretation of the bedrock at Big Falls County Park suggests gneissic banding was inherited from igneous layering from a layered mafic intrusion (Cummings, 1984). This study describes field and petrographic observations that indicate intense ductile fabric development during shearing such as asymmetric inclusions, pressure shadows, and feldspar grain-boundary migration within the gneiss (Figure 1A, 1C). Comparison of these textures with other sheared amphibolites and amphibolitic gneisses support the interpretation that banding is, at least in part, caused by intense shearing (e.g. Bozkurt et al., 1997). Geochemical analysis of the Big Falls Region shows a hydrated, mantle-derived signature closely related to an E-MORB oceanic-arc system, indicating that these magmas are not derived from an Archean, continental fragment (Weber et al. 2023). Structural analysis of these rocks shows extensive fabric shearing and deformation, supporting the theory that these rocks are structurally emplaced. Petrographic and outcrop analysis of the tonalite intrusion and “lensoidal amphibolite” in the Little Falls region indicates the gneissic fabrics and banding are migmatites. Two possible theories for the genesis of the migmatites are suggested: anataxis of the amphibolite and granulite facies metamorphism (Ashworth 2011) or melt injection from the tonalite intrusion during the Penokean deformation. Previous interpretations of the Little Falls region indicated three episodes of metamorphism (Cummings 1984). Evidence of a weak foliation and recrystallization in the tonalite intrusion (Figure 1D) containing xenoliths of banded amphibolite gneiss suggests at least two metamorphic events. However, its relationship to the thermal event that formed the migmatites is uncertain. 92 �A B C D Figure 1: A: Outcrop photo of amphibolitic bands and sheared garnet-hornblende porphyroblasts in banded gneiss, Big Falls County Park, Wisconsin. B: Outcrop photo of migmatite at Little Falls County Park, Wisconsin. The neosome consists of quartz, plagioclase, and feldspars while the paleosome consists of hornblende, biotite, and chlorite in the picture. C: Photomicrograph (XPL, 10x) of strained feldspar crystal with grain boundary migration, and an amphibole band being deflected via shearing above the feldspar grain. D: Photomicrograph (XPL, 10x) of the tonalite intrusion at Little Falls which contains quartz, plagioclase, biotite, hornblende, and chlorite. References Ashworth, J.R., 2011, Migmatites. New York, NY, Springer, 371 pp. Bozkurt, E., and Park, R.G., 1997, Microstructures of deformed grains in the augen gneisses of southern Menderes Massif (western Turkey) and their tectonic significance: Geologische Rundschau: Zeitschrift für allgemeine Geologie, v. 86, p. 103–119. Cummings, M.L., 1984, The Eau Claire River complex: A metamorphosed Precambrian mafic intrusion in western Wisconsin: Geological Society of America bulletin, v. 95, p. 75. Lodge, R.W.D., Weber, E.M., and Hooper, R.L., 2023, Precambrian Geology of the Eau Claire River Valley: Re-discovering the Eau Claire Volcanic Complex, in Lodge, R.W.D. ed., Institute on Lake Superior Geology Proceedings, 69th Annual Meeting, Eau Claire, Wisconsin, Part 2 – Field Trip Guidebooks, p.47-70. Schulz, K.J., and Cannon, W.F., 2007, The Penokean orogeny in the Lake Superior region: Precambrian research, v. 157, p. 4–25. Weber, E.M., Lodge, R.W.D., and Marsh, J.H., 2023, U/Pb geochronology and zircon petrochronology of Paleoproterozoic magmas from the Marshfield terrane, Institute on Lake Superior Geology Proceedings, 69th Annual Meeting, Eau Claire, Wisconsin, Part 1-Program and Abstracts, p. 9798. 93 �Geochemistry and Petrology of the Eagle’s Nest Intrusion, McFaulds Lake Greenstone Belt, Northern Ontario SHESHNEV, Vlad1, HOLLINGS, Peter1, PHILLIPS, Noah1, WESTON, Ryan2, DELLER, Matt2, CAMPBELL, Dana2 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada Wyloo Metals, 1-1127 Premier Way, Thunder Bay, ON, P7B 0A3, Canada 2 The Eagle’s Nest intrusion hosts economically significant orthomagmatic Ni-Cu-(PGE) mineralization located in the McFaulds Lake greenstone belt within the northern regions of the Archean Superior province, approximately 500km northeast of Thunder Bay, Ontario. The Eagle’s Nest intrusion is part of the 2734 Ma ultramafic-dominated Koper Lake subsuite of the larger Ring of Fire intrusive suite (Metsaranta et al., 2015; Houlé et al., 2020). The mineralized ore body of the Eagle’s Nest intrusion is zoned, with massive sulfide mineralization at its northwestern extent that gradationally becomes semi-massive, net-textured and disseminated to the southeast (Zucceralli et al., 2022). Mineralization consists of 11.1 million tonnes of proven and probable mineral reserves containing 1.68% Ni, 0.87% Cu, 0.89g/t Pt, 3.09g/t Pd and 0.18g/t Au (Burgess et al., 2012). The intrusion was emplaced along a sub-horizontal conduit, forming a blade-shaped dike (Barnes and Mungall, 2018). Mineralization is consistent with gravitational sulfide segregation at the basal, northwestern contact of the intrusion. Subsequent deformation rotated the intrusion into its present day, subvertical orientation, with a width of ~500m, thickness of ~150m and vertical extent >1600m. Parental magma composition of the Eagle’s Nest intrusion has been determined on a number of occasions but with contrasting outcomes. A low MgO komatiitic magma composition with ~22% MgO and ~12% total FeO was proposed by Mungall et al. (2010). In contrast, Zuccarelli et al. (2022) reported olivine compositions of Fo82-86 consistent with picritic parental magmas containing moderate MgO (10-20%) and high total FeO (>12%). The contrasting results means that parental magma composition of the Eagle’s Nest intrusion needs to be further constrained. The objectives of this study are to petrographically and geochemically characterize the (1) unmineralized portions of the Eagle’s Nest intrusion and (2) associated offshoot dikes in the vicinity, and (3) constrain the parental magma characteristics by using geochemical, petrographic, mineral chemistry, and radiogenic isotope techniques. A total of 136 samples were collected for this study. Forty-four samples came from offshoot dikes consisting of fine- to medium-grained mafic to ultramafic units. Eighty-seven samples were collected from within the intrusion comprising peridotite (Fig. 1), gabbro, and chilled margin samples. Lastly, five samples were collected from the host wall-rock of the intrusion which consists of tonalite. One-hundred and twenty-one samples were analyzed for major and trace elements using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS), respectively. An initial batch of 30 thin sections was prepared consisting of 15 offshoot dikes, eight contacts, and seven peridotite samples. Twenty samples from the intrusion were selected for Sm-Nd isotopes. To constrain the parental magma composition, three approaches will be considered: (1) examination of preserved chilled margins along the length of the chonolith, (2) examination of chilled margins preserved in the magmatic breccia matrix within the stratigraphic hanging-wall 94 �of the intrusion, and (3) mineral chemistry of fresh olivine preserved within the peridotite horizons of the intrusion. The parental magma composition obtained from these three methods will be further compared to ensure consistency between the methods. The intrusion’s magmatic history will be constrained using the obtained Sm-Nd isotope data, which will provide further insights into the mantle source and contamination history of the melts that formed the Eagle’s Nest intrusion. Figure 1. Photomicrograph in crossed-polarized light (XPL) of a peridotite sample containing serpentinized cumulus olivine and poikilitic orthopyroxene with fresh olivine within the oikocryst. References Barnes, S.J. and Mungall, J.E. 2018 Blade-shaped dikes and nickel sulfide deposits: A model for the emplacement of ore-bearing small intrusions: Economic Geology, v. 113, p. 789 – 798. Burgess, H., Gowans, R., Jacobs, C., Murahwi, C. and Damjanović, B. 2012. Noront Resources Ltd.—NI 43–101 technical report feasibility study—McFaulds Lake Property, Eagle’s Nest Project, James Bay Lowlands, Ontario, Canada: Micon International Ltd., 197p. Houlé, M.G., Lesher, C.M., Metsaranta, R.T., Sappin, A.-A., Carson, H.J.E., Schetselaar, E.M., McNicoll, V., and Laudadio, A., 2020. Magmatic architecture of the Esker intrusive complex in the Ring of Fire intrusive suite, McFaulds Lake greenstone belt, Superior Province, Ontario: Implications for the genesis of Cr and Ni-Cu-(PGE) mineralization in an inflationary dyke-chonolith-sill complex: Geological Survey of Canada, Open File 8722, p. 141–163. Metsaranta, R.T., Houlé, M.G., McNicoll, V.J., and Kamo, S.L., 2015. Revised geological framework for the McFaulds Lake greenstone belt, Ontario: Geological Survey of Canada, Open File 7856, p. 61–73. Mungall, J.E., Harvey, J.D., Balch, S.J., Azar, B., Atkinson, J., and Hamilton, M.A., 2010, Eagle’s Nest: A magmatic Ni-sulfide deposit in the James Bay lowlands, Ontario, Canada: Society of Economic Geologists Special Publication, v. 15, p. 539–557. Zuccarelli, N., Lesher, C.M., Houlé, M.G., Weston, R. and Barnes, S.J. 2022. The diversity of nettextured sulfides in Magmatic Sulfide Deposits: Insights from the Eagle’s Nest Ni-Cu-(PGE) Deposit, McFaulds Lake greenstone belt, Superior Province, Canada: Economic Geology, v. 117 (8), p. 1731 – 1759. 95 �New LA-ICP-MS U-Pb geochronology of Archean rocks, central Upper Peninsula, Michigan, USA: a step toward refining the final assembly of the Superior craton SOUDERS, A.K.1, CANNON, W.F.2, DRENTH, B.J.1, SALERNO, R.A.2, THOMPSON, J.M.1, SYLVESTER, P.J.3 1 U.S. Geological Survey, Denver, CO 80225 USA (asouders@usgs.gov) U.S. Geological Survey, Reston, VA 20192 USA 3 Texas Tech University, Lubbock, TX 79409 USA 2 The central Upper Peninsula, Michigan consists of two contrasting Archean terranes of the Superior Province: the granite-greenstone terrane of the Wawa-Abitibi Subprovince in the north (Northern Complex) and the gneisses of the Minnesota River Valley Subprovince (MRVS) in the south (Southern Complex). The two terranes are separated by the Great Lakes Tectonic Zone (GLTZ). The suturing of the MRVS to the southern margin of the Superior Province and development of the GLTZ, long interpreted to have occurred at about 2.69 Ga, has more recently been suggested to be related to the ca. 2.58 - 2.6 Ga Sacred Heart Orogeny (Schmitz et al. 2018; Cannon et al. 2024, this volume), a proposal that we are still evaluating. In this study we present new LA-ICP-MS U-Pb geochronology for Archean crystalline rocks from both the Northern Complex and Southern Complex, across the GLTZ. This age characterization is essential to define/refine the regional geochronologic framework of ‘basement’ rocks in the central Upper Peninsula. This is essential to understand subsequent geologic processes. Heavy mineral separates were produced using Electro Pulse Dissagregation (EPD) followed by heavy liquid separation at Zirchron (AZ, USA). Individual zircon grains were hand-picked and mounted in 25 mm epoxy resin mounts and polished to a 1 µm finish. All samples were imaged via cathodoluminescence in the Denver Microbeam Lab (USGS) using the JEOL 5800 LV SEM. LA-ICP-MS analyses were made using a Nu AttoM sector field ICP-MS coupled to a NWR 193 ArF excimer laser system in the Mineral Isotope Laser Laboratory (MILL) at Texas Tech University. Typical laser ablation conditions during all analytical runs were a fluence of 3 J/cm2, 8 Hz, and 240 laser pulses using a 15 µm laser spot. Data was reduced using Iolite v.4 (Paton et al. 2011) and final age calculations were made using IsoplotR (Vermeesch, 2018). We are presently working on zircon LA-MC-ICP-MS Hf isotope analyses to characterize the source components of Archean crystalline rocks from the Northern Complex and Southern Complex. Zircon grains from nine rocks in the Southern Complex and six rocks in the Northern Complex were targeted for LA-ICP-MS U-Pb analysis. Examples of samples analyzed from Southern Complex granitic gneisses are shown in Figure 1. For all samples, the age spectrum of concordant grains is complicated with multiple inherited populations within a single sample. This observation is like that presented by Ayuso et al. (2018) for the ca. 2700 Ma Carney Lake gneiss and ca. 2750 Ma Hardwood gneiss, south of our current study area. Examples of crystalline samples analyzed from the Northern Complex are shown in Figure 2. A single age population for zircon grains analyzed with little to no evidence of Early or Middle Archean inheritance is common for Northern Complex basement samples. These data support the fundamental difference between the primitive volcanic/plutonic terrane of the Northern Complex and an older Archean continental crustal component in the Southern Complex. 96 �Figure 1. Examples of a subset of Archean granitic rocks sampled from the Southern Complex, MI Figure 2. Examples of a subset of Archean granitic rocks sampled from the Northern Complex, MI References Ayuso, R.A, Schulz, K.J., Cannon, W.F., Woodruff, L.G., Vazquez, J.A., Foley, N.K., Jackson J. (2018) New U-Pb Zircon Ages for Rocks from the Granite-Gneiss Terrane in Northern Michigan: Evidence for Events at ~3750, 2750, and 1850 Ma. ILSG, Proceedings 64th Annual Meeting. Cannon, W.F., Souders, A.K., Drenth, B.J., Ayuso, R.A. (2024) The Sacred Hearth Orogeny in Michigan: Latest Archean Granites and the Great Lakes Tectonic Zone. ILSG, Proceedings 70th Annual Meeting. Paton, C., Hellstrom, J., Paul, B.,Woodhead, J. and Hergt, J. (2011) Iolite: Freeware for the visualisation and processing of mass spectrometric data. JAAS. doi:10.1039/c1ja10172b. Schmitz, M.D., Southwick, D.L., Bickford, M.E., Mueller, P.A., Samson, S.D. (2018) Neoarchean and Paleoproterozoic events in the Minnesota River Valley subprovince, with implications for southern Superior craton evolution and correlation. Precambrian Research, v.316, p. 206-226. Vermeesch, P. (2018) IsoplotR: a free and open toolbox for geochronology. Geoscience Frontiers. doi: 10.1016/j.gsf.2018.04.001. 97 �Geochemical fingerprints from the late Mesoproterozoic epeiric seaway of the Nonesuch Formation, Wisconsin and Michigan, USA STEWART, Esther K.1,2, TAPPA, Michael1, BAUER, Ann1, PRAVE, Anthony3, and BRENGMAN, Latisha4 1 Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA Wisconsin Geological and Natural History Survey, UW-Madison Division of Extension, Madison, Wisconsin 53705, USA 3 School of Earth and Environmental Sciences, University of St. Andrews KY16 9TS, Scotland/UK 4 Department of Earth and Environmental Sciences, University of Minnesota-Duluth, Duluth, Minnesota 55812, USA 2 The Oronto Group (Copper Harbor Conglomerate, Nonesuch Formation, and Freda Formation) of the southern Lake Superior Region preserves an exceptional record of late Mesoproterozoic environments and associated microfossils (e.g., Cumming et al., 2013; Fedorchuk et al., 2016; Strother and Wellman, 2021). A lacustrine rift basin is often cited as the most plausible depositional setting for the ca. 1080 Ma Nonesuch Formation because of its association with alluvial deposits of the underlying Copper Harbor Conglomerate and overlying Freda Formation and because of its location within interior Laurentia (Elmore et al., 1989; Slotznick et al., 2023). Our recent sedimentologic and stratigraphic evidence demonstrates deposition of the lower Oronto Group within a tide- and wave-influenced estuary (Stewart et al., 2023, see also Hieshima and Pratt, 1991; Jones et al., 2020). New geochemical results from Nonesuch Formation carbonates (Figure 1), including strontium (Sr), carbon (C), and oxygen (O) isotope compositions, rare earth element - yttrium (REY) patterns, and trace element ratios complement and add new dimension to this environmental interpretation. Strontium isotope compositions refine the Precambrian marine 87Sr/86Sr curve (Chen et al., 2021), with the Nonesuch recording relatively radiogenic compositions at ca. 1080 Ma between previously reported values of 0.706600 at ca. 1109 Ma and 0.705965 at ca. 1058 Ma. Most shale-normalized REY patterns from Nonesuch Formation carbonates are characterized by positive lanthanum anomalies and elevated yttrium: holmium (Y/Ho) ratios. Many of the same samples are also enriched in heavy REE, while others record light REE enrichment. These patterns indicate Nonesuch Formation carbonates precipitated from brackish water, consistent with REY patterns observed in modern estuaries (Lawrence and Kamber, 2006). One sample has a flat shale-normalized REY distribution and likely precipitated within part of the estuary dominated by fluvial input. The combined geochemical evidence suggests Nonesuch Formation carbonates were minimally altered by diagenesis, and diagenetic alteration was dependent on sedimentary facies. While C and O isotopes are uncorrelated, initial Sr isotope compositions correlate positively with O isotopes, and C isotopes group by sedimentary facies. Minor diagenetic alteration thus resulted in less radiogenic Sr isotope compositions and did not impact C isotope compositions, which instead reflect facies-dependent incorporation of remineralized, isotopically light organic carbon during deposition or early diagenesis. Although ƩREY correlates with initial Sr isotope composition, there is no covariation between initial Sr isotope composition and lanthanide anomalies or Y/Ho ratios. This implies that carbonates likely precipitated in shallow pore waters where ƩREY was modified by contribution from surrounding detrital material, and REY profiles were determined by original pore water chemistry in connection with the overlying water body. 98 �Figure 1. Example of carbonate sampled for geochemistry from the Nonesuch Formation. (a) core scan showing fine-grained siliciclastic sediment (dark gray) and carbonate (light cray). Note molar tooth crack cross-cutting laminae. Core is 1 inch (2.54 cm) wide. (b) photomicrograph showing carbonate spar (light color) infilling molar tooth crack. Plain polars, scale bar is 1000 µm. Modified from Stewart et al. (2023). Chen, X., Zhou, Y., Shields, G.A., 2022. Progress towards an improved Precambrian seawater 87Sr/86Sr curve. Earth-Science Reviews 224, 103869. Cumming, V.M., Poulton, S.W., Rooney, A.D., Selby, D., 2013. Anoxia in the terrestrial environment during the late Mesoproterozoic. Geology 41(5), 583-586. Elmore, R.D., Milavec, G.J., Imbus, S.W., Engel, M.H., 1989. The Precambrian Nonesuch Formation of the North American mid-continent rift, sedimentology and organic geochemical aspects of lacustrine deposition. Precambrian Research 43(3), 191-213. Fedorchuk, N.D., Dornbos, S.Q., Corsetti, F.A., Isbell, J.L., Petryshyn, V.A., Bowles, J.A., Wilmeth, D.T., 2016. Early non-marine life: evaluating the biogenicity of Mesoproterozoic fluvial-lacustrine stromatolites. Precambrian Research 275, 105-118. Hieshima, G., Pratt, L., 1991. Sulfur/carbon ratios and extractable organic matter of the middle Proterozoic Nonesuch Formation, North American Midcontinent rift. Precambrian research 54(1), 65-79. Jones, S., Prave, A., Raub, T., Cloutier, J., Stüeken, E., Rose, C., Linnekogel, S., Nazarov, K., 2020. A marine origin for the late Mesoproterozoic Copper Harbor and Nonesuch Formations of the Midcontinent Rift of Laurentia. Precambrian Research 336, 105510. Slotznick, S.P., Swanson-Hysell, N.L., Zhang, Y., Clayton, K.E., Wellman, C.H., Tosca, N.J., Strother, P.K., 2024. Reconstructing the paleoenvironment of an oxygenated Mesoproterozoic shoreline and its record of life. Bulletin 136(3-4), 1628-1650. Stewart, E.K., Bauer, A.M., Prave, A.R., 2023. End-Mesoproterozoic (ca. 1.08 Ga) epeiric seaway of the Nonesuch Formation, Wisconsin and Michigan, USA. Geological Society of America Bulletin. Strother, P.K., Wellman, C.H., 2021. The Nonesuch Formation Lagerstätte: a rare window into freshwater life one billion years ago. Journal of the Geological Society 178(2). 99 �Characteristics of graphitization across a metamorphic gradient in the Michigamme Formation of the Marquette Trough and Baraga Basin, MI STOKES, Rebecca1, CANNON, William1, SALERNO, Ross1 1 U.S. Geological Survey, Geology, Energy and Minerals Science Center, Reston, VA 20192 Graphitization of carbonaceous material (CM) occurs by the progressive aromatization of carbon, expulsion of heteroatoms, and three-dimensional stacking of graphene layers — a process that alters the structure, isotopic, and trace element chemistry of the residual CM. Graphitization is generally considered thermally driven and has been extensively studied in the context of predictable changes in crystallinity as measured by Raman spectroscopy or X-ray diffraction, ultimately yielding the development of several graphite geothermometers (Henry et al., 2019). More broadly, crystalline graphite is an industrial mineral used in lithium-ion batteries and critical for the energy transition away from fossil fuels. The efficacy of graphite in the anode of batteries is directly related to its physicochemical properties which are a function of its geologic origin. The variably metamorphosed and deformed black shales and slates of the Michigamme Formation provide a natural laboratory to revisit our understanding of the graphitization process and the associated changes in structure and chemistry of CM in the context of technological applications. The Michigamme Formation is a Paleoproterozoic metasedimentary and metavolcanic sequence that is widespread in the Upper Peninsula of Michigan. The lower part of the formation is finer-grained black shale and siltstone, typically with a prominent slaty cleavage. In the Marquette Trough and Baraga Basin, the focus areas of our study, this fine-grained sequence has been mapped and formally designated the Lower Slate Member of the Michigamme Formation. Highly carbonaceous units are ubiquitous near the middle of this member and vary from ~70 meters on average along the Marquette Trough to 150 meters or more in the Baraga Basin. Along the Marquette Trough, the outcrop trace of the Lower Slate transects a regional metamorphic gradient from chlorite to staurolite grade whereas the metamorphic grade in the Baraga Basin is uniformly low, within the chlorite zone. In the Marquette Trough, the graphitic beds sampled for this study are along the north limb of this complex syncline, mostly dip steeply southward, and have a well-developed slaty cleavage that is axial planar to the larger structure of the trough. Deformational features diminish to the north and the northernmost of our samples, in the Baraga Basin, are from nearly flat lying beds with no penetrative structural features. An important and widely accepted distinction of the Michigamme Formation is that the development of the penetrative regional cleavage predates the final metamorphic event. Thus, graphitization of the CM occurred under both stressed (tectonic) and static conditions. A suite of thirteen core samples from the Upper Peninsula Geological Repository and three outcrop samples, all from carbonaceous sections of the Michigamme Formation, were selected for detailed evaluation (Figure 1). In all samples, CM occurs as disseminated and elongated fine-grained (<20 µm) particles that tend to be concentrated in bands parallel to the metamorphic fabric defined by phyllosilicates and quartz. Analysis of total carbon on decarbonated samples yielded values ranging from 1 to 24 wt.% C, with an average value of 7 100 �wt.%. Carbon isotopic analysis from decarbonated samples yielded a general trend towards heavier δ13C values with increasing metamorphic grade, ranging from -32.05‰ (Sample 4) to 21.85‰ (Sample 13). Raman spectroscopic analysis of CM yielded a similar trend with metamorphic grade across the sample suite. The R2 ratio, which is one parameter used to evaluate metamorphic grade, decreases from 0.64 in Sample 15 to 0.35 in Sample 8. These R2 values correspond to a temperature range from ~325°C to ~500°C using the empirical geothermometer from Aoya et al. (2010). Notably, Samples 11 and 13 from the garnet zone are significant outliers in both the Raman and C isotope datasets. Combining these results with additional data from scanning electron microscope imaging, X-ray diffraction mineralogical analysis, and laser ablation ICP-MS analysis of CM concentrates will yield a more detailed look at the evolution of CM with metamorphism and deformation. These results will help refine our understanding of the geologic processes that lead to economic graphite deposits and fine-tune graphite deposit models with an application focus. Figure 1. Geologic map and metamorphic isograds (red lines) of the field area in the Upper Peninsula region of Michigan. Map is generalized from Cannon and Ottke (1999). Core samples are noted with white dots, and blue dots for outcrop samples. References Aoya, M., Kouketsu, Y., Endo, S., Shimizu, H., Mizukami, T., Nakamura, D., Wallis, S., 2010. Extending the applicability of the Raman carbonaceous material geothermometer using data from contact metamorphic rocks. Journal of Metamorphic Geology, 28: 895–914. Cannon, W. F., and Ottke, D., 1999. Preliminary digital geologic map of the Penokean (early Proterozoic) continental margin in northern Michigan and Wisconsin: U.S. Geological Survey Open-File Report 99-547. Henry, D. G., Jarvis, I., Gillmore, G., & Stephenson, M. (2019). Raman spectroscopy as a tool to determine the thermal maturity of organic matter: Application to sedimentary, metamorphic and structural geology. Earth-Science Reviews, 198: 102936. 101 �Sulfur-isotope ratios in Paleoproterozoic Michigamme Formation at the Lake Superior Region: Implications on basin evolution and ambient seawater composition in the Greater Animikie Basin THAKURTA, Joyashish 1, and HAAG, Beau 2 1 Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Hermantown, MN 55811, USA 2 Niblack Project LLC, 136 River Street, Elko, Nevada 89801, USA A considerable variation in δ34S-ratios of sulfide minerals has been found in a sulfide-mineralrich succession of slate, metasiltstone, and metagreywacke in the Paleoproterozoic Michigamme Formation located within the Baraga Basin at the eastern edges of the Greater Animikie Basin in the Lake Superior Region (Ojakangas, Morey, and Southwick, 2001). Sulfide minerals such as pyrite, chalcopyrite and pyrrhotite display δ34S-values ranging between 2 and 40‰ (V-CDT) and appear to systematically vary with respect to the stratigraphic intervals (Figure 1). This variability is gradational and devoid of any anomalous spike. It is predominantly a function of stratigraphic location and it shows no relationship with the type of sulfide mineral or the textural mode of occurrence. This observation rejects the possibility that the δ34S values were influenced by selective infiltration of externally derived sulfur-rich fluids along the stratigraphic layers of the Michigamme Formation. It also overrules the possibility that the observed δ34S values were caused by low-grade metamorphism and recrystallization of the sedimentary rocks of the Michigamme Formation. The values are consistent with primary δ34S ratios in sulfide minerals which were precipitated from intergranular fluids within a sequence of clastic sediments in a basin shortly after deposition. Consequently, the measured δ34S ratios in the stratigraphic horizons represent Sisotopic signatures inherited from the S-reservoir in the ambient basin, as well as changes introduced by basin-evolution and diagenesis of the siliciclastic sediments in a marine foreland depositional environment along the eastern portion of the Greater Animikie Basin. While the observed general trend of gradual increase in δ34S-values in the lower and upper members of the sequence can be explained by a systematic chronological trend in the global seawater composition (Paiste et al., 2020), a significant rise and fall in δ34S-values in the Lower Slate and Upper Greywacke Members can be attributed to a limited-term separation of the Baraga Basin from an open-ocean circulation to an isolated basin environment in response to structural adjustments caused by the formation of a fold and thrust belt along the southern shore line of the foreland basin during the waning stages of the Penokean Orogeny (Schulz and Cannon, 2007). In this period of isolation, the sulfur-isotope composition was primarily controlled by intrabasinal bacterial fractionation leading to a significant rise in the δ34S values up to 40‰ in the observed sediments. Upon subsequent erosional removal of the thrust sequence, and associated structural readjustments, the connection of the Baraga Basin to an open ocean was restored and the δ34S-values in the new sedimentary rocks mimic values that are consistent with deposition in a larger open ocean setting. 102 �Figure 1: Observed variation in δ34S-ratios. Stratigraphy adapted from Rossell and Coombes (2005) References: Ojakangas, R.W., Morey, G.B. and Southwick, D.L., 2001. Paleoproterozoic basin development and sedimentation in the Lake Superior region, North America. Sedimentary Geology 141-142: 319-341. Paiste, K., Lepland, A., Zerkle, A.L., Kirsimae, K, Kreitsmann, T, Mand, K., Romashin, A.E., Rychanchik, D.V. and Prave, A.R., 2020. Identifying global vs. basinal controls on Paleoproterozoic organic carbon and sulfur isotope records. Earth-Science Reviews 207: 01320. Rossell, D. and Coombes, S., 2005. The Geology of the Eagle Nickel-Copper Deposit Michigan, USA. Report for Kennecott Exploration, dated April 29, 2005, 35 p. Schulz, K.J. and Cannon, W.F., 2007, The Penokean orogeny in the Lake Superior region: Precambrian Research, 157: 4-25. 103 �An evaluation of structural and mineralogical controls on gold mineralization on the GoldRich property in the Abbie Lake area, Wawa, Ontario THIBODEAU-BELLO, Demily, HILL, Mary Louise, CONLY, Andrew Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada The GoldRich property in the Abbie Lake area is an active gold prospect in the Michipicoten greenstone belt, within the Wawa subprovince of the Archean Superior province of the Canadian shield. The property is located 30 km northeast of Wesdome’s Eagle River mine and 10 km northeast of Wesdome’s Mishi property in northern Ontario. The Main Shear trench on the GoldRich property is an area of regional metamorphism dominated by ductile deformation hosting orogenic gold. The Main Shear trench hosts mylonites of felsic and intermediate composition with varying strain intensities; all lithologies strike east-west. The mylonite of intermediate composition is characterized by having en-echelon quartz veins perpendicular to the foliation. Gold occurrences have been found in zones of high strain, in both felsic and intermediate mylonite lithologies. This HBSc thesis project relies on detailed trench mapping, microstructural and petrographic analyses, and geochemical methods to discover how gold is hosted on this property. Understanding the controls on gold mineralization will guide future exploration. Gold mineralization in the Main Shear trench is related to deformation. Based on geochemical and petrographic analysis there is no correlation observed between alteration mineralogy and gold mineralization. Microstructural analysis revealed pervasive subgrain-rotation quartz recrystallization in both mylonitic lithologies indicating that the Main Shear trench has undergone deformation at the upper greenschist to lower amphibolite facies regional metamorphic conditions. Gold is associated with deformation, microstructurally related to recrystallized quartz grain boundaries, boudinaged veins, and shear bands. Based on these results, it is recommended that further exploration on this property should be focused on locating zones of similar structure and strain intensity, including areas of boudinage, regardless of lithology, to continue building the gold prospect. 104 �Petrology and Geochemistry of Felsic Magmatism in the Paleoproterzoic Eau Claire Volcanic Complex, Northcentral Wisconsin VICKERS, Lyndsie A. 1, LODGE, Robert W.D.1 1 Department of Geology and Environmental Sciences, University of Wisconsin-Eau Claire, Eau Claire, WI 54701 USA The 1.8-1.9 Ga Eau Claire Volcanic Complex (ECVC) (Figure 1) is the type locality for Penokean-age magmatism formed on an Archean crustal block (~2.6-3.0 Ga) called the Marshfield Terrane during the Penokean Orogen (Sims et al., 1989; Schulz & Cannon, 2007) and has influenced historic tectonic models and terrane-boundary maps. The other volcanic terrane within the Penokean Orogen, the Pembine-Wausau terrane (PWT), is interpreted to have formed with minimal influence of older crust and hosts about 150 million tonnes of volcanogenic massive sulfide (VMS) ores. The ‘continental’ setting of the Marshfield Terrane assumes a different metallogenetic system than the ‘oceanic’ setting of PWT and may be less prospective for the same VMS mineralization. However, recent U/Pb isotopic and other geochemical data (Lodge et al., 2023; Weber et al, 2023) indicates parts of the ECVC were mantle-derived and not contaminated by older Archean crust and challenges this ‘continental’ model. The ECVC is challenging to study because of a lack of mineral exploration (and drilling) coupled with rare outcrop exposure due to glacial/fluvial sediment and Paleozoic rock cover. This project studies remote, inaccessible outcrops along the Eau Claire River to refine the tectonic model and terrane boundaries of the southern Penokean Orogen. Samples obtained from mapping were petrographically characterized and analyzed for major and trace element geochemistry. Thirteen samples were analyzed for major and trace elements via WD-XRF and compiled with other geochemical datasets from the region. Trace element geochemical data are used to determine magmatic and tectonic settings of these rocks and improve regional tectonic models for the ECVC. Felsic magmatism in the region consists of fine-grained quartz-muscovite schists (Figure 1A) and banded quartzofeldspathic gneisses interpreted to be felsic volcanism and mediumgrained massive granodiorite (Figure 1B). Fine-grained quartz-muscovite schist are characterized by approximately 5% quartz porphyroclasts that are 1-3mm in size. The matrix is very finegrained quartz, feldspar, and muscovite that can have variable amounts of chlorite. Banded quartzofeldspathic gneisses have fine grained biotite and amphibole in quartz and feldspar-rich matrix that may define volcaniclastic textures at the outcrop-scale. Medium-grained, massive granodiorite is 15% biotite/hornblende and is characterized by weak to minimal foliation. These granodiorites are interpreted to be intruding felsic volcanic rocks, amphibolites, and metasedimentary units (Figure 1C). At one outcrop, the contact region is exposed revealing the formation of a megabreccia matrix, incorporating gneiss fragments that can reach up to 1 meter in size. These gneissic metasedimentary rocks are fine-grained with alternating bands of light and dark layers, ranging from straight to intensely folded. Samples from the ECVC on Zr/Ti vs. Nb/Y classification diagrams reveal a bimodal magmatic suite which is commonly associated with extensional tectonic settings. Felsic volcanic and intrusive rocks on Nb vs. Y discrimination plots suggest that felsic magmatism was likely formed in syn-collisional or volcanic arc settings. Rhyolite fertility discrimination diagrams (Zr/Y vs. Y) show that both felsic volcanic and intrusive suites from the ECVC are FII-type rhyolites, typical of upper-crustal melting in rift zones. Therefore, the ECVC region may be prospective for VMS mineralization and the tectonic setting should be further evaluated. 105 �Figure 1. Bedrock geologic map of the Eau Claire River region in northcentral Wisconsin showing extent of Precambrian bedrock. Map from Mudrey & Brown (1982). (A) Poorly exposed quartz-phyric micaceous schist near Rock Dam, WI interpreted as a metarhyolite. (B) Weathered surface of granodiorite intrusion along bank of Eau Claire River, (C) Contact region between granodiorite and dark gneiss or metasedimentary rocks. References Lodge, RWD, Weber, EM, Hooper, RL (2023), Precambrian Geology of the Eau Claire River Valley: Re-discovering the Eau Claire Volcanic Complex. in Lodge, RWD (Ed.), Institute on Lake Superior Geology Proceedings, 69th Annual Meeting, Eau Claire, Wisconsin, Part 2 – Field Trip Guidebooks. v.69, part 2, p.47-70. Mudrey, M.G., Jr., Brown, B. A., Greenberg, J. K. "Bedrock Geologic Map of Wisconsin." Wisconsin Geological and Natural History Survey, 1982. Schulz, K.J., and Cannon, W.F., 2007, The Penokean orogeny in the Lake Superior region: Precambrian Research, v. 157, p. 4–25. Sims, P. K., Van Schmus, W. R., Schulz, K. J., and Peterman, Z. E., 1989, Tectonostratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of the Penokean orogen: Canadian Journal of Earth Sciences, v. 26, p. 2145-2158. Weber, EM, Lodge, RWD, Marsh, JH (2023). U/Pb geochronology and zircon petrochronology of Paleoproterozoic magmas from the Marshfield terrane, Penokean Orogen, Wisconsin. Institute on Lake Superior Geology Proceedings, 69th Annual Meeting, Eau Claire, Wisconsin, Part 1-Program and Abstracts, p. 97-98. 106 �The Keweenaw Geoheritage Summer Internship Experience VYE, Erika1, LIZZADRO-MCPHERSON, Daniel2, and JUIP, James2 1 Great Lakes Research Center, Michigan Technological University, 1400 Townsend Drive, Houghton, MI, 49931 2 Geospatial Research Facility, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931 Earth science education benefits from holistic interpretation of geologic features, processes, and landscapes through multiple ways of knowing (Deloria & Wildcat, 2001; Morton & Gawboy, 2003; Ricci & Riggs, 2019; Semken, 2005). Geoheritage is an evolving field that emphasizes the importance of the varied personal values people have for geologic features and explores the wide-ranging relationships we have with landscapes; as such it is an excellent place-based education tool to explore connections to our underpinning geology (Semken et al, 2017; Tormey, 2019). In this project setting, the Keweenaw region of Michigan’s Upper Peninsula on Lake Superior, we demonstrate a regional, place-based approach to help deepen participant understanding of the billion-year-old geologic processes at the heart of the Midcontinent Rift system. These processes created both the Lake Superior basin and the largest known native copper deposit on Earth in a region further defined as the ancestral and contemporary homelands and waters of the Keweenaw Bay Indian Community (KBIC) on Lake Superior. The Keweenaw Geoheritage Summer Internship Experience was created in partnership with the Keweenaw Bay Indian Community's Natural Resources Department (KBIC NRD) and Tribal Historic Preservation Office (THPO), and Michigan Tech’s Great Lakes Research Center (GLRC) and Geospatial Research Facility (GRF). The experience was created to support intergenerational and multicultural learning about the Keweenaw landscape, its stories, and geology. Tribal and non-tribal high school student interns, community partners, and knowledge holders spent time together reading the landscape and sharing reflections on our varied relationships with land and water. Week 1 entailed a 5-day field experience visiting valued sites of the Keweenaw Bay Indian Community that also teach us how geology impacts land, life, and culture in our place asking “what gifts does geology offer us? what are the relationships with land and water in this place?”. During the field experience, youth interns collected and documented local knowledge by engaging in multi-sensory and multimedia documentation strategies to record their experiences (photos, audio recordings, drawing, 360 virtual reality images, etc.). Sacred Anishinaabe knowledge was not sought or shared in these experiences. Data collected during the field experience was then used as the foundation for a 5-day geospatial workshop following the field experience. The goal of the workshop was for youth interns to design, create, and publish ARC GIS StoryMaps depicting their personal reflections of the diverse relationships with local landscape and understanding of its formation. ARC GIS StoryMaps is a story-authoring, web-based application that enables sharing of maps in the context of narrative text and other multimedia content. Workshop participants worked with the data they collected during the field experience in combination with supporting data layers and 107 �maps specifically created for the experience. Interns were mentored by the geospatial research team who helped students develop digital storytelling skills, inspired brainstorming sessions for topics to explore in the maps, and facilitated peer-review of StoryMap content prior to publication. Upon completion, students presented their work at a community open house; all StoryMaps created have been peer-reviewed by all project partners and are now published. The StoryMaps reflect a deepened understanding of relationships between geology, mining, and current environmental justice issues within our community. In the context of the Keweenaw, the European copper mining boom is most prominently interpreted in our place; students also reflected on the long history of mining, ways of mining, changing narratives, and missing human stories seen and experienced when visiting our landscape. Of note, respect, gratitude, and deepened relationships with land and water featured in all StoryMaps. Students shared reflections on reciprocity and their responsibility to help steward their place. Figure 1: Left - students deepen their understanding of mining impacts to Buffalo Reef and our local communities; Right: students brainstorm story arcs for the foundation of their StoryMap References [1] Deloria, V. and Wildcat, D. R. (2001). Power and place: Indian education in America. Fulcrum Publishing: Golden, Colorado. [2] Morton, R. and Gawboy, C. (2003). Talking Rocks: Geology and 10,000 Years of Native American Tradition in the Lake Superior Region. University of Minnesota Press. [3] Ricci, J. and Riggs, E.M. (2019). Making a Connection to Field Geoscience for Native American Youth through Culture, Nature and Community. Journal of Geoscience Education, special theme issues on Diversity in the Geosciences, DOI:10.1080/10899995.2019.1616273. [4] Semken, S. (2005). Sense of place and place-based introductory geoscience teaching for American Indian and Alaska Native undergraduates. Journal of Geoscience Education, 53 (2), 149-157. [5] Semken, S., Geraghty Ward, E., Moosavi, S. and Chinn, P.W.U (2017). Place-Based Education in Geoscience: Theory, Research, Practice, and Assessment. Journal of Geoscience Education, 65:4, 542562, DOI: 10.5408/17-276.1. [6] Tormey, D. (2019). New approaches to communication and education through geoheritage. International Journal of Geoheritage and Parks, 7 (4), 192-198, ISSN 2577-4441, https://doi.org/10.1016/j.ijgeop.2020.01.001. 108 �R.W. Boyle’s History of Geochemistry and Cosmochemistry WILSON, Graham C.1, BUTT, Charles R.M.2, GARRETT, Robert G.3 and ROBINSON, Heather A.4 1 Turnstone Geological Services Limited. P.O. Box 1000, Campbellford, ON K0L 1L0 Canada, CSIRO Minerals Resources, Kensington, Western Australia; 3 Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8 Canada, 4 25 Chester Crescent, Ottawa. ON K2J 2J6 Canada 2 Robert W. Boyle (1920-2003) was a well-respected geochemist with a long career at the Geological Survey of Canada. He is perhaps best remembered for geological and geochemical studies of gold, silver and other commodities, and for his association with mining camps, such as Yellowknife, N.W.T. and Keno Hill, Yukon. Retiring in 1985, Boyle devoted much of his final two decades to trips to far-flung libraries, gathering information exotic and/or obscure, and penning a major 3-volume review of the evolution of human knowledge of the nature and use of metals and other materials, and of diverse fields within geochemistry, cosmochemistry and biogeochemistry. The result, with copious help in compilation, was almost 2,000 pages of discussion (90% of it typed, fortunately!), backed by a formidable bibliography of almost 3,000 references. It was essentially complete at the time of his death, but altogether lacking in illustration. In time, his G.S.C. colleague Bob Garrett made some editorial notes, and then, in 2011, Charles Butt assessed the manuscript and made a detailed scientific and editorial review, heavy in marginalia. However, the work evidently arrived too late for the glory days of Survey and Society printing budgets, and it sat upon the shelf. In 2015, Ryan Noble, of the Association of Applied Geochemists, broadcast the existence of the manuscript, which attracted Wilson, who undertook to advance the work of the earlier editors, with encouragement from Boyle’s daughter, biochemist Heather Robinson. Volume 1 of the trilogy is set for publication in 2024 (Boyle, 2024). It covers the vast span of human time from the inception of mining and agriculture to the fall of Rome in the West (476 A.D.), and so ventures onto ground traditionally left to aspects of the Classics, Ancient History and Archaeology. Despite the western time frame, it is a worldwide review, covering, besides Europe and the Middle East, India and China and the Americas, every part of the globe where Boyle found relevant knowledge to impart. The intended volumes 2 and 3 explore, respectively, history through the critical 19th century, and then the 20th century (and so to the present). Volume 1 traverses the long development of early thought on the nature of matter. In addition to the various, often conflicting strains of philosophy, there is an equal treatment of the harnessing of materials (Stone, Bronze and Iron ages), and the early stages of the broad swathe of Earth sciences, mining and metallurgy. Early practical ideas on “Earth, air, fire and water” are discussed, e.g., geochemistry and mineralogy, cosmochemistry (meteorites), and early ideas on the hydrosphere and atmosphere. How was the raw manuscript processed? In brief, Wilson: a) ported Boyle’s references into a database, the easier to split up the long bibliography by chapter, rendering each section and volume a stand-alone story; b) utilised his MINLIB bibliography to update Boyle’s references, which for Volume 1 had ended in 1987; c) split up the seminal chapter 3, which provides reviews for some 29 metals and commodity groups (e.g., Au, Ag, Cu; Fe; Sn, Pb; industrial minerals, gemstones and organics); d) added a third layer of editing and consistency checks; and e) 109 �ultimately added 132 individual or composite illustrations in 93 numbered figures, including two original versions of the periodic table. Some of the additions (mostly to post-1987 research) are inserted in the text, others are collected in endnotes to each section. Some of the additions may be skating on thin ice (in which case, it is Wilson who falls through), a problem that one suspects would not have unduly worried the author of the original text. Boyle himself travelled widely across Canada, and the world. In terms of the Lake Superior region, Volume 1 has multiple references to the native copper and native silver of the Mesoproterozoic Midcontinent Rift (Fig. 1; see, e.g., Bornhorst and Barron, 2013; Wilson, 2023). One of two additional text boxes is devoted to native copper, while the other concerns the wider literature on the chemical elements, including some of the most accessible, popular titles. An explicit reference is made to native elements, of which Boyle was fond (e.g., Zn, Pb), including the obvious starting point of Au, Ag and Cu, and listing some 30 elements (many of them very rare in their unalloyed forms). Figure 1. Samples from famous occurrences of native metals in the Lake Superior region. Left: A spectacular, 4,264-kilogram mass of native copper, the exterior coloured by secondary Cu salts (Calumet, Keweenaw peninsula, Michigan). Right: native silver revealed in sawn and polished faces of calcite-veined fractured diabase from the Silver Islet mine in northwestern Ontario, a rich but short-lived venture on the east side of the Sibley peninsula, east of Thunder Bay, in Lake Superior. References Bornhorst, T.J. and Barron, R.J. (2013) Geologic overview of the Keweenaw peninsula, Michigan. Institute on Lake Superior Geology, v. 59, part 2: 1-42, Houghton, MI. Boyle, R.W. (2024) A History of Geochemistry and Cosmochemistry. Prehistory to the end of the Classical Period. Cambridge Scholars Publishing, Newcastle upon Tyne, England (Wilson, G.C., Butt, C.R.M., Garrett, R.G. and Robinson, H.A., editors), circa 600pp., in press. Wilson, W.E. (editor) (2023) Michigan Copper Country II. Mineralogical Record, v. 54 no.1: 196pp. 110 �Cooling of an Archean metamorphic terrane: garnet diffusion study of the Quetico Subprovince, Canada XU, Yiruo1 and HOLDER, Robert1 1 Department of Earth and Environmental Sciences, University of Michigan, 1100 North University Avenue, Ann Arbor, MI 48109 United States Archean metamorphic terranes are traditionally suggested to have cooled significantly slower than their Phanerozoic counterparts. Many have argued that the contrast in metamorphic timescale reflects changes in Earth’s tectonic regime. However, diffusion chronometry-based cooling rate data on Precambrian rocks are very limited. We present a case study of metamorphic timescales on the Neoarchean Quetico metasedimentary belt of the Superior Province, which has been hypothesized to represent a fore-arc accretionary prism. We combine conventional thermobarometry and phase-equilibrium modeling to constrain the peak temperature and pressure and estimate metamorphic cooling rates from major element diffusion in garnet. We then compare cooling rates across the Quetico Subprovince and with those from Phanerozoic metamorphic terranes of similar conditions. The results will contribute to the diffusion chronometry data available on Precambrian orogens for assessing any fundamental change in global tectonics. 111 � https://digitalcollections.lakeheadu.ca/files/original/462b1934e715e86830529c08f5039d63.pdf d45ee759251a8ad2fecc3255f6c3c6ab PDF Text Text 70th Annual Meeting Institute on Lake Superior Geology Houghton, Michigan May 15-18, 2024 Proceedings Volume 70 Part 2 - Field Trip Guidebook �70th Annual Meeting Institute on Lake Superior Geology Houghton, Michigan May 15-18, 2024 Sponsored by: A. E. Seaman Mineral Museum Great Lakes Research Center Department of Geological and Mining Engineering and Sciences Michigan Technological University Meeting Co-Chairs Theodore J. Bornhorst, Erika Vye, Patrice Cobin, and James DeGraff Proceedings Volume 70 Part 2: Field Trip Guidebook Compiled by Patrice F. Cobin and Theodore J. Bornhorst Cover Photo: The only known color photograph of in situ colorless calcite crystals with inclusions of native copper. Vug is about 15 cm across and 30 cm deep; located at the top of the Knowlton basalt lava flow at the 4th level, 850 ft stope, of the Caledonia Mine, Michigan. Photo taken in 1994 soon after the vug was blasted open. Native copper in the calcite crystals has not been visibly altered despite being about 1 billion years old. Photograph by Theodore J. Bornhorst i ��70th Institute on Lake Superior Geology Volume 70 consists of: Part 1: Program and Abstracts Part 2: Field Trip Guidebook Trip 1: Mesoproterozoic Midcontinent Rift-filling Strata and Native Copper Deposits of the Keweenaw Peninsula, Michigan Trip 2: Mining History and Geology of the Quincy Mine, Keweenaw Peninsula Native Copper District, Michigan Trip 3: Geoheritage of Buffalo Reef: Industrial Impact on Land, Culture, and Fish Sovereignty Trip 4: Keweenaw Fault System Geometry and Kinematics: Clues to Its Nature and Origin Trip 5: Geology and History of a Native Copper Mine: Adventure Mine, Ontonagon County, Michigan Trip 6: Southern Complex Granitoids, Gneisses, and Migmatites: New Data, Discoveries, and Perspectives Trip 7: Landslides on the Ontonagon River at Military Hill Reference to material in Part 2 should follow the example below: Authors, 2024, field trip title, 70th Institute on Lake Superior Geology, Abstracts and Proceedings, v. 70, Part 2, Field Trip Guidebook, p. xx-xx. Proceedings Volume 70, Part 1: Program and Abstracts and Part 2: Field Trip Guidebook are published by the 70th Institute on Lake Superior Geology and distributed by the Institute Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca Some figures in this volume were submitted by authors in color but are printed grayscale to conserve printing costs. Full color imagery will appear in the digital version of the volume when it is available on-line at: http://www.lakesuperiorgeology.org ISSN 1042-99 ii ��Part 2: Field Trip Guidebook Table of Contents Trip 1: Mesoproterozoic Midcontinent Rift-filling Strata and Native Copper Deposits of the Keweenaw Peninsula, Michigan………………………...1 Trip 2: Mining History and Geology of the Quincy Mine, Keweenaw Peninsula Native Copper District, Michigan……………………………………………………….55 Trip 3: Geoheritage of Buffalo Reef: Industrial Impact on Land, Culture, and Fish Sovereignty…………………………………………………………………….79 Trip 4: Keweenaw Fault System Geometry and Kinematics: Clues to Its Nature and Origin………………………………………………………………………………..97 Trip 5: Geology and History of a Native Copper Mine: Adventure Mine, Ontonagon County, Michigan.…………………………………………………………137 Trip 6: Southern Complex Granitoids, Gneisses, and Migmatites: New Data, Discoveries, and Perspectives…………………………………..………….157 Trip 7: Landslides on the Ontonagon River at Military Hill…………………………………..173 iii ��Field Trip 1 Mesoproterozoic Midcontinent Rift-filling Strata and Native Copper Deposits of the Keweenaw Peninsula, Michigan Theodore J. Bornhorst A.E. Seaman Mineral Museum, Michigan Technological University, 1404 E. Sharon Avenue, Houghton, MI 49931 Introduction The geology of the far western Upper Peninsula of Michigan consists of three temporally distinct episodes. During the Mesoproterozoic, about 1.1 Ga, up to 30 km of Keweenaw Supergroup volcanics and clastic sediments filled an intracratonic rift, the Midcontinent Rift (MCR) (Figures 1 and 2) (Cannon et al., 1989). After about 500 million years of erosion, the MCR rocks were buried by Phanerozoic sedimentary rocks from about 500 Ma to 175 Ma (Catacosinos et al., 2001). There are no exposures of rocks in the interval between 175 Ma to about 2.5 Ma (Velbel, 2009). Pleistocene continental glaciations, beginning about 2 million years ago, removed the Phanerozoic rocks from the Keweenaw Peninsula leaving only a few Phanerozoic outliers. About 10,000 years ago glaciers retreated from the Lake Superior basin and left behind a variety of unconsolidated clastic sediments. The geologic evolution of the far western Upper Peninsula is illustrated in Figure 3. Figure 1: Generalized geologic map of the Midcontinent Rift showing Grenville tectonic zone with interpretative cross-section in Figure 2. Modified from Bornhorst and Barron (2013). 1 �Figure 2: Generalized bedrock map showing the exposed rocks of the Midcontinent Rift around Lake Superior, the native copper occurrences, and the bedrock of the Upper Peninsula of Michigan modified from Bornhorst and Barron (2013). Interpretative cross section from Cannon et al. (1989). 2 �Figure 3: Cartoon NW to SE cross sections from Minnesota (left) to the Upper Peninsula (right) illustrating the progressive geologic evolution. Modified from Bornhorst and Lankton (2009). In the strictest sense, the geographic area of the Keweenaw Peninsula proper extends from L’Anse northwest to Lake Superior perpendicular to strike of the strata, however, the term Keweenaw Peninsula has also been applied to the area containing MCR rocks farther to the south from L’Anse to the White Pine area (Figure 4 and 5). The geologic descriptions in this field trip guide are mostly restricted to the Keweenaw Peninsula proper. The descriptions provided here were modified from a combination of Bornhorst and Barron (2011, 2013), Bornhorst and Lankton (2009), Bornhorst and Rose (1994) and Bornhorst et al. (1983). These sources are mostly used here without specific citation or quotation. 3 �Figure 4: Bedrock geologic may of the western Upper Peninsula of Michigan showing area of the Keweenaw Peninsula native copper district (from Bornhorst and Barron, 2013). Figure 5: Stratigraphic column the Keweenaw Peninsula, Michigan. 4 �Midcontinent Rift Strata The Keweenaw Peninsula is located on the southern margin of the Lake Superior segment of the MCR (Figures 1and 2). The rock units that are associated with the MCR have been termed the Keweenawan Supergroup (Figure 5). These rocks were deposited about 1.1 Ga (Heaman et al., 2007; Davis and Paces, 1990; Cannon et al., 1989). The MCR beneath Lake Superior is filled with up to about 30 km of volcanic rocks (Figures 2 and 3) (Hinze et al., 1990; Cannon et al., 1989). The MCR geology of the Keweenaw Peninsula can be divided into northwest-dipping, rift-filling volcanic and clastic sedimentary rocks under the central highlands and northwest flank of the Keweenaw Peninsula (Figure 4) and flat to low-dipping, rift-flanking clastic sedimentary rocks located on the southeast side. The Keweenaw Fault separates the rift-filling and rift-flanking strata. The rift-filling strata are subdivided into volcanic-dominated and clastic sedimentary rockdominated lithologies. (Figure 5). Portage Lake Volcanics The Portage Lake Volcanics of the Keweenaw Peninsula (Figures 4 and 5) are a 2,500 to 5,200 m thick formation dominantly composed of subaerial basalt lava flows with less than 1 % by volume intermediate to felsic volcanic and subvolcanic rocks which are located stratigraphically near the base of the exposed formation. Interflow reddish-colored conglomerate and sandstone layers are less than 5 % by volume and are stratigraphically scattered throughout the Portage Lake Volcanics although greater in abundance towards the top of the formation (Butler and Burbank, 1929; White, 1968). The base of the formation is truncated by the Keweenaw Fault. The lavas flowed from fissure vents that tended to be located near the axis of the rift zone which produced a layered succession of flood basalts comparable to the rift zones of East Africa and Iceland (e.g., Nicholson et al., 1997 and reference therein). Much of the Portage Lake Volcanics erupted over 2 to 3 million years from 1,096.2+/-1.8 (Copper City flow, Figure 6) to 1,094.0+/-1.5 (Greenstone flow, Figure 6) (Paces and Miller, 1993; Davis and Paces, 1990). There are more than 200 individual basaltic lava flows in the exposed Portage Lake Volcanics which are typically aphyric, Mg-rich, high-Al olivine tholeiites (Paces, 1988). The most abundant type of basalt flows are olivine tholeiites, followed by primitive olivine tholeiites and quartz tholeiites. Iron-rich olivine tholeiites are generally lesser in abundance (Table 1). The thicker lava flows are compositionally stratified due to magmatic differentiation after eruption. Magmatic differentiation after eruption is especially significant in the Greenstone flow, which is the thickest individual flow in the formation (Cornwall, 1951a and b; Broderick, 1935; Broderick and Hohl, 1935). The composition of the basalts of the Portage Lake Volcanics is cyclical with minor and major cycles superimposed on an overall trend. The basalt magmas were derived by partial melting of sub-continental upper mantle with an overall compositional trend towards younger more primitive basalt compositions as a result of less crustal contamination (Paces, 1988; Paces and Bell, 1989). The repeated magmatism at the rift axis and progressive crustal thinning provided pathways for magma with less extended contact with crustal rocks. The youngest rocks of the Portage Lake Volcanics in the Keweenaw Peninsula have compositions similar to MORB suggesting the MCR nearly formed an ocean basin. The major geochemical cycles are due to fractional crystallization 5 �and replenishment in large magma chambers near the crust/mantle interface whereas the minor cycles are due to closed system fractional crystallization in small magma chambers within the crust (Paces, 1988). The Portage Lake Volcanics were likely derived by partial melting of enriched plume-related mantle (Nicholson et al., 1997; Nicholson and Shirey, 1990; Paces and Bell, 1989). All observed basalt lava flows in the Portage Lake Volcanics were erupted subaerially and consist of a massive (vesicle-free) interior capped by a vesicular and/or brecciated flow top. There is one thin hyaloclastic unit in the upper part of the formation (Johnson, 1985). Subaerial eruption resulted in degassing of volatiles, notably SO2 (Cornwall, 1951c). The lava flows range in thickness from 1 to 450 m with most of them between 10 to 20 m thick (Paces, 1988; White, 1960). Most of the lava flows cannot be traced along strike with confidence although a few such as the Scales Creek, Kearsarge, and Greenstone flows have well documented lateral continuity (Figure 6). The Greenstone flow has been correlated down dip across the Lake Superior syncline to Isle Royale (Longo, 1982; Huber, 1975). Table 1: Average representative geochemical data for least altered lavas of the Portage Lake Volcanics (from Paces, 1988). Tholeiites were grouped by Ni content. Primitive olivine tholeiite Intermediate olivine tholeiite Iron-rich olivine and quartz tholeiites Olivine tholeiite Olivine tholeiite 400-300 300250 250200 200-100 100-15 n=5 n=9 n=14 n=8 SiO₂ 47.82 47.34 48.03 Al₂O₃ 15.89 15.27 FeOt 9.77 MgO Andesite Dacite Rhyolite n=6 n=1 n=1 n=1 48.55 49.94 56.39 68.44 77.89 15.32 15.12 13.28 13.78 15.17 12.77 11.82 12.32 12.86 14.91 9.87 4.46 1.11 12.44 11.69 9.85 9.06 7.78 5.52 1.14 0.17 CaO 10.58 10.24 10.16 9.65 6.64 5.10 1.40 0.04 Na₂O 2.04 2.10 2.25 2.31 2.91 3.94 4.74 3.67 K₂O 0.19 0.22 0.33 0.42 1.43 2.27 3.86 4.28 TiO₂ 0.98 1.13 1.35 1.60 2.34 1.83 0.51 0.08 P₂O₅ 0.16 0.19 0.22 0.25 0.36 1.00 0.19 0.01 MnO 0.14 0.16 0.16 0.18 0.24 0.30 0.08 0.01 Ni 326 279 231 172 54 10 7 5 Cu 37 51 73 86 126 5 13 61 Zr 78 85 101 126 212 430 573 145 Ni (ppm) Wt.% PPM FeOt=total Fe as FeO 6 �The uppermost 5 to 20% of the tops of most individual lava flows are vesicular with between 5 and 50% vesicles (White, 1968). The tops of 21 % of the flows are brecciated with clasts of vesicular basalt. The vesicles in most lava flows within the Portage Lake Volcanics are largely filled with secondary minerals, except for the stratigraphically uppermost lava flows; the filled vesicles are amygdules. Thus, local terminology is to call lava flows with vesicle-only tops, amygdaloids and those with brecciated tops fragmental amygdaloids. There are minor amounts of andesite, dacite, and rhyolite lava flows and subvolcanic plutons that interfinger with and cross-cut the basalts of the Portage Lake Volcanics (Table 1). Most of these occur in the stratigraphically lowermost portion of the Portage Lake Volcanics. A few dikes of intermediate composition and a diorite stock at Mt. Bohemia intrude into the exposed Portage Lake Volcanics. The rhyolitic volcanic setting is analogous to the shield-type central volcanoes of Iceland (Nicholson, 1991). Interflow clastic sedimentary rocks layers of the Portage Lake Volcanics are recognized as informal members since they are important stratigraphic markers in an otherwise monotonous succession of basalt lava flows. Many of them are given informal names (Figure 6). A few of them can be traced along strike for large distances, up to 90 km. These interflow sedimentary rock layers consist of redcolored conglomerates with lesser amounts of interbedded sandstone and occasional significant amounts of siltstone and shale. These informal members range in thickness from a few cm up to about 40 m (Merk and Jirsa, 1982; White, 1968; Butler and Burbank, 1927). The typical conglomerate is characterized by sub-rounded to angular pebbles in a sandy matrix. Clast size varies from granules to boulders and clast lithologies are predominantly felsic, although there is considerable variation within and between specific beds reflecting diversity in source terrane. Within the interflow Calumet and Hecla conglomerate, Kalliokoski and Welch (1985) interpreted a subunit as a caliche soil profile. The interflow clastic sedimentary beds were deposited during intervals of volcanic quiescence, as terrestrial alluvial fans in an arid to sub-arid climate. Deposition was on top of the shallow-dipping to flat-lying lava flows by streams flowing from the topographic high on the margins of the MCR toward the center of the rift basin (now under Lake Superior) (White, 1968). Copper Harbor Conglomerate The Copper Harbor Conglomerate is the oldest formation of rift-filling clastic sedimentary rocks and conformably overlies and interfingers with the top of the Portage Lake Volcanics (Figures 4 and 5). It consists of red-brown clastic sedimentary rocks with a maximum exposed thickness 2,000 m. Conglomerates and sandstones are the dominant lithologies in the Copper Harbor Conglomerate. The formation fines distally and up section, reflecting a waning sediment supply due to progressive erosion of the source area (Elmore, 1984). The poorly-sorted clasts in the conglomerates range in size from granules to boulders that are subrounded to rounded and are mostly volcanic in origin and have a ratio of mafic-to-intermediate + silicic composition of about 2:1 (Daniels, 1982). The conglomerates include clast-supported and matrix- supported varieties; some of the latter are diamictites. The conglomerates are interpreted as high-energy channel deposits on coalescing alluvial fans (Elmore, 1984). The diamictites are debris flow in origin. Sandstone interbeds are more common in the upper 2/3 of the formation. Sandstones are predominantly red-brown, subangular-to- 7 �angular lithic graywackes with volcanic lithic fragments. The sandstones exhibit current-ripples, trough-cross beds, current and parting lineations, and reduction spots. Abundant calcite cement in select conglomerate and coarse sandstone layers was probably deposited as vadose carbonate or caliche (Kalliokoski, 1986). Thin red-colored siltstone and shale interbeds have desiccation cracks and are interpreted as periodic drying of the surface. In the Copper Harbor area, there are also laminated cryptoalgal carbonate beds and ooid lenses. These are laterally-linked contorted layers in shale-siltstone that are draped over cobbles and are found as poorly developed mats in coarse sandstone (Elmore, 1983). The laminated carbonate beds are algal stromatolites (genus Colleria). Figure 6: Generalized stratigraphy of the Portage Lake Volcanics in a strike parallel (longitudinal) section. Modified from Stoiber and Davidson (1959). Figure 4 shows location of Greenland-Mass subdistrict (Michigan, Caledonia, Mass, Adventure Mines). For decades The Copper Harbor Conglomerate CHC and overlying Nonesuch Formation (Figure 5) have been interpreted by many geologists as non-marine. Elmore (1984) interpreted the environment as a prograding coalescing non-marine alluvial fan complex with proximal-to-distal braided stream and sheet flood facies on the alluvial fans to distal sand flats and flood plain facies (Elmore, 1984). However, a number of sedimentological features could be interpreted as either non-marine or marine and thus, non-marine interpretations often relied on other evidence (Jones et al., 2020). In the stromatolite interval, Jones et al. (2020) cite bimodal (herring-bone) transport 8 �directions indicated by ripple marks that are mud draped and reactivated as evidence of a shallow marine environment. Hummocky cross stratification suggests waves on a marine shelf generated by storms (Jones et al., 2020). Periodic to rhythmic sedimentological features are indicative of “cyclical periodicity” of tidal deposition on a marine shoreline and are among evidence cited by Jones et al. (2020). Jones et al. (2020) conclude that the Copper Harbor Conglomerate and overlying Nonesuch Formation were deposited in a “braided fluvial-evaporitic shoreline-marine embayment” rather than fluvial-non-marine lacustrine setting. Geochemical evidence provided by Stüeken et al. (2020) also supports a marine estuary. The climate was probably arid with flashy seasonal streams. The highlands to the southeast from which the Copper Harbor Conglomerate was derived are now buried under the Jacobsville Sandstone. The Copper Harbor Conglomerate in the Keweenaw Peninsula includes a succession of subaerially deposited lava flows. Lane (1911) used the name, Lake Shore Traps, for this informal member (Figure 5). This member is well exposed near the tip of the Keweenaw Peninsula where the unit is composed of 31 lava flows and one interflow conglomerate with a maximum thickness of about 600 m (Paces and Bornhorst, 1985). The composition of the Lake Shore Traps is different than the underlying Portage Lake Volcanics reflecting the change from active rift-filling magmatism to passive subsidence with rift-filling clastic sedimentation and little to no magmatism except for the Lake Shore Traps. These subaerial lava flows range from Fe-rich olivine tholeiitic basalt at the base to Fe-rich olivine-bearing tholeiitic basaltic andesites and tholeiitic andesites and are likely a shield volcano. Geochemical data are best explained by a combination of fractional crystallization, parental magma replenishment, and wall rock assimilation (Paces and Bornhorst, 1985). Davis and Paces (1990) report a U-Pb age on zircon of 1087.2 +/- 1.6 Ma for the Lake Shore Traps. Nonesuch Formation The Nonesuch Formation conformably overlies and locally interfingers with the Copper Harbor Conglomerate (Figures 4 and 5). It consists of dominantly black-to-gray-to-green to redgray siltstone and shale with a maximum thickness 240 m. Bornhorst and Williams (2013) provide a stratigraphic column of the entire Nonesuch Formation just south of the Porcupine Mountains State Park from exploration drilling. Exposures of the Nonesuch Formation in the Keweenaw Peninsula proper are limited with the best exposure at the Hancock campground and boat launch on M-203 (Stop 14). There are excellent exposures of the Nonesuch Formation along the Big Iron and Presque Isle rivers in the White Pine area (Woodruff et al., 2013). In areas with thicker stratigraphic section, siltstone and shale are the dominant lithologies with lesser very-fine sandstone and minor carbonate laminates. While gray (reduced) color characterizes most of this formation, the stratigraphic upper beds have more red-brown colors (Bornhorst and Williams, 2013). Well-laminated to massive black to dark-gray siltstone and shale are the dominant lithologies near the base of the Nonesuch Formation. The base of the Nonesuch Formation hosted economic quantities of chalcocite and native copper at the now closed White Pine Mine (Mauk et al., 1992) and chalcocite at the Copperwood project (Bornhorst and Williams, 2013; Williams and Bornhorst, 2023). A thin carbonate laminate yielded a Pb-Pb isochron age of 1,081 ± 9 Ma (Ohr,1993). The environmental setting of the Nonesuch is described above under the Copper Harbor Conglomerate. 9 �Freda Sandstone In the Keweenaw Peninsula the Freda Sandstone is the youngest rift-filling clastic sedimentary rock formations (Figures 4 and 5). The contact between the lower most Freda Sandstone and Nonesuch Formation is gradational (Bornhorst and Williams, 2013). The exposed thickness is greater than 3,700 m, with the top of the formation submerged beneath Lake Superior. The Freda Sandstone is generally poorly exposed except along the Lake Superior shoreline. This field guide provides an optional stop 13 at the McLain State Park where exposures of the Freda are visible during times of low levels of Lake Superior. Angular and tabular specimens are obtainable at the beach from outcrops just offshore. The last MCR magmatism was Bear Lake, an intrusive-extrusive dome of alkaline trachyandesite was emplaced near the middle of the exposed Freda Sandstone (Kulakov et al., 2018). Red-brown fine to very-fine sandstone, siltstone, and mudstone are the dominant lithologies in the Freda Sandstone. Fining-upward sequences occur on the scale of a few meters. The Freda Sandstone was deposited in an environment characterized by shallow meandering streams (Daniels, 1982). Based on regional correlations the Freda was likely deposited between 1,080 to 1,060 Ma. Jacobsville Sandstone The Jacobsville Sandstone was deposited in a rift-flanking basin (Figure 3D) and is outside the scope of this field guide. Its stratigraphic relationship with other formations is not determined. It occurs in a contiguous geographic region bound on the northwest side by the Keweenaw Fault and on the southeast by an unconformable contact with Paleoproterozoic and Archean basement rocks (Figure 4). The Jacobsville Sandstone is estimated to be more than 2,900 m thick and the top is not exposed (Kalliokoski, 1982). Red to red-brown sandstone is the dominant lithology with lesser amounts of red-brown conglomerate, siltstone, and shale. The sandstone varies from subarkose to quartz sublithic arenite although there are some beds of arkose and quartz arenite (Kalliokoski, 1982). The Jacobsville Sandstone was deposited in an environment characterized by shallow meandering streams (Kalliokoski, 1988). Faults, Folds, Fractures The last episode of the Midcontinent Rift was characterized by post-rift compressional inversion that facilitated hydrothermal formation of native copper deposits (Woodruff et al., 2020; Bornhorst, 1997). This compression transformed original normal faults into reverse faults, reactivated other extensional rift-related faults/fractures, and produced new compression-only faults/fractures and folds. Rather than being an inverted rift-related normal fault, the Keweenaw fault was likely a detached thrust (DeGraff and Carter, 2023). Cannon et al. (1993) have determined that compression occurred at about 1,060+/-20 Ma. The probable cause of this event was continental collision along the Grenville front (Figure 1) beginning as early as 1.08 Ga (Cannon, 1994; Cannon and Hinze, 1992; Hoffman, 1989). Final inversion of the MCR by compression during Grenville orogeny occurred between 1,010-980 Ma (Hodgin et al., 2022). 10 �The Mesoproterozoic Midcontinent Rift-filling strata of the Keweenaw Peninsula dip moderately toward the center of the rift with the angle of dip increasing toward Keweenaw fault where the stratigraphic base is truncated (Figure 7). The dip of the strata is interpreted as a combination of syn-depositional downwarpage and structural tilting in response to reverse faulting caused by regional continental compression (Woodruff et al., 2020; Cannon, 1994). There are many faults/fractures in the Mesoproterozoic rocks of the Keweenaw Peninsula. Some of these were exclusively formed during extension of the Midcontinent Rift when grabenbounding normal faulting was prominent along the margin (Figure 3). However, most faults/fractures were likely either reactivated by or directly produced by the regional compressional event. The Keweenaw Fault strikes and dips more or less parallel to the bedding of the truncated Portage Lake Volcanics (Figure 7) and is not necessarily one fault, as it is a zone with branches up to 0.8 km from the main fault (Butler and Burbank, 1929). It is a detached thrust fault related to regional compression. Although the Keweenaw Fault would make an ideal conduit for movement of hydrothermal fluids, there are no native copper deposits along it similar to other ore-bearing districts where the main faults are not well mineralized. However, the rocks within and adjacent to the fault are altered by late-stage hydrothermal fluids. Figure 7: Simplified geologic map showing the location of the major deposits within the Keweenaw Peninsula native copper district, Michigan. Table 2 provides the names and production for the numbered deposits. The areas shown on the map are the mined out down-dip portion projected to the surface. All of the native copper mines are hosted by the Portage Lake Volcanics. Modified from Bornhorst and Barron (2011). 11 �Table 2: Production from 1845 to 1968 of refined copper from native copper deposits (after Weege and Pollock, 1971). Million lbs Produced Refined Copper Location Number see Figure 7 Calumet & Hecla Conglomerate 4,229 7 Kearsarge Flow Top 2,263 3 Baltic Flow Top 1,845 12 Pewabic Flow Top 1,077 9 Osceola Flow Top 578 8 Isle Royale Flow Top 341 10 Atlantic Ashbed 143 11 Allouez Conglomerate 73 6 Houghton Conglomerate 38 4 Kingston Conglomerate 20 Greenland-Mass Subdistrict 72 5 See Figure 3 Other Flow Top and Conglomerate Deposits 137 Cliff Fissure 38 1 Central Fissure 53 2 Other Fissure Deposits 123 Name of Deposit District Total 11,030 Several faults occur oblique to the strike of bedding. In the Eagle River area, fault-controlled native copper veins are common in association with high-angle faults whose displacement is from 0 to 200 m, (Figure 7; see also Figure 19; Butler and Burbank, 1929). The Allouez Gap fault (Figure 7) bisects the largest lava flow top hosted native copper deposit in the district (see Figure 16) and was likely a significant conduit for native copper mineralizing hydrothermal fluids (Bornhorst, 1997). Correspondence between the thickness of the Kearsarge lava flow and the Allouez Gap fault suggests this fault was active during deposition of the Portage Lake Volcanics. It is interpreted as having been reactivated during regional compression. Faults were the principal pathway for the upward movement and focusing of ore fluids into the stratabound lava flow tops in the Baltic and Isle Royale deposits as well as those in the Greenland-Mass subdistrict (Broderick, 1931). Faulting occurred before, during, and after deposition of native copper and its associated alteration minerals based on fault brecciated and recemented alteration minerals. There is a close relationship between faulting/fracturing produced by or reactivated by compression and native copper deposits. The compressional structures acted as pathways for mineralizing hydrothermal fluids (Bornhorst 1997). 12 �Broad open synclines and anticlines, with wavelengths of around 10 km and various orientations, are superimposed on the regional dip. Faults with displacement and mineralized tension breaks are common near the crests of anticlines (Butler and Burbank, 1929). These post-depositional folds are likely related to the late regional compression (White, 1968). Keweenaw Peninsula Native Copper District Active copper mining occurred from 1845 to 1968 in the Keweenaw Peninsula native copper district. The estimated pre-mining geologic resource for the district is ~20 billion lbs of copper (Bornhorst and Barron, 2011). Small quantities of native silver are temporally and spatially associated with the native copper. The major ore producing horizons are located in a 45 km-long belt in the Keweenaw Peninsula (Figures 4 and 7) and in a subdistrict to the southwest. Native copper and silver were the only economic metallic minerals and were co-precipitated with a suite of nonmetallic alteration minerals (Figure 8). Sulfide minerals, such as chalcocite, are uncommon in native copper deposits and when present only occur in trace amounts. Sulfide minerals occur in latestage veins (Figure 8). Several chalcocite deposits of unknown connection to the native copper deposits are hosted by the stratigraphically older Portage Lake Volcanics; the largest of these contains roughly 230 million lbs of copper (Woodruff et al., 2020; Maki and Bornhorst, 1999); these will not be discussed here. Native Copper Ore Bodies Ore bodies in the Keweenaw Peninsula are tabular, stratabound concentrations of native copper hosted by the Portage Lake Volcanics where there is sufficient original porosity including brecciated and amygdaloidal flow tops (58.5% of production) and interflow conglomerate beds (39.5% of production). Secondary porosity occurs along fractures/faults which host veins (about 2% of production). Since the deposits represent important stratigraphic horizons, the host rocks were given informal member names (Butler and Burbank, 1929). Several mines with different names often worked the same deposit/ lithostratigraphic unit. About 85% of the total district production came from four deposits: Calumet and Hecla Conglomerate, top of the Kearsarge lava flow, top of the Baltic lava flow, and the top of the Pewabic lava flow (Table 2). The most common host rocks for native copper deposits are brecciated flow tops (fragmental amygdaloid) as their original porosity was typically much greater than vesicular (amygdaloidal) flow tops (White, 1968). The stratabound flow top deposits are “sandwiched” between a footwall consisting of barren massive basalt of the same flow as the mineralized flow top and hanging wall interior of the succeeding lava flow. Native copper is often more abundant near the top and bottom of the brecciated/fragmental amygdaloid interval of the flow top, however, in rich ore shoots, the entire brecciated/fragmental amygdaloid flow top contains significant amounts of copper. As brecciated/fragmental amygdaloidal transitions downward into massive basalt, it becomes deficient in native copper. In some cases, ore shoots are located in tongues of brecciated flow tops within massive basalt (Weege and Schillinger, 1962). The lateral and vertical distribution of brecciated/fragmental amygdaloid within the top of a lava flow is irregular and hence, so is the grade of copper. In general, mined stope heights are from 3 to 5 m. Ore shoots are elongated, but 13 �also occur in a wide variety of shapes, with widths of 30 to 150 m and down dip lengths from 50 m to 600 m (White, 1968). The strike length for major ore bodies ranges from 1.5 to 11 km with down dip mineralization extending from 1.5 to 2.6 km on the incline below the surface (Butler and Burbank, 1929; White, 1968). Although interflow conglomerate beds make up only a small volume of the Portage Lake Volcanics, about 40 % of the district production of refined copper were hosted by them. These deposits were tabular and stratabound, just like the flow top deposits. They are “sandwiched” between a footwall consisting of the top of the underlying lava flow and hanging wall of barren massive basalt interior in the overlying lava flow. The porosity of underlying brecciated/fragmental amygdaloid lava flow top is often greatly decreased by silt and sand filling the primary open space between fragments of the flow top hence, the originally porous flow top underlying a conglomerate bed acts more like an aquiclude in the paleohydrologic hydrothermal system. Native copper tends to be concentrated along specific stratigraphic bands within the conglomerate that are 0.5 to 5 m thick (Weege et al., 1972). The Calumet and Hecla Conglomerate was by far the largest single native copper deposit in the district producing 4.2 billion lbs. as compared to the next largest deposit, the Kearsarge flow top which produced 2.3 billion lbs. from a fragmental amygdaloid (Figure 7 and Table 2). The Calumet and Hecla Conglomerate was mined along a strike length of 4.9 km, and down-dip 2.8 km. The productive area corresponds to a thickening of the conglomerate from less than 1 m up to 6 m (Butler and Burbank, 1929; Weege et al., 1972). Ore grades decrease with depth where the width of the conglomerate is greater; essentially the same amount of copper is distributed throughout a greater volume. (Butler and Burbank, 1929). The highest grades correspond to beds where there is relatively little fine interstitial material in the clastic sedimentary host rock or where interstitial spaces are filled with coarse sand or small pebbles (Weege et al., 1972). Thus, localization of native copper ore is dependent on sedimentary environmental factors. The first mines in the district were developed on tabular steeply dipping deposits that cross-cut bedding at high angles. However, overall, the vein deposits are of slight economic importance in the district. The veins have widths of up to 3 m or more (Butler and Burbank, 1929). Veins are not single tabular bodies, but rather a series of parallel anastomosing filled open spaces. While brecciation within vein deposits is common, gouge is not present (Butler and Burbank, 1929). The lava flow tops and conglomerates adjacent to the vein are mineralized. The distribution of native copper in veins is more erratic than in either lava flow top or conglomerate deposits. The richest ore veins tend to be spatially associated with the intersections of the vein and well-oxidized lava flow tops (Butler and Burbank, 1929). Native copper occurs as both finely disseminated and as masses weighing many tons. The grade of native copper in the veins has the nugget-effect making determination of grade difficult. Several small vein deposits are localized just beneath the thickest basalt flow in the district, the Greenstone flow. For these veins the hydrothermal fluids moved up along the cross fractures until blocked by the very thick impermeable massive interior of the Greenstone Flow. There are veins spatially and genetically associated with the stratabound lava flow top or conglomerate deposits; these veins occur along faults that intersect major deposits such as the Baltic 14 �and Isle Royale faults (Broderick, 1931). This suggests that ore fluids moved upward along faults and outward into the permeable flow tops. The intersection of subsidiary faults with locally thick permeable horizons is a key factor in concentrating ore such as the Kearsage deposit (see Figure 16). White (1968) suggested that for the movement of ore fluids to occur, permeability due to fracturing was more important than primary permeability. Faults and small fractures cutting massive interior of lava flows were also likely important for upward transport of ore fluids. Overlapping of successive lava flows and minor unconformities suggests that simple up-dip movement of ore fluids was not likely without a network of fractures (Bornhorst, 1997). Hydrothermal Minerals The rocks within the Keweenaw Peninsula native copper district were pervasively altered by lowtemperature, low-pressure hydrothermal/burial metamorphic fluids. Alteration was most intensely associated with the native copper deposits although to some degree secondary hydrothermal minerals occur in all rocks of the Portage Lake Volcanics. Areas in the Keweenaw Peninsula more distal to the area of major native copper deposits rocks were less altered at lower temperature. The intensity and degree of alteration also varies as a function of position within lava flows; the massive interiors of lava flows are much less altered whereas the lava flow tops are relatively more altered. Lava flows in close proximity to cross cutting features tend to be more altered. The minerals occur as amygdule and vein fillings, and as whole rock replacements. Within the Portage Lake Volcanics, some original igneous minerals are present in the massive interiors of some flows, but secondary minerals exist in the massive interiors of all flows regardless of their thickness. While the thicker massive interiors of lava flows contain secondary minerals, their original igneous geochemical composition is often only slightly or essentially not modified by secondary hydrothermal processes. There are more than 50 different secondary alteration minerals in the Keweenaw Peninsula; most of them are related to hydrothermal processes and some are related to supergene processes. Only about 20 alteration minerals are major to less common minerals (Figure 8). Native copper with small quantities of native silver represents over 99% of the metallic minerals in the mined ore bodies of the district. Most of the native copper carries a small amount of arsenic in solid solution (typically less than 0.2 % arsenic of total copper + silver + arsenic; Broderick, 1929). Copper-nickel arsenides occur in veins that are paragenetically late (Moore, 1971; Stoiber and Davidson, 1959; Butler and Burbank, 1929). There is a district wide temporal (paragenetic) and spatial variation in the assemblage of alteration minerals which was first well described by Butler and Burbank (1929) and later summarized by White (1968). Recently Bodden et al. (2022) have refined the paragenetic and spatial variation of the hydrothermal minerals (excluding igneous and supergene related minerals) (Figure 8). The hydrothermal alteration minerals can be subdivided into main-stage which paragenetically overlap with the precipitation of native copper (Figure 8). While district-wide there is a well-defined mineral paragenesis, individual deposits may not exactly follow the district-wide timing of precipitation (compare Figure 8 to Stop 5). The main-stage is interpreted by Bodden et al. (2022) as formed during a continuous hydrothermal event. 15 �Figure 8: Paragenesis and relative abundance of secondary hydrothermal alteration minerals in the Keweenaw Peninsula native copper district. After Bodden et al. (2022). The late-stage minerals are widespread but volumetrically minor. They commonly occur in small veins/fractures which cross-cut the main stage minerals or as coatings on main-stage vug filling minerals. Late-stage alteration minerals are notably more abundant near the Keweenaw fault. The suite of late-stage minerals are distinguishable by the occurrence of sulfur-bearing minerals, sulfides and sulfates, and by an assemblage of lower temperature minerals in areas where they overprint an assemblage of main-stage minerals formed at higher temperatures. The timing of the late-stage hydrothermal event is uncertain. There could have been no time break or a major time break between the main-stage and late-stage hydrothermal events. Bodden et al. (2022) suggested that the main-stage and late-stage hydrothermal events are practically continuous with each other. Main-stage alteration minerals are spatially zoned perpendicular to stratigraphic strike as demonstrated for the Calumet area of the district (Figure 9). Epidote and the appearance of quartz are spatially associated with major native copper deposits (Stoiber and Davidson, 1959). A detailed study by Stoiber and Davidson (1959) of the Kearsarge deposit shows that native copper is much more irregularly distributed than secondary mineral zones, but there is a general correlation with the abundance of native copper associated with the variation of quartz and microcline (Stop 5). The alteration mineral zones of the Portage Lake Volcanics are similar to the North Shore Volcanic Group of Minnesota (Schmidt and Robinson, 1997). Bodden et al. (2022) mapped the occurrence of 16 �Figure 9: Distribution of prominent secondary hydrothermal alteration minerals in the Portage Lake Volcanics in a cross-section in vicinity of Calumet at the center of the major deposits of the Keweenaw Peninsula native copper district modified from Bornhorst and Rose (1994). Figure 10: Main-stage hybrid metamorphogenic hydrothermal mineral zones of the Keweenaw Peninsula (modified from Bodden et al., 2022) 17 �alteration minerals of the Keweenaw Peninsula into those zones used for the North Shore Volcanic Group (Figure 10; Schmidt and Robinson, 1997). These zones can be equated to the temperatures of mineral formation (Bodden et al., 2022). The spatial zoning of alteration minerals is consistent with a thermal high associated with the major native copper deposits (compare Figure 4 and 10). The mineral zones dip more gently towards Lake Superior than the strata, implying that the strata were at least somewhat tilted prior to main-stage hydrothermal alteration (Livnat, 1983; Broderick, 1929). Native copper mineralization is younger than the Copper Harbor Conglomerate, which hosts rare veins of calcite and native copper (see Stop 7). White (1968) interpreted the age of native copper mineralization as after the deposition of parts or all of the Freda Sandstone. The Bear Lake igneous body within the Freda Sandstone is native copper mineralized. Minor amounts of native copper occur within the lower beds of the Jacobsville Sandstone near Rice Lake (Calumet and Hecla unpublished drill core log). Based on field relations, hydrothermal alteration is younger than deposition of rift-filling strata and at least some of the rift-flanking Jacobsville Sandstone. The absolute age of hydrothermal alteration is between 1060 and 1050 Ma (+/- ~ 20 Ma) (Bornhorst et al., 1988). This age is consistent with the approximate age of 1060+/-20 Ma for regional continental compression that caused thrust faulting along the Keweenaw Fault (Cannon et al., 1993). Thus, the age of main-stage hydrothermal alteration is about 1060 to 1050 Ma contemporaneous with regional continental compression and some 30 million years after eruption of the Portage Lake Volcanics. Genesis of the Main-Stage Native Copper Deposits This section is summarized from Bornhorst and Mathur (2017) and Bodden et al. (2022) and illustrated in Figure 11. Native copper occurs throughout the MCR in Wisconsin, Minnesota, and Ontario (Figure 2). It formed during a regional hydrothermal event from about 1060 to 1050 Ma (Bornhorst et al., 1988). The regional Cu-bearing hydrothermal fluids are best explained as generated during burial metamorphism of rift-filling basalts with temperatures reaching a thermal maximum approximately 30 million years after the end (~1085 Ma) of widespread rift magmatism. The suite of main-stage hydrothermal minerals precipitated, except native copper, (Figure 7 and 8) is similar to those found elsewhere rocks have undergone very low to low grade burial metamorphism at less than about 300OC. The coincidence of regional continental compression with a burial thermal maximum (Woodruff et al., 1995) provided an integrated paleohydrologic system through reactivated and new faults and fractures. This allowed the upward movement of hydrothermal fluids to focus in sites of future copper deposits at the very time of greatest fluid availability (Bornhorst 1997). During generation of the regional burial metamorphogenic hydrothermal ore fluids, copper was leached at depth from the rift-filling basalt strata (Bornhorst and Mathur, 2017, 2018). More than sufficient amount of copper was available to have been leached from the buried rift-filling basalts. 18 �Figure 11: Cartoon cross sections showing conceptual genetic model of the native copper deposits of the Keweenaw Peninsula formed at about 1070 to 1040 million years ago. Modified from Bodden et al. (2022). A. Marine incursions and seawater penetration during deposition of volcanic and sedimentary rocks in MCR. B. Area prior to burial metamorphism with sulfur depleted evolved seawater providing salinity for ore-forming fluids. C. Burial metamorphism with generation of burial metamorphic hydrothermal fluids. Mixing the burial metamorphic fluids with evolved seawater produces copperbearing hybrid metamorphogenic ore-forming hydrothermal fluids. D. Precipitation of main-stage minerals as a result of mixing of the ore-forming fluids with meteoric water, decreasing temperature, and water-rock reactions. 19 �The rift-filling volcanic rocks were low in sulfur when they erupted and the little available sulfur was degassed prior to solidification (Bodden et al., 2022; Bornhorst and Mathur, 2017). These lowsulfur rift-filling volcanic rocks were buried into the source zone where burial metamorphogenic hydrothermal fluids were generated (Figure 3 and 11C). Since the very low sulfur rift-filling basalts in the source zone were the same as those in the fluid pathways to the zone of precipitation, the fluids remained sulfur poor. Since the native copper ore host rocks were again the same rift-filling very low sulfur volcanic rocks, the burial metamorphogenic hydrothermal fluids remained depleted in sulfur. The lack of sulfur resulted in the precipitation of native copper rather than copper sulfides. While the metamorphogenic hydrothermal fluids lacked sulfur, several studies (Kelly, 2022; Kelly, 2020; Püschner, 2001; Brown, 2006; Livnat, 1983; Jolly, 1974) have suggested that the main-stage hydrothermal fluids had at least moderate degree of salinity. The possible sources of salinity were evaluated by Bornhorst and Mathur (2017), Bornhorst (2021), and Bodden et al. (2022). Viable sources of salinity for the hydrothermal fluids need to also satisfy the constraint of very low sulfur. Bornhorst (2021) hypothesized that evolved formation water derived from sulfur depleted seawater could have been the source of salinity. As seawater penetrates midocean ridge basalts it is heated, reacts with the host basalt, and as a result of precipitation of minerals it becomes depleted in sulfur. If the sulfur content of the Mesoproterozoic seawater was low in sulfur, as proposed by Blattlet et al. (2022), then less depletion of sulfur would have been needed. During deposition of the youngest Portage Lake Volcanics and overlying Copper Harbor and Nonesuch formations there were probable incursions of an arm of the sea into the rift for a significant amount of time. This could have resulted in seawater deeply penetrating into the underlying rift-filling volcanic rocks (Figure 11A; Bornhorst, 2021). During burial, the riftfilling volcanic and clastic sedimentary rocks and contained seawater was progressively heated and thereby evolved to be depleted in sulfur (Figure 11B). Continued heating during burial resulted in burial metamorphic hydrothermal fluids which then thoroughly mixed with the evolved seawater to form a hybrid metamorphogenic-dominated ore-forming fluid (Figure 11C; Bodden et al., 2022). These main-stage ore-forming hydrothermal fluids moved upwards from the source zone through the same very sulfur poor strata as in the source rocks (Figure 11D). As they moved upwards they cooled, interacted with host rocks, and in the relatively shallow zone of precipitation they variably mixed with sulfur-poor, low salinity, reduced meteoric water (Figure 11D; Bodden et. al, 2022). These processes resulted in precipitation of native copper and main-stage hydrothermal minerals. Higher temperature main-stage mineral assemblages are spatially associated with the area of native copper deposits where the thermal anomaly was greatest because of focused hydrothermal fluids (Figure 10). The possible depth of the zone of precipitation is poorly estimated with a best guess at this time of 10 to 15 km (Kelly et al., 2022; Kelly, 2020). Within the native copper district, the suite of main-stage minerals, including native copper, is followed by late-stage minerals precipitated at lower temperatures than the main-stage hydrothermal fluids coincident with the native copper district. 20 �Late-Stage Hydrothermal Minerals The suite of late-stage minerals is widespread and similar throughout the Keweenaw Peninsula. The late-stage suite is readily distinguished in the main area of the native copper district since late-stage minerals are lower temperature (100 to 150OC) than the main-stage minerals in district itself. However, outside of the native copper district where main-stage minerals are expected to be formed at lower temperature, the main-stage and late stage are indistinguishable. Bodden et al. (2022) suggested that late-stage fluids are a variable mixture of hybrid metamorphogenic hydrothermal fluids, meteoric water, and shallow seawater, the latter being a source of sulfur in the late-stage fluids. Phanerozoic The Keweenaw Peninsula was subjected to a 500-million-year period of erosion, from about 1 Ga to 0.5 Ga (500 Ma) and multiple kilometers of rock were eroded exposing the native copper deposits at the surface (Figure 3). Downward percolating groundwaters supergene altered native copper and produced a suite of including cuprite, tenorite, malachite, and chrysocolla (Bornhorst and Robinson, 2004). The rocks of the Keweenaw Peninsula were subsequently buried by Paleozoic sedimentary rocks associated with the Michigan basin beginning about 500 Ma (Figure 3) and ending Precambrian supergene alteration. Over the past two million years, the Keweenaw Peninsula was subjected to several continental glacial periods which removed all of the overlying Paleozoic sedimentary rocks with the exception of Paleozoic outliers slightly south of the Keweenaw Peninsula (Figure 3). The last glacial episode exposed the native copper deposits at roughly the same erosional level as at 500 Ma or the end of the Precambrian (Bornhorst and Robinson, 2004). The continental glaciers sculpted the bedrock of the Keweenaw Peninsula and when the last glacier retreated about 10,000 years ago, it left behind a variety of unconsolidated glacial-related sediments that included entrained boulders of native copper. The glaciers carved out the topographic low the Lake Superior basin corresponding to the less competent clastic sedimentary rocks under the center of the MCR. After the glaciers retreated, very large volumes of water filled this topographic low and initially all but the highest land elevations were submerged under a large glacial lake. The glacial lake levels successively dropped over time to the current level of Lake Superior (Farrand 1960). As the lake levels receded humans populated the area. 21 �Figure 12: Geologic map of the far western part of the Upper Peninsula of Michigan showing field trip stops. Objectives of Field Trip This field trip is designed to provide an overview of the Mesoproterozoic Midcontinent Rift-filling strata and native copper deposits of the Keweenaw Peninsula (Figure 12). There are four rift-filling formations: Portage Lake Volcanics, Copper Harbor Conglomerate, Nonesuch Formation, and Freda Sandstone. The Nonesuch and Freda formations are poorly exposed in the Keweenaw Peninsula thus, only two optional stops are included in this field guide. The Jacobsville Sandstone is a rift-flanking formation and is outside the scope of this field trip. The rift-filling strata are overlain by unconsolidated Pleistocene glacial sediments. There is one glacial related stop. IMPORTANT NOTE TO ALL READERS: Many of the field trip stop descriptions and significant parts of the introductory geologic overview have been previously published especially in other Institute on Lake Superior guidebooks e.g., Bornhorst and Barron (2013) and the extensive guides by Bornhorst and Rose (1994) and Bornhorst et al. (1883). The stop descriptions in this field guide, as compared to previously published guides range from exact wording to significantly modified wording without specific citation. Two of the stops in this field guide, Stops 2 and 8, were not visited by previous field guides involving Bornhorst. These stop descriptions are new. 22 �Stop 1: Subaerial basalt lava flow cross-section at South Range Quarry Latitude: 47.07750N; Longitude: -88.64240W Directions: Drive west through downtown Houghton on US-41 to south M26. Drive about 4.5 miles to unmarked road on west side of M-26 just before church on south side of unmarked road. Proceed on road to tree line. Walk NE uphill to quarry. THIS STOP IS ON PRIVATE PROPERTY. PLEASE GET PERMISSION TO ENTER PROPERTY. Volcanic textures and structures typical of moderate-to-thick subaerial lava flows within the Portage Lake Volcanics are well exposed in this old quarry (Figure 13). As one traverses up the hill to the quarry along the rubbly path there is a low-profile exposure of a 4 m thick interflow conglomerate bed which is also exposed laterally along the SE slope face of this knob. The conglomerate layer is stratigraphically the National Sandstone member and is approximately near the middle of the exposed Portage Lake Volcanics stratigraphic section (Figure 6). The conglomerate is overlain by an 18 m thick lava flow (A). Figure 13: Geologic cross section of the South Range Quarry (modified from White, 1971). Laterally continuous interflow sedimentary beds provide critical stratigraphic markers within the Portage Lake Volcanics, an otherwise monotonous volcanic pile with many laterally discontinuous lava flows. The sedimentary unit exposed below the quarry has been correlated with the National Sandstone, a marker bed in the Mass-Rockland area (Figure 6). At South Range Quarry, the National Sandstone is a massively bedded, pebble-cobble framework conglomerate, composed of silicic with subordinate mafic volcanic clasts that are subangular to subrounded, within a matrix of poorly-sorted medium-to-coarse sand of similar composition. The Portage Lake Volcanics basalts in this part of the stratigraphic section are mainly olivine tholeiites and erupted as subaerial lava sheets. The principal lava flow exposed in the quarry walls illustrates many of the volcanological features of Portage Lake Volcanics lava flows. The top and bottom of the South Range Quarry lava flow (B) are exposed. Flow B was deposited directly on top of Flow A and consists of aphanitic chilled basalt. The base of Flow B occurs where amygdules disappear abruptly in the top of the underlying flow. 23 �The upper surface of the main flow (B) was brecciated slightly by movement of lava after the formation of an upper crust. The flow top breccia (locally termed fragmental amygdaloid) is laterally discontinuous. The fragmental amygdaloid rapidly grades downward to an unbrecciated, highly amygdaloidal (vesicular) flow top. Note the variation in vesicle size and shape downward in the flow. There are numerous layers of flattened amygdules (vesicles) in the flow top with their orientation parallel to the top and bottom of the flow (B). The orientation of these layers may represent laminar flow planes within the flow top. The section was tilted after emplacement. Slow cooling of the lava flow caused solidification toward the flow interior at a rate which allowed development of subophitic to ophitic textures (ophitic texture denoted by large oikocrysts of clinopyroxene enclosing a felted framework of An-rich plagioclase and intergranular olivine). The resulting massive, non-vesicular flow interior constitutes about two-thirds of the flow (B). Before final solidification, small amounts of volatile-rich, differentiated residual liquid were likely injected into thin discontinuous layers and lenses (tabular openings produced during cooling). Most of these pegmatoid layers are subparallel to the bottom and top surfaces of the flow. A typical pegmatoid zone consists of a 5 to 10 cm border zone at the top and bottom which is composed of a medium-to-coarse grained aggregate of Ab-rich plagioclase, prisms of Fe-rich clinopyroxene and abundant Fe-Ti oxides, as well as accessory minerals such as apatite and zircon (Cornwall, 1951c). The cores of the pegmatoid zones are 5 cm to 1.2 m thick consisting of a green vesicular basaltic rock. Zircons extracted from pegmatoids within thick Portage Lake Volcanics basalt flows have yielded high-precision U-Pb dates (e.g., Davis and Paces, 1990). There are thin tabular layers and flattened amygdules (vesicles) in the top of the flow and in the massive interior of the flow (B) that are composed of a brown and sometimes green fine-grained material that have been described interpreted by White (1971) as detrital material. Alternatively, this green-to-red cherty rock, could be simply alteration minerals (quartz, prehnite, epidote, and pumpellyite) filling fractures and nearly complete pseudomorphic replacement of basalt. Numerous pegmatoid layers are exposed in the quarry walls, as well as in the glacially-polished surfaces above and to the north of the quarry. The effects of regional hydrothermal alteration can be observed within the vesicular flow top and pegmatoid zones. Vesicles are filled with a variety of secondary minerals including quartz, epidote (olive green), prehnite (waxy light green), calcite, pumpellyite (pale bluish green), chlorite (dark green to black) and traces of native copper (pinkish color). Pseudomorphic replacement of basalt by fine-grained secondary minerals is most intense where permeability was highest. The massive interior of the flow is only a little visibly altered, however plagioclase is altered to albite and pyroxene is altered to chlorite. In the vicinity of selected fractures there can be intense epidote or prehnite alteration. The massive interior was a relatively impermeable horizon in the paleohydrothermal system. Fracturing during late compression integrated the system and provided limited pathways for upward movement of ore fluids. 24 �Stop 2: Subaerial basalt lava flow, eastbound US-41 Houghton Latitude: 47.12144N; Longitude: -88.56474W Directions: Drive west through downtown Houghton and loop around (Yooper Loop) to head east for 0.8 miles along US-41 or Montezuma Avenue (one-way two-lane highway). Stay left (north side) as though going back through downtown and turn into unmarked gravel lot just before large outcrops on left (north). Use sidewalk to walk to outcrop. BE CAREFUL OF TRAFFIC. The rock cut at the southeast end of downtown Houghton provides an excellent example of the characteristics of Portage Lake Volcanics subaerial basalt lava flows (Figure 14). There is a sidewalk providing access to the Stop 2 south-facing road cut. While there is also a sidewalk on the other side of the road crossing the road is discouraged. These other outcrops can be accessed by parking uphill from them on the other side of the road. There are also north-facing exposures of this same stratigraphic interval on west bound US-41 (Shelden Avenue) east of the Houghton U.S. Post Office. The exposures at Stop 2 are located stratigraphically above the Scales Creek flow and below the Kearsarge flow, slightly closer to the Scales Creek flow (Figure 6). The lava flows of the exposure and vicinity strike approximately N30oE and dip toward the center of the rift (Lake Superior) at about 55o northwest (White, 1956). Figure 14: Geologic cross section of road cut eastbound US-41, southeast end of downtown Houghton. There are parts of two lava flows exposed at Stop 2 (Figure 14). The base of the stratigraphically lower of these two lava flows (A) is not exposed on the east end of the Stop 2 exposures. It is also not exposed on the other side of the road. The top of Flow A is well exposed and is overlain by massive basalt of Flow B. The contact itself is denoted by an abrupt change from underlying highly altered greenish, slightly brecciated amygdaloidal basalt of Flow A (about 0.5 m thick) that is overlain by massive dark grey to black basalt of Flow B. This greenish zone of the top of Flow A is underlain by about 3.5 m of amygdaloidal basalt with slightly fragmental (brecciated) basalt, also Flow A. The slightly fragmental basalt is gradational downward (east) towards the center of Flow A where the abundance of amygdules (filled vesicles) is sufficient to call the rock amygdaloidal basalt. Amygdaloidal basalt lacking fragments is about 4 m thick. There is an arbitrary boundary where the 25 �abundance of amygdules is too low to call the rock amygdaloidal, although it contains some amygdules. The abundance of amygdules progressively decreases towards the center of Flow A, the porous and permeable top of Flow A is about 8 m thick. Flow A is greater than 14 m thick as its base is not exposed. Notably, flattened amygdules (vesicles) occur along planar layers in the flow A top with their orientation roughly parallel to the top of the flow. Larger flattened amygdules are about 2 by 2 by 1 cm and between them, appearing to connect larger amygdules, are much smaller amygdules 1 to 2 mm thick. The orientation of these layers may represent laminar flow planes within the flow top. Layers of flattened amygdules are also numerous at Stop 1. Flow A is overlain by Flow B and the top of Flow B is not exposed at Stop 2. However, its top is poorly exposed on the other (south) side of the road. Flow B is about 30 m thick. There is a pegmatoid layer in the massive basalt interior of Flow B (Figure 14). Flow B is thicker than the average flow. The pegmatoid is distinguished as notably amygdaloidal. Pegmatoids are discussed further at Stop 1. The effects of regional hydrothermal alteration can be observed within the top of Flow A and the pegmatoid layer in Flow B. The massive basalt interior of Flow A and B are much less altered than the flow top. However, the primary magmatic plagioclase in the massive basalt has been replaced by albite and the primary mafic minerals are replaced by chlorite, pumpellyite, and iron oxides. The intensely altered basalt at the very top of Flow A is largely replaced by hydrothermal alteration minerals including epidote, prehnite, pumpellyite, quartz, chlorite, calcite, and trace native copper. Amygdules are frequently filled with colorless to white quartz. a mixture of prehnite pumpellyite and quartz, a mixture of pumpellyite, a mixture of quartz with lesser calcite, only quartz, and only pink inclusions of native copper in milky or colorless quartz. There is visible native copper in some amygdules. The massive interior of the flow is much less altered than the flow top and represents a relatively impermeable horizon in the paleohydrothermal system. In contrast, the flow top was a pathway for movement of hydrothermal fluids. 26 �Stop 3: Overview at Bumbletown Hill Latitude: 47.290100N; Longitude: -88.417250W Directions: From Portage Lift Bridge follow US-41 towards Copper Harbor and proceed through Calumet towards Allouez for about 15.5 miles to Bumbletown Road. Turn left (west) and proceed one mile up to the top of Bumbletown Hill via Cedar Street. Walk around outside of communication tower fence to get excellent views as described below. Figure 15: Geologic sketch map of Bumbletown Hill modified from White (1971). From the overlook on a clear day, Isle Royale may be seen 80 km to the northwest and the Huron Mountains may be seen beyond Keweenaw Bay, 60 km to the southeast. From the top of Bumbletown Hill the land slopes very gradually to the northwest toward Lake Superior. This slope is similar throughout much of the northwestern side of the Keweenaw Peninsula. The area is underlain mainly by conglomerates and sandstones of the Copper Harbor Conglomerate dipping at about 20 to 30 degrees NW. The southeastern flank of the Keweenaw Peninsula has a steeper slope at the skyline, following approximately the line of the Keweenaw fault. The low-lying plain between the fault and Keweenaw Bay is underlain by flat-lying Jacobsville Sandstone. Next to the Keweenaw fault beds of the Jacobsville Sandstone can be steeply dipping. Looking northeast along the strike of the Portage Lake Volcanics, one can see the cuesta form of the ridge underlain by the Greenstone flow. At Bumbletown Hill, the Greenstone flow is only 75 m thick (Figure 15), but the flow thickens abruptly to more than 400 m near this end of the cuesta ridge. The Greenstone flow dips northward at about 25o toward the center of the Lake Superior. It can be traced along much of the Keweenaw Peninsula (Figure 6) and has been stratigraphically and 27 �geochemically correlated with a similar unit on Isle Royale, 90 km away, on the opposite side of the rift. Thus, the areal extent of this great flow exceeds 5,000 km2, and its volume is on the order of 800 to 1,500 km3 (Longo, 1983; White, 1960). The Greenstone Flow is an enormous lava flow. It is possible that rather than having been a lava flow the Greenstone Flow was a lava lake. Regardless, the Greenstone Flow perhaps is the greatest single continuous outpouring of lava on Earth. Very slow solidification of this great mass of magma allowed extensive in-situ magmatic differentiation (Cornwall (1951a, 1951b). Magmatic differentiation resulted in a massive, ophitic (lath-shaped plagioclase surrounded by large irregular masses of clinopyroxene) base of the flow; an overlying zone of intercalated subophitic and pegmatoidal layers; an upper ophitic zone; and a finegrained, vesicular flow top. The lower ophitic zone experienced rates of undercooling low enough to allow growth of clinopyroxene oikocrysts up to 5 cm in diameter. The geochemical composition of the Greenstone Flow magma is more evolved than typical olivine tholeiites; which constitute the greatest volume of the Portage Lake Volcanics. Primitive olivine tholeiite and quartz tholeiite occur between the Greenstone Flow and the top of the Portage Lake Volcanics. Generally, magmas of the Portage Lake Volcanics become more primitive and less crustal contamination with time during the development of the Midcontinent Rift (Paces, 1988). At Bumbletown Hill, the Greenstone Flow is only 75 m thick and is composed of a thick amygdaloidal flow top with some exposures of fine-grained columnar basalt. To the left of the cuesta ridge the rocks consist of the top of the Portage Lake Volcanics and the bottom of the Copper Harbor Conglomerate. To the right of the ridge, the more distant hills are formed by lava flows near the base of the Portage Lake Volcanics. Bumbletown Hill is located on the southwest side of Allouez Gap, a NW- to SE-trending valley (see Figure 7). The valley follows the Allouez Gap fault, a zone of faults and fractures, along which the Portage Lake Volcanics and Keweenaw fault, are offset. At this gap, the strike of the Portage Lake Volcanics swings from about N35oE to N50oE (Figure 7). Almost every permeable horizon near the Allouez Gap fault contains above average amounts of native copper; nowhere else in the district are there so many mineralized beds (Figure 7). About 60% of the district production can be linked to the fault as a primary pathway for ore fluids. The fault bisects the Kearsarge deposit (see Figure 16), which was the second largest copper producer in the native copper district. There was a readily visible line of poor rock piles, a little more than 1,500 m southeast of Bumbletown Hill, from the many mines which were producing native copper from the Kearsarge deposit. Many of these piles are now gone as they have been crushed for aggregate. The line is still visible in the fall when leaves are not on the trees. About 1,200 m N65oE of the hilltop, the Houghton conglomerate and the stratigraphically lower Iroquois flow produced 33 million pounds of copper. East of Bumbletown Hill but no longer visible is the Kingston Mine, one of the most recent native copper mines to open and last to close. It was discovered by exploration near the Allouez Gap fault. It only produced 20 million pounds of copper from 1963 to 1968. 28 �Stop 4: Interflow Conglomerate at Bumbletown Hill Latitude: 47.287136N; Longitude: -88.415365W Directions: From top of Bumbletown Hill turn around and head downhill 0.5 miles to pull over on left just before dirt road. Specimens of Allouez Conglomerate are scattered about the southeast flank of base of Bumbletown Hill. The remnants of Allouez Conglomerate poor rock piles are private property (Figure 15). Near this pullover there is the opportunity to collect specimens of the Allouez Conglomerate. The Allouez Conglomerate is one of a small number of interflow clastic sedimentary horizons within the Portage Lake Volcanics and is visible on the lower slopes southeast of Bumbletown Hill. Conglomerate layers within the Portage Lake Volcanics are important stratigraphic marker horizons (Figure 6). Correlation of basaltic lava flows along strike would be difficult without clastic sedimentary marker beds deposited during periodic waning of volcanism. The Allouez Conglomerate can be traced more than 120 km along strike from Mass to Delaware (Figure 6). The Allouez conglomerate is just below the Greenstone flow. At Bumbletown Hill the Allouez Conglomerate was mined for native copper (Figure 15) albeit it only yielded about 75 million lbs of refined copper (Table 2). Elsewhere the Allouez Conglomerate has yielded additional native copper. The Allouez Conglomerate consists of mostly red-colored conglomerate with lesser amounts of sandstone and siltstone. The largest clasts at this locality are about 65 cm in diameter and the median size is about 8 cm. A pebble count of boulders more than 20 cm across by White (1971) gave the following results: 16% basalt, mostly amygdaloidal; 36% quartz porphyritic rhyolite; 11%, feldspar porphyritic rhyolite; and 37% felsic granophyre. The Houghton Conglomerate is almost entirely clasts of quartz porphyry as is the Kingston Conglomerate. There are conglomerates within the lower Portage Lake Volcanics near the tip of the Keweenaw Peninsula whose clasts are clearly sourced from an extrusive dome of rhyolite. This could be the explanation for the uniformity of clasts in the Houghton and Kingston Conglomerates. In contrast, the heterogeneity of the Allouez Conglomerate clasts suggests a less restricted source area (White, 1971). Evidence of native copper mineralization can be seen in some rocks of the Allouez Conglomerate at nearby this stop. Occasionally, one can find a specimen with native copper filling the void space between clasts and grains. Calcite and chlorite are the dominant pore-filling secondary minerals visible on this rock pile. Thin black veinlets cutting the Allouez conglomerate consist of calcite with chalcocite “dust.” Chalcocite is a product of late- stage hydrothermal fluids. Supergene alteration resulting from the downward percolation of groundwater is not common at depth in most the native copper deposits. Supergene alteration products are likely common when native copper ore bodies are at or very near the surface. At this stop, supergene alteration minerals are common including chrysocolla, malachite, and cuprite. The depth of occurrence is unknown although given the widespread distribution in rocks of the poor rock piles it seems likely at least some supergene alteration occurred at depth as documented elsewhere. The occurrence of supergene alteration minerals at depth in the native copper mines was used by Bornhorst and Robinson (2004) to hypothesize that at least some supergene alteration was Precambrian in age. 29 �Stop 5: Seneca Mine Rock Pile Latitude: 47.311915N; Longitude: -88.365818W Directions: At the intersection of Bumbletown Road and US-41, turn left (northeast). Drive about 2.9 miles almost through Mohawk to 1st Street when you will turn left (northwest). Proceed on 1st Street/Seneca Location Road 0.3 miles to gated road. Walk to rock piles. THIS STOP IS ON PRIVATE PROPERTY. PLEASE GET PERMISSION TO ENTER PROPERTY. The Kearsarge lode was worked by the Seneca Mine, one of multiple mines which produced native copper from the top of the Kearsarge lava flow over a strike length of more than 12 km and downdip as much as 2,500 m (Figure 16). About 1,026 million kg of refined copper were produced at an average grade of 1.05% Cu, making the Kearsarge deposit the largest flow top hosted deposit and the second largest producer in the district behind the Calumet & Hecla Conglomerate mines (Table 2). Production of copper from the Kearsarge lode began in 1887 and stopped in 1967. The Kearsarge lava flow has been recognized for a distance of about 55 km along strike and dips between 35 and 40o NW (Figures 6 and 16). It lies directly above the Wolverine Sandstone (Figure 6). The amygdaloidal and/or brecciated top of the Kearsarge flow ranges from near zero up to 10 m in thickness. The productive top has an average thickness of around 2 m and consists of brecciated basalt (individual fragments of amygdaloidal basalt are generally less than 15 cm in greatest dimension). The brecciated basalt grades downward into amygdaloidal basalt with amygdules concentrated in layers. Further downward, the top grades into a zone of fewer and larger amygdules, and then into massive basalt in the interior of the flow. Just below the brecciated and/or amygdaloidal top of the flow, there is distinct plagioclase porphyritic basalt. The abundance and size of the plagioclase phenocrysts in this zone are variable, but they can make up a large percentage and phenocrysts are up to 2.5 cm in length. This zone is probably the result in situ floating of plagioclase during surface crystallization of the flow. The phenocrysts likely formed in a shallow magma chamber. Specimens with abundant plagioclase phenocrysts are common on this rock pile. The basalt of the Kearsarge flow is well oxidized. Albitized and pumpellyitized basalt consists of pseudomorphically replaced plagioclase set in a fine-grained to cryptocrystalline groundmass. Original igneous minerals were replaced in areas where alteration was intense. Olivine is almost invariably completely replaced while other igneous minerals are replaced by alteration minerals to varying degrees. The amygdule and interfragment spaces are filled with (in order of most to least abundant): calcite, epidote, K-feldspar, quartz, and lesser amounts of chlorite, prehnite, pumpellyite, laumontite, and sericite. Native copper is closely associated in time and space with the secondary amygdule minerals (Stoiber and Davidson, 1959). Paragenetically, chlorite; epidote; microcline; and prehnite are early-formed minerals, and the latest-formed minerals are quartz; native copper; calcite; and chlorite (Figure 17). A zonal stratabound arrangement of amygdule minerals in the Kearsarge deposit is seen in the Ahmeek Shaft No. 3 (Figure 18). The zoning may be explained by deposition of secondary minerals from a hydrothermal solution moving along a permeable channel. Chlorite and microcline would have been deposited first, along the outer limits of the solution channel; 30 �followed by quartz and epidote in the center of the channel; and finally, deposition of calcite in the remaining openings. This is consistent with the paragenetic relationships seen in individual samples from the rock pile. No strict correlation exists between the stratabound zoning and the grade of native-copper mineralization (Stoiber and Davidson, 1959). The amygdule minerals and grade of copper mineralization vary with depth. Within the upper limit of quartz (Figure 16), the quartz content is typically about 15 % of open space fillings although it is considerably less than 10% at shallower depths. The lower limit of microcline may also mark the limit of significant copper mineralization. The amount of native copper present is much more irregular than variation of the mineral zones. Figure 16: Thickness of the Kearsarge lava flow showing the productive area to be the thickest in the top diagram modified from Butler and Burbank (1929). The Kearsarge flow top ore body is bisected by the Allouez Gap fault. Bottom diagram shows strike parallel down-dip projection to vertical showing distribution of higher-grade native copper ore and occurrence of important alteration minerals modified from Stoiber and Davidson (1959). Abundance of quartz in amygdules is greater than 10 % on the downdip side of the line (lower) and K-feldspar is absent on the down-dip side lower line shown. The Kearsarge flow dips about 35 to 40o NW. Mine names and shaft numbers are noted. 31 �Figure 17: Paragenesis of secondary hydrothermal alteration minerals in the Kearsarge deposit at the Wolverine No. 2 Mine. Figure 18: Cross section of the top of the Kearsarge lava flow (amygdaloid) deposit showing the distribution of secondary hydrothermal amygdule-filling alteration minerals at the Ahmeek Mine, 35th level, 400 to 500 ft south of the shaft. Modified from Stoiber and Davidson (1959). Data from the back and walls are projected to a horizontal plane. There is a barren laumontite-quartz-calcite zone not shown here. 32 �The Allouez Gap Fault bisects the thickest segment of the Kearsarge Flow along its 55 km strike length (Figure 16). Higher grades and production occur northeast of the fault where fractures with orientations that parallel the fault are more abundant. Within the Allouez Gap Fault zone, early epidote and quartz were brecciated and recemented by calcite, quartz, and native copper. After another episode of brecciation, the fault zone was recemented again with calcite; quartz; and lesser laumontite (Butler and Burbank, 1929). The latter may be late-stage. Movement along the fault occurred before, during, and after deposition of native copper. The fault apparently was a conduit for transport of ore fluids to the permeable flow top. The coincidence of this fault with the relatively thick flow top resulted in the second largest deposit in the district. The Seneca Mine is an excellent locality to study the character of a representative basaltic flow top hosted native copper deposit. Specimens of massive basalt, massive basalt with abundant plagioclase phenocrysts, and amygdaloidal basalt can be found on this rock pile. Masses of native copper are readily collectable especially when using a metal detector. Open-space filling minerals (amydgules and between breccia fragments) that occur in the lode can be found on the rock pile. Stoiber and Davidson (unpublished data) made a quantitative analysis of open-space filling minerals for the Seneca Mine rock pile and found open-space filling minerals consisted of: calcite, 57%; red feldspar 8% (microcline); pink feldspar (adularia) 15%; epidote, 17%; prehnite, trace; pumpellyite, trace and quartz, trace. Many specimens contain multiple minerals and illustrate paragenetic relationships. Stop 6: Eagle River Falls Latitude: 47°24'44.9N; Longitude: - 88°17'47.3W Directions: Return to US-41 from Seneca Mine and turn left (northeast) continuing 7.2 miles to junction of US-41 and M-26. Turn left (north) on M-26 towards Eagle River and proceed 2.3 miles across bridge and immediately right after the bridge into the parking lot. The waterfalls of Eagle River are near the contact between the top of the Portage Lake Volcanics and the base of the Copper Harbor Conglomerate (Figure 5). The contact dips about 30o NNW. The beds strike roughly parallel to the shoreline of Lake Superior; the orientation of the Keweenaw Peninsula changes from NE in vicinity of Houghton to ENE at Eagle River to E-W near the tip of the peninsula. The tholeiitic basalt subaerial lava flows just below the contact are pahoehoe type with a ropy upper surface. The orientation of the ropes indicates that the flow erupted from a vent to the north geographically under Lake Superior. That the ropy flow top is preserved suggests that little erosion occurred between deposition of the last of the lava flows of the Portage Lake Volcanics and the Copper Harbor Conglomerate. The Copper Harbor Conglomerate consists of red-brown rhyolite-pebble conglomerate but includes many sandstones and even some shale beds. Under the bridge, one can get a good view of the lithology of the lower part of the Copper Harbor Conglomerate. The environmental setting of the Copper Harbor Conglomerate is discussed further at Stop 8. This contact marks an abrupt change in the geologic evolution of the Midcontinent rift. Below this contact the dominant strata is a very thick succession of subaerial basalt lava flows erupted from fissure vents under Lake Superior that filled the progressively extended and down dropped rift basin 33 �during active Midcontinent rifting. Below this contact the Portage Lake Volcanics consists of more than 200 individual lava flows with a cumulative exposed thickness of about 5,000 m; the base is fault truncated (Figure 4 and 5). Abruptly above the contact lava flows are strikingly absent and clastic sedimentary rocks are the dominant strata deposited in a sagging basin after active extension ended. The clastic sedimentary strata above this contact consists mostly of conglomerate and sandstone with a cumulative exposed thickness of more than 5,700 m; the top is not exposed (Figure 4 and 5). While generally absent, a thin package of mafic to intermediate volcanic rocks (Lake Shore Traps) deposited as a shield volcano interfingers with clastic sedimentary rocks of the Copper Harbor Conglomerate. The very last magmatic activity in the MCR is the lone Bear Lake alkaline igneous body near the middle of the clastic sedimentary strata. Stop 7: Great Sand Bay Latitude: 47. 446140N; Longitude: -88. 216411W Directions: Continue driving northeast (right from parking area) on M-26 for 4.5 miles until the Great Sand Bay paved pullover with overview and beach access. The Great Sand Bay overlook provides a beautiful view of Lake Superior (Figure 19). Very large volumes of water filled the Lake Superior basin as a result of melting of the glaciers, turning it into a glacial lake. The levels of the glacial lakes depended on the position of the ice front, outlets, and crustal rebound. There are 15 lake stages recognized in the Lake Superior basin (Farrand 1960). As the lake levels receded to the current level of Lake Superior, more and more of the Keweenaw Peninsula emerged. At the road level, the sand dunes are remains of the Lake Nipissing Stage (4,000 to 5,000 years ago) when the lake level was about 9 m (30 feet) higher than today. After lake stages at about 3,200, 2,000, and 1,000 years ago, the waters receded toward the present level termed Lake Superior. The underlying bedrock is the Copper Harbor Conglomerate. In the Keweenaw Peninsula the Lake Shore Traps are interbedded near the middle of the Copper Harbor Conglomerate (see Stop 9). The massive interiors of these lava flows are more resistant to erosion than the underlying and overlying conglomerates and sandstones of the Copper Harbor Conglomerate. As a result, harbors such as those at Eagle Harbor and Copper Harbor are maintained by lava flows visible at their mouths. While not visible, lava flows occur at the mouth of Great Sand Bay too. There are many extensive underwater fissure vein deposits which crosscut the Eagle River shoals located about 0.5 to 1 km offshore. To date, there have been a total of 36 underwater copper veins discovered from the eastern tip of Great Sand Bay (visible at this stop) to Eagle River, about 3.2 km west. Some of these submerged veins likely connect with veins recognized from on land exposures (Figure 19). The native copper that is naturally on the bottom lands of Lake Superior are grouped as “lake copper.” The largest lake copper specimen ever recovered underwater was a massive 19-ton unattached copper slab in July of 2001. It was recovered from one of these vein deposits north of Jacobs Creek in about 9 m of water. This large underwater native copper vein is on display at the A.E. Seaman Mineral Museum in the outside copper pavilion. 34 �Many of the submerged veins are often quite rich in native copper and can contain long continuous stringers protruding up to 1.5 m in height and extending almost 6 meters in length. Most of the veins are less than 50 cm in width and are primarily composed of quartz or calcite with minor amounts of laumontite, datolite, prehnite, and traces of silver. Veins will locally contain clay pockets associated with well-defined copper crystal specimens. Bornhorst and Barron (2017) provide additional information about the Guiness World record tabular 19-ton native copper mass. Figure 19. Geologic and location map of the 19-ton submerged native copper vein recovered from Great Sand Bay, Keweenaw Peninsula, Michigan (from Bornhorst and Barron, 2017). 35 �Stop 8: Copper Harbor Conglomerate, J. & M. Lizzadro Lakeshore Preserve By Daniel J. Lizzadro-McPherson Latitude: 47. 479128N; Longitude: -87.979576W Directions: Continue east-northeast on M-26 from Great Sand Bay to Eagle Harbor. Continue on M-26 towards Copper Harbor for 9.2 miles (14.8km) until the sign for the preserve appears on the lakeward side of the road. Park at the pull-off on the north-side of M-26 for beach access. PLEASE DO NOT DAMAGE THE OUTCROP. NO HAMMERS ALLOWED. COLLECTION OF BEACH ROCK ONLY. Figure 20: Overview of the J. & M. Lizzadro Lakeshore Preserve, which encompasses the entire shoreline between private properties (shaded, cross-hatched areas) along M26. The Joseph & Mary Lizzadro Lakeshore Preserve is located near the northern-most point on the Keweenaw Peninsula and is host to prominent outcrops of Copper Harbor Conglomerate, colorful cobble-pebble beaches, and iconic views of Lake Superior. This site is notable due to the diverse facies of CHC, rare occurrences of stromatolites (genus Colleria) and raindrop impressions found among the wave-washed bedrock exposures. Established as a preserve in 2003 by the Houghton Keweenaw Conservation District and the Keweenaw Land Trust, the Lizzadro Lakeshore Preserve protects over 640-feet of undeveloped shoreline and several small islands (Figure 20). The preserve showcases one of only three A-ranked Michigan occurrences of the globally rare imperilied plant community-type (G2) bedrock beaches. Thanks to a generous donation by Gina Nicholas, the preserve was named in honor of my grandparents, Joseph & Mary Lizzadro, with the hope that this significant Geoheritage site remains protected for many future generations to enjoy. 36 �The bedrock geology of the preserve consists of red-colored clastic sedimentary rocks grouped under the stratigraphic formation of the Copper Harbor Conglomerate (Figure 5). The Copper Harbor Conglomerate. ranges from 490 m thick near the Wisconsin border, to about 1,310 m thick on the Keweenaw Peninsula, reaching a maximum exposed thickness of 2,000 m on the shores of Isle Royale. Overall, the Copper Harbor Conglomerate is generally a medium reddishbrown colored wedge of fluvial siliciclastic conglomerates and sandstones that rapidly filled-in the rift basin as volcanism waned and subsequently terminated (Daniels, 1982; Cannon and Nicholson, 2000; Woodruff et al., 2020). The base of the Copper Harbor Conglomerate. locally interfingers with the uppermost subaerial basaltic lava flows of the Portage Lake Volcanics (Figure 5). Near the stratigraphic lower-third of the Copper Harbor Conglomerate there is a succession of basaltic to intermediate subaerial lava flows, informally named the Lakeshore Traps (Figure 5). Outcrops of Copper Harbor Conglomerate. exposed along the Lake Superior shoreline at the Lizzadro Lakeshore Preserve (Figure 21) are stratigraphically above Lakeshore Traps (Cornwall, 1954). Figure 11: Overview of geologic features present at the J. & M. Lizzadro Lakeshore Preserve. 37 �Nearby at well-studied Dan’s Point, outcrops of Copper Harbor Conglomerate exhibit lithologic facies that are characteristic of the upper two-thirds of the formation. The conglomeratic facies (Figures 21 and 22: Points A, C, and F) are primarily clast-supported and comprised of rounded to well-rounded poorly sorted clasts, consisting of a 2:1 silicic-to-mafic volcanic rock fragment ratio with minor pyroclastic, plutonic, and metamorphic rocks (Elmore, 1984). The conglomerate matrix is comprised of coarse sand-sized subangular grains cemented with carbonate and iron oxides. The sandstone facies (Figures 21 and 22: Points B and E) are predominantly subangular to angular, lithic graywackes which exhibit a number of sedimentary structures including: current-ripples, cross beds, and parting lineations. Many of the coarser sandstones and conglomerates in the upper section have calcite-rich cement in the matrix, consistent with a vadose or semiarid, caliche-style environment (Kalliokoski, 1986). Two noteworthy outcrop features include: 1) a thin continuous zone of laminated cryptoalgal carbonate where laterallylinked stromatolite are draped over cobbles and over contorted sandy siltstone layers (Figures 21 and 22: Point D); and 2) raindrop imprints preserved in fine- to medium-grained sandstone lenses (Figure 21: Point G). At the time of deposition, the region was nearly equatorial in geographic position and the climate was likely arid with seasonal rainfall patterns conducive to flashfloods and the development of vadose carbonate (Elmore and Vander Voo, 1982; Kalliokoski, 1986). The raindrop imprints in sandstone (Figures 21 and 22: Point G) support this paleoclimate assumption. Details surrounding the origin and depositional setting of the Copper Harbor Conglomerate and overlying Nonesuch Formation (Figure 5) have recently been reevaluated. The long-held inference of the origin for these two formations has been interpreted as a non-marine, fluvial-tolacustrine couplet (White and Wright, 1960; Elmore, 1983, 1984; Ojakangas et al., 2001). The Copper Harbor Conglomerate closely resembles modern-day examples of coalescing fluvial and prograding alluvial fan deposits with varying facies that exhibit proximal-to-distal braided streams, sheet flooding and sand flat features (Elmore, 1984). Isolated cryptoalgal carbonate and ooid lenses are interpreted by Elmore (1983) to have formed in shallow, medial fan lakes and possibly in abandoned or low-water stream channels with limited sediment input (Elmore, 1983). However, marine sedimentological features often resemble and can be easily mistaken for nonmarine features unless contextualized with additional evidence, such as isotopic geochemistry. New research presents evidence for a shallow-marine estuarine origin for at least the upper-third of Copper Harbor Conglomerate and overlying Nonesuch Formation based on new sedimentological observations (Jones et al., 2023) and geochemical analyses (Stüeken et al., 2020; Jones et al., 2020). Sedimentological observations include periodic to rhythmic flaserwavy-linsen-pinstripe bedding, superimposed sets of ripple cross-laminations with bimodal (herring-bone) sediment transport directions, desiccation cracks and hummocky crossstratification (Jones et al., 2023). Periodic to rhythmic textures are indicative of tidal-influenced marine depositional settings. Geochemical evidence indicates that gypsum evaporite fabrics have a marine sulfur isotopic composition (Stueken et al., 2020) and that pseudomorphs after gypsum formed in a saline-to-brackish waterbody (Jones et al., 2020). Both studies conclude that the upper section of the Copper Harbor Conglomerate and overlying Nonesuch formation were deposited in a braided fluvial-evaporitic, sabkha-like, tidally-influenced shallow marine embayment rather than fluvial-lacustrine non-marine depositional setting. 38 �Figure 22: Lithologic column of measured section at the Lizzadro Lakeshore Preserve (data collected by Lizzadro-McPherson, Bornhorst, and Vye (2023). Bedding about E-W strike and 35 to 40o N dip. 39 �Stop 9: Copper Harbor Conglomerate and interbedded Lakeshore Traps at Hunter’s Point Park Latitude: 47.474355N; Longitude: -87.899199W Directions: Continue driving east on M-26 for 3.7 miles to North Coast Road. Turn left (northwest) on North Coast Road and proceed 0.3 miles to Harbor Coast Lane. Turn right and drive 0.3 miles to the parking area for Hunter’s Point Park at end of the road. Figure 23: Geologic map of the Copper Harbor area taken directly from Cornwall (1955) showing the location of Hunter’s Point (Stop 9) and Brockway Nose (part of Stop 10). Geology from Cornwall (1954). Hunter’s Point Park was established in 2005 when funding provided by the Michigan Natural Resources Trust Fund and many generous private donors (www.hunters-point.org) allowed the land to be purchased (Figure 23). Prior to becoming an official park the point was a popular hiking destination for visitors. The landowners subdivided the area for residential housing which would have restricted public access without its conversion into a park. The origin of the name Hunter’s Point is uncertain, but it could have been named after A.W. Hunter, an early resident in the town of Copper Harbor who purchased the point from the U.S. Government. The Copper Harbor Conglomerate is overall composed of volcanogenic clastic sedimentary rocks, dominantly conglomerates with lesser sandstone, siltstone, and shale such as observed at Stop 8. These rocks were deposited in a fining upward prograding alluvial fan complex (Elmore, 1984). Typically conglomerates are composed of clasts with a ratio of mafic-tointermediate+felsic composition of about 2:1 (Daniels, 1982). Towards the tip of the Keweenaw Peninsula, the Copper Harbor Conglomerate is informally subdivided into an inner (land side) “member” and an outer (lake side) “member.” Between these two “members” there is a thin 40 �succession of interbedded lava flows collectively known as the Lake Shore Traps (Figure 23). The Lake Shore Traps consist of Fe-rich olivine tholeiite, basaltic andesite, and andesite lava that were erupted during a time of waning volcanism within the MCR at 1087.2 +/- 1.6 Ma (Davis and Paces, 1990). The thickest section of the Lake Shore Traps is about 15 km to the east at the tip of the peninsula. Volcanologically, the lower lava flows are interpreted as erupted as ponded sheets while the upper lava flows erupted on a low positive slope such as a shield volcano. The Lake Shore Traps were subaerially erupted pahoehoe lava flows. At Hunter’s Point, the top of the andesitic lava flows of Lake Shore Traps are conformably overlain by conglomerates of the Copper Harbor Conglomerate (Figure 23). The strike of bedding is about E-W and dip is about 36o to the north (towards the lake). The orientation of the contact is roughly parallel to the orientation of Hunter’s Point. From the Hunter’s Point parking lot, follow the walkway to beach towards the west side of the point. As the walkway ends, you will be on outcrops of lava flows of the Lake Shore Traps. Walking to the east, the beach gives way to a rocky shoreline. In erosional coves, you can see contacts between lava flows, represented by vesicular to amygdalodoidal andesitic lava flow top overlain by massive interior of the overlying lava flow. The massive lava flow interiors within the Lake Shore Traps often retain relict olivine and interstitial glass due to the overall low degree of alteration (hydrothermal and weathering). In contrast, in massive lava flow interiors within the Portage Lake Volcanics the olivine and interstitial glass are completely replaced by Mg-Fe phyllosilicates and amygdule filling minerals are equivalent to higher degree of metamorphism. Secondary minerals filling amygdules include agate, chalcedony, quartz, laumontite, analcite, calcite, and smectite. The Lake Shore Traps are geographically more distal to the thermal high and increased hydrothermal activity that resulted in the native copper deposits, hence, lower degree and grade of burial metamorphic/hydrothermal alteration. Highly visible red hematitic bands form circular patterns within the massive interior; this banding is interpreted to be the result of alteration related to weathering rather than hydrothermal fluids. To the west from the walkway, you can see a rocky point extending towards Lake Superior, the rocks in this point are conglomerates of the Copper Harbor Conglomerate. The sharp contact between the uppermost lava flow of the Lake Shore Traps and the conglomerates can be viewed on the eastern edge of this rocky point. The conglomerate above the contact is dominated by rounded to sub-rounded boulders that are matrix-supported. There are proportionately more basaltic and andesitic clasts in this conglomerate bed than stratigraphically higher elsewhere along the Lake Superior shoreline such as at Stop 8 as these clasts are likely derived from erosion of strat equivalent to the Lake Shore Traps updip towards the highlands of the Keweenaw Peninsula on the edge of the rift (the updip rocks are now missing having been removed by erosion). The very poor sorting and fine matrix-supporting the clasts suggest this conglomerate could have been deposited as a debris flow. Sedimentary debris flows are common in alluvial fan depositional environments. The Copper Harbor Conglomerate was deposited in an alluvial fan derived from highlands to the southeast in the vicinity of Keweenaw Bay. 41 �Additional outcrops of the Copper Harbor Conglomerate can be seen on the far western end of the pebble to cobble beach. These outcrops consist of interbedded conglomerates and sandstone that are typical of the formation as a whole. These conglomerates are similar to those described at Stop 8. There are several prominent, white-colored calcite-filled fractures (calcite veins) within these outcrops. The calcite veins are northerly oriented consistent with the orientation of faults cutting the Portage Lake Volcanics about 5 km to the south. Calcite veins are a common occurrence in the Copper Harbor Conglomerate and some of them contain native copper such as those described at Stop 7, Great Sand Bay. Stop 10: Overview at Brockway Nose Latitude: 47.467061N; Longitude: -87.898581W Directions: Return from Hunter’s Point to M-26. Turn left on M-26 towards Copper Harbor and drive 0.3 miles (0.5km) to Brockway Mountain Drive. Turn right, uphill, on Brockway and proceed 0.6 miles (1.0km) to Brockway Nose pullover on the left. Brockway Nose provides an excellent view of Copper Harbor and Lake Fanny Hooe (Figure 23). Copper Harbor and several other harbors between here and Eagle River have the Lake Shore Traps at the harbor entrance as the dipping massive interiors of these basaltic to andesitic lava flows are relatively more resistant to erosion. From Brockway Nose viewpoint, the town of Copper Harbor is a prominent visible feature. The town of Copper Harbor began as a boom town in 1843, following the nearby discovery of native copper. Porter's Island, at the mouth of Copper Harbor on the west side of the harbor's Lake Superior entrance (left) was the site of the first government land office. Hunter's Point is west of Porter's Island (Figure 23). On the east side of the mouth of Copper Harbor, the Copper Harbor Lighthouse, built in 1866, is visible. Near the lighthouse on the Lake Superior shoreline is the famous "green rock". The "green rock" is a vein that was described by Douglass Houghton. The vein contained native copper and secondary copper alteration minerals. This location and others in the Keweenaw Peninsula became the foundation of the geological investigations of Douglass Houghton. Houghton's report to the Michigan legislature that sparked the first major mining rush in North America to the Keweenaw Peninsula. Lake Fanny Hooe is located southeast of Copper Harbor. Fort Wilkins is located on the north shore of Lake Fanny Hooe on the thin strip of land between the lake and harbor. Nearby, the Estivant Pines is a 0.8 mi2 nature sanctuary established in 1973, containing one the last stands of virgin white pines in the Midwest and the last stand in the Upper Peninsula. Some of the trees are up to 600 years old (www.michigannature.org). In 1955, the white pine was designated the state tree of Michigan. 42 �Stop 11: Overview at Brockway Mountain Latitude: 47.464260N; Longitude: -87.969506W Directions: Continue uphill and towards Brockway Mountain for 3.4 miles (5.5km). The top of Brockway Mountain is accessed by continuing upwards from Brockway Nose. Brockway Mountain is a conglomerate ridge that reaches an elevation of over 400 m, with excellent views of the ridge and valley topography of the northern shore of the Keweenaw Peninsula. At Brockway Mountain, the Lake Superior shoreline is oriented about east-west From the Brockway Mountain viewpoint there are an excellent 360o views. Underfoot, the Copper Harbor Conglomerate dips about 20o to the north. Near the base of the ridge on the south side, opposite Lake Superior, there is an exposure of a single basaltic lava flow erupted as part of the Lake Shore Traps. With care, at the southwest end of the rock wall, one can view the dipping conglomerates of the Copper Harbor Conglomerates and see the lava flow near the base of the ridge. To the west, the Lake Shore Traps form island chains and a prominent ridge in the vicinity of Agate Harbor and Esrey Park. The ridges of the Lake Shore Traps and Copper Harbor Conglomerate along the Keweenaw Peninsula’s north shore are also the site of numerous shipwrecks. Lake Bailey (with the small island) and Lake Upson occupy a topographically low valley underlain by a finer-grained clastic horizon (sandstone and siltstone) within the Copper Harbor Conglomerate which was easier to erode by the glaciers than conglomerates. Just to the south of Lake Bailey, is the ridge of Mt. Lookout, marking the contact between the basal conglomerates of the Copper Harbor Conglomerate and the uppermost basalt lava flows of the Portage Lake Volcanics. This contact was viewed at Stop 6. The inland lake almost directly south, is Lake Medora, and just before the lake is a prominent ridge which marks the stratigraphic position of the Greenstone flow as also seen at Stop 3. In the distance, farther to the south across Lake Medora, is Mount Bohemia, a dioritic stock-sized intrusion within the lower section of the Portage Lake Volcanics. To the southwest, a distant ridge is Gratiot Mountain, which is a small shallow rhyolite intrusive body that cuts the Portage Lake Volcanics. To the east are the communities of Copper Harbor and Lake Fanny Hooe not easily viewed from Brockway Mountain (better viewed from Brockway Nose). Just south of Copper Harbor is a golf course that is part of Brockway Mountain lodge. Brockway Mountain lodge was built during the Great Depression in the 1930’s by the WPA. To the north, Lake Superior is the prominent feature. On the skyline roughly 50 miles (80km) away, is Isle Royale National Park, which can be visible on a clear day. The skyline of Isle Royale is formed by the Greenstone Flow, as it is on the Peninsula. The beds on Isle Royale dip towards the Keweenaw Peninsula forming the Lake Superior “syncline.” Viewed from here, the Midcontinent 43 �Rift proper extends from the Keweenaw Fault, near the edge of the rift, just south of Mt. Bohemia to the Isle Royale Fault, also originally a graben bounding fault on the edge of the rift on the other side of Lake Superior, just northwest of Isle Royale. Glacial erosion exposed Keweenawan and pre-Keweenawan relatively hard and competent bedrock on the edges of the MCR. Dipping well-cemented conglomerates of the Copper Harbor Formation are exposed at Brockway Mountain and basaltic lava flows of the Portage Lake Volcanics are exposed when viewing south. Both are relatively resistant to glacial erosion. On Isle Royale, on the southeast (Keweenaw side) are exposed the same conglomerates of the Copper Harbor Formation and on the northwest side, there are exposed basaltic lava flows of the Portage Lake Volcanics. In the center of what is now Lake Superior, much less competent, nearly flat lying, very fine sandstone and siltstone of the Freda Formation was at the bedrock surface. The latest glacial advance(s) preferentially eroded out the less competent rocks in the center of the rift, resulting in present day Lake Superior following the horseshoe shape of the 1.1 billion year old MCR. Very large volumes of water filled the Lake Superior basin as a result of melting of the glaciers, turning it into a glacial lake. The Duluth Glacial Lake was the largest of these glacial lakes and only elevations above roughly 400 m (1,300 ft) were emergent such as here at Brockway Mountain and the visible Mt. Bohemia. Stop 12: Float Copper at US-41 Calumet Latitude: 47.241989N; Longitude: -88.448427W Directions: Follow US-41 from Copper Harbor to Calumet. Across the street from the headquarters of the Keweenaw National Historical Park. Google Maps shows the float copper as “Float copper memorial.” Figure 24: Float copper exhibit along US-41 near headquarters of the Keweenaw National Historical Park. 44 �A glacially transported native copper mass is on exhibit at this stop (Figure 24). It weighs 4,263 kg (9,392 lbs) and was found about 7 miles SW of Calumet in less than three feet of surficial sediment/soil. Native copper deposits of the Keweenaw Peninsula were exposed at the bedrock surface at the time of Pleistocene glaciations. The glacial ice entrained masses laying at the surface from previous erosion and plucked masses of malleable native copper from the tabular lodes and veins/fissures. These originally irregular masses were largely cleaned of other minerals and were smoothed and flattened by abrasion from other rocks carried by the glacial ice. The native copper masses were "floating" in the glacial ice, hence locally called “float” copper. When the glaciers retreated about 10,000 years ago, unconsolidated rock debris was left behind by the melting ice as deposits of gravel, sand, and clay. Masses of native copper were scattered among the other sediments carried by the glacier. While some of the rocks in the glacial deposits are from far north of the Keweenaw Peninsula, most of them are recognizable as from local strata exposed in the Keweenaw Peninsula. Most of the large float copper masses did not move far from their bedrock source in the Keweenaw Peninsula, but smaller masses have been transported quite far and have been found southward in Lower Michigan, Indiana, and Illinois (Bornhorst, 2017). The largest known float copper was discovered in the early 2000s and weighed about 35 tons (~70,000 lbs) near the Houghton County airport; it was cut into smaller masses and sold to be smelted and refined. Most pieces of float copper are small, ranging from a few to 50 cm across. The world’s largest existing float copper weighs 26.6 tons (53,200 lbs) was discovered on Quincy Mine claims near Hancock. It is exhibited at a museum in China. The famous Ontonagon boulder was a 1,700 kg (3,708 lbs) float copper mass much smaller than the float copper on exhibit at this stop. The Ontonagon boulder was visited by numerous early European explorers. After Michigan became a territory, Henry Rowe Schoolcraft led an expedition in 1820 with a special goal of seeing the Ontonagon boulder. In 1831, Douglass Houghton accompanied Schoolcraft and visited the boulder too. Pieces of native copper were hacked off of the boulder by Houghton and one of these pieces is part of the University of Michigan mineral collection held by the A. E. Seaman Mineral Museum under the Michigan Mineral Alliance. The Ontonagon boulder was removed from the Keweenaw Peninsula to the nation’s capital in 1843 and is now part of the National Museum of Natural History, Smithsonian Institution’s collection. Float copper masses were altered by oxygenated groundwater and precipitation since the glaciers retreated. Many masses likely had smoothed fresh copper surfaces as abrasion in the glacial ice cleaned off and smoothed the surfaces. The alteration of these surfaces would have occurred in the last 10,000 years. This supergene alteration produced a surface coating on the native copper consisting of varying amounts of cuprite (copper oxide; Cu2O), tenorite (copper oxide; CuO), malachite (hydrated copper carbonate; (Cu2(CO3)(OH)2) and rarely azurite (hydrated copper carbonate, (Cu3(CO3)2(OH)2) (Figure 24). When small 10s of cm sized masses of float copper are cut, the typical surface alteration is often less than several mm thick. While native copper is not stable in contact with water at typical oxidizing surface conditions, the coating of copper oxides, in particular cuprite, inhibits surface oxidation and thereby protects the native copper from extensive alteration. An open access article provides more about float copper (Bornhorst, 2017). The basalt mine rock buildings are part of the Keweenaw National Historical Park. They were once all part of the Calumet and Hecla Mining Company (Bornhorst and Molloy, 2017). The Calumet and Hecla Mining Company was incorporated in 1871 as a consolidation of the Calumet 45 �(formed in 1865), Hecla (formed in 1866), Portland, and Scott Mining Companies. The buildings are built, as are many of the buildings, of local materials, including rock from the Calumet & Hecla Mining Company mines. The national park was established on October 27, 1992, by U. S. Congress Public Law 102-543. The enabling legislation ascertained that the Keweenaw was nationally significant because of its unique geology, the prehistoric use of its copper by Native Americans, the importance of the region as a past leading copper producer and developer of new technologies, its long history of corporate paternalism, and because it became home to so many European ethnic groups that immigrated to the United States. Older mining districts, such as the Keweenaw Peninsula, typically had only single-industry economies and when the mines shut down, the communities suffered major contraction. In 1910, nearly 40,000 people resided within a few miles of this stop whereas now, fewer people live in all of Houghton County. Behind the float copper stands the statue of Alexander Agassiz. Alexander was the son of famous Harvard biologist Louis Agassiz. Alexander was the president of the Calumet & Hecla Mining Company for over 40 years. The statue was moved here from its previous location near Agassiz Park near downtown Calumet in the 1960s. It now stands in front of the Keweenaw History Center, the location of the archives of the Keweenaw National Historical Park. This building was the Calumet & Hecla Library. It is said that at one time this library had more volumes in its collection than the Michigan State Library. Built in 1898, it served as an employee library and bathhouse. The baths were in the basement, until a new bathhouse was constructed in 1911 allowing the basement to be remodeled into additional library space. Stop 13: Freda Sandstone at McLain State Park Latitude: 47.238371N; Longitude: -88.613116W Directions: Follow US-41 to M-203 to McLain State Park. There is an entrance fee. From the pay station turn right towards the campground. Park near the large open area and walk towards the covered shelter and gazebo. Walk down a sandy slope from the gazebo to the lake shore and look around for blocks and slabs of red-colored sandstone. The Freda Sandstone is generally poorly exposed in the Keweenaw Peninsula except for numerous cliff exposures along the shore of Lake Superior southwest of this stop. At this stop, depending on the level of Lake Superior, slabs and blocks of Fred Sandstone can be found along the beach. If the lake level is low enough, then at the shoreline the bedrock of the Freda Sandstone is partially exposed in shallow water and just off shore. The Freda consists of red-colored fine sandstone and siltstone. The red is interrupted by whiteish reduced zones and spots. There are occasional outcrops of Freda Sandstone landward of the Lake Superior shoreline in the area north and south of Portage Channel which are shown on U.S. Geological Survey geologic quadrangle maps and generally located along creeks (Cornwall and Wright, 1956). There are good exposures of Freda Sandstone on the Lake shore between McLain State Park and Porcupine Mountains State Park. The bedding of the Freda at McLain State Park dips about 5OW as compared to the underlying Nonesuch at Hancock Campground and boat launch (Stop 14) where it dips 25OW. This shallowing of dip up-section is typical of the rift-filling strata, and is mostly due to syn-depositional down warping of the rift-filling strata. The Freda Sandstone is generally fine sandstone which is interpreted to have been deposited in a shallow fluvial environment. 46 �There are multiple excellent well-described stops in the vicinity of White Pine, Michigan to examine the Freda Sandstone and these stops are well described by Woodruff et al. (2013). Stop 14: Nonesuch Formation at Hancock Campground and Boat Launch Latitude: 47.133755N; Longitude: -88.620581W Directions: Follow US-41 to M-203 to Hancock Boat Launch and Campground. Drive towards the boat launch and park. At the shoreline you will find a small outcrop of Nonesuch Formation. Walk towards the tree area approximately perpendicular to the Portage Canal shore line and boat launch. About 150 ft from the pavement, you will find the long ago abandoned rock quarry. The Nonesuch Formation is generally poorly exposed in the Keweenaw Peninsula. At this stop the Nonesuch Formation crops out around the margin of a historic rock quarry which is northeast of the Hancock boat launch. Here the Nonesuch Formation is a fine- to-medium grained, gray-to reddish brown sandstone with subordinate interbedded reddish-brown laminated siltstone and shale. The attitude of bedding is about N30OE and 25OW (Cornwall and Wright, 1956). Overall, the Nonesuch Formation consists primarily of siltstone and shale with subordinate amounts of sandstone. At Hancock campground area the formation is coarser grained since this locality is on the northern fringe of the depositional basin centered some 60 km southwest of this stop near White Pine, Michigan. The Nonesuch can be distinguished from the formations below and above by its generally grayish color. Most Nonesuch is a ripple, laminated siltstone with reddish-gray partings. Siltstones and sandstones of the Nonesuch are composed of around 30 to 40 % rock fragments and 60 to 70 % mineral grains. The rock fragments are mostly volcanic with a 2:1 ratio of mafic-tosilicic + intermediate composition (Daniels, 1982). There are multiple excellent well-described stops in the vicinity of White Pine, Michigan to examine the Nonesuch Formation and these stops are well described by Woodruff et al. (2013). Acknowledgments I thank Allan Blaske for his review of this field guide. His comments made this a better guide. References Cited Blattler, C.L., Bergmann, K.D., Kah, L.C., Gomez-Perez, I., and Higgins, J.A., 2020 Constraints on Meso-Neoproterozoic ancient evaporite deposits: Earth and Planetary Science Letters, 532:115951. 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Paces, J.B., and Bell, K., 1989, Non-depleted sub-continental mantle beneath the Superior Province of the Canadian Shield: Nd-Sr isotopic and trace element evidence from Midcontinent rift basalts: Geochimica Cosmochima Acta, v. 53, p. 2023-2035. Paces, J.B., and Bornhorst, T.J., 1985, Geology and geochemistry of lava flows within the Copper Harbor Conglomerate, Keweenaw Peninsula, Michigan: 31st Annual Institute on Lake Superior Geology Proceedings (Kenora, Ontario), p. 71-72. Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of the Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagmatic, and tectnomagmatic processes associated with the 1.1 Ga Midcontinent Rift system: Journals of Geophysical Research, v. 98, p. 13,997-14,013. 52 �Püeschner, U.R. Very low-grade metamorphism in the Portage Lake Volcanics on the Keweenaw Peninsula, Michigan, USA. Ph.D. Dissertation, University of Basel, Basel, Switzerland, 2001; pp. 1– 81. Schmidt, S.T.; and Robinson, D. Metamorphic grade and porosity and permeability controls on mafic phyllosilicate distributions in a regional zeolite to greenschist facies transition of the North Shore Volcanic Group, Minnesota. Geological Society of America Bulletin 1997, p. 683-697. Stoiber, R.E., and Davidson, E.S., 1959, Amygdule mineral zoning in the Portage Lake Lava Series, Michigan copper district: Economic Geology, v. 54, p. 1250-1277, p. 1444-1460. Stüeken, E.E., Jones, S., Raub, T.D., Prave, A.R., Rose, C.V., Linnekogel, S., and Cloutier, J., 2020, Geochemical fingerprints of seawater in the Late Mesoproterozoic Midcontinent Rift, North America: Life at the marine-land divide: Chemical Geology, v. 553, p. 119812 Velbel, M.A., 2009, The “Lost Interval”: Geology from the Permian to the Pliocene: in Schaetzl, R., Darden, J., and Brandt, D. (eds.), Michigan Geography and Geology, Pearson Custom Publishing, New York, p. 60-68. Weege, R.J., and Pollack, J.P., 1971, Recent developments in native-copper district of Michigan: Society of Economic Geologists Field Conference, Michigan Copper District, September 30 - October 2, 1971, p. 18-43. Weege, R.J., Pollock, J.P., and the Calumet Division Geological Staff, 1972, The geology of two new mines in the native copper district: Economic Geology, v. 67, p. 622-633. Weege, R.J., and Schillinger, A.W., 1962, Footwall mineralization in Osceola amygdaloid, Michigan native copper district: A.I.M.E. Transactions, v. 223, p. 344-350. White, W.S., 1960, The Keweenawan lavas of Lake Superior, an example of flood basalts: American Journal of Science, v. 258A, p. 367-374. White, W.S., 1968, The native-copper deposits of northern Michigan: in Ridge, J.D., ed., Ore Deposits of the United States, 1933-1967 (the Graton Sales volume), American Institute of Mining, Metallurgical, and Petroleum Engineering, New York, p. 303-325. White, W.S., 1971, Field Trip A-2 – Houghton to Calumet via South Range quarry and Eagle River: Society of Economic Geologists, Guidebook for field conference, Michigan copper district, Sept. 30-Oct. 2, 1971, p. 68-75. White, W.S. and Wright, J.C., 1960. Lithofacies of the Copper Harbor conglomerate, northern Michigan. U.S. Geological Survey Prof. Paper 400-B, 5-7. Williams, W.C. and Bornhorst, 2023, Controls on the stratiform copper mineralization in the western syncline, Upper Peninsula, Michigan: Minerals, v. 13, p. 927-950. https://doi.org/10.3390/min13070927 53 �Woodruff, L.G., Cannon, W.F., Nicholson, S.W., and Schulz, K.J., 2013, Geology of Keweenawan Supergroup, Porcupine Mountains, Ontonagon and Gogebic Counties, Michigan: 59th Institute on Lake Superior Geology Proceedings, v. 59, Part 2, Field Trip Guidebook, p. 69-96. Woodruff, L.G., Daines, M.J., Cannon, W.F., and Nicholson, S.W., 1995, The thermal history of the Midcontinent Rift in the Lake Superior region: implications for mineralization and partial melting: in International Geological Correlation Program, Field Conference and Symposium on the Petrology and metallogeny of volcanic and intrusive rocks of the Midcontinent rift system, Duluth, Minnesota, v. 336, p. 213-214. Woodruff, L.G., Schulz, K.J., Nicholson, S.W., and Dicken, C.L., 2020, Mineral deposits of the Mesoproterozoic Midcontinent Rift system in the Lake Superior region — A space and time classification: Ore Geology Reviews, v. 126, p. 1–21, https://doi.org /10.1016/j.oregeorev .2020 .103716. 54 �Field Trip 2 Mining History and Geology of the Quincy Mine, Keweenaw Peninsula Native Copper District, Michigan Theodore J. Bornhorst1, James M. DeGraff2, Tom Wright3, and Katherine Langfield2 1 Department of Geological and Mining Engineering and Sciences and A.E. Seaman Mineral Museum, Michigan Technological University, 1404 E. Sharon Avenue, Houghton, MI 49931 2 Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 1404 E. Sharon Avenue, Houghton, MI 49931 3 Quincy Mine Hoist Association, 49750 US-41, Hancock, MI 49930 Objectives of Field Trip The historic Quincy Mine was the fourth largest copper mine in the Keweenaw Peninsula native copper district. From 1851 to 1967, mining and processing of native copper ore produced ca. 488 million kilograms of refined copper via several shafts sunk along the top of the Pewabic basaltic lava flows. The Quincy Mine is interpreted and made accessible to the public by the Quincy Mine Hoist Association. The field excursion include tours of: the Quincy No. 2 shaft-rockhouse, the largest steam-driven mine hoist in the world, geology exposed along 675 m of an adit that intersects the historic Quincy Mine workings at the No. 5 shaft on the 7th level (107 ft below surface), the Quincy smelter works on the shore of Portage Lake, and two sites on Torch Lake for extraction of native copper from mined ore. Dress appropriately for the underground portion of the tour where the temperature averages 7o C (45 o F). Walk-in is on a wet surface and will access areas not normally visited by the public. Hard hats and lights will be provided. Brief history of the Quincy Mine Douglass Houghton's copper report in 1841 sparked the beginning of the first mining rush of North America to the Keweenaw Peninsula. This human event has its true beginning in the origin of the rocks and the native copper they host. The human story begins with native people's exploitation of native copper and their introduction of native copper to European explorers. Houghton's copper report led to human migration to the Keweenaw Peninsula, the discovery of the first native copper mine in 1845, the Cliff Mine, and the beginning of modern mining. The mining district quickly developed into the most significant copper district in the United States with peak copper production between 1890 and 1915. The district began to decline from 1920 to the end of mining in 1968 and the decline was accompanied by human exodus; the local population continues to decline today. The Keweenaw Peninsula exhibits the classic boom and bust cycle that is often associated with mining. In 1992, a national park was created to preserve and interpret the rich human and geologic history of the now dormant native copper district. 55 �The Quincy Mining Company was established in 1846 and was termed “Old Reliable” for its reliable payment of dividends to its shareholders. From 1866 to 1890 the Quincy Mine area developed into a robust mine. The Quincy Mine was one of the first mines in the Keweenaw Peninsula to evaluate their operations to maximize efficiency and hence, maximize profits. In 1866 they were the third mine to install a man-engine. They experimented with Burleigh power drills in 1868, but the large and cumbersome drills proved a failure. They built a rock house complete with rock crushers in 1873 which eliminated the time and labor involved in the old kiln houses. In 1879, black powder was replaced by dynamite and most hand drilling was eliminated when they adopted Rand Company's "Little Giant" power drills. While these innovations were adapted to increase efficiency, other changes were made to solve problems. The Quincy Mine could not find enough water in the abandoned shafts for its steam plant near the mine up on the top of Quincy hill which led to installation of a pumping station to pump water uphill from Portage Lake. In 1887, they closed their stamp mill on Portage Lake as the tailings they deposited into the lake began to hinder navigation. Quincy proceeded to build a new mill on Torch Lake. By building a new mill they were able to adopt steam-powered stamps rather than the old Cornish drop stamp technology (Lankton and Hyde, 1982). Quincy was able to continue to adapt because they had sufficient copper ore to mine. The Quincy Mining Company began to rebuild itself during the boom years for the whole Keweenaw Peninsula native copper district from 1890 to 1915. It acquired more powerful engines and hoists and began to raise two skips at a time in balance with each other. In 1891, they built the photogenic and functional No. 6 combined shaft and rockhouse. The man-engine reached its maximum depth in 1892 and as a result Quincy began using man-cars to raise and lower workers using the same hoist they used for ore skips. They built their own smelter on Portage Lake on the old Pewabic Mill site in 1898. In 1905 they began to replace their 2-cylinder engines with 4-cylinder engines to get better use of the steam and reduce coal consumption. Underground mining was changing too. Miners switched from candles to paraffin-based lamps in 1896-97 and to the less-smoky calcium carbide lamps in 1912-14. Miners began using machines to cut up masses in 1906 rather than drills. Quincy became the first mine in the district to modernize tramming and hauling when they installed a haulage system with a GE batterystorage locomotive in 1901. Quincy designed and patented their own side-dumping tramcars. By 1903 Quincy had 15 electric locomotives. They experimented without success with power shovels underground. In 1905, they reduced the wait time to dump cars underground by digging 500-ton underground storage bins into the hanging wall above the inclined shafts and thereby were able to hoist 25 % more ore to the surface. In 1910, Quincy operated 160 two-man drills, each of which weighed 245 pounds. The lighter one-man drills significantly decreased the cost of mining but this new technology left a miner alone underground which became one issue of the 1913 strike (Lankton, 1991). The Keweenaw Peninsula native copper mining district never fully recovered after the 1913 strike. The Quincy Mining Company continued to move forward after the strike, and they installed the world's largest steam hoist. Quincy constructed the No. 2 hoist house in 1918 and began operating the world’s largest steam hoist in 1920 as the shaft reached an incline depth of 7750 (2360 m) feet. In 1920 Quincy increased efficiency of processing and smelting by adding 56 �Wilfley tables to their stamp mill and adding a casting wheel at the smelter to eliminate hand pouring of ingots. As the underground workings went deeper, mining became much more difficult. In 1926, they had to install additional pumps to reduce the time used waiting for hoisting water-bailing skips rather than rock skips. In 1927, there was a major underground fire that impacted production. As the mine went deeper, they had to install fans to reduce the temperature at the bottom where miners still sometimes had to work at temperatures of 98oF (Lankton and Hyde, 1982). The Great Depression closed most Keweenaw mines. The Quincy Mine closed in 1930. The Calumet & Hecla Mining Company closed most of its mines except for a lucky few that operated on reduced shifts. The Quincy Mining Company reopened the No. 6 and No. 8 shafts on a limited basis in 1937. It wasn't until World War II and the advent of price controls that a larger production schedule was resumed. District-wide production resumed on a broader scale when price controls were lifted on August 31, 1945 and Quincy permanently ended its underground mining. The reclamation mill continued to operate and make a profit. In 1948, on its 100th anniversary, Quincy paid a dividend of 25 cents per share due only to the copper produced by the reclamation from tailings in Torch Lake plant (Lankton and Hyde, 1992). The Quincy smelter reopened in the mid-1950s after being closed for about 15 years as Quincy could no longer send reclamation concentrates to the Calumet & Hecla Mining Company smelter. The Quincy smelter remained open until the reclamation plant closed in 1967. The mines were allowed to begin flooding in 1970 (Thurner, 1994). The Keweenaw Peninsula native copper mining district has remained dormant in the 54 years to follow except for several episodes of exploration. Highland Copper Company, from 2011 to 2015, has been the latest explorer to attempt to reopen native copper mines in the Keweenaw Peninsula. The economies of mining districts are typically sustained by one principal industry, mining. When the mines are profitable and expanding the local communities also do well and when the mines suffer decline the local communities also suffer decline. This creates the boom to bust mining cycle. The local population follows this boom-and-bust trend. During the boom of mining Calumet was a vibrant city and since the mines have closed it has contracted. The last value obtained from the mining industry is selling the useful equipment to other mines followed by the dismantling of buildings to sell for scrap. Evidence of mining such as shaft-rockhouses and industrial buildings disappeared too. The surface rock piles left from mining once dotted the landscape of the Keweenaw Peninsula, but over time they too are disappearing as they are an inexpensive source of crushed stone for roads and other purposes. In the late 1980s Calumet community leaders envisioned that the past might be the key to the future of Calumet and they sought development of historical tourism. This led to creation of the Keweenaw National Historical Park in 1992. The park consists of limited park owned lands/structures and multiple Keweenaw Heritage Sites which are public, private, and non-profit. Together the park and its cooperative sites preserve and interpret the mining history of the Keweenaw Peninsula and supports historical tourism. The brief history of the Quincy Mine was slightly modified from text written by Larry Molloy, published by Bornhorst and Molloy (2016), and republished by Bornhorst (2022). 57 �Geologic Overview of the Quincy Mine Readers are referred to Bornhorst (this volume) and Bornhorst and Lankton (2009) for the geologic setting of the Quincy Mine and its native copper deposits hosted by the Portage Lake Volcanics. This overview of the geology of the Quincy Mine is from Bornhorst and McDowell (1992), Bornhorst et al. (1986), and Butler and Burbank (1929). The Portage Lake Volcanics comprise several hundred subaerial lava flows erupted within the Midcontinent Rift of North America about 1.1 billion years ago. There are occasional interbedded sedimentary rock layers dominated by conglomerate (Bornhorst, this volume). The native copper ores at the Quincy Mine occur in tabular bodies hosted by the originally porous and permeable tops of subaerial basalt lava flows. Ore-forming hydrothermal fluids precipitated native copper in the open spaces some 30 million years after the lava flows were erupted. About 50 lava flows of the Portage Lake Volcanics are exposed along the adit, twelve of them beneath the interflow sedimentary layer termed the Allouez conglomerate. The Allouez conglomerate and overlying Greenstone flow can be traced from the Houghton-Hancock area to the tip of the Keweenaw Peninsula (Bornhorst, this volume). A clay gouge along a bedding plane fault with undetermined slip occurs at the top of the Allouez conglomerate (Bornhorst and McDowell, 1992). Similar bedding plane faults are common at the tops and bases of conglomerate layers throughout the district. Native copper ore at the Quincy Mine is hosted by a group of relatively thin porous tops of lava flows that are difficult to correlate laterally without mapping the flows in detail (Butler and Burbank, 1929). The productive tops of the lava flows at Quincy, termed the Pewabic amygdaloids, are not brecciated as is common in the larger flow-top mines in the district. Cavernous zones within the Pewabic flows began as open spaces and subsequently were filled with hydrothermal minerals described above. There is much variability within the tabular lode from well to poorly banded and from high-grade of copper ore to practically barren poor rock (Butler and Burbank, 1929). Stratigraphically, about 12 lava flows with a cumulative thickness of about 100 m occur between the Allouez conglomerate and the overlying Pewabic flows which are the host rocks for the native copper deposits at the Quincy Mine. However, at this location along the adit, the Pewabic flows are not mineralized as they are on the northwest side of the Hancock fault. There are about 14 additional lava flows until the Hancock fault is reached. The Hancock fault is marked by a distinctive clay gouge, almost pure corrensite, and a green corrensite-rich brecciated mineralized zone adjacent to the gouge (Bornhorst and McDowell, 1992). The Hancock Mine produced native copper from a mineralized segment of the Hancock fault, suggesting that it may have been a feeder of ore-forming hydrothermal fluid into void spaces in the tops of the Pewabic flows (Bornhorst and McDowell, 1992). The amygdaloidal flow tops exposed by the adit are filled with a number of hydrothermal alteration minerals. Butler and Burbank (1929) describe the main-stage hydrothermal minerals at the Quincy Mine: quartz and calcite are abundant throughout the lode, commonly as euhedral crystals in open cavities; pumpellyite is less abundant but is present throughout the lode; epidote is less abundant than 58 �pumpellyite but is a common hydrothermal mineral; chlorite is particularly abundant in amygdules near the bases of Pewabic lava flows and is locally replaced by quartz and calcite; prehnite is present but not common; and datolite is present only in upper levels of the mine. The Pewabic lode of the Quincy Mine is notable for spectacular euhedral calcite with visibly unaltered pink to rose colored inclusions of native copper, which are highly sought by mineral collectors. Native silver is closely associated with native copper, although abundance is low. Laumontite is sparse and associated with small fissures. Thus, it may be a late-stage hydrothermal mineral rather than main-stage (Bornhorst, this volume). Butler and Burbank described early pumpellyite and epidote followed by quartz, calcite, and native copper. Bumgarner (1980) described amygdules indicating that prehnite and chlorite were early hydrothermal minerals followed by quartz, then chlorite, and lastly calcite. Overview from native copper ore to copper products At the Quincy Mine and elsewhere in the Keweenaw Peninsula native copper was extracted from tabular ore bodies by underground mining methods. Quincy Mine yielded about 42,870,000 tons of ore (1 ton = 2,000 lbs) with recovered refined copper totaling 1,077,000,000 lbs at an average grade of 1.26 % copper per ton of ore. The amount of silver in the native copper ore can be estimated from incomplete production statistics in Butler and Burbank (1929) to be roughly 0.2 oz of silver per ton of ore. The ore is blasted underground into small enough size to be able to be hoisted to the surface. Rock lacking sufficient native copper was sent to the poor rock pile or into abandoned underground mine openings. Prior to being sent to the mineral processing plant to recover native copper, the broken ore is sized by a slatted grating, “grizzly”. The broken ore passing through the grating (most of the ore) is sent to the mineral processing plant and those fragments too large are broken further at the surface near the shaft rock-house with a steam driven hammer. One reason a fragment could be too large is because it is mostly native copper. Native copper is malleable and large masses are not readily fragmented by underground blasting. Once most of the rock was removed from the larger fragments, the copper-dominant fragments were put into barrels (barrel copper) or, if they were too big, they were put onto a flat rail car and shipped directly to the smelter instead of to the mineral processing plant. The ore from the Quincy Mine was processed to separate native copper from the barren host rock and barren minerals in order to produce a product sufficiently enriched in copper to be smelted. The ruins of the Quincy processing plant will be visited at Stop 4. The first essential step in processing was crushing the ore into sand and smaller sized fragments using stamp mills. The crushing aims to produce fragments which are mostly native copper or mostly rock and thereby liberates the native copper from the rock. Because native copper is much denser than the host rock (about 3 times denser than the barren host rock), fragments of native copper can be separated from the barren host rock using water and gravity methods such as jigging. Hence, the mineral processing plant usually was constructed near a body of water. The mineral processing plant produced a “concentrate” which was composed of sand-sized and finer native copper with some fragments or partial fragments of barren rock because no separation method is able to completely separate every particle. The ore 59 �mined at Quincy averaged about 25 lbs of copper in a ton of ore. The copper concentrate was more than 50 % copper, hence most of the sand and smaller size fragments from the crusher ended up being waste, which still contained some copper. This waste, called tailings, was transported by water slurry and dumped into lakes, especially Torch Lake. The amount of native copper in a sand-sized fragment may have been so low that it was correctly separated into the tailings. In other fragments there was enough copper in them but they were incorrectly separated into the tailings. For example, the copper could have been finer grain size than the fragment and it was diluted by barren host rock and minerals. By crushing such fragments to a finer grain size, more copper could have been liberated from the barren host material. Quincy Mine kept track of the lost copper as it dumped tailings into Torch Lake. They later went back and recovered much of the lost copper by reprocessing the tailings using newer more efficient technology. The mineral processing plant and recovery of lost copper are discussed at Stop 4. The copper concentrate from the processing mill was shipped to the Quincy Smelter Works which is discussed further at Stop 2. The smelting and refining process resulted in solid Quincy copper ingots of up to 99.8 % pure copper. The copper ingots transported to markets by ships. The copper mined and processed by Quincy was sufficiently pure to be fabricated into a variety of usable copper products such as electrical wire. Copper ore to copper products text from previously published field trip guide by Bornhorst (2022) with modification. Field Trip Stops Figure 1: Map showing approximate stop locations of Field Trip 2 to the Quincy Mine. 60 �Stop 1: Quincy Mine, Keweenaw Heritage Site of the Keweenaw National Historical Park Latitude: 47.137137N; Longitude: -88.574875W Directions: From Michigan Tech drive west on US-41 (left from parking lot by the MUB) and continue through downtown Houghton across the Portage Lake lift bridge through downtown Hancock and uphill to the Quincy Mine No. 2 shaft rock-house turning right into the parking lot just past the shaft rock-house. The Quincy Mining Company was incorporated in 1846 and operated until 1967. Quincy mined underground from nine shafts on the Pewabic flow top and there was an industrial complex associated with mining (Figure 2). Throughout its history, Quincy Mining Company paid dividends on such a regular basis it was nicknamed "Old Reliable". Quincy Mining Company produced a total of 1.08 billion pounds of refined copper and approximately 100 million oz of silver from approximately 43 million tons of ore at an average grade of 25.1 lbs. of copper per 2000 lb. ton of ore (including copper reclaimed from Quincy tailings). The Quincy Mine ranks as the fourth largest mine in the native copper district. In 1921 the No. 2 shaft was the world's deepest. The Nordberg steam hoist is the world’s largest. Figure 2: The Quincy Mining Company complex ca. 1900. The No. 2 shaft-rockhouse is near the center of the drawing. From Molloy (2011) with permission. 61 �The No. 2 shaft of the Quincy Mine opened in 1858. At the beginning of mining a simple house was built over the shaft. By 1892, Quincy introduced the concept of hoisting the ore and doing initial crushing and sorting of the ore in the same building, a shaft-rock house. The current Quincy No. 2 shaft rock house was built in 1908 (Figure 3). The Quincy No. 2 shaft rock house is 147 feet (45 m) tall and the angle on the side of the building facing US-41 is at the dip angle of the native copper deposit. Behind the shaft rock house, there are two of the original eight pulley stands and stanchions that were used to support a steel cable extending to the No. 2 hoist house built in 1919. Mining at the No. 2 shaft ended in 1931. Figure 3: Quincy Mine No. 2 shaft-rockhouse. This drawing is based on Historic American Engineering Record drawing, MI- 2,19/34, Durward W. Potter, Jr., 1978 and Richard K. Anderson, Jr., 1979. From Molloy (2007). By 1917, the No. 2 shaft had reached such great depths that the hoist engine housed in the 1895 hoist house was no longer adequate. The Quincy Mine needed a large and faster hoist to continue its production. In 1918, the No. 2 hoist house was constructed but World War I delayed 62 �delivery of a new hoist until 1919. The Nordberg hoist began operating in 1920 as the shaft reached an incline depth of 7750 (2360 m) feet. The Nordberg hoist consists of four cross-compound steam engines that work as one (Figure 4a and 4b). The new hoist could move an ore skip carrying 10 tons of rock (13 tons total weight) up at 3200 feet per minute (36 miles per hour) and was more energy efficient than the hoist it replaced. The Nordberg hoist, the world's largest, operated 24 hours per day for 11 years until mining ended in 1931; to a depth of over 9000 feet (2743 m) on the incline (Molloy, 2007). Stairs - down Entrance from the 1895 Hoist House Down Hoist Rope Slots DisplaysModel of #6, Mine Cross Sections Up Overhead Crane Low Pressure Cylinder High Pressure Receiver Oil For Hydraulics Stored Under Here Low Pressure Receiver High Pressure Cylinder Condenser Under Here Oiler's Gallery Hoisting Drum 30' Maximum, 16' Minimum Diameter D i s p l a y s Vacuum Pump Low Pressure receiver Miniatures High Pressure Cylinder Displays High Pressure Receiver Operator's Platform Overhead Door Stairs to Platform Top Of Water Circulating Pump Lily Hoist Controler Displays Oil Pump Drive Low Pressure Cylinders Up Corliss Steam Engine Flywheel Display Of Large Tools Down Figure 4a: Quincy Mining Company No. 2 shaft Nordberg hoist diagram. This drawing is based on HAER drawing, MI-2,14/34, Durward W. Potter, Jr., 1978. From Molloy (2007) A steam engine functions by using alternating intake and exhaust valves to allow steam to enter a chamber, expand inside of the chamber, and use the force of the expansion to push a piston as the steam expands. The double-acting pistons used here can be pushed both up and down in the cylinder by the expanding steam. The entire engine occupies 60 feet by 54 feet of floor space and is 60 feet tall. It is a cross-compound steam engine, an engine where steam is used twice. This common technique accounts for the “choo-choo-choo-choo” sound one hears near steam powered trains. Today the hoist remains an engineering marvel and is still the world’s largest steam mine hoist. 63 �Miniatures H ois ting D rum Operator's Platf orm Brak e Main C rank Oiler's Gallery Low Pres s ure Throttle H igh Pres s ure Throttle Steam Supply Line H igh Pres s ure Low Ex haus t to Pres s ure Equalizing Line Pres s ure C ondens er C y linder C y linder H igh Press ure Low Pres s ure Steam R ec eiv er Steam R ec eiv er Figure 4b: This diagram illustrates the major features of the hoist and traces the flow of steam through the hoist. It is based on HAER drawing, MI-2,13/34, Jon R. Carter, 1978. From Molloy (2007). At the Quincy Mine and elsewhere in the Keweenaw Peninsula native copper was extracted from tabular ore bodies by underground mining methods. The ore is blasted underground into small enough size to be able to be hoisted to the surface. Prior to being sent to the mineral processing plant to recover native copper, the broken ore is sized by a slatted grating, “grizzly.” Those fragments too large are broken further by steam hammers. The native copper-dominant fragments were put into barrels (barrel copper) or, if they were too big, they were put onto flat rail cars. The barrel copper and larger masses were shipped directly to the smelter located downhill from the shaft rock-house. Next to the Quincy Mine No. 2 shaft rock house is a specimen of mass copper weighing hundreds of lbs. that would have been shipped directly to the smelter. Text from previously published field trip guide by Bornhorst (2022) with limited modification. 64 �Stop 2: Quincy Smelter Works Latitude: 47.126688N; Longitude: -88.565290W Directions: From Stop 1 turn left from Quincy Mine returning downhill to Hancock, just before the Portage Lake lift bridge follow M-26 through underpass and from bridge continue 0.4 miles (0.65 km) to the Quincy Smelter Works. The Quincy Smelter was constructed in 1898 and initially consisted of four or five reverberatory furnaces until switching to two larger furnaces in 1920. The purpose of the smelter was to produce bars of copper with as few as possible impurities. Copper with low enough impurities was ready to be made into copper products. The copper concentrate from the mill, barrel copper, and larger masses are melted, turned from solid to liquid in a furnace. Smelting is melting at temperatures higher than the melting point. By being higher than the melting point the liquid metal separates from the liquid silicate rock. Since the density of metallic copper is much higher than other components in the liquid rock, the liquid copper sinks to the bottom of the furnace while the other components, molten slag (waste liquid) float to the top. Men skimmed off the molten slag and it was dumped on a pile where it was left to cool into glass and minerals. Across from the parking lot opposite the agent’s house/office is a pile of solidified slag. The molten copper was tapped from the bottom of the furnace into a refining furnace where it was rabbled and poled. Rabbling (stirring) agitates the molten copper and allows the introduction of small amounts of air which oxidizes the impurities (copper has less tendency to oxidize than impurities). The oxidized impurities are lighter than liquid copper and thus, rise to the surface and are skimmed off the molten copper. After the impurities are skimmed off of the molten copper, green sapling poles are inserted into the molten copper (poling). Poling introduces carbon (wood) which reduces the amount of copper oxides in the molten copper. The refined copper was ladled and poured into molds and sprayed with water to cool it. The solid copper shapes were then dumped into a tank of water to complete cooling. Quincy cast copper ingots of several shapes and sizes (bars, wedges, and cakes). Refined copper from Quincy was up to 99.8 % copper which was sufficient for fabrication of copper products in the early 1900s. Some copper ingots had to undergo further refining. Today, the copper would undergo electrolytic refining to make it more than 99.99 % pure copper. Quincy constructed the smelter to be able to refine and ship its own copper. It also accepted custom work from neighboring mining operations. While underground mining ended in 1945, the Quincy smelter remained open until 1971. After closure of the underground mine, Quincy was actively recovering copper from reprocessing of tailings from Torch Lake. Fortunately, the smelter site remained intact after closing and is now open for guided tours. In 2014, the Keweenaw National Historical Park Advisory Commission acquired the smelter from Franklin Township (Figure 5). Text from previously published field trip guide by Bornhorst (2022) with limited modification. 65 �Figure 5: The historic Quincy Smelter Works. View July 2022 towards the north from Houghton waterfront. Stop 3: Quincy Mine Adit to 7th Level Underground Workings Latitude: 47.130383°N; Longitude: 88.573824°W Directions: Turn left onto M-26 heading west and drive 0.5 mi (0.8 km) to its junction with US41 north of the Portage Lake lift bridge. At the stop sign, merge onto US-41 and continue west for two blocks (~0.1 mi, ~0.15 km) to Dunstan Street on the right before a BP gas station. Turn right onto Dunstan and drive uphill for three blocks on the right (~0.15 mi, ~0.25 km) to Mason Avenue. Turn right onto Mason and drive east about 0.2 mi (0.3 km) to near a sharp right-hand curve. Just before the curve, turn left to enter an unpaved driveway that leads to the adit. The Quincy adit was first opened in 1892 by the Quincy Mining Company and subsequently widened in the 1970s by Michigan Tech mining engineering faculty and students for an underground educational and research facility. The adit enters the south side of Quincy hill at a point located ~690 m SSE of the iconic Quincy No. 2 shaft house along US-41 at the top of the hill. From the portal, the adit follows an azimuth of 331° for 675 m, where it intersects the 7th level workings at the Quincy No. 5 shaft (Bumgarner, 1980). The average strike of PLV layers here is 213° (right-hand rule), which means that the adit cuts across them in a direction 30° clockwise from the dip direction. Approximately fifty lava flows and one conglomerate layer, dipping on average 50° NW, are exposed along the adit (Bumgarner, 1980; Bornhorst et al, 1986). Geologic sites of interest in the adit and side passages are described as sites 3A to 3E in the following text and shown in Figure 6. Underground workings of the Quincy Mine were developed along a series of parallel, relatively thin, lava flow tops, referred to collectively as the Pewabic flow top within the Portage Lake Volcanics. The mine was developed to a vertical depth of 1897 m (6225 ft) comprising 92 levels. The ore body decreases in dip from 54o at the surface to 32o at the bottom levels. Pewabic flows are characterized by large cavernous zones up to 1.5 m thick, interpreted by Butler and Burbank (1929) as coalescing vesicles and large gas cavities. Alternatively, the large cavities could have 66 �been caves or lava tubes formed by lava drainage beneath a solidified flow surface. Openings were connected up to 100s m along formation strike and, where especially well developed, they may host 2 to 10 high-grade copper zones. An interconnected series of such openings would have provided continuous flow paths for ore-forming hydrothermal solutions. Several prominent and steeply dipping veins extend throughout the mine. They probably helped to integrate the hydrothermal system as their mineralogy is similar to that of the hydrothermal mineral assemblage in the flow tops. Figure 6: Map of Quincy adit and 7th level mine workings (modified from Bumgarner, 1980). Six geological sites of interest are labeled 3A through 3F. Numbers 1 – 19 along adit are distances in hundreds of feet. 67 �Site 3A: Contact between two basaltic lava flows. [~75 m from portal] The contact between the two lava flows at this site shows many characteristics that are common in the Portage Lake Volcanics of the Keweenaw Peninsula. Contacts between successive subaerial lava flows are recognized by textural and color differences between the top of the older layer and base of the younger one, as well as by the geometry of their boundary. Massive flow interiors grade into margins with finer grain size and abundant vesicles often arranged in bands parallel to flow contacts. Flow tops have the highest abundance of vesicles. Upper flow surfaces were exposed to the atmosphere and escaping gases during and after emplacement, and later to hydrothermal fluids permeating along flow tops that filled many voids (amygdules) with an assemblage of minerals (Butler and Burbank, 1929; Stoiber and Davidson, 1959; White, 1968; Bornhorst, 1997), thus modifying their color as seen here. Looking at the adit’s northeast wall (right side walking in), the top of the older flow toward the southeast is highly vesicular and has a lighter greenish tone relative to the younger to the northwest. In this case, the distinctive lighter tone and greenish tinge of the flow-top rock results from secondary epidote, pumpellyite, and various copper minerals, which are not present in the base of the overlying flow. The adit cuts nearly perpendicular across the contact between the flows, such that their boundary can be traced from one side of the adit across its ceiling to the other side. This exposure is nearly ideal for measuring strike and dip of the contact because it is exposed in 3-D over a sufficient distance to allow visual averaging of irregularities that are typical of flow contacts. Site 3B: Allouez conglomerate and the Greenstone flow. [~290 m from portal] The Allouez conglomerate and overlying Greenstone flow are well correlated stratigraphic units along the strike of the historic copper mining district (Butler and Burbank, 1929; Stoiber and Davidson, 1959). They can be traced from near the tip of the Keweenaw Peninsula to southwest of the Quincy Mine, a strike length of 80 kilometers. In the Quincy adit, the Allouez conglomerate is a 2-m-thick layer and the overlying Greenstone flow is only 10-15 m thick, far less than its maximum thickness elsewhere. The conglomerate here is typical of most conglomerates interbedded with mafic lava flows of the Portage Lake Volcanics. It is largely clast-supported and contains rounded to subrounded cobbles and pebbles of felsic volcanic rocks and subordinate mafic rocks. At this site, the bedding surface between conglomerate and basalt is marked by a clay seam, or “fluccan” following Cornish mining terminology (Hubbard, 1898). Hubbard (1898) systematically documented such clay seams in mines accessible at the time and he noted that they typically follow contacts between interflow sedimentary layers and lava flows, but also occur at contacts between flows. The clay seams are commonly associated with polished surfaces, fault gouge, brecciation and alteration of adjacent units, and secondary mineralization in fractured zones generally less than a couple of meters thick. As noted by Hubbard (1898), these phenomena indicate that many layer boundaries in the Portage Lake Volcanics have slipped during one or more deformation events. Such surfaces are essentially layer-parallel faults that slipped because of their weaker mechanical strength relative to the layers themselves. 68 �Site 3C: Hancock fault at the adit and in drift to the northeast. [~570 m from portal] The Hancock fault is a major splay of the Keweenaw fault system that intersects the main Keweenaw fault about 14.5 km northeast of here and downdip at a depth of around 3 km (Cannon and Nicholson, 2001; DeGraff and Carter, 2023). Over its 18-km length, the Hancock fault strikes 234° (right-hand rule) and cuts obliquely across the exposed Portage Lake Volcanics in a direction that is ~25° clockwise from formation strike and dips northwest at a steeper angle than layering. It cuts the Pewabic flows of the Quincy and Hancock mines. The Hancock fault had a strong influence on the distribution of copper in these two mines as follows: 1. A portion of the fault in the Hancock Mine had copper ore that was exploited; 2. Copper is also abundant along the Hancock fault in the Quincy Mine; 3. Copper mineralization in Pewabic beds of the Quincy Mine is restricted to the hanging wall northwest of the fault. The Hancock fault has been proposed as an important pathway for ore-forming fluids to access porous zones at the Quincy and Hancock Mines (Bornhorst et al., 1986). Figure 7: Cross-sectional view of the Hancock fault looking northeast at the wall of the adit. Height of the view is about two meters. The slip surface is marked by a clay seam. 69 �The Hancock fault is best exposed and investigated in the Quincy Mine workings, where it is cut by the adit and drifts leading northeast and southwest from the adit (Figure 7). At this site, the Hancock fault is observed crossing the adit and following the northeast drift for about 50 meters, so that measuring its orientation is easy. At the adit, the fault strikes 239° and dips 54° NW, in contrast to stratigraphic layering that strikes 213° and dips a bit less than layering. The fault zone here has a thin (~3 cm) medial clay seam, marking the main slip surface, that is contained within a breccia envelope of disaggregated fragments. Outside of the breccia envelope, country rock is highly fractured near the fault but the rock is still intact. As the fault is followed northeastward along the drifts orientation varies and the thicknesses of its clay seam and surrounding breccia increase and decrease and are not always symmetrically arranged. Site 3D: Ropy pahoehoe at the top of a Pewabic flow in the first northeast drift. [~570 m from portal; ~75 m along drift] Farther along the same drift at Site 3D is a good exposure of the surface of one of the Pewabic flows (Figure 8). Like subaerial mafic lava flows elsewhere, the tops of Portage Lake Volcanics flows show characteristics of two fundamental types – a smooth or ropy geometry (pahoehoe, locally termed amygdaloid) and a rough blocky geometry (aa, locally termed fragmental amygdaloid). Most Portage Lake Volcanics lava flows have smooth flow tops and yet ropy pahoehoe is rarely observed, probably because plan views of flow surfaces are uncommon and because of degradation of flow surfaces between successive extrusive events. The ropy pahoehoe observed on the northwest side of the drift is well preserved. The view here is upward at the base of an overlying flow, and so the pahoehoe feature is really a mold of the upper surface of the underlying flow top that has been removed along the drift. The curved geometry of pahoehoe ropes results from local movement of partly solidified lava crust that forms arcuate patterns that are convex in the local direction of flow (Fink and Fletcher, 1978; Self et al., 1998). The local flow direction indicated by the convexity of the ropes seen here is updip and toward the southwest, i.e. from the center towards the edge of the rift. Most flow-top native copper ore bodies are hosted by brecciated flow tops because their relatively large volume of open space was well connected and provided relatively easy movement of ore-forming fluids (Butler and Burbank, 1929; White, 1968; Bornhorst, 1997). Flows with smooth to ropy pahoehoe tops are usually not favorable because vesicles have limited volume of not well connected open space which hindered movement of ore-forming fluids. However, Pewabic flows with pahoehoe tops had economic grades of copper mineralization because they had large connected openings up to 1.5 m wide and extending up to 100s m along strike. Butler and Burbank (1929) proposed that the exceptional Pewabic lode was the result of an extremely gaseous flow that produced a great abundance of vesicles that coalesced to produce large connected voids. An alternative hypothesis is that the large openings resulted from lava draining from pools and tubes beneath a solidified flow surface. 70 �Figure 8: Ropy pahoehoe in the hanging wall of the first drift northeast from the adit. View is to the northwest. The two highlighted patterns indicate flow updip and toward the southwest. Height of the view is about six meters. Site 3E: Hancock fault in drift to the southwest. [~675 m from portal; ~55 m along drift] This site provides another opportunity to examine the Hancock fault where it follows much of the drift leading southwest from the No. 5 shaft to the No. 7 shaft (Figure 6). From Site 3C at the adit, the fault angles across the corner of unmined rock between the adit and southwest drift and is next exposed on the southeast wall of the drift at Site 3E. From here, the fault trace rises up the southeast wall, gradually angles across the ceiling, and descends the northwest wall where it enters the drift’s hanging wall before reaching the No. 7 shaft. Native copper is frequently found in the fault’s hanging wall up to the breccia zone but has not been found in the footwall. Here, the “medial” clay seam is generally thicker than at Site 3C but its thickness varies considerably along the fault trace, as does the thickness of the disaggregated breccia envelope. It is difficult to estimate the thickness of the still-intact zone of fractured rock 71 �that encloses the breccia zone because, as seen at other faults examined in detail (Caine et al., 1996; Caine and Forster, 1999), its intensity gradually decreases away from the fault to a background value for the system. Study of exposures of the Hancock fault and nearby satellite faults in the Quincy Mine is needed to better understand the slip characteristics and wall-rock modification of the Hancock Fault. This could also help understand the influence of the Hancock fault on copper mineralization. Fault slip is being investigated by the measurement and analysis of slip indicators – slickenlines and steps – on the surfaces of the many small faults associated with the Hancock fault. Observations of how rocks along the Hancock fault were modified physically and chemically have been mostly qualitative so far, though with some exceptions (Bornhorst and McDowell, 1992; Langfield et al., 2023). Site 3F: Hanging wall of Pewabic flow in drift northeast of No. 5 shaft. [~675 m from portal; ~30 m along drift] Along the drift leading northeast from the No. 5 shaft to the No. 4 shaft (Figure 6), the hanging wall of the drift and attached stope exhibit slickenlines directed parallel to dip of the lava flows. One may need to search a little and use oblique lighting to see these fault surface features, which manifest layer-parallel dip slip on the boundary between two lava flows. Another surface with similarly oriented slickenlines occurs in a parallel drift a few tens of meters northwest beyond the hanging wall of this drift. As discussed at Site 3B, Hubbard (1898) documented 12 layer boundaries with layer-parallel slip, but this surface and the other northwest of here were not among them. He mentioned polishing and slickensides on such surfaces but did not specify their orientation. It is likely that such layer-parallel slip is far more common than has been observed because, like the ropy pahoehoe texture, it can only be observed where an ideal exposure permits viewing. The documented layer-parallel slip within the PLV section is strong evidence for a detached style of thrusting, which was used recently to model the cross-sectional geometry of the Keweenaw and Hancock faults (DeGraff and Carter, 2023). 72 �Stop 4: Quincy Mining Company Processing Plant and Dredge Latitude: 47.146295N; Longitude: -88.460474W, Directions: From Quincy Smelter Works turn right (east) and continue 5.7 miles (9.2 km) to ruins of the Quincy processing plant on left carefully pulling into open lot on west side of ruins. The Quincy Mine dredge No. 2 is located on the edge of Torch Lake on the other side of the road. Figure 9: The Quincy Mining Company mills, 1890-1928. Star shows the location of the Quincy dredge No. 2. From Molloy (2011) with permission. The Quincy Mining Company had to move their mill from its Portage Lake site below Quincy Hill because tailings deposited in the lake were beginning to hinder navigation. Construction began on the Quincy mill at this site in 1888 (Figure 9). Mill No. 1 began with three rock crushing stamps and two additional were added in 1892. The building closest to the dredge on the north side of the road contained the 1890 mill which was modified over time. The square building adjacent to it was a turbine building that was built in 1921. As production increased, Stamp Mill No. 2 was built to the north of the No. 1 mill in 1900 and had three stamps. Underground mining activities at the Quincy Mine focused on mining ore (rock with sufficient recoverable copper to make a profit). Inevitably the mine also produced waste (uneconomic) rock along with the ore rock. Some of this waste rock can be seen today throughout the Keweenaw Peninsula as poor rock piles. Some of the waste rock was used underground as fill for already mined out stopes. The broken ore from underground blasting that is small enough to pass through a coarse grating was sent by train to the processing mill. 73 �The purpose of the mill was to remove as much rock as possible from the native copper producing a product where the percent of contained copper is up to 20 times higher than the percent of copper in the ore. The copper-rich product is sand to smaller sized fragments of native copper mixed with similar sized rock and mineral fragments (termed “copper concentrate”). The inefficiencies in separating native copper from rock and minerals fragment results in some amount of waste rock and minerals in the copper concentrate. At the end of processing in the mill, most of the rock and minerals end up as fine sand to sand of waste containing only a small amount of copper (termed tailings). The Quincy Mill used several techniques to separate the copper from the rock and minerals. The essential first step in milling begins with crushing the ore rock to a small enough size to liberate the unwanted rock and minerals from the native copper. The ore rock and minerals containing disseminated copper of various sizes was crushed by steam stamps. Much of the rock was liberated from the native copper by crushing the broken ore fragments derived from mining to fragments 0.0165 to 0.188 inches in diameter (fine to very fine sand size). Larger masses of native copper were removed prior to crushing at the mine and sent directly to the smelter. Since the density of native copper is much higher than the liberated particles of rock and mineral, gravity and water methods could be used to separate the waste rock and minerals (tailings) from the copper. Early mines used jigging to concentrate the native copper from the tailings. Jigging was a well-developed mineral separation technology prior to the first mining of native copper in the Keweenaw Peninsula beginning in 1845. Jigging is accomplished by placing the sand sized particles on a screen where pulsating water allows the heavier copper particles to settle while the lighter particles rise to the top and overflow the screen or are skimmed off the top as waste tailings. Jigging resulted in a “concentrate” of sand sized native copper particles with some rock and minerals that were not copper-bearing (tailings), about 50 % copper and the rest waste rock and mineral. The concentrate was sent to the smelter to be turned into nearly pure copper. The waste tailings were dumped into nearby Torch Lake. However, using jigging up to 25 % of the native copper was incorrectly classified as waste (tailings) and dumped the copper was lost as the tailings were dumped into nearby Torch Lake. Over time new technologies were introduced into the mills by Quincy to increase efficiency and loose less copper into the tailings. Quincy used froth flotation in its processing circuit as early as 1920s. Violent agitation of copper and rock/mineral particles in an oily water along with frothing agents and other chemicals cause bubbles to rise to the surface with copper particles attached to their surface. The bubbles and copper particles were captured from the top of the flotation tank while the waster rock/mineral (tailings) sank to the bottom. and was slurried to Torch Lake. The heavier copper particles were carried upward by the floatation bubbles rather than sinking due to gravity by jigging. The mineral processing circuit could begin with jigging and with the tailings after jigging sent to floatation cells to recover more copper. Later Wilfley tables were added to the mineral processing circuit to help minimize loss of copper from finer grain sizes. A Wilfley table utilizes gravity and water to separate denser particles. A shaking ribbed table with a film of water flowing along the long axis results in the higher density copper particles concentrating in beds behind the riffles. 74 �The Quincy Mining Company knew that the tailings contained a lot of copper that they were unable to recover with technology available at that time. The company had considerable foresight and kept detailed maps each year of the copper content of the tailings that were deposited into the lake. As technology improved, they were able to reclaim copper from the tailings at a profit. Quincy built a special reclamation processing mill near here in 1942-43 for $1.2 million. The main building, 124'x255', had six Harding ball mills to grind the tailings even finer than the stamp mills in order to release more fine particles of copper from the rock/mineral and facilitate addition of Wilfley tables and flotation cells to mineral processing circuit. Across the road is Quincy Mining Company dredge No. 2 on the shore of Torch Lake. Torch Lake was filled with several 100 millions of tons of tailings since not only did Quincy Mine operate a mill on its margin so did many other companies, especially Calumet and Hecla Mining Company. The tailings were sucked up via the dredge and sent to the reclamation mill to recover more copper from the tailings. The reprocessed tailings were redeposited back into Torch Lake. This dredge was built in 1913 by Calumet & Hecla Mining Company. In 1951, the Quincy Mining Company purchased the dredge and it became known as Quincy Dredge No. 2. It could process over 10,000 tons of tailings per day and it had 141 ft suction pipe that could work 115 ft below the surface of the lake (Figure 10). Tailings were conveyed to the mill via a tube held up with pontoons. Quincy recovered 100 million lbs. of copper from tailings in Torch Lake from 1943 until it closed in 1967 or about 10 % of total production. In the 1800s and early 1900s, depositing 100s of millions of tons of tailings into Torch Lake was acceptable practice and the environmental consequences were not considered. Today, the environmental impact of tailings must be carefully considered to obtain a permit, "social license" to operate a mine. The mining companies did not just put tailings into Torch Lake, they also used it to dispose of other waste such as that from electrical systems and barrels filled with chemicals. The early processing plants used only water to separate the copper from the rock. Later floatation cells were used in processing ore from the mine and in processing reclaimed copper from the tailings. The floatation cells used chemicals in water. These chemicals along with the tailings were put into Torch Lake. Discovery of fish with tumors in Torch Lake led to its being designated an U.S. Environmental Protection Agency Superfund site. Some of these chemicals were biodegradable and are no longer present in the Torch Lake water but would have been present decades ago, thereby could have readily caused tumors in older fish. There are far too many tons of tailings in Torch Lake to remove them and thus, the mitigation strategy is to simply cover them with soil. Fortunately, the copper ores of the Keweenaw Peninsula lack pyrite that is known under certain environmental settings to produce acid drainage and in turn can result in significant environmental impact. The lack of acid generating potential and the otherwise mostly inert minerals in the waste rock has greatly lessened the environmental impact of the tailings themselves. Today crushed poor rock from the mines is used directly as aggregate and is also crushed for use as aggregate. Text from previously published field trip guide by Bornhorst (2022) with limited modification. 75 �Figure 10: Quincy Mining Company dredge No. 2 working on Torch Lake, ca. 1920s. From Molloy (2011). Stop 5: Ahmeek Mining Company Stamp Mill Latitude: 47.168633N; Longitude: -88.435810W Directions: From Quincy Mill Ruins and Dredge carefully pull out of gravel lot and continue east on M-26 for 1.9 miles (3 km) to stamp mills of the Ahmeek Mining Company. This is the only remaining steam-powered stamp in the Keweenaw Peninsula. The Ahmeek Mining Company had four of these Nordberg compound steam-powered stamps installed in 1910, and four additional stamps were added in 1914. Rail cars brought rock to the mill using the trestle, located above the trees across the street from the stamp. The compound-expansion nature of the stamps represented a major improvement in processing of copper ore. The stamp could strike approximately 104 24-inch blows per minute. The mill could process approximately 7,000 tons of ore in a 24-hour period. This is not part of the Quincy Mine but we stop here to see a steam-powered stamp used for crushing the native copper ore. Text from previously published field trip guide by Bornhorst (2022) with limited modification. 76 �References Cited Bornhorst, T.J., this volume, 2024, Mesoproterozoic Midcontinent Rift-filling Strata and Native Copper Deposits of the Keweenaw Peninsula, Michigan Field Trip 1: Institute on Lake Superior Geology, Field Trip Guidebook, 70th Annual Meeting, Houghton, MI v. 70, part 2, p. this volume. Bornhorst, T. J., 1997, Tectonic context of native copper deposits of the North American Midcontinent Rift system: Geological Society of America Special Paper 312, p. 127-136. Bornhorst, T.J., 2022, Field guide to the geology and native copper mining history of the Keweenaw Peninsula: Field Trip Guidebook for American Institute of Professional Geologists 2022 National Conference held in Marquette, Michigan August 6-9, 68p. Bornhorst, T.J., Kalliokoski, J.O., and Paces, J.B., 1986, The Keweenaw native-copper district: in Brown, A.C. and Kirkham, R.V. (eds.), Proterozoic Sediment-hosted Stratiform Copper Deposits of Upper Michigan and Belt Supergroup of Idaho and Montana: Geological Survey of Canada, Contribution Series 13386, p. 21-36. Bornhorst, T. J., and Lankton, L. D., 2009, Copper mining: A billion years of geologic and human history: in Schaetzl, R., Darden, J., and Brandt, D. (eds.), Michigan Geography and Geology, Pearson Custom Publishing, New York, p. 69-90. Bornhorst, T.J., and McDowell, S.D., 1992, Michigan Tech Earth Science Laboratory and Experimental Mine connecting with the Quincy native copper mine, Michigan: Society of Economic Geologists Field Conference Guidebook Series, v. 13, p. 100-104. Bornhorst, T.J., and Molloy, L.J., 2016, Geological and Historical Field Trip to the Keweenaw Peninsula: A Tribute to Douglass Houghton "Michigan's Pioneer Geologist". Michigan Basin Geological Society, 65 p. Bornhorst, T. J., and Molloy L.J., 2016, Geological and historical field trip to the Keweenaw Peninsula, A tribute to Douglass Houghton: “Michigan’s Pioneer Geologist: Michigan Basin Geological Society Geological and Historical Excursion September 10th-12th, 89 p. Bumgarner, E.L., 1980, The Geology of the Portage Lake Volcanics in the M.T.U. Mining Laboratory, Hancock, Michigan: Michigan Technological University, M.S. thesis, 138 p. Butler, B. S., and Burbank, W. S., 1929, The copper deposits of Michigan: U.S. Geological Survey Professional Paper 144, 238 p. Caine, J.S., Evans, J.P., and Forster, C.B., 1996, Fault zone architecture and permeability structure: Geology, v. 24, no. 11, p. 1025-1028. Caine, J.S. and Forster, C.B., 1999, Fault zone architecture and fluid flow: Insights from field data and numerical modeling: in Faults and Subsurface Fluid Flow in the Shallow Crust, American Geophysical Union, Geophysical Monograph 113, p. 101-128. Cannon, W.F. and Nicholson, S.W., 2001, Geologic Map of the Keweenaw Peninsula and Adjacent Area, Michigan: United States Geological Survey, Map I-2696, Scale = 1:100,000. 77 �DeGraff, J.M. and Carter, B.T., 2023, Detached structural model of the Keweenaw fault system, Lake Superior region, North America: Implications for its origin and relationship to the Midcontinent Rift System: Geological Society of America Bulletin, v. 135, no. 1/2, p. 449–466. https://doi.org/10.1130/B36186.1 Fink, J.H. and Fletcher, R.C., 1978, Ropy pahoehoe: surface folding of a viscous fluid: Journal of Volcanology and Geothermal Research, v. 4, p. 151–170. Hubbard, L.L., 1898, Keweenaw Point with particular reference to the felsites and their associated rocks: Geol. Survey Michigan, v. 6, part 2, 155 p. Langfield, K.M., DeGraff, J.M., and Gamet, N.G., 2023, Slip kinematics of the Keweenaw and Hancock faults within the Midcontinent Rift System, Upper Peninsula of Michigan: Institute on Lake Superior Geology, 69th Annual Meeting, Eau Claire, Wisconsin, Part 1 – Program and Abstracts, v. 69, p. 5051. Lankton, L.D., 1991, Cradle to Grave: Life, Work, and Death at the Lake Superior Copper Mines, Oxford University Press, 319 p. Lankton, L.D. and Hyde, C. K., 1982, Old Reliable: An Illustrated History of the Quincy Mining Company, Quincy Mine Hoist Association, 159 p. Molloy, L. J., 2011, A guide to Michigan's historic Keweenaw copper district: published by Great Lakes Geoscience LLC, 122 p. Molloy, L. J., 2007, A Visitor's Guide to the Historic Quincy Mine: published by Great Lakes Geoscience LLC, 61 p. Self, S., Keszthelyi, L., and Thordarson, T., 1998, The importance of pahoehoe: Annual Review of Earth and Planetary Sciences, v. 26, p. 81-110. Stoiber, R.E. and Davidson, E.S., 1959, Amygdule mineral zoning in the Portage Lake Lava Series, Michigan copper district: Economic Geology, v. 54, p. 1250-1277, p. 1444-1460. Thurner, A. W., 1994, Strangers and Sojourners: A History of Michigan's Keweenaw Peninsula, Wayne State University Press, 404 p. White, W. S., 1968, The native-copper deposits of northern Michigan: in Ridge, J.D., ed., Ore Deposits of the United States, 1933-1967 (the Graton Sales volume), American Institute of Mining, Metallurgical, and Petroleum Engineering, New York, p. 303-325. 78 �Field Trip 3 Geoheritage of Buffalo Reef: Industrial Impact on Land, Culture, and Fish Sovereignty Erika Vye Great Lakes Research Center, Michigan Technological University, 1404 E. Sharon Avenue, Houghton, MI 49931 Charlie Kerfoot Great Lakes Research Center, Michigan Technological University, 1404 E. Sharon Avenue, Houghton, MI 49931 Stephanie Swart Michigan Department of Environment, Great Lakes, and Energy, 525 West Allegan Street, Lansing, MI, 48909 Dione Price Keweenaw Bay Indian Community, Natural Resources Department, 14359 Pequaming Road, L’Anse, MI 49946 Evelyn Ravindran Keweenaw Bay Indian Community, Natural Resources Department, 14359 Pequaming Road, L’Anse, MI 49946 INTRODUCTION Buffalo Reef is a geoheritage site with scientific, educational, cultural, and aesthetic value. This field trip explores the relationship between geology, mining waste, and culture of Buffalo Reef a 2,200-acre natural cobble feature of Lake Superior’s lakebed southeast of the Keweenaw Peninsula and about 20 miles northeast of Houghton. Finely crushed waste rock - stamp sand from copper ore milling operations at the community of Gay have been moved by currents along the shoreline of the Keweenaw Peninsula to Big Traverse Bay, thus covering this highly productive spawning ground for lake trout and whitefish. This has negative implications for commercial fisheries, local economies, subsistence uses, and the spiritual, physical, and cultural well-being of tribal nations that identify as fishing people. Further, this has adverse impacts on tribes’ ability to exercise their treaty rights to fish in this area (Buffalo Reef Task Force, 2024). Geoheritage is a nascent yet evolving field in the United States that considers the protection, interpretation, and management of geologic features with significant scientific, educational, cultural, or aesthetic value (Brocx & Semeniuk, 2007; Geological Society of America, 2017; National Park Service & American Geosciences Institute, 2015; Reynard & Brilha, 2017). More 79 �distinctively, geoheritage emphasizes the importance of the varied personal values people have for geologic features and explores the wide-ranging relationships people have with landscape. The rich geodiversity of the Keweenaw has fostered relationships for millennia and has imbued our place with significant sites that provide opportunities to broaden both Earth science and cultural literacy (Rose & Vye with Martin, 2017; Vye, 2016). The rich geosites of the Keweenaw provide an accessible platform for people to learn about deep time, Earth's dynamic processes, and importantly, the diverse relationships and reciprocity people have with our geologic underpinnings. The Keweenaw is renowned for Earth's largest native copper deposits that became the first great copper mining district in the United States. From 1845 to 1968, over 11 billion pounds of refined copper were produced in Keweenaw mines, making it a cornerstone of the American economy in the 19th and 20th centuries (Bornhorst & Lankton, 2009; Bornhorst & Barron, 2011). Interpretations and public educational programming depicting the relationship between people and geology have largely focused on stories and heritage associated with the European diaspora that fueled the Copper Boom of 1845-1968 and how copper’s prolific use for transatlantic cables, telegraphs, electricity, and the auto industry ultimately modernized the country (Bornhorst & Lankton, 2009). Less interpreted, yet central to our history, is how geology has shaped, and continues to shape, the ancestral, traditional, and contemporary lands, waters, and livelihoods of the Anishinaabeg- the Three Fires Confederacy of Ojibwe, Odawa, and Potawatomi peoples and their many more-than-human relatives. As such, this field trip visits three stops (Figure 1) to explore mining impacts on culture, subsistence uses, and the wellness of all beings. Figure 1: Map of field trip site locations, courtesy of D.J. Lizzadro-McPherson, MTU 80 �Stop 1 - Mohawk, MI: The trip begins at the poor rock pile where the Mohawk No. 4 mine shaft once stood - a native copper host rock from which some of the Gay stamp sands were generated. Stop 2 - Gay, MI: Participants will then travel to the town of Gay to walk the stamp sands and learn how Tribal, State, Federal, and academic partnerships are collaborating to mitigate environmental damage and ultimately restore Buffalo Reef to the ecological resource that has sustained both tribal and non-tribal communities for generations. Stop 3 - Big Traverse Bay & Buffalo Reef, MI: From Gay, we will travel to Big Traverse Bay to learn more about why shoreline and habitat restoration efforts are necessary and how stakeholders and rights holders are working together to address this environmental justice issue in our community. Participants will benefit from visiting Buffalo Reef aboard Michigan Tech’s Research Vessel Agassiz. This part of the trip enables participants to compare healthy parts of the reef with areas that have been inundated with stamp sands. STOP 1 - MOHAWK, MI At this site, we will learn more about the source rock for the Gay stamp sands after a brief overview of the geological history of the Keweenaw region. Participants will have time to explore the poor rock pile before departing for Gay. Directions: Leave downtown Houghton and head north on U.S. 41 towards Calumet. Stay on U.S. 41 until you reach Mohawk; turn right on 6th street, just past E Phoenix Street there is a place to park on the left-hand side of the road. This space provides access to the Mohawk No. 4 poor rock pile (Figure 2). Coordinates: 47.301203, 88.363369 Figure 2: Parking location to visit Mohawk No.4 Geological Origin Story Michigan’s Keweenaw Peninsula has a rich and globally significant geodiversity in tandem with a fascinating cultural story. This is the site of the largest native copper dominated deposits known on Earth, the oldest metal workings in the Western Hemisphere, and a recent diaspora of 81 �European cultures that flocked to the region for copper mining in the late 1800s – this has shaped an entangled mosaic of cultural, mining, and industrial heritage. Importantly, the Keweenaw Peninsula offers an important window to Earth’s past, exposing the heart of the Mesoproterozoic Mid-Continent Rift. Located on Lake Superior in Michigan’s Upper Peninsula (Figure 3); the abundant geodiversity sites are the result of a flood basalt sequence comprised of hundreds of voluminous lava flows, interbedded and covered by a redbed clastic sedimentary rocks. An upwelling of heat and magma from a hot spot initiated great lava flows erupting from the rifting of supercontinent Rodinia. The rift created a ~3000 km U-shaped feature in the center of North America that extends from Kansas, through what is now Lake Superior and terminated in what is now Michigan (Cannon, 1994, Cannon and Nicholson, 2001, Stein et al., 2015). Figure 3: Extent of rifting associated with the Mid-Continent Rift (K. Schulz, USGS) Some of the largest lava flows on Earth erupted and ponded in the Mid-Continent rift over many centuries (Huber, 1983). During quiet times, red-brown conglomerates and sandstone were deposited between flows in high-energy alluvial fans (Elmore, 1984). The interbedded lava flows and sedimentary layers were normally faulted resulting in a syncline feature that extends from Isle Royale to the Keweenaw Peninsula, now the basin for Lake Superior (Figure 4). Copper was brought to the surface by hydrothermal systems mineralizing permeable layers such as the amygdaloids of lava flow tops and the cracks and crevices of conglomerate rocks (Bornhorst & Barron, 2011, Nicholson et al, 1997, Cannon, 1984). Figure 4: Cartoon depicting interbedded lava flows and minor sedimentary rocks (Huber, 1983. 82 �The Keweenaw is noted for mining nearly 9000 years ago (Martin, 1995, 1999). The valued red metal has since been traded up and down the Mississippi by the Ojibwa caretakers of this landscape and used in ceremony today. From 1845 to 1968, ~11 billion pounds of refined copper were produced from Keweenaw Peninsula mines making it one of the cornerstones of the American economy and the first great metal mining district in the United States (Bornhorst and Barron, 2011, Bornhorst and Lankton, 2009). The region has been extensively mapped and researched since the mid-1800s because of the discovery of copper and the subsequent mining boom. This field trip explores the impacts related to one of the many mining sites in operation from the late 1800s to mid-1900s in the Keweenaw region - Mohawk Mine, a native copper host rock from which some of the Gay stamp sands were derived. Mohawk No. 4 This field stop explores the site of the Mohawk mine, specifically the poor rock pile associated with the Mohawk No. 4 shaft (Figure 5). Rock from this site is the main source for the Gay stamp sands. Following the discovery of copper on the property in 1896 by lumberman Ernest Koch, the Mohawk Mining Company was founded. It was incorporated in 1898 and lasted until 1932 (John Stanton as president, later replaced by Joseph Gay). In 1923 the Mohawk Mining Company took over the neighboring Wolverine Figure 5: A group of Keweenaw youth explore the poor rock Copper Mining Company and the pile at Mohawk No. 4 (photo credit Erika Vye) Michigan Copper Mining Company. In 1934 the company was purchased by the Copper Range Company (Molloy, 2008). The Mohawk Mine had six shafts numbered 1 through 6 running from north to south along what is now US 41. The was a profitable mine paying out over $15 million in shareholder dividends between 1906 and 1932 (Clark, 1978). The No. 4 shaft of the Mohawk Mine was constructed at this site in 1901 and stayed open until the mine closed in 1932 (Figure 6). In the early days of operation, the shaft reached a depth of 501 feet. Technological advances in 1904 led to equipping the No. 1, 2, and 4 shafts with Nordberg Conical Drum Hoists (Figure 7) enabling depths of up to 6000 feet. With this technology, the shaft reached a depth of 900 feet by 1906, 1,175 feet by 1908, and by 1922 a final depth of 2,832 feet. In 1914, the No. 4 shaft was producing between 450 and 500 tons of ore per day. Mining continued from the No. 4 shaft until 1924, with a brief lull in operations until it was reopened in 1926, eventually closing in 1932 (Clark, 1978). 83 �Figure 6: No 4 shaft in Mohawk, MI (photo courtesy of the MTU Archives) Figure 7: Image of Nordberg hoist; the world's largest Nordberg hoist can be visited at the Quincy Mine in Hancock, MI, pictured here (photo courtesy of the MTU Archives) 84 �Ore Deposit and Significant Minerals The Kearsarge lode at Mohawk is a native copper-dominated deposit with minor native silver hosted by basalt lava flows of the Portage Lake Volcanics (about 1.1 billion years old). The strike of the Kearsarge orebody is N45E, dips at 35W, and is 2.9 meters thick, 1,490 meters wide, with a total length of 3,250 meters spanning an area of 250 hectares. The productive part of the fragmental amygdaloidal lode was richer at greater depths than nearer to the surface where the lode consisted of more massive basalt. The Kearsarge lode was the second largest in the district and averaged 6 to 13 feet thick (Butler & Burbank, 1929; White et al, 1953; and USGS, 2005). The fragmental amygdaloid ore of the Mohawk mines is associated with arsenic-rich minerals, more common than in most of the other Keweenaw deposits. The Mohawk mine is well known among mineral collectors for its large occurrence of mohawkite - a mixture of algodonite, domeykite, and copper (Moore, 1971). Mohawkite was first found on the property in 1901, on the first level north of the No. 1 shaft. In addition to copper, the mine also produced a small amount of native silver (Figure 8). Figure 8: Left -Polished mohawkite, a rare mixture of copper and copper arsenides, is named after the Mohawk-Ahmeek area of the Keweenaw (Photo by Robert M. Lavinsky); Right – native copper in amygdaloidal basalt. Geologic features of note include the occurrence of the Mohawkite Fissure that crossed the Kearsarge lode. From 1900-1901, 105 metric tons of mohawkite ore was produced from the Mohawk Mine at the No. 1 shaft. From 1902 through 1925 the No. 1 shaft produced about 117,000 metric tons of refined copper. Additional veins were discovered south of the No. 2 shaft in 1901 resulting in 230,000 pounds of mohawkite (Butler and Burbank, 1929). Mohawkite proved to be challenging in the Keweenaw as smelters were not able to process it on account of the high arsenic levels and the deadly fumes produced. A unique smelter was built in Hackensack Meadows, New Jersey for the specific purpose of processing the ore; it became 85 �operational in 1901. The mohawkite ore contained mostly copper and arsenic, it also contained small amounts of nickel and cobalt, as well as about 20 ounces of silver per ton of ore. (Stevens, 1902) Copper-bearing rocks were transported 11 miles by rail to the town of Gay for milling near the mouth of the Tobacco River on Traverse Bay. This is the next stop on the field trip. STOP 2 – GAY, MI This part of the field trip begins at the Gay Sands sign just outside of the town of Gay. This stop is intended to engage participants in a discussion of how the stamp sands were generated and to learn how Tribal, State, Federal, and academic partnerships are collaborating to mitigate environmental damage and ultimately restore Buffalo Reef to the ecological resource that has sustained both tribal and non-tribal communities for generations. Participants will have an opportunity to walk the stamp sands (accessed just south of the Gay Sands sign): note that at times large trucks are moving through this area; always exercise caution and stay together. Directions: From Mohawk, travel to the town of Gay; follow the Mohawk-Gay Road. When you arrive in Gay, turn right on Main Street, then left on 2nd Street (which becomes the Gay Lac la Belle Rd), park on the right-hand side of the road beside the smokestack and local signage that shares information about the Gay stamp sands (Figure 9). Coordinates: 47.226766, -88.161933 Figure 9: Left - Gay Sands signage; Right – Gay mill smokestack (photo credit Erika Vye) How were the stamp sands created? Liberating the copper from the host rock required a large water source - in this instance, Lake Superior. In 1900, the Wolverine mine and mill opened in Gay. This piqued the interests of New York financiers of the Central, Atlantic, and Baltic mining companies and shortly thereafter the Mohawk mines and mills were opened. The two mills were built in Gay by the Mohawk and Wolverine mining companies enabling the processing of copper ore. A railroad was built to 86 �transport the copper-bearing rocks from Mohawk to Gay; poor rock was left at the mine sites in Mohawk remaining in piles today. A dock was built nearby to import coal to power operations and to load copper on ships to transport to other places (Keweenaw County Historical Society). Note: A QR is included in the resources section of this guide leading to the recently developed Gay Mills Exhibit created by the Keweenaw County Historical Society. An unusual landscape & vastly different soundscape When visiting this site take note of what you hear; try to imagine what the soundscape of this place might have been like in the early 1900s. At that time, the stamps at both mills were running constantly. Imagine the ground shaking with over 70 stamps per minute crushing pieces of rock brought up from underground to free the copper from the matrix. The next step required washing the material to separate the copper and produce a copper-rich mineral concentrate to then be sent to the smelter and formed into copper ingots (Lankton, 2005). The material was brought to the top of the mill with a railway line. To process the copper and create the concentrate, water was pumped from Lake Superior, and the tailings and waste slurry were dumped back into the lake with conveyors. The length continued to grow as the tailings built up along the shoreline (Keweenaw County Historical Society). Figure 10 shows an image of this process on the left, with what remains of this structure today. Figure 11 illustrates the milling process. Figure 10: Left - Conveyor sending tailings slurry back to the lake (Photo credit: MTU Archives) during milling operations in Gay; Right - wood beams left from conveyor structure as sands are continually swept away by wind and currents over time (Photo by Erika Vye) What is happening to the sands? From 1900-1930 the mills flushed over 22.7 million metric tons of tailings to Lake Superior leaving a large bank of black sand, the consistency of kitty litter, along the shore. This finely crushed waste rock has moved, and continues to move, along the southeast shoreline of the Keweenaw Peninsula to Grand Traverse Bay Harbor threatening the nearby Buffalo Reef. Since the 1930s Lake Superior’s fierce storms and strong currents have eroded the stamp sand bank, pushing the tailings into the lake and gradually moving them southward along the shoreline to 87 �the Traverse River. These stamp sands contain high amounts of copper and cover 1,426 acres of shoreline and lakebed. Currently, 30% of the reef has been impacted by stamp sands; modeling predicts that by 2025 stamp sands will impact 60% of the reef. The next stop focuses on the importance of understanding the impacts of these predictions and what is happening for restoration. Before traveling to the last stop we will have lunch at the Gay Park. This site also hosts the Gay Historic School and Museum open from 1 PM to 4 PM on Wednesday and Saturday. 88 �Figure 11: This image describes how copper was processed in the mills - first by gravity and large amounts of water through steam stamps, the rock was funneled into a trommel that sorted and classified the pieces by size. A jig then separated the copper from the mine rock. Next, a series of vibrating Wilfley tables separated the tailings from the small-sized copper. This image was recently modified for a Keweenaw County Historical Society exhibit on the Gay Mills (a link for the exhibit is found in the Resources section of this guide). Original delineation by the Historic American Engineering Record, Heritage Conservation & Recreation Service, Eric M. Hansen, 1978. Modification by David A. Vago, MTU, for the Houghton County Historical Society, 2005; updated 2021. STOP 3 - BIG TRAVERSE BAY & BUFFALO REEF. MI At this site, we will explore how the Gay sands are impacting life and culture in the region, learn about remediation efforts, and how we can share in educating ourselves and others about this environmental justice issue. Directions: From the Gay Park by the Historic School and Museum travel left on Lake Street, then right on 1st Street (becomes the Lake Linden-Gay Rd). Turn left onto Rice Lake Rd, which becomes the Big Traverse Rd. Turn left on the Traverse River Rd, please drive slowly along this road as there are families and small children frequently playing here. This takes approximately 15 min from Gay to Big Traverse. Note that the dock is on the other side of the harbor; to get there simply head back to Big Traverse Rd, then turn left. The dock is about ¼ mile ahead. Coordinates: Big Traverse Bay 47.189857, -88.236644, Dock to board Agassiz 47.190769, 88.237154 What is Buffalo Reef? Buffalo Reef is a natural cobble 1 feature in Lake Superior, located just off the eastern edge of the Keweenaw Peninsula in the U.P. of Michigan (Figure 12). The reef has historically maintained an invaluable spawning habitat for fish species such as lake trout and lake whitefish. Buffalo Reef is estimated to supply 33% of the tribal harvest of lake trout and lake whitefish from Michigan waters of Lake Superior (i.e. 136,375 pounds of whitefish and 61,830 pounds of lake trout average annual yield 2001-2016). Figure 11: Location of Buffalo Reef, also not the locations of Additionally, if the stamp sands both Field Trip Stop 2 and 3, marked by stars. migrate south of Grand Traverse Harbor, they will threaten the undisturbed native sand that serves as habitat for juvenile whitefish (Kerfoot et al, 2012 & 2021). Juvenile whitefish produced on Buffalo Reef migrate south of 89 �Grand Traverse Harbor to sandy, shallow-water habitat where they feed before migrating to the deeper water they inhabit as adults. Without this habitat, whitefish recruitment in the vicinity of Buffalo Reef would be greatly diminished. Whitefish provide the bulk of the commercial, cultural, and spiritual value for the tribal communities that use this resource (BRTF, 2024). How are the stamp sands impacting Buffalo Reef? Most of the cobbles are glacial rocks, scattered around the Jacobsville sandstone bedrock highs of the reef. During spawning, fish drop eggs into the crevices between rocks. Cobbles are coated with a natural organic film, the basis of a food web for feeding fish. Stamp sands move into the northern cobble field of Buffalo Reef burying cobbles and killing living communities on rocks along the leading edge of the sands. (Figure 13). These impacts to fish habitat create a risk for a decline in commercially and culturally important fish keynote species. Importantly, a decline in species impacts the ability to exercise treaty rights. The sands also have high enough copper concentration (about two-eighths of a percent) to be toxic to fish and other organisms that live in Grand Traverse Bay. There are additional impacts to human health through fish consumption. Other concerns include the impacts of the copper-toxic sands on coastal wetlands medicine chests to the KBIC - as the material migrates up streams thereby affecting plant and amphibian habitat (Kerfoot et al, 2012, 2021, in prep). The sands also impact the aesthetics and sense of place inspired by the landscape; the aerial photos in Figure 14 illustrate the movement of the stamp sands inundating white sand Figure 12: Healthy reef habitat progressively beaches along the south shore of the inundated with stamp sands until covered completely Keweenaw Peninsula. The stamp sands also (Photos by Charlie Kerfoot) create challenges for people coming and going from the Big Traverse Bay Harbor; the sands have repeatedly blocked the harbor stopping boat traffic. For example, in 2015, Big Traverse Bay Harbor was dredged removing 4,500 cubic yards of stamp sand. Dredging happens repeatedly here, without this continued maintenance there is a great risk of harming the region’s fishing industry. As such, local stakeholders are working with the Army Corps of Engineers on a long-term solution. 90 �Figure 13 Figure 14: Aerial views of the entry of Traverse River into Lake Superior. The regular movement of stamp sands over the top of the breakwater requires annual or biennial dredging to keep the small Traverse River Harbor, home of a commercial fishing fleet, open to entry. Note the black sands on the left to the north of the channel compared to the white (natural) sands to the right. Left photo credit: Neil Harri; Right photo credit: Charlie Kerfoot Impacts on Fish Sovereignty Buffalo Reef lies within the ceded-territory homelands established by the Treaty of 1842 where 11 Lake Superior Bands of Ojibwa retain rights and responsibilities to harvest fish from the Michigan waters of Lake Superior. Buffalo Reef has always been considered a culturally significant harvesting ground for local communities. Fishing is the strand of the cultural core that ties history to the present day and the future; it is a vital part of the foundation for cultural beliefs and values, traditional lifeways, and even individual identity (Gagnon, 2018; KBIC, 2017). The KBIC identify as fishing people and have always had a strong focus on cultivating and protecting relationships within ecosystems that support healthy food sovereignty initiatives within the community (Figure 15). The Anishinaabeg teachings and ways of life emphasize that landscapes and waters are an intricate system of diverse relationships, and interconnected rights and responsibilities rooted in an acknowledgment and understanding that humans and nature are relatives. These sentient elements have been honored in ceremony since time immemorial and these important teachings Figure 14: Fresh catch on Keweenaw Bay 91 �and traditions continue today. These relationships are founded in the long-standing nation-tonation agreements between the Anishinaabe Ojibwa and all orders of creation from rock, water, fire, and wind; the physical world of sun, stars, moon and earth; plant beings; animal beings; and human beings - all rooted in the First Treaty with Gichi Manidoo (the Creator), also known as Sacred Law, Original Instructions, and Natural Law (Johnston, 1976; Keweenaw Bay Indian Community, 2021). Today, Tribal, State, Federal, and Academic partnerships are combining efforts to mitigate damages and ultimately restore Buffalo Reef as the ecological resource that has sustained both tribal and non-tribal communities for generations. Buffalo Reef Remediation Efforts This is a complex environmental justice issue requiring local, regional, and state stakeholders and rights holders to work together on research and restoration efforts for Buffalo Reef. It was tribal fishermen who first alerted our communities of this issue, underscoring the vital need to bridge knowledges for a holistic understanding of what is happening within our landscape and to recognize the gifts that different knowledges share. Research over the past years has included: a) modeling wave action, currents, and how the sands are migrating across the coastal shelf and along the shoreline; b) use of LiDAR and sonar assessments to understand the extent of the sands underneath water; c) submerging remotely operated underwater vehicles to take pictures of the boulder and cobble fields to assess the quality of the remaining fish habitat; d) researching environmental impacts on the benthic and fish communities; e) exploring considerations for building a revetment to hold back stamp sands from further entering the bay; f) conducting archaeology to understand industrial heritage, mining legacies and Native American uses of the area prior to colonization; and f) and meeting with community members to understand social and economic impacts on the communities involved. After years of observation, scientific study, and dredging at Big Traverse, the EPA funded a feasibility study for a long-term solution. In 2017 the USEPA endorsed the formation of a Buffalo Reef Task Force (BRTF) comprised of multiple state, federal, and tribal agencies. In addition, several academic institutions and private entities have joined the team, recognizing that this issue is larger than any single entity can accomplish on its own. The Buffalo Reef Final Alternatives Analysis Report (Report) was recently on public notice from January 30 to March 1, 2024. This report provides an overview of efforts made by the BRTF “in 2019, the BRTF identified 13 potential alternatives to remediate the stamp sands and restore the habitat. The alternatives were screened based on constructability, operation and maintenance requirements, environmental impact, ecological sustainability, initial costs and legacy costs, regulatory requirements, public input and opinion, time needed for implementation, impact to local populations, and potential for beneficial use of the stamp sands”. Three alternatives were selected for further consideration that included the dredging and disposal of stamp sands in the following locations: 1) White Pine Mine, a closed tailings basin; 2) a Lakeside Placement site; and 3) an Upland Placement site. In 2022, a public meeting was held to discuss the one alternative that would be feasible - the removal of the stamp sands to a regulated and lined landfill to be constructed near Gay. These remediation efforts require land ownership, 92 �both at the shore for a proposed jetty to impede migration of the stamp sands and an upland placement area to move the mining waste. The Report indicates that the BRTF’s preferred remedial alternative and “Potential Plan” identifies the “Upland Placement Alternative” as the BRTF’s preferred remedial alternative (Figure 16). The study compares three scales of implementation for the Upland Alternative are compared in the report. “The scale of implementation differs based on the volume of stamp sands to be dredged and disposed of during the implementation phase. Additionally, the study provides an assessment of the impact the volume of stamp sands removed during implementation has on the cost and duration of the operation, maintenance, repair, replacement, and rehabilitation phase.” Figure 16: Upland Placement Alternative: Small, Medium, All Dredging Scales represented (Left to Right). S Shoreline – South Shoreline; M Shoreline – Middle Shoreline; and N Shoreline – North Shoreline. The Upland Placement footprint required to dispose of the volume of stamp sands removed during project implementation varies per dredging scale. In the placement site measure, the solid white line represents the estimated placement site footprint to contain the stamp sands dredged during the project implementation phase. The dashed white line represents the estimated placement site footprint to contain stamp sands dredged during the OMRR&R project phase (sourced from page ES-5 of the Buffalo Reef Final Alternatives Analysis Report). RETURN TO HOUGHTON Billion-year-old geologic processes brought copper to the surface where people could find it - a gift from the deep Earth. Relationships with the red metal have varied over millennia resulting in different values and actions within our shared lands and waters. Further efforts are required by all stakeholders and rights holders to communicate and elevate the importance of this environmental issue with decision-makers, government officials, and environmental advocacy groups. This field trip is meant to deepen our understanding of these relationships and to reflect on the following questions: What are the relationships between people, Earth, and Buffalo Reef? Why does this matter? How have people impacted Buffalo Reef in the past, and how will they in the future? What can you do to help with the remediation efforts of this place? 93 �Directions: From Big Traverse Bay Harbor, head north on S Big Traverse Rd and continue along Rice Lake Rd. Take M-26 S to US-41 S in Houghton. EDUCATIONAL RESOURCES Access the following websites and videos by following the QR codes. Saving Buffalo Reef Website (DNR) Saving Buffalo Reef Video (GLIFWC) Buffalo Reef Restoration (Great Lakes Now, Ep. 1006 Segment 3) Gay Milling at Gay A Lake Superior Story - REFERENCES CITED Bornhorst, T. J. and Barron, R. J. (2011). Copper deposits of the western Upper Peninsula of Michigan. Geologic Society of America, Field Guide, 24, 83-99. Bornhorst, T. J. and Lankton, T. J. (2009). Copper Mining: A Billion Years of Geologic and Human History, in Schaetzl, R., Darden, J. and Brandt, D. (eds.) Michigan Geography and Geology. United States of America: Pearson Custom Publishing, 150-173. Brocx, M. and Semeniuk, V. (2007). Geoheritage and geoconservation - history, definition, scope and scale. Journal of the Royal Society of Western Australia, 90, 53-87. Buffalo Reef Task Force (2024). DRAFT Buffalo Reef – Final Alternatives Analysis. Retrieved from: https://www.michigan.gov/dnr/-/media/Project/Websites/dnr/Documents/Fisheries/BuffaloReef/00DRAFT-Buffalo-Reef-Main-ReportJAN2024.pdf?rev=1ebf68cee9ae428881d8d6991b4d7471&hash=48BD2B6EA77CD5C430FD63757863 2B28 94 �Butler, B.S. & Burbank, W.S. (1929). The Copper Deposits of Michigan. USGS Professional Paper No. 144 Cannon, W. F. (1994). Closing of the Midcontinent Rift: a far-field effect of Grenvillian compression. Geology 22, pp. 155-158. Cannon, W. F. and Nicholson, S. W. (2001). Geologic Map of the Keweenaw and Adjacent Area Michigan 1:100,000. USGS Map I-2696 Clarke, D. (1978). Copper mines of Keweenaw; no. 12: Mohawk Mining Company. ASIN B0066RONSQ. Elmore, R. D. (1984). The Copper Harbor Conglomerate: A late Precambrian fining-upward alluvial fan sequence in northern Michigan. Geological Society of America Bulletin 95, pp. 610-617. Gagnon (2018). A Tribute to Our Fisherman -Minaadowenjigaaziwaat Gidoo giigoonkeninii-minaanik. Retrieved from: https://nrd.kbicnsn.gov/sites/default/files/A%20Tribute%20to%20our%20Fishermen%20-%20handout.pdf Geological Society of America (2017). GSA Position Statement: Geoheritage. Retrieved from: https://www.geosociety.org/documents/gsa/positions/pos20_Geoheritage.pdf. Huber, N. K. (1983). The geologic story of Isle Royale National Park. United States Geologic Survey Bulletin (1309), pp. 66. Johnston, B. (1976). Ojibway Heritage. Toronto: McClelland and Stewart. Kerfoot C., Yousef F., Green A., Regis R., Shuchman R., Brooks N., Sayers M., Sabol B., and Graves M. (2012). LiDAR (Light Detection and Ranging) and multispectral studies of disturbed Lake Superior coastal environments. Limnol. Oceanogr. 57: 749–771. https://doi.org/10.4319/lo.2012.57.3.0749 Kerfoot C., Hobmeier M., Swain G., Regis R., Raman V., Brooks C., Grimm A., Cook C., Shuchman R., and Reif M. (2021). Coastal Remote Sensing: Merging Physical, Chemical, and Biological Data as Tailings Drift onto Buffalo Reef, Lake Superior. Remote Sensing. 2021, 13 (13), 2434. https://doi.org/10.3390/rs13132434 Kerfoot. C., Swain, G., Regis, R., Raman, V.K., Brooks, C., Cook, C and Reif, M. (in prep). Coastal Copper Tailings Dispersal: 3D Mapping and Shoreline Impacts, Particle Migration, Leaching, And Toxicity. Remote Sensing, 2024, 16. Keweenaw Bay Indian Community (2017). Testimony of Warren C. Swartz Jr. Before the Senate Committee on Commerce, Science & Transportation. Retrieved from: https://www.michigan.gov/dnr//media/Project/Websites/dnr/Documents/Fisheries/BuffaloReef/TribalTestimonyADA1.pdf?rev=2c9d195 864e44557bc3bc315ed93e4a6 Keweenaw Bay Indian Community (2021). Who We Are. Keweenaw Bay Indian Community Natural Resources Department. Retrieved from http://nrd.kbic-nsn.gov/about-us. Keweenaw County Historical Society (2022). Copper Milling at Gay - A Lake Superior Story. Retrieved from: https://www.keweenawhistory.org/Copper-Milling-At-Gay 95 �Lankton, L. (2005). Keweenaw National Historical Park Historic Resource Study. Prepared for the National Park Service, United States Department of the Interior. Retrieved from: http://npshistory.com/publications/kewe/hrs.pdf Martin, S. R. (1995). Michigan prehistory facts: The state of our knowledge about ancient copper mining in Michigan. The Michigan Archaeologist, 41(2-3), pp. 119-138. Martin, S. R. (1999) Wonderful Power: The Story of Ancient Copper Working in the Lake Superior Basin Wayne State University Press, Detroit, MI, p. 284 Molloy, Lawrence J. (2008). A Guide to Michigan's Historic Keweenaw Copper District: Photographs, Maps, and Tours of the Keweenaw, Past and Present. Hubbell, Michigan: Great Lakes GeoScience. p. 66. ISBN 978-0-979-1772-1-7. Moore, P. (1971). Copper-Nickel Arsenides of the Mohawk No. 2 Mine, Mohawk, Keweenaw Co., Michigan. American Mineralogist, Volume 56, pages 1319-1331. National Park Service and American Geosciences Institute (2015). America’s Geologic Heritage: An Invitation to Leadership. NPS 999/129325. National Park Service, Denver, Colorado. Nicholson, S.W., Shirey, S.B., Schulz, K.J., and Green, J.C. (1997). Rift-wide correlation of 1.1 Ga Midcontinent Rift System basalts; implications for multiple mantle sources during rift development. Can. J. Earth Sci., v. 34, pp. 504-520. Reynard, E. and Brilha, J. (2017). Geoheritage: Assessment, protection, and management. Elsevier, ISBN: 9780128095317. Rose, W.I. and Vye, E. with Martin, V. (2017). How the Rock Connects Us: A Geoheritage Guide to Michigan’s Keweenaw Peninsula and Isle Royale. Isle Royale and Keweenaw Parks Association, ISBN 9780935289213. Stein, C. A., Kley, J., Stein, S., Hindle, D. and Keller, G. R. (2015). North America’s Midcontinent Rift: When Rift met LIP. Geosphere, 11(5), pp. 1607-1616. Stevens, Horace J. (1902). The Copper Handbook: A Manual of the Copper Industry of the United States and Foreign Countries. Vol. II. Houghton, Michigan: Mines Publications. US Geological Survey (USGS) (2005). Mineral Resources Data System (MRDS). Vye, E. (2016). Geoheritage of the Keweenaw Peninsula (Doctoral dissertation). Michigan Technological University. White, W.S., Cornwall, H.R., & Swanson, R.W. (1953). Bedrock Geology of The Ahmeek Quadrangle. USGS Map GQ-27, Scale 1:24000 96 �Field Trip 4 Keweenaw Fault System Geometry and Kinematics: Clues to Its Nature and Origin James M. DeGraff, Katherine M. Langfield Department of Geological and Mining Engineering and Sciences Daniel J. Lizzadro-McPherson Geospatial Research Facility, Great Lakes Research Center Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931 Objectives This field trip is designed to demonstrate four fundamental attributes of the Keweenaw fault system along the Keweenaw Peninsula: (1) geometry of the fault network and individual faults; (2) style of deformation of hanging-wall and footwall strata; (3) zonation of deformed rocks in fault zones; and (3) kinematic slip indicators on individual faults. Stops in this guide were chosen and grouped into four areas to illustrate the segmented geometry of the Keweenaw fault system and to point out fault blocks defined by intersections between faults in three main directions. Kinematic slip indicators observed at some stops manifest a preponderance of oblique slip with components of dextral strike slip and northwest-side-up reverse slip. Based on this information and fault network geometry, the overall nature of the fault system is interpreted to be transpressional and consistent with forcing by the Grenville orogeny. Introduction The Keweenaw fault extends southwest from the tip of the Keweenaw Peninsula in Michigan’s Upper Peninsula to near Ashland, Wisconsin (Fig. 1), a distance of about 250 kilometers. It is one of two faults along the south shore of western Lake Superior with reverse movement that juxtaposes Mesoproterozoic volcanic layers against Meso-Neoproterozoic clastic sedimentary strata, the other being the Douglas fault in Wisconsin and Minnesota. Irving and Chamberlin (1885) first established the existence of the Keweenaw fault by combining detailed observations in outcrops and trenches with astute reasoning to settle a long-running debate about the nature of the contact between volcanic layers to the northwest and sandstone strata to the southeast. Prior to their work, Wadsworth (1884) had argued that the sandstone was older and dipped northwest beneath the volcanic layers based on their similar dip at some locations along their contact. Afterward, this idea was resurrected intermittently until the mid-1970s, when deep drilling for oil and gas established the younger age of sandstone in the Keweenaw fault’s footwall once and for all. 97 �Figure 1: Mesoproterozoic Keweenawan Supergroup of the Midcontinent Rift System (inset map modified from Stein) in the Lake Superior region. For sources of geologic units and faults, see DeGraff and Carter (2023). KF‒Keweenaw fault; LOF‒Lake Owen fault, DF‒Douglas fault; IRF‒ Isle Royale fault, MIF‒Michipicoten Island fault, TF‒Thiel fault; MF‒Munising fault (provisionally named). Red “U” on upthrown sides of faults. Tectonic Evolution The Keweenaw fault and related major faults along the southern edge of the western Lake Superior basin cut rocks of the Midcontinent Rift System (Fig. 1) and have slipped several kilometers in a reverse sense (Cannon and Nicholson, 2000, 2001; DeGraff and Carter, 2023). Ideas to explain the origin and evolution of the Keweenaw fault differ in relation to the roles of rifting and orogenesis on fault initiation and movement over time. The Midcontinent Rift System (MRS), formed by extension of Laurentian crust in Mesoproterozoic time, was accompanied by voluminous mafic volcanism followed by a period of crustal sag with siliciclastic sediment filling the resulting basin (Cannon et al., 1989; Cannon, 1992; Hinze et al., 1990; Stein et al., 2015). Woodruff et al. (2020) defined stages of magmatism and tectonism in MRS evolution as follows: (1) Plateau Stage with widespread volcanism and distributed minor extension (c.1112 to c.1105 Ma); (2) Rift Stage when volcanism and extension reached maximum intensity along a central subsiding rift basin (c.1102 to c.1090 Ma); (3) Late-Rift Stage with declining volcanism transitioning to sedimentary infill of the rift basin (c.1090 to c.1083 Ma); and (4) Post-Rift Stage with sedimentary infill of a sag basin (c.1083 to c. 1060 Ma). Prior to recognition of the MRS, the Keweenaw fault was interpreted as a thrust that formed during an unspecified compressional event following eruption of lava flows and deposition of siliciclastic strata in the Lake Superior basin (Irving and Chamberlin, 1885; Butler and Burbank, 1929, White, 1968). After recognition of the rift, a new model for formation and evolution of the Keweenaw fault and others around the Lake Superior basin proposed that they formed as normal faults during stages 1 and 2, and then were inverted as reverse faults by post-rift Grenville compression (Cannon et al., 1989; Hinze et al., 1990; Cannon, 1994), i.e. the fifth 98 �Compressional Stage of Woodruff et al. (2020). A recent model, discussed below in relation to the inversion model, is that the Keweenaw fault formed by thrusting during post-rift Grenville compression and did not exist as a normal fault during MRS extension or sag (DeGraff and Carter, 2023). In some ways, the latter model reverts to earlier ideas about the Keweenaw fault. Ideas about the timing of reverse slip on the Keweenaw fault have evolved from the original ideas of Irving and Chamberlin (1885), who inferred two episodes of movement – one prior to deposition of the footwall sandstone and a second afterward. Early thrusting elevated the region northwest of the fault, creating a highland of mafic volcanic layers that shed debris into the tectonic basin southeast of the fault. A second episode of thrusting occurred after most of the sandstone deposition based on deformed footwall strata with steep to overturned attitudes toward the southeast. Later workers have suggested quasi-continuous fault slip that began before Jacobsville deposition and continued during most or all of its deposition (Cannon and Nicholson, 2000), and others have argued for a late reactivation of the fault system during one or more phases of the Appalachian orogeny (Craddock et al., 1997). Stratigraphy Extension during MRS evolution followed by compression during the Grenville Orogeny produced a broad syncline with strata dipping toward an axis beneath Lake Superior (Fig. 1). The related rocks in the vicinity of the Keweenaw Peninsula are well described by Cannon and Nicholson (2000, 2001), whose stratigraphic column and nomenclature are adopted here (Fig. 2). Here, the main geologic units of the Keweenawan Supergroup are from oldest to youngest: (1) Siemens Creek Volcanics and Kallander Creek Volcanics (Powder Mill Group) with basaltic to andesitic lava flows; (2) Portage Lake Volcanics (Bergland Group) mostly composed of basaltic to andesitic flows with lesser conglomerate and sandstone layers; (3) Oronto Group strata beginning with the Copper Harbor Conglomerate and locally interbedded basalt-andesite flows, continuing with the Nonesuch Formation composed of siltstone and shale, and ending with the Freda Sandstone composed of lithic sandstone and siltstone; and (4) Jacobsville Sandstone mostly composed of quartzose to subarkosic sandstone and generally correlated with the Bayfield Group in Wisconsin, USA. Archean granite and gneiss of the Superior craton, with a Paleoproterozoic cover of graywacke and slate (Michigamme Formation), lie beneath the Keweenawan rocks south of Keweenaw Bay. The following descriptions focus on the two geologic units that are juxtaposed along the Keweenaw fault system, namely the Portage Lake Volcanics in the hanging wall and the younger Jacobsville Sandstone in the footwall (Fig. 3). Both units are stratified and have compositional and textural variations between layers that likely influenced their mechanical behavior, which in turn influenced fault initiation and propagation. A more complete description of other Precambrian units along the Keweenaw Peninsula is provided elsewhere in this field trip volume (see Field Trip 1 in this volume). 99 �Figure 2: Left. Regional stratigraphy following Cannon and Nicholson (2001). For sources of radiometric ages in red italics, see DeGraff and Carter (2023). Right. Stratigraphy of the Portage Lake Volcanics from USGS bedrock geology maps of the Keweenaw Peninsula. Intense green units are major lava flows: pcc‒Copper City; pgf‒Gratiot; psc‒Scales Creek; pk‒Kearsarge; pg‒ Greenstone. Letter codes on left side are sedimentary layers, except for one: pu‒unnamed conglomerate (cgl), pbc-pl‒Baltic-Lac La Belle cgl; ps‒St. Louis cgl; pb‒Bohemia cgl; poc‒Old Colony sandstone; pw‒Wolverine sandstone; pkc‒Kingston cgl; pc‒Calumet and Hecla cgl; ph‒ Houghton cgl; pa‒Allouez cgl; pp‒Pewabic West cgl; paf‒ashbed flow top; phc‒Hancock cgl. Bold red bars mark units with layer-parallel slip observed in mines and trenches. Letters on right side indicate where slip occurred (T‒top; B‒bottom; A‒top and bottom). The Portage Lake Volcanics (PLV) exposed along the Keweenaw Peninsula is truncated on the southeast by the Keweenaw fault system, leaving an undetermined thickness of PLV in the footwall beneath Jacobsville Sandstone. The PLV section in the fault’s hanging wall is estimated to be 3000 to 5000 m thick and to contain about 300 flows (Cannon and Nicholson, 2000). It mostly consists of subaerial basaltic flows with less abundant andesitic flows, rhyolitic to dacitic extrusive domes, and associated pyroclastic layers (Butler and Burbank, 1929; White, 1968; Cannon and Nicholson, 2000, 2001). Six named basalt flows are sufficiently traceable along strike due to their 100 �great thickness and distinctive texture to serve as marker units (Fig. 2). The thickest of these, the Greenstone flow, in the upper part of the section is traceable for 80 kilometers in outcrop and drill holes and has a precise radiometric age date of 1091.6 ± 1.3 Ma (Swanson-Hysell et al., 2019; cf. 1094.0 ± 1.5 Ma, Davis and Paces, 1990). The next thickest flow, the Copper City flow, in the lower part of the section is traceable for 25 kilometers and is precisely dated at 1093.4 ± 1.4 Ma (Swanson-Hysell et al., 2019; cf. 1096.2 ± 1.8 Ma of Davis and Paces, 1990). About 3% of the PLV section consists of conglomerate and sandstone deposited between some of the lava flows (Butler and Burbank, 1929; White, 1952; Merk and Jirsa, 1982). Eleven named interflow units (Fig. 2) are traceable along strike for considerable distances up to 90 km (Bornhorst and Barron, 2011), and thus are useful for correlation. Interflow sedimentary layers mostly consist of clast-supported, pebble-to-cobble conglomerate with a gravelly to sandy matrix, which is well indurated and tends to form prominent strike ridges. Conglomerate layers are coarsely bedded and poorly stratified, except where sandy lenses or conglomeratic sandstone occur. Less common are sandstone and siltstone that may occur as interbeds within a conglomerate layer or locally may make up an entire interflow layer. Interflow sedimentary layers may be up to 40 m thick and locally may pinch to near zero thickness (Butler and Burbank, 1929; Merk and Jirsa, 1982; White, 1968). Clasts and matrix grains in the interflow layers were derived from volcanic rocks of the rift basin, with felsic material generally being far more abundant than expected from the small proportion of felsic volcanic rocks that make up the PLV section (< 1% according to Nicholson, 1992). These so-called felsic conglomerates may occur by themselves or in association with mafic conglomerates, in which case the felsic layer tends to lie atop the mafic layer lying atop the underlying lava flow (Butler and Burbank, 1929). Stops of this field trip are all within the lower part of the PLV section exposed in the fault’s hanging wall. For the first half of the trip, the thick Copper City flow is an important stratigraphic reference that be traced for 25 km along strike. Another important stratigraphic reference with a greater strike extent is the St. Louis conglomerate (#6 on USGS bedrock geology maps), which lies at the base of the Copper City flow in many places but elsewhere is separated from it by a thinner basalt flow. For the second half of the trip, the key stratigraphic reference is the Lac La Belle conglomerate (#3 on USGS maps), which lies 580 m to 1050 m stratigraphically below the St. Louis conglomerate. Other lava flows and interflow sedimentary layers in the lower part of the PLV have not yet been correlated well enough to be useful as regional stratigraphic references. A few intrusions cut the lowermost PLV layers along the Keweenaw Peninsula, such as the Mt. Bohemia syenodiorite stock (Cornwall, 1954a) and scattered dikes largely of intermediate composition (Robertson, 1975). Jacobsville Sandstone (JS) in the Keweenaw fault’s footwall fills a sub-basin that extends southeast beneath Keweenaw Bay to an onlap with Paleoproterozoic metasedimentary rocks, northeast to near Stannard Rock, and southwest to Lake Gogebic (Fig. 1). The unit may reach to 23 kilometers in depth based on gravity modeling and early seismic reflection data (Bacon, 1966; Aho, 1969; Kalliokoski, 1982). An exploratory drill hole (Mayflower #41) through the fault near Calumet penetrated JS to a total depth of 803 m (White, 1985), and a deeper drill hole 8.6 km southeast of the fault (Rice Lake #1) reached a total depth in sandstone at 1106 m (Keweenaw NHP, 2016). Regionally correlated units are not well defined for the unit because of lateral facies variability and lack of marker beds. However, Hamblin (1958) recognized the following 101 �succession: (1) basal “conglomerate facies” with locally-sourced clasts deposited adjacent to topographic highs; (2) “lenticular sandstone” facies dominated by planar and trough cross-bedding; (3) “massive sandstone” facies with laterally persistent layers and occasional planar cross-bedding; and (4) “red siltstone” facies with thinly bedded, fine-grained, siliciclastic layers and local trough cross-bedding. Based on this facies succession, Hamblin (1958) interpreted Jacobsville depositional environments as transitioning from (1) alluvial-fluvial at the base, (2) to dominantly fluvial in the lower cross-bedded part of the unit, (3) to dominantly lacustrine in the upper massive part of the unit, and (4) to variably lacustrine and fluvial in the uppermost thinly bedded part of the unit. He inferred that the general sequence of facies and depositional environments was time transgressive, representing both a vertical time sequence at any given point as well as a basinward change of depositional environment at any given time. The JS section along the Keweenaw Peninsula consists of siliciclastic strata that generally conform to the facies of Hamblin (1958) but with some variations. Proximal to the fault system, the lower parts of exposed sections commonly have brownish conglomerate layers consisting of subangular volcanic clasts chaotically dispersed in a poorly indurated muddy matrix, and interbedded with soft, fissile, reddish-brown siltstone and shale (Irving and Chamberlin, 1885; Hamblin, 1958; DeGraff, 1976; Brojanigo, 1984). In such sections with muddy conglomerate and shaly strata, interbeds of whitish to orangish to pinkish, fine- to medium-grained sandstone occur as isolated thin beds near the base and become more abundant and thicker upward in the section until muddy strata are rare. The quartzose to subarkosic sandstones are locally conglomeratic and often crossbedded, which is typical of most JS strata exposed away from the fault system to the southeast. Sand-prone JS strata also occur near the fault system where the muddy conglomerate and shaly facies is absent. Brojanigo (1984) interpreted the muddy conglomerate layers as debris flows derived from an upland of PLV rocks to the northwest, and the more mature sandy strata as fluvial deposits derived from older quartzo-feldspathic rocks to the south. We interpret this bimodal alluvial-fluvial assemblage to be basal Jacobsville, whose PLV-derived detritus is evidence of prior reverse slip on the Keweenaw fault system that elevated the northwest hanging wall and allowed volcanics there to be weathered, eroded, and transported southeastward into the JS sub-basin. The time span of JS deposition has been difficult to determine due to a lack of fossils and igneous rocks interbedded with or crosscutting Jacobsville strata (Kalliokoski, 1982). Deposition probably did not occur prior to reverse slip on the Keweenaw fault system (KFS) because conglomerate low in the JS section near the KFS contains PLV detritus sourced from an elevated region to the northwest (Brojanigo, 1984; Cannon and Nicholson, 2000). The earliest slip on the fault system is generally taken to be ~1060 Ma based on Rb-Sr ages of gangue minerals in fractures associated with native copper mineralization of the Keweenaw Peninsula (Bornhorst et al., 1988) and reset biotite ages in Archean granite gneiss uplifted along related faults west of Lake Gogebic (Cannon et al., 1993). Later reverse slip on the Keweenaw fault that deformed adjacent Jacobsville strata is relatively well constrained at 985 ± 30 Ma by a U-Pb date for late-kinematic vein calcite in the fault zone, in combination with a maximum depositional age of ~993 Ma for deformed strata high in the Jacobsville section (Hodgin et al., 2022). Most Jacobsville deposition probably occurred in the period 1060-985 Ma, although somewhat younger layers could have been deposited in the region (Craddock et al., 2013; Malone et al., 2016). 102 �Keweenaw Fault System – Key Observations and Concepts The Keweenaw fault has been described as a strike fault because of the general parallelism between its map trace and PLV layers in its hanging wall (Fig. 3). Butler and Burbank (1929) also noted a remarkable similarity in dip of hanging-wall layers and the fault, such that PLV layers change dip along strike by about the same amount and in the same sense as changes in the underlying fault surface. Where differences in dip exist, the fault cuts 5° to 17° more steeply and upward across PLV layers in a southeasterly direction, i.e. in the thrusting direction (DeGraff and Carter, 2023). Related to these surface observations, Hubbard (1898) had systematically catalogued in mine workings many layer-parallel faults along contacts between volcanic and interflow sedimentary layers (Fig. 2). These bedding-plane faults are manifested by fault gouge, polished surfaces, breccia with alteration, and secondary mineralization in fractured zones up to 8 m thick (65% ≤ 1.5 m). Hubbard’s observations indicate that PLV layer boundaries were relatively weak and became detached during one or more deformation episodes. Furthermore, his observations along the Keweenaw Peninsula mining district northeast of Portage Lake document layer-parallel slip throughout the PLV section. The surface and subsurface evidence of layer-parallel slip within the PLV section is consistent with knowledge about PLV stratigraphy. The lava flows have variations in texture and mineralogy across their thickness that likely influence their mechanical properties. Massive holocrystalline flow interiors grade into margins with finer grain size and abundant vesicles and amygdules, often arranged in bands parallel to the contacts. Flow tops have the highest abundance of amygdules, are commonly brecciated, and often have been modified by weathering between eruptions or by mineralizing fluids that later migrated along these once-permeable zones (Butler and Burbank, 1929; Stoiber and Davidson, 1959; White, 1968; Bornhorst, 1997). Where one flow lies directly atop another, a significant mechanical contrast exists across their comparatively weak contact. Contrasts in layer characteristics and properties are even more significant at contacts between interflow sedimentary layers and lava flows. Altogether, the evidence just summarized implies a detached style of formation for the Keweenaw fault and related splays (DeGraff and Carter, 2023). Although the Keweenaw fault has been described as a high-angle reverse fault, its northwesterly dip at the surface varies from as low as 20° up to 70° (Butler and Burbank, 1929). Cross-section construction along a transect with abundant constraining data from mining operations and surface mapping implies that the fault is nearly horizontal northwest of its surface trace at Hungarian Falls (DeGraff and Carter, 2023, their Fig. 6), one of the stops on this trip (see Stop 2-1, Fig. 8). Several cross-sections on USGS bedrock geology map sheets from the 1950s show the fault dipping ≤ 25° or show nearly horizontal PLV layers near surface that imply a similarly-dipping fault below (e.g., Laurium, Ahmeek, Mohawk, and Bruneau Creek quadrangles). Thus, the fault and overlying PLV strata dip between nearly horizontal up to 25° NW at several locations and, there, the hanging-wall PLV block has overridden footwall JS strata by as much as 2.5 kilometers. Between 2017 and 2022, three mapping projects funded by the USGS EDMAP program have refined fault and stratal geometries along the Keweenaw fault, and the results indicate that what was largely considered to be a single fault is better characterized as a fault system (Tyrrell, 2019; Mueller, 2021; Lizzadro-McPherson, 2023; Gamet, 2023, Langfield, 2024). The Keweenaw fault 103 �system (KFS), as used here, refers to an ensemble of fault segments whose collective motion has a significant component of dextral strike slip in addition to the long-recognized component of reverse slip with northwest side up. The fault system consists of segments that define three main directional sets: (1) a dominant set that defines the trend of the fault system and locally separates more steeply dipping PLV layers to the northwest from less steeply dipping PLV layers to the southeast; (2) splay faults that are angled 15-30° clockwise from set 1; and (3) connector faults angled 35-75° counterclockwise from set 1 that join footwall splays to the main fault trend (Fig. 3). The three directional fault sets maintain these angular relationships among each other as the curved KFS changes direction from a 35° azimuth near Houghton to a 95° azimuth near the tip of the peninsula. The interconnected nature of the three fault sets defines fault blocks with long dimensions parallel to the local trend of the KFS. Fault-slip indicators measured on ~400 fault surfaces show that the collective ratio of reverse slip to dextral slip on the KFS varies from 1:1 near Houghton to 1:2 or more near the tip of the peninsula. In other words, strike slip is twice as large as dip slip on the KFS near the tip of the peninsula where fault azimuth is 60° clockwise from its azimuth near Houghton (Tyrrell, 2019; Mueller, 2021; Lizzadro-McPherson, 2023; Gamet, 2023; Langfield, 2024). This change in the ratio of strike slip to dip slip is expected as the fault’s azimuth approaches the estimated 105° azimuth of maximum compressive stress that is attributed to the Grenville orogeny. The leftstepping arrangement of footwall splay faults and the fault blocks they help to define is consistent with a transpressional fault system as indicated by the fault-slip data. The relatively short, north- to northeast-trending, connector faults (set 3) are associated with similar-trending fold axes at relatively high angles to the estimated maximum compression direction and shortening direction. The connector faults are inferred to have mostly reverse slip with west side up and to accommodate northeast to east transport of footwall fault blocks associated with dextral transpressional slip of the entire KFS. Why concern ourselves with the geometric details and timing of slip on the KFS? From a science perspective, the arrangement of fault segments in the system along with fault-slip indicators like slickenlines provide clues to the mechanics of fault initiation and kinematics of fault slip. Improved understanding of these aspects of the KFS should apply to similar faults around Lake Superior and help to understand their causative tectonic events and stress regimes. From a practical perspective, faulting along the Keweenaw Peninsula and on strike to the southwest provided pathways for upward migration of mineralizing fluids (Bornhorst, 1997), leading to copper deposits that once supported a thriving mining industry and are still prospective today. Native copper is commonly found along major and minor faults in the mining district. The Hancock fault cutting the Quincy and Hancock mines is an example of a major fault with copper mineralization concentrated in its hanging wall. Another example is the Allouez Gap fault that bisects the Kearsarge flow-top copper deposit, the largest of this type in the district (Bornhorst, 1997). Faults likely provided pathways for upward-moving ore fluids into vesicular flow tops at the Baltic and Isle Royale deposits and others in the Greenland-Mass subdistrict (Broderick, 1931). The earliest native copper deposits exploited in the district east of Eagle River were along subvertical fracture zones and minor transverse faults with displacements less than 200 m (Butler and Burbank, 1929). Therefore, a better understanding of fault geometry and timing may provide insights about the distribution of known deposits and about the possible presence of undiscovered deposits. 104 �Figure 3: Geologic map of the Keweenaw Peninsula showing the Keweenaw fault system north of Portage Lake and field trip stops within the four focus areas. Faults are indicated by curved black lines. Field Trip Stops The field trip stops described below are grouped into four areas to illustrate themes related to fault system geometry, stratal relationships and deformation, fault blocks, deformation fabric in fault zones, and slip kinematics. Each area has an introductory overview of the key themes and nature of the stops, followed by individual stop descriptions. Reported strike and dip values follow a right-hand-rule convention. The dip value is followed by letters indicating the cardinal direction of dip, which is redundant but makes for clarity. About half of the stops in this guide are on public land. Those on private land are specified in the stop descriptions as requiring permission from the owners to access. 105 �Figure 4: Geologic map of Area 1 with the Keweenaw fault system (KFS) and field trip stops 1-1 to 1-3. Geologic unit codes: pbc – Baltic conglomerate, ps – St. Louis conglomerate, pb – Bohemia conglomerate, psc – Scales Creek flow, pk – Kearsarge flow with Wolverine sandstone at its base. Teeth along reverse faults are on upthrust sides. 106 �Area 1: Hungarian Fault Block ‒ Southwest End STOPS IN THIS AREA ARE ON PRIVATE LAND AND REQUIRE PERMISSION TO ACCESS. The three stops in Area 1 are within the vehicle testing grounds of Michigan Tech’s Keweenaw Research Center (KRC), under lease from the Houghton County Memorial Airport (Fig. 4). From west to east, the stops are on the main Keweenaw fault zone (1-1), in a basalt quarry within a faultbounded block (1-2), and on a footwall splay fault (1-3) that passes through Hungarian Falls to the northeast in Area 2. Within the fault-bounded block, an unconformity between the PLV and JS is interpreted to dip shallowly southwest. These three stops illustrate an en echelon, overlapping, fault geometry that define a fault block whose long dimension is parallel to the trend of the Keweenaw fault system (KFS). Recent mapping shows that such fault patterns and fault-bounded blocks are repeated along the KFS north of Portage Lake (Fig. 3). A similar configuration of faults with the enclosed block having a west-dipping PLV-JS unconformity occurs in Area 4 (Figs. 3 and 13). Stop 1-1: Gooseneck Creek fault exposure Directions: From the Portage Lake lift bridge between Houghton and Hancock, follow US-41 north for 6.7 mi (10.8 km) to Airpark Boulevard on the right. Turn right toward Houghton County Memorial Airport and drive 0.5 mi (0.8 km) to the Keweenaw Research Center (KRC) on the right. Enter the KRC parking area and wait in vehicles. We may be escorted by KRC staff for 1.5 mi (2.4 km) to the south edge of their vehicle testing grounds, where we will park and walk. [Lat: 47° 9.259'N | Lon: 88° 29.895'W] The fault exposure on Gooseneck Creek is part of the main Keweenaw fault zone that is traced by means of outcrops, drill holes, and water wells along a line bearing 36° from the Michigan Tech campus to the intersection of Airport Park and Forsman roads, about one kilometer southwest of this stop (Fig. 4). Northeast from that intersection, the fault’s map trace curves eastward so that here it trends 72°, as do the hanging-wall PLV strata. This stop is the most northeasterly site where this segment of the KFS can be observed because the flat upland to the northeast, where the airport is located, has little to no bedrock exposure. Hubbard (1898) seems to be the first geologist to describe this site. While his report puts the outcrop about 80 m south of its actual location, his geologic description is very similar to what was observed and measured during the 2021-2022 EdMap project (Langfield, 2024). “A conglomerate here, underlain by trap, strikes N. 72° E., and dips northerly 44°, the trap being in contact on the south with the sandstone, which is much broken and disturbed but appears to dip rather flat to the N. E.” (Hubbard, 1898) The fault’s main slip surface is not exposed but its position is determined to within a couple of meters by the proximity of hanging-wall PLV outcrops to footwall JS outcrops. PLV strata in the hanging wall, striking 252° and dipping 40-44° NW, change stratigraphically upward from amygdaloidal basalt at the fault at creek level to a felsic pebble conglomerate, and then to a series of basaltic flows for as far as outcrop exists to the north and northwest. The stratigraphic 107 �position of the conglomerate layer suggests that it is the Baltic (#3) conglomerate shown on the USGS Hancock and Chassell bedrock geology maps (Cornwall, 1956a; White, 1956). Abundant fractures within the basalt flows are not obviously systematic, although detailed work might reveal dominant sets. Jacobsville strata in the footwall commonly appear massive due to being thickly bedded to locally cross-bedded, which makes their attitude difficult to measure. In general, JS strata dip gently both toward and away from the fault, suggesting an open anticlinal structure with an axis that trends approximately east-west (Fig. 4). Figure 5: Gooseneck Creek cross-section in progress using outcrop and drill hole data (Langfield, 2024). Red lines below topography are New Arcadian drill hole trajectories. Colors and letter codes of geologic units are explained in Figure 2. KFS-P = Keweenaw fault system - Portage segment; KFS-H = Keweenaw fault system - Hungarian segment. Dashed orange line above KFSP marks what may be the Baltic (#3) conglomerate. Diamond drill holes (DDH) of the Calumet and Hecla Consolidated Copper Company provide data that are critical for the interpretation of this site, situated on a NW-trending line of four New Arcadian holes drilled in 1911-1912 (Fig. 4). Southeast of here, three drill holes penetrated the following beneath glacial overburden (hole depth converted to vertical depth): DDH #17 about 140 m SE cut 46 m of JS sandstone followed by 109 m of PLV basalt; DDH #15 about 205 m SE cut 69 m of JS sandstone and conglomerate followed by 14 m of PLV basalt; DDH #13 about 355 m SE cut 37 m of JS sandstone (Fig 5). Southeast of the fault, therefore, drilling reveals a veneer of JS strata less than 70 m thick overlying PLV basaltic lava flows. About 135 m northwest of the fault, DDH #11 inclines 52° toward this site and reaches a total depth of 457 m. It cuts a single felsic conglomerate layer between depths of 93 m and 109 m (16 m apparent thickness) that we correlate to the 6-m-thick conglomerate layer seen here in outcrop, which yields an average stratal dip of 53° NW. Below the conglomerate, DDH #11 penetrated ~2 108 �meters of sludge that we interpret as fault gouge, and then continued through basaltic flows to its bottom at a vertical depth of 363 m below the DDH #17 surface location to the southeast. The position of the fault below the conglomerate in DDH #11 and not far below the conglomerate at the surface indicates that the fault dips nearly parallel to hanging-wall PLV strata. While at the surface the fault juxtaposes PLV strata against JS strata in the footwall, at depth in DDH #11 the fault’s hanging wall and footwall both have PLV strata, which is consistent with the thin veneer of JS strata penetrated by DDH #17 and DDH #15 southeast of here. Stop 1-2: Basalt quarry of the Keweenaw Research Center Directions: Return to vehicles and drive north 0.15 mi (0.25 km) to KRC perimeter road. Turn right and drive southeast for 0.75 mi (1.2 km) to a basalt quarry on the right. Pull off road into a gravel terrace on the right and park. [Lat: 47° 9.144'N | Lon: 88° 29.023'W] The KRC basalt quarry was opened in 2018-2019 to provide materials for expansion of their vehicle testing facilities. The quarry lies between the fault segment just visited at Gooseneck Creek and a parallel segment to be visited at the next stop (Fig. 4). The quarry exposes portions of at least two basalt lava flows that dip shallowly southwest. The shallow dip is roughly manifested by subhorizontal benches of the quarry and can be measured on the first dry bench west of the quarry pond, where the upper surface of a lava flow has been exposed. Inspection of the irregular subhorizontal surface reveals isolated patches of sediment whose stratification yields an average strike of 125° and dip of 19° SW. The shallow southwesterly dip of PLV strata is important to the understanding of structural geometry. Early geologists noted that older PLV strata near the Keweenaw fault and its splays commonly have anomalous orientations relative to younger PLV strata away from the fault zone, which have a well-defined regional trend (Hubbard, 1898; Butler and Burbank, 1929). In most of the PLV section, strata generally strike northeast to east and dip 35° – 55° northwest to north. At this stop and for ~3 kilometers north and northeast, PLV strata dip less than 25° in various directions, including counter-regional to the southeast. Such anomalies are clues to the structural configuration of the area along the Keweenaw fault system. Specific to this stop, the shallow southwesterly dip of PLV stratal is consistent with data from DDHs #17 and #15, where a thin veneer of JS strata overlies PLV basaltic rocks. The nature of the PLV-JS contact is not described in the core logs, but the normal stratigraphic sequence indicates that it is an unconformity. The unconformity dips shallowly southwest to south based on the two cited DDHs and DDH #20 located ~510 m southwest, which penetrates the unconformity ~35 m lower relative to a common datum. Projecting the unconformity updip brings it to the surface southwest of this stop. Therefore, we interpret that basalt at the quarry roughly correlates with basalt below JS strata in the three DDHs by passing beneath an erosional wedge of JS strata on a SW-dipping unconformity (Langfield, 2024). This new interpretation differs from one involving a transverse fault shown on USGS bedrock geology maps for the Hancock and Laurium quadrangles (Cornwall and Wright, 1956a, 1956b). 109 �Stop 1-3: East branch of Quincy Creek fault exposure Directions: Return to vehicles and drive east on the perimeter road for 0.25 mi (0.4 km) to an intersection with a gravel road on the left. Turn left and drive ~0.35 mi (~0.6 km) to a clearing on the right. Turn into the clearing and park. [Lat: 47° 9.282'N | Lon: 88° 28.548'W] The east and west branches of Quincy Creek cross a segment of the Keweenaw fault system that is about 750 meters southeast of the fault segment at Gooseneck Creek and subparallel to it. Both branches of Quincy Creek constrain the position of the fault to within 8-10 m by the proximity of hanging-wall PLV outcrops to footwall JS outcrops, but the fault zone itself is not exposed. Outcrops along the west branch of Quincy Creek are the most southwesterly constraint on the position of this fault segment because of glacial deposits that cover bedrock southwest of here. However, we interpret the fault to continue southwesterly to an acute intersection with the fault segment seen on Gooseneck Creek at a point about halfway to Portage Lake (Fig. 4). Northeast from this stop, the fault segment is easily traced by means of outcrops and water wells along an azimuth of 42° for a distance of 3 km to the upper Hungarian Falls in Area 2. We will focus on the east branch of Quincy Creek because it provides better exposures near the fault. Again, Hubbard (1898) appears to have been the first geologist to describe outcrops at this site as well as other outcrops in stream valleys to the northeast that cross the fault line. Although his report puts the outcrop 150-175 meters north-northeast of its actual location, his geologic description of part of the outcrop is very similar to what was observed and measured during the 2021-2022 EdMap project (Langfield, 2024). “In Sec. 22 . . . occur outcrops of sandstone and of a conglomerate with a very sandy matrix. The pebbles in the conglomerate are subangular and some of them are of quartz porphyry. The dip is about 50°-54° N. W., strike about N. 45°-50° E.” (Hubbard, 1898) The outcrop just described begins 60 meters downstream from the gravel access road and extends another 30 m downstream. The sedimentary layers here are well indurated and well stratified, having an average strike of 225° and dip of 35° NW. A reddish-brown sublithic sandstone is the dominant rock type along the semi-continuously exposed section in the creek bed, with subordinate conglomeratic sandstone and pebble-to-granule conglomerate that ranges from matrix-supported to clast-supported. Clasts are mostly subrounded to subangular and have a variety of compositions but are dominantly felsic. Whereas Hubbard (1898) thought that these were JS strata, we interpret them as PLV strata because of their induration, lithic nature, and attitude that are similar to other PLV sedimentary units in the area and differ from JS sandstone strata to be seen downstream. The south end of the 30-m extent of indurated sandstone and lesser conglomerate coincides with a sharp deflection in the creek by ~20 meters east before the creek resumes its southerly course at a sharp right bend. Downstream from this point for ~30 meters, intermittently exposed sandstone and minor conglomerate differ in many aspects from the sedimentary strata upstream of the creek’s deflection. The sandstone is lighter toned and locally streaked off-white to beige, is less 110 �indurated and commonly friable, and has a more quartzose composition. The pebble-to-granule conglomerate layer in the first outcrop downstream from the creek’s deflection has a muddy to silty matrix, is poorly indurated, and has subrounded to rounded clasts with a variety of compositions. These characteristics of the sandstone and one conglomerate layer are typical of Jacobsville strata observed elsewhere in fault juxtaposition with hanging-wall PLV strata. The sharp deflection in the creek is interpreted, therefore, as marking a fault contact between PLV sedimentary strata upstream and JS strata downstream. This interpretation is supported by structural observations. Strata in the first outcrop south of the creek’s deflection strike 220° and dip 65° NW (overturned), but become less steeply dipping over a short distance as the creek is followed downstream to the south. These stratal attitudes and their significant change going downstream from the creek’s deflection contrast with stratal orientation and its consistency over a similar distance upstream of the deflection. About 15 meters south of the creek’s deflection, a small fault striking 225° and dipping 80° N cuts thickly bedded JS strata in a larger outcrop along the west bank of the creek. South of the fault, deformation bands in the sandstone are expressed as quasi-linear to broadly sinuous ridges on the outcrop surface. Deformation bands are cataclastic shear zones that develop during compression of partly indurated, porous, clastic material (Fossen et al., 2007). They are generally more cemented than the host material, accounting for their raised relief on erosional surfaces, and have not been observed in PLV sedimentary strata. Farther downstream, JS strata are abundantly displayed for a distance of 1.4 km, nearly down to the derelict Quincy Mining Company stamp mill along state highway M-26. Over that distance, JS strata generally dip less than 15° NW and display a few broad open folds with NNE-trending axes. Area 2: Hungarian Fault Block ‒ Northeast End The first two stops in Area 2 are on the same fault segment seen at Quincy Creek (2-1) and on its curved portion (2-2) that connects back to the main Keweenaw fault zone to the north (Fig. 6). A PLV sedimentary layer, usually conglomerate but locally sandstone, is traceable in the hanging wall from Quincy Creek through the east branch of Dover Creek near Hungarian Falls, where it begins a smooth northward curve from 42° to 345° azimuth, a change of over 55°. From this curved stratal geometry in map view, we infer a single smoothly curved fault along the southeast and east edges of the Hungarian fault block. The long straight part of the fault is the block’s southeast edge that parallels the overall KFS, whereas the short curved part is the block’s east edge and connects the fault segment back to the main Keweenaw fault zone. This geometry and a northwesterly decrease in stratal dip based on drill hole data define a single scoop-shaped fault rather than an intersection of two distinct faults. Stop 2-2 illustrates a common dynamic of the KFS – northeast and east edges of fault-bounded blocks were thrust eastward along west-dipping reverse faults. The third stop in Area 2 is on the main Keweenaw fault zone (2-3), which aligns with its counterpart in Area 1 (Fig. 3). The relationship between the main fault zone and the connector fault may involve the connector fault terminating against a main fault zone that continues to the southwest (Fig. 6 at “?”). Another option is that the hanging-wall sedimentary layer is continuous from Stop 2-2 to Stop 2-3 and is draped over a lateral ramp in the KFS that steps up in stratigraphy to the northeast. The stratigraphic relationships across the Hancock fault are discussed in the Stop 2-3 description. 111 �Figure 6: Geologic map of Area 2 with the Keweenaw fault system (KFS) and field trip stops 2-1 to2-3. Geologic unit codes: ps – St. Louis conglomerate, pcc – Copper City flow; pb – Bohemia conglomerate, psc – Scales Creek flow, poc – Old Colony sandstone; pk – Kearsarge flow with Wolverine sandstone at its base. Teeth along reverse faults are on upthrust sides. See Figures 4 or 13 for symbology. 112 �Stop 2-1: Dover Creek fault exposure at Upper Hungarian Falls Directions: Return back to the KRC perimeter road, then turn right and follow it back to the KRC entrance on Airpark Boulevard. Turn left and drive 0.5 mi (0.8 km) to US-41. Turn right and drive 1.6 mi (2.5 km) to Oneco Road. Turn right onto Oneco and drive 3.1 mi (5.0 km) to Amygdaloid Street. Turn left and follow Amygdaloid and Tamarack Hill Road for 0.3 mi (0.5 km) to M-26. Turn left and drive 0.35 mi (0.56 km) to 6th Street in Hubbell. Turn left and then, after the second house on the left, veer left onto Golf Course Road going uphill. Drive 0.5 mi (0.8 km) to a rutted gravel road on the left. Depending on conditions, we may turn left and take the gravel road. Otherwise, park along Golf Course Road and walk to the stop at Upper Hungarian Falls. [Lat: 47° 10.440'N | Lon: 88° 27.086'W] The fault segment crossed by Dover Creek is near the northeast end of the segment seen at Quincy Creek, and is where the fault’s surface trace begins to curve northward (Fig. 6). Dover Creek has three sets of falls that are worthwhile visiting. The two downstream falls have larger drops entirely over Jacobsville Sandstone. We will visit the upper set of falls at the fault contact between PLV strata upstream in the hanging wall and JS strata in the footwall. This site has been visited by geologists since the mid-1800s and is considered a classic exposure of the Keweenaw fault (original sense). The most insightful description by early geologists is in the work of Irving and Chamberlin (1885), whose diligent field work, keen observations, and logical reasoning convinced the geological community of the time that the contact between PLV strata and JS strata was a large fault. Prior to their work, some geologists argued that JS strata lay stratigraphically below PLV strata based in part on their similar dip here and at Houghton-Douglass Falls on Hammell Creek (Stop 2-3). Of historical interest, Roland Duer Irving is this year’s nominee for recognition by the ILSG as a Pioneer of Lake Superior Geology, and Thomas Chrowder Chamberlin is another wellknown geologist of his time, famous for his classic 1890 work “The method of multiple working hypotheses” published in Science. Both were contemporaries of John Wesley Powell, a U.S. Army officer during the American Civil War, famed explorer of the American west, and second director of the U.S. Geological Survey from 1881–1894. The faulted relationship between PLV and JS strata at this stop was revealed by excavations made by a “force of miners” hired to trench across the PLV-JS contact at three locations on the southwest valley wall (Fig. 7). The trenches exposed a fault zone dipping 30-35° NW along the contact, which has the following internal zonation from hanging wall to footwall, as summarized from Irving and Chamberlin (1885) and converted to true thickness. 1. Trap [basalt]: highly fractured but in place. (2.57 m) 2. Trap debris: disintegrated basaltic fragments in a lumpy crudely laminated clay, having a transitional boundary with zone 1. (0.39 m) 3. Clay: red and “shaly” with light grayish-green spots, some sandy seams, and occasional lumps of disintegrated trap. (0.13 m) 4. Trap debris: similar to zone 2 but with more clayey material, whose dark color contrasts with adjacent red clay. (0.13 m) 113 �5. Trap debris mixed with red clay: trap debris is similar to zone 4 but red clay and minor sand gives zone 5 a reddish tint. (0.17 m) 6. Sandstone: light reddish and quartzose. (0.51 m) Footwall JS strata near the fault contact dip 10° or less, but at the fault contact they are bent downward to be nearly parallel with the fault surface. The trench observations indicate that the fault propagated upward across subhorizontal JS strata and the overriding PLV strata crushed and abraded the truncated edges of JS strata. Figure 7: Trenches at upper Hungarian Falls that exposed relationships across the fault, marked by the red lines (Irving and Chamberlin, 1885). Downstream from the trenches, JS strata generally dip 10-20° NW toward the fault but exhibit a few broad open folds similar to what is observed downstream along Quincy Creek. Upstream from the trenches, PLV strata in the hanging wall begin with a fractured basalt flow at the minor falls below the main falls, and then progress stratigraphically upward to a felsic cobble-pebble conglomerate at the main falls, followed by a series of basaltic flows intermittently exposed for over two kilometers upstream. Stratal dip decreases upstream from 25-30° NW at the main falls to flat-lying and then to 15° SE to define an open syncline, which is succeeded upstream by an open anticline before reaching the Hancock fault (Fig. 8). A well-laminated, 1-m-thick sandstone layer at the base of the 6-m-thick conglomerate layer provides a confident formation strike of 205° and dip of 30° NW, which is essentially parallel to the underlying fault surface. This geometry indicates that the hanging-wall PLV section at this location became detached along 114 �layering somewhere downdip to the northwest and was thrust southeast and upward to its current position along a ramp that cuts upward across younger layers. The fault segment exposed at this stop is penetrated by the vertical Oneco #9 DDH located 1.7 km west-northwest, at a depth of 556 m, which gives an average fault dip over this distance of 19° NW after accounting for topography (Fig. 8; DeGraff and Carter, 2023). Because the fault dips 30-35° NW at the surface, it must dip less than 19° NW over some portion of its trajectory between here and the Oneco #9 DDH. That is, its dip must shallow going in a northwesterly direction similar to the shallowing of PLV stratal dips observed upstream along Dover Creek, which is characteristic of a detached style of faulting with layer-parallel slip. Figure 8: Cross-section along Dover Creek based on outcrop and drill hole data (DeGraff and Carter, 2023). Long bar inclination of L-shaped symbols at land surface shows apparent dip. Thin black lines below topography are drill hole trajectories. Colors and letter codes of geologic units are explained in Figure 2. KF‒Keweenaw fault; HF‒Hancock fault; B‒Bacon (1966) seismic experiment. Stop 2-2: Beaudoin Creek fault exposure Directions: Return to Golf Course Road and turn left to go uphill. Drive 0.5 mi (0.8 km) on Golf Course Road to Beaudoin Creek. We will park along the west shoulder of the road north of the creek culvert. This will require turning vehicles around at the first driveway north of the creek. [Lat: 47° 10.806'N | Lon: 88° 27.049'W] THIS STOP IS ON PRIVATE PROPERTY. PERMISSION IS REQUIRED TO OBTAIN ACCESS. 115 �The small creek crossed by Golf Course Road is unofficially called Beaudoin Creek after the property owner, who also owns the Wood’n Spoon specialty food and gift shop in Mohawk, MI. We will walk upstream to the west for 300 meters to outcrops first described by Hubbard (1898), who traced the PLV-JS fault contact throughout this area and noted remarkable changes in its strike. Between Quincy Creek and the west branch of Dover creek (not visited), the Hungarian fault segment has an azimuth of 42°, but it begins to curve northward at the east branch of Dover Creek, where it has an azimuth of 25° (Fig. 6). The gradual change in fault direction continues from the last stop to this one such that here the fault trace and hanging-wall PLV strata have an azimuth of 345°, which completes a total change in fault strike of 55-60° along a smooth arc. The NNW-trending portion of the fault segment extends more than a kilometer north to a point along another smooth curve of the fault toward the northeast, which re-aligns the fault with the trend of the Portage fault segment. It is unlikely that the position of the second broad curve in the fault north of here is a coincidence, and more likely that it manifests in some way a continuation of the KFS-Portage segment seen at Stop 1-1. Along Beaudoin creek east of the fault, JS strata are intermittently exposed over a distance of 170 meters and mostly consist of yellowish quartzose sandstone with occasional layers of reddish siltstone and conglomerate. Over most of this distance, JS strata dip less than 15° W toward the fault, but within 20 meters of the fault they dip more steeply at 70° E to vertical. The fault contact between PLV basaltic rock to the west and JS sandstone to the east is located to within a meter by the exposures, but the fault zone is not well exposed due to the degraded basaltic rock in the hanging wall. PLV stratigraphy here is very similar to that observed at Hungarian Falls, beginning with a basaltic lava flow of low relief that extends upstream to a small pond below a 6-m-tall waterfall. The waterfall is over a NNW-trending ridge of felsic cobble-pebble conglomerate ~10 m thick that has a meter-thick basal layer of siltstone to fine-grained sandstone, as seen at the previous stop. West of the conglomerate ridge, a series of basaltic flows crops out along the creek bed for another 200 meters. A reliable measurement of PLV strata orientation from the basal unit of the conglomerate layer gives a strike of 170° and dip of 45° W, which probably is also the attitude of the fault surface by analogy with Hungarian Falls and based on the interpretation of a detached fault system. The smooth curve of the fault segment and subparallel PLV strata that are traceable from southwest of Stop 2-1 to north of Stop 2-2, combined with the decrease in dip of the fault and PLV strata toward the Oneco #9 DDH, imply a curved fault surface in three dimensions. A number of geometries involving smaller faults and folds are possible, but the overall geometry implies a larger scoop-shaped fault surface that plunges between southwest and west. Further work to integrate surface mapping with subsurface DDH data are needed to fully define the fault and stratal geometries in this area. For now, we interpret the curved fault segment as part of the Hungarian segment of the KFS, which defines the long southeast edge and shorter east edge of the Hungarian fault block (Fig. 6) with relatively shallow dipping PLV strata. 116 �Stop 2-3: Hammell Creek fault exposure at Houghton-Douglass Falls Directions: Drive southeast on Golf Course Road 1.0 mi (1.6 km) to M-26. Turn left and drive 2.6 mi (4.2 km) to 10th Street in Lake Linden, where M-26 makes a 90° left turn. Turn left and drive 2.0 mi (3.2 km) to a parking area on the right. Turn in and park. [Lat: 47° 12.413'N | Lon: 88° 25.611'W] Houghton-Douglass Falls (a.k.a. Douglass Houghton Falls) was purchased by the State of Michigan in 2018 and, as of this writing, is being converted into a day park with walking trails and signage. The work is not yet complete and access is still somewhat limited but allowed. We will view the site from overlooks at the top of the steep canyon walls and will not descend to the fault contact near the base of the falls due to time constraints. The total vertical drop of 34 m (110 feet) makes Houghton-Douglass Falls the tallest in Michigan. This stop is another classic exposure of the Keweenaw fault (original sense) that has been investigated by geologists since the mid-1800s. The trail to the overlook area crosses the Hancock fault, not yet found in outcrop, and PLV strata in the Keweenaw fault’s hanging wall. The Hancock fault has been traced 16 km on an azimuth of 55° by means of drill holes from the Hancock and Quincy mines to an intersection with the Keweenaw fault one kilometer northeast of the falls (Fig. 3). The Hancock fault seems to have had an important role in the occurrence and distribution of native copper at those two mines (Bornhorst et al., 1986; Field Trip 2 in this volume). Copper mineralization at the base of Houghton-Douglass Falls next to the Keweenaw fault was investigated in an adit opened by the Douglass Houghton Mining Company, organized in 1845 (Stephens, 1902), and recently observed in veins (Gamet, 2023). The geometry of two PLV layers near the intersection of the Hancock and Keweenaw faults is critical to understanding geologic relationships at Houghton-Douglass Falls (Fig. 6). The Laurium bedrock geology map shows the St. Louis (#6) conglomerate and overlying Copper City flow in the hanging walls of the Hancock and Keweenaw faults north and west of their intersection but not to the southwest in the acute fault wedge containing Houghton-Douglass Falls (Cornwall and Wright, 1956b). The two units are easily recognized and correlated in drill holes and outcrops from many kilometers northeast of Calumet-Laurium in a southwesterly direction to the Hancock fault. The St. Louis conglomerate is a felsic, pebble-cobble, clast-supported conglomerate with locally significant lithic and conglomeratic sandstone that is associated with rhyolitic rocks along strike (Hubbard, 1898; White et al., 1953; Nicholson, 1992; Gamet, 2023). The Copper City flow is recognized for its anomalous thickness of 180 m at a point 7 kilometers to the northeast, coarse grain size, and pegmatitic segregations that have been precisely dated (Fig. 2). Recent mapping aided by drill hole data has allowed these two units to be correlated across the Hancock fault into the acute fault wedge, where they are identified in outcrop at Houghton-Douglass Falls (Fig. 6). The main drop at Houghton-Douglass Falls is over the Copper City flow, which extends from upstream of the waterfall down to a ledge ~10 meters above its base. From the north side of the gorge, a planar fabric with meter-scale spacing over much of the flow thickness dips gently 117 �upstream. Observations at the top of the waterfall suggest that this fabric results from amygdule layers that dip 20-25° NW. Below the main waterfall on the ledge above the base, a 3-m-thick sedimentary layer, first noted by Hubbard (1898), consists of a felsic pebble-granule conglomerate and coarse lithic sandstone whose layering provides a reliable strike of 225° and dip of 20° NW, i.e. parallel to amygdule layers at the top of the waterfall. This is interpreted to be the St. Louis (#6) conglomerate below the anomalously thick Copper City flow. Both layers project northeast along strike to intersect the Hancock fault at a point where their map offsets across the fault match the offsets of PLV layers previously correlated across the fault (Fig. 6). Below the St. Louis conglomerate is a basalt lava flow that becomes increasingly fractured at the foot of the falls, at which point sheared basaltic rock is observed along the south face of the gorge. Figure 9: Trenches below Houghton-Douglass Falls that exposed relationships across the fault, marked by the bold red line (Irving and Chamberlin, 1885). Irving and Chamberlin (1885) focused their investigation of Houghton-Douglass Falls along the bottom and walls of the gorge below the falls, again combining careful field observations with trenching across the fault surface. Two trenches running up and down the south valley wall and a third trench along the fault surface (Fig. 9) revealed similar relationships to those observed in the trenches at Hungarian Falls but with some important differences. Basaltic rock in the hanging wall has a clay seam at the fault surface that transitions upward to a clayey breccia and then to fractured but intact basalt as seen in the Hungarian trenches. However, footwall JS strata here are generally poorly indurated, red, shaly conglomerate with subordinate whitish quartzose sandstone layers, which is the opposite relationship of dominant to subordinate lithologies at Hungarian Falls. The downward flexing of subhorizontal JS strata to dips of 20-30° close to the fault surface is again 118 �observed, but the flexing is spread over a larger horizontal distance than at Hungarian Falls. The wider zone of flexure here may result from the poor induration and ductility of the red shaly conglomerate here relative to the moderately indurated sandstone strata at Hungarian Falls. Irving and Chamberlin (1885) estimated a fault dip of 25-30° NW based on their excavations, which is similar to their estimate of PLV stratal dip of 25° NW in the hanging wall. Recent field work yielded a fault dip of 20° NW based on a three-point method using surveyed points across the gorge and by visually siting upward along the fault surface (Gamet, 2023). This result for fault dip matches the PLV stratal dip measured on the St. Louis conglomerate and on amygdule layers in the Copper City flow. While earlier and recent dip values are slightly different, both studies agree that hanging-wall PLV strata are essentially parallel to the underlying fault surface, which is indicative of a detached thrust system. A recent cross-section through Houghton-Douglass Falls uses concepts related to detached thrusting, conservation of volume, and ductile portions of the JS section to model fault geometry and deformation of strata in the hanging wall and footwall of the fault system (Fig. 10). Figure 10: Cross-section along Hammell Creek at Houghton-Douglass Falls based on outcrop data and drill hole correlations (Gamet, 2023). Geologic unit codes: pcc – Copper City flow; psc – Scales Creek flow, pk – Kearsarge flow; pg – Greenstone flow. KFS-M = Keweenaw fault system Mayflower segment; HF = Hancock fault. Area 3: Snake Creek Fault Block – Keweenaw Fault Zone at Lake Gratiot The three stops in Area 3 (Fig. 11) are on the east boundary fault of the Snake Creek block (3-1), on the main Keweenaw fault zone north of Lake Gratiot (3-2), and at the intersection of these two fault trends (3-3). The Keweenaw fault zone roughly parallels the north edge of Lake Gratiot and, relative to Areas 1 and 2, it trends more easterly with an azimuth of 72° and dips more steeply northwest. Other fault segments in Area 3 are similarly oriented clockwise relative to their counterparts in Areas 1 and 2. South of the main Keweenaw fault zone, two fault-bounded blocks have long dimensions oriented parallel to the KFS (Fig.3). West of Lake Gratiot, the Snake Creek block is bounded on the southeast by a NE-trending connector fault, along which PLV layers are 119 �thrust southeast over JS strata (Fig. 11). Deformed hanging-wall PLV strata and footwall JS strata have NE-trending fold axes parallel to their fault contact. Fold axes in the hanging wall plunge 23° with an azimuth of 44° (Mueller, 2021). The southern boundary fault of the Snake Creek block is an ESE-trending footwall splay of the main Keweenaw fault zone (Fig. 3) that juxtaposes PLV strata on the north against JS strata to the south. Similar to the northeast end of the Hungarian block (Fig. 6), this footwall fault splay appears to curve north and merge seamlessly into the connector fault along the southeast edge of the Snake Creek block. Figure 11: Geologic map of Area 3 with the Keweenaw fault system (KFS) and field trip stops 3-1 to 33. Geologic unit codes: pb – Bohemia conglomerate, pgf – Gratiot flow. Teeth along reverse faults are on upthrust sides. See Figures 4 or 13 for symbology. The main Keweenaw fault zone in this area is well exposed along several creeks that empty into Lake Gratiot and, based on outcrop relationships and cross-section models, it consists of two parallel branches (Fig. 11). The northern branch has a well-developed gouge and breccia zone that is at least 20 m wide in places and perhaps as much as 45 m wide. This branch juxtaposes PLV basaltic flows on the north against presumably younger basaltic flows on the south, whereas the southern branch juxtaposes basaltic flows on its north against JS strata on the south. The double fault zone north of Lake Gratiot continues in a west-southwest direction past the termination of the 120 �connector fault intersecting from the southwest and forms the northern boundary of the Snake Creek block. This main fault zone trajectory differs from the 1950s USGS bedrock geology maps and was first proposed by Cannon and Nicholson (2001) in their map compilation. We have adjusted the position of their proposed fault based on a combination of outcrop relationships, drill hole data, and topographic features, and we consider evidence for its existence to be compelling. Stop 3-1: Unnamed creek fault exposure along Iron Gate Road Directions: Continue driving north on M-26 for 2.0 mi (3.2 km) to Hecla Street. Turn right and drive through historic downtown Laurium for four blocks (0.5 mi, 0.8 km) to 1st Street, a.k.a. School Street. Turn left and drive 0.35 mi (0.56 km) to US-41. Turn right and drive 17.6 mi (28.3 km) past the road to Eagle Harbor to historic Central Location. Turn right onto Gratiot Lake Road and drive ~4.8 mi (~7.7 km) to unpaved Iron Gate Road on the right. Turn right and drive 0.5 mi (0.8 km) to an unimproved dirt road on the right. Turn into the side road and park. [Lat: 47° 21.222'N | Lon: 88° 9.340'W] This stop will be a quick show-and-tell to explain fault and stratal geometries at the east edge of the Snake Creek fault block that extends 5 kilometers to the west (Fig. 11). A short walk northwest along the two-track road leads to a 3-m-tall outcrop of JS in the creek northeast of the road. This outcrop is much larger than other JS outcrops sometimes exposed for 35 meters upstream in the creek bed, depending on spring run-off. About 80 meters upstream is the first PLV basalt outcrop where the creek emerges from the upland to the northwest. The boundary between the upland with PLV bedrock and the lower flatter area to the southeast with JS bedrock has a trend of 42° and it extends ~3 kilometers from the southeastern rounded corner of the Snake Creek fault block to the main Keweenaw fault zone at Nine Thirty Two Creek (Stop 3-3). Several small creeks flowing southeast from the upland toward Lake Gratiot cross this boundary and expose bedrock, constraining the position of the geologic contact but not exposing it. The sandstone strata here strike 36° and dip 63° SE in the inferred direction of stratigraphic up, indicating that they have been rotated down to the southeast (Lizzadro-McPherson, 2023). Similar orientations of JS strata are noted at the mouths of other creek valleys where they emerge from the upland. Scattered JS outcrops to the southeast have nearly flat-lying strata, indicating that tilting of JS strata is negligible beyond 100 to 150 meters from the PLV-JS contact. In the upland northwest of the contact, fractured PLV strata with veins of secondary minerals generally do not present good opportunities to determine strike and dip. Where possible to measure, consistent northeasterly strikes with dips both to the northwest and southeast define an anticlinesyncline pair with NE-trending fold axes that parallel the PLV-JS contact. The relationships along the east edge of the Snake Creek block are evidence of a NE-trending fault along which PLV strata to the northwest were thrust over JS strata to the southeast. Similar to the curved fault at the northeast end of the Hungarian block, the fault at the east end of the Snake Creek block also has a curved geometry where it wraps around the block’s southeast corner and gradually changes direction by 55° to a west-northwesterly trend. Along the fault’s 121 �NE-trending section, its position and nature in the latest map (Fig. 11) do not differ much from what is shown on the Bruneau Creek bedrock geology map (Wright and Cornwall, 1954). However, the earlier map labels this fault the “Keweenaw fault,” whereas we interpret it to be part of a curved splay in the footwall of the main Keweenaw fault that lies to the north, which we will visit at the next stop. Stop 3-2: Main Keweenaw fault zone at Eister Creek and Falls Directions: Return 0.5 mi (0.8 km) on Iron Gate Road back to Gratiot Lake Road. Turn left and drive 0.2 mi (0.3 km) to East Gratiot Lake Road. Turn right and drive 1.0 mi (1.6 km) to a parking area along the road. [Lat: 47° 21.975'N | Lon: 88° 7.967'W] Eister Creek near the falls provides excellent exposures across the northern branch of the main Keweenaw fault zone where PLV strata in the hanging wall are juxtaposed against presumably younger PLV strata in the footwall (Figs. 11 and 12). Jacobsville strata that commonly occur in the footwall of the main fault zone are not present here, though they may constitute bedrock south of the southern branch of the main fault zone. Elsewhere nearby in the footwall, the JS unit has thicknesses ranging up to 130 m confirmed in outcrop and greater than 135 m in a water well northwest of Lake Gratiot. In general, the thickness of the JS unit in this area appears to be much less than to the southwest near Houghton, where the unit is known to be at least 785 m thick at the fault system and at least 1,100 m thick away from it to the southeast, but could be 2,000 to 3,000 m thick based on geophysical data. Figure 12: Cross-section along Fault Creek east of Eister Creek (Lizzadro-McPherson, 2023). Geologic unit codes: pb – Bohemia conglomerate; pgf – Gratiot flow; psc – Scales Creek flow. KFS = Keweenaw fault system. Walking north up Eister Creek from the parking area, scattered outcrops of PLV basaltic lava are first encountered at the mouth of the incised creek valley. About 80 meters into the narrow gorge 122 �where it becomes deeper, the basaltic rocks become more fractured and are locally sheared, brecciated, and gougy. Over the next 40-50 m upstream to the base of Eister Falls, fracture intensity increases and cataclastic shearing becomes more prominent until the rock is essentially a breccia cut through by shear zones. The area around the base of the falls is at the core of the main Keweenaw fault zone that is up to 45 meters wide along the creek, depending on how its northern and southern edges are defined. A rock wall perhaps 4 meters wide that projects from the eastern wall of the gorge consists of fault breccia and gouge that is better indurated that the surrounding material, which is also brecciated and gougy. This ridge is inclined steeply upstream, and is taken to define the orientation of the fault zone as striking 255° and dipping 76° N. Upstream from the projecting wall of breccia, the long gradual rise of Eister Falls exposes highly fractured basaltic rocks with some shear zones, but the rock mass is mostly intact and unlike the completely brecciated fault core. The fault-core relationships seen at Eister Creek are even better displayed along another creek located ~800 m east-northeast of here at a locality informally named “Fault creek” (Fig. 12). Stop 3-3: Nine Thirty Two Creek Directions: Return 1.0 mi (1.6 km) on East Gratiot Lake Road to Gratiot Lake Road. Turn right and drive 0.4 mi (0.6 km) to a narrow driveway on the right. Either turn into the driveway and park where possible or park along the right shoulder of the paved road. [Lat: 47° 21.711'N | Lon: 88° 8.730'W] THIS STOP IS ON PRIVATE PROPERTY. PERMISSION IS REQUIRED TO OBTAIN ACCESS. A walk of about 300 meters down the overgrown old road to Lake Gratiot leads to a hairpin turn in Nine Thirty Two Creek that marks the intersection of two fault trends (Fig. 11). The south branch of the main Keweenaw fault zone generally follows the creek valley upstream from the hairpin turn toward the west and, in the opposite direction, it follows an ENE-trending path to the north shore of Lake Gratiot. The thrust that defines the eastern edge of the Snake Creek block follows the western side of the creek downstream from the hairpin turn and terminates northward against the main Keweenaw fault zone. The Keweenaw fault zone upstream of the hairpin turn is manifested by basaltic rocks that are fractured, brecciated, and altered for a few hundred meters to the west. Its east-northeast path is marked by a linear depression that connects to the neighboring creek valley where altered basalt occurs in a cutbank. PLV strata north of the Keweenaw fault zone generally strike east-west and dip moderately north based on two nearby creek traverses. These hanging-wall strata are juxtaposed against presumably younger footwall PLV strata west of the creek’s hairpin turn and against footwall JS strata east of it (Fig. 11). This change in juxtaposition of geologic units along the Keweenaw fault zone occurs because the thrust fault intersecting it on the footwall side raises PLV strata on the west over JS strata to the east. South of the creek’s hairpin turn, JS strata crop out for 235 meters along the creek bed and valley walls and consist of yellowish to reddish, medium-grained, quartzose sandstone that is poorly 123 �laminated along with subordinate reddish, muddy-to-silty, clast-supported conglomerate that is poorly indurated. The exposed JS strata are generally subhorizontal but their dip increases to 69° SE with a strike of 58° within 30 meters of the hairpin turn. The rotation of JS strata downward to the southeast is consistent with components of reverse slip on the intersecting faults that contain the sandstone within their obtuse angle (Fig. 11). Above the sandstone outcrops to the west, the upland caprock is a layer of trachyandesite overlying a basaltic layer, whose contact dips about 10° NE. This stratal orientation may represent a plunging fold axis that is parallel to fold axes near the southeast corner of the Snake Creek block. Some important questions arise from the geometric relationships observed at the intersection of the two faults. For example, what happens to the main Keweenaw fault zone west and south of this fault intersection? Wright and Cornwall (1954) show the Keweenaw fault coming from Eister Creek as bending southwest to follow the PLV-JS fault contact discussed at Stop 3-1. They also show a splay of the Keweenaw fault continuing along Nine Thirty Two Creek as far as Gratiot Lake Road. Cannon and Nicholson (2001) continued this splay fault in a broad arc parallel to regional strike to a reconnection with the Keweenaw fault of Wright and Cornwall (1954) about 6 kilometers east of Mohawk. Based on new mapping and integration of DDH data, we have modified the trajectory of the fault extension proposed by Cannon and Nicholson (2001) and we propose that this is actually the main Keweenaw fault zone. The corollary to this interpretation is that the thrust fault along the east edge of the Snake Creek block is a footwall splay of the main fault zone. In other words, we think that the main Keweenaw fault zone is not always the one that juxtaposes PLV strata in the hanging wall against JS strata in the footwall. Area 4: Keweenaw Fault Zone with Footwall Splays, Lac La Belle to Bête Grise The three stops in Area 4 are along part of the KFS that changes direction from a 72° azimuth to nearly east-west (Fig. 3). The first stop is in the deformed hanging wall of the main Keweenaw fault zone (4-1) that runs along the northwest edge of Lac La Belle, past the southern base of Mt. Bohemia, and along most of the paved road to Bête Grise Bay east of here (Fig. 13). In a westerly direction, the main fault zone forms the northern boundary of the Deer Lake block, whose long dimension is again parallel to the KFS. The Deer Lake block is limited on the south by a footwall splay that diverges from the main fault zone north of Lake Gratiot and juxtaposes PLV basaltic flows on the north against nearly vertical JS strata at the Little Gratiot River (Lizzadro-McPherson, 2023; DeGraff, 1976). Based on diamond drill hole data, the eastern edge of this fault-bounded block is a connector fault where PLV strata are thrust eastward over JS strata, whereas the block’s western edge has a thin cover of Jacobsville Sandstone unconformably overlying weathered basaltic rock. Field relationships and magnetic data suggest that the footwall fault splay curves north and merges into the connector fault along the east edge of the Deer Lake block. The other two stops in Area 4 are at historic localities on the shore of Bête Grise Bay that Irving and Chamberlin (1885) investigated as part of their USGS Bulletin 23 titled “Observations on the junction between the eastern sandstone and the Keweenaw series on Keweenaw Point, Lake Superior”. The Bête Grise block is defined by the main Keweenaw fault zone on the north, by a footwall splay that diverges from it onshore and passes offshore while remaining visible in shallow 124 �Figure 13: Geologic map of Area 4 with the Keweenaw fault system and field trip stops 4-1 to 4-3. Teeth along reverse faults are on upthrust sides. 125 �water (4-2), and by an inferred thrust along the east edge of the block (4-3). The thrust is inferred from a NE-trending belt of intense fracturing and veining in altered basaltic rock that follows the shoreline parallel to the strike of the PLV-JS unconformity exposed to the east along the shore. Thrusting of hanging-wall PLV strata toward the southeast has tilted footwall PLV strata and JS strata above the unconformity 50° SE. Stop 4-1: Haven Falls Directions: Get back onto Gratiot Lake Road and drive 4.3 mi (6.9 km) back to US-41. Turn right and drive 5.7 mi (9.2 km) to Lac La Belle Road. Turn right and drive 4.2 mi (6.8 km) to a Yintersection. Veer right and then through the sharp right curve for 0.5 mi (0.8 km) to Haven Falls Park on the right. Enter the park and park. [Lat: 47° 22.913'N | 88° 1.716'W] The small but beautiful park at Haven Falls spans a terrace formed by a previous higher stand of Lake Superior that was about 10 meters above the current lake level. The cliff of felsic conglomerate over which Haven Creek flows was probably a shoreline cliff at the time of that higher lake level. This stop is in the proximal hanging wall of the main Keweenaw fault zone (Fig. 13) and its stratigraphy is exposed almost continuously from south of Lac La Belle Road, which is privately owned, to well upstream of the falls. About 650 meters east of here and south of the main fault zone, an east-directed thrust fault, cored by the Deer Lake #2 DDH, defines the east edge of the Deer Lake block (Fig. 13A). Slip on this thrust fault has pushed PLV basaltic lavas on the west up and over JS strata to the east (Lizzadro-McPherson, 2023) in a manner similar to what occurs at the east edge of the Snake Creek block. The Haven Falls stop is, therefore, analogous to the Eister Creek stop except that the fault core here is not as well exposed south of the paved road (cf. Figs. 12 and 14). The stratigraphic sequence in the hanging wall begins with a highly fractured, veined, and locally brecciated and sheared lava flow that crops out on both sides of the paved road, but please stay on the north side to respect the private property on the south side. This strongly deformed lava flow lies along a topographic step up from the lowland at the lakeshore to the old lake terrace, and it probably marks the northern edge of the core of the fault zone. Upstream from this first lava flow, a 12-m-wide belt of conglomerate is followed by a 30-m-wide belt of ophitic basalt that locally is highly fractured, veined, and sheared. The felsic cobble-pebble conglomerate at the main falls is largely clast-supported and has a massive appearance. It extends for a horizontal distance of 16 meters along the creek to well above the top of the falls, where it is brecciated along a fault zone that cuts slightly up section toward the east. A reliable formation strike of 245° and dip of 65° NW comes from a sandstone layer above the falls near the northern edge of the second conglomerate layer. The two conglomerate layers here are collectively known as the Lac La Belle conglomerate (Cornwall, 1954a) and have been tentatively correlated with the Baltic (#3) conglomerate near Houghton. Although there is considerable uncertainty about this correlation due to the distance involved, the Lac La Belle conglomerate lies well below the St. Louis (#6) conglomerate seen at many of the earlier stops of this field trip. This means that the Keweenaw fault system here cuts the PLV section at a significantly deeper level than near Calumet, Laurium, and Lake Linden. 126 �The stratigraphic units at the falls have been traced by detailed mapping along strike in both directions. About 350 meters east-northeast, the two conglomerate layers merge into one layer where the intervening basalt flow pinches out, but otherwise the layers can be traced continuously along strike. The only structural complications observed along strike are a few faults that cut at acute angles upward across layers toward the east and have small reverse offsets, similar to the fault at the top of Haven Falls. Detailed mapping in 2019-2020 did not find evidence of four transverse faults shown on the Delaware bedrock geology map as offsetting the Lac La Belle conglomerate, neither in terms of offsets nor enhanced fracturing (LizzadroMcPherson, 2023). In fact, the kinematics of slip along the main Keweenaw fault zone would argue for a system of smaller subparallel faults rather than a series of faults normal to the main fault zone. Figure 14: Cross-section along Haven Creek at the falls (Lizzadro-McPherson, 2023). Geologic unit codes: pb – Bohemia conglomerate; psc – Scales Creek flow. KFS = Keweenaw fault system. THE NEXT TWO STOPS ARE ALONG APACHE LANE, WHICH IS PRIVATELY OWNED AS ARE ALL PROPERTIES ALONG IT. PERMISSION IS REQUIRED TO OBTAIN ACCESS. Stop 4-2: Bête Grise Shoreline, Irving & Chamberlin Historic Site Directions: Exit Haven Falls Park and drive east back to the stop sign at the Y-intersection. Turn sharply right onto Bête Grise Road and drive 3.2 mi (5.2 km) to Apache Lane on the left. [Geology Note: at 1.9 mi / 3.1 km along BG Road, an outcrop to the north is the location of the dated calcite vein in the fault zone.] Turn left, drive 0.5 mi (0.8 km), and then park along the right side of the road. [Lat: 47° 23.345'N | Lon: 87° 56.905'W] We will walk down a moderately steep slope to the shore, where Irving and Chamberlin (1885) used a “force of miners” to strip the shoreline bare in order to better expose the contact between PLV layers on the north and JS strata to the south (Figs. 13B and 15). This is probably not a field 127 �practice we could get away with today even if the site was not privately owned. Depending on the lake level, we may be able to see the contact between highly fractured, veined, and partly altered basaltic rock at the base of the shoreline scarp and JS strata that strike 105° and dip 55° S. An aerial image of the shoreline at this stop shows a sharply defined line with an azimuth of 105° that separates uniformly dark-toned PLV basaltic rocks on the lake bottom and along the shore to the east from variably lighter-toned layers of JS strata to the south and west (Fig. 16). This is the fault line that may be observed onshore or can be closely constrained by nearby outcrops. The aerial image shows many small faults as darker lines and narrow zones that cut JS strata at angles approaching 90° and offset strata by less than a meter or two. Near the fault line, splay faults break the JS unit into blocks up to 30 meters long parallel to the fault line and 4 meters wide. Figure 15: Cross-sectional view of a segment of the KFS exposed by excavation along the Bête Grise Bay shoreline. Fault strike = 100°, dip = 55° S (Irving and Chamberlin, 1885). Circle with black dot indicates movement toward viewer; circle with cross indicates movement away. Onshore north of the fault line, PLV strata are highly fractured, veined, and sheared for at least 55 meters along the shoreline to the east, which is about 25 meters perpendicular to the fault. Basaltic outcrops along Apache Lane and to the north do not exhibit such deformation. Following the shore to the west and south, JS strata change in terms of both facies and structural orientation. The basal part of the section consists of reddish, thin-bedded, siltstone and mudstone with minor fine-grained sandstone (6-7 m true thickness), followed upward by a lighter-toned package of silty to fine-grained sandstone with minor silty pebble conglomerate that forms a resistant ridge (~2 m), and then an interval of reddish siltstone and muddy conglomerate (~4 m). 128 �From this point southward and up section, JS strata tend to be sand-prone but alternate between: (1) pinkish to orange, fine to medium grained, quartzose sandstone; (2) red, thinly bedded, siltstone to mudstone; and (3) muddy to silty, poorly indurated, conglomerate layers. In summary, the JS unit tends to clean upward from a silt- and mud-dominated basal section with conglomeratic units near the fault to a quartzose sand-prone section away from the fault. Along with these stratigraphic changes, the dip of JS strata decrease from 55° S near the fault to about 20° S at a perpendicular distance of 60 meters from the fault, and presumably becomes subhorizontal not much further to the south. Figure 16: Aerial image of Stop 4-2 showing the south boundary fault of the Bête Grise block. The fault is interpreted to have dominant strike slip. Stratigraphic up in the JS is to the south. It is clear that movement along the fault has juxtaposed older PLV strata to the north against younger JS strata to the south and that the north side has a component of upward movement, as noted elsewhere. However, what is the nature of this fault? The work by Irving and Chamberlin (1885) and their team exposed a fault surface that dips about 55° S, essentially parallel to adjacent JS strata (Fig. 15). Based on textbook definitions, this would be a normal fault with younger JS strata in the hanging wall to the south above older PLV strata in the footwall to the north. Something about this interpretation seems paradoxical, however, because at previous stops 129 �the main fault zone dipped north or northwest and also because the tectonic setting of the Keweenaw fault system was compressional. Part of the paradox results from textbook definitions of reverse and normal faults that are based on idealized planar fault surfaces that have mostly dip slip. In the case of curved or corrugated fault surfaces with mostly strike slip, the textbook nomenclature for dip-slip faults may lead to confusion. Depending on the portion of a corrugated fault surface that is exposed, a mostly strike-slip fault with a smaller dip-slip component may exhibit an apparent normal or reverse component of dip-slip even though the actual dip-slip component is the same everywhere along the fault. The Keweenaw fault system in this area trends nearly east-west and is dominated by right-lateral strike slip with lesser north-side-up dip slip (2:1 ratio of strike-to-dip slip), and we infer that motion on this fault is dominantly right-lateral strike slip. The fault forms the southern edge of a fault-bounded block (Fig. 13B) whose east side will be visited next. Stop 4-3: Bête Grise Shore, PLV-JS Unconformity and Fault Directions: Continue driving east on Apache Lane for 0.4 mi (0.6 km) to the end of the road. Park where space allows. [Lat: 47° 23.437'N | Lon: 87° 56.350'W] We will walk 100 meters east and 50 meters south to access the shoreline, where geologists traveling along the coast by boat in the mid-1800s described impressive layers of sandstone on the rocky bottom of Lake Superior (Figs. 13B and 17). This area was examined later by none other than Irving and Chamberlin (1885) and then by Cornwall (1954b). The aerial image of the shoreline and offshore region reveals what the early explorers reported, a set of well-defined parallel layers that curve sharply from NE-trending layers on the western side to EW-trending layers along the shore to the east. The shoreline stop is at the western edge of the JS strata where an unconformity between PLV and JS strata is tilted about 50° SE. Depending on water level and shoreline erosion, we may be able to see the unconformity between saprolitized PLV basaltic lava to the northwest and JS strata to the southeast. If the saprolite is exposed, please do not disturb it by digging, picking at it, or walking on it. The basaltic protolith has been completely converted to clay minerals and still retains its original textures, including whitish veins that are approximately normal to the tilted unconformity. Northwest of the unconformity, i.e. deeper below the paleosurface, PLV basaltic rocks do not exhibit such alteration. Saprolitic basaltic rock that retains original textures, such as ophitic texture, has been observed elsewhere in the area where the PLV-JS unconformity is inferred, such as the east end of the Deer Lake block (Lizzadro-McPherson, 2023; DeGraff, 1976). East of the unconformity along the shore, a recessive basal part of the JS section consists of thinly bedded, reddish siltstone and mudstone with minor interbedded fine-grained sandstone. The recessive strata here strike northeast and dip moderately southeast over a distance of 25 meters to the first small resistant sandstone layer (1-2 m thick). Next in the section is another recessive interval (6-7 m wide) of reddish siltstone with interbeds of poorly indurated muddy conglomerate, followed by a larger ridge of resistant sandstone that begins a sequence of 130 �alternating resistant sandstone beds, recessive reddish siltstone, and reddish poorly indurated conglomerate. Thus, the basal JS section here is very similar to the section at Stop 4-2. The main difference is in their structural attitude, which differs in strike by 55-60°. At Stop 4-2, the dip direction of JS strata is 195° and away from the inferred strike-slip fault on the south side of the Bête Grise block, whereas the dip azimuth of JS strata on the east side of the fault block is 138° and away from the tilted PLV-JS unconformity (Fig. 13B). We infer that southeast tilting of the PLV-JS unconformity resulted from southeast thrusting along the east edge of the Bête Grise fault block, based in part on intense fracturing observed along the shoreline northwest of the unconformity at stop 4-3. Figure 17: Aerial image of Stop 4-3 showing the north and east boundary faults of the Bête Grise block. The east boundary fault is interpreted to have dominant dip slip with west side thrust eastward, whereas the north boundary fault is interpreted to have dominant strike slip. Stratigraphic up in the JS is to the southeast and south. If time permits, we will ascend the shoreline scarp to the flat bench above and walk another 100 meters to reach the eastern edge of JS outcrop along the shore. Here, JS strata strike nearly eastwest and are vertical to slightly overturned to the south, forming narrow ridges and eroded furrows and clefts due to differential erosion of the resistant and recessive layers (Fig. 17). Near the eastern edge of JS outcrop, vertical JS strata are flanked on the north by PLV strata that begin 131 �with a felsic conglomerate and continue upslope with PLV basaltic lavas. The contact between PLV strata on the north and vertical JS strata to the south is a major fault that intersects the shoreline, where a fault breccia is exposed several meters east of the last onshore JS outcrop. From this point, the fault turns eastward and runs along the shoreline for 200 meters before continuing offshore and splitting into two branches. This is the last stop and we hope that you had a good experience that will help you to understand other fault systems. Thank you for your participation! Acknowledgements We thank the following M.S. graduates and their assistants, whose field mapping was funded by U.S. Geological Survey EDMAP projects G17AC00115, G19AC00140, and G21AC10681: Colin Tyrrell (M.S.), Sophie Mueller (M.S.), Nolan Gamet (M.S.), Graham Hubbard, Ian Gannon, Ginny Hemmila, Gabe Ahrendt, Jack Hawes, Braxton Murphy, Breeanne Heusdens, and Dillon Breen. We also thank many who have expressed interest in this work and have provided helpful comments. References Cited Aho, G.D., 1969, A Reflection Seismic Investigation of Thickness and Structure of the Jacobsville Sandstone, Keweenaw Peninsula, Michigan: Michigan Technological University, MS thesis, 104 p. Bacon, L.O., 1966, Geological structure east and south of the Keweenaw fault on the basis of geophysical evidence: in Steinhart, J.S. and Smith, T.J. (eds), The Earth Beneath the Continents: Am. Geophys. Union, Geophysical Monograph 10, p. 42-55. Bornhorst, T.J., 1997, Tectonic context of native copper deposits of the North American Midcontinent Rift System: in Ojakangas, R.W., Dickas, A.B., and Green, J.C. (eds.), Middle Proterozoic to Cambrian Rifting, Central North America: Boulder, CO, GSA Special Paper 312, p. 127-136. Bornhorst, T.J., and Barron, R.J., 2011, Copper deposits of the western Upper Peninsula of Michigan: Geological Society of America Field Guide, v. 24, p. 83-99. Bornhorst, T.J., Paces, J.B., Grant, N.K., Obradovich, J.D., and Huber, N.K., 1988, Age of native copper mineralization, Keweenaw Peninsula, Michigan: Economic Geology, v. 83, p. 619-625. Broderick, T.M., 1931, Fissure vein and lode relations in Michigan copper deposits: Econ. 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Cannon, W.F., Peterman, Z.E., and Sims, P.K. 1993, Crustal scale thrusting and origin of the Montreal River monocline – a 35-km-thick cross section of the Midcontinent rift in northern Michigan and Wisconsin: Tectonics, v. 12, p. 728 - 744. Cannon, W. F., 1994, Closing of the Midcontinent Rift - A far field effect of Grenvillian contraction: Geology, v. 22, p. 155-158. Cannon, W.F. and Nicholson, S.W., 2000, Geologic Map of the Keweenaw Peninsula and Adjacent Area, Michigan to accompany Map I-2696: U.S.G.S Pamphlet, 7 p. Cannon, W.F. and Nicholson, S.W., 2001, Geologic Map of the Keweenaw Peninsula and Adjacent Area, Michigan: United States Geological Survey, Map I-2696, Scale = 1:100,000. Cornwall, H.R., 1954a, Bedrock Geology of the Delaware Quadrangle, Michigan: U.S. Geological Survey, Washington, D.C., Geologic Quadrangle Map GQ-51, scale 1:24,000. Cornwall, H.R. and Wright, J.C., 1956a, Geologic Map of the Hancock Quadrangle, Michigan: U.S. Geological Survey, Washington, D.C., Mineral Investigations Field Studies Map MF-46, scale 1:24,000. Cornwall, H.R. and Wright, J.C., 1956b, Geologic Map of the Laurium Quadrangle, Michigan: U.S. Geological Survey, Washington, D.C., Mineral Investigations Field Studies Map MF-47, scale 1:24,000. Craddock, J.P., Pearson, A., McGovern, M., Kropf, E., Moshoian, A., and Donnelly, K., 1997, Postextension shortening strains preserved in calcites of the Midcontinent Rift: Geological Society of America, Special Paper 312, p. 115-126. https://doi.org/10.1130/0-8137-2312-4.115 Craddock, J.P., Konstantinou, A., Vervoort, J.D., Wirth, K.R., Davidson, C., Finley-Blasi, L., Juda, N.A., and Walker, E., 2013, Detrital zircon provenance of the Mesoproterozoic Midcontinent Rift, Lake Superior region, USA: Journal of Geology, v. 121, no. 1, p. 57-73. Davis, D.W., and Paces, J.B., 1990, Time resolution of geologic events on the Keweenaw Peninsula and implications for development of the Midcontinent Rift system: Earth and Planetary Science Letters, v. 97, p. 54-64. DeGraff, J.M., 1976, Structural and Age Relationships of Rocks Associated with the Lac La Belle Magnetic Anomaly, Keweenaw County, Michigan: Michigan Technological University, M.S. thesis, 130 p. 133 �DeGraff, J.M. and Carter, B.T., 2023, Detached structural model of the Keweenaw fault system, Lake Superior region, North America: Implications for its origin and relationship to the Midcontinent Rift System: Geological Society of America Bulletin, v. 51, no. 1, p. 449–466. https://doi.org/10.1130/B36186.1 Fossen, H., Schultz, R.A., Shipton, Z.K., and Mair, K., 2007, Deformation bands in sandstone: a review: Journal of the Geological Society, v. 164, p. 755-769. Gamet, N.G., 2023, Structural Analysis and Interpretation of Deformation along the Keweenaw Fault System from Lake Linden to Mohawk, Michigan: Michigan Technological University, M.S. thesis, 122 p. Hamblin, W.K., 1958, The Cambrian Sandstones of Northern Michigan: Michigan Department of Conservation, Geological Survey Division, Lansing, MI, Publication 51, 55 p. Hinze, W.J., Braile, L.W., and Chandler, V.W., 1990, A geophysical profile of the southern margin of the Midcontinent Rift System in western Lake Superior: Tectonics, v. 9, no. 2, p. 303-310. Hodgin, E.B., Swanson-Hysell, N.L., DeGraff, J.M., Kylander-Clark, A.R.C., Schmitz, M.D., Turner, A.C., Zhang, Y., and Stolper, D.A., 2022, Final inversion of the Midcontinent Rift during the Rigolet Phase of the Grenvillian orogeny: Geology, v. 50, no. 5, p. 547-551. Hubbard, L.L., 1898, Keweenaw Point with particular reference to the felsites and their associated rocks: Geological Survey of Michigan, v. 6, part 2, 155 p. Irving, R.D. and Chamberlin, T.C., 1885, Observations on the junction between the eastern sandstone and the Keweenaw series on Keweenaw Point, Lake Superior: U.S. Government Printing Office, Washington, D.C., U.S. Geological Survey, Bulletin No. 23, 58 p. Kalliokoski, J., 1982, Jacobsville Sandstone: in Wold, R.J. and Hinze, W.J. (eds.), The Geology and Tectonics of the Lake Superior Basin, Geological Society of America Memoir, No. 156, p. 147-155. Keweenaw National Historical Park, 2016, Calumet & Hecla Records – 00019/004.02.01.03-007 Microfiche Drill Core Log Library: Calumet, Michigan, National Park Service, U.S. Department of the Interior, on microfiche. Langfield, K.M., 2024, Slip Kinematics and Structural Analysis of the Keweenaw Fault System from Lake Linden to Hancock, Michigan: Michigan Technological University, M.S. thesis, in preparation. Lizzadro-McPherson, D.J., 2023, Structural Analysis and Slip Kinematics of the Keweenaw Fault System between Bête Grise Bay and Gratiot Lake, Keweenaw County, Michigan: Michigan Technological University, M.S. thesis, 140 p. Malone, D.H., Stein, C.A., Craddock, J.P., Kley, J., Stein, S., and Malone, J.E., 2016, Maximum depositional age of the Neoproterozoic Jacobsville Sandstone, Michigan: implications for the evolution of the Midcontinent Rift: Geosphere, v. 12, no. 4, p. 1271-1282. 134 �Merk, G.P., and Jirsa, M.A., 1982, Provenance and tectonic significance of the Keweenawan interflow sedimentary rocks: Geological Society of America Memoir 156, p. 97-105. Mueller, S.A., 2021, Structural Analysis and Interpretation of Deformation Along the Keweenaw Fault System West of Lake Gratiot, Keweenaw County, Michigan: Michigan Technological University, M.S. thesis, 69 p. Nicholson, S.W., 1992, Geochemistry, Petrography, and Volcanology of Rhyolites of the Portage Lake Volcanics, Keweenaw Peninsula, Michigan: U.S.G.S. Bulletin 1970, Chapter B, p. B1-B57. Robertson, J. M, 1975, Geology and mineralogy of some copper sulfide deposits near Mount Bohemia, Keweenaw County, Michigan: Economic Geology 70 (7) 1202-1224. Stein, C.A., Kley, J., Stein, S., Hindle, D., and Keller, G. R., 2015, North America’s Midcontinent Rift: When rift met LIP: Geosphere, v. 11, no. 5, p. 1607-1616. Stoiber, R.E. and Davidson, E.S., 1959, Amygdule mineral zoning in the Portage Lake Lava Series, Michigan copper district, parts I and II: Economic Geology, v. 54, p. 1250–1277 and 1444-1460. doi:10.2113/gsecongeo.54.7.1250/ Swanson-Hysell, N.L., Ramezani, J., Fairchild, L.M., and Rose, I.R., 2019, Failed rifting and fast drifting: Midcontinent Rift development, Laurentia’s rapid motion and the driver of Grenvillian orogenesis: Geological Society of America Bulletin, v. 131, nos. 5-6, p. 913-940. Tyrrell, C.W., 2019, Keweenaw Fault Geometry and Slip Kinematics – Bête Grise Bay, Keweenaw Peninsula, Michigan: Michigan Technological University, M.S. thesis, 30 p. Wadsworth, M.E., 1884, On the relation of the Keweenawan series to the eastern sandstone in the vicinity of Torch Lake, Michigan: Boston Soc. Natural Hist. Proc., v. 23, p. 172-180. White, W.S., 1952, Imbrication and initial dip in a Keweenawan conglomerate bed, J. Sediment. Petrol., v. 22, pp. 189-199. White, W.S., 1956, Geologic Map of the Chassell Quadrangle, Michigan: U.S. Geological Survey, Washington, D.C., Mineral Investigations Field Studies Map MF-43, scale 1:24,000. White, W.S., 1968, The native copper deposits of northern Michigan: in Ridge, J.D., ed., Ore Deposits of the United States, 1933-1967 (Graton-Sales Vol.): New York, Am. Inst. Mining Metall. Petroleum Engineers, v. 1, p. 301-325. White, W.S., 1985, “Unpublished diamond drillhole core logs”: U.S. Geological Survey, Field Records Collection, Boxes 282, 287-290. Woodruff, L.G., Schulz, K.J., Nicholson, S.W., and Dicken, C.L., 2020, Mineral deposits of the Mesoproterozoic Midcontinent Rift system in the Lake Superior region – A space and time classification: Ore Geology Reviews, v. 126, p. 1-21. 135 �136 �Field Trip 5 Geology and History of a Native Copper Mine: Adventure Mine, Ontonagon County, Michigan Theodore J. Bornhorst Department of Geological and Mining Engineering and Sciences and A.E. Seaman Mineral Museum, Michigan Technological University, 1404 E. Sharon Avenue, Houghton, MI 49931 Matt Portfleet Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931 [Latitude: 46.777224; Longitude: -89.081906] Directions: Leave downtown Houghton and head south on M-26 towards South Range. Stay on M-26(highway turns into M-38 past the Mass City turnoff). Stay on M-38 towards Ontonagon. Follow signs to Adventure Mine. The Adventure Mine is privately owned and operated as a publicly available mine tour Introduction The historic Adventure Mine is part of the Greenland-Mass subdistrict of the Keweenaw Peninsula native copper district of the western Upper Peninsula of Michigan (Figures 1 and 2). The Adventure Mining Company began mining native copper in 1850. It permanently ceased mining in 1917 at the time when most small mines of the subdistrict ceased mining operations. From the 1980s to today, the mining activities at the Adventure Mine have yielded specimens of massive native copper and copper crystals for purchase by tourists and mineral collectors. The Adventure Mine is a Keweenaw Heritage Site of the Keweenaw National Historical Park. It is located in the Greenland-Mass subdistrict about 40 km southwest of the Baltic Mine in Painesdale, the southernmost major native copper mine in the district (Figure 2). The subdistrict yielded about 85 million lbs (39 million kg) of refined copper at grades ranging from 0.5 to 1.25 percent (Butler and Burbank, 1929; Weege and Pollock, 1971). The Adventure Mine was the second largest producer in the subdistrict, yielding about 11 million lbs. (5 million kg) of refined copper from the tops of five different basaltic lava flows (Butler and Burbank, 1929). Despite the relatively low production of copper from the Greenland-Mass subdistrict (~0.8 % of the total district production of ~ 5 billion kg (11 billion lbs), the geologic characteristics of the Greenland-Mass subdistrict deposits are typical of the native copper deposits elsewhere in the Keweenaw Peninsula (e.g., Stoiber and Davidson, 1959; Butler and Burbank, 1929; etc.). Since 137 �mining in the Keweenaw Peninsula native copper district ceased in 1968, underground access to observe or study the native copper deposits has been limited. Currently there is access to only two mines in the main area of the district (Quincy and Delaware) and two mines in the Greenland-Mass subdistrict (Adventure and Caledonia). These mines are operated as underground experiences for tourists, except for Caledonia. This field trip guide relies on existing publications by Bornhorst and Whiteman (1995); Bornhorst et al. (2013); Bornhorst and Barron (2011); and Butler and Burbank (1929). The overview of the geologic and part of the human history is summarized from Bornhorst and Lankton (2009), Bornhorst and Mathur (2016), and Bodden et al. (2022). Brandon Erickson prepared a brief history of the Adventure Mine which is included in this guide. Figure 1: Simplified bedrock geology of the Mesoproterozoic Midcontinent Rift around Lake Superior. Modified after Bornhorst et al., 2013) Overview of Geologic History The largest known accumulation of native copper in the world is the Keweenaw Peninsula native copper district (Figure 2). In comparison to other copper mining districts where major copper ore 138 �minerals are sulfides, nearly all of the copper in the district occurs as native copper. About 5 billion kg (11 billion lbs) of refined copper were extracted from about 380 million tons of ore between 1845 and 1968 via underground mines (Weege and Pollock, 1971). Small quantities of native silver occur with the native copper. An estimate using incomplete records suggests the amount of silver was between .05 to .5 oz per ton of ore. Native copper and silver were coprecipitated. The native copper deposits are hosted by the Mesoproterozoic Midcontinent Rift (MCR) (Figures 1 and 2). More than 25 km of volcanic rocks and 8 km of clastic sedimentary rocks fill the center of the MCR (Cannon et al., 1989 and 1993). These rift-filling rocks were emplaced between about 1.15 to ~1 Ga (Cannon et al., 1989; Davis and Paces, 1990; Kulakov et al., 2018; Heaman et al., 2007). Figure 2: Simplified geologic map of the Keweenaw Peninsula and vicinity modified from Bornhorst et al. (2013). At the White Pine mine “Keweenawan” native copper cuts across diagenetic the shale-hosted chalcocite deposit. Early eruptions of MCR basaltic lava flows were scattered on a broad land area above a developing mantle plume. These early eruptions were followed by many eruptions from fissure vents concentrated in the center part of the MCR (now buried under the center of Lake Superior). These eruptions were dominated by fissure volcanoes along linear faults. The Portage Lake Volcanics and Porcupine Volcanics are rift-filling basaltic volcanic rocks erupted about 1.1 billion years ago 139 �during the active rifting of the MCR (Figure 3). The MCR was bounded on the edges by down dropped normal faults resulting in the MCR being a faulted basin. The basin progressively dropped down by stretching and by magma erupted at the surface during active rifting. Eruptions of basalts of the Portage Lake Volcanics were on land surface (subaerial). Subaerially erupted basalt lava flows have either a vesicular or brecciated and vesicular top (pahoehoe or aa lava flow). Mineral filled vesicles are termed amygdules and flow tops dominated by vesicles are termed amygdaloids and those dominated by breccia clasts of amygdaloidal basalt are termed fragmental amygdaloids. The top of a subaerial lava flow is underlain by a massive (relatively vesicle-free) basalt. Massive basalt flow interior in thinner flows is fine grained and in thicker flows it is coarse-grained (ophitic). Thin flows can be vesicular throughout with vesicles more abundant at the top. The typical flow is 10 to 20 m thick. Eruptions of basaltic lavas were cyclical and during eruptive hiatuses minor gravel and sand were deposited on top and infiltrated in the tops of occasional lava flows. These clastic sedimentary layers are overlain by basalt lava flows. A desert environment 1.1 billion years ago resulted in red coloration of these clastic sedimentary rocks. Figure 3: Stratigraphic column for the Adventure Mine region with approximate ages. Active rifting and basaltic volcanic activity ended over a short period of time, but the rift basin continued to passively sag and was progressively filled with clastic sedimentary rocks from the Copper Harbor Conglomerate to the Freda Sandstone (Figure 3). In the Adventure Mine region, the 140 �rift-filling volcanic rocks (Porcupine Volcanics and Portage Lake Volcanics, Figure 3) were first covered by red-colored gravels and sands (Copper Harbor Conglomerate. Overlying the Copper Harbor Formation are black- to gray-colored muds and silts (Nonesuch Formation). Lastly the rift was filled with a thick section of red-colored fine sandstones (Freda Sandstone). Today, the rocks of the Keweenaw Peninsula and Adventure Mine region span the edge of the MCR, and consist of a thick section of rift-filling subaerial basaltic lava flows overlain by a thick section of rift-filling clastic sedimentary rocks (Figures 1, 2, and 3). The last and final phase of the MCR resulted from a regional compressional event due to collision of continental land mass along the eastern edge of North America at that time (Grenville Orogeny, Cannon, 1994). Compression resulted in reverse and thrust faults as well as folding and fracturing of rift-filling volcanic and clastic sedimentary rocks. Native copper and related minerals were emplaced during this regional compressional event (Bornhorst, 1997). From the beginning of regional compression at about 1.06 to 1 billion years ago the Jacobsville Sandstone was deposited in a rift-flanking basin until about 1 billion years ago (Figure 2). There were no recorded geologic events by rocks of Michigan’s Upper Peninsula from about 1.0 billion years ago to 500 million years ago. Erosion likely exposed the native copper deposits at the surface and downward percolating oxidizing groundwaters had access to alter the native copper (Bornhorst and Robinson, 2004). The MCR rocks were buried by Phanerozoic rocks deposited from 500 to 175 million years ago (Catacosinos and others 2001). The native copper deposits of the Keweenaw Peninsula were again at the surface after erosion of overlying rocks by Pleistocene glaciers over the last 2.5 million years. Native Copper Deposits of the Keweenaw Peninsula The cumulative pre-mining geologic copper resource of the Keweenaw Peninsula native copper district totaled about 9 billion kg of copper (20 billion lbs; Bornhorst and Barron, 2011). About ½ of the geologic resource was recovered. Speculative concentrations of copper in rocks are even greater. These concentrations have not been mined for various reasons such as too low of grade or too deep below the surface. Permeable and porous primary geologic settings that have sufficient open spaces that were sufficiently connected with each other facilitated the movement of ore-forming hot waters (hydrothermal fluids) from which native copper and other minerals were precipitated. Compression of the MCR strata integrated the primary permeability and porosity of the hydrothermal plumbing system with compression generated faults/fractures (Bornhorst, 1997). The absolute age of the main-stage of hydrothermal activity coincides with the age of the regional compressional event at about 1.06 to 1.04 billion years ago (Bornhorst et al., 1988). The permeable and porous tops of amygdaloidals and fragmental amygdaloids hosted ~58.5% of produced native copper. Horizons of conglomerate and sandstone between lava flows, which are also permeable and porous, hosted ~39.5% of produced native copper. Native copper ore bodies are "sandwiched" on the bottom side by the massive basalt interior of the flow whose top hosts the native copper ore body. On the top side the ore bodies are sandwiched by massive basalt of the overlying lava flow. The sandwiched ore-bodies are geometrically approximately tabular 141 �(called lode) with a thickness between 3 and 5 m and the same orientation as surrounding host lava flows. The typical lode extends down-dip 1.5 to 2.6 km and has a lateral extent of 1.5 to 11 km (Butler and Burbank, 1929; White, 1968). Open spaces in amygdaloidal lava flow tops (vesicles) and sandstones/conglomerates are typically up to a cm across. They are typically filled dominantly by gangue minerals and lesser native copper. Less frequently the entire open space was filled with masses of native copper. Open spaces between breccia fragments in the top of a lava flow (fragmental amygdaloid) or between clasts in conglomerate will tend to have larger masses of native copper and can weigh up to several lbs, to tens of lbs to hundreds of lbs. and rarely weighing tons. A minor amount of the total produced native copper, ~ 2%, was from sub vertical tabular open spaces (veins when filled with minerals) that follow faults and fissures that perpendicularly cut across the volcanic-dominated strata. Ore-forming hydrothermal fluids readily moved along faults and fissures since they have a relatively large amount of interconnected open-space; these ore-bodies are also tabular lodes. Since the size of open space is large the corresponding size of masses of native copper can also be large weighing multiple tons with the largest masses being several hundred tons. Native copper is closely associated with about 22 common and many more uncommon minerals (Bodden et al., 2022; Butler and Burbank, 1929; White, 1968). These minerals fill the same open spaces along with and instead of native copper. The suite of minerals is similar to those found where rocks have undergone very low to low grade burial metamorphism at less than about < 300OC (Bodden et al., 2022). Thermal modeling suggests that peak burial metamorphic conditions at depth were between 400 to 500oC (Woodruff, 1995). Burial metamorphic processes resulted in ore-forming hydrothermal fluids carrying copper leached from the tops of buried riftfilling basalt lava flows. Batches of metamorphogenic-dominated hydrothermal fluid generated over time were similar to one another. The rift-filling volcanic rocks were very low in sulfur when they erupted, and the little contained sulfur degassed into the atmosphere. During subsequent deposition of rift-filling clastic sedimentary rocks there was an incursion of seawater into the rift for a significant amount of time. This resulted in seawater deeply penetrating into the underlying rift-filling volcanic rocks (Figure 4A). During burial, the rift-filling volcanic and clastic sedimentary rocks were progressively heated and during initial heating the seawater evolved to be depleted in sulfur similar to expelled modern sea floor hydrothermal fluids (Figure 4B). Continued heating during burial resulted in burial metamorphic-dominated hydrothermal fluids with copper leached from the rift-filling volcanic rocks. These fluids were well mixed with the evolved seawater resulting in hybrid metamorphic-dominated ore-forming fluids (Figure 4C). These main-stage hybrid metamorphic-dominated ore-forming hydrothermal fluids moved upwards from the source zone through the same very sulfur poor strata as in the source rocks (Figure 4D). As they moved upwards they cooled, interacted with host rocks, and in the relatively shallow zone of precipitation they variably mixed with sulfur-poor reduced meteoric water (Figure 4D; Bodden et. al, 2022). These processes resulted in precipitation of native copper and main-stage hydrothermal minerals. Higher temperature main-stage mineral assemblages are spatially associated with the area of native copper deposits where the thermal anomaly was greatest 142 �because of focused hydrothermal fluids. Within the native copper district, the suite of main-stage minerals that is associated with native copper and is followed by late-stage minerals precipitated at lower temperature than the main-stage. Figure 4: Cartoon cross sections showing conceptual genetic model of the native copper deposits of the Keweenaw Peninsula formed at about 1060 to 1040 million years ago. Modified from Bodden et al. (2022). A. Marine incursions and seawater penetration during deposition of volcanic and sedimentary rocks in MCR. B. Area prior to burial metamorphism with sulfur depleted evolved seawater providing salinity for ore-forming fluids. C. Burial metamorphic fluids mixing with evolved seawater produce hybrid ore-forming fluids. D. Precipitation of main-stage minerals, including native copper, as a result of mixing of ore-forming fluids with meteoric water, decreasing temperature, and water-rock reactions. 143 �Figure 5: Bedrock geologic map and cross sections of the Greenland-Mass subdistrict of the Keweenaw Peninsula Native Copper District. Modified from Whitlow (1974). 144 �The Evergreen Succession Butler and Burbank (1929) recognized the Evergreen lava flow and a succeeding number of lava flows of the Portage Lake Volcanics as having distinctive lithologies and hosting the native copper deposits in the Greenland-Mass subdistrict (Figure 3 and 5). These are informally termed the Evergreen Succession (Figure 3). The Evergreen Succession is stratigraphically about 150 m (500 ft) above the Bohemia (No. 8) conglomerate (Butler and Burbank, 1929). The Evergreen Succession basaltic lava flows are slightly more intermediate in composition than other lava flows within the PLV (Butler and Burbank, 1929). They are characterized by porphyritic or glomerporphyritic texture although thicker flows are ophitic. It is difficult to correlate individual lava flow with one another except in developed areas for mining of native copper where individual lava flow can be traced along strike from mine to mine. In the Greenland-Mass subdistrict most of the native copper was produced from the tops of lava flows of the Evergreen Succession. Those flow tops hosting native copper are generally fragmental amygdaloidal lodes with the best areas for native copper being where the flow top is thicker. Thin amygdaloidal only flow tops or those with areas of massive basalt mixed in the flow top are typically lower grade. The Evergreen Succession is at a similar stratigraphic position as those lava flows developed at the Isle Royale Mine to the north of the subdistrict in the Houghton area of the main district (Butler and Burbank, 1929). The Evergreen flows were also developed for native copper in the Winona area in the middle between the main district and the subdistrict. The individual copper-rich lava flows within the Evergreen succession were each informally named (Figure 3). The Evergreen flow is a 3 to 15 m thick plagioclase porphyritic lava flow. The Ogima flow is a 30 to 43 m thick slightly plagioclase glomerophyritic basalt lava flow. The Butler flow is a 15 to 27 m thick plagioclase glomerporphyritic basalt lava flow. The Mass and Merchant flows are up to about 25 m thick. The South Knowlton flow is up to 15 m and is a plagioclase glomeroporphyritic basalt. At the top of the Evergreen succession is the Knowlton flow which is a 9 to 21 m thick plagioclase glomeroporphyritic basalt. Between the Butler and Knowlton flows there are a number of thin flows of plagioclase glomeroporphryitic basalt with total thickness of 75 to 90 m thick (Calumet and Hecla, 1958). The tops of the Evergreen lava flows were productive over a strike length of about 5 km. Native copper mined from the Evergreen Succession was extracted from many different mines with some of them connecting with others. The Butler flow top yielded the most copper followed by the Evergreen and Knowlton flow tops which also yielded significant amounts of copper. Vesicle- and inter-fragment void-fillings consist of quartz, calcite, K-feldspar, epidote, prehnite, pumpellyite, and chlorite (Table 1). Less abundant main-stage minerals are native copper, native silver, and datolite. Laumontite and adularia are common late-stage minerals. 145 �Table 1: Percent amygdule-filling minerals estimated from rock piles adjacent to mines/shafts of the Greenland-mass subdistrict. Unpublished data by Stoiber and Davidson 1959). % Amygdule-Filling Mineral Quartz Calcite Mine/Shaft Adventure #1 Adventure #2 Adventure #3 Adventure #4 National #2 Old Mass Mass C Mass 1 &2 Mass B Mass A Michigan Michigan Flintsteel #1 Flintsteel #2 Butler Knowlton Red KFeldspar Epidote Prehnite Pumpellyite Chlorite 52 5 6 26 2 9 tr 30 20 5 40 0 1 4 11 13 30 37 5 1 3 36 22 17 22 55 19 45 63 21 18 23 20 30 27 17 21 10 23 22 16 27 30 52 45 45 38 0 3 40 45 0 40 trace trace 35 18 18 20 15 2 5 15 19 22 18 35 5 3 5 5 5 6 24 53 4 0 0 0 trace 5 10 0 0 0 8 10 0 2 3 trace trace 3 0 0 trace 0 10 0 1 0 1 1 trace 1 1 0 0 7 9 0 8 The Evergreen Succession in the Greenland-Mass subdistrict dips about 45o NW and forms a local broad open anticline (Fig. 6). The largest mine, the Mass Mine, occurs near the maximum bend in this anticline. Most faults have displacement of < 1 m while those faults with significant vertical displacement are uncommon. There are multiple veins in tension fractures in the area of maximum bend that cut perpendicular across the lava flows (Butler and Burbank, 1929). There are some veins that are parallel to the strike of the lava flows but dip in the opposite direction. In the stratigraphically equivalent Isle Royale lode, Broderick (1931) describes similar strike parallel veins which he interpreted to be feeders of ore-forming fluid into the top of the lava flow. The Adventure Mine The Adventure Mine was very small, producing only about 5 million kg (11 million lbs) of refined copper, in context of all mines in the district which produced about 5 billion kg (11 billion lbs. Most of the production of native copper from the Adventure Mine came from the top of the Knowlton basalt lava flow (Knowlton lode; Figure 6 and 7). There was also significant 146 �production from the Butler lode and minor production from the Evergreen, Ogima, and Merchant lodes (Figure 6 and 7). Figure 6: Historic 1902 sketch cross section showing the lodes of the Evergreen succession at the Adventure Mine. Adits perpendicular to strike of the lava flows are shown and drifts parallel to the strike are indicated by black squares. The field excursion utilizes the adit near the No. 1 shaft. Figure 7: Longitudinal sections (parallel to strike) of the Evergreen succession native copper lodes showing underground workings (openings) for the Adventure Mine shafts #1 to #4. Section from Butler and Burbank (1929). 147 �The Knowlton was the focus of native copper mining at the Adventure Mine. The Knowlton lava flow top is a fragmental amygdaloid. In the subdistrict, the Knowlton flow top was developed for about 3000 m along strike and to a maximum depth of about 375 m. At the nearby Mass Mine (Fig 5), the Bulter lava flow top was the principal focus of native copper mining. It was the second focus of mining at the Adventure Mine. In the subdistrict the Butler lava flow top was developed for about 2000 m along strike and to a maximum depth of 300 m down dip. The most abundant secondary minerals in the Butler are quartz and calcite with slightly lesser amounts of K-feldspar and epidote (Table 1). Prehnite and pumpellyite are usually much less abundant and chlorite is present in amounts < 1 %. The Butler contains a high number of veins. Usually, the veins strike subparallel to the strike of the Butler lava flow top and have dips both similar to the dip of bedding and at a high angle to bedding (Butler and Burbank, 1929). The average thickness of the Knowlton lava flow top is about 2.5 m but locally it can thicken to around 6 m (Calumet and Hecla, 1958). In general, a thicker flow top results in better ore. While most of the ore occurs in the top of the Knowlton lava flow top, there are pockets of ore that extend into the underlying Knowlton massive flow interior (footwall) and are closely associated with strikeparallel fractures and veins which were likely feeders of hydrothermal fluids (Bornhorst et al, 2013). At the Adventure Mine, on average the most abundant main-stage minerals filling amygdules and spaces between fragments is quartz which is closely followed by epidote and then calcite and red K-feldspar (Table 1). There are lesser amounts of prehnite, pumpellyite, and chlorite. Native copper is present in small amounts with average grades of between 0.5 to 1.25 % copper with native copper associated epidote, quartz, and calcite. Native silver and datolite are present in much lesser amounts. Least abundant are the late-stage hydrothermal minerals precipitated after native copper that occur in open space fillings as coatings on earlier formed minerals; late-stage minerals include calcite, laumontite, and adularia and in cross cutting fractures and veins. Alteration of hydrothermal mineral is most obvious for native copper. Tenorite and cuprite (Cu oxide) often but not always occurs as a thin coating on native copper that is found in open space fillings. The tenorite and cuprite likely formed by downward-percolating groundwater when the native copper deposits were sufficiently near the surface (supergene alteration) as they are today. In addition to tenorite and cuprite, there are occasional copper carbonate minerals (such as malachite), brochantite (hydrated Cu sulfate) and atacamite (hydrated Cu chloride). These are likely to be supergene in origin. At least one mineral, gerhardtite (hydrated Cu nitrate) is the result of chemical reactions involving explosives. At the Adventure Mine a near horizontal cross-cut adit beginning at Shaft No. 2 connects to the near horizontal Butler drift (Figure 8). To the southeast the Butler drift daylights at the Overview Entrance/Exit. To the northwest the Butler drift connects with the Shaft No. 1 cross-cut adit and with the cross-cut adit to the Ogima lode where there is a large, in place mass, of native copper (Figure 8). The underground of the Adventure Mine is at a stable temperature of about 6oC and is relatively dry and regular field shoes are usually sufficient; hard hats and lights are required and provided by Adventure Mining Company (tour operator). The field trip involves an easy walk 148 �underground to observe the character of native copper mineralization in a horizontal adit that cross cuts the lava flows and a horizontal drift that parallels the strike of the top of the Butler lava flow which hosts a tabular native copper ore body (lode) (Figure 8). The character of native copper mineralization is readily observable in adits, drifts, and stopes (Figure 9). Figure 8: A. Longitudinal section of the Butler lode at the Adventure mine showing workings/openings projected to the vertical and locations for the field trip. B. Geologic map of the Adventure Mine showing workings/openings projected to the horizontal and locations for the field trip. Modified from Butler and Burbank (1929). 149 �Figure 9: Cross section sketch of the topography at the Adventure Mine showing top of the Butler lava flow and the Shaft No. 1 crosscut adit. Overview of the Human History As the land surface of the Keweenaw Peninsula emerged above the progressively retreating glacial lake levels by ca. 7,000 years ago native people took an interest in native copper since its malleability facilitated making tools. At first native peoples likely found boulders of native copper deposited from the glaciers (locally termed float copper) along with gravels and sands derived from erosion of local bedrock and bedrock north of Lake Superior in Canada. These boulders of native copper have a distinctive weathered surface crust of malachite, a green copper-bearing mineral. The green color would have made the relatively infrequent float copper boulders stand out among the other brown, gray, red, and white rocks. The float copper would have also been much heavier than other rocks of the same size. The native people shaped the native copper into tools and decorations. After they depleted the float copper boulders on the surface, they needed a new source of native copper to be able to continue making and trading these items. The native peoples likely found native copper in bedrock because of the green coloration as compared to black- or red-colored host rocks and then became prehistoric miners. There are many shallow mine pits throughout the Greenland-Mass subdistrict. Early European explorers were shown specimens of float copper by native inhabitants which created interest in the Keweenaw Peninsula. In 1841, Douglass Houghton’s report to the Michigan legislature (Michigan’s first state geologist) sparked the first major mining rush in North America. The first significant discovery of native copper was in 1845 at the Cliff Mine which in 1849 became the first profitable native copper mine in the Keweenaw Peninsula native copper district. There were only a few profitable native copper mines from 1845 to the early 1860s. The Minesota Mine, in the Greenland-Mass subdistrict (Figure 6) southwest of the Adventure Mine, became profitable a few years after the Cliff Mine. Many discoveries led to the opening of many mines in the early 1860s. But by 1880, the fate of most of these mines was the same as described below. Initial excitement of possible riches from mining copper was promoted by discoveries of mass copper that implied high grade ore (Figure 10C). Unfortunately, the existence of masses of copper did not necessarily indicate high grade ore. These masses represent the sampling problem termed the “nugget effect.” When the deposit formed there was a clustering of copper in distinct parts of the ore body into a “nugget”. If the mass of copper was missed during exploration the estimate of the grade of the ore body could be far too low and if a 150 �Figure 10: Historic photos from the Adventure Mine. mass is found the grade can be far too high. Mining decisions, such as putting in a shaft or constructing an oversized mill, made from discovery of masses of copper can be costly mistakes especially when funds are limited. Many of the mining projects were underfunded making it difficult to succeed and as a result of lack of funds the operations had to frequently close and in many cases the company merged with another company if they could convince shareholders of potential to discover a large ore body with high grade. There were also frequent shareholder assessments. Rather than paying a dividend to each shareholder from excess funds (profits) an 151 �assessment is the opposite and required each shareholder to pay the company a fee for each share. The fate of the Adventure Mine briefly described below follows this fate and was permanently closed and abandoned by 1917. Adventure Mine History (modified with permission from text provided by Brandon Erickson) In 1848 the Adventure Mining Company began exploration for native copper in the GreenlandMass subdistrict. Exploration activities were focused on a topographic bluff where several different lodes were exposed at the surface. By 1850, the Butler lode appeared to have best potential and the first production of native copper began in 1850. Despite the initial promise the mine struggled to turn a profit as the Butler lode was very rich in copper in some areas and in other areas it was barren. By 1855, the Adventure Mining Company itself ceased mining and to survive in 1855 the company introduced a tributing system. Under tributing, miners would receive a percentage of copper profits instead of a daily wage. In 1856, during financial troubles, a water-powered stamp mill was built along nearby Adventure Creek. By the start of the Civil War in 1861, the richest known ore shoots had been mined out and in 1864 the mine was sold to new investors. The new Adventure Copper Company explored the Butler lode on the eastern side of the bluff but also started an exploration crosscut on the northern slope. The purpose of this adit was to cut across all the lodes. Today’s mine tours enter the mine using this adit (Figure 8, Shaft No. 1 crosscut.) By 1869, the adit had reached the Butler lode and the miners commenced drifting along the lode. Several years later this zone proved rich enough to warrant the sinking of a shaft from the top of the bluff, which is seen on today’s tours as the “skylight stope.” (Figure 8). Mining was centered around this area until an economic downturn in 1877 forced the mine to cut costs and once again only support a small handful of tribute miners. In 1890, the tribute miners uncovered the Knowlton lode, which, unlike the others, cropped out at the base of the bluff. The deeping of shaft No. 1 started immediately and soon reached sufficient depth to begin drifting along the Knowlton lode. The Knowlton lode was very rich in stamp rock (fine sand sized copper disseminated throughout the ore body and lacking the nugget effect). The high-grade copper ore incentivized the miners to continue sinking the shaft to 200 feet deep. At these depths, work was severely hindered by the lack of more modern equipment, and without financial backing the Knowlton efforts ceased by 1893. Adventure Mine was revived again in 1898, as the Adventure Consolidated Mining Company and listed on the Boston stock exchange. This new company was backed by 2.5 million dollars perhaps prompted by ”riches” from mass copper (Figure 10C). The new investment at Adventure Mine was sufficient to build a company town, put in a railroad spur, purchase modern drills, construct and equip a state-of-the-art stamp mill, and install an electric tram line. The company first dewatered the No. 1 shaft and started sinking No. 2 shaft. The promising initial results prompted the company to start a new fully modern third No. 3 shaft (Figure 10A and 10B). This shaft is near the present-day parking lot. By 1903, the No. 1 shaft production had declined and was abandoned at a depth of 700 feet. In response, the company started a fourth shaft on the 152 �eastern limit of their lands (Figure 8). This shaft was a failure and was abandoned after a short time. Shipments of ore from the No. 3 shaft (Figure 10A and 10B) to the stamp mill declined in tonnage and grade. The mine was forced to use diamond drilling to explore for a new ore shoot. In 1909, they found a series of promising new lodes, but to reach them a vertical shaft would need to be sunk. This endeavor around 1910 was Adventure Mines’s “ last hope.” The Adventure Consolidated Mining Company diverted all resources to sink a shaft to 1,500 feet and explored several different lodes along the way. Unfortunately, this exploration was a failure as the ore bodies were not large enough or with high enough grade to make them profitable. The Adventure Mine was more or less abandoned in 1910 but one last attempt was made in 1916 when copper prices increased. The No. 3 Shaft (Figure 10A and 10B) was dewatered down to 700 feet and soon after they were shipping 300 tons of concentrate per day to the smelter. This effort was short-lived and on October 27, 1917 the company ceased all mining and processing activities. Keweenaw National Historical Park The historical significance of the Keweenaw Peninsula native copper district can readily be deduced from the fact that 80% of the new copper for the entire United States was produced from the district in 1880. This was the peak of significance of native copper mining in the district. By 1900, the Keweenaw Peninsula produced only 25 % of the United States new copper. However, absolute copper production from the district peaked in 1916 at an annual production of 121 million kg (267 million pounds). Mining ended in the Keweenaw Peninsula native copper district in 1968. The Adventure Mine peaked in production of copper between 1902 and 1907 at a total of about 3.9 million kg (8.5 million lbs.). The Keweenaw National Historical Park was created in 1992 to preserve and interpret the historical importance of native copper mining to the history of the U.S. The national park visitors center in Calumet provides an excellent overview of the historical significance of the district. Keweenaw Heritage Sites are affiliated with and support activities of the national park. The A.E. Seaman Mineral Museum and Quincy Mine are heritage sites and support the activities of the national park. The Adventure mine is also a Keweenaw Heritage Site and welcomes tourists and visitors seeking an underground mining experience. ACKNOWLEDGMENTS We thank Brandon Erickson for his brief Adventure Mine history summary which we modified for this field guide. We thank Allan Blaske for his review of this field guide that provided significant improvements to this guide. REFERENCES CITED Bodden, T.J., Bornhorst, T.J., Bégué, F., and Deering, C., 2022, Sources of hydrothermal fluids inferred from oxygen and carbon isotope composition of calcite, Keweenaw Peninsula native copper district, Michigan, USA: Minerals, v. 12, 474. https://doi.org:10.3390/min12040474 153 �Bornhorst, T.J., 1997, Tectonic context of native copper deposits of the North American Midcontinent Rift System: Geological Society of America Special Paper 312, p. 127-136. Bornhorst, T.J., and Barron, R.J., 2011, Copper deposits of the western Upper Peninsula of Michigan: Geological Society of America Field Guide, v. 24, p. 83-99. Bornhorst, T.J., Barron, R.J., and Whiteman R.C., 2013, Caledonia Mine, Keweenaw Peninsula native copper district, Ontonagon County, Michigan: 59th Institute on Lake Superior Geology Proceedings, v. 59, part 2, p. 43-57. Bornhorst, T.J., and Lankton, L.D., 2009, Copper mining: A billion years of geologic and human history: in Schaetzl, R., Darden, J., and Brandt, D., eds, Michigan Geography and Geology, Pearson Custom Publishing, New York, p. 150-173. Bornhorst, T.J. and Mathur, R., 2017, Copper isotope constraints on the genesis of the Keweenaw Peninsula native copper district, Michigan USA: Minerals, v. 7, 185, https://doi.org:10.3390/min7100185 Bornhorst, T.J., Paces, J.B., Grant, N.K., Obradovich, J.D., and Huber, N.K. 1988. Age of native copper mineralization, Keweenaw Peninsula, Michigan: Economic Geology, v. 83, p. 619-625. Bornhorst, T. J., and Robinson, G.W., 2004, Precambrian aged supergene alteration of native copper deposits in the Keweenaw Peninsula: Michigan; Institute on Lake Superior Geology Proceedings and Abstracts, v. 50, part 1, p. 40-41. Bornhorst, T.J., and Whiteman, R.C., 1995, Native copper and associated minerals in basalts at the Caledonia Mine, western Upper Michigan: 41st Institute on Lake Superior Geology Proceedings, v. 41, part 1, p. 34. Bornhorst, T.J., and Whiteman, R.C., 1992, The Caledonia native copper mine, Michigan: Society of Economic Geologists Guidebook Series, v. 13, p. 139-144. Butler, B.S., and Burbank, W.S., 1929, The copper deposits of Michigan: U.S. Geological Survey Professional Paper 144, 238 p. Calumet and Hecla, 1958, Unpublished report for Defense Minerals Exploration Administration, 29p. Cannon, W.F., 1994, Closing of the Midcontinent Rift – A far field effect of Grenvillian contraction: Geology. 22, p. 155-158. Cannon, W. F., Green, A. G., Hutchinson, D. R., Lee, M.W., Milkereit, B., Behrendt, J.C., Halls, H.C., Green, J.C., Dickas, A.B., Morey, G.B., Sutcliffe, R., and Spencer, C., 1989, The North American mid-continent rift beneath Lake Superior from Glimpse seismic reflection profiling: Tectonics, v. 8, p. 305-332. Cannon, W. F., Peterman, Z.E., and Sims, P.K. 1993, Crustal-scale thrusting and origin of the Montreal River monocline - A 35-km-thick cross section of the Midcontinent Rift in northern Michigan and Wisconsin: Tectonics, v. 12, p. 728-744. 154 �Catacossinos, P.A., Harrison, W.B., Reynolds, R.F., Westjohn, D.B., and Wollensak, M.S., 2001, Stratigraphic lexicon for Michigan: Michigan Department of Environmental Quality, Geologic Survey Division Bulletin 8. Lansing, MI. Davis, D.W., and Paces, J.B., 1990, Time resolution of geologic events on the Keweenaw Peninsula and implications for development of the Midcontinent Rift system: Earth and Planetary Science Letters, v. 97, p. 54-64. Heaman, L.M., Easton, R.M., Hart, T.M., MacDonald, C.A., Hollings, P., and Smyk, M., 2007, Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario: Canadian Journal of Earth Sciences, v. 44, p. 1055-1086. Stoiber, R.E., and Davidson, E.S., 1959, Amygdule mineral zoning in the Portage Lake Lava Series, Michigan copper district: Economic Geology, v. 54, p. 1250-1277, p. 1444-1460. Weege, R.J., and Pollack, J.P., 1971, Recent developments in native-copper district of Michigan: Society of Economic Geologists Field Conference, Michigan Copper District, September 30 - October 2, 1971, p. 18-43. White, W.S. 1968, The native-copper deposits of northern Michigan: in Ridge, J.D., ed., Ore Deposits of the United States, 1933-1967 (the Graton Sales volume). American Institute of Mining, Metallurgical, and Petroleum Engineering, New York: p. 303-325. Whitlow, 1974, Geologic map of the Greenland and Rockland quadrangles, Ontonagon County, Michigan: U.S. Geological Survey Miscellaneous Field Studies Map MF-596. Woodruff, L.G.; Daines, M.J.; Cannon, W.F.; Nicholson, S.W., 1995, The thermal history of the Midcontinent Rift in the Lake Superior region: implications for mineralization and partial melting: in International Geological Correlation Program, Field Conference and Symposium on the Petrology and metallogeny of volcanic and intrusive rocks of the Midcontinent rift system, Duluth, Minnesota, v. 336, p. 213-214. 155 �156 �Field Trip 6 Southern Complex Granitoids, Gneisses, and Migmatites: New Data, Discoveries, and Perspectives Chad D. Deering Michigan Technological University, 1400 Townsend Dr., Houghton, MI 49931 Introduction The Superior Province is part of the Archean Canadian Shield in North America and represents one of the oldest and most stable cratonic regions on Earth, encompassing parts of Canada and the United States, including Michigan, Wisconsin, and Minnesota. The Archean craton in Northern Michigan is divided into a Northern Complex and Southern Complex, which are separated by the Great Lakes Tectonic Zone (GLTZ) (Morey and Sims, 1976; Sims et al., 1980). The northern portion of the Southern Complex consists of classic ‘dome-and-keel’ structures characterized by domes of Archean basement surrounded by keels of Paleoproterozoic Marquette Range Supergroup lithologies (Annhaeusser et al., 1969). The Archean rocks include a complex assemblage of granitoids, gneisses, and migmatites intruded by numerous mafic dikes and/or sills. Our recent research on the area has revealed new aspects of the igneous and metamorphic evolution that have improved our understanding of the assembly of this large igneous-metamorphic complex through the Archean-Proterozoic transition, but numerous questions regarding the origin evolution of these rocks remain unresolved. This field excursion will include the exploration of a number of different terrains representative of the magmatic, metamorphic, and structural evolution of the Southern Complex and the overlying metasedimentary rocks; highlighting new discoveries while at the same time providing an opportunity to investigate still unresolved questions regarding the geologic evolution of the region. Figure 1. Regional map outlining the location of the Southern Complex Mineral District near Marquette, Michigan. 157 �Evolution of the Superior Craton Mesoarchean The Mesoarchean evolution of the Superior Craton in North America encompasses a critical period of crustal evolution between 3.2 to 2.8 billion years ago. During this time, significant tectonic and magmatic processes shaped the early Earth's crust and laid the foundation for the stable continental core we recognize today. In the early Mesoarchean, around 3.2 billion years ago, smaller continental fragments began to accrete and amalgamate due to tectonic processes related to subduction and associated magmatic activity (Percival et al., 2012; Thurston et al., 2008; Wyman, 2010). These proto-continents served as the building blocks for the Superior Craton (Percival et al., 2012). Intense magmatic activity occurred, leading to the generation and growth of the continental crust within the Superior Craton, as magma intruded into and solidified within the existing crust (King et al., 1998). Greenstone belts, characterized by volcanic and sedimentary rocks, also began to form during this period. These belts, such as the Abitibi and Wawa greenstone belts of Canada (Thurston, 2002) and the Ishpeming greenstone belt of the Upper Peninsula, Michigan, USA (Bornhorst and Johnson, 1993), are important features of the Superior Craton and provide insights into early Earth processes, including volcanic activity and the nature of oceanic environments. The rocks of the Superior Craton underwent significant metamorphism and deformation during the Mesoarchean and high temperatures and pressures caused by tectonic activity led to the development of foliations and other prominent structural features in the rocks. Neoarchean During the Neoarchean Eon, which lasted from approximately 2.8 to 2.5 billion years ago, the crust of the Superior Craton underwent further significant geological evolution. This involved the continued accretion of smaller continental blocks and terranes through tectonic processes such as subduction, collision, and magmatic activity (Mole et al., 2021). In particular, the Minnesotan orogeny occurred around 2.7 to 2.6 billion years ago. This was a period of intense tectonic activity characterized by the collision and amalgamation of various smaller continental blocks and island arcs, leading to the formation of a larger continental mass (Schmitz et al., 2018). The collisional process resulted in the growth of the Superior Province and the formation of the granite-greenstone terranes that comprise much of the region. The amalgamation of these crustal fragments contributed to the expansion and stabilization of the Superior Craton. Neoarchean rocks include extensive granitic intrusions, which formed through the partial melting of existing crustal rocks or through the emplacement of mantle-derived magmas (Mole et al., 2021). These granitic intrusions contributed to the growth of the continental crust and are often associated with mineralization and hydrothermal activity (Mole et al., 2021). Greenstone belts, characterized by volcanic and sedimentary rocks, continued to develop during the Neoarchean within the Superior Craton (Polat et al, 1998; Polat and Kerrich, 2000). These belts represent ancient oceanic crust and island arc environments, and they are interspersed with granitic intrusions. Metamorphic processes affected both the greenstone belts and the granitic intrusions within the craton. Hydrothermal activity continued to play a significant role in the formation of mineral deposits within the Superior Craton during the Neoarchean. Ore deposits such as gold, iron, and copper formed in association with granitic intrusions, greenstone belts, and hydrothermal alteration zones, contributing to the economic significance of the region (Mole et al., 2022). 158 �Southern Complex, Marquette District: Compeau Creek and Bell Creek batholith The southern complex is located south of the Marquette synclinorium and is dominated by the Archean Bell Creek batholith, which consists primarily of coarse-grained megacrystic, high-K igneous rocks with minor amounts of mafic gneiss and metasedimentary rock layers typically found concordant with the foliation of the gneiss. Bell Creek includes trondhjemite-tonalitegranodiorite (TTG), granites, gneisses, and migmatites. Migmatite comprises only a small portion of the complex, distributed at irregular intervals throughout the region and is assigned to a unit referred to as Compeau Creek. The age and origin of the Southern complex of the Marquette region has been debated for decades. It was originally thought to be genetically related to similar lithologies found in the nearby Northern Complex (Cannon and Simmons, 1973; Van Schmus and Woolsey, 1975). However, the Southern Complex is separated from the Northern complex by the Great Lakes Tectonic Zone (GLTZ), which is a continental scale suture/fault zone (Morey and Sims, 1976; Sims et al., 1991). The only age information available before our study of the Bell Creek batholith was obtained by Tinkham (1997) from a single zircon with a U-Pb age of ~2.61 Ga. This period marks the onset of the Archean-Proterozoic transition, which is associated with a shift from the production of dominantly mantle-derived magmas that differentiated to form new continental crust to the early stages of significant recycling of crustal material (Taylor & McLennan, 1995; Valley, 2005). This is a crucial period in Earth’s history due to a decrease in global heat flow, and potentially the onset of ‘modern style’ subduction (Brown et al., 2020) and an increased number of sedimentary environments (Taylor and McLennan, 1995). The origin of trondjemite-tonalite-granodiorite (TTG) and high-K granites during the Archean Eon, including those produced in the Bell Creek batholith, is closely linked to the evolution of continental crust. Several models have been proposed to explain the formation of TTGs during the Archean: 1) Partial melting of pre-existing continental crust. During the Archean, the Earth's crust was thicker and more mafic (rich in magnesium and iron) compared to modern crust (Tang et al., 2016). As a result, when portions of this crust were subjected to high temperatures and pressures, particularly in subduction zones or during collisional events, they could undergo partial melting to produce high potassium granites. 2) Partial melting of the mantle. In this scenario, mantle-derived melts ascend through the crust, assimilating and interacting with continental crust along the way. These interactions can lead to the enrichment of potassium and other incompatible elements in the melts, ultimately resulting in the formation of high potassium granites. 3) Derivation from hybrid sources. It is also possible that the formation of high potassium granites during the Archean involved a combination of crustal and mantle processes. This hybrid model suggests that both the crust and mantle contributed to the source materials for the granites, with melting and mixing occurring at various depths within the Earth's crust. New petrogenetic insights on the origin of Bell Creek batholith Our new bulk-rock major and trace element data and U-Pb zircon dates (including oxygen and LuHf isotopes) provide insight into several aspects of the origin of these rocks. First, extensive U-Pb zircon dating of Bell Creek and Compeau Creek rocks from several recent Michigan Tech geology graduate student studies (Table 1; Petryk, 2019 and Barth, 2023) indicates that the bulk of the magmas were emplaced during a single tectonic event between ~2.4 to 2.6 Ga and is attributed to the collision of the Paleoarchean Minnesota River Valley Terrane (MRVT). Second, bulk-rock 159 �major and trace element compositions of the Compeau Creek migmatite/gneiss and Bell Creek granitoids/gneisses are consistent with formation in a continental arc type tectonic environment (Figure 2). Table 1. Age summary for Bell Creek granitoids (data from Petryk, 2019) Bell Creek granitoid subgroups Sample U-Pb crystallization age (Ga) Inherited grains (Ga) Metamorphic grains (Ga) CLG-14B 2.42 ±0.042 2.8-3.6 2.3 CCG-12A BCG-4A BCG-1A BCG-8B CCG-9D BCG-7C CCG-9E 2.43 ±0.160 2.51 ±0.021 2.58 ±0.056 2.54 ±0.040 2.59 ±0.028 2.61 ±0.047 2.56 ±0.038 2.7 2.8-3.9 (4.2) 2.7-2.8 3.1 3.3-3.5 2.7-3.3 2.7-2.8 2.0 2.3, 2.4 2.3, 2.4 Fine-grained CCG-6A 2.53 ±0.070 - 2.3, 2.4 Clotted BCG-8A CCG-1A 2.55 ±0.017 2.55 ±0.033 2.7-2.9 2.7-3.2 2.2, 2.3 1.8, 2.3 Normal' Foliated 2.3 Figure 2. Left: Tectonic discrimination diagram showing a volcanic arc to syn-collision tectonic setting for Bell Creek granitoids and gneissic rocks (blue) and two younger alkaline granites consistent that formed within-plate following the emplacement of the Bell Creek batholith. Right: Arc rock discrimination diagram showing the high-K nature of most of the Bell Creek and Compeau Creek rocks. Note that the mafic rocks plotted here represent dikes/sills of varying and relatively unknown age. 160 �Third, the oxygen isotopic composition of zircon from the granitoids is dominated by mantle-like values (5.3±0.3) indicating additional of a significant amount of juvenile magma to the crust; however, the hafnium isotopic compositions of the same zircons are heterogeneous and have model ages that indicate the involvement of older Mesoarchean crustal lithologies at the source and during emplacement in the mid- to upper-crust. Our current interpretation is that the granitoids, gneisses, and migmatites were formed through a hybrid process, initially by partial melting of an isotopically heterogeneous Mesoarchean lower crust followed by assimilation of mid- to uppercrustal lithologies. Continental arc subduction produces TTGs when high heat flow facilitates partial melting of lower crust and potentially later as the magma reaches the level of final emplacement in the mid- to upper-crust. Interestingly, the high oxygen isotopes (δ18O > 6‰) and εHf isotopes, with a range between -2 and -24, together indicate a potentially greater role for older Mesoarchean crust in the formation of the Southern Complex magmas than what has been found in the Canadian portion of the Superior craton. Therefore, it appears as if the evolution of the southernmost region of the Superior Craton involved a much greater contribution from crustal lithologies than what has currently been found in the well-studied Canadian segment of the Superior Craton (Mole et al., 2019). Figure 3. Hf-O isotopic compositions of zircon from Bell Creek TTGs Mid- to upper-crustal level mixing and assimilation The Archean granitoids of the Bell Creek batholith also display field evidence of assimilation and mafic-felsic magma mixing at the level of emplacement. Small (up to a cm or two) xenoliths that appear as clots throughout the granitoids in the area consist of biotite, chlorite, garnet and quartz indicative of assimilation of pelitic crust. However, basement crustal lithologies of this type are apparently not well exposed. In our investigation we have identified several outcrops that have quartzite and schist in contact with, or within, gneissic or granitoid host rocks, but the ages have yet to be determined (in progress). These metasedimentary rocks outcrop within the igneous intrusions and are considered to be the best possible candidates for remnants of Archean supracrustal material that was assimilated into the felsic magmas. In addition, our new O and Hf isotope data from single, inherited zircons indicate incorporation of Mesoarchean age crustal lithologies (Table 1 and Figure 3). Major and trace element data reveal a slightly peraluminous 161 �character (Figure 4), high-K (Figure 2), and enrichment of incompatible trace elements (Figure 2) and are best explained as reflecting the assimilation of metasedimentary crustal lithologies. The mafic intrusions also show evidence of felsic ‘blebs’ and complex interactions with the host granitoids throughout the region and are interpreted to reflect magma mixing, which would indicate that at least some of them are sills rather than dikes. However, later generations of mafic intrusions clearly crosscut the dominant foliation, have sheared boundaries, and/or sharp contacts with the host rock and represent at least four to five distinct episodes of magmatism related to younger events unrelated to the granitoids. Figure 4. Shand's index for peraluminosity of Compeau Creek and Bell Creek granitoid, gneissic and migmatitic rocks. Paleoproterozoic During the Paleoproterozoic Eon (roughly 2.5 to 1.6 billion years ago), the Superior Craton underwent significant geological evolution, marked by a series of tectonic, magmatic, and metamorphic events. The early Paleoproterozoic was dominated by the development of rift basins during the breakup of the Superia supercraton during rifting that began ~2.1 Ga, which separated the Wyoming Province from the Superior Province (Drenth et al., 2021). These rift basins accumulated thick sequences of sedimentary rocks, including sandstones, shales, and iron formations that are today known in the Lake Superior region as the Marquette Range Supergroup and Huronian Supergroup (Ojakangas et al., 2001). The Penokean Orogeny, which occurred around 1.85 to 1.75 billion years ago, resulted from the collision of the Superior Craton with other continental blocks that contributed to the growth of Laurentia, leading to crustal thickening, mountain-building, and metamorphism (Schulz and Cannon, 2007). Hydrothermal activity during the Paleoproterozoic played a significant role in the formation of mineral deposits that include iron-rich sedimentary deposits, such as banded iron formations (BIFs), which were deposited during this time, along with important mineral deposits such as iron, copper, and gold (DeMatties, 2022). Following the main tectonic events of the Paleoproterozoic, the Superior Craton experienced episodes of post-orogenic magmatism, marked by the emplacement of large igneous provinces and granitic intrusions. 162 �Field Trip Objectives This field trip is designed to provide a geologic overview of the formation of the Neoarchean Bell Creek batholith including associated metasedimentary rocks and the Paleoproterozoic metasedimentary rocks that filled the deep basins. Figure 5. Generalized geologic map of the Southern Complex Mineral District. 163 �Stop 1: Paleoproterozoic Negaunee Iron Formation (FR/Xn) and mafic dike Directions: Leaving Michigan Technological University drive south along US-41 ~63 miles to the Michigamme Roadside Park. N 46° 32’ 20” W88° 5’ 31” The outcrop is prominently exposed along the northern side of US-41 across from the Michigamme Roadside Park, which overlooks Lake Michigamme to the south. Paleoproterozoic Negaunee Iron Formation that is weakly magnetic containing hematite-goethite with banded chert as fine laminations. The formation is part of a westward plunging syncline conformably overlying Siamo Slate or Ajibik quartzite with a gradational contact and lies unconformably over the Archean basement complex (Gair and Thaden, 1968). This unit includes sideritic slates, grunerite-magnetite-schists; ferruginous slates, ferruginous cherts and jaspilite (Van Hise and Bayley, 1895; Cannon and Gair, 1970). Here, the iron formation is dipping ~65° to the SW and is in sharp contact with the adjacent massive, medium-grained to porphyritic amphibolite dike. A younger, near vertical dike can be observed on the east side of the outcrop in sharp contact with the older amphibolite dike/sill. Stop 2: Paleoproterozoic Ajibik quartzite (Xa) Directions: Heading east along US-41 ~0.5 miles exposure of Ajibik quartzite outcrops along the northern edge of the road. N 46° 32’ 35” W88° 4’ 16” The Ajibik consists of basal conglomerates, slates, and graywackes that grade into the overlying quartzite. At this location, the small exposure of the Ajibik is massive, thick bedded white to buff orthoquartzite. Some relic cross-bedding may be present, but clear evidence of ripple marks or other features reflecting the original depositional environment are not present. A mafic dike crosscuts the quartzite roughly NW-SE and is highly sheared along a sharp contact. Bedding is difficult to identify, but it appears to be dipping ~60° to the SW. Stop 3: Paleoproterozoic Michigamme formation (Xms) Directions: Head east on US-41 S toward Orange Rd. 8.4 miles. Turn right onto M-95 S, and drive ~7.3 miles, crossing the Michigamme River basin to destination. N 46° 24’ 37” W87° 59’ 45” This exposure is of the lower slate member, which includes laminated iron-rich rock consisting of biotite-garnet-cummingtonite-quartz schist with thin beds of quartzite. Minor fold axes are prominent here as chevron folds plunging to the NW and reflect the regional fold axis orientation. Boudinage, which form when single, competent layers are stretched into separate pieces through plastic and/or brittle deformation mechanisms and reflect the presence of minor quartzite layers or lenses within the schist. 164 �Stop 4: Archean Bell Creek batholith granitoid (Wbcf) Directions: Head southwest on M-95 N ~0.5 miles to destination. N 46° 24’ 13” W87° 59’ 52” The most common form of Bell Creek batholith is exposed here as a medium to coarse-grained granitoid that is locally porphyritic with sparse megacrysts of alkali-feldspar (up to several cms). Minerals include alkali-feldspar, oligoclase, biotite, oxides with apatite and zircon as common accessory phases. The abundance of mafic xenoliths (clots) (Figure 6) within the granitoids is correlated with the degree of peraluminosity of the host rock. Some minor pegmatitic veins are also present. Small mafic injections are in sharp contact with the granitoid indicating emplacement that post-dates the main magmatic episode. There is some weak alignment of alkali-feldspar megacrysts that can be best observed on the top of the outcrop. Kfs Grt Iron-Chl 250 μm Figure 6. Left: Mafic clot containing Fe-chlorite or biotite, K-feldspar, and garnet (almandine). Garnet-biotite geothermometry yields an equilibration temperature range between 450°C and 550° C. Right: Example of mafic xenoliths clustered within granitoid. 165 �Stop 5: Undivided granitic rocks (Wgu) Directions: Head south on M-95 toward Co Rd 601 ~4.8 miles to destination. Outcrop is located just south of the city park along the Michigamme river. N 46° 20’ 13” W087° 58’ 22” A metasedimentary sequence is exposed here, which doesn’t appear on the regional geologic map. Blonde and gray/black feldspathic quartzite forms alternating beds with gradational contacts dipping to the north (Figure 7). The blonde quartzite is dominated by quartz, with minor amounts of feldspar and sparse garnet (1-2mm). The gray/black quartzite has similar proportions of quartz and feldspar to the blonde quartzite but includes muscovite, biotite and oxides nearing the abundance of feldspar. Complex interaction with mafic dikes/sills also appear in this outcrop with minor ptygmatic folding and mafic enclaves throughout. Vertical dike contacts are sharp, whereas an apparently older generation of mafic intrusions have more complex contact margins with the host rock. Near vertical mafic dikes on both ends of the outcrop have sharp contacts with the host rock penetrating locally as splays or fingers. Figure 7. Metasedimentary sequence dominated by blonde quartzite and a grey to black quartzite. Lunch Stop: Leif Erickson Roadside Park along the Michigamme River 166 �Stop 6: Archean Bell Creek batholith and Compeau Creek gneiss/migmatite (Wbcf/Wccg) Directions: Drive north along M-95 towards Welsh’s Rd. ~5.7 miles to destination. Outcrop exposed prominently on the east and west sides of the road. N 46° 24’ 54” W87° 59’ 31” The contact between Compeau Creek to the south and Bell Creek batholith to the north is clearly exposed at this outcrop. A zircon U-Pb date of 2426±160 Ma was obtained for the Bell Creek granitoid at this location. The granitoid is fine- to medium-grained, typically equigranular and has an apparently gradational contact with the adjacent migmatite/gneiss. Migmatite/gneiss with ptygmatic folding is best exposed in the outcrop on the west side of the road, but the highly deformed continuation of this rock type is prominently exposed with mafic intrusions on the east side of the road. Stop 7: Archean Bell Creek batholith (Wbcg) Directions: Drive north along M-95 ~2.1 miles to destination. N 46° 26’ 16” W87° 57’ 48” Exposure of Bell Creek megacrystic granitoid similar to Stop 4. Minerals include alkali-feldspar, oligoclase, biotite, oxides and apatite with apatite and zircon as common accessory phases. A strong foliation is not apparent here, but the mafic xenoliths (clots) are well represented as mm- to cm-sized dark spots typically clustered and randomly oriented. Pegmatitic veins are common and vary in width up to tens of centimeters. Stop 8: Archean Migmatite (Wbsc); Compeau Creek? Directions: Drive north along M-95 ~0.8 miles to destination. N 46° 26’ 53.7” W087° 57’ 20.0” This exposure is likely the Compeau Creek quartofeldspathic migmatite/gneiss dipping to the south. The highly deformed migmatite/gneiss includes numerous mafic inclusions and enclaves (Figure 8). However, it is unclear how much of the mafic-felsic segregations are representative of the initial magma mixing followed by deformation or melanosome-leucosome complementary rocks derived by melting. Ptygmatic folding is prominently displayed and likely represents felsic segregations that have buckled during deformation. A zircon U-Pb date of 2525±70 Ma was obtained for the gneiss, which overlaps in time with the dates obtained for other Bell Creek granitoids along the M-95 corridor. 167 �Figure 8. Near vertical mafic dike cross-cutting migmatite with chaotic mixture of mafic-felsic components. Stop 9: Archean Compeau Creek migmatite/gneiss (Wccg) Directions: Drive north along M-95 ~1.4 miles to destination. N46° 27’42” W87° 56’ 05” This outcrop is mapped as Compeau Creek migmatite/gneiss that is presumed to be older than the adjacent Bell Creek granitoid/gneiss. A zircon U-Pb date for the migmatite is slightly older (2633±46 Ma) than the pink, clotted granite, which has been dated to 2550±33 Ma. Inherited zircon grains range in age from 2717 to 3200 Ma. The Compeau Creek gneiss/migmatite on the northern end of this outcrop appears to include a sliver of what might be Archean quartzite (Figure 9); note that the Paleoproterozoic Goodrich quartzite is mapped directly to the north and would, therefore, be in direct contact with the migmatite/gneiss. There are numerous mafic dikes/sills that manifest as either single generation dikes that clearly cross-cut the existing foliation or more complex dismembered dikes (possibly sills) associated with mixing at the time of felsic magma emplacement (Figure 10). Inclusions of felsic material (either bulk rock or individual feldspar crystals) can be found in some of the composite dikes/sills indicative of magma mixing that may have occurred contemporaneously between the mafic magma and a liquid dominant felsic host magma. The foliation is steeply south-dipping (~70°). 168 �Figure 9. Band of quartzite concordant with the foliation of gneiss/migmatite fabric. Relict feldspar crystals ~ 4 cm Figure 10. Mafic intrusion with felsic xenocrysts/xenoliths interpreted to have likely formed through mixing with the host magma during emplacement. This particular intrusion has a Nd model age of ~2.57 Ga, which is within error of the age obtained for the Bell Creek gneiss at this location. 169 �References Barth, Elana G. Age and chemistry of Bell Creek batholith (MS report), Michigan Technological University (2023) Bornhorst, Theodore J., and Rodney C. Johnson. Geology of volcanic rocks in the south half of the Ishpeming greenstone belt, Michigan. No. 1904. US Government Printing Office, 1993. Brown, Michael, Tim Johnson, and Nicholas J. Gardiner. "Plate tectonics and the Archean Earth." Annual Review of Earth and Planetary Sciences 48 (2020): 291-320. Cannon, W. F., and J. E. Gair. "A revision of stratigraphic nomenclature for middle Precambrian rocks in northern Michigan." Geological Society of America Bulletin 81, no. 9 (1970): 2843-2846. Cannon, W. F., and George C. Simmons. "Geology of part of the southern complex, Marquette district, Michigan." J. Res. US Geol. Surv 1 (1973): 165-172. DeMatties, Theodore A. "Exploration-resource assessment of productive felsic volcanic centers in the Paleoproterozoic Penokean Volcanic Belt of northern Wisconsin, Michigan and east-central Minnesota, USA." Ore Geology Reviews 141 (2022): 104489. Gair, Jacob Eugene, and Robert E. Thaden. Geology of the Marquette and Sands Quadrangles, Marquette County, Michigan. No. 397. 1968. King, Elizabeth M., John W. Valley, Don W. Davis, and Garth R. Edwards. "Oxygen isotope ratios of Archean plutonic zircons from granite–greenstone belts of the Superior Province: indicator of magmatic source." Precambrian Research 92, no. 4 (1998): 365-387. Mole, D. R., C. L. Kirkland, M. L. Fiorentini, S. J. Barnes, K. F. Cassidy, C. Isaac, E. A. Belousova, M. Hartnady, and N. Thebaud. "Time-space evolution of an Archean craton: A Hfisotope window into continent formation." Earth-Science Reviews 196 (2019): 102831. Mole, D. R., P. C. Thurston, J. H. Marsh, R. A. Stern, J. A. Ayer, L. A. J. Martin, and Y. J. Lu. "The formation of Neoarchean continental crust in the south-east Superior Craton by two distinct geodynamic processes." Precambrian Research 356 (2021): 106104. Mole, D. R., B. M. Frieman, P. C. Thurston, J. H. Marsh, T. R. C. Jørgensen, R. A. Stern, L. A. J. Martin, Y. J. Lu, and H. L. Gibson. "Crustal architecture of the south-east Superior Craton and controls on mineral systems." Ore Geology Reviews 148 (2022): 105017. Morey, G. B., and P. K. Sims. "Boundary between two Precambrian W terranes in Minnesota and its geologic significance." Geological Society of America Bulletin 87, no. 1 (1976): 141-152. Ojakangas, R. W., G. B. Morey, and D. L. Southwick. "Paleoproterozoic basin development and sedimentation in the Lake Superior region, North America." Sedimentary Geology 141 (2001): 319-341. 170 �Percival, John A., Tom Skulski, Mary Sanborn-Barrie, Greg M. Stott, Alain D. Leclair, M. Tim Corkery, and Michel Boily. "Geology and tectonic evolution of the Superior Province, Canada." In Tectonic styles in Canada: the LITHOPROBE perspective, vol. 49, pp. 321-378. Saint‐John's, Newfoundland: Geological Association of Canada, 2012. Petryk, Brandi. "The origin of an Archean batholith in Michigan’s Upper Peninsula.”. (MS thesis) Michigan Technological University (2019). Polat, Ali, and Robert Kerrich. "Archean greenstone belt magmatism and the continental growth– mantle evolution connection: constraints from Th–U–Nb–LREE systematics of the 2.7 Ga Wawa subprovince, Superior Province, Canada." Earth and Planetary Science Letters 175, no. 1-2 (2000): 41-54. Polat, A., R. Kerrich, and D. A. Wyman. "The late Archean Schreiber–Hemlo and White River– Dayohessarah greenstone belts, Superior Province: collages of oceanic plateaus, oceanic arcs, and subduction–accretion complexes." Tectonophysics 289, no. 4 (1998): 295-326. Schmitz, M. D., D. L. Southwick, M. E. Bickford, P. A. Mueller, and Scott Douglas Samson. "Neoarchean and Paleoproterozoic events in the Minnesota River Valley subprovince, with implications for southern Superior craton evolution and correlation." Precambrian Research 316 (2018): 206-226. Sims, Paul Kibler. Great Lakes tectonic zone in Marquette area, Michigan: implications for Archean tectonics in north-central United States. No. 1904. US Government Printing Office, 1991. Tinkham, D. K. Tectonic Evolution of the Southern Complex Regiong of the Penokean Orogenic Belt, Upper Peninsula Michigan: The Formation of Precambrian Dome-and-Keel Architecture (MS), University of Illinois. (1997). Tang, Ming, Kang Chen, and Roberta L. Rudnick. "Archean upper crust transition from mafic to felsic marks the onset of plate tectonics." Science 351, no. 6271 (2016): 372-375. Taylor, Stuart Ross, and Scott M. McLennan. "The geochemical evolution of the continental crust." Reviews of geophysics 33, no. 2 (1995): 241-265. Thurston, P. C. "Autochthonous development of Superior Province greenstone belts?." Precambrian Research 115, no. 1-4 (2002): 11-36. Van Hise, Charles Richard, and William Shirley Bayley. Preliminary report on the Marquette ironbearing district of Michigan. US Government Printing Office, 1895. Valley, J. W., J. S. Lackey, A. J. Cavosie, C. C. Clechenko, M. J. Spicuzza, Miguel Angelo Stipp Basei, I. N. Bindeman et al. "4.4 billion years of crustal maturation: oxygen isotope ratios of magmatic zircon." Contributions to Mineralogy and Petrology 150 (2005): 561-580. 171 �Wyman, Derek, and Robert Kerrich. "Mantle plume–volcanic arc interaction: consequences for magmatism, metallogeny, and cratonization in the Abitibi and Wawa subprovinces, Canada." Canadian Journal of Earth Sciences 47, no. 5 (2010): 565-589. 172 �Field Trip 7 Landslides on the Ontonagon River at Military Hill Stanley J. Vitton Civil, Environmental, and Geospatial Engineering, Professor Emeritus, Geological and Mining Engineering and Sciences, Adjunct Professor Emeritus, Michigan Technological University, Houghton, MI 49931 Mohammad Sadeghi Civil, Environmental, and Geospatial Engineering, Assistant Professor, Geological and Mining Engineering and Sciences, Affiliated Assistant Professor Michigan Technological University, Houghton, MI 49931 Figure 1: 2003 landslide east of US-45 on the East Branch of the Ontonagon River. 173 �Introduction An impressive sight, at least to geologists and engineers, are the landslides along US-45 as it descends into the Ontonagon River Valley at Military Hill. US-45 follows the trail used by Indigenous peoples such as the Menominee, Dakota and Anishinaabe (Ojibwe/Chippewa) tribes, and later fur traders making their way to and from to the mouth of the Ontonagon River at Lake Superior. In 1844, U.S. Secretary of War William Wilkins, proposed a road between Fort Howard in Green Bay, WI to Fort Wilkins just north of Copper Harbor, MI to create a supply route to the Army posts and to improve access to frontier communities such as Copper Harbor and other neighboring mine sites. Congress, however, did not fund the road. In 1862, with British troops in nearby Ontario and the prospect of Great Britian entering the war on the side of the confederacy, the road became a national security issue. According to a Michigan historian, Le Roy Barnett, “More than half the copper used in the United States came from mines along the proposed frontier passage. As a report of the U.S. Senate Committee on Military Affairs noted, "In case of hostilities with Great Britain, and a descent upon this important portion of our lake coast, there would be no means of affording succor without a road [to the region], and these valuable deposits of copper and other ores might be lost to us. Senator Jacob Howard pressed the point, explaining that if the Soo Canal were "seized and closed ... there would be no means of getting into the [Keweenaw] country or out of the country, either with troops or munitions of war or without them, except by means of some such road as this." (The Free Library, 2024) On March 3, 1863, Abraham Lincoln signed an Act of Congress authorizing construction of the military road. Road construction was started in 1863 but was not completed until 1872. Following construction, the road was used primarily for timber and other commercial interests until the railroad network in the western Upper Peninsula became operational a few years later. Again, according to LeRoy Barnett, “Despite the years of work put into the route, it never became the avenue of settlement and commerce its supporters anticipated. As legislators in the early 1860s had feared, railroads--built shortly after the road's completion--soon siphoned traffic. Furthermore, parts of the road were poorly maintained. "There was no surfacing on most of the road," wrote Upper Peninsula author/historian Knox Jamison of a trip in 1915, "and if it had rained recently, you had better not try it." Getting up and down the Military Hill--a massive body of slippery, wet clay through which the Ontonagon River had cut a 300foot gorge--took him at least two hours.” In 1923, the Michigan Highway Department added Military Road into the state’s transportation network as a section of State Highway M-26 but did not appreciably improve the road, leaving it with a gravel surface and in general following the route’s topography. In 1957, the Michigan State Highway Department added this section of M-26 to the federal highway system as US-45. 174 �Becoming a federal highway requires the road to meet both state and federal road design standards, such as the highway having a maximum grade of less than 8%. This required significant excavation of the glacial clay slopes and most likely the start of the Military Hill landslides. Construction was completed in 1959 with a concrete pavement and a grade meeting state and federal specifications. While the Military Hill clay soils are known for their difficult road construction issues, the highway department most likely did not fully appreciate the difficulty with the excavation and long-term behavior of the slopes. An investigation, either upstream or downstream of the US-45 river crossing, for example, would have shown many old and new landslides not to mention the red silt laden river itself that never changes throughout the seasons. The landslides are in the glacial lacustrine and till sediments in the Ontonagon Basin formed during the area’s deglaciation. One of the more recent landslides near the river crossing, which occurred in 2003, is shown in Figure 1, is just upstream of US-45 and one of the sites we will visit. Figure 2 shows the US-45-Ontonagon River crossing and the location of the landslide north of the confluence of the East and Middle Branches of the Ontonagon River. The steep slopes and the meandering of the river are evident. Figure 2 also shows the stops that we will be making at Military Hill. The clays in the Military Hill area were deposited in the final stages of the Late Wisconsin glacial period, estimated to be at its maximum at about 26,000 BP with deglaciation ending about 9,500 years BP when the Superior Lobe of the Laurentide Ice Sheet retreated into the Superior Basin for the last time (Attig et al., 1985). Fortunately, the northern Great Lakes region is known as the “type area” for the stratigraphic subdivisions of the late-glacial period of North America so the area has been investigated in some detail (Evenson et al., 1976). The first and the most detailed investigation of the glacial sediments in the Ontonagon area was conducted between 1905 and 1919 by Frank Leverett who identified the area’s main glacial features including Glacial Lake Ontonagon and Duluth (Leverett, 1928). Later, Hatch (1965) investigated the postglacial drainage evolution and stream geometry in the Ontonagon area while detailed mapping was conducted by Peterson (1985 and 1986). The area’s prominent feature is the Copper Range, a topographic ridge formed by the Portage Lake Volcanics. The Copper Range was influential in the formation of the Ontonagon River Basin shown in Figure 3 and Figure 4. As shown in these figures, the Ontonagon River System converges at a gap along the Copper Range. According to Leverett (1928), the Copper Range formed an ice-margin boundary during the last couple of glacial advances resulting in the formation of a series of proglacial lakes shown in Figure 5. The proglacial lakes Ontonagon, Ashland, Brule and Duluth formed along this ice-margin with drainage flowing westward to the St. Croix River. As isostatic rebound occurred and the Superior Lobe retreated into the Superior Basin, the drainage merged through a gap in the Copper Range. Leverett notes that it was probable that Lake Goebic was in a pre-glacial valley that extended northward to Lake Superior but was prevented from draining into Lake Superior by a moraine that filled the gap. Thus, the West and South Branch of the Ontonagon River flowed eastward along the Copper Range to where it drained into the present Ontonagon River. 175 �Another distinct feature of the area is a series of parallel rivers that formed between Copper Range and Lake Superior as shown in Figure 6 and 7. Hatch (1965) investigated the drainage system indicating that “it was strongly grooved by glacial flutings parallel to the direction of ice motion. In places the grooves are buried by lacustrine sediments of glacial Lake Duluth.” It is possible that the Ontonagon River, as it began flowing through the Copper Range gap at Military Hill, followed one of the parallel drainages. Due to a greater volume of water, however, it has significantly cut through the Lake Ontonagon and Duluth sediments as shown in Figure 6. Figure 2: Military Hill along US-45 showing Field Trip Stops (Google Maps, 2024). Figure 3: Ontonagon River Basin location (USACE, 2010). 176 �Figure 4: Ontonagon drainage basin showing river recreation, science, and wild designations (Ontonagon, 2023). Figure 5: Development of proglacial lakes along the Superior Lob (Farran and Drexler, 1985). Figure 6: Parallel drainage system on the Ontonagon Plain by glacial grooving (Hatch, 1965). 177 �Figure 7: Aerial photo of the parallel river systems on the Ontonagon till plain. Figure 8: Bedrock geology of the Military Hill area (Cannon et al., 1995). 178 �Background Information Bedrock Geology The landslides at Military Hill are located over the Jacobsville Sandstone, just south of the Keweenaw Fault, as shown in Figure 8. Ontonagon River Watershed Overview The Ontonagon River Watershed, at 1,348 sq. miles, is the second largest watershed entering Lake Superior behind the St. Louis watershed west of Duluth, MN, at 3,584 sq. miles. What makes the Ontonagon River watershed distinctive, however, is the large amount of red sediments that continually flows into Lake Superior. In 1987, the National Geography Magazine, ran an article on “The Great Lakes Trouble Waters” showing an aerial view of the sediment discharge from the Ontonagon River into Lake Superior with the caption, “Each year millions of tons of sediment – like this red clay silt spilling into Lake Superior from Michigan’s Ontonagon River – enters the lakes from tributary stream. Many contaminated with agricultural chemicals and industrial wastes (National Geographic, 1987).” The cover and photo of the Ontonagon River are shown in Figure 9. In the early 2000s, the US Corps of Engineers (USACE, 2010) conducted a 516e sediment study on the Ontonagon River to determine the source of this large quantity of sediments to estimate future dredging requirements. After its investigation, the USACE reported: “The Ontonagon River watershed is primarily undeveloped and consists of forested land uses with little urbanization or agriculture. Much of the watershed experiences significant erosion of the incised valley walls due to the highly erodible soils associated with the lacustrine geology of the lower reaches. Based on the comparison of historic and present-day river morphology it is concluded that the valley walls and river banks have been a large contributor of sediment long before logging or other anthropogenic disturbances were present in the watershed. Therefore, the primary contributor of sediment yield in the watershed is natural processes, rather than anthropogenic alterations. Based on this conclusion, a quantification of geologic time scale sediment yields was conducted.” The report went on the state: There is evidence that the deep valleys and steep walls that exist today are due to natural sediment transport processes and landscape scale geomorphic evolution that have carved out the valleys and deepened the Ontonagon River and its tributaries. Frequent examples of mass wasting of the valley walls exist throughout the downstream reaches of the Ontonagon River and a flat terrace with steep valley walls adjacent to a meandering channel with a narrow beltwidth are typical in the lacustrine areas of the watershed…..Moreover, an average sediment yield of 2.4 million tons per year is more than an order of magnitude greater than current sediment yields of similar watershed sizes in the Great Lakes.” The USACE report did not investigate which branch of the river produced the greatest amount of 179 �sediments. Based on a study by Weidner et al., (2019), however, it would appear the East and Middle Branches produce the most sediment based on the frequency of observable landslides. This would be constitent with Peterson’s 1985 mapping showing that the East and Middle Branches are mostly located in the Glacial Lake Ontonagon sediments as seen in Figure 10, which is a portion of Peterson’s glacial geology map. The length of the East and Middle Branches of the Ontonagon River can also be seen in the soils map from the USDA Natural Resources Conservation Service landform map shown in Figure 11 where the East and Middle Branch extend to the southern end of the lake sediments. Figure 9: July 1987 issue of the National Geographic Magazine (1987). Review of Landslide Activity on the Ontonagon River Travelers along US-45 can view a landslide just to the west of the US-45 – Ontonagon River crossing. A 1,000-foot walk upstream on the East branch of the Ontonagon River will provide the opportunity to inspect a large-scale landslide that occurred in 2003. If you continue to walk upstream from the 2003 landslide, about every bend in the river will show landslide activity, either current or past as shown in Figure 12. Weidner et al., (2019) studied the landslide activity in the Military Hill area (Figure 13a) for the development of a landslide susceptibility map based on riverbank erosion-triggering. Additional landslide studies were conducted by Koons (1965), Dyl (1979), and Smith (2012), all master theses or reports at Michigan Tech. The Weidner et al., study utilized aerial and satellite imagery from the United States Geological Survey (USGS) EarthExplorer web tool for the years 1992 through 2016 identifying 21 landslides. The landslide locations in the study area are shown in Figure 13(a). 180 �Figure 10: A portion of the USGS map of the glacial history of Iron River 1° x 2° Quadrangle (Peterson, 1985). Figure 11: Landforms map of the study area adapted from Jerome (2006), showing the main soil regimes. River hydraulic data were obtained from a USGS gauging station downstream. The USGS Scoops3D limit-equilibrium analysis software was used to develop a “factor of safety” map of the study area, which is shown in Figure 13(c). A significant portion of the Ontonagon River slopes have a factor of safety less than one, which is supported by the observable landslide activity shown in Figure 13(b) through the lacustrine sediments of Glacial Lake Ontonagon and Duluth. 181 �Figure 12: Landslide activity upstream of US-45 on the East Branch of the Ontonagon River (Google Map, 2024). Figure 13: Weidner et al., Military Hill landslide investigation, (a) study area, (b) observed landslides between 1992 and 2016, and (c) Scoops3D factor of safety assessment. 182 �Objectives of the field trip: The objective of the field trip is to investigate landslides in the sediments deposited in the former Glacial Lakes Ontonagon and Duluth. In general, the sediments consist of lacustrine sediments deposited in Glacial Lake Ontonagon and later in Lake Duluth which overlay older till deposits. The first stop, however, will be at Quincy Hill, north of Hancock, MI to observe glacial grooving and striations in the Portage Lake Volcanics indicating the direction of glacial lobe movement across the Keweenaw Peninsula. The second and third stops will be in the US-45 Military Hill area. A summary of the stops are as follows: • Stop 1: Top of Quincy Hill, north of Hancock, to view glacial grooving in the Portage Lake Volcanics indicating the direction of Keweenaw Bay Lobe movement westward into the Ontonagon Lobe that formed Lake Ontonagon. • Stop 2A: The north side of Military Hill to observe the effects of vegetation on limiting landslide development. • Stop 2B: US-45 Military Hills Roadside Park: 2003 Large landslide on the East Branch of the Ontonagon River • Stop 2C: Lower Military Hill Erosion with Slope Movement • Stop 2D: Middle Military Hill with Partial Vegetation and Some Slope Movement • Stop 2E: Middle Military Hill with More Vegetation and Limited Slope Movement • Stop 2F: Middle Military Hill with Some Vegetation and Varved Clay Slope Movement • Stop 2G: Upper Military Hill with Active Slope Movement on both Sides of Highway in non-varved clay • Stop 3: Slope movement two miles south of Military Hill, across from Primrose Acres, where a recently excavated slope on the east side of US-45 has been moving for about eight years. Stop 1: Glacial Grooves at the Quincy Hill Historic Park Lookout Directions: From Michigan Tech drive west through Houghton on US-41 and cross the Houghton-Hancock Bridge, staying on US-41 going into Hancock. In Hancock, go straight uphill, passing Hancock’s main street, onto East White Street. East White Street will take you to US-41 bypassing downtown Hancock. At the US-41 stop, take a right turn, going uphill to the Quincy Mine Hoist. Directly across from the entrance from the Quincy Mine Hoist take a left turn onto No. 2 Road. Follow No. 2 Road a short distance until you come to a two-track road on your left that takes you to the Keweenaw National Historic Park’s “Quincy Mine Dryhouse Ruins” parking lot. The glacial grooves are a short distance from the parking lot. From Michigan Tech to the parking lot is 3.6 miles. Lat: 47.135904°, Lon: -88.577986° 183 �Figure 14: View, looking west, of glacial grooves sculptured into the Portage Lake Volcanics at the Keweenaw National Historic Park. Glacial grooving and striations are common along the Keweenaw Peninsula as the glaciers moved over the Portage Lava Volcanics, which were resistant to glacial erosion. It is generally assumed glaciers came from northeastern Canada moving south to southwest but are surprised to see grooves heading due west as shown in Figure 14. The reason for this direction is the movement of the Keweenaw Bay Lobe filled Keweenaw Bay and then moved west, south, and east as shown in Figure 15. The Keweenaw Bay Lobe then was stopped by the Ontonagon Lobe and to the west by the Michigamme Lobe to the east. Figure 15: Location of glacial lobes in the western Lake Superior Basin (from Attig et al., 2013). Stop 2: Military Hill – Seven Stops – Safety instruction will be provided at the site Directions: From Michigan Tech drive west through Houghton on US-41 to M-26. At the Houghton-Hancock Bridge go straight through the intersection to access M-26. Stay on M-26. Going southwest 37.8 miles to the M-26/M-38 intersection. Take a left turn, staying on M-26 through Mass City, to the M-26/US-45 intersection, 5.5 miles. Take a left turn at the M-26/US-45 intersection headed south 1.8 miles to Stop 2A. Lat: 46.705726°, Lon: -89.159581° 184 �Stop 2A: North US-45 - Slope with Vegetation and Small Slope Movement In the Military Hill area, the north side slopes required less excavation than on the south side and therefore there is less slope movement. In addition, where the slopes were excavated, they tend to be more vegetated as seen in Figure 16. This is due (possibly) to the north facing slope losing the spring snow before the south facing slopes and thus having longer growing season. Figure 16: Stop 1 (a) descending Military Hill going south and (b) vegetated slopes with some slope movement. Stop 2B: US-45 Military Hill Roadside Park: 2003 Large landslide on the East Branch of the Ontonagon River Directions: From Stop 2A to Stop 2B go 0.4 miles south on US-45 to the Military Hills Roadside Park parking lot on the east side of US-45. Starting at the rest area facilities (outhouses) on the northside of the parking lot, walk northeast through the woods to the landslide. There is no path, but the woods are relatively easy to walk through. The 2003 large scale landslide is approximately 1,400 ft (411 m) northeast of the parking lot as shown in Figure 17. There is a smaller landslide north of the path, which has been active for many years, but is difficult to access. The slope debris, below the landslide, has a thick undergrowth, which is difficult to walk through. Military Hills Roadside Park: Lat: 46.699650°, Lon: -89.158627° Military Hill 2003 Landslide: Lat: 46.703234°, Lon: -89.154132° In 2003, a large-scale landslide occurred on the East Branch of the Ontonagon River as shown in Figure 1 and Figure 17. The landslide’s stratigraphy consisted of lacustrine varved clay over a clean alluvial sand that grades downward into a red silty sand, silt and then clay till as illustrated in Figure 18 and Figure 19. While the failure mechanism is unknown, it is speculated that the spring runoff and possibly high-water table caused the alluvial sand to liquefy causing the massive landslide. As the liquefied sand lost strength, it started to flow outward into the river channel as shown in Figure 20(b) and (c). During the slope’s collapse, a sliding plane formed in the varved clay as shown in Figure 20(a) allowing the varved clay to fail over the alluvial sand. Liquefaction boils are shown in Figure 20(b) and (d). 185 �It will take about 20 minutes to walk to this landslide through the woods from the roadside park. While there is no path, the woods are fairly open and easy to walk in. However, the landslide itself is more difficult due to the landslide debris and the vegetation that has developed over the years. Caution must be observed when walking over this site and it is highly recommended that the landslide slopes are not accessed. Figure 17: Stop 2B Military Hills Roadside Park and path to the 2003 landslide. Figure 18: Assumed stratigraphy of the 2003 landslide (Smith, 2012). 186 �Figure 19: 2003 landslide illustrating the varved clay directly over an alluvial sand. Figure 20: Landslide illustrating (a) a sliding plane on the varved clay, (b) liquefaction boils in the varved clay, (c) the flow debris into the river's channel, and (d) a large sand boil. 187 �Stop 2C: Lower Military Hill Erosion with Slope Movement Directions: From Stop 2B go 0.8 miles south on US-45 across the US-45 bridge to Stop 2C, which is the start of the lower landslides with significant erosion. Lat: 46.690087°, Lon: -89.165513° At the following five stops we will travel up Military Hill’s southside. In general, the soils traveling up the south side of Military Hill are like the sediments seen at Stop 2B except for the alluvial sands. The varved clays overlie a sandy brown silt, which have significant erosion (Figure 21). The base of the varved clay can be seen moving over the sandy silt. As we moved up US-45 at Stop 2C a short distance, MDOT has been attempting to stabilize a small landslide just below the base of the varved clay as shown in Figure 22. The clay content appears to be higher in this area. On the west side of US-45 you can observe the steep US-45 embankment that descends down to Sandstone Creek, exposing the Jacobsville Sandstone. Figure 21 Stop 2C showing the contact between the varved clay and a brown/red sandy silt, which is eroding. Figure 22 Stop 2C small landslide. 188 �Figure 23 Stop 2C showing the US-45 embankment that descends to Sandstone Creek on the Jacobsville Sandstone. Stop 2D: Middle Military Hill with Partial Vegetation and Some Slope Movement Directions: From Stop 2C go 0.23 miles south on US-45 Stop 2D, which is at the middle landslides. Lat: 46.686869°, Lon: -89.164320° Stop 2D is a short distance up US-45 and is in the same sediments as Stop 2C, which also are sloping uphill as seen in Figure 24. Movement of the varved clay base can be seen in the upper portion of Figure 24. Figure 24 Stop 2D varve clay movement over the lower sandy silt but without the erosion seen at Stop 2C. 189 �Stop 2E: Middle Military Hill with More Vegetation and Limited Slope Movement Directions: From Stop 2D go 0.30 miles south on US-45 Stop 2E, an excavated slope that is now partially vegetated. Lat: 46.682564°, Lon: -89.164857° At Stop 2E, the slope has much more vegetation with limited apparent slope movement. Figure 25 Stop 2E, slope with more vegetation and limited slope movement. Stop 2F: Middle Military Hill with Some Vegetation and Slope Movement Directions: From Stop 2E go 0.34 miles south on US-45 Stop 2F, a slope with vegetation that had was excavated for the construction of US-45. Lat: 46.677892°, Lon -89.166195° At Stop 2F, there is again less vegetation but more slope movement Figure 26. The clay soils at this elevation are not varved. Figure 26: Stop 2F showing partial vegetation and upper slope movement. 190 �Stop 2G: Upper Military Hill with Active Slope Movement on both Sides of Highway Directions: From Stop 2F go 0.23 miles south on US-45 Stop 2G, a slope with active slope movement. Lat: 46.674640°, Lon -89.167126° At Stop 2G, landslides occur on both sides of the highway as shown in Figure 27. Figure 27(a) shows slope movement on the west side of US-45, which has less movement than on the eastside of US-45 shown in Figure 27 (b), (c) and (d). These landslides have been moving for many years and appear to be in a non-varved clay. While limited soil investigation has been conducted in these sediments, it appears that at the higher elevations in the Military Hill area, a non-varved lacustrine soil overlies the varved clay soils indicating that it possible that this sequence might indicate when Glacial Lake Duluth and Lake Ontonagon combined. An interesting feature of Glacial Lake Ontonagon is that it is at a much higher elevation at 1,320 feet than Glacial Lake Duluth. Isostatic rebound would have had to occur for the lakes to combine. Figure 27 Stop 2G near the top of Military Hill showing slope movements on both sides of the highway with (a) west side, (b) top of east side, (c) south section of slope movement on east side, and (d) north portion of slope movement on the east side. 191 �Stop 3: US-45 Recent Excavation and Resulting Slope Movement Directions: From Stop 2G go 3.4 miles south on US-45 Stop 3, a slope with active slope movement. Lat: 46.627240°, Lon -89.178520° Stop 3 is at a recent location where MDOT excavated a natural slope to improve drainage along the eastside of US-45. The natural slope was excavated at a 2H:1V angle, the same slope as the highway embankment as can be seen in Figure 28. Soon after excavation, however, the slope started to move and has been moving ever since. Figure 29 was taken one year after the photo in Figure 26. The US-45 embankment was constructed with the local clay soil but was compacted, whereas the natural slope was not. The clay soils at this site and at Stop 2G have relatively high plasticity and are at the interface of the low (CL) and high (CH) plasticity soils defined by the Unified Soil Classification System (USCS) with moisture contents in the 20 to 25% range. As we saw at Stop 2F, the lacustrine clay soils are not stable at the angles at which they were excavated, unless compacted. Figure 28 Stop 3 showing a recent excavation into the clay soil at a 2(H):1(V) angle. Figure 29 Stop 3 - excavated slope one year after the photo in Figure 26 was taken. 192 �References Cited Attig, J.W., Clayton, L., and D.M. Mickelson, 1985. Correlation of late Wisconsin glacial phases in the western Great Lakes area, Geological Society of America Bulletin, v. 96, p. 1585-1593. Black, R.F., 1969, Valderan glaciation in Western Upper Peninsula, Proc. 12th Conf. Great Lakes Res., Internat. Assoc. Great Lakes Res., p. 116-123. Cannon, W.F, Nicholson, L.G., Woodruff, C.A, Hedgman, C.A. and K.J. Schulz, 1995. Geologic Map of the Ontonagon and Part of the Wakefield 30’ x 60’ Quadrangle, Michigan, USGS. Creech, C., Selegean, J., and T. Dahl, 2010. Historic and modern sediment yield from s forested watershed, 2nd Joint Federal Interagency Conference, Las Vegas, NV, June 27 - July 1. Dyl, Stanley, 1979. Engineering geologic factors affecting the stability of slopes in the Ontonagon Clay at the Military Hill Slide, U.S. Highway 45, Ontonagon County, Mi. In: M.S. Thesis. Michigan Technological University, pp. 92. Farran, W.R and C.W. Drexler. 1985. Late Wisconsinan and Holocene History of the Lake Superior basin. In Quaternary Evolution of the Great Lakes, eds. P.F. Karrow and P.E. Calkin, Geological Association of Canada Special Paper 30:17–32. Gunderman, B.J. and E. A. Baker, 2008. Ontonagon River Assessment, Michigan DNR, Fisheries Division, Special Report 46, Hack, J.T. Postglacial drainage evolution and stream geometry in the Ontonagon area, Michigan. US Geological Survey, 1965. Jerome, D.S., 2006. Landforms of the Upper Peninsula of Michigan. 1:750,000. USDA Natural Resources Conservation Service, pp. 17. Koons, G.J., 1969. Some geologic and engineering properties of the Pleistocene Ontonagon Clays at Victoria, Ontonagon County, Mi. In: M.S. Thesis. Michigan Technological University, pp. 110. Leverett, Frank, 1928, Moraines and shorelines of the Lake Superior region: U.S. G. S. Prof. Paper 154A, p. 1-72. Ontonagon River. (2023, Nov. 12). In Wikipedia. https://en.wikipedia.org/wiki/Ontonagon_River Peterson, W. L., 1985, Surficial geologic map of the Iron River 1° x 2° quadrangle, Michigan and Wisconsin: U.S. Geological Survey Miscellaneous Investigations Series Map I-1360-C, scale 1:250,000. Peterson, W.L., 1986. Late Wisconsinan glacial history of northeastern Wisconsin and western upper Michigan. USGS Bulletin 1652, p. 1-14. Smith, J., 2012. Large scale landslide on the Ontonagon River, Michigan. In: M.S. Report. Michigan Technological University. http://digitalcommons.mtu.edu/etdrestricted/146. The Free Library. S.V. 2024. Through the wilderness, Retrieved Mar 19 2024 from https://www.thefreelibrary.com/Through+the+wilderness.-a0452051967. USACE, 2010. Ontonagon River Watershed 516e Sediment Study, USACE Great Lakes Hydraulics and Hydrology Office, Detroit District, p. 90. 193 �Weidner, L. DePrekel, K., Oommen, T. and Vitton, S., 2019, Investigating large landslides along a river valley using combined physical, statistical, and hydrologic modeling. Engineering Geology, v. 259, p. 1-12. http://doi.org/10.1016/j.enggeo.2019.1051169 194 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2024 Description An account of the resource Institute on Lake Superior Geology. Houghton, Michigan. May 15-18, 2024. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2024-05 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English Type The nature or genre of the resource Text https://digitalcollections.lakeheadu.ca/files/original/99c083bfbf57b97590b6050acae4bd4e.pdf f1d247daa107d03ceffc4ef05679054d PDF Text Text 71st Annual Meeting Proceedings Volume 71 Part 1 – Program and abstracts Mountain Iron, Minnesota, May 14-17, 2025 �71st Annual Meeting Institute on Lake Superior Geology Mountain Iron, Minnesota May 14-17, 2025 Meeting Co-Chairs Amy Radakovich, Allison Severson, Eric Nowariak, Stacy Saari, Aaron Hirsch Proceedings Volume 71 Part 1: Program and Abstracts Edited by Co-Chairs i �71st Institute on Lake Superior Geology Volume 71 consists of: Part 1: Program and Abstracts Part 2: Field Trip Guidebook Trip 1: Transect of the Quetico subprovince Trip 2: Drill Core from three Cu-Ni deposits of the Duluth Complex Trip 3: How do you make iron and/or manganese in Proterozoic Iron Formation? Trip 4: New Geological Insights into the genesis of iron ores at Lake Vermillion – Soudan Underground Mine State Park Trip 5: Neoarchean alkalic intrusions in the Wawa and Quetico subprovinces Trip 6: Unique Keweenawan inclusion (Colvin Creek) in the Duluth Complex Trip 7: Classic outcrops of Northeastern Minnesota Trip 8: Glacial Lake Norwood and the Koochiching Lobe Reference to material in Part 1 & 2 should follow the examples below: Authors, 2025, Title in Institute on Lake Superior Geology, 71st Annual Meeting, Mountain Iron, Minnesota, Part 1 - Abstracts and Program, v. 71, part 1, p. xx-xx. Authors, 2025, Field Trip title in Institute on Lake Superior Geology, 71st Annual Meetings, Mountain Iron, Minnesota, Part 2 – Field Trip Guidebook, v. 71, part 2, p. xx-xx. Proceedings Volume 71, Part 1: Program and Abstracts and Part 2: Field Trip Guidebook are published by the 71st Institute on Lake Superior Geology and distributed by the Institute Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca Some figures in this volume were submitted by authors in color but are printed black and white. Full color imagery will appear in the digital version of the volume when it is available on-line at: http://www.lakesuperiorgeology.org ISSN 1042-99 ii �Table of Contents Table of Contents ..................................................................................................................................... iii Institutes on Lake Superior Geology, 1955-2025 .................................................................................... iv Sam Goldich and the Goldich Medal ...................................................................................................... vii Goldich Medal Guidelines ....................................................................................................................... ix Goldich Medalists .................................................................................................................................... xi Citation for the 2025 Goldich Medal Recipient ...................................................................................... xii Honoring the Pioneers of Lake Superior Geology ................................................................................. xiv Pioneers of Lake Superior Geology ....................................................................................................... xiv 2025 Citation for Robert Bell (1841-1917)............................................................................................. xv Eisenbrey Student Travel Awards ........................................................................................................... xx Joe Mancuso Student Research Awards ................................................................................................. xxi Doug Duskin Student Paper Awards ..................................................................................................... xxii Board of Directors ................................................................................................................................ xxiii 2025 ILSG Meeting Volunteers ........................................................................................................... xxiv 2025 ILSG Meeting Session Chairs ..................................................................................................... xxiv Field Trip Leaders and Guidebook Authors .......................................................................................... xxv Mine to Mountain Bike Mecca: ........................................................................................................... xxvi Report of the chairs of the 70th annual meeting .................................................................................. xxvii Donations to Support the Annual Meeting ........................................................................................... xxxi TECHNICAL PROGRAM ................................................................................................................ xxxiii ABSTRACTS........................................................................................................................................ xliii iii �Institutes on Lake Superior Geology, 1955-2025 # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Date 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 Place Minneapolis, Minnesota Houghton, Michigan East Lansing, Michigan Duluth, Minnesota Minneapolis, Minnesota Madison, Wisconsin Port Arthur, Ontario Houghton, Michigan Duluth, Minnesota Ishpeming, Michigan St. Paul, Minnesota Sault Ste. Marie, Michigan East Lansing, Michigan Superior, Wisconsin Oshkosh, Wisconsin Thunder Bay, Ontario Duluth, Minnesota Houghton, Michigan Madison, Wisconsin Sault Ste. Marie, Ontario Marquette, Michigan iv Chairs C.E. Dutton A.K. Snelgrove B.T. Sandefur R.W. Marsden G.M. Schwartz & C. Craddock E.N. Cameron E.G. Pye A.K. Snelgrove H. Lepp A.T. Broderick P.K. Sims & R.K. Hogberg R.W. White W.J. Hinze A.B. Dickas G.L. LaBerge M.W. Bartley & E. Mercy D.M. Davidson J. Kalliokoski M.E. Ostrom P.E. Giblin J.D. Hughes �# 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 Date 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 Place St. Paul, Minnesota Thunder Bay, Ontario Milwaukee, Wisconsin Duluth, Minnesota Eau Claire, Wisconsin East Lansing, Michigan International Falls, Minnesota Houghton, Michigan Wausau, Wisconsin Kenora, Ontario Wisconsin Rapids, Wisconsin Wawa, Ontario Marquette, Michigan Duluth, Minnesota Thunder Bay, Ontario Eau Claire, Wisconsin Hurley, Wisconsin Eveleth, Minnesota Houghton, Michigan Marathon, Ontario Cable, Wisconsin Sudbury, Ontario Minneapolis, Minnesota Marquette, Michigan Thunder Bay, Ontario Madison, Wisconsin Kenora, Ontario Iron Mountain, Michigan Duluth, Minnesota Nipigon, Ontario Sault Ste. Marie, Ontario Lutsen, Minnesota Marquette, Michigan Ely, Minnesota International Falls, Minnesota 57 58 59 60 61 62 2011 2012 2013 2014 2015 2016 Ashland, Wisconsin Thunder Bay, Ontario Houghton, Michigan Hibbing, Minnesota Dryden, Ontario Duluth, Minnesota v Chairs M. Walton M.M. Kehlenbeck G. Mursky D.M. Davidson P.E. Myers W.C. Cambray D.L. Southwick T.J. Bornhorst G.L. LaBerge C.E. Blackburn J.K. Greenberg E.D. Frey & R.P. Sage J. S. Klasner J.C. Green M.M. Kehlenbeck P.E. Myers A.B. Dickas D.L. Southwick T.J. Bornhorst M.C. Smyk L.G. Woodruff R.P. Sage & W. Meyer J.D. Miller & M.A. Jirsa T.J. Bornhorst & R.S. Regis S.A. Kissin & P. Fralick M.G. Mudrey & Jr., B.A. Brown P. Hinz & R.C. Beard L. Woodruff & W.F. Cannon S. Hauck & M. Severson M. Smyk & P. Hollings A. Wilson & R. Sage L. Woodruff & J. Miller T.J. Bornhorst & J. Klasner J. Miller, G. Hudak, D. Peterson M. Jirsa, P. Hollings & T. Boerboom, P. Hinz & M.Smyk T. Fitz P. Hollings T.J. Bornhorst & A. Blaske J. Miller & M. Jirsa R. Cundari & P. Hinz J. Miller, C. Schardt & D. Peterson �# 63 Date 2017 Place Wawa, Ontario 64 2018 Iron Mountain, Michigan 65 66 2019 2020 Terrace Bay, Ontario Meeting cancelled 67 68 69 2021 2022 2023 Virtual meeting Sudbury, Ontario Eau Claire, Wisconsin 70 2024 Houghton, Michigan 71 2025 Mountain Iron, Minnesota vi Chairs A. Pace, A. Wilson & T.J. Bornhorst L. Woodruff, W. Cannon & E.K. Stewart P. Hollings & M.C. Smyk Cancelled by the COVID-19 pandemic M. Jirsa, M. Smyk & P. Hollings R.M. Easton & W. Bleeker R. Lodge, E.K. Stewart, & C. Ames T.J. Bornhorst, E. Vye, P. Cobin, & J. Degraff A. Radakovich, A. Severson, E. Nowariak, S. Saari, A.C. Hirsch �Sam Goldich and the Goldich Medal Sam Goldich received an A.B. from the University of Minnesota in 1929, a M.A. from Syracuse University in 1930, and a Ph.D. from the University of Minnesota in 1936. During World War II Sam worked for the U.S. Geological Survey in mineral exploration. In 1948, Sam returned to the University of Minnesota, and became Professor and Director of the Rock Analysis Laboratory the following year. He rejoined the U.S. Geological Survey in 1959 and was appointed as the first Branch Chief of the Branch of Isotope Geology. Sam returned to academia in 1964 when he went to Pennsylvania State University. He left PSU in 1965 and moved to the State University of New York at Stony Brook, where he stayed for 3 years. Restless yet again, he moved to Northern Illinois University in 1968 where he was a professor until his retirement in 1977. Sam’s final move was to Denver where he became an emeritus at the Colorado School of Mines. Sam died in 2000, less than a month before his 92nd birthday. In the late 1970s, Geological Society of America Special Paper 182, which included seminal geochronological studies by Sam Goldich and coworkers on the Archean rocks of the Minnesota River Valley, was nearing completion. At this time various ILSG regulars began discussing the possibility of recognizing Sam for his pioneering work on the resolution of age relationships and thus the geology of Precambrian rocks in the Lake Superior region. Three members, R.W. Ojakangas, J.O. Kalliokoski, and G.B. Morey, presented the idea to the ILSG Board of Directors in 1978. The Board approved the creation of an award, provided funding could be obtained. It was suggested that collecting one or two dollars at registration for a dedicated account would provide resources for striking the medal. A general request was made to the ILSG membership for donations and Sam himself offered a challenge grant to match the contributions. In total, $4,000 was collected and thus began the work of creating the Goldich Medal. The initial Goldich Award was presented to Sam by G.B. Morey in 1979 and consisted of a large paper proclamation. For the actual medal, G.B. Morey consulted with the foundry on production details, while Dick Ojakangas and Jorma Kalliokoski worked on the design of the award, suggesting that it be given for “outstanding contributions to the geology of the Lake Superior region.” Simultaneously, a committee of J.O. Kalliokoski, W.F. Cannon, M.M Kehlenbeck, G.B. Morey, and G. Mursky developed the Award Guidelines that were approved by the ILSG Board. By 1981 all the elements of the Goldich Award had come together, and the second recipient, Carl E. Dutton, Jr., received the Goldich Medal for 50 years of significant contributions to the understanding of the geology of the Lake Superior region. Since the beginning, the Awards Committee has consisted of individuals representing industry, government and academia, with each member of the Committee serving for three years. The medal is now awarded every year at the annual ILSG meeting. Reference: Morey, G.B. and Hanson, G.N. (editors). 1980. Selected studies of Archean gneisses and Lower Proterozoic rocks, southern Canadian Shield. Geological Society of America, Special Paper 182, 175 p. Prepared by various Goldich Medal Awardees, 2007 vii �Institute on Lake Superior Geology Goldich Medal viii �Goldich Medal Guidelines (Adopted by the Board of Directors, 1981; amended 1999) Preamble The Institute on Lake Superior Geology was born in 1955, as documented by the fact that the 27th annual meeting was held in 1981. The Institute’s continuing objectives are to deal with those aspects of geology that are related geographically to Lake Superior; to encourage the discussion of subjects and sponsoring field trips that will bring together geologists from academia, government surveys, and industry; and to maintain an informal but highly effective mode of operation. During the course of its existence, the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have made particularly noteworthy and meritorious contributions to the understanding of Lake Superior geology and mineral deposits. The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years. Subsequent medalists and this year’s recipient are listed in the table below. Award Guidelines 1) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution to, the geology of the Lake Superior region. 2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of three members, one to serve for three years, one for two years, and one for one year. The member with the briefest incumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In his/her third year this member shall be the chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSG membership. The out-going, senior member of the Board of Directors shall act as liaison between the Board and the Committee for a period of one year. 3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of the Board of Directors, who will then inform the Board of the nominee. 4) The Board of Directors normally will accept the nominee of the Committee, inform the medalist, and have one medal engraved appropriately for presentation at the next meeting of the Institute. 5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award. Nominating Procedures ix �1) The deadline for nominations is November 1. Nominations shall be taken at any time by the Goldich Medal Committee. Committee members may themselves nominate candidates; however, Board members may not solicit for or support individual nominees. 2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation, lists of publications, curriculum vitas, and evidence of contributions to Lake Superior geology and to the Institute. 3) Nominations are not restricted to Institute attendees but are open to anyone who has worked on and contributed to the understanding of Lake Superior geology. Selection Guidelines 1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including: a) importance of relevant publications; b) promotion of discovery and utilization of natural resources; c) contributions to understanding of the natural history and environment of the region; d) generation of new ideas and concepts; and e) contributions to the training and education of geoscientists and the public. 2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institute meetings, presentation of talks and posters, and service on Institute boards, committees, and field trips. 3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of the Committee members. 4) There are several points to be considered by the Goldich Medal Committee: a) An attempt should be made to maintain a balance of medal recipients from each of the three estates—industry, academia, and government. b) It must be noted that industry geoscientists are at a disadvantage in that much of their work in not published. 5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedly one of the Institute’s great strengths and should be nurtured by equitable recognition of excellence in both countries. x �Goldich Medalists 1979 Samuel S. Goldich 1998 Zell Peterman 2016 Mark A. Jirsa 1980 not awarded 1999 Tsu-Ming Han 2017 Philip Fralick 1981 Carl E. Dutton, Jr 2000 John C. Green 2018 Val W. Chandler 1982 Ralph W. Marsden 2001 John S. Klasner 2019 Mark Severson 1983 Burton Boyum 2002 Ernest K. Lehmann 2020 not awarded 1984 Richard W. Ojakangas 2003 Klaus J. Schulz 2021 Alan MacTavish 1985 Paul K. Sims 2004 Paul Weiblen 2022 Terrence J. Boerboom 1986 G.B. Morey 2005 Mark Smyk 2023 Peter Hollings 1987 Henry H. Halls 2006 Michael G. Mudrey 2024 Suzanne W. Nicholson 1988 Walter S. White 2007 Joseph Mancuso 2025 Robert Michael Easton 1989 Jorma Kalliokoski 2008 Theodore J. Bornhorst 1990 Kenneth C. Card 2009 L. Gordon Medaris, Jr 1991 William Hinze 2010 William D. Addison & 1992 William F. Cannon 1993 Donald W. Davis 2011 Dean M. Rossell 1994 Cedric Iverson 2012 James D. Miller 1995 Gene La Berge 2013 Tom Waggoner 1996 David L. Southwick 2014 Laurel Woodruff 1997 Ronald P. Sage 2015 Rodney J. Ikola Gregory R. Brumpton 2025 GOLDICH MEDAL RECIPIENT Robert Micheal Easton Goldich Medal Committee Serving through the meeting year shown in parentheses. Dean Peterson (2022-2025) Big Rock Exploration, Industry Member (Committee Chair) Marcia Bjornerud (2023-2026) Lawrence University, Academic Member Robert Cundari (2025 - 2028) OGS, Government Member xi �Citation for the 2025 Goldich Medal Recipient Robert Michael Easton It is a great pleasure and honor to present the 2025 Goldich Medal to Dr. Robert Michael Easton, a highly respected senior scientist at the Ontario Geological Survey, in Sudbury, Ontario. Michael Easton, or ‘Mike’ as we know him, has been and is, without any doubt, among the leading and most productive geoscientists at the Geological Survey of Ontario (OGS) where he has spent much of his geological career (1982–2025). His curriculum vitae and publication list provide evidence for >600 publications and significant contributions — way too many to cite here. Even a short list of publications most relevant to the interests of the Institute and the geology of the Midcontinent Rift (MCR) spans four pages. The highlights include: • • • • a large number of peer-reviewed papers and reports; numerous extended abstracts in ILSG Proceedings volumes spanning the years from 1985 to 2023; meticulous editing of various ILSG Proceedings volumes; and the writing and editing of several comprehensive ILSG field trip guidebooks. In 2022, Mike co-lead and co-organized the 68th ILSG meeting in Sudbury, the first post-“peak COVID” meeting. We had proposed organizing this Sudbury meeting years earlier, an idea cooked up at another ILSG meeting in Terrace Bay, … but then COVID hit! It was a pleasure to organize this highly successful meeting with Mike, as one can always be 100% sure Mike will come through with everything. Although it was a joint effort, Mike took care of all the editing of both Proceedings volumes (Part I and II), and a fair bit of the local logistics. Born and raised in Ontario, Mike started his geology career with a BSc Honours degree (1976) at the University of Western Ontario, London, with a thesis titled "Geobotanical Studies in the Back River Volcanic Complex, NWT." He then moved on to the University of Hawaii, Honolulu, where he graduated (1978) with an MSc thesis on the "Stratigraphy and Petrology of the Hilina Formation: The oldest exposed lavas of Kilauea Volcano, Hawaii". In the late 1980s, I remember studying a treatise and guidebook on volcanology that was influential at the time, and this was authored by Mike Easton and his wife Monica (Easton & Easton, 1985)! Mike completed his graduate studies with a PhD from Memorial University (1982), in Newfoundland, with a thesis titled "Tectonic Significance of the Akaitcho Group, Wopmay Orogen, NWT." These studies brought him to the Slave craton of northern Canada, and its western active margin, the Paleoproterozoic Great Bear Magmatic Zone. His studies of these ancient terranes, sponsored in part by the Geological Survey of Canada, prepared him well for the complex geology of the Canadian Shield in Ontario. He then joined the OGS, where over the years he has taken on more and more senior roles but never gotten away from doing fieldwork. At the OGS, Mike has mentored and supervised numerous students and junior colleagues, including a good number of them working in areas along the northern xii �shore of Lake Superior. He has also taken on more and more editorial roles for various OGS publications, maps, and datasets. From 2002 to 2007, Mike was one of the scientific leads of the Lake Nipigon Geoscience Initiative (LNGI), and provided oversight on OGS mapping projects in the region. He was directly involved in some of the mapping, and particularly the geochronology sampling. He handled numerous publications for this large project and was a guest-editor on the final volume that published many of the LNGI results (Easton et al., 2007). Since then, Mike has been a frequent collaborator on other projects either directly or indirectly relevant to Lake Superior area geology. In the early 1990s, together with Terry Carter, Mike investigated the basement geology beneath the Paleozoic cover in SW Ontario (e.g., Easton & Carter, 1991, 1994, 1995), using geophysical data, and drill cores and cuttings, to locate the Grenville Front and the extension of the MCR in Ontario and into Michigan. Notably, they found that the Grenville Front was located some 100 km to the east of where previous interpretations had located it, and that metamorphosed equivalents of MCR rocks were likely present in the Grenville Front tectonic zone in Essex County. They were among the first to hypothesize that the final stages of MCR rifting and inversion were connected to the main tectonic phases of the Grenville orogeny. From 2002 to 2010, Mike was involved with the MCR digital data and publication collaboration between the OGS, the Minnesota Geological Survey, and the United States Geological Survey, specifically the compilation of Ontario geological, mineral deposit, geochemical, and geochronological data in GIS-compatible formats to allow incorporation into the USGS-led cross-border compilation for the Midcontinent Rift. In addition, also in 2010, he was a co-organizer and editor of four guidebooks for the 11th International Platinum Symposium (June 2010, Sudbury), a meeting that had a strong focus on the MCR, including a week-long field trip visiting deposits around Lake Superior. None of this would have ever happened without Mike’s efforts and contributions. Among his many other contributions to ILSG over the years, Mike also served (and still serves) as a board member for the Institute (2022-2025). After spending the last 53 summers doing fieldwork and research in the Grenville, the Southern Province, the Lake Superior area, or elsewhere in Ontario, Mike retired in March 2025. Given his outstanding accomplishments and amazing productivity over the years, either for the OGS or for various extra-curricular projects such as ILSG meetings, leading field trips, time-consuming editorial jobs, teaching as an adjunct professor, or supervising and mentoring many students (and never missing a beat!), it is a great honour to present Mike with the Goldich Medal Citation by: Wouter Bleeker, Senior Research Scientist, Geological Survey of Canada, Ottawa xiii �Honoring the Pioneers of Lake Superior Geology (Adopted by the Board of Directors, 2016) Preamble At the suggestion of Gene LaBerge, the 2016 executive board agreed to implement a program to recognize historic pioneers in the understanding of geology in the Lake Superior region. Beginning with the 2017 annual meeting, nominations will be accepted from the membership for geologists whose work was conducted primarily before the inception of the Institute in 1955. Biographical sketches of those pioneers will be presented at future annual meetings so that all may appreciate the value of their contributions. Selection of nominees will be decided in part by the organizing committee of each year's annual meeting, in consultation with the Board, to ensure equitable geographic representation in the selection process. Award Guidelines 1) Nominations from the membership will be submitted via the Institute web site and forwarded to the Chair of the next Annual Meeting. The nominations will be no more than half a page in length and will summarize the contribution of the nominee. 2) The Organizing Committee will select one or two individuals to be highlighted at the next Annual meeting and submit those names to the Board for approval. 3) The nominator will be requested to prepare a brief presentation to be given during the next annual meeting with a summary to be included in the Proceedings volume. 4) Unsuccessful nominations will be kept by the Secretary for two years and forwarded to the next meeting Chair; these nominations may be resubmitted at a later date. The Board will review this award every five years. Pioneers of Lake Superior Geology 2017 Douglass Houghton (1809-1845) 2018-20 not presented 2021 Newton Horace Winchell (1839-1914) 2022 Thomas Leslie Tanton (1890-1971) 2023 Thomas Benton Brooks (1836-1900) 2024 Roland Duer Irving (1847-1888) 2025 Robert Bell (1841-1917) xiv �2025 Citation for Robert Bell (1841-1917) Pioneer of Lake Superior Geology Robert Bell had a decided taste for the natural sciences, especially for geology. In 1856, at the age of 15, he secured a temporary position with the Geological Survey of Canada. He assisted Sir William Logan, the Survey’s Director, beginning an illustrious career with the GSC that would span half a century. While working summers for the Survey, Bell graduated in 1861 from McGill College and received the Governor General’s Medal. Two years later, after study at the University of Edinburgh, he joined the faculty of Queen’s College. All the while, he spent summers with the GSC and was made a permanent officer in 1869, named Assistant Director in 1877, Chief Geologist in 1890, and, finally, Acting Director in 1901. He also earned a medical degree in 1878 so that he was prepared for any mishap in the field. Bell is best-remembered for his extensive explorations in northern Quebec, Ontario, Manitoba and the eastern Arctic in the 1870s and 1880s. He mapped the rivers between Hudson Bay and Lake Superior and reconnoitred part of the route that would be adopted for the National Transcontinental Railway. In 1859, Bell assisted in mapping the north shore of Lake Huron and first visited the Lake Superior region in 1860, west of Sault Ste. Marie. His Report on the Geology of the Northwest Side of Lake Superior and of the Nipigon District was published in 1870. In 1870 and 1871, he continued to work north of Lake Superior. In 1872 and 1873, he assisted GSC Director Alfred Selwyn on a preliminary exploration westward from Lake Superior to Fort Garry (now Winnipeg). In 1876, Bell examined the eastern shore of Lake Superior, as well as the Garden River and Echo Lake areas and the northeastern shore of Georgian Bay. A reconnaissance survey was undertaken between Parry Sound and the Ottawa River. In 1881, Bell carried out additional surveys in the Hudson Bay basin and in the Lake Superior region. During 1883 and 1884, Bell continued work near Lake of the Woods. In 1887, he continued a survey, started in 1886, between the Montreal River and Lake Huron to clarify the nature of the Huronian, especially in connection with its mineral deposits. He served as a member of the Royal Commission on the Mineral Resources of Ontario from 1888 to 1889. Between 1888 and 1892, Bell mapped the Sudbury and French River areas. His 1890 paper, On Glacial Phenomena in Canada, was regarded as the most significant advance in Canadian glaciology since Logan’s first acceptance of glacial action in Canada in 1847. Bell was the first to recognize ice streaming in the Laurentide ice sheet and also noted the occurrence of diamonds in glacial drift from Ohio through Indiana, Michigan and Wisconsin, suggesting a possible provenance in Ontario. Of particular interest to the ILSG is Bell’s involvement on a Special Committee created in 1903 on the nomenclature and correlation of the Lake Superior region geology of the United States and Canada. Its findings led to the first joint report by geologists of the two countries. The Committee comprised C.R. Van Hise and C.K. Leith of the United States Geological Survey, A.O. Lane, State Geologist of Michigan; Robert Bell and Frank D. Adams of the GSC, and W.G. Miller, Provincial Geologist of Ontario. In August, 1904, the committee met in the Marquette district, and, during the six weeks following, visited the Gogebic, Mesabi, Vermilion, Rainy Lake, Lake of the Woods, Animikie, and xv �Huronian districts. As a result, both countries adopted a common stratigraphy and nomenclature that served as a basis for later iterations and our current stratigraphic framework. Bell also made field notes on flora and fauna, forests, climate, soil, indigenous people, ethnology and resources. He performed much of his field work without maps and had to do topographical surveys as he went along. It is estimated that Bell named over 3000 geographical features, prompting colleagues to call him the “Father of Canadian Place-Names”. Bell authored 32 GSC reports, 111 journal papers, 17 solo-authored geological maps, 46 other geological, topographical, and cadastral maps as senior author, and 38 maps as junior author. He gave numerous lectures to natural history, historical, and charitable societies. In 1865, at the age of 23, he was elected a fellow of the Geological Society of London. A chartermember of the Royal Society of Canada (1882), he became a fellow of the Royal Society of London in 1897. In 1903, he was made a companion of the Imperial Service Order and in 1906 was awarded both the Patron’s Medal of the Royal Geographical Society of London and the Cullum Geographical Medal of the American Geographical Society of New York. Bell retired from the GSC in 1908. His career exemplified the wide-ranging reconnaissance work performed by the government geologist in the late 19th century. He was a generalist who valued field work over more detailed, specialized study. Few could match Bell’s travels, eclectic interests, and length of service. As Ami (1927) memorialized, “Bell was especially fond of investigating and exploring regions hitherto untraversed. Pioneer work of this nature can scarcely be appreciated today, when newer and more up-to-date methods of examining a hitherto-unknown territory are employed.” Citation by: Mark Smyk (Lakehead University / Ontario Geological Survey (retired)) References Adams, F.D., Bell, R., Lane, A.C., Leith. C.K., Miller, W.G. and Van Hise, C.R. 1905. Report of International Committee on Lake Superior Geology; Journal of Geology, February-March, 1905; in Precambrian nomenclature; Ontario Bureau of Mines, Report for 1905, v.4, part 1, 1905, pp.269-277. Ami, H.M. 1927. Memorial of Robert Bell. Bulletin of the Geological Society of America, v.38, pp. 18-33; PLS. 1- https://archive.org/details/sim_geological-society-of-americabulletin_1927_38/page/n41/mode/2up?view=theater Brookes, I.A. 2016. All that glitters… The Scientific and Financial Ambitions of Robert Bell at the Geological Survey of Canada; Geoscience Canada, v. 43, pp. 147–158; http://www.dx.doi.org/10.12789/geocanj.2016.43.098. Waiser, W.A. 1998. Robert Bell. Dictionary of Canadian Biography, v. XIV (1911-1920), https://www.biographi.ca/en/bio/bell_robert_1841_1917_14E.html. xvi �In Memoriam James M. Franklin (November 9, 1942 – June 19, 2024) We note the passing, on June 19, 2024, of long-term member and former SEG President (2000) James (Jim) M. Franklin. Jim had a long and productive career in academia, government, and industry. He made landmark scientific contributions to our understanding of volcanogenic massive sulfide (VMS) deposits, black smokers and sea-floor massive sulfide (SMS) deposits, and the metallogeny of the Precambrian orogenic belts. Jim’s work with the Ontario Department of Mines in 1966 along the north shore of Lake Superior led to his PhD study of the Proterozoic geology and metallogeny of the Thunder Bay area. He was the first Professor of Economic Geology at Lakehead University (1970–1976) and was later named as a Fellow of Lakehead University in 2017 in recognition of his many contributions to that institution and to its fledgling Geology Department. He then spent more than 20 years at the Geological Survey of Canada where he led the marine minerals program and ongoing work on VMS deposits on land, such as at Sturgeon Lake. Late in his career at the GSC, he was Chief Scientist and responsible for the day-to-day scientific direction of the organization and helping to inform and educate politicians and bureaucrats on the importance of science to the economy and well-being of Canada. In 1998, after retiring from the GSC, he established Franklin Geosciences and had a highly successful career as a consultant and contributed to the discovery of mineral resources globally, while serving as a director and advisor to numerous companies and scientific organizations, including SEG. Jim was very generous with his time and provided guidance and mentorship to students and professionals alike. He received numerous recognitions for his contributions, including Fellow of the Royal Society of Canada, member of the Canadian Mining Hall of Fame, and recipient of the Logan Medal recipient the Geological Association of Canada and the R.A.F. Penrose Gold Medal from SEG. He supported ILSG as field trip leader, Proceedings editor and banquet speaker. xvii �In Memoriam Jorma “Joe” Kalliokosk (November 23, 1923 – June 3, 2024) Jorma “Joe” Kalliokoski passed away on June 3, 2024, at age 100. He was born in Harma, Finland on Nov 23, 1923. In 1931 his family moved to Sudbury, ON, and later to Timmins, ON. He graduated from Western University in London, ON, and later received his PhD from Princeton University. Joe started as a geologist with the Geological Survey of Canada and later worked for Newmont Exploration. He was hired by Princeton’s Department of Geology in 1956. In 1968, he moved to Michigan Tech as a Professor and Head of the Department of Geology and Geological Engineering, where he remained until his retirement in 1988. He was very proud of the department’s growth in research papers and in research funding during his tenure. During his long geology career, he had many travel adventures from the wilds of Canada, to remote areas of South America, and various locations in Europe. He had the ability to make new friends everywhere he went. Joe was a Fellow of SEG for a noteworthy 60 years, from 1958 to 2018. He served the Society in a number of volunteer positions, including SEG Councillor (1972–1974) and SEG President (1980). He served as Trustee of SEG Foundation, Associate Editor for Economic Geology, and Business Editor (1971–1977) and Director for the Economic Geology Publishing Company (PUBCO), the company that was established to publish the journal and later merged with SEG. Joe was also an active member and supporter of ILSG, serving as its Secretary-Treasurer and Chair of the Goldich Medal Committee. He delivered papers at ILSG on various topics, including unconformitytype Proterozoic uranium deposit potential in northern Michigan; the Jacobsville sandstone and tectonic activity; and new Precambrian geology mapping of the Upper Peninsula. He Chaired the 1972 meeting in Houghton and was awarded the Goldich Medal in 1989. xviii �In Memoriam James Alexander Grant (October 3, 1935 — October 3, 2024) James Alexander Grant died on October 3, 2024 – coincidentally also his birthday – at the age of 89. James “Jim” Grant was born in Inverness, Scotland, in 1935. After graduating from the University of Aberdeen, he left Scotland for Canada where he earned his M.S. at Queens University and then his Ph.D. at the California Institute of Technology (Caltech). After graduating from Caltech, Jim took a job in Minneapolis as a geology professor with the University of Minnesota. Jim and his family moved to Duluth in 1969 and he joined the geology department at the University of Minnesota-Duluth, where he would work with his beloved colleagues and students for the next 35 years. In the early 1970s, he helped launch UMD’s still-running geology summer field camp in Park City, Utah, bringing undergrad students out to the mountains for many years. Jim’s groundbreaking work in the 1980s on the isocon diagram is now used by geologists the world over. Over the course of his career, Jim made substantial contributions to the geology of the Lake Superior region. His seminal mapping of the Minnesota River Valley subprovince is still referenced today by the dozens who have since worked in the region. Jim taught hundreds of students over the course of his career who have gone on to contribute in many ways to the geology of the Lake Superior region. Among the most memorable experiences for his students were Jim’s metamorphic petrology trips through Michigan’s Upper Peninsula. xix �Eisenbrey Student Travel Awards The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name “Eisenbrey” was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. “Ned” Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader - he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The number of awards and value are determined by the annual Chair in consultation with the Secretary and Treasurer. Recipients will be announced at the end of the annual meeting. The following general criteria will be considered by the annual Chair, who is responsible for the selection: 1) The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. 2) Students who are the senior author on either an oral or poster paper will be given favored consideration. 3) It is desirable for two or more students to jointly request travel assistance. 4) In general, priority will be given to those in the Institute region who are farthest away from the meeting location. 5) Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. Successful applicants will receive their awards during the meeting. xx �Joe Mancuso Student Research Awards The 2005 Board of Directors established the ILSG Student Research Fund with $10,000 US from the Institute’s general fund to encourage student research on the geology of the Lake Superior region. A minimum of two awards of $500 US each for research expenses (but not travel expenses) will be made each year. Students are expected to present their research orally or during a poster session at an ILSG meeting. The award winners will also be automatically eligible for the Eisenbrey Travel Awards. To allow the fund to grow, the Fund will receive one-half of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. • The ILSG Board of Directors will be responsible for selecting a minimum of two awards each year. The ILSG Treasurer will issue the awards. • The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. • The applications are due to the ILSG Secretary by August 31st of each year. Awards will be made by October 1st of each year. • Names of the award recipients will be announced at the next annual meeting and posted on the ILSG website. • Details of the application process can be found on the ILSG web site. • The proposal will need to be signed by the researcher’s supervisor. The 2012 Board of Directors approved modification of the fund’s name, adding “Mancuso” to reflect the many contributions of Joseph Mancuso to the organization and sizeable donations made in his name. “Doc Joe,” as he was known by his students, taught geology for 36 years at Bowling Green State University, Ohio. He advised many graduate students in field-oriented research, and frequently brought them to Institute meetings. Joe was the 2007 Goldich Medalist. In fall 2024, the ILSG Board of Directors selected two students to be granted research funding of $500 each from the Joe Mancuso Student Research Fund. The awardees were: Zsuzsanna P. Allerton, University of Minnesota- Twin Cities Omar Khalil Droubi, University of Wisconsin - Madison xxi �Doug Duskin Student Paper Awards Each year, the Institute selects the best of student presentations and honors the presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting, and from generous donations to the fund in honor of Doug Duskin—an exploration geologist and long- time friend of the Institute. The 2012 ILSG Board of Directors approved adding Doug’s name to the award to acknowledge his contributions and distribute those donations in a manner that would have pleased him. The Duskin Student Paper Committee is appointed by the Meeting Chair. Criteria for best student paper—last modified by the Board in 2001—follow: 1) The contribution must be demonstrably the work of the student. 2) The student must present the contribution in-person. 3) The Student Paper Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. 4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. 5) The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). 6) The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required but is left to the discretion of the Committee. 7) The names of award recipients shall be included as part of the annual Chair’s report that appears in the next volume of the Institute. Student papers will be noted on the Program. 2025 Student Paper Awards Committee Aaron Hirsch – Minnesota Geological Survey (Committee Chair) Carsyn Ames – Wisconsin Geological and Natural History Survey Paula Leier-Englehardt – HydroGeo Solutions LLC, Wisconsin Ross Salerno – United States Geological Survey Esther Stewart – Wisconsin Geological and Natural History Survey Nick Swanson-Hysell – University of Minnesota xxii �Board of Directors Amy Radakovich, Chair (2025-2028) - Minnesota Geological Survey Peter Hollings, Secretary (2019-2027) — Lakehead University Mark A. Jirsa, Treasurer (2022-2025) — Minnesota Geological Survey Mike Easton (2022-2025) — Ontario Geological Survey Carysn Ames (2023-2026) — Wisconsin Geological and Natural History Survey Theodore J. Bornhorst, (2024-2027) — Michigan Technological University Board members serve through the close of the meeting year shown in parentheses. xxiii �2025 ILSG Meeting Volunteers Angela Sipila - Mesabi Range Geological Society Henry Djerlev - Mesabi Range Geological Society Kim Berry - Mesabi Range Geological Society William Daniels - Mesabi Range Geological Society Ann Marie Prue - MN Department of Natural Resources 2025 ILSG Meeting Session Chairs Aaron Hirsch, Minnesota Geological Survey Robert Lodge, University of Wisconsin, Eau Claire Eric Nowariak, Minnesota Geological Survey Amy Radakovich, Minnesota Geological Survey Stacy Saari, Minnesota Department of Natural Resources, Lands and Minerals Allison Severson, Minnesota Geological Survey xxiv �Field Trip Leaders and Guidebook Authors Field trips have been the mainstay of the ILSG since its inception 71 years ago. We give special thanks to the field trip leaders and guidebook authors who volunteered their time and talent in carrying that tradition forward. Trip 1: Transect through the Quetico subprovince of northern Minnesota – Eric Nowariak (Minnesota Geological Survey), Mark Jirsa (Minnesota Geological Survey, retired) Trip 2: Drill Core from three Cu-Ni deposits of the Duluth Complex - Mark Severson (Natural Resources Research Institute, Teck Retired), Cullen Phillips (New Range Copper Nickel), Kevin Boerst (Twin Metals Minnesota) Trip 3: How do you make iron and/or manganese in Proterozoic Iron Formation? - Alex Steiner (Big Rock Exploration), Latisha Brengman (University of Minnesota, Duluth), Dean Peterson (Big Rock Exploration) Trip 4: New Geological Insights into the genesis of iron ores at Lake Vermillion – Soudan Underground Mine State Park - George J. Hudak (University of Minnesota, George Hudak Geosciences P.L.L.C.), Zsuzsanna P. Allerton (University of Minnesota), Annia Fayon (University of Minnesota) Trip 5: Neoarchean alkalic intrusions in the Wawa and Quetico subprovinces - Terry Boerboom (Minnesota Geological Survey, retired), Amy Radakovich (Minnesota Geological Survey) Trip 6: Unique Keweenawan inclusion (Colvin Creek) in the Duluth Complex - Mark Severson (Natural Resources Research Institute, Teck, retired), Allison Severson (Minnesota Geological Survey), Lauri Severson (Earth Science teacher, retired) Trip 7: Classic outcrops of Northeastern Minnesota - Dean M. Peterson (Big Rock Exploration), George J. Hudak (University of Minnesota, George Hudak Geosciences P.L.L.C.) Trip 8: Glacial Lake Norwood and the Koochiching Lobe - Phillip Larson (Vesterheim Geoscience PLC), Andrew Breckinridge (University of Wisconsin-Superior), Howard Mooers (University of Minnesota, Duluth) xxv �Mine to Mountain Bike Mecca: The story of the Redhead Mountain Bike Park Pete Kero PE, Senior Environmental Engineer Barr Engineering Co. Pete Kero, PE, is an environmental engineer and Vice President with Barr Engineering Co. He has over 30 years of experience in mine permitting, water management, reclamation, and repurposing across the United States. He was the visionary behind the award-winning Redhead Mountain Bike Park in Chisholm, Minnesota which repurposed several former iron mine pits and stockpiles into a destination-quality regional park for mountain biking, hiking, water recreation and all-terrain vehicles. The project has been featured by Outside Magazine, the Sierra Club and the nation-wide documentary film Biketown. Pete’s book Minescapes: Reclaiming Minnesota’s Mined Lands, which was published by the Minnesota Historical Society Press, won a 2024 Minnesota Book Award. This talk will describe the transformation of ten idled open pit iron ore mines in northeastern Minnesota into a world-class recreation destination for mountain biking, hiking and paddling. In addition to describing how and why the trails were built, the presentation will include technical details on sustainable trail design, the concept of intermediate recreational use, changes to mine pit fencing laws that allow for government-sanctioned recreational use of mine lands and the early results and benefits from the first 5 years of the park’s operation. xxvi �Report of the Chairs of the 70th Annual Meeting Theodore J. Bornhorst, Erika C. Vye and Patrice F. Cobin Houghton, Michigan The 70th Institute on Lake Superior Geology (ILSG) was held May 15 to 18, 2024 in Houghton, Michigan, with the meeting headquartered at the Memorial Union Building on the campus of Michigan Technological University. The meeting was sponsored by the A. E. Seaman Mineral Museum, the Great Lakes Research Center, and the Department of Geological and Mining Engineering and Sciences - all units of Michigan Technological University. The meeting was co-chaired by Ted Bornhorst (principal cochair), Erika Vye, Patrice Cobin, and Jim DeGraff; all co-chairs are affiliated with Michigan Technological University. In addition to being a co-chair Patrice Cobin and Julie Stark served as registrars for the 70th annual meeting. The institute was attended by a total of 182 participants of which 40 were students. The meeting consisted of two full days of technical sessions from Thursday morning 16th of May through Friday afternoon 17th of May, and two days for field trips, pre-and post-meeting. A total of 57 presentations were subdivided into 8 technical sessions; 6 technical sessions for 30 oral presentations (of which 5 were presented by students), and 2 poster technical sessions with a total of 27 poster presentations (of which 16 were presented by students). Three presentations were withdrawn. Since past meetings have not included a dedicated technical session for poster presentations, the chairs opted to include two poster sessions for the 70th meeting. We believe this facilitated more time for attendees to review the posters and facilitated interaction between the authors of posters and attendees. The technical sessions of the 70th annual meeting of ILSG were published in 2024 as Part 1 of Proceedings Volume 70 (111 pages). As is customary with ILSG meetings, the field trips were a highlight of the 70th ILSG. The meeting offered 7 field trips with 3 pre-meeting on Wednesday May 15, and 4 post-meeting trips on Saturday May 18. Overall, the field trips were well attended. There were 145 registrants for the 5 field trips that were able to be run. Demand for 4 of the trips exceeded capacity resulting in wait lists. Pre-meeting trip 1 was led by Ted Bornhorst (Michigan Tech) and focused on Mesoproterozoic “Midcontinent Rift-filling Strata and Native Copper Deposits of the Keweenaw Peninsula, Michigan.” Pre-meeting trip 2 was led by Tom Wright (Quincy Mine Hoist Association) and Jim DeGraff and Katherine Langfield (Michigan Tech) and focused on the “Mining History and Geology of the Quincy Mine, Keweenaw Peninsula Native Copper District, Michigan.” Pre-meeting trip 3 focusing on “Geoheritage of Buffalo Reef: Industrial Impact on Land, Culture, and Fish Sovereignty” was scheduled to be led by Erika Vye, Charlie Kerfoot (Michigan Tech), Stephanie Swart (Michigan Department of Environmental Quality), and Dione Price and Evelyn Ravindran (Keweenaw Bay Indian Community). However, the trip could not be run because of low water levels and shifting sediment impeding access to the harbor. xxvii �Post-meeting trip 4 was led by Jim DeGraff, Katherine Langfield, and Dan Lizzadro-McPherson (Michigan Tech) and focused on “Keweenaw Fault Geometry and Kinematics: Clues to Its Nature and Origin.” Post-meeting trip 5 was led by Matt Portfleet (Adventure Mining Company) and Ted Bornhorst (Michigan Tech) and focused on the “Adventure Mine, Ontonagon County, Michigan: Geology and History of a Native Copper Mine.” Post-meeting trip 6 led by Chad Deering (Michigan Tech) ventured outside of the Keweenaw rift to investigate “Southern Complex Granitoids, Gneisses, and Migmatites: New Data, Discoveries, and Perspectives.” Field trip 7 led by Stan Vitton and Mohammad Sadeghi (Michigan Technological University) was scheduled to investigate “Landslides in the Glacial Lake Ontonagon Sediments,” but had to be cancelled due to lack of registrations. Field trip guides were published in 2024 as Part 2 of the Proceedings Volume 70 (194 pages). Five Doug Duskin Best Student Paper Awards were given for student oral and poster presentations as judged by the 2024 Student Paper Awards Committee chaired by Stacy Saari (Minnesota Department of Natural Resources). Zsusanna Allerton was awarded the best oral presentation. The best graduate student poster presentation was awarded to Yirou Xu. The best undergraduate student poster presentation was awarded to Lyndsie Vickers. Alice Martin and Alexander Lawrence were awarded the runner-up for graduate student poster and for undergraduate student poster respectively. The 70th ILSG awarded 14 Student Travel and Participation Awards to help defray the cost of presentations of their research and participation in the ILSG professional meeting. The eligibility of costs, as designated by the Eisenbrey Award, were expanded for the 70th ILSG Student Travel and Participation Awards. We thank the donors for supporting the student awards. The awards were made possible by the generous financial support from our corporate sponsor Eagle Mine – Lundin Mining, the Geological Society of Minnesota, and 23 individual donors. The awardees were Zsuzsanna Allerton, Farhan Ahmed Bhuiyan, Andrea Paola Corredor Bravo, Kevin Mexia Duran, Trent Ediger, Alex Lawrence, Jordan Peterzon, Lucas Robarg, Daniel Shakked, Vlad Sheshnev, Demily Thibodeau-Bello, Adam Vanderkin, Lyndsie Vickers, and Yiruo Xu. There were 6 Michigan Tech students whose registration fees were waived because they volunteered with logistics for the meeting. The ILSG social and banquet was hosted at the Memorial Union Building on Thursday evening May 15. There were 120 people at the annual banquet. Ted Bornhorst served as master of ceremonies for the postbanquet program. After the introductions, Peter Hinz gave a short presentation about a geological excursion to Hawaii. Amy Radovich announced the location of the 2025 meeting as Mountain Iron. The program continued with ILSG awarding the prestigious Goldich Medal to Suzanne W. Nicholson (recently retired from the U.S. Geological Survey). Laurel Woodruff (U.S. Geological Survey and Goldich Medalist in 2014) provided the citation for Suzanne. The co-chairs and the A. E. Seaman Mineral Museum recognized Ted Bornhorst with a plaque for his distinguished service to ILSG. A highlight of the banquet was the keynote presentation by Robert Hazen (Carnegie Institution for Science), an internationally recognized and distinguished mineralogist. His thought-provoking presentation was on “Mineral Informatics: A New Frontier in Understanding Earth.” The keynote presentation ended the banquet program. Hazen’s presentation was made possible by joint funding between the 70th ILSG and the A. E. Seaman Mineral Museum of Michigan Tech. Hazen gave a second presentation on Friday evening for the general public and as a bonus for ILSG participants. This presentation was the A. E. xxviii �Seaman Mineral Museum’s 2024 Edith D. and E. Wm Heinrich Lecture titled “Mineral Evolution: A case study of a new natural law.” The first presentation of the technical sessions was given by Jim Miller (Goldich Medalist in 2012) who gave the citation for Roland Duer Irving as the 2024 Pioneer of Lake Superior Geology. Irving is the 5th person to be recognized for their contributions to Lake Superior Geology prior to the initiation of the ILSG. The Institute’s Board of Directors met on Thursday May 16, 2024 to discuss ILSG business and approve the 2025 meeting location. The meeting was attended by Ted Bornhorst (Board Chair), Carysn Ames, Mark Smyk, Peter Hollings (Secretary), and Mark Jirsa (Treasurer). Guests at the meeting were the meeting co-chairs Patrice Cobin, Erika Vye, and Jim DeGraff, and Amy Radakovich (Assistant Treasurer) and also the Chair of the proposed 2025 Mountain Iron meeting (approved by the board see below). Stacy Saari, Alli Severson, Eric Nowariak, and Aaron Hirsch were additional guests supporting the proposed Mountain Iron 71st ILSG. Institute’s Board of Directors meeting notes were taken by ILSG Secretary Hollings, which are as follows: Accepted report of the Chairs for the 69th ILSG, as published in the Proceedings volume, and minutes of last Board meeting, May, 2023 (Hollings). 2. Received, discussed, and accepted 2023-2024 ILSG Financial Summary (Jirsa). 3. Received, discussed, and accepted 2023-2024 report of the Secretary (Hollings). 4. Approved Ted Bornhorst as on-going ILSG Board member and Amy Radakovich as Chair. 5. Discussed and approved replacing Dorothy Campbell as the “member from government” on Goldich Committee (end of term 2024) with Robert Cundari. 6. Approved Mt Iron as the site for the 71st annual ILSG meeting. The meeting will be Chaired by Amy Radakovich and hosted by the Minnesota Geological Survey. 7. A number of future meeting locations were discussed. Peter Hinz has offered Kenora as a future site, while Mark Puumala has offered Thunder Bay. 8. The confusion over the appointment of the Board Chair was discussed and it was agreed we would follow the Constitution with the incoming Meeting Chair assuming the role of Board Chair. 9. It was agreed that the purchase of additional safety equipment would be postponed for now. 10. The Secretary agreed to revamp the boilerplate material for the volumes to make it easier for the organisers of subsequent meetings. Carsyn agreed to revamp the Eisenbrey and Mancuso award applications. Bornhorst agreed to rewrite the Eisenbrey award document for Board consideration. The allowable expenses will be broadened so the award will be more than travel. 11. Discussed and approved renewal of Pete Hollings as Institute Secretary (end of term 2027). This was later approved by a vote of the membership. 12. Hollings mentioned that the ILSG proceeding volumes standing order sales remain the same as the recent past with only 5 institutions receiving them plus one sent to GeoRef. 1. xxix �13. The co-Chairs would like to thank all those who helped make the 70th annual meeting a success such as judging student papers, chairing sessions, leading field trips, driving for field trips, staffing the registration desk, caring for the projectors, general logistics and more. A special thank you goes to Julie Stark, who played a key role in online and onsite registration. The 70th ILSG was a milestone for a professional organization, as noted by Pete Hollings in a recently published article on ILSG in the Lake Superior Magazine - “not a lot of groups hang around 70 years.” Forty years ago, Ted Bornhorst chaired the annual meeting and Board of Directors. At this time the board had serious concerns about the survival of the organization. We are happy to report that ILSG continues to thrive and has done so by being a small, but vibrant organization. We believe that the combination of collegial, friendly, and open discussion and exchange of ideas on geology of the Lake Superior region between government, industry, and academic geologists has played a major role in ILSG’s survival for 70 years. We strongly believe that field relations are the foundation of geologic interpretation. The depth, breadth, and quality of ILSG field trips is another reason ILSG continues to thrive. What makes ILSG field trips special is that trip leaders are open to debate on their interpretation of an outcrop. Open - but not competitive - discussion is a hallmark of both ILSG field trips and technical sessions. Lastly, meetings would not be possible without people willing to serve as chair or co-chair and people willing to organize the annual conference, to lead field trips, and to serve on local committees. Chairing an ILSG meeting involves personal time, extra work, and a bit of extra stress as attested to by anyone who has risen to this challenge in past years. One of us (Bornhorst) has been principal chair for 6 meetings over 41 years, from 1983 to 2024. He agreed to be Chair one last time to mentor Erika and Patty with the hope that one day, one or both of them, will chair a future annual meeting, contributing to the continuation of ILSG. We hope that ISLG survives for many decades and into the next century and beyond. We are gratified by the positive comments by participants and are happy to have served the Lake Superior geological community. We look forward to the 2025 Mountain Iron ILSG meeting when we can be much more relaxed! Respectfully submitted, Theodore (Ted) Bornhorst, Erika Vye, and Patrice (Patty) Cobin Co-chairs, 70th Institute on Lake Superior Geology xxx �Donations to Support the Annual Meeting of the Institute on Lake Superior Geology A special thank you to our individual contributors Roger Anderson Allan MacTavish Dave Dahl xxxi �Donations to Support Student Participation at the Annual Meeting of the Institute on Lake Superior Geology A special thank you to our individual contributors Kate Clover Jim and Isagel DeGraff Tom Erickson Tom Fitz Aaron Hirsch Paula Leier-Engelhardt Bob Mahin Vince and Susan Mathews Jim Miller Allison Severson Mark and Lauri Severson John Verhoeven Gerry White xxxii �TECHNICAL PROGRAM xxxiii �Wednesday May 14, 2025 All field trips begin and end at the Mountain Iron Community Center Pre-meeting Field Trips May 14, 2024 8:00 am - 5:00 pm PRE-MEETING FIELD TRIPS Trip 1: Transect through the Quetico subprovince of northern Minnesota – Eric Nowariak (Minnesota Geological Survey), Mark Jirsa (Minnesota Geological Survey, retired) Trip 2: Drill Core from three Cu-Ni deposits of the Duluth Complex - Mark Severson (Natural Resources Research Institute, Teck Retired), Cullen Phillips (New Range Copper Nickel), Kevin Boerst (Twin Metals Minnesota) Trip 3: How do you make iron and/or manganese in Proterozoic Iron Formation? - Alex Steiner (Big Rock Exploration), Latisha Brengman (University of Minnesota, Duluth), Dean Peterson (Big Rock Exploration) Trip 4: New Geological Insights into the genesis of iron ores at Lake Vermillion – Soudan Underground Mine State Park - George J. Hudak (University of Minnesota, George Hudak Geosciences P.L.L.C.), Zsuzsanna P. Allerton (University of Minnesota), Annia Fayon (University of Minnesota) Wednesday evening May 14, 2025 5:00 pm - 8:00 pm Registration (Mountain Iron Community Center) 6:00 pm - 8:00 pm Poster Setup and Viewing (Mountain Iron Community Center) 6:00 pm - 8:00 pm Welcoming Reception (Mountain Iron Community Center) * Denotes a student eligible for Best Student Paper Award. To be eligible students must have graduated no more than one month before the ILSG meeting, be first author, and present the paper at the meeting + Denotes author that will present the paper if different than the first author. Thursday - May 15, 2025 7:15 am – 12:00 pm 8:00 am. Registration (Mountain Iron Community Center) Opening remarks (Mountain Iron Community Center) Amy Radakovich, Allison Severson, Eric Nowariak, Stacy Saari, Aaron C. Hirsch Co-Chairs, 2025 ILSG xxxiv �TECHNICAL SESSION I – ORAL PRESENTATIONS Session Chair: Amy Radakovich 8:20 Mark SMYK Robert Bell - Pioneer of Lake Superior geology 8:40 William J. HINZE and Mark B. LONGACRE Revisiting Gravity and Magnetic Anomalies of the Baraboo Range 9:00 Huifang XU and Tianyu ZHOU Battle between the bands: competitive precipitations lead to bands in banded iron formations 9:20 Howard MOOERS, Mark SEVERSON, Peter JONGEWAARD, and Phillip LARSON US Steel Corporation / Ralph W. Marsden iron ore collection 9:40 Matt CARTER Updates on the Minnesota Department of Natural Resource’s Drill Core Library 10:00 END OF TECHNICAL SESSION I 10:00-10:20 COFFEE BREAK TECHNICAL SESSION II – ORAL PRESENTATIONS Session Chair: Stacy Saari 10:20 Alan AUBUT A Contrarian View: Thoughts on the Genesis of the Tamarack Ni-Cu Deposit 10:40Cory PALIEWICZ and Joyashish THAKURTA Lithogeochemical Characterization of Manganese Mineralization at the Cuyuna Range, Central Minnesota 11:00 Guy N. EVANS and William E. SEYFRIED JR. Experimental Reproduction of Acidic Mafic-Ultramafic Hydrothermal Fluids with Implications for Linking Seafloor Lithology to Ore Mineral Solubility and Novel Geochemical Trapping Mechanisms 11:20 Wyatt BAIN, James TOLLEY, and Peter HOLLINGS An overview of the geology, tectonic setting, and occurrence of sulphide mineralization in the Lac Des Iles Intrusive Suite 11:40 Thomas BUCHHOLZ, Alexander FALSTER, and William SIMMONS A complex F-rich alkalic pegmatite in the pyroxene syenites of the stettin complex, Wausau Complex, Marathon county, Wisconsin 12:00 END OF TECHNICAL SESSION II xxxv �12:00-1:30 LUNCH BREAK and ILSG BOARD OF DIRECTORS MEETING - Buffet lunch provided- TECHNICAL SESSION III- POSTER PRESENTATIONS Session Chair: Robert Lodge 1:30-3:00 AUTHORS PRESENT AT THEIR POSTERS 2:40-3:00 COFFEE BREAK 3:00 END OF TECHNICAL SESSION III TECHNICAL SESSION IV – ORAL PRESENTATIONS Session Chair: Allison Severson 3:00 James V. JONES, Ross SALERNO, William F. CANNON, and Pau O’SULLIVAN Geologic implications of detrital zircon U-Pb ages from Archean and Paleoproterozoic strata in central Minnesota and the Gogebic Range of Wisconsin and Michigan, USA 3:20 R. SALERNO, W.F. CANNON, A. SOUDERS, J.M. THOMPSON, and J. VERVOORT Constraining the timing of crustal exhumation following the Penokean orogeny using U-Pb, SmNd, and Lu-Hf geochronology and microstructural analysis 3:40 James DeGRAFF, Chad DEERING, and James JONES III The Archean Carney Lake gneiss complex in Michigan’s Upper Peninsula: Preliminary subdivisions with age constraints 4:00 *Omar Khalil DROUBI, Erik SCHOONOVER, Mona-Liza SIRBESCU, Joshua GARBER, and Chlo BONAMICI Geochronology of lithium mineralization in the Florence pegmatite field, WI, USA 4:20 END OF TECHNICAL SESSION IV xxxvi �Thursday evening May 15, 2024 5:30 pm RECEPTION AND CASH BAR (Mountain Iron Convention Center) 6:30 pm ANNUAL BANQUET (Mountain Iron Convention Center) 2025 Goldich Medal Recipient: Robert Michael Easton Banquet Speaker: Pete Kero, Mine to Mountain Bike Mecca: The story of the Redhead Mountain Bike Park Friday - May 16, 2025 8:15 INTRODUCTORY REMARKS AND UPDATES (Mountain Iron Community Center) Amy Radakovich, Allison Severson, Eric Nowariak, Stacy Saari, Aaron C. Hirsch; Co-Chairs, 2025 ILSG TECHNICAL SESSION V – ORAL PRESENTATIONS Session Chair: Aaron Hirsch 8:20 Wouter BLEEKER, Michael HAMILTON, and Sandra KAMO Paleoproterozoic mantle plume tracks shaping the southern margin of the Superior craton and the geology of the Lake Superior region 8:40 Max ROHRMAN Plume control on the initiation of Mid-Continent Rift breakup using Unconformities: Implications for the Tectono-magmatic evolution and mineral deposits 9:00 James TOLLEY, Pete HOLLINGS, Kevin MEXIA DURAN, and Myles HARDING Evaluating Ni in Olivine as a Prospectivity Indicator for Magmatic Ni-Cu-(PGE) Deposits: A Preliminary Study from the Midcontinent Rift System. 9:20 James TOLLEY, Jacob HANLEY, James CROWLEY, Sasha TSAY, Zoltan ZAJACZ, and Pete HOLLINGS A Porphyry in a Rift? Constraining the Petrogenesis of the Jogran Porphyry, Mamainse Point, Ontario, Canada: Insights from Zircon and Melt Inclusion Geochemistry 9:40 Nicholas SWANSON-HYSELL, Eben B. HODGIN, Tadesse ALEMU, Anthony FUENTES, Yiming ZHANG, Sarah SLOTZNICK, and Luke FAIRCHILD Midcontinent Rift extension ceased and the rift inverted due to the Grenvillian orogeny 10:00 END OF TECHNICAL SESSION V xxxvii �10:00-10:20 COFFEE BREAK – Sponsored by MRGS TECHNICAL SESSION VI – POSTER PRESENTATIONS Session Chair: Robert Lodge 10:00-11:30 AUTHORS PRESENT AT THEIR POSTERS 11:30 END OF TECHNICAL SESSION VI 11:30-1:00 LUNCH BREAK - Buffet lunch provided- TECHNICAL SESSION VII – ORAL PRESENTATIONS Session Chair: Eric Nowariak 1:00 Steven D.J. BAUMANN Pembine-Wausau Terrane as an Icelandic style island overthrust onto Archean basement, instead of an island arc or continental fragment accretion 1:20 Jiří1 ŽÁK, Filip TOMEK, Václav KACHLÍK, František VACEK, Martin SVOJTKA, and Lukáš ACKERMAN Broadly coeval but migrating deformation, plutonism and deposition in the northeastern Superior Province, Québec: evidence of hot accretionary orogeny and oroclinal folding in the late Archean? 1:40 Mark SMYK, Pete HOLLINGS, Riku METSARANTA, Robert CUNDARI, Stephen KISSIN, and Colleen KURCINKA Basaltic rocks of the Animikie Group in Ontario: Geochemical characteristics and tectonic significance 2:00 W. F. CANNON, M. Rebecca STOKES, Ross A. SALERNO Micromineralogy and textures in the Sudbury impact layer on the Mesabi Iron Range, Minnesota: record of processes in the proximal-distal ejecta transition zone 2:20 END OF TECHNICAL SESSION VII 2:20 COFFEE BREAK xxxviii �TECHNICAL SESSION VIII – ORAL PRESENTATIONS Session Chair: Amy Radakovich 2:40 J.D. VERHOEVEN and Tim ZOWADA Origin of magnetic black sand found on the south Shore of Lake Superior 3:00 Erika VYE and Daniel LIZZADRO-MCPHERSON Geospatial Learning Resources to Explore Relationships with Keweenaw Geology 3:20 Allan MACTAVISH, Peter HINZ, +George HUDAK, Phil LARSON, Allan AUBUT, Terry BOERBOOM, Vern CHILTON, Jim DeGRAFF, Tom ERICKSON, Barb FAULKNER, Isabel SERRANO, Larry and ZANKO An informal review of the ILSG field trip to Hawaii: January and February 2025 4:00 END OF TECHNICAL SESSION VIII 4:00 Presentation of Student Awards Best Student Paper Awards – Student award committee Student Travel/Participation Awards – Amy Radakovich MRGS Awards – Mark Severson 4:30 Concluding Remarks and Field Trips Amy Radakovich, Allison Severson, Eric Nowariak, Stacy Saari, Aaron C. Hirsch; Co-Chairs, 2025 ILSG END OF TECHNICAL SESSIONS OF THE 71st ANNUAL MEETING xxxix �Saturday May 17, 2025 Field trips begin and end at the Mountain Iron Community Center 8:00 am – 5:00 pm POST-MEETING FIELD TRIPS Trip 5: Neoarchean alkalic intrusions in the Wawa and Quetico subprovinces Terry Boerboom (Minnesota Geological Survey, retired); Amy Radakovich (Minnesota Geological Survey) Trip 6: Unique Keweenawan inclusion (Colvin Creek) in the Duluth Complex Mark Severson (Natural Resources Research Institute, Teck, retired); Allison Severson (Minnesota Geological Survey); Lauri Severson (Earth Science teacher, retired) Trip 7: Classic outcrops of Northeastern Minnesota Dean M. Peterson (Big Rock Exploration); George J. Hudak (University of Minnesota, George Hudak Geosciences P.L.L.C.) Trip 8: Glacial Lake Norwood and the Koochiching Lobe Phillip Larson (Vesterheim Geoscience PLC); Andrew Breckinridge (University of Wisconsin-Superior); Howard Mooers (University of Minnesota, Duluth) xl �POSTER PRESENTATIONS * Denotes a student eligible for Best Student Paper Award. To be eligible students must have graduated no more than one month before the ILSG meeting, be first author, and present the paper at the meeting + Denotes author that will present the paper if different than the first author. Numbered Posters and Abstracts are in sequential order 1. *Zsuzsanna ALLERTON, George HUDAK, Guy EVANS, Xinyuan ZHENG, and Christian TEYSSIER Geochemical analyses of banded iron formations and formerly mined iron ore in the Lake Vermilion-Soudan Underground Mine State Park, NE Minnesota 2. *Madelyn BANKS, Latisha BRENGMAN, Athena EYSTER Linking whole rock geochemical data with micro-scale mineral characterization of oxidation reactions in the Biwabik Iron Formation, MN, USA 3. Howard MOOERS, Mark SEVERSON, Peter JONGEWAARD, and Phillip LARSON US Steel Corporation / Ralph W. Marsden iron ore collection 4. *Sarah JAROZEWSKI, Paige DUFFY, Cole BARRÉ, Latisha BRENGMAN, and Athena EYSTER Mapping oxidation reactions in iron-rich rocks from northeast Minnesota, USA. 5. *Celia L. CORTOPASSI, Zsuzsanna P. ALLERTON, Joshua M. FEINBERG Alteration of magnetic mineralogy in the Giants Range Batholith by the Duluth Complex 6. *Samara GRIES, Robert W.D. LODGE, Sara HANEL, and Robert HOOPER Rare-element Geochemistry of the Eau Claire River Complex Pegmatites 7. *Linsey HULA and Dyanna CZECK Emplacement of the Mesoproterozoic Wausau Syenite Complex, Wisconsin 8. *Renee O. JEUTTER and Robert W.D. LODGE Geology and Geochemistry of the Mesoproterozoic Round Lake Intrusion and associated TiMineralization, Northern Wisconsin 9. *Bekah R. THOMPSON and Robert W.D. LODGE Ni-Cu-PGE Mineralization at the Mineral Lake Intrusive Complex, northern Wisconsin 10. *Lyndsie A. VICKERS and Robert W.D. LODGE Zircon Petrochronology of the Eau Claire Volcanic Complex in the Marshfield Terrane of the Penokean Orogen, Northcentral Wisconsin 11. *Andrew A. CASPER and Robert W.D. LODGE R Geology and Mineralization of the Plover Au Prospect, Marathon County, Wisconsin xli �12. William FITZPATRICK Textural and chemical analysis of sphalerite ores from the Highland Subdistrict, Upper Mississippi Valley Zinc-Lead District, Wisconsin 13. *Haley P. JOHANNESEN and Robert W.D. LODGE Geology and Geochemistry of the Ritche Creek Cu-Zn deposit, North central Wisconsin 14. *Aidan O. KWIATKOWSKI and Robert W.D. LODGE Zircon Petrochronology of Wisconsin’s Volcanogenic Massive Sulfide Deposits, Northcentral Wisconsin 15. Sara PEARSON, Nolan GAMET, Molly SHALIFOE, Ashley QUIGLEY, and Robert MAHIN Michigan Geological Survey’s Contributions to the USGS Earth MRI National Mine Waste Inventory Effort 16. Ashley K. QUIGLEY, Robert A. MAHIN, and Nolan G. GAMET Critical Mineral Potential of the Northern Margin of the Watersmeet Gneiss Dome, MI USA 17. *MaryElizabeth SHALIFOE and Peter VOICE Identifying Abandoned Mine Surficial Features Using Mask R-CNN, Upper Peninsula Michigan. 18. Sophie CHURCHLEY and Philip FRALICK Unusual early diagenetic structures in the Paleoproterozoic Gunflint Formation, Ontario, Canada 19. Gordon MEDARIS Jr. and Dave MALONE Post-Penokean and Pre-Yavapai Magmatism and Sedimentation in Central Wisconsin (Southern Lake Superior Region) 20. Esther K. STEWART, Michael TAPPA, Ann BAUER, Latisha BRENGMAN, and Anthony PRAVE Sedimentologic and geochemical evidence of marine incursion to the Oronto Group basin, southern Lake Superior region, at ca. 1.08 Ga 21. Carsyn AMES and Brad GOTTSCHALK High resolution thin-section scanning and metadata capture- WGNHS Data Preservation Project 2024 early efforts 22. Nate DANIELS, Grace MCELLISTREM, Raeann VOGEL, and Michael BRAUNAGEL Architecture of the Douglas Fault damage zone, northwest Wisconsin 23. Mark B. LONGACRE and William J. HINZE, William Geologic Interpretation of Filtered Gravity and Magnetic Anomalies of the Baraboo Range 24. Jack MALONE, David MALONE, Raymond ANDERSON, Ryan CLARK Refining the Age and Occurrence of Basement Rocks in Northwest Iowa: Implications for Precambrian Tectonics and Magmatic Evolution of the Laurentian Midcontinent xlii �ABSTRACTS xliii �Geochemical analyses of banded iron formations and formerly mined iron ore in the Lake Vermilion-Soudan Underground Mine State Park, NE Minnesota ALLERTON, Zsuzsanna1, HUDAK, George1,2,3, EVANS, Guy1, ZHENG, Xinyuan1, and TEYSSIER, Christian1 1 Earth & Environmental Sciences, University of Minnesota, Minneapolis, MN 55455, USA Earth and Environmental Sciences, University of Minnesota, Duluth, MN 55812, USA 3 George Hudak Geosciences P.L.L.C., Duluth, MN 55804, USA 2 The Lake Vermilion-Soudan Underground Mine State Park in northeastern Minnesota is known for its underground tours in the former iron mine that was operational between 1884-1962 (Klinger, 1960). The mine contains lenticular-shaped ore bodies enclosed in variably altered banded iron formations (BIFs) that were upgraded to massive hematite iron ore during replacement-style hydrothermal alteration (Gruner, 1926; Klinger, 1960; Thompson, 2015). The timing of ore mineralization is constrained to 1.8-1.6 Ga (Allerton, 2024b). The widely accepted simplified genetic model for these ore deposits involves hydrothermal fluids that leached silica from BIFs and concentrated iron as hematite. Here we utilize historic and recently acquired whole rock major, trace and rare earth element lithogeochemical analyses to perform mass balance evaluations via the isocon method (Grant, 2005) and iron stable isotope geochemistry to propose a new hydrothermal model to better constrain the transition from BIF to iron ore. Eight BIF and twelve ore samples from Thompson (2015) were utilized for this study. BIFs show varying degrees of alteration adjacent to the orebodies, whereas iron ore samples comprise massive hematite ± chlorite. Our data include eight additional samples; four least altered and two hematite-altered BIFs collected from surface outcrops, and two iron ore samples that are 1) high-grade hematite ore with primary phase microcrystalline hematite-martite (MCHMT) with minimal chlorite and 2) lower grade ore with abundant secondary quartz and microplaty hematite (MPH; Allerton, 2024b). The Fe isotope analysis incorporates variably deformed gabbroic rocks and chlorite schist adjacent to the ore bodies as well. Lithogeochemical analyses of 14 BIFs and 14 iron ore samples indicate inverse correlation between SiO2 and Fe2O3(total); BIF has high SiO2 and low Fe2O3(total) contents, whereas iron ore displays low in SiO2 and Fe2O3(total). Statistical evaluations suggest that high strength field elements (HFSE) are immobile and therefore have been selected for isocon analysis. Utilizing a HFSE ‘best fit’ isocon, the system shows almost complete SiO2-loss (99%) and 54% Fe2O3-loss from least altered BIF to high-grade ore (Fig. 1A), suggesting that greater loss of silica relative to iron has resulted in a net concentration of iron. Moreover, there is secondary quartz and MPH in the lower grade ore based on petrography, indicating the lower-grade ore postdates the high-grade ore. Isocon analysis shows SiO2-gain and continued Fe3O2-loss from high-grade to lower grade ore (Fig. 1B). The Fe stable isotope results attest to this by presenting higher δ56Fe values for BIFs that decrease from less to more altered BIF. MCH-MT in highgrade ore displays even lower δ56Fe values, and chlorite within fractures of high-grade ore shows similar values to secondary quartz and MPH in heterogeneous ore, suggestive of paragenesis of two different hematite phases and gangue minerals (Fig. 2). Our new hydrothermal model proposes continuous removal of Fe, entailing coeval Si mobilization and removal from BIF to high-grade MCH-MT ore and re-deposition into lower grade MPH ore. These hypotheses are supported by detailed analyses of lithogeochemistry, mineral textures, and Fe stable isotopes. 1 �Figure 1: A) Diagram displays major oxides of least altered BIF (x-axis) against MCH-MT ore (yaxis) and B) MCTMT ore (x-axis) against MPH ore (y-axis). Ratios show gains and losses are calculated based on the slope of HFSE best fit isocons. Figure 2: Diagram shows decreasing δ57Fe/ δ56Fe values of less and more BIFs, MCH-MT and MPH ore samples, and lithologies adjacent to the ore bodies in Soudan; foliated gabbro, gabbroderived schist, chlorite schist. Values are calibrated to BHVO-2 iron standard commonly used in Fe stable isotope geochemistry. REFERENCES Allerton, Z., Hudak, G., Teyssier, C., Fayon, A., Daniŝik, M., Courtney-Davies, L, and Larson, P., 2024b. Geochronology and geochemistry of hematite ore in northeastern Minnesota: Institute on Lake Superior Geology, Proceedings Volume 70, Part 1 – Program and Abstracts, p. 4-5. Grant, J.A., 2005. Isocon analysis: A brief review of the method and applications: Physics and Chemistry of the Earth, Parts A/B/C, v. 30, p. 997–1004, doi: 10.1016/j.pce.2004.11.003. Gruner, J. W., 1926. Hydrothermal alteration of iron ores of the Lake Superior type—a modified theory: Economic Geology, v. 32, p.121-130. Klinger, F.L., 1960. Geology and ore deposits of the Soudan mine, St. Louis County, Minnesota [thesis]. Thompson, A., 2015. A hydrothermal model for metasomatism of Neoarchean Algoma-Type banded iron formation to massive hematite ore at the Soudan Mine, NE Minnesota [thesis]. 2 �High resolution thin-section scanning and metadata capture- WGNHS Data Preservation Project 2024 early efforts AMES, Carsyn1 and GOTTSCHALK, Brad1 1 Wisconsin Geological and Natural History Survey, UW-Madison, 3817 Mineral Point Rd, Madison, WI 53704 USA The Wisconsin Geological and Natural History Survey (WGNHS) has recently undertaken an effort to scan approximately 3800 of the 4800 historical thin sections held in WGNHS collections as part of the USGS-National Geological and Geophysical Data Preservation Program (NGGDPP). This work builds upon a number of previous projects including: a 2011 NGGDPP project to inventory all thin sections in the WGNHS collections, an internal project to catalog fields notebooks and refine locations of recorded samples, a pilot study to develop a workflow for scanning and editing high resolution photos of thin sections, and a project to inventory and collect metadata from an extensive collection of samples donated to WGNHS by Gene LaBerge (UW-Oshkosh). Building on the lessons learned from these prior studies and methods outlined in Leung and Mcdonald (2023), we have developed a workflow to scan thin sections using a Plustek OpticFilm 8200i film scanner (Figures 1a and 1c) and SilverFastSE Plus software with settings shown in Figure 1b. Forty-eight-bit raw images are produced in both plane, non-polarized light and cross-polarized light (Figure 2). Images are edited post scanning in Adobe Lightroom to enhance the sharpness and exposure to better replicate what users see when viewing thin sections with a petrographic microscope. Scanning and editing images takes approximately 10 minutes per thin section. Photos are stored in TIFF format and are intended to be served on the WGNHS Dataviewer for public access. Thin sections included in this project capture a wide range of lithologies from several Wisconsin counties. Many samples represent some of the first efforts to survey the natural resources and map the geology of northern Wisconsin. The original data associated with the thin sections is archived in historic field notebooks archived at the WGNHS and includes documentation of geomorphology, bedrock and glacial geology, and magnetic susceptibilities of encountered bedrock units. Locations are recorded in Public Land Survey System (PLSS) notation. Samples with at least section level location information were included in this project; many of the locations given in the field notes can be narrowed down to quarter-section designation with certainty. In the initial phase of this ongoing project we have focused on scanning and entering metatdata for samples in and around Florence County, Wisconsin. Precambrian iron formation in this area was mined from 1880-1931 to produce some three million tons of hematite and limonite ore (Brown B., 2021). This project has focused on capturing lithological information from samples in this area, which is characterized by complex Precambrian stratigraphy and structure. Upon project completion, high resolution thin section images will be made publicly available online using the WGNHS Dataviewer. Additionally, all metadata will be uploaded to the USGS’s ReSciColl collection and the WGNHS internal database (Geobase). 3 �Figure 1: A) Plustek Optic Film 8200i scanner and acompyning film tray. B) Scanner settings to be used during the proposed project. Note the 600 ppi preset and further 7,200 ppi adjustable resolution. Thin sections are scanned in 48-bit HDR Raw C) Tray with card stock paper cut to better hold thin sections. Note the two slots on the right are fitted with linear polarizing screens that sandwich the thin sections. The polarized screens are oriented to cross polarize the light when scanning. Figure 2: High resolution thin section images scanned as part of the pilot project. A. and C. were scanned using plane, non-polarized light; B. and D. were scanned using cross polarized light. REFERENCES Brown, B., 2021. Florence Iron Mine: Historical Maps Showing Location of Surface Development, Regional Setting, and Underground Workings. Wisconsin Geological and Natural History Survey WOFR2018-03: 5. Leung, D. D.V., and Mcdonald, A.M., 2023. Picture-perfect petrography: affordable thin-section scanning for geoscientists in the digital era. The Canadian Journal of Mineralogy and Petrology, 61: 10451050. 4 �A Contrarian View: Thoughts on the Genesis of the Tamarack Ni-Cu Deposit. AUBUT, Alan1 1 Sibley Basin Group Ltd., PO Box 304, Nipigon, ON P0T 2J0.Canada The Tamarack Ni-Cu deposit has been attributed as being of intrusive origin (Goldner, 2021; Taranovic et al., 2018). There are many nickel deposits hosted by ultramafic bodies that display clear evidence of being the product of extrusive flows, often exhibiting the same key features used to invoke an intrusive origin (e.g. Arndt, 1975; Hill et al., 1995; Hubbert and Sparks, 1985; Marston et al.,1981). This includes the nickel deposits of the Kambalda district of Australia, Pechenga in the Kola Peninsula of western Russia, Raglan in northern Quebec and Thompson in northern Manitoba. All have been, or currently are, attributed to the intrusion of ultramafic sills (e.g. Bleeker, 1990; Marston et al., 1981; Melezhik et al., 1994). Key evidence in support of this model is that the ultramafic bodies typically exhibit at least some differentiation and are sub-concordant to the host sediments. This tendency to default to an intrusion model now includes the Tamarack deposit in Minnesota even though an extrusion model is more valid. The major komatiite hosted nickel deposits listed above share common features: 1) the nickel mineralisation is hosted by ultramafic rocks; 2) the sulphides are at the stratigraphic base of the host ultramafics; 3) the ultramafic rocks are hosted by, or in contact with, sulphidic and carbonaceous argillaceous rocks; 4) the ultramafic bodies are stratabound and generally conformable to the host lithology; and 5) they are hosted within extensional basins usually with a significant sedimentary component with Kambalda being the one exception. But there is a density “problem” in that ultramafic magmas are typically denser than the host rocks, especially when they are sedimentary. When rocks melt, they become about 10% less dense. In the case of ultramafic rocks, the average density is about 3.0 g/cc (Nisbet et al., 1993) while the crust has a density of 2.7 g/cc or less. To move upward from the mantle through the crust there must have been a mechanism other than buoyancy. “Overpressure” is a valid explanation (Sleep, 1974, 1992). Magma plumes in a mantle plume move upward due to buoyancy to the Mantle-Crust boundary. There it collects and then moves laterally thus creating extensional forces in the overlying crust. This accumulating magma would be constrained by the overlying lithostatic load and in doing so would build up overpressure. If the crust thins enough vertical fractures can form allowing the trapped magma to escape due to the built-up overpressure exceeding the lithostatic load. At surface the hot, dense ultramafic magma would then flow over, and into, deep water sediments where the magma would mechanically and thermally erode and assimilate sulphide rich sediments. Tamarack shows all the same characteristics as other Ni-Cu deposits associated with rift basins and features that are more easily explained by extrusive flow of komatiitic magma. As such the intrusive emplacement model currently favoured should be reviewed and serious consideration given to emplacement by extrusion of a high-density magma driven by overpressure. 5 �REFERENCES Arndt, N.T., 1975. Ultramafic rocks of Munro Township and their volcanic setting; Unpub. Ph.D. Thesis, Univ. Toronto. Bleeker, W., 1990. New Structural-Metamorphic constraints on Early Proterozoic oblique collision along the Thompson Nickel Belt, Manitoba, Canada; In Lewry, J.F. and Stauffer, M.R., eds., The Early Proterozoic Trans-Hudson Orogen of North America: Geological Association of Canada, Special Paper 37, p. 57-73. Goldner, B.D., 2011. Igneous Petrology of the Ni-Cu-PGE Mineralized Tamarack Intrusion; Unpub. M.Sc. Thesis, Univ. Minesota. Aitkin and Carlton Counties, Minnesota; Canadian Journal of Earth Sciences, 44, 1087-1110. Hill, R.E.T., Barnes, S.J., Gole, M.J. and Dowling, S.E., 1995. The volcanology of komatiites as deduced from field relationships in the Norseman-Wiluna greenstone belt, Western Australia; Lithos 34, p. 159-188. Huppert, H.E. and Sparks, R.S.J., 1985, Komatiites I: Eruption and Flow; Journal of Petrology, Vol. 26, Part 3, pp. 694-725. Marston, R.J., Groves, D.I., Hudson, D.R. and Ross, J.R., 1981, Nickel sulfide deposits in Western Australia: a review; Economic Geology, Vol. 76, pp. 1330-1363. Melezhik, V.A., Hudson-Edwards, K.A., Skuf'in, P.K and Nilsson, L.P., 1994a, Pechenga Area, Russia Part 1: geological setting and comparison with Pasvik, Norway; Transactions of Institution of Mining and Metallurgy (Sect. B: Applied Earth Science), Vol. 103, p B129-B145. Nisbet, E. G., Cheadle, M. J., Arndt, N. T., & Bickle, M. J. (1993). Constraining the potential temperature of the Archaean mantle: a review of the evidence from komatiites. Lithos, 30(3-4), 291-307. Sleep, N. H., 1974. Segregation of Magma in the Ascending Mantle. The Journal of Geology, 82(2), 131– 142. Sleep, N. H., 1992. Time Dependence of Kilauea Volcano Structure from Hotspots to Trench Due to Overpressure in the Asthenosphere. Journal of Geophysical Research: Solid Earth, 97(B8), 11773– 11782. Taranovic, V., Ripley, E.M., Li, C. and Shirey, S.B., 2018. S, O, and Re-Os Isotope Studies of the Tamarack Igneous Complex: Melt-Rock Interaction During the Early Stage of Midcontinent Rift Development; Economic Geology, v. 113, no. 5, pp. 1161-1179. 6 �An overview of the geology, tectonic setting, and occurrence of sulphide mineralization in the Lac Des Iles Intrusive Suite BAIN, Wyatt1, TOLLEY, James 2, and HOLLINGS, Peter 2 1 Department of Earth Sciences, Western University, 1151 Richmond St, London, ON N6A 5B7 Canada 2 Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada The Lac des Iles (LDI) mafic-ultramafic complex hosts a world-class platinum group element (PGE) deposit and is spatially associated with a suite of mafic-ultramafic satellite intrusions (i.e. the LDI-intrusive suite; LDI-IS). The intrusions are hosted in the crystalline rocks of the Wabigoon subprovince, along its eastern contact with the sedimentary and volcanic rocks of the Quetico subprovince. Previous work identified textural and geochemical similarities between the LDI-IS and the mineralized rocks of the LDI complex that likely reflect a temporal and genetic association, and perhaps a similar degree of prospectivity for PGE mineralization (Stone et al., 2003). Here, we present an overview of the geology and setting of the LDI-IS, as well as new geochronology, isotopic data, and parental melt modelling. The LDI-IS (Tib Lake, Legris Lake, Wakinoo Lake, Demars Lake, Dog River, Taman Lake, and Buck Lake; Fig. 1a) are mostly leucogabbro to gabbronorite in composition but commonly include hornblende gabbro, hornblendite, and minor peridotite and pyroxenite. Zircon U-Pb ages for mineralized gabbro from the Buck Lake (2698.1 ± 1.6 Ma), Wakinoo Lake (2696.6 ± 0.8 Ma), Demars Lake (2694.1 ± 1.5 Ma), Legris Lake (2690.6 ± 0.8 Ma), Dog River (2689.9 ± 0.7 Ma), and Tib Lake (2685.9 ± 1.6 Ma) intrusions show a spatial trend of younging to the north and demonstrate a temporal association with the Lac des Iles Mine Block intrusion (2689.0±1.0 Ma; Stone, 2010; Fig 1 b). Trace element profiles for modelled parental melts are similar across most of the LDI-IS and are consistent with an arc setting and a common parental magma source reservoir. However, modelled REE profiles for some cyclic units in the Tib lake intrusion were more evolved and enriched in light rare earth elements. Similar patterns are reported in modelled parental melts from North LDI and are consistent with mixing between primitive and more evolved, siliceous magmas (Djon et al., 2017). Though magma mixing influenced the geochemical evolution of the Tib lake intrusion, cyclic units with more evolved signatures were not significantly mineralized. Whole rock εNdT values of gabbroic rocks from the LDI-IS and the Lac des Iles complex overlap with the tonalitic rocks of the Wabigoon subprovince in older intrusions and trend toward increasingly negative values in younger intrusions (Fig. 1c). This suggests assimilation of Wabigoon tonalite by LDI-IS parental magmas early in the formation of this magmatic system, and greater degrees of contamination by Quetico metasediment over time. Magmatic sulphides from the Legris Lake intrusion have δ34S values that overlap the mantle range but trend toward the composition of Wabigoon tonalite (Bain et al., 2023). This suggests that external S or Si addition from the tantalite drove sulphide saturation during its formation. However, a comparison of whole rock S/Se and Cu/Pd ratios of mineralized lithologies across the LDI-IS suggest that sulphide melt retention during emplacement was a more crucial control on the occurrence of PGE-bearing sulphide mineralization than the source of S or the timing of sulphide saturation. 7 �Figure 1: a. Regional geologic map showing locations of Thunder Bay, the Lac des Iles mine (in red), and the Lac des Iles intrusive suite (in blue). b. U-Pb ages for individual intrusions in the Lac des Iles intrusive suite. c. Whole-rock εNdT values for the LDI intrusive suite and host rock lithologies. North LDI and South LDI data from Brügmann et al. 1997 REFERENCES Bain, W.M., Hollings, P.N., Djon, M.L., Brzozowski, M.J., Layton-Matthews, D., Dobosz, A., and Stern, R.A., 2024. Geochemical evolution and parental magma of the Lake Legris mafic-ultramafic complex, Ontario. Mineralium Deposita 59:85-108 Brügmann, G.E., Reischmann, T., Naldrett, A.J., and Sutcliffe, R.H., 1997. Roots of an Archean volcanic arc complex: The Lac des Iles area in Ontario, Canada. Precambrian Research, 81: 223−239. Djon, M.L., Olivo, G.R., Miller, J.D., and Peck, D.C., 2017. Stratiform platinum-group element mineralization in the layered northern ultramafic center of the Lac des Iles Intrusive Complex, Ontario, Canada. Ore Geology Reviews, doi: 10.1016/j.oregeorev.2017.03.011. Stone, D., Lavigne, M.J., Schnieders, B., Scott, J., and Wagner, D., 2003. Regional geology of the Lac des Iles area. Ontario Geological Survey, Open File Report 6120: 15–25. Stone, D. 2010. Precambrian geology of the central Wabigoon Subprovince area, northwestern Ontario. Ontario Geological Survey, Open File Report 5422:1-130. 8 �Linking whole rock geochemical data with micro-scale mineral characterization of oxidation reactions in the Biwabik Iron Formation, MN, USA BANKS, Madelyn1, BRENGMAN, Latisha1, EYSTER, Athena2 1 Department of Earth and Environmental Sciences, University of Minnesota Duluth, Heller Hall, 1114 Kirby Drive, Duluth, MN 55812, USA 2 Department of Earth and Climate Sciences, Tufts University, Lane Hall, 2 North Hill Road, Medford, MA 02155, USA Oxidation and hydration reactions in iron-rich chemical sedimentary rocks are of critical interest because they signify post-depositional changes often linked to later weathering and fluid alteration. Evaluating oxidation and hydration reactions present in iron formations is therefore required to separate out depositional signals in mineralogical and geochemical data from those that link to post-depositional mineral reactions and enrichment processes (Geymond et al., 2022). The Biwabik iron formation is a part of a well-preserved, sub-greenschist lithologic assemblage containing three major meta-sedimentary formations known together as the Animike Group (e.g. Severson, 2009 and references therein). Previous work (e.g. Duncanson et al., 2024 and references therein) demonstrated the preservation of numerous mineral reactions in the Biwabik iron formation, making it an ideal location to test how mineral reactions link directly to whole rock geochemical signals. To evaluate the relative timing of different oxidation and hydration reactions and how they link to whole rock geochemical data, we integrate core, petrographic, and scanning electron microscope observations with whole rock digestion ICP-MS geochemical datasets from core LWD-99-01 (n = 60) of the Biwabik iron formation. Two key oxidation reactions identified in this work include (1) magnetite to hematite and (2) carbonate to magnetite. Mineral reactions are documented by cross-cutting relationships (Figure 1A-D). The mineral reaction of magnetite to hematite (possibly via the recrystallization of metastable maghemite, 2(αFe3O4) + H2O ↔ 3(γFe2O3) + H2); Geymond et al., 2023) is present in all four informal lithologic subunits of the Biwabik iron formation, occurring in 48% (n = 23) of samples (n = 48) across these units. The mineral reaction of carbonate to magnetite (3FeCO3 +H2O → Fe3O4 + 3CO2 + H2, Duncanson et al., 2024) is also present in all four informal lithologic subunits of the Biwabik Iron Formation, occurring in 77% (n = 37) of samples (n = 48) across these units. Based on 64 EDS point analyses of 4 representative samples from each subunit of the Biwabik iron formation, dominant carbonate minerals range from siderite at the base of the stratigraphy, to ankerite, dolomite, and calcite towards the top. Combined, carbonate compositional variability and zonation indicate element exchange during multiple generations of post-depositional fluid alteration, and cross-cutting relationships between carbonate-magnetite, and magnetite-hematite indicate post-depositional oxidation via fluid interaction with pre-existing reduced iron phases. Dissolution of carbonate may have created porosity providing pathways for oxidizing fluids, and further oxidation. Despite these later oxidation reactions, whole rock geochemical data preserves lithology specific signals of oxic vs. anoxic conditions, independent of the presence of the post-formational reactions outlined above. Lower stratigraphic units preserve oxic signals even with ferrous iron phases like siderite and greenalite preserved, while upper stratigraphic units preserve anoxic signals, despite the presence of hematite. Overall, bulk geochemical data from lithologic subunits of the Biwabik Iron Formation do not preserve clear signals associated with post-depositional mineral assemblage modification and oxidation documented by detailed petrographic work. 9 �Figure 1: LWD-99-01 reflected light photomicrographs documenting cross-cutting relationships between mineral phases. A. Upper Slaty sample MIR-17-15 carbonate granule cross-cut by euhedral magnetite (mag) in 20x. B. Lower Cherty sample MIR-19-14 carbonate granule (carb) cross-cut by euhedral magnetite (mag) in 5x. C. Upper Cherty sample MIR-19-15 magnetite crystal (mag) cross-cut by platy hematite (hem) in 20x. D. Lower Slaty sample U-05 magnetite crystals (mag) crosscut by platy hematite (hem) at the edge of a silicate granule in 10x. REFERENCES Duncanson, S., Brengman, L., Johnson, J., Eyster, A., Fournelle, J., Moy, A., 2024. Reconstructing diagenetic mineral reactions from silicified horizons of the Paleoproterozoic Biwabik Iron Formation, Minnesota. American Mineralogist, 109, 339-358. Geymond, U., Briolet, T,. Combaudon, V., Sissmann, O., Martinez, I., Duttine, M., Moretti, I., 2023. Reassessing the role of magnetite during natural hydrogen generation. Front. Earth Sci., 11, 1169356. Geymond, U., Ramanaidou, E., Lévy, D., Ouaya, A., Moretti, I., 2022. Can Weathering of Banded Iron Formations Generate Natural Hydrogen? Evidence from Australia, Brazil and South Africa. Minerals, 12, 163. Severson, M., Heine, J., Patelke, M., 2009. Geologic and Stratigraphic Controls of the Biwabik Iron Formation and the Aggregate Potential of the Mesabi Iron Range, Minnesota. University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR- 2009/09, 173, 37 plates. 10 �Pembine-Wausau Terrane as an Icelandic style island overthrust onto Archean basement, instead of an island arc or continental fragment accretion BAUMANN, Steven D.J. Midwest Institute of Geosciences and Engineering Since at least the 1960s, we have thought of the Pembine-Wausau Terrane (PWT) as an island arc or continental fragment accretion, smashed between the Superior Craton to the north and the Marshfield Terrane to the south. We all have seen a fault zone appear on geologic maps of the border between the Upper Peninsula of Michigan and northeast Wisconsin called the Niagara Fault Zone (NFZ). There is only one major problem, no one has ever found the NFZ. It doesn’t outcrop anywhere, it does not appear in well records, nor clearly on gravity maps, or magnetic maps. Often where it is inferred it can be interpreted other ways. And the NFZ isn’t reflected in any smaller chronostratigraphically equivalent structures that do outcrop. I have found white unbaked quartzite pebbles (the Sturgeon Quartzite) north of the NFZ (fig. 1). The host rock of these pebbles according to maps, are metamorphic rocks that supposedly have an igneous protolith. I find that very hard to reconcile with present modeling. As I have looked at the highly deformed rocks of the Florence Wisconsin, Iron Mountain Michigan, and Norway Michigan areas, I have come to the conclusion that many rocks mapped as metaigneous, are in fact, metasedimentary. I have been working on a local cross section for several years with my observations. I am coming to the conclusion that the mafic rocks and metasediments to the north of where the NFZ has traditionally been mapped, are more or less continuous and correlative to the mafic and metasedimentary rocks to the south of it. Interpretation of the rocks is understandably very difficult as the rocks are highly metamorphosed and deformed. This work is preliminary. My interpretations could change. But this is where the evidence is leading me thus far. So, if the area that is mapped as the NFZ is not a fault zone, what is it? I see it as one of two possibilities. It could be more of a shear zone formed from a more lateral accretion of a volcanically active, partially rifted Archean sliver, similar in appearance to Baja California. Sheering would be hard to see expressed in the rocks, just as it is for other covered shear zones further north. The second possibility is that the PWT was originally an Icelandic style island on a spreading center that would eventually become subducted under the Superior Craton, similar to the East Pacific Rise, before subduction switched to the south as the Marshfield Terrane approached. Its suspected Archean basement could be explained by a thin skinned over thrusting of the PWT over a small sliver of Archean crust, while volcanism was ongoing. The age of the xenocryst zircons expected to be Archean are only 2,607+22 Ma (VanWyck and Johnson. 1997). This is similar to many Archean ages of the Superior Craton. It is still a possibility the Penokean was a continental fragment like the Marshfield Terrane, only far more incomplete and still covered with younger deposits, but this cannot be the default without understand the nature of the NFZ, if it even exists. The Archean basement of the PWT could also be some sort of an extension of nearly in situ Superior Craton, that hosted the PWT as it formed, or it was overridden by the PWT. I am currently favoring the second interpretation. In this case no NFZ is needed to explain anything observed, at least locally. Everything can be explained by dominantly ductile deformation, at least in the upper crust. This is really reflected in the rocks at Piers Gorge and in the local Michigamme Formation, which locally do not express any Penokean aged faults of any 11 �significance. It also would explain the contemporaneous crustal thinning to the east in the Sudbury area if we had a subducting rift. This is something that forearc extension and island arc accretion cannot explain on their own. This would also put the continental suture further south, at the Eau Claire Sheer Zone. Figure 1: Adapted from Baumann, 2021 REFERENCES Baumann, S.D.J., 2021. The Misunderstood Penokean Orogeny. Midwest Institute of Geosciences and Engineering, publication G-102021-1A VanWyck, N. and Johnson, C.M., 1997. Common lead, Sm-Nd, and U-Pb constraints on petrogenesis, crustal architecture, and tectonic setting of the Penokean orogeny (Paleoproterozoic) in Wisconsin. GSA Bulletin; July 1997; v. 109; no. 7; p. 799–808; 8 figures, 2 tables 12 �Paleoproterozoic mantle plume tracks shaping the southern margin of the Superior craton and the geology of the Lake Superior region BLEEKER, Wouter1, HAMILTON, Michael 2, and KAMO, Sandra 2 1 Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8, Canada; wouter.bleeker@canada.ca 2 Jack Satterly Geochronology Laboratory, Department of Earth Sciences, University of Toronto, 22 Ursula Franklin Street, Toronto, ON M5S 3B1, Canada All Archean cratons are fragments of late Archean “supercratons”, i.e. the larger landmasses to which these craton fragments trace their origin (Bleeker, 2023). At least two large independent supercratons, Superia and Sclavia, named after their well-preserved internal fragments, had formed by the late Archean and underwent progressive breakup during the early Paleoproterozoic, from ca. 2.2 Ga to 1.9 Ga. Based on well-populated apparent polar wander paths, these supercratons moved independently; hence, the mythical notion of a single, longlived, late Archean supercontinent “Kenorland” is incorrect, aside from being untestable. Craton fragments can be correlated and put back together again by matching distinctive basement geology, by correlating overlying pre-breakup basin stratigraphies, and by correlating remnants of pre- to syn-breakup large igneous provinces, particularly their dyke swarms (Bleeker and Ernst, 2006). Of all the dispersed Archean craton fragments, more than 10 trace their origin back to supercraton Superia, representing “nearest neighbour” fragments to the Superior: Karelia, Kola, Wyoming, Hearne, Kaapvaal, Pilbara, Yilgarn, Zimbabwe, North Atlantic craton, and possibly Dharwar. Kaapvaal-Pilbara joined a growing Superia late in the game, at ca. 2650 Ma, separating again ~600–700 Myr later, leaving the ancient Minnesota River Valley terrane behind. With a robust reconstruction of Superia, numerous other important insights follow, including that of ancient mantle plume tracks (Figure 1). Here we discuss evidence for two major plume tracks that shaped the southern margin of the Superior craton, the 2480-2440 Ma “Matachewan” plume track, and the 2125-2050 Ma “Marathon” plume track. Both these plume tracks started within Superia’s core, before crossing over to then-contiguous crust of “greater Karelia” and Kaapvaal, respectively. The Matachewan plume track was initiated in the Sudbury area with a suite of 2480-2472 Ma mafic layered intrusions emplaced at the base of the Huronian Supergroup. It then triggered the ca. 2461 Ma giant Matachewan dyke swarm, which converge to a magmatic centre well to the south. At ca. 2450 Ma, the plume crossed over to then-contiguous “greater Karelia” (Davey et al., 2020) where it spawned additional dyke swarms and a flare-up of large layered intrusions, some as young as 2440 Ma. The much younger Marathon plume was initiated at ca. 2125 Ma with a giant radiating mafic dyke swarm, the Marathon dykes, with a focal point in the eastern Lake Superior area. It then spawned progressively younger mafic dyke swarms to the southwest before crossing over to the contiguous Kaapvaal craton where it spawned carbonatites at 2060 Ma, and finally the emplacement of the Bushveld Complex at 2056 Ma, the long axis of which is aligned with the plume track (Figure 1). Both plume tracks show well-defined age progressions indicating plate velocities of ~1–5 cm/yr. SOME REFERENCES Bleeker, W., 2003. The late Archean record: a puzzle in ca. 35 pieces. Lithos 71(2-4): 99-134. Bleeker, W. and Ernst, R.E., 2006. Short-lived mantle generated magmatic events and their dyke swarms: The key unlocking Earth's palaeogeographic record back to 2.6 Ga. In: Hanski, E., Mertanen, S., 13 �Rämö, T., Vuollo, J. (Eds.) Dyke Swarms—Time Markers of Crustal Evolution, AA Balkema, Rotterdam, p. 3-26. Davey, S.C., Bleeker, W., Kamo, S.L., Vuollo, J., Ernst, R.E., and Cousens, B.L., 2020. Archean block rotation in Western Karelia: Resolving dyke swarm patterns in metacraton Karelia-Kola for a refined paleogeographic reconstruction of supercraton Superia. Lithos 368: 105553. Fiorentini, M.L., O’Neill, C., Giuliani, A., Choi, E., Maas, R., Pirajno, F., and Foley, S., 2020. Bushveld superplume drove Proterozoic magmatism and metallogenesis in Australia. Scientific Report 10(1): 19729. Figure 1. Paleogeographic reconstruction of late Archean–early Paleoproterozoic supercraton Superia, involving >10 of the better-known Archean craton fragments from around the world, with the wellpreserved Superior craton as its signature internal fragment. Vaalbara and several other cratons (e.g., Wyoming) formed a single, large, ancient superterrane that collided with the southern margin of growing Superia at ca. 2650 Ma. After a period of stasis, supercraton Superia underwent progressive rifting and breakup from ca. 2.2 Ga to 1.9 Ga. Selected Paleoproterozoic mafic magmatic events are shown, with a focus on two well-defined mantle plume tracks, the “Matachewan” plume track (bold grey arrow) and the “Marathon” plume track (bold purple arrow), both with clear age progression. The Marathon plume track, which initiated at 2125 Ma with a giant radiating dyke swarm, crossed over into the adjacent Kaapvaal craton where it culminated in the emplacement of the Bushveld Complex. The actively rifting Superia plate was likely at a stand-still at Bushveld time (ca. 2056 Ma), allowing the plume tail to erode and dramatically thin the Kaapvaal lithosphere and setting up the conditions for the emplacement of Earth’s largest mafic layered intrusive complex. Ponding of voluminous sublithospheric plume magma resulted in outflow to distal localities (dashed arrows), possibly as far as Karelia-Kola (e.g., Kevitsa, 2058 Ma) and the Yilgarn (e.g., Mount Weld, ca. 2060 Ma; cf. Fiorentini et al., 2020). 14 �A COMPLEX F-RICH ALKALIC PEGMATITE IN THE PYROXENE SYENITES OF THE STETTIN COMPLEX, WAUSAU COMPLEX, MARATHON COUNTY, WISCONSIN BUCHHOLZ, Thomas1, FALSTER, Alexander2, and SIMMONS2, William 1 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494, 2MP2 Research Group, Maine Mineral and Gem Museum, PO Box 500, 99 Main Street, Bethel, Maine 04217, USA The Stettin Complex is the oldest (1565 +3-5 Ma, Van Wyck 1994) and most alkalic of the four intrusions that comprise the Wausau Syenite Complex, and is composed of various syenite phases. This abstract is an update to a study of this dike in ILSG 2024. The sub-horizontal pegmatite is weathered, mineralogically and texturally zoned, and includes numerous syenite screens. Thin 2-3 cm reaction zones are common at contacts, with small miaroles, scattered patches of abundant, tiny pink zircons, fergusonite-(Y), and other minerals. Small miarolitic cavities are common throughout the dike. Overall mineralogy is complex, typical for fractionated alkalic pegmatites. Pyroxenes are largely absent except for highly altered replacements and sparse unaltered remnants of hedenbergite. Early formed pyroxene(s) appear to have been destabilized by later oxidation, altering Fe2+-rich pyroxenes to quartz and smectite-group clays ± goethite with sparse remnants of hedenbergite, and allowing crystallization of more oxidized (Fe3+ rich) magnetite and arfvedsonite. Similar reactions may have altered early-crystallizing chevkinite(Ce) (or a similar LREE-Ti species) and possibly aeschynite-(Ce), to an unidentified Ti-Ce4+-Fe phase: relatively common soft, pale yellow to creamy to brown grains of varying morphologies typically containing high Ti-Ce-Fe contents with traces of other elements. Cerium is likely present as Ce4+ based on the absence of associated LREE3+ (La, Nd, Pr). Alteration under oxidizing conditions may have removed LREE3+, Si and other elements, leaving immobile Ti, Ce4+, minor Fe3+ and trace amounts of other elements. Fluorapatite occurs as abundant hexagonal prisms in intermediate zones of the dike; generally highly altered with elevated to very high LREE (Ce-dominant) and Si contents, while similar reddish crystals in pegmatite units near the lower contact show more typical very low LREE contents. This may be the result of alteration/partial replacement of fluorapatite by fluorbritholite as discussed by Betkowski et al (2016). Work also continues on a rare unidentified Ba-silicate mineral, where lack of Al precludes Ba-feldspars. Several small-volume units and isolated occurrences contain minerals not normally found in alkalic pegmatites, including cassiterite, Hf-enriched zircons (up to 5.5 wt.% HfO2, vs 1.48 wt. % HfO2 in pegmatite margin zircons), fluorcalciomicrolite (D-site occupancy Ta 1.05, Nb 0.65, Ti 0.30; Σ 2), tantalite-(Mn), and barite. Sphalerite in unweathered lower portions of the dike is notable in containing about 0.7 wt. % Indium. Later oxidizing conditions are evident in late crystallization of siderite (now goethite), and LREE fluocarbonates. Crystallization of fergusonite-(Y) (to date Nb-dominant, ≈Nb 1.96, Ta 0.04; Σ 2), being rich in MREE and HREE and lacking redox sensitive Ce, appears to have continued throughout dike crystallization. 15 �REFERENCES Betkowski, Wladyslaw B., Harlov, Daniel E. and Rakovan, John F., 2016. Hydrothermal mineral replacement reactions for an apatite-monazite assemblage in alkali-rich fluids at 300-600° C and 100 MPa, American Mineralogist 101, 2620-2637. Van Wyck, N. 1994. The Wolf River A-type magmatic event in Wisconsin: U/Pb and Sm/Nd constraints on timing and petrogenesis (abstract): Institute on Lake Superior Geology, 40th Annual Meeting, Part 1, Program and Abstracts, 81-82. Aeschynite-(Ce) Albite Anorthoclase Barite Bavenite(?) Bertrandite Calcite Cassiterite Columbite-(Fe) Fayalite Fergusonite-(Y) Fluoro-arfvedsonite Fluorannite Fluorapatite Fluorite Fluorcalciomicrolite Fluorcalciopyrochlore Graphite Hedenbergite Ilmenite K-feldspar Kainosite-(Y) Magnetite Molybdenite Monazite-(Ce) Niocalite? Phenacite Quartz Siderite Sphalerite Thorite Titanite Zinnwaldite Zircon Zircon (metamict) (Ce,Ca,Fe,Th)(Ti,Nb)2(O,OH)6 NaAlSi3O8 (Na,K)AlSi3O8 BaSO4 Ca4Be2Al2Si9O26(OH)2 Be4(Si2O7)(OH)2 CaCO3 SnO2 Fe2+Nb2O6 Fe2+2SiO4 YNbO4 [Na][Na2][Fe2+4Fe3+]Si8O22F2 KFe2+3(Si3Al)O10F2 Ca5(PO4)3F CaF2 (Ca,Na)2(Ta,Nb)2O6F (Ca,Na)2(Nb, Ti)2O6F C CaFe2+Si2O6 Fe2+TiO3 KAlSi3O8 Ca2(Y,Ce)2(Si4O12)(CO3) · H2O Fe2+Fe3+2O4 MoS2 Ce(PO4) (Ca,Nb)4(Si2O7)(O,OH,F)2 Be2SiO4 SiO2 FeCO3 ZnS Th(SiO4) CaTi(SiO4)O KFe22+Al(Al2Si2O10)(OH)2 to KLi2Al(Si4O10)(F,OH)2 Zr(SiO4) Zr(SiO4) Table 1. Dike Mineralogy 16 Common Rock-forming Rock-forming Rare Rare Rare Common Uncommon Rare Rare Common Rock-forming Rock-forming Common Common Rare Rare Rare Uncommon Common Uncommon Rare Common Uncommon Common Rare Rare Rock forming Common Rare Rare Uncommon Uncommon Very common Common �Micromineralogy and textures in the Sudbury impact layer on the Mesabi Iron Range, Minnesota: record of processes in the proximal-distal ejecta transition zone CANNON, W. F., STOKES, M. Rebecca, SALERNO, Ross A. U.S. Geological Survey, Geology, Energy & Minerals Science Center, Mail Stop 954, Reston, VA 20192 The Sudbury Impact Layer (SIL) (1849 Ma), deposited here within hours of the giant meteor impact at Sudbury, Ontario, is known from drill core at four locations on the Mesabi Iron Range (Fig. 1) along a trajectory distance as great as 980 kilometers from the impact point. It records an instant of high energy deposition of about one meter of mixed ejecta and local bedrock within an otherwise quiescent sequence of siltstone and iron formation. Optical, scanning electron microscope, and Raman spectroscopy data provide details of the SIL that reveal some of the complexities of ejecta transport and deposition. Data presented here are from the Nashwauk occurrence where four drill holes provide continuous samples across the layer. Figure 1. Geologic map of the Mesabi Iron Range showing the four locations where the Sudbury Impact Layer (SIL) has been observed: Coleraine (Huber, et al., 2014; Nashwauk (Cannon, et al., 2017, this study); Eveleth (Addison, et al., 2005, this study); Erie (this study). The SIL on the Mesabi Iron Range consists of millimeter-scale ejecta particles expelled from the large crater near Sudbury, and coarser fragments, of sedimentary rocks, some greater than 3 cm diameter, which were derived locally. The ejecta can be subdivided into two categories: A- devitrified glass (Fig. 2), and B- millimeter-scale mineral grains and rock fragments displaying shock metamorphic features (Fig. 3). Figure 2. A-microtektite in matrix of coarse secondary dolomite. Original glass devitrified to K-mica. Vesicles are filled with dolomite. B-delicate bubble structures preserved in secondary dolomite. Bubble walls are mostly K-mica and chlorite. C-angular fragment of flattened vesicular glass, now mostly chlorite. D-rounded particle composed of fine K-mica. Spheres of vesicular glass and their fragments are common (Fig. 2A), including thinwalled hollow structures (Fig. 2B). They are now composed of micron-scale K-mica and chlorite. Irregularly shaped glass shards, mostly composed of chlorite, are also abundant (Fig. 2C). Many are larger than typical spherules and are probably far-flung bits of impact melt rather than broken spheres. Most are flattened into bedding. Also common are rounded grains composed of sub-micron K-mica with relict vesicles (Fig. 2D). These are distinct in having been 17 �sufficiently strong to have avoided flattening. Other glass particles are molded around them. They were likely droplets of melt with very uniform K-Al-Si composition. Quartz and feldspar grains with multiple sets of planar deformation features and zones of devitrified impact glass attest to the intense shock unique to meteor impacts. Small rock fragments with intense shock features are also common (Fig. 3). Figure 3. A-quartz with one well-developed set of planar deformation features and two weaker sets (red lines). B- intensely shocked quartz with “toasted” appearance and zones of devitrified glass. Ccathodoluminesence image of B showing complex shock-induced internal features. D-intensely shocked polycrystalline orthoquartzite fragment. Abundant glass spherules in the Nashwauk ejecta appear to be microtectites. Such particles are widely interpreted to form by condensation from impact vapor plumes above the atmosphere and can be distributed worldwide. At Nashwauk they are mixed with small rock and mineral particles from the outermost margins of the impact ejecta curtain. These were transported either (or both) on ballistic trajectories, or by intense impact-generated winds beyond the ejecta curtain. Many have strongly developed shock features attesting to their derivation by crater excavation near Sudbury. Notably missing from the SIL on the Mesabi Iron Range are accretionary lapilli, a hallmark of more proximal sites where ballistic ejecta and ground surges were the dominant transport mechanism for ejecta. The SIL at Nashwauk is very similar to that at the three other occurrences along the Mesabi Iron Range which, together, document a broad transition zone, between about 900 to 1000 kilometers from the impact point. Here the most distal ballistic ejecta persisted as millimeter-scale grains into a zone where ejecta plume material was becoming dominant. Along the Mesabi Iron Range ejecta was deposited in a shallow sea where fine-grained laminated silt and chert were being deposited, both before and after the impact. Strong impact-generated tsunamis reworked the ejecta and underlying sediments within hours or days of the impact to produce the intermixing of fine-grained ejecta particles with much coarser rip-up clasts from the pre-impact seabed. REFERENCES Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, Davis, D.W, Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005, Discovery of distal ejecta from the 1850 Ma Sudbury impact event. Geology, v. 33, p.193–196. doi: https://doi.org/10.1130/G21048.1 Cannon, W.F., Woodruff, L. J., Jirsa, M., and Everett, W, 2017, New observations on distal ejecta from the Sudbury impact in the central Mesabi Iron Range, northern Minnesota, Institute on Lake Superior Geology, v. 63, Proceedings Part 1, Program with abstracts, p. 19-20. Huber, M.S., McDonald, I. and Koeberl, C., 2014, Petrography and geochemistry of ejecta from the Sudbury impact event, Meteoritic and Planetary Science: v. 49. p. 17491768. https://doi.org/10.1111/maps.12352 Jirsa, Mark, Chandler, V.W., and Lively, R. S., 2005, Bedrock geologic map of the Mesabi Iron Range Minnesota, Minnesota Geological Survey Miscellaneous Map Series map M-163. 18 �Updates on the Minnesota Department of Natural Resource’s Drill Core Library CARTER, Matt1 1 Minnesota Department of Natural Resource, Division of Lands and Minerals, 1525 3rd Ave E, Hibbing, MN, 55746 USA The Minnesota Department of Natural Resource’s (DNR) Drill Core Library (DCL) in Hibbing, MN is the only state-owned facility for archiving drill cores and other geological materials from Minnesota. The DCL was first established in 1972 when Building 1 (B1) was constructed. It was expanded in 1979 when Building 2 (B2) was constructed. Building 3 was first constructed in 1989 and expanded in 1995 and 2009. The facility currently stores around 3.5 million linear feet of drill core and contains material and/or data for over 20,000 drillholes. In 2023, it was identified that original shelving units installed in B1 and B2 needed to be replaced, and other safety issues needed to be resolved. The DNR diligently prepared to move all drill cores and other noncore geological materials from B1 to replace the shelving units. This was accomplished by assessing materials for deaccession, creating a box index of its holdings, applying barcodes to over 38,000 drill core boxes, as well as inventorying and barcoding noncore materials. Boxes were moved box by box by hand onto roller tables to an intake station where tracking information and digital images were captured. Boxes were then palletized and placed into temporary storage, which involved 712 pallets and 40 storage containers. The captured digital images have created a new and accessible digital record for B1 cores. Once the original shelving units were removed from B1, the DNR upgraded its lighting, and a new racking system was installed. Reverse flow of materials to B1 is anticipated to be completed by the end of May. Similar preparation activities are being applied on materials in B2. Instead of placing materials into temporary storage, rack space will be freed up through deaccession activities and materials will be rearranged within B2 to create new egress space. Over 210,000 iron ore boxes will be repackaged and moved onto new rack units. Lighting upgrades have been implemented in portions of B2, with the remaining lights to be replaced later this year. The DCL remains partially open to visitors, but users should be aware that materials from B1 and B2 may be unavailable until the project is completed on or before June 30, 2026. The DCL is nearing its facility-wide storage capacity and there is very limited space to accept additional materials. The DNR is only accepting deliveries on a case-by-case basis, and it is expected that any materials turned over to the state will become public upon delivery. In 2017, recognizing that the DCL was rapidly filling up, the DNR designed and actively sought funding for a fourth building to double the facility-wide storage capacity and quadruple view room space. This project is shovel-ready, but construction remains on hold until funding is legislatively secured. 19 �20 �Geology and Mineralization of the Plover Au Prospect, Marathon County, Wisconsin CASPER, Andrew A.1, LODGE, Robert W.D.1 1 Department of Geology and Environmental Sciences, University of Wisconsin-Eau Claire, Eau Claire, WI 54701 USA The Plover Au Prospect, located in Marathon County, WI, is hosted in Paleoproterozoic metaandesites, schist, and felsic/mafic intrusive units of the Wausau Volcanic Complex (LaBerge & Myers, 1983). Gold prospects in this region are bound to the southeast and northwest by large faults within the Eau Claire Deformation zone and the Wolf River Batholith (Lynott et al, 2022) (Figure 1). Rocks have undergone potassic and sericite alteration, greenschist to amphibolite grade metamorphism, and multiple stages of deformation. Research on the formational history of gold mineralization, in combination with its geochemical footprint, is essential for establishing a regional geologic setting of gold-forming events. Previous mineral exploration on this area has focused on exploring high concentrations of gold (Au) within volcanic units and sulfide vein networks at the Reef Deposit (Figure 1). With its proximity to the larger Reef Deposit, a more complete understanding of the Plover Prospect can add to a better regional context to the Au mineralizing system and potentially improve mineral exploration models. For this study, two holes (PL-76-1 & PL-76-4), totaling ~1,180 linear feet of core were chosen based on their relative locations and lithologic variation to fully characterize the range of units hosting mineralization. Representative volcanic strata and intrusive rocks were sampled and characterized through petrographic and geochemical analyses. The Plover deposit is primarily composed of andesitic/basaltic volcanics and gabbro/diorite intrusive units deposited sequentially showing sharp and, in some cases, brecciated contacts with one another. Brittleductile deformation is indicated by zones of brecciation present within the volcanic units. These structures include vein networks containing boudins and vugs containing sulfides and calcitechlorite alteration. It is probable that multiple deformational events occurred due to veins crosscutting foliation locally, and variation in the internal composition of veins. Hydrothermal alteration is suggested based on the presence of potassic alteration within the basaltic foliation and sericite-chlorite alteration in layers. Pyrite, chalcopyrite and pyrrhotite occur within vein networks. Since high Au concentrations are typically present within massive/semi-massive sulfide veins which contain brittle to brittle-ductile deformation, this mineralization likely occurred after Penokean deformation and metamorphism that formed the primary structural fabric in the rocks. The Reef gold-copper deposit has been researched extensively by various exploration companies since the 1990’s (Lynott et al, 2022). The deposit is located <1 mi east of the Plover deposit and has shown significantly higher Au concentrations. The deposit has been broadly classified as orogenic in origin and is claimed to have produced shear hosted vein-type gold and copper occurrences. Gold/copper mineralization occurs within stacked and relatively thin zones of quartz-sulfide veins and lenses; and sericite alteration within vein selvage typically accompanies gold mineralization within these areas. The primary lithology between the two deposits is similar, however, the Reef deposits proximity to the Wolf River batholith potentially influenced the degree of deformation and sericite, talc, tremolite, and pyrrhotite alteration. Future research should focus on more detailed comparisons between the Reef and Plover gold systems to better constrain potential genetic links between them. 21 �Figure 1. The relative geographic locations of the Plover and Reef deposits in the Penokean Volcanic Belt (PVB), central Wisconsin. Plover Au prospect is bounded to the east by the Eau Claire fault zone and the Wolf River Batholith. Figure has been adapted from Dematties (2022) and Lynott et al, (2022). REFERENCES LaBerge, G.L., and Myers, P.E., 1983a, Precambrian geology of Marathon County, Wisconsin: Information Circular, v. 45. Lynott, J.S., and Dematties, T.A., 2022, An Evaluation of the Reef Gold-Copper Deposit, Marathon County, Wisconsin, USA, NI 43-101 Technical Report, 402p. DeMatties, T.A., 2022, Exploration-resource assessment of productive felsic volcanic centers in the paleoproterozoic penokean volcanic belt of northern Wisconsin, Michigan and East-central Minnesota, USA: Ore Geology Reviews, v. 141, p. 104489. 22 �Unusual early diagenetic structures in the Paleoproterozoic Gunflint Formation, Ontario, Canada CHURCHLEY, Sophie1, FRALICK, Philip2 1 Ontario Geological Survey, 435 James St S, Suite B002, ON P7E 6S7 Canada 2 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada Newly identified microbial and diagenetic structures in the Gunflint Formation from the Thunder Bay area provide additional information to further our understanding of the environment in which these sediments were deposited and the diagenetic processes that affected them. Along the Current River, a horizon of carbonate lenses outcrop within a shale sequence. The structures are oblate in shape and range from approximately 0.5-1 m in diameter and 20-30 cm in height. Internally, the lenses are mostly calcite that displaces fine-grained siliciclastic laminae and preserves several interesting structures including cone-in-cone, inverted cuspate fenestrae and feather-like and braided fabrics. Some faces also display features that are more ‘fern-like’ in appearance and migrate up and across the surface of the oblate carbonate pods. We propose that these are oblate carbonate concretions formed via carbonate precipiation during diagenesis within a sequence of organic-rich laminated shales and siltstones. Several important features observed in the Gunflint Formation suggest that these structures formed via diagenetic processes including the similarities in stratigraphic placement within organic-rich shaley horizons and the light δ13Сcarb values recorded in the carbonate fraction. Cone-in-cone structures have been identified in both hand sample and thin section, displaying similarities to structures described from other locales (Figure 1A,B). Cone-in-cone structures occur in calcite-cemented sandstones or at the edges of disc-like to ellipsoidal concretions ranging in size from decimeters to meters long within shale beds in Erfoud, Morocco (Lugli et al. 2005). This stratigraphic positioning and size is consistent with what has been observed in the Gunflint Formation oblate concretions that host the cone-in-cone layers. Likewise, jagged ‘sawtoothed’ draping laminae that were identified in the Gunflint Formation are similar in appearance to those identified from cone-in-cone structure in the Devonian Middle Timan Formation in Russia (Figure 1C,D) (Shumilov 2020). The formation of cone-in-cone structure is still not well understood and numerous hypotheses have been proposed (see Lugli et al. 2005 and references therein). It is commonly associated with concretions and organic-rich sediments. The oblate carbonate horizons are located stratigraphically within a sequence of carbonaceous black shales near the base of the Upper Member of the Gunflint Formation with abundant evidence of microbially induced sedimentary structures (MISS) (Fischer and Fralick 2020). δ13Сcarb analyzed from the carbonate fraction in the Gunflint Formation samples displayed light values ranging from -12.29‰ to -0.17‰ with most values clustering near -10‰. These values are coincident with precipitation occurring in the zones of Fe, Mn, and/or sulfate reduction with relatively low rates of organic-carbonate oxidation and are consistent with ferruginous, reducing conditions (Mozley and Burns 1993). 23 �Figure 1. A. 1-3 cm scale cone-in-cone structure consisting of beige fibrous calicite draped by thin black siliciclastic films. B. Thin section close up of cone-in-cone structure from Permian carbonates in Thailand that is similar in appearance to those observed in the Gunflint Formation (see figure 3C; Chenrai et al. 2022). C. Hand sample of a carbonate-rich horizon displaying jagged ‘saw-toothed’ laminae near the top. D. Close-up image of similar jagged laminae from the Devonian Middle Timan Formation in Russia (see figure 9A; Shumilov 2020). At this location, the jagged laminae are associated with cone-in-cone structure. References Chenrai P., Assawincharoenkij T., Warren J., Sa-nguankaew S., Meepring S., Laitrakull K. and Cartwright I. (2022) The Occurrence of Bedding-Parallel Fibrous Calcite Veins in Permian Siliciclastic and Carbonate Rocks in Central Thailand. Front. Earth Sci. 9:781782. doi: 10.3389/feart.2021.781782. Fischer, S. and Fralick, P. (2020) Biological mats in siliciclastic sediments of the Paleoproterozoic Gunflint Formation, northwestern Ontario, Canada. Can. J. Earth Sci. 57: 947–953. Lugli, S., Reimold, W. and Koeberl, C. (2005). Silicified Cone-in-Cone Structures from Erfoud (Morocco): A Comparison with Impact-Generated Shatter Cones. doi: 10.1007/3-540-27548-7_3. Mozley, P.S. and Burns, S.J. (1993) Oxygen and carbon isotopic composition of marine carbonate concretions: an overview. Journal of Sedimentary Research 63, 73–83. Shumilov I.Kh. (2020) Сone-in-cone structure: New data. Litosfera, 20(1), 76-92. doi: 10.24930/16819004-2020-20-1-76-92. 24 �Alteration of magnetic mineralogy in the Giants Range Batholith by the Duluth Complex CORTOPASSI, Celia L., ALLERTON, Zsuzsanna P., FEINBERG, Joshua M. Department of Earth and Environmental Sciences, University of Minnesota, Suite 150, 116 Church St SE, Minneapolis MN 55455 During the Midcontinent Rift event (ca. 1.1 Ga) of the North American craton, the Duluth Complex (DC), a large mafic igneous intrusion, was emplaced into the Neoarchean Giants Range Batholith (GRB; ca. 2.7 Ga) in northeastern Minnesota, thermally altering the granitic country rock (Allison, 1925).The basal mineralized zone of the DC has been well-studied with regard to sulfide deposits, but the extent of alteration within the GRB footwall has not been as well constrained. Previous research has indicated the presence of sulfides at the DC-GRB contact, extending about hundred meters into the GRB (Steiner, 2014), and prior petrographic analysis has revealed textures consistent with contact metamorphism that diminish with distance from the contact (Pardi, 2024). This project seeks to define the magnitude of alteration within the GRB and to further characterize the orientation of the intrusion. This project utilizes a profile of 13 outcrop samples from the GRB that were collected systematically at distances between 100 and 4500 meters from the DC-GRB contact. We characterize changes in magnetic mineralogy as a function of distance from the DC-GRB contact using measured optical microscopy, electron microscopy, and magnetic properties (susceptibility and parameters calculated from hysteresis loops and backfield curves). These data reveal a distinguishable and consistent pattern in magnetic properties as a function of distance from the contact and distinct zones of textural alteration in oxide minerals (Figure 1). Patterns in smallscale magnetic properties broadly align with the large-scale trends seen in aeromagnetic data (Minnesota Geological Survey, n.d.), including changes in magnetic properties co-located with mapped faults (Jirsa et al., 2011). The orientation of the DC-GRB contact was examined using information from previously-drilled exploration drill holes (Minnesota Department of Health, n.d.) that penetrated through the DC and into the GRB. These observations, as well as outcrop measurements of modal layering and igneous foliation within the DC (Minnesota Geological Survey, 2023), constrain the orientation of the present-day DC-GRB contact to between 16-24° towards the east. The original depth of the modern day exposure of the DC-GRB remains unknown, as does any component of subsidence that occurred since the Midcontinent Rift event. Future work may include the collection of oriented samples for paleomagnetic studies, which would help constrain both the extent of thermal reheating of the GRB and postemplacement subsidence. Thermal modeling of the subsurface DC-GRB contact at various depths, alongside observed patterns in oxide mineral textures, could produce estimates of the extent of subsurface contact metamorphism. With these methods, we hope to better understand the conditions under which the DC was emplaced and accommodated, as well as estimate the thickness of Precambrian rock that has since been eroded away. 25 �Figure 1. A: Distribution of identified textures (A-E) as a function of distance from the DC-GRB contact. B: Measured magnetic properties as a function of distance from the DC-GRB contact. REFERENCES Allison, I. S.,1925. The Giants Range Batholith of Minnesota. The Journal of Geology, 33(5), 488–508. https://www.jstor.org/stable/30057863. Jirsa, M., Boerboom, T., Chandler, V. W., Mossler, J., Runkel, A., & Setterholm, D., 2011. S-21 Geologic Map of Minnesota-Bedrock Geology. https://conservancy.umn.edu/items/96de8d96-46ba441c-94ca-41080b4335be Minnesota Department of Health, n.d.. Minnesota Well Index (MWI). https://mnwellindex.web.health.state.mn.us/. Minnesota Geological Survey, n.d.. Collection of aeromagnetic data from Minnesota. https://doi.org/10.5066/P14LP38P. Minnesota Geological Survey, 2023. D-06, Structure Database. https://arcg.is/jfCLD. Pardi, L., 2024. Petrographic Analysis of the Giants Range Batholith in Northeastern Minnesota. Steiner, R. A., 2014. Genesis of sulfide mineralization within the granite footwall of the Maturi deposit of the South Kawishiwi intrusion, Duluth Complex, NE Minnesota. https://hdl.handle.net/11299/169376. 26 �Architecture of the Douglas Fault damage zone, northwest Wisconsin DANIELS, Nate, MCELLISTREM, Grace, VOGEL, Raeann, and BRAUNAGEL, Michael Department of Earth & Environmental Sciences, University of Minnesota Duluth, 1114 Kirby Drive Duluth, MN 55812 USA Major faults in the upper crust can be divided between the fault core, where most of the displacement is accommodated, and a surrounding damage zone (Faulkner et al., 2010). Fracturing in this damage zone occurs across a range of scales and intensity, varying from regularly spaced joint or deformation band sets to pervasive pulverization of the host rock. As such, fault damage zones can serve as fluid pathways, which control the migration of hydrothermal fluids and can alter the frictional strength of seismogenic fault systems. A number of processes are responsible for formation and evolution of a fault’s damage zone, including microfracturing within the process zone during fault propagation, localized wear along irregular fault surfaces, and volumetric changes associated with dynamic rupture propagation (Mitchell & Faulkner, 2009). As each process leaves a unique record in the fault system, the distribution and intensity of fault damage zones can provide insight into past fault activity and its relationship to fluid flow in the crust (Blenkinsop, 2008). This study presents preliminary observations of the fault-related damage surrounding the Douglas Fault from Amnicon and Pattison State Parks in northwestern Wisconsin. The Douglas Fault was activated during structural inversion of the Midcontinent Rift and previous work estimates its vertical displacement at ≳10 km (Grant, 1901; Cannon, 1994; Nicholson et al., 2006; Hodgin et al., 2024). At our study sites, the fault places basalts of the mid-continental rift Chengwatana volcanic group over post-rift siliciclastic sandstones of the Bayfield Group. Field and thin section observations along the fault system reveal pronounced damage zone asymmetry, with a hanging wall damage zone that is several times the width of the damage zone in the footwall. Chengwatana volcanics in the hanging wall are intensely fractured at the grain scale and cut by multiple generations of primarily calcite-filled opening mode veins. These veins and fractures broadly show two distinct orientations; one set striking generally NE to SW and the second characterized by NW to SE strikes. The damage-zone width is constrained by identifying changes in the slope of cumulative damage frequency plots, which shows high deformation frequency as a steep slope within an inner damage zone and less deformation decaying to background levels as a gentle slope in the outer damage zone of the Douglas Fault. Collectively, the full thickness of the hanging wall damage zone is >100 m (Grant, 1901). In contrast, sandstones of the Bayfield Group in the footwall exhibit lower frequency fracturing at the outcrop scale, no apparent grain-scale fracturing in thin section, and compressional deformation bands defined by porosity reduction. Bayfield sandstones in the footwall at these sites are also deformed by faultpropagation and drag folding that extend for tens of meters beyond the fault contact (Hodgin et al., 2024). Field and thin section scale observations of fault damage in both units are consistent with ultrasonic pulse velocities measured in samples collected from the fault zone with a Proceq Pundit Lab system. 27 �REFERENCES Blenkinsop, T.G., 2008. Relationships between faults, extension fracture and veins, and stress. Journal of Structural Geology, 30 (5), 622-632. Cannon, W.F., 1994. Closing of the Midcontinent Rift - A far-field effect of Grenvillian compression. Geology, 22 (2), 155-158. Faulkner, D.R., Jackson, C.A.L., Lunn, R.J., Schlische, R.W., Shipton, Z.K., Wibberley, C.A.J., and Withjack, M.O., 2010. A review of recent developments concerning the structure, mechanics and fluid flow properties of fault zones. Journal of Structural Geology, 32 (11), 1557-1575. Grant, U.S., 1901. Preliminary report on the copper-bearing rocks of Douglas County, Wisconsin (No. 3). Hodgin, E.B., Swanson-Hysell, N.L., Kylander-Clark, A.R.C., Turner, A.C., Stolper, D.A., Ibarra, D.E., Schmitz, M.D., Zhang, Y., Fairchild, L.M., and Fuentes, A.J., 2024. One billion years of stability in the North American midcontinent following two-stage Grenvillian structural inversion. Tectonics, 43 (9). Mitchell, T.M., and Faulkner, D.R., 2009. The nature and origin of off-fault damage surrounding strikeslip fault zones with a wide range of displacements: A field study from the Atacama fault system, northern Chile. Journal of Structural Geology, 31 (8), 802-816. Nicholson, S.W., Cannon, W.F., Woodruff, L.G., and Dicken, C., 2006. Bedrock geologic map of the Port Wing, Solon Springs, and parts of the Duluth and Sandstone 30’x60’ Quadrangles, US Geological Survey. 28 �The Archean Carney Lake gneiss complex in Michigan’s Upper Peninsula: Preliminary subdivisions with age constraints DeGRAFF, James1, DEERING, Chad1, and JONES III2, James 1 Department of Geological & Mining Engineering & Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931 U.S.A. 2 U.S. Geological Survey, Alaska Science Center, 4210 University Drive, Anchorage, AK 99508 U.S.A. Much of the Precambrian bedrock in Michigan’s Upper Peninsula was last mapped at 1:24000 scale prior to modern tectonic concepts and advances in understanding related structural, magmatic, and metamorphic processes. A later decline in base and ferrous metal mining in the region reduced interest in commercial and scientific investigations, however recent concerns about the supply of critical minerals has renewed interest in developing an improved geologic framework for ore deposit exploration. The Archean Carney Lake gneiss complex (CLGC) and other granitegneiss complexes south of the Great Lakes tectonic zone are in the Minnesota River Valley subprovince of the southern Superior craton (Sims and Day, 1993). The CLGC, like the other complexes, is surrounded by Paleoproterozoic continental margin strata, partly older than and partly coeval with Penokean orogenesis (~1.85 Ga) that deformed the region (Schulz and Cannon, 2007). Bayley et al. (1966) describe the CLGC as predominantly felsic gneiss but with ~10% mafic inclusions and ~5% younger granodiorite and syenite intrusions by area. Mapping funded by the USGS Earth MRI program has revealed a wider variety of rocks than previously reported, differences in metamorphic grade, and new structural relationships (DeGraff et al., 2023). Felsic intrusions with little to no foliation are more abundant and varied than previously thought, ranging from granitic to tonalitic to locally syenitic. The original classification of gneiss based on mineralogy has been revised by also considering fabric characteristics. Consequently, we have identified an older EW-elongate core of thickly banded (≥2 cm) poly-deformed gneiss characterized by tightly folded banding, discordant banding across shear zones, and dismembered mafic pods (Fig. 1, area 1). Younger, less deformed, Archean rocks flank the older terrane, except on the north, and include the widespread felsic intrusions and thinly banded (≤1 cm), quartzo-feldspathic, gneissic rocks. The latter have quasi-planar, laterally continuous banding and local textures resembling cross-bedding and relict grains, suggesting derivation from a siliciclastic protolith. Boundaries between the older deformed gneiss terrane, the younger gneissic terrane with relict features, and areas with felsic intrusions are not yet well defined nor is their nature well understood. In addition to the above, at least four generations of mafic to ultramafic magmas have intruded the CLGC up to the late Mesoproterozoic. Our results, combined with those of others, indicate a long and complex tectonomagmatic history for the CLGC and adjacent rock units. The poly-deformed gneiss terrane includes rocks with inherited zircon cores dated at ca. 3750 Ma (Eoarchean) and recrystallized zircons and overgrowths dated at ca. 2750 Ma, the latter having formed during a Neoarchean thermal event (Ayuso et al., 2018). Neoarchean metamorphism of the Eoarchean gneiss, and perhaps much of its deformation, was accompanied by widespread felsic intrusions based on new zircon LA-ICPMS U-Pb dates ranging from ca. 2810 Ma to 2670 Ma (8 sites). Zircon trace-element analysis indicates that these magmas came from a hydrous oxidizing source and were contaminated while passing through a relatively thick crust, as is typical of magma generated during modern subduction. At the northern and eastern margins of the CLGC, relatively undeformed gneissic rocks were probably derived in part from siliciclastic protoliths of Neoarchean age. At the northern margin, 29 �however, NE-dipping beds of Paleoproterozoic Sturgeon Quartzite are parallel to well-defined layers of quartzo-feldspathic gneissic rocks along strike to the east. Field relationships and detrital zircon analysis suggest two scenarios: 1) a lateral facies change within Sturgeon Quartzite from meta-arkose on the east to meta-sandstone on the west, or 2) an onlapping relationship between younger quartzite and its parent Neoarchean meta-arkose. Figure 1: Preliminary subdivisions of the Archean Carney Lake gneiss complex (CLGC = 1, 2a, 2b, 3, Agu_clg). 1 = poly-deformed; 2 = metaigneous, 3 = meta-sedimentary; Agu_clg = undifferentiated; Xmrs = Paleoproterozoic Marquette Range Supergroup; Pz = Paleozoic clastic strata. Study area outlined in purple. REFERENCES Ayuso, R.A., Schulz, K.J., Cannon, W.F., Woodruff, L.G., Vazquez, J.A., Foley, N.K., and Jackson, J., 2018. New U-Pb zircon ages for rocks from the granite-gneiss terrane in northern Michigan: evidence for events at ~3750, 2750, and 1850 Ma. Institute on Lake Superior Geology, 64th Annual Meeting Proceedings, Part 1-Program and Abstracts, 64: 7-8. Bayley, R.W., Dutton, C.E., and Lamey, C.A., 1966. Geology of the Menominee Iron-Bearing District, Dickinson County, Michigan and Florence and Marinette Counties, Wisconsin. U.S. Geological Survey, Professional Paper 513: 1-96. DeGraff, J.M., Gannon, I.M., Deering, C.D., Smirnov, A.V., 2023. Bedrock geology of southeastern Dickinson County, Michigan: Vulcan 7.5’ quadrangle and adjacent parts of the Carney Lake, Cunard, Faithorn, Felch, Foster City, and Waucedah 7.5’ quadrangles. Michigan Geological Survey, Bedrock Geologic Map, 1:25,000 scale map sheet with explanatory text. Schulz, K.J. and Cannon, W.F., 2007. The Penokean orogeny in the Lake Superior region. Precambrian Research, 157: 4-25. Sims, P.K. and Day, W.C., 1993. Great Lakes tectonic zone – revisited. U.S. Geological Survey, Bulletin 1904-S: S1-S11. 30 �Geochronology of lithium mineralization in the Florence pegmatite field, WI, USA DROUBI, Omar Khalil1, SCHOONOVER, Erik2, SIRBESCU, Mona-Liza3, GARBER, Joshua2, BONAMICI, Chloë1 Department of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton Street, Madison, Wisconsin, 53706, USA 2 Department of Geosciences, The Pennsylvania State University, University Park, PA, USA 3 Geology Department, Central Michigan University, 314 Brooks Hall, Mount Pleasant, MI 48859, USA 1 Global and national progression toward decreasing reliance on fossil fuels will correlate with increasing the supply of mineral resources that contain high concentrations of elements like lithium, copper, or rare earth elements– “critical minerals” deemed essential for building low-CO2 technologies. Lithium-cesium-tantalum (LCT) pegmatites are a significant component of global lithium production; despite their importance, the tectonomagmatic mechanisms by which these pegmatites form are not completely understood. The two main models for LCT pegmatite formation are 1) as late-stage fractionation products of nearby peraluminous granites (“fractionation origin”) (e.g., Černý, 1991) or 2) directly from partial melts of Li-bearing highgrade metamorphic rocks (“anatectic origin”) (e.g., Knoll et al., 2023; Koopmans et al., 2023). These models have implications for LCT pegmatite exploration (i.e., mapping outward from plutons or anatectic zones in mountain belts) and testing them requires precise age estimates for the pegmatites and their neighboring magmatic and metamorphic rocks. This study provides new age constraints for models of LCT pegmatite mineralization in Florence County, WI, USA. (Falster et al., 2005; Falster et al., 1996; Sirbescu et al., 2008) by applying LA-ICP-MS U-Pb geochronology and trace-element analysis to apatite crystallized within the LCT pegmatites and titanite in the proximal wall rock and a separate, non-mineralized granitic pegmatite located >1.8 km away (Figure 1). The Florence LCT pegmatites were emplaced <2.5 km south of the Niagara Fault Zone and are hypothesized to be fractionated products from the nearby Bush Lake granite (undated, but hypothesized ~1835 Ma; Sims et al., 1985) or anatectic melts of the wall rock, the metavolcanic/metasedimentary Quinnesec Fm. (~1866 Ma; Sims et al., 1985). These models suggest pegmatite emplacement is broadly bracketed in space and time by the end of the Penokean orogeny (~1835 Ma) and Yavapai arc accretion (~1750–1700 Ma) (Figure 1). The apatite grains from the King’s X and Animikie Red Ace LCT pegmatites have U-Pb dates of 1446 ± 6 [29] Ma and 1432 ± 4 [29] Ma, respectively. The targeted apatite grains, which nucleated in the pegmatite chilled margin at the wall rock contact, are interpreted as magmatic based on oscillatory cathodoluminescence zoning (Sirbescu et al., 2009). Xenoblastic titanite from the Quinnesec Fm., sampled at distances <1 cm to ~150 m from the pegmatites, have U-Pb ages of 1473 ± 7 [29] Ma (<1 cm), 1466 ± 7 [29] Ma (<5 m), 1436 ± 13 [29] Ma (60 m), and 1471 ± 8 [29] Ma (150 m), but euhedral titanite grains from the non-mineralized granitic pegmatite have a U-Pb age of 1811 ± 10 [36] Ma. Our data indicate that the Florence LCT pegmatites did not result from fractionation of the Bush Lake granite nor anatexis during the Penokean or Yavapai orogenies and are instead coeval with emplacement of the ~1476 Ma Wolf River batholith further south. A revised age model for lithium mineralization in northern WI suggests involvement of the Wolf River batholith or far-field influence of the Mesoproterozoic Pinware-Baraboo-Picuris orogeny. 31 �Figure 1. Conceptual cross section (not to scale) showing age constraints for the Florence pegmatite field. Hypothesized ages based on the following references: Bush Lake granite and Quinnesec Fm. (Sims et al., 1985), metagabbro (Guice et al., 2023). U-Pb dates reported as: date ± internal 2s [external uncertainty=2% of date]. REFERENCES Bradley, D.C., McCauley, A.D., and Stillings, L.M., (2017), Mineral-deposit model for lithium-cesiumtantalum pegmatites: U.S. Geological Survey Scientific Investigations Report 2010–5070–O, 48 p., https://doi.org/10.3133/sir20105070O. Černý, P., 1991, Rare-element Granitic Pegmatites. Part II: Regional to Global Environments and Petrogenesis: Geoscience Canada, v. 18, p. 68–81, Falster, A. U., Simmons, W.B., and Webber, K.L. (2005), Origin of the pegmatites in the Hoskin Lake pegmatite field, Florence Co., Wisconsin, in Crystallization Processes in Granitic Pegmatites, International Meeting in Cavoli, Elba Island, Italy, May 23–28, 2005, edited by F. Pezzotta, Mineral. Soc. of Am., Chantilly, Va. Falster, A. U.; Simmons, Wm. B.; and Webber, K. L. (1996) The Mineralogy and Geochemistry of the Animikie Red Ace Pegmatite, Florence County, Wisconsin. In Pandalai, S. G., ed., Recent Research Developments in Mineralogy, 7-67. Guice, G. L., Viete, D. R., Holder, R. M., & Roy, S. (2023). A c. 1900 Ma Tethyan-type ophiolite in the Penokean Orogen, Pembine, Wisconsin (USA): Insights from the volcanic stratigraphy. Precambrian Research, 399, 107223. Knoll, T., Huet, B., Schuster, R., Mali, H., Ntaflos, T., & Hauzenberger, C. (2023). Lithium pegmatite of anatectic origin-A case study from the Austroalpine Unit Pegmatite Province (Eastern European Alps): geological data and geochemical model. Ore geology reviews, 105298 Koopmans, L., Martins, T., Linnen, R., Gardiner, N.J., Breasley, C.M., Palin, R.M., Groat, L.A., Silva, D., and Robb, L.J., (2023). The formation of lithium-rich pegmatites through multi-stage melting. Geology. Sims, P. K., Peterman, Z. E., & Schulz, K. J. (1985). The Dunbar Gneiss-granitoid dome: Implications for early Proterozoic tectonic evolution of northern Wisconsin. Geological Society of America Bulletin, 96(9), 1101-1112. Sirbescu, M. L. C., Hartwick, E. E., & Student, J. J. (2008). Rapid crystallization of the Animikie Red Ace Pegmatite, Florence county, northeastern Wisconsin: inclusion microthermometry and conductivecooling modeling. Contributions to Mineralogy and Petrology, 156, 289-305. Sirbescu, M. L. C., Leatherman, M. A., Student, J. J., & Beehr, A. R. (2009). Apatite textures and compositions as records of crystallization processes in the Animikie Red Ace pegmatite dike, Wisconsin, USA. The Canadian Mineralogist, 47(4), 725-743. 32 �Experimental Reproduction of Acidic Mafic-Ultramafic Hydrothermal Fluids with Implications for Linking Seafloor Lithology to Ore Mineral Solubility and Novel Geochemical Trapping Mechanisms EVANS, Guy N.1 and SEYFRIED JR., William E.1 1 Department of Earth and Environmental Sciences, University of Minnesota, 116 Church St SE, Minneapolis, MN, 55455, United States Ultramafic-hosted seafloor massive sulfide (UM-SMS) deposits constitute a distinct class of CuZn-Co-Ni-Au-rich seafloor hydrothermal deposits (Fouquet et al., 2010). However, ultramafichosted volcanogenic massive sulfide (UM-VMS) deposits have been historically overlooked, in part because the formation of UM-VMS deposits differs from traditional VMS genetic models based on basalt-hosted SMS deposits (Pattern et al., 2022). Adding to this complexity, ultramafic-hosted seafloor hydrothermal fluids span nearly the full range of pH and metal concentrations observed at active seafloor hydrothermal vents, from highly acidic (pH= 2.8), metal-rich (Fe > 20 mmol/kg) fluids observed at Rainbow Hydrothermal Field (Douville et al., 2002), to alkaline (pH = 10.5), metal-poor (Fe < .02 mmol/kg) fluids observed at Lost City Hydrothermal Field (Kelley et al., 2005; Evans et al., 2024). Here, we present results from recent experiments conducted at the University of Minnesota that for the first time reproduce acidic hydrothermal fluids from mixed mafic-ultramafic source minerals. The observed acidity of these fluids results from temperature-dependent fluid-rock reactions and superimposed geochemical and physical processes. We further highlight the implications of these findings for UM-VMS deposit models, including novel geochemical trapping mechanisms potentially relevant in areas exhibiting significant ultramafic volcanic/intrusive rocks. Regional examples include the Newton Belt (northeast Minnesota), Shebandowan Belt (northwestern Ontario), and Kidd-Monroe assemblage (eastern Ontario and Quebec). REFERENCES Douville, E., et al., (2002). The rainbow vent fluids (36 14′ N, MAR): the influence of ultramafic rocks and phase separation on trace metal content in Mid-Atlantic Ridge hydrothermal fluids. Chemical Geology, 184(1-2), 37-48. Evans, G. N. et al. (2024). Transition metals in alkaline Lost City vent fluids are sufficient for early-life metabolisms. Geochimica et Cosmochimica Acta, 385, 61-73. Fouquet, Y. et al. (2010). Geodiversity of hydrothermal processes along the Mid‐Atlantic Ridge and ultramafic‐hosted mineralization: A new type of oceanic Cu‐Zn‐Co‐Au volcanogenic massive sulfide deposit. Diversity of hydrothermal systems on slow spreading ocean ridges, 188, 321-367. Kelley, D. S. et al. (2005). A serpentinite-hosted ecosystem: the Lost City hydrothermal field. Science, 307(5714), 1428-1434. Patten, C. G. et al. (2022). Ultramafic-hosted volcanogenic massive sulfide deposits: an overlooked subclass of VMS deposit forming in complex tectonic environments. Earth-Science Reviews, 224, 103891. 33 �34 �Textural and chemical analysis of sphalerite ores from the Highland Subdistrict, Upper Mississippi Valley Zinc-Lead District, Wisconsin FITZPATRICK, William1 1 Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd. Madison, WI, USA Lead + zinc ± barite ± copper deposits are widespread in Ordovician carbonate rocks of southwestern Wisconsin and the bordering areas of Illinois and Iowa, commonly referred to as the Upper Mississippi Valley zinc-lead district (UMVD). Sphalerite, the primary zinc ore mineral in the UMVD, is known from other mining districts to contain valuable byproduct commodities as trace elements such as gallium, germanium, cadmium and silver. Several previous studies have examined sphalerite from the UMVD, but focused on samples from the southern part of the district (Hall and Heyl, 1968, McLimans and others, 1980). Zinc ores from other areas of the UMVD have received less attention, and little is known of their textural character and trace element content. This study presents new trace element data and textural observations of sphalerite ores from the Highland Subdistrict, the northernmost mining center in the UMVD. Two hand-picked sphalerite concentrates and thirty-five bulk ore samples were analyzed by whole rock geochemical methods, complemented by 282 in situ electron microprobe analyses on six thin sections from a mix of vein and disseminated ores. Textures in the sphalerite ores were also documented through scanning electron microscopy with the aim of understanding mechanisms that localized sulfide mineralization. Sphalerite from the Highland Subdistrict is characterized by alternating sequences of lighter, honey-colored bands and darker, reddish-brown bands in both the disseminated and vein hosted ores (Fig. 1). Microprobe analysis shows that darker bands tend to localize elevated iron and lower cadmium relative to lighter bands (Fig. 1). Silver content is variable, but tends to be higher in lighter bands, especially in the cores of disseminated grains. Comparing results from the whole rock and microprobe analyses from the Highland Subdistrict to sphalerite analyzed elsewhere in the UMVD, iron and cadmium are within known ranges, but silver is enriched to a significant degree (Hall and Heyl, 1968). Gallium and germanium abundances were too low to be detected in microprobe analyses, but whole rock analysis indicates they are towards the low end of the range observed in sphalerite from the UMVD (Hall and Heyl, 1968). Scanning electron microscope observation discovered abundant, texturally early framboidal pyrite intergrown with marcasite that is enveloped by later sphalerite. Framboidal pyrite has a well-documented association with sulfate reducing bacteria (e.g. Maclean and others, 2008), indicating bacterial processes were likely important in creating a reservoir of reduced sulfur within the carbonate host rocks. This in turn may have acted as a chemical trap for metals in migrating connate brines to form the zinc deposits. REFERENCES Hall, W.E., and Heyl, A.V., 1968, Distribution of Minor Elements in Ore and Host Rock, IllinoisKentucky Fluorite District and Upper Mississippi Valley Zinc-Lead District. Economic Geology, 63, 655-670. Maclean, L., Tyliszczak, T., Gilbert, P., Zhou, D., Pray, T., Onstott, T., and Southam, G., 2008, A high resolution chemical and structural study of framboidal pyrite formed within a low-temperature bacterial biofilm. Geobiology, 6, 471-480. McLimans, R.K., Barnes, H.L., and Ohmoto, H., 1980, Sphalerite Stratigraphy of the Upper Mississippi Valley Zinc-Lead District, Southwest Wisconsin. Economic Geology, 75, 351-361. 35 �Figure 1. Plots of iron, cadmium and silver along linear traverses through banded sphalerite crystals from the Highland Subdistrict. Top panel shows scans of the thin sections analyzed and locations of the analyses. Note the concentrically zoned disseminated grain (left) vs vein (right). 36 �Rare-element Geochemistry of the Eau Claire River Complex Pegmatites GRIES, Samara1, LODGE, Robert W.D1, HANEL, Sara1,2, HOOPER, Robert1 1 Department of Geology and Environmental Sciences, University of Wisconsin-Eau Claire, Eau Claire, WI 54701 USA 2 Current Affiliation: Department of Earth and Environmental Sciences, University of Minnesota Twin Cities, Suite 150, 116 Church St. SE, Minneapolis, MN 55455 Minerals, such as monazite and xenotime, are an important source of rare earth (La, Ce, Nd) and high field strength (Th, Nb, Zr) elements which are essential for modern energy, communication, and military technologies. These critical minerals are often sourced in pegmatites and are important exploration targets worldwide (Haque et al, 2014). The Paleoproterozoic Eau Claire Volcanic Complex (ECVC) is intruded by granitic pegmatite dikes that postdate peak metamorphism (Lodge et al, 2023), indicating they are unrelated to Penokeanaged orogenic events. The ECVC pegmatites are highly fractionated, garnet bearing, and contain a high concentration U, Th, La, Ce, and other rare earth elements. Based on major and trace element associations, the pegmatites in the ECVC are classified as NYF family pegmatites that contain Nb>Ta, REE, U, Th, Zr and are A- to I- types with peralkaline relationship (Cerny and Ercit, 2005). This study collected bedrock samples from several locations across the ECVC (Little Falls, North Fork, Muskeg Creek) (Figure 1). The pegmatite dikes can range in size from a few meters to 100 m in width near the North Fork of the Eau Claire River. They mainly intrude foliated and metamorphosed Paleoproterozoic to Archean tonalites, amphibolites, and gneisses. Samples from these pegmatites were analyzed for whole rock and mineral chemistries. Whole rock chemistry was analyzed on XRF and ICPMS whereas mineral chemistry was determined using SEM-EDS. All three locations have quartz, feldspar, plagioclase, biotite, and muscovite. The main mineralogy of Little Falls samples are albite and muscovite. Trace mineralogy of the Little Falls samples include Fe- and Mn-garnet, samarskite, columbite, zircon, and xenotime. Muskeg Creek samples contains both orthoclase and albite with biotite instead of muscovite. Trace mineralogy of the Muskeg Creek samples includes xenotime, monazite, and barite. The North Fork samples mainly contain albite with minor orthoclase and biotite. Trace mineralogy of the pegmatites in the North Fork area include in Fe- and Mn-garnets, monazite, xenotime, and thorite. The pegmatites from the ECVC are all low in Ca and have trace minerals with rare earth elements. They all contain with albite with low quantities of orthoclase and almost no anorthite. The North Fork and Muskeg Creek samples have more barium-rich minerals than Little Falls, which may be the result of fractionation of feldspars and plagioclase (Yu et al, 2007). Ba-rich minerals can also be a product of hydrothermal activity (Hanor, 2000), but there is no evidence of syn- to post-hydrothermal alteration of the pegmatites. Mn-rich garnets at Little Falls and North Fork indicate a higher degree of fractionation relative to Fe-garnets at Muskeg Creek (Hernández-Filiberto et al, 2021). North Fork and Muskeg Creek also had the largest crystals reaching over 20 cm in size. The North Fork is enriched in the heavy rare earth elements, U, and Th. In comparison, Little Falls has more light rare earth elements. Muskeg is also enriched in light rare earth elements in addition to an increased enrichment of heavy rare earth minerals like Gd and Dy. All three locations contain other metals such as Nb, Zr, Hf. 37 �Figure 1. Bedrock geologic map of the Eau Claire Volcanic Complex with site locations. North Fork depicts a pink pegmatite intruding into a grey tonalite. Muskeg Creek depicts a 6 m pegmatite dike with zoning. Little Falls shows a 13 m pegmatite dike. Map from Mudrey & Brown (1982). REFERENCES Cerny, P., and Ercit,T., 2005. The classification of granitic pegmatites revisited. The Canadian Mineralogist, 43: 2005-2026. Hanor, J.S., 2000, Barite-celestine geochemistry and environments of formation. In Alpers, C.N., Jambor, J.L., Nordstrom, D.K., eds. Reviews in Mineralogy and Geochemistry, 40: p. 193-275 Haque, N., Hughes, A.., Lim, S., Vernon, C., 2014, Rare Earth Elements: Overview of Mining, Mineralogy, Uses, Sustainability, and Environmental Impact: Resources, 3, p. 614-635 Hernández-Filiberto, L., Roda-Robles, E., Simmons, W.B., Webber, K.L., 2021, Garnet as Indicator of Pegmatites from the Oxford Pegmatite Field (Maine, USA): Minerals, 11(8), 802 Lodge, RWD, Weber, EM, Hooper, RL, 2023, Precambrian Geology of the Eau Claire River Valley: Rediscovering the Eau Claire Volcanic Complex. in Lodge, RWD (Ed.), Institute on Lake Superior Geology Proceedings, 69th Annual Meeting, Eau Claire, Wisconsin, Part 2 – Field Trip Guidebooks. v.69, part 2, p.47-70. Mudrey, M.G., Jr., Brown, B. A., Greenberg, J. K., 1982, "Bedrock Geologic Map of Wisconsin." Wisconsin Geological and Natural History Survey, scale 1:1,000,000 Yu, J.-H., O’Reilly, S. Y., Zhao, L., Griffin, W. L., Zhang, M., Zhou, X., Jiang, S.-Y., Wang, S.-Y., Wang, R.-C., 2007, Origin and evolution of topaz-bearing granites from the Nanling Range, South China: a geochemical and Sr-Nd-Hf isotopic study: Minerology and Petrology, 90, p. 271-300 38 �Revisiting Gravity and Magnetic Anomalies of the Baraboo Range HINZE, William J.1 and LONGACRE, Mark B.2 1 Purdue University, 30 Brook Hollow Ln., West Lafayette, IN 47906 2 MBL, Inc., 51 Captain Perry Dr., Phippsburg, ME 04562 The efficacy of gravity and magnetic methods of geological exploration have increased greatly since they were first used to investigate the Baraboo Synclinorium of Wisconsin nearly 75 years ago (Ostenso, 1953; Hinze, 1959). These methods and their associated technology are used for the first time since then to investigate the geology of the Mesoproterozoic Baraboo Synclinorium, its regional basement, and to illustrate the importance of modern data sets and analysis and interpretation methods. The latter are the result of computers for analysis, interpretation, and presentation of anomalies that were unavailable when the geophysical methods were first applied to mapping the Synclinorium. Analysis and interpretation of current gravity and magnetic anomaly data sets (Figure 1) indicate that the negative gravity anomaly associated with the Baraboo Synclinorium is not unique to the Synclinorium but is the southern termination of the Wisconsin Gravity Minimum (WGM) that covers a large portion of central Wisconsin including the Wolf River Batholith (WRB). The WGM is derived largely from felsic plutons in the upper crust extending outward from the ~1.5 Ga WRB. The lower density of the plutons compared to the metamorphosed orogenic rocks of the upper crust is the likely source of the negative gravity anomaly. The Synclinorium, located along an east-northeast trending gravity and magnetic lineament within the Yavapai orogenic province, occurs in a syncline of largely felsic volcanic rocks (Figures 2 and 3), the Sauk Syncline, that was likely deformed along with the Baraboo Synclinorium by south and southeast-verging thrusting during the Mazatzal and Picuris-BarabooPinware Orogenic events. Variations in thrusting has led to significant differences in the eastern and western portions of the Baraboo Synclinorium. Key results of the gravity and magnetic anomaly data analysis of the Synclinorium include: (1) The Baraboo Synclinorium’s negative gravity anomaly originates in upper crustal Yavapai and Wolf River Batholith felsic plutons that are the source of the Wisconsin Gravity Minimum. (2) The Synclinorium occurs within a synclinal structure resulting from deformation related to generally south-verging, thin-skinned thrust faulting that also produced the Baraboo Synclinorium. (3) The structure of the eastern and western portions of the Baraboo Synclinorium differ likely as a result of variations in the direction and intensity of thrusting during the Mazatzal and PicurisBaraboo-Pinware Orogenies (~1.63-1.41 Ga). REFERENCES Hinze, W.J., 1959. A gravity investigation of the Baraboo Syncline region. The Journal of Geology, 67(4), 417-446. Ostenso, Ned, 1953. Magnetic studies of the Baraboo Syncline. Unpublished M.A. thesis, University of Wisconsin-Madison. 39 �Figure 1. Gravity and magnetic anomaly maps of the Baraboo Synclinorium. Reduced to pole (RTP) total magnetic intensity anomaly map (right) eliminates the effect of the inclined earth’s magnetic field on the induced magnetization of the crustal rocks and the vertical gradient Bouguer gravity anomaly map of the Baraboo Synclinorium region (left) minimizes the regional gravity anomaly. The boundaries of the counties are indicated and the outline of the boundary of the Baraboo Synclinorium is the dashed white line. The white line interior to the Synclinorium is the boundary of the Freedom Formation. Color coding of both figures is non-linear. Figure 2. Tilt derivative of the Bouguer gravity anomaly map of the Baraboo Synclinorium region showing the outline of the Sauk Syncline in thick dashed white lines interpreted from the gravity and magnetic anomaly maps. The outline of the Baraboo Synclinorium is the thin dashed white line. The white line interior to the Synclinorium is the boundary of the Freedom Formation. Color coding is non-linear. Figure 3. High pass 10-km RTP magnetic anomaly map of the Baraboo Synclinorium region showing the outline of the Sauk Syncline in thick dashed white line interpreted from the gravity and magnetic anomaly maps. The outline of the Baraboo Synclinorium is the thin dashed white line. The white line interior to the Synclinorium is the boundary of the Freedom Formation. Color coding is non-linear. 40 �Emplacement of the Mesoproterozoic Wausau Syenite Complex, Wisconsin HULA, Linsey1 and CZECK, Dyanna1 1 Department of Geosciences, University of Wisconsin Milwaukee, Lapham Hall, Room 366, 3209 N. Maryland Ave. Milwaukee, WI 53211 The Wausau Syenite Complex (WSC) in Marathon County, Wisconsin is an intrusive complex of granitoids emplaced approximately 1.5 Ga (Dewane and Van Schmus, 2007). It is traditionally considered part of a major anorogenic ferroan granite magmatic event that affected the southern margin of Laurentia circa 1.4 Ga. Recent studies have recognized a Laurentian-scale accretionary margin between 1520-1340 Ma (Fig. 1), including the Pinware Orogeny in the northeast, the Picuris Orogeny in the southwest, and the most recently attributed section, the Baraboo Orogeny centered in Wisconsin (Daniel et al., 2023). This new hypothesis provides intriguing opportunities to reconsider the origin and tectonic setting of WSC emplacement as well as other Mesoproterozoic granitoids in Wisconsin, including the larger 1.4 Ga Wolf River Batholith (Dewane and Van Schmus, 2007). This research project will use the orientation of magnetic fabrics within the WSC to better understand how the batholith was emplaced. Figure 1: Simplified geologic map of Precambrian crustal provinces including the Mesoproterozoic accretionary margin of the Picuris, Baraboo, and Pinware Orogenies. The 1.48-1.35 Ga ferroan granites, including the Wolf River Batholith, are highlighted in white and the ~1.5 Ga Wausau Syenite Complex is added. Modified from Medaris et al., 2021, originally based on (Whitmeyer and Karlstrom, 2007). The project will consist of an anisotropy of magnetic susceptibility (AMS) survey and thin section analysis of each granitoid within the WSC. With these data, the magmatic flow directions and any subsequent tectonic overprint can be determined, which can be used to constrain the location of the magmatic feeder and the tectonic environment of emplacement. For the purpose of this abstract, three possible outcomes are proposed: 41 �1. Radial magmatic fabrics are preserved, indicating that the WSC was emplaced and cooled prior to the Baraboo Orogeny, with deformation accommodated by the surrounding weaker country rock (Fig. 2A). 2. Magmatic fabrics show a preferential flow pattern parallel to the tectonic margin caused by differential stress from the Baraboo Orogeny, suggesting syntectonic emplacement (Fig. 2B). 3. Only solid-state deformation fabrics are present, implying that the WSC was emplaced before or at the onset of the Baraboo Orogeny and had fully cooled before significant deformation occurred (Fig. 2C). By focusing on these oldest known Mesoproterozoic ferroan granites in the region, we can learn about the timing and geometry of the earliest Baraboo orogenesis. This study will address the question of how these enigmatic granites fit into the overall tectonic history of the Great Lakes Region. Figure 1: Schematic diagram of the WSC showing three possible outcomes of the AMS study. A) Radial magmatic fabric. B) Magmatic fabric with preferential flow parallel to the tectonic boundary. C) Solid state fabric. REFERENCES Daniel, C.G., Indares, A., Medaris Jr., L.G., Aronoff, R., Malone, D., and Schwartz, J., 2023. Linking the Pinware, Baraboo, and Picuris orogens: Recognition of a trans-Laurentian ca. 1520–1340 Ma orogenic belt, in Whitmeyer, S.J., Williams, M.L., Kellett, D.A., and Tikoff, B. eds., Laurentia: Turning Points in the Evolution of a Continent, Geological Society of America, 175–190. Dewane, T.J., and Van Schmus, W.R., 2007. U–Pb geochronology of the Wolf River batholith, northcentral Wisconsin: Evidence for successive magmatism between 1484Ma and 1468Ma: Precambrian Research, 157, 215–234. Medaris, L.G., Singer, B.S., Jicha, B.R., Malone, D.H., Schwartz, J.J., Stewart, E.K., Van Lankvelt, A., Williams, M.L., and Reiners, P.W., 2021. Early Mesoproterozoic evolution of midcontinental Laurentia: Defining the geon 14 Baraboo orogeny: Geoscience Frontiers, 12, 101174. Whitmeyer, S.J., and Karlstrom, K.E., 2007. Tectonic model for the Proterozoic growth of North America: Geosphere, 3, 220–259. 42 �Mapping oxidation reactions in iron-rich rocks from northeast Minnesota, USA. JAROZEWSKI, Sarah1, DUFFY, Paige1, BARRÉ, Cole1, BRENGMAN1, Latisha, EYSTER2, Athena 1 Department of Earth and Environmental Sciences, University of Minnesota Duluth, Heller Hall, 1114 Kirby Drive, Duluth, MN 55812, USA 2 Department of Earth and Climate Sciences, Tufts University, Lane Hall, 2 North Hill Road, Medford, MA 02155, USA Aqueous alteration and post-depositional mineral assemblage modification in Precambrian terranes are ubiquitous, but clear accounting of the relative timing of oxidation reactions at the landscape scale is limited. Here we synthesize observations of oxidation and hydration reactions in three iron-rich lithologies in northeast Minnesota, the Soudan Iron Formation, the Cuyuna Iron Formation, and the Partridge River Intrusion of the Duluth complex to contribute to building a compiled relative mineral redox history for the landscape. The Soudan Iron Formation (~2.7 Ga) is a greenstone-hosted metamorphosed chemical sedimentary unit primarily composed of alternating bands of magnetite, hematite and microquartz affected by at least two Archean deformation events, and later fluid alteration (Thompson, 2015). The Soudan Iron Formation primarily consists of mm-scale bands of authigenic microquartz and iron oxides that preserve in outcrop samples distal to the ore zone, and within the ore horizon at Soudan underground mine. Reflected light petrography of oxides in outcrop samples reveals magnetite replacement by hematite (Figure 1A). Within ore zone samples, complete hematite replacement and large platy hematite is common, similar to previous observations (Thompson, 2015). Clear metamorphic minerals that could indicate high temperatures, pressures, and P-T-path histories are absent from the unit. The younger Cuyuna Iron Formation (~1.9 Ga) is part of an intensely folded metamorphosed sedimentary sequence deformed during the Penokean orogeny (Schmidt, R.G., 1963). Combining new observations and previous data from historic samples (Melcher et al., 1996), oxidation of magnetite is prevalent but limited in samples from the Gloria drill hole (Figure 1B). Metamorphic stilpnomelane is common in the Cuyuna iron formation in contrast to the Soudan Iron Formation. Documented differences in metamorphic silicate mineralogy between these two iron formations may indicate key differences in precursor phases, as both units were affected by significant metamorphic deformation events. Yet, for both, oxidation of magnetite and replacement by hematite indicate both iron formations are similarly affected by post-depositional fluid alteration and oxidation. The Partridge River Intrusion (PRI) is part of the layered series troctolitic intrusions that form the base of the Duluth complex (Tyson and Chang, 1984). In its present geometry, the magmatic layered series PRI now intersects the current land surface. Clear evidence of aqueous alteration in the first few hundred feet of drill core 17700 includes mineral transformation of biotite and olivine to hydrous ferric oxides and secondary iron silicates (Figure 1C). These replacement reactions are limited in scale, and primary igneous mineralogy is still preserved. Leveraging cross-temporal comparisons of current iron-rich bedrock outcrop exposures and drill cores in north-east Minnesota to identify formational vs. post-formational mineralogy will allow for landscape-scale mapping of oxidation reactions and their extent in the subsurface. 43 �Figure 1. Reflected light photomicrographs of the Soudan iron formation, Cuyuna Iron formation and back-scatter electron image (BSE) of the Patridge River Intrusion of the Duluth complex. (A) Magnetite is partially replaced by hematite in the Soudan iron formation. (B) Magnetite oxidation to hematite in the Cuyuna iron formation. (C) Back-scatter electron image of altered olivine in the Partridge River Intrusion from the UMTC EPMA lab, CHARFAC facility.. REFERENCES Melcher, F., Morey, G. B., McSwiggen, P. L., Cleland, J. M., & Brink, S. E. 1996. RI-46 Hydrothermal Systems in Manganese-Rich Iron-Formation Of the Cuyuna North Range, Minnesota: Geochemical and Mineralogical Study of the Gloria Drill Core. Report of Investigations 46, ISSN 0076-9177, 1 - 45. Schmidt, R. G. 1963. Geology and ore deposits of the Cuyuna North range, Minnesota. U.S. Geological Survey Professional Paper 407, p. 96. Taylor, Richard B., 1964. Geology of the Duluth Gabbro Complex near Duluth, Minnesota. Bulletin No. 44. Minnesota Geological Survey, University Digital Conservancy. Thompson, A. 2015. A hydrothermal model for metasomatism of neoarchean Algoma-Type banded iron formation to massive hematite ore at the Soudan Mine, NE Minnesota. University of Minnesota, Duluth. P. 1-59. Tyson, R. M., and Chang, L, L, Y. 1984. The Petrology and sulfide mineralization of the Partridge River Troctolite, Duluth Complex, Minnesota. Canadian Mineralogist, v. 22, p 23-38. 44 �Geology and Geochemistry of the Mesoproterozoic Round Lake Intrusion and associated Ti-Mineralization, Northern Wisconsin JEUTTER, Renee O.1, LODGE, Robert W.D.1 1 Department of Geology & Environmental Science, University of Wisconsin-Eau Claire, 105 Garfield Avenue, Eau Claire, WI 54701, USA Modern technology and renewable energy require large amounts of metals that are currently imported, and there is a tremendous effort to domesticate our mineral extraction and processing. Several of these critical minerals, such as Ti, are found in Wisconsin, but little data is available to guide future mineral exploration efforts. The Mesoproterozoic Mid-Continent Rift and its satellite intrusions are known to host Ti-Fe oxide mineralization and Ni-Cu-PGE magmatic sulfide deposits (Woodruff, 2020). During the development of the Mid-Continent Rift, there is a temporal evolution of occurrences of mineral deposits. The plateau stage typically created Ni-Cu-PGE sulfide deposits, layered Ti-Fe oxide deposits, and alkalic hosted U-Nb deposits. The Round Lake Intrusion, like other intrusions discovered through a strong aeromagnetic anomaly (Mudrey et al. 2003), is an example of a layered Ti-Fe oxide deposit. Anorthosite layers alternate with magnetite troctolite layers approximately every 100 ft. Fractional crystallization throughout the evolution of the magma created the alternating “layers” of plagioclase rich and plagioclase poor segments but has minimal additional differences in mineral composition and presence (Stuhr, 1976). This study describes the petrology and geochemistry of the Round Lake intrusion and Timineralization using historic drill cores stored at the Wisconsin Geological and Natural History Survey core repository. Two holes were relogged, totaling ~1787 feet, and representative samples were obtained of host intrusive phases and mineralization types. The intrusion was characterized via transmitted-light petrography and whole rock geochemistry was determined via WD-XRF. Mineral chemistry of the intrusion and mineralization was determined using SEDEDS. The intrusion segregated into layers: anorthosite, upper magnetite troctolite, middle magnetite troctolite, magnetite, and lower magnetite troctolite, crosscut by an intrusive gabbro dike (Stuhr, 1976). The main intrusion hosting mineralization magnetite-ilmenite rich troctolite, ranging from 35-60% intergrown magnetite-ilmenite and 5-20% coarse grained plagioclase laths (Figure 1). Movement and flow of magmas during emplacement are indicated trachytic flow textures of aligned plagioclase crystals. The anorthosite has 55-90% euhedral plagioclase, 10-15% magnetite, and 5-15% clinopyroxene. The magnetite-ilmenite rich troctolite and anorthosite are crosscut by fine-grained gabbroic dikes. Within the magnetite-ilmenite troctolite unit, magnetitetitanomagnetite and lesser ilmenite assumes interstitial growth between silicates (Figure 1). Apatite is variably present. Olivine is variably altered to iddingsite and serpentine strips of magnetite forming within fractures in the crystal. Both the Round Lake intrusion and Clam Lake intrusion are intrusions rich in magnetite associated with the Mid-Continent Rift, and both are likely to be hosts of Ti-Fe ± V deposits and are known to contain large amounts of titanomagnetite with approximately 1.5% V (Woodruff, 2020). Future work is recommended on the Round Lake Intrusion and Ti-mineralization to better constrain the layering and economic potential of Ti-mineralization. 45 �Figure 1: (A) Geologic map of Northwestern Wisconsin region surrounding the Round Lake Intrusion, Digitized from Stuhr (1976). (B) Image of Magnetite-Ilmenite rich troctolite core sample showing textures and magnetite matrix filling features. (C) Image from SEM showing major magnetite and olivine textures within a sample. Magnetite matrix filling texture and fracture filling within olivine fractures. REFERENCES Stuhr, S. W., 1976, Geology of the Round Lake Intrusion, Sawyer County, Wisconsin [Master’s Thesis]: Madison, University of Wisconsin, 148 p. Woodruff, L. G., Schulz, K. J., Nicholson, S. W., Dicken, C. L., 2020, Mineral Deposits of the Mesoproterozoic Midcontinent Rift system in the Lake Superior region – A space and time classification: Ore Geology Reviews, v. 126, p. 1-21. Mudrey Jr., M.G., Ervin, C.P., Olmsted, J.F., 2003, Middle Keweenawan Basin Evolution Inferred from Geophysical Analysis of a Strongly Magnetic Intrusion, Clam Lake, Wisconsin: Wisconsin Geological and Natural History Survey, Open-file Report 2003-04, 17 p. 46 �Geology and Geochemistry of the Ritche Creek Cu-Zn deposit, North central Wisconsin JOHANNESEN, Haley P. 1, LODGE, Robert W.D.1 1 Department of Geology and Environmental Sciences, University of Wisconsin-Eau Claire, Eau Claire, WI 54701 USA The Ritchie Creek deposit is a Volcanogenic Massive Sulfide (VMS) deposit located within the Paleoproterozoic Penokean Volcanic Belt (PVB) of northcentral Wisconsin (Figure 1). Mineral exploration efforts have demonstrated that the VMS mineralization at this site is concentrated on the western edge of a felsic volcanic center that is interpreted to have formed in a back arc or intra-arc rift environment within bimodal volcanic sequences (DeMatties, 1990). These interpretations were based on physical descriptions of units intersected in drill core and comparisons to other VMS deposits regionally and globally. Like many VMS deposits in the PVB, little research has been done at the Ritchie Creek prospect to link petrogenesis with largerscale VMS environments. This research aims to characterize and refine the geological and geochemical characteristics of the Ritchie Creek Cu-Zn deposit by re-examining historic drill core and representative volcanic stratigraphic units and provide a more comprehensive understanding of the tectonic environment that influenced mineralization. The study involved logging 1,000 linear feet of historic drill core from two holes and collecting 22 core samples from representative stratigraphic units for petrographic and geochemical characterization. The four sampled units include: (1) a medium grey, fine grained quartz mica schist with alternating coarse-grained quartz and K-feldspar bands and disseminated sulfides including pyrite and chalcopyrite, (2) a light green quartz mica schist, strongly altered by sericite and chlorite, containing disseminated chalcopyrite and pyrite, (3) an intermediate metafelsite unit, characterized by sericite and biotite, that grades into a rhyolitic tuff with angular felsic fragments, localized sulfide blebs, and quartz veins, and (4) a semi-massive to massive sulfide unit consists mainly of pyrite with minor chalcopyrite, in a sheared and brecciated matrix. Major and trace element geochemical data was generated via WD-XRF at the Material Science Center at the University of Wisconsin-Eau Claire. Major element geochemistry is highly variable because of varying degrees of hydrothermal alteration. Therefore, immobile trace elements are used to classify protoliths and discriminate tectonic settings. Least-altered volcanic strata were chemically classified as mafic volcanics (based on low Zr/Ti, high Cr), intermediate volcanics (based on elevated Zr/Ti), and felsic volcanics (based on high Zr/Ti). Mafic volcanic strata have high Zr, consistent with calc-alkalic magmatic affinities. Felsic volcanic strata are FII type felsic magmas and have low Nb and Y consistent with volcanic-arc felsic magmas. The quartz-sericite altered rocks have trace element chemistry consistent with the intermediate volcanic strata and alteration indices indicate a potassic-dominated alteration. These characteristics suggest an oceanic arc-backarc bimodal-mafic petrochemical association (Piercey, 2011) and provide a more comprehensive understanding of VMS mineralization in the PVB. 47 �(A) (B) Figure 1. (A) A regional map of the Ritche Creek VMS Deposit located in North Central Wisconsin. (B) A Cross-section view of the Ritche Creek VMS deposit, showing drill hole locations and stratigraphic units, faulting and alteration zones. This cross section focuses on (RC5) a drill holes that intersects significant sericite alteration and massive sulfide mineralization zones. REFERENCES DeMatties, T.A., (1990), The Ritchie Creek Main Zone: A Lower Proterozoic CopperGold Volcanogenic Massive Sulfide Deposit in Northern Wisconsin. Economic Geology Vol. 85, 1990, pp. DeMatties, T.A., (2018), Effects of paleoweathering and supergene activity on volcanogenic massive sulfide (VMS) mineralization in the Penokean Volcanic Belt, northern Wisconsin, Michigan and east- central Minnesota, USA: Implications for future exploration: Ore Geology Reviews, v. 95, p. 216–237. DeMatties, T.A., (2022), Exploration-resource assessment of productive felsic volcanic centers in the Paleoproterozoic Penokean Volcanic Belt of northern Wisconsin, Michigan, and east-central Minnesota, USA: Ore Geology Reviews, v. 141, p. 104489. Piercey SJ (2011) The setting, style, and role of magmatism in the formation of volcanogenic massive sulfide deposits. Mineralium Deposita 46:449-471. 48 �Geologic implications of detrital zircon U-Pb ages from Archean and Paleoproterozoic strata in central Minnesota and the Gogebic Range of Wisconsin and Michigan, USA JONES, James V.1, SALERNO, Ross2, CANNON, William F.2, and O’SULLIVAN, Paul4 1 U.S. Geological Survey, Anchorage, AK 99508, USA jvjones@usgs.gov U.S. Geological Survey, Reston, VA 20192, USA; 3 U.S. Geological Survey, Denver, CO 80225, USA 4 GeoSep Services LLC, Moscow, ID 83843, USA 2 Archean and Paleoproterozoic metasedimentary successions in the Lake Superior region of the northern United States record the assembly and breakup of southern Superia and the subsequent transition to long-lived accretionary orogenesis along the southern Laurentia margin. The successions are difficult to correlate for reasons that include contrasts in thickness, facies, and variable amounts of erosion, similarities in depositional environment through hundreds of millions of years of sedimentation, and variable overprinting by younger tectonic events. Detrital zircon U-Pb geochronology is useful for correlating siliciclastic strata and for identifying provenance patterns that reflect past tectonic and sedimentary interactions. We present new data for samples collected from ca. 2.6–1.8 Ga strata from across the Lake Superior region that provide key insights into regional correlations and local to global tectonic histories. In the eastern Gogebic Range of Michigan, Archean volcanic and volcaniclastic rocks are mapped in a fault-bounded panel between the Watersmeet gneiss dome to the southeast and Neoarchean Puritan batholith to the northwest. One new sample of Archean metagraywacke from within the supracrustal succession yielded only Neoarchean detrital zircon with age populations ranging from ca. 2740 to 2590 Ma, indicating derivation from nearby gneisses but not from older sources such as the early Paleoarchean Watersmeet gneiss. A younger succession of Paleoproterozoic metavolcanic and metasedimentary rocks near Lake Gogebic overlies the Archean gneisses and supracrustal rocks. One sample of fine- to medium-grained slate and metagraywacke from the Copps Formation yielded a mixture of Archean and Paleoproterozoic detrital zircon dates. Archean grains were minor and included age populations of ca. 2649 and 2553 Ma that match the nearby Neoarchean metagraywacke and gneiss domains. Paleoproterozoic grains defined a ca. 1846 Ma age peak and a maximum depositional age of ca. 1829 Ma. We also collected samples of the Paleoproterozoic Palms and Tyler Formations that overlie Archean domains in the western part of the Gogebic Range. Fine-grained gray quartzite of the Palms Formation yielded detrital zircon age populations ranging from ca. 2976 to 2458 Ma and a prominent peak at ca. 2675 Ma. The age spectrum indicates input and (or) recycling of Archean sources and an absence of coeval magmatic sources in the region. In contrast, fine-grained argillaceous sandstone of the overlying Tyler Formation contained mostly Paleoproterozoic detrital zircon with major age peaks at ca. 1863 and 1827 Ma together with minor older age populations ranging from ca. 2780 to 1953 Ma. In central Minnesota, new samples were collected from the Paleoproterozoic Denham and Little Falls Formations. The Denham Formation sample was collected on the northern side of the McGrath gneiss dome and consisted of fine-grained biotite argillite with 1-2 mm horizons of coarser sandstone. The sample yielded chiefly Archean detrital zircon with a dominant age population at ca. 2603 Ma and minor older populations ranging from ca. 3409 and 2789 Ma. Archean age populations match previously published data from nearby samples of basal arkose and dolomitic arkose from the same unit (Craddock et al., 2013). However, that basal arkose also contained a distinct ca. 2101 Ma age population that established a potential correlation between the Denham Formation and the East Branch Arkose of the Dickinson Group in Michigan. The 49 �Little Falls Formation sample of garnet-staurolite-biotite schist was collected from the southern side of the McGrath dome, and it predominately contained Paleoproterozoic detrital zircon that define a dominant unimodal age population at ca. 1846 Ma. The age spectrum for the Little Falls sample is nearly identical our data from the Copps Formation and is also like our Tyler Formation data and to previously published data for other parts of the upper Animikie Group. The marked difference in the proportion of Archean and Paleoproterozoic grains between the Little Falls and Denham Formations suggests a major change in provenance across their contact. The Denham Formation appears to have been derived from the underlying Archean gneiss dome with lesser contribution from older gneisses elsewhere in the region. Circa 2.1 Ga sources are rare in the region but are found locally to the east in Dickinson County, Michigan. The Paleoproterozoic age population that dominates the Little Falls Formation indicates derivation from the Wisconsin magmatic terrane that was approaching from the south (present coordinates) prior to collision that defines the Penokean orogenic cycle in the region. Additionally, our data indicate a maximum depositional age of ca. 1846 Ma for the Little Falls Formation that contrasts with the inferred ca. 2101 Ma age of the underlying Denham Formation reported by Craddock et al. (2013). Published observations suggest a gradational contact between schist of the Little Falls Formation and dolomitic marble of the underlying Denham Formation (Boerboom and Chandler in Bauer et al., 2022). Boerboom and Chandler (2022) noted a 1-meter graphitic/carbonaceous argillite at the base of the Little Falls Formation that could represent a hiatus and then a major change in depositional environment above the arkosic conglomerate and dolostone. We previously reported similar geologic and provenance patterns from the Dickinson Group approximately 600 km to the east in Michigan (Jones et al., 2024). In that area, the East Branch Arkose contains a similar distribution of DZ ages: a mixture of Archean detrital zircon and a distinctive ca. 2099 Ma age population interpreted to have been derived from local granitic sources. The overlying Solberg Schist is made up of biotite-staurolite schist that contains prominent ca. 1.86–1.84 Ga age populations together with minor ca 2.5 and 2.3 Ga age populations. More work is needed to better constrain the stratigraphic position of the depositional age and provenance shifts in both successions and to better understand the tectonic setting and significance of the subtle unconformities and pronounced shift in zircon sources. Preliminary observations and data suggest that the two successions are regionally similar but also distinct from surrounding strata. Thus, the Denham and Little Falls Formations may provide a distinctive and unique record of the transition from Superia rifting to Penokean orogenesis. REFERENCES Bauer, Emily J; Chandler, V.W.; Boerboom, Terrence J; Knaeble, Alan R; Nguyen, Maurice K; Lively, R. S.; Setterholm, Dale R; Steenberg, Julia R. (2022). C-52, Geologic Atlas of Aitkin County, Minnesota. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/253808. Craddock, J.P., Rainbird, R.H., Davis, W.J., Davidson, C., Vervoort, J.D., Konstantinou, A., Boerboom, T., Vorhies, S., Kerber, L., and Lundquist, B., 2013, Detrital zircon geochronology and provenance of the Paleoproterozoic Huron (~2.4-2.2 Ga) and Animikie (~2.2-1.8 Ga) basins, southern Superior Province: Journal of Geology, v. 121, p. 623–644, https://doi.org/10.1086/673265. Jones, J., Cannon, B., Drenth, B., and O’Sullivan, P., 2024, Geologic and tectonic implications of detrital zircon U-Pb age from the Dickinson Group in the western Upper Peninsula of Michigan, USA: Institute on Lake Superior Geology, “Institute on Lake Superior Geology: Proceedings, 2024,” Archives & Digital Collections at Lakehead University Library, accessed April 9, 2025, https://digitalcollections.lakeheadu.ca/items/show/10352. 50 �Zircon Petrochronology of Wisconsin’s Volcanogenic Massive Sulfide Deposits, Northcentral Wisconsin KWIATKOWSKI, Aidan O. 1, LODGE, Robert W.D.1 1 Department of Geology and Environmental Sciences, University of Wisconsin-Eau Claire, Eau Claire, WI 54701 USA Northern Wisconsin’s Paleoproterozoic Penokean Orogen, one of the classic Precambrian orogenic belts in North America, is known to host multiple volcanogenic massive sulfide (VMS) deposits which are important sources of Cu, Zn, Pb, Ag, and Au globally. Despite known large and potentially economic VMS deposits, limited outcrop exposure has hindered detailed reconstructions of the VMS-hosting environment to guide future exploration. Historic U-Pb geochronology indicates that volcanism occurred between 1889-1835 Ma (Sims et al. 1989) with the majority of VMS-hosting strata constrained between 1875-1873 Ma (Quigley 2016). Schultz & Cannon (2007) attribute the main VMS forming event ca. 1875 Ma to extension in a developing back arc basin, with a second later magmatic pulse around 1830 Ma being attributed to post-tectonic stitching plutons. However, a newer model by Zi et al. (2022) shows two VMS forming events around 1875 Ma and 1845 Ma suggesting a regime consisting of alternating compressional and extensional environments caused shifting subduction angles. Zircon petrochronology (U/Pb, Lu/Hf isotopic data and trace elements) can not only better constrain the timing of VMS formation but can also allow for a more complete understanding of the geological evolution and metallogeny of Wisconsin VMS deposits. This study sampled felsic igneous rocks from several VMS deposits to determine the timing and tectonic settings of VMS environments in the western Penokean Orogen. Samples were studied from the Flambeau, Eisenbrey, and Lynne deposits of the Ladysmith-Rhinelander Volcanic Belt (Figure 1a). Samples from the Flambeau and Eisenbrey deposits consist of felsic volcaniclastic units associated with sulfide mineralization and the sample from the Lynne deposits consists of a granodiorite which has intruded into the VMS deposit and volcanic strata. Samples were pulverized and heavy mineral separates were obtained by various magnetic and density separation techniques. The zircon mineral grains were imaged by cathodoluminescence prior to isotopic (U/Pb, Lu-Hf) and trace element analyses via LA-ICPMS at the Mineral Exploration Research Centre at Laurentian University, Sudbury, Ontario. U/Pb isotopic data constrains timing of magmatism. Trace elements and Lu-Hf data constrain the tectonic setting and crustal architecture. Preliminary results indicate two distinct VMS-forming magmatic events during the Penokean Orogeny that have similar tectonic and magmatic styles. All samples show a bimodal distribution of U/Pb ages centered on 1830-1835 Ma and 1870-1875 Ma (Figure 1). Trace element geochemistry of zircons reveals little petrogenetic difference between the magmatic events. Negative ƐHf(i) values, indicating interaction with Archean basement, is consistent amongst all samples and between magmatic events. The VMS-forming extensional event at ca. 1835 Ma contradicts the Schulz and Cannon (2007) model where collisional tectonics are dominant at this time. While the timing of VMS-formation more closely aligns with the Zi et al. (2022) model, the accordion-like tectonics cannot explain the lack of variation in magmatic setting or crustal architecture observed in our data. Therefore, additional data is needed to fully understand the tectonic and metallogenic significance of this younger extensional event. 51 �Figure 1. A) Generalized geologic map of the Penokean Orogen illustrating major tectonostratigraphic subdivisions and the location of sampled and major VMS occurrences. Figure modified from Schulz and Cannon (2007) and DeMatties (1994). Subdivisions of Pembine-Wausau terrane from DeMatties (1994, 2018). LRVC = Ladysmith-Rhinelander volcanic complex. B) U/Pb concordia diagram of older magmatic zircons. C) U/Pb concordia diagram of younger magmatic zircons interpreted to be crystallization age of sample. D) Weighted mean diagram distribution of ages and analyzed grains. Inset image shows frequency distribution of ages in samples. REFERENCES DeMatties TA (1994) Early Proterozoic volcanogenic massive sulfide deposits in Wisconsin: An overview. Econ Geol 89: 1122-1151. DeMatties TA (2018) Effects of paleoweathering and supergene activity on volcanogenic massive sulfide (VMS) mineralization in the Penokean Volcanic Belt, northern Wisconsin, Michigan and east-central Minnesota, USA: Implications for future exploration. Ore Geol Rev 95: 216-237. Quigley A (2016) Setting of the volcanogenic massive sulfide deposits in the Penokean Volcanic belt, Great Lakes region, USA. Colorado School of Mines, Masters Thesis. 95 p. Schulz KJ, Cannon WF (2007) The Penokean orogeny in the Lake Superior region. Precambrian Res 157: 4-25. Sims PK, Van Schmus WR, Schulz KJ, Peterman ZE (1989) Tectonostratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of the Penokean orogen. Can J of Earth Sci 26: 2145-2158. Zi J-W, Sheppard S, Muhling JR, Rasmussen B (2021) Refining the Paleoproterozoic tectonothermal history of the Penokean Orogen: New U/Pb age constraints from the Pembine-Wausau terrane, Wisconsin, USA. Geol Soc Am Bull 134: 776-790. 52 �Geologic Interpretation of Filtered Gravity and Magnetic Anomalies of the Baraboo Range LONGACRE, Mark B.1 and HINZE, William J.2 1 MBL, Inc., 51 Captain Perry Dr., Phippsburg, ME 04562 Purdue University, 30 Brook Hollow Ln., West Lafayette, IN 47907 2 Investigations over the past decade have made significant advances in our geologic knowledge of the Mesoproterozoic Baraboo Synclinorium and adjacent region of south-central Wisconsin (e.g., Medaris, Jr. et al., 2021; Stewart et al., 2021; Marshak et al., 2023). To further the geologic information of this feature and nearby region we have filtered their gravity and magnetic anomaly maps to identify geologic formations and structures in the crystalline basement. The filtered maps isolate specific attributes of the anomaly fields which are useful in interpretation especially when combined with constraining geological information. These maps have identified a buried geologic structure to the east of and immediately adjacent to and along strike of the Sauk Syncline (Figures 1 and 2) which encompass the Baraboo Synclinorium. The buried structure is a near mirror image of the Sauk Syncline and thus is referred to as the Twin Syncline. Unlike the Sauk Syncline and the Baraboo Synclinorium the eastern structure is south rather than north of a geological lineament within the Yavapai orogenic province that marks the southern boundary of the Wisconsin Gravity Minimum. The Twin Syncline is notable in the magnetic anomaly map because of the positive anomaly associated with a magnetite-rich formation that is likely an extension of the lower portion of the Freedom Formation of the Baraboo Synclinorium. The elliptical trace of this anomaly and the steep gradients of the outer margin support the synclinal nature of the structure. We interpret this structure to be a result of south-verging thrusting with steeply dipping thrusts along the northern and southern margins of the Twin Syncline similar to the situation of the Sauk Syncline. This structure is not as tightly folded as the Baraboo Synclinorium suggesting that the thrusting to the east of Baraboo Synclinorium was less intense. The Syncline can be identified on the filtered maps as can other quartzite synclinoriums of the region by the subdued geophysical anomalies of the underlying felsic volcanic rocks because of their burial beneath the non-magnetic quartzite. The magnetic anomalies of the magnetic lower half of the Freedom Formation are also a useful marker for detailing the structure within the Baraboo Synclinorium and defining the limits of the Freedom Formation within it. Additionally, two parallel intrusives on strike with the Denzer Diorite that crops out along the southwestern margin of the Synclinorium extend northnortheasterly within the sub-quartzite basement across the western portion of the Synclinorium. They are associated with anticlines within the Synclinorium that may have resulted from differential deformation caused by variations in the rheology of the basement rocks. These and other interpretations of the filtered gravity and magnetic anomalies suggest that revisiting the studies of basement rocks of Wisconsin and adjacent regions is in order using the available improved analysis, interpretation, and presentation methods and modern data gravity and magnetic data sets. REFERENCES Marshak, S., Wilkerson, M.S., and DeFrates, J., 2023. Kinematic and tectonic implications of crenulation cleavage, kink bands, and mesoscopic folds in the Baraboo Syncline, Wisconsin (∼1.45 Ga Picuris Orogen). Journal of Structural Geology, 178, 105007. Medaris, Jr, L.G., Singer, B.S., Jicha, B.R., Malone, D.H., Schwartz, J.J., Stewart, E.K., Van Lankvelt, A., Williams, M.L., and Reiners, P.W., 2021. Early Mesoproterozoic evolution of midcontinental Laurentia: Defining the geon 14 Baraboo orogeny. Geoscience Frontiers,12(5), 101174, 17 p. 53 �Stewart, E.K., Brengman, L.A., and Stewart, E.D., 2021. Revised provenance, depositional environment, and maximum depositional age for the Baraboo (< ca. 1714 Ma) and Dake (< ca. 1630 Ma) Quartzites, Baraboo Hills, Wisconsin. The Journal of Geology, 129, 1-31. Figure 1. Total horizontal derivative of the RTP total magnetic anomaly map of south-central Wisconsin. The outlines of the Sauk (left) and Twin (right) Synclines are shown by the thin dashed white lines and the Baraboo Synclinorium by a dashed white line. The Baraboo Lineament of the Yavapai province is shown by the wide broadly dashed line. Color coding is non-linear. Figure 2. Tilt derivative of the Bouguer gravity anomaly map of south-central Wisconsin that emphasizes the short wavelength components. The outlines of the Sauk (left) and Twin (right) Synclines are shown by the dashed white lines and the Baraboo Synclinorium by a thin dashed white line. The Baraboo Lineament of the Yavapai province is shown by the wide broadly dashed line. Color coding is non-linear. 54 �An Informal Review of the ILSG Field Excursion to Hawaii, January – February, 2025 MACTAVISH, Allan1, HINZ, Peter1, HUDAK, George1, LARSON, Phil1, AUBUT, Allan1, BOERBOOM, Terry1, CHILTON, Vern1, DeGRAFF, Jim1, ERICKSON, Tom1, FAULKNER, Barb1, SERRANO, Isabel1, and ZANKO, Larry1 1 Members of the 2025 ILSG Field Trip to Hawaii, 2025 Between January 24, 2025 and February 5, 2025, twelve members of the Institute on Lake Superior Geology participated in a geological field excursion to investigate the geology of the island of Hawaii, with a focus on observing field relationships, outcrop characteristics and geomorphology to better understand the characteristics of modern basaltic volcanism in a hotspot environment. The field excursion was led by Allan MacTavish, Peter Hinz, George Hudak and Phil Larson. A new field trip guidebook and glossary of geological terms (MacTavish and Hudak, 2024) was prepared and utilized during the thirteen-day long trip. This presentation will review key features and take-aways from the excursion, which included investigations of five of the seven volcanoes associated with the island of Hawaii. Investigations took place via examinations of various outcrops, hikes through the Hawaiian wilderness, and a helicopter tour. Various eruption types, volcano types, coherent (lava flow) and volcaniclastic deposit types and features, different types of volcanic products and hydrothermal alteration facies, and observations of historical and cultural artifacts and natural phenomena will be discussed. Challenges and surprises associated with field studies of Hawaii will also be presented. REFERENCES MacTavish, A., and Hudak, G., 2024, The Volcanoes of the Island of Hawaii – Field Trip Guide: Institute on Lake Superior Geology Special Publication 3, 200 p. 55 �56 �Refining the Age and Occurrence of Basement Rocks in Northwest Iowa: Implications for Precambrian Tectonics and Magmatic Evolution of the Laurentian Midcontinent MALONE, Jack1, MALONE, David2, ANDERSON, Raymond1, CLARK, Ryan1 1 Iowa Geological Survey, University of Iowa, Iowa City, IA 52242 USA 2 Geography-Geology, Illinois State University, Normal, Illinois 61790 Precambrian basement rocks in northwest Iowa reveal an Archean and two Paleoproterozoic tectonic sutures (Figure 1; 1.9-1.8 Ga Trans-Hudson/Penokean and 1.8-1.7 Ga Yavapai; Bickford et al., 1986; Holm et al., 2007). Here we present four new U-Pb (LA-ICPMS) ages for drill cores of basement rocks along the Transcontinental Arch in northwest Iowa, USA (Figure 2). The cores are on repository at the Iowa Geological Survey. The Camp Quest migmatite gneiss was sampled at a depth of 1,078 ft from the Camp Quest D-21 core (W25498; z=38). The weighted mean and Concordia ages were both 1845 Ma, which is the first TransHudson/Penokean age recognized in Iowa. This core is located south of the Spirit Lake tectonic zone (SLTZ) which is interpreted as the suture between Yavapai terrane rocks to the south and Archean Superior province rocks to the north. Nine inherited zircons are mostly Archean in age and interpreted as xenocrysts, indicating Archean crust occurs at depth south of the SLTZ. Granite was also sampled at a depth of 660 ft from the Hawarden D-7 core (W27270; z=35). The zircon age spectrum reveals three age clusters at ~2895, ~2683, and ~1800 Ma. The older, inherited age clusters are consistent with ages of the Minnesota River Valley terrane and the greater Superior Province, respectively. The ~1800 Ma age is similar to the nearby 1803-1810 Ma Matlock “keratophyre” and the distant 1805 Ma Humboldt granite (northern Michigan), representing the initiation of north-directed Yavapai subduction and granitic melt production into Archean and previously accreted Trans-Hudson/Penokean rocks north of the SLTZ (Kilburg, 2024). Granodiorite was sampled at a depth of 915 ft from the Harris D-13 core (W27270; z=41). The weighted mean and Concordia ages are ~1780 Ma, suggesting that Yavapai rocks intrude older Trans-Hudson/Penokean or Archean rocks north of the SLTZ. A late-stage granitic dike was sampled at a depth of 1,611 ft from the Spencer BX-2 core (W16223; z=7), which is from a tabular noritic body within the Spencer intrusive complex just south of the SLTZ. The sampled interval yielded sparse zircons; however, the weighted mean age of 1238 Ma is the first Grenville age recognized in Iowa. This age suggests an obscure early Grenvillian thermal resetting or reactivation in the upper Midcontinent which postdates anorthositic/noritic magnetism concentrated along the SLTZ at Spencer. New complementary whole rock WDXRF major oxide and ICP-OES trace element geochemical analyses (n=163) from Precambrian units in northwest Iowa reveal a complex tectonic and crustal growth configuration. Intermediate to felsic intrusions are generally LREEenriched and have I-type volcanic arc-like trace element patterns. The origin of anorthositic to mafic-ultramafic occurrences are less straightforward but are characterized by slight to significant Ce, Sm, Eu, and Lu anomalies, indicating basaltic to mantle fractionation, differential partial melting at depth, and/or derivation from Fe-rich residual melts. These new results provide significant insight into the tectonomagmatic evolution of the southernmost Superior Province during the final assembly of the Laurentian craton. 57 �Figure 1: Geological map of Precambrian basement rocks in the northern midcontinent and northwest Iowa. Top: Red dots indicate previously published U-Pb ages and white dots are new (this study). Bottom: New geochronologic ages indicated with stars are CQ = Camp Quest, HW = Hawarden, HA = Harris, SP = Spencer. Figure 2: Weighted mean, probability density, and Concordia plots of newly dated Precambrian units in northwest Iowa. REFERENCES Bickford, M.E., Van Schmus, W.R., and Zeitz, I., 1986. Proterozoic history of the midcontinent region of North America. Geology, 14(6), 492-496. Holm, D.K., Anderson, R., Boerboom, T.J., Cannon, W.F., Chandler, V., Jirsa, M., Miller, J., Schneider, D.A., Schulz, K.J., & Van Schmus, W.R., 2007. Reinterpretation of Paleoproterozoic accretionary boundaries of the north-central United States based on a new aeromagnetic-geologic compilation. Precambrian Research, 157(1-4), 71–79. Kilburg, N., 2024. Age and petrogenesis of the Matlock ‘Keratophyre’ in northwest Iowa [M.S. Thesis]: Iowa City, University of Iowa, 129 p. 58 �Post-Penokean and Pre-Yavapai Magmatism and Sedimentation in Central Wisconsin (Southern Lake Superior Region) MEDARIS, Gordon Jr.1 and MALONE, Dave2 1 Dept. of Geoscience, University of Wisconsin-Madison, Madison, WI 53706 2 Dept. of Geography, Geology, and the Environment, Illinois State University, Normal, IL 61790 The principal Precambrian domains in Wisconsin are the Penokean Province, consisting of the Marshfield Terrane, Wausau-Pembine Terrane, and Craton margin, which include 2450-1770 Ma craton margin and foreland basin sediments and 1890-1830 Ma volcanic arc associations, 1760 Ma rhyolite and granite of the Yavapai Province, <1643 Ma quartzite of the Baraboo Interval, 1484-1468 Ma granitic rocks of the Wolf River batholith, and 1109-960 Ma igneous and sedimentary rocks of the Midcontinent Rift (Fig. 1). In addition to these five major domains, small outcrops of post-Penokean and preYavapai igneous and sedimentary rocks are scattered across central Wisconsin, which have been investigated in detail at Hamilton Mounds (Medaris et al., 2007), Biron Dam (Holm et al., 2020), and Brokaw (this report) (Fig. 1). Two sedimentary successions occur at Hamilton Mounds: an older arkose and a younger quartzite correlative with the Baraboo quartzite. The arkose is a gray, fine- to medium-grained, feldspathic sandstone (CIA = 59.0; Fig. 2). Detrital monazite in arkose yields a total Pb median age of 1850 Ma, with the youngest detrital grain at 1757 Ma, signifying post-Penokean deposition of the arkose. An upper age for the arkose is provided by the intrusion of 1762 ± 7 Ma granite (Yavapai), whose age is within error of the youngest detrital monazite grain. Muscovite in the younger quartzite yields a 40Ar/39Ar plateau age of 1470 ± 11 Ma, reflecting the widespread thermal effect of the Wolf River batholith throughout central and southern Wisconsin. At Biron Dam, trachybasaltic diabase dikes (Fig. 2) intruded Archean gneiss and Penokean tonalite, granodiorite, and granite. Zircon grains in three samples of diabase yield 207 Pb/206Pb ages within error of each other, with a weighted mean age of 1817 ± 2 Ma, which demonstrates post-Penokean and pre-Yavapai emplacement of the dikes. The diabase dikes have been metamorphosed under amphibolite-facies conditions; hornblende in metadiabase yields a 40 Ar/39Ar plateau age of 1672 ± Ma, possibly representing a Mazatzal influence. At Brokaw, polymictic conglomerate, feldspathic sandstone (CIA = 59.0; Fig. 2) and siltstone were intruded by rhyolite, which contains inherited zircon with ages between 2125 Ma and 3565 Ma. The sandstone contains detrital zircon with a 207Pb/206Pb median age of 1850 Ma and an age of 1810 Ma for the youngest subset of grains with overlapping errors, demonstrating post-Penokean deposition of the Brokaw sedimentary rocks. Primary structures and textures of the Brokaw igneous and sedimentary rocks have been preserved on the macroscopic scale, but such rocks have been pervasively recrystallized to greenschist-facies mineral assemblages on the microscopic scale, as seen for example in rhyolite, in which plagioclase was replaced by albite and epidote, and hornblende, by epidote (Fig. 3). The age of such recrystallization has not yet been determined, but is presumed to be related to the nearby Wolf river batholith. It is now recognized that igneous rocks were emplaced and sedimentary rocks were deposited over much of central Wisconsin in the interval 1817-1757 Ma after the Penokean orogeny, perhaps as a precursor to the Yavapai orogeny. 59 �Figure 2. Chemical compositions of Biron Dam diabase, Brokaw rhyolite and sandstone, and Hamilton Mounds sandstone in terms of Al (Al2O3), Ca* (CaO), N (Na2O), and K (K2O); CIA: Chemical Index of Alteration. Figure 1. Map of the major Precambrian geological units in the southern Lake Superior region. Star symbols: Brokaw (BK), Biron Dam (BD), and Hamilton Mounds (HM) localities; B: Baraboo Interval sedimentary rocks. Figure 3. Photomicrograph (crossed polarizers) of recrystallized Brokaw rhyolite; ab, albite; ep, epidote REFERENCES Medaris, L.G. Jr., Van Schmus, W.R., Loofboro, J., Stonier, P.J., Zhang, X., Holm, D.K., Singer, B.S., and Dott, R.H. Jr., 2007. Two Paleoproterozoic (Statherian) siliciclastic metasedimentary sequences in central Wisconsin. Precambrian Research, 157, 188-202. Holm, D., Medaris, L.G. Jr., McDannell, K.T., Schneider, D.A., Schulz, K., Singer, B.S., and Jicha, B.R., 2020. Growth, overprinting, and stabilization of Proterozoic Provinces in the southern Lake Superior region. Precambrian Research, 339, Article 105587. 60 �US Steel Corporation / Ralph W. Marsden iron ore collection MOOERS, Howard1, SEVERSON, Mark2, JONGEWAARD, Peter3, LARSON, Phillip4 1 Department of Earth and Environmental Sciences, University of Minnesota Duluth, Duluth, MN 55812 2 2122 W 22nd St., Duluth, MN 55811, USA 3 7009 Three Lake Rd., Canyon, MN 55717, USA 4 1613 14th Ave. East, Hibbing, MN 55746, USA By the time Ralph W. Marsden joined Oliver Iron Mining Division of US Steel Corporation (USSC) in 1951 he was already one of the World’s experts on iron ore. From 1953-1964 he managed the Geologic Investigations Unit in Duluth, MN. During this time, Ralph was one of the co-founders of the Institute on Lake Superior Geology in 1954. In 1964 Ralph was transferred to the Pittsburgh corporate office as Manager of Geologic Investigations, Iron Ore, however, Ralph wanted to return to Minnesota, and in 1967 he left USSC and moved to the University of Minnesota Duluth (UMD) Department of Geology as Professor and Head. USSC had an active, worldwide exploration program for iron ore from the 1920s into the 1960s, and a large number of the samples collected were housed in Duluth, MN. When USSC closed its Duluth, MN, office, this iron ore sample collection was to be discarded. Ralph “rescued” the collection of iron ore samples and moved them to the University of Minnesota Duluth. In 1986, Ralph died suddenly while attending the Geological Society of America Annual Meeting in San Antonio, TX. The collection of iron ore samples sat in a service tunnel at UMD for 40 years. This globally significant collection of iron ore samples was recently inventoried, photographed, and placed in storage containers that are readily accessible. The inventory of the 483 samples, complete with photographs, is cataloged on the University of Minnesota Digital Conservancy (https://hdl.handle.net/11299/265081). Many of these samples are from localities that are no longer accessible, are from closed mines, or are from areas of the World that simply cannot be visited because of political and social issues. This collection of iron ore samples dates from 1926 to the 1960s and has samples from 25 countries and 30 US states and Canadian provinces. The individual sample boxes are labeled, and many have great detail on the origin of the samples. Most of the samples are also individually labeled, with sample numbers and descriptions. There are photographs of the contents of each box, and where possible supporting documents are shown in the photos. For further information or to request access to samples contact the Department of Earth and Environmental Sciences, University of Minnesota Duluth or Howard Mooers (hmooers@d.umn.edu). 61 �Countries represented: USA, Angola, Australia, Brazil, Canada, Chile, Colombia, Congo, Costa Rica, Cuba, Gabon, Germany, Guatemala, Honduras, India, Ivory Coast, Liberia, Mexico, Nicaragua, Namibia, Portugal, South Africa, Sudan, Sweden, Venezuela. US States and Canadian Provinces represented: Alabama, Alberta, Arizona, British Columbia, California, Idaho, Illinois, Massachusetts, Michigan, Minnesota, Missouri, Montana, Nevada, New Jersey, New Mexico, New York, Newfoundland, North Carolina, North Dakota, Ontario, Oregon, Puerto Rico, Quebec, South Dakota, Utah, Virginia, Washington, Wisconsin, Wyoming. Figure 1. Example of samples from Liberia, West Africa, with supporting documentation. REFERENCES University Digital Conservancy, University of Minnesota Duluth, (2024). List of Samples for US Steel Corporation / Ralph W. Marsden Iron Ore Collection. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/266399. 62 �Lithogeochemical Characterization of Manganese Mineralization at the Cuyuna Range, Central Minnesota PALIEWICZ, Cory1, THAKURTA, Joyashish1 1 Natural Resources Research Institute (NRRI), University of Minnesota Duluth, 5013 Miller Trunk Hwy, Duluth, MN 55811 The Paleoproterozoic Cuyuna Range of central Minnesota contains elevated levels of manganese when compared to other Banded Iron Formations in the Lake Superior region. The total tonnage is estimated at 49 million metric tons at 7.84 percent Mn (Kilgore and Thomas, 1982). The Cuyuna Range consists of a Penokean fold-and-thrust belt divided into the Emily District, North Range, and South Range. These are separated by structural and stratigraphic discontinuities which make each area geologically distinct (Southwick et al., 1988; Morey, 1990). Although prior work has documented a variety of textural and sedimentary associations, this study will provide new lithogeochemical data to further characterize the manganese-bearing lithologies across the Cuyuna Range in support of ongoing research for manganese and other critical minerals in Minnesota as part of the USGS Earth MRI program. A total of 201 drill core samples were collected from 37 drill holes across the Emily District, North Range, South Range, and Glen Lake Sulfide Deposit (Figure 1). To date, all samples have been studied in hand-sample and sent for bulk geochemical analysis, 40 samples have been analyzed in thin section, and whole-rock geochemical results of 60 samples from 16 drill holes have been received from the USGS. Although lithologic features of both ironformations and non-iron-formations are variable across the range, the deposits also share many attributes. As such, we find it useful to texturally classify the collected samples into granular, banded, and irregular types while still recognizing the special characteristics of each individual mineral association. This study will present petrographic and whole-rock geochemical data, with particular emphasis on rocks from the Emily District, which from past studies is known to be mostenriched in Mn-content. In addition, Mn-bearing country rocks throughout the Cuyuna Range are also characterized and compared to historic drill logs and prior work (e.g., Morey et al., 1991, Dahl et al., 1992). In this way, new insights on lithological variation, manganese distribution, and other potential critical minerals at the Cuyuna Range may further be addressed and incorporated during the Earth MRI program. 63 �Drill Hole Sampled Figure 1: Regional geologic map of the Cuyuna Range showing approximate drill hole locations sampled for this study. Modified from Southwick et al., 1988 and Cleland et al., 1996. REFERENCES Dahl, L.J., Brink, S.E., Blake, R.L., Tuzinski, P.A., and Adamson, N.R., 1992, Site characterization of Minnesota manganese deposits to evaluate the potential for in-situ leach mining: Littleton, Colorado, Society for Mining, Metallurgy and Exploration, Inc. Preprint 92-243, 31 p. Cleland, J.M., Morey, G.B., and McSwiggen, P.L., 1996, Significance of tourmaline-rich rocks in the North Range Group of the Cuyuna Iron Range, east-central Minnesota: Economic Geology, v. 91, no. 7, p. 1282-1291 Kilgore, C.C., and Thomas, P.R., 1982, Manganese availability-Domestic: U.S. Bureau of Mines Information Circular 8889, 14 p. Morey, G.B., 1990, Geology and manganese resources of the Cuyuna iron range, east-central Minnesota: Minnesota Geological Survey Information Circular 32, 28 p. Morey, G.B., D.L. Southwick, and S.P. Schottler, 1991, “Manganiferous Zones in Early Proterozoic Iron Formation in the Emily District, Cuyuna Range, East Central Minnesota.” Minnesota Geological Survey Report of Investigations 39. 42 pp. Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, Geologic map (scale 1:250,000) of the Penokean orogen, central and eastern Minnesota, and accompanying text: Minnesota Geological Survey Report of Investigations 37, 25 p., 1 pl. 64 �Michigan Geological Survey’s Contributions to the USGS Earth MRI National Mine Waste Inventory Effort PEARSON, Sara1, GAMET, Nolan2, SHALIFOE, Molly 1, QUIGLEY, Ashley2, and MAHIN, Robert2 1 Michigan Geological Survey, Western Michigan University, 5272 W. Michigan Ave. Kalamazoo, MI 49009 2 Michigan Geological Survey, Western Michigan University, 416 Avenue C Gwinn, MI 49841 In the mid-19th century, the discovery of rich copper and iron deposits in Michigan’s Upper Peninsula (U.P.) led to intense mining, resulting in hundreds of abandoned mine waste sites. Both published and unpublished geological literature suggests that some of these legacy mine waste sites have the potential to host critical minerals, such as manganese and graphite, that were previously overlooked during production. The Michigan Geological Survey (MGS) is contributing to the United States Geological Survey’s (USGS) national effort to build a comprehensive national inventory of mine wastes, their compositions, and potential critical minerals. The MGS team has completed an inventory and submitted 120 mine waste sites from 6 counties across the western U.P. to the USGS for a final review and inclusion in the national mine waste database (Figure 1). These 120 sites are further subdivided into 216 individual mine waste features that met the minimum 2,000m2 size requirement. Finalized point and polygon layers for each mine site were accompanied by corresponding geology, resource, and reference attribute tables. The process consisted of creating an ArcGIS Pro project, adding all available mine-related state and federal datasets, LiDAR-derived DEMs (digital elevation models), published maps, and an ArcGIS geodatabase template containing feature classes and related attribute tables required by the USGS. Initial mine waste inventory work focused on searching for and digitizing mine waste features throughout the western U.P that exceeded the 2,000m2 size requirement. The MGS team originally located and digitized 441 mine waste features by utilizing LiDAR-derived, 1-meter DEMs, 2024 ESRI areal imagery, and published geologic maps. This process is depicted by a simplified workflow shown in Figure 2. The mine waste features were then filtered based on their size. Those smaller than 2,000m2 were omitted from the master dataset. Corresponding attribute tables were then populated with data from publicly available literature, websites, state and federal datasets, and information archived in the state’s drill core repositories. The final databases will ultimately comprise the most up-to-date record of the volume, tonnage, grade, and mineralogy of Michigan’s legacy mine waste sites. Future MGS work within the scope of the Earth MRI Mine Waste Cooperative Agreement is a mine waste characterization effort, which aims to sample and evaluate nonfuel mine waste sites that potentially contain critical minerals. This project will begin in 2025 and continue through 2026. 65 �Figure 1. Map displaying all mine waste features inventoried and submitted to the USGS for the fiscal year 2023 Priority 1 funding represented as purple points and polygons. Figure 2. Simplified process to locate and digitize the mine waste features using ArcGIS Pro coupled with online sources. A.) ESRI imagery (Esri, 2024); B.) Bedrock geology of central Dickinson County, MI (James and others, 1961); C.) 1-m QL2 LiDAR DEM model; D.) Digitization of mine waste features. REFERENCES Esri, 2024, World imagery: Esri, https://services.arcgisonline.com/ArcGIS/rest/services/ World_Imagery/MapServer. James, H.L., Clark, L.D., Lamey, C.A., and Pettijohn, F.J., 1961, Geology of central Dickinson County, Michigan, U.S. Geological Survey, Professional Paper 310, 1:24,000. 66 �Critical Mineral Potential of the Northern Margin of the Watersmeet Gneiss Dome, MI USA QUIGLEY, Ashley K.1, MAHIN, Robert A. 1, and GAMET, Nolan G. 1 1 Michigan Geological Survey, Western Michigan University, 416 Avenue C Gwinn, MI 49841 Precambrian gneisses and schists on the northern margin of the Watersmeet Dome in Michigan have been shown to be unusually enriched in rare earth elements, fluorite and incompatible elements including U, Th, Hf, and Zr (Barovich et al., 1991; Sims, 1990). The area is within two Earth Mapping Resources Initiative (EMRI) critical mineral focus areas for IOCG/IOA and magmatic REE deposits (Dicken and others, 2022). To further assess the potential for critical minerals, the Michigan Geological Survey (MGS) is conducting detailed geologic mapping and sampling, as well as collecting geophysical and geochronological data. The project area is roughly 36 square kilometers on the border of Gogebic and Ontonagon Counties and 10 kilometers northwest of the town of Watersmeet, MI. Field work began in July of 2024 with a projected completion date in early 2026. During the 2024 field season, the MGS mapped, described and recorded 620 outcrops in the project area using ArcGIS Field Maps and submitted 124 samples for whole rock and trace element geochemistry. An RS-230 BGO gamma-ray spectrometer was used to take over 600 total gamma (K/U/Th) measurements from outcrop. Additionally, a drone-borne, high resolution magnetic survey was flown over areas where permission was granted by landowners. Preliminary field observations include the presence of fluorite in outcrop spatially associated with magnetic and gamma count anomalies. The results of the lithogeochemistry show a strong spatial correlation between fluorine, uranium, thorium, and total REEs. When rare earth element concentrations were converted to industry standard rare earth oxides (REOs), 14 samples had total rare earth oxide (TREO) values greater than 1,000 ppm (Hellman and Duncan, 2018). Geophysical, analytical, and field data also identified an anomalous magnetic high approximately 500m x 300m associated with previously undescribed REE-bearing, magnetic, fine-grained schists. In 1982, Rocky Mountain Energy (RME) conducted exploration drilling for uranium based on anomalous gamma radiation in outcrop. A reexamination of the core found chalcopyrite in close association with fluorite. The presence of anomalous F, Cu, U, REE and magnetite is suggestive of an IOA/IOCG footprint (Hitzman, 2000). This will be investigated using IOCG discrimination diagrams such as Montreuil and others (2013). Barovich et al. (1991) observed that the elevated REEs, fluorite and incompatible elements were tied to a gneiss and schist unit with an interpreted Paleoproterozoic age between 1.9 and 1.7 Ga, much younger than the Archean aged rock units that make up most of the Watersmeet gneiss dome. Because of the apparent link between rock age and critical minerals, confirming existing ages with modern U-Pb dating techniques, as well as adding ages from new locations, is an important piece of this study. Five samples were submitted for U-Pb geochronology of zircon grains. Results are pending. 67 �REFERENCES Barovich, K.M., Patchett, P.J., Peterman, Z.E., and Sims, P.K., 1991. Neodymium Isotopic Evidence for Early Proterozoic Units in the Watersmeet Gneiss Dome, Northern Michigan. U.S. Geological Survey Bulletin 1904-G: G1-G7. Dicken, C.L., Woodruff, L.G., Hammarstrom, J.M., and Crocker, K.E., 2022, GIS, supplemental data table, and references for focus areas of potential domestic resources of critical minerals and related commodities in the United States and Puerto Rico (ver. 2.0, April 2024): U.S. Geological Survey data release, https://doi.org/10.5066/P9DIZ9N8. Hellman, P.L. and Duncan, R.K., 2018, Evaluating Rare Earth Element Deposits. ASEG Extended Abstracts. 2018. 1. 10.1071/ASEG2018abT4_3E. Hitzman, M.W., 2000, Iron oxide-Cu-Au deposit: What, where, when, and why, in Porter, T.M., ed., Hydrothermal iron oxide copper-gold and related deposits a global perspective: Adelaide, Australian Mineral Foundation, p.9–26. Montreuil J-F., Corriveau L., Grunsky E., 2013. Compositional data analysis of IOCG systems, Great Bear magmatic zone, Canada: To each alteration types its own geochemical signature. Geochem. Explor. Environ. Anal. 13:219–247. Sims, P.K., 1990, Geologic map of Precambrian rocks, Marenisco, Thayer, and Watersmeet 15-minute quadrangles, Gogebic and Ontonagon counties, Michigan, and Vilas County, Wisconsin: U.S. Geological Survey Miscellaneous Investigations Series Map I-2093, scale 1:62,500. 68 �Plume control on the initiation of Mid-Continent Rift breakup using Unconformities: Implications for the Tectono-magmatic evolution and mineral deposits ROHRMAN, Max1 1 DECAN Geosolutions, PO Box 131148, Houston, TX 77219 Regional unconformities from the stratigraphic record interpreted on existing Multi Channel Seismic (MCS) data obtained by Grant Norpac/Argonne (red numbered) and the GLIMPCE program (red lettered) (Figure 1A), are used for temporal and spatial control on MidContinent Rift (MCR) evolution. This allows identification of key events in the evolution of the rift, whereas potential field data, seismic refraction and Rayleigh waves, help constrain spatial and quantitative constraints. Based on magmatic stage definition, two regional unconformities were interpreted from MCS data: MU (Magmatic Unconformity), at the top of the Main stage (~ 1100 – 1089 Ma), signaling the end of major flood basalt magmatism, and BU (Breakup Unconformity) representing the Latent stage (~ 1104 – 1100 Ma). The latter is observed as a sequence at Mamainse Point (Figure 1), rather than an unconformity, stressing the importance of spatial control on events. Magmatic crustal thicknesses and lower crustal seismic velocities obtained from MCS and refraction data (Shay and Trehu, 1993) are used to constrain relative importance of important parameters in melt production, such as: potential temperature, active mantle upwelling and lithospheric thinning. Together, these data suggest that the MCR originated from an earlier NW-SE pre- or proto-rift (blue, Figure 2A) recognized from outcrop (Figure 1A) and MCS, further reconstructed by aligning Archean granitic blocks such as White Ridge (WR), Grand Marais (GM) and Wawa-Abitibi (purple, WA) from gravity lows (Figure 1B, 2A). The area was affected by a plume constrained by a Rayleigh Wave Low Velocity Anomaly (RWLVA) (Foster et al., 2020) (Figure 1A). This generated uplift in central Lake Superior focused on a region around the Coldwell Complex (Figure 1A). Subsequently, Earlystage (~ 1110 - 1104 Ma) magmatism in the proto-rift generated by NE-SW extension along strike slip faults such as the Thiel Fault (TF) (Figure 1A), in the central and eastern arm of the MCR. By the end of the Early-stage, the plume was deeply embedded in the lithosphere and initiated the start of a thick N-S crustal ridge or proto-hotspot track in central Lake Superior during the late Early- to Latent stage (Figure 2B,C). After a break in activity recorded by the Breakup Unconformity (BU), the plume moved relatively southward during the Main-stage and possibly influenced stress re-orientation to N-S (Figure 2D). This locked the eastern arm and locally, new thick oceanic crust formed along the syncline in central Lake Superior, generating the western rift arm. However, magmatism and breakup terminated shortly after as a result of Grenvillian compression, evidenced by the Magmatic Unconformity (MU). During the Main stage, active upwelling and anomalously thick oceanic crust formation was highest on the crustal ridge (black dash-dot line, Figure 2D), measured at line A, just north of the WA block (purple arrow, Figure 2D) and decreasing toward line C (purple arrow). Further west, at St Croix, upwelling rates approach unity and no oceanic crust formation took place. Pulsing and waning of the plume stem/conduit through time (Figure 2) is recorded in the unconformities, suggesting a drop in potential temperature and upwelling rate around BU time (Latent stage) (Figure 2C). 69 �Figure 1: A. Geological map with seismic lines (red). Numbering refers to onshore geological sections. B. Gravity map. Abbreviations: MB Marquette Basin, KP Keweenaw Peninsula, HVB High Velocity Body. Figure 2: Tectono-magmatic evolution. South shore (between yellow cubes) is mobile, North shore is kept fixed. EPC Early Plume center, LPC Latent Plume Center, MPC Main Plume Center. REFERENCES Foster, A., Darbyshire, F., and Schaeffer, A., 2020. Anisotropic structure of the central North American Craton surrounding the Mid-Continent Rift: evidence from Rayleigh waves. Precambrian Research, 342: 105662. Shay, J., and Trehu, A., 1993. Crustal structure of the central graben of the Midcontinent Rift beneath Lake Superior. Tectonophysics, 225: 301-335. 70 �Constraining the timing of crustal exhumation following the Penokean orogeny using U-Pb, Sm-Nd, and Lu-Hf geochronology and microstructural analysis SALERNO, R.,1 CANNON, W.F.,1 SOUDERS, A.,2 THOMPSON, J. M.,2 VERVOORT, J.,3 1 U.S. Geological Survey, Reston, VA 20192, 2U.S. Geological Survey, Denver, CO 80225, 3Washington State University, Pullman, WA 99164. Precambrian terranes in the Lake Superior region have complex igneous, metamorphic, and deformational histories spanning the Eoarchean to the Neoproterozoic. In this sequence, the Penokean orogeny (1880–1830 Ma) is the first collisional event in a long-lived subduction system on Laurentia’s southern margin, marking a transition in the style of Laurentian assembly from the amalgamation of disparate Archean cratons to growth by accretion of juvenile arcs. The metamorphic and structural history of the corridor of Archean gneiss domes south of Lake Superior is typically attributed to the Penokean orogeny. However, recent 40Ar/39Ar geochronology calls this relationship into question as ~1760 Ma cooling ages across the region indicate the deformation and metamorphism coincident with dome uplift is markedly younger (Schneider et al., 2004; Tinkham and Marshak, 2004; Holm et al., 2005; Schulz and Cannon, 2007). To correctly distinguish the effects of the Penokean orogeny and more accurately reconstruct the Paleoproterozoic tectonic history of the Upper Midwest, we present new U-Pb, Sm-Nd, and LuHf geochronology and microstructural analyses for a suite of metamorphosed and deformed rocks within and adjacent to several gneiss domes (Fig. 1). Titanite U-Pb ages and trace element compositions reflect Archean metamorphism at 2550 ± 46 Ma (2SE), and variable degrees of recrystallization in the Paleoproterozoic (Fig. 2). Apatite and monazite U-Pb ages, along with garnet Lu-Hf ages of metamorphosed supracrustal rocks directly outside of domes, record the onset of peak conditions by 1837 ± 7 Ma that continued beyond the end of the Penokean orogeny until 1782 ± 15 Ma. The garnet Sm-Nd ages of several samples are ~70 Ma younger than the Lu-Hf ages, reflecting a period of cooling and exhumation between 1752 ± 10 and 1738 ± 9 Ma. This exhumation interval overlaps with the U-Pb ages of synkinematic titanite at 1713 ± 32 Ma and the 1750 ± 6 Ma Lu-Hf age of re-equilibrated pre-kinematic garnets. U-Pb ages of apatite in one sample reflect much later reheating of the system at 1592 ± 26 Ma. These data show that deformation and metamorphism related to the uplift of gneiss domes in the Lake Superior region can only be partially linked to tectonic events between 1880–1830 Ma. Peak metamorphic conditions lasting until 1782 Ma indicate the persistence of thick orogenic crust well after the end of the Penokean orogeny—perhaps supported by continued convergence or an unrecognized collisional event along the margin. Exhumation beginning at 1752 Ma coincided with subduction farther south during the Yavapai orogeny (1760-1720 Ma), whereas uplift may be related to crustal extension above the downgoing slab, aided in part by gravitational forces acting on overthickened crust. Extension during this time would also have played a role in the generation and spatial accommodation of Yavapai-age granite intrusions across the region (e.g., East-Central Minnesota batholith). The youngest apatite U-Pb age at 1592 Ma likely represents distal thermal effects of the Mazatzal orogeny (1650–1600 Ma) farther south. These data reveal the gneiss dome structures in the Upper Midwest are the result of a protracted history including several Paleoproterozoic metamorphic, deformational, and uplift events spanning more than 70 m.y.. 71 �Figure 1: Left, geologic map showing gneiss domes in northern Michigan with geochronology sample sites. Cities shown – Marquette (M), Watersmeet (W), Republic (R), and Hardwood (H). Modified from Tinkham and Marshak (2004). Figure 2: Right, ages at 2SE precision. Vertical bars represent the timing of the Sacred Heart (S), Penokean (P), Yavapai (Y), and Mazatzal (M) orogenies. Hatched fields represent durations of metamorphic prograde and cooling intervals. Sm-Nd ages of UPMI 10 23 and UPMI 8 23 have high uncertainties from mineral inclusions that could not be removed prior to analyses and therefore are not used to define the duration of the cooling interval. Diagrams below show the Archean-Mesoproterozoic tectonic evolution of southern Laurentia. Yellow star shows study area location. REFERENCES Holm. D., Van Schmus, W., MacNeill, L., Boerboom, T., Schweitzer, D., Schneider, D., 2005, U-Pb zircon geochronology of Paleoproterozoic plutons from the northern mid-continent, USA: Evidence for subduction flip and continued convergence after geon 18 Penokean orogenesis: Geol. Soc. Am. Bull. 117, 259-275. Schneider. S., Holm. D., O’Boyle. C., Hamilton. M., Jercinovic. M., 2004, Paleoproterozoic development of a gneiss dome corridor in the southern Lake Superior region, USA: GSA Special Paper 380, 339357. Schulz. K., Cannon. W., 2007, The Penokean orogeny in the Lake Superior Region: Precambrian Research, 157, 4-5. Tinkham. D., Marshak. S., 2004, Precambrian dome and keel structure in the Penokean orogenic belt of northern Michigan, USA: GSA Special Paper 380, 321-338. 72 �Identifying Abandoned Mine Surficial Features Using Mask R-CNN, Upper Peninsula Michigan. SHALIFOE, MaryElizabeth1, VOICE, Peter1 1 Department of Geological and Environmental Sciences and Michigan Geological Survey, Western Michigan University, 1903 W Michigan Ave, Kalamazoo MI, 49008-5241, USA From the 1840s to the 1980s, iron, and copper mining in Michigan's Upper Peninsula thrived, leaving behind numerous surficial features from the early underground mining practices. Even today the Eagle Mine located in Marquette County is still active Mining both copper and nickel. Today's demand for rare earth minerals has sparked interest in exploring locations near these primary ores including the tailing piles (Demas A., 2023). Mapping old mine features using optical satellite imagery is challenging in Michigan's Upper Peninsula due to dense vegetation and snow cover – instead we need to use techniques that allow us to see through this cover. This study aims to assess the performance of object detection Deep Learning Models (DLMs) in mapping potential mine features using high-resolution terrain data (LIDAR-derived 1meter Digital Elevation Models) produced through the 3D Elevation Program. Dickinson County was chosen as the study area due to its rich history of 52 known abandoned mines within the East Menominee Iron Range (Figure 1). This study targeted various features, such as prospect pits, open pits, lateral ditches, and waste piles, resulting in a total of 946 identified features used for training the DLMs. The object detection methods available within ArcGIS software were evaluated including Feature Classifier, Faster R-CNN, and Mask R-CNN. Our initial evaluation has shown that Mask R-CNN performed better than the other methods, due to the Mask R-CNN method that enables pixel-level segmentation in addition to object detection (Maxwell A. E., et al., 2020. Our ongoing work is focused on the refinement of the model parameters to better locate surface features related to historic mining. Once the model is completed, it will be tested on various locations in northern Michigan within the mining ranges of the Marquette Iron Range, Menominee Iron Range, Gogebic Iron Range, and the Copper Ranges within Ontonagon and through the Keweenaw. This will then be ground-truthed, by going out into the field to verify the locations of the features or using historical topographic maps to verify the existence of features that may be inaccessible. REFERENCES Department of Environment, Great Lakes, and Energy, (2024) EGLE Geowebface; mining and minerals, State of Michigan, https://www.egle.state.mi.us/geowebface/#btnToolNavInfo Demas A. (2023). Bipartisan Infrastructure Law Funds Geologic Mapping in Michigan, by Bipartisan Infrastructure Law Investments, USGS, https://www.usgs.gov/special-topics/bipartisaninfrastructure-law-investments/news/bipartisan-infrastructure-law-funds-6 Maxwell, A. E., Pourmohammadi, P., & Poyner, J. D. (2020). Mapping the Topographic Features of Mining-Related Valley Fills Using Mask R-CNN Deep Learning and Digital Elevation Data. Remote Sensing, 12(3), 547. https://doi.org/10.3390/rs12030547 73 �Figure 1: Study Area in Dickinson County, showing the distribution of underground mines (Department of Environment, Great Lakes, and Energy, 2024). Figure 2: Mine Features located near East Central Vulcan Mine in Dickinson County, DEM sourced USGS TNM, 2016. (Department of Environment, Great Lakes, and Energy, 2024). 74 �Basaltic rocks of the Animikie Group in Ontario: Geochemical characteristics and tectonic significance SMYK, Mark1,3, HOLLINGS, Pete1, METSARANTA, Riku2, CUNDARI, Robert3, KISSIN, Stephen1 and KURCINKA, Colleen3 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada 2 Ontario Geological Survey, Ministry of Mines, 933 Ramsey Lake Road, Sudbury, ON P3E 6B5 Canada 3 Ontario Geological Survey, Ministry of Mines, 435 James St. South, Thunder Bay, ON P7E 6S7 Canada The Paleoproterozoic Animikie Group in Ontario records a history of continental sedimentation and minor volcanism on the southern margin of the Superior Craton between ca. 1.88 Ga and 1.82 Ga. Both the chemical sedimentary rock-dominated Gunflint Formation and overlying, siliciclastic sedimentary rock-dominated Rove Formation contain significant intervals of tuffs and basalt flows. Copper-bearing amygdaloidal basalts were noted in Crooks and Blake townships (Coleman 1900); basalt flows and tuffs were identified by Gill (1925) and Goodwin (1960) in the Mink Mountain area, and by Tanton (1931) in Oliver Township. Tanton (1936) mapped “Rove basalt” in Devon Township. In 2022, a 774 m diamond drill hole (DDH ST-2201), completed by Metal Energy Corp. in Hartington Township, provided a complete section from Rove Formation into Archean basement. New geochemical, petrographic and stratigraphic data gleaned from this drill core and recent field work have provided insights into the nature of the basaltic rocks. The lowermost volcanic unit occurs in the middle of the Gunflint Formation, exposed near Mink Mountain; its base is ~53 m above Archean basement. Approximately 21 m thick, it consists of several distinctive, typically massive, locally pillowed, vesicular/spherulitic basalt flows. An isolated outcrop of amygdaloidal basalt in Oliver Township, approximately 40 km northeast of Mink Mountain, shares similar petrographic characteristics, stratigraphic position and geochemistry. Limited geochemical data gleaned from amygdaloidal basalts in Crooks Township are similar to those of the aforementioned Gunflint lavas. Further work is required to elucidate the nature and stratigraphic position of these flows. Basaltic flows, exposed on top of Rove shales and wackes in Devon Township (Cundari, 2010) had recently been considered part of the Mesoproterozoic Midcontinent Rift, based mainly on a Keweenawan reversed paleomagnetic mean direction and equivocal stratigraphic constraints (Cundari et al., 2012). However, a mafic interval, approximately 510 m above Archean basement and ~4 m thick, occurs within DDH ST-22-01 and displays a variolitic, chilled basal contact and spherulitic, vesicle-like features, similar to those displayed by the lowermost Devon flows. Similar trace element geochemistry further supports the contention that the mafic rocks intersected in drilling may be correlative with the Devon basalts and with other, similar rocks exposed in an isolated outcrop in Hardwick Township, ~30 km northwest of the Devon basalts. The Gunflint basalts are characterized by moderate La/SmCN ratios (~1.9 to 3.9), negative Nb-Ta and Ti anomalies and relatively flat Gd/YbCN ratios (~1.3 to 1.7). The Devon basalts are characterized by moderate La/SmCN ratios (~2.8 to 3.5), negative Nb and Ti anomalies and moderate Gd/YbCN ratios (~3.0-3.6). In a Penokean tectonic context, the Gunflint basalts may represent limited back-arc volcanism (cf. Kissin and Fralick, 1994), contemporaneous with the older phase of volcanism in the Pembine domain of the Pembine-Wausau terrane (PWT; ca. 1875 Ma, Zi et al. 2022). The Devon basalts may represent relatively deeply sourced, crustally contaminated, OIB-like magmas generated after ca. 1840 Ma, at the same time as renewed volcanism in the PWT. 75 �REFERENCES Coleman, A.P. 1900. Copper and iron regions of Ontario; in Ninth Report of the Bureau of Mines, 1900; Ontario Bureau of Mines, Annual Report, pp.143-191. Cundari, R. 2010. Geology and Geochemistry of the Devon volcanics, south of Thunder Bay, Ontario; unpublished HBSc. thesis, Lakehead University, Thunder Bay, 68p. Cundari R., Piispa, E., Smirnov, A.V., Pesonen, L.J., Hollings P. and Smyk, M. 2012. Geochemistry and paleomagnetism of the Devon township basalt, Ontario, Canada; in Mertanen, S., Pesonen, L. J. and Sangchan, P. (eds.). Supercontinent Symposium 2012 – Programme and Abstracts; Geological Survey of Finland, Espoo, Finland, p.30-31. Gill, J. E. 1925. Gunflint iron-bearing formation; Geological Survey of Canada, Summary Report 1924, pt.C, pp.28-88; https://doi.org/10.4095/103167. Goodwin, A.M. 1960. Gunflint iron formation of the Whitefish Lake area; Ontario Department of Mines, Annual Report, vol.69, pt.7, pp.41-63. Kissin, S.A. and Fralick, P.W. 1994. Early Proterozoic volcanics of the Animikie Group, Ontario and Michigan, and their tectonic significance; 40th annual Institute on Lake Superior Geology, Houghton, MI, Proceedings, vol.40, pp.18-19. Tanton, T.L. 1936. Pigeon River area, Thunder Bay District; Geological Survey of Canada, Map 354A, sheet 1, scale 1:63 360; https://doi.org/10.4095/107549. Tanton, T. L. 1931. Fort William and Port Arthur, and Thunder Cape map areas, Thunder Bay District, Ontario; Geological Survey of Canada, Memoir, 167, 222. https://doi.org/10.4095/100799. Zi, J.-W., Sheppard, S., Muhling, J.R. and Rasmussen, B. 2021. Refining the Paleoproterozoic tectonothermal history of the Penokean Orogen: New U-Pb age constraints from the PembineWausau terrane, Wisconsin, USA; GSA Bulletin; March/April 2022; v. 134; no. 3/4; p. 776–790; https://doi.org/10.1130/B36114.1; 8 figures; 1 supplemental file. published online 1 July 2021. 76 �Sedimentologic and geochemical evidence of marine incursion to the Oronto Group basin, southern Lake Superior region, at ca. 1.08 Ga STEWART, Esther K.1, 2, TAPPA, Michael 1, BAUER, Ann1, BRENGMAN, Latisha3, and PRAVE, Anthony 4 1 Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin 53705 2 Wisconsin Geological and Natural History Survey, University of Wisconsin-Madison Division of Extension, Madison, Wisconsin 53705 3 Department of Earth and Environmental Sciences, University of Minnesota-Duluth, Duluth, Minnesota 55812 4 School of Earth and Environmental Sciences, University of St. Andrews KY16 9TS, Scotland/UK The late Mesoproterozoic Oronto Group (Copper Harbor Conglomerate, Nonesuch, and Freda Formations), Wisconsin and Michigan, preserves a continuous record of depositional environment and related microbial habitat. Over three kilometers of siliciclastic sediments with minor authigenic, molar tooth calcite record physical and biogeochemical processes acting within the Oronto Group basin at the time of deposition and early diagenesis. Combined sedimentologic and geochemical evidence motivates reevaluation and refinement of evolving depositional conditions (Stewart, 2025). Sedimentary facies indicate a shallow marine, tidal influence on deposition, requiring marine incursion to the Laurentian interior at ca. 1.08 Ga (Stewart et al., 2024). The degree of marine connectivity is investigated using C, O, and Rb-Sr isotope compositions of calcite microspar in molar tooth structures and carbonate laminae of the Nonesuch Formation. Molar tooth structures and laminae were milled from thick sections, and one split of sample powder was analyzed for C and O isotopes while the other underwent a multistep chemical separation process to isolate Rb-Sr isotopes from calcite. Carbon isotope (δ13C) values (-3.9 to -2.0‰) of earliest diagenetic calcite reflect organic matter remineralization driven by in situ microbial carbon cycling (e.g. Gilleaudeau and Kah, 2013). Values of δ18O (-6.7 to -3.6‰) measured in the calcite microspar of molar tooth structures overlap the isotopic signature of marine carbonates from other late Mesoproterozoic evaporative marine basins (e.g. Kah, 2000). The 87Sr/86Sr of least-altered calcite (~0.7068 to 0.7069) reflects marine mixing with continental runoff. Combined, these data reflect deposition within a restricted-marine epeiric setting. In addition to isotopic evidence for marine connectivity, conditions of salinity, redox, and productivity are evaluated using whole rock geochemistry of fine-grained siliciclastics and rare earth element + yttrium (REY) distributions of calcite microspar. Whole rock geochemistry was compiled from published sources and new data was collected from two cores in Wisconsin. Calcite REY distributions were analyzed from aliquots of the same sample material processed for Rb-Sr isotopes. Shale geochemistry, including Mo and U enrichment and stratigraphic trends in proxies for detrital input (Zr, Al), redox (S, TOC) and productivity (Ba, P) reveal deposition within an oxidized basin with a deep, fluctuating chemocline and expansion of anoxic and euxinic conditions during maximum flooding and base level lowstand. REY distributions of calcite microspar preserve an early diagenetic estuarine signal characterized by muted, positive La and Y/Ho anomalies and heavy REE enrichment. Shale geochemistry and carbonate REY distributions bring into focus the prevalence of particle shuttling between the water column and shallow sediments that likely enhanced and focused nutrient P bioavailability, analogous to modern estuarine nutrient cycling. Collectively, these data provide a richer understanding of late Mesoproterozoic environmental conditions that influenced early eukaryote ecology. 77 �Figure 1: Images from core (A, C-D, F) and thin section (B, E) of the Nonesuch Formation highlighting sedimentary structures indicative of tidal influence and molar tooth calcite microspar targeted for geochemistry. A & C: photos and line drawings showing close association of fine-grained sandstone (light color) and shale (dark color). Note mud drapes on bi-directional ripple laminae (red arrows, A), flame structures (red arrow, C), and structureless mud layers indicative of fluid mud deposits. B: Photomicrograph (cross-polarized light) showing bedding deflecting around molar tooth structure (arrow) and brittle deformation of molar tooth structures (1 displaced from 2). D: molar tooth structure (MT) cross-cutting carbonate-rich layers (CR) in drill core. E: Photomicrograph (plane-polarized light) highlighting characteristic molar tooth microspar texture. F: Core photo showing ~2 cm diameter mud ball with subangular rhyolite clast at its core. Scale bars are 1 cm unless otherwise noted. REFERENCES Gilleaudeau, G. J., and Kah, L. C., 2013. Carbon isotope records in a Mesoproterozoic epicratonic sea: carbon cycling in a low-oxygen world. Precambrian Research, 228, 85-101. Kah, L. C., 2000. Depositional δ18O signatures in Proterozoic dolostones: constraints on seawater chemistry and early diagenesis. SEPM Special Publication 67, 346 – 360. Stewart, E. K., Bauer, A. M., and Prave, A. R., 2024. End-Mesoproterozoic (ca. 1.08 Ga) epeiric seaway of the Nonesuch Formation, Wisconsin and Michigan, USA. Geological Society of America Bulletin, 136, 7-8, 2940-2960. https://doi.org/10.1130/B37060.1 Stewart, E.K., 2025. Sedimentologic and geochemical markers of marine incursion to the interior Laurentian Oronto Group basin at ca. 1.08 Ga. Ph.D. dissertation, University of WisconsinMadison. 78 �Midcontinent Rift extension ceased and the rift inverted due to the Grenvillian orogeny 1 2 3 SWANSON-HYSELL, Nicholas , HODGIN, Eben B. , ALEMU, Tadesse , FUENTES, 4 2 5 4 Anthony , ZHANG, Yiming , SLOTZNICK, Sarah and FAIRCHILD, Luke 1 Department of Earth and Environmental Sciences, University of Minnesota, Minneapolis, MN, USA Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USA 3 Department of Geology and Environmental Science, University of Wisconsin, Eau Claire, WI, USA 4 Department of Earth and Planetary Science, University of California, Berkeley, CA, USA 5 Department of Earth Sciences, Dartmouth College, Hanover, NH, USA 2 The cessation of rifting within the Midcontinent Rift was a key event in the evolution of the Lake Superior region. If rifting had continued and led to the formation of an ocean basin, the subsequent geologic and paleogeographic history would have been profoundly different. In a 1994 paper, Bill Cannon used emerging geochronology from the Midcontinent Rift and the Grenville orogen to conclude that the closing of the Midcontinent Rift was a far-field effect of compression associated with the Grenvillian orogeny (Cannon, 1994). An alternative proposal was put forward by Stein et al. (2014) who proposed that the Midcontinent Rift is an abandoned rift segment associated with successful rifting along Laurentia’s margin. In this contribution, we leverage improved chronostratigraphy within the volcanics and sedimentary rocks of the Midcontinent Rift (e.g. Fairchild et al., 2017; Hodgin et al., 2024) combined with rich new records of metamorphic chronology associated with the Grenvillian orogeny (reviewed in Swanson-Hysell et al., 2023) to revisit this question and gain fresh insight. The transition from active rift extension to post-rift thermal subsidence is recorded by the Brownstone Falls angular unconformity in northern Wisconsin. The thinning of the Copper Harbor Conglomerate from >2,200 m thick on the Keweenaw Peninsula of Michigan to pinching out against the unconformity implies topographic relief at the onset of post-rift sedimentation that is comparable to that in the modern-day East African rift. The end of active extension (ca. 1090 to 1085 Ma) is coincident with early prograde metamorphism associated with the Grenvillian orogeny, whose metamorphic imprint extends from the Blue Ridge inliers of the eastern US up through the Grenville Province of eastern Canada. This timing is consistent with the onset of continent-continent collision resulting in the cessation of extension in the rift. Figure 1: The start and end of Midcontinent Rift extension compared with U-Pb dates from Grenville Province metamorphic chronometers (blue diamonds: zircon; red pentagons: monazite). The rift developed during an interval of tectonic quiescence on the margin. Extension ceased with the onset of the Grenvillian orogeny and the rift contractionally inverted during the peak of the Ottawan stage. 79 �Following the end of Midcontinent Rift extension, deposition of the Oronto Group continued until ca. 1045 Ma (Hodgin et al., 2024; Fuentes et al, in review). This deposition resulted from post-rift thermal subsidence prior to contractional deformation associated with the Grenvillian orogeny propagating into the continental interior. Paleomagnetic records from the Oronto Group, including recently published data from the Nonesuch Formation (Slotznick et al., 2024) and new unpublished data from the upper Freda Formation, reveal that Laurentia’s plate motion dramatically slowed coincident with the onset of Grenvillian orogenesis. Preceding rapid motion was associated with ocean basin closure leading up to continent-continent collision that changed the force balance and slowed the plate. Oronto Group deposition ended when contractional deformation associated with the Grenvillian orogeny propagated into the Midcontinent. This deformation occurred in two phases with major exhumation occurring during the peak of the Ottawan phase of the Grenvillian orogeny and a second more minor phase of ca. 1000 to 980 Ma contraction associated with the Rigolet phase (Hodgin et al., 2024). This final interval of contraction is associated with the ca. 990 Ma deposition of the Jacobsville-Bayfield Group (Hodgin et al., 2022; Alemu et al., 2023). Following 130 Myr of tectonic excitement from ca. 1110 to 980 Ma, stability returned to Laurentia’s Midcontinent region. While the comings and goings of inland seas and the occasional impact crater have left their mark on the geological record, there has been only very minor tectonism over the past billion years. REFERENCES Alemu, T.B., Hodgin, E.B., and Swanson-Hysell, N.L., 2023. Grooving in the midcontinent: A tectonic origin for the mysterious striations of L’Anse Bay, Michigan, USA. Geosphere, 19(5), 1291–1299. Cannon, W.F., 1994. Closing of the Midcontinent Rift—A far-field effect of Grenvillian compression. Geology, 22(2), 155–158. Fairchild, L.M., Swanson-Hysell, N.L., Ramezani, J., Sprain, C.J., and Bowring, S.A., 2017. The end of Midcontinent Rift magmatism and the paleogeography of Laurentia. Lithosphere, 9(1), 117–133. Hodgin, E.B., Swanson-Hysell, N.L., DeGraff, J.M., Kylander-Clark, A.R.C., Schmitz, M.D., Turner, A.C., Zhang, Y., and Stolper, D.A., 2022. Final inversion of the Midcontinent Rift during the Rigolet Phase of the Grenvillian orogeny. Geology, 50(5), 547–551. Hodgin, E.B., Swanson-Hysell, N.L., Kylander-Clark, A.R.C., Turner, A.C., Stolper, D.A., Ibarra, D.E., Schmitz, M.D., Zhang, Y., Fairchild, L.M., and Fuentes, A.J., 2024. One billion years of stability in the North American Midcontinent following two-stage Grenvillian structural inversion. Tectonics, 43(9). Slotznick, S.P., Swanson-Hysell, N.L., Zhang, Y., Clayton, K.E., Wellman, C.H., Tosca, N.J., and Strother, P.K., 2024. Reconstructing the paleoenvironment of an oxygenated Mesoproterozoic shoreline and its record of life. Geological Society of America Bulletin, 136(3–4), 1628–1642. Stein, C.A., Stein, S., Merino, M., Keller, R.G., Flesch, L.M., and Jurdy, D.M., 2014. Was the Midcontinent Rift part of a successful seafloor-spreading episode? Geophysical Research Letters, 41(5), 1465–1470. Swanson-Hysell, N.L., Rivers, T., and van der Lee, S., 2023. The late Mesoproterozoic to early Neoproterozoic Grenvillian orogeny and the assembly of Rodinia: Turning point in the tectonic evolution of Laurentia. In: Whitmeyer, S.J., Kellett, D.A., Tikoff, B., and Williams, M.L. (Eds.), Laurentia: Turning Points in the Evolution of a Continent. Geological Society of America Memoir 220, 337–356. 80 �Ni-Cu-PGE Mineralization at the Mineral Lake Intrusive Complex, northern Wisconsin THOMPSON, Bekah R. 1, LODGE, Robert W.D.1 1 Department of Geology & Environmental Science, University of Wisconsin-Eau Claire, 105 Garfield Avenue, Eau Claire, WI 54701, USA The Mineral Lake Intrusive Complex (MLIC), near Mellen, Wisconsin, is a 1.1 Ga layered and differentiated mafic intrusive complex within the Mesoproterozoic Mid-Continent Rift in the Lake Superior region (Siefert et al., 1992). This intrusive complex hosts Ni-Cu-PGE mineralization discovered in the 1960’s via electromagnetic geophysical surveys and at least 16 drill holes were completed (Bakheit, 1981). With an increase in demand for domestic critical minerals to supply metals for energy, communication, and military infrastructure, underexplored prospects like the Mineral Lake Ni-Cu-PGE prospect are increasingly important. This project aims to describe the mineralogy of the sulfide inclusions and the host intrusion geochemistry to better understand the geological characteristics of PGE-mineralization within the MLIC. Two drill holes were re-logged (WIS-12 and WIS-11), totaling ~950 linear feet of core, and sixteen samples were collected from representative intrusive phases and mineralization types. Micron-scale PGE-bearing mineral phases are described using the SEM-EDS. Whole rock geochemistry of the MLIC was completed via X-ray Fluorescence (WD-XRF). Silicate and sulfide mineralogy was determined by transmitted and reflected light petrography. Mineralization is hosted in either medium-grained, equigranular olivine gabbro, olivine norite and troctolite phases of the intrusion and are found as mm-scale sulfide segregations composing 1-10% of the rock. Weak foliation and alteration along fractures are observed along brittle-ductile shears resulting in serpentinization of olivine. Contacts between intrusive phases are generally gradational over a few centimeters. Sulfide inclusions contain varying amounts of chalcopyrite, pyrrhotite, and pentlandite and are not obviously correlated with any specific intrusive phase. Graphite, both fracture-associated and matrix-associated, were observed in the Troctolite and Olivine norite phases. Sulfide inclusions are comprised of primarily pyrrhotite with variable amounts of chalcopyrite and pentlandite. Analysis on the SEM-EDS has shown PGE mineralization is commonly hosted as micron-scale inclusions within pyrrhotite and pentlandite. These PGEbearing mineral phases include rhenium-bearing molybdenite (Mo,Re)S2, padmaite (PdBiSe) (found within silicates), argentopentlandite Ag(Fe,Ni)8S8 , sperrylite (PtAs2), rhenite (ReS2), naldrettite (Pd2Sb). PGE minerals are typically ~5 microns. Notably large, 30-micron sperrylite (PtAs2) grains and 25-micron rhenite (ReS2) grains were observed (Figures 1C and 1D). PGE’s are most abundant hosted in sulfide minerals whereas the notable critical elements (Bi, Mo, Sb, Te, Ob, Se) tend to be hosted in the silicates. These results are comparable to other conduit-type and contact-type MCR intrusions, although the age of the MLIC is coeval with contact-type mineralization. Dunka road of the Duluth complex is a contact type Ni-Cu-PGE sulfide deposit. Phases include norite-hosted disseminated sulfides, troctolite-hosted disseminated sulfides, PGE-rich disseminated sulfides, and chalcopyrite rich disseminated sulfides (Theriault and Barnes, 1998). Since the MLIC is a large, differentiated intrusion that is coeval with other contact-type mineralization in the MCR, future exploration efforts and research should focus on the lower parts of the intrusion where dense sulfides may accumulate. 81 �Figure 1. (A) Regional map of Mineral Lake area. Map modified from Cannon and Ottke (1999). Inset map from Mudrey & Brown (1982). (B) Rhenium-bearing molybdenite (Mo,Re)S2 (white) under SEMEDS, (C) Rhenite (ReS2) (white) under SEM-EDS, (D) Sperrylite (PtAs2) under SEM-EDS (white). REFERENCES Bakheit, A.K., 1981. Petrography of Cu-Ni mineralization in the Mineral Lake area, Ashland County, Wisconsin. Unpublished M.S. thesis, University of Wisconsin-Madison. Cannon, W.F. and Ottke, D., 1999. Preliminary digital geologic map of the Penokean (Early Proterozoic) continental margin in northern Michigan and Wisconsin (No. 99-547). The Geological Survey of America. Middlemost EAK (1994) Naming materials in the magma/igneous rocks system. Earth Sci Rev 37:215– 224. doi:10.1016/0012-8252(94)90029-9 Siefert, K.E., Peterman, Z.E., Thieben, S.E. 1992. Possible crustal contamination of the Midcontinent Rift igneous rocks: examples from the Mineral Lake intrusions, Wisconsin. Canadian Journal of Earth Science, 29. 1140-1153. Thériault, R.D., Barnes, S.-J., 1998. Compositional variations in Cu-Ni-PGE sulfides of the Dunka Road deposit, Duluth complex, Minnesota: the importance of combined assimilation and magmatic processes. Can. Mineral. 36, 869–886 82 �A Porphyry in a Rift? Constraining the Petrogenesis of the Jogran Porphyry, Mamainse Point, Ontario, Canada: Insights from Zircon and Melt Inclusion Geochemistry. TOLLEY, James1, HANLEY, Jacob2, CROWLEY, James3, TSAY Sasha4, ZAJACZ Zoltan4, and HOLLINGS, Pete1 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada. Department of Geology, Saint Mary’s University, 923 Robie Street, Halifax, Nova Scotia, B3L 2Y5, Canada. 3 Isotope Geology Lab, Department of Geosciences, Boise State University, 1910 University Drive, Boise, Idaho, 83725-1535, USA. 4 Department of Earth Sciences, University of Geneva, Rue des Maraichers 13, Geneva, 1205, Switzerland. 2 The Jogran quartz-monzonite porphyry, located near Mamainse Point, Ontario, Canada, on the northeastern shoulder of the ~1.1 Ga Midcontinent Rift System (MRS), hosts unique porphyrystyle Cu-(Mo) mineralization in an intra-plate, rift-related large igneous province setting (Perelló et al., 2020). Combining high precision zircon geochronology with zircon and melt inclusion (MI) geochemistry refines the timing of emplacement and offers constraints on the crystallization temperature, oxygen fugacity (fO2), and melt composition (including ore metal tenor) during the magmatic evolution of the deposit. A new high precision 206Pb/238U zircon age of 1090.90 ± 1.27 Ma (CA-TIMS) constrains the formation of the Jogran porphyry to the waning of the main Rift Stage (1102-1090 Ma) and synchronous with the transition to the Late-Rift stage (1090-1083 Ma), as defined by Woodruff et al., (2020). Zircon geothermometry (Crisp et al., 2023) and oxybarometry (Loucks et al., 2020) suggest crystallisation conditions of 900-670 °C and a fO2 range of ∆FMQ = -1.3 to +0.6. As temperatures decrease, ΔFMQ values increase along a trend subparallel to the SO₂-H₂S buffer. The presence of sulfide inclusions in zircon, confirms sulfide saturation during crystallization. The analysed zircon crystals are zoned. They display an increase in [Yb/Gd]n ratios (1220) and concomitant depletion in Th/U (1.0-0.4) in the rims relative to the cores ([Yb/Gd]n = <12; [Th/U] = >1). This zonation infers that the parental magma underwent a single stage of fractionation and crystallisation upon emplacement. Melt inclusions (MIs) range in composition from 65-70 wt.% SiO₂ with 5.5-8.3 wt.% K₂O and K₂O/Na₂O ratios of ~1.5-3.5, suggesting the parental melt was alkalic to shoshonitic. Low Cs concentrations, coupled with high Rb, Ba, and Nb, in MIs indicate minimal crystal fractionation of a near-primitive, mantle-derived composition. In contrast, whole-rock data show lower alkali contents (4.0 wt.% K2O) and have a subalkalic affinity, suggesting crustal contamination or alteration obscured the primitive magmatic signature. A new, precise U-Pb zircon age constrains the felsic magmatism and porphyry-style mineralization at Jogran to the period of maximum lithospheric weakening/crustal thinning during the shift from extensional tectonics to thermal subsidence in the late stages of the MRS. This study suggests that early partitioning of metals and sulfur into magmatic fluids played a key role in ore formation. However, the conditions required remain ambiguous, as the tectonic environment at Jogran differs markedly from the subduction-related settings upon which most porphyry models are based. Porphyry deposits are increasingly recognised across a broader range of tectonic settings (e.g., southeast China [Richards, 2021]; and central Europe [Drew, 2006]). Jogran highlights the potential for porphyry-style mineralisation in non-subduction tectonic contexts and underscores the need to better understand metallogenic pathways beyond the traditional subduction models. 83 �Quartz-Feldspar Porphyry (K-Ar) 1 Tribag Breccia (K-Ar) 2 Mamainse Point Rhyolites (Rb-Sr) 3 Mamainse Point Volcanics (U-Pb) 4 Mamainse Point Tuff (U-Pb) 5 Jogran Porphyry Error bars represent the reported uncertainties in respective studies. Satellite Mineralisation (Re-Os) 6 Porphyry Stock Mineralisation (Re-Os) 6 References 1 Norman and Sawkins (1985) 2 Roscoe (1965) 3 Van Schmus (1971) 4 Davies et al. (1995) 5 Swanson-Hysell et al. (2014) 6 Perelló et al. (2020) Figure 1: New U-Pb zircon age (1090.90 ± 1.27 Ma) for the Jogran porphyry (diamond), published age data (circles) and MRS stages defined by Woodruff et al., (2020) – Early (green), 1109–1104 Ma; Latent (orange), 1104–1098 Ma; Main (blue), 1098–1090 Ma; and Late (purple), 1090–1083 Ma. REFERENCES Drew, L.J. (2005). A tectonic model for the spatial occurrence of porphyry copper and polymetallic vein deposits - Applications to central Europe: U.S. Geological Survey Scientific Investigations Report 2005-5272. Crisp, L. J., Berry, A. J., Burnham, A. D., Miller, L. A. & Newville, M. (2023). The Ti-in-zircon thermometer revised: The effect of pressure on the Ti site in zircon. Geochimica et Cosmochimica Acta 360, 241–258. Loucks, R. R., Fiorentini, M. L. & Henríquez, G. J. (2020). New magmatic oxybarometer using trace elements in zircon. Journal of Petrology, 61, egaa034. Perelló, J., Sillitoe, R. H. & Creaser, R. A. (2020). Mesoproterozoic porphyry copper mineralization at Mamainse Point, Ontario, Canada in the context of Midcontinent rift metallogeny. Ore Geology Reviews, 127, 103831. Richards, J.P, (2021). Porphyry copper deposit formation in arcs: What are the odds? Geosphere, 18, 130– 155. Woodruff, L. G., Schulz, K. J., Nicholson, S. W., and Dicken, C. L. (2020). Mineral deposits of the Mesoproterozoic Midcontinent Rift system in the Lake Superior region - A space and time classification. Ore Geology Reviews, 126, 103716. 84 �Evaluating Ni in Olivine as a Prospectivity Indicator for Magmatic Ni-Cu-(PGE) Deposits: A Preliminary Study from the Midcontinent Rift System. TOLLEY, James1, HOLLINGS, Pete1, MEXIA DURAN, Kevin1 and HARDING, Myles1 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1, Canada. Nickel content of olivine [(Mg,Fe)2SiO4] can serve as an important petrogenetic marker in mafic igneous systems. Nickel’s concentration in olivine is controlled by several factors, including: (1) the Ni content of the parental magma; (2) the partition coefficient of Ni between olivine and the silicate melt; and (3) variable parameters such as temperature, pressure and fO2 of the melt (Li et al., 2007). More recently, Ni content in olivine has been studied as a potential fertility indicator for magmatic Ni-Cu sulfide deposits as well as providing information about the original composition of the magma. Olivine crystallizing from sulfide-saturated magmas will exhibit lower Ni contents relative to olivine crystallized from sulfide-undersaturated melts. This premise was assessed by Barnes et al. (2023) across Ni-Cu-(PGE) deposits globally, but there was a notable paucity of olivine data from Ni-Cu deposits within the Midcontinent Rift System (MRS). This study presents 700 new electron probe microanalyses (EPMA) of Ni and other major elements in olivine from five magmatic Ni-Cu-(PGE) deposits in the MRS: Sunday Lake, Steepledge, Escape, Current and Hele. These data have been integrated with published datasets from the mineralised Seagull, Eagle and East Eagle intrusions to produce the first regional-scale dataset of olivine chemistry from the MRS. Curation of this data aims to assess: (1) the deposit scale variability of olivine chemistry across the MRS; (2) the utility of Ni in olivine as a regional prospectivity indicator for Ni-Cu deposits within the MRS; and (3) the implications for primary melt evolution across the MRS. Preliminary results show that olivine forsterite (Fo) contents (i.e., 100*Mg/[Mg+Fetotal], mol %) range from Fo72.5-85 across most intrusions, except for the Hele intrusion, which has a much wider range (Fo44.0-82.5; Fig. 1). Across the entire dataset, Ni concentrations in olivine vary significantly (600-2500 ppm) and generally increase with higher Fo values. The range of Ni in olivine values can be vary up to 1000 ppm from a single deposit, over a narrow Fo range (e.g., Current Intrusion – Fo79.7-81.7). Furthermore, concentric zoning between Mg-rich cores relative to the Mg-depleted rims is frequently observed – most notably at Eagle East, where an average core analysis displays Fo80 vs. average rim value of Fo77. This preliminary compilation of olivine compositions across the MRS both reveals the variability of olivine compositions within a single intrusive complex and highlights fractionation trends regionally. The integration of the MRS data with the global compilation of Barnes et al. (2023) highlights the similarities between the signatures of unmineralized and mineralized intrusions and that there is no universal evidence for consistent Ni depletion in olivine from mineralised deposits. Placing the MRS olivine data within the context of other Ni-Cu-(PGE) systems may elucidate previously unrecognized potential within the MRS, and similarly these data can contribute to the global understanding of magmatic processes that culminate in economically viable deposits. 85 �Figure 1: Ni concentrations (ppm) in olivine as a function of forsterite content (Fo#) from a suite of maficultramafic Ni-Cu intrusions located in the Midcontinent Rift System. Grey field denotes the global array of ‘barren’ intrusions as defined by Barnes et al. (2023). Published datasets comprise: (1) Eagle and East Eagle Intrusion – Ding et al. (2010); (2) Seagull Intrusion – Heggie (2005); (3) Coldwell, Two Duck Gabbro – Good (1992); (4) Coldwell, Eastern Gabbro – Shaw (1997). REFERENCES Barnes, S. J., Yao, Z. S., Mao, Y. J., Jesus, A. P., Yang, S., Taranovic, V., & Maier, W. D. (2023). Nickel in olivine as an exploration indicator for magmatic Ni-Cu sulfide deposits: A data review and reevaluation. American Mineralogist, 108, 1-17. Ding, X., Li, C., Ripley, E. M., Rossell, D., & Kamo, S. (2010). The Eagle and East Eagle sulfide ore‐ bearing mafic‐ultramafic intrusions in the Midcontinent Rift System, upper Michigan: Geochronology and petrologic evolution. Geochemistry, Geophysics, Geosystems, 11(3). Good, D.J. (1992). Genesis of copper-precious metal sulphide deposits in the Port Coldwell Alkalic Complex, Ontario; unpublished Ph.D. thesis, McMaster University, Hamilton, Ontario, 203p. Heggie, G.J. (2005). Whole rock geochemistry, mineral chemistry, petrology and Pt, Pd mineralization of the Seagull Intrusion, northwestern Ontario. Unpublished M.Sc. thesis, Lakehead University, Thunder Bay, Ontario, 156. Li, C., Naldrett, A. J. & Ripley, E. M. (2007). Controls on the Fo and Ni Contents of Olivine in Sulfidebearing Mafic/Ultramafic Intrusions: Principles, Modeling, and Examples from Voisey’s Bay. Earth Science Frontiers 14, 177–183. Shaw, C. S. (1997). The petrology of the layered gabbro intrusion, eastern gabbro, Coldwell alkaline complex, Northwestern Ontario, Canada: evidence for multiple phases of intrusion in a ring dyke. Lithos, 40(2-4), 243-259. 86 �Origin of magnetic black sand found on the south Shore of Lake Superior Verhoeven, J.D1., and Zowada, Tim2 1 Iowa State University, Emeritus Prof., Iowa State University, Levering MI 49755, jver@iastate.edu, 2 Custom Knifemaker, Boyne Falls, MI, timzowada@gmail.com Many of the beaches on the shores of Lake Superior contain black sand which is magnetic. This sand can be smelted into iron using the ancient bloomery process which produces small chunks of iron called blooms. They consist of iron containing a low level of carbon. The chunk of iron is filled with cavities containing remnant slag produced in the smelting process. Recent experiments [1] have shown that often but not always the resultant iron of the blooms contain significantly levels of Ti and that one of the microconstituents in the slag is the mineral ulvöspinel. The authors of [1] had assumed that the magnetic black sand came from erosion of banded hematite-magnetite iron formations (BIF) which are the source of the iron mined in the Lake Superior region. Finding Ti in some of the blooms shows that there is likely an alternate source of the iron in the black sands, namely the Fe–Ti oxide-bearing ultramafic intrusions (OUIs) deposited in the lake bottom from the 1.1Ga Midcontinent Rift (MCR) that runs through the lake region. This talk presents a comparison of the composition of the ulvöspinel constituent found in bloom slags of black sand smelts with the composition of the ulvöspinel constituents found in a recent study [2] of drillings from the Coldwell Complex region located at the north central region of Lake Superior which contain Fe-Ti magnetite-ilmenite intergrow deposits from the MCR. The results present strong evidence that the some of the magnetic black sand on Lake Superior’s shores comes from source rocks of MCR deposits in the Coldwell Complex and some from BIF deposits in the lake bottom. Additional evidence that the Fe-Ti source rock is the Coldwell Complex is that the location of the black sand used in the study is in the same region of the south shore of Lake Superior near White Fish Point where yooperlite rocks have been found. Literature data [3] shows that the source rock of the yooperlite is the Coldwell Complex. REFERENCES 1 Zowada T., Straszheim W., Chumbley S. and Verhoeven. J.D., 2025. A study of the carbon distribution and alloy composition of iron blooms made from two different batches of black sand collected from Lake Superior, accepted for publication in JMMA. 2 Brzozowski M.J., Samson I.M., Gagnon J.E., Linnen R.L. and Good D.J., 2021. Effects of fluid-induced oxidation on the composition of Fe–Ti oxides in the Eastern Gabbro, Coldwell Complex, Canada: implications for the application of Fe–Ti oxides to petrogenesis and mineral exploration, Mineralium Deposita 56, 601–618. 3 Laughlin, R. and Carlson A., 1987. A new find of fluorescent sodalite, Mineral News 34, no 5. 87 �88 �Zircon Petrochronology of the Eau Claire Volcanic Complex in the Marshfield Terrane of the Penokean Orogen, Northcentral Wisconsin VICKERS, Lyndsie A.1, LODGE, Robert W.D.1 1 Department of Geology and Environmental Sciences, University of Wisconsin-Eau Claire, Eau Claire, WI 54701 USA The Eau Claire Volcanic Complex (ECVC) serves as a type locality for Penokean-age magmatism and volcanism associated with the Marshfield terrane in the Penokean Orogeny (Figure 1A). This volcanic event is central to tectonic models that describe the collision of the Pembine-Wausau oceanic arc terrane and Archean crustal fragments of the Marshfield terrane with the southern margin of the Superior Craton (Shultz and Cannon 2007). A defining feature of these models is the proposed "double" subduction zone system, which is thought to have overprinted the Archean Marshfield terrane with younger Penokean volcanism and magmatism during ocean closure. Newer tectonic models suggesting accordion-like tectonics (Zi et al, 2022) still rely on historic interpretations of the ECVC where only physical outcrop descriptions in the literature (Myers et al, 1980). Despite its significance, the ECVC has remained understudied due to extensive Paleozoic and Quaternary cover which obscures outcrops and little mineral exploration and drilling. To address these challenges, this study focused on remote outcrops of the ECVC along the Eau Claire River in Wisconsin, aiming to better constrain tectonic models and clarify terrane boundaries in the southern Penokean Orogen. Field mapping yielded samples that were processed to isolate zircon grains for U/Pb radiometric dating and petrochronological analyses. These zircons were analyzed using a Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA-ICP-MS) at Laurentian University, providing the only modern geochronological and petrochronological data (U/Pb, Lu/Hf, zircon trace elements) from this region in the orogen. The results challenge long-standing interpretations of the area’s stratigraphy. Rocks previously classified as Paleoproterozoic volcanic units have Archean U/Pb ages and are now redefined as part of an Archean greenstone belt, significantly altering the geological narrative of the region. This study confirmed the presence of Paleoproterozoic intrusions (Figure 1C), but Lu-Hf isotopic analyses revealed that magmas did not have isotopic inheritance from the Archean basement (Figure 1D). This suggests the Paleoproterozoic magmas are in structural contact with Archean rocks. Additionally, Paleoproterozoic metasedimentary samples exhibited a diverse array of sedimentary sources (Figure 1-B), including Penokean, Marshfield, and a 2.2 Ga provenance, hinting at potential links to the Chocolay and Huronian groups which are continental rift assemblages formed during the breakup of an Archean supercontinent (Shultz & Cannon, 2007). As the first comprehensive petrochronological dataset from the Penokean Orogen, this study not only redefines the age and origin of key outcrops but also shows the complexity of the region’s tectonic and magmatic evolution. The discovery of previously unrecognized Archean basement rocks necessitates a reassessment of regional stratigraphy, particularly for classic outcrops historically attributed to Paleoproterozoic activity. Furthermore, the potential connection between the Marshfield terrane’s sedimentary sources and those of the Superior Craton’s rift assemblages raises questions about the terrane’s origins, suggesting it may represent a southernmost fragment of the Superior Craton. 89 �Figure 1: (A) Geologic map of the North Fork of the Eau Claire River adapted from Brown (1988). (B) Histogram displaying the distribution of zircon ages from a metasedimentary sample (C) Weighted mean diagram for intrusive sample showing a uniform range of zircon 207Pb/206Pb ages. Grey bars represent outliers and were excluded from age calculation. (D) ƐHf(i) versus 207Pb/206Pb age comparing ECVC intrusion to other Penokean intrusions in the Marshfield Terrane (Weber et al., 2023). REFERENCES Brown, B.A., 1988. Bedrock Geology Map of Wisconsin (Regional Map Series: West-Central Sheet), University of Wisconsin-Extension Geological and Natural History Survey, Scale: 1:250,000. Schulz K.J., Cannon W.F., 2007. The Penokean orogeny in the Lake Superior region. Precambrian Research 157:4-25. Weber, E.M., Lodge, R.W.D., Marsh, J.H., 2023. U/Pb geochronology and zircon petrochronology of Paleoproterozoic magmas from the Marshfield terrane, Penokean Orogen, Wisconsin. Institute on Lake Superior Geology Proceedings, 69th Annual Meeting, Eau Claire, Wisconsin, Part 1-Program and Abstracts, p. 97-98. Zi, J.W., Sheppard, S., Muhling, J.R., and Rasmussen, B., 2021. Refining the Paleoproterozoic Tectonothermal History of the Penokean Orogen: New U-Pb Age Constraints from the PembineWausau terrane, Wisconsin, USA: GSA Bulletin, v. 134, p. 776–790. Myers, P. E., Cummings, M. L., and Wurdinger, S. R., 1980. Precambrian geology of the Chippewa Valley, Wisconsin, Institute of Lake Superior Geology 26th Annual Meeting, Eau Claire, Wisconsin, Field Trip Guidebook 1, 123 p 90 �Geospatial Learning Resources to Explore Relationships with Keweenaw Geology VYE, Erika1, and LIZZADRO-MCPHERSON, Daniel2 1 Great Lakes Research Center, Michigan Technological University, 1400 Townsend Drive, Houghton, MI, 49931, United States 2 Geospatial Research Facility, Michigan Technological University, 1400 Townsend Drive, Houghton, MI, 49931, United States The globally significant geologic processes and features of the Keweenaw have fostered relationships with land and water for millennia. We have created three geospatial, digital resources that express the deep relationships between the underpinning geology and the scientific, educational, cultural, economic, and aesthetic significance of publicly accessible geosites in the Keweenaw region. These geospatial resources serve as living databases that will evolve over time in order to support formal and informal learners in understanding the fundamental role geology plays in our varied relationships with land and water. All resources are hosted and shared publicly on the Geospatial Research Facilities’ Enterprise Geospatial Research Portal at Michigan Technological University. 1) The Keweenaw Coastal Geoheritage StoryMap was created as a teaching and learning resource for local K-12 educators to explore the rock types of the Keweenaw at geosites along the shores of Lake Superior (Fig. 1). This resource: a) provides an overview of the main lithologies in the Keweenaw region, b) shares where federal, state, local government, and nonprofit organizations are working to preserve the rich geologic landscape and fragile wetlands of the Keweenaw, and c) provides a virtual learning experience to explore over 30 geologically significant sites along Lake Superior (Lizzadro-McPherson & Vye, 2023). 2) The Keweenaw Geoheritage Geoatlas is a knowledge directed exploration geospatial data hub that integrates physiographic landscape-wide feature coverages with a variety of downloadable GIS datasets. The repository of maps and data articulate the geoheritage of the region; the data hub supports educators, students, the scientific community, local tourist entities, land use planners, and the broader public in learning more about specific geosites in the Keweenaw region (Cowling, et al., 2024). 3) The Keweenaw Geoheritage geodatabase and web-viewer provide an innovative way of exploring the relationships between the bedrock geology and how this influences current and future education, conservation, and sustainable economic development initiatives in the Keweenaw region (Fig. 2). Each site expresses: a) a brief description of how the site contributes to the rich geoheritage of the Keweenaw, b) a 360-photo, and c) a description of the scientific (specific to the geologic phenomena), educational, cultural, economic, and aesthetic significance of the site (Lizzadro-McPherson & Vye, 2024). These resources are intended to support the co-stewardship of cultural heritage, restoration of legacy mining sites, conservation issues, and the development of sustainable economic opportunities based on the region’s globally significant geologic underpinnings. Further, they serve as the foundation for an evolving community participatory geoheritage mapping project in the Keweenaw. Through innovative, interactive geospatial resources we aspire to engage the broader public in sharing and exploring their relationships with the Keweenaw landscape (e.g. stories, valued geosites, photos, and curiosities). 91 �REFERENCES Cowling, R., Lizzadro-McPherson, D.J., Verissimo, L. & Vye, E.C. (2023). Keweenaw Geoheritage Geoatlas. DOI: 10.13140/RG.2.2.30945.28005 Lizzadro-McPherson, D.J., and Vye, E.C. (2024). Keweenaw Geoheritage Geodatabase. Michigan State Geological Survey; U.S. Geological Survey, National Cooperative Geologic Mapping Program (Award #G23AC00285 FY23). Lizzadro-McPherson, D. J. & Vye, E.C. (2023). Keweenaw Coastal Geoheritage StoryMap. DOI: 10.13140/RG.2.2.12680.74242 Fig. 1: Keweenaw Coastal Geoheritage StoryMap Fig. 2: Keweenaw Geoheritage Viewer 92 �Battle between the bands: competitive precipitations lead to bands in banded iron formations Xu, Huifang, and Zhou, Tianyu Department of Geoscience, University of Wisconsin–Madison, Madison, WI 53706, USA Banded iron formations (BIFs) are massive chemical deposits composed of alternating layers of chert and iron-rich minerals (such as hematite, magnetite and siderite), with three scales of bandings: microbands, mesobands (1 mm - 10 cm) and macrobands. Their abundance in the Archaean/early Proterozoic era and their absence thereafter suggest that chemical conditions and iron transport pathways on the early Earth surface were different from those after 1.7 billion years ago. Thermodynamic calculations show that Fe-silicate metal complex can be generated by hydrothermal leaching of low-Al oceanic crustal rocks such as komatiites, which suggest that the presence of low-Al ultramafic rocks (for example, komatiitic rocks) in the early oceanic crust were the reason for both the formation of BIFs and their abundance in the Archaean/early Proterozoic era (Wang et al., 2009). This is consistent with the findings that the ages of komatiites are correlated strongly, at the 99% confidence level, with the ages of BIFs (Isley and Abbott, 1999). We used the PHREEQC geochemical modeling package was used to test the chemical reactions that may have led to the banding pattern in the BIFs based on competitive precipitation of ferrihydrite (precursor of hematite and magnetite) and silica gel (precursor of chert) (Zhou et al., 2024). After aqueous ferrous silicate decomposition in O2-sufficient condition (pO2 ≥ 10-4), the faster Fe2+ oxidation and precipitation rate led to Fe-rich layer preceding Si-rich layer with ferrihydrite and amorphous silica as the precursor to the hematite and quartz, respectively. Episodic Fe(H3SiO4)2 input resulted in successive cycles of layering (Fig. 1). O2-deficient environments (pO2 < 10-4) results in jaspilite (no bands). The kinetic model also works well for the formation of siderite bands under O2-deficient environments when pCO2 is high. In summary, the precipitation process model proposed in this study offers an alternative abiotic explanation for the formation of distinct bands within the BIFs. 93 �Figure 1: Schematic depositional model of felsic volcanism associated BIF-like Iron Formations under different surface oxygen levels in a shallow hot spring lake (O2-deficient: pO2 < 10-4; O2-sufficient: pO2 ≥ 10-4). When the O2 level is high, the mix of aerobic lake water and Fe(H3SiO4)2-bearing spring fluid leads to the ferrihydrite-rich layer and amorphous silica-rich layer precipitating successively. But ferrihydrite and silica coprecipitate when O2 is deficient and there is no layering. The ferrihydrite-rich layer would convert to hematite-rich layer and amorphous silica-rich layer transforms into Si-rich layer. The surface water level is regulated by precipitation, evaporation and seepage from surrounding rock without visible inflow or outflow. DOI:10.1016/j.chemgeo.2024.122091) REFERENCES Isley, A. E. & Abbott, D. H., 1999. Plume-related mafic volcanism and the deposition of banded iron formation. J. Geophys. Res. 44, 15461-15477. Wang Y., Xu, H., Merino, E., and Konishi, H., 2009. Generation of banded iron formations by internal dynamics and leaching of oceanic crust. Nature Geoscience, 2, 781-784. Zhou, T., Hill, T., Roden, E. E., and Xu, H., 2024. The Felsic Volcanism Associated BIF-like Iron Formations: Their Origin and Implication for BIFs. Chemical Geology, 656, 122091. 94 �Broadly coeval but migrating deformation, plutonism and deposition in the northeastern Superior Province, Québec: evidence of hot accretionary orogeny and oroclinal folding in the late Archean? ŽÁK, Jiří1, TOMEK, Filip 1, 2, KACHLÍK, Václav 1, VACEK, František 1, 3 SVOJTKA, Martin 2, and ACKERMAN, Lukáš 2 1 Institute of Geology and Paleontology, Faculty of Science, Charles University, Albertov 6, Prague, 12843, Czech Republic 2 Institute of Geology of the Czech Academy of Sciences, Rozvojová 269, Prague, 16500, Czech Republic 3 Czech Geological Survey, Klárov 3, Prague, 11821, Czech Republic The James Bay Road in Québec provides a unique crustal-scale transect across several principal lithotectonic belts of the northeastern Superior Province. From north to south, these belts are Bienville (plutonic), La Grande (ʽgrayʼ gneisses, metaplutonic), Opinaca–Némiscau (metasedimentary), and Opatica (mostly volcano-plutonic). This assemblage has been controversially interpreted to record non-plate vertical tectonics driven by mantle plume activity or as resulting from the step-wise accretion of these belts to the northerly proto-cratonic core. We present here new structural and anisotropy of magnetic susceptibility (AMS) data from all the units along the James Bay Road transect. The data indicate a multistage fabric evolution: (1) an early fabric F1 is preserved only in isolated domains across the La Grande and Opinaca belts and is at a high angle to boundaries between the individual belts; (2) the F2 fabric seems to record a progressive reorientation (folding) towards an E–W direction; (3) the regionally dominant F3 fabric indicates regional NNE–SSW shortening across all units and is coeval with pluton emplacement and anatexis; (4) the last major ductile event is represented by localized dextral shear zones. The AMS indicates that magnetic foliations in general match well the mesoscopic foliations, whereas magnetic lineations vary from steeply plunging to subhorizontal, interpreted as recording a transition from vertical stretching during folding to horizontal stretching during shearing. The latter interpretation is further supported by a more detailed analysis of the ca. 2712–2697 Ma Radisson pluton, which is a syntectonic intrusion at the Bienville–La Grande boundary. Its magmatic to solid-state fabrics analyzed through the AMS also suggest a strain evolution from vertical magma stretching during regional shortening overprinted by later dextral shearing. In conjunction with the previously published U–Pb geochronology, the structural data suggest a short time span and north-to-south migration of plutonism, deposition, and contractional/transpressional deformation, altogether favoring a modern-style plate tectonics operating in the NE Superior Province in the late Archean. Furthermore, the relict F1 and F2 fabrics overprinted by F3 are interpreted as being compatible with changing block/microplate convergence vectors and crustal-scale folding of the outboard La Grande and Opinaca–Némiscau belts. In conclusion, the northeastern Superior Province may have been assembled as large, hot accretionary supra-subduction orogen, oroclinally folded, and finally dextrally sheared. Were this interpretation correct, a key question arises what was the geodynamic cause and mode of the oroclinal folding, whether with or without hard collision, taking the Alaskan terrane wreck or Mongolian orocline as prime examples, respectively. 95 � https://digitalcollections.lakeheadu.ca/files/original/772524cda5601210f7360a628832f9e3.pdf ed3bb63e47c2b1557a98f43e22c97dfb PDF Text Text Volume 71, Part 2 71st ANNUAL MEETING Mountain Iron, Minnesota, May 14-17, 2025 PART 2—Field Trip Guidebook �Meeting Co-Chairs Amy Radakovich, Allison Severson, Eric Nowariak, Stacy Saari, Aaron Hirsch Special thanks to field trip leaders: Zsuzsanna Allerton Terry Boerboom Kevin Boerst Latisha Brengman Annia Fayon George Hudak Mark Jirsa Phil Larson Dean Peterson Cullen Phillips Laurie Severson Mark Severson Alex Steiner i �71st Institute on Lake Superior Geology Volume 71 consists of: Field Trip 1 – Transect of the Quetico Subprovince ................................................................................... 1 Field Trip 2 – Drill Core from three Cu-Ni Deposits of the Duluth Complex .......................................... 15 Field Trip 3 – How Do You Make Iron and/or Manganese Ores in Proterozoic Iron Formation?............ 46 Field Trip 4 – New Geological Insights into the Genesis of Iron Ores at Lake Vermilion – Soudan Underground Mine State Park..................................................................................................................... 74 Field Trip 5 – Neoarchean Alkalic Intrusions in the Wawa and Quetico Subprovinces ......................... 108 Field Trip 6 – Unique Keweenawan Inclusion (Colvin Creek) in the Duluth Complex ......................... 136 Field Trip 7 – Classic Outcrops of Northeastern Minnesota ................................................................... 151 Field Trip 8 – Glacial Lake Norwood and the Koochiching Lobe…. ..................................................... 188 ii �Trip 1 – Quetico FIELD TRIP 1 Transect of the Quetico Subprovince Eric Nowariak1 and Mark Jirsa (retired)1 1 Minnesota Geological Survey, College of Science and Engineering, University of Minnesota, 2609 Territorial Road, St. Paul, MN 55114 Introduction This trip will examine exposures of the metasedimentary, migmatitic, and intrusive rocks of the Neoarchean Quetico subprovince from north of Mountain Iron to near Crane Lake and along part of the Echo Trail. It will attempt to “unpack” the primary components of deposition, magmatism, deformation, and metamorphism that likely spanned 40 million years (~2700-2660 Ma). The latter is based in part on newly acquired geochronologic analyses (Jirsa and others, 2020; Salerno, 2017). The trip will also address the challenge of creating meaningful geologic maps of this and similarly complex terranes, and the apparent lithologic and temporal link between Quetico metasediments and those associated with successor basins in the region. Figure 1-1. Complex migmatite exposed at field trip stop # 3. 1 �Trip 1 – Quetico Figure 1-2. Geologic Map of Central St. Louis County. This draft version of the St Louis County Precambrian bedrock map (superceded by Jirsa, 2020) portrays parts of the Neoarchean Wawa and Quetico subprovinces of Superior Province, and the approximate location of field trip stops. Wawa subprovince colors: greens=volcanic and volcaniclastic rocks; blues=metasedimentary rocks (primarily metagraywacke); reds=iron-formation; pinks=granitoid rocks; yellows=epiclastic and volcaniclastic sedimentary rocks. Quetico subprovince is labeled: BS=biotite schist (metagraywacke); SM=schist-rich migmatite; GM=granite-rich migmatite; TM=tonalite-rich migmatite. Pale pink Lac La Croix granite is more magnetic, darker pink is less so. Bold line marks the approximate boundary between subprovinces—a fault in some places, an inferred unconformity in others. 2 �Trip 1 – Quetico GEOLOGIC SETTING The Quetico is one of a number of east-trending, largely metasedimentary subprovinces in the Superior Province. It is bounded on the south by the Wawa volcanoplutonic subprovince, and on the north by the Wabigoon subprovince. It consists of schist derived from turbiditic sedimentary rocks and a complex suite of granitic intrusions and associated migmatite. In northeastern Minnesota, the subprovince displays a roughly symmetrical distribution of metasedimentary rocks on the north and south, that grade irregularly through zones of schist-rich migmatite, to a central axial zone composed largely of polyphase granitoid migmatite and younger granite. To some extent, metamorphic grade mimics this symmetry, with generally higher grade rocks in the central axis and lower grade near the bounding volcanoplutonic subprovinces. The accretionary prism model of Williams (1990) implies deposition of sediment shed from the craton to the north, and the subducting island arc to the south by submarine fans and abyssal turbidites. The Rainy Lake-Seine River Fault zone at the southern margin of the Wabigoon subprovince is thought to mimic the subduction front. The southern boundary against the Wawa subprovince is interpreted as an unconformity in some locales, and a fault in others. DEPOSITIONAL HISTORY Timing of deposition of the clastic sedimentary rocks of the Quetico subprovince in Minnesota has been constrained by populations of the youngest detrital zircons at 2690 +/- 12 Ma (Salerno, 2017). Similarly, geochronologic studies of the Quetico metasedimentary sequences in Canada has been constrained to 2698 Ma near Atikokan, ON (Davis and others, 1990) and <2690 according to Zaleski and others (1999) near the Manitowage Greenstone Belt of the Wawa subprovince. Zaleski and others (1999) also proved deposition of graywacke units within the Manitowage Greenstone Belt were contemporaneous, if not genetically related. A similar temporal and possible genetic link between the metasedimentary rocks of the Lake Vermilion formation and turbiditic metasediments of the Quetico in Northern Minnesota, wherein immature, volcaniclastic rocks of the Lake Vermilion formation gave way to silicic, clastic sedimentation observed in the Quetico subprovince as the basin evolved from alluvial fan deposits to a deep-water, active margin depocenter as the Quetico basin developed (Davis and others, 1990). In addition to Neoarchean zircons, small populations of older zircons including Mesoarchean zircons have also been recognized (Salerno, 2017; Davis and others, 1990). Probable sources of these older zircons have identified from multiple terranes in the southern Superior province and a proximal source for the sediments is inferred. The composition of the Quetico metasedimentary rocks suggests the source region was shedding sediment from a mixture of sialic plutonic terranes and lesser juvenile volcanic terranes. In addition to the clastic metasedimentary rocks that dominate the bulk of the Quetico subprovince, thin, discontinuous amphibolitic layers are found interbedded in many areas; likely representing volcanoclastic deposits and rare flows from active volcanism occurring near the margins of the subprovince. VERMILION GRANITIC COMPLEX The migmatitic and plutonic rocks in the axial zone are known collectively in Minnesota as the Vermilion Granitic Complex (Southwick and Sims, 1980). Southwick and Ojakangas (1979) subdivided migmatite for mapping purposes as schist-rich and granite-rich components, depending on the ratio of paleosome to neosome. Subsequent mapping by Jirsa (2011) and Jirsa and others (2014), applied the same terms, based instead on the extent to which the predominant fabric in the rock is controlled by neosome vs. paleosome and further distinguished units based on neosome composition. In this nomenclatural system, schist-rich migmatite is schist containing intrusions of granitoid neosome as both delaminating and crosscutting bodies; granite-rich migmatite is neosome with inclusions of paleosome. These can be equated generally with the terms metatexite (low degree of partial melting) and diatexite (nearly complete fusion), respectively, of Sawyer, (2008). Field relationships within the complex (Southwick, 1991) indicate that earliest granitoid phases are leucogranite, tonalite, granodiorite, and trondhjemite, which make up the leucosome of a broad area of migmatite across the western portion of the Vermilion Granitic Complex. For this field trip guide, these granitoids, broadly of TTG affinity, are referred to as “neosome 1”. The 3 �Trip 1 – Quetico migmatite is interlayered at all scales with paleosomes of biotite schist, paragneiss, orthogneiss, and amphibolite and often carries an internal fabric similar to the paleosome. Salerno (2017) obtained a discordant U-Pb zircon age of a granodiorite phase of neosome 1 at 2684 +/- 23 Ma. The migmatite is cut by poorly to non-foliated dikes, sills, and irregular masses of two mica leucogranite with accessory garnet, and a slightly younger biotite granite and pegmatite that contain minor magnetite. The presence of magnetite within the granitic rocks related pegmatites has proved to be an important mapping tool as these intrusive bodies create conspicuous aeromagnetic anomalies (Fig. 3b). The latter forms a large granitic mass, known as the Lac La Croix granite nearest the US/Canadian border, and apophosial intrusions that flare and pinch westward, producing aeromagnetic anomalies that highlight broad fold structures. These intrusive “fingers” generally decrease in thickness and continuity westward, suggesting that the western portion of the complex may represent the roof- or floor-zone of the batholith cored by massive granite. For simplicity within this field guide, the leucogranitic and granitic rocks of the Lac La Croix granite are referred to as “neosome 2” and represents the youngest granitic intrusive units of the Vermilion Granitic Complex. Geochronologic analyses of leucogranite and granite of the Lac La Croix granite has dated the crystallization of this unit with U-Pb zircon ages of 2658.71 +/- 0.47 Ma and 2668 +/10 Ma (Jirsa and others, 2014; Salerno, 2017). a b Figure 1-3. Maps of the Crane Lake and Brule Narrows 30’X60’ quadrangles (US and Canada) illustrating the connections between attributes of structure, lithology, topography, and magnetite content in this area of abundant near-surface bedrock. (a) 30m lidar land surface topographic grid; low areas darker. Topography defines major fold structures, and massive granitic vs. foliated orthogneissic and schistose bedrock. Prominent NNW-trending linear low areas are fault and fracture systems, many of which are occupied by rivers and lakes (named). (b) First vertical derivative map of aeromagnetic data. Magnetic highs (lighter colored) typically are more granitic; lows, more schist-rich. Like the topographic map, the magnetic data identify folds and faults. Linear, NW-trending highs are normally polarized diabase dikes of the Paleoproterozic Kenora-Kabetogama dike swarm. Linear lows are coincident with topographic lows, implying oxidation by meteoric, or more likely hydrothermal fluids along fractures. Some field evidence indicates that rock adjacent to fractures is chemically weathered, and hence more easily eroded. The subparallelism of dikes with fractures may indicate that oxidizing hydrothermal fluids were temporally related to dike emplacement. Neosome 1 and neosome 2 are readily distinguished in the field based on mineralogy and textural characteristics described above. Day and Weiblen (1986) used simple geochemical plots and CIPW normative mineralogy to visualize these differences (Fig. 1-4). Both plutonic suites are characterized as calc-alkaline, metaluminous to weakly peraluminous, magnesian granitoids. Neosome 1 tonalitetrondhjemite-granodiorite intrusions are inferred to have been sourced from partial melting of mafic crust. 4 �Trip 1 – Quetico Geochemical evidence indicates that the early neosome 2 migmatite was derived from partial melting of a metasedimentary protolith (Day and Weiblen, 1986). Southwick (1991) and Day and Weiblen (1986) suggested that the younger Lac La Croix-type granite of neosome 2 may represent further distillation of granitic liquid from partial melting of the combined older migmatite and metasedimentary rocks. Figure 1-4. From Day and Weiblen (1986). (A) CIPW normative mineralogy for Vermilion Granitic Complex. Q – quartz; Pl – albite+anorthite; Or – orthoclase. “Early Plutonic Suite” is equivalent to neosome 1 of this guidebook. (B) AFM diagram of same data (Irvine and Baragar, 1971). DEFORMATION AND METAMORPHIC HISTORY The Quetico subprovince has undergone a complex deformation history over a relatively contracted tectonic history between deposition of sediments ca. 2690 Ma and intrusion of the Lac La Croix Granite related pegmatite dikes ca. 2658 Ma. As summarized by Bauer and others (2011), three main phases of deformation have been recognized. D1 produced tight to isoclinal, recumbant folds plunging to the southwest and locally overturned to the southeast and produced a weak, bedding parallel axial planar foliation. Hinges of these recumbent folds are rare, and recognition of this event are often limited to overturned bedding and bedding-parallel foliation that is crenulated by subsequent deformational events. This event may locally have produced recumbent folds over a broad region (Bauer, 1985; Poulsen and others, 1980). It occurred shortly after deposition and involved burial to produce metamorphic conditions of moderate pressure and temperature (Valli and others, 2004). Fralick and others (2006) suggested D1 deformation was contemporaneous with development of the Quetico basin as an accretionary wedge. D2 deformation was synchronous with peak regional metamorphism to upper greenschist facies in the Wabigoon subprovince and amphibolite facies in the adjacent Quetico subprovince, and produced the dominant structural grain observed in the Minnesota segment of the Quetico subprovince. Folding associated with D2 deformation produced tight to isoclinal upright folds that plunge to the E-NE 10-30°. The intrusion of neosome 1 occurred slightly prior to or contemporaneous with D2 as veins of neosome 1 are commonly folded and occupy gently to moderately plunging D2 related fold hinges. Peak metamorphism presumed to be contemporaneous with D2 deformation within the Vermilion Granitic Complex has been dated by U-Pb monazite geochronology by Salerno (2017) and has constrained to ca. 2675 Ma. Continued contractional and transpressional deformation during D3 has been noted as ductile, eastnortheast trending transpressional shear zones and coaxial refolds of D2 related structures. Folding associated with D3 deformation is better developed near plutons of the Lac La Croix granite and related granitoids, suggesting early stages of neosome 2 intrusions were contemporaneous with deformation or used these structures as conduits (Bauer and others, 1992). D2-D3 is interpreted to be a result of the accretion of the Quetico subprovince to the Wabigoon subprovince to the north. Metamorphic indicator minerals within the Vermilion Granitic Complex including garnet, sillimanite, and locally cordierite have been well documented (Day, 1990; Tabor, 1988; Salerno, 2017). Limited thermobarometric studies have determined peak metamorphism reached amphibolite facies in the axial core of the Minnesota segment of the Quetico subprovince and upper greenschist facies along the northern margin of the subprovince, with garnet-biotite thermometry revealing metamorphic temperatures between 500-600°C and 430-475°C, 5 �Trip 1 – Quetico respectively (Bauer and others, 1992; Salerno, 2017). It is unknown whether the intrusion of the Lac La Croix granite and other neosome 2 intrusions produced a significant metamorphic overprint, however Tabor (1988) recognized kyanite in metamorphic assemblages of the Quetico subprovince along its northern boundary along the Rainy Lake-Seine Fault; which may represent an earlier, relatively higher pressure metamorphic regime prior to intrusion of the Lac La Croix granite. Subsequent deformation including brittle-ductile faulting with associated planar fabrics developed locally near fault zones and minor open folds reorienting existing structures has been ascribed to continued contraction post-dating metamorphism and major plutonism has been ascribed to D4 deformation. One of the most prominent and through-going features of the Quetico subprovince in Minnesota is the Vermilion Fault—a northwest-trending structure that truncates metamorphic zones and folds that are apparent on aeromagnetic maps is likely a product of late D3 and/or D4 deformation. Based largely on geophysical maps, dextral offset along this fault is on the order of 40 km. The fault can be traced from the extreme NW corner of the state for some 250 miles southeastward to near Ely, Minnesota. There it appears to veer to the northeast, manifest as a complexly splayed, post-metamorphic thrust-system known collectively as the Burntside Lake Fault. A summary of the deformational and metamorphic features observed in the Minnesota segment of the Quetico subprovince is shown in Table 1-1 below. Event General Features Associated Fabrics Metamorphic Features Timing Source(s) D1 Recumbant folds Bedding parallel foliation N/A ca. 2690 Ma Fralick and others (2006), Bauer (1985) D2 Upright to inclined tight to isoclinal folds, axes plunge to the northeast and southwest Axial planar cleavage, strong hinge-parallel lineation Upper greenschist to amphibolite facies ca. 2675 Ma Bauer and others (1992) and references therein, Salerno (2017) D3 Upright tight to isoclinal folds – coaxial to D2 folding, east-northeast trending shear zones Axial planar cleavage, strong hinge-parallel lineation, shear fabrics proximal to fault zones Amphibolite Facies 2675-2668 Ma Bauer and others (1992) and references therein, Jirsa (2014) D4 Brittle-ductile faulting, broad folding Planar fabrics proximal to shear zones Hydrothermal alteration along fault zones </=2668 Ma Bauer and others (1992), Recent unpublished mapping Table 1-1. Summary of deformational and metamorphic features observed in the Quetico subprovince within Minnesota. CONSIDERATIONS FOR GEOLOGIC MAPPING Complex geologic terranes recording multiple, interdependent geologic processes including sedimentation, multiple phases of deformation, and diverse polyphase intrusive histories like that of the Quetico subprovince represent a unique challenge in creating meaningful, consistent geologic maps and map units. Multiple attempts to properly portray the complicated geology of the Minnesota segment of the Quetico subprovince have used varied approaches, which have proved to require the incorporation field observation, petrography, aeromagnetic and gravity anomalies, LiDAR and aerial photography, and magnetic susceptibility measurements. Early iterations of geologic maps in the area focused on the proportional differences between the paleosomatic and neosomatic components of the migmatitic rocks within the subprovince to distinguish geologic units (Southwick and Ojakangas, 1979), while other authors have decided to incorporate the textural, compositional, and petrophysical characteristics of paleosomes and neosomes to further distinguish coherent map units (Jirsa, 2011; Jirsa and others, 2014). In addition to traditional field observations, thousands of magnetic susceptibility measurements recorded for units across the subprovince have been used to varied effect (Chandler and Lively, 2014). While petrophysical characteristics of the host 6 �Trip 1 – Quetico rocks are not sufficient to determine many units, the extent and morphology of some distinct units, namely late magnetite bearing granites and pegmatites, have been found to correlate with higher magnetic susceptibilities and resultant aeromagnetic anomalies (Fig. 1-3b). The structural complexities observed in this trip are preserved from the outcrop to map-scale. Many map-scale structures are discernable in aeromagnetic derivative maps and have been used in conjunction with the magnetic characteristics described above to outline geometric and temporal relationships between deformation and intrusive intervals where outcrop exposure is insufficient. Careful observations at individual outcrops has been found to be beneficial in comparison to regional lithologic mapping. Many geologic structures may be more readily identified by field checking and correlating the roughness and patterns of exposed outcrops using lidar and aerial photos. Recognition of post-intrusive faults visible in LiDAR derived maps and as linear magnetic lows in aeromagnetic maps have also helped reconcile locales where map patterns would be otherwise difficult to align with the known structural character of the area. FIELD TRIP STOP DESCRIPTIONS It should be noted that this trip derives from several years of field work to produce two geologic maps of the western-most exposed portions of the Quetico subprovince in Minnesota (Jirsa, 2011; Jirsa and others, 2014); and refinement by more recent field work to create maps of St. Louis and Koochiching Counties (Jirsa and others, 2020; Nowariak and others, in preparation). Mapping focused largely on structural and magnetic attributes that could yield a “meaningful” geologic map of this very complex terrane, and little analytical work was conducted; though ongoing work in Koochiching county has begun to tackle this. As a result, this field trip lacks details of metamorphism, petrology, and geochemisty. Instead, the focus was largely structural, in an attempt to reconcile prominent geophysical anomalies and topographic trends with field observations. The associated maps incorporate structural data, field relationships, and thousands of magnetic susceptibility measurements to ascertain the connections between lithology and magnetite content. Because glacial sediments are thin to absent in much of the area, mapping was also influenced by 10-meter (and subsequent 1-meter, for more recent mapping) LiDAR imagery (Fig. 1-3a). Mapping in the Quetico subprovince on which this field trip is based was supported by grants from the U.S. Geological Survey STATEMAP element of the National Geologic Mapping program, and by the Minnesota Environmental and Natural Resources Trust Fund. NOTE: All locations are denoted in UTM coordinates, NAD 83, Zone 15N STOP 1 – Feldspathic graywacke of the Lake Vermilion Formation Location: 526342E/5288565N, (47.74991°, 92.64856°), Highway 53 Northbound, 0.4 miles north of Heino Road (County 467) Description: This stop examines the feldspathic metasedimentary and meta-volcanogenic sediments of the Lake Vermilion formation, formally part of the Wawa Subprovince. Here, the Lake Vermilion formation is composed of feldspathic graywackes and tuffaceous slates and wackes. The stratigraphy Figure 1-5. Pavement exposure of laminated feldspathic metagraywacke of the Lake Vermilion generally tops to the north and is folded, with locally formation. well-developed axial planar cleavage and thin shear bands. This stop serves as a reference in comparing the composition and character of the metasedimentary 7 �Trip 1 – Quetico rocks of the uppermost units of the Wawa subprovince and the metasedimentary rocks of the Quetico subprovince. Directions: From the Mountain Iron Community Center, head east on highway 169 and turn north onto highway 53, continue north 20 miles and pull-off on the right side of the highway. STOP 2 – Alkalic and Lamprophyric Intrusive Rocks, Gheen Pluton Area Location: 515162E/5306034N (47.74992°, -92.64856°) (2a); 514380E/5306809N (-92.80754°, 47.91444°) (2c); Highway 53, ~6.5 miles northwest of Cook, MN Description: This series of outcrops examines exposures of alkalic granitoids and lamprophyric rocks intruded into metasediments of the Quetico Subprovince. Stop 2a (515162E/5306034N): This outcrop preserves outstanding porphyritic textures within pyroxene syenite and syenodiorite of the Gheen Pluton (Fig. 1-6). Evidence for multiple phases of intrusion and magma mingling are observed throughout. Very coarse phenocrysts exhibit compositional zoning and local magmaticflow features. The main phase of syenite and syenodiorite contains inclusions of, and is cross-cut by medium grained, amphibole-phyric gabbro and pyroxenite. Late aplitic and pegmatitic dikes represent the youngest intrusive components of the Figure 1-6. Porphyritic syenodiorite with abundant outcrop. Chloritic slickensides are apparent on feldspar phenocrysts at stop 2a. fracture faces, locally. Stop 2b (514530E/5306605N): This outcrop of the west side of the highway, USE CAUTION WHEN CROSSING THE ROAD. Here, pyroxene-biotite phyric lamprophyric rocks are exposed (Fig. 1-7). Beyond the dominant biotite and pyroxene, the mineralogy includes prismatic hornblende, feldspar, apatite, and trace chalcopyrite. Limited work to characterize these rocks has determined they are best described as augite bearing kersantites and spessartites (Le Bas, 2007). The mineralogy and texture vary within the a b Figure 1-7. Representative examples of lamprophyric rocks exposed at stops 2b-2d. (a) Biotite-pyroxene bearing kersantite with inclusions of wallrock. (b) Pyroxene-hornblende phyric spessartite with plagioclase dominated groundmass. Blocky, prismatic pyroxene dominates the modal mineralogy here. 8 �Trip 1 – Quetico outcrop at multiple scales, where complex structural and intrusive relationships juxtapose and include multiple mineralogic and lithologic phases. Stop 2c (514380E/5306809N): This outcrop, on the east side of the highway, is composed of similar lamprophyric rocks as stop 2b, but include blocks of lamprophyric rocks of varied composition and the schist wall-rock. The schist here is commonly altered and is cross-cut by small dikes and veinlets of lamprophyric mineralogy. Schist inclusions become more abundant to the north. Stop 2d (514262/5306920): Continuing to the north from stop 2c, the dominant lithology transitions to well foliated biotite-muscovite schist cross-cut by sulfide bearing quartz veins and discontinuous dikes and veinlets of lamproid parallel to and cross-cutting foliation. Directions: From the Stop 1, continue north along highway 53, continue north ~14 miles and pull-off on the side of the highway. STOP 3 – Polyphase, granitoid rich migmatite Location: 512642E/5316320N, (48.00005°, -92.83053°), Highway 53, ~2.5 miles north of Gheen Corner Figure 1-8. Multiphase migmatite at stop 3 showing representative intrusive relationships between the host biotite schist (dark-grey to black), tonalitic neosome 1 (grey),and granitic neosome 2 (tan-pink). Schist preserves crude structural grain. 9 �Trip 1 – Quetico Description: This extensive roadcut exhibits the complex features common throughout much of the migmatitic core of the Quetico subprovince. Here, we will observe and discuss the structural and intrusive relationships and geophysical properties between the metasedimentary quartz-biotite schist paleosome, early granodioritic and tonalitic neosome 1 intrusions, and granitic neosome 2 intrusions. The complexity observed here begs the question of how to create coherent geologic maps in similarly complex regions across the central Quetico Subprovince. Stop 3a (512642E/5316320N) Here, paleosomes of biotite schist have been strongly recrystallized and exhibit a granoblastic texture with faint foliation defined by biotite orientation. Multiple phases of neosome intrusions, both mafic and felsic, include lenses and irregular, blobby bodies of biotite-hornblende granodiorite ascribed to neosome 1 affinity. All units are cross-cut by pink, coarse-grained biotite granite and syenogranite with abundant pegmatitic veins and segregations. The intrusive relationships seen here generally apply to the regional evolution of magmatic rocks within the Vermilion Granitic Complex and Quetico Subprovince, at large. Stop 3b (512645E/5316400N) The agmatic migmatite here includes mafic and silicic paleosome blocks which are disaggregated by the intrusion of both neosome 1 and neosome 2 (Fig. 1-8). Although intrusive phases of the migmatite dominate the outcrop, the structural grain of the paleosomes is preserved as relict bedding and faint foliations. One may note that the paleosomes of differing compositions are difficult to distinguish on the outcrop. Silicic, quartz-biotite schist paleosomes are strongly recrystallized and exhibit an almost massive granular texture. Mafic paleosomes are locally present and are characterized by poorly foliated hornblende (+/- pyroxene) bearing assemblages along with coarsened biotite. Mafic paleosomes seen here may represent thin layers of primary, mafic protoliths or may be restitic components of in-situ melting of the migmatitic host rock. The exposure here is representative of many of the outcrops within the migmatitic core of the Quetico subprovince and highlights the difficulty of creating meaningful geologic maps in the region. How would you map this outcrop? Stop 3c (512644E/4135316N) Small, biotite-pyroxene lamproid intrusion, similar to those inspected at stop 2. Here, acicular, prismatic pyroxene is supported in potassium feldspar-rich segregations (Fig. 1-9). Chalcopyrite is present in trace amounts. Stop 3d (512645E/5316450N) Throughout the core of the Quetico Subprovince, migmatites are locally associated with pyroxenite and pyroxene-hornblende rich gabbroic dikes. Here, a set of pyroxenite dikes with sheared, biotite rich margins crosscut the biotite schist and neosome 1 wallrock and have mutually cross-cutting relationships with granitic neosome 2 intrusions. Stop 3e (512612E/5316833N) On the northern end of the roadcut, the complex multi-stage migmatite gives way to bedded biotite schist with graded beds and crosscutting dikes of late neosome 2 granite and Figure 1-9. Acicular, prismatic pyroxene within pegmatite. Beds here are stratigraphically facing up, potassium feldspar-rich matrix at stop 3c. This small based on fining upward sequences in graded beds, intrusion is similar to alkalic rocks observed at stop 2. and dip 45 to the south-southeast. Directions: From stop 2, continue north along highway 53, continue north ~6.5 miles and pull-off on the right side of the highway. 10 �Trip 1 – Quetico STOP 4 – Schist and schist-rich rich migmatite near Myrtle Lake Location: 523750E/5324590N, (-92.68116°, 48.07414°), Highway 23, ~7.5 miles east of Orr Description: Here, quartz-feldsparbiotite schists and schist-rich migmatite of the Quetico subprovince are exposed. This outcrop preserves moderately dipping beds (30° to the E-SE) of turbiditic metasedimentary rocks with graded beds. Though obscured by metamorphism, bedding here is interpreted to be upright with graded beds observed as decimeter scale, subtle, rhythmic changes in the amount of micaceous minerals. The base of individual beds is marked by coarse grained sandy layers, which transition to biotite rich schist marking the top of the beds. Fine grains of garnet are present locally in beds Figure 1-10. Biotite schist with faint, relict bedding intruded by with appropriate composition. The schist is boudinaged and lit-par-lit dikelets of tonalitic neosome 1. intruded by boudinaged dikes and veins up to 1 meter thick and lit-par-lit injections of tonalitic and granitic neosome 1 (Fig. 1-10). Rare dikes of coarse-grained to pegmatitic, pink, granitic neosome 2 crosscut bedding and dominant fabric of the outcrop and mark the latest intrusive event. Directions: From stop 3, continue north on Highway 53 to the town of Orr and make a right turn on OrrBuyck Road (Highway 23) and continue 7.5 miles east to the roadcut. LUNCH AND STOP 5 – Vermilion Falls ***No Hammers*** Location: 531860E/5345460N, (48.26155°, -92.57072°), Picnic area off Vermilion Falls Rd (USFS 491) Description: This picturesque waterfall cuts through quartz-plagioclase-biotite schist and schist rich migmatite. Both upstream and downstream of the falls, the Vermilion River runs parallel to the dominant regional fabric defined by the orientation of the underlying bedded and foliated metasedimentary rocks and generally foliation parallel intrusions of neosome 1 before draining into Crane Lake. Vermilion Falls occupies a N-NW trending, post-metamorphic and post-intrusive fracture and fault zone orthogonal to the dominant internal fabric of the Precambrian bedrock (Fig. 1-3, Fig. 1-11). These fracture and fault networks are ubiquitous throughout the Vermilion Granitic Complex and the Quetico Subprovince and strongly influence the surface topography and outcrop exposure in the area. Little is known about the timing and relative offsets along these fault zones, though correlation of map units on either side of these features suggests only minor relative motion. Near the upper portion of the falls, tonalitic neosome 1 dikes and sills delaminate the schist along bedding and sub-parallel foliation planes and occupy mesoscopic fold hinges. Downstream, tonalitic neosome 1 dikes are discordant and cross-cut the dominant fabric in the rock. 11 �Trip 1 – Quetico Figure 1-11. Geologic map of the Vermilion Falls area, after Jirsa and others (2011) draped over LiDAR hillshade. NW trending, post-metamorphic and post-intrusive fractures and faults have been highlighted with dashed lines. "GM" - granite rich migmatite, "SM” – schist rich migmatite, “LLC” – Lac La Croix granite, “TTG” – tonalitetrondhjemite-granodiorite gneiss. Directions: From Stop 4, continue east on Orr-Buyck Road (Highway 23) for 8.5 miles, continue straight along Crane Lake Road (Highway 24) at the village of Buyck for 9.5 miles, turn left onto Vermilion Falls Road (USFS 491) for 5.6 miles, turn left onto single-lane access road to turnaround at the picnic area. STOP 6 – Echo Lake Quarry Location: 549810E/5324630N, (48.07300°, -92.33132°), Quarry off USFS 200 Description: NOTE: This is an active quarry, permission to access this site needs to be granted from the quarry operator prior to visiting. The photogenic exposures at this dimension stone quarry include washed, glacially scoured outcrops and fresh blast faces of taxitic, red-pink to pinkish grey granitic gneiss and granite with abundant mafic inclusions (Fig. 1-12). This unit is mapped as a gneissic phase of the Lac La Croix of the Vermilion Granitic Complex, temporally related to neosome 2 seen at other stops (Jirsa, 2011). Gneissic layering here is chaotic and boundaries between gneissic phases are diffuse. Abundant mafic inclusions and schlieren ranging from a few centimeters to multiple meters in size and are randomly oriented. Mafic inclusions are delaminated along planar features and have diffuse, fringed boundaries. Increases in the abundance of biotite and 12 �Trip 1 – Quetico hornblende on the margins of mafic inclusions represent restitic rinds developed during assimilation and interaction with the host granitic melt. Directions: From Stop 5, return to Crane Lake Road (Highway 24) along Vermilion Falls Road (USFS 491) and turn right. Continue southward on Crane Lake Road (Highway 24) for 5.5 miles and turn left onto Echo Trail. Continue along Echo Trail for 8.3 miles and turn right onto USFS 200 for 5 miles. Turn Left onto unnamed forest road near Gustafson Lake. RETURN TO MOUNTAIN IRON COMMUNITY CENTER Figure 1-12. Gneissic granitoid with partially digested amphibolite inclusion. Directions: From Stop 6, return to Echo Trail via USFS 200 and turn left on Crane Lake Road. Continue south on Crane Lake Road/Orr-Buyck Road to the town of Orr. Turn left onto Highway 53 and continue 44.1 miles to the Highway 169 exit ramp. Turn left onto Enterprise Drive. 13 �Trip 1 – Quetico REFERENCES Bauer, R.L., 1985, Correlation of early recumbent and younger upright folding across the boundary between an Archean gneiss belt and greenstone terrane, northeastern Minnesota: Geology, v. 13, p. 657-660. Bauer, R.L., Czeck, D.M., Hudleston, P.J., and Tickoff, B., 2011, Structural geology of the subprovince boundaries in the Archean Superior Province of northern Minnesota and adjacent Ontario: Geological Society of America Field Guide 24, p. 203-241. Bauer, R.L., Hudleston, P.J., and Southwick, D.L., 1992, Deformation across the western Quetico subprovince and adjacent boundary regions in Minnesota: Canadian Journal of Earth Sciences, v. 29, p. 2087-2103. Chandler, V.W., and Lively, 2014, Rock Properties Database; Minnesota Geological Survey web-accessible file data (http://www.mngs.umn.edu/). Davis, D.W., Pezzutto, F. and Ojakangas, R.W., 1990. The age and provenance of metasedimentary rocks in the Quetico Subprovince, Ontario, from single zircon analyses: implications for Archean sedimentation and tectonics in the Superior Province. Earth and Planetary Science Letters, 99(3), pp.195-205. Day, W.C., 1990, Bedrock geologic map of the Rainy Lake area, northern Minnesota: U.S. Geological Survey Miscellaneous Investigations Series I-1927, scale 1:50,000. Day, W.C. and Weiblen, P.W., 1986. Origin of late Archean granite: geochemical evidence from the Vermilion Granitic Complex of northern Minnesota. Contributions to Mineralogy and Petrology, 93(3), pp.283-296. Fralick, P., Purdon, R.H., and Davis, D.W., 2006, Neoarchean trans-subprovince sediment transport in southwestern Superior Province: sedimentalogical, geochemical, and geochronological evidence: Canadian Journal of Earth Sciences, v.43, p. 1055-1070. Jirsa, M.A., 2011, Bedrock geology of the Crane Lake and Brule Narrows 30’X60’ quadrangles, northern Minnesota: Minnesota Geological Survey, Miscellaneous Map M-192, scale 1:100,000. Jirsa, M.S., Block, A.R., Boerboom, Chandler, V.W., and Peterson, D.M., 2020, Bedrock geology of St. Louis County, Minnesota: Minnesota Geological Survey County Geologic Atlas C-51, Part A, Plate 2—Bedrock Geology; scale 1:200,000. [contains ancillary digital files including geophysics and geochronology] Jirsa, M.A., Boerboon, T.J., and Chandler, V.W., 2014, Bedrock geology of the International Falls-Little Fork 30’X60’ quadrangles, northern Minnesota: Minnesota Geological Survey Miscellaneous Map M-197, scale 1:100,000. Le Bas, M., 2007. Igneous rock classification revisited 4: Lamprophyres. Geology Today, 23(5), pp.167-168. Poulsen, K.H., Borradaile, G.J., and Kehlenbeck, M.M. 1980. An inverted Archean succession at Rainy Lake, Ontario: Canadian Journal of Earth Sciences, v. 17, p. 1358-1369 Salerno, R.A., 2017. Neoarchean Deposition, Metamorphism, And Intrusion In Rapid Succession, Vermilion Granitic Complex, Superior Province Of Northern Minnesota. Master's thesis, University of Minnesota – Duluth. Sawyer, E.W., 2008. Atlas of migmatites (Vol. 9). NRC Research press. Southwick, D.L., 1991, On the genesis of Archean granite through two-stage melting of the Quetico accretionary prism at a transpressional plate boundary: Geological Society of America Bulletin v. 103, p. 1385-1394. Southwick, D.L., and Ojakangas, R.W., 1979, Geologic map of Minnesota, International Falls sheet: Minnesota Geological Survey, scale 1:250,000. Southwick, D.L., and Sims, P.K., 1980, The Vermilion Granitic Complex—A new name for old rocks in northern Minnesota: U.S. Geological Survey Professional Paper 1124A, p. A1-A11. Tabor, J.R., 1988, Deformational and metamorphic history of Archean rocks in the Rainy Lake District, Northern Minnesota, [Ph.D. thesis]: Minneapolis, University of Minnesota, 224 p. Valli, F., Guillot, S., and Hattori, K.H., 2004, Source and tectono-metamorphic evolution of mafic and pelitic metasedimentary rocks from the central Quetico metasedimentary belt, Archean Superior Province of Canada: Precambrian Research, v. 132, p. 155-177. Williams, H.R., 1990, Subprovince accretion tectonics in the south-central Superior Province: Canadian Journal of Earth Sciences, v. 27, p. 571-581. Zaleski, E., van Breemen, O. and Peterson, V.L., 1999. Geological evolution of the Manitouwadge greenstone belt and Wawa-Quetico subprovince boundary, Superior Province, Ontario, constrained by U-Pb zircon dates of supracrustal and plutonic rocks. Canadian Journal of Earth Sciences, 36(6), pp.945-966. 14 �Trip 2 – Cu-Ni Duluth Complex FIELD TRIP 2 Drill Core from three Cu-Ni Deposits of the Duluth Complex Mark Severson1,2 (retired), Cullen Phillips3, and Kevin Boerst4 1 (1988–2012) Natural Resources Research Institute, University of Minnesota, Duluth, 5013 Miller Trunk Hwy, Duluth, MN 55811 2 (2013–2018) Previously Teck American, then Teck Resources Unlimited, now NewRange (joint venture between Teck and PolyMet Mining Inc.) 3 NewRange Copper Nickel, 6500 Kensington Dr., Hoyt Lakes, MN 55750 4 Twin Metals Minnesota, 400 Miners Drive East, P.O. Box 329, Ely, MN 55731 Diagram from Peterson (2010) modified from plots of Eckstrand and Hulbert (2007). This guidebook is modified and updated from a guidebook published in 2016 for the 62nd Institute on Lake Superior Geology (pdf). Severson, M., Ware, A., Boerst, K., and Geerts, S., 2016, Cu-Ni-PGE Deposits of the Duluth Complex. Proceedings of the Institute on Lake Superior Geology, Volume 62, Part 2-Field Trip Guidebook, Trip 3, P. 27-78 15 �Trip 2 – Cu-Ni Duluth Complex INTRODUCTION The Duluth Complex, located in northeastern Minnesota, is a series of tholeiitic intrusions of Keweenawan age (1.1 billion years ago) that formed with coeval flood basalts along a portion of the Midcontinent Rift. The Midcontinent Rift system developed during crustal extension during the Mesoproterozoic era and is traceable in a broad arc that begins in northeastern Kansas extending northward through the axis of Lake Superior and then southeastward into Michigan. The Duluth Complex and associated Keweenawan intrusions constitute one of the largest mafic complexes in the world. These rocks cover an arcuate area over 3,000 square miles (5,000 square kilometers) extending from the city of Duluth northward 170 miles (275 km) to the Canadian border. The northwest, convex edge of the complex defines its basal contact, which dips to the southeast towards the rift. Along this contact the complex is successively underlain by Neoarchean granites (Giants Range granitic rocks) and greenstones (Vermilion District) to the north, and Paleoproterozoic sediments (Virginia Formation and Biwabik Iron Formation) to the south. Roof rocks to the Duluth Complex consist of Mesoproterozoic intrusive and volcanic rocks of the Beaver Bay Complex and North Shore Volcanic Group, respectively. Once recognized as a single large lopolithic intrusion, the complex has since been established to be collectively comprised of numerous smaller subintrusions (Figure 2-1) that were episodically emplaced into the base of a comagmatic volcanic edifice between 1108 and 1098 million years ago. Figure 2-1. Generalized geologic map of northeastern Minnesota (modified from Miller et al., 2002). 16 �Trip 2 – Cu-Ni Duluth Complex The Duluth Complex hosts several known large low-grade disseminated Cu-Ni occurrences (Figure 2-2), all of which are located within the basal portions of the Partridge River (PRI), Bathtub (BTI) and South Kawishiwi (SKI) sub-intrusions. A cursory study by Listerud and Meineke (1977) estimated 4.4 billion tons of material averaging 0.66% Cu and 0.20% Ni, using a 0.5% Cu cutoff, in at least nine deposits. Five of these Cu-Ni deposits have recent NI 43-101 reports that estimate a combined mineral inventory well over that amount using lower cutoff values. Known resources (Measured and Indicated, and Inferred) for several of the deposits in Duluth Complex are shown in Table 2-1. Copper-to-nickel ratios generally range from 3:1 to 4:1. Primary mineralization is magmatic. Sulfur source is probably both local (from the footwall sediments) and magmatic. Sulfur isotope studies indicate that most of the sulfur was derived from the Virginia Formation. Most of the mineralization is in the basal portions of these intrusions but there are also local disseminated zones higher in the intrusions. The latter tend to be much more discontinuous except for continuous mineralized horizons, termed “Magenta” style mineralization, that are present at NorthMet and Mesaba, and to a lesser degree, at South Filson Creek. The mineralization styles at each of the Cu-Ni deposits are varied. The general geology and mineralization of the Partridge River, Bathtub, and South Kawishiwi intrusions, as well as the deposits that they host, are presented below. Figure 2-2. Distribution of Cu-NiPGE deposits (in red) and potential titaniumenriched ultramafic pipes (OUIs in blue) in the Partridge River, Bathtub, and South Kawishiwi intrusions. Note that the Mesaba deposit is mostly contained in the Bathtub intrusion. 17 �Trip 2 – Cu-Ni Duluth Complex Table 2-1. Known Resources for the Various Duluth Complex Cu-Ni-PGE Deposits at various Cut-Offs. Average values for Co and Ag are available for some of the deposits but are not shown in the table. Deposit Tons (st) millions Cu % Ni % Pd ppb Pt ppb Au ppb Maturi – Measured and Indicated 1,233 0.58 0.19 334 147 80 0.30% Cu Twin Metals Minnesota 43-101 AMEC Oct-14 Maturi – Inferred 563 0.49 0.16 305 134 68 0.30% Cu Twin Metals Minnesota 43-101 AMEC Oct-14 Birch Lake – Indicated 100 0.52 0.16 515 235 115 0.30% Cu Twin Metals Minnesota 43-101 AMEC Oct-15 Birch Lake – Inferred 239 0.46 0.15 370 180 87 0.30% Cu Twin Metals Minnesota 43-101 AMEC Oct-14 480 0.43 0.16 0.30% Cu Twin Metals Minnesota 43-101 AMEC Oct-14 425 0.41 0.14 0.20% Cu Encampment Resources Non 43101 Amax 1979 NorthMet – Measured and indicated 702 Open Pit 0.25 0.07 234 67 34 0.20% Cu New Range Cu-Ni 43-101F1 M3/HRC Dec-22 NorthMet – 441 Open Pit 0.25 0.07 243 67 34 0.20% Cu /New Range Cu-Ni 43-101F1 M3/HRC Dec-22 Mesaba – Measured and Indicated 2,207 Open Pit 0.43 0.10 97 34 25 NSR $12/ton New Range Cu-Ni 43-101F1 IMC/JDS Nov-22 Mesaba – Inferred 1,423 Open Pit 0.37 0.09 143 43 26 NSR $12/ton New Range Cu-Ni 43-101F1 IMC/JDS Nov -22 Spruce Road – Inferred Serpentine Inferred Cut-Off Company Source Partridge River intrusion The Partridge River intrusion (PRI) is exposed in an arc-shaped area ~10x20 miles (16x32 km) that extends from the southern edge of the Mesaba deposit on the northeast to the Water Hen deposit on the southwest as shown in Figure 2-2. Footwall rocks include the Virginia Formation and very locally the Biwabik Iron Formation. The basal stratigraphic section (Figure 2-3) was first described by Severson and Hauck (1990) and is briefly summarized below. Unit I (PR1) The lowest troctolitic unit of the PRI consists of intermixed troctolite and augite troctolite that locally grade to olivine gabbro. Most of the unit is sulfide-bearing with a PGE-bearing horizon at the top (Red Horizon of Geerts, 1991, 1994). Unique to PR1 are extreme variations in modal mineral percentage and average grain size. Due to this heterogeneous texture, numerous internal contacts divide PR1 into several subunits that are probably related to continuous magma replenishment. Hornfels inclusions of the Virginia Formation are most commonly present within PR1. Near the basal contact the intrusive rocks of PR1 have undergone silica contamination and norite and gabbronorite are often the dominant rock type in the bottom zone. Unit II (PR2) This unit is characterized by sulfide-poor, texturally-homogenous, troctolite that locally grades to augite troctolite and leucotroctolite. PR2 grades downward into a persistent ultramafic horizon(s) defined by melatroctolite, with local peridotite zones, that generally exhibits a sharp contact with PR1. 18 �Trip 2 – Cu-Ni Duluth Complex Figure 2-3. Stratigraphy of the Partridge River intrusion at the Mesaba, NorthMet, Wetlegs, and Wyman Creek deposits (note that the Bathtub intrusion is denoted by the BT-series units in the lower right corner). From Severson and Hauck (2008). Unit III (PR3) Unit III is the most distinctive “marker bed” of the PRI at the NorthMet, Wetlegs, and southern Mesaba deposits. This unit is fine-grained and is characterized by leucotroctolite that locally grades to troctolite and augite troctolite. In all cases, the rock presents a mottled appearance due to the presence of very coarse-grained (>2 cm) olivine oikocrysts that are irregularly distributed throughout the rock. This mottled-texture and fine-grained nature make PR3 unique relative to all the other units of the PRI. PR3 exhibits variable thicknesses at each of the Cu-Ni deposits and pinches out to the west of Wetlegs and to the southeast of Mesaba. The extreme thickness range for PR3 and its physical attributes (poikilitic) have suggested to several geologists that it may be associated with an earlier Anorthositic Series intrusive phase. In this scenario, PR3 may have been intruded earliest along the Virginia Formation-North Shore Volcanic contact and was later underplated by PR1 and PR2 in an early-formed magma chamber subject to continuous magma replenishment. Unit IV (PR4) Unit IV of the PRI is characterized by thick intervals of texturally-homogeneous troctolite and/or augite troctolite. In many areas, PR4 grades upward into a persistent zone of augite-rich augite troctolite and olivine gabbro, which in turn, grades upward into leucotroctolite that is characteristic of PR5. At its base, PR4 has a semi-persistent ultramafic horizon that contains one or more melatroctolite and/or peridotite layers. In some areas a thin semi-massive oxide layer containing very fine-grained chromium titanomagnetite is present immediately above the upper contact of PR3. Unit V (PR5) Unit V is generally an easily recognizable unit in that it is characterized by thick intervals of texturally-homogeneous, medium- to coarse-grained leucotroctolite (dominantly anorthositic troctolite). Another feature that aids in distinguishing PR5 is a highly gradational bottom contact into augite troctolite at the top of PR4. The upper contact of PR5 is sharp against one or more ultramafic horizons that mark the base of the overlying PR6. 19 �Trip 2 – Cu-Ni Duluth Complex Unit VI (PR6) Leucotroctolite (anorthositic troctolite to troctolitic anorthosite) is the most common rock type in PR6. However, near equal amounts of troctolite and augite troctolite are more common in some drill holes at Mesaba. Overall, PR6 becomes more heterogeneous, consisting of multiple rock types, toward the southern and eastern margins of the Mesaba deposit. The base of PR6 is usually marked by a fairly persistent ultramafic horizon. Unit VII (PR7) This unit consists almost wholly of homogeneous leucotroctolite at the NorthMet deposit, but it is characterized by a potpourri of rock types at Mesaba with leucotroctolite being slightly more common. Overall, PR7 becomes more heterogeneous, consisting of multiple rock types, toward the southern and eastern margin of the Mesaba deposit. PR7 contains a basal ultramafic horizon(s) in most drill holes. Unit VIII (PR8) The uppermost PRI unit that has been drilled at Mesaba is referred to as PR8 that consists of a multitude of rock types with no consistent pattern except that leucotroctolite is slightly more dominant. PR3-like inclusions are excessively common to this unit. Oxide-bearing Ultramafic Intrusions (OUIs) Several plug-like, late-stage, oxide-bearing ultramafic intrusions have been delineated in the PRI, the Bathtub intrusion (BTI), and elsewhere within the Duluth Complex (Figure 2-4). The OUIs are intrusive into all units of the PRI and BTI and range in size from large bodies (>200 feet thick, >60 meters thick) to small bodies/lenses (<30 feet thick, <9 meters thick). Rock types are characterized by coarse- to very coarse-grained peridotite and dunite to pegmatitic clinopyroxenite and locally minor orthopyroxenite. These rock types contain varying amounts of ilmenite and titanomagnetite ranging from 5% to massive oxide zones (>80% oxides). The OUIs are in sharp contact with the surrounding troctolitic rocks and are clearly younger. In almost all instances the OUIs are spatially arranged along linear trends suggesting that structural control was important to their genesis. Two of the OUIs are currently being evaluated for their titanium potential and include: 1. Longnose with a NI 43-101 inferred resource of 65.3 million tonnes of 16.4 TiO2; and 2. Titac with a NI 43-101 inferred resource of 45.1 million tonnes of 15% TiO2 (Farrow, 2012). A third OUI, Skibo, is being evaluated for its high-grade Cu-Ni-PGE potential where two vein stockwork zones have been identified by historic drill holes (Inco – up to 6.42% Ni in a 1 foot-thick massive sulfide and other intervals in the hole) and recent drilling by Encampment Minerals (Green Bridge Metals press release, Feb. 6, 2024 at www.greenbridgemetals. com). NorthMet Deposit (NewRange Copper Nickel) The NorthMet deposit is located in the PRI as shown in Figures 2-2 and 2-5. This deposit was initially drilled by United States Steel Corporation (USSC) at what they called the Dunka Road deposit. More recent drilling was conducted by PolyMet Mining Incorporated at the now renamed NorthMet deposit that is being developed by NewRange Copper Nickel (Glencore and Teck joint venture). The geology of the deposit consists of seven igneous units, originally defined by Severson and Hauck (1990) as shown in Figure 2-6. 20 �Trip 2 – Cu-Ni Duluth Complex Mineralization Trends at NorthMet Two open pits are currently planned at the NorthMet deposit - an East Pit and a West Pit (shown in Figure 2-7). The majority of economic mineralization at NorthMet occurs in three scenarios: 1. All of Unit I is mineralized at the East Pit (see cross-section in Figure 2-8); 2. mostly the upper portion of Unit I is the best mineralized in the West Pit (see cross-section in Figure 2-9) and the bottom portions of Unit I will not be mined; and 3. the cross-cutting Magenta Zone (Figs. 2-9 and 2-10) is located well above the basal contact. Grades are generally highest at the top of Unit I and decrease going down hole. However, there are exceptions, and the middle of Unit I contains the highest grades in the center of the deposit. PGE-enriched zones at NorthMet Geerts (1991, 1994) found that the top of Unit I often hosts a PGE-bearing zone that he referred to as the Red Horizon (also referred to as Red Zone) which was determined to be approximately 10 meters thick with an average of 0.57% Cu and 986 ppb Pt+Pd. Geerts also found two more PGE-bearing zones within Unit I referred to as Orange and Yellow horizons. All three of these horizons are positioned beneath ultramafic layers suggesting that they are the result of recharge events and magma mixing whereby a new influx of primitive, PGE-bearing magma was injected into the chamber creating Figure 2-4. Distribution of the Oxide-bearing Ultramafic the ultramafic layers (crystal settling) before Intrusions (OUIs) within the Partridge River, Western mixing with the resident magma (sulfideMargin and Boulder Lake intrusions. Note the linear bearing) and forming the PGE-enriched zones arrangement of OUI along various trends. beneath them. The continuity of these three PGE-bearing zones in more recent PolyMet/NewRange drilled holes is unknown and the three zones are not specifically mentioned in any NI 43-101 reports or field trip guidebooks. It is important to note that the Red Horizon/Zone has been documented to be present at the top of PR1 at Mesaba (Severson and Hauck, 2003). Mineralized Magenta Zone at NorthMet In addition to the Red Horizon, at the top of PR1, there is another PGE-bearing horizon that has been referred to as the Magenta Horizon (or Magenta Zone). This zone is unique in that it crosses several lithologic contacts and progressively downcuts through Units 6, 5, 4, and 3 in a northerly direction (Figure 2-10). The total resource volume of the Magenta zone relative to the rest of the deposit has not been documented in any NI 43-101 reports. Initially, Geerts (1991, 1994) found the Magenta Zone in six holes wherein it averaged about 0.72% Cu and 1,488 ppb Pd+Pt in an over 8 meters thick zone. Cu:Ni ratios in the Magenta Zone are reported to be 3.9-4.1:1. More recent drilling by PolyMet and NewRange has documented the presence of the PGE-enriched Magenta Zone in additional drill holes that are positioned in 21 �Trip 2 – Cu-Ni Duluth Complex the western half of the deposit as shown in Figure 2-8. The Magenta Zone is also present in the PRI along the southern edge of Mesaba deposit to the east where it is referred to as the PRU zone by NewRange CuNi. Figure 2-5. Location and geology of the NorthMet deposit relative to the nearby Mesaba deposit. Modified from combined maps of Miller and Severson (2005) and Severson and Miller (2005). Mesaba Deposit and the Bathtub intrusion (NewRange Copper Nickel) In 1990, the Natural Resources Research Institute (NRRI) was the first to define and describe the igneous stratigraphy of the PRI (Severson and Hauck, 1990). This same stratigraphy was documented to be present in portions of the Mesaba deposit in 1995 (then referred to as the Babbitt deposit). However, this stratigraphy applied to only the deep drill holes along the extreme southern portion of Mesaba and all 22 �Trip 2 – Cu-Ni Duluth Complex Figure 2-6. Igneous stratigraphic section recognized by NewRange Copper Nickel at their NorthMet deposit (not that these same units are also present along the southern margin of the Mesaba deposit where they are referred to as PR1, PR2, PR3 etc.) 23 �Trip 2 – Cu-Ni Duluth Complex Figure 2-7. Geologic map of the NorthMet deposit showing outlines of the two planned open pits. The location of the mineralized Magenta zone is present in the southern half of the West Pit. Figure 2-8. Cross-section illustrating mineralization trends in NorthMet’s East Pit. Note that all of Unit I is mineralized down to the footwall Virginia Formation. 24 �Trip 2 – Cu-Ni Duluth Complex Figure 2-9. Cross-section illustrating mineralization trends in NorthMet’s West Pit. Note that the top portion of Unit I will be mined as it exhibits the best mineralization. Note also that the Magenta mineralized zone is present in a downcutting relationship in Units 5, 4, and 3. Figure 2-10. Typical cross-section at NorthMet (facing east) showing mineralized zones and modeled units. The “Upper Zone Mineralization” in this diagram is also referred to as the Magenta zone. Note how this zone progressively transects downward into the lower geologic units in a northerly direction. 25 �Trip 2 – Cu-Ni Duluth Complex Figure 2-11. Geologic map (circa 2015) showing distribution of major igneous units in the Bathtub intrusion (BTI) and adjacent Partridge River intrusion (PRI) of the Mesaba deposit. attempts to carry this stratigraphy to the north into the majority of Mesaba were not conclusive. Through several iterative follow-up logging campaigns by the NRRI, the Bathtub intrusion (BTI) was finally recognized as a separate intrusion (Severson and Hauck, 2008). A geologic map of the Mesaba deposit showing the geologic units (per the igneous stratigraphy) is shown in Figure 2-11. At least five criterions, listed below and discussed in Severson and Hauck (2008), were initially used to help separate the PRI from the newly named BTI: 1. Abrupt terminus of the PR3 Unit (major marker bed in the PRI) northward into the BTI at the Mesaba deposit 2. Thicker sections of heterogeneous-textured rock in the BTI relative to the adjacent PRI 3. Lack of PGE-enrichment at the top of a specific unit in the BTI (BT1 Unit) relative to PGEenrichment at the top of a similar unit (PR1) in the adjacent PRI 4. The best mineralization at Mesaba is near the base of the BTI (base of the BT1 unit) where it is characterized by high Cu grades associated with pyrrhotite- and cubanite-rich zones. In contrast, the best mineralization at the majority of the NorthMet deposit is present at the top of the PR1 unit where it is associated with chalcopyrite-rich zones 5. Use of a hornfels-rich zone, termed the Hidden Rise, was used to help separate the BTI from the PRI. The current igneous stratigraphy of the Bathtub intrusion, as defined by the NRRI, is summarized in Figure 2-12, and is described in the sections below. 26 �Trip 2 – Cu-Ni Duluth Complex Figure 2-12. Stratigraphic section at the Mesaba deposit showing the relationships between major units (and their corresponding subunits) in the BTI and PRI. Modified from Severson and Hauk (2008). BT1 Unit The lowest unit of the BTI consists of intermixed troctolite and augite troctolite with localized leucotroctolite zones. Unique to BT1 are extreme variations in modal mineral percentage and average grain size; both change rapidly over zones that vary from a few feet to tens of feet thick. Due to this heterogeneous texture, numerous internal contacts subdivide the BT1 into several subunits that often cannot be correlated from drill hole to drill hole. Thus, BT1 is a mixture of various troctolitic subunits that are probably related to continuous magma replenishment. Most of this unit is sulfide bearing. Hornfels inclusions of Virginia Formation are common within the BT1 Unit, especially closer to the basal contact. The BT1 Unit has been further subdivided into several internal subunits based on the dominant presence of one rock type over other rock types. Contacts between these rock types vary from highly gradational to abrupt with locally measurable sharp contacts. The various subunits of the BT1 Unit, and hornfels-rich zones in the BT1, are presented in Figure 2-12 and some are briefly discussed below. • BT1-c – At the base of the BT1 unit there is significant silica contamination of the magma, due to assimilation of the footwall rocks, and orthopyroxene rather than olivine crystallized to produce noritic rocks. Thus, rock types that dominate in the BT1-c subunit range from norite to gabbro norite; especially near either the basal contact or surrounding common hornfels inclusions. Overall, the BT1-c subunit spatially occurs as a rind or coating along the basal contact of the BTI where it ranges anywhere from a foot-thick to over 650 feet-thick. • “The Rise” – along the extreme northern edge of the entire Mesaba deposit, the basal contact of the BTI rises steeply toward the surface. However, in one area, called “The Rise,” the basal contact actually subcrops at the surface and then drops off again in a northerly direction beneath the South Kawishiwi intrusion (see Figure 2-11 for location); A pyrrhotite-rich and graphite-rich unit within the Virginia Formation in “the Rise” has been informally termed the Bdd Po or BDPO unit. • The “Hidden Rise” – the “Hidden Rise” unit is a loosely-defined zone situated along the crest of the Local Boy anticline (Figures 2-11, 2-12 and 2-13) wherein scattered hornfels inclusions, and associated noritic rocks, are fairly common. Like the BT1-c unit, the Hidden Rise shows evidence 27 �Trip 2 – Cu-Ni Duluth Complex of mixing and contamination with the Virginia Formation. This unit is indicative of strong magma contamination and assimilation of what once may have been a magma chamber wall initially separating the BTI and PRI. Thus, the Hidden Rise is used to both define this hornfelsbearing zone and to artistically, and conveniently, divide the BTI from the PRI. Figure 2-23. Projected distribution of the Hidden Rise at Mesaba relative to structural features. The projected location of the shaft and drifts of the Local Boy ore zone are shown in red. The southern edge of the Hidden Rise is approximated due to a paucity of drill holes. BT4 Unit The uppermost unit of the Bathtub intrusion is referred to as the BT4 Unit. It was originally correlated with PR4 of the PRI. However, BT4 is distinctly different from PR4 in that the BT4 Unit is heterogeneous-textured at all scales (though less heterogeneous than BT1 overall), composed of many alternating rock types, and is locally sulfide-bearing. The BT4 Unit appears to grade into thicker, more homogenous troctolitic packages toward the extreme east of the deposit. The BT4 Unit has been further subdivided into several more internal subunits based on the dominant presence of one rock type over other rock types. All these various subdivisions of the BT4 Unit are shown Figure 2-12 and are discussed below. • “± Picrite – the base of the BT4 is defined by a semi-persistent ultramafic layer and/or package, consisting of melatroctolite to peridotite ± troctolitic beds that is referred to as the "± Picrite.” The "± Picrite is present in about 60% of the drill holes in the Bathtub Intrusion and acts as a local horizon that defines the BT1-BT4 contact; however, in many instances the "± Picrite is absent and the BT1-BT4 contact is arbitrarily chosen based on its presence in nearby drill holes. 28 �Trip 2 – Cu-Ni Duluth Complex • Bathtub Layered Interval (BTLI) – this subunit designates zones (see Figures 2-12 and 2-14) where ultramafic layers are extremely common within the BT4 Unit. The ultramafic layers may represent repetitious cyclic layers and can be correlated in drill holes as an overall rock package. The inclination of internal contacts and modal bedding associated with the ultramafic layers are highly variable, ranging from 5° to 80° (even within a single drill hole). Individual ultramafic beds cannot be traced with certainty between drill holes; however, correlations of packages of the BTLI can be traced. This dichotomy for individual ultramafic beds indicates that the bedding relationships are extremely complex in the third dimension and may be related to rapid pinch-out of individual beds. In addition, the BTLI package fades out to the north with increased distance away from the Hidden Rise. If the Hidden Rise represents a magma chamber wall, the BTLI may have crystallized against it via either a static crystallization method or by current-driven crystal settling against the wall. Figure 2-34. Spatial distribution of the BTLI (in solid green hatch) relative to Bathtub syncline and the Hidden Rise (cross-hatched zone). This map is circa 2015 and changes have been made by NewRange Copper Nickel based on newer information. 29 �Trip 2 – Cu-Ni Duluth Complex Footwall Rocks The footwall rock types at both the NorthMet and Mesaba deposits consist mainly of the Virginia Formation, Biwabik Iron Formation (BIF), and very locally the Pokegama Quartzite. All are Paleoproterozoic in age (approximately 1.9-1.8 billion years ago) and collectively comprise the Animikie Group. The rock types of the Virginia Formation and BIF have undergone metamorphism and partial melting that was produced during emplacement of the Duluth Complex. The metamorphic variants of the footwall rocks are schematically portrayed in Figure 2-15 but are not discussed individually herein. Figure 2-45. General Relationships of the Metamorphosed Footwall Rocks beneath the Duluth Complex at the Mesaba, NorthMet, Wetlegs, and Serpentine Deposits. See Severson and Hauck (2008) for more information. Structural Features There are several prominent structural features at Mesaba that were important to formation of the BTI and possibly to mineralization trends. These major features are shown in Figure 2-16 and discussed below (features such as the Rise and the Hidden Rise have been discussed previously). Local Boy anticline and Bathtub syncline The most prominent structural features at the Mesaba deposit are a pair of east-west trending parallel folds, defined by contouring the top of the footwall Biwabik Iron Formation (Figure 2-17), that are informally referred to as the Local Boy anticline and Bathtub syncline. Both of these folds probably exerted strong controls on the style of emplacement of the BTI and its basal contact mimics the form of the anticline and syncline. The trend of the Hidden Rise, the possible wall once separating the BTI and PRI, also correlates with these two fold axes. The structural history regarding the Local Boy anticline and Bathtub syncline appears to be extremely complicated and long lived. Grano Fault Along the far eastern edge of the Mesaba deposit is the north-trending Grano Fault (Fig. 2-16), so named for the abundant and sometimes voluminous amounts of associated late granitoid lenses and OUIs that are associated with the fault zone (Severson, 1994). Both types of late intrusive lenses are interpreted to be steeply oriented and to have been injected along subsidiary fault zones parallel to, and immediately west of the Grano Fault. These late intrusives cross-cut the troctolitic rocks and thus, demonstrate that the 30 �Trip 2 – Cu-Ni Duluth Complex Figure 2-56. Major structural features at the Mesaba deposit. Note the orange-outlined zone to the immediate west of the Grano Fault is a zone wherein late stage subvertical lenses of granitoid and OUI (cyan outlines) commonly cross-cut the troctolitic rocks of the PRI and BTI. OUI (outlined in cyan) are also common along the inferred trace of the South Minnamax Fault. Figure 2-67. Contoured top of the footwall beneath the Mesaba deposit relative to sea level. The contour interval is 100 feet. Note that the contoured lines in this map are derived from Severson and others (1994) and do not take into account any of the more recent drill holes; however, the overall trends would remain basically the same. 31 �Trip 2 – Cu-Ni Duluth Complex fault was active during and after emplacement of the PRI, BTI and SKI. The Grano Fault is thought to be a primary feeder structure for the BTI and possibly the massive sulfides at the Local Boy ore zone (Severson and Hauck, 2008). South Minnamax Fault The South Minnamax Fault is an east-west trending fault along the extreme southern edge of the Mesaba deposit. Several OUIs occur at the surface along the trend of the fault (Figure 2-16). Displacement of the fault, based on correlations and projections of units between only six drill holes, is generally 100200 feet (30-61 meters), but in one area a displacement of over 400 feet (122 meters) is indicated. Mineralization The Mesaba deposit is characterized by disseminated sulfide mineralization that occurs most commonly as fine- to coarse-grained, intercumulus disseminations of chalcopyrite, cubanite, pentlandite, and pyrrhotite. The most important continually mineralized zone at Mesaba is a basal zone with disseminated sulfides that is present within all or portions of the BT1 unit, and locally in the bottom of the BT4 unit (Figure 2-18). This zone commonly ranges between 200 and 600 ft (61 to 183 m) in thickness. Higher in the intrusive package, often overlapping the BT1-BT4 unit boundary, are thinner, secondary zones of erratic and discontinuous, disseminated sulfide mineralization referred to as “cloud zones.” Increased sulfide contents with depth are obvious in drill core and are manifested mainly by increasing amounts of pyrrhotite and cubanite. This dramatic increase in pyrrhotite and cubanite with depth appears to be related to contamination from the footwall rocks and has been classified as occurring mainly in the basal contaminated BT1-c unit but there are exceptions. Talnakhite [Cu9(Fe,Ni)8S16] is present in numerous holes coincident with the axis of the Bathtub syncline (and north of the Hidden Rise), as well as, in the massive sulfides at Local Boy, as shown in Figure 2-19. Talnakhite occurs as exsolution lamellae with chalcopyrite and cubanite. Talnakhite is difficult to distinguish from chalcopyrite in freshly drilled core. However, talnakhite tarnishes rapidly, sometimes within 10-15 minutes depending on the relative humidity, to a purplish brown or peacock blue similar to bornite (orange-brown color in polished sections). Figure 2-78. Typical cross-section at the Mesaba deposit showing mineralization throughout most of BT1 and in portions of BT4. Note the discontinuous “cloud zone” occurrences in the upper portions of the BT4 unit. 32 �Trip 2 – Cu-Ni Duluth Complex Figure 2-89. Distribution of holes that contain significant amounts of Talnakhite based on tarnished relationships observed on drill core. This map is circa 2015. Note that the Local Boy ore zone, shown in lower right red ovoid, also contains significant talnakhite. Massive Sulfides at the Local Boy Ore Zone of the Mesaba Deposit Cu-rich massive sulfides near the basal contact of the Complex are locally present at the Mesaba deposit in a small zone referred to as the Local Boy ore zone. In 1976, AMAX Inc. completed a 1,700foot-deep exploratory shaft (Minnamax shaft), and in 1977, completed four drifts (A, B, C, and D; Figures 2-20 and 2-21). Underground Fan drilling (217 holes) was completed in 1978 to further define the massive sulfide distribution. Potential ore resources for Local Boy are presented in Table 2-2; high PGE values (up to 11 ppm Pd and up to 8 ppm Pt) are locally present in the ore. Sulfide minerals include pyrrhotite, pentlandite, chalcopyrite, talnakhite, cubanite, maucherite (nickel arsenide), sphalerite, bornite, late mackinawite, chalcocite, covellite, godlevskite, and native silver (Severson and Barnes, 1991). Table 2-2. Grade/tonnage data for Cu and Ni in the Local Boy ore zone. These values are for geologic resources, not mineable ore. From Severson and Barnes, 1991. The Local Boy ore zone is also situated over the Local Boy anticline. The majority of massive sulfide ore zones, hosted mainly by the Virginia Formation (Severson and Barnes, 1991), are broadly 33 �Trip 2 – Cu-Ni Duluth Complex coincident with the axis of the anticline. The contoured top of the BIF in the Local Boy area is shown in Figure 2-20 (left). Similar anticline geometries are also present for the basal contact as shown in Figure 220 (right). All the data indicate that an EW-trending anticline is the major structural feature present within the footwall rocks of the Local Boy area. Figure 2-20. Contoured top of the Biwabik Iron Formation at Local Boy (left) and the contoured top of the basal contact between the Virginia Formation and the intrusive rocks at Local Boy (right). Mineralization Trends in the Massive Sulfide at the Local Boy Ore Zone The vast majority of massive sulfides at Local Boy are contained within the Paleoproterozoic Virginia Formation. Even though the massive sulfides straddle the basal contact, most of the massive sulfides are associated with either hornfelsed sedimentary inclusions above the contact or with footwall rocks below the contact while the interfingering intrusive rocks (mostly norite) are relatively barren of massive sulfides (Severson and Barnes, 1991). This suggests that the massive sulfide ores were not formed in this area by the gravitational settling of sulfides, but rather, the ores formed by injection of an immiscible sulfide melt into structurally prepared areas within the footwall rocks along the Local Boy anticline in a vein-like setting. A similar mechanism is proposed for the Norilsk-Talnakh deposits in Russia. Even though the basal contact of the Complex with the Virginia Formation is highly undulatory, the massive sulfides exhibit a definite top and bottom. Figure 2-21 is an attempt to show, in plan view, where massive sulfide zones are present. Also shown in the figure are the different massive sulfide types (ranging from pyrrhotite-dominant to Cu-rich) relative to structural features. The relationships shown in Figure 2-21 indicate that the massive sulfides show a progressive change in an east-to-west direction from Cu-poor massive sulfides to Cu-rich massive sulfides in the vicinity of the Local Boy anticline. These relationships suggest that the injected immiscible sulfide melt underwent fractional crystallization and progressively became more Cu and PGE enriched as it moved through the footwall rocks in an east-to-west direction. 34 �Trip 2 – Cu-Ni Duluth Complex Figure 2-29. Potential distribution of semi- massive to massive sulfide types (Cu-poor versus Cu-rich) at the Local Boy ore zone (left); and an isopach map of cumulative thickness of the massive sulfides (right). Note that the massive sulfides are not present as a continuous blanket, but rather, as one or more stacked disjointed/separated multiple horizons near the basal contact. A possible feeder vent for the sulfide injection event may have been the Grano Fault, which was repeatedly reactivated during emplacement of the Complex. Other data that indicates that the Grano Fault was a potential feeder vent include: 1) the massive sulfides are more common, and thicker (Figure 2-21 right), close to the Grano Fault (feeder) and along the axis of the Local Boy anticline (structurally-prepared site); 2) the VirgSill, at the base of the Virginia Formation, rarely contains significant amounts of disseminated sulfides – except near the Grano Fault; and 3) the Biwabik Iron Formation rarely contains sulfides – except near the Grano Fault. In summary, the massive sulfides at the Local Boy ore zone are interpreted to be structurally controlled in that they are situated along the axis of the Local Boy anticline. The massive sulfides are Curich (5-25% Cu) and are almost exclusively hosted by the Virginia Formation. Sulfide textures suggest that the massive sulfides were injected as an immiscible sulfide melt into the footwall rocks. The overall pattern of sulfide types and PGE contents suggest that the sulfides formed via a process of fractional crystallization of an immiscible sulfide melt as it migrated into the footwall rocks. The Grano Fault is inferred to represent the potential feeder zone in this scenario. 35 �Trip 2 – Cu-Ni Duluth Complex Wetlegs Deposit The Wetlegs deposit (Figures 2-2 and 2-3) was drilled by Bear Creek (13 holes) and Exxon (12 holes). Exxon determined that there were 38 million tons of material at a 0.57% Cu equivalent (files at DNR) but details regarding their cursory calculations are unknown. Most of the igneous units that are present at NorthMet are also present at Wetlegs except: Unit II thins down to a single ultramafic horizon positioned immediately below Unit III (Figure 2-3), and Unit I contains abundant ultramafic layers that are referred to as the Wetlegs Layered interval (Miller and others, 2002). The top of Unit I (aka Red Horizon of the NorthMet deposit) contains scattered anomalous concentrations of PGEs up to 3,132 ppb Pd+Pt (Severson and Hauck, 2003). The Magenta Zone is also present at Wetlegs, but is only known in one drill hole (A4-11) with up to 6,072 ppb Pd+Pt. No work has been conducted on this property since 1998. Wyman Creek Deposit (Encampment Minerals) The Wyman Creek deposit (Figures 2-2 and 2-3) is located at a turning point in the basal contact of the PRI – the contact trends northeast to the east of Wyman Creek and then exhibits a drastic change to a north-south orientation to the south of the deposit. This area was initially drilled by Bear Creek, followed by more extensive drilling (21 holes) by United States Steel Corp. (USSC), and very limited drilling by Exxon. USSC determined (literally a back-of-the-envelope calculation) an open pit potential of 14 million tons of material containing 0.30% Cu and 0.18% Ni (Severson and Heine, 2007). South Kawishiwi intrusion The South Kawishiwi intrusion (SKI) is exposed in an arc-shaped area ~5x20 square miles (8x32 square km) that extends from the Serpentine deposit on the southwest to the Spruce Road deposit on the northeast as shown in Figure 2-2. Footwall rocks include the Virginia Formation, Biwabik Iron Formation, and granitic rocks of the Neoarchean Giants Range granitic complex; the latter is the dominant footwall rock type. The basal stratigraphic section (shown in Figure 2-22) is known in detail from studies of historic drill core (Severson, 1994; Zanko and others, 1994) and is divided into 17 different units that are present over a strike-length of 19 miles (31 kilometers). Figure 2-102. Generalized igneous stratigraphy of the basal zone of the SKI (Severson, 1994). The Lowermost units are BAN = Basal Augite Troctolite and Norite; BH = Basal Heterogeneous; U3 = Ultramafic 3; PEG = Pegmatitic unit of Foose (1984); U2 = Ultramafic 2; U1 = Ultramafic 1; AT-T = Anorthositic Troctolite to Troctolite; UW = Up dip Wedge; Main AGT = Main Augite Troctolite; AN-G Group = Anorthositic Series inclusion with internal gabbroic lenses. 36 �Trip 2 – Cu-Ni Duluth Complex The lowermost units are unevenly distributed along the strike-length of the intrusion in a compartmentalized fashion, suggesting a complicated intrusive history. The stratigraphy, as defined by Severson (1994), has been documented to be present in all the holes drilled recently by Twin Metals Minnesota (TMM) at the Birch Lake and Maturi deposits but it has since been simplified by TMM. According to Severson and Hauck (2008), a few salient features of the SKI include: • • • • • The vast majority of sulfide mineralization is confined to the BH, BAN, and U3 units - all of these are collectively referred to as BMZ by TMM at the Maturi deposit). The PEG unit, though not particularly mineralized except locally, is also included in the BMZ by TMM Major marker beds include three horizons that contain abundant ultramafic layers (U1, U2 and U3) and a pegmatite-bearing unit (PEG unit – originally recognized by Foose, 1984). The U3 unit is unique in that it contains several massive oxide pods (titanomagnetite-rich and locally Cr-bearing) as well as recognizable inclusions of bedded Biwabik Iron Formation. The spatial correspondence between the U3 unit and footwall iron-formation suggests that most of the massive oxide pods are iron-rich “restite” produced by assimilation and a high degree of partial melting of the iron-formation. This relationship is the most prevalent at Birch Lake The U3 unit contains the vast majority of high PGE values; however, high PGE values are locally present in the overlying PEG unit. High PGE values are also present well above the base of the SKI at the South Filson Creek deposit A large inclusion of anorthosite of formidable size (3,500 feet thick) is present at Maturi and was referred to as the AN-G Group by Severson (1994). Peterson (2001) suggested that the high PGE contents within the BMZ unit (beneath the inclusion) formed as a result of confined turbulent magma flow, and thus an increased R-factor, beneath a “pillar” of anorthosite. Peterson (2001) further hypothesized that a portion of a Nickel Lake Macrodike, which served as a feeder to the nearby Bald Eagle intrusion, may have projected beneath the Maturi deposit and also served as a feeder to the SKI Four of the Cu-Ni deposits within the SKI historically held by TMM are shown in Figure 2-23. These four deposits and others within the SKI are discussed in the following sub-sections. The information given for each of the deposits is based on various NI 43-101 reports, field trip guidebooks (Patelke and others, 2009; and Severson and others, 2016), various NRRI reports, oral presentations at professional meetings, and personal knowledge. Maturi Deposit (Twin Metals Minerals) The very first exploration drill hole in search of Cu-Ni deposits in the Duluth Complex was cored in the Maturi deposit by Fred S. Childers (prospector) and Roger V. Whiteside (investor) in 1951. Eventually, the International Nickel Company (Inco) picked up the property and outlined a sizeable, but low grade, Cu-Ni deposit. A shaft was sunk on the property during 1966 to 1967 to collect material for metallurgical tests. Inco took their bulk sample from pyrrhotite-rich material, which is more prevalent near the basal contact, and decided the grade was too low to support an underground mine. Three other companies (Bear Creek, Duval and Newmont) put down several scattered drill holes on the periphery of the deposit and intersected good mineralization at great depth but also determined it was too low grade to support an underground mine. 37 �Trip 2 – Cu-Ni Duluth Complex Figure 2-113. Twin Metals Minnesota (TMM) deposits and Resource Classification. Mineralization at Maturi, and an extension referred to as Maturi SW, is present in the lower portion of the South Kawishiwi intrusion (SKI) in what TMM refers to as the Basal Mineralized Zone (BMZ). Note that the BMZ collectively consists of the PEG, U3, BH and BAN units of Severson (1994). While the Severson units are recognized by TMM at Maturi they are not consistently present throughout the deposit and for simplicity-sake TMM combined them into the BMZ unit. In 2008, Dean Peterson from Duluth Metals (the original company from which TMM was created) theorized that the initial SKI magmas at Maturi were intruded as sulfide-bearing, crystal-laden (olivine- and plagioclase-rich), crystal slurries. Based on this new interpretation, TMM combined Severson’s (1994) basal units into the BMZ unit that they believed originated by physical sorting of the crystal slurries (possibly top down) and by melting of the footwall granitic rocks (bottom up) to create the heterogeneous lithologies and textures of the BMZ as is shown in Figure 2-24. Mineralization at the Maturi deposit consists of a tabular sheet of disseminated Cu-Ni-Fe sulfides that averages 215 feet thick (65 meters) with a range of 5 to 865 feet thick (1.5 to 260 meters) with the thickest range towards the north end of the deposit. Dips of the BMZ vary from 35 to 55 degrees with a N60E plunge along the contact. Higher grades are concentrated in the upper 100 feet (30 meters) of a zone that has been traced laterally by drilling for approximately 2.2 miles (3.5 km) and open at depth. While mineralization is mostly restricted to the BMZ, exceptions are locally present in the overlying PEG unit and in the footwall granitic rock. TMM reports that mineralization within the footwall granite occurs in appr38 �Trip 2 – Cu-Ni Duluth Complex Figure 2-124. Simplified crystal-liquid slurry model for the SKI in the Maturi area. oximately one-quarter of the holes drilled to date with about 80% of these holes showing mineralization in the overlying BMZ that continues directly downward into the footwall mineralization with little or no breaks. Mineralization typically consists of 1-5% disseminated chalcopyrite, talnakhite, cubanite, pyrrhotite, and pentlandite. Bornite, covellite, and millerite occur in subordinate amounts. Better grades of Cu, Ni and PGE are associated with more mafic units located near the top of the BMZ. Modeling of the ore deposits by Duluth Metals, TMM, and AMEC indicated that the mineralization at Maturi can be characterized by several distinct patterns as shown in Figure 2-25. Figure 2-135. Mineralization trends of the BMZ and adjacent rocks within the Maturi area. 39 �Trip 2 – Cu-Ni Duluth Complex The three stages of mineralization within Maturi’s BMZ zone include: • Stage 1 Mineralization (S1, with top and bottom zones): barren to very low-grade mineralization showing low variability. • Stage 2 Mineralization (S2, with top and bottom zones): moderate grade mineralized intervals showing low variability. Cu:Ni ratios are 3.0 to 3.2:1. Cu-sulfides are the dominant sulfide but pyrrhotite becomes increasingly present with depth. Cubanite and pentlandite decrease in abundance with depth. Normalized chalcopyrite/chalcopyrite+cubanite ratios are approximately 0.61 to 0.65 (Hoffmann and others, 2015). • Stage 3 Mineralization (S3): higher grade mineralized intervals that are commonly bounded by low grade selvages and, interestingly, contains ultramafic units (aka U3 unit of Severson, 1994). Cu:Ni ratios are 3.0 to 3.2:1. Cu-sulfides are the dominant sulfide. Normalized chalcopyrite/chalcopyrite+cubanite ratios are approximately 0.58 to 0.67 (Hoffmann and others, 2015). Birch Lake Deposit (Twin Metals Minerals) The Birch Lake deposit lies to the south of Maturi (Figures 2-2 and 2-23), with mineralization also hosted at the bottom of the SKI. The area was first drilled by Duval in the 1970s but remained dormant until high PGE values were found in drill hole Du-15 by state agencies in the mid-1980s (Sabelin and Iwasaki, 1985). This discovery marked the start of serious PGE exploration in the Duluth Complex. Ernest Lehmann formed several joint ventures, the last known as Franconia Minerals LLC or Beaver Bay Joint Venture, and several holes were drilled on the property intermittently during 1988 through 2010. TMM acquired the property in 2011 and drilled 30 holes from 2011 to 2012. A total of 114 holes have been drilled at Birch Lake (excluding 154 wedge holes that were drilled mainly to obtain material for metallurgical testing). The geology is very similar to Maturi except for the common occurrence of more (and often thicker) ultramafic layers, assimilated BIF inclusions, and discontinuous oxide-rich horizons/pods that are inferred to represent BIF “restites”, all of which are present in the U3 unit. The continuity of these U3 rock types is extremely heterogeneous in 3D as revealed by wedge drilling. Mineralization is associated with what TMM also refers to as the BMZ which consists of the U3, BH, and BAN units of Severson (1994). The BMZ averages about 100 feet thick (30 meters) but is as thick as 515 feet (157 meters). The main footwall unit at Birch Lake is the Neoarchean Giants Range granitic complex, but Paleoproterozoic rocks are exposed at the surface in the Dunka Pit mine located <1 km to the southwest. The four inferred mineralization types at Birch Lake in the BMZ and GRB are shown in Figure 2-26 (non-mineralized material below the GRB_M is identified as GRB_B for barren footwall rocks). Figure 2-26. Igneous stratigraphy according to mineralization trends at Birch Lake. 40 �Trip 2 – Cu-Ni Duluth Complex Mineralization trends at Birch Lake are very similar to Maturi, with four inferred types: 1. Melatroctolite / BL_MT Unit (similar to S3 at Maturi and U3 unit of Severson): an upper melatroctolite to mafic troctolite unit that hosts the highest grade mineralization and is correlative across the deposit. The top and bottom of this unit are typically based on high Mg contents with values generally greater than 6% Mg. The Cu:Ni ratio is about 3.3. The base of the BL_MT unit is gradational downward into the BL_T unit. Almost all of the significant Cu-Ni and precious metal mineralization is hosted by this unit but the total volume, or percentage of the mineral resource, has not been published 2. Troctolite / BL_T Unit (similar to S1 at Maturi and BH and BAN units of Severson): a lower troctolitic unit with lower grades that is also correlative across the deposit. Mg contents are in the 3.5-4.5% range. Locally the top of BL_T is more mineralized and there are small, mineralized zones near the base 3. Basal Hybrid Zone / BL_HX: a basal hybrid rock sequence, with localized oxide-rich layers, that shows similarities to both BL_T and underlying metasomatized Giants Range granitic rocks. This hybrid sequence is marked by an abrupt increase in P and erratic Sr, Ba, Mg, and V concentrations. Iron ranges from 2% Fe to upwards of 45% Fe (largely because of assimilated BIF inclusions) 4. Mineralized GRB / GRB_M: consists locally of mineralized Giants Range granitic rocks as well as locally mineralized Virginia Formation and BIF. Average grade is about 0.28% Cu and 0.16% Ni with a Cu:Ni ratio of about 2.3. Local massive sulfide bodies are present and contribute significantly to the average grade The thickness of the four units is quite variable, but the stratigraphic succession does not vary across the deposit. Any one or more of the units, however, can be missing locally from a specific drill hole. Geologic modeling indicated that there is a sinuous, channel-like body of persistent and higher Cu grades that traverse the length of the deposit and follows the thickest portion of the BL_MT unit as shown in Figure 2-27. The origin of the channel is not well understood but it may be related to a magma conduit. Spruce Road Deposit (Twin Metals Minerals) The Spruce Road deposit lies to the northeast of Maturi (Figures 2-2 and 2-23). Mineralization is also present at the base of the SKI. It was at this deposit that the first good indications of Cu-Ni mineralization were uncovered while constructing a forest access road in 1948. From 1954 to 1971, Inco drilled the deposit on 200-foot centers (61 meters) for a total of 232 holes (the vast majority of which are no longer preserved after they were destroyed in a fire at Sudbury, Ontario). In 1997, Inco’s subsidiary, American Copper and Figure 2-147. Birch Lake magma channel superimposed on average copper grade base map. 41 �Trip 2 – Cu-Ni Duluth Complex Nickel Company (ACNC), joint ventured the property with Wallbridge Mineral Company Limited (from which Duluth Metals was eventually created). Wallbridge eventually drilled two holes on the property during 1999 to 2000 in search of high-grade footwall veins but failed to find significant mineralization in the footwall rocks. In 2002, Franconia Minerals Corp. entered into an agreement with Beaver Bay Joint Venture to acquire the Spruce Road and Maturi properties from ACNC but conducted no work. TMM acquired the property in 2011 and drilled 57 drill holes totaling 65,635.5 ft between September 2012 and January 2014 and a prefeasibility study technical report was issued on the TMM project in August 2014. The geology at Spruce Road is vastly different than at either Maturi or Birch Lake. Publiclypreserved core from historic drill holes are extremely limited for this deposit in that only six Inco holes are preserved along with two Wallbridge holes. From this limited data, Severson (1994) determined that most of the igneous units that typify the SKI elsewhere are not present at Spruce Road. Rather, the mineralization appears to be present in a much thicker BH unit (also referred to as the BMZ unit by TMM) consisting of a heterogeneous mix of troctolitic rocks with common hornfelsed inclusions of basalt (North Shore Volcanic Group). Also present are extremely localized noritic rocks associated with hornfelsed sedimentary rocks (Virginia Formation and Biwabik Iron Formation) and at the basal contact with the Giants Range granitic complex. The U3 unit and massive oxide zones are also locally present. Mineralization does not appear to correlate with any specific igneous lithology and there are no known marker horizons. Historic drill logs indicate that there may be an igneous mega-breccia unit that is referred to as “Spruce Road breccia.” South Filson Creek Deposit (Encampment Minerals) The South Filson Creek deposit, located to the east of Spruce Road, as previously presented in Figure 2-2, was initially drilled by the Hanna Mining Company (23 holes) in the late 1960s. There, the CuNi mineralization is hosted by troctolitic rocks (AT&T unit of Severson, 1994), both in outcrop and in the tops of several drill holes situated well above the basal contact. In 1987, encouraging high PGE values (>1.0 ppm) were reported in these “cloud” zone sulfides by Steve Hauck of the NRRI. A subsequent study of the PGE mineralization (Kuhns and others, 1990) indicated that the PGE were concentrated by a latestage hydrothermal event that concentrated the PGE in extremely fine, discontinuous, microscopic veinlets that were inferred to be associated with a NE-trending fault zone. Encampment Minerals drilled an additional 27 holes on the property, but results are largely unknown. Serpentine Deposit (Encampment Minerals) The Serpentine deposit, shown in Figure 2-26, is located to the north of the Mesaba deposit. The deposit was initially discovered by Bear Creek Mining Company in 1967 as part of a follow-up drilling campaign of an airborne electromagnetic conductor (Kulas, 1979). The name “Serpentine” was chosen for this deposit due to the presence of a sinuous-trending massive sulfide located at the base of the SKI. When the next owner, Amax, began working on the deposit, they calculated that the deposit contained 250 million tons of resources (not NI 43-101 compliant) grading 0.41% Cu, 0.14% Ni and 1.96% S at a 0.20% copper cut-off, with a higher-grade portion of over 7 million tons, at a 0.60% copper cut-off, with a grade of 0.88% Cu, 0.30% Ni and 5.67% S (Kulas, 1979, Zanko and others, 1994). The presence of such voluminous pyrrhotite-rich massive to semi-massive sulfide at the basal contact at Serpentine makes this an unusual deposit (Figure 2-29). There, the massive sulfide is closely related to the BDPO which provided a local sulfur source. Empirical evidence in drill core is evidenced by partially melted BDPO (present in the footwall and in inclusions) that transitions upwards and downwards into massive sulfide near the basal contact. The massive sulfide is also located close to the projected location of the Grano Fault that may have played a role in its origin. Another feature that may be related to the Grano Fault at Serpentine is a northerly-trending zone wherein subvertical olivine-rich ultramafic dikes were emp42 �Trip 2 – Cu-Ni Duluth Complex -laced in the troctolitic host rocks while the host rocks were still solidifying. Encampment Minerals drilled eight holes at the Serpentine deposit, but no data are known regarding their results. Figure 2-168. Location of the Serpentine deposit in relation to the Mesaba deposit. Note drill holes posted in red are holes that intersected massive sulfides at or slightly above the basal contact (larger red dots intersected significantly more and thicker massive sulfide zones). Figure 2-159. Trend of BDPO unit relative to basal massive sulfide mineralization at the Serpentine deposit (from Zanko and others, 1994). 43 �Trip 2 – Cu-Ni Duluth Complex Field Trip Stops Drill core will be displayed at NewRange’s Babbitt core facility, for both the NorthMet and Mesaba deposits, and at Twin Metals Ely office for the Maturi deposit. Cross-sections displaying the geology will be posted, as well as Cu-Ni-PGE grades, for the appropriate holes. At this point in time, the core displayed will be determined by the companies. NewRange core facility: 578810 / 5284970, (47.71330°, -91.94930°) Twin Metals Ely office: 585230 / 5306550, (47.90661°, -91.85949°) References Barber, J., Parker, H., Frost, D., Hartley, J., White, T., Martin, C., Sterrett, R., Poeck, J., Eggleston, T., Gormely, L., Allard, S., Annavarapu, S., Radue, T.,Malgensini, M and Pierce, M., 2014:, Twin Metals Minnesota Project, Ely, Minnesota, USA: NI 43-101 Technical Report on Pre-feasibility Study prepared by AMEC E&C Services, Inc. for Duluth Metals Limited, October 2014, Project 176916. Bennett, A., Dempers, N., Neff, D., Radue, P.E., Roth, D., Schwering, R., Tahija, L., Uble, J.S. and Welhener, H.E., 2022, NorthMet Copper-Nickel Project, Feasibility Update National Instrument 43-101F1 prepared for PolyMet Minerals Corp. by M3 Engineering and Technology Corp., Project M3-PN220283, Dec. 30th, 2022, 248 p. Eckstrand, 0.R., and Hulbert, L.J., 2007, Magmatic Nickel-Copper-Platinum Group Element Deposits; in Goodfellow,W.D., ed., Mineral Deposits of Canada: A synthesis of Major Types, District Metallogeny, The Evolution of Geological Provinces, and Exploration Methods: Geological Association of Canada, Mineral Deposits Division, Special Publication No.5, p. 205-222. Farrow, D, and Johnson, M, 2011, January 2012 National Instrument 43-101 Technical Report on the Titac Ilmenite Exploration Project, Minnesota, USA. SRK Consulting (Canada) Inc. SRK Project Number 2CC031.004. Cardero Resources Corp. Farrow, D, and Johnson, M, 2012, January 2012 National Instrument 43-101 Technical Report on the Longnose Ilmenite Exploration Project, Minnesota, USA. SRK Consulting (Canada) Inc. SRK Project Number 2CC031.004. Cardero Resources Corp. Foose, M., 1984, Logs and correlation of drill holes within the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: United States Geological Survey, Open-file Report 84-14 Geerts, S.D., 1991, Geology, stratigraphy, and mineralization of the Dunka Road Cu-Ni prospect, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Duluth, MN, Technical Report NRRI/TR-91/14, 63 p. Geerts, S.D., 1994, Petrography and geochemistry of a platinum group element-bearing horizon in the Dunka Road prospect, (Keweenawan) Duluth Complex, northeastern Minnesota: University of Minnesota Duluth, Unpublished M.S. thesis, 100 p. Kuhns, M.P, Hauck, S.A, and Barnes, R.J, 1990, Origin and occurrence of platinum group elements, gold and silver, in the South Filson Creek copper-nickel deposit, Lake County, Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Duluth, MN, Technical Report NRRI/GMIN-TR-89-15, 60 p. Kulas, J.E., 1979, Serpentine Reserve – Minnamax project: unpublished AMAX Company report on file at the Minnesota Department of Natural Resources, Lands and Minerals Division, Hibbing, MN, 5 p. Listerud, W.H., and Meineke, D.G., 1977, Mineral resources of a portion of the Duluth Complex and adjacent rocks in St. Louis and Lake Counties, northeastern Minnesota: Minnesota Department of Natural Resources, Div. of Minerals, Hibbing, MN, Report 93, 49 p. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geologic Map of the Duluth Complex and Related Rocks: Minnesota Geological Survey, Miscellaneous Map Series, Map M-119. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northern Minnesota: Minnesota Geological Survey, Report of Investigations RI-58, 207 p. Miller, J.D., and Severson, M.J., 2005, Bedrock Geology of the Babbitt Southwest Quadrangle, St. Louis County. Minnesota: Minnesota Geological Survey, University of Minnesota, Miscellaneous Map Series, M-161. 44 �Trip 2 – Cu-Ni Duluth Complex Patelke, R, Peterson, D, Severson, M, Jefferson, T, and Lehmann, E., 2009, Cu-Ni-PGE Deposits of the Duluth Complex, Geology and Development; 55th Annual Institute on Lake Superior Geology, Ely, MN, Part 2 Field Trip Guidebook, p. 1-80. Peterson, D.M., 2001, Development of a conceptual model of Cu-Ni-PGE mineralization in a portion of the South Kawishiwi Intrusion, Duluth Complex, Minnesota: Laurentian University – Society of Economic Geologists, Second Annual PGE Workshop, Sudbury, Ontario. Peterson, D.M., 2010, The Nokomis Cu-Ni-PGE Deposit, Minnesota. Prospectors and Developers Association of Canada. Annual Meeting, Powerpoint presentation. Sabelin, T., and Iwasaki, I., 1985, Metallurgical evaluation of chromium-bearing drill core samples from the Duluth Complex (Contract report of Minnesota Department of Natural Resources, Division of Minerals): Minerals Resources Research Center, University of Minnesota, Minneapolis, MN, 58 p. Sabelin, T., and Iwasaki, I., 1986, Evaluation of platinum group metal occurrence in Duval 15 drill core from the Duluth Complex: Internal report, Minerals Resources Research Center, University of Minnesota, Minneapolis, MN, 23 p. Severson, M.J., 1994, Igneous stratigraphy of the South Kawishiwi intrusion: Duluth Complex, northeastern Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Duluth, MN, Technical Report NRRI/TR-93/34, 210 p. Severson, M.J. and Barnes, R.J., 1991, Geology, mineralization and geostatistics of the Minnamax/Babbitt Cu-Ni deposit (Local Boy area), Minnesota, Part II: Mineralization and geostatistics: Natural Resources Research Institute, University of Minnesota, Duluth, Technical Report, NRRI/TR-91/13b, 216 p. Severson, M.J. and Hauck, S.A., 1990, Geology, geochemistry, and stratigraphy of a portion of the Partridge River intrusion: Natural Resources Research Institute, University of Minnesota Duluth, Duluth, MN, Technical Report NRRI/GMIN-TR-99-11, 236 p. Severson, M.J. and Hauck, S.A., 1997, Igneous stratigraphy and mineralization in the basal portion of the Partridge River intrusion, Duluth Complex, Allen Quadrangle, Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Technical Report NRRI/TR-97/19, 102 p. Severson, M.J. and Hauck, S.A., 2003, Platinum group elements (PGEs) and platinum group minerals (PGMs in the Duluth Complex, Natural Resources Research Institute, University of Minnesota Duluth, Technical Report NRRI/TR-2003/37. 296 p. Severson, M.J. and Hauck, S.A., 2008, Finish Logging of Duluth Complex Drill Core (And a Reinterpretation of the Geology at the Mesaba (Babbitt) Deposit): Natural Resources Research Institute, University of Minnesota Duluth, Duluth, MN Technical Report NRRI/TR-2008/17, 68 p. + 94 plates. Severson, M.J. and Heine, J.J., 2007, Data compilation of United States Steel Corporation (USSC) exploration records in Minnesota; Natural Resources Research Institute, University of Minnesota Duluth, Duluth, MN, Technical Report NRRI/TR-2007/25, 98 p. Severson, M.J. and Miller, J.D., 2005, Bedrock Geology of the Babbitt Quadrangle, St. Louis County. Minnesota: Minnesota Geological Survey, University of Minnesota, Miscellaneous Map Series, M=159. Severson, M.J., Patelke, R.L., Hauck, S.A., and Zanko, L.M., 1994, The Babbitt copper-nickel deposit, Part B: Structural datums: Natural Resources Research Institute, University of Minnesota Duluth, Duluth, MN, Technical Report NRRI/TR-94/21b, 48 p. Severson, M, Ware, A, Boerst, K, and Peterson, D, 2016, Cu-Ni-PGE Deposits of the Duluth Complex, Geology and Development; 62nd Annual Institute on Lake Superior Geology, Duluth, MN, Part 2 Field Trip Guidebook, p, 27-78. Welhener, H. and Crowie, S.T., 2022, NI 43-101F1 Technical Report on Mesaba Project, Mineral Resource Statement prepared for PolyMet Mining Corp. by Independent Mining Consultants, Inc. and JDS Energy and Mining, Inc., November 2022 Zanko, L.M., Severson, M.J., and Ripley, E.M., 1994, Geology and mineralization of the Serpentine copper-nickel deposit, Duluth Complex, Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, Duluth, MN, Technical Report NRRI/GMIN-TR93-52, 90 p. 45 �Trip 3 – Proterozoic Fe & Mn Formations FIELD TRIP 3 How Do You Make Iron and/or Manganese Ores in Proterozoic Iron Formation? Dean Peterson1, Alex Steiner1, and Latisha Brengman2 1 Big Rock Exploration, 2505 W. Superior St., Duluth, MN 55806 Earth and Environmental Sciences, Swenson College of Science and Engineering, University of Minnesota, Duluth, 1114 Kirby Dr., Heller Hall 229, Duluth, MN 55812 2 Introduction Iron formations are among the most important rocks for our modern industrial world. Their extraordinary iron content facilitates the manufacture of steel, while their manganese content is of crucial importance as a steel-alloy product and a critical component of battery technologies. Fueling modern technology requires efficient production of iron resources, exploration of manganese resources, and determination of enrichment processes that lead to ore formation. This field trip will explore ore-formation processes that turn otherwise uneconomic iron formations into valuable resources of iron and manganese. The trip may include an optional stop at the Hibbing Core Library where participants will examine drill core of the Biwabik taconite ores. Participants will explore sedimentary features, diagenetic reactions, and weathering reactions that contribute to iron grade and iron distribution within ore-horizons of the Biwabik. We will then travel to the North Star Manganese/Electric Metals core logging facility in Emily, MN to look at four recent (2023 drilling) drillholes where we will discuss the formation and subsequent redistribution of manganese within the Emily Iron Formation. Participants will have the opportunity to observe highgrade manganese oxide drill core, primary iron-manganese carbonate facies iron formation, and breccia horizons possibly associated with the 1.85 Ga. Sudbury impact. The trip will then proceed to the Mary Ellen mine, a former natural ore pit, where the oxidation and weathering of the Biwabik was central to ore formation and early mining efforts on the range. Participants can observe primary features such as stromatolites and sedimentary structures as well as oxidation-weathering features. If time allows, we will wrap up the trip at the Biwabik outcrops in Virginia near the new Highway 53 bridge over the historic Rouchleau natural ore (hematite) mine before heading back to Mountain Iron. Regional Geologic Setting To gain a true understanding of the geology and origin of the high-grade Paleoproterozoic iron and manganese resources of the Mesabi and Cuyuna Ranges of northern Minnesota (Fig. 3-1), it is best to start with an understanding of the regional-scale geologic setting and its contained ferrous mineral resources. These Paleoproterozoic iron ranges include several categories of marine chemocline mineral systems outlined in recent USGS publications (Schulz et al., 2017 and Hofstra and Kreiner, 2020). These categories include: 1) Superior-iron deposits (Mesabi Iron Range and the Emily District of the Cuyuna Iron Range) and 2) Algoma-type iron-manganese deposits (Cuyuna North and South Iron Ranges). Superior Type Iron Resources of the Mesabi Iron Range Superior type iron formation resources of Minnesota are exemplified by the long-standing mining of iron resources of the Biwabik Iron Formation along the length of the Mesabi Iron Range. The Mesabi Iron Range is largely located in St. Louis and Itasca counties and has been the most important iron ore district in the United States since ~1900. The Mesabi Iron Range is 120 miles long, averages one to two miles wide, and is comprised of rocks of the Paleoproterozoic Animikie Group. The Animikie Group on 46 �Trip 3 – Proterozoic Fe & Mn Formations the Mesabi Iron Range consists of three major conformable formations: Pokegama Formation at the base; Biwabik Iron Formation in the middle; and the overlying Virginia Formation. On the Mesabi Iron Range, these three formations generally dip gently to the southeast at angles of 3-15 degrees. Figure 3-1. Location map of identified ferrous mineral resources in Minnesota. Since the early 20th century, the Biwabik Iron Formation has been subdivided into four informal members referred to as (from bottom to top): Lower Cherty member, Lower Slaty member, Upper Cherty member, and Upper Slaty member (Wolff, 1917). The cherty members are typically characterized by a granular (sand-sized) texture and thick-bedding (beds ≥ several inches thick); whereas the slaty members are typically fine-grained (mud-sized) and thin-bedded (≤1 cm thick beds). The cherty members are largely composed of chert and iron oxides (with zones rich in iron silicate minerals), while the slaty members are composed of iron silicates and iron carbonates with local chert beds. Both cherty and slaty iron-formation 47 �Trip 3 – Proterozoic Fe & Mn Formations types are interlayered at all scales, but one rock type or the other predominates in each of the four informal members, and they are so-named for this dominance Severson et. al. (2009). Leached and iron enriched direct ores (or natural ores) were the first materials mined, with the first shipments beginning in 1892, from strongly oxidized pockets along fault and fracture zones and the blanket oxidation of the iron formation at the surface. Taconite, which is the material that is mined today using magnetic separation methods, constitutes most of the iron formation and pertains to the hard, non-oxidized portions of the iron-formation. Production has been dominantly controlled by vertically integrated steelmakers since 1901, and therefore the mining and utilization of these ores have been dictated largely by US ironmaking capacity and demand. Taconite typically contains 30-35% iron and 40-50% SiO2, plus other components (Morey, 1992). The Biwabik Iron Formation is around 175-300 feet thick in the extreme eastern end of the Mesabi Iron Range at Dunka Pit, 730-780 feet thick in the central Mesabi Iron Range/Virginia Horn area near Eveleth, around 500 feet thick in the western Mesabi Iron Range near Coleraine, and eventually exhibits a “nebulous ending about 15 miles southwest of Grand Rapids” (Marsden et al., 1968) on the extreme western end of the Mesabi Iron Range. Maps of currently active taconite mining operations on the Mesabi Iron Range are presented in Figure 3-2 and compiled grade/tonnage ore reserve calculations for these operations are given in Table 3-1. Table 3-1. Reported grade/tonnage of active taconite mines. operations. Geology of the Cuyuna Iron Range The Cuyuna iron range is about 160 km west-southwest of Duluth in Aitkin, Cass, Crow Wing, and Morrison Counties (Fig. 3-1). It is part of an Early Proterozoic geologic terrane which occupies much of east-central Minnesota. The Cuyuna iron range is traditionally divided into three districts, the Emily district, the North range, and the South range (Fig. 3-3). The Emily district extends from the Mississippi River northward through Crow Wing County and into southern Cass County and comprises an area of about 1,165 square kilometers. Although exploration drilling has been extensive in the Emily district, mining never commenced. The North range, a much smaller area about 19 km long and 8 km wide, is near the cities of Crosby and Ironton in Crow Wing County. 48 �Trip 3 – Proterozoic Fe & Mn Formations Figure 3-2. Bedrock geology and iron mining features of the Mesabi Iron Range. 49 �Trip 3 – Proterozoic Fe & Mn Formations Figure 3-3. Bedrock geologic map of the Cuyuna Iron Range of Minnesota, illustrating the locations of the Emily District, North Range, and South Range. Since their discovery in 1904, it has been recognized that the iron-formations and associated ore deposits of the Cuyuna iron range in east-central Minnesota contained appreciable quantities of manganese, and large quantities of manganese were extracted as ferromanganese ores from several mines on the North range from 1911 to 1984. The presence of this manganese resource sets the Cuyuna range apart from other iron-mining districts of the Lake Superior region. Although relatively small, the North range was the principal site of mining activity (Fig. 3-4), which had largely ceased by 1970. The South range, where one small open pit and only a few underground mines were operated, in the 1910s and 20s, comprises an area of northeast-trending, generally parallel belts of iron-formation extending from near Randall in Morrison County northeast for about 100 km. In addition to the three named districts, numerous linear magnetic anomalies occur east of the range proper, and may indicate other, but currently poorly defined, beds of iron-formation. Three major insights regarding the geology of the Cuyuna range have emerged from the geologic mapping (Schmidt, 1963) and associated studies which utilized geophysical and drilling data (Southwick et al., 1988). First, there is clear evidence that iron sedimentation occurred at several different times and under varying geological conditions. This observation invalidates the stratigraphic premises of Morey (1978). Major iron-formations are associated stratigraphically with volcanic rocks in the South range, with black shale, argillite and rare volcanic rocks in the North range, and with shallow-water deposits of sandstone and siltstone in the Emily district. 50 �Trip 3 – Proterozoic Fe & Mn Formations Figure 3-4. Bedrock geology and open pit Fe-Mn mine map of the North Range of the Cuyuna Iron Range. Second, the iron-rich strata of the Emily district are correlative with the Biwabik Iron Formation of the Mesabi Range, as inferred by Marsden (1972) and Morey (1978). However, they and the other sedimentary rocks of the well-known Animikie Group occur above a major deformed unconformity that cuts across previously deformed, somewhat older sedimentary and volcanic rocks of the North range. There, a prominent iron-rich unit named the Trommald Formation, as well as several other units beneath the unconformity, forms part of a locally twice-deformed sequence. Therefore, the rocks of the North range and the Emily district cannot be correlative but are separate stratigraphic entities. Because the stratigraphic succession of folded sedimentary rocks on the North range comprises a distinct stratigraphic entity, Southwick et al., (1988) referred to it informally as the North Range group with the understanding that a formal name may be justified later. As defined by Schmidt (1963), the stratigraphic sequence in the North range consists of a quartz-rich lower sedimentary unit named the Mahnomen Formation, a middle iron- and locally manganese-rich sequence assigned to the Trommald Formation, and an upper greywacke shale interval called the Rabbit Lake Formation. Third, Southwick et al., (1988) recognized several geophysically defined structural discontinuities in the southern part of the Cuyuna iron range, within and southeast of the South range. These discontinuities are marked by demonstrable contrasts in metamorphic grade, by differing structural styles, and by different lithic components. One of the most pronounced of these, the Serpent Lake structural discontinuity, passes along the south edge of the North range. This discontinuity is interpreted as a tectonic boundary, probably involving major thrust faults between slices of folded rocks. Thus, it seems certain that the iron-rich strata of the South range are not correlative with either the Trommald Formation of the North range or the ironrich strata of the Emily district. The fact that iron-formation occurs within three different stratigraphic and structural contexts in the Cuyuna iron range is of considerable importance to the ultimate development of manganese resources. Since we now recognize that the Emily district, the North range, and the South range are separate entities, we can no longer develop regional syntheses that extrapolate mineralogical and structural attributes from one entity to another. 51 �Trip 3 – Proterozoic Fe & Mn Formations Cuyuna Iron Range Manganese Resources Several attempts have been made over the last 70 years to estimate the size of the manganese resources of the Cuyuna iron range. For example, Lewis (1951) estimated that 455 million metric tons of manganiferous iron-formation containing from 2 to 10 percent manganese were available to open-pit mining to a depth of 45 meters. Dorr et al., (1973) used that estimate to establish that the Cuyuna range contains approximately 46 percent of known manganese resources in the United States. US Steel geologist Richard Strong (1959) estimated iron and manganese resources from several well-drilled deposits in the Emily District and Beltrame et al., (1981) estimated a minimum of 170 million metric tons of manganiferous rock with an average grade of 10.46 weight percent manganese. All historic grade/tonnage estimates (Lewis, 1951, Strong, 1959, and Beltrame et al., 1981) should be considered with a certain amount of skepticism for at least two reasons. First, the manganese data used to make these estimates were, for the most part, by-products of data that were acquired originally by various mining companies as they explored for iron. Second, the various estimates were prepared for different reasons at different times, using different databases and different methodologies. Therefore, the results of these estimates are neither comparable, nor do they necessarily reflect the actual resource. A table listing the grade and tonnage from properties that Strong (1959) and Beltrami et al. (1981) estimated manganese resources is given in Table 3-2 and a location map of these properties is presented in Figure 3-5. Table 3-2. Manganese grade and tonnage estimates from reports by Strong (1959) and Beltrame et al. (1981). 52 �Trip 3 – Proterozoic Fe & Mn Formations Despite their problematic nature, the estimates of Lewis (1951) and Beltrame et al., (1981) do show that the Cuyuna range contains a large, but low- to moderate-grade manganese resources remaining. This large size, combined with the fact that the manganese deposits are in an established mining district, makes the Cuyuna range an ideal place to study geological and technological factors needed to evaluate this and other sedimentary manganese deposits in the United States. Especially important are studies of the geologic habit of the manganese and the controls on its distribution and subsequent concentration into deposits of minable size. Figure 3-5. Bedrock geology and location map of properties outlined in reports by Strong (1959) and Beltrame et al. (1981) that includes manganese grade-tonnage estimates. Labeled parcels correlate with the MAP ID column in Table 3-2 and are those with an estimated resource greater than 100,000,000 pounds of manganese metal. 53 �Trip 3 – Proterozoic Fe & Mn Formations Penokean Orogeny The Penokean orogeny began at about 1880 Ma when an oceanic arc, the Paleoproterozoic Pembine–Wausau terrane, collided with the southern margin of the Archean Superior (Laurentia) craton marking the end of a period of south-directed subduction. The docking of the buoyant craton to the arc resulted in a subduction jump to the south and development of back-arc extension both in the initial arc and adjacent craton margin to the north. Synchronous extension and subsidence of the Laurentia craton resulted in the development of broad shallow seas overlapping the Archean craton. The classic Superior-type banded iron-formations of the Lake Superior District, including those in the Marquette, Gogebic, Mesabi, and Gunflint Iron Ranges, formed in that sea. The newly established subduction zone caused continued arc volcanism until about 1850 Ma when a fragment of Archean crust, now the basement of the Marshfield terrane, arrived at the subduction zone. The convergence of Archean blocks of the Superior and Marshfield cratons resulted in the major contractional phase of the Penokean orogeny. Rocks of the Pembine–Wausau arc were thrust northward onto the Superior craton causing subsidence of a foreland basin in which sedimentation began at about 1850 Ma in the south (Baraga Group rocks) and 1835 Ma in the north (Rove Formation). A thick succession of arc-derived turbidites constitutes most of the foreland basin-fill along with lesser volcanic rocks. In the southern fold and thrust belt, tectonic thickening resulted in high-grade metamorphism of the sediments by 1830 Ma. At this same time, a suite of post-tectonic plutons intruded the deformed sedimentary sequence and accreted arc terranes marking the end of the Penokean orogeny. A regional geologic map of the Penokean orogen, modified from Schulz and Cannon (2007), is given in Figure 3-5. Figure 3-5. Generalized geologic map of the Penokean orogen. Abbreviations: ECMB - East-central Minnesota batholith; EPSZ - Eau Pleine shear zone; MD - Malmo discontinuity; NFZ - Niagara fault zone. Modified from Schulz and Cannon, 2007. The Penokean deformation in Minnesota includes a southern intensely and complexly deformed series of thrust panels (Cuyuna North, Cuyuna South, Moose Lake, McGrath-Little Falls panels) that gives way northward to progressively more weakly and simply deformed rocks (Emily District) across a belt 54 �Trip 3 – Proterozoic Fe & Mn Formations about 100 km wide. Farther north strata in the Mesabi and Gunflint Iron Ranges are essentially undeformed (Holst, 1991). It should be noted that the “more weakly and simply deformed rocks” of the Emily District have been shortened ~250% into a series of shallowly east-plunging anticlines and synclines. Substantial progress has been made in deciphering the structure of the poorly exposed rocks of the Minnesota foreland through the use of aeromagnetic and gravity data and drillhole information. Southwick and Morey (1991) and Southwick et al. (1988) have presented syntheses of this information. The complex thrust panels on the south, like comparable structures in Michigan, appear to be thinskinned slices without Archean basement. However, as in Michigan, this area of thin-skinned thrusting is also the area where Archean-cored gneiss domes developed during post orogenic collapse of the Penokean orogen (Holm and Lux, 1996; Schneider et al., 2004). Farther north, basement-cover relations are not well known except for the Mesabi Range where Paleoproterozoic strata are mostly nearly flat lying above an undisturbed unconformity with Archean basement rocks. A schematic north-south geologic cross section of the Penokean orogeny in Minnesota, modified from Southwick and Morey (1991) is presented in Figure 3-6. Figure 3-6. Schematic diagram illustrating the interpreted tectonic setting of the Penokean orogen in Minnesota. A) continental margin sedimentation, and B) thin-skinned thrusting and deformation related to the Penokean orogeny. Modified from Southwick & Morey, 1991. Post Penokean Weathering and Erosion Perhaps the most important component in the formation of the high-grade iron and manganese ores on the Mesabi and Cuyuna ranges is the vast amount of time (measured in hundreds of millions of years) upon which the newly-formed and uplifted Penokean mountains of the southern Laurentia craton weathered and eroded. As plate tectonic forces moved Laurentia across the globe to its current position on planet Earth, there were long periods of time when it resided within the tropical weathering zone (+30° to -30° latitude) near the Earth’s equator. It is believed that the supergene enrichment of iron (to >60 wt.% elemental Fe) and manganese (to >50 wt.% elemental Mn) on the Mesabi and Cuyuna largely formed during the protracted periods of time that the area resided within the tropical weathering zone. A paleogeographic reconstruction of the location of Laurentia on planet Earth is given in Figure 3-7. 55 �Trip 3 – Proterozoic Fe & Mn Formations Figure 3-7. Paleogeographic reconstruction of the Laurentia craton from the Paleoproterozoic to present times. FIELD TRIP STOPS Four field trip stops have been selected to showcase selected geological features associated with supergene weathering of primary Paleoproterozoic Superior-type iron formation into high-grade of Fe and Mn ores. The locations of the field trip stops are shown in Figure 3-8 and briefly described below. 1) DNR drillcore library in Hibbing: Drillcore review of unoxidized Biwabik Iron Formation, 2) Emily deposit core shed: Drillcore review of high-grade supergene Mn ores, primary Mn-Fe carbonate-facies iron formation, Overlying Sudbury Impact breccias & accretionary lapilli? 3) Mary Ellen Mine: Walk into a historic natural-ore (hematite) open pit iron mine, sampling of the classic Mary Ellen stromatolites, and 4) Large roadcut of the partially oxidized Biwabik Iron Formation adjacent to the historic naturalore Rouchleau Mine Complex. Stop 1: DNR Drillcore Library, Hibbing Minnesota Longitude/Latitude: 47.432412°N, -92.941811E UTM NAD 83 Zone 15N: 504388E, 5253220N At our first stop on this field trip, we will examine sections of two different drill cores of the Biwabik iron formation to directly compare depositional features to post-depositional features. The Drill Core Library is maintained by the Minnesota Department of Natural Resources, Lands and Minerals Division, and provides direct access for visitors to examine publicly owned geologic materials and exploration data. This incredible 56 �Trip 3 – Proterozoic Fe & Mn Formations Figure 3-8. Location map of field trip stops and the collar location of the two drill holes looked at Stop 1. repository contains over 7,000 mineral exploration cores, 1,500 roadway and bridge foundation cores, and 500 cores collected during scientific, governmental, and academic research, and their curation activities support researchers, exploration geologists, and engineers from around the world. Depositional features and mineralogy in Precambrian chemical sedimentary rocks like iron formations have long been of interest to the scientific community as they may record information about Earth’s early surface conditions. However, recovering data that links to depositional conditions requires the reconstruction of post-depositional mineral reactions and quantification of geochemical exchange. For this reason, the original mineralogy and geochemistry of iron formations has been the subject of numerous investigations. Critical early investigations recognized the mineral greenalite - postulating its authigenic origin as a “chemical oceanic precipitate”, and hypothesizing its role in forming the iron ore deposits in Minnesota, USA (Irving, 1886; Irving and Van Hise, 1892; Leith, 1903; Van Hise and Leith, 1911; Aldrich 1929; Gruner, 1946; Tyler, 1949; James, 1954; White, 1954; Goodwin, 1956; Gundarson and Schwartz, 1962; LaBerge; 1964). Key initial evidence for a primary or early origin for greenalite in the Superior craton included: (1) its abundance in low temperature, well-preserved assemblages, and comparative absence in metamorphosed iron formations in the same region (LaBerge, 1964); (2) the existence of submicroscopic 57 �Trip 3 – Proterozoic Fe & Mn Formations greenalite in the cores of circular to elliptical sand-sized grains as either the main mineral phase, <0.05 mm spherules, or as dusty nano-scale (submicroscopic) particles (Goodwin, 1956); intergranular relationships between phases where greenalite is crosscut by other phases in the same sample; (LaBerge, 1964; French, 1973; Klein and Fink, 1976), and even distribution throughout primary bedding (LaBerge, 1964). Recent mineralogical investigations (e.g, Duncanson et al., 2024, Muhling et al., 2025 and references therein) highlight that the earliest forming minerals in iron formation are commonly found within silica-cemented horizons, where abundant chert cement silicified the sediments at or near the sediment water interface. Such silica-cemented horizons preserve incomplete reactions and allow for identification of direct mineral relationships and local element exchange. Common mineral reactions observed in the Biwabik iron formation include the transformation of greenalite to minnesotaite, minnesotatite to stilpnomelane, greenalite to magnetite, siderite to magnetite, and magnetite to hematite. Of the mineral reactions that commonly occur in iron formation, transformations of Fe2+-containing silicates like greenalite to mixed valence state minerals like magnetite and further oxidation of magnetite to hematite, contributed to formation of iron ores in the Biwabik. To illustrate some of the many post-depositional reactions that occur in iron formations worldwide, we will examine a small section of two drill cores, MGS 8 from the western end of the Mesabi iron range, and LWD-99-01 from near the Virigina horn area (see Figure 3-8 for locations). LWD-99-01 clearly preserves depositional features, while MGS-8 documents abundant post-depositional oxidation throughout. Stop 2: North Star Manganese Inc Drillcore Shed, Emily Minnesota Longitude/Latitude: 46.753571°N, -93.973496E UTM NAD 83 Zone 15N: 425650E, 5178240N Historic exploration and drilling in the 1940’s and 1950’s by Pickands Mather and US Steel identified iron and manganese-bearing mineralization within the Emily Iron Formation. US Steel developed but did not implement a preliminary mine plan for mining of the Emily Deposit. Following approximately 50 years of inactivity, Cooperative Mineral Resources (subsidiary of Crow Wing Power) pursued a pilot mining operation using pressurized water that was ultimately unsuccessful. As a follow up investigation into the outcomes of pilot mining, a small-scale drill program was accomplished in 2010-2012. A drilling program was designed and executed by Big Rock Exploration, LLC, in 2022-2023. A total of 29 drill holes were completed to extend mineralization and refine the previous resource estimates. A total of 13,107 feet of drilling was completed for this program. Data collected for this project includes lithological, structural, geotechnical, geochemical and geophysical data from the drill core. Through interpretation of legacy, recent and new drilling data, Big Rock Exploration identified coherent zones of high-grade manganese mineralization (30 to ≥40 wt.% Mn) over a 1.25-kilometer strike length. Mineralization is comprised of horizons of secondary manganese oxide minerals, as well as locally present primary iron-manganese carbonate mineralization. An ore deposit model has been developed that incorporates the oxidation of primary thin-bedded manganese-iron carbonates into massive manganese oxide through early folding and prolonged periods of weathering, oxidation, and erosion. This ore deposit model and associated geological model have been used to support an updated and expanded mineral resource estimate (Table 3-3) for the Emily Deposit that was completed and published by Forte Dynamics (Hulse et al., 2024) on May 24, 2024. We’ll first begin the review of important geological features revealed in the four Emily Mn deposit drillholes on display in the core shed by elucidating our current understanding of the geology and structure for the whole Cuyuna Iron Range and then focus specifically on the stratigraphy and ore-forming processes at Emily through a series of figures and descriptive text written into a technical report (Steiner et al., 2024) and orally presented at the 2024 ILSG conference (Peterson and Steiner, 2024). 58 �Trip 3 – Proterozoic Fe & Mn Formations Table 3-3. Mineral resource estimate for the Emily Manganese deposit. Figure 3-9. Continent-scale initial condition framework of Paleoproterozoic iron formations of Minnesota. Initial Conditions Deposition of Paleoproterozoic iron formations of the Lake Superior district all owe their origins to the ~2.4 – 2.1 Ga. rifting of the Wyoming Province craton off of the southern Superior Province craton (Figure 3-9). This rifting set the stage for the development of environments of deposition conducive to the formation of thick sequences of both Algoma- and Superior-type iron formations (Figure 3-10). During the Penokean orogeny (see Figure 3-6) these variable environments of iron formation deposition were transposed northwestward via thin-skinned tectonics into a fold & thrust belt (Cuyuna North and South range thrust panels) over a series of thrust-front folds (Emily District) that was bounded by a basal decollement. Outcomes of the Penokean orogeny in the Cuyuna Range of central Minnesota are shown in an idealized cross sectional view Figure 3-11 and how ~1.8 billion years of erosion has left it today in Figure 3-12. 59 �Trip 3 – Proterozoic Fe & Mn Formations Figure 3-10. Schematic model for the variable environments of deposition of Paleoproterozoic iron formations of the Cuyuna and Mesabi ranges of Minnesota. Figure 3-11. Idealized cross section of Minnesota’s Penokean Mountains of central Minnesota approximately 1.83 billion years ago. Figure 3-12. Schematic representation of the results of deep weathering and erosion of the Penokean Mountains in central Minnesota. 60 �Trip 3 – Proterozoic Fe & Mn Formations Geologic Maps Geologic maps are the foundation upon which geologists interpret Earth processes and depict on a piece of paper the final outcomes of such processes in plan form. As such, the authors have added annotated bedrock geology map of the Cuyuna Range in Figure 3-13 and more specifically for the Emily District of the Cuyuna Range in Figure 3-14. Figure 3-13. Annotated bedrock geologic map of a portion of the Cuyuna Range, central Minnesota. Clipped from the map of Peterson (2022). Figure 3-14. Annotated bedrock geologic map of the Emily District, Cuyuna Range, central Minnesota. Modified after Peterson, 2022. 61 �Trip 3 – Proterozoic Fe & Mn Formations Figure 3-15. Schematic stratigraphic section through the Emily Manganese deposit, after Peterson & Steiner, 2024. Stratigraphy Relogging of historic drill core and logging of new core drilled during the 2023 exploration drilling program has led to the identification of a predictable stratigraphic sequence at the Emily Deposit (Fig. 315). The four formations recognized at the Emily Deposit include three units of the Paleoproterozoic Animikie Basin unconformably overlain by Quaternary glacial drift. Descriptions of these stratigraphic units from the oldest to youngest are given below: 1. Pokegama Formation – White-grey, tan, or cream-colored indurated argillite and quartzite. The Pokegama formation at the Emily deposit is typically a clayey siltstone, though mudstones and quartz-arenites are common. Bedding is quite variable ranging from massive siltstones and quartzarenites to thick, medium, and thin bedded or even finely laminated clayey-siltstones and mudstones. The Pokegama formation does not host significant manganese mineralization but when observed manganese minerals occur in trace amounts in veinlets or as small patches with iron oxides. 2. Emily Iron Formation – See below for subdivision descriptions. 3. Virginia Formation – Grey-brown in color, red when oxidized, fine-grained, well bedded clastic sediments commonly forming turbidite sequences. Fine-scale bedding, graded beds, and sandy lenses are common. Thin horizons of lean iron formation (subunit Pvif) composed of ferruginous chert occur locally. The basal 20-40 feet is characterized by highly disrupted and fragmented turbidite clasts sed in a poorly sorted massive matrix. This horizon has been hypothesized to be landslides associated with the Sudbury Impact. 4. Glacial Overburden – Unconsolidated glacial material including well sorted sands, lacustrine clays, and unsorted glacial till. The preservation of earthy hematite and saprolitic materials immediately below the basal angular unconformity indicates that overlying Laurentide ice sheet was not eroding its base in the immediate deposit area. Composition of the overburden is inferred from drill returns during tri-cone drilling. Emily Iron Formation is further divided into five sub-units. Criteria for subunit designation requires that a given interval be sufficiently distinctive in petrologic character to be easily identified, and laterally extensive enough to be intercepted in multiple boreholes. Distinctive petrologic characteristics may be texture (e.g., banded or granular iron formation, composition (e.g., chert, carbonate), or unique 62 �Trip 3 – Proterozoic Fe & Mn Formations characteristics such as stromatolites. The subdivisions of the Emily Iron Formation are as follows from bottom to top: 1. Peif1 – This unit is located as the base of the Emily Iron Formation lies conformably atop the Pokegama quartzite. The base is commonly cherty or stromatolitic before giving way to grain stones. The majority of the unit is a red-brown to black, granular iron formation (GIF) or ferruginous quartzose sandstone. The upper part (above the Peif1r marker, see below) is granular iron formation composed of silicious, hematitic granules that range from <1 to 2mm in size. Granular iron formation is weakly bedded. The lower part of Peif1 (below the Peif1r marker horizon) contains abundant quartzose sands cemented by iron oxides giving the rock a purple appearance. Sands are composed of well-rounded fine-grained quartz and are interbedded with granular iron formation. Manganese oxide mineralization is most intense within the Peif1 unit. Manganese oxides occur in multiple styles including massive Mn-oxide that replaces all original textures (may be within a bed or cross bedding), interstitial to grains (replacing the original iron cement?), and as veinlets. The most intense manganese oxide mineralization within Peif1 occurs adjacent to (above and below) the Peif1r marker horizon, though mineralization may occur throughout the unit. The lower Peif1 commonly exhibits a pock-mark texture when strongly mineralized. A thin stromatolite horizon (Peifbs) typically occurs at the base of Peif1. 2. Peif1r – This unit is located within the Peif1 unit. Usually <3m thick, the Peif1r is composed of finegrained hematite-chert banded iron formation. The base of the Peif1r hosts distinctive digitate stromatolites. 3. Peif2 – This unit lies conformably atop Peif1, usually gradually transitioning from granular iron formation (Peif1) to banded iron formation (Peif2) over a meter. The Peif2 is characterized by finegrained well-bedded banded iron formation though the composition of the iron formation is variable. The most common composition for Peif2 is a hematite-chert banded iron formation, though this appears to be a secondary, altered composition. The primary composition is iron-manganese carbonate facies type iron formation. The carbonate facies are cream to greenish and gradually becomes red with increased oxidation. Fresh carbonate facies contain much more manganese than oxidized material, with the manganese found in rhodochrosite. This represents a very different manganese host than the oxide mineralization found in the granular iron formation units (Peif1 and Peif3). 4. Peif3 – Peif3 is characterized by interbedded medium to fine grained GIF and fine grained, narrow BIF lenses. This unit lies conformably atop Peif2 where the contact is a graduation from BIF to GIF dominant facies. Both GIF and BIF are weakly to moderately strongly bedded and silicious in composition. Peif3 is commonly mineralized manganese oxides, only subordinate in manganese endowment to Peif1. 5. Peif4 – White to grey, massive chert with mottled patches of iron-oxides. The sharp basal contact with the underlying Peif3 is often “sheared” possibly from bedding parallel slip. Some parts of the massive chert contain faint outlines of granules while most is simply massive white chert. The mottled iron oxides include large irregular pods and stringers, sometimes reaching a meter in width. Manganese is rarely found within the oxide pods. 6. Peif5 – Well-bedded banded iron formation consisting of 2-5cm beds of chert and iron oxide in gradational contact with the underlying Peif4 massive chert. This unit is the least spatially consistent, due to a lack of drillhole intercepts and difficulty identifying it as a result of intense oxidation. Supergene Enrichment of Manganese The unique manganese endowment of the Emily Iron Formation is attributed to the primary deposition of Mn-carbonates in a shallow water environment (Figures 3-10, 3-16 and 3-17). However, 63 �Trip 3 – Proterozoic Fe & Mn Formations subsequent weathering, erosion, and oxidation has redistributed much of the manganese from the carbonate unit to other areas. The majority of the manganese mineralized material at the Emily deposit is composed of manganese oxides including manganite, jacobsite, and cryptolomene/hollandite. However, these phases are not the stable manganese phase predicted by geochemical modelling of early oceans (Mitra et al., 2022). Instead, manganese carbonates are the predicted stable phase. Therefore, the manganese oxide minerals that constitute the majority of the orebody must have formed at a later stage. The tectonics during and immediately after deposition of the Animikie basin sediments provide a plausible explanation for the extremely high-grade manganese oxide formation. Figure 3-16. Carbonate facies iron formation where the gradual oxidation of primary carbonates can be observed from left to right. Note the increasingly hematite rich BIF from left to right. Primary silicate and carbonate minerals in iron formations are well documented to be unstable under oxidizing, near surface conditions. For example, the formation of direct ship ores of the Biwabik Iron Formation on the Mesabi Iron Range has been ascribed to deep weathering of primary Fe-minerals (e.g., greenalite and siderite) over hundreds of millions or a billion years. The direct ship ores were composed of hematite and goethite. The Emily Iron Formation, having been deposited contemporaneously with the Biwabik Iron Formation, would have endured at least as much weathering over that period. The weathering and subsequent supergene enrichment of manganese is related to the hydrogeologic and geochemical interaction between interbedded banded (BIF) and granular (GIF) iron formation. Manganese in the Emily Figure 3-17. Emily deposit drill core that seemingly documents that the oxidation of carbonate-facies (rhodochrosite-siderite-chert) BIF generates classic thin-bedded hematite-jasper BIF as well as being the primary source of Mn3+ that forms the massive manganese oxide zones in permeable GIF horizons. 64 �Trip 3 – Proterozoic Fe & Mn Formations Iron Formation was originally co-precipitated with iron carbonate minerals (rhodochrosite MnCO3 and siderite FeCO3) within the banded iron formation lithotype (Peif2 subunit). Primary carbonates are observed at various stages of oxidation in several boreholes (e.g., NSC-23005). Like the Biwabik Iron Formation, the Emily Iron Formation underwent a protracted period of weathering and oxidation. Exposure of carbonate facies iron formation to oxidizing waters over that period is hypothesized to be the causative mechanism for supergene manganese enrichment at the Emily deposit. Oxidized meteoric water percolating through the carbonate iron formation reacts with and dissolves the carbonates, liberating manganese from rhodochrosite and converting siderite to hematite. The restite lithology appears very similar to hematiterich banded iron formation (Fig. 3-17). The now manganese enriched waters redistribute manganese downslope to other subunits of the Emily Iron formation. The second important litho-type, granular iron formation, is recognized as the primary manganeseoxide ore hosts at the Emily Deposit. Granular iron formation is composed of granules of varying compositions (e.g., Fe-silicate, chert, Fe-carbonates) with pore space found between the granules. That pore space creates permeability that drives fluid flow through the granular iron formation thereby moving and redepositing manganese from the enriched waters leaving the carbonate facies banded iron formation. The observations from drillcore logging at Emily indicate that manganese oxides are not found in significant concentrations within the banded iron formations, but manganese oxides are abundant in the granular iron formation. The migration of manganese-rich waters from the banded iron formation into the granular units is the primary redistribution mechanism for manganese. Once manganese enriched waters enter the granular units, it is unclear by what mechanism the precipitation of manganese occurs. However, manganese oxide minerals are observed in the interstices between granules indicate direct precipitation from the pore fluids. It is unclear whether this interstitial manganese is the result of filling otherwise empty pore space if it is the result of replacement of prior GIF matrix. Additionally, pock marked textures in manganese-rich units suggest that granules may be replaced by the manganese-rich fluids, though the mechanism by which this may occur is unclear due to a lack of mineralogical constraints. Stratigraphic and Structural Controls on the Distribution of Secondary Manganese The compression associated with the Penokean Orogen uplifted the rocks Emily deposit. Folding and subsequent uplift of these originally shallow water subaqueous rocks into the Penokean mountains has important hydrogeological implications by greatly lowering the water table and exposing the Emily Iron Formation to oxidizing meteoric waters. The Emily deposit is on the northernmost anticline of the Penokean fold and thrust belt, specifically within a parasitic syncline along the norther limb of the larger anticline. The structural geometry established during the Penokean provides a hydrogeologic “slope” that meteoric waters can migrate down under the influence of gravity. In particular, the parasitic syncline that hosts the Emily Deposit (see Figure 3-14), likely acted like a funnel or gutter that focused fluid flow through the rocks that now constitute the deposit. A schematic stepwise ore genesis model for the Emily deposit is presented in Figure 3-18. On a deposit scale and within this structural setting, the two major litho-types play an important role in the movement of fluids due to their contrasting hydrogeological characteristics. In particular, the Peif1r stromatolite horizon represents an aquitard that seemingly focused fluid along its margins. The focusing of fluids along these margin manifests as exceptionally high Mn-grades (often massive manganese oxides) at the upper and lower contacts with flanks GIF of Peif1. Similarly, but to a lesser extent, the basal contact with the Pokegama formation and the contact between Peif2 and Peif3 represent areas of contrasting hydrogeologic characteristics that may concentrate mineralizing fluids. Both areas are observed to host massive manganese oxide mineralization, supporting such a relationship. 65 �Trip 3 – Proterozoic Fe & Mn Formations Figure 3-18. Emily deposit Mn-Oxide ore deposit model that incorporates stratigraphy, permeability, processes, and time, after Peterson & Steiner, 2024. Drillholes on Display Four drillholes from the 2023 exploration program at Emily will be on display for the 2025 ILSG field trip. These holes include: 1) NSC-23002A - high-grade Mn-oxide ores at the bedrock interface in Peif1, 2) NSC-23004 - an almost complete stratigraphic section through the Emily IF, 3) NSC-23005 - Peif2 with carbonate facies IF and Sudbury Impact breccias in unit Pvf, and 4) NSC-23050 – The Western-most drillhole, nearly complete section of the Emily IF. A detailed bedrock geology and drillhole location map of North Star Manganese Inc’s Emily Project is presented in Figure 3-19, and striplogs of the four holes on display are given in Figures 3-20, 3-21, 3-22, and 3-23. 66 �Trip 3 – Proterozoic Fe & Mn Formations Figure 3-19. Detailed bedrock geology and drillhole location map of North Star Manganese Inc’s Emily project. Note that the collar location of the four drillholes on display are highlighted by the small yellow circles. Modified after Steiner et al., 2024. 67 �Trip 3 – Proterozoic Fe & Mn Formations Figure 3-20. Striplog for drillhole NSC-23002A. Figure 3-21. Striplog for drillhole NSC-23004. 68 �Trip 3 – Proterozoic Fe & Mn Formations Figure 3-22. Striplog for drillhole NSC-23005. Figure 3-23. Striplog for drillhole NSC-23050. 69 �Trip 3 – Proterozoic Fe & Mn Formations Stop 3: Mary Ellen Mine, Biwabik Minnesota Longitude/Latitude: 47.527677°N, -92.366645E UTM NAD 83 Zone 15N: 547675E, 5264000N The historic natural ore Mary Ellen mine (Figure 3-24) near Biwabik is probably most well-known today as the source of Mary Ellen Jasper, a world-class type-locality of Precambrian stromatolites. Rocks and polished slabs of stromatolites from the Mary Ellen mine can be found in natural history museums throughout the world, and you’ll get to find and take-home pieces yourself during this field trip. According to several annual mining directories, the Stanley Iron Mining Company operated the Mary Ellen Mine between 1924 and 1928, with stockpile shipments occurring in 1929 and 1930. It actively mined the property again between 1948 and 1951. Beginning in 1952, the Pioneer Mining Company worked the mine, continuing to do so through 1961. The Pittsburgh Pacific Company operated it for one final year, in 1962. Its cumulative output of natural ore was 4,574,973 long tons, again according to an annual mining directory. An interesting quote from the 2015 book titled: Stromatolites Ancient, Beautiful and EarthAltering, by Bruce Stinchcomb & Bob Leis copied below strongly hints that some oxidation-related processes that formed the high-grade earthy hematitic iron ores were similar to those outlined for the highgrade manganese oxide ores at Emily. The quote is as follows, "In the early days of iron mining in Minnesota, the location of stromatolite material would indicate that an iron rich vein was close. The Mary Ellen Stromatolite material could be as much as 15 feet thick and would have to be removed before mining could commence. To the miners this material was considered a nuisance and a waste product." Figure 3-24. Simplified bedrock geology and iron mine map of the Mary Ellen mine area. 70 �Trip 3 – Proterozoic Fe & Mn Formations Stop 4: Rouchleau Mine Complex Bridge, Virginia Minnesota Longitude/Latitude: 47.516178 °N, -92.518663 E UTM NAD 83 Zone 15N: 536240 E, 5262640 N The Thomas Rukavina Memorial Bridge carries U.S. 53 over the Rouchleau Mine pit connecting Virginia, Minnesota with other cities to the south. U.S. 53 continues through Virginia to International Falls and Canada; International Falls is about 100 miles (160 km) north of Virginia. The bridge, opened in 2017, was named after Tom Rukavina in 2021 following his death in 2019. Rukavina was a state legislator from the Iron Range. At 204 feet tall, it is the tallest bridge in Minnesota. This bridge carries a traffic volume of about 22,200 cars per day, making it one of the most-traveled highway segments on the Iron Range. The bridge also features a bike lane and pedestrian walkway (the Mesabi Trail) that leads to trails connecting Gilbert and Virginia. In 1960, the state of Minnesota and the mining companies in the area came to an agreement that allowed the construction of U.S. 53 across lands held by the mining company without the state paying anything for the land. The agreement stipulated that after 1987, the state would be responsible for the costs involved with moving the roadway to allow for mining after given advance notice by the mining companies. The two owners of the land notified MnDOT of their intent to mine the site in 2010 which gave the state until 2017 to move the roadway. Cliffs Natural Resources, which had a nearby active mine, hoped to begin mining the site by 2017. After evaluating several more expensive options that involved longer bridges or routing US 53 across an active mine pit, an alignment was selected that resulted in the highest bridge in Minnesota. A route on level ground away from the mining formation was identified as too disruptive to development patterns in the area. The entire project estimated to cost $220 million with $159 million for construction of the bridge and diverted roadway. The bridge crosses the Rouchleau Mine pit.[9] The water filled pit also serves as Virginia's water supply. The final cost was $230 million with $30 million coming from the federal government and the remaining from the state. To prevent the need to move the bridge in the future, the state purchased the mineral rights for the land beneath roadway for $15 million. References Aftabi, A., Atapour, H., Mohseni, S., and Babaki, A., 2021, Geochemical discrimination among different types of banded iron formations (BIFs): A comparative review, Ore Geology Reviews, Volume 136. Aldrich, H. R., 1929, The geology of the Gogebic Iron Range of Wisconsin: Wisconsin Geol. Survey Bull. 21, 279 p. Beltrame, R.J., Holtzman, R.C., and Wahl, T.E., 1981, Manganese resources of the Cuyuna range, east-central Minnesota: Minnesota Geological Survey Report of Investigations 24, 22 p. Berg, T., Peterson, D.M., and Sweet, G., 2022, The Emily Manganese Deposit, Crow Wing County, Minnesota: A mineral resource evaluation for North Star Manganese Inc, Big Rock Exploration technical report BRE-TR2022-02, 33 pages, 3 appendices. Dorr, J.VN., II, Crittenden, M.D., Jr., and Worl, RG., 1973, Manganese, in Probst, D.A., and Pratt, W.P., eds., United States Mineral Resources: U.S. Geological Survey Professional Paper 820, p. 385-399. Duncanson S and 5 coauthors (2024) Reconstructing diagenetic mineral reactions from silicified horizons of the Paleoproterozoic Biwabik Iron Formation, Minnesota. American Mineralogist 109: 339-358, doi: 10.2138/am2022-8776. Goodwin, A.M., 1956, Facies relations in the Gunflint iron-formation: Ecox. GEOL., v. 51, p. 565-595. Gruner, 1946, Mineralogy and geology of the Mesabi range: Iron Range Resources and Rehabilitation, St. Paul, Minn., 127 p. Gunderson, J. N., and Schwartz, G. M., 1962, The geology of the metamorphosed Biwabik iron-formation, Eastern Mesabi District, Minnesota: Minnesota Geol. Survey Bull. 43, 139 p. Hofstra, A.H., and Kreiner, D.C., Systems-Deposits-Commodities-Critical Minerals Table for the Earth Mapping Resource Initiative: U.S. Geological Survey Open-File Report 2020-1042, 24 p. Holm, D.K., Lux, D.R., 1996, Core complex model proposed for gneiss dome development during collapse of the Paleoproterozoic Penokean orogen, Minnesota, Geology 24, 343–346. 71 �Trip 3 – Proterozoic Fe & Mn Formations Holst, T.B., 1991, The Penokean orogeny in Minnesota and Upper Michigan, U.S. Geological Survey Bulletin 1904D, 10 pages. Hulse, D.E., Irons, A., and Malhotra, D., 2024, Electric Metals (USA) Limited Emily Manganese Project, NI 43-101 Technical Report, Project No. 219001, Forte Dynamics, 89 pages. Irving, R. D., 1886, Origin of the ferruginous schists and iron ores of the Lake Superior region: Am. Jour. Sci., v. 32 p, 255-272. Irving and Van Hise, C. R., 1892, The Penokee iron-bearing series of Michigan and Wisconsin: U.S. Geol. Survey Mon. 19, 534 p. James, H. L., 1954. Sedimentary facies of iron-formation. Economic Geology, 49(3), 235–293. Klein, C., and Fink, R.P., 1976, Petrology of the Sokoman Iron Formation in the Howells River area, at the western edge of the Labrador trough: Economic Geology, v. 71, p. 453–487. LaBerge, G.L., 1964, Development of magnetite in iron-formations of the Lake Superior Region: Econ. Geol., V. 59, p. 1313-1342. Leith, C. K., 1903, The Mesabi iron-bearing district of Minnesota: U.S. Geol. Sur. Mono. 43, 316 p. Lewis, W.E., 1951, Relationship of the Cuyuna manganiferous resources to others in the United States, in Geology of the Cuyuna Range Mining Geology Symposium, 3rd, Hibbing, Minnesota, Proceedings: Minneapolis, University of Minnesota, Center for Continuation Study, p. 30-43. Marsden, R.W., 1972, Cuyuna district, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 227-239. Marsden, R.W., Emanuelson, J.W., Owens, J.S., Walker, N.E., and Werner, R.F., 1968, The Mesabi Iron Range, Minnesota, in Ridge, J.D. (ed.), Ore Deposits of the United States, 1933-1967: New York, American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc., The Grafton-Sales Volume, v. 1, p. 518-537. Mitra, Kaushik, Eleanor L. Moreland, Greg J. Ledingham, and Jeffrey G. Catalano, 2023, Formation of manganese oxides on early Mars due to active halogen cycling, Nature Geoscience 16, no. 2, p. 133-139. Morey, G.B., 1978, Lower and Middle Precambrian stratigraphic nomenclature for east-central Minnesota: Minnesota Geological Survey Report of Investigations 21, 52 p., 1 pIate. Morey, G.B., 1992, Chemical composition of the eastern Biwabik Iron Formation (Early Proterozoic), Mesabi Iron Range, Minnesota: Economic Geology, v. 87, p. 1649-1658. Muhling, J., Brengman, L., & Johnson, J. Greenalite (2025, June issue): Cryptic mineral of ancient ferruginous oceans. Elements: Greenalite - Tiny crystal with a big story, Elements (in press). Peterson, D.M., 2022, Bedrock geology and manganese mineral resource assessment map of the Emily Manganese Deposit, Big Rock Exploration map BRE-MAP-2022-02, 1:50,000 scale. Peterson, D.M., and Steiner, A., 2024, The geology, history, and ore deposit model of the high-grade Emily Manganese Deposit, Cuyuna Range, Minnesota: Oral presentation, Institute on Lake Superior Geology conference, Houghton, Michigan. Schmidt, R.G., 1963, Geology and ore deposits of Cuyuna North range, Minnesota: U.S. Geological Survey Professional Paper 407, 96 p. Schneider, D.A., Holm, D.K., O’Boyle, C., Hamilton, M., Jercinovic, M., 2004, Paleoproterozoic development of a gneiss dome corridor in the southern Lake Superior region, U.S.A. In: Whitney, D.L., Teyssier, C., Siddoway, C.S. (Eds.), Gneiss Domes in Orogeny. Geol. Soc. Am. Spec. Pap. 380, pp. 339–357. Schulz, K.J. and Cannon, W.F., 2007, The Penokean orogeny in the Lake Superior region, Precambrian Research, 157, p. 4–25. Schulz, K.J., DeYoung, J.H., Jr., Seal, R.R., II, and Bradley, D.C., eds., 2017, Critical mineral resources of the United States - Economic and environmental geology and prospects for future supply: U.S. Geological Survey Professional Paper 1802, 797 p., https://doi.org/10.3133/pp1802. Severson, M.J., Heine, J.J., and Patelke, M.M., 2009, Geologic and Stratigraphic Controls of the Biwabik Iron Formation and the Aggregate Potential of the Mesabi Iron Range, Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR- 2009/09, 173 p. + 37 plates. Southwick, D.L. and Morey, G.B., 1991, Tectonic imbrication and foredeep development in the Penokean orogen, east-central Minnesota; an interpretation based on regional geophysics and results of test drilling, U.S. Geological Survey Bulletin 1904-C, pp. C1–C17. Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, Geologic map (scale 1:250,000) of the Penokean orogen, central and eastern Minnesota, and accompanying text: Minnesota Geological Survey Report of Investigations 37, 25 p., 1 pIate. 72 �Trip 3 – Proterozoic Fe & Mn Formations Steiner, A., Peterson, D.M., Berg, E., Solie, J., Larson, M., Schaefbauer, E., and Sweet, G., 2024, The North Star Emily Manganese Deposit: Observations, Interpretations, and Recommendations Following the Initial 2023 Drilling Campaign, Big Rock Exploration Technical Report BRE-TR-2023-01, 47 pages, 4 appendices, 1 plate. Strong, R., 1959, Report on Geological Investigation of the Cuyuna District, Minnesota, 1949-1959, US Steel Internal Report, 318 pages. Tyler, S.A., 1949, Development of Lake Superior soft iron ores from metamorphosed information: Geol. Soc. Am. Bull., v. 60, p. 1101-1024. Van Hise, C. R., and Leith, C. K., 1911, Geology of the Lake Superior region. U.S. Geol. Survey Mon. 52, 641 p. White, D. A., 1954, The stratigraphy and structure of the Mesabi Range, Minnesota: Minnesota Geol. Survey Bull. 38, 92 p. Wolff, J.E., 1917, Recent geologic developments on the Mesabi Iron Range, Minnesota: American Institute of Mining and Metallurgical Engineers, Transactions, v. 56, p. 229-257. 73 �Trip 4 – Soudan FIELD TRIP 4 New Geological Insights into the Genesis of Iron Ores at Lake Vermilion – Soudan Underground Mine State Park George J. Hudak1,2,3, Zsuzsanna P. Allerton1, and Annia Fayon1 1 Department of Earth and Environmental Sciences, University of Minnesota Twin Cities, 116 Church Street SE, Minneapolis, MN 55455 2 Department of Earth and Environmental Sciences, University of Minnesota Duluth, 1114 Kirby Drive, Duluth, MN 55812 3 George Hudak Geosciences P.L.L.C., Duluth, MN 55804 Introduction The Vermilion District of northeastern Minnesota contains one of the classic greenstone belts in the United States. The district comprises the southwestern part of the Wawa-Abitibi Terrane (Stott et al., 2007; Stott and Mueller, 2009) which encompasses Neoarchean metavolcanic, metasedimentary, and metaintrusive rocks that extend northeastward through northwestern Ontario and Quebec (Figure 4-1). In Canada, this terrane hosts numerous volcanogenic massive sulfide deposits (e.g. Winston Lake, Geco, Noranda), gold-rich volcanogenic massive sulfide deposits (Horne (Noranda camp), Bousquet 2 – LaRonde 1, LaRonde-Penna; Mercier-Langevin et al., 2010), as well as a large number of lode (orogenic) gold deposits (for example, in the Hemlo, Timmins, and Kirkland Lake camps). The Vermilion District is known for its numerous, previously mined massive hematitic iron ore deposits (including the Pioneer Mine in Ely and the Soudan Mine in Soudan) which locally occur within regional extensive Algoma-type banded iron formations cut by Neoarchean shear zones. To date, no volcanogenic massive sulfide, gold-rich volcanogenic massive sulfide, or lode gold deposits have been discovered in the Vermilion District, although several studies (Peterson and Jirsa, 1999; Peterson, 2001; Hudak et al., 2002a; Peterson and Patelke, 2003; Hoffman, 2007; Hudak et al., 2007; Hudak et al., 2012; Lodge et al., 2013; Lodge et at., 2015; Thompson, 2015) have indicated that evidence for volcanic, hydrothermal, and structural processes associated with these types of mineral deposits is present throughout the Vermilion District. The Vermilion District’s iron ore mining heritage is currently preserved at Lake Vermilion / Soudan Underground Mine State Park located near Soudan, Minnesota as well as within several historic mines west of and within Ely, Minnesota. The Soudan mine operated from 1882 until December, 1962 and produced approximately 15.5 tons of hematic iron ore. With the donation of land and infrastructure associated with the former Oliver Iron Mining Division’s Soudan Mine by United States Steel to the State of Minnesota in 1965, Soudan Underground Mine State Park was established. This state park currently preserves the historical surface and underground workings from, as well as the wilderness adjacent to, Minnesota’s oldest iron ore mine, the Soudan Mine. The mine previously hosted several underground physics laboratories, including: 1) Soudan 1 (23rd level) which studied neutrino decay; 2) Soudan 2 (27th level), also to study neutrino decay; and 3) the MINOS (Main Injector Neutrino Oscillation Search) lab, which was built on the 27th level adjacent to Soudan 1 and studied the decay of neutrinos within the earth as they passed from Fermilab to Soudan. This popular tourist site continues to be the focus of a wide variety of research related to geology, geochemistry, hydrogeology, biology, biochemistry and physics. Lake Vermilion/Soudan Underground Mine State Park is Minnesota’s newest state park. In 2008, Minnesota State Legislature set aside $20 million in bonding authority to buy, plan, and develop the park, which is located immediately east of the former Soudan Underground Mine State Park. Lake Vermilion/Soudan Underground Mine State Park was established in June 2010 after over 3,000 acres land was purchased from U. S. Steel Corporation (Bakst, 2013). At the present time, considerable development 74 �Trip 4 – Soudan Figure 4-1. Regional geology of the Lake Superior region illustrating the wide variety of mineral deposit types (modified from Hudak and Peterson, 2014; D.M. Peterson, personal communication, 2013). has taken place in the eastern part of the park, including the establishment of trails, roads, and campsites. The park boasts a rich natural and human history, including a wide variety of ~2.7 billion year old rocks that were formed by a wide variety of genetic process, abundant wildlife, as well as archaeological evidence for human habitation dating back over 6,000 years. Additionally, considerable evidence for recent (within the past 140 years) mineral exploration efforts can be readily identified in the park. Since the late 1990’s considerable geological research has been conducted in the region between Tower, MN (in the west) to Ely, MN (in the east) within the Vermilion District. Much of this research has been conducted to better understand the stratigraphy, structural geology, and economic geology of the belt. This research is summarized in several recent Institute on Lake Superior Geology (ILSG) field trips (Hudak et al., 2004; Jirsa et al., 2004; Peterson and Patelke, 2003; Larson and Mooers, 2009; Peterson et al., 2009a; Jirsa and Hillman, 2009; Peterson et al., 2009b), as well as in a few recent journal publications (Lodge et al., 2013; Lodge et al., 2015). In 2010 and 2011, students and faculty from the University of Minnesota Duluth Precambrian Research Center conducted new, 1:5000 scale mapping of this park and several maps and reports were produced (Radakovich et al., 2010; Vallowe et al., 2010; Heim et al., 2011; Baumgardner et al., 2013; Hudak et al., 2016; Peterson et al., 2016). These findings are summarized in Hudak et al., 2014. In addition, geologists from the Natural Resources Research Institute (NRRI), the Minnesota Geological Survey (MGS), and the University of Wisconsin Eau Claire produced a 1:10000-scale map of the park as well as a project report and accompanying spatial databases (Peterson et al., 2016; Hudak et al., 2016). Over the past several years, students and faculty from the University of Minnesota Twin Cities Advanced Field Camp have refined the geological map in an area approximately one-half mile east of the Soudan Mine headframe. 75 �Trip 4 – Soudan The results of these studies have provided a solid foundation for geological research that is currently taking place in Lake Vermilion/Soudan Underground Mine State Park (e.g.). Recent masters and doctoral studies from the University of Minnesota Duluth (Thompson, 2015) and the University of Minnesota Twin Cities (Allerton, in prep.; Allerton et al., 2024a; Allerton et al., 2024b; Allerton et al., in review) have focused their research on understanding the absolute age of massive hematite mineralization at the Soudan Mine. This is a problem that has baffled geoscientists for over a century (e.g. Gruner, 1926; Klinger, 1960). In addition, students and faculty from the University of Minnesota Twin Cities Advanced Field Camp have conducted more recent geological mapping (1:5000 scale) in an area approximately one-half mile east of the Soudan Mine headframe for the past several years. This mapping has led to minor reinterpretations of the geology in the central part of Lake Vermilion/Soudan Underground Mine State Park that was depicted by Peterson et al. (2016). Recently, a grant from the Leaonardt Foundation was awarded to one of the co-authors (Fayon) to develop a new trail in the park that will focus on public education related to the ancient geology and geological processes that have taken place in the park. K-12 teachers are playing a major role in developing the curriculum and lessons that will be part of this trail project. The goals of this field guide are to illustrate to field trip participants the wide variety of geological processes that have taken place within Lake Vermilion/Soudan Underground Mine State Park. Morning field trip stops will focus on understanding the stratigraphy, structure, hydrothermal alteration and mineralization closely associated with the Soudan iron orebodies. The afternoon will focus on observing both geological features of the massive hematite orebodies, as well as recent advances in our geochemical and geochronological understanding of these iron ore deposits in the Montana stope, located on the 27th level of the Soudan Mine. Figure 4-2. Simplified correlation map of Neoarchean assemblages in Minnesota and northwestern Ontario (after Peterson et al., 2001; Hudak and Peterson, 2014). Inset map illustrates location of the Wawa-Abitibi Terrane in Minnesota and northwestern Ontario (Stott et al., 2007). The Leach Lake structural discontinuity is illustrated in red. The red star symbols indicate location of Lake Vermilion State Park. 76 �Trip 4 – Soudan Regional Geologic Setting A simplified regional geological map of the Neoarchean terranes of northeastern Minnesota and adjacent Ontario is presented in Figure 4-2. Supracrustal rocks in the Vermilion district consist of volcanicdominated stratigraphic sequences of the Wawa Abitibi Terrane within the Superior Province of the Canadian Shield. Rocks of the Wawa Abitibi Terrane in northern Minnesota are divided based on stratigraphic and structural setting into: (1) the Soudan belt, to the south, and (2) the Newton belt, to the north (Jirsa et al., 1992; Southwick et al., 1998). The boundary between these contrasting structural panels can be traced geophysically across the width of Minnesota and was informally designated the Leech Lake structural discontinuity (Jirsa et al., 1992). In the region west and north of Lake Vermilion/Soudan Underground Mine State Park, the Leech Lake structural discontinuity occurs along the Mud Creek shear zone (Hudleston et al., 1988), small segments of the Vermilion and Wolf Lake faults (Sims and Southwick, 1985), and the Bear River fault (Jirsa et al., 1992). The Soudan belt (Figure 4-3) contains large, broad generally east-west trending folds involving calc-alkalic and tholeiitic volcanic strata overlain by, and locally interlayered with, turbiditic rocks. In contrast, the Newton belt consists of elongate, northeast-trending, and mostly northward-younging volcanic and volcaniclastic sequences. Volcanic rocks of the Newton belt differ from those of the Soudan belt in containing locally abundant komatiite/basaltic komatiite flows and peridotite sills. The two belts are faultbounded, and the relationships between stratigraphic units within each belt are largely conformable (although faults obscure contacts locally). In its eastern extension, the Soudan belt is continuous with the Saganagons assemblage in Ontario and terminates against the Saganaga pluton and Northern Light Gneiss. The Newton belt extends discontinuously eastward into the Shebandowan District of Ontario to form the Greenwater and Burchell assemblages. Intrusive rocks in both belts vary from gabbroic and felsic porphyries demonstrably related to volcanism, to large plutons emplaced post-tectonically. Both districts contain unconformable, Timiskaming-type sequences composed of calc-alkalic volcanic rocks, conglomerates, and finer grained sedimentary rocks. Lithostratigraphic units in the western Vermilion district (Table 4-1) include: (1) the Lower member, Soudan Iron-Formation member, and Upper member (Upper Ely) of the Ely Greenstone Formation, the Lake Vermilion Formation (including the informally named Britt and Gafvert Lake sequences), and the Knife Lake Group of the Soudan belt; (2) the Bass Lake sequence (Peterson and Jirsa, 1999) and the Newton Lake Formation of the Newton belt; and, (3) syn- to post-tectonic granitoid intrusions of the Giants Range batholith, and a suite of post-tectonic alkalic stocks and plutons. Contacts between the different units are typically conformable, although considerable overlap in time and space is documented between volcanic and sedimentary sequences (Southwick, 1993). Regional chronostratigraphic correlations between the Vermilion district, the Wawa Greenstone (northwestern Ontario) and the Abitibi greenstone belt (eastern Ontario and Quebec) are indicated in Figure 4-4. Geochronological information for supracrustal and intrusive lithologies in the Vermilion District is relatively sparse (Figure 4-4). Peterson et al. (2001) obtained a U-Pb zircon age date of 2722 ± 0.9 Ma from a quartz-phyric rhyolite dome in the Fivemile Lake Sequence of the Lower Member of the Ely Greenstone Formation. Lodge et al. (2013) obtained a U-Pb zircon date of 2689.7 ± 0.8 Ma for a Gafvert Lake Sequence dacitic tuff breccia that occurs approximately 2m north of the contact with the Soudan Iron-Formation member of the Ely Greenstone Formation. As well, Lodge et al. (2013) obtained detrital zircon dates ranging from 2680-2690 Ma from greywackes that comprise the Lake Vermilion Formation. This date confirms the source of the detritus in the Lake Vermilion Formation was derived locally from the volcaniclastic rocks comprising the Gafvert Lake Sequence. Jirsa et al. (2012) obtained a U-Pb age of 2690.7 ± 0.6 Ma for synvolcanic intrusions that cross-cut volcaniclastic rocks that comprise the Knife Lake Group. The upper part of the Knife Lake Group includes conglomerates which contain clasts derived from Table 4-1. Lithostratigraphic units within the western Vermilion District (modified after Peterson and Jirsa, 1999; Peterson et al., 2009; Hudak et al., 2012). 77 �Trip 4 – Soudan Intrusive Rocks Late Intrusions Plutons and stocks of syenite, monzonite, diorite, and lamprophyre. A U-Pb zircon age date of a non-foliated feldspar porphyry intrusion in the Newton belt is 2683 ± 1.4 Ma (Peterson et al., 2001). Vermilion Granitic Complex Granite, schist, amphibolite, and schist-rich migmatite Giants Range Batholith Granite, granodiorite, monzodiorite, and schist-rich migmatite. U-Pb zircon dates indicate a crystallization age ranging from 2640-2777Ma (Allerton et al., 2024a). Supracrustal Rocks Newton Belt Newton Lake Formation Tholeiitic and komatiitic basalt lava flows, intrusions, and clastic strata (deep subaqueous?) Bass Lake Sequence Tholeiitic basalt lava flows, iron-formation, and felsic porphyries (deep subaqueuous) Soudan Belt Knife Lake Group Graywacke, slate, conglomerate, and sheared equivalents Lake Vermilion Formation Graywacke, slate, dacitic tuff, minor conglomerate. Detrital zircons from planar bedded, normal-graded resedimented volcaniclastic rocks have UPb age dates of 2680-2690 Ma (Lodge et al., 2013; subaerial to subaquous) Gafvert Lake Sequence Dacitic to rhyodacitic tuff, lapilli-tuff, tuff-breccia, and iron-formation. Basal dacite tuff-breccia deposits in Lake Vermilion State Park have UPb age date of 2689.7 ± 0.8 Ma (Lodge et al., 2013; subaerial to subaqeous) Britt Sequence Tholeiitic basalt lava flows (deep subaqueous?) Upper Member – Ely Greenstone Tholeiitic basalt lava flows and iron-formation (deep subaqueous?) Soudan Member – Ely Greenstone Oxide-facies iron formation with intercollated basalt lava flows and felsic volcaniclastic rocks (deep subaqueous) Lower Member – Ely Greenstone Calc-alkaline and tholeiitic basalt-rhyolite lava flows, tuffs, epiclastic rocks, and minor iron-formation (shallow- to deep subaqueous) Central Basalt Sequence Calc-alkaline to tholeiitic sparsely amygdaloidal basalt and minor basaltic andesite lava flows with MORB-like or back arc basin-like chemical affinities within 100-200 meters of the overlying Soudan Member iron-formation; FII- and FIIIa-type felsic volcanic and volcaniclastic rocks (transition from shallow- to deep water environment) Fivemile Lake Sequence Calc-alkaline to transitional moderately to highly vesicular basalt and andesite lava flows and volcaniclastic rocks with arc-like chemical affinities: FI-, FII-, and FIV-type felsic volcanic and volcaniclastic rocks. Rhyolite dome at near Fivemile Lake has U-Pb age date of 2722.6 ± 0.9 Ma (Peterson et al., 2001). Epithermal-like zinc stringer mineralization is present near Fivemile Lake (Hudak et al., 2002a; interpreted as shallow subaqueous environment). Eagles Nest Sequence Algoma-type iron formation, basalt-andesite lava flows, hydrothermal exhalites, felsic tuffs. 78 �Trip 4 – Soudan Figure 4-3. Generalized geology of the Soudan belt in the vicinity of the Tower-Soudan anticline (modified after Peterson, 2001; Hudak et al., 2014; Hudak and Peterson, 2014). Locations, ages, and sources of U-Pb ages dates within the district are noted in the callout boxes. Generalized lithologies for each of the groups, formations or sequences are also noted. The outline of the Lake Vermilion section of Lake Vermilion/Soudan Underground Mine State Park is shown in green. 79 �Trip 4 – Soudan Figure 4-4. Regional chronostratigraphic correlations between the Vermilion district (Minnesota), the Wawa greenstone belt (northwestern Ontario), and the Abitibi greenstone belt (eastern Ontario and Quebec; after Ayer et al., 2010). the Saganaga Tonalite, which has been dated by Driese et al. (2011) at 2690.83 ± 0.26 Ma. Peterson et al. (2001) also dated a non-foliated feldspar porphyry intruded into Newton Belt strata at 2683.1 +1/-4 Ma. This date provides a minimum age for the regional D2 deformation event that is described below. Structural Geology The structural geology of the Vermilion District has been well described by Peterson et al. (2009). Periods of generally N-S directed compression resulted in three major regional deformation events in the Neoarchean terranes of northern Minnesota. The earliest deformation event (D1) produced broad, locally recumbent folds within the Soudan belt and major fault zones throughout the region. In the Newton belt, D1 was accommodated by thrust imbrication of large crustal blocks, resulting in mainly northward stratigraphic facing. Field relationships indicate that uplift, faulting, and the deposition of Timiskamingtype clastic sedimentary sequences in local fault-bounded basins occurred late in D1 deformation (Jirsa, 2000). A large, map-scale structure related to D1 deformation in the western Vermilion District is the Tower-Soudan Anticline, which is a west-plunging anticline within which the axis and plunge changes orientation along strike from nearly vertical in basalts to shallow NE plunging in the western sedimentary rocks. Axial-planar cleavage associated with this early fold typically is lacking, although Bauer (1985), Hooper and Ojakangas (1971), Hudleston (1976), and Jirsa et al. (1992) have described early cleavage (S1) locally. A second deformation event (D2) associated with synchronous regional metamorphism resulted in foliation development and structures exhibiting dominantly dextral asymmetry. D2 is constrained in the Vermilion District to the time period 2674 to 2685 Ma (Boerboom and Zartman, 1993), and between about 2680 and 2685 Ma in the Shebandowan (Corfu and Stott, 1998). Because D2 deformation affected all the supracrustal rocks in the area and is reasonably constrained by geochronology, the regional foliation (S2) can be used in the field to temporally relate other structural, intrusive, and deformation events. The relationship between S2 fabric and shear structures indicates that most shearing occurred relatively late in the D2 event. Major shearing that produced the Mud Creek and related shear zones is attributed to the late stages of D2 dextral transpression (Peterson, 2001; Hudak et al., 2004; Peterson et al., 2009). 80 �Trip 4 – Soudan The third deformation event (D3) is believed to be associated with the juxtaposition of the Wawa Abitibi and Quetico terranes (Peterson and Patelke, 2003). Structures associated with D3 include abundant NE- and NW-trending faults that dissect the stratigraphic assemblages and include the NE-trending Waasa and Camp Rivard faults east of the Soudan Mine area, and the WNW-trending, crustal-scale Vermilion and related faults that form the Wawa-Quetico Subprovince boundary. Geology of Lake Vermilion/Soudan Underground Mine State Park Lake Vermilion State Park contains a variety of supracrustal and intrusive lithological units (Figure 4-5). Supracrustal rocks that can be observed in the park include the Lower Member of the Ely Greenstone Formation (both the Fivemile Lake and Central Basalt Sequences), the Soudan Member of the Ely Greenstone Formation, and the Gafvert Lake Sequence of the Lake Vermilion Formation. Additionally, a wide variety of syn- and post-volcanic mafic and felsic intrusive rocks and several varieties of sheared rocks crop out in the park (Peterson and Patelke, 2003; Radakovich et al., 2010; Heim et al., 2011; Hudak et al., 2016; Peterson et al., 2016). These various lithologies are described below. Lithology Supracrustal rocks in Lake Vermilion/Soudan Underground Mine State Park were described by Hudak et al. (2016) based on lithological types rather than lithostratigraphic members and/or formations. Their lithological descriptions are included below. A summary of mafic supracrustal rocks that occur within the park include: • undivided mafic volcanic rocks, including gray-green to green massive basalt, pillow basalt, basalt tuff, bedded scoria tuff and lapilli-tuff, and foliated basalt rocks • massive basalt comprising green to dark green, aphyric to sparsely plagioclase-phyric basalt • • • • pillow basalt, including gray-green to green bun, mattress, and lobe morphologies using the pillow lava classification of Dimroth et al., (1978) basalt tuff, including green, massive to bedded, aphyric to sparsely plagioclase-phyric tuff. bedded scoria tuff and lapilli-tuff, composed of green, thin- to very thick-bedded, poorly-sorted, typically poorly-graded tuff and lapilli-tuff containing up to 65% <1-20cm scoria lapilli foliated basaltic rocks, made up of green, fine-grained, moderately to strongly foliated basalt comprising anastomosing bands of chlorite-rich phyllite separating domains of less deformed basalt Felsic volcanic rocks within the park include: • • • • epiclastic, intermediate-felsic volcanic-derived sedimentary rocks, composed of light gray to brownish gray polymict volcaniclastic matrix-supported conglomerates and sandstones containing clasts of felsic volcanic and volcanic strata, oxide facies iron formation, and chertrich iron formation. laminated felsic tuff, made up of white to dark gray, laminated- to very thinly bedded, aphyric to sparsely quartz- ± plagioclase-phyric dacite to rhyolite tuff. felsic tuff breccia, comprising light gray, very thickly bedded to massive, matrix-supported quartz- and plagioclase-phyric polymict dacite to rhyodacite tuff breccia containing 10-20% 110 cm quartz and plagioclase-phyric coherent dacite lapilli and blocks, 5-7% lens-shaped quartz- and plagioclase-phyric pumice lapilli up to 3 cm in diameter, 1% light- to dark-gray chert lapilli up to 3 cm in diameter, and 1-3% 0.5-5.0cm diameter black to dark gray to red magnetite-rich, hematite-rich, or jasper-rich banded iron formation lapilli. massive felsic lava flows composed of light gray to greenish gray, fine-grained, massive, aphyric to quartz-phyric rhyodacite to rhyolite lava flows. 81 �Trip 4 – Soudan Figure 4-5. Geologic map of Lake Vermilion/Soudan Underground Mine State Park (after Peterson et al., 2016). Detailed maps showing locations of field trip stops are provided in Figure 4-8, and optional stops are shown in Figure 4-12. 82 �Trip 4 – Soudan Figure 4-6. Regional stratigraphic correlations across the Vermilion District (after Hudak et al., 2012; Hudak et al., 2014; Hudak and Peterson, 2014). The sections are hung on the base of the Soudan Member of the Ely Greenstone Formation 83 �Trip 4 – Soudan • felsic tuff, made up of gray to tan, fine-grained, aphyric to quartz ± plagioclase-phyric rhyodacite to rhyolite tuff. Clastic sedimentary rocks within the park include: • graywacke-slate, made up of light gray, fine- to medium-grained, thin- to medium-bedded graywacke containing up to 3% <1-2mm quartz and plagioclase grains that are interbedded with dark gray, laminated to thin-bedded mudstone/slate. • graphitic argillite, composed of dark gray to black, laminated to thin-bedded graphite-bearing argillite Chemical sedimentary rocks that occur in the park include: • oxide-facies iron formation, made up of black (magnetite-rich), dark gray (magnetite- and/or hematite-rich, red (jasper-rich or hematite-rich), or gray (chert-rich) laminated to medium-bedded, planar bedded to chaotically soft-sediment folded, banded iron formation. Hydrothermal alteration of the oxide-facies iron formations has resulted in the genesis of the massive hematite ores that make up the numerous iron ore lenses of the Soudan Mine (Gruner, 1926; Klinger, 1960; Thompson, 2015; Allerton, 2024a, 2024b). • chert-rich iron formation, composed of light gray to black laminated to very thin bedded chert that is locally interbedded with subordinate laminated to very thin bedded oxide facies iron formation Both mafic and felsic intrusive rocks have been identified in the park. Mafic intrusive rocks include: • lamprophyre intrusions, including 1) massive gray-green intrusions containing scoria, chert and granite clasts within a fine- to medium-grained groundmass composed of up to 85% acicular amphibole; and 2) black, fine-grained massive hornblende-plagioclase-bearing intrusions containing up to 15% fine-grained hornblende needles and local rounded granite blocks greater than 25cm in diameter in a fine-grained gray-black to red groundmass (Peterson and Patelke, 2003). Felsic intrusive rocks in the park include: • diorite, comprising gray to gray-green, fine- to medium-grained, plagioclase- and hornblendephyric equigranular diorite (actinolite pseudomorphs of hornblende are common) • granodiorite, made up of whitish-pink to green-gray, medium-grained granodiorite and hornblende granodiorite that locally contains xenoliths of oxide-facied banded iron formation, chert, felsic epiclastic rocks, and mafic volcanic and volcaniclastic rocks • feldspar porphyry, composed of white to whitish-pink, medium-grained, holocrystalline dacite with 5-12% 1-4mm subhedral to euhedral tabular plagioclase feldspar phenocrysts, and locally, 2-5% 13mm dark green actinolite pseudomorphs of hornblende (Radakovich et al., 2010) • quartz feldspar porphyry, characterized by white to whitish-pink, light gray to pale green-gray porphyritic dacite and rhyodacite that contains 20-25% 1-5mm diameter subhedral to euhedral plagioclase feldspar phenocrysts and 5-15% 1-3mm diameter subhedral to euhedral pale gray to gray-blue quartz phenocrysts Sheared rocks that crop out in Lake Vermilion/Soudan Underground Mine State Park include: • chlorite-dominant schist, composed of dark green very fine- to fine-grained chlorite phyllite and schist (Peterson and Patelke, 2003) • sericite-dominant schist, made up of pale yellow to yellow-gray to yellow-green very fine- to finegrained sericite-bearing phyllite and schist (Peterson and Patelke, 2003) • green mica (fuchsite)-dominant schist, comprising pale yellow to yellow gray, very fine- to finegrained sericite-bearing phyllite that contains up to 20% emerald green disseminated porphyroblasts of green mica that are up to 5mm in length • Schist ‘n’ BIF, an enigmatic unit made up of interlayered chlorite-dominant phyllite and schists and sericite-dominant phyllites and schists that contain lens-shaped clasts of oxide facies iron formation ranging from 1mm – 1 meter in length 84 �Trip 4 – Soudan Stratigraphic correlations across the central part of the Vermilion district are illustrated in Figure 4-6 (Hudak et al., 2012). Structure Three distinctive types of fault zones have been identified during geological mapping within Lake Vermilion/Soudan Underground Mine State Park. These structures include: • • • Synvolcanic fault zones (D0), which formed at the time of volcanism associated with the genesis of the volcanic rocks in the State Park, and which possess higher concentrations of synvolcanic hydrothermal alteration mineral assemblages proximal to the synvolcanic structures (see Gibson et al. (1999) and Hudak et al. (2014) for a detailed explanation of synvolcanic fault zones). Two potential synvolcanic fault zones have been described in the north-central part of the former Lake Vermilion State Park by Hudak et al. (2014); Shear zones that are associated with the regional D2 deformation, and are characterized by linear zones of sheared rocks including chlorite-dominant schist, sericite-dominant schist, fuchsite (green mica)-dominant schist, and schist ‘n’ BIF. The Mine Trend and Murray shear zones (Peterson and Patelke, 2003; Peterson et al., 2016; Table 4-2) are examples of D2associated shear zones within the bounds of Lake Vermilion/Soudan Underground Mine State Park. Late faults are characterized by brittle deformation and associated offset of adjacent lithological units. Within Lake Vermilion/Soudan Underground Mine State Park, these D3associated structures are commonly expressed as northwest- to northeast-trending, minor displacement (generally less than one meter) brittle faults that offset sedimentary bedding and / or contacts between adjacent lithological units (D3-associated faults are clearly evident at field trip stop 1). Table 4-2. Calculated displacements among the Mine Trend and Murray Shear zones (Peterson and Patelke, 2003). Ranges of values were calculated geometrically by using the average plunges of lineations associated with the shear zones, and two measured lines of possible correlative stratigraphy offset by the bounding shear zones. See Peterson and Patelke (2003) for further details. Measurements of other planar (e.g. bedding orientation, orientations of geological contacts, foliation measurements) and linear (e.g. mineral lineations, glacial striations) geological structures were recorded during field mapping, and are included on the new geologic map of Lake Vermilion/Soudan Underground Mine State Park (Peterson et al., 2016; see Figure 4-5). Geochronology Geochronological information for supracrustal and intrusive lithologies in the Vermilion District is relatively sparse (refer back to Figure 4-4). Peterson et al. (2001) obtained a U-Pb zircon age date of 2722 ± 0.9 Ma from a quartz-phyric rhyolite dome in the Fivemile Lake Sequence of the Lower Member of the Ely Greenstone Formation. Allerton et al. (2024a) obtained a crystallization age of 2708 ± 25 Ma for the Purvis Pluton, which intrudes the Eagles Nest Succession of the Lower Ely Member and has been interpreted as a synvolcanic intrusion (Peterson, 2001). The age of the Upper Member of the Ely Greenstone formation is currently unknown. Jirsa (2016) obtained an age of 2715.74 ± 0.50 Ma for a felsic volcanic 85 �Trip 4 – Soudan unit within the Newton Lake Formation (Boerboom, T. J., 2020). Lodge et al. (2013) obtained a U-Pb zircon date of 2689.7 ± 0.8 Ma for a Gafvert Lake Sequence dacitic tuff breccia that occurs approximately 2 meters north of the contact with the Soudan Iron-Formation member of the Ely Greenstone Formation. As well, Lodge et al. (2013) obtained detrital zircon dates ranging from 2680-2690 Ma from greywackes that comprise the Lake Vermilion Formation. This date confirms the source of the detritus in the Lake Vermilion Formation was derived locally from the volcaniclastic rocks comprising the Gafvert Lake Sequence. The age of the orebodies at the Soudan Mine has eluded geologists for nearly a century. The genesis of the massive hematite orebodies was previously interpreted to be syn- or post-depositional to the formation of the Soudan Member of the Lower Ely Greenstone Formation (Gruner, 1926; Klinger, 1960; Thompson, 2015). Gruner (1926) believed the ores could be as young as the Mesoproterozoic Duluth Complex. Klinger (1960) found abundant evidence for post-iron formation depositional genesis of the massive hematite ores, but he could not determine a specific date for mineralization and concluded the orebodies were formed along with or after shear zones that are spatially associated with the ores. Thompson (2015) speculated based on geological, structural, and lithogeochemical data, that the ores were formed during the D2 deformation, but could not determine a specific date for the massive hematite mineralization. Recent U/Pb and (U-Th)/He radiometric dating of hematite by Allerton (2024b) suggests the massive hematite orebodies at Soudan formed during Paleoproterozoic time (1640.8 ± 47.2 Ma – 1740.4 ± 72.5 Ma) and have been overprinted by a Mesoproterozoic hydrothermal event at approximately 1100 Ma (1093.1 ± 16.4 Ma). Terminology Used for This Field Trip The terminology used on this field trip will be consistent with the terminology used by Hudak et al. (2014) for their “Walk in the Park” ILSG field trip and is described below. All stop locations for this field trip are given in Universal Transverse Mercator (UTM) coordinates, Zone 15N, using the North American Datum of 1983 (NAD83) as well as latitude/longitude. Section subdivisions read from smallest to largest quarter (e.g., “NW, SE” should be read “NW quarter of the SE quarter”). A geologic map with stop locations is given in Figure 4-8. A map of optional field trip stops is given in Figure 4-12. It is important to note the terminology utilized in this field trip guide for: 1) volcaniclastic rocks; and 2) bedding characteristics. Use of consistent terminology is required to facilitate consistent and accurate describe these geological features. Volcaniclastic rocks contain abundant volcanic material irrespective of their origin or depositional environment (Fisher, 1966). Such rocks can form directly from volcanic eruptions (whether subaerial or subaqueous), resedimentation of non-lithified volcanic deposits (for example, resedimentation of pyroclasts prior to lithification), or weathering and resedimentation of pre-existing lithified volcanic rocks. Primary (juvenile) volcaniclastic particles result directly from eruptive processes, and are of three types: • • • Pyroclasts, which form by explosive fragmentation of magma into particles (including ash, highly vesiculated glass (pumice, scoria), crystals and crystal fragments, and lithic fragments); Hydroclasts, which form by explosive interaction with external water (via phreatic (steam only) and/or phreatomagmatic (steam and magma) explosions) or by non-explosive quenching and granulation of lava (for example, the formation of hyaloclastite fragments on the margins of submarine lava flows or intrusions into wet sediments); and Autoclasts, which form by frictional breakage of moving viscous lava flows (for example, to form carapace breccias on the margins of subaerial lava flows). Based on these different types of fragmentation, four types of primary volcaniclastic deposits have been identified by White and Houghton (2006): 86 �Trip 4 – Soudan • • • • Pyroclastic deposits, which are generated from volcanic plumes and jets or pyroclastic density currents as particles first come to rest. Deposition mechanisms associated with these processes include suspension settling, traction, or en masse freezing; Autoclastic deposits, which are generated during effusive volcanism when lava cools and fragments as a result of thermal processes, or recently cooled lava breaks during flow. Deposition for these types of rocks is under the influence of continued lava flowage; Hyaloclastite deposits, which are generated during effusive volcanism when magma or flowing lava is chilled and fragmented due to contact with water. Deposition of such deposits is is influenced by the continued emplacement of the lava in the presence of water, and the thicknesses of the hyaloclastite deposits can be dictated by the temperature of the magma, the effusion rate, and the distance from the volcanic vent (Cas and Wright, 1987; Gibson et al., 1999; Newkirk et al., 2001); and Peperite deposits, which are generated when magma intrudes into unconsolidated clastic material and mingles with (generally wet) debris to form a volcaniclastic deposit (McPhie et al., 1993). Deposition of peperite deposits takes place essentially in-situ. Secondary volcaniclastic particles are known as epiclasts: • Epiclasts are lithic clasts and/or crystals derived from physical weathering and erosion of preexisting lithified rocks. Epiclasts are volcaniclasts when the pre-existing rocks are volcanic. The terminology for volcaniclastic rocks has historically been somewhat confusing because many different classification schemes have been developed (for example Fisher, 1961; Fisher 1966; Schmid, 1981; Cas and Wright, 1987; McPhie et al., 1993; White and Houghton, 2006), and different classification schemes are preferentially used in different parts of the world. As a result, the terminology relating to volcaniclastic rocks is commonly misused or misinterpreted. Four classification schemes that have been used most in the recent geological literature include: • • • • Fisher (1961, 1966) – Classification based on particle size, particle formation, or particle fragmentation mechanism; Schmid (1981) – Particle type within the deposit; Cas and Wright (1987) – Mode of fragmentation and deposition; and McPhie et al. (1993) – Transport and deposition mechanisms. According to R. V. Fisher (1998), the difficulties with volcaniclastic rock classification can be understood because “volcaniclastic rocks are essentially igneous on the way up and sedimentary on the way down”. In fact, Fisher’s thesis advisor, when observing the volcaniclastic rocks that were the focus of his thesis studies, indicated that they were “the ugliest and most undistinguished rocks I’ve seen in my 30 years of petrology!” Classification is also especially difficult in ancient volcaniclastic rocks because key aspects of classification can be obscured by subsequent hydrothermal alteration, metamorphism and/or structural deformation (e.g. particle type, particle size) or because genetic processes cannot be ascertained unambiguously (e.g. transport and deposition mechanism, fragmentation mechanisms). For this field trip guidebook, we will utilize Fisher’s (1966) classification (Figure 4-7) for volcaniclastic rocks. This classification scheme is based on the relative proportions of ash-sized material (< 2mm), lapilli-sized material (2-64mm), and blocks/bomb sized material (>64mm) in the rock. Both Gibson et al. (1999) and Mueller and White (2004) suggest that this classification be used for field-based rock classification (mapping, diamond drill core logging, petrography) of ancient volcaniclastic deposits for the following reasons: 87 �Trip 4 – Soudan • • • The classification scheme is “field-user friendly” because it accommodates both the historically important pyroclastic rock names and enables comparison at both the hand sample and thin section scale (Mueller and White, 2004); It is a Wentworth-based scale, and thus enables comparison of volcaniclastic deposits to sedimentary deposits; and Rock classification does not require knowledge of the specific transport mechanism or depositional processes involved with the genesis of the deposit. More recently, White and Houghton (2006) have developed a modified version of Fisher’s (1966) volcaniclastic classification scheme (Figure 4-7). The scheme is essentially equivalent to the Fisher (1966) scheme, with the exception that the lapill-tuff field in the White and Houghton (2006) classification comprises the lapilli-tuff and lapillistone fields of Fisher’s (1966). Figure 4-7. Volcaniclastic rock classification schemes of Fisher (1966) and White and Houghton (2006). This field trip guidebook will classify volcaniclastic rocks using Fisher’s (1966) classification scheme. Specific terms for bedding thicknesses are also used in this guidebook. The terminology for bedding thickness has been adopted from McPhie et al. (1993) and includes: • • • • • • Laminated Very thinly bedded Thinly bedded Medium bedded Thickly bedded Very thickly bedded <1 centimeters thick 1-3 centimeters thick 3-10 centimeters thick 10-30 centimeters thick 30-100 centimeters thick >100 centimeters thick 88 �Trip 4 – Soudan FIELD TRIP OVERVIEW NOTE: This field trip will require hiking along trails and through the bush in Lake Vermilion/ Soudan Underground State Park and includes observations on the 27th level of the Soudan Mine. Hiking boots and safety eyewear are strongly encouraged as traverses in the park may encounter slippery conditions and vegetation which can cause eye injuries. Field trip participants should plan to wear a jacket and gloves while underground as the temperatures in this location are typically around 50°F (10°C). Upon arriving at Lake Vermilion/Soudan Underground Mine State Park, we will park in the main parking lot located near the Park Manager’s office. After a coffee break, we will strap on our hiking boots and spend the remainder of the morning making field trip stops in the central part of Lake Vermilion/Soudan Underground Mine State Park along a more-or-less north-south traverse. These field trip stops (Figure 4-8) will illustrate the stratigraphy, structural, and hydrothermal alteration features associated with the massive hematite ores at the Soudan Mine. We will then head back to the visitor center and have lunch overlooking one of the original Soudan Mine ore pits. After lunch, we plan to continue the field trip by going underground. We will proceed to the mine shaft and travel 2341 feet underground to the 27th Level of the Soudan Mine. We will board a train (converted ore cars) and travel west for approximately three-quarters of a mile to the Montana Stope, the last active part of the mine. At this point we will climb vertically approximately 30 feet using a very tight spiral staircase. One in the Montana Stope, we will observe the massive hematite ore, the transitional region of non-ore iron formation, and will observe massive chlorite-rich schists associated with approximately east-west-trending D2-associated shear zones. Here we plan to discuss recent geological research that has been conducted to determine the absolute age of the massive hematite ores that reside there (Allerton et al., 2024a, 2024b; Allerton et al., in review). At the end of the tour we will proceed back down to the 27th level drift, board the train, and head back east through the drift to the shaft station where we will proceed back to the surface. Field trip participants may not be able to access the 27th level of the Soudan Mine due to flooding that occurred during summer, 2024. Should this happen, afternoon field trip stops will investigate outcrops that illustrate the rarely exposed geological contact between the Soudan Member iron formation and Gafvert Lake Sequence tuffs, lapilli-tuff and tuff-breccias, Gafvert Lake Sequence tuffs and lapilli-tuffs, and subvolcanic intrusive rocks related to the Gafvert Lake Sequence that occur in the northeastern part of Lake Vermilion/Soudan Underground Mine State Park. Descriptions of these outcrops are included in a section below called “Optional Outcrop Stops” which have been taken from a recent ILSG field trip titled “Field Trip 2 - A Walk in the Park: Neoarchean Geology of Lake Vermilion State Park” (Hudak et al., 2014). Following the completion of the field trip, we will board the vehicles and proceed back to the Mountain Iron Community Center, where the field trip will end. FIELD TRIP From the Mountain Iron Community Center, proceed 0.2west on Enterprise Drive S to Emerald Avenue. Turn north on Emerald Avenue and go 0.05 miles to Highway 169. Turn east on Hwy 160 and proceed 1.5 miles to the turn off for Hwy 169/Hwy 53N. Take Hwy169/Hwy53 approximately 6.1 miles, bear right, and continue north on Hwy1/169 toward Ely. Continue north/northeast on Hwy 1/169 for approximately 23.75 miles to the first turn-off to Soudan (this will be Main Street and you will see an ore car and a sign for the Soudan Mine at the intersection). Proceed north for 0.4 miles on Main Street, turn right, and continue on Main Street for approximately 0.9 miles until it intersects 1st Ave./Stuntz Bay Road. Turn north on 1st Ave/Stuntz Bay Road and proceed for approximately 0.4 miles until you see the dirt road (McKinley Park Road) that is the east entrance to the Soudan Mine. Turn west on the dirt road, go about 0.05 miles, and park near the Lake Vermilion/Soudan Underground Mine State Park Manager’s office. Here 89 �Trip 4 – Soudan we will check in with the Manager. Turn left (south) on the dirt road and follow it around the old mine infrastructure, parking near the intersection of McKinley Park Road and Stuntz Bay Road (approximately 0.1 miles). From our vehicles parked at the intersection of Mckinley Park Road and Stuntz Bay Road, we will walk north approximately 175 meters up 1st Ave/Township Highway 4598 to field trip stop 1. Please walk against traffic as we proceed to and from this location. Stop 1: Soudan Member Banded Iron Formation Longitude/Latitude: 47.820074°N, -92.2365908°E UTM NAD 83 Zone 15N: 557144E, 5296585N (NOTE: From Peterson et al., 2009A; Hudak and Peterson, 2014) The Soudan Member of the Ely Greenstone Formation is dominantly composed of laminated to thinly bedded Algoma-type oxide facies banded iron-formation, with subordinate, locally interstratified, sparsely amygdaloidal massive to pillowed basalt lava flows and resedimented felsic tuff deposits. Regionally, the stratigraphic thickness of the Soudan Member of the Ely Greenstone Formation varies from 50-3,000 meters, with an average stratigraphic thickness of approximately 700 meters (Peterson et al., 2009). Within Lake Vermilion State Park, the Soudan Member ranges in stratigraphic thickness from approximately 300 – 680 meters in thickness. Individual horizons of oxide-facies iron formation range from approximately 70-345 meters thick, whereas the Soudan basalt lava flow units range from approximately 60-300 meters in thickness. This classic exposure of the Soudan member of the Ely Greenstone Formation lies on the north limb of the Tower-Soudan anticline approximately 75 meters north of the stratigraphic contact with the Lower member of the Ely Greenstone. The outcrop displays two generations of tight folding in delicate laminae of chert (creamy white), chert-hematite jasper (red), and magnetite-chert (black to silver-colored). The second generation of folds (F2) is tectonic in origin, having subvertical axial surfaces that trend east, and steeply plunging axes. Most display Z-asymmetry. The earlier folds (F0-1) appear to have been sharply refolded to produce complex interference patterns. Lundy (1985) studied folding at this locality and concluded that some of the apparent interference structures are the product of early-formed sheath folds that did not involve refolding by D2. The F1 structures are predominantly intrafolial, and exhibit a great variety of styles and orientations; implying they formed by layer-parallel, soft-sediment slumping (Fig. 49). Lundy’s mapping of this outcrop is an interesting demonstration of how unraveling details at a single outcrop that led to recognition that D1 deformation was not systematic here, but likely the result of soft sediment folding. It is interesting to observe the rhythmic microlaminae (1 mm or so thick) in various cherty beds exposed here and speculate about the paleoenvironment - that is, whether these represent daily heating/cooling, tidal, climatic, annual, or some other repetitive influence (e.g. waxing/waning of a hydrothermal system) in the depositional environment. What is known about units of iron-formation in the Ely Greenstone, of which there are many, is that deposition occurred in deep water (below wave base) during periods of relative volcanic and tectonic quiescence by the slow subaqueous precipitation of chemical sediments. The deep excavations in this area are the early workings of the Soudan iron mine, the first in Minnesota. The mine produced about 16 mt of high-grade hematite ore (60-63 percent ironconverted to a park. Although some early production came from open pits, most of the ore was extracted from underground workings that began here in 1900, and which now can be visited on guided tours. The mine previously housed several underground physics research facilities. These include Soudan 1 (23rd level) which studied neutrino decay; 2) Soudan 2 (27th level), also to study neutrino decay; and 3) the MINOS (Main Injector 90 �Trip 4 – Soudan Figure 4-8. Geologic map of the central part of Lake Vermilion/Soudan Underground Mine State Park (after Peterson et al., 2016) illustrating locations of field trip stops. See Figure 4-5 for the description of map units. 91 �Trip 4 – Soudan Figure 4-9. Outcrop map showing bedding trajectories and multiple generations of folds and faults (from Lundy, 1985). F1 folds are non-systematic and include both nappe- and sheath fold geometries. Neutrino Oscillation Search) lab, which was built on the 27th level adjacent to Soudan 1 and studied the decay of neutrinos within the earth as they passed from Fermilab to Soudan. Follow the field trip leaders south along First Avenue/Township Highway 4598 to the blacktop-paved Mesabi Trail. Turn to the east and walk along the paved Mesabi trail for approximately 800m. There, turn north and proceed up the unpaved trail approximately 350 meters, where the trail intersects another trail that goes northeast. Turn right and proceed northeast along the trail for about 90 meters. We will then head into the bush and hike approximately 200 meters northeast to field trip stop 2. Stop 2: Soudan Member Basalt Pillow Lavas Longitude/Latitude: 47.82544775°N, -92.22434651°E UTM NAD 83 Zone 15N: 558055E, 5297191N Detailed mapping in the park by Peterson and Jirsa (1999), Peterson and Patelke (2003), Hoffman (2007), Radakovich et al. (2010), Vallowe et al. (2010), Heim et al. (2011), and Baumgardner et al. (2013) has shown that the Soudan member is dominantly composed of oxide facies iron formation horizons that are locally interlayered with massive and pillowed mafic lava flows and associated volcaniclastic rocks (e.g. pillow breccias). Basalt lava flows associated with the Soudan Member of the Lower Ely Greenstone Formation are characterized by a medium green to dark green color. They are typically aphyric- to sparsely plagioclase ± pyroxene (now actinolite)-phyric. Plagioclase phenocrysts vary from subhedral to euhedral tabular in morphology, are typically less than or equal to 1mm in length and are locally present in abundances up to 3%. Locally, 5-7% dark green actinolite pseudomorphs of pyroxene phenocrysts may be present. Where amygdaloidal, the unit contains up to 7% oval to round, light gray quartz-filled amygdules ranging from <1-4mm in diameter. At this outcrop we will observe well-preserved 0.5-2m long, aphyric- to sparsely plagioclasephyric, massive- to sparsely amygdaloidal bun- and mattress-shaped pillow lavas. These pillows dip steeply to the north and strike approximately east-west. Interpillow hyaloclastite is locally well-preserved and is 92 �Trip 4 – Soudan composed of <1-5mm chlorite-rich cuspate shards that are pseudomorphs of original volcanic glass formed by quenching of the mafic magma by water. These pillow lavas share many characteristics with the underlying Central Basalt Sequence mafic lava flows that comprise the uppermost part of the Lower Ely Member of the Ely Greenstone Formation. Such characteristics include exceptional preservation of primary volcanic textures, medium- to dark green color, sparsely plagioclase ± pyroxene-phyric, and low vesicularity. Based on these features, the Soudan Member pillow basalts at this location and are interpreted to have formed in a “deep water” (e.g. below wave based) volcanic environment. Proceed approximately 200 meters southwest to the northeast-southwest trending trail. Walk approximately 90 meters southwest to intersect the main north-south trail that intersects the Mesabi Trail. Walk approximately 350 meters south back to the paved Mesabi Trail. Turn to the east and follow the field trip leaders through the bush for about 140 meters to field trip stop 3. Stop 3: Soudan Member Oxide Facies Banded Iron Formation Longitude/Latitude: 47.82139974°N, -92.225809591°E UTM NAD 83 Zone 15N: 557990E, 5296775N Within Lake Vermilion/Soudan Underground Mine State Park, the Soudan Member oxide-facies banded iron-formation is generally planar laminated to medium-bedded, with black magnetite-rich horizons, light gray to black chert horizons, red to blueish-black hematite-rich horizons, and red jasper horizons defining the bedding. Locally, very tight, chaotically oriented folds, resulting from syndepositional soft sediment deformation and subsequent tectonic deformation, are present. As indicated above, these iron formation deposits are locally intimately interbedded with basalt lava flows such that mapping individual iron-formation and basalt horizons is often impossible at 1:5000 scale (Peterson and Patelke, 2003; Hudak and Peterson, 2014; Hudak et al., 2016). This outcrop is composed of slightly- to moderately hematite-altered Soudan member oxide facies banded iron formation. The rock varies from locally non-magnetic to slightly magnetic due to alteration of magnetite to hematite/martite. Such alteration is common in areas within a few hundred meters of massive hematite ore and is commonly found in close proximity to D2-associated shear zones. The closest previously mined massive hematite orebody was located approximately 250 meters west-southwest of this location in an existing mine pit. D2-associated shear zones have been identified approximately 25 meters north and south of this outcrop. Here, the oxide-facies banded iron formation comprises interlayered planar horizons of gray oxiderich (hematite ± magnetite), red jasper-rich, and pale white (silica (chert)-rich that are laminated, thinly bedded, and locally medium bedded. Bedding orientations generally strike more or less east-west, although locally contorted layers may vary significantly in strike direction. Dips are generally steep (>75°) to the north, although locally dips may be steep to the south. Follow the field trip leaders southwest for about 85 meters to field trip stop 4. Stop 4: Mine Trend Shear Zone “Schist ‘n’ BIF” Longitude/Latitude: 47.82126659°N, -92.22607876°E UTM NAD 83 Zone 15N: 557725E, 5296740N The “Schist ‘n’ BIF” units at this location (Figure 4-10) are composed of sheared rocks comprising chlorite schist that are interlayered with, and locally contain fragments of red, jasper-rich banded iron formation and light gray to white chert. The chlorite schist is fine-grained (<1 mm) with a tan (chloriteankerite) to green (chlorite-rich) weathered surface. Common minerals include chlorite, ankerite, sericite, 93 �Trip 4 – Soudan Figure 4-10. Photographs of outcrop exposures at Stop 4. A) Image is showing the southern face of the “Schist ‘n’ BIF” unit. The greenish tan rock is chlorite schist, and the red and white layers are banded iron formation. B) This image is north of image A and is the near horizontal exposure of the “Shist ‘n’ BIF” outcrop. The dark red inset in this figure is the location of the photographic in Figure 4-10C. C) This image shows a sigma clast of banded iron formation enclosed by chorite schist. Although the banded iron formation pieces are locally broken off, the clast can be traced, and a sketch of the clast is shown in Figure 4-10D). D) Sketch of a somewhat intact banded iron formation clast (dark pink) surrounded by silicates and other chert fragments (light pink) and enclosed by green and tan chlorite ± ankerite schist. and siderite. Banded iron formation fragments occur as clasts or thin bedded layers between foliation planes of the schist. The foliation here strikes east-west and dips near-vertically. The previously mentioned D2 associated shearing has a dextral or right lateral sense of shear that is approximately east-west trending on a regional scale, although locally sinistral shear sense indicators are present locally. This outcrop is located southwest of the previously visited oxide facies banded iron formation. The construction of a new paved road in 2020 exposed the now southern face of the unit (Figure 4-10A), providing access to three planes for structural measurements. The shear plane (Figure 4-10B) contains banded iron formation/chert clasts that serve as kinematic indicators and are located conveniently under our feet due to the dip of the schist layers. Outcrop-scale kinematic indicators of sigma and delta clasts (Figures 4-10C and 4-10D) trend mostly east-west with dextral sense of shear, mimicking regional deformation trends. Follow the field trip leaders and walk west-southwest for approximately 800 meters along the paved Mesabi back to First Avenue/Township Highway 4598. We will then proceed back to the vehicles and head to the 94 �Trip 4 – Soudan main parking lot for the Soudan Mine. We will eat lunch at the Soudan Mine visitors’ center (bathrooms are available in the visitors’ center). Stop 5: Soudan Underground Mine Shaft Longitude/Latitude: 47.82126659°N, -92.22607876°E UTM NAD 83 Zone 15N: 556765E, 5296536N Following lunch, we will watch a short video, pick up hard hats from state park staff, and proceed to the Soudan Mine shaft. Make sure to bring a jacket and gloves underground as the temperature there is approximately 50°F (10°C). Once in the cages, we will travel 2341 feet underground to the 27th Level of the Soudan Mine. After exiting the cage we will board a train (converted ore cars) and travel west for approximately three-quarters of a mile to the Montana Stope, the last active part of the mine. It is important for everyone’s safety to stay in the train car for the duration of the trip, and to not raise your hands while traveling in the train car. At this point we will exit the train, and after a short walk, climb vertically approximately 30 feet using a very tight spiral staircase to the Montana Stope. The Montana Stope is the last active part of the Soudan Mine. Thompson (2015) conducted detailed mapping of the Montana ore zone (Figure 4-11) and noted the presence of several rock types, including: • • • • • • • Chlorite-dominant schist, which locally replaces sericite-dominant schist proximal to the ore (unit 5c) Chlorite + sericite schist, which locally replaces sericite-dominant schist (unit 5cs) Sericite-dominant schist composed of sericite + paragonite ± pyrophyllite that has a mylonitic texture and occurs intermediate to ore breccia zones (unit 5s) Sericite-dominant schist that is locally silicified and occurs in well foliated zones at the margins of ore bodies that locally contain disseminated iron-rich chlorite domains (unit 5sc) Hematite ore, predominantly composed of specular hematite with microplaty hematite occurring locally within fractures and vugs (unit 4o) Hematite ore breccia, composed of hematite-rich banded iron formation and brecciated massive hematite ore with abundant milky “bull” quartz and disseminated sulfides (pyrite ± chalcopyrite; unit Fbx) Hematite-jasper banded iron formation, which retains many of its primary sedimentary textures and represents an intermediate rock between fresh Soudan Member oxide facies banded iron formation and the altered hematite-rich iron ore (unit 4a) The absolute age and geological processes associated with the genesis of the Soudan (and other Vermilion district) massive hematite ores have baffled geoscientists for over a century (e.g. Gruner, 1926; Klinger, 1960). Gruner (1926) proposed that massive hematite mineralization occurred after deposition and lithification of the Soudan Member oxide facies banded iron formation and proposed that the mineralization occurred resulted from alteration of the original banded iron formation by ascending upwelling hydrothermal solutions that oxidized most of the iron, dissolved quartz, and precipitated secondary carbonates and sulfides. He did not specify an exact age for this mineralization process. Klinger (1960) noted the close association of massive hematite ore zones at the Soudan Mine to faults (shear zones) within the mine. He states that “the orebodies occur in the iron formation and their dimensions are controlled by its structure”. He also noted that the massive hematite ores showed little evidence of deformation and concluded that “a second generation of hematite appears to post-date structural movements which occurred after the main ore-forming period. These movements, and later hematite, are 95 �Trip 4 – Soudan Figure 4-11. Geological map of the Montana stope (modified from Thompson, 2015). both later in time than a sericite rock that has been dated at 1.67 billion years by the A40/K40 method”. He also indicated that “at least some of the hematite is younger than 1.67 billion years” and concluded that the ores were “post-Huronian to pre-Keewenawan” in age. Recent masters and doctoral studies from the University of Minnesota Duluth (Thompson, 2015) and the University of Minnesota Twin Cities (Allerton, in prep.; Allerton et al., 2024a; Allerton et al., 2024b; Allerton et al., in review) have focused their research on understanding the absolute age of massive hematite mineralization at the Soudan Mine. Based on detailed mapping, petrographic studies, and lithogeochemical studies, Thompson (2015) suggested that the massive hematite ores at Soudan were formed from a multi-stage process involving alteration of the original oxide-facies banded iron formation by a fluid-dominated synvolcanic sea-floor hydrothermal system followed by interaction with hydrothermal metamorphic fluids associated with the subduction of strata within the Vermilion district. Therefore, his model for the genesis of the Soudan massive hematite ores suggests a Neoarchean age ranging from the time of the original deposition of the oxide-facies banded iron formations (~2720 Ma) to the time spanning the D2 deformation (2674-2685 Ma (Boerboom and Zartman, 1993) which is likely associated transpression and the development of the D2 shear zones in which the ores occur. New research (Allerton et al., in review) utilizing petrographic observations and electron microprobe analyses shows that the massive hematite ore can be divided further into two distinct ore 96 �Trip 4 – Soudan textures comprising: 1) homogenous microcrystalline hematite-martite; and 2) heterogenous microcrystalline hematite. The fine-grained microcrystalline hematite-martite (martite comprises hematite pseudomorph replacing magnetite) locally contains minute vug spaces and larger fractures that are filled by microplaty hematite and silicates. The heterogeneous ore contains a minor amount of metallic and/or earthy microcrystalline hematite, but is predominantly composed of coarser-grained microplaty hematite and silicates. Microcrystalline hematite-martite predates microplaty hematite and silicates based on crosscutting relationships. U-Pb radiometric dating of hematite was used to establish the timing of ore mineralization at ca. 1.8-1.6 Ga. Our new model for the genesis of Soudan massive ore suggests that hydrothermal alteration related to mineralization is coeval with orogenic events generated by Proterozoic terrain accretion and associated magmatism. Additional geochemical analyses involving mass-balance calculations and Fe stable iron isotopes indicate that the upgrade of BIF to hematite ore was a two-stage process. Dense microcrystalline hematitemartite matrix yielding a homogeneous texture was produced during the first stage. The second stage resulted in the formation of a heterogeneous texture containing microplaty hematite and silicates in larger vugs and fractures in the microcrystalline hematite-martite ore. At the end of this stop, we will proceed back to the 27th level drift using another tight spiral staircase. We will board the train and proceed east back to the shaft station where we will board the cages and return to the surface. At the surface, we will reboard the vehicles and proceed back to the Mountain Iron Community Center via the directions below. From our parking spot at Soudan Mine, proceed down the hill on McKinley Park Road for approximately 0.4 miles to the intersection with Main Street. Turn south and drive for approximately 0.4 miles to the intersection with Hwy 1/169. Turn west and Hwy 1/169 and drive for about 23.7 miles and merge onto Hwy 53/169 South. Follow Hwy 1/169 the intersection with Hwy 53/Hwy 169. Merge on to Hwy 169 south and proceed for 1.5 miles to Emerald Avenue. Turn south and proceed on Emerald Avenue for approximately 0.1 mile. Turn east and proceed for approximately 0.2 miles back to the Mountain Iron Community Center. OPTIONAL OUTCROPS The following field trip stop descriptions have been taken from the 2014 ILSG Field Trip 2 “A Walk in the Park – Neoarchean Geology of Lake Vermilion State Park” (Hudak et al., 2014). A map showing the locations of the optional field trip stops is shown in Figure 4-12. From the original parking spot near the Manager’s office at Soudan Mine, proceed approximately 0.2 miles south on Stuntz Bay Road/1st Avenue to the intersection with Jasper Street. Go southeast on Jasper Street for about -.5 miles to the intersection of Hwy 1/169. Proceed east on Hwy 1/169 for 0.75miles to Vermilion Park Drive (this is the eastern entrance to Lake Vermilion/Soudan Underground Mine State Park and allows access to campsite near Cable Bay). Turn north on to Vermilion Park drive and proceed for 2.9 miles to Old Hwy 169. Turn west (left) on to Old Hwy 169 and follow it for 0.8 miles to Vermilion Ridge Road. Turn west on Vermilion Ridge Road and proceed for approximately 0.5 miles. Turn right and park near the restroom east of Cable Bay. We will depart the vehicles here and walk across the street to the Crosscut Trail. walk southeast along the Crosscut Trail for about 2200 meters. We will then take a short hike (approximately 30 meters) up the hill to Stop 6o. 97 �Trip 4 – Soudan Figure 4-12. Geologic map of the northeastern part of Lake Vermilion/Soudan Underground Mine State Park (after Peterson et al., 2016) illustrating locations of optional field trip stops. See Figure 4-5 for the description of map units. 98 �Trip 4 – Soudan Stop 6o (Optional)” Contact” Between Soudan Member Banded Iron Formation and Gafvert Lake Sequence Rhyodacite Polymict Lapilli-tuff/Tuff-breccia Longitude/Latitude: 47.834710°N, -92.211647°E UTM NAD 83 Zone 15N: 558,995E / 5,298,230N Here we will see one of the few places where the nature of the contact between the Soudan IronFormation Member oxide facies iron-formation and the overlying dacitic to rhyodacitic volcaniclastic rocks associated within the informally named Gafvert Lake Sequence can be observed (Figure 4-13). The Gafvert Lake Sequence (mapped as the “Upper Sequence” by Peterson and Patelke, 2003; Radakovich et al., 2010: and Heim et al., 2011) comprises dacitic to rhyodacitic volcaniclastic and epiclastic rocks that are locally interbedded with Algoma-type banded iron-formation and chert deposits. This sequence is part of the Lake Vermilion Formation. Within Lake Vermilion State Park, the overall stratigraphic thickness of the Gafvert Lake Sequence is up to approximately 1300 meters thick, with individual felsic volcaniclastic deposits having stratigraphic thicknesses ranging from approximately 75 – 400 meters thick, and individual Algomatype oxide facies banded iron formations and associated massive- to bedded chert deposits ranging from 25-250 meters and up to 175 meters in stratigraphic thickness, respectively. Northwest of the Soudan Mine, the Gafvert Lake Sequence is locally interlayered with, and overlain by, greywacke deposits associated with the Lake Vermilion Formation. Within Lake Vermilion State Park, several lithofacies comprise the Gafvert Lake Sequence. The basal member of this sequence comprises massive, very-thickly bedded, quartz- and plagioclase-phyric polymict dacitic to rhyodacitic tuff, lapilli-tuff, and tuff-breccia deposits. These light gray, non-sorted, nongraded, matrix-supported deposits contain 3-8% 1-2mm (rare 3mm) pale gray anhedral to subhedral quartz phenocrysts, 10-15% <1-2mm subhedral to euhedral tabular plagioclase phenocrysts, and a wide variety of lapilli- to block-sized clasts including: 1) 10-20% 1-10 cm quartz- and plagioclase-phyric coherent dacite to rhyodacite lapilli and blocks; 2) 5-7% <3cm diameter pale gray-green lens-shaped, locally quartz- and plagioclase-phyric pumice lapilli; 3) up to 1% dark gray to light gray angular chert lapilli ranging from 0.53cm in diameter; and 4) 1-3% 0.5-5cm dark gray to black to red magnetite-rich, hematite-rich, or jasperrich banded iron formation lapilli. These deposits are overlain by, and interbedded with, light gray, matrixsupported, non-sorted and non-graded quartz- and plagioclase-phyric dacitic to rhyodacitic tuff deposits (Figure 4-14) which contain 10-25% 1-3mm subhedral to euhedral tabular plagioclase phenocrysts, 1-3% 1-3mm subhedral to anhedral, commonly broken, quartz phenocrysts, as well as 10-15% subangular quartzand plagioclase-phyric coherent dacite to rhyodacite lapilli and up to 5% locally quartz- and plagioclasephyric pumice lapilli. Spectacular felsic epiclastic deposits comprising polymict volcaniclastic conglomerates and lithic sandstones are also present in the Gafvert Lake Sequence and crop out west of Lake Vermilion State Park in Stunz Bay (Radakovich et al., 2010). Based on regional mapping, Sims and Southwick (1980), Southwick (1993), and Southwick et al. (1998) have suggested that the contact between the underlying Soudan Iron-Formation Member of the Ely Greenstone Formation and the overlying Lake Vermilion Formation is locally an unconformity. Geochronological work in the Vermilion District (Peterson et al., 2001. Lodge et al., 2013), combined with detailed field mapping in the limited number of locations where the contact between the Soudan IronFormation Member and the Lake Vermilion Formation occurs, bears out this interpretation. Based on field relationships recognized by Radakovich et al. (2010), Lodge et al. (2013) collected a sample of the basal part of the Gafvert Lake polymict dacite- to rhyodacite lapilli-tuff / tuff-breccia deposits that occur at this outcrop in order to determine the age of volcanism of the Gafvert Lake Sequence relative to the ages of the Lower and Soudan Iron-Formation members of the Ely Greenstone Formation. Zircons from the sample of polymict rhyodacite tuff-breccia from this outcrop approximately 2m north of the contact with the Soudan Iron-Formation Member at this field trip stop produced a high precision U-Pb date of 2689.7 ±0.8 Ma using 99 �Trip 4 – Soudan Figure 4-13. Detailed (1:5000 scale) map (after Hudak et al., 2014) illustrating the disconformable contact between the Soudan Member Algoma-type banded iron-formation (unit S4a) and the Gafvert Lake Sequence quartz- and plagioclase-phyric polymict dacite-rhyodacite tuff-breccia / lapilli-tuff deposits (unit US2eh). We will start our investigation where Stop 6o is indicated, and traverse along the bedding and are locally folded. path indicated by the red dashed line over a series of outcrops. We will assemble on the two-track trail where indicated by the star symbol before proceeding to Stop 7o. Figure 4-14. Quartz- and plagioclase-phyric polymict dacite-rhyodacite tuff-breccia / lapilli-tuff from the Gafvert Lake Sequence. A) Typical appearance of very thickly bedded quartz- and plagioclase-phyric polymict daciterhyodacite lapilli-tuff. B) Close-up of unit illustrating tannish-white subhedral to euhedral tabular plagioclase phenocrysts, gray to gray-blue anhedral quartz phenocrysts, and 1cm diameter angular accidental fragment composed of jasper-rich banded iron formation. 100 �Trip 4 – Soudan hermal ionization mass spectrometry (Lodge et al., 2013). Given that the basal Gafvert Lake Sequence deposits contain angular intraclasts of chert and banded iron formation, and that there appears to be no intense structural fabric in either the Soudan Iron-Formation Member or the Gafvert Lake volcaniclastic rocks, Lodge et al. (2013) interpreted the contact here to represent a disconformity, a type of unconformity characterized by strata that are essentially parallel on either side of the erosional or non-depositional surface. Several outcrops occur at this location (refer back to Figure 4-13). The largest part of the outcrop, which extends east up the hill, is composed of laminated to medium bedded Soudan Iron-Formation Member. Alternating magnetite-rich horizons, chert horizons, and jasper horizons display planar bedding and are locally folded. Moving toward the northwest part of this outcrop, we observe a small break in the outcrop exposure. This break occurs directly above the contact between the Soudan Member iron formation (to the south) and the Gafvert Lake volcaniclastic rocks (to the north). In this area, note the lack of deformation in both lithological units. The lack of structural deformation at this contact, as well as geochronological data obtained from the Gafvert Lake volcaniclastic rocks near this contact (Lodge et al., 2013), supports the interpretation of a disconformity. Moving to the northwest, we observe the basal several meters of the Gafvert Lake Succession volcaniclastic rocks. Here, the rock is composed of a very thickly bedded quartz- and plagioclase-phyric polymict dacite-rhyodacite tuff-breccia / lapilli-tuff. The rock is characterized by up to 5% 1-3mm diameter subhedral to euhedral gray to blue-gray quartz phenocrysts and locally, 5-10% subhedral to euhedral light gray to tan tabular plagioclase phenocrysts set in a fine-grained quartzo-felspathic matrix that is locally sericite altered. Accidental fragments comprising laplli-sized light gray to grayish black angular to subangular chert, gray to dark gray subangular to angular banded iron formation (Figure 4-14), and rare angular to subangular reddish brown jasper fragments are present. As well, juvenile fragments comprising lapilli- to locally block-sized pumice are present. Lapilli- to block-sized accessory fragments of quartz- and plagioclase-phyric coherent dacite and rhyodacite are also present, in abundances up to 5%. As we move northwest then north down the hill, we will traverse several outcrops composed of Gafvert Lake Sequence tuff-breccia and lapilli-tuff deposits. We will traverse northwest then north down the hill (as shown in Figure 4-13) for about 80 meters back to the Crosscut Trail. We will then head northeast along the Crosscut Trail for approximately 900 meters. We will then traverse southeast through the bush for about 45 meters to Stop 7o. Stop 7o (Optional): Gafvert Lake Sequence Tuffs and Lapilli -tuffs Longitude/Latitude: 47.838875°N, -92.202496°E UTM NAD 83 Zone 15N: 559,675E / 5,298,700N We will stop here to observe several small outcrops of the Gafvert Lake Sequence tuffs and lapillituffs. These deposits comprise very thickly bedded, light gray, quartz- and plagioclase-phyric dacitic to rhyodacitic tuffs and lapilli-tuffs. The light gray recrystallized matrix generally contains 10-15% <1-2mm subhedral to euhedral tabular plagioclase phenocrysts which locally appear to be broken, as well as 3-8% <1-2mm pale gray anhedral, locally broken, anhedral to subhedral quartz phenocrysts. Various types of lapilli may be observed, including: 1) 10-20% 1-3cm diameter quartz- and plagioclase-phyric coherent dacite to rhyodacite lapilli; 2) 5-7% <3cm diameter pale gray green, lens-shaped, locally quartz- and plagioclase-phyric pumice lapilli; 3) <1mm dark gray to light gray angular chert lapilli ranging from 0.53cm in diameter; and 4) 1-3% 0.5-5cm dark gray to black magnetite-rich banded iron formation lapilli. We will traverse northwest for about 45 meters back to the Crosscut Trail. We will then proceed northeast along the Crosscut Trail for approximately 750 meters, then turn north for about 35 meters to Stop 8o. 101 �Trip 4 – Soudan Stop 8o (Optional): Quartz- ± Plagioclase-phyric Rhyodacite Sill (informally named the Gafvert Lake Intrusive Complex) Longitude/Latitude: 47.843117°N, -92.197887°E UTM NAD 83 Zone 15N: 560,015E / 5,299,175N At this location we will observe a spectacular light gray, massive, quartz- ± plagioclase-phyric coherent rhyodacite which, based on regional mapping (Peterson and Jirsa, 1999; Peterson, 2001; Hudak et al., 2002b; Heim et al., 2011) comprises a sill-dike complex that extends from the northern extents of Lake Vermilion State Park over 20km eastward to Mitchell Lake. This intrusion is most prevalent in the vicinity of Gafvert Lake, where it comprises several sills and dikes that intrude into the thickest section of Gafvert Lake Sequence volcaniclastic rocks. Based on the distribution of sills and dikes, coherent-facies Gafvert Lake Sequence deposits, and an abundance of coarse polymict breccias in this region, Peterson (2001) has interpreted this area to be the remnants of a stratovolcano that produced the Gafvert Lake Sequence dacitic to rhyodacitic volcaniclastic rocks. For this reason, this unique quartz-feldspar porphyry intrusion has been informally named the Gafvert Lake Intrusive Complex (GLIC). Lithogeochemical work recently completed at the University of Wisconsin Eau Claire (Schwierske et al., 2014; Figure 4-15) indicates that the GLIC and Gafvert Lake volcaniclastic rocks have very similar major, trace and rare earth element characteristics suggesting that they may be genetically related. However, geochronological studies will need to be performed to determine unambiguously if the GLIC and Gafvert Lake volcaniclastic rocks are indeed genetically related. The GLIC comprises light gray, massive, quartz ± plagioclase-phyric coherent rhyodacite. The light gray aphanitic groundmass contains 3-7% gray to light blue subhedral rounded to euhedral square quartz phenocrysts that range from 3-10mm in diameter, and 2-10% pale gray to tan, subhedral to euhedral tabular plagioclase phenocrysts ranging from 1-4mm in length. A variety of xenoliths may be found in this intrusion, including: 1) brown mudstone lapilli; 2) green to gray-green massive and/or amygdaloidal basalt lapilli; and 3) light gray aphyric coherent rhyodacite lapilli. In the field, the presence of large 5mm-10mm diameter gray to blue gray quartz phenocrysts distinguishes the GLIC from other quartz-feldspar-porphyry intrusions in the Vermilion District. 102 �Trip 4 – Soudan Figure 4-15. Chemical classification of various lithologies within Lake Vermilion State Park (Schwierske et al., 2014) using the immobile element classification scheme of Winchester and Floyd (1977). Open triangles represent samples from a quartz- ± plagioclase-phyric rhyodacite/dacite sill in the northeastern part of Lake Vermilion State Park. The black squares, large black diamonds, and small black diamonds represent various Gafvert Lake Succession volcaniclastic and epiclastic rock units. We will return to the Crosscut Trail and proceed northeast on the trail back to the vehicles. Upon loading the vehicles, we will return to the Mountain Iron Community Center. Travel east on Vermilion Ridge Road for approximately 0.5 miles to the intersection with Old Highway 169. Turn right (south) and continue east on Old Highway 169 for 0.8 miles. Turn south at the intersection with Vermilion Park Drive and proceed south for 2.9 miles to the intersection with Hwy 1/169. Turn west and Hwy 1/169 and drive for about 25.4 miles and merge onto Hwy 53/169 South. Follow Hwy 1/169 the intersection with Hwy 53/Hwy 169. Merge on to Hwy 169 south and proceed for 1.5 miles to Emerald Avenue. Turn south and proceed on Emerald Avenue for approximately 0.1 mile. Turn east and proceed for approximately 0.2 miles back to the Mountain Iron Community Center. END OF FIELD TRIP 103 �Trip 4 – Soudan Acknowledgements The authors would like to thank Jim Essig (Manager, Lake Vermilion/Soudan Underground Mine State Park), James Pointer (former Interpretive Supervisor, Lake Vermilion/Soudan Underground Mine State Park), and Jim DeVries (Assistant Manager, Lake Vermilion/Soudan Underground Mine State Park) for their assistance over the past two decades while the authors have conducted research, teaching, and numerous field trips within the park. References Allerton, Z., in prep., Thermal and Hydrothermal Effects of Proterozoic Events in the Archean Terrain of Northeastern Minnesota: unpublished Ph. D. dissertation, University of Minnesota Twin Cities. Allerton, Z., Hudak, G., Teyssier, C., Fayon, A., Daniŝik, M., Courtney-Davies, L, and Larson, P., 2024b, Geochronology campaign in northeastern Minnesota: Institute on Lake Superior Geology, Proceedings Volume 70, Part 1 – Program and Abstracts, p. 2-3. Allerton, Z., Hudak, G., Teyssier, C., Fayon, A., Daniŝik, M., Courtney-Davies, L, and Larson, P., 2024b, Geochronology and geochemistry of hematite ore in northeastern Minnesota: Institute on Lake Superior Geology, Proceedings Volume 70, Part 1 – Program and Abstracts, p. 4-5. Allerton, Z. P., Courtney-Davies, L., Daniŝik, M., Hudak, G. J., Teyssier, C., Mitchell, J. T., and Larson, P., in review, Hematite double-dating defines Proterozoic mineralization and thermal history of Archean banded iron formations, NE Minnesota, USA: submitted to Geology. Ayer, J. A., Goutier, J., Thurston, P. C., Dube, B., and Kamber, B. S., 2010, Tectonic and Metallogenic Evolution of the Abitibi and Wawa subprovinces: Summary of Field Work and Other Activities, 2010, Ontario Geological Survey Open File Report 6260, p. 3-1 – 3.6. Bakst, B., 2013, Minnesota’s Lake Vermilion State Park evolving at a cautious pace: TwinCities.com, Pioneer Press, http://www.twincities.com/localnews/ci_23939683/lake-vermilion-state-park-minnesotas-newest-evolving-at. Bauer, R. L., 1985, Correlation of early recumbent and younger upright folding across the boundary between an Archean gneiss belt and greenstone terrane, northeastern Minnesota: Geology, v. 13, p. 657-660. Baumgardner, M., Brown, N., Jacobson, A., Kendall, J., Ostwald, C., Schriner, N., White, J., and Peterson, D., 2013, Bedrock geologic map of the Gafvert Lake area, St. Louis County, northeastern Minnesota: Precambrian Research Center Map Series, PRC/Map-2013-04, 1:10,000 scale. Boerboom, T. J., 2020, D-07, Geochronology Database: Minnesota Geological Survey – Open Data Site, https://mngsumn.opendata.arcgis.com/datasets/d7903b83244a4878bcfb31f362bf5787_0/explore?location=46.147285%2C92.317888%2C7.04. Boerboom, T. J., and Zartman, R. E., 1993, Geology, geochemistry, and geochronology of the central Giants Range Batholith, northeastern Minnesota: Canadian Journal of Earth Science, v. 30, p. 2510-2522. Cas, R. A. F., and Wright, J. V., 1987, Volcanic Successions – Modern and Ancient: George Allen and Unwin, London, 528 p. Corfu, F. and Stott, G. M., 1998, Shebandowan greenstone belt, western Superior Province: U-Pb ages, tectonic implications, and correlations: Geological Society of America Bulletin, v. 110, p. 1467-1484. Dimroth, E., Cousineau, P., Leduc, M., and Sanschagrin, Y., 1978, Structure and organization of Archean subaqueous basalt flows, Rouyn-Noranda area, Quebec: Canadian Journal of Earth Sciences, v. 15, p. 902-918. Driese, S. G., Jirsa, M. A, Ren, M., Brantley, S. L., Sheldon, N. D., Parker, D., and Schmitz, M. D., 2011, Neoarchean paleoweathering of tonalite and metabasalt: Implications for reconstructions of 2.69 Ga early terrestrial ecosystems and paleoatmospheric chemistry: Precambrian Research, v. 189, p. 1-17. Fisher, R. V., 1961, Proposed classification of volcaniclastic sediments and rocks: Geological Society of America Bulletin, v. 72, p. 1409-1414. Fisher, R. V., 1966, Rocks composed of volcanic fragments and their classification: Earth Science Reviews, v. 1, p. 287-298. Fisher, R. V., 1998, Out of the Crater: Princeton University Press, 180 p. Gibson, H. L., Morton, R. L., and Hudak, G. J., 1999, Submarine volcanic processes, deposits, and environments favorable for the location of volcanic-associated massive sulfide deposits: Reviews in Economic Geology, v. 8, p. 13-51. 104 �Trip 4 – Soudan Gruner, J. W., 1926, Hydrothermal alteration of iron ores of the Lake Superior type – a modified theory: Economic Geology, v. 32, p. 121-130. Heim, N., Scott, H., Kilduff, R., Rahtz, C., Vial, A., Young, S., Mahr, C., and Hudak, G., 2011, Preliminary bedrock geology map of the eastern part of Lake Vermilion State Park, St. Louis County, NE Minnesota: Precambrian Research Center Map Series Map PRC/Map – 2010-01, 1:5000 scale. Hoffman, A. T., 2007. Lithostratigraphy, Hydrothermal Alteration, and Lithogeochemistry of Neoarchean Rocks in the Lower and Soudan Members of the Ely Greenstone Formation, Vermilion District, NE Minnesota: Implications for Volcanogenic Massive Sulfide Deposits: unpublished M. S. thesis, University of Minnesota – Duluth, 295 p. Hooper, P., and Ojakangas, R., 1971, Multiple deformation in the Vermilion District, Minnesota: Canadian Journal of Earth Sciences, v. 8, p. 423-434. Hudak, G. J., Heine, J., Hocker, S. M., and Hauck, S., 2002b, Geological mapping of the Needleboy Lake – Sixmile Lake area, northeastern Minnesota: a summary of volcanogenic massive sulfide potential: Natural Resources Research Institute Report of Investigation NRRI/RI-2002/14, 15 p. Hudak, G.J., Heine, J., Jirsa, M.A., and Peterson, D.M., 2004, Field Trip 1 - Volcanic stratigraphy, hydrothermal alteration, and VMS potential of the Lower Ely Greenstone, Fivemile Lake to Sixmile Lake area: 50th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 50, Part 2 – Field Trip Guidebook, p. 1-45. Hudak, G. J., Heine, J., Lodge, R. W. D., and Jansen, A., 2012, Recent developments understanding the volcanic, magmatic, tectonic, and metallogenic evolution of the Ely Greenstone Formation, Vermilion District, NE Minnesota: Geological Association of Canada – Mineralogical Association of Canada, Abstracts and Program, v. 35, p. 59. Hudak, G. J., Heine, J., Newkirk, T., Odette, J., and Hauck, S., 2002a, Comparative geology, stratigraphy, and lithogeochemistry of the Fivemile Lake, Quartz Hill, and Skeleton Lake VMS occurrences, Vermilion district, NE Minnesota: A report to the Minerals Coordinating Committee, DNR, Minerals Division, State of Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2002/03, 390 pages. Hudak, G. J., Hoffman, A. T., Peterson, D. M., and Heine, J., 2007, Recent developments understanding the volcanic, magmatic, tectonic, and metallogenic evolution of the Ely Greenstone Formation, Vermilion District, NE Minnesota: 53rd Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 53, Part 1 – Program and Abstracts, p. 42-43. Hudak, G. J., and Peterson, D. M., 2014, Non-Ferrous Mineralization Associated with the Wawa-Abitibi Terrane and Duluth Complex Cu-Ni-PGM Deposits, Northeastern Minnesota: Society of Economic Geologists, Guidebook Series, v. 47, 150 p. Hudak, G. J., Peterson, D. M., Radakovich, A, Pignotta, G., Schwierske, K., and the students from the 20120-2013 Precambrian Research Center Field Camp, 2016, Bedrock Geology of Lake Vermilion/Soudan Underground Mine State Park: Natural Resources Research Institute Technical Report NRRI/TR-2016-20, 23 p. Hudak, G. J., Radakovich, A., Pignotta, G., and Schwierske, K., 2014, Field Trip 2 – A Walk in the Park – Neoarchean Geology of Lake Vermilion State Park: Institute on Lake Superior Geology, Proceedings Volume 60, Part 2 – Field Trip Guidebook, p. 37-75. Hudleston, P. J., Schultz-Ela, D., and Southwick, D. L., 1988, Transpression in an Archean greenstone belt, northern Minnesota: Canadian Journal of Earth Sciences, v. 25, p. 1060-1068. Hudleston, P.J., 1976, Early deformational history of Archean rocks in the Vermilion district, north-eastern Minnesota: Canadian Journal of Earth Sciences, v. 13, p. 579-592. Jansen, A. C., Hudak, G. J., Heine, J. J., and Peterson, D. M., 1999, Lithogeochemical evaluation of Neoarchean mafic volcanic rocks comprising the footwall to the Soudan Member of the Ely Greenstone Formation, northeastern Minnesota: 55th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 55, Part 1 – Program and Abstracts, p. 46-47. Jirsa, M. A., 2000. The Midway sequence: a Timiskaming-type pull-apart basin deposit in the western Wawa Subprovince, Minnesota: Canadian Journal of Earth Sciences, v. 37, p. 1-15. Jirsa, M.A., 2016, Preliminary geologic maps of Lake and St. Louis counties, northeastern Minnesota: Minnesota Geological Survey Open File Report OFR 16-4, https://conservancy.umn.edu/items/a68ec090-cf02-463a-8fcbe3c05926d80e. Jirsa, M. A., Boerboom, T. J., Green, J. C., Miller, J. D., Morey, G. B., Ojakangas, R. W., and Peterson, D. M., 2004, Field Trip 5 – Classic outcrops of northeastern Minnesota: 50th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 50, Part 2 – Field Trip Guidebook, p. 129-169. 105 �Trip 4 – Soudan Jirsa, M., and Hillman, M., 2009, Field Trip 4 – Pioneer Mine (Miners Lake) Canoe Excursion: 55th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 55, Part 2 – Field Trip Guidebook, p. 110115. Jirsa, M. A., Starns, E. C., and Schmitz, M. D., 2012, Bedrock geologic map of the 2006 Cavity Lake forest fire area, Boundary Waters Canoe Area Wilderness, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map M-193, 1:24,000 scale. Jirsa, M.A., Southwick, D.L., and Boerboom, T.J., 1992, Structural evolution of Archean rocks in the western Wawa subprovince, Minnesota: refolding of precleavage nappes during D2 transpression: Canadian Journal of Earth Sciences 29:2146-2155. Klinger, F. L., 1960, Geology and ore deposits of the Soudan Mine, St. Louis County, Minnesota: unpublished Ph. D. dissertation, University of Wisconsin, Madison, 96 p. Larson, P., and Mooers, H., 2009, Field Trip 2 – Glacial geology of the Vermilion Moraine: 55th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 55, Part 2 – Field Trip Guidebook, p. 81-99. Lodge, R.W.D., Gibson, H. L., Stott, G. M., Franklin, J. M., and Hudak, G. J., 2015, Geodynamic setting, crustal architecture, and VMS metallogeny of ca 2720 Ma greenstone belt assemblages of the northern Wawa Subprovince, Superior Province, Canadian Journal of Earth Sciences, v. 52, p. 196 – 214. Lodge, R. W. D., Gibson, H. L., Stott, G. M., Hudak, G. J., Jirsa, M. A., and Hamilton, M. A., 2013, New U-Pb geochronology from the Timiskaming-type assemblages in the Shebandowan and Vermilion greenstone belts, Wawa Subprovince, Superior Craton: Implications for the Neoarchean development of the southwestern Superior Province: Precambrian Research, v. 235, p. 264-277. Lundy, J. R., 1985, Clues to structural history in the minor folds of the Soudan Iron Formation, northeastern Minnesota: Unpublished M. S. thesis, University of Minnesota, Minneapolis, 144 p. McPhie, J., Doyle, M., and Allen, R., 1993, Volcanic Textures: A Guide to the Interpretation of Textures in Volcanic Rocks: CODES Key Centre, University of Tasmania, Hobart, Tasmania, 198 p. Mercier-Langevin, P., Hannington, M. D., Dubé, B., and Bécu, V., 2010, The gold content of volcanogenic massive sulfide deposits: Mineralium Deposita, v. 46, p. 509-539. Mueller, W. U., and White, J. D. L., 2004, 4.2 – Terminology of Volcanic and Volcaniclastic Rocks: in Eriksson, P. G., Altermann, W., Nelson, D. R., Mueller, W. U., and Catuneanu, O., (eds.), The Precambrian Earth: Tempos and Events: Developments in Precambrian Geology, v. 12, Elsevier, Amsterdam, p. 273-277. Newkirk, T., Hudak, G. J., and Hauck, S. A., 2001, Preliminary lava flow morphology studies at the Five Mile Lake VMS prospect, Vermilion District, NE Minnesota: Implications for volcanic processes, volcanic paleoenvironments, and VMS exploration [abstract/poster]: Geological Society of America Abstracts and Programs V. 33, No. 6. Peterson, D. M., 2001, Development of Archean lode-gold and massive sulfide deposit exploration models using geographic information system applications: targeting mineral exploration in northeastern Minnesota from analysis of analog Canadian mining camps: unpublished Ph. D. dissertation, University of Minnesota, Duluth, Minnesota, 503 p. Peterson, D. M., Gallup, C., Jirsa, M. A., and Davis, D. W., 2001, Correlation of Archean assemblages across the U.S.- Canadian border: Phase I geochronology: 47th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 47, Part 1 – Programs and Abstracts, p. 77-78. Peterson, D.M., Hudak, G.J., Radakovich, A., Pignotta, G., and Schwierske, K., 2016, Geologic Map of Lake Vermilion/Soudan Underground Mine State Park: Precambrian Research Center Map PRC/Map-2016-01, 1:10,000 scale. Peterson, D. M., and Jirsa, M.A., 1999, Bedrock geologic map and mineral exploration data, western Vermilion district, St. Louis and Lake Counties, northeastern Minnesota: MGS Miscellaneous Map M-98, scale 1:48,000. Peterson, D., Jirsa, M., and Hudak, G., 2009a, Field Trip 7 – Architecture of an Archean Greenstone Belt: Stratigraphy, Structure, Mineralization: 55th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 55, Part 2 – Field Trip Guidebook, p. 178-215. Peterson, D. M., and Patelke, R. L., 2003, National Underground Science and Engineering Laboratory (NUSEL): Geological site investigation for the Soudan Mine, northeastern Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2003/29, 88 p.a Peterson, D. M., and Patelke, R. L., 2004, Field Trip 7 – Economic geology of Archean gold occurrences in the Vermilion District, northeast of Soudan, Minnesota: 50th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 50, Part 2 – Field Trip Guidebook, p. 200-226. 106 �Trip 4 – Soudan Peterson, D. M., Pointer, J., and Marshak, M., 2009b, Field Trip 3 – Soudan Iron Mine and Physics Lab Tour: 55th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 55, Part 2 – Field Trip Guidebook, p. 100-109. Radakovich, A. L., Parent, C. T., Partridge, M. E., Ritts, A. D., Pierce, R., and Hudak, G. J., 2010, Reconnaissance bedrock geological map of the northern part of Soudan Underground Mine State Park and the northwestern part of Lake Vermilion State Park, St. Louis County, Minnesota: Precambrian Research Center Map Series Map PRC/Map – 2010-04, 1:5000 scale. Schmid, R., 1981, Descriptive nomenclature and classification of pyroclastic deposits and fragments; recommendations of the IUGS subcommission on the systematics of igneous rocks: Geology, v. 9, p. 41-43. Schwierske, K.L., Pignotta, G. S., and Hudak, G. J., 2014, The 2.7 billion year old Mt. St. Helens of northern Minnesota: Petrography, geochemistry, and economic significance of the Neoarchean Gafvert Lake Sequence: 60th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 60, Part 1 – Programs and Abstracts, p. 113-114. Sims, P. K., and Southwick, D. L., 1985, Geologic map of Archean rocks, western Vermilion district, northern Minnesota: U. S. Geological Survey, Miscellaneous Investigations Map I-1527, scale 1:48,000. Southwick, D. L., (compiler), 1993, Bedrock geologic map of the Soudan-Bigfork area, northern Minnesota: Minnesota Geological Survey, Miscellaneous Map M-79, scale 1:100,000. Southwick, D. L., Boerboom, T. J., and Jirsa, M. A., 1998, Geologic setting and descriptive geochemistry of Archean supracrustal and hypabyssal rocks, Soudan-Bigfork area, northern Minnesota: implications for metallic mineral exploration: Minnesota Geological Survey, Report of Investigations 51, 69 p. Stott, G., Corkery, T., Leclair, A., Boily, M., and Percival, J., 2007, A revised terrane map for the Superior Province as interpreted from Aeromagnetic Data: 53rd Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 53, Part 1 – Program and Abstracts, p. 74-76. Stott, G. and Mueller, W., 2009, Superior Province: The nature and evolution of the Archean continental lithosphere: Precambrian Research, v. 168, p. 1-3. Thompson, A., 2015, A Hydrothermal Model for Metasomatism, of Neoarchean Algoma-Type Banded Iron Formation to Massive Hematite Ore at the Soudan Mine, NE Minnesota: unpublished M. S. Thesis, University of Minnesota Duluth, 59 p. Vallowe, A. M., Thalhamer, E. J., Rhoades, D. L., and Peterson, D. M., 2010, Surface and subsurface geologic maps of the Soudan Underground Mine State Park, St. Louis County, northeastern, Minnesota: Precambrian Research Center Map Series Map PRC/Map – 2010-01, 1:2500 and 1:5000 scale. White, J. D. L., and Houghton, B. F., 2006, Primary volcaniclastic rocks: Geology, v. 34, no. 8, p. 677-680. Winchester, J. A., and Floyd, P. A., 1977, Geochemical discrimination of different magma series and the differentiation products using immobile elements: Chemical Geology, v. 20, p. 325-343. 107 �Trip 5 – Alkalic plutons FIELD TRIP 5 Neoarchean Alkalic Intrusions in the Wawa and Quetico Subprovinces Terry Boerboom (retired)1 and Amy Radakovich1 1 Minnesota Geological Survey, College of Science and Engineering, University of Minnesota, 2609 Territorial Road, St. Paul, MN 55114 This field trip will visit several alkalic intrusions that have been mapped from a combination of outcrop, drilling, and geophysical data. Refer to Figure 5-1 for the pluton names and generalized regional geology. The stop descriptions are very brief, but a more thorough description of each pluton (as well as others not visited on this trip) are contained in the introductory text. These plutons were emplaced mainly into the Lake Vermilion Formation which is composed of volcanogenic sedimentary rocks sourced from Gafvert Lk rhyodacite tuff (2689.7±0.8 Ma) and also likely from felsic tuffs at the south limb of the Britt structure (2689.6±0.5 Ma). The Linden pluton has an age of 2681.00±0.29 Ma, and the Lost Lake pluton an age of 2675.1±0.5 (Boerboom et al., 2022). These ages are slightly older than the large Shannon Lake granite (2674±5 and 2674±27; Boerboom and Zartman 1993), and straddle two ages obtained on the Britt granodiorite (2681±4 and 2685±4 Ma; Boerboom and Zartman 1993). The Idington pluton is intruded by the Shannon Lake granite, consistent with the aforementioned age dates. The suite of alkalic plutons we will visit are located mainly in the Wawa subprovince but one (Gheen) is in the Quetico subprovince and will include the Side Lake, Morcom, Linden, Gheen, Idington, and Lost Lake plutons (Figure 5-1). All of these intrusions are similar in mineralogy with varied ratios of perthitic to antiperthitic feldspar and Na-plagioclase, hence are divided into those that are more syenitic vs. monzodioritic. All contain Na-rich aegirine/aegirine-augite with variable proportions of primary and secondary-deuteric hornblende, titanite, biotite, and minor oxides and apatite. Textures vary from uniformly medium-coarse grained to strongly and coarsely porphyritic, and they typically exhibit a flow-foliation defined by feldspar and subprismatic pyroxene and/or hornblende. All except the Linden are multi-phase with variations from ultramafic pyroxenite/hornblendite to intermediate syenite/monzodiorite, with relatively minor late-phase felsic phases; where multi-phase they generally show complex and conflicting intrusive relationships between the various phases. The following discussion, modified from Minnesota Geological Survey Report of Investigations 43 (Boerboom, 1994), covers the plutons we will visit as well as others that we will not visit. Note that some of the ideas presented in this report may have been modified or discredited based on newer geochronological data. 108 �Trip 5 – Alkalic plutons ALKALIC PLUTONS OF NORTHEASTERN MINNESOTA Minnesota Geological Survey Report of Investigations 43 By T. Boerboom ABSTRACT A series of alkalic plutons in northeastern Minnesota intrude metamorphosed sedimentary and volcanic rocks in the Wawa and Quetico subprovinces of the Archean Superior Province. The plutons generally fall into one of three categories-a syenitic clan, a monzodioritic clan, and a granitic clan. The main rock phases of the syenitic and monzodioritic clans are strongly porphyritic, coarse-grained, green and pink, quartz-poor syenite and diorite. Na-rich pyroxene is the predominant mafic mineral in these intrusions, and titanite is prominent in hand sample. Some of the syenitic intrusions contain melanite garnet, and at least one contains the feldspathoids nepheline and cancrinite. The granitic intrusions consist of variably porphyritic, coarse-grained, pink granite and monzonite, with hornblende as the dominant mafic mineral. Whereas these granitic plutons tend to be uniform in texture and composition, the syenite and monzodiorite plutons are characterized by abrupt internal variations in rock type ranging from dark-colored pyroxenite to light-pink leucocratic granite, syenite, and trondhjemite. The alkalic plutons range in size from 1.5 to 60 mi2 (3.9 – 155 km2) are oval to amoeboid in shape and elongate to the northeast, and are eroded to middle and upper levels. All of the alkalic plutons produce positive aeromagnetic anomalies; outcrops, although limited, confirm that these anomalies reflect the shapes of the plutons. Several unexposed plutons, whose shapes are inferred from aeromagnetic data, have been verified by test drilling. Field relationships show that these plutons are post-tectonic. Although chemical data are not available for all of the plutons, those with analyses plot as alkalic in terms of Na2O + K2O vs SiO2, but as mainly calc-alkalic on an AFM diagram. The syenitic and monzodioritic clans are generally neither nepheline-normative to neither quartz- nor nepheline-normative, with the exception of minor leucocratic phases. All are characterized by steep REE patterns and exceptionally high concentrations of Ba and Sr. INTRODUCTION Recent mapping in northern and northeastern Minnesota, including the Koochiching-ItascaBeltrami County area (Jirsa and Boerboom, 1990) and western St. Louis County (Jirsa and others, 1991), has delineated several previously unrecognized subalkalic to alkalic plutons. This report summarizes the lithological and intrusive relationships of several of these alkalic intrusions and briefly summarizes their geochemical characteristics. Some plutons in this group, such as the Snowbank and Kekekabic stocks and the Daisy Bay and Dead River plutons have been previously described (Geldon, 1972; Sims and Mudrey, 1972) and are not included in this report. Others (Coon Lake, Linden, Lost Lake plutons) have been briefly described in the literature (Sims and others, 1970, 1972; Sims and Mudrey, 1972), but are detailed here, as are others which have no published information or were unknown (Fig. 5-1A). Alkalic rock complexes similar to these are well known in Ontario (for example Sage, 1988a, 1988b, 1988c), but few have been described from Minnesota. Several other small alkalic plutons are inferred from aeromagnetic data, but are not exposed or have not been drilled (Jirsa and others, 1991). 109 �Trip 5 – Alkalic plutons Characteristics of the Alkalic Rock Suite The alkalic intrusions fall into three general categories – a syenitic group comprising the Coon Lake, Linden, Gheen, and Baudette plutons; a monzodioritic group including the Side Lake, Morcom, Idington, and Cook plutons; and a granitoid group containing the Bello Lake, Stingy Lake, and Rice River plutons (Fig. 5-1). Although most classify into one of the three clans, the many phases in each pluton (Table 5-1) produce considerable overlap. The syenitic and monzodioritic intrusions consist mainly of medium- to coarse-grained, porphyritic, pink and green syenite and monzodiorite, whereas the granitoid intrusions are typically medium-grained, variably porphyritic, pink quartz monzonite or granodiorite. The syenite and especially the monzodiorite plutons contain multiple erratic melanocratic to felsic phases with aegirineaugite as the predominant mafic mineral, whereas the granitoid plutons generally lack multiple phases, are more uniform in texture, and contain mainly hornblende as the mafic phase. Most of the alkalic plutons intrude metamorphosed volcanic and sedimentary rocks in the western Wawa subprovince, but some are within the Quetico subprovince (Card and Ciesielski, 1986; Fig. 5-1A). All were emplaced in the latest stages of the last major regional deformational event (Jirsa and others, 1992) or after it. Several of the alkalic plutons are cut by northwest-trending Late Proterozoic diabase dikes of the KenoraKabetogama swarm, which have been dated al 2,125 Ma (Rb-Sr; Beck, 1988) [NOTE: more recent U-Pb geochron ages of ca. 2067 -2070 Ma (Chamberlain and others, 2015; Schmitz and others 2006; Wirth and others , 1995). The plutons are exposed at various levels, and many, such as the Gheen, Side Lake, and Lost Lake, are exposed close to their roof zones. All of the alkalic plutons produce positive aeromagnetic anomalies which generally conform to the pluton shape (Fig. 5-1B). The major-element geochemistry of the alkalic plutonic rocks varies greatly as a result of their diverse mineralogy. However, except for minor proportions of felsic differentiates, they are low in SiO2 (49-62 wt. % for syenites, 47 to 58 wt. % for monzodiorites, 61-70 wt. % for granites; Table 5-2), and are mostly metaluminous to weakly peralkalic in composition (Fig. 5-2). The syenitic and monzodioritic rocks are generally quartz-free to nepheline-normative, whereas the granite from the Bello Lake pluton is mostly quartz-normative (Fig. 5-3). Except for one of the granites and a leucocratic differentiate of the Idington pluton, all plot as alkalic in terms of Na20 + K20 vs Si02, but as calc-alkalic to weakly alkalic on an AFM diagram. In all the plutons, Ba and Sr are in general highly enriched, but vary between the different phases. However, the Coon Lake pluton although slightly enriched, is surprisingly low in Ba and Sr, considering its extremely alkalic composition. The Linden pluton is extremely enriched in Ba and Sr, with Ba values of up to 13,000 ppm and Sr values up to 8,100 ppm reported from company drill cores. Chondrite-normalized REE patterns for the syenites and monzodiorites are fairly consistent, with moderately steep slopes and negligible Eu anomalies (Fig. 5 6). No REE data are available for any of the granitoid plutons. Table 5-1 (next page). Modal analyses of alkalic plutonic rocks; results in volume percent. Linden analyses from Sims and others (1972, p. 161); samples with KIB and CD prefixes from drill cores, all others from outcrops. Pyroxene includes aegirine to augite; n, points counted; est, estimate 110 �Trip 5 – Alkalic plutons Sample Quartz K-feldspar Plagioclase Pyroxene Hornblende Biotite Muscovite Chlorite Epidote Apatite Sphene Opaques Calcite, Fl* Nepheline Cancrinite Melanite n KIB-7 20 43 32 2 1 tr tr tr 2 est Bello Lake Coon Lake KIB-39 KIB-40 DL-61 KIB-6 3 32 46 50 79 53 40 9 tr 8 5 2 1 8 1 tr tr tr 1 tr tr tr tr tr tr 1 tr tr tr 3 3 tr tr tr 33 15 2 2 2 est est 1143 est Gheen Idington Linden C027 C029 C551X C552B C650 C561A Gnw7A Msw2A Gnw7-2 tr 41 2 tr 8 61 26 3 tr 3 77.6 56.2 37.1 21 7 29 55 32 88 1.9 5.6 0.4 12 31 49 16.8 25.9 57.1 46 19 8 6 7 5 7 4.1 0.4 4 4 1 1 tr tr 2 3 2 5 2 2 1368 1114 989 2 Fl* 1157 tr tr 2 2 tr 999 1 1 1 0.8 2.9 1.1 3.7 3.4 2.7 2.3 946 Side Lake satellites Morcom Stingy Rice R. Cook Side Lake Linden L-Sat Sample Gnw-7B CD-4 1242 C706B C533A C534A C543A C564A C603A CD-7** CD-7 CD-9 CD-17 CD-19 Quartz 26 13 K-feldspar 56.3 28 31 tr 13 2 28 1 29 7 20 22 30 Plagioclase 0.9 52 30 65 47 33 55 44 20 54 55 47 43 67 Pyroxene 27.1 16 17 22 26 30 47 21 5 Hypersthene 14 8 Hornblende 2 34 16 9 3 8 8 3 6 Biotite 9 7 4 1 3 20 1 7 10 1 Muscovite 1 14 3 1 1 4 Chlorite Epidote 5 1 1 1 2 183 Apatite 5.2 1 tr tr 2 2 1 1 0.5 tr Sphene 1.5 2 2 I 1 1 0.5 1 0.5 tr Opaques 1 1 tr 0.5 tr Calcite 1 n 1166 1151 est 976 1188 947 1115 1137 988 1152 836 960 est * Fluorite in sample C552B; ** Poikilitic and non-poikilitic phases, sample CD-7; L-Sat is intrusion beween Linden and Gheen 111 �Trip 5 – Alkalic plutons Figure 1-1. 112 �Trip 5 – Alkalic plutons SYENITIC PLUTONS Most of the syenitic plutons are northwest of the other plutons (Fig. 5-1A). The Gheen and Baudette plutons are within the Quetico subprovince, the Coon Lake pluton is in the Wawa subprovince, and the Linden pluton straddles the subprovince border. These plutons are distinguished by a preponderance of K-rich perthite, typically as trachytic, blocky phenocrysts in an aegirine-rich groundmass, or an amphibole-rich groundmass in the case of the Gheen pluton. . The Gheen and Linden plutons contain quartz, chlorite, apatite, epidote, titanite, opaque oxides, and pyrite as ubiquitous but generally minor constituents. The Coon Lake pluton differs from all others in that it contains substantial nepheline and cancrinite; the Baudette pluton lacks both feldspathoids and quartz. Melanite garnet is present in the Coon Lake and Baudette plutons, and in some phases of the Linden. A distinctive phase of spotted monzodiorite with centimeter-size poikilitic feldspar enclosing pyroxene, hornblende, plagioclase, biotite, and sphene is present in both the Linden and Gheen plutons and in plutons of the monzodiorite clan. Although the Gheen and parts of the Linden plutons are texturally similar to rocks of the monzodiorite clan, they differ by having phenocrysts of pink perthite instead of gray antiperthite. Trachytic fabric in the syenitic intrusions conforms to the pluton edges and dips steeply toward the pluton centers. However, outcrops are generally limited to the pluton borders, and the Baudette and Linden satellite intrusions are seen only in drill core. Aeromagnetic signatures correspond with intrusion shapes, whether it be a consistent oval like the Baudette, Coon Lake, and Linden plutons, or irregular and amoeboid like the Gheen and Linden satellite plutons (Fig. 5-l B). Coon Lake Pluton The Coon Lake pluton (Fig. 5-l; Jirsa, 1990; Jirsa and Boerboom, 1990) is a 48-mi2 subcircular pluton which intrudes mafic to felsic volcanic rocks metamorphosed to greenschist grade. A narrow aureole of amphibolite-grade metamorphism accompanied pluton emplacement. The pluton has a strongly magnetic border and internal lithological zonation is indicated by a circular, weakly positive magnetic anomaly within the pluton. Its north and northeast edges are exposed in scattered outcrops, and a single 10-foot-long vertical drill core was obtained from the pluton center (Boerboom and others, 1989). The main rock type in the exposed and cored portions of the Coon Lake pluton is pink to gray, medium- to very coarse grained, slightly to strongly porphyritic nepheline syenite, 50-79% microperthite, 15-33% nepheline {Ne76-80) 2-5% aegirine (Ac23Wo18En7Fs52), and as much as 9% plagioclase, 2% cancrinite, and 3% melanite, together with accessory sphene, apatite , biotite, magnetite, muscovite , and zircon (Tables 5-1 and 5-3, Fig. 5-7). Minor proportions of pyroxenite occur in ill-defined dikelets. String- and braid-textured microperthite forms rectangular crystals with minor inclusions of aegirine, sphene, cancrinite, and nepheline. Nepheline is typically anhedral but locally euhedral, up to 2 mm in size, and ranges from fresh to moderately altered to an unknown fibrous mineral of low birefringence. Prismatic, grass- green aegirine formed early in the crystallization sequence and is trachytic. Plagioclase and cancrinite are interstitial, the latter as colorless, highly birefringent fibrous grains. Melanite garnet forms subhedral, dark-brown grains up to 1 cm across with inclusions of aegirine and altered feldspar. In places the syenite consists of trachytic, purplishbrown, rectangular perthite crystals up to 7 cm in length, with minor nepheline, melanite, biotite, and aegirine. A syenite dike that cuts mafic volcanic rocks outboard of the main pluton contains an estimated 1% scolecite and a trace of blue corundum. Netlike anastomosing veinlets of white nepheline parallel to the vertical trachytic fabric of the feldspar in drill core from the center of the pluton imply that a late influx of volatiles affected the magnetic signature of the pluton's interior. 113 �Trip 5 – Alkalic plutons Table 5-2. Major- and select minor-element geochemical analyses of alkalic rocks [Major elements in wt% oxides, minor elements in ppm, blank – nod determined. See Boerboom 1994 for more information. Bello Lake Coon Lake Gheen Morcom Sample KIB-7 KIB-40 SiO2 Al2O3 CaO MgO Na2O K2O Fe2O3t FeO Fe2O3c MnO TiO2 P2O5 LOI Total Rb Sr Y Zr Nb Ba Ni Cu Zn Cs La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf V Cr Li B 69.7 15.7 1.87 0.51 5.67 3.51 1.84 0.4 1.4 0.04 0.2 0.09 0.7 100.2 165 1210 <10 90 <10 1340 63.9 17.4 2.16 1.28 6.13 4.47 3.32 1 2.21 0.07 0.35 0.18 0.77 100.4 135 1340 41 303 14 1470 F KIB39 60.2 17.2 3.55 1.85 6.3 4.4 3.94 1 2.83 0.08 0.42 0.32 1 99.8 129 2180 17 253 14 1970 KIB-6 CLP-1 I-561A DL-61 C029 C027 12, IC-2 CD-7 57.7 17.8 5.45 0.42 5.75 5.8 2.71 0.3 2.38 0.07 0.26 0.11 2.77 99.6 100 3700 81 64 15 2530 62.3 18.9 0.33 0.35 5.32 9.3 2.18 1.6 0.76 0.05 0.18 0.02 0.85 100.1 246 1300 <10 118 24 676 8 8.7 70.3 9 17.5 33 60.3 22.4 1.69 0.28 7.1 4.13 2.54 1.7 1.78 175 ppm 0.19 <10 ppm 1.31 100.1 132 806 10 404 27 500 <l 9.8 46.3 3 79.3 130 12.8 39.7 5.5 1.45 3.9 0.5 2.6 0.49 1.4 0.5 1.4 0.19 9 45 29 74 41 55.65 21.88 1.65 1.01 8.12 7.28 3.67 6.08 5 0.08 0.44 0.08 0.37 100.38 100 926 2 119 49.2 9.52 12.3 10.5 1.51 2.4 10.2 49.65 9.27 13.08 8.89 2.38 1.71 11.76 58.5 l5.4 5.01 3.58 5.9 3.8 5.22 0.16 0.88 0.33 2.39 99.5 60 290 14 84 13 697 94 40.8 80.1 2 20.8 46 0.56 0.89 0.06 1.99 99.93 79 1470 <10 0.1 0.48 0.29 1.08 99.7 215 7 16 30 4.5 27 58 56 14.1 6.48 2.55 3.57 6.56 6.19 3.2 1.66 0.12 0.71 1.03 1.85 99.9 120 2520 47 331 24 3710 78 5.5 84.5 1 130 328 33 5.1 1.21 186 34.7 9.3 24 5.7 1.9 0.4 2.3 0.7 0.65 0.1 4 2.3 0.2 8 1.7 0.2 3 11 39 <10 67 <10 <10 630 <10 <10 160 850 11.2 8.18 11 1.7 0.4 <0.5 22 26 36 <10 0.2 <0.1 4 20 10 18 30 114 Cs �Trip 5 – Alkalic plutons Idington Sample C552B SiO2 Al2O3 CaO MgO Na2O K2O Fe2O3t FeO Fe2O3c MnO TiO2 P2O5 LOI Total Rb Sr y Zr Nb Ba Ni Cu Zn Cs La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf V Cr Li B F 74.5 14.8 0.08 0.09 8.8 1.08 0.51 0.03 0.03 0.02 0.16 100.1 97 43 <10 36 30 81 <1 1.5 28.6 7 5.9 10 <5 0.2 0.2 <0.5 <0.2 <0.1 2 <10 4 <10 20 10, IC2- 11, IC-2 47.27 6.99 20.49 9.18 1.91 0.89 9.15 5.44 3.1 0.3 0.67 1.62 1.33 99.28 50.27 7.61 13.87 7.92 2.41 3.63 10.68 7.04 2.86 0.2 1.51 1.46 0.84 99.91 Linden L’ndn Sat Side Lk-Sat Side Lake Cook C650A C551X 8, IC-2 MN-10 CD-4-92 C564A 1242 C706B CD-19 48.3 7.24 16.6 9.58 2.12 1.23 IO.IO 6 3.43 0.19 1 1.25 0.77 98.6 59 772 37 262 15 817 76 185 125 9 164 348 41.4 171 27.9 6.99 18.2 2 8.8 1.46 3.2 0.3 2.6 0.32 7.6 256 140 231 14 2900 54 13.1 8.16 4.56 4.24 3.86 7.05 3.3 3.38 0.13 0.96 0.75 1.23 98.5 57 1770 26 181 9 1830 64 54.9 120 1 134 278 30.8 124 19.2 5.14 11.8 1.3 6.4 1.01 1.9 0.2 1.6 0.22 5 148 120 76 14 1400 60.21 16.28 4.76 2.21 3.78 6.32 4.29 1.62 2.49 0.08 0.56 0.23 0.73 99.76 57.1 10.2 10.1 4.43 2.64 6.52 6.51 2.83 3.36 0.15 0.64 1.07 62.2 15 4.09 1.62 6.57 4.73 3.98 1.4 2.42 0.09 0.48 0.24 0.54 99.9 94 510 10 246 14 2290 52.2 11.7 9.89 7.08 4.04 2.22 10.3 55.2 14.6 7.38 6.89 3.42 2.95 7.93 0.19 0.81 0.45 1.08 100.2 67 1080 <10 102 21 985 57 108 128 2 50.6 101 0.14 0.77 0.39 0.23 100.2 60 1030 20 30 10 1220 53.5 15.4 7.68 6.25 3.79 2.21 8.37 5.7 2.04 0.15 0.76 0.39 0.47 99.3 31 1190 20 82 2 1530 61 20.4 108 1 43.2 88 11.3 48.8 9.3 2.81 6.4 0.8 4.1 0.73 1.8 0.2 1.6 0.23 2.2 208 180 26 <10 680 53.6 17.7 7.59 3.26 5.4 1.79 7.07 2.6 4.18 0.13 0.71 0.36 1.54 99.5 39 1950 <10 110 15 882 99.1 164 2924 3574 406 183 28.1 6.62 16.6 46 8.9 2.4 0.6 1.53 0.262 115 1.4 0.2 4 210 80 84 <10 �Trip 5 – Alkalic plutons Figure 5-3. Modal (A) and normative (B) compositions of the alkalic plutonic rocks. Compositional fields from Streckeisen (1973), except “P” corner, which consists of albite and anorthite, used here to emphasize variations in K content. Q, quartz; F, feldspathoids; A, alkali feldspars; P, plagioclase. Circled symbols are feldspathoidal, not quartz. Figure 5-4. Geochemical discrimination diagrams. (A) Alkalic versus subalkalic discrimination diagram; modified from Irvine and Baragar (1971). (B) AFM diagram for the alkalic plutonic rocks; modified from Barker and Arth (1976). Figure 5-5. Harker diagrams for the alkalic plutonic 116 rocks, in wt % oxides recalculated to no loss on ignition. �Trip 5 – Alkalic plutons Figure 5-6. Chondrite-normalized rare-earth-element patterns for the alkalic plutonic rocks for which analyses are available. Figure 5-7. Pyroxene compositions from the Coon Like and Linden plutons. Pyroxene compositions from Poohbah Lake (Sage, 1988a) and compositions of pure aegirine (Deer and others, 1966, p. 107) shown for comparison. 117 �Trip 5 – Alkalic plutons Table 5-3. Microprobe analyses of minerals from the Coon Lake and Linden plutons. [Linden results from Sims and others (1972). Chemical analyses in weight percent oxides. Cancrinite totals low due to abundance of volatiles. Table is continued on next page. Biotite Aegirine Coon Lake Linden Coon Lake Sphene Linden Coon Lake Linden SiO2 51.49 51.62 51.8 53 46.85 37.16 35.69 44 43 30.01 30.65 31 TiO2 0.58 0.54 0.48 0.5 2.13 2.66 3.13 0.5 0.5 37.17 34.62 31 Al2O3 1.35 1.34 1.38 1 2.5 13.96 12.34 11 13 0.55 0.57 3.5 FeO 25.43 25.36 25.74 14.8 16.97 21.02 18.42 18 14 2.06 2.34 3 MnO 0.33 0.41 0.44 0.31 1.13 1.11 0.01 0.06 MgO 1.96 2.00 1.85 8.6 8.17 9.32 10.03 0.02 0.02 CaO 6.74 6.92 6.29 18.8 17.85 0 0.01 26.51 25.44 29 Na2O 9.74 9.51 9.88 3.5 2.49 0.14 0.22 1 1 0.29 0.33 3.3 K2O 0.00 0 0.00 0.5 0.5 9.21 9.34 10.5 11.5 0.01 0.01 Cr203 0.00 0.02 0.00 nd 0.01 0 0.00 0 Total 97.62 97.70 97.85 99.84 94.60 90.28 96.62 94.04 100.80 100.7 Number of cations based on 6 oxygen 14 99.0 16.2 99.20 Number of cations based on 24 oxygen Si 2.1 2.1 2.1 2.02 1.88 6.31 6.33 6.96 6.71 4.89 5.11 4.91 Ti 0.02 0.02 0.01 0.01 0.06 0.34 0.42 0.06 0.06 4.55 4.34 3.69 Al 0.06 0.06 0.07 0.04 0.12 2.79 2.58 2.05 2.39 0.11 0.11 0.65 Fe 0.87 0.86 0.87 0.47 0.57 2.98 2.73 2.38 1.83 0.28 0.33 0.40 Mn 0.01 0.01 O.D2 0 0.01 0.16 0.17 0.00 0.01 Mg 0.12 0.12 0.11 0.49 0.49 2.36 2.65 3.3 3.77 0.01 0.01 Ca 0.29 0.3 0.27 0.77 0.77 0 0.00 0.00 0.00 4.63 4.54 4.92 Na 0.77 0.75 0.78 0.26 0.19 0.05 0.08 0.31 0.3 0.09 0.11 1.01 K 0 0 0 0.02 0,03 1.99 2.1I 2.12 2.29 0.00 0.00 Cr 0 0 0 0.00 0.00 0.00 0.00 0.00 0.00 118 �Trip 5 – Alkalic plutons Table 5-3 continued Exsolved albite Perthite Nepheline Coon Lake Linden Cancrinite Coon Lake Coon Lake SiO2 72.37 63.422 66.725 67.493 66.235 61.5 46.153 46.641 48.068 46.297 45.345 37.455 37.613 37.472 Al2O3 19.868 15.347 17.527 18.889 18.612 19 33.061 34.701 35.152 34.217 33.123 28.053 26.956 28.416 BaO 0.000 0.000 0.132 0.000 0.104 0.000 0.000 0.000 0.028 0.000 0.122 0 0 CaO 0.031 0.000 0.018 0.001 0.000 0.5 0.084 0.076 0.116 0.102 0.484 5.583 5.432 5.348 Na2O 10.58 0.646 3.055 3.785 0.716 1.5 15.947 14.848 13.198 15.917 13.574 18.776 18.468 18.201 K 2O 1.268 12.139 12.521 12.016 15.932 15.8 6.186 6.211 5.877 6.133 6.068 0.04 0.052 0.206 Total 104.118 91.553 99.977 102.185 101.598 99.8 101.43 102.476 102.412 102.693 98.594 90.029 88.52 89.643 Number of cations based on 8 oxygen Number of cations based on 32 oxygen No. of cations based on 12 O Si 3.03 3.13 3.04 3 3.01 2.92 8.67 8.62 8.79 8.59 8.70 3.02 3.08 3.02 Al 0.98 0.89 0.94 0.99 1 1.06 7.33 7.56 7.58 7.48 7.49 2.67 2.6 2.7 Ba 0 0 0.01 0 0.01 0 0 0 0.01 0 0.01 0 0 Ca 0 0 0 0 0 0.03 0.02 0.02 0.02 0.02 0.10 0.48 0.48 0.46 Na 0.86 0.06 0.27 0.33 0.06 0.14 5.81 5.32 4.68 5.72 5.05 2.94 ·2.93 2.85 K 0,07 0.76 0.73 0.68 0.92 0.96 1.48 1.46 1.37 1.45 1.49 0 0.01 0.02 119 �Trip 5 – Alkalic plutons Linden Pluton The Linden pluton [2681.00±0.29 Ma] and its smaller satellite to the east intrude mafic to felsic volcaniclastic and sedimentary rocks that are metamorphosed to the sillimanite grade at the north edge of the pluton and to chlorite grade at the southern edge. A narrow amphibolite-grade metamorphic aureole surrounds the pluton (Jirsa and others, 1992), as is evident in drill core LF-1 (Fig. 5-1). In this core, thin syenitic dikelets cut biotite-amphibole schist that has centimeter-thick green bands dominated by bright-green sodic amphibole and brown bands dominated by biotite. The country rock here is of sillimanite grade, and the aureole along the north edge of the Linden pluton may reflect retrogression. Sims and others (1972), who describe amphibolite-grade contact metamorphism of mafic volcanic rocks adjacent to the western margin of the pluton, suggest that the foliation at a high angle to the regional fabric is the result of forcible pluton emplacement. The Linden pluton is roughly 54 mi2 in size and elongate to the northwest, whereas the satellitic intrusion is about 4.5 mi2 in size and elongate to the northeast (Fig. 5-1). Exposures are limited to the pluton edges, but ten drill cores from the pluton were obtained by various private and governmental agencies. Records of these cores and the cores themselves are on file at the Minnesota Department of Natural Resources, Division of minerals in Hibbing. A summary of the cores is given in Table 5-4. The LP-series of drill cores were subsequently examined by Himmelberg (1973), and thin sections from these cores were briefly reexamined in conjunction with this report. The LP-series descriptions are directly from company logs, and the OB­ series descriptions are from the Minnesota Department of Natural Resources, Division of Minerals (Martin and others, 1988). The target of company drilling is not known, but complete metals analyses, together with Na, K, Al, Ca, Ba, and Sr abundances, were obtained by the explorationists. The satellite intrusion is not exposed, but one short drill core was obtained by the Minnesota Geological Survey (Meints and others, 1993). The main Linden pluton is generally uniform in composition within the exposed portions and in the drill cores. The typical phase consists of trachytic, variably porphyritic, salmon-pink and greenish-black, medium-to coarse-grained aegirine-augite syenite with conspicuous dark-brown sphene and centimeter-scale elliptical pyroxenite clots. As reported by Sims and others (1972), and confirmed here, the dominant minerals of the syenite are braidtextured perthite and dark-green aegirine-augite (Ac7Wo41En26Fs25, Table 5-3 and Fig. 5-7), with variable but lesser amounts of plagioclase, sphene, apatite, biotite, hornblende, magnetite, and epidote. Modal analyses and compositions of selected minerals are summarized in Tables 51 and 5-3. Complete descriptions of exposures are given in Sims and others (1972), and the drill cores are summarized in Table 5-4. Textures in the groundmass of drill holes CD-13 (Meints and others, 1993) and LF-2 are suggestive of cataclasis, yet other features in these cores, such as tabular plagioclase, blocky pseudomorphic biotite and epidote (presumably after pyroxene), and euhedral diamond-shaped sphene, show no evidence of brittle deformation. Thus the granoblastic fabric of the groundmass is most likely the result of plastic flow deformation of a viscous, mostly crystallized magma, in conjunction with late deuteric fluids. Drill hole CD-4 in the Linden satellite also contains zones of moderately sheared syenite characterized by rounded, rolled feldspar phenocrysts, suggestions of C­ S fabric, and streaky pink and gray segregations. Shear bands <I inch to 10 feet thick are foliated parallel to the trachytic fabric of unsheared portions, and the mineralogy of the sheared rock in thin section is identical to that of the undeformed portions (Table 5-1). As is the case in the Linden pluton proper, the annealed texture implies that deformation occurred in a hot, 120 �Trip 5 – Alkalic plutons semiplastic state, under near-magmatic temperatures. These submagmatic deformation features are also present in the Idington and Coon Lake plutons. Gheen Pluton The Gheen pluton, some 3 miles east of the Linden pluton, intrudes sillimanite-grade metasedimentary rocks. It is currently exposed at a high level, and its magnetic signature conforms to the long, sinuous, northeast­ elongate shape deduced from scattered outcrops along the length of the body. Local trachytic fabric defined by tabular perthite phenocrysts is steep and subconformable to the pluton contacts and the pluton shape at both map and outcrop scale is subcordant to the schistosity of the host supracrustal rocks. The pluton is chiefly mesocratic, pink and dark-green, porphyritic syenite and ranges to dark-greenish-black, coarse-grained pyroxenite and leucocratic, pink, coarse­ grained alkalifeldspar syenite. Conflicting internal intrusive relationships are common, with melanocratic phases occurring both as inclusions and as dikes in the porphyritic phase. However, pink leucosyenite dikelets cross all other phases. Mesocratic syenite phases contain 1- to 3-cm tabular perthite phenocrysts in a groundmass of fibrous amphibole, euhedral sphene, blocky oxides, stubby prismatic apatite, and minor calcite and epidote. Trace amounts of dark-green pyroxene are present, but most has been deuterically altered to bright-green fibrous amphibole. Calcite occurs both in irregular veinlets with amphibole and as magmatic, interstitial grains against sharp comers of feldspar phenocrysts. The melanocratic monzodiorite phase consists predominantly of relict pale-green pyroxene up to 2.5 mm across and lesser amounts of sericitized plagioclase, finegrained, euhedral sphene, and apatite. The pyroxene is variably replaced by euhedral, pale-green hornblende. Biotite occurs as brown books within hornblende, and is slightly altered to chlorite. Unaltered microperthite occupies a late anhedral interstitial position. Sills of pink, leucocratic, coarse-grained syenite up to 10 feet wide emanate from the Gheen pluton and cut adjacent metasedimentary rocks. This phase is characterized by irregular microcline phenocrysts in a seriate groundmass of macroscopically identified, pink microcline, fine-grained biotite, and minor white albite. Local planar miariolitic cavities lined with K-feldspar crystals are consistent with the interpretation that the pluton is exposed at a high level. The Gheen pluton differs from the other alkalic intrusions by its pervasive deuteric alteration and relatively abundant chalcopyrite. Modal analyses of the melanocratic and porphyritic mesocratic phases are listed in Table 5-1 and shown on Figure 5-3. 121 �Trip 5 – Alkalic plutons Table 5-4. Descriptions of cores from the Linden pluton; Dominant lithology in bold type. Drill Description Hole Pink, slightly porphyritic, medium-grained homogeneous leuco alkalifeldspar syenite. Trachytic foliation defined by aligned mafic minerals. LF-2 Feldspar varies from coarse blocky phenocrysts with recrystallized edges to granoblastic groundmass. LF-3 LF-4 LF-5 Dark-gray, slightly porphyritic, fine- to medium-grained heterogeneous poikilitic syenite to monzonite. Poikilitic feldspar encloses pyroxene, biotite, and sphene. Weak trachytic fabric defined by aligned pyroxenes and feldspar oikocrysts. Feldspathic dikelets and aegirine veinlets. Light-grayish-white, medium-grained, granular to hypidiomorphic hornblende monzonite with 1-cm mafic segregations. Deuteric pyroxene alteration. Light-pinkish-gray, medium- to coarse-grained, moderately porphyritic syenite; 2- to 4-cm mafic segregations of slightly porphyritic, euhedral aegirine in groundmass of feldspar, hornblende, sphene, etc. Mineralogy Perthite, biotite, muscovite, aegerine, melanite, sphene, calcite, epidote, oxides, pyrite. Microperthite that grades to antiperthite, pyroxene, biotite, sphene, apatite Perthite, antiperthite, hornblende-biotite-oxide clusters after pyroxene, epidote, sphene, apatite, calcite in brittle veinlets. Perthite, aegirine, sphene, apatite, oxides, hornblende, biotite, chlorite. Perthite, aegirine-augite, biotite, sphene, apatite, oxides. Red-stained perthite, fine granular plagioclase, stilpnomelane after biotite, chlorite after hornblende or pyroxene, leucoxene after sphene. Very coarse feldspar, 7-80% aegirine, sphene. Not described. OB-207 Pinkish-gray, coarse-grained, trachytic, aegirine syenite. OB-212 Coarse-grained, green and pink, trachytic syenite. Deuteric alteration of mafic minerals. LP-1 Pink, coarse-grained syenite with erratic distribution of mafic minerals. Contains a 1-foot-wide dike of melasyenite (biotite pyroxene-carbonate). LP-2 Upper 20 feet, leucosyenite with 75-90° dipping trachytic fabric; rest is pink and green mesocratic syenite with biotite segregations. Not described CD-13 Pink and green, coarse-grained, weakly trachytic syenite with annealed cataclastic texture. Microperthite phenocrysts, pyroxene altered to tabular clusters of biotite plus epidote. Foliated matrix of fine-grained plagioclase. Microperthite, plagioclase, biotite, epidote, melanite, sphene, sericite. CD--4 Linden Satellite Gray, coarse-grained, trachytic, porphyritic syenite with narrow pink and green, fine-grained shear bands. Granoblastic-recrystallized texture in shear bands grades into unsheared rock, contains rolled feldspar phenocrysts. Sheared portions of same mineralogy as unsheared. In unsheared portion, tabular perthite rimmed by granular plagioclase, zoned euhedral aegirine-augite rimmed by pale-green fibrous amphibole. Apatite, chlorite, sphene, allanite, oxides. 122 �Trip 5 – Alkalic plutons Baudette pluton, Lake of the Woods County The Baudette pluton is not exposed, but is seen in a drill core obtained by the Minnesota Geological Survey (hole 1986-CUSMAP-1; Mills and others, 1987). It is located 8 miles south of the town of Baudette, in Lake of the Woods County, and intrudes felsic schists. As judged from geophysical data, the pluton is approximately 1 mile long and half a mile wide, but the best resolution of available geophysics is only 1:250,000 (USGS data in Chandler, 1991). The core consists of coarse-grained, green and pink, porphyritic garnet-biotite syenite. Trachytoid phenocrysts of pink perthite up to 2 cm long with irregular granular borders are in a groundmass of green biotite, melanite garnet, plagioclase, lesser epidote, sphene, and aegirine­ augite, and accessory apatite and zircon or monazite. The brownish-yellow melanite varies from small euhedral crystals to large granular masses enclosed within coarser green biotite. The biotite varies greatly in grain size from fine-grained mats to medium-grained books with a decussate intergrown fabric. MONZODIORITE CLAN The monzodioritic group includes the Side Lake, Morcom, Idington, Lost Lake, and Cook plutons, as well as the Daisy Bay pluton (Sims and Mudrey, 1972), which is shown on Figure 5-1 but not discussed here. All are within the Wawa subprovince, adjacent to the north edge of the Shannon Lake granite phase of the Giants Range batholith (Jirsa and others, 1991). These plutons tend to be irregular in shape and elongate to the northeast, parallel to the regional D2 fabric (Jirsa and others, 1992) of the supracrustal country rocks. The rock is largely pink and green porphyritic monzodiorite, but varies erratically to dark-green pyroxenite and pink granite, granodiorite, and Na-rich trondhjemite. The pyroxenite tends to occur in small irregular pods and segregations, whereas the felsic differentiates occur in thin, straight dikes and in larger segregations. In addition, the monzodiorite group contains minor dark-green poikilitic phases in which antiperthite is grown over pyroxene, sphene, apatite, and hornblende. Porphyritic phases typically have strong trachytoid fabrics, defined by aligned feldspar phenocrysts that are subconformable to the borders but more erratic in the centers of the intrusions. The typical porphyritic phase is characterized by coarse, blocky, pink to gray phenocrysts of Na-rich antiperthite in a groundrnass of predominantly fine-grained euhedral aegirine-augite, along with sphene, perthite, polygonal plagioclase, lesser proportions of hornblende, biotite, apatite, epidote, chlorite, opaque oxides, and rare quartz. Feldspar phenocrysts are typically antiperthitic, but range from albitic plagioclase to strongly perthitic K-feldspar. The normative composition is commonly midway between the K-rich and Na-rich end members in contrast to the compositions implied by point counting, because of difficulties in properly quantifying modal abundances of the strongly exsolved feldspars (Fig. 5-3B). Idington Pluton The Idington pluton is located in west-central St. Louis County near the former village of Idington. Its irregular horseshoe shape, roughly 8 mi2 in size, is elongate to the northeast (Fig. 5-1). Trachytic foliations are predominantly northeast-oriented, subcordant to the pluton boundary, and dip generally more than 70°. However, exposures are limited to central parts of the pluton, where trachytic fabrics are less likely to conform to the pluton shape. The pluton has a rather irregular magnetic anomaly (Fig. 5-1), but the magnetic pattern has been somewhat obscured by a 150-foot-wide, strongly magnetic diabase dike and possibly by late north-trending brittle faults. No contact relationships with country rocks were observed, except at the southwestern edge of the pluton, where it is intruded by the 2,674-Ma Shannon Lake granite of the Giants Range batholith (Jirsa and others, 1991; Boerboom and Zartman, 1993). 123 �Trip 5 – Alkalic plutons Rock types in the Idington pluton are consistent in mineralogy but extremely erratic in modal proportions. Mesocratic, porphyritic pyroxene monzonite predominates, but dark-green aegirine-augite pyroxenite is common, and a small proportion of pink, sodic leucotrondhjemite occurs in aplopegmatite dikes 1 to 10 cm wide in the heart of the pluton. A mappable segregation of leucotrondhjemite exposed at the pluton's northeast corner contains minor flat-lying vuggy fractures lined with purple fluorite and a dark brown translucent tetragonal mineral tentatively identified as zircon or cassiterite. Modal analyses from four samples of the Idington pluton (porphyritic phase-C.551.X, pyroxenite phase­ C.650.A, and two felsic differentiates-C.552.B and C.561.A) are listed in Table 51. In the porphyritic phase, feldspar phenocrysts are mostly gray, rectangular, 1- to 4- cm crystals of coarsely exsolved antiperthite, but small intergrown polygonal grains of plagioclase and perthite also are abundant in the groundmass. Aegirine-augite crystals are prismatic, weakly pleochroic, and zoned with darker green rims. Apatite inclusions are common near the edges of pyroxene crystals. Euhedral, dark-brown sphene is prominent in hand sample and is microscopically associated with biotite. Melanocratic diorite is medium to coarse grained, with trachytically aligned aegirine-augite prisms in a groundmass dominated by fresh, zoned, anhedral plagioclase. Hornblende in this phase occurs as dark-green patches of secondary origin within pyroxene and as larger subpoikilitic grains with inclusions of apatite, pyroxene, and plagioclase. Green biotite forms clusters of euhedral blocky grains aligned parallel to the trachytic fabric. Sphene is mostly euhedral, but locally is subpoikilitic-anhedral and partially encloses pyroxene and other mafic minerals. Accessory minerals include allanite and secondary chlorite, calcite, and epidote. The leucocratic differentiate contains albitic feldspar as large as 40 cm across, which is characterized by graphic Intergrowths with quartz. Aplitic parts of the leuco-phase contain radial-plumose sheaves of albite as long as 3 cm. The erratic distribution between phases, which typifies exposures of this intrusion, can be documented on an outcrop scale to have formed by filter-pressing of a mafic liquid out of a feldsparphenocryst slurry. The groundmass in the porphyry is identical to the melanocratic pyroxenite, which occurs as irregular amoeboid to net-vein segregations as large as several feet. However, at the southwestern end of the pluton, cumulus modal layering is also present in the form of melanocratic layers tens of centimeters thick interlayered with mesocratic, porphyritic diorite. This diorite itself shows layering by changes in phenocryst size and abundance and trachytoid foliation parallel to layering. The layering and trachytic fabric have been drag-folded into widely spaced, crosscutting ductile shear bands, which lack cleavage or schistosity, but instead have an annealed, granoblastic habit similar to that in the Linden pluton. Pink granite pegmatite dikelets (Shannon Lake granite?) commonly occupy these shear planes. In these bands, elliptical deformed relict feldspar phenocrysts are recrystallized into granoblastic aggregates, and the pyroxenes have been replaced by bright-green hornblende. The annealed textures and lack of through-going fabric development imply that ductile deformation, recrystallization, and annealment, caused by self-induced strain during emplacement or the last vestiges of regional deformation, occurred while the rock was still hot. In a series of exposures along Highway 53 at the south edge of the Idington pluton a sharp line of demarcation exists whereby outcrops of Shannon Lake granite have inclusions only of Idington monzodiorite north of an east-west line, and those south of it have inclusions only of granodiorite derived from the Britt pluton, an early, D2-deformed intrusion (Jirsa and others, 1991, 1992). This implies that the earlier intrusive contact of the Idington pluton into the Britt granodiorite was overprinted but preserved by upward stoping of the Shannon Lake granite (Boerboom and Zartman, 1993). Side Lake Pluton The long, arcuate Side Lake pluton, roughly 27 mi2 in dimension, extends eastward from Side Lake in western St. Louis County. The pluton crops out only at its very western and eastern limits. Two exposed satellitic plugs off the eastern tip of the main pluton (Fig. 5-1) are identical in mineralogy to, and 124 �Trip 5 – Alkalic plutons conterminous with, the eastern end of the Side Lake pluton. These satellites have complex intrusive relationships with the country rocks, and are described in a separate section below. The main pluton intrudes metamorphosed basaltic volcanic and felsic sedimentary rocks along most of its length. It is just north of the Shannon Lake granite, but intrusive relationships with the granite are unknown because of lack of exposure. Crosscutting Proterozoic diabase dikes have lowered the magnetism along their length, in contrast to the Idington pluton, where the diabase dikes have enhanced the magnetism. Rocks from the exposures of the Side Lake pluton range from mesocratic biotite-hyperstheneclinopyroxene diorite on the west to hornblende monzodiorite on the east Trachytic fabrics in most outcrops are generally conformable to the margins of the pluton. Segregations of melanocratic pyroxenite to hornblendite occur throughout the intrusion, but are more abundant to the west, where they occur as irregular dikelets, segregations, and small inclusions in mesocratic diorite. In addition, dikes of diorite and pyroxenite up to 150 feet wide, which emanate from the western margin of the pluton, are parallel to the pluton boundary and intrude metabasaltic rocks. These dikes are clearly discordant to the regional metamorphic fabric in the intruded basalts; some of them contain wispy felsic stringers parallel to their walls produced by flow segregation. Thin, straight, late-stage pink granitic to syenitic dikelets are common within and adjacent to the pluton. Mesocratic, medium-grained, pinkish-gray diorite, which predominates at the western end, contains 5-10% tiny euhedral grains of pleochroic pale-pink to green hypersthene, and at least 20% euhedral, palegreen clinopyroxene with light-colored rims. These pyroxenes range in size from less than 1 to 3 mm; the hypersthene is generally finer grained, and the clinopyroxene is variably phenocrystic. Plagioclase is the predominant feldspar. A sample from the pluton 200 feet from the western contact contains strongly zoned, subhedral, trachytic plagioclase, with fuzzy grain boundaries. Another thin section 500 feet from the contact has fine-grained granoblastic plagioclase, orthoclase, and quartz between larger augite phenocrysts. Apatite is abundant as fine-grained euhedral prismatic grains included within pyroxene and feldspar. Oxides occur both within augite as wormy blebs of apparent secondary origin and as scattered blocky, fine-grained crystals. Sphene is rare or lacking at the western end. Brown biotite is a generally minor component at the western end of the pluton, but in some outcrops composes up to 5% of the rock. It locally forms vertically oriented poikilitic plates that are up to 2 cm long and oriented parallel to the trachytic fabric of the monzodiorite. The eastern outcrops consist of moderately heterogeneous, medium- to coarse-grained, pink and green pyroxene-hornblende monzodiorite. Here moderately developed trachytic foliation plunges 2030° to the souL'1west, down the axis of the pluton. Fine-grained, centimeter-sized, angular cognate xenoliths of mafic monzodiorite, in addition to dark-green mafic stringers, are common. Side Lake Pluton Satellites Two small plugs are exposed northeast of the Side Lake pluton. One, about 1/4 mile east of the Side Lake pluton, is round and 3/4 mile in diameter. It consists of medium- to coarse-grained, trachytic, weakly porphyritic, green and pink hornblende-pyroxene monzodiorite. Subhedral 5-mm orange-white antiperthite and scattered 2-mm aegirine-augite phenocrysts are set in a fine-grained groundmass of green prismatic pyroxene, fine-grained anhedral feldspar of unknown composition, minor hornblende, and brownish-green biotite mostly replaced by chlorite. Apatite, biotite, opaques (oxides and pyrite), and sphene each compose about 1% (Table 5-1). Clots and irregular segregations of pyroxenite are common, as are late dikelets of pink syenite up to 10 cm wide which cut across trachytic fabrics. The trachytic fabric is locally variable and inconsistent in orientation, but generally has a shallow southwest plunge toward the main Side Lake pluton. The next satellite is a horseshoe-shaped body, 0.75 mi2 in size, located 1.5 miles east of the Side Lake pluton (Fig. 5-1). Its linear trachytic fabrics are subconformable to the edges of the plug, and again plunge shallowly southwest toward the Side Lake pluton. This small intrusion is highly varied in 125 �Trip 5 – Alkalic plutons texture and mafic content, but dark-green poikilitic diorite and white leucocratic monzodiorite with small inclusions of pyroxenite predominate. The poikilitic diorite has 1-cm, oval-shaped antiperthitic to perthitic poikilitic feldspar with inclusions of aegirine-augite, biotite, sphene, and apatite. The long axes of the poikilitic feldspars and prismatic pyroxenes are parallel and define a primary trachytic fabric. The dark poikilitic phase is sharply cut by sills of the white monzodiorite. However at one location, the two rocks are commingled in a pillow-like fashion that suggests mixing of immiscible liquids. Thus the field relationships, as well as mineralogy, indicate that the dark poikilitic and leucocratic phases are comagmatic. Relationship of Satellitic Intrusions to the Side Lake Pluton Their similar lithological and textural attributes and aligned trachytic fabrics imply that the Side Lake pluton and the two satellites are derived from a common source at depth to the west. Further evidence of comagmatism is the presence of numerous thin anastamosing sills of white monzodiorite, identical to the white phase in the small plugs, which are parallel to the schistosity of the surrounding metasedimentary country rocks and intercalated with them. The intercalated monzodiorite and schist define a mappable unit along a discrete zone (dashed area on Fig. 5-1 that links the two satellitic plugs to the Side Lake pluton. This zone continues past the eastern satellite for at least 2.5 miles, where it merges back into a magnetic anomaly interpreted as another alkalic intrusion (Jirsa and others, 1991). At the intersection of Highway 73 and the Sturgeon River east of the Side Lake pluton, the intercalated monzodiorite and metasedirnentary rocks are transected by a late, north-trending brittle shear zone, which has minimal offset but has reduced the rocks to a fine-grained cataclasite. Morcom Pluton [Thin section CD-7] The Morcom pluton is just north of the Side Lake pluton and may be related to it at depth, because a large positive gravity anomaly underlies the area between the two. The Morcom pluton, which intrudes metasedimentary rocks, has a bulbous shape with a long narrow appendage to the east (Fig. 5-1). Scattered outcrops exist at the eastern limit of the pluton, and a drill core was obtained from the western end, near the north side. Trachytic foliation near the southeast edge dips 80°N, and at the eastern tip plunges 40°SW. In the drill core the foliation dips 40-45°, presumably toward the pluton center. Rock types are similar in the Morcom pluton, the eastern Side Lake pluton and its satellites, and the Idington pluton. However, the major-element geochemistry of the Morcom is very similar to that of the Linden pluton (Fig. 5-5). The drill core consists of multiphase, medium-grained, weakly porphyritic biotite-hornblende-pyroxene monzodiorite, with a trachytoid foliation defined by alignment of plagioclase phenocrysts and prismatic mafic minerals. Dark-green, poikilitic monzodiorite occurs in the core as 15-cm inclusions or layers; small miariolitic cavities lined with fine-grained crystalline biotite, pyroxene, and pyrite are also present. Late brittle slickensided faults and fractures, oriented obliquely to foliation, locally transect the core. Feldspar compositions vary from clean plagioclase with narrow twin lamellae to untwinned plagioclase, and from antiperthite to perthite, the latter confined to anhedral grains in the groundmass. Euhedral, light-green, aegirine-augite has hornblende rims and alteration patches; hornblende is also present as subhedral to prismatic, brownish- to bluish-green grains with patchy color zonation and rare deuteric overgrowths of colorless actinolite. Biotite is dark green and pleochroic, and is associated with hornblende. Accessory minerals include sphene, allanite, apatite, epidote, calcite, and minor secondary oxides within pyroxene. The nonpoikilitic and poikilitic phases have similar mineralogy (Table 5-1). Exposures at the eastern tip of the pluton consist of pink to gray, medium-grained monzodiorite with abundant 5- to 10-cm, elongate xenoliths of foliated felsic to pelitic schist, together with cognate xenoliths of fine-grained melanocratic monzodiorite. Some of the intrusive-breccia xenoliths are themselves an intrusive breccia. Pink monzonitic dikelets are abundant and cut all the earlier intrusive phases and xenoliths. The monzodiorite contains scattered sericitized plagioclase phenocrysts in a fine­ grained groundmass consisting of up to 5% quartz intergrown with granular K-feldspar and plagioclase, 126 �Trip 5 – Alkalic plutons along with hornblende, actinolite, sphene, chlorite, apatite, and minor oxides and secondary calcite. Melanocratic clots are of similar mineralogy but with a higher proportion of mafic minerals. Elsewhere at the eastern terminus, the rock lacks xenolithic inclusions but is still heterogeneous and cut by late, pink felsic differentiates. This inclusion­ free monzodiorite has a trachytic fabric defined by aligned feldspars and mafic clots, and is similar in mineralogy to the core from the western end of the pluton. Lost Lake Pluton (2675.1±0.5 Ma) The Lost Lake pluton, the "pluton southwest of Lost Lake" of Sims and Mudrey (1972), was described as a circular pluton composed of heterogeneous syenite with a local, conspicuously porphyritic facies, a pegmatitic facies with miariolitic cavities, and small bodies of pyroxene­ biotite lamprophyre. They noted that the borders of the pluton tend to be quartzose and contain small angular inclusions of metagraywacke and slate of the Lake Vermilion Formation. Based on detailed remapping, the authors have redefined the shape of the pluton as a long, sinuous and bulbous, northeast-trending body that is 1 mile or less wide but approximately 9 mi2 in size. The eastern tip of the pluton lies 1/4 mile south of Lost Lake, and the western terminus is just south of Angora on State Highway 53, about half a mile north of the Idington pluton (Fig. 5-1). The uniform magnetic signature of the pluton has been lowered locally by late, north-south, brittle faults which have minimal offset. The western end of the Lost Lake pluton is not exposed and its shape is inferred from aeromagnetic data, whereas the central portion is well exposed, and scattered outcrops exist over the eastern end, mainly adjacent to more resistant crosscutting Proterozoic diabase dikes. Two mappable intrusions of quartz monzonite 1/4 mile in diameter occur adjacent to the main body (Jirsa and others, 1991). These small plugs are similar in composition to leucocratic dikes within the main pluton, and are related to the pluton. Subvertical trachytic fabric, which is defined by both phenocrysts and elongate poikilitic feldspar, strikes generally northeast, subparallel to the length of the intrusion. The small, separate bodies of pink monzonite also possess a northeast-oriented trachytic fabric, defined by orbicular clots of biotite, disseminated biotite, or aligned feldspar crystals. The Lost Lake pluton is mineralogically similar to the Idington and eastern Side Lake plutons, but contains a higher proportion of pink leucocratic phases. The main rock types range from pink and green, porphyritic monzodiorite to dark-green, poikilitic biotite-pyroxene monzodiorite to pink monzonite, syenite, and quartz monzonite. Pyroxenite occurs in small segregations, in the same fashion as in the Idington pluton. In general, the poikilitic and porphyritic phases are earliest and are cut by the pink rock varieties. Small dikes of pink granitic pegmatite cut all other rock types, but it is unclear whether these dikes are related to the pluton or are from an external source, such as the Shannon Lake granite of the Giants Range batholith. The pink monzodiorite and syenite differentiates are medium grained, equigranular to weakly porphyritic, and commonly aplitic to pegmatitic, with pyroxene, hornblende, and biotite as the predominant mafic phases. The two small felsic plugs of quartz monzonite to granodiorite contain a mafic mineral assemblage of varied proportions of biotite, chlorite, and hornblende, and up to 30% quartz. The margins of these plugs contain abundant inclusions of felsic volcanic country rocks up to 15 feet across which were clearly deformed prior to incorporation, and small dikes emanating from these plugs cut across fold axes in the supracrustal rocks. One of the plugs contains a unique medium-grained, pink, orbicular granodiorite with trachytically aligned discs of black, concentrically foliated biotite that are as much as 0.5 cm thick and 5 cm long. The biotite orbs which contain intergrown sphene, apatite, plagioclase, quartz, and magnetite, compose as much as 15% of the rock. The orbicular rock grades into a non-orbicular phase with the same proportion of biotite, but as medium-grained, uniformly disseminated flakes. In addition to the typical phases, related rocks in the small plugs include coarse-grained, dark-green biotite-hornblende lamprophyre; green poikilitic monzodiorite; and pink monzonitic pegmatite. 127 �Trip 5 – Alkalic plutons Cook Pluton The Cook pluton (Cook Airport pluton on Southwick, 1993), 1 mile south of the town of Cook (Fig. 5-1), is inferred from aeromagnetic data to be 1.5 mi2 in size, elongate to the east. No outcrops of this pluton are known, but a drill hole in the western margin of the pluton recovered core of uniformly coarse-grained, peppery, dark-greenish-black and light-green, epidote-altered hornblende-biotite diorite. Strong trachytic foliation, which is defined by tabular plagioclase and mafic minerals, dips 50° from horizontal. One fine-grained cognate xenolith, 1 cm x 3 cm in size, is present near the bottom of the 10-foot core. The rock has a primary hypidiomorphic-granular texture, but pervasive, small euhedral crystals of secondary epidote are overprinted on all primary minerals, preferentially in the cores of plagioclase, and as fine-grained granular masses in biotite. Pale-green hornblende is rimmed by green biotite, and zoned plagioclase is clean and well-twinned, despite the pervasive epidote alteration. Accessory minerals include apatite, sphene, oxides, calcite, and interstitial orthoclase. Scattered late, brittle fractures which dip as much as 20° from horizontal are lined with coarse, lineated chlorite, pinkaltered feldspar, and a crust of epidote and white carbonate. The pristine trachytic igneous texture and lack of metamorphic fabric indicate that the pervasive epidotization is the result of deuteric alteration, rather than regional metamorphism. GRANITOID PLUTONS The granitoid group includes the Stingy Lake, Rice River, and Bello Lake plutons, all within the Wawa subprovince. These plutons tend to be oval in shape and elongate to the northeast. Rocks in this group are characterized by substantial quantities of quartz and are vaguely to strongly porphyritic and trachytic. Hornblende is the predominant mafic phase, along with biotite and rare pyroxene. These plutons are considered part of the alkalic group on the basis of their similarity to the other alkalic plutons in size, shape, high Ba and Sr content, magnetic signature, and trachytic fabric. Stingy Lake Pluton The Stingy Lake pluton is a 9 mi 2 circular pluton located 3 miles south of Sturgeon Lake, adjacent to the Giants Range granite, and is inferred to intrude mafic volcanic rocks (Fig. 5-1). Although unexposed, its round shape is well defined by its aeromagnetic anomaly (magnetic rim and nonmagnetic core). A 10-foot drill core was obtained from the northwest side of the pluton (Meints and others, 1993). The intrusion is cut by two Proterozoic diabase dikes. The rock in the core is uniformly coarse-grained, porphyritic, pink granodiorite to quartz monzodiorite. Tabular phenocrysts of string-and-braid microperthite up to 7 mm long, together with weakly zoned plagioclase up to 2 mm long having narrow twin lamellae and weakly sericitized cores, define the 45°-dipping trachytoid foliation. The perthite contains small blocky plagioclase inclusions, and the areas between abutting perthite grains are also stuffed with small blocky plagioclase grains. Lightgray anhedral interstitial quartz with shadowy extinction has been recrystallized into coarse polycrystalline aggregates. Hornblende is mostly altered to green biotite, epidote, and granular oxides; however, fresh, dark-green, euhedral hornblende is locally preserved within quartz and feldspar. Accessory minerals include blocky primary oxides, sphene, zircon, and apatite (Table 5-1). Late closely spaced, vertical brittle fractures lined with epidote and chlorite are pervasive in the 10-foot core. Rice River Pluton The Rice River pluton, about 5 miles west of Cook, intrudes metamorphosed sedimentary rocks. It is inferred to be approximately 15 mi2 in size, although its magnetic signature (Fig. 5-1B) of magnetic rim and nonmagnetic core is irregular and overprinted at the western edge by a north­ trending Proterozoic diabase dike. A drill hole in the magnetic eastern rim of the pluton recovered core of gray, coarse-grained, porphyritic quartz monzonite to monzodiorite. Steeply inclined trachytic foliation is 128 �Trip 5 – Alkalic plutons defined by euhedral, strongly zoned, 1- to 2-cm perthite phenocrysts and small elliptical melanocratic clots that are fine-grained cognate xenoliths. Groundmass to the phenocrysts consists of 3- to 6-mm subhedral microcline, zoned plagioclase, hornblende, and anhedral interstitial quartz; the phenocrysts are rimmed by fine-grained plagioclase and myrmekitic quartz-feldspar intergrowths. Plagioclase grains are heavily sericitized, preferentially in the cores. Hornblende is weakly zoned and slightly altered to biotite, chlorite, and opaques. Euhedral sphene, allanite rimmed by epidote, and secondary calcite occur in minor proportions. Scattered chlorite-pyrite veinlets dip 5-10° from horizontal and occupy brittle fractures; some have slickensides that dip shallowly in the fracture planes. Bello Lake Pluton The Bello Lake pluton (Jirsa, 1990; Jirsa and Boerboom, 1990) is just southwest of the Coon Lake pluton. It is approximately 60 mi2 in size, elongate to the northeast parallel to the regional strike of the mafic to felsic supracrustal rocks that it intrudes. The Bello Lake pluton is unexposed, but three 10-foot drill cores were obtained by the Minnesota Geological Survey, two near the western end, and one near the eastern end of the pluton (Fig. 5-1). The intrusion is magnetically quiet relative to the mafic volcanic rocks around it, but the pluton margins are strongly magnetic locally. As judged from the cores, the Bello Lake pluton is uniform in color and texture, but moderately variable in composition, ranging from pyroxene monzonite to hornblende granite. The pyroxenebearing phase occurs close to the pluton margin, whereas the hornblende monzonite occurs near the center of the pluton at its western end, and the hornblende granite is near the eastern end of the pluton. Data are insufficient to properly judge spatial variation of rock types, but the observed distribution suggests that the pluton may be zoned from a pyroxene-bearing phase at the rim, to a more differentiated, quartz-bearing phase near the center. Pyroxene monzodiorite near the pluton border (KIB-39; Table 5-1) is green and pink, medium grained, and seriate in texture with a strong trachytic fabric defined by rectangular, zoned plagioclase and subhedral, weakly uralitized augite crystals. Accessory oxides, sphene, hornblende, chlorite, biotite, and apatite all formed late in the crystallization sequence, and tend to occur together. Green and pink, medium-grained, slightly porphyritic hornblende monzonite (KIB-40; Table 5-1) on the western side of the pluton contains subhedral-prismatic hornblende and small phenocrysts of grayish-pink, blocky microperthite in an allotriomorphic-granular to weakly seriate groundmass of plagioclase, perthite, and minor quartz. This rock is similar to the pyroxene monzodiorite in hole KIB-39, except that hornblende occupies the position of pyroxene. Hornblende granite from the eastern part of the pluton (drill hole KIB-7; Table 5-1) is characterized by strongly zoned, blocky plagioclase with sericitized cores surrounded by poikilitic microperthite. Myrmekitic feldspar-quartz intergrowths occur along perthite-plagioclase grain boundaries. Quartz is coarse and anhedral interstitial, and hornblende forms dark-green, irregular grains with abundant tiny quartz inclusions near the edges and granular oxide inclusions in the cores. Accessory green biotite, epidote, and chlorite are associated with hornblende as alteration products, and blocky apatite crystals are associated with mafic phases. PETROGENETIC AND GEOCHRONOLOGICAL STUDIES Arth and Hanson (1975), using data on major, trace, and rare earth elements, and isotopic data from the Linden pluton, concluded that the magma formed from 5 to 10% partial melting of a mixed eclogite and garnet peridotite source at mantle depth. Stern and others (1989) believe that the Linden originated by partial melting of a LILE­ enriched mantle peridotite at shallow depths under hydrous conditions created by mantle metasomatism from rapid subduction of oceanic 129 �Trip 5 – Alkalic plutons lithosphere. However, they have lumped the Linden pluton in with the "sanukatoid suite," a very broad suite of rocks of variable size, timing, and associations throughout the Superior Province. The age of D2 deformation of t h e supracrustal rocks at the southern edge of the area from Cook to Side Lake (Jirsa and others, 1991) was bracketed by U-Pb zircon geochronology to between 2,685 and 2,669 Ma (Boerboom and Zartman, 1993). The alkalic plutons lack significant D 2 fabrics, and thus could not have been emplaced until approximately 2,669 Ma. [NOTE: This has been discredited by the new ages of 2681.00±0.29 Ma on the Linden and 2675.1±0.05 Ma on the Lost Lake plutons – Boerboom and others, 2022] The Idington pluton is intruded by a granite pegmatite inferred to have originated from the Shannon Lake granite, which was dated by Boerboom and Zartman (1993) at 2,674 ± 5, or a minimum of 2,669. Catanzaro and Hanson (1971) obtained a discordant Pb207/Pb206 age of 2,740 ± 10 Ma on sphene f r o m the Linden pluton. Prince and Hanson (1972) obtained a similar age of 2,740 Ma, based on a Rb/Sr isochron through apatite and two whole-rock samples. These older ages on the Linden pluton relative to the younger age implied for the ldington pluton indicate that the syenitic rocks may be slightly older than the monzodioritic group, or that pluton emplacement may have progressed from north to south. Clearly, modern high-precision U-Pb zircon dates on the alkalic plutons are needed. [NOTE: Recent age of 207Pb/206Pb 2681.00±0.29 Ma (Boerboom and others, 2022)] CONCLUSIONS The alkalic intrusions in northern Minnesota can be generally subdivided into a syenitic group, a monzodioritic group, and a granitoid group. The syenitic plutons are somewhat north of the monzodioritic intrusions, whereas the granitoid plutons are interspersed with the monzodiorites. Although these groups differ in mineralogy, they are all similar in terms of size, texture, map pattern, geochemistry (e.g., high Ba and Sr), aeromagnetic signature, and timing of emplacement All of the alkalic plutons have porphyritic textures, and the syenitic and monzodioritic plutons typically contain abrupt phase transitions from predominantly mesocratic, porphyritic rocks to dark-green pyroxenites and pink felsic differentiates. The granitoid plutons are more uniform in composition and texture. The plutons are eroded to various levels. The northeastward-elongation and en-echelon map pattern of the Gheen and Lost Lake plutons and the eastern Side Lake pluton and its satellites indicate exposure at high levels, whereas the broad, rounded map shapes of the Linden, Coon Lake, and Bello Lake plutons indicate a deeper level of erosion. The map patterns of the relatively well exposed Side Lake, Idington, and Lost Lake plutons indicate a similar style of emplacement, in which the plutons have penetrated the supracrustal rocks to different levels. The Side Lake pluton plunges to the west, as indicated by the deeper level of erosion at the western end of the pluton and the west-plunging linear trachytic fabrics in the Side Lake satellites. This westward plunge may be either a primary emplacement feature or the result of tilting of the pluton prior to unroofing. Several other plutons of alkalic affinity are suggested by the aeromagnetic data, but they are not exposed and their existence has not been verified by drilling. ACKNOWLEDGMENTS Field work and geochemical analyses for this project were funded by the Minerals Diversification Program administered by the Minerals .Coordinating Committee for the Minnesota Legislature. The Minerals Division of the Minnesota Natural Resources Research Institute (also supported by the Minerals Diversification Program) coordinated the analytical work for several of the geochemical samples. 130 �Trip 5 – Alkalic plutons FIELD TRIP STOPS These stop descriptions are brief – see introductory section for more detail about the individual plutons. Directions: To stop 1: Drive west on Highway 169 to Highway 5 at Chisholm • go ~15 miles north on Hwy. 5 to road 915/McCarthy Beach road the •go west on McCarthy Beach road between Side and Sturgeon Lakes, this turns into Link Lake trail—stay on for a total of ~7 miles to small trail (491988, 5283636) • walk SW on trail ~ 1000’ / 300m and go W-SW to ridge where there are some peeled outcrops. Work your way back NE along top of ridge for more outcrop back to road. Stop 1. (NAD83: 491723, 5283344) (47.70606°, -93.10680°) Side Lake pluton – multiphase Side Lake Pluton ultramafic to felsic. NEXT: Head back east on Link Lake Trail • at about one mile turn left on Beatrice Lake road at sharp bend in trail. • Take Beatrice Lake road ~1.7 miles to intersection with Snake Trail and turn right. • Follow Snake Trail ~2.5 miles to Hwy. 5. • Go right/south on Hwy. 5 for ~1.8 miles. • Go left on Hwy. 65/Perch Lake road for 1.7 miles then • take a left / north on Dean Forest Road (268). • Follow Dean Forest Road (276) ~4.6 miles through a series of jogs to a small road on the right / south (Mud Hole Road # 276). • Drive ~500 feet and park next to knob on the west side of the road (504781, 5284935), go up on outcrop knob to east. Multiple peels. Stop 2. (NAD83: 504781, 5284935) (47.71778°, -92.93625°) Roof zone of Side Lake Pluton. Many dikes of multiphase monzonitic rocks cut high-grade garnet-staurolite-sillimanite bearing metasedimentary rocks of the Lake Vermilion Formation. Some dikes may be unrelated tonalite. One smaller outcrop near road along south edge of knob has 3-5 x 7-15 cm mafic enclaves in quartz tonalite to monzonite. This lies in what is interpreted as the roof zone of the Side Lake Pluton. NEXT: Go back to road # 276 • turn right / east and drive ~3.4 miles to Highway 73. • Turn left / north on Hwy 73 for 4 miles to Hwy. 22 • turn left / west for 3 miles to road 931. • Turn left / south on 931 for 0.5 miles to crest of small hill, outcrop in the east side of road. Stop 3. ***Private Property please be respectful*** (NAD83: 504882, 5290921) (47.77164°, -92.93484°) Morcom Pluton – Monzdioritic intrusive breccia of widely variable grain size bearing many inclusions of different phases of itself that range from intermediate-porphyritic to ultramafic. One thin section was made from this outcrop and in it the mafic phase is dominantly hornblende, in contrast to samples from other parts of the pluton which contain abundant green Na-pyroxene in addition to hornblende. Sphene, magnetite, and apatite are also relatively abundant. This pluton is not well exposed with only a few outcrops in this vicinity on the east end and a drill hole on the west end; extent outlined via aeromag data. NEXT: Head back east to Hwy. 73 • turn left / north for 5 miles to Highway 1 • Turn left / west on Hwy. 1 for 3.4 miles to large outcrop ridge and find a safe place to park... Stop 4. Linden Pluton (NAD83: 504222, 5301045) (47.86273°, -92.94355°) (2681.00±0.29 Ma) Brownish-pink medium- to coarse-grained, strongly foliated, moderately porphyritic pyroxene syenite. Tabular crystals of gray perthite and prismatic dark green pyroxene phenocryst are surrounded by a pink groundmass composed mainly of fine granular albitic plagioclase. The foliation (and weak subvertical lineation), interpreted as magmatic, is defined by the phenocrysts of microcline and 131 �Trip 5 – Alkalic plutons pyroxene, and dips steeply to the northwest parallel to the pluton margin. The syenite also contains cm-scale ellipsoidal ultramafic pyroxenite enclaves that are flattened parallel to the main foliation. Dominantly composed of perthitic alkali feldspar, albitic plagioclase, and dark-green prismatic aegirine-augite (Ac7Wo41En26Fs25, Table 5-3 and Fig. 5-7), with lesser amounts of sphene, apatite, biotite, hornblende, magnetite, and epidote. Relatively coarse reddish-brown titanite/sphene is readily visible in hand sample. Plagioclase is generally restricted to the groundmass as very fine-grained granoblastic grains. In thin section the pyroxene is very fresh, bright green, sub-euhedral, and weakly zoned with roundish lighter green cores. Sphene forms small euhedral crystals, apatite forms thick irregular to subprismatic crystals, and minor proportions of biotite form strongly pleochroic light brown to deep brownish-green irregular books commonly intergrown with or included in pyroxene. Strongly aligned perthitic orthoclase forms blocky-rectangular crystals up to 7mm in length that are commonly Carlsbadtwinned. The groundmass matrix between the orthoclase and pyroxene is composed of fine-grained granoblastic feldspar that appears to have undergone brittle deformation; however within this granulated matrix are pristine pyroxene, sphene, and apatite crystals that show no evidence of shearing or rotation. This coupled with the apparent lack of shear bands on the outcrop implies that the granulation of the groundmass may have occurred during emplacement by semi-plastic deformation during upward flowage of the magma. Just north of the highway at the lowermost east end of this outcrop is an old adit that goes straight into the hillside; not sure as to when or why this was made. NEXT: Head back east on Hwy. 1 to Hwy. 73 • turn left / north for 5.2 miles to Highway 53. • Hang a right (go SW) on Hwy. 53 for 2 miles to where there are outcrops on the northeast side of the road. There is a driveway adjacent to this outcrop (on the north end) that would be a good place to park. Stop 5. ************WATCH OUT FOR TRAFFIC THIS IS A BUSY ROAD************ Gheen Pluton (NAD83: 515162, 5306034) (47.90745°, -92.79711°) The Gheen pluton is a spectacular example of multi-phase magma mingling textures. Strongly and coarsely porphyritic pyroxene syenite grades into, is cut by, and has inclusions of, medium-grained dark green hornblende gabbro to pyroxenite. Pink aplite and pegmatite forms the latest phase as small dikes that cross the other phases, and seems to have preferentially permeated the more mafic phases. Bluish chloritic slickenside surfaces look similar to those in the Linden pluton. The phenocrysts in the porphyritic phase at this stop are composed of braid-textured perthite to antiperthite in a matrix of bluish-green hornblende and prismatic actinolitic amphibole, abundant sphene, magnetite, and apatite, and anhedral to subpoikilitic saussuritized plagioclase. The aphyric mafic phases are composed varied combinations of pale green augite magmatic hornblende, secondary actinolitic amphibole, biotite, and accessory sphene, apatite, magnetite, and calcite. NEXT: Continue southeast on Hwy. 53 for about 14 miles, through the town of Cook, to County Road 467. • Turn left / east for 0.6 miles then veer right to stay on 467. Continue on 467 to railroad crossing; from there go another 0.75 miles to Forest Road 258D • Either drive or walk south on this road for 0.5 mile to an outcrop on the left / east in an overgrown clearcut, next to a logging trail that goes east. Stop 6. Idington Pluton (NAD83: 529108, 5287200) (47.73752°, -92.61175°) 132 �Trip 5 – Alkalic plutons The Idington (eye-ding-ton) pluton as not been dated, but it is intruded by the 2,674-Ma Shannon Lake granite along the southwestern margin of the pluton. This pluton is characterized by its coarsely porphyritic character as dramatically shown at this stop, and the phenocrysts are typically aligned by magmatic flow (trachytoid) in a crystal mush. In places the phenocrysts are randomly oriented, and in others oriented in a circular fashion that implies they were caught in an eddy. The mafic matrix is composed dominantly of aegirine or aegirine-augite and commonly has been squeezed out (i.e. filter-pressed) of the crystal mush to form small to large (10’s of meters) zones of an ultramafic phase. White ‘aplite’ dikes and some larger segregations cut the syenite and pyroxenite phases; these are interpreted as late residual differentiated melts that were squirted around through the semi-solid pluton. Very fine, delicate compositional zonation is visible in some of the phenocrysts here at this stop, where the rock is properly weathered. For a more thorough description of the pluton as a whole refer to the appropriate section of the introduction. The strikingly porphyritic syenite at this stop is typical of the Idington pluton although the phenocrysts are larger than normal. The phenocrysts are composed of perthite / antiperthite and NEXT: Go back north to the main road (467) and head east for 2.25 miles then follow road around bend to north (turns into County road 381) • Continue on 381 for 2.75 miles to Highway 1 • Turn right / east on Hwy. 1 for 3.25 miles to County Road 361 • Turn left / north and drive 1.5 miles to a small flat outcrop in the east ditch. There are also outcrops along the road on the way here one can stop at. Stop 7. Lost Lake Pluton (NAD83: 537263, 5293105) (47.79023°, -92.50248°) 2675.1±0.5 Ma The Lost Lake pluton is an irregularly-shaped intrusion that elongate to the east-northeast. The small outcrop in the road ditch shows mafic pyroxene-rich enclaves in pink syenitic phase. Outcrops nearby in the woods to the east demonstrate many different phases ranging from uniform pink to coarsely porphyritic to dark green and ultramafic. The ultramafic phases/enclaves in the road ditch outcrop are composed primarily of deep green (in thin section) aegirine as small equant to subprismatic crystals, a lesser proportion of larger blocky to subpoikitic biotite, and accessory sphene and apatite in a groundmass of poikilitic calcite (it fizzes) and minor sodic plagioclase. The pink portion is composed of allotriomorphic-granular mosaic of anhedral sodic plagioclase, perthite to antiperthite (commonly poikilitic), prismatic green aegirine, apatite, biotite, and interstitial calcite. At this stop we also will display some drill core from the Lost Lake pluton which demonstrates the multiple phases and diversity within this unit. NEXT: End of trip. Go back south to Highway 1 then east to Highway 169, then south back to Mountain Iron. Thank you for attending. 133 �Trip 5 – Alkalic plutons REFERENCES CITED Arth, J.G., and Hanson, G.N., 1975, Geochemistry and origin of the early Precambrian crust of northeastern Minnesota: Geochimica et Cosmochimica Acta, v. 39, p. 325-362. Barker, J.G., and Arth, J.G., 1976, Generation of trondhjemite-tonalite liquids and Archean bimodal trondhjemite-basalt suites, Geology, v. 4, p. 596-600. Beck, J.W., 1988, Implications for Early Proterozoic tectonics and the origin of continental flood basalts, based on combined trace element and neodymium/strontium isotopic studies of mafic igneous rocks of the Penokean Lake Superior belt, Minnesota, Wisconsin, and Michigan: Unpublished Ph.D. dissertation, University of Minnesota, Minneapolis. Boerboom, T.J., Jirsa, M.A., Southwick, D.L., Meints, J.P., and Campbell, F.K., 1989, Scientific core drilling in parts of Koochiching, Itasca, and Beltrami Counties, north-central Minnesota, 1987-1989: Summary of lithological, geochemical, and geophysical results: Minnesota Geological Survey Information Circular 26, 159 p. Boerboom, T.J., and Zartman, R.E., 1993, Geology, geochemistry, and geochronology of the central Giants Range Batholith, northeastern Minnesota: Canadian Journal of Earth Sciences, v. 30, p. 2510-2522. Boerboom, T.J., Block, Amy Radakovich, Jirsa, M.A., Chandler, V.W., and Peterson, D.M., 2022, Bedrock Geology, pl. 2 of Jirsa, M.A., project manager, Geologic Atlas of Lake County, Minnesota: Minnesota Geological Survey County Atlas C-54, pt. A, 6 pls., scale 1:200,000 Card, K.D., and Ciesielski, A., 1986, DNAG #1. Subdivisions of the Superior Province of the Canadian Shield: Geoscience Canada, v. 13, p. 5-13. Catanzaro, E.J., and Hanson, G.N., 1971, U-Pb ages for sphene in northeastern Minnesota-northwestern Ontario: Canadian Journal of Earth Sciences, v. 8, p. 1319-1324. Chamberlain, K.R., Boerboom, T.J, and Bleeker, W., 2015: 2070 Ma dyke of southern Superior Province: a test of the radiating dyke model for the Kenora-Kabetogama/Fort Frances swarm, in Reconstruction of supercontinents back to 2.7 Ga using the large igneous province (LIP) record: with implications for mineral deposit targeting, hydrocarbon resource exploration, and earth system evolution; Supercontinent.org report number A194, 9 p. Chandler, V.W., 1991, Aeromagnetic map of Minnesota: Minnesota Geological Survey State Map Series S-17, scale 1:500,000. Deer, W.A., Howie, R.A., and Zussman, J, 1966, An introduction to the rock-forming minerals: London, Longman Group Limited, 528 p. Geldon, A.L., 1972, Petrology of the larnprophyre pluton near Dead River, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 153-159. Himmelberg, G.R., 1973, Geologic descriptions of drill core from greenstone belts in northeastern Minnesota: Minnesota Geological Survey Open-File Report Irvine, T.N., and Baragar, W.R.A., 1971, A guide to the chemical classification of the common volcanic rocks: Canadian Journal of Earth Sciences, v. 8, p. 523-548. Jirsa, M.A., 1990, Bedrock geologic map of northeastern Itasca County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-68, scale 1:48,000. Jirsa, M.A., and Boerboom, T.J., 1990, Bedrock geologic map of parts of Koochiching, Itasca, and Beltrami Counties, north-central Minnesota: Minnesota Geological Survey Miscellaneous Map series M-67, scale 1:250,000 / Jirsa, M.A., Boerboom, T.J., Chandler, V.W., and McSwiggen, P.L., 1991, Bedrock geologic map of the Cook to Side Lake area, St. Louis and Itasca Counties, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-75, scale 1:48,000. Jirsa, M.A., Southwick, D.L., and Boerboom, T.J., 1992, Structural evolution of Archean rocks in the western Wawa subprovince Minnesota: Refolding of pre­ cleavage nappes during D2 transpression: Canadian Journal of Earth Sciences, v. 29, p. 2146-2155. Martin, D.P., Meyer, G.N., Lawler, T.L., Chandler, V.W., and Malmquist, K.L., 1988, Regional survey of buried glacial drift geochemistry over Archean terrane in northern Minnesota: Minnesota Department of Natural Resources, Division of Minerals Report 252, V. 1, 74 p.; V. 2, 386 p. Meints, J.P., Jirsa, M.A., Chandler, V.W., and Miller, J.D., Jr., 1993, Scientific core drilling in parts of Itasca, St. Louis, and Lake Counties, northeastern Minnesota, 1989-1991: Summary of lithologic, geochemical, and geophysical results: Minnesota Geological Survey Information Circular 37, 159 p. 134 �Trip 5 – Alkalic plutons Mills, SJ., Southwick, D.L., and Meyer, G.N., 1987, Scientific core drilling in north-central Minnesota: Summary of 1986 lithologic and geochemical results: Minnesota Geological Survey Information Circular 24, 48 p. Prince, L.A., and Hanson, G.N., 1972, Rb-Sr isochron ages for the Giants Range granite, northeastern Minnesota: Geological Society of America Memoir 135, p. 217-225. Ruotsala, A.P., and Tufford, S.P., 1965, Chemical analyses of igneous rocks: Minnesota Geological Survey Information Circular 2, 87 p. Sage, R.P., 1988a, Geology of carbonatite-alkalic rock complexes in Ontario: ·Poohbah Lake alkalic rock complex, district of Rainy River: Ontario Geological Survey Study 48, 68 p. Sage, R.P., 1988b, Geology of carbonatite-alkalic rock complexes in Ontario: Sturgeon Narrows and Squaw Lake alkalic rock complexes, district of Thunder Bay: Ontario Geological Survey Study 49, 117 p. Sage, R.P., 1988c, Geology of carbonatite-alkalic rock complexes in Ontario: Wapikopa Lake alkalic rock complex, district of Kenora: Ontario Geological Survey Study 52, 63 p. Schmitz, M.D., Bowring, S.A., Southwick, D.L., Boerboom, T.J., and Wirth, K.R., 2006, High-precision U-Pb geochronology in the Minnesota River Valley subprovince and its bearing on the Neoarchean to Paleoproterozoic evolution of the southern Superior Province: Geological Society of America Bulletin, v. 118, p. 82-93. Sims, P.K., and Mudrey, M.G., Jr., 1972, Syenitic plutons and associated lamprophyres: in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 140-152. Sims, P.K., Morey, G.B., Ojakangas, R.W., and Viswanathan, S., 1970, Geologic map of Minnesota, Hibbing Sheet: Minnesota Geological Survey, scale 1:250,000. Sims, P.K., Sinclair, D., and Mudrey, M.G., Jr., 1972, Linden pluton: in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 160-162. Southwick, D.L., 1993, Geologic map of Archean bedrock, Soudan to Bigfork area, northern Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-79, scale 1:100,000. Stern, R.A., Hanson, G.N., and Shirey, S.B., 1989, Petrogenesis of mantle-derived LILE-enriched Archean monzodiorites and trachyandesites (sanukitoids, in southwestern Superior Province: Canadian Journal of Earth Sciences, v. 26, p. 1688-1712. Streckeisen, A.L., 1973, Plutonic rocks: Classification and nomenclature recommended by the IUGS Subcommission on the Systematics of Igneous Rocks: Geotimes, v. 18, no. 10, p. 26-30. Wirth, K.R., Vervoort, J.D., and Heaman, L.M., 1995, Nd isotopic constraints on mantle and crustal contributions to 2.08 Ga diabase dykes of the southern Superior Province (abstract), Program & Abstracts for the Third International Dyke Conference, Sept. 4-8, 1995, Jerusalem, Israel, A. Agnon, G. Baer, 84, 1995. 135 �Trip 6 – Colvin Creek FIELD TRIP 6 Unique Keweenawan Inclusion (Colvin Creek) in the Duluth Complex Mark Severson (retired)1,2, Allison Severson3 and Laurie Severson (retired)4 1 (1988–2012) Natural Resources Research Institute, University of Minnesota, Duluth, 5013 Miller Trunk Hwy, Duluth, MN 55811 2 (2013–2018) Previously Teck American, then Teck Resources Unlimited, now NewRange (joint venture between Teck and PolyMet Mining Inc.) 3 Minnesota Geological Survey, College of Science and Engineering, University of Minnesota, 2609 Territorial Road, St. Paul, MN 55114 4 Earth Science Teacher, Woodland Middle School, ISD 709, Duluth, MN 55811 In memory of Richard Patelke 1957-2011 “Well, it ain’t my truck” 136 �Trip 6 – Colvin Creek INTRODUCTION Magnetic basalt inclusions within the Duluth Complex were first described by Bonnichsen (1974). Most of that description pertained to limited outcrops in what is now informally referred to as the South Colvin Creek Hornfels (Fig.6-1). Later, Tyson (1976) looked at four basalt inclusions including three nonmagnetic basalt inclusions in railroad cuts to the north, as well as the South Colvin Creek Hornfels. He concluded that the South Colvin Creek Hornfels was different and theorized that the magnetic basalts were derived from weathered and oxidized basalt flows, correlative with the North Shore Volcanic Group, and metamorphosed by the Duluth Complex. This field trip will visit the North Colvin Creek Hornfels (NCCH), shown in Figure 6-1, which is better exposed, contains several internal mappable units, and was first described by Severson and Hauck (1990). There they found a unique very fine-grained and cross-bedded unit, of gabbroic composition, within the inclusion. They initially theorized that the NCCH was formed as a result of magmatic currents (a concept they no longer support). It was also theorized that the cross-bedded unit represents a portion of a shear zone (Ojakangas and Holst, pers. com., sited in Patelke (1996)). This theory is also no longer deemed viable. Lastly, Patelke (1996) mapped and described the NCCH in more detail and proposed that it was an inclusion containing both metavolcanic and metasedimentary rocks that can be correlated with the North Shore Volcanic Group. This field trip will visit the NCCH which is referred to simply as the Colvin Creek Inclusion for the remainder of this guide. The thesis by Patelke (1996) is the source of almost all of this guide. GEOLOGIC SETTING The Colvin Creek inclusion is a large inclusion (2,500 X 800 meters), associated with a magnetic high, that has been rotated to a near vertical position and exhibits stratigraphic tops to the northwest as defined by pipe amygdules, sheeted amygdules, local convoluted flow bases and flow tops, and crossbedding. Patelke (1996) subdivided the inclusion into five major mappable units that include: two granoblastic, fine-grained metavolcanic units; two gabbroic sill units that bound the inclusion on the north and south; and a 350-meter-thick, cross-bedded, granoblastic, fine-grained metasedimentary unit of gabbroic composition. Overall, the inclusion strikes about N60°E with dips of 70-90° to the northwest. The entire inclusion has been metamorphosed to pyroxene grade facies and rotated to a subvertical position by the Duluth Complex. According to Miller and Severson (2005), the Colvin Creek inclusion is situated near the bottom of a “heterogeneous upper troctolitic cumulate” of the Partridge River intrusion (PRI). Geochemical work by Patelke (1996) indicate that metamorphism of the magnetic metabasalt units was isochemical and that they are probably equivalent to intermediate olivine tholeiites of the North Shore Volcanic Group (NSVG). The metasedimentary rocks are more problematic in that they are not analogous to any of the interflow sandstones of the NSVG as described by Jirsa (1980, 1984). At about 350 meters thick they are as thick as the total measured section of the NSVG interflow sedimentary rocks and show: no rock fragments; NO quartz, no conglomeratic horizons, and no intercalated volcanic rocks. Patelke (1996) suggested that the cross-bedded rocks were most likely deposited in a restricted basin as an eolian sediment that was derived from a strictly basaltic terrain – thus no quartz. Similar inclusions of crossbedded sediments with a gabbroic composition have been found at six locations within the Duluth Complex (i.e., geologic maps of the Babbitt SE and Babbitt SW quadrangles). Patelke (1996) thought that the informal “Phantom Lake sandstone,” an inclusion in the Whyte Quadrangle to the north of Two Harbors, was the most similar to the sediments in the Colvin Creek inclusion. While Patelke (1996) felt that these two units were similar, he concluded that neither of them can be strictly correlated with any other of the interflow sandstones in the Keweenawan system. 137 �Trip 6 – Colvin Creek Figure 6-1. Generalized geologic map of a portion of the Partridge River intrusion showing locations of the Colvin Creek Hornfels inclusions (gray) relative to the known Cu-Ni deposits. Base map from Miller and others (2001). GEOLOGY OF THE COLVIN CREEK INCLUSION Patelke (1996) mapped six major units associated with the Northern Colvin Creek Hornfels inclusion (exposed in Sections 27, 28, 33, and 34, T.59N., R.13W.). These units are briefly described below, and their distribution is shown in the geology map of Figure 6-2. The six units are, from south to north (also stratigraphically younging to the north) labeled as: MCC, AMG, AA, XBB, and GOG. These names are acronyms for field textures observed by Severson and Hauck (1990), and while these are not appropriate rock names, Patelke (1996) retained them in his thesis. MCC (Massive Colvin Creek unit) The MCC unit is a plagioclase-augite-oxide (titanomagnetite>ilmenite) rock, with local orthopyroxene and/or olivine, that under Phinney’s classification system (1972) is an oxide-bearing gabbro to augite troctolite. The MCC displays a massive, fine- to medium-grained texture similar to the units that stratigraphically overlie it but also shows primary decussate igneous texture. It is variably ophitic, and locally porphyritic. Granoblastic triple point junctions are reasonably well developed where the feldspar is 138 �Trip 6 – Colvin Creek Figure 6-2. Geology of the Northern Colvin Creek inclusion from Patelke, 1996. equant. Locally, there are ovoid clots of granular plagioclase that could be interpreted as amygdule infillings. The bottom contact of the MCC unit is not exposed. The upper contact with the AMG unit consists of rock types attributable to both MCC and AMG units within a 3-meter zone. For this reason, Patelke (1996) suggests that the MCC was injected sill-like and was mixed into the AMG while both were in a plastic state. Unfortunately, this particular exposure will not be visited during this trip. AMG (Amygdaloidal Gabbro unit) The AMG is stratigraphically above the MCC unit and is interpreted to be a subaerial metavolcanic unit with recrystallized amygdules. The rock is classed as an oxide melagabbro to augite troctolite. In the vast majority of the exposures, it is fine- to medium-grained and composed of plagioclase, augite, and oxide (titanomagnetite>ilmenite) with local orthopyroxene and poikilitic olivine. The AMG unit shows a persistent fine-grained, polygonal-granoblastic, sugary texture. In a few instances this texture is interrupted by clusters of plagioclase and by rounded to amoeboidal clot-like segregations of augite; both of which are lengthened parallel to the overall strike of bedding. These layers of pyroxene-rich segregations are interpreted to be recrystallized amygdules within relict volcanic flowtops. In areas of outcrop with common pyroxene-rich layers, the spacing of layers indicates flow thicknesses of 0.5 to 3.5 meters. The upper contact with the AA unit is exposed in only one outcrop (Fig. 6-3 - not visited this trip) wherein a black pyroxenemagnetite rich convoluted flowtop of the AMG is overlain by the base of a flow in the AA unit. 139 �Trip 6 – Colvin Creek Figure 6-3. Contact between the AMG (bottom, dark) and AA (top, light) units. The dark portion of the image is related to increased pyroxene and oxide content and interpreted to be a rubbly flow top that is abruptly overlain by the AA unit. MGC (Medium-Grained Gabbro unit) The MGC is a sill of very limited extent in the SW end of the Colvin Creek inclusion (Fig. 6-2). The rock is a medium- to coarse-grained orthopyroxene-bearing anorthositic gabbro according to the classification system of Phinney (1972). The sill crosscuts only the AMG unit, exhibits apparent chilled margins, and is estimated to be about two meters thick. Exposures of this unit will not be visited. AA (Amoeboidal Augite unit) The AA unit overlies the AMG unit and is also a fine- to medium-grained, massive, granoblastic magnetic basalt unit. At the outcrop scale, the AA is distinguished from the AMG by increased amounts pyroxene-filled avoids (amygdules) and by elongate pyroxene segregations that are interpreted as recrystallized pipe amygdules. Petrographically, the AA and AMG are very similar. Mineralogy consists of plagioclase, diopsidic augite, and titanomagnetite>ilmenite with local orthopyroxene (a major constituent in one outcrop). Plagioclase has a bimodal grain distribution consisting of fine-grained equant to stubby grains (0.25-1 mm) and patchy distributed laths (2-7 mm). The modal layering within this unit is defined by pyroxene stringers and ovoid clots that are interpreted to define both lava flow bases and amygdaloidal tops. Individual flows range from 0.5 to several meters thick. Elongate pyroxene masses lying perpendicular to strike are thought to be recrystallized pipe amygdules near the flow base (Fig. 6-4). 140 �Trip 6 – Colvin Creek Figure 6-4. Two flow units in the AA unit. Base of a single flow (6 inches to left of hammer head) with recrystallized, coalescing upward-trending pipe vesicles. Black wavy lines in extreme upper right of photo is the base of a third lava flow. The location of this outcrop is not documented. “Pyroxene interval” At the very top of the AA unit is a 0-2 meter thick, black, melagabbro unit, or “pyroxene interval” as mapped by Patelke (1996) in a few scattered outcrops. This unit consists of fine- to coarse-grained ferrosalite pyroxene and plagioclase. At one locality this unit contains: 1-10% brown garnet, 2-5% ilmenite>>titanomagnetite, and trace amounts of cordierite and hercynite. At one exposure (Fig. 6-5 – to be visited), there are several “veins” of potassium feldspar masses (up to 20-40 cm long by 1-10 cm wide), or tension gashes according to Patelke (1996). These “veins” are perpendicular to, and truncated by, the upper contact with the overlying XBB unit. The base of the XBB unit often exhibits a trough-like morphology downwards towards these feldspar masses. At several locations where this “pyroxene interval” is present, Patelke (1996) thought that there was some evidence of left-lateral tectonic movement. The tension gashes are one of his lines of evidence. Overall, Patelke (1996) thought that the “pyroxene interval” represents a deeply weathered flow top or soil developed on the AA unit and the effects of faulting are secondary. 141 �Trip 6 – Colvin Creek At another outcrop along the contact between the AA and XBB units (Stop 5 - to be visited), the “pyroxene interval” is absent. In its place are several sigmoidal-shaped pyroxene-rich lenses. Patelke (1996) thought that these lenses were developed along a bedding parallel fault. Figure 6-5. “Pyroxene Interval” (bottom 2/3rds of photo) between the AA and XBB units. Note convolute contact and k-spar-filled “tension gashes” as described by Patelke (1996). Figure 6-6. Typical cross-bedding exhibited by the XBB unit in a flat-laying outcrop. 142 �Trip 6 – Colvin Creek XBB (Cross-Bedded Belt) To the north of, and overlying the magnetic basalt units, is the Cross-Bedded Belt unit of roughly gabbroic composition. The rock is composed of fine-grained (1mm average) plagioclase-diopsideorthopyroxene-titanomagnetite>ilmenite with minor amount of orthopyroxene, hematite, hercynite, and geikielite. The rock exhibits beautiful bedding, cross-bedding (Fig. 6-6), density graded modal layering, and concave upward cross-beds along with scour and fill structures. Throughout the unit are localized minor biotite. There are several intervals, 0.5-3.0 meters thick, that are located near the base of the XBB that contain poikiloblastic pyroxene, up to several inches long (Fig. 6-7) that appear to have grown along bedding planes. The density graded modal layering consists of oxide- and pyroxene-rich basal layers grading upward into plagioclase-rich layers. Grain size for any individual mineral (1 mm) remains constant throughout the bed thickness. Angles of bedding and cross-bedding change over short distances in most outcrops. In some areas, the bedding exhibits a weak convolution or deformation (Fig. 6-7); possibly due to either soft sediment deformation and/or partial melting by the Duluth Complex. GOG (Gabbro-Olivine Gabbro unit) A gabbro to olivine gabbro unit, labelled as GOG, bounds the Colvin Creek inclusion at its upper (northwest) contact. The GOG was classed as a unit of the inclusion because it contains contact parallel layering (as does the inclusion) and shares strike-length and general tabular form with the other units of the inclusion. The GOG is medium to coarse-grained and composed of plagioclase (37-72%), augite (9-42%), olivine (0-23%), ilmenite (3-12%), and titanomagnetite (2-8%). The GOG unit is best exposed at the northwest end of the Colvin Creek inclusion where four contact-parallel zones were described by Severson and Hauck (1990) and by Patelke (1996). These zones (Fig. 6-2) are: A. weakly modally layered granular-textured augite troctolite zone with a plagioclase foliation B. phenocryst-rich gabbroic zone with anorthositic inclusions up to 5 inches across; C. a zone of heterogeneous gabbroic rocks with local inch-scale layering and cross-bedding (Fig. 6-8) indicative of magmatic currents; and D. a zone of anorthositic gabbro grading upward to gabbro with olivine gabbro interbeds. The trend of all of these zones parallel the contact trends of the underlying Colvin Creek inclusion. Unfortunately, this unit is too far away to bushwhack to Figure 6-7. Poikiloblastic pyroxene (black squares) in XBB unit. Note strange gain access and convolutions of bedding. visit during this trip. 143 �Trip 6 – Colvin Creek Common Characteristics of Colvin Creek Hornfels Listed below are characteristics common to all rock types of the Colvin Creek Hornfels: • All of the units are strongly magnetic and microscopically exhibit polygonal/granoblastic triple point junctions • Thin veins and later pods of pyroxene and/or massive magnetite are locally common. They are arranged in both parallel sets and discontinuous cross-cutting stringers • Thin, brown hornblende and/or orthopyroxene rims around titanomagnetite are commonly seen in thin-section • Sericitized plagioclase is seldom seen • Olivine, where present, is usually fresh and never serpentinized • Biotite is generally absent except in the XBB just above the “pyroxene interval” • The titanomagnetite is titanium-rich and the ilmenites are magnesium-rich. Figure 6-8. Rhythmic layering in GOG unit (subzone C) consisting of alternating olivine-rich and olivinepoor layers. Upper massive gabbro (hammer) truncates bed sets. FIELD TRIP STOPS Access starting from Mountain Iron will be heading south on Highway 53 through Virginia. Shortly after crossing the Tony Rukavina Bridge, over the Rouchleau Mine, turn and head east on Road 135 through the towns of Gilbert, Biwabik, and Aurora. Within Aurora, turn right at the stop sign and head south on CSAH 100, cross the railroad tracks, proceed to a stop sign and turn left on CSAH 110. Proceed to Hoyt Lakes on this highway. Within Hoyt Lakes continue straight through two stop signs and head out of town on Highway 110 (also called Skibo Vista Road). for about 4 miles. Just after passing the Bird Lake Recreation area, turn left onto road UT9235 (also called 569/Skibo Rd). Proceed down this road about 2.7 miles, cross the railroad tracks and continue east for another 1.9 miles. Turn left (north) on forest road 113 (yellow “share the road” sign at this intersection). Go 5.9 miles north on 113 to an unmarked logging road (another yellow ”share the road” sign at this intersection). Turn left on unmarked logging road (Figure 69) and head west about 1.5 miles depending on road conditions. Turn vehicles around, park as best as possible, and walk about 0.2 miles to the west (through an old beaver pond) to the first stop. Locations of the trip stops are shown in Figures 6-9 and 6-10. 144 �Trip 6 – Colvin Creek Figure 6-9. Access to Colvin Creek hornfels area and trip stops via remote logging road. Figure 6-10. Field trip stops (black dots) relative to mapped geology (modified from Patelke, 1996). 145 �Trip 6 – Colvin Creek Stop 1: MCC (Massive Colvin Creek unit) (NAD83: 577246E/5268002N) (47.56084°, -91.97315°) The MCC unit at this exposure is enigmatic. The rock is massive, lacks modal layering and regular concentrations of minerals. It is classed as a gabbro to augite troctolite composed of plagioclase, augite, orthopyroxene, olivine, and oxide (titanomagnetite>ilmenite). It is fine- to medium-grained with granoblastic triple point junctions. Locally, there are clots of granular plagioclase that could be interpreted as amygdule infillings, as in the overlying basaltic units. However, the MCC also displays primary decussate igneous textures, is variably ophitic, and locally porphyritic. For this reason, the distinction between the MCC and overlying AMG are often unclear. Patelke (1996) felt that the portions of the MCC were injected sill-like into the base of the inclusion while they were both in a plastic state. Directions: Continue down the road for about 2 minutes to a flagged trail off to the north. Follow the trail for another 5 minutes to Stop 2. Stop 2: AMG (Amygdaloidal Gabbro unit) (NAD83: 577230E/5268130N) (47.56199°, -91.97334°) At first glance, the AMG unit at this exposure is similar to the previous stop in that it consists mostly of massive, fine- to medium-grained “oxide gabbro.” However, within this exposure are several localized dark-gray, very fine-grained internal patches of basalt, that contain unquestionable plagioclasefilled amygdules. These patches exhibit gradational contacts with the surrounding medium-grained “oxide gabbro.” Thus, both fine-grained basalt and medium grained “gabbro” are present here (best seen after peeling a large area of the exposure). It is unknown whether these basalt patches represent true inclusions or are remnant unmetamorphosed patches in a rock that has undergone various degrees of partial melting to produce the “gabbroic” portions. Directions: Return to fork in flagged trail and continue north a few minutes to Stop 3. Stop 3: AA (Amoeboidal Augite unit) (NAD83: 577094E/5268083N) (47.56159°, -91.97515°) The AA unit overlies the AMG unit and is also a fine- to medium-grained, massive, granoblastic magnetic basalt unit. At the outcrop scale, the AA is similar to the AMG except for zones that contain common pyroxene-filled ovoids (recrystallized amygdules) and by pyroxene-rich horizons that are interpreted as sheeted amygdules and/or flow tops. Petrographically, the AA and AMG are very similar. Mineralogy consists of plagioclase, diopsidic augite, and titanomagnetite>ilmenite. Several basalt flows can be distinguished in portions of this outcrop based on massive flows grading upward (northward) into amygdule-rich basalt that in turn grades into pyroxene-rich flow tops. Individual flows range from over several meters to less than one meter thick. Note the presence of a cluster of coarse-grained pyroxene with minor K-spar (similar features will be seen at stop 6). Directions: Return to road and proceed further west for about 1-2 minutes to another flagged trail leading to the north. The Stop 4 exposure is about 100 feet north of the road on this trail. Stop 4: XBB Unit (Cross-Bedded Belt) (NAD83: 577065E/5268019N) (47.56101°, -91.97555°) To the north of, and overlying the magnetic basalt units, is the Cross-Bedded Belt unit of roughly gabbroic composition. This is the first of several exposures of the XBB unit that will be viewed during this trip. The rock is composed of fine-grained (1mm average) plagioclase-diopside-orthopyroxenetitanomagnetite>ilmenite. The rock exhibits beautiful bedding, cross-bedding, density graded modal layering, and concave upward cross-beds along with scour and fill structures. Note that NO quartz has ever been noted in this unit! 146 �Trip 6 – Colvin Creek There appears to be small-scale convolutions in the bedding trends that may be related to either soft-sediment slump or folding during intrusion of the Duluth Complex and subsequent rotation of the Colvin Creek inclusion. The location of this exposure along a curved mapped contact (Fig. 6-9) suggests that there is a small open fold between the AA and XBB units as suggested by Patelke (1996). Directions: Return to the road and head 3 minutes to the west to Stop 5 (about 50 feet north of the road). Stop 5: Contact of XBB and AA units (below photo) (NAD83: 576907E/5267935N) (47.56028°, 91.97767°) Both the AA and XBB units are present in this exposure. At the southern end of the exposure is a massive basalt unit that grades upward (northward) into a rock that contains abundant pyroxene-filled amygdules, which in turn, contains several pyroxene-rich lenses that represent sheeted amygdules and flow tops. Several flows are defined in the outcrop and the contact with the XBB unit is well defined (see Fig. 6-11). The overlying XBB unit consists of a fine-grained gabbroic rock with bedding planes similar to the previous stop but actual cross-bedding is not as striking. In regard to the contact between the two units, the intervening “pyroxene interval” is largely absent except for thin irregular pyroxene-rich lenses that display sigmoidal shapes. Patelke (1996) thought that sigmoidal-shaped pyroxene-rich lenses were developed along a bedding parallel fault with left-lateral movement. At the extreme north end of the exposure is an irregular, cross-cutting, massive oxide vein up to 3 inches wide. Figure 6-11. Contact between AA (left) and XBB (right) units with very poorly defined “pyroxene interval” in the contact zone. Note sigmoidal shapes of pyroxene layers at the contact. To the left of the contact (not in photo) are 2-3 trough-shaped zones (less than 2x3 feet) that contain bedded sediments similar to the XBB unit. Whether these zones are sedimentary interbeds or enfolded patches is unknown. 147 �Trip 6 – Colvin Creek Directions: Return to the road and proceed further west for about 10 minutes to Stop 6 on the southern edge of the road. On the way to Stop 6 there are numerous pavement road-crop exposures that consist mostly of massive magnetic basalt with local amygdules. Stop 6: AA (Amoeboidal Augite Unit) (NAD83: 576560E/5267549N) (47.55684°, -91.98234°) This outcrop is situated about 2,000 feet down the road from Stop 5 and serves more as a rest and regrouping stop. At this locale, the unit is massive and grades upwards (toward the road) into typical amygdaloidal basalt. Directions: Proceed down the road 350 feet and follow a flagged trail through the woods for about 840 feet westward (10 minutes). Stop 7: Contact of XBB & underlying AA unit (NAD83: 576259E/5267482N) (47.55628°, -91.98636°) This is the best exposure of “pyroxene interval” along the contact (see Figure 6-5 and description in text). Perpendicular to the contact, and wholly within the “pyroxene interval,” are at several “vein-like” potassium feldspar veins (up to 20-40 cm long by 1-4 cm wide) and a mass about 4 ft long by 1.5 ft wide. Patelke (1996) felt that the veins are tension gashes formed by lateral movement along the contact. The contact between the XBB and “pyroxene interval” exhibits some folding (soft-sediment?) with small-scale V-shaped troughs projecting downward into the “pyroxene interval.” Some of these troughs exhibit truncated bedding of the XBB against the “pyroxene interval” At one of the “V’s”, biotite, garnet and cordierite have been identified by Patelke (1996). At the extreme east end of the exposure, it appears that a bed of the XBB is folded(?) downward into the “pyroxene interval.” Patelke (1996) thought that the “pyroxene interval” represents a deeply weathered flow top or soil developed on the AA unit. Directions: At the west end of the Stop 7 exposure proceed northward for short distances (<100 feet) to several outstanding outcrops of the XBB unit of Stop 8. Stop 8: XBB Unit (Cross-Bedded Belt) (NAD83: 576248E/5267546N) (47.55685°, -91.98649°) Numerous exposures of beautifully cross-bedded XBB unit are present on the top of this hill. The rock is a very fine-grained granoblastic rock with a general modal composition of oxide-bearing anorthositic gabbro to gabbroic anorthosite. It is composed of plagioclase, diopsitic augite, and various iron-titanium-manganese oxides making up to 8-15% of the rock, NO quartz has ever been documented. As shown in Figures 6-6, 6-7 and 6-11, the rock is bedded and cross-bedded, exhibits density graded modal layering, concave upward cross beds, and scour and fill features. Some of the cross-beds show an unusually high angle of repose over very short distances possibly related to the environment of deposit (aeolian). 148 �Trip 6 – Colvin Creek Figure 6-12. Classic exposure of the XBB unit. Bedding tops to the north (right). Directions: Return to Stop 7 and proceed west for about 5 minutes to large exposures of the XBB and AA units. Note that between stops 8 and 9 is a glacial erratic of the GOG unit with stupendous inch-scale layering. This erratic is a good example of the GOG unit (otherwise inaccessible on this field trip). Stop 9: AA (Amoeboidal Augite Unit) (NAD83: 576167E/5267425N) (47.55578°, -91.98759°) After crossing over a large outcrop of the XBB unit, proceed southward a short distancer to a large tip over exposure (uprooted and wind-fallen tree) of the AA unit consisting of multiple basalt flows with ropey tops. This outcrop is present near the upper contact of the unit and small exposures of the XBB are present to the north and west. Pipe vesicles are present in one small area of the AA unit. Also present is a very small exposure of the “pyroxene interval.” Directions: Return to vehicles. Return to Mountain Iron Community Center (47.51869°, -92.58997°). References Bonnichsen, B., 1972, Southern Part of the Duluth Complex. In: Sims, P.K. and Morey, G.B. (eds), Geology of Minnesota – A Centennial Volume, Minnesota Geological Survey, p. 361-388. Jirsa, M.A., 1980, The Petrology and Tectonic Significance of Interflow Sediments in the North Shore Volcanic Group, Northeastern Minnesota, unpublished M.S. Thesis, University of Minnesota Duluth, 125 pages. Jirsa, M.A., 1984, Interflow Sedimentary Rocks in the Keweenawan North Shore Volcanic Group, Northeastern Minnesota: Minnesota Geological Survey, Report of Investigations 30, 20 p. 149 �Trip 6 – Colvin Creek Miller, J.D., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geological map of the Duluth Complex and related rocks, Northeastern Minnesota; Minnesota Geological Survey, Miscellaneous Map M119, scale 1:200,000. Miller, J.D., Jr. and Severson, M.J., 2002, Geology of the Duluth Complex in Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002a, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations RI-58, p. 106-143. Miller, J.D., Jr. and Severson, M.J., 2004, Geology and Mineralization of the Western Contact of the Duluth Complex, Partridge River and South Kawishiwi intrusions, Northeastern Minnesota: Institute on Lake Superior Geology, 50th Annual Meeting, Duluth, MN, Part II: Field Trip Guidebook, p. 227-258. Miller, J.D., Jr. and Severson, M.J., 2005, Patelke, R.L., 1996, The Colvin Creek Body, A Metavolcanic and Metasedimentary Mafic Inclusion in the Keweenawan Duluth Complex, northeastern Minnesota: unpublished M.S. Thesis, University of Minnesota, 232 p. Phinney, W.C., 1972, Duluth Complex, history and nomenclature, in Sims, P. K., and Morey, G. B., eds., Geology of Minnesota: A Centennial Volume: Minn. Geol. Survey, pp. 333-334. Severson, M.J., and Hauck, S.A., 1990, Geology, geochemistry, and stratigraphy of a portion of the Partridge River intrusion: Natural Resources Research Institute, University of Minnesota-Duluth, Technical Report, NRRI/GMIN-TR-89-11, 236p. (with plates). Tyson, R. M., 1976, Hornfelsed Basalts in the Duluth Complex: unpublished M.S. Thesis, Cornell University, Ithaca, New York, 85 p. 150 �Trip 7 – Classic Outcrops FIELD TRIP 7 Classic Outcrops of Northeastern Minnesota Dean M. Peterson1 and George J. Hudak2,3 1 Big Rock Exploration, 2505 W. Superior St., Duluth, MN 55806 George Hudak Geosciences P.L.L.C., Duluth, MN 55804 3 Department of Earth and Environmental Sciences, University of Minnesota Twin Cities, 116 Church Street SE, Minneapolis, MN 55455 2 Introduction This field trip will investigate a wide variety of Neoarchean, Paleoproterozoic and Mesoproterozoic rocks that illustrate the diversity of Precambrian rocks in northeastern Minnesota. The field trip is an updated version of “Field Trip 5 – Classic Outcrops of Northeastern Minnesota” that was run during the 50th Annual Meeting of the Institute on Lake Superior Geology that took place in Duluth, Minnesota during May, 2004. As such, several of the field trip stop descriptions in this guidebook are derived from this earlier field trip guide, with updates based on recent geological studies. Generalized Stratigraphy of Northeastern Minnesota Neoarchean Vermilion District Supracrustal rocks in the Vermilion district consist of volcanic-dominated stratigraphic sequences of the Wawa-Abitibi Terrane within the Superior Province of the Canadian Shield. Rocks of the WawaAbitibi Terrane in northern Minnesota are divided on the basis of stratigraphic and structural setting into: (1) the Soudan belt, to the south, and (2) the Newton belt, to the north (Jirsa et al., 1992; Southwick et al., 1998). The boundary between these contrasting structural panels can be traced geophysically across the width of Minnesota and was informally designated the Leech Lake structural discontinuity (Jirsa et al., 1992). In the region west and north of the Lake Vermilion State Park, the Leech Lake structural discontinuity occurs along the Mud Creek shear zone (Hudleston et al., 1988), small segments of the Vermilion and Wolf Lake faults (Sims and Southwick, 1985), and the Bear River fault (Jirsa et al., 1992). A simplified regional geological map of the Neo-Archean terranes of northeastern Minnesota and adjacent Ontario is presented in Figure 7-1. The Soudan belt (Figures 7-1 and 7-2) contains large, broad folds involving calc-alkalic and tholeiitic volcanic strata overlain by, and locally interdigitated with, turbiditic rocks. In contrast, the Newton belt consists of elongate, northeast-trending, and mostly northward-younging volcanic and volcaniclastic sequences. Volcanic rocks of the Newton belt differ from those of the Soudan belt in containing locally abundant komatiitic flows and peridotitic sills. The two belts are fault-bounded, and the relationships between stratigraphic units within each belt are largely conformable (although faults obscure contacts locally). In its eastern extension, the Soudan belt is continuous with the Saganagons assemblage in Ontario and terminates against the Saganaga pluton and Northern Light Gneiss. The Newton belt extends discontinuously eastward into the Shebandowan District of Ontario to form the Greenwater and Burchell assemblages. Intrusive rocks in both belts vary from gabbroic and felsic porphyries demonstrably related to volcanism, to large plutons emplaced post-tectonically. Both districts contain unconformable, Timiskaming-type sequences composed of calc-alkalic volcanic rocks, conglomerates, and finer grained sedimentary rocks. Lithostratigraphic units in the western Vermilion district (Table 7-1) include: (1) the Lower member, Soudan Iron-Formation member, and Upper member (Upper Ely) of the Ely Greenstone 151 �Trip 7 – Classic Outcrops Figure 7-1. Simplified correlation map of Neoarchean assemblages in Minnesota and northwestern Ontario (after Peterson et al., 2001). Inset map illustrates location of the Wawa-Abitibi Terrane in Minnesota and northwestern Ontario (Stott et al., 2007). The Leach Lake structural discontinuity is illustrated in red. Figure 7-2. Generalized geology and geochronology of the Vermilion District in the vicinity of the Tower-Soudan anticline (modified after Peterson, 2001; Hudak et al., 2014). 152 �Trip 7 – Classic Outcrops Formation, the Lake Vermilion Formation (including the informally named Britt and Gafvert Lake sequences), and the Knife Lake Group of the Soudan belt; (2) the Bass Lake sequence (Peterson and Jirsa, 1999, Peterson, 2001) and the Newton Lake Formation of the Newton belt; and, (3) syn- to post-tectonic granitoid intrusions of the Giants Range batholith, and a suite of post-tectonic alkalic stocks and plutons. Contacts between the different units are typically conformable, although considerable overlap in time and space is documented between volcanic and sedimentary sequences (Southwick, 1993). Regional chronostratigraphic correlations between the Wawa Greenstone (northwestern Ontario) and the Abitibi greenstone belt (eastern Ontario and Quebec) are indicated in Figure 7-3. Geochronological information for supracrustal and intrusive lithologies in the Vermilion District is relatively sparse (Figure 7-3). Peterson et al. (2001) obtained a U-Pb zircon age of 2722 ± 0.9 Ma from a quartz-phyric rhyolite dome in the Fivemile Lake Sequence of the Lower Member of the Ely Greenstone Formation. Allerton et al. (2024a) obtained a crystallization age of 2708 ± 25 Ma for the Purvis Pluton, which intrudes the Eagles Nest Succession of the Lower Ely Member, and has been interpreted as a synvolcanic intrusion (Peterson, 2001). The age of the Upper Member of the Ely Greenstone formation is currently unknown. Jirsa (2016) obtained an age of 2715.74 ± 0.50 Ma for a felsic volcanic unit within the Newton Lake Formation (Boerboom, T. J., 2020). Lodge et al. (2013) obtained a U-Pb zircon age of 2689.7 ± 0.8 Ma for a Gafvert Lake Sequence dacitic tuff breccia that occurs approximately 2m north of the contact with the Soudan Iron-Formation member of the Ely Greenstone Formation. As well, Lodge et al. (2013) obtained detrital zircon ages ranging from 2680-2690 Ma from greywackes that comprise the Lake Vermilion formation. These dates confirm the source of the detritus in the Lake Vermilion Formation was derived locally from the volcaniclastic rocks comprising the Gafvert Lake Sequence. Figure 7-3. Regional chronostratigraphic correlations between the Vermilion district (Minnesota), the Wawa greenstone belt (northwestern Ontario), and the Abitibi greenstone belt (eastern Ontario and Quebec; after Ayer, 2010). Table 7-1. Lithostratigraphic units within the western Vermilion District (modified after Peterson and Jirsa, 1999; Peterson et al., 2009; Hudak et al., 2012). 153 �Trip 7 – Classic Outcrops Intrusive Rocks Late Intrusions Plutons and stocks of syenite, monzonite, diorite, and lamprophyre. A U-Pb zircon age date of a non-foliated feldspar porphyry intrusion in the Newton belt is 2683 ± 1.4 Ma (Peterson et al., 2001). Vermilion Granitic Complex Granite, schist, amphibolite, and schist-rich migmatite Giants Range Batholith Granite, granodiorite, monzodiorite, and schist-rich migmatite. U-Pb zircon dates indicate a crystallization age ranging from 2640-2777Ma (Allerton et al., 2024a). Supracrustal Rocks Newton Belt Newton Lake Formation Tholeiitic and komatiitic basalt lava flows, intrusions, and clastic strata (deep subaqueous?) Bass Lake Sequence Tholeiitic basalt lava flows, iron-formation, and felsic porphyries (deep subaqueous) Soudan Belt Knife Lake Group Graywacke, slate, conglomerate, and sheared equivalents Lake Vermilion Formation Graywacke, slate, dacitic tuff, minor conglomerate. Detrital zircons from planar bedded, normal-graded resedimented volcaniclastic rocks have UPb age dates of 2680-2690 Ma (Lodge et al., 2013; subaerial to subaqueous) Gafvert Lake Sequence Dacitic to rhyodacitic tuff, lapilli-tuff, tuff-breccia, and iron-formation. Basal dacite tuff-breccia deposits in Lake Vermilion State Park have UPb age date of 2689.7 ± 0.8 Ma (Lodge et al., 2013; subaerial to subaqueous) Britt Sequence Tholeiitic basalt lava flows (deep subaqueous?) Upper Member – Ely Greenstone Tholeiitic basalt lava flows and iron-formation (deep subaqueous?) Soudan Member – Ely Greenstone Oxide-facies iron formation with intercalated basalt lava flows and felsic volcaniclastic rocks (deep subaqueous) Lower Member – Ely Greenstone Calc-alkaline and tholeiitic basalt-rhyolite lava flows, tuffs, epiclastic rocks, and minor iron-formation (shallow- to deep subaqueous) Central Basalt Sequence Calc-alkaline to tholeiitic sparsely amygdaloidal basalt and minor basaltic andesite lava flows with MORB-like or back arc basin-like chemical affinities within 100-200 meters of the overlying Soudan Member iron-formation; FII- and FIIIa-type felsic volcanic and volcaniclastic rocks (transition from shallow- to deep water environment) Fivemile Lake Sequence Calc-alkaline to transitional moderately to highly vesicular basalt and andesite lava flows and volcaniclastic rocks with arc-like chemical affinities: FI-, FII-, and FIV-type felsic volcanic and volcaniclastic rocks. Rhyolite dome at near Fivemile Lake has U-Pb age date of 2722.6 ± 0.9 Ma (Peterson et al., 2001). Epithermal-like zinc stringer mineralization is present near Fivemile Lake (Hudak et al., 2002a; interpreted as shallow subaqueous environment). Eagles Nest Sequence Algoma-type iron formation, basalt-andesite lava flows, hydrothermal exhalites, felsic tuffs. 154 �Trip 7 – Classic Outcrops The upper part of the Knife Lake Group includes conglomerates which contain clasts derived from the Saganaga Tonalite, which has been dated by Driese et al. (2011) at 2690.83 ± 0.26 Ma. Jirsa et al. (2012) obtained a U-Pb age of 2690.7 ± 0.6 Ma for synvolcanic intrusions that cross-cut volcaniclastic rocks that comprise the Knife Lake Group. Peterson et al. (2001) also dated a non-foliated feldspar porphyry intruded into Newton Belt strata at 2683.1 +1/-4 Ma. This date provides a minimum age for the regional D2 deformation event that is described below. The age of the orebodies at the Soudan Mine were previously interpreted to be syn- or postdepositional to the precipitation of the Soudan Member of the Lower Ely Greenstone Formation (Gruner, 1926; Klinger, 1960; Thompson, 2015). Recent U/Pb and (U-Th)/He radiometric dating by Allerton (2024b) suggest the massive hematite orebodies at Soudan formed during Paleoproterozoic time (1640.8 ± 47.2 Ma – 1740.4 ± 72.5 Ma) and have been overprinted by a Mesoproterozoic hydrothermal event at approximately 1100 Ma (1093.1 ± 16.4 Ma). Structural Geology The structural geology of the Vermilion District has been well described by Peterson et al. (2009) and is reproduced below. Periods of generally N-S directed compression resulted in three major regional deformation events in the Neoarchean terranes of northern Minnesota. The earliest deformation event (D1) produced broad, locally recumbent folds within the Soudan belt and major fault zones throughout the region. In the Newton belt, D1 was accommodated by thrust imbrication of large crustal blocks, resulting in mainly northward stratigraphic facing. Field relationships indicate that uplift, faulting, and the deposition of Timiskamingtype clastic sedimentary sequences in local fault- bounded basins occurred late in D1 deformation (Jirsa, 2000). A large, map-scale structure related to D1 deformation in the western Vermilion District is the Tower-Soudan Anticline, which is a west-plunging anticline within which the axis and plunge changes orientation along strike from nearly vertical in basalts to shallow NE plunging in the western sedimentary rocks (Figure 7-2). Axial-planar cleavage associated with this early fold typically is lacking, although Bauer (1985), Hooper and Ojakangas (1971), Hudleston (1976), and Jirsa et al. (1992) have described early cleavage (S1) locally. A second deformation event (D2) associated with synchronous regional metamorphism resulted in foliation development and structures having largely dextral asymmetry. D2 is constrained in the Vermilion District to the time period 2674 to 2685 Ma (Boerboom and Zartman, 1993), and between about 2680 and 2685 Ma in the Shebandowan (Corfu and Stott, 1998). Because D2 deformation affected all of the supracrustal rocks in the area and is reasonably constrained by geochronology, the regional foliation (S2) can be used in the field to temporally relate other structural, intrusive, and deformation events. The relationship between S2 fabric and shear structures indicates that most shearing occurred relatively late in the D2 event. Major shearing that produced the Mud Creek and related shear zones is attributed to the late stages of D2 dextral transpression. Structures related to the third deformation event (D3), which led to juxtaposition of the Wawa Abitibi and Quetico terranes (Peterson and Patelke, 2003) include abundant NE- and NW-trending faults that dissect the stratigraphic assemblages. Named structures related to D3 include the NE-trending Waasa and Camp Rivard faults east of the Soudan Mine area, and the WNW-trending, crustal-scale Vermilion and related faults that form the Wawa-Quetico Subprovince boundary. Paleoproterozoic Superior Type Iron Resources of the Mesabi Iron Range Superior type iron formation resources of Minnesota are exemplified by the long-standing mining of iron resources of the Biwabik Iron Formation along the length of the Mesabi Iron Range. The Mesabi Iron Range is largely located in St. Louis and Itasca counties and has been the most important iron ore district in the United States since ~1900. The Mesabi Iron Range is 120 miles long, averages one to two miles wide, and is comprised of rocks of the Paleoproterozoic Animikie Group. The Animikie Group on 155 �Trip 7 – Classic Outcrops the Mesabi Iron Range consists of three major conformable formations: Pokegama Formation at the base; Biwabik Iron Formation in the middle; and the overlying Virginia Formation. On the Mesabi Iron Range, these three formations generally dip gently to the southeast at angles of 3-15 degrees. Since the early 20th century, the Biwabik Iron Formation has been subdivided into four informal members referred to as (from bottom to top): Lower Cherty member, Lower Slaty member, Upper Cherty member, and Upper Slaty member (Wolff, 1917). The cherty members are typically characterized by a granular (sand-sized) texture and thick-bedding (beds ≥ several inches thick); whereas the slaty members are typically fine-grained (mud-sized) and thin-bedded (≤1 cm thick beds). The cherty members are largely composed of chert and iron oxides (with zones rich in iron silicate minerals), while the slaty members are composed of iron silicates and iron carbonates with local chert beds. Both cherty and slaty iron-formation types are interlayered at all scales, but one rock type or the other predominates in each of the four informal members, and they are so-named for this dominance Severson et. al. (2009). Leached and iron enriched direct ores (or natural ores) were the first materials mined, with the first shipments beginning in 1892, from strongly oxidized pockets along fault and fracture zones and the blanket oxidation of the iron formation at the surface. Taconite, which is the material that is mined today using magnetic separation methods, constitutes most of the iron formation and pertains to the hard, non-oxidized portions of the iron-formation. Production has been dominantly controlled by vertically integrated steelmakers since 1901, and therefore the mining and utilization of these ores have been dictated largely by US ironmaking capacity and demand. The taconite typically contains 30-35% iron and 40-50% SiO2, plus other components (Morey, 1992). The Biwabik Iron Formation is around 175-300 feet thick in the extreme eastern end of the Mesabi Iron Range at Dunka Pit, 730-780 feet thick in the central Mesabi Iron Range/Virginia Horn area near Eveleth, around 500 feet thick in the western Mesabi Iron Range near Coleraine, and eventually exhibits a “nebulous ending about 15 miles southwest of Grand Rapids” (Marsden et al., 1968) on the extreme western end of the Mesabi Iron Range. Maps of currently active taconite mining operations on the Mesabi Iron Range are presented in Figure 7-4. Mesoproterozoic Duluth Complex The Duluth Complex and associated intrusions of Keweenawan age (~1.1 billion years) in northeastern Minnesota constitute one of the largest mafic intrusive complexes in the world, second only to the Bushveld Complex of South Africa (Miller et al., 2002). These rocks cover a 2,200 square mile (5,700 square km) arcuate area associated with the two strongest gravity anomalies (+50 and +70 milligals) in North America, implying intrusive roots over 8 miles (13 km) deep (Allen and others, 1997). The comagmatic flood basalts and intrusive rocks underlying much of northeastern Minnesota were emplaced during development of the Mesoproterozoic Midcontinent rift, which can be traced geophysically from exposures in the Lake Superior region along a 1250 mile (2,000 km) long, segmented, arcuate path to Kansas and Lower Michigan. The Duluth Complex is defined as the more or less continuous mass of mafic to felsic plutonic rocks that extends for >170 miles (275 km) in an arcuate fashion from Duluth nearly to Grand Portage (Figure 7-5). It is bounded by a footwall of Paleoproterozoic sedimentary rocks and Archean granite-greenstone terranes (Peterson and Severson, 2002), and a hanging wall largely of comagmatic, rift-related flood basalts and hypabyssal intrusions of the Beaver Bay Complex. In genetic terms, the Duluth Complex is composed of multiple discrete intrusions of mafic to felsic tholeiitic magmas that were episodically emplaced into the base of a volcanic edifice between 1108 and 1098 Ma. The geology of the Duluth Complex and adjacent areas has been described in two major publications by the Minnesota Geological Survey (MGS). These include a 1:200,000 scale regional bedrock geological map of northeastern Minnesota (Miller et al., 2001), and a comprehensive written description of the geology depicted on this map (Miller et al., 2002), commonly referred to as the “bible” by geologists working on Duluth Complex geology. Within the nearly continuous mass of intrusive igneous rock forming the Duluth Complex, four general rock series are distinguished on the basis of age, dominant lithology, internal structure, and structural position within the complex. 156 �Trip 7 – Classic Outcrops Figure 7-4. Bedrock geology and iron mining features of the Mesabi Iron Range. Felsic series—Massive granophyric granite and smaller amounts of intermediate rock that occur as a semicontinuous mass of intrusions strung along the eastern and central roof zone of the complex, that were emplaced during early-stage magmatism (~1108 Ma). Early gabbro series—Layered sequences of dominantly gabbroic rocks that occur along the northeastern contact of the Duluth Complex, emplaced during early-stage magmatism (~1108 Ma). 157 �Trip 7 – Classic Outcrops Anorthositic series—Structurally complex suite of foliated, but rarely layered, plagioclase-rich gabbroic anorthosite emplaced throughout the complex during main stage magmatism (~1099 Ma). Layered series—Suite of stratiform troctolitic intrusions that comprises at least 11 variably differentiated mafic layered intrusions that occur mostly along the base of the Duluth Complex. These intrusions were emplaced shortly after the Anorthositic series (~1099 Ma). South Kawishiwi Intrusion The South Kawishiwi intrusion (SKI), together with the similar sized Partridge River intrusion (PRI) immediately to the south, are most renowned for hosting the largest tonnage of Cu-Ni sulfide mineralization in the world (Naldrett, 1997). The realization that the SKI hosts vast quantities of Cu-Ni mineralization over 50 years ago has led to the publication of numerous geologic maps, (Green et al., 1966, Bonnichsen, 1974, Foose and Cooper, 1974, Miller et al., 2001, Peterson, 2002e, f, Peterson et al., 2004, Peterson, 2006b, Peterson et al., 2006), articles (Bonnichsen et al., 1980, Weiblen and Morey, 1980, Ripley, 1986, Chandler and Ferderer, 1989, Lee and Ripley, 1996, Hauck et al., 1997, Peterson, 2001b) theses (Weiblen, 1965, Vislova, 2003, Marma, 2003, Gal, 2008, White, 2010), and reports (Phinney, 1969, Phinney, 1972, Listerude and Meineke, 1977, Morey and Cooper, 1977, Foose, 1984, Dahlberg, 1987, Figure 7-5. Geologic map of northeastern Minnesota. 158 �Trip 7 – Classic Outcrops Dahlberg et al., 1989, Kuhns et al., 1990, Severson, 1994, Zanko et al., 1994, Hauck et al., 1997, Peterson, 1997, Peterson, 2001c, Miller et al., 2002, Peterson, 2002d, Patelke, 2003, Severson and Hauck, 2003). The SKI is shallow dipping (~20º to the east-southeast) sill-like intrusion dominantly composed of troctolitic cumulates that are exposed in an 8 x 32-km arcuate band along the northwestern margin of the Duluth Complex. Footwall rocks include the Paleoproterozoic Virginia Formation in the Serpentine and Dunka Pit deposits, the Paleoproterozoic Biwabik Iron Formation in the Dunka Pit and Birch Lake deposits, and the Archean Giants Range batholith from the northern Birch Lake deposit north to the Spruce Road deposit. The presence of shallow-dipping Biwabik Iron Formation inclusions as far north as the Spruce Road deposit indicates that the majority of Paleoproterozoic units were assimilated and removed from the footwall during emplacement of the SKI, leaving the Giants Range batholith as the dominant footwall rock type. Alternately, the Virginia and Biwabik Iron Formations may simply have been largely eroded prior to the development of the Mid-Continent Rift. Also present as inclusions in the SKI are mafic volcanic hornfels (North Shore Volcanic Group), quartz sandstone hornfels (either the Puckwunge or Nopeming sandstones), and anorthosite (of the Anorthosite series). Anorthositic series rocks about the SKI on the northeast – and enclose an interpreted SKI feeder dike (the NLM) that extends farther northeast – the PRI forms the southern sidewall of the SKI, and the BEI and Anorthositic series rocks overlie the SKI to the east. On the regional Duluth Complex map of Miller et al. (2001), the SKI is subdivided into five major map units. These are, from the base upward, 1. Heterogeneous sulfide-bearing troctolite, gabbro, and norite with localized hornfels inclusions, 2. A thick unit of subophitic to ophitic augite troctolite, 3. Discontinuous and localized layers of poikilitic leucotroctolite, 4. A thick homogeneous sequence of ophitic troctolite, and 5. A thick uppermost sequence of homogeneous troctolite that contains numerous anorthositic layers. Severson (1994) and Zanko et al. (1994) further subdivided the SKI into 17 different lithostratigraphic units that are present in over 180 drill holes over a strike length of 31 kilometers. Sulfide mineralization is confined to the BH, BAN, UW, and U3 units near the base of the intrusion, and to a lesser extent the U1, U2, and PEG units. Major marker horizons that are correlated in drill holes include three horizons with abundant cyclic ultramafic layers (U1, U2, and U3 units) and a pegmatite-bearing unit (PEG unit) that was initially recognized by Foose (1984). The understanding of the significance of a large anorthositic inclusion, originally intersected in six deep drill holes east of the Maturi deposit, and its role in magma dynamics of the SKI has been a key feature in the development of an exploration model for Duluth Metals Limited’s Maturi Extension deposit (Peterson, 2001c). Terminology It is important to note the terminology utilized in this field trip guide for: 1) volcaniclastic rocks; 2) bedding characteristics; and 3) description and unit coding of outcrops in the Duluth Complex. Use of consistent terminology is required in order to accurately describe these geological features. Volcaniclastic rocks contain abundant volcanic material irrespective of their origin or depositional environment. Such rocks can be formed directly from volcanic eruptions (whether subaerial or subaqueous), result from resedimentation of non-lithified volcanic deposits (for example, resedimentation of pyroclasts prior to lithification), or result from weathering and resedimentation of pre-existing lithified volcanic rocks. Primary (juvenile) volcaniclastic particles result directly from eruptive processes, and are of three types: • Pyroclasts, which form by explosive fragmentation of magma into particles (including ash, highly vesiculated glass (pumice, scoria), crystals and crystal fragments, and lithic fragments); 159 �Trip 7 – Classic Outcrops • • Hydroclasts, which form by explosive interaction with external water (via phreatic (steam only) and/or phreatomagmatic (steam and magma) explosions) or by non-explosive quenching and granulation of lava (for example, the formation of hyaloclastite fragments on the margins of submarine lava flows or intrusions into wet sediments); and Autoclasts, which form by frictional breakage of moving viscous lava flows (for example, to form carapace breccias on the margins of subaerial lava flows). Based on these different types of fragmentation, four types of primary volcaniclastic deposits have been identified by White and Houghton (2006): • • • • Pyroclastic deposits, which are generated from volcanic plumes and jets or pyroclastic density currents as particles first come to rest. Deposition mechanisms associated with these processes include suspension settling, traction, or en masse freezing; Autoclastic deposits, which are generated during effusive volcanism when lava cools and fragments as a result of thermal processes, or recently cooled lava breaks during flow. Deposition for these types of rocks is under the influence of continued lava flowage; Hyaloclastite deposits, which are generated during effusive volcanism when magma or flowing lava is chilled and fragmented due to contact with water. Deposition of such deposits is is influenced by the continued emplacement of the lava in the presence of water, and the thicknesses of the hyaloclastite deposits can be dictated by the temperature of the magma, the effusion rate, and the distance from the volcanic vent (Cas and Wright, 1987; Gibson et al., 1999; Newkirk et al., 2001a, 2001b); and Peperite deposits, which are generated when magma intrudes into unconsolidated clastic material and mingles with (generally wet) debris to form a volcaniclastic deposit (McPhie et al., 1993). Deposition of peperite deposits takes place essentially in-situ. Secondary volcaniclastic particles are known as epiclasts: • Epiclasts are lithic clasts and/or crystals derived from physical weathering and erosion of preexisting rocks. Epiclasts are volcaniclasts when the pre-existing rocks are volcanic. The terminology for volcaniclastic rocks has historically been somewhat confusing because many different classification schemes have been developed (for example Fisher, 1961; Fisher 1966; Schmid, 1981; Cas and Wright, 1987; McPhie et al., 1993; White and Houghton, 2006), and different classification schemes are preferentially used in different parts of the world. As a result, the terminology relating to volcaniclastic rocks is commonly misused or misinterpreted. Four classification schemes that have been used most in the recent geological literature include: • • • • Fisher (1961, 1966) – Classification based on particle size, particle formation, or particle fragmentation mechanism; Schmid (1981) – Particle type within the deposit; Cas and Wright (1987) – Mode of fragmentation and deposition; and McPhie et al. (1993) – Transport and deposition mechanisms. According to R. V. Fisher (1998), the difficulties with volcaniclastic rock classification can be understood because “volcaniclastic rocks are essentially igneous on the way up and sedimentary on the way down”. In fact, Fisher’s thesis advisor, when observing the volcaniclastic rocks that were the focus of his thesis studies, indicated that they were “the ugliest and most undistinguished rocks I’ve seen in my 30 years of petrology!” Also, classification is especially difficult in ancient volcaniclastic rocks because key aspects 160 �Trip 7 – Classic Outcrops of classification can be obscured by subsequent metamorphism and/or structural deformation (e.g. particle type, particle size) or because genetic processes cannot be ascertained unambiguously (e.g. transport and deposition mechanism, fragmentation mechanisms). For this field trip guidebook, we will utilize Fisher’s (1966) classification (Figure 7-6) for volcaniclastic rocks. This classification scheme is based on the relative proportions of ash-sized material (< 2mm), lapilli-sized material (2-64mm), and blocks/bomb sized material (>64mm) in the rock. Both Gibson et al. (1999) and Mueller and White (2004) suggest that this classification be used for field-based rock classification (mapping, diamond drill core logging, petrography) of ancient volcaniclastic deposits for the following reasons: • • • The classification scheme is “field-user friendly” because it accommodates both the historically important pyroclastic rock names and enables comparison at both the hand sample and thin section scale (Mueller and White, 2004); It is a Wentworth-based scale, and thus enables comparison of volcaniclastic deposits to sedimentary deposits; and Rock classification does not require knowledge of the specific transport mechanism or depositional processes involved with the genesis of the deposit. Figure 7-6. Volcaniclastic rock classification schemes of Fisher (1966) and White and Houghton (2006). This field trip guidebook will classify volcaniclastic rocks using Fisher’s (1966) classification scheme. More recently, White and Houghton (2006) have developed a modified version of Fisher’s (1966) volcaniclastic classification scheme (Figure 7-6). The scheme is essentially equivalent to the Fisher (1966) scheme, with the exception that the lapill-tuff field in the White and Houghton (2006) classification comprises the lapilli-tuff and lapillistone fields of Fisher’s (1966). Specific terms for bedding thicknesses are also used in this guidebook. The terminology for bedding thickness has been adopted from McPhie et al. (1993) and includes: • • • • Laminated Very thinly bedded Thinly bedded Medium bedded <1 centimeters thick 1-3 centimeters thick 3-10 centimeters thick 10-30 centimeters thick 161 �Trip 7 – Classic Outcrops • • Thickly bedded Very thickly bedded 30-100 centimeters thick >100 centimeters thick Figure 7-7. The classification scheme used to describe and code mafic intrusive rocks within the Duluth Complex, modified after Phinney, 1972. Classification of outcrops and map units within the Duluth Complex have relied on the early work of William Phinney (Green et al., 1966; Phinney 1969, and Phinney, 1972) and is given in Figure 7-7. 162 �Trip 7 – Classic Outcrops FIELD TRIP STOPS Table 7-2. Simplified description of the twenty-field trip stops presented in this guidebook. Also included are the coordinates (UTM, Nad83, Zone 15N and Lat-Long), mileage along the route to the stop, and the age of features that will be observed and discussed on the outcrops. Stop 1: Laurentian Divide at Confusion Hill Longitude/Latitude: 47.51868699°N, -92.58996713E UTM NAD 83 Zone 15N: 530870E, 5262888N Exposed near this wayside and in road cuts on both sides of the highway is an array of variably layered intrusions having both tonalitic (white) and dioritic (black) compositions. A cursory look shows intrusive relationships that conclusively demonstrate that diorite was emplaced into tonalite at one locality, and at another, tonalite was emplaced into diorite. In detail, all compositions intermediate between the two end members are also present locally. Although the dioritic component is abundant here, the bulk of the mapped unit is tonalitic. Emplacement of this unit, now known as the Lookout Mountain tonalite, probably involved some degree of magma mingling. Dikes of tonalite that cut the adjacent high-grade supracrustal rocks of the Minntac sequence contain metamorphic fabrics, yet little evidence of metamorphic origin can be seen in the interior of the body, implying it is syntectonic with respect to D2 deformation. U-Pb zircon dates (Boerboom and Zartman, 1993) of two components of the batholith exposed to the north bracket the age of D2 deformation between about 2674 and 2682 Ma. Exposures at Confusion Hill are a small part of the Giants Range batholith, which forms the core bedrock of the Laurentian (drainage) divide. The batholith is a 40-mile wide belt of intrusions that can be traced on geophysical maps and outcrop east to the Mesoproterozoic Duluth Complex, and west beyond the western border of Minnesota. It separates Archean supracrustal sequences in the Virginia horn from those of the Tower-Soudan area - making stratigraphic correlation between the two districts speculative. Return to bus. 163 �Trip 7 – Classic Outcrops Figure 7-7. Simplified bedrock geology map overlain by the field trip stops and traveled route. Stop 2: Pike River Dam Greywackes Longitude/Latitude: 47.57736062°N, -92.54367719E UTM NAD 83 Zone 15N: 534317E, 5269428N This glacially scoured outcrop exposes a nearly perfect cross-section of straight-bedded, variably graded, feldspathic graywacke and black slate. The feldspar-rich, dacitic composition of sandy textured beds is presumed to represent derivation from the Gafvert Lake felsic volcanic sequence exposed to the east in the Soudan area. Regionally, a series of outcrops from Gafvert Lake westward shows an irregular transition from proximal, possibly subaerial deposition on the east, to distal submarine turbiditic fan deposition to the west. The beds contain numerous "soft-sediment" deformation features including load structures, flames, intrafolial slump folds, and possibly some of the cross-stratal faulting. Bedding is nearly vertical, and graded beds indicate stratigraphic younging to the south. This topping direction, and the presence of a weak D2 cleavage that is left of bedding, indicate westward structural facing in the cleavage; consistent with a position on the south limb of a large, south-overturned, regional, D1 fold structure—the western extension of the Tower–Soudan Anticline. Northeast-trending kink bands, fault zones, and raised quartz veins traversing the outcrop. One of the truly classic outcrops of greywacke of the Lake Vermilion Formation is beautifully exposed at this stop. Prior to about the 1950s, no depositional mechanism could satisfactorily explain the coincidence in graywacke of; 1) coarse sand derived from a source many kilometers distant and having an altered clayey matrix; 2) interbedded black slate; and 3) the lack of evidence for reworking in shallow water (indicative of deposition below wave base). This was changed when the concept of turbidity currents was introduced to the geological profession by Kuenen and Migliorini (1950). Despite widespread publication on turbidites in more modern geologic settings through the 1950s and 1960s, the facies model was not 164 �Trip 7 – Classic Outcrops refined and applied to Archean and Proterozoic strata in the Lake Superior region until somewhat later (Morey, 1965; Ojakangas, 1966). Return to bus. Stop 3: Gafvert Lake Sequence Volcaniclastic Rocks Longitude/Latitude: 47.80135914°N, -92.28615141E UTM NAD 83 Zone 15N: 553454E, 5294469N This relatively new roadcut (approximately 15 years old) exposes rhyodacitic to dacitic composition Gafvert Lake Sequence lapilli tuffs and tuff breccias. The deposits have tentatively been interpreted to represent mass flow units produced by slumping of volcanic and volcaniclastic material from the Gafvert Lake volcano into an adjacent, probably submarine basin. Close inspection of the unit indicates the presence of a variety of lapilli and blocks including: 1) subrounded to subangular plagioclase ± quartz-phyric coherent dacite to rhyodacite; 2) subrounded to subangular pumice; 3) angular carbonate-rich fragments; 4) angular chert fragments; and 5) local subangular to angular massive sulfide fragments. Locally, abundant (up to 10%) <1mm euhedral pyrite cubes are disseminated in the matrix. The presence of both carbonate and massive sulfide fragments, as well as plagioclase- and quartz phyric coherent rhyodacite to dacite lapilli, may suggest the slumps are derived from a Gafvert Lake sequence subaqueous lava dome that was affected by local hydrothermal alteration and the deposition of chemical exhalates (e.g. carbonate, chert and massive sulfide fragments). Structurally, this outcrop occurs on the southern margin of an east-southeast – west-northwest trending D2-associated structure that extends from Pike Bay (northwest) to south of Putnam Lake (southeast). Here one can observe a strong, steeply dipping E-NE foliation and a well-developed lineation that plunges approximately 70° E. Return to bus. Stop 4: Soudan Member Banded Iron Formation Longitude/Latitude: 47.820074°N, -92.2365908E UTM NAD 83 Zone 15N: 557144E, 5296585N (NOTE: Modified from Peterson et al., 2009 and Hudak and Peterson, 2014.) This classic exposure of the Soudan iron-formation member of the Ely Greenstone Formation lies on the north limb of the Tower-Soudan anticline approximately 75 meters north of the stratigraphic top of the volcanic sequences known collectively as the Lower member of the Ely Greenstone. The outcrop displays two generations of tight folding in delicate laminae of chert (creamy white), chert-hematite jasper (red), and magnetite-chert (black to silver-colored). The second generation of folds (F2) is tectonic in origin, having subvertical axial surfaces that trend east, and steeply plunging axes. Most display Z-asymmetry. The earlier folds (F0-1) appear to have been sharply refolded to produce complex interference patterns. Lundy (1985) studied folding at this locality and concluded that some of the apparent interference structures are the product of early-formed sheath folds that did not involve refolding by D2. The F1 structures are predominantly intrafolial and exhibit a great variety of style and orientation; implying they formed by layerparallel, soft-sediment slumping (Fig. 7-8). Lundy’s mapping of this outcrop is an interesting demonstration of unraveling details at a single outcrop that led to recognition that D1 deformation was not systematic here, but likely soft sediment. Furthermore, it is a microcosm of regional-scale deformation. 165 �Trip 7 – Classic Outcrops It is interesting to observe the rhythmic microlaminae (1 mm or so thick) in various cherty beds exposed here and speculate about the paleoenvironment—that is, whether these represent daily heating/cooling, tidal, climatic, annual, or some other repetitive influence (e.g waxing/waning of a hydrothermal system) in the depositional environment. What is known about units of iron-formation in the Ely Greenstone, of which there are many, is that deposition occurred in deep water (below wave base) during periods of relative volcanic and tectonic quiescence by the slow subaqueous “rain” of chemical precipitates. The deep excavations in this area are the early workings of the Soudan iron mine, the first in Minnesota. The mine produced about 16 mt of high-grade hematite ore (60-63 percent iron converted to a park. Most of the production came from underground workings that began here in 1900, and which now can be visited on guided tours. The mine previously housed several underground physics research facilities. These include Soudan 1 (23rd level) which studied neutrino decay; 2) Soudan 2 (27th level), also to study neutrino decay; and 3) the MINOS (Main Injector Neutrino Oscillation Search) lab, which was built on the 27th level adjacent to Soudan 1 and studied the decay of neutrinos within the earth as they passed from Fermilab to Soudan (Peterson et al., 2009b). Figure 7-8. Outcrop map showing bedding trajectories and multiple generations of folds and faults (from Lundy, 1985). F1 folds are non-systematic and include both nappe- and sheath fold geometries. Return to bus. Stop 5: Murray Shear Zone Along Hwy 1/169 Longitude/Latitude: 47.81809993°N, -92.20694376E UTM NAD 83 Zone 15N: 559366E, 5296388N A series of roadcuts along Highway 169 expose a transect through the northern edge of the Murray Shear Zone, which is one of the most striking Neoarchean structural features in the Tower-Soudan area 166 �Trip 7 – Classic Outcrops (Peterson and Patelke, 2003). This series of outcrops perfectly display a classic feature of Neoarchean ductile (shear zone) structures - strain partitioning (Figure 7-9). A close look at these outcrops also gives one hints of broader economic geology implications via the presence of carbonate alteration of the chlorite schists. The carbonate (ankerite and/or ferro-dolomite) strain hardens the ductile deformed schistose rocks and allows for subsequent brittle deformation (and perhaps orogenic gold mineralization in cross-cutting quartz-ankerite-sulfide veins. On the larger scale, the D2 Murray shear zone transposes rocks 3-5 km eastwards in the zone bounded by its northern and southern strain partitioned boundaries. The overall geometry of this panel of Figure 7-9. Scanned image of the field sheet used to map outcrop OC-567. On the right are digital photographs of outcrop OC-567: A) the overall outcrop view looking WNW; B) view to the north of steeply east plunging, lineated and rod-shaped pillowed andesite (rock hammer 68cm for scale); and C) close-up view of rock sample S-604, taken from the west side of the outcrop (bright zone on the left side of picture A). Data from Peterson & Patelke, 2003. 167 �Trip 7 – Classic Outcrops rocks is like the geometry of “wedge-shaped shear zones” described in detail by Ramsey and Huber (1987). Peterson’s mapping and collection of structural data in the Murray shear zone panel is largely confined to a series of outcrops along the northern edge of the zone. Field observations of these outcrops indicate that the strain symmetry along this boundary is largely constrictional, with a dominant steeply east-dipping, elongate and rod-shaped structural fabric. A stereonet projection of planar and linear structural features within the Murray panel is shown in Figure 7-10. The mean value of the strike and dip of planar features is 282°/82°, and the trend and plunge of linear features has a mean orientation of 87°/71°. The overall mapscale internal geometry of the Murray panel clearly shows dextral asymmetry, with a strong sigmoidal wrapping of iron-formation (see field trip geologic maps) to the northeast. Figure 7-10. Stereonet projections of foliation, shear fabrics, and linear features from the Murray shear zone. An estimate of the amount of displacement of the rocks within the panel of rocks bounded by the Murray shear zone is given in Table 7-3. These values were calculated geometrically by using the average plunge of measured lineations (71°) and two measured lines of possible correlative stratigraphy offset by the bounding shear zones. The calculated total displacement values (net slip) are quite large (up to 13.8 km, or 43,000 feet of net slip), but the displaced rocks would still fall within the range of depth generally associated with greenschist facies metamorphism. Table 7-3. Calculated displacement along the Murray shear zone Strike Slip Lineation Plunge Dip Slip Net Slip 71° 4.5 13.1 13.8 71° 3.0 8.7 9.2 Return to bus. 168 �Trip 7 – Classic Outcrops Stop 6: Lower Ely Member (Central Basalt) Sheet and Pillowed Flows Longitude/Latitude: 47.8306566°N, -92.17157352E UTM NAD 83 Zone 15N: 561999E, 5297811N (NOTE: Modified from Field Trips of Hudak et al., 2004, 2014; Peterson et al., 2005, 2009). This classic outcrop has been visited during field trips associated with the 2004, 2009 and 2014 ILSG conferences (Hudak et al., 2004; Peterson et al., 2009; Hudak et al., 2014). This is a no-hammer outcrop, as the preservation of the delicate textures here rivals those observed in other classic Neoarchean camps in the Superior Province containing well-preserved volcanic textures such as Noranda, Quebec and Timmins, Ontario. The description and figure below have been modified from Peterson et al. (2009) and Hudak et al. (2014). The Central Basalt sequence (Peterson and Patelke, 2003, Peterson, 2005) comprises a steeply north-dipping (75°- vertical), north-facing sequence of sparsely amygdaloidal pillowed and massive lava flows of basalt andesite to basalt composition that are believed to be correlative with the tholeiitic Armstrong Lake volcanic sequence mapped in the Eagles Nest quadrangle (Jirsa et al., 2001), approximately 11km to the east. Hudak et al. (2007), Jansen et al. (2009), and Hudak et al. (2012) have shown that the lowermost sections of the Central Basalt Sequence are composed of submarine basaltic andesite to basalt lava flows that have rare earth element lithogeochemical patterns similar to mafic rocks in oceanic volcanic arcs. However, locally, submarine basalt lava flows that occur within 50-200m stratigraphically below the contact between the Central Basalt Sequence and the overlying Soudan Member of the Ely Greenstone Formation illustrate MORB-like or back-arc basin-like lithogeochemical patterns. This change in rare earth element characteristics may be interpreted to indicate a change from an oceanic arc to back-arc environment immediately prior to the deposition of the Soudan Member. Relative to massive and pillowed basalt and andesite flows in the Fivemile Lake sequence, Central Basalt sequence lava flows are notably less amygdaloidal, and lack multiple pillow rind structures. In addition, the Central Basalt sequence lacks the thick sequences of scoria-rich basalt-andesite lapilli tuffs that are commonly interstratified with lava flows in the Fivemile Lake sequence. These characteristics of the Central Basalt sequence indicate eruption and deposition in a deeper submarine environment than the stratigraphically older Fivemile Lake sequence and suggest overall increasing water depth during the temporal development of the Lower Ely. Deepening of the water column could be accommodated by extensional tectonics and normal faulting associated with the development of the proposed back-arc environment. At this stop, the outcrop comprises two east-southeast striking massive basalt flows, ranging from at least five to nine meters in thickness, that are separated by a ten-meter-thick flow unit comprising pillows and pillow lobes (Fig. 7-11). All three lava flows at this vicinity illustrate tholeiitic, MORB-like lithogeochemistries (Hudak et al., 2007). Flow 1, at the southern part of the outcrop, is composed of a pale- to dark green, faintly feldspar-phyric (~10% 0.5-1 mm laths), sparsely amygdaloidal, basalt sheet flow that locally exhibits tortoise-shell jointing formed in response to contraction during cooling. The uppermost 10-40 cm of the coherent part of Flow 1 is generally silicified and epidotized. Petrographic observations indicate that this section of the flow also contains up to 70% <0.1 cm round spherulites. An irregular contact occurs between the coherent basalt flow and an overlying one- to two-meter-thick unit of dark green, exceptionally well-preserved perlitic in-situ hyaloclastite and associated self-peperite (c.f. Batiza and White, 2000). The hyaloclastite formed from non-explosive fracturing of the basalt glass developed on the flow top due to quenching by water, whereas the perlite formed following deposition by hydration of volcanic glass. An irregular contact occurs between the hyaloclastite and Flow 2, which is composed of north-facing mattress- to bun- shaped pillow lavas and pillow lobes with numerous “neck and knob” structures. Individual Pillow structures have well developed perlitic hyaloclastite margins that range from 1-4 cm in 169 �Trip 7 – Classic Outcrops Figure 7-11. Detailed geological map of sheet flows, pillow lavas, and associated hyaloclastite deposits at Field Trip 3, Stop 1 (after Hudak et al., 2014; Hudak and Peterson, 2014). width. Pillow buds indicate propagation from east to west, suggesting the volcanic vent was located east of this location. The coherent pillows and lobes are overlain by up to 2.5 meters of hyaloclastite breccia that contains 20-40% subrounded to subangular pale gray green basalt lapilli in a jigsaw puzzle-fit dark green perlitic hyaloclastite matrix. The upper contact of Flow 2 and the overlying basalt sheet flow (Flow 3) is irregular, and is marked by thin (1-8 cm thick), sheet- like basalt fragments that are up to 1.6 meters in length. These fragments locally appear to be isoclinally folded about an east-west-trending fold hinge. Although the genesis of this structure is currently not well understood, it may be due to syneruptive deformation of either thin slabs of hot, basal flow margin crust from the overlying flow, or thin injections of basalt magma into the hyaloclastite from either the underlying pillows or the overlying sheet flow. Flow 170 �Trip 7 – Classic Outcrops 3 comprises an at least ten-meter-thick pale green-gray, slightly feldspar-phyric, sparsely amygdaloidal sheet flow. Steep, NNE-trending west dipping D3 joints are well developed in this unit, as are lens-shaped psuedo-pillows that are up to 50 cm in diameter. Return to the bus by walking back down the hill. Stop 7: Mud Creek Shear Zone Longitude/Latitude: 47.87440908°N, -92.14025702E UTM NAD 83 Zone 15N: 585753E, 5309482N This outcrop shows highly strained rocks in the Mud Creek shear zone. The rock type is a quartziron carbonate-sericite schist, having quartz and tourmaline knots, abundant pyrite, and trace amounts of gold. Its protolith is unknown, because of the intense deformation, but could be any of several rock types in the region, including quartzofeldspathic porphyry, basaltic metavolcanic rocks, or graywacke. The shear fabric trends east-northeast, and lineations plunge at shallow angles to the east. Development of this shear zone, which occupies most of the valley of Mud Creek, is a product of largely dextral transpressive deformation that has been partitioned into discrete zones, presumably late in D2 deformation. It is generally believed that gold-bearing mineralization was introduced during these later deformation events, and the Mud Creek shear zone and environs have attracted considerable attention as a gold target (Peterson, 2001, Peterson and Patelke, 2004a, 2004b). The Mud Creek shear is a broad, anastomosing zone that forms the boundary between rocks of the Ely Greenstone and Lake Vermilion Formation on the south, and volcanic and iron formation-bearing rocks known informally as the Bass Lake sequence on the north. The Bass Lake rocks may be equivalent to parts of the Newton Lake Formation exposed north of Ely, but a complex series of faults in the intervening area makes this correlation speculative. Return to bus. Stop 8: Newton Lake Formation Variolitic Pillow Lavas and Hyaloclastite Longitude/Latitude: 47.93291301°N, -91.85191152E UTM NAD 83 Zone 15N: 585753E, 5309482N (NOTE: Modified from Field Trip Stop 5-16 (Jirsa et al., 2004), and Field Trip Stop ET-1 (Peterson et al., 2009)). The Neoarchean Newton Lake Formation is composed primarily of tholeiitic and komatiitic pillowed mafic lava, diabasic gabbro, differentiated mafic-ultramafic sills, intermediate-mafic pyroclastic rocks and siliceous marble with minor felsic-intermediate volcaniclastic rocks and lava flows. This formation is approximately 2,350 m thick. The unit overlies the Knife Lake Group in central part of the Vermilion district and the Lake Vermilion Formation in western part of Vermilion district (USGS National Geologic Map Database, https://ngmdb.usgs.gov/Geolex/UnitRefs/NewtonLakeRefs_9525.html). The Newton Lake Formation differs from the Ely Greenstone Formation in that the former contains a high proportion of mafic-ultramafic sills and lava flows, abundant diabasic sills and rare iron-formation. Lava flows in the Newton Lake Formation typically have larger MgO and incompatible element contents than those of the Ely Greenstone Formation, and some are classified as komatiites and komatiitic basalt (Schulz, 1980; Jirsa et al., 2004; Grotte and Hudak, 2014). The Newton Lake Formation (and possibly equivalent Bass Lake sequence) appears to be younger than the Lower Ely Member (~2723 MA; Peterson et al., 2001) with an age date of ~2715 MA (Jirsa, 2016) and was previously interpreted to be the youngest Archean supracrustal sequence in the Vermilion district until the Gafvert Lake Sequence was dated at 171 �Trip 7 – Classic Outcrops approximately 2689 MA (Lodge et al., 2013). Rocks having nearly identical composition and stratigraphic/structural setting occur in Itasca County some 80 kilometers to the west (Jirsa, 1990; Jirsa et al., 2004). A sequence of exceptionally well-preserved steeply-dipping, south-topping, lower greenschist facies metamorphosed Newton Lake Formation variolitic pillow lavas is exposed along a series of outcrops located on the west side of the road approximately one-half mile north of CR-88 on the Echo Trail. Variolites are defined as “a spherulite-like radiating aggregate composed of feathery, needle-like crystals of plagioclase and pyroxene that occur in mafic volcanic rocks (typically basalt). Variolites may result from devitrification but are commonly believed to be formed in subaqueous rocks by quench-induced crystallization (Cas and Wright, 1987, p. 420). According to Arndt and Fowler (2004), variolites result from either magma mingling or blotchy alteration, or they are a type of plagioclase spherulite. A generalized cross-section through these pillows from a detailed field and petrographic study of this outcrop (Grotte and Hudak, 2014) is presented in Figure 7-12A. Pillows vary from “bun-” to “mattress” shaped (Dimroth et al., 1978) and range from <1 to >2.5 meters in diameter. Pillow shapes, as well as the local occurrence of quartz-filled vacuoles within individual pillows, indicate younging directions to the south. Pillow cores tend to be dark green to pale yellow-green in color depending upon the abundance of secondary epidote alteration. The pillow cores commonly contain massive, globular variolites with local <1cm diameter spherical variolites., and are locally variolitic (Figure 7-12B). Pillow selveges are well preserved and commonly contain concentric zones globular to spherical Figure 7-12. Summary of field and petrographic observations of Newton Lake Formation variolitic pillow lavas at this location (from Grotte and Hudak, 2014). A) Generalized cross-section through a Newton Lake Formation pillow lava at this location. B) Outcrop photo of the margin of a pillow lava at this location noting the transition from well-preserved interpillow hyaloclastite into a variolitic pillow selvege. C) Thin section scan illustrating the well preserved cuspate, angular shards comprising the interpillow hyaloclastite. Dark spherical shapes on the right half of the photo are variolites. 172 �Trip 7 – Classic Outcrops variolites that mimic individual pillow shapes. Interpillow hyaloclastite is extremely well preserved and is composed of jigsaw-puzzle-fit angular cuspate shards that were originally glass but are now composed of fine-grained alteration minerals (Figure 7-12C). Petrographic observations (Grotte and Hudak, 2014) indicate that variolites in this exceptional exposure of Newton Lake Formation pillow lavas are dominantly composed of rounded to oval, radiating plagioclase spherulites with rare, axiolitic plagioclase spherulites locally present. The presence of needlelike to acicular skeletal plagioclase crystals and absence of phenocrysts suggest that the pillow lava flows at this location erupted at temperatures above the liquidus and experienced relatively large degrees of undercooling before undergoing rapid crystallization on the Neoarchean seafloor. As indicated in Jirsa et al. (2004), the Newton Lake is separated from the Ely Greenstone to the south by a complex zone of faulting (Shagawa Lake and Sibley faults) developed within sedimentary rocks of the Knife Lake Group. Although relatively undeformed conglomerate and sedimentary rocks of the Knife Lake Group are exposed just a few miles to the east, they are typically so sheared and altered in this area as to obscure lithologic and sedimentary interpretations. Return to bus Stop 9: Giants Range Batholith Longitude/Latitude: 47.81587746°N, -91.79083789E UTM NAD 83 Zone 15N: 590518E, 5296544N (NOTE: Modified from Hudak and Peterson, 2014). Footwall rocks to the northern part of the South Kawishiwi Intrusion are part of the Neoarchean Giants Range batholith (GRB). At this exposure along Highway 1, the GRB consists of porphyritic hornblende quartz monzonite that contains distinctive 1-2cm diameter potassium feldspar phenocrysts. One may also observe a distinctive foliation represented by alignment of black to dark green amphiboles and locally dark brown biotite. The massive nature of this unit creates an excellent footwall for Duluth Complex-associated intrusions and associated Cu-Ni-PGE deposits as the GRB lacks bedding and thus rare (if ever) gets incorporated into the mineralized zone as barren xenoliths. Additionally, melting of the GRB beneath longlived magma channels (Peterson and Boerst, 2013) at the base of the Maturi deposit has contaminated the South Kawishiwi intrusion, inducing additional sulfide immiscibility and the genesis of Ni- and Co-rich massive sulfide bodies. Return to bus Stop 10: Maturi SW Roadcuts of BH and U3 Units Longitude/Latitude: 47.78505228°N, -91.79056387E UTM NAD 83 Zone 15N: 590592E, 5293118N Classic roadside exposures of heterogeneous sulfide-bearing troctolite and layered melatroctolite of Severson’s (1994) Basal Heterogeneous (BH) and Ultramafic 3 (U3) units of the SKI. A large core-stone is well exposed in the weakly saprolitic heterogeneous troctolite outcrop. Several small xenoliths of finegrained troctolite can be observed on top of the outcrop and are interpreted as Stage 1 chilled margin autoliths (Peterson and Boerst, 2013). Within the exposure of the overlying U3 layered melatroctolite, olivine layers strike 17° and dip steeply 51° to the ESE. The steep dip is apparently associated with two 173 �Trip 7 – Classic Outcrops defined north-south trending faults east of these exposures. Recent drilling by Twin Metals Minnesota in this area has led to the definition of the Maturi SW deposit. Return to bus Stop 11: SKI Magmatic Slurry Igneous Breccia Longitude/Latitude: 47.780584°N, -91.79273111E UTM NAD 83 Zone 15N: 590437E, 5292619N At this stop, we’ll examine perhaps the best exposure of Severson’s (1994) BH unit in the whole of the SKI. The heterogeneous troctolitic rocks at this stop are generally poorly mineralized and thus lack a gossanous saprolitic weathering profile which lets one see the true nature of the heterogeneity within the troctolite. We believe that all geologists who log drill core within the Cu-Ni-PGE deposits of the Duluth Complex (or who attempt to model such deposits for mine planning purposes) should be required to spend several days examining the rocks within the area around both Stops 10 and 11. All participants should imagine a drill core cutting this exposure and how they would interpret the geology of that core without first examining this outcrop. Such thoughts are why the Precambrian Research Center’s field camp had for many years its students complete a 1:5,000-scale bedrock geology map of this area. Return to bus Stop 12: Main AGT Longitude/Latitude: 47.81303067°N, -91.73468023E UTM NAD 83 Zone 15N: 594727E, 5296295N Recent road cut along the south side of Minnesota Highway #1 of massive, extremely homogeneous augite troctolite of the Main AGT unit of Severson (1994). Troctolite of the Main AGT unit differs from the Middle and Upper SKI troctolite in two distinctive ways: 1) ophitic augite crystals are black, distinctly associated with Fe-Ti oxides + apatite, and occur as high-density ophitic crystals from 1 to 3 inches in diameter. In the Middle and Upper SKI, ophitic augite crystals are brown, not associated with Fe-Ti oxides, and occur as large (up to 15 inches) low-density grains; and 2) The Main AGT is never layered. Geologists at Duluth Metals interpret the units’ homogeneity and lack of layering as evidence that the Main AGT magma lacked phenocrysts of olivine and plagioclase and represents the end product of topdown and bottom-up solidification of a basaltic liquid. We currently interpret the Main AGT as the solidification of much of the “carrier liquid” of the underlying sulfide-bearing BMZ magmatic slurry. Return to bus Stop 13: Spruce Road Bulk Sample Site/Discovery Burrow Pit Longitude/Latitude: 47.83271644°N, -91.67864227E UTM NAD 83 Zone 15N: 598885E, 5298553N Beginning in the late 1940s, the U.S. Forest Service utilized locally derived glacial tills and weathered bedrock gossans as road building materials during the construction of the Spruce Road. As we take a short hike into one of these borrow pits, we will walk by the 1973 INCO bulk sample site in the Spruce Road deposit and visit several outcrops with fresh Cu-Ni sulfide minerals. This short stop will examine the bottom of an old borrow pit where participants can walk on and sample sulfide-bearing 174 �Trip 7 – Classic Outcrops troctolite gossans. Please note the friable nature of the rocks in the weakly saprolitic exposure and look for rounded core-stones where weathering over the eons was less intense. Return to bus Stop 14: Nickel Lake Macrodike Longitude/Latitude: 47.83079527°N, -91.63760896E UTM NAD 83 Zone 15N: 601959E, 5298393N The Nickel Lake Macrodike (NLM) is a northwest to southwest-trending, steeply dipping, asymmetric troctolitic and gabbroic intrusion interpreted to be a feeder dike for the northern portions of the SKI. The macrodike is interpreted to be located within a major rift-parallel normal fault (down to the southeast) now obscured by intrusion of NLM igneous rocks. Regional southward tilting (based on the deep level of erosion of the northern Bald Eagle Intrusion directly east of this area) leads to the interpretation that the southwest end of the NLM (near Omaday Lake) is structurally higher than the northeastern portion of the dike, and represents the location where magma flow changed from dike-like to sill-like, as it exited Figure 7-13. Bedrock geology map of the southwestern end of the Nickel Lake Macrodike. 175 �Trip 7 – Classic Outcrops the dike – thus the magma velocity slowed – and entered the growing SKI magma chamber. Excellent potential exists for Ni-Cu rich massive sulfide at the basal contact where the dike enters the SKI (Section 31, T62N, R10W). The 6.5km long by 1.0 km wide macrodike is composed of three main units: 1) inclusion-rich, locally sulfide-bearing, heterogeneous troctolite (unit Mpth) along the northwestern margin; 2) layered troctolite, melatroctolite, and dunite (unit Mltmt) along the southeastern margin; and 3) a late, cross-cutting, coarse-grained to pegmatitic oxide-rich, olivine-gabbro to melagabbro (unit Mxog) traversing generally through the center. Small (< 1m) to large (hundreds of meters long) xenoliths include Mesoproterozoic Anorthositic Series wall rocks (unit Mai) and North Shore Volcanic Group basalts (unit Mhb), and Paleoproterozoic Biwabik Iron Formation (unit Pifs) and Virginia Formation (unit Pvf). For this field trip we are simply going to take some walks in the bush, mostly along logging roads and snowmobile trails as time allows and look at numerous outcrops of the NML and adjacent rocks and discuss geology as we see it. Numerous detailed bedrock geology maps, reports, and presentations of the NLM and adjacent areas have been published over the last couple of decades (Peterson, 2002a, 2002b, 2002c, 2006a, 2006b, 2006c, 2008, Peterson and Albers, 2007) and a compilation of detailed geologic mapping data for the southwestern NLM is given in Figure 7-13. Return to bus Stop 15: Remnant Saprolite, Middle SKI Longitude/Latitude: 47.77089981°N, -91.66297244E UTM NAD 83 Zone 15N: 600176E, 5291703N A short field trip stop to examine locally layered troctolitic rocks of the Upper SKI of Peterson and Boerst (2013). This outcrops in this area epitomizes the “Sea of Troctolite” that occurs throughout the vast majority of the SKI (Middle and Upper SKI of Peterson and Boerst, 2013). Careful attention will be given to an outcrop next to the bus where spheroidal weathering of the troctolite is forming rounded core stones of troctolite, which we’ll see once again at stop 18. Return to bus Stop 16: Anorthosite Series Roadcut Longitude/Latitude:: 47.75915521°N, -91.64719916E UTM NAD 83 Zone 15N: 601381E, 5290418N Large, glacially polished roadside outcrop of the gabbroic and troctolitic anorthosites of the Anorthositic Series of the Duluth Complex. At this location these anorthositic rocks form the eastern sidewall of the SKI and are cut by a series of northeast-striking valleys. The valleys were interpreted by geologists of Duluth Metals Limited as steeply west-dipping reverse faults that were formed by emplacement of the SKI immediately to the west. Approximately 2.5 km to the southwest of this roadcut Cold Spring Granite Company quarries a large gabbroic anorthosite xenolith similar to this stop in their Mesabi Black quarry. Return to bus 176 �Trip 7 – Classic Outcrops Stop 17: Bald Eagle Intrusion Longitude/Latitude: 47.7385175°N, -91.6405279E UTM NAD 83 Zone 15N: 601921E, 5288133N A quick stop to observe a roadside outcrop of troctolite of the Bald Eagle Intrusion (BEI). The BEI is a large (4.5 to 16.5 km x 31 km) troctolitic to gabbroic body that was emplaced partially within Anorthositic series rocks, the SKI, and the Greenwood Lake Intrusion (see BEI on Figure 7-7). Weiblen (1965) mapped the well-exposed northern portion of the intrusion and showed that it is funnel-shaped and consists of an outer zone of troctolite and an inner zone of olivine gabbro. In the poorly exposed southwestern portions of the intrusion, field mapping by Green et al., (1966) and Foose and Cooper (1978) showed the BEI and SKI in direct conformable contact. Steep foliation and modal layering (Weiblen, 1965, Green et al., 1966) integrated with a distinct gravity anomaly over the northern BEI imply that the northern part of this intrusion is funnel shaped and necks down to a steep feeder dike. Weiblen and Morey (1980) interpreted the limited cryptic variation (Weiblen, 1965), the steep dip of lamination and layering, and adcumulate nature of the BEI as indicative of its being an open conduit to higher intrusions and perhaps volcanic flows. Petrologic observations and geophysical interpretations (Chandler, 1990, Chandler and Ferderer, 1989) suggest that the BEI and SKI were emplaced by successive overplating of magmas from a common feeder centered on the northern BEI and extending along the trace of the NLM that links the BEI and SKI. In a related analogy, Cartwright and Møller-Hansen (2006) have shown that interconnected sill complexes transect the middle to upper crust over a vertical distance of 8-12 km offshore of Norway. The geometry of the gravity and magnetic anomalies of the BEI, as well as the overall Midcontinent Rift is very similar to the pattern of the seismic reflections profiles of active ridge systems (Vislova, 2003). In detail, the geophysical expressions of the BEI have the same shape and dimensions as the “bulls’ eye” pattern of low velocity seismic reflection anomalies along the East Pacific Rise. These anomalies are interpreted to define regions of melt concentrations, i.e., active magma chambers. These data suggest that the BEI could be a “frozen” dynamic magma chamber (Weiblen et al., 2005, Peterson and Hauck, 2005). Eight exploration holes drilled by Duluth Metals Limited in 2011 revealed several new distinct features of the BEI. All of these holes encountered chromitite layers within horizontally layered troctolites with many of the chromitite horizons occurring as “rip up” clasts within troctolites. Duluth Metals Limited’s hole LOD-06, drilled 12km SSW of this field trip stop, encountered flowing gas at a depth of 1,778 feet. The gas was analyzed and found to contain >10% helium. This is the site where Pulsar is currently exploring with the aim of producing helium gas. Return to bus Stop 18: Vermilion Moraine Longitude/Latitude: 47.6918526°N, -91.81063993E UTM NAD 83 Zone 15N: 589247E,5282737N In common with most of the high latitude regions of North America, northeastern Minnesota was repeatedly glaciated during the ice ages of the Pleistocene Epoch. Glaciogenic sediments and landforms in this 2025 ILSG field trip area are associated with the Rainy Lobe of the Laurentide ice sheet. While there are a number of possible definitions of what constitutes the Rainy Lobe – sedimentological, textural, compositional, and association with particular geomorphic features – a definition rooted in glacial dynamics perhaps works best. In this sense, the Rainy Lobe refers to that portion of the Laurentide ice sheet lying northwest of Lake Superior (occupied by the Superior Lobe), and east of the Winnipeg Basin and Red River Valley (occupied by the Red River Lobe). In common, Rainy Lobe landforms and glaciogenic sediments 177 �Trip 7 – Classic Outcrops reflect a general northeast to southwest ice flow direction, and a Labradoran (northeastern) sediment provenance. In common with much the Canadian Shield, glacial erosion has nearly completely stripped preglacial regolith from bedrock north of the Laurentian Divide. However, preglacial saprolites are a common occurrence underlying glaciogenic sediments in central and western Minnesota; the nearest such occurrences are exposed in open pit mines of the Mesabi Range, on the south flank of the Giant’s Range. Approximately 12,400 years ago, the retreating Rainy Lobe made a last stand in northern Minnesota to form the West-Northwest to East-Southeast trending Vermilion Moraine. This stop includes a quick walk over the end of the Vermilion Moraine and a view to the south over a glacial lake plain (Fig. 7-14). Return to bus Figure 7-14. Annotated lidar digital elevation model showing glaciogenic landforms in the stop 18 area. Stop 19: Contaminated Basal SKI, Dunka Pit Area Longitude/Latitude: 47.69423099°N, -91.85803438E UTM NAD 83 Zone 15N: 585687E, 5282948N The recently permitted extension of Cliffs Natural Resources Northshore mine required the rerouting of St. Louis County Road 623 to the north. The building of the new road resulted in the exposure of rocks of the ~1.85 Ga. Biwabik Iron Formation and the 1.1 Ga. South Kawishiwi intrusion. This short stop will include the examination of three new roadside outcrop areas, including: 1) Metamorphosed Biwabik Iron Formation, 2) Sulfide-poor gabbroic rocks, and 3) Sulfide-rich (pyrrhotite-dominant) 178 �Trip 7 – Classic Outcrops contaminated noritic rocks. Geochemical analyses of rock samples from these three outcrop areas (completed by Duluth Metals in 2012) are given in Table 7-4. Table 7-4. Geochemical analyses of rock samples taken from the roadside outcrops of Stop 19. Sample ID DMR0446 DMR0447 DMR0164 DMR0165 DMR0448 DMR0449 DMR0450 DMR0451 Rock Type Iron Formation Iron Formation Olivine Gabbro Biotitic Gabbro Sulfidic Norite Sulfidic Norite Sulfidic Norite Sulfidic Norite Outcrop # 1 1 2 2 3 3 3 3 Cu (ppm) 4 0 357 218 3960 2630 4380 4270 Ni (ppm) 0 0 82 87 1230 761 1030 1120 Co (ppm) 2 13 44 53 191 117 168 183 Pt (ppb) 5 1 1 1 9 20 6 16 Pd (ppb) 1 3 1 2 53 40 58 60 Au (ppb) 1 1 1 1 18 15 23 23 S (%) -0.01 -0.01 0.02 -0.01 2.61 1.62 1.58 1.79 SiO2 (%) 51.35 38.71 48.14 47.28 42.67 51.62 41.25 43.53 Al2O3 (%) 0.25 0.45 15.21 15.06 15.02 17.56 13.81 13.71 Fe2O3 (%) 40.11 55.01 15.64 16.35 21.04 11.98 22.12 21.34 CaO (%) 6.07 3.33 7.87 8.00 7.51 5.39 6.29 6.90 MgO (%) 1.93 1.52 4.93 5.47 7.15 5.13 6.01 6.97 Na2O (%)t 0.06 0.06 2.90 2.62 2.25 2.72 1.99 2.34 K2O (%) 0.01 0.01 1.12 1.10 0.68 2.54 0.56 0.63 Cr2O3 (%) -0.01 -0.01 0.02 0.01 0.02 0.03 0.02 0.02 TiO2 (%) -0.01 0.05 3.12 3.47 1.77 0.76 1.90 2.31 MnO (%) 0.97 0.51 0.19 0.20 0.16 0.08 0.14 0.18 P2O5 (%) 0.06 0.07 0.43 0.34 0.23 0.05 0.26 0.26 -0.95 -1.37 0.05 -0.09 1.25 1.80 5.13 1.02 LOI (%) Photographs of the Biwabik Iron Formation (outcrop #1) and sulfidic norite of the South Kawishiwi intrusion (outcrop #3) are presented in Figure 7-15. 179 �Trip 7 – Classic Outcrops Figure 7-15. Field photographs of roadside outcrops of stop 5. (A) outcrop of the Biwabik Iron Formation, (B) closeup shot of bedding in granular iron formation (GIF), (C) rusty weathering and gossanous outcrop of the basal mineralized zone of the South Kawishiwi intrusion, and (D) pyrrhotite-rich norite. Return to bus Stop 20: Giants Range Batholith Migmatite/Pyroxenite-Lamprophyre Dike Longitude/Latitude: 47.68689429°N, -92.05199159E UTM NAD 83 Zone 15N: 571144E, 5281936N This field trip ends where we began, within the Neoarchean Giants Range Batholith. This roadside outcrop of the Embarrass tonalite, an early phase of the GRB that was first mapped by Griffin and Morey (1969) and later remapped by Terry Boerboom in 2015. The outcrop, as mapped by Terry Boerboom, consists of intermixed migmatitic biotite-schist and tonalitic gneiss crosscut by a lamprophyre/pyroxenite dike. Approximately 4-miles to the west of this outcrop the Embarrass Tonalite was originally dated by UPb zircon at 2718 ± 67 Ma by Peterman in Southwick (1994). This data has been superseded by a second U-Pb zircon age of 2687 ± 0.6 Ma by Jirsa (2016). RETURN TO MOUNTAIN IRON COMMUNITY CENTER 180 �Trip 7 – Classic Outcrops Acknowledgements Characterizing and evaluating the detailed geology of northeastern Minnesota has been a team effort involving former NRRI geologists, former and current Minnesota Geological Survey geologists and geophysicists, personnel from the Minnesota Department of Natural Resources and students and faculty from the Precambrian Research Center Field Camp, the University of Minnesota Duluth, the University of Minnesota Twin Cities, and the University of Wisconsin Eau Claire. Their efforts are appreciated. As well, permission to map private properties that was granted by local landowners and mineral exploration/mining companies is much appreciated. The authors would like to thank Jim Essig (Manager, Lake Vermilion / Soudan Underground Mine State Park) and James Pointer (Interpretive Supervisor, Lake Vermilion / Soudan Underground Mine State Park) from the MDNR for their support, assistance, and guidance while planning and conducting detailed geological mapping by the NRRI geologists during the DUSEL project and PRC students and faculty in Lake Vermilion State Park in 2010 and 2011. Funding from the Minerals Coordinating Committee, the University of Minnesota Permanent University Trust Fund, the National Science Foundation, the University of Minnesota Duluth Undergraduate Research Opportunities Program, the University of Minnesota Duluth Graduate School, The University of Wisconsin Oshkosh StudentFaculty Research Program, and many mineral exploration companies also enabled geological research in northeastern Minnesota. 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Hauck, S., Severson, M., Ripley, E., Goldberg, S., and Alapieti, T., 1997, Geology and Cr-PGE mineralization of the Birch Lake area, South Kawishiwi intrusion, Duluth Complex: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report, NRRI/TR-97/13, 32 p. Hauck, S.A., Severson, M.J., Zanko, L.M., Barnes, S.-J., Morton, P., Aliminas, H.V., Foord, E.E., and Dahlberg, E.H., 1997, An overview of the geology and oxide, sulfide, and platinum-group element mineralization along the western and northern contacts of the Duluth Complex, in Ojakangas, R.W., Dickas, A.B., and Green, J.C., eds., Middle Proterozoic to Cambrian rifting, central North America: Geological Society of America Special Paper 312, p. 137-185. Hooper, P., and Ojakangas, R., 1971, Multiple deformation in the Vermilion District, Minnesota: Canadian Journal of Earth Sciences, v. 8, p. 423-434. Hudak, G. J., Heine, J., Newkirk, T., Odette, J., and Hauck, S., 2002a, Comparative geology, stratigraphy, and lithogeochemistry of the Fivemile Lake, Quartz Hill, and Skeleton Lake VMS occurrences, Vermilion district, NE Minnesota: A report to the Minerals Coordinating Committee, DNR, Minerals Division, State of Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2002/03, 390 pages. Hudak, G. J., and Peterson, D. M., 2014, Non-Ferrous Mineralization Associated with the Wawa-Abitibi Terrane and Duluth Complex Cu-Ni-PGM Deposits, Northeastern Minnesota: Society of Economic Geologists, Guidebook Series, v. 47, 150 p. 182 �Trip 7 – Classic Outcrops Hudak, G. J., Heine, J., Lodge, R. W. D., and Jansen, A., 2012, Recent developments understanding the volcanic, magmatic, tectonic, and metallogenic evolution of the Ely Greenstone Formation, Vermilion District, NE Minnesota: Geological Association of Canada – Mineralogical Association of Canada, Abstracts and Program, v. 35, p. 59. 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L., 1988, Transpression in an Archean greenstone belt, northern Minnesota: Canadian Journal of Earth Sciences, v. 25, p. 1060-1068. Hudleston, P.J., 1976, Early deformational history of Archean rocks in the Vermilion district, north-eastern Minnesota: Canadian Journal of Earth Sciences, v. 13, p. 579-592. Jansen, A. C., Hudak, G. J., Heine, J. J., and Peterson, D. M., 2009, Lithogeochemical evaluation of Neoarchean mafic volcanic rocks comprising the footwall to the Soudan Member of the Ely Greenstone Formation, northeastern Minnesota: 55th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 55, Part 1 – Program and Abstracts, p. 46-47. Jirsa, M. A., 2000. The Midway sequence: a Timiskaming-type pull-apart basin deposit in the western Wawa Subprovince, Minnesota: Canadian Journal of Earth Sciences, v. 37, p. 1-15. Jirsa, M. A., Boerboom, T. J., and Peterson, D. 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Jirsa, M.A., and Boerboom, T.J., and Peterson, D.M., 2001, Bedrock Geologic Map of the Eagles Nest Quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-114, scale 1:24,000. Jirsa, M.A., Southwick, D.L., and Boerboom, T.J., 1992, Structural evolution of Archean rocks in the western Wawa subprovince, Minnesota: refolding of precleavage nappes during D2 transpression: Canadian Journal of Earth Sciences 29:2146-2155. Klinger, F. L., 1960, Geology and ore deposits of the Soudan Mine, St. Louis County, Minnesota: unpublished Ph. D. dissertation, University of Wisconsin, Madison, 96 p. Kuenen, P.H., and Migliorini, C., 1950, Turbidity currents as a cause of graded bedding: Journal of Geology, 58:91127. Kuhns, M.J., Hauck, S.A., and Barnes, R.J., 1990, Origin and occurrence of platinum group elements, gold and silver in the South Filson Creek copper-nickel mineral deposit, Lake County, Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/GMIN-TR-89-15, 60 p., 3 pls. Lee, I., and Ripley, E.M., 1996, Mineralogic and oxygen isotopic studies of open system magmatic processes in the South Kawishiwi intrusion, Spruce Road area, Duluth Complex, Minnesota: Journal of Petrology, v. 37, no. 6, p. 1437-1461. Listerud, W.H., and Meineke, D.G., 1977, Mineral resources of a portion of the Duluth Complex and adjacent rocks in St. Louis and Lake Counties, northeastern Minnesota: Hibbing, Minnesota Department of Natural Resources, Division of Minerals, Report 93, 74 p. Lodge, R. W. D., Gibson, H. L., Stott, G. M., Hudak, G. J., Jirsa, M. A., and Hamilton, M. 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(ed.), Ore Deposits of the United States, 1933-1967: New York, American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc., The Grafton-Sales Volume, v. 1, p. 518-537. McPhie, J., Doyle, M., and Allen, R., 1993, Volcanic Textures: A Guide to the Interpretation of Textures in Volcanic Rocks: CODES Key Centre, University of Tasmania, Hobart, Tasmania, 198 p. Miller, J.D., Green, J.C., Severson, M.J., Chandler, V.W., and Peterson, D.M., 2001, Geological map of the Duluth Complex and related rocks, Northeastern Minnesota, Minnesota Geological Survey, Miscellaneous Map M119, scale 1:200,000. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minnesota Geological Survey Report of Investigations RI-58, 207 p. Morey, G.B., 1965, The sedimentology of the Precambrian Rove Formation in northeastern Minnesota (abs.): Institute on Lake Superior Geology, 11th Annual Meeting, St. Paul, Minnesota, Proceedings p. 25-26. Morey, G.B., 1992, Chemical composition of the eastern Biwabik Iron Formation (Early Proterozoic), Mesabi Iron Range, Minnesota: Economic Geology, v. 87, p. 1649-1658. Morey, G.B., and Cooper, R.W., 1977, Bedrock geology of the Hoyt Lakes–Kawishiwi area, St. Louis and Lake Counties, northeastern Minnesota: Minnesota Geological Survey, Open-File Report, scale 1:48,000. Mueller, W. U., and White, J. D. L., 2004, 4.2 – Terminology of Volcanic and Volcaniclastic Rocks: in Eriksson, P. G., Altermann, W., Nelson, D. R., Mueller, W. U., and Catuneanu, O., (eds.), The Precambrian Earth: Tempos and Events: Developments in Precambrian Geology, v. 12, Elsevier, Amsterdam, p. 273-277. Naldrett, A.J., 1997, Key factors in the genesis of Noril’sk, Sudbury, Jinchuan, Voisey’s Bay and other world-class Ni-Cu-PGE deposits: Implications for exploration: Australian Journal of Earth Sciences, v. 44, no. 3, p. 283-315. Newkirk, T., Hudak, G. J., and Hauck, S. A., 2001a, Preliminary lava flow morphology studies at the Five Mile Lake VMS prospect, Vermilion District, NE Minnesota: Implications for volcanic processes, volcanic paleoenvironments, and VMS exploration: Institute on Lake Superior Geology, 47th Annual Meeting, Proceedings Volume 47, Part 1- Program and Abstracts, p. 69-70. Newkirk, T., Hudak, G. J., and Hauck, S. A., 2001b, Preliminary lava flow morphology studies at the Five Mile Lake VMS prospect, Vermilion District, NE Minnesota: Implications for volcanic processes, volcanic paleoenvironments, and VMS exploration: Geological Society of America Abstracts and Programs Volume 33, No. 6, p. A-398. Ojakangas, R.W., 1966, Precambrian stratigraphy and structure of the Tower, Minnesota quadrangle (abs): Institute on Lake Superior Geology Proceedings, 12th Annual Meeting, Sault Ste. Marie, Michigan, Proceedings p. 17. Patelke, R.L., 2003, Exploration drill hole lithology, geologic unit, copper-nickel assay, and location database for the Keweenawan Duluth Complex, northeastern Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report, NRRI/TR-2003/21, 97 pages, 1 CD. Peterson, D.M., 1997, Ore deposit modeling of the footwall mineralization of the Duluth Complex: Minnesota Department of Natural Resources, Division of Minerals, Project 317, 55 p., 46 pls. Peterson, D.M., 2001, Development of Archean lode-gold and massive sulfide deposit exploration models using geographic information system applications: targeting mineral exploration in northeastern Minnesota from analysis of analog Canadian mining camps: unpublished Ph. D. dissertation, University of Minnesota, Duluth, Minnesota, 503 p. Peterson, D.M., 2001b, Development of a conceptual model of Cu-Ni-PGE mineralization in a portion of the South Kawishiwi intrusion, Duluth Complex, Minnesota: Society of Economic Geologists, Second Annual PGE Workshop, Sudbury, Ontario, 3 p. Peterson, D.M., 2001c, Copper-Nickel-PGE mineral potential of the eastward extension of the Maturi Cu-Ni deposit, Duluth Complex, Lake County, Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Confidential Report of Investigations NRRI/RI-2001-02, 29 pages, 15 plates, 1 CD. Peterson, D.M., 2002a, 3-Dimensional view through a mineralized system: the South Kawishiwi intrusion, Duluth Complex: Institute on Lake Superior Geology, 48th Annual Meeting, Thunder Bay, Ontario, v. 48. 184 �Trip 7 – Classic Outcrops Peterson, D.M., 2002b, Cu-Ni-PGE mineralization in the South Kawishiwi intrusion, northeastern Minnesota, Variation due to magmatic processes: Institute on Lake Superior Geology, 48th Annual Meeting, Thunder Bay, Ontario, v. 48. Peterson, D.M., 2002c, Variation in the Cu-Ni-PGE mineralization in the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: 9th International Platinum Symposium, Billings, Montana, USA, July 21-25. Peterson, D.M., 2002d, Copper-Nickel grade maps for the Spruce Road deposit, South Kawishiwi intrusion, Duluth Complex: University of Minnesota Duluth, Natural Resources Research Institute, Report of Investigations NRRI/RI-2002/03, 99 p. Peterson, D.M., 2002e, Shaded relief map of the basal contact of the South Kawishiwi intrusion, Duluth Complex, northeastern, Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Map Series NRRI/MAP-2002-01, scale 1:75,000. Peterson, D.M., 2002f, Bedrock geology, sample location, and property position maps of the west Birch Lake area, South Kawishiwi intrusion, Duluth Complex, northeastern, Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Map Series NRRI/MAP-2002-02, scale 1:10,000. Peterson, D.M., 2005, Bedrock Geologic and Volcanogenic Massive Sulfide Deposit Mineral Potential Map of the Lower Ely Greenstone and Adjacent Areas: Soudan, Eagles Nest, and Bear Island 7.5° Quadrangles, St. Louis County, Northeastern Minnesota: Unpublished geological map, North-Central Section of the Geological Society of America Meetings, Minneapolis, 1:20,000 scale. Peterson, D.M., 2006a, 3D Visualizations of mafic Intrusions in the Duluth Complex, northeastern Minnesota, Institute on Lake Superior Geology, 52nd Annual Meeting, Sault Ste Marie, Ontario, May 8-12, 2006, Minnesota, v. 52. Peterson, D.M., 2006b, Digital base for geological mapping within the northern South Kawishiwi intrusion: Lake and St. Louis Counties, northeastern Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Map Series NRRI/MAP-2006-01, scale 1:20,000. Peterson, D.M., 2006c, New ideas on mineralization in the Duluth Complex, Oral presentation and online pdf file to the Mesabi Range Geological Society, December 20, 66 pages. Peterson, D.M., 2008, Bedrock geologic map of the Duluth Complex in the northern South Kawishiwi intrusion and surrounding area, Lake and St. Louis Counties, Minnesota: Natural Resources Research Institute, Map Series NRRI/MAP-2008-01, scale 1:20,000. Peterson, D.M. and Albers, P.B., 2007, South Kawishiwi Intrusion Cu-Ni-PGE mineralization in association with the Nickel Lake Macrodike, Institute on Lake Superior Geology, 53rd Annual Meeting, Field Trip Guidebook, Lutsen, Minnesota, Volume 53. Peterson, D.M., Albers, P.B., and White, C.R., 2006, Bedrock geology of the Nickel Lake macrodike and adjacent Areas, Lake County, northeastern Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Map Series NRRI/MAP-2006-04, scale 1:10,000. Peterson, D.M. and Boerst, K., 2013, Twin Metals Minnesota’s Maturi Deposit, in Severson, M.J., Peterson, D.M., Ware, A., and Boerst, K., 2013, Cu-Ni-PGE Deposits of the Duluth Complex, Geology and Development: Precambrian Research Center, Workshop on the Copper, Nickel, Platinum Group Element Deposits of the Lake Superior Region, October 6-13, 2013, Field Trip Guidebook, pp. 45- 57. Peterson, D.M., Gallup, C., Jirsa, M.A., and Davis, D.W., 2001, Correlation of Archean assemblages across the U.S.-Canadian border: Phase I geochronology (abs): Institute on Lake Superior Geology, 47th Annual Meeting, Madison, Wisconsin, Proceedings v. 47, Part 1, p. 77-78. Peterson, D.M., and Hauck, S.A., 2005, Visualization of "Frozen" dynamic magma chambers in the Duluth Complex, northeastern Minnesota: Eos Trans. AGU 86(52), Fall Meet. Suppl., Abstract V23A-0680. Peterson, D.M., and Jirsa, M.A., compilers, 1999, Bedrock geologic map and mineral exploration data, western Vermilion district, St. Louis and Lake Counties, northeastern Minnesota: Minnesota Geological Survey Miscellaneous Map M-98, scale 1:48,000. Peterson, D.M., Jirsa, M. A., and Hudak, G. J., 2005. Field Trip 9: Architecture of an Archean Greenstone Belt: Stratigraphy, Structure and Mineralization: in Robinson, L., ed., 2005, Field Trip Guidebook for Selected Geology in Minnesota and Wisconsin: Minnesota Geological Survey Guidebook 21, p. 154-180. Peterson, D.M., Jirsa, M., and Hudak, G., 2009, Field Trip 7 – Architecture of an Archean Greenstone Belt: Stratigraphy, Structure, Mineralization: 55th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 55, Part 2 – Field Trip Guidebook, p. 178-215. Peterson, D.M., and Patelke, R. L., 2003, National Underground Science and Engineering Laboratory (NUSEL): Geological site investigation for the Soudan Mine, northeastern Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2003/29, 88 p. 185 �Trip 7 – Classic Outcrops Peterson, D.M., and Patelke, R. L., 2004a, Bedrock Geology and Lode Gold Prospect Data Map of the Mud Creek Road Area, Northern St. Louis County, Minnesota: Natural Resource Research Institute Geologic Map NRRI/MAP-2004/01, 1:12,00 scale, available for free download at http://www.nrri.umn.edu/egg/REPORTS/MAP200401/MAP200401.html. Peterson, D.M., and Patelke, R. L., 2004b, Field Trip 7 – Economic geology of Archean gold occurrences in the Vermilion District, northeast of Soudan, Minnesota: 50th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 50, Part 2 – Field Trip Guidebook, p. 200-226. Peterson, D.M., Patelke, R.L., and Severson, M.J., 2004, Bedrock geology map and Cu-Ni mineralization data for the basal contact of the Duluth Complex west of Birch Lake, St. Louis and Lake Counties, northeastern Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Map Series NRRI/MAP2004-02, scale 1:10,000. Peterson, D.M., Pointer, J., and Marshak, M., 2009b, Field Trip 3 – Soudan Iron Mine and Physics Lab Tour: 55th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 55, Part 2 – Field Trip Guidebook, p. 100-109. Peterson, D.M. and Severson, M.J., 2002, Chapter 4, Archean and Paleoproterozoic rocks forming the footwall of the Duluth Complex, in Geology and mineral potential of the Duluth Complex and related intrusions of northeastern Minnesota, Minnesota Geological Survey, Report of Investigations 58, pp. 76-93. Phinney, W.C., 1969, The Duluth Complex in the Gabbro Lake quadrangle, Minnesota: Minnesota Geological Survey, Report of Investigations 9, 20 p. Phinney, W.C., 1972, Duluth Complex, history and nomenclature, in Sims, P. K., and Morey, G. B., eds., Geology of Minnesota: A Centennial Volume: Minn. Geol. Survey, pp. 333-334. Phinney, W.C., 1972, Northwestern part of Duluth Complex, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 335-345. Ramsey, J.G., and Huber, M.I., 1987, The Techniques of Modern Structural Geology, Academic Press Inc. (London) Ltd. Ripley, E.M., 1986, Origin and concentration mechanisms of copper and nickel in Duluth Complex sulfide zones – a dilemma: Economic Geology, v. 81, p. 974-978. Schmid, R., 1981, Descriptive nomenclature and classification of pyroclastic deposits and fragments; recommendations of the IUGS subcommission on the systematics of igneous rocks: Geology, v. 9, p. 41-43. Schulz, K.J., 1980, The magmatic evolution of the Vermilion greenstone belt, NE Minnesota: Precambrian Research 11:215-245. Severson, M.J., 1994, Igneous stratigraphy of the South Kawishiwi intrusion, Duluth Complex, northeastern Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR93/34, 210 p., 15 plates. Severson, M.J., and Hauck, S.A., 2003, Platinum-group elements (PGEs) and platinum-group minerals (PGMs) in the Duluth Complex: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report, NRRI/TR-2003/37, 296 p., 1 CD. Severson, M.J., Heine, J.J., and Patelke, M.M., 2009, Geologic and Stratigraphic Controls of the Biwabik Iron Formation and the Aggregate Potential of the Mesabi Iron Range, Minnesota: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR- 2009/09, 173 p. + 37 plates. Sims, P. K., and Southwick, D. L., 1985, Geologic map of Archean rocks, western Vermilion district, northern Minnesota: U. S. Geological Survey, Miscellaneous Investigations Map I-1527, scale 1:48,000. Southwick, D. L., (compiler), 1993, Bedrock geologic map of the Soudan-Bigfork area, northern Minnesota: Minnesota Geological Survey, Miscellaneous Map M-79, scale 1:100,000. Southwick, D. L., Boerboom, T. J., and Jirsa, M. A., 1998, Geologic setting and descriptive geochemistry of Archean supracrustal and hypabyssal rocks, Soudan-Bigfork area, northern Minnesota: implications for metallic mineral exploration: Minnesota Geological Survey, Report of Investigations 51, 69 p. Stott, G., Corkery, T., Leclair, A., Boily, M., and Percival, J., 2007, A revised terrane map for the Superior Province as interpreted from Aeromagnetic Data: 53rd Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 53, Part 1 – Program and Abstracts, p. 74-76. Thompson, A., 2015, A hydrothermal model for metasomatism of Neoarchean Algoma-type banded iron formation to massive hematite ore at the Soudan Mine, NE Minnesota: unpublished M. S., thesis, University of Minnesota Duluth, 59 p. Vislova, T., 2003, Petrology of the Bald Eagle intrusion and associated rocks and its relevance to crystallization in dynamic magma chambers in the Midcontinent Rift: Unpublished Ph.D. Thesis, University of Minnesota. 186 �Trip 7 – Classic Outcrops Weiblen, P.W., 1965, A funnel-shaped, gabbro-troctolite intrusion in the Duluth Complex, Lake County Minnesota: Unpublished Ph.D. Thesis, University of Minnesota, 161 p. Weiblen, P.W., Morey, G. B., 1980, A summary of the stratigraphy, petrology, and structure of the Duluth Complex: American Journal of Science, vol. 280A, Part I, p 88-133. Weiblen, P.W., Peterson, D.M., and Vislova, T., 2005, Implications of Midcontinent Rift and oceanic ridges analogies and 3-D interpretations of the subsurface structure of the Bald Eagle intrusion in the Duluth Complex and the East Pacific Rise: Institute on Lake Superior Geology, 51st Annual Meeting, Sault Ste Marie, Ontario, v. 51, 3 p. White, C., 2010, The Nokomis Deposit, a Masters of Geology thesis: University of Minnesota, Duluth. White, J. D. L., and Houghton, B. F., 2006, Primary volcaniclastic rocks: Geology, v. 34, no. 8, p. 677-680. Wolff, J.F., 1917, Recent geologic developments on the Mesabi range, Minnesota: American Institute of Mining and Metallurgical Engineers Transactions, v. 56, p. 142-169. Zanko, L.M., Severson, M.J., and Ripley, E.M., 1994, Geology and mineralization of the Serpentine copper-nickel deposit: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR93/52, 90 p., 3 pls. 187 �Trip 8 – Glacial FIELD TRIP 8 Glacial Lake Norwood and the Koochiching Lobe Phil Larson1, Andrew Breckinridge2, and Howard Mooers3 1 Vesterheim Geoscience PLC Natural Sciences Department, University of Wisconsin Superior, 202 Barstow Hall, Superior, WI 54880 3 Department of Earth and Environmental Sciences, University of Minnesota Duluth, 1114 Kirby Drive, Duluth, MN 55812 2 Introduction The region north of the Giants Range is draped by sediment deposited during the final retreat of the Laurentide ice sheet from northeastern Minnesota. These sediments record the retreat of the Rainy Lobe ice margin to the northeast, the formation of Glacial Lake Norwood (GLN), two successive advances of the Koochiching Lobe from the northwest, and the opening of a western outlet of GLN and its succession by Glacial Lake Agassiz, all over the span of a few thousand years. Historically, the Quaternary geology of this region has received scant attention. However, recent work integrating varve chronology, high resolution LiDAR digital terrain models, till geochemistry, rotasonic drilling, and mapping has resulted in substantially improved and nuanced understanding of the sedimentary processes active, and the sequence of events, during deglaciation. A key finding is that GLN was of significantly longer duration than previously believed, and consequently a stronger control on sediment and landform distribution in the region. Within the footprint of GLN, there is scant evidence for preservation of glacigenic sediments predating the Late Wisconsinan. Interbedded till and glaciolacustrine sediment thicknesses up to 70 m thick preserve evidence of extremely high sedimentation rates in a dynamic sediment system. High rates of sediment delivery by the Koochiching Lobe and analogues from the west served as the dominant sediment source, while intense reworking by wave action in GLN was a dominant control on sediment distribution. Historical Background The earliest formal studies of glacial deposits in northeastern Minnesota were conducted by Upham (1894) who identified a series of moraines across Minnesota. He identified the Vermillion moraine as the 12th moraine in the deglaciation sequence, although he did not define its entire length. Elftman (1898) suggested two lobes for the northeastern portion of Minnesota because of observed till differences and provenances; he named these the Superior and Rainy lobes; the Rainy lobe referring to the ice flowing from the Rainy River areaWinchell (1899) compiled Upham, Elftman, and his own observations into a map of large portions of northeastern Minnesota and description of the surficial deposits. Winchell (1900) described evidence for glacial lakes in Minnesota, including naming Glacial Lake Norwood. Notably, he did not recognize the full extent of Glacial Lake Norwood, assigning portions of the Norwood basin to other glacial lakes. Leverett (1932), based mostly on the work of his predecessors, proposed that northeastern Minnesota was glaciated by three separate lobes of ice. He recognized that the earliest drift in the area was the result of ice flowing from the Patrician [Labradoran] ice center located in the Hudson Bay Lowlands between the Keewatin and Labradorean ice accumulation centers. 188 �Trip 8 – Glacial Figure 8-1. Extent of Glacial Lake Norwood (light green-blue). The lack of modern lakes (blue) in the GLN basin highlight area of significant glaciolacustrine sediment thickness. Major Rainy lobe recessional moraines (brown lines). Proglacial Lake Northofnashwauk is the high-level (elev. >1500’) proglacial lake predating GLN dammed by the St. Louis sublobe. Modern understanding of the surficial geology of northeastern Minnesota began with Wright Wright (1956), who was the first to conduct systematic fieldwork in the area between the border lakes and Lake Superior. Wright and Watts (1969) reconstructed the postglacial vegetational history of northeastern Minnesota, and established the first regional deglaciation chronology, including use of radiocarbon dates to establish absolute ages to deglaciation. These early efforts were summarized in the comprehensive general glacial geologic framework of Minnesota Wright (1972). The United States Geological Survey conducted a comprehensive study of surficial geology and groundwater availability on the Mesabi Iron Range. An initial map (Cotter, Young, and Winter 1964) was later followed by additional publications on the glaciation sequence (Winter 1971) glacial sediment composition (Winter, Cotter, and Young 1973), and groundwater hydrology in glacial drift-hosted aquifers (Winter 1973). Hobbs (1983) provided the first comprehensive account of Glacial Lake Norwood’s extent and history. At that time, he rechristened GLN as Glacial Lake Koochiching, not recognizing the continuity with Winchell’s (1900) definition of GLN. In this respect he was hampered by the paucity of well-defined strandlines for the upper levels of GLN in the rocky meltout till underlying much of the southern portion of the basin. He also posited a late, lower elevation outlet for Glacial Lake Koochiching southward along the Prairie River; recent (2012) LiDAR elevation data (MNDNR 2012) combined with a better defined isostatic rebound reconstruction (Breckenridge 2015) suggest the existence of a southern outlet to GLN untenable. Significantly, Hobbs recognized that Glacial Lake Norwood expanded westward, ultimately establishing an outlet via the McIntosh Channel into Glacial Lake Climax. Continued retreat of the Red River lobe ice margin resulted in coalescence of Glacial Lake Climax and GLN into Glacial Lake Agassiz at the Herman level at 13.9±0.3 cal kyr BP (Lepper et al. 2007). 189 �Trip 8 – Glacial Björck (1990) expanded Wright and Watts (1969) pioneering work by collecting radiocarbon dates north of the Giants Range. He obtained basal radiocarbon dates from Sabin Lake (located in the outflow to GLN) of 10,230±230 and 10,320±170 14C kyr BP. Bjorck’s oldest date was 12,100±150 14C kyr BP from Heikkila Lake, located within the Big Rice moraine. Lowell et al. (2009) reported a radiocarbon date from north of the Vermilion moraine of 12,000±85 14C kyr BP, assigning a minimum age to the Vermilion phase and the moraines to the south of 13.9±0.2 cal kyr BP. These dates establish that Glacial Lake Norwood and drainage through the Embarrass Gap persisted long after the Laurentide ice sheet margin retreated from the Vermilion moraine. Johnson et al. (2016) assigned the glacigenic deposits in northeastern Minnesota to a formal statewide lithostratigraphic framework. Essentially all the aforementioned published work was opened to critical re-examination and revision upon release of 1m resolution LiDAR-derived digital terrain models in the spring of 2012 (MDNR 2012). This data provides resolution orders of magnitude greater than previous topographic models, allowing for vastly improved recognition of some classes of glacigenic landforms, and recognition for the first time of entire new classes of landforms. These advances allowed for significant refinement in mapping of glacial landforms, interpretation of sediment-landform relationships, and development of deglaciation process models. Breckenridge (2015) mapped glacial lake strandlines and developed an isostatic rebound model for much of northern Minnesota, including much of the Glacial Lake Norwood basin, demonstrating the previously unrecognized widespread extent of both Glacial Lake Norwood and the early, high levels of Glacial Lake Agassiz. Bauer et al. (2022) published a surficial geologic map and Quaternary stratigraphic interpretation of much of the GLN basin, relying primarily on the lithostratigraphic mapping approach favored by the Minnesota Geological Survey, but also incorporating landform interpretation based on the 2012 LiDAR data. Glacial History Northeastern Minnesota was continuously covered by ice from the earliest Late Wisconsin ice advance approximately 28 kyr bp until about 11 kyr bp by the Rainy lobe of the Laurentide ice sheet (Clayton and Moran 1982); (Mooers and Lehr 1997)). Although the Glacial Lake Norwood basin was subjected to multiple glacial cycles, the vast majority of glacigenic sediment was deposited during the last retreat of the Laurentide ice sheet during the Late Wisconsinan (<15 kyr bp). The Pleistocene stratigraphic record therefore principally reflects retreat of the ice sheet, and is composed of glacigenic sediment deposited at or near the ice margin. Bedrock Geology and Preglacial Regolith The GLN basin underlain by greenstone (metavolcanic and metasedimentary rocks) and granitoids of the ~2.7 Ga Wawa-Shebandowan Subprovince. The craton was intruded by mafic intrusives of the 2076 Ma Kenora-Kabetogama dike swarm (Southwick and Halls 1987; Buchan, Halls, and Mortensen 1996), while contact relationships indicate the Archean craton was peneplained by the time arenites, ironformation, greywacke, and argillite of ~1.85 Ga Animikie Basin were deposited to the south. Minor mafic dikes related to the 1.1 Ga Midcontinent Rift are known to intrude the Archean craton north of the Giants Range; it is probable that additional similar intrusives have yet to be recognized or mapped. Subsequent to cessation of the Midcontinent Rift, Precambrian bedrock in the GLN basin was subject to a nearly 1 billion year period of chemical weathering and saprolite formation. Saprolite formation was preferentially, but not necessarily, focused along joints, faults, and less weathering-resistant lithologies, forming deep linear weathering pendants beneath a more widespread blanket of saprolite. Commencement of glaciation at the beginning of the Pleistocene ~3 Ma subjected the Superior craton to significant physical erosion for the first time in nearly a billion years. Successive glacial cycles 190 �Trip 8 – Glacial preferentially eroded unconsolidated saprolite, removing first the extensive saprolite blanket, and then excavating saprolite from deep weathering pendants. To the southwest of the GLN basin (central Minnesota), the preglacial saprolite is largely intact beneath Pleistocene glacigenic sediment. To the northeast (northwestern Ontario), preglacial saprolite has been essentially completely removed. Here, the rugged ‘glacially sculpted’ shield terrain characteristic of this region is better explained as the unweathered bedrock surface forming the base of the preglacial saprolite; bedrock has undergone relatively little actual glacial weathering. In the GLN basin proper, preglacial saprolite removal by glacial erosion is incomplete, leaving patchy remnants of unconsolidated preglacial saprolite on the bedrock surface. Saprolite is occasionally intercepted in boreholes. Saprolite and incipient pendant weathering have been encountered associated with joints and fractures as deep as 100 m. Pre-Late Wisconsinan The overlying preglacial saprolite was removed by repeated cycles of erosion and deposition during the Pleistocene. Saprolite eroded as the ice sheet grew (relative early in a glacial cycle) was transported to the margin. The remnant saprolite was blanketed by glacigenic sediment as the ice margin receded (late in the glacial cycle. Subsequent glacial cycles removed both the older glacigenic sediment and additional saprolite. Winter (1971) and Winter, Cotter, and Young (1973) described a dark-colored, sandy-silty calcareous till in exposures in open pit mines on the Mesabi Iron Range. Since this till, where present, occurred immediately above bedrock, they referred to it as the “basal till”. Stark (1977) and (Lehr and Hobbs 1992) described occurrences of Winter’s basal till in exposures in the Dunka Mine. The matrix of Winter’s basal till is calcareous, and the pebble fraction contains carbonate clasts in addition to the granitic and metamorphic lithologies typical of Rainy lobe tills. A northeast-southwest pebble fabric in Winter’s basal till strongly supports a northeastern provenance for this till, indicating the carbonate in pebbles and till matrix is derived from Paleozoic carbonates in the Hudson Bay Lowlands (HBL). A distinctive greywacke lithology (Prest, Donaldson, and Mooers 2000) associated with carbonate-bearing tills has been recovered from glacigenic sediments north of the Giants Range (this author), indicating older carbonatebearing glacigenic sediment was actively reworked during the last retreat of the Laurentide ice sheet. Additional occurrences of this calcareous basal till have been intercepted in boreholes elsewhere in the GLN basin, indicating patchy remnants of preglacial saprolite and older (carbonate-bearing) glacigenic sediment are present beneath the relatively continuous blanket of glacigenic sediment deposited between ca. 15 kyr bp and 10 kyr bp during the last retreat of the Laurentide ice sheet. Post-Last Glacial Maximum – Rainy Lobe Recession of the Laurentide ice sheet margin following its last glacial maximum extent at ca. 20 kyr bp was characterized by rapid melting of ice during summer months followed by stabilization and minor re-advance during the winter. This process formed a series of small, annual recessional moraines, spaced 25-75 m apart, reflecting the long-term retreat rate of the ice sheet. To a significant degree, glacigenic sediment deposited by the Rainy lobe of the Laurentide ice sheet during retreat of its margin from the southwest to northeast is the oldest Pleistocene sediment preserved in the GLN basin. Post-LGM Rainy lobe sediments are typically comprised predominantly of sediment eroded locally from Archean greenstone and granitoid lithologies; this results in significant lithologic and geochemical compositional variability(Larson, 2004; Larson & Mooers, 2004). Lodgment tills are commonly ~2 m thick, while sand and gravel deposited in subaqueous recessional moraines commonly form sharp-crested ridges 5-40 m thick. Distal glaciolacustrine sand and silt commonly drapes older basal lodgment tills and recessional moraines. 191 �Trip 8 – Glacial This process of gradual ice margin retreat was punctuated by surges, episodes of major re-advance and stagnation. These surges resulted in deposition of moraines significantly broader and thicker than annual recessional moraines. The surges may not reflect re-advance of the ice sheet as a whole, but were likely restricted to sectors of the ice margin on the order of 100s of km. They may therefore not be a direct physical reflection of climate fluctuations, but rather reflect internal ice sheet dynamics. Most of the area exposed by ice margin retreat from the Giants Range was inundated in proglacial lakes, successively by Glacial Lake Nashwauk, Glacial Lake Norwood, and finally by Glacial Lake Agassiz. The extended interval between ice margin retreat, lake drainage, and establishment of terrestrial vegetation over most of this area significantly hinders the ability to establish a precise deglaciation chronology (compare Björck (1990)). Allen Phase The oldest major surge-stagnation moraine recognized in the GLN basin is the Allen moraine, which forms a WNW-ESE trending belt of stagnation topography (ice-walled lake plains, meltout tills, etc.), passing through the Embarrass Gap. Ice flow during the Allen phase was generally toward the SSW (bearing 190°). Ice margin retreat from the Allen moraine and opening of meltwater drainage through the Embarrass Gap was the event that by definition resulted in formation of Glacial Lake Norwood. Further ice margin recession and deposition of annual recessional moraines suggests about 150 years before the next major surge-stagnation event. Big Rice and Wahlsten Phases The second major surge-stagnation moraine recognized in the GLN basin is the Big Rice moraine, a W-E trending belt of thick meltout till and stagnation topography. Ice flow during the Big Rice phase was generally toward the SSW (bearing 190°). The third major moraine recognized in the TMM AOI is the Wahlsten moraine, an E-W trending belt of thick meltout till and stagnant ice topography. Ice flow during the Wampus phase was generally toward the S (bearing 180-190°), reflecting a significant reorientation in ice flow of the Laurentide ice sheet. Annual recessional moraines associated with the Wampus and Wahlsten phases consist of both subaqueous moraines composed of sand, gravel, and meltout tills deposited in Glacial Lake Norwood, and subaerial moraines predominantly composed of meltout tills. Vermilion Phase The fourth and final major moraine recognized in the GLN basin is the Vermilion moraine, a 40 m high, 1-2 km wide, WNW-ESE trending belt of thicker meltout till, stagnant ice topography, and subaqueous debris flow fans. Ice flow during the Vermilion phase was generally toward the SSW (bearing 195-205°). The Vermilion moraine truncates the eastern extent of the Wahlsten moraine, reflecting a further significant reorientation in ice flow of the Laurentide ice sheet. The next moraine formed by a major surgestagnation event lies >100 km to the northeast, suggesting an interval of >1000 years of gradual ice margin retreat after the Vermilion phase. Glacial Lake Norwood Retreat of the Rainy lobe margin north of the continental height of land at the Giants Ridge dramatically changed the character of sedimentation associated with the Laurentide ice sheet. South of the divide, meltwater generally flowed downslope away from the margin, depositing outwash in channels and as outwash plains with intervening rolling plains of subglacial lodgment till or moraines composed of hummocky supraglacial meltout till. Immediately upon marginal retreat north of the divide, ponding of meltwater against the ice sheet formed the first of a nearly continuous succession of proglacial lakes. Glacial 192 �Trip 8 – Glacial meltwater and other precipitation ponded against the ice sheet overflowed to the south through a series of successively lower outlets over the height of land, a process that continued until final collapse of the ice sheet in Hudson Bay. Initially, a series of ephemeral lakes formed in stagnant ice north of the divide. These lakes were dammed in part by the advance of the St. Louis Sublobe into the Glacial Lake Upham I basin (see Knaeble et al. (2005) and Larson et al. (2014)) Associated strandlines and meltwater channels are only poorly defined, and meltwater likely drained southward through stagnant Rainy lobe ice karst and St. Louis sublobe ice. Once the active Rainy lobe ice margin receded to the Allen moraine, a stable, relatively long-lived meltwater outlet was established through the Embarrass Gap. By definition, the first proglacial lake located north of the Laurentian Divide that drained through the Embarrass Gap is referred to as Glacial Lake Norwood. Three well-developed outlets to Glacial Lake Norwood are recognized, corresponding to relatively stable, long-lived lake levels. These are herein referred to as Glacial Lakes Norwood I, II, and III, corresponding to successively older and lower lake levels. The initial stable lake level (Glacial Lake Norwood I) was controlled by an outlet channel with a modern floor elevation of about 450 m amsl. This channel was bounded by the Giants Range ridge to the south, and the Allen moraine to the north. The Allen moraine at this location is a major recessional moraine, approximately 500 m wide with in excess of 15 m of vertical relief above the meltwater channel. The ice-cored Allen moraine formed an effective barrier to meltwater drainage blocking most of the Embarrass Gap until after the ice sheet margin retreated from the Vermilion moraine, a time interval of 100s to 1000s of years. Incision of the Glacial Lake Norwood I outlet was inhibited during this time interval in part because the channel was graded to its downstream inlet into Glacial Lake Upham II; only after drainage of this lake was further significant erosion and channel incision in the Embarrass Gap initiated (Larson and Mooers 2009). Gradual collapse of the Allen moraine due to ice melt led to resulted in an episode of collapse and downcutting of the moraine dam, and establishment of a second, lower stable outlet level for Glacial Lake Norwood II in the Embarrass Gap at a modern floor elevation of about 443 m amsl. Paleoislands of outwash and esker sediment located in Glacial Lake Norwood considerable distances north of the Vermilion moraine display well-developed shoreline features corresponding to this outlet, indicating that the downcutting episode occurred well after ice margin retreat from the Vermilion moraine, and that Glacial Lake Norwood II stood at this stable lake level for a relatively long time interval. A second collapse and downcutting episode through the Big Rice moraine led to establishment of the third, and final, lower stable outlet level corresponding to Glacial Lake Norwood III in the Embarrass Gap. An outlet with a modern floor elevation of about 433 m amsl corresponds to a second, lower welldeveloped strandline on esker and outwash paleoislands to the north. During its relatively long history, Glacial Lake Norwood expanded along the receding ice margin to form a lake that ultimately extended ~400 km E-W and in excess of 100 km N-S. The lake experienced two major ice re-advances into its western arm, evidenced by thick (>70 m) accumulations of glaciolacustrine sediment and till. The large fetch of the lake resulted in vigorous wave erosion along its shoreline and the considerable fraction of the lakebed situated above wave base. Final drainage of Glacial Lake Norwood III occurred when a western outlet (the McIntosh spillway) flowing into an early (Herman) level of Glacial Lake Agassiz formed in the vicinity of Trail, MN, 260 km to the west of the Embarrass Gap. 193 �Trip 8 – Glacial Figure 8-2. Outline of maximum extent of Glacial Lake Norwood in northern Minnesota. The lake extended over 350 km from east to west. The final outflow was westward into Glacial Lake Climax near Trail, MN. Glacial Lake Norwood sediments generally consist of gravels and sands in littoral (shallow) environments, reflecting local reworking of till and outwash by wave action, and silt and clay in benthic environments, reflecting settling of suspended fine-grained sediment from the water column. In general, Glacial Lake Norwood sediment sequences fine upward, reflecting diminished wave erosion and the increasing distance of the primary sediment source (the receding ice margin). Koochiching Lobe Subsequent to retreat of the Rainy lobe from the Vermilion moraine, Koochiching lobe (KL) ice re-advanced into the Glacial Lake Norwood basin, this time from the west and the Red River lobe (Meyer, 1993). In marked contrast to the sandy-textured till and glaciofluvial sediment associated with the Rainy lobe, KL diamicton is calcareous, and distinctly finer grained than Rainy lobe till; these sediments are placed in the Blackduck Formation in the MGS lithostratigraphic framework (Johnson et al., 2016). Based on rotosonic drilling, diamictons associated with at least two distinct advances into the GLN basin are present in the field trip area, separated by fine-grained glaciolacustrine sediment. The genesis of these diamictons – till or subaqueous debris flow – are enigmatic; fine-grained lacustrine sediment may grade upward into normally consolidated diamicton, which may grade upward into overconsolidated diamicton of similar composition. The Koochiching lobe advances overran older Rainy lobe landforms, including the Vermilion moraine. There is little evidence for erosion and entrainment of older glacigenic sediment by the KL, and no well-defined moraines or other landforms define the limits of the advances. Sediment was deposited from suspended sediment plumes or debris flows in the proglacial GLN, or as subglacial lodgment till. Although KL ice thickness is unknown, it was sufficiently thick relative to the depth of GLN to preclude development of a calving margin. 194 �Trip 8 – Glacial The majority of sediment shed by the advancing Koochiching lobe was deposited as glaciolacustrine sediment in GLN, and subject to a high degree of reworking in the lacustrine environment. Bedrock highs – shallow areas in GLN – are typically devoid of either older Rainy lobe or KL sediments. In places, a thin boulder lag containing limestone and dolomite clasts attests to the former presence of KL diamicton. In contrast, thicknesses of up to 70 m of till and glaciolacustrine sediment have been reported in intervening bedrock lows. Two distinct till compositions attesting to two distinct source areas have been reported in KL sediments. The younger, overlapping KL till bears greater similarity to calcareous Red River and Des Moines lobe tills elsewhere in Minnesota. In contrast, an older KL till is characterized by a distinctly higher Na2O content, similar to tills exposed at surface in Hubbard and Wadena Counties. Even as the Laurentide ice sheet margin was broadly retreating from Minnesota, both from the Red River Valley and from the Arrowhead, the advances of the Koochiching lobe into the GLN served to block development of meltwater outlets to the north and west. Ultimately, stagnation and wasting of the KL led to westward propagation of GLN until development of the McIntosh spillway. The massive sediment accumulations associated with the KL – up to 70 m in places as previously noted – were deposited over a time interval on the order of 1000 years. Description of Field Trip Stops Stop 1: Glacial Lake Norwood strandline 498550E/5283740N (UTM Zone 15, NAD83) (47.70704, -93.0193) Side Lake 7.5’ USGS Quadrangle This site is located on the uppermost relatively well-developed beach associated with Glacial Lake Norwood. A well-developed boulder lag and wave-cut notch attest to a relatively long-lived stable lake at this level characterized by energetic wave action. To the south, ice collapse pits in the subaqueous deposited Big Rice moraine evidence long lived stagnant ice along this moraine trend. Locally, the Big Rice and other moraines served as ice-cored dams preventing southern outflow. Stop 2: Gravel pit in minor Rainy lobe recessional moraine 498480E/5294980N (UTM Zone 15, NAD83) (47.80817, -93.0203014) Bear River 7.5’ USGS Quadrangle Here a small, sharp-crested subaqueous deposited recessional moraine has been developed into a gravel pit. The flanks of the moraine are draped by fine-grained glaciolacustrine sediment deposited in Glacial Lake Norwood. 195 �Trip 8 – Glacial Stop 3: Gravel pit in large Rainy lobe recessional moraine 495260E/5302290N (UTM Zone 15, NAD83) (47.8739285, -93.0633884) Bear River 7.5’ USGS Quadrangle This gravel pit is developed in a large subaqueous ice marginal fan(?) deposited at the margin of the retreating Rainy lobe. The fan was of sufficient height that its surface was above the GLN wave base, precluding deposition of finer-grained glaciolacustrine sediment. The presence of limestone and dolomite boulders on the fan surface indicate that this area was overrun by Koochiching lobe ice. Stop 4: Wave-washed bedrock high 492880E/5301180N (UTM Zone 15, NAD83) (47.8639194, -93.095198) Bear River 7.5’ USGS Quadrangle This wave-scoured bedrock high evidences the intensity of wave action in Glacial Lake Norwood. Rainy lobe sediment has been almost completely washed away, no Koochiching lobe sediment is preserved, and no glaciolacustrine sediment has been deposited. The very large boulder – a Rainy lobe erratic - attests to the ‘minimum’ particle size of this ‘boulder lag’. Stop 5: Glacial striae and grooves 489180E/5304010N (UTM Zone 15, NAD83) (47.8893303, -93.1447397) Rauch 7.5’ USGS Quadrangle Glacial striae and grooves on outcrop on either side of the road at this stop preserve evidence of ice flow directions for both the Rainy lobe (bearing 190° and 205°) and the later Koochiching lobe (bearing 140°). This indicates that Rainy lobe sediment was largely stripped from bedrock highs by wave action in Glacial Lake Norwood prior to advance of the Koochiching lobe from the west. 196 �Trip 8 – Glacial Stop 6: Borrow pit in reworked calcareous Koochiching lobe drift 490750E/5305460N (UTM Zone 15, NAD83) (47.9024009, -93.1237689) Silverdale 7.5’ USGS Quadrangle This small borrow pit on the margin of a wave-scoured bedrock high contains abundant carbonate pebbles and cobbles. These originated from calcareous Koochiching lobe till deposited on the bedrock high and later eroded by wave action. Stop 7: Slumping Koochiching lobe till and Glacial Lake Norwood 491460E/5311020N (UTM Zone 15, NAD83) (47.9524357, -93.114379) Silverdale 7.5’ USGS Quadrangle This site exposes a sequence of interbedded Koochiching lobe diamicton (till and debris flows(?)) and fine-grained glaciolacustrine sediment adjacent to the Littlefork River. The slope, already prone to slumping by stream erosion at the toe, was further destabilized by construction of the road. In the near vicinity to the southwest, an exploration rotosonic borehole intercepted around 70 m of such sediment. Stop 8: Samuelson Park 492580E/5310600N (UTM Zone 15, NAD83) (47.9459716, -93.0993661) Silverdale 7.5’ USGS Quadrangle Bedrock underlying the small waterfall in the Littlefork River has been striated by the Rainy lobe (bearing 196°). In the upstream direction, boulders eroded from the basal Rainy lobe lodgment till are visible in the stream bed and banks. Such bedrock and boulder lags serve as knickpoints defining the bed of the Littlefork River; steep and commonly slumping slopes adjacent to the river attest to the significant erosion of Koochiching lobe and Glacial Lake Norwood sediment during the Holocene. 197 �Trip 8 – Glacial Stop 9: Embarrass Gap 9A: 551980/5270340N (UTM Zone 15, NAD83) (47.583892, -92.3087077) 9B: 552150/5272700N (UTM Zone 15, NAD83) (47.6056089, -92.3061663) 9C: 552760/5272950N (UTM Zone 15, NAD83) (47.6078088, -92.2980211) Biwabik 7.5’ USGS Quadrangle These three stops are in the three successive major outlet channels for Glacial Lake Norwood. Stop 9A (elevation 450 m) is in a meltwater channel developed at the margin of the Rainy lobe, perhaps against an active ice margin. Stops 9B (elevation 443 m) and 9C (elevation 433 m) are two successively lower major outlets formed as the ice-cored Allen moraine collapsed over a time interval on the order of 1000 years. The outlet at 9C served as the stable outlet to Glacial Lake Norwood until opening of its final lower outlet to the west, through the McIntosh spillway in the vicinity of Trail, Minnesota. 198 �Trip 8 – Glacial REFERENCES Bauer, Emily J., Mark A. Jirsa, Amy Radakovich Block, Terrence J. Boerboom, Val W. Chandler, Dean M Peterson, Kaleb G. Wagner, Elizabeth L. McDonald, Jennifer M. Dengler, Gary N. Meyer, and Jacqueline D. Hamilton. 2022. “Geologic Atlas of St. Louis County, Minnesota.” Minnesota Geological Survey County Atlas Series C–51. Björck, Svante. 1990. “Late Wisconsin History North of the Giants Range, Northern Minnesota, Inferred from Complex Stratigraphy.” Quaternary Research 33:18–36. Breckenridge, Andrew J. 2015. “The Tintah-Campbell Gap and Implications for Glacial Lake Agassiz Drainage during the Younger Dryas Cold Interval.” Quaternary Science Reviews 117:124–34. https://doi.org/10.1016/j.quascirev.2015.04.009. Buchan, Kenneth L., Henry C. Halls, and James K. Mortensen. 1996. “Paleomagnetism, U-Pb Geochronology, and Geochemistry of Marathon Dykes, Superior Province, and Comparison with the Fort Frances Swarm.” Canadian Journal of Earth Sciences 33:1583–95. Clayton, Lee, and Stephen R. Moran. 1982. “Chronology of Late Wisconsinan Glaciation in Middle North America.” Quaternary Science Reviews 1:55–82. Cotter, Ralph D., H.L. Young, and Thomas C. Winter. 1964. “Preliminary Surficial Geologic Map of the Mesabi-Vermilion Iron Range Area, Minnesota.” USGS Miscellaneous Geologic Investigations Map I-403. Elftman, A.H. 1898. “The Geology of the Keweenawan Area in Northeastern Minnesota, Part I.” The American Geologist 21:90–109. Hobbs, Howard C. 1983. “Drainage Relationships of Glacial Lakes Aitkin and Upham and Early Lake Agassiz in Northeastern Minnesota.” Edited by James T. Teller and Lee Clayton. Geological Association of Canada Special Paper 26:245–59. Johnson, Mark D., Roberta S. Adams, Angela S. Gowan, Kenneth L. Harris, Howard C. Hobbs, Carrie E. Jennings, Alan R. Knaeble, Barbara A. Lusardi, and Gary N. Meyer. 2016. “Quaternary Lithostratigraphic Units of Minnesota.” Minnesota Geological Survey Report of Investigations 68:262. Knaeble, Alan R., Gary N. Meyer, Lisa M. Marlow, Phillip C. Larson, and Howard D. Mooers. 2005. “Deposits and Landforms in the Region Glaciated by the St. Louis Sublobe.” In Field Trip Guidebook for Selected Geology in Minnesota and Wisconsin, edited by Lori Robinson, Guidebook, 40–79. Minneapolis: Minnesota Geological Survey. Larson, Phillip C., Alan R. Knaeble, Howard D. Mooers, and Lisa M. Marlow. 2014. “The St. Louis Sublobe and Glacial Lake Upham.” Institute on Lake Superior Geology Field Trip Guidebook 60:102–18. Larson, Phillip C., and Howard D. Mooers. 2009. “Glacial Geology of the Vermilion Moraine.” Institute on Lake Superior Geology Field Trip Guidebook 55 (2): 81–99. Lehr, James D., and Howard C. Hobbs. 1992. “Glacial Geology of the Laurentian Divide Area, St. Louis and Lake Counties, Minnesota.” Minnesota Geological Survey Guidebook 18:82. Lepper, Kenneth, Timothy G. Fisher, Irka Hajdas, and Thomas V. Lowell. 2007. “Ages for the Big Stone Moraine and the Oldest Beaches of Glacial Lake Agassiz : Implications for Deglaciation Chronology.” Geology 35 (7): 667–70. https://doi.org/10.1130/G23665A.1. Leverett, Frank. 1932. “Quaternary Geology of Minnesota and Parts of Adjacent States.” USGS Professional Paper 161:149. 199 �Trip 8 – Glacial Lowell, Thomas V., Timothy G. Fisher, Irka Hajdas, K. Glover, Henry M. Loope, and T. Henry. 2009. “Radiocarbon Deglaciation Chronology of the Thunder Bay, Ontario Area and Implications for Ice Sheet Retreat Patterns.” Quaternary Science Reviews 28 (17–18): 1597–1607. https://doi.org/10.1016/j.quascirev.2009.02.025. MNDNR. 2012. “LiDAR Elevation, Arrowhead Region, NE Minnesota, 2011.” Minnesota Department of Natural Resources. ftp://ftp.gisdata.mn.gov/pub/gdrs/data/pub/us_mn_state_mngeo/elev_lidar_arrowhead2011/metadata/meta data.html. Mooers, Howard D., and James D. Lehr. 1997. “Terrestrial Record of Laurentide Ice Sheet Reorganization during Heinrich Events.” Geology, no. 11, 987–90. Prest, Victor K., J. Allan Donaldson, and Howard D. Mooers. 2000. “The Omar Story: The Role of Omars in Assessing Glacial History of West-Central North America.” Géographie Physique et Quaternaire 54:257–70. Southwick, David L., and Henry C. Halls. 1987. “Compositional Characteristics of the Kenora-Kabetogama Dyke Swarm (Early Proterozoic), Minnesota and Ontario.” Canadian Journal of Earth Sciences 24:2197– 2205. Stark, James R. 1977. “Surficial Geology and Ground-Water Geology of the Babbitt-Kawishiwi Area, Northeastern Minnesota with Planning Implications.” M.S. Thesis. M.S. Thesis, University of Wisconsin. Upham, Warren. 1894. “Preliminary Report of the Field Work during 1893 in Northeastern Minnesota, Chiefly Relating to the Glacial Drift.” Geological and Natural History Survey of Minnesota Annual Report 22:18–86. Winchell, Newton H. 1899. “The Geology of the North Part of St. Louis County.” Geological and Natural History Survey of Minnesota 4:222–65. ———. 1900. “Glacial Lakes of Minnesota.” Geological Society of America Bulletin 12:109–28. Winter, Thomas C. 1971. “Sequence of Glaciation in the Mesabi-Vermilion Iron Range Area, Northeastern Minnesota.” USGS Professional Paper 750–C:C82–88. ———. 1973. “Hydrogeology of Glacial Drift, Mesabi Iron Range, Northeastern Minnesota.” USGS Water Supply Paper 2029-A:31. Winter, Thomas C., Ralph D. Cotter, and H.L. Young. 1973. “Petrography and Stratigraphy of Glacial Drift, Iron Range Area, Northeastern Minnesota.” USGS Bulletin 1331–C:50. Wright, Herbert E. 1956. “Sequence of Glaciation in Eastern Minnesota.” Geological Society of America Guidebook 3:1–24. ———. 1972. “Quaternary History of Minnesota.” In Geology of Minnesota: A Centennial Volume, edited by Paul K. Sims and G.B. Morey, 515–47. St. Paul, Minnesota. Wright, Herbert E., and William A. Watts. 1969. “Glacial and Vegetational History of Northeastern Minnesota.” Minnesota Geological Survey Special Publication 11. 200 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2025 Description An account of the resource Institute on Lake Superior Geology. Mountain Iron, Minnesota. May 14-17, 2025. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2025-05-14 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English Type The nature or genre of the resource Text https://digitalcollections.lakeheadu.ca/files/original/72cc0c183e5d4fc8ab0811c66edf140d.pdf 7527b93a85865027caf582e7cb0434fb PDF Text Text 72nd Annual Meeting Thunder Bay, Ontario - May 21-22, 2026 Institute on Lake Superior Geology Part 1 – Program and Abstracts �Thank you to our sponsors! �65th Annual Meeting Institute on Lake Superior Geology May 21-22, 2026 Thunder Bay, Ontario HOSTED BY: Mark Puumala and Peter Hinz Co-Chairs Ontario Geological Survey (Retired) Proceedings - Volume 72 Part 1 – Program and Abstracts Compiled and edited by Pete Hollings & Mark Smyk Cover Photos: Top: Amethyst veins in Rossport Formation at the Blue Points Amethyst Mine, north of Highway 11-17 near Big Pearl Lake.Middle: Neoarchean mafic metavolcanic rocks, Highway 102 at the intersection with Mud Lake Road. Bottom: Corestones of the McKenzie Granite at the Archean-Paleoproterozic unconformity, Highway 11-17 near Crystal Beach. All photos courtesy Mark Puumala. �72nd Institute on Lake Superior Geology Volume 72 consists of: Part 1: Program and Abstracts Part 2: Field Trip Guidebook Trips 1 & 4: “Classic” Geological Sites in the Thunder Bay Area Trip 2: Geology of the Quetico Supprovince North of Thunder Bay Trip 3: Gold Deposits of the Shebandowan Greenstone Belt Trip 5: Structural Geology and Gold Mineralisation of the Mine Centre Area Trip 6: Amethyst Deposits of Thunder Bay Reference to material in Part 1 should follow the example below: Akin, K. and Swanson-Hysell, N., 2026. Constraining the 3-D Geometry of the Duluth Complex, MN, Using Magnetic Fabrics and Paleomagnetic Data. In; Hollings, P. and Smyk,, M., (Eds.), Institute on Lake Superior Geology Proceedings, 72nd Annual Meeting, Thunder Bay, Ontario, Part 1 - Abstracts and Proceedings. v.71, part 1, 1-2. Published by the 72nd Institute on Lake Superior Geology and distributed by the ILSG Secretary: Pete Hollings - ILSG Secretary Department of Geology Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Email: peter.hollings@lakeheadu.ca ILSG website: www.lakesuperiorgeology.org ISSN 1042-9964 �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Table of Contents Institutes on Lake Superior Geology, 1955-2026............................................................... ii Sam Goldich and the Goldich Medal................................................................................. iv Goldich Medal Guidelines................................................................................................. iv Institute on Lake Superior Geology Goldich Medal............................................................v Goldich Medalists.............................................................................................................. vi Sam Goldich and the Goldich Medal................................................................................ vii Goldich Medal Guidelines............................................................................................... viii Goldich Medal Committee ................................................................................................ ix 2026 Goldich Medal Recipient.......................................................................................... ix Citation for Goldich Medal Recipient..................................................................................x Honoring the Pioneers of Lake Superior Geology............................................................. xi In Memoria........................................................................................................................ xii Report of the Chair of the 71st Annual Meeting .............................................................. xvi Eisenbrey Student Travel Awards.................................................................................... xix Joe Mancuso Student Research Awards.............................................................................xx Doug Duskin Student Paper Awards..................................................................................xx 2026 Student Paper Awards Committee........................................................................... xxi Board of Directors............................................................................................................ xxi Local Committee.............................................................................................................. xxi Field Trip Leaders and Guidebook Authors.................................................................... xxii Index..................................................................................................................................88 -i- �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Institutes on Lake Superior Geology, 1955-2026 # Date Place Chairs 1 1955 Minneapolis, Minnesota C.E. Dutton 2 1956 Houghton, Michigan A.K. Snelgrove 3 1957 East Lansing, Michigan B.T. Sandefur 4 1958 Duluth, Minnesota R.W. Marsden 5 1959 Minneapolis, Minnesota G.M. Schwartz & C. Craddock 6 1960 Madison, Wisconsin E.N. Cameron 7 1961 Port Arthur, Ontario E.G. Pye 8 1962 Houghton, Michigan A.K. Snelgrove 9 1963 Duluth, Minnesota H. Lepp 10 1964 Ishpeming, Michigan A.T. Broderick 11 1965 St. Paul, Minnesota P.K. Sims & R.K. Hogberg 12 1966 Sault Ste. Marie, Michigan R.W. White 13 1967 East Lansing, Michigan W.J. Hinze 14 1968 Superior, Wisconsin A.B. Dickas 15 1969 Oshkosh, Wisconsin G.L. LaBerge 16 1970 Thunder Bay, Ontario M.W. Bartley & E. Mercy 17 1971 Duluth, Minnesota D.M. Davidson 18 1972 Houghton, Michigan J. Kalliokoski - ii - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 19 1973 Madison, Wisconsin M.E. Ostrom 20 1974 Sault Ste. Marie, Ontario P.E. Giblin 21 1975 Marquette, Michigan J.D. Hughes 22 1976 St. Paul, Minnesota M. Walton 23 1977 Thunder Bay, Ontario M.M. Kehlenbeck 24 1978 Milwaukee, Wisconsin G. Mursky 25 1979 Duluth, Minnesota D.M. Davidson 26 1980 Eau Claire, Wisconsin P.E. Myers 27 1981 East Lansing, Michigan W.C. Cambray 28 1982 International Falls, Minnesota D.L. Southwick 29 1983 Houghton, Michigan T.J. Bornhorst 30 1984 Wausau, Wisconsin G.L. LaBerge 31 1985 Kenora, Ontario C.E. Blackburn 32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg 33 1987 Wawa, Ontario E.D. Frey & R.P. Sage 34 1988 Marquette, Michigan J. S. Klasner 35 1989 Duluth, Minnesota J.C. Green 36 1990 Thunder Bay, Ontario M.M. Kehlenbeck 37 1991 Eau Claire, Wisconsin P.E. Myers 38 1992 Hurley, Wisconsin A.B. Dickas 39 1993 Eveleth, Minnesota D.L. Southwick 40 1994 Houghton, Michigan T.J. Bornhorst 41 1995 Marathon, Ontario M.C. Smyk 42 1996 Cable, Wisconsin L.G. Woodruff 43 1997 Sudbury, Ontario R.P. Sage & W. Meyer 44 1998 Minneapolis, Minnesota J.D. Miller & M.A. Jirsa 45 1999 Marquette, Michigan T.J. Bornhorst & R.S. Regis 46 2000 Thunder Bay, Ontario S.A. Kissin & P. Fralick 47 2001 Madison, Wisconsin M.G. Mudrey & Jr., B.A. Brown 48 2002 Kenora, Ontario P. Hinz & R.C. Beard 49 2003 Iron Mountain, Michigan L. Woodruff & W.F. Cannon 50 2004 Duluth, Minnesota S. Hauck & M. Severson 51 2005 Nipigon, Ontario M. Smyk & P. Hollings 52 2006 Sault Ste. Marie, Ontario A. Wilson & R.Sage 53 2007 Lutsen, Minnesota L. Woodruff & J. Miller 54 2008 Marquette, Michigan T. Bornhorst & J. Klasner - iii - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 55 2009 Ely, Minnesota J. Miller, G. Hudak & D. Peterson 56 2010 International Falls, Minnesota M. Jirsa, P. Hollings, T. Boerboom, P. Hinz & M. Smyk 57 2011 Ashland, Wisconsin T. Fitz 58 2012 Thunder Bay, Ontario P. Hollings 59 2013 Houghton, Michigan T.J. Bornhorst & A. Blaske 60 2014 Hibbing, Minnesota J. Miller & M. Jirsa 61 2015 Dryden, Ontario R. Cundari & P. Hinz 62 2016 Duluth, Minnesota J. Miller, C. Schardt & D. Peterson 63 2017 Wawa, Ontario A. Pace, A. Wilson & T.J. Bornhorst 64 2018 Iron Mountain, Michigan L. Woodruff, W. Cannon & E.K. Stewart 65 2019 Terrace Bay, Ontario P. Hollings & M.C. Smyk 66 2020 Meeting cancelled Cancelled by the COVID-19 pandemic 67 2021 Virtual meeting M. Jirsa, M. Smyk & P. Hollings 68 2022 Sudbury, Ontario R.M. Easton & W. Bleeker 69 2023 Eau Claire, Wisconsin R. Lodge, E.K. Stewart, & C. Ames 70 2024 Houghton, Michigan T.J. Bornhorst, E. Vye, P. Cobin, & J. Degraff 71 2025 Mountain Iron, Minnesota A. Radakovich, A. Severson, E. Nowariak, S. Saari, A.C. Hirsch 72 2026 Thunder Bay, Ontario P. Hinz and M. Puumala - iv - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Institute on Lake Superior Geology Goldich Medal -v- �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Goldich Medalists 1979 Samuel S. Goldich 1996 David L. Southwick 2012 James D. Miller 1980 not awarded 1997 Ronald P. Sage 2013 Tom Waggoner 1981 Carl E. Dutton, Jr 1998 Zell Peterman 2014 Laurel Woodruff 1982 Ralph W. Marsden 1999 Tsu-Ming Han 2015 Rodney J. Ikola 1983 Burton Boyum 2000 John C. Green 2016 Mark A. Jirsa 1984 Richard W. Ojakangas 2001 John S. Klasner 2017 Philip Fralick 1985 Paul K. Sims 2002 Ernest K. Lehmann 2018 Val W. Chandler 1986 G.B. Morey 2003 Klaus J. Schulz 2019 Mark Severson 1987 Henry H. Halls 2004 Paul Weiblen 2020 not awarded 1988 Walter S. White 2005 Mark Smyk 2021 Allan MacTavish 1989 Jorma Kalliokoski 2006 Michael G. Mudrey 2022 Terrence J. Boerboom 1990 Kenneth C. Card 2007 Joseph Mancuso 2023 Peter Hollings 1991 William Hinze 2008 Theodore J. Bornhorst 2024 Suzanne W. Nicholson 1992 William F. Cannon 2009 L. Gordon Medaris, Jr 2025 Robert Michael Easton 1993 Donald W. Davis 2010 William D. Addison & 2026 William (Bill) Rose 1994 Cedric Iverson 1995 Gene La Berge Gregory R. Brumpton 2011 Dean M. Rossell - vi - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Sam Goldich and the Goldich Medal Sam Goldich received an A.B. from the University of Minnesota in 1929, a M.A. from Syracuse University in 1930, and a Ph.D. from the University of Minnesota in 1936. During World War II Sam worked for the U.S. Geological Survey in mineral exploration. In 1948, Sam returned to the University of Minnesota, and became Professor and Director of the Rock Analysis Laboratory the following year. He rejoined the U.S. Geological Survey in 1959 and was appointed as the first Branch Chief of the Branch of Isotope Geology. Sam returned to academia in 1964 when he went to Pennsylvania State University. He left PSU in 1965 and moved to the State University of New York at Stony Brook, where he stayed for 3 years. Restless yet again, he moved to Northern Illinois University in 1968 where he was a professor until his retirement in 1977. Sam’s final move was to Denver where he became an emeritus at the Colorado School of Mines. Sam died in 2000, less than a month before his 92nd birthday. In the late 1970’s, Geological Society of America Special Paper 182, which included seminal geochronological studies by Sam Goldich and coworkers on the Archean rocks of the Minnesota River Valley, was nearing completion. At this time various ILSG regulars began discussing the possibility of recognizing Sam for his pioneering work on the resolution of age relationships and thus the geology of Precambrian rocks in the Lake Superior region. Three members, R.W. Ojakangas, J.O. Kalliokoski and G.B. Morey, presented the idea to the ILSG Board of Directors in 1978. The Board approved the creation of an award, provided funding could be obtained. It was suggested that collecting one or two dollars at registration for a dedicated account would provide resources for striking the medal. A general request was made to the ILSG membership for donations and Sam himself offered a challenge grant to match the contributions. In total $4,000 was collected and thus began the work of creating the Goldich Medal. The initial Goldich Award was presented to Sam by G.B. Morey in 1979 and consisted of a large paper proclamation. For the actual medal, G.B. Morey consulted with the foundry on production details, while Dick Ojakangas and Jorma Kalliokoski worked on the design of the award, suggesting that it be given for “outstanding contributions to the geology of the Lake Superior region.” Simultaneously, a committee of J.O. Kalliokosi, W.F. Cannon, M.M Kehlenbeck, G.B. Morey, and G. Mursky developed the Award Guidelines that were approved by the ILSG Board. By 1981 all the elements of the Goldich Award had come together, and the second recipient, Carl E. Dutton, Jr., received the Goldich Medal for 50 years of significant contributions to the understanding of the geology of the Lake Superior region. Since the beginning, the Awards Committee has consisted of individuals representing industry, government and academia, with each member of the Committee serving for three years. The medal is now awarded every year at the annual ILSG meeting. Reference: Morey, G.B. and Hanson, G.N. (editors). 1980. Selected studies of Archean gneisses and Lower Proterozoic rocks, southern Canadian Shield. Geological Society of America, Special Paper 182, 175 p. Prepared by various Goldich Medal Awardees, 2007 - vii - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Goldich Medal Guidelines (Adopted by the Board of Directors, 1981; amended 1999) Preamble The Institute on Lake Superior Geology was born in 1955, as documented by the fact that the 27th annual meeting was held in 1981. The Institute’s continuing objectives are to deal with those aspects of geology that are related geographically to Lake Superior; to encourage the discussion of subjects and sponsoring field trips that will bring together geologists from academia, government surveys, and industry; and to maintain an informal but highly effective mode of operation. During the course of its existence, the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have made particularly noteworthy and meritorious contributions to the understanding of Lake Superior geology and mineral deposits. The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years. Subsequent medallists and this year’s recipient are listed in the table below. Award Guidelines 1) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution to, the geology of the Lake Superior region. 2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of three members, one to serve for three years, one for two years, and one for one year. The member with the briefest incumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In his/her third year this member shall be the chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSG membership. The out-going, senior member of the Board of Directors shall act as liaison between the Board and the Committee for a period of one year. 3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of the Board of Directors, who will then inform the Board of the nominee. 4) The Board of Directors normally will accept the nominee of the Committee, inform the medallist, and have one medal engraved appropriately for presentation at the next meeting of the Institute. 5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award. Nominating Procedures 1) The deadline for nominations is November 1. Nominations shall be taken at any time by the Goldich Medal Committee. Committee members may themselves nominate candidates; however, Board members may not solicit for or support individual nominees. 2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation, lists of publications, curriculum vita’s, and evidence of contributions to Lake Superior geology and to the Institute. 3) Nominations are not restricted to Institute attendees, but are open to anyone who has worked on and contributed to the understanding of Lake Superior geology. - viii - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Selection Guidelines 1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including: a) importance of relevant publications; b) promotion of discovery and utilization of natural resources; c) contributions to understanding of the natural history and environment of the region; d) generation of new ideas and concepts; and e) contributions to the training and education of geoscientists and the public. 2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institute meetings, presentation of talks and posters, and service on Institute boards, committees, and field trips. 3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of the Committee members. 4) There are several points to be considered by the Goldich Medal Committee: a) An attempt should be made to maintain a balance of medal recipients from each of the three estates— industry, academia, and government. b) It must be noted that industry geoscientists are at a disadvantage in that much of their work in not published. 5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedly one of the Institute’s great strengths and should be nurtured by equitable recognition of excellence in both countries. Goldich Medal Committee Serving through the meeting year shown in parentheses Marcia Bjornerud, Academic member - Chair (2023-2026) Robert Cundari, Government member (2024-2027) Phil Larson, Industry member (2025-2028) 2026 Goldich Medal Recipient William (Bill) I. Rose Michigan Tech University, Houghton, Michigan - ix - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Citation for Goldich Medal Recipient William I. Rose It is my heartfelt honor to present the late William I. Rose (Bill) with the 2026 Institute on Lake Superior Geology’s Goldich Medal. Bill has made tremendous contributions to the field of geoheritage and to increasing public understanding of the value and global importance of Lake Superior geology. This highly significant phase of his career, despite being retired for most of it, came from a genuine desire to encourage people to “get outside and love it”, to increase their Earth science literacy, and to deepen their love of Lake Superior. This work is strongly aligned with the criteria and spirit of this distinguished award. Bill served for 41 years as a professor of geology and volcanology at Michigan Technological University, working alongside scientists from around the world. He mentored countless graduate students, many of whom became close friends and respected colleagues. He took immense pride in their accomplishments and in his role advancing global volcano research. The volcanology program he helped build at Michigan Tech has become one of the world’s leading departments, drawing students from around the globe and producing leaders in the field. In his transition to retirement, Bill’s research focus shifted to geoscience education and outreach. This new direction was rooted in his dedication to K-12 educators through projects like the Michigan Teachers Excellence Program (MiTEP) and other teacher professional development in the Keweenaw that focused on Lake Superior geology. Bill held teachers in very high esteem, recognizing them as multipliers and the heart of essential knowledge growth. Working with educators helped launch Bill’s commitment to the field of geoheritage, inspiring the multitude of initiatives and learning resources that he developed for both formal and informal learners within the Keweenaw and Lake Superior regions. The thoughtful design of these programs yielded a vast inventory of Keweenaw geosites that could be used to explore the ways Lake Superior geology guides and influences our lives and culture. Bill shared countless “geostories” with the Keweenaw community - an expression he coined, along with “geopoetry”. His enthusiasm and energy never waned, and he never told a story the same way twice. His stories have made the global significance of Lake Superior geology accessible to people and have helped them to see how geology has shaped their own identity, history, and culture - the very essence of geoheritage. These stories resonated with people, inspiring a sense of pride rooted in the geology of our place and understanding just how fascinating Lake Superior geology is. His stories have inspired others to share their own geostories in the Keweenaw community, such as the Keweenaw National Historical Park and the Carnegie Museum. Bill shared every geoheritage outreach resource he created for zero profit in order to help the Keweenaw thrive and to promote greater understanding of Lake Superior geology. Signage, books, geotours, boulder gardens, museum exhibits, concerts in the belly of an abandoned copper mine, geologic contributions to federal grant applications to support local conservation efforts, and the labyrinth Keweenaw Geoheritage website - all of these were given freely to support our community shift from an extractive economic past and to be forward-thinking and supportive of conservation, education, recreation tourism opportunities - all rooted in our rich geology. This generosity is punctuated by the family gift of Silver Island to the Keweenaw Land Trust - an example of Bill’s strong advocacy for the protection of Lake Superior geosites and the promise of continued public access and education. -x- �Proceedings of the 72nd ILSG Annual Meeting - Part 1 At the very heart of Bill’s education and outreach work is community. Through this work he fostered relationships at the local, national, and global level. At the local level his efforts have united teachers, artists, scientists, outdoor recreation enthusiasts, conservation organizations, and tourists, all drawn together by a common curiosity of Lake Superior geology. Nationally, Bill played a vital and formative role in shaping the vision for geoheritage in the United States, contributing to numerous workshops hosted by the U.S. Committee for Geoheritage and Geoparks and the Geological Society of America. At the global level, the Keweenaw has achieved recognition as a leader in the US geoheritage movement through Bill’s pursuit of prestigious global designations. He spearheaded the designation of the Jacobsville Sandstone as one of the first Global Heritage Stone Resources in the world and the first in the United States, recognized by the International Union of Geological Sciences (IUGS) and the UNESCO’s International Geoscience Program. Bill also promoted the Keweenaw as a strong candidate to become the first UNESCO Global Geopark in the United States. Within both global and national communities, the Keweenaw is largely viewed as a Geopark. Bill’s active membership with the ILSG served as a bridge between the professional geoscience community and the broader public. He was a first or co-author on numerous abstracts and field guides presented at ILSG meetings, including the Geological Field Trip, Eastern Isle Royale, Michigan (2013) and the Self-guided geological field trip to the Keweenaw Peninsula, Michigan (1994). Bill was visionary and big thinking; this is clearly reflected in his research and many contributions to the training and education of both geoscientists and the broader public. Bill’s leadership in geoheritage and passion for education and outreach has deepened public understanding, appreciation, and desire to protect Lake Superior geology. I am brimming with gratitude to see Bill’s service honored with the prestigious Goldich Medal award. Submitted by Erika Vye Great Lakes Research Center, MTU Honoring the Pioneers of Lake Superior Geology (Adopted by the Board of Directors, 2016) Preamble At the suggestion of Gene LaBerge, the 2016 executive board agreed to implement a program to recognize historic pioneers in the understanding of geology in the Lake Superior region. Beginning with the 2017 annual meeting, nominations will be accepted from the membership for geologists whose work was conducted primarily before the inception of the Institute in 1955. Biographical sketches of those pioneers will be presented at future annual meetings so that all may appreciate the value of their contributions. Selection of nominees will be decided in part by the organizing committee of each year’s annual meeting, in consultation with the Board, to ensure equitable geographic representation in the selection process. Award Guidelines 1) Nominations from the membership will be submitted via the Institute web site and forwarded to the Chair of the next Annual Meeting. The nominations will be no more than half a page in length and will summarize the contribution of the nominee. 2) The Organizing Committee will select one or two individuals to be highlighted at the next Annual meeting and submit those names to the Board for approval. - xi - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 3) The nominator will be requested to prepare a brief presentation to be given during the next annual meeting with a summary to be included in the Proceedings volume. 4) Unsuccessful nominations will be kept by the Secretary for two years and forwarded to the next meeting Chair; these nominations may be resubmitted at a later date. The Board will review this award every five years. Pioneers of Lake Superior Geology 2017 Douglass Houghton (1809-1845) 2018-20 not presented 2021 Newton Horace Winchell (1839-1914) 2022 Thomas Leslie Tanton (1890-1971) 2023 Thomas Benton Brooks (1836-1900) 2024 Roland Duer Irving (1847-1888) 2025 Robert Bell (1841-1917) In Memoria William Ingersoll Rose (1944-2025) William Ingersoll Rose, aged 81, died at his home in Eagle Harbor, Michigan, on July 17, 2025. Born in Detroit, Bill moved with his family at age five to New Mexico, where his love of rocks and the Earth began. Bill spent his childhood exploring the desert, riding horses, swimming in the neighborhood pool, and working at a local television station. New Mexico planted the seeds of a lifelong fascination with geology. After high school, Bill attended Dartmouth College, where he received Bachelor’s and Ph.D. degrees. Professor Dick Stoiber, one of the pioneers of volcano research, recognized potential in the unpolished young man and offered him an opportunity to study volcanoes in Guatemala—a pivotal experience that would shape Bill’s life. Bill and his wife, Nanno, settled in Houghton in 1970 where Bill joined the faculty of Michigan Tech. Bill’s work as a volcanologist took him across the globe and occasionally, Nanno and his two sons were able to come along. Following that first trip to Guatemala, Bill developed a deep passion for understanding volcanic eruptions. His adventures throughout Central America, along with his love of its people and landscapes, led him to speak Spanish and immerse himself in local cultures. He devoted himself to forecasting volcanic eruptions to help protect people living near volcanoes. Bill served for 41 years as a Professor of geology and volcanology at Michigan Tech, working alongside scientists from around the world. He mentored countless graduate students, many of whom became close friends and respected colleagues. He took immense pride in their accomplishments and in his role advancing global volcano research. The volcanology program he helped build at MTU has become one of the world’s leading departments, drawing students from around the globe and producing leaders in the field. He was instrumental in establishing signature programs such as the International Masters in Volcanology and Geotechniques and the Peace Corps Master’s International program in Mitigation of Geologic Natural Hazards. In retirement, Bill remained active and engaged. He developed geoheritage materials, led tours of Isle Royale and the Keweenaw Peninsula, and supported teachers, artists, kayakers, hikers, bicyclists, and tourists in learning - xii - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 about the region’s rich geological and cultural history. The Keweenaw has achieved recognition as a leader in the US geoheritage movement through Bill’s pursuit of prestigious global designations. He spearheaded the designation of the Jacobsville Sandstone as one of the first Global Heritage Stone Resources in the world and the first in the United States, recognized by the International Union of Geological Sciences and the UNESCO’s International Geoscience Programme. Bill also promoted the Keweenaw to become the first UNESCO Global Geopark in the United States. Due to his efforts, within both global and national communities, the Keweenaw is viewed as a Geopark by definition. Many of these geoheritage initiatives were presented at ILSG. His Field Trip Guidebook for Isle Royale: Keweenawan Rift Geology, co-authored with Justin Olson, is one of ILSG’s Special Publications. For his remarkable work in the Lake Superior region, his teaching, supervisory and outreach efforts, and support of ILSG, Bill was posthumously awarded the Samuel S. Goldich Medal in 2026. Bill treasured time with his children and grandchildren, especially during family vacations in Eagle Harbor. Always curious, he took the road less-traveled and delighted in whatever he discovered along the way. Richard Wayne (Dick) Ojakangas (1932 - 2025) Dr. Richard (Dick) Wayne Ojakangas died peacefully in his sleep on December 16, 2025 at the age of 93. Dick was born November 20, 1932, in Moose Lake, Minnesota, and grew up in Kettle River and Warba. He was very proud of his 100% Finnish heritage. After graduating from Grand Rapids High School, he enrolled as a business major at the University of Minnesota Duluth (UMD), intending to take over the family store in Warba after graduation. However, during his senior year, he took an introductory geology class from Dr. Robert Heller, and switched his major after the first lecture to geology. He joined the Reserve Officer’s Training Corps because he felt it was his patriotic duty to serve his country. After graduation, he was assigned to the USAF base in Upper Heyford, England. He married Finnish beauty Beatrice (Peaches) Luoma and they moved to England within one week after their wedding. Due to his geological expertise, Dick was assigned to be a Petroleum Supply Officer, fueling jets with highly toxic JP4 jet fuel. After two years in the Air Force, he continued his studies in geology, earning a master’s degree from the University of Missouri, and a PhD from Stanford University. Returning to Duluth, “Dr. OJ” enthusiastically taught geology at UMD for 38 years, where he was beloved by many hundreds of students. Dick was renowned as an entertaining and exceptional geology professor. He began each lecture with a Finn joke, and the punchlines were meticulously written on his calendar. His colleagues in the Geology Department were also his extended family and lifelong friends. He retired in 2002. Dick wrote or collaborated on more than 60 scientific publications and several books, including the highly acclaimed Minnesota’s Geology, and Roadside Geology of Minnesota. He was awarded the prestigious Horace T. Morse Award for Distinguished Teachers from the U of M, and received an honorary PhD from the University of Helsinki, Finland. A long-time member of the ILSG, Dick was awarded the Samuel S. Goldich Medal by the Institute in 1984. He was a fixture at annual meetings, leading field trips and giving presentations, either as himself or as one of his alter egos, like the Old Prospector or Herr Dr. Direktor Professor Wolfgang von Schlummerklutz from the World Panzerenkotklotzen Institute in Europe! Dick was a passionate traveler and photographer, doing geological research on all seven continents. His work in Antarctica as part of the United States Antarctic Research Program resulted in having Mount Ojakangas being named after him. In India, he found evidence of the first Archaean glaciation ever discovered. In Finland, he and a colleague were the first to determine the direction that glaciers moved through northern Europe. As a worldrespected geologist and an engaging, highly understandable speaker, he spread his enthusiasm for science by giving lectures on cruise ships from 1978 to 2017, feeding his obsession with traveling the world. Curious and inquisitive, Dick entertained many interests and hobbies. He lived an active life - running several - xiii - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Grandma’s Marathon’s (all without training) and cross-country skiing (doing the Birkebeiner 54 km race in Wisconsin many times, also without training). Dick was an avid mushroom hunter on all continents. His wife said that he could ‘find mushrooms, whether they were there or not!’ Known for his generosity, humor, and warm personality, Dick made sure everyone felt included and always sought out strangers, who promptly became his friends. His trademark greeting “Hiya!” and farewell “Cheers!” are remembered fondly by his family and friends. Paul Willard Weiblen (1927 – 2025) Professor Paul Willard Weiblen, 98 years old, died peacefully in the presence of family on Tuesday, December 23, 2025 in St. Paul, Minnesota. PW, to friends, colleagues and students, was born in Miller, South Dakota in 1927. After graduation from high school in 1945, he entered the U.S. Army. After military service he returned to college and earned a B.A. degree at Wartburg College in Waverly, Iowa (1950), and an M.A. in History at the University of Minnesota (1952). PW came into geology in a roundabout way. Apparently, he was in Istanbul, Turkey working as a travel agent for American Express when he encountered a geologist exploring the world for uranium deposits. Consequently, he returned to the University of Minnesota in 1959 to pursue geology. He focused on the metamorphism of the Paleoproterozoic Thomson Formation of east-central Minnesota for his Master’s thesis (1962) and on the geology and petrology of the Bald Eagle intrusion of the Duluth Complex in northeastern Minnesota for his Ph.D. (1965). He stayed at the University in the Geology and Geophysics Department as an Assistant Professor (1965), Associate Professor (1969), Professor (1980), and Professor Emeritus (1997), teaching Minnesota geology and characterizing the minerals of the Duluth Complex with the Minnesota Geological Survey. He was hired specifically to organize and supervise the Department’s new Electron Microprobe Laboratory (1965-1980) in the Space Science Centre. He also served as Curator of the petrology collection (1970-1997) and supervisor of the scanning electron microscope facility (1970-1997). Over his 32 years as a faculty member, Paul’s analytical expertise and unbridled curiosity led him to pursue, and engage others, in many areas of research. The principal focus of his research was on the petrology and mineral deposits of the Duluth Complex. A highlight was a 1980 American Journal of Science paper, co-authored by Minnesota Geological Survey Chief Geologist G.B. Morey, that summarized the stratigraphy, petrology and structure of the Duluth Complex. Another significant area of interest in Paul’s career was lunar petrology. In the early 1970s, he and Edwin Roedder (USGS) confirmed the phenomenon of silicate liquid immiscibility by examining lunar glasses, and terrestrial basalts. In 1978-79, Paul served as lead curator of NASA’s Washington, D.C. lunar sample collection. The focus of Paul’s research in the latter part of his academic career and into his retirement was building and promoting the electric pulse disaggregator (EPD, or “the Zapper”). He was introduced to the EPD and its inventor, Nikolay S. Rudashevsky, during a visit to Russia in 1991.   Recognizing the potential of this instrument to create ultraclean mineral separates for a variety of applications, PW built and installed an EPD at U of M in 1992. He actively promoted it to other scientists who have used it to prepare samples for detrital zircon dating, mineral liberation analyses, and microfossil studies. As a teacher and student advisor, PW was engaging, approachable, and deeply committed to his students. He routinely taught undergraduate and graduate level Igneous Petrology and Optical Mineralogy and offered hands-on classes on electron microprobe analysis. His annual petrology field trips up the Gunflint Trail were legendary. As a graduate advisor, he was open to letting his 11 PhD and 13 MS students develop their own thesis projects, with topics that included igneous petrology; volcanology; structural, metamorphic and field geology; geochemistry, mineralogy, petrography, and economic geology. A particular source of pride was that all but one graduate thesis was based on the geology of Minnesota. PW was awarded the Goldich Medal from the Institute on Lake Superior Geology in 2004 for his lifelong commitment to promoting geologic studies of Minnesota. - xiv - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Ronald Parker Sage (1938 - 2026) Ronald Parker Sage, 87, of Kingsford, MI, passed away peacefully on January 28, 2026, after a battle with numerous health conditions. Ron was born on August 4, 1938, in Pontiac, Michigan. Ron graduated in 1960 with a BSc degree in geological engineering from Michigan Technological University in Houghton. While at MTU, Ron spent most of his free time collecting rocks and minerals in the copper and iron mining districts, earning him the nickname “Rocky”. He was a student of Kiril Spiroff, the “Mad Russian”. Ron held his Alma Mater in high esteem. In the early 1960s, Ron worked for three years as an engineer for the Shell Oil Company in west Texas. His duties included well siting, well logging, well workovers, and other production-related activities. In 1966, Ron graduated with a Master’s degree in Geology from the Colorado School of Mines. It was here that he was first exposed to alkalic rocks, his study topic and thesis being “Geology and Mineralogy of the Cripple Creek Syenite Stock, Teller County, Colorado”. This led to employment with Anaconda American Brass Ltd to investigate CuNi-PGE minerals in the Port Coldwell alkalic complex near Marathon, Ontario. During the summer of 1967, Ron searched for base metals in the Ely greenstone belt in northern Minnesota for Bear Creek Mining. It was in that year that he first met two other greats of Lake Superior geology, Ned Eisenbrey and Gene LaBerge. In 1969, Ron again worked for Anaconda American Brass Ltd., this time north of Lake Superior in the Schreiber greenstone belt, searching for gold and base metals. In the fall of 1969, Ron joined the Ontario Geological Survey, his professional home for over 30 years. Ron’s work for the OGS took him to many parts of the province, but never very far, and never for very long, from Lake Superior. He spent four years on a helicopter reconnaissance in Northern Ontario, then mapped the Slate Islands in Lake Superior, and next worked on a multi-year program to study alkalic rocks north of Port Coldwell along the northern extension of the Trans Superior Tectonic Zone and along the Kapuskasing Structural Zone. In 1978, Ron was assigned to the Michipicoten greenstone belt. Here he spent 10 years mapping Archean supracrustal rocks over approximately 540 square miles, with emphasis on the gold and base metal potential. In 1993, Ron was assigned to a province-wide program of kimberlite documentation to stimulate diamond exploration. Some of this work was again in the Michipicoten area, where diamond-bearing rocks had recently been discovered. Despite his busy professional schedule, Ron was able to complete his PhD degree in 1986 for a thesis submitted to Carleton University in Ottawa, entitled “Alkalic Rock Complexes and Carbonatites of Northern Ontario, and their Economic Potential.” Ron was a long-time ILSG supporter, giving presentations, leading field trips and Co-Chairing the annual meetings in 1987, 1997 and 2006. In 1997, Ron was recognized for his many contributions when he received the Samuel S. Goldich Medal from the Institute on Lake Superior Geology. Charles Edward (Charlie) Blackburn (1940 - 2026) Charlie passed away on Friday March 6, 2026 at the Royal Jubilee Hospital, Victoria, BC. Although he identified himself as a Welshman, having grown up in a small village near Cardiff, Wales, by an accident of fate during the early days of World War II, Charlie was actually born in Kidderminster, England. He went to Swansea University to complete his Bachelor of Science degree in geology. Charlie loved the summer field mapping excursions in Northern - xv - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Norway working towards his goal of becoming a professional geologist. Geology was always his passion and drawing maps, his gift. Charlie emigrated from Wales to Canada and undertook a Master of Science degree at the University of Western Ontario, London, Ontario where he met his wife of sixty years Christine (nee Spence) – also a recent UK immigrant. It was love at first sight. The couple were married after a two-month courtship. They left for Italy where Charlie studied the metamorphic puzzle of the Seisia-Lanzo zone in the Valle d’Aosta at the University of Padua, Italy. He liked that his office was right opposite the Cappella Della Scrovegni – famous for its Giotto frescoes. His time in Italy left him with an enduring love for the people and culture there. Charlie returned to Canada in 1969 and in 1970 accepted a position as a mapping geologist with the Ontario Geological Survey (OGS) in Toronto. As a field geologist, Charlie spent summers of his early career in the bush of northern Ontario and didn’t see too much of his family. As the children grew, Charlie saw the need to be with his family more and so accepted the position of Resident Geologist in Kenora, Ontario. Many of his over 75 OGS publications resulted from his mapping efforts in the Archean greenstone belts of the western Wabigoon Subprovince and other areas in the Kenora District. He was the lead author of the seminal review of the Wabigoon in the 1991 compendium, Geology of Ontario. In the early 90’s, he took a sabbatical from his Resident Geologist duties to return to mapping the Separation Lake area. Charlie retired from the OGS at age 60 after 30 years of service and, with his wife Christine, became co-founder of their consulting company, Blackburn Geological Services. Charlie Chaired the 1985 ILSG annual meeting in Kenora and was on the organizing committee for the 2002 annual meeting, also held in Kenora. He delivered papers and chaired sessions at many ILSG meetings and led field trips to the Separation Rapids rare-element pegmatite field and other locations in the Kenora District, of which he had an encyclopedic knowledge due to his years of mapping and documenting mineral occurrences in the Superior Province. Report of the Chair of the 71st Annual Meeting Amy Radakovich, Allison Severson, Eric Nowariak, Aaron Hirsch, Stacy Saari Mountain Iron, Minnesota The 71st Institute on Lake Superior Geology (ILSG) was held May 14 to 17, 2025 in Mountain Iron, Minnesota at the Mountain Iron Community Center. The meeting was sponsored by the State of Minnesota’s Iron Range Resources and Rehabilitation agency, Bayside Geoscience, the Geological Society of Minnesota, the Mesabi Range Geological Society, George Hudak Geosciences, PLLC, and the University of Minnesota Duluth’s (UMD) Swenson College of Science and Engineering Earth and Environmental Sciences department, as well as individual contributors Roger Anderson, Allan MacTavish, Dave Dahl, Tom Erickson, and Barry Frey. The meeting was cochaired by Amy Radakovich, Allison Severson, Eric Nowariak, and Aaron Hirsch of the Minnesota Geological Survey (MGS), and Stacy Saari of the Minnesota Department of Natural Resources (MNDNR). Patrice Cobin and Julie Stark of Michigan Technological University served as registrars for the meeting. The institute was attended by a total of 137 participants of which 25 were students. Generous donations from the following individuals helped provide a reduced registration and field trip price for students: Kate Clover, Jim and Isabel DeGraff, Tom Erickson, Tom Fitz, Aaron Hirsch, Paula Leier-Engelhardt, Bob Mahin, Vince and Susan Matthews, Jim Miller, Allison Severson, Mark and Lauri Severson, John Verhoeven, and Gerry White. The 71st meeting consisted of two full days of technical sessions, which ran from Thursday morning, May 15 through Friday afternoon, May 16th. The meeting also held pre-and post-meetingfield trips on May 14th and May 17th. A total of 51 presentations were subdivided into 8 technical sessions; 6 technical sessions for 26 oral presentations (of which 1 was presented by a student), and 2 poster technical sessions with a total of 23 poster presentations (of which 14 were presented by students). The chairs continued the previous meeting’s precedent of including two poster sessions to allow both attendees and judges more time to review the posters. The first th - xvi - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 presentation of the technical sessions was given by Mark Smyk (OGS - retired; Goldich Medalist in 2005) who gave the citation for Robert Bell, the 2025 Pioneer of Lake Superior Geology. Bell is the 6th person to be recognized for their contributions to Lake Superior Geology prior to the initiation of the ILSG. The technical sessions of the 71st annual meeting of ILSG were published in 2025 as Part 1 of Proceedings Volume 71 (95 pages). Five Doug Duskin Best Student Paper Awards were given for student oral and poster presentations as judged by the 2024 Student Paper Awards Committee chaired by Aaron Hirsch (MGS). PhD student poster awards were given to Zsusanna Allerton and Madelyn Banks. Undergraduate student poster awards were given to Celia Cortopassi and Lyndsie Vickers. Omar Khali Droubi received the best oral presentation award. The 71st ILSG also awarded 12 Eisenbrey Student Travel and Participation Awards to help defray the cost of travel to and participation in the ILSG professional meeting for undergraduate and graduate students. The awardees were Drew Casper, Haley Johannesen, Mary Elizabeth Shalifoe, Linsey Hula, Omar Khalil Droubi, Samara Gries, Renee Jeutter, Aidan Kwiatkowski, Celia Cortopassi, Lyndsie Vickers, Zsuzsanna Allerton, and Bekah Thomson. As usual, field trips were a highlight of the 71st ILSG. Mountain Iron’s close proximity to exposures of Archean, Paleoproterozoic, and Mesoproterozoic rocks made it a prime location to run numerous excellent field trips. The meeting offered 8 field trips which included 4 pre-meeting trips on Wednesday May 14, and 4 post-meeting trips on Saturday May 17. Seven field trips focused on the varied Precambrian geology of northeastern Minnesota, and one trip highlighted the unique Quaternary features of the region. Seven of the eight field trips were able to run, with one cancelled due to active wildfires in the field trip area. The remaining 7 field trips were well attended. There were 130 registrants for the field trips, excluding leaders, representing over 100 different individuals (some registrants took multiple trips). Pre-meeting trip 1 was a “Transect through the Quetico subprovince of northern Minnesota,” led by Eric Nowariak (MGS) and Mark Jirsa (MGS-retired). Pre-meeting trip 2 was led by Mark Severson (Natural Resources Research Institute, Teck - retired), Cullen Phillips (New Range Copper Nickel), and Kevin Boerst (Twin Metals Minnesota) and highlighted “Drill Core from three Cu-Ni deposits of the Duluth Complex.” Pre-meeting trip 3 asked the question “How do you make iron and/or manganese in Proterozoic iron formation?” and was led by Alex Steiner and Dean Peterson (Big Rock Exploration) and Latisha Brengman (University of Minnesota Duluth [UMD]). Pre-meeting trip 4 was led by George J. Hudak (University of Minnesota; George Hudak Geosciences, P.L.L.C) and Zsuzsanna Allerton and Annia Fayon (University of Minnesota) and highlighted “New geological insights into the genesis of iron ores at Lake Vermillion-Soudan Underground Mine State Park.” Post-meeting trip 5 traveled to numerous “Neoarchean alkalic intrusions in the Wawa and Quetico subprovinces” and was led by Terry Boerboom (MGS-retired) and Amy Radakovich (MGS). Mark (NRRI, Teck - retired), Allison (MGS), and Lauri (earth science teacher - retired) Severson planned to lead post-meeting trip 6 focused on a “Unique Keweenawan inclusion (Colvin Creek) in the Duluth Complex.” However, the trip was cancelled due to wildfire conditions, and participants were invited to join other trips or receive a refund. Post-meeting trip 7 led by Dean Peterson (Big Rock Exploration) and George Hudak (University of Minnesota; George Hudak Geosciences, P.L.L.C) visited numerous “Classic outcrops of northeastern Minnesota” Field trip 8 was led by Phil Larson (Vesterheim Geoscience, PLC), Andrew Breckinridge (University of Wisconsin - Superior), and Howard Mooers (UMD) and focused on Glacial Lake Norwood and the Koochiching Lobe.” Field trip guides were published in 2025 as Part 2 of the Proceedings Volume 71 (200 pages). A catered welcome reception was held at the Mountain Iron Community Center on Wednesday evening, May 14, after all of the pre-trips returned. The event was well attended, and offered a chance for meeting attendees to reconnect with colleagues and friends prior to the start of technical sessions. Steve Solkela provided entertainment for a portion of the evening. The annual ILSG social and banquet were hosted at the Mountain Iron Community Center on Thursday evening, May 15, 2025. Ninety-three people were in attendance at the sold-out banquet. After introductions - xvii - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 and announcements, Mark Puumala announced the location of the 2026 meeting as Thunder Bay, Ontario. The program continued with ILSG awarding the prestigious Goldich Medal to the very deserving Robert Michael (Mike) Easton (Ontario Geological Survey), who unfortunately could not be present at the meeting. Wouter Bleeker (Geological Survey of Canada) provided the citation for Mike, highlighting Mike’s long tenure with the OGS, his impressive publication record, and his contributions to ILSG. Another highlight of the banquet was the keynote presentation by Pete Kero, P.E., Senior Environmental Engineer with Barr Engineering Co and visionary behind the award-winning Redhead Mountain Bike Park in Chisholm, Minnesota. His fascinating talk entitled “Mine to Mountain Bike Mecca: The story of the Redhead Mountain Bike Park” detailed the transformation of ten idled open pit iron mines in northeast Minnesota into a world-class recreation destination for mountain biking, hiking, and paddling. Kero fielded many questions from the engaged audience and sold and autographed his book Minescapes: Reclaiming Minnesota’s Mined Lands after the keynote presentation, which ended the banquet program. The Institute’s Board of Directors met on Thursday May 15, 2025 to discuss ILSG business and approve the 2026 meeting location. The meeting was attended by Amy Radakovich (Board Chair and Assistant Treasurer), Ted Bornhorst, Carsyn Ames, Peter Hollings (Secretary), and Mark Jirsa (Treasurer). Guests at the meeting were the meeting co-chairs Allison Severson, Eric Nowariak, Aaron Hirsch, and Stacy Saari and also Mark Puumala, the Chair of the proposed 2026 Thunder Bay meeting (approved by the board - see below). Michael Easton was unable to attend. Institute’s Board of Directors meeting notes were taken by ILSG Secretary Hollings, which are as follows: 1. Accepted report of the Chairs for the 70th ILSG, as published in the Proceedings volume, and minutes of last Board meeting, May, 2024 (Hollings). 2. Received and discussed 2024-2025 ILSG Financial Summary (Jirsa/Radakovich). Final approval tabled for Email vote after necessary revisions are made to balances as listed 3. Received, discussed, and accepted 2024-2025 report of the Secretary (Hollings). 4. Approved Alli Severson as on-going ILSG Board member and Pete Hinz and Mark Puumala as coChairs. 5. Discussed and approved appointing Amy Radakovich as the Institute Treasurer. This was subsequently approved by the Membership. Mark Jirsa was thanked for his 31 year service to the Institute. 6. Discussed and approved replacing Dean Peterson as the “member from industry” on Goldich Committee (end of term 2025) with Phil Larson. 7. Approved Thunder Bay as the site for the 72nd annual ILSG meeting. The meeting will be Chaired by Pete Hinz and Mark Puumala with tentative dates of May 19 to 23. 8. A number of future meeting locations were discussed including Grand Marais (Jim Miller), Baraboo (Esther Stewart & Carsyn Ames) and Marquette. 9. The revised Eisenbrey guidelines were discussed and approved with edits. Changes expand the list of expenses which are eligible for reimbursement from the Eisenbrey award (ex: registration fees, meals, lodging, and transportation are all now included) 10. There was discussion over the format and page limits for the abstracts. It was agreed that the two page limit would be maintained. 11. The cost of hosting the meeting registration through MTU was discussed. MTU currently charges 12% of the total registration sales as their fee. It was agreed that the hosts of each meeting would evaluate possible hosting options and pick the one that worked best for them. Puumala indicated that next year the hosts would likely go with a Canadian registrar so that registration fees could be charged in Canadian dollars 12. The cost of printing the Proceedings and Field Guide volumes was discussed. It was agreed that future meeting Chairs would explore the possibility of making the full printed volumes a paid option for participants - xviii - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 and providing only the guides for individual trips. 13. The ongoing storage of ILSG poster boards and easels was discussed. MTU has stored them for the last ~10 years but can no longer offer to do that. Boards and easels were stored for the past year at the Minnesota Geological Survey, but there is no room for permanent storage there. It was suggested that the storage and transport of the posters and easels become the responsibility of the meeting hosts, such that after each ILSG meeting, the boards and easels would leave with the host of the following year’s meeting. This way storage and transport costs can be built into the next year’s meeting costs. Thunder Bay hosts do not need boards next year and did not want to take them across the border given recent border crossing issues. It was suggested that ILSG perhaps have two sets of boards and two sets of easels - one that resides in Canada and one that resides in the USA. Carsyn Ames volunteered to store the boards and easels at the Wisconsin Geological Survey for the next year, delaying the need to make a final decision. Our large, five-person committee allowed us to divide-and-conquer the innumerable tasks to make The 71st annual ILSG meeting a great success. We were proud to continue the long-standing tradition of bringing people together from many states and provinces to share and learn about the fascinating geology of the Lake Superior region, both in the meeting and ‘on the rocks.’ The co-chairs would like to thank the many people and organizations who made the meeting possible, including the Mesabi Range Geological Society and UMD students who ran the registration table and helped with merchandise sales, and the numerous individuals who offered to drive rental or personal vehicles on our fieldtrips. The Sawmill supplied all meeting and field trip food, Caribou provided coffee and tea for the field trips, and Peplinjack’s Bakery supplied the delicious field trip pastries. Lastly, we would like to thank the numerous generous donors who donated hundreds of rock and mineral specimens, books, and maps that made up the biggest and most profitable book sale and silent auction in ILSG memory. The sale and auction netted a total of approximately $4,500 which will be used to fund student participation at subsequent meetings. We look forward to seeing everyone next year in Thunder Bay! Respectfully submitted, Amy Radakovich, Allison Severson, Eric Nowariak, Aaron Hirsch, and Stacy Saari Co-chairs, 71st Institute on Lake Superior Geology Eisenbrey Student Travel Awards The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name “Eisenbrey” was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. “Ned” Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader - he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The number of awards and value are determined by the annual Chair in consultation with the Secretary and Treasurer. Recipients will be announced at the end of the annual meeting. The following general criteria will be considered by the annual Chair, who is responsible for the selection: 1) The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. 2) Students who are the senior author on either an oral or poster paper will be given favored consideration. 3) It is desirable for two or more students to jointly request travel assistance. 4) In general, priority will be given to those in the Institute region who are farthest away from the meeting location. - xix - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 5) Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. Successful applicants will receive their awards during the meeting. Joe Mancuso Student Research Awards The 2005 Board of Directors established the ILSG Student Research Fund with $10,000 US from the Institute’s general fund to encourage student research on the geology of the Lake Superior region. A minimum of two awards of $500 US each for research expenses (but not travel expenses) will be made each year. Students are expected to present their research orally or during a poster session at an ILSG meeting. The award winners will also be automatically eligible for the Eisenbrey Travel Awards. To allow the fund to grow, the Fund will receive one-half of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. • The ILSG Board of Directors will be responsible for selecting a minimum of two awards each year. The ILSG Treasurer will issue the awards. • The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. • The applications are due to the ILSG Secretary by August 31st of each year. Awards will be made by October 1st of each year. • Names of the award recipients will be announced at the next annual meeting and posted on the ILSG website. • Details of the application process can be found on the ILSG web site. • The proposal will need to be signed by the researcher’s supervisor. The 2012 Board of Directors approved modification of the fund’s name, adding “Mancuso” to reflect the many contributions of Joseph Mancuso to the organization and sizeable donations made in his name. “Doc Joe,” as he was known by his students, taught geology for 36 years at Bowling Green State University, Ohio. He advised many graduate students in field-oriented research, and frequently brought them to Institute meetings. Joe was the 2007 Goldich Medalist. In fall 2025, the ILSG Board of Directors selected two students to be granted research funding of $500 each from the Joe Mancuso Student Research Fund. The awardees were: Kathryn Akin, University of Minnesota- Twin Cities Alyssa Hellrung, University of Wisconsin Doug Duskin Student Paper Awards Each year, the Institute selects the best of student presentations and honors the presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting, and from generous donations to the fund in honor of Doug Duskin—an exploration geologist and long- time friend of the Institute. The 2012 ILSG Board of Directors approved adding Doug’s name to the award to acknowledge his contributions and distribute those donations in a manner that would have pleased him. The Duskin Student Paper Committee is appointed by the Meeting Chair. Criteria for best student paper—last modified by the Board in 2001—follow: 1) The contribution must be demonstrably the work of the student. 2) The student must present the contribution in-person. 3) The Student Paper Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. - xx - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. 5) The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). 6) The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required but is left to the discretion of the Committee. 7) The names of award recipients shall be included as part of the annual Chair’s report that appears in the next volume of the Institute. Student papers will be noted on the Program. 2026 Student Paper Awards Committee Emily Smyk - Bayside Geoscience Justin Jonsson - Ontario Geological Survey Nick Swanson-Hysell - University of Minnesota Board of Directors Board appointment continues through the close of the meeting year shown in parentheses, or until a successor is selected Peter Hinz and Mark Puumala, Co-Chairs (2026-2029) - Ontario Geological Survey, Retired Alli Severson (2025-2028) - Minnesota Geological Survey Ted Bornhorst (2024-2027) - Michigan Tech, Houghton Carsyn Ames (2023-2026) - Wisconsin Geological & Natural History Survey, Madison Amy Radakovich, Treasurer (2025-2028) - Minnesota Geological Survey Peter Hollings, Secretary (2024-2027) - Lakehead University Local Committee Chairs Peter Hinz and Mark Puumala - Ontario Geological Survey, Retired Organising Committee Robert Cundari - Ontario Geological Survey, Thunder Bay, Ontario Al MacTavish - Thunder Bay, Ontario Mark Smyk - Lakehead University, Thunder Bay, Ontario Pete Hollings - Lakehead University, Thunder Bay, Ontario Jim Miller - Thunder Bay, Ontario - xxi - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Field Trip Leaders and Guidebook Authors Field trips have been the mainstay of the ILSG since its inception 72 years ago. We give special thanks to the field trip leaders and guidebook authors who volunteered their time and talent in carrying that tradition forward. Trips 1 & 4: Classic” Geological Sites in the Thunder Bay Area - Mark Smyk (Lakehead University) and Mark Puumala (Geological Consultant) Trip 2: Geology of the Quetico Subprovince and Shebandowan greenstone belt north of Thunder Bay - Riku Metsaranta and Gaetan Launay (Ontario Geological Survey) Trip 3: Geological assemblages, regional structural framework and tectonic evolution of the Neoarchean Shebandowan greenstone belt - Dorothy Campbell, Justin Jonsson and Vittoria D’Angelo (OGS Resident Geologist Program) Trip 5: Archean Geology and Metallogeny of the Rainy Lake Wrench Zone - K. Howard Poulsen (Geological Consultant) Trip 6: Amethyst Deposits of Thunder Bay - Steve Kissin (Lakehead University) and Greg Paju (OGS Resident Geologist Program) - xxii - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Technical Program Wednesday May 20 (Parking Lot G14, Lakehead University) 8:00 a.m. Field Trip 1: “Classic” Geological Sites in the Thunder Bay Area Leaders: Mark Smyk and Mark Puumala 8:00 a.m. Field Trip 2: Geology of the Quetico Subprovince and Shebandowan greenstone belt north of Thunder Bay Leaders: Riku Metsaranta and Gaetan Launay 8:00 a.m. Field Trip 3: Geological assemblages, regional structural framework and tectonic evolution of the Neoarchean Shebandowan greenstone belt Leaders: Justin Jonsson and Vittoria D’Angelo 5:00 p.m. Return of Trips 1-3 4:00 p.m. - 8.00 p.m. Registration (Faculty Lounge, Lakehead University) 6:00 p.m. - 9.00 p.m. Ice Breaker Social, Poster Setup and Core Shack (Faculty Lounge, Lakehead University) Thursday May 21 7:30 a.m. - 4:00 p.m. Registration (Faculty Lounge, Lakehead University) 8:30a.m. - 9:00 a.m. Introductory Remarks (Room UC0050, Lakehead University) Technical Session I NOTE: Asterisk * denotes a student eligible for a Best Student Paper Award Session Chairs: Mark Puumala and Jim Miller 9:00 a.m. Stephan, T., Phillips, N., and Hollings, P. Timing and conditions of magmatism, metamorphism, and strain partitioning in the western Shebandowan Greenstone Belt (Superior Province) 9:20 a.m. MacDonald, P., Hastie, E., Malegus, P., Kamo, S., Hamilton, M. and Marsh, J. Implications of recent geochronology on the regional geology and timing of gold mineralization in the Red Lake greenstone belt, Ontario 9:40 a.m. Hollings, P., Vrzovski, J., Cooke, D. and Gorner, E. Using epidote and chlorite mineral chemistry to extend the alteration footprint around the Hemlo Au deposit, N. Ontario 10:00 a.m. - 10:30 a.m. Coffee Break, Poster Session and Core Shack 10:30 a.m. Tiitto*, H., Phillips, N., and Stephan, T. Deformation processes in a mid-crustal strike-slip shear zone: Insights from the Archean Quetico Shear Zone, Superior Province, Canada 10:50 a.m. Sheshnev*, V., Hollings, P., Tolley, J., Angombe, M., Deller, M. and Stern, R. Whole Rock and Mineral Chemistry of the Eagle’s Nest Intrusion, McFaulds Lake Greenstone Belt, Ontario, Canada: Insights into the Origin and Paragenesis - xxiii - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 11:10 a.m. Carlton*, K., Tikoff, B. and Nachlas, W. An introduction to the northwestern Huron Mountains of the Upper Peninsula, Michigan: field relations and preliminary structural interpretations 11:30 p.m. - 1:00 p.m. Lunch Break, Poster Session and Core Shack (ILSG Board Meeting by invitation) Technical Session II Session Chairs: Esther Stewart and Phil Larson 1:00 p.m. Salerno, R., Cannon, W. F., Thompson, J., Souders, A., Vervoort J. and Hillenbrand, I. Reassessing variations in metamorphism across the Penokean orogen in Northern Michigan: Part 1, new Pressure-Temperature-Time-Deformation constraints 1:20 p.m. Cannon, W. F., Salerno, R., Drenth, B. and Bedrosian, P. Reassessing variations in metamorphism across the Penokean orogen in Northern Michigan: Part 2, Reinterpreting metamorphic nodes 1:40p.m. Hirsch, A. Can we improve the bouguer gravity resolution in the Cuyuna Range? Increasing gravity measurements in a region of high gravity station density. 2:00 p.m. - 2:30 p.m. Coffee Break, Poster Session and Core Shack 2:30 p.m. Allerton, P. and Hudak, G. Characterization of hematite ore from former Ely mines, NE Minnesota 2:50 p.m. Steiner, R.A., Watson, N., Riley, J., Hammer, M., Thole, J., Feinberg, J., Sandri, H. and Savage, B. Oxidation to Ores: Petrological Insights into Supergene Manganese Enrichment at the Emily Deposit, Minnesota 3:10 p.m. Hagedorn, G. Ice flow history, surficial geology, and till composition of Georgia Lake area, northwestern Ontario Poster Session 3:30 - 5:00 p.m. 6:00 p.m Annual Banquet and Award Presentation (Faculty Lounge, Lakehead University) Announcement of 73rd Annual Meeting Location 2026 Goldich Award Presentation to Bill Rose 2026 Quiz night Meeting participants not registered for the banquet are welcome to attend the quiz night - xxiv - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Friday May 22 9:00 a.m. - 12:00 p.m. Registration Technical Session III Session Chairs: Shannon Zurevinski and Therese Pettigrew 8:30 a.m. Beyer, S., Cutts, J., Hnatyshin, D., Powell, J., Camacho, A., Cawood, T. and Drever, G. Preliminary geochronology of lithium pegmatites and host rocks, Archean Superior Province, northwestern Ontario 8:50 a.m. Quigley, A., Mahin, R., and Gamet, N. Critical Mineral Potential of the Watersmeet Gneiss Dome, MI USA 9:10 a.m. Bleeker, W. and Wodicka, N. Improved Precision and Better Accuracy: SHRIMP-II Detrital Zircon Analysis of Samples Across the Stratigraphy of the Midcontinent Rift 9:30 a.m. Easton, R.M. and Kamo, S. The Badgerow complex, a Midcontinent Rift-related REE-Zr-rich peralkaline intrusion in the Grenville Province near Verner, Ontario 9:50 a.m. - 10:20 a.m. Coffee Break, Poster Session and Core Shack 10:20 a.m. Nitescu, B., Torres, D., and Gaona, J.. Models of the regional gravity and magnetic anomalies associated with the Nipigon Embayment 10:40 a.m. Bain, W. and Hollings, P. Coeval silicate melt and PGE-bearing salt melt inclusions in the Thunder and Seagull intrusions, Ontario: An overview of evidence and data processing challenges 11:00 a.m. Drost, A. and Heggie, G. A new look at the Seagull mafic-ultramafic Intrusion and potential hydrogen and helium accumulations 11:20 a.m. Swanson-Hysell, N., Zhang, Y., Mohr, M. and Schmitz, M. Linking the Southwestern Laurentia large igneous province and rapid Duluth Complex emplacement through mantle plume dynamics 11:40 p.m. - 1:00 p.m. Lunch Break, Poster Session and Core Shack Technical Session IV Session Chairs: Wouter Bleeker and Peter Hinz 1:00 p.m. Smith, J., Kaski, K., Tschirhart, V. and Enkin, R. Integrating petrophysical data with full tensor magnetic gradiometry for improved interpretation and modelling of remanently magnetized intrusions in the Midcontinent Rift 1:20 p.m. Peterson, D., Steiner, A., Sweet, G. and Boucher, C. Physical Magmatic System Interpretation of the Marathon Cu-Pd Deposit, Coldwell Complex, Ontario 1:40 p.m. Smyk, E., Dolega, S., Churchley, J. and Flank, S. Optimizing data collection for better geological interpretations and adding value to your project - xxv - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 2:00 p.m. Lizzadro-McPherson, D., Vye, E., Degraff, J., and Rose, W. Interactive Geospatial Geoheritage: Efforts to Support Place-based Exploration and Digitally Preserve Keweenaw’s Geoheritage 2:20 p.m. - 2:50 p.m. Coffee Break, Poster Session and Core Shack 2:50 p.m. Degraff, J., Hiltunen, L., Lafreniere, D., Lizzadro-McPherson, D., Vye, E., Cowling, B., Bornhorst, T. and Rose, W. Digital Preservation and Enhanced Utility of Exploration Core Descriptions from the Keweenaw Copper District, Michigan: Progress toward a Map-based Web Portal 3:10 p.m. Stone, A., Lizzadro-McPherson, D. amd Vye E. Rocks and Roots: The Role of Geoheritage in Biodiversity Stewardship 3:30 p.m. Smyk, M., Hodge, J. and Robillard, C. Pukaskwa Redux: Revisiting and Reconnecting with Superior’s Wild North Shore 3:50 p.m Presentation of Best Student Paper Award and Eisenbrey Awards 5:00 p.m. Field Trip 5: Archean Geology and Metallogeny of the Rainy Lake Wrench Zone Leader: Howard Poulsen Parking Lot G14, Lakehead University Poster Presentations Akin*, K. and Swanson-Hysell, N. Constraining the 3-D Geometry of the Duluth Complex, MN, Using Magnetic Fabrics and Paleomagnetic Data Angombe, M., Phillips, N., Hollings, P., Stephan T., Sheshnev, V., Deller, M. and Smith, A. Decoding Shear Zone Evolution in the McFaulds Lake Greenstone Belt, Ontario: Constraints on CrystalPlastic Deformation and Timing from in-situ Titanite U–Pb Thermochronology Bilboe*, M., Zurevinski, S. and Conly, A. Quartz Trace Element and TEM Analysis of Selected Economic LCT Pegmatites Buchholz, T., Falster, A. and Simmons, W. Update to: a complex F-rich alkalic pegmatite in the pyroxene syenites of the Stettin Complex, Wausau Complex, Marathon County, Wisconsin Chaisson*, A., Smyk, M. and Zurevinski, S. Petrography and Geochemistry of the Mound Lake Pluton, Northwestern Ontario Duffy*, P., Brengman, L. and Eyster, A. Integrated X-Ray Diffraction and Petrography Document Carbonate Mineral Heterogeneity and Hematite Mineralization in the Upper Biwabik Iron Formation, MN Ellison*, K ., Cisneros, J., Eyster, A. and Brengman1, L. Comparing mineralogy along a surface to depth transect of the ~2.7 Ga North Limb Soudan Iron Formation, NE Minnesota - xxvi - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Erickson, S., Fayon, A., Allerton, Z. and Hudak, G. Middle school virtual field trip lessons materials for Archean formations of Lake Vermilion-Soudan Underground Mine State Park Gilberg*, N., Fralick, P. and Li, Z. Geochemical Constraints on Mn Cycling in the Paleoproterozoic Gunflint Formation Gosai*, M., Fralick, P. and Li, Z. Modified Sequential Iron Extraction Method for Analyzing Rare Earth Elements in Banded Iron Formations Grauch, V. and Heller, S. Time-to-depth conversion of seismic-reflection data from eastern Lake Superior and implications for the eastern arm of the Midcontinent Rift Harding*, M. and Hollings, P. Geochemistry, Petrogenesis, and Mineralization of the Makwa Deposit, Bird River Sill Hellrung*, A., Droubi, O., Ruggles, C. and Bonamici, C. Using Anisotropy of Magnetic Susceptibility and U-Pb Geochronology from the Bush Lake Granite, Florence County, WI to Understand Post-Penokean Continental Growth Jonsson, J. and Li, Z. Petrographic Study of Granular Iron Formation in the Gunflint Formation: Evidence for Well-Oxygenated Surface Waters Marin López*, V., Brengman, L., Eyster, A., Mitchell, J., Pu, X., Mangum, J. and Walker, P. Quantitative analysis of iron mineral composition and crystal sizes in the contact metamorphosed Biwabik iron formation and the Bald Eagle intrusion, NE, MN, USA Nowak*, R., Deering, C. and Essig, E. Origin of the World-Class Eagle, Eagle East, and Tamarack Ni-Cu-PGE Deposits and comparative analysis with other Midcontinent Rift- and Siberian Trap-related intrusions Nowariak, E. and Severson, A. Bedrock Geology of the Ericsburg NW, Ericsburg NE, Ray SW, and Ray SE Quadrangles, St. Louis and Koochiching Counties, Minnesota Paliewicz, C., Post, S. and Thakurta, J. Petrographic, geochemical, and mineralogical analyses of manganiferous iron formations and associated lithologies at the Cuyuna Range, central Minnesota Saini-Eidukat, B., Chittick, S. and Nesheim, T. Current geologic and geophysical research on the Precambrian basement of eastern North Dakota, USA Stewart, E., McNall, N., Hart, D., Ames, C., Chase, P., Stewart, E. and Graham, G. Subsurface mapping of the late Ordovician Maquoketa Group in eastern Wisconsin using airborne electromagnetic and well data Tolley, J. and Hollings, P. Variations in Olivine Major Element Composition Across the Midcontinent Rift System NOTE: Asterisk * denotes a student eligible for a Best Student Paper Award - xxvii - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Saturday May 23 (Parking Lot G14, Lakehead University) 8:00 a.m. Field Trip 4: “Classic” Geological Sites in the Thunder Bay Area Leaders: Mark Smyk and Mark Puumala 8:00 a.m. Field Trip 6: Amethyst Deposits of Thunder Bay Leaders: Steve Kissin and Greg Paju 5.00 p.m. Return of Trips 4 & 6 Sunday May 24 (Parking Lot G14, Lakehead University) 5.00 p.m. Return of Trip 5 - xxviii - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Constraining the 3-D Geometry of the Duluth Complex, MN, Using Magnetic Fabrics and Paleomagnetic Data AKIN, Kathryn1 and SWANSON-HYSELL, Nicholas1 1 Department of Earth and Environmental Sciences, University of Minnesota, Minneapolis, MN, USA The Midcontinent Rift developed within the interior of Laurentia during a period of extension and magmatism from 1109 Ma to 1084 Ma (Swanson-Hysell et al., 2019). Emplaced during the development of the Midcontinent Rift, the Duluth Complex is interpreted as the second-largest exposed mafic intrusive complex on Earth. The Duluth Complex is composed of an anorthositic series and a layered series of gabbro and troctolite cumulates (Figure 1; Miller et al., 2002). Many studies have been conducted on the geology, mineralization, structure, timing, and mechanisms of emplacement of the Duluth Complex and nearby Beaver Bay Complex and North Shore Volcanic Group, but there is still some uncertainty surrounding the thickness, and therefore overall volume, of the Duluth Complex. Figure 1: Map of the Duluth Complex field location in northeastern Minnesota. Red diamonds represent sampling locations from the August 2025 field season. Geological map data from Bauer (2022). The tilt of the Duluth Complex is not well-constrained in the anorthositic series, given the absence of macroscopic igneous foliation, so this research is focused on developing data on the magnetic fabrics of the Duluth Complex along a transect to constrain the igneous foliation and to use these data to develop new estimates of the tilt and thickness of the intrusion. Anisotropy of magnetic susceptibility (AMS) is sensitive to changes in mineral alignment and, therefore, is used to constrain igneous foliation, especially in samples that do not display an obvious macroscopic fabric in the field (Schmidt et al., 2007). Remanent magnetization data collected and compared with the expected directions of contemporaneous volcanics can also provide further insight into tilt. Together, the new susceptibility and remanence data will provide important petrophysical information for interpreting upcoming USGS aeromagnetic surveys currently being flown in -1- �Proceedings of the 72nd ILSG Annual Meeting - Part 1 northeastern Minnesota. New constraints on the intensity of remanent magnetization and its ratio with susceptibility (the Koenigsberger ratio) will be added to the Rock Properties database maintained by the Minnesota Geological Survey (Chandler et al., 2011). REFERENCES Bauer, E.J., Jirsa, M.A., Block, A.R., Boerboom, T.J., Chandler, V.W., Peterson, D.M., Wagner, K.G., McDonald, J.M., Dengler, E.L., Meyer, G.N., and Hamilton, J.D., 2022, C-54, Geologic Atlas of Lake County, Minnesota: Minnesota Geological Survey: University of Minnesota Digital Conservancy, https://hdl.handle.net/11299/254822. Chandler, V.W., and Lively, R.S., 2011, Density, Magnetic Susceptibility, and Natural Remanent Magnetization of Rocks in Minnesota: An MGS Rock Properties Database: Minnesota Geological Survey, https://hdl.handle.net/11299/175580 Miller, J.D., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E., 2002, RI-58 Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota:, https://hdl.handle. net/11299/58804. Schmidt, P.W., McEnroe, S.A., Clark, D.A., and Robinson, P., 2007, Magnetic properties and potential field modeling of the Peculiar Knob metamorphosed iron formation, South Australia: An analog for the source of the intense Martian magnetic anomalies? Journal of Geophysical Research: solid Earth, v. 112, doi:10.1029/2006JB004495. Swanson-Hysell, N. L., Ramezani, J., Fairchild, L. M., and Rose, I. R., 2019, Failed rifting and fast drifting: Midcontinent Rift development, Laurentia’s rapid motion and the driver of Grenvillian orogenesis: GSA Bulletin, vol. 131, pp. 913–940, doi:10.1130/b31944.1. Swanson-Hysell, N.L., Hoaglund, S.A., Crowley, J.L., Schmitz, M.D., Zhang, Y., and Miller Jr., J.D., 2021, Rapid emplacement of massive Duluth Complex intrusions within the North American Midcontinental Rift: Geology, vol. 49, pp. 185-189, https://doi.org/10.1130/G47873.1. -2- �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Characterization of hematite ore from former Ely mines, NE Minnesota ALLERTON, P. Zsuzsanna1 and HUDAK, J. George1,2,3 1 2 3 Earth and Environmental Sciences, University of Minnesota, Minneapolis, MN 55455, USA Earth and Environmental Sciences, University of Minnesota, Duluth, MN 55812, USA George Hudak Geosciences P.L.L.C., Duluth, MN 55804, USA The hematite ore deposits located in the Vermilion Range in Ely, northeastern Minnesota, represent some of the highest-grade iron ores ever mined in the United States. These deposits occur within Neoarchean (~2.7 Ga) Algoma-type banded iron formations (BIFs) in the Ely Greenstone belt which is dominantly composed of greenschist facies metamorphosed volcanic, sedimentary and intrusive rocks. The ore bodies, exploited in underground mines such as the Zenith, Pioneer, Sibley and others, consist of steeply dipping, tabular to lens-shaped masses of massive hematite that replace jaspilitic BIF. These bodies are enclosed within greenstone wall rocks and are often localized along brecciated zones within a complex regional fold structure. Machamer’s 1968 study of the Zenith mine details the textural varieties of high-grade hematite ore formed by hypogene hydrothermal replacement of jaspilitic BIF. His petrographic and field descriptions identify five prominent ore textures that reflect stages of replacement, brecciation, cementation, and zoning. The characterization and documentation of these five textures at the Pioneer and Sibley mines are the focus of this research. Hematite ore samples utilized for this study were obtained from the Minnesota DNR Hibbing Core Library. Zenith mine ore samples were not available for re-analysis. Ore texture types described are consistent with the nomenclature developed by Machamer (1968). Type 1, the most abundant texture, is a dense, uniform material composed almost entirely of crystalline hematite, representing the primary massive replacement ore (Figure 1A). Type 2 texture consists of brecciated fragments of type 1 ore cemented by a later generation of secondary crystalline hematite, which commonly contains minute vugs lined with small hematite crystals and appear in a reticulated pattern resembling a boxwork (Figure 1B). Type 3 texture is similar to type 2 but features a cement composed dominantly of carbonate minerals (primarily ankerite or siderite) rather than hematite (Figure 1C). Type 4 texture is composed largely of carbonate minerals; it may contain fragments of earlier type 1 hematite material as well as earlier-formed carbonates, reflecting deeper or more advanced carbonate replacement (Figure 1D). Type 5 texture consists principally of magnetite with variable amounts of carbonate minerals, hausmannite (manganese oxide, Mn3O4) and pyrite; this type is generally non-merchantable due to its lower iron content or higher sulfur. The great bulk of the ore mined at Zenith (and similarly at Sibley mine) consisted of types 1 and 2, with lesser amounts of type 3. Many of the types preserve faint layering parallel to the ore-body walls, produced by alternating textural variations in hematite or by interlayering of massive hematite with more porous hematite or carbonates. Texture types 4 and 5 become more abundant with depth in the Zenith mine (Machamer, 1968). These five textures record a progressive hypogene upgrade of BIF to hematite ore that is generally similar to what has been observed in recent research at the Soudan mine (Allerton, 2025; Allerton et al., 2025). Upgrade processes include initial silica replacement by massive hematite, followed by repeated brecciation and multi-stage cementation, and downward transition to carbonate assemblages, producing the dense, low-impurity ore that made the Ely deposits economically significant (Machamer, 1968). -3- �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Figure 1: Hematite ore textures from the Sibley and Zenith mines, Ely, MN. A) Type 1 texture showing dense crystalline “matrix” with primary hematite aggregates (white to off white, Hem 1) and vug spaces (black, V). B) Type 2 texture exhibiting brecciated Type 1 material (Hem 1 fragment outlined with white dashed line) cemented by secondary hematite crystals (Hem 2) with reticulated pattern and occasional minute silicates (light gray). C) Type 3 texture displaying brecciated Type 1 material (Hem 1) cemented by mostly carbonates (patchy dark gray, Crb) and some silicates (light gray, Sil). D) Type 4 texture presenting mainly carbonates (patchy light and dark gray, Crb), sporadic silicates (light gray, Sil), and hematite aggregates (Hem 1 fragment outlined with white dashed line) and stingers. REFERENCES Allerton, Z.P., 2025. Thermal and hydrothermal effects of Proterozoic events on Archean rocks in northeastern Minnesota, USA: University of Minnesota ProQuest Dissertations & Theses [Ph.D. thesis]. Allerton, Z.P., Courtney-Davies, L., Danišík, M., Hudak, G.J., Teyssier, C., Mitchell, J.T., and Larson, P., 2025. Hematite double-dating defines Proterozoic mineralization and thermal history of Archean banded iron formations in northeastern Minnesota, USA: Geology, https://doi .org /10.1130 /G53517.1. Machamer, J. F., 1968. Geology and origin of the iron ore deposits of the Zenith Mine, Vermilion District, Minnesota (Special Publication SP-2). Minnesota Geological Survey. -4- �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Decoding Shear Zone Evolution in the McFaulds Lake Greenstone Belt, Ontario: Constraints on Crystal-Plastic Deformation and Timing from in-situ Titanite U–Pb Thermochronology ANGOMBE, Moses1, PHILLIPS, Noah2, HOLLINGS, Pete1, STEPHAN, Tobias1, SHESHNEV, Vlad1, DELLER, Mathew3 and SMITH, Andrew3 1 Department of Geology, Lakehead University, 955 Oliver Rd, Thunder Bay, P7B5E1, ON, Canada Department of Earth Sciences, University of Southern California, 3651 Trousdale Pkwy, Los Angeles, 90089, California, United States of America 2 3 Wyloo, 1127 Premier Way unit 1, Thunder Bay, 90089, P7B 0A3, ON, Canada Constraining deformation conditions, kinematics and timing of shear zone activity is essential for determining whether mechanical processes concentrate and localize metal deposits. The McFaulds Lake Greenstone Belt in northern Ontario hosts some of Canada’s most prospective mineralization, including magmatic sulphide, chromite and volcanogenic massive sulphide (VMS)–type deposits. A robust reconstruction of the belt’s deformation history is hindered by an understudied, poorly exposed, arcuate, regionally extensive, dextral shear system including, the Webequie, Triple‑J, and McFaulds shear zones. This study integrates field-based structural observations, microstructural analysis, and in-situ titanite U–Pb geochronology to (1) resolve the kinematic architecture of the major shear zones, (2) constrain the crystal-plastic deformation mechanisms, and (3) determine the temperature and timing of deformation. Newly acquired kinematic results derived from field outcrop‑scale S–C fabrics and asymmetrically rotated porphyroclast microstructures indicate that the NW‑striking Webequie Shear Zone accommodated dextral‑reverse displacement, while the NE‑striking McFaulds and Triple-J Shear Zone are characterized by a dextral‑normal sense of shear. Deformed quartz in phyllonites and mylonites from all shear zones exhibits fine‑grained polygonal aggregates with a few subgrains and a weak crystallographic preferred orientation. These textures indicate that shearing was accommodated predominantly through diffusion‑creep–assisted grain‑boundary sliding processes. Five deformed titanite grains from mylonitic tonalite associated with the Triple‑J shear zone yielded U–Pb dates of ~2775 Ma and Zr‑in‑titanite temperatures of 530–640 °C. In contrast, eighteen euhedral to subhedral titanite grains yield dates between ~2768 and ~2812 Ma and Zr‑in‑titanite temperatures of 650–900 °C. All analyzed titanite grains show no significant difference in temperature or U–Pb dates between rims and cores. We infer that the younger U–Pb dates (~2775 Ma) recorded in deformed titanite constrains the timing of crystal‑plastic deformation, whereas the older, higher‑temperature dates (~2768–2812 Ma) from intact titanites reflect either metamorphic or crystallization. The overlap in deformation and crystallization ages for both deformed and undeformed titanites suggests that shearing in the McFaulds Lake Greenstone Belt was broadly synchronous with emplacement of the regional tonalite suite. These preliminary results show that both shear deformation and magmatism play a critical role in forming the McFaulds Lake Greenstone Belt and its critical mineral deposits. -5- �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Coeval silicate melt and PGE-bearing salt melt inclusions in the Thunder and Seagull intrusions, Ontario: An overview of evidence and data processing challenges. BAIN, Wyatt1 and HOLLINGS, Pete 2 1 2 Department of Earth Sciences, Western University, 1151 Richmond St, London, ON N6A 5B7 Canada Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada The Seagull (~90km north-northeast of Thunder Bay) and Thunder (~12 km north-northwest of Thunder Bay) intrusions are two magmatic sulphide-bearing mafic-to-ultramafic intrusions formed during the early stages of Midcontinent rift (MCR) formation. Investigation of olivine crystals from both intrusions reveals abundant assemblages of polycrystalline silicate inclusions and coeval assemblages of hypersaline inclusions. Both inclusion types occur along primary growth zones in their host crystals and undergo partial homogenization at >700 °C. This indicates that these inclusions contain primary, orthomagmatic fluids trapped at magmatic conditions (i.e., immiscible silicate and salt melt). Scanning electron microscope (SEM) analysis shows that the silicate melt inclusions from both intrusions have similar bulk chemistry and host assemblages of feldspar-apatite-phlogopitebiotite-ilmenite-pyrrhotite with a coexisting volatile phase. Similarly, salt melt inclusions from both intrusions also had similar bulk compositions and comprise mixtures of NaCl-KCl with variable amounts of C- and B-bearing salts. The time-resolved laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) Figure 1: a., b., Photomicrograph of olivine-hosted coeval assemblages of silicate (SMI) and salt melt (HIS) inclusions from the Thunder (a) and Seagull (b) intrusions. c. Annotated backscatter electron (BSE) image of a silicate melt inclusion exposed at the surface of an olivine crystal. Alb=Albite; Bio=Biotite; Phl=Phlogopite; Apt=Apatite; Hbl=Hornblende; Po=Pyrhotite; Ill=Illmenite; Ol=Olivene d. BSE image of a salt melt inclusion exposed at the surface of an olivine crystal and accompanying energy dispersive spectroscopy maps showing the distribution of selected elements for the same area (right). -6- �Proceedings of the 72nd ILSG Annual Meeting - Part 1 signals from unhomogenized salt melt inclusions from both intrusions consistently showed unambiguous, overlapping peaks for the following element groups: K-P-La-Ce-Ta-U-Th-Nb-RbSr-Ba-Nd-Li, Co-Ni-Cu-Zn-Ag-Pb-S, and Pd-Pt-Au-Sb-Bi. The overlapping peaks for base metals and S likely reflect the presence of crystalline sulphides. Likewise, the overlap of the PGE+Au and Sb-Bi suggests the presence of PGE-bearing antimonide and bismuthide minerals (i.e. PGM). This indicates that salt melts coexisted with silicate melts during the emplacement of both intrusions and were significantly enriched in base metals and PGEs. This data, along with observations of salt melt inclusions in other mafic-ultramafic intrusions (Mcfall et al., 2021; 2023), suggests that these fluids may be important transport media for Ni-Cu-PGE in orthomagmatic environments. Salt melt compositions derived from LA-ICPMS data had unusually high PGE concentrations in the 10s to 100s of ppm. These results should be treated critically, as reducing data from salt melt inclusions presents several technical challenges. These include uncertainty in determining a major element internal standard for salt inclusions and matrix mismatch between the inclusions and the external standard. ICPMS systems are also typically limited in their ability to analyze halogens, C, and S, which are typically major element components of salt melt inclusions (e.g. Xu et al, 2024; Bain et al., 2022). This talk will provide an overview of the geology of the Seagull and Thunder intrusions, present textural and geochemical data from coeval polycrystalline silicate melt and salt melt inclusion in both and discuss the various data reduction schemes being used on this data set. This talk will also discuss a general workflow for salt melt analysis using SEM and LA-ICPMS techniques. REFERENCES Bain, W.M., Lecumberri-Sanchez, P., Marsh, E.E., and Steele-MacInnis, M., 2022. Fluids and melts at the magmatichydrothermal transition, recorded by unidirectional solidification textures at Saginaw Hill, Arizona, USA. Economic Geology, doi:10.5382/econgeo.4952 McFall, K.A., McDonald, I., Yudovskaya, M.A., Kinnaird, J., Hanley, J.J., Kerr, M., and Tattitch, B., 2023. Carbonatedominated hypersaline brines and their importance for metal transport in magmatic and magmatic-hydrothermal critical mineral systems. AGU Fall Meeting, San Francisco, Volume of Abstracts, V44A-08 McFall, K.A., McDonald, I., Yudovskaya, M.A., Kinnaird, J., Hanley, J.J., Kerr, M., and Tattitch, B., 2022. High temperature (> 800° C) brine and sulphide melt interaction during the formation of Northern Bushveld magmatic sulphide Cu-NiPGE deposits. Goldschmidt Conference, Hawaii, Volume of Abstracts, #9496 Xu, X., Bain, W.M., Tornos, T., Hanchar, J.M., Lamadrid, H.M., Lehman, B., Xu, X., Steadman, J.A., Bottrill, R.S., Soleymani, M., Rajabi, A., Li, P., Tan, T., Shihong Xu, S., Locock, A.J., Steele-MacInnis, M., 2024. Magnetiteapatite ores record widespread involvement of molten salts. Geology. 52, 417-422. doi:10.1130/G51887.1 . -7- �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Preliminary geochronology of lithium pegmatites and host rocks, Archean Superior Province, northwestern Ontario BEYER, Steve1, CUTTS, Jamie1, HNATYSHIN, Danny1, POWELL, Jeremy1, CAMACHO, Alfredo2, CAWOOD, Tarryn3, and DREVER, Garth4 1 2 3 4 Natural Resources Canada, Geological Survey of Canada, 601 Booth Street Ottawa, ON K1A 0E8 Canada University of Manitoba, 125 Dysart Rd Winnipeg, MB R3T 2N2 Canada University of British Columbia-Okanagan, 3247 University Way Kelowna, BC V1V 1V7 Canada Frontier Lithium Inc., 2614 Belisle Drive Val Caron, ON P3N 1B3 Canada With a combined resource estimated at 50 million tonnes Li grading 1.6% Li2O [1], the PAK and Spark lithium-cesium-tantalum (LCT) pegmatites in northwestern Ontario represent a major potential source of Li, as well as other rare metals such as Nb, Sn, Ta, Rb, and Cs. Together with other Li pegmatite showings in the region (Fig. 1), this suggests high Li prospectivity for the northwestern Superior Province. Better understanding of these significant but understudied pegmatites, together with their peripheral peraluminous granites and other host rocks, will help refine models of rare-metal-enriched pegmatite formation in Archean terranes, and lead to improved discovery success. Here we present multi-mineral geochronological data for the pegmatites and host rocks to clarify connections between pegmatite emplacement and regional tectonics. The crystallization ages of pegmatites and host rocks were investigated using U and Pb isotopes in zircon and monazite measured by SHRIMP. The oldest rock in the area is gabbro that hosts the Spark pegmatite, in which zircon gives an age of 2861 ±3 Ma. Although this unit is mapped as the 2925 Ma Setting Net assemblage of the Favourable Lake greenstone belt, the age is instead within error of the younger 2858 ±5 Ma Eastern Trout assemblage [2]. Zircon in the Pakeagama Lake granite, a biotite-muscovite-garnet peraluminous granite that hosts the PAK pegmatite, gives an age of 2727 ±4 Ma, the first reported age for this pluton. Zircon from coarse K-feldspar-muscovite-apatite-quartz pegmatite at PAK, and zircon from tonalite that hosts the Pennock Lake pegmatite 20 km northwest of PAK, yield ages of 2727 ±1 and 2728 ±4 Ma, respectively, which are the same age as the Pakeagama Lake granite within error. Similar Th/U ratios, indistinguishable ages, and some textural evidence suggests that PAK pegmatite zircon may be inherited from the Pakeagama Lake granite. An overgrowth on one zircon in Spark gabbro gives an age of 2683 ±6 Ma. Isotopes of Hf are used to trace the source of the melt from which the zircon crystallized, and were measured in situ using LA-MC-ICPMS in the same location as the SHRIMP spots. Zircon from gabbro hosting the Spark pegmatite have the most radiogenic εHf values of 5.30 ±0.33, intersecting the value of depleted mantle at 2.86 Ga. Zircon from the PAK pegmatite and tonalite hosting the Pennock Lake pegmatite are less radiogenic, having εHf values of 2.21 ±0.35 and 1.25 ±0.25, respectively, possibly suggesting mixing with older continental crust. Lastly, we examine the thermochronology of muscovite in pegmatite zones, and biotite and hornblende in host rocks and contact zones using Ar-Ar isotope systematics. Step heating age spectra for muscovite (n=10) in the PAK, Spark, and Pennock pegmatites, and the Pakeagama Lake granite, are all disturbed and yield integrated ages between 2532 and 2174 Ma. Hornblende (n=1) in gabbro at Spark gives a slightly disturbed age spectrum with a pseudo-plateau age of 2805 ±4 Ma. Biotite (n=3) in metavolcanics at Spark, and at the contact between the Spark pegmatite and metavolcanics, yield pseudo-plateau ages of 2447 and 2446 ±1 Ma, respectively, whereas biotite in the Pakeagama Lake granite yields a pseudo-plateau age of 1955 ±10 Ma, possibly suggesting partial disturbance of Ar systematics during the Trans-Hudson orogeny. In situ Ar-Ar ages in transects from grain edge to center in muscovite from the Spark pegmatite range from 2687 ± 16 Ma to 1933 ±42 Ma, the oldest age indistinguishable from the U-Pb zircon overgrowth age of 2683 ±6 Ma in Spark gabbro. It is possible this age (~2685 Ma) represents the emplacement of the Spark pegmatite. Taken collectively, these data indicate that the host rocks comprise both ~2861 Ma gabbro and ~2727 Ma granite and tonalite. Although pegmatite emplacement has not yet been directly constrained, it may have occurred together with a thermal pulse at ~2685 Ma, as recorded by Ar-Ar dates from muscovite in the Spark pegmatite, -8- �Proceedings of the 72nd ILSG Annual Meeting - Part 1 and a zircon overgrowth in the host rock. Figure 1. Map showing the location of LCT pegmatites in northwestern Ontario and their host rocks. The area shown in the main map is indicated by the red box in the location map. LCT = lithium-cesium-tantalum; NRCan MRDEM = Natural Resources Canada medium resolution digital elevation model REFERENCES Accad, E., Bisaillon, C., Gagnon, D., Ibrango, S., Liskovych, V., Prévost, G., Sellars, E., and Vasquez, L., 2025. NI 43-101 Technical Report Feasibility Study – PAK Lithium Project, Mine and Mill in Northwestern Ontario, Canada. DRA Americas Inc. Corfu, F., Davis, D.W., Stone, D., and Moore, M.L., 1998. Chronostratigraphic constraints on the genesis of Archean greenstone belts, northwestern Superior Province, Ontario, Canada. Precambrian Research, 92, 277–295. -9- �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Quartz Trace Element and TEM Analysis of Selected Economic LCT Pegmatites BILBOE, Michael1, ZUREVINSKI, Shannon1, and CONLY, Andrew1 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1, Canada. This study assesses geochemical and textural trends of economic LCT-type pegmatitic quartz using different analytical applications, namely laser ablation- inductively coupled plasma mass spectrometry (LA-ICP MS) trace element geochemistry, microscope-based laser induced breakdown spectroscopy (LIBS) and high-resolution transmission electron microscopy with- energy dispersive X-ray (TEMEDX) analyses. The study utilized samples from well-documented economic LCT pegmatites (Northwestern Ontario and Manitoba) to assess a variety of modern questions relating to trace element geochemistry. Specifically, this study observes geochemical trends in trace element composition of quartz that display spodumene-quartz intergrowth (SQUI) textures, extrapolates classification of SQUI textures (after Breasley, 2025) to the economic Pakeagama pegmatite (Ontario) and utilizes HR-TEM techniques to image potential Li-bearing nano inclusions hosted within quartz. Breasley (2021) outlined varieties of SQUI originating from unique crystallization sequences. In this study, SQUI from the Pakeagama pegmatite was compared to the recent proposed classifications to ensure consistency in texture classification can be met in different pegmatite systems. Few studies have targeted quartz trace element trends in Group 1 SQUI-bearing pegmatites. Trace element trends in SQUI should be properly understood to avoid improper conclusions when inferring mineralization trends outlined by Müller et al. (2021). Trends in SQUI-associated quartz trace elements were analyzed and compared with non-SQUI pegmatite quartz trace element trends using LA-ICP-MS and LIBS. It was found that few groups of trace elements, particularly Na and Ge, show weak to moderately depleted values with respect to the ratio of Li/Al specifically in quartz grains associated with SQUI (Figure 1). This is interpreted to be the result of trace elements present in the parent mineral (petalite) preferentially incorporating into spodumene rather than quartz during SQUI formation. Additionally, LIBS analysis suggests that elevated concentrations of Li are incorporated into micas and feldspars in the North Aubry sample, likely related to elevated trace element incorporation seen in quartz. Nanoinclusions (fluid and mineral) are thought to be a major contributor to trace element incorporation in quartz (Shah et al., 2022). TEM-EDX analysis was conducted to document and image potential nanoinclusions hosted in quartz. The analyzed portion of the North Aubry sample did not host nanoinclusions displaying any detected Li signatures, however, a decrepitated nanofluid inclusion, with detected sodium and chlorine, was identified (Figure 2). The results suggest that nanoinclusions, while present, may not necessarily contribute significantly to trace element concentrations of Li, Ti, Ge or Be in pegmatitic quartz (possibly due to their presence below detection limits), however, the observed nanoinclusions could suggest the potential Li-brine fluid fluid inclusions and this may be contributing to well-documented quartz trace element concentrations in quartz. REFERENCES Breasley, C. (2021). Lithium aluminosilicate formation and textural origins in evolved pegmatites: Insights from the Tanco Pegmatite, Manitoba and Prof Pegmatite, British Columbia. Doctoral Thesis, University of British Columbia. Müller, A., Keyser, W., Simmons, W. B., Webber, K., Wise, M., Beurlen, H., Garate-Olave, I., Roda-Robles, E., & Galliski, M. Á. (2021). Quartz chemistry of granitic pegmatites: Implications for classification, genesis and exploration. Chemical Geology, 584, 120507. Shah, S. A., Shao, Y., Zhang, Y., Zhao, H., & Zhao, L. (2022). Texture and Trace Element Geochemistry of Quartz: A Review. Minerals, 12(8), 1042. Young, T. (2023). Trace Element Geochemistry of Pegmatitic Quartz from the Superior Province, ON HBSc thesis, Lakehead University. - 10 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Figure 1: Ge (PPM) Versus Li/Al ratios in analyzed samples. Samples Pakeagama, Tanco and Frontier are SQUI-hosted quartz analyses. Data from three additional non-SQUI samples, Seymour, Georgia and Mavis Lake, were included to better highlight the role SQUI has on quartz trace element incorporation (Seymour, Georgia and Mavis Lake data from Young, 2024). Figure 2: TEM image of a nanoinclusion in quartz, identified in the North Aubry sample. EDX mapping of the inclusion detected Na and Cl. - 11 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Improved Precision and Better Accuracy: SHRIMP-II Detrital Zircon Analysis of Samples Across the Stratigraphy of the Midcontinent Rift BLEEKER, Wouter1 and WODICKA, Natasha1 1 Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8, Canada As part of on-going research on the evolution of North America’s Midcontinent Rift (MCR), its stratigraphy, and the detailed setting of its mineral systems, we continue our efforts to improve the age constraints on key geological features of the rift. In addition to many new and improved U-Pb ages on igneous units [e.g., 1,2], we are also undertaking detrital zircon dating of key stratigraphic units across the MCR stratigraphy, from bottom to top (Fig. 1), to resolve remaining questions of depositional ages and sediment provenance. We do so by using the SHRIMP-II ionprobe at the GSC in Ottawa (Fig. 2). With typical spot sizes of ~13x16 μm, fewer corrections during data processing, no down-hole parent-daughter fractionation, and the ability to do multiple, carefully placed spots (away from cracks Figure 1: Generalized stratigraphy of the MCR. Many key ages and mineral systems are indicated. Small red squares identify our detrital samples analyzed by SHRIMP. Figure 2: The SHRIMP-II lab at the Geological Survey of Canada, Ottawa. (SHRIMP: sensitive highresolution ion microprobe.) or other complexities) on grains of particular interest, the SHRIMP-II ionprobe yields significantly more precise and accurate data than more rapid laser ablation analysis, and a more rigorous check on concordancy [3,4]. A typical sample run will analyze 80–100 grains, with >90% of the results falling within the 95–105% concordancy interval (accuracy) used in final interpretation. With multiple spots (n=3–5) on key grains, the 2s uncertainty of weighted mean ages can be improved to ±5–15 Ma (precision). All of this does take a fair amount of machine time, with a typical spot analysis taking ~15 mins, and an entire sample run, including calibration on well-characterized zircon reference materials, more than 24 hrs. Analysis is done on polished grain mounts that are imaged in both BSE (backscatter) and CL (cathodoluminescence) mode prior to analysis to guide grain selection and spot location. Here we briefly discuss some initial results. One such result, on the high-energy “event layer” near the top of the Gunflint Formation, was presented at an earlier ILSG meeting [5]. It confirmed that this layer contains ejecta material from the Sudbury target area in the form of ca. 2460–2450 Ma zircons from the Creighton Granite and Copper Cliff Rhyolite. In the next sample up (Rove Formation greywackes), our results fail to identify any age peaks younger than ca. 1845 Ma, which we consider the maximum depositional age for the Rove Formation [cf. 6], i.e. entirely a Penokean foreland basin. - 12 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 There are hints of some younger grains, perhaps to as young as ca. 1805 Ma, but this requires further work. Interestingly, in addition to some Archean input, there is also one grain at ca. 2311 Ma of the reworked felsic ash material known from the upper Huronian Supergroup [see also ref. 5]. Thin sandstone layers intercalated with the Pillar Lake Volcanics basalt flows, near Armstrong, show youngest grains at ca. 1500 Ma, similar to our Sibley Group sandstone samples, and do not contain any of the abundant younger grains (and peaks) prominent in the basal MCR sandstones discussed below (see Fig. 3). This confirms our interpretation that these thin sandstone beds and the Pillar Lake Volcanics are part of basal Sibley Group rift volcanism and sedimentation at ca. 15001480 Ma, not a northern outlier of ca. 1.11 Ga MCR stratigraphy sensu stricto [cf. 7]. Three samples of the sandstone/quartzites (Bessemer, Nopeming, and Puckwunge formations) immediately below the onset of “Early Stage” basaltic volcanism yield generally similar results with youngest grains in the 1135–1100 Ma age range (weighted means), and strong peaks (modes) at ca. 1125 Ma, 1160–1140 Ma and various older ages (e.g., 1470 Ma, Wolf River Batholith), all the way to 3.3 Ga (Fig. 3). These are just some initial results and a full and complete analysis of all 12 samples will be presented elsewhere. SOME REFERENCES [1] [2] [3] [4] [5] [6] [7] Bleeker, W., Smith, J., Hamilton, M., Kamo, S., Liikane, D., Hollings, P., Cundari, R., Easton, M., and Davis, D., 2020. Geological Survey of Canada, Open File 8722, p. 7–35. DOI: 10.4095/326880. Smith, J., Bleeker, W., and Hamilton, M., 2026. GSA Bulletin, v. 138(3–4), p. 1419–1438. DOI: 10.1130/B37649.1. Stern, R.A., 1997. Geological Survey of Canada, Current Research 1997-F, p. 1–31. DOI: 10.4095/209089. Stern, R.A., and Amelin, Y., 2003. Chemical Geology, v. 197, p. 111–146. DOI: 10.1016/S0009-2541(02)00320-0. Bleeker, W., Wodicka, N., Kamo, S., Hamilton, M., Emon, Q., and Smith, J., 2024. 70th ILSG Meeting, Proceedings & Abstracts, Part I, p. 11–12. Heaman, L., and Easton R.M., 2006. Ontario Geological Survey, Miscellaneous Release, MRD-191, 78 p. Hollings, P., Smyk, M., Bleeker, W., Hamilton, M., Cundari, R., and Easton, M., 2021. Canadian Journal of Earth Sciences, v. 58(10), p. 1116–1131. DOI: 10.1139/cjes-2021-0012. Figure 3: Example of our SHRIMP-II detrital zircon results: probability density plot for the Bessemer Quartzite (BNB-18022), sampled just below the onset of basalt flows. Inset: images of youngest grains with 3 spots (repeated analyses at the same locality), yielding a weighted mean age of 1101±14 Ma. - 13 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Update to: a complex F-rich alkalic pegmatite in the pyroxene syenites of the Stettin Complex, Wausau Complex, Marathon County, Wisconsin BUCHHOLZ, Thomas1, FALSTER, Alexander2, and SIMMONS, William2 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494, 1 MP2 Research Group, Maine Mineral and Gem Museum, PO Box 500, 99 Main Street, Bethel, Maine 04217, USA 2 The Stettin Complex is the oldest (1565 +3-5 Ma, Van Wyck 1994) and most alkalic of the four intrusions that comprise the Wausau Syenite Complex, and is composed of various syenite phases. This abstract is an update to studies of this dike reported in ILSG 2024 and 2025; interested readers are referred to those abstracts. As noted by Buchholz et al. (2025) relatively common soft, pale yellow to creamy to brown grains typically contain high Ti-Ce-Fe contents with traces of other elements, and were suspected to consist of an unidentified Ti-Ce4+-Fe phase. Hand-picked grains from several visually identical samples were analyzed using powder XRD to determine crystalline phases present. Results indicate the presence of only three crystalline phases: arfvedsonite, lucasite-(Ce), (CeTi2(O,OH)6) and cerianite(Ce), (Ce4+,Th)O2. To balance charges in lucasite-(Ce), Ce is likely present as Ce4+ and OH probably absent or negligible. The altered grains may have originally been a LREE-Ti-rich mineral such as chevkinite-(Ce) or aeschynite-(Ce) that were subsequently altered under oxidizing conditions, removing LREE3+ and Si (and altering Ce3+ to Ce4+), thus allowing the crystallization of lucasite-(Ce) and cerianite-(Ce). Oxidation states appear to have fluctuated during pegmatite crystallization, as Ce3+ rich minerals such as synchysite/parisite, britholite-group minerals, monazite-(Ce) and indeed sparse remnants of chevkinite-(Ce) are present in later crystallizing portions of the dike. The potential for britholite-group minerals was discussed by Buchholz et al. (2025), and since then two group minerals have been identified: fluorbritholite-(Ce) and britholite-(Ce). Both form small pale pink to whitish rounded masses in pockets and vugs. At a minimum EDS analysis is required to distinguish these two species, as well as distinguish them from visually similar synchysite/parisite series minerals. Although bismuthinite is known from thin veinlets crosscutting the pegmatite (Buchholz et al., 2025), native Bi has subsequently been found as masses in small interior zone vugs in the pegmatite. Standards-based EDS indicates the Bi contains small amounts of Te; approximately 2-3.5 wt. %. The Te (as Te2-) is probably present as small admixed grains of a Bi-Te mineral such as tellurobismutite, hedleyite or another Bi-Te species. Possible nacareniobsite-(Y) was found as an inclusion in a small aggregate of fergusonite-(Y). Standards-based EDS data show good agreement with the published composition of the species, but the small size of the grain (approx. 25 µm) and the scarcity of the mineral suggest more examples should be sought to confirm this data. Nacareniobsite-(Y) was first described in 2023 and is so far a one-locality mineral, suggesting this may be the second locality for this species. Recent thorough cleaning of fresher exposures has revealed that parallel joints or fractures are closely spaced across much of the pit exposure. All are parallel, near-vertical and roughly oriented WNW-ESE. Possible displacement is unknown at this time, but they suggest a degree of oriented stress may have affected portions of the pluton late in its cooling history or at sometime thereafter. REFERENCES: Buchholz, Thomas, Falster, Alexander, and Simmons, Wm, 2024. Preliminary mineralogy of a pegmatite in the pyroxene syenites of the Stettin Complex, Wausau Complex, Marathon County, Wisconsin (Abstract): Institute on Lake Superior Geology, 70th Annual Meeting, Part I, Program and Abstracts, 19-20. - 14 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Buchholz, Thomas, Falster, Alexander, and Simmons, William, 2025. A complex F-rich alkalic pegmatite in the pyroxene syenites of the Stettin Complex, Wausau Complex, Marathon County, Wisconsin (abstract): Institute on Lake Superior Geology, 71st Annual Meeting, Part I, Program and Abstracts, 15-16. Van Wyck, N. 1994. The Wolf River A-type magmatic event in Wisconsin: U/Pb and Sm/Nd constraints on timing and petrogenesis (abstract): Institute on Lake Superior Geology, 40th Annual Meeting, Part 1, Program and Abstracts, 81-82. - 15 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Reassessing variations in metamorphism across the Penokean orogen in Northern Michigan: Part 2, Reinterpreting metamorphic nodes CANNON, W. F.1, SALERNO, R.1, DRENTH, Benjiman J.2 and BEDROSIAN, Paul A.2 1 2 U.S. Geological Survey, Reston, VA U.S. Geological Survey, Denver, CO A classic study of regional metamorphism (James, 1955) documented variations in metamorphic grade in Paleoproterozoic sedimentary rocks across the Upper Peninsula of Michigan. James interpreted the spatial variations of index minerals as four discrete nodes of metamorphism with concentric zones, defined in pelitic rocks, ranging from chlorite to sillimanite grade (Figure 1). Those isograds are widely used up to the present day to characterize the Penokean metamorphism of the region. These concentric nodes imply localized sources of heat across the region rather than a more widespread source related to regional orogenic processes. Figure 1. Map showing isograds interpreted by James (1955) and distribution of metamorphic index minerals from James and later studies. Compilation of metamorphic index minerals in northern Wisconsin indicates that the high-grade metamorphism extends well west of the Watersmeet node as mapped by James. Widespread garnet occurrences observed in core drilled through Paleozoic cover rocks also show that metamorphism to at least garnet grade extends far east of the exposed Peavy node. Patterned area is proposed allochthon(s) which include the Iron River-Crystal Falls and Menominee iron ranges. Gray shaded region in SE is area of Paleozoic cover. We propose an alternative interpretation for the Watersmeet and Peavy metamorphic nodes and their implied discrete heat sources. The index mineral occurrences in Figure 1 show a belt, at least 250 km long, of metamorphism to garnet or higher grade including scattered occurrences of kyanite to about 50 km north of the Niagara fault. That belt is broken by a gap of about 50 km between the Watersmeet and Peavy nodes where rocks are mostly chlorite-grade sedimentary rocks. We suggest that the gap is a result of post-metamorphic northward emplacement of allochthons of low-grade rocks over the more highly metamorphosed rocks, and that the belt of high-grade rocks is continuous beneath the allochthons. The belt of high metamorphic grade rocks, thus, is a result of regional tectonic burial to mid- to lower crustal depths, and related heating during the climactic closing phase of the Penokean orogeny, rather than to largely speculative individual heat sources. More localized - 16 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 heating by contemporaneous intrusions likely caused some magnification of regional heating such as in the Peavy node (Roy, et al., 2025) The allochthonous nature of the Paleoproterozoic rocks was first proposed by Sims (1992) and supported by more recent work (i.e. Cannon and Ottke, 1999), and recently acquired aeromagnetic and electromagnetic data. The very close spacing of isograds inferred by James (1955), such as the southeastern edge of the Watersmeet node and southwestern edge of the Peavy node, would require extreme lateral temperature gradients that are difficult to reconcile with progressive heating from a central source. Those abrupt lateral changes in metamorphic temperatures are more consistent with a tectonic contact between the high-grade rocks and overthrust low-grade rocks. If that is correct, it has significant implications for the age of allochthon emplacement and the nature of post-Penokean tectonism in the region. The peak metamorphism of the Watersmeet and Peavy nodes is well constrained to 1837-1825 Ma at depths of 30-35 km (Roy, et al., 2025: Salerno, et al., in press). Emplacement of allochthons with low metamorphic grade directly atop these mid- to lower-crustal rocks implies that the high-grade rocks were largely exhumed before emplacement, and that overthrusting must have been a post-Penokean event. Rapid exhumation of active orogens has been documented in many places globally with rates measured in kilometers/million years, so exhumation observed in Michigan could have been accomplished in 10 million years or less. Thus, the suggested overthrusting could be only slightly younger than the generally accepted ~1830 Ma date for termination of Penokean deformation, nevertheless recording continued post-Penokean regional compressive tectonism in the region. REFERENCES Cannon, W.F., and Ottke, D., 1999. Preliminary digital geologic map of the Penokean (early Proterozoic) continental margin of Northern Michigan: U.S. Geological Survey Open-File report 99-547. James, H.L., 1955. Zones of regional metamorphism in northern Michigan: Geological Society of America Bulletin, v. 66, p. 1465-1488. Roy, Supratik, Holder, R.M., Jahandar, R., Brenner, D.C., Nelson, L.L. and Viete, D.R., 2025. Mantle heating drove shortduration Barrovian-type regional metamorphism during the Penokean orogeny, Michigan (USA) Geological Society of America Bulletin, https://doi.org/10.1130/B38653.1 Salerno, R., Cannon, W.F., Thompson, J., Souders, A., Vervoort, J., and Hillenbrand, I., in press. Unraveling protracted modification of Archean and Paleoproterozoic crust in central Laurentia, Penokean orogen, with garnet and accessory mineral geochronology and microstructural analysis: Geological Society of America Bulletin. Sims, P.K., 1992. Geologic map of Precambrian rocks, southern Lake Superior region, Wisconsin and northern Michigan: U. S. Geological Survey Miscellaneous Investigations Map I-2185, scale 1:500,000. - 17 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 An introduction to the northwestern Huron Mountains of the Upper Peninsula, Michigan: field relations and preliminary structural interpretations CARLTON, Kenz M.1, TIKOFF, Basil1, and NACHLAS, William O.1 University of Wisconsin–Madison, Department of Geoscience, 1215 West Dayton Street, Madison, Wisconsin 53706, USA 1 The Huron Mountains of the Upper Peninsula, Michigan, are part of a granite-greenstone terrane and likely represent part of the southern extent of the Superior Craton. Recent field mapping and microstructural analysis indicate the existence of an amphibolite basement intruded by compositionally variable granitoids. The amphibolite basement is a banded schist with a high amphibole content that may represent a strongly metamorphosed mafic protolith. The two plutons of this site each have rapidly varying appearances and expressions of fabrics, banding that varies from non-existent to thick gneissic, and variable compositions from monzogranite to quartz-rich tonalite lithologies. The contacts between the schist and granitoid plutons of this site vary in expression over relatively short distances and, in some cases, can be traced from a planar feature into a 50+ m wide transition zone. The relation between the granitoid and the amphibolites is intrusive, as a range of sizes of amphibolite inclusions can be found within the plutons, usually near the contacts. Mafic and felsic dikes are both abundant. Ongoing work to analyze bulk and trace element geochemistry and U-Pb geochronology will constrain the timeframe of geologic events, the tectonic origin of the groundmass (i.e., which terrane, protolith), and the source of plutonism. The pervasive regional fabric displays a general northwest strike/northeast dip; however, the foliation expression in outcrop is frequently inconsistent, with tens of degrees of difference in both strike and dip possible within 30 meters or less. In general, traceable exposures of the schist-pluton contacts are parallel or subparallel to foliation. Additional structures found in outcrops include mesoand micro-scale faults and meso-scale or larger shear zone features. In thin section, microstructures indicate solid-state deformation, including myrmekite, cuspate-lobate grain boundaries, and internal grain deformation. These analyses support the model of emplacement of quartz-rich plutons into a meta-mafic basement during regional shearing, in the northwestern Huron Mountains. - 18 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Petrography and Geochemistry of the Mound Lake Pluton, Northwestern Ontario CHAISSON, Amy1, SMYK, Mark1, and ZUREVINSKI, Shannon1 1 Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Mound Lake lies approximately 90 km north-northeast of Thunder Bay and 25 km northwest of Nipigon. The Mound Lake Pluton is a 7 km-wide, ovoid, muscovite-bearing granite that has intruded Quetico metasedimentary rocks. It was first described by Hart (2005) and Hart et al. (2005) and thought to be a prospective fertile granite, capable of hosting or spawning rare metal mineralization (cf. Breaks et al., 2005). This study documents the petrography, mineral chemistry, and whole-rock geochemistry of the Mound Lake granitic rocks. The study utilized samples from an initial geochemical and geological reconnaissance program (Smyk, 2022). Thirty-one samples were collected from the pluton and another was collected from from a granitic pegmatite dyke in andalusite schist from the shore of Frazer Lake. Analytical methods included transmitted light microscopy, major and trace element whole-rock geochemistry, and quantitative mineral compositional analyses using Scanning Electron Microscopy- Energy Dispersive X-ray Spectroscopy (SEM-EDX) with Back Scattered Electron (BSE) imaging to characterize mineral textures and compositions. The Mound Lake granitic rocks host irregular pegmatitic patches and miarolitic cavities containing quartz and large, drusy K-feldspar crystals. Massive, medium-grained granitic rocks are crosscut by a variety of aplitic and pegmatitic dykes. Petrographic and mineral compositional analysis identifies the pluton as a two-mica granite, composed of K-feldspar, quartz, muscovite, biotite, and plagioclase, with accessory zircon, apatite, monazite, tourmaline and thorite. Plagioclase, whose compositions range from albite to oligioclase, locally exhibited Na-rich, albite rims. Biotite compositions were found to represent annite/siderophyllite endmembers. Perthitic exsolution and granophyric intergrowths exemplify late-stage crystallization, while sericitization and chlorite alteration are related to post-magmatic hydrothermal activity. The presence of granophyric intergrowths suggests that at least some portions of the magma experienced pronounced undercooling during the final stages of crystallization. Geochemical data confirm a peraluminous, S-type affinity (Alumina Silica Index of 1.05–1.31) with trace element signatures plotting in the Volcanic Arc Granite (VAG) and syn-collisional fields. Granitic rocks display moderate LREE enrichment and HREE depletion, with variable Eu anomalies reflecting the relative role of plagioclase fractionation and accumulation. The Mound Lake Pluton shows increased Li and Cs (+ Ce, Ta and Be) concentrations along its northern contact (Figure 1). Elevated Ce concentrations correlate with samples with higher monazite content. The consistently peraluminous nature and mineralogy (muscovite + biotite, + garnet) support the contention that the pluton is a product of metasedimentary melting, likely triggered by thermal relaxation following oblique accretion in the Superior Province (Chappell, 1999). A spodumene-bearing, granitic pegmatite dyke, discovered in 2023 (https://www.geologyontario.mines.gov.on.ca/mineral-inventory/ MDI000000003501), approximately 3 km north of the northern contact of the pluton, attests to the fertility of local granitic rocks. - 19 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Figure 1. Li (left) and Cs concentrations (right) across the Mound Lake pluton (data from Smyk, 2022). REFERENCES Breaks, F. W., Selway, J. B., and Tindle, A. G. (2005). Fertile Peraluminous Granites and Related Rare-Element Pegmatites, Superior Province of Ontario. Short Course Notes, 17, pp.87–125. Chappell, B. W. (1999). Aluminium saturation in I- and S-type granites and the characterization of fractionated haplogranites. Lithos, 46(3), pp.535–551. https://doi.org/10.1016/S0024-4937(98)00086-3. Hart, T.R. 2005. Precambrian geology of the southern Black Sturgeon River and Seagull Lake area, Nipigon Embayment, northwestern Ontario; Ontario Geological Survey, Open File Report 6165, 63p. Hart, T.R., Whaley, A.G. and Pace, A. J. 2005. Precambrian Geology of the Southern Black Sturgeon River–Seagull Lake– Disraeli Lake Area, Nipigon Embayment, Northwestern Ontario; Ontario Geological Survey, Preliminary Map P.3562, scale 1:50 000. Smyk, M. C. (2022). NI 43-101 Early-Stage Exploration Property Report, Mound Lake Property, Thunder Bay District, Ontario, Canada; Technical Report, 107p. https://www.geologyontario.mines.gov.on.ca/persistentlinking?assessment=20000022160. - 20 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Digital Preservation and Enhanced Utility of Exploration Core Descriptions from the Keweenaw Copper District, Michigan: Progress toward a Map-based Web Portal DeGRAFF, James, HILTUNEN, Lindsay, LAFRENIERE, Don, LIZZADRO-McPHERSON, Dan, VYE, Erika, COWLING, Bob, BORNHORST, Theodore, J., and ROSE, William (deceased) Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931 U.S.A. The Michigan copper rush starting in 1843 at Copper Harbor (Fig. 1) led to 150 years of mining that produced ~7.5 x 106 MT of copper (1), attracted ~100,000 persons from 40 countries, and profoundly influenced understanding of Lake Superior geology, advances in mining technology, and the region’s pattern of life. Companies invested significantly in trenching, coring, and mining operations that generated an enormous body of geologic information. The U.S. Geological Survey (USGS) compiled much of this information in the 1950s as bedrock geology maps with supporting cross sections and reports. Available online in digital form, these map products are derived in large part from a substantial quantity of detailed paper records that are not easily accessed, including core descriptions from exploratory holes drilled from 1899 through the 1970s. Drilling records produced after the 1950s generally have not been used in later investigations also because of difficulty of access. Paper records and microfiche that degrade with time are stored at various locations (2-4), further complicating their use. A few years ago, we began a volunteer project to identify and gather such information into a digital image repository, to extract it into tabular databases, and to explore how to make it available (5) for use by scientists, industry, land-use planners, and the general public (Fig. 2). These early efforts led to a two-year project funded by a Save America’s Treasures grant (ST-256897-OMS-24) through the National Park Service, focused on drilling records in the Michigan Figure 1: Michigan’s native copper mining district with exploratory diamond-drill holes (DDHs) coded by information that is available. TBD – to be determined; WUP – Western Upper Peninsula. Technological University Archives. The current project has three phases: 1) scan all paper records of core descriptions, drafted vertical sections, and drilling metadata; 2) convert scanned records to character data and store in files with - 21 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 tabular formats; 3) create an online, GIS-based, search tool to provide access to the materials. Phase 1 of the project, now complete, has produced scanned core logs for 801 diamond-drill holes from 64 series. After the project was terminated in April 2025 and then reinstated in June, we prioritized Phase 3 to develop the online GIS-based search and delivery tool for scanned files in case funding was lost again. Functional design work is complete and implementation is being tested. Drill hole locations for the GIS-based map were digitized from USGS maps of the Keweenaw Peninsula and supplemented with data from Michigan’s EGLE website. A drillhole attribute table contains positional data, hole direction, total depth, and drilling metadata. Phase 2 of the project is ongoing and involves extracting character data from PDF files and creating tabular data for each core description. We are investigating optical character recognition to extract character data combined with AI tools to organize the data into prescribed tabular formats. This has proven successful for high-fidelity records but requires human checking and editing to ensure the accuracy of extracted data. Less well preserved records may require humans to transcribe them and manually enter characters into the tables. Upon making these MTU records available to others in an online format, we hope to extend this work to similar records in Acknowledgements: We thank the U.S. National Park Service for the grant that makes this work possible. Casey Koch and Gwen Martin performed nearly all of the document scanning. This work is possible because of the foresight of many late geologists who gathered and preserved the original paper records. Figure 2: Potential uses of the database upon completion. the other archives. REFERENCES 1. 2. 3. 4. 5. Bornhorst, T.J. and Barron, R.J., 2011, Copper deposits of the western Upper Peninsula of Michigan, in Miller, J.D., Hudak, G.J., Wittkop, C., and McLaughlin, P.I., eds., Archean to Anthropocene: Field Guides to the Geology of the Midcontinent of North America: Geological Society of America Field Guide 24, p. 83–99, doi:10.1130/2011.0024(05). Keweenaw National Historical Park, 2016, Calumet & Hecla Records – 00019/004.02.01.03-007 Microfiche Drill Core Log Library: Calumet, Michigan, U.S. Department of the Interior, National Park Service, on microfiche (accessed August 2016). White, W.S., 1985, “Unpublished diamond drillhole core logs”: U.S. Geological Survey, Field Records Collection, Boxes 282, 287-290. Michigan Technological University Archives, 2025, Major Mining Company Collections MS-001, MS-002, MS080, MS-635: J. Robert Van Pelt and John and Ruanne Opie Library, Houghton, Michigan (accessed December 2025). DeGraff, J.M. and Rose, W.I., 2020, Digital capture and preservation of historic mining data from the Keweenaw copper district, Michigan: GSA Abstracts with Programs, v. 52, no. 5, doi: 10.1130/abs/2020NC-348035. - 22 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 A new look at the Seagull mafic-ultramafic Intrusion and potential hydrogen and helium accumulations DROST, Abraham 1 and HEGGIE, Geoff 2 1 2 Rift Minerals Inc. 1113 Jade Court, #102, Thunder Bay, Ontario P7B-6V3 Canada Pursuit Geosciences, 245 Nicholetts Road, Murillo, Ontario P0T-2G0 Canada The mafic-ultramafic Seagull intrusion located approximately 80km northeast of Thunder Bay, Ontario and forms part of the Paleoproterozoic 1.1 Ga Midcontinental Rift which extends in an arcuate shape from Iowa through Lake Superior into Michigan (Fig. 1). The intrusion was intruded into the Archean Quetico Metasedimentary Terrain and transects a portion of the Sibley Group Metasedimentary rocks. The Quetico Terrain is dominated by deep water turbidites accumulated in a forearc basin between adjacent volcanic terranes, that underwent inversion during crustal accretion. Partial melting of the Quetico Terrane at depth resulted in the generation and emplacement of S-type melts at shallower levels with both uranium occurrences and LCT pegmatites present (Fig. 2). Figure 1. Geological and geophysical interpreted extent of the 1.1 Ga Midcontinent Rift centered on Lake Superior. Distribution of major rock types shown along with location of Seagull Project (Rift Minerals) and Topaz Project (helium: Pulsar Helium) Figure 2. Geology map of the Lake Nipigon area. Archean basement terrains shown in the legend. 1.1Ga Midcontinent Rift rocks shown in purple with early olivine bearing intrusions outlined in red. Uranium occurrences identified are demarked by yellow and orange circles from Ontario OMI database. Historic exploration between 1998 and 2012 on the Seagull Intrusion included airborne and ground geophysical surveys and approximately 20,000m of diamond drilling. The geology of the Seagull intrusion is characterized by mafic-ultramafic rocks, with in-excess of 700 m of variously serpentinized olivine cumulate rocks, predominantly lherzolites and pyroxenites (Fig. 3). This exploration work identified disseminated to semi-massive sulphide mineralization containing nickel, copper and platinum group elements along parts of the intrusion’s basal contact and as reef-type mineralization. Additionally, the exploration operator at the time reported the presence of naturally occurring gases at pressure. Histoically, the intrusion was targeted for orthomagmatic mineralization, without attention being paid to the presence of gas. With the discovery of an unconventional helium reservoir within the MCR, the prospectivity of the area has pivoted, resulting in new ideas in explored areas. Serpentinization is well known as an alteration process that generates hydrogen. The presence of ubiquitous uranium and LCT pegmatite occurrences in the Archean basement metasedimentary rocks of the Quetico Terrain are a potential source of helium (Fig. 2). Lithostatic pressures, structural plumbing and concentration gradients can potentially result in downward migration of generated gases (Strauch et al, 2023). - 23 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Figure 3. East-West cross-section through the Seagull intrusion as interpreted from diamond drilling. Modified from East West Resources (2002). Figure 4. Cross section through inversion model of the Ambient noise tomography (ANT) survey completed by Sisprobe (2024). Historical drill traces shown in white. Cross section at AZ of 027° facing NNW. In 2024, Rift Minerals completed an ambient noise tomography (ANT) survey with Sisprobe to refine the internal geometry of the Seagull intrusion and to identify subsurface velocity contrasts interpreted to reflect lithological and alteration variations. Integrated interpretation of drilling and geophysical data sets, including ANT velocity modelling, has been used by Rift to refine the interpreted geometry of the Seagull intrusion and underlying basement. The ANT velocity section (Fig. 4) is of high statistical quality and agrees well with stratigraphic variations identified in drilling. An unexplained low velocity interval within or beneath high velocity Quetico basement rocks below the Seagull Intrusion, topping at ~1250m, is being targeted for high pressure gas reservoir potential (Fig. 4). Rift Minerals and its funding partner Anteros Metals Inc. initiated a drill program in 2026 to test the deep lower velocity feature with drill hole RM26-01. The drill hole intersected disseminated to locally weakly net-textured, orthomagmatic sulphide mineralization in the basal cumulate sequence of the Seagull intrusion grading:* • 7.25 metres from 587.00 to 594.25 m grading 1.58 g/t Pt+Pd (0.72 part per million Pt and 0.86 ppm Pd), with 294 ppm copper and 2,168 ppm nickel; • 1.00 m from 606.25 to 607.25 m grading 2.27 g/t Pt+Pd (1.02 ppm Pt and 1.25 ppm Pd), with 1,660 ppm Cu and 2,080 ppm Ni. * Weighted-average results using a 0.5-gram-per-tonne-platinum-plus-palladium cut-off During the drilling of hole RM26-01 pressurized gas was encountered at a depth of approximately 877m within a narrow fault zone in the Quetico basement rocks. The 877-metre occurrence is located approximately 100m southwest from drill hole WM01-08, which reportedly encountered pressurized and flammable gas at a similar stratigraphic level when drilled in 2001. The significance, continuity and composition of the gas remain under evaluation. REFERENCES Strauch, B., Pilz, P., Zimmer, M and Hierold, J., 2023. Hydrogen Migration through natural rocks – an experimental approach. Harvard University – EGU23, the 25th EGU General Assembly, held 23-28 April, 2023 in Vienna, Austria (https://egu23.eu) - 24 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Integrated X-Ray Diffraction and Petrography Document Carbonate Mineral Heterogeneity and Hematite Mineralization in the Upper Biwabik Iron Formation, MN DUFFY, Paige1, BRENGMAN, Latisha1, and EYSTER, Athena2 Department of Earth and Environmental Sciences, University of Minnesota Duluth, Heller Hall 1114 Kirby Drive, Duluth, MN 55812, USA 1 Department of Earth and Climate Sciences, Tufts University, Lane Hall, 2 North Hill Road, Medford, MA 02155, USA 2 Core LWD-99-1 preserves the ~1.9 Ga Biwabik Iron Formation located near the Virginia horn, outside the contact metamorphic aureole associated with the intrusion of the Duluth Complex ca. 1.1 Ga. In this study, X-ray diffraction (XRD) and petrographic data are used to: (1) characterize carbonate mineral heterogeneity; and (2) evaluate depositional and post-depositional mineral assemblages. Emphasis was placed on identification of Fe2+ bearing carbonates (e.g., ankerite, siderite) and hematite-magnetite relationships. To minimize contamination and weathering effects, outer surfaces were removed during processing, and veins were avoided. Samples were cut, dried, then crushed to a uniform powder using a SPEX ShatterBox. XRD analyses were performed using a PANalytical X’Pert diffractometer, with data collected in θ-2θ geometry over a range of 5-65 degrees, sufficient to capture all minerals of interest. Analysis of diffraction data was done using X’Pert HighScore (Malvern PANanalytical) software. XRD analysis of 24 samples documents the presence of multiple different carbonate and oxide minerals throughout core LWD-99-1. Carbon was detected in 92% of analyzed samples, with 77% associated with carbonate minerals. All carbonate phases identified petrographically and with scanning electron microscopy (Duncanson et al., 2024) were also detected by XRD, indicating strong agreement between methods. Siderite is the most common carbonate phase, occurring in 41.7% of samples, followed by ankerite at 33.3%, dolomite in 20.8%, kutnohorite in 16.7%, and calcite in 6.3%. Carbonate mineral distribution greatly varies by informal stratigraphic unit. Siderite is prevalent in the Lower Slaty, Lower Cherty, and Upper Slaty, whereas kutnohorite (a calcium manganese carbonate) only occurs in the Upper Cherty. Calcite is restricted to the uppermost part of the Upper Slaty while dolomite and ankerite are most abundant in the Upper Cherty but also appear once in the Lower Cherty and twice in the Upper Slaty. Overall, carbonates are more abundant in the Upper Slaty and Upper Cherty compared to the Lower Slaty and Lower Cherty. Within the Upper Slaty, siderite occurs in 42.9% of samples, ankerite in 28.9%, dolomite in 28.6%, and calcite in 14.3%. In the Upper Cherty, siderite and ankerite each occur in 50% of the samples, dolomite in 20%, and kutnohorite in 40%. These distributions highlight a clear variation of carbonate minerals in the upper portions of the stratigraphy. Additionally, preliminary XRD and petrographic observations of iron oxide minerals suggest an overall increase in hematite occurrence in the Upper Cherty and the Lower Cherty, with petrographic data indicating magnetite is more prevalent in slaty units. Such transitions towards increasing carbonate mineral diversity and increasing hematite up section could link to depositional changes in the system or post-depositional oxidation reactions. Post-depositional mineral reactions and accounting of ferrous: ferric iron ratios are of critical interest as they preserve a record of fluid: rock interaction driven by multiple geologic events. Some redox reactions that involve siderite and magnetite are of broader interest for tracking hydrogen production or stimulation potential (Geymond et al., 2023; 2025). Ongoing work includes detailed accounting and mapping of mineral distributions and mineral reactions across the lateral extent of the iron formation. - 25 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Figure 1: XRD analysis of LWD-99-01 Sample MIR 17-11 taken from the Upper Cherty showing variation in carbonate mineralogy (ankerite (00-033-0282), dolomite (00-036-0426), kutnohorite (00-043-0695), and siderite (00-029-0696)). REFERENCES Duncanson, S., Brengman, L., Johnson, J., Eyster, A., Fournelle, J., Moy, A., 2024. Reconstructing diagenetic mineral reactions from silicified horizons of the Paleoproterozoic Biwabik Iron Formation, Minnesota. American Mineralogist, 109, 339-358. Geymond, U., Briolet, T,. Combaudon, V., Sissmann, O., Martinez, I., Duttine, M., Moretti, I., 2023. Reassessing the role of magnetite during natural hydrogen generation. Front. Earth Sci. 11, 1169356. Geymond, U., Truche, L., Sissmann, O., Kubaniova, D., Recham, N., Martinez, I., 2025. Mineralogical changes and H2 generation yield during hydrothermal alteration of a magnetite-siderite assemblage. Journal of Geophysical Research: Solid Earth, 130, 8. - 26 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 The Badgerow complex, a Midcontinent Rift-related REE-Zr-rich peralkaline intrusion in the Grenville Province near Verner, Ontario EASTON, Robert Michael1 and KAMO, Sandra L.2 Earth Resources and Geoscience Mapping Section, Ontario Geological Survey, retired, 933 Ramsey Lake Road, Sudbury, Ontario P3E 4W1 1 Jack Satterly Geochronology Laboratory, Department of Earth Sciences, University of Toronto, Toronto, Ontario M5S 3B1 2 The Badgerow complex (Lumbers 1975; Easton 2025) is located approximately 8.5 km north of the community of Verner within the northern Nepewassi domain of the Grenville Province (Easton 1992). The main part of the complex is roughly circular, approximately 4.5 by 3.5 km in size (Figure 1), and is only weakly deformed, with a narrow gneissic margin and a massive to slightly foliated interior. It consists predominantly of pink weathering, medium-grained monzogranite with less than 5% mafic minerals (sample 24RME-3047). Near the eastern margin of the complex, fine-grained monzogranite veins crosscut medium-grained gabbro of the complex containing relict pyroxene cores rimmed by amphibole. The monzogranite was sampled for geochemistry and U-Pb geochronology because of the relatively undeformed nature of these rocks, and the fact that the complex is the only near-circular pluton within Nepewassi domain. Approximately 600 m northeast of the near-circular body, Lumbers (1975) included an approximately 6 km long, up to 1 km wide, lens of gneissic syenite as part of the Badgerow complex. Well-exposed along Highway 575 (Figure 1), the lens is a homogeneous, medium-grained, gneissic amphibole syenite (sample 24RME-3052) hosted by migmatites. Given its mineralogy, and its greater degree of deformation, the lens was assumed to be older than the granitic rocks. It is unclear why Lumbers (1975) included it in the Badgerow complex. Preliminary geochemical results from the complex were reported in Easton (2025, 2026). Sample 24RME-3052 (Figure 2) is peralkaline and has niobium, yttrium, zirconium and total rare earth Figure 1. Simplified geological map of the Badgerow complex in the Grenville Province north of Verner (from Easton 2025). Sites sampled for geochemistry and for U/ Pb geochronology are indicated. Figure 2. Chondrite-normalized rare earth element plot for granitoid samples mentioned in the text (from Easton 2025). Remember the y-axis scale is logarithmic, so the difference between samples 24RME-3047 and 24RME-3052 is larger than it might appear (e.g., La normalized is 97 ppm for sample 24RME-3047 but 1866 ppm for sample 24RME-3052). Sample 24RME-1114 is an undeformed monzogranite exposed near Noelville. Normalizing values of Sun and McDonough (1989) were used. - 27 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 contents (164, 155, 4800 and 1936 ppm, respectively) that are some of the highest recorded for any igneous rock sample from the Grenville Province of Ontario. There are some similarities between sample 24RME-3052 and the West Bay migmatitic monzonite body to the south of Verner (Easton 2014). Key differences are that the West Bay body samples do not show a europium anomaly (Figure 2) nor are they peralkaline. It is unclear if the gneissic syenite was originally an intrusive or a volcanic rock. If volcanic, it has a comendite composition. In contrast, granite sample 24RME-3047 has a much lower total rare earth content (Figure 2). Preliminary U–Pb chemically abraded-isotope dilution thermal ionization mass spectrometric results on zircons have been obtained from samples 24RME-3047 and 24RME-3052. Zircons from sample 24RME-3047 are discordant and lie along a reference line anchored between 1097 and 2700 Ma. The most concordant zircon from sample 24RME-3052 gives a 207Pb/206Pb age of 1106 Ma (igneous based on Th/U). This age is older than Grenvillian metamorphism in Nepewassi domain (1030-980 Ma, Easton 2026), but similar to the Early Stage of Midcontinent Rift magmatism (11101104 Ma, Smith et al. 2026) and the Rb-Sr age of a mantle-xenolith bearing lamprophyric breccia at Elliot Lake (1112.8±4.95 Ma, Legros et al. 2024). Both these potential Midcontinent Rift-related intrusions lie along the northwest-southeast rifting trend of the Early Stage of magmatism, despite their location east of any previously described Midcontinent Rift magmatism. These new results suggest that other Midcontinent Rift-related intrusions may be present in the Sault Ste-Marie to North Bay area. REFERENCES Easton, R.M. 1992. The Grenville Province; Chapter 19 in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 2, p.713-904. ——— 2014. Geology and mineral potential of the Nepewassi domain, Central Gneiss Belt, Grenville Province; in Summary of Field Work and Other Activities, 2014; Ontario Geological Survey, Open File Report 6300, p.16-1 to 16-12. ——— 2025. Zirconium and rare-earth element potential of a Grenville Province gneiss north of Verner, northeastern Ontario; in Summary of Field Work and Other Activities, 2025; Ontario Geological Survey, Open File Report 6421, p.10-1 to 10-7. ——— 2026. Geological, geochemical, geophysical and petrographic data from the Wanup area, Grenville Province, northeastern Ontario; Ontario Geological Survey, Miscellaneous Release—Data 397. Legros, H., Czas, J., Luo, Y., Woodland, S., Sarkar, C., Shirey, S.B., Schulze, D, and Pearson, D.G. 2024. Post‑Archean Nb‑REE‑U enrichment in the Superior craton recorded in metasomatised mantle rocks erupted in the 1.1 Ga Midcontinental Rift event; Mineralium Deposita, v.59, p.373-396. Lumbers, S.B. 1975. Burwash area, districts of Nipissing, Parry Sound and Sudbury; Ontario Department of Mines, Geological Report 116, 158p. Accompanied by Map 2271, scale 1:126 720. Smith, J.W., Bleeker, W. and Hamilton, M. 2026. The 1093 Ma Crystal Lake Intrusion: A nickel-copper mineralized intrusion emplaced during the younger southwest–northeast rift phase of the Midcontinent Rift (North America); Geological Society of America, Bulletin, published online Oct 15, 2025, 20p. Sun, S-S. and McDonough, W-F. 1989. Chemical and isotopic systematics of oceanic basalts: Implications for mantle compositions and processes; in Geological Society of London, Special Publication No.42, p.313-345. - 28 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Comparing mineralogy along a surface to depth transect of the ~2.7 Ga North Limb Soudan Iron Formation, NE Minnesota. ELLISON, Kimberly1, CISNEROS, John Alex1, EYSTER, Athena2, and BRENGMAN, Latisha1 Department of Earth and Environmental Sciences, University of Minnesota Duluth, Heller Hall, 1114 Kirby Drive, Duluth, MN 55812, USA 1 Department of Earth and Climate Sciences, Tufts University, Lane Hall, 2 North Hill Road, Medford, MA 02155, USA 2 The Lake Vermilion-Soudan Underground Mine State Park in Northeast Minnesota is home to the classic 2.7 Ga Algoma-type Banded Iron Formation - a type of authigenic, chemical sedimentary rock known to record past ocean chemistry. Here, we compare multiple generations of mineralization in the Soudan Iron formation to evaluate the relative timing of oxidation reactions. Recent U-Pb and (U-Th)/He hematite geochronology places new age constraints on iron mineralization of microplaty hematite, documenting that this generation of hematite post-dates initial deposition by over 1 billion years (Allerton et al., 2025). Distinguishing between initial mineral formation and later overprinting is critical for reconstructing paleowater-rock interactions within the Soudan system. The goal of this work is to compare drill core and outcrop records from the north limb of the Soudan fold to samples from the mineralized portion of the Soudan mine, with a focus on building a spatial map documenting these oxidation reactions. To evaluate mineral reactions in the Soudan Iron formation, we combine transmitted and reflected light petrography with X-Ray Diffraction (XRD), focusing on a vertical transect of samples from drill core 26501 from the north limb of the Soudan Iron Formation, comparing these samples to nearby surface outcrop samples, and mine samples from the fold hinge to the west. Preliminary results indicate shallow core samples (28.5 to 95 feet) contain dominant mineral assemblages of quartz, calcite, magnetite, hematite, and minimal iron silicates, while deeper samples (129 to 394 feet) mainly contain quartz, magnetite, carbonate, chalcopyrite, and iron silicate assemblages, lacking visible hematite. To compare optical data to XRD data, we prepared powdered whole rock samples using standard cutting and crushing techniques for XRD scanning at positions ranging from 2θ = 5° to 2θ = 65°. Resulting peaks were matched to mineral reference patterns provided by the X’Pert HighScore analysis software and compared to observed mineralogy of thin section samples from drill core 26501. XRD results confirm the presence of quartz, magnetite, and hematite in shallow drill core samples, and an assemblage of quartz, magnetite, and iron silicates in deeper drill core samples. Combined, petrographic data and XRD data indicate hematite is confined to shallow drill core samples. This observed trend continues in petrographic data from surface outcrop samples near the same location, which also contain abundant hematite. The absence of hematite at depth in drill core samples, combined with the top-down nature of the hematite distribution, could indicate minor amounts of hematite locally formed from surface oxidation distal to the mine site. Next steps include more detailed paragenesis work in combination with larger-scale mapping of the spatial distribution of hematite in drill core along the north limb of the Soudan iron formation towards the historic mine site which sits at the fold hinge. Mapping the extent of multiple generations of oxidation reactions can help document past fluid-rock interactions and allows for identification of preserved ferrous iron-containing assemblages at depth in the iron formation. Such ferrous-iron-containing phases may record depositional information and are of interest for potential natural or stimulated hydrogen generation. - 29 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Figure 1.) Petrographic photomicrographs and X-ray Diffraction patterns of the Soudan iron formation from core 26501. (A) Reflected light image of sample 26501-28.5, a shallow (28 feet depth) banded iron formation sample with quartz and hematite. (B) Cross-polarized light image of sample 26501-191, a deeper iron formation sample (191 feet), that contains quartz, calcite, magnetite, and iron-silicate mineral phases. (C) XRD analysis of sample 26501-28.5 (28 feet depth) with peaks that match mineral reference patterns of quartz, magnetite, and hematite. (D) XRD analysis of sample 26501-242 (242 feet depth) with peaks that match mineral reference patterns of quartz, magnetite, and iron silicates. REFERENCES Duncanson, S., Brengman, L., Johnson, J., Eyster, A., Fournelle, J., Moy, A., 2024. “Reconstructing diagenetic mineral reactions from silicified horizons of the Paleoproterozoic Biwabik Iron Formation, Minnesota”. Mineralogical Society of America, Volume 109, Number 2, American Mineralogist, https://doi.org/10.2138/am-2022-8776. Geymond, Ugo, Briolet, T., Combaudon, V., Sissmann, O., Martinez, I., Duttine, M., Moretti, I., 2023. “Reassessing the Role of Magnetite during Natural Hydrogen Generation”. Frontiers in Earth Science, Volume 11, Frontiers, 10.3389/ feart.2023.1169356. Zsuzanna, P. Allerton, Courtney-Davies, L., Danisik, M., Hudak, G., Teyssier, C., Mitchell, J., Larson, P., 2025. “Hematite double-dating defines Proterozoic mineralization and thermal history of Archean banded iron formations in Northeastern Minnesota, USA”. Geology, Volume 53, page 11, Geological Society of America, https://doi. org/10.1130/G53517.1. - 30 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Middle school virtual field trip lessons materials for Archean formations of Lake VermilionSoudan Underground Mine State Park ERICKSON, Stephanie S.1, FAYON, Annia2 , ALLERTON, Zsuzsanna1,2, and HUDAK, George2,3,4 1 2 3 4 Curriculum and Instruction, University of Minnesota School of Earth and Environmental Science, University of Minnesota, Minneapolis, MN 55455, USA School of Earth and Environmental Science, University of Minnesota, Duluth, MN 55812, USA George Hudak Geosciences P.L.L.C., Duluth, MN 55804, USA The Lake Vermilion-Soudan Underground Mine State Park located in St. Louis County, Minnesota provides unique opportunities to learn about Archean geology and mineral resources of northern Minnesota. Archean rocks exposed in the park consist of a series of mafic lava flows and intrusive rocks interlayered with classic banded iron formation, iron ore, felsic tuffs, and chloritesericite schists (Hudak et al., 2014, Hudak and Peterson, 2014; Peterson et al., 2016) and record deformation associated with the accretionary growth of the Superior craton. A cross-section through the stratigraphy can be observed along a trail through part of the east side of the park. The trail is in the planning stages and is in collaboration with the state park. The purpose of this project is to enhance formal and informal Earth science education in Minnesota. After consultation with local secondary teachers the project expanded to include a virtual field trip with an accompanying lesson as part of the formal education portion of the project. In 2019 Minnesota revised their science standards (Minnesota Department of Education, 2019). These changes marked a significant change in the pedagogical practices aligned with national trends such as Next Generation Science Standards (NGGS) (NGSS Lead States, 2013). There are a number of shifts in instruction teachers are challenged to make when implementing these standards including using phenomenon based instruction (BSCS Science Learning, 2017; Reiser et al., 2021). Phenomenon based instruction engages students in a series of lessons arranged in a cohort storyline around a real world, observable events. An additional challenge facing Minnesota educators was moving Earth science in from 8th grade to 6th grade. According to survey data collected from the Minnesota Earth Science Teachers Association many 6th grade teachers did not feel prepared to teach Earth science content. A combination of lack of high quality instructional materials for Minnesota phenomenon and gaps in the required background knowledge are some factors contributing to these findings. This project provided teachers with high quality instructional materials that are aligned with the 2019 Minnesota State Science Standards for 6th grade teachers. Three, 45-minute lessons were designed to address the stratigraphy standard. The goal for the students is to tell the geological story of the park. The first lessons take students on a virtual walk through the park stopping at six significant outcrops along the way (Figure 1). At each stop students are making observations of the rock outcrops and hand samples while also asking questions. The second lesson, using information about rock formation, processes the map and picture of core samples taken from locations in the park (Figure 2) while applying principles of deformation and stratigraphy. formations and thus the early geologic history of the Earth. The lessons conclude with students writing a story of the Archean formations and thus the early geologic history of the Earth. - 31 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Figure 1: The first two stops orient students to the park and where they learn about the park’s iron mining history including an open pit and a trip down to a deep mine. After emerging from the underground mine they make three stops at outcrops: the classic BIF outcrop, the schist in BIF outcrop, and Ely Greenstone pillow basalts. The final stop is to make observations of tuff and the lower section of the Ely greenstone from rocks found on the “ground.” (after Peterson et al., 2016 ) Figure 2: Virtual core samples that students use to correlate and deduce the order the rocks are formed in. Each core sample comes from a point of the map in figure 1. These are not actual core samples rather simplified samples that allow students to correlate the stratigraphy of the area. REFERENCES BSCS Science Learning. (2017). Guidelines for Assessing Instructional Materials that Exemplify the NGSS. https://bscs.org/ reports/guidelines-for-assessing-instructional-materials-that-exemplify-the-ngss/ Hudak, G. J., and Peterson, D. M., 2014, Non-Ferrous Mineralization Associated with the Wawa-Abitibi Terrane and Duluth Complex Cu-Ni-PGM Deposits, Northeastern Minnesota: Society of Economic Geologists, Guidebook Series, v. 47, 150 p. Hudak, G. J., Radakovich, A., Pignotta, G., and Schwierske, K., 2014, Field Trip 2 – A Walk in the Park – Neoarchean Geology of Lake Vermilion State Park: Institute on Lake Superior Geology, Proceedings Volume 60, Part 2 – Field Trip Guidebook, p. 37-75. Minnesota Department of Education. (2019). 2019 Minnesota Academic Standards in Science. https:// education.mn.gov/mdeprod/idcplg?IdcService=GET_FILE&dDocName=MDE086711 &RevisionSelectionMethod=latestReleased&Rendition=primary NGSS Lead States. (2013). Next generation science standards: For states, by state. The National Academies Press. Washington D.C. Peterson, D.M., Hudak, G.J., Radakovich, A., Pignotta, G., and Schwierske, K., 2016, Geologic Map of Lake Vermilion/ Soudan Underground Mine State Park: Precambrian Research Center Map PRC/Map-2016-01, 1:10,000 scale. Reiser, B. J., Novak, M., McGill, T. A. W., & Penuel, W. R. (2021). Storyline units: An instructional model to support coherence from the students’ perspective. Journal of Science Teacher Education, 32(7), 805–829. https://doi.org/10 .1080/1046560X.2021.1884784 - 32 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Geochemical Constraints on Mn Cycling in the Paleoproterozoic Gunflint Formation GILBERG, Nolan1, FRALICK, Philip1, and LI, Zhiquan1 1 Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Iron formations (IFs) are iron-rich (>15% Fe) and siliceous (>20 wt.% SiO2) chemical sedimentary rocks that precipitated from seawater. Most IFs were deposited between 2.80 and 1.85 Ga during the Neoarchean and Paleoproterozoic, followed by a near one-billion-year hiatus before reappearing in the Neoproterozoic. The Gunflint Formation in the Animikie Basin, overlain by the siliciclastic Rove Formation, is composed mainly of IFs, chert, carbonates, and minor siliciclastic sediments deposited during the Paleoproterozoic (~1.88 Ga), and represents the final major episode of IF deposition. Therefore, investigating the source materials and redox conditions of the Gunflint Formation is key to understanding this transitional period in the marine environment. This study conducts a high-resolution stratigraphy and chemostratigraphy study of a 142.9-meterdeep drill hole (MC-1-89), located south of Thunder Bay in the Gunflint Iron Range. Samples were taken in short intervals of ~1-5 meters along the drill core, where 55 samples were analyzed for major, trace and rare earth (REE+Y) element concentrations through ICP-OES and MS. All samples from drill core MC-1-89 consists of IFs (often magnetite, hematite rich, or jaspilite), chert, carbonates, and siliciclastic rocks (often fine sandstone and argillaceous mudstone). IFs contain a total Fe content ranging from 15-36%. MnO values are enriched in the upper and lower portion of the hole (0.30, 0.57 wt.% respectively), while depths 30-90m show an average of 0.10 wt.%. Samples with >1 wt.% Al2O3 and >0.1 wt.% TiO2 are excluded for REE+Y analysis due to potential detrital contamination. The rest of the samples do not show correlation of REE+Y with Al2O3 + TiO2 (R2 < 0.1), suggesting the REE+Y system is authigenic. All samples display positive Eu/Eu* (1.18 – 3.14, average ~1.83), suggesting a strong hydrothermal input. Moreover, most of the samples display a depletion of LREE, enrichment of HREE, along with high Y/Ho ratios (average of 30.4), suggesting marine signatures. All these features are typical of global Paleoproterozoic IFs. A key distinction between the Gunflint Formation and other Paleoproterozoic IFs is the presence of positive Ce anomalies in many samples, which contrasts with most Archean and Paleoproterozoic IFs. Ce/Ce* values decline with depth (0.35 – 1.92, average =1.32). The elevated Ce might be related to the cycling of Mn oxides in the water column, but further detailed work is still needed to better constrain the mechanism. - 33 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Modified Sequential Iron Extraction Method for Analyzing Rare Earth Elements in Banded Iron Formations GOSAI, Meghna, FRALICK, Philip, and LI, Zhiquan Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada Banded iron formations (BIFs) are chemically precipitated sedimentary rocks characterized by alternating iron-rich and silica-rich layers, formed predominantly in Precambrian marine environments. Rare earth elements (REEs) are among the most used geochemical tools for understanding the origin and deposition of iron formations and other iron oxide–rich sedimentary rocks, because the precipitation of ferric iron oxyhydroxides can adsorb signatures from the water column and thus preserve a seawater REE signature. However, BIFs that formed in shallow-marine settings often contain detrital material, thereby affecting the bulk rock geochemistry. For instance, detrital input may elevate light REEs and suppress yttrium (Y) anomalies, complicating interpretation. Sequential extraction of different iron phases (e.g., magnetite, iron carbonates, and iron sulphides), developed by Poulton and Canfield (2004), was used to accurately determine the composition of ironbearing minerals without interference from detrital materials. However, the chemical solutions used in this process introduce additional dissolved ions, thereby increasing total dissolved solids (TDS) and making it difficult to analyze low REE concentrations using ICP-MS. Therefore, this study aims to develop a method to reduce the introduced TDS while still extracting enough REEs for detection by ICP-MS. Six concentrations (10%, 20%, 40%, 60%, 80%, and 100%) of an ammonium oxalate monohydrate and oxalic acid solution were used for sequential extraction. This solution was used to selectively extract magnetite from two types of samples: (1) magnetically selected magnetite grains, and (2) bulk rock powder from the same sample. The extracted iron solutions were then analyzed for REE anomalies for interpretation. Sample patterns were compared to determine the minimum concentration required to introduce additional elements into the solution without resulting in a high dilution factor. The patterns were also compared with those from Dolega’s (2018) bulk-rock acid digestion to assess any improvements in REE patterns. The results and comparison indicate that the REE patterns show the greatest improvement at a solution concentration of 40%. However, one concern is the absence of a positive Y anomaly, which differs from the original bulk rock data (Dolega, 2018). It is likely that reprecipitation causes the interference with Y, but further work is still needed for this investigation. REFERENCES Dolega, S., 2018. Geochemistry of Shallow and Deep Water Archean Meta-Iron Formations and Their Post-depositional Alteration in Western Superior Province, Canada. Unpbl. MSc thesis, Lakehead University, Department of Geology. Poulton, S.W., and Canfield, D.E., 2005. Development of a Sequential Extraction Procedure for Iron: Implications for Iron Partitioning in Continentally Derived Particulates. Chemical Geology, 214, 209–221. - 34 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Time-to-depth conversion of seismic-reflection data from eastern Lake Superior and implications for the eastern arm of the Midcontinent Rift GRAUCH, V.J.S.1, and HELLER, Samuel J.2 1 2 U.S. Geological Survey, MS 973, Federal Center, Denver, CO 80225 U.S. Geological Survey, MS 939, Federal Center, Denver, CO 80225 Seismic-reflection data were acquired in the mid 1980s along several lines across eastern Lake Superior by industry and the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE) (Fig. 1). The lines form part of a larger network of crossing lines over the entire lake, which can be used to develop three-dimensional geologic models of the Mesoproterozoic Midcontinent Rift that lies below. To better interpret these lines, we developed velocity models to convert seismic reflections versus two-way travel time (TWTT) to reflections versus depth. In addition, the velocity models themselves provide insights into the structure of the Midcontinent Rift by recognizing common velocity ranges for certain rock types (Grauch, 2023). Figure 1. Seismic-reflection lines overlain on Bouguer gravity for eastern Lake Superior. Gravity map from Anderson and Grauch (2018) is displayed in color shaded-relief, with illumination from the northeast. Lake Superior shores are outlined in black. Digital data are publicly available for lines A, F, and G, collected as part of GLIMPCE. Digital data were derived for the industry lines (LS-15, LS-25, LS-26, and LS-36) by scanning published images from McGinnis and Mudrey (2003) and estimating the location parameters. Velocity model development was guided by (1) bathymetric data, providing thickness of the lowvelocity water column; (2) previous shallow seismic-reflection studies targeting the top of bedrock below glacial till and lake sediments; (3) previous refraction studies, which provide information on depth and compressional velocity at interfaces of large velocity contrast; and (4) correlations across multiple lines, allowing independent constraints on individual lines to influence modeling on crossing lines. In addition, digital data for the GLIMPCE lines were analyzed using common midpoint gathers to check the accuracy of the modeled velocities. Gravity anomalies provided qualitative guidance on broad velocity variations. The velocity models consist of intervals of constant velocity bounded by prominent horizons recognized in the seismic-reflection TWTT sections before time-to-depth conversion. After - 35 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 conversion, the resulting reflection sections versus depth show similar overall geometry compared to the TWTT sections, although structural relief is more subdued. Thus, several qualitative aspects of the results are similar to those observed by previous workers (e.g., Cannon et al., 1989; Mariano and Hinze, 1994; Samson and West, 1994). For example, lines that cross the lake from SW to NE are interpreted to show a symmetric basin of fairly uniform basalt thickness except at the edges of the basin, where the basalts rise and thin and are expressed by pronounced gravity highs (Fig. 1). The thickness of the overlying sedimentary section increases toward the middle of the basin to 7–9 km and the underlying volcanic section is locally folded. In contrast, the velocities derived from the modeling indicate different rock types than anticipated from the previous interpretations at the edges of the basin. The upturned basalt edges have been previously interpreted as basalt layers thrust over the younger Jacobsville Sandstone, with sharply rounded reflection patterns considered as thrust rollovers on lines LS-26 and LS-36 between the crossings with LS-15 and LS-25 (Mariano and Hinze, 1994). Where these authors interpret Jacobsville Sandstone under thrust faults, the models indicate velocities on the order of 6.0 km/s instead of the expected velocity range of 3.0–4.5 km/s for this unit (Grauch, 2023). The higher velocities are consistent with those of igneous or basement rocks instead. An alternate interpretation is that the upturned edges represent the vestiges of magmatic feeder zones and the sharply rounded reflection patterns represent igneous intrusions. The zones may be faulted and folded due to the later compressional regime that affected the region. REFERENCES Anderson, E.D., and Grauch, V.J.S., 2018, Updated aeromagnetic and gravity anomaly compilations and elevationbathymetry models over Lake Superior: U.S. Geological Survey data release, https://doi.org/10.5066/F7F18X8S. Cannon, W.F., Green, A.C., Hutchinson, D.R., Lee, M.W., Milkereit, B., Behrendt, J.C., Halls, H.C., Green, J.C., Dickas, A.B., Morey, G.B., Sutcliffe, R.H., and Spencer, C., 1989, The North American Midcontinent rift beneath Lake Superior from GLIMPCE seismic reflection profiling: Tectonics, v. 8, p. 305–332. doi: 10.1029/TC008i002p00305. Grauch, V.J.S., 2023, Compressional-wave seismic velocity, bulk density, and their empirical relations for geophysical modeling of the Midcontinent Rift system in the Lake Superior region: U.S. Geological Survey Scientific Investigations Report 2023-5061, 60 p. https://doi.org/10.3133/sir20235061. Mariano, J., and Hinze, W. J., 1994, Structural interpretation of the Midcontinent Rift in eastern Lake Superior from seismic reflection and potential-field studies: Canadian Journal of Earth Sciences, v. 30, p. 619–628. McGinnis, L.D., and Mudrey, M.G., Jr., 2003, Seismic reflection profiling and tectonic evolution of the Midcontinent rift in Lake Superior: Wisconsin Geological and Natural History Survey MP 91-2. https://wgnhs.wisc.edu/pubs/000480/. Samson, C., and West, G. F., 1994, Detailed basin structure and tectonic evolution of the Midcontinent Rift System in eastern Lake Superior from reprocessing of GLIMPCE deep reflection seismic data: Canadian Journal of Earth Sciences, v. 31, p. 629–639. - 36 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Ice flow history, surficial geology, and till composition of Georgia Lake area, northwestern Ontario HAGEDORN, Grant1 Ontario Geological Survey, Ministry of Energy and Mines, 933 Ramsey Lake Road, Sudbury, ON P3E 6B5 Canada 1 During the last glaciation, the Lake Superior basin was covered by the Laurentide Ice Sheet. The ice sheet advanced over the landscape, eroding the substrate and depositing a variety of sediments including till (a common sample medium for mineral exploration) and glaciofluvial sand and gravel (a common source of aggregates). During deglaciation, glacial lakes inundated the landscape depositing successions of silt and clay, which can act as a barrier for mineral exploration and infrastructure development. As such, the Ontario Geological Survey completed a three-year field mapping program which measured striations and landforms to decipher different ice flow directions, mapped the surficial geology around the Georgia lake pegmatite, and collected regional scale till samples to identify mineral prospectivity (Figure 1). These data hold broad applications for regional mineral exploration and land use planning / resource management decisions for local communities. Striation and landform mapping were used to determine the relative age and direction of ice flow over the region. A southwest flow is pervasive across mafic uplands, suggesting this was the paleoflow direction during thickest ice cover (Arrows labeled 1 in Figure 1). As the ice sheet thinned, it became more topographically-controlled resulting in southward ice flow in lowlands, and westward ice flow on mafic uplands (Arrows labeled 2 in Figure 1). Finally, a late-stage re-advance out of the Lake Superior basin created northwestward striations and landforms in the areas around Thunder Bay (Arrows labeled 2 in Figure 1). Surficial mapping completed in the Georgia Lake area indicate more sediment than previously identified although the sediments are mostly thin (>2 m). Till is common at surface and many new small eskers have been mapped. Glacial lake sediments are present, and at a higher elevation than previously indicated. Postglacial organic accumulations are also abundant over the landscape, specifically over poorly-drained substrates like till and glaciolacustrine silt and clay. Till samples were also collected as part of the project and analyzed for till matrix geochemistry and indicator minerals. Till compositions indicate two units differentiated based on bedrock provenance. One till contains southwest transported carbonate material while the other contains locally sourced bedrock material. Lithium material transported southwest from the Georgia Lake pegmatite is also clearly identified in both the geochemistry and indicator mineral data. Further work is being completed on the till samples to indicate prospectivity of the region for other deposits. - 37 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Figure 1: Study area for the project. Highways and towns are labeled. Ice flow directions indicated by white arrows with the corresponding flow event as the number beside (1: older, 2: younger). Surficial geology mapping area is indicated by the dash box. Till sample locations are circles. - 38 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Geochemistry, Petrogenesis, and Mineralization of the Makwa Deposit, Bird River Sill HARDING, Myles1 and HOLLINGS, Pete1 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, On P7B 1J4, Canada. The Maskwa West-Dumbarton layered mafic-ultramafic intrusion is located approximately 145 km northeast of Winnipeg, Manitoba and is host to the Ni-Cu-PGE Makwa Deposit. The intrusion is related to the 2743 ± 0.5 Ma Bird River Sill (BRS; Scoates and Scoates, 2013) which is approximately 15-25km long and is made up of several separated ~800m thick differentiated mafic-ultramafic intrusive bodies. The Maskwa West-Dumbarton intrusion is emplaced into the mafic metavolcanic MORB-type massive to pillowed basalt Northern Lamprey Falls Formation (Mealin, 2008, Duguet et al., 2009). After the discovery of the Maskwa deposit in 1975, a year later 332,000 tonnes of nickel copper ore was mined in a shallow open pit (Grid Metals Corporation, 2024). In 2004 Mustang Minerals (now Grid Metals Corporation) acquired the property and have since completed extensive drilling and geophysical surveys targeting PGE mineralization. The approximately 5 km long Maskwa West-Dumbarton intrusion is composed of a ~500m thick upper gabbro-anorthositic section and a ~500m thick lower section of metaperidotite-metapyroxenite (Mustang Minerals Corp., 2014). The intrusion has been metamorphosed to the lower amphibolite facies (Coats and Buchan, 1979) with primary igneous textures Maskwa West-Dumbarton obscured or completely overprinted by alteration. The Makwa deposit is a conventional basal accumulation type magmatic sulphide deposit with the highest grade mineralization hosted within the lowest portion of the ultramafic series (Grid Metals Corporation, 2024). The deposit is comprised of a magmatic assemblage of disseminated to net textured and semi-massive pyrrhotite-pentlandite-chalcopyrite as well as low sulphide platinum group minerals (PGM) mineralization (Grid Metals Corporation, 2024). The open pit resources at Makwa are indicated to be 14.2 million tonnes with 0.48% nickel, 0.11% copper, 0.02% cobalt, 0.37 g/t palladium, and 0.10 g/t platinum (Grid Metals Corporation, 2024). The most recent up to date resource estimate for the high-grade zone indicates 4.8 million tonnes with a grade of 0.89% nickel and a 1.26% nickel equivalent (Grid Metals Corporation, 2024). The purpose of this project is to characterize the stratigraphy of the Maskwa-Dumbarton body and Ni-Cu-PGE mineralization. Assess the effects of alteration on the mineralogy, trace element geochemistry, and ore remobilization. A fence of five drill holes covering the stratigraphy of the intrusion were selected for this project where 151 core samples were collected. Forty polished thin sections were cut in representative areas for petrographic and scanning electron microscope (SEM) analysis. 141 of those samples were selected for whole rock geochemical analysis. A combination of petrographic and geochemical analysis was used to characterize the Makwa mafic and ultramafic rocks. Primary mineralogy is almost entirely replaced (Fig. 1) therefore preserved relict cumulus textures along with whole rock geochemistry are utilized to determine primary mineralogical composition. The Makwa ultramafic samples dominantly plot as Mg-rich cumulates within the komatiite field (Fig. 2) displaying a trend of Fe-enrichment highlighting strong fractionation. The results of this study will be used to determine the evolution, geotectonic setting, and sulfur source of the sulphides. REFERENCES Coats, C. J. A., & Buchan, R. (1979). Petrology of serpentinized metamorphic olivine, Bird River Sill, Manitoba. Canadian Mineralogist, 17, 847–855. Duguet, M., Gilbert, H.P., Corkery, M.T. and Lin, S. (2009): Geology and structure of the Bird River Belt, southeastern Manitoba (NTS 52L5 and 6): reprinted with revisions; in Report of Activities 2006, Manitoba Science, Technology, Energy and Mines, Manitoba Geological Survey, p. 170–183. Grid Metals Corp. – Combined Makwa and Mayville Project, Technical Report NI 43-101 – June 14, 2024 - 39 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Mealin, C. A., & University of Waterloo. Department of Earth Sciences. (2008). Geology, geochemistry and Cr-Ni-Cu-PGE mineralization of the Bird River sill evidence for a multiple intrusion model. University of Waterloo. Mustang Minerals Corp. – Combined Makwa and Mayville Project, #2098 Technical Report NI 43-101 – April 30, 2014 Scoates, J. S., & Scoates, R. F. J. (2013). Age of the Bird River Sill, southeastern Manitoba, Canada, with implications for the secular variation of layered intrusion-hosted stratiform chromite mineralization. Economic Geology and the Bulletin of the Society of Economic Geologists, 108(4), 895–907. Figure 1. Photomicrograph (XPL) of Makwa peridotite displaying mesh-textured serpentine replacing metamorphic blade shaped olivine in net-textured sulphides. Figure 2. Jensen Cation Plot highlighting Makwa Mg-rich cumulates dominantly within the komatiite field displaying Fe and Al-enrichment trends highlighting strong fractionation. - 40 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Using Anisotropy of Magnetic Susceptibility and U-Pb Geochronology from the Bush Lake Granite, Florence County, WI to Understand Post-Penokean Continental Growth HELLRUNG, Alyssa1, DROUBI, Omar Khalil1, RUGGLES, Claire1, and BONAMICI, Chloë1 Department of Geosciences, University of Wisconsin-Madison, 1215 W. Dayton Street, Madison, Wisconsin, 53706, USA 1 The Bush Lake granite in Florence County, Wisconsin, is well suited to constrain the timing of granitic magmatism relative to Proterozoic deformation events as the youngest intrusion in the Dunbar Gneiss Dome. The Dunbar Gneiss Dome is south of the Niagara fault zone, which marks the suture of the Pembine-Wausau terrane to the Superior craton during the 1.85 Ga Penokean orogeny (Schulz and Cannon, 2007). This suture may have been reactivated during later orogenic events, such as the ca. 1.75 Ga Yavapai orogeny, the ca. 1.65 Ga Mazatzal orogeny, and/or the ca. 1.45 Ga Baraboo orogeny. Emplacement and deformation of the Bush Lake granite determined through U-Pb geochronology, microstructural analysis, and anisotropy of magnetic susceptibility (AMS) fabric data provides insight into the tectonic history of the region. The Bush Lake granite is a weakly peraluminous biotite granite that contains quartz, plagioclase, megacrystic alkali feldspar, and accessory allanite, zircon, titanite, and apatite. Microstructures in the Bush Lake granite indicate variable solid-state deformation, including interlobate grain boundaries and undulose extinction in quartz, as well as grain size reduction of quartz and feldspar. Magnetic mineralogy, which informs the AMS fabric, is dominated by paramagnetic biotite with trace magnetic oxides. AMS fabrics generally record NW-SE striking foliations and moderately plunging to subvertical lineations (Figure 1), which are consistent with predominantly NE-SW shortening at a high angle to the Niagara fault zone and associated vertical thickening of the crust. Based on solid-state deformation microstructures, this magnetic fabric formed after emplacement and crystallization of the unit and records a younger period of deformation than previously thought. Cathodoluminescence (CL) imaging shows that most Bush Lake zircons preserve oscillatory zoning of likely magmatic origin, though many zircon crystals also have irregular, disturbed zoning and low-CL regions consistent with alteration. The Bush Lake granite was previously interpreted to have intruded at ~1835 Ma as a late-stage intrusion of the Paleoproterozoic Penokean orogeny, coeval with other nearby granites (Sims et al., 1985). Based on U-Pb SIMS analyses of zircon, the Bush Lake granite is interpreted to have emplaced at 1749 ± 1 Ma, making it coeval with the 1754 ± 11 Ma Amberg granite, ~28 km southwest (Holm et al., 2005), rather than the more proximal ~1835 Ma granites in the Dunbar Gneiss Dome. Solid-state deformation recorded by the Bush Lake granite may signify a broader regional deformation event in northern Wisconsin after 1750 Ma, possibly related to re-activation of the Niagara Fault Zone during the Yavapai orogeny or later events. REFERENCES Holm, D. K., Van Schmus, W. R., MacNeil, L. C., Boerboom, T. J., Schweitzer, D., and Schneider, D., 2005. U-Pb zircon geochronology of Paleoproterozoic plutons from the northern midcontinent, USA: Evidence for subduction flip and continued convergence after geon 18 Penokean orogenesis. Geological Society of America, 117(3/4), 259-275. Schulz, K. J., and Cannon, W. F., 2007. The Penokean orogeny in the Lake Superior region. Precambrian Research, 157(14), 4-25. Sims, P. K., Peterman, Z. E., and Schulz, K. J., 1985. The Dunbar Gneiss-granitoid dome: Implications for early Proterozoic tectonic evolution of northern Wisconsin. Geological Society of America Bulletin, 96, 1101-1112. - 41 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Figure 1: Simplified geologic map of the Bush Lake pluton (pink) in Florence, Wisconsin, with sample locations plotted and colored by average magnetic susceptibility [SI]. Lower hemisphere equal area net projections bordering the map show the AMS foliation plane and lineation at each site for each specimen. At each site, ≥ 2 rock samples are collected from different parts of the outcrop to test for slumping. Sample BL07 is an example of a failed test with two distinct sample groupings and does not provide reproducible data. - 42 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Can we improve the bouguer gravity resolution in the Cuyuna Range? Increasing gravity measurements in a region of high gravity station density. HIRSCH, Aaron1 1 Minnesota Geological Survey, University of Minnesota, 2609 Territorial Road, St. Paul MN 55114 In East-central Minnesota, the Cuyuna-Penokean orogen is made up of deformed Precambrian rocks of the Penokean-Fold-Thrust belt and adjacent terranes. This complexly folded and thermally overprinted region hosts the 2nd largest known manganese occurrences in the US (Cannon et al., 2017) and has been mined intermittently since the early 1900s. Despite decades of mining, mapping the geology is difficult with most of the bedrock overlain by thick glacial sediments from multiple glacial advances. Mapping of this critical resource and the surrounding region has relied on very limited outcrops, historical mining records, drill core, and geophysical datasets. The Minnesota Geological Survey (MGS) houses state-wide aeromagnetic, gravity, and rock property geophysical datasets that are a key tool in mapping the bedrock geology. The MGS gravity database consists of over 60,000 variably spaced measurements (Chandler et al., 2010). In the Cuyuna-Penokean area, specifically the areas around the Emily District, North Range, and parts of the South Range, the average gravity measurement spacing is ~1.6km with select areas at 0.8-1km. Station spacing of this density is generally considered very good coverage for regional geologic modeling. Due to the complex geology of the area, the MGS set out to determine if increased gravity data will further improve the geophysical resolution and subsequent geologic mapping. Over three field seasons, as part of an Earth Mapping Resources Initiative (Earth MRI) funded project, 210 new gravity points were measured, processed, and added to the gravity database. Gravity stations were tied to an existing base station, and three new field base stations were created in the area to reduce gravity loops. Measurements were prioritized along five transects perpendicular to structure: 2 North-South and 3 Northwest-Southeast profiles. Due to the varying age and accuracy of the gravity database and base stations, tie-point measurements were made at existing gravity station locations for comparison and if any corrections were needed. Multiple comparisons were made between the original and updated datasets with raw 2D Bouguer gravity profiles and gridded Bouguer gravity and second vertical derivatives analyzed (Blakely, 1996). An increase in gravity measurement density resulted in variable differences along profiles resulted in less smoothing and small shifts in slope in some regions but little to no difference in others. Both Bouguer and 2nd vertical derivative gravity grids showed significantly less variability due to inherent smoothing from the minimum curvature gridding process. Two-dimensional modeling was also performed to assess the impact of increased gravity measurement density to geologic mapping. REFERENCES Blakely, R. J.,1996, Potential Theory in Gravity and Magnetic Applications (441 p.). Cambridge: Cambridge University Press. Cannon, W.F., Kimball, B.E., and Corathers, L.A., 2017, Manganese, in chapter L of Schulz, K.J., DeYounge, J.H., Jr., Seal, R.R., II, and Bradley, D.C., eds., Critical mineral resources of the United States – Economic and environmental geology and prospects for future supply: USGS Professional Paper 1802, p. L1–L28. Chandler, V. W., Lively, R. S., and Wahl, T. E., 2010, Gravity and Aeromagnetic Data Grids of Minnesota, Minnesota Geological Survey, http://purl.umn.edu/92939 - 43 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Figure 1: Bouguer gravity map of the Emily District, North Range, and South Range. Black dots are the existing gravity stations. Triangles are the new gravity stations. Circle in bottom left corner is the base station used for this study. Gravity values range from -14.9 - -67.8 mGals. - 44 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Using epidote and chlorite mineral chemistry to extend the alteration footprint around the Hemlo Au deposit, N. Ontario HOLLINGS, Pete1, VRZOVSKI, Joseph1, COOKE, David2, and GORNER, Emily1 1 2 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, P7B 5E1, Canada CODES, University of Tasmania, Private Bag 79, 7001, Hobart, Australia The Hemlo deposit is a world class Archean Au deposit situated in Northern Ontario, Canada with historic production of >21 Moz of Au over 35 years of continuous operation. The deposit has a strike length of ~3 km with a well-documented alteration footprint surrounding mineralization. LA-ICPMS analyses of epidote, chlorite and pyrite from within and surrounding the deposit (Fig. 1) have identified major and trace element variations in mineral chemistry that allow for the discrimination of deposit-proximal and deposit-distal signatures. Epidote compositions vary with distance from Hemlo, with the highest concentrations of As and Sb in epidote proximal to the mineralized zones. Anomalous trace element compositions in epidote can be detected up to 1.5 km further than the mapped alteration footprint. Chlorite also displayed variation in trace elements with deposit-proximal chlorite displaying exponentially higher Ti/Sr and V/Co values than deposit-distal and intrusion-related chlorite. The Ti/Sr ratio for chlorite expanded the geochemical footprint of the Hemlo deposit by up to 1 km. Pyrite displayed anomalous enrichments in a number of elements, with Au, Te and As proving to be the most effective pathfinder elements in pyrite as they were detected at anomalous concentrations up to 2.5 km from the deposit. Several post-mineralization intrusions that surround the deposit were evaluated using epidote and chlorite chemistry to assess whether they generated any false positive geochemical anomalies. The distal post-mineralization intrusions have epidote with consistently low As and Sb concentrations and elevated Fe/Al values relative to deposit-related epidote and can be easily distinguished. Intrusion- Figure 1. Location of samples collected for this study - 45 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 related chlorite displayed low Ti/Sr and V/Co values relative to the deposit chlorite and was also found to be more enriched in Fe relative to deposit-proximal chlorite. These results indicate that the postmineralization intrusions did not produce false positive mineral chemistry anomalies. Variations in chlorite Fe-Mg content can be tracked spectrally using the position of the diagnostic 2250 nm absorption feature. Chlorite displays a range of wavelengths from 2240 – 2256 nm throughout the Hemlo district. Chlorite with lower wavelengths (< 2248 nm) display lower average Fe/Mg (<1) values whereas chlorite with longer wavelengths (> 2252 nm) display higher Fe/ Mg (>1) values. Spectral variations 1550 nm absorption feature of epidote can be used to track compositional variations between the Fe-(epidote) and Al-(clinozoisite) epidote group endmembers. Epidote throughout the Hemlo area displayed a range of wavelengths from 1540 – 1564 nm. These variations in spectral features of epidote could be correlated to epidote major element variations with wavelengths > 1550 nm having on average lower Fe/Al values (< 0.8), whereas wavelengths < 1448 nm displayed average Fe/Al values of ~1. The systematic variations in syn-mineralisation epidote and chlorite compositions around Hemlo suggests that methods developed for investigating geochemical footprints defined by green rock alteration around porphyry systems may also be applicable to Archean orogenic gold deposits. - 46 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Petrographic Study of Granular Iron Formation in the Gunflint Formation: Evidence for WellOxygenated Surface Waters JONSSON, Justin1 and LI, Zhiquan2 Ontario Geological Survey, Ministry of Energy and Mines, Suite B002, 435 James St. South Thunder Bay, ON P7E 6S7 Canada 1 2 Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Granular iron formation (GIF) exhibits distinct features compared to banded iron formation (BIF), being characterized by granule-rich textures and commonly interpreted as detrital, with some grains derived from sedimentary reworking of iron-rich clays, mudstones, arenites, and even stromatolites. Other granules consist of concentric hematite cortices that likely precipitated from Fe(II)-rich waters upon interaction with oxygenated shallow seawater. Previous studies have demonstrated that GIF provides valuable insights into shallow marine environments, as physical energy from waves, tides, and storms is largely restricted to depths above the storm wave base. The 1.88 Ga Gunflint Formation comprises both BIF and GIF, along with chert, carbonates, and minor siliciclastic materials, deposited on a storm-dominated continental shelf. In this study, we examine the petrography of GIF from the lower Gunflint Formation to identify evidence for redox variations in a shallow marine environment. Thin sections of the GIF commonly exhibit oolitic textures, with subordinate peloids and oncoids. Ooids and oncoids are typically composed of hematite, whereas peloids commonly consist of a chert core with hematite rims. Most granules display well-developed concentric hematite cortices, suggesting that iron oxides were directly precipitated from an Fe(II)-rich water column. The grains are not uniformly in contact with one another; instead, many appear to be suspended within the matrix, indicating co-deposition of granules with silica gel. Approximately 30% of the matrix consists of carbonate material, which is randomly distributed within the chert matrix. Hematite grains in the GIF exhibit platy to needle-like morphologies, with grain sizes generally less than 15 µm. Most grains fall within the 1–5 µm range, suggesting an authigenic origin. Some ooids contain manganese carbonates within their inner rims, similar to those observed in the matrix, indicating Mn enrichment in bottom sediments. Our findings suggest that during the early depositional stage of the Gunflint Formation, bottom sediments of the surface water were enriched in Mn, indicating that surface waters were sufficiently oxidizing to promote the precipitation of Mn oxides. However, subsequent early burial of organic matter may have facilitated Mn reduction. Importantly, redox conditions in the shallow marine environment appear to have been oxidizing enough to preserve Fe oxides, but not sufficiently oxidizing to retain Mn oxides. The occurrence of bacterial reduction suggests an increase in organic carbon burial during this time, potentially associated with enhanced primary productivity; however, further investigation is required. - 47 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Interactive Geospatial Geoheritage: Efforts to Support Place-based Exploration and Digitally Preserve Keweenaw’s Geoheritage LIZZADRO-MCPHERSON, Daniel J.1, VYE, Erika C.1, 2, DeGRAFF, James M.2, and ROSE, William I.2 The Great Lakes Research Center, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931 USA 1 Department of Geological and Mining Engineering Sciences, Michigan Technological University, 630 Dow Environmental Sciences, 1400 Townsend Drive, Houghton, MI 49931 USA 2 The Keweenaw Peninsula, renowned for many superlatives – world’s largest native copper deposit, first major industrial mining complex in the United States – continues to inspire scientists, historians, and the general public. Ongoing geoheritage efforts enable these groups to explore the deep connections between the underlying geology, landforms, mining industry, and the people working and living on this land for over a millennia. Geoheritage uses a structured approach to identify, manage, and protect geosites and areas with geologic features of significant scientific, educational, cultural, or aesthetic value. Grassroots efforts, spearheaded by Bill Rose, have raised awareness and elevated the prestige of Keweenaw Geoheritage on the global stage despite lacking any formal designation. Bill’s efforts with others to create the first U.S. Geoheritage Park is still in the development stage, while other efforts led by Michigan Technological University (MTU) personnel are helping to bring Bill’s dream to fruition through two geospatial projects: 1) the Keweenaw Geoheritage Geodatabase and companion webGIS-viewer; and 2) Preservation, Indexing, and Enhanced Utility of Historic Copper Mining Drill Hole Records. The Keweenaw Geoheritage geodatabase and webGIS-viewer serve as a living atlas designed to facilitate ways of understanding relationships people hold with the Keweenaw’s geology. The publicly accessible interactive map explores how geology influences education, conservation, and sustainable economic development initiatives in the region (Fig. 1). Each geosite provides a) a brief description of how the site contributes to Keweenaw’s Geoheritage, b) a 360-view, and c) a description of the scientific, educational, cultural, economic, and aesthetic significance of the site (Lizzadro-McPherson & Vye, 2024). This effort supports the co-stewardship of cultural heritage, restoration of legacy mining sites, conservation issues, and the development of economic opportunities based on the region’s globally significant geology. The diamond drill hole (DDH) project aims to digitally preserve at-risk paper core logs, map DDH locations and details, and produce a robust database with a webGIS-based finding aid. The DDH core records document the more recent history of exploratory drilling by the copper mining industry (1899-1970) and contain information still relevant to geological research and exploration for critical minerals. The inventory of records is a tabular database of transcriptions of down-hole data from each scanned core log. An interactive webmap-based finding aid with PDF records and tables of interval descriptions on an open access data portal is in development. These innovative, interactive, geospatial resources aim to enhance scientific inquiry and broaden public engagement and exploration of Keweenaw’s iconic geologic landscape. REFERENCES Lizzadro-McPherson, D.J., and Vye, E.C. (2024). Keweenaw Geoheritage Geodatabase. Michigan State Geological Survey; U.S. Geological Survey, National Cooperative Geologic Mapping Program (Award #G23AC00285 FY23). - 48 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Fig. 2: Diamond drill hole record (left) and mapped surface location with metadata for Suffolk Exploration drilling campaign (right). Fig. 1: Keweenaw Geoheritage Viewer with pop-up displaying the core geoheritage values of the geosite at Great Sand Bay, Keweenaw County, MI. Fig. 2: Diamond drill hole record (left) and mapped surface location with metadata for Suffolk Exploration drilling campaign (right). - 49 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Implications of recent geochronology on the regional geology and timing of gold mineralization in the Red Lake greenstone belt, Ontario MACDONALD, Peter1, HASTIE, Evan1, MALEGUS, Paul2, KAMO, Sandra3, HAMILTON, Mike3 and MARSH, Jeff4 Earth Resources and Geoscience Mapping Section, Ontario Geological Survey, 933 Ramsey Lake Rd, Sudbury, ON P3E 6B5, Canada 1 2 Resident Geologist Program, Ontario Geological Survey, 227 Howey St, Red Lake, ON P0V 2M0, Canada Jack Satterly Geochronology Laboratory, Department of Earth Sciences, University of Toronto, 22 Ursula Franklin St, Toronto, ON, M5S 3B1, Canada 3 Mineral Exploration Research Centre, Harquail School of Earth Sciences, Laurentian University, 935 Ramsey Lake Rd, Sudbury, ON P3E 2C6, Canada 4 As part of the Ontario Geological Survey’s Red Lake bedrock mapping compilation project, geochronology samples were collected from the Red Lake gold camp to improve the ages of volcanic assemblages, sedimentary units and intrusive suites. Eighteen samples were analyzed using ID-TIMS and LA‑ICP‑MS uranium/lead methods on zircon grains. The new ages suggest significant revisions to the geographic presence and/or stratigraphy of the Balmer, Ball, Trout Bay and Confederation assemblages; as well as expanding the regional presence of the Huston conglomerates and identifying the presence of English River terrane sedimentation in the Uchi Subprovince. Newly dated intrusions from throughout the belt refine the timing of synvolcanic, syntectonic, and post‑tectonic magmatism, along with improving the known timing of early gold mineralization and later remobilization. Geochronology from the LP Fault highlights a sequence of felsic and porphyritic intrusive magmatism that is coeval with known gold mineralizing events in the main camp. - 50 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Quantitative analysis of iron mineral composition and crystal sizes in the contact metamorphosed Biwabik iron formation and the Bald Eagle intrusion, NE, MN, USA. MARIN LÓPEZ, Valentina1, BRENGMAN, Latisha1, EYSTER, Athena,2 MITCHELL, Jennifer3, PU, Xiaofei4, MANGUM, John4, and WALKER, Patrick4 Department of Earth and Environmental Sciences, University of Minnesota Duluth, Heller Hall, 1114 Kirby Drive, Duluth, MN 55812, USA 1 2 Department of Earth and Climate Sciences, Tufts University, Lane Hall, 2 North Hill Road, Medford, MA 02155, USA Characterization Facility and the Department of Earth and Environmental Science, University of Minnesota, Twin Cities, S-104 John T. Tate Hall, 116 Church Street Se, Minneapolis, MN 55455, USA 3 4 The National Laboratory of the Rockies, 15013 Denver West Parkway, Golden, CO 80401 Integrated experimental, theoretical, and field data demonstrate the potential viability of hydrogen production via subsurface fluid-rock interaction in systems with significant ferrous iron content (Mayhew et al., 2018; Ellison et al., 2021; Geymond et al., 2022; 2023; 2025; Templeton et al., 2024). As olivine is a key mineral of interest for hydrogen generation either through natural water-rock interaction, or engineered production, we focus on quantifying mineral compositions, crystal size distributions, and modal mineralogy in lithologic units from northeast Minnesota to enable future quantification of hydrogen production feasibility. Units of focus are the troctolitic portion of the Bald Eagle Intrusion (BEI; drill core LOD-6, n=16 samples), and the olivine-rich contact metamorphosed Biwabik iron formation (drill cores 8041 and 8016, n=12 and 13 samples respectively). Olivine and serpentine crystal size distributions (CSD) were quantified using image-based analysis. Combining 2D CSD measurements from BEI depths 970, 1091, and 1212.5 feet (n = 407 crystals from 3 samples; Figure 1A) yielded 8.0% partially serpentinized olivine, and 35.2% fully serpentinized olivine, with the remaining 56.8% of the sample composed of plagioclase, oxides, and minor phases external to olivine crystals. Olivine compositions (Fo76) are similar across BEI samples from multiple depths. In addition to olivine, BEI samples contain pyroxenes, labradorite, and titanium-bearing magnetite and ilmenite, with serpentine-group minerals present along key fracture sets. Reaction boundaries between olivine and serpentine were observed using transmission electron microscopy (TEM; Figure 1B, C). Serpentines are either amorphous or nano-crystalline with variations in crystallinity dependent on orientation in the fracture. Banding was observed in both Focused Ion Beam sections within serpentines proximal to olivine edges (Figure 1C). In metamorphosed Biwabik iron formation samples, olivine compositions are iron-rich (Fo12). In addition to olivine, meta-iron-formation samples contain quartz, oxides, sulfides, pyroxenes, amphiboles, chlorite, mica, calcite, with minor amounts of garnet, plagioclase, serpentine, and accessory phases. CSD analysis of metamorphosed iron formation sample 8016-271 (n = 190 crystals from 1 sample) yielded 55.3% olivine. Next steps include comparison of 2D CSD analyses with 3D X-ray computed tomography data. Overall, the presence of abundant olivine indicates the area could be of interest for future hydrogen generation. To quantify hydrogen generation potential, heterogeneity between serpentinized and un-serpentinized zones should be quantified to extend data from the mineral to the intrusion and formation scale, in addition to connecting hydrologic, geomechanical, and geochemical parameters to mineral data. Next steps include workflow modifications to improve scalability, and application of the workflow to the contact metamorphism Biwabik iron formation. - 51 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Figure 1. Image-based CSD analysis and Transmission electron microscopy images for sample LOD-6-1212.5. A) Traces of olivine and serpentine crystals in thin section with mineral proportions calculated using CSD analysis after Higgins, 2000. B) STEM image of reaction boundary between olivine and serpentine. C) TEM image of serpentine and olivine boundary. Top right diffraction pattern of serpentine with a green oval around planes (001) and (002) where the brightest area shows direction of growth of serpentine. Green arrows show crystal orientation. Bottom right diffraction pattern of olivine. REFERENCES Ellison, E. T., Templeton, A. S., Zeigler, S. D., Mayhew, L. E., Kelemen, P. B., Matter, J. M., et al. (2021). Low-temperature hydrogen formation during aqueous alteration of serpentinized peridotite in the Samail ophiolite. J. Geophys. Res. Solid Earth 126, e2021JB021981. doi:10.1029/2021JB021981. Geymond, U., Briolet, T., Combaudon, V., Sissmann, O., Martinez, I., Duttine, M., & Moretti, I. (2023). Reassessing the role of magnetite during natural hydrogen generation. Frontiers in Earth Science (Lausanne), 11. https://doi.org/10.3389/ feart.2023.1169356 Geymond, U., Truche, L., Sissmann, O., Kubániová, D., Recham, N., & Martinez, I. (2025). Mineralogical changes and H2 generation yield during hydrothermal alteration of a magnetite-siderite assemblage. Journal of Geophysical Research: Solid Earth, 130, e2024JB030724. https://doi.org/10.1029/2024JB030724 Higgins, M. (2000). Measurement of crystal size distributions. American Mineralogist , 85 (9): 1105–1116. https://doi. org/10.2138/am-2000-8-901 Mayhew, L. E., Ellison, E. T., Miller, H. M., Kelemen, P. B., and Templeton, A. S. (2018). Iron transformations during low temperature alteration of variably serpentinized rocks from the Samail ophiolite, Oman. Geochimica Cosmochimica Acta 222, 704–728. doi:10.1016/j.gca.2017.11.023 Templeton, A. S., Ellison, E. T., Kelemen, P. B., Leong, J., Boyd, E. S., Colman, D. R., & Matter, J. M. (2024). Low-temperature hydrogen production and consumption in partially-hydrated peridotites in Oman: implications for stimulated geological hydrogen production. Frontiers in Geochemistry, 2. https://doi.org/10.3389/fgeoc.2024.1366268. - 52 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Models of the regional gravity and magnetic anomalies associated with the Nipigon Embayment NITESCU, Bogdan1, TORRES, David Santiago1, and GAONA, Jorge Mario1 1 Department of Geosciences, Universidad de los Andes, Cra. 1 Nº 18A - 12 Bogotá, Colombia The Nipigon Embayment, a region dominated by Proterozoic rocks around Lake Nipigon, extends northward for approx. 150 km into the Superior craton from the Nipigon/Thunder Bay region on the northern shore of Lake Superior. The Embayment is characterized by the presence of intruded maficultramafic rocks and diabase sills dating from the early magmatic stage of Keweenawan rifting in Lake Superior (Heaman et al., 2007). The relationship between the Nipigon Embayment and the MCR has long been a topic of scientific investigation. Various researchers proposed that the Nipigon Embayment represents a viable candidate for a possible third branch of the MCR system (e.g., Hinze and Chandler, 2020), based on various lines of evidence, such as the existence of mafic-ultramafic igneous rocks in the upper crust with geochemical and geochronological similarities to the MCR rocks (e.g., Heaman et al. 2007; Hollings et al., 2007), and the anomalous upper mantle beneath the region reflected in weak seismic anisotropy (Ola et al., 2016), low velocity (Frederiksen et al., 2007; 2013; Foster et al., 2020), and electrical resistivity (Ferguson et al., 2005). However, some investigators suggest that the Nipigon Embayment is related to pre-existing structures, arguing against this region representing a third branch of the MCR due to its lack of Keweenawan extensional features (e.g., Hart and MacDonald, 2007). In this contribution, the gravity and magnetic regional anomalies associated with parts of the Nipigon Embayment are evaluated, both qualitatively, using various filters, and quantitatively, using 2.5D forward modelling. The positive mass anomalies that account for the regional gravity highs in the area covered by the Nipigon sills are equivocal and could be related either to Nipigon magmatic rocks or to covered older rocks bodies, such as Archean mafic-ultramafic intrusions and greenstone belts. If it is assumed that some of these anomalies are related to the Nipigon magmatic rocks, then the gravity models suggest the existence of structures that may have acted as feeders for the emplacement of the Nipigon Embayment mafic-ultramafic intrusive bodies and diabase sills. The Figure 1: 2.5D forward model of the Bouguer gravity anomaly along an W-E profile in the northern part of the Nipigon Embayment, assuming that the cause of the anomaly is related to Nipigon magmatic rocks. Density values: Nipigon magmatic rocks 2.89 g/cc; greenstone belt mafic rocks 2.9 g/cc; granitoid and tonalite 2.64 g/cc; background 2.67 g/cc. - 53 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 magnetic models of the regional magnetic anomalies indicate the presence of a significant subsurface volume of highly magnetic rocks within the Nipigon Embayment crust. These results are compatible with the interpretation of this region as a segment of the crust affected by magmatism in the initiation stage of the MCR, possibly as an incipient, undeveloped part of the rift controlled by pre-existing structures. REFERENCES Ferguson, I.J., Craven, J.A., Kurtz, R.D., Boerner, D.E., Bailey, R.C., Wu, X., Orellana, M.R., Spratt, J., Wennberg, G., Norton, M., 2005. Geoelectric response of Archean lithosphere in the western Superior Province, central Canada. Phy. Earth Planet Int. 150, 123–142. https://doi.org/10.1016/j.pepi.2004.08.025 Foster, A., Darbyshire, F., Schaeffer, A., 2020. Anisotropic structure of the central North American Craton surrounding the Mid-Continent Rift: Evidence from Rayleigh waves. Prec. Res. 342, 105662. https://doi.org/10.1016/j. precamres.2020.105662. Frederiksen, A.W., Miong, S.K., Darbyshire, F.A., Eaton, D.W., Rondenay, S., Sol, S., 2007. Lithospheric variations across the Superior Province Ontario, Canada: Evidence from tomography and shear wave splitting. J. Geophys. Res-Earth 112, 1–20. https://doi.org/10.1029/2006JB004861. Frederiksen, A.W., Bollmann, T., Darbyshire, F., van der Lee, S., 2013. Modification of continental lithosphere by tectonic processes: A tomographic image of central North America. J. Geophys. Res-Earth 118, 1051–1066. https://doi. org/10.1002/jgrb.50060. Hart, T.R., MacDonald, C.A., 2007. Proterozoic and Archean geology of the Nipigon Embayment: Implications for emplacement of the Mesoproterozoic Nipigon diabase sills and mafic to ultramafic intrusions. Can. J. Earth Sci. 44, 1021–1040. https://doi.org/10.1139/e07-026. Heaman, L.M., Easton, R.M., Hart, T., MacDonald, C.A., Hollings, P., Smyk, M., 2007. Further refinement to the timing of Mesoproterozoic magmatism Lake Nipigon region, Ontario. Can. J. Earth Sci. 44, 1055–1086. https://doi. org/10.1139/e06-117. Hinze, W.J., Chandler, V.W., 2020. Reviewing the configuration and extent of the Midcontinent rift system. Prec.Res. 342, 105688. https://doi.org/10.1016/j.precamres.2020.105688. Hollings, P., Hart, T., Richardson, A., MacDonald, C.A., 2007a. Geochemistry of the Mesoproterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario: Evaluating the earliest phases of rift development. Can. J. Earth Sci. 44, 1087–1110. https://doi.org/10.1139/e06-127. Ola, O., Frederiksen, A.W., Bollmann, T., van der Lee, S., Darbyshire, F., Wolin, E., Revenaugh, J., Stein, C., Stein, S., Wysession, M., 2016. Anisotropic zonation in the lithosphere of Central North America: Influence of a strong cratonic lithosphere on the Mid-Continent Rift. Tectonophysics 683, 367–381. https://doi.org/10.1016/j.tecto.2016.06.031. - 54 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Origin of the World-Class Eagle, Eagle East, and Tamarack Ni-Cu-PGE Deposits and comparative analysis with other Midcontinent Rift- and Siberian Trap-related intrusions NOWAK, Robert1, DEERING, Chad1 , and ESSIG, Espree1 Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA 1 The 1.1 Ga Mesoproterozoic Midcontinent rift hosts the Eagle, Eagle East, and Tamarack Ni-CuPGE deposits and Embayment Prospect. These deposits are hosted by ultramafic igneous rocks and have some of the highest Ni-Cu grades on Earth. We use new bulk-rock data and published datasets (bulk-rock, mineral chemistry, and isotopic analyses) to examine major, minor, and trace element trends of both Midcontinent rift-related alkaline and tholeiitic intrusions (Nowak et al., 2025). In addition, we compare the geochemical data to local kimberlite-hosted lower-crustal xenoliths and local igneous (Archean) and sedimentary (Paleoproterozoic) country rocks. We found the peridotite magma compositions dominantly consist of primitive mantle compositions with varying abundances of subduction-related components, alkaline-transitional melts, and local country rock contaminates (e.g., Baraga and Animikie Basin sediments). The subduction-related components are interpreted to be derived from previous Archean and Paleoproterozoic subduction events and likely hosted within the sub-continental lithospheric mantle. Importantly, these subduction-related components are also interpreted to have acted as oxidizing agents within the melt, stabilizing sulfate (+2 FMQ (fayalite–magnetite–quartz) to FMQ) while inhibiting sulfide crystallization as the magma ascended through ~50 km of the Superior craton. This study largely corroborates the previous findings with respect to the contribution of local country rock contamination to the Eagle–Tamarack peridotite host rocks, which is estimated to be minimal (<5%). However, the incorporation of <5% reductive pelitic siltstone contamination results in strong shifts in the oxygen fugacity of the peridotite melt, from +2 FMQ to slightly below FMQ, as determined from spinel Fe3+/∑Fe ratios (Figure 1). This shift in oxygen fugacity resulted in the transition from total sulfate (+2 FMQ) to sulfate + sulfide (<+2 FMQ to FMQ) to total sulfide (<FMQ). This shift in oxygen fugacity is a key contributor to the formation of Ni-Cu-PGE-rich massive sulfides within the Eagle peridotite. This study presents an expanded geochemical interpretation for the exploration of Midcontinent rift-related Ni-Cu-PGE deposits to include peridotites with subduction-like signatures and contaminated via <5% reductive sedimentary country rocks. Based on these findings, we also comparatively analyze geochemical samples from Figure 1: Downhole profiles of drillhole 03EA034 of Fe3+/∑Fe ratios of spinel (Ding et al., 2010); with oxygen fugacity estimates (relative to FMQ) this study), Ni, Cu, and S (all in wt%; this study), and the relative proportion (%) of compositions (eclogite, amphibolite, subducted sediment, alkaline-transitional, and Baraga Basin sediments) used to reconstruct the multielement compositions of Eagle peridotite. Analytical error (accuracy; 1σ) is estimated to be smaller than the symbol size. - 55 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Midcontinent rift-related prospective intrusions and Siberian-Trap-related intrusions in order to better determine economic vs. subeconomic host rock signatures. REFERENCES Ding, X., Li, C., Ripley, E.M., Rossell, D., Kamo, S., 2010, The Eagle and East Eagle sulfide ore-bearing mafic-ultramafic intrusions in the Midcontinent Rift System, upper Michigan. Geochronology and petrologic evolution. G3 Geochem. Geophys. Geosyst., 11, p.1-22. Nowak, R., Deering, C., and Essig, E., 2025, Origin of the World-Class Eagle, Eagle East, and Tamarack Ni-Cu-PGE Deposits. Minerals, 15, 871. https://doi.org/10.3390/min15080871 - 56 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 BEDROCK GEOLOGY OF THE ERICSBURG NW, ERICSBURG NE, RAY SW, AND RAY SE QUADRANGLES, ST. LOUIS AND KOOCHICHING COUNTIES, MINNESOTA NOWARIAK, Eric and SEVERSON, Allison Minnesota Geological Survey, University of Minnesota – Twin Cities, 2609 Territorial Road St. Paul, MN, USA New geologic mapping presented here portrays the Precambrian bedrock geology and tectonic history of the axial zone of the Quetico subprovince across four 7.5’ quadrangles in portions of eastcentral Koochiching County and far western St. Louis County, Minnesota. The map records the Neoarchean deposition, deformation, metamorphism, and migmatization of turbiditic sediments, along with the intrusion of the granitic rocks of the Vermilion Granitic Complex during the accretion of the Wawa subprovince to the southern margin of the Superior Province, and continuing through the intrusion of the Paleoproterozoic Fort Frances dike swarm. The metasedimentary rocks of the Quetico subprovince, now predominantly biotite schist, granofels, and migmatite, have been subject to at least four successive contractional and transpressional deformation styles documented in the map area. The map pattern and dominant structural grain of bedrock is controlled by structures associated with D2 and D3 deformation. D2 deformation produced map-scale, tight to isoclinal F2 folds with well-developed ENE-WSW-striking subvertical S2 axial-planar foliation. D3 deformation coincides with the development of ENE- and NW-trending ductile shear zones with dextral motion. F3 folds are coaxial to F2 folds and manifest as isoclinal refolds and reorientations of D2 structures. D4 deformation post-dates the dominant D2 and D3 deformational events and is represented by steeply plunging broad, open folds and NNW-trending fault and fracture zones. New geochemical analyses illustrate rocks of the Vermilion Granitic Complex are generally calkalkaline, weakly peraluminous to metaluminous, magnesian granitoids with minor amphibole-rich dioritic to gabbroic rocks. Based on geological and geochemical features, the Vermilion Granitic Complex can be subdivided into groups with distinct lithologies, geochemistry, and magma sources. Tonalites, trondhjemites, and granodiorites (TTG) of the Early Magmatic Suite are distinctly more sodic than the younger Lac La Croix Suite granitoids. Compared to the Early Magmatic Suite, Lac La Croix Suite granitoids are relatively more alkalic, more aluminous, and have steeper REE profiles. Quetico metasedimentary rocks and the Vermilion Granitic Complex have been subject to at least two metamorphic events recording the burial, uplift, and intrusive history of the subprovince. M1 metamorphism is likely contemporaneous with D2 and early D3 deformation, based on the presence of syn- and post-kinematic, inclusion-rich porphyroblasts. Peak metamorphic conditions reached amphibolite facies during M1 and have been constrained to 525-575°C with pressures exceeding 6 kbars based on phase equilibrium modeling outside the thermal influence of the Lac La Croix Suite. Proximal to the Lac La Croix granite, M1 metamorphic features have been overprinted by a hightemperature, low-pressure event, M2, presumably due to the intrusion of voluminous granitoids of the Lac La Croix Suite during the waning stages of D3 deformation. M2 metamorphism manifests as inclusion-poor garnet, sillimanite-, cordierite-, and andalusite-bearing assemblages in metasediments and granitic orthogneisses. - 57 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Figure 1. A. Plutonic rock classification of igneous rocks in this study, after Enrique and Esteve, 2019. B. 2ACNK (2* molar Al2O5/(CaO+Na2O+K2O), Na2O/K2O, 2 FMSB (2*(FeOtot+MgO)wt.%*(Sr+Ba)wt.%) source identification diagram, of Laurent and others (2014). Fields for TTG (T), continental or C-type (C), and metasomatized mantle or M-type (M) granitoids from Moyen (2019) have been added. C. Alumina Saturation plot after Barton and Young (2002) for all intrusive units within the map area. - 58 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Petrographic, geochemical, and mineralogical analyses of manganiferous iron formations and associated lithologies at the Cuyuna Range, central Minnesota PALIEWICZ, Cory1, POST, Sara1, and THAKURTA, Joyashish1 Natural Resources Research Institute (NRRI), University of Minnesota Duluth, 5013 Miller Trunk Hwy, Duluth, MN 55811 USA 1 The Paleoproterozoic Cuyuna Range of central Minnesota hosts one of two significant manganese deposits in the United States and contains anomalously high manganese concentrations (up to ~50 Wt. % Mn) when compared to other Banded Iron Formations in the Lake Superior region (Cannon et al., 2017). The area was highly deformed and metamorphosed during the Penokean Orogeny and encompasses the Emily District at edge of the Animikie Basin to the north, and the North and South ranges which occur within the older fold and thrust belt to the south (Boerboom and Chandler, 2004; Southwick et al., 1988; Morey, 1990). Although the area has a rich history of iron mining and ongoing manganese exploration, many questions remain regarding the occurrence, nature, and mechanisms of manganese mineralization. This work includes new petrographic, lithogeochemical, and mineralogical data collected and analyzed from 201 drill core samples from 37 drill holes across the Emily District, North Range, and South Range (Figure 1). The regional pilot study is part of a larger USGS Earth Mapping Resources Initiative to map and better-constrain the mineral potential of the region. We emphasize the lithologic variability of mineralized iron formations throughout the range, but especially within the Emily District, which from past studies is known to be most-enriched in Mn-content. Cuyuna iron formations generally range from cherty to slaty (thick bedded to thin bedded / granular to non-granular) with manganiferous units extending from enriched (5-10% Mn), manganiferous (>10 % Mn) and highly manganiferous (>35 % Mn). Although textural and mineralogical differences of mineralized units vary widely with increasing grade, the variability and significance of non-enriched lithologies throughout the Cuyuna Range also offer insights regarding possible sources or mechanisms of mineralization, especially when taken within the context of recently integrated historic drill logs and prior works (e.g., McSwiggen et al., 1995). Textural and mineralogical variation among mineralized units exhibit many signs of hydrothermal modification during manganese enrichment. Many grains have been replaced with manganese oxides and hydroxides in both cherty and slaty iron formations and the occurrence of vugs associated with other hydrothermal accessory minerals such as carbonates, epidote, micas, and clays, along with abundant sieved and altered grains indicate that many pulses of variable hydrothermal activity likely resulted in disequilibrium of most preserved mineral assemblages. Non-mineralized units such as graywackes are typically highly altered to sericite and kaolinite and pyritic graphitic argillites have been observed to exhibit non isochemical characteristics illustrating high mobility of Fe and Mn. The occurrence of silicified and oxidized zones as they relate to variations of grade are also characterized along with variability of downhole changes of Mn, Fe, SiO2, Al2O3, and LOI plotted as split logs which also show changes of Co, Cu, and Zn to further assess the possibility of other critical minerals associated with manganese. - 59 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Figure 1: Geologic map after Boerboom and Chandler (2004; 2022) showing drill hole locations sampled in Crow Wing and Aitkin Counties, central Minnesota. REFERENCES Boerboom, T.J., and Chandler, V. W., 2004, Plate 2 - Bedrock Geology, in Setterholm, D. R. Geologic atlas of Crow Wing County, Minnesota, MGS County Geologic Atlas, C-16 Part A, 1:100,000. Boerboom, T.J., and Chandler, V. W., 2022, Plate 2 - Bedrock Geology, in Bauer, et al., 2022. Geologic atlas of Aitkin County, Minnesota, MGS County Geologic Atlas, C-52 Part A, 1:200,000. Cannon, W.F., Kimball, B.E., and Corathers, L.A., 2017, Manganese, in chap. L of Schulz, K.J., DeYoung, J.H., Jr., Seal, R.R., II, and Bradley, D.C., eds., Critical mineral resources of the United States—Economic and environmental geology and prospects for future supply: USGS Professional Paper 1802, p. L1–L28. McSwiggen, P.L., Morey, G.B., and Cleland, J.M., 1995, Iron-formation protolith and genesis, Cuyuna range, Minnesota: Minnesota Geological Survey Report of Investigations 45, 54 p. Morey, G.B., 1990, Geology and manganese resources of the Cuyuna iron range, east-central Minnesota: Minnesota Geological Survey Information Circular 32, 28 p. Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, Geologic map (scale 1:250,000) of the Penokean orogen, central and eastern Minnesota, and accompanying text: Minnesota Geological Survey Report of Investigations 37, 25 p., 1 pl. - 60 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Physical Magmatic System Interpretation of the Marathon Cu-Pd Deposit, Coldwell Complex, Ontario PETERSON, Dean1, STEINER, R. Alex1, SWEET, Gabriel1, and BOUCHER, Chanelle2 1 2 Big Rock Exploration, 2505 West Superior Street, Duluth, MN 55806. Generation PGM Inc., 100 King Street West, Toronto, ON M5X 1B1. The goal of geologic mapping and/or drill core logging in mafic magmatic ore deposits is to not just know what the lithology is at a specific outcrop and/or drill hole interval, but to know with some confidence where you are in the mineralized intrusion, i.e., within the overall magmatic system. Generation Mining (GenM) contracted Big Rock Exploration (BRE) to reevaluate the Coldwell Complex hosted Marathon Cu-Pd deposit using a magmatic system approach. Mineralized mafic intrusions are typically composed of three principal minerals, plagioclase-olivinepyroxene along with various proportions of apatite, Fe-Ti oxides, and Fe-Cu-Ni sulfides. Variations in mineralogic estimates of the three principal minerals by many geologists over decades of time can be the difference between calling a rock an anorthosite, a gabbro, a troctolite, or a peridotite. In deposit areas with decades upon decades of exploration history, these basic lithologic calls by many different geologists can directly influence how a mafic magmatic ore deposit is interpreted and/or modeled. Problems in interpretation can come to the forefront when drill hole intervals are logged strictly by lithology and subsequently digitally assigned a LithCode. Another method of logging and interpreting mineralized mafic intrusions is to approach it from the physical process side, i.e., as a magmatic system. Utilizing a magmatic system approach begins with an understanding of the initial conditions of the system. Initial conditions include the intrusive geometry and Figure 1. Schematic model of the magmatic architecture of the Marathon Cu-Pd deposit. - 61 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 flow paths, the lithology of the footwall, hangingwall and sidewall rocks, and the magmas composition, crystallinity, plagioclase-olivine phenocryst content, trace element signature and sulfide content. In general terms, mafic intrusions have slower moving marginal boundaries, which are commonly xenolithrich, surrounding a faster flowing and xenolith-poor ‘clean’ central core. Magmatic shearing is induced by the differential velocity, from margin to core, in which magmas intrude can lead to pronounced local mineralogical variability in the outcome. For example, phenocryst sorting leads to modal layering, and kinetic sieving processes raises large particles, which can be phenocrysts, autoliths and/or xenoliths, upwards in the intrusion. The rocks formed in these mafic magmatic systems, though largely governed by the initial conditions, locally can vary by associated chemical, thermal, and momentum boundaries. BRE coupled these magmatic first principals with GenM assisted field work and drillhole relogging to reevaluate the Marathon Cu-Pd deposit magmatic system. A schematic model of the interpreted magmatic system at the Marathon Cu-Pd deposit is presented in Figure 1, and stratigraphic profiles depicting the historic lithology-based coding (Lith Codes) and recently proposed magmatic systems approach coding (Unit Codes) is given in Figure 2. This talk will highlight many of BRE’s research findings on the Marathon Cu-Pd deposit magmatic system. Figure 2. The proposed magmatic system approach Unit Codes (left) versus the historic GenM Lith Codes (right) assigned to the rocks of the Marathon Cu-Pd deposit. Rectangular arrows point to how logged lithologies can be assembled into discrete magmatic system units of the Marathon Series. - 62 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Critical Mineral Potential of the Watersmeet Gneiss Dome, MI USA QUIGLEY, Ashley1, MAHIN, Robert1, and GAMET, Nolan1 1 Michigan Geological Survey, 416 Avenue C, Gwinn, MI 49841U.S.A. Precambrian gneisses and schists on the northern margin of the Watersmeet Dome in Michigan’s Upper Peninsula are unusually enriched in rare earth elements, fluorite and incompatible elements including U, Th, Hf, and Zr (Barovich et al., 1991; Sims, 1990). Rocks are mainly Archean gneisses and amphibolites although elevated REEs, fluorite and incompatible elements are associated with a gneiss and schist unit of possible Paleoproterozoic age (Barovich et al., 1991). The area is within Earth Mapping Resources Initiative (EMRI) critical mineral focus areas for both IOCG/ IOA and Magmatic REE deposits (Dicken and others, 2022). The Michigan Geological Survey (MGS) conducted detailed geologic mapping and sampling, as well as collected geophysical and geochronological data. An RS-230 BGO gamma-ray spectrometer was used to take over 600 total gamma (K/U/Th) measurements from outcrops. Additionally, an unmanned aerial vehicle (UAV), high resolution magnetic survey was flown. A previously undescribed magnetic, fine-grained schist comprised 85% of the highest total REE samples (high of 1659 ppm TREE). The schists are associated with magnetite and fluorite and coincide with a kilometer-wide central magnetic anomaly, as well as a three kilometer, roughly east-west trending, sinuous anomaly. In plots, granitoids, gneisses, and schists show three distinct populations. Group 1 clusters in the VAG-syn/COLG field, has no europium anomaly and average 72 ppm TREE. Group 2 is transitional between VAG-syn/ COLG and WPG, has a marked europium depletion, and contains an average of 136 ppm TREE. Group 3 is enriched in REE with an average 711 ppm TREE, plots in the WPG/A-Type granite field and has moderate europium depletion. All three groups are peraluminous. Group 3 rocks include enriched REE magnetic schists, magnetic granitoids, and gneisses all of which are located in proximity to each other as well as to magnetic highs. Highly fractionated, non-peralkaline felsic granites can have geochemical characteristics which overlap those for typical A-type granites Figure 1: Map showing the location of the Watersmeet project area. - 63 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 (Whalen and others, 1987). Some fractionation is indicated in Group 1 and Group 2 rocks by a semicontinuous trend of decreasing Zr, Nb, Ce, and Y. Group 3, however, displays no such evidence of fractionation, which is typical of A-Type granites. Numerous REE, F, Th, and/or U-bearing silicate, oxide and carbonate minerals including fluorite, thorite, pyrochlore, allanite, columbite, parasite, and yttrialite were identified using SEM within alteration halos along fractures and occasionally within veins. Zircons with strong pleochroic halos are common, particularly within biotite grains but also observed with amphiboles. The presence of fluorite and REE bearing-fluorocarbonates indicate that REE enrichment was facilitated, at least in part, by fluorine-rich hydrothermal fluids. Preliminary, unpublished U-Pb zircon geochronology indicate that all units are Archean and the previous proposed Paleoproterozoic ages may represent a thermal resetting event. REFERENCES Barovich, K.M., Patchett, P.J., Peterman, Z.E., and Sims, P.K., 1991. Neodymium Isotopic​Evidence for Early Proterozoic Units in the Watersmeet Gneiss Dome, Northern​Michigan. U.S. Geological Survey Bulletin 1904-G: G1-G7. ​ Dicken, C.L., Woodruff, L.G., Hammarstrom, J.M., and Crocker, K.E., 2022, GIS,​supplemental data table, and references for focus areas of potential domestic resources​of critical minerals and related commodities in the United States and Puerto Rico (ver.2.0, April 2024): U.S. Geological Survey data release, https://doi.org/10.5066/P9DIZ9N8. Pearce, Julian & Harris, Nigel & Tindle, Andrew. (1984). Trace Element Discrimination Diagrams for the Tectonic Interpretation of Granitic Rocks. Journal of Petrology. 25. 956-983. 10.1093/petrology/25.4.956.​ Sims, P.K., 1990, Geologic map of Precambrian rocks, Marenisco, Thayer, and​ Watersmeet 15-minute quadrangles, Gogebic and Ontonagon counties, Michigan, and​ Vilas County, Wisconsin: U.S. Geological Survey Miscellaneous Investigations Series Map​I-2093, scale 1:62,500.​ Whalen, J.B., Currie, K.L. & Chappell, B.W. A-type granites: geochemical characteristics, discrimination and petrogenesis. Contrib Mineral Petrol 95, 407–419 (1987). https://doi.org/10.1007/BF0040220. - 64 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Current geologic and geophysical research on the Precambrian basement of eastern North Dakota, USA SAINI-EIDUKAT, Bernhardt1, CHITTICK, Steve2, and NESHEIM, Timothy2 1 2 Dept. of Chemistry and Biochemistry, North Dakota State University, Fargo, ND 58102 USA North Dakota Geological Survey, Grand Forks, ND 58202 USA In the entirety of the state of North Dakota, no crystalline basement is exposed due to Phanerozoic sedimentary cover. Regional geophysical mapping, combined with lithological data and radiometric dates, have correlated the Wabigoon and Wawa subprovinces of the Superior Craton into eastern North Dakota (Figure 1). However, understanding of the geologic and the geophysical characteristics of the basement in this region is, with some exceptions, relatively poor compared to many other areas (Figure 2). Figure 1: Map of North Dakota Precambrian geology, RRVD drill core locations, and proposed survey area (black outline). Open symbols: geochronology samples. Base map from Sims et al. (1991). Figure 2: Regional aeromagnetic map, and proposed survey area (blue outline), showing the difference in resolution between ND and MN. (The NE corner of ND does already have higher quality aeromagnetic data.) - 65 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 The North Dakota Geological Survey (NDGS), working with the Earth Mapping Resources Initiative (Earth MRI) of the U.S Geological Survey (USGS) (www.usgs.gov/special-topics/earthmri), and North Dakota State University are undertaking a renewed initiative to obtain high quality geochronologic, geochemical, geophysical, and radiometric data over eastern North Dakota. Depth to basement is on the order of a few hundred meters in eastern ND, but increases to thousands of meters westward underneath the Williston Basin. For that reason, the focus of the initiative is on the eastern region where depth to basement is less than 1000 m. As part of Earth MRI, the USGS is planning to carry out a high-resolution airborne magnetic and radiometric survey in eastern North Dakota, to be flown in 2026-27. The survey will be designed to meet complementary needs related to geologic mapping and mineral resource research. The survey design is being coordinated with the NDGS to provide complete coverage of a region that crosses the boundaries of multiple subprovinces and greenstone belts within the Archean Superior Province. The mineral systems of interest in the survey area include Mafic magmatic, Porphyry Sn, and Metamorphic. Potential critical mineral commodities include Cr, PGE, Au, Co, graphite, REE, Li, Ta, and Sn. There is additional potential for Mn, Ni, Cu, Fe, Mg, and Cs. Samples of drill core from the 1977 Red River scientific drilling project (Moore, 1978; Kelley, 1980; Beaudry et al., 2024, Pereira et al., 2024), and from other cores, will undergo geochemical, geochronological, petrological, and geophysical investigation. Portable XRF analysis for trace elements is underway, as is a gravimetric survey of eastern ND by the NDGS. REFERENCES Beaudry, C., Hess, M., Pereira, C., Saini-Eidukat, B., 2024, Petrology and geochemistry of Precambrian basement rocks in Walsh County, North Dakota. ILSG Abstr. and Proc., v.70, part 1, p. 6-7. Kelley, L.I., 1980, Kaolinitic weathering zone on Precambrian basement rocks, Red River Valley, eastern North Dakota and northwestern Minnesota. M.S. Thesis, University of North Dakota. 85 pp. Moore, W. L., 1978. A preliminary report on the geology of the Red River Valley Drilling Project, eastern North Dakota and northwestern Minnesota: Bendix Field Engineering Company Subcontract H77-059-E, 292p. https://www.osti.gov/ biblio/6538603 doi:10.2172/6538603 Pereira, C., Nesheim, T., Vervoort, J.D., and Saini-Eidukat, B., 2024, Major element geochemistry and first zircon U-Pb age dates of Precambrian basement rocks in eastern North Dakota. ILSG Abstracts and Proceedings, v.70, part 1, p.74-75. Sims, P.K., Peterman, Z.E., Hildenbrand, T.G., and Mahan, S., 1991, Precambrian Basement Map of the Trans-Hudson Orogen and adjacent terranes, northern Great Plains, U.S.A.: USGS Miscellaneous Investigations Series Map, I-2214. DOI: 10.3133/i2214 - 66 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Reassessing variations in metamorphism across the Penokean orogen in Northern Michigan: Part 1, new Pressure-Temperature-Time-Deformation constraints SALERNO, R.1, CANNON, W. F.1, THOMPSON, J. M.2, SOUDERS, A. K.2, VERVOORT J.3, and HILLENBRAND, I.2 1 2 3 U.S. Geological Survey, Reston, VA 20192, USA U.S. Geological Survey, Denver, CO 80225, USA Washington State University, Pullman, WA 99163, USA The Penokean orogeny (1890-1830 Ma) represents the earliest collisional event in a long subduction sequence active throughout the Paleoproterozoic to Mesoproterozoic along Laurentia’s southern margin. Traditionally, spatial variations in metamorphic grade in the Penokean orogenic belt were described as three “nodes” (Fig. 1) and ascribed to the main accretionary phase which ended at 1830 Ma. However, the swath of younger 40Ar/39Ar cooling ages at ~1750 Ma across the terrane suggests later collisional episodes also played an important role in modifying the Penokean orogenic belt (Schneider et al., 1996). This observation, coupled with newly mapped younger structures by recent geophysical surveys, raises questions about which features are truly Penokean in origin, and which reflect later overprinting by younger tectonic events (Drenth et al., 2021). Elucidating the causes and timing of post-Penokean modification of crust in central Laurentia is key for accurately reconstructing the outward growth of proto-North America throughout the Proterozoic. We have used multiple geochronometers and isotope systems to unravel the metamorphic evolution of the Penokean orogenic belt. New geochronology and thermodynamic modeling of metasedimentary rocks reveal variations in the timing of metamorphism and subsequent cooling histories between metamorphic nodes (Fig. 1). Directly adjacent to the Niagara fault zone, rocks in the Peavy node have garnet Lu-Hf ages of 1837±7 Ma, reflecting the age of granulite facies metamorphism in the lower crust. Overlapping garnet Sm-Nd (1830±65 Ma) and apatite U-Pb (1822±28 Ma) ages indicate rapid exhumation of these lower crustal rocks near the end of the Penokean orogeny. In contrast, rocks in the Watersmeet and Republic nodes, located farther inboard from the paleomargin, reflect later lowergrade amphibolite facies regional metamorphism after the end of the Penokean orogeny, from 1825±5 to 1782±15 Ma. Unlike the Peavy node, these samples have offset Lu-Hf and Sm-Nd ages reflecting the different closure temperatures of the two isotope systems in garnet. Dispersed Lu-Hf and Sm-Nd ages indicate prolonged residence of these rocks at mid-crustal depths and correspond with protracted cooling paths of 1-3°C/Mya, until final exhumation began at ~1750 Ma. Our results illustrate that the metamorphic nodes in the Penokean orogenic belt do not reflect the same conditions or cooling histories, and do not all represent the same tectonic event. Instead, our data reveal a sequence where early granulite facies metamorphism and rapid exhumation are linked with the end stages of the Penokean orogeny and are restricted to the belt of high-grade rocks north of the Niagara fault. Regional amphibolite facies metamorphism persisted after, requiring continued crustal thickening following both the accretionary phase of the Penokean orogeny and exhumation of deep crustal rocks. The implications of this are two-fold. 1) The metamorphic nodes in the Penokean orogenic belt are not cogenetic but rather reflect different tectonic events and different times. 2) Post-Penokean regional metamorphism followed by widespread uplift and cooling after ~1750 Ma represent significant modification of the Penokean orogenic belt throughout Geon-17. More broadly, younger overprinting on this terrane reveals that outboard tectonic activity following the Penokean orogeny played a major role in the modification of Paleoproterozoic and Archean crust in central Laurentia. - 67 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Figure 1: Top, generalized geologic map showing metamorphic nodes in the Penokean orogen in northern Michigan and sample locations in our study (map after Tinkham and Marshak, 2004). Bottom, temperature-time diagrams showing cooling histories of garnet-bearing rocks in three metamorphic nodes. Microstructures indicate deformation during uplift at ~1750 Ma proceeded after peak metamorphism. 40Ar/39Ar data are from previous studies and references are compiled in Salerno et al. (2026). REFERENCES Drenth, B.J., Cannon, W.F., Schulz, K.J., and Ayuso, R.A., 2021, Geophysical insights into Paleoproterozoic tectonics along the southern margin of the Superior Province, central Upper Peninsula, Michigan, USA: Precambrian Research, v. 359, doi:10.1016/j.precamres.2021.106205. Salerno, R., Cannon, W.F., Thompson, J.M., Souders, A.K., Vervoort, J., Hillenbrand, I., 2026, Unraveling protracted modification of Archean and Paleoproterozoic crust in central Laurentia, Penokean orogen, with garnet and accessory mineral geochronology and microstructural analysis: Geological Society of America Bulletin, in press. Schneider, D., Holm, D., and Lux, D., 1996, On the origin of Early Proterozoic gneiss domes and metamorphic nodes, northern Michigan: Canadian Journal of Earth Sciences, v. 33, p. 1053–1053, doi:10.1139/e96-080. Tinkham, D.K., and Marshak, S., 2004, Precambrian dome-and-keel structure in the Penokean orogenic belt of northern Michigan, USA, in Whitney, D.L., Teyssier, C., and Siddoway, C.S., eds., Gneiss Domes in Orogeny: Geological Society of America Special Paper, v. 380, p. 321-338, doi:10.1130/0-8137-2380-9.321. - 68 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Whole Rock and Mineral Chemistry of the Eagle’s Nest Intrusion, McFaulds Lake Greenstone Belt, Ontario, Canada: Insights into the Origin and Paragenesis SHESHNEV, Vlad1, HOLLINGS, Pete1, TOLLEY, James1, ANGOMBE, Moses1, DELLER, Matt2, and STERN, Richard3 1 2 3 Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada Wyloo, Thunder Bay, Ontario, Canada Canadian Centre for Isotopic Microanalysis, University of Alberta, Edmonton, Alberta, Canada Orthomagmatic Ni-Cu-(PGE) deposits originate in the mantle, where source composition and degree of partial melting are the first-order controls on composition and metal fertility of the derived magmas (Naldrett, 2011). During ascent, these magmas undergo differentiation, producing more evolved compositions that reflect both the characteristics of the mantle source and subsequent magmatic processes (Barnes, 2023; Smith et al., 2024). The Eagle’s Nest intrusion is a maficultramafic, blade-shaped dike, which is host to the only known economically significant Ni-Cu-(PGE) mineralization within Meso- to Neoarchean McFaulds Lake Greenstone Belt. The Eagle’s Nest is part of the mafic to ultramafic magmatism of the Koper Lake subsuite, of the larger Ring of Fire Intrusive Suite (ca. 2736–2732 Ma; Houlé et al., 2020; Metsaranta and Houlé, 2020). Two different parental magma compositions have been proposed for the Eagle’s Nest intrusion, including a low- and a highMg komatiitic magma, both of which are inconsistent with the observed mineralogy of the intrusion (Mungall et al., 2010; Zuccarelli, 2020). To better understand the origin and nature of the Eagle’s Nest intrusion, this study integrated petrography, whole-rock geochemistry, mineral chemistry, as well as radiogenic and stable isotope systematics. The Eagle’s Nest intrusion can be subdivided into the marginal and inner zones. The marginal zone comprises mafic intrusive rock in contact with the wall rock tonalite, exhibiting the most evolved mineralogical and geochemical characteristics. The marginal zone gradationally transitions into the inner zone, which consists of ortho- to mesocumulate ultramafic rocks with more primitive compositions, reflecting the accumulation of olivine and chromite in cotectic proportions, along with variable amounts of intercumulus silicate phases and interstitial sulfides. Using the whole rock geochemistry of olivine-chromite cotectic cumulate rocks, combined with olivine and chromite mineral chemistry, a new parental magma composition was determined for the Eagle’s Nest intrusion. The new estimate suggests a komatiitic basalt magma that contained ~11 wt% FeOt and ~15 wt% MgO. The new parental magma estimate is more evolved than previously proposed compositions, however, it is consistent with the composition of identified chilled margins, associated mafic dikes, and olivine from the Eagle’s Nest intrusion. Using the newly obtained estimate, the petrographically determined crystallization sequence was recreated at low pressures, suggesting the Eagle’s Nest formed in shallow crustal levels. Whole-rock geochemistry and Sm-Nd isotopes indicate that the Eagle’s Nest magma was derived from a depleted mantle source above the garnet stability field. During transport, this magma underwent crustal contamination by the host tonalite and older supracrustal rocks. Assimilation of sulfur-bearing supracrustal material likely triggered sulfide saturation, supported by the mass-independent fractionation values of the measured Δ³³S. The intrusion’s distinct petrological and metallogenic features likely reflect both the emplacement dynamics and the parental magma composition, resulting in its unique metal endowments within the greenstone belt. REFERENCES Barnes, S.J., 2023. Lithogeochemistry in exploration for intrusion-hosted magmatic Ni-Cu-Co deposits. Geochemistry: Exploration, Environment, Analysis, vol. 23(1), pp. geochem2022–025. Houlé, M.G., Lesher, C.M., Metsaranta, R.T., Sappin, A.-A., Carson, H.J.E., Schetselaar, E.M., McNicoll, V.J., and Laudadio, A., 2020. Magmatic architecture of the Esker intrusive complex in the Ring of Fire intrusive suite, McFaulds Lake - 69 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 greenstone belt, Superior Province, Ontario: Implications for the genesis of Cr and Ni-Cu-(PGE) mineralization in an inflationary dyke-chonolith-sill complex, in Bleeker, W., and Houlé M.G. (eds). Targeted Geoscience Initiative 5, Geological Survey of Canada, Open File 8722, pp. 141–163. Metsaranta, R.T., and Houlé, M.G., 2020. Precambrian geology of the McFaulds Lake “Ring of Fire” region, northern Ontario. Ontario Geological Survey, Open File Report 6359, 260 p. Mungall, J.E., Harvey, J.D., Balch, S.J., Azar, B., Atkinson, J., and Hamilton, M.A., 2010. Eagle’s Nest a Magmatic NiSulfide Deposit in the James Bay Lowlands, Ontario, Canada, in The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries, Volume II: Zinc-Lead, Nickel-Copper-PGE, and Uranium. Society of Economic Geologists, Special Publication 15, pp. 539–557. Naldrett, A.J., 2011, Fundamentals of Magmatic Sulfide Deposits. Reviews in Economic Geology, vol. 17, pp. 1–50. Smith, W.D., Jenkins, C.M., Augustin, C.T., Virtanen, V.J., Vukmanovic, Z., and O’Driscoll, B., 2024. Layered intrusions in the Precambrian: Observations and perspectives. Precambrian Research, 50th Anniversary Invited Review, vol. 415, 107615. Zuccarelli, N., 2020. Sulfide textures, geochemistry, and genesis of the Komatiite-Associated Eagle’s Nest Ni-Cu-(PGE) Deposit, McFaulds Lake Greenstone Belt, Superior Province, Ontario. MSc Thesis, Laurentian University, Sudbury, Ontario, Canada, 108 p. - 70 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Integrating petrophysical data with full tensor magnetic gradiometry for improved interpretation and modelling of remanently magnetized intrusions in the Midcontinent Rift SMITH, Jennifer1, KASKI, Krista1, TSCHIRHART, Victoria1, and ENKIN, Randy1. 1 Natural Resources Canada, Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8 Magnetic surveys are widely used in mineral exploration to detect and delineate subsurface structures and ore-bearing systems. As near-surface, high-grade deposits become increasingly rare, exploration is shifting toward deeper targets and more complex geological settings. Full tensor magnetic gradiometry (FTMG), particularly when deployed with highly sensitive SQUID-based quantum sensors, provides high-resolution measurements of all components of the magnetic field gradient tensor, offering enhanced imaging of subtle geological structures and ore bodies that conventional total magnetic intensity (TMI) surveys may not resolve (Rudd et al., 2022). FTMG reduces the influence of regional magnetic fields, diurnal variations, and cultural noise, supporting more robust 3D inversion and geological interpretation. Despite these advantages, adoption of FTMG has been limited by logistical complexity, depth constraints, and a lack of publicly available datasets particularly in geologically complex or remanently magnetized areas. To address this, the Geological Survey of Canada is acquiring and openly disseminating precompetitive SQUID-based FTMG datasets (e.g. Fig. 1), providing real-world data for benchmarking inversion workflows and testing emerging quantum sensors. Figure 1: Maps of the total magnetic intensity (TMI) (a), and three components of the magnetic gradient tensor: Bxx (b), Byy (c) and Bzz (d) over the Escape Intrusion within the Thunder Bay North Intrusive Complex of the Midcontinent Rift. The Midcontinent Rift (MCR) provides a geologically complex environment to evaluate FTMG in remanently magnetized settings. Mafic-ultramafic conduit-type intrusions in this region, including the Escape and Current intrusions of the Thunder Bay North Intrusive Complex (TBNIC), exhibit strong remanent magnetization, generating distinct and heterogeneous magnetic anomalies (Kaski et al., 2024; Fig. 1). These characteristics make the MCR an ideal setting to assess how FTMG resolves both induced and remanent magnetic components. In this study, we integrate SQUID-based FTMG inversions with petrophysical, petrographic, and geochemical data, including magnetic susceptibility, natural remanent magnetization, and mineralogical composition, to examine how lithologic variability, serpentinization, and magnetic mineral development influence the intensity and orientation of remanent magnetization, providing a more geologically realistic framework for interpretation and modeling. Preliminary results show that integrating FTMG with rock property data improves resolution of key geological contacts and remanent magnetic sources, enabling more robust 3D modeling of conduithosted Ni-Cu-PGE systems. This study highlights the value of combining high-resolution geophysical and petrophysical datasets for interpreting complex magnetic anomalies. - 71 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 REFERENCES Kaski, K., Smith, J., Tschirhart, V.L., and Heggie, G., 2024, 3D magnetic-susceptibility and magnetization vector inversions of remanently magnetized conduit-type Ni deposits: a case study from the Thunder Bay North intrusive complex, Ontario: Geological Survey of Canada, Open File 9209, 25 p, https://doi.org/10.4095/pkwpmf1tju Rudd, J., Chubak, G., LaNier, H., Stolz, R., Schiffler, M., Zakosarenko, V., Schneider, M., Schulz, M., Meyer, M., 2022, Commercial operation of a SQUID-based airborne magnetic gradiometer: Leading Edge. https://doi.org/10.1190/ tle41070486.1 - 72 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Optimizing data collection for better geological interpretations and adding value to your project SMYK, Emily1, DOLEGA, Simon1, CHURCHLEY, Jeffrey1, and FLANK, Steven1 1 Bayside Geoscience Inc., 1179 Carrick St. Thunder Bay, ON P7B 6M3 “Data are disembodied information. Data are not the same as knowledge.” ~ W. Olsen (2012) A well-designed field or drill program is developed from the beginning to produce substantiated, appropriate and robust datasets. However, data are commonly considered interchangeable with interpretations and are often misreported to fit a geologist’s bias within the context of a project. Common instances of data distortion include: (1) identifying and classifying rocks as pre-named units with assumed occurrences; (2) designating altered rocks as separate lithologies; (3) recording qualitative descriptions rather than quantitative variables; (4) not standardizing all aspects of data collection; and (5) generating incomplete geochemical datasets in the pursuit of select geochemical data. It is a human instinct to apply human interpretations to systematic rocks and processes, but collecting purely observational, quantified geologic data can provide significantly more flexible information during later interpretation. Some findings may emerge from a dataset without being expected or predicted in advance (Olsen, 2012). More ‘expected’ findings might follow the usual predictable patterns, but unpredictable trends may be obfuscated by unintentionally engineered data biases. The most impactful approach to optimize data collection procedures is standardizing all data input for recording rock identification and descriptions, photos, and QA/QC practices. Collecting alteration, mineralization, and structural data as separate data to the lithology, rather than integrated into lithology names (e.g., carbonatized basalt), allows for separation of different datasets for multiple applications and discourages segregating single rock units due to varying characteristics. Many issues are resolved by generating mandatory fields that can only be populated by standardized terms using drop-down menus. Another approach is quantifying and binning as many descriptors as possible. A simple change is including mineral abundance ranges in mineral description fields. For example, describing weak epidote alteration as ‘Weak (2-5%)’ provides a quantitative visual cue to the core logger/mapper, ensuring consistent descriptions and binning similar mineral percentages together. Another consideration is developing sampling programs that submit all samples for consistent analytical packages. Cost-saving measures are often implemented by selectively submitting samples for different packages or only submitting samples that are anticipated to return good assay data. These practices can identify high-grade samples, but can also miss secondary, unpredictable mineralization. Without a range of geochemical data, it is impossible to assess truly elevated values from background values. Purely objective geologic data can provide new interpretations depending on the approach/aims of the geologist. Consistent and comprehensive data collection may produce unexpected results and provide a valuable final product – a strong asset that increases the value of a project, property, or deposit. REFERENCES Olsen, W. (2012). Data Collection: Key Debates and Methods in Social Research. Sage Publications Ltd. - 73 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Pukaskwa Redux: Revisiting and Reconnecting with Superior’s Wild North Shore SMYK, Mark1, HODGE, Joanna2 and ROBILLARD, Carly3 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada Canadian Federation of Earth Sciences, University of Ottawa, 150 Louis Pasteur Private, Ottawa ON K1N 6N5 Canada 2 3 Parks Canada, Pukaskwa National Park, PO Box 212, Heron Bay, ON, P0T 1R0 Canada In August, 2025, the Senior Author served as Geologist-in-Residence (GIR) at Pukaskwa National Park, on Lake Superior near Marathon. The GIR program at Pukaskwa is a partnership between the Canadian Federation of Earth Sciences and Parks Canada, with volunteer expenses funded by the APGO Education Foundation. It is a two-week, volunteer position that started at Pukaskwa in 2022. The role of the GIR is to highlight Pukaskwa’s remarkable geological features and to educate park visitors and Parks Canada interpretive staff about the local geology. Guided hikes, “walk and talk” sessions, drop-in opportunities and presentations were employed to convey knowledge and messaging. As a result of the 2025 GIR program, ideas are being considered to develop a self-guided geology field trip for the readily accessible “front country” trails at Pukaskwa that expose a variety of Neoarchean supracrustal rocks of the Schreiber-Hemlo greenstone belt. Its “back country”, featuring the Coastal Hiking Trail, is underlain mainly by Neoarchean granitoids of the Pukaskwa Batholith. Archean rocks are intruded by Paleoproterozoic and Mesoproterozoic diabase dykes, the latter of which are associated with Midcontinent Rift magmatism. There are numerous features attributed to Quaternary glaciation, including prominent roches moutonnées (Figure 1), potholes and glacial polish/ striae. Modern shoreline and aeolian processes continue to redistribute sediment and create unique and critical habitats for rare and endangered plant species. The GIR program serves to remind us of the importance and value of participating in outreach activities, sharing information and underscoring the critical role that geology plays in ecological processes. The program is expanding to Fundy National Park in 2026 with the hope that further National Parks will be added in the future to provide more opportunities for geoscience outreach and education to a broader audience. Figure 1: Geologist-in-Residence, Mark Smyk, pointing out a prominent roche moutonnée at Horseshoe Beach during a guided hike of the Southern Headland Trail, Pukaskwa National Park, August, 2025 - 74 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Oxidation to Ores: Petrological Insights into Supergene Manganese Enrichment at the Emily Deposit, Minnesota STEINER, R. Alex¹, WATSON, Noa2, RILEY, Jack2, HAMMER, Mikala3, THOLE, Jeff2, FEINBERG, Josh3, SANDRI, Henry4, and SAVAGE, Brian4 ¹Big Rock Exploration LLC, 2505 W Superior Street, Duluth, MN, 55803 USA 2 3 4 Macalester College, 1600 Grand Ave, St. Paul, MN 55105 USA University of Minnesota, 116 Church Street SE, Suite 150, Minneapolis, MN 55455 USA Electric Metals (USA) Limited, 109 West 13th Street Wilmington, DE 19801 USA Electric Metals (USA) Limited’s Emily Deposit in Minnesota’s historic Cuyuna Iron Range contains zones reaching +50 wt. % manganese, making it the highest-grade manganese resource in North America and one of the highest-grade manganese deposits in the world. Manganese-oxide ores of the Emily Deposit are proposed to have formed through supergene enrichment due to deep, potentially protracted weathering of folded iron formation strata during the deposit’s 1.9-billion-year history. Weathering of manganese-bearing carbonate facies oxidizes the original rhodochrosite, drawing the manganese into solution. The manganese enriched groundwaters then migrate down-dip, along stratigraphic boundaries before redepositing manganese as oxides in the porous grainstones of the iron formation. The recent exploration drilling campaign by Electric Metals USA Limited and Big Rock Exploration provided a wealth of geologic, geochemical, and microscopic data that may be used to evaluate the hypothesized ore genesis mechanism on a deposit scale and constrain the metallurgical behavior of the ores. Here we present an analysis of a large exploration geochemical dataset using deposit-wide mass-balance calculations to determine the element mobility within the iron formation. The geochemical results are then contextualized within geology by combining optical and X-ray microscopy to identify mineral phases and phase transitions, as well as intergrowths of secondary minerals. Mass balance calculations show depletions in manganese from the weathered carbonate facies of the Emily Iron Formation and parallel enrichment of manganese into the grainstones. Integration of preliminary optical and X-ray microscopy shows a breakdown of early-formed minerals in the source carbonates and replacement by Fe-oxides and oxyhydroxides along bedding and fractures. Secondary manganese minerals appear to surround primary grains in the grainstones and may be replacing early formed ferruginous cements. These observations support the hypothesized ore-genesis model and provide the necessary information for subsequent metallurgical evaluation of the Emily Deposit including the manganese-iron-silicate mineral associations that may impact ore upgrading, grinding, and hydrometallurgical outcomes. REFERENCES Steiner, R. A., Peterson, D., Berg, T., Solie, J., Larson, M., Schaefbauer, E., Sweet, G., 2024, North Star Emily Manganese Deposit, Crow Wing County, Minnesota: Observations Interpretations, and Recommendations Following the Initial 2023 Drilling Campaign, January 17, 2024. Big Rock Exploration. - 75 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Figure 1 – Full section reflected light (above) and X-ray map showing texture of iron and manganese minerals. Areas with mixed iron and manganese minerals and pure, coarse grained manganese species are highlighted. - 76 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Timing and conditions of magmatism, metamorphism, and strain partitioning in the western Shebandowan Greenstone Belt (Superior Province) STEPHAN, Tobias1, PHILLIPS, Noah1,2, and HOLLINGS, Pete1 1 Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Department of Earth Sciences, University of Southern California, 3651 Trousdale Pkwy., Los Angeles, CA, 90089-0740, United States 2 The Shebandowan Greenstone Belt is an Archean granite–greenstone terrane within the Wawa subprovince of the Superior Province, comprising calc-alkaline to tholeiitic, felsic to ultramafic supracrustal metavolcanic rocks, synvolcanic to late intrusive suites, and felsic hypabyssal dikes and sills. Despite its economic and tectonic significance, the timing and conditions of magmatism, metamorphism, and deformation remain incompletely constrained. Here, we integrate structural geology, high-precision geochronology, metamorphic petrology, and microstructural analyses to establish a coherent tectonometamorphic framework for the western belt. Strain varies from weakly deformed domains (e.g., felsic intrusions and pillow basalts) to highstrain mylonitic zones, mainly affecting diorites and metavolcanic rocks. The orientation of the main ductile foliation orientation is relatively consistent across the study area, while stretching lineations range from shallow to steep. These variations correlate with spatial changes in vorticity, reflecting strain partitioning between high-strain shear zones and coarse-grained, feldspar-rich, and thus, mechanically strong intrusive bodies (Stephan et al. 2025). Peak metamorphic conditions of ~600–700 °C are constrained by pseudosection modeling and conventional thermometry, consistent with Zr-in-titanite temperatures (570–700 °C). Retrograde conditions of ~400–500 °C are preserved in post-kinematic assemblages. Quartz microstructures, crystallographic preferred orientations, and grain-size piezometry indicate deformation at ~400–600 °C and differential stresses of ~20–60 MPa, suggesting deformation near the brittle–ductile transition. CA-ID-TIMS U-Pb zircon geochronology identifies two magmatic phases based on concordant ages: an intrusive phase at 2718 Ma (e.g. felsic intrusion of Moss Lake Stock and Obadinaw Stock) and a younger phase at 2707 Ma (e.g. Greenwater Stock). An upper intercept age constrains volcanism at 2712 Ma in the metavolcanic sequences. In situ U–Pb titanite dates of 2711±76 Ma (2σ) and 2672±100 Ma record metamorphic events spanning greenschist- to amphibolite-facies conditions. A Re-Os molybdenite age of 2708±12 Ma overlaps with both magmatism and metamorphism, linking mineralization to tectonometamorphic processes. These results indicate synkinematic magmatism and amphibolite-facies deformation under predominantly horizontal tectonics. Strain was strongly partitioned due to competency contrasts between coarse-grained intrusive rocks and fine-grained metavolcanic units. This integrated dataset provides new constraints on the coupling between magmatism, deformation, metamorphism, and mineralization in Archean granite–greenstone belts. REFERENCES Stephan, T., Phillips, N., Tiitto, H., Perez, A., Nwakanma, M., Creaser, R., and Hollings, P. 2025. Going with the flow — Changes of vorticity control gold enrichment in Archean shear zones (Shebandowan Greenstone Belt, Superior Province, Canada). Journal of Structural Geology, 201, 105542. https://doi.org/10.1016/j.jsg.2025.105542 - 77 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Subsurface mapping of the late Ordovician Maquoketa Group in eastern Wisconsin using airborne electromagnetic and well data STEWART, Esther K.1, McNALL, Natalie1, 2, HART, Dave1, AMES, Carsyn 1, CHASE, Pete1, STEWART, Eric1, and GRAHAM, G.1 Wisconsin Geological and Natural History Survey, University of Wisconsin-Madison Division of Extension, Madison, Wisconsin 53705 1 2 Department of Geosciences, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53211 The late Ordovician Maquoketa Group is a fine-grained unit and regional aquitard separating the upper, fractured Silurian dolostone aquifer from the deep, Cambrian-Ordovician sandstone-dolomite aquifer in eastern Wisconsin. Here, the Maquoketa Group lithostratigraphy includes, from top to bottom, the Brainard Formation (marls and shale), Ft Atkinson Formation (carbonate wackstonethrough grainstone and marls), and Scales Formation (black shales and marls). The shale-rich composition of the Maquoketa Group is readily distinguished from the overlying Silurian dolostone by airborne electromagnetic (AEM) data (Minsley et al., 2022). We undertook subsurface mapping and characterization of the Maquoketa Group to address regional issues of groundwater quantity and quality. For Wisconsin users, the resulting 3D surfaces can be used as inputs to groundwater models and aid land-use decisions by providing information on the depths, thickness, and rock properties of this aquitard. We used AEM data tied to borehole logs and core to generate raster surfaces and understand facies changes and structures across study area (Figure 1). Despite cultural interference mainly from roads, the AEM data nicely imaged the top of the Maquoketa Group aquitard. The Maquoketa Group basal surface and its internal formations were imaged by the AEM data but with greater uncertainty, and the base of the unit dipped below the penetration depth of the AEM data to the east. The Maquoketa Group extends from about 850 feet (259 m) above sea level near its western subcrop extent to 100 feet (31 m) above sea level at the eastern edge of the map area, with thicknesses between about 220 – 450 ft (67 – 137 m). The north-south strike of depth-structure elevation contours is abruptly offset in three locations, labeled on Figure 1. One of these (location 2) corresponds to the Precambrian Spirit Lake Tectonic zone (Holm et al., 2007) and fault offset of Silurian bedrock (Luczaj, 2011). Several new and existing drill core tie to the AEM data in the western study area, and lithologic variation in the cores corresponds to vertical changes in the resistivity profile of the Maquoketa Group. Internal variability in the resistivity of the Maquoketa, as imaged by the AEM data, apparently decreases to the east. Future air rotary drilling will test whether this signal is due to decreased data resolution as these units dip eastward, or whether it reflects an increase in shaley facies to the east. REFERENCES Holm, D.K., Anderson, R., Boerboom, T.J., Cannon, W.F., Chandler, V., Jirsa, M., Miller, J., Schneider, D.A., Schulz, K.J. and Van Schmus, W.R., 2007. Reinterpretation of Paleoproterozoic accretionary boundaries of the north-central United States based on a new aeromagnetic-geologic compilation. Precambrian Research, 157, 71-79. Luczaj, J.A., 2011. Preliminary Geologic Map of the Buried Bedrock Surface, Brown County, Wisconsin. Wisconsin Geological and Natural History Survey Open File Report 2011-02. Minsley, B.J, Bloss, B.R., Hart, D.J., Fitzpatrick, W., Muldoon, M.A., Stewart, E.K., Hunt, R.J., James, S.R., Foks, N.L., and Komiskey, M.J., 2022. Airborne electromagnetic and magnetic survey data, northeast Wisconsin. U.S. Geological Survey data release, https://doi.org/10.5066/P93SY9LI. - 78 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Figure 1: Maps showing the elevation and thickness of the Maquoketa Group (top) and an example AEM line (below). The inset map of Wisconsin (left) shows counties outlined in black and the eastern Wisconsin study area outlined in orange. Circled numbers to the left of the top Maquoketa elevation map locate offsets in depthstructure contours. The star locates the Krepline core on the map and AEM line, and formation contacts from core are tied to AEM line. Roads and railroads, symbolized above the line, cause cultural interference with the AEM signal. - 79 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Rocks and Roots: The Role of Geoheritage in Biodiversity Stewardship STONE, Abraham1, LIZZADRO-McPHERSON, Dan2, and VYE, Erika3 Michigan Natural Features Inventory, Deborah A. Stabenow Building, 1st Floor, 525 W. Allegan St., Lansing, MI 48933, United States 1 Geospatial Research Facility, Michigan Technological University, 1400 Townsend Dr., Houghton, MI 49931, United States 2 Great Lakes Research Center, Michigan Technological University, 1400 Townsend Dr., Houghton, MI 49931, United States 3 Conservation of natural surficial landforms with regional, scientific, or cultural significance has long been an intrinsic component used by scientists and educators who follow the principles of geoheritage. On the Keweenaw Peninsula, intact outcrops of Copper Harbor Conglomerate, Portage Lake Volcanics, and Jacobsville Sandstone each provide accessible learning opportunities to both students and citizens and create spaces for deeper emotional connections to the landscape. Culturally and geologically important sites are currently used in both educational tools and to increase community-wide engagement in geologic studies (Cowling et al. 2023; Lizzadro-McPherson and Vye 2023). The principles of natural heritage, hereto referred also as ‘bioheritage’, strongly overlap with that of geoheritage. As geoheritage promotes connection to landscape via geological features, bioheritage facilitates connection through valuable natural features – ecosystems, flora and fauna – and encourages the conservation of landscapes that promote biodiversity. Sites that are identified by bioheritage ecologists, botanists, zoologists, and geographers as being of regional, scientific, or cultural significance often coincide with areas of high geodiversity. Categorization of these natural features show geographies dependent on both surface geology and glacial landforms; for example, the statewide distribution of volcanic bedrock lakeshore (Fig. 1), an imperiled natural community in Michigan, is wholly limited to surface-level exposures of Keweenawan rocks (Cohen et al. 2013) and supports a series of rare plants and animals found nowhere else in the state (Albert et al. 1997; MNFI 2026). Conservation of one outcrop for geological reasoning can therefore work beneficially for bioheritage, and vice versa. In the summer of 2025, we conducted interdisciplinary research highlighting the natural connections between underlying geological formations, community ecology, and rare plant Figure 1: Portage Lake Volcanics featured prominently along a high-quality volcanic bedrock lakeshore natural community recognized under both geoheritage and natural heritage. Figure 2: Pilot data examining ecological structure of bedrock lakeshore systems. Different zones of bedrock exposure promote plant communities of unique species composition. - 80 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 distributions on volcanic bedrock lakeshores of the Keweenaw Peninsula. The project involved collaboration between botanists, geologists, geographers, and conservationists. Pilot data yielded significant plant community differences between bedrock types and microhabitats, and the summer was documented in an educational StoryMap (Stone et al. 2025) (Fig. 2). Meandering transects outlining subtle distinctions in ecosystem processes based on geological formation identified multiple new rare plant populations, including the discovery of red anemone (Anemone multifida) on the Keweenaw Peninsula (Fig. 3). The project has since led to multiple additional collaborations in the Western Upper Peninsula focused on geoand bio-education. Partnerships between bioheritage and geoheritage scientists can be valuable sources of interdisciplinary research and collaboration. Despite originating in disparate academic fields, the two disciplines can work in tandem to increase scientific understanding of our geological features while producing valuable teaching tools. Future research and educational opportunities are plentiful as the two worlds of geoheritage and bioheritage establish common ground. REFERENCES Figure 3: Red anemone (Anemone multifida), a rare plant identified during field research on the Keweenaw Peninsula. Albert, D.A., Comer, P., Cuthrell, D., Hyde, D., MacKinnon, W., Penskar, M., & Rabe, M., 1997. The Great Lakes Bedrock Lakeshores of Michigan. Michigan Natural Features Inventory, Lansing, MI. 218 pp. Cohen, J.G., Kost, A., Slaughter, B.A., & Albert, D.A., 2015. A Field Guide to the Natural Communities of Michigan. Michigan State University Press. 362 pp. Cowling, R., Lizzadro-McPherson, D.J., Verissimo, L. & Vye, E.C., 2023. Keweenaw Geoheritage Geoatlas. DOI: 10.13140/RG.2.2.30945.28005 Lizzadro-McPherson, D. J. & Vye, E.C., 2023. Keweenaw Coastal Geoheritage Story Map. DOI: 10.13140/ RG.2.2.12680.74242 Michigan Natural Heritage Database (MNFI), 2026. Michigan Natural Heritage Database. Lansing, MI. Stone, A.F., Lizzadro-McPherson, D.J., and Vye, E.C., 2025. Rocks and Roots: A Keweenawan Love Story. StoryMap. https://storymaps.arcgis.com/stories/7d9a428effe04dc4923736310182d52f - 81 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Linking the Southwestern Laurentia large igneous province and rapid Duluth Complex emplacement through mantle plume dynamics SWANSON-HYSELL, Nicholas L.1, ZHANG, Yiming1, MOHR, Michael T.2, and SCHMITZ, Mark D.2 1 2 Department of Earth and Environmental Sciences, University of Minnesota, Minneapolis, MN, USA Department of Geosciences, Boise State University, Boise, ID, USA Midcontinent Rift volcanism was protracted, spanning from ca. 1109 to 1084 Ma with major magmatic pulses separated by ~10 Myr and >30° of latitudinal plate motion (Figure 1). The long duration of magmatism and large spatial displacement of the continent are difficult to reconcile with a single stationary mantle plume beneath the rift. A corresponding question is what caused the renewal of voluminous magmatism ca. 1096 Ma that produced the massive Duluth Complex layered mafic intrusions and comagmatic lavas of the North Shore Volcanic Group after a period of relative magmatic dormancy (Miller and Vervoort, 1996), and after Laurentia had drifted >3000 km since the rift’s initiation (Swanson-Hysell et al., 2019, 2021). Figure 1: The plate motion of Laurentia reconstructed from Midcontinent Rift paleomagnetic data revealing large-scale latitudinal change between the start of early phase volcanism and the major pulse of magmatism that emplaced the Duluth Complex ca. 1096 Ma. The red dot indicates the location of the Lake Superior region in each reconstruction. High-precision ²06Pb/²38U zircon dates developed through CA-ID-TIMS geochronology have resolved temporally distinct pulses of magmatism across Laurentia’s interior. In southwestern Laurentia, the Southwestern Laurentia large igneous province (SWLLIP) encompasses >750,000 km² of ca. 1.1 Ga mafic sills, dikes, and lava flows. New dates from SWLLIP mafic rocks reveal a rapid, voluminous magmatic pulse at ca. 1098 Ma, with thick sills emplaced across Death Valley, the Grand Canyon, and central Arizona within ≤0.25 Myr (Mohr et al., 2024). Approximately 2 Myr later, the bulk of the Duluth Complex anorthositic and layered series was emplaced ca. 1096 Ma in <1 Myr (500 ± 260 kyr; Swanson-Hysell et al., 2021). Both pulses were rapid and voluminous, characteristic of plume-related large igneous provinces. The close temporal and spatial relationship between the ca. 1098 Ma SWLLIP pulse and the ca. 1096 Ma Duluth Complex pulse supports a geodynamic link through lateral plume spreading. Rates of lateral plume spread predicted by mantle plume lubrication theory (Sleep, 1997) are consistent with a model in which a plume derived from the deep mantle impinged beneath southwestern Laurentia, then spread to the thinned Midcontinent Rift lithosphere over ~2 Myr, elevating mantle temperatures and generating melt. Buoyant plume material would have been directed to the rift through “upsidedown drainage” at the base of the Laurentian lithosphere (Sleep, 1997; Swanson-Hysell et al., 2021), - 82 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 wherein material flows along the topography of the lithosphere–asthenosphere boundary from thick to thin lithosphere. This hypothesis reconciles the close temporal relationships between voluminous magmatism across Laurentia and provides an explanation for the anomalous renewal of high magmatic flux within the protracted magmatic history of the Midcontinent Rift. REFERENCES Miller Jr., J.D., and Vervoort, J.D., 1996. The latent magmatic stage of the Midcontinent rift: a period of magmatic underplating and melting of the lower crust. In: Inst. Lake Superior Geol., 42nd Ann. Mtg., Proceedings, vol. 42, pp. 33–35. Mohr, M.T., Schmitz, M.D., Swanson-Hysell, N.L., Karlstrom, K.E., Macdonald, F.A., Holland, M.E., Zhang, Y., and Anderson, N.S., 2024. High-precision U-Pb geochronology links magmatism in the Southwestern Laurentia large igneous province and Midcontinent Rift. Geology, doi:10.1130/G51786.1. Sleep, N.H., 1997. Lateral flow and ponding of starting plume material. Journal of Geophysical Research, 102, 10,001– 10,012, doi:10.1029/97JB00551 Swanson-Hysell, N.L., Hoaglund, S.A., Crowley, J.L., Schmitz, M.D., Zhang, Y., and Miller, J.D., 2021. Rapid emplacement of massive Duluth Complex intrusions within the North American Midcontinent Rift. Geology, 49, doi:10.1130/ G47873.1. Swanson-Hysell, N.L., Ramezani, J., Fairchild, L.M., and Rose, I.R., 2019. Failed rifting and fast drifting: Midcontinent Rift development, Laurentia’s rapid motion and the driver of Grenvillian orogenesis. GSA Bulletin, 131(5–6), 913–940, doi:10.1130/B31944.1. - 83 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Deformation processes in a mid-crustal strike-slip shear zone: Insights from the Archean Quetico Shear Zone, Superior Province, Canada TIITTO, Hanna1, PHILLIPS, Noah1, 2, and STEPHAN, Tobias1 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7C 5E1, Canada Department of Earth Sciences, University of Southern California, 3651 Trousdale Pkwy., Los Angeles, CA, 90089-0740, United States 2 The brittle-ductile transition, where most earthquakes nucleate, occurs at ~10-15 km depth in the crust (Sibson, 1983). The structures produced at the brittle-ductile transition in active shear zones are challenging to study as they occur at depth. To further understand shear zone structures at depth, this study focuses on an analogue structure for active strike-slip systems, the Quetico Shear Zone, due to its estimated erosional depths of 10-15 kms, which exposes the Archean brittle-ductile transition zone (Percival et al., 2012). The Quetico Shear Zone is a right-lateral, strike-slip shear zone located within the Wabigoon and Quetico subprovinces and has a strike length of at least 400 km (Kennedy, 1984). This project focuses on the eastern extent of the shear zone, north of Thunder Bay, and aims to constrain the kinematics, structures, conditions, and timing of deformation processes within the shear zone and adjacent to paleo-earthquake surfaces. The extent of deformation from the shear zone was analyzed through macro- and microstructures using field mapping and microscopy. The conditions of deformation were constrained using paleopiezometry through electron backscattered diffraction of recrystallized quartz (Cross et al., 2017) and Ti-in-quartz geothermometry through secondary ion mass spectrometry measurements of recrystallized quartz (Wark and Watson, 2006). To constrain the timing of deformation, laser-ablation split-stream inductively coupled plasma mass spectrometry of apatite, monazite, titanite, and zircon was performed to produce U-Pb dates (Kylander-Clark, 2017). We found that Quetico Shear Zone deformation is characterized by increased mylonitization and brittle deformation with increasing proximity to the shear zone trace (within 500 m) where paleoearthquake surfaces (i.e., pseudotachylite veins) were found (Fig. 1). Mylonitization produces recrystallized quartz ribbons and a strong foliation unique to the Quetico Shear Zone (stronger than regional Quetico Subprovince transpressional structures), particularly in granitic units (Fig. 1C-E). Non-granitic rock units within the core of the shear zone display pervasive brittle deformation with numerous faults (Fig. 1A). Granitic rock types display more variable orientations due to the isolated quartz ribbons deforming around larger feldspar grains. The recrystallized quartz grain sizes do not correlate with increased mylonitization and proximity to the shear zone. Recrystallized quartz grain sizes remained constant within error, with calculated stress values ranging from 69 to 116 MPa, with a median of 80 MPa. The temperatures of quartz recrystallization range from 457 to 589°C, with a median of 487°C, with no clear evolution with increasing proximity to the Quetico Shear Zone trace. Apatite and titanite provided the best ages for deformation, mainly producing interpreted ages younger than the Quetico subprovince metamorphism. The interpreted Quetico Shear Zone deformation ages are approximately from 2620 to 2600 Ma. The exhumed Quetico Shear Zone appears to be deformed at a constant stress shortly after the Kenoran orogeny. REFERENCES Cross, A.J., Prior, D.J., Stipp, M., & Kidder, S., 2017. The recrystallized grain size piezometer for quartz: An EBSD-based calibration. Geophysical Research Letters, 44, 6667-6674. Kennedy, M.C., 1984. The Quetico Fault in the Superior Province of the Southern Canadian Sheild [MSc]: Lakehead University, 323. Kylander-Clark, A.R.C., 2017. Petrochronology Laser-Ablation Inductively Coupled Plasma Mass Spectrometry. Reviews in Mineralogy and Geochemistry, 83, 183-198. Percival, J.A., Skulski, T., Sanborn-Barrie, M., Stott, G.M., Leclair, A.D., Corkery, M.T., Boily, M., 2012. Geology and tectonic evolution of the Superior Province, Canada. Chapter 6 In Tectonic Styles in Canada: The Lithoprobe - 84 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Perspective. Geological Association of Canada, Special Paper 49, 321-378. Sibson, R.H., 1983. Continental fault structure and the shallow earthquake source. Journal of Geological Society, 140, 741767. Wark, D.A., and Watson, E.B., 2006. TitaniQ: a titanium-in-quartz geothermometer. Contributions to Mineralogy and Petrology, 152, 743-754. Figure 1: Quetico Shear Zone structures proximal to pseudotachylite veins: A: Plane-polarized light photomicrograph displaying pseudotachylite veins (medium brown layers cutting the white to light brown mylonitic fabric) from the core of the shear zone. Co-seismic injection veins are highlighted with white arrows. Right-lateral, late brittle faults are indicated by kinematic arrows. B: Magnified view of a pseudotachylite that has been viscously deformed. Cross-polarized light photomicrographs showing quartz microstructures of quartz-rich metamorphic rocks from increasing distance from the pseudotachylites: C: Extremely fine-grained quartz ribbons with minor feldspar porphyroclasts within a mylonite. D: Very fine-grained quartz within a quartz ribbon adjacent to fine-grained quartz in a protomylonitic granite. E: Fine- to mediumgrained recrystallized quartz within a weakly deformed granite. Note that the recrystallized grain size is consistent in C-E. - 85 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Variations in Olivine Major Element Composition Across the Midcontinent Rift System TOLLEY, James1 and HOLLINGS, Pete1 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1, Canada. Olivine [(Mg,Fe)2SiO4] is an early crystallising phase in mafic–ultramafic Ni–Cu–(PGE) deposits and a sensitive recorder of mantle melting history. Its forsterite content reflects the parental melt composition, while the Ni concentration and trace element ratios can be used to constrain petrogenetic processes and the physicochemical conditions of melting. However, the plutonic nature of these deposits means primary compositions can be overprinted by sub-solidus re-equilibration and latestage fluid interaction, complicating the recovery of primary magmatic signals. Deconvoluting these signatures is critical to understanding melt generation, fractionation, and ultimately the mineralisation processes that govern the formation of these deposits. The Midcontinent Rift System (MRS) one of the most extensively mineralised large igneous provinces and renowned for its magmatic Ni–Cu–(PGE) deposits. Despite this, olivine compositional data is sparse. We present new and collated major element olivine data from multiple Ni–Cu–(PGE) deposits across the MRS to evaluate regional-scale trends in forsterite and Ni contents. We examine deposit-scale variability and explore broader implications for the underlying magmatic architecture of the rift system. This study builds on previously collected electron probe microanalyses (EPMA) of olivine from mineralised magmatic Ni–Cu–(PGE) deposits of the MRS within Canada e.g., Sunday Lake (Durán, 2025), Steepledge (Harding, 2024), Escape Lake, Current and Hele, and contributes new olivine compositional data from several unmineralized intrusions, namely Inspiration Sill, St. Ignace Island and Nipigon Sills. These data are further supplemented by olivine compositions from USA-based mineralised Ni–Cu deposits e.g., Tamarack (Goldner, 2011; Taranovic, 2015) and the Duluth Complex (Peterson, 2025). Together, this data constitutes the first regional-scale compilation of olivine chemistry across the MRS. Figure 1: Simplified geological map of the Midcontinent Rift System highlighting the distribution of major rock types. Locations of the mafic–ultramafic intrusions sampled in this study are denoted by stars (red = data collected in this study; blue = literature data). Modified after: Good et al. (2015). - 86 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 REFERENCES Ding, X., Li, C., Ripley, E. M., Rossell, D., & Kamo, S. (2010). The Eagle and East Eagle sulfide ore‐bearing maficultramafic intrusions in the Midcontinent Rift System, upper Michigan: Geochronology and petrologic evolution. Geochemistry, Geophysics, Geosystems, 11(3). Durán, K. M. (2025), Petrogenesis of the Sunday Lake Intrusion, Jacques Township, Ontario, Canada. M.Sc. thesis Lakehead University, Thunder Bay, Ontario, 222p. Goldner, B.D. (2011). Igneous petrology of the Ni–Cu–PGE mineralized Tamarack intrusion, Aitkin and Carlton Counties, Minnesota; M.Sc. thesis, University of Minnesota, Minneapolis, 156p. Good, D.J. (1992). Genesis of copper-precious metal sulphide deposits in the Port Coldwell Alkalic Complex, Ontario; unpublished Ph.D. thesis, McMaster University, Hamilton, Ontario, 203p. Good, D. J., Epstein, R., McLean, K., Linnen, R. L. and Samson, I. M. (2015). Evolution of the Main Zone at the Marathon Cu–PGE sulfide deposit, Midcontinent Rift, Canada: Spatial relationships in a magma conduit setting. Economic Geology, 110(4), 983–1008p. Harding, M. F., (2024). Olivine Geochemistry of the Current and Escape Lake (Steepledge) intrusions, Thunder Bay North Intrusive Complex. HBSc. Thesis, Lakehead University, Thunder Bay Ontario. Heggie, G.J. (2005). Whole rock geochemistry, mineral chemistry, petrology and Pt, Pd mineralization of the Seagull Intrusion, northwestern Ontario. M.Sc. thesis, Lakehead University, Thunder Bay, Ontario, 156p. Peterson, D.M., (2025). Compilation of electron probe microanalyses of Olivine from the Duluth Complex, Minnesota, USA [Unpublished Dataset – personal communication]. Shaw, C. S. (1997). The petrology of the layered gabbro intrusion, eastern gabbro, Coldwell alkaline complex, Northwestern Ontario, Canada: evidence for multiple phases of intrusion in a ring dyke. Lithos. 40(2-4), 243–259. Taranovic, V., Ripley, E.M., Li, C. and Rossell, D., (2015). Petrogenesis of the Ni–Cu–PGE sulfide-bearing Tamarack Intrusive Complex, Midcontinent Rift System, Minnesota. Lithos, 212, 16–31p. - 87 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 Index AKIN, Kathryn�����������������������������������������������������1 ALLERTON, Zsuzsanna���������������������������������3, 31 AMES, Carsyn����������������������������������������������������78 ANGOMBE, Moses����������������������������������������5, 69 BAIN, Wyatt���������������������������������������������������������6 BEDROSIAN, Paul A.����������������������������������������16 BEYER, Steve�������������������������������������������������������8 BILBOE, Michael�����������������������������������������������10 BLEEKER, Wouter���������������������������������������������12 BONAMICI, Chloë���������������������������������������������41 BORNHORST, Theodore�����������������������������������21 BOUCHER, Chanelle�����������������������������������������61 BRENGMAN, Latisha����������������������������25, 29, 51 BUCHHOLZ, Thomas����������������������������������������14 CAMACHO, Alfredo��������������������������������������������8 CANNON, W. F.�������������������������������������������16, 67 CARLTON, Kenz M.������������������������������������������18 CAWOOD, Tarryn������������������������������������������������8 CHAISSON, Amy�����������������������������������������������19 CHASE, Pete������������������������������������������������������78 CHITTICK, Steve�����������������������������������������������65 CHURCHLEY, Jeffrey����������������������������������������73 CISNEROS, John Alex���������������������������������������29 CONLY, Andrew�������������������������������������������������10 COOKE, David���������������������������������������������������45 COWLING, Bob�������������������������������������������������21 CUTTS, Jamie�������������������������������������������������������8 DEERING, Chad�������������������������������������������������55 DeGRAFF, James�����������������������������������������21, 48 DELLER, Matt������������������������������������������������5, 69 DOLEGA, Simon������������������������������������������������73 DRENTH, Benjiman J.���������������������������������������16 DREVER, Garth���������������������������������������������������8 DROST, Abraham�����������������������������������������������23 DROUBI, Omar��������������������������������������������������41 DUFFY, Paige�����������������������������������������������������25 EASTON, Robert Michael����������������������������������27 ELLISON, Kimberly�������������������������������������������29 ENKIN, Randy����������������������������������������������������71 ERICKSON, Stephanie���������������������������������������31 ESSIG, Espree�����������������������������������������������������55 EYSTER, Athena������������������������������������25, 29, 51 FALSTER, Alexander�����������������������������������������14 FAYON, Annia����������������������������������������������������31 FEINBERG, Josh������������������������������������������������75 FLANK, Steven��������������������������������������������������73 FRALICK, Philip������������������������������������������33, 34 GAMET, Nolan���������������������������������������������������63 GAONA, Jorge Mario�����������������������������������������53 GILBERG, Nolan�����������������������������������������������33 GORNER, Emily������������������������������������������������45 GOSAI, Meghna�������������������������������������������������34 GRAHAM, G.�����������������������������������������������������78 GRAUCH, V.J.S��������������������������������������������������35 HAGEDORN, Grant�������������������������������������������37 HAKURTA, Joyashish����������������������������������������59 HAMILTON, Mike���������������������������������������������50 HAMMER, Mikala���������������������������������������������75 HARDING, Myles����������������������������������������������39 HART, Dave��������������������������������������������������������78 HASTIE, Evan����������������������������������������������������50 HEGGIE, Geoff��������������������������������������������������23 HELLER, Samuel J.��������������������������������������������35 HELLRUNG, Alyssa������������������������������������������41 HILLENBRAND, I.��������������������������������������������67 HILLIPS, Noah���������������������������������������������������84 HILTUNEN, Lindsay������������������������������������������21 HIRSCH, Aaron��������������������������������������������������43 HODGE, Joanna�������������������������������������������������74 HOLLINGS, Pete����������������5, 6, 39, 45, 69, 77, 86 HOMPSON, J. M������������������������������������������������67 HUDAK, George��������������������������������������������3, 31 JONSSON, Justin������������������������������������������������47 KAMO, Sandra���������������������������������������������27, 50 KASKI, Krista�����������������������������������������������������71 LAFRENIERE, Don�������������������������������������������21 LI, Zhiquan���������������������������������������������33, 34, 47 LIZZADRO-McPHERSON, Dan�����������21, 48, 80 MACDONALD, Peter����������������������������������������50 MAHIN, Robert��������������������������������������������������63 MALEGUS, Paul������������������������������������������������50 MANGUM, John������������������������������������������������51 MARIN LÓPEZ, Valentina���������������������������������51 MARSH, Jeff������������������������������������������������������50 McNALL, Natalie�����������������������������������������������78 MITCHELL, Jennifer�����������������������������������������51 MOHR, Michael�������������������������������������������������82 NACHLAS, William O���������������������������������������18 NESHEIM, Timothy�������������������������������������������65 NITESCU, Bogdan���������������������������������������������53 NOWAK, Robert�������������������������������������������������55 NOWARIAK, Eric����������������������������������������������57 OST, Sara������������������������������������������������������������59 PALIEWICZ, Cory���������������������������������������������59 - 88 - �Proceedings of the 72nd ILSG Annual Meeting - Part 1 PETERSON, Dean����������������������������������������������61 PHILLIPS, Noah���������������������������������������������5, 77 POWELL, Jeremy�������������������������������������������������8 PU, Xiaofei���������������������������������������������������������51 QUIGLEY, Ashley����������������������������������������������63 RILEY, Jack��������������������������������������������������������75 ROBILLARD, Carly�������������������������������������������74 ROSE, William���������������������������������������������21, 48 RUGGLES, Claire����������������������������������������������41 SAINI-EIDUKAT, Bernhardt�����������������������������65 SALERNO, R�����������������������������������������������16, 67 SANDRI, Henry��������������������������������������������������75 SAVAGE, Brian��������������������������������������������������75 SCHMITZ, Mark������������������������������������������������82 SEVERSON, Allison������������������������������������������57 SHESHNEV, Vlad������������������������������������������5, 69 SIMMONS, William�������������������������������������������14 SMITH, Andrew���������������������������������������������������5 SMITH, Jennifer�������������������������������������������������71 SMYK, Emily�����������������������������������������������������73 SMYK, Mark������������������������������������������������19, 74 SOUDERS, A. K�������������������������������������������������67 STEINER, R. Alex����������������������������������������61, 75 STEPHAN, Tobias������������������������������������5, 77, 84 STERN, Richard�������������������������������������������������69 STEWART, Eric��������������������������������������������������78 STEWART, Esther����������������������������������������������78 STONE, Abraham�����������������������������������������������80 SWANSON-HYSELL, Nicholas��������������������1, 82 SWEET, Gabriel�������������������������������������������������61 THOLE, Jeff�������������������������������������������������������75 TIITTO, Hanna���������������������������������������������������84 TIKOFF, Basil�����������������������������������������������������18 TOLLEY, James��������������������������������������������69, 86 TORRES, David Santiago����������������������������������53 TSCHIRHART, Victoria�������������������������������������71 VERVOORT J.����������������������������������������������������67 VRZOVSKI, Joseph�������������������������������������������45 VYE, Erika����������������������������������������������21, 48, 80 WALKER, Patrick�����������������������������������������������51 WATSON, Noa����������������������������������������������������75 WODICKA, Natasha������������������������������������������12 ZHANG, Yiming�������������������������������������������������82 ZUREVINSKI, Shannon�������������������������������10, 19 - 89 - � https://digitalcollections.lakeheadu.ca/files/original/9c27d09e8827f7b36e6389fd03c441fd.pdf ed3efa960579e362e6c602cd6bb9be41 PDF Text Text 72nd Annual Meeting Thunder Bay, Ontario - May 21-22, 2026 Institute on Lake Superior Geology Part 2 – Field Trip Guidebook �Thank you to our sponsors! �72tnd Annual Meeting Institute on Lake Superior Geology May 21-22, 2026 Thunder Bay, Ontario HOSTED BY: Mark Puumala and Peter Hinz Co-Chairs Ontario Geological Survey (Retired) Proceedings - Volume 72 Part 2 – Field Trip Guidebook Compiled and edited by Pete Hollings Cover Photos: Top - Keweenawan diabase dyke on Lake Superior shoreline near Thunder Bay, Middle Archean-Paleoproterozoic unconformity, Highway 11-17, near Pass Lake turnoff, Bottom - Colloform stromatolite, Gunflint Formation, Kakabeka Falls �72nd Institute on Lake Superior Geology Volume 72 consists of: Part 1: Program and Abstracts Part 2: Field Trip Guidebook Trips 1 & 4: “Classic” Geological Sites in the Thunder Bay Area Trip 2: Geology of the Quetico Supprovince North of Thunder Bay Trip 3: Gold Deposits of the Shebandowan Greenstone Belt Trip 5: Structural Geology and Gold Mineralisation of the Mine Centre Area Trip 6: Amethyst Deposits of Thunder Bay Reference to material in Part 2 should follow the example below: Poulsen, K.H., 2026. Archean Geology and Metallogeny of the Rainy Lake Wrench Zone. In; Hollings, P. (Ed.), Institute on Lake Superior Geology Proceedings, 72nd Annual Meeting, Thunder Bay, Ontario, Part 2 Field trip guidebook, v.72, part 2, 3-31. Published by the 72nd Institute on Lake Superior Geology and distributed by the ILSG Secretary: Pete Hollings - ILSG Secretary Department of Geology Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Email: peter.hollings@lakeheadu.ca ILSG website: www.lakesuperiorgeology.org ISSN 1042-9964 �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Table of Contents Introduction - considerations and acknowledgements.........................................................1 Trips 1 & 4 - “Classic” Geological Sites in the Thunder Bay Area.....................................2 Trip 2 - Geology of the Quetico Subprovince and Shebandowan greenstone belt north of Thunder Bay...............................................................................................................44 Trip 3 - Geological assemblages, regional structural framework and tectonic evolution of the Neoarchean Shebandowan greenstone belt..........................................................67 Trip 5 - Archean Geology and Metallogeny of the Rainy Lake Wrench Zone..................82 Trip 6 - Amethyst Deposits of Thunder Bay....................................................................126 -i- �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Introduction - considerations and acknowledgements Peter Hinz and Mark Puumala Resident Geologist Program, Ontario Geological Survey, Ministry of Energy & Mines, Thunder Bay, Ontario (Retired) This volume is intended to serve not only as a guide for the 72nd ILSG field trip participants but also as a reference for those interested in reprising the trips at a future date. In order to facilitate this, trip leaders have provided UTM coordinates in the NAD 83 datum for stops, as well as plain word descriptions for locating each trip stop. It should be noted that some stops are located on private land or registered mining claims. As such, individuals visiting these stops are advised to obtain the land holders’ permission prior to entering their property. If in doubt, we recommend contacting the Resident Geologist Program office in Thunder Bay for further information about current property ownership. This year’s slate of field trips include stops located either on provincial highways or busy logging roads which can create safety issues. For those participating in facilitated trips at this year’s meeting, make sure to pay attention to the trip leaders’ safety orientation at the start of the trip, and follow any stop-specific safety instructions. For individuals using the field trip guide for future private tours it is advisable to be wary of road traffic and exercise extreme caution. Please take care when crossing or parking at the sides of these roads. The organizing committee would like to thank all the field trip leaders who authored and contributed to this field guide along with those who provided comments and/or assisted with the running of the trips themselves. Field trip leaders and authors include Howard Poulsen, Riku Metsaranta, Gaetan Launay, Dorothy Campbell, Justin Jonsson, Vittoria D’Angelo, Mark Smyk, Mark Puumala, Steve Kissin and Greg Paju. The Committee thanks participating exploration companies and mine operators for their cooperation and assistance in providing access and information in regards to their properties, as well as their staff time for leading the tour participants on their respective properties. Participating companies include Delta Resources, Gold X2 Mining, Amethyst Mine Panorama and Diamond Willow Amethyst Mine. Figure 1. Map illustrating general locations of ILSG 2026 field trips. Symbols are labelled with numbers that correspond to the trip numbers (1 to 6) used in the meeting program and field trip guidebook. -1- �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Trips 1 & 4 - “Classic” Geological Sites in the Thunder Bay Area Mark Smyk Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada and Mark Puumala Geological Consultant, 370 Crossbow Court, Thunder Bay, Ontario, P7G 1H5 Canada Introduction The geology of the Thunder Bay area features a variety of Archean and Proterozoic rocks of the Superior and Southern Provinces of the Canadian Shield, respectively, as well as unconsolidated deposits and landforms associated with Quaternary glacial and post-glacial processes. This field trip features examples of many of these rocks and features, providing an overview of the varied geology the area has to offer. A number of field guides (e.g., Pye, 1969; Kustra et al., 1977; Franklin et al., 1982) have covered the Thunder Bay area, including those written for the 46th (e.g. Pufahl et al., 2000; Phillips et al., 2000) and 58th Institute on Lake Superior Geology annual meetings (e.g. Fralick et al., 2012; Smyk, 2012; Phillips et al., 2012; Cundari et al., 2012). These guides contain descriptions of some of the field trip stops covered in this guide and they will be referenced appropriately. This guide also benefits from ongoing local research and mapping conducted by the Ontario Geological Survey, Geological Survey of Canada and Lakehead University. Day One of this trip features exposures north and east of Thunder Bay, while those of Day Two are located south and west of the City. Bear in mind that this trip marks the first time that many of these stops have been visited and described as part of a formal field trip. This is especially true of stops along Highway 11-17, whose expansion ca. 2010-2012 produced many remarkable new exposures. Please exercise caution when stopping and viewing roadside outcrops. Permission or admittance may need to be obtained to visit some stops; this will be outlined in the guide when necessary. Regional Geology Overview Precambrian Geology The Thunder Bay area straddles the boundary between Archean rocks of the Superior Province and Proterozoic rocks of the Southern Province (Figure 1). In the vicinity of Thunder Bay, Superior Province rocks comprise volcano-plutonic rocks of the Neoarchean Wawa Subprovince and metasedimentary and granitoid rocks of the Neoarchean Quetico Subprovince, bounding the Wawa to the north. Locally, the supracrustal rocks of the Wawa Subprovince have been subdivided into the Greenwater and Shebandowan assemblages (Williams et al., 1991). The ca. 2.72 Ga Greenwater assemblage consists of a north-younging sequence of mafic to felsic metavolcanic rocks with subordinate interbedded clastic and chemical metasedimentary rocks. Mafic metavolcanic rocks within this assemblage consist predominantly of tholeiitic to calc-alkalic pillowed flows. The intermediate and felsic metavolcanic sequences are calc-alkalic and consist predominantly of coarse-grained pyroclastic deposits and massive to feldspar-phyric flows. The ca. 2.69 Ga Shebandowan assemblage is a younger, possibly fault-bounded (Williams et al., 1991) sequence of sub-alkalic to alkalic, predominantly coarse-grained pyroclastic metavolcanic rocks with interbedded coarse- to fine-grained, commonly well-preserved, proximal metasedimentary rocks. Intrusions within the Wawa Subprovince supracrustal assemblages consist of narrow felsic dikes, syenitic to tonalitic pre- to syntectonic plutons, minor gabbro bodies and scattered narrow mafic dikes. Rocks of the Neoarchean Quetico Subprovince abut the Wawa Subprovince to the north. They consist mainly of clastic metsedimentary rocks (turbiditic wacke, arkose, quartz arenite, slate and argillite) as well as post- to syn-deformational, syenitoid to granitoid plutons (cf. Metsaranta, 2022; Metsaranta and Walker, 2019). Migmatization becomes common in the rocks towards the northern portion of the area as metamorphic grade increases (Williams, 1991). -2- The Southern Province consists of Proterozoic �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 1. Generalized geology of the Thunder Bay area; geology from Ontario Geological Survey Map 2542, Bedrock Geology of Ontario, West-Central Sheet, scale 1:1 000 000 (1991). rocks which unconformably overlie or intrude Archean basement rocks of the southern Superior Province (cf. Tanton, 1931; Pye, 1969). North and west of Lake Superior, the Southern Province comprises: 1) Paleoproterozoic (ca. 1.8 Ga) Animikie Group sedimentary and minor volcanic rocks; 2) Mesoproterozoic (ca. 1.4 Ga) Sibley Group sedimentary rocks; and -3- �Proceedings of the 72nd ILSG Annual Meeting - Part 2 3) Mesoproterozoic (ca. 1.1 Ga) Midcontinent Rift volcanic and intrusive rocks. The Animikie Group, exposed in Ontario, Minnesota, Wisconsin and Michigan, is represented locally by the Gunflint Formation and overlying Rove Formation. These dominantly sedimentary formations constitute a largely unmetamorphosed, undeformed, homoclinal succession which dips shallowly to the southeast. The Gunflint Formation is a chemicalclastic assemblage which yielded a U-Pb age from reworked volcanic ash of 1878.3 ± 1.3 Ma (Fralick et al., 2002). Rocks containing intraformational breccias, accretionary lapilli, spherules and shocked quartz that occur near the top of the Gunflint Formation are interpreted to represent ejecta from the Sudbury impact event that occurred at circa 1850 Ma (Addison et al., 2005; Krogh, Davis and Corfu 1984). The Gunflint Formation grades upward into turbiditic sandstone and shales of the Rove Formation south of Thunder Bay. U-Pb zircon ages from ash beds in the basal Rove Formation yielded 1836+5 and 1832+3 Ma (Addison et al., 2005). A sandstone sample from the submarine fan portion of this succession yielded a youngest detrital zircon U-Pb age of approximately 1780 Ma (Heaman and Easton, 2006), but this relatively young age is widely considered to be problematic and not reflective of the true age of these rocks. Sedimentation in this part of the Animikie basin, widely thought to represent the distal foreland of the Penokean Orogen, likely ended ca. 1800 Ma or earlier. However, Maric (2006) suggested that the Rove (and correlative Virginia) Formation represents the transition from a sedimentstarved basin, with exceedingly slow deposition rates, to active deltaic progradation with sediment probably derived from the Trans-Hudson orogenic zone to the north. The Sibley Group, exposed on the Sibley Peninsula and farther north, has been subdivided into five formations; detailed descriptions of each formation have been reported previously (Franklin et al., 1980; Cheadle, 1986; Rogala, 2003; Rogala et al., 2005, 2007). The overall sedimentary environment indicates a fluctuating climatic scenario, in which the Sibley Group was deposited in a lacustrine system (Pass Lake Formation) that gradually evolved into a saline playa lake environment (Rossport Formation). As the climate progressively became drier, a sabkha-type environment developed (Fire Hill Member of the Rossport Formation). After a break in time, the Kama Hill and Outan Island formations represent outbuilding of a large deltaic complex to the north (Jones et al., 2022), and the Nipigon Bay Formation represents an aeolian environment (Rogala, 2003; Rogala et al., 2007). The depositional age for much of the Sibley Group had been constrained between ~1340 and 1450 Ma. The northern margin of the Midcontinent Rift is dominated by mafic hypabyssal rocks of the Midcontinent Rift Intrusive Supersuite (Miller et al. 2002), which intrude all Proterozoic rocks and Archean basement. South of Thunder Bay, Logan (1106.3+2.0 Ma; Smith et al., 2025) diabase sills predominate. Nipigon diabase sills (1108.2+0.9 Ma; Bleeker et al., 2020) occur in and north of the City, and form the bulk of the Nipigon Embayment. Volcanic and minor sedimentary rocks of the ca. 1108 to 1105 Ma Osler Group (Davis and Sutcliffe, 1985; Davis and Green, 1997) are exposed to the east on Black Bay Peninsula and on offshore islands in Lake Superior. While all aforementioned rocks are related to the Early Magmatic Stage (ca. 1110–1103 Ma; Miller and Nicholson, 2013) of Midcontinent Rift development, younger intrusions (ca. 1097-1092 Ma; Smith et al., 2025) are associated with a magmatic episode that followed the emplacement of the 1099 Ma Duluth Complex. Three main domains were suggested by Smith et al. (2025) in describing the northern flank of the Midcontinent Rift west of Thunder Bay and Lake Nipigon, namely, from south to north: (1) a gently south-(southeast-)tilted Midcontinent Rift margin; (2) a pronounced basement arch just north of Thunder Bay, likely representing a flexural bulge; and (3) the erosional remnant of the Lake Nipigon rift-and-sag basin, preserving the Sibley Group intruded by extensive Nipigon diabase sills (Figure 2). Quaternary Geology The first Pleistocene ice sheet in the Thunder Bay region, ca. 1 Ma, moved over and stripped a deeply weathered, relatively flat bedrock landscape (Zaniewski et al., 2020). During the Pleistocene, possibly ten or more major advances and retreats of ice took place, each with its own history of advancing and retreating lobes of ice. The region’s present landscape is the product of interplay between three major ice lobes (i.e. Patricia or Rainy River Lobe, from the north; the Hudson Bay Lobe, from the northeast; and the Superior Lobe, from the east) originating from three accumulation centers -4- �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 2. West-looking cross section through the northern flank of the Midcontinent Rift, just west of Thunder Bay and Lake Nipigon, from Smith et al. (2025) during Wisconsin glaciation. Only the final event, comprising the Marquette Readvance (ca. 11 500 Ka) and its subsequent retreat, is understood in local detail (ibid). remains of the toolkit of these people other than a variety of knapped lithic tools made from taconitic chert that occurs in the local Gunflint Formation (cf. Hamilton, 1996). Starting about 11,000 years ago (Ka), Wisconsin ice melted back from its position in central Minnesota and Wisconsin, and quickly exhumed the Thunder Bay region, forming recessional moraines during brief stillstand periods (Phillips, 2004; Phillips et al., 1994). The Lake Superior basin was occupied by Early Lake Minong, the shoreline of which is found close to the 1400-foot (427m) contour in the borderland area. About 10 Ka, ice re-advanced from north of Lake Nipigon, sweeping across the Superior Basin (Marquette Readvance). As that ice began to melt, glacial lakes were formed between the moraines and the retreating ice margins. As Superior ice melted, water levels progressively lowered, forming a series of shoreline features down-slope and depositing thick lacustrine clays. Superior ice withdrew to the north of Lake Nipigon around 9.5 Ka, and for the first time since the Marquette Re-advance, the Superior basin was occupied by a single lake, Lake Minong. This lake level extended up the Kaministiquia embayment to Rosslyn, where a large delta structure was built. The Minong shoreline runs through the upper part of the city, being particularly evident in Boulevard Park where river mouth bars and terraces of the Current River are seen. The Minong shoreline in the city is strongly associated with Palaeo-Indian sites, the Cummins Site being the best-known. It is likely that as water levels fell, these early people moved down from the Arrow-Whitefish Lakes area into the Kaministiquia embayment. Little Field Trip Stop Descriptions - Day One Day One begins with visits to a number of locations northeast of the City, clustered around the northern end of Thunder Bay of Lake Superior (Figure 3) and ends near and within the City (Figure 4). This small area is underlain by a variety of rocks that record almost three billion years of local geologic history, spanning from the Neoarchean (ca. 2.7 Ga) to the Paleoproterozoic (ca. 1.8 Ga) and Mesoproterozoic (ca. 1.4 and 1.1 Ga) and perhaps to the Mesozoic (ca. 100 Ma). Unconsolidated glacial and post-glacial deposits and features attest to a long-lived, Pleistocene glaciation record. All GPS coordinates are NAD83, UTM Zone 16. STOP 1-1: Blende Lake Unconformity (0367703 E / 5383357N) This exceptional highway rock cut, like many others on this stretch of Highway 11-17, was exposed by new highway excavations ca. 2012. This ~700 m-long exposure features the unconformity between Neoarchean Wawa metavolcanic and gabbroic rocks and Paleoproterozoic sedimentary rocks of the lower Gunflint Formation (cf. Scott, 1990; Figure 5). Basement rocks here have also been described by Landman (2021) as coarse-grained amphibolite, interpreted as a mafic intrusion which has undergone amphibolite-facies metamorphism. -5- �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 3. Generalized geology of the northern end of Thunder Bay of Lake Superior, showing the first 9 field trip stop locations of Day 1. Geology from Map M2232 (Carter et al., 1973). BLF – Blende Lake Fault This exposure was described by Metsaranta and Kurcinka (2022), as part of an ongoing Ontario Geological Survey (OGS) bedrock mapping project of the Animikie Basin near Thunder Bay: “…chloritized Archean felsic to intermediate intrusive rocks are locally overlain by at least 4 stromatolite mounds comprising thinly laminated black to red chert [Figure 6]. The stromatolite mounds have a height of up to 30 to 50 cm and similar widths. The top of one mound is marked by a thin stylolitic band [Figure 7]. The areas between stromatolite mounds comprise silicified grainstones that locally contain sulphide nodules up to 5 cm in diameter. The grainstones enclosing the stromatolite form medium to thick beds characterized by medium- to large-scale trough cross-stratification. At this locality, an east-dipping and roughly north-striking small displacement thrust fault puts Archean basement rocks above Gunflint Formation rocks. The fault appears to displace Midcontinent Rift–related quartz-carbonate-sulphide veins indicating Figure 4. Field trip stop location map, showing Day 1 stops 1-10 and 1-11 and Day Two stops. (See Figure 3 for map legend). -6- �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 5. Paleoproterozoic Gunflint Formation sedimentary rocks unconformably overlying Archean basement, east side of Highway 11-17, STOP 1-1. that the thrusts may be related to rift inversion; however, this field relationship is equivocal.” These ferroan dolomite and siderite grainstones (medium-grained, sand-sized iron carbonates), referred to as granular iron formation, are common in the Thunder Bay region, dominating the near-shore of the Animikie Basin (see Fralick et al., 2012; STOP 1-4). The thin basal conglomerate (aka Kakabeka Member, Figure 8) of the Gunflint Formation is discontinuously distributed along the paleosurface. As described by Metsaranta and Kurcinka (2022), the conglomerate has a clast-supported texture, consisting of coarse-grained sand, granules, pebbles and rare cobbles in a sandy matrix. Quartz, pink granitoid, clastic metasedimentary and mafic metavolcanic clasts were noted. Figure 6. Mound-shaped stromatolites, with onlapping grainstones, resting on chloritized Archean basement, STOP 1-1. The stromatolite has a black chert core, and red, jaspilitic outer layers. Photo from Metsaranta and Kurcinka (2022). varies from 0 to 30 cm in thickness here, and is usually absent from the local topographic “highs” (i.e. knobs or ridges of the Archean basement), but may thicken in depressions in the paleosurface. The basal conglomerate lag may contain large, well-rounded boulders, up to 0.5 m in diameter; these have been observed only on the northwest side of the highway, opposite STOP 1-1. Black, cherty bands (0.1–1.5 cm) occur locally in parts of the Kakabeka conglomerate (ibid). Recent geochronologic study of the conglomerate shows that the main population of detrital zircons is consistent with derivation from local Neoarchean intrusions (R. Kup et al. (2025) also noted that the conglomerate Figure 7. Stylolites in Gunflint Formation grainstone, STOP 1-1, visible as a black serrated band to the right of the scale card. Figure 8: Quartz pebble-rich Kakabeka conglomerate, STOP 1-1. Photo from Metsaranta and Kurcinka (2022). -7- �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Metsaranta, personal communication, 2026) like the Mackenzie granite (ca. 2672 Ma; Puumala et al., 2015). The stromatolites which occur in the basal unit of this Gunflint section (Unit 1 of Kup et al., 2025) were described by Kup et al. (2025): “Unit 1 of the Gunflint Formation is characterized by the common occurrence of cherty, domical to columnar stromatolites within the basal 2 m of the formation. Relatively large stromatolite heads tend to occur preferentially on knobs or ridges of the Archean basement, usually metres apart from one another. Smaller stromatolite heads (5–15 cm in diameter or height) are present locally, even on top of the Kakabeka conglomerate. Tabular, finely undulatory, microbialite-like structures (commonly <10 cm thick) may occur in places, or laterally connected to stromatolite heads. The microbialite-like structures may weather to a reddish colour, similar to some of the weathered stromatolite heads. Overall, the colour of the domical to columnar cherty stromatolites ranges from red, yellow, white, grey and black, depending on the iron content and the nature of weathering. Relatively large (usually fresh) stromatolite heads tend to be jet black near their centre and change to lighter grey and white toward their edges; however, edges themselves are commonly marked by red and/or yellow banding. The stromatolites are most easily seen and accessible toward the southern ends of the outcrop, directly above the road ditch level.” galena, have been noted. The fault is also exposed on the other side of the highway; Gunflint rocks are folded next to the fault there as well. The east-northeast orientation of the Blende Lake Fault is similar to other structures on and north of the Sibley Peninsula, some of which host gabbroic dykes Figure 9. Rock cut exposure of the Blende Lake Fault, east side of Highway 11-17 (STOP 1-2), separating folded Gunflint Formation rocks (left) from Neoarchean basement (right). The fault zone is cored by calcite vein / vein breccia; fault gouge flanks the vein. Field notebook for scale. Optical and SEM imaging data collected by Kup et al. (2025) suggest that well-preserved Gunflint-type microfossils (both filamentous and coccoid types) tend to occur in sporadic pockets in samples collected from this locality. attributed to the waning stages of Mesoproterozoic Midcontinent Rift magmatism. Scott (1990) noted that Gunflint rocks, normally flat-lying or gently southeastdipping, are folded and brecciated to a large extent in the area between Blende Lake and O’Connor Point on Lake Superior. Folding described by Moorhouse (1960) east of Blende Lake, was attributed to farfield Penokean fold-and-thrust deformation by Hill and Smyk (2005), prior to the recognition of the Sudbury Impact Layer and associated deformation in the Animikie Basin. Koroscil (2013) noted that thrust faults, once ascribed to Penokean deformation, cut the SIL at the Terry Fox Monument (STOP 1-10) and thus may post-date the Penokean. Landman (2021) noted that: STOP 1-2: Blende Lake Fault (0368005 E / 5383802N) The northern end of the same rock cut, approximately 500 m north-northeast of STOP 1-1, exposes the eastnortheast-striking Blende Lake Fault (cf. Scott, 1990). Rusty fault gouge occurs between Archean gabbroic rocks to the south and folded, flaser-bedded Gunflint wacke and siltstone, cored by a 3 m-wide calcite + quartz vein / vein breccia which contains Gunflint fragments (Figure 9). Base metal sulphides, including -8- “Later Proterozoic features, including the Blende Lake fault, have a common strike of eastnortheast, which aligns with the orientation of the 1.1 Ga Mid-Continent Rift in Thunder Bay. This similarity is further reflected by the Blende Lake fault being oriented subparallel to silver veins related to the Mid-Continent Rift. Similarities between orientations of brittle structures in the [Neoarchean] amphibolite and Gunflint Formation suggest that the Mid-Continent Rift �Proceedings of the 72nd ILSG Annual Meeting - Part 2 in Thunder Bay may have reactivated some Archean-aged, orogenic-related faults and shear fractures. Minor folding in the Gunflint Formation truncated by the Blende Lake fault, as well as reverse reactivation along the plane, may be evidence of compression during the later stage of the Mid-Continent Rift.” STOP 1-3: Mirror Lake Turn-off (0370744 E / 5387262N) This stop displays a number of quite enigmatic features that are still being evaluated in the context of evolving local geologic ideas. The hillside exposes iron-rich, folded and brecciated Gunflint Formation sedimentary rocks that have been intruded by a Nipigon diabase sill. Tight to isoclinal, plunging to recumbent folds have developed in certain parts of the otherwise ~flat-lying, thinly to thickly bedded, taconitic, martite(?)-bearing grainstones (Figure 10). As is the case at STOP 1-2, the cause of the deformation in the Gunflint rocks is a matter of debate. There is growing support that local folding and brecciation may be related to far-field effects generated by the Sudbury meteor impact ca. 1850 Ma., postdating the end of Gunflint deposition by perhaps ca. 20 My and preceding the onset of Rove sedimentation. Rocks interpreted as being part of the Sudbury Impact Layer were noted in geotechnical drilling ~ 1 km north of this location (P. Fralick, personal communication, 2025). Despite the fact that the Sudbury impact structure is ~660 km away and that the most dramatic deformation is usually crater-proximal (i.e. within ~5 crater radii), Addison and Brumpton (2012) noted that the Thunder Bay area would have still experienced dramatic impact-induced effects, including magnitude 10.7 earthquakes and likely tsunamis. Alternatively, it has also been suggested that some of this deformation may be related to the dominantly extensional stress regime associated with Midcontinent rifting. Local compressional (contractional) structures may form within relay zones between overlapping normal fault tips, particularly as the fault segments grow, interact, and prepare to connect. While normal fault systems are dominated by horizontal extension (pulling apart), the 3D interaction and rotation of blocks in the relay zone (or “relay ramp”) can create local stress perturbations that lead to shortening, folding, and antithetic faulting (cf. Camanni et al., 2023). Further work is required to better map the extent and character of folding and brecciation in order to suggest deformation mechanisms and causative factors. Rove shales and wackes (e.g. STOP 1-5), mapped ~2.5 km south of here by McIlwaine (1975), are not deformed. They overlie Gunflint rocks and are disconformably overlain by sandstones of the Pass Lake Formation of the Sibley Group (STOP 1-6). Figure 10. Recumbent fold in Gunflint Formation chertcarbonate rocks, STOP 1-3. Folded bedding planes are traced by dashed lines. A thin veneer of Phanerozoic(?) conglomerate (cgl) occurs on outcrop surfaces and in crevices. Two other enigmatic rocks are exposed at this location; both are conglomerates. One conglomerate occurs as thin coatings plastered on exposed outcrop surfaces and in fractures in the folded Gunflint rocks (Figures 10 and 11). It is a brown, poorly sorted, sandy, matrix-supported unit. Sibley Group (ca. 1.4 Ga) sedimentary rock clasts, ranging from sub-angular to rounded pebbles and cobbles, predominate. Most recognizable are rust-red calcareous siltstones of the Rossport Formation (with their characteristic pale reduction spots) the base of which occurs approximately 75 m stratigraphically above the Gunflint exposed here. It must also be noted that medium-grained mafic igneous clasts appear to be ca. 1.1 Ga Nipigon diabase -9- Given the presence of Sibley and Nipigon diabase �Proceedings of the 72nd ILSG Annual Meeting - Part 2 and/or enrichment (cf. Fralick and Riding, 2015). Interestingly, the exposure of stromatolitic Gunflint rocks on the west side of Highway 11-17, just opposite the Mirror Lake turnoff, only 200 m away from STOP 1-3, is notably rusty and apparently more oxidized than the vast majority of Gunflint rocks. This may represent Mesozoic paleoweathering, raising the possibility that Cretaceous deposits and paleoweathering effects may have once extended as far east as the Thunder Bay area. Any rocks that survived Pleistocene glaciation may survive in isolated patches or have been as yet unrecognized. The second conglomerate (Figures 12A and 12B) is similarly exposed as a plastered veneer draped on the exposed outcrop face. Unlike the other unit, it appears to be clast-supported and monomictic; angular, dark grey, fine-grained, shaly Gunflint fragments are cemented by calcite. This monomictic clast population suggests local derivation, perhaps a talus deposit created and cemented during the Pleistocene. Figure 11. Close-up of thin veneer of conglomerate on Gunflint substrate, showing reddish-orange Rossport Formation siltstone and Nipigon diabase fragments. clasts, the conglomerate must postdate at least Midcontinent rifting, the hitherto youngest geologic event in the local lithologic record. Although no Phanerozoic rocks have been documented in the Thunder Bay region, Cretaceous rocks have long been known to overlie the Biwabik Formation (time-correlative with the Gunflint) on the Mesabi iron range of northern Minnesota (e.g. Bergquist, 1944); “soft” iron ores there formed there during the Cretaceous. Paleomagnetic studies by Purucker (1983) in the Eldorado Beach – Nelson roads area, ~6.5 km southwest of this location, suggested that secondary enrichment of Gunflint and Mesabi iron ores took place at approximately the same time between Aptian and Cenomanian time (ca. 12594 Ma). In their study of anthraxolite in the Gunflint Formation in the Kakabeka Falls area, Hayatsu et al. Remnants of a Nipigon diabase sill form prominent, cuesta-like hills in the vicinity of this stop and around Deception and Mirror lakes (McIlwaine, 1975). Smooth, glacially polished surfaces with striae are visible at the road level in this cliffside exposure. STOP 1-4: Gunflint Formation, Blende Creek area (0369581E / 5383837N) This stop description, featuring deformed chertcarbonate units in the Gunflint Formation (Figure 13), is taken from Fralick et al. (2012): (1983) identified two very distinct macromolecular materials. These two hydrocarbon fractions were thought to represent derivation from sediments of two vastly different ages: an older one, characterized by heavier aromatic ring compounds, derived from Gunflint-aged organic remains; and another, aliphatic fraction derived from Cretaceous (or possibly Jurassic) sediments. Cretaceous microfossils were described in lateritic “buckshot” ore in the Archean Steep Rock Lake iron deposit near Atikokan (Machado, 1987) that underwent Mesozoic karstification, weathering - 10 - “Along Highway 587, rock cuts display thinly bedded, generally flat-lying sedimentary rocks of the Gunflint Formation. The outcrops we have driven past are composed of ankerite and siderite grainstones (medium-grained, sandsized iron carbonates) referred to as granular iron formation (GIF). These are common in the Thunder Bay region, dominating the near-shore of the Animikie Basin. The iron carbonate grains were produced by wave erosion of carbonate precipitates and represent storm deposits in the near-shore. The iron may have precipitated as a carbonate in this shore-proximal zone due to photosynthesizing bacteria removing CO2 from the water and thus increasing the pH and driving the carbonate phase into supersaturation. The outcrop we are looking at has these carbonate �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 12. A (left). Clast-supported, calcite- cemented conglomerate veneer over Gunflint, STOP 1-3. B (right). Close-up view of conglomerate / breccia in Figure 12A. grainstones weathering orangey-brown alternating with white chert layers. In places the chert can be seen replacing the carbonate but other layers appear to be primary chert. In the older literature an outcrop such as this would be ascribed to deeper water due to less evidence of current activity. However, because of its shore proximal location it probably formed in a Figure 13. Folded Gunflint Formation grainstones, north side of Highway 587, STOP 1-4, with locally axial-planar quartzcarbonate veins. - 11 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 quieter water location near the strand-line, i.e., a sheltered lagoonal area behind on offshore bar. These exposures are somewhat unique in that the rocks are folded; elsewhere, they are undeformed. The hinge zones, where the majority of stress is focused, are commonly fractured. These fractures may be occupied by quartzcalcite veins following the vertical axial plane. The outcrop to the west hosts numerous veins and vein breccias that strike between 40° and 45° and dip almost vertically to the southeast. These breccias contain sparry calcite, drusy quartz and also altered shale fragments, suggesting that these Rove Formation rocks likely occurred above this section during vein emplacement. A thin, northwest-dipping diabase dyke intrudes the Gunflint rocks at this location and is, in turn, cut by these veins.” Deformational features in Gunflint Formation rocks near Pass Lake have been previously ascribed to Penokean fold-and-thrust activity in the foreland (i.e. passive margin Archean basement + Gunflint Formation; Hill and Smyk, 2005). These include discrete bedding-plane faults with locally developed gouge and breccia that can be traced laterally into horizontal, hanging wall ramps with associated fault-bend folding. Previous workers had also ascribed folding to synsedimentary slumping and Keweenawan diabase sill emplacement and thought that they were attributable to local, rather than regional-scale, deformation. As introduced at STOP 1-3, there is growing support for the contention that such deformation may be related to the Sudbury impact event ca. 1850 Ma. Figure 14: Flowerpot-shaped Rove Formation concretion on wall of inactive shale quarry at STOP 1-5. a piece of organic material or other foreign object, which creates a perturbation in fluid flow with a distinct chemistry. Because the cementing agent in this case is more resistant to weathering, these concretions stand out of the soft shale and may commonly completely detach form their host rock. Groundwater and surficial water flow through the shale has led to the dissolution and subsequent precipitation of a variety of low-temperature minerals (e.g. carbonates, sulphates, hydroxides) that occur as white and yellow encrustations on the bedrock surface. One of the more unusual of these secondary minerals is yellow magnesium aluminocopiaptite ((Mg,Al) (Fe,Al)4(SO4)6(OH)2.20H2O; Resident Geologist’s Files, Thunder Bay).” STOP 1-5: “Devil’s Flower Pots” (Rove Formation concretions) 0370841E / 5382426N This stop description is taken from Fralick et al. (2012): “Just north of Highway 587, a quarry face exposure of black, fissile Rove Formation shale displays lenticular and elliptical concretions, flattened along bedding planes [Figure 14]. These structures form during diagenesis, following initial compaction and dewatering of the sediments. They represent a concentration of a cementing agent (e.g., silica, calcite) focused during the migration of fluid through the sediments. They often are nucleated around STOP 1-6: Edwards Road section (Pass Lake and Rove formations) 0371737E / 5382460N (n.b. private property; permission is required to access) This stop provides us with an excellent stratigraphic section that extends upward from the top of the Animikie Rove Formation into the Pass Lake Formation, the lowermost formation of the Sibley - 12 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 and sandstones of the Loon Lake Member, Pass Lake Formation, overlie the Rove. The conglomerate and sandstone layers are laterally discontinuous, with some conglomerates in clast-support (fluvial deposits) and some in matrix-support (sub-aerial debris-flow deposits). Successions such as this in the Sibley are typical of arid to semi-arid alluvial fans (Cheadle 1986), though this would have been a very small one. The abundant hematite probably denotes a deep water table. Clasts are locally derived from Group. The youngest detrital zircons in the Sibley Group are ca. 1.4 Ga (Rogala et al., 2007). An Rb– Sr isochron age of 1339 ± 33 Ma was determined on dolomitic mudstones from the Rossport and Kama Hill formations (Franklin 1978; Franklin et al. 1980). Recent studies of some of the concretions (quartzcarbonate + various very fine-grained impurities and inclusions) in the Pass Lake Formation, associated with late advanced diagenesis, had enough uranium to generate an age of 1483+4 Ma (W. Bleeker and H. Rochin-Banaga et al., unpublished data / personal communication, 2025; Figure 15). Together with the ca. 1500 Ma youngest detrital zircons (SHRIMP data on 3 samples; ibid), this provides a greatly improved age constraint, just marginally younger than 1500 Ma, on the deposition of the lower part of the Sibley Group. This stop description is taken from Fralick et al. (2012): “A private access road extending up the mesa provides an excellent 150 m long section exposing the disconformity between the Rove Formation and the overlying basal conglomerate and sandstones of the Pass Lake Formation [Figures 16 and 17]. The Rove shales immediately below the contact were subject to Mesoproterozoic weathering. Geochemical investigations have outlined an oxidized zone below the contact grading to a more reduced zone with abundant chlorite a few tens of centimeters lower in the section. In one area what may be a dewatering or degassing structure strongly deforms the shale. Very immature, iron oxide-rich conglomerates Figure 16. Cobble-sized clast of Gunflint Formation stromatolitic jasper/chert visible in the Loon Lake Member conglomerate exposed along the Edwards Road section, STOP 1-6. Figure 15. U-Pb concordia diagram presenting geochronological data for the Sibley Group (W. Bleeker and H. Rochin-Banaga et al., unpublished data, 2025) Figure 17. Disconformable contact (just above hammer) between weathered green Rove Formation shales and hematite-rich, basal conglomerate and sandstone of the 6-7 m thick, Loon Lake Member (Pass Lake Formation), STOP 1-6. Overlying, well-sorted, buff sandstones of the Fork Bay Member form the top of the exposure. - 13 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 the erosion of underlying units. This is sharply overlain by mature, well-sorted, medium-grained sandstones of the Fork Bay Member, Pass Lake Formation. Detrital zircon geochronology and paleocurrents (Cheadle 1986; Rogala et al. 2007) indicate that the major source of this sediment was the Trans-Hudson highlands. The travel distance accounts for its maturity compared to the locally derived underlying conglomerates. The sandstone was deposited as sheet flows into the shallow nearshore of a lacustrine system that had flooded the area (Cheadle 1986; Rogala 2003; Metsaranta 2006; Rogala et al. 2007). These sandstone layers are laterally continuous, massive to parallel-laminated, in places with trough cross-stratified or rippled tops [Figure 18]. Rare, odd features are present both in crosssectional and bedding plane views in this outcrop. These may be dewatering pipes.” exposed on this outcrop surface. These include two sets of glacial striae at 040˚ and ~060˚ and subparallel arrays of crescentic gouges and chatter marks/lunate fractures (Figure 19). En route to STOP 1-7, the highway traverses a series of baymouth bars that formed as lake levels fell from the Lake Beaver Bay stage (ca. 11 to 10.5 Ka) to the Lake Minong stage (ca. 10.5 to 8.5 Ka), connecting what had been the “island of Sibley” to the mainland near Pass Lake (Zaniewski et al., 2020; Geddes et al., 1987; see Fralick et al., 2012). This new connection formed an ideal natural trap for Palaeo-Indian hunters to use. The materials excavated at the Brohm archaeological site, on the top of the main baymouth bar, were all hunting-related projectiles and scrapers, many made onsite from chunks of jasper taconite that they carried with them from quarry sources (Zaniewski et al., 2020; MacNeish 1952). Fralick et al. (2012) noted that the matrix-supported conglomerate was probably deposited as a high-density mass-flow while the boulder-cobble, matrix-supported conglomerate probably represents a very high-viscosity mass flow as the larger clasts were suspended near the top of the flow. Upper flow regime, parallel-laminated sandstones were probably deposited by sheet-floods on an alluvial fan’s surface and are interbedded with clastsupported fluvial conglomerate. The top of the hill affords a tremendous view of Thunder Bay, Sibley Peninsula and offshore islands. A south-dipping diabase sill forms the prominent cuesta of Caribou Island. Glacial erosional features are Figure 18. Medium- to coarse-grained, well-sorted sandstone bed of the Fork Bay Member, top of hill, STOP 1-6. The majority of the bed is upper flow regime parallel laminated, with a reworked, cross-stratified top. Figure 19. Glacial erosional features exposed in the sandstone outcrop surface at the top of the hill, STOP 1-6. These include glacial striae (dashed arrows), concave up-ice crescentic gouges (CG) and concave down-ice chatter marks (CM). - 14 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 STOP 1-7: Pass Lake section (Loon Member conglomerate) 372282E / 5380560N The cliffs adjacent to the abandoned railway at Pass Lake is the type section for the Pass Lake Formation. Exposure is almost continuous for 3.2 km along the tracks and provides a ~50 m-thick stratigraphic section. Rove Formation shales, exposed at the northwestern end of the cliff exposure, disconformably underlie the Pass Lake Formation but are not exposed here. This cliff face, a popular destination for local rock climbers, exposes the basal Loon Member conglomerate and overlying, buff sandstones of the Fork Bay Member (Pass Lake Formation; Figure 20). A description was provided by Fralick et al. (2012): “The basal conglomerate thins and thickens laterally, pinching down to pebbly sandstone in places. Clasts are generally surrounded and dominated by local Gunflint Formation lithologies. The matrix is poorly sorted. The conglomerates are overlain by a thinningupward sequence of sandstone beds capped by siltstones on the top of the cliff. Individual beds are reasonably laterally continuous though sometimes lens out. They are dominated by upper flow regime parallel lamination with occasional ripples and small-scale dunes on their tops. Figure 20. Pass Lake section exposure of Loon Lake Member conglomerate overlain by Fork Bay Member sandstone at STOP 1-7. STOP 1-8: Neoarchean Pyroclastic and Clastic Sedimentary Rocks 360568E / 5379943 Both alluvial fan-braided fluvial and shallow lacustrine (Cheadle, 1986; Franklin et al., 1980, respectively) depositional environments have been proposed. The bedding organization of the conglomerates exposed here is somewhat different than those observed earlier. This opens the possibility that the conglomerates at this location were reworked by wave activity during initial lacustrine flooding. This 100 m-long rock cut on the northwest side of Highway 11-17 was exposed by highway construction ca. 2012. It features a remarkable exposure of Neoarchean pyroclastic and clastic sedimentary rocks of the Shebandowan greenstone belt that strike ~140˚ and dip steeply northeast (Figure 21). These supracrustal rocks are intruded by granitoid rocks of the McKenzie granite and may represent a large pendant within the intrusion. The exposure was the subject of an undergraduate thesis by Bjorkman (2014), from which most of the descriptions will be gleaned. The sandstone beds again represent sheetfloods, forming sand-flats in the shallow lake. The thinning- and fining-upward sequence of sandstone beds is a classic example of a transgressive succession showing decreased sand supply through time as the shoreline moves further away from the area.” Bjorkman (2014) identified 13 lithofacies/lithologic units in this complex section: Overlying red-orange Rossport Formation siltstones begin to outcrop approximately 1.2 km east of STOP 1-7 (see Fralick et al., 2012 for stop descriptions). This exposure exemplifies the close connection between Neoarchean pyroclastic activity and sedimentation (Figure 22). Bjorkman (2014) suggested that these rocks were deposited in a vent-proximal environment, a contention supported by the presence of graded ash beds, high-velocity base surge deposits and impact structures from pyroclastic bombs (Figure 23). - 15 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Lithofacies / Interpretation 1 Lahar – debris flow 2 Ash fall 3 Fluvial reworking and base surge deposits 4 Ash fall 5 Lag deposit 6 Slump 7 Lahar 8 Lahar / channelized debris flow 9 Ash tuff 10 Hornblendite 11 Hornblendite 12 Hornblendite 13 Syenite Description Unsorted, clast-supported, conglomerate; clasts are pebble-sized to small boulder-sized within a medium-grained, sand-sized matrix Fine-grained to medium-grained, sandy, continuous beds; parallel laminated with no crossbedding, with an average thickness of 1 cm or less Cross-bedded and graded, medium-grained sandstone beds, which occur in alternating sequences; this unit has a sharp basal contact with the lower Lithofacies 2 and a poorly defined contact with upper units. The transitions from the graded beds to the cross-stratified beds are distinct. Parallel-laminated, continuous layers of medium-grained, graded beds, more rarely observed to be cross-bedded at very low angles. Clast-supported conglomerate in which clasts are very uniform and commonly cobble-sized. The clasts are flattened and oval-ellipsoidal, with rounded edges. The long axes of the clasts occasionally have tail-like tips. Disturbed beds of material very similar to that found within Lithofacies 4. The parallel-laminated strata are disrupted by failure of the slope and are truncated by an angular disconformity of the overlying unit. There are rubble blocks adjacent to the truncated strata. These blocks of failed beds lie along the base of this facies, with no evidence of sorting after the failure. There is no grading in the matrix, which is massive, medium-grained sandstone. Repetitive sequences of normally graded, medium-grained sandstone beds, gravelly matrix supported beds, and non-graded massive beds composed of medium-grained sand. Discontinuous, lens-shaped beds are very common. The normal graded beds are composed of coarse-grained sandstone, which grades into fine-grained tops of beds. These often have eroded, scoured tops, with very distinctly defined bases. Cross-bedding is common. Massive graded conglomerate with angular to very rounded and moderately flattened clasts. The clasts appear monomictic and range in size between 5-15 cm, the majority being 12 cm by 7 cm. The clasts make up 70% of the total composition, while the matrix is mostly a uniform mediumgrained sandy composition, with 10% coarser sand-sized fragments. The unit is on average 3-5 m wide. Fine-grained, sand-sized matrix with medium-grained and subhedral porphyritic feldspar crystals. The weathering surface is very irregular as the feldspars stand-out from the matrix. The unit occurs sporadically, locally intruded by dark green material. The average thickness varies between 50 cm to 100 cm. There is no grading throughout this unit, and the unit conforms to the same stratigraphy as the surrounding units, which is most often Lithofacies Association 1. Medium-grained dyke which crosscuts stratigraphy. The largest of the intrusive dikes, it can be traced through the entire outcrop. It is distinguished by the very irregular shape of its contacts with the host rock. The matrix consists of green equigranular, subhedral crystals in the middle of the intrusion and lighter altered plagioclase crystals along finer-grained contacts. The body intrudes (brecciates) itself where the dike dilates. Other smaller dikes crosscut this one. Cobbleto boulder-sized xenoliths were noted. Medium-grained, green-grey mafic dyke, 5-7 cm wide, with equal amounts of mafic and felsic minerals. The dike cuts through the green intrusive veinlets. A set of dark green-grey dykes, up to 0.5 m in width, striking approximately the same direction as Lithofacies 10; may be sill-like intrusions, wispy and infiltrating intrusions which engulf clastic material. This unit commonly contains wall rock xenoliths, which are very sharp and angular. Medium- to coarse-grained, subvertical and east-southeast-striking dykes. The dykes are the youngest rock type in the outcrop and are noted regionally. They are composed of red feldspar, amphibole, biotite, and quartz. The red feldspar gives the rock a brick-red colour. A combination of subaerial and shallow subaqueous conditions likely existed at the time of deposition, with fluvial reworking and deposition occurring during periods of volcanic dormancy. Phreatomagmatic processes, similar to those that produce maar craters, likely predominated. The calc-alkalic geochemistry (Figure 24), pyroclastic volcanism and subaerial/shallow water to fluvial clastic sedimentary rocks suggest that these rocks are part of the younger Shebandowan assemblage - 16 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 21. Map of the main outcrop, STOP 1-8 (Bjorkman, 2014). Lithofacies Association 1 through 9 are (resedimented) pyroclastic and clastic sedimentary units; lithofacies association 10 through 14 are crosscutting dykes. of the Wawa subprovince. Shebandowan rocks (ca. 2690-2680 Ma), unconformably overlying the older (ca. 2720 Ma) Greenwater assemblage rocks, were deposited in fault-bounded, pull-apart basins during regional transpressive (D2) deformation. North and south of the main, supracrustal-dominated outcrop, pink granitoid rocks associated with the McKenzie granite occur (Figure 25). The McKenzie granite is approximately 22 km long (east-west) by 3.2 km wide (north-south) and has been divided into two segments that are separated by a fault (Scott, 1990). Based on the mapping of Scott (1990), and the interpretation of aeromagnetic data, Metsaranta (2015) suggested that the McKenzie granite comprises multiple distinct intrusive bodies, and referred to the western segment as the Mount Baldy intrusion. Figure 22. Resedimented pyroclastic material as conglomeratic beds and lenses, STOP 1-8. - 17 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 24: AFM plot of samples collected and analyzed by Bjorkman (2014), showing the calc-alkaline nature of the resedimented pyroclastic rocks at STOP 1-8. Figure 23: Large pyroclasts, commonly with attendant bomb sags, STOP 1-8. The Neoarchean, S-type McKenzie granite (Hughes, 2016) is primarily a peraluminous quartz monzonite, with mineral assemblages characterized by microcline-plagioclase-quartzmuscovite-biotite with minor amounts of inequigranular hornblende, chlorite, titanite and rarely calcite. The McKenzie granite exhibits a peraluminous geochemistry, with SiO2 contents ranging from 63.8 to 68.2 weight % along with enrichment in light rare earth elements and fractionated heavy rare earth elements, decreasing trends of major oxides, transition metals and high field strength elements. Scattering of the large ion lithophile elements on discrimination diagrams is likely due to remobilization during chlorite, sericite and carbonate alteration (Hughes et al., 2017). It is proposed to have formed in a similar way to the model proposed for the later stages of the Figure 25. Photo illustrating cross-cutting relationships at STOP 1-8. The granitoid dyke in the bottom half of the photo that cross-cuts all lithologies, including a hornblendite dyke (top center), is associated with the nearby McKenzie granite. genesis of the nearby Dog Lake Granite Chain, which involved partial melting of a mantle wedge beneath the Wawa-Abitibi island arc. The proposed late-stage emplacement model is consistent with recent U-Pb geochronology (Puumala et al., 2015) that indicated that the McKenzie granite was emplaced at 2672.6 ± 1.5 Ma (zircon, U/Pb thermal ionization mass spectrometry). These S-type melts, formed from the partial melting of metasedimentary rocks, may have interacted with I-type melts, allowing for the variations in geochemical and petrological data that are observed in the McKenzie granite, such as the presence of hornblende, that are not common for standard S-type granites. - 18 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 STOP 1-9: Gunflint / Archean Unconformity, Crystal Beach 358661E / 5378895N This road cut on the northwest side of Highway 1117 near Crystal Beach provides another outstanding exposure of the contact between Archean basement and unconformably overlying Paleoproterozoic Gunflint Formation, similar to that exposed at STOP 1-1. However, there are a number of unique features here that warrant description and examination. The basement at this location is the Neoarchean McKenzie granite (2672 Ma) which has been conspicuously altered 1 to 3 m below the unconformity. The original pink granite has been altered to dark green chlorite (+ clays?) up to 2 m; alteration intensity increases upward towards the unconformity. Unaltered pink, K-spar-phyric granite gives way to altered versions in which the matrix is incipiently to completely chloritized, leaving relict, unaltered K-spar phenocrysts. The phenocrysts have also been replaced (saussurite + clays + chlorite) in the most intensely altered granite, leaving only relict quartz (Figure 26). The correlation between alteration intensity and proximity to the unconformity suggests that the altered rocks may represent a regolith/saprolite. Such alteration is often interpreted as a combination of ancient subaerial weathering (true paleosols) and later fluid migration from the overlying iron formation. Geochemical studies by Yip (2016) and Fralick (personal communication, 2024) at this location suggest that iron-rich Gunflint fluids replaced and masked the geochemical signature of the original paleosol. Similar alteration characteristics were described by Kronberg and Fralick (1992), who noted that alteration of ferromagnesian minerals in felsic Archean rocks Figure 26. Selected hand samples of McKenzie granite from STOP 1-9, showing progressive alteration (chloritization) from unaltered (left) through incipient and pervasive matrix replacement (second and third from left, respectively) to complete replacement of matrix and K-spar phenocrysts (far right). southwest of Thunder Bay was apparently due to diffusion of iron-rich, Gunflint-derived fluids across the Proterozoic -Archean unconformity, consistent with slow mineral-fluid exchanges under diagenetic or low-grade metamorphic conditions. Chemical changes in mafic minerals include additions of iron, manganese, and water and losses of silica, calcium, and magnesium. They concluded that these chemical changes occurred as Gunflint fluids diffused into underlying rock over a time frame of 105-107 years. Spalling of overlying Gunflint rocks has exposed a section of smooth, bare basement paleosurface (Figure 27). The contention of Pre-Gunflint weathering is supported by the occurrence of boulder-sized, spheroidally weathered, altered granitic corestones on the paleosurface, where they are enveloped by Kakabeka Member (basal) conglomerate and saprolite/ regolith, and are draped by Gunflint grainstones (Figure 28). Kakabeka conglomerate infills depressions in the paleosurface and fractures that extend down into weathered basement. The conglomerate here consists largely of resistate quartz pebbles in a chloritic, Figure 27: Smooth, curved, bare Archean basement paleosurface (accentuated in half-shadow above yellow field notebook), exposed below overlying, draped Gunflint grainstones, STOP 1-9. - 19 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 saprolitic/regolith matrix. Although this is perhaps the first documented example of corestones in the Gunflint or Biwabik formations, Paleoproterozoic (ca. 1.85 Ga) weathering-produced corestones in the Flin FlonCreighton area of Manitoba and Saskatchewan were documented by Sindol et al. (2020). Many of the joint surfaces and fractures in this exposure are covered and infilled by vein minerals, mainly quartz/amethyst, barite, fluorite, calcite with rare base metal sulphides (pyrite, chalcopyrite, galena, acanthite). These veins constitute the 7Z amethyst occurrence (Figure 29), first explored ca. 1890 (Ontario Mineral Inventory, https://www.geologyontario.mines. gov.on.ca/mineral-inventory/MDI52A10SW00007). The following description of the occurrence is excerpted from Puumala et al. (2015). Figure 29. Amethyst-bearing vein hosted in the Gunflint Formation at the 7Z occurrence. unconformity and are hosted by both Gunflint and granitic rocks. Gunflint rocks are strongly silicified adjacent to the veins. The exposed width of the vein system is approximately 10 m. The 7Z amethyst occurrence is hosted in a vein system and/or breccia zone that strikes 050º and is located approximately at the unconformity between sedimentary rocks of the Paleoproterozoic Gunflint Formation and Neoarchean intrusive rocks of the McKenzie granite stock. The amethyst-bearing vein system has been exposed in a series of 3 historic trenches over a strike length of 180 m. The majority of the amethyst-bearing veins strike 050º (i.e., parallel to the breccia zone) with near-vertical dips. The vein widths are variable, ranging from centimetre- to metrescale. In the Gunflint Formation rocks, a nearhorizontal set of narrow veins also occurs along bedding plane fractures. A third set of narrow, approximately north-striking veins, was also observed immediately to the south of the main breccia zone in road cuts along the north side of Highway 11-17. The portion of the vein system exposed in the southwestern and central trenches is hosted by rocks of the Gunflint Formation, while the veins exposed in the northeastern trench occur at the Figure 28. Spheroidally weathered, chloritized Neoarchean granitic corestone boulders resting on the paleosurface at the Paleoproterozoic-Archean unconformity, STOP 1-9. The corestones are enveloped by saprolitic sediments and conglomerate/regolith, and overlain by draping Gunflint grainstones. The amethystine quartz in this vein system shows a wide variation in colour, ranging from light pink (i.e., rose quartz) through to deep purple. Colourless to white quartz and smoky quartz are also abundant. Veins hosted by granite tend to contain lighter coloured amethyst, while deep purple amethyst and smoky quartz are most likely to be found in the southwestern trench, which is hosted by Gunflint Formation rocks. Most amethyst crystal points are on the order of 1 cm wide. However, much larger crystals were observed in some vugs. Crystals hosted in the Gunflint Formation rocks commonly have a surface coating of hematite. Although recent sampling has reported no significant silver values, a local newspaper reported in 1890 that 7Z was “a veritable mountain of amethyst with rich surface signs of silver” (ibid). This vein system is an - 20 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 example of a broad group of silver-bearing, carbonatequartz veins that typically occur in Animikie Group sedimentary rocks, often in close association with the Archean-Proterozoic unconformity and Midcontinent Rift-related diabase sills (Oja, 1967; Franklin et al., 1986; Kissin, 1992). They likely formed from metamorphically generated fluid from in the Midcontinent Rift and expelled along rift-bounding faults (Smyk and Frankin, 2007). largely based on a former outcrop exposure that was removed during highway reconstruction in 2011. “This is the only outcrop showing a complete ~ 3 m cross-section of the ejecta-bearing debrisite layer extending from Gunflint chert-carbonate up into the basal Rove Formation, which is overlain STOP 1-10: Terry Fox National Historic Monument 339836E / 5372406N This stop includes opportunities to view outcrop exposures near the Terry Fox National Historic Monument and lookout that commemorates Terry Fox’s 143-day, 5373-km Marathon of Hope run to raise money for cancer research in 1980, which continues to inspire global fundraising efforts. A number of rock types and features are exposed in the road cuts that flank the highway and access road near the monument (Figure 30). A prominent, columnarjointed Nipigon diabase sill (Terry Fox sill; Magnus, 2012; Magnus and Kissin, 2010) intrudes and caps these Rove and Gunflint formation sedimentary rocks. As a result, this site displays a complete stratigraphic section from the Gunflint Formation, through the Sudbury Impact Layer (SIL) and up into the overlying Rove Formation. Disconformities appear at both the base and top of the SIL (Addison and Brumpton, 2012). Figure 31. Rocks of the Sudbury Impact Layer (grey) and Rove formation (black) are visible in this photo from STOP 1-10. Geologist’s hand is located at the top of the Sudbury Impact Layer. The description of the SIL (Figures 31 and 32) at this location by Addison and Brumpton (2012) was Figure 30. This quarried rock face adjacent to the Terry Fox Lookout road displays a cross-section that includes (bottom left to top right) the Gunflint Formation, Sudbury Impact Layer (SIL), Rove Formation and Nipigon diabase. A Midcontinent Rift-related normal fault exhibiting approximately 4 to 5 metres of vertical displacement is visible near the left margin of the quarry face and is highlighted with a dashed line. Figure 32. The weathered outcrop (now gone) at STOP 1-10 in 2010 (Addison and Brumpton, 2012). Carbonatereplaced devitrified vesicular impact glass shapes and tektites were then visible on the weathered surface. The Ocean Transgression Sequence is composed of ankerite grainstones identical to those of the Gunflint and probably represents a limited transgression millions of years prior to the deposition of the Rove Formation (P. Fralick, personal communication, 2026). - 21 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 in turn by a diabase sill. An iron-rich alteration profile, heavily replaced by secondary pyrite, lies ~ 1 m below the base of the debrisite and a few metres northeast of the main outcrop. weave through this spherule-rich material but on a much finer scale than at Hillcrest Park. Red-brown agate 3-8 cm thick lies on top of the spherule-rich layer. Laterally discontinuous vertical digitate projections extend down from the top and project up from the base of this agate layer. They are similar in shape and size to the agate stalactites in vugs at Hillcrest Park, except that in this case the spaces between the projections were subsequently infilled by more agate. The red-brown colour is similar to that of the iron-rich alteration profile overlying it but it is a less saturated hue. The basal SIL is a recessively weathering, locally sheared, clastic layer about 0.5 m thick containing crushed spherule clusters, some of which are aligned subvertically instead of in the usual subhorizontal position. Several sets of subhorizontal slickensides, whose striae are aligned at a 140º azimuth, are found at various levels within this layer. Postdepositional anastomosing chert has replaced much of this basal sheared layer, obliterating considerable structural detail. Non-ejecta features include centimetre to millimetre-sized angular chert clasts and angular, subrounded to round Gunflint Formation iron carbonate clasts plus two rounded crystalline rocks with prominent alteration rinds. The presence of clasts with weathering rinds reinforces the idea that Gunflint clasts lacking such rinds were freshly fractured by impactgenerated earthquakes before being incorporated into the debrisite. An iron-rich alteration profile on top of the spherule-rich layer, consisting of hematite has been largely replaced by secondary pyrite. Prominent deformed spherule clusters are locally present. The total thickness of all these ejectabearing layers is 3 m. The top of this iron-rich layer marks a return to carbonate deposition. The basal 10-15 cm of this 80-100 cm thick carbonate zone is unstratified and shows dark, angular, commonly rectangular, millimetre-centimetre-sized rip-up mudstone clasts and probable Gunflint Formations clasts. This is followed by millimetre- to centimetre-scale layered carbonate strata topped by a zone with a few poorly defined, laterally discontinuous beds containing centimetre-scale, angular carbonate clasts. The main body of the 2.2 m thick debrisite lies in sharp contact over the basal sheared clastic unit. It is so heavily replaced by recrystallized dolomite that any possible ejecta features are only seen as vaguely outlined shapes on weathered surfaces or in thin section. Almost all detail, including any vesicles in possible DVIG- [devitrified vesicular impact glass] shaped clasts, has been destroyed. Tektites and microtektites may be present, based upon shape and rare faint devitrification textures. A single, polycrystalline, rounded quartz grain shows faint planar features. Both angular and rounded millimetre-scale chert clasts are also present, but not common. A 5-20 cm thick undulating, dark brown, recessively weathering, spherule-cluster-rich layer appears as a groove across the cliff face at the top of the dolomite-replaced debrisite. This mass of spherule clusters is much more concentrated than seen at any other location or than is suggested by faint shapes in the main dolomite-replaced layer immediately beneath it. These concentrated clusters seem to be the residuum of a thicker layer. Plentiful, thin anastomosing post-depositional chert strands The carbonate then makes an abrupt transition to 10-15 cm of gray siltstone and is overtopped by 10-15 cm of black, rusty weathering shale characteristic of the Rove Formation. The black shale is interrupted by 5 cm of chert before returning to 0.9-1.2 m of black, rusty weathering shale which is overlain in turn by a diabase sill more than 8 m thick. The shale is less friable than typical lower Rove shale, probably the result of low-grade metamorphism induced by the overlying sill.” A 050˚-060˚-striking normal fault, perhaps related to Midcontinent Rift-related extension, has displaced Animikie rocks and diabase 4 to 5 m. Koroscil (2013) identified thrust faults, mainly expressed as small discrete bedding plane faults with few kinematic indicators or piercing points to quantify the displacement. Thrust faults within the SIL were - 22 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 identified by slickenlines or slickenfibres on fault plane surfaces. The faults can be traced along strike until they are either covered by overburden or cut by a prominent normal fault which displaces all units in the hanging wall down to the south several metres (ibid). Reid-Sharp (2016) described faults, related damage zones and calcite vein breccias along the highway ~2 km northeast of STOP 1-10. Normal faults that transect Gunflint Formation + Archean basement rocks, strike east-northeast and dip to the southeast, were also ascribed to extension during Midcontinent rifting. The past-producing Thunder Bay Silver Mine is situated between the highway and the Terry Fox access road. Discovered in 1866 by P. McKellar, it was mined underground until 1874 via four shallow (8 to 21 m deep) shafts (Ontario Mineral Inventory, https://www. geologyontario.mines.gov.on.ca/mineral-inventory/ MDI52A06NE00005). Mineralization occurs in calcite-quartz veins that are hosted in chert-carbonates (Gunflint Formation) and shales (Rove Formation). The host rocks strike 034/22 southeast in the vicinity of the vein but subhorizontally 30.5 m to the northwest beneath a diabase sill 12.2 m thick. A 3 m wide composite vein or stockwork system consisting of 2.5 cm wide quartz-carbonate veinlets lies within and parallel to a fault that also strikes 034/65 northwest. Ore was mined locally over the total length of 182.9 m. Native silver and acanthite occurred in pockets 7.6 - 45.7 cm thick by 1.8 -12.2 m in length, the silver being in leaves and grains irregularly distributed in a gangue of quartz, with some calcite, galena, sphalerite, and pyrite. A second vein of calcite occurs in a parallel fault 6.1 m southeast of the composite vein (ibid; Sergiades, 1968). When first opened, two orebodies were found, one next to the north or hanging-wall and one in the middle (Tanton, 1931). The ore was brought to a stamp mill at the mouth of the Current River, 4 km south of the mine (Figure 33). Production totaled an estimated $20 000 (Bowen, 1911), or approximately 15 000 ounces of silver. Figure 33. Stamp mill of the Thunder Bay Silver Mine at the mouth of the Current River, ca. 1880. Shegelski (1982; Figure 34) and later described in the context of impact-related brecciation by Addison et al. (2010) and Addison and Brumpton (2012, Figure 35): “A bedrock exposure, about 5 m by 15 m, in a private yard in Thunder Bay contains a spectacular debrisite exposure composed mainly of Gunflint chert-carbonate breccia and ejecta, primarily DVIG [devitrified vesicular impact glass], which is surrounded and partially replaced by blocky calcite cement. The debrisite remnant preserved here is 0-0.5 m thick and unconformably overlies stromatolites and chloritic grainstone of the uppermost Gunflint Formation. An iron-rich alteration zone exists approximately 30 cm below the erosive contact between the debrisite and the Gunflint bedrock. DVIG clasts are up to 2 cm across. Vesicles range from round to ovoid to nearly flat. Angular quartz and feldspar grains, chert shards, and chloritic granules are also present. Quartz grains with PDFs have not been found here.” The SIL is also exposed nearby at Hillcrest Park and along Banning Street. STOP 1-11: Sudbury Impact Layer, Markland and Hill Streets 334163E / 5366301N (n.b. Private Property, ask for permission to access. Be very careful not to step on any plants. No hammers are allowed.) Another spectacular debrisite breccia of the Sudbury Impact Layer is exposed at the corner of Markland and Hill streets. This outcrop was mapped in detail by - 23 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 34. Detailed map of the debrisite breccia outcrop at STOP 1-11 by Shegelski (1982) Figure 35. (from Addison and Brumpton, 2012) A – Gunflint Formation stromatolites exposed on a glacially truncated surface, STOP 1-11. While it is not recognizable in the photo, debrisite lies over stromatolites at upper right of the photo. B – Angular to slightly subangular clast-supported Gunflint Formation breccia with a finer DVIG-rich and calcite-rich matrix, all of which lies directly on Gunflint stromatolites, STOP 1-11. The angular clasts suggest a short travel distance from their point of origin. C – DVIG clasts within a recrystallized calcite matrix, STOP 1-11. The silicate devitrification product supports growth of a black lichen, whereas calcite prevents lichen growth. The vesicles are calcite infilled. D – Orange, weathered accretionary lapilli in a recrystallized carbonate matrix, Hillcrest Park. - 24 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Field Trip Stop Descriptions - Day Two Day Two comprises stops to a variety of locations south and west of Thunder Bay (Figure 36). This area is underlain by a variety of rocks that record almost three billion years of local geologic history, spanning from the Neoarchean (ca. 2.7 Ga) to the Paleoproterozoic (ca. 1.8 Ga) and Mesoproterozoic (ca. 1.4 and 1.1 Ga). Unconsolidated glacial and post-glacial deposits and features attest to a long-lived, Pleistocene glaciation record. All GPS coordinates are NAD83, UTM Zone 16. Figure 36. Generalized geology of the Thunder Bay area, showing Day Two field trip stop locations. Geology from Map M2232 (Carter et al., 1973). STOP 2-1: Mount McKay Lookout (Anemki Wajiw) 0331126E / 5357384N (n.b. admission via a gate operated by Fort William First Nation) Our first stop provides not only a panoramic view of Thunder Bay and surrounding area, but also stacked Logan sills which have produced the iconic mesa topography of Mount McKay and other similar mesas to the south and west in the Animikie-underlain Logan basin, collectively known as the Nor’westers. This location had previously been described by Cundari et al. (2012). Mount McKay is also known as Anemki Wajiw (“Thunder Mountain”) in Ojibwe. The summit, at 482 m ASL, is approximately 300 m higher than Lake Superior. The stop is centered on the lookout area (Figure 37), which represents the top of the lower sill at approximately 337 m ASL. The upper, ~60 m thick, columnar-jointed sill and adjacent, hornfelsed Figure 37. View of the top of Mount McKay, capped by the ~60 m-thick, upper Logan diabase sill. The Lookout level is underlain by the top of the lower sill. - 25 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Rove Formation wacke can be accessed by way of a hiking trail which leads to the summit. Although stacked sills have been encountered in drilling, few examples exist in surface exposures. As many at 14 sills were reported, for example, in a 705 m-deep drill hole in central Pardee Township by Dumont Nickel Inc. (Assessment Files, Thunder Bay South Resident Geologist’s District, Thunder Bay). al., 2007). Resampling of Zr-enriched, pegmatoidal, upper portion of the Logan Sill capping Mount McKay and re-analysis of baddeleyite and magmatic zircon yielded an age of 1106.3 + 2.0 Ma (Bleeker et al., 2020). This, and similar ages elsewhere, led Bleeker et al. (2020) to favour a relatively sharp onset of high-volume mafic-ultramafic magmatism in the Midcontinent Rift at ca. 1110 to 1106 Ma. The rugged topography (Figure 38) has produced extensive colluvial deposits and talus slopes. Unconsolidated, sandy lacustrine and fluviolacustrine deposits occur below the bedrock- and colluvium-predominated slopes. Abandoned shoreline escarpments and beach bars, visible between the lookout and Lake Superior, reflect higher post-glacial lake levels (Burwasser, 1977). Feldspar-phyric patches, common near upper chilled sill contacts, are present in an exposure of the upper, chilled contact of the lower sill along a path to the west of the clearing (Figure 39). A tentative age of 1114.7 ± 1.1 Ma was determined from a Logan sill on Mount McKay, using a limited selection of very small baddeleyite grains (Heaman et Figure 39. Polygonal jointing in upper chilled surface of lower diabase sill, STOP 2-1. Figure 38. Shaded relief LiDAR image of area south of Thunder Bay, showing topographic relief (i.e. gently southdipping mesas/cuestas) resulting from erosion-resistant mafic sills and, to a lesser extent, siliceous wackes in the Rove Formation (data from https://www.arcgis.com/apps/ mapviewer/index.html?url=https://ws.geoservices.lrc.gov. on.ca/arcgis5/rest/services/Elevation/FRI_DTM_SPL/ ImageServer). STOPS 2-1 and 2-2 are also shown. Logan sills generally consist of fine- to coarsegrained, ophitic to intergranular, quartz tholeiitic diabase/gabbro (Smith and Sutcliffe, 1987; Geul, 1970, 1973). Coarse-grained, intergranular gabbro, locally rich in granophyric mesostasis, is common in the interior of the thicker sills. Geochemical data from sampling of the upper and lower sills by Hart and Magyarosi (2004) are provided in Figure 40. These sills represent the northernmost known extent of Logan diabase sills near Thunder Bay. Nipigon diabase sills occur within the city and extend northward to the Nipigon Embayment. - 26 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 40. Primitive mantle-normalized trace element plots for upper and lower sills at Mount McKay with Nipigon sill sample for comparison. Data from Hart and Magyarosi (2004) and Hollings et al. (2011). Normalizing values from Sun and McDonough (1989). Nipigon sills are characterized by generally lower incompatible trace element abundances, lower TiO2 content, and a distinct negative Nb–Ta anomaly. They typically have lower Gd/Ybcn ratios compared to Logan sills. Logan Sills are characterized by higher TiO2 and higher Gd/Ybcn ratios, indicating a greater degree of heavy rare earth element fractionation (cf. Hollings et al., 2010). Riverdale sills (STOP 2-2) are geochemically distinguishable from both Nipigon and Logan diabase (Figure 41). STOP 2-2: Riverdale Quarry 322418E / 5355233N (n.b. Private property; permission is required to access. Caution advised on site due to slip and fall risks associated with steep slopes and vertical rock faces) This former shale quarry exposes a ~20 m-thick section of the lower Rove Formation, overlain by a ~12 m-thick Riverdale, columnar-jointed, gabbronorite sill related to Midcontinent Rift magmatism (Figure 41). This location was previously described by Cundari et al. (2012): “Sampling by Smyk and Hollings (2007) identified this as a Riverdale gabbronorite Figure 41. Discrimination diagrams for mafic and ultramafic intrusions near Thunder Bay (from Cundari et al. 2012). Data are from Hollings et al. (2007a) and Puchalski (2010). Normalizing values from Sun and McDonough (1989). - 27 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 lie within the major element abundances; olivine gabbros are lower in SiO2 and elevated in MgO, Cr, Co, and Ni compared to the gabbronorite samples. The sill does not display any evidence for Figure 42. Riverdale gabbronorite sill capping section of Rove Formation clastic sedimentary rocks, Riverdale Quarry. (Photo taken in 2008. Arrow points to geologist for scale.) sill in Rove Formation shale, wacke and minor tuffaceous units. Subsequent detailed petrographic and geochemical analyses were carried out by Puchalski (2010). Samples were taken through stratigraphy at the quarry to investigate composition and contamination, as well as to test whether the sill had undergone differentiation. The following section provides a concise summary of those findings. differentiation as shown by the erratic trends of MgO, SiO2, TiO2, Cr and Ni through stratigraphy. An olivine gabbro in the center of the sill displays elevated MgO, Cr, and Ni values as well as a lower abundance of silica when compared to the surrounding samples. This is likely the result of a slightly more primitive magma intruding the center of the sill. The lack of chilled margins between the olivine gabbro and the gabbronorite suggest that the sill had not fully crystallized when the second pulse intruded. A sample of a 60 cm wide north-trending diabase dyke which intrudes the sill near the western end of the quarry is geochemically comparable to the surrounding Logan sills. Contamination by the Rove shale is evident in samples taken from close to the contact (<1 m above the contact). These samples display higher SiO2 values as well as lower Nb/Nb* and Gd/Ybn values than the rest of the unit. As the Rove shale displays significantly lower Nb/Nb* and Gd/Ybn values than that of the surrounding gabbronorite. The Rove shale is the likely source of this contamination signature. Two different pulses of magma are recognized within the Riverdale sill, based on contamination signatures denoted by negative niobium anomalies. The less-contaminated samples are typically found towards the core of the intrusion with rocks above and below displaying a greater degree of contamination. Samples taken within 60 cm of a shale xenolith do not display a distinct negative niobium anomaly. This shows that the source of contamination responsible for the negative niobium anomaly is not the Rove shale but is likely a crustal component from depth. εNd (T=1100 Ma) values of -1.6 to -1.9 for the Riverdale Sill are consistent with this model (Smyk and Hollings, 2009). The mafic intrusive rocks within the quarry are dominantly classified as gabbronorites with olivine gabbro present towards the center of the sill. The gabbronorites are generally fine-grained with plagioclase occurring as subhedral laths. Orthopyroxene is present in greater abundance than clinopyroxene, occurring as anhedral to subhedral crystals. Varying degrees of alteration are manifested as sericitization of plagioclase and chloritization of pyroxene. The olivine gabbro is texturally similar to the gabbronorite, albeit with a higher modal percentage of fine-grained, anhedral to euhedral olivine. In most samples, olivine is replaced by serpentine, producing secondary quartz and calcite, as well as minor magnetite. Alteration is significantly greater in the narrow, chilled margin at the contact. Pyrite occurs throughout the unit; minor chalcopyrite has also been noted. Sampling for whole rock major and trace element geochemistry was undertaken by Puchalski (2010) throughout the 10 m exposure at 1-m intervals. Olivine gabbro samples display broadly similar trace element characteristics to those of the gabbronorite samples. Differences Although the Riverdale sill is located near Logan sills, it remains petrographically and geochemically distinct from them [Figure 42]. Geochemical discrimination based on La/Smn (LREE) vs. Gd/Ybn (HREE) shows characteristics - 28 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 similar to those for the ultramafic units of the Nipigon Embayment (e.g., Disraeli, Kitto, Hele and Seagull), closely resembling the mafic to ultramafic Jackfish sill. The Jackfish sill is finergrained and displays a higher modal abundance of olivine than the Nipigon sills surrounding it (Hollings et al., 2007a). This suggests that the Riverdale sill may be genetically related to the ultramafic and mafic to ultramafic units of the Nipigon Embayment. This is consistent with the reversed polarity of the Riverdale sill (Hollings et al., 2010).” A number of features are visible at or near the exposed upper and lower sill contacts (Figure 43). Calcite-filled vesicles define a crudely developed Figure 43. Lower gabbronorite sill contact. (A) Delamination of Rove shales by injection of gabbronorite sill magma; (B) Chilled margin of gabbronorite sill against hornfelsed Rove shale. layer/joint filling(?) in medium-grained gabbronorite, ~1 m above the lower sill contact. Stoping and delamination of Rove shales is also evident here. Thin, parallel chilled margins, perhaps representing multiple influxes of magma, occur above the lower sill contact. A narrow (75 cm) diabase dyke with Logan sill-like geochemistry intrudes the Riverdale gabbronorite sill near the western end of the quarry exposure (Figure 44). Glacial striae are visible on exposed outcrop surfaces at 060˚ and 075˚. STOP 2-3: Sudbury Impact Layer, Highway 588 0307539E / 5357977N Figure 44. Narrow diabase dyke with Logan sill-like geochemistry intruding Riverdale gabbronorite sill, Riverdale Quarry. Scale card straddles the eastern dyke contact. This stop, while having lost much of the best exposure of the Sudbury Impact Layer (SIL) due to ongoing highway construction, still provides an opportunity to view some of the features associated with the SIL in the affected ankeritic, Gunflint Formation chertcarbonate rocks. The SIL was also intersected a few metres below surface in a shallow drill hole, collared in Rove Formation shales in an abandoned quarry approximately 300 m south-southwest of STOP 2-3. This location was previously described by Addison and Brumpton (2012): - 29 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 “When first observed in 2000, the Hwy 588 outcrop was a bedrock exposure in the ditch on the northwest side of the highway, 2.4 km southwest of the hamlet of Stanley. It was a glacially polished and striated surface showing erosively truncated stromatolites up to 0.5 m diameter, some of which were surrounded by accretionary lapilli 3-25 mm in diameter [Figure 45]. Ankeritic grainstone and chloritic grainstone surrounded other stromatolites. This exposure was subsequently blasted to deepen the ditch and the blasted rock now lines the ditch slopes, giving a highly fragmented cross-section and plan view of the exposure. Since then, we have exposed bedrock in the ditch about 50 m southwest of the first exposure. It shows a glacially striated surface of exposed stromatolites and shattered, but insitu black chert with an ankeritic grainstone filling in the cracks. The chert is assumed to have fractured during the compressional stage of impact-triggered earthquake waves with the fractures then opening during the dilational wave phase. Fine granular material then fell into the openings, preventing them from closing and subsequently the material was lithified. material show a variety of ejecta features, the most obvious being accretionary lapilli which have yielded quartz and feldspar grains showing planar deformation features (PDFs) and planar fractures. Planar features have not been found in larger subrounded and angular quartz and feldspar grains contained within the debrisite generally as opposed to within accretionary lapilli. This is the only site in which DVIG [devitrified vesicular impact glass], is not the most obvious ejecta feature within the debrisite. In fact, no DVIG has been observed, however carbonate and silica replaced clusters of spherules are present. Non-ejecta features include subrounded to round chert grains in carbonate cement, subcentimetre stromatolite fragments and mudstone and shale rip-ups. Chloritic, blotchy, black Gunflint Formation granules, similar in shape and size to microtektites, are present within the carbonate cement. Carbonate-replaced microtektite shapes are present but since they lack residual internal structure, it is impossible to determine if they were microtektites or carbonatereplaced Gunflint chlorite granules.” Thin sections prepared from the blasted STOP 2-3: Kakabeka Falls Provincial Park 0305738E / 5364400N; 0305178E / 5364663 (n.b. Entry/parking fee is required in Kakabeka Falls Provincial Park. Sample collecting and hammers are NOT permitted.) Two stops at Kakabeka Falls provide an opportunity to see both a thick section of Gunflint Formation rocks exposed in the gorge of the Kaministiquia River, and the basal, stromatolite-bearing units of the Gunflint unconformably overlying Neoarchean granitoid basement. This location was previously described by Pufahl et al. (2000) and Smyk (2012). Figure 45. Accretionary and armored lapilli draped unconformably over a stromatolite, composed of silicified carbonate, which was abraded to its present configuration likely by a base surge immediately preceding the deposition of the lapilli; STOP 2-3, polished surface. The gray component is primarily fine-grained, angular, fractured carbonate clasts whose individual crystals are usually <10 μm. These clasts are typically <50 μm but they may be as large as 500 μm. Quartz and feldspar grains are a minor component among the carbonate clasts within the lapilli. (caption modified from Addison and Brumpton, 2012). The park is dominated by a single, spectacular feature, Kakabeka Falls, which drops 39 m over sheer cliffs in Gunflint Formation sedimentary rocks (Figure 46). Kakabeka is an aboriginal word meaning “steep cliffs”. The age of the river gorge below the falls is still debated. If none of it existed prior to the glacial Lake Beaver Bay stage, then it is less than ca. 9700 years old. The portage around the falls contains artifacts ranging from the Paleoindian to the historic (fur trade) periods. - 30 - The falls owes its existence to the thin chert- �Proceedings of the 72nd ILSG Annual Meeting - Part 2 are often attributed to deposition in intertidal or lagoonal subtidal environments (Pufahl et al., 2000). Figure 46. Kakabeka Falls and gorge, cut into flat-lying Gunflint Formation shales. Photo from https://hikebiketravel. com/a-trip-to-kakabeka-falls-near-thunder-bay/. carbonate bed which forms a resistant cap rock to the softer underlying shales. The river gorge is composed of a sequence of volcaniclastic shales (lessresistant, darker units) and tuffs (more-resistant, lighter coloured units). This sequence represents the major volcaniclastic horizon in the upper Gunflint Formation that is traceable to the south through the Mesabi Range. Note that shale is the predominant lithology in the Kaministiquia sections and this is, in fact, typical for the Gunflint Formation in general throughout the Thunder Bay region. Samples of lapilli tuff and reworked tuffs from the middle of the Gunflint Formation, collected by Fralick et al. (2002) at Kakabeka Falls yielded a euhedral zircon population with a U-Pb age of 1878.3 ± 1.3 Ma, believed to be nearly synchronous with the depositional age. The outcrop on the northern edge of the parking lot contains layers of banded/ribbon chert-carbonate within black, fissile shale. The alternating, dark grey chert and brown siderite-ankerite layers display slump and soft-sediment deformation features. Microscopic examination of banded chert-carbonates reveals delicate lamination in the chert which resembles the “ribbon texture” of algal mats. The interlayered carbonate bands contain complex, microspherical structures which likely resulted by nucleation from a gel state. Local thick beds of carbonaceous siderite (2-3 wt% carbon) form carbonate iron formation; contemporaneous deposition of carbon and carbonate suggests biological activity during iron deposition. Studies of the Gunflint Formation have described this type of sediment as forming in a deep, quiet water environment. However, similar carbonate sequences The rapids visible north of the highway bridge are formed by Archean granitoids. The slow-water area to the south is underlain by the Gunflint Formation. The basal conglomerate (Kakabeka member) is patchily preserved on Archean basement here. Silicified stromatolites are developed on the conglomerate or directly on the basement. This is the location from which samples collected from silicified stromatolites in the 1950’s yielded the first documented Gunflint cyanobacteria (Tyler and Barghoorn, 1954). The rock cut on Highway 590 immediately south of the intersection with Highway 11-17, west of the Kaministiquia River bridge, expose cherty carbonates at the base of the Gunflint Formation where it rests unconformably over Neorchean granitoid basement. Large-form stromatolites are developed at the unconformity (Figures 47 and 48). The stromatolitic, ribbon carbonates are abruptly overlain by a grainstone succession. Black anthraxolite veinlets and void fillings occur with vein quartz in the chert-carbonate rocks. Anthraxolite and pyrobitumen in the Gunflint Formation (Figure 49) has been noted and studied numerous researchers, including Tanton (1931), Ellesworth (1934), Goodwin (1956), Kwiatkowski (1975), Barghoorn et al. (1977), Hayatsu et al (1983), Mancuso et al. (1989), Rutter (2014), Rasmussen and Muhling (2019), and Rasmussen et al. (2021). Figure 47. Colloform stromatolite (left of hammer) in Ferich carbonate grainstones, Highway 590 exposure, STOP 2-4. The stromatolite was situated approximately 40 cm above the unconformity with Neoarchean basement granitoid rocks. Unfortunately, the stromatolite spalled from the outcrop face ca. 2016. Coin is 2.5 cm in diameter. - 31 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 49. Void-filling, conchoidal anthraxolite and quartz in sideritic Gunflint grainstone, Highway 590 exposure, STOP 2-4. of Superior lobe drift. The exposed sequence consists of approximately 6 to 7 m of well sorted, steeply-dipping sand and gravel of Superior provenance overlain by 2 to 3 m of silty Superior lobe till [Figure 50]. Clasts in the lower glaciofluvial unit consist primarily of Proterozoic metasedimentary rocks. Numerous cobbles and boulders belonging to the Gunflint Formation and Sibley Group are recognizable. Foreset beds dip steeply to the north and are likely of deltaic origin. The delta was probably built proglacially into an early phase of glacial Lake Kaministikwia. The feature therefore represents a location at which the advancing Superior Lobe stalled prior to reaching its maximum position at the Marks Moraine. Figure 48. Detailed view of the large, silicified, colloform stromatolite in Figure 47. Rasmussen et al. (2021) suggested that stromatolitic, black Gunflint cherts were saturated in syn-sedimentary oil. Thermally altered oil (pyrobitumen) occurs in the stromatolites and intercolumn sediments, fills pores and fractures, and coats detrital and diagenetic grain surfaces. Hayatsu et al. (1983) described two very distinct macromolecular materials in the Gunflint anthraxolite that suggested that the Thunder Bay area was once covered by Cretaceous or Jurassic marine sediments, similar to those documented in the Mesabi range of Minnesota. The delta is actually located within only 3 km of the Superior lobe limit and occurs at an elevation of about 375 m asl, 85 m below the maximum elevation of Lake Kaministikwia. The delta was STOP 2-5: Briggs Drive Gravel Pit 0304790E / 5369390N (n.b. Private Property, contact Township of Conmee for access permission) This stop not only highlights some interesting glacial sediments, but also provides an opportunity to examine large boulders of a variety of local rock types that have been transported by glacial ice and meltwater. This location was described by Bajc (2000): “At this stop, we will be looking at a section Figure 50. View, looking west, of steeply dipping, gravelly foreset beds of a delta constructed along the margin of the advancing Superior lobe, Briggs Drive gravel pit, STOP 2-5. Silty Superior lobe till caps the sequence. - 32 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 overridden by the Superior lobe resulting in the truncation of the foreset unit and removal of the topset beds. Several metres of silty, Superior Lobe subglacial till was deposited on top of the sands and gravels. “seven lenticular masses of brecciated, banded iron formation, in which pyrite has replaced a considerable part of the rock” (Carter 1990). The largest of these masses has a maximum width of 23 m and is 244 m long. Other discoveries include a 21-m wide body of pyrite containing magnetite and pyrrhotite and a 9 m wide by 15 m long zone of magnetite-pyrite-jasper ironstone. It is possible that the boulders found within the gravel pit [Figure 51] were derived from this area and Of particular significance is the occurrence of large angular to rounded boulders on the pit floor. The boulders were extracted from the lower glaciofluvial unit and, in some cases, do not appear to have been transported very far. Some of the larger boulders measure several metres across and still display striated surfaces. The boulders are derived from both Archean and Proterozoic source rocks. Several boulders of sulphidized iron formation and massive pyrite of Archean age were discovered in the boulder piles. One of the boulders measured over 1 m in diameter and consisted of massive pyrite and magnetite with 10 to 15% sphalerite disseminated in pyriterich sections. Sphalerite was also concentrated along fractures and adjacent to quartz veinlets throughout the rock. Two samples from the pyriterich zones returned values of: 1) 5.13% Zn, 18 ppm Cu, 19 ppm Pb, 260 ppb Au and 0.5 ppm Ag; and 2) 2.85% Zn, 16 ppm Cu, 20 ppm Pb, 245 ppb Au and 0.5 ppm Ag. A sample from the magnetiterich zone returned values of 850 ppm Zn, 25 ppm Cu, 5 ppm Pb, 25 ppb Au and <0.2 ppm Ag. A second sulphidized iron formation boulder measuring approximately 0.5 m in diameter and consisting almost exclusively of pyrite, returned values of 140 ppm Zn, 8 ppm Cu, 11 ppm Pb, 710 ppb Au and <0.2 ppm Ag. There are two possible source areas for the boulders. Superior lobe striae in the immediate vicinity of the pit are oriented at 320 to 330° Az. If the boulders were eroded and transported by Superior ice, then there is a 5 km window towards the southeast from which they could have been derived. Proterozoic metasedimentary rocks outcrop beyond the 5 km limit. Alternatively, the boulders could have initially been eroded by northern ice from a source to the north-northeast of the pit then remobilized by the Superior lobe. Exploration work during the early 1900s along the lower reaches of Brule Creek, 4 to 5 km northnortheast of the gravel pit, by B.L. Morrison, the Davis Sulphur Company and General Chemical Company resulted in the discovery of Figure 51. Pile of oversized boulders of a variety of local Archean and Proterozoic rock types, Briggs Drive gravel pit, STOP 2-5. Reddish silty Superior lobe till is visible at the top of the pit wall. Photo taken ca. 1999. that sphalerite was not recognized in the rock. It is not yet clear whether the sulphides indicate proximity to a VMS style zone of mineralization. Further work is required to assess the mineral potential of this area.” STOP 2-6: Temiskaming sedimentary rocks, Finmark 293709E / 5383903N This stop, having long been a “must-see” for local geology students, has benefited from new exposures - 33 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 on the south side of Highway 11-17 that were created during highway expansion and ballast quarry development ca. 2019. The outcrops on both sides of the highway expose excellent examples of clastic Neoarchean “Temiskaming-type” metasedimentary rocks (ca. 2690-2695 Ma) of the Shebandowan assemblage that still display many primary sedimentary features that provide clues as to the depositional environment. The Timiskaming-type successions of the SGB are interpreted to have been deposited in subaerial to shallow marine environments (Shegelski, 1980). Highway 11-17 by Koebernick and Fralick (1995) and Koebernick (1996) documented sedimentary structures and bed sequences consistent with shallow water, coastal sedimentation in three major depositional environments: tidal strandline, the shoreface, and the offshore (e.g. Figure 52). Koebernick (1996) noted: “The three environments and associated sub-environments record processes reflective of differing current activity which controlled and The ballast quarry immediately to the south has been developed in mafic, Neoarchean metavolcanic rocks of the Greenwater assemblage (ca. 2720 Ma) which presumably underlie the clastic rocks unconformably or are in fault contact with them. Parker (1980) noted that reversals of top directions and the presence of both easterly and westerly plunging minor folds, suggest that one or more episodes of folding have occurred. Detrital zircon geochronology by Corfu and Stott (1998) confirmed that the metasedimentary rocks in the Finmark area (<2691+3 Ma) and in the southern part of Adrian Township (<2700+4 Ma) are younger than the Greenwater assemblage. Because of the remarkable preservation of primary sedimentary features, this stop has been the focus of study for many years, including theses by Parker (1980) and Koebernick (1996). The metasedimentary sequence here comprises interbedded sandstonesiltstone-mudstone sequences which alternate with thick deposits of cross-stratified sandstones (Parker, 1980). The interlayered sequences contain many of the primary sedimentary structures characteristic of tidal flat deposits, such as flaser bedding, lenticular bedding, herringbone cross-bedding, mud cracks, mud drapes, and bipolar paleocurrent indicators. Parker (1980) noted that the clastic sedimentary rocks are composed of feldspar, rock fragments, quartz, and some mafic minerals. Modal analysis revealed that most of the sandstones in the area are arkosic arenites. Lithic fragments are felsic to intermediate and predominantly calc-alkalic volcanics, with lesser amounts of other igneous grains and sedimentary rock fragments. This led Parker (1980) to suggest that the clastic rocks probably represent immature detritus from proximal volcanic centers. A detailed study of the “Temiskaming-type” clastic rocks at this location and along this section of Figure 52. Stacked, trough cross-bedded sandstone beds with tangential foreset laminae, south side of Highway 1117, STOP 2-6. Ripples are preserved on bedding surfaces (lower photo). - 34 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 53. Dark alteration holes defining herringbone cross bedding in sandstone, north side of Highway 11-17, STOP 2-6. influenced deposition. The tidal environment was dominated by bidirectional tidal currents [Figure 53]. Deposition In the shoreface was predominated by unidirectional wave-produced currents which overprinted prevailing tidal current activity, in the distal portions of the shoreface environment though, deposition was once again controlled by tidal currents. In the offshore, deposition was controlled by storm currents which generated distinctive beds of hummocky cross-stratification. The tidal environment is composed of many sedimentary structures similar to those present in Phanerozoic and present-day tidal sequences. In the tidal flat sub-environment, vertical sequences of flaser, lenticular, wavy and coarsely interlayered bedding reflect current velocity fluctuations Intimately tied to spring - neap tidal cycles. The tidal channel sub-environment lacks many of the features characteristic of tidal channels described in the literature; such as extensive point bar development. Instead, the tidal channels of the study area appear to represent sequences deposited in relatively straight channels. Migration of sand waves and dune fields deposited the cross-stratified lithofacies of the shoreface environment. Similar to a highenergy, non-barred coastline, the proximal portion of the shoreface lacks any evidence of beach development. Instead, the shoreface records a rapid and discontinuous transition from the tidal strandline environment. Hummocky cross-stratification (HCS) [Figure 54], parallel- Figure 54. Hummocky cross-stratification, which is only formed and preserved by storm waves in depths between fair weather wave base and storm wave base. laminated and massive sandstone beds as well as siltstone and mudstone beds typify the offshore environment [Figure 55]. The HCS differs greatly in thickness and internal structure from HCS described in the literature. The HCS in the study area reflects restricted and/or variable sediment supply and flow conditions. A paleotidal range was determined from the sediments of the tidal environment. The range indicated a mesotidal Figure 55. Thinning- and fining-upward sequence, showing transition from medium-bedded sandstones to a mudstone/ siltstone-dominated package with thin sandstone interbeds, south side of highway, STOP 2-6. This likely represents deepening of the water, going from nearshore coarse-grained sands moved around on the bottom as dunes by fair-weather waves and currents, to deeper water deposits below fairweather wave base, representing tempestites (i.e. hummocky cross-stratified storm deposits and graded beds formed below storm wave base by the same geostrophic flows that formed hummocky cross-stratification in shallower water; P. Fralick, personal communication, 2026). - 35 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 56. Stratigraphic column of outcrops on north side of Highway 11-17, STOP 2-6, showing primary sedimentary features and paleocurrent measurements (P. Fralick, personal communication, 2026). environment and is comparable to Precambrian tidal ranges reported in the literature. Tidal rhythmites, present on the tidal flats, suggest a length of 26 days for the Neoarchean lunar month. Currents which deposited the tidal rhythmites produced both semi-diurnal and diurnal sediment sequences [Figure 56].” reversals. Evidence of shearing and brittle deformation, including quartz-carbonate veining, can also be observed, especially in the easternmost portions of the outcrop exposure on the south side of the highway (Figures 57 and 58). As noted above, in spite of the remarkable preservation of sedimentary structures, the Shebandowan assemblage sedimentary rocks in this area have experienced tectonic deformation and display features that include minor folds and younging Bedding-cleavage relationships indicative of folding are visible in the outcrops at this location. Bedding orientations vary from approximately 325/70 northeast on the north side of the highway to 110/70 south on the south side. The cleavage-bedding relationship is most easily observed in the thin-bedded mudstones and siltstones south of the highway, where the cleavage - 36 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 57. Quartz-carbonate veining in altered and deformed Figure 58. Photo illustrating small-scale folds, shears and rocks at the east end of the outcrop area on the south side of sigmoidal tension fractures in thin-bedded siltstone and mudstone at STOP 2-6. Highway 11-17 at STOP 2-6. orientation is approximately 085/85 south. The change in bedding orientation relative to cleavage, when combined with northward-younging indicators (e.g., graded bedding), indicate the probable presence of an anticline axis a short distance to the north. This interpretation is consistent with previous geological mapping completed by Carter (1985). The structures observed here may have developed during the same tectonic events that gave rise to orogenic gold mineralization at the nearby Eureka Gold Deposit, which is currently being explored by Delta Resources Limited. Eureka is located approximately 4 km to the west-northwest of here, and the deposit occurs within a structural corridor known as the “Shebandowan structural zone.” Gold mineralization at Eureka also has a close spatial association with the unconformity between the Greenwater and Shebandowan assemblages. Delta Resources has outlined the Eureka Gold Deposit over a 2.5-kilometre strike length, and to a vertical depth of 300 metres. Mineralization occurs over true widths ranging from 10 to 100 metres, and the deposit remains open in all directions. Gold is hosted by multiple generations of quartzankerite-pyrite veinlets that generally range from 1 mm to 10 cm wide and cross-cut multiple lithologies. Wider quartz veins up to 4.5 metres wide, and goldbearing silica-flooding zones are also found within the deposit. Host rock alteration is characterized by intense, texture-destructive ankeritization, silicification, albitization and sericitization combined with trace to 2% disseminated pyrite and trace arsenopyrite. The altered rocks typically contain anomalous gold. Feldspar-phyric monzonite to diorite dikes also have a close spatial association with the mineralization and are locally altered (https://www.deltaresources.ca/ delta-1-gold-project/). STOP 2-7: Pillowed Basalt, Mud Lake 315029E / 5376770N No trip would be complete without pillowed basalt! These roadside exposures along Highway 102 near Mud Lake display tholeiitic, mafic to intermediate volcanic Figure 59. Pillowed basalt flow, STOP 2-7, showing wellpreserved, close-packed pillows and hyaloclastite-filled inter-pillow spaces. - 37 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 rocks of the Neoarchean Greenwater assemblage, situated near the Quetico–Wawa subprovince boundary (Brown, 1995; Brown and Fogal, 1995). In this area, the degree of pillow preservation varies considerably. Figure 60. Close-up of pillow, STOP 2-7, showing contact of chilled upper selvage (large dashed line). However, well-formed, close-packed pillows, ranging from 10 by 15 cm to 30 by 60 cm in size, are locally preserved (Figure 59). Where discernible, younging directions within this unit are consistently to the north. Carbonate-filled amygdules, generally less than or equal to 1 mm in size and constituting up to 10% of the pillows by volume, are commonly present, radiating outward from the core of the pillows (ibid; Figure 60). North-younging, pillowed flows exposed at STOP 2-7 display well-preserved primary features, including autoclastic breccias (e.g. pillow breccia, inter-pillow hyaloclastite; Figure 61), close packing and pillow cusps, and calcite-filled amygdules. Larger, ovoid amygdules occur sparingly in the cores of pillows, while smaller, more numerous, pipe-like amygdules tend to be concentrated near pillow selvages. Pillows typically range between 25 cm and ~1m in size. The Mud Lake Cu-Zn occurrence (Ontario Mineral Inventory, https://www.geologyontario.mines.gov. on.ca/mineral-inventory/MDI000000002310) can be observed in a roadside outcrop located a few hundred metres northwest of STOP 2-7 along the highway. Pyrite, minor chalcopyrite and rare sphalerite are finely disseminated throughout, and adjacent to, a sericitized northeast-trending zone of shearing hosted within chemical metasedimentary rocks interbedded with felsic and intermediate pyroclastic metavolcanic rocks (Brown, 1995; Brown and Fogal, 1995). The mineralization was first uncovered during construction along Highway 102 in the mid-1970s. A grab sample collected in 1975 by staff of the Resident Geologist’s office, Thunder Bay, yielded values of 0.24% Cu, 0.87% Zn, 0.12 ounces Ag per ton and 0.005 ounces Au per ton (Fenwick and Scott, 1976). The felsic metavolcanic rock unit adjacent to the copper- and zinc-mineralized horizon yielded a U-Pb age of 2718+3 Ma (Corfu and Stott, 1998). A magnetic lamprophyre dyke, < 2m wide, crosscuts the pillowed flows at 065˚-080˚ and dips steeply north. Some of these late Neoarchean intrusions in this area were classified as kersantites (i.e. calc-alkaline, biotiteplagioclase-bearing lamprophyre) by Brown (1995). ACKNOWLEDGEMENTS Figure 61. Isolated-pillow breccia, STOP 2-7, showing amoeboid to angular pillow fragments in hyaloclastite-rich matrix. The authors would like to acknowledge the support and guidance of many former and present colleagues at the Ontario Geological Survey, Lakehead University and the Geological Survey Canada over the past several decades. This field guide has benefitted greatly from the comments, information and suggestions provided by Dr. Phil Fralick (Lakehead University), Riku Metsaranta (Ontario Geological Survey) and Dr. Wouter Bleeker (Geological Survey of Canada). Pete - 38 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Hollings assembled the final manuscript. We would also like to thank property owners who have provided permission to access several sites for the purposes of this field trip. 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Fort William and Port Arthur, and Thunder Cape map-areas, Thunder Bay District, Ontario; Geological Survey of Canada, Memoir 167, 222p. Tyler, S.A. and Barghoorn, E.S., 1954. Occurrence of structurally preserved plants in Precambrian rocks of the Canadian Shield; Science v. 199, p.606-608. Williams, H.R. 1991. Quetico Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, pt.1, p.383-403. Williams, H.R., Stott, G.M., Heather, K.B., Muir, T.L. and Sage, R.P. 1991. Wawa Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, p.485-539. Yip, C.I., 2016. Sedimentology and geochemistry of regressive and transgressive surfaces in the Gunflint Formation, northwestern Ontario; unpublished M.Sc. thesis, Lakehead University, Thunder Bay ON, 321 p. Zaniewski, K., Phillips, B. and Dean, F. 2020. Northwestern Ontario: The Thunder Bay region; February 2020; DOI:10.1007/978-3-030-35137-3_6; In book: Landscapes and Landforms of Eastern Canada, pp.159-177. Smyk, M.C., Hollings P. and Heaman, L.M. 2006. Preliminary investigations of the petrology, geochemistry and geochronology of the St. Ignace Island Complex, Midcontinent Rift, northern Lake Superior, Ontario; Institute on Lake Superior Geology, 52nd Annual Meeting, Sault Ste. Marie, ON, Program with Abstracts, v. 52, p.61-62. - 43 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Trip 2 - Geology of the Quetico Subprovince and Shebandowan greenstone belt north of Thunder Bay Riku Metsaranta and Gaetan Launay Earth Resources and Geoscience Mapping Section, Ontario Geological Survey, Sudbury, Ontario, P3E 6B5 Introduction This field trip examines the geology of the southern Quetico Subprovince (QS) and its tectonically intercalated contact with the Shebandowan greenstone belt (SGB) north and west of the City of Thunder Bay. Much of the content of this guidebook is informed by a multiyear, 1:50 000 scale bedrock mapping project that is being carried out in the area by the Ontario Geological Survey. The area encompassed by this guidebook represents the southern half of the multiyear bedrock mapping project area (see Figure 1). At the time of this field trip, a new bedrock geology map of the southern half of the project area and associated data are in preparation. Fieldwork on the northern half of the larger project area should be completed during the summer of 2026. In total, the new mapping will cover an area of approximately 4200 km2 of which approximately 70% had never been mapped at the 1:50 000 scale prior to this work. Some of the results of this bedrock mapping are summarized in interim publications (Metsaranta 2015; Metsaranta and Walker 2019; Metsaranta and Hamilton 2020, Metsaranta and Kamo 2021, Metsaranta 2022, Launay and Metsaranta 2023, Launay and Metsaranta 2024). The field trip will focus on the Archean geology of the area depicted on Figure 1. Although this is a oneday field trip, we have included 15 stops dispersed over a large area. We will not be able to visit all stops in one day. The order of the stops is organized from west to east in the SGB, followed by a south to north traverse along Highway 527 across the QS. Stops are labelled as “Optional” or “Planned”. “Planned” outcrops are the stops we will endeavour to visit during the field trip. Optional stops are included to put into perspective many of the “Planned” stops. As they are easily accessible, participants can visit these “Optional” outcrops on their own. As we are attempting to visit a high number of outcrops over a large area in one day, time spent on each outcrop may be limited. UTM coordinates used throughout the guidebook are NAD 83 Zone 16. Background Regional Geological Context The Quetico Subprovince is a vast geological entity that extends, at minimum, from central Minnesota to western Quebec. In “subprovince-style” subdivisions of the Superior Province (e.g. Card and Cielieski 1986, Williams 1991) the QS is bounded to the north by the Wabigoon Subprovince and to the south by the Wawa Subprovince. In more recent subdivisions (e.g. Percival et al. 2006, 2012; Stott et al. 2010) of the Superior Province into “terranes” and “domains” the Quetico Subprovince is commonly referred to as the Quetico basin or Quetico terrane and it is bounded to the south by the Wawa-Abitibi terrane and to the north by the Western and Eastern Wabigoon terranes and the Marmion terrane. In this guidebook, we will refer to the “Quetico” as the Quetico Subprovince (QS) to avoid any interpretive tectonic implications. Similarly, rather than discussing subprovinces or terranes bounding the QS to the south, we will simply refer to the Shebandowan greenstone belt (SGB). The detailed geology of the QS (as a whole) is somewhat poorly understood. Systematic OGS mapping of large portions of the QS has not been carried out previously at 1:50 000 or 1:20 000 scale. Consequently, accurate bedrock geology maps of much of the QS do not exist, nor do large scale regional geochronology or geochemistry datasets that are tied to geological mapping. General regional geological syntheses of the QS are provided by Percival (1989) and Williams (1991). Additional synoptic descriptions of the QS are included in Percival et al. (2006, 2012) and these include a summary of existing geochronological constraints. Additional influential studies on the metamorphic history of the QS include Pan, Fleet and Heaman (1996); Valli et al. (2004) and a recent PhD study by Rehm (2025) among others. The QS has historically been interpreted to have been deposited in a fore-arc setting (e.g., Percival 1989, Williams 1991). In reality, the tectonic setting is likely more complex. Geochronology indicates most of the QS was deposited after circa 2700 Ma. However, - 44 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 1 (A) Total magnetic field image of the map area (Ontario Geological Survey 2017), underlain with lidar imagery (Ministry of Natural Resources and Forestry 2023). (B) Geological map (modified from Launay and Metsaranta, 2023) of the field trip area showing the location of stops presented in this guidebook. Note that Stops 13-15 are just to the north of the area portrayed by this map. Geological abbreviations: BLI, Barnum Lake intrusion; CCF, Crayfish Creek fault; CLI; HLG, Hilma Lake granite; HLI, Hadwen Lake intrusion; HLIC, Hades Lake intrusive complex; KF, Kingfisher fault; MFP, Moving Post fault PLIC, Penassen Lakes intrusive complex; QDZ, Quetico deformation zone; RLIC, Roll Lake intrusive complex; SFIC, Silver Falls intrusive complex; SIC, Shabaqua intrusive complex; TBLLF, Thunder Bay–Loon Lake fault. constraints vary by location (see discussion in Percival et al. 2006; and references therein) with some authors indicating deposition between approximately 2698 Ma and 2696 Ma and others indicating deposition after approximately 2692 Ma. Maximum depositional ages are poorly constrained because of the limited availability of representative and consistent detrital zircon datasets across the Quetico Subprovince, making stratigraphic and tectonic interpretations challenging. A framework for deformation and metamorphism summarized in - 45 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Percival et al. (2006 and references therein) suggests polyphase deformation and metamorphism spanned from close to the time of deposition to approximately 2650 Ma. Williams (1991) described four discrete deformation episodes affecting the QS. Although work on the metamorphic history of the Quetico differs regarding details, most work converges on the prevalence of subprovince-wide high temperature, low pressure metamorphism, which was likely preceded in some places by early medium-pressure and temperature metamorphism. Metamorphic grade increases in the eastern part of the QS where it reaches granulite-facies conditions (see Pan et al. 1998). Intrusive rocks are abundant in the QS, their characteristics are explored in the field guidebook and general characteristics are summarized by Williams (1991). Much attention has been paid to the boundary between the QS and the Wabigoon subprovince (e.g. the Beardmore-Geraldton greenstone belt), however, much less has been paid to the geology of the southern boundary. The SGB (and correlative greenstone belts in Minnesota) is a relatively narrow, arcuate greenstone belt, that extends from the Pass Lake area (east of Thunder Bay) to Northern Minnesota. The SGB is described in detail in Williams et al. (1991). A more recent geochronology based tectonostratigraphic framework for the SGB was proposed by Corfu and Stott (1998) and this remains in common usage as a stratigraphic and structural framework. Minor modifications to the Corfu and Stott (1998) framework have been added by Lodge (2016). In contrast to the QS, much of the SGB has been mapped by the OGS at 1:20 000 scale. However, much of this mapping was carried out prior to technological advancements like the widespread use of U-Pb geochronology, routine high precision trace element geochemistry, access to lidar imagery and high-resolution airborne magnetic data. That said, the OGS also has an on-going multiyear bedrock mapping project in progress to map much of the eastern part of the SGB at 1:20 000 scale (e.g., Lodge 2014; Ratcliffe 2016, 2017, 2019). The general greenstone belt-wide tectonostratigraphic framework for the SGB described by Corfu and Stott (1998) includes circa 2720 Ma aged rocks of the Greenwater assemblage (mainly tholeiitic mafic metavolcanic rocks and lesser ultramafic metavolcanic rocks, mafic-ultramafic intrusive rocks and metasedimentary rocks), circa 2718 Ma age rocks of the Burchell assemblage (mainly calc-alkalic felsic to intermediate metavolcanic rocks), circa 2695 Ma aged rocks of the Kashabowie assemblage (mainly calc-alkalic intermediate metavolcanic and metasedimentary rocks), circa 2690Ma aged rocks of the Shebandowan assemblage (calc-alkalic intermediate metavolcanic rocks, shallow marine and fluvial metasedimentary rocks) and younger than circa 2682 Ma aged rocks of the Auto Road assemblage (conglomerate). Corfu and Stott (1998) envisaged a structural history of D1 thrusting that interleaved the Greenwater assemblage along with the Burchell assemblage with the Kashabowie assemblage followed by a regional unconformity overlain by younger rocks of the Shebandowan and Auto Road assemblages deposited during D2 transpression. D2 transpression ceased by about 2680 Ma. According to the framework of Corfu and Stott (1998) intrusive rocks in the SGB include older gneissic tonalitic rocks to the south of the belt with ages as old as approximately 2750Ma, syn-Greenwater assemblage mafic-ultramafic intrusions, syn-Kashabowie assemblage tonalite, syn-Shebandowan assemblage monzodiorite-granite (Tower stock) and post-tectonic circa 2680 Ma aged intrusions of biotite-hornblende bearing diorite to granodiorite. Locally, evidence for magmatic rocks with ages around 2710 Ma are present in some parts of the SGB, e.g. Kabaigon porphyry (Corfu and Stott 1998). Geology of field trip area This fieldtrip will examine the geology of four distinct geological domains (see Figure 1): 1) the northeastern part of the Shebandowan greenstone belt, 2) the Lappe domain, 3) the southern Quetico domain and 4) the Dog Lake injection complex (Figure 1). Figure 2 is a geological timeline summarizing important events affecting the different geological units in the QS-SGB boundary zone and southern QS compiled from unpublished OGS data and various other sources. Synoptic reviews of these domains are given below. Additional details are provided in the field trip stop descriptions and will be augmented by discussions in the field. - 46 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 2 Geological timeline summarizing the main volcanic, sedimentary, intrusive, and structural events affecting the Shebandowan greenstone belt and the Quetico Subprovince. Ages are compiled from Corfu and Stott, 1998; Corfu, 2000; Kamo, 2013; Wang et al., 2020 and preliminary OGS data. Age bars include analytical uncertainties. Shebandowan greenstone belt (Stops 1, 2, 3, 5 and 6) This field trip guidebook only examines a narrow portion of the northern margin of the SGB that was covered by our mapping (Figure 1). In this area, the northern boundary of the SGB is generally eaststriking dextral shear zone interpreted to be the eastward extension of the Crayfish Creek fault. (Figure 1). Within this area of the SGB, we recognize four mappable units at the 1:50 000 scale which are subdivided into two informal groups, each comprising two informal formations. The older Greenwater group (a less repetitive Group-level name should be devised) consists of the Greenwater and Mud Lake formations and the younger Shebandowan group consists of the Strawberry Hill and Auto Road formations. Although not specifically identified in our area of mapping, we would include the “Kashabowie assemblage” (e.g. Corfu and Stott 1998) with the Shebandowan group. These units correspond with the “older” and “younger” portions of the SGB as described by previous workers (e.g., Corfu and Stott 1998, Lodge 2016) in most respects. However, we feel that moving towards a “sub-assemblage level” nomenclature is warranted to begin a framework for more detailed characterization of supracrustal rock variability at the regional scale. This is particularly true for rocks of the Shebandowan assemblage (in the sense defined by previous workers) which is lithologically heterogeneous. Greenwater group The Greenwater formation (Stops 1 and 6) consists mainly of tholeiitic mafic metavolcanic rocks, minor ultramafic metavolcanic rocks, synvolcanic gabbroic intrusions and minor clastic and chemical metasedimentary rocks. We do not have specific age constraints on the Greenwater formation; however, we infer that it is part of the “older” SGB based on lithological similarities with rocks of the Greenwater assemblage sensu stricto. At more detailed mapping scales, the Greenwater formation could likely be further subdivided (e.g. mafic dominated vs ultramafic dominated portions). The Mud Lake formation (Stop 1) consists mainly of calc-alkalic fragmental volcaniclastic rocks and locally coherent flows of felsic to intermediate composition. In the area, Corfu and Stott (1998) determined the age of this unit to be 2718 +/- 3 Ma. These are likely equivalent to Burchell assemblage of Lodge (2016). - 47 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Shebandowan Group The Strawberry Hill formation (SHF, Stop 2) typically consists of coarse-grained polymictic breccias characterized by hornblende phenocrysts in a dark matrix. The SHF is typically mafic to intermediate and calc-alkalic. It typically has elevated magnetic susceptibility and displays pink to red hematite alteration and/or pale green epidote alteration. Locally, it also occurs as more massive, hornblende-phyric mafic flows or shallow intrusions. At some localities, breccias or flows are associated with thinly layered tuffs, and reworked tuffs with clear pyroclastic textures such as bombs that deform layering. Geochronology by Corfu and Stott (1998) and our own work indicate deposition/eruption of the SHF at circa 2690 Ma. Although the bulk of SHF rocks are undeformed, they are locally cut by narrow ductile shear zones, and it is in sheared contact with older rocks. The SHF forms a thin, but important marker unit that can be correlated across much of the central SGB in the area depicted by Figure 1. Breccias of the SHF are compositionally similar to, and of the same age as the Tower stock (just west of the area depicted in Figure 1) which hosts low-grade disseminated, intrusion related gold mineralization and includes marginal breccias similar to the SHF (e.g. Carter 1992). The Auto Road formation (ARF, Stops 3 and 5) comprises mainly polymictic, matrix supported conglomerate. Local occurrences of trough crossstratified, likely fluvial sandstones, are also considered part of the ARF. Calc-alkalic mafic metavolcanic rocks, including apparently pillowed flows are locally intercalated with ARF sedimentary rocks. The ARF was deposited after 2682 ± 3 Ma based on geochronology in Corfu and Stott (1998) and as such, it clearly postdates the SHF. Dextral shear zones are a common feature of the SGB. Although it is not clear in Figure 1, geophysical patterns in adjacent areas like the LD and the SGB outside of our mapping area suggest that D2 shear zones post date D1 thrust faults. Sinistral northeast-trending shear zones in the SGB such as the Kingfisher and Thunder Bay-Loon Lake faults are relatively younger than the dextral shear zones based on mapping inferred off-sets. Potassium-rich calc-alkalic suite intrusions (PRCAS) form a minor component of the SGB as shown on Figure 1. These include massive to weakly foliated hornblende-biotite-magnetite quartz monzonite to monzogranite dominated intrusions of unknown age. These may be related to similar intrusions in the SGB like the Kekekaub pluton (circa 2680 Ma) or perhaps correlate with the Tower stock (circa 2690Ma). Lappe Domain (Stops 4, 7 and 9) The Lappe domain comprises mainly metasedimentary rocks (wacke and siltstone) similar to those of the southern Quetico subprovince to the north. Its southern boundary is the Crayfish creek fault whereas it is bounded to the north by the Moving Post fault. The LD is characterized by thin fault bounded panels of mafic metavolcanic and mafic intrusive rocks comparable to the Greenwater formation intercalated with the metasedimentary rocks. Where best preserved, these mafic panels contain pillowed mafic flows, local banded iron formations, local thin ultramafic schists (sheared flows or thin sills) and local sulfidic mudstones. The margins of the mafic panels are commonly sheared and often preserve well developed steeply plunging stretching lineations indicating dip-slip, probable thrust motion. Locally LD rocks are folded, however we do not have a sufficient coverage of detailed younging data or outcrop scale fold observations to determine the nature of folding in the LD. Although we do not have direct age constraints on metavolcanic rocks in the mafic panels, preliminary data suggests some thin gabbro bodies in LD metasedimentary rocks are intrusive (i.e. not structurally interleaved). These provide minimum age constraints for LD sedimentation; combined with preliminary detrital zircon data, and considering analytical uncertainties, LD sedimentation is bracketed between about 2698 and 2689 Ma. At another locality, Corfu (2000) determined and age of circa 2718 Ma for a gabbro within one of the mafic panels in Ware township. If this age is reliable, then some of the mafic rocks in the LD correlate with the Greenwater formation. Another hypothesis to consider is that this age could reflect zircon inheritance. Regardless of the age of the mafic panels and whether they represent tectonic slivers of older rocks, or if they are part of the “stratigraphy”, observed thrusts faults indicate that the boundary between the SGB and the QS likely represents a zone of fold-thrust deformation. The timing of LD sedimentation corresponds with the timing of deposition of the Kashabowie assemblage in the SGB (see Corfu and Stott 1998). Observed thrust - 48 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 faults in the LD may therefore correspond to “D1” of Corfu and Stott (1998) and this interpretation links the timing of early QS deformation with early deformation in the SGB. East of the Kingfisher fault, large, multiphase, PRCAS intrusive complexes, (the Penassen Lakes intrusive complex and the Roll Lake intrusive complex) were emplaced into the LD. These intrusions consist of an early mafic phase (hornblendite to monzogabbro) roughly coeval with intermediate a hornblende-biotitemagnetite monzodiorite to quartz monzonite phase and a late phase that is volumetrically dominant and composed of biotite monzogranite. Compositionally similar rocks are found to the north in the QS and the Dog Lake injection complex and ages determined for the different phases are consistent regionally as summarized in Figure 2. Locally, peraluminous granitic pegmatites occur in the northern LD (e.g. Walkinshaw pegmatites) but are not associated with any obvious parental granite. LD metasedimentary rocks are typically low metamorphic grade, dominated by biotite or chlorite, quartz, plagioclase assemblages. However, near intrusions they locally contain contact metamorphic porphyroblasts of andalusite and/or cordierite and in some areas experienced partial melting. Southern Quetico domain (Stops 10, 11 and 12) The southern Quetico domain comprises mainly metasedimentary rocks (wacke and minor siltstone) with rare intermediate tuffaceous horizons, rare mafic tuff (possibly boninitic). Bedding and foliations in the southern QS are typically east to northeast trending and “D2” folds typically have east to northeast trending axial surfaces. Southern QS metasedimentary rocks gradually become more recrystallized towards the north. Metamorphic assemblages are typically dominated by biotite and locally biotite-garnet. Andalusite- and cordierite-bearing metamorphic assemblages are also relatively common and may related to proximity to intrusions. Staurolite-bearing assemblages are present locally but rare. The southern QS was intruded by numerous potassium-rich calc-alkalic suite intrusions (PRCAS). These are depicted on Figure 1 and include from west to east, the Shabaqua intrusive complex, the Silver Falls intrusive complex, the Trout Lake intrusion, the Barnum Lake intrusion, the Whitelily Lake intrusive complex and the Hades Lake intrusive complex. These intrusions and intrusive complexes are variably complex mixtures of mafic (hornblendite-monzogabbro), intermediate (monzodiorite-quartz monzonite) and felsic (monzogranite) phases. S-type granites are also common in the southern QS, these include the Hilma Lake granite, the Voutilainen intrusion and the Hadwen Lake intrusion. Peraluminous granitic pegmatites are also abundant and spatially related to S-type granite bodies. Dog Lake injection complex and Quetico deformation zone (Stops 12, 13, 14 and 15) Strain and degree of metasedimentary rock recrystallization increase abruptly in the vicinity of the Quetico deformation zone (QDZ) in the northern part of the area. In this domain, QS metasedimentary rocks are recrystallized and comprised mainly of biotite-quartzfeldspar+/- magnetite paragneiss and locally also sillimanite-cordierite-garnet bearing paragneiss. The Dog Lake injection complex (DLIC) is characterized by paragneiss intruded by a high volume of both peraluminous and HPCAS intrusions (injections) that were emplaced synchronously with intense dextral transpression along the QDZ. Intrusions of both suites are commonly schlieric with strong fabrics defined by schlieren and magmatic minerals. At the contact with HPCAS intrusions, paragneisses are commonly strongly magnetic which likely resulted from their oxidation by fluids exsolved from these HPCAS intrusion triggering the crystallisation of magnetite. These zones are also particularly rich in biotite which locally give the paragneisses the appearance of melanosome. Migmatitic rocks are a volumetrically minor component of the DLIC and comprise mainly patchy metatexite likely related to heating by the high volume of intrusive rocks in the area. Syn-tectonic fabrics are ubiquitous in intrusive rocks of the DLIC. Strong fabrics are generally steep and east striking to east-northeast striking. Dextral shear bands with strikes of approximately N290-300 are common as are approximately N40 striking and N15-20 striking subvertical sinistral shear bands. C-S fabrics are common and suggest syn-emplacement dextral strike slip and locally north side up thrust components of motion. Shearing related fabrics in the paragneisses have the same orientations and kinematics and syn-tectonic emplacement fabrics in the granitoid rocks. Folding of gneissosity is common and folding - 49 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 tends to be gently plunging. Intense, steeply dipping, east striking mylonitic zones are present locally. Late brittle-ductile strike slip and thrust motion occurred along the QDZ locally overprinting ductile fabrics. This pattern is repeated at map scale where the sinistral north to northeast-trending shear zones branches onto dextral east-trending QDZ in the Dog Lake area (see Figure 1). These observations suggest a transition from an earlier ductile transpressive deformation to later more brittle-ductile deformation. Field trip stop descriptions Stop 1 (Optional) - Mafic metavolcanic rocks of the Greenwater formation and felsic to intermediate metavolcanic rocks of the Mud Lake formation 315073E 5376695N (Greenwater formation) 314610E 5377220N (Mud Lake formation) Park on the shoulder of Mud Lake Road. Wear reflective vests, stay on shoulder or in the ditch. Traffic is heavy on Hwy 102 so be very cautious crossing the highway. A series of outcrops in this area shows mafic metavolcanic rocks considered to be part of the Greenwater formation and felsic metavolcanic rocks of the Mud Lake formation. These two formations are typical of the older part of the Shebandowan greenstone belt. From the junction of Hwy 102 and Mud Lake Road, outcrops immediately to the east and west are mainly mafic metavolcanic rocks of the Greenwater formation. These rocks display well preserved volcanic features such as prominent pillows (Figure 3A) and local pillow breccias. The pillowed flows are subvertical and striking to N75. Based on pillow cusps, they appear to young northward. Quartzepidote veining, local red-pinkish alteration and east striking brittle-ductile shear zones can also be seen at this outcrop. Younger, mica-phyric mafic lamprophyre dikes are also present. Farther west, closer to the north end of Mokomon Lake (314610E, 5377220N) felsic metavolcanic rocks of the Mud Lake formation comprising tuff breccias, lapilli tuffs and locally flows are present in outcrops located on the north side of the highway. These felsic metavolcanic rocks host a thin, semi-massive sulfide (sphalerite, pyrite, pyrrhotite) horizon known as the Mud Lake VMS occurrence (Figure 3B). The rocks Figure 3. Metavolcanic rocks of the Greenwater and Mud Lake formations. A) Large pillows in mafic flow, Greenwater formation. B) Sulfide mineralization in felsic metavolcanic rocks, Mud Lake formation. here are strongly foliated (260/75) with local shearing (234/80) and locally cut by rusty shallow dipping faults. Felsic rocks from this outcrop have an age of 2718 +/- 3 Ma based on Corfu and Stott (1998). The age of Greenwater formation (Greenwater assemblage) rocks throughout the greenstone belt is mainly inferred from adjacent felsic to intermediate units and maficultramafic intrusions. Although the contact between these two units appears sharp at this location, mafic rocks of the Greenwater formation are intercalated with felsic to intermediate rocks of the Mud Lake formation in other locations, suggesting that the contact may have originally been gradational. - 50 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Stop 2 (planned) - Strawberry Hill formation, Shebandowan group 312462E 5378109N Park along the access road to the quarry on the southern side of Dawson Road. Participants must always wear reflective vests and remain on the road shoulder, as traffic along this section is relatively dense and driver visibility is poor. This stop exposes a representative outcrop of the Strawberry Hill formation of the Shebandowan group. It consists of a massive, undeformed, mafic to intermediate calcalkaline, hornblende-phyric, matrix-supported breccia (Figure 4A). The breccia is polymictic, containing angular to subrounded clasts composed predominantly of plagioclase-phyric, medium- to coarse-grained pink monzonite, along with subordinate clasts of mafic and intermediate volcanic rocks, all set within a fine-grained, dark-green matrix. Clasts of monzonitic composition locally preserve an internal magmatic foliation, highlighted by the alignment of plagioclase phenocrysts. This magmatic fabric suggests that the intrusion from which the clasts were derived was likely emplaced syn-tectonically. The absence of visible bedding, combined with the generally angular to sub-rounded nature of the clasts, suggests formation in a high-energy volcanic environment, possibly associated with a cryptodome, and likely proximal to the volcanic source. Alternatively, this unit may represent a subvolcanic magmatic breccia, comparable to the breccias around the Tower stock described by Carter (1992). Based on TIMS U–Pb ages reported by Corfu and Stott (1998), magmatism related to this breccia likely occurred at approximately 2692±6 Ma. Thus, the SHF is likely contemporaneous with the 2690 ± 3 Ma Tower stock (Corfu and Stott 1998) Although the unit is largely massive and lacks visible foliation, discrete mylonitic shear bands are locally present (Figure 4B). These shear bands are commonly associated with carbonate alteration and quartz–carbonate veining. The shear zones generally strike northeast and locally preserve kinematic indicators consistent with a thrust motion, indicating a northsideup sense of shearing (Figure 4B). The breccia is also cut by multiple generations of quartz–carbonate veins, indicating postdepositional brittle deformation coeval with hydrothermal activity. These veins locally offset both clasts and matrix but do not significantly disrupt the overall massive character of the breccia. Stop 3 (Planned) - Auto Road formation, Shebandowan group 316851E 5378641N Park at the entrance of the private dirt road south of Korpela Road. As Korpela Road is narrow, please ensure that your vehicle does not obstruct traffic or restrict access along the road. Figure 4 Representative photographs of the Strawberry Hill formation outcrop (Stop 2). (A) Massive poorly sorted matrix supported polymictic intermediate breccia. (B) Discrete mylonitic shear band with asymmetric kinematic indicators (C-S fabric) indicating a north-side-up sense of shearing. This stop exposes a representative outcrop of the Auto Road formation of the Shebandowan group. It consists of a strongly foliated, poorly sorted, matrixsupported polymictic conglomerate. The conglomerate contains predominantly pebble to boulder-sized clasts of plagioclasephyric, medium to coarse-grained pink monzonite and monzogranite, many of which are - 51 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Road formation deformation event to approximately 2688 ± 0.8 Ma, whereas TIMS U–Pb ages reported by Corfu and Stott (1998) indicate a maximum depositional age of 2682 ± 2 Ma for the conglomerate. Together, these ages suggest that the intrusions from which the clasts were derived were deformed after 2688 Ma, prior to erosion and deposition of the conglomerate sometime after 2682 Ma. The conglomerate was subsequently deformed and affected by dextral shearing likely related to the Crayfish Creek fault. Stop 4a (Planned) - Sheared mafic rocks, mediumbedded wackes and thrust deformation at the Northern boundary of Lappe domain. 326238E/ 5383267N Park along Moving Post Road or in the parking lot of the old Lappe Store if permission is obtained. Wear reflective vests, be mindful of traffic, shoulders of the road are narrow and visibility is poor. Figure 5 Representative photographs of the Auto Road formation outcrop (Stop 3). (A) Strongly foliated polymictic conglomerate. (B) Dextral asymmetrical C-S fabric wrapping around a clast of pink monzogranite. Note the discordant internal foliation within the clast. strongly flattened parallel to the foliation (Figure 5A). The matrix is sandy, medium to coarse-grained, and characterized by a darkgreen color. Several monzonitic and monzogranitic clasts preserve an internal foliation that is discordant with the matrix foliation (Figure 5B), indicating that these intrusive rocks were deformed prior to erosion and deposition. This relationship suggests a preAuto Road formation deformation event affecting the source intrusions. The conglomerate exhibits a strong east–west trending foliation and is overprinted by a northwesttrending (approximately N300°) dextral shearing, highlighted by the development of asymmetric C–S fabrics wrapping around the clasts (Figure 5B). Preliminary ages obtained from a foliated intrusive clast constrain the maximum age of the preAuto This outcrop illustrates strongly deformed, tholeiitic mafic metavolcanic rocks (Figure 6A) of the northernmost mafic metavolcanic unit of the Lappe domain. Mafic metavolcanic rocks at this exposure are characterized by N250 striking north-dipping strong foliation and well-developed northward plunging lineation. The apparent dip-slip motion along the shear zone is north-side-down. In its present geometry, the true kinematics of the dip-slip motion along this structure is equivocal (Figure 6B). However, an interesting and commonly repeating pattern along strike is that metamorphosed north dipping wackes located north and south of this mafic metavolcanic unit consistently young southward indicating that the whole stratigraphic succession is overturned. This pattern was also observed in the past OGS mapping campaign of MacDonald (1939, observe printed version of old map). If our interpretation of the kinematics of this structure and the facing direction in the bounding metagreywacke units are correct, a possible interpretation of this structure is that it represents an originally northward verging thrust that was subsequently steepened and overturned. The stratigraphic relationship between the mafic metavolcanic rocks at this locality and surrounding wackes is not well constrained. Commonly both contacts of the mafic unit are sheared and there is a paucity of rocks suitable for geochronology in the exposures that we have mapped. Reliable younging - 52 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Stop 4b (Optional) - Southward younging Lappe domain metasedimentary rocks 326207E 5382509N This optional outcrop is located approximately 750 m south of Stop 4a along Dog Lake Road. It exposes mediumbedded wackes displaying a southward younging direction, as indicated by graded bedding and load casts (Figure 7A and 7B). The southward younging of these clastic metasedimentary rocks occurs near the mafic metavolcanic rocks and associated thrust fault observed at Stop 4a, providing important constraints on local stratigraphic facing and structural relationships. Figure 6 Representative photographs of sheared mafic volcanic flow from the Lappe Domain (Stop 4). (A) Strongly foliated and sheared mafic volcanic flow. (B) C-S fabric wrapping around quartz eyes indicating north-side down sense of shearing indicators within the mafic metavolcanic unit are almost nonexistent. At one locality, farher to the east, southward younging pillows were observed along a similar structure in a similar setting. We have not observed strong evidence of stratigraphic continuity between the surrounding wackes and the mafic unit. In this case, observed younging directions do not argue against stratigraphic continuity, however the sheared nature of contacts makes interpretation difficult. A thin, mica-phyric lamprophyre dike is also present at this locality. Mafic lamprophyre dikes are common throughout the area in the SGB, LD and QS. They have not been successfully dated and their contact relationships and relationships to structures and other intrusive suites is difficult to interpret as in many places relative age relationships are contradictory. This may Figure 7 Clastic sedimentary rocks of the Lappe Domain indicate multiple generations of lamprophyric mafic (Stop 4b). (A) Medium bedded wackes with graded beds. (B) intrusion are present regionally. Flame structure showing a southward younging direction. - 53 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Stop 5 (Optional) - Cross bedded sandstone, Auto Road formation, Shebandowan group 329269E 5379186N Park along the south side of Peterson Road, near the entrance to the private residential access road located on the curve. This outcrop is situated on private property. Authorization from the landowner is required prior to accessing the outcrop, and participants must ensure that permission has been obtained before entering on the property. This large outcrop exposes a well-preserved section of sandstone in sheared contact with a calc-alkaline mafic volcanic flow (Figure 8A). The sandstone displays a variety of well-developed sedimentary structures, including crossbedding and channelized geometries, indicative of a fluvial depositional environment (Figure 8B). The sandstone locally contains plagioclase phenocrysts, suggesting a potential juvenile volcaniclastic component. The mafic lava flow is locally pillowed and occurs in sheared contact with the sandstone (Figure 8C). The shear zone is characterized by a penetrative east– west-striking foliation that dips steeply to the south. A well-developed stretching lineation, plunging steeply (~55°) to the east, is observed on foliation planes. Locally, kinematic indicators are preserved and indicate a dextral sense of shearing. Together with the steeply plunging lineation, these observations suggest a dextral transpressional deformation with a top-to-the northwest thrust component. Younging directions determined from cross bedding indicate a consistent southward younging across the outcrop. The sandstone is also crosscut by late, narrow mafic dikes, indicating post-depositional magmatic activity (Figure 8D). Although samples collected for U–Pb geochronology Figure 8 Outcrop of cross-bedded sandstone of the Auto Road formation (Stop 5). (A) Aerial drone photograph showing crossbedding and channel structures. (B) Close up on cross-stratified sandstone with southward younging direction. (C) Strongly sheared and foliated mafic volcanic flow occurring within the sandstone. (D) Late mafic dikes crosscutting sandstone. - 54 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 did not yield datable mineral phases, the sandstone is interpreted to be part of the Shebandowan group, most likely the Auto Road formation, based on its characteristic fluvial depositional environment. Stop 6 (Planned) - Greenwater assemblage mafic metavolcanic rocks, eastern extension of Crayfish Creek deformation zone 340868E 5376210N Park on the shoulder of Mount Baldy Road at its junction with Hwy 527. Wear high visibility vests. The shoulder here is narrow and logging truck traffic can be heavy. Be careful if crossing the highway. Footing is uneven and there are commonly garbage and glass in the ditches. This outcrop represents the sheared contact between the Shebandowan greenstone belt (to the south) and the Lappe domain (to the north) and may represent the eastward extension of the Crayfish Creek deformation zone. The northern part of the outcrop consists mainly of east-striking, steeply south-dipping, sheared, ankerite altered, magnesium-rich mafic rocks (Figure 9A). During mapping, kinematics of the shearing at this locality were not determined confidently. Precise identification of protoliths in the northern part of the outcrop is problematic as most primary features were obliterated. However, towards the south, rock types are well preserved and include massive, fine- to mediumgrained mafic volcanic flows (Figure 9B), mafic pillowed flows, a thin pyrite-bearing nodule black mudstone, and quartz-feldspar porphyritic intermediate dikes. Enigmatic weakly boudinaged dikes of mafic to ultramafic composition locally cut the main shearing fabric in the northern part of the outcrop. Minor Proterozoic calcite veins are also present. Lappe domain metasedimentary rocks to the north were deposited after circa 2700 Ma and perhaps after circa 2690 Ma, depending on the interpretation of preliminary detrital zircon data. Based on geochronology performed elsewhere in the SGB, the Greenwater formation is inferred here to have an age of circa 2720 Ma. Therefore, this shear zone juxtaposes rocks that differ in age by at least 20 million years. At this locality, shearing could represent a transpressive dextral reactivation of an earlier shear zone that interleaved units of disparate age. Note that there does not appear to be a rhyolitic unit equivalent to the Mud Lake formation north of the Greenwater formation as seen in Stop 1. This feature could be attributable to either a fault-related subtraction or a lateral stratigraphic discontinuity. The “QFP” dikes cutting the Greenwater formation here have the same appearance as dikes commonly observed in the Lappe domain and in the southern Quetico. Unfortunately, these dikes have proven difficult to date due to the lack of mineral phases amenable to U-Pb geochronology. Pyritic black shales like those at this locality occur locally in the Greenwater and Mud Lake formations. Figure 9. Greenwater formation metavolcanic rocks (Stop 6). (A) ankerite altered mafic-ultrmafic schist. (B) Massive, plagioclase-phyric mafic flow. - 55 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Stop 7 (Optional) - Lappe domain metasedimentary rocks 341256E 5378262N Park at the “weigh-scale” on the west side of Highway 527. Wear high visibility vests and use a high level of caution crossing the highway to the outcrop. Ditches also have uneven footing and garbage. This outcrop illustrates low metamorphic grade rocks of the southern Lappe domain. Here ~ 250° striking, north dipping but southward younging metasedimentary rocks include thin- to mediumbedded intercalated siltstone and sandstone (Figure 10A) overlain by very thickly bedded poorly sorted volcaniclastic sandstone (Figure 10B). Near the base of the thick sandstone bed, there is a folded horizon of thinly interbedded sandstone and siltstone. This may represent syn-depositional soft deformation. Locally, sparse carbonate nodules are present in the outcrop; these have been interpreted as early diagenetic features. Farther north, calc-silicate nodules (amphibole, epidote, locally garnet) are common in the QS and probably represent more metamorphosed equivalents of these carbonate nodules. As alluded to in the description of Stop 6, Laserablation-ICP-MS zircon geochronology was carried out on samples from this outcrop. This data suggest deposition of these rocks about the same time as much of the Shebandowan group. However, analytical precision does not permit precise chronostratigraphic correlation with the Strawberry Hill or Auto Road formations. Maximum depositional ages for tidally influenced shallow marine sediments in the Finmark area have maximum depositional ages of about 2691 Ma based on limited population, single crystal IDTIMS geochronology reported by Corfu and Stott (1998). The Finmark metasedimentary rocks occur in a similar structural setting based on geophysical interpretation. Stop 8 (Optional) - Northern margin of the Penassen Lakes intrusive complex 345318E 5385843N This stop requires parking on the shoulder of Hwy 527. Use hazard lights and traffic cones to increase visibility. Wear reflective vests and be mindful again of the traffic on Hwy 527. Be also mindful of soft shoulders when parking and walking. This outcrop represents part of the northern contact of the Penassen Lakes intrusive complex. At this outcrop early hornblende monzodiorite is crosscut by intermediate aged hornblende quartz monzonite, which is cut by late pink leucocratic biotite monzogranite dikes (Figure 11A and 11B). Also visible are narrow mica-phyric mafic lamprophyre dikes and xenoliths of metawacke and possibly mafic metavolcanic rocks. Figure 10. Lappe domain metasedimentary rocks at Stop 7. (A) steeply dipping, southward younging, thin- to mediumbedded, low metamorphic grade siltstone and sandstone. (B) Poorly sorted, lapilli and intraformational-sedimentary-clast bearing thick-bedded volcaniclastic sandstone. Multiphase, oxidized (magnetite bearing) intrusive complexes are a common feature of the SGB, LD and QS (see Figures 1 and 2). Early mafic phases of these intrusive complexes were emplaced between about 2677 and 2670 Ma, whereas later pink monzogranite phases appear to be about 5 million years younger (Figure 2). Although relative and absolute age differences between different phases of these intrusions are observed, we refer to them collectively as the potassium-rich calcalkalic suite. - 56 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Stop 9 (Planned) - Lappe Domain metasedimentary rocks and Walkinshaw peraluminous granitic pegmatites 346697E 5388265N Turn to the east on side road located just south of Stop 9 coordinate (Seagris Rd; no sign). If this side road is too rough for vehicles, use the road leading to summer camps on the northern end of Walkinshaw Lake, farther north. Wear high visibility vests, stay on shoulder or in ditch. Use extreme caution when crossing the road. Outcrops are relatively tall at this locality, be mindful of the potential for falling rocks in places. Figure 11. Northern margin of the Penassen Lakes intrusive complex at Stop 8. (A) Metasedimentary country rock fragments intruded by grey monzodiorite, crosscut by pink quartz monzonite to monzogranite dikes. (B) Amphibolephyric quartz monzonite with xenoliths of metamorphosed wacke. We consider this area part of the Lappe domain as we are south of the northernmost mapped mafic metavolcanic panel and the inferred eastward extension of the Moving Post fault. The Penassen Lakes intrusive complex and the similar Roll Lake intrusive complex to the north appear to post date the Moving Post fault based on geophysical interpretation and the presence of amphibolitic mafic pillowed flows occurring as large, strongly deformed inliers within the RLC and the emergence of a thin shear zone bounded mafic panel on the east side of the RLIC (Figure 1). This provides a clear relative age constraint on the Moving Post fault. These rocks are not, however, post-tectonic as we will see in later stops. This long outcrop contains relatively undeformed metasedimentary rocks (Figure 12A and 12B) in the northern part of the Lappe domain along with peraluminous granitic pegmatite dikes locally containing green mica, black tourmaline (Figure 12C and 12D) and disseminated molybdenite. These pegmatites we refer to as the Walkinshaw pegmatites. This area of metasedimentary rocks is surrounded by several large intrusive complexes (Potasssium-rich calc-alkalic suite), the Whitelilly Lake, Roll Lake and Penassen intrusive complexes, as well as several minor, sub-concordant, pink monzogranite bodies that are too small to map at 1:50 000 scale. At this locality sedimentary features are well preserved (see Figure 12A) and the overall strain appears low. Foliation-bedding orientation relationships and measured intersection lineations suggest that folds in this area likely plunge steeply. Locally, thin shear zones have steep lineation plunges. Finer-grained beds locally have well developed porphyroblasts of andalusite (Figure 12B) and some cordierite, both are commonly replaced by muscovite. These assemblages are consistent with high temperature-low pressure metamorphism. At this locality we interpret that the observed metamorphic assemblage results from proximity to the many large intrusions in the area. The Walkinshaw pegmatites are somewhat enigmatic. They occur in relatively low metamorphic grade rocks and there is no clear peraluminous “parent” granite nearby. A speculative explanation could be that small volume melts were locally produced by partially melting adjacent to contacts of nearby intrusions (PRCAS). There is local evidence for such melts, but only in very small volumes. - 57 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 12. Lappe domain metasedimentary rocks and the “Walkinshaw” peraluminous granitic pegmatites (Stop 9). (A) northward younging wacke bed based on scouring and normal grading. (B) Andalusite porphyroblast-rich bed. (C) and (D) Black-tourmaline-rich subconcordant muscovite pegmatite dike. Stop 10 (Planned) - Onion Lake pegmatites and folded Quetico metawacke 346817E 5398027N Turn east on Cliff Rd (no sign) and drive about 250m down the dirt logging road and park to one side. Be aware of potential for logging traffic or other road users. Wood ticks are common at this site in spring and early summer. Use caution while walking around on the uneven ground. This outcrop shows a clean exposure of a peraluminous granitic pegmatite typical of pegmatites we refer to as the Onion Lake pegmatites. At this stop, a biotite-muscovite-garnet bearing pegmatite-aplite dike (Figure 13A and 13B) strikes roughly northeast and appears to post-date a strong foliation affecting typical Quetico Subprovince metasedimentary rocks. The pegmatite displays prominent interlayering of pegmatitic and aplitic rock and unidirectional solidification textures (Figure 13A). This pegmatite is not far south of the contact of a peraluminous granite we refer to as the Voutilainen intrusion. Several large “whalebacks” of pegmatite are present in this area. Although not entirely clear at this locality, the Onion Lake pegmatites are boudinaged and commonly have internal fabrics defined by magmatic phases implying a syn-tectonic emplacement. Regionally, the northeastward strike of pegmatite contacts is parallel to sinistral shear zones which are interpreted to be antithetic structures related to the overall dextral shearing related to the Quetico deformation zone. Granites in this area occur near the southern margin of prominent deformation related to the QDZ. At least two distinct generations of peraluminous granitic pegmatites are present in the southern Quetico. A second generation of pegmatites, younger than the one at this locality crosscut at a high angle the foliation related to the Quetico deformation zone and therefore are post-tectonic. These later pegmatites are much less abundant, and we will not be able to examine the younger generation of pegmatites on this trip. Metasedimentary rocks at this locality are strongly deformed. Relatively steeply plunging, east-northeast striking z- to m-folding is revealed by prominent quartz-feldspar veins (Figure 13C). These quartzfeldspar veins are very common in the southern QS. - 58 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 13. Onion Lake peraluminous granitic pegmatite and folded Quetico metasedimentary rocks (Stop 10). (A) Pegmatite-aplite layering, unidirectional solidification textures, biotite-muscovite pegmatite and garnet aplite (B) Close-up of abundant garnet in aplitic phase, (C) boudinaged S-type granite dike sub-parallel to axial plane of “D2” folds in Quetico metawacke. Based on cross-cutting relationships they appear to predate most of the mapped intrusive suites. Note the narrow, boudinaged peraluminous granitic dike emplaced sub parallel to the axial plane of the folds appearing to crosscut the veining (Figure 13C). Outcrop scale folds of sedimentary layering are relatively uncommon in the southern Quetico making overall understanding of fold-geometries somewhat difficult. These folds are likely “D2” in the nomenclature of Williams (1991) and limbs of similarly oriented folds are elsewhere postdated by dextral shearing (D3). A prominent linear magnetic anomaly, caused by a relatively magnetic wacke unit, shows a clear regional z-folding pattern and this is likely the general geometry of “D2” folding in the southern QS. In this area, many traverses across-strike documented well preserved younging indicators that show multiple reversals over relatively short distances. These reversals likely represent parasitic folds in hinge zones of larger z-fold enveloping surfaces. Towards the north, in the Quetico deformation zone, fold orientations change and tend to be east-striking, upright or slightly inclined with gentle plunges. These folds have been interpreted as being part of the D3 event. Stop 11 (Optional) - Mylonite and late brittle-ductile deformation Quetico deformation zone 348059E 5401951N Pull vehicles over near the north end of long outcrop and park on the shoulder of Hwy 527. Keep the duration of stop relatively short. If longer stop required park on logging road just to the south. Wear reflective vests, use hazard lights. Traffic can be heavy. At this locality, highly deformed metasedimentary rocks and sheared and boudinaged muscovite pegmatites are present. Locally, north dipping brittle structures offset pegmatite dike contacts with a north over south sense of displacement (Figure 14A). Pale - 59 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 QS domain to the south and the Dog Lake injection complex to the north. Grey amphibole-biotite-magnetite quartz monzonite at this outcrop is crosscut by later pink, moderately magnetic biotite monzogranite (Figure 15A and 15B). The quartz monzonite is moderately foliated and syn-tectonic. The quartz monzonite and the biotite monzogranite are compositionally similar to bigger, typically less strained intrusions located farther south e. Conversely, these types of intrusions are highly strained to the north in the Dog Lake injection complex. Preliminary geochronology suggests that despite highly variable degrees of strain, PRCAS intrusions have comparable ages in the Lappe domain, southern Quetico Subprovince and in the Dog Lake injection complex. Weakly deformed intrusions like the Trout Lake and Barnum Lake intrusions are essentially contemporaneous with highly sheared syn-tectonic equivalents in the DLIC. Figure 14. Sheared Quetico metasedimentary rocks and peraluminous granitic pegmatites, Quetico deformation zone (Stop 11). (A) brittle minor off-set thrust fault post-dates dextral shearing related to the ductile phase of the QDZ. (B) Pale green, siliceous mylonite related to the Quetico deformation zone. green siliceous rocks exposed at the north end of the outcrop are likely a mylonitic band (Figure 14B) within the larger Quetico deformation zone. To the south, dark grey rocks are strongly deformed wacke. Stop 12 (Planned) - Intermediate and felsic phases of potassium-rich calc-alkalic intrusive suite and syn-tectonic schlieric S-type granite 348541E 5403237N Pull over on the shoulder of Hwy 527. Use hazard lights, wear reflective vests, use extreme caution when crossing the highway. This large outcrop illustrates contact relationships between intermediate and felsic phases of the potassium-rich calc-alkalic intrusive suite (PRCAS) and a schlieric biotite-rich peraluminous granite. This outcrop represents the transition from the southern At the north end of the outcrop, syn-tectonic, schlieric, low magnetic susceptibility peraluminous leucogranites are in contact with rocks of the PRCAS intrusions. C-S fabrics in the granite, and narrow sheared bands (Figure 15C and 15D) indicate north side up thrusting with a dextral strike-slip horizontal component during emplacement. East-northeast striking foliations locally bear shallowly plunging stretching and mineral lineations indicating strike slip motion. These lineations may have formed under brittleductile conditions after the main phase of ductile syngranite shearing. The relative age of the peraluminous intrusion and the intermediate PRCAS phase is not immediately clear at this exposure. Regionally, the intermediate phase of the PRCAS slightly predates the bulk of S-type granite intrusions, and pink biotite monzogranites are typically younger. In some areas, hybridization of magmas have been documented. Stop 13 (Planned) - Shear-hosted leucogranitic injections, Dog Lake injection complex 346928E/ 5407112N (this stop is not on Figure 1) From Highway 527, turn right onto Hiccup Road (a logging road). Park on the curve near the stack of logs. This road is not active at the time of writing, and traffic is expected to be minimal. This outcrop exposes paragneiss of the Dog Lake complex intruded and variably digested by large volumes of syntectonic leucogranitic injections (Figure - 60 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 15. Potassium rich calc-alkalic suite intrusions and schlieric peraluminous granite at Stop 12. (A) Grey, foliated, feldspar porphyritic, hornblende-biotite quartz monzonite cross but by dikes of pink biotite monzogranite; (B) Porphyritic texture in quartz monzonite. (C) and D) syn-emplacement C-S fabrics defined by biotitic schlieren in biotite-rich peraluminous granite indicate a north-side up thrust component of motion (C) and dextral strike slip component of motion (D) during peraluminous granite emplacement. 16A), producing raft and schollen textures that can be easily confused with diatexite, and may therefore lead to misleading interpretations. The outcrop displays variable degrees of assimilation and digestion of the sedimentary host rocks by granitic melts, locally producing biotite schlieren within the leucogranite. Leucogranite injections are heterogeneous, ranging from coarse-grained to porphyritic textures (Figure 16B). They occur as sheeted to irregular intrusions that clearly exploit east-west trending foliation planes and northwest-trending dextral shear bands within the paragneiss host rock. This strong structural control indicates syntectonic emplacement facilitated by regional dextral zones related to the Quetico deformation zone. Locally, antithetic sinistral shear bands are also observed. Wider and more homogeneous granite injections locally contain garnet and pegmatitic segregations (Figure 16C and 16D). The presence of these pegmatitic segregations indicate a high degree of melt fractionation, which is incompatible with in situ partial melting. This interpretation is supported by chondritenormalized REE patterns, which display a strongly fractionated geochemical signature and variably developed negative Eu anomalies (Figure 17), comparable to those observed in the S-type granite intrusions of the Quetico subprovince. These features indicate that granitic melts had already undergone significant plagioclase fractionation at depth forming cumulates before extraction of the residual melt along shear zones. Although some textures locally resemble those seen in migmatite, their interpretation as true anatectic melts is not supported by the mineralogy. - 61 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 16 Photographs of shear-hosted leucogranitic injections of the Dog Lake injection complex (Stop 13). (A) Dense network of leucogranitic injections emplaced along foliation planes and dextral shear bands. Note the presence of paragneiss rafts within wider injections (B) Close-up view of dextral shear bands. Note the presence of biotite-rich schlieren in some leucogranitic injections. (C) Schlieric garnet-bearing dyke of leucogranite. Note the presence of pegmatitic pods indicating fluids segregation. (D) Close up on garnets within the dyke of leucogranitic. Melanosome-looking parts are mostly only composed of biotite and lack typical peritectic mineral phases (e.g., garnet, cordierite, sillimanite) expected from insitu partial melting of metasedimentary protoliths. Instead, this outcrop is interpreted as a migration zone for granitic melts generated deeper in the crust, which were channeled upward along regional dextral shear zones. During ascent, these melts assimilated sedimentary host rocks, resulting in the development of characteristic schollen and schlieric textures observed at this outcrop. The large volume of ascending granitic melt likely induced high-temperature, low-pressure metamorphic conditions in the surrounding paragneiss, as evidenced by the development of sillimanite–cordierite assemblages. Locally, the paragneiss also experienced limited partial melting, interpreted to have been induced by thermal input associated with the emplacement of this large volume of granitic melts. Stop 14 (Planned) - Patchy metatexites, Dog Lake injection complex 346711E 5406946N (this stop is not on Figure 1) From the previous stop, return to Highway 527 and cross the highway onto Doodie Road. Park at the entrance of the road. The outcrop is located on the north side of the road. This outcrop exposes an interlayered biotite–quartz– feldspar and garnet–biotite–quartz–feldspar migmatitic paragneiss, interpreted as a patchy metatexites (Figure 18A). Peritectic garnet is present within leucosome patches, providing clear evidence for insitu partial melting of fertile pelitic layers. The estimated melt proportion is relatively low (approximately 5–10%). - 62 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 17 Chondrite normalized rare earth element (REE) patterns of leucogranitic injections from the Dog Lake injection Complex compared with Stype granite intrusions occurring in the Quetico Subprovince. Bedding is largely preserved, indicating low melt connectivity and limited melt extraction (Figure 18B). This migmatitic paragneiss is gently folded, with fold axes plunging eastward. The east–westtrending foliation dips steeply to the north. The paragneiss is cut by boudinaged dikes of white, coarse-grained leucogranite, similar to the leucogranitic injections observed at the previous stop. This outcrop highlights a significant volumetric contrast between the limited amount of melt generated in situ within the paragneiss and the much larger volume of granitic injections observed at the previous stop (Stop 13), despite that the two localities are separated by only ~300 m. This contrast is a strong indication that the granitic melts observed elsewhere in the Dog Lake injection complex were not produced in situ but instead represent migrated and fractionated melts generated at deeper crustal levels, which were subsequently channeled upward along regional shear zones. The spatially restricted partial melting and migmatitization observed at this outcrop therefore do not represent the source of the Stype granites but rather reflect a thermal response to advected heat associated with the emplacement of large volumes of granitic melt in nearby shear corridors, rather than a widespread regional anatexis. Stop 15 (Optional) - Syn-tectonic schlieric pink monzogranite, Dog Lake injection complex 347031E 5405626N (this stop is not on Figure 1) Continue driving south along Doodie Road for approximately 1.5 km. This road is not active at the time of writing; however, road shoulders may be narrow or unstable, so vehicles should be parked directly on the road where it is safe to do so. This final stop exposes a representative outcrop of syntectonic, magnetitebearing pink monzogranite (Figure 19). The intrusion displays a heterogeneous texture, ranging from porphyritic to locally pegmatitic, highlighting the high fluid content of the granitic melt during its emplacement. The granite contains biotite schlieren, resulting of the complete digestion of the paragneiss host rock. Locally, relict structures of the - 63 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 19 Photograph of syntectonic, schlieric biotite pink monzogranite (Stop 15). Note the presence of dextral and antithetic sinistral shear bands. shear corridors. Melt migration was facilitated and channelized by regional transpressional deformation acting in the Quetico Subprovince and its adjacent greenstone belts. REFERENCES Card, K.D., and Ciesielski, A. 1986. DNAG No. 1: subdivisions of the Superior Province of the Canadian Shield. Geoscience Canada, 13: 5–13. Figure 18 Representative photographs of garnet bearing, patchy metatexites from the Dog Lake Injection Complex (Stop 14). (A) In situ partial melting localized within fertile pelitic layers of the paragneiss. Note that primary bedding structures are largely preserved. (B) Close up view of photo A. The upper layer shows a clear lack of segregation between leucosome and melanosome indicating low melt connectivity. original sedimentary bedding can still be inferred within these schlierenrich domains. The intrusion is strongly foliated and affected by welldeveloped dextral shear bands and antithetic sinistral shear bands, along which pegmatitic pods are locally emplaced. This outcrop illustrates how regional dextral shear zones within the Dog Lake complex have acted as efficient pathways for multiple types of granitic melts, which derived from different sources in the lower crust. The pink monzogranite suite is interpreted as the final, most fractionated product of the potassiumrich calcalkaline intrusive suite. These relationships reinforce the interpretation that the Dog Lake complex represents a major migration zone, where granitic magmas produced at depth were focused, transported, and emplaced along Carter, M. W., 1992, Geology and mineral potential of the Tower syenite stock, Conmee Township, District of Thunder Bay, in Dressler, B. O., Baker, C. L., and Blackwell, B., eds., Summary of field work and other activities 1992: Ontario Geological Survey Miscellaneous Paper 160, p. 60–63. Corfu, F., 2000. Extraction of Pb with artificially too-old ages during stepwise dissolution experiments on Archean zircon. Lithos, 53, nos. 3–4, p. 279–291. Corfu, F. and Stott, G.M. 1998. Shebandowan greenstone belt, western Superior Province: U-Pb ages, tectonic implications, and correlations; Geological Society of America Bulletin, v.110, p.1467-1484. Kamo, S.L. 2013. Report on U-Pb geochronology (CA-IDTIMS and LA-ICPMS) of rocks from the Grenville and Superior provinces of Ontario; internal report prepared for the Ontario Geological Survey, Jack Satterly Geochronology Laboratory, University of Toronto, Toronto, Ontario, 50p. Launay, G.A. and Metsaranta, R.T. 2023. Precambrian bedrock geology mapping in the Onion Lake and Sunshine areas, Quetico and Wawa Subprovinces, northwestern Ontario; in Summary of Field Work and Other Activities, 2023, Ontario Geological Survey, Open File Report 6405, p.11-1 to 11-12. - 64 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Launay, G.A. and Metsaranta, R.T. 2024. Mapping regional fractionation patterns in S-type peraluminous granite and pegmatite intrusions in the southern Quetico Subprovince; in Summary of Field Work and Other Activities, 2024, Ontario Geological Survey, Open File Report 6413, p.9-1 to 9-11. Lodge, R.W.D. 2014. Precambrian geology of Aldina Township; Ontario Geological Survey, Preliminary Map P.3776, scale 1:20 000. Lodge, R.W.D., 2016. Petrogenesis of intermediate volcanic assemblages from the Shebandowan Greenstone Belt, Superior Province: evidence for subduction during the Neoarchean. Precambrian Research, 272, p. 150–167. MacDonald, R.D. 1939. Gorham Township and vicinity, District of Thunder Bay, Ontario; Ontario Department of Mines, Map 48C, scale 1:63 360. Ministry of Natural Resources and Forestry 2023. Forest Resources Inventory leaf-on LiDAR; Ministry of Natural Resources and Forestry, Science and Research Branch, Forest Resource Information Unit, online data, April 10, 2022 update, https://geohub.lio. gov.on.ca/maps/lio::forest-resources-inventory-leafon-lidar/about. [accessed April 27, 2023] Metsaranta, R.T. 2015. Preliminary results from geological mapping of the Quetico Subprovince, the Shebandowan greenstone belt and Proterozoic rocks north of Thunder Bay; in Summary of Field Work and Other Activities, 2015, Ontario Geological Survey, Open File Report 6313, p.15-1 to 15-20. Metsaranta, R.T. 2022. Highlights of bedrock geology mapping in the Quetico Subprovince, north of Thunder Bay, northwestern Ontario; in Summary of Field Work and Other Activities, 2022, Ontario Geological Survey, Open File Report 6380, p.9-1 to 9-9. Metsaranta, R.T. and Walker, J.A. 2019. Precambrian geology of western McGregor Township and adjacent areas, northeast of Thunder Bay; in Summary of Field Work and Other Activities, 2019, Ontario Geological Survey, Open File Report 6360, p.11-1 to 11-10. Metsaranta, R.T. and Hamilton, M.A. 2020. A precise U/ Pb age for a north-trending mafic dike from the western flank of the Marathon swarm, East Bay area, northwestern Ontario; in Summary of Field Work and Other Activities, 2020, Ontario Geological Survey, Open File Report 6370, p.7-1 to 7-9. Metsaranta, R.T. and Kamo, S.L. 2021. A uranium–lead baddeleyite age for the Midcontinent Rift–related Lone Island Lake intrusion, northwestern Ontario; in Summary of Field Work and Other Activities, 2021, Ontario Geological Survey, Open File Report 6380, p.12-1 to 12-8. Ontario Geological Survey 2017. Ontario airborne geophysical surveys, magnetic data, grid data (ASCII and Geosoft® formats), magnetic supergrids; Ontario Geological Survey, Geophysical Data Set 1037— Revised. Pan, Y., Fleet, M.E., and Heaman, L. 1998. Thermo‑tectonic evolution of an Archean accretionary complex: U–Pb geochronological constraints on granulites from the Quetico Subprovince, Ontario, Canada. Precambrian Research, 92: 117-128. Percival, J.A. 1989. Late Archean Quetico accretionary complex, Superior Province, Canada. Geology, 17: 23–25. Percival, J.A., Sanborn‑Barrie, M., Skulski, T., Stott, G.M., Leclair, A.D., and Corkery, M.T. 2006. Tectonic evolution of the western Superior Province from NATMAP and Lithoprobe studies. Canadian Journal of Earth Sciences, 43: 1085–1115. Percival, J.A., Skulski, T., Sanborn‑Barrie, M., Stott, G.M., Leclair, A.D., Corkery, M.T., and Boily, M. 2012. Geology and tectonic evolution of the Superior Province, Canada. In: Tectonic styles in Canada: the Lithoprobe perspective. Geological Association of Canada, Special Paper 49, p. 321–378 Ratcliffe, L.M. 2016. Precambrian geology of Sackville Township, Shebandowan greenstone belt, Wawa– Abitibi terrane; Ontario Geological Survey, Preliminary Map P.3802, scale 1:20 000.Ratcliffe, L.M. 2017. Precambrian geology of Adrian Township, Shebandowan greenstone belt, Wawa–Abitibi terrane; Ontario Geological Survey, Preliminary Map P.3813, scale 1:20 000. Ratcliffe L.M. 2019. Precambrian geology of Marks Township, Shebandowan greenstone belt, Wawa– Abitibi terrane, northwestern Ontario; Ontario Geological Survey, Preliminary Map P.3830, scale 1:20 000. Rehm, A. G. 2025. “Tectonometamorphic Evolution, Fluid Production, and Evaluation of Gold Liberation in the Quetico Metasedimentary Belt, Canada.” Ph.D., Laurentian University Sudbury, Ontario. Stott, G.M., Corkery, M.T., Percival, J.A., Simard, M. and Goutier, J. 2010. A revised terrane subdivision of the Superior Province; in Summary of Field Work and Other Activities, 2010, Ontario Geological Survey, Open File Report 6260, p.20-1 to 20-10. Valli, F., Guillot, S., and Hattori, K.H. 2004. Source and tectono‑metamorphic evolution of mafic and pelitic metasedimentary rocks from the central Quetico metasedimentary belt, Archean Superior Province of Canada. Precambrian Research, 132: 53–72. Wang, S., Kuzmich, B., Hollings, P., Zhou, T. and Wang, F. 2020. Petrogenesis of the Dog Lake Granite Chain, Quetico Basin, Superior Province, Canada: Implications for Neoarchean crustal growth. - 65 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Precambrian Research, 346: 105828. 4, Part 1, p.485-541. Williams, H.R. 1991. Quetico Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.383-403. Williams, H.R., Stott, G.M., Heather, K.B., Muir, T.L. and Sage, R.P. 1991. Wawa Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume - 66 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Trip 3 - Geological assemblages, regional structural framework and tectonic evolution of the Neoarchean Shebandowan greenstone belt Dorothy Campbell, P.Geo and Justin Jonsson P.Geo Resident Geologist Program, Ontario Geological Survey, Ministry of Energy and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Introduction This trip provides an overview of the geological assemblages, regional structural framework, and tectonic evolution of the Neoarchean Shebandowan Greenstone Belt (SGB) and their relationship to gold and base metal mineralization. The SGB is situated in the western Wawa Subprovince (Superior Province) and extends 150 km from the Ontario–Minnesota border in the west to northeast of Thunder Bay in the east (Figure 1). The SGB is locally in fault contact with the Quetico Subprovince to the north and bounded by the older (2750 Ma) Northern Light–Perching Gull Lakes batholith (tonalitic gneiss) and younger granitic intrusions to the south (Lodge 2016). The SGB is characterized by a complex history of early rifting, subduction-driven volcanism, tectonic accretion, and later transpressional deformation. The SGB comprises three main assemblages and two primary deformation events (Williams et al. 1991; Stott and Corfu 1991; Corfu and Stott 1998; Percival 2006; Lodge 2016; Reynolds et al. 2023; Dorval et al. 2026): BLF=Burchell Lake fault; USSZ=Upper Shebandowan Lake shear zone; SGFZ=Squeers Lake-Greenwater Lake fault zone; TLFZ=Tinto Lake fault zone; CCF=Crayfish Creek fault; LSSZ=Lower Shebandowan Lake shear zone; MLS=Moss Lake stock; BLS=Burchell Lake stock; HGC=Haines gabbroic complex; HS=Hermia stock; HLS=Hood Lake stock; GLS=Greenwater Lake stock; LGP=Little Greenwater Lake pluton; PCS=Pinecone stock; KS=Kekekuab stock; PS=Peewatai stock; SS=Shebandowan stock; TS=Tower Stock Figure 1. Regional Geology of the Shebandowan greenstone belt (modified from Kuster, Lesher and Houlé, 2022; modified from Sotiriou et al 2019; Lodge 2016; Osmani 1997a; Corfu and Stott, 1998). - 67 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Greenwater assemblage (2722-2719 Ma): volcanic suites characterized by thick sequences of tholeiitic mafic volcanic rocks, ultramafic flows (komatiites) and sills, iron formations, mafic intrusions, and minor FII- and FIII-type felsic volcanic rocks. Burchell assemblage (2719-2716 Ma): calcalkalic, dominantly intermediate volcanic rocks and lesser FI-type felsic volcanic rocks and with no known ultramafic sills or intrusions. This subdivision was first defined by Williams (1991) on the basis of younging directions but rejected by Corfu and Stott (1998) due to lack of chronological distinction and re-interpretation of structural architecture. Lodge (2016) interpreted more recent higher-precision geochronology as supporting a similar subdivision to that of Williams (1991). Kashabowie assemblage (2695 Ma): syn-D1, represents renewed activity on the SGB after a long hiatus. It is less voluminous and more evolved than Greenwater assemblage. Calc-alkaline to alkalic intermediate/felsic volcanic rocks, associated diorites, tonalites (e.g., Shebandowan Lake Pluton), tectonically interleaved with older 2720 Ma volcanic suites. This subdivision was first introduced by Corfu and Stott (1998) as part of their re-interpretation of older assemblage classifications. D1 Compressional Deformation event (2695 and 2690 Ma): associated with calc-alkaline magmatism and intra-arc deformation (thruststacking and interleaving). Shebandowan assemblage (2690-2680 Ma): syn-D2 Timiskaming-type assemblage, unconformably overlies older Greenwater assemblage, composed of calc-alkalic to alkalic volcanic rocks and associated coarse clastic Timiskaming-type sedimentary rocks, iron formation and late sanukitoid plutons. D2 Transpressional Deformation event (2685– 2680 Ma): marked the final accretionary phase of the Wawa subprovince evolution of the Superior Craton, termed the Shebandowanian phase of the Kenoran Orogeny (Stott and Corfu 1991). This late-stage dextral transpression and obliqueslip deformation represents the development of Timiskaming-type pull-apart basins and regional Timiskaming-aged structures. Auto Road assemblage (<2682 Ma): distinctly younger sedimentary assemblage in the SGB, dominated by conglomerate-sandstone units (with clasts of volcanic and granitoid origin). Corfu and Stott (1998) describe the assemblage as a small sedimentary basin, informally termed the “Auto Road assemblage”. The western limb of the SGB is often divided from the central and eastern portions of the belt by an informal north-south boundary roughly, at the town of Kashabowie (Figure 1). There are differences in the distribution of assemblages between the west and east sides: the Kashabowie assemblage is situated mostly along the western limb, the Shebandowan assemblage is situated on the central-eastern side, and the Auto Road assemblage is restricted to a small area on the eastern side. The Greenwater/Burchell assemblage(s) make up the large majority of the preserved supracrustal rocks, despite comprising just ~6 million years of the >40 million-year evolution of the SGB (Figure 1). The volcanism recorded by these assemblages appears to have been two-stage: an extensional plume-rift setting recorded by the Greenwater assemblage followed by a compressional subduction-arc setting recorded by the Burchell assemblage (Figures 2, 3; Lodge 2016). The deposition of chemically distinct Kashabowie assemblage volcanic rocks occurred after a ~21 million-year hiatus, recording a later compressional subduction-arc setting (Figure 3). These rocks are contemporaneous with the D1 structural event, which involved the interleaving and thrust-stacking of the Kashabowie and Greenwater units (Reynolds et al. 2023). Subsequently, the Shebandowan assemblage, represents the final stages of the Shebandowan accretionary event. These “Timiskaming-type,” deposits unconformably overlie the Greenwater assemblage. They are characterized by a mix of clastic sediments (conglomerate, sandstone), iron formation, and calc-alkalic to alkalic volcanic rocks (Figure 3), interpreted to have formed in transtensional, pullapart basins along the flanks of transpressional uplifts (Reynolds et al. 2023). Due to their tectonic setting, these rocks are strongly associated with structurally controlled, late-orogenic gold mineralization (Figure 4). - 68 - Based on the spatial distribution of southwest- �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 2. Schematic illustration of possible tectonic evolution of the Shebandowan greenstone belt in both plan view and crustal cross section (from Lodge 2016). Colors of units correspond to legends in Figure 1. Note sketch is not to scale. (A) Initial plume-dominated tectonic setting forming Greenwater assemblage. (B) Subduction-dominated tectonic setting forming Burchell Assemblage. A change in plate motion results in the initiation of subduction and formation of a calc-alkalic arc dominated by andesitic strata. Subduction of ridge results in high geothermal gradient and melting of slab to produce adakitic melts. Hybridization of mantle and slab derived melts results in magnesian andesites (Mg# > 50) from Lodge 2016. trending metavolcanic rocks on the western limb, Osmani (1997a) defined three distinct geological units that remain in use by current explorers (Figure 6): • Central Felsic Belt (CFB): a <5 km-wide core of the Burchell/Kashabowie assemblage. • Northern Mafic Belt (NMB) and Southern Mafic Belt (SMB): mafic metavolcanic rocks of the Greenwater assemblage, flanking the CFB to the north and south respectively. There are three past-producing mines in the SGB: the North Coldstream copper mine (1957-1967) and the Ardeen gold mine (1932-1936, 1942) in the western part of the belt, and the Shebandowan nickel-copperPGE-cobalt mine (1971-1998) in the eastern part of the belt (Figure 5). Tectonic associations provide spatial context for mineral prospectivity in the SGB (e.g. Lodge et al. 2015, Lodge 2016, Reynolds et al. 2023). Magmatic Ni-Cu-PGE mineralization occurs in the Greenwater assemblage mafic-ultramafic intrusive rocks, notably the sill-hosted deposit comprising the past-producing Shebandowan mine. The mine operated for most of 1971-1998, producing 9.29 Mt at 1.75% Ni, 0.88% Cu, 0.06% Co and 1.83 g/t PGEs. Clusters of magmatic sulfide occurrences also occur in the Haines gabbro (~7 km northwest of the mine) and in the Bateman Lake area (~40 km east of the mine). Although no economic deposits that are definitively of volcanogenic massive sulfide (VMS) affinity exist in the SGB, several prospects exist in spatial association with Greenwater/Burchell assemblage felsic metavolcanic rocks. The North Coldstream copper-gold-silver deposit, located 10 km southwest of Kashabowie on the western arm of the SGB, is a - 69 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 past-producing (2.48 Mt at 1.87% Cu, 0.28 g/t Au, and 5.53 g/t Ag from 1957-1967) atypical deposit variably interpreted to be intrusion-related (e.g. Farrow 1994) or volcanogenic (Reynolds et al. 2023); it is currently being explored by Gold X2 Mining Inc., who tentatively interpret the deposit as a sheared, remobilized VMS system. In more recent years, orogenic gold has become the main focus of mineral exploration in the SGB. Gold is primarily controlled by late tectonic D2 structural zones, in contrast to lithologically controlled magmatic and VMS mineralization associated with Greenwater/ Burchell assemblages. Gold mineralization is generally hosted within ductile-brittle shear zones, particularly near regional fault zones (Figure 4) or adjacent to “Timiskaming-type” unconformities (Figure 14). Gold is typically hosted by quartz-carbonate-pyrite veins and veinlet networks cross-cutting all lithologies. On this field trip, we will look at some specific examples these structural zones: • Moss Gold Deposit (Gold X2 Mining Inc.) and the 111 Zone (Bold Ventures Inc.): gold mineralization occurs near regional fault zones, within sheared diorites, felsic dykes/sills and mafic to intermediate metavolcanic rocks. • I-Zone (Delta Resources Limited): gold-bearing quartz ladder veins within a felsic dyke (brittle, extensional), intruding Timiskaming iron formation. • Eureka Zone (Delta Resources Limited): a key target for gold exploration at the unconformity Figure 3. Evolution of Greenwater/Burchell, Kashabowie, and Shebandowan assemblages (from Reynolds et al. 2023). Figure 4. Schematic section of western Shebandowan greenstone belt (from Reynolds et al. 2023). - 70 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 5. Geology map of the Shebandowan greenstone belt showing location of field trip stops. NCM: North Coldstream Mine, See Figure 1 for all other abbreviations. - 71 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 between the Greenwater and Shebandowan assemblages, marking a “Timiskaming-type” unconformity. Stops 1 to 2 - Moss Gold Deposit (Gold X2 Mining Inc.) Permission is required from company to access sites Gold X2 Mining Inc. (Gold X2) is exploring the Moss gold deposit, a high-tonnage low-grade deposit (Figure 6), located 100 km west of Thunder Bay on the western limb of the SGB. Gold X2 recently completed a Preliminary Economic Assessment, releasing an updated resource estimate as of January 16, 2026, (Dorval et al. 2026) for the deposit as follows: • Indicated: 2.125 Moz Au at 1.03 g/t with 3.160 Moz Ag at 1.53 g/t • Inferred: 3.910 Moz Au at 0.97 g/t Au with 6.273 Moz Ag at 1.55 g/t Engineering trade-off studies & design work is underway and a feasibility study is anticipated for Q3 2027 (Gold X2 Mining Inc., Corporate Presentation, April 12, 2026). The Moss deposit is structurally controlled and situated within intermediate to felsic metavolcanic rocks of the Central Felsic Belt (CFB; Figure 6). Primarily hosted by sheared diorite (Figure 7), the deposit developed during and after intense ductile deformation, with 2 distinct tectonic-hydrothermal events identified (Reynolds et al 2023; Dorval et al. 2026). Alteration occurs in different styles and intensities but is generally composed of albite, biotite, sericite, chlorite, carbonate, epidote and pyrite (typically 2-10% of the rock; locally up to 15%). Gold mineralization occurs in complex arrays of smallscale quartz-carbonate-pyrite veinlets, breccias, and stockworks with higher grades within more intense, narrow shear zones (Nwakanma 2024; Dorval et al. 2026). The sulfide assemblage is dominated by pyrite, with minor chalcopyrite, sphalerite, and molybdenite. Rare, high-grade tellurides are associated with the high-grade gold mineralization (Reynolds et al. 2023; Dorval et al. 2026). Stop 1. Moss Gold Deposit (Portal) N83 Z15 U 668730E 5379177N At this stop, highly sheared diorite and feldspar porphyry has been variably silicified, chloritized, hematized, sericitized and sulphidized (Figure 7). The outcrop is highly fractured and exhibits a network of narrow quartz-carbonate-pyrite veinlets. The now closed-off portal, developed in the mid-1980s by Tandem Resources Limited and Storimin Exploration Limited, lead to historical underground workings and gold zones at the 230-foot (70 m) level (Figure 8). Figure 6. Geology map shear hosted Moss Lake Deposit (in red) modified from Dorval et al. (2026). - 72 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 8. Underground plan of the 230-foot (70 m) level, showing gold-bearing zones of the Moss deposit (from Osmani 1997a; modified from an underground plan of Tandem Resources Limited - Storimin Exploration Limited, 1989). Figure 7. Gold-mineralized diorite at the Moss deposit that has been variably silicified, chloritized, hematized, sericitized and sulphidized. Stop 2. Discovery Outcrop The Moss property has a long history of exploration dating back to 1936, when Mining Corporation of Canada completed 5 trenches that exposed a zone of mineralization later known as the Main Zone (often referred to in historical records as the Snodgrass showing). Gold was initially identified in a mineralized zone hosted by sheared dacite and felspar porphyry near the northern contact with diorite. The zone measured approximately 25 feet (7.6 m) in width and 600 feet (180 m) in length. In 1945, Lobanor Gold Mines Limited followed up with 12 diamond drill holes which ultimately led to the development of the Moss deposit (Figure 9). Subsequently, more than 30 companies explored various smaller sections of the property that were consolidated in 2014-2016 by Wesdome Gold Mines Ltd. In May 2021, Gold X2’s predecessor (Goldshore Resources Inc.) acquired the Moss Gold property from Figure 9. Map showing location of initial trenches, drill holes and gold assay results by Lobanor Gold Mines (1945) (from Harris 1970). - 73 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Wesdome. In 2024-2025, the property was further expanded by: i) purchasing the “Coldstream claims” and acquiring Kesselrun Resources Ltd., whose Huronian project claims include the past-producing Ardeen mine, ii) staking the Hillcrest property (Crayfish Creek Fault extension) and claims covering the Squeers-Greenwater Fault Zone extension, to the north and south of the Moss deposit, respectively, and iii) optioning Sky Gold’s Star Lake property, based on OGS gold-in-till anomalies (Figure 10). These expanded land holdings are strategic and a testament to the importance of regional structures for gold exploration. Stop 3. 111 Au Zone Trench - Burchell Lake Au-Cu Property (Bold Ventures Inc.) N83 Z15 U 676840E 5380320N Permission is required from company to access site The Burchell Lake Au-Cu property, located 95 km west of Thunder Bay, is adjacent to Gold X2’s Moss property to the west. While the property hosts multiple Au and Au-Cu showings, this stop focuses on Bold’s newly discovered 111 Au Zone (Figure 11 and 12). Initial grab samples returned 59.9 g/t and 68 g/t Au (Figure 11). Sampling at the 111 Au Zone by the Regional Resident Geologist (2025) returned up to 61.2 g/t Au and >1.2% Cu. Early assay results from 2026 drilling at the 111 Au Zone, BL-26-001 returned 0.42 g/t Au over 19 m, including 1.1 g/t Au over 5.0 m, and 2.7 g/t Au over 1 m. At this stop, silicified mafic to intermediate metavolcanic rocks are crosscut by a northeasttrending anastomosing shear zone (Figures 11, 12). A 14 m-wide halo of anomalous gold (see red dotted outline on Figure 12) has been outlined, flanked with zinc and copper mineralization. Gold mineralization is associated with disseminated pyrite and stringers of chalcopyrite, hosted in strongly silica‑ and sericite‑altered metavolcanic rocks. Locally, the rock is characterized by intense shearing and alteration obscuring the protolith, potentially a sheared and highly silicified metavolcanic rock or diorite. A narrow, relatively undeformed felspar porphyry occurs adjacent to the shear zone (Figure 12). Osmani (1997b) mapped this location as a felsic metavolcanic-dominated portion of the Southern Mafic Belt, though there was no outcrop exposure at the 111 Au Zone at the time of his mapping. Corfu and Stott (1998) reported a U–Pb zircon age of 2721 ± 1 Ma from a felsic metavolcanic flow less than 2 km to the northeast, interpreted to represent its eruption age. The mafic metavolcanic Figure 10. Map showing Gold X2’s 2025-2026 land acquisitions: Kesselrun’s Huronian project with the past-producing Ardeen Mine (red oval), Hillcrest and Squeers-Greenwater projects (yellow ovals), and Sky Gold’s Star Lake property (blue) with a cluster of gold in-till anomalies (orange and yellow dots). - 74 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 11. Map of land position, major showings, and 111 Au Zone trench highlighted with red oval (from Bold Ventures Inc., news release, October 20, 2025). Figure 12. Geology map showing the 111 Au Zone with channel sample results for gold, copper and zinc (from Bold Ventures Inc, news release, October 20, 2025). - 75 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 rocks here may either be tectonically interleaved with or conformable with the felsic metavolcanic rocks mapped by Osmani (1997b). Stop 4. Pillowed vesicular basalt at Swamp River N83 Z15 U 714018E 5390896N This outcrop is an example of typical Greenwater assemblage tholeiitic mafic metavolcanic rocks. The outcrop is glacially polished with well-defined striations that trend 25°. Glassy pillow selvages and abundant vesicles are well preserved at this location. Pillows are deformed (~10:1 aspect ratio) in the same orientation as foliation, striking 100° and dipping steeply south. Original mineralogy is replaced by a typical greenschist facies assemblage of chlorite, hornblende, sericite, saussurite, and carbonate (Morin, 1973). Pillow selvages appear to have been loci for fluid movement, as evidence by localization of pyrite and carbonate to the selvages. Morin (1973) mapped these pillows as younging to the north-northeast – can you see this? Stop 5. Timiskaming-type conglomerate N83 Z15 U 715392E 5387505N From Aubet and Campbell (2012). At this location two facies of the epiclastic suite of Timiskaming-type rocks are exposed. The dominant rock type is poorly sorted, highly foliated conglomerate (Figure 13). Note the heterolithic nature of the fragments, including minor Keewatin-type red jasper fragments. This particular outcrop is highly deformed with the clasts being stretched, forming a welldeveloped lineation plunging steeply to the southeast. Note the abundant iron carbonate alteration within the sandy matrix. In fault contact with the conglomerate to the west are mudstone and siltstone. Here we have near vertical mineral lineations normal to rolls on the bedding planes. This unit is finely bedded with grading, although present, obscured by the deformation. Stop 6. Autoclastite N83 Z15 U 715698E 5385810N This location is an example of ultramafic metavolcanic rocks of the Greenwater assemblage, featuring a flow-top breccia with a mixture of transported sub-angular, blocky clasts exhibiting some nice examples of random spinifex-textures and Figure 13. Timiskaming Conglomerate variolitic textures. The clasts range in size from 0.5 cm to 20 cm in diameter. Although this particular outcrop was not mapped by Rogers (1995), Rogers and Berger (1995) reported other nearby ultramafic metavolcanic units to be generally narrow (<50 m thick) and discontinuous (<1 km along-strike). Both olivine and pyroxene spinifex have been reported in the eastern SGB (e.g. Hinz 2018). Stops 7 to 10. Delta-1 Au Property (Delta Resources Limited) Permission required from company to access sites The Delta-1 Gold property (formerly Shabaqua Gold Project) is located near Shabaqua, 50 km west of Thunder Bay. The area has a long history of exploration dating back to 1930s, where numerous companies and prospectors carried out prospecting, geological, geochemical, and geophysical surveys, trenching, sampling and diamond drilling programs. While the Eureka deposit is the company’s flagship, Delta Resources significantly expanded the Delta-1 property in 2024 by acquiring more than a dozen properties from numerous companies and prospectors. The Delta-1 property now has multiple gold prospects and occurrences covering a 35-km strike extent of several regional-scale structural zones, near or at the unconformity between Shebandowan (Temiskaming- - 76 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 type) metasedimentary rocks and Greenwater metavolcanic rocks (see black dotted lines in Figure 14). (Portofino Resources Inc., news release, November 17, 2020). Stop 7. I-Zone gold-bearing quartz ladder veins N83 Z15 U 714705E 5382490N Modified from Aubet and Campbell (2012) The I-Zone (and associated gold showings) is an exploration target situated proximal to the Crayfish Creek Fault (Figure 14), a major regional structure currently presenting as brittle but likely with a protracted brittle-ductile history. The I-Zone gold occurrence consists of felsic dikes intruding Timiskaming oxide facies iron formation intercalated with argillite. The felsic dikes are host to gold-bearing quartz-tension/ ladder veins with 3%-5% pyrite and localized visible gold (Figure 15). Fractures opened up in the dike due to the ductility contrast of the enclosing ironrich argillites and the felsic dike. Later hydrothermal fluids, likely carrying gold reacted with the iron oxides resulting in the formation of pyrite and precipitation of native gold. Historical findings at the site include Landore Resources’ 1995 drill program, which intersected 4.32 g/t Au over 41 m, 4.53 g/t Au over 14.4 m, and 4.36 g/t Au over 20.4 m. Additionally, a 2008 mini-bulk sample conducted by Mengold Resources yielded an average grade of 9.9 g/t Au. Portofino Resources Inc. reported 2020 sampling at the I-Zone returned up to 45.9 g/t Au with 6 of 14 samples returning more that 5 g/t Au Figure 15. Simplified geology at the I-Zone (modified from Aubut et al., 1990). Stop 8 Eureka Zone (2024 Delta-1 Eureka Trench above drill hole D1-23-60) N83 Z16 U 290200E 5385348N In 2017, Doug Parker and Barbara D’Silva generated renewed interest in gold exploration in the Shabaqua area, on the eastern limb of the SGB. The ParkerD’Silva team followed up on historical data and OGS gold-in-till anomalies with prospecting, mechanical stripping, and rock sampling, which successfully led to the discovery of the Eureka Gold Zone (Figure 16). In 2019, Mr. Parker optioned the property to Delta Resources Inc. (Delta). Since then, Delta has advanced the project with 140 diamond drill holes (totaling more 40 000 m), confirming a mineralized zone for more than 2.5 km along strike and to a depth of 400 m (Figure 17). The Eureka Zone is situated adjacent to the unconformity between Shebandowan (<2690 Ma) and Greenwater (2720 Ma) assemblages. The Figure 14. Geology map showing Delta 1 Gold property (black outline), regional structural zones (black dashed lines) and gold showings (red stars) situated at or near the uniformity between Greenwater-Shebandowan assemblages (from Delta Resources Inc. website, Projects; Delta-1; Regional and Property Geology, April 2026). - 77 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 ankerite-pyrite veinlets. The quartz-ankerite-pyrite gold veinlets crosscut all lithologies and are hosted within a 300-400 m wide corridor of ankerite-silicasericite altered rocks. The Greenwater assemblage host rocks at this stop are comprised of mafic metavolcanic and ultramafic flows, weathered to a dark rusty brown with rock textures nearly or completely obliterated (Figure 18). Stop 9. Bylund Trench N83 Z15 290490E 5385211N Figure 16. Doug Parker’s 2017-18 prospecting and mechanical trenching programs generated renewed interest in gold exploration in the Shabaqua area. unconformity between the Greenwater (ultramafic and mafic to intermediate metavolcanic rocks) and Shebandowan (Temiskaming-type metavolcanic and metasedimentary rocks) assemblages has a close spatial association with gold occurrences, widely known as prospective for gold exploration (Figure 14). At this stop, Delta’s 2024 trenching program exposed an 11 m surface section of the Eureka Gold Zone, directly above drill hole D1-23-60. This drill hole returned an intersection of 1.79 g/t Au over 128.5 m (including 2.16 g/t Au over 97.5 m), while channel sampling from the surface trench returned an average grade of 1.23 g/t Au over 11 m (Delta Resources Inc., news releases, September 12, 2023, and September 25, 2024). Gold mineralization at the Eureka Zone is hosted by a stockwork of 1 mm to 10 cm wide quartz- At this stop, stockworks of gold-bearing quartzankerite-pyrite veinlets are situated within a broader 300-400 m carbonate-sericite-silica-altered halo that hosts anomalous/low-grade gold mineralization. The mineralized trend strikes ~110° and dips approximately 50-55° north. Mineralization is hosted within Greenwater assemblage rocks – most commonly, a feldspar-phyric tholeiitic basalt. That unit is not seen at this trench; what we see here a silica-rich rock that has historically been interpreted as chert but is being re-evaluated by the company as at least partially comprising highly silicified metavolcanic rocks. On the northeastern end of the trench, silicified komatiite or komatiitic basalt is present, displaying beautiful spinifex texture. Intense pyrite-ankerite alteration is widespread, but gold grades from this trench are relatively low. Three generations of quartz-carbonate veins are visible at this trench: i) NE-trending, steeply NW- Figure 17. Longitudinal section of the Eureka Zone - 78 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 18. Eureka Zone stockwork of 1 mm to 10 cm wide gold-bearing quartz-ankerite-pyrite veinlets hosted by Greenwater assemblage rocks are weathered to dark rusty brown with rock textures nearly or completely obliterated. dipping, ii) NW-trending, steeply dipping, and iii) NE-trending, shallowly dipping. Vein sets 1 and 2 are conjugate and are post-dated by vein set 3. Vein set 1 is the main gold-bearing set. Stop 10. Finmark Metasedimentary Rocks N83 Z16 U 293525E 5383950N From Puumala and Cundari (2023) At this stop we will have an opportunity to view a remarkably well-preserved roadside exposure of Shebandowan assemblage clastic metasedimentary rocks. The following description of these rocks is provided by Carter (1990). The rocks are mainly thinly bedded, the beds ranging in thickness from 5 cm to 12 cm. Primary sedimentary structures comprising load casts and flame structures, small-scale ripple structures, and cross bedding, are well developed in these rocks in the road exposures along Highway 11- 17 about 2.5 km west of the eastern boundary of Horne Township, and in the outcrops immediately southeast of these. Parker (1980) indicates that the “Finmark metasedimentary belt” consists of sandstone-siltstonemudstone sequences that alternate with thick units of cross-stratified sandstone. These sequences display many of the primary sedimentary structures that are characteristic of tidal flat (e.g., rhythmic layering, lenticular, wavy and flaser bedding) and tidal channel (e.g., herringbone cross stratification, large scale crossstratification) depositional environments respectively. Petrology of these rocks indicates that the primary sediment source was a felsic to intermediate volcanic terrain (Parker 1980). Characteristics of the clasts and rock fragments are consistent with a proximal Shebandowan assemblage sediment source. - 79 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Stop 11. Auto Road Assemblage (Optional) mafic clasts to nearly undeformed. N83 Z16 U 313838E 5377726N The Auto Road assemblage comprises a small sedimentary basin in south-central Ware township. It was first provisionally subdivided by Corfu and Stott (1998) on the basis of a U-Pb in youngest detrital zircon age of 2682±3 Ma – this is 9 m.y. younger (and outside of error provisions) than the youngest detrital minerals (zircon and titanite) in the Shebandowan assemblage. The assemblage is affected by D2 deformation and therefore provides a lower constraint on both sedimentation and regional transpression in the SGB. Corfu and Stott (1998) comment: The map pattern suggests that this conglomerate-sandstone unit is interbedded with Greenwater assemblage basaltic units (Brown, 1995), yet the polymictic conglomerate includes feldspar-hornblende-phyric volcanic clasts typically found within the Shebandowan assemblage. Also common are coarse granitoid cobbles as well as clasts of various volcanic lithologies. The results for sandstone sample Au presented below demonstrate that this is indeed one of the youngest supracrustal units of the Shebandowan greenstone belt as well as of the neighboring Quetico Subprovince, justifying its separate designation. Acknowledgements We would like to thank Gold X2 Mining Inc., Bold Ventures Inc., and Delta Resources Limited for permission to access parts of their properties and for their generous time, knowledge and support in preparing for this field trip. REFERENCES Aubet, A. and Campbell, D. 2012. Field trip 4 - Shebandowan Mine Area In; Hollings, P., MacTavish, A. and Addison, W. (Eds.), Institute on Lake Superior Geology Proceedings, 58th Annual Meeting, Thunder Bay, Ontario, Part 2 - Field trip guidebook, v.58, part 2, 2-26. Aubut, A., Lavigne Jr., M.J., Scott, J. And Kita, J. 1990. Metallogeny, Stratigraphy and Structure of the Shebandowan Greenstone Belt; Field Trip 3 Guidebook, Mineral Deposits of Central Canada, CIM Thunder Bay Branch. Campbell, D.A. and Rainsford, D.R.B. 2020. Nickelcopper-cobalt-PGE potential in the Shebandowan greenstone belt; in Ontario Geological Survey, Resident Geologist Program, Recommendations for Exploration 2019–2020, p.69-74 At this location, felsic intrusive, chert, and felsic to mafic intrusive clasts of up to 40 cm in size are deformed (up to ~5:1 aspect ratio) by D2 transpression. Mafic clasts are deformed to a roughly uniform degree, while felsic clasts vary from similarly deformed as Carter, M.W. 1990. Geology of Goldie and Horne townships; Ontario Geological Survey, Open File Report 5720, 189p. Corfu, F. and Stott, G.M. 1998. Shebandowan greenstone belt, western SuperiorProvince: U–Pb ages, tectonic implications, and correlations. GSA Bulletin 110,1467–1484. Dorval, A., Lussier, D., Michaud, C., Taschereau, C., Vanier-Larrivée, N., Shankie, S. 2026. Preliminary Economic Assessment NI 43-101 Technical Report, Moss Gold Project, Ontario Canada, prepared for Gold X Mining Inc. by G. Mining Services Inc. Farrow, C.E.G. 1994. Base metal mineralization, Shebandowan greenstone belt, District of Thunder Bay in Summary of Field Work and Other Activities 1994, Ontario Geological Survey, Miscellaneous Paper 163, p. 22-97 to 22-104 Harris, F.R. 1970. Geology of the Moss Lake area, Ontario Geological Survey, Geological R085, 89p. Hinz, S.L.K. 2018. Geochemistry and petrography of the ultramafic metavolcanic rocks in the eastern portion of the Shebandowan greenstone belt, northwestern Ontario; Lakehead University, unpublished MSc thesis, 157p. Figure 19. Polymictic conglomerate of the Auto Road assemblage. Inco Limited Ontario Division 2001. Shebandowan Mine closure plan Part I of II: unpublished report; Ministry of Energy and Mines, Thunder Bay Mining Division; - 80 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Thunder Bay District, 84p. Survey, Special Vol. 4, Part 1, pp.145-238. Kuster, K., Lesher C.M., and Houlé, M.G. 2022. Geology and geochemistry of mafic and ultramafic bodies in the Shebandowan mine area, Wawa-Abitibi terrane: implications for Ni-Cu-(PGE) and Cr-(PGE) mineralization, Ontario and Quebec, Geological Survey of Canada Scientific Presentation 130, 25p. Lodge, R.W.D., Gibson, H.L., Stott, G.M., Franklin, J.M. and Hudak, G. 2016. Geodynamic setting, crustal architecture, and VMS metallogeny of ca. 2720 Ma greenstone belt assemblages of the northern Wawa subprovince, Superior Province. Canadian Journal of Earth Sciences, vol. 52, p. 196-214. Lodge, R.W.D. 2016. Petrogenesis of intermediate volcanic assemblages from the Shebandowan greenstone belt, Superior Province: Evidence for subduction during the Neoarchean: Precambrian Research, v.272, p.150–167. Morin, J.A. 1973. Geology of the Lower Shebandowan Lake area, District of Thunder Bay. Ontario Geological Survey, Report 110, 45p. Nwakanma, M.U. 2024. Characterization of alteration and mineralization of the Moss gold deposit, Shebandowan greenstone belt, Northwestern Ontario, Lakehead University, Department of Geology, Masters Thesis, 173p. Osmani, I.A., 1997a. Geology and mineral potential: Greenwater Lake area, west-central Shebandowan greenstone belt; Ontario Geological Survey, Report 296, 135p. Osmani, I.A. 1997b. Precambrian Geology, BurchellGreenwater Lakes area, west half; Ontario Geological Survey, Map 2622, 1: 20 000. Parker J. R. 1980: The Structure and Environment of Deposition of the Finmark metasediments, Thunder Bay, Ontario. Unpublished Hon.B.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 90 p. Percival, J.A., Sanborn-Barrie, Skulski, T., M., Stott, G.M., Helmstaedt, H., and White, D.J. 2006. Tectonic evolution of the western Superior Province from NATMAP and Lithoprobe studies. Geological Survey of Canada, NRC Research Press Web site at http:// cjes.nrc.ca.on 4 September 2006. Puumala, M. and Cundari, R. 2023. Geological highlights of the Thunder Bay area, Thunder Bay South Resident Geologist’s Office, unpublished field trip guide, 23p. Reynolds, N., Field, M., Fung, N., Peruse, C., Raponi, R., Ugarte, E., Gupta, N. 2023. NI 43-101 Technical report mineral resource estimates for the Moss Gold and East Coldstream deposit, Ontario, Canada, prepared for: Goldshore Resources Inc., 285p. Rogers, M.C. 1995. Precambrian geology, Duckworth township; Ontario Geological Survey, Map 2621, 1:20 000. Rogers, M.C. and Berger, B.R. 1995. Precambrian geology, Adrian, Marks, Sackville, Aldina and Duckworth townships. Ontario Geological Survey, Geological Report 295, 66p. Williams, H.R., Stott, G.M., Heather, K.B., Muir, T.L. and Sage, R.P. 1991. Wawa Subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p.485-541. Sotiriou, P., Polat, A., Frei, R. 2019. Petrogenesis and geodynamic setting of the Neoarchean Haines Gabbroic Complex and Shebandowan greenstone belt, southwestern Superior Province, Ontario, Canada: Lithos, v.324-325, p.1–19. Stott, G.M. and Corfu, F.1991.Uchi subprovince. In Geology of Ontario. Edited by P.C. Thurston, H.R. Williams, R.H. Sutcliffe, and G.M. Stott. Ontario Geological - 81 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Trip 5 - Archean Geology and Metallogeny of the Rainy Lake Wrench Zone K. Howard Poulsen Geological Consultant USA. It includes approximately 2600 km of rocky shoreline plus more than 1600 islands and covers an area of approximately 930 square kilometers. Rainy Lake is fed from the east by the Seine River waterway and is drained westward by the Rainy River which leads to the even larger Lake of the Woods and the Winnipeg River system (Fig. 1). For centuries it has been part of the historic water link between the Atlantic and Arctic watersheds: it was known as Tekamaniwen to the indigenous inhabitants of the region and as Lac a la Pluie to the French voyageurs and fur traders. Rainy Lake and Lake of the Woods are remnants of the vast glacial Lake Agassiz which formed by melting of the Wisconsin continental ice sheet approximately 13,000 years ago. The predominately Archean bedrock in the Rainy Lake region (Figs. 1, 2) is now exposed in arched, glacially-sculpted outcrops within areas of generally thin and discontinuous surficial cover overgrown by boreal forest. A little learning is a dangerous thing; Drink deep, or taste not the Pierian spring: There shallow draughts intoxicate the brain, And drinking largely sobers us again. Fired at first sight with what the Muse imparts, In fearless youth we tempt the heights of Arts, While from the bounded level of our mind Short views we take, nor see the lengths behind; But more advanced, behold with strange surprise New distant scenes of endless science rise! So pleased at first the towering Alps we try, Mount o’er the vales, and seem to tread the sky, The eternal snows appear already past, And the first clouds and mountains seem the last; But, those attained, we tremble to survey The growing labors of the lengthened way, The increasing prospects tire our wandering eyes, Hills peep o’er hills, and Alps on Alps arise! Alexander Pope, 1711 Foreword Rainy Lake is a body of fresh water which straddles the border between Ontario, Canada and Minnesota, My first visit to Rainy Lake was in summer 1966 when I helped a geophysical operator evaluate the longwire electromagnetic survey method for our employer Dr. Ray Oja, a geological consultant working out of Thunder Bay. The equipment test was focused on the Grassy Portage Bay property of Noranda Mines Ltd. which included copper mineralization on their Halkirk Figure 1: Southwestern Superior Province with locations of selected mineral deposits. The area of Figure 2 is outlined by the dashed rectangle. - 82 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 2: Rainy Lake Geology – Watten (Northrock) prospect: C.J. Hodgson who later became one of my thesis supervisors at Queen’s had completed his MSc thesis on this deposit in 1959. I returned to the property in the early 1970’s with Dr. Mel Bartley who was then consulting for Northrock Mines and I helped him log a section of diamond drill core from the deposit which is located on the south flank of a feature known as the Rice Bay Dome (Fig. 2). Mel, who was a well-regarded geologist and one of the founders of Lakehead University, also consulted around that time for George Armstrong of Fort Frances. George was a successful highway construction contractor who, along with Mike Hupchuk, was also an avid part-time prospector. They had discovered Zn mineralization in 1971 near Pocket Pond east of Rice Bay. Mel and I made a site visit to Pocket Pond in fall 1972 and I prepared a report on the geophysical data for the property (Poulsen, 1973). At that time, I noted the existence of abundant outcrops along the drill roads near Armstrong’s trenches which were extremely large for the time - they had been excavated by his road construction crew! Jim Franklin, for whom I had been a research assistant at Lakehead University, left in July 1975 to join the GSC in Ottawa while I became a full-time technician in the geology department. I also began an independent look at roadside outcrops around Thunder Bay with a view toward identifying a possible thesis topic with Dick Ojakangas, the well-regarded sedimentologist at Duluth who was also famous for his Finn jokes. Around the same time, however, I told Jim about the interesting geology and the massive sulfide mineralization at Rainy Lake and we decided to investigate further. Armed with Andrew C. Lawson’s classic GSC Memoir 40 as well as Fred Harris’ more recent Ontario geology maps as guides, we visited several outcrops on Rainy Lake in summer 1976. In particular we revisited the Pocket Pond property where we confirmed my original observation that, based on pillow shapes, the strata appeared to be overturned. We also visited Lawson’s classic outcrops at Bear’s Passage using a Zodiak boat only to realize when we got there that they now are located at a boat launch site which is easily accessible by road! We nonetheless had also found the key outcrops and agreed that the staurolite-bearing metasedimentary rocks are also overturned. We later met John Wood of the Ontario Geological Survey who was mapping to the east at Mine Centre and he showed us outcrops of Lawson’s Seine conglomerate and of gold-bearing quartz veins near Bad Vermilion Lake. Once Lakehead University gained approval for its own M.Sc. program in geology, I applied to become the first (part-time) graduate geology student at Lakehead to study the problem of the apparently - 83 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 overturned stratigraphy around the Rice Bay Dome and Bear’s Passage. The thesis work began in 1978 under the supervision of Manfred Kehlenbeck and the newly hired structural geology professor, Graham Borradaile. The resulting structural analysis showed that, contrary to the long-standing interpretation of the Rice Bay Dome as originally proposed by Lawson, the evidence was clearly in favour of downward facing folds and extensive stratigraphic inversion but it was not easy to convince others of this: I had, somewhat unwittingly, stumbled into a larger problem that had already dominated the discussion of the Archean rocks northwest of Lake Superior for many decades. The Institute on Lake Superior Geology was initiated in 1955 as an annual meeting, most times with companion field trips, to discuss developments in geological understanding of both the U.S. and Canadian side of Lake Superior. Early meetings emphasized iron ore deposits which were of the greatest economic importance at that time but evolved into an exploration of a much more eclectic range of topics. Mel Bartley and Ed Pye of Port Arthur were among the early participants and Samuel S. Goldich, a pioneer of geochronology, was a founder and frequent contributor. I attended my first ILSG meeting at Madison, Wisconsin in 1973. Among the speakers were Paul Sims and Klaus Schultz of the U.S. Geological Survey and Don Davidson and John Green of the University of Minnesota at Duluth. A memorable and perhaps prophetic moment came during a presentation on Proterozoic stratigraphy of the Lake Superior area by the mild-mannered John Green who suggested that the Puckwunge Formation should be excluded from the Keweenawan Group. The proposal brought loud and angry condemnation by the short, red-faced Sam Goldich even before the talk was completed. As it turns out, although Goldich was a painfully shy and quiet individual in social situations, he was equally fierce and combative in professional settings. This proved to be the case again in 1976 at the ILSG meeting at St. Paul Minnesota. I attended Goldich’s excellent field trip to the Archean gneisses of the Minnesota River Valley, including a visit to a small outcrop in a swamp where he believed he had sampled and analysed the oldest rock on Earth as reported by that time. When one of his former graduate students questioned the statistical validity of Goldich’s data regression, the offender was told in no uncertain terms that, if he didn’t like the method, he could just leave the field trip immediately! At that time Goldich was a member in high standing of an international group of geologists with a strong interest in Precambrian geochemistry and geochronology. The Canadian leader within this group was Alan M. Goodwin of the University of Toronto who had conceived of and organized the multidisciplinary Superior Geotraverse Project which ran from 1970 to 1978. Near the project’s end Goodwin organized the Archean Geochemistry Conference in summer 1978 to highlight the significant results. This meeting involved the “who’s who” of Precambrian geochemistry at the time and, after a series of conference presentations at the Quetico Centre, the group headed west to Fort Frances-International Falls. Sam Goldich and Zell Peterman led a one-day field trip on route to illustrate aspects of the geology at Rainy Lake. Peterman had completed his MS thesis with Goldich in 1959 on the metasedimentary rocks of the Rice Bay Dome and now was a geochemist with the US Geological Survey in Denver. With prior arrangement by Jim Franklin who was a formal participant, I was able to tag along unofficially and silently as a beginning graduate student. The emphasis at each stop was placed on the chemical composition of rock units as recorded on hand-written file cards which Goldich drew from a deck at each outcrop. At one exposure there was considerable debate about whether a xenolith-rich lamprophyre dike might actually be a new locality of the Seine conglomerate but the important localities at Bear Passage and Pocket Pond were not part of the field trip. The classical stratigraphic interpretation of the eminent geologist Andrew C. Lawson was adhered to and I was not in any position to offer an objection. I made my first formal presentation on “Polyphase Deformation of Archean Rocks at Rainy Lake, Ontario” on May 10th of the following year at the Institute of Lake Superior Geology meeting at Duluth where I made the case for overturned strata around the Rice Bay Dome, contrary to Lawson’s original interpretations. Sam Goldich, who often referred reverently to “Professor Lawson”, was upset by this, so much so that he was unable to speak to me about it in person: he sent a delegation of Zell Peterman and Paul Sims instead to voice his displeasure. Both were apologetic and conciliatory and asked if it would be possible for me to arrange a field trip to visit the outcrops in question and this was set for later in the summer. In addition to Goldich, Sims and Peterman, the field trip participants included Dick Ojakangas - 84 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 and David Southwick from Minnesota as well as the Ontario geologists John Wood, Charlie Blackburn and Dick Beard. The event did not start well with Goldich clearly muttering something to the effect of “young punks don’t know anything” but the situation improved somewhat with successive stops. The second to last was at Lawson’s famous Bear’s Passage exposure: the group hadn’t fully had a chance to look closely at the outcrop before Dick Ojakangas, with a pointing of his thumb, indicated the southwestward direction of younging of the northwestward-dipping graded beds. This prompted Goldich to become agitated and declare that he didn’t believe graded bedding was a reliable criterion: Ojakangas, who had completed his PhD on the Cretaceous turbidites of the Great Valley Sequence in California, responded calmly that he had measured at least a thousand similar beds there without conflict. The final stop was at the exposure of pillow basalt near Pocket Pond where the field relationships proved to be even more convincing. Paul Sims, a nononsense individual whose standard dinner included a martini, a rare steak and a salad, followed by a footlong cigar, had remained quiet throughout the day but, when confronted with the outcrop, he turned to Goldich and said “there’s no question about this Sam, the section is overturned”. This prompted Goldich to smile, walk over and shake my hand, saying “well young man, you showed me something important today that I didn’t know before – let’s go back to Fort Frances and drink some of that “Canadian” beer”. He was always cordial to me from that point onward and made a point of connecting again in the field the following season. That one-day field trip was essential in demonstrating the credibility of the field observations and a second presentation on overturned Archean successions at the 1980 ILSG meeting at Eau Claire met with little resistance. A companion journal paper which previously had been rejected by the editor was ultimately accepted for publication with minor revisions by the Canadian Journal of Earth Sciences on June 17, 1980. The involvement of representatives from the Ontario and Minnesota geological surveys also proved to be important. Charlie Blackburn and John Wood later asked me to incorporate many of the field stops into one leg of a multi-day OGS-led excursion on Western Wabigoon Geology for the May 1982 GAC Meeting at Winnipeg. Dick Beard also lobbied hard for the funding of my subsequent work for Sandy Colvine of the Mineral Deposits Section of the OGS on the mineral deposits of the Mine Centre-Fort Frances area. Much of the field work for the OGS had been completed by 1981 and formed the basis of a third ILSG presentation at International Falls in 1982. Dave Southwick of the Minnesota survey was the organizer of the meeting and late in 1981 asked me if I would lead a related field trip focussed on the mineral deposits of the area. I agreed and we set a limit of 25 participants but this was a period of renewed in interest in gold exploration so registration quickly filled up. Dave contacted me again in the New Year and asked if we could double the limit to 50 participants and I reluctantly agreed but that limit was also reached in a short time so Dave developed a waiting list which grew to more than 20 requests. He contacted me once more to ask whether I would accommodate additional participants if he could find a Greyhound bus and provide shuttle vans and drivers to speed up the logistics at some field stops. I once again agreed and an exhausting, but gratifying, one-day, 10-stop field trip was delivered to 77 participants on May 5, 1982. Two weeks later, John Wood and I also led a field trip with a structural-stratigraphic focus as part of the larger excursion organized by Charlie Blackburn for the Winnipeg meeting. What follows is an attempt to illustrate the historical development of ideas about Rainy Lake geology using outcrops in the Mine Centre – Fort Frances corridor (Fig. 2). It includes a concise historical overview and a summary account of the highlights of the regional geology followed by updated descriptions of representative field localities. The motivation for doing this involves three main considerations. The first is purely practical and involves the precision and accuracy of outcrop locations. After the passage of more than forty years, some of the sites described in the guidebooks of 1982 are difficult to re-locate, especially for someone without prior knowledge of the area. Most mapping at that time was largely carried out on nonrectified aerial photographs with considerable local distortion and the modern digital tools of geographic positioning were not available. Furthermore, destruction of vegetation in some areas and new growth in others has rendered past bush trails and landmarks to be obscure, if not impossible, to recognize, even for the author of the guidebooks. Abandonment of old bush roads has also given way to new gravel access roads and the widening of highways has compromised some outcrops while exposing new ones. A second consideration is the currency of information and ideas. - 85 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 The work in the late 1970’s and early 1980’s was done within a limited context without much consideration of comparable situations globally: this is somewhat ironic because one of Lawson’s goals had been to use this region as a global type example of an Archean granitegreenstone belt. It is also important to recognize that new contributions have been made to the understanding of the geology of this area since the original guidebooks were written. The final consideration is historical. In its time the “Seine-Coutchiching problem”, rooted in the simple but laborious task of geological field mapping, influenced discussions among geologists world-wide but many of the details about the background to the debate have not been adequately recorded, especially in the context of the outcrops themselves. Every effort has been made to avoid duplication of points which are adequately covered in the past guidebooks for this region and the material presented below is meant to serve mainly as a source of information for field trip leaders and new researchers to draw on to supplement the existing documents. The Seine-Coutchiching Problem The discovery of gold at Lake of the Woods in 1878 followed by the building of the C.P.R. line prompted Arthur Selwyn, director of the Geological Survey of Canada, to instruct one of its senior mappers to begin a survey of the geology of this area. Part of the task given to Robert Bell and his young assistants, A.C. Lawson and J.W. Tyrell, was to have the geology carefully worked out as a type locality for the “socalled Huronian system” (Zaslow, 1975, p. 184). Bell left Lawson and Tyrell at Bigstone Bay in spring 1883 to map the shoreline geology and topography while he Figure 3: Simplified geology of the Coutchiching Rapids area using the colour scheme of Figure 2. surveyed a line northward toward Red Lake. When he returned, he checked their results and directed them to work separately, Lawson on geology and Tyrell on topography, before they all reconvened at Rat Portage (now Kenora) at the end of the season. By the beginning of the 1884 field season, Andrew Cowper Lawson had graduated from the University of Toronto with the gold medal in natural science and was put in charge of the project. By then, at age 23, he had been taken on staff at the Geological Survey of Canada and, during that field season, he continued the geological work at Lake of the Woods. A.E. Barlow and W.H.C. Smith independently mapped the topography southward toward Rainy Lake. Lawson’s geological report and maps resulting from the work conducted at Lake of the Woods were published in 1885, questioning the approach the Survey had taken in the mapping Precambrian rocks up to that time (Zaslow, 1975). Lawson (1885) argued that the greenstone which he termed “Keewatin” is clearly intruded by foliated granitoid rocks. He termed these “Laurentian” in keeping with the original GSC terminology introduced by its first director W.E. Logan to denote quartzo-feldspathic basement gneiss upon which all supracrustal rocks had been deposited. Although Lawson’s productivity was admired, his geological interpretations were doubted by more senior geologists. Perhaps because of the attention he gained and the fact that he had by now received an M.A. from the University of Toronto, Lawson was able to prevail on Selwyn to support his further academic advancement. He was allowed to attend courses during the winter in the U.S.A.: he was the first of many GSC geologists to follow this course of action for decades to come. Lawson, with Smith as topographer, began by mapping the canoe routes between Lake of the Woods and Rainy Lake in 1885 and in 1886 focussed on systematic mapping of the Rainy Lake area (Fig. 2). The following season, Lawson filled in the details in the Rainy Lake district while Smith moved eastward along the Seine River Route and southward to the Canada-U.S. border. Lawson recognized a series of metasedimentary rocks exposed along the Coutchiching rapids at the outlet of Rainy Lake into Rainy River at modern-day Fort Frances and International Falls (Fig. 3). He traced these rocks farther northeastward from the type locality into the Rice Bay and Bear’s Passage areas (Figs. 4, 5) where the evidence suggested that rocks of the Coutchiching series are even older than the Keewatin (Fig. 6a). This - 86 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 4: Simplified geology of the Rice Bay area only added further to his dispute with GSC management leading to the heavy editing of his map and first report on Rainy Lake geology (Lawson, 1888). He also provided additional evidence that, rather than being a fundamental basement gneiss, the Laurentian rocks are actually deformed and metamorphosed intrusions that show evidence of cross-cutting the supracrustal rocks. A hand-written version of the report was also submitted for his PhD thesis at John’s Hopkins University (Lawson (1888) where he applied the relatively new technique of optical petrographic description to thin sections from his field samples. Although the accomplishments of Lawson and his colleague W.H.C. Smith were significant, it was the resulting geological interpretation that met continued Figure 5: Simplified geology of the Bear’s Passage area. Note that “Bear’s Passage” refers to the strait linking Swell Bay to Redgut Bay but the terms “Bear Pass” or “Bear Passage” have also been used over time to describe the nearby area. resistance from management and the 1887 report was heavily edited (Saslow, 1975). Lawson, however, largely prevailed and showcased his results at the Fourth International Geological Congress at London in 1888 and the American Association for the Advancement of Science meeting at Toronto in 1889. During the 1889 field season Lawson completed the mapping of the Hunter Island Sheet southeastward of Rainy Lake with Smith but resigned from the Geological Survey of Canada in spring 1890. He worked for a while as a geological consultant in Vancouver but soon accepted a faculty position at the University of California at Berkley, where he pursued an illustrious career for the Figure 6: Portrayals of stratigraphic order at Rainy Lake (1887-1999). - 87 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 next sixty years. Although increasingly accepted overall, some aspects of Lawson’s interpretation of Rainy Lake geology continued to be questioned by his peers. In particular, Coleman (1898) noted that the sedimentary sequence at Shoal Lake (Fig. 7), portrayed by Lawson as belonging to the Coutchiching, includes conglomerate with abundant rounded clasts of both Keewatin greenstone and Laurentian granitoid rocks which are also exposed nearby. The U.S. Survey, which was responsible for mapping the southward extensions of the area covered by Lawson and Smit, took particular exception to Lawson’s interpretations. A special committee on stratigraphic nomenclature for the Lake Superior region was therefore convened by the U.S. Geological Survey and the Geological Survey of Canada and the resulting report was published in the Journal of Geology (Adams et al., 1905). It was critical of Lawson’s interpretation and suggested that there is evidence for the Coutchiching rocks to be younger than the Keewatin, a point that was later re-affirmed Minnesota by Van Hise and Leith (1909). Lawson was irate over the findings of the special committee and R.W. Brock, who was by then the director of the Geological Survey of Canada, invited Lawson to re-study the key parts of his original Rainy Lake map sheet in 1911. Lawson was also given the mandate to examine the rocks farther east along the Seine River toward Steeprock Lake and Sapawe. Several practical developments had ensued since the first mapping, including a gold rush to Mine Centre in the 1890’s, construction of the CNR south line through the area circa 1906 and major forest fires in the region in 1910, generating much new bedrock exposure. By then Lawson was in mid-career and had gained pre-eminence in many aspects of geological science in the western U.S. so that, when he produced his famous Geological Survey of Canada Memoir 40 in 1913 and an accompanying map in 1914, they were accepted without revision. In the memoir he reaffirmed his interpretation of the field relationships in the Rice Bay and Bear Passage areas where he observed the Coutchiching metasedimentary rocks to dip at moderate angles below Keewatin strata. He also issued a bitter challenge to the members of the special committee (Memoir 40, p.13-14): “The facts here recited in regard to this line of contact, particularly near the railway on the shores of Bear Passage and the south end of Redgut Bay, taken in connexion with the relations of the Coutchiching to the granite, appear to me to prove conclusively the superposition of the Keewatin upon the rocks mapped by me as Coutchiching in the report of 1887. I invite the attention of the International Committee and of the U.S. Geological Survey to this section and challenge them in view of the facts there Figure 7: Bad Vermilion Lake area - 88 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 apparent and easily accessible, to deny the relations of the Keewatin and Coutchiching as I mapped and described them a quarter of a century ago. The fact that these eminent authorities have denied in toto the existence of the Coutchiching series as a constituent member of the Archean below the Keewatin, without any attempt to verify the very explicit statement of the evidence in regard to this section contained in the report of 1887 places them in a curious light from the point of view of scientific method.” While forcefully retaining his contention that the Coutchiching rocks at Rice Bay (Fig. 4) and Bear’s Passage (Fig. 5) are positioned below the Keewatin, Lawson also admitted to an error in the Bad Vermilion Lake area (Fig. 7). There he proposed the new term “Seine Series” for the conglomeratic metasedimentary rocks that he had previously included as a basal part of the Keewatin. He now confidently placed the basal Seine conglomerate unconformably above both the Laurentian granitoid rocks and the Keewatin metavolcanic rocks in that area (Fig. 6b). He also noted the presence of trough crossbeds in the sandy portions of the Seine series near Old Mine Centre (Fig. 7) and used the newly recognized criterion of determining the directions of stratigraphic younging using their shapes. This allowed him to define a synclinal fold within the Seine sedimentary sequence in a narrow belt extending eastward along the Seine River (Fig. 2). He also recognized that the Seine Series locally extended farther eastward beyond the Rainy Lake area. In so doing, he mistakenly classified sedimentary rocks at Sapawe (Fig. 1, then known as Iron Spur) as part of the Seine Series. The granitoid Blalock stock cuts metasedimentary rocks discordantly at that locality so Lawson introduced the new term “Algoman” for such intrusions which he believed to be generally younger than the Seine (Fig. 6b). Although subsequent studies at Sapawe support Lawson’s contention of a late-tectonic intrusion, they also have consistently portrayed the intruded sedimentary rocks there as part of the Coutchiching rather than the Seine. Nonetheless, Lawson offered other acceptable field and petrographic distinctions that argue for the existence of a younger set of Algoman granitoid intrusions in the Rainy Lake area proper. They tend to contain higher proportions of K-feldspar than the dominantly sodic tonalitic rocks which comprise the Laurentian. He also mapped a narrow band of conglomerate near Hopkins Bay, west of Rice Bay (Fig. 2), and tentatively correlated it with the Seine sequence: at that locality he also presented strong evidence that the conglomerate is cut by younger Algoman granitoid rocks. As he had in the 1880’s Lawson used the International Geological Congress, this time at Ottawa in 1913, to promote his revised view of Rainy Lake geology and Precambrian stratigraphy in general. A debate was staged between Lawson and C.K. Leith to present arguments for and against the findings of the special committee: as later recalled by Leith, Lawson had a “slashing style” and “while I came out feeling I had presented the facts, I also felt Lawson had chewed me up and thrown me to the wolves” (Dott, 2001, p.1007). Lawson’s views were further solidified during the 1913 International Congress field trip that he led on Sunday, August 17th for approximately 90 participants who had traveled by C.N.R. to the Mine Centre and Bear’s Passage train stations after similar visits at Iron Spur and Steep Rock Lake the day before. At Mine Centre, participants were given the option of riding in horsedrawn wagons from the station to the Golden Star Mine along the Shoal Lake Road or taking a short boat ride across Bad Vermilion Lake to a walking trail leading to the mine (Fig. 7). At Bear’s Passage, one group was assigned to a boat trip which visited lakeshore outcrops along Redgut Bay and Bear’s Passage (Fig. 5) while others made a traverse though a similar geological section exposed relatively new rock cuts along the C.N.R. railway line. A carefully prepared itinerary and field guide for both of the historic localities (Uglow, 1913) allowed Lawson to illustrate the nature of each of his five stratigraphic units and his observations on the relationships among them. Lawson’s revised interpretation of the geology of the Lake of the Woods and Rainy Lake regions prevailed for another decade (Bruce, 1925) before the idea that there was still a “problem” was revived by F. F. Grout of the Minnesota Geological Survey. Grout (1925) reviewed the field relationships at the type locality of the Coutchiching near International Falls (Fig. 3) and at outcrops which Lawson had assigned to the Seine in the area of Neil Point farther to the east. Grout affirmed Lawson’s use of cross-bedding as an indicator of stratigraphic younging at Neil Point but offered an alternative overall interpretation which placed both the Seine and Coutchiching above the Keewatin. He then moved northeastward across the international boundary to a locality south of Bear’s Passage known as Morton Island (Fig. 5). There he used the newly recognized - 89 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 field criterion of graded bedding to deduce that the direction of stratigraphic younging in the Coutchiching is northward and away from the Keewatin volcanic rocks. He also visited the key localities at Rice Bay, Bear Passage, Shoal Lake and also at Jackfish Lake southwest of Steeprock Lake. In all cases he raised objections to Lawson’s positioning of the Coutchiching and placed it above both the Keewatin and the Seine (Fig. 6c). The problem expanded in scope when, during a subsequent field examination with T.L. Tanton of the Geological Survey of Canada, it became apparent that Grout had made a significant observational error on the Minnesota side of the boundary by misinterpreting an intrusion breccia to be a conglomerate of sedimentary origin. Tanton (1927) took great pains to publicly point this out at a Geological Society of America Precambrian Symposium, noting that the error was made by “a Minnesota geologist”. Grout, in a discussion of Tanton’s paper, duly acknowledged his own mistake but also stated combatively that a “structure section sketched in the field by Tanton shows more errors than any before”. Rather than resolving the problem, these exchanges only served to accentuate it. Some years later, Tanton (1936) made a comparable error by misinterpreting the Seine conglomerate at Shoal Lake to have been intruded by the Laurentian granitoid rocks rather than being deposited above it (Fig. 6d). Apart from his local mistake, Grout’s overall arguments found some traction and provided the impetus for additional field work. J.E. Hawley, a graduate of the University of Wisconsin and a professor at Queen’s University, was well versed in stratigraphy and structural geology. Along with a review of the Shoal Lake area, he conducted a study of the Seine and Coutchiching eastward though Jackfish Lake and past Sapawe. He concluded (Hawley, 1930) that part of the problem was, in some localities at least, that the contacts between the Coutchiching and Keewatin are arguably occupied by faults so that attitudes of strata alone provide inconclusive evidence of stratigraphic order. F.F. Grout also remained influential at that time and recommended the Seine-Coutchiching Problem to P.L. Merritt who conducted a study of the entire corridor from Rainy Lake eastward along the Seine River watershed for his Ph.D. thesis at Columbia University (Merritt, 1934). His conclusions concerning the two metasedimentary sequences supported Grout’s interpretation (Fig. 6c) and he suggested that, with the notable exception the clastic rocks at Rice Bay and Bear’s Passage, the term Coutchiching should be abandoned altogether and that all other sedimentary units should be included in the post-Keewatin, Seine Series above the basal conglomerate. Like Hawley, Merritt also provided detailed documentation of a fault contact between the Keewatin and sedimentary units at various localities and traced a continuous fault from Calm Lake eastward through Sapawe as far as Dog Lake, 60 km north of Thunder Bay: this is known today as the Quetico Fault. He also proposed (Merritt, 1934, p. 371) that “the fault movement along the contact is believed to combine a horizontal shear with an associated overthrust to the south”. Grout had a further influence on the expanding Seine-Coutchiching problem in that he inspired Francis Pettijohn, his field assistant during the work at Rainy Lake, to take on pioneering work in the study of Archean sedimentary rocks in general. Pettijohn did his undergraduate and graduate work at the University of Minnesota and, in accepting the Penrose Medal for 1975 from the Geological Society of America, he acknowledged the importance of Grout’s tutelage as well as the short time that he spent studying with A.C. Lawson at Berkley in 1927-28 to learn more about the alternative view. Pettijohn’s Ph. D. thesis documented the Abram Lake conglomerate in the Minnitaki Lake area in part because it resembled both the Seine conglomerate at Rainy Lake and the Ogishke conglomerate at Knife Lake Minnesota (Fig. 1). He later summarized his findings at all three of these localities as well as at several others in the northern Lake Superior region (Pettijohn, 1937) to also conclude that the majority of sedimentary units are arguably younger than the Keewatin. He also raised the possibility, however, that not all of the units included in the Keewatin need be of the same age and that local intercalation between volcanic and sedimentary rocks may locally be possible. As important as these insights ultimately proved to be, Pettijohn’s resolution of the Seine-Coutchiching problem also called for the abandonment of both of Lawson’s local units (Seine and Coutchiching) in favour of an overarching “Knife Lake Series” composed of similar rocks which had precedence of definition in the geological literature in Minnesota. This also reinforced Grout’s views and a summary paper on the topic (Grout et al., 1951) notably included a section entitled “No Coutchiching Recognized in Minnesota”. Nonetheless, the “SeineCoutchiching problem” was kept alive intermittently well into the 1970’s even to the extreme point of academic speculations that no unconformities existed - 90 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 at all and that there was simply a proximal to distal lateral facies equivalence (Fig. 6e) among broadly age-equivalent Keewatin, Seine and Coutchiching rocks (Bass, 1961; Ayres, 1971; Mackasey et al., 1974; Goodwin, 1977). Sam Goldich who was a graduate of the University of Minnesota rejoined that institution in 1948 as a professor and director of the Rock Analysis Laboratory where he and Alfred Nier gained international reputations as pioneers in isotope geochemistry and geochronology. An outcome of that work was the application of geochronological methods to the resolution of stratigraphic problems in the Lake Superior region (Goldich et al., 1961; Goldich, 1968). Goldich compromised on the question of the SeineCoutchiching problem by favouring the term “Knife Lake Group” over “Seine Group” above the Keewatin but also allowed for the possibility (using a question mark for emphasis) of the existence of Coutchiching metasedimentary rocks below it. Goldich tackled the geology of the Rainy Lake area head on by supporting three field-based M.S. theses at the University of Minnesota (Alt, 1959; Frye, 1959; Peterman, 1959) and the resulting maps and samples became the basis for on-going geochronology and geochemical studies (Peterman et al, 1972; Goldich and Peterman, 1980). With time, Lawson’s original terminology and interpretation of stratigraphic order was largely supported by the data but with the added implication that all of the constituent rock-forming events took place in less than 100 million years with only local evidence for younger post-metamorphic retrogression. At the time of the studies by Goldich and his colleagues an important fact remained: apart from Grout’s work in Minnesota, no geologist other than A.C. Lawson had mapped systematically in the Rainy Lake area. This had been undertaken by him at a scale of one inch to four miles in 1885-87 and, with the assistance of H.C. Cooke and R.C. Wallace, at one inch to one mile in 1911. New mapping in greater detail was therefore ultimately undertaken by the Ontario Division of Mines beginning in the 1970’s (Davies, 1973; Blackburn, 1973; Harris, 1974; Wood et al., 1980 a, b; Fumerton, 1985). The outcrop mapping of Fred Harris is perhaps the most notable because it provided an advanced level of lithostratigraphic detail, at a scale of one inch to ½ mile, while covering the historically controversial Rice Bay and Bear’s Passage areas. He also provided new local evidence for stratigraphic younging in the Keewatin strata including the first recorded use of the shapes to pillows in basaltic flows in this area. Harris (1974) avoided the use of the historical stratigraphic terms but his table of formations tends to support Lawson’s original interpretation of metasedimentary biotite schists at the base of the sequence. John Wood provided a comparable level of mapping at Mine Centre (Wood et al., 1980 a, b) with a focus on the Seine and Coutchiching metasedimentary rocks (Wood, 1980). Companion studies of the geology in the Minnesota portion of the Rainy Lake area were conducted under the auspices of the Minnesota Geological Survey and the US Geological Survey (Ojakangas, 1972; Southwick, 1972; Southwick and Ojakangas, 1979; Southwick and Sims, 1980). Poulsen (1980) made extensive use of the report and maps of Harris (1974) as a foundation for structural and metamorphic studies in the Rice Bay and Bear’s Passage areas. It eventually became clear, however, that one of the flaws in Lawson’s original interpretation was that it relied on the assumption that structural superposition of the Keewatin above the Coutchiching equates to stratigraphic superposition as well in rock packages that are arguably overturned (Poulsen et al., 1981). This led to a revised interpretation of stratigraphic order (Fig. 6f) but one without geochronological constraint. The full essence of the Seine-Coutchiching problem was ultimately clarified by application progressively improved methods of U-Pb analysis of zircons (Davis, 2023). Strategic sampling of each of Lawson’s five lithostratigraphic units across several sites where field relationships had been established (Davis et al., 1989; Davis et al., 1990; Fralick and Davis,1999) provided the results that form the basis of the current chronostratigraphic chart for the area (Fig. 6f). Keewatin metavolcanic rocks and Laurentian metaplutonic rocks were shown to be of similar age (circa 2727 Ma) whereas detrital zircons from the Coutchiching suggested a younger age (circa 2700 Ma) and a granitoid clast and detrital zircons from the Seine conglomerate even younger (<2693 Ma). The age of crystallization of the Algoman intrusions was estimated to be in the range of 2693 to 2684 Ma. In total, the geochronological constraints have shown that most of the historical interpretations, including those of Lawson, had both merits as well as flaws whereas the lateral facies concepts which were so widely and uncritically accepted in the 1970’s have proven to be entirely untenable. The net result, however, is that - 91 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Lawson’s placement of the Coutchiching beneath the Keewatin on the grounds of the dip of strata alone was the main source of geological error. It was not that he did not understand the difference because, in Memoir 40, he appears to have been the first geologist to have compare directions of dip to the directions of younging (way-up or bedding top) in cross-bedded arenite of the Seine sedimentary unit. Lawson (1913) did acknowledge that he had learned about the utility of truncated cross-bedding from his field assistant J.D. Trueman, then a graduate student who in turn had been taught this by W.O. Hotchkiss at the University of Wisconsin (Dott, 2001). Hotchkiss was also familiar with upward-fining grain size variation in sandstone mudstone sequences but this was not yet in common use and therefore the significance of graded bedding, as preserved at Bear Passage area, was not yet appreciated by Lawson in 1913. It took the work of F.F. Grout (1925) to demonstrate that the graded beds at Morton Island indicate that the Coutchiching beds there stratigraphically overlie the Keewatin. Lawson also certainly would have been aware of the stratigraphic use of pillowed volcanic flows, as advocated by Morley E. Wilson (1913) in Memoir 39 of the Geological Survey of Canada, but was of the opinion that this method was unsound because he believed that pillows, then referred to as ellipsoidal structures, were of intrusive origin (Lawson, 1912). As one looks back, the Timiskaming-Keewatin problem evolved along similar paths as another great geological debate that played out during much the same time frame, the Highlands controversy of Scotland. That problem also involved many observers who were focussed on small, geographically separated parts of a bigger problem and it has been said that, in many cases, they did not know what they did not know (Oldroyd, 1990). A case in point is the famous anecdote concerning T.L. Tanton of the Geological Survey of Canada and E.B. Bailey of the British Geological Survey (Dott, 2001). Tanton, who had graduated from the University of Wisconsin in 1915 under the supervision of C.K. Leith, led a group of Princeton geologists on a tour of Rainy Lake as part of their geological trip across Canada by rail in 1927. The group included two eminent guests from overseas, L.W. Collett from Switzerland and E.B. Bailey of Britain (Bailey, 1927). Tanton demonstrated the utility of cross-bedding and graded bedding to determine wayup in metasedimentary rocks, using Lawson’s examples from the Seine Group at Shoal Lake and Grout’s Coutchiching outcrops at Morton Island respectively. This resulted in Tanton’s inclusion as a participant on a reciprocal visit to Scotland where he convinced Bailey that the Dalradian strata at Ballachulish are overturned (Tanton, 1930; Bailley, 1930, Dott, 2001). Another point of communality between the two controversies is that the reputations of the observers, especially Sir R.I. Murchison in the Highlands and A.C. Lawson at Rainy Lake, tended to get in the way of the geological facts. This should not overshadow the reality, however, that in his first, youthful burst of mapping from 1882 to 1889 and in his mid-career re-study from 1911 to1913, Lawson identified the five lithological building blocks which are representative of the architecture of virtually every Archean greenstone belt in the world. As Oldroyd (1990) has pointed out for the Scottish Highlands controversy, it is not only about who was right and who was wrong but it is also about the process of narrowing in on a consensus view based on the facts at hand. Similar sentiments were expressed by Lawson himself in the introduction to his 1913 report which fuelled the Seine-Coutchiching problem in the first place. The overall lesson of the problem seems to be that: “Science is never ‘settled’ but evolves by the accumulation of facts, new ideas and vigorous open discussion and debate. Consensus is irrelevant in science; only truth matters.” (Dewey and Ryan, 2022, p.1834). Rainy Lake Wrench Zone Poulsen (1986b) introduced the term “Rainy Lake wrench zone” to distinguish rocks between the E-W Quetico Fault and the ENE Rainy Lake – Seine River Fault from the Quetico metasedimentary belt to the south and the main mass of the Wabigoon granite-greenstone belt to the north (Fig. 8). The rationale for highlighting the wrench zone involved many different geological aspects (Poulsen, 1986b) but the most prominent are the distinctive lenticular. s-shaped lithostratigraphic domains which merge with the discordant boundary faults. Broadly similar patterns are also evident in the steep metamorphic foliation which affects the Seine as well all of the older lithostratigraphic units. This is also the area in which the Seine – Coutchiching problem mainly played out and where generations of geologists contributed to the understanding of diverse aspects of its geology. It is also the focus of this field guide which can be used to illustrate the major lithostratigraphic - 92 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 8: Simplified geological map of the Rainy Lake Wrench Zone. units that comprise the wrench zone as well as the related topics of deformation, metamorphism and metallogeny. Keewatin Lawson applied the term “Keewatin Series” to all of the Archean metavolcanic rocks in the Rainy Lake area mainly to distinguish them from foliated quartzo-feldspathic rocks of probable plutonic origin. He initially did this in a descriptive way (Lawson, 1885) but his Rainy Lake reports (Lawson, 1887; Lawson, 1913) also provided petrographic detail and genetic interpretation. The Keewatin rocks within the wrench zone include lithofacies which are common to Archean greenstone belts in general. Mafic volcanic rocks predominate in the northwestern part of the zone, particularly at Windy Point, Nickel Lake and Pocket Pond. The rocks at these locations were metamorphosed to amphibolite facies assemblages so that primary features are difficult to document in the resulting foliated mafic tectonites. In places where strain is moderate it is relatively easy to identify pillows and varioles but the level of distortion in many places (Fig. 10a) makes it difficult to confidently use the shapes of pillow to confidently define directions of younging (Borradaile and Poulsen, 1981). A notable exception is at Pocket Pond (Fig. 9a) where adequate evidence of stratigraphic polarity is preserved (Fig. 10b). Felsic metavolcanic rocks predominate in the southeastern part of the wrench zone where they are commonly intercalated with rocks of andesitic composition (Fig.10c), The rhyolitic rocks are commonly quartzphyric and included both coherent (Fig. 10d) and volcaniclastic (Fig. 10e) facies. Figure 9: Pocket Pond locality - 93 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 A B v C D E F Figure 10: a) westward plunging shape lineation defined by deformed pillows in metabasalt, Nickel Lake area; a) Pillow basalt, Pocket Pond; c) Amygdaloidal basalt, Port Arthur Copper; d) spherulitic rhyolite, Ottertail east; e) Rhyolite Breccia, Wind Bay; f) volcaniclastic ferropicrite, Belacoma area. One outstanding unit within the Keewatin sequence is composed of a distinctive ultramafic volcaniclastic rock (Fig. 10f) which is exposed in the Grassy Portage Bay area (Fig. 4). The unit was first recognized by Harris (1974) who classified it as an intermediate volcanic rock, mainly because of its common bright green, chloritic appearance along with volcaniclastic textures. Poulsen (1980) prosaically termed it “magnetic green rock” which is composed mainly of Mg-chlorite plus actinolite and magnetite. He provided lithogeochemical analyses to show that the rock has an ultramafic bulk composition but incorrectly classified it as a komatiite, a rock type with which it shares only some chemical similarities. He also compared the unit, both chemically and texturally to the betterknown picritic Steep Rock Ashrock approximately 100 km to the east and suggested that their separation might be due to dextral displacement on the Quetico Fault (Fig. 1). Steve Schaefer conducted a study of the ultramafic units at both localities and confirmed their volcanic origins (Schaefer and Morton, 1991). He also provided the acronym GUP (Grassy Portage - 94 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Ultramafic Pyroclastic) for the rocks at the Rainy Lake locality. Goldstein and Francis (2008) pointed out the differences in the chemical composition of this unit compared to komatiites: the GUP shows higher FeO, TiO2 and incompatible elements (e.g., Nb) as well as displaying fractionated rather than flat rare earth element patterns. Goldstein and Francis (2008) reclassified the rocks as pyroclastic ferropicrites, noting that they are examples of relatively rare Ferich volcanic varieties that were likely derived from partial melting a mantle source that was enriched in Ti and rare earth elements. A further characteristic of the GUP is that it contains microdiamonds which were discovered in 2008 by MetalCORP Limited at the Beaver Pond Occurrence (Hinz et al., 2010). All of these observations have significantly improved the understanding of this unusual volcanic unit but questions remain regarding its stratigraphic position and regional significance. Despite the remarkable similarity to the Dismal Ashrock at Steeprock, the notion of a strike-slip separation of the same unit is still feasible but not fully demonstrated. Tomlinson et al., (2003) reported a maximum age of 2780.4 +/-1.4 Ma for the Dismal Ashrock based on analyses of inherited zircons and argued that it is feasible for it and overlying basalts (Witch Bay formation) to be as young as other sequences in the Western Wabigoon Subprovince: by extension, this would include the mafic-ultramafic volcanic units in the Grassy Portage Bay area. If the ages of the ultramafic volcanic rocks at the two distant localities prove to be different, however, it would mean that an alternative explanation for their similarity would have to involve operation of similar processes at different times. In that case the communality might be sought in the mantle composition and depth that led to the formation and deposition of these unusual rocks. Interflow metasedimentary rocks comprise a common but volumetrically small component of the Keewatin sequence. Although in places these rocks could be mistaken as providing evidence for interdigitation with Coutchiching biotite schists or with volcaniclastic rocks of intermediate composition, in most cases, they are arguably metalliferous, synvolcanic sedimentary units which range from pyritic mudstone and minor sandstone, to chert-magnetite banded iron-formation (Fig. 11a) and pyritic massive sulfide deposits (also termed sulfide facies ironformation). The sulfide-bearing varieties were targets for possible sulfur production in the period around World War I when, particularly at Nickel Lake, they were noted to contain anomalous concentrations of CoNi-Zn-Cu. In places zinc is also a locally anomalous component and, at Pocket Pond, the small sphalerite lenses discovered in the 1970’s are associated with iron-formation intercalated with metabasalt (Fig. 9). From a strictly geological perspective the presence of laterally extensive interflow units proves valuable for establishing a sense of stratification within the Keewatin because they not only can be mapped discontinuously in outcrop and drill core but cane be easily traced accurately by magnetic and electromagnetic surveys. A.C. Lawson’s 1914 geological map of Rainy Lake also includes two mafic plutonic rock types which he regarded to be part of the Keewatin sequence: extensive units of what he termed hornblende gabbro in the Grassy Portage Bay area (Fig. 4) and anorthosite in the Bad Vermilion Lake area (Fig. 5). Subsequent mapping has shown that he underestimated the total volumes of mafic plutonic rock in both cases and this was with good reason. It is now well-understood that thick, mafic submarine lava flows are capable of slow cooling rates to produce what can easily be accepted as a “gabbro-textured” facies that grades vertically and laterally over short distances into finer grained basaltic rocks, making their visual distinction from plutonic equivalents difficult. Furthermore, where amphibolite facies metamorphism has affected mafic volcanic rocks, recrystallization tends to coarsen the texture and obscure primary features: this is certainly the case in the northwestern western part of the Rainy Lake Wrench Zone. Finally, considerable local variations in textural detail are common in layered mafic intrusions (Fig. 11b, c, d) that that are difficult to map at a reconnaissance scale. What Lawson did map, however, were two of most extensive, distinctive and homogeneous plutonic phases, leucogabbro in the Grassy Portage Intrusion (Fig. 11e) and coarse anorthosite in the Bad Vermilion intrusion (Fig. 11 f). Detailed mapping by Hodgson (1959) at Grassy Portage Bay and by Harris (1974) at both localities provided much better definition of the full extents of these intrusions. Ashwal et al. (1983) undertook a more advanced petrological and geochemical study of the Bad Vermilion anorthosite and concluded that it represents the remnants of a subvolcanic magma chamber from which aliquots of magma had been extracted as extrusive lava flows. Poulsen and Hodgson (1985) reviewed the disposition of the different phases of both intrusions and the sulfide - 95 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 A B D C D F E Figure 11: a) chert-magnetite iron-formation, Pocket Pond; b) thin-layered gabbro, Grassy Portage intrusion, west side of Redgut Bay; c) thick-layered gabbro-melagabbro, Northrock East trenches, Grassy Portage intrusion; d), Pegmatitic Gabbro, Northrock E trenches, Grassy Portage intrusion; e) leucogabbro, Grassy Portage intrusion; f) coarse anorthosite, Bad Vermilion intrusion, Scott Islands (the edge of the compass in the bottom left measures 10 cm) and oxide mineralization within them, providing support to the idea that they are examples of synvolcanic layered intrusions resulting from cumulus growth and magma fractionation. Both intrusions also received attention for their economic potential during exploration programs for Cu-Ni-PGE sulfide and Ti-V oxide mineralization (Hinz et al., 2010). The Bad Vermilion Intrusive complex and the surrounding metavolcanic rocks have recently been described as an arc-related “ophiolite” sequence (Wu et al., 2016) but this is highly unlikely given the dominance of rhyolite in the volcanic section and the absence of both peridotite and sheeted dikes. The ages of the Keewatin units were largely unknown until the mid-1970’s despite many attempts to apply modern geochronological methods (Goldich, 1968; Tilton and Grunenfelder, 1968; Hart and Davis, 1969; Peterman et al., 1972). At that point improvement in analytical precision and accuracy allowed U-Pb geochronology on carefully constrained samples to impact stratigraphic interpretations (Davis, 2023). Davis et al. (1988) applied these methods in the Rainy Lake Wrench zone to show that the units which were historically classified as Keewatin formed around 2727 - 96 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Ma in a remarkably short interval of five to six million years (Fig. 5f). This included direct analysis of rhyolitic rocks from both the northwestern and southeastern part of the zone as well as the indirect constraint of mafic (Grassy Portage Gabbro) and felsic (Mud Lake trondhjemite) that cut the volcanic rocks. More recent attempts to provide additional ages of volcanic rocks near the Bad Vermilion Intrusion have proven to be unsuccessful in light of the lower analytical precision and accuracy of the methods employed (Wu et al. 2016). Coutchiching Lawson’s 1914 map of Rainy Lake outlines three areas of Coutchiching rocks labeled as “mica schist, paragneiss and phyllite”. The most extensive area occurs south of the Seine River Fault in Quetico Subprovince where the term Quetico metasedimentary rocks also applies (Fig. 12a). The other two major localities are located within the Rainy Lake Wrench Zone: a southern belt extending from Fort FrancesInternational Falls northeastward along Swell Bay and a northern one as a partially annular zone within the Rice Bay Dome (Figs. 2, 3, 4). Grout (1925) A B C D E F Figure 12: a) Quetico metasedimentary rocks, Bleak Bay area; b) thick-bedded wacke, Sandpoint Island c) graded beds cut by ENE cleavage, Morton Island (N to top of photo); d) knotty biotite schist containing retrograded staurolite and garnet, Great River Road; e) graded beds, Bear’s Passage boat launch; f) graded beds in greenschist facies turbidites, Old Station Road (diagonal lines are glacial striae). - 97 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 who was the first to recognize graded bedding in the metasedimentary rocks in the Swell Bay belt (Fig. 12b, c) and to apply it to establish local stratigraphic polarity at Morton Island (Fig. 13). This was confirmed by Merritt (1934) who also notably interpreted the colour banding of graded rocks in the Swell Bay as “varves”. R.W. Ojakangas who often lamented that the Canadian glaciers had been cruel to Minnesota re-examined the sparse exposures of the Coutchiching rocks at Ranier, Minnesota near Lawson’s type locality (Fig. 3). He described the rocks there as metagreywacke noting that they originally consisted of alternating beds of greywacke sandstone (or simply wacke) and mudstone deposited, not by glacial processes but by turbidity currents on submarine fans (Ojakangas, 1972; 1982). Most authors who have studied the belt of Coutchiching rocks along Swell Bay have also noted that they been clearly intruded by younger granitoid rocks (Algoman) and that the rocks on the northern shore of Swell Bay display amphibolite facies, pelitic metamorphic assemblages (Fig. 12d) involving biotite, muscovite, garnet, cordierite and staurolite (Ojakangas, 1982; Poulsen, 1980). The higher metamorphic grade has also been implicated by many authors for obscuring primary features such as graded bedding in the metasedimentary rocks. The most contentious interpretations of the stratigraphic significance the Coutchiching rocks in the Swell Bay corridor result from observations in the Bear’s Passage area (Figs. 4, 14). The Keewatin at this locality consists of a northwestward-dipping section composed of the upper part of the southeastwardyounging Grassy Portage layered intrusion overlain by a thin unit of what are arguably mafic metavolcanic Figure 13: Morton Island locality (adapted from Poulsen and Wood, 1982) rocks. The staurolite-bearing metasedimentary rocks, although locally folded, also dip to the northwest and are cut discordantly by granodiorite of the Bear’s Passage intrusion (Fig. 4). Although minor reversals in polarity of grading suggest local folding within the Coutchiching rocks in the Bear Passage area, a good quality exposure at their contact with Keewatin strata (Fig. 12e) demonstrates that the metasedimentary rocks are overturned (Poulsen, 1980). This plus the observations at Pocket Pond (Fig. 9) and Morton Island (Fig. 13), provides the geological evidence in favour of the Coutchiching being younger than the Keewatin. The most conclusive evidence, however, ultimately came from U-Pb analyses of detrital zircon in biotite schist near Tunnel Bay and in well preserved greenschist grade metagreywacke (Fig. 12f) northeast of Shelter Cove (Fig. 6) which represents the northeastward extension of the exposures at Morton Island (Davis et al. 1989). The age of the Coutchiching is constrained by the youngest detrital zircon grains at approximately 2704 Ma and by the circa 2692 Ma age of across-cutting felsic dike (Fig. 5 f). The presence of much older detrital grains (2930, 2940 and 3060 Ma) also suggested a potential contribution of detritus to the Coutchiching from a source area comparable to the Marmion domain north of the Quetico Fault extending in the Steeprock Lake area (Fig. 1). Similar conclusions were reached for the Quetico metasedimentary rocks by Davis et al. (1990). Laurentian Lawson (1887) used the term Laurentian for variably foliated granitoid rocks in general but by 1913 he only applied it at only three localities, Bad Vermilion lake, Figure 14: Bear’s Passage locality - 98 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Rice Bay and Grassy Island near Neil Point (Fig. 2). The Laurentian granitoid rocks at Bad Vermilion Lake occupy three sinuous bodies that are broadly co-spatial with the Bad Vermillion layered intrusion (Fig. 7). The sodic granitoid rocks range from tonalite (Fig. 12a, b) to trondhjemite (Fig. 12c) in composition (Goldich and Peterman, 1980) and likely occupy the remnants of sills that are broadly concordant with stratigraphic layering in the surrounding northward-younging Keewatin volcanic rocks (Fig. 7). It is noteworthy that Lawson (1887) was the first to suggest that they might be syn-volcanic, subvolcanic intrusions. This point was verified by Davis et al. (1989) who established nearly identical ages of 2728 Ma for the intrusive Mud Lake trondhjemite near the Stellar gold deposit and an overlying rhyolite west of the Port Arthur copper deposit. The Laurentian rocks at Grassy Island likely represent an isolated remnant of the same stratigraphic section to the southwest (Fig. 2). A noteworthy characteristic of some outcrops of tonalite, particularly near gold-bearing quartz veins, is a quartz-rich sericitic rock (Fig. 12b) that was termed “protogene” by the early gold explorers in the region and results from plagioclase-destructive metamomatism related to carbonatization associated with brittle-ductile shear zones in the tonalite (Diamond and Marshall, 1990). The Laurentian rocks exposed in the core of the Rice Bay Dome (Fig. 4) are much more difficult to interpret, in part due to overprinting deformation and amphibolite facies metamorphism. Lawson’s 1914 map classified them to include as an inner body of granite and granite gneiss with an intrusive relationship with an outer annulus of Coutchiching biotite schist. Subsequent petrographic studies documented the distinctions among the lithologies (Frye, 1959; Peterman, 1959) and the term “paragneiss” was ultimately given to the innermost rocks. The existence of a large pluton was questioned and both the paragneiss and biotite schist were considered to be different components of the Coutchiching (Peterman, 1959; Peterman et al. 1972). By the same token, however, a small volume of deformed quartz-feldspar dikes and sills were shown to cut the paragneiss within the dome (Peterman et al. 1972). Harris (1974) took much the same approach and, apart from areas where the minor granitoid dikes and sills were particularly abundant, he mapped most of the interior of the Rice Bay Dome as being composed mainly of “biotite-feldspar-quartz schist” which he also assigned to the lower metasedimentary unit (i.e. the Coutchiching). Goldich and Peterman (1980) continued to view the rocks in the interior of the Rice Bay Dome as being composed of paragneiss derived from epiclastic sedimentary rocks but they also presented chemical data to show that they are different from the Coutchiching biotite schists and metagreywackes. Poulsen (1980) used the non-genetic term “grey gneiss” for rocks in the interior of the Rice Bay Dome (Fig. 15d, e) and also showed that they are fundamentally different in chemical composition from the annulus of biotite schist that envelopes them (Fig. 4). The minor deformed quartz-feldspar porphyry dikes (Fig. 15 e, f) are in, turn, different in chemical composition from both the grey gneiss and biotite schists (Poulsen. 1980; Goldich and Peterman, 1980). Dick Ojakangas (personal communication, circa 1980) provided the novel suggestion that some of the grey gneisses actually may have been felsic volcanic rocks rather than felsic intrusions. This prompted Poulsen (1984) to opt for the uninspiring descriptive term quartzo-feldspathic gneiss to distinguish the Laurentian rocks from the Coutchiching biotite schists. Davis et al., (1989) reported a U-Pb zircon age of 2725+/-2 Ma from a sample of the quartzofeldspathic gneiss near Moran’s Bay (Fig. 4) to demonstrate its probable chronological equivalence with both the Keewatin rhyolite and the Laurentian Mud Lake trondhjemite in the Bad Vermilion Lake area. One of the notable lithogeochemical attributes of the biotite-rich Laurentian gneisses within the Rice Bay dome is their local deficiency in Na and Ca and their excess in Mg and Fe relative to their high silica and low Ti contents (Goldich and Peterman, 1980; Poulsen, 1980). One explanation for this is that they were locally subjected to plagioclase-destructive metasomatism which would also explain the presence of staurolite, andalusite and/or cordierite within them at specific sites. Such alteration in well-known in the environments of volcanic-associated massive sulfide deposits. Beakhouse (1984) evaluated this possibility in the western part of the Rice Bay dome where he identified the metamorphic assemblage quartzchlorite-garnet-anthophyllite-staurolite with possible large relict grains of cordierite at one locality and common garnet over a larger area. Teck Corporation subsequently verified these mineralogical anomalies with further mapping and lithogeochemical surveys to conclude that the alteration is likely related to pyritic massive sulfide mineralization within an iron- - 99 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 A B C D E F Figure 15: a) Bad Vermilion tonalite, Mine Centre; b) Sericitized tonalite (“protogene”), Mine Centre c) Mud Lake trondhjemite, Stellar gold property; d) grey quartzo-feldspathic gneiss cut by leucocratic dikes, Rice Bay, e) quartz-phyric grey gneiss cut by quartz-feldspar-phyric dike, Laurentian gneiss unit, Moran’s Bay; f) deformed quartz (dark) and feldspar phenocrysts in qfp dike, Moran’s Bay formation unit near the outer part of the Rice Bay dome (Alderman, 1988). In summary, despite incremental advances in establishing the geological facts concerning the Laurentian gneiss of the Rice Bay dome, considerable uncertainty remains about its origin. It has been established to be age equivalent and compositionally similar to both the Keewatin and Laurentian rocks in the Bad Vermilion Lake area but much study is required to establish its stratigraphic significance with respect to the Coutchiching and Keewatin rocks which structurally overlie them. The weights of evidence suggest, however, that the definitively intrusive aspects of the dome are attributable to the minor volume dikes and sills for no absolute ages have been established. On lithogeochemical grounds they may represent a phase on the younger Algoman intrusive suite (Goldich and Peterman, 1980). - 100 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Seine Of all of Lawson’s many achievements at Rainy Lake, it was arguably the recognition of the sedimentary rocks of the Seine Series, the interpretation of their probable depositional paleoenvironment and the demonstration of a high-angle unconformity beneath them that have best withstood the test of time. Although the overall level of exposure is uneven, the critical localities where this is best illustrated are located between Shoal Lake and Bad Vermilion Lake in the Mine Centre area (Fig. 7). In particular, exposures of the basal conglomerate (which Lawson termed “fanglomerate”) near the Golden Star Mine (Fig. 16) and the overlying arenite facies exposed on islands in Shoal Lake to the south provided the diagnostic evidence for Lawson’s arguments. Figure 16: Simplified geology of the Golden Star locality. Rocks of the Seine series occupy the area to the southeast of the trace of the unconformity and the underlying rocks of the Laurentian and Keewatin are located to the northwest. The S-shaped configuration of the unconformity trace is likely meaningful, not only because it mimics larger patterns in the wrench zone as a whole (Fig. 8) but also because the northsouth segment reflects lower than average intensity of superimposed strain. Lawson was the first to note that this is in part responsible for the convincing preservation of contact relationships. The basal Seine conglomerate dips shallowly southeastward whereas Lawson showed that an interflow chert-carbonate unit within the Keewatin dips moderately northward. A relatively minor refinement (Pouslen and Wood, 1982) is that pillowed metabasalt overlies the chertcarbonate marker and indicates a northwestwardyounging for the Keewatin rocks. In other words, there is evidence for back-to-back younging across a highangle unconformity. Although weakly aligned due to overprinting strain, a critical point of observation is that clasts in the basal conglomerate show no evidence of a prior metamorphic foliation (Fig. 17a). The derivation of the coarse gritty matrix of the basal conglomerate from the underlying Laurentian tonalite is also clearly evident when compared to the intrusive rocks below the nonconformity. The shallow dipping basal conglomerate (Fig. 17b) to which Lawson ascribed an alluvial origin has been mapped along a persistent ridge of fair outcrop (Fig. 16) but topographically recessive arenite which overlies it to the east is poorly exposed. The Seine arenite unit is well-exposed at Shoal Lake where cross-bedded sandstone (Fig. 17c) provides stratigraphic polarity as well as supporting the common interpretation of a fluviatile origin. Cross-bedded sandstone (Fig. 17d) also can be traced farther eastward along the Seine River (Fig. 2) where it can be demonstrated to be overlain by an upper unit of coarse, polymictic conglomerate (Fig. 17e) and, in some cases, intercalated with it (Fig. 17f). The uppermost conglomerate unit is notable for an abundance of granitoid clasts and Davis et al. (1989) reported an age of 2696.1+5/-3 demonstrating that it was sourced in a granitoid body that was much younger than the Laurentian which provided detritus for the basal Seine Conglomerate. Davis (1990) further constrained the depositional age of the sandsized fraction from arenite at Horsecollar Junction (Fig. 2) by noting the presence of abundant detrital zircons with a U-Pb age of approximately 2693 Ma., effectively the same age as the Bear Pass pluton. This fact contradicted Lawson’s original contention that all of the Algoman intrusions could be defined on the basis of the fact that they are younger than the Seine (see below). Nonetheless, Lawson’s original interpretation of the Seine Series mainly as a product of Archean alluvial and fluvial sedimentary processes has been reinforced and elaborated upon by several authors (Ojakangas, 1972; Wood, 1980; Fralick and Davis, 1999; Czech and Fralick, 2002). Although his language was somewhat dense, the overall message of Lawson’s paleoenvironmental interpretation is paraphrased as follows: “it seems a fair inference that the conglomerate represents a gravelly flood plain… The distribution of the conglomerate … indicates the course of a river (following) the dominant structural lines … at a time which antedates the intense complication which folded - 101 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 A B C D E F Figure 17: a) basal Seine conglomerate inter-clast quartz grit derived from underlying tonalite, Golden Star arear; b) stratified basal conglomerate, Golden Star area; c) trough cross-bedded Seine arenite in plan view, Shoal Lake showing younging toward the top of the photo; d) deformed cross-bedded Seine arenite and pebble conglomerate in cross-section view, Seine River Bridge; e) deformed polymictic conglomerate in cross section view, east of Mine Centre; f) sandstone interbed in coarse upper Seine conglomerate in plan view west of Wild Potato Lake. Note the angle between bedding (arrows) and foliation. and deformed the conglomerate” (Lawson, 1913, p.62). In other words, he envisioned the location of Seine conglomerate and arenite to have been controlled by syn-sedimentary faults to account in part for it’s elongate map pattern (Fig. 2). Algoman Lawson’s 1914 map portrays five different varieties of intrusive rocks at Rainy Lake in decreasing order of perceived age which he classified with the term Algoman: basic facies of syenite, syenite gneiss, granite and granite gneiss, banded and streaked gneiss and porphyroid gneiss. Harris (1974) made similar distinctions which allowed the least deformed Algoman rocks to be discussed in terms of three distinct spatial and compositional suites: the Rocky Islet Bay complex, the Swell Bay intrusions and the large and conspicuous Ottertail Lake Intrusion (Fig. 3). The first of these are dominated by quartz monzonite syenite and mafic syenite and are commonly feldspar-phyric (Fig. 18a, - 102 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 A B C D E F Figure 18: a) feldspar-phyric quartz monzonite, Raspberry Island; b) porphyritic quartz monzonite, Rocky Islet Bay; c) granodiorite cut by vertical sheeted quatz-pyrite veins, Bear’s Passage; d) xenolithic monzodiorite, Ottertail Lake intrusion; e) Intrusion breccia with granitoid matrix, western Ottertail Lake intrusion; f) incipient brecciation and granitoid infilling of metamorphic tectonite, Ottertail Lake intrusion. b), The Swell Bay intrusions, exemplified by the Bear Pass Pluton (Fig. 18c) are composed mainly of quartz monzonite and granodiorite (Goldich and Peterman; 1980). Some of these intrusions are compositionally zoned with mafic to intermediate margins and felsic interiors (Cram, 1923; Harris, 1974). The Ottertail Lake intrusion is also compositionally zoned from marginal hornblende-biotite quartz monzonite to interior leucocratic quartz monzonite in the interior (Goldich and Peterman, 1980): wallrock xenoliths are common in the marginal phase (Fig. 18d) and internal magmatic breccias (Fig. 18e, f) are well developed in what Lawson interpreted to be roof pendants of deformed and metamorphosed Keewatin rocks. Goldich and Peterman (1980) demonstrated that the Algoman intrusive rocks commonly contain abundant K-feldspar and have much higher Sr contents than Laurentian tonalite and trondhjemite. The Ottertail Lake intrusion, also with high overall Sr content, displays a fractionation trend of increasing Rb:Sr - 103 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 ratio toward its interior. Shirey and Hanson (1984, 1986) and Stern et al. (1989) further defined specific lithogeochemical characteristics of the Algoman rocks at Rainy Lake to show that they are also distinctive from other granitoid rocks at a global scale. Relative to their intermediate silica content (55-60%), they contain anomalous Mg, Sr, Ba, Ni, Cr and are strongly enriched in light rare earth elements. Stern et al. (1989) proposed their formation from hydrous melting of mantle that had been enriched large ion lithophile elements though prior metasomatism. Davis (1990) provided an estimate of 2693 +/- 2 Ma age for the Bear Pass pluton and coupled with the 2686+2/-1 Ma age of the Ottertail Lake intrusion (Davis et al. 1989), demonstrated that the sanukitoid magmatism spanned the time bracket for inferred for deposition of the Seine conglomerate and arenite above a profound angular unconformity. A recent study by Bjorkman et al. (2024) has demonstrated the widespread distribution of the sanukitoid suite of rocks across the Wabigoon Subprovince, including the Ottertail Lake Intrusion. This has been interpreted to represent a significant shift in magmatism at approximately 2690 Ma that can be explained by metasomatism and magmatism in a suprasubduction setting leading to collisional deformation and metamorphism that is commonly attributable to the Kenoran Orogeny. Deformation and Metamorphism The emphasis on protoliths and stratigraphic relationships that has historically dominated the discussion of the geology does not outweigh the fact that most of the rocks are clearly metamorphic tectonites as well. Lawson (1913) recognized this and attributed commonly observed foliation and lineation (“pencilling” in his terminology) to compressive deformation related temporally to the Algoman granitoid suite. Rocks in the southeastern part of the wrench zone have been metamorphosed to greenschist facies mineral assemblages and rocks of the amphibolite facies are dominant in the northwest (Fig. 8). Significant areas of retrograde metamorphism have also been noted (Peterman et al. 1972; Poulsen, 1984) and this has been taken to be the explanation why most geochronological approaches have yielded unreliable protolith ages. It is likely that the overall distribution of preserved prograde assemblages is the result a combination of both local contact and regional dynamothermal metamorphism. The common existence of minor structures of dynamothermal metamorphic origin such as foliation (Fig. 12c, 17f, minor folds (Fig. 19a, b) and lineation (Fig. 10a) are reflections of local strain. Rheological contrasts within and among lithological units have also been well established to be important in controlling the local strain intensity in the Rainy Lake area, particularly in the Seine conglomerate (Hsu, 1971; Jackson, 1982; Czeck et al., 2009). The highest strains are also common in features which are arguably shear zones in which strong foliation is accompanied by asymmetric distribution of foliation (Fig. 19c, d, e, f) that mimics the overall structural pattern in the wrench zone as a whole (Fig. 8). Following the lead of Peter Hudleston (1986) in the Vermilion district of Minnesota, dynamic interpretations invoking dextral transpression have been invoked by several authors to explain the overall structural style of the Rainy Lake wrench zone (Poulsen, 1986b; Borradaile et al., 1988; Poulsen et al., 1992; Czeck and Hudleston, 2003; Fernandez et al., 2013). Beyond the local importance of dynamothermal metamorphic fabrics, however, the larger structural features in the wrench zone also of considerable interest. Foremost among these is the angular unconformity at the base of the Seine sedimentary sequence in the southeastern part of the zone (Fig. 20a) and it also provides an ideal temporal reference point for understanding the deformational and metamorphic history of the area. As illustrated above, the fact that lithic clasts in the basal conglomerate above the unconformity show no evidence of pre-depositional metamorphic fabrics yet clasts throughout the Seine have been variably strained during post-depositional dynamothermal metamorphism is an important one. It illustrates the insufficiency of using the development of foliation alone as a means of tracking a protracted structural history. A second notable structural aspect at Rainy Lake is the stratigraphic evidence for significant overturning of beds in the northwestern part of the zone (Fig. 20b). Poulsen (1980) suggested that this might have resulted from the overprinting of early recumbent folds by younger upright ones but, given the observation that the first-formed foliation in these rocks is also folded in the Rice Bay dome, the possibility of late-overturning of what may have been at one time steep strata can’t be entirely ruled out. A third topic of importance is the fact that, since their recognition in the 1930’s, there also has been a great deal of attention paid to the major faults that define the boundaries of - 104 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 A B C D E F Figure 19: a) shortening and transposition of felsic dikes cutting Coutchiching biotite schist, north of Noden Causeway; b) folded felsic sills, Great River Road; c) asymmetric boudinage of the interior of a mafic dike relative to its foliated margins, Noden Causeway; d)) asymmetric boudinage in felsic metavolcanic rocks south of the Olive gold mine, e) asymmetric shapes of clasts in Seine meta-conglomerate adjacent to the Rainy Lake – Seine River Fault south of Seine River Bridge; f) tight asymmetric folds in mylonite, Little Turtle Lake landing. the wrench zone. The rocks that now help to define the Quetico Fault at Rainy Lake were originally mapped by Lawson (1913) as part of a narrow belt of “porphyroid gneiss” extending westward from Little Turtle Lake at Mine Centre to Cheery Island. He recognized that the red porphyroid gneiss “has a pronounced cataclastic structure and that the schistosity of the rock is referable to deformation involving shearing of the mass” (Lawson, 1913, p.94). He stopped short of relating the rocks to a fault, however, interpreting them instead to represent the deformed southern margin of a large granitoid batholith: this is somewhat ironic because he is the geologist who, by this time, had named the San Andreas Fault and had compiled the definitive technical report on the Great San Franciso Earthquake of 1906. By the time F.R. Harris remapped the area, however, it had been recognized that the rocks here belong to the greater than 350 km long Quetico Fault based on the interpretation linears on air photo mosaics (Parkinson, 1962). Harris (1974) went on to describe the rocks in the fault as crushed granite, augen gneiss and mylonite and, like Lawson before him, locally showed gradational contacts with adjacent banded gneissic rocks which he termed migmatite. Kennedy (1984) studied 14 sites along the entire Quetico Fault, including 3 in the Rainy Lake wrench zone, and concluded that the mylonitic foliation on average resulted, not strictly from cataclastic processes, but from ductile flattening - 105 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 A B Figure 20: Schematic cross-sections through a) Rice Bay – Bear’s Passage and b) the Bad Vermilion – Shoal Lake areas. See Figure 8 for the locations of the sections (adapted from Davis et al., 1989). The Quetico fault is located at the northern end of both sections. based on measured axial ratios of deformed mineral aggregates and object-object strain estimates. She also used quartz c-axis fabric measurements and analysis of brittle micro-faults and ductile shear zones to argue for overall dextral displacement on the fault. Kennedy (1984) showed that the microfaults and minor shear zones dominantly strike NW and have dextral shear sense. She further argued that transition from ductile behaviour (mylonite) to brittle is consistent with the current level of exposure representing deformation at a crustal depth of 10-15 km. Borrradaile and Kennedy (1982) also showed evidence of flow-banding in veins of pseudotachylite at Crowrock Inlet as evidence of frictional melting in the fault zone. Peterman and Day (1989) reported a Rb-Sr isochron age of 1947+/23 Ma to suggest that the pseudotachylite from both the Quetico and Seine River faults resulted from Proterozoic reactivation of the Archean faults. Metallogeny A commonly understated geological feature of the Rainy Lake wrench zone is the simple abundance of mineral occurrences within it in comparison to the adjacent areas on either side. Poulsen (2000b) enumerated 88 of them in total and demonstrated that they include examples that are representative of multiple deposit types (Figs. 8, 21) which, in turn, are thought to relate to multiple geological processes. Syngenetic deposits include stratabound metalliferous sediments in the mafic sections of the Keewatin including banded iron formation, pyritic massive sulfide deposits with locally anomalous zinc sulfides (Nickel Lake and Pocket Pond). Numeous Zn-Cu occurrences (Port Arthur Copper, Lochart Lake, Wind Bay, Gagne Lake, Pidgeon) demonstrably possess the descriptive of volcanic-associated massive sulfide deposits in general. Basal Cu+/-Ni sulfide mineralization (North Rock) and magnetite+/ilmenite mineralization (Seine Bay, Mironsky) is clearly associated with the Grassy Portage and Bad Vermilion Lake layered gabbroic intrusions (Poulsen and Hodgson, 1984). Quartzpyrite-molybdenite veins show a spatial association with Algoman granitoid rocks and sheeted veins of this type within the Bear Pass Pluton are similar in style to those in the deeper parts of granitoid-related - 106 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 main geological features. The western starting point of the road log Km 0.0 (92.4) is at the lookout tower on the waterfront in Fort Frances and the eastern ending point Km 92.4 (0.0) is at the highway bridge across the Seine River near Crilly. The highway distances are approximate and, although the stops are described from west to east, they can be visited in any order depending on topical interests. Figure 21: Metallogenic Summary of the Rainy Lake Wrench Zone Phanerozoic vein and stockwork deposits. The goldbearing quartz veins in the Mine Centre area which were the focus of a gold rush in the 1890’s (Coleman, 1894; Winchell and Grant, 1895), are readily classified in modern terms as “orogenic” deposits characterized by ribbon quartz, carbonate-sericite alteration and spatial control by minor shear zones (Poulsen, 1986a). A recurring question about the metallogeny of the Rainy Lake wrench zone concerns the apparent absence of economically viable mineral deposits compared to the numerous occurrences. While it is true the there is strong similarity between the make-up of the rocks in the Rainy Lake wrench zone and the central volcanic complexes at Chibougamau, Val d’Or and Noranda in the Abitibi subrprovince, the discrepancy in metal endowment may simply be explained in the context of the geological deposit types. For example, the metal endowment of syngenetic massive sulfide systems is thought to be negatively influenced by shallow water environments, the lack of a well-defined lithocap or by cooler upwelling fluids and this might apply to Rainy Lake. A notable characteristic of the orogenic Auquartz veins at Mine Centre the kinematic evidence for strike-slip stress conditions for vein formation at Rainy Lake in contrast to conditions for reverse faulting allowing for higher fluid pressure at Val d’Or in the Abitibi Subprovince (Poulsen et al., 1992). Road Log and Field Stops A traverse which follows Highway 11 along the Rainy Lake wrench zone provides an opportunity to examine representative outcrops which illustrate its The lookout tower at Fort Frances is located on the north shore of the Rainy River near its outlet from Rainy Lake (Fig. 22). The rock exposures which Lawson (1887) originally chose as a type locality of the Archean metasedimentary biotite schist at the Coutchiching Rapids were flooded upon construction of the power dam to the west of here circa 1906. Since then, representative outcrops that illustrate the Coutchiching Group have been described nearby at Ranier, Minnesota by Ojakangas et al. (1982, Stop 1) and Jirsa and Hemstad (2010, Stop 6-2). Drive east along Front Street and join Highway 11 and continuel eastbound from Fort Frances. Lake Road intersects the highway at Km 1.9 (90.5). Continue through the land of the Couchiching First Nation past Couchiching Drive at Km 3.3 (89.1). Note the discrepancy between the modern spelling compared that of the geological unit which was based on the version used topographically circa 1887. Continue past the C.N.R. Railway Crossing (Km 5.5 (86.9)) and over the crest of the Noden Causeway bridge and continue past the intersection with a side road to the north marked “Scenic Lookout”. This sideroad (Km 8.0, 84.4) leads to stop 13 of Czeck and Poulsen (2010). Continue eastward on Highway 11 and turn in to the next (unmarked) sideroad (Km 8.8, 83.6) which leads northward to a parking area beneath the hydro tower. This is STOP NC (Noden Causeway). This is an instructive stop (Fig. 23) in that this is one of the many islands in Rainy that would have been mapped both topographically by triangulation by W.H.C. Smith and geologically by Andrew C. Lawson in the 1880’s. The rocks here consist mainly of foliated quartz monzonite of which Lawson first assigned to the Laurentian but later revised to the Algoman intrusive suite which he described as “mica syenite” belonging to a larger Pukamo Island intrusion (Lawson, 1913). Harris (1974) correlated these rocks with the Rocky Islet Bay Complex west of Rice Bay which are comprised mainly of felsic to intermediate granitoid rocks of variable composition. The main unit - 107 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 22: Simplified geology of the Fort Frances segment. Field stops NC – Noden Causeway; GA – George Armstrong Drive Figure 23: Noden Causeway stop (NC) - 108 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 here is cut by a variety of dikes which offer contrasts in structural competence compared to the surrounding granitoid rock. Note the s-shaped asymmetric foliation pattern in one of the mafic dikes (site 1) that mimics the regional structural pattern of the Rainy Lake Wrench Zone as a whole (19c). Return to Highway 11 to resume the road log. Km 11.8 (80.6) – George Armstrong Drive intersects highway 11 from the east; turn in and park near the mailboxes to examine STOP GA (George Armstrong). This is also stop 11 of Czeck and Poulsen (2010) and Point of Interest 27 described in Pye (1968). This is the first area of significant exposure of the Coutchiching rocks northeast of their type locality at Fort Frances. Although the nature of the contact with the Keewatin rocks is obscure (Fig. 24), it is still a good place to examine the differences between the metasedimentary biotite schists which are cut by felsic intrusive rocks (site 1) and the metavolcanic Figure 24: George Amstrong Drive stop (GA) amphibole-biotite schists (site 3). Both units are now metamorphic tectonites which exhibit moderate to high strain but the variability of layer thickness in the metasedimentary units is consistent with their inferred origin as submarine turbidites (Ojakangas et al., 1982). Further evidence for the superimposed strain is evident at (site 2) where at least four generations of dikes cut the metasedimentary rocks and display the variable effects of folding and boudinage depending on their structural competence and pre-strain orientation with respect to bedding (see also Czeck and Poulsen (2010) and Druguet et al. (2008). Continue eastward along Highway 11 past Commissioners Bay which is the location of a zircon sample from a Keewatin felsic which yielded a U-Pb age of approximately 2727 Ma (Davis et al., 1989). Km 18.4 (74.0) – Windy Point Bridge Km 21.1 (71.3) – outcrops on both sides of Highway 11. This is STOP SM (Sims) and corresponds in part to the Windy Point locality described by Pye (1968). The outcrops here) display deformed pillowed and variolitic metabasalt which is a dominant lithology within the Keewatin volcanic sequence on the flanks of the Rice Bay Dome (Fig. 25). It is important to examine the exposure (site 1, Fig. 26)) carefully in three dimensions because primary pillow shapes which are inherently variable are further distorted by superposition of a moderate amount of tectonic strain. This result is log-shaped pillows with long axes that plunge moderately westward (Fig, 10a). The effects of the strain can be further appreciated by examining the cm-scale light-coloured patches that stand out against the darker amphibolitic background of the metabasalt (especially at site 2). They are varioles which predictably would have formed originally as spherical patches due to devitrification of glassy volcanic rock but here their shapes reflect their tectonic distortion with a flat aspect corresponding to a foliation and a long axis which plunges westward in the foliation. Note also that the dark pillow selvedges offer rheological contrasts with the rest of the basaltic material so that the down-plunge elongation is also expressed in places in the outcrops by boudinage of individual pillows. Pye (1968) described these outcrops without reference to their volcanic origins at all while still emphasizing the lineation and the sets of joints perpendicular to it. Even where the pillows are clearly defined the considerable strain makes it difficult to draw satisfactory conclusions about primary stratification and directions of younging. Harris (1974) and Poulsen (1980) suggested, albeit with some doubt, that the stratigraphic section in this area faces downward and eastward. Km 24.9 (67.5) – The Nickle [sic] Lake Shores Road which intersects Highway 11 from the south leads to STOP NL (Nickel Lake). This was stop 1 of Poulsen (1982). This area illustrates the fact that, although the term Keewatin is synonymous with metavolcanic protoliths, it also contains clastic and chemical interflow sedimentary units which include oxide, sulfide, carbonate and silicate facies of iron-formation. These rocks are important from at structural point of view in that they typically have sharp magnetic and - 109 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 25: Simplified geology of the Swell Bay segment. Field Stops: SM-Sims; NL-Nickel Lake; MB – Moran’s Bay; GR-Great River Rd.; PP-Pocket Pond; BC- Belacoma; GP-Grassy Portage; BL- Bear’s Passage boat launch; BB- Bear’s Passage bridge; TB-Tunnel Bay Figure 26: Sims stop (SM). - 110 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 electromagnetic geophysical responses which aids in the definition of their position in areas of poor exposure. The rock here (Fig. 27, site 1) is typically referred to as chert-magnetite, banded iron formation (BIF) and is a lithology that is commonly folded at all scales. At Nickel Lake the iron-formation defines a structural synform (historically the Nickel Lake Syncline) which plunges shallowly westward along axes coincident with those of the minor folds and with the axes of maximum elongation in the adjacent volcanic rocks. The curved traces of folds observed here in a downplunge view was originally interpreted by Poulsen (1980) to present a type 3 (coaxial) fold interference pattern. It is equally possible, however, that they result from a single deformation with a strong westward plunging linear component of strain (i.e. L-s tectonite). potentially represent coeval subvolcanic intrusions or younger sills Algoman which are responsible for the cross-cutting relationships. Both lithofacies display prominent polycrystalline quartz aggregates which are likely deformed phenocrysts which help define both the tectonic foliation and a prominent lineation which plunges shallowly eastward at this locality (Fig. 15f). Figure 28: Moran’s Bay stop (MB) Km 29.0 (63.4) intersection between Highway 11 and Highway 502 (Fig. 25). This is STOP GR (Great River Rd.) which corresponds to Stop 10 of Czeck and Poulsen (2010). Figure 27: Nickel Lake stop (NL). Km 26.9 (65.5) – outcrops on both sides of highway 11 but a particularly large one on the south side. This is STOP MB (Moran’s Bay) and is described as Stop D.1 in Poulsen and Wood (1982). The outcrop is located on the south limb of the prominent antiformal Rice Bay Dome (25). It provides ample illustration of the rocks Lawson (1914) mapped as Laurentian granite and granite gneiss in the interior of the dome (Fig. 28). Both Lawson (1913) and Harris (1974) interpreted the unit to be at least in part intrusive into the mantling Coutchiching metasedimentary rocks but the details remain in considerable doubt. R.W. Ojakangas was the first to suggest that the wispy banded, quartz-phyric, grey, foliated quartzofeldspathic can also be interpreted as a deformed rhyolite. This unit yielded a U-PB zircon age of 2725+/-2 Ma (Davis et al., 1989). It is cut by more competent sheets of coarser quartz-feldspar porphyry (Fig. 15e) which could Folded quartz-phyric intrusions on the north side of Highway 11 west of the intersection (site 1, Fig. 29) cut amphibole-biotite schists containing local ironformation which were included with the Coutchiching biotite schist on the maps of Lawson (1914) and Harris (1974) but which show greater similarity to Keewatin units elsewhere. The porphyritic felsic intrusions have been generally included in the suite of Laurentian intrusions but the molybdenite-bearing quartz veins exposed here are also a characteristic of Algoman intrusions elsewhere. Despite these uncertainties of interpretation and the somewhat transitional nature of the contacts, it is clear these rocks serve to separate the inner core of the Rice Bay dome from a structurally higher annular band of moderately southeastwarddipping Couchiching biotite schists which are well exposed approximately east of the intersection (site 2). It is also possible to make a short side-trip form this intersection northward along highway 502 for 2.2 km to its intersection with the Baseline Bay side road which enters from the east. This is STOP PP (Pocket Pond) and corresponds to Stop D.2 of Poulsen and Wood (1982). - 111 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Km 30.2 (62.2) – bush road and outcrops on north side of Highway 11. This is STOP BC (Belacoma) corresponding to stop 3 of Poulsen (1982, stop D.3 of Poulsen and Wood (1982) and stop 5 of Hinz (2010). Figure 29: Great River Road stop (GR) The critical outcrops (Fig. 30, site 1) that demonstrate the overturned stratigraphic section on the northern limb of the Rice Bay dome are now heavily overgrown and no longer instructive. A good sense of the nature of the northeastward-dipping contact between the Coutchiching metapelites and the distinctive green, magnetic ultramafic unit which here represents the Keewatin volcanic rocks can still be observed along Highway 502 (site 2). Continuity of the lithostratigraphic units and their moderate northeasterly dips in this area were established with the assistance of ground magnetic and electromagnetic surveys and by diamond drilling which targeted Cu-Zn mineralization associated with the interflow iron-formation units in the section. Although the contacts among the units are sharp and well defined there is no conclusive evidence for them to be erosional-depositional in origin but the evidence for an overturned volcanic sequence is sound (Fig. 30). This is a continuation of the Coutchiching-Keewatin contact which extends southward from Pocket Pond and westward to Nickel Lake and sharply defines the eastern closure of the Rice Bay Dome. The volcaniclastic ferropicrite unit here is exposed over a wider area than at Pocket Pond and the full nature of the contact is uncertain. The ultramafic rocks near the contact with the structurally underlying Coutchiching biotite schists (Fig. 31, site 1) are foliated as but appear to be progressively less deformed eastward (sites 2 and 3). Nonetheless, graded bedding of reasonable quality suggests the Coutchichiing strata are overturned in support of the observations at Pocket Pond. The cluster of outcrops near the beaver pond (site 3) have been documented by Schaefer and Morton (1991), Goldstein and Franceis (2008) and Hinz (2010) and the inference is that this unit is composed of relatively rare mantlederived ultramafic coherent and pyroclastic rocks that locally contain well-preserved accretionary lapilli (Fig. 10f). Return to Highway 11 and resume the road log. Figure 31: Belacoma stop (BC) Km 31.3 (61.1) C.N.R. overpass Km 31.9 (60.5) – numerous outcrops on both sides of Highway 11; safe parking is available beneath the powerline on the west side of the highway (Fig. 32). This is Stop GP (Grassy Portage) and corresponds to Stop D.4 of Poulsen and Wood (1982). Figure 30: Pocket Pond stop (PP) The gabbroic rocks exposed here are part of the metamorphosed Grassy Portage layered mafic intrusion and include plagioclase-rich leucogabbro (site 1) that gives way southward to garnet-bearing quartz- 112 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 ferrodiorite (site 2). The garnets are metamorphic porphyroblasts that likely crystallized owing to the favourable bulk composition of the diorite which has a higher Fe/Mg ratio and silica content than the leucogabbro. Lawson’s 1914 map of the area portrayed the leucogabbro as “hornblende gabbro” alone as an intrusion within the Keewatin while including the gabbro and melagabbro to the north and the garnetbearing quartz diorite to the south as Keewatin metavolcanic rocks. This inferred symmetry led to his interpretation of a synclinal axis centred on the leucogabbro but Harris (I974), Poulsen (1980) and Poulsen and Hodgson (1986) recognized all three lithofacies as distinctively different phases of a single layered mafic intrusion that shows progressive southward, upward in a stratigraphic sense, chemical and mineralogical fractionation. contact (site 3) is consistent with southward younging in the meta-turbidites and contradicts the structural order of the rocks based on dip alone. It is, however, consistent with the southward younging implied by the fractionation within the Grassy Portage layered mafic intrusion. Return to Highway 11 to resume the road log Figure 33: Bear’s Passage Boat Launch stop (BL) Km 36.4 (56.0) – Taylor’s Road intersects Highway 11 from the north Km 37.0 (55.4) – parking area and scenic view on South side of the highway (Fig. 34). This is STOP BB (Bear’s Passage Bridge) corresponding to Stop 7 of Poulsen (1982) and Point of Interest 2 of Pye (1968). Figure 32: Grassy Portage stop (GP) Km 33.6 (58.8) - the side road on the east side of Highway 11 leads to the boat launch at Bear’s Passage where parking is available at the lakeside (Fig. 33). This is STOP BL (Bear’s Passage Boat Launch) corresponding to Stop D.5 of Poulsen and Wood (1982) and locality 20 of Uglow (1913). This critical area of outcrop illustrates one of the most contentious points of the Seine-Coutchiching problem. The Keewatin rocks which are cut locally by a foliated lamprophyre dike structurally overlie gabbroic rocks of the Grassy Portage layered intrusion (site 1.) The Coutchiching rocks are staurolite-bearing biotite schists (site 2) and locally display evidence of primary graded bedding with is enhanced by the distribution of porphyroclasts in upper parts of individual beds. Graded bedding which can be observed directly adjacent to the relatively sharp Keewatin-Coutchiching The eastward dipping Coutchiching biotite schist, as exposed on the north side of the highway (site 2). is cut by granodiorite of the Bear Pass Pluton which contains sheeted quartz-pyrite-molybdenite veins which are exposed on both sides of the bridge (sites 1 and 3) The view southward from the lookout features Swell Bay and the belt of Keewatin volcanic rocks to the south of it. The Keewatin-Coutchiching contact is located on Morton Island to the southwest. Km 37.7 (54.7) – Bear Pass Road intersects Highway 11 from the north. From this location it is possible to make a side trip to STOP TB (Tunnel Bay) by driving northward for 1.3 km to the C.N.R. tracks and taking the first dirt road uphill to an exposure of Coutchiching metasedimentary rocks (Fig. 35). This area is near Tunnel Bay and localities 5 and 6 of Uglow (1913). These outcrops are located on the eastern limb of the antiformal culmination in the Bear’s Passage area. The demonstration of the existence of the antiform - 113 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 OS (Old Station Road) where the field relationships are comparable to those at Morton Island (Stop D.6 of Poulsen and Wood (1982)). Figure 34: Bear’s Passage bridge stop (BB) was essential to Lawson’s (1913) interpretation of the Coutchiching strata in the interior of this structure. It is also the location where D.W. Davis first demonstrated the effects of zircon inheritance from the Coutchiching metasedimentary rocks by felsic dikes related to the Bear Pass Pluton. One of the dikes near the stop of a steep outcrop can be viewed to the east at (site 2). An additional point of interest in these exposures (sites 1 and3) is that the main foliation is locally crenulated by a steep, northwest striking, transecting cleavage (S3 of Poulsen, 1980), which is particularly prominent in a 2 km-wide northwesterly trending corridor through this area. Although locally dominant at the mesoscopic and mircroscopic scales, where crenulation of the main biotite-rich foliation and rotation of metamorphic porphyroblasts are both evident, the effects of this deformation at the macroscopic scale are negligible. Return to Highway 11 to resume the road log Km 40.8 (51.6) – Old Station Road intersects Highway 11 from the north (Fig. 36). This is STOP Figure 35: Tunnel Bay stop (TB) Highway 11 at this locality (Fig. 37) is approximately parallel to the strike of stratification in the Coutchiching meta-sdedimentary rocks as well as to their mapped contact with Keewatin meta-volcanic rocks (Harris, 1974). The overall dip of bedding is steep to the southeast and in places a steep cleavage with a more northerly strike transects bedding to form a moderately eastward plunging intersection lineations. Exposure is plentiful but the clearest features of the Coutchiching beds are illustrated in flat outcrops on the south side of Highway 11 (site 1). The rocks here are metamorphosed to greenschist facies assemblages and primary features are reasonably well preserved: polarity in graded beds consistently indicate a northward direction of younging which is away from Keewatin volcanic rocks which are exposed at the shore of Rainy Lake south of here. Km 43.3 (47.9)) – Ottertail Landing Road intersects Highway 11 from the north. Km 46.2 (46.2) – a side road to a communications tower intersects Highway 11 from the north: turn in and park (Fig. 38). This is STOP OW (Ottertail West). The field relationships exposed in the outcrops east of the intersection on the north side of Highway 11 are comparable to those at stop 1 of Czeck and Poulsen (2010) which is located approximately 1 km to the west. This is an area in which a roof pendant composed of foliated metavolcanic and metasedimentary schists has been variably incorporated into granitoid rocks of the Ottertail Lake intrusion. The outcrops here provide a rare case where highway improvement has also resulted in outcrop improvement. A marginal phase of the Ottertail Lake intrusion (site 1) is composed of diorite containing abundant mafic xenoliths (Fig. 18d). Magmatic breccias (Fig. 18e) are well exposed along the highway to the east (site 2) and, at one location nearby, a narrow NNE-striking mylonitic zone cuts the intrusive rocks. The most critical point made by Lawson (1913) and most observers since is that there is abundant visual evidence for intrusion of felsic magma into previously foliated metamorphic tectonites. Km 47.5 (44.9) – the outcrop on the north side of the road was sampled by D.W. Davis to yield a U-Pb zircon age of 2686+/-3 Ma for this part of the Ottertail Lake intrusion. - 114 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 36: Simplified geology of the Ottertail Lake segment. Field Stops: OS- Old Station Rd; OW: Ottertail Lake West; OE-Ottertail East Figure 37: Old Station Road stop (OS) - 115 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Turtle River Road. Km 58.8 (33.6) Patten Park picnic area Km 62.0 (30.4) – Low outcrops are present on both sides of highway (Fig. 40) and a rusty waste dump is visible across a marshy area on the north side of Highway 11. This is STOP PA (Port Arthur Copper) which was stop 9 of Poulsen (1982) and Point of Interest 22 in Pye (1968). Figure 38: Ottertail West stop (OW) Km 48.2 (46.2) – Pearson’s Road intersects Highway 11 from the north Km 53.9 (38.5) – the outcrops of Ottertail Lake intrusive rocks exposed here are described as Stop 2 in Czeck and Poulsen (2010). Km 56.1 (36.3) – outcrops on both sides of the road expose the eastern margin of the Ottertail Lake Intrusion. This is STOP OE (Ottertail East) and corresponds in part to Stop D.7 of Poulsen and Wood (1982). The easternmost outcrop on the north side of Highway 11 (site 1, Fig. 39) exposes deformed spherulitic and flow-banded rhyolite that is common in the Keewatin volcanic section in this part of the belt. It is cut by granitoid phases or the Ottertail Lake Intrusion, including a distinctive feldspar-phryic variety containing xenoliths (site 2). The abundance of xenoliths decreases westward in these outcrops (site 3). Km 56.4 (36.0) intersection of Highway 11 and Figure 39: Ottertail East stop (OE) Access the rusty area from the west side of the water and cross a small Beaver Dam to reach the large area of exposure (Fig. 41). The main mineralized lithology is composed of foliated amygdaloidal andesite (Fig. 10c) containing disseminated and semi-massive lenses of pyrite, chalcopyrite and sphalerite (site 1). Stratified, rusty felsic volcanic rocks are exposed on the north side of the outcrop area (site 2). This is but one of several occurrences of syngenetic sulfide deposits hosted by the felsic portions of the Keewatin volcanic section extending more than 25 km southwestward beyond Wind Bay. It is also noteworthy that the base metal deposits are located up-section northward from the syn-volcanic Bad Vermilion Lake intrusive complex (Fig. 40). Km 63.4 (29.0 side road intersects Highway 11 from the north Km 67.0 (25.5) -the Mine Centre Road intersects Highway 11. This road can be followed north to Little Turtle Lake by travelling for 1.0 km to Government Road and continuing .5 km to the C.N.R. tracks. Bear right at the intersection with Queen St. and follow the dirt road to the public boat launch site. The outcrops near the shoreline constitute STOP LT (Little Turtle Landing) which corresponds to Stop D.9 of Poulsen and Wood, 1982). Lawson (1913) mapped the rocks that are exposed here as a distinctive lithological unit which he described as “porphyroid gneiss”. In doing so, he effectively defined a 60 km E-W segment of what is now known as the Quetico Fault without explicit reference to faults but certainly recognized the overall significance of the rock type in “that it has a pronounced cataclastic structure and that the schistosity of the rock is referable to deformation involving shearing of the mass” (Lawson, 1913. P.94). Today the lithologies which he described are regarded as variably deformed fault rocks which include protomylonite (site 1) which is exposed in the outcrop east of the parking area and mylonite (site 2) along the shore of Little Turtle Lake. - 116 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 40: Simpified geology of the Mine Centre segment. Field Trip Stops: PA-Port Arthur Copper; LT-Little Turtle landing; FG: Ferguson; GS-Golden Star; WC-Windy City Rd. Regrettably, a recently constructed dock partially obscures the best exposure of the folded mylonite (Fig. 19f) as described in Poulsen and Wood (1982). Km 68.2 (24.2) The Shoal Lake Road meets Highway 11 from the south (Fig. 40). This road leads to what is arguably the most significant geological feature in the entire belt – the angular unconformity at the base of the Seine Group metasedimentary rocks. Follow the (in places rough) Shoal Lake public road southward for 3.3 km to a point where it is met from the east by a recently constructed but as yet uncompleted sideroad. The is STOP FG (Ferguson) and outcrops in this area Figure 41: Port Arthur Copper stop (PA) Figure 42: Little Turtle Landing stop (LT) Return to Highway 11 and resume the road log - 117 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 offer a good view of the Bad Vermilion tonalite which Lawson assigned to his Laurentian suite of granitoid rocks. The recent excavation (site 1) has exposed the tonalite and small quartz veins with adjacent sericiteankerite alteration of the style exposed at the Ferguson gold prospect to the north (site 2). Lawson (1913) demonstrated conclusively that the tonalite cuts both the Bad Vermillion gabbro-anorthosite to the west and Keewatin volcanic rocks to the north which include moderately northward dipping interflow chertcarbonate units and a northward younging unit of pillow basalt. conglomerate (Fig. 17a, b) it is also clear that even the least competent lithic clasts possessed no tectonic fabric at the time of deposition across strata with a preexisting steep dip. Return northward to Highway 11 and continue eastward along it. Km 76.5 (15.9) an unmarked bush road meets Turn around and proceed back northward along the Shoal Lake Road for 2.2 km to a small rise with a low outcrop on the east side; pull to the right side of the road and park as safely as possible (Fig. 44). This is Figure 44: Golden Star stop (GS) Figure 43: Ferguson stop (FG) STOP GS (Golden Star) and the site of Stop D.10 of Poulsen and Wood (1982) and the contact described by Uglow (1913) as being marked by “brown flags” for the International Geological Congress Field Trip led by Lawson. The field relationships here have also been described and discussed more recently as Stop 1 of Czeck and Fralick (2020). The base of the Seine Group here dips gently eastward at high angle to stratification in the Keewatin rocks. Much of the outcrop (site 1) is now grown over but five small patches have been recently cleaned to clearly show the west to east transition from quartzbearing tonalite a), tonalite sand with rare clasts (b) to angular conglomerate (c) with interstitial sand (fanglomerate of Lawson) to polymictic pebble and cobble conglomerate (d, e). Although there is evidence of a weak tectonic foliation superimposed on the Highway 11 on the south side; pull in and park. This is STOP WC (Windy City road). The increasingly overgrown leads southward from here for approximately 500 metres to a sign which explains how a windstorm in 1988 flattened trees over a seven km2 area resulting in its nickname of “Windy City”. Reclamation of the area resulted in local removal of shallow glacial overburden to produce two-dimensional pavement exposures of cobble to boulder conglomerate which show the rheological effects of superimposed strain. These outcrops comprise the “Forest Tour” Stop 5 of Czeck and Poulsen (2010)) can be reached by continuing another 250 m southward beyond the sign and following the second sideroad to the southwest (approximate UTM NAD 83 Zone15 N: 536 800E, 5 398 500N). The outcrops exposed at the intersection along highway at its intersection with the Forest Tour Road, however, make for a good and easily accessible substitute stop. The outcrops occur along both sides of the highway and serve to illustrate three important aspects of Seine Group as a whole. The first is the distinction between the two main lithofacies: polymictic clast-supported conglomerate (site 1) versus thick-bedded, locally cross-bedded, arenaceous sandstone (site 2) which occupies the middle part of the Seine stratigraphic - 118 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 consistent with that of rocks which are regarded to be part of the Algoman suite (Davis et al., 1989). Km 78.4 (14.0) the Manion Lake Road meets Highway 11 from the north (Fig. 46). Km 82.2 (10.8) Horsecollar Junction – the road to the south leads to the Seine River village and the outcrops the deformed conglomeratic facies of the Seine Group on the north side of the highway constitute Stop 5 of Czeck and Fralick (2002). Km 92.1 (0.3) the Crilly Road meets Highway 11 from the north. Figure 45: Windy City Road stop. section – most evidence suggests that the strata young southward toward the polymictic conglomerate units. Second, a good three-dimensional view of the shape fabrics shows both elements of both foliation and eastward plunging lineation as well as the rheological differences in response to the bulk strain by clasts of different original composition and grain size. Third, a population of granitoid clasts is particularly noticeable in this part of the Seine stratigraphic section and these were commonly assumed to have been sourced in the Laurentian granitoid suite. A sample from this area (site 3) was collected and analysed by D.W. Davis to demonstrate that the age of a granitoid clast was actually Km 92.4 (0.0) Highway bridge across the Seine River (Fig. 47). This is STOP SR (Seine River Bridge) and is also described as Stop D12 of Poulsen and Wood (1982) and Stop 5 of Czeck and Fralick (2002). The outcrop southeast of the bridge provides an excellent visual representation in cross-section of the mixed arenite-conglomerate facies of the Seine Series. The overprinting steep foliation corresponds to pronounced shape fabrics in clasts at high angle to bedding in pebble conglomerate and the comparable shortening across the foliation is manifested by steepening and distortion of the foresets in the crossbedded sandstone units. The beds dip shallowly northward and this also corresponds to the inferred Figure 46: Seine River segment - 119 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Geological Magazine, v. 67, p. 77-92. Bailey, E.B., 1927, Across Canada with Princeton; Nature, v.120, p.673-675. Bass, M. N., 1961, Regional tectonics of part of the southern Canadian Shield; Journal of Geology, v. 69, p. 668702. 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Zhou, S., Polat, Ali, Longstaffe, F., Yang, K.G., Fryer, B.J. and Weisener, C., 2016, Formation of the Neoarchean Bad Vermillion Lake Anorthosite Complex and Spatially Associated Granitic Rocks at a Convergent Plate Margin, Superior Province, Western Ontario, - 124 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Canada; Earth Sciences Publications, v. 10 (https:// ir.lib.uwo.ca/earthpub/10) - 125 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Trip 6 - Amethyst Deposits of Thunder Bay Stephen Kissin Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada and Greg Paju Resident Geologist Program, Ontario Geological Survey, Ministry of Energy and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Canada Introduction Properties of Amethyst Amethyst, occurring in abundance in the Thunder Bay region, is purple gemstone variety of α-quartz. It has been known for some time that an iron impurity in quartz is the underlying source of amethyst coloration (Holden, 1925). However, incorporation of iron of alone cannot account for the formation of amethyst, as many varieties of quartz contain trace amounts of iron, yet amethyst is relatively rare, and large deposits of amethyst are very rare. interstitial sites. The color of amethyst is produced by absorptions of light in the visible region of the spectrum owing to the presence of Fe4+, as originally shown by Cox (1977). The proposed mechanism requires the coincidence of four geological conditions for the formation of amethyst: (1) The incorporation of Fe and Al, as well as Na or Li. This is not a limiting condition, as the small concentrations of these trace elements are readily available in hydrothermal solutions. (2) A source of ionizing radiation, either from U and Th or 40K in order to produce the defects in Fe and Al. (3) Deposition at generally rather shallow depth such that oxidizing conditions prevail and iron is in the form of Fe3+. (4) Deposition with a temperature range for the stability of Fe4+, the source of amethyst coloration. In a series of papers by Cohen and coworkers, culminating in a summary in Cohen (1989), a simultaneous sequence of reactions was proposed for the formation of amethyst. (1) (Al–O)- → (Al–O)° + eIonizing radiation forms a hole center from oxidizing the substitutional Al-O bond. (2) Na+ + e- → Na° Electron from step 1 is trapped by an interstitial alkali metal ion. (3) Fe3+int → Fe4+int + eInduced ionizing radiation forms a trapped hole center via oxidizing the interstitial Fe3+. (4) (Al–O)°+e- → (Al–O)Trapped hole center is satiated as [AlO°] is reduced via gaining the electron from step 3. The presence of iron is positions interstitial with respect to the SiO4 framework was established by Adekeye and Cohen (1986), in noting its correlation with pervasive Brazil law twinning in colored sectors of amethyst crystals. Data on incorporation of the alkalis Na, K and Li and trivalent Al and Fe in quartz were reported by Deer et al. (1963), who further noted that the incorporation of Al3+ (and presumably Fe3+), is compensated by the incorporation of Na+ or Li+ The mechanism proposed above is consistent with observed data and provides a logical mechanism for the formation of amethyst. However, Rossman (1994) noted that there are unestablished factors in the model such that its acceptance is tentative. Crystal forms expressed in amethyst are invariably simple, consisting only of combined positive {101} and negative {011} rhombohedra. The faces of one of the forms are generally largely and are designated as the major rhombohedron r, and the other form is designated as the minor rhombohedron z (Fig. 1). The only other form occasionally observed is the ditrigonal prism m (Frondel, 1962). Amethystine coloration is unevenly distributed in the crystal, generally with concentration in the major rhombohedral forms, in which Brazil law twins are also concentrated (Fig. 2; Frondel, 1962). The orientation of Brazil law twins in Figure 2, is typical - 126 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 2. Etched basal α-quartz illustrating the typical occurrence of Brazil law twinning in which alternate bands contain left- and right-handed α-quartz (after Frondel, 1962). Figure 1. A typical amethyst crystal viewed perpendicular to the c-axis, illustrating the combination of positive {101 ̅1} and negative {011 ̅1} rhombohedra. of their occurrence in α-quartz; however, in amethyst the twins are polysynthetic with a typical width of 0.1 mm (McLaren and Pitkethly 1982). The twin plane of the Brazil law is {101}, which separates right-handed and left-handed orientations of quartz. McLaren and Pitkethly (1982) demonstrated that the composition plane of the Brazil law twin provides space for incorporation of Fe3+ and that iron is preferentially concentrated along this composition plane in amethyst. Amethyst’s Name and Colour Origins The word amethyst has its origins from the ancient Greek word amethystos which may be translated as “not drunken”, from the Greek a-, “not” + methustos, “intoxicated”, as the gemstone was believed to prevent or lessen the effects of drinking alcohol. There is a common theme regarding the mythological origin of amethyst’s purple colouration. Bacchus (Dionysus to the Romans); the Greek god of winemaking, orchards, fruit, vegetation, fertility, festivity, insanity, ritual madness, religious ecstasy, and theatre, pursuing a maiden named Amethyste, who was refusing his affections. Amethyste prayed to the gods to remain chaste, a prayer answered by the goddess Artemis (Diana to the Romans), who transformed her into a white stone. Bacchus humbled by Amethyste’s desire to remain chaste, poured wine over the stone as an offering, dyeing the crystals purple. In another variation the god was insulted by a mortal, and vowing to slay the next mortal who crossed his path in retaliation created fierce tigers to carry out his wrath. The hapless mortal a young woman, Amethystos, was on her way to the shrine of the goddess Diana, when the tigers fell upon her. Her life was spared by the goddess, but the price was being transformed into a statue of pure quartz. Seeing what his anger had done, a remorseful Dionysus was so moved that tears of wine poured from his eyes onto Amethystos, staining her stature purple. Despite the belief in this origin story, there are no ancient texts supporting the myth, as compared to the ancient period that supposed birthed this story, it’s quite recent as it was written in 1569 by the French Renaissance poet Rémi Belleau (1528–1577), in the poem “L’Amethyste, ou les Amours de Bacchus et d’Amethyste” (Amethyst or the loves of Bacchus and Amethyste; Belleau, 1576). Amethyst Deposits in the Thunder Bay Area In his summary of the history of amethyst in the Thunder Bay area, Patterson (1985) reported that as early as 1642, Radisson described the use of - 127 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 “torquoise” as a gemstone by local indigenous peoples. Amethyst was an associated mineral in most of the lead-zinc and silver mines, and attracted the interest of a few prospectors. In the early 1860s, the McEachern brothers prospected for amethyst in the Thunder and Black Bay areas. In 1862, they mined two tons of amethyst crystals, which they barged to Toronto to sell in that city. About the same time, a shipment of amethyst from the Thunder Bay area was sold in Montreal. The success of the mineral as a valued item for sale even in an unprocessed state and the ease of mining encouraged other prospectors and developers to try searching for and producing amethyst (Garland 1994).In the 1880s, amethyst was mined northeast of Thunder Bay in a place now called Amethyst Harbour. Interest in Thunder Bay amethyst declined around the turn of the century with the development of deposits of high quality and inexpensive amethyst from Brazil. The deposit that became known as the Amethyst Mine Panorama was originally discovered in 1935. When the fire tower was built in the 1950s, near Elbow Lake in McTavish Township, the large amethyst veins were uncovered by the roadbuilders. In the early 1960s, the area was staked, and trenches exposed the veins in what is now the open pit for the mine. In large vugs near the surface of the vein deposit, amethyst crystal of spectacular size were obtained. The development of the deposit with wide-spread sales and distribution of specimens revitalized interest in amethyst in the region (Sinkankas, 1976; Garland, 1994). The interest and activity in amethyst deposits in the Thunder Bay area led to the proclamation in 1975 designating amethyst as Ontario’s provincial gemstone (Patterson 1985). A comprehensive report on amethyst deposits and mining activity in the Thunder Bay area was completed by Garland (1994). The interest and activity in amethyst deposits in the Thunder Bay area led to the Mineral Emblem Act in 1975 designating amethyst as Ontario’s provincial Mineral Emblem (Ontario, 1990; Patterson, 1985), with the 50th anniversary taking place in 2025. There are currently 15 amethyst quarries authorized to produce under the Ontario Ministry of Nature Resources Aggregate Resources Act within two areas northeast of Thunder Bay (Campbell et al. 2024). Twelve of these authorized amethyst extraction sites are in McTavish Township and are accessible from Highway 11-17. The other three authorized quarries are located in the Tartan Lake Area (north of MacGregor Township) in an area that is accessed via the Magone Lake Road from Highway 527. Four quarries operate as tourist attractions that are open to the public on a seasonal basis. A listing of these amethyst quarries, including information about their products and services (where available), is provided in Table 1 (Campbell et al., 2024). Geology Of Amethyst Mine Panorama (Thunder Bay Amethyst Mine) Geologic Setting The geological setting of the mine is complex, as an Archean and a Proterozoic record are preserved in the area. This record has been recently reviewed by Sutcliffe (1991) with an update by Addison et al. (2010) and will not be repeated in detail here. The Amethyst Mine Panorama (Thunder Bay Amethyst Mine) is hosted in the Archean Hilma Lake granite of McCrank et al. (1981). This pluton lies on the boundary of the Quetico Subprovince and the Wawa Subprovince, with typical greenstone lithologies on its southern margin and gneissic metasedimentary rocks on the northern margin. The Hilma Lake granite in the vicinity of the mine consists predominantly of monzonite, with compositional variation along the trend monzonitequartz monzonite-granite-granodiorite and pegmatite and pegmatitic textural variants (Jennings, 1985). Jennings’ study indicates that monzonite had been cut first by granodiorite, then by pegmatite, with some metasomatic alteration of early monzonite toward granodioritic composition. At the Greenwich Lake uranium occurrence, a vein-type occurrence located 10 km to the northwest, Franklin (1978) noted the presence of quartz monzonitic pegmatites containing 60-100 ppm U in the form of uraninite. As these pegmatites are apparently comagmatic with the Hilma Lake granite, its uranium-rich character is likely a general feature. The Proterozoic rocks were deposited on the eroded Archean surface; however, the Animike Group (Gunflint and Rove Formations) is missing in the vicinity of the amethyst mine. As indicated by Franklin et al. (1980), the Mesoproterozoic Sibley Group progressively onlaps Archean terrain in a northerly direction. The Sibley Group is presently absent in the vicinity of the Amethyst Mine Panorama, although its presence as abundant fragments in mineralized breccias within the vein system indicates that these sediments - 128 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Table 1. Amethyst quarries in the Thunder Bay Area authorized to produce under the Aggregate Resources Act (from Campbell et al. 2024) Deposit Name and Ownership Amethyst Mine Panorama Precious Purple Gemstones Ltd. Location (Licence) McTavish Township (622921) Products and Services Tourist attraction (pick-your-own and mine tours), specimens, decorative and landscaping stone, and tumbling stone, jewellery, giftware, carvings, faceted gemstones www.amethystmine.com/ Blue Points Amethyst Mine Jordan Vivian McTavish Township (624926) Tourist attraction (pick-your-own), specimens, decorative stone, aquarium stone www.tripadvisor.ca/Attraction_Reviewg155017-d3334892- ReviewsThe_Blue_Point_Amethyst_MineThunder_Bay_Thunder_Bay_District_Ontario.ht ml (lynswan@lakenet.com – email) Diamond Willow Amethyst Mine Big Pearl, Sward Lake B. Leroux and C. Fayle McTavish Township 3 permitted quarries, (626151, 625922, 626134) Tourist attraction (pick-your-own and mine tours), specimens, decorative and landscaping stone, slabs, tumbling stone, jewellery and giftware www.diamondwillowamethyst.com/ Keetch Quarry / Boulder Creek Amethyst Quarry L. Harasym McTavish Township (77956) Tourist attraction (pick-your-own), specimens https://mininglifeonline.net/company_page_487.html Assiniboia Amethyst Mine P. and T. Smitham McTavish Township (626091) Not open to the public, but may be visited by invitation only. Contact: https://assiniboiaamethystmine.weebly.com/ Bill’s Old Amethyst Mine K. Zytaruk McTavish Township (607322) Not advertised Canadian Shield Amethyst Mine K. Zytaruk McTavish Township (616261) Not advertised Tartan Lake Danbill Mine Auralite 23 Mine and Company Area (20227) Inc. Specimens, polished and tumbled stone, jewellery, tiles and countertop stone www.auralite23canada.com/home.html Gunnard Project M. Noyes and J.A. Gavin McTavish Township (625989) Not advertised Loon Lake Technical Services Quarry Loon Lake Technical Services McTavish Township (625067) Not advertised Tartan Lake Area Purple Haze Mine Auralite 23 Mine and Company (624879) Inc. Specimens, giftware, jewellery, decorative and landscaping stone from former owners at www.purplehazeamethyst.com/. Transferred to new ownership in late 2022. Roll Lake Amethyst Tartan Lake Area Auralite 23 Mine and Company (624838) Inc. McTavish Windy Ridge Amethyst L. Kowtuski Township (625831) Specimens, polished and tumbled stone, jewellery, tiles and countertop stone www.auralite23canada.com/home.html - 129 - Email: windyridge@live.ca �Proceedings of the 72nd ILSG Annual Meeting - Part 2 were present as basement cover during the forming of the deposit. The significance of the Sibley Group is unclear in the face of contradictory evidence concerning its age and depositional setting. Franklin et al. (1980) noted that the Sibley Group isdeposited at the location of a failed arm of an r-r-r triple junction, although they admitted to uncertainty as to the contemporaneity of sedimentation and rifting. Although some features of the Sibley Group are suggestive of a rift-filling deposit, the whole-rock Rb/Sr age of 1339±33 Ma (Franklin, 1978b) is approximately 200 Ma prior to the main stage of rifting of the Midcontinent (Keweenawan) Rift (Van Schmus et al., 1982). Cheadle (1986), however, concluded on the basis of sedimentological studies that the Sibley Group was not deposited in a classical aulocogen, but represents a deposit on a sagging crust preceding rifting. The Sibley Group was more recently dated by U/Pb geochronology in zircons in a basal rhyolite unit at 1537+10/-2 Ma (Davis and Sutcliffe, 1985). This timing makes a relationship with the Midcontinent Rift event unlikely, and Hollings et al. (2004) proposed that the Sibley Basin formed due to effects of a plume track that created an infracratonic basin. by breccias of granitic country rock and Sibley Group sedimentary rocks with large proportions of void space. The brecciated fault was subsequently mineralized by hydrothermal solutions. At least two periods of mineralization occurred, as an early generation of amethyst was clearly brecciated and subsequently coated by a second generation of amethyst. Figure 4 is an illustration of the state of the mine in 1987. At present, the main pit configuration is basically the same but has been deepened. In that year, an extension of the vein system to the east was developed, offset to the north by a few metres strike-slip fault. Jennings (1985) subdivided the mineralization patterns into three basic types: (i) open fracture fillings, (ii) breccias with tectonic and collapse subtypes, and (iii) “honeycomb” veins. Other deposits located at or near the Sibley -Archean unconformity include the Dorion lead -zinc -barite veins (Fig. 3). The ore-depositing solution was considered to be a basinal, connate brine by Franklin and Mitchell (1977), an interpretation supported by the fluid-inclusion studies of Haynes (1988). As illustrated in Figure 3, there is a close spatial relationship between the lead-zinc-barite veins and the amethyst, and both are spatially related to the Sibley-Archean unconformity. Geological features of the mine Amethyst Mine Panorama is located within a firstorder strike-slip fault, which strikes at 90- 100º and dips steeply to the south. This fault is roughly parallel to one 2.1 krn to the south, which strikes east-northeasterly (McIlwaine, 1971) and has a vertical displacement of at least 125 m (Jennings, 1985), forming a major boundary to the Sibley Group’s depositional basin. The strike-slip fault hosting the amethyst deposit is offset by seven first-order strike-slip faults, five of which are illustrated in Figure 4, which strike 162 - 150° and dip vertically producing en echelon, pull-apart structures in the main fault. These structures are filled Figure 3. Local geology and location map of amethyst deposits and lead-zinc-barite deposits, showing the relationship of the former to the margin of the Sibley Group outcrop and the Hilma Lake granite. The location of the producing Thunder Bay Amethyst Mine’s are indicated by stars. Bedrock geology and mineral occurrence locations modified from Ontario Geological Survey (2011; 2026). - 130 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 4. Diagram of the main pit of Amethyst Mine Panorama (Thunder Bay Amethyst Mine). The strike directions of these veins are strongly clustered in two groups, one slightly west of north and parallel to the second stage of strike-slip faulting, and one easterly, parallel to the principal directions of faulting. Open fracture fillings are common in the shallower zones of the deposit where low lithostatic pressure permitted the maintenance of open fissures following faulting. The veins are widest near the edges of collapsed breccias and at the intersections of oblique shears with the main fault zone. Fracture-fill mineralization occurred at the crystal-fluid interface as quartz crystals grew outward from the fracture walls. The crystals formed as parallel to radial growths with long crystal axes oriented perpendicular or subperpendicular to the growth surface. The crystal size invariably increases outward, and outward growth from opposite fractures resulted in an interlocking comb structure of euhedrally terminated crystals. This vein type may also contain vugs up to 2-3 m in diameter with large quartz crystals up to 10-15 cm in prism diameter. Tectonic breccias are here attributed to fault movement, as opposed to brecciation caused by collapse with variable degrees of fluid action. Some breccia fragments are surrounded only by a later portion of the paragenetic sequence, suggesting that multiple fault motion during the mineralizing event has occurred. Breccia fragments of this type are invariably angular and may consist of fragments of earlier deposited vein material, which may have been thermally bleached. Collapse brecciation is not always differentiated from tectonic brecciation, and some collapse breccias have undergone subsequent tectonic brecciation and vice versa. Evidence of collapse brecciation is seen in the occurrence of Sibley Group lithologies not present in the mine area now, together with granite and diabase as breccia fragments. Sibley Group fragments are particularly abundant within channel- or pipe-like structures in which fluid transport and abrasion have produced subangular to subrounded fragments, which have undergone an appreciable degree of sorting. Collapse-breccia fragments are typically coated with successive layers of chalcedony, colorless quartz, and amethyst, producing a cockade structure. Vugs have developed in open space produced in the breccia in which crystals with prism diameters of up to 10 cm have grown. Honeycomb veins are the result of quartz crystallization that has occurred in all directions from small nuclei, usually chalcedony, hematite, or silicified granite fragments, rather than from a fracture wall. The amethyst and quartz are more massive than in the other types of veins, but the growth is chaotic. - 131 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Mineralogy Amethyst and other varieties of quartz. Several varieties of quartz occur in Amethyst Mine Panorama, including colorless quartz; chalcedony; amethyst; the yellowish variety, citrine; and the greenish variety, prasiolite or “greened amethyst”. Smoky quartz has very limited development . The only variety of gemstone interest is amethyst, although the occurrence of the other varieties has aided in establishing the sequence of deposition. A grading system based on estimated intensity of coloration and clarity of specimens is in use at the mine, and this system has also aided in establishing the paragenetic sequence. Thus, the intensity of coloration may be from I (lightest) to IV (darkest) and clarity from a (clear) to f (opaque). Table 2 lists typical paragenetic sequences in an older sequence, which is present as breccia fragments in a younger sequence presently occupying the veins. The prasiolite in stages 4 and 5 of the older sequence appears to be thermally bleached amethyst on the basis of both its appearance and experimental evidence that heat-treated amethyst can be transformed to prasiolite (Lehmann and Bambauer, 1973). In the younger sequence, late-stage variations are noted, particularly as cappings to stage 5. A distinctive variety called “black gem”, a dark, brownish-black amethyst, is apparently characteristic of larger crystals grown in vugs in which iron-enriched, late-stage fluids were trapped. These frequently have final growth zone that contains abundant hematite inclusions, such that recent sales of such material has been called “Thunder Bay red”. It was this material, recovered in the early development of the deposit that led to the notorious statement by Sinkankas (1976, p. 204): “By far most of the amethyst is unsuited for lapidary purposes, with very little being free from flaws and hence useless for faceted gems or even baroques.” Figure 5 illustrating cut and faceted gemstone demonstrates the error in Sinkankas’ statement. The compositions of specimens of amethyst by neutron activation analysis for selected trace elements (Table 3) revealed the presence of subequal concentrations of Fe and Al. As well, low concentrations of Ge were sought based on absorption spectra that indicated its presence. The low Ti concentrations are perhaps related to the spotty occurrence of rutile needles in the amethyst, needles occurring when concentrations are relatively greater. Figure 5. Cut and faceted smoky quartz (top left) and amethyst from Amethyst Mine Panorama (Thunder Bay Amethyst Mine). Photo by S. Kissin. Table 2. Paragenetic sequences observed in the veins of Amethyst Mine Panorama Notes: Variations observed include (i) late-stage greenish and yellowish-amethyst; (ii) late-stage smoky quartz; (iii) discontinuous hematitic and milky quartz capping to crystal terminations; and (iv) development of black gem in crystals, deposited in vugs. - 132 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Table 3. Analyses of r-zones of amethyst for selected trace elements (in ppm; Kissin, 1997) Sample No. Fe Al Ge Ti DZS1* 217 447 0.5 n.d. LZS2 102 393 0.5 0.01 BSS3 273 369 1.0 n.d. TPS4 368 249 0.5 n.d. *DZS1 evidently contains solid inclusions, as high concentrations (in ppm) were noted; e.g. Ta 0.329, W 0.38, Eu 0.349, Sr 89.43, Zr 1.02, Nb 0.13, Ba 2985.26, La 3.85, Ce 0.35, U 0.387. All samples contain small, but detectable quantities of Co, Ni, Ga, Rb, Nb, Zr, Mo, Sn, Sb, Cs, La, Pr, Nd and U. Other non-sulfide minerals. Barite is rare in the veins at the Amethyst Mine Panorama, although it is abundant in other amethyst mines of the district, where it follows the final stage of quartz deposition. It was not observed in the course of the present study, but has been noted in the mine. Calcite is fairly common in thin, monomineralic veins, but was not observed within the amethyst-bearing veins. The genetic link between the calcite veins and amethyst veins, if any, is unclear. Hematite is abundant as minute- (< 0.1 mm diam.) solid inclusions in stage 5 of amethyst deposition and occurs sporadically at other stages of deposition as well. Hematite occasionally occurs as a daughter mineral in fluid inclusions, particularly in stage 5 of crystallization. Rutile occurs as needles that transect the growth zones of the quartz in scattered locations within the mine. The orientations of the needles are apparently random; however, the possibility of crystallographically controlled growth directions has not been considered in detail. Native copper occurs in association with copper and copperiron sulfides. Sulfides The common base-metal sulfides pyrite, chalcopyrite, galena, and sphalerite occur in small amounts throughout the vein succession and as veinlets and replacement bodies in altered granitic wall rock. Copper -iron sulfides, however, are predominant, and a sequence of the minerals cuprite-native copperchalcocite-covellite associated with hematite and pyrite was documented by McArthur et al. (1993). The copper -iron sulfides exhibit typical replacement textures (atoll structures, core-and-rim relationships) in occurrences both in amethyst growth stages and in wall rock. The assemblages bornite+pyrite and chalcopyrite+pyrite and chalcopyrite+pyrite occur in wall rock only; however, spatial relations of wallrock sulfides to the veins do not reveal any pattern, owing in part to their scarcity. Malachite is present as a supergene product derived from these hypogene copper minerals. Wall-rock alteration mineralogy. Hematitization, chloritization, and kaolinitization are prominent in envelopes surrounding the veins within zones of brecciated granite; however, the alteration extends only a few centimetres into the granites outside of the zone of brecciation. Intense hematitization occurs fairly generally in altered rock nearest the amethyst veins. The strongly hematitized zone is generally only a few centimetres thick, but weaker hematitization is notable throughout the altered zone. Outward from the hematized zone is an irregular zone of highly chloritized rock ranging from a few to a few tens of centimetres thick. Sometimes associated with the chloritization is diffuse epidotization, which produced a pistachio green tint over zones up to a metre wide. Kaolinitization is widespread and pervasive through the breccia zone, imparting a chalky, white appearance to relict feldspars. Other clay minerals, e.g., montmorillonite and illite, may also be present; however, they have not been sought in a detailed examination. The pervasive kaolinitization has allowed weathering to penetrate into the brecciated zone, resulting in a soft and loosely aggregated matrix in which the near-surface exposures of the amethyst are contained. The nature of this matrix has enabled a good deal of the amethyst to be mined with a minimum of blasting. The hematite-chlorite-epidote alteration assemblages in the presence of ubiquitous quartz are characteristic of the propylitic alteration typical in many hydrothermal ore deposits. The kaolinite and other clay minerals are characteristic of the argillic - 133 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 alteration of hydrothermal ore deposits. The two alteration types are analogous at least in their relative timing to early peripheral propylitic alteration, which is overprinted by argillic alteration stemming from downward-infiltrating meteoric water. Genesis of the deposit The genesis of the deposits of the Amethyst Mine Panorama were discussed in detail by McArthur et al. (1993) in the light of evidence obtained in their study. The conclusions of their study are given below; however, for details of the evidence, their paper should be consulted. Genetic speculations on the Amethyst Mine Panorama are hampered at the outset by questions as to the timing of amethyst deposition, as discussed in the Introduction. The spatial and geochemical affinities of the amethyst deposits with the Dorion lead-zinc-barite veins and the relationships of both to the depositional margin of the Sibley Group sediments suggest that all three are interrelated. Franklin and Mitchell (1977) proposed that the lead -zinc -barite veins formed when, during diagenesis and settling of the Sibley Group sediments, metal-bearing brines were formed when expelled connate waters mobilized metals from the Sibley Group sediments and(or) weathered granitic basement rocks below the Archean-Proterozoic unconformity. The solutions thus formed would have hypothetically migrated through the basal Pass Lake Formation aquifer to escape at basin-marginal faults. Precipitation of sulfide, carried in chloride- and sulfatebearing solution, occurred because of mixing of the relatively oxidized solution with H2S gas trapped at the Pass Lake Formation pinch-out. The amethyst deposits seem to be a variant of the lead-zinc-barite type of deposit in which the temperature was lower than that of the sulfide-rich lead-zinc-barite veins. The initially oxidizing to later reducing character of the solution is similar to that proposed for the lead-zinc-barite veins, but the relation to Pass Lake Formation pinch-outs is not present in most amethyst deposits. Rather, the amethyst deposits are generally hosted in granitic basement often with no Sibley Group sediments present. The amethyst deposits are richer in dissolved silica, having gained this component through the kaolinitization of feldspar during hydrothermal alteration of granitic country rock. As the amethyst deposits formed near the present or former unconformity with the Sibley Group, local reduction of the solution would have tended to occur as H2S was released during thermal breakdown of organic matter in the sediments. The quantity of sulfides precipitated would have been limited not only by the relatively small amount of H2S produced but also by the lower metal content of the solutions as compared with those depositing the Dorion lead-zinc-barite veins. The latter characteristic is inferred by a comparison of the results of this study with those of Haynes (1988) on the Dorion lead-zinc-barite veins. He found that fluid inclusions from these deposits are NaC1-CaC12H2O type on the basis of microthermometry and direct analysis of decrepitates. However, the fluid inclusions depositing sulfides are significantly more saline than those at the Amethyst Mine Panorama in that they contain daughter salts. The more saline and higher temperature (105-203°C) fluid inclusions indicate that solutions that they represent would have had a better metal carrying capacity as chloride complexes. The similarity of the solution components to those at Amethyst Mine Panorama lends support to the idea that the same event formed both types of deposits. The solutions depositing amethyst would have been cooler and less saline variants of those that formed the lead-zinc-barite veins. If the two types of deposit are genetically linked, both suffer from the problem of lack of knowledge of the timing of ore deposition. The maximum age of both is 1339 Ma, the whole rock Rb/Sr age of the Sibley Group (Franklin, 1978b), as both types of veins cut Sibley Group rocks and contain breccia fragments of them. Franklin and Mitchell (1977) did not suggest a specific timing for formation of the Dorion lead-zinc-barite veins; however, their suggested mechanisms for creation of the deposit favor a timing soon after the deposition of the Sibley Group sediments. The expulsion of pore water called upon would presumably occur during late diagenesis. However, as there is no evidence to suggest that the Sibley Group sediments have ever been deeply buried, the source of heat is a problem. If the timing of deposition were close to the formation of the Sibley depositional basin, it is possible that a thermal anomaly, perhaps augmented by seismic pumping, in the lower crust was responsible for both phenomena. Haynes (1988) suggested that the Dorion leadzinc-barite veins formed either in the environment of Keweenawan rifting or later, possibly in the Paleozoic. There is no geological evidence for activity in the Paleozoic in the western Lake Superior region, and the style of mineralization associated with Keweenawan - 134 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 events is different (silver deposits associated in part with Ni-Co arsenides; Franklin et al. 1986). Our preferred hypothesis is that the lead-zinc-barite veins and amethyst veins are associated with the timing of formation of and deposition in the Sibley basin. We, therefore, believe that these deposits are distinct from silver deposits in the Thunder Bay area and formed at a somewhat earlier time. The timing is, however, not at all certain. An attempt was made to directly date amethyst deposition by U/Pb age determinations on rutile needle inclusions in amethyst (Heaman and Easton, 2006). An age of 887±40 Ma with 68.6% discordance was determined on a very small sample with low uranium content. The authors indicated that these results should be viewed with caution as a large lead correction was needed. This age does not coincide with any known geological events in the area. As well, an attempt was made to date the cross-cutting diabase dikes, but was unable to yield any results. Summary Field and laboratory studies of the Amethyst Mine Panorama reveal the following: (3) Sulfide minerals including pyrite, chalcopyrite, galena, and sphalerite accompany amethyst deposition as small mineral inclusions and occur, as well, as veinlets and replacement bodies in altered granitic wall rock. Copper and copper-iron sulfides are most abundant and, together with native copper and cuprite, Eh-pH relationships indicate that the solutions forming the deposit were initially rather oxidizing and weakly acidic. In the course of crystallization, the solution became more reducing and slightly more acidic. (4) Fluid-inclusion studies indicate that in the younger sequence of quartz deposition, homogenization temperatures range from 146.5 to 114.7°C (mean 132.1°C) as contrasted with 91.2-40.9°C (mean 68.4°C) for amethyst. Eutectic temperatures of frozen inclusions indicate that the solution was of the NaClCaCl-H2O system, with possible concentration of an additional halide salt component in late-stage fluids. Few inclusions contain daughter minerals, and those found are hematite and sphalerite in late-stage fluids. Final melting temperatures indicate a trend of decreasing salinity in later growth stages. (5) Oxygen isotopic determinations on quartz indicate a range of δ180 outside that of juvenile (1) The vein system hosting amethyst deposits was formed by mineralization of an east-west-striking, waters and end-member basinal brines. Progressive steeply dipping strike-slip fault, opened into en mixing of basinal brine with local meteoric water echelon pull-apart structures by a series of later strike- is suggested. slip faults, also dipping steeply and intersecting the (6) Sulfur isotopic analyses of pyrite yield δ34S first-formed fault at high angles. Much open space of -0.4 to 0.6 ‰ and -1.4 ‰ in chalcopyrite. These with brecciated and vuggy textures resulted. Breccia volumes are consistent with derivation from H S 2 fragments include granitic host rock and Sibley Group gas liberated by thermal action protection on sedimentary rocks, implying that the latter were present organic material involving iron. The values are as a thin cover at the time of mineralization, although similar to those of the sulfur contained in sulfides they are erosionally removed from the mine area at in the Dorion lead-zinc-barite veins. present. At least one early generation of amethyst is included as breccia fragments, indicating that fault movement continued during mineralization. (2) At least two phases of amethyst crystallization separated by a period of brecciation are present. The older sequence contains five stages of quartz growth, the latter two of which were originally amethyst, but were thermally bleached to prasiolite by the influx of hot solutions that deposited the younger sequence of quartz. The younger sequence contains five and occasionally six stages of deposition, beginning with a stage of chalcedony and a stage of colorless quartz, followed by amethyst. Both sequences of deposition are traceable throughout the mine. (7) The presence Sibley breccia fragments cemented by quartz indicates that the veins cannot be older than 1339 Ma, the Rb/Sr age of the unit. However, a younger limit cannot be established at present. (8) On grounds of similarity in geological setting, proximity, composition of the ore-depositing solution, and sulfur isotopic composition, the amethyst veins are believed to be genetically related to the Dorion lead-zinc-barite veins. Both are believed to have been formed by solutions expelled and mobilized during diagenesis and compaction of the Sibley Group. The lead-zinc-barite veins formed in fractures at or near the margin of the Sibley depositional basin from solutions that were both hotter and more saline than those - 135 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 depositing amethyst. Amethyst-depositing solutions travelled longer distances in granitic basement, dissolving silica by alteration of feldspar. Although the amethyst-depositing solutions probably carried less metal as chloride complexes than did the solutions forming the lead-zinc-barite veins, less H2S at the site of deposition was probably the most significant factor causing a low sulfide content in the amethyst veins. (9) The temperature conditions under which amethyst forms appear to have a high temperature limit; at the Amethyst Mine Panorama this limit is no higher than approximately 115°C and may be as low as approximately 90°C. Temperatures as high as approximately 145°C but possibly as low as 115°C may be sufficient to thermally bleach earlier generations of amethyst in the influx of hot solutions. However, this theory of thermal bleaching has been recently criticized by Herbert and Rossman (2008), who attributed the development of greenish-grey to greenish quartz to the presence of H2O in the crystal. Our work (Klarner and Kissin, 201l) confirms the presence of water in IR absorption spectra; however, the water is largely contained in fluid inclusions, which are abundant and of secondary origin. Use of the highly focus beam of an FTIR microscope has shown that molecular water is of low and nearly identical concentration in both amethyst and “greened amethyst”. Experiments by Goetz (2014) demonstrated that heating at 250ºC for extended periods did not result in bleaching of amethyst, disproving that the 145ºC temperature caused bleaching of amethyst. This problem is unresolved at present. Geology of the Amethyst Mine Diamond Willow Unlike the years of extensive research undertaken at Amethyst Mine Panorama, the other known amethyst deposits in the Thunder Bay region are not well studied and with most information available being from Garland (1994) The original Diamond Willow Amethyst Mine was staked by Gunnard Noyes, in the 1960s with the mine initially operating in the 1970s. Following Gunnard’s passing in 1988, the mining leases were split into two parcels per inheritance and turning the original mine into the current Diamond Willow and Blue Points Amethyst Mines, owned by a son and daughter, respectively. The Diamond Willow Amethyst Mine operated until 2007 and was subsequently closed until 2015 (Garland, 1994). The currently producing Blue Points Amethyst Mine is the eastern extension of the original Diamond Willow mine and operates two and three-quarters of the four pits situated along the breccia zone (Fig. 6). The centre pit is the original and the largest, almost 60 m long and 4 m deep. A fence divides the pit between the two mines. The current Diamond Willow Amethyst Mine; the western extension of the original namesake mine site, operates one and one quarter of the four pits situated along this breccia zone (Fig. 7; Garland, 1994). This mineralized and well developed breccia zone occupies a vertically dipping fault zone, trending approximately 090° and extends for almost a kilometre. The fault separates Sibley Group conglomerates of the Pass Lake Formation from Sibley Group mudstone of the Rossport Formation (Garland, 1994). The current Diamond Willow Amethyst Mine is located at the western end of this fault/breccia zone, separating the Rossport Formation mudstones on the south from the Pass Lake Formation conglomerates on the north side. Both the mudstone and the conglomerate are well-layered, giving them a blocky appearance (Garland, 1994). The breccia zone varies from 1 to 5 m wide, and is characterized by a quartz­rich core and fragments of wall-rock material. In general, the fragment density increases away from the core, but is always matrix supported, the fragments are angular and representative of the wall rocks. Within the breccia, amethyst filled vugs can attain sizes of over 1 m and are lined with large, dark purple crystal points up to approximately 7.5 cm in diameter. The vugs also tend to be filled with a dense red clay; fault gouge, consisting of finely ground quartz, feldspar, chlorite, and biotite (Vos, 1982; Patterson, 1985; Garland, 1994). which must be removed in order to mine the amethyst. Light violet to a very dark, nearly black purple amethyst forms an extensive druse covering along the south wall, crystallizing between the mudstone and the breccia. The crystal points in this druse tend to be small, but are very well-formed, yielding excellent mineral specimens. Like Amethyst Mine Panorama, the amethyst crystals are sometimes coated with a layer of reddish brown hematite. Galena occurs as seams of crystals 1 cm in size, within the quartz at the west end of the exposed breccia zone, chalcopyrite-rich zones - 136 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Figure 6. Plan view of the Blue Points amethyst mine. Figure 7. Plan view of the Diamond Willow amethyst mine, refer to Figure 6 for legend. are associated with rusty stained or clear quartz crystals (Vos, 182; Garland, 1994). - 137 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Road Log Lakehead University to Amethyst Mine Panorama and Diamond Willow Amethyst Mine Leaving Lakehead University, we will follow the portion of the Trans-Canada Highway 11-17 which is the Thunder Bay Expressway. The flat terrain is the remnant of the bottom of the Nipissing stage of ancestral Lake Superior, and proceeding northeasterly, we pass upward through strandlines of the receding Pleistocene lake. Lakehead University itself is underlain by the Gunflint Formation at or near the top of the unit. Shaly rocks near the top of the formation are exposed in the bed of the McIntyre River that flows through the campus; however, in recent years blocks of rock containing the Sudbury ejecta debrisite were excavated during construction of new student residences. These placed in various places around the campus as ornamentation or barriers to vehicular traffic. Continuing, outcrops of a Logan sill diabase are exposed on the left side of the expressway. These sills form the caps of the prominent mesas south of town and underlie the high ground in the northern section of Thunder Bay, formerly the city of Port Arthur. Passing the junction of Red River Road (Highway 102), the expressway is on a level stretch marking the top of a Logan sill. The expressway then passes downhill to the Current River. In proceeding downhill outcrops of Logan sill diabase, Gunflint shale and Gunflint carbonate are successively exposed. The carbonate is ankeritic and is oxidized to yellowish orange. Climbing uphill from the Current River bridge, the highway is again cutting into diabase sill. A fault trends along the highway offsetting the sill on opposite sides of the highway. A few hundred metres farther along the highway, the sill is dropped downward by a fault trending perpendicularly to the highway. Recent work has shown that this sill, known locally as the Terry Fox sill, is a Nipigon sill (Magnus and Kissin 2010). Nipigon sills, which occur from here northeasterly to the Lake Nipigon area, are somewhat younger than Logan sills and can be distinguished on the basis of their trace element composition. Proceeding downhill and rounding a curve to the left, there is a high bluff on the left capped by a prominent diabase sill. The sill has intruded the top of the Gunflint Formation and the overlying Sudbury debrisite layer, which is capped by a thin remnant of Rove Formation shale. This is the only outcrop known in the Thunder Bay area that contains the complete debrisite layer. The east end of this outcrop is bounded by a fault that dropped down the section. Continuing onward, high ground on both sides of the highway are capped by sills; the sill on the right was extensively quarried for railway bed ballast and large stone for construction of the breakwall in the harbor. After the junction with Highway 527, the highway climbs the hill locally known as KOA hill. Prior to a widening of the highway about a decade ago, the angular unconformity between the Gunflint Formation and steeply dipping Archean metavolcanics was exposed on the left of the highway. The hill is formed by the outcrop of the Mackenzie granite, an unmetamorphosed and undeformed, late Archean pluton. The highway continues on top the of granite, which contains occasional roof pendants of Archean metavolcanics. After crossing the Mackenzie River sparse outcrops of granite are replaced by poorly exposed Gunflint Formation until just past the junction with Highway 587. Here, well-bedded red-stained carbonates of the Gunflint Formation crop out beside the highway. Passing onward to the East Loon Road, turn left onto the road, then right on Bass Lake Road. Continue to the turn off on the right to the private road to the mine. Proceeding along the mine road, it climbs steeply up from the Sibley basin onto the Archean Hilma Lake granite, ascending along a border fault surface. At the top of the grade, there is a chance to view Lake Superior with Black Bay, the Black Bay Peninsula and the Sibley Peninsula, clear weather permitting. A few more kilometres brings the road to the mine. To get to the Diamond Willow Mine, head back to Highway 11-17, and turn east towards Nipigon. Travel for approximately 13.4 km and turn left onto 5 Rd S, then make a right and drive to 5 Rd N,, crossing the railbed and make a left onto a private dirt road. Continue on this road staying right for 2.56 km, until a “Y” junction is reached and stay left until you reach the Diamond Willow Amethyst Mine parking area. - 138 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Amethyst Mine Tours amethystine color in quartz. Mineralogical Record, v. 20, p. 365-367. Note: Safety boots or shoes recommended. No sandals or open-toed shoes. Cox, R.T., 1977, Optical absorption of the d4 ion Fe4+ in pleochroic amethyst quartz, Journal of Physics C: Solid State Physics, v. 10, p. 4631-4643. The tours will pass through the operating mining areas, which is not available to ordinary tourists. No collecting is allowed in these areas After visiting the mining area, there will be an opportunity to look for specimens in a designated collecting area. The charge for specimens is by weight. Hammering or chiseling is not permitted in Amethyst Mine Panorama’s collecting area, hammering and chiseling are only permitted at Diamond Willow Amethyst Mine; however, only hammers up to 2 lb. max, are permitted and absolutely no sledge hammers and safety glasses must be worn when using hammers or tools while collecting. Specimens are also for sale in the shops. Davis, D.W., and Sutcliffe, R.H., 1985, U-Pb ages from the Nipigon Plate and northern Lake Superior. Geological Society of America Bulletin, v. 96, p. 1572-1579. References Addison, W.D., Brumpton, G.R, Davis, D.W., Fralick, P.W., and Kissin, S.A., 2010, Debrisites from the Sudbury impact event in Ontario,north of Lake Superior, and a new age constraint: Are they base-surge deposits or tsunami deposits? In W.U.Reimold, and R.L. Gibson, eds., Large Meteorite Impacts and Planetary Evolution IV: Geological Society of America Special Paper 465, p. 245-268. Deer, W.A., Howie, R.A., and Zussman, J. 1963, RockForming Minerals,Vol. 4 Framework Silicates. John Wiley and Sons, Inc., New York, 435 p. Franklin, J.M. 1978a, Uranium mineralization in the Nipigon area,Thunder Bay District, Ontario. in Current Research, Part A. Geological Survey of Canada, Paper 78-lA, pp. 275-282. Franklin, J.M., 1978b, The Sibley Group, Ontario, in Rubidium strontrium isochron age studies report 2. Edited by R.K. Wanless and W.D. Loveridge. Geological Survey of Canada, Paper 77-14, p. 31-34. Franklin, J.M., and Mitchell, R.H., 1977, Lead-zinc -barite veins of the Dorion area, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, v. 14, p. 1963-1979. Franklin, J.M., McIlwaine, W.H., Poulsen, K.H., and Wanless, R.K., 1980, Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada. Canadian Journal of Earth Science, v. 17, p. 633-651. Adekeye, J.I.D., and Cohen, A.J., 1986, Correlation of Fe4+ optical anisotropy, Brazil twinning and channels in the basal plane of amethyst quartz, Applied Geochemistry, v. 1, p.153-160. Franklin, J.M., Kissin, S.A., Smyk, M.C., and Scott, S.D., 1986, Silver deposits associated with the Proterozoic rocks of the Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, v. 23, p. 1576-1591. Belleau, Rémi., 1576. Les amours et nouveaux eschanges des pierres précieuses : vertus et proprietez d’icelles ; Discours de la vanité, pris de l’Ecclesiaste ; Eclogues sacrees, prises du Cantique des Cantiques ([Reprod.]) / par Remy Belleau. Published by M. PatissonM. Patisson (Paris). Accessed from BnF Gallica: https:// gallica.bnf.fr/ark:/12148/bpt6k522648/f21.image. Last Accessed on November 14, 2025 Frondel, C. 1962, The System of Mineralogy, 7th edition, Vol. III Silica Minerals. John Wiley & Sons, New York and London, 334 p. Campbell, D.A., Jonsson, J.R.B., Kurcinka, C.E., Hinz, S.L.K., Sabiri, N., Meyer, G., McEachern, A.D. and Smith, A.M. 2024. Report of Activities 2024, Resident Geologist Program, Thunder Bay South Regional Resident Geologist Report: Thunder Bay South District; Ontario Geological Survey, Open File Report 6417, 128p. Cheadle, B.A., 1986, Alluvial-playa sedimentation in the Lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Science, v. 23, p. 527-541. Cohen, A.J., 1989, New data on the cause of smoky and Garland, M.I., 1994. Amethyst in the Thunder Bay area. Ontario Geological Survey, Open-file Report 5891, 197 p. Goetz, M.M. 2014. Heating Experiments of Amthyst from Thunder Bay Amethyst Mine. HBSc thesis, Lakehead University, 63 p. Haynes, F.M., 1988, Fluid-inclusion evidence of basinal brines in Archean basement, Thunder Bay Pb-Zn-Ba district, Ontario, Canada. Canadian Journal of Earth Sciences, v. 25, p. 1884-1894. Heaman, L.M., and Easton, R.M., 2006, Preliminary U/ Pb geochronology results: Lake Nipigon Region Geoscience Initiative. Ontario Geological Survey Miscellaneous Release – Data 191, 79 p. Hebert, L.B., and Rossman, G.R., 2008, Greenish quartz from the Thunder Bay Amethyst Mine Panorama, Thunder - 139 - �Proceedings of the 72nd ILSG Annual Meeting - Part 2 Bay, Ontario, Canada. Canadian Mineralogist, v. 46, p. 111-124. Holden, E.F., 1925, The cause of color in smoky quartz and amethyst, American Mineralogist, v. 10, p. 203-252. Hollings, P., Fralick, P., and Kissin, S., 2004, Geochemistry and geodynamic implications of the Mesoproterozoic English Bay granite-rhyolite complex, northwestern Ontario. Canadian Journal of Earth Science, v. 41, p. 1329-1338. Jennings, E.A., 1985, Geology of the Thunder Bay Amethyst Mine and Precious Purple Gemstone claims. Report to Precious Purple Gemstones Ltd., Thunder Bay, Ont., 65 p. Kissin, S.A., 1997, Comprehensive research to colour enhance Canadian amethyst by heat treatment and irradiation. Final Report, Amsearch Colour Project, Northern Ontario Development Agreement SSC File #015SQ-2223440-2-9243, 36 p. and appendix. Klarner, J.M., and Kissin, S.A., 2011, Hydrothermal bleaching of amethyst at the Thunder Bay Amethyst Mine, Ontario. Geological Society of America Annual Meeting, Minneapolis, Paper No. 44-11. Lehmann, G., and Bambauer, H.U., 1973, Quartz crystals and their colors. Angewendtede Chemie International Edition, v. 12, p. 283-291. Magnus, S., and Kissin, S., 2010, Assimilation and petrogenesis in the Navilus and Terry Fox sills, Thunder Bay, Ontario; in Institute on Lake Superior Geology, Proceedings and Abstracts, v. 56, part 1, p. 36-37. McArthur, J.R., Jennings, E.A., Kissin, S.A., and Sherlock, R.L., 1993, Stable-isotope, fluid-inclusion, and mineralogical studies relating to the genesis of amethyst, Thunder Bay Amethyst Mine, Ontario. Canadian Journal of Earth Science, v. 30, p. 19551969. McLaren, A.C., and Pitkethly, D.R., 1982, The twinning microstructure and growth of amethyst quartz. Physics and Chemistry of Minerals, v. 8, p. 128-135. Ontario 1990. Mineral Emblem Act, 1990, c. M.13, s. 1 Ontario Geological Survey. 2011. 1:250 000 scale bedrock geology of Ontario; Ontario Geological Survey, Miscellaneous Release— Data 126 – Revision 1. Ontario Geological Survey, 2026. Ontario Mineral Inventory; Ontario Geological Survey, Ontario Mineral Inventory, online database (March 2026 update). Patterson, G.C., 1985, Amethyst in the Thunder Bay area of Ontario, Canadian Gemologist, V. 6, p. 104-116. Rossman, G.R., 1994, Colored varieties of the silica minerals, in P.J. Heaney, C.T. Prewitt, and G.V. Gibbs, eds., Silica: Physical Behavior, Geochemistry and Materials Applications, Mineralogical Society of America, Reviews in Mineralogy, v. 29, p. 433-467. Sinkankas, J., 1976, Gemstones of North America, Vol, II. D. Van Nostrand Company, Inc., New York, 494 p. Sutcliffe, R.H., 1991, Proterozoic geology of the Lake Superior area in P.C. Thurston, H.R. Williams, R.H. Sutcliffe, and G.M.Stott eds., Geology of Ontario. Ontario Geological Survey, Special Volume 4, Part 1, p. 627-660. Van Schmus, W.R., Green, J.C., and Halls, H.C., 1982, Geochronology of Keweenawan rocks of the Lake Superior region: A summary, in R.J. Wold and W.H. Hinze, eds., Geology and Tectonics of the Lake Superior Basin, Geological Society of America, Memoir 156, p. 165-171. Vos, M.A., Abolins, T., and Smith, V. 1982: Industrial. Minerals of Northern Ontario- Supplement 1, Ontario Geological Survey Open File Report 5388, 344 p. McCrank, G.F.D., Misiura, J.D., and Brown, P.A., 1981, Plutonic rocks in Ontario. Geological Survey of Canada, Paper 80-23, 171 p. - 140 - � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2026 Description An account of the resource Institute on Lake Superior Geology. Thunder Bay, Ontario. May 21-22, 2026. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2026-05-21 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English Type The nature or genre of the resource Text https://digitalcollections.lakeheadu.ca/files/original/19e11aefc78ed911ffd2919a7a4f0e47.pdf 408bdbe9402ec8dd1840b6f48eed31bc PDF Text Text luoty !big Aoooal Meetio Institute on Lake Ihooder Hay, Ootariu Superior Geology �PROCEEDINGS tWENTY - THIRD ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY HELD AT THE AIRLANE MOTOR HOTEL THUNDER BAY ONTARIO MAY 2 — 8, 1977 SPONSORED BY THE ONTARIO DIVISION OF MINES AND LAKEHEAD UNIVERSITY THUNDER BAY.. ONTARIO M.M. Keh1enbeck,:S.A. Kissin, R.H. General Editors Mi t che 11 �This page intentionally left blank �TABLE OF CONTENTS GENERAL INFORMATION . INSTITUTE EOARD OF DIRECTORS LQCAL COMMITTEE Vi . SESSIONS CHAIRMEN . . ANNUAL BANQUET SPEAKER. ACKNOWLEDGEMENTS V . ,.. ... . . . . Vii Viii . ix CALENDAR OF EVENTSAND. PROGRAM ABSTRACTS FIELD TRIPS . . 51 A. COLDWELL COMPLEX, MARATHON, ONTARIO. 51 B. PROTEROZOIC ROCKS OF THE THUNDER BAY AREA 52 MATTABI, IGNACE, ONTARTO 53 C. iii �L GENERAL INFORMATION 23rd "ANNUAL INSTITUTE. ON. LAKE SUPERIOR GEOLOGY AIRLANE MOTOR HOTEL THUNDER BAY MAY 2. -: 8, 1977: SPONSORED BY THE ONTARIO DIVISION OF MINES AND LAKEHEAD UNIVERSITY THUNDER,. BA, 'ONTARIO INSTITUTE BOARD OF DIRECTORS P.E. Giblin, Ontario Division of Mines, Ministry Resources,. Sault Ste. Marie, Ontario. of Natural J.D. Hughes, Department of Geography., Earth .Science,and Conservation, 'Northern Michigan University, Marquette, Michigan. . M.M. $ Kehlenbeck, Department of Geology, L.akehead',Iiniversity, Thunder Bay, Ontario. R.C. Reed' (Secretary-Treasurer), Geological Survey Division, Department of Natural Resources, Lansing, Michigan. M.S. Walton, Minnesota Geological Survey, University of Minnesota, St. Paul, Minnesota. v �Trip C - Mattabi James M Franklin, Geological Survey of Canada, Ottawa, Ontario. WallyGibb, Mattabi Mines Ltd, Ignacé, Ontario. Howard Poulsen, Department of Geology, Läkehead University, Thunder Bay, Ontario. Paul Severin, Sturgeon Lake Mines Ltd. , Ontario. Adel Tammán, Mattabj Mines Ltd. , Ignace, Ignace, Ontario. Banquet Chairman John S. Mothersill, Dean of S,cience, Lakehead University, Thunder Bay, Ontario. Session Chairmen S.S. Goldich1, Department of Geology, University of Northern Illinois, DeKalb, Illinois. H.C. Halls, Dgpartment of Geology, University of Toronto, Toronto, Ontario. Bram Janse, Selco Mining Corp., Tpronto, Ontario. R.H. McNutt, Department of Geology, •McMaster University, Hamilton, Ontario. G.B. Morey, lvlinnesota Geologidal Survey, University of Minnesota, St.: Paul, Minnesota. R.W. Ojakangas, Department of Geology, University of Minnesota, Duluth, Minnesota. H. Walton, Minnesota Geo'ogical Survey, University of Minnesota, St. PaUl, Minnesota. G.M. Young, Department of Geology,. University of Western Ontario, London., Ontario. vii �LOCAL COMMITTEE General Chairman Hanf red M. Kehlenbeck, Department of Geology, Lakehead University, Thunder Bay, Ontario. Technical' Program Stephen A. Kissin, Department of'Geoiogy, Lakehead University,, Thunder Bay, Ontario. Roger H. Mitchell,' Department of Geology, Lakehead University, Thunder Bay, Ontarip; Field Trips Trip A - Coldwell Complex Roger H., Mitchell, Department of Geology, Lakehead University, Thunder Bay, Ontario. R. Garth Platt, Department of Geology, Lakehead University, Thunder Bay, Ontario. Trip B - Proterozoic Rocks of the Thunder. Bay Area Kenneth G. Fenwick, Ontario Division of Mines., Thunder Bay, Ontario. Clarence R.. Kustra,: Ontario Division of Mines, Toronto, Ontario. William " ' H. Mcllwaine, Petrologic 'Ltd. ,,thunder' Bay, Ontario. John F. Scott, Ontario Division Of Mines, Thunder Bay, Ontario. vi �Annual Banquet Guest Speaker Dr. J. Tuzo Wilson, Department of Physibs Geophysics Division, University of Toronto) Toronto, Ontario Acknowledgements The organizing committee for the 23rd Annual Meeting of the Institute on Lake Superior Geology gratefully acknowledges the work of Wendy Bons and Cathy LeBrun for typing the final, manuscripts of the field trip guidebooks and proceedings volume. Special thanks to Sam Spivak wh prepared the many figures, maps, and cover illustrations'. viii �CALENDER OF EVENTS AND PROGRAM MONDAY, MAY 2, 1977 1:00 p.m.- 330 p.m. Early Registration .AirianeMotor Hotel-Lobby 4O0 p.m. Pre—Institute field trip ACoidwell Complex departs from the Airlane parking lot for Marathon, Ontario: 8:00 p.m.—10:00 p.m. Early Registration Airlane Motor Hotel-Lobby TUESDAY, MAY 3, 1977 7:00 a.m.- 8:00a.m. 8:00 a.m. 5:00 p.m. Early RegiStration Airlane Motor Botel-Lobby :pre....Institute fièldtrip B— (part 1)—Proterozoic Rocks of the Thunder Bay area departs from the Airlane parking lot. Field trip B (part 1) returns to Airlane Motor Hotel. ix �WEDNESDAY, MAY 4, 1977 Pre—Institute field trip B (part 2)—Proterozoic rocks 8:00 a.m. of the Thund'ei Bay area. departs Airlane parking lot. 5:00 p.m.— 9: 00 p.m. Registration 5:00 p.m. Pre—Institute Airlane Motor Hotel—Lobby field trip B returns to Airlane (part 2) Motor Hotel. 5:00 p.m. Pre—Institute field trip A returns from Marathon to Airlane Motor Hotel. 8:00 p.m. Conference Smoker (cash bar) Tiberio Room - Airlane Motor Hotel THURSDAY, MAY 7:30 5, 1977 Registration Airlane Motor Hotel-Lobby a.rn.— 9:30 a.m. 8:30 a.m..-12:00 a.m. Technical Session 1 (see page xii) 1:30 p.m.-' 6:00 p.m. Technica' Session 2 (see page xiii) 7:00 p.m.— 8:00 p.m. Cocktail Hour (cash bar) Tiberio 1oom - Airlane Motor Hotel Annual Banquet - 8:00 p.m. x Tiberio Room �- FRIDAY, NAY 6, 1977 - 8:30 a.m.—12:;00 noon Technical Session 3 (seepage. xiv) l:30 p-.m.— 5:40 p.m. Technical Session 4 (see page xv) .1 6: 30 p.m. Post—Institute field trip C— Mattabi. departs Airlane parking lot for Ignace. Trip C will return on Sunday, May 8,1977 by 1:00 p.m. to Airlane Motor Hotel. 8:00 p.m. Northwoods MOtel, Ignace Informal discussion period in preparation for field trip.. xi �$7SSZON 2 1Morning' Thursday, May 5th, 2977 8.20 Opening Remarks 8.30 Meineke, D.G., Valdis, M.K. & Klaysmat. A.W. - 8:30 - 22:00 a.m. Organic-rich lake sediment exploration geochemical survey of eastern Lake Verraillion-Ely Minnesota. area, Northeastern 8.50 Beard, R.C.. Urani'vM deposits of the Kenora area. 9.10 0jakanas, R.W. Proterozoic pitchblende vein potential in Minnesota: Theory and Speculation. 9.36 Cannon, Two-hi 1 lion-year-old sedimentary phosphorite deposits in •the precambrian of northern Michigan. IV. F. 9. 50. - 10.00 t4ôrey, G.B.. Preliminary manganese resource for the Cuyuna District: approach. Mudrey, M.G., Massive sulphide deposits Beltrarne, R.J., I-Ioltzman, 10.20 £offe_ByLeak C. estimates A statistical in Wisconsin. Ostrom, M.E. Reinke, G. 10.40 Booy, E. Engineering problems in glacial soils near the Canadian-United States border. 11.00 Morey, G.B. Stratigraphic and tectonic history of lower and middle precambrian rocks in east-centra7 Minnesota. LaBerge, GIL. Major structural fsatures in Central Wisconsin and their implications on the 11.20 $ Animike Basin. 11.40 Davidson, D.M. 12.00 -. Paleostrain i.30 Lunch jj S analyvis: That, How and Why? �SESSION 2 'A Lt erno on" Thursday, l30 Ma_yb 5th, 19?? Birk, D. McNutt, R .H. - 1.30 - 6.00 p.m1 Rb/Sr geochronolo,yof Wabigoon Belt Granitoids, Northwestern Ontario. Rare earth element geochemistry of 'Archean anrphiboli tes, tona lites, granites and paragneisses in the eastern Lac Seul area, Ontario. 1.50 thou, C.L. 2.10 Mcbennan, S.M., Fryer, B.J. ' An estimate of the rare earth element'' distribution in Post-Kenoran upper crust, Young, north 2.30 G.M. of Lake Huron. The occurénce" and some nobel metal concentrations in selected komatiitic' ultramafic volcanic rocks from Munro McCrae, W.E. & Crocket, J.H. Township, Ontario. 2.50 3.00 3.90 goffe_Bea& Geochemistry of early proterozoic of Lake Huron, Ontario. Fryer, B.J. north 3.20 Longstaffe, F.J.,, 180/160 results for Archean plutonic rocks, Lake Despair 'area, Northwestern Ontario. Schwarcz,H:P. 3.40 4.00 ' 4.20 Review of Occygen isotope geochemistry of Ahinad, SN Perry, E.G. Jr. some precambrian Mitchell, R.H Platt, R.G.' CoQper, ' Mafic iron formations. mineralogy of ferroaugite syenite from the coldwell Complex, Marathon, Ontario. ' Weiblen, P.W. paleosols Shape, sixe, cznd cooling history of trochtolitic—gabbroic rocks in the Duluth E, R.W. complex. 4.40 McMaster, B., 1'icNutt, R.H. Archean volcanism Washeigamaga' Lake area, Wabigoon Subprovince, Northwest Ontario. 5.00 Blackburn, G.E., Identification 5.20 Pilatzke, R.H., Petrology and trend curface,analjgsis of two lake-stage gra'nodioritic plutons, Northern . Er Karner, F.R. Er Peterson, W.M. '5.40 Morris, W.J., E Wilband, J.T, of archean calc-alkaline volcanid centres in the Ma'nitou Lakes, area, Northwestern Ontario Lake of the'Woods region, Ontario. Geochemistry of the Yellow Dog Plains., Marquette County, Michigan. peridotite, xiii �SESSION 3 'Morning,' Friday, May ôf/j, 8.30 Weber, R.E. 1977 — 8.30 - 12.00 a.rn. The petrology and sedimentation of the upper precambrian sioux quartzite. of Minnesota South Dakota, and Iowa. 8.50 Morey, G.B Schulz, K.J. 9.10 Young, G.M., Long, D.G.F Petrographic and chemical attributes of some lower and middle precambrian gray-wacke-shale sequences in northern Minnesota. Deltaid deposits in tlze upper Pecors, Espanola and Gowanda formations (Huronian). McLennan, S.M. 9.30 Mancuso, J.J., Seavoy, R.E. Lougheed, M.S. Strati graphy of tle Baraga Basin metasediment, Michigan; 9.50 - 10.00. Coffle Break Lougheed, M.S. f Mancuso, J.J. Fossil collectibles from the Gunf lint 10.20 Shegeiski, R.J. Evidence fbr archean turbidite and submarine fan sedimentation from the Savant Lake greenstene terrain, N.W. Ontario. 10;40 Frost, B.R. Some comment; on the metamorphism of iron-formati 'n$. 11.00 Gower, C.F. Clifford, P.M. Metamorphism in the English River Feixzn, W.C. The stratigraphy and petrology of the archean volcanic rocks at Jasper Lake, eastern Vermilion District, Cook County, Minnesota. Maas, R.S., Medaris, L.G. El VanSchmus, W.R. Penokean structures and 10.00 Jl.20 11.40 12.00 - formatiiin. sub- province near Kenora, Northwest Ontario. Wisconsin. 1.30 Lunch xiv plutpnic rocks iv �SESSION 4 'Afternoon 1.30 Friday, May 6th, 1977 ' - 5.40 p.m. 1,30 Eyerson.C,I, Drift lithology in relation to bedrothk geology, Long Is land Lake Quadrangle, Cook County, Minnesota. 1.50 Zarth, R. Sedimentary facies associated with late 2.10 2.30 Wisconsin Glacial Lake. Duluth, Wrenshall 'areas Minnesota. Welke, C.J., Nebriga, E.t. Meyer, R.P. Swrficial Green, J.C. Environmental sediment analyse,s offshore of the copper-bearing provinc of Keweenaw Point, geotogy 'of the North Shore'a coastal zone management project. Lso - 3,00 43.00 Upper Michigan. Mothersill, j.s. Coffiae Break Post-glacial Canadian 3.20 the The application of linear topographic features a glaciated precainbiian terráine in Cooper, R.W. e Morey, G.B. t.o structural interpretation of northeas tern 3.40 sediment 'distribution ih of Lake. SuperiorS portion Minnesota. Geophysical' studies of peridotite dikes, Yellow Dog Plains, Northern Michigan. Snider, D.W,, Kiasner, J.S, Quam, S., Lilienthal, R. Geraci, P. G Grosz, A. 4.00 4.20 Dugan, J.P. Jr. Ervin, C.P. Geophysical study 'of a gabbroic' intrusion, Klasñer,' J.S., Bouguer gravity anomaly map of northern peninsula of Michigan, Lake Superior, and Hinze, W.J.,Bacon, L.0. 4.40 E O'Hara, N.W. Chandler, v.w., Hinze, w.j. Braile, L.W. ' ' 5.00 5.20 environs. Analytical'correlation magnetic data in of gravity and the North American Midcontinent. studies of a regional'gravity model anomaly in norther'n'Michigan and Wisconsin, .extent.of anomaly, and its relationship to near surfac geology. D.' Pesonén, LJ . Halls, 4Ij.C. Lake, Wisconsin. Crustal Klasner, J.s. Bomke, Clam " PaleomagnetidAtiñdpaleointensity studies of normal and reversed keweenawan 'rocks - implications North xv for the polar wander path of America. �Str cts �This page intentionally left blank �REVIEW OF OXYGEN ISOTOPE GEOCHEMISTRY OF SOME PRECAMBRIAN IRON FORMATIONS S.N. Ahmad and E.C. Perry, Jr. ABSTRACT A review of published values for oxygen isotope data for quartz and magnetite from the Hamersley Iron Formation and the eastern portion of the Biwabik Iron Formation indicates that consistent trend lines appear if ol8o of quartz and magnetite are plotted v5AQM. This implies that during metamorphism these iron fprmations behaved as closed systems on same scale and that the system as a whole records isotopic information not present iii any one sample. The Hamersley and Biwabik trend lines intersect one another at a olSo for quartz of about 24.0 0/00., a value close to that observed for pure chert horizons in the Hamersley Iron Formation and the Gunflint Iron Formation (correlative with the Biwabik). Subject to certain assumptions, the intersectiônof the Hamersley and Biwabik oxygen isotope trend lines permits us to estimate that the temperature oX precipitation or diagenesis of these iron formations was about 22°C. A continuing study of other iron formations of low metamorphic grade may show whether this temperature estimate is reliable. It may also permit evaluation of reactions proposed by several authors for conversion of iron carbonate to magnetite during diagenesis and metamorphism. 3 �URANIUM DEPOSITS O,F'TH:IENORP AREA R. C. Beard, Ont. Div. of Mines, MNR, Kenora, Ontario Uranium deposits were first recognized in the Kenora area in 1949 and exploration has been carried out on these occurrences during two periods, 1952-57 and 1965-67. Recent increases in the price of uranium have stimulated a new cycle of exploration in the area, and numerous programs, both detailed examinations of previously known occurrences and grassroots exploration for new deposits, are currently under way. Past work on the various properties consisted of tren— ching, geophysical surveys, and diamcnd.drilling. An exception, the flew Campbell Island Mining and Exploration Ltd. deposit in MacNicol Twp., has been explored by underground development on two levels. Over thirty deposits have been documented in the KenoraDryden area; there is evidence to suggest that many more have been discovered which have not yet found their way into the public record. They tend to be concentrated in two general areas: a) near Vermilion Bay, 40 miles east of Kenora, associated with a narrow "greenstone" belt and, b) north of Kenora within the English ,River Gneiss Belt. The uranium occurrences are (typibally) asociated with pegmatitic phases of anatectically—derived, rather than plutonic, granitoid rocks. Supracrustal rocks of Archean age, exhibiting remelting and assimilation features to varying degrees, are associated with almost all deposits. These are quartz-biotite paragneisses of sedimentary derivation although some deposits are associated with amphibolitized mafic volcanic rocks. 'Uranium occurs as fine grains of uraninite disseminated in the granitoid rocks which are usually, but not always, It js frequently associated with one pegmatitic in texture. of the following accessory minerals: biotite, magnetite, sulphides, or apatite. Minor leaching and redeposition as uranophane along near surface fractures is common to many of Grab sample assays from radioactive zones range the deposits. from 0.5 or less to 1.5 'lbs. U 08 per ton; with occasional grab samples assaying over 10 bs. per ton. It is suggested that the urahiferous pegmatites of the area are, to a certain extent, stratigráphically controlled,: having been anatectically derived from supracrustal rocks which contained ahomalous amounts of uranium. Study should be directed toward the identification of these "source beds", so that exploration may then be directed to these more favorable areas. Reconnaissance mapping by the Ontario Div. of Mines (Breaks et al, 1975) suggests one such area near Umfre— ville Lake, north of Kenora. 4 �PRELIMINARY MANGANESE RESOURCE ESTIMATES OR THE CUYUNA DISTRICT: .A STATISTICAL APPROACH R.3. Beltrame, Richard C.' Holtzman, and G.B. Morey, Minnesota Geologiëal Survey, University of Minnesota, St. Paul, Minnesota. The Cuyuna range in east-central Minnesota has produced over 105 million tons of iron ore and manganiferous-iron ore during the past 62 years. Although iron ore reserves are nearly exhausted and all mining activity, has ceased, significant amounts of manganese beating materials are presently available. The occurtence of high-grade rnanganiferous material (>5 weight % Mn) is generally limited to the Emily iron-formation member of the Rabbit Lake Formation and more commonly to the Trommald Formation. In both iron-rich units the manganese is present in both the original protolithic iron-formation (10-30% Fe) and the so-called secondarily enriched "natural ores" (40-60% Fe). In the Trommald Formation of the North range,most of the manganese occurs in the transistion zone between the thick-bedded (granule chert layers 1-100 cm thick) and thin-bedded (laminae ci cm thick) facies rocks. Due to the complexity of the stratigraphic and structural relations and because of the enormous amount of available drill hole data, preliminary manganese resource estimates were statistically calculated for 69 uniquely defined deposits. A deposit was defined, for statistical reasons, as a legal land section. The location coordinates, collar elevations, and chemical assay data for 5,045 drill holes were entered into a computerized storage and retrival system. Statistical methods involved calculating an area of influence for each given Mn assay value for each drill hole. These areas of influence were calculated based on the spatial distribution of drill holes in each deposit and the location of the drill hole relative to the section boundaries. Computer-generated data location and drift thickness maps were produced as were grade-quantity estimates of manganese resources. Manganese quantity resource estimates were calculated for five gcade classes (13%, 3-5%, 5-10%, 10-15%,and >15%) for each of five depth intervals (<30, <60, <90, <120, and < 150 meters below the surface) for each of the 69 deposits.' Resource estimates for three deposits in the Emily district, 39 deposits in the North range, and 27 deposits in the South range accounted for 6%, 77%, and 17% of the total Cuyuna district resource estimate respectively. A total of 22.4 billion metric tons of manganiferous material (>1% Mn) was talculated 'to a depth of 150 meters for the entire Cuyuna district. More realistic values are 2.3 billion metric tons at>5% Mn, and 887 million metric tons at >10% Mn, calculated to a depth of 60 meters. 5 �Rb/Sr GEOCHRONOLOGY OF WABIGOON BELT GRANITOIDS, NORTHWESTERN ONTARIO DIETER BIRK and POI3ERT H. McNUTT Department of Geology, McMaster University. Hamilton, Ontario L8S 4M1 granitoid ThirtyLóne whole rocksamples from five piutons, intrusive intomet- volcanics- of the Wabigoon Greenstone Belt, generate a composite Rb/Sr errorchro (MSWD = 2,34): Age = 2621 ± 42 m.y'. (2°) k0 = :7007 ± 4 (2e) Linear regression of togenetic samples generate Rb/Sr isochrons sensustricto for each pluton as follows: Pluton (No. Of Samples Analysed) Age in m.y. Lithology (± 2a) - Initial Location Ratio (± 2a) (NTS) Burditt Lake (9) granodiorite 2598 ± 45 .7009 ± 6 52C/13 Esox Lake (4) quartz-feldspar porphyry 2572 ± 42 •.7003 ± S -52F/3 Flora Lake (5) granite-monzodiorite 2636 ± 63 .7017 ± 52F/S Taylor Lake (5) granodiori'te-monzonite 2640 ± 31 .7005 ± 3 52F/7E- Ryckman Lake (7) granodiorite-.mónzodiorite 2609 ± 63 :7001 ± 52C/lS 2230 ± 55 Ryckman Lake (5) For the Ryckman Lake Stock, the lower S .7012 ± 2- 52C/1S age, is fro'm a "pseudoisochron" caused- by the fortuitous alignment of five data points beyond that expected from known analytical error. The seven point isochron represents a wider range of rock chemtstry a-nd more meaningful age and-intercept. Isochron data must be tested by several linear regression techniques to expose such "pseudoisochrons'1. i These Wabigoon granitoid isochrons, when compared with published isochrons from the Rainy Lake area, suggest juvenescence of granitoid plutonism from north to south. This may relate genetically to the presence of the Quetico-Wabigoon Belt interface near Rainy Lake. The low 875r/86Sr ratios for all the late-kinematic granitoids implies a source region of low Rb/Sr. Partial melting of upper mantle rather than older sialic material is indicated, 6 �IDENTIFICATION OF ARCHEANS CALC-ALKALINE VOLCANIC CENTRES IN THE MANITOU LAKES AREA,. NORTHWESTERN ONTARIO C. E. BLACKBURN Ontario Division o: Mifles, Queen Park, Toronto A B S T R A C T Centres of felsic to intermediate vo1Oanisn within Archean volcanic-sedimentary belts have long remained enigmatic. In their detection reliance has frequently been made upon physical parameters in pyroclastic rocks (eg. coarsening towards vents), without special attention to overall volcanic and subvolcanic stratigraphy. In the Manitou Lakes area detailed geological mapping has pinpointed a number of vent areas, both simple and compOund. Within the study area a thick submarine basaltic flow sequence of tholeiitic affinity was built up, followed by eruption of a caic—alkaline sequence composed predominantly oE dacitic to andesitic coarse pyroclastics. The tholeiitic baa1t sequence was intruded by quartz-feldspar porphyry plugs, at Sunshine Lake and at Thundercloud and Washeibemaga. Lakes (McMaster and McNutt, this volume). The plug at Sunshine Lake is the subvolcanic equivalent of rhyolitic flows that occur within the pyroclastic sequence. This plug was in turn intruded by irregular lamprophyric dikes and sills that are subvolcanic equivalents of a calc-alkaline to alkaline mafic flow that terminated volcanism in this part of the area. Southwest of Cane Lake, elongate to lenticular quartz-feldspar porphyry bodies within the pyroclastic sequence variously have subvolcanic and volcanic characteristics, suggesting theY. location of a felsic vent in this vicinity. A mafic sill correlatable with the late mafic phase at Sunshine Lake occurs in close spatial association with these porphyries. None of these vents has been directly identified as supplying dacitic to andesitic pyroclastic debris. However, one vent that did supply such coarse pyroclastic material has been identified at Frenchman Island, Upper Manitou Lake, where a. subvolcanic porphyritic to inequigranular plug intrudes coarse pyroclastics of comparable chemical and.. mineralogical composition. 7 �ENGINEERING PROBLEMS IN GLACIAL SOILS NEAR TILE CANADIAN-UNITED STATES BORDER Emmy Booy Department of Geology ar3d Geological Engineering Michigan Technological University Houghton Michigan 49931 ABSTRACT The borderlands of the United States and Canada ranging from Lake Superior eastward to the Gulf of St. Lawrence are characterized by the presence of glacial deposits which cause problems in safe utilization of the land. At various locations, but particularly along the valley of the St. Lawrence River and its tributaries and along the southern margin of Lake Superior, slope failures have caused expensive losses in property and hazards to human life. Because glacial clays in northern climates have been .known for cén— tunes to be prone to failure, they have been studied by engineers and geologists to determine the causes of their failures and means for controlling them. Most of these investigations have concentrated on the so—called "quick clays", glacial clays laid down in marine environments. There have been relatively few studies of fresh—water glacial deposits and their relationship, if any,,to the marine and estuarine deposits. At the present state of knowledge, there is no definitive explanation for the sudden outflows of clay which are characterized as quick clay failures. It should be noted that a variety of types of failures are observad in all the glacial deposits. Many of them appear to be a result f excess hydrostatjc pressure in the coarser strata of varved deposits. These occur in both the fresh water and marine deposits. Classic slump failures have, also been described in both fresh and salt—water deposits. The only major difference in slope stability between the more easterly marine deposits of glacial soils and those deposited in the fresh water predecessors of Lake Superior, is the apparent lack of quick clay flows in the latter. These flows have been reported from Scandinavia, Alaska, and eastern Canada as occurring suddenlyon slopes as low as a few degrees. The failed material has an extremely low viscosity and may require hours to regain significant shear strength. There has been a significant lack of reports in the literature of similar failures in freh—water glacial deposits. It appears likely that there is a significant difference in material between those soils which fail as quick clays and those which do. not. It is generally agreed that there is a major rearrangement of soil fabric during quick clay failure which is held responsible for the variation in shear strength between failed and unfailed portions- of a deposit. Itis equally possible that the "cement" which is alleged to give quick clays their original shear strength is significantly different i-n marine and fresh water glacial deposits. S �Two—billion—year—old sedimentary phosphorite deposits in the Precambrian of northern Michigan 1/ by W. F. Cannon S. Geological Survey Reston, Virginia 22092 U. Abstract Phosphate—rich beds have recently been found at five localities in 2—billion—. year—old metasedimentary rocks of the t4arquette Range Supergroup in northern Michigan (Cannon and Klasner, 1976). All occurrences are near the unconformable base of the supergroup within 100 meters stratigraphically above older gneisè. Four occurrences are in the Michigamme Formation, part of the Baraga Group; the fifth is in the older Ajibik Quartzite, part of the Menominee Group. as The phosphatic minerals occur in two ways: 1) thin beds of apatite, mostly associated with lean carbonate iron—formation, and 2) as pebbles of apatite in conglomerate. Two thin—bedded occurrences in the Michigannne Formation, first reported by Mancuso and others (1975), have not been evaluated for grade and extent. Of the three new occurrences reported by Cannon and Kiasner (1976), two are low grade and consist of scattered pebbles of apatite in basal Michigamme and Ajibik conglomerates, and a few thin beds of apatite generally less than 1 cm thick. The third contains thick conglomerate beds, including a bed about 15 m thick that averages about 15% P205, and many thinner beds of comparable grade. Because outcrops are very limited in *the area, the grade and extent of the deposits are impossible to determine without subsurface data, but the economic potential of these deposits warrants further evaluation. 'The area has never been systematically explored for phosphate minerals. The five known occurrences were found by only a cursory examination of field notes and hand specimens; none of these rather cryptic deposits was identified in the field. Precambrian sedimentary rocks have not been considered a likely host for' economic phosphate deposits in the United States, and these deposits are the richest so far known in the Precambrian of this country. Because five localities have been found without a thorough field search in an area that has very sparse outcrops, a good possibility exists for undiscovered phosphate deposits in the region. - References cannon, W. F., and Klasner, J. 5., 1976, Phosphorite and other apatite—bearing sedimentary rocks in the Precambrian of northern Michigan: U. S. Geol. Survey Circ. 746, 6 p. Mancuso, J. J., Lougheed, M. 5;, and Shaw, R., 1975, Carbonate—apatite in Precambrian cherty iron—formation, Baraga County, Michigan: Econ. Geology, v. 70, no. 3, p. 583—586. 1/ Prepared in cooperation with •the Geological Survey Division, Michigan Department of Natural Resources 9 �ANALYTICAL CORRELATION OF GRAVITY AND. MAGNETIC DATA IN THE NORTH AMERICAN NIDCONTCNtNT V.W. Chandlr, W.J. Hinze and L.W. raile Department of Geosciences Purdue University. West Lafayette, Indiana 47907 The correlation of gravity ane mágñetic data isone of the most commonly used geophysical techniques in basement. This usage is especially vital in the geological studies. Midcontinent area of North America where direct observation of the basement complex is essentially prohibited by a In the past the àorrelation blanket of Phanerozoic strata. of.gravity and magnetic data has usually been carried out by purely visual method:; or by restricted applications of Visual theoretical methods such as Poisson's theorem. methods are hampered by their subjectie nature wherea the classical application of Poisson's theorem is often voided Recent studies have by necessary theoretical assumptions. investigated several computerized apprc'aches to the correlaOne of these recently tion of gravity and magnetic data. developed techniques, internal correspondence, has been investigated through model studies and has been shown to be a potentially valuable supplement to ccmbined gravity and magnetic interpretation. The technique '3 involves a moving window analysis Df anotalies of the fir;t vertical derivative A least squares of gravity and iragnetics reduced to the pole. linear regression is conducted between the two data sets for each window position with a subsequent creation of three Considregression coefficient arrays over the data space. eratic:n of Poisson's theorem shows that the slope coefficient array is equivalent to a continuous estimate of magnetization-density ratios for anomalous sources. The intercept coefficient array yields valuable information regarding The correlation coefficient array anomaly base levels. expresses the significance of the linear fit for each window position. Analysis of gravity and riagnetic data from. the Midcontinent region of North America demonstrate that the internal correspondence approach yieldE useful constraints in local as well as regional geophysical irterpretation of the basennnt complex. - 10 �RARE EARTH ELEMENT GEOCHEMISTRY OF ARCHEAN AMPHIBOLITES, TONALITES, GRANITES AND PARAGNEISSES IN TIlE EASTERN LAC SElL AREA, ONTARIO C.—L. Chou, Department of Geology and Erindale College, University of Toronto, Mississauga, Ontario, Canada LSL1C6 ABSTRACT Using neutron activation techniques twenty samples from the eastern Lac Seul area of the English River gneiss belt have been analyzed for twenty—eight major and 'trace elements (A12O3, total Fe, MgO, CaO, Na2O, K2O, T1O2, MnO, Sc, V, Cr, Co, Zn,'Rb, Zr,. Ba, La, Nd, Sm, Eu, Tb, Dy, Yb, Lu, Hf, Ta and Th). Variations of Mn, Sc, Co and Cr are related to total Fe content, their concentrations decrease in the order of amphibolites, tonalites paragneisses, leucosome, trondhjemite, granites, muscovite granites, and peginatite. Distinct rare earth element (REE) patterns are found for. various rock types. Two amphibolites have flat REE patterns and total REE contents about l0—l2X chondritic abundances, similar to Archean basalts. A third amphibolite (T91) is enriched in La and Ce relative to other amphibolites, probably due to metamorphism. Six tonalites can be separated into two groups based on their REE patterns. Type—A tonalites (4 samples) have smooth and steep—sloped REE patterns with remarkable enrichment of lIght REE and depletion of Ce, heavy REE (LaE F =71—135 and YbE F 3—11). Type—B tonalites (two samples) have higher total REE contents than type—A, negative anomalies and flat heavy REE abundances (LaEF = 106—112 and Type—A tonalites may have derived from garnet = 22—39) . eclogites, with garnet as a residual phase during partial melting, Eu whereas type—B tonalites are probably derived from dioritic source with amphibole and plagioclase as residual phases. Granites have smooth and steep—sloped REE patterns. Muscovite granites have lower light REE contents than granites and negative Eu anomalies. Both granites and muscovite granites may have originated from sedimentary rocks by crustalanatexis. The REE patterns of garnet paragneiss and biotite paragneiss are. similar to type—A tonalites, suggesting that tonalitic rocks are the dominant source of meta— sediments. 'I �THE APPLICATION OF LINEAR TOPOGRAPHIC I'EATURES TO STRUCTURAL INTERPRETATION OF A GLACIATED PRECAMBRIAN TERRANE IN NORTHEASTERN MINNESOTA R.W. Cooper and G.l3. Morey, Minnesota Geological Survey, tiniMersity of Minnesota, St. Paul, Mjnnesota The application of linear topographic features to various kinds of structural interpretations has become increasingly popular since the advent of spacecraft and high-altitude imagery. Although many types of lineaments can be recognized in Minnesota, a major question remains as to their usefulness in structural studies, particularly in areas where glacial activity has obscured many fundamental bedrock attributes. We have analyzed in detail an area of 3600 sq. km. in parts of northern St. any, exists Louis and Cook Counties. Minnesota to determine what correlatAu, between the bedrock geology and topographic linearrients. The bedrock geology of our study area may be divided into four strato-tectonic units: (1) Lower Precambrian metavolcanic and metasedirnentary rocks of Jhe Vernlion distct having a &redeminance of Saults trending approximately N.20 E., (2) Lower Precambrian "granitig" N.33 E., N.55 -6o°E;, N.70 -75 E. and N.85 E.; rocks of the Vermilion massif and Giants Range batholith having faults trending N.20 40 E.; (3) Middle Precambrian sedimentary rocks of the Animikie Group having a few northwest- and north-northeast- trending faults; and (4) various kinds of Upper Precambrian mafic rocks assignable to the Duluth Complex. lm\ addition, the northern part of the study area is covered by thin (c6 meters), discontinuous patches of Quaternary materials, 'whereas a thick, more- or less continuous mantle of these materials obscures bedrock relationships in the southern part of the area. The southern part of ths stu0dy area is characterized by numerous topographic lineãments trending in a N.35 -40 E. direction; a direction parallel to movement of the Rainy lobe in this area. In contrast, lineaments in the northern part of the study area exhibit a number of divora !irect½r.a. ".r.tn alysis o these lineaments indicates that bedrock structural features exerted a profound influence on 8resent-y lend forms. or eample, an excellent correlation exists between N.20 E., N.35 -40 F_.. N.55 -60 E. and N.85 E. trending lineaments and ground-mapped faults in areas underlain by Lower Precambrian metavolcanic and etaedimentary rocks. Similarly there is a marked correspondence between N.25 -40 E. trending lineaments and ground-mapped faults in areas underlain by the Lower Precambrian Vermilion massif and Giant's Range batholith. FUrthermore faulting or fracturing in a northeast direction may be much more prevalent in the "granitic" rocks than present mapping indicates 'ecause of many codirectional lineaments that are not ssociated with areas of known faulting. Previous mapping has documented only a few northeast- and northwest-trending faults in the Duluth Complex. However this part of the study area is characterized by nhim,rn,,c northeast-trending topographic lineaments. Thus an area of approximately 20 sq. km. was mapped in detail to determine if the lineaments could be related to any strtktural features in the bedrock. As a result of this mapping several faults having unknown amounts of displacement were recognized that correspond to major lineament trends. Subsequently it was recognized that many topographic lineaments also coihcide with northeast-trending aeromagnet ic lineaments and with disrupted struc- tural elements such as contacts between, and o1Lsiü .. n vnap units. This suggests that faulting in a northeast direction was a majar tectonic process during and after.intrusion of the Duluth Complex. A few northwest-trending faults have been inferred, primarily on the basis of subsurface inforrnaiiui, tn nit the western margin of the Duluth Complex. In places these faults correspond to mapped faults in the Middle Precambrian Animikie Group, whereas in other places they correspond to well-defined northwest-trending aeromagnetic gradients. However other magnetic gradients have no known geologic expression. We infer from these data that northwest-trending faults may he more numerous than present mapping indicates. The results of our lineament analysis suggest that any pre-glacial topographic expression of the. northwest-trending faults was eliminated by the. southward flow of glatial ice which at the same time enhanced the topographic expression of the northeast-trending bedrock structures. Thus although topographic lineaments are a useful adjunct to structural studies in nqrthern Minnesota, they must be carefully interpreted in terms of the glacial history. �PALEOSTRAIN ANALYSIS: WHAT, HOW AND WHY? Donald M. Davidson, Jr., Geology Department, University of Minnesota,. Duluth, 55812 ABSTRACT WHAT? Several excellent techniquest for 3lantitatively'analyzing natural strain (paleostrain) in deformed rock units have appeared in the geological literature within the past decade. HOW? Samples are collected and slabbed along orthoganal planes ot photographs taken of the unit viewing three such planes. Deformed structures within rocks, such as fossils, oolites, concretions, phenocrysts, or reduction spots, often The major and minor axes of these ellipses may be have elliptical shapes. measured along with angular relationship of the axes (0) to some fundamental directional property in the rock (bedding, cleavage, foliation, lineation). The particular method used in treating the data is dependent upon certain assumptions fundamental to the mathematical techniques employed, although virtually Other all methods assume that deformation involved finite, homogeneous strain. assumptions relate to.knbwledge of the initial shape of the deformed object (circular or otherwise) tr knowledge of the orientation of primary planar features such as bedding ,within the deformed unit. The procedures of Ramsay, Elliott, Dunnet, Hsu and Matthews and a new method currently being developed at the.University of Toronto will be reviewed, WHY? Paleostrain techniques are powerful tools in direcdy analyzing strain history in rocks and shear zones, in preferentially.discriminating between defor— mational models or in treating sedimentary fabrics.. These methods warrant' serious. consideration by geologists working in the Lake Superior region. REFERENCES Barr, M. and Coward, M. P., 1974, A method for the, measurement of volume change, Ceol. Nag., v. 111, p. 293—296. Boulter, C. A., 1976, Sedimentary fabrics and their relation to strain— analysis methods: Geology, v. 4, p. 141—146. Coward, M. P., 1976, Strain within ductile shear zones? Tectonophysics, v. 34', p. 181—197. Coward, N. 1?. and James, P.. R. 1974, The deformation of two' Archaean greenstone belts in Rhodesia and Botswana: Precambrian Res., v. 1, p. 235—258. Dunnet, D. and Siddans, A.W.B., 1971, Non—random sedimentary fabrics and their Tectonophysics, v. 12, p. 307-325. modification by strain: Elliott, D.,' 1970, DeterminatiOn of finite strain and' initial shape from deformed elliptical objects: Geol. Soc. Amer. Bull: v. 81, p. 2221—2236. Hobbs, B. E. and Talbot,' J. 1., 1966, The analysis of strain in deformed rocks. Jour. Ceol: v. 74, p.'. 500—512. Matthews, P. E., Bond, R.A.B; and Van Den Berg, J. J., 1974, An algebraic method of strain analysis using elliptical markers: Tectonophysics, v. 24, p. 31—67. Owens, W. H., 1974, Representation of finite ,strain state by three—axis planar diagrams: G.S.A. Bull., v. 85, p. 307—310. Ramsay, J. G., 1967, Folding and fracturing of rocks: McGraw—Hill, New 'York, p. 103—120, 134—142, 200—221. Talbot, C. J., 1969, The minimum strain ellipsoid using deformed quartz veins: Tectonophysics, v. 9, p. 47—76. Tobisch, 0. T., et al., 1977, Strain' in metamorphosed volcaniclastic rocks and its bearing on the evolutiQn of 'orogenic belts: G.S.'A.B., v. 88, p. 23—40. ' , 1.3 . ' �GEOPHYSICAL STUDY OF A GABBROIC INTRUSION, CLAM LAKE, WISCONSIN Joseph Patrick Dugan, Jr., and C. Patrick Erviri, Dept. of Geology,Northerrt Illinois University, DeKalb, IllinOis 60115 ABSTRACt Aeromagnetic maps recently released by the &Wisconsin Geologiàal àhd Natural History Survey contain a sharp, 7000 gamma magnetic anomaly near the town of Clam Lake, Ashland County, in the northwestern part of the state. The anomaly has a wavelength of only 5.5 km, suggesting a shallow source. A coincident, but less well-defined, Bouguer gravity anomaly of 15-20 mgals is also present. Inland Steel Company drilled a 104 m hole in the center of the anomaly using a diamond drill. The lithologic log shows 29 meters of drift overlying a fresh, unaltered gabbroic sequence with zones containing up to 60% magnetite and ilmenite. Preliminary analysis of the potential field anomalies suggests that the source is a vertical body that is circular-to-elliptical in horizontal section. Since the Mellen Gabbro Complex lies only about 8 •km to the north, the Clam Lake Anomaly may be caused by an intrusive offshoot at depth.. 14 �DRIFT LIThOLOGY IN RELATION TO BEDROCK GEOLOGY, LONG ISLAND LAKE QUADRANGLE, COOK COUNTY, MINNESOTA Curtis I. Everson, Department of Geology, University of .Minnesota, Duluth Lithologic studies in northeastern Minnesota suggest that drift prospecting is a useful tool for mappIng drift—covered bedrock. A detailed study of till clasts composition in the Long Island Lake quadrangle revealed a significant relationship between drift lithology and bedrock geology. The Long Island Lake quadrangle. is a suitable area for this study for 1) outcrops are numerous enough to have allowed the following reasons: the construction of a detailed geologic map, 2) the area contains eight 3) the local bedrock experiended glacial erosion,. distinctive rock unit, indicated by the existence of glacially abraded and quarrIed outcrops. The distribution of glacial sediments, mainly till and outwash,&were mapped,. and one hundred and one samples of drift were collected along traverses parallel to ice flow (perpendicular to strike of the bedrock). Both till and outwash contain a large quantity of local bedrock clasts in the size ranges greater than 2 mm in diameter. Clasts smaller than 2 mm are mainly the mineral quartz, and therefore not so diagnostic of local As a test, boulders greater than 1 meter in diameter were used bedrock. in the field for inferring bedrock contacts. These contacts were found to be within ± 60 meters (200 ft.) of cQntacts placed by outcrop mapping.. tack of local bedrock clasts in the smaller size fractions indicate either high resistance of local bedrock to crushing, or lack of opportunity for crushing because of short residence .time in the glacial system (short distance transport). In either case, the fine—grained fraction therefore represents a eontribution to the glacial load from more distance sources. 15 �THE STRATIGRAPHY AND PETROLOGY OF THE ARCHEAN VOLCANIC ROCKS AT JASPER LAKE, EASTERN VERMILION DISTRICT, COOK COUNTY, MINNESOTA William C. Feirn, Geology Department, University of Minnesota, 55812 Duluth, Minnesota ABSTRACT The Jasper Lake area, located within the eastern Vermilion district in Cook County, northeastern Minnesota, represents the basal portion of a thick metavolcanic-metasedimentary sequence. Gruner (1941) found the area to contain threedominantly.ign'eoas units: a greenstone unit, an "agglomerate-conglomerate" unit, and an "andesite intrusive" unit., These rocks were shown t& have been complexly faulted and isoclinally folded. All units have been metamorphosed to greenschist facies. The oldest unit consists of predominantly massive metavolcanics (including basalt, diabase, andesite, and lesser dacite) and is herein referred to as the Jasper Lake greenstone. Du'e to the presence of pillow structures and quench textures observed at several localities, thse rocks are interpreted, as subaqueous The unit is linear in outline, 1000—1500 meters thick flows. (exposed), and trends east-west, and is probably continuous with the Chub Lake Volcanic Complex of Morey, Weiblen, and others (1971) to the east. The SaganaEa tonalite intruded the greenstone unit along its northern margin, locally metamorphosing it to amphibolite n'ade along a 30-60 meter wide zone. The "agglomerate-conglomerate", herein referred to as the Jasper Lake pyroclastic unit, and the associated "andesite intrusive" conformably overlie the greenstone. The pyroclastip unit consists mainly of volcanic breccias, tuffs, and lesser amounts of epiclastic volcanic breccias, conglomerates,, and metaClasts range from 0.1 mm to 1.2 meters in diameter graywacke. and are composed of dominantly porphyritic andesite with very minor amounts of basalt, dacite, and tuff. Some of the basaltic clasts may have been derived from the older greenstone unit. The Jasper Lake andesite unit (Gruner's "andesite intrusive") is composed of predominantly porphyritic augite andesite with lesser amounts of massive, porphyritic hornblende andesite-dacite. The rock is typically fine-grained to aphanitic, and vesicular to amygdaloidal, thereby representing a shallow hypabyssal intrusion which may have reached the surface locally. These rocks are conformably overlain by a well-bedded, graded graywacke-slate unit, greater than 1.6 kilometers thick. Detailed study in the area shows that the volcanic rocks at Jasper Lake represent the oldest portion of the regional volcanic pile. They trend west-northwest and are faulted off to the west by northeast-trending units which are clearly younger, as they contain clasts of the Saganaga tonalite, which intrudes the greenstone. �SOME COMMENTS ON THE METAMORPHISM OF IRON—FORMATIONS B. Ronald Frost, Department of Geology, University of Minnesota, Duluth, Mn. 55812 ABSTRACT Preliminary work on the metamorphism of iron—formations shows that the rocks can be modeled by the system Fe—Si—O—C—H. In the typical assemblage of Fe—silicate-- quartz—magnetite, the fluid composition is controlled by a reaction of the form: Fe—silicate + 02 = magnetite t quartz ± H2Q When fayalite i present the oxygen fugacity is controlled by the QFM' buffet1 and it deviates increasingly from the buffer when it is controlled by increasingly lower—temperature Fe—silicates. It is conceivable that at very low temperatures, of the range of diagenesis, the oxygen fugacity buffered by the greenalite + those quartz + Fe—oxide assemblage is high enough to make the coexisting Fe—oxide hematite. The presence of siderite requires the consideration of carbon in the fluid Fortunately, the oxygen fugacities of an Fe—silicate—quartz—magnetite rock seem to be high enough to allow CO2 to exist as the major carbon—bearing component in the fluid instead of CH4. Under such conditions siderite will.break down to magnetite by the reaction: phase. 6 siderite 1- 0.2: = 2 magnetite Siderite + 6. CO2 will also react to form an Fe—silicate by the reaction: siderite + quartz +H20 = Fe—silicate Simple topological calculations show that the assemblage siderite ± magnetite + Fe—silicate + quartz will be isobarically, isothermally invariant, indicating that at fixed T and P the fugacities of O2 H2O, and CO2 will be fixed.. Furthermore, the same diagrams show that the breakdown of siderite to magnetite canoccur at constant f0 if there is a gradient of H20 present; 2 model can be used to explain the origin of magnetite in siderite—bearing taconite formations. If the rock originally consisted of alternating layers rich This in siderite with those containing iron—silicate + quartz, each layer would be Equilibration of the fluids across the layers would cause the chemical potential of CO2 in the siderite layer to decrease, and induce the formation of magnetite without introduction of oxygen from outside buffered to a.different. fluid co:mposition. the system. . . 11 . ,. �Geochemistry of Early Proterozoic PaleosoU; North ofLakeHuron,bntir±o B. J, Fryer, Department of Geology University of Western Ontario London, Ontario, Canada The contact between the reHuroniàrI and Hurbniañ tôcks, north pt Lake Huron, Ontario, is often marked byresidual weathering products or paleosols. These -are characterized by extensive leaching of Na, Ca, Mg, Fe, Mn and Si inthOir upper parts and conctmtration of K, Al and Ti. Even-when developed on mafic volcanics, these paleosOls approach a sériciteatitanium dioxide mineralogy. With increasing depth in the páleosois a very iroa—rich chlorite abruptly jOins the sericite and titanium oxide assemblage. This transitionpossIbly represents the permanent paleowater table. The behavior of the rare earth eleMents in these plebsols indicates that the ground waters responsible for these weathered deposits were of significantly higher pH (greater than 8) than at present. A direct consequence of this,is that the-high K contents of these paleosols are almost certainly original features produced during weathering and not a result of later metasomatism. This is substantiated by the behavior of Rh and Ba. The behavior of all elements, whether major or trace, appears to be dominated by the reducing and high pH nature of the ground waters. These results suggest that element solubilities and hence concentrations during Early Proterozoic surficial prOcesses nay have been considerably different: thn previously assumed 18 �?ETAM0PPHISM IN THE ENGLISH rv:u Gower, C.F. ançi SUBPROVINCE NEAP KENORA-,. NOPtHWT ONTAPTO Cliffqrd1 P4M., Dept. of Geplo, MoMaster University, Hamilton, Ontario Detailed petrograpitic investigations on 300 thift sections of gneissic and associated rocks from 200 6q.km. of the EnglIsh River Subprovince near Kenora have enabled two metemorphic episodes to be. defined. N1 metamorphism attained uppermost amphibolite facies and, using mineral assefnbiages together with whole rock chemistry in calcic pods, amphibolites and metasedimgnts, it is estimated that the P-P conditions were 5.25 ±0.75 kb and 650 ± 40 C. Prograde reactions during this metamorphism generated garnet from hornblende and biotite in amphibolite and tonalitic gneisses respectively. The distribution of garnets can be closely correlated with U) alkali feldspar mega-cryst distribution, (ii) lowest Fe20 /FeO bulk composition ratios in amphibolite and tonalitic gneisses, (iii) deepes structural level. The garnets are .undeform— ed and show no correlation with F? fold trends suggesting that the earliet recognizable metamorphism is late or pOst-f2. The N9 metamorphism is retrograde and, using mineral assemblages in amphi— bolites and metasediments, appears to have taken place under greenschist facies conditions. P—T conditions cannot be closely0defined from petrographic evidence but are estimated as 2.25 ± 1 kb and 400 ± 50 C. Potassium has behaved as a mobil,! component during both phases of metamorphism and is extensively involved in (i) asa reactant with hornblende to give biotite and epidote/clinozoisite, (ii) as a product, together with magnetite, from the oxidation of biotite, (iii) as a roduct from the reaction of biotite to give garnet. The presence of megacrysts in both gneisses and granitoid rocks is suggested as an expression of this mobility. 19 �ENVIRONMENTAL GEOLOGY OF THE NORTH SHORE A COASTAL ZONE MANAGEMENT PROJECT by John C. Gren, Gedlogy Department, TJniversity-df Minnesota, Duluth ABSTRACT During the jast two years the Minnesota Geblogicàl -Survey, on contrac -from the State Planning Agency, has undertaken a study of the environmental geology of the state's Lake Superior shore as an element of the Federal Coastal Zone Management Program. Two field seasons were devoted to mapping, with sample analysis, literature research, map development,, and report writing during the academic year. Besides the author, two graduate students (C. Moss and M. Jirsa) and five undergraduates (C. Baker, M. Gasser, K. Husby, and K. Peterson) were involved. The products, are a set of 13 maps,' covering the entire shore at a scale of 1:24,000, of each of 5 types (Bedrock geology, Surf icial geology, Depth t' bedrock, Landforms, and Economic Geology), plus a report which includes interpretations. Geologic processes currently active! geologic hazards, opportunities and resources offered, and land—use constraints are treated for the major surf icial material types and landform units. In this aréa the major geology—basèd lthid—use constraints are imposed by (a) geologic processes such as wave processes-and stream erosion and flooding, (b) soil suitability related to clayey glacial lake deposits and to shallow and exposed bedrock, and (c) economic resource protection, particularly gravel. in abandoned deltas of higher lake levels. 20 �CRUSTAL MODEL STUDIES OFA REGIONAL GRAVITY ANOMALY IN NORTHERN MICHIGAN AND WISCONSIN, EXTENT OF ANOMALY, AND ITS RELATIONSHIP TO NEAR SURFACE GEOLOGY J. S. Kl'asner and D'. Bbmke, Department of Geology, Western Illinois University, Macomb, Illinois 61455 The Bouguer gravity anomaly map oP northern Michigan •and Wisconsin has a broad, long wavelength gravity maximum that extends in an east—west direction for about 800 km from near the eastern end of the northern peninsula of Michigan into north-central Wisconsin. This anomaly may be part of a generally continuous gravity maximum that extends along the Southern Province of the Canadian Shield, except where it is overprinted by the gravity expression of the midcontinent gravity high.. It is truncated in South Dakota by a gravity maximum of similar width and amplitude that extends 'around the western and northern edge of the' Superior province. It is truncated on the eastern end by the gravity expression o'f the Grenville..orogenic belt. Two dimensional gravity models were constructed over the' regional anomaly in The models consider mass variations within the upper 20 km Michigan and Wisconsin. of the crust and consist of primarily two layers with a density of 2.80 gm/cc for the upper layer and 2.94 gm/cc for the lower layer. Th'ey show that the upper layer is thinnest beneath the middle Precambrian (X) basins and troughs such as the Marquette Trough and it reaches a thickness of about 16 km in central Wisconsin. In Michigan and Wisconsin several important geologic and economic features are associated with the regional gravity anomaly. For example, middle Precambrian (X) basins and troughs, which cause relatively short (a few kilometers or less) wavelength gravity anomalies, are located over the regional anomaly. Middle Precambrian (X) volcanic accumulations are generally located along the edge of the regional anomaly or within the middle Precambrian (X) basins. The recently discovered boundary between gneiss and greenstone terrane (Sims, 1976) is located roughly near the northern edge of the regional gravity anomaly. Regional metamorphic zones are generally located near the edge of the gravity anomaly, or, where superimposed upon the anomaly, cause gravity minima within the regional anomaly. The recently discovered massive sulfide deposits in Wisconsin seem to occur ajong the edge of the regional gravity anomaly or along prominent gravity features that cut the regional anomaly. The above data suggest a genetic relationship between the thinning of the uppermost (2.81) gm/cc) crustal layer and the formation of the middle Precambrian (Xe) basins and troughs, the accumulation of volcanic deposits, the formation of regional metamorphic zones, and possibly the accumulation of the sulfide deposits in northern Wisconsin. Perhaps the v,olcanic deposits, the sulfide deposits, and the igneous intrusions that supplied the heat for the formation of the metamorphic zones are all differentiates of the lower (2.94 gm/cc) crustal layer. They were intruded and extruded through fracture zones that formed in the 'uppermost layer during the Penokean orogeny. Sims (1976) has suggested that the middle Precambrian (X) basins were developed over, and approximately parallel to, the boundary between gneiss and greenstone terranes. The regional gravity maxima lie within the eugeosynclinal zone that Sims has postulated for this area'. Sims, P. K. 1976, Precambrian Tectonics and MineràT Deposits, Lake Superior Region, Presidential Address: Econ. Geol. V. .71 , M6, p. 1092-1110. 21 �BOUGIJER GRAVITY ANOMALY MAP OF, NORTHERN PENINSULA OF MICHIGAN, LAKE SUPERIOR, AND ENVIRONS J. S. Klasner, U.S. Geological $urvey and Western Illinois University, "acomb, 61455, William J. Hinze, Purdue University, Lafayette, Indiana 47907, .L. 0. Bacon, Michigan Technological University, Houghton, Michigan 49931, arid N. W. OtHara, Florida Institute 'of Technology, Melbourne, Florida 32901 Illinois ABSTRACT A prepublicatin version of the Boug1uer gravity anomaly map of the northern peninsula of Michigin, Lake Superior, and adjacent parts of Michigan, Lake Huron and Lake Michigan, Ls presented for discussion purposes. The map, which is at, 1:500,000 scale and has a 5-mga. contour interval, is a compilation of data collected since 1951 f±om several sources. Personnel from the U.S. Geological Survey tied each of the individual surveys to the 1971 base reference datum and made additional observations in areas that lacked gravity coverage. Data were reduced and compiled on digital tape by the Defense Mapping Agency, Aerospaae Center, St. Louis, Missouri, using the 1967 international gravity formula, sea-level datum and a 2.67-gm/cc reduction density. Terrain corrections were applied to selected sections in the Porcupine Mountain area only. Because of high station density in the western part of northern Michigan, the Bouguer gravity data were contoured using values selected frpm 1-minute quadrilaterals. On most of the map, station spacing is broader than the 1-minute interval, so that all stations are represented. Although the geologic implications' of many of the individual anomalies on the map h:tve been discussed in the literature, this map provides a comprehensive integrated view of the gravity field, which can be used in geologic and strutural anaLysis. The geologic Fources of a few of the gravity anomalies are discussed in this context. Over the Keweenawan-Lake Superior basin, a pronowiced g .'avity low is found along the center of the lake, and gravity maxima parallel ;he shoreline and join together at both ends of the lake to connect the midcontinent gravity high with the mid-Michigan gravity maxima. In general the gravity maxima reflect near-surface accumulations of relatively dense mafic volcanic and plutonic rock and the gravity mnima reflect thick less dense clastic rock. In northern Michigan, middle Frecambrian (x) basins and troughs such as the Marquette trough have east-trending positive anomalies. Lower Precambrian (W) granitic terranes commonly have gravity minima. A broad gravity maximum extends east across the northern peninsula. It has no apparent surficial origin and is believed to be caused by deep crustal or upper mantle mass variations.. 22 �Major Structural Features in Central Wisconsin and Their Implications on the Animikie Basin by Gene L. LaBerge university of Wisconsin—Oshkosh, Oshkósh, WI, 54901 EXPLANATION Abstract PRECAMBRIAN Paleotoic Niddle Precambrian batholith comprising numerous in composition from quartz diorite to granite intrusive sedimentary pile in Central Wisconsin has been outlined LaBerge and Myers. This batholith lies on the southern west trending sedimentary-volcanic (Animikie) basin. - IN) C.s.) A epizonal plutons ranging into a complex volcanicby recent mapping by margin of the large east— WISCONSIN LATE PREcAMBRIAN I Broad steeply dipping cataclastic zones separate the composite batholith and its greenschist facies roof pendants from upper amphibolite gneisses, migmatites and amphibolites that flank the batholith on the north and south. The scale of cataclasis, presence of ultramafic bodies along the zones, and the marked difference in metamorphic grade across the shear zones indicates deformation on a crustal scale. The gneissic areas appear to be horsts, and the batholith a graben-like structure. Field relations along comparable shear zones within the batholith indicate a long and complex history of cataclasis during emplacement of the batholith, and, by inference, during the history of the Animikie Basin. of [ :::::::::::: Bayf ld Group Oronto Group I! :::: ::::: Wetf RiVer aatholith Qqartslte MIDDLE P}EECLMBRIAN The present distribution of Precambrian rocks in Wisconsin is one of east— west trending belts of Middle Precambrian sedimentary-volcanic-plutonic rocks alternating with Early Precambrian(?) gneissic rocks. This has disrupted the basin into a series of horst-like and graben-like blocks. Field relations in Central Wisconsin indicate that at least part of this deformation occurred during the. tectonic history of the basin, and is consistent with Cannon's (1973) interpretation in the Marquette District. This suggests that the Animikie Basin was characterized by vertically moving blocks, which may have provided local sediment sources within the basin and also produced local strongly reducing troughs, one of which may have resulted in the highly graphitic "Flambeau Anomaly." Granitic Ro49 Iron-Formation Dominantly Matasedimeckary Rocks 1Ta tlyMt 1 Rok EARLY PREcAMBRIAN Granitic Reeks Metavolcanic Rocks GneisSic Rocks (Modified from Sims, 1976) Gee Eases Creenschtst Gneisses kigeaEttas )taa,rphism Pligeatitea MiphtkCliteI Episenal Plutats Asphibolita - Greenschist Matarerphisa Gneisses Pligmatitis Asphibolltss �18 0/ 15 0 Results for Archean Plutonic Rocks, Lake Despair Area, Northwestern Ontario F. J. Longstaffe and R. H. McNutt, H. P. Schwarcz Department of Geology, McMaster University, Hamilton, Ontario An oxygen isotope study of the Jackfish Lake nlutonic complex and the Burditt Lake stock, (Ivabigoon granite-greenstone belt), has indicated the importance of nagmatic-autometasomatic fluid activity and/or hydrothermalmeteoric water interaction in their crystallization and alteration. o18o values of the main phase of the Burditt Lake granodiorite are relatively constant across the stock (8.00 ± 0.33 0/00); microcline megacryst hearing phases, as well as late stage anlitic rocks have higher 6180 values (8.95 ± 0.35 0/00); Depletion in lo occurs in t e volcaniclastic county rocks as the granodiorite contact is approached (11.35 to 8.00 0/00). The movement of mac'matic water unward from hotter, deeper portions of the stock through the roof zone into the country rocks can cause such isotopic variations. The high water/rock ratios required at the contact by such a model appear reasonable, as considerable chemical modifications of the country rock has occurred in the vicinity of the contact. The Jackfish Lake Complex. as exnosed in the Lake Despair area, is composed nrelominantly of diorite and monzodiorite, with lesser volumes of quartz diorite, miaocline megacryst hearing granodierite and soda syenite. The samnles Thich preserve the highest and the most concordant oxygen isotopic terneratures are located within 50 meters of the contact with the mafic amphibnlite country rock (which itself has been enriched in 18o from 5.7 to 8.0 otoo). Apnarently, the oxygen isotopes have been quenched more rapidly and nore comnletely in the outer margins of the body. Elsewhere in the Complex, discordant oxygen isotope mineral-pair temperatures indicate varying degrees of isotopi disequilibrium. Th"se disturbances can he largely attributed to late stage deuterir alteration and continuing subIn snite of such pe'turhations, some primary solidus isotonic exchange. isotopic trends are still discernible; 6180 values of cluartz, plagioclase, hornhlende and hiotite decrease gradually with increa;ing degree of differentiation of the rock type. This hehavinur nrohahly reflects the relative imnoverishment of the remaining melt in ISO as Iarc'e amounts of 180-rich mineral rhases begin to crystallize during the formation of the late stac'e srnll volume granodiorite. The preservntion of citch trends. as ''ll as the nrrnnl a18o enrichnont pattnn from diorite tO granodiorite observed in rock samples swgests cln.scl system isotonic exchange in these rocks. The southern boundary of the Jackfish Lake Complex is formed by a major fault. Cranodiorite located near the shear zone is altered, showing Fe staining arid large scale saussuritization of felclspars. Chemically, such samples are enriched in lirht rare earths, Zr, Ni, Fe. Ti, P, K and Rh, and denleted in Na, Sr and Ha. These rocks are also denleted in 18o (5.41 O/oo The depleted 750 meters from the fault; 7.80 °/oo 2400 meters distant). rocks contain minRral phases which are grossly out of isotopic equilibrium and depleted in 1o0 relative to "unaltered" granodiorites from the Complex. Such behaviour is best explained by hydrot'iermal-meteoric water interaction ir anisotopically open system. 24 �FOSSIL COLLECTIBLES FROM THE GUNFLIN'P FO1MATION S. Lougheed and J. J. Mancuso, Department of Geology, Bowling Green State University, Bowling Green, Ohio 43403 P4. ABSTRACT Iron—formation to be. of economic value is dependent upon a succession of processes that transform the initial biogenic particulate material, produced on a gently sloping marine shelf zone There are five great fossil collectibles into iron-rich, minerals. of initial material bccurring in the Gunf lint; they are, siliceous and carbonate shells, blue—green algae, greenaloid and bacterial framboidal pyrite. Two outstanding areas for collecting fossil materials occur at Kakabeka Falls and in the vicinity of Schreiber. Excellent specimens of ooids are found at the falls, but more remarkably, many chalcedonic chert laminae contain totally or almost completely dissolved ooids so that only the nuclei remain. The nuclei are varied in their structural pattern but commonly appear as ellipsoids, spheres, or as spheres with concentric laminae. The structures are small, generally less than thirty microns in diameter. The ellipsoid structures and probably some of the spherical structures are siliceous shells of microorganisms. Some, if not most, of the spherical nuclei are silicified shells of calcar— Both siliceous and calcareous shells are eous microorgansisms. fairly common in organically pigmented chalcedonic chert. Although bacterial carbonate, is common in specimens from Kakabeka Falls particularly those specimens rich in pyrite, the best specimens are found in the Schreiber area, where the micron sized carbonate crystals occur in the cortex of oncolites. The; carbonate bacteria produce ammonia as a by-product from their metabolism of expired algal laminae in the cortex, and carbonate is therefore precipitated in the microdomain of high alkalinity., The columnar stromatolites in the Sbhreiber area are noted for the fidelity of preservation of the filimentous and coccoid algae in a siliceous matrix and therefore are not associated with bacterial carbonate. We find the best specimens of bacterial carbonate produced in columnar stromatolites occurring in the Biwabic iron—formation, however good specimens 'may be found in the road cut at the junction of highways 590 and 17-11 near Kakabeka Falls. Greenaloid, the gel-like material composed of silica, and sapropel complexed with ferrous iron, is best collebted as matrix material in laminae of ooids or as matrix material occurring with tuffball laminae at Many specimens show transitional steps in the' Kakabeka Falls,. oxidation of greenaloid to greenalite and/or magnetite and less commonly to hematite. 2.5 �PENOKEAN STRUCTURES AND PLUTONIC ROCKS IN WISCONSIN K. S. Maass'and L. G. Nedaris, Jr.,' DepartmerIt'of Geology and Geophysics, University of Wisconsin, Madison 53706 W. R. Van Schmus, Department of Geology, University of Kansas, Lawrence, Kansas 66044 ABSTRACT Last •year we reported on the occurrence of Penokean structures and plutonic rocks in early Precambrian gneiss in Portage and Wood Counties, Wisconsin. We now have completed more detail3d structural and isotopic studies on these occurrences and have extende tour investigations westward about 100 miles to include localities in Clark, Jackson, and Chip— pewa Counties. The Early Precambrian gneiss, formed about 2.8 by. ago (Van Schmüs and Anderson, 1977) and domposed of quartzofeldspathic gneiss, amphibo— lite, and migtnatite, contains three sets of folds: first, penetrative isoclinal folds; second, non—penetrative S-- and Z—folds; and third, non— penetrative broad, open folds. The axial surfaces of the second and third fold sets are discordant -to those of the first set, but the fold axes of all three are colinear, Well developed lineations, defined by the dimensional orientation of elongate minerals and trains of mineral In all the localities examined so grains, are parallel to fold axes. far, fold axes and lineations plunge steeply, from 45° to 900. • The gneiss has been intruded by two different tonalites: an earlier medium—grained tonalite, which contains a strong foliation and lineation, and a later fine—grained tonalite, which contains a weak foliation and strong lineation. Lineations in both tonalites are colinear with those in the gneiss. The tonalites as well as the gneiss have been recrystal— 1ied under middle—grade metamorphic conditions. Petrofabric analyses have been completed for samples along the Wisconsin River, including three samples of quartzofeldspathic gneiss, one Measure—, -of medium—grained tonalite,- and two of fine—grained tonalite. ments of [0001] in quartz have given a similar pattern for all five samples: a girdle normal to the h fabric axis.- Thus, the tonalites contain structural elements in common with some of those in the gneiss, on both mesoscopic and microscopic scales. - U—Pb analyses of zircofl'have yielded ages of 2800'm.y. 'for the gneiss, 1850 ± 25m.y.- for the medium—grained tonalite, and 1800 ± 25 m.y. for the fine—grained tonalite. We believe that the zircon ages for the tonalites represent their times of emplacement and, consequently, that the structures within them and some of the structures within the gneiss were produced during the Penokean orogeny. -Further investigations of this type shouli give a 'more complete un— - derstancling of the Penokean orogeny in Wisconsin and provide a basis for comparison between gneisses of Early Precambrian age in Wisconsin and 'those of the Minnesota River Valley terrane. 26 �THE OCCURENCE AND SOME NOBLE METAL CONCENTRATIONS IN SELECTED KOMATIITIC ULTRAMAFIC VOLCANIC ROCKS FROM MUNRO TOWNSHIP, ONTARIO. ' MacRae, William E., and Crocket, James I-I., Department of Geology, McMaster University, Hamilton, Ontarjo. Munro township, situated in 'the Abitibi volcariic belt, Is the location of a sequence of well documented komatiitic ultramafic voldanics. The rocks are well exposed and the area has been subjected to only very low' grade metamorphism. The Jcomatiitic volcanic rocksrange from peçidotitic through pyroxenitic to basaltic in composition'. Samples taken from peridotitic flows at the base of Centre, Hill have been'analysed for gold, platinum, palladium, and iridium by neutron activation analysis. Four lithologic units were sampled from two flows. The results pf the analysis are summarized below: Lithologic Unit Au 'Pt" Pd Ir 1.6 2.5 8.9 1.1 Spinifex zoné(3) 2.1' 14.3* 10.7' 0.8 Foliated zone(1) 3.3 '— '7.5 0.4 Cumulate zone(4) 3.6 ll,.l. 6.3 1.5. ' (Flow) Chilled marins(3) ' , (ppb) *(3) Number of samples for zone. There appears to be a slight enrichment of'gold in the' cumulate zone as well as'iridium relative to the spinifex zone. This is probably due to the settling of immiscible sulphides as well as olivine before the formation of the spinifex. Platinum and palladium increase in the spinifex zone and were possibly enriched in the molten silicate phase. The average of the cumulate and the spinifex zone for palladium and iridium are the" same as the chilled zone, while the 'values: fr gold and platinum are lower. The latter values are possibly due to seawater leaching. The average concentration of gold (2.7 ppb) is not significantly higher in peridotitic komatiites 'than' in other major rock types and do not appear to contain enough gold to make them a source 'rock for 'gold deposits. 27 �Stratigraph of the Dar1aga' Bain Metasédiments; Michigan J• J. Mncu'èo, R.- E. éavoy, M. S. Louc4heed Bowling Green State University Bowling Green, Dhio 43403 ABSTRACT The BaragaBasin is located- in eastern Baraga and northern Marquette Counties, Michigan. It is 30 miles long by 8 miles wide and is filled with approximately 1400 feet of mildly deformed Middle Precambrian metasediments. Lower Precambrian basement. rocks ecposed around the perimeter of the basin are crystalline granites and gneisses which unbonformably underly the metasediments. The lowermost Middle Precambrian unit in the basin is a white vitreous quartzite which appears to be limited to the western and central portibn of the basin. A basal quartz-pebble conglomerate is exposed at Pikes Peak in sec. 11, T. 51 .N., R. 32 W. Overlying the quartzite is a chert-carbonate iron-formation and a volcaniclastic sequence. Recent phosphate discoveries occur within this unit (Mancuso, Lougheed, Shaw, 1975; Cannon and Klasner, 1976). More than 1100 feet of graphitic slates and a thick meta-arkose make up the rest of the section. Flat lying Cambrian(?) Jacobsville Sandstone unconformably overlies the Precambrian rock section. . -The white basal quartzite -is correlated with the Goodrich Formation while the iron-formation, volcaniclastic. sequence, the. black slates and the meta—arkose are correlative with the Greenwood Iron-formation Member, the çlArksburg Volcanics Member, and the Lower Slate member of theMichigarnme1ormationin.the.western part of the . Marquette Basn,. . . References, Cited. Mancuso, J. •J., Lougheed, M. S., and Shaw,R., l975,Carbonate apatite in Precambrian cherty iron-formation, Baraga County, Michigan: Econ. Geology, v. 70, no. 3, p. 583-586. Cannon, W. F., and Klasner, 3. 5., 1976, •Phosphorite and other apatite bearing sedimentary •rocks in the Precambrian of Northern Michigan: U.S. Geol.. Survey Circular 746, 6 p. 28 �AN ESTIMATE OF THE RARE EARTH ELEMENT DISTRIBUTION IN POST-KENORAN UPPER CRUST, NORTH OF LAKE HURON McLenrian,. Scott M., Fryer,B.J.., and Young,. Grant 1W., Department' of ceology, University of Western Ontario., London, .Qntario., N6A 5.B7. Rare earth analyses havë been made on'.sampls of tillite matrix from the Gowganda' Formation, north of Lake Huron. Agrandmean based on averages.from the. Cobalt, Quirke Lake and Esp-anola'- SudburSr Areas is considered to berepresentative of upper cnistal abundances for a large area. northof Lake Huron. The three districts were given equal weight in the estimate. The ovtrall abundances are('ih ppm): La, 24,;' 'Ce, 55 Nd, 23; Sm, 43; Eu, 1.2; Gd, 3.8; Dy,. 31 Er, 2.0. These data are compatible with analyses of granitic and volcanic rocks typical of the surrounding areas. Values are also in line with estimate's 'pf Canadian Precambrian crust and post-Arch'ean crustal abundance.s in Australia, though relative distributions have significant 'dirferenoes. Analyses of Gowganda argillites and sandy argillites intimately associated with the tillites have- similar patterns tji those of the tillites. Absolute magnitudes, however, are consistentlyhigher(by. a factor of about 1.3) than the tillites. A possible explanation for this could be concentration of clay minerals in the argillites. This may suggest that other estimates of crustal abundances which are based, in part, 'on analyses of fine grained sedimentary rocks are systematically high by a similar proportion. 29 �Archean Volcanism Washeibamaga Lake Area, Wabigoon -ubprovinc?, Northwest Ontario. G.E. McMaster and Departmento R.!J. McNutt Geology, McMaster 'University. The Washeibamaga-Thind'rcloüdtâkes area' )f the Wabigoon Subprovince, can be subdivided into three faàies; I) The Lower volcanic sequence of metabaèalts (lower greenschist facies) is preserved. as a:-stenly-dipping, north-facing- homoclinal volcanic -pile six kilometres thick Nb, rr, Ni,-Ba .Pb, Sri', They show trace, element (Y, geochemical. similrities tomoderxi ocear. --floor. tholeiitic basalts. 2) The Thundercloud Lake Quartz-Porphyry intrudes the lower sequence-and is helieved-to..represent a vent-plug filling a late-stage felsic volcano. Accompanying explosive vojcanism produced 'a threekilometre. thick sequence of coarse pyroclastic rocks an&-tuffs.. Associated dacitic and autobrecciated. rhyolltic flows have calc .alkaline affinties and are chemically 'distinct from both volcanic sequences and appear not to be a differentiated product but to have originated as a seperate magma. 3) The Upper volcanic sequence of metabasalts is composed of tightly folded, massive to pii.lowed flows. The contact with t le unde: 'lying epiclastic and pyroclastic rocks is at ah angle c' thirt-- degrees-, implying either profound angular unconformity or a fau'±t dont ct. -The upper sequence is chemically distinct from the lower sequence. K,-Rb-, Sr, Ba, abundances suggest similariie with rirodern Island Arc'tho3eiites 30 �ORGANIC-RICH 'lAKE SEDIMI4T EXPLORATION GEOCHEMICAL SURVEY OF EASTERN LAKE VERMILION—ELY AREA, NORTIIEASTERN MINNESOTA D. G. Meineke, M. K. Vadis and A. W. Klaysmat Minerals Exploration Section, Division of Minerals, Minnespta Department of Natural Resources, Hibbing, Minnesota 55746 ABSTRACT An organic rich (gyttja) lake sediment geochemical survey was conducted over Lower Precambrian volcanic and associated rocks in northeastern Minnesota for the purpose of determining the applicability of this method for evaluation of mineral resource potential and, reconnaissance exploration. Two hundred and seventy samples were collected from 75 lakes over an area of 200 square miles (520 sq. km.). A weak aqua regia leach on unignited gyttja produced the best contrast over background. Statistical analysis of the data indicates that trace element distributions are greatly dependent upon the limnological environment of each lake; trace elements tend to be concentrated in the organic and/or inorganic fractions of the gyttja; and, of all parameters considered, LOl (loss on ignitipn) is the best single 'indicator of limno— logical environment. Due to the variations in lake environments and trace eLement accumulation inthe gyttja, parameters other than the element concentrations were considered. However, the study indicated, even though a perfect datum for comparing lakes was not possible, •the element concentrations for arsenic, cobalt, copper, nickel,'lead and zinc provided the best datum for comparing all 75 lakes. Several significant anomalieth were located by the survey. Anomalous copper was found in a lake near an interesting copper prospect. Copper, lead, titanium and zinc appear to reflect bedrock composition; chromium, magnesium and nickel reflect bth bedrock domposition ,and glacial dispersion. 31 �MAFIC MINERALOGY OF FERROAUGITE SYENITE PROM THE COLDWELJ1 ALKALINE COMPLEX ROGER H. MITCHELL and R. GARTH PLATT DEPT. OF GEOLOGY LAKEHEAD UNIVERSItY ,. THUNDR BAY ,ONTARIO Plutonic Center 1 of the Coldwell alkaline complex is dominantely ferroaugite syenite associated with minor A c. 2000 m. amounts of earlier hypersthene gabbro. section of ferroaugite syenite exposed on the lake shore between Marathon and the eastern margins of the complex exhibits well defined igneous layering in the eastern portion of the sequence. The layered syenites grade into syenites with poorly defined diffuse turbulent layering and thSe in turn into coarse syenites conCryptic layering taining patch and sheet pegmatites. is well developed in the sequence and indicates this portion of Center 1 ferroaugite syenite is a small intrusion in which crystallization occurred simultaneously Olivines range in composition from at the roof and base. Fa3 to Fa03. Pyroxenes initially belong to the diopside— hedenbergiCe series (Di42HdagAc3.to Di10Hd85Ac5 and grade into members of thp acmie-hedenbergite series (Di10H95 Ac5-Di5Hd45Ac0) Pyroxene c npostional trends are similar to tháse observed in peralkaline igneous rocks and in particular to those of the undersaturated Fivegroups of amphiboles are Ilimaussaq intrusion. present; 1 — ferroedenite-hastingsite; 2 — sub-aluminousr ferroedenite; 3 - aluminous ferrorichterite - ferrorichterite; Amphibole compositional 4 — arfvedsonite; 5 - ferroactinolite. trends parallel those of the pyroxene in showing dcreasing Al and Ca with increasing Na and extreme iron enrichment. Oxides minerals in the earliest stages of crystallization were Fe—Ti oxides and baddeleyite, these were replaced as. liquidus phase by aenigmatite and zircon respectively as Residual liquids, as represented by the magam evolved. the pegmatites crystallized ferrorichterite, feldspar, zircon and quartz. The ferroaugite syenite magma evolved along an oversaturated peralkaline trend characterized by extreme iron enrichment under conditions pf low oxygen fugacity at high silica activity. 32. �STRATIGRAPHIC AND TECTONIC HISTORY OF LOWER AND MIDDLE PRECAMBRIAN ROCKS IN EAST-CENTRAL MINNESOTA G.B. Morey, Minnesota Geological Survey, University of Minnesota, St. Paul, Minnesota It has been recognized for nearly 70 years +that a great diversity of Precambrian rock types crop out in east-central Minnesota. However the rocks are poorly exposed and an understanding of their geologic history has been hampered by a lack of definitive geologic data from which age and spatial relationships can be deduced. Nonetheless, recent geologic studies utilizing conventional mapping techniques in conjunction with subsurface, magnetic, and recently acquired gravity data have more precisely defined the spatial relationships of various rock units and have led to a more complete understanding of their stratigraphic and tectonic histories. The Lower and Middle Precambrian rocks in east-central Minnesota are divisible into three distinct terranes: (I) a diverse Lower Precambrian terrane; (2) overlain on the north by a thick sequence of folded and metamorphosed Middle Precambrian stratified rocks; and (3) intruded on the south by a variety of Middle Precambrian plutonic rocks. All of these rocks are overlain by generally flat-lying sedimentary rocks of Late Precambrian, Cambrian and Cretaceous age. Two presumably high_angle, east-trending faults of Early Precambrian age divide the Lower Precambrian terrane into three lithotectonic segments. The southernmost segment consists dominantly of quartzófeldspathic gneisses metamorphosed to the 'upper. amphibolite or granulite grade. Granite and lesser a,mounts of rnetasedimentary and metavolcanic rocks assignable to the greenstone-granite belts of northern Minnesota comprie the northern mast segment. Substantive data bearing on the lithic attributes in the middle segment are lacking, The segment may consist of either cataclasized gneissic rocks or metagraywacke and slate similar to that in northernmost segment. However, regardlessof their original age and character, the rocks in the middle segmen't forrrt a discrete zbne separating two considerably different Lower Precambrian lithotectonic units. The Middle Precambrian strt-ified 'rocks occur within an intracratonic baih centered over and approximately parallel to the boundary zone between the Lower Precambrian gneissic 'and greenstone-granite segments. The stratified rocks are divisible into two 'groups separated by an unconformity. The older group consists dominantly of quartzose rocks of clastic and perhaps volcanogenic origin. Mappable ynits of metabasalt, mafic 'tuft, oxide- to carbon,ate-facies iron-formation and carbonaceoui mudstone are abundant near the base, whereas carbonate rocks occur as mappable beds near 'the top of the group. The younger group is similar to, an,d correlative with, the well-known Animikie Group of northern'Minnesota and Ontario. Sedimentation was either terminated or cldsely followed by a period of regional deformation and metamorphism assignable to the Penokean orogeny. The dominant Penokean structure is an eastward-plunging synchnorium bounded on the north, west, and.south by"Lower Precambrian rocks. However the extent to which the rocks were' deformed varies from place to place ,within the synclinorium and the style of deformation is attributable to the tectonic behavior of contrasting kinds of Lower Precambrian rocks. Where they overlie granitic basement rocks, the stratified rocks dip gently southward and the basal contact is relatively undisturbed.' ln contrast, where they overlie gneissic or metasedimentarv basement rocks, the stratified rocks are complexly infolded into a number of large anticlines and synclines having numerous coaxial. second- and third-order folds on their limbs. The metamorphic grade of the stratified rocks increases from north' to south. To the north, argillaceous rocks contain minerals indicative of high-grade diagenesis or zeolite-facies metamorphism, whereas to, the south they contain minerals indicative of the lower atnphibolite facies. Metamorphic mineral isograds conform in a general way to the fold geometry, but in detail they transect fold axes implying that deformation ' nd metamorphism were discrete events. - Teètonic instability during the Penokean orogeny was manifested principally by vertical uplift of the gneissic basement rocks and the development of a mantled gneiss dome along the south edge of the Penokean synclinorium. The virtual coincidence of bedding in the stratified rocks that surround the gneiss dome with cataclastic foliations within the gneiss dome suggests that folding and uplift occurred contemporaneously. In addition, the spatial coincidence of high-grade metamorphic rocks along, the flanks of the gneiss dome suggests that the gneissic terrane' was characterized by relatively high heat flow during deformation. The Middle Precambrian plutonic rocks are confined to that part of east-central Minnesota underlain by gneissic rocks. Most' of the plutonic rocks are post-tectonic in. age as evidenced by cross-cutting relationships with the mantled gneiss dome,' and by their relatively homogeneous and undeformed nature. Igneous activity of calc-alkaline affinity began with the emplacement of dike-like bodies of quartz diorite. This was followed by the emplacement of small, to large plutons of granodiorite and quartz monzonite, which in turn was follOwed by the emplacement of various sized plutons.of granite. Quartz monzonitic rocks having rapakivi-like textures occur locally as border phases to some of the granite plutons. . ' Erosion, following uplift along major northwest-trending faults, exposed the 5gneissic and' plutonic rocks prior' to the deppsition of Upper Precambrian sedimentary -rocks. ' - - �PETROGRAPHIC AND CHEMICAL ATTRIBUTES OF SOME LOWER AND MIDDLE PRECAMBRIAN GRAY WACK E-SH ALE SEQUENCES tN NORTHERN MINNESOTA G.B. Morey and NI Schulz, Minnesbta Geological ,$UrVey, University of Minnesota, Si. Paul, Minhesota. Graywacke-shale sequences comprise a significant proportion of the Lower and Middle Precambrian rock record in northern Minnesota. Although the petrographfc character of these rocks has been evaluated in detail, little use has been made oftheir bulk chemical compositions, particularly in classification and provenance studies. We suggest however that the chemical data, when used with petrographic data, provide useful new insights regarding the sedimeritological history of these rocks. According to the classification scheme of Crook (1974), which considers only th framework grains, the Lower Precambrian graywackes are quartz-poor to quartzintermediate in composition and are indicative of a tectonically active island-arc environment. They contain 2 5-50 percent dacitic to rhyodacitic rock fragments, 10-36 percent sodic plagioclase, trace amounts to 12 percent volcanic quartz, and as much as 22 percent labile components such as hornblende. In contrast, the Middle Precambrian graywackes are quartz-intermediate to quartz-rich in composition and are indicative of deposition under tectonically stable conditions. They contain: 15-90 percent plutonic quartz, 1-36 percent feldspar, and as, much as 7 percent rock fragments of mostly granitic composition. The bulk chemistry of the two sequences emphasizes the fact that they are different chemical entities. The_Middle Precambrian graywackes contain more silica (X= 75% vs. 62%) and less K20 (X=l.43% vs. 1.90%) and Na 0 (X=2.42% vs. 4.06) than do the Lower Precambrian graywackes. The former also Jhibit a narrower range of Na2O/K.,O values (1:2'to 2:1) than do the latter (1:1 to 13:1). These differences can be related 'to the, mineralogy of the framework grains. Quartz is the dominant mineralogic variable in the Middle Precambrian gçaywackes, and its abundance exerts a major influence on the amount of Si02 in the analyzed samples. In contrast, dacitiç to rhyodacitic rock fragments dominate the framework grains of ' the! Lower Precambrian graywackes 'and exert a strong influence on the. Na20/K20 ratios. Very little is known about the petrography of intercalated shale units iri either sequence. However the bulk chemical data suggest that the tower Precambrian shales are fine-grained equivalents of the graywackés, whereas the Middle Precambrian shales are discrete chemical entities not related to graywackes simply ,by the relative abundances of framework grains. The bulk chemistry of the Lower Precambrian graywackes suggests that they were derived from: a dacitic to rhyodacitic source with little attendent chemical alteration. Thus these rocks were not 'markedly affected by .post-depositiona . processes; the framework grains reflect the comp'osition of the source area. However neither the Middle Precambrian graywackes nor their' intercalated shales can be derived chemically. from a simple granitic source; chemical-mixing calculations indicate a complex source of consisting of quartz monzqnitic, rhyodacitic, and basaltic1 rocks. The mixing calculations'also imply that the Middle 'Precambrian sediments were, derived from a considerably weathered terrane and subjected to post-depositional processes which considerably modified the original framework mineralogy. 3.4 �-GEOCHEMISTRY OF THE YELLOW DOG PLAINS PERIDOTITE, MARQUETTE COUNTY, MICHIGAN W. J. Norris and J. T. Wilband, Geology Department, Michigan State University, East Lansing, NI 48824; P. W. Snider, Geological Survey Division, Michigan Department of Natural Resources, Lansing, MI. 48909 . A relatively fresh, previously undescribed,.peridotite body outcrops in an area locally known as the Yellow Dog Plains, adjacent to county road MA in the Champion quadrangle, Marquette County, Michigan. A small exposure south of the road and a roughly oval shaped "plug", 120 meters wide by 190 meters long, intrude the Precambrian X Michigaimne Slate which underlies most of the Yellow Dog Plains. The larger outcrop stands 15 meters above the plain at its highest point. Recent paleomagnetic data indiqate the, intrusive is of lower Keweenawan (Precambrian Y) age (K. Books, U.S.G.S., personal 'communication, 1977). The fact that the Yellow Dog Plains peridotite is located in an extensive east—west trending magnetic belt suggests it maybe' genetically related to the exposed east—west,trending Keweenawap diabase dikes to the south -which-haye been saippied and analyzed for comparisons. The peridotite contains up to. 50 percent olivine, as much as 30- percent pyroxene (both clino— and orthopyroxene), approximately 10 percent plagioclase, and less than 10 percent opaque minerals. A dark red pleochroic biotite (Cl percent) is common in most specimens. Preliminary microprbbe analysis of unserpentinized olivine and plagioclase give Fo80 and An8.65, respectively. The sulfide minerals pyrite, pyrrhotite, chalcopyrite, cubanite (?), pentlandite, and bornite are present in small amounts mostly associated wit-h magnetite. Major oxides, Cu, Ni, Cr, Zp,. Co, and several rare earth elements were analyzed from 22 specimens. The average values for the oxides are as follows: 5i02 = 42.46%,' A1203 4.24%, Fé203 = 5.65%, FeO = 8.71%, MgO = 26.19%, CaO = 4.40%, Na20 = 0.49%, K20 = 0.24%, 1120 = 6.72%, Ti02 = 0.71%, P.20s 0.10%, MnO = 0.18%. Samples from a 30'meter vertical drill core in the large exposure show a continuous increase in Ni with depth. MgO, FeO, and Cr have the same trend as Ni., A break' in. the alkali values, which otherwise consistently decrease with depth, suggests. the intrusion maybe layered. Layering has not been confirmed by modal data. - 35 �Post-Glacial Sediment Distribution in the Canadian Portion. of .-Läkë SupérioJ J. S. Mothersill, Lakehead Univerity The Canadian portion of Lake Superior covers an area •of approximately 29,882 km2 -of a -total lake area of 82,375 km2. The drainage area of the Canadian portion of the lake, excluding the lake and the Lake--Nipigon drainage basin, is approximately72,,000 km2. The drainage basin which was covered .by a virgin forest up:until a century ago, is'still mainly covered by boreal forest. Based on radiocarbon dating of the lake-sediments, glacial retreat from the-northern part- of Lake Superior occurred about 11,600 yrs. B.P. Since the time of glacial retreat, approximately 12,545x106 m3 of sediment have been deposited in the lake proper and adjacent -bay areas. The post-glacial sediments tendto have been deposited in topographic basins with the thickest sequences occurring in Thunder Bay, Nipigon Bay and Black Bay where up to 12 m, 12 m and 14 m respectively have been deposited in topographic basins. In the lake proper, the. maximum thickness of post— glacial sediments is only in the -order of S m. The average rate of sediment reaching the. lake since glacial retreat has been about l.08x106m3/yr. The sediments consist ofquartzarenite to arkosic.sandswhich occüradjacent to the shore and the islands-and a silty clay, sequence that occurs in the topographic basins. The bulk of the sediments (>99% percent) are formed of silty- clay'-.which'is comprised of major amounts of quartz, K—feldspar and plagioclase, subordinate amounts of chlorite, ililte and kaolin añd minor amounts of amphibole and an interbedding 'Of vermiculite artdsmectite. - The average sediment yield fromthe drainaqebasin was about 15 m3/km2/yr. or a total-of l74;000 m3/km Sinde- glacial retreat'. 3- �MSSIVE SULFllTh 1)E1OSITh IN WISO0N4TN M. G. Mudres', Jr., K. K. Ostrom, Wiscons in Geological and Na Lu,-a I Ill s)ot'y Survey, l815 University Avenue, Madison, Wisrons in 5371)6, antI Gordon Reinke, Wisconsin Department of Natural Resources, 4610 University Avenue, Madison, Wisconsin 53702. ABS TRACT Since 1968 with tile discovery near Ladysmith in Husk County of n massive sulfide ore body, over three dozen mining conpanies have at one time or another explored for non—ferrous massive sulfide deposits in the Precambrian of northern Wisconsin. The most significant find to date is by the Exxon Company, U. 5. A., of a 60-million ton deposit of zinc and copper south of Crandon in Forest County. Exploration activity has precipitated numerous studies by the state, including geological, geophysical and hydrotogical surveys, review of legislation, and social and economic ,.impact analysis. Exploration has been concentrated in a 100—km wide hand from Ladysmith in the west, through the ghinelander-Crandon area to the Pembine area in the east, a distance of 350 kilometers. Available outcrop data, gravity compilations, and state—acquired aeromagnetic data, coupled with isotopic studies by the U. S. -Geological Survey and the University of Kansas, suggest that this terrane is a middle Precambrian volcanic belt surrounded by early Precambrian gneisses, Detailed geology is known only for the Ladysmith deposit. This deposit, owned by the Kennecott Copper Qorporat ion, is essentially a verti— cally—oriented, lens-shaped pod 15 m wide, 720 m long, and 240 m deep. Country rock consists of intermediate to felsic volcanic rocks of anda— lusite metamorphic grade. Economic minerals found within a pyritized quartz—serieite schist are a supergene enriched blanket of chalcocite and bornite of pre—Late Cambrian age, overlying primary chalcopyrite. The 6-million ton body averages three and one-half to Tour percent copper. Favorable terrane was identified by an INPUT survey in 1967, and follow—up drilling in 1969—1970. sulfidc ore hodics in North A:aerjca. Wisconsin environmental laws are administered by the state Department of Natural Resources. Present regulations require an:environme,ntal impact assessment of all proposed mining operations. Experience has shown that for significant new mines this assessment will invariably result in preparation if an Environmental Impact Stateaent by the Department. New mines are also required to obtain a mining permit which in— Mine operators cludes a reclamation plan approved by the Department. are required to post a bond to cover the cost of reclamation, Uining &mmpanies must also obtain other permits required by thc Department for the protection of the environment. - Several pieces of legislation that were introduced in the spring 1977 legislative session would: (I) replice present mineral taxes with a graduated severance tax on net proceeds; (2) require the registrótion Cf exploration companies in Wisconsin with the additional requirement that some kinds of company—acquired geologic data be turned over to the state after exploration; (3) require the registration of severed mineral rights and ultimate aquisition of orphan mineral rights; (4) set limits on the duration of mineral exploration leases; (5) set a cooling—off peiod during which n exploration lease could be broken; and (6) eliminate the mine'al depletion atlowande. - It is reasonable to expect that more deposits of massive sulfide ore will be identified in future years in Wisconsin. Whether or not the deposits are developed depends on cots, mctal prices,- environmental constraints at each individual prospect, the tax climate, and the mineral policy of the state. - Tn 1974, Noranda Exploration, Ihc. announced the discovery of a small zinc—copper body on 'the Pelican River east of Rhinelander in Oneida County, The deposit consists of three zones. The total deposit consists of 2.3 million tons at an average grade of one percent copper and four and one— half percent zinc. The deposit is 300 a long, 15 m wide and 203 m deep. It was identified by combined geologic and INPiJI' surveys. The generally small- size and uncertain mine development climate, preclude the immediate develop'nent of the property. Noranda is continuing exploration activities in the area. In May 1976, the Exxon-company-U. S. A., announced the discovery of a zinc-copper body south of Crandon. The body appears to be slightly less than 2 km long, at least 480 m deep, and about 69 a wide, Preiiminarv drilling suggests 60—million tops of ore sveraging six and one—half percent zinc and one percent copper, making this nnc of the five largest massive - �PROTEROZOIC PITCHBLENDE flIN POTENTIAL IN MINNESOTA:: THEORY 4ND SPECULATION Richard W. Ojakangas, Department of Geology,.University of Minnesota, Duluth ABSTRACT Several major Proterozbic unconformitie-s are present within the rock column of Minnesota. (1) The MPG Animikian formations (the Pokegama Quartzite and older units) rest upon LPG granitic and volcanic rocks. (2) The UPG Sioux Quartzite overlies LPG rocks in southwestern Minnesota and adjacent South Dakota and Iowa. (3) The UPG Puckwunge Formation overlies the MPG Rove Formation in northeastern Minnesota, and in adjacent Ontario the correlative Sibley Formation overlies the Rove and older Anits. (4) The UPG "Nopeming Quartzite" overlies the MPG Thomson Formation near Duluth. (5) The UPG Fond du Lac Formation rests upon the MPG Thomson Formation in east—central Minnesota. In addition, Upper Cambrian rocks of southeastern Minnesota unconformably overlie LPG, MPG, and UPG units and Cretaceous deposits overlie LPG, MPG and UPG rock units over the westernhalf of the state. Two localities which have abnormal total radioactivity may be related tc nearby unconformities. One is in the LPG McGrath Gneiss, nearly adjacent to moderately dipping unnamed MPG roék units. The second is in a probable shear zone in the MPG Thomson Formation, a few miles from the UPG Fond du Lac Formation. - At Beaverlodge, Northern Saskatchewan, the MPG Martin Formation overlies crystalline basement rocks; pitchblende veins in the area may be related to this unconformity (e.g., Langford, 1977). Further south, the UPG Athabasca Sandstone unconformably overlies a similar basement and uranium mineralization appears to be related to the unconformity. Discoveries in northern Australia are of a similar nature. No promising uranium shows have as yet been discovered in Minnesota. However, the stratigraphic—structural relationships, coupled with the supergene pitchblende vein model, callsforcIetailed exploration along the cited uncon— * formities. Refer en a e Langford, F. F., 1977, Surficial origin of North American pitchbiende and related uranium deposits: American Assoc. Petroleum Geologists Bull., v. 61, p. 28—42. 38 �PALEOMAGNETIC AND PALLOINTENSITY STUDIES OF NORMAL AND REVERSED KEWEENAWAN ROCKS IMPLICATIONS FOR THE' POLAR WANDER PATH OF NORTH AMERICA Lauri J. PeAOnen and Henry C. Halls Department of Geology, Erindale College University of Toronto, TOronto, Ontario KB S TRA c t Paleomagnetic' studies on Keweenawan rocks (1200 - 1000 my) have revealed a well—defined magnetic stratigraphy composed of units with both normal and 'reversed polarity. There are at least two polarity changes in the Keweenawan sequence of which the younger One (from reversed to normal polarity) has been detected throughout the Lake Superior region. A characteristic feature of this reversal is its asymmetry: the reversed magnetization always has a much steeper (upward), inclination than the normal (downward) one; resulting'in a difference of 300 in their paleopoles. Of particular concern in the interpretation of Keweenawan paleomagnetism is whether this asymmetry in reversal is caused by ,a secondary remagnetization component or whether it is the signature of apparent polar wander during Keweenawan igneous activity. , ' , - Detailed thermal and alternating field demagnetizaticin "studies On both igneous and baked Keweenawan rocks do not, however, reveal any systematic secondary component but rather the difference in inclination between reversed and normal rocks A possibility remains throughout the blocking temperature and coercivity spectra. exists that these demagnetization techniques are unable to detect the secondary component. If a 'non—removable' secondary component indeed is present in all Keweenawan rock units", it would result in a lower Thellier-type paleointensity still determination for the reversed rocks compared to that predicted for the normal ones. On the other hand, if the apparent polar wander 'interpretation is correct, and the Earth's magnetic field was dipolar during the Keweenawan, an enhanced paleo— field'value would be obtained for the reversed rocks because they have a significantly higher paleolatitude than do the normal ones. In order to test the credibility 'of the above models, we have conducted about luo Thellier—Thellier paleointensity measurements on Keweenawan intrusives of both polarities, and adjacent baked contact rocks from the Sibley and Rove formations. These results suggest a higher paleo— field for the reversed epoch compared to that for the normal one. Moreover if the paleofield data are reduced to the paleoequator, this difference in paleointensity between reversed and normal rocks disappears. Both paleomagnetic and paleointensity data therefore cast doubt on the hypothesis that a secondary component has caused the Keweenawan asymmetric reversal. The results, however, are 'consistent with apparent polar wander during Keweenawan tin'e. 39 �PETROLOGY AND TREND SURFACE ANALYSIS OF TWO LATE-STAGE GRANODIORITIC PLUTONS, NORTHERN LAKE OF THE WOODS REGION, ONTARIO Pilatzke, Richard H.; Karner, Frank R.; and Peterson, William M., Geolc5gy Department, University of North Dakota, Grand Forks, North Dakota 58202 Trend Surface analysis of modal data for two small plutons in the Keno±a block of the Superior Province show similar concentric patterns of mineral abundance. Alkali feldspa ith concentrated at the margins of the plutons and oligoclase in the cores. The Indian Reserve pluton Outcrops about one km northeast of Kejick at the north end of Shoal Lake at latitude 49°38'N and longitude 95°04'W. the area of exposure is about 6 km2 and has an elliptical shape about 4.0 km by 1.6 km with the major axis trending E-W. •Field study at 60 locations and point-count analysis of 30 thin sections shows that the rock is typically a pink, mediuiñgrained, hypidiomorphic granular granodiorite with minor oligoclase phenocrysts and scattered, small, greenstone xenoliths. The average composition is 50% oligoclase, 26% quartz, 13% slightly perthitic microcline, 4% biotite, 3% sericite, 2% epidote and minor opaque minerals, sphene and apatite. The oligoclase typically contains two or three, thin, euhedral.to subhedral, in€eLrnal alteration zones marked by a concentration of fine—grained sericite and epidot.e. The Dogtooth pluton. outcrops about 16 km east of Kenora at latitude 49°l''N The area studied is 4 km2 and is irregular in shape and longitude 94°l3'W. with its long axis oriented NE—SW. It appears to be a texturally distinct lobe of a larger granodioritic pluton to the east.. Field study at 115 locations and point-count analysis of 86 thin sections indicates that the rock is typically a pink, medium-grained, hypidiomorphic granular granodiorite characterized by polycrystalline quarta aggregates, protoclastically deformed oligoclase, and ve;y low total mafic mineral content. The average composition is 47% oligoclase, 26% quartz, 20% slightly perthitic microcline, 1% biotite and 4% epidote, sericite, chlorite, opaque minerals and accessories. In these rock?s oligoclase typically varies from 40% to 60% and microcline Trend surface analysis of mineral distributions shows similar from. 5% to 25%. NE-SW trends for first and second—order surfaces for oligoclase and alkali feldspar with oligoclase increasing to the SE and inward and alkali feldspar Higher degree surfaces show increasingly increasing to the NW and outward. complex, concentric patterns. Quartz surfaces show mpre irregular patterns with higher order surfaces showing marginal, alternating highs and lows. Biotite surfaces for the Indian Reserve pluton follow the pattern of. alkali feldspar surfaces. We interpret the striking concentric patterns of the feldspar distributions to be related to the cooling and crystallization histories of the plutons. The linear trends for lower order surfaces and the axes of elongation of the The. southeastward concentric patterns ref léct regional structural trends. increase of oligoèlase and the northwestward increase of alkali feldspar shown on lower order surfaces may reflect a fundamental assymmetry of the plutons or their regional tectonic framework relative to the present erosional Both plutons nay be on the, southern lint of major synclinal features. surface. 40 �.EVIDENCE FOR ARCHEAN TURBIDITE AND SUBMARINE FAN SEDIMENTATION FROM THE SAVANT LM GREENSIONE TERRAIN, N. W. ONTARIO R.J. SHEGELISKI LSKEHEAD UNIVERSfl'Y Results from an investigation of vertithliy dipping Archeah netaseditrnts in the Savant area have outlined the presence of four basic sedimentary facies: • 1. 2. 3. 4. Graded-stratified cohglomerates of submarine fan association. Graded greywacke—siltstone beds of turbidite association. Stratified-laminated mudstones of pelitic association. Laminated oxide iron formati6i of chemical association. The graded—stratified conglomerates are corrnDnly associated wfth graded greywacke—siltstone beds and form a coarse-grained l.—2. facies group. The oxide iron formation and mudstones re also associated with greywacke—siltstone sequences and form a finer—grained 2.-3.-4. facies group. The sequence of deposition of metasediments in the north arm of Savant Lake is that of coarse— grained l.—2. facies group overlain by the finer grained 2.—3.—4. facies group, thereby forming a mega—fining upward cycle. Facies Group l.—2. Detailed field mapping of this conglomerate—rich group reveals major fining—upward cycles within the group. Such features nay be indicative of fan—channel abandonment. A predominanbe of well—rounded clasts within the conglomerate suggest efficient abrasion of clasts in a shallow—water, highenergy environment, prior to final deposition, via turbid flow, in a deepwater environment. The l.-2. facies group is therefore considered to represent a portion of a submarine fan systen composed of residimented conglomeratçs and inter layered turbidites. * Facies Group 2.-3.-4. Detailed mapping indicates that the overlying greywacke—silstone and mudstone facies contain several sedimentary structures and textures of the deep—water turbidite association. The presence of interlayered iron—rich and chemical iron formation indicate extrenely quiet periods between mudstones turbidite deposition. This facies group is therefore considered to represent elastic and chemical accumulation in a portion of a restricted, deep—water turbidite basin. The interpretation that the clastic metasec.iments are coexisting proximal coarse—grained submarine fan facies and dist al finer—grained turbidite basin facies requires a deep—water environment for tIe acccmulat ion of oxide iron formation as well. This interpretation sheds dcubt upon the ccrmnn belief that Archean oxide facies iron formations are products of shallow water deposition. 41 �GEOPHYSICAL STUDIES OF PERID0TITE DIKES, YELLOW DOG PLAINS, NORThERN MICHIGAN W. Snider, Michigan Dept. of Natural Resources, Lansing, Michigan 48926, 3. S. Klasner, U.S. Geological Survey and Western Illinois University, Macomb, 61455, S. Quam, Western Illinois University, Macomb, Illinois 61455, R. Lilienthal, Michigan Dept. of Natural Resources, Lansing, Michigan 48926, and P. Geraci and A. Grosz, U.S. Geological Survey, Reston, Virginia 22092 D. Illinois ABSTRACT Very low frequency electromagnetic, gravity, and ground magnetic studies indicate that peridotite exposed in two outcrops within the Pleistocene outwash of the Yellow Dog Plains is part of a dike swarm that extends in a westnorthwest direction for about 20 km beneath the Pleistocene drift cover. Rocks at the two outcrops contain small quantities of copper— and nickel-bearing sulfide minerals and have slightly anomalous copper content. Paleomagnetic studies by Kenneth Books of the U.S. Geological Survey show that the perido- .tite has a remnant pole position typical of lower Keweenawan rocks from throughout the region. Analyses of the three types of geophysical data in sec. 11 and 12, T. 50 N., R. 29 W., where the peridotite crops out, indicate that several dikes are present. The dikes are intruded into middle Precambrian (x) metasedimentary rocks within a structural trough in lower Precambrian (W) rocks. Gravity data suggest that a steep, west-trending fault with the downdropped side to the south lies beneath the southernmost dike in secs. 11 and 12. The fault offsets the contact between lower and middle Precambrian rocks and may have been a channelway for intrusion of the dikes. Northwest-trending faults offset both the dikes and the west-trending fault. Filtered VLF-EM data combined with ground magnetic data suggest the presence of two different types of dikes. Negative VLF-EM anomalies and associated large-magnitude positive magnetic anomalies occur at the peridotite outcrops. In addition, positive VLF-EM anomalies cannot be attributed to nearsurface conductors or fault zones, and therefore suggest the presence of subsurface conductors. Gravity studies indicate the presence of dikes in the NW) 1W* sec. 12 and NW NW sec. 11, T. 50 N., R. 29 W. but no magnetic anomalies were found. Two positive VLF-flt anomalies were also found there. We believe that these are attractive exploration targets for sulfide mineralization and warrant further study. 42 �TIlE PETROLOGY AND SEDIMENTATION OF THE UPPER PRECAMBRIAN SIOUX QUARTZITE OF MINNESOTA, SOUTH DAKOTA, AND IOWA Richard E. Weber, Department of Geology, University of Minnesota, Duluth, Duluth, Minnesota 55812 ABSTRACT The Upper Precambrian Sioux Quartzite.is exposed at along several locations an east—west trend 175 miles long and 30 miles wide between Mitchell, South Dakota and New Ulm, Minnesota. It rests unconformably on Lower Precambrian rocks and is overlain by Cretáceous sediments and Pleistocene drift. The formation consists of over 1600 meters of orthoquartzite sandThe conglomerates stone with minor interbedded conglomerates and mudstones. are present in the lower two—thirds of the section and mipor thin mudstones occur in the upper third. The pebbles of the conglomerates consist of vein A coarse basal conquartz, hematitic chert, iron formation and quartzite. glomerate is exposed at New Ulth, Minnesota where it crops out 110 meters from the underlying granite. The mature orthoquartzite is composed almost exëlusively of well rounded, moderately sorted, monocrystalline quartz. Detrital chert and jasper are common in some samp1e. Grains are coated with a thin film of iron oxide and cemented by quartz overgrowths that are locally partially replaced by secondary diaspore and sericite. Rounded zircon and tourmaline. are the only common nonopaque detrital heavy minerals. Measurements of. 856 cross—beds and 491 ripple marks show paleocurrent directions to the south and southeast; no major vertical or lateral changes in trends were observed. Paleocurrent patterns are unimodal throughout most of the unit but some bimodal patterns occur in the upper part of the section. The crossbedding consists predominantly, of narrow troughs 60 to 140 cm wide and 15 to 30 cm thick. Asymmetric current ripple marks are common, but both small— and large—scale symmetrical ripple marks are also present. The abundance of crossbedding and current ripple marks indicates vigorous current action. Mudcracks and mudchip conglomerates suggest periodic exposure and fluctuating current strength. These structures may suggest in part a fluvial origin but herring—bone cross—beds and reactivation surfaces, structures commonly associated with tidal deposits, are present in a few areas in the upper third of the section. It is intruded by diabase at The Sioux Quartzite is gently folded. Corson, South Dakota. A rhyolite interbedded with the quartzite in a well at Hull, Iowa has been dated at 1470±. 50 m.y. (Lidiak, 1971). REFERENCES Lidiak, E. G., 1971, Buried Precambrian rocks of: South Dakota: Ceol. Spc. America Bull., v. 82, p. 1411—1420. 43 �SHAPE, SIZE, AND;COOLING HISTORY OF TROCTOLITIC-GABBROIC ROCKS IN THE DULUTH COMPLEX by PW. Weiblen and R.W. Cooper Data on mineral proportions and chemistry have been obtained on randomly oriented thin sections of troctolitic-gabbroic rocks along a 10 km traverse normal to the contact in the central part of the Duluth Complex in N.E. Minnesota. The data provide, new insights into the shape, size, and cooling history of individual intrusions. The spread in data at - any locality on olivine (fig. I), plagioclase, and to a lesser extent clinopyroxene may be correlated with degree of layering in the rocks. The data suggest a regular increase in mineral layering away from the contact. Data on biotite (fig. 2) sulfides, iron oxides and orthopyroxene show an expontential decrease away from the contact. These data suggest a diffusion controlled equilibration of basaltic magma with pelitic country rocks and introduction of K, H20, and S into the magma. The above data combined with geological and geophysical data on textural relations, faulting, and aeromagnetic anomalies suggest the shape and size of individual troctilitic-gabbroic intrusions as shown in fig. 3. These intrusions are distinctly asymetric and show a continuous variation betweçn. flow (region A fig. 2) and gravity (region B, fig. 2) layered rocks. HIGHWAY HIGHWAY TRAVERSE I TRAVERSE !! +\ ;L_• :i - Fig. Di%1.r. FaOI C1RC1tVM; I '•' 4+ OSTPtLC( FRO CONTPCT IKM 1. Olivine vs distance. Fig. 2. Biotite - - - vs distance. Inclusions A Curved lines - Flow pattern Blaèk areas 5 KK Fault shown is normal to the probable transform fault direction in the kidcontinent Rift.. IN Fig. 3. Three dimensional view qf proposed magma chamber for troctolitic—gabbroic intrusions in the Duluth Complex. 44- �Surf icial Sediment Analyses Offshore of the Copper—Bearing Province of Keweenaw Point, Upper Michigan C. J. Welkie, E. L. Nebrija, R. P. Meyer, Geophysical and Polar Research Center, Department of Geology & Geophysics, University of Wisconsin, Madison, WI 53706 Surficial bottom samples collected in 1974 and 1975 around Keweenaw Point were analyzed for selected trace elements and textural parameters as indicators of depositional processes following the methods of Moore and Welkie (1976). The distribution of the concentrations of Cu, Zn, Ni, Co, Mn and Fe were compared for Five Mile Point (north of the peninsula) and Bete Grise bay (south). The test of log— normality was applied (Ahrens, 1954) and the number of statistical geochemical populations for each element determined. For Cu, three statistical populations were found in both sites. Regression curves were drawn for all possible pairs of trace elements and a least—squares fit determined. The slopes of the regression lines support the contention of Smith and Moore (1972) that the sediments north of the Keweenaw represent a separate grouping of popuiations from those to the south. The contours fo copper distribution off Five Mile Point generally parallel the shore, with three areas of high concentration (295 to 175 ppm) which are uncorrelated to bathymetry. In Bete Grise, six samples containing anomalous values were found (145 to 175 ppm) and these clustered in two areas, both occurring in an elongate bathymetric low corresponding to a postulated ancient channel of the Montreal River Thus, the samples are anomalous which drains copper—bearing rocks (Goodden, 1974). according to the criteria established by Bolviken (1971), i.e., values exceeding two standard deviations from the arithmetic mean over all samples. After correlation of copper content with all other variables, multiple linear— regression analysis showed 91% of the variation in Cu at Five Mile Point could be and explained by the variables Zn, Ni, Fe, Mn, by percent 3.5 , percent 4.0 At Bete Grise, only 72% of the variation percent 4:5 $ grain size, and by bathymetry. in the copper concentrations could be explained by these variables; either the relationships between these parameters are nonlinear, or other vari4bles as yet undetermined enter into the linear model. 3.5 Kc seismic profiles and towed electrical resistivity profiling and sounding failed to correlate with the areal extent of the placer deposits as determined from physical sampling in Bete Grise, implying that the anomalous values do not continue to depth or that these geophysical techniques as applied had insufficient resolution. References: Ahrens, L. H., 1954, The lognormal distribution of the elements, Geochim. Cosmochim. Açta, 5, 49—73. Bolviken, B., 1971, A statistical approach to the problem of interpretation in geochemical prospecting, Geochemical Exploration (Boyle, Tech. Ed.),Special Vol. No. 11, Canadian Institute of Mining and Metallurgy, 564—567. Goodden, J.J., 1974, Sedimentological aspects of underwater copper exploration in Lake Superior, M.S. Thesis, University of Wisconsin, Madison, Wisconsin. Moore, J. R., and C. J. Welkie, 1976, Metal—bearing sediments of economic interest, coastal Bering Sea, Symposium Proc., Alaska Geol. Society, Recent & Ancient Sed. Envir. in Alaska, pp. K—l to K—17. Smith, P. A., and J. R. Moore, 1972, The distribution of trace metals in the surficial sediments surrounding Keweenaw Point, Upper Michigan, Sea Grant College Reprint, WIS—SG—73—341, 383—393. 45 �DELTAIC DEPOSIPS IN TITLE UPPER PECORS,ESPANOLA AND GOWGANDA FORMATIONS (HIJRONIAN) G. IC Young, D. G. F. Long. ad S. N. NcLennan Dept. of Geology, -University of Western Ontario, London, Ont. The cyclical repetitibn of mixtite, siltstone, sandstone is the hall-mark of much of the Huronian succession. Little attention has been given to the finer grained units (Pecors, Espanola and upper Gowganda Formations). This report deals mainly with the upper parts. of these units in the southern part of the Huronian outcrop belt. The upper parts of the Pecors and Gowganda Formations constitute complex coarsening upward sequences with many of the attributes of the classical prograding deltaic sequence. Both units are composed mainly of muddy and silty argillite. The prodelta deposits consist of laminated, in some cases graded siltstone-mudstone couplets, some of which may be varves. The delta slope is represented by finely interbedded mudstonés and wavy, laminated and cross laminated,siltstones. Slope instability is evidenced by The presence of abundant asymmetrical flame, and ball and pillow structures. Thin—to—thick massive units of siltstone-fine sandstone.with rip-up clasts and erosive bases are considered to have been resedimented by downslope mass movement. Clastic dykes are present in the Pecors Formation. The delta slope deposits pass rapidly upwards into fluvial(?) sandstones of the Nississagi and Lorrain Formations which appear to have been derived predominantly from the northwest. The upper Espanola Formation differs from the other two units in containing much more sandstone3 anc carbonate-rich units.In some areas the upper Espanola Formation contains abundant fining upward sequences(one to seeral metres thick) like those of both fluvial and tidal channel deposits. The interpretation is favoured because of the presence oL bimodal-bipolar(NW-SE oriented) cross bedding distributions in some units. This interpretation is important because it implies a tide-dominated and therefore marine environment in the upper Espanola Formation. This unit passes upward into the fluvial (part eolian?) sandstones of the Serpent Formation. The Pecors and upper Gowganda Formations are interpreted as prograding muddy delta deposits whereas the upper Espanola appears to have accumulated in a higher energy, tide-dominated delta platform. The reasons for this difference are not understood, but might have been caused.by greater rates of subsidence or fluvial advance in the Pecors and Gowganda than in the case. of the Espanola Formation. latter o oO 00 46 �SEDIMENTARY FACIES ASSOCIATED WITH LATE WISCONSIN GLACIAL LAKE DULUTH, WRENSI-LALL AREA, MINNESOtA. Randee Zarth, Geology Dept., University of Minnesota, Duluth, Mn, 55812 ABSTRACT Study of Late Wisconin glacial deposits southwest of Duluth suests a revised model for the late'- and postglacial history of the area. Two major sedimentary environments, are distinguishedi (1) an ice-disintegration environment and (2) a glaciolacustrine environment associated with Glacial Lake Duluth. Sediments produced by ice-disintegration are stratified, 'moderately- to poorly-sorted sand, and gravel, with clasts predominately Of Precambrian. sandstone, volcanics, gr'anite, and slate; and minor bodies of'laminated silt and clay.. Topographically, these sediments comprise a wide belt of 'kettles, kames, disintegration ridges; and outwash plains that are dissected locally by meltwater channels and' tunnel valleys, some of which contain eskers. The lacustrine environment contains the following facies: (1) thick, flat-bedded sands, (2) cross—bedded sands, (3) parallel laminated silt and clay, (k), massive clay, and (5) massive' and stratified drop stone deposits. In the nearshore environment. are found moderately—sorted and well-rounded sand grains (0.25 mm) with boulders at' the shoreline. At 305 to 31k meters in elevätion,,the sand grades rather abruptly to massive clay. The sand facies overlies the silts and clays indidating progradation into Glacial Laket Duluth by nearshóre currents. The highest strandline features occur 'at elevations near 335 meters. They are expressed primarily as beach scarps and other welldeveloped shoreline features, such as several spits and a delta. A prominent linear, northeast trending scarp between 305 and 31k meters previously considered to be a strandline, is here interpreted, to be the depositional front of. a coarse-grained shelf deposited into Glacial Lake Duluth as it stood near its highest stage (335 meters). This indicates what wa previously considered to be two stages Glacial Lake Nemadji and Glacial Lake Duluth is actually a single stage of Glacial Lake Duluth, The following, late- ard early postglacial history is" indcateds (1) Ice from the last advance of the Superior Lobe stagnated along the margin of the Lake Superior Basin, resulting in the development of an ice-disintegration complex and stratified glacial deposits. (2) Meltwater from the disintegrating ice, the retreating Superior Lobe in the basin, and from more distant upland sources, along with runoff from the hydrologic cycle, were ponded in fron.t of the retreating ice to form Glacial Lake Duluth. (3) A lake level rise to 335 meters is represented by a transgressive sequence of sediments. (k) The lake stabilized long enough to develop strong beach features. Sediment supplied, to the lake at this stage appears to have been mainly derived from the ice-disintegration complex with minor contributions 'from ice rafting, (5) Progradation of the shallbw water facies over the deep water facies was the result of sediment laden streams, meltwater, and, other runoff enterinE the lake. '(6) The lack of a regressive facies indicates a rapid drop in the lake level as a lower outlet was uncovered by the retreating ice front.. 47" �!field rs Copies of the guidebooks Department of may be obtained frOm: Geology Lakehead University Thunder Bay, Ontario PTh 5E1 Price $5.90 Canadian. Make checks payable to Lake SUperiOr Institute. �FIELD TRIP A 'COLDWELL COMPLEX LEADERS: R.H. Mitchell and R.G. Platt DATE: May 2 -4, 1977. The Coldwell Complex is a large Proterozoic alkaline igneous complex containing saturated, oversaturated, and under-saturated syenites. Visits will be made to exposures of all the major rock types found within the complex and to areas which illustrate the relationships between the magma types and the mechanisms of intrusion of the. complex. 1. Depart Thunder Bay on Monday May 2 at4:00 p.m. Return Thunder Bay Wednesday May 4 by 5:00p.m. All 'day Tuesday May .3 and the morning of. Wednesday May 4 will be spent examining the complex. 2. The cost is $7O.O0. and includes: a) 2 nights accomodation (double) at Marathon. b) Transportation to and from Thunder Bay and during the. excursion. c) Guidebook. Maximum costs for meals Cost does not include meals. in Marathon are about $12.00 per day,. 3. Accomodation will be in rñotels in Marathon (with restaurants). Costs are based upon double occupancy of motel units. Persons requiring single occupancy must notify the organizers in advance and be prepared to pay $10.00 extra. 4. Limited to a maximum of 45 persons. 51 �FIELD TRIP PROTEROZOIC ROCKS OF THE THUNDER BAY AREA LEADERS: K.G. Fenwick, C.R. Kustra, W.H. Mcllwalne, J.F. Scott. DATE: May 3: and 4, 1977. A two day field trip will cover the Proterozoic (Middle to Lake Precambrian) rocks of the Thunder Bay area. Day one will cover selected stratigraphic units of lower and upper members of the Gunflint Formation and the overlying Rove Formation. On the second day, outcrops of the Sibley Group will be examined. The stops are designed to illustrate the stratigraphic relationships of the three fold division of the Sibley Group into formations. Side trips to Ouimet Canyon and the Thunder Bay Amethyst Mine are also planned. leave the Airlane Motor Hotel, Thunder Bay, on Tuesday May 3rd and on Wednesday May 4th at 8:00 a.m. The bus will return each day by late afternoon. 1. Field Trip B will, 2. The costs for participants is $4O.OQper person. This fee includes bus transportaion, lunch each day, literature, and guide to field stops1. It does not include lodging. 3. Limited to a maximum of 45persons. 52 �FIEL,D TRIP C STURGEON. LAKE LEADERS: W. Gibh, P. Severin, A. Tarnman, H. Poulsen, J. Franklin. DATE: May 6 - 8, 1977. A one ay field trip to the Sturgeon Lake area will include the examination Qf two open-pit mines (Mattabi and Sturgeon Lake Mines Ltd.) and outcrops representative of the volcanic stratigraphy of the lower portion of the pile.. The Mattabi and Sturgeon Lake Mines are typical volcanogenic massive sulphide deposits. Tour stops within the mines will include an examination of both massive and stringer ore and various types of alteration The associated with the footwall stringer suiphides. regional stops will examine a variety of felsic and mafic pyroclastic, flow,, and epiqlastic rocks,. and two subvolcanic intrusive bodies. 1. Participants will depart by bus from Thunder Bay at A approximately.6:30 p.rp. on Friday, May 6. discussion period, will be held in Ignace that evening. As the tour will be rather lengthy, participants will stay in Ignacethe evening of Saturday, May 7. Buses will reach Thunder Bay and Dryden on Sunday, 'May 8 in order to connect with mid-day planes. 2.. A fee of $75.00 will include transportation, accomodatjon,, meals, and guidebook. 3. Limited to a maximum of 45 persons. 53 � https://digitalcollections.lakeheadu.ca/files/original/b2bf913cd5f79f3b89a1b7891165c6b1.pdf c40ef197359f1bb47e247efcc376edec PDF Text Text a I*eHty Third Alinoal MeeliHg Thunder ilay, Ontario Ii r Institute on Lake Superior Geology S I I I I. I I I I I I P.! I Li 1 COLD WELL TRIP COLDWELL U �r I I FIELD GUIDE GUIDE TO TO ASPECTS ASPECTS OF OF THE THE FIELD I GEOLOGY OF OF THE THE COLOWELL COLDWELL ALKALINE ALKALINE COMPLEX COMPLEX GEOLOGY I I ROGER ROGER H. MITCHELL AND R. GARTH PLATT H. MITCHELL AND R. GARTH PLATT I I Department of Geology Lakehead University Thunder Bay I I I I I I I I I Twenty Third Third Annual Annual Meeting, Meeting, Institute Institute on Lake Twenty on Lake Superior Geology, Marathon, May 1977 Superior Geology, Marathon, May 1977 �THE COLDWELL ALKALINE CuwLEX COMPLEX na LULIDWELL ". I I I I I I I I I I I I I I I The Coldwell complex located 1 o - y is 410c-~-d on the north shore shore Superior between the Pie of Lake Superior Pic and and Little Little Pie Pic Rivers. Rivers. The community The community of Marathon rathon is is located located on the the eastern eastern side side of the the complex. complex. This circular complex with a diameter of 25 25 km. of km. is the largest alkaline alkaline intrusion intrusion in in North North America and is is unusual in that oversaturated, oversaturated, under— undersaturated and saturated magmatism magmatism is saturated is present. present. The alkaline (1000 m.y. ) The alkaline rocks rocks are areof ofNeohelikian Neohelikianage. age(1000 m.y.) are emplaced in Archean rocks of the Superior Province and are emplaced in Archean rocks of the Superior Province of Canadian Shield, f the Canadian Shield, which in this area form form an north— essentially east—west trending ssentially east-west trending greenstone greenstone belt. belt. A northeasterly bifurcation of this belt originates originates in the easterly Marathon area area and it is is at this this point that the the alkaline alkaline rocks have have been The Archean rocks, rocks, which rocks been emplaced. emplaced. The include basic and acidic volcanics and greywackes have been metamorphosed to greenschist and amphibolite amphibolite grade, grad been subjected to at least two periods of folding and intruded by Archean granites Little is granites and and syenites. syenites. Little is known of the the Archean geology although although some some information information known can be found in Puskas Puskas (1967) (1967) Milne Milne (1967), (1967), Walker Walker (1967), (1967)., Ayres et al. (1970), (1970), Thompson Thompson (1931), (1931), and Einarsson (1972). (1972). A general geological map of the complex, complex, together together with an aeromagnetic map is figures lA with is given given in infigures 1A and and lB. IB. The geological (1967), geological map is is based on the the work of Puskas Puskas (1967), together with our own observations observations and re—interpretation together re-interpretation of the sequence sequence of igneous It should should be be noted igneous events. events. It that simplication of the geology of hat figure 1A is an over simplication the area. area. In detail, detail, relationships relationships are the are extremely extremely mapping, coupled with complicated and very detailed mapping, extensive mineralogical mineralogical studies, is required required before before studiesis extensive anything approaching an accurate geological map can be nything appr produced. reduced. S- �-2- I - 2 - P I I1 II I I Ij I I I I II I I I I I The structure The structure of of the the complex complex is is poorly poorly known because of insufficient insufficient geophysical geophysical and and structural structural observations. observations. Puskas Puskas (1967) believed that the the complex complex was was aa lopolith lopolith but our recent work does does not support this this concept of a single differentiated intrusion. single differentiated intrusion. Lilley (1964) (1964) considers conside that the bulk of the intrusion intrusion is a funnel funnel shaped shaped body of gabbro gabhro and ferroaugite of ferroaugite syenite syenite which has been intruded intruded by nepheline nepheline syenites. syenites. Our recent studies studies indicate indicate that that several present, and that an several centers centers of intrusion intrusion may be present, an area Pic River, area bounded by the the Little Little Pic River, Redsucker Cove Cove and Geordie Geordie Lake Lake may be be aa downfaulted downfaulted block. block. Rocks Rocks within within this area are characterized characterized by by the occurrenceof occurrerueof multiple this breccias breccias and and metasomatism metasomatism and and may represent represent rocks rocks which which were were originally originally close close to to the the roof roof of of the the complex. complex. Rocks Rocks of the the eastern eastern portion of the the intrusion intrusion are are in in contrast contrast less ss complex complex and and relatively relatively xenolith xenolith free. free. Petrologically we have Petrologically have recognized recognized three three distinct distinct intrusive magmatic episodes, episodes, each being characterized trusive magmatic characterized by In order differentiationtrend. by a distinct differentiationtrend. order of of^ intrusion these are: intrusion these are: CENTER 11 CENTER -— Saturated Saturated rocks with peralkaline alkaline rocks peralkaline oversaturated residua. oversaturated residua. CENTER Miascitic alkaline alkaline rocks rocks with with under— un CENTER 22 -- Miascitic saturated saturated residua. residua. CENTER 3 - Alkaline rocks with oversaturated rsaturate residua. - . ., Gabbro,, ferroaugite ferroaugite syenite I CENTER CENTER 1 - I I I I I I I The The oldest unit of the the complex comulex is is represented represented by by eastern border These rocks the eastern border gabbros gabbros (figure (figure lÀ). 1A). These rocks are Igneous layering layering are intruded intruded by ferroaugite ferroaugite syenites. syenites. Igneous Several centers is characteristic characteristic of of both both units. units. Several centers of o intrusion may be present in in the the ferroaugite ferroaugite syenites syenite , . �r —3— I I 1 I I I 1 I I I I I I I which typically typically exhibit exhibit extreme extreme iron iron enrichment enrichment and and , Characteristic differentiate to quartz differentiate quartz bearing bearing residua. residua. Characteristic minerals of minerals of the the ferroaugite ferroaugite syenite syenite are are fayalite, fayalite, ferroaugite, ferroaugite, ferrorichterite, ferrorichterite, ferroedenite ferroedenite and aenigmatite. aenigmatite. CENTER 2 Biotite gabbro, gabbro, nepheline and natrolite syenites 2 —- Biotite syenites CENTER outcrops is an arcuate Alkaline biotite gabbro outcrops arcuate ring pattern on the Coldwell Penninsula Penninsula and we believe believe that this this together with nepheline syenite syenite defines defines an undersaturated undersaturated nepheline intrusive center center (figure (figure lA). 1A). AA second intrusion intrusion of ne~heline Nepheline syenites syenite may be located syenite located on on Pic Pic Island. Island. Nepheline syenites are are characterised by characterised by moderate moderate iron iron enrichment, enrichment,alurninous aluminous amphiholes amphiboles and acmitic acmitic pyroxenes. pyroxenes. Titanium in these rocks enters amphibole and pyroxenes rather than forming forming aenigmatite aenigmatite as in Center Center 1. 1. of Centers Centers 11 and and The distinctly differentiation differentiation trends trends of 2 are well illustrated illustrated by the trends trends in pyroxene compositions compositions Platt, 1977) illustrated below. illustrated below. (Mitchell and Platt, 1977) ACM ITE ACMITE DIOPSIDE HEDENBERGITE �a -4— ifi I Figure - lÀ - Geological Geological map of thq the Coldwll Coidwell alkaline Department of complex based upon Ontario Department Preliminary Map P114 Mines Preliminary P114 (Puskas (Puskas 1967) 1967) together ogether with our own observations and re—interpretation of the sequence of re-interpretation igneous events. igneous events. 1 ' 1 Figure alkaline Based upon Ontario Department of Mines Aeromagnetic Maps 2146G, 2147G, 2156G, 2157G. I I 1 1 I I I I I I I I I - lB - Magnetic Magnetic expression of the Coldwell r complex. �+ 0 z In -4 C-) + �GEOLOGICAL S— MAP —a I 2 4 I 3 6 4 6 Gabbro Ferroaugite Syenite Biotite-Gabbro +-LIIIJ Acid Metavolcanics & Metasediments LZ1II Basic Vojcanjcs & Metcisediments [±IEI Ultrabasjc Intrusives LII1 Granite Gneisses 1IIIJ Basic Xenoljths (metavolcanics) PAA Nepheline Syenite Syenite - Syenodiorife 2 MILES 8 KILQMETRES 5 ++ Granite ,Quartz - Syerüte, Hybrid Syenites I 2 LEGEND o o MARATHON AREA COLDWELL COMPLEX a VICINITY _____ ______ LAKE SUPERIOR �-7- 1 1 S N 1 I ' CENTER 3 - Syenite, quartz syenites thewestern westernportion portion of ofthe thecomplex complexare arefound founda a InInthe widevariety varietyofofsyenites, syenites,quartz quartzsyenites syenitesand and granitic granitic wide The quartz rockswhose whosepetrology petrologyisispoorly poorlyknown. known. The quartz rocks syenites syeniteshave havebeen beenfound foundto to intrude intrudeall allearlier earlierrocks rocks by an Theserocksare arecharacterized characterized by an Center2.2. These.rOcks ofofCenter abundanceof ofzircon, zircon,paucity paucity of ofpyroxene, arfvedsonitic abundance amphiboles, amphiboles,fluorite fluori and quartz. MINORINTRUSIONS INTRUSIONS MINOR Theplutonic plutonicrocks are cut by two groups of minor The intrusions. (a) (a)diatremes diatremes (b) (b) dikes dikes intrusions. DIATREMES Threediatreifles diatremesare areknown knownin inthe theColdwell Coldwellregion, region Three I oneof ofwhich whichcuts cutsthe the intrusive intrusiverocks rocksof of the the only y one Thisdiatreme diatremelocated locatedon on the the west west side sideof ofthe t complex. This complex. ColdwellPenninsula Penninsulacontains containshornfelsed hornfelsedmetasediments metasediment Coidwell I I I and and inclusions inclusionsof ofCenter Center 22rocks rocksas asxenoliths xenoliths (Balint (Balint rocks, such as Diatremesin inthe theArchean Archean rocks, such.asthe the 1977). Diatremes 1977). be Deadhorseand andMcKellar McKellarCreek Creekdiatremes, diatremes,may may be Deadhorse Coldwell rocks have contemporaneous, yet been been contemporaneous,but but no no Coldwell rocks haveyet amongtheir theirxenolith xenolithsuites. suites. found among DIKE ROCKS s I widevariety varietyofof dike dike rocks rockscut cutthe thecomplex complexand and AA wide of These dikes In order surroundingcountry countryrocks. roc surrounding I vedabundance abundanceare:— are: observed 1 1 I 1) ocellular �--S C 2) analcite tinguaites 1 3) porphyritic (Al—Cr—cpx) lamprophyres 4) 4) glomeroporphyritic glomeroporphyritic and and alkali alkali basaltic basaltic dikes dikes (? Pukasaw swarm) ( ? Pukasaw swarm) . I ' 5) porphyritic (resorbed 5) (resorbed quartz) quartz) lamprophyres lamprophyres 6) nepheline nepheline syenite 6) syenite 7) rhyolitic 7) rhyolitic dikes dikes 8) syenites with a 8 ) syenites a high organic organic content content TECTONIC TECTONIC SETTING SETTING I I I I I I I I The complex The complex is is aa part part of of the the Keweenawan Keweenawan igneous igneous activity activity centered centered around around Lake Lake Superior Superior which which includes includ the Keweenawan basalts, basalts, the Duluth Complex Complex and the the the Logan sills. summary of the the regional regional geology and and Logan sills. A summary tectonic framework is 2. The complex complex tectonic framework is given given in in figure figure 2. is located at the Thinge 'hinge point' of of two two belts belts of of essentially tholeiitic essentially tholeiitic volcanics, volcanlcs, i.e. i.e. the the North North Shore— ShoreOsler volcanics and the Mamainse-Michipicoten Mamainse—Michipicoten volcanics Osier north—south and is itself the southern most member of a north-south trending belt of belt of of trending of alkaline alkaline intrusions. intrusions. A belt alkaline intrusions, alkaline intrusions, some some being being contemporaneous contemporaneous with with the Coldwell Coidwell Complex, Complex, is found along the "Kapuskasing the "Kapuskasing High" but no no petrological or or tectonic tectonic connection connection between these two The tectonic two belts belts is is known known to to exist. exist. The setting and type setting type of of igneous igneous activity activity is is similar similar to to that that found in the Kangerdlugssqaq Kangerdlugssuaq area of East Greenland and the Gregory-Kavirondo Gregory—Kavirondo Rifts the Rifts of of East East Africa. Africa. Both Both of of these areas these areas have been considered considered to to be be the the sites sites of of plume plume generated generated triple triple junctions, junctions, the the alkaline alkaline rocks rocks being associated with the failed ed arm of the spreading center. I I I �1 V I I I i I I 1 I I I I I I I I I I Figure Figure 22 -- Tectonic Tectonic setting setting of of the the Coidwell Coldwell alkaline alkaline from data data given given by by Card Card complex, complied compiled from complex, et et al a1 (1972), (1972). Currie Currie (1976), (1976), Gittins Gittins et et a1 (1967), (1967). Halls Halls and and West West (1971). (1971). Alkaline Alkaline complexes complexes and and carbonatites carbonatites are are designated designated ** and and their their radiometric radiometric ages ages (mostly (mostly K—Ar) K-Ar) are of years. are given given in in millions mill years. ~, �I I! 1 .m A ARCHEAN ARCHEAN 0 50 50 1do 00 150 ISO 00 200 KILOMETRES 200 KILOMETRES 150 MILES Ar, A SOUTH RANGE TRAPS KEWEENAWAN BASIN STRUCTURALAXIS AXIS OF OF STRUCTURAL MAJOR KEWEENAWAN MAmR KEWEENAWAN INTRUSIVES I EARLY EARLY PRECAMBRiAN PRECAMBRIAN UNCONFORMITY UNCONFORMITY A - LOWER KEWEENAWAN LOWER KEWEENAWAN UNCONFORMITY UNCONFORMITY MIDDLE PRECAMBRIAN t MIDDLE KEWEENAWAN LL.. CAMBRIAN CAMBRIAN UPPER KEWEENAWAN UPPER KEWEENAWAN U. U. KEWEENAWAN KEWEENAWAN SILURIAN SILURIAN - U. U. CAMBRIAN CAMBRIAN UNCONFORMITY UNCONFORMITY PRECAMBRIAN f LATE PALEOZOIC - A P LEGEND _7_ Ar NIPIGON NIflGON PLATE PLATE MICHIGAN PUCKASAW :/ / I / j Seabrook Lake 1100 Lackner Lake 1090 SAULT STE. MARIE Mamainse Car gantua F. Ar 1/ " Nemegosenda 1010 Portage 1090 /1 'i/la ía' Is I0Io*I Goldray 1695 Valentine Tp** 4Herman Lake.) Borden * Arg2655 * Sextant Rapids * Teetzel Tp. 1155 GRAVITY HIGH KAPUSKASING CargilI 1740* Clay — Howel Firesand 1048/ lola Lake 1185 1000 Coldwell 1 000 * Chipman Lake see �r I STOP I I ' I I I I I I I 1 Two EXPLOSION DIATREME - DEADHOBSE CREEK small explosion diatremes (Deadhorse Creek & McKeller Creek) are located in the Arehean greenstone belt just to the west of the Coldwell alkaline complex. A third subcircular diatreme (The Neys Diatreme) cuts rocks of the Coidwell complex. Located on the west side of the Coldwell Peninsula, this latter diatreme has been studied by Balint (1977). Neither diatreme in the greenstone belt has been studied in detail. This stop examines the small diatrerne exposed on the Ministry of Natural Resources access road which parallels Deadhorse Creek. Here the diatreme cuts Archean metavolcanics and pyroclastics. cross— The matrix of the diatreme, when unweathered, is dark green in colour and consists of carbonate and a greenish amphibole. Embedded in this are clasts of varying size and angularity. By far the most prominent are fragments from the greenstone belt. Of regional geological interest are occassional clasts of orthoquartzite. Similar clasts, together with red— purple shales, are found in greater abundance in the McKeller Creek diatreme. These clasts closely resemble rocks formed extensively in the paleohelikian Sibley l I I I I Group. Until now, the most easterly extension of this group of rocks was thought to he some forty miles to the west in the vicinity of Rossport. �p —12— I I 1 I I 1 I 1 I I I I I I I I I Fragments similar in appearance to certain felsic Fngments similar in appearance to certain felsic porphyries of the Keweenawan Osier volcanic rocks are porphyries of the Keweenawan Osler volcanic rocks are present. This may indfcate an easterly extension of present. This may indicate an easterly extension of Keweenawan volcanism, although the seeming total lack Keweenawan volcanism, although the seeming total lack Keweenawan basaltic rocks makes this assumption ofof Keweenawan basaltic rocks makes this assumption problematical. problematical �I I I I 1 ' I I I I I I I I I I I I -22 STOP STOP WESTERN MARGIN MARGIN OF OP THE WESTERN THE ALKALINE ALKALINECOMPLEX COMPLEX This stop investigates This investigates the the complexities complexities of of the the western stern contact region of the Coldwell Complex as exposed in the outcrops outcrops and road cuts exposed cuts adjacent adjacent to to Hwy. 17. region, the border intrusive Hwy. 17. In this region, intrusive rocks rocks of the complex are in contact with folded Archean of metasediments. metasediments. The rocks of the intrusion exposed in this region The are extremely extremely varied, varied, ranging ranging from from ultramafic ultramafic cumulates, cumulates, olivine gabbros olivine gabbros and syenodiorites syenodiorites to to nepheline nepheline syenites, syenites, quartz pegmatites with with and without without uartz syenites and syenitic pegmatites natrolite. atrolite. Later diabasic and lamprophyric dykes also cross ross cut the the region. region. The interrelationships interrelationships between these these various rock rock types ypes is still somewhat problematical as as is is their their exact exact relationship to the intrusive history of the complex in general. Webelieve, We believe, however, however, that the geographic general. relationships of the major intrusive relationships intrusive phases of the the contact contact zone are at least in in part fault fault controlled. controlled. It convenient, for the purposes of this stop, stop, to It is convenient, body of the intrusion traverse the contact zone from the body towards the purposes, we out towards out the contact. contact. For descriptive purposes, will consider consider the the rocks rocks exposed exposed in in three three major major zones. zones. These are outlined outlined as the accompanying sketch map map and These described below: below: described Zone 1 The main Coldwell Coldwell rock of this zone The zone is is a banded syenodiorite consisting consisting of subequal amounts of oligoclase syenodiorite and alkali feldspars, feldspars, the latter showing incipient exsolution. Apatite is ubiquitous and the mafic minerals minerals exsolution. consist of ferroaugite, ferroaugite, fayalitic olivine olivine and exsolved' exsolved consist ilmeno—magnetite. Thick ultramafic bands develop by the ilmeno-magnetite. �r I I I I I I I I I 1 I I I I I accumulation minerals, particularly particularly the accumulation of the mafic minerals, ilmeno-magnetite. ilmeno-magnetite. wide Cross cutting the syenodiorite is a relatively wide Cross syenite, most probably dyke or sheet of nepheline syenite, associated with with Center 2. Alkalic feldspar feldspar is is the the preassociated Center 2. dominant felsic phase with nepheline occurring in the interlath regions. interlath regions. Green (acmitic) (acmitic) pyroxene pyroxene is is the the major major mafic phase, while opaque minerals and accessory fluorite mafic phase, fluorite make up the the remaining remaining mineral mineral phases. phases. Cutting both the syenodiorite Cutting syenodiorite and the nepheline nepheline syenite is syenite is a coarse—grained coarse-grained natrolite natrolite syenite syenite pegmatite. pegmatite. In this, this, the natrolite In natrolite is is seen seen as as large large reddish reddish patches. patches. At least two thin lamprophyres intrude the rocks of of At this zone. zone. this These zone These zone 11 rocks rocks have not been recognized along along the the coastal section of the contact region lying lying some some 1 1 mile Here gabbros the south south of our to the to our present present location. location. Here gabbros of of with ferroaugite ferroaugite syenites, syenites, as shown zone 2 are in contact with the geological, map of the margin of themargin of the on the peolWica1 map ofsouthwestern the southwestern complex below below (Aubut (Aubut 1977). 1977). Zone 2 A zone of banded olivine gabbro intruded by syenite natrolite—bearing syenitic and natrolite-bearing syenitic pegmatites. pegmatites. The gabbros show considerable evidence of textural The plagioclase mineralogical readjustment. and mineralogical readjustment. Invariably the plagiocla crystals have been granulated and recrystallized giving crystals olivines rise to microscopic 'augen'—like rise 'augenl-liketextures. textures. The divines by coronas of of amphibole and are commonly surrounded by are mica and in many instances instances the original olivine olivine is is now represented by somewhat rounded replacement zones of 1 I �-15- fl I I green-blue green-blue amphibole amphibole and and pale pale green—brown green-brown mica. mica. Thin Thin , microscopic microscopic shear shear zones zones cross cross cut cut the the gabbro. gabbro. I I I I I I Again, later Again, later lamprophyric lamprophyric dykes dykes intrude intrude the the main main Coidwell Coldwell intrusive intrusive rocks. rocks. Along the Along the coastal coastal section section lying lying to to the the south, south, this this combination combination of olivine olivine gabbro gabbro intruded intruded by syenitic syenitic pegmatites pegmatites can can also also be be identified. identified. Here Here the the gabbros gabbros are somewhat somewhat coarser coarser than than those those seen seen on on the the highway. highway. The The olivines divines in in general are are fresh, fresh, although the the plagioclase crystals crystals still still show show considerable considerable evidence evidence of of readjustment. readjustment. On On the the coast, coast, the the gabbros gabbros are are in in direct direct intrusive intrusive contact contact with Archean metasediments and often often contain contain inclusions inclusions of the the latter. latter. (Aubut, (Aubut, 1977) 1977) The The Highway section section however, however, shows shows aa third third zone zone of of rocks rocks lying lying between between the the gabbro—pegmatite gabbro-pegmatite grouping grouping and and the the Archean Archean country countrv rocks. rocks. Zone 3 I This This zone zone consists consists of of quartz quartz syenite syenite which which is is often often seen seen intruding intruding aa 'hybrid' 'hybrid' rock rock of of overall overall syenitic syenitic mineralogy. mineralogy. I The The quartz quartz syenite syenite is is yellowish yellowish to to pink pink in in colour colour and and consists consists predominantly predominantly of of perthite perthite with with interstitial interstitial quartz quartz and and minor proportions proportions of of biotite, biotite, amphibole, amphibole, zircon zircon and and fluorite. fluorite. As yet, yet, we we do do not know know if if there there is is more I than than one one generation generation of of quartz quartz syenite. syenite. Thin Thin veins veins and and dykelets dykelets are are seen seen invading invading the the country country rock. rock. These These syenites 3. syenites have have been been ascribed ascribed to to Center Center 3. The The colour colour of of the the 'hybrid' 'hybrid' syenite syenite of of this this zone zone is is I I I I generally generally purple—brown, purple-brown, although although this this varies, varies, as as does does the degree to to which which it it is is invaded invaded by by the the quartz quartz syenite. syenite. �— 16 . I I I I I I 1 I I I I I I . - . . This 'hybrid' 'hybrid' syenite syenite is Us This .. . is of problematic problematic origin. origin. Lts:.. ..-$< . , , ' mineralogy is is syenitic syenitic consisting consisting predominantly of perthitic alkali alkali feldspars. feldspars. The large large red—pink red-pink alkali alkali invariably feldspar crystals crystals visible in hand specimens specimens invariably have remnant cores have cores of of plagioclase. plagioclase. The visible mafic spots, common spots, common throughout throughout the the rock, rock, consist consist of of biotite biotite and/or amphibole. Texturally the and/or amphibole. the rock rock is is hornielsic. hornfelsic. No later dykes dykes are are seen seen to to cut cut this this zone. zone. of quartz quartz syenitelhybrid syenite/hybrid We believe that this zone of syenite is in fault contact with the syenite the banded gabbro— gabbropegmatite complex pegmatite complex of of zone dbne 2. 2. We also feel that the the quartz quartz syenite rocks represent a higher structural structural level of the intrusion intrusion and that that the the hybrid syenite syenite represents aa block highlymetasomatised metasomatised country country rock. rock. represents block of ofhighly �I-> I I I S Xs S S I I I I I I I I 1 I I I I 000 0000 000 Syenodiorite iiIlllllIIIii' Nepheline Syenite / I -\ , \\'i../V '-1 I Olivine Gabbro ((banded) " J. Olivine *' Gabbro ban 'I ; Xxxx X X Syenite xx xx Quartz Syenite ¥  •Â0•.•• •0•5 •5 0 ''Hybrid Hybrid Syenite' syenitel -------- - Archean Sediments --- Archean Sediments Dyke(s) D NP NP Natrolife Pegmatite Natrolite Pegmatite (s) sP SP Syenite Pegmafite (s) Syenite Pegmatite ($1 w 'v\/- '\/\- Fault Fault o0 400 400 800 800 2000 FEET 1200 200 1600 600 2000 FEET M 600 METRES 600 METRES 400 200 0 I I I • I I Western Central Stop 2. Stop 2. Western Central Region. Region r %______ �-- I unsubdivided {b) -—--l porphyritic (Al—Cr Augite) (c) 2--—--2 acellular (a) == 4 6--—-6 layering (in intrusive racks bedding laps unknawn (inclined breccia zone (abundant xenoliths) sa L o ke Superior 2.. a SOUTHWEST MARGIN of the COLDWELL COMPLEX Sand and Gravel Pyraxene Horntels (including xenaliths Gabbra Augite Syenite Red Syenile Pegrnatite calloidal residua gabbro Cd) 3—3 parphyritic (resarbed quartz) 4--—--4 porphyritic (feldspar) 5--—-5 lamprophyre DYKES —- _____ ' iaI 23 _ a ac after A. Aubut Z.3 2(0 50 90° 1977 sa 200 METRES . an ;_ a us �I STOP STOP 3 LITTLE LITTLE PlC PTC LOOKOUT LOOKOUT 3 I j Parking Parking Lot Lot To seen cliffs cliffs of of xenolith xenolith free free To the the southwest southwest can can he be seen I .. $ I ferroaugite ferroaugite syenite syenite along along the the west west bank bank of of the the Little Little Plc Pic River. River. The The river river probably probably occupies occupies aa fault fault zone, zone, the down faulted faulted block block of of Center Center -- 22 the east east bank bank being being aa down and and 33 rocks rocks from from higher higher levels levels of of the the intrusion. intrusion. To To the the south south lies lies the the Coldwell Coldwell Penninsula Penninsula and and Pie Pic Island. Island. Densely Densely wooded wooded shores shores are are alkali alkali gabbro gabbro and and nepheline nepheline syenite. syenite. The The distant distant barren barren shores shores are are syenite syenite and and quartz quartz syenites. syenites. I I I I I I I I I I I Highway Highway Cuts Cuts The The highway cuts cuts on on the the north north side side of of highway highway 17 17 provide provide excellent excellent examples examples of of the the complex complex multiple multiple igneous whiah are aTe characteristic characteristic of of the the Little Little igneous breccias breccias which Pie Pic - Redsucker Redsucker Cove Cove block. block. The The oldest oldest breccias breccias are are of of - Center rocks, alkali alkali gabbro gabbro and and nepheline nepheline syenites syenites Center 2 rocks, similar similar to to those those exposed exposed on on the the West West side side of of the the These breccias Coidwell Coldwell Penninsula. Penninsula. These breccia* are are found found as a large large xenoliths xenoliths in in the the later later Center Center 33 quartz quartz syenite syenite Xenoliths Xenoliths in in the the quartz quartz syenite syenite are are oligoclase oligocla basalts basalts showing showing all all stages stages of of assimilation assimilation from from relatively sericitized basalt basalt to to almost almost relatively unaltered unaltered sericitized completely completely digested digested xenoliths xenoliths of of amphibolite amphibolite mineralogy. mineralogy. Development Development of of "clots" r'clots''of of biotite biotite and and amphibole amphibole is is aa breccias. breccias. characteristic characteristic metasomatic metasomatic feature feature of of the the xenoliths. xenoliths. The The oligoclase oligoclase basalts basalts probably probably are are remnants remnants of of Proterozoic Proterozoic extrusives extrusives which which originally originally capped capped the the complex. complex. These outcrops These outcrops demonstrate demonstrate conclusively conclusively that that Center Center 3 quartz syenites syenites are 3 quartz are younger younger than than Center Center 22 undersaturated undersaturated �g I ii I I rocks. rocks. , Two types types of of lamprophyre l a m p r o p h y ~ ecan can be be found found crosscutting crosscutting Two the breccias. the (a) porphyritic porphyritic lamprophyre, lamprophyre, characterized characterized by by (a) greenish phenocrysts phenocrysts of of Al—Cr Al-Cr augite, augite, possibly possibly greenish of high high pressure pressure origin. origin. of (b) ocellular ocellular lamprophyre, lamprophyre, characterized characterized by by ocelli ocelli (b) I 1 I I I I I I 1 I I I I I of carbonate, carbonate, quartz quartz and and fluorite. fluorite. of , �r I I I A A hi, I, I I hi p S p. I I I I Red quartz syenite with angular to rounded xenoliths of oligoclase basalt. Xenoliths show all stages of assimilation from sericitizecj basalt to almost completely digested ,9host xenoliths of amphibolite. Prominent biotite — amphibole clots" of metasomatic origin. Nepheline syenite xenoliths. A p. Large xenolith of Centre 2 rocks, biotite gabbo Large xenolith of Centre 2 rocks, vetned by nepheline syenites, cutbiotite by pegmatitic gabbro veined by nepheline syenites, cut by pegmatitic notrolite syenite dikes. In thin section biotite natrolite syenite In thincorono section biotite gabbro showsdikes. extensie structures. •'55 . gabbro shows extensive corona structures. • •• .S . ' doo_/ a Centre 2 rocks veined by quartz syenite. <0 I. I A I I I I I I I © 4\ ' 4 Red to ye1 low quartz Red to yellow quartz syenite breccia, syenite breccia. ow Porphyritic lamprophyre. Alurninous - Porphyritic lamprophyre. Aluminous in a cpq cpx phemcrysts biotite cpx phenocrysts feldspar matrix. in a cpx — biotite — feldspar matrix. - Ocellular lompophyres. Ocel li ore corbonoteOcellular lamprophyres. Ocelli are carbonate— quartz fluorite, matrix is amphibole, cpx , quartz — fluorite, matrix is amphibole, cpx, feldspar, biotite. feldspar, biotite. - 0 50, 100 200 FEET 150 I- 0 25 50 METRES Stop 3.3. Little Little Plc Pic River River Lookout. Lookout. Stop \ quartz syenites quartz syenites �STOP STOP 44 - - BRECCIA BRECCI DIKES I I I I I I I I I I I I I I At At this this locality locality are are found found several several natrolite natrolite syenite These dikes dikes are are of of variable variable syenite breccia breccia dikes. dikes. These thickness, thickness, they pinch and and swell swell and and terminate terminate in in thin thin natrolite natrolite syenite syenite veins. veins. Bifuroations Bifurcations of of the the dikes dikes are are common. common. The The outcrop outcrop of the the dikes dikes are are sinuous sinuous and give impreesion that they may have been em— emgive the the impression placed placed in in relatively relatively plastic plastic host host rocks. rocks. The The thicker thicker portions of the the dikes dikes are are crowded crowded with dark grey xenoliths xenoliths set set in in aa fine fine grained grained reddish reddish natrolite natrolite syenite. syenite. As the the dikes dikes thin the amount of xenolith decreases decreases and the terminating veins are composed composed of xenolith coarse grained xenolith free coarse grained natrolite natrolite syenite. syenite. The The xenoliths xenoliths are are rounded rounded to to very very irregular irregular in in shape. shape. Crenulated margins Crenulated margins are are typical. typical. No angular angular xenoliths xenoliths are present although the wedging action of the syenite syenite on comonly on the xenolith causing causing further further fragmentation fragmentation is commonly visible. visible. The The shape shape of ~f the the xenoliths xenoliths is is considered considered to to be the result result of brecciation and corrosion both in situ situ and and during during transport. transport. The The xenoliths xenoliths are are of of two two types, types, the the most abundant abundant being being a fine fine grained dark grey rock which in thin section section is seen of amphibole and mica, mica, alkali composedo€amphibo alkali seen to to be be composed sericitized feldspar feldspar and plagioclase. plagioclase. Rare relict sericitized phenocrysts The xenolith xenolith phenocrysts of of plagioclase plagioclase are are present. present. The margins margins are enriched in amphibole and mica relative relative to the 5 mm. in the interior. interior. Rounded aggregates of mica up to 5 diameter diameter are are common. common. Although the the xenoliths xenoliths have have been been bear a resemblance extensively extensively metasomatized they be+r resemblance to the the western end metavolcanic xenoliths metavolcanic xenoliths seen seen at at Stop Stop 3. 3. At the western of of the the outcrop outcrop occur xenoliths which consists consists of rounded aggregates of greenish mica set in a matrix of pale green aggregates pyroxene clinopyroxene and clinopyroxene and minor minor oligoclase. oligoclase. Rare euhedral pyroxen I I -22- �1 -23- I phenocrysts are are found. found. The The mica mica "rosettes" "rosettes" are are the the result result phenocrysts No comparable comparable rocks rocks of intense intense metasomatism metas~matism ofpyroxenite. pyroxenite. No of of I I I I I I I I I I I I I I I are known known elsewhere elsewherein inthe theintrusion. intrusion. are The The matrix matrix of of the the dikes dikes is is aa leucocratic leucocratic natrolite natrolite syenite composed composed of of alkali alkali feldspar feldspar (patch (patch perthites) perthites) syenite with with albite albite replacements, replacements,natrolite, natrolite,minor minor green green alkali alkali amphibole amphibole and and accesory accesory zircon zircon and and fluorite. fluorite. The host host rock rock of of the the dikes dikes is is aa leucocratic leucocratic syenite syenite The composed perthite, minor minor amphibole amphibole and and accessory accessory composed of of patch patch perthite, zircon and and fluorite. fluorite. Although Although natrolite natrolite has has not not yet yet been been zircon observed observed these these rocks rocks bear bear aa remarkable remarkable mineralogical mineralogical These rocks rocks have have similarily to to the the matrix matrix of of the thedikes. dikes. These similarily been been intruded intruded by by aa very very dark dark quartz quartz syenite. syenite. The breccia breccia dikes dikes are are considered considered to to be be intrusive intrusive The breccias,rather rather than than multiple multiple intrusions, intrusions, connected connected breccias, tabular lamprophyre lamprophyre dike dike can can activity. AA tabular with Center Center 22activity. with be be observed observed at at the the eastern eastern end end of of the the outcrop. outcrop. �r STOP STOP 55 - -- MINK MINK CREEK CREEK -- REDSUCKER REDSUCKER COVE COVE -- NEPHELINE NEPHELINE SYENITES SYENITES - -- r This This area area is is located located at at the the eastern eastern margin margin of of the the I I I faulted faulted block block characterized characterized by by extensive extensive igneous igneous breccia breccia development. development. Here Here the the arcuate arcuate structure structure defined defined by by the the gabbros gabbros and and by Center by Center 33 nepheline nepheline syenites syenites of of Center Center 22 is is truncated truncated quartz syenite. quartz syenite. The The Center Center 22 rocks rocks contain contain abundant abundant xenoliths xenoliths of of earlier earlier rocks, rocks, whilst whilst the the Center Center 33 rocks rocks are are relatively relatively xenolith xenolith free. free. Biotite Biotite gahbros gabbros are are the the oldest oldest rocks rocks at at this this locality locality and are found as greenish massive coarse grained to and are found as greenish massive coarse grained to pegmatitic pegmatitic rocks rocks which which in in many many places places are are commonly commonly brecciated brecciated and and veined veined by by natrolite—nepheline natrolite-nepheline syenites. syenites. The The gabbros gabbros are are composed composed of of hortonolitic hortonolitic olivines, olivines,augite, augite, plagioclase plagioclase (andesine—labradotite) (andesine-labradotite) biotite biotite and and alkali alkali feldspar feldspar which which in in some some examples examples becomes becomes sufficiently sufficiently I I I I I I abundant abundant that that the the rocks rocks should should be be termed termedsyenodiorite, syenodiorite. Corona Corona structures structures of of alkali alkali amphibole amphibole and and biotite biotite are are comnionlv developed around commonly developed around olivine olivine and and augite. augite. The The nepheline nepheline syenites syenites are are leucocratic leucocratic rocks rocks containing containing patch patch perthites, perthites, nepheline nepheline and/or and/or natrolite natrolite together together with with acicular acicular crystals crystals of of hastingsitic hastingsitic amphiboles. Quartz Quartz syenites syenites in in this this area area are are reddish reddish rocks rocks which which have have been been extensively extensively brecciated brecciatedand andsheared. sheared. At At the the localities localities shown shown can can be be found found the the following:— following:A. A. of of carbonate carbonate ocelli ocelli into into the the upper upper portions portions of of the the dike, dike, aa characteristic characteristic feature feature of of many many of of the the lamprophyres lamprophyres in in this thisarea. area. I B. B. I AA lamprophyre lamurophyre dike dike which which illustrates illustrates the the segregation segregation AA lamprophyre lampro~hyredike dike which which illustrates illustratesthe the intense intense metasomatism metasomatism associated associated with with many many of of the the Coldwell Coldwell minor minor intrusions. intrusions. The The metasomatism metasomatism is is manifested manifested I I -24- �—25— I C. C. D. D. E. I I I I I I I I I I I I I I by by aa "reddening" "reddening" of of the the host host rock rock feldspars. feldspars. Widths widths & of of the the metasomatic metasomatic zones zones are are commonly commonly much greater greater than than the the width width of of the the dike dike causing causing the the alteration. alteration. Igneous Igneous breccia. breccia. Xenoliths Xenoliths of of greenish greenish biotite biotite gabbro gabbro in in natrolite—nepheline natrolite-nepheline syenite. syenite. Hybrid Hybrid grey grey syenites. syenites. Coarse grained amphibole—nepheline—natrolite syenites. �P I I I I I I I I I I I I I U . .. . . . . .. . . .. .. . . . I 1*: :.•..I QUARTZ SYENITE NEPHELINE SYENITE SYENITE with gabbro gabbro and and NEPHELINE metavolcanic metavolcanic xenohths xenoliths j £j -- BIOTITE GABBRO as massive rock or in nepheline syenite — — Lineaments Lineaments I I I I I 0 - 0 1 200 I 400 I 600 METRES Redsucker Cove Stop 5. Mink Creek — . ..... .. . .. : ., .' ... .. - : . .... .. .. . ..... .. . ... . :.. . . .....1 ' . /2 MILE 1(4 Redsucker Cove. n ' sTS �r STOP 66 -STOP - FERROAUGITE SYENITE SYENITE QUARRY AND ROAD CUTS, HIGHWAY 17 FERROAUGITE - QUARRY AND ROAD CUTS, HIGHWAY 17 -- I I Ferroaugite syenite was formerly quarried at Marathon Ferroaugjte syenite was formerly quarried at Marathon for use use as as aa building building stone stone under under the the name "laurvekite", a for name ttlaurvekite, a term first first used used by by Kerr Kerr (1910) (1910) because because of the supposed term of the supposed I similarity between between the the Coidwell Coldwell complex complex and rocks of the similarity and rocks of the Oslo igneous province. Unfortunately this term has has permeated permeated Oslo igneous province. Unfortunately this term much of of the the geological geological literature literature concerning concerning the Coldwell much the Coldwell I complex.. complex.. j I I U I ~3, The only similarity in fact between the Coldwel The only similarity in fact between the Coldwell ferroaugite syenites and the Oslo larvikites is the ferroaugite syenites and the Oslo larvikites is the presence of cryptoperthitic intergrowths which impart an presence of cryptoperthitic intergrowths which impart an intense schiller schiller to to the thefeldspars. feldspars. The Oslo larvikites intense The Oslo larvikites are monzonitic monzonitic rocks rocks which which grade gradeinto intonepheline nephelineplagi— plagi are foyaite (lardalite). (lardalite). They do not show extreme iron foyaite They do not show extreme iron chment nor nor do do they they differentiate differentiateto tooversaturated oversaturatedresidua. enrichment residua. The quarry at this stop exposes highly weathered The quarry at this stop exposes highly weathered oaugite syenite and illustrates the typical deep1 ferroaugite syenite and illustrates the typical deeply weathered friable friableappearance appearance of of ferroaugite ferroaugite syenite weathered syenite away from the polished glaciated outcrops on the lake away from the polished glaciated outcrops on the lake shore (Stop (Stop8). 8). shore The fresh ferroaugite syenite exposed in the roa The fresh ferroaugite syenite exposed in the road cut to the east east of of the the quarry quarry is is an example of one of cut to the an example of one of I the most highly differentiated .portionsof the ferrothe most highly differentiated portions of the ferroaugitesyenite. syenite. Olivines are fayalite (FaQ4Tp4F02), augite Olivines are fayalite (Fa94Tp4Fo2), pyroxenes are light greenish brown ferroaueite (Di pyroxenes are light greenish brown ferroaugite (Di10Hd85 1oHd85 I I I zoned to to acmitic—hedenlergite acmitic-hedenbergite(Ac50Hd50). (Ac50Hd 50) . Amphiboles Ac 5 ) zoned Ac5) Amphiboles are light green ferrorichterite (Na2CaFe5Si8On2(OH),,) are light green ferrorichterite (Na2CaFe5Si8o22(O}J)2) with minor minor mantles mantles of of arfvedsonite arfvedsonite (Na3Fe5Si8O29(0H)2) (Na3Fe,Si8022(OH)2) with Aenigmatite (Na2Fe5TiSi6O20) is abundant and calcite and quartz can he found as interstitial residual phases. I I I I — 27 — �STOP STOP 77 - - GABBRO GABBRO p I I The The arcuate arcuate mass mass of of basic basic rocks rocks which which define define the the eastern eastern margin margin of of the the complex complex is is commonly commonly referred referred to to as as the the eastern eastern gabbro gabbro to to distinguish distinguish it it from from the the alkaline 2. This This eastern eastern gabbro gabbro is is alkaline gabbro gabbro of of Center Center 2. considered considered to to belong belong to to Center Center 11 activity activity as as it it is is intruded intruded in in many many places places by by ferroaugite ferroaugitesyenite. syenite. The The petrological petrological relationship relationship between between the the two two magmas magmas is is however unclear. however unclear. Ferroaugite Ferroaugite syenite syenite is is unlikely unlikely to to be b aa direct direct differentiate differentiate of of the the gabbro gabbro because because of of the the greater greater volume volume of of the the former former and and lack lack of of mineralogical mineralogical gradations gradations between between the the two two rock rock types. tyoes. The The zone zone of of I I I I I I gabbro gabbro defines defines aa prominent prominent magnetic magnetic low low on on figure figure lB 1B and and is is considered considered by by Lilley Lilley (1964) (1964) to to be be due due to to reversed reversed magnetization magnetization of of the the gabbros. gabbros. The The gabbros gabbros are are composed composed of plagioclase (An6035)) and ) , augite, augite, plagioclase and of olivine olivine (Fo67_43), minor minor orthopyroxene orthopyroxene (Efls5rc) (En55-..c ) (Lum, (Lum,1973). 1973). The The ortho— orthopyroxene pyroxene may may be be aa product product of of assimilation assimilation of of Archean Archean metasediments, metasediments, aa xenocryst xenocryst derived derived frpm from the the pyroxene pyroxene hornfels hornfels thermal thermal aureole aureole or or aa relict relict high high pressure pressure phase. phase. The The gabbro gabbro has has been been extensively extensively prospected prospected with with regard regard to to its its copper copper potential potential as as accumulations accumulations of of pyrrhotite pyrrhotite and and chalcopyrite chalcopyrite with with minor minor pentlandite, pentlandite, cubanite, cubanite, pyrite, pyrite, bornite, bornite, arsenopyrite arsenopyrite and and mackinawite (vatkinson (Watkinsonet et al. al. 1973, 1973, Lum, Lum, 1973) 1973) are are common. common. The The excursion excursion stop stop is is close close to to the the contact contact between between the the gabbro gabbro and and the the ferroaugite ferroaugitesyenite. syenite. Many Many pegmatites pecmatites of of ferroaugite ferroaugitesyenite syenitecut cut the the gabbro gabbroat at this thislocality locality and and demonstrate demonstrate that that the the gabbro gabbro is is the the earliest earliest activity activity present present in in the thecomplex. complex. The The gabbro gabbro is is widely widely variable variable in in appearance appearance due due to to the the presence presence of of variable variable amounts amounts of of Archean Archean xenoliths. xenoliths. At At this this location location the the gabbro gabbro shows shows all all transitions transitions from from massive massive homogenous homogenousgabbro gabbroto torocks rocks I I I -28- �1 I- -3 F-Q CD O P CO -C CD CD CD P o c-f CD 0 0 H- CO C p H- Q o ow ci- CD CD .w wc p P0 <p CO CD H CD H, o ci- H- OqGq lipli (t ci- CD — CD ot (DO -C -C CD CD 0 ci- w CD — — CD P' CD o c-I- c-I-p c* 0 c* Elill — F" tt pa ____ ____ ____ w i t h w e l l developed igneous l a y e r i n g . The l a y e r s a r e not t r a c a b l e o v e r l a r g e d i s t a n c e s and do not s e r v e t o o u t l i n e t h e s t r u c t u r e o f t h e gabbro i n t r u s i o n . �a STOP STOP 88 -- LAKE LAKE SUPERIOR SUPERIOR SHORE SHORE LINE LINE CENTER CENTER 11 - PLUTONIC PLUTONIC - ROCKS ROCKS AND AND MINOR MINOR INTRUSIONS INTRUSIONS I Proceed Proceed fron from the the parking parking lot lot at at the the foot foot of of Howe Howe St., St., Marathon Marathon along along the the trail trail through through the the woods woods to to avoid av the the boulder boulder beach. beach. The The trail trail emerges emerges at at location location F, F , from from that that point point follow foil the coast to to location A. I Location AA Location ~Hornfelsed Hornfelsed Archean Archean metasediment metasediment cut cut by by analcite I , I tinguaite tinguaite dikes. dikes. These These rocks rocks were were initially initially described described by by Coleman Coleman (1900) (1900) as as heronites. heronites. The The tinguaites, tinguaites, after after I lamprophyres, lamprophyres, are are the the second second most most abundant abundant type type of of minor minor intrusion intrusion at at Coidwell Coldwell and and are are probably probably associated associated with magmatism. Xenoliths Xenoliths with the the undersaturated undersaturated Center Center 22 magmatism. I of of coarse coarse grained grained nepheline nepheline syenite syenite can can be be found foundin in some some examples examples at at Heron HeronBay. Bay. The The majority majority of of the the tinguaites tinguaites I I are are intensely intensely hematized hematized and and carbonatized, carbonatized,are are very very fine fin grained grained and and brick brick red red to to dark dark reddish—brown reddish-brownin incolor. color. At At this this locality locality is is found found aa relatively relatively fresh fresh 3—4 3-4 ft. ft. wide vertical vertical dike. dike. Black Black margins margins with with conchoidal conchoidalfractures fractures may may represent represent an an original original chilled chilled glassy glassy margin. margin. The The tinguaite tinguaite is is porphyritic porphyritic with with phenocrysts phenocrysts of of pale pale green green I I ferroaugite ferroaugite with with titan—acmite titan-acmite rims, rims, brown brown hastingsite hastingsite and and anorthoclase anorthoclase set set in in very very fine fine grained grained groundmass groundmassof of apatite, apatite, acicular acicular pyroxene, pyroxene, hematized hematized feldspar, feldspar,fluorite fluorite and and analcite. analcite. I I Location_B Location B - Glomeroporphyritic Glomeroporphyritic diabase diabase representative renresentativeof of the the post-Coldwell post-Coldwell alkali alkali basaltic basalticmagma magma activity. activity. The The glomeroporphyritic glomeroporvhyritic feldspars feldspars are are labradorite labradoriteset set in in aa I groundmass groundmass of of andesine andesine and and aluminous aluminousaugite augite(8% (8%A1203). Alp03). Several Several thin thin ocellular ocellular lamprophyre lamprophyredikes dikescan canbe be I I -30- , �-3'- P observed between locations A and B. I I I Location C Location I Ocelli in these observed between locations A and B. rocks contain quartz plus calcite orOcelli dolomite. in these rocks contain quartz plus calcite or dolomite. Extensive deposits of sand and gravel cover the Extensive deposits of sand and and the gravel contact between the intrusion Archean covercountry the contact between the intrusion and the rocks and no outcrops are found between locations B Archean country rocks and no outcrops are found between and C. The area however presents excellent exposures locations B and C. The area however presents excellent of the lowest of the six beach terraces at exposures Marathon. of the lowest of the six beach terraces at Marathon c Xenolith bearing gabbro considered to be equivalent Xenolith bearinggabbro gabbroobserved considered to be7.equivalent to the hypersthene at Stop to the hypersthene gahbro observed at Stop 7. Location - Location I I I I D 0 Fayalite-ferroaugite syenite with well dev Fayalite_ferroaugite with well developed igneous layering defined syenite by the mafic minerals. Crossigneous layering defined by the mafic minerals. bedding, slump structures, and diffuse turbulentCross— layering bedding, slump structures, and diffuse are all well developed in this area. turbulent layering are all well developed in this area. The mafic minerals ferroaugite, and amphiboles belonging toare thefayalite, ferroedenite-hastingsite amphiboles belonging to the ferroedenite_hastingsite series (NaCa2Fe5Si~102~OH)2-NaCa2Fe5SinA1202(OH)2). series (NaCa2Fe5si7Alo22(OH)_NacaFesiAlo(0H)) Location E Location E Ferroaugite syenites representative of the more I I I I I I Ferroaugite syenites representative of the extreme differentiates of this magma. Pyroxenes more are extreme differentiates of this magma. members of the acmite-hedenbergite series and amphiboles Pyroxenes are members of the acmite_hedenbergite series and amphiboles are subaluminous ferroedenite (NaCa2Fe5Si7.5Alo. are suhaluminous ferroedenite (NaCa2Fe5si7Alo(OH)) or ferrorichterite (Na2CaFe3Si8o2 (OH) (Na2Fe5Tisi6O) is ) abundant. Aenigmatite �— 32 — Location FF Location Ferroaugite syenite cut by very coarse patch and Ferroaugite syenite cut by very coarse patch and I sheet pegmatites. The pegmatites illustrate the oversheet pegmatites. The pegmatites illustrate the oversaturated nature of the ferroaugite syenite differentiation saturated nature of the ferroaugite syenite differentiation trend, and contain ferrorichterite altering to ferrotrend, and contain ferrorichterite altering to ferro— I actinolite, i I I I I I I I I I I I I I C feldspars, quartz and zircon. �I parking lot ii I I I I I I I I I I I I I I I I F FERRQ4UGITE SYENITE C, / / / 0 I- 0 1/2 500 4- J 1000 Mile 1500 Metres 4RCHE4N Stop 8. Layered Ferroaugite__Syenites. �. r ~ . REFERENCES REFERENCES I I • • I I I I I I I I I Aubut, Aubut, A.J. A.J. (1977): (1977): Geology Geology of of the the southwestern southwestern margin margin of of the the Coidwell Coldwell alkaline alkaline complex, complex. Northwestern Northwestern Ontario. Ontario. H.B.Sc. H.B.Sc. Thesis, Thesis, Lakehead Lakehead Univ., Univ., Thunder Thunder Bay, Bay, Ontario. Ontario. Ayres, Ayres, L.D., L.D., Lumbers, Lumbers, S.B., S.B., Mime, Milne,V.G., V.G.,and andRoherson, Roberson, D.W. D.W. (1970): (1970): Ontario Ontario Geological Geological Map. Map. East East Central 2198a. Central Sheet, Sheet, Ontario Ontario Dept. Dept. Mines Mines Map, Map, 2198a. Balint, 1 dwell alkaline Balint,F. F. (1977): (1977): The The Neys Neys diatreme, diatreme, Co Coldwell alkaline complex, o. H.B.Sc. complex, Northwestern Northwestern Ontari Ontario. H.B.Sc. Thesis, Thesis, Lakehead Univ., Thunder Bay, Ontario. Lakehead Univ., Thunder Bay, Ontario. Card, Card, K.D K.D., Church, Church. W.R., W.R., Franklin, Franklin,J.M., J.M., Frarey, Frarey, M.J., M.J., Robertson, J.S., West, West, G.F., G.F., and and Young, Young, G.M. G.M. Robertson,J.S. (1972): hem Province. Variations (1972): The The Sout Southern Province. In In : Variations in in Tectonic Tectonic Style Style in in Canada. Canada. Eds,, Eds., Price, Price,R.A., R.A., and and Douglas, Douglas.J.W. J.W. Geol. Geol. Assoc. Assoc. Canada Canada Spec. Spec. Paper Paper 11, 11,335—380. 335-380. : Coleman, Coleman, A.P. A.P. (1900): (1900): ileronite Heronite or analcite analcite tinguaite. tinguaite. Ann. Rept. Bur. Mines Ontario 9,186—191. 186-191. Ann. Rept. Bur. Mines Ontario 9, Currie, Currie,K.L. K.L. (1976): (1976): The The alkaline alkaline rocks rocksof of Canada. Canada.Geol. Geol. Surv. Surv. Canada CanadaBull. Bull., 239. 239. , Einarrson, G.W. G.W. (1973): (1973): Variations Variations in in the the style styleof of metamorphism in Archean supracrustal units metamorphism in Archean supracrustal units Einarrson, of of the the Superior Superior Province. Province. H.B.Sc. H.B.Sc.Thesis, Thesis, Lakehead Univ., Thunder Bay, Lakehead Univ., Thunder Bay,Ontario. Ontario. Gittins, Gittins,J., J., MacIntyre, R.M., and York, D. (1967): The ages ages of of carbonatite carbonatite complexes complexes in in eastern eastern Canada. J. Earth EarthSci. Sci. 4, 4,651—655 651-655. Canada. Canad. Canad.J. Halls, Halls,H.C., H.C.,and and West, West.G.F. G.F.(1971): (1971): AA seismic seismic refraction refraction survey survey in in Lake Lake Superior. Superior. Canad. Canad. J. J. Earth Earth Sd. Sci. 8, 8,610—630. 610-630. Kerr, Kerr,H.L. H.L.(1910): (1910): Nepheline Nepheline syenites syenites of of Port Port ColdwelL Coldwell. Ann. Ann. Rept. Rept. Bur. Bur. Mines Mines Ontario, Ontario,19, 19,194-232. 194-232. I I I — 33 — �ri Lilley, Lilley, F.E.M. F.E.M. (1964); (1964): An analysis analysis of of the the magnetic magnetic An features features of of the the Port Port Coidwell Coldwell intrusive, intrusive. M.Sc, M.Sc. Thesis. Thesis. Univ. Univ. Western Western Ontario, Ontario. London, London. I I Ontario. Ontario. Lum, Lum, H.K. H.K. (1973): (1973): Petrology Petrology of of the the eastern eastern gabbro gabbro and and associated associated sulphide sulphide mineralization mineralization of of the the Coldwell Coldwell alkalic alkaliccomplex. complex. B.Sc. B.Sc. Thesis, Thesis, Carleton, Carleton, Univ. Univ., Ottawa, Ottawa, Ontario, Ontario. , I ' Milne, Milne,V.G. V.G. (1967): (1967): Geology Geology of of the the Cirrus Cirrus Lake—Bamoos Lake-Bamoos Lake Lake area. area. Ontario Ontario Dept. Dept. Mines, Mines,Rpt. Rpt.43. 43. Mitchell, and Platt, P1att, R.G. Mitchell,11.11., R.H., and R.G. (1977): (1977): Mafic mineralogy mineralogy Mafic of of ferroaugite ferroaugite syenite syenite from from the the Coidwell Coldwell alkaline alkaline I I I I I I complex! complex. 23rd 23rd Ann. Ann. Instit. Instit.Lake LakeSuperior SuperiorGeology, Geology, Thunder ThunderBay Bay (abstract). (abstract). Puskas, Puskas, P.P. F.P. (1967): (1967): The The geology geology of of the the Port PortColdwell Coldwellarea area Ontario .No. Ontario Dept. Dept. Mines Mines Open Open File FileRpt Rpt. No.5014, 5014, Thunder Thunder Bay, Bay,Ontario. Ontario. Thompson, Thompson, J.E. J.E. (1931) (1931)- Geology Geology of of the theHeron HeronBay Bayarea. area. Ann. Rept. Ontario Dept. Mines, 40, 21—39. Ann. Rept. Ontario Dept. Mines. 40. 21-39. Walker, Walker,J.W.R. J.W.R. (1967): (1967): Geology Geology of of the theJackfish-Middleton Jackfish-Middl area, area,Ontario OntarioDept. Dept.Mines MinesGeol. Geol.Rpt. Rpt.50. 50. Watkinson, ,D.H., D.H.,Mainwaring, Mainwaring,P.R., P.R.,and andLum, Lum,H.K. H.K.(1973): (1973): Petrology Petrology and and copper copper mineralization mineralizationof ofthe the Coldwell Coldwellcomplex, complex.Ontario. Ontario. Geol. Geol.Soc. Soc.Amer. Amer. Abs. Abs. Ann. Ann.Mtg. Mtg. 5, 5,856. 856. ACKNOWLEDGEMENTS ACKNOWLEDGEMENTS I I I I Research on the thepetrogenesis petrogenesisof ofalkaline alkalineintrusions intrusions in in Northwestern Northwestern Ontario Ontariois issupported supportedby by the theNational National Research ResearchCouncil CouncilofofCanada. Canada. Sam SamSpivak Spivakis isthanked thankedfor fordrafting draftin services involved in in the theproduction productionof ofthis thisguide guidebook. book. �a 8 (4) a; -- - a__fl -. AIRLANE MOTOR HOTEL LOTUS INN CROSSROADS MOTOR INN RED OAK INN p � https://digitalcollections.lakeheadu.ca/files/original/511ac78db0b56dd6f5327fb751cf7e10.pdf 3e8cc58a84a56f5723808e43eaafc4d9 PDF Text Text Iwety Ihipti Aonoal Institute Meeting Thunder Bay, Ontario Superior PROTEHOZOIC TRIP �PROTEROZOIC ROCKS OF THE THUNDER BAY AREA NORTHWESTERN ONTARIO May 3—4, 11.977 FIELD EXCURSION GUIDE PREPARED BY C.R. Kustra, Ontario Ministry of Natural Resources W.H. Mcllwaine, Petro-logic Limited, Thunder Bay K.G. Fenwick, Ontario Ministry of Natural Resources J. Scott, Ontario Ministry of Natural Resources �1 Guide to the Proterozoic Rocks of the Thunder Bay Area, Northwestern Ontario INTRODUCTION: The Proterozoic rocks of Northwestern Ontario, which form part of the itAnimikiet and !TKeweenawantt units, represent one of the most complete geological records of Middle and Late Precambrian sedimentation and igneous activity in eastern North America. These rocks are virtually unmetamorphosed and relitively undeformed. Mineral depsits in the Proterozoic rocks include silver in Keweenawan dikes and the Rove Formation, iron in the Gunf lint Formation, nickel in mafic intrusive rocks, copper in various volcanic and sedimentary strata, and lead-zinc-barite, amethyst and uranium associated with the Sibley Group. During the last century, the famous Silver Islet mine produced over 2.8 million ounces of silver. REGIONAL GEOLOGY The Proterozoic rocks lie unconformably on the peneplained Early Early Precambrian (Archean) surface of the Superior Structural Province. Precambrian rocks form several northeast-trending "belts" of metamorphosed and complexely deformed metavolcanic and metasedimentary rocks intruded by felsic, and intermediate to ultramafic intrusive rocks. The lithostratigraphy of the Proterozoic rocks is shown in Table 1. API-JEBIAN The Gunflint Formation (Fig. 2) has been studied by Goodwin (1956, 1960) and Moorhouse (1960). The petrology has been studied in detail by Floran and Papike (1975). Detailed descriptions of fossils from the Gunf lint Formation are recorded by Barghoorn and Tyler (1965), Barghoorit (1971) and Edhorn (1973). The Rove Formation has been described by Morey (1969) and Geul (1970, 1972). Much of the descriptive detail is taken from these authors. Gunf lint Formation (adapted from Goodwin, 1956) The Gunflint Formation extends continuously for 110 miles (177km) from Gunflint Lake east to beyond Thunder Bay, from where it can be traced intermittently to the Slate Islands, (Sage et al 1975), southeast of Schreiber. It averages 400 feet (122 m) in thickness (Goodwin, 1956) Except for local faulting and brecciation caused by intrusive activity and slumping, the Gunf lint Formation is structurally simple and uncomplicated, with an average southeast dip of 5°. �2 TABLE 1 Proterozoic Stratigraphy of Northwestern Ontario Neohelikian Osler Group: basalt, minor rhyolite and sedimentary rocks Intrusive Rocks: gabbro stocks undersaturated stocks Iayered bodies northeast trending dykes Logan diabase sills Paleohelikian Sibley Group: Kama Hill Formation Rossport Formation Pass Lake Formation Aphebian Animikie Group: Rove Formation: shale, greywacke Gunflint Formation: iron formation (taconite) algal chert, limestone, tuffaceous shale. TABLE 2 Stratigraphy of the Gunf lint Formation (modified from Goodwin, 1956) Limestone-dolomite Member Upper Member Taconite-chert carbonate submember: taconite (west) facies chert carbonate (east) facies Tuffaceous shale submember Algal chert submember Lower Member Taconite-chert carbonate submember: Tuffaceous shale submember Algal chert submember Kakabeka Conglomerate Member west taconite facies chert carbonate facies east taconite 1 acies �Fig. I LII UPPER TACONITE i — - 5 0 10 10 20 • ••.• I S •••• :.:... •. : : .••: : : SS --- ALGAL CHERT 30 KILOMETRES MILES 0 501 0 I0 1- 20 oo-f 30 F 40 I5O FEET METRES LOWER EAST TACONITE - !JPER CHEAT CARBONATE •: Thunder Bay Slate River UPPER TUFF ARGILL1TE — ... •••• •' ::•::•:••::• •: •s,••••I•s,s••,•• .• • 'a' '::•::: 'c.:.: :•.•.:•: :•.• . ..... fg'\ k'..... IUI Northeast Longitudinal section of the Gunf lint formation. (after Goodwin ,1956).(Rove fm. added by author). Stops described in field guide BASAL CONGLOMERATE MEMBER LOWER WEST TACONITE —urrLn P4L%)ML .ncni-— MEMBER ci'- LOWER MEMBER UPPER Southwest �__ ___ ____ r.fl77' f ////// /////,../// ////. / ______ ____________ ____ ___ + + . + ÷ (4 + + 44+ * + ÷/+ + r+___.r4.m.y4.T.rn Urrn + - ÷ + ÷;' ÷ + + ÷ t-cj_XY +t.tt___ + 4 9....... I + - . ++ I LEGEND I I I j.-I + + I DIABASE I ri ROVE FORMATION THUNDER ÷ ÷ . L9÷.t_t ÷+÷+÷+÷: + + + + + + + + K K FHWH GUNFLINT FORMATION - ÷ . 1'' 1 LH1 I 7' - GRANITE - H + + HH H + H +/i;H I / H 'HJ 0/0 T -- FAULT / N-_--- FaMERS ::-: . + I,Hi NOLKLU : .... I I :'- :- . . T:.. . (I I 7-• :- -. . . . ,- w :' o . .-- -a.- .-'- . .: 1-k .•[ S .. . ,-7 c—cs : - ,j, I . - - :HYl t,-nPY -. . -0 •j/ ) - SLATE RIVER HWY 61, SW. OF CLOUD LAKE ROAD I 2 5 KAKABEKA FALLS (SWIM AREA) KAKABEKA FALLS RIM (WEST SIDE) KAKABEKA FALLS (HYDRO SPILLWAY) 6 DICKSONS QUARRY (BELROSE ROAD) 7 —S, HILLCREST PARK (HIGH STREET) BOULEVARD LAKE ( LYONS BRIDGE) THUNDER BAY LOOKOUT (EXPRESSWAY) 4 8 9 -. -' 0 4 2 6 - 0 . , .7' F7 (. . •----\ STOP LOCATION FIELD STOPS 3 A L- H- •- .-Z,, c-.i U -: ... . Yk * 2 4 6 8 10 12 KILOMETRES L)/ C Fig 2 Geology and Field Stop Locations a �5 Deposition of the Gunflint Formation was in part cyclical. A basal conglomerate member is overlain by two members each composed of chert, tuffaceous shale, and carbonate-taconite submembers. These members are in turn overlain by a discontinuous limestone member, (Fig. 1 and Table 2). Although no isotopic investigation has satisfactorily established the time of sedimentation, various studies suggest that deposition of Animikie sediments took place 2,000 m.y. B.P. (Floran and Papike, 1975). Stratigraphic Descriptions (a) Basal Kakabeka Conglomerate Member This member ranges to five feet (1.5 m) in thickness and is composed of polymictic conglomerLte. Clasts of Archean volcanic rocks, granite and quartz are cemented in a matrix of chlorite and quartz. The unit is highly irregular in thi.kness but persistent. (b) Lower Member The lower algal chert submember (Fig. 1) consists of reef-like mounds of finely banded black, red, and white oolite chert. These mounds This submember forms the are intergrown and cemented in dolomite. western margin of Gunflint outcrop (Fig. 1), but is continuous to just It contains abundant microflora remains west of Kakabeka Falls. (Barghoorn and Tyler, 1965, Barghoorn, 1971, Edhorn, 1973). The lower tuffaceous shale (lower tuff argillite, Fig. 1) submember It overlies the lower algal chert subranges to 20 feet (6 m) thick. member in the area west of Kakabeka Falls and is composed of fissile black shale containing much volcanic ash. The uppermost submunber of the lower member is subdivided into three facies (Fig. 1). The lower west taconite facies, which is 150 feet (45 m) thick, extends northeast from Gunf lint Lake to Kakabeka Falls and is composed of wavy-banded granular chert, carbonate, and oxides. The lower half contains disseminated greenalite granules in pale grey chert; siderite forms local beds. The upper half contains increasing amounts of hematite and magnetite. This facies grades upward into jaspilitic upper algal chert and grades laterally into the lower banded chert-carbonate facies. The lower banded chert-carbonate facies extends from Kakabeka Falls to Thunder Bay city, and consists of 4 to 6 inch (10 to 15 cm) siderite beds, with interbedded 2 to 6 inch (5 to 15 cm) grey cherty beds. Carbonaceous material and pyrite are common in shale interbeds. This facies grades into granular taconite towards the northeast. The lower east granular taconite facies extends from Thunder Bay to The basal 2 to 6 feet (60 to 180 cm) are formed of interbedded granular chert and ankerite. The upper 10 to 20 feet (3 to 6 m) consist of interbedded red to green mottled chert and dolomitic limestone. This facies grades upwards into the tuffaceous shale (upper tuff argillite, Fig. 1) submember of the upper member. Loon Lake. �6 (c) Ujper Member The upper algal c iert submember extends west from Nolalu to Gunf lint Lake and consists of b sal granular chert overlain by algal chert and, in the Mink Mountain area, amygdular basalt flows. The flows and algal chert are overlain by granular chert and bedded jasper. Jasper beds grade into tuffaceous shale of the overlying submember. The upper tuffaceous shale (upper tuff argillite) is the only continuous submember in the Gunf lint Formation and forms a key stratiIt ranges to 100 feet (30 m) thick and thins graphic marker (Fig. 2). laterally in either direction from Kakabeka Falls. It consists of black tuffaceous shale and siltstone with interbedded siderite and pyrite and The ash contains ellipsoidal, aceretionary extensive beds of volcanic ash. lapilli and concentric layers of small angular tuff fragments, arranged Similar lapilli have formed due to the about a larger central fragment. accumulation of volcanic dust in water droplets and on water coated shards (Moore and Peck, 1962). The upper tuffaceous shale submember grades into the upper taconite and chert-carbonate submember. The upper taconite facies extends from Gunflint Lake to Thunder Ba, (Fig. 1), and is composed of wavy bands of granular greenalite-bearing chert. The greenaTite-bearing granules are round to oval, evenly distributed throughout a layer, and appear to have The unit exhibits a rusty weathering, contains abundant formed "in situ". hematite and magnetite in granules towards the top, and grades laterally (Fig. 1) into the upper banded chert-carbonate facies which extends from west of Thunder Bay to Loon Lake. The latter facies consists of interbedded grey chert and brown carbonate (siderite with lesser dolomite arid Brecciation and folding, apparently contemporaneous with ankerite). deposition, are common. (d) Upper Limestone Member The upper limestone member marks the top of the Gunflint Formation. Minor chert beds, illite and volcanic shards are present, and tuffaceous shale is most prevalent in the eastern area of Gunflint outcrop. Stratigraphic Interpretation Goodwin (1956) concluded that Gunf lint deposition occurred in a After shallow basin which had limited circulation with an open sea. initial algal activity in the neritic zone, volcanic activity (tuffaceous Silicate-bearing material shale) was accompanied by sinking of the basin. (taconite) was deposited in the deepest portions while in the neritic, or intertidal zone (between Kakabeka Falls and Thunder Bay) banded chertFarther to the northeast, the lower east taconite facies carbonate formed. formed in agitated, oxygenated waters. As the basin filled, conditions of algal growth returned, initiating the "Upper Gunflint" cycle. Volcanic activity, marked by local basalt flows, terminated the upper algal chert deposition and resulted in widespread distribution of pyroclastics of the upper tuffaceous shale. Downwarping resulted in deposition �7 of granular iron silicate rocks in the deeper, southwest portion of the basin, while on the shallow northeast shore, chert carbonate was deposited. As the basin filled, sporadic but violent volcanic activity was accompanied by the entry of sea water, resulting in formation of the upper limestone. Goodwin (1956) in drawing an analogy with the Santorin volcano of the Aegean Sea, suggests that volcanism was the chief source of iron and silica. Alternatively, Hough (1958) suggests deposition in a fresh water basin, with material derived through weathering of an adjacent landmass, and deposition controlled by limnic cycles. Rove Formation The Rove Formation conformably overlies the Gunflint Formation. In the Thunder Bay and Pigeon River areas, it may attain a thickness of at least 1,250 feet (380 m), and possibly more than 2,000 (610 m), Geul (1970). The formation consists of three lithologic units (Geul, 1970) which are, from base upwards: (1) black pyritic shale and argillite (base) (2) interbedded argillite and greywacke and shale (transition sequence of Morey, 1969) (3) quartzitic greywacke with argillite interbeds (top) The lower argillite is the dominantly exposed unit in Ontario and commonly enclosed carbonate concretions of varying size and complex in The transition sequence consists of thin-bedded greyform and texture. wacke, consisting of grey to pink greywacke and sandstone, is the thickest unit of the Rove Formation and is exposed mostly in northeastern Minnesota. The metamorphic age of the Rove Formation is considered to be 1.7 billion years (Peterman, 1966). Morey (1969) notes that the detrital matenal comprising the Rove Formation was derived from Archean terrain to the north. PALEOHELI KlAN The Sibley Group This section is adapted from Franklin et al (in press). Introduction: The distinctive red and white sandstone and dolomite of the Sibley Group were first described by Logan (1863) who considered them as part of his "Upper Copper-Rearing Series". Logan included the Gunflint and Rove Formations, as well as the Sibley rocks, in this "series". �8 Robert Bell (1872), in completing the first comprehensive description of these rocks, considered them as the Upper Group of the Upper CopperBearing Series. He compared the Sibley Group with rocks of Nova Scotia and concluded (p. 321) that "they (Sibley rocks and the overlying ()sler volcanic sequence) may now be considered as of Permianand Triassic age". Logan (1872), in strongly reprimanding Bell, outlined evidence for a pre-Silurian age. T. Sterry Hunt (1873) divided Logan's Upper Copper-Bearing Series into Animikie (now the Gunflint and Rove Formations) and Keweenawañ Groups, and assigned the Sibley to the latter Group. Uilson (1910, p. 69) indicated that these rocks occupy a "trough-like depression between two areas of Archean rocks" in the western Lake Nipigon area, and tentatively assigned them to the Paleozoic. The name "Sibley" was first assigned to this group by Tanton (1931). Hs description of the "Sibley Series" of the Sibley Peninsula is accurate, but unfortunately he did not have an opportunity to map the more northern areas, and thus his stratigraphic analysis is of somewhat limited value. Although local studies of the Sibley Group rocks were undertaken in subsequent years by Hawley (l)29) and Moorhouse (1960) the first comprehensive mapping was completed by Coates (1972) and Mcllwaine (l971a, 197lb) Recent geophysical studies by duBois (1962) and Robertson (1973) have underscored the need for a comprehensive study of the stratigraphy and paleogeography of the unit. Red-bed sequences like the Sibley Group are important environmental indicators and in addition this occurrence may have tectonic implications. Age: The age of the Sibley Group is a key question with respect to the position of these rocks within the framework of the Keweenawan tectonic event, and the Helikian polar wandering curve. A rubidiumstrontium whole rock isochron was completed in the laboratory of the Geological Survey of Canada. The analytical procedure is outlined by Wanless and Loveridge (1972). The Geological Survey of Canada uses an R87 decay constant of 1.47 x l0-1yr, as physically determined and outlined in the aforementioned paper. The widely used 1.39 x 10-11 yr'. a 'geologically determined' decay constant results in ages approximately 5.75% older. The age, using the 1.47 x l0-11yr constant, is 1294 t 31 m.y. Samples were selected from the Kama Hill and Red Rock formation; the areas near diabase sills were avoided as their metamorphic effects are extensive (Robertson, 1973). The Kama Hill section is cut by multiple thin sills; the data for one of the samples probably reflects this metamorphism, and should be discounted. The Sibley Group is apparently older (by at least 150 m.y.) than the accepted age of the Keiceenawan igneous event as typified by the �9 Duluth complex (1,115 m.y. RbSr, Faure et al 1969) and the.Keweenawan extrusive rocks (1,142 m.y. RbSr Faure and Chaudhuri, 1967). However well documented comparable age determinations have notbeen published on either the Logan sills or the lower Osler group; These latter units are magnetically reversed (duBois, 1962) and are the oldest Keweenawan igneous rocks in this area. K-Ar dates of 1,060 m.y. on Logan sills (Franklin, 1970) probably represent a minimum age due to possible argon leakage. Unpublished RbSr data indicates that the sills may be as old as 1298 ± 33 m.y. (Robertson and Fahrig, 1971), and thus very close in age to the Sibley Group. The Sibley Group exhibits both normal and reversed magnetic polarity (Robertson 1973); igneous rocks of similar paleomagnetic characteristics have a very similar age to that obtained for the Sibley Group (Peterman et al, 1968, Murthey et al, 1968). Thus the determined RbSr age of the Sibley Group is probably reliable. Stratigraphy of the Sibley Group The Proterozoic rocks of the Southern Province of the northern Lake Superior region have been described in a general way by Card et al (1972). Although the Sibley Group has not been heretofore formally subdivided, subdivision into formations is possible. The subdivisions are given in Table and locations of type sections are included in the formation descriptions. The Pass Lake Formation The type section for the Pass Lake Formation is exposed along the Canadian National Railway tracks in southern McTavish Twp. (Mcllwaine, 1974) near the north shore of Pass Lake where it is SO m thick; it continues eastward along the tracks for about 3 kms where it is overlain by the Red Rock Formation. Reference sections are located under the type section of the Red Rock Formation at Red Rock cuesta, at Mousseau Mountain north of Nipigon and on Quarry and Channel Islands in Lake Superior near Rossport. The formation is composed of lensoid basal conglomerate overlain by quartz-rich arenites. It thins rapidly northward; scattered patches are present along the eastern edge of the Sibley basin, but the unit is absent along the western basin margin. The Pass Lake Formation overlies the Rove Formation in the south and to the north lies unconformably on Archean Rocks. The contact with the Rove is exposed over a distance of about 20 m at Pass Lake; the Rove has been altered for about 50 cm from the contact. This normally black shale has been partially oxidized to a dark reddish brown. Where the Pass Lake Formation lies directly on the Archean there is little or no evidence of a reaction; at the site of the old Enterprise Mine in McTavish Township (Mcllwaine 1971b) the sandstone rests directly on quartz monzonite with no apparent affect. �1 U Overlying the basal conglomerate with sharp contact is a succession of arenites which are generally buff to pale pink with a minor number of red interbeds. The arenites are commOnly very thickly bedded (greater than 1 m) at the base of the sequence and range to very thinly bedded (1-3 cm) with increasing stratigraphic height. The sharpness of bedding boundaries also increase with stratigraphic height. They 'are commonly fine-grained (2-3 phi) but are locally very fine-grained (3-4 phi), with generally moderate sorting and locally poorly (especially at Pass Lake) and moderately well sorted (e.g. Mousseau Mountain). There appears to be no systematic variation in grain size or sorting with stratigraphic. height Thin section examination and modal analyses indiëate the rocks range from quartzose arenite to quartz arenite1 in composition. Detrital grains range from angular to well rounded but are generally subrounded with larger grains tending to be more rounded than finer grains. Quartz grains with undulatory extinction are more abundant than quartz with straight extinction. Total quartz at Pass Lake appears to be lower in the upper half than the lower half but this is mainly a function of cement content. For the most part there is little or no cement in the lower half and induration isapparently due to conpaction. Feldspar is a mino:' constituent, especially at Pass Lake. There is a suggestion of a systeiiatic decrease in feldspar with stratigraphic height in the Rossport section. Chert is present in all areas. Chert content is greater in the top half of the formation than in the bottom half at Pass Lake, other sections exhibit no apparent variation. Cementing material includes carbonate, mainly at Pass Lake, Red Rock and Rossport, and silica, which is more common at Mousseau Mountain. The matrix, generaUy fine mica and clay, forms consistently less than 15%; this is the content generally accepted as the dividing line between arenite and wacke. The Rossport Formation The type section of the Rossport Formation, is exposed on the shore of Channel Island, near the Village of Rossport. This formation overlies the Pass Lake Formation disconformably through much of the area, and unconformably on Copper Island near Rossport. In the northern area it lies nonconformably on the Archean basement. The Rossport Formation is distinguished by its brick red color, high dolomite content, and concoidal fracture. The formation maintains a relatively constant thickness of about lOOm in all measured sections except on the Sibley Peninsula, where it thins to approximately 2Om. In the southern area of the basin the formation may be divided into three members (a) lower dolomite, (b) central chert-carb.onate (stromatolite to the north) and (c) upper dolomite. In the northern area these members are less clearly distinguishable due to the lack of exposure. Much of the description is thus based on the more southerly sections. 1 . . . . . The limits used here are based on Pettijohn's (1957) classification but his rock names have been changed. �11 The lower and upper members exhibit enly minor lateral facies changes in the east and west direction, but to the north both members become distinctly more clastic. The central member is lithologically distinct and forms a lateral (east/west) continuous 'marker bed traceable from Rossport to Sibley Peninsula. The upper and lower members can be distinguished on the basis of mineralogical composition and bedding development. The lower member has distinct bedding, whereas the upper is. more massive. The lower is richer in carbonate and quartz whereas the upper member is clearly richer in clay and feldspar. Dolomite is the dominant carbonate mineral present. No easy distinction between the lower and upper members on the basis of calcite-dolomite ratio may be made, although in general. the calcite-dolomite ratio is higher in the upper unit than the lower. iKama Hill Formation The type section of the Kama Hill Formation is located along the northern-most powerline, on the west side of Kama Hill, 17 miles (27 km) east of Nipigon. Reference sections are available at Albert Lake, Stewart Lake and Channel Island. The boundary with the underlying Red Rock Formation is placed at the disappearance of carbonate and the change in color. The maximum preserved thickness is 50 m. The Kama Hill Formation is distinguished by its deep red-purple colour, silt to clay sized particles, thin bedding, moderately well developed fissility, its mineralogy and distinctive structures. Due to its fine-grained nature, mineralogical analysis is possible only by x-ray diffraction. Two clay minerals, quartz, microline and minor calcite and hematite constitute the mineralogy. Colour in outcrop varies from deep red to deep purple and is duç to hematite. The amount of hematite varies widely betweei 'beds from less than 1% in a few course silt beds to over 90% in a fw clay rich layers. It normally constitutes approximately 4% of the rock. The coloration is quite homogeneous within individual beds. Conspicuous "bleaching" is present only along a few bedding plane fractures and joint planes. Microscopic variation in coloration intensity is related to grain size, with finer clay rich beds containing more hematite. The lower portion of the formation has up to 10% white spherical "reduction" spots; these are much less common in the upper portion of the unit. Hematite occurs as evenly disseminated very fine grained aggregates. In the coarse silty beds it forms a coating and is interstitial to the clasts. Bedding is very difficult to distinguish in outcrop but is readily apparent in thin section and cut surfaces. The three types of beds which may be distinguished are: �12 (a) regular finely laminated clay rich regular beds. (b) irregular partially reworked course silty beds. (c) stromatolitic beds. Types (a) and (b) are intermixed but the type (a) beds appear more dominant with stratigraphic height. Bed types (a) and (b) are distinguished by their lack of carbonate. Beds of type (c) are confined to the lower part of the formation. * Sedimentary Structures Mud cracks are pervasive and a characteristic featur• of the Kama Hill Formation regardless of the bed composition (sand, silt, or clay). Linear and polygonal cracks occur on most bed surfaces. Small scale erosional features are common. Disturbed bedding is locally present. Ripple marks are present only in the sand and silt rich beds. These are symmetrical current ripples, with wave lengths of 1.0 to 3.5 cm and amplitudes of 0.5 to 1.5 cm, covered with fine mud; ripple surfaces have small spindle shaped flute casts, superimposed at an oblique angle to the ripple axis. Interference ripples are most common in the more coarse-grained beds on Sibley Peninsula. Rain-print surfaces occur rarely at Kama Hill. Evaporite casts (probably halite) are present, and are particularly evident at Stewart Lake. $ Summary of the Depositional History The Sibley Group was depositod in a elongate, north/south basin The which was initially probably deepest in the south. basin formed relatively rapidly and along its margins fans of locally The derived and rapidly deposited conglomerate formed. initial period of rapid deposition gave way quickly to relatively slow deposition of the arenites of the Pas's Lake Formation. The basin transgressed northwards towards the end of the deposition of the Pass Lake Formation and extended far to the north (at least to southwestern Lake Nipigon) during deposition of the lower member of the Red Rock Formation. Moderately rapid regression marked the middle stage of Red Rock deposition, accompanied by increased clastic deposition and stromatolite growth to the north of Nipigon and chert precipitation to the south. Transgression followed as the basin extended northward at least to Armstrong. The basin depth was relatively constant, and it slowly filled with clay-rich dolomites. * The transition to the Kama Hill Formation marks a change from predominantly sub-aqueous to predominantly sub-aerial deposition. Deposition of the Kama Hill Formation continued in an extremely consistent very quiet mud flat environment. Primitive life flourished during quiescent periods of deposition of the Kama i-Jill sediments. Tectonic Implications $ The relation of the Sibley Group in any tectonic reconstruction can only be reviewed in the context of the entire Helikian history of eastern �13 North America. The dominant tectonic event in the Lake Superior area was the development of the Keweenawan rift zone which forms an inverted U, extending roughly from the northern area of the Michigan basin, around Lake Superior and continuing southwest from Duluth as the mid-continent gravity high. Although nomajor "opening ocean" event has been firmly documented during the Helikian, Baer (1974) in summarizing papers present at the Grenville symposium (Ottawa, Feb. 1974) indicates that a pre-Grenvillian orogenic event may have initiated with a divergent phaseat 1,300 m.y. As part of this rifting, the Keweenawan arm may have developed at this time (Burke and Dewey, 1973). This arm ultimately (at 1,100 m.y.) underwent limited spreading (Burke and Dewey, 1973). It is probable that igneous activity was initially very limited, but rifting occurred by means of crestal rifts about which developed "rrr" triple junctions. These rifts meet at 120°, and are located at major strike changes of a rift valley (Burke and Dewey, 1973). The major flexure in the Keweenawan rift valley occurs immediately south of Nipigon. Should a triple junction have formed in association with the Keweenawan rifting, the Nipigon area would be the most probable area of development. The 'failed' arm would thus extend north from Nipigon. The Sibley group occupies a N-S block which is the result of a failed arm developed about the Nipigon crestal rift. Later reactivation of the Keweenawan rift was accompanied by intrusion of the Logan sills into the same failed arm. Many problems related to the time of development remain unsolved. The Sibley Group is similar in age to the proposed age of initial spreading of the Grenvillian orogen (Baer, 1974). However, little evidence related to time of initial development of the other ultimately actively spreading arms of the Keweenawan rift is available. Minor igneous activity has been recorded at 1400 m.y. (Books 1969). Sibley sedimentation appears to have been controlled by a rapidly developing fault scarp in the southern area, as indicated by a rapid increase in coarse clastic material near the basin margin. Limited exposure of the Sibley Group in the northern area precludes examination of the nature of the basin margins here. The elongate basin shape is suggestive of rift-valley filling, but the possibility remains that the Sibley basin has been preserved in a failed arm, rather than the 'arm' actually controlling sedimentation. �14 Description of Stops and Road Log for the Gunf lint and Rove Formations Time and seasonal water level conditions may prevent access to all stops indicated on Figures 1 and 2. For alternate and addItional stops of the Gunflint and Rove Formations, the reader is referred to Franklin and Kustira (1972). Mileage count begins at the intersection of Highways 11-17 and 61, near the Airlane Motor Hotel, Thunder Bay. Proceed south on Highway 61. Figures in brackets record accumulated mileage. MILES KM. 00. 0.0 The prominent Intersection Highways 17-11 and 61. hill to the southeast is Mount McKay, the most northerly of the "Norwesters" range of hills. Towering over the Kaministikwa River delta, it is 1,581 feet (481 m) above sea level and 978 feet Mount McKay is a large (298 m) above Lake Superior. mesa, made up of shales and greywackes of the Rove Formation overlain by a hard, protective 200 foot (61 m) thick capping of diabase (Pye, 1969, p. 39). The upper half of the mesa is 3,000 feet (914 m) long and has a maximum width of 1,100 feet (336m). A second sill, about 15 feet (4.6 m) thick is found in the Rove sedimentary rocks and is 474 feet (141.5 m) It forms the base of a below the top of the hill. wide and prominent terrace to which the tourist may drive his automobile for a magnificent view of the City of Thunder Bay (Pye, 1969, p. 39). 5.9 9.5 7.1 11.4 20th Si.deroad; turn right (north). Riverdale Road, turn left (west) and follow to end of road. 8.5 13.7 Parking spot; follow cottage road (J.C. Kirkup); this is private property and permission for access to the Slate River must be obtained from the owner. �15 BASAL CONCRETION-BEARING SHALE OF Th ROVE FOkMATION, SLATE RIVER CANYON (FIG. 1). STOP 1 The black, graphitic, fissile Rove shale contains These vary an abundance of carbonate concretions. from a few inches (cm) to 8 feet (2.4 m) in size Although commonly in the shape of in diameter. oblate spheroids, they display a complexity in shape and texture. Some show radial septarian cracks on their surfaces. The concretions form bedded unitsin the canyon walls. They are in various stages of weathering out of the host shale, some slumping into the river bed and many others lying in the river bed arranged in imbriShale bedding is warped around the cate fashion. top and bottom of the cpncretions. Moorhouse (1963) describes these concretions in detail. Return to Highway 61. 0.0 (11.1) 0.0 (17.9) Intersection of Highway 61 and 20th Sideroad. Continue south on Highway 61. The range of hills to the south are the Nor'westers. 5.4 (19.1) 19.4 (33.1) STOP 2 8.7 Highway 130; continue on Highway 61. (30.7) 31.2 (53.3) Entrance to abandoned quarry, west side of Highway 61, approximately 1,600 feet (410 m) beyond Cloud Bay Road. UPPER THICK-BEDDED QUARTZITIC GREYWACKE, ROVE FORMATION, (FIG. 1). Quarry operations have exposed thin to thick bedded quartzitic greywacke interbedded with lesser amounts Greywacke displays sole of black fissile argillite. markings and graded bedding. The quarry walls are bounded by two dikes bifurcating from a single olivine diabase dike of Geul's (1973) Pigeon River intrusions. The west dike is approximately SO feet (15 m) wide and vertically dipping; the narrower east dike dips steeply southeast, its attitude well exposed at the back of the qvarry. �16 Due to the development of a closely spaced fracture pattern in the Rove Formation, the quarry walls and hack may be unstable; extreme caution is advised. Return to Highway 130. (47.1) 0.0 (75.8) Intersection of Highways 130 and 61. Proceed north on Highway 130; continue to end of road. 3.2 (50.3) 5.1 (81.0) Paipoonge Concession 1 Road. Turn left (west). Proceed for 1.8 miles (2.9 km) to unmarked gravel •road; turn right (north). Continue for 3;4 miles (5.5 km) to steel bridge over Kaministikwa River. Turn right and proceed over bridge on paved road (Highway 588) for 1.2 miles (1.9 km) to Highway Turn •left (west) and proceed for approxi11-17. mately 3.5 miles (5.6 km) through village of Kakabeka Falls into Kakabeka Falls Provincial Park. Turn right at the park gatehouse, before crossing the old bridge in the park, and follow road under Highway 17, to its end at the swimming area. Proceed on foot past cabins to the Kaministikwa River shore. 0.0 STOP 3 BASAL CONGLOMERATE AND LOWER ALGAL CHERT, GUNFLINT FORMATION, (FIG. 1). Here, the basal conglomerate and algal chert mounds rest directly on Archean granitic gneiss basement. Basal conglomerate may be seen in place only at very low water. However, large, angular, locally derived Note the blocks of conglomerate are abundant. angularity and polymictic nature of the pebbles. All can be assigned to various Archean rocks to the west and north. The Ep-Archean Note also the absence of a paleosol. interval, here occupying 800 million years, is represented by little or no "in-situ" weathered basement, suggesting absence of normal weathering conditions, or pre-Gunflint strong fluvial or glacial transport action, The algal mounds here are similar to those found 1.8 miles (2.9 km) west of Nolalu (Franklin and Kustra, 1972, p. 31). Return to park gate house, turn right and cross the bridge. Proceed to parking area, thence by foot to the rim of the falls. �17 STOP 4 UPPER TUFFACEOUS SHALE SUBMEMBER (UPPER TUFF ARGILLITE) AND OVERLYING UPPER CHERT CARBONATE, GUNFLINT FORMATION, FIG. i. Kakabeka Falls drops 128 feet (39 m) into a gorge formed in fissile, thinly bedded upper tuffaceous shale submember (Goodwin, 1956) A more resistant massive two-foot bed of thinly banded chert-carbonate caps the escarpment. Note apparent cross lamination in the chert carbonate and the undulating nature of the surface. The chert carbonate capping is overlain by tuff argillite and a second chert carbonate bed, exposed in a parking lot on the east side of the gorge. A bed of lower chert carbonate occurs at the base of the falls, (Fenwick, personal communication). Return to Highway 17, turn right and proceed approximately 1/3 mile (0.5 km) Ontario Hydro station access road imrpediately west of the Kakabeka Falls Motel. Turn right (south) and proceed to parking lot by the generating station. Access to STOP 5 is through the genenting station to its west side. Permission must be obtained from The spillway serves as a the station supervisor. safety feature to bleed off excess water in the event of generator failure at the power station. Follow the river bank for approximately 600 feet (183 mj to the"spillway" cut. Beware of Poison Ivy. STOP 5 UPPER TUFFACEOUS SHALE SUBMEMBER (UPPER TUFF ARGILLITE) GUNFLINT FORMATION (FIG. 1). The best section of upper tuffaceous shale submember is exposed at this locality. Pyrite-bearing chert, possibly of the upper algal chert submember; occurs It is overlain by shale at the base of the section. containing pyrite nodules and calcareoüs concretions, interbedded shale and tuff and a cap of thinly bedded upper chert-carbonate. �_____— 18 One of the best exposures of "mud ball tuff" in the shtle occurs near the bottom of the section; the tuff is formed of closely packed accretionziry lap ill F elongated a long the bedding. ellipsoids contain mdiv idual small, angular fragments of uniform size, grouped concentrically around a larger shard fragment. The remainder of the material comprising the beds consists of volcanic fragments in a groundmass of green illite: Higher than background radioactivity (0.004% U3O8 has been noted in the tuff argillite by Fenwick (personal communication). Note downwarping of beds on the west side of the exposure and the fault filled with quartz-carbonate and anthraxolite for which Kwiatkowski (1975) reported a 0.2% nickel content. Return to Higiway 11-17 and proceed east. 0.0 (64.7) 0.0 (104.1) 9.8 15.8 (74.5) (119.9) 10.9 17.5 (75.6) (l2l7) 16.3 (81.0) STOP 6 26.2 (130.4) Junction Highway 11-17 and Oliver Road (formerly Highway 590),. Proceed on Oliver Road. Thunder Bay city limit. Good ''iew of a series of mesa type hills, the Nor'westers, all capped by Logan diabase. Junction of Oliver Road with Highway 130; continue east. Belrose road. Turn left, proceed 0.5 miles (0.8 knflto quarry on west side of road, (Dickson's Quarry). UPPER TACQNITE SUBMEMBER, GUNFLINT FORMATION, (FIG. 1) In this quarry wavy-banded, red jàspilitic and darker greena],ite-bearing taconite is capped by a Neohelikian (Logan) diabase sill. The taconite contains approximately 50 percent shale, interbedded with 6 to 12 inch (15 to 30 cm) irregular taconite beds that are best exposed at the north end of the outcrop where quarry operations exposed the taconite at a lower stratigraphic level. The diabase sheet displays an irregular, undulating �19 bottom surface at a slight angular discordaneL The upper surface is polygonally jointed and contains occasional patches of a thin veneer of argitlite. Return to Highway 130, turn left (east), and proceed across Highway 11-1.7 and past Lakehead University to High Street. 19.8 31.9 (83.5) (134.4) Intersection Highway.130 (Oliver Road) and High Street. Turn left at the traffic lights and proceed up High Street. 20.4 (84.1) Entrance to Hillcrest Park. 32.8 (135.4) STOP 7 UPPER LIMESTONE MEMBER, GUNFLINT FORMATION, (FIG. 1). Hillcrest Park stands about 160 feet (48 m) above the level of Lake Superior and offers a panoramic view the of Thunder Bay harbour, the Seqping Giant, WelcomeIslands, Pie Island and the Nor'westers. Dolomitic limestone and chert layers arc exposed at the base of the flag pole and bell. Follow stairs to base of hill where fragmental limestone (upper limestone member) is exposed. The rock consists of many angular tci rounded chert fragments in a matrix of coarsely crystalline, iron-bearing carbonate, and thin chert interbeds. Volcanic shards and fragments occur in the limestone (Goodwin, 1956). Proceed north on High Street. 35.2 21.9 (85.6) (137.8) Intersection with Balsam Street: Turn lefton 22.5 36.1 (86.2) (138.7) Huron Street, 300feet (90 m) south of Highway 17-11. Turn right on Huron Street, then Balsani Street. immediate left. 23.9 38.4 (87.6) (141.0) Bridge over Current River, cross bridge, turn right into Boulevard Lake Park and proceed 0.3 miles (0.5 km); park on right side of road. Traverse begins in river bed. �20 STOP S UPPER CHERT CARBONATE FACT ES, GUNFL I N FORMATION (FIG. 1) The upper chert carbonate fades is overlain by the Rove Formation. An upstream traverse encounters ferruginous carbonate, interrupted by thin layers and lenses of granular and algal chert, dark, fissile shale and dolomitic limestone. At the beginning of the traverse, note the rounded chert lenses showing concretionary structures, attributed to action of algae. Features to observe include stylolite surfaces lined with anthraxolite, pyrite veinlets, imbrication of thin chert layers and the striking weathered app3arance of the rock. Under the bridge, a bed of gray, massive limestone, possibly the Upper Limestone member, contains pancake-like lenses of serpentine material, and is interrupted by a thin band of pyritic and pyrrhotitic Note the hununocky upper surface of the chert. limestone at the shale-limestone interface. The overlying shale is probably Rove Formation, Several hundred feet north of the bridge, at the lookout, a diabase sheet caps the shale. Heat from the cooling of this sheet metamorphosed the limestone, forming serpentine and pyrrhotite. East:of the bridge, in the picnic area, several well developed river terraces are preserved. From bridge, proceed east along Arundel Street. 40.7 25.3 (90.0) (144.9) Intersection, Arundel Street and fodder Avenue. Turn left on fodder Avenue at the fodder Avenue Hotel. Highway 17-11: Turn right. 26.9 43.3 (91.6) (147.4) Scenic lookout. Park View of Thunder Bay harbour. car and walk 500 feet (150 m) east to roadcut on north side of road. �21 SioP 9 UPPER LIMESTONE MEMBER, GUNFLINT FORMATION, OVERLAIN BY DJABASE, (PIG. 1). Sill of Logan diabase overlies argillite and fragmental limestone of the upper limestone member; The contact i gently undulating and visible effects of contact metamorphism are little In this section, however, a microporphyevident. roblastic texture is developed in the argillite. Pyrite is altered to pyrrhotite. Note the lenticular chert patches within the limestone, some veined with pyrrhotite, exhibiting agate textures. Additional End of Animikie portion of trip. points of interest concerning a more complete picture of Gunf lint Formation stratigraphy are given in Fra1lin and Kustra (1972), �______ INSET SCALE 4 KM. Stop Locations Pass Lake Railroad tracks east of Pass Lake No. 5 Road (Pass Lake Area) Ouimet Canyon Kama Fiji Thunder Bay Amythest SIBLEY THUNDER - BAY 40 KILOMETRES Locations. �_____ _____ 23 A A an • - . •. -H-—:- + + + - -:- + + + + + + ARCHEAN + + + + + + ÷ + ÷ FT. 00-, t .+ + + + + + + + B ÷ + + + ÷ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + SANDSTONE ÷ + + + + + CONGL)MERATE + + + + +__+ + + + + + 20 KM. I0 0 B' '10 MILES •.7CSLTSTONE_1-SHALE.T . 7 -. . . . BRECCIA . ..... RED' QOLOSTONE / SANDSTONE ONGLOMERATE ROVE FM, KAMA HILL FM. ( Dots indicate increased silt-sand content ROSSPORT PASS Fig. FM. LAKE FM. 4. Longitudinal and Cross-sections of the Sibley Group. t ÷ 25 METRES 'O q ROVE FM + + �24 Description of Stops and Roadlog for the Sibley Groip Mileage count begins at the junction of Highways 11-17 ad 61 near the Thunder Bay Airport. Figures in brackets recprd accumulated mileages. MILES KM. 0.0 0.0 Proceed north along Highway 11-17. * 1.9 3.1 Golf Links. Road. 2.9 4.7 Oliver Road (Highway 130). 4.9 7.9 5.6 9.0 7.1 11.4 Balsam Street. 9.7 15.6 Hodder Avenue. 19.3 16.6 Scenic Lookout. 12.1 19.5 Spruce River Road (Highway 527 - 21.6 34.8 Lakeshore Drive. 31.2 50.2 Highway 587: turn right and proced southeast. 35.0 56.3 A large azea of outcrop extends along the north side of the C.N.R. railway tracks and Highway 587 where they parallel Pass Lake, * John street. Red River Rowi (Highway 102). formerly 800). �25 This cliff is the type section for the Pass Lake Formation. Exposure is almost continuous for about 2 miles (3.2 km) along the tracks and gives a stratigraphic thickness of 164 feet (50 metres). STOP 1 At the western end of this outcrop, a sandstone quarry provides an excellent exposure of Sibley sandstone. In the railway cut at the western edge of the quarry, Rove shale is altered.to a reddish colour. This alteration affected the Rove for several fçet below its contact with the Sibley Group. Also present is a thin lens of basal conlomerate. PLEASE EXERCISE EXTREME CAUTION CLIMBING ON THE DEBRIS. 35.3 56.8 The conglomerate is better exposed behind'the railway shed at the east end of Pass Lake. Clasts in the basal polymictic conglomerate are composed of 93 per cent Gunflint iron formation, 6 per cent quartz and 1 per cent granite. Boulders are of variable size and angularity, The contact and are cemented in a sandy matrix. with overlying sandstone is sharp; only a few pebbles are found in the base of the overlying unit. The sandstone is moderately to poorly indurated, thick bedded at the bottom of the section, and composed of quartz, with minor chert and feldspar, in a calcite matrix. Continue along Highway 587. 35.7 57.5 Pass Lake East road: 37.6 60.5 No. 4 Road: 37.9 61.0 Y Junction - bear left. 38.6 62.1 Field on left side (west) of road: park here and proceed on foot across field and through bush to CNR tracks - about 1/4 jnije (400 metres); turn left and proceed east. turn left and proceed north. �26 this outcrop is red intraformational breccia Angular fragments vary of the Rossport Formation. from 0.4 in. to 15.7 in. (1 cm up to about 40 cm.) Several clastic dikes are also present. STOP 2 Much of' The lithology is generally a red sandy limestone. Return. to No. 4 Road and continue north. 39.9 64.2 Gravel Pit - for turning vehicle around. back to Pass Lake East Road. 42.3 68.1 Pass Lake East Road - turn Proceed left. *********************** 0.0 0.0 Proceed along Pass Lake Eas; Road from Junction with No. 4 Road. 0.8 (43.1) 1.3 (69.4) Right angle bend to left. 1.5 (43.8) 2.4 (70.5) No. S Road - 2.1 (44.4) 3.4 (71.5) Juncticn - 4.0 (46.3) 6.4 (74.5) Area of outcrop. STOP 3 turn turn left. left. KAMA HILL SANDSTONE Poorly exposed reddish-brown to reddish-purple sandstone of theKama Hill Formation. Generally Although fine- to medium-grained sandstone. not evident at this location mudcracks and ripple marks are common in this unit. These occur along the shore of Lake Superior to the east, (see The main difference between Mcllwaine, 1972). this sandstone and sandstone of the Rossport Formation is the lack of carbonate in the Kama Hill Formation. �27 Continue north along No. S Road. 15.9 9.9 (52.2) (84.0) 10.3 (52.6) (84.7) 16.6 Cross CNR Tracks. Highway 11-17 - Turn right. *********************** 0.0 0.0 (84.7) Proceed northeast along Highway 11-17. (52.6) 2.0 (54.6) 3.2 (87.9) Road to Enterprise Mine. 7.8 (60.4) 12.6 (97.2) Road to Ouimet Canyon - Turn left. 8.4 (61.0) 13.5 (98.2) Y Junction - 9.2 (61.7) 14.8 bear left. * Sharp right turn. (99.3) 9.7 15.6 (62.3) (100.3) Sharp left turn. 10.2 16.4 (62.8) (101.1) Junction - proceed straight. 10.7 17.2 (63.3) (101.9) Junction - proceed straight. 12.8 20.6 (65.4) (105.3) Junction - 13.3 21.4 (65.9) (106.1) Turn off to Gulch Lake Picnic Grounds to Ouimet Canyon. 15.2 turn 24.5 (67.8) (109.1) Bridge. 15.4 24.8 (68.0) (109.4) Quimet Canyon. right. turn left �28 Ouimet Canyon, a spectacular steep-walled gorge, is located in a thick Keweenawan (Logan) di;tbase sill. The canyon can be traced for over two miLes (3.2 km) and is approximately 600 feet (183 m) wide at its It has a maximum depth. of 400 feet, southern end. STOP 4 (122 in). Although diabase is the most conspicuous rock type, a pink quartz monzonite occurs in the south central portion of the canyon, and a calcareous red mudstone of the Sibley Group has been noted localLy at the base of the west wall of the canyon (here bleached grey) and surrounding the southern portion of Gulch Lake. Mapping along the western rim and wall oF the canyon indicates the presence of two major, continuous joint sets and three less-continuou.; minor It appears that the canyon sets (Stacey, 1976, p.3). is a deep erosional depression, carved out of the. diabase along two major joints by the action of glacialice, running water and freeze-thaw action. An interesting feature within the canyon is a prominent diabase pinnacle, referred to as the Indian Head, which has been isolated from the west rim by erosion. The canyon has been declared a Provincial, Park under the Quetico Nature Reserves system. Return to Highway 11-17. Highway 11-17 - 38.2 23.8 (76.4) (123.0) turn left. *********************** 0.0 0.0 (76.4) (123.0) 9.0 5.6 (82.0) (132.0) 12.4 7.7 (84.1) (135.3) Proceed northeast along Highway 11-17. . Bridge over Wolf River. Road to Stewart Lake at end of Itinerary)'. (see extra stop descriptions �29' 20.4 32.8 (96.8) (155.8) Highway 627 - proceed straight. 21.3 34.3 (97.7) (157.2) Highway plaque describing Red Rock Cuesta. The cuesta features a thick sequence of red sandy limestone capped by a diabase sill. 22.6 (99.0) 36.4 (159.3) Road cut here shows a diabase sill cutting Archean rocks and Sibley Group. 25.7 41.4. (102.1) (164.0) Road to Mousseau Mountain - (see extra slop descriptions at end of Itinerary). 28.3 45.5 (104.7) (168.5) Junction of Highways 11 and 17, - Proceed straight along Highway 17. 40.6 65.3 (117.0) (187.3) Junction of Domtar Road - (see extra stop descriptions at end of Itinerary). 66.5 41.4 (117.8) (189.6) First Lookout, Kaina Hill. STOP SA ROSSPORT FORMATION OVERLAIN 'BY KAMA HILL FORMATION WITH A CAPPING OF DIABASE. A broad anticline of sandy red carbonate is exposed in the prominent road cut to the north of this lookout. This represents the lowest member of the Rossport Formation. Soft-sediment deformation may have produced this structure. Three thin diabase sheets follow bedding planes; the sills pinch out, and locally Thi: cut across bedding at a high angle homogeneous, calcareous sandy mudstone :orms a distinct horizon at the base of the sand red mudstone unit. Follow the road (south) to the distinctive red and white interbedded sandy mudstone. Sand:.tone beds (white) are lenticular in shape, anti are interbedded with red, sandy mudstone. �30 42.1 67.8 (118.5) (190.7) Second Lookout, Kama Hill. In the roadcut to the north of the second lookout, the following features may be observed: (1) Two thin Keweenawan diabase sills, partially replaced by carbonate, cut across the poorly developed bedding place at a low angle. (2) Finely laminated chert of the chert-stromatolite unit is exposed below the lower sill. Up to six inches (15 cm) of anthraxolite carbonate has accumulated at the base of the chert. An oily smell may be detected when this anthraxolite is freshly broken. (3) Limey red mudstone above this unit is marked by many cream-coloured spots, (average diameter 0.5 inch, 13 mm). Similar spots are evident throughout this unit, and commonly have a small amount of graphite or hydrocarbon at the centre. In thin section, the only apparent mineralogical change in the spots is the lack, of hematite coating on clay and carbonate grains. (4) Irregular, flame-shaped, bleached zones follow structures and bedding plane cleavage in the Leaching of hematite and red limey mudstone. destruction of clay mjnerals and feldspar has occurred along the fractures. (5) Above the road cut and overlying the talus It is slope, the purple mudstone crops out. more highly fissile, and contains approximately 4 per cent hematite, which coats very fine grained corrensite and microcline, and forms Bleaching blades of specularite in tiny vugs. along fractures is common in this rock. Purple mudstone contains abundant syneresis cracks, and to the west, at Stewart Lake, contains thin stromatolite beds. 119.6 192.5 Outcrop on east side of Highway. �31 STOP SB Red Shale of the Kama Hill Formation is exposed here underneath a cap of diabase. Turning Point - Proceed back towards Thunder Bay. * * * * * * * * * * * * * *+ * * * * * * * * * 0.0 0.0 (119.6) (192.5) Proceed west along Highway 17. 55.2 88.8 (174.8) (281.3) East Loon Road - turn right (north) and proceed to end of road to Thunder Bay Amethyst Mining Company Limited. STOP 5 THUNDER BAY AMETHYST MINING COMPANY LIMITED The Thunder Bay Amethyst Mine is the largest producing amethyst mine in Ontario and is open annually to the public, from May 1st to November 1st. The property has been in production since 1962. The number of rockhounds and tourists visiting the mine site has increased steadily from 900 visitors There are in 1967 to 24,396 visitors in 1976. sufficient reserves to give the mine an expected life of 65 years at current mining rates (R. Hartviksen, Mine Manager, personal communication). The amethyst deposit is located in an east-west fault zone cutting an intrusive body of massi'e, mediumgrained, red to pink Archean quartz monzonite. Spectacular breccia is noted in the floor of the quarry and in large blocks in the display area exhibiting fragments of unaltered quartz monzonite and highly silicified dolomite of the Sibley Group. Quarrying, diamond drilling and stripping has delineated the deposit for a length of approximately 1000 feet (305 m) and for a width of over 80 feet (24th). Individual amethyst veins vary in width from 1/4 inch (7.6 mm) to 4 feet (1.2 m) and include numerous cavities lined with purple crystals. Well formed crystals (points) line the cavities and vary in size from 1/4 inch (7.6 mm) diameter to large crystals measuring 9 inches (22.9 cm) from tip to root and 6 inches (15.3 cm) in diameter. �32 Coloration of crystals and-the more massive material is dark purple. Variations in intensity of purple colour occur, and locally, colourless am! smokycoloured quartz is found. Crystals are )ccasionally coated with a reddish brown hematite. The colour of ametht results from substitution of small quantities of ferric iron for silicon followed by irradation, (Dennen and Puckett, 1972, p.448). The deposits of the producing amethyst mLnes in McTavi;h Township are found either in fradtures that extend below the.Sibley-Archean unconformity or. at the coittact of the Sibley Group with Archean granite. END OF ITINERARY For anyone interested in a more complete view of the Sibley Group, the following additional areas may be visited. From Rossport, a boat trip to Quarry Channel and Wilson Islands, which lie one to two miles off shore, will allow the visitor On Quarry to see an almost complete section of Sibley rocks. Island, Rove shale is overlain by a thick section of Pass Lake sandstone. Here, crossbeds and ripple marks are abundant. 1. On Channel Island, the upper part of the sandstone unit, sandy red mudstone units are all exposed. The latter is disconformably overlain by Osler volcanic rocks. The type section of the Kama Hill Formation at the top of Kama Hill. The best access is provided by following the Domtar Logging Road (0.8 mi., 1.3 km west of the first lookout at The Kama Hill) for 0.3 mi (0.5 km) to the first powerline. section is at the top of the hill and is exposed on the powerline. 2. 3. - good section of Pass Lake sandstone is exposed along the road up to Mousseau Mountain. The top of this hill also provides one of the best views of Lake Superior and the surrounding country. A MILES KM. 0.0 0.0 Leave Highway 11-17 point). (see Itinerary fox Junction �33 turn 1.5 2.4 Road junction - 1.9. 3.1 Road junction - turn left. Proceed along this road to Mousseau Mountain (see Coates 1972). left. 4. A further section of Kama Hill Shale may be viewed at Stewart Lake, salt casts may be found here. 5. The stromatolites near Disraeli Lake may be reached by following the Armstrong road north from Hurkett for 21.6 miles, (34.7 kin) to the Disraeli Lake road, which connects the Armstrong road with the Spruce River Road (Hwy. 527). Follow the Disraeli Lake road west for 22.2 miles (35.7 km) past Shillaber and Seagull Creeks to the Disraeli campground road. Proeed for 3/4 of a mile (1.2 km) beyond this, to the first bush ro.sd leading north. Follow this road for two miles (3.2 km). Blocks of stromatolite are strewn along side the road for some distance. Stromatolite blocks are common throughout the Disraeli area, and may be found in outcrop and float along most of :he bush roads. AC KNOWLEDGEMENTS The authors wish to acknowledge the assistance of Mi. S. Spivak who compiled and drafted the figures and Mrs. Cathy LeBnn for typing the manuscript. The cover plate was taken by J.F. Scott. �34 SELECTED BIBLIOGRAPHY OF ThE PROTEROZOIC ROCKS 'OF THE THUNDER BAY AREA * Abelson, P.H. and Hare, P.E. 1968: Recent Amino Acids in the Gunf lint Chert; Carnegie Inst. Washington, Yearbook 67, pp. 208-210. Alexandrov, Eugene A. 1955: Contribution to Studies of Origin of Precambrian banded Iron Ores; Econ. Geol., Vol. 50, pp. 459-468. 40Ar39Ar Studies of Precambrian Alexander, E. Colvin, Jr. 1975: Cherts: An unsuccessful attempt to measure the time evolution of the atmospheric 4OAr/36Ar Ratio; Precambrian Research, Vol. 2, pp. 329-344. Proterozoic Flood Básalts of Eastern Lake Annells, R.N. 1973: Superior: The Keweenawan Vblcanic Rocks of the Mamainse Point Area, Ontario; Geol. Surv. Canada, Paper.72-1O, Slp. Keweenawan Volcanic Rocks of Michipicoten Annells, R.N. 1974: Island, Lake Superior, Ontario; An Eruptive Centre of Proterozoic Age; Geol. Surv. Canada, Bull. 218, PUp. Geoscience Baer, A.J. 1974: Grenville Geology and Plate Tectonics; Canada, 1, pp. 54-60. Diffusion in gatO Point Vitrophyres; Amer. Jour. Sci., Vol. 211, pp. 74-88. Bain, George IV. 1926: Barghoorn, E.S. 1971: No. 5, pp. 30-42. The Oldest Fossils; Sci. Amer., tol. 224, Microorganisms Three Barghoorn, Elso S., Schopf, William, J. 1966: Billion Years Old from the Precambrian of South Africa; Science, Vol. 152, pp. 758-763. * Barghoorn, Elso Microorganisms from S., Tyler, Stanley, A. 1965: the Gunflint Chert; Science, Vol. 147, No. 3658, pp. 563-577. Barghoorn, Elso.S., Tyler, Stanley A. 1965: Mitroorganisms of Middle Precambrian Age from the Animikie Series, Ontario, Canada; Chap. 3, in Current aspects of exobiology, Calif. Technol., Jet PropulsionLab., Fasadena, pp. 93-118. '[he Animikie Sea; a talk given at Institute On Bartley, M.W. 1958: Lake Superior Geology, Minneapolis, Univ. Minn. Center Continuation Study, 9p. * * All * references are on file with Regiona' Geologist, Ontario Ministry of Natural Resources, Ontario Government Building, 435 James St. S., Thunder Bay, P7C 5G6. �35 1960: Magnetization of Volcanic Rocks in the Lake Superior Geosyncline; U.S. Geol. Surv. Prof. Paper 400-B, Bath,. Gordon, D. pp: B'212-B213. Minera1oy and S.tdimentation in the Kama Hill Battrum, D.D. 1975: Formation of the Sibley Group, Northwestern Ontario; unpubl. B.Sc. Thesis, Lakehead University., Thunder Bay, Ontario, 141 p. Bayley, R.W. and James, ILL. 1973: Precambrian Iron-Formations of the United States; Econ. Geol.. Vol. 68, No. 7, pp. 934-959. Paleomagnetism of Keweenawan Intrusive Beck, Myrl E., Jr. 1970: Rocks, Minnesota; J. Geophys. Res., Vol. 75, No. 26, PP.. 49854996, On the Geology and Economic Minerals on the NorthBell, R. 1870: east Coast of Lake Superior and Adjoining Country from Pigeon River to Black Bay, Black Sturgeon River, Nipigon River and Lake Geol. Surv. Can., Report of Progress 1866-1869, Pt. IX. Nipigon; The Iron Formation Syndrome; Econ;.Geol., Beutner, E.L. 1972: Vol. 67, PP. 254-255. Differentiation and Assimilation in the Logan Blackadar, R.G. 1956: Sills, Lake Superior District, Ontario; Amer. Jour. Sci., Vol. 254, pp. 625-645. Metamorphic Pyroxenes and Amphiboles in Bonnichsen, Bill, 1969: the Biwabik Iron Formation, Dunka River Area, Minnesota; Mineral.Soc. Amer. Spec. Paper 2, pp. 217-239. The Duluth Complex; Geo. Soc. America, Bonnichsen, Bill, 1972: Abstracts with Programs, Vol. 4, No. 7, pp. 453-454. Magnetization of the Lowermost Keweenawan Books, Kenneth G. 1968: Lava Flows in the Lake Superior Area: U,S. Geol. Survey Prof.. Paper 600-D, pp. D248-D254. Paleomagnetism of some Lake Superior Books, Kenneth G. 1972: Keweenawan Rocks; U.S. Geol. Survey Prof. Paper 760, 42 p. Further Paleomagnetic Books, Kenneth C. and Green John C. 1972:. Data for Keweenáwan Rocks in the Western Lake Superior Area; Geo. Soc. Amer., Abstracts with Programs, Vol. 4, No. 7 P. 454. Concerning "EvIdence of Liquid. Immiscibility in Bowen, N.L. 1926: a Silicate Magma, Agate Point, Ontario", Jour. Geol., Vol. 34, pp. 71-73. Economic Geology and Stratigraphy of the Broderick, T.M. 1920: Günflint Iron District, Minnesota; Econ. Geol., Vol. 15, pp. 422452. �36 Broughton, Paul L. 1975: The Thunder Bay Amethyst Mine; Rock and Gem, July, pp. 58-62. Burke, K. and Dewey, J.F. 1973: Plume generated triple junctions: Key indicators in applying plate tectonics to old rocks; Jour. Geol., Vol. 81, pp. 406-433. Campling, Neil R. 1973: Some Geological and Environmental Aspects of Remnant Pre-Gunflint and. Pre-Sibley Weathered Profiles; urtpubl. B.Sc. Thesis, Lakehead University, Thunder Bay, Ontario. 43 p. Card, K.D., Church, W.R., Franklin, J.M., Frarey, M.J., Robertson, J.A., West, G.F., and Young, G.M. 1972: The Southern Province; in Variations in Tectonic Styles in Canada, R.A. Price, ed., Geol. Assoc. Canada, Special Paper 11, pp. 336-380. Chase, Clement G. and Gilmer, Todd H. 1973: Precambrian Plate Tectonics: The Midcontinent Gravity High; Earth Planet. Sci. Letters, Vol. 21, pp. 70-78. Cloud, Preston E. Jr. 1942: Vol. 240, pp. 363-379. Notes on Stromalites; Amer. Jour. Sci., Cloud, Preston E. Jr. 1965: Significance of the Gunflint (Precambrian) Microflora; Science, Vol. 148, No. 3666, pp. 27-35. Cloud, Preston E. Jr., and Hagen, Hannelore, 1965: Electron Microscopy of the Gunflint Microflora: Preliminary Results; Natl. Acad. Sci. Proc., Vol. 54, No. 1, pp. 1-8. Cloud, P.E. Jr., and Semikhatov, M.A. 1969: Proterozoic Stromatolite Zonation; Amer. J. Sci., Vol. 267, pp. 1017-1061. Coates, M.E. 1972: Geology of the Black Sturgeon River Area, District of Thunder Bay; Ontario Dept. Mines and Northern Affairs, GR 98, Accompanied by Maps 2233, 2234, 2235, 2236, scale 1 inch to 41 p. 1 mile. Cooke, H.C. 1931: Studies of Physiography of the Canadian Shield: III. The pre-Pliocene physiographies, as inferred from the geologic record; Trans. Roy. Soc. Canada, Vol. 25, Sect. 4, pp. 127-180. Cornwall, HenryR. 1951: Ilmenite, Magnetite, Hematite and Copper in Lavas of the Keweenawan Series; Econ. Geol., Vol. 46, pp. 51-67. Cornwall, Henry R. 1951: Differentiation in Lavas of the Keweenawan Series and the Origin of the Copper Deposits of Michigan; Geol. Soc. Amer. Bull., Vol. 62, pp. 159-202. Cornwall, Henry R. 1951: Differentiation in Magnias of the Keweenàwan Series; Jour. Geol., Vol. 59, pp. 151-172. �37 Courtis, W.M. 1887: The Animikie Rocks and their Vein Phenomena as shown at Duncan Mine, Lake Superior; Amer. Inst. Mi Eng., Transactions, Vol. 15, pp. 671-677. Craddock, Campbell, 1972: StructUral Evolution of the Keweenawan Province; Geol. Soc. Amer., Abstracts with Programs, Vol. 4, No. 7,, pp. 715-716; Davies, F. Bryan, and Windley, Brian, F. 1976: Significance of major Proterozoic high grade linear belts in continental evolution; Nature, Vol. 263, pp. 383-385. Dennen, William, i-I. and Puckett, Anita M. 1972: On the Chemistry and colour of Amethyst; Can. Mineralogist, Vol. 11, No. 2, pp. 448-456. Dott, R.H. 1972: A Post-Animikean - Pre-Keweenawan Transgressive Sand Blanket over the Lake Superior Region; Geo. Soc. Amer., Abstracts with Programs, Vol. 4, No. 7, pp. 490-491. Drever, James I. 1974: Geochemical Model fOr the Origin of Precambrian Banded Iron Formations; Geol. Soc. Amer. Bull, Vol. 85, pp. 10991106. Correlation of Keweenawan Rocks of Lake Superior District by Palaeomagnetic Methods; Geol. Asoc. Canada, Vol. 11, pp. 115-128. flu Bois, P.M. 1959: Palaeomagnetism and Correlation of Keweenawan Rocks; Geol. Surv. Canada, Bull: 71, 75p. flu Bois, P.M. 1962: Further investigations of fossils from the Edhorn, Anna-Stina, 1973: Animikie, Thunder Bay, Ontario; Proc. Geol. Assoc. Canada, Vol. 25, pp. 37-66. Nickeliferous and liraniferous Anthraxolite from Ellsworth, H.V. 1934: Port Arthur, Ontario., Amer. Mm., Vol. 19, No. 9, pp. 426-428. Age and significance of Diabase Fahrig, W.F. and Wanless, R.K. 1963: Dyke Swarms of the Canadian Shield; Nature, Vol. 200, pp. 934-937. Faure, G. and Chaudhuri, 5. 1967: The geochronology of +the Keweenawan Dept. Geol. Rocks of Michigan and Origin of the copper deposits. Ohio State Univ.,Lab. Isotope, Geol. and Gepchem. Dept. No. 1. Ages of the Duluth Faure, G., Chaudhuri, S., and Fenton, M.D. 1969: Gabbro complex and of the Endion Sill, Duluth,Minnesota; J. Geophys. Red., 74, pp. 720-725. The Age of the Gunflint Iron Faure, Gunter; Kovach, Jack, 1969: Formation of the Animikie Series in Ontario, Canada; Bull. Geol. Soc. Amer., Vol. 80, pp. 1725-1736. �38 Faure, Gunter; Kovaçh, Jack, 1969: The Age of the Gunflint Iron Formation of the Animikie Series in Ontario, Canada; Ohio State University, Laboratory for Isotope Geology and Geochemistry, Contribution No. 8, 23p. Floran, R.J. and Papike, J.J. 1975: Petrology of the Low-Grade Rocks of the Gunflint Iron Formation, Ontario-Minnesota; Geol. Soc. Amer. Bull., Vol. 86, pp. 1169-1190. Franklin, J.M. 1970: Metallogeny of the Proterozoic Rocks of the Thunder Bay District, Ontario; unpubl. Ph.D. Thesis, Western Univ., London, Ontario, 317p. Franklin, J.M. and Kustra, C.R. 1970: Proterozoic rocks in the Thunder Bay Area; Institute on Lake Superior Geology, 16th Annual Meeting, Thunder Bay, Guidebook, pp. 48-68. Franklin, J.M. and Kustra, C.R. 1972: The Proterozoic Rocks of the Lake Superior Area, Northwestern Ontario; p.20-46 in Guidebook for Field Excursion C34. International Geological Congress, Twenty-fourth Session, Canada 1972, 74 p. Franklin, J.M., Poulsen, K.H., and Mcllwaine, W.H 1972: Stratigraphy of the Sibley Group, A Helikian Red Bed Sequence; Gecl. Soc. Amer., Abstracts with Programs, Vol. 4, No. 7, p. 509. Franklin, James, H. (in press): Interpretation of the Rb/Sr isochrons of metamorphosed and unmetamorphosed Rove shale. Franklin, J.M. Mcllwaine, W.H., Poulsen, K.H. and Wanless, R.K. Stratigraphy and Sedimentation of the Sibley Group; 37 p. (in prep.): Franklin, J.M. and Mitchell, R.H. (in prep.): Lead-Zinc-Barite Veins of the Dorion Area, Thunder Bay District, Ontario; 25 p. French, Bevan M. 1973: Mineral Assemblages in Diagenetic and Low Grade Metamorphic Iron-Formation; Econ. Geol. Vol. 68, pp. 1063-1074. French, William A. 1976: Silver Mining in the Thunder Bay Region 18451891: An Examination of its Economic Viability; Unpubl. B.A. Thesis, Lakehead University, Thunder Bay, 44p. Garrels, R.M., Perry, E.A. Jr. and MacKenzie. F.T. 1973: Genesis of Precambrian Iron Formations and the Development of Atmospheric Oxygen; Econ. Geol., Vol. 68, pp. 1173-1179. Guel, J.J.C. 1970: Geology of Devon and Pardee Townships and the Stuart Location; Ontario Dept. of Mines and Northern Affairs, GR 87, 52 p. Accompanied by Map 2207, scale 1 inch to 1/2 mile. Guel, J.J.C. 1973: Geology of Crooks Township, Jarvis and Prince Locations, and Offshore Islands, District of Thunder Bay; Ontario Div. Mines, GR 102, 46 p. Accompanied by Map 2250, scale l inch to 1/2 mile. * �39 Giguere, J.F. 1975: Geology of St. Ignace Island and Adjacent Islands, District of Thunder Bay; Ontario Div. Mines, GR 118, 35p. Accompanied by Map 2285, scale linch to 1 mile. Gill, J.E. 1926: Gunflint Iron-Bearing Formation; Geol. Surv Canada, Summ. Rept., 1924, Pt. C, pp. 28c-88c. Gill, J.E. 1927: Origin of the Gunflint Iron-Bearing Formation; Econ. Geol., Vol. 22, No. 7, pp. 687-728. Gill, J.E. 1957: Summary and Discussions; Pp. 183-191 in the Proterozoic in Canada, Roy. Soc. Canada, Spec. Pubi. No. 2, 191 p. Glaessner, M.F. 1968: Biological events and the Precambrian time scale; Can J. Earth Sci., Vol. 5, No. 3, PP. 585-590. Geochronology in the Lake Superior region; Can. Goldich, S.S., 1968: Jour. Earth Sci., Vol. 5, No; 3, Pt. 2, PP. 715-724. Goldich, S.S., 1973: Ages of Precambrian Banded Iron Formation; Econ. Geol., Vol. 68, PP. 1126-1134. Facies Relations in the Gunflint Iron Formation; Goodwin, A.M. 1956: Econ. Geol., Vol. 51, No. 6, PP. 565-595. Goodwin, A.M. 1960: Gunflint Iron Formation of the Whitefish Lake Area; Ontario Dept. Mines, Vol. 69, Pt. 7, Pp. 41-63. Origin of Banded Iron Formation; Geol. Soc. Amer Govett, G.J.S., 1966: Bull., Vol. 77, pp. 1191-1212. Field Trip Guide Book for Precambrian North Shore Green, John C. 1972: Volcanic Group, Northeastern Minnesota; Annual Meeting, Geol. Soc. Amer., Minneapolis, Minnesota, 36 P. Metamorphism of Iron Formations and its Bearing on• Gross, G.A. 1961: their Benefication; Can. Mm. Met. Bull., Vol. 54, PP. 30-37. Gross, G.A. 1965: Geology of Iron Deposits in Canada; Vol. 1, General Geology and Evaluation of Iron Deposits; Geol. Surv. Canada, Econ. Geol. Report No. 22, 181 P. Gross, G.A. 1972: Primary Features in Cherty Iron Formations; Sediment. Geol., Vol.7, pp. 241-261. The Grout, Frank F., Sharp, Robert P., and Schwartz, George M., 1959: Geology of Cook County, Minnesota; Minn. Geol. Survey, Bull. 39, 16 3p. The Origin of the Sedimentary Iron Formations: Gruner, John W. 1922: The Biwabik Formation of the Mesabi Range; Econ. Geol., Vol. 17, pp. 407-460. �40 Halls, H.C. 1966: A Review of the Keweenawan Geology of the Lake Superior Region; in The Earth beneath the continents (edited by J.S. Steinhart and T.H. Smith), Am. Geophys. Union, Monograph 10, pp. 3-27. Halls, H.C. 1969: Compressional wave velocities of Keweenawan roèk specimens from the Lake Superior region; Can. Jour. Barth Sci., Vol. 6, pp. 555-568. Halls, H.C. 1972: Geophysical Studies in Northern Lake Superior; Geo. Soc. Amer., Abstracts with Programs,. Vol. 4, No. 7, pp.525: Halls, H.C. 1972: Magnetic Studies in Northern Lake Superior; Can. J. Earth Sci., Vol. 9, No. 11, pp. 1349-1367. Halls, H.C. 1974: A Paleomagnetic Reversal in the Osler Volcanic Group, Northern Lake Superior; Can. J. Earth Sci., Vol. 11, No. pp. 1200-1207. 9, Halls, H.C. 1975: Shock induced remanent magnetization in late Precambrian rocks from Lake Superior; Nature, Vol. 225, pp. 692-695. Halls, H.C., and West, G.F. 1971: A Seismic Refraction Survey in Lake Superior; Can. J. Earth Sbi., Vol. 8, No. 6, pp. 610-630. Hamblin, William Kenneth, 1961: Palaeographic Evolution of the Lake Superior Region from Late Keweenawan to Late Cambrian Time; Bull. Geol. Soc. Am., Vol. 72, pp. 1-18. Hanson, Gilbert N. 1975: 4OAr/39Ar Spectrum Ages of E4ogan Intrusions, a Lower Keweenawan Flow, and Mafic Dikes in Northeastern MinnesotaNorthwestern Ontario; Can. J. Earth Sci., Vol. 12, pp. 821-835. Hanson, G.N., and Malhotra, R. 1971: K-Ar Ages of Mafic Dikes and Evidence for Low-Grade Metamorphism in Northeastern Minnesota; Geol. Soc. Amer. Bull., Vol. 82, pp. 1107-1113. Harris, F.R. and Kustra, C.R. 1968: Field Excursion Guide: Gunflint Iron Formation - Sibley Group; Ontario Dept. Mines Geol. Field Trip, Canadian Lakehead Area, 11 p. Hawley, J.E. 1929: Lead and zinc Deposits, Dorion and McTavish Townships, Thunder Bay District. Ontario Dept. Mines, Vol. 38, pt. 6, pp. 58-85. Accompanied by Map 38f, scale 1 inch to 1 mile. Heslop, John Boyd, 1968: Mineralogy and Textural Relationships of the Mount Mollie Sulphides, Pine Bay Area, Thunder Bay District; Unpubi. B.Sc. Thesis, University of Western Ontario, London, Ontario. 68p. Hinze, W.J., Roy, R.F. and Davidson, D.M. 1972: The Origin of Late Precambrian Rifts; Geol. Soc. Amer. Abstracts with Programs, Vol. 4, No. 7, pp.723. �41 Hofmann, H.J. 1969: Stromatolites from the Proterozoic Animikie and Sibley Groups, Ontario; Geol. Surv. Canada, Paper 68-69, 77p. Hofmann, H.J. 1969: Attributes of Stromatolites; Geol. Surv. Canada, Paper 69-39, 58p. Hofmann, H.J. 1971: Precambrian Fossils, Pseudofossils, and Problematica in Canada; Geol. Surv. Canada, Bull. 189, l46p. Holden, Edward F. 1925: The cause of color in smoky quartz and amethyst; The American Mineralogist, Vol. 10, No. 9, pp. 203-252. Hotchkiss, W.O. 1923: Amer., Vol. 34, The Lake Superior Geosyncline; Bull. Gepl. Soc. pp. 669-678. Fresh-water Environment of Deposition of PreHough, J.L. 1958: cambrian Banded Iron Formations; Jour. Sed. Pet.,V. 28, Np. pp. 414-430. 4, Intensity of the Geomagnetic Hubbard, T.P. and Ade-Hall, J.M. 1972: Field during the Extrusion of the Keweenawan Lavas; Geol. Soc., Amer., Abstracts with Program, Vol. 4, No. 7, pp. 546. Keweenawan Geology of Isle Royale, Michigan; Huber, N.King, 1972: Geo. Soc. Amer., Abstracts with Program, Vol. 4, No. 7, p. 546. The Portage Lake Volcanics (Middle Keweenawan) Huber, N.K., 1973: on Isle Royale, Michigan; Geol. Survey Prof. Paper 754-C, 32p. Hunt, T.S. 1873: The Geonostical History of the Metals; Trans. Am. Inst. Mi Eng., 1., pp. 331-345. Hurley, P.M., Fairbairn, H.W., Pinson, W.H. Jr., Hower, J. 1962: linmetamorphosed minerals in the Gunf lint Formation used to test the age of the Animikie; Jour. Geology, Vol. 70, No. 4, pp. 489492. Ingall, E.D. 1887: Report on mines and mining on Lake Superior (published 1888); Geol. and Nat. Hist. Survey Canada, Ann. Rept., Vol. 3, pt.H, 124p. Sedimentary Facies of Iron Formation; Econ. James, Harold J. 1954: Geol., Vol. 49, No. 3, pp. 235-293. Keeler, R.G. 1971: The Petrology of the Moss Lake Noritic Intrusion, District of Thunder Bay, Ontario; unpubi. B.Sc. Thesis, University of Toronto, 53p. Changes in Mineral Assemblages with Klein, Cornelis, Jr. 1973: Metamorphism of some Banded Precambrian Iron Formations; Econ. Geol., Vol. 48, pp. 1075-1088. �42 Konda, Tadashi, and Green, John C. 1974: Clinopyroxenes from the Keweenawan Lavas of Minnesota; Am. Mineralogist, Vol. 59, pp. 1190-1197. Kwiatkowski, Dennis 1975: Geology and Geochemistry of the Kakabeka Falls Anthraxolite; unpubl. B.Sc. Thesis, Lakehead University, 103p. LaBerge, Gene L. 1964: Development of Magnetite in Iron Fbrmations of the Lake Superior Region; Econ. Geol., Vol.59, pp. 1313-1342. LaBerge, Gene L. 1967: Microfossils and Precambrian Iron Formations; Geol. Soc. Amer. Bull., Vol. 78, pp. 331-342. LaBerge, Gene L. 1973: Possible Biological Origin of Precambrian Iron Formations; Econ. Geol., Vol. 68, No. 7, pp. 1098-1109. Lepp, Henry and Goldich, Samuel 5. Origin of Precambrian Iron 1964: Formations; Econ. Geol., Vol. 59, pp. 1025-1060. Logan, Sir. W.E. 1863: The Geology of Canada; Geol. Surv. Can. Rept. of Progress from Commencement of 1863. Logan, B.W., Rezak, R. and Ginsburg, R.N. 1964: Classification and Environmental Significance of Algal Stromatolites; J. Geol., Vol. 72, No. 1, pp. 68-83. The Sulphide Assemblage of The Great Lakes Mainwaring, Paul R. 1968: Nickel Intrusion; unpubl. B.Sc. Thesis, University of Western Ontario, London, Ontario. 63p. McCuaig, James Auley, 1950: A Copper-Nickel Occurrence in Pardee Township, Thunder Bay District, Ontario; unpubl. M.Sc. Thesis, McGill University, Montreal, 61p. McTavish Township (West Part of North Half) Mcllwaine, W.H. 1971a: District of Thunder Bay; Ont. Dept. Mines and Northern Affairs, Prelim. Geol. Ser. Map P.720, scale 1 inch to 1/4 mile. Mcllwaine, W.H. 1971b. McTavish Township (East Part of North Half) District of Thunder Bay, Ont. Dept. Mines and Northern Affairs, Preliin. Geol. Ser. Map P.721, scale 1 inch to 1/4 mile. Mcllwaine, W.H. 1975: MeTavish Township (South Half) District of Thunder Bay. Ont. Dept. Mines and Northern Affairs, Prelim. Geol. Ser. Map P.990, scale 1 inch to 1/4 mile. Stratigraphy, Petrography, Mcllwaine, W.H. and Wallace, Henry, 1972: and Chemistry of the Late Precambrian Osler Group, District of Thunder Bay, Ontario; Geol. Soc. Amer. Abstracts with Programs, Vol. 4, No. 7, p. 590. Geology of the Black Bay Mcllwaine, W.H. and Wallace, Henry, 1976: Peninsula Area, District of Thunder Bay; Ontario Div. Mines, Accompanied by Map 2304, scale 1 inch to 1 mile. GR 133, 54p. �43 Mcllwaine, W.H., Wallace, Henry, Franklin, J.M. and Poulsen, K.H., 1974: Stratigraphy and Tectonic Setting of the Late Precambrian. (Helikian) of Northwestern Ontario; Geol. Assoc. Canada Mm. Assoc. Canada, Annual Meeting, Program Abstracts pp. 60-61. Mcllwaine, W.H. and Tihor, L.A. 1975: Dorion-Wolf Lake Area (Western Part)., District of Thunder Bay; Ontario Div. Mines, Prelim. Map 994, Geol. Ser., Scale 1 inch to 1/4 mile or 1:15,840. Geology 1972. Moore, J.G. and Peck. D;L. 1962: Accretionary Lapilli in Volcanic Rocks of the Western Continental U.S.; Jour. Geol., V 10, No. pp. 182-193. 2, Moorhouse, W.W. 1957: The Proterozoic of Port Arthur and Lake Nipigon Regions, Ontario; pp. 67-76, in The Proterozoic in Canada, Roy. Soc. Canada, Special Publication No. 2, l9lp. Moorhouse, W.W. 1960: Gunflint Iron Range in the Vicinity of Port Arthur; Ontario Dept. Mines, Vol. 69, pt. 7, pp. 1-40. Accompanied by 7 maps, Scale 1 inch to 1/2 mile. Moorhouse, W.W. 1963: Concretions from the Animikie of the Port Arthur Region, Ontario; Proc. Geol. Assoc. Canada, Vol. 15, pp. 43-59. Moorhouse, W.W. and Beales, F.W. 1962: Fossils from the ithimikie, Port Arthur, Ontario; Trans. Roy. Soc. Canada, Vol. 56, Series 3, pp. 97-110. Morey, G.B. 1967: Stratigraphy and Sedimentology of the Middle Precambrian Rove Formation in Northeastern Minnesota; Jour. Sed. Pet., Vol. 37, No. 4, pp. 1154-1162. Morey, G.B. 1969: The Geology of the Middle Precambrian Rove Formation in Northeastern Minnesota; Minnesota Geological Survey Sp-7, Special Publication Series, University of Minnesota, Minheapolis, 62p. Morey, G.B. 1972: Gunflint Range; inSims, P.K., and Morey, G.B.,eds., Geology of Minnesota - A Centennial Volume (Schwarcz Vol.), Minnesota Geol. Survey, pp. 218-225. Morey, G.B. 1973: Mesabi, Gunflint and Cuyuna Ranges, Minnesota; Unesco 1973, Genesis of Precambrian Iron and Manganese Deposits, Proc. Kiev. Symp., 1970 Earth Sciences, Vol. 9, pp. 193-207. Morey, G.B. and Sims, P.K. 1976: Boundary Between Two Precambrian W. Terranes in Minnesota and its Geologic Significance; Geol. Soc. Amer. Bull., Vol. 87, pp. 141-152. Mudrey, M.G., Jr. 1976: Late Precambrian Structural Evolution of Pigeon Point, Minnesota and Relations to the Lake Superior Syncline; Can. J. Earth Sci., Vol. 13, pp. 877-888. �44 Mudrey, M.G., Jr. and Weiblen, P.W. 1972: Diabase Intrusions of Northeastern Minnesota: Part 1; Geo. Soc. Amer., Abstracts with Programs, Vol. 4, No. 7, pp. 606. Murthy, G.S., Fahrig, W.F. and Jones, D.L. 1968: The Paleomagnetism of the Michikamau anorthositic intrusion, Labrador;, Can. J. Earth Sci., 5, pp. 1139-1144. Palmer, H.C. 1970: Paleomagnetism and Correlation Of Some Middle Keweenawan Rocks, Lake Superior; Can. J. Earth Scia, Vol. 7, No. 6, pp. 1410-1436. Peterman, Zell E., 1966: Rb-Sr Dating of Middle Precambrian Metasedimentary Rocks of Minnesota; Geol. Soc. Amer. Bull., Vol. 77, pp. 1031-1044. Peterman, Z.E., Hedge, C.E. and Braddock, W.A. 1968: Age of Precambrian events in the Northeastern Front Range, Colorado.; J. Geophys., Res., 73, pp. 2277-2296. Phinney, Wm. C., 1968: Guide for Field Trip in the Duluth Complex near Ely, Minnesota; Instit. Lake Superior Geology, May 1968, 6p. Pye, E.G. 1953: A Petrographic Study of the Textures of Basic and Ultrabasic Igneous Rocks, unpubl. Ph.D. Thesis, University of Toronto, Toronto, Ontario, 93p. Pye, E.G. 1964: A Preliminary Study of the Shapes of Plagioclase Feldspars in Some Basic Rocks; Proc. Geol. Assoc. Canada, Vol. 15, Pt. 2, pp. 9-25. * Pye, E.G. 1968: Geology and Scenery, Rainy Lake and East to Lake Superior; Ontario Dept. Mines, Geol. Guide Book No. 1, ll4p. Pye, E.G., 1969: Geology and Scenery, North Shore of Lake Superior, Ontario Dept. Mines, Geol. Guide Book No. 2, 144 p. (reprinted 1975). Reeve, Edward John, 1969: Petrology and Mineralogy of a Gabbroic Intrusion in Pardee Township Near Port Arthur, Ontario; Unpubl. M.Sc. Thesis, University of Wisconsin, Milwaukee, Wis., 79 p. Robertson, W.A. 1973: Pole Positions from the Mamainse Point Lavas and Their Bearing on a Keweenawan Pole Path and Polarity Sequence; Can. J. Earth Sci., Vol. 10, No. 10, pp. 1541-1555. Robertson, W.A. 1973: Pole Position From Thermally Cleaned Sibley Group Sediments and its Relevance to Proterozoic Magnetic Stratigraphy; Can. J. Earth Sci., Vol. 10, No. 1, pp. 180-193. * Robertson, W.A. and Fahrig, W.F., 1971: The Great Logan Paleomagnetic Loop - The Polar Wandering Path from Canadian Shield Rocks During the Neohelikian Era; Can. J. Earth Sci., Vol. 8, pp. 1355-1372. �45 Sage, R.P., Treacher, K., Meloche, D., and Bathe, D., 1975: Slate Islands, District of Thunder Bay; Ontario Div. Mines, Prelim. Map P.997, Geol. Ser., Scale 1 inch to 660 feet or 1:7,920. Geology and compilation, 1974. Sakainoto, Takao, 1950: The Origin of the Pre-Cambrian Banded Iron Ores; Amer. Jour. Sci., Vol., 248, No. 7, pp. 449-474. Schwarçz, G.M. 1942: Concretions of the Thomson Formation, Minnesota; Amer,. Jour, Sci., Vol. 240, pp. 491-499. Schwarcz, George M., and Sandberg, Adolph E., 1940: Rock Series in Diabase Sills at Duluth, Minnesota; Bull. Geol. Soc. Amer., Vol. 51, pp. 1135-1172. Silver, L.P. 1906: The Animikie Iron Range; Annual Report, Ontario Bur. Mines., Vol. 15, pt. 1, No. 5, pp. 156-172. Silver, Leon T., and Green, John C. Time Constants for 1972: Keweenawan Igneous Activity; Geol. Soc. Amer., Abstracts with Programs, Vol. 4, No. 7, pp. 665-666. Phase Relations Simmons, E.C., Lindsley, D.H. and Papike, J.J. 1974: and Crystallization Sequence in a Contact-Metamorphosed Rock from the Gunf lint Iron Formation, Minnesota; Jour. Petrology, Vol. 15, pt. 3, pp. 539-565. Sims, Paul K. 1976: Early Precambrian Tectonic-Igneous Evolution in the Vermillion District, Northeastern Minnesota; Geol. Soc. Amer. Bull., Vol. 87, pp. 379-389. Sims, P.K., 1976: Precambrian Tectonics and Mineral Deposits, Lake Superior Region; Econ. Geol., Vol. 71, No. 6, pp. 1092-1118. Smith, W.N., 1905: Loon Lake Iron-Bearing District; Annual Report, Ontario Bur. Mines, Vol. 14, Pt. 1, pp. 254-260. Smith, T. Jefferson, Steinhart, John S., and Aldrich. L.T. 1966: Lake Superior Crustal Structure; Jour. Geophys. Research, Vol. 71, No. 4, pp. 1142-1172. Spall, Henry 1971: Evidence Precambrian Apparent Polar Wandering: from North America; Earth Planet. Sci. Letters, Vol. 10, pp. 273280. Stacey, P.E., 1976: Report to Division of Parks, Ontario Ministry 'of Natural Resources on the Stability Aspects of the Development of Ouimet Canyon; Golder Associates, Vancouver, B.C., 14p. Steacy, Harold R. 1974: Our Beautiful Little Known Gemstones; Can. Geog. J., Vol. 89, No. '6, pp. 4-13. �46 Stewart, John H. 1976: Late Precambrian Evolution of North America: Plate Tectonics Implications; Geology, Vol. 4, No. 1, pp. 11-15. Geochronology of Stratified Rocks of the Stockwell, C.H. 1968: Canadian Shield; Can. J. Earth Sd., Vol. 5, pp. 693-698. Sutton, J. and Watson, J.V. 1974: Tectonic Evolution of Continents in Early Proterozoic Times; Nature, Vol. 247, Feb. 13, pp. 433-435. A Paleomagnetic Study of the Gunflint, Mesabi, Symons, D.T.A., 1966: and Cuyuna Iron Ranges in the Lake Superior Region; Econ. Geol. Vol. 61, pp. 1336-1361. Iron Formation at Gravel Lake, Thunder Bay District, Tanton, T.L. 1923: Ontario; Geol. Survey Suimnary Report., Pt. Cl, pp. 1-5. * Tanton, T.L. 1925: Evidence of Liquid Immiscibility in a Silicate Magma, Agate Point,*Ontario; Jour. Geol., Vol. 33, pp. 629-641. Stratigraphy of the Northern Subprovince of the Tanton, T.L. 1927: Lake Superior Region; Bull. Geol. Soc. Amer., Vol. 33, pp. 731-748. Tanton, T.L. 1928: pp. 66-68. Emulsions of Silicates; Amer. Jour. Sci., Vol. 15, Tanton, T.L. 1931: Fort William and Port Arthur and Thunder Cap Map Areas: Thunder Bay District, Ontario; Geol. Surv. Can., Mem. 167, 222 p. Tanton, T.L. 1935: Copper-Nickel Mineral Occurrences in Pigeon Area, Ontario; Canada Dept. Mines., Bur. Econ. Geol., Paper 35-1, llp. Radioactive Nodules in Sediments of the Sibley Tanton, T.L. 1948: Series, Nipigon, Ontario; Trans. Roy. Soc. Canada, 3rd Series, Vol. 42, Section 4, pp. 69-75. The Origin of Iron Range Rocks; Trans. Roy. Soc. Tanton, T.L. 1950: Canada, Vol. 44, Series 3, pp. 1-19. Three Great Basins of Precambrian Banded Iron Trendall, A.F., 1968: Formation Deposition: A Systematic Comparison; Geol. Soc. Amer. Bull., Vol. 79, pp. 1527-1544. Development of Lake Superior Soft Iron Ores Tyler, Stanley A., 1949: from Metamorphosed Iron Formation; Bull. Geol. Soc. Amer., Vol.60, pp. 1101-1124. Tyler,. Stanley A. and Barghoorn, Elso 5., 1954: Occurrence of Structurally Preserved Plants i Pre-Cambrian Rocks of the Canadian Shield; Science, Vol. 119, No. 3096, pp. 606-608. Studies Tyler, S.A., Mardsen, R.W., Grout, F.F. and Thiel, G*.A. 1940: of the Lake Superior Pre-Cambrian by Accessory-Mineral Methods; Bull. Geol. Soc. Amer., Vol. 51, pp. 1429-1538. �4 Van Lewen, Melvin C., 1957: The Geology of St. Ignace Jsland, Ontario and a Correlation of the Keweenawan Series of the Lake Superior Region; pnpubl. B.Sc., Mich. College Mining Tech., Michigan, 67 p. Van Schmus, W.R. 1976: Early and Middle Proterozoic History of the Great Lakes Area, North America; in A Discussion on Global Tectonics in Proterozoic Times, Roy. Soc. (London) Phil. Trans. A. Vol. 280, pp. 605-628. Vos, M.A. 1976: Amethyst Deposits of Ontario; Ontario [liv. Mines, Mm. Nat. Res., Geol. Guidebook No. 5, 99p. Wallace, Henry, 1972: Differentiation Trends in Osler V'olcanics, Shesheeb Bay Section; unpubl. M.Sc. Thesis, University of Toronto, Toronto, Ontario, lO9p. Walter, M.R. 1972: A Hot Spring Analog for the Depositional Environment of Precambrian Iron FormatiOns of the Lake Superior Region; Econ. Geol., Vol. 67, pp. 969-971. Logan Intrusions; Weiblen, P.W., Mathez, E.A. and Morey, G.B. 1972: in Geology of Minnesota, A centennial Volume, P.K. Sims and G.B. Morey (Eds.), Minn. Geol. Surv., pp. 394-406. White, Walter 5., 1960: The Keweenawan Lavas of Lake Superior, an Example of Flood Basalts; Amer. Jour. Sci., Vol. .258-A, (Bradley Vol.), pp. 367-374. White, Walter 5., 1966: Geologic Evidence for Crustal Structure in the Western Lake Superior Basin; in The Earth Beneath the Continents (edited by J.S. Steinhart and T.J. Smith), Amer. Geophys. Union, Geophys. Monograph 10, pp. 28-41. White, W.S. 1966: Tectonics of the Keweenawan Basin, Western Lake Superior Region; U.S. Geol. Survey Prof. Paper 524-E, 23p. White, W.S. 1972: Keweenawan Flood Basalts and Continental Rifting; Geo. Soc. Amer., Vol. 4, No. 7, pp. 732-734. Woolnough, W.G., 1941: Origin of Banded Iron Deposits - A Suggestion; Econ. Geol., Vol. 36, No. 5, pp. 465-489. Proterozoic Ensialic Orogenesis: The Millipede Model of Ductile Plate Tectonics; Amer. Jour. Sci., Vol. 276, No. 8, pp. 927-953. Wynne-Edwards, FJ.R., 1976: � https://digitalcollections.lakeheadu.ca/files/original/45b75880cbd97514ac8a952d95a23e85.pdf 6473b0dd1b5d2b22dfbe572b53d991de PDF Text Text ������������������������������������������������������������������������������ Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 1977 Description An account of the resource Institute on Lake Superior Geology. Thunder Bay, Ontario. May 2-8, 1977. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology. Date A point or period of time associated with an event in the lifecycle of the resource 1977 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/795d9a865d55c3086bcda137ccad1c7f.pdf 59d34dc03c74139c37d2d5698efda839 PDF Text Text Volume 67, Part 1* 67th ANNUAL MEETING VIRTUAL EVENTS May 11, 14, 18, 21, 2021 Hosted by: Peter Hollings, Mark Smyk, and Mark Jirsa Co-chairs Lakehead University and Minnesota Geological Survey The Sleeping Giant 66th *The Annual meeting originally planned for 2020 in Mountain Iron, Minnesota, was cancelled due to the Covid-19 pandemic, and thus, no Proceedings Volume 66 was produced. To mitigate potential impact from the ongoing pandemic, the 67th Annual meeting was held in a virtual format. As a result, there were no fieldtrips, and only this Part 1 of Proceedings Volume 67 was produced. We would like to thank the Lakehead University Technology Services Centre, particularly Blain Boyd, Shawn Hartviksen and John Bonofiglio, for providing the Zoom account and technical support for this virtual meeting. i �Proceedings Volume 67, Part 1: Program and Abstracts Table of Contents Institutes on Lake Superior Geology, 1955-2019 iii Sam Goldich and the Goldich Medal Goldich Medal Guidelines Goldich Medalists and Goldich Medal Committee Citation for Goldich Medal Award to Allan MacTavish v vii ix x Honoring the Pioneers of Lake Superior Geology-Newton Horace Winchell xii Memoriams for John Klasner, Thomas Waggoner, Ronald Seavoy, and John Heine xvi Eisenbrey Student Travel Awards xvii Joe Mancuso Student Research Awards xviii Doug Duskin Student Paper Awards and Award Committee Institute Board of Directors Report of the 65th Annual Meeting (2019) TECHNICAL PROGRAM ABSTRACTS xix xx xxi xxv 1-72 Reference to material in Part 1 should follow the example below: Authors(s), 2021, abstract title. 67th Institute on Lake Superior Geology Proceedings v. 67, Part 1Program and Abstracts, p. XX Published by the 67th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Peter Hollings Department of Geology Lakehead University Thunder Bay, ON P7B 5E1 CANADA peter.hollings@lakeheadu.ca Digital version of the volumes are available on-line at http://www.lakesuperiorgeology.org. lSSN 1042-99 ii �Institutes on Lake Superior Geology, 1955-2019 # Year Meeting Location Chairs 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 Minneapolis, Minnesota Houghton, Michigan East Lansing, Michigan Duluth, Minnesota Minneapolis, Minnesota Madison, Wisconsin Port Arthur, Ontario Houghton, Michigan Duluth, Minnesota Ishpeming, Michigan St. Paul, Minnesota Sault Ste. Marie, Michigan East Lansing, Michigan Superior, Wisconsin Oshkosh, Wisconsin Thunder Bay, Ontario Duluth, Minnesota Houghton, Michigan Madison, Wisconsin Sault Ste. Marie, Ontario Marquette, Michigan St. Paul, Minnesota Thunder Bay, Ontario C.E. Dutton A.K. Snelgrove B.T. Sandefur R.W. Marsden G.M. Schwartz & C. Craddock E.N. Cameron E.G. Pye A.K. Snelgrove H. Lepp A.T. Broderick P.K. Sims & R.K. Hogberg R.W. White W.J. Hinze A.B. Dickas G.L. LaBerge M.W. Bartley & E. Mercy D.M. Davidson J. Kalliokoski M.E. Ostrom P.E. Giblin J.D. Hughes M. Walton M.M. Kehlenbeck iii �# 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 Year 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 56 2010 57 58 59 60 61 62 63 64 2011 2012 2013 2014 2015 2016 2017 2018 65 66 67 2019 2020 2021 Meeting Location Milwaukee, Wisconsin Duluth, Minnesota Eau Claire, Wisconsin East Lansing, Michigan International Falls, Minnesota Houghton, Michigan Wausau, Wisconsin Kenora, Ontario Wisconsin Rapids, Wisconsin Wawa, Ontario Marquette, Michigan Duluth, Minnesota Thunder Bay, Ontario Eau Claire, Wisconsin Hurley, Wisconsin Eveleth, Minnesota Houghton, Michigan Marathon, Ontario Cable, Wisconsin Sudbury, Ontario Minneapolis, Minnesota Marquette, Michigan Thunder Bay, Ontario Madison, Wisconsin Kenora, Ontario Iron Mountain, Michigan Duluth, Minnesota Nipigon, Ontario Sault Ste. Marie, Ontario Lutsen, Minnesota Marquette, Michigan Ely, Minnesota Chairs G. Mursky D.M. Davidson P.E. Myers W.C. Cambray D.L. Southwick T.J. Bornhorst G.L. LaBerge C.E. Blackburn J.K. Greenberg E.D. Frey & R.P. Sage J. S. Klasner J.C. Green M.M. Kehlenbeck P.E. Myers A.B. Dickas D.L. Southwick T.J. Bornhorst M.C. Smyk L.G. Woodruff R.P. Sage & W. Meyer J.D. Miller & M.A. Jirsa T.J. Bornhorst & R.S. Regis S.A. Kissin & P. Fralick M.G. Mudrey & Jr., B.A. Brown P. Hinz & R.C. Beard L. Woodruff & W.F. Cannon S. Hauck & M. Severson M. Smyk & P. Hollings A. Wilson & R. Sage L. Woodruff & J. Miller T.J. Bornhorst & J. Klasner J. Miller, G. Hudak, & D. Peterson International Falls, Minnesota M. Jirsa, P. Hollings, T. Boerboom, P. Hinz & M.Smyk Ashland, Wisconsin T. Fitz Thunder Bay, Ontario P. Hollings Houghton, Michigan T.J. Bornhorst & A. Blaske Hibbing, Minnesota J. Miller & M. Jirsa Dryden, Ontario R. Cundari & P. Hinz Duluth, Minnesota J. Miller, C. Schardt, & D. Peterson Wawa, Ontario A. Pace, A. Wilson, & T.J. Bornhorst Iron Mountain, Michigan L. Woodruff, W. Cannon, & E.K. Stewart Terrace Bay, Ontario P. Hollings & M.C. Smyk Meeting cancelled due to Covid-19 pandemic Virtual meeting P. Hollings, M. Smyk, M. Jirsa iv �Sam Goldich and the Goldich Medal Sam Goldich received an A.B. from the University of Minnesota in 1929, a M.A. from Syracuse University in 1930, and a Ph.D. from the University of Minnesota in 1936. During World War II Sam worked for the U.S. Geological Survey in mineral exploration. In 1948, Sam returned to the University of Minnesota, and became Professor and Director of the Rock Analysis Laboratory the following year. He rejoined the U.S. Geological Survey in 1959 and was appointed as the first Branch Chief of the Branch of Isotope Geology. Sam returned to academia in 1964 when he went to Pennsylvania State University. He left PSU in 1965 and moved to the State University of New York at Stony Brook, where he stayed for 3 years. Restless yet again, he moved to Northern Illinois University in 1968 where he was a professor until his retirement in 1977. Sam’s final move was to Denver where he became an emeritus at the Colorado School of Mines. Sam died in 2000, less than a month before his 92nd birthday. In the late 1970’s, Geological Society of America Special Paper 182, which included seminal geochronological studies by Sam Goldich and coworkers on the Archean rocks of the Minnesota River Valley, was nearing completion. At this time various ILSG regulars began discussing the possibility of recognizing Sam for his pioneering work on the resolution of age relationships and thus the geology of Precambrian rocks in the Lake Superior region. Three members, R.W. Ojakangas, J.O. Kalliokoski and G.B. Morey, presented the idea to the ILSG Board of Directors in 1978. The Board approved the creation of an award, provided funding could be obtained. It was suggested that collecting one or two dollars at registration for a dedicated account would provide resources for striking the medal. A general request was made to the ILSG membership for donations and Sam himself offered a challenge grant to match the contributions. In total $4,000 was collected and thus began the work of creating the Goldich Medal. The initial Goldich Award was presented to Sam by G.B. Morey in 1979 and consisted of a large paper proclamation. For the actual medal, G.B. Morey consulted with the foundry on production details, while Dick Ojakangas and Jorma Kalliokoski worked on the design of the award, suggesting that it be given for “outstanding contributions to the geology of the Lake Superior region.” Simultaneously, a committee of J.O. Kalliokosi, W.F. Cannon, M.M Kehlenbeck, G.B. Morey, and G. Mursky developed the Award Guidelines that were approved by the ILSG Board. By 1981 all the elements of the Goldich Award had come together, and the second recipient, Carl E. Dutton, Jr., received the Goldich Medal for 50 years of significant contributions to the understanding of the geology of the Lake Superior region. Since the beginning, the Awards Committee has consisted of individuals representing industry, government and academia, with each member of the Committee serving for three years. The medal is now awarded every year at the annual ILSG meeting. Reference: Morey, G.B. and Hanson, G.N. (editors). 1980. Selected studies of Archean gneisses and Lower Proterozoic rocks, southern Canadian Shield. Geological Society of America, Special Paper 182, 175 p. Prepared by various Goldich Medal Awardees, 2007 v �INSTITUTE ON LAKE SUPERIOR GEOLOGY GOLDICH MEDALLION �Goldich Medal Guidelines (Adopted by the Board of Directors, 1981; amended 1999) Preamble The Institute on Lake Superior Geology was born in 1955, as documented by the fact that the 27th annual meeting was held in 1981. The Institute’s continuing objectives are to deal with those aspects of geology that are related geographically to Lake Superior; to encourage the discussion of subjects and sponsoring field trips that will bring together geologists from academia, government surveys, and industry; and to maintain an informal but highly effective mode of operation. During the course of its existence, the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have made particularly noteworthy and meritorious contributions to the understanding of Lake Superior geology and mineral deposits. The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years. Subsequent medallists and this year’s recipient are listed in the table below. Award Guidelines 1) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution to, the geology of the Lake Superior region. 2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of three members, one to serve for three years, one for two years, and one for one year. The member with the briefest incumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In his/her third year this member shall be the chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSG membership. The out-going, senior member of the Board of Directors shall act as liaison between the Board and the Committee for a period of one year. 3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of the Board of Directors, who will then inform the Board of the nominee. 4) The Board of Directors normally will accept the nominee of the Committee, inform the medallist, and have one medal engraved appropriately for presentation at the next meeting of the Institute. 5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award. Nominating Procedures 1) The deadline for nominations is November 1. Nominations shall be taken at any time by the Goldich Medal Committee. Committee members may themselves nominate candidates; however, Board members may not solicit for or support individual nominees. vii �2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation, lists of publications, curriculum vita’s, and evidence of contributions to Lake Superior geology and to the Institute. 3) Nominations are not restricted to Institute attendees, but are open to anyone who has worked on and contributed to the understanding of Lake Superior geology. Selection Guidelines 1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including: a) importance of relevant publications; b) promotion of discovery and utilization of natural resources; c) contributions to understanding of the natural history and environment of the region; d) generation of new ideas and concepts; and e) contributions to the training and education of geoscientists and the public. 2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institute meetings, presentation of talks and posters, and service on Institute boards, committees, and field trips. 3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of the Committee members. 4) There are several points to be considered by the Goldich Medal Committee: a) An attempt should be made to maintain a balance of medal recipients from each of the three estates—industry, academia, and government. b) It must be noted that industry geoscientists are at a disadvantage in that much of their work in not published. 5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedly one of the Institute’s great strengths and should be nurtured by equitable recognition of excellence in both countries. viii �Goldich Medalists 1979 Samuel S. Goldich 1998 Zell Peterman 2016 Mark A. Jirsa 1980 not awarded 1999 Tsu-Ming Han 2017 Philip Fralick 1981 Carl E. Dutton, Jr 2000 John C. Green 2018 Val W. Chandler 1982 Ralph W. Marsden 2001 John S. Klasner 2019 Mark Severson 1983 Burton Boyum 2002 Ernest K. Lehmann 2021 Allan MacTavish 1984 Richard W. Ojakangas 2003 Klaus J. Schulz 1985 Paul K. Sims 2004 Paul Weiblen 1986 G.B. Morey 2005 Mark Smyk 1987 Henry H. Halls 2006 Michael G. Mudrey 1988 Walter S. White 2007 Joseph Mancuso 1989 Jorma Kalliokoski 2008 Theodore J. Bornhorst 1990 Kenneth C. Card 2009 L. Gordon Medaris, Jr 1991 William Hinze 2010 William D. Addison & 1992 William F. Cannon (Not awarded 2020) Gregory R. Brumpton 1993 Donald W. Davis 2011 Dean M. Rossell 1994 Cedric Iverson 2012 James D. Miller 1995 Gene La Berge 2013 Tom Waggoner 1996 David L. Southwick 2014 Laurel Woodruff 1997 Ronald P. Sage 2015 Rodney J. Ikola Goldich Medal Committee Serving through the meeting year shown in parentheses. Dan England (2017-2022) Eveleth Fee Office (Committee Chair) Steve Kissin (2018-2023) Lakehead University Dorothy Campbell (2020-2024) Ontario Geological Survey Terms of the committee members have been extended 2 years due to cancelation of the 2020 meeting and the logistical difficulties of voting during the 2021 virtual meeting. Presentation of the award to Allan MacTavish was to have occurred at the 2020 meeting, but was delayed one year. 2021 GOLDICH MEDAL RECIPIENT ALLAN MACTAVISH ix �Citation for Allan MacTavish ILSG members, Goldich Medal recipients and guests, it is my privilege to present the citation for this year’s recipient of the Goldich Medal, Allan MacTavish. Allan received his Bachelor of Science (B.Sc.) degree in geology from Laurentian University in 1977. Over the past 44 years, he has spent much of his career working as an economic geologist in the Lake Superior region, both in the mineral exploration industry and with the Ontario government. Early on, Allan also somehow found the time to be a part-time graduate student at Lakehead University, where he completed a Master of Science (M.Sc.) thesis entitled “The Geology, Petrology, Sulphide and Platinum-Group Element Mineralization of the Quetico Intrusions, Northwestern Ontario” in 1992. Since completing this project, much of Allan’s work has been focussed on the evaluation of copper-nickel-platinum group element (Cu-Ni-PGE) deposits. While working in the private sector, Allan has led numerous field exploration programs in the Lake Superior region that have targeted Cu-Ni-PGE, volcanogenic massive sulphide and gold deposits. Most notably, he has served since 2007 as Exploration Manager for Magma Metals (Canada) Ltd./Panoramic PGMs (Canada) Ltd. and most recently as Vice President Project Manager with Clean Air Metals Inc., with a primary focus on the Thunder Bay North Cu-Ni-PGE project. At Thunder Bay North, Allan has led teams of geoscientists from the grassroots exploration phase, through to mineral resource delineation. The initial exploration success at Thunder Bay North, largely attributed to Allan’s leadership, sparked a staking rush in the Nipigon Embayment and consequently, a flurry of exploration activity and investment in the region. Although exploration activity in the project area has waxed and waned over the past 14 years, Allan’s persistence is now paying off, with Clean Air Metals having reported a significant mineral resource expansion during the winter of 2021, along with an exciting new intersection of highgrade massive sulphides. Furthermore, his work on the Thunder Bay North project, and his support of academic research work into the Current Lake intrusive complex, has made significant contributions to our understanding of chonolith-hosted magmatic sulphide deposits and the greater Midcontinent Rift system. One of Allan’s most notable skills is his competence as a field mapping geologist, an aspect of geology that has largely become a lost art. This skill was developed through his long list of positions, including a stint as a Field Geoscientist with the Ontario Geological Survey, during which he mapped the Montcalm greenstone belt. Even in the private sector, any project under Allan’s watch can be counted on to include solid geological mapping. This steadfast belief in the core fundamentals of geology has led to a significant advancement in our understanding of many geological districts in the Lake Superior region, including the Hemlo gold camp, the Coldwell x �complex, the Nipigon Embayment, the Atikokan-Quetico district and the Abitibi greenstone belt. Understanding the importance of a solid foundation in geological mapping as part of the training for future geoscientists, Allan has also been a long-time supporter of the Precambrian Research Centre (PRC), focussing his contributions toward the PRCs mapping school and the students therein. It was during Allan’s time working on his M.Sc. project, when I was a fellow graduate student, that I first got to know him. I quickly gained an appreciation for his knowledge and enthusiasm for the science of geology. I consider myself fortunate that, shortly after graduation, I became one of the many aspiring geoscientists who have benefitted from Allan’s mentorship when he hired me as a field assistant for a short-term project. More recently, the Lakehead University Student Chapter of the Society of Economic Geologists (SEG) has also been privy to Allan’s wisdom and knowledge. Since its inception in 2014, he has served as the Industry Representative for the SEG student chapter, assisting the group with fundraising and field trip endeavours, most notably trips to Arizona and Ireland. The Geology Department at Lakehead University has long known the benefit of having Allan as an associate, whether as a voice for industry to help direct departmental decisions, or as an employer hiring a geology student during or immediately following their studies. Allan’s contributions to the Institute on Lake Superior geology are many. He has tirelessly supported the ILSG by serving as a Board member, Goldich Award Committee member, Annual Conference planning committee member, and as an individual contributor to the ILSG Student Travel Scholarship. Dating back to 1985, Allan is credited as an author for 7 abstracts and 4 field trip guides, plus as an editor for 3 field trip guidebooks. Allan has led field trips for ILSG annual meetings, most recently the Coldwell Complex trip at the 2019 Terrace Bay meeting. He was also the co-organizer and field trip leader for the 2017 ILSG Iceland Geological Field Trip, and organized and led the 2020 Hawaii Geological Field Trip, which also happened to be the last ILSG in-person event to occur prior to the onset of the global Covid-19 pandemic (let’s all hope that we can once again get together for ILSG field trips in 2022). Anyone who has had the pleasure of knowing Allan over the years immediately becomes aware of his enthusiasm for geoscience and his deep knowledge of the rocks of the Lake Superior region. Allan’s contributions to the understanding of the geology of the Lake Superior region, as well as his unwavering support of the Institute on Lake Superior Geology, make him a worthy recipient of the 2021 Goldich Medal. Submitted by Mark Puumala, M.Sc., P.Geo. Senior Manager, Resident Geologist Program Ontario Geological Survey xi �Honoring the Pioneers of Lake Superior Geology (Adopted by the Board of Directors, 2016) Preamble At the suggestion of Gene LaBerge, the 2016 executive board agreed to implement a program to recognize historic pioneers in the understanding of geology in the Lake Superior region. Beginning with the 2017 annual meeting, nominations will be accepted from the membership for geologists whose work was conducted primarily before inception of the institute in 1955. Biographical sketches of those pioneers will be presented at future annual meetings so that all might appreciate the value of their contributions. Selection of nominees will be decided in part by the organizing committee of each year's annual meeting, in consultation with the Board, to ensure equitable geographic representation in the selection process. Award Guidelines 1) Nominations from the membership will be submitted via the Institute web site and forwarded to the Chair of the next Annual Meeting. The nominations will be no more than half a page in length and will summarise the contribution of the nominee. 2) The Organising Committee will select one or two individuals to be highlighted at the next Annual meeting and submit those names to the Board for approval. 3) The nominator will be requested to prepare a brief presentation to be given during the next annual meeting with a summary to be included in the Proceedings volume. 4) Unsuccessful nominations will be kept by the Secretary for two years and forwarded to the next meeting Chair; these nominations may be resubmitted at a later date. The Board will review this award every five years. Pioneers of Lake Superior Geology 2017 Douglass Houghton (1809-1845) 2018-2020 not presented 2021 Newton Horace Winchell (1839-1914) xii �2021 Pioneer of Lake Superior Geology Newton Horace Winchell (1839-1914) “Minnesota’s Geologist” Citation by Jim Miller, Former Associate Professor, University of Minnesota Duluth and Senior Geologist, Minnesota Geological Survey, with insights from author Sue Leaf. Newton Horace Winchell was first director of the Minnesota Geological and Natural History Survey at the University of Minnesota from 1872 to 1898. During his 26 year tenure, Winchell and his staff visited every corner of the state to document its bedrock geology, mineral deposits, quarry stone resources, glacial deposits and landforms, paleontological record, modern flora and fauna, and hydrologic framework. The survey’s findings were recorded in 24 Annual Reports, which summarized each year’s accomplishments and commonly totaled hundreds of pages, and in a six-volume final report. Pdfs of these reports are all available from the University of Minnesota Digital Conservancy (https://conservancy.umn.edu/handle/11299/708). As skillfully told in Sue Leaf’s (2020) engaging and intimate biography of Winchell’s private and professional life, Newton Horace Winchell (NHW) was born and raised as the middle child in a large family near the New York-Massachusetts-Connecticut borders. At the age of 15 after 8th grade, he began teaching in small schools in the area. In 1857, he moved to Ann Arbor, Michigan to live with his older (by 15 years) brother Alexander, who was a professor of natural history at the University of Michigan and director of the Michigan Geological Survey. Over the next 12 years, NHW worked as an elementary school teacher/principal in various southern Michigan towns, got married, had two children, and also intermittently took classes at the U of Michigan. In the summer of 1860, he was hired by the Michigan Geological Survey as a botanist and explored the coastlines of Lakes Huron and Michigan. NHW completed his BA degree in 1866, committed to geology as a profession, and began to work more regularly for the Michigan Survey. In 1869, he completed his MS degree at U of Michigan and became Assistant State Geologist under his brother. In 1870, the Michigan legislature defunded the geological survey, which prompted NHW to work the summers of 1871 and 1872 for the Ohio Geological Survey. In July 1872, University of Minnesota president, William Folwell, offered NHW the position of director of the newly created Minnesota Geological and Natural History Survey. With an initial legislative appropriation of $1000 per year, Folwell estimated that the survey would need 20 years to accomplish its task of conducting “a thorough geological and natural history survey of the State". In addition, he would be responsible for teaching two geology courses each year and organizing a natural history museum. NWH accepted the offer, and began field work that fall. The first six field seasons of the Survey (1872-1877), NWH and his small staff focused their attention on the geology, geomorphology, paleontology, and economic geology of southern Minnesota counties. In 1876, Winchell’s responsibility for the University museum was relieved by the hiring of zoologist, Clarence Herrick, as museum director. HNW’s other non-survey obligation of teaching college classes was removed just before the 1879 field season by the hiring of another zoologist, Christopher Hall, who was also appointed assistant director of the survey. Additionally significant hires in 1879 were those of glacial geologist Warren Upham and paleontologist Edward Urlich. Although NHW was well versed in both these disciplines, their contributions and other staff hires in the 1880’s, freed Winchell to devote many months of field xiii �work per year over the next decade and to accomplish his most challenging task –deciphering the Precambrian geology of northeastern Minnesota. The Paleozoic, Cretaceous and Quaternary geology of southern and western Minnesota was summarized in Volumes 1 (1884; 673 p.) and 2 (1888; 671 p.) of the final report written by NHW and Upham. The paleontology of Ordovician and Cambrian rocks (at the time interpreted to be Upper and Lower Silurian) and of Cretaceous rocks was summarized in the 1200+ page Volume 3 that was published in two parts (pt. 1 – 1895; Pt. 2 - 1897). The task of mapping and deciphering the Precambrian geology of northeastern Minnesota started in the summer of 1878 when NWH and three others paddled along the Lake Superior shoreline from Duluth to the Canadian border. In all, they collected over 300 samples. Then, from September to November of that same year, NHW and two Ojibwe guides paddled and portaged from Grand Portage, to Lake Vermilion, down the Embarrass, St. Louis and ultimately the Mississippi rivers to Minneapolis – a 500+ mile trek! Winchell devoted most of his remaining field seasons to northern Minnesota geology with the able assistance of his son (Horace), his sonin-law (Uly), and his brother (Alexander). With iron mining taking off on the Vermilion Range (1st ore shipped -1884), and then the Mesabi Range (1st ore shipped - 1892), much of NHW’s attention was focused on that geology. Also, beginning the mid-1880’s, Winchell regularly employed petrography to better understand the granite, gabbro, greenstone, and basalt that he encountered. In fact, in 1895, he took a year-long “sabbatical” to Paris to learn petrographic methods from the French petrographers Fonque and Michel-Levy. Still, as I highlighted in an ILSG talk in 2004, Winchell struggled mightily to understand the genesis of Precambrian crystalline rocks. In the introduction of Volume 4 of the Final Report (1899), wherein he summarizes his studies of northern Minnesota geology, NWH comments: "Here [among the crystalline rocks] the geologist is deprived of his usual guides and guys, and finds himself floundering in a muddy sea of innumerable conflicting currents". Some other notable aspects of Winchell’s remarkable career include his role in the development of the first scientific journal exclusively devoted to geology – The American Geologist. He served as its managing editor and publisher from its inception in 1888 to its merger with Economic Geology in 1905. Part of his motivation for creating this journal was to give a voice to state survey geologists which had the potential of being dismissed by academics and experienced overreach by the USGS. Also, as a founding fellow, NHW was instrumental in the creation of the Geological Society of America, also starting in 1888. After the Minnesota survey ended in 1899 and he got closure on Volumes 5 (1900, Uly Grant’s PhD thesis) and 6 (1901, atlas of county geologic maps), NHW focused the final decade of his life on archeological studies with the Minnesota Historical Society, particularly as they applied to native peoples who lived in the midcontinent before, during and after the last glacial episode. He published Aborigines of Minnesota in 1911 (743 p. 642 figures, 32 plates). As Sue Leaf points out, this well received book was “comprehensive in scope, methodical in approach, and meticulous in detail; a worthy companion to the voluminous Geology of Minnesota.” Winchell died on May 2, 1914 due to surgical complications at the age of 74. Newton Horace Winchell’s geological survey took the blank canvas of Minnesota’s landscape (Fig. 1) and began the process of revealing the rich diverse geology that we now recognize (Fig. 2) and continue to embellish and improve upon. As Sue Leaf’s book proclaims in its title, Newton Horace Winchell indeed deserves the designation as “Minnesota’s Geologist,” and as one of the great pioneers of Lake Superior geology. xiv �Figure 1 – Geological map of Minnesota in 1872 at the beginning of the Minnesota Geological and Natural History Survey References (Annual Report #1). Figure 2 – Geological map of Minnesota in 1900 at the end of the Minnesota Geological and Natural History Survey (Final Report, V. 6). References Leaf, Sue, 2020, Minnesota’s Geologist - The Life of Newton Horace Winchell. University of Minnesota Press, Minneapolis, MN, 261p. Miller, James D., 2004, N.H. Winchell's study of the Keweenawan Supergroup rocks of northeastern Minnesota, 1872-1900. Proceedings and Abstracts, 50th Institute of Lake Superior Geology, Duluth, MN, p. 117-118. xv �In Memoriams The ILSG lost four of its’ long-time members since our last in-person meeting in 2019. Brief descriptions of their professional lives follow, excerpted from online obituaries and tributes. They were active members throughout their professional careers and beyond. They all made significant contributions to the science and to the Institute. They will be missed. Ronald Seavoy passed away on March 25, 2020. He was born July 6, 1931 in New York City and was raised in Chicago. He received his Bachelor of Science degree in Geology in 1953 from the University of Michigan, and his Master of Arts (1963) and PhD (1969) in History from Michigan. He served in the U.S. Army from 1953-1955. He was hired by the Department of History at Bowling Green State University in 1965 where he taught U.S. Constitutional History, U.S. Business History, as well as numerous survey courses until his retirement in 1991 as Professor Emeritus. Ron is the author of ten books and nineteen articles on the subjects of American business history, famine in developing countries, political economy, and mining exploration. Before and while employed as a historian, Ron worked as an exploration geologist for Canadian Johns Manville, International Nickel Company, Alcoa, Burwest, Western Nuclear, and Cleveland Cliffs Mining. He was a member of the Organization of American Historians, the Society of Economic Geologists, and the Institute of Lake Superior Geology. Over the many years of Ron’s involvement with ILSG, he donated thousands of dollars to the Mancuso Student Research Award, which was named for Ron’s good friend and colleague. John Klasner passed away on July 15, 2020. John was born June 22, 1935 in Flint, Michigan. He attended Michigan State University and Michigan Technological University where he graduated with degrees in geology and geophysics. He worked in mineral and oil exploration in the US, Canada, Africa, and the Middle East. John then became a geology professor and directed the honors program at Western Illinois University. He also was employed by the USGS for mapping in the Upper Peninsula of Michigan and Northern Wisconsin. John was a member of several professional organizations, including the Institute on Lake Superior Geology. John Heine passed away August 17, 2020 in Duluth. He was born in Minneapolis and graduated from University of St. Thomas in St. Paul. John moved to Duluth and pursued graduate studies in the Geology Department at the University of Minnesota Duluth. During his 30+ year career with the Natural Resources Research Institute (NRRI), Minerals & Metallurgy Strategic Research Initiative, he worked on a wide variety of field- and laboratory-based geological studies to better understand the mineral characteristics and mineral potential of the state. He retired from the NRRI in 2020 after an accomplished career. John loved field work, and he never stopped learning and teaching. Thomas Waggoner of Negaunee, Michigan, died December 7, 2020. Tom acquired an MSc in geology at Michigan State University, and worked for Cleveland Cliffs Mining. He helped Cliffs make the transition from underground mining of direct shipping ore to surface mining and production of taconite pellets, which are now the life blood of iron mining in the Lake Superior region. The success of his contributions is best measured by 40 years of continual production of pellets from the Empire and Tilden mines. During his long career with Cliffs (1965-1997), Tom actively participated in the teaching function of the Institute on Lake Superior Geology. He led 45 field trips for various organizations, and made numerous poster and oral presentations at annual meetings of the Institute. After his retirement in 1997, he continued to serve the mining industry. Tom remained active in ILSG, leading a field trip during the Iron Mountain meeting in 2018 and attending the Terrace Bay meeting in 2019. We all benefited from his knowledge, enthusiasm, and support. He will be sorely missed by all who knew him. (excerpted from a tribute by Alan Strandlie, December 10, 2020) xvi �Eisenbrey Student Travel Awards Because the 2020 meeting was canceled, and the 2021 meeting is virtual, no awards are granted. The program will continue when it’s safe to travel and meet in-person. The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name “Eisenbrey” was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. “Ned” Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The number of awards and value are determined by the annual Chair in consultation with the Secretary and Treasurer. Recipients will be announced at the annual banquet. The following general criteria will be considered by the annual Chair, who is responsible for the selection: 1) The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. 2) Students who are the senior author on either an oral or poster paper will be given favored consideration. 3) It is desirable for two or more students to jointly request travel assistance. 4) In general, priority will be given to those in the Institute region who are farthest away from the meeting location. 5) Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. Successful applicants will receive their awards during the meeting. xvii �Joe Mancuso Student Research Awards The 2005 Board of Directors established the ILSG Student Research Fund with $10,000 US from the Institute’s general fund to encourage student research on the geology of the Lake Superior region. A minimum of two awards of $500 US each for research expenses (but not travel expenses) will be made each year. Students are expected to present their research orally or during a poster session at an ILSG meeting. The award winners will also be automatically eligible for the Eisenbrey Travel Awards. To allow the fund to grow, the Fund will receive one-half of any additional proceeds from each annual meeting, after all other commitments and expenses are covered. • The ILSG Board of Directors will be responsible for selecting a minimum of two awards each year. The ILSG Treasurer will issue the awards. • The ILSG Student Research Fund is available for undergraduate or graduate students working on geology in the Lake Superior region. • The applications are due to the ILSG Secretary by August 31st of each year. Awards will be made by October 1st of each year. • Names of the award recipients will be announced at the next annual meeting and posted on the ILSG website. • Details of the application process can be found on the ILSG web site. • The proposal will need to be signed by the researcher’s supervisor. The 2012 Board of Directors approved modification of the fund’s name, adding “Mancuso” to reflect the many contributions of Joseph Mancuso to the organization and sizeable donations made in his name by Ronald Seavoy, Joe’s colleague and friend. “Doc Joe,” as he was known by his students, taught geology for 36 years at Bowling Green State University, Ohio. He advised many graduate students in field-oriented research, and frequently brought them to Institute meetings. Joe was the 2007 Goldich Medalist. In fall 2020, the ILSG Board of Directors selected one student to be granted research funding of $1000.00 from the Joe Mancuso Student Research Fund. The awardee was: Ann Marie Prue University of Minnesota-Duluth, MSc, Department of Earth and Environmental Sciences, TOPIC: Multi-method Geochemical Investigation of the Neoarchean Soudan Iron Formation, NE, Minnesota. [editor’s note: She is presenting her research at this meeting] xviii �Doug Duskin Student Paper Awards Because the virtual format of this meeting would likely complicate the process of judging student presentations, no awards are granted. The awards program will continue when it’s safe to travel and meet in-person. Each year, the Institute selects the best of student presentations and honors the presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting, and from generous donations to the fund in honor of Doug Duskin—an exploration geologist and longtime friend of the Institute. The 2012 ILSG Board of Directors approved adding Doug’s name to the award to acknowledge his contributions, and distribute those donations in a manner that would have pleased him. The Duskin Student Paper Committee is appointed by the Meeting Chair. Criteria for best student paper—last modified by the Board in 2001—follow: 1) The contribution must be demonstrably the work of the student. 2) The student must present the contribution in-person. 3) The Student Paper Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations. 4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution. 5) The total amount of the awards is left to the discretion of the meeting Chair in conjunction with the Secretary, but typically is in the amount of about $500 US (increase approved by Board, 10/01). 6) The Secretary maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee. 7) The names of award recipients shall be included as part of the annual Chair’s report that appears in the next volume of the Institute. Student papers will be noted on the Program. xix �Board of Directors Board appointment continues through the close of the meeting year shown in parentheses, or until a successor is selected. The terms of Board members have been extended 2 years due to cancellation of the 2020 meeting, and the difficulties of virtual voting by the membership during the 2021 meeting. Mark Smyk (2019-2024) – Lakehead University Esther Stewart (2018-2023) – Wisconsin Geological & Natural History Survey Anthony Pace (2017-2022) – Ontario Ministry of Energy, Northern Development and Mines Pete Hollings - Secretary (2016-2024) – Lakehead University Mark Jirsa – Treasurer (2017-2022) – Minnesota Geological Survey (retired) xx �REPORT OF THE 65th ANNUAL MEETING OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY Terrace Bay, Ontario Lakehead University and the Ontario Geological Survey (OGS), hosted the 65th Annual Institute on Lake Superior Geology on May 07 – 10, 2019 at the Terrace Bay Cultural Centre, Terrace Bay, Ontario. The meeting consisted of two days of technical sessions with pre- and posttechnical session field trips. Mark Smyk (OGS) and Pete Hollings (Lakehead University) were cochairs for the 2019 meeting. Invaluable advice and logistical support was provided by Dean Main and Michelle Malashewski of the Township of Terrace Bay’s Community Development department. Access to the Terrace Bay Public Library was graciously provided by Mary Deschatelets, Library CEO. Patty Cobin and Ted Bornhorst (A.E. Seaman Mineral Museum, Michigan Technological University) handled pre-meeting registration. Ted also supplied the poster boards. Generous contributions to the ILSG general fund and in support of 2019 student travel scholarships came from: Abitibi Geophysics, Barrick Gold Corporation, Benton Resources Corp., Geological Society of Minnesota, Greenstone Gold Mines, Landore Resources Canada Inc., Noront Resources Ltd., North American Palladium Ltd., Northwestern Ontario Prospectors Association, Panoramic PGMs (Canada) Limited, Premier Gold Mines Limited, Stillwater Canada Inc., Superior Lake Resources Limited, Thunder Bay Community Economic Development Commission, Wesdome Gold Mines Ltd., and Wolfden Resources Corporation. Individual Contributors to Student Travel Scholarship were: Al MacTavish, Mary Kay Arthur, L. Gordon Medaris, Jr. and Nick Swanson-Hysell. Total meeting registration was 114, which included 15 students. Welcoming remarks were provided by Terrace Bay Mayor Jody Davis. Elder Raymond Goodchild of Pays Plat First Nation provided traditional blessings for the meeting. Proceedings Volume 65 was published in two parts. Part 1 – Program and Abstracts, compiled and edited by Mark Puumala (OGS), contains 51 published abstracts for 32 oral and 19 poster presentations. Students presented 6 oral and 3 poster presentations. Part 2 – Field Trip Guidebooks, was compiled and edited by Al MacTavish and Pete Hollings. It contains descriptions of four premeeting and four post-meeting field trips. The 65th ILSG marked the first time in the Institute’s long history that its annual meeting was held in Terrace Bay. The meeting location enabled organizers to offer trips that showcased a variety of Archean and Proterozoic rocks along the north shore of Lake Superior. Although the majority of the field trips had been offered at previous ILSG annual meetings (e.g. Marathon 1995; Nipigon 2005), they greatly benefitted from the new mapping, research, discoveries and interpretations that had taken place in the intervening years. Local exploration companies graciously provided information and access to their properties. Unfortunately, a late spring precluded visiting the Slate Islands (lake ice) and the Midcontinent Rift-related carbonatites and diatremes in the Dead Horse Road area (snow). Slate Islands trip delegates were provided with an impromptu, “eclectic” field trip along Highway 17 between Schreiber and Marathon by Mark Smyk. Those who planned to see the MCR carbonatites and diatremes switched to other available trips. xxi �A list of field trips is provided below: Pre-meeting field trips (and leaders) on Tuesday, May 07: 1) Slate Islands (Pete Hollings, Lakehead University)* 2) Midcontinent Rift-related carbonatites and diatremes (Shannon Zurevinski, Lakehead University)* 3) Geology of the western Schreiber-Hemlo greenstone belt (Seamus Magnus, Ontario Geological Survey) 4) Geology of the Nipigon area (Philip Fralick, Lakehead University and Rob Cundari, Ontario Geological Survey) (*denotes cancelled due to snow/ice conditions) Post-meeting field trips on Friday, May10: 5) A stratigraphic transect across the northern flank of the Midcontinent Rift near Rossport (Pete Hollings, Lakehead University) 6) Geology of the Coldwell alkaline complex (Allan MacTavish, Panoramic PGMs (Canada) Limited and David Good, Western University) 7) Building and ornamental stone sites of the Marathon area (Peter Hinz, Ministry of Energy, Northern Development and Mines) 8) Geology of the past-producing Winston Lake Cu-Zn Mine (Mark Puumala and Mark Smyk, Ontario Geological Survey). Pre- and post-meeting field trips attracted 172 registrants, once again reaffirming their integral role in the allure and success of ILSG annual meetings. The vast majority of registrants and invited guests attended the annual ILSG banquet on Wednesday night. Although a Homer Award overview presentation was given, no “recipients” were identified during the 2019 annual meeting! As always, a highlight of the post-banquet activities was presentation of the 2019 Goldich Medal. This year’s very deserving recipient was Mark Severson. Mark’s wife, Laurie, and daughter, Allison, attended the banquet and award ceremony. The Goldich Medal citation was presented by George Hudak, his colleague for many years. George described Mark’s contributions to ILSG and to the greater understanding of Minnesota’s geology over several decades during his time as a student, in his role at the Natural Resources Research Institute-University of Minnesota Duluth, in his collaborations with the Minnesota Geological Survey and later in the mining and exploration industry. Mark is indeed a worthy recipient of this prestigious award. The 65th ILSG included a radical departure from the usual post-banquet guest speaker tradition. In lieu of a guest speaker, Master of Ceremonies, Mark Smyk, moderated a trivia contest entitled “ILSG Geo-Pardy.” It tested our delegates’ knowledge of ILSG history (e.g. past host cities, Goldich Medalists), geology in pop culture and “fun facts” about Lake Superior geology. Scores were tabulated and winners were announced the following day. Donated prizes and bragging rights attended the “awards ceremony.” In 2019, the student paper committee had its usual difficult job of selecting the best among six excellent oral presentations and three poster presentations for the Doug Duskin Student Paper Awards. The 2019 committee comprised Katarina Bjorkman (Bjorkman Prospecting), George Hudak (Natural Resources Research Institute–UMD) and David Good (Western University). The committee awarded three prizes with best talk going to Sophie Kurucz for her talk on “Paleoproterozoic snowball earth? Sedimentology and geochemistry of a Huronian glacial cycle” xxii �and runner up prizes going to Kira Arnold (Geology and geochemistry of the Terrace Bay Batholith, N. Ontario) and Munira Afroz (Sulfur, Carbon, and Oxygen Isotope Geochemistry of ~2.93 Ga Mesoarchean Chemical Sedimentary rocks in the Red Lake Area, Ontario) Eisenbrey Student Travel Grants were given to nine students: Paul Bielski (Lakehead) $200, Kira Arnold (Lakehead) $200, Jaqueline Drazan (UMD) $400, Jackie Wrage (University of Michigan) $600, Brittany Ramsay (Lakehead) $200, Munira Afroz (Lakehead) $200, Thomas Bodden (MTU) $500, Sophie Kurucz (Lakehead) $200 and Chanelle Boucher (Lakehead) $200. The Institute’s Board of Directors met on Wednesday, May 8, 2019 and a brief overview of the meeting is provided below: 1. Accepted report of the Chairs for the 64th ILSG, Iron Mountain, Michigan; as printed in the 2019 Proceedings Volume, and minutes of last Board meeting, May 16, 2018 (Hollings) 2. Received, discussed, and accepted 2018-2019 ILSG Financial Summary (Jirsa). 3. Received, discussed, and accepted 2018-2019 Report of the Secretary (Hollings). 4. Approved Mark Smyk as on-going ILSG Board member 5. Approved Mountain Iron, Minnesota as the site for the 66th annual ILSG meeting, hosted by Mark Jirsa, with help from Amy Radakovich and Terry Boerboom, all of the Minnesota Geological Survey. The dates of the meeting were set for May 12-15, 2020. 6. There was discussion as to future meeting locations, with Sudbury being suggested as a possibility. In addition, Esther Stewart has been in discussion with Rob Lodge and Dyanna Czeck about hosting a meeting in Wisconsin. 7. Discussed and approved replacing Klaus Schultz as the “member from government” on Goldich Committee (end of term 2019) with Dorothy Campbell (OGS). 8. Liability Insurance for the Institute was discussed and approved by the Board as an ongoing, annual expense. The Treasurer will investigate further to ensure that insurance is in place for the 2020 meeting. 9. It was approved that the Treasurer should purchase 10 Goldich medals, as only one remains. 10. Discussed and approved renewal of Pete Hollings as Institute Secretary (end of term 2022). This was later approved by a vote of the membership. The 65th ILSG meeting was a great success and we wish to thank all the people who contributed to that success, including staff of the Ontario Geological Survey and Lakehead University who were pressed into action as editors, field trip leaders and drivers. The Township of Terrace Bay was extremely hospitable in providing facilities (including the Cultural Centre, Library and Bowling Alley), suggestions and liaison services. We enlisted five different businesses to admirably handle our many catering needs in trying to “spread business around” in both Terrace Bay and neighbouring Schreiber. We heard many favourable comments from local people and businesses, not only about our patronage, but also about their interactions with ILSG delegates. They were thrilled to have us. We even received coverage in the local newspaper, provided by a reporter who tagged along on a field trip and popped by the Technical Sessions. We were happy that we were finally able to host the annual meeting in Terrace Bay. It was well worth the wait! Like most ILSG annual meetings hosted in smaller communities in Ontario, attendance was lower in 2019. However, those who did attend thoroughly enjoyed the meeting, facilities and field trips. Always flexible and understanding, our delegates took any changes in programming in stride. xxiii �We continue to encourage student attendance and new delegates in ensuring a healthy future for the Institute. We appreciate the support of our respective organizations, the Ontario Geological Survey and Lakehead University, and recognize the efforts that everyone made in travelling, attending, presenting and participating in activities that made the a 65th Annual Meeting what we feel was a great success. We encourage others to consider hosting and contributing to future meetings. It is truly a rewarding experience. Mark Smyk and Pete Hollings Co-Chairs, 65th Institute on Lake Superior Geology xxiv �TECHNICAL PROGRAM (Times are EDT) TUESDAY, MAY 11, 2021 SESSION 1 9:45-10:00 Welcome, introduction, and instructions (meeting chairs) 10:00-10:20 Paul Bielski and Philip Fralick Comparisons of Archean Iron Formations deposited at Shallow vs Deeper Depths 10:20-10:40 Bailey Drover, Mary Louise Hill, and Shannon Zurevinski The deformation and alteration of granitoid plutons in the Wabigoon subprovince 10:40-11:00 Manuel Duguet Geochemistry and Age of Archean Volcaniclastic and Mafic Intrusive Rocks, Georgia Lake Area, Quetico Subprovince, Northwestern Ontario 11:00-11:20 Seamus Magnus Chemostratigraphy of the western Schreiber-Hemlo Greenstone Belt—Results and Regional Implications 11:20-11:40 Brittany Ramsay, Philip Fralick, Stefan Lalonde, Paul Bielski, and Laureline Patry Environmental Control of Seawater Geochemistry in a Mesoarchean Peritidal System, Woman Lake, Superior Province 11:40-12:00 Michael Tamosauskas, Robert Lodge, M.A. Chong, Rasmus Haugaard, and Ross Sherlock The provenance, depositional environment and metallogenic implications of the Ament Bay Metasedimentary Assemblage, Sturgeon Lake Greenstone Belt, Northwest Ontario 12:00-12:20 Val W. Chandler V. W and Mark Jirsa Three-dimensional geologic mapping of Precambrian rocks in Minnesota: The creation of “removeable” geologic layers using gravity and magnetic data interpretation 12:20-12:40 Planavsky, Noah Negative Carbon Dioxide Emissions through Enhanced Silicate Weathering and the Lake Superior Region xxv �FRIDAY, MAY 14, 2021 SESSION 2 9:45-10:00 Introduction and instructions 10:00-10:20 Benjamin Drenth, Bill Cannon, Klaus Schulz, and Robert Ayuso Geophysical insights into Paleoproterozoic tectonics along the southern margin of the Superior Province, central Upper Peninsula, Michigan 10:20-10:40 Megan Landman and Mary Louise Hill Archean Orogenesis to Proterozoic Rifting: A structural history of Pass Lake, Thunder Bay, Ontario 10:40-11:00 Seamus Magnus Proterozoic Geology of the Schreiber–Terrace Bay Area 11:00-11:20 Ann Marie Prue and Latisha Brengman Preliminary pXRF results from Precambrian rocks of northern Minnesota 11:20-11:40 Nicholas Swanson-Hysell, Margaret Avery, Yiming Zhang, Eben Hodgin, and Terry Boerboom The paleogeography of Laurentia in its early years: new constraints from the Paleoproterozoic East-Central Minnesota batholith 11:40-12:00 Michael Easton The Critical Mineral potential of carbonatite and alkalic complexes in Ontario 12:00-12:20 Mark Puumala and Robert Cundari Critical Minerals Exploration and Development Potential in Ontario 12:20-12:40 Robert Cundari, Mark Puumala, Riku Metsaranta, and Dorothy Campbell Critical Mineral Potential on the North Shore of Lake Superior; Elements to ‘Structure’ our future POSTER SESSION 14:00-14:05 Introduction to 5-minute author summaries 14:05-14:10 Benjamin Gallagher and Marcia Bjornerud Reconstructing the hydration and carbonation history of the Presque Isle peridotite, Marquette Michigan: Insights into mechanisms of carbon sequestration in ultramafic rocks 14:10-14:15 Tara Lemke, Evan Weber, and Robert Lodge Characterization of Hydrothermal Alteration and Sulfide Ores at the Lynne ZnCu-Pb Deposit, Oneida Co. WI. xxvi �14:15-14:20 Trevor Nelson, Rory Johnson, Robert Lodge, Chong MA, Jeffery Marsh U-Pb geochronology and zircon trace-element geochemistry from granitoid plutons in the Neoarchean Sturgeon Lake greenstone belt, Ontario, Canada 14:20-14:25 Dana Peterson, P. Bedrosian and C. Finn 3-D Modeling of the Duluth Complex from geophysical data 14:25-14:30 Rebecca Price, Shannon Zurevinski, and Roger Mitchell A newly discovered orbicular occurrence within the Good Hope carbonatite, north of Marathon, Ontario 14:30-14:35 Shelby Short, Lillian Glodowski, and Robert Lodge Geochemistry and Petrography of Volcanic and Intrusive Rocks Hosting the Lynne Cu-Zn-Pb Deposit, Oneida County, WI 14:35-14:40 Esther Stewart, Billy Fitzpatrick, and Eric Stewart Geologic mapping identifies bedrock folds that may be significant for increased probability of arsenic detection in water wells 14:40-15:30 Breakout room discussions with Poster Presenters TUESDAY, MAY 18, 2021 SESSION 3 9:45-10:00 Introduction and instructions 10:00-10:20 Paul Bedrosian, Max Pace, and Katerina Zamudio Geophysical mapping of the eastern arm of the Midcontinent Rift in Upper Michigan 10:20-10:40 Sophie Mueller, J. Degraff, and D. Lizzadro-Mcpherson Structural Analysis and Interpretation of Deformation along the Keweenaw Fault System West of Lake Gratiot, Keweenaw County, Michigan 10:40-11:00 Nicholas Craik and Phil Fralick [presentation cancelled] Tracing redox pathways of the Mesoproterozoic Copper Harbour Conglomerate, Michigan 11:00-11:20 Ted Bornhorst Evolved seawater as the source of salinity for metamorphic-dominated oreforming hydrothermal fluids of the Keweenaw Peninsula native copper district, Michigan xxvii �11:20-11:40 Tien Grauch and S.J. Heller Integration of geophysical evidence indicates that anorthosite composes a significant portion of Grand Marais ridge, an inferred basement high in western Lake Superior 11:40-12:00 Laurel Woodruff and Tien Grauch Possible implications of a non-Archean Grand Marais Ridge, western Lake Superior 12:00-12:20 Eben (Blake) Hodgin, Nicholas Swanson-Hysell, Daniel Stolper, Andrew Turner, James DeGraff, Andrew Kylander-Clark, and Mark Schmitz Final inversion of the Midcontinent Rift during the Rigolet Phase of the Grenvillian orogeny FRIDAY, MAY 21 2021 SESSION 4 9:45-10:00 Introduction and instructions 10:00-10:20 Matthew Brzozowski, Peter Hollings, Allan MacTavish, Dawn EvansLamswood, Abraham Drost, and Derek Wilton Mineralizing processes in the Current Lake and Escape Lake conduit-type PGE– Cu–Ni deposits of the Thunder Bay North igneous complex, northwestern Ontario, Canada 10:20-10:40 David Good A Practical Approach to Using Incompatible Elements to Define Geochemical Correlation and a Common Mantle Source: Example Crystal Lake Gabbro and the Duluth Complex 10:40-11:00 Kyle Lachance, Jackie Kleinsasser, Adam Simon, Dean Peterson, and George Hudak Identifying the genesis of Fe-Ti oxide- and sulfide-bearing ultramafic intrusions in the Duluth Complex through sulfide geochemical analysis 11:00-11:20 Jackie Kleinsasser, Adam Simon, Dean Peterson, and George Hudak Textures and geochemistry of ilmenite and titanomagnetite in Fe-Ti oxide-bearing ultramafic intrusions of the Western Margin of the Duluth Complex, Minnesota 11:20-11:40 Amartya Kattemalavadi, Jackie Kleinsasser, Adam Simon, Dean Peterson, and George Hudak Olivine Geochemistry from Fe-Ti Oxide-Bearing Ultramafic Intrusions in the Duluth Complex, MN xxviii �11:40-12:00 Dave Dahl, Stacy Saari, and Thomas Lee Availability of Historical Airborne Geophysical Survey Data at Minnesota DNR 12:00-12:20 Allan MacTavish, Peter Hinz, Mary Kay Arthur, Robert Chataway, Jim Edberg, Tom Erickson, Steve Fox, Joan Furlong, Jim Gerlich, Lindsay Smith, and David Wilhelm Island of Hawaii Field Trip, February 11-21, 2020 12:20-12:30 Closing remarks MAY 21-AWARDS EVENING 19:00-20:30 • • • • • • Words of welcome (chairs) ILSG updates Goldich Medal presentation to Allan MacTavish 2022 ILSG meeting plans—Sudbury Ontario Honoring the Pioneers of Lake Superior Geology—Newton Horace Winchell (by Jim Miller and Sue Leaf) Closing remarks xxix �ABSTRACTS xxx �Geophysical mapping of the eastern arm of the Midcontinent Rift in Upper Michigan BEDROSIAN, Paul A, PACE, Max, and ZAMUDIO, Katrina D. Geology, Geophysics and Geochemistry Science Center, U.S. Geological Survey, Denver, CO The western arm of the Midcontinent Rift system (MRS), extending from western Lake Superior into Kansas, is somewhat understood due to exposures of MRS igneous and sedimentary rocks, scattered drillholes, and seismic imaging. By comparison, the eastern rift arm (ERA), extending from eastern Lake Superior through lower Michigan is poorly understood, with almost no outcropping volcanic rocks south of Michipicoten and Caribou Islands in Lake Superior, few boreholes that intercept MRS rocks, and the majority of the rift concealed beneath the Michigan basin. Seismic reflection sections in the eastern Lake Superior basin (Behrendt et al., 1990; Mariano and Hinze, 1994) and in Lake Michigan (Cannon et al., 1991) provide the primary constraints on the gross structure of the ERA. Unlike the western rift arm, which has undergone considerable post-rift shortening (Cannon et al., 1993), the ERA appears less deformed, with perhaps as much as 30 km of rift clastic and volcanic rocks preserved within a broadly symmetric basin (Behrendt et al., 1988) and about 5 km of anticlinal relief on the top of the volcanics (Mariano and Hinze, 1994). Under Paleozoic cover in the eastern part of Michigan’s Upper Peninsula, the ERA western margin is inferred from linear northwest-trending potential-field anomalies (Fig. 1a, b) and the occurrence of MRS basalt and rhyolite beneath ~2 km of Paleozoic and MRS clastic rocks in the St. Amour drillhole (Ojakangas and Dickas, 2002). In this examination of the ERA, we present results of a recent magnetotelluric survey over the ERA and a 3D electrical resistivity model derived from these data. The ERA is clearly imaged as a 150-km wide conductive rift basin with a sharp western margin and an irregular eastern margin that traces the distribution of mapped MRS clastics in Ontario (Fig. 1c). The lowest resistivities (10 Ω·m) correspond to the Jacobsville Ss, imaged as an east-dipping horizon extending to 2-3 km depth beneath the edge of the Michigan basin. Underlying the Jacobsville are moderately conductive rocks (100-500 Ω·m) that thicken basinward to 8-10 km depth with a nested inner basin that may be 20 km or more deep. The rift-fill rocks are attributed to a combination of Oronto Grp. clastics and MRS basalts, which cannot be separately distinguished within the resistivity model. In map view, the resistivity model at 10 km depth (Fig. 1d) shows along-rift changes in resistivity with a wavelength of ~50 km. Though data coverage is limited, both magnetic and gravity data (extending north into Lake Superior) show a similar pattern of highs and lows. A seismic reflection profile parallel to the rift axis (LS-15; Mariano and Hinze, 1994, their Fig. 7) interprets folding of the rift sedimentary and volcanic section and infers several kilometers of relief along strike. The linearity of the western rift margin suggests that both the thickest sections of rift basalt and Jacobsville are fault bounded. Together with the evidence for along-strike folding and faulting, we suggest the current structure of the ERA reflects post-rift NW-SE compression oriented along the rift axis, with the western margin marked by a transform fault (likely a reactivated normal fault of MRS age). Extrapolating farther north, we speculate that left-lateral offsets within the broad magnetic high that follows the western side of the ERA reflect a series of en echelon faults that together conspired to shorten the ERA by several tens of kilometers or more. �Figure 1. (a) magnetics, (b) gravity, (c) resistivity at 1 km depth and (d) resistivity at 10 km depth. Shaded regions show mapped or inferred MRS clastic rocks. White circles, stars, and lines indicate magnetotelluric stations, deep drillholes, and seismic reflection lines, respectively. Faults are shown in black. White dashed lines show interpreted structures. Potential field data are from Anderson and Grauch (2018). Anderson, E.D., and Grauch, V.J.S., 2018, Updated gravity stations and anomaly compilation over Lake Superior: U.S. Geological Survey data release, doi:10.5066/F7F18X8S. Behrendt, J.C., Green, A.G., Cannon, W.F., Hutchinson, D.R., Lee, M.W., Milkereit, B., Agena, W.F., and Spencer, C., 1988, Crustal structure of the Midcontinent Rift system: Results from GLIMPCE deep seismic reflection profiles: Geology, v. 16, p. 81, doi:10.1130/0091-7613(1988)016<0081:CSOTMR>2.3.CO;2. Behrendt, J., Hutchinson, D., Lee, M., Thornber, C., Trehu, A., Cannon, W., and Green, A., 1990, GLIMPCE seismic reflection evidence of deep—Crustal and upper-mantle intrusions and magmatic underplating: Tectonophysics, v. 173, p. 595–615. Cannon, W.F., Lee, M.W., Hinze, W.J., Schulz, K.J., and Green, A.G., 1991, Deep crustal structure of the Precambrian basement beneath northern Lake Michigan, midcontinent North America: Geology, v. 19, p. 207–210. Cannon, W.F., Peterman, Z.E., and Sims, P.K., 1993, Crustal-scale thrusting and origin of the Montreal River monocline-A 35-km-thick cross section of the midcontinent rift in northern Michigan and Wisconsin: Tectonics, v. 12, p. 728–744, doi:10.1029/93TC00204. Mariano, J., and Hinze, W.J., 1994, Structural interpretation of the Midcontinent Rift in eastern Lake Superior from seismic reflection and potential-field studies: Canadian Journal of Earth Sciences, v. 31, p. 619–628. Ojakangas, R.W., and Dickas, A.B., 2002, The 1.1-Ga Midcontinent Rift System, central North America: sedimentology of two deep boreholes, Lake Superior region: Sedimentary Geol., v. 147, p. 13–36. 2 �Comparisons of Archean Iron Formations deposited at Shallow vs Deeper Depths BIELSKI, Paul and FRALICK, Philip Department of Geology, Lakehead University, Thunder Bay, ON, Canada The occurrence of iron formation (IF) during the Archean is well documented, however the mechanisms of their genesis are poorly understood within shallow waters and even more so within the deep-ocean. At the same time our understanding of Archean deep-ocean chemistry is also limited and poorly constrained. To address these issues, multiple shallow and deeper water iron formations were analysed by coarse and fine-scale geochemical analysis. The fine-scale method entailed using Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS) alongside Scanning X-Ray Fluorescence (XRF), while the coarse-scale analysis is done with bulk rock geochemistry. The IFs used in this study were all deposited at approximately 2.7 Ga in different depositional settings. The shallow water IF is from Beardmore and Lake St. Joseph, which are both instances of deltaic sediments intermingled with sections of cherty-IF, and magnetite layers directly within deltaic distributary mouth bars. The deeper water IFs are from Temagami and Timmins in Ontario and Soudan, Minnesota. These IFs contain no discrete siliciclastic strata within the main body of IF, where chert and iron oxides are the dominant lithology. While the true depths of these deeper water formations are unknown, the lack of non-chemical sedimentary rock, excluding thin slate strata, and no storm-related sedimentary structures within layering suggests some distance from landmasses and depths from below storm wave base to abyssal. Siliciclastic turbidites from Beardmore have been included in this study to assist in identifying trends related to detrital contamination. Rare earth element (REE) concentrations, normalized with post-Archean Australian shale (PAAS), and analysed by bulk rock geochemistry, show similarities between deeper and shallow water deposits. The deeper water IFs contain pronounced HREE enrichment with prominent Eu and Y anomalies while shallow water IF do not show consistent Y anomalies but do contain relatively smaller Eu anomalies (Figure 1) despite siliciclastic contamination. Y/Ho is largest within the deep water IF with mainly supercondritic ratios from 28 to 55. Shallow water IF contains near chondritic (~28) ratios. The smaller Eu and Y anomalies are interpreted to be a combination of weaker Eu anomalies present in shallower water (cf. Planavsky et al., 2010) and contamination by siliciclastics, which have non-anomalous values. The lack of superchondritic Y/Ho in the shallow water IF samples is odd as Y anomalies are thought to be largest in shallow water during the Archean (cf. Planavasky et al., 2010) which may relate to a scavenging disequilibrium with the surrounding seawater as deeper water IF contain a consistent Y anomaly (Figure 1). Trace element analysis of redox sensitive elements U, V, and Cr have a strong relationship with Al2O3, indicating significant contributions of these elements from detrital material, especially within shallow water IF. However, V/Fe and Cr/Fe ratios in deeper water IF are comparable to shallow water ratios while having much lower levels of detrital contamination (i.e., Al2O3) (Figure 2). This indicates Cr and V were enriched in the precipitated phase, adding extra Cr and V to the sediment. These elements are more soluble in oxidizing fluids, but their solubility greatly decreases in reducing environments, e.g. the Archean deeper ocean. Thus, they required the presence of some oxygen in the weathering environment, but were reduced and precipitated in the Archean deeper ocean. Th/U ratios of IF samples may be used to identify enrichments of U, or depletions of Th, 3 �relative to the ratio found within igneous rock at a given time. At 2.7 Ga, the Th/U ratio within crustal rock was approximately 3.9, which is imprinted into the seawater ratio from the weathering and breakdown of minerals containing this ratio. Deep water IF contains low Th/U ratios in bulk rock geochemistry (Figure 2) and has even lower ratios (<1) in laser ablation data, which is less affected by detrital contamination. Shallow water IF bulk geochemistry shows values near that of average crustal rock, with variance around this value (~3.9). A more detailed understanding of differences between shallow and deeper water IF is obscured by the inevitable presence of detrital contamination. However, it is evident that Eu anomalies are larger and redox elements are enriched in deeper water IF. Shallow water IF may be more susceptible to changes in water chemistry within the deltaic depositional environment causing anomalous and/or variable REE and trace element geochemistry compared to the similar geochemistry between different deeper water IFs. Taylor-McLennan 1985-REEs 80 70 60 1 50 Y/Ho Rock/Post-Arch. Aust. Shale-PAAS 10 40 30 .1 20 10 .01 Ce La Nd Pr Eu Sm Tb Gd Y Dy Er Ho 0 .01 Yb Tm .1 1 Al 2 O 3 Lu 10 100 Figure 1. REE analysis. Note that the flatter pattern of the shallow IF is due to contamination. 5 100 4 10 3 Th/U Al 2 O 3 1 2 .1 1 0 .01 .1 1 10 .01 .01 .1 Th 1 V/Fe 2 O 3 10 100 Figure 2. Redox element analysis. Shallow water IF and turbidites correlate while deep water IF contains similar V/Fe2O3 ratios despite much lower amounts of Al2O3 (detritus). References Planavsky, N., Bekker, A., Rouxel, O. J., Kamber, B., Hofmann, A., Knudsen, A., & Lyons, T. W. (2010). Rare earth element and yttrium compositions of Archean and Paleoproterozoic Fe formations revisited: new perspectives on their significance and mechanisms of deposition. Geochimica et Cosmochimica Acta, 74(22), 6387-6405. 4 �Evolved seawater as the source of salinity for metamorphic-dominated ore-forming hydrothermal fluids of the Keweenaw Peninsula native copper district, Michigan BORNHORST, Theodore J. Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, Michigan 49931 The Mesoproterozoic Midcontinent rift of Michigan’s Keweenaw Peninsula hosts the world’s largest accumulation of native copper. For more than 60 years, it has been generally accepted that burial metamorphic processes generated ore-forming hydrothermal fluids. However, Brown (2006) reasoned that metamorphic processes could not generate high enough salinity. This constraint requires an outside source of salinity. He proposed a hybrid hydrodynamic evolved meteoric water and metamorphic ore-forming fluid at depth and down-dip below the native copper deposits in the fluid source zone. Brown’s envisioned meteoric waters evolved during descent through the Oronto Group clastic sedimentary rocks and the salinity was dissolved from postulated evaporite horizons and/or scattered cement. An additional genetic constraint is that the ore-forming fluids were abnormally low in sulfur leading to the precipitation of native copper rather than copper sulfides (White, 1967). Because of very low sulfur magmas and degassing of sulfur upon eruption, the rift-filling Portage Lake Volcanics (PLV), host rocks of the native-copper deposits, and stratigraphically equivalent rocks of the source zone are sulfur-poor. Metamorphism of the source zone rocks resulted in sulfur-poor metamorphic ore-forming fluids. Brown’s hydrodynamic evolved meteoric water model lacks a mechanism to satisfy the constraint of very low sulfur fluids. Further, the reasoning presented by Bornhorst and Mathur (2017 and 2018) suggests that hydrodynamic evolved meteoric water had limited to no significant role in the ore-forming fluids. A role for seawater in generation of the native copper ore-forming fluids was proposed by Livnat (1983). However, seawater has not been widely accepted since the PLV were subaerially deposited and until recently there was only the debatable possibility of an incursion of seawater into the rift during deposition of the Nonesuch Formation. Recent evidence (Jones et al., 2019) suggests that the Nonesuch and underlying Copper Harbor Formations were deposited in a “braided fluvial-evaporitic shoreline-marine embayment triplet.” Johnson (1985) documented an extensive subaqueous emplaced volcanic layer in the Keweenaw Peninsula and Isle Royale in the upper part of the PLV basalts which are otherwise subaerially deposited. This layer may have been deposited during an incursion of seawater into the rift. Incursions of seawater are also possible during times of PLV interbedded minor clastic sediments when volcanism waned. Thus, there is likely a long history of seawater incursion into the Midcontinent rift and a significant volume of seawater could have penetrated deeply into the PLV. As the PLV was buried into the fluid source zone, the contained seawater (formation water) evolved. A plausible mechanism is needed to satisfy the constraint of very low sulfur fluids. Blättler et al. (2020) have shown that low sulfate levels in seawater occurred during the late-Mesoproterozoic which would have required less depletion of sulfur. When seawater penetrates mid-ocean basalts in vicinity of hydrothermal vents it becomes heated and evolves as it reacts with and alters the host basalt. At 130-150oC anhydrite begins to precipitate; the solubility of anhydrite is retrograde (Antonelli et al., 2017). The precipitation of anhydrite (CaSO4) depletes sulfate from seawater and if there is sufficient Ca, precipitation of anhydrite removes most of the sulfate. As the partially evolved seawater and its host basalts continue to be heated, albitization of basalt releases Ca into the seawater which results in complete removal of any remaining sulfate. Breakdown of olivine and pyroxene releases Mg into the fluid but it is bound up in secondary Mg minerals, such as chlorite, leaving the fluid both sulfur- and Mg-poor. At yet higher temperatures 5 �(> 250oC) the water-rock reactions continue to release Ca to the fluid, which accumulates, producing a Ca-rich brine (Antonelli et al., 2017; Tivey, 2007). Oceanic and continental rifts undergoing greenschist to amphibolite facies metamorphism are known for CaCl2-rich, Mg-poor brines, also enriched in leached Na, K, and Cu among other elements (Tivey, 2007; Hardie, 1983). This model for modern mid-ocean basalts is applicable to the generation of native copper ore-forming fluids. The PLV basalts are tholeiitic, similar to mid-ocean ridge basalts, and have undergone extensive albitization. The rocks in the source zone have been subjected to temperatures exceeding the temperature of precipitation of anhydrite, hence the seawater could have become depleted in sulfur and enriched in Ca before undergoing burial metamorphism. Kelly (2020) demonstrated that probable primary fluid inclusions formed during precipitation of native copper are filled by Ca-rich fluids with up to 30 equivalent weight percent CaCl2. Published light stable isotope data are permissive of a hybrid evolved seawater and metamorphic-dominated ore-forming fluid originating from the source zone. Such a hybrid cannot be isotopically distinguished from metamorphic-only fluid. The combination of low sulfate seawater and anhydrite precipitation provides a viable mechanism to generate evolved seawater that provides an outside source of salinity for the ore-forming fluids without sulfur. Lengthy incursions of seawater into the Midcontinent rift supports the hypothesis that evolved seawater may have played an important role in the generation of burial metamorphic-dominated native copper ore-forming fluids. References: Antonelli, M.A., Pester, N.J. Brown, S.T, DePaolo, D.J., 2017, Effect of paleoseawater composition on hydrothermal exchange in midocean ridges, Proceedings of the National Academy of Sciences,114: 12413-12418. Blättler, C.L., Bergmann, K.D., Kah, L.C., Gómez-Pérez, I., Higgins, J.A., 2020, Constraints on MesoNeoproterozoic seawater from ancient evaporite deposits, Earth and Planetary Science Letters, 532: 115951. Bornhorst, T.J, Mathur, R., 2017 and 2018, Copper isotope constraints on the genesis of the Keweenaw Peninsula native copper district, Michigan, USA, Minerals: 7, 185 and 8, 508 Brown, A.C., 2006, Genesis of native copper lodes in the Keweenaw Peninsula, northern Michigan: A hybrid evolved meteoric and metamorphogenic model, Economic Geology, 101: 1437–1444. Hardie, L. A., 1983, Origin of CaCl2 brines by basalt-seawater interaction; insights provided by some simple mass balance calculations, Contributions to Mineralogy and Petrology: 82, 205-213. Johnson, R.C., 1985, Documentation of a subaqueously emplaced volcanic horizon in the upper Portage Lake Volcanics, Keweenaw Peninsula, Michigan, ILSG Proceedings, 31, part 1: 38-39. Kelly, David., 2020, Fluid inclusion study of selected calcite associated with native copper, Quincy mine, Keweenaw Peninsula, Michigan, Open Access M.S. Report, Michigan Technological University, 165. Livnat, A., 1983, Metamorphism and Copper Mineralization of the Portage Lake Lava Series, Northern Michigan. Ph.D. Dissertation, University of Michigan, Ann Arbor, MI, USA; 1–292. Tivey, M.K., 2007, Generation of seafloor hydrothermal vent fluids and associated mineral deposits, Oceanography, 20: 50-65. White, W.S., 1968, The native-copper deposits of northern Michigan, In Ore Deposits of the United States, 1933–1967 (Graton Sales Volume), Ridge, J.D., Ed.; American Institute of Mining, Metallurgical, and Petroleum Engineers: New York, NY, USA, 303–325. 6 �Mineralizing processes in the Current Lake and Escape Lake conduit-type PGE–Cu–Ni deposits of the Thunder Bay North igneous complex, northwestern Ontario, Canada BRZOZOWSKI, Matthew1, HOLLINGS, Peter1, MACTAVISH, Allan2, EVANS– LAMSWOOD, Dawn2, DROST, Abraham2, WILTON, Derek3. 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada Clean Air Metals, 1004 Alloy Drive, Thunder Bay, ON, P7B 6A5, Canada 3 Earth Sciences, Memorial University, 230 Elizabeth Avenue, St. John’s, NL, A1C 5S7, Canada 2 The Thunder Bay North igneous complex (TBNIC), located approximately 50 km northeast of Thunder Bay, comprises a series of small mafic–ultramafic intrusions that occur in the vicinity of several other similar intrusions, including the Sunday Lake, Saturday Night, and Thunder intrusions to the southwest, and the Seagull, Disraeli, and Hele intrusions to the northeast. The intrusions of the TBNIC comprise the 1106.6 ± 1.6 Ma Current Lake (Bleeker et al., 2020), Escape Lake, Lone Island Lake, and 025 intrusions, all of which were emplaced proximal to the east–west trending Quetico Fault System. Of these intrusions, the Current Lake (CL) and Escape Lake (EL) chonoliths host significant orthomagmatic Ni, Cu, and platinum-group element (PGE) mineralization, and have been in active exploration since 2005. Although few theses have been completed on the Current Lake (Chaffee, 2015) and Escape Lake (D’Angelo, 2013) chonoliths, the mechanisms that generated the variable styles of PGE–Cu–Ni mineralization in these spatially associated chonoliths, and their genetic relationships have never been formally characterized. This presentation highlights some of the preliminary results from a three-year collaborative effort between Clean Air Metals, Lakehead University, and Memorial University, with the ultimate goals of developing robust mineralization models for and characterizing the genetic relationships between the CL and EL intrusions. Although both the CL and EL chonoliths have been actively explored, the most extensive work has been conducted on the CL chonolith, resulting in an indicated resource of 12 Mt at 1.48 g/t Pt, 1.4 g/t Pd, 0.28% Cu, and 0.17% Ni for CL, and 4.3 Mt at 0.92 g/t Pt, 1.18 g/t Pd, 0.52% Cu, and 0.28% Ni for EL (Kuntz and Jones, 2021). The CL chonolith contains five variably mineralized zones (from north to south) — the Current Lake, Bridge, Beaver Lake, Cloud, and 437 zones. The southernmost portion of the conduit is represented by the unmineralized Southeast Anomaly. The EL chonolith has been divided into two sections (from north to south) — the Escape and Steepledge Lake sections. Both conduits dip gently to the south, with their lowest points being adjacent to the E–W trending Escape Lake Fault. Primary base-metal sulfides (BMS) in both systems largely comprise pyrrhotite, pentlandite, and chalcopyrite; BMS occur in the CL system occur throughout the chonolith in the Current Lake and Bridge zones, and at the base and top of the chonolith in the Beaver Lake and Cloud zones, respectively. Metal tenors in the CL and EL systems are similar, being enriched in Cu and the Pt-group PGEs relative to Ni and the Ir-group PGEs, and exhibiting a distinct negative Ru anomaly, a feature not commonly observed in other Ni–Cu–PGE systems globally (e.g., Eagle and Jinchuan). The anomalously low Ru concentrations are likely the result of clinopyroxene fractionation at depth. Copper/Pd ratios in the Current Lake and Beaver Lake zones of the CL chonolith, and Escape Lake and Steepledge Lake sections of the EL chonolith, are indistinguishable and largely within the range of mantle values, suggesting that significant amounts of sulfide were not removed at depth. Although most of the samples from both systems have mantle-like Cu/Pd and Pd/Pt ratios, data 7 �from the Southeast Anomaly, Escape Lake, and Steepledge Lake zones trend towards high Cu/Pd and low Pd/Pt ratios, suggesting that some Pd may have been removed by segregation of limited sulfide liquid at depth. If the Southeast Anomaly is considered the feeder to the CL system, then this may suggest that sulfides are located deeper in the feeder system. Numerical modeling suggests that, although the CL system is characterized by slightly higher R factors than the EL system (~12,000 vs. ~7,000), there was not a significant variation in R factor along the length of the conduits, with variations in bulk-rock metal concentrations largely representing variable accumulations of sulfide. In both systems, the Pt-group and Ir-group PGEs, and Au correlate with Pd, suggesting that the physical separation of residual Cu-rich liquid from monosulfide solid solution did not play a role in generating the mineralization. Variations in Ni/Cu–Mg and Pd/Pt– Pd/Ir ratios suggest that the concentrations of Ni and Ir in the CL system appear to be controlled, in part, by the accumulation of olivine towards the Southeast Anomaly. The fact that rocks in this zone are olivine-poor suggests that olivine may be located at depth in the feeder system. Systematic inverse correlations between Fe–S–Cu–Ni–Pd–Pt (+ spikes) and Mg–Ca–Cr–Ti (– spikes) throughout the thickness of the Current Lake Zone suggest multiple influxes of metal-rich magma into the system. The abundance of these inverse correlations at the base and top of the Beaver Lake–Cloud zones suggest that the dynamics of magma flow varied along the length of the conduit, likely as a result of the variable conduit morphology. Bulk assimilation of the granitic and sedimentary country rocks is unlikely to have played a role in sulfide saturation as there are no systematic mixing trends in La/Zr–Th/Nb between the host and country rocks. Although it is possible that the large variation in S/Se values could be the result of contamination by BMS from the granitic and sedimentary country rocks, it is also possible that the variation resulted from S loss and very low R factors (< ~1,000). The importance of local S addition for, and the timing of, sulfide liquation remains unclear, but is one of the critical questions that will be assessed in the Clean Air Metals–Lakehead University–Memorial University partnership. References Bleeker, W., Smith, J., Hamilton, M., Kamo, S., Liikane, M., Hollings, P., Cundari, R., Easton, M., Davis, D., 2020. The Midcontinent Rift and its mineral systems: Overview and temporal constraints of Ni-CuPGE mineralized intrusions (No. 8722), Targeted Geoscience Initiative 5: Advances in the understanding of Canadian Ni-Cu-PGE and Cr ore systems - Examples from the Midcontinent Rift, the Circum-Superior Belt, the Archean Superior Province, and Cordilleran Alaskan-type intrusions. Chaffee, M.R., 2015. Petrographic and Geochemical Study of the Hybrid Rock Unit Associated with the Current Lake Intrusive Complex (MSc). University of Minnesota. D’Angelo, M., 2013. Igneous textures and mineralogy of the Steepledge Intrusion, Northern Ontario (BSc). Lakehead University. Kuntz, G., Jones, L., 2021. NI 43-101 Technical report and mineral resource estimate for the Thunder Bay North project, Thunder Bay, Ontario (NI 43-101). 8 �Three-dimensional geologic mapping of Precambrian rocks in Minnesota: The creation of “removeable” geologic layers using gravity and magnetic data interpretation CHANDLER V. W. (chand004@umn.edu) and JIRSA M.A.(jirsa001@umn.edu) Minnesota Geological Survey (retired), 2609 Territorial Road, St, Paul, MN 55114 -1009 As part of the National Geological and Geophysical Data Preservation Program (NGGDP) of the U. S. Geological Survey, the Minnesota Geological Survey (MGS) compiled geophysical models from past and on-going investigations to create three-dimensional representations of several Proterozoic basins, consisting of elevation contours and inferred bedrock geology of the underlying basement surfaces. Thus defined, the Proterozoic basins are represented as removeable, three-dimensional layers in the Precambrian bedrock map of the state. The NGGDP-MGS work focused on several basins including those associated with the Sioux Quartzite, the Animikie Group, the sedimentary rocks of the Midcontinent rift system, and the somewhat more complicated basin enclosing the North Shore Volcanic Group and the Duluth Complex (NSVG-DC). In this presentation we will focus on the results from the main bowl of the Animikie basin and the adjoining NSVG-DC basin. Figure 1 presents the elevations estimated for the basement of these two basins. The basement elevations estimated for the main bowl of the Animikie basin are primarily based on Euler analysis, a semi-automated depth interpretation scheme, in which a non-magnetic basin sequence is assumed to overlie an assortment of anomaly sources that suitably approximates a basin floor. Correspondingly much of the Animike sequence consists of non-magnetic slates and grauwackes, with anomaly sources restricted to iron-formation near the base of the sequence, or to sources within the underlying Archean basement. The Euler results were cross-checked in a few areas by conventional, two-dimensional modeling, and the geology of the inferred basin floor is based on extrapolating the bedrock geology of adjacent areas, as guided by gravity and magnetic anomaly signatures. The results (Figure 1) indicate that northern and western margins of the Animike basin are rimmed by a shallow (0-1.5 km elevation) shelf. The northern part of the shelf has been previously inferred from earlier investigations, but the western shelf is somewhat novel, and it appears to be significantly controlled by NW-striking structures. Several prominent sources underlying the Animikie sequence are interpreted to be strongly magnetic, consistent with ironformation-bearing horizons, and the steep northward dips interpreted for some of these sources are most consistent with Archean rocks in the region. The deeper parts of the basin are locally below -4 km elevation, including the area along the basal contact of the NSVG-DC basin. This ~5 km.thick Animikie sequence could presumably continue to the east in some fashion beneath the NSVG-DC sequence. Such a scenario has implications for the metallogenesis and upper crustal geology of the region. Euler analysis cannot resolve the base of a magnetic sequence, so the base of the NSVG-DC basin is estimated along twenty two-dimensional gravity and magnetic models that transect the complex in strategic areas. Of these, ten models have been recovered and revised from earlier investigations and ten models have been created specifically for this study. Wherever possible, modeling has been constrained by geology mapped at the bedrock surface (Jirsa and others, 2012) and by rock-property data (Chandler and Lively, 2011). The modeling results were used to compile basement elevation contours beneath the land surface, and these contours were smoothed and merged with elevation contours beneath Lake Superior, which were compiled from 3-d gravity modeling and seismic reflection interpretations (Allen and others, 1997). The two lowest parts of the NSVG-DC sequence are interpreted lie ~-15 km. below MSL, and are plausible candidates for major feeder zones. A prominent basement high separates these two lows, and it appears to 9 �ultimately connect with major basement ridges beneath Lake Superior, including Walter White ridge to the south, and with the Grand Marais ridge to the east. Estimated dips along the base of the NSVG-DC basin generally range from 25 to 60 degrees, with the steepest dips inferred along the northern and western basal contacts. A shelf-like structure inferred beneath the northwestern margin of the NSVG-DC basin lies roughly along-strike with the shelf inferred beneath the northern margin of the Animikie basin. The interpretations presented here represent our current “best guess” of subsurface structure, based on available data, and many improvements should be possible in the future. As such, the interpretations presented here should serve as a helpful starting point for future three-dimension investigations, including the gravity, magnetic, and electromagnetic studies that has been recently initiated by the U. S. Geological Survey in the region. References cited: Allen, D. J., Hinze, W. J., Dickas, A. B., Mudrey Jr., M. G., 1997, Integrated geophysical modeling of the North American Midcontinent Rift System: new interpretations for western Lake Superior, northwestern Wisconsin, and eastern Minnesota, in, Ojakangas, R. W., Dickas, A. B., and Green, J.C., eds., Middle Proterozoic to Cambrian Rifting, Central North America: Geological Society of America Special Paper 312, p 47-72. Chandler, V.W.; Lively, R.S.. (2011). Density, Magnetic Susceptibility, and Natural Remanent Magnetization of Rocks in Minnesota: An MGS Rock Properties Database. Minnesota Geological Survey. Retrieved from the University of Minnesota Digital Conservancy, http://hdl.handle.net/11299/175580. Jirsa, M.A.; Boerboom, T.J.; Chandler, V.W.. (2012). S-22, Geologic Map of Minnesota, Precambrian Bedrock Geology. Retrieved from the University of Minnesota Digital Conservancy, http://hdl.handle.net/11299/154540. Figure 1. Estimated elevation of the basement surface below the main bowl of Animikie basin and the combined igneous sequences of the North Shore volcanic Group and the Duluth Complex. Elevations are relative to mean sea level, and range from 0 to 2,000 m. (yellow), -2,000 to -5,000 m. (light green), -5,000 to -15,000 m. (green), and <15,000 m. (blue). Elevations beneath the North Shore Volcanic Group- Duluth Complex basin are highlighted with 1000 m. contours. 10 �Tracing redox pathways of the Mesoproterozoic Copper Harbour Conglomerate, Michigan CRAIK, Nicholas1 and FRALICK, Philip1 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada The history of Earth’s oxygenation is key to understanding the evolution of life, the emplacement of important mineral deposits, and the cycling of oxidative weathering products through time. Many studies have been particularly focused on atmospheric and oceanic oxygenation during the Archean to Paleoproterozoic – denoting the Mesoproterozoic as part of the ‘Boring Billion’, a time of general stability or stagnation of many Earth processes. During the ‘Boring Billion’ oxygen was thought to have been stalled at low concentrations relative to today. This idea has largely been put forth by researchers utilizing deep sea black shales as a proxy for continental weathering patterns Many studies of atmospheric oxygen within the Mesoproterozoic are contradictory to one another, likely due to sampling of deep ocean settings where oxygen cannot penetrate, and lack of abundant studies on terrestrial sediment. To reach results more representative of the ancient atmosphere, choosing a depositional setting that has been well documented as subaerial, and/or being able to constrain the weathering history of the system is imperative. In this work sedimentological and geochemical evidence from the subaerial red-bed deposits and carbonates of the unmetamorphosed Copper Harbour Conglomerate were collected with the aim of better understanding and constraining the spatial and temporal fluctuations of atmospheric oxygen levels during the Mesoproterozoic. The Copper Harbour Conglomerate of the Keweenaw Peninsula, Michigan is a progradational alluvial to lacustrine sedimentary sequence representing the first continuous infilling of the Midcontinent Rift basin after the cessation of associated volcanic activity at ~1.1 Ga. Within the formation are pervasive carbonate cement (calcrete) lenses as well as a stromatolitic horizon located in its uppermost portion. The purpose of this study was to examine the oxidative weathering products (siliciclastics), groundwater-precipitated carbonates (calcrete), and pond/lacustrine-precipitated carbonates (non-biogenic precipitates and stromatolites) found within the Copper Harbour Conglomerate to determine the relative oxygenation of the atmosphere at the time of deposition (~1085 Ma). ICP-MS and ICP-AES methods, both whole rock and preferential carbonate extraction, were utilized to determine the whole rock and carbonate geochemistry of the rock types within the upper Copper Harbour Conglomerate. By analyzing the redox sensitive metals (e.g., Fe, Mn, V, and U) and rare earth elements (e.g., Ce anomaly), theoretical constructions of the hydrological pathways of these elements can be developed. This allows understanding of the redox environment during early diagenesis. 11 �Figure 1 shows the Ce and La anomalies of the samples taken in the study area. The calcrete samples have the strongest negative Ce anomalies, indicating they have the least abundance of Ce and therefore have previously had the greatest interaction with oxygen, either due to abundance of oxygen or elapsed reaction time. This makes sense because calcrete is formed by the evaporation of groundwater near surface (i.e., subaerial) and takes thousands of years to form. The stromatolite Figure 1: La and Ce anomalies (PAAS normalized) of samples show a slight negative Ce calcrete, stromatolite, and siliciclastic samples in the anomaly which indicates the fluids study area. they formed from had previously interacted with oxygen. The anomaly for the stromatolites is less than that of the calcrete which can be attributed to the lower residency time of surface water (in which the stromatolites form) compared to groundwater. The siliciclastic samples do not show a positive or negative Ce anomaly. This is likely due to abundance of primary, unweathered material contained within the samples themselves. Similar results were obtained from the redox sensitive elements Fe, Mn, V, and U. Surficial weathering of the basaltic sediment induced by significant atmospheric oxygen levels resulted in the development of Fe hydroxides and oxyhydroxides pervasive throughout the sediment of the Copper Harbour Conglomerate. In this system Mn was staying in solution for a longer time, on average, than Fe and became concentrated within the groundwater and precipitated out with the calcrete. The oxidized forms of V and U were readily weathered from the basalt and held in solution until encountering the reducing biochemical reactions of the decaying bacterial component of the stromatolites or becoming oversaturated within the groundwater and precipitating out with the calcrete. By breaking down the sub-environments of the three sample types in this study, a clear progression and hydrologic geochemical evolution of the broader depositional environment could be traced out. There was enough oxygen in the atmosphere for pervasive oxidative weathering at surface and for the studied redox sensitive elements to maintain their oxidized state through surface and groundwater transportation – only being removed through oversaturation or local reduction via organic decomposition. These results indicate that atmospheric oxygen was present in greater concentrations ~1085 Ga than previously thought. This also leads into the idea that Earth’s oxygenation may have been variable in relatively short time spans. 12 �Critical Mineral Potential on the North Shore of Lake Superior; Elements to ‘Structure’ our future CUNDARI, Robert1, PUUMALA, Mark1, METSARANTA, Riku2 and CAMPBELL, Dorothy1 1 Resident Geologist Program, Ontario Geological Survey, Thunder Bay, ON, P7E 6S7 2 Earth Resources and Geoscience Mapping Section, Ontario Geological Survey, Sudbury, ON P3E 6B5 In March 2021, the Ontario Government released a Critical Minerals Framework Discussion Paper which outlines Ontario’s proposal for developing a critical minerals strategy as well as a draft critical mineral list. Critical minerals are defined as “….raw materials needed to produce many products and specialized technologies. The minerals that a jurisdiction deems “critical” depends on its geology, as well as its own domestic and economic priorities.” (https://www.ontario.ca/page/critical-minerals). This abstract and associated presentation provides a general synopsis of the geology and existing models for several critical mineral enriched mineralization styles (“deposits” sensu lato) related to the Proterozoic Midcontinent Rift (MCR) along the northwestern shore of Lake Superior. The specific deposit types highlighted are: orthomagmatic platinum group elements deposits in MCR-related intrusions (PGE-Ni-Cu-Cr-Co) , polymetallic hydrothermal vein deposits (Ag-rich, Pb-Zn-Cu-rich U-rich types; U-Ni-Zn-Cu-Co-Sb-Bibarite-fluorite) and hydrothermal polymetallic breccia complex related deposits (Deadhorse Creek diatreme type; U-Be-Zr-Hf-Y-Ce-Nb-Ti-V-Cr-REE). Platinum Group Elements (PGE) have long been a subject of exploration interest north of Lake Superior. Since the discovery of PGE mineralization at the Thunder Bay North (TBN) project, the region has seen relatively steady exploration efforts targeting similar chonolith-hosted magmatic sulphide deposits. Larger MCR-related mafic-ultramafic intrusions, including Sunday Lake and Seagull, have also been explored for magmatic sulphides. These PGE prospective intrusions are interpreted to have been emplaced early in MCR history through crustal-scale structures (Bleeker et al., 2020). Hart and MacDonald (2007) stated the importance of deep-seated structures, specifically the Black Sturgeon fault, as conduits for early, primitive melts and subsequent emplacement of the early mafic-ultramafic units (e.g. Hele, Seagull, Kitto, Disraeli). Furthermore, Heggie (2012) recognized that the emplacement of the TBN complex was primarily controlled by a macro-structure (i.e. Archean Quetico fault) during MCR magmatism which provided a zone of weakness for the intrusive complex to develop. Many of these mineralized intrusions are relatively removed spatially from the historically mapped extent of rocks related to the MCR highlighting the larger structural footprint of the rift and its related fault systems. The silver-bearing veins in the Thunder Bay area have been subject to considerable historical mining and more recently, renewed exploration interest. The critical mineral potential of these vein systems is not well established, although examples of similar vein systems (e.g. deposits at Cobalt) were a source of critical minerals (e.g. Kissin, 1992). The vein systems are organized in 2 groups known as the Mainland and Island belts (cf. Franklin et al., 1986). The Mainland Belt deposits (e.g., Beaver, Badger, Silver Mountain) occur in Rove Formation sedimentary rocks immediately below the contact with Logan diabase sills. The veins locally extend upwards into the sills, with silver-rich portions of the systems bound by locally silicified shale. The Island Group deposits (e.g. Silver Islet Mine) are characterized by fracturefilled vein systems oriented perpendicular to a northeast-trending swarm of gabbroic (Pigeon River) dykes. The Mainland and Island group veins contain similar mineral assemblages of acanthite and native silver associated with base-metal sulphides, fluorite, barite, quartz, and calcite. The Island group differs from the Mainland group in that they locally contain a nickel-cobalt sulpharsenide suite of minerals often termed 5elements veins (Ag-Bi-Co-Ni-As). The Island and Mainland group are present in or near crustal-scale, extensional listric faults that formed throughout the main episode of MCR extension which then became reverse faults at the termination of that event (Smyk and Franklin, 2006). Similar to the mafic-ultramafic 13 �intrusions hosting PGE mineralization, polymetallic vein systems have a wider spatial distribution than the mapped extent of MCR related rocks and thus may serve as a tool for mapping rift related structures and hydrothermal systems. Uraninite (pitchblende) was documented by Franklin (1978) and occurs in quartz veins and vein breccias in association with hematite, magnetite and/or pyrite within the Archean basement rocks of the Sibley basin (Sutcliffe, 1991). Uranium mineralization is associated with fracture zones related to north- to northwest-striking regional structures such as the Black Sturgeon fault, and has been dated at 1094 Ma (Ruzicka and LeCheminant, 1984), supporting a model similar to the genesis of lead-zinc veins in the Dorion area (Franklin and Mitchell, 1977; Smyk and Franklin 2007). The model invokes mineralizing hydrothermal fluids generated by MCR-heating inducing leaching of uranium and other metals from uraniferous basement pegmatites or basal sedimentary strata in the Sibley Group (ibid). The fluids which migrated through permeable Pass Lake formation sandstone precipitated metals in stratigraphic traps at or near the Archean unconformity (ibid). Although the timing of these veins is considered broadly coeval with the Ag-rich veins of the Mainland and Island silver-belts, the variable metal signatures of different vein systems (Ag-rich vs U-rich vs Pb-Zn-Cu-rich) remains relatively poorly understood. In addition, their trace critical mineral contents have not been investigated extensively. The Dead Horse Creek volcaniclastic breccia complex is host to a diverse group of highly altered heterolithic breccias and mafic to intermediate dikes that have been explored for rare earth elements, uranium, yttrium, cerium, zirconium and hafnium (Zurevinski et al., 2019). The Dead Horse Creek Complex has been divided into 5 subcomplexes, referred to as North, South, East, West and Central and described as altered heterolithic breccias that have undergone varying degrees of alteration and are variably radioactive (Sage 1982; Smyk et al., 1993). Exploration has mainly been focused on the mineralized zones at the West Dead Horse and the North Dead Horse subcomplexes. The West Dead Horse subcomplex has been described as “diverse, exotic, hydrothermally altered and rare metal mineralized” (ibid). The main mineralized zone at the West Dead Horse Subcomplex is small; however, the grade and style of mineralization highlights the importance of following up on larger, underexplored structures elsewhere in the Dead Horse Complex. Uranium-beryllium-zirconium mineralization was introduced via A-type granitic fluids along fault structures that crosscut the genetically unrelated breccias. Subsequently, niobiumtitanium-vanadium-chromium-bearing alkaline fluids were introduced into the same fault system. These reacted with the pre-existing mineral assemblage and created the observed exotic mineralogy (Potter and Mitchell 2005). The recognition of structures that were formed or re-activated during the Midcontinent Rift event is integral to understanding the location and timing of all the deposit types noted above. Bleeker et al, (2020) have made significant advances in the understanding of the timing of intrusions and subsequently the overall geodynamic evolution of the MCR. It has been noted that younger and more voluminous magmatism was focussed into the central rift as the lithosphere thinned and rifted apart, whereas the older intrusions (e.g. Thunder Bay North, Tamarack and others) were emplaced farther afield. This lithospheric thinning may have been an important factor in the location of structurally controlled hydrothermal deposits surrounding the Lake Superior basin as the majority of the vein systems are hosted in faults parallel to the rift axis, proximal to the rift margin. Distance from rift axis may have also had a bearing on the type of mineralization present in the vein systems as specific arrays of deposits appear to have differing metal signatures. The structural controls on the deposits, whether deposit controlling faults are primary MCR-related or reactivated Archean structures, remain important to understanding the structural evolution of the MCR and may help to further elucidate the nature of metallogeny in the region. Apart from the intrusion-hosted PGE deposits, the polymetallic vein deposits outlined here (Ag-rich, U-rich, Pb-Zn rich, Deadhorse Creek-type) are all generated by fluids derived from heat related to the MCR (Smyk and Franklin, 2007). The composition of the polymetallic veins is a product of the nature of the diverse ore-related fluids, and the geochemistry of the host rocks and their relative interactions and contributions of metals. Understanding 14 �the structures which host all the deposit types outlined here and their relative timing, along with refining tectonic/structural evolution of the MCR, will be integral to furthering our understanding on the controls on these critical deposit types. References Bleeker, W., Smith, J., Hamilton, M., Kamo, S., Liikane, D., Hollings, P., Cundari, R., Easton, M., and Davis, D., 2020. The Midcontinent Rift and its mineral systems: Overview and temporal constraints of Ni-Cu-PGE mineralized intrusions; in Targeted Geoscience Initiative 5: Advances in the understanding of Canadian Ni-CuPGE and Cr ore systems – Examples from the Midcontinent Rift, the Circum-Superior Belt, the Archean Superior Province, and Cordilleran Alaskan-type intrusions, (ed.) W. Bleeker and M.G. Houle; Geological Survey of Canada, Open File 8722, p. 7–35. https://doi.org/10.4095/326880. Franklin, J.M., Kissing, S.A., Smyk, C. and Scottt, S.D. 1986. Silver deposits associated with the Proterozoic rocks of the Thunder Bay district, Ontario. Canadian Journal of Earth Sciences 23, p. 1576- 1591. Franklin, J.M. and Mitchell, R.H. 1977. Lead-zinc-barite veins of the Dorion area, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, 14: 1963-1979. Franklin, J.M. 1978. Uranium mineralization in the Nipigon area, Thunder Bay District, Ontario; in Current Research, Part A, Geological Survey of Canada, Paper 78-1 A, p.275-282. Heggie, G., MacTavish, A., Johnson, J., Weston, R., and Ma, L., 2012. Structural control on the emplacement of the TBN-Igneous Complex in Institute on Lake Superior Geology Proceedings, 58th Annual Meeting, Thunder Bay, Ontario, Part 1 – Proceedings and Abstracts, v.65, p.37-38. Hart, T.R. and MacDonald, C.A., 2007. Proterozoic and Archean Geology of the Nipigon Embayment: implications for emplacement of the Mesoproterozoic Nipigon diabase sills and mafic to ultramafic intrusions. Canadian Journal of Earth Sciences 44: 1021-1040. Kissin, S.A., 1992. Five-element (Ni-Co-As-Ag-Bi) veins. Geosci. Can. 19, 113–124. Potter, E.G. and Mitchell, R.H. 2005. Mineralogy of the Dead Horse creek volcaniclastic breccia complex, Northwestern Ontario, Canada; Contributions to Mineralogy and Petrology, v.150, p.212-229. Ruzinka, V. and LeCheminant, G.M. 1984. Uranium deposit research, 1983, in Current research, part A. Geological Survey of Canada, Paper 84-1A, p39-51. Sage, R.P. 1982. Mineralization in diatreme structures north of Lake Superior, Ontario Geological Survey, Study 27, Ontario Ministry of Natural Resources, Toronto, 79p. Smyk, M.C. and Franklin, J.M. 2007. A synopsis of mineral deposits in the Archean and Proterozoic rocks of the Lake Nipigon Region, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences 44, p. 1041-1053. Smyk, M.C., Taylor, R.P., Jones, P.C. and Kingston, D.M. 1993. Geology and geochemistry of the West Deaf Horse Creek rare metal occurrence, northwestern Ontario; Exploration and Mining Geology, v.2, p.245- 251. Sutcliffe, R.H. 1991. Proterozoic Geology of the Lake Superior Area in Geology of Ontario. Edited by P.C. Thurston, H.R. Williams, R.H. Sutcliffe and G.M. Stott. Ontario Geological Survey, Special Volume 4 Part 1. P. 627-672. Zurevinski, S., Campbell, D. and Puumala, M. 2019. Midcontinent Rift-related carbonatites and diatremes; in Institute on Lake Superior Geology Proceedings, 65th Annual Meeting, Terrace Bay, Ontario, Part 2 - Field Trip Guidebook, v.65, part 2, p.2-10. 15 �Availability of Historical Airborne Geophysical Survey Data at Minnesota DNR DAHL, David1, SAARI, Stacy1, and LEE, Thomas1 Minnesota Department of Natural Resources – Lands and Minerals Division, 1525 3rd Ave. East, Hibbing, MN 55746 USA dave.dahl@state.mn.us stacy.saari@state.mn.us thomas.lee@state.mn.us 1 The collections of exploration and scientific research data housed at the Minnesota Department of Natural Resources include a large volume of unpublished airborne geophysical survey deliverables, including maps, flight line profiles, flight records, interpretation reports, and other materials. An effort to discover, organize and make accessible these materials has uncovered 131 non-government pre-GPS mineral exploration airborne geophysical surveys representing 156,192 line kilometers of acquired data. The effort has accomplished several goals: • From a collections perspective, segregation of these airborne survey deliverables from other exploration project records has succeeded in greatly reducing the physical volume of mineral exploration materials remaining to be handled in other curation and cataloging activities. Large maps, typically on Mylar, have been indexed and collected into vertical map cabinets. Folders of flight line profiles have been indexed at the survey level and stored in archival quality office boxes on a space efficient mobile shelving system. Contractor technical reports and irregular documents such as 35mm strip films and nine-track magnetic tapes have also been indexed, boxed and stored in the mobile shelving system. This single episode of curation activity has addressed some 30% of the volume in the collections. • The effort has introduced the geography of mineral exploration data collections to newer mineral potential geoscientists in the Division, and provided big-picture orientation to historical exploration program data that is often reused in support of geologic survey mapping, university research, and natural resource land management activities. • Scanned products of the organizing effort include 1) PDF/A archive format files of all maps, reports and similar documents, 2) color-indexed TIFF image files of all inventoried airborne geophysical survey map sets, and 3) georeferencing links files for all inventoried map images (georeferenced to UTM zone 15, NAD83). These files provide a critical backup to otherwise irreplaceable hard copy. An effort to make these and other collections products more easily accessed, viewed, searched and downloaded is currently in development. • Four index files have been developed: 1) a surveys index based on flight line extents contains acquisition footprints (multi-part polygons) and survey attributes for the 131 surveys, 2) a maps index based on the 860 maps contains map tile footprints and map tile attributes (some survey map sets have up to 18 tiles, with up to 4 map sheets per tile), 3) a flight lines index contains line traces and line attributes for the 8,778 flight lines represented on the 129 map sets, and 4) a documents index spreadsheet based on inventory of the document collection contains metadata for each of the 1,089 documents curated during the effort. Availability of these indexes serves dual purpose, first as a finding aid, and secondly as an orientation and quality control aid for the curation of much of the follow up geologic, ground geophysical, and geochemical mineral exploration data sets archived at Minnesota DNR. Mineral exploration magnetic-EM survey data comprise 89% of flight line content (138,786 line km). Magnetics-only surveys represent 10% of the data, and 1% is from a regional AFMAG (passive EM) reconnaissance survey over the Duluth Complex. Locations in Minnesota targeted by these surveys include the Archean Wawa and Wabigoon Subprovinces, Proterozoic bedrock on 16 �the Mesabi and Cuyuna iron ranges and in areas of west-central, east-central, and southwestern Minnesota. Most of the surveys are based on ¼ mile flight line spacing. For geologic mappers these surveys provide context for site surveys and drill core locations, and additional information about Minnesota’s concealed bedrock terranes. For instance, many of the surveys have flight orientations different than government sponsored magnetic surveys; and formational EM conductors mapped in areas such as the southern part of the Animikie basin may enhance geologic interpretation in areas mostly devoid of magnetic anomaly detail. EM data may also provide information on surficial material features such as gravels or conductive clays. Obviously, more work can be done to round out the inventory of non-government airborne geophysical surveys in Minnesota. GPS-based airborne survey data sets, which are digital and less susceptible to physical loss, are yet to be indexed, and indexing staff are aware of about 20 additional historical non-government airborne surveys in Minnesota that might potentially be targets for future donation (if they still exist). These additional historical airborne surveys are partially evidenced, either through coarse index maps, or clusters of named ground target sites, or staff knowledge of survey area vicinities for which no footprints are found in the collections. The flight line profiles, which hold the actual magnetic, EM, and occasional radiometric survey data are now curated at the survey level and indexed at the flight line level. But they are still without backup and vulnerable to physical loss. These flight line profiles are an obvious target for a future National Geological and Geophysical Data Preservation Program (NGGDPP) proposal to scan and convert the hard copy records to PDF/A format, for preservation and access. Figure 1: Acquisition footprints for the non-government pre-GPS airborne geophysical surveys inventoried during current effort. Bedrock geology - Minnesota Geological Survey map S-22. 17 �Geophysical insights into Paleoproterozoic tectonics along the southern margin of the Superior Province, central Upper Peninsula, Michigan DRENTH, Benjamin J.1, CANNON, William F.2, SCHULZ, Klaus J.2, and AYUSO, Robert A.2 1 U.S. Geological Survey, PO Box 25046, MS 964, Denver Federal Center, Denver, CO 80225 U.S. Geological Survey, 12201 Sunrise Valley Dr., MS 954, Reston, VA 20192 2 The southern margin of the Archean Superior Province in the central Upper Peninsula of Michigan was a nexus for key Paleoproterozoic tectonic events involved in the ~2.1 Ga rifting of proposed Archean supercraton Superia and subsequent assembly of Laurentia. Interpretations of the region’s tectonic history have historically been hampered by extensive Pleistocene glacial and Paleozoic sedimentary cover and a lack of appropriate geophysical data. These rifting and orogenic events formed geologic effects that are readily mappable with modern geophysical methods. New aeromagnetic and gravity data provide a critical means of mapping and interpreting the complex geological framework through cover, allowing development of significantly richer geographical and process-based perspectives on all these tectonic events (Drenth et al., in press). Existing interpretations of Archean and Paleoproterozoic contacts and structure (e.g., James et al., 1961; Bayley et al., 1966) are carried >30 km eastward under Paleozoic cover (Fig. 1). The progression of ~2.1 Ga rift-related sedimentation and magmatism recorded in rocks of the Dickinson Group are clearly expressed as a geographically extensive and largely concealed tectonic feature of the southern Superior Province. Geophysical interpretations provide evidence for plausible ~2.1 Ga intrusive magmatism, such as a previously unrecognized swarm of mafic northeast-striking dikes cutting Archean rocks and multiple granitic bodies. Effects of the ~1.87-1.83 Ga Penokean orogeny include the clearly imaged Niagara fault zone suture, abundant evidence for thin-skinned thrusting and folding in the Menominee iron district, and speculative emplacement of an allochthonous sedimentary sequence in the Calumet trough. Numerous east-west trending structures likely originated, or were significantly reactivated, by post-Penokean deformation. Metamorphic events at ~1.76 Ga and ~1.65 Ga may correspond to orogenies involving younger, outboard Paleoproterozoic crustal provinces recognized in southern Laurentia. For example, the previously unrecognized West Branch fault, separating the Dickinson Group from Archean rocks, is shown to be a major structure in the region and is a proposed expression of ~1.76 Ga thick-skinned deformation documented elsewhere in the region. Oblique disruptions of crudely east-west striking structures have robust geophysical expressions and are speculatively connected to transpressive deformation at ~1.65 Ga. References Bayley, R. W., Dutton, C. E., Lamey, C. A., and Treves, S. B., 1966, Geology of the Menominee ironbearing district Dickinson County, Michigan and Florence and Marinette Counties, Wisconsin: U.S. Geological Survey Professional Paper 513, 96 p. Drenth, B. J., Cannon, W. F., Schulz, K. J., and Ayuso, R. A., in press, Geophysical insights into Paleoproterozoic tectonics along the southern margin of the Superior Province, central Upper Peninsula, Michigan, USA: Precambrian Research. James, H. L., Clark, L. D., Lamey, C. A., and Pettijohn, F. J., 1961, Geology of central Dickinson County, Michigan: U.S. Geological Survey Professional Paper 310, 176 p. 18 �Figure 1: Plan view of interpretations (Drenth et al., in press). All units Paleoproterozoic unless otherwise noted. 19 �The deformation and alteration of granitoid plutons in the Wabigoon subprovince DROVER, Bailey, HILL, Mary Louise, ZUREVINSKI, Shannon Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Ongoing research suggests that the brittle deformation, ductile deformation and associated metasomatism/hydrothermal alteration studied in 12 granitoid plutons across the Wabigoon subprovince provide evidence for coeval brittle-ductile deformation at the amphibolite facies of metamorphism consistant with Archean-aged oblique convergence (transpression). Approximate outcrop locations where the brittle and/or ductile deformation were studied are shown in figure 1. Four hundred and two structural measurements of fractures and faults along with approximately 95 thin sections were used to assess the degree of deformation /metamorphism and the metasomatism of various granitoid plutons across the subprovince. Figure 1. Modified map from Blackburn et al. (1991) showing the approximate outcrop locations where the brittle and/or ductile deformation of the granitoid plutons were studied. Brittle deformation features common to all plutons include chlorite ± epidote shear veins and unfilled fractures. Shear veins show a conjugate strike-slip relationship in the field and are seen with dips ranging from 15° to 90°. Average and median dips of the shear veins is roughly 70° ± 10° for each pluton studied. These shear veins are seen with sub-horizontal slickenlines that have pitches ranging from 00° to 33°. Although conjugate relationships are noted in the field, the orientation of the shear veins are quite variable across outcrops of individual plutons and the subprovince. These shear veins are also commonly seen defining the fabric in narrow shear zones present in the plutons across the Wabigoon subprovince. Many of the plutons also have quartz veins that have been subsequently ductiley deformed. These ductiley deformed quartz veins are commonly associated with narrow shear zones and are typically boudinaged with strong undulose extinction, subgrains and serrated grain boundaries. The ductile overprint of the quartz veins demonstrates coeval brittle-ductile deformation and is seen in the Sabaskong batholith, the Dryberry batholith, the Ottertail pluton, the Atikwa batholith and the Revell batholith. Although the plutons studied appear to lack a notable macroscopic deformation fabric, thin section analysis shows that dislocation creep mechanisms were active in both quartz and feldspar mineral phases during peak metamorphic conditions. Deformation microstructures commonly seen 20 �in quartz across all plutons studied include undulose extinction, serrated grain boundaries and subgrains. Feldspars have undulose extinction, subgrains, serrated grain boundaries, intragranular microfractures and deformation twins. Evidence for dislocation creep in feldspars indicates that the metamorphic grade of the plutons is consistent with the amphibolite facies of metamorphism. Alteration of the plutons is directly related to brittle fracturing. Alteration minerals commonly seen adjacent to brittle fractures include chlorite, epidote group minerals, biotite, sphene, calcite, sericite, and muscovite with lesser amounts of hematite. The degree of alteration is most notable near the margins of the plutons where strain is concentrated. REFERENCES Blackburn, C.E., John, G.W., Ayer, J., Davis, D.W., 1991, Wabigoon Subprovince. Ontario Geological Survey Special Volume 4, Pt. 1, pp. 303-383. 21 �Geochemistry and Age of Archean Volcaniclastic and Mafic Intrusive Rocks, Georgia Lake Area, Quetico Subprovince, Northwestern Ontario DUGUET, Manuel Earth Resources and Geoscience Mapping Section, Ontario Geological Survey, Sudbury, Ontario manuel.duuet@ontario.ca The metaturbidites of the Georgia Lake area in the Quetico Subprovince host 2 peculiar types of rock: volcaniclastic rocks that are mafic to intermediate in composition and mafic intrusive rocks that likely were emplaced when the turbidites were unconsolidated. The volcaniclastic rocks were described first on the shoreline of Lake Jean by Williams (1988), who interpreted them as sedimentary rocks derived from the erosion of nearby ultramafic bodies. A similar interpretation was given by Valli, Guillot and Hattori (2004), but they suggested a komatiite basalt as the source. Field work conducted during the summer 2019 showed that these rocks are very likely the result of deposition and local reworking of either ash from a volcanic plume or a distal part of a pyroclastic flow. Analyses of newly acquired and historical geochemical data were performed to, first, assess the level of contamination of these volcaniclastic rocks by the surrounding turbidites and, second, to compare it with possible source in the area such as the numerous Archean mafic sills and veins that intruded the turbidites. The least contaminated members of the volcaniclastic rocks fall in the fields of boninites and siliceous high-magnesium basalts in various discriminant diagrams such as SiO2 versus MgO, TiO2 versus MgO, SiO2 versus Cr and TiO2 versus Cr (Pearce and Reagan 2019). TiO2 content of the mafic intrusive rocks compared to that of the volcaniclastic rocks (respectively >1 wt.% and less than 0.6 wt. %) show that the former are unlikely to be the source of the volcaniclastic rocks. On primitive mantle– normalized incompatible element diagram, volcaniclastic rock compositions show moderate to strong enrichment in light rare earth elements (LREE) and depletion in heavy rare earth elements (HREE), with negative anomalies in niobium, tantalum and titanium and positive anomalies in zirconium and hafnium, as summarized in Duguet (2020). Although being more enriched in REE, the mafic intrusive rocks display similar fractionation pattern than those of the volcaniclastic rocks but with negative anomalies in zirconium and hafnium. LA-ICP-MS U-Pb dating on zircon of a volcaniclastic rock yielded a mean age of 2702 ± 3 Ma that is interpreted as its depositional age (Figure 1). These volcaniclastic rocks came either from the Wabigoon Subprovince to the north or the Wawa Subprovince to the south. Such rocks remain to be identified in these areas and the closest candidates may be the andesitic volcanic rocks of the Shebandowan greenstone belt to the south. Different process can generate boninitic magmas, but based on the geological setting and previous studies, an arc/fore-arc environment is favoured. The mafic intrusive rocks are also very interesting because they were intruded at the time when the sediments were deposited. This would rule out an accretionary prism environment for the Quetico Subprovince because active volcanism coeval with turbidite sedimentation is more common in back-arc and/or rifted arc settings. However, the calc-alkaline chemistry, which also display similarities with sanukitoid plutons in the area, contradicts this interpretation. The geodynamical implications of this remain to be investigated and a more complex scenario involving either rifted fore-arc/arc or subduction polarity flips should be investigated. 22 �Figure 1. Concordia plot for a volcaniclastic rock from the Georgia Lake area showing the main zircon population at 2702±3 Ma (n=28) reflecting the age of deposition. Geon 28, 29 and 30 grains are also present. Data from Sutcliffe (2020). Duguet, M. 2020. Geochemistry of Archean volcaniclastic and mafic intrusive rocks, Georgia Lake area, Quetico subprovince, northwestern Ontario; in Summary of Field Work and Other Activities, 2020; Ontario Geological Survey, Open File Report 6370: 8-1 to 8-10. Pearce, J.A. and Reagan, M.K. 2019. Identification, classification, and interpretation of boninites from Anthropocene to Eoarchean using Si-Mg-Ti systematics; Geosphere, 15: 1008-1037. doi.org/10.1130/GES01661.1 Sutcliffe, C.N. 2020. U-Pb geochronology by LA-ICP-MS in samples from northern Ontario; internal report prepared for the Ontario Geological Survey, Jack Satterly Geochronology Laboratory, Ontario, 128p. Valli, F., Guillot, S. and Hattori, K.H. 2004. Source and tectono-metamorphic evolution of mafic and pelitic metasedimentary rocks from the central Quetico metasedimentary belt, Archean Superior Province of Canada. Precambrian Research, 132: 155-177. Williams, H.R. 1988. Geological studies in the Wawa, Quetico, and Wabigoon subprovinces, with emphasis on structure and tectonic development; in Summary of Field Work and Other Activities, 1988, Ontario Geological Survey, Miscellaneous Paper, 141: 169-172. 23 �The Critical Mineral potential of carbonatite and alkalic complexes in Ontario EASTON, Robert Michael Earth Resources and Geoscience Mapping Section, Ontario Geological Ontario P3E 6B5 mike.easton@ontario.ca Survey, Sudbury, There are more than 50 carbonatite and/or alkalic complexes in Ontario ranging in age from Neoarchean to Jurassic (Figure 1). Although there are numerous reports and compendia on them (Sage 1991; Woolley and Kjarsgaard 2008), from a mineral exploration standpoint, there is little guidance available with respect to determining which complexes might have higher mineral potential than the others, especially for niobium and the rare earth elements (REE). Recent academic studies on alkaline and carbonatitic magmas by Nabyl et al. (2020) and Ballouard et al. (2020) indicate that calcio-carbonatites associated with coeval feldspathoidal-dominated silicate rocks (e.g., ijolite, nepheline syenite) are more likely to host niobium and/or REE mineralization than are calcio-carbonatites found with other igneous rock associations. This presentation examines this suggestion with respect to carbonatite and alkalic complexes in Ontario; more details are found in (Easton 2020). It should be noted that many of the complexes in Ontario, especially the carbonatite complexes, are typically poorly exposed, known only through limited diamonddrill hole data, and only major element geochemical data are available for most. Neoarchean alkaline intrusions in Ontario do not have phonolite to phono-trachyitic rock compositions and thus are not considered as having high REE potential. This also applies to the circa 1090-1065 Ma Kensington-Skootamatta intrusive suite in the Grenville Province. In contrast, mafic end-members of the Frontenac intrusive suite (circa 1180-1155 Ma) in the Grenville Province are characterized by high TiO2, P2O5 and alkali (Ba, K, Rb, Sr) contents, have bulk rock compositions of basaltic trachyandesite to trachyandesite, and have the potential to host iron-oxide (P-rich) mineralization (Easton 2019). Using the suggestion that phonolite to phono-trachyitic rock compositions may be a predictor or favourable rare element mineralization in carbonatite and alkalic complexes reduces the number of potential targets from more than 40 to ~10. The more favourable complexes are associated with feldspathoidal-bearing intrusive rocks and known pyrochlore and/or significant apatite occurrences. Most occur in 3 main areas of the province associated with areas of crustal extension and/or mantle upwelling. Specifically, 1) the Paleoproterozoic Argor Complex is proximal to the Circum-Superior Orogen; 2) the Manitou Island, Iron Island and Burritt Island complexes, and the Lavergne fenite, are all associated with the Ottawa–Bonnechere graben where it impinges on the former trace of the Sudbury dike swarm mantle plume at circa 1238 Ma; and, 3) Although proximal to the Midcontinent Rift (MCR), the Firesand River, Good Hope and Prairie Lake complexes were emplaced between 1163 and 1143 Ma, suggesting that they may be a far-field effect of the Grenville Orogen rather than the MCR. The Mesoproterozoic Lackner, Nemegosenda and Seabrook complexes along the Kapuskasing Structural Zone also have exploration potential. Acquisition of modern trace element geochemistry from these 10 complexes may help to further refine their potential to host rare element mineralization. Ballouard et al. 2020; Earth Science Reviews, v.203, article 103115, 31p. Easton, R.M. 2019. in Ontario Geological Survey, Miscellaneous Release—Data 351 Easton, R.M. 2020. in Ontario Geological Survey, Open File Report 6370, p.9-1 to 9-10. Nabyl, Z. et al. 2020. Geochimica et Cosmochimica Acta, v.282, p.297-323. Sage, R.P. 1991. Chapter 18, in Ontario Geological Survey, Special Volume 4, Part 1, p.682-709. Woolley, A.R. and Kjarsgaard, B.A. 2008. Geological Survey of Canada, Open File 5796. 24 �Figure 1. Locations of carbonatite complexes, alkalic complexes, aeromagnetically inferred alkalic and/or carbonatite complexes and areas of fenite and carbonatite dikes in Ontario. Also indicated are the traces of the southern limit of the Circum-Superior Orogen, the Kapuskasing Structural Zone, the Midcontinent Rift, and the Ottawa–Bonnechere graben. Complexes with higher estimated exploration potential are indicated in red. Figure modified from Sage (1991). 25 �Reconstructing the hydration and carbonation history of the Presque Isle peridotite, Marquette Michigan: Insights into mechanisms of carbon sequestration in ultramafic rocks GALLAGHER, Benjamin and BJORNERUD, Marcia Geosciences Department, Lawrence University, 711 East Boldt Way, Appleton WI 54911 USA The Presque Isle peridotite is a serpentinized lherzolite that occupies about 500,000 m2 on the northern outskirts of Marquette, Michigan. The age of the Presque Isle peridotite is not known, although it and a similar body, the Deer Lake Peridotite, exposed about 25 km to the WSW, have been assumed to be Neoarchean (Sims, 1991; Sasso, 2016). Both ultramafic masses lie within an area of Neoarchean (2.7 Ga) rocks -- the tonalitic to granodioritic Compeau Creek Gneiss -- near the southern edge of the Superior Craton, about 10 km north of the Great Lakes Tectonic Zone. If the peridotites are Neoarchean, they may represent cumulate bodies related to metabasalt/ greenstone units mapped as the Mona and Kitchi Schists in the Marquette region. However, there is another ultramafic body in the area, the mineralized Yellow Dog peridotite found 40 km northwest of Marquette, that is related to the ca. 1.0 Ga Midcontinent Rift – part of a dike swarm emplaced into Paleoproterozoic metasedimentary rocks of the Baraga Group. In the absence of direct geochronologic constraints, a Mesoproterozoic age for the Presque Isle and Deer Lake peridotites cannot be ruled out, although their irregular, non-tabular shapes are quite different from the fracture-controlled form of the Yellow Dog intrusions. The Presque Isle peridotite is overlain nonconformably by a unit that has been mapped as the ca. 1.0 Ga Jacobsville Sandstone (a late rift-filling unit). However, the sedimentary material just above the unconformity is a rubbly conglomerate about 2 m thick with a greenish matrix and irregularly shaped redorange clasts of chalcedony -- very different in character from the typically well sorted, quartzose Jacobsville Fm. In places along the contact, it is in fact difficult to distinguish the highly altered peridotite from the overlying sedimentary rocks. Despite the uncertainties about the igneous age and origin of the Presque Isle peridotite and the time at which it was exposed to weathering in the geologic past, extensive unvegetated exposures of the peridotite at the northern tip of Presque Isle Park make it possible to reconstruct a detailed relative chronology of fluid flow, deformation and weathering processes that altered the rock over time. Based on field, thin section, XRD and stable isotope analyses, we infer the following sequence of events, recording progressive cooling and exhumation of the peridotite from depth. 1) Percolative infiltration by water-rich fluids The least altered parts of the Presque Isle peridotite still have recognizable igneous textures and record an early period of pervasive, percolative flow by aqueous fluids that altered the original olivine crystals to lizardite along grain boundaries and transgranular cracks. Phase relations for ultramafic rocks (Philpotts and Ague, 2009) suggest this happened at temperatures below 500-600°C, depending on depth. XRD analysis also indicates the presence of minor talc in these rocks, which could indicate metamorphism at slightly higher temperatures (up to ca. 650°). Serpentine also occurs in centimeter-scale curviplanar seams within the Presque Isle peridotite. These seams are typically meters apart but variably oriented and intersecting, and thus do not appear to have been influenced by a consistent external stress regime. Although the seams represent a morphologically distinct style of serpentinization, they may be temporally linked with the grain-scale alteration of the rocks. The serpentine seams exhibit small-scale transverse fractures whose spacing is comparable to their width; such features have been interpreted in other peridotites to reflect the significant volume increase (close to 40%; 26 �Klein and Le Roux, 2020) that occurs with serpentinization. This early serpentinization event does not seem to have been associated with significant deformation. 2) Tectonically-influenced hydrofracturing and reaction-induced fracturing by CO2-rich fluids Carbonate material in different forms cuts across serpentinized areas of the peridotite. The simplest carbonate occurrences are NW-striking, vertical dolomite-serpentine veins with fibrous crack-seal textures indicating incremental, dilational opening. These are spatially associated with vertical WNW-trending dextral faults with dolomite slickenfibers. The orientation of the veins relative to the faults is consistent with their formation as tensile fractures in a dextral shear zone. The crack-seal texture of the vein fillings suggests repeated, perhaps seismically induced, fluid flow and hydrofracturing events at depths shallow enough that the rocks deformed brittlely. In a few samples, carbonate crystals have terminated faces, suggesting they grew into open space or a dense vapor phase; this too implies shallow depths of formation. More geometrically complex carbonate masses are 5-10 cm thick ‘rinds’ around pods of less altered peridotite. These have crude ‘onion-skin’ layers and transverse veins that create a mesh-like appearance, and are similar to features attributed to reaction-induced fracturing by Jamtveit et al (2008). Like the crackseal veins, the carbonate meshes consist mainly of dolomite. Oxygen isotope values for both the vein and mesh carbonates are consistently negative (18O VPDB between -6.57 and -11.89) and suggest that the mineralizing fluids came from meteoric water. These fluids must have carried enough dissolved CO2 to cause reactions such as: Mg2SiO4 (ol) + CaMgSi2O6 (cpx) + CO2 + H2O → Mg3Si2O5(OH)4 (serp) + 2CaMgCO3 3) Deep near-surface weathering under high CO2 conditions Beginning about 5 m below the unconformity, the peridotite has a strikingly different color and composition. The rock mass consists of at least 30% carbonate (dolomite, magnesite, rhodochrosite), some of it massive, with intervening areas of reddish material that represents extremely altered olivine and serpentine. As one approaches the unconformity, even the serpentine has been altered to a residuum of fine-grained quartz or chalcedony and opaque minerals, likely Fe and Mg oxides. This deep weathering may reflect long-term leaching by surface fluids in late Proterozoic time under higher atmospheric CO2 levels than at present. The large amount of carbonate near the unconformity indicates that the rock continued to be reactive even after the original olivine had already been altered to serpentine. The complex hydration and carbonation history of the Presque Isle peridotite provides a natural analog for physical and chemical processes that might be emulated for carbon sequestration in ultramafic rocks if carbon capture becomes technologically and economically feasible. References cited Jamtveit, B., Putnis, C., and Malthe-Sorenson, A., 2008. Reaction induced fracturing during replacement process. Contributions to Mineralogy and Petrology, 157: 127-133. Klein F. and Le Roux, V., 2020. Quantifying the volume increase and chemical exchange during serpentinization. Geology, 48: 552-556. Philpotts, A. and Ague, J., 2009. Principles of Igneous and Metamorphic Petrology. Cambridge Univ Press. Sasso, A., 2016. Geochemical and Petrological Characterizations of Peridotite and Related Rocks in Marquette County, Michigan. Master’s Thesis, Western Michigan University. 100 pp. Sims, P.K., 1991, Great Lakes Tectonic Zone in Marquette Area, Michigan Implications for Archean Tectonics in the North-Central United States. US Geological Survey Rep. 1904-E. 27 �A Practical Approach to Using Incompatible Elements to Define Geochemical Correlation and a Common Mantle Source: Example Crystal Lake Gabbro and the Duluth Complex GOOD, David Department of Earth Sciences, Western University, London, ON N6A 5B7 Canada This presentation focuses on fundamental principles of igneous geochemistry to provide a practical approach for identifying geochemical correlations between mafic rock units in the MCR. A common approach will aid in development of a rift-wide classification strategy and knowledge of the geographic extent of defined mafic rock series. This level of information is key to understanding the nature of mantle sources across the rift. The ultimate goal is to provide useful criteria to guide ongoing exploration for Cu-Ni-PGE deposits in the MCR. There is a seemingly infinite array of methods to combine geochemical criteria and trace element data to derive useful petrogenetic models to explain mafic rocks, but most are not necessary for a regional scale study such as this. By defining a clear hypothesis, it is possible to simplify the number of elements used to a manageable and reliable few. In this case, a widely known concept that igneous rock sequences which have not been contaminated by crustal material, and that have similar incompatible trace element patterns were derived from a similar mantle source. With this objective in mind, it is not necessary to consider elements that are susceptible to hydrothermal alteration or are associated with plagioclase. Thus, U, Th, Ta, Nb, Zr and the REE provide a definitive range of elements for making comparisons and drawing conclusions regarding correlation (but do not necessarily show rocks are co-genetic). In those rare instances where there is evidence for mantle metasomatism, K, Ba and Sr abundances should be considered. A practical approach applies 4 steps: 1) elimination of samples considered to have undergone crustal contamination by evaluation Th/Nb (>0.2), 2) cleaning the data set of unrelated or outlier samples using REE patterns or the λ1 vs λ2 plot, 3) testing for evidence of clinopyroxene fractionation (La vs. Sm), and 4) inspection and comparison of twice normalized (primitive mantle and Lu) extended trace element diagrams. Two examples are presented: 1) the Babbitt intrusion of the Duluth Complex is compared to the Crystal Lake gabbro, and 2) the Current Lake intrusion is compared to the Lower Suite (Simpson Island traverse) of the Osler Volcanic Group. Sources: Current Lake: Cundari, R.M., Puumala, M.A., Smyk, M.C. and Hollings, P.N. 2021. OGS MR Data 308 Osler Volcanic Group: Barnes et al., 2021, Magmas Through Time, J. Petrology, in press. Crystal Lake Gabbro: O’Brien, S. 2018, MSc Thesis, Lakehead University. Duluth Complex: Ripley et al., 1998, Geochimica et Cosmochimica Acta 62, 3349-3365. λ-λ plot: O’Neill, H. 2016, J. Pet. 57, 1463-1508 28 �Integration of geophysical evidence indicates that anorthosite composes a significant portion of Grand Marais ridge, an inferred basement high in western Lake Superior GRAUCH, V.J.S. 1 and HELLER, S.J. 2 1 U.S. Geological Survey, MS 973, Federal Center, Denver, CO 80225 U.S. Geological Survey, MS 939, Federal Center, Denver, CO 80225 2 The Midcontinent Rift System (MRS) is expressed geophysically by a semi-linear, regional gravity high that trends across the Midcontinent and Great Lakes region of North America. The gravity high is interrupted by two prominent, semicircular gravity lows, which have been interpreted from modeling and seismic-reflection sections as basement highs of Archean granite (Allen et al., 1997). One is centered southwest of Isle Royale in western Lake Superior (Grand Marais ridge) and the other over Bayfield Peninsula (White’s Ridge). Allen et al. (1997) suggest that the Archean granite highs were pre-rift features that remained high while lava basins of the MRS subsided adjacent to them. Hart et al. (1994) questioned the presence of granitic rocks underlying Grand Marais ridge (GMR) because heat flow measurements there are much lower than is typical for Archean granitic upper crust. They argued that the region must instead be underlain by rocks of low radiogenic heat production, such as gabbro, extending to at least 15 km depth. However, gabbro has high densities and would not produce the observed gravity low. Thus, the geophysical observations appear contradictory. Figure 1. Physical properties of rock types compared to the ones inferred for Grand Marais ridge. (A) All velocitydensity point data are from Christensen and Mooney (1995), representing means at 5, 10, 15, 20, 25 km depths, except for two additional sites for anorthosite (orange), which are from Birch (1961), measured at 2 kbars. The regression line representing Keweenawan basalt and diabase is from Halls (1969). Typical radiogenic heat production by rock type (in brackets) in µW/m3 are from Hasterok et al. (2018) except for anorthosite, which is from Roy et al. (2021). (B) Statistics for Duluth Complex and Animikie Basin densities were analyzed from the database of Chandler and Lively (2011). Representative densities for the NE Grenville and Egersund massifs are from Keary and Thomas (1979) and Smithson and Ramberg (1979), respectively. N is the number of samples measured. Bouguer gravity, seismic-reflection, seismic-refraction, and heat-flow data suggest that the rock types composing GMR to a depth of about 15 km have densities of 2,650-2,720 kg/m3 (Hutchinson et al., 1990; Allen et al., 1997), compressional-wave velocities of about 6.5 km/s (Luetgert and Meyer, 1982; Hutchinson et al., 1990; our own velocity analyses), and radiogenic heat production 29 �values of no more than 0.7 µW/m3 (Hart et al., 1994). These inferred GMR physical properties are compared to the same properties compiled globally and measured locally for rock types likely to occupy the subsurface of western Lake Superior (Fig. 1). As already noted, the granitic rocks have densities and velocities closest to the GMR suite of properties, but the heat production is too high. The mafic and intermediate rocks (Fig. 1A) and metasedimentary rocks of the Animikie Basin (Fig. 1B) are too dense. The latter also are likely to have heat production that is too high (noted next to MGW on Fig. 1A). If the values compiled for anorthosite (Fig. 1A) are representative for the MRS, the low heat production fits but velocities are too high for anorthosite to be the sole rock type underlying GMR. However, if added in significant volume to felsic rock types, for example, the velocity and the heat generation could be combined to result in the desired physical properties for the whole rock column. Alternatively, the lower densities for anorthosite xenoliths and massifs elsewhere (Fig. 1B) hint that the compiled values are not representative; the xenolith source may have velocities that match those of GMR. In any case, anorthosite likely constitutes a significant volume of GMR. References Allen, D.J., Hinze, W. J., Dickas, A. B., and Mudrey, M. G., Jr., 1997, Integrated geophysical modeling of the North American Midcontinent Rift System: New interpretations for western Lake Superior, northwestern Wisconsin, and eastern Minnesota: Geological Society of America Special Paper 312, p. 47-72. doi: 10.1130/0-8137-23124.47 Birch, F., 1961, The velocity of compressional waves in rocks to 10 kilobars, Part 2: Journal of Geophysical Research, v. 66, no. 7, p. 2199-2224. Chandler, V. W., R. S. and Lively, 2011, Density, magnetic susceptibility, and natural remanent magnetization of rocks in Minnesota: Minnesota Geological Survey online services, http://dx.doi.org/10.13020/D63S3D, accessed 22 January 2018. Christensen, N.I., and Mooney, W.D., 1995, Seismic velocity structure and composition of the continental crust: A global view: Journal of Geophysical Research, v. 100, no. B6, p. 9761-9788. doi:10.1029/95JB00259. Halls, H.C., 1969, Compressional wave velocities of Keweenawan rock specimens from the Lake Superior region: Canadian Journal of Earth Sciences, v. 6, p. 555-568. Hart, S.R., Steinhart, J.S., and Smith, T.J., 1994, Terrestrial heat flow in Lake Superior: Canadian Journal of Earth Sciences, v. 31, p. 698-708. Hasterok, D., Gard, M., and Webb, J., 2018, On the radiogenic heat production of metamorphic, igneous, and sedimentary rocks: Geoscience Frontiers, v. 9, p. 1777-1794. Hutchinson, D.R., White, R.S., Cannon, W.R., and Schulz, K.J., 1990, Keweenaw hot spot: Geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent Rift system: Journal of Geophysical Research, v. 95, no. B7, p. 10,869-10,884. Keary, P., and Thomas, M.D., 1979, Interpretation of the gravity field of the Lac Fournier and Romaine River anorthosite massifs, eastern Grenville Province: Significance to the origin of anorthosite: Journal of the Geological Society of London, v. 136, p. 725-736. Luetgert, J. H., and Meyer, R. P., 1982, Structure of the western basin of Lake Superior from cross structure refraction profiles, in Wold, R. J., and Hinze, W. J., eds., Geology and Tectonics of the Lake Superior Basin, Geological Society of America Memoir 156, p. 245-255. Roy, D.J., Merriman, J.D., Whittington, A.G., Hofmeister, A.M., 2021, Thermal properties of carbonatite and anorthosite from the Superior Province, Ontario, and implications for non-magmatic local thermal effects of these intrusions: International Journal of Earth Sciences, published online 29 March 2021, doi: 10.1007/s00531-02102032-w. Smithson, S.B. and Ramberg, I.B., 1979, Gravity interpretation of the Egersund anorthosite complex, Norway: Its petrological and geothermal significance: Geological Society of America Bulletin, v. 90, p. 199-204. 30 �Final inversion of the Midcontinent Rift during the Rigolet Phase of the Grenvillian orogeny HODGIN, Eben B., SWANSON-HYSELL, Nicholas L., STOLPER, Daniel A., TURNER, Andrew C. Department of Earth and Planetary Science, University of California, Berkeley, CA, USA DeGRAFF, James M., Department of Geological and Mining Engineering and Sciences, Michigan Technical University, Houghton, MI, USA KYLANDER-CLARK, Andrew Department of Earth and Planetary Science, University of California, Santa Barbara, CA, USA SCHMITZ, Mark D. Department of Geosciences, Boise State University, Boise, ID, USA The Keweenaw Fault is a major structure associated with inversion of the Midcontinent Rift; it thrusts ca. 1093 to <1086 Ma rift volcanic and sedimentary rocks atop the post-rift Jacobsville Sandstone in northern Michigan. Given the position of folded Jacobsville Sandstone in the footwall of the fault, its depositional age provides a maximum age constraint on faulting. Previous detrital zircon U-Pb geochronology used laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) that led to an interpretation of a ~950 Ma maximum depositional age for the Jacobsville Sandstone and to argue that motion on the Keweenaw Fault must postdate the ca. 1080 to 980 Grenvillian orogeny (Craddock et al., 2013; Malone et al., 2016). In this study, we reproduced similar low-precision maximum age constraints on detrital zircons using LA-ICPMS, then we dated the youngest detrital grains at high-precision by chemical abrasion-isotope dilutionthermal ionization mass spectrometry (CA-ID-TIMS). Our revised maximum depositional age is based on the youngest detrital zircon dated by CA-ID-TIMS and contemporaneous with the 1000– 980 Ma Rigolet Phase of the Grenvillian orogeny. This revised maximum age constraint overlaps with U-Pb LA-ICPMS dates on syn- to post-kinematic calcite veins within the brecciated fault zone of the Keweenaw Fault that yield lower intercept dates that are also contemporaneous with the Rigolet Phase of the Grenvillian orogeny. From these age constraints, we infer that the Jacobsville Sandstone was deposited during the final inversion of the Midcontinent Rift during the Rigolet Phase of the Grenvillian orogeny, just before reverse motion on the Keweenaw Fault. This deposition and subsequent reverse faulting is contemporaneous with development of the Grenville Front during the Rigolet phase of the Grenvillian orogeny. These data demonstrate the propagation of Rigolet-aged orogenesis into the continental interior ca. 580 km from the Grenville Front. Craddock, J.P., Konstantinou, A., Vervoort, J.D., Wirth, K.R., Davidson, C., Finley-Blasi, L., Juda, N.A., & Walker, E. (2013). Detrital zircon provenance of the Mesoproterozoic Midcontinent Rift, Lake Superior region, USA. The Journal of Geology, 121(1), 57–73. doi:10.1086/668635 Malone, D.H., Stein, C.A., Craddock, J.P., Kley, J., Stein, S., & Malone, J.E. (2016). Maximum depositional age of the Neoproterozoic Jacobsville Sandstone, Michigan: Implications for the evolution of the Midcontinent Rift. Geosphere, 12(4), 1271–1282. 31 �Olivine Geochemistry from Fe-Ti Oxide-Bearing Ultramafic Intrusions in the Duluth Complex, MN KATTEMALAVADI, Amartya , KLEINSASSER, Jackie , SIMON, Adam , PETERSON, Dean , and HUDAK, George 1 2 1 2 1 1 2 Department of Earth & Environmental Sciences, University of Michigan Natural Resources Research Institute, University of Minnesota–Duluth The Duluth Complex is a series of mafic intrusions in northeastern Minnesota which formed 1.1 billion years ago as part of the Midcontinent Rift System. On the southern side of the Complex, specifically on the western margin, it hosts 14 different ultramafic intrusions which bear Fe-Ti oxides, known as oxide bearing ultramafic intrusions, or OUIs. There are currently three competing hypotheses about how these OUIs formed, but none of them have been rigorously tested. However, there are many minerals present in these OUIs which can be used as proxies to Figure 1: Depth vs Forsterite content in Titac and Longnose. Both discover more about their the surrounding host troctolites and the OUI rocks themselves were genesis, such as olivine. analyzed, and results can be seen here. Since olivine is a solid solution mineral and is typically one of the first minerals to crystalize from a melt phase, we can gain a lot of information about the way the magma formed from its composition alone. Two OUIs in particular, Titac and Longnose, were studied in thin section using a variety of tools, including a petrographic microscope, a scanning electron microscope (SEM), and an electron probe microanalyzer (EPMA). The petrographic microscope and SEM showed the different textures in which olivine formed. In Titac, olivine appears to be tubular in shape and fractured. In Longnose, olivine appears to form in a ropy shape, and is heavily striated. There is evidence of fracturing, but not as much as in Titac. In addition, there is evidence of deformation, shown by grain boundary bulging of Fe-Ti oxides and plagioclase present. In both intrusions, the olivine is heavily serpentinized. Under the EPMA, compositions of olivine were measured, of olivine in OUIs as well as olivine in surrounding host rocks (troctolites). The results of this analysis can be seen in Figure 1. In the OUI rocks, it was found that forsterite content was overall higher than the host rock in both intrusions (~Fo60 to ~Fo70), with Longnose having a higher content than Titac. It also appears to be very consistent throughout the drill core for both. In addition, the host rocks have much lower amounts of forsterite (~Fo60 to ~Fo35), with the Longnose (both OUI rock and host rock) having a higher forsterite content overall. The higher forsterite content in the OUI rocks can likely be attributed to the abundance of Fe-Ti oxide minerals, which leach out iron from the area, leaving the olivine with a higher forsterite content. The higher forsterite content in Longnose 32 �indicates that the magma which formed it is relatively more primitive than the magma which formed Titac. The forsterite content is consistent with the findings of Miner (1995) in her thesis about the Longnose body1. We also compare the forsterite content to the content of nickel in the olivine (trace). We can see that there is overall more nickel in the Longnose OUI, and this is most likely due to the lower modal abundance of sulfide minerals compared to Titac. 1 Miner, G. C., 1995. Aspects of the Petrogenesis of the Longnose Fe-Ti-Oxide-Rich Ultramafic Body, Duluth Complex, MN. Washington University Department of Earth and Planetary Sciences. 33 �Textures and geochemistry of ilmenite and titanomagnetite in Fe-Ti oxide-bearing ultramafic intrusions of the Western Margin of the Duluth Complex, Minnesota KLEINSASSER, Jackie1, SIMON, Adam1, PETERSON, Dean2, and HUDAK, George2 1 Department of Earth & Environmental Sciences, University of Michigan, Ann Arbor, MI, USA Natural Resources Research Institute, University of Minnesota–Duluth, Duluth, MN, USA 2 The Duluth Complex hosts numerous undeveloped mineral deposit types, including CuNi, PGE, Mn, Fe-Ti±V, and others. Significant Fe-Ti±V mineralization has remained particularly unstudied over the past 20 years and represents one of the United States’ most promising domestic resources of these energy- and infrastructure-critical metals. Fe-Ti±V mineralization is hosted in ~14 Fe-Ti oxide-bearing ultramafic intrusions (known in the literature as OUIs) along the Western Margin. Although most are broadly similar in mineralogy, three contrasting genetic models have been proposed to explain the source of Fe and how the deposits formed. Because there exists incredibly scarce geochemical evidence to (a) support or refute any of the three models, ilm and the main objective of this project is to unravel the genesis of Fe-Ti±V oxidebearing ultramafic intrusions. pln ilm Focusing on the Longnose and Titac (Section 34) OUIs, two well-drilled intrusions ~40 km apart hosted in the ttm Partridge River and Western Margin intrusions, respectively, we used detailed optical and microbeam methods to compare the textures and compositions of 100µm Fe-Ti oxides (ilmenite, titanomagnetite, and magnetite) to gain insight into their (b) origin and cooling history. Both intrusions show similar complex textures despite ttm differing proportions of ilmenite and titanomagnetite, with Longnose typically having higher amounts of ilmenite. ‘dog tooth’ Titanomagnetite contains extensive pln ilmenite and pleonaste ((Fe,Mg)Al2O4). ilm Ilmenite is present as primary grains and as ‘oxy-exsolution’ lamellae in titanomagnetite (e.g., Buddington and Lindsley, 1964) as thin trellis-type 100µm lamellae, thicker bands of sandwich-type lamellae, and titanomagnetite-hosted Figure 1 – BSE images of Fe-Ti oxides from Longnose (a) and granular exsolutions (Fig. 1). Pleonaste is Titac (b). Multiple types of exsolution and oxy-exsolution are present in ilmenite at or near the contacts present. Ilmenite – ilm; titanomagnetite – ttm; pleonaste – between titanomagnetite and ilmenite as pln. large, granular blebs and as ‘dog tooth’ symplectites that are triangular intergrowths with ilmenite (Fig. 1b). In titanomagnetite, pleonaste exsolution is present as lamellae and cubic granules and within and around ilmenite sandwich-type 34 �lamellae as both vermicular symplectites (Fig. 2a) and blebs. Hematite lamellae are present in ilmenite from both OUIs, which indicates the original ilmenite-hematitess decomposed during cooling below 750–675℃ to form the hematite exsolution rods (Lindsley 1991). These textures indicate that both magnetite-ulvöspinelss and ilmenite-hematitess were present and that cooling and oxidation-reduction reactions induced the various exsolution textures observed, starting at >900℃ (Turnock and Eugster 1962). (a) (b) ilm pln hem pln ttm ilm 10µ m 30µ m Figure 2 – BSE images of finer exsolution textures from Titac. In (a), ilmenite oxyexsolution bands contain vermicular pleonaste symplectites while titanomagnetite contains pleonaste exsolution lamellae. In (b), the presence of hematite lamellae that exsolved during cooling through decomposition of an original ilmenite-hematitess below 750–675℃. Ilmenite – ilm; titanomagnetite – ttm; pleonaste – pln, hematite – hem. Major and minor element compositions of titanomagnetite and ilmenite are broadly similar between Longnose and Titac, with the possibility of remobilization and distribution of V and Co during alteration in Titac. The initial results of this study point towards a similar complex cooling history for Fe-Ti oxide liquids in both the Longnose and Titac OUIs, despite being emplaced in different rock types and hosted in different intrusions. Future work will involve measuring stable Fe and Ti isotopes of the ilmenite and titanomagnetite to fingerprint the metal sources and directly assess whether assimilation of the Biwabik Iron Formation is necessary to form the OUIs. Another avenue for future work involves using stable O isotopes to test whether volatiles released during the assimilation of Virginia Formation footwall were responsible for transporting metals and the serpentinization of olivine. Buddington, A., and Lindsley, D., 1964. Iron-Titanium oxide minerals and synthetic equivalents. Journal of Petrology, 5: 698–719. Lindsley, D., 1963. Equilibrium relations of coexisting pairs of Fe-Ti oxides. Carnegie Institution of Washington Year Book, 62: 60–66. Turnock, A. and Eugster, H., 1962. Fe-Al oxides: Phase relationships below 1000. Journal of Petrology, 3: 533–565. 35 �Identifying the genesis of Fe-Ti oxide- and sulfide-bearing ultramafic intrusions in the Duluth Complex through sulfide geochemical analysis LACHANCE, Kyle1, KLEINSASSER, Jackie1, SIMON, Adam1, PETERSON, Dean2, and HUDAK, George2 1 Department of Earth & Environmental Sciences, University of Michigan Natural Resources Research Institute, University of Minnesota–Duluth 2 During the formation of the 1.1 Ga Midcontinent Rift System, a series of massive mafic intrusions were emplaced in northeast Minnesota and are referred to as the Duluth Complex. Rock types include a series of gabbroic, troctolitic and anorthositic intrusions that protruded into the Archean and Paleoproterozoic footwall, which comprises banded iron formation and other metasedimentary rocks. The western margin of the complex is home to 14 different Fe-Ti oxidebearing bodies containing ilmenite, magnetite and titanomagnetite. While these oxide-bearing ultramafic intrusions (OUIs) have been identified Figure 1: (Femol+ Nimol)/Smol vs. Smol ratio plot demonstrating the distribution of electron probe in field studies, their exact genesis is unknown. microanalysis data for both deposits Three hypotheses have emerged that attempt to explain the origin of the OUIs, none of which has been rigorously tested. To investigate the origin of the OUIs, we studied the sulfide geochemistry of two spatially distant OUIs: Longnose, which is located in the Partridge River Intrusion, and Titac (sec. 34), which is located in the Western Margin Intrusion. Published compositional data for sulfides from these OUIs is sparse, with most studies reporting a very brief overview of the sulfide abundances, mineralogies and stable sulfur isotopes. One study, for instance, reports modal abundances of sulfides within Longnose ranging from 0.2-5%, 99% of which being chalcopyrite (Linscheid 1991). Our petrographic study of samples from Titac and Longnose revealed a greater abundance and variety of sulfides in samples from both deposits. As the first systematic geochemical comparison of sulfides between OUIs, our research began with scanning electron microscopy (SEM) to image the sulfide grains, followed by energy dispersive spectroscopy (EDS) to identify a range of different sulfides present within the OUI, and finally electron probe microanalysis (EPMA) to quantitatively determine the elemental makeup of the sulfides. Initial analysis of these data demonstrates that chalcopyrite is the modally dominant sulfide in both Titac and Longnose, with trace bornite, sphalerite and millerite found in both deposits as well. Pyrite was found only in the Titac samples, whereas chalcocite was found only at Longnose. Using the electron probe data, metal/sulfur ratio plots can be produced, such as in figure 1, to visualize the full spectrum of data including non-stoichiometric points that do not distribute into the expected mineral phases. As seen in this figure, which takes the ratio of moles of Fe and Ni to moles of S, the data indicates areas of points consistent with the Fe+Ni values for sulfides of interest, such as chalcopyrite around the value of 0.5 and bornite around 0.25. Many points were present that did not match any known mineral elemental compositions, suggesting points of exsolution, mixing or inclusions. Some Co-rich data points were also present that we were unable to characterize, so further analysis will be required to identify, if any, Co-sulfides within the deposits. 36 �With similar bulk sulfide content across the two deposits, these initial results suggest a consequently similar genesis across both of the OUIs. Future work regarding the stable sulfur isotopes will be necessary to identify the origin of the sulfur content to determine whether it is magmatic or through assimilation from the nearby Virginia Formation. Due to the heavier footprint of the Virginia Formation sulfur, an increase in the sulfur isotope ratio in the deposits would indicate some form of assimilation rather than solely a magmatic origin. Furthermore, a complete study on trace elements, such as the Pt group elements, will be necessary to further this project. Reference Linscheid, E., 1991. The Petrography of the Longnose Peridotite and Its Relationship to the Duluth Complex: MSc thesis, University of Minnesota Duluth, 121p. 37 �Archean Orogenesis to Proterozoic Rifting: A structural history of Pass Lake, Thunder Bay, Ontario LANDMAN, Megan and HILL, Mary Louise Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1 Canada New roadside outcrops along the Trans-Canada Highway 11/17 near Pass Lake, ON, expose the basal unconformity between Archean basement rock and the Proterozoic Gunflint Formation. Shear fractures, joints, and the Blende Lake Fault damage zone seen in these two outcrops record the brittle deformation history of the area before and after the Gunflint Formation was deposited. The unconformity represents a temporal gap in the stratigraphic sequence of at least 800 million years. The Archean rock belongs to the Wawa subprovince of the Superior province, in close proximity to the Wawa-Quetico subprovince boundary. Structural measurements, stereographic projections, and qualitative observations have allowed deeper insight and analysis of how structural controls have changed over an approximately 2.7 billion year-old geological history. The Archean basement unit underneath the unconformity is a coarse-grained amphibolite that contains accessory epidote and biotite, and is homogeneous throughout the length of both high-standing outcrops. This is interpreted to be a mafic pluton that has undergone amphibolitefacies metamorphism. The amphibolite records a scatter in orientation of joints and shear fractures, but some trends align well with data from Mackenzie River granite plutons, including an overall east-northeast and west-southwest strike. Later post-Proterozoic features, including the Blende Lake fault, have a common strike of east-northeast, which closely align with the orientation of the 1.1 Ga Mid-Continent Rift in Thunder Bay. This similarity is further reflected by the Blende Lake fault being oriented subparallel to the Mid-Continent Rift related silver veins. Similarities between trends in the amphibolite and Gunflint Formation suggest that the Mid-Continent Rift in Thunder Bay may have reactivated some Archean-aged orogenic-related faults and shear fractures. Minor folding in the Gunflint Formation directly adjacent to the Blende Lake Fault may be evidence of compression during the later stage of the Mid-Continent Rift. 38 �Characterization of Hydrothermal Alteration and Sulfide Ores at the Lynne Zn-Cu-Pb Deposit, Oneida Co. WI. LEMKE, Tara C., WEBER, Evan M., and LODGE, Robert W.D. Department of Geology, University of Wisconsin-Eau Claire, Eau Claire, WI The volcanogenic massive sulfide (VMS) Lynne Zn-Cu-Pb deposit was discovered by Noranda Exploration in 1990 within Oneida County, WI (Dematties, 1994; Adams, 1996). The Lynne deposit is one of many of VMS deposits across northern Wisconsin found in the Pembine-Wausau terrane. The Pembine-Wausau terrane is an accreted volcanic arc that formed during the Paleoproterozoic Penokean Orogeny. VMS deposits form from metallic-rich fluid exhaling on or near the ocean floor during extensional submarine volcanism (Galley et al., 2007). Interestingly, the hydrothermal alteration at the Lynne deposit contains talc-carbonate and calc-silicate mineral assemblages (Adams, 1996) and is unique compared to other deposits in Wisconsin (DeMatties, 1994). In addition, the sulfide ores contain more Pb than other deposits (DeMatties, 1994). The purpose of this study is to better characterize these unique hydrothermal rocks to improve our knowledge of the ore-forming environment at the Lynne deposit. The drill core from the Lynne deposit was obtained from the Natural Resources Research Institute in Duluth, Minnesota, and brought to Eau Claire where it was re-logged and sampled. Samples taken from the hydrothermal alteration and mineralized zones were cut into thin sections for petrographic analyses using transmitted- and reflected-light microscopes and scanning electron microscope (SEM) to determine the paragenetic sequence of ore minerals. Major and trace element geochemistry of altered rocks were determined using X-Ray Fluorescence (XRF) at UW-Eau Claire to characterize the element mobility during alteration and to determine the nature of the protoliths. The main sulfide mineralization within the Lynne deposit with abundance in decreasing order includes sphalerite, pyrrhotite, pyrite, galena, and chalcopyrite. Gold, silver, and copper are also found but in more localized sections within the Lynne deposit (Adams, 1996). The mineralization can be further divided into units A, B, and C, based on their stratigraphic position and compositional differences (Adams, 1996). Sphalerite is formed early on and is replaced by other minerals over time. In the deeper ore zones, pyrite tends to form relatively early and is replaced over time but in the ore zones closer to the surface, pyrite forms later. Chalcopyrite forms later in the ore zones replacing other minerals as it grows. Galena, pyrrhotite, and magnetite form in the middle replacing some minerals such as sphalerite and getting replaced by others such as pyrrhotite over time. The main alteration types found within the Lynne contain are carbonate and talc alteration. The unusually high amounts of carbonate material - unique to VMS deposits - found within the alteration zones are credited to CO2-rich hydrothermal vent fluid (Adams, 1996). The talc alteration is highly variable throughout the deposit but is largely found within two major zones. There is also unique skarn-type alteration is noted by chloritized diopside and garnet. A brecciated alteration texture is also notable as it shows progressive replacement of volcanic rocks with alteration minerals. Major chemistry indicates Mg-metasomatism with MgO values up to 30 wt% and five times that of the hosting volcanic rocks. Trace element chemistry reveals that these unique alteration assemblages had felsic protoliths with similar trace element abundances as the host rhyolites. 39 �Figure 1: A cross section of the Lynne deposit shows the distinct sulfide units. A) massive sphalerite and pyrite, B) disseminated sphalerite in carbonate host rock, C) massive sphalerite with semi-massive pyrite and disseminated galena, D) talc alteration, E) carbonate alteration, and F) skarn alteration (Adams, 1996). References: Adams, G.W., 1996, Geology of the Lynne base-metal deposit, north-central Wisconsin, U.S.A., in LaBerge, G.L., ed., Volcanogenic massive sulfide deposits of northern Wisconsin: A commemorative volume: Institute on Lake Superior Geology Proceedings, 42nd Annual Meeting, Cable, WI, v. 42, part 2, p. 161-179. DeMatties, T.A., 1994, Early Proterozoic volcanogenic massive sulfide deposits in Wisconsin: An overview. Economic Geology, 89: 1122-1151. Galley, A., et al., 2007, Volcanogenic massive sulfide deposits in Goodfellow W.D., ed., Mineral Deposits of Canada, Special Publication 5, p. 141-161. 40 �Island of Hawaii Field Trip, February 11-21, 2020 MACTAVISH(1), Allan, HINZ(2), Peter, ARTHUR(3), Mary Kay, CHATAWAY(4), Robert, EDBERG(3), Jim, ERICKSON(3), Tom, FOX(3), Steve, FURLONG(3), Joan, GERLICH(3), Jim, SMITH(5), Lindsay, WILHELM(3), David 1 Clean Air Metals Inc., Thunder Bay, ON; 2Ontario Ministry of Energy, Northern Development and Mines, Thunder Bay, ON; 3Minnesota Geological Society, Minneapolis, MN; 4Consulting Geologist, Thunder Bay, ON; 5Big Rock Exploration, Vancouver, BC Between February 11 and 21, 2020 the Institute on Lake Superior Geology facilitated a field trip to the Island of Hawaiʻi. This 10-day trip was led by Allan MacTavish and Peter Hinz, and consisted of 11 participants that included 4 professional geologists and 7 members of the Minnesota Geological Society. The trip circumnavigated the entirety of the Big Island of Hawaiʻi (Figures 1 and 2) and visited all 5 exposed volcanoes on the island. The aims of the field trip were to observe the characteristics of modern volcanoes in an intra-plate, non-rift environment (see) and to provide contrast and comparison to modern and ancient rift environments (i.e. the MCR and Iceland; Figure 3). The Big Island is made up of five distinct volcanoes: Kohala, Hualālai, Mauna Kea, Mauna Loa and Kīlauea. A sixth volcano associated with the Big Island, Loʻihi, is an active undersea volcano located approximately 20 km (12mi) south of Kīlauea. Stops were made at all five (5) extinct, dormant, or active volcanoes with the intent to focus on volcanology, igneous petrology, and geomorphology as well as a few stops at Hawaiʻian cultural and historical locations The 9 main islands comprising the Hawaiʻian islands are part of a chain of over 100 individual volcanoes in various states or formation and erosion (Lockwood and Hazlett, 2010). The Hawaiʻian-Emperor Chain is composed of eighteen islands, atolls, and seamounts stretching for greater than 6000km from the Island of Hawaiʻi and Loihi Seamount, in the east, northwestward and then north-northwestward across the Pacific Ocean to the Aleutian Trench (Lockwood and Hazlett, 2010; Best, 2003). It is postulated that this chain of islands was formed over a greater than 65 million year period as the Pacific Ocean plate migrated over a mantle “hotspot” initially in a north-northwest direction (Emperor Chain) and later in a northwesterly direction (Hawaiʻian Chain) (Barker, 1983). The volcanism creating the islands was, and is, primarily basaltic in chemistry with the occurrence of both effusive (dominant) and explosive eruptions (Lockwood and Hazlett, 2010). Explosive activity is primarily due to the interaction between basaltic magma and ground or sea water. This presentation will summarize the highlights of our trip and provide comparisons of Hawaiʻian geology, the Midcontinent rift, and Iceland (?). Featured highlights will include historical eruptive features from the eruptions of Kohala (100,000BP). Hualālai (1801 To 1802), Mauna Kea, and the 2018 Kīlauea Lower East Rift Zone eruption (Fissures 8 and 9). The presentation will also display local features such as the Koaʻe fault zone, Hōlei Pali (cliff), Mauna Ulu, the Pu‘u ‘Ō‘ō eruption lava field, Kīlauea Iki, fossil footprints in 18th century Kīlauea ash, the 2018 Kīlauea caldera collapse (Halemaʻumaʻu), and the Papakōlea Green Sand Beach. 41 �Kaua‘i Ni‘ihau Oahu Ka‘ula Molokaʻ i Maui Lanaʻi Kahoʻolaw e Hawai‘i Figure 1: Topographic relief and bathymetric map of the nine Hawai‘ian Islands. Modified from Eakins et al., 2003. Figure 2: Daily ILSG 2020 Field Trip routes, Island of Hawai‘i. Modified from Hazlett and Hyndman, 1996. 42 �Iceland MCR Figure 3: Location of the Hawai‘ian Islands with respect to world tectonic plates, the MCR, and Iceland. Modified after Tilling et al., 2010. REFERENCES Barker, D.S. 1983. Igneous Rocks; Prentis-Hall Inc., 417p. Best, Myron, G. 2003. Igneous and Metamorphic Petrology; Blackwell Science Ltd., 729p. Eakins, Barry W.; Robinson, Joel E.; Kanamatsu, Toshiya; Naka, Jiro; Smith, John R.; Takahashi, Eiichi; Clague, David A. 2003. Hawaii's volcanoes revealed: U.S. Geological Survey Geologic Investigations Series Map I-2809, 1 plate, https://pubs.usgs.gov/imap/2809/. Hazlett, R.W. and Hyndman, D.W. 1996. Roadside Geology of Hawaiʻi; Mountain Press Publishing Company, MT, 308p. Lockwood, Jock P. and Hazlett, Richard W. 2010. Volcanoes: Global Perspectives; Wiley-Blackwell, 536p. Tilling, R.I., Heliker, C., and Swanson, D.A. 2010. Eruptions of Hawaiian Volcanoes – Past, Present, and Future: U.SW. Geological Survey General Information Product 117, 63p. 43 �Chemostratigraphy of the western Schreiber-Hemlo Greenstone Belt, Results and Regional Implications MAGNUS, Seamus Ontario Geological Survey, Ministry of Energy, Northern Development and Mines, 933 Ramsey Lake Road, Sudbury, ON P3E 6B5 Canada seamus.magnus@ontario.ca The Neoarchean Schreiber–Hemlo greenstone belt is located within the Wawa–Abitibi terrane of the Superior Province. The western (Schreiber) and eastern (Hemlo) parts of the belt are separated by the cross-cutting Mesoproterozoic Coldwell Alkalic Complex. Recent mapping by the Ontario Geological Survey in the western part of the belt was conducted to produce an updated genetic model for the western part of the greenstone belt and to better understand its relationship with the rocks in the Hemlo area and surrounding greenstone belts (Hastie and Magnus, 2021a; Magnus 2021a, 2019a, 2017). New geochemical and geochronological data (Hastie and Magnus, 2021b; Magnus 2021b, 2019b, 2018) have changed the model presented by the author in a field trip guide for the 2019 ILSG conference (Magnus, 2019c). The supracrustal rocks in the western Schreiber–Hemlo greenstone belt are arranged in 4 depositional packages composed of metavolcanic and metasedimentary rocks with distinct petrographic and geochemical characteristics. Chemical and clastic metasedimentary rocks mark major disconformities between the depositional packages. For the sake of brevity, the clastic and chemical metasedimentary rocks of the greenstone belt will not be discussed in this presentation. The oldest recognized supracrustal rocks are exposed south of the Town of Schreiber, west of the Terrace Bay Pluton. In this package, predominantly tholeiitic mafic flows are interbedded with plagioclase porphyritic felsic flows and volcaniclastic breccias, wackes and mudstones. One mafic flow has major and trace element concentrations that are transitional between tholeiitic and calc-alkalic mafic volcanic rocks. Felsic volcaniclastic rocks near the top of this package are crosscut by a quartz porphyritic felsic intrusive rock west of Worthington Bay, dated at 2722.6 ±1.1 Ma by zircon (Kamo, 2019), which constrains this depositional package to >2720 Ma. Quartz porphyritic felsic volcanic and volcaniclastic rocks with U-Pb zircon ages of circa 2720 Ma occur along the entire length of the greenstone belt and along the shores of Lake Superior in Tuuri and Walsh townships (Hastie and Magnus, 2021; Magnus, 2021a, 2019a, 2017). These felsic rocks are closely associated with mafic flows that commonly contain an abundance of both equant plagioclase phenocrysts and coarse amygdules. These flows contain major and trace element concentrations that are transitional between tholeiitic and calc-alkalic volcanic rocks. Unlike the single flow of “transitional” mafic rock in the older package, the transitional mafic rocks in this package are the overwhelming majority. No contacts between the >2720 Ma and the circa 2720 Ma depositional packages have been observed. Both packages are disconformably overlain by an extensive package of tholeiitic mafic flows with distinct variolitic textures and trace element geochemistry consistent with a back-arcbasin volcanic environment. These rocks are cross-cut by quartz and feldspar porphyritic felsic dikes dated 2698.1±4.5 Ma (Sutcliffe and Davis, 2019), constraining their deposition to between circa 2720 Ma and circa 2700 Ma. The disconformity between the younger tholeiitic rocks and the older transitional mafic rocks is not easily distinguishable by geophysics or by field observations, especially where the rocks are deformed. The distribution of geochemical samples in this study area has helped trace this disconformity and locate cryptic folds along that contact that otherwise would not have been identified. This geochemical mapping technique may be helpful for delineating the stratigraphy in the nearby greenstone belts, it could be critical for reconciling the stratigraphy between the east 44 �and west parts of the Schreiber–Hemlo greenstone belt, and it may be applicable to other nearby greenstone belts in the western Wawa–Abitibi Terrane. In addition to the major and trace element geochemical analyses, 27 samples of mafic, intermediate and felsic metavolcanic rocks from the previously described packages and 12 samples of the varied granitoid plutons that cross-cut and surround the greenstone belt were submitted for Sm-Nd isotope analysis (Magnus, 2021b). εNd values for all of the volcanic and intrusive rocks cluster around the value of the depleted mantle at circa 2700 Ma, which supports the model that the volcanic rocks of the Wawa-Abitibi Terrane were generated in a juvenile arc magmatic system. Figure 1. Simplified geological map of the western Schreiber–Hemlo greenstone belt, highlighting the three metavolcanic packages described in this abstract References Hastie, E.C.G. and Magnus, S.J. 2021. Ontario Geological Survey, Preliminary Map P.3846, scale 1:20 000. Hastie, E.C.G. and Magnus, S.J. 2021b. Ontario Geological Survey, Miscellaneous Release—Data 382. Kamo, S.L. 2019. Internal report for the Ontario Geological Survey; Jack Satterly Geochronology Laboratory, University of Toronto, Toronto, Ontario, 4p. Magnus, S.J. 2017. Ontario Geological Survey, Preliminary Map P.3812, scale 1:20 000. Magnus, S.J. 2018. Ontario Geological Survey, Miscellaneous Release—Data 361. Magnus, S.J. 2019a. Ontario Geological Survey, Preliminary Map P.3826, scale 1:20 000. Magnus, S.J. 2019b. Ontario Geological Survey, Open File Report 6357, 41p. Magnus, S.J. 2019c. Ontario Geological Survey, Open File Report 6357, 41p. Magnus, S.J. 2021a. Ontario Geological Survey, Preliminary Map P.3845, scale 1:20 000. Magnus, S.J. 2021b. Ontario Geological Survey, Miscellaneous Release—Data 381. Sutcliffe, C.N. and Davis, D.W. 2019. Internal report for the Ontario Geological Survey; Jack Satterly Geochronology Laboratory, University of Toronto, Toronto, Ontario, 146p. 45 �Proterozoic Geology of the Schreiber–Terrace Bay Area MAGNUS, Seamus Ontario Geological Survey, Ministry of Energy, Northern Development and Mines, 933 Ramsey Lake Road, Sudbury, ON P3E 6B5 Canada seamus.magnus@ontario.ca The Mesoproterozoic Keweenawan Midcontinent Rift event emplaced a multitude of intrusive and volcanic igneous rocks around present-day Lake Superior. Previous work on these rocks along the north shore of Lake Superior has mainly focused on the Thunder Bay, Lake Nipigon and Marathon areas. Recent mapping by the Ontario Geological Survey along the north shore in the Schreiber–Terrace Bay area was conducted, in part, to bridge this gap (Hastie and Magnus, 2021a; Magnus, 2017, 2019a, 2021a). Dikes with orientations and geochemical and geophysical signatures consistent with known Paleoproterozoic dike swarms, including the Matachewan, Biscotasing and Marathon dike swarms, have been identified in the Schreiber–Terrace Bay area. Similarly, dikes with other orientations and geochemical and geophysical signatures, assumed to be Mesoproterozoic and belong to the Midcontinent Rift event. A total of 156 samples of Mesoproterozoic diabase and lamprophyre dikes were submitted for whole rock geochemical analysis throughout this project (Hastie and Magnus, 2021b; Magnus, 2018, 2021b). The results of mapping and geochemical analysis suggest potential correlation between several smaller populations of dikes and known Keweenawan units. East-northeast trending dikes in the study area are parallel to, along strike with (separated by 200 km), and share similar wholerock geochemistry with the Pigeon River dikes of the Thunder Bay area (Cundari et al. 2021). Dikes of varied orientation share similar whole-rock geochemistry with Osler Group I and Group II volcanic rocks (Hollings et al., 2007). A newly recognized, circular intrusion north of the town of Terrace Bay, dated 1107.9±1.4 Ma by baddeleyite (Kamo, 2019), also shares similar geochemistry with Osler Group II volcanic rocks. The results of mapping and geochemical analysis have revealed one new and significant population of diabase dikes. These west-northwest trending dikes with reversely polarized aeromagnetic signature cross-cut the entire study area. In the east, they were observed crosscutting, and being cross-cut by, syenite at the western edge of the circa 1108 Ma Coldwell Alkalic Complex, which constrains them to emplacement at around the same time. The dikes consist of an ophitic to sub-ophitic textured olivine gabbro composed of augite, plagioclase and generally fayalitic olivine (average composition Fo30, as low as Fo10, up to Fo70), which is an expected composition of olivine for an alkalic rock. The dikes have alkalic major element concentrations and enriched, “ocean island basalt”-like trace element concentrations and chondritic to slightly depleted Sm-Nd and Sr-Rb isotopic values. There are two small subsets of these dikes – one that is plagioclase megacrystic and is geochemically indistinguishable from the main population; and one that is heavily depleted in calcium, strontium, barium and phosphorous, consistent with apatite fractionation, and which displays a “trachytic” texture with abundant plagioclase laths. South of the town of Schreiber a parallel set of alkalic dikes with normally-polarized aeromagnetic signature cross-cut the peninsula between the Schreiber Channel and Worthington Bay. Geophysical data, air photos and previous maps indicate that these dikes continue along strike westward, cross-cutting the islands in Schreiber Channel and south of Nipigon Bay. A targeted 46 �sampling program in these islands would be helpful to study the extent of these normally-polarized alkalic dikes. The petrographic and geochemical characteristics of the alkalic diabase are similar to the Wolfcamp Lake alkalic basalts that overlie the Coldwell Alkalic Complex (Davis et al., 2017), the Geordie Lake gabbro, which cross-cuts the complex (Meghji, 2016), and a subset of dikes that crop out south of the complex in Pukaswka National Park (Cundari et al., 2021). Altogether, the mafic alkalic magmatism in the northeast part of Lake Superior represents an emergent and potentially important part of the Keweenawan geological history (Good et al., in review). References Cundari, R.M., Puumala, M.A., Smyk, M.C. and Hollings, P. 2021. New and compiled whole-rock geochemical and isotope data of Midcontinent Rift–related rocks, Thunder Bay area; Ontario Geological Survey, Miscellaneous Release—Data 308 – Revised. Davis, S., Hollings, P. and Cundari, R.M. 2017. Geochemistry of the Mesoproterozoic Wolfcamp Lake basalts, northwestern Ontario; Ontario Geological Survey, Miscellaneous Release—Data 345. Hastie, E.C.G. and Magnus, S.J. 2021a. Precambrian geology of Strey Township, northwestern Ontario; Ontario Geological Survey, Preliminary Map P.3846, scale 1:20 000. Hastie, E.C.G. and Magnus, S.J. 2021b. Geological, geochemical and petrographic data from Strey Township, western Schreiber–Hemlo greenstone belt, Wawa–Abitibi terrane, Superior Province, northwestern Ontario; Ontario Geological Survey, Miscellaneous Release—Data 382. Hollings, P., Fralick, P and Cousens, B. 2007. Early history of the Midcontinent Rift inferred from geochemistry and sedimentology of the Mesoproterozoic Osler Group, northwestern Ontario; Canadian Journal of Earth Sciences, v.44, p.389-412. Good, D., Hollings, P., Dunning, G., Epstein, R., McBride, J., Jedemann, A., Magnus, S., Bohay, T. and Shore, G. in review. A new model for the Coldwell Complex and associated dykes in the Midcontinent Rift, Canada; Journal of Petrology, 43p. Kamo, S.L. 2019. Part A: Report on U-Pb ID-TIMS geochronology for the Ontario Geological Survey: Bedrock Mapping Projects, Ontario, Year 4: 2018-2019, internal report for the Ontario Geological Survey, Jack Satterly Geochronology Laboratory, University of Toronto, Toronto, Ontario, 26p. Magnus, S.J. 2017. Precambrian geology of Tuuri and Walsh townships, northwestern Ontario; Ontario Geological Survey, Preliminary Map P.3812, scale 1:20 000. Magnus, S.J. 2018. Geological, geochemical, geophysical and petrographic data from Tuuri and Walsh townships, Schreiber–Hemlo greenstone belt, Wawa–Abitibi terrane, Superior Province; Ontario Geological Survey, Miscellaneous Release—Data 361. Magnus, S.J. 2019a. Precambrian geology, Syine Township; Ontario Geological Survey, Preliminary Map P.3826, scale 1:20 000. Magnus, S.J. 2019b. Geological, geochemical and petrographic data from Syine township, Western Schreiber–Hemlo greenstone belt, Wawa–Abitibi terrane, Superior Province, Northwestern Ontario; Ontario Geological Survey, Miscellaneous Release—Data 375. Magnus, S.J. 2021a. Precambrian geology of Priske township; Ontario Geological Survey, Preliminary Map P.3845, scale 1:20 000. Magnus, S.J. 2021b. Geological, geochemical and petrographic data from Priske Township and Nd, Sm and Sr isotopic data from Priske, Strey, Syine, Tuuri and Walsh townships, Western Schreiber–Hemlo Greenstone Belt, Wawa–Abitibi Terrane, Superior Province, Northwestern Ontario; Ontario Geological Survey, Miscellaneous Release—Data 381. Meghji, I. 2016. The character and distribution of Cu-PGE mineralization at the Geordie Lake Deposit within the Coldwell Complex, Ontario. Unpublished M.Sc. thesis, University of Western, Ontario, London, Canada, 336 p. 47 �Structural Analysis and Interpretation of Deformation along the Keweenaw Fault System West of Lake Gratiot, Keweenaw County, Michigan MUELLER, S.A., DEGRAFF, J.M., and LIZZADRO-MCPHERSON D.J. Michigan Technological University, Houghton, MI 49931 The Keweenaw fault extends along the southern margin of the Midcontinent Rift System from northwest Wisconsin to near Keweenaw Point in Michigan, making it longest known fault associated with the rift. Reverse movement on the NW-dipping fault has thrust Portage Lake Volcanics (PLV, 1.1 Ga) over younger, mostly flat-lying Jacobsville Sandstone (JS) (Fig. 1), regionally tilting PLV strata northwestward on the southeast limb of the Lake Superior syncline (1). Published geologic maps from the 1950s (2-4) depict the fault as a single sinuous trace parallel to strike of hanging-wall PLV strata that are juxtaposed against JS nearly everywhere. Published cross-sections generally show hanging-wall strata facing northwest with a simple listric geometry and footwall strata tilted steeply southeast near the fault. The maps and crosssections show a few complications along the fault trace (anomalous sinuosity) and in its hanging-wall (lesser faults, folds), hints of deformation complexity that may reveal the nature and cause of faulting. Midway along the Keweenaw Peninsula, published maps show a prominent reentrant of JS into PLV along the fault and hint at a major splay in the Keweenaw fault’s hanging wall (3-4). New mapping near Lake Gratiot and Lac La Belle, combined with drill core logs, suggests that the postulated splay in the Keweenaw fault’s hanging wall is instead the main fault of an orderly fault system (Fig. 2). The Keweenaw fault system here consists of: 1) segments striking east-northeast with steep northerly dip (main trend), 2) segments striking east-southeast also dipping steeply north, and 3) segments striking north-northeast with moderate-to-shallow westerly dip. Members of these fault sets define a multistranded fault system and several, large, PLV blocks southeast of the main fault. Set 2 faults have a left-stepping arrangement similar to analogous faults mapped in a previous project to the east along Bête Grise Bay (5). Mapping also has revealed folding in hanging-wall Portage Lake Volcanics and has resolved fold geometry in footwall Jacobsville Sandstone. Fold axes are generally subparallel to adjacent faults and therefore are probably related to fault movement. A Set 3 fault crossing Bruneau Creek is associated with a faulted anticline in hanging-wall PLV and multiple anticlines and synclines in the footwall JS, indicating significant shortening across a west-dipping thrust fault. A Set 2 fault crossing a Snake Creek tributary has an anticline-syncline pair in hanging-wall PLV and a single asymmetric syncline in footwall JS, resulting from mostly right-lateral strike slip and lesser north-side-up reverse slip on a steeply dipping fault. Shortening across this ESE-trending fault is relatively minor because of its inferred steep dip and dominant strike slip motion. The pattern and relationships of faults and folds suggest a fault system dominated by dextral shear rather than by reverse movement as in the conventional model. Indicators of slip direction and sense measured on 55 small faults demonstrate that the fault system here is in fact dominated by strike slip, with a 2.5:1 ratio of strike-to-dip slip. Inversion of fault-slip data yields a maximum shortening direction of 285°105° during faulting, which implies overall dextral shear on the fault system and differences in slip kinematics between fault sets. Set 1 faults defining the overall trend of the Keweenaw fault system in this area have strike slip > dip slip; Set 2 faults with ESE-trend have strike slip >> dip slip; Set 3 faults with N to NE trends have dip slip >> strike slip. New mapping and structural analyses in this area have revealed a multistranded Keweenaw fault system that is transpressional in nature, dominated by dextral strike slip, and has lesser reverse slip with north side up. Fault-bounded blocks with ENE-oriented long dimensions generally moved eastward along set 1 and 2 faults, as thrusting of Portage Lake Volcanics over Jacobsville Sandstone occurred on set 3 faults. The estimated WNW-ESE maximum shortening direction associated with fault movement strongly suggests that the Grenville Orogeny was primarily responsible for movement of the Keweenaw fault system, with possible reactivation occurring during the Appalachian Orogeny. 48 �Acknowledgements: We appreciate primary funding by the USGS EDMAP program, sponsorship by the Michigan Geological Survey, a Michigan Space Grant Consortium award to the lead author, and a generous donation by the Keweenaw Community Forest Company. Figure 1 (left): Keweenaw Peninsula where Portage Lake Volcanics are thrust over Jacobsville Sandstone. Green rectangle near middle of peninsula marks area of Figure 2. (adapted from 1). Figure 2 (below): Study area along Keweenaw fault between Mohawk and Bête Grise Bay. Main units: PLV mafic = greens; PLV felsic = reds; JS = yellow. Red line marks newly interpreted main trace of the Keweenaw fault system. Other major faults as black solid and dashed lines. References 1. 2. 3. 4. 5. Cannon, W.F. and Nicholson, S.W., 2001, Geologic Map of the Keweenaw Peninsula and Adjacent Area, Michigan: United States Geological Survey, Map I-2696, Scale = 1:100,000. Cornwall, H.R., 1954, Bedrock Geology of the Delaware Quadrangle, Michigan: U.S. Geological Survey, Washington, D.C., Geologic Quadrangle Map GQ-51, scale 1:24,000. Davidson, E.S., Espenshade, G.H., White, W.S. and Wright, J.C., 1955, Bedrock Geology of the Mohawk Quadrangle, Michigan: U.S. Geol. Survey, Washington, D.C., Geologic Quadrangle Map GQ-54, scale 1:24,000. Wright, J.C. and Cornwall, H.R., 1954, Bedrock Geology of the Bruneau Creek Quadrangle, Michigan: U.S. Geological Survey, Washington, D.C., Geologic Quadrangle Map GQ-35, scale 1:24,000. DeGraff, J.M., Tyrrell, C.W., and Hubbell, G.E., 2018, Keweenaw Fault Geometry, Secondary Structures, and Slip Kinematics along the Bête Grise Bay Shoreline: U.S. Geological Survey, Final Technical Report, 21 p. 49 �U-Pb geochronology and zircon trace-element geochemistry from granitoid plutons in the Neoarchean Sturgeon Lake greenstone belt, Ontario, Canada NELSON, Trevor J.1, JOHNSON, Rory M.1, LODGE, Robert W.D.1, MA, Chong2, MARSH, Jeffery H.2 1 Department of Geology, University of Wisconsin-Eau Claire, Eau Claire, WI 54701 USA Metal Earth, Mineral Exploration Research Centre, Harquail School of Earth Sciences, Laurentian University, Sudbury, Ontario, Canada P3E 2C6 2 The Sturgeon Lake granite-greenstone belt is in the eastern portion of the Western Wabigoon subprovince of the Superior province and is bordered to the north, east, and south by the Winnipeg River terrane. The belt is comprised of felsic to mafic volcanic successions, synorogenic sedimentary assemblages, and calc-alkaline to alkalic plutons (Sanborn-Barrie and Skulski, 2005; Lodge et al., 2019). The emplacement of granitoid plutons in the Sturgeon greenstone belt can provide a more complete magmatic and deformational history than volcanic assemblages and sedimentary rocks. The origin and timing of their melts, mantle-crust interaction, and subsequent magmatic-hydrothermal activity record important tectonic events throughout the petrogenetic history of Sturgeon Lake greenstone belt and the western Superior Province. The primary objective of this project is to better constrain the magmatic and tectonic history of the Sturgeon Lake greenstone belt through U-Pb geochronology and trace element geochemistry of zircons from the granitoid plutons. Previous mapping in the Sturgeon Lake greenstone belt (Sanborn-Barrie and Skulski, 2005) constrained spatial and temporal relationships between plutonic rock and supracrustal assemblages largely by describing the geometry of plutons through aeromagnetic surveys and their relationship with regional foliations in the surrounding rocks. However, geochronologic data was largely constrained to the volcanic and sedimentary assemblages and the timing of plutonic events were mostly estimated. Plutons were broadly characterized into three categories based on these relationships: synvolcanic, early- to syntectonic, and late- to post-tectonic (Sanborn-Barrie and Skulski, 2005). This study obtained representative samples from the different plutons along the Metal Earth Sturgeon transect. After petrographic and whole-rock geochemical characterization (Nelson et al., 2020), zircons were separated for further analyses. Zircon grains were analyzed using Laser Ablation Split Stream Inductively Coupled Plasma Mass Spectrometry (LASS-ICP-MS) at Laurentian University for simultaneous collection of U-Pb isotopic abundances and trace-element geochemistry. The interpreted U-Pb ages of the sampled plutons allowed for refinement of the prior tectonic classifications. Some plutons were re-assigned to refined classifications based on the interpreted age: those that formed at the same time as the greenstone belt volcanic assemblages (syn-volcanic), those that formed during major deformation events (syn-tectonic), and those that post-date most deformation events (post-tectonic) (Figure 1). Trace-element geochemistry of the zircons not only supports the updated plutonic classifications, but also reveals a temporal evolution in the petrogenesis of granitoid plutons from anhydrous melting of mantle-derived sources to hydrous, oxidized melts that have significant interaction with the evolved crust. (Figure 1, inset). 50 �Figure 1: Regional geology of Sturgeon Lake granite-greenstone belt modified from Sanborn-Barrie and Skulski (2005) with new U-Pb ages of granitoid plutons and their timing relative to major tectonic events. Inset diagram displays U/Yb versus 207Pb/206Pb age displaying increasing role of evolved crust in the genesis of felsic melts over time. REFERENCES Lodge, RWD., Ma, C, Etienne, M, Nelson, TJ, Chandler, MD, Brock, NM (2019). Geologic Overview and petrogenetic history of the Sturgeon Lake Transect, Western Wabigoon. Summary of Field Work and Other Activities, Ontario Geological Survey Open File Report 6260. Nelson, TJ, Lodge, RWD, Ma, C (2020). Geologic overview of Sturgeon Lake greenstone belt granitoid plutons, Western Wabigoon Subprovince. Geological Society of America Abstracts with Programs, 52(6), paper 97-7. Sanborn-Barrie, M, and Skulski, T (2005) Geology, Sturgeon Lake greenstone belt, western Superior Province, Ontario; Geological Survey of Canada, Open File 1763, scale 1:100 000. 51 �3-D Modeling of the Duluth Complex from geophysical data PETERSON, D.E., BEDROSIAN, P.A. and FINN, C.A. U.S. Geological Survey, MS 973, Federal Center, Denver, CO 80225 The Mesoproterozoic Duluth Complex in northeastern Minnesota is one of the major plutonic components of the Midcontinent Rift System and hosts a variety of copper-nickel sulfide and platinum-group element deposits. The Duluth Complex is composed of a series of individual mafic and felsic intrusions emplaced 1110-1098 Ma within Paleoproterozoic sedimentary rocks of the Animikie basin and volcanic flows of the Midcontinent Rift. Prior work has included 2-D modeling and qualitative geologic interpretations of gravity and magnetic data (e.g., Chandler, 1990; Chandler and Ferderer, 1989), much of which is still preliminary (V. Chandler, written commun., 2020). Three-dimensional modeling has been limited, with only one 3-D model created using Bouguer gravity data constrained by seismic-reflection interpretations as part of a PhD thesis (Allen, 1994). Given the complex geology of the area, 3-D modeling is useful for providing a complete picture of the variable densities, susceptibilities, and electrical resistivities throughout the Duluth Complex and associated volcanic rocks as well as their depth extent beneath sedimentary cover. Models of these geophysical properties at depth enable more accurate geologic mapping in the subsurface which can lead to an improved understanding of the formation history of the Duluth Complex. In this study, we use aeromagnetic data acquired between 1979-1991 (Chandler, 2007), Bouguer gravity data collected since 1950 (Chandler and Lively, 2019), and magnetotelluric data collected in 2019 to create new 2-D and 3-D geophysical models of the Duluth Complex constrained by seismic reflection, geologic, and rock property data. An inversion of the Bouguer gravity data for thickness of the Duluth Complex using constant densities of 3110 kg/m3 and 2670 kg/m3 for the Duluth Complex and surrounding crustal rocks, respectively, results in thicknesses ranging from ~3-28 km for the Duluth Complex and related intrusions and volcanic rocks (Figure 1A). A 3-D model of the magnetotelluric data reveals low resistivity anomalies at ~5-10 km depth below the northern margin of the Duluth Complex and below the Greenwood Lake intrusion (Figure 1B). We expect to encounter low resistivities at depth associated with the Paleoproterozoic Animikie basin, which makes up the floor of the Duluth Complex, and therefore interpret these anomalies as either the base of the complex or as fragments of Animikie sediments interfingered with igneous intrusive rocks. Finally, 3-D voxel models of density and susceptibility illuminate the subsurface distribution of rock properties below the Duluth Complex which, in combination with resistivity and thickness models, can be used to create a 3-D geologic map of this area. 52 �Figure 1. Geophysical models of the Duluth Complex. A) Modeled thickness of the Duluth Complex and associated igneous rocks (dashed black outline in A and B) resulting from constant density inversion of Bouguer gravity data. B) Depth slice through 3-D resistivity model at 5 km below the surface. GL=Greenwood Lake intrusion. References Allen, D. J., 1994, An integrated geophysical investigation of the midcontinent rift system: Western Lake Superior, Minnesota, and Wisconsin, [PhD thesis]: Purdue University, West Lafayette, Indiana, 267 pp. Chandler, V. W., 1990, Geologic interpretation of gravity and magnetic data over the central part of the Duluth Complex, northeastern Minnesota: Economic Geology, v. 85, p. 816-829. Chandler, V. W., 2007, Upgrade of aeromagnetic databases and processing: Minnesota Geological Survey Open File Report OFR07_06. Chandler, V. W., and Ferderer, R. J., 1989, Copper-nickel mineralization of the Duluth Complex, Minnesota; a gravity and magnetic perspective: Economic Geology, v. 84, p. 1690-1696. Chandler, V. W., and Lively, R. S., 2019, Upgrade of the gravity database at the Minnesota Geological Survey: Retrieved from the University of Minnesota Digital Conservancy, 28 August 2020. 53 �Negative Carbon Dioxide Emissions through Enhanced Silicate Weathering and the Lake Superior Region PLANAVSKY, Noah Department of Earth and Planetary Sciences, Yale University, 210 Whitney Avenue, New Haven, CT, USA Carbon capture is now widely recognized as necessary to stabilize atmospheric carbon dioxide to keep global warming below 1.5˚C, a temperature threshold mandated by the 2015 Paris Agreement. The Intergovernmental Panel on Climate Change (IPCC) projects significant degradation of livelihoods, food security, and water supply, if this threshold is exceeded. There are multiple proposed mitigation approaches that could curb climate catastrophe. Although no one carbon capture approach is perfect, all are on the table by necessity. Each has specific capture potential in terms of uptake rate, storage capacity, and storage longevity. I will give an overview of work determining whether adding basalt to agricultural lands is an effective, low-risk carbon capture strategy—and make a case that Lake Superior region is great place to implement this means of climate change mitigation. The idea behind this carbon dioxide removal (CDR) strategy is simple. Carbon dioxide in the atmosphere chemically reacts with silicates minerals—and reacts especially rapidly with mafic minerals. The byproducts of this reaction lead to marine carbonate precipitation, which transfers carbon from the atmosphere into the rock record. This is, on geologic timescales, how Earth has sequestered almost all the carbon continuously sourced to the atmosphere from Earth’s interior (e.g., volcanic carbon dioxide outgassing). In principle, carbon capture through mineral weathering involves enhancing the rate of a process that the Earth does naturally. In practice, the overriding controls on this process are complex. Carbon capture rates are highly variable and dependent on local conditions and implementation strategy. Nonetheless it is clear that capture through weathering can be greatly accelerated by milling of mafic minerals to increase its surface exposure. This approach holds great potential and is certain to be carbon negative. IPCC estimates are that the agricultural sector contributes about 25 percent of total greenhouse gas emissions worldwide. I will present modeling work and empirical studies that suggests addition of mafic rocks to agricultural lands has the potential to simultaneously offset a significant component of this anthropogenic greenhouse gas source and increase agricultural yields and pest resistance. If subsidized, this process could provide a stable source of income for farmers increasingly vulnerable to climate extremes and promote regional economic growth. The Lake Superior region is the ideal area to carry out this carbon capture process and widespread implementation could revitalize the economy of Lake Superior region towns that are (or were formerly) heavily dependent on mining. Compilations of the extensive knowledge of Lake Superior Region geology are essential to support that enhanced silicate weathering is a viable means climate change mitigation, sway policy makers, and foster investment in case studies of this process. 54 �A newly discovered orbicular occurrence within the Good Hope carbonatite, north of Marathon, Ontario PRICE, Rebecca, ZUREVINSKI, Shannon, and MITCHELL, Roger H. Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada The Good Hope carbonatite is emplaced within Archean gneisses of the Wawa Subprovince. This occurrence is located along the northwest margin of the Prairie Lake carbonatite complex (1160 Ma, Wu et al. 2017), although their genetic relationship remains speculative (Mitchell et al. 2020). The Good Hope carbonatite is a niobium occurrence with the niobium primarily occurring within pyrochlore-group minerals, minor niobium in ferrocolumbite, and consists of multiple generations of carbonatite that include calcite-, dolomite-, ferrodolomite-, and quartz fluorite-carbonatite (Mitchell et al. 2020). The purpose of this study is to characterize the orbicular occurrence and suggest a possible petrogenesis for this rare textural occurrence. The orbicular occurrence was first identified from the drill core, where it occurs as narrow bands or possible clasts within the carbonatite groundmass. These orbicules are mm-scale, oblate to spherical in shape with variable degrees of monomineralic shell development ranging from welldeveloped shells to spherical aggregates of the minerals which define the orbs (Figure 1). The orbicules consist of biotite, Ti-bearing aegerine, and magnetite with ilmenite lamellae within an allotriomorphic calcite carbonatite groundmass that contains some Fe-dolomite. The orbicules commonly consist of a ‘core’ of magnetite + biotite followed by a shell of aegirine and one or more outer shells of monomineralic biotite and/or magnetite. Within these orbicules there are variable amounts of calcite and Fe-dolomite, which can occur both as monomineralic shells, commonly associated with the outer shells, as well as entrained within the ‘core’ of the orbicules. This type of texture has been observed within other carbonatites (eg. Haggerty and Fung, 2006), but is more commonly observed within granitic and dioritic compositions (eg. Smillie and Turnbull, 2013). It is largely accepted that the orbicular textures are of magmatic origin, however a variety of genetic models exist, due to the unique nature of each occurrence. In general, these models broadly fit into three groups with orbicular formation controlled by (1) liquid immiscibility (eg. Ai et al. 2020) (2) superheating of the magma that lowers the nucleation rate and increases the growth rate (eg. Lindh and Näsström, 2006) or (3) quenched recrystallization following the injection of magma (eg. Zurevinski and Mitchell, 2015). Orbicular textures within carbonatites are commonly modelled by liquid immiscibility, given this model lends itself to the orbicules having a different composition than the groundmass. The second group has variations in the compositions of the shells and groundmass that lead to variability between the models in the source of the ‘orbicular magma’, the cause of the increased temperature within the magma, and the process of crystallization of the orbicule shells. The third group also has variability, with the orbicule and groundmass being either the same composition (Zurevinski and Mitchell, 2015) or different compositions (Zhang and Lee, 2020) due to the composition of the initial magma and the latter injected magma. The Prairie Lake orbicular occurrence mimics ‘contact metamorphism-type’ recrystallization of the orbicules within a compositionally identical groundmass (Zurevinski and Mitchell, 2015). The Good Hope occurrence differs significantly from the Prairie Lake occurrence; the composition of the orbicules and the groundmass are distinctly different, the orbicules are variably oblate indicating soft-shell deformation, and they do not exhibit quenched or recrystallization textures. 55 �Figure 1: Scan of the standard polished petrographic thin section of the orbicular occurrence of the Good Hope carbonatite, highlighting the predominantly oblate, mm-sized orbicules. References Ai, J., Lu, X., Li, Z., and Wu, Y. 2020. Genesis of the graphite orbicules in the Huangyangshan graphite deposit, Xinjiang, China: Evidence from geochemical, isotopic and fluid inclusion data; in Ore Geology Reviews, 122: 103505 Haggerty, S. E., and Fung, A. 2006. Orbicular oxides in carbonatitic kimberlites; in American Mineralogist, 91: 1461-1472 Lindh, A., and Näsström, H. 2006. Crystallization of orbicular rocks exemplified by the Slättemossa occurrence, southeastern Sweden; in Geological Magazine, 143: 713-722 Mitchell, R.H., Wahl, R., and Cohen, A. 2020. Mineralogy and genesis of pyrochlore apatitite from the Good Hope Carbonatite, Ontario: A potential niobium deposit; in Mineralogical magazine, 84: 8191 Smillie, R. W., and Turnbull, R. E. 2013. Field and petrographical insight into the formation of orbicular granitoids from the Bonney Pluton, southern Victoria Land, Antarctica; in Geological Magazine, 151 (3): 534-549 Wu, F., Mitchell, R.H., Li, Q., Zhang, C., and Yang, Y. 2017. Emplacement age and isotopic composition of the Prairie Lake carbonatite complex, Northwestern Ontario, Canada; in Geological Magazine, 154: 217-236 Zhang, J., and Lee, C. A. 2020. Disequilibrium crystallization and rapid crystal growth: acase study of orbicular granitoids of magmatic origin; in International Geology Review Zurevinski, S. E., and Mitchell, R. H. 2015. Petrogenesis of orbicular ijolites from the Prairie Lake complex, Marathon, Ontario: Textural evidence from rare processes of carbonatitic magmatism; in Lithos, 239: 234-244 56 �Preliminary pXRF results from Precambrian rocks of northern Minnesota PRUE, Ann Marie1 and BRENGMAN, Latisha1 1 Department of Earth and Environmental Sciences, University of Minnesota-Duluth, 229 Heller Hall, 1114 Kirby Drive, Duluth, MN 55812 Precambrian iron-rich chemical sedimentary rocks are an important archive of early seawater chemistry (Konhauser et al., 2017). Interpreting the origin of these iron-rich rocks is complicated because they can form at the intersection of sedimentary and hydrothermal activity. One tool that can be used to identify signatures of past seawater and hydrothermal fluids are major-, trace-, and rare-earth-elements. Drawbacks to laboratory-based geochemical approaches include extensive sample preparation and destruction of the material. Advances in XRF technology have led to the development of portable x-ray fluorescence (pXRF) instruments capable of producing laboratory-quality element analyses in the field when used with a custombuilt, rock-type-specific calibration (Steiner et al., 2017; Al-Musawi and Kaczmarek, 2020). With the ability to create a custom calibration, it is possible to measure major- and trace- elements in the field or in drill core. One goal of this project is to create a customized, rock-type specific calibration for the Bruker 5g Tracer pXRF instrument to analyze iron-rich rocks that outcrop near Soudan, MN. Field and drill-core geochemical analysis can help identify units of unknown origin, while aiding in the identification of key samples for additional destructive analyses. To create the custom iron formation calibration for the pXRF instrument, we first need an internal reference material. We analyzed select iron formation samples from the Soudan iron formation for a suite of 46 major, trace-, and rare-earth-elements using inductively coupled plasma optical emission spectrometry (ICPOES) and inductively coupled plasma mass spectrometry (ICP-MS) methods following the procedures outlined in Brengman et al. (2020). We can directly compare elements analyzed using ICP-OES and ICPMS to the pXRF data (Figure 1A). The pXRF instrument was set up in tabletop working mode, and powdered samples were analyzed in standard XRF mount cups. To test the effect of pressing the sample into the mount cup, we performed a comparison test on one sample (PL-19-3c; Figure 1B) using the internal mudrock calibration. First, the un-pressed powder was run, then the pressed powder was run 1.5 hr. later, with five minutes between each analysis to allow for cooling. Measurements for the element silicon (Si) in the three un-pressed powers ranged from 23.075 wt. % (±0.083) to 23.197 (±0.083) wt. % (Figure 1B). The same three powders were pressed, and measurements were repeated using the same analytical setup. Measurements for the element silicon (Si) in the three samples (now pressed powders) ranged from 23.733 (± 0.084) to 23.891 (±0.0847) wt. %. We observe a difference of ~0.7 wt. % between Si measured in the un-pressed and pressed powders and recommend pressing powders to ensure uniform measurements. Previous studies identified a “warm-up” period in which the pXRF instrument took roughly 1 hour to stabilize. To test whether the Bruker Tracer 5g instrument also required this “warm-up” period, we conducted continuous acquisition of one sample (V-19-05A) for 21 consecutive runs, with 5 minute-spacing between analyses. At this point, a second “split” of the same initial powder (V-19-05B) was analyzed in the same way to determine if there was a measurable difference between the two splits of the same sample (V-19-05A and V-19-05B). Data for Si (wt. %) and Ca (wt. %) from sample V-19-05A and B are reported in Figure 1C, and D. We identified a steady increase (Figure 1C, D) until the instrument stabilized (run #110 for Si and #102 for Ca). We determined the instrument requires a significant “warm-up” period to ensure consistency between analytical runs. Future work will focus on comparison of ICP-OES and ICPMS data to pXRF data to build the custom-calibration for powdered, and non-powdered sample materials. 57 �Figure 1: Preliminary geochemical data from iron-rich rocks near Soudan, MN. (A) Rare-earth-element data for iron formation (V-19-05) and regional greywacke (PL-19-3c). (B) Initial test run of pressed versus un-pressed powders using the pXRF instrument. (C, D) Consecutive pXRF analyses of iron formation sample V-19-05 to determine the duration of the “warm-up” period for the instrument. REFERENCES Al-Musawi, M., Kaczmarek, S., 2020. A new carbonate-specific quantification procedure for determining elemental concentrations from portable energy-dispersive X-ray fluorescence (PXRF) data. Applied Geochemistry 113. Brengman, L.A., Fedo, C.M., Whitehouse, M.J., Jabeen, I., Banerjee, N.R. 2020. Textural, geochemical, and isotopic data from silicified rocks and associated chemical sedimentary rocks in the ~ 2.7 Ga Abitibi greenstone belt, Canada: Insight into the role of silicification, Precambrian Research, 351. Konhauser, K.O., Planavsky, N.J., Hardisty, D.S., Robbins, L.J., Warchola, T.J., Haugaard, R., Lalonde, S. V., Partin, C.A., Oonk, P.B.H., Tsikos, H., Lyons, T.W., Bekker, A., Johnson, C.M., 2017. Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history. Earth-Science Reviews 172, 140–177. Steiner, A. E., Conrey, R. M., Wolff, J. A. 2017. PXRF calibrations for volcanic rocks and the application of in-field analysis to the geosciences. Chemical Geology, 453: 35-54. 58 �Critical Minerals Exploration and Development Potential in Ontario PUUMALA, Mark and CUNDARI, Robert Ontario Geological Survey, Ministry of Energy, Northern Development and Mines, Resident Geologist Program, Suite B002, 435 James Street South, Thunder Bay, Ontario, P7E 6S7 On March 10, 2021, Ontario released a discussion paper that outlines the province’s proposal to develop a critical minerals strategy (Ministry of Energy, Northern Development and Mines 2021). The discussion paper is focused on: 1. Supporting partnership opportunities with Indigenous peoples; 2. Finalizing an Ontario critical minerals list; 3. Enhancing investment in mineral exploration and development; 4. Regulatory and policy reform; and, 5. Supply chain and manufacturing opportunities. Public consultation is underway, with the strategy scheduled for release by the end of 2021. While there is no universal definition of critical minerals and various jurisdictions define them differently, the term generally applies to minerals that have specific industrial, technological and strategic applications for which there are few viable substitutions. These minerals are also at higher supply risk due to geopolitical considerations and market demand. Ontario is uniquely positioned to meet rising global demand for critical minerals and can attract potential investment opportunities by creating a provincial critical minerals list to guide mineral exploration and development. An Ontario critical minerals list will be of interest to jurisdictions that are seeking to secure a reliable supply of raw materials for their own domestic markets, such as the United States, the European Union, Japan and South Korea. Ontario has created a draft list of critical minerals that includes commodities that are currently being produced commercially at existing mines, commodities that have a reasonable prospect of being developed in the near term (i.e., advanced projects that could produce within 5 years), commodities that have demonstrated exploration potential, and commodities that were not originally mined in Ontario, but are being smelted or refined here. The entire list is provided in Table 1, with the critical minerals that are in common with the United States draft list (Fortier et al. 2018) indicated in bold type. Ontario’s current critical mineral production is derived from magmatic copper-nickelcobalt-platinum group element (Cu-Ni-Co-PGE) deposits (8 Sudbury mines and Lac des Iles) and volcanogenic massive sulphide zinc-copper (Kidd Creek) deposits. The Sudbury mines also produce selenium and tellurium as by-products, while indium is a by-product at Kidd Creek. Due to current high prices for platinum group elements (most notably palladium) and battery metals (which include nickel and cobalt), current exploration interest in magmatic Cu-Ni-Co-PGE deposits is strong in Ontario, with numerous active projects. These include several in the Midcontinent Rift area that are primarily targeting PGEs, including the Clean Air Metals Inc. Thunder Bay North project, where the Escape Lake deposit is currently being delineated, and Generation Mining Limited’s Marathon project, which is undergoing an Environmental Assessment to support future mine development. Another notable advanced-stage magmatic sulphide project is Canada Nickel Company’s Crawford Nickel-Cobalt project near Timmins, where a high-tonnage low-grade resource has been defined in an Archean ultramafic intrusion. Strong interest in battery metals has also fuelled significant exploration interest in lithium and cobalt in recent years. Numerous areas in the Superior Province of northwestern Ontario are known to host lithium-bearing rare element pegmatites. These deposits are associated with fertile peraluminous granites (Breaks et al. 2003), with notable examples currently moving toward 59 �development in the Pakeagama Lake (Frontier Lithium Inc.), Separation Rapids (Avalon Advanced Materials Inc.) and Georgia Lake (Rock Tech Lithium Inc.) areas. Recent cobalt exploration has largely been focussed on the Proterozoic 5-element vein systems of the Cobalt Embayment, where First Cobalt Corp. is developing a refinery to produce battery-grade cobalt sulphate. Some cobalt exploration has also occurred in the Thunder Bay area, where Honey Badger Exploration Inc. recently discovered a new style of disseminated cobalt mineralization in close proximity to silver-bearing veins at the Beaver Mine, near the contact between the Rove Formation and a diabase sill (Puumala et al. 2019). Another previously unrecognized style of cobalt-coppernickel mineralization that has been described as a possible skarn or IOCG occurrence is hosted in altered calcareous sedimentary rocks of the Rossport Formation north of Thunder Bay, near Disraeli Lake (Ontario Geological Survey 2019). Both deposit types present new critical mineral research and exploration opportunities in Ontario. Table 1. Ontario’s draft list of critical minerals. Asterisk indicates commodity also processed in Ontario. Critical minerals listed in bold type are also found on the United States draft list (Fortier et al. 2018). Producing Cobalt * Copper * Indium Nickel * Platinum Group Elements * Selenium * Tellurium * Zinc Advanced Projects Barite Chromite Graphite Lithium Magnesium Niobium Exploration Potential Antimony Beryllium Bismuth Cesium Fluorspar Manganese Molybdenum Phosphate Rare Earth Elements Tantalum Tin Titanium Tungsten Vanadium Zirconium Processing Only Uranium References Breaks, F.W., Selway, J.B. and Tindle, A.G. 2003. Fertile peraluminous granites and related rare-element mineralization in pegmatites, Superior Province, northwest and northeast Ontario: Operation Treasure Hunt; Ontario Geological Survey, Open File Report 6099, 179p. Fortier, S.M, Nasser, N.T., Lederer, G.W., Brainard, J., Gambogi, J. and McCullough, E.A. 2018. Draft critical mineral list – summary of methodology and background information; United States Geological Survey, Open File Report 2018-1021, 15p. Ministry of Energy, Northern Development and Mines 2021. Critical minerals framework discussion paper; Queen’s Printer for Ontario, 32p. Ontario Geological Survey 2019. Caro Lake; Mineral Deposit Inventory Record No. MDI000000002293. Puumala, M.A., Campbell, D.A., Tuomi, R.D., Fudge, S.P., Pettigrew, T.K. and Hinz, S.L.K. 2019. Report of Activities 2018, Resident Geologist Program, Thunder Bay South Regional Resident Geologist Report: Thunder Bay South District; Ontario Geological Survey, Open File Report 6353, 109p. 60 �Environmental Control of Seawater Geochemistry in a Mesoarchean Peritidal System, Woman Lake, Superior Province 1 RAMSAY, Brittany, 1FRALICK, Philip, 2LALONDE, Stefan, 1BIELSKI, Paul, and 2PATRY, Laureline 1Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada 2Ocean Geosciences Laboratory, European Institute for Marine Studies, Technopôle Brest, Place Nicolas Copernic, 29280 Plouzané, France The 2.857±5 Ga (this study) carbonate platform at Woman Lake, Ontario, Canada, presents a unique opportunity to fill a 130 million year knowledge gap on early carbonate sedimentology and ocean chemistry between similar platform occurrences at Steep Rock Lake (2.80Ga)(Fralick & Riding, 2015) and Red Lake (2.93Ga)(MacIntyre & Fralick (2017). Woman Lake carbonates are among the few very early and thick carbonate platforms to develop in the Mesoarchean. Field, petrographic, and geochemical investigations were performed on the limestone sequence to better understand the paleoenvironmental context of this understudied, 90-meter-thick succession. At the base of the carbonate platform, lying atop felsic subaerial Archean tuff, are stratiform stromatolites interbedded with thin beds of massive carbonate grainstone, followed by laterally linked low domal stromatolites, which gradually become larger domes, then bioherms with walled pseudocolumnar stromatolites. They are overlain by cross-stratified and parallel laminated carbonate grainstones and more pseudocolumnar stromatolites. A variety of fenestral microbialites overly this unit, including thrombolites, stromatactis-bearing low domal stromatolites, and narrow isolated columnar stromatolites. This is followed by a cyclic succession of low domal stromatolites alternating with microbial carbonate and carbonate grainstone. Three main stromatolitic morphologies exist and represent a range of low to moderate current energies from upper intertidal to subtidal environments. They are: 1) low relief stratiform to undulating stromatolites 2) laterally linked low domal and pseudocolumnar stromatolites, and 3) isolated to locally isolated domes and narrow columnar stromatolites. Evidence here supports mainly peritidal environments on a carbonate platform with fluctuating sea-level and water energies in an overall deepening succession. The diverse carbonate facies are comprised of geochemical features reminiscent of both Archean and modern signatures in shale normalized REE patterns. Trace elements indicate that the carbonates precipitated from a mixture of two different fluids: anoxic seawater that carried a positive Eu anomaly, and oxygenated waters that imparted significant negative Ce anomalies. On a microscopic scale, using LA-ICP-MS, there is less compositional contrast between carbonate phases, which indicates that dissolution and precipitation on a small spatial scale homogenized localized areas, but did not affect changes on a metric scale. Geochemical trends paired with stratigraphic depth show decameter cycles of gradual declines in Mg, Fe, Mn, Ba and Sr substitution into the calcite lattice followed by sharp increases throughout the platform’s deposition, possibly reflecting changing accommodation space effecting precipitation rate (Fig. 1). Typical Archean values for δ13C ranging from -3.83‰ to 1.30‰, with an average of 0.53‰ (±0.59, n=31) occur with Y/Ho ratios ranging from 27 to 117 and 87Sr/86Sr isotopic values from 0.700346 to 0.711313 (±0.00098 (1σ)). The observed trends suggest that the precipitating carbonates were able to record and retain the effects of an evolving water column had in the local environment. Importantly, the Woman Lake carbonate platform provides context for, and evidence of, free oxygen approximately 500 million years before the Great Oxygenation Event, during a relatively undocumented period in time. 61 �Fig. 1. Five meter moving averages of molar weight ratios normalized to Ca are plotted against stratigraphic depth. Ratios are of common carbonate group elements that substitute into various carbonate minerals. Three cycles of increasing and decreasing concentrations are evident. Decreasing trends represent times of higher substitution rates and an increase represents lower substitution rates relative to Ca (numerator). Fralick, P., & Riding, R. (2015). Steep Rock Lake: Sedimentology and geochemistry of an Archean carbonate platform. Earth-Science Reviews, 151, 132–175. https://doi.org/10.1016/j.earscirev.2015.10.006 MacIntyre, T., & Fralick, P. (2017). Sedimentology and Geochemistry of the 2930 Ma Red LakeWallace Lake Carbonate Platform, Western Superior Province, Canada. The Depositional Record, 3(2), 258–287. https://doi.org/10.1002/dep2.36 62 �Geochemistry and Petrography of Volcanic and Intrusive Rocks Hosting the Lynne Cu-ZnPb Deposit, Oneida County, WI SHORT, Shelby R., GLODOWSKI, Lillian N., and LODGE, Robert W.D. Department of Geology, University of Wisconsin-Eau Claire, Eau Claire, WI The Lynne Zn-Cu-Pb deposit was first discovered in 1990 by Noranda Exploration in Oneida County, WI and is one many volcanogenic massive sulfide (VMS) deposits in northern Wisconsin (DeMatties, 1994). These deposits formed 1.8 to 1.9 Ga when the Pembine-Wausau Terrane collided and accreted onto the Superior Craton during the Paleoproterozoic Penokean Orogeny. VMS deposits are created in extensional submarine environments that are preserved in the rock record in a variety of tectonic settings. The VMS-forming environment in Wisconsin has previously been interpreted to be back-arc rifting in a continental setting during the collision of the Archean Marshfield Terrane onto the Pembine-Wausau Terrane and the rest of the Superior Craton (Schulz and Cannon, 2007). However, studies at other deposits throughout Wisconsin have highlighted variable volcanic and tectonic settings (e.g. Jackson et al, 2016; Jacobson and Lodge, 2018). The purpose of this study is to better constrain the volcanic and tectonic setting at the Lynne deposit and improve tectonic and metallogenic models across the Penokean Orogen. Research on Penokean VMS systems depends on drill core as outcrop exposure in the region is limited due to a thick cover of glacial deposits and Paleozoic Sedimentary strata (Dematties, 1994; Adams, 1996). The Noranda drill cores from the Lynne deposit were obtained from the archives at the Natural Resources Research Institute of Duluth, Minnesota and re-logged and sampled at an off-campus lab associated with UW-Eau Claire. Core samples from the leastaltered felsic host rocks and various intrusive rocks were then processed for petrographic and geochemical analyses. Major and trace elements were analyzed using X-ray Fluorescence at the Materials Science Center at UW-Eau Claire. Interpretations from this new geochemical data yielded an improved volcanic stratigraphy and tectonic model that describes the volcanic system forming the Lynne VMS deposit. The stratigraphy of the Lynne deposit was described by Adams (1996) based on drilling during the exploration program. The stratigraphy consists of the Upper Rhyolite, Upper Dacite, Upper Volcaniclastic (VCS), Lower Rhyolite, Lower Dacite, and Lower Volcaniclastic based on their relative position to the ore horizon (Adams, 1996). After a geochemical analysis of the felsic volcanic units was conducted, it revealed that there are no chemical distinction between the upper and lower horizons. Therefore, unit descriptions are constrained by composition and not stratigraphy. Rhyolites are crystal rich lapilli tuff to ash with plagioclase being the most abundant crystal. The VCS unit is greywackes and siltstones with some bedding and localized alteration. The Dacite unit is a crystal to crystal-lithic lapilli tuff with plagioclase crystals. The Lynne drill core also intersects felsic and mafic dykes, along with a large granophyre/granodiorite pluton at the bottom of the deposit which is geochemically indistinguishable from intersecting felsic dykes. The felsic dykes are fine grained and light to medium grey in color and contain plagioclase and quartz veins. The mafic dykes appear fine grained, dark grey in color, and contain plagioclase phenocrysts. The granodiorite/granophyre contains medium- to coarse-grained quartz and feldspars which vary between the phenocrystic granophyre near the contact to equigranular away from the contact region. The geochemical signature of the intrusive units of the Lynne deposit indicate that they are derived from two distinct 63 �sources that formed in a volcanic arc setting much like the extrusive volcanic stratigraphy. Therefore, it is likely that they are genetically related. REFERENCES Figure 1: Generalized stratigraphic column of the Lynne Deposit (from Quigley, 2016) with core pictures from representative volcanic and intrusive units. A. Upper Dacite B. Upper Rhyolite C. Upper VCS D. Felsic Dyke E. Lower VCS F. Lower Rhyolite Adams, G.W., 1996, Geology of the Lynne base-metal deposit, north-central Wisconsin, U.S.A., in LaBerge, G.L., ed., Volcanogenic massive sulfide deposits of northern Wisconsin: A commemorative volume: Institute on Lake Superior Geology Proceedings, 42nd Annual Meeting, Cable, WI, v. 42, part 2, p. 161-179. DeMatties, T.A., 1994, Early Proterozoic volcanogenic massive sulfide deposits in Wisconsin: An overview. Economic Geology, 89: 1122-1151. Jackson, N.R., Moura Merss, B.H., and Lodge, R.W.D., 2016, Lithostratigraphy and Ore Petrology of the Eisenbrey Zn-Cu-Pb Deposit, Rusk County, Wisconsin: Institute on Lake Superior Geology Proceedings, 62 nd Annual Meeting, Duluth, MN. Jacobson, R.E. and Lodge, R.W.D., 2018, Reconstructing Paleoproterozoic volcanism in northwestern Wisconsin: Geochemistry of the Flambeau Cu-Zn-Au Mine: Institute on Lake Superior Geology Proceedings, 64 th Annual Meeting, Iron Mountain, MI. Schulz, K.J. and Cannon, W.F., 2007, The Penokean orogeny in the Lake Superior region. Precambrian Research 157: 4-25. Quigley, P.O., 2016, The Spectrum of Ore Deposit Types, their Alteration and Volcanic Setting in the Penokean Volcanic Belt, Great Lakes Region, USA, Colorado School of Mines, Master’s Thesis, 2016 64 �Geologic mapping identifies bedrock folds that may be significant for increased probability of arsenic detection in water wells STEWART, Esther K.1, FITZPATRICK, Billy1, and STEWART, Eric D.1 1 Wisconsin Geological & Natural History Survey, 3817 Mineral Point Road, Madison, WI 53705 Recently completed bedrock mapping of Dodge County, southeast Wisconsin, provides an improved geologic framework that informs the region’s geologic history and adds to our understanding of groundwater resources (Stewart, E.D. et al., 2021; Stewart, E.K. 2021). Dodge County is underlain by glacial sediments, Precambrian Quartzite, and Cambrian through Silurian siliciclastics and carbonates and is situated proximal to the WI Arch, a basement high that influenced Paleozoic basin development. We identify bedrock folds that likely increase the probability of arsenic (As) detection in groundwater wells. New petrographic observations constrain the As host and suggest processes that resulted in As mineralization. Mapping indicates episodic movement along the WI Arch influenced deposition of Paleozoic strata. Most significant deformation during early Ordovician time resulted in localized uplift and erosion of the dolomitic Prairie du Chien Group prior to deposition of the overlying siliciclastic Ancell Group. Continued deformation is associated with folding of the overlying Sinnipee Group, fracture development, and mineralization. Marcasite and carbonate-filled fractures in one locality along an anticlinal axis show well developed cataclasis evidencing deformation during and after formation of trace MVT occurrences. Most elevated hydraulic conductivity (K) estimates from the Ancell Group are within 2 km of mapped folds, indicating fractures associated with folds locally increases groundwater flow (Stewart, E.D. et al., 2021). Water wells completed in the Ancell Group in Dodge County have increased likelihood of detecting As close to folds, suggesting fold induced fractures promoted a focus or change in As host minerals that increased the probability of arsenic release. No As was detected in SEM-EDS analysis of oxide phases, while two sulfide As hosts were identified in mineralized core samples: (1) Small (10-20 µm), cubic, Ni-Co-Fe sulfides with up to 3 wt.% As are scattered within marcasite that infills localized fractures and sulfide cement in Cambrian Elk Mound Group sandstone . These crystals are similar in morphology, size, and chemistry to Ni-Co-Fe sulfides identified as VaesiteBravoite from trace MVT-mineralized samples in Brown and Oconto Counties reported by Luczaj et al., 2016. (2) Tiny (3-5 µm), circular, rimmed Fe-sulfides with 1.4-2.2 wt.% As are only present within thin clay rims on quartz grains in the Ancell Group sandstone , in contrast to all other sulfides observed from the same sample, which lack detectable As. Rimmed grains may be the mineralized remains of specialized microbacterial colonies that metabolically reduced dissolved aresenate and sulfate to produce As-bearing Fe-sulfide (e.g. Stolz et al., 2006). Observation of Fesulfide frambroids supports a biologic control on sulfide mineralization. High-resolution SEMBSE imaging reveals common As-poor, Fe-sulfides present as hexagonal, small to large, euhedral marcasite plates and subordinate framboidal clusters of tiny Fe-sulfide crystals . Framboids are common as isolated clusters encased within larger euhedral sulfide or as nucleation points for single grains of euhedral sulfide. Framboidal sulfide clusters are commonly interpreted as mineralized remains of sulfate-reducing microbacterial colonies (e.g. Folk, 2005), and sulfatereducing bacterial colonies are known to seed adjacent areas with reduced sulfur allowing for inorganic precipitation of larger volumes of sulfide cement (Schieber, 2002). Bacterial reduction 65 �of sulfate sourced from regionally migrating hydrothermal brines coming out of the Michigan Basin is proposed as a broader mechanism that drove precipitation of Fe-sulfides forming the bulk of mineralization within the trace MVT occurrences of Dodge County. References: Folk, R.L., 2005, Nanobacteria and the formation of framboidal pyrite: Textual evidence: Earth System Science, v. 114, no. 3, p. 369-374. Luczaj, J.A., McIntire, M.J., and Olson-Hunt, M.J., 2016, Geochemical characterization of trace MVT mineralization in Paleozoic sedimentary rocks of northeastern Wisconsin, USA: Geosciences, v. 6, no. 29. Schieber, J., 2002, The role of an organic slime matrix in the formation of pyritized burrow trails and pyrite concretions: PALAIOS, v. 17, p. 104-109. Stewart, E.D., Stewart, E.K., Bradbury, K.R., and Fitzpatrick, W., 2021, Correlating bedrock folds to higher rates of arsenic detection in groundwater, SE Wisconsin, USA, Groundwater. Stewart, E.K., 2021, Bedrock geology of Dodge County, 1:100,000-scale. Wisconsin Geological and Natural History Survey map series, plate 1. Stolz, J.F., Basu, P., Santini, J.M., and Oreland, R.S., 2006, Aresenic and selenium in microbial metabolism: Annual Reviews in Microbiology, v. 60, p.107-130. Figure 1: (A) Electron backscatter image of marcasite-cemented vertical zone within Elk Mound group. Frame is centered on an unusual cubic Fe-Ni-Co-As sulfide grain. (B) Electron backscatter image of unusual rimmed, circular As-bearing Fe-sulfides from Tonti Member of the St. Peter Formation, Ancell Group. Smaller, unrimmed grains outboard of main rimmed sulfides contain detectable As but at reduced concentration relative to main rimmed grain (<.5 wt%). Platy medium gray matrix enclosing sulfides is a thin rim of kaolinitic clays. Light gray at bottom right of image is edge of detrital quartz grain. (C) Electron backscatter image of single framboidal cluster at high magnification illustrating tiny individual grains making up structure. (D) Electron backscatter image of cluster of sulfide grains within Tonti Member of St. Peter. Note how framboidal clusters appear to be serving as nucleation points for larger, euhedral hexagonal sulfide grains. 66 �The paleogeography of Laurentia in its early years: new constraints from the Paleoproterozoic East-Central Minnesota batholith SWANSON-HYSELL, Nicholas L., AVERY, Margaret S., ZHANG, Yiming, HODGIN, Eben B. Department of Earth and Planetary Science, University of California, Berkeley BOERBOOM, Terrence, J. Minnesota Geological Survey, St. Paul, MN, USA The ca. 1.83 Ga Trans-Hudson orogeny resulted from collision of an upper plate consisting of the Hearne, Rae, and Slave provinces with a lower plate consisting of the Superior province. While the geologic record of ca. 1.83 Ga peak metamorphism within the orogen suggests that these provinces were a single amalgamated craton from this time onward, a lack of paleomagnetic poles from the Superior province following Trans-Hudson orogenesis has made this coherency difficult to test. Figure 2: Map of Laurentia showing the location of Archean provinces and younger Proterozoic crust (simplified from Whitmeyer and Karlstrom 2007)). The localities of paleomagnetic poles that constrain Laurentia’s position just after its amalgamation are shown with stars including the new pole from this study developed from the EastCentral Minnesota Batholith (ECMB). This right map shows interpreted Precambrian geology for the state of Minnesota (simplified from Jirsa et al., 2012)) including in regions covered by Phanerozoic sedimentary rocks where the bedrock is inferred from geophysical data and drill cores. We develop a high-quality paleomagnetic pole for northeast-trending diabase dikes of the post-Penokean orogen East-Central Minnesota Batholith (Holm et al., whose age we constrain to be 1779.1 ± 2.3 Ma (95% CI) with new U-Pb dates. Demagnetization and low-temperature magnetometry experiments establish dike remanence be held by low-Ti titanomagnetite. Thermochronology data constrain the intrusions to have cooled below magnetite blocking temperatures upon initial emplacement with a mild subsequent thermal history within the stable craton. The similarity of this new Superior province pole with poles from the Slave and Rae provinces establishes the coherency of Laurentia following Trans-Hudson orogenesis. This consistency supports interpretations that older discrepant 2.22 to 1.87 Ga pole positions between the provinces are the result of differential motion through mobile-lid plate tectonics. The new pole supports the NENA connection between the Laurentia and Fennoscandia cratons. The pole can be used to jointly reconstruct these cratons ca. 1780 Ma thereby strengthening the paleogeographic position of these major constituents of the hypothesized late Paleoproterozoic supercontinent Nuna. 67 �Figure 2: Paleogeographic reconstructions at five different times in the Paleoproterozoic and the position of the provinces at present. Paleomagnetic poles within 20 Myr of the given time (10 Myr for 1888 and 1868 Ma) are shown from the compilation of Evans et al. (2021) as well as the new ECMB pole. These data illustrate differential plate motion between the Superior and Slave Provinces that is required by the data leading up to the closure of the Manikewan Ocean and the assembly of Laurentia during the Trans-Hudson orogeny. The ECMB pole is consistent with an assembled Laurentia following the Trans-Hudson orogeny which contrasts with the disparate orientations and paleolatitudes between Laurentia’s constituent provinces prior to the orogeny. Evans, D. A. D., Pesonen, L. J., Eglington, B. M., Elming, S.-Å., Gong, Z., Li, Z.-X., … Zhang, S. (2021). An expanding list of reliable paleomagnetic poles for Precambrian tectonic reconstructions. In Ancient supercontinents and the paleogeography of the Earth. Holm, D. K., Van Schmus, W. R., MacNeill, L. C., Boerboom, T. J., Schweitzer, D., & Schneider, D. (2005). U-Pb zircon geochronology of Paleoproterozoic plutons from the northern midcontinent, USA: Evidence for subduction flip and continued convergence after geon 18 Penokean orogenesis. Geological Society of America Bulletin, 117(3), 259–275. doi: 10.1130/b25395.1 Jirsa, M., Boerboom, T., & Chandler, V. (2012). S-22, Geologic Map of Minnesota, Precambrian Bedrock Geology (Tech. Rep.). Minnesota Geological Survey. Whitmeyer, S., & Karlstrom, K. (2007). Tectonic model for the Proterozoic growth of North America. Geosphere, 3(4), 220–259. doi: 10.1130/GES00055.1 68 �The provenance, depositional environment and metallogenic implications of the Ament Bay Metasedimentary Assemblage, Sturgeon Lake Greenstone Belt, Northwest Ontario TAMOSAUSKAS, Michael1, LODGE, Robert2, MA, Chong1, HAUGAARD, Rasmus, SHERLOCK, Ross1 1 Mineral Exploration Research Centre (MERC), Harquail School of Earth Sciences, Goodman School of Mines, Laurentian University, 935 Ramsey Lake Rd., Sudbury, ON, P3E 2C6, Canada 2 Department of Geology, University of Wisconsin-Eau Claire, 105 Garfield Ave, Eau Claire, WI 54701, United States The Ament Bay Metasedimentary Assemblage (ABMA) is the youngest supracrustal assemblage in the Savant Lake-Sturgeon Lake greenstone belt (SSGB), and has previously been interpreted to have alluvial-fluvial origin (Sanborn-Barrie and Skulski, 2005). However, its timing, tectonic setting, and metallogenic significance is unclear. The SSGB makes up the easternmost part of the Western Wabigoon terrane of the Superior Province. The supracrustal assemblages in this belt form a regional-scale syncline, with the ABMA exposed along the hinge, and is intersected by the Sturgeon Lake fault zone (fig. 1). The ABMA is surrounded by felsic and mafic volcanic rocks that compose the Central and South Sturgeon Assemblages, however its contact relationships are poorly understood (Sanborn-Barrie and Skulski, 2005). Figure 1. Regional geology of the Sturgeon Lake greenstone belt. Modified after Sanborn-Barrie and Skulski (2005). Inset map showing close-up view of central portion of ABMA (outlined). 69 The goal of this study is to determine if the ABMA represents a Timiskaming-type basin. Timiskamingtype basins are found in various Neoarchean greenstone belts throughout the Superior. These are synorogenic basins which consist of clastic rocks with fluvial and marine �origins representing molasse basin-fill successions (Hyde, 1980; Thurston and Chivers, 1990; Mueller and Corcoran, 1998). These basins are commonly coeval with alkalic magmatism, as alkalic rocks can be both interbedded and intruded into the sediments, however they are also incorporated as clasts. Timiskaming-type basins are of interest because of their association with gold mineralization, as numerous prolific gold occurrences in the Abitibi greenstone belt are hosted within Timiskaming Group sediments (Bleeker, 2012). The ABMA consists of many features consistent with Timiskaming-type basins, as it is interpreted as a molasse basin-fill succession and Figure 2. Close-up view of central portion of the ABMA and its interpreted lithofaices. is spatially associated with alkalic magmatism. The ABMA consists of mostly weakly foliated, massive- to- planar bedded polymictic conglomerates and arkosic- to- lithic-rich arenites and greywackes. Based on petrographic analysis and lithology distribution, three lithofacies have been attributed to the ABMA: (i) arkosic greywacke lithofacies; (ii) polymictic conglomerate-sandstone lithofacies; (iii) lithic greywackemafic-rich conglomerate lithofacies (fig. 2). These are comparable to some lithofacies which make up other molasse basin-fill successions, some of which are Timiskaming-type (Corcoran and Mueller, 2007). Despite having similar characteristics with Timiskaming-type basins, the ABMA is poorly endowed relative to some of the gold-rich Timiskaming Group sediments in the Abitibi greenstone belt. This study explores possible factors which hindered gold mineralization in the Sturgeon Lake greenstone belt. References Bleeker, W., 2012. Lode gold deposits in Deformed and metamorphosed terranes: The role of Extension in the Formation of Timiskaming Basins and Large Gold Deposits, Abitibi Greenstone Belt–A Discussion, in Parker, J., ed., Summary of Field Work and Other Activities 2012: Ontario Geological Survey, Open File Report 6280, p. 47-41 to 47-12 Corcoran, P., & and Mueller, W., 2007. Time-Transgressive Archean Unconformities Underlying Molasse Basin-Fill Successions of Dissected Oceanic Arcs, Superior Province, Canada. Journal of Geology, volume 115, p. 655–674. Hyde, R.S., 1980. Sedimentary facies in the Archean Timiskaming Group and their tectonic implications, Abitibi greenstone belt, northeastern Ontario, Canada; Precambrian Research, v.12, p.161-195. Mueller, W.U. and Corcoran, P.L., 1998. Late-orogenic basins in the Archean Superior Province, Canada: Characteristics and inferences; Sedimentary Geology, v.120, p.177-203. Sanborn-Barrie, M. and Skulski, T. 2005. Geology, Sturgeon Lake greenstone belt, western Superior Province, Ontario; Geological Survey of Canada, Open File 1763, scale 1:100 000. Thurston, P.C., Chivers, K.M., 1990. Secular variation in greenstone sequence development emphasizing Superior Province, Canada. Precambrian Res. 46, 21 58. 70 �Possible implications of a non-Archean Grand Marais Ridge, western Lake Superior WOODRUFF, L.G. 1, and GRAUCH, V.J.S. 2 1 U.S. Geological Survey, 2280 Woodale Drive, St. Paul, MN, 55112 U.S. Geological Survey, MS 973, Federal Center, Denver, CO, 80225 2 The Grand Marias Ridge (GMR) is a gravity low but a structural high hidden beneath the waters of western Lake Superior. It has been interpreted as a preserved basement block of low-density Archean granite that remained a positive topographic high as adjacent volcanic basins subsided (Allen et al., 1997). Recent geophysical analysis (Grauch and Heller, 2021, this volume), however, suggests that the GMR likely comprises a significant volume of anorthosite, possibly mixed with felsic rocks. This part of the Midcontinent Rift System (MRS) in western Lake Superior is an area where many unusual features occur, including extraordinarily large concentrations of rapidly intruded mafic intrusions of the Duluth and Beaver Bay Complexes (DC and BBC), and a high percentage of felsic rocks in the North Shore Volcanic Group (NSVG) compared to other MRS volcanic sections. The thick basalt and overlying sedimentary packages in western Lake Superior also occur in a series of sag basins rather than the graben-like basins that characterize the MRS in the nearby St. Croix horst and the western arm extending away from Lake Superior. Here we address implications of recent modeling results and speculate on the potential significance of a GMR composed of anorthosite and felsic rocks, as well as the evolution of the MRS in this unique part of the Lake Superior region. Perhaps the most challenging aspect of this new compositional interpretation is how the GMR came to be a topographic high if it is not a block of remnant Archean basement. 1)What is the origin of anorthosite in the GMR? Option 1. Anorthosite represents pre-MRS basement rocks (e.g., Allen et al., 1997). This age is possible but unlikely as the regional Archean basement is granite-greenstone with little reported major units of anorthosite, and thus does not match the observed geophysical characteristics. Option 2. Anorthosite is part of the large complex suite of plagioclase-rich cumulate intrusions of the DC. Anorthosite intrusions are widespread throughout the DC, from the exposed western margin to where it disappears under the younger BBC and the NSVG. It is plausible that anorthosite in the GMR is an eastern extension of the DC, although it is challenging to determine how an older, deeper DC anorthosite pluton would now represent a high structural block. Option 3. The striking presence of abundant and, in places, very large anorthosite inclusions in the BBC Beaver River gabbro has been attributed to incorporation at depth of older DC anorthosite (Miller and Chandler, 1997). A recent paleomagnetic intensity study by Zhang (2020) found that anorthosite inclusions and host Beaver River diabase have similar paleomagnetic poles, which suggests that inclusions could be related to the BBC rather than the slightly older DC. Thus, anorthosite in the GMR could be products of BBC igneous events. In either case, anorthosite inclusions appear to be related to the MRS rather than pre-MRS basement and may be evidence of significant anorthosite accumulation at some depth near the GMR. 2)What is the origin of the felsic component in the GMR? Option 1. Archean granitic rocks that were intruded by plagioclase-rich magma during MRS time. Option 2. Partial melting of older basement rocks caused by staging of mafic magmas at depth during MRS time would lead to large accumulations of felsic magma (Vervoort and Green, 1997). Felsic magma could have been incorporated into an anorthosite cumulate melt and intruded higher in the crust. Option 3. The GMR is located just offshore of two of the largest rhyolite lava or ash flows in the NSVG (Green and Fitz, 1993), all temporally related to the DC. Nicholson et al. (1997) suggested that the proximity of large felsic flows on land and the GMR was not coincidental. It is possible that the GMR was a felsic volcanic center during DC time that erupted these large lava flows along with other regional felsic volcanics. 71 �3)What dynamic processes could have created the GMR anorthosite/felsic structural high? Option 1. Anorthosite and layered troctolite-gabbro plutons were intruded as inclined sheets between a footwall of Paleoproterozoic Animikie Group sedimentary rocks and Archean granites and a hangingwall of the NSVG (Miller et al., 2002). Thus, an Archean GMR would have to have been a topographic block within the Animikie Basin, as suggested by Allen et al. (1997). This basement structure could have been intruded by one of the many anorthositic igneous bodies generated during DC magmatic events as well as by felsic magmas derived from partial melting of some basement rocks. Option 2. Recent high precision 206Pb/238U zircon dates show that multiple intrusions of DC were emplaced together around 1096 Ma within less than 1 million years (Swanson-Hysell et al., 2020), suggesting that magma supply from deep crustal chambers for both intrusions and overlying volcanics was effectively continuous. Rapid, repetitive magmatic pulses would have elevated crustal temperatures around DC feeder zones and crustal magma chambers. Vervoort and Green (1997) suggested that the large rhyolite flows of the NSVG were products of crustal melting produced by the large thermal flux of DC magmatism. Perhaps remnants of a felsic volcanic center are located above a now crystallized plagioclase-rich magma chamber in the GMR, although the relative geometry of a middle-crust anorthosite intrusion, an upper crust felsic center, and a resulting GMR topographic high is perplexing. Option 3. A large volcanic sag basin just offshore from the BBC is bounded on the northeast by the GMR and on the southwest by the Archean (probable) structural block of White’s Ridge. The ~14 km-thick basalt section that fills this bowl-shaped basin, which is not fault-bounded as shown by Grant-Norpac legacy seismic profiles, was likely erupted from the curvilinear Beaver River diabase dike and related sheets that acted as major conduits for igneous activity (Miller and Chandler, 1997). Loading by a rapid succession of relatively dense basalt flows into an initially thermally subsiding basin could create an unstable environment (e.g., Mukherjee et al., 2020) where less dense underlying plagioclase-rich cumulates could be forced upward into a zone of weakness created by earlier high temperature felsic volcanism. This scenario, however, would require ductile behavior relatively high in the crust, but could explain how MRS anorthosite and felsic rocks became a topographic high as adjacent volcanic basins subsided. Perhaps localization of magmatic events over a short duration in this area created this seemingly implausible scenario. Allen, D. J., Hinze, W. J., Dickas, A. B., Mudrey, M. G., Jr., 1997, Integrated geophysical modeling of the North American Midcontinent Rift System: New interpretations for western Lake Superior, northwestern Wisconsin, and eastern Minnesota: Geol. Soc. Amer. Special Paper 312, 47-72. Grauch, V.J.S., Heller, S., 2021, Integration of geophysical evidence indicates that anorthosite composes a significant portion of Grand Marais ridge, an inferred basement high in western Lake Superior: this volume. Green, J.C., Fitz, T.J., III, 1993, Extensive felsic lava and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota: petrographic and field recognition: Jour. Volcan. Geotherm. Res., 54, 177-196. Miller, J.D., Jr., Chandler, V.W., 1997, Geology, petrology, and tectonic significance of the Beaver Bay Complex, northeastern Minnesota: Geol. Soc. Amer. Special Paper 312, 73-96. Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota: Minn. Geol. Sur. Report Invest. 58, 207 p. Mukherjee, A.B., Das, S., Sen, D., Bhattacharya, B., 2020, Buoyant rise of anorthosite from a layered basic complex triggered by Rayleigh-Taylor instability: Insights from a numerical modeling study: Amer. Min., 105, 437-446. Nicholson, S.W., Shirey, S.B., Schulz, K.J., Green, J.C., 1997, Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development: Can. Jour. Earth Sci., 34, 504520. Swanson-Hysell, N.L., Hoaglund, S.A., Crowley, J.L., Schmitz, M.D., Zhang, Y., Miller, J.D., Jr., 2020, Rapid emplacement of massive Duluth Complex intrusions within the North American Midcontinent Rift: Geol., 49, 185-189. Vervoort, J.D., Green, J.C., 1997, Origin of evolved magmas in the Midcontinent rift system, northeastern Minnesota: Nd-isotope evidence for melting of the Archean crust: Can. Jour. Earth Sci., 34, 521-535. Zhang, Y., 2020, Pairing paleointensity results with coercivity spectra: providing support for selection criteria: Inst. Rock Mag. Quar., 30, 2-4. 72 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings 2021 Description An account of the resource Institute on Lake Superior Geology. Virtual meeting. May 11, 14, 18, 21, 2021. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology. Date A point or period of time associated with an event in the lifecycle of the resource 2021 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/c8c4c3103eec111ade66a79fd1788f93.pdf 21961203f4cc0eba4f8e4a0fe3ee69de https://digitalcollections.lakeheadu.ca/files/original/522bc7409122f20f5388a823c7517dab.pdf 665265caad9a54ea311fb8a943ae7153 PDF Text Text 68th ANNUAL MEETING Sudbury, Ontario — May 10-11, 2022 INSTITUTE ON LAKE SUPERIOR GEOLOGY Part 2 — Field Trip Guidebook �Thank you to our sponsors! INDIVIDUAL CONTRIBUTORS TO STUDENT TRAVEL SCHOLARSHIP: MARY KAY ARTHUR, AL MACTAVISH, MARK & LAURIE SEVERSON JIM DEGRAFF, MICHAEL & MONICA EASTON, DICK HEGLUND JIM DEGRAFF, BOB MAHIN, MIKE BEAUREGARD JOANNA HODGE, TERRY BOERBOOM, JIM MILLER BEN BERGER, DEAN PETERSON, GRAHAM WILSON �Proceedings of the 68th ILSG Annual Meeting – Part 2 68th ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY May 10-11, 2022 Sudbury, Ontario HOSTED BY Michael Easton and Wouter Bleeker Co-Chairs Ontario Geological Survey and Geological Survey of Canada Proceedings - Volume 68 Part 2 – Field Trip Guidebook Compiled and edited by Michael Easton Cover Photos. Upper Left — Signage at the Sudbury ore discovery site (Trips 1 and 4). Upper Right— Footwall Breccia in the Crean Hill Mine area (Trip 3). Lower Left — Arkosic sandstone overlain by matrix- to clast-supported conglomerate, Gowganda Formation. On Highway 108 north of Elliot Lake (Trip 5). Lower Right — Deformed, migmatitic gneiss in the Grenville Front tectonic zone showing garnet dispersed throughout the rock. Scale card is 9 cm long. On Highway 537 southeast of Sudbury (Trip 2). �Proceedings of the 68th ILSG Annual Meeting – Part 2 �Proceedings of the 68th ILSG Annual Meeting – Part 2 68th INSTITUTE ON LAKE SUPERIOR GEOLOGY VOLUME 68 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD T RIP GUIDEBOOK Trip 1: A TRAVERSE ACROSS THE SUDBURY IMPACT STRUCTURE Trip 2: GEOLOGY OF THE GRENVILLE FRONT AND THE GRENVILLE FRONT TECTONIC ZONE IN THE SUDBURY AREA Trip 3: MAGMATISM AND BRECCIATION IN THE FOOTWALL ROCKS IN THE SOUTHWESTERN SUDBURY STRUCTURE Trip 4: AN OVERVIEW OF THE GEOLOGY OF THE SUDBURY STRUCTURE Trip 5: A CROSS-SECTION THROUGH THE HURONIAN SUPERGROUP AT ELLIOT LAKE, ONTARIO Reference to material in Part 2 should follow the example below: Gordon, C., Généraux, C-A. and Clarke, B. 2022. Magmatism and Brecciation in the Footwall rocks of the southwestern Sudbury Structure; in Easton, R.M. (Ed.), Institute on Lake Superior Geology Proceedings, 68th Annual Meeting, Sudbury, Ontario, Part 2 – Field trip guidebook. v.68, part 2, p.147-180. Published by the 68th Institute on Lake Superior Geology and distributed by the ILSG Secretary: Pete Hollings - ILSG Secretary Department of Geology Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Email: peter.hollings@lakeheadu.ca ILSG website: www.lakesuperiorgeology.org ISSN 1042-9964 �Proceedings of the 68th ILSG Annual Meeting – Part 2 Part 2: Field Trip Guidebook Table of Contents Introduction, safety considerations and acknowledgements 1 Field trip 1 — A traverse across the Sudbury Structure Earth’s largest preserved impact crater (2 days) 2 Field trip 2 — Geology of the Grenville Front and the Grenville Front tectonic zone in the Sudbury area 102 Field trip 3 — Magmatism and brecciation in the Footwall rocks of the southwestern Sudbury Structure 147 Field trip 4 — Overview of the Sudbury Structure 182 Field trip 5 — A cross-section through the Huronian Supergroup at Elliot Lake 200 Figure 1. Map showing the location of the five field trips offered in 2022. vi �Introduction, safety considerations and acknowledgements Michael Easton Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5 and Wouter Bleeker Geological Survey of Canada, 601 Booth Street Ottawa, Ontario K1A 0E8 Sudbury is located near the boundary between 3 major geological provinces (the Archean Superior Province, the Paleoproterozoic Southern Province, and the Mesoproterozoic Grenville Province) and the largest preserved ancient impact crater on Earth. Consequently, it is an ideal setting for geological field trips. Despite the impact of Covid19 related health-measures on meeting planning and organization, 3 pre-meeting and 2 postmeeting field trips were available for delegates to the 68th Annual Institute on Lake Superior Geology (ILSG) meeting in 2022 (see Figure 1, opposite). In the case of Trips 1 and 3, some stops are on property owned by mining companies, who granted special permission to the ILSG trip leaders and participants to access these properties. It will not be possible for the average guidebook user to revisit these stops. We would like to thank all the other authors who contributed to this field guide, all those who provided comments and/or assisted with the running of the field trips themselves (Manuel Duguet, Peter MacDonald, Alinda Aubin, Matthew Eles, and Monica Easton). In addition, the efforts of Johanne Roux and Carlo Casrechino in producing the guidebooks in a timely fashion is greatly appreciated. This volume is intended to serve not only as a guide for 68th ILSG field trip participants but also as a reference for those planning to revisit these areas at a later date. Consequently, we have included UTM coordinates in the NAD 83 datum for stops, as well as instructions on how to reach them. As some of the stops are on private and/or staked land, please be sure to obtain the land owners’ permission before entering their land. Contact the staff of the Resident Geologist Program of the Ontario Geological Survey in Sudbury for current ownership information. We also appreciate the assistance and cooperation of the exploration and mining companies in providing access and information concerning their properties, particularly Vale Canada, Limited Crean Hill Mine, (Whistle Mine), Lonmin PLC, Wallbridge Mining Company Limited (Parkin, Trill, Hess), SPC Nickel Corporation (Worthington), KGHM International (Podolosky), Sudbury Integrated Nickel Operations (a Glencore Company) and North American Nickel (distal Whistle). We also thank the City of Greater Sudbury for providing access to a trip stop on field trip 4. The field trips, for the most part, will be visiting stops along either major highways or municipal roads. Please take care when crossing or parking along these roads. 1 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Frontispiece: Classical “shatter cones” in the shocked footwall and target rocks of the 1850 Ma Sudbury impact crater, well developed in quartzites of the circa 2.4 Ga Mississagi Formation. With the newly recognized knowledge, in the mid- to late-1950s, that these conical, radiating fracture surfaces represent unique “trace fossils” for the high-velocity, extremely high-pressure shock waves associated with meteorite impacts (Dietz, 1959), Sudbury was quickly recognized, in 1962, as an astrobleme—the scar of an ancient impact crater (see Dietz, 1964; and Dietz and Butler, 1964). 2 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Field Trip 1 – A Traverse Across the Sudbury Structure—Earth’s Largest Preserved Impact Crater Wouter Bleeker Ontario Geological Survey of Canada, 601 Booth Street Ottawa, Ontario K1A 0E8 Sandra Kamo Jack Satterly Geochronology Laboratory, University of Toronto, 22 Ursula Franklin Street, Toronto, Ontario M5S 3B1 Henning Seibel and Michael Lesher Mineral Exploration Research Centre, Harquail School of Earth Sciences, Laurentian University, 935 Ramsey Lake Road, Sudbury, ON P3E 2C6 This two-day field trip involves a traverse across the deformed 1850 Ma Sudbury impact structure, from the older country rocks and brecciated target rocks to the south of the preserved melt sheet, across the entire folded impact structure, the melt sheet, and crater fill, and onto its northern rim. Day 1 will concentrate on the regional cross-section/traverse and will introduce many of the key aspects—and controversies— associated with the structure. Day 2 will finish the regional traverse and allow time to examine some of the ore environments in more detail, along the basal contact of the differentiated melt sheet and into the brecciated footwall. Note 1: Ore environments to be examined are dependent on company approval to access properties, as well as the evolving situation regarding COVID-19. Note 2: This trip is a more detailed examination of the Sudbury impact structure than that offered by post-meeting Trip #4. Trip #4 provides more of an overview of the structure. 3 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Day 1 Wouter Bleeker1 and Sandra Kamo2 1 2 Geological Survey of Canada, Ottawa Jack Satterly Geochronology Laboratory, University of Toronto 1. Introduction This two-day fieldtrip will involve a traverse across the deformed Sudbury structure, one of the world’s largest and oldest preserved meteorite impact structures: from deformed and brecciated older country rocks in the south, across the folded impact structure, the melt sheet and crater fill, onto the northern rim, and into the footwall rocks below (Figure 1). Day 1 will focus on the regional cross-section and traverse and will introduce many of the key aspects—and controversies—associated with this unique structure. Day 2 will finish the regional traverse and allow time to examine some of the ore environments in more detail, along the basal contact of the differentiated melt sheet and into the brecciated footwall. Due to the on-going but hopefully waning Covid-19 situation, the issue of access to company properties remains somewhat fluid and some last-minute changes may have to be made in terms of field trip stops and localities. Nevertheless, this two-day trip will allow most major points of interest regarding the Sudbury impact structure to be addressed and discussed. With well over a century of geological research in the area, the literature on the Sudbury structure is voluminous (e.g., Coleman, 1905; Thomson, 1956; Hawley, 1962; Dietz, 1964; Souch et al., 1969; Naldrett et al., 1970; Brocoum and Dalziel, 1974; Krogh et al., 1982; Pye et al., 1984 and all contributions therein; Faggart et al., 1985; Grieve et al., 1991; Dickin et al., 1992; Butler, 1994; Wu et al., 1995; Spray et al., 2004; Lightfoot and Zotov, 2005; Zieg and Marsh, 2005; Ames et al., 2002, 2008a,b; Bleeker et al., 2015; Papapavlou et al., 2018; and numerous other papers listed in the reference list). Yet many key questions remain. For instance, how big was the original impact structure? And where was “Ground Zero”, i.e. the centre of the impact? Is there a global Sudbury fall-out layer and, if so, where is it? How much of the melted footwall geological heterogeneity has been inherited in the differentiated melt sheet? Given the size of the structure and its transient crater, did it trigger any mantle melting? And, with respect to the complex spectrum of observations on the ores and the footwall rocks: where do impact processes and cratermodification processes stop, and where do regional deformation and hydrothermal-metamorphic processes take over? In the context of an Institute of Lake Superior Geology (ILSG) fieldtrip, one of the questions raised above is particularly pertinent: is the accretionary lapilli layer recognized at the disturbed top of the Gunflint Formation in the Lake Superior area indeed the Sudbury event and fall-out layer (Figure 2), as is permissible and as has been argued based on current evidence (e.g., Addison et al., 2005, 2010; Cannon et al., 2010), or does that horizon represent a different event layer? From an ECREE perspective ("extraordinary claims require extraordinary evidence", Carl Sagan), a unique link of this prominent event layer to the Sudbury area still remains to be documented, and other localities of an 1850 Ma global event layer need to be demonstrated (Figure 2). Among the largest preserved impact craters on Earth (Dietz, 1964; Dietz and Butler, 1964), Sudbury’s very thick (~3–5 km) and differentiated melt sheet (the “Sudbury Igneous Complex”, hereafter SIC; Pye et al., 1984 and contributions therein) is unique. Why? Perhaps this can be reasoned away by other comparable melt sheets not being preserved (e.g., Vredefort) or not yet fully explored (Chicxulub), or perhaps was Sudbury bigger than the current consensus (~200 km final crater diameter; see Grieve et al., 1991; Grieve, 1994; Butler, 1994; Spray et al., 2004), and is it in a class of its own? 4 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 1: Geological map of the Sudbury area centered on the erosional remnants of the deformed Sudbury impact structure, the Sudbury Igneous Complex (SIC); after Bleeker et al. (2015), and adapted from Dressler (1984) and Ames et al. (2005). Day 1 fieldtrip stops are indicated, as is the area we will visit on Day 2 (and a possible alternate area for Day 2). The arcuate red dashed line marks the outer limit of observed shatter cones in the footwall. The circa 1.0 Ga Grenville Front truncates the area in the southeast. 5 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Finally, Sudbury is fascinating from a science history perspective: how it was discovered and how its interpretation was slow to change from, originally, a largely mafic igneous complex or lopolith — “the Sudbury Irruptive” — in part or wholly derived from mantle melting (e.g., Wilson, 1956; prior to the work by R. Dietz in the early 1960s); to a hybrid igneous and impact-generated structure (see, for instance, the various contributions in Pye et al., 1984; see also Dietz, 1964 1); and, finally, to an impact-only crater, melt sheet, and crater fill complex with little to no mantle input and merely modified by deformation (e.g., Stöffler et al., 1989; Grieve et al., 1991; Grieve, 1994). With respect to the latter perspective, the pendulum only changed in the late 1980s and early 1990s when detailed isotopic data became available showing that most if not all of the melt sheet was derived from melting of the crust (e.g., Faggart et al., 1985; Stöffler et al., 1989; Walker et al., 1991; Dickin et al., 1992; Deutsch, 1994 and references therein). From this slowly evolving historical perspective, the brilliant early 1960s papers by Robert Dietz, who in 1962 quickly confirmed “shatter cones” in the footwall rocks around the Sudbury complex (see frontispiece of this guidebook), and from there confidently posited a meteorite impact origin, stand out as even more remarkable. 2 This field trip will allow participants to become familiar with this amazing structure and to discuss and debate all these first-order questions. The current fieldtrip guidebook should be viewed as a preliminary offering, as many details remain to be expanded on. 2. The Sudbury Structure: Geological Setting The deformed Sudbury impact structure, now preserved as a broadly doubly plunging, synclinal, erosional remnant ~60 km long by ~30 km wide, is situated in the southern Canadian Shield, approximately on the boundary between two main structural provinces, the Archean Superior Province to the north and the Paleoproterozoic Southern Province to the south (Figures 1 and 2). The former is represented by the circa 2.85-2.64 Ga granite-greenstone terrain of the southern Superior craton, whereas the latter is dominated by moderately to tightly folded and faulted Paleoproterozoic strata of the circa 2.50-2.30 Ga Huronian Supergroup (Young, 1973 and contributions therein; Bennett et al., 1991; Young et al., 2001; Rasmussen et al., 2013) overlying Superior craton basement (Figures 3 and 4). The Southern Province trends roughly E-W and, to the west, extends into the Lake Superior area where it is known as the Penokean fold belt, part of the larger 1.87-1.83 Ga Penokean orogen (e.g., Brocoum and Dalziel, 1974; Schulz and Cannon, 2007). Just to the south of the city of Sudbury, the Paleoproterozoic Southern Province fold belt is truncated by the relatively sharply defined deformation front of the circa 1.1-1.0 Ga Grenville Front sensu stricto (see front #4, Figure 2). This front trends from northeast to southwest and represents the northern boundary of the complex, multi-cyclic and very extensive Grenville orogen that rims Proterozoic Laurentia to the southeast and played major role in building the late Proterozoic supercontinent Rodinia. In more detail, the Grenville Front sensu stricto represents, in the Sudbury area, the relatively well-defined deformation front of the terminal collisional phase of the Grenville orogen, an orogen that also involves older Paleo- to Mesorproterozoic events, some of which are represented by rocks units right along the Grenville Front (e.g., see front #3 in Figure 2). Although quickly recognizing the fundamental impact origin of the structure in 1962, based on his identification of shatter cones in the footwall rocks, in his seminal 1964 paper Dietz maintains an intrusive origin for much of the Sudbury Igneous Complex. 1 2 A bibliography of relevant papers by Dietz is included at the beginning of the reference list and they make for an interesting read. After convincing himself, in the mid-1950s, that shatter cones were a key indicator for high-energy meteorite impacts, he quickly clarifies a large number of impact structures, such as Sudbury, which were previously seen as enigmatic and attributed to “crypto-volcanic” (i.e. endogenic) processes. In 1961 he coined the term “astroblemes” for these structures (i.e., “star wounds” or impact scars; Dietz, 1961) to highlight the fundamental role of impact processes here on Earth. 6 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 2: General location of the deformed Sudbury impact structure on the approximate boundary of the Archean Superior Province and the Paleoproterozoic Southern Province. The latter is broadly part of the “Penokean fold belt” and the associated orogen that rims the southern margin of the Superior craton (e.g., Card et al., 1972; Brocoum and Dalziel, 1974; Card et al., 1984; Holm et al., 2007; Schulz and Cannon, 2007). Structural fronts younger than the circa 1.87-1.84 Ga Penokean sensu stricto (front #1) also contributed to the deformation of the Southern Province (i.e. fronts #2 and 3). South of Sudbury, the Southern Province is truncated by the main circa 1.1-1.0 Ga Grenville Front (front #4; Davidson, 1997). Also shown is the approximate footprint of the circa 0.6–0.5 Ga Ottawa-Bonnechere Graben (in light grey), which is manifested in the Sudbury area by E-W-trending olivine-bearing diabase dykes. Red star symbols indicate the localities where the accretionary lapilli event layer at the top of the Gunflint Formation and correlative strata has been identified (e.g., Cannon et al., 2010). The figure also draws attention to other circa 1870-1840 Ma sequences preserved in the Canadian Shield, only at marginally larger distances than the Mesabi Range locality, where a comparable Sudbury fall-out layer remains to be identified. Map adapted from Wheeler et al. (1996), Young (1983), and Cannon et al. (2010). 7 �Proceedings of the 68th ILSG Annual Meeting - Part 2 A simplified NNW-SSE cross-section through the area illustrates the main pre-impact, litho-tectonic elements that define the wider Sudbury area (Figure 3). A summary stratigraphic column, highlighting the main features of the Paleoproterozoic Huronian Supergroup in more detail, is shown in Figure 4. The age range of the Huronian Supergroup is constrained by the onset of rifting and associated large-scale mafic magmatism at circa 2500-2480 Ma (Krogh et al., 1984), defining its base, and final emplacement of extensive mafic sill complexes at circa 2250 Ma (May Township sills, Bleeker et al., in prep.) and circa 2217 Ma (Nipissing sills; Corfu and Andrews, 1986; Noble and Lightfoot, 1992; Bleeker et al., 2015; Davey et al., 2019). The latter intrusive episodes provide a minimum age for the supergroup. Thin felsic ash layers in the fine-grained sedimentary rocks of the Gordon Lake Formation, in the upper part of the supergroup, have an approximate age of circa 2308 Ma, based on SHRIMP dating of zircons (Rasmussen et al., 2013) on a sample obtained from drill core through the formation. Figure 3: A simplified cross-section through the Sudbury area from NNW to SSE, showing the main lithotectonic elements that define the geology of the area, prior to Penokean and younger deformation and prior to the 1850 Ma impact. Note the lower Huronian rift structure and associated rift fill (the Elliot Lake Group, E), which is well developed and exposed in the Sudbury area, in part due to impact-induced uplift and exhumation. The estimated position of “Ground Zero” is indicated above the section, based on both ringlike structures in the foreland (Butler, 1994) and a statistical intersection of shatter cone axes (Bleeker, unpublished). A circa 2.0 Ga passive margin sequence, comparable to the Gunflint Formation in the Lake Superior area, is indicated for general comparison only but is not preserved in the Sudbury area. Postimpact, the area was covered by the depositional wedge of Penokean foreland basin deposits, the upper shale dominated Onwatin Formation and the turbiditic Chelmsford Formation, which are preserved only within the doubly plunging syncline of the “Sudbury Basin”. These turbiditic deposits are comparable and likely directly correlative to the Rove Formation of the Lake Superior area. The three stars on the right side of the section indicate glacially-influenced formations of diamictite, from bottom to top: the Ramsay Lake, Bruce, and Gowganda Formation diamictites. Based on global correlations and U-Pb age dating in the Transvaal Basin of South Africa, we know that the Ramsay Lake Formation glacial episode ended at circa 2426 Ma (Gumsley et al., 2017). 8 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 4: Summary stratigraphic column of the 2.50-2.30 Ga Paleoproterozoic Huronian Supergroup, unconformably overlying Archean basement of the southern Superior craton. Compiled from numerous sources: Young (1973) and contributions therein; Bennett et al. (1991); Young et al. (2001); Gumsley et al. (2017). Age data from: Krogh et al. (1982) and Krogh et al. (1984); Corfu and Andrews (1986); Noble and Lightfoot (1992); Heaman (1997); Rasmussen et al. (2013); Bleeker et al. (2015); Davey et al. (2019). 9 �Proceedings of the 68th ILSG Annual Meeting - Part 2 3. More on the 2.50-2.30 Huronian Supergroup Age range, thickness, Archean-Proterozoic boundary The Huronian Supergroup (Figure 4) is of more than just local interest. With its extended age range and ~6–12 km total stratigraphic thickness, it represents one of the principal, and best-preserved Paleoproterozoic stratigraphic records in the world, perhaps only rivalled by the Transvaal Supergroup of southern Africa, and correlative units of the Hamersley Basin overlying the Pilbara craton. Its basal unconformity and low-grade metamorphic state, overlying Archean granite-greenstone terrain, played a major role in the debate and final definition of the Archean–Proterozoic boundary, one of the most fundamental boundaries of the terrestrial geological time scale. GOE: the Great Oxidation Event, pyritic placer deposits, continental red beds Equally important from a historical point of view, the Huronian Supergroup straddles the circa 2.4–2.3 Ga “Great Oxidation Event” (GOE, see Figure 4; e.g., Roscoe, 1973 3; Holland, 1978, 1984, 2002; Prasad and Roscoe, 1996; Bekker et al., 2004; Gumsley et al., 2017), which marks the initial rise of atmospheric oxygen across the critical threshold above which oxidized surface environments promoted the deposition of hematite-stained red sandstones (“red beds”). The world’s oldest genuine terrestrial red beds occur in the Cobalt Group of the upper Huronian, specifically the Gowganda and overlying Lorrain formations. The elevated oxygen levels no longer allowed prolonged preservation of detrital pyrite and associated heavy minerals, in terrestrial sedimentary environments, thus resulting in pyrite placer deposits largely disappearing from the geological record. Hence, no such deposits are known from the upper Huronian Supergroup, but, with associated gold and uraninite, they form minor but important components of the lower part of the supergroup that was deposited prior to the GOE transition. Pyrite-uraninite placers form important placer deposits in the basal Matinenda Formation (Figures 3 and 4) and were mined extensively for uranium in the Elliot Lake area west of Sudbury. Gold-bearing pyrite placers are currently the subject of active exploration northeast of Sudbury and are thought to be part of the conglomerates and cross-bedded sandstones at the base of the Mississagi Formation (Long et al., 2011; Whymark and Frimmel, 2018). The emergence of these important insights involving atmospheric evolution, increasing oxygen levels, detrital pyrite, and the first red beds were largely based on work in the Huronian Supergroup and correlated sequences, such as the Snowy Pass Supergroup overlying the Wyoming craton (e.g., Roscoe and Card, 1992, 1993; Prasad and Roscoe, 1996). GOE: disappearance of the mass-independent sulphur isotope fractionation signature Correlated with the observable transition “detrital pyrite out, red bed sandstones in”, more recent research has shown that there is also a fundamental shift in global S-isotopic signatures, particularly the disappearance of anomalous “mass-independent fractionation” (MIF) of the 33S isotope relative to 32S and 34 S isotopes (Farquhar et al., 2000; Farquhar and Wing, 2003). This MIF signature of 33S is understood to result from UV-induced processes in the upper atmosphere and this MIF signal can only survive and be transmitted to the sedimentary record at very low total oxygen atmospheric levels, estimated at <10-5 times present atmospheric level (PAL) of oxygen (Farquhar and Wing, 2003). These insights have revolutionized the understanding of both the shallow and deep sulphur cycles in the last two decades and shed new light 3 Although now widely referred to as the “Great Oxidation Event” (GOE), following Holland and others, Stew Roscoe through his work in the Elliot Lake area, on the pyrite-uraninite placer deposits there, was one of the first to draw attention to this important transition and called it the “oxyatmoversion”. In his 1973 paper he also draws attention to the obvious potential of this transition in terms of a first-order time scale boundary. 10 �Proceedings of the 68th ILSG Annual Meeting - Part 2 on a host of fundamental recycling processes. As shown in Figure 4, detrital pyrite and the MIF-S signature are absent above the Mississagi Formation. Glacial episodes, Snowball Earth events, cap carbonates Stratigraphic and sedimentological studies of the Huronian Supergroup also played a key role in the recognition of early Precambrian glacial deposits, and possibly global-scale glacial events. Unsorted diamictite deposits, almost certainly tillites or reworked tillites, are recognized at three stratigraphic levels within the Huronian Supergroup (Figure 3 and 4): in the Ramsay Lake Formation, the Bruce Formation, and in the post-GOE Gowganda Formation. Some of these diamictite deposits have other associated features that confirm a glacial origin, such as dropstones in overlying varve-like siltstones, and glacially polished clasts with striae. This is certainly the case for the Gowganda Formation (Young, 1983; Young and Nesbitt, 1985; Young et al., 2001). Paleomagnetic evidence places the Superior craton at low latitudes in the earliest Paleoproterozoic (e.g., Evans and Hall, 2010; Salminen et al., 2014). This, together with glacial deposits in what was at least in part a marine basin, thus places glacial deposits at sea level, and close to the equator and far from the poles. In turn, observations such as these argue for glacial events of global significance, and events that can be correlated to other cratons and similar sedimentary successions such as the Transvaal basin of southern Africa (e.g., Gumsley et al., 2017). This is particularly relevant for the second of the three glacial events represented by the Bruce Formation diamictites. This formation is overlain by the only carbonates in the Huronian succession, the Espanola Formation (see Figure 4). These carbonates may represent deposits that are thought to have formed in response to rapid deglaciation of global ice cover and, in this context, are referred to as “cap carbonates” (e.g., Kirschvink, 1992; Hoffman et al., 1998; Hoffman and Shrag, 2000; Kirschvink et al., 2000). If so, these carbonates would allow global correlations and constitute an ideal, globally synchronized time scale boundary, as they do in the Neoproterozoic. Epicontinental rift and sag succession or passive margin sequence? Various authors have, somewhat uncritically, referred to the Huronian Supergroup as a rift and passive margin sequence, associated with the breakup of the Superior craton. That implies that the Superior craton broke up along its southern margin sometime around 2.4 Ga following the formation of the lower Huronian rift succession. There is, however, little evidence for this early breakup. Clearly the Superior craton, with its present outline, is just a fragment of a much larger ancestral Archean supercraton, which Bleeker (2003, 2004) has referred to as “Superia”. Breakup of Superia was clearly progressive but may have only started with the very extensive circa 2.22 Ga Nipissing and related mafic magmatic events or large igneous provinces (LIPs), and likely even later along the southern margin of the craton. The specific LIP event that most likely represents initiation of breakup along the southern margin of the Superior is the 2125-2100 Ma Marathon event (Halls et al., 2008; Davey et al., 2020, 2022), with very extensive mafic dyke swarms projecting north into the craton and bimodal magmatism along the southern margin of the craton. The Wyoming and Karelia-Kola cratons are among the cratonic fragments that broke away at that time, as they can be matched to the southern Superior, based on multiple lines of evidence, prior to 2.1 Ga breakup (e.g., Roscoe and Card, 1993; Bleeker and Ernst, 2006; Kilian et al., 2016a, b). This alternative scenario argues for the entire Huronian Supergroup to represent a long-lived intra- and epicontinental rift and sag basin, without a proximal passive margin prism. Consequently, this puts the Huronian Supergroup in a different perspective and has implications for global reconstructions and compilations, such as those of successions thought to represent passive margin sequences through time (Bradley, 2008). Similar arguments very likely apply to other Proterozoic sequences that have been too easily characterized as passive margins. 11 �Proceedings of the 68th ILSG Annual Meeting - Part 2 4. Large Igneous Provinces (LIPs) and Mafic Magmatic Events of the Wider Sudbury Area Mafic magmatic events Here we briefly list and describe some of the large mafic magmatic events in the wider Sudbury area. These events are important as temporal and structural markers, either pre-dating certain structural events, or the impact event itself, or post-dating such events and thus providing minimum age constraints. From old to young, a brief summary of these magmatic events would include (e.g., Krogh et al., 1984; Kamo et al., 1995; Heaman, 1997; Noble and Lightfoot, 1992; Ernst and Bleeker, 2010; Bleeker et al., 2015): • • Major pyroxenite dykes: circa 2507 Ma, perhaps part of the broader Mistassini event? These dykes, which occur just north of the SIC, may indicate the onset of the Huronian rifting event (Bleeker et al., 2015). Matachewan-I: circa 2480 Ma layered intrusions and sills, dykes; the East Bull Lake Suite. Matachewan-II: circa 2460 Ma major diabase swarm, main pulse of Matachewan dykes. May Township sill complex: circa 2250 Ma, large sills to the west of the Sudbury area. Nipissing sills and Senneterre dykes: circa 2217 Ma, volumetrically important in the immediate Sudbury area. Biscotasing dykes: 2167 Ma dykes, mostly north of Sudbury. Marathon dykes: circa 2110 Ma dykes, mostly west of Sudbury. Lauzon Lake dykes: circa 1950 Ma, major NW-trending dykes west of Sudbury. Alkaline dykes, lamprophyres and carbonatite intrusions, circa 1880 Ma; e.g. the Spanish River Complex north of Sudbury. Thin mafic sills/subhorizontal sheets: undated and of yet unknown significance. Lamprophyre dykes: undated and of yet unknown significance; probably more than one event. • Sudbury impact event and associated dykes: 1850-1849 Ma. • Trap dykes: circa 1750 Ma, numerous E-W trending diabase dykes cutting across the Sudbury South Range, still affected by metamorphism. Alkaline intrusions in the wider area, e.g. the Croker Island Intrusion, southwest of Sudbury, Mesoproterozoic in age. Larder Lake dykes: circa 1270 Ma, undeformed olivine diabase dykes similar to Sudbury dykes. Sudbury dykes: circa 1235 Ma, abundant NW-trending undeformed olivine diabase dykes cutting across the area and the Sudbury structure, but themselves cut and truncated by the Grenville Front. Grenville dykes: circa 590 Ma, E-W-trending dykes cutting across the Sudbury structure and Grenville Front, and a manifestation of the extensive Ottawa-Bonnechere Graben system. Kimberlites and associated intrusions in the wider area, Jurassic to Cretaceous. • • • • • • • • • • • • • • Some of the pre-impact mafic magmatic events are rather voluminous in the Sudbury area, and thus a significant component of the overall target rocks. This is particularly true for: 1) the early Matachewan (I) mafic rocks, which form mid-sized layered intrusions in the general area (e.g., James et al., 2002), at or close to the Archean–Paleoproterozoic (i.e. basal Huronian) unconformity (Figure 3), and less voluminous dykes and sills; and 2) the very extensive Nipissing Diabase sill complex, which invaded large parts of the Huronian Supergroup stratigraphy as well as the basement immediately below the unconformity (see Figures 3 and 4). Some of these units have minor, to locally significant, Ni-Cu-PGE magmatic sulphide mineralization; for instance, the Shakespeare Intrusion that is part of the Nipissing magmatic event (e.g., Sproule et al., 12 �Proceedings of the 68th ILSG Annual Meeting - Part 2 2007; Davey et al., 2019). For these reasons (overall volume, sulphur (S), metals (Ni, Cu, PGEs)), they have featured and continue to feature in the debate on metal sources and elemental mass balances in the differentiated Sudbury melt sheet and the associated magmatic sulphide ores. In other words, did certain geological units in the target rocks play an important role in the sulphur and metal budgets of the impact melt sheet, and thus in early sulphur saturation and in the overall metal endowment in the orebodies, or are such roles insignificant in the overall melt sheet evolution and the formation and segregation of the ores? Clearly, immediately following the impact, and the generation of a superheated melt sheet, early sulphide saturation was critical and can be clearly demonstrated (Figure 5); but overall metal budgets may not require enriched source rocks if sulphur saturation was indeed early and given the enormous volume of very hot impact melt available. In the latter scenario, it is simply all about the efficiency of early sulphide saturation, exsolution and segregation, sulphide droplet formation, and the “rain out” of sulphide globules enriched in chalcophile metals. Figure 5: A polished slab of Fe-Cu-Ni magmatic sulphide droplets, millimetres to centimetres in size, in ore from the proximal part of the Copper Cliff “offset dyke”. This dyke was injected into the footwall of the impact crater, during an early stage of the melt sheet evolution. The host rock consists of relatively unfractionated early mafic melt sheet material formed near the base of the melt sheet. Sulphide droplets were actively “raining out” (down in the picture) and physically interacting with mafic inclusions on their way down. Textures such as these provide clear evidence for early sulphide saturation in the melt sheet and that the “rain out” of exsolved sulphide melt droplets was the first-order process collecting magmatic sulphide ores at or near the basal contact of the melt sheet. Note the dumbbell structure of two merging sulphide globules near the top of the sample. 13 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Several of the intrusive events are also of key importance as structural markers. For instance, the extensive Nipissing Diabase sills within the Huronian succession, and locally in basement just below the Huronian unconformity, are folded with the strata of the Huronian Supergroup and, where well-exposed, sill contacts are largely concordant with the sedimentary layering in the immediate host rocks; although locally the sills are cross-cutting and dyke-like due to a “saucer-shaped” sill geometry (see Figure 3). However, the largely concordant nature of the observed contacts of many of the sills, together with a notable absence of clearly documented examples of such sills cutting across both limbs of previously folded Huronian strata, strongly favours an interpretation of the Huronian being largely unfolded, or perhaps only very weakly folded and/or locally tilted, at the time of widespread Nipissing sill emplacement at circa 2217 Ma (Bleeker et al., 2015). This important observation has major implications for the concept of the “Blezardian orogeny” and, ultimately, how to interpret the Sudbury structure (see below), and is fully supported by observations on the orientation of shatter cones. 5. The “Blezardian Orogeny”, Fact or Fiction? The lowermost Huronian Supergroup is intruded by a number of granite plutons in the Sudbury area, one of the major plutons being the Creighton Granite on the South Range, just west of Sudbury (Figure 1). As part of early attempts to systematize the geology of the Canadian Shield and divide this enormous territory into “structural provinces” based on regional deformation patterns and tectono-magmatic events (e.g., Wilson, 1949; Stockwell, 1982), late-stage granites were generally seen, and indeed correctly in many places, as the terminal phase of a major orogenic episode. As it had been clear since the early mapping of the southern Canadian Shield that the folded Huronian Supergroup unconformably overlies Archean basement that had previously been affected by major late Archean deformation and metamorphism, this grew into the concept of the two structural provinces: the Archean Superior Province to the north, affected and shaped by a terminal “Kenoran orogeny”, and a younger Proterozoic Southern Province and fold belt to the south, affected by deformation attributed to a “Blezardian orogeny” dated by the intrusion of the Creighton Granite and similar granite plutons. Early zircon dating attempts of these (shock-deformed!) granite bodies, based on large, multigrain, highly discordant zircon fractions (unabraded), suggested interpreted upper intercept ages of circa 2.35 Ga (Frarey et al., 1982); i.e. a circa 2.35 Ga Blezardian orogeny associated with granite magmatism that terminated the Huronian cycle of sedimentation and associated deformation. Perhaps reasonable at the time, we now know this interpretation is no longer tenable for the following reasons: 1) the granite plutons are lower Huronian rift-related A-type granites, dated at 2455–2460 Ma (Bleeker et al., 2015), not collisional granites as part of a terminal orogenic phase; 2) they only intrude the lowermost Huronian rift volcanics and are deformed with the lower Huronian strata they intrude; and 3) as explained above, the Huronian strata are extensively intruded by Nipissing Diabase sills at 2217 Ma, with most or all of the deformation post-dating the emplacement of the sills. From Bleeker et al. (2015): “A tightly folded Nipissing Diabase sill has been dated at 2215 ±1 Ma. It is fully conformable with surrounding Huronian strata on the South Range, inconsistent with the concept of a pre-Nipissing “Blezardian orogeny”. The main rationale for the Blezardian orogeny was the idea that deformation and intrusion of granite plutons, such as the Creighton Granite, thought to be circa 2.35 Ga in age, terminated the depositional history of the Huronian succession (Frarey et al., 1982; Stockwell, 1982). None of these ideas are supported by present evidence. The Creighton Granite is an early Huronian 2455– 2460 Ma rift-related granite, not an orogenic granite pluton; folding of the Huronian succession did not commence until well after emplacement of Nipissing Diabase sills and sheets with the onset of Penokean accretion and collision events at circa 1860 Ma. Other observations that have contributed to the concept of 14 �Proceedings of the 68th ILSG Annual Meeting - Part 2 a Blezardian orogeny can all be explained without a significant pre-Nipissing deformation event. For instance, saucer-shaped Nipissing sills locally may appear to crosscut Huronian strata and, after superimposed Penokean deformation, could easily lead to confusing field relationships.” With these caveats, does this leave any room for pre-Nipissing deformation of the Huronian? It seems reasonable that there was some local deformation and/or tilting related to movement on rift faults, perhaps local inversion. As shown in Figures 3 and 4, the Huronian succession consists of several major groups, and some of the boundaries between these groups may represent second order sequence boundaries and local, relatively low-angle unconformities. In this context, the basal contact of the aerially extensive Cobalt Group, overlain by coarse conglomerate and diamictite, is particularly relevant. The Cobalt Group far oversteps the lower groups of the Huronian and onlaps onto the Superior craton far to the north. In some areas northeast of Sudbury, careful examination of township geological maps suggests that a low-angle unconformity separates the Cobalt Group from the lower Huronian groups (Hough Lake and Quirke Lake Groups), as depicted in the sections of Figures 3 and 4 (see also, Meyn, 1973). These interesting relationships do not equate with a Blezardian orogeny, however. 6. Post-impact Deformation Beyond the observations discussed above, it is clear that most of the deformation and metamorphism of the Southern Province and the Sudbury area, locally intense, post-dates all of the Huronian Supergroup, the Nipissing Diabase sill emplacement, and also the final settling of the differentiated Sudbury melt sheet, as well as the deposition of the overlying Whitewater Group (Figure 6; see also Figure 3). The major deformation that folds the Sudbury structure, melt sheet, crater fill, and overlying Whitewater Group into a regional-scale, doubly plunging synclinal structure can be largely attributed to the Penokean orogeny (e.g., Card et al., 1972; Brocoum and Dalziel, 1974), amplified and overprinted to varying degrees, particularly on the South Range and toward the Grenville Front, by younger Proterozoic events (Shanks and Schwerdtner, 1991; Bailey et al., 2004; Papapavlou et al., 2017). Figure 6 (next page): Simplified lithologic-stratigraphic column for the Sudbury impact structure, its differentiated melt sheet, and the overlying Whitewater Group. Adapted and compiled from various sources, including Naldrett and Hewins (1984), Grieve et al. (1991), Zieg and Marsh (2005), Ames et al. (2005), Bleeker et al. (2015), and Lightfoot (2016). In the context of the present discussion, note the uppermost units of the Whitewater Group, which represent the foreland depositional wedge of the Penokean foreland basin. These sedimentary formations are only preserved in the regional scale, doubly plunging, Sudbury Basin syncline. The turbiditic Chelmsford Formation is similar in age and character to the Rove (Virginia) Formation of the Lake Superior area. Note the very tight age control on the differentiated melt sheet, including a new age of last crystallizing basal granophyres at 1850.0±0.9 Ma based on chemically abraded, fully concordant zircon data. Numbers highlight the different orebody settings and types: 1) disseminated sulphides in mafic norite near the base of the SIC; 2 and 3) disseminated to semi-massive sulphides in the Sublayer and along the footwall contact; 4) sulphides infiltrated in the footwall breccia; 5) semi-massive to massive sulphides in footwall rocks, variably fractionated and enriched in Cu; 6) massive sulphide sills deeper within the footwall, (sub)parallel to the footwall contact, very Cu-rich; 7) deeper remobilized veins, with a transition to hydrothermal processes; 8) major sulphide concentrations in funnel-like embayments; 9) sulphide ore within inclusion-bearing diorite (IQD) injected into footwall; and 10) globules to semi-massive sulphides, fractionated and Cu-rich, injected deep into footwall dykes. 15 �Proceedings of the 68th ILSG Annual Meeting - Part 2 16 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Early publications, at a time when the Grenville orogenic front was already well recognized, attributed the folding of the Sudbury structure to Grenvillian deformation (e.g., Dietz, 1964). More recent mapping and geochronological data suggest, however, that the actual Grenville Front sensu stricto. is rather sharply defined and that circa 1.0 Ga deformation did not significantly extend far into the immediate foreland and had little effect on the Sudbury structure (e.g., Easton, 1992; Davidson, 1997; Easton et al., 1999). Instead, there is increasing evidence for significant post-Penokean but pre-Grenville deformation, broadly correlated with the Yavapai and Mazatzal (or Labradorian?) orogenic belts/episodes defined in the southwest USA, i.e. broadly in the interval 1.80-1.60 Ga (e.g., Bailey et al., 2004). However, at a somewhat larger scale, Grenvillian deformation, specifically loading of the crust by the Grenvillian thrust stack, is likely to have contributed regional tilting of the crust to the south, causing further uplift of the Sudbury area and the Archean craton to the north. The precisely dated Sudbury impact event (Krogh et al., 1982, 1984; Corfu and Lightfoot, 1996; Davis, 2008; Bleeker et al., 2015; Bleeker and Kamo, in prep.), based on multiple high-precision U-Pb ages on units of the SIC, at 1850 Ma, occurred as a sharply (seconds and minutes!) timed event during the circa 1860–1840 Ma Penokean orogeny. From a broader regional perspective, the onset of the major Penokean orogenic event at circa 1870–1860 Ma predates the impact event (e.g., Holm et al., 2007). In the Sudbury area, this can be demonstrated, perhaps, by Huronian rocks being folded to some degree prior to emplacement of the melt sheet and associated dykes. However, this requires de-convolving the potentially very complex deformation associated with the impact, the immediately post-impact collapse along ring faults, and the large-scale in-flow of material into the transient crater (e.g., see modelling studies on large impact studies and the spectacular deformation, and folding, it induces in the central uplift and the surrounding annulus; Ivanov, 2005). These latter processes would undoubtedly involve tight folding of Huronian strata in places and satisfy the apparent timing relationships. The most pertinent structures in this respect occur to the north of the preserved SIC, where melt sheet injection dykes (known locally as “offset dykes”, e.g. the Hess offset dyke) appear to cut both limbs of fold structures in synclinal outliers of Huronian strata. Is this apparent folding pre-impact (e.g., as assumed by Mungall and Hanley, 2004, using an outdated model of the Blezardian orogeny), or syn-impact and due to large-scale in-flow of material into the annulus around the rebounding central uplift? In any case, these structures were tightened with present dips of Huronian units locally being steep. They were thus likely tightened to some degree by Penokean deformation extending into the foreland. This is the main reason the “Penokean front” in Figure 2 is placed to the north of the preserved SIC, and north of these deformed and folded Huronian outliers. Zooming back out, most of the more intense and final Penokean deformation post-dates the impact structure and also the emplacement of the foreland depositional wedge of the Whitewater Group turbiditic sediments that must have covered the area and which are now preserved only in the keel of the doubly plunging Sudbury Basin syncline (see map of Figure 1). Figure 7 presents a NNW-SSE cross-section through the western half of the Sudbury structure, as constrained by map and outcrop data, as well as a down-plunge projection of the western fold closure of the Sudbury basin (Bleeker et al., 2014). 17 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 7: NNW-SSE cross-section through the western half of the Sudbury structure, illustrating the firstorder regional scale synclinal fold structure, with more intense deformation and tilting of the southern flank (the “South Range”). This structure is largely Penokean in origin, but, particularly on its southern flank, was further amplified and shortened by younger post-Penokean deformation. Overall shortening on the South Range may reach ~50%, with the basal contact of the SIC being steep to subvertical, and locally overturned. Discrete later shear zones, with south-over-north displacement, further imbricated this steepened southern flank of the structure. However, at the scale of the section, there is no large-scale offset on major discrete shear zones. The folded melt sheet must have extended well to the north, to allow injection of the most distal offset dykes, and also to the south. However, all of the southern half of the folded melt sheet has been removed by erosion. The major structural front in the south is the circa 1.0 Grenville Front, with large scale thrust movement. Abbreviations: CG, Creighton Granite; CC, Copper Cliff Rhyolite; EM, Elsie Mt. Formation; MK, McKim greywacke turbidites; MS, Mississagi Formation quartzites; N, Nipissing Diabase sills; R, Ramsay Lake Formation; S, Stobie Formation; SIC, Sudbury Igneous Complex. This cross-section clearly demonstrates the overall north-verging synclinal structure with the younger than 1850 Ma Chelmsford Formation (circa 1840 Ma?) preserved in the core of the syncline. Much of this deformation is likely Penokean, but significantly overprinted by post-Penokean tightening and further shortening on the southern limb of the asymmetric syncline, during deformation associated with the 1.751.65 Ga South Range Deformation Zone. The swarm of mafic “Trap dykes”, which was emplaced at circa 1750 Ma along the South Range (Bleeker et al., 2015; see also the “quartz diabase dykes” of Cochrane, 1984) cuts much of the (Penokean) deformation, but is itself weakly deformed and metamorphosed to upper greenschist facies. It provides an important temporal and structural marker for this post-Penokean interval. The section demonstrates the intensification of structures and overall shortening on the locally steeply dipping southern flank of the large-scale syncline. Here, bulk shortening may locally reach ~40-50%. Much of this deformation was likely Penokean, but was amplified by younger circa 1.80–1.60 Ga deformation that is manifested by a system of southeast-dipping, south-side up reverse shear zones known as the South Range Shear Zone (Shanks and Schwerdtner, 1991; Bailey et al., 2004). Metamorphic titanites in these sheared rocks, which reached epidote amphibolite facies metamorphic grade, date this overprinting deformation as post-Penokean, in the time range of 1.80-1.60 Ga (e.g., Bailey et al., 2004; Papapavlou et al., 2017). Hence, these events have been broadly correlated with Yavapai and Mazatzal orogenic events, as they have in the broader Lake Superior area. Indeed, some of the orebodies on the steep South Range of the SIC are cross-cut and displaced by discrete and well-defined southeast-dipping shear zones. This has been clearly demonstrated in the Thayer Lindsley Mine, and also in the Creighton Mine. This will be discussed during the fieldtrip. 18 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Important as these younger structures are, on the scale of the folded Sudbury structure (i.e. Figure 7), this entire shear zone system only causes minor offsets of the SIC, and the first-order continuity of the SIC at both the western and eastern closures of the folded structure rules out very significant, discrete, fault offsets as has been suggested in some of the published literature (e.g., Milkereit and Green, 1992; Wu et al., 1995; Cowan et al., 1999). The section of Figure 7 is thus best interpreted as a largely Penokean fold structure, moderately overprinted and further shortened by post-Penokean bulk shortening deformation (at the scale of the section), with the final shortening on the South Range reaching perhaps ~50%. 7. Where Was “Ground Zero” and How Big Was the Final Impact Structure? Given the final deformational state of the very large Sudbury impact structure (Figure 7), a first-order question is: where was the geometrical centre of the impact; or in other words, where was “Ground Zero”, relative to the preserved erosional remnants of the structure and those of the melt sheet? An accurate answer to this question has major implications on the interpretation of the size of the impact structure, the volume of impact melt generated, and thus also affects overall mass balance calculations of elements in the melt sheet and the orebodies. Several important datasets may provide an answer to this all-important question: 1) The analysis of preserved ring structures in the relatively undeformed foreland to the north of the deformed Sudbury structure (Butler, 1994; Spray et al., 2004; see also Grieve et al., 1991). 2) An analysis of the statistical focal point of all well-preserved and least deformed (and possibly somewhat re-oriented) shatter cones (Bleeker, in preparation). As presented by Butler (1994), a careful analysis of lineaments and possible impact ring structures in the foreland of the Superior craton places the geometrical centre well to the south of Sudbury by as much as 10-15 km. Butler’s analysis is supported by a statistical analysis of all intersections of shatter cone axes (see also Guy-Bray et al., 1966), which also focus along the South Range, approximately in the Copper Cliff area, or slightly to the northeast in the Frood and Stobie mines area. These findings are summarized in Figure 8. Superimposed on the diagram of Figure 8 is a model of progressive restoration of the preserved outline of the Sudbury structure (i.e. the base of the SIC), in 4-5 steps, including minor deformation in the foreland, 15-20% shortening in the northern half of the SIC syncline, and up to 50% shortening of the southern half of the structure. This restoration of realistic amounts of shortening places the trace of the South Range, and the original focal point of shatter cone axes, just in range of the Butler’s geometrical centre defined by his most robust, lineament-constrained “Ring 3” which is shown in red on Figure 8. A significant conclusion from this work is that all of the preserved SIC represents only a portion of the northern half of the preserved melt sheet. Hence, there is no a priori symmetry between the preserved North Range and the South Range, but rather all kinds of asymmetry: the North Range preserves a thinning northern lobe (but not the edge!) of the original melt sheet, whereas the South Range preserves a more central portion of the melt sheet that was onlapping onto, and partially overlapping the collapsed central uplift. All of the southern part of the melt has been removed by uplift and erosion. These constraints and conclusions are further summarized in Figure 9. These findings can be modified to some extent by increasing the shortening deformation somewhat, moving the restored South Range of the SIC a bit farther south, but without going to unreasonable shortening estimates it will not change the first-order conclusion that the preserved SIC is all from the northern half of the folded melt sheet and thus preserves a relatively small, fundamentally asymmetric sample of the original melt sheet. 19 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 8: Summary map of the size and location of the preserved Sudbury structure relative to the main structural deformation fronts, and the impact ring structures defined by Butler (1994) based on a detailed lineament analysis (here only lineaments related to Ring 3 are shown for illustration, and some to the northeast of the structure where there is some discordance between lineaments and Ring 4). Of these rings, the one shown in red is the most robust (see Butler, 1994) and it, together with the other rings, defines the centre of the ring structures. This is “Ground Zero” and is shown with the small red circle south of Sudbury. This impact centre is pinned to the foreland as it is largely based on Ring 3. The purple star represents the statistical focal point of best-preserved shatter cone axes and is also located south of the preserved structure. Restoring the shortening deformation in 4-5 progressive steps, including 50% shortening of the southern half, less in the northern half, and ~5% in the foreland up to a radius of 50 km, places the original South Range just in contact with Butler’s “Ground Zero” (see the blue trace). Hence, all of the preserved melt sheet represents only part of the northern half of the original melt sheet. Only ~10% of the melt sheet is presently preserved in the erosional remnant of the SIC. 20 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 9: Two critical stages in the formation of the Sudbury impact structure (adapted and simplified after Grieve, 1994): A) early during the impact phase and formation of the transient cavity, as shockwaves are transmitted into the target; and B) after collapse and rebound, and final settling of the melt sheet. Note the estimated position of the North Range (NR) and South Range (SR) in figure B. The diagrams also illustrate the general pattern of shatter cones in the target rocks around an impact crater during its formation and excavation (A) and after rebound (B). A shatter cone (S, outlined by square) forms in response to the shockwaves radiating out from the focus of the impact. When the floor of the transient crater rebounds, the cone and the cone and its axis are rotated up and the rocks may also move inwards due to collapse and large-scale material in-flow into the transient crater. In figure B, the concentric Hess Offset dyke is show, at ~50 km distance from “Ground Zero”. The Foy Offset dyke, more or less parallel to the section, is not shown but extends out to ~65 km (see Figure 8). The Hess Offset dyke cuts small synclinally folded outliers of Huronian rocks that help to define the down-folded and down-faulted annulus surrounding a broad area of central uplift. The overall extent of the final melt sheet, after settling, likely reached the well-defined Ring 3 with a radius of ~67 km. This conclusion is supported by at least one of the injection dykes, the Foy Offset, reaching close to this Ring 3, although the originally overlying melt sheet from which it was injected down has been removed by post-Penokean uplift and erosion. Thus, with an estimated radius of 67 km, and an average thickness of the melt sheet of 2.5 km (see also Grieve et al., 1991 and Naldrett and Hewins, 1984: 2–3 km on the preserved North Range, 3–5 km on the South Range), this suggests a final melt sheet volume (πr2 x d) of ~35 x103 km3. This estimate does not include the considerable melt component preserved in the ~2 km-thick Onaping Formation or within the ejecta that were blown far beyond the crater (tektites?) and into space. Thus, it is likely an underestimate. Nevertheless, reasonable uncertainty estimates put this total impact melt volume in the range of 25–50 x103 km3, which could be used as an input parameter into various scaling models and models of elemental mass balances. 21 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Of this original volume of settled impact melt sheet, the thin edge of which likely extended out to ~67– 70 km radius, only ~2.5–5.0 x103 km3 is preserved in the current extent of the SIC. This estimate can be derived by measuring the cross-sectional area of a central cross-section through the SIC (similar to Figure 7 but a bit farther east), multiplied by 0.5x the preserved width (long axis) of ~60 km. In other words, only on the order of 10% of the original melt sheet is represented by the currently preserved erosional remnant of the SIC, and all of this represents the proximal part of the northwestern half or lobe of the original melt sheet (see Figure 9), perhaps including a thicker “moat” north of the collapsed central uplift. As deformational structures, both large and small, typically nucleate on and amplify original “seed structures” or weaknesses, the curved main synclinal axis of the preserved SIC may be inherited to some degree from such a moat surrounding the core of the central uplift. This is where the melt sheet may be thickest and even larger orebodies could have collected. All of these key points should be considered in any interpretation of the Sudbury structure and its endowment of magmatic sulphide orebodies. Perhaps one of the more robust features of the final impact structure, as presently preserved in the foreland, and in relation to overall size estimates, is the down-folded and faulted annulus of Huronian outliers to the north of the SIC, with a pattern of synclinal fold traces that curves around the entire northern half of the structure, including around the western and eastern first-order fold closures of the structure (Figure 8). This down-folded annulus fits a ring structure with a radius of ~50–55 km. The annulus thus has an apparent diameter of 100-110 km, which is larger than the well-defined annulus of the circa 2023 Ma Vredefort impact structure in South Africa, where it is ~90 km in diameter. This can be scaled to a final crater diameter. The broad, collapsed central uplift must fit within this annulus and therefore has a total radius and diameter of ~45-47 km, and ~90–100 km, respectively, and includes all of the South Range area and the deeply exhumed Huronian to the south, and also the Levack Gneisses in the north. In large complex terrestrial impact craters the diameter of the broad central uplift area is roughly 1/3 of the final crater diameter (Therriault et al., 1997), the scaling relationship being: Dcu = 0.31 Df1.02 where Dcu is the diameter of the central uplift, and Df is the diameter of the final crater rim (Therriault et al., 1997). This would suggest that the final Sudbury structure may have had a diameter closer to 300 km and somewhat bigger than most estimates. Based on the estimate of the annulus diameter alone, it is clear that Sudbury is the largest among known terrestrial impact structures, just slightly larger than the more deeply eroded (no melt sheet preserved) Vredefort structure. Major Sudbury Breccia pseudotachylite occurrences reaching out to a radius of ~120 km supports such a larger estimate based on the scaling relationship of Stöffler et al. (1988): Dpst . 0.8 Df Where Dpst is the diameter of significant pseudotachylite breccia occurrences, i.e. ~240 km. This would result in a similar estimate of ~300 km for the final crater diameter (Df). 22 �Proceedings of the 68th ILSG Annual Meeting - Part 2 8. Conclusions The Sudbury structure represents the deformed erosional remnants of a very large 1850.0±0.9 Ma meteorite (or comet?) impact crater. The current consensus is that the final collapsed crater reached ~200– 260 km (final crater diameter, Df), but it may have been larger based on a robust size estimate of the downfolded and faulted circular annulus, and the most distal observations of significant pseudotachylite occurrences (Sudbury Breccia), some of which occur well beyond the 100 km-radius ring structure (Ring 4 of Butler (1994; see also Thompson and Spray, 1994)). We thus estimate the final crater diameter at ~300 km, which makes it the largest known terrestrial impact crater. The volume of impact melt generated was ~35 x103 km3. Once the crater had collapsed and rebounded, and the melt sheet had settled across a complex peak-ring crater, on a time-scale of mere hours (!) (e.g., see Grieve, 1994), the sheet of impact melt reached out to ~67–70 km, and had an average thickness of ~2.5 km. It may locally have reached 5 km or more (Figure 10). A large proportion of this melt sheet was initially superheated and underwent rapid homogenization and differentiation. Zieg and Marsh (2005; see also Golightly, 1994) point out some of the very complex processes involved as the final melting front burned into the footwall and different blobs of melt or partial melt were generated and may not have fully mixed and homogenized by turbulent convection. There likely was an early separation in i) less dense, more felsic melts, and ii) denser mafic melts, the first floating to the top of the melt sheet pool to form or contribute to the Granophyre of the Main Mass of the SIC, whereas the latter collected towards the base to form the Norite, and Sublayer. The Sublayer represents a complex boundary layer with remnant mafic and ultramafic fragments that collected along the base of the SIC, together with a rain-out of sulphides globules. The Transition Zone Gabbro represent the differentiated top of the lower mafic section, enriched in incompatible elements. The density contrast between the Granophyre and Norite (including the gabbro) is significant, and large enough such that the basal granophyre contact would have equilibrated in a near horizontal position, providing an important reference plane for evaluating the effects of later structural deformation. One of the important implications of this is that it allows a semi-quantitative reconstruction of the footwall topography of the SIC (Figure 10) and aids in general structural reconstructions (e.g., Figure 7). Some of the enormous volume of ejecta fell back into the crater, or was washed back in by various processes and formed the Onaping Formation. In a general sense, the upwards fining stratigraphy of the Onaping Formation and increasing carbon content reflect the waning energy levels and somewhat longer time scales of deposition, with the uppermost and finer-grained “Black Member” material transitioning into post-impact sedimentary and volcano-exhalative processes of the Vermillion member. The impact struck at a target site with complex geology, on the boundary of the rifted Superior craton and the developing Penokean fold belt of the Southern Province. The rebounded and settled crater, and the crater-fill deposits, were then covered by the expanding wedge of foreland basin sediments, first deeper water mud- and siltstones (foredeep?) and finally the greywacke turbidites of Chelmsford Formation at circa 1850–1840 Ma. Soon after, the crater, the melt sheet, and overlying deposits were deformed and shortened by the climax of Penokean deformation, which transformed the area into a doubly plunging regional syncline. The ~3-5 km-thick SIC melt sheet formed a very thick competent layer during this deformation, which resulted in a single large wavelength open fold structure. This syncline, particularly its southern limb, was further modified and shortened by post-Penokean deformation associated with the circa 1.80-1.60 Ga South Range Shear Zone. 23 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 10: A simplified, first-order reconstruction of the South Range melt sheet with its inherent thickness variations and footwall topography. It highlights the embayment structures where most of the significant orebodies collected (see Pye et al., 1984, and contributions therein), as well as the lateral thickness variations which suggest a second-order “peak-ring” on the overall central uplift, where the mafic section of the melt sheet (norites plus gabbro) is much thinner than in the “central puddle” that approximately overlapped “Ground Zero”. The dashed horizontal line indicates the level of the topographic highs of the secondary peak-ring. The section was constructed by first restoring minor second-order folding along strike of the South Range, then correcting from apparent thicknesses to true thicknesses, and finally hanging the section segments from the paleo-horizontal reference plane, i.e. the base of the significantly less dense granophyre upper section of the melt sheet. From west to east, star symbols identify the major magmatic sulphide deposits and producers: Vi, Victoria mine; AK, Aer-Kidd; To, Totten; Lo, Lockerby; Ge, Gertrude; Cr, Creighton; CC, Copper Cliff; CCN and CCS, Copper Cliff North and South mines; Mu, Murray; LS, Little Stobie; Fr, Frood; St, Stobie; Li, Thayer Lindsley; Ga, Garson; and Fa, Falconbridge. Of these Creighton and the Copper Cliff system are among the largest deposits known, and associated with the deepest embayments or funnels, in the floor of what appears to be a deep central puddle in the melt sheet, approximately overlying the centre of the collapsed central uplift. Following various stages of regional uplift and erosion, the end result is that only ~10% to 20% of the original melt sheet is preserved in the current erosional remnant of the SIC. Both the analysis of lineaments and ring structures, and the analysis of shatter cones, suggest that the preserved, folded melt sheet (the SIC) represent parts of the northern half of the original melt sheet and associated crater, with “Ground Zero” being situated south of the main footwall contact of the South Range. This is significant as most studies have implicitly assumed that “Ground Zero” was underneath the preserved SIC and that the preserved SIC (e.g., Golightly, 1994) more or less preserved a symmetrical patch of the original melt sheet and impact structure centered on “Ground Zero”. If the ring structure and shatter cone analysis is correct, this is clearly not the case and there is no inherent basis for symmetry between the North and South Ranges. Rather, the North and South Ranges represent fundamentally different parts of the melt sheet and the collapsed crater; the former represents a more distal northern part of the structure, and the latter a thick, more central part of the melt sheet lapping onto and overlying the rebounded and collapsed central uplift of the final peak-ring crater (see Figure 9). Petrologists had long recognized that something was odd about the Sudbury structure and its igneous rocks, the “Sudbury Irruptive”. In contrast with other large mafic igneous complexes, there was no layered lower section of mafic and ultramafic cumulates near the base, and way too much granitic granophyre near the top (~40% of the volume, rather than ~10% in a fully differentiated mafic intrusive complex). And the 24 �Proceedings of the 68th ILSG Annual Meeting - Part 2 mafic lower part had an unusual SiO2 content (~55–58 wt%), dominantly characterized by norites rather than gabbros 4. The regional deformation was also puzzling and under-appreciated, feeding into interpretations of the whole complex as a concave-upwards igneous lopolith. It took a marine geologist from California, who somehow had solved the mystery of shatter cones in a small number of suspected impact craters in the 1940–1950s (e.g., Dietz, 1947, 1959) 5, and who also was reflecting on the lessons learned from observing the lunar surface (Dietz, 1946), to come and check out Sudbury, and within days confirm his suspicion that it was, fundamentally, a large impact structure with major shock damage in the footwall and no damage in the melt rocks (the igneous rocks of the SIC). Being less familiar with the ores, he still chose to hedge his bets on some of the details, such as the origin of the ores, or the exact nature of the igneous rocks, settling on a hybrid model of a large impact crater that was then intruded by the igneous rocks of the irruptive. As late as 1970, the debate on Sudbury was summarized as follows in a major paper on the structure (see Naldrett et al., 1970): “There are two main theories of origin for the Sudbury structure. These are very different from one another and hinge on the interpretation of the Onaping formation and certain unusual pre-irruptive breccia dikes. Speers (1956, 1957), pointed out that the Sudbury Irruptive lies at the apex of a broad dome some sixty miles in diameter involving Huronian and older rocks. He postulated that uplift of the dome occurred in response to pressure exerted by igneous magma. Successive episodes of uplift, followed by tensional release, gave rise to the breccia dikes and finally resulted in caldera collapse at the apex of the dome. Magma escaping around the rim of the caldera and flowing into the center of collapse produced the Onaping formation. The Irruptive was intruded subsequently, spreading out along the base of this formation. According to this hypothesis, the Nickel Irruptive is a later plutonic manifestation of the igneous activity which previously had given rise to the extrusive Onaping formation. In opposition to this hypothesis, Dietz (1964), suggested that the circularity and brecciation characteristic of the Sudbury structure could best be explained by an explosive meteorite impact, an interpretation for which his own discovery of shatter cones gave support. This theory led French (1967) to find, in inclusions in the Onaping formation, microscopic features characteristic of shock metamorphism. Subsequent work has shown that shock metamorphism, a typical feature of impact sites but unknown in volcanic rocks (French and Short, 1968), is widespread and common in the Onaping formation; it also is found in footwall rocks adjacent to the Irruptive and in fragments in Sudbury breccia. According to the meteorite impact theory the sequence of events was as follows: shock waves radiating from the point of impact produced brecciation, melting, microscopic shock features and shatter cones, and excavated a circular crater; part of the material blasted from this crater fell back as a poorly, sorted See, for instance, the papers by Wilson (1956) and Hamilton (1960), written and published just prior to Dietz’ shatter cone revolution, to appreciate the conceptual struggles that petrologist and geologist were dealing with to explain major aspects of the Sudbury structure. In his paper, Hamilton is inching towards an essentially extrusive interpretation for the Sudbury lopolith. 5 See the amazing review paper by Bourgeois and Koppes (1998) to better understand the historical development of these ideas, and all the players involved, including of course the life and career of Dietz himself. Originally from New Jersey, he was broadly educated with degrees from the University of Illinois and having spent time at the Scripps Institution for Oceanography. He was also well-travelled. A professional posting in Europe during the 1950s had also allowed him to visit the Steinheim and Ries basins in Germany. 4 25 �Proceedings of the 68th ILSG Annual Meeting - Part 2 breccia—the Onaping formation; fracturing and heating of the rocks, and reduction of pressure in the upper mantle below the crater, triggered the evolution of the Nickel Irruptive; the magma was emplaced in the breccia zone beneath the center of the structure.” [End of quote.] It would take another 15–20 years for clarity to emerge. Once the isotopic evidence became available showing that essentially all the igneous rocks, including the more mafic ones, have bulk Nd isotopic signatures that reflect melting of the crust, rather than melting of the mantle, the pendulum finally swung to an impact-only interpretation (Stöffler et al., 1989; Grieve et al., 1991). Bulk sample isotopic values and mixing equations may still hide a very small mantle component into some of the melts but, to date, no conclusive evidence for this has emerged. What lessons can be learned from all of this? One is that it is critical to think “big”, always, and broaden one’s horizon, and to to reflect on new ideas from related or not-so-related sciences. The other is that, as our understanding of the impact record grows, particularly on nearby planetary surfaces, there got to be other Sudbury’s out there, with perhaps less than ~10% preservation: just the odd bit of breccia; or a poorly preserved shatter cone here or there; or some odd dyke of quartz diorite with some sulphides in it, and which was just a little too hot for your average diabase dyke! Remember, every breccia is an interesting breccia! Acknowledgements The first author (WB) was first introduced to some aspects of Sudbury geology in 1987, during a fall fieldtrip of the Canadian Tectonics Group. He has worked, on and off, on Sudbury geology since the early 1990s, first as a researcher for Falconbridge Ltd. at Onaping Mine, and later for the Geological Survey of Canada. The second author (SK) has been involved in precise U-Pb dating of many of the rocks in and around Sudbury. The geology of this unique area never stops to fascinate. WB would like to thank coleaders for helping to put this fieldtrip together, and also those who gave the guidebook a proof-read to catch some of the imperfections. 26 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Excursion Stops, Day 1: Bedding at this locality dips and youngs to the southeast. The shatter cones are well developed and enough of the cone surfaces are visible, such that their axes can be measured accurately. These axes point up towards the north-northwest (~340º/+45º, i.e. up), and they plunge up more or less parallel to the southeast dipping bedding surface, perhaps just a bit shallower than the bedding dip angle (~45– 55º). Although there is plenty of impact brecciation in these footwall rocks, locally chaotic, these outcrops are not chaotic and align with the regional pattern. The shatter cone axes therefore point approximately to “Ground Zero”, the centre of the impact structure. Stop 1: Well-developed shatter cones in Mississagi Formation quartzites, south of Sudbury 46.432448° N, 81.072267° W 494448E 5142100N This is one of the classic shatter cone localities south of the Sudbury structure, and probably one seen by Dietz early on during his 1962 fieldtrip to Sudbury (Figure 11). The conical, striated fracture surfaces can be seen both in the blasted roadside outcrop on the south side of the gravel road (Gibson Road), but also on top on natural outcrop surfaces. Typically, shatter cones are more obvious in blasted outcrops where the rock surface has been opened up, and are more easily missed on natural surfaces. We will see examples of that on this fieldtrip, but this is not a limitation here. Spectacular shatter cones are visible south of the road, on top of the outcrop. In meteorite impacts, the shockwaves travel out from the centre of the transient crater, outwards, and interaction of the high-velocity shock waves with imperfections in the rocks nucleate the conical fracture surfaces. The apparent point of origin of the shockwaves is below ground, as the crater is being excavated and the ground is depressed. Therefore, away from “Ground Zero”, shatter Figure 11: Classical “shatter cones” in the shocked footwall and target rocks of the 1850 Ma Sudbury impact crater, well developed in quartzites of the circa 2.4 Ga Mississagi Formation. With the newly recognized knowledge, in the mid- to late-1950s, that these conical, radiating fracture surfaces represent unique “trace fossils” for high-velocity, very high pressure shock waves associated with meteorite impacts (Dietz, 1959), Sudbury was quickly recognized as an astrobleme—the scar of an ancient impact crater (1962; see Dietz, 1964; and Dietz and Butler, 1964). 27 �Proceedings of the 68th ILSG Annual Meeting - Part 2 cones will fan outward and their axes will be shallow. On rebound of the crater floor, the central uplift moves up and rotates the cones, with their axes now pointing up (see Figure 9). If marginal rocks are turned up or even flipped over during crater formation, the cones may point outwards in rocks that are flipped over (as in the collar around the Vredefort central uplift). Here this is not the case, the rocks are right-way up, dipping moderately to the southeast, and the cones plunge up to the north-northwest. Hence, just from this outcrop alone, it seems “Ground Zero” was to the north, and their dip was subhorizontal when the shockwaves hit. The cones can be examined on top and in section, to determine their axes, as will be demonstrated during the visit. Stop 2: Well-developed shatter cones in blasted Mississagi Formation quartzites, south of Sudbury 46.422030° N, 81.086392° W 493367E, 5140943N This roadside outcrop is again in Mississagi quartzites, somewhat to the southwest of the previous locality. Here beds dip and young towards the north, so we have travelled across one of the many folds in the Mississagi Formation. Several reasonably developed shatter cones are visible in the rocks on the east side of the road (Figure 12). The cone axes here plunge down and to the north, in approximately identical relative orientation to the bedding surface, but now plunging down! Figure 12: Shatter cones plunging down to the north-northwest, on the southern limb of the local syncline. Both bedding and the shatter cone axes have been re-oriented by the Penokean folding. 1) The local Huronian strata were (sub)horizontal at the time of impact; 3) The shockwave traveled from north to south and formed more or less flat lying shatter cones with their apices plunging gently to the north; 4) And both bedding and cones were affected by the folding that affected the south range of the Sudbury structure. Duplicating this exercise at as many as possible localities, and then intersecting all cone axes statistically, looking for a maximum of intersections, can define the origin of the shockwaves, i.e. “Ground Zero”, or perhaps a point below “Ground Zero”. Just from these two outcrops alone, it is clear that there is a correlation between final bedding attitude and final attitude of the cones: both have been affected by moderately tight folding that produced the local synclines and anticlines in the Mississagi Formation. Unfolding of these folds and restoring bedding to approximately horizontal will also align the cones between Stop 1 and Stop 2. This general story is repeated all through the area and the conclusion must therefore be: 28 �Proceedings of the 68th ILSG Annual Meeting - Part 2 This outcrop also demonstrates that shatter cones are rare on naturally weathered outcrop surfaces but are more clearly developed or visible on certain broken rock surfaces. scale of this dyke emplacement, relative to “time zero” could be years to thousand of years? Here the dyke does not show evidence for superheating and melting of adjacent country rocks, but elsewhere melting can be observed along the contact. Stop 3: Southern extent of the Copper Cliff “Offset dyke” cutting across Pecors and Mississagi Formation strata After emplacement and cooling, the dyke was moderately deformed together with the country rocks during the largely Penokean shortening and folding deformation. 46.434569° N, 81.068932° W 494704E, 5142335N The sedimentary rocks on the south side of this Gibson Road locality represent the transition from the finer-grained Pecors Formation to the quartzites (quartz arenites) of the Mississagi Formation. Dips and younging direction are to the south, as at Stop 1, which is just along strike. The Huronian strata are cut by a ~25 m-wide mafic dyke, which at first sight looks not unlike a diabase dyke. This dyke is subvertical and trends south, and its contact relationships look rather typical for a diabase dyke, wandering a little bit and stepping sideways a little bit here and there. There is, however, no known swarm of this trend and/or the right age to explain this dyke. Stop 4: Unsorted matrix-supported conglomerate (reworked diamictite) of the Ramsay Lake Formation 46.433508° N, 81.077564° W 4947041E, 5142218N This glacially polished outcrop is on the north side of the Highway 17 Bypass just south of Kelly Lake. Note that this is a very busy highway with heavy, high-speed traffic. The shoulder of the highway allows cars to stop safely here but make sure to pull well on to the shoulder. This is an outcrop of poorly sorted, matrixsupported, sandy conglomerate typical for the lowermost of the three glacial formations in the Huronian succession. The Ramsay Lake Formation is the basal unit of the Hough Lake Group (see Figure 4), which overlies the rift and rift-fill succession of the Elliot Lake Group, locally with a sharp contact. Petrography shows it to be a medium-grained quartz diorite, with some chilling near the margin. A quartz diorite is rather atypical for a regional diabase swarm, which are essentially all basaltic. It is perfectly along strike of the Copper Cliff Offset dyke to the north, on the north side of Kelly Lake, which can be mapped into the base of the Sudbury Igneous Complex (SIC). This outcrop shows the unsorted nature of the conglomerate and on an outcrop like this one could debate the evidence for a glacial origin. Some bedding surfaces and cross-bedding are visible. Long (2009) describes these rocks as sub-glacial melt-out till. Elsewhere the Ramsay Lake Formation, which typically is thick to very thickly bedded, or even massive, has a finer-grained, darker matrix and looks more like a typical diamictite (Figure 13). Here we are at the top of the Formation, where the till material was reworked and sorted to some degree by sub-glacial processes. It is tempting to think of these “radial offset dykes” as having been emplaced laterally from the north, but that is not the right interpretation. Almost certainly, these dykes of basal melt rock from the SIC were injected downwards from the overlying melt sheet into active fracture planes in the deforming footwall, during the overall crater modification processes and settling and early differentiation of the melt sheet. Overall homogeneity of these quartz diorite dykes indicates that the melt sheet had already mixed and homogenized to some extent. The overall time 29 �Proceedings of the 68th ILSG Annual Meeting - Part 2 The brecciation, locally approaching large-scale chaos, is typical for many areas in the footwall. Dykes and veins, and more irregular domains of fine-grained, dark pseudotachylitic rocks, are what is referred to as “Sudbury Breccia”, indicating intense brecciation and pulverization of the target rocks during impact and intense deformation associated with rebound and collapse of the central uplift. All these rocks are cross-cut by an irregular dyke-like body of quartz diorite emanating from the Copper Cliff funnel. The outer contact of this quartz diorite dyke induced melting and mixed with the adjacent rhyolite (Figure 16). The quartz diorite itself shows typical quench textures: more or less spheroidal structures of radiating, fine acicular pyroxene/amphibole crystals. Together, these observations show that this quartz diorite was injected in a superheated state and then quickly cooled due to interaction with wall rocks. Locally the quartz diorite dyke rock interacted with dark Sudbury Breccia pseudotachylitic material, overall showing that the latter was marginally older but not yet acting fully lithified or brittle. Figure 13: Unsorted more typical diamictite of the Ramsay Lake Formation, with a dark fine-grained matrix (not this outcrop). Pebbles and cobbles are mostly granitoid rocks. Dark patches with striae are a rare example of how shatter cones are exposed on naturally weathered (and polished by Pleistocene ice movement) surfaces. Stop 5: Mineralized Copper Cliff Offset dyke, up from the walking trail at Copper Cliff 46.470977° N, 81.075562° W 494199E, 5146381N The outer phase of the dyke is known as typical quartz diorite, or “QD”, and is generally similar to quartz diorite of many of the offset dykes (e.g., see Stop 3). It generally does not contain sulphides, and does not weather rusty. This outer QD was intruded by one or more phases of dyke injections that are characterized by carrying along inclusions of varying size (inclusion-bearing quartz diorite, or “IQD”), many of which are more mafic, together with variable amounts of sulphides, either disseminated or as conspicuous cm-size globules. A central phase of the dyke carries semi-massive sulphides, which are enriched in Cu. Hence, in these outcrops here, three successive phases of dyke injection can be demonstrated, which mark different stages in the evolution of the melt sheet: This series of outcrops on the side of a hill overlooking the urban area of Copper Cliff exposes parts of the major Copper Cliff Offset dyke which can be mapped to the north into a major “funnel” or narrow embayment structure at the base of the SIC. Major Cu-Ni sulphide mineralization occurs in this funnel structure and into the dyke (Figures 14 and 15). Walking up toward the dyke, one walks across heavily brecciated wackes and arenites at the base of the McKim Formation, and into the top of the Copper Cliff Rhyolite Formation. Bedding features at the base of the McKim Formation, such as graded bedding, scours, and truncated bedding indicate younging is towards the southeast in steeply southeast dipping strata, i.e. stratified sediments that overlie the rhyolites to the north. The latter show quartz phenocrysts and beautiful flow lamination in places. 30 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 14: Map and corresponding longitudinal section, looking west, of the Copper Cliff Offset dyke and its associated orebodies, supporting several active mines. North is to the right in the map figure. Modified after Cochrane (1984) and Farrow and Lightfoot (2002), based on mine and exploration sections. 1) Initial homogenization and injection of unmineralized quartz diorite, probably just prior to sulphide saturation having taken place. 2) Renewed injection from near the base of the melt sheet, with abundant inclusions and sulphide globules being entrained. 3) A final phase of injection, after sulphides had collected and had become enriched in Cu due to Fe-rich and Cu-poor monosulphide solid solution (MSS) having separated out, enriching residual sulphide liquid in Cu and other MSS-incompatible elements (e.g., Craig and Kullerud, 1969). Elsewhere, a fourth and final phase of injection can be recognized, consisting of plagioclasephyric, sulphide-free quartz diorite, representing differentiated norite from the main melt sheet, making it into the footwall. On Figure 14, the mineralized bodies are shown in magenta along the extent of the offset dyke (after Cochrane, 1984). In the corresponding longitudinal section, looking west, the overall extent of the sulphide bodies in the dyke are shown, forming kmscale steeply plunging “fingers”, generally along the centre of the dyke, but sometimes along the margin. These steeply plunging fingers of mineralized IQD clearly indicate emplacement was downwards from the base of the overlying SIC (now eroded away) where sulphides had collected. The vertical plunge of these IQD “dyke in dyke” injections was further amplified by N-S shortening during folding, and vertical extension, but deformation intensity is insufficient to explain the observed aspect ratios. So, the long axis of the orebodies, well defined by drilling and mining, indicates the injection direction: i.e., down from an overlying but now removed melt sheet undergoing critical stages in magmatic evolution: 31 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 15: Longitudinal section similar to but expanded from Figure 14, with full interpretation. Note the full extend of the Copper Cliff Offset dyke with an initial “leading sheet” of relatively homogeneous quartz diorite, followed by later injections of mineralized inclusion-bearing quartz diorite and entrained sulphides, all down from the overlying but now eroded Sudbury melt sheet, which underwent final differentiation into a thick basal norite section (~3 km), a thin transitional quartz gabbro section (~500 m, TZG in blue), and an upper “granophyre” section of broadly granitic composition (~1–2 km, pink). As shown on the figure, the overall ore formation process can be divided into six stages: 1) sulphide saturation, 2) growth and sinking of sulphide globules, 3) collection of sulphides along the basal contact, particularly in topographic low or “embayments”, 4) the onset of liquid fractionation of the sulphide melt due to MSS fractional crystallization, and progressive enrichment of the residual sulphide melt in Cu, 5) episodic injection of dense melts into footwall fractures, and 6) further sub-solidus remobilization of ore components during later deformation. The dip of the SIC is due to subsequent deformation. 1) Initial homogenization due to rapid convection in a superheated impact melt; 2) Rapid sulphide saturation; 3) Collection of basal sulphides and entrained inclusions in a still very hot magma; 4) And repeated injection of basal phases of the evolving melt sheet into footwall fractures forced open by i) on-going movements in the adjusting footwall, and ii) fluid pressure of the magma. The density of the overall “dioritic” (more or less, average crust, well mixed) is about 2.8 g/cm3 and is denser than the average density of the footwall rocks. Figure 15 above shows the completed section and interpretation. In many ways this one section tells much of the story of the Sudbury structure, its evolving melt sheet and its orebodies. 32 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 16: Some key features associated with the quartz diorite offset dykes. A) Margin of the Copper Cliff quartz diorite dyke, against melted rocks of the Copper Cliff Rhyolite country rocks. This degree of melting is highly unusual for normal mafic dykes of this size and clearly indicates the superheated state of the first injections of the Copper Cliff Offset dyke. B) Quench textures in outer (unmineralized) quartz diorite indicating rapid cooling from a superheated states (no crystallization nuclei), followed by rapid crystallization. C) Inner inclusion-bearing quartz diorite with rusty sulphide blebs, cut by a last phase of plagioclase-phyric quartz diorite dyking along the core of the Worthington Offset dyke, Aer-Kidd Mine area. 33 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Stop 6: Extensively developed Sudbury Breccia developed near the top of the Copper Cliff Rhyolite Formation, Lively Arena Robert Dietz, being exposed to this relatively new science of lunar geology in the 1950s, and also having carefully read the early papers/reports by Boon and Albritton (1936, 1937, 1938), quickly broke through this stasis in geological thinking. 6 46.427506° N, 81.144626° W 488888E, 5141558N There is a general lesson here: always broaden your horizon, think out of the box, and listen to what may be totally new views out of left field. This glacially polished outcrop shows spectacular development, essentially chaotic at the ~5–10 m scale, of Sudbury Breccia (often abbreviated to SBX). Metre-size clasts, some rounded, float around in a dark matrix of pulverized rock flower that in some cases may have melted. Many of the fragments can be linked to the surrounding Copper Cliff Rhyolite Formation, the rhyolite that formed the uppermost felsic component of the basal rift succession of the Huronian. However, other fragments are more “exotic”, relative to local outcrops, and illustrate significant movement of breccia material, and injection for some distance into dilating fractures. One final anecdote: One of us remembers, as a 21-year-old undergraduate student back in Amsterdam, in 1980, sitting through a (very) long lecture by an eminent metamorphic petrologist talking about the Vredefort Dome in South Africa, and how it could only be an endogenic cryptoexplosion domal structure based on this or that metamorphic reaction ... . This was 20 years after Hargraves (1961) and Dietz (1961) showed a systematic pattern of shatter cones around the dome, which even then was long known to be riddled with large pseudotachylite bodies and dykes (Shand, 1916). This outcrop illustrates an important truism: any kind of breccia is an interesting breccia! Stop 7: Pillow lavas of the Elsie Mountain Formation, lower Huronian volcanic rocks Seeing this amount of breccia, and the dynamic processes that must have been involved, requires a generative process with sufficient cause and energy, and volcanism clearly is not it. Although this particular outcrop is very spectacular, similar breccia bodies, dykes, and veins occur all over the Sudbury area (in the footwall), and even early workers were familiar with the fact that SBX occurred up ~60–70 km away from the Sudbury Igneous Complex. 46.442417° N, 81.147691° W 488655E, 5143216N In these roadside outcrops along the main road north out of Lively, the mafic volcanic rocks of the lowermost Huronian rift succession are well exposed. Plagioclase-phyric basaltic pillow lavas of the lowermost Elsie Mountain Formation are steeply dipping and facing south (Figure 17). The pillows show well developed rims and, although moderately flattened, show enough asymmetry to determine top directions, to the south. In hindsight, and with our present understanding of Solar System geology, it is easy to see that only an ancient meteorite impact had sufficient energy to do this much damage, and at this scale. As stated in the introduction, it is perplexing how early workers clung to the cryptovolcanic explosion model for that long, perhaps largely due to nonfamiliarity with the emerging knowledge of the geology of the Moon and other planetary bodies. These lavas, together with the Copper Cliff Rhyolite, comprise a bimodal succession typical for continental rifts. Their overall age is 2460–2480 Ma. The mafic lavas, with their prominent plagioclase crystals, were fed by the similarly plagioclase-phyric diabase dykes of the circa 2460 The short and clearly written papers by Boon and Albritton in the 1930s could be characterized as “Sleeping Beauties” (van Raan, 2004; see also Ke et al., 2015; Miura et al., 2021), i.e. papers that were ahead of their time, and went largely unnoticed until they were “discovered” decades later. Boon and Albritton (1937, 1938) add Vredefort to their growing list of suspected impact craters. 6 34 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Ma Matachewan swarm (e.g., Heaman, 1997), which are of the same age and which riddle the Archean basement to the north. None of these dykes cut across the uppermost volcanic unit, the Copper Cliff Rhyolite, where they would be easily noted. Hence, the Copper Cliff Rhyolite marks the final, felsic phase of this rift magmatism. Together with the subvolcanic A-type granite bodies of the Creighton Granite, and similar plutons along strike, this final phase has been dated at circa 2460 Ma (2459±7: Bleeker et al., 2015). Figure 17: Pillow lavas near the top of the Elsie Mountain Formation, steeply dipping and younging to the south. Stop 8: Creighton Mine, among the largest and deepest mines of the structure Overall, the structure of Creighton Mine is typical for the more strongly deformed South Range of the Sudbury structure, with the basal contact of the SIC dipping ~45–50º at surface and steepening at depth, with second-order structures superimposed. At depth, the steep basal contact is cut and offset by a discrete south-dipping shear zone, with south-side up displacement (e.g., Papapavlou et al., 2018), which is part of the postPenokean “South Range Shear Zone” deformation that has further shortened the South Range. 46.461018° N, 81.176757° W 486427E, 51415287N We will make a brief stop here on the access road to the Creighton Mine, one of the largest and deepest mines of the Sudbury structure. Various magmatic sulphide deposits occur at or near the basal contact of the SIC in what is one of the more prominent “embayments” along the basal contact. Creighton Mine geology and structure will be introduced and discussed by means of the composite cross-section shown below (Figure 18). 35 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 18: A) Composite cross-section of the Creighton Mine with its various ore lenses (projected onto a common section). Creighton Mine is one of the largest producers and it is getting very deep. As shown on the section, there is the typical variety of ore types, including more fractionated Cu-rich ore in the footwall, often controlled to some degree by local structures. Section modified after various published sources and original Inco mine sections. Because of the large depth and deep mine infrastructure, the mine also hosts one of the major neutrino labs in the world, the SnoLab, at about 2 km depth. Research at this lab was among the work that has demonstrated some of the fundamental characteristics of these elementary particles. 36 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 18: B) Photo of an active mining face in a Cu-rich orebody in the footwall complex below the SIC at Creighton Mine. The vein-like body is several metres wide and shows sharp, in part structurallycontrolled, contacts. Inclusions of wall rock are suspended in the sulphide matrix and in various states of dismemberment by both physical and magmatic processes. Stop 9: Creighton Granite, gabbro enclaves, and Sudbury Breccia The somewhat coarser grained gabbro enclave at the east end of the road section has been dated at circa 2479 Ma (Bleeker et al., 2015), thus being part of the early Matachewan (Matachewan I) event, which also emplaced larger layered intrusions at the base of the Huronian section, at or near the unconformity with Archean basement. 46.453751° N, 81.186197° W 485700E, 5144481N To the south of Creighton Mine, a ~300 m-long section of the Lively regional road exposes superb outcrops of the Creighton Granite pluton, variably affected by dykes and veins of dark Sudbury Breccia (SBX). The granite hosts major enclaves of early Huronian gabbro/diabase sills or dykes, some with very prominent zone calcic plagioclase megacrysts, which clearly link them to the Matachewan magmatism and large igneous province. Overall, it is rather challenging to get highly precise U-Pb ages on many of units in this area, due mainly to two related reasons: 1) all the pre-1850 Ma zircons are shocked and disturbed, and 2) the zircons are generally altered. Results on the Creighton Granite and the Copper Cliff Rhyolite are shown in Figure 19, showing the scatter and general “pull down” due to shock-induced Pb loss at 1850 Ma, with superimposed younger Pb loss. Many individual analyses, whether single grain or multigrain, are almost meaningless due to these There are also remnants of lower Huronian volcanics with interlayered sandstone layers, which here show graded bedding suggesting tops are to the north, not to the south. 37 �Proceedings of the 68th ILSG Annual Meeting - Part 2 combined Pb loss effects. This resulted in the circa 2350 Ma early age estimates for the Creighton Granite (Frarey et al., 1982), a result based on several un-abraded multigrain zircon fractions that would have averaged out all these effects and plot in the middle of the Pb loss array or triangle (see Figure 19). The current apex of this Pb loss triangle is constrained by analyses of single, small, best- preserved zircon grain fragments pre-treated by chemical abrasion. Collectively the data indicate a minimum age of 2455 Ma for the Creighton Granite and the co-magmatic Copper Cliff Rhyolite, with a most likely upper intercept age of 2459 +7/-4 Ma. Figure 19: U-Pb concordia diagram of the combined results on Creighton Granite and Copper Cliff Rhyolite samples, showing the complex Pb loss array or “triangle” formed by various Pb loss processes. Shock effects of the Sudbury impact have damaged most if not all of the zircon crystals to varying degrees (e.g., Krogh et al., 1996), with results being pulled down to an 1850 Ma lower intercept. The variably altered zircons were then affected by varying stages and degrees of younger Pb loss, including recent Pb loss. Large multigrain fractions, non-abraded, from the early Frarey et al. (1982) study, plot in the middle of the triangle, having averaged out all the various Pb loss processes. Only tiny, best-preserved, single zircon fragments pre-treated by chemical abrasion (CA) from recent studies (Bleeker et al., 2015) plot near the apex of the triangle, constraining both a minimum (2455 Ma) and most likely upper intercept age of 2459+7/-4 Ma for the felsic magmatism. In contrast, comparatively large laser ablation spots on these complicated zircons, without CA pre-treatment, will simply sample all this complexity in Pb loss and result in meaningless upper intercept ages. 38 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Stop 10: Magma mingling structures between Creighton Granite and mafic magmas Stop 11: Basal contact of the Sudbury Igneous Complex 46.459667° N, 81.203009° W 46.450530° N, 81.195107° W 484411E, 5145142N 480515E, 5144125N Roadcuts along the Highway 144 Bypass expose the basal contact of the SIC on underlying Elsie Mountain dark, mafic volcanics and gabbro sills. As was shown at Stop 9, younging in the volcanic section is most likely to the north here, into the base of the SIC. These outcrops show how difficult it is to actually put one’s finger on the lower contact of the SIC, due to the nature of immediate footwall rocks. The section is locally cut by felsic dykes, which are “rheomorphic” dykes of melted footwall that were back-intruded into the base of the SIC. These dykes are of interest in terms of constraining final melt formation and migration in the footwall rocks, but they are very hard to date because they contain essentially all xenocrystic zircons from the underlying rocks (e.g., Creighton Granite), with all their shock damage, and very few newly grown zircon crystals. A couple of hundred metres farther to the north, there are the first outcrops of typical, massive, homogeneous norite from near the base of the Main Mass of the SIC. Just south of the intersection of the Lively regional road with the Highway 144 Bypass, on the east side of the highway, are roadcuts through the Creighton Granite with classic magma mingling structures: rounded blobs of mafic magma suspended in surrounding Creighton Granite. This is significant in the sense that it clearly demonstrates contemporaneous mafic and felsic magmatism in an overall bimodal magmatic system. This is relevant to the interpretation of the Creighton Granite and the confusion about the “Blezardian orogeny”. The Creighton Granite is not a terminal collisional granite, i.e. the interpretation that fed the idea of a Blezardian orogeny, but rather an early A-type granite associated with the final rift-related magmatism at the base of the Huronian Supergroup. Stop 12: Top of the norite section, across the transition zone gabbro, and into the base of the granophyre section 46.487202° N, 81.207053° W 484109E, 5148202N About 3 km north of Stop 11, just north of the powerline and in a lazy curve of the Highway 144 Bypass, occurs the transition zone from uppermost norite, into a ~500 m thick gabbro section where augite becomes the dominant pyroxene, rather than hypersthene, and into the base of the thick granitic granophyre section. Figure 21 shows the typical variation in modal mineralogy across the SIC, as compiled from various sources. The “Transition Zone Gabbro” crystallized oxides and apatite, giving it a much higher magnetic susceptibility. Figure 22 shows some petrographic details of the “black norite”. Figure 20: Typical magma co-mingling structure of rounded blobs of mafic magma interacting with K-feldspar porphyritic granitoid magma of the Creighton Granite. 39 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 21: First-order variations in modal mineralogy, density, and whole-rock chemistry across the Main Mass of the SIC. Figure A and B from Naldrett and Hewins (1984); C from observations by the first author; D, E, F and G from Zieg and Marsh (2005), North Range; F and H from Lightfoot and Zotov (2005), South Range along Highway 144. Profiles resized to a common scale to highlight first-order features. Note the interesting spike in Ni values in the South Range “black norite” (see star in H), where samples also show large scatter in La (see G)), approaching values in basal norite. This area in South Range norites also shows shallow mineral lamination, suggesting a major structure may repeat the basal norites (see section of Figure 18). Grey bars are: in H, initial Ni values in early quartz diorite, highlighting the large Ni depletion in much of the SIC; the dashed line indicates Ni values in glassy melt fragments in the Onaping Formation (Ames et al., 2002); in F and G, upper crustal average values from Rudnick and Gao (2005). Based on normal liquidus and solidus temperatures for these various compositions, one predicts that the base of the granophyre would crystallize last (see Figure 6). Overall heat loss would be highest from the roof of the SIC, and less so into the footwall of the SIC. Crystallization of the norites and gabbro would add latent heat of crystallization into the base of the granophyre section, thus keeping it hot and molten until the upper crystallization front closed in on it from above. Toward the top of the gabbro section occurs a ~50 m-wide zone of very coarse-textured “Crowsfoot Granophyre” (Figure 23). This probably reflects the final accumulation of H2O in an evolved residual gabbro magma, promoting coarse crystal growth in the residual melt. So, although this unit is called “granophyre” it is probably better seen as the top of the gabbro section. Up from here, one enters the base of the “Granophyre” proper, which is of broadly granitic composition and fairly evenly grained and homogeneous. 40 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 22: Thin section image (SEM, back-scattered electron map) of “black norite” showing typical mineral lamination of platy feldspars; typical zircon and baddeleyite crystals are highlighted and shown in close-ups on the right. Note the characteristic dendritic or skeletal habit of the zircons, typical for the norites, suggesting crystallization in a rapidly cooling melt, originally superheated with no nuclei. Zircons from this sample were dated using the Pb evaporation technique, resulting in an age of 1849.7±0.2 (Bleeker et al., 2015). With these expectations, we attempted to precisely date final crystallization of the Main Mass and this resulted in a precise and concordant zircon age of 1850.0±0.9 Ma, which fully overlaps with the best results on ages for the norites. From this we can conclude that crystallization of the entire SIC melt sheet was all within a million years, and likely well within the current resolution of the best available ages. As our ages get better and more precise, this age range of crystallization and cooling may shrink further. The boundary between the top of the Transition Zone Gabbros and the base of the Granophyre represents a major boundary in physical parameters, among them density (Figure 21). The granophyres are on average 1.4 g/cm3 less dense than the norite section and much less dense than the oxide-rich gabbros. The base of the thick granophyre section, also last to crystallize, would thus have equilibrated in an essentially horizontal position after settling of the thick melt sheet. It thus represents an important “paleo-horizontal” reference surface when thinking about the overall structure. 41 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 23: Coarse-grained radiating textures of the “crows foot granophyre”, which probably represents the H2O-rich residual liquid at the top of the Transition Zone Gabbro section. It resembles typical pegmatitic zones at the top of other large differentiated gabbro sill complexes (e.g., Nipissing Diabase sills). If so, this is indeed better interpreted as the top of the gabbros, rather than the base of the granophyre section of the SIC. Stop 13: Typical granophyre, lower half of the Granophyre section Stop 14: Basal section of the Onaping Formation, “Grey Member” rich in angular to rounded Huronian quartzite fragments 46.498015° N, 81.204066° W 46.524529° N, 81.195052° W 484341E, 5149403N 485040E, 5152347N A large pull-out on the west side of the Highway 144 Bypass provides an easy place to examine typical granophyre about 1 km above the transition with the gabbros. Here the granophyre is medium grained, and evenly textured. This spectacular outcrop shows a perfect example of the clast-rich base of the Onaping Formation, with a clast population dominated by Huronian quartzite fragments 1–20 cm in size, somewhat rounded to angular. The fragmental material is tightly compacted into a “suevite breccia”, the latter name used when impact melt clots can be recognized. Early workers on the “Sudbury Irruptive”, among them petrologists, had noted of course that the SIC was unusual in two main ways: 1) the basal norites being more silica-rich (~56–58 wt%) than other large mafic intrusions, and 2) being characterized by a very thick granitic “granophyre” section. Typical large layered intrusion would differentiate into a mafic-ultramafic layered base, and overall perhaps ~10% of evolved granitic granophyric material underneath the chilled roof section. Clearly something was odd about Sudbury! The fragmental rocks are moderately to strongly deformed with a strong, southeast-dipping schistosity/cleavage that is axial planar to the overall syncline of the Sudbury structure, with a stretching lineation that plunges down-dip (on the cleavage plane). Although these structural elements are largely Penokean in origin, they were likely amplified by the younger deformation of the South Range Shear Zone (Shanks and Schwerdtner, 1991). 42 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Stop 15: Upper member of the Onaping Formation (“Black Member”) core of the doubly-plunging Sudbury Basin syncline, which preserved the folded foreland sedimentary wedge from regional uplift and erosion. 46.535044° N, 81.185639° W 485765E, 5153514N Prior to the regional uplift, over the course of the Proterozoic, the Chelmsford and Rove Formation foreland wedges may well have connected. With extensive rock cuts on both sides of the Highway Bypass, this is a good outcrop to examine the upper part of the Onaping Formation, which is finer grained and overall upward fining (see Figure 6), darker, and traditionally was called the “Black Member” of the “fall-back breccias”. The darker colour of the breccias going up-section reflects, in part, a finer-grained matrix, and also an increasing carbon content. Bedding in the Chelmsford Formation is varied, with locally thick sandy turbidite beds. Bedding is folded and overprinted by the regional cleavage that is perfectly aligned with the axial plane of the overall fold structure. Large carbonate concretions are deformed and give an indication of the finite strain. In a general sense this “Grey Member” and “Black Member” terminology is still useful, but more detailed mapping in the last two decades has shown a more complex system of different depositional units including volcanic deposits due to venting of a still active melt sheet, and debris flows (see the work by Ames et al. (2008a,b) and references therein). So only part of the Onaping Formation represents true suevitic fall-back breccias. Other parts were reworked or washed back into the crater by processes other than strict fall back. Here along the highway, and the railway cut above, the Black Member of the Onaping Formation is well exposed in large rock cuts. Micro-diamonds have been reported from these rocks (Masaitis et al., 1999; see also French, 2004). Stop 16: Greywacke turbidites of the uppermost Whitewater Group, the Chelmsford Formation Stop 18: Transition Zone Gabbro, North Range, at Highway 144 – Highway 8 intersection Stop 17: Black Member of the Onaping Formation on the North Range, along Highway 144 at Onaping Falls 46.589851° N, 81.382453° W 470702E, 5159658N 46.617271° N, 81.413896° W 46.575577° N, 81.289467° W 468309E, 5162717N 477820E, 5158042N Outcrops near this intersection expose the Transition Zone Gabbro. The gabbro is medium grained, and characterized by augite being the main pyroxene, with little or no hypersthene. The gabbros also show a spike in oxide (Fe, Ti) and apatite (P) crystallization, typical of the “peak” in relative Fe-Ti concentrations during progressive crystallization, as seen in AFM diagrams of relatively reduced and anhydrous mafic magmas. This is reflected in a spike in magnetic susceptibility, with values increasing by an order of magnitude. The transition zone gabbro is several hundred metres thick and upward transitions into the base of the granophyre section. This roadside outcrop shows the folded and cleaved greywacke turbidites of the Chelmsford Formation, the uppermost formation preserved in the core of the Sudbury Basin syncline. The approximate age of these turbidites is circa 1840 Ma and they represent the foreland depositional wedge of the Penokean orogen. Participants of the fieldtrip who have seen the Rove Formation in the Thunder Bay area will recognize the great similarities and, indeed, these two formations are broadly related. The reason these turbidites are only locally preserved has to do with the fairly high-amplitude down folding in the 43 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Stop 19: Felsic norite towards top of norite section on North Range plagioclase pheno/megacrysts. These dykes are circa 2460 Ma in age and part of the giant Matachewan (II) dyke swarm. They represent the feeders to the plagioclase-phyric basalts of the lower Huronian mafic volcanic rocks seen at Stop 7 north of Lively. They are numerous in this part of the Superior Province and provide an important time marker. As pointed out earlier, they do not cut the Copper Cliff Rhyolite, which represents the terminal phase of the Huronian rift magmatism and volcanism. 46.615361° N, 81.428390° W 467198E, 5162571N These outcrops in the broad curve of the highway at the bottom of the hill expose the top of the North Range norites, referred to as “Felsic Norite” (e.g., Naldrett and Hewins, 1984). The norite is massive and homogeneous, as is typical for most of the SIC rocks. SiO2 contents of these norites are ~58 wt%. Ni values are ~20–30 ppm, which is significantly depleted from values of what are thought to have been primary Ni values in the early undifferentiated impact melts that were perhaps as high as ~100– 200 ppm (Zieg and Marsh, 2004; Lightfoot and Zotov, 2005; Lightfoot, 2006). Both the gneisses and the Matachewan dykes are overprinted by Sudbury Breccia, and shatter cones can be seen at several localities, with the cone axes projecting to the southeast. One well-developed cone in the gneisses has a cone axis of ~120º/+20º (up). A subtle mineral lamination, formed mainly by alignment of platy plagioclase crystals, can be seen and dips moderately to the south, parallel to the attitude of the basal SIC contact in this part of the North Range. Magnetic susceptibility values are ~7.5±1.0 x10-3 SI units. Well-preserved pseudotachylite of the Sudbury Breccia bodies, locally up to ~1 m wide, is very dark in colour here below the North Range, and only weakly recrystallized. This contrasts with pseudotachylite on the South Range (e.g., Stop 9), where it is typically strongly recrystallized and metamorphosed to epidote amphibolite facies and shows cleavage/foliation development due to the more intense deformation of the South Range. Stop 20: Levack Gneisses, cut by Matachewan diabase dykes, all overprinted by Sudbury Breccia and southeast pointing shatter cones 46.624698° N, 81.444722° W Given this relatively good state of preservation of the black pseudotachylite matrix here at these outcrops, it is unlikely that the Levack Gneisses were at a lower crustal level, and thus hot, at the time of impact. The gneisses were likely exhumed to shallower crustal levels during the latest Archean or earliest Paleoproterozoic, well prior to the impact (e.g., James et al., 1992). This point has been debated in the Sudbury literature and is relevant to the question of where “Ground Zero” is located. To the northeast of the Sudbury structure, Levack Gneisses occur in close proximity to the unconformity with the Huronian succession. 465954E, 5163556N Roadcuts on both sides of the highway show typical “Levack Gneisses”, high grade migmatitic gneisses that characterize the Archean basement to the north of the SIC. These rocks reached pyroxene granulite facies grade, before being retrogressed to amphibolite facies in the latest Archean (e.g., Prevec et al., 2005, and references therein). The gneisses contrast with more homogeneous late Archean granites farther north, the Cartier Granites, dated at circa 2640 Ma (Meldrum et al., 1997). The gneisses are cut by large, SSE-trending, subvertical mafic dykes with conspicuous 44 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 24: Shatter cones and Sudbury Breccia in the Levack Gneisses ~0.5 km below the basal contact of the SIC on the North Range. A) Photo of typical, heterogeneous, coarse-textured Levack Gneiss. B) Relatively well-developed partial shatter cone in Levack Gneiss, ~0.5 m in size. C) Shocked zircon with multiple sets of planar deformation features and fractures, from the original study of U-Pb dating of these rocks (see Krogh et al., 1984). D) Well-developed Sudbury Breccia with displaced and variably rounded gneiss fragments floating around in a black pseudotachylite matrix. E) Small shatter cones, ~10-25 cm in size, developed within the Matachewan dyke at this stop, with cone axes focusing toward the southeast (to the right in this picture). 45 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Stop 22: Discovery outcrop along railway cuts, just east of Murray Mine, South Range Stop 21: Cartier Granites, with locally conspicuous Sudbury Breccia, and rare shatter cones 46.521393° N, 81.052983° W 46.668937° N, 81.532490° W 495936E, 5151982N 459268E, 5168513N At this final stop we visit the approximate “discovery outcrop” just east of the Murray Mine. When in 1883 a railway line was cut through this area, Fe-sulphides and some chalcopyrite were noted in the rock cuts. This led to more prospecting and, eventually, the first mining operations in the area. At the time, the main interest was in Cu, as the application of Ni to steel manufacturing had not yet been invented. This final stop or stops, ~10 km out (horizontally) from the footwall contact of the SIC, will examine the more homogeneous late Archean granites of the “Cartier Batholith”, part of the “Algoman granites” to the south of the main Abitibi granite-greenstone terrane. Although somewhat variable and locally showing relict layering, typical parts of the Cartier Granite are homogeneous, relatively massive, pink, and relatively K-feldsparrich late granites with little structure. However, despite locally conspicuous K-feldspar, petrographically the granites are largely monzogranitic to granodioritic in composition, typical for late Archean granites. They contrast with and intrude the older Levack Gneisses. These late-stage granites, reflecting final re-melting of earlier tonalite-trondhjemite-granodiorite (TTG)dominated granite-greenstone crust, have been dated at circa 2640 Ma (Krogh et al., 1984; Meldrum et al., 1997)). They represent a final stage in the Archean crustal evolution of the southern Superior craton prior to stabilization and “cratonization”. The original rock cut was a bit farther to the west, but both the railway and the roads were moved east to allow for the development of the Murray open pit, which is located on the other side of the highway. In the present railway cut, semi-massive sulphide veins anastomose around somewhat deformed mafic fragments, some containing minor disseminated sulphides, others with no sulphides. This kind of material is typical for the basal contact of the SIC and is generally referred to as the “Sublayer” or “Contact Sublayer” (e.g., Pattison, 1979). To the west, these minor sulphide stringers broaden out into the orebodies of the Murray Mine (Figure 25). Occasional shatter cones can be seen in the road cuts, all pointing to the southeast (e.g., GPS waypoint #2686). And there is abundant Sudbury Breccia in places (GPS #2522), forming veins, dykes, and larger breccia bodies with rounded fragments of granite in a black pseudotachylite matrix. Similar Sudbury Breccia occurrences can be mapped radially outwards for another ~75 km, out to ~120 km from “Ground Zero” (see Figure 8; see also Butler, 1994; and Thompson and Spray, 1994) 7. Figure 25 shows a composite section across the basal contact of the SIC in this area, illustrating the SIC dipping to the north at ~40–45º. Relationships are approximately similar to that shown in the Creighton Mine section. The Murray Mine area could be described as yet another minor embayment, just to the east of the major Copper Cliff funnel structure. Confusion is possible with pseudotachylite veinlets formed in relation to regional faults, unrelated in time to the Sudbury structure. However, such veins are typically 0.5–2 cm in width and can mapped along or in proximity to observed fault or slip planes. Almost all Sudbury Breccia, such as discussed here, is developed at a different scale, often as dykes or bodies up to 10-100 cm wide. 7 46 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 25: Composite cross-section, looking west, across the footwall contact of the SIC illustrating the first-order relationships at the Murray Mine and open pit, just to the west of the “Discovery Outcrop” locality along the railway bed, and the Thayer Lindsley Mine farther to the east. Sections of both mines are integrated at the same scale. The Murray Mine section is from old Inco data, as published by Naldrett (1984), and differentiates some of the ore types, all in close proximity to or right along the footwall contact. The outline of the open pit is schematic. The Lindsley Mine section is from old Falconbridge data (see Binney et al., 1994), as published by Bailey et al. (2004). It is one of the localities where the significant offsets due to structures associated with the South Range Shear Zone was first recognized. Ore bodies along this shear zone were highly deformed. A Cu-rich ore body occurred within the footwall complex, dominated by Murray Granite. 47 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Papers by Robert Dietz and related literature (in chronological order) Incorporated into the section of Figure 25, at the same scale, is a section across the Thayer Lindsley Mine. This latter mine is located ~5 km to eastnortheast along strike and represents a next shallow embayment along the footwall contact of the SIC. Papers by Dietz Dietz, R.S. 1959. Shatter cones in cryptoexplosion structures (meteorite impact?). The Journal of Geology, v.67 (5), p.496-505. At the Lindsley Mine, at depth, the basal contact of the SIC is offset by a well-defined, discrete, shear zone that dips to the southeast and shows significant south-side up displacement (Binney et al., 1994). Metamorphic grade in this shear zone is lower amphibolite facies and titanite crystals from sheared norite show two distinct growth phase, brown titanite overgrown by colourless titanite (Bailey et al., 2004). U-Pb data for two brown titanite fractions suggest they grew at circa 1815±15 Ma, whereas the colourless titanites, interpreted to be syntectonic relative to the shear zone fabric, record an imprecise but somewhat younger age, circa 1670±70 Ma (Bailey et al., 2004). Dietz, R.S. 1960. Meteorite impact suggested by shatter cones in rock: Three cryptoexplosion structures yield new evidence of natural hypervelocity shocks. Science, v. 31 (3416), p.1781-1784. Dietz, R.S. 1961. Vredefort Ring structure: meteorite impact scar?. The Journal of Geology, v.69 (5), p.499-516. Dietz, R.S. 1961. Astroblemes. Scientific American, v.205 (2), p.0–59. Dietz, R.S. 1962. Vredefort Ring structure—a reply. The Journal of Geology, v.70, p.502-504. Dietz, R.S. 1963. Astroblemes: ancient meteorite impact scars on earth. In: The Solar System, Volume 4, University of Chicago Press, Chicago. The latter age dates this marked shear zone, which is part of the South Range Shear Zone system that further shortened and imbricated the South Range of the SIC. Dietz, R.S. 1963. Collapsing continental rises: an actualistic concept of geosynclines and mountain building. The Journal of Geology, v.71, p.314-332. Dietz, R.S. 1964. Sudbury structure as an astrobleme. The Journal of Geology, v.72 (4), p.412-434. End of Road Log for Day 1 Dietz, R.S. and Butler, L.W. 1964. Shatter-cone orientation at Sudbury, Canada. Nature, v204 (4955), p.280-281. Dietz, R.S. 1970. Cosmogenic ores at Sudbury astrobleme?. Meteoritics, v.5, p. 91-192. Dietz, R.S. 1971. Shatter cones (shock fractures) in astroblemes. Meteoritics, v.6, p. 58-259. Dietz, R.S. 1971. Sudbury astrobleme: A review. Meteoritics, v.6, p.259-260. Dietz, R.S. 1972. Sudbury astrobleme, splash emplaced sub-layer and possible cosmogenic ores. In: New Developments in Sudbury Geology, J.V. Guy-Bray (ed.), Geological Association of Canada, Special Paper 10, p. 29-40. 48 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Some directly related literature Ames, D.E., Golightly, J.P., Lightfoot, P.C. and Gibson, H.L. 2002. Vitric compositions in the Onaping Formation and their relationship to the Sudbury Igneous Complex, Sudbury Structure. Economic Geology, v.97, p.1541-1562. Shand, S.J. 1916. The pseudotachylyte of Parijs (Orange free State), and its relation to ‘Trap-Shotten Gneiss’ and ‘Flinty Crush-rock’. Quarterly Journal of the Geological Society, v.72 (1-4), p.198–221. Ames, D.E., Buckle, J., Davidson, A. and Card, K. 2005. Sudbury bedrock compilation. Geological Survey of Canada Open File 4570, geology, color map, and digital tables, scale 1:50,000. Boon, J.D. and Albritton, Jr, C.C. 1936. Meteorite craters and their possible relationship to "cryptovolcanic structures". Field and Laboratory, v.5, p.1–9. Ames, D.E., Card, K., Wodicka, N. and Davidson, A. 2008a. 100K Geological map of the Sudbury mining camp and Surrounding area, Ontario, Canada; Supplement to Ames, D.E. and Wodicka, N., 2008, Geology of the Giant Sudbury Polymetallic Mining Camp, Ontario, Canada. Economic Geology, v.103, (5), p.1057-1077. Boon, J.D. and Albritton Jr, C.C. 1937. Meteorite scars in ancient rocks. Field and Laboratory, v.5 (2), p.5364. Boon, J.D. and Albritton Jr, C.C. 1938. Established and supposed examples of meteoritic craters and structures. Field and Laboratory, v.6 (2), p. 4–56. Ames, D.E., Davidson, A. and Wodicka, N. 2008b. Geology of the giant Sudbury polymetallic mining camp, Ontario, Canada. Economic Geology, v.103 (5), p.1057–1077. Boon, J.D. and Albritton Jr, C.C. 1942. Deformation of rock strata by explosions. Science, v.96 (2496), p.402-403. Hargraves, R.B. 1961. Shatter cones in the rocks of the Vredefort Ring. Transactions of the Geological Society of South Africa, v. 4, p.147-161. Bailey, J., LaFrance, B., McDonald, A.M., Fedorowich, J.S., Kamo, S. and Archibald, D.A. 2004. MazatzalLabradorian-age (1.7-1.6 Ga) ductile deformation of the South Range Sudbury impact structure at the Thayer Lindsley mine, Ontario. Canadian Journal of Earth Sciences, v.41, p.1491-1505. DOI: 10.1139/e04-098. Guy-Bray, J.V. and Geological Staff 1966. Shatter cones at Sudbury. The Journal of Geology, v.74, p.243245. French, B.M. 1967. Sudbury structure, Ontario. Some petrographic evidence for an origin by meteorite impact. Goddard S pace Flight Centre, Maryland, Publication X-641-67-67, p.1-56. Bekker, A., Holland, H.D., Wang, P.L., Rumble, D.I.I.I., Stein, H.J., Hannah, J.L., Coetzee, L.L. and Beukes, N.J. 2004. Dating the rise of atmospheric oxygen. Nature, v.427 (6970), p.117-120. Bourgeois, J. and Koppes, S.,1998. Robert S. Dietz and the recognition of impact structures on Earth. Earth sciences history, v.17 (2), p.139-156. Bennett, G., Dressler, B.O. and Robertson, J.A. 1991. The Huronian Supergroup and associated intrusive rocks. In: Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p. 549-592. References related to Day 1 Addison, W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S., Fralick, P.W. and Hammond, A.L. 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact event. Geology, v.33 (3), p. 93-196. Binney, W.P., Poulin, R.Y., Sweeney, J.M. and Halladay, S.H. 1994. The Lindsley Ni–Cu–PGE Deposit and its Geological Setting. In: Proceedings of the Sudbury–Noril’sk Symposium. Ontario Geological Survey, Special Volume 5, p.91-103. Addison, W.D., Brumpton, G.R., Davis, D.W., Fralick, P.W. and Kissin, S.A.,2010. 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Canadian Mining and Metallurgy Bulletin, v.42 (451), p.547-554. Wu, J., Milkereit, B. and Boerner, D.E. 1995. Seismic imaging of the enigmatic Sudbury Structure. Journal of Geophysical Research, v.100 (B3), p.4117-4130. Sullivan, R.W. and Davidson, A. 1993. Monazite age of 1747 Ma confirms post-Penokean age for the Eden Lake complex, Southern Province, Ontario. In: Radiogenic Age and Isotopic Studies: Report 7, Geological Survey of Canada, Paper 93-2, p. 45-48. Young, G.M. (editor) 1973. Huronian Stratigraphy and Sedimentation. Geological Association of Canada, Special Paper 12, 27 p. Therriault, A.M., Fowler, A.D. and Grieve, R.A.F. 2002. The Sudbury Igneous Complex: a differentiated 55 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Chai, G. and Eckstrand, R. 1994. Rare-earth element characteristics and origin of the Sudbury Igneous Complex, Ontario, Canada. Chemical Geology, v.113, p.221-244. DOI: 10.1016/00092541(94)90068-X. Young, G.M. 1983. Tectono-sedimentary history of early Proterozoic rocks of the northern Great Lakes region. In: Geological Society of America Memoir 160, p.15-32. Young, G.M. and Nesbitt, H.W. 1985. The Gowganda Formation in the southern part of the Huronian outcrop belt, Ontario, Canada: stratigraphy, depositional environments and regional tectonic significance. Precambrian Research, v.29 (1-3), p.265-301. Darling, J.R., Hawkesworth, C.J., Lightfoot, P.C., Storey, C.D. and Tremblay, E. 2010. Isotopic heterogeneity in the Sudbury impact melt sheet. Earth and Planetary Science Letters, v.289 (3-4), p.347-356. DOI: 10.1016/j.epsl.2009.11.023. Davis, W.J., Jones, A.G., Bleeker, W. and Grütter, H. 2003. Development of the lithosphere below the Slave Province. Lithos, v.71 (2-4), p.575-589. Young, G.M., Long, D.G., Fedo, C.M. and Nesbitt, H.W. 2001. Paleoproterozoic Huronian basin: product of a Wilson cycle punctuated by glaciations and a meteorite impact. Sedimentary Geology, v.141, p.233-254. Dickin, A.P., Artan, M.A. and Crocket, J.H. 1996. Isotopic evidence for distinct crustal sources of North and South Range ores, Sudbury Igneous Complex. Geochimica et Cosmochimica Acta, v.60, p.1605-1613. DOI:10.1016/0016-7037(96)00044-0. Zieg, M.J. and Marsh, B.D. 2005. The Sudbury Igneous Complex: Viscous emulsion differentiation of a superheated impact melt sheet. Geological Society of America Bulletin, v.117, p.1427-1450. Dickin, A.P., Nguyen, T. and Crocket, J.H. 1999. Isotopic evidence for a single impact melting origin of the Sudbury Igneous Complex. In: Large meteorite impacts and planetary evolution II, Geological Society of America, Special Paper 339, p. 361-371. Other relevant references Ames, D.E., Watkinson, D.H. and Parrish, R.R, 1998, Dating of a regional hydrothermal system induced by the 1850 Ma Sudbury impact event. Geology, v.26, p.447–450. Gariépy, C. and Allègre, C.J. 1985. The lead isotope geochemistry and geochronology of late-kinematic intrusives from the Abitibi greenstone belt, and the implications for late Archaean crustal evolution. Geochimica et Cosmochimica Acta, v.49 (11), p. 2371-2383. DOI: 10.1016/0016-7037(85)90237-6. Anders, D., Osinski, G.R., Grieve, R.A.F., Pilles, E.A., Pentek, A. and Smith, D. 2020. Origin and formation of Metabreccia in the Parkin Offset Dike, Sudbury impact structure, Canada. Canadian Journal of Earth Sciences, v.57 (11), p.1324-1336. Bailey, J., McDonald, A.M., Lafrance, B. and Fedorowich, J.S. 2006. Variations in Ni content in sheared magmatic sulfide ore at the Thayer Lindsley mine, Sudbury, Ontario. The Canadian Mineralogist, v.44 (5), p.1063-1077. Holm, D.K., Van Schmus, W.R., MacNeill, L.C., Boerboom, T.J., Schweitzer, D. and Schneider, D. 2005. U-Pb zircon geochronology of Paleoproterozoic plutons from the northern midcontinent, USA: Evidence for subduction flip and continued convergence after geon 18 Penokean orogenesis. Geological Society of America Bulletin, v.117 (3-4), p.259-275. Bleeker, W., Kamo, S. and Ames, D.E. 2013. New field observations and U-Pb age data for footwall (target) rocks at Sudbury: Towards a detailed cross-section through the Sudbury Structure. In: Large Meteorite Impacts and Planetary Evolution V Meeting, 5–8 August, Sudbury, Ontario. Extended abstract, Lunar Planetary Institute contribution no. 1737, p. 13. Ivanov, B.A. and Deutsch, A, 1997. Sudbury impact event: cratering mechanics and thermal history. In: Large Meteorite Impacts and Planetary Evolution, LPI Contribution no. 922, p. 26. Brocoum, S.J. and Dalziel, I.W. 1976. The Sudbury Basin, the Southern province, the Grenville Front, and the Penokean orogeny; Discussion and reply: Reply. Geological Society of America Bulletin, v.87 (6), p.958-958. Kawohl, A., Frimmel, H.E., Bite, A., Whymark, W. and Debaille, V. 2019. Very distant Sudbury impact dykes revealed by drilling the Temagami geophysical anomaly. Precambrian Research, v.324, p.220-235. DOI: 10.1016/j.precamres.2019.02.014. 56 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Keays, R.R. and Lightfoot, P.C., 2004. Formation of NiCu-PGE sulphide mineralization in the Sudbury Impact Melt Sheet. Mineralogy and Petrology, v.82, p.217-258. Mungall, J.E., Ames, D.E, and Hanley, J.J. 2004. Geochemical evidence from the Sudbury structure for crustal redistribution by large bolide impacts. Nature, v.429 (6991), p.546-548. Keays, R.R. and Lightfoot, P.C. 2004. Mafic intrusions in the footwall of the Sudbury Igneous Complex: Origin of the Sudbury impact melt sheet and its associated ore deposits. Ore Geology Reviews, v.120, article 103435. DOI: 10.1016/j.oregeorev. 2020.103435. Prevec, S.A., Lightfoot, P.C, and Keays, R.R. 2000. Evolution of the Sublayer of the Sudbury Igneous Complex: geochemical, Sm-Nd and petrologic evidence. Lithos, v.51, p.271-292. Rousell, H.D. 1972. The Chelmsford Formation of the Sudbury Basin—a Precambrian turbidite. In: New Developments in Sudbury Geology, Geological Association of Canada, Special Paper 10, p.79-91. Ketchum, K.Y., Heaman, L.M., Bennett, G. and Hughes, D.J. 2013. Age, petrogenesis and tectonic setting of the Thessalon volcanic rocks, Huronian Supergroup, Canada. Precambrian Research, v.233, p.144-172. Rousell, H.D. 1975. The origin of foliation and lineation in the Onaping Formation and the deformation of the Sudbury Basin. Canadian Journal of Earth Sciences, v.12, p.1379-1395. Lightfoot, P.C., Keays, R.R. and Doherty, W. 2001. Chemical evolution and origin of nickel sulfide mineralization in the Sudbury Igneous Complex, Ontario, Canada. Economic Geology, v.96, p.18551875. Rousell, H.D. 1984a. Structural geology of the Sudbury Basin. In: The Geology and Ore Deposits of the Sudbury Structure, Ontario Geological Survey, Special Volume 1, p. 83-95. Lightfoot, P.C., Keays, R.R., Morrison, G.G., Bite, A. and Farrell, K. 1997. Geologic and geochemical relationships between the Contact Sublayer, inclusions, and the Main Mass of the Sudbury Igneous Complex: A case study of the Whistle Mine embayment. Economic Geology, v.92, p.647-673. Rousell, H.D. 1984b. Onwatin and Chelmsford Formations. In: The Geology and Ore Deposits of the Sudbury Structure, Ontario Geological Survey, Special Volume 1, p. 211-218. Shanks, W.S. and Schwerdtner, W.M., 1991. Crude quantitative estimates of the original northwest– southeast dimension of the Sudbury Structure, southcentral Canadian Shield. Canadian Journal of Earth Sciences, v.28 (10), p.1677-1686. Lightfoot, P.C., Keays, RR., Morrison, G.G., Bite, A., and Farrell, K., 1997. Geochemical relationships in the Sudbury Igneous Complex: Origin of the Main Mass and Offset dikes. Economic Geology, v.92, p.289-307. Wieland, F., Gibson, R.L. and Reimold, W.U. 2005. Structural analysis of the collar of the Vredefort Dome, South Africa—Significance for impact‐ related deformation and central uplift formation. Meteoritics & Planetary Science, v.40 (9–10), p.1537-1554. Lightfoot, P.C. and Farrow, C.E.G. 2002. Geology, geochemistry, and mineralogy of the Worthington offset dike: a genetic model for offset dike mineralization in the Sudbury Igneous Complex. Economic Geology, v.97, p.1419-1446. DOI: 10.2113/gsecongeo.97.7.1419. Zolnai, A.I., Price, R.A. and Helmstaedt, H. 1984. Regional cross section of the Southern Province adjacent to Lake Huron, Ontario: implications for the tectonic significance of the Murray Fault Zone. Canadian Journal of Earth Sciences, v.21, p.447-456. Marsh, B.D. and Zieg, M.J. 1999. Melt sheet madness: superheated emulsion differentiation. Geological association of Canada–Mineralogical Association of Canada, Sudbury 1999, Abstracts v.24, p.78 Morrison, G.G. 1984. Morphological features of the Sudbury Structure in relation to an impact origin. In: The Geology and Ore Deposits of the Sudbury Structure, Ontario Geological Survey, Special Volume 1, p. 513-522. 57 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Day 2 Sudbury Ore Environments and Offset Dikes – Examples from Whistle and Parkin, NE Sudbury Henning Seibel and Michael Lesher Mineral Exploration Research Centre, Harquail School of Earth Sciences, Laurentian University, 935 Ramsey Lake Road, Sudbury, ON P3E 2C6 1. Organization of Day 2 The second day of pre-meeting Trip 1 will introduce participants to the ore environments at the base of the Sudbury Igneous Complex (SIC) and to the general geology of offset dikes that are exposed on mechanically and hydraulically stripped outcrops in the northeast corner of the Sudbury basin. The day starts adjacent to the former Whistle mine, mined by Inco (now Vale) from 1993 to 1997, where large outcrops highlight the complexity of Superior Province footwall rocks near the contact with the overlying SIC and the transition from mineralized Sublayer through anatectic breccias to traditional offset dike lithologies. The Podolsky North Zone outcrop to the northeast contains footwall-style mineralization on surface, extensions of which were mined underground by FNX/KGHM from 2008 to 2013. In the afternoon, outcrops south (distal Whistle) and north (proximal Parkin) of the Post Creek fault will be compared. The field trip will end with a visit to Rocky’s Restaurant on Lake Wanapitei. In case we are not able to access the Whistle mine outcrops, several interesting and well-preserved outcrops of the Worthington, Trill, and Hess offset dikes to the southwest and northwest of the Sudbury Structure will be visited. Participants will be introduced to typical characteristics of offset dikes, emplacement mechanisms and formation models. 2. Introduction The Sudbury mining camp is the one of the largest magmatic Ni-Cu-PGE mining camps in the world (Fig. 2.1) and has been mined for over 135 years (see review by Lightfoot, 2016). Mineralization is associated mainly with breccias along and near the lower contact of the Main Mass of the Sudbury Igneous Complex (SIC) and within associated offset dikes (Fig. 3.1). Breccias in the Sudbury Structure include 1) pre-impact magmatic breccias (e.g., Levack Breccia), 2) syn-impact pseudotachylitic breccias, locally referred to as Sudbury Breccia (SUBX; e.g., Rousell et al., 2003), 3) syn- to post-impact magmatic breccias directly derived from the SIC, such as Inclusion-Bearing Quartz Diorite (IQD; e.g., Grant and Bite, 1984) and inclusion-bearing Sublayer Norite (SLNR; e.g., Lightfoot and Farrow, 2002; Lightfoot et al., 1997a), and 4) contact metamorphosed and/or partially melted (anatectic) breccias, variably referred to as Footwall Breccia (FWBX; e.g., McCormick et al., 2002) and “Metabreccia” (MTBX; e.g., Lafrance et al., 2014). The ore deposits in the Sudbury Structure occur in two distinct environments (Fig. 3.1): 1) mineralization along or near the basal contact of the Main Mass, including a) disseminated to semi-massive Fe-Ni-Cu sulfides in SLNR and FWBX, and b) veins and disseminations of Fe-Cu-Ni sulfides in underlying SUBX and associated footwall rocks, and 2) disseminated-blebby to semi-massive Fe-Ni-Cu and Fe-Cu-Ni sulfides in offset dikes (e.g., Souch et al. 1969; Naldrett, 2004; Lightfoot, 2016). The Whistle embayment and Whistle-Parkin offset dike at the northeastern corner of the SIC contain elements of both environments, providing an excellent introduction to Sudbury ore systems. 58 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 2.1: Pre-mining Ni resources (past production + current resources) and grades of the world’s largest magmatic Cu-Ni (circles) and PGE (triangle) deposits. Modified from Naldrett (2004). Figure 3.1: Distribution of contact, footwall, and offset dike deposits and occurrences in the Sudbury Impact Structure. Note that locations for deeper ore bodies are projected to surface. Simplified after Ames et al. (2008). 3. Sudbury Ore Environments Ore deposits and occurrences occur all around the Sudbury Structure (Fig. 3.1), but some areas, such as Levack in the North Range and Frood-Stobie, Creighton, and Copper Cliff in the South Range, are much better endowed. 59 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 3.2: A) Inclusion-bearing massive sulfide from the Stobie Mine (contact deposit). B) Sharp-walled Cu vein in contact with tonalitic country rock from the Strathcona mine (footwall deposit). C) Disseminated sulfide blebs in variable altered inclusion-bearing quartz diorite from the Copper Cliff North Mine (offset deposit). Courtesy of Harquail School of Earth Sciences (Laurentian University). Contact-Footwall Environment Contact ores occur discontinuously along the SIC-footwall contact and are typically hosted by SLNR and FWBX (also referred to as Granite Breccia or Late Granite Breccia on the North Range). Mineralization typically occurs in funnels (e.g., Whistle, Foy, Copper Cliff), troughs (e.g., Creighton), and embayments (e.g., Levack) (Fig. 3.3) along the basal contact of the SIC, and are subeconomic or absent outside of those features. Mineralization typically grades downward from sparse disseminated sulfides in overlying Main Mass norite through fine and coarse (blebby) disseminated sulfides in Sublayer to semi-massive sulfides in Footwall Breccia (Fig. 3.2A) (see review by Lightfoot, 2016). Contact ores contain a typical magmatic 60 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 3.3.: Distribution of contact, footwall, and offset dike deposits and occurrences in the Sudbury Impact Structure. Note that locations for deeper ore bodies are projected to surface. Simplified after Ames et al. (2008). sulfide assemblage of pyrrhotite (containing up to 1% Ni) > pentlandite > chalcopyrite with minor magnetite and platinum-group minerals (PGMs) (see review by Ames et al., 2008). Ore tenors (metals in 100% sulfides) vary widely depending on the composition of the magma, magma:sulfide mass ratio (R factor: Naldrett et al., 1979), and degree of MSS fractionation (e.g., Li and Naldrett, 1994), but typically range from 3.9–6.1% Ni100, 1.3–7.1% Cu100 and 0.7–5.6 ppm (Pd100+Pt100) (Naldrett, 2004). Some contact deposits in the South Range (e.g., Thayer Lindsley: Bailey et al., 2004; Garson: Mukwakwami et al., 2014) are faulted, sheared, and remobilized. Footwall ores appear to occur only below contact deposits, up to 700m (Golightly, 2009) but more typically up to 200–300m (Farrow and Lightfoot, 2002) from the base of the SIC (Fig. 3.3). They are common in the North and East Ranges, but rare in the South Range (Fig. 3.1). Farrow et al. (2005) discriminate three types of footwall mineralization: 1) sharp-walled veins that can reach up to several meters in thickness with predominant chalcopyrite and lesser pentlandite, millerite, and cubanite grading downward and outward into bornite ± millerite veins (Fig. 3.2B), 2) disseminated sulfides often with high PGE/S ratios, and 3) a hybrid type containing both mineralization styles. Ore tenors typically range from 3.5–8.7% Ni100, 28.8–38.3% Cu100, and 13.4–33.5 ppm (Pt100+Pd100) (Naldrett, 2004). A gradual transition from Fe-Co-(Ni)-IPGE-rich contact mineralization to Cu-(Ni)-PPGE-Au-rich footwall mineralization at several deposits (e.g., Frood: Hawley, 1965; Strathcona: Li and Naldrett, 1994; McCreedy East: Gregory, 2006; Levack-Morrison: Nelles, 2012; Nickel Rim South: Glencore Ltd., unpubl.; Podolsky, KGHM unpubl.) is consistent with fractional crystallization and accumulation of Fe-Co-IPGE-rich monosulfide solid solution (MSS) in contact ores and segregation of Cu-rich intermediate solid solution (ISS) and Cu-PPGE-Au-rich residual sulfide liquid in footwall ores (e.g., Mungall, 2007). The formation of bornite-millerite ores across a thermal divide in the Fe-Cu-S system appears to require reactions with wall rocks (Nelles, 2012; see also Lesher, 2017). The formation of distal disseminated PGE-Au rich sulfides appears to require deposition from hydrothermal fluids (e.g., Hanley et al., 2004; Stout, 2009). 61 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 3.4: Schematic drawing of typical relationships between marginal QD and interior (mineralized) IQD) in offset dikes as well as several footwall-offset dike relationships in the Worthington offset dike. After Lightfoot and Farrow (2002). Offset Dike Environment Offset dikes are sub-vertical radial or concentric quartz monzodioritic lithologies (historically referred to as quartz diorite) that extend up to 20 km into the underlying footwall rocks from 300–500m-wide funnels at the contact with Main Mass (Fig. 3.1; see review by Lightfoot, 2016). The funnels typically contain SLNR ± MTBX ± IQD-QD pods and grade downward/outward into inclusion- and sulfide-poor Quartz Diorite (QD) margins with inclusion- and sulfide-rich Quartz Diorite cores (IQD; Fig. 3.4; e.g., Pattison, 1979; Grant and Bite, 1984; Lightfoot and Farrow, 2002; Murphy and Spray, 2002; Tuchscherer and Spray, 2002). Inclusions comprise angular to subrounded xenoliths derived from local country rocks, anteliths derived from the offset dike lithologies (i.e., QD clasts in IQD), and ultramafic xenoliths derived from deeper crustal lithologies (Wang et al., 2020). They vary in size from microscopic to tens of meters, and can reach up to 90% in volume (Grant and Bite, 1984). Sulfide contents vary from negligible to massive and are generally linked to the presence of (ultra)-mafic inclusions (Pattison, 1979). Ni-Cu-PGE mineralization ranges from finely disseminated to blebby (Fig. 3.2C), semi-massive, and massive coarse-grained pyrrhotite with variable amounts of pentlandite and chalcopyrite in steeply plunging ore bodies (Cochrane, 1984; Farrow and Lightfoot, 2002). Ore tenors typically range from 3.2–6.5% Ni100, 2.6–12.8% Cu100, and 1.2–28.3 ppm (Pt100+Pd100) (Naldrett, 2004). Ore bodies are often associated with large (ultra)-mafic clasts (e.g., Totten, Podolsky), changes in strike (e.g., Copper Cliff), or cross-cutting faults (Cochrane, 1984). Endowment varies significantly between dikes located in the North Range and South Range, and with proximity to the SIC (Fig. 3.1). Most economic offset deposits (historic and current) are located in South Range dikes (e.g., Frood-Stobie, Copper Cliff). Small PGE-Cu-(Ni) occurrences in the recent discovered Rathbun offset dike indicate potential for untypical (footwall-style) mineralization in distal offset dikes (Kawohl et al., 2020). A gradual transition from Ni-rich contact mineralization to Curich footwall mineralization in some offset deposits (e.g., Frood: Hawley, 1965) is consistent with fractional crystallization of Ni-IPGE-rich MSS to produce residual Cu-PPGE-rich sulfide liquid. 62 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Ore Genesis The ultimate sources of S and metals in the SIC are Fe ± Cu ± Ni-sulfide bearing Archean mafic gneisses, Huronian mafic volcanic rocks, East Bull Lake Suite intrusions, and Nipissing Suite intrusions (Lightfoot et al., 1997, 2001; Keays and Lightfoot, 2004), all of which contain significant amounts of sulfides and the latter two of which contain significant amounts of Ni-Cu-PGE mineralization (e.g., James et al., 2002; Sproule et al., 2007; Holwell and Keays, 2014). However, that leaves two end-member models for the generation of the ores in the SIC (Fig. 3.5): 1) dissolution of Fe ± Cu ± Ni sulfides in the superheated impact melt followed by exsolution and settling during cooling (e.g., Lightfoot et al., 2001; Keays and Lightfoot, 2004) and 2) impact devolatilization of the majority of the S from the impact melt and incorporation of Fe ± Cu ± Ni sulfide xenomelts during thermomechanical erosion of footwall rocks (Lesher, 2019). The latter appears to be more consistent with very consistent Hf isotopic composition of the Main Mass and more heterogeneous Pb-S-Os isotopic compositions of the ores (see review by Wang et al., in press), indicating complete retention and homogenization of refractory Hf (Kenny et al., 2017) but significant loss of more-volatile Pb (O’Sullivan et al., 2016; Kenny et al., 2017; McNamara et al., 2017), Sb (O’Sullivan et al., 2016 GCA), Zn‐Cd‐Rb‐Cs (Kamber and Schoenberg, 2020), and therefore also much/most of the highly volatile S‐Se‐Bi and significant amounts of moderately volatile Ag‐Cu‐Au‐As (Lesher, 2019). Figure 3.5: Schematic representations of end-member sulfide generation and localization models (from Wang et al., in press). Model A: exsolution and convective settling of molten sulfide droplets (T1) followed by gravity flow into embayments, troughs, and funnels (T2). Model B: volatilization of most of the S from the impact melt (T1) followed by generation of sulfide xenomelts by local thermomechanical erosion (T2). 63 �Proceedings of the 68th ILSG Annual Meeting - Part 2 The short interval between emplacement of inclusion- and sulfide-free QD margins and inclusion ± sulfide bearing IQD cores of nested offset dikes (<1–5 days: Wang et al., in press) provides insufficient time for exsolution and settling of sulfide droplets through the 2–5 km-thick impact melt, as dissolution, exsolution, and settling are all inherently slow processes (months: see discussion by Robertson et al., 2015, 2016), whereas the transfer of metals between the magma and sulfide droplets is much faster (hours-days: Yao and Mungall, 2021). Together, available data favor a model involving impact devolatilization of S and other volatile elements, rapid thermomechanical erosion of impact debris by the superheated impact melt, and dynamic upgrading of Fe ± Cu ± Ni sulfide xenomelts (Fig. 3.5). 4. Offset Dike Emplacement Extensive impact melt-bearing dikes are only known from the two largest terrestrial impact structures, Sudbury and Vredefort (Dressler and Reimold, 2004; Osinski et al., 2018, and references therein). Granophyre dikes are the only remnant of an impact melt at the 2023 Ma (Kamo et al., 1996) Vredefort impact crater. They are spatially related to the centrally uplifted dome structure of the deeply eroded impact site (Reimold and Gibson, 2006). Similar to Sudbury offset dikes, the granophyre dikes have a radial and concentrical distribution, widths of 10–50m, lengths of up to 10km, crosscutting relationships with pseudotachylitic breccia, spherulitic textures, and more rarely fragment-poor granophyre margins as well as fragment-rich interiors (Reimold and Gibson, 2006; Osinski et al., 2018; Huber et al., 2022). Granophyre dikes have a homogeneous chemical composition similar to the upper continental crust which could represent the undifferentiated impact melt (Dressler and Reimold, 2004; Huber et al., 2020). Offset Dike Characteristics Quartz Diorite (QD) is predominantly composed of a medium-grained, homogenous matrix with acicular plagioclase, acicular sometimes radiating amphiboles (after pyroxene), variable amounts of quartz and minor biotite, granophyric quartz-alkali feldspar intergrowths, and secondary amphibole. Aphanitic and spherulitic QD margins are common in the intermediate and distal segments of offset dikes and their apophyses (e.g., Hess, Trill). Inclusion-bearing QD (IQD) is mineralogically similar to QD, but typically finer grained with a more granular texture and less pronounced acicular amphibole. Contacts between QD and IQD are often sharp, but sometimes gradational (Fig. 4.3). QD and IQD samples from the same dike are remarkably similar in major and trace element geochemistry, indicating similar melt source and a short succession of injection (Lightfoot et al., 1997a; Pilles et al., 2017). Trace element differences between North Range and South Range dikes appear to reflect incorporation of different amounts of local footwall rocks (Lightfoot et al., 1997a). Mechanisms of Emplacement Two general mechanisms have been proposed to explain the geochemical, petrological, and spatial characteristics of QD and IQD. Some authors (e.g., Lightfoot and Farrow, 2002; Riller, 2005, Prevec and Büttner, 2018) prefer a multi-phase emplacement of two or more melts (Fig. 4.1), whereas others (e.g., Grant and Bite, 1984; Pilles et al, 2018) favor a single injection and flowage differentiation (e.g., Barrière, 1976) to explain the characteristics of some dikes (e.g., Foy). In a multi-phase injection model, an initial phase of sulfide-poor, inclusion-poor QD melt is followed by injection of a second core phase of sulfide-rich, IQD melt in the center of the dikes (Fig. 4.1; e.g., Lightfoot and Farrow, 2002; Prevec and Büttner, 2018). Further injections at later stages to form the more evolved Pele and Cascaden dikes are required, but they do not contain any sulfides (Pilles et al., 2018a). This model is supported by the common presence of inclusions of QD within IQD, common sharp contacts between QD and IQD (Fig. 4.3A), and the spatial relationship between marginal QD and interior IQD. 64 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 4.1: Schematic diagram of multi-stage emplacement model. Injection of an initial phase of sulfidepoor, inclusion-poor QD melt is followed by injection of a second phase of sulfide-rich IQD melt in the center of the dikes. Modified after Prevec and Büttner (2018). In a single-stage injection model, IQD melt is injected into the fractured country rock with flowage differentiation producing marginal QD and interior IQD (Fig. 4.2). This model is supported by local gradational transitions between QD and IQD (Fig. 4.3B), rare IQD inclusions in IQD, increasing clast diameters towards the center of the dike, and clast alignments parallel to dike margins in the Foy offset dike (Pilles et al., 2018b). Figure 4.2: Schematic diagram of single-stage model. IQD melt is injected into dilating fractures with flowage differentiation producing marginal QD and interior IQD. Modified after Pilles et al. (2018b) The spatial relationships between marginal QD and internal IQD can be explained by flowage differentiation or by multi-stage emplacement if IQD intruded before QD had completely solidified. However, flowage differentiation and multiple injections should produce different inclusion types and contact relationships. For example, flowage differentiation cannot easily produce the commonly observed sharp contacts between QD and IQD or the frequent inclusions of QD in IQD. In contrast, the common sharp contacts can be produced if IQD intrudes QD, the rare gradational contacts if IQD melts QD (see discussion by Huppert & Sparks, 1985), and the rare inclusions of IQD in IQD by multiple phases of injection or by radial dikes crosscutting/intruding concentric dikes. The weight of evidence presently favors a multiple injection model (Fig. 4.4 and 4.5). 65 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 4.3: Sharp (A) and well-defined but gradational (B) contact of medium-grained QD and IQD from two stripped outcrops of the Foy offset dike. Contact is defined by grain size differences as well as abundance of inclusions and blebby sulfides. Image widths are ca. 40cm. Timing of Emplacement Impact cratering can be subdivided into three stages: 1) contact and compression, 2) crater excavation, and 3) crater modification (Gault et al., 1968; Osinski and Pierazzo, 2012). The timing of emplacement of the Sudbury offset dikes proposed by different workers varies from during impact excavation to tens of thousands of years after impact: 1) dilation during transient cavity formation, 1 second to 1 minute after impact (e.g., Lightfoot and Farrow, 2002; QD: Wang et al., in press); 2) dilation during rebound and central uplift, minutes to days after impact (e.g., Tuchscherer and Spray, 2002; IQD: Wang et al., in press); 3) dilation during crater wall collapse, leading to injection of melt into transfer faults (e.g., Scott and Benn, 2002); 4) dilation during isostatic uplift, up to 10,000 years after impact (e.g., Wichman and Schultz, 1993); 5) dilation after melt pressure increase as a result of a coherent roof, 1500–130,000 years after impact (e.g., Prevec and Büttner, 2018); 6) dilation during cooling and subsequent contraction of footwall rocks, >10,000 years after impact (e.g., Riller, 2005); 7) dilation during cooling of the Main Mass, 10,000s to 100,000s of years after impact (Mathieu et al., 2021); 8) dilation during readjustment and late tectonic deformation, >10,000 years after impact (e.g., Therriault et al., 2002). 1) The presence of only sparse local xenoliths and sulfides, and the presence of aphanitic, spherulitic, and radiating “quench” pyroxene textures in the distal parts of the dikes require the impact melt to have been superheated when QD was emplaced, so this precludes all of the models involving emplacement during or after crystallization of the Main Mass. 2) The nested QD/IQD relationships with no occurrences of IQD without QD (with the possible exception of the South Range Breccia Belt) require emplacement while the cores of the QD dikes were still weak, which has been modelled by to have been within 2–5 days (Wang et al., in press). 3) The presence of inclusions and sulfides in IQD require the impact melt to have reached sulfide saturation within that time interval and for IQD to have been forcibly emplaced within that interval. 4) The two events most likely to have caused rapid basin-wide sequential injection of QD and IQD are excavation and impact (Fig. 4.4). 5) The process most likely to have driven the impact melt from a superheated, sulfide-undersaturated state to a liquidus, sulfide saturated state is assimilation of fragments generated by collapse of the peak ring. 66 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 4.4: Schematic representation of complex crater-forming and -modification events leading to injection of inclusion- and sulfide-poor marginal QD (T1), generation of inclusion- and sulfide-rich IQD (T3), and melting of inclusions and generation of mineralized FWBX (T4). FWBX – Footwall Breccia, IQD – Inclusion-bearing Quartz Diorite, SLNR – Sublayer Norite, QD – Quartz Diorite. From Wang et al. (in press), as modified after Melosh (1989). Figure 4.5: Inferred geological history of the Sudbury structure, highlighting major events related to the formation of QD, IQD and Sublayer, Footwall Breccia, and associated Ni-Cu-PGE mineralization. From Wang et al. (in press), as modified from Lightfoot, 2016). 67 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Excursion Stops, Day 2: Ore Environments Geology of the Whistle and Parkin Area Footwall Rocks The Whistle funnel and Whistle-Parkin offset dike are located at the northeastern lobe of the Main Mass of the Sudbury Igneous Complex in the Norman and Parkin townships (see regional map from Day 1). The footwall comprises Archean rocks of the Superior Province close to the Whistle embayment and Paleoproterozoic greenschist facies to amphibolite facies metamorphosed volcanic and sedimentary rocks of the Huronian Supergroup in the distal Parkin offset dike (Fig. 5.1; Ames et al., 2008; Lightfoot, 2016 and references therein). With ages of 2725 to 2703 Ma (Nunes and Pykes, 1980) felsic to intermediate volcanic rocks as well as feldspar and quartz-feldspar porphyritic rocks of the Benny greenstone belt are the oldest rocks in the area. (Meyn, 1970). Units of the Levack Gneiss Complex are not shown in Fig. 5.1, but occur as tens of meter size bodies close to the Sudbury Igneous Complex and consist of migmatitic tonalite orthogneiss, biotite paragneiss, mafic to felsic gneiss, and gabbros (Meldrum et al., 1997; Murphy and Spray, 2002). Krogh et al. (1984) established an age of 2711 Ma for leucosomes of a tonalitic orthogneiss. Figure 5.1: Simplified geological map of NE Sudbury after Murphy and Spray (2002). Inset: satellite image of the NE Sudbury area showing the field trip stops on the stripped outcrops at the Whistle funnel and Whistle-Parkin offset dike. MM – Milnet Mine, MMFZ – Milnet Mine Fault Zone, NP – Norman Project, NWP – Norman West Project, PCFZ – Post Creek Fault Zone, PM – Podolsky Mine, WM – Whistle Mine. 68 �Proceedings of the 68th ILSG Annual Meeting - Part 2 The 2642 Ma (Meldrum et al., 1997) Cartier batholith is part of the Algoma plutonic domain and consists of medium- to coarse-grained subporphyritic granite. It intruded and at least partially melted parts of the Levack Gneiss Complex (Langford, 1960; Dressler, 1984b; Ames et al., 2008), resulting in local breccia formation (“Levack breccia”) which can resemble Footwall Breccia. Aphyric to plagioclase glomeroporphyritic diabase and gabbros of the 2473 Ma (Heaman, 1997) Matachewan dike swarm crosscut gneissic and granitic units (Meldrum et al., 1997). North of the Milnet Mine fault, Huronian metasediments of the Quirke Lake and Cobalt Group dominate. In addition, diabase and gabbros of the 2.2 Ga (see Lightfoot, 2016 (p. 88) and references therein) Nipissing mafic intrusive suite occur conformable in the Huronian Supergroup. Whistle Funnel and Contact Mineralization The funnel-shaped Whistle embayment is located at the base of the Main Mass and is overlain by continuous layers of micropegmatite, transition quartz gabbro, and felsic norite, and a thin discontinuous layer of mafic norite (Fig. 5.2; Pattison, 1979; Lightfoot et al., 1997a; Lightfoot et al., 1997b). The dip of the contact changes from ~45 degrees south along the northern side of the funnel to ~70 degrees west along the eastern side of the funnel. First published descriptions of surface and drill core data by Pattison (1979) indicated a zonation of orthopyroxene-rich SLNR at the center of the embayment and gradually more siliceous igneous-textured SLNR matrix towards the margins. Here, the Sublayer transitions into FWBX (Fig. 5.2). In a detailed study of SLNR from the open pit, Lightfoot et al. (1997b) identified several lithologies: The central part of the embayment hosts a two pyroxenite SLNR, whereas orthopyroxene-rich SLNR, olivine norite and leucocratic norite occur at the margins or as centimeter-sized to tens of metersized pods within the others. Sublayer matrix is typically non-poikilitic and hosts disseminated to blebby sulfides. Inclusions can range from millimeters to meters and are described as diabase inclusions, anorthositic to gabbroic inclusions and melanorite inclusions or segregations with gradational contacts (Lightfoot et al., 1997b). The Whistle mine was operated by INCO Limited (now Vale) from 1988–1991 and 1994–1997, producing 5.7 Mt of ore grading 1% Ni and 0.3% Cu (Carter et al., 2009). Inclusion-rich massive sulfides (2–3% Ni, >0.2% Cu, <500ppm Pt+Pd) occur at the Sublayer-Footwall breccia contact and show a fractionation to more Cu-rich ore towards the base of the embayment (Lightfoot et al., 1997b). Whistle Dike and Footwall Mineralization After the Whistle Mine closure, acid-generating waste rocks that had been stored to the northeast of the embayment were mechanically and hydraulically stripped during the backfilling of the open pit, allowing subsequent detailed mapping and studies of the proximal Whistle offset dike (Fig. 5.2). The main outcrop was mapped in 2003 by FNX Mining Company (now KGHM), who were exploring in the proximal Whistle dike for Footwall-type mineralization, as well as locally in more detail by Carter et al. (2009) in 2003, and the Podolsky North Zone by Lafrance et al. (2014) in ~2008–2009 during petrographic-geochemical studies of the “metabreccias” and quartz diorite lithologies. The Whistle dike emerges from the funnel and can be traced for 2km to the northeast where it terminates in a breccia zone at the Post Creek fault (Fig. 5.1; Lightfoot et al., 1997b). MTBX is the dominant lithology with meter- to tens of meters-sized inclusion-poor and inclusion-rich quartz diorite pods more rarely occurring close to the dike margins. Magnetic lineation measurements suggest lateral emplacement with locally downward-directed flow for the dike lithologies and subsequent downward-directed sinking of massive sulfides (Giroux and Benn, 2005). 69 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 5.2: Simplified geological map and cross-section of the funnel-shaped Whistle embayment and proximal Whistle dike (after Farrow et al., 2005). Two Footwall-style Cu-(Ni)-PGE ore bodies occur within MTBX and IQD in the proximal segment of the Whistle dike: the Podolsky 2000 Deposit, which occurs at depth near the northwestern margin of the dike, and the Podolsky North deposit, which extends to the surface northeast of the main outcrop (Fig. 5.2). Mineralization in the Podolsky 2000 deposit occurs in the form of disseminated/blebby sulfides and “lowsulfide” stockwork veins in the host rocks, as well as breccia sulfide veins and “sharp-walled” massive sulfide veins in a large gabbroic inclusion (Farrow et al., 2005). The latter mineralization appears to cut the others. Chalcopyrite and millerite are the dominant Cu and Ni ore minerals. 2.24 Mt ore at 4.2% Cu and 0.4% Ni were mined between 2008–2013 (Lightfoot, 2016). A stripped outcrop (field trip stop 2-C1; Fig. 5.1) of the distal Whistle segment located just south of the Post Creek fault indicates that metabreccia with disseminated sulfides is still the dominant lithology. Parkin Dike and Offset Dike Mineralization The Parkin segment of the offset dikes appears north of the Post Creek fault, 2km displaced from the Whistle segment, and can be traced for another 12km to the northeast (Fig. 5.1). It is hosted by units of the Archean Benny greenstone belt for the first 4km and by the metasediments and metavolcanics of the Huronian Supergroup past the Milnet Mine fault. Exploration by Wallbridge Mining in 2014–2015 led to the stripping of several large outcrops of the offset dike close to the Post Creek fault. Here, QD and IQD are the common offset dike lithologies and MTBX occurs mainly as pods or inclusions in the quartz diorite lithologies (Anders et al., 2020) or as parallel bodies adjacent to the IQD (Murphy and Spray, 2002). QD is commonly located at the margins of the dike and in sharp contact with the interior IQD. Disseminated sulfides and sulfide stringers are mainly associated with inclusion-bearing lithologies in the dike center. 70 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Rock Descriptions The most common units/lithologies in the field area are listed in Table 5.1 with short descriptions, the most important of lithologies are discussed in more detail below. Table5.1: Rock units and lithologies to be encountered on the Field Trip. Unit/Lithology Abbreviation Description Levack Gneiss Complex MGN, IGN, and FGN Cartier Granite GR Medium- to fine-grained, weakly to strongly foliated, mafic (MGN), intermediate (IGN) and felsic (FGN) gneisses, gabbros and migmatites Medium- to coarse-grained, alkali feldspar-megacrystic granites to granodiorites and equigranular monzogranites Matachewan Intrusives Gabbro GAB Diabase DIA Impact-Related Rocks Sudbury Breccia SUBX Quartz Diorite QD Medium- to fine-grained, green, magnetic gabbro and diorite bodies intruding other footwall rocks Fine- to medium-grained, northwest striking, plagioclase glomeroporphyritic dikes intruding other footwall rocks Polymictic, matrix-supported breccia with aphanitic to very finegrained, black to dark grey groundmass that supports subrounded heterolithic footwall fragments (millimeter to tens of meters) Medium-grained, leucocratic, homogeneous, igneous textured granodioritic matrix with tabular to acicular and sometimes radiating amphibole (after pyroxene) laths; interpreted to represent variably contaminated impact melt Magmatic Impact-Related Breccias Inclusion Quartz Diorite IQD Polymictic, homogeneous, matrix-supported breccia with finegrained, grey to “salt-and-pepper” igneous-textured groundmass; local and exotic (mainly ultramafic) inclusions; often with <2% disseminated sulfides Sublayer Norite SLNR Polymictic, homogeneous, matrix-supported breccia with fine- to medium-grained, subophitic, noritic groundmass; abundant local and exotic (mainly ultramafic) inclusions; often with <5% disseminated sulfides Metamorphic-Anatectic Impact-Related Breccias Footwall Breccia FWBX Polymictic, heterogeneous, matrix-supported breccia with finegrained, pinkish white, granitic groundmass; subrounded inclusions (Granite Breccia) (GRBX) of local footwall rocks and SUBX Metabreccia MTBX Polymictic, heterogeneous, matrix- to clast-supported breccia with fine-grained dark grey to pinkish-grey, recrystallized groundmass; >50% inclusions of local footwall rocks ± pods of QD and IQD Late Dikes Olivine diabase UM East-west trending, up to 7m wide, dark, fine-grained dikes crosscutting all other lithologies in the area 71 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Breccia Classification Pre-impact Archean Breccia (“Levack Breccia”) Intrusion of the 2642 Ma Cartier Batholith brecciated many parts of the Levack Gneiss Complex (Card and Innes, 1981; Dressler, 1984b; Meldrum et al., 1997). Levack Breccia is characterised by a medium- to coarse-grained, locally alkali feldspar-megacrystic, granodioritic to monzogranitic matrix with angular to rounded, cm- to tens of meter-sized mafic, gneissic, and migmatitic inclusions (Fig. 5.3A). The distribution and abundance of Levack Breccia is not well known, but there are several areas on the Whistle outcrop where this breccia is present. Sudbury Breccia Pseudotachylitic SUBX appears to be the earliest formed impact breccia as it is crosscut by all other impact-related lithologies. Shock compression and cataclasis during crater formation led to shattering, pulverization, and frictional melting of footwall rocks, forming centimeter- to meter-sized veins and more irregular bodies of intense brecciation, often at structurally weakened zones and at lithology boundaries (e.g., Speers, 1957; Dressler, 1984a; Rousell et al., 2003; Lafrance et al., 2008; Lafrance and Kamber, 2010). It is characterized by a black to dark grey, aphanitic to very fine-grained matrix with (sub)-rounded centimeter- to meter-sized clast of immediate country rock lithologies (Fig. 5.3B). Sudbury breccia occurs frequently in the vicinity of the SIC but has been described up to distances of 50 km or more (Dressler, 1984a). Close to the SIC, contact metamorphism led to a grain size increase (often accompanied by lighter matrix colours), growth of biotite porphyroblasts, and partial melting of felsic mineral clasts. Inclusion-Bearing Quartz Diorite IQD is a polymictic breccia constraint to the offset dikes. It typically consists of a homogeneous, equigranular, fine- to medium-grained groundmass with a “salt-and-pepper” appearance (Fig. 5.3C). Inclusion sizes and abundancies vary from millimeter to tens of meters and <10% up to 90%, respectively. The matrix typically consists of tabular plagioclase with lesser alkali feldspar, quartz, and amphiboles. Radial amphiboles, which are typical for QD, occur less frequent in IQD. Sublayer Norite SLNR forms a discontinuous layer at the base of the Sudbury Igneous Complex normally occurring within funnels, troughs, and embayments. It is a variably mineralized polymictic breccia with centimeterto meter-size inclusions of Ol melanorite anteliths, local xenoliths, and exotic ultramafic xenoliths set in a fine- to medium noritic matrix (Fig. 5.3D; e.g., Pattison, 1979; Naldrett et al., 1984; Wang et al., 2018, 2020). Footwall Breccia FWBX (also termed “leucocratic breccia” and “late granite breccia”) is a polymictic breccia containing inclusions of local footwall rocks, SUBX, Main Mass norite, and exotic ultramafic inclusions (e.g., Pattison, 1979; Coats and Snajdr, 1984; Dressler, 1984a; Lakomy, 1990; McCormick et al., 2002; Wang et al., 2020). Grain sizes, compositions, and textures of the matrix are highly variable and dependent on proximity to the SIC contact and mineralization: the matrix is coarser-grained, igneous-textured and of dioritic to tonalitic composition closer to the SIC (Fig. 5.3E) and finer-grained, metamorphic-textured (recrystallized) and of granitic composition further into the footwall (Lakomy, 1990; McCormick et al., 2002). In proximity to mineralization, FWBX is often characterized by a grey colour (Greenman, 1970). 72 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Several modes of formation have been proposed, including a) contact metamorphism of impactbrecciated footwall during cooling of impact melt (Dressler, 1984a), b) partial melting and anatexis of felsic layers in the Levack Gneiss Complex due to heat conduction of cooling impact melt (Coats and Snajdr, 1984), and c) contact metamorphism during injection of Sublayer norite (Pattison, 1979). Metabreccia The term “metabreccia” was first used by INCO mining geologists in the 1970’s to describe thermal metamorphism of SUBX in proximity to the superheated melt sheet (E.F. Pattison, 2019, pers. comm.), which has been adopted by some of the other mining companies (e.g., Poulin et al., 2009). Other workers have interpreted MTBX to represent variable amounts of impact melt (QD/IQD) and partially melted footwall rocks (FWBX) mobilized into the dike structure (Murphy and Spray, 2002; Giroux and Benn, 2005; Lafrance et al., 2014; Carter et al., 2009; Anders et al., 2020). Farrow et al. (2005) were the first to introduce “metabreccia” as a term in the published literature, but different terminology exists by further authors, such as diatexite, metatexite (Lightfoot, 2016), radial breccia, mafic sulfide-bearing breccia (both Murphy and Spray, 2002), metamorphic leucocratic breccia/Footwall Breccia (Carter, 2005; Carter et al., 2009), and recrystallised Footwall Breccia (Grant and Bite, 1984). Research on metabreccia (or its synonyms) is restricted to the Whistle funnel and WhistleParkin offset dike, Ministic, Foy and Trill offset dikes – all located in the North Range. Metabreccia is a heterogeneous, grey pinkish, polymictic breccia with abundant millimeter- to centimeter-sized and lesser meter-sized footwall inclusions (Fig. 5.3F). Textures and mineralogy vary from igneous-textured with similar modal abundancies as QD and IQD (Lafrance et al., 2014) to dynamically recrystallized with a fine-grained quartz-feldspar-rich matrix (Anders et al., 2020). MTBX is the dominant lithology in the Whistle dike, whereas it occurs as pods in quartz diorite in Parkin. 73 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 5.3. A) Levack breccia near Whistle funnel. B) Metamorphosed SUBX near Whistle funnel (field trip stop 2-A2. C) IQD in Whistle funnel (field trip stop 2-A4). D) SLNR in Whistle funnel (field trip stop 2-A3). E) FWBX near Coleman Mine. F) MTBX in Whistle offset dike. 74 �Proceedings of the 68th ILSG Annual Meeting - Part 2 A. Whistle Embayment and Proximal Offset Dike The stripped outcrops of the Whistle funnel provide excellent exposure and a wealth of interesting field relationships and textures. Key aspects are highlighted on the satellite image (Fig. 5.4) and on the geological map (Fig. 5.5), and are described in more detail below. Figure 5.4: Satellite image of the filled and reclaimed Whistle open pit (prior to re-greening stage), showing field trip stops on the associated stripped outcrops. The Podolsky Mine is in the upper right part of the image. 75 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 5.5: Map of the Whistle outcrop (simplified from FNX Mining Company) showing field trip stops. Dashed lines delineate the footwall-funnel/dike contact. 76 �Proceedings of the 68th ILSG Annual Meeting - Part 2 2-A1) SLNR-GRBX contact (NAD83 17N, 509053, 5179920) The first stop is located at the western flank of the embayment near the reclaimed open pit. SLNR is in contact with FWBX over a few centimeters. SLNR has a fine- to medium-grained, dark grey to black matrix with minor rugged sulfide blebs and some subrounded inclusions (Fig. 5.6A–C). FWBX has a pinkish grey, fine- to medium-grained, heterogenous matrix with abundant strongly intergrown quartz-feldspar and variable amounts of interstitial green amphibole. Inclusions are typically subangular and of mafic to intermediate composition (Fig. 5.6D–F). Figure 5.6: High-resolution sample scans, plane-polarized and cross-polarized images of SLNR (A-C) and FWBX (D-F) from the eastern funnel flank. 77 �Proceedings of the 68th ILSG Annual Meeting - Part 2 2-A2) SUBX and footwall rocks (NAD83 17N, 509289, 5180141) This 170m x 120m stripped outcrop is located in the footwall just west of the former open pit. Meter- to tens of meters-scale blocks of various country rocks are enveloped and truncated by cm- to dm-wide pseudotachylitic SUBX veins and bodies (Fig. 5.7A). Walking from the NW end of the outcrop towards the embayment (ESE direction), a slight gradational change of color, grain size and clast angularity are noticeable in the SUBX veins and might reflect a baking of the breccia closer to the former impact melt. The matrix changes from a black/dark grey to a medium grey, coarsens slightly, biotite porphyroblasts are observable and small felsic clasts are getting wispier closer to the former impact melt. Small SUBX bodies are intruded by a pinkish medium- to coarse-grained feldspar-rich granitoid in some areas, which might indicate the formation of localized feldspar-rich FWBX (Fig. 5.7B). Figure 5.7: A) Clast of a pre-impact Levack Breccia consisting of sub-angular to -rounded dm-sized gabbro in a granitic matrix, surrounded by SUBX. B) SUDBX fragment (?) wrapped around subrounded, 30cm large gabbro inclusion itself intruded by feldspar-rich granitoid (FWBX?). 2-A3) Sublayer norite (NAD83 17N, 509540, 5180011) SLNR is only exposed at the SW end of the 300m x 300m main outcrop. It typically displays a rustybrown weathering surface sulfide oxidation (Fig. 5.3). The noritic matrix is typically homogeneous, fineto medium-grained, and displays equigranular textures. It contains abundant country rock inclusions, minor dark green/brown ultramafic inclusions, and QD-type inclusions. Globular and ragged sulfide blebs up to 2cm in size are common. 2-A4) IQD-MTBX (NAD83 17N, 509563, 5180046) To the NE, the SLNR transitions into FWBX/MTBX in the central part of the main outcrop, indicating the beginning transition from an embayment environment to an offset environment. At the margins of the FWBX/MTBX, bodies or pods of leucocratic QD and IQD occur (Fig. 5.5), often in sharp contact with FWBX/MTBX as shown here. The IQD is characterized by a homogeneous fine-to medium-grained matrix with abundant amphiboles needles and less than 20% (sub)-rounded inclusions. FWBX/MTBX is appearing more heterogenous and rich in inclusions. 78 �Proceedings of the 68th ILSG Annual Meeting - Part 2 2-A5) SLNR-IQD-MTBX-QD (NAD83 17N, 509600, 5179940) This area on the southeastern flank of the embayment, which was mapped in detail by Carter et al. (2009), features excellent exposure of contact relationships between leucocratic QD, MTBX/FWBX, IQD, and SLNR. It also highlights the complexity and variability in breccia matrix composition in a small area (Fig. 5.8 and Fig. 5.9). Footwall gabbros and granitoids are crosscut by decimeter wide SUBX veins (e.g., location 4 in Fig. 5.9). Abundant subrounded inclusions from local footwall rocks are set in an aphanitic to very fine-grained groundmass which is, independent of the host unit, composed of predominantly chlorite and epidote. Leucocratic QD occurs as a large body in contact with the footwall (location 1 in Fig. 5.9). It is mediumgrained with abundant amphibole and plagioclase laths as well as oikocrystic quartz, feldspar, and granophyric quartz-feldspar intergrowth (Fig. 5.8B–C). Figure 5.8: A) Sharp contact between LQD (left) and polymictic MTBX/FWBX (right). See Fig. 5.9 for location. Plane- (B) and cross-polarized (C) images of the leucocratic QD matrix. 79 �Proceedings of the 68th ILSG Annual Meeting - Part 2 MTBX/FWBX (depending on the map) is fine-grained, equigranular, polymictic, and in sharp contact with leucocratic QD at location 1. The polymictic breccia is typically inclusion-rich (>50%) with inclusion sizes ranging from millimeters to several meters. Feldspar with lesser quartz and interstitial amphibole are the dominant minerals in the dark grey, fine-grained breccia groundmass. In contrast, MTBX/FWBX at location 3 (Fig. 5.9), 5m away from location 1, displays typical grey-pinkish colors, is inclusion-rich and appears recrystallized on fresh surfaces. The poikilitic groundmass consists of highly altered equant plagioclase and tabular amphibole chadacrysts in coarser, oikicrystic quartz-(feldspar). IQD at location 2 (Fig. 5.9) is characterized by a homogeneous matrix with a typical “salt-and-pepper” color on fresh surfaces. It is polymictic with <25% inclusions and minor sulfide blebs. The matrix is fineto medium-grained and poikilitic. Variably altered, equant plagioclase and rare amphibole needles are set in oikocrystic quartz with minor feldspar and granophyric intergrowth. Macroscopically, the different breccia units appear to be quite different and easily distinguishable. SUBX adjacent to the funnel/offset dike displays distinct grain size differences, is strongly altered and of a more mafic matrix composition. Groundmass compositions of the other breccia units are remarkably similar (see also Lafrance et al., 2014). 80 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 5.9: Detailed map of the southeastern flank with SUBX, SLNR, IQD, MTBX/FWBX, leucocratic QD, and footwall rocks. Locations of breccia samples are shown in the map. The leucocratic QDMTBX/FWBX contact is shown in Fig. 5.8. Images of slabs and thin sections are described in the text. 81 �Proceedings of the 68th ILSG Annual Meeting - Part 2 82 �Proceedings of the 68th ILSG Annual Meeting - Part 2 2-A6-1 GRBX-Cartier Batholith contact (NAD83 17N, 509538, 5179867) Located at the SE flank of the embayment just meters away from the covered and re-greened open pit. FWBX is in a well-defined contact over 1–2cm with Cartier granite. 2-A6-2 IQD with QD Inclusions (NAD83 17N, 509552, 5179875) Sharp contact between QD and IQD (SLNR?). QD is leucocratic, medium-grained and displays well developed amphibole needles <1cm. IQD is characterized by a finer-grained, homogeneous matrix with salt-and-pepper texture, 10–40% inclusions and disseminated blebby sulfides. Several subrounded QD pods or inclusions are visible and highlighted by their more leucocratic appearance (Fig. 5.10A). 2-A7 Leucocratic QD with local footwall clasts (NAD83 17N, 509693, 5180036) Coarse-grained, leucocratic QD with up to 2cm long amphibole needles and angular, greenish-altered gabbro inclusions (Fig. 5.10B). Located just 1m away from the SE dike margin right at the contact between Cartier batholith and gabbro, this area indicates only a short lateral transport of the gabbro inclusions. Country rock inclusions at QD margins is also documented in other offset dikes, such as Foy, Hess, and Worthington. Figure 5:10: A) IQD with three (leucocratic) QD pods/inclusions in sharp contact with (leucocratic) QD at 2-A6-2. B) QD with large amphibole needles and local clasts at 2-A7. Image width is approximately 75 cm. 2-A8 IQD-MTBX (NAD83 17N, 509799, 5180205) This area has been mapped in detail by Carter et al. (2009) and shows some excellent IQDMTBX/FWBX features on glacially polished surfaces. The IQD-MTBX/FWBX contact can be traced over several meters and is generally sharp. MTBX/FWBX is dark grey on fresh surfaces, inclusion-rich (ca. 50%), and closely associated with large (>5m) Cartier granite clasts. Small (<3cm), rounded, red feldspar inclusions are common with mafic inclusions typically being smaller and less common. IQD is leucocratic, medium-grained with well-developed amphibole needles and typically less than 10% inclusions. 83 �Proceedings of the 68th ILSG Annual Meeting - Part 2 B. Intermediate Whistle Dike 2-B1 Podolsky IQD-MTBX-Sulfides (NAD83 17N, 509937, 5180490) The Podolsky outcrop located 500m NE of the Whistle embayment (Fig. 5.4), features the surface exposure of sharp-walled Cu-PGE-rich sulfide veins from the Podolsky North Zone. Steeply dipping chalcopyrite-rich veins are locally up to 3m wide but range more commonly on a cm- to dm-scale. They follow lithological boundaries as well as crosscuts IQD and MTBX (Fig. 5.11B–C). Flow laminations can be observed in the veins (Lafrance et al., 2014). IQD predominates in the southern part of the outcrop and MTBX in the northern part. QD pods or inclusions (typically <3m) are more abundant in contact with IQD than with MTBX and display several centimeter long radiating amphibole needles (after pyroxene). Contacts between QD and the other units can be sharp or gradational over a few centimeters (Fig. 5.11A). IQD and MTBX have similar clast compositions, but the latter is generally more enriched. Detailed descriptions of the lithologies can be found in Lafrance et al., (2014). 84 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 5.11: Geological map of the Podolsky outcrop (simplified after Carter et al., 2009). A) Sharp contact between MTBX and QD. B) Chalcopyrite-rich sample with inclusions. C) Drone image of the Cu-vein with gneissic clast. 85 �Proceedings of the 68th ILSG Annual Meeting - Part 2 C. Distal Whistle Dike 2-C1 (NAD83 17N, 510293, 5181352) This stripped outcrop 1.5km northeast of the Whistle embayment (Fig. 5.1) belongs to claims of North American Nickel Inc. and displays the northern most segment of the Whistle dike before it is offset by the Post Creek fault. The eastern part of the outcrop consists of Cartier granite. MTBX is the dominant lithology in the central part. It is very fine-grained, grey pinkish with abundant small feldspar inclusions and <2% disseminated sulfides (Fig. 5.12A–B). Meter-sized inclusions of subangular to subrounded diabase, granite and QD inclusions are common (Fig. 5.12C). Figure 5.12: A) close-up image of MTBX matrix. B) polished slab of MTBX with abundant feldspar clasts. C) QD clast in contact with MTBX at the distal Whistle outcrop 2-C1. D) Well defined contact between inclusion-poor and inclusion-rich quartz diorite at the proximal Parkin outcrop 2-D1. Hammer head for scale 86 �Proceedings of the 68th ILSG Annual Meeting - Part 2 D. Proximal Parkin Dike Wallbridge Mining´s proximal Parkin properties are located just north of the Post Creek Fault (Fig. 5.1). Mechanical stripping, mapping, and drilling of 63 drill holes as well as geophysical surveys were conducted in 2015 and 2016. Outcrops maps are shown in Fig. 5.13 and Fig. 5.14. 2-D1 Northern Part (NAD83 17N, 509199, 5183212) The northern stripped outcrops show large sections of the central portion of the Parkin offset dike. IQD is the dominant lithology and incorporates millimeter to tens of meter sized country rock inclusions as well as disseminated and stringer sulfides. QD can locally be observed at the margins of the dike and the outcrops. There are several areas where a sharp contact between QD and IQD can be observed (Fig. 5.12D). MTBX can be observed as inclusions and irregular pods in QD and IQD, often with well-defined contacts over <1mm and more rarely, with gradational contacts as indicated by changes in inclusion abundance (see Anders et al., 2020). 2-D2 Southern Part (NAD83 17N, 509104, 5183025) The southern stripped outcrop displays similar features as the northern outcrops. In addition, two prominent large (˃10m) felsic gneiss inclusions occur in the central portion of the dike. These potential Levack Gneiss inclusions are not from the immediate country rocks, thus indicating the high energy of the quartz dioritic melt during injection. Similar observations were made by Murphy and Spray (2002) 1km to the northeast of this outcrop. 87 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 5:13: Map of Wallbridge Mining´s proximal Parkin stripped outcrops (northern part, from Wallbridge Mining Assessment Report on the Parkin Property 2015-16). 88 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 5:14: Map of Wallbridge Mining´s proximal Parkin stripped outcrops (northern part, from Wallbridge Mining Assessment Report on the Parkin Property 2015-16). 89 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Alternative Field Trip Stops: Worthington, Trill East, and Hess Ermatinger Worthington The Worthington offset dike is located at the SW lobe of the SIC and is connected to the Main Mass of the SIC through the Victoria embayment (Fig. 2.1). The dike crosscuts and incorporates inclusions of metasediments of the Huronian Supergroup as well as Nipissing diabase. Several historical mines targeted small ore bodies close to the surface over the last century. Vale´s Totten mine, in operation since 2014, is the latest addition to Sudbury operations and the only active mine at the Worthington dike. Nevertheless, there are several exploration projects in the area. KGHM´s Victoria project is located close to the embayment and SPC Nickel is conducting an exploration and drilling program on their AER-KIDD property (Fig. 6.1), located between Victoria to the northeast and Totten to the southwest. Figure 6.1: Satellite image with outcrop locations of SPC Nickel’s AER-KIDD exploration project. Outcrop 1 is located in the southwest, outcrop 2 in the center and outcrops 3 and 4 in the northeast of the map. 90 �Proceedings of the 68th ILSG Annual Meeting - Part 2 AER-KIDD Outcrop 1 Outcrop 1 displays the eastern contact between the Worthington offset dike and Footwall metasediments of the McKim Formation, which are in sharp contact with very fine-grained to spherulitic QD (Fig. 6.2A– B). QD matrix grain size gradually increases over 2–3m towards the center where it is in a well-defined contact with finer grained IQD (Fig. 6.2A–B). The contact is characterized by a) the grain size difference, b) inclusions, and c) disseminated sulfides. Centimeter- to decimeter-sized inclusions are generally aligned parallel to the strike of the dike and are comprised of footwall rock assemblages, i.e., gabbros and amphibolites presumably of the Nipissing mafic suite and McKim Formation metasediments. Subrounded, meter-sized inclusions of QD in IQD are common. AER-KIDD Outcrop 2 Outcrop 2 is adjacent to the former small Robinson mine and displays similar field relationships as outcrop 1, although less well defined due to abundant sulfide oxidation. In addition to QD and IQD, the most central part of the dike (and most westerly part of the outcrop before the fenced-off mine) is comprised of an amphibolite inclusion-rich quartz diorite (AIQD) which can grade into amphibolite-bearing sulfide matrix breccia. Amphibolite inclusions are typically larger in AIQD than in IQD. AER-KIDD Outcrop 3 and 4 Outcrops 3 and 4 are located 300m northeast of outcrop 2 and display similar features as outcrop 1 (Fig. 6.3). QD-IQD contacts are generally well defined over a few millimeters and characterized by grain size differences as well as inclusion and sulfide abundances (Fig. 6.3A). In some areas, rounded inclusions of McKim Formation are incorporated into QD close to the footwall contact. Also, “mushroom-like” bulging of McKim Formation into QD can be observed (Fig. 6.3B). 91 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 6.2: Geological map, measurements, and field images of the AER-KIDD outcrop 1. Most country rock clast in the IQD are rotated parallel to the IQD-QD and QD-Footwall contact. A-B) QD-IQD contact is well-defined and characterized by grain size differences as well as inclusion and sulfide content. 92 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 6.3: Geological map and field images of AER-KIDD outcrops 3 and 4. A) Sharp QD-IQD contact defined by grain size differences, inclusions and disseminated blebby sulfides. B) McKim Fm. “intruding” QD. 93 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 6.4: Geological map of Trill East outcrop (simplified after Wallbridge Mining Assessment Report on the Trill Property 2014). A) SUBX vein crosscut by QD. MTBX might be present at the contact. B) Close-up of MTBX inclusion/pod in QD. 94 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Trill East The 3–20m wide, radial Trill offset dike is located southwest of the Sudbury Igneous Complex, just north of the boundary between Superior province and Southern province (Fig. 2.1). Exploration by Wallbridge Mining in 2005 led to the discovery of a 65m x 5m long sulfide mineralization 4km west of the SIC. Mechanical stripping of several outcrops revealed several offset dike units: QD, IQD, spherulitic QD and glassy QD. Common QD is typically found at the dike margin and IQD in the center. Glassy and spherulitic QD are restricted to thin apophyses in the footwall (Klimesch, 2009). The Trill East outcrop, ca. 500m west of the SIC, features the northern dike-footwall contact (Fig. 6.4). SUBX crosscuts the Cartier granite in fine veinlets as well as in a thicker zone at the eastern end of the outcrop and is itself crosscut by the offset dike (Fig. 6.4A). Dark grey QD is the only offset dike unit present. Several inclusions or pods of MTBX occur within QD (Fig. 6.4B) or between the QD-footwall contact. MTBX is fine-grained, has a light grey appearance and is enriched in small (mm–cm) plagioclase-quartz, alkali feldspar mineral, and mafic clasts (Anders, 2016). Hess Ermatinger The concentric Hess offset dike follows the northern outline of the SIC in ca. 15km distance. It has been traced from E of the Foy-Hess intersection to the Ermatinger township WNW of Sudbury (Fig. 2.1). Exploration efforts by Wallbridge Mining in 2010–2012 led to the mechanical stripping and drilling of several outcrops with one in particular displaying excellent exposures of offset dike-footwall interactions, QD and IQD. The footwall contact of the steeply SE dipping offset dike can be traced for 40m and is characterized by a decimeter-wide zone of QD chilled against Cartier granite (Fig. 6.5). The contact is often sharp but can be irregular in zones of QD bulging into the footwall (Fig. 6.5C). Granite inclusions are abundant in the chilled QD, vary in size from cm-dm and are sometimes partially digested (Fig. 6.5D). Several smaller and larger apophysis are cutting through the footwall granite and diabase dikes. QD grain sizes gradationally increase from chilled to medium-grained over 5m. The contact between medium-grained QD and fine-grained IQD is predominantly defined by the abrupt change in grain size (Fig. 6.5A–B). IQD is characterized by a fine-grained, sulfide-poor (<2%), inclusion-poor (<10%) IQD. In comparison to the local inclusions in the chilled QD, inclusions in IQD are of gabbroic, dioritic and ultramafic composition not directly associated with the local footwall. Disseminated blebby sulfides occur in patches and are composed of a typical magmatic sulfide assemblage. Acknowledgements The authors for Day 2 would like to thank the mining companies for access to their properties: Whistle mine (Vale); Podolsky (KGHM); Parkin, Trill and Hess (Wallbridge Mining); Worthington (SPC Nickel); distal Whistle (North American Nickel). We are grateful to Wouter Bleeker for organizing both field days and to Mike Easton for editorial handling. Safety gear was kindly provided by the Harquail School of Earth Sciences at Laurentian University. 95 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 6.5: Geological map of the Hess Ermatinger outcrop. A) Northern contact (dashed lines) between medium-grained QD and fine-grained IQD. Rusty spots in IQD indicate presence of sulfide blebs. B) Southern QD-IQD contact with fresh and weathered surfaces. Gabbro clast in IQD is not from the local footwall. C) Chilled QD intrudes into Cartier granite. D) Dm-sized granite clast showing partial digestion. Diabase dikes are crosscut by QD. 96 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Literature related to Day 2 Davis, D.W. 2008. Sub-million-year age resolution of Precambrian igneous events by thermal extraction– thermal ionization mass spectrometer Pb dating of zircon: application to crystallization of the Sudbury impact melt sheet: Geology, v.36, p.383-386. Ames, D.E., Davidson, A. and Wodicka, N. 2008. Geology of the giant Sudbury polymetallic mining camp, Ontario, Canada: Economic Geology, v.103, p.1057-1077. Anders, D. 2016. The Sudbury Impact Structure – new insights into the origin and emplacement of the Basal Onaping Intrusion and the Parkin, Trill and Foy offset dykes of the North Range: Unpublished PhD thesis, The University of Western Ontario, 183p. Dressler, B.O. 1984a. 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Geology, geochemistry, and mineralogy of the Worthington offset dike: a genetic model for offset dike mineralization in the Sudbury Igneous Complex: Economic Geology, v.97, p.1419-1446. Morrison, G.G. 1984. Morphological Features of the Sudbury Structure in Relation to an Impact Origin, in The Geology and Ore Deposits of the Sudbury Structure, Special Volume 1, p.513-520. Lightfoot, P.C., Keays, R.R. and Doherty, W. 2001. Chemical evolution and origin of nickel sulfide 99 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Osinski, G.R. and Pierazzo, E. 2012. Impact cratering: processes and products, in Osinski, G. R., and Pierazzo, E., eds., Impact Cratering, p.1-20. Mukwakwami, J., Lafrance, B., Lesher, C.M., Tinkham, D., Rayner, N.M. and Ames, D.E. 2014. Deformation, metamorphism, and mobilization of Ni–Cu–PGE sulfide ores at Garson Mine, Sudbury: Mineralium Deposita, v.49, p. 75-198. O’Sullivan, E.M., Goodhue, R., Ames, D.E. and Kamber, B.S. 2016. Chemostratigraphy of the Sudbury impact basin fill: Volatile metal loss and post-impact evolution of a submarine impact basin: Geochimica et Cosmochimica Acta, v.183, p.198233. Mungall, J.E. 2007. Crystallization of magmatic sulfides: An empirical model and application to Sudbury ores: Geochimica et Cosmochimica Acta, v.71, p.2809-2819. Pattison, E.F. 1979. The Sudbury Sublayer: The Canadian Mineralogist, v.17, p.257-274. Murphy, A.J. and Spray, J.G. 2002. Geology, mineralization, and emplacement of the WhistleParkin offset dike, Sudbury: Economic Geology, v.97, p.1399-1418. Pilles, E.A., Osinski, G.R., Grieve, R.A F., Smith, D.A. and Bailey, J.M. 2017. Chemical variations and genetic relationships between the Hess and Foy offset dikes at the Sudbury impact structure: Meteoritics & Planetary Science, v.52, p.2647-2671. Naldrett, A.J. 2004. Magmatic Sulfide Deposits: Geology, Geochemistry and Exploration: Berlin, Heidelberg, Springer Berlin Heidelberg, 727p. Pilles, E.A., Osinski, G.R., Grieve, R.A F., Coulter, A.B., Smith, D. and Bailey, J. 2018a. The Pele offset dykes, Sudbury Impact Structure, Canada: Canadian Journal of Earth Sciences, v. 55, p. 230-240. Naldrett, A.J., Asif, M., Schandl, E., Searcy, T., Morrison, G.G., Binney, W.P. and Moore, C., 1999, Platinum-group elements in the Sudbury ores; significance with respect to the origin of different ore zones and to the exploration for footwall orebodies: Economic Geology, v. 94, p. 185-210. Pilles, E.A., Osinski, G.R., Grieve, R.A.F., Smith, D. and Bailey, J. 2018b. Formation of large-scale impact melt dikes: A case study of the Foy offset dike at the Sudbury Impact Structure, Canada: Earth and Planetary Science Letters, v.495, p.224-233. Naldrett, A. J., Hewins, R. H., Dressler, B. O., Rao, B.V. and Pye, E.G. 1984. The Contact Sublayer of the Sudbury Igneous Complex, in The Geology and Ore Deposits of the Sudbury Structure: Ontario Geological Survey, Special Volume 1, p. 253-274. Poulin, R., Dunlop, S. and Everest, J.O. 2009. Podolsky field guide - Podolsky mine property and Whistle pit, Norman Twp., Sudbury mining district, Ontario, FNX Mining Company Ltd., 8p. Naldrett, A.J., Hoffman, E.L., Green, A.H., Chou, C.L., Naldrett, S.R. and Alcock, R.A. 1979, The composition of Ni-sulfide ores, with particular reference to their content of PGE and Au: The Canadian Mineralogist, v. 17, p. 403-415. Prevec, S.A. and Büttner, S.H. 2018. Multiphase emplacement of impact melt sheet into the footwall: offset dykes of the Sudbury Igneous Complex, Canada: Meteoritics & Planetary Science, v.53, p.1301-1322. Nelles, E.W. 2012. Genesis of Cu-PGE-rich Footwalltype mineralization in the Morrison Deposit, Sudbury: Unpublished MSc thesis, Laurentian University, 87p. Reimold, W.U. and Gibson, R.L. 2006. The melt rocks of the Vredefort impact structure–Vredefort Granophyre and pseudotachylitic breccias: implications for impact cratering and the evolution of the Witwatersrand Basin: Geochemistry, v.6, p.135. Nunes, P. and Pyke, D. 1980. Geochronology of the Archean metavolcanic belt, Timmins-Matachewan area—Progress report; in Summary of Field Work and Other activities, Ontario Geological Survey, Miscellaneous Paper 92, p. 34-39. Riller, U. 2005. Structural characteristics of the Sudbury Impact Structure, Canada: impact-induced versus orogenic deformation—a review: Meteoritics & Planetary Science, v.40, p.1723-1740. Osinski, G.R., Grieve, R.A.F., Bleacher, J.E., Neish, C.D., Pilles, E.A. and Tornabene, L.L. 2018. Igneous rocks formed by hypervelocity impact: Journal of Volcanology and Geothermal Research, v.353, p.2554. Robertson, J., Ripley, E.M., Barnes, S.J. and Li, C. 2015. Sulfur liberation from country rocks and 100 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Stout, A.E. 2009. Geology, mineralogy, and geochemistry of the McCreedy East 153 Cu-Ni-PGE Deposit, Sudbury, Ontario: Unpublished MSc thesis, Utrecht University, 39p. incorporation in mafic magmas: Economic Geology, v.110, p.1111-1123. Robertson, J.C., Barnes, S.J. and Le Vaillant, M. 2016. Dynamics of magmatic sulphide droplets during transport in silicate melts and implications for magmatic sulphide ore formation: Journal of Petrology, v.56, p.2445-2472. Therriault, A.M., Fowler, A.D. and Grieve, R.A.F. 2002. The Sudbury Igneous Complex: a differentiated impact melt sheet: Economic Geology, v.97, p.15211540. Rousell, D.H., Fedorowich, J.S. and Dressler, B.O. 2003, Sudbury Breccia (Canada): a product of the 1850 Ma Sudbury event and host to footwall Cu–Ni– PGE deposits: Earth-Science Reviews, v.60, p.147174. Tuchscherer, M.G. and Spray, J.G. 2002. Geology, mineralization, and emplacement of the Foy offset dike, Sudbury Impact Structure: Economic Geology, v.97, p.1377-1397. Wang, Y., Lesher, C.M., Lightfoot, P.C., Pattison, E.F. and Golightly, J.P. 2018. Shock metamorphic features in mafic and ultramafic inclusions in the Sudbury Igneous Complex: Implications for their origin and impact excavation: Geology, v.46, p. 43446. Scott, R.G. and Benn, K. 2002. Emplacement of sulfide deposits in the Copper Cliff offset dike during collapse of the Sudbury crater rim: evidence from magnetic fabric studies: Economic Geology, v.97, p.1447-1458. Souch, B E., Podolsky, T. and Wilson, H.D.B. 1969. The sulfide ores of Sudbury: Their particular relationship to a distinctive inclusion-bearing facies of the Nickel Irruptive, Magmatic Ore Deposits, 4, Society of Economic Geologists. Wang, Y., Lesher, C.M., Lightfoot, P.C., Pattison, E.F. and Golightly, J.P. 2020. Geochemistry and petrogenesis of mafic and ultramafic inclusions in Sublayer and Offset Dikes, Sudbury Igneous Complex, Canada: Journal of Petrology, v.61. Speers, E.C. 1957. The age relation and origin of common Sudbury Breccia: The Journal of Geology, v.65, p.497-514. Wang, Y., Lesher, C.M., Lightfoot, P.C., Pattison, E.F. and Golightly, J.P. (in press). Genesis of Sublayer, Footwall Breccia, and associated Ni-Cu-PGE Mineralization in the Sudbury Igneous Complex: Economic Geology. Spray, J.G., Butler, H.R. and Thompson, L.M. 2004. Tectonic influences on the morphometry of the Sudbury Impact Structure: implications for terrestrial cratering and modeling: Meteoritics & Planetary Science, v.39, p.287-301. Wichman, R.W. and Schultz, P.H. 1993. Floor-fractured crater models of the Sudbury Structure, Canada: implications for initial crater size and crater modification: Meteoritics, v.28, p.222-231. Sproule, R.A., Sutcliffe, R., Tracanelli, H. and Lesher, C. M. 2007. Palaeoproterozoic Ni–Cu–PGE mineralisation in the Shakespeare intrusion, Ontario, Canada: a new style of Nipissing gabbro-hosted mineralisation: Applied Earth Science, v.116, p.188200. Yao, Z.-s. and Mungall, J.E. 2021. Kinetic controls on the sulfide mineralization of komatiite-associated Ni-Cu-(PGE) deposits: Geochimica et Cosmochimica Acta, v. 05, p.185-211. 101 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Location of stops for ILSG Field Trip 2. Trip starts and leaves from Science North (upper left). Stops 1 to 5 are accessed along Highway 17 and the Highway 17 bypass. Stops 6 is on Highway 537. Stops 7 to 14 are accessed from Estaire Road (formerly Highway 69). 102 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Field Trip 2 – Geology of the Grenville Front and the Grenville Front Tectonic Zone in the Sudbury area R.M. Easton Earth Resources and Geoscience Mapping Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5 Introduction For metamorphic rocks, mineral prefixes are listed in order of relative abundance, starting with least abundant first. Mineral abbreviations follow Whitney and Evans (2010). The following conventions are used regarding descriptive adjectives. A gneissic granite is a meta-igneous rock of granitic composition. A granitic gneiss, a granite gneiss, or a gneiss of granitic composition may be either a meta-igneous or a metasedimentary rock. Similarly, a tonalitic gneiss or a tonalite gneiss is a gneiss of tonalite modal composition but may be of either meta-igneous or metasedimentary origin. A gneissic meta-arkose is a metasedimentary gneiss of overall granitic composition. The term metamorphic grade is used where bulk-rock composition or other factors prevent a more detailed assignment of metamorphic conditions. Where metamorphic conditions can be outlined, metamorphic facies terminology is used. The field trip uses road accessible outcrops. All of the road stops can be accessed using a 2-wheel drive vehicle. Unless otherwise stated, all UTM coordinates are in Zone 17, datum NAD 83. Safety Many of the field trip stops are located on highways that are especially busy during the summer season. Care should always be exercised when parking, exiting vehicles, and crossing the roads. Use of safety vests and/or bright clothing is recommended, in order to improve your visibility to motorists. Most of the trip routes are on Crown land or public roadways, but access is on or near private property in some cases. As in all such situations, please respect the property rights of others, so as to maintain good relationships, so that future access for geologists is not adversely affected. Many rocks in the Grenville Province were subjected to extreme ductile deformation and subsequently recrystallized, and can be described either as tectonites or gneissic mylonites. Several field-based terms have been proposed to describe these gneissic mylonites including the terms straight gneiss, block gneiss, and porphyroclastic gneiss (e.g., Davidson et al. 1982; Hanmer and Ciesielski 1984). Terminology A number of terms used in this report are outlined below. Rock Classification Layering thickness terms used in this report are listed below. These terms apply to bedded, layered and gneissic rocks. Very thinly layered Thinly layered Medium layered Thickly layered Very thickly layered Extremely thickly layered A migmatite is a heterogeneous rock composed of two or more components, one generally quartzofeldspathic in composition (leucosome or neosome) and the other more mafic in composition (paleosome or mesosome). Within the field trip area, such rocks are commonly layered, and in many instances, are formed by partial melting during high-grade regional metamorphism. <3 cm 3 to 10 cm 10 to 30 cm 30 to 100 cm 1 to 3 m >3 m Terminology for plutonic rocks follows that of Streckeisen (1976) and LeMaitre et al. (2002). 103 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Geological Setting Descriptive terminology for these rocks follows Sawyer (2008) and Mehnert (1971). Migmatites collectively display a wide variety of features depending on the degree of partial melting and deformation during development. The first-order division of migmatites, based on morphology and proportion of leucosome, results in 2 types: metatexite and diatexite. The division between the 2 is based on the relative amount of melt (leucosome) in the rock. The Ontario Geological Survey uses a boundary of 20% leucosome between metatextite and diatextite, which is near the minimum value suggested by Sawyer (2008) but does not require the same precision in estimating leucosome content as the use of 16% would require. The 20% boundary also accounts for the fact that initial bulk-rock composition of the protolith is a factor in the amount of partial melt that can be produced, and thus is better suited for a wide range of bulk-rock compositions. Proterozoic Rocks in the Sudbury area Proterozoic rocks in the Sudbury area are assigned to either the Paleoproterozoic Southern Province or the Mesoproterozoic Grenville Province (cf. Wynne-Edwards 1972; Easton 1992). The Southern Province in Ontario comprises Paleoproterozoic metasedimentary and metavolcanic rocks of the Huronian Supergroup and gabbroic intrusions of the Nipissing gabbro suite. Also included in the Southern Province are the Sudbury Igneous Complex and the Whitewater Group; plutonic and minor volcanic rocks of the Killarney Magmatic Belt; and rocks of the Sudbury diabase dike swarm (Figure 1) (Bennett, Dressler and Robertson 1991). The Huronian Supergroup (Figure 2) was deposited unconformably on Archean plutonic and supracrustal rocks of the Superior Province. The lowest unit, the Elliot Lake Group, consists of both metavolcanic and metasedimentary rocks (Figure 2). In the Sudbury area, the metavolcanic units include tholeiitic basalts of the Elsie Mountain Formation, evolved tholeiitic basalts, dacites, and metasedimentary rocks of the Stobie Formation, and dacites and rhyolites of the Copper Cliff Formation (circa 2460 Ma). The latter are likely coeval with the Murray and Creighton granites (Bennett, Dressler and Robertson 1991; Bleeker et al. 2015). The metavolcanic units interfinger with, and are overlain, by the Matinenda Formation in the west and the McKim Formation in the east. Geochemical data reported by Innes (1972, 1977), Easton (1998), Gordon (2021) indicate a tholeiitic affinity for the Stobie Formation, whereas felsic metavolcanic rocks of the Copper Cliff Formation and the Murray and Creighton granites show calcalkalic signatures. Purpose The purpose of the trip is two-fold. First, is to examine the nature of the Grenville Front and the associated Grenville Front tectonic zone using a variety of exposures in the Sudbury area. Second, is to examine outcrops mapped in the fall of 2021 between Wanup and Estaire. This new mapping indicates that Nepewassi domain rocks occur closer (by approximately 5-10 km) to the Grenville Front then previously recognized. Also, although not specifically part of the field trip route, the new field data also suggests the presence of a major north-northwest-trending structure along the Wanapitei River near Wanup, with the character of the rocks in the Grenville Front tectonic zone being very different in lithology and structural character on either side of the structure (see section on “The GFTZ zone near Wanup”). 104 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 1. Geology of the northern Central Gneiss Belt of the Grenville Province and the Grenville Front region in Ontario. Locations of mapping areas described by Easton (2014) and Van de Kerckhove (2014) are indicated by dashed-line boxes. Abbreviations: C, Cosby pluton; SF, Sturgeon Falls batholith; WB, West Bay batholith; and WC, Wanapitei complex. Figure modified from Easton (1992, p.755). At the base of the Huronian Supergroup in the Elliot Lake, Agnew Lake and Sudbury areas are several layered gabbro to anorthosite intrusions referred to as the East Bull Lake intrusive suite (Peck et al. 1993; James et al. 2002a, 2002b). These bodies have ages of circa 2475 Ma (Clough and Hamilton 2017; Krogh et al. 1984) and appear to be slightly older than the rocks of the Elliot Lake Group. and sandstone units are interpreted to represent deposition during warmer intraglacial or postglacial periods in either fluvial or marine environments (Junnila and Young 1995; Fralick and Miall 1989). Huronian Supergroup deposition was complete by 2217 Ma, the age of the Nipissing gabbro (Davey et al. 2019; Corfu and Andrews 1986; Noble and Lightfoot 1992). The Huronian Supergroup has been interpreted to represent a Wilson cycle, starting from a rifting phase represented by the Elliot Lake, Hough Lake and Quirke Lake groups; followed by a passive margin sequence (Cobalt Group); and concluded by a continent-arc collision between the SuperiorSouthern provinces and the Wisconsin Magmatic Arc Terrane (e.g., Young 1983; Hoffman 1989; Bennett et al. 1991). Each of the 3 groups overlying the Elliot Lake Group consists of sedimentary cycles of conglomerate, mudstone, siltstone or carbonate, capped by crossbedded sandstone (Bennett et al. 1991) (Figure 2). Conglomerate units (e.g., Ramsey Lake, Bruce and Gowganda formations) in each of the cycles have been interpreted as being glaciogenic in origin, likely deposited in a marine environment adjacent to an ice shelf. The siltstone 105 �Proceedings of the 68th ILSG Annual Meeting - Part 2 This collisional event at 1870 to 1835 Ma, termed the Penokean Orogeny, is believed to be responsible for most of the metamorphism and deformation present in the Huronian Supergroup. The scale and intensity of the Penokean Orogeny remains a subject of debate (Davidson et al. 1992; Card 1992; Raharimahefa et al. 2014; Holm et al. 2018; Zi et al. 2022), in part because the Penokean Orogeny has no associated plutonism in Ontario. In contrast, Riller et al. (1999) attributed deformation and peak metamorphism of the Huronian Supergroup to the Blezardian Orogeny (2470-2220 Ma), with subsequent transpressional deformation during the Penokean. These divergent views reflect the lack of constraints on the age of Huronian Supergroup metamorphism and deformation. The Sudbury Igneous Complex was emplaced at 1850 Ma (Krogh et al. 1984; Davis 2008) and consists of a lower, ore-bearing sublayer, a main mass of norite, and an upper granophyre (e.g., Dressler et al. 1991). Associated with the Sudbury Igneous Complex are brecciated rocks, termed the Sudbury breccias (e.g., Dressler et al. 1991), consisting of randomly oriented blocks of country rock in a fine-grained, pseudotachylite matrix. The breccias occur up to 200 km from Sudbury but are most abundant near Sudbury. The Sudbury Igneous Complex and related rocks have been variously interpreted as originating from meteorite impact, impact-induced plutonism and volcanism, and volcanism (see reviews in Pye et al. 1984). The southern part of the Sudbury Igneous Complex was weakly metamorphosed by an event that also retrograded metamorphosed rocks of the Huronian Supergroup. Regional sodium and potassium metasomatism and silicification have intensely altered rocks locally within the Huronian Supergroup, especially along faults, at circa 1700 Ma (Meyer et al. 1990; Gates 1991; Schandl et al. 1994; Easton et al. 1996; Fedo et al. 1997). Significant magmatism occurred again at 1750 to 1730 Ma and at 1500 to 1450 Ma in the Killarney Magmatic Belt (van Breemen and Davidson 1988; Davidson and van Breemen 1994; Krogh 1994). Figure 2. Idealized Huronian Supergroup stratigraphy as utilized by the Ontario Geological Survey based on Robertson, Card and Frarey (1969). Yellow units are sandstone dominated, blue units are carbonate rocks (limestone or dolostone), brown units are mudstones and/or turbidites (lined) or conglomerates. Green units are volcanic rocks. 106 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Table 1. Timing of major geological events and summary of age constraints on the main rock units present in the Sudbury area. Event and/or Map Unit Age Constraint (Ma) Comment and/or Source Grenville dike swarm 586±4 Kamo, Krogh and Kumarapeli (1995) Pegmatite vein emplacement 989±2 Corfu and Easton (2000) Age of peak metamorphism in the hangingwall of the Grenville Front tectonic zone 1000 to 990 Corfu and Easton (2000), this study Age of peak Grenvillian metamorphism in the Central Gneiss Belt 1040 to 1030 Carr et al. (2000) Sudbury dike swarm 1238±4 emplaced in or along northwest-trending faults in the Southern Province, deformed and metamorphosed in the Grenville Province, Krogh et al. (1987). Killarney magmatic belt second-stage magmatism, coincident with magmatism in the Eastern Granite Rhyolite Province and in the Central Gneiss Belt 1471±3 van Breemen and Davidson (1988) Regional albitization metasomatic event 1701±4 U/Pb monazite, Schandl, Gorton and Davis (1994); fluid focussed along northwest faults Killarney magmatic belt volcanism and high-level plutonism 1740, 1747±3, 1749±12 van Breemen and Davidson (1988); Sullivan and Davidson (1993); Davidson and van Breemen (1994) Northwest-trending regional faults Pre-1700, post-1850 Faults cut Sudbury Structure Penokean orogeny (folding and metamorphism of Huronian Supergroup rocks?) 1775±10 ~1835 Peak deformation. Zi et al. (2022) Peak metamorphism. Holm et al. (2001) Impact event and formation of Sudbury breccia 1850±1 Krogh, Davis and Corfu (1984); Davis (2008) Penokean arc formation 1880-1870, 1845-1830 Zi et al. (2022) Thrust faulting post-F2 pre-regional faulting Sudbury breccia localized along these faults, suggesting they are pre-Sudbury Structure F2 folding post-2200, pre-1700, pre 1850? Pre-regional faulting, Nipissing sills axial planar to folds F1 folding pre-2200 Nipissing sills folded or intruded into early folds Emplacement of Nipissing gabbro sills 2217±4 Davey et al. (2019); Corfu and Andrews (1986); Noble and Lightfoot (1992) Huronian Supergroup sedimentation >2220 but <2460 Youngest detrital grains in Bar River Fm are 2306 Ma (Hill, Davis and Corcoran 2018) Huronian Supergroup felsic volcanism and related plutonic rocks, including the Matachewan dike swarm ~2477 to 2375 (2450±25, 2460±20, 2477±9, 2415±5 Krogh, Davis and Corfu (1984), Heaman (1997); Corfu and Easton (2000), Krogh, Kamo and Bohor (1996), Smith (2002); Bleeker et al. (2015) Emplacement of East Bull Lake intrusive suite rocks 2475±2 Heaman (geochronologist, University of Alberta, personal communication, 1999); Clough and Hamilton (2017) Emplacement of orthopyroxene hornblendite bodies (East Bull Lake suite) 2468±5 Corfu and Easton (2000) Emplacement of alkali feldspar granite and megacrystic granodiorite near River Valley 2660 to 2665 Bodies intrude Crerar and Pardo gneiss, Easton (2003) High-grade Archean metamorphism and migmatization 2647±4 Krogh, Davis and Corfu (1984); Wodicka and Card (1995); Ames et al. (2005) Emplacement ages of Archean units in the Sudbury area 2711±7 to 2642±1 see also Table 5 Krogh, Davis and Corfu (1984); Wodicka and Card (1995); Chen, Krogh and Lumbers (1995); Meldrum et al. (1997); Ames et al. (2005) 107 �Proceedings of the 68th ILSG Annual Meeting - Part 2 The last major magmatic activity in the Southern Province occurred at circa 1240 Ma with the emplacement of the northwest-trending Sudbury diabase dike swarm (Krogh et al. 1987). This event is noteworthy, as rocks of this dike swarm can be traced across the Grenville Front into the Grenville Front tectonic zone, providing an important marker horizon (e.g., Bethune 1997). (Figure 3). movement would be necessary to juxtapose granulites of the Levack gneiss complex against higher-crustal-level rocks to the east, however, if the Upper Wanapitei River fault re-activated an older listric fault system, then considerably less relative uplift across the fault may be present. Murray fault system Significant changes in stratigraphic thickness within the Huronian Supergroup occur across the Murray fault system (e.g., Bennett et al. 1991, and references therein). North of the Murray fault, the McKim Formation is tens of metres thick, whereas south of the fault, it probably exceeds 1000 m. The thickness and facies variations across the Murray fault system suggest that the faults represent south side down, syn-sedimentary, growth faults that were reactivated during compression attributed to the Penokean Orogeny (e.g., Zolnai et al. 1984). Significant changes in thickness within the Huronian Supergroup occur east and west of a line roughly coincident with the trace of the northtrending Upper Wanapitei River fault. Debicki (1990) estimated the thickness the Huronian Supergroup at Sudbury to be approximately 10,350 m, 85% of which consists of the lower 3 groups. In contrast, east and northeast of Wanapitei Lake, the thickness of the Huronian Supergroup is approximately 6,250 m, 75% of which consists of the Cobalt Group. The Elliot Lake, Hough Lake and Quirke Lake groups are all considerably thinner east of Wanapitei Lake, and the McKim and Ramsey Lake formations are apparently absent (Easton and Murphy 2002). In addition to stratigraphic thickness variations, the Murray fault system also marks profound changes in structural style, metamorphic grade and magmatic associations (Card et al. 1972). Deformation is more complicated and of greater intensity south of the fault. Likewise, metamorphic grade is higher immediately south of the fault (amphibolite facies transitional southward to greenschist facies) than to the north (greenschist to subgreenschist facies). South of the fault, there are several 1750 Ma and younger granitoid complexes (e.g., Cutler batholith) (Davidson and van Breemen 1994), whereas, north of the fault, there are no such intrusions. Major Fault Systems Upper Wanapitei River fault The Upper Wanapitei River fault has had a protracted deformation history, exhibiting at least 7 to 8 km of left-lateral movement between 2170 and 1850 Ma (Buchan and Ernst 1994), and at least 3 km of left-lateral movement post-1040 Ma (Easton and Murphy 2002). According to Easton (2000), the north-trending Upper Wanapitei River fault apparently divides the Archean rocks in the Elliot Lake to North Bay area into two domains, with the boundary between these domains passing through Street Township. The eastern domain, which includes the River Valley–Hagar area consists of supracrustal and metaplutonic rocks, with deeper levels in the crust being exposed to the south, likely due to Grenville orogenesis. In contrast, the western domain is pluton-dominated, with deeper levels of the crust, being exposed to the east. The amount of vertical movement across the fault is unknown. Significant vertical Northeast of Coniston, the Grenville Front boundary fault and the Murray fault system are thought to merge into the Wanapitei fault (Davidson 1997), which can be traced into Street Township. This fault is then offset to the north by the Upper Wanapitei River fault and continues eastward as the Ess Creek and Grenville Front boundary faults along the trend of the Kabikotitwia and Sturgeon rivers (Easton and Murphy 2002). Thus, Huronian Supergroup strata located west and northwest of the Wanapitei and Ess Creek faults occur north of the Murray fault system, whereas 108 �Proceedings of the 68th ILSG Annual Meeting - Part 2 any Huronian Supergroup rocks preserved within the Grenville Province would have originally been deposited south of the Murray fault system. These rocks are considered parautochthonous in the sense that they were once part of the autochthon, having been subsequently transposed and uplifted northwestward. It is difficult to recognize such rocks directly in many places along the Grenville Front This is due not only to the effects of intense reworking within the Grenville orogen, but also to juxtaposition, on opposing sides of the front, of rocks that were originally at different crustal levels, thus exhibiting different states of deformation and metamorphism, and not necessarily representing the same lithologic units. Geochronology has been of inestimable value in making broad correlations across the front. Lack of identification of deformed and metamorphosed equivalents in the Grenville Province of the flatlying supracrustal rocks (e.g., Huronian Supergroup) northwest of the front is interpreted to be due to uplift and erosion of these successions, so that only their substrate is preserved. This field trip specifically examines these issues. The Grenville Province Introduction Rocks of the Grenville Province in Ontario range in age from circa 2690 to 990 Ma. All rocks older than 1300 Ma are pre-Grenvillian, whereas those younger than 1300 Ma are Grenvillian. With respect to nomenclature, a variety of subdivisions are in use for the Grenville Province in Ontario and fall into 2 broad groups: those that are lithologically based, commonly with a long history of usage (e.g., Wynne-Edwards 1972); and those that are more tectonic or interpretative in character, generally of more recent vintage (e.g., Rivers et al. 1989; Carr et al. 2000). Geological domains and their boundaries between the different types do not always coincide from one scheme to another (e.g., the Central Gneiss Belt contains paraautochthonous and allochthonous rocks), however, both approaches are valid, and usage is based on needs (e.g., lithologic- and historic-based terminology may be used more on detailed maps (<1:50 000 scale), tectonic-based terminology may be used on regional maps and in academic literature). Key divisions of the Grenville Province are listed in Table 2. The extent of Superior and Southern province rocks within the Grenville orogen can be documented by Nd depleted mantle model ages. Dickin and McNutt (1989) found that Archean and Paleoproterozoic model ages of gneisses are restricted to northwest of a line extending from Key Harbour to Timiskaming, well southeast of the GFTZ and some 60 km from the Grenville Front. It can be argued, however, that meta-sedimentary rocks younger than the Huronian Supergroup (>2.2 Ga) could equally well have had Archean provenance. Gneisses southeast of this line have distinctly younger Nd model ages (circa 1.9 Ga). The field trip route mostly lies within the northernmost part of the Grenville Front tectonic zone (GFTZ), but also includes some rocks in northernmost Nepewassi domain. Both are part of the parautochthonous belt or the Laurentian Margin, which is defined as that part of the Grenville Province in which the rocks, although thoroughly reworked during the Grenvillian and/or earlier orogenies, can be reasonably equated with rocks of older Shield provinces to the northwest (Rivers et al. 1989; Carr et al. 2000). 109 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 3. Distribution of 1.24 Ga Sudbury diabase and metadiabase, -1.17–1.15 Ga coronitic olivine metagabbro, and eclogitic rocks in the Central Gneiss Belt, Ontario and westernmost Quebec. The broken line near the Grenville Front is the southeast margin of the Grenville Front tectonic zone (Wynne-Edwards 1972). Figure from Ketchum and Davidson (2000). 110 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Table 2. Key divisions and boundaries within the Grenville Province in Ontario. Historic/Lithologic Regional Tectonic Local Tectonic/Historic Grenville Front Tectonic Zone (GFTZ) Para-autochthonous belt (Rivers et al. 1989) or Laurentian margin 1 (Carr et al. 2000) Segments 1, 2, 3 Central Gneiss Belt (CGB) (WynneEdwards 1972; Easton 1992) Para-autochthonous and/or allochthonous belt (Rivers et al. 1989), Laurentian margin 2 and 3 (Carr et al. 2000) Parry Sound, Algonquin, Tomiko, Beaverstone terranes Britt, Fishog, Go Home (lower), Go Home (upper), Huntsville, Kiosk, McCraney, McClintock, Moon River, Nepewassi, Novar, Powassan, Shawanaga, Sequin, Tilden Lake domains Central Metasedimentary Belt (CMB) (Wynne-Edwards 1972; Easton 1992) Composite Arc Belt (CAB) and Frontenac-Adirondack Belt (FAB) (Carr et al. 2000) Bancroft, Elzevir, Frontenac terranes (Elzevir contains Anstruther, Belmont, Grimsthorpe, Mazinaw, Sharbot Lake domains), Adirondack Lowlands and Highlands Grenville Front (Wynne-Edwards 1972; Easton 1992) North limit of Grenville metamorphism and penetrative deformation (locally migmatite front) Grenville Front boundary fault (GFBF) Allochthon Boundary Thrust (ABT) (Rivers et al. 1989) Separates para-autochthonous and allochthonous rocks (Rivers et al. 1989; Carr et al. 2000) a.k.a. central Britt shear zone, Shawanga shear zone Laurentian Margin - Composite Arc Belt boundary (Carr et al. 2000) Composite Arc boundary zone (CABZ) (Carr et al. 2000) Central Metasedimentary boundary zone (CMBBZ), a.k.a Central Metasedimentary Belt boundary thrust zone (CMBbtz) Composite Arc Belt – FrontenacAdirondack Belt boundary (Carr et al. 2000) Frontenac-Adirondack boundary zone (FABZ) (Carr et al. 2000) a.k.a. Maberly shear zone, Sharbot LakeFrontenac boundary Important Boundaries Nepewassi domain eastward continuation into the Grenville Province of the main igneous components of the Killarney Magmatic Belt, which is straddled by the Grenville Front in the Killarney area. Table 3 summarizes the ages from plutons of both suites in the Killarney Magmatic Belt, the Grenville Front tectonic zone, and the Nepewassi domain. Leucogabbro to anorthosite of the St. Charles and Mercer intrusions cut the West Bay batholith and were emplaced at circa 1225 Ma (Prevec 2004). The Nepewassi domain (Easton 1992) was discriminated from its neighbours on the basis of structural trends as well as rock types. The area underlain by the Nepewassi domain in the field trip area was mapped by Lumbers (1975) at 1:126 720 scale. The Nepewassi domain is underlain by compositionally heterogeneous migmatitic gneisses which have a polycyclic history. Plutonic rocks in the domain form 2 suites which are less deformed than their typically migmatitic host rocks (Lumbers 1975): an older, granite-monzogranite suite circa 1740 Ma that includes the migmatitic West Bay and the Sturgeon Falls batholiths, and a younger, non-migmatitic suite, circa 1450 to 1420 Ma, that includes the Cosby pluton. Near Alban, the Cosby pluton intruded a thick sequence of quartzite, known as the French River quartzite (cf. Lumbers 1975). Both plutonic suites represent an Limited geochronological data are available from the Nepewassi domain (Table 3). Tonalitetrondhjemite gneisses exposed between Hagar and Warren yielded U/Pb zircon ages of 2678 to 2683 Ma, with titanite indicating that regional metamorphism of these same gneisses occurred between 996 and 975 Ma (Chen et al. 1989). A grey gneiss located on Highway 535 north of Noelville yielded a similar age of 2680±11 Ma by laser 111 �Proceedings of the 68th ILSG Annual Meeting - Part 2 ablation on zircon (Van de Kerckhove 2016). These zircon ages are consistent with the Nd/Sm and Pb/Pb model ages of Dickin (1998a, 1998b) which indicate the presence of Archean crust throughout much of Nepewassi domain. (Lumbers 1975, p.98), but neither the details on the age determination nor the location of the sample are available. Aldis (2016) reported a laser U/Pb zircon age of circa 1434 Ma from the Cosby pluton near Noelville, similar to the age reported by Lumbers (1975). Along the western margin of the Nepewassi domain and the Grenville Front tectonic zone, a layered gneiss unit, the French River “paragneiss”, yielded a zircon population with an age of 1744±11 Ma (Krogh 1989). The homogeneous nature of the zircon population (Krogh 1989) suggested that the French River “paragneiss” is not a typical clastic metasedimentary rock, but rather that it may have been derived from metamorphosed volcanic and/or volcaniclastic rocks. Alternatively, it may be a highly strained orthogneiss. In contrast, the French River quartzite contained only Archean zircons, with monazite giving a metamorphic age of 1062±15 Ma (Krogh 1989). Quartzites examined by Van de Kerckhove (2016) northeast of Noelville had detrital zircon populations ranging from 2563 to 2962, with peak populations between 2686-2702 Ma, consistent with detrital zircon populations from Huronian Supergroup rocks in the Southern Province (see summary in Easton 2019). Van de Kerckhove (2016) also reported metamorphic zircon and monazite ages of 1755±11 and 1761 Ma, respectively, from the same area, suggesting a regional metamorphic event coincident with Killarney belt magmatism. New observations from the northwestern Nepewassi domain, which will be seen during the field trip. include the identification of granulitefacies, green and pink, garnet-bearing, potassium feldspar megacrystic granodioritic gneiss of the Estaire pluton and incorporated pods of hypersthene-bearing gneissic diorite, a likely comagmatic phase (Stop 7, 8). The Estaire pluton granodiorites are characterized by high Ba (>2000 ppm) and high Zr (>500 ppm) contents similar the those found in the West Bay pluton, south of Verner. In addition, quartzite (Stop 9), possibly correlative with the French River quartzite, occurs as a 2.2 km long, up to 300 m wide, belt on the south side of the Wanapitei River, only 2.2 km south of the boundary with the GFTZ. The Grenville Front The Grenville Front itself is a zone of southeastdipping faults and mylonites and has generally been placed at the southeast limit of recognizable Southern Province rocks (e.g., Lumbers 1975; Davidson 1997). Locally there are complications that have led to many debates concerning the identity of the Grenville Front and its distinction from other faults that intersect, merge with or are parallel to the Front (see discussion in Davidson 1997). In central Street Township the Grenville Front (which is coincident with the Wanapitei fault) has been displaced to the north by the younger, north-trending, Upper Wanapitei River fault by at least 850 m of sinistral and west-side-up movement (Easton and Murphy 2000, 2002). This displacement likely occurred after circa 590 Ma, as a Grenville swarm diabase dike in northern Henry and Loughrin townships is also displaced by northtrending faults. The age of the major plutonic units in Nepewassi domain is poorly known, but many probably have affinities to the Killarney magmatic suite (circa 1740) (Easton 2014). The migmatitic, French River granite, located along the western boundary between the Nepewassi domain and the Grenville Front tectonic zone, and intruded into the French River “paragneiss”, gave a robust Rb/Sr age as well as a U/Pb zircon age of circa 1700 Ma (Krogh and Davis 1969, 1972). The age of the texturally similar West Bay batholith near Lavigne has not been determined reliably, although a poor-quality laser U/Pb zircon age of circa 1255 was reported by Aldis (2016). A U/Pb zircon age of circa 1420 Ma has been reported from the Cosby pluton 112 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Table 3. Summary of geochronological data for Killarney area magmatic rocks, the Wanapitei complex and the Nepewassi domain. Age (in Ma) Unit Comment Killarney Area Magmatic Rocks 1749+12/–8 biotite granodiorite North of Chief Lake granite 1747±3 granodiorite Eden Lake complex 1744±29 granodiorite Eden Lake complex 1704±13 granitic dike 1596±39 granitic dike 1746+16/–6 granodiorite Cuts fabric and shear zone in Huronian Supergroup rocks near McFarlane Lake Cuts fabrics in Eden Lake complex Cutler batholith 1742±1.4 1464±2 1467±18 granite granite granite Killarney granite Chief Lake granite Chief Lake granite 1429 1447 1464 pegmatite dike pegmatite dike pegmatite dike South of GF, Chief Lake area South of GF, Chief Lake area South of GF, Chief Lake area Wanapitei Complex 1746+12/–6 quartz monzonite dike 1707±17 quartz monzonite dike 1746+6/–5, hornblende 996 metanorite Wanapitei complex, cuts metagabbro Wanapitei complex, cuts metagabbro Wanapitei complex, lower intercept age of metamorphism Wanapitei complex Method Source U/Pb TIMS zircon U/Pb TIMS monazite U/Pb LA-ICP–MS zircon U/Pb LA-ICP–MS zircon Davidson and van Bremen (1994) Sullivan and Davidson (1993) Raharimahefa, Lafrance and Tinkham (2014) Raharimahefa, Lafrance and Tinkham (2014) U/Pb LA-ICP–MS zircon U/Pb TIMS zircon Raharimahefa, Lafrance and Tinkham (2014) Davidson, van Breemen and Sullivan (1992) van Breemen and Davidson (1988) Davidson and van Bremen (1994) Raharimahefa, Lafrance and Tinkham (2014) Krogh (1994) Krogh (1994) Krogh (1994) U/Pb TIMS zircon U/Pb TIMS zircon U/Pb LA-ICP–MS zircon U/Pb TIMS zircon U/Pb TIMS titanite U/Pb TIMS monazite U/Pb TIMS zircon U/Pb LA-ICP–MS zircon U/Pb TIMS zircon Davidson, p.39 in Easton, Davidson and Murphy (1999) Rousell et al. (2012) Prevec (1993, 1992) U/Pb LA-ICP–MS zircon U/Pb LA-ICP–MS zircon Rousell et al. (2012) Nepewassi Domain French River 1744±11 “paragneiss” U/Pb TIMS zircon Krogh (1989) circa 1700, 1689±16 U/Pb TIMS zircon, Rb/Sr whole rock Krogh and Davis (1969, 1972) U/Pb TIMS zircon Lumbers (1975) U/Pb TIMS monazite Krogh (1989) U/Pb TIMS zircon Prevec (2004, 1992) U/Pb SHRIMP zircon Prevec (2004, 1993) U/Pb TIMS zircon Chen, Krogh and Lumbers (1995); Van de Kerckhove (2016) Chen, Krogh and Lumbers (1995) 1735±3 garnet metagabbro 1694±7, 1640±10 garnetiferous mafic Wanapitei complex, dike cuts other units, 2 populations 1420 1062±15 1245±48 1244±100 2678 to 2683 975 to 996 GFTZ near western boundary of Cosby subdomain, 2 sample sites, single population GFTZ near western boundary of French River Cosby subdomain, migmatitic, “granite” cuts French River “paragneiss” Cosby subdomain, Cosby pluton no location given French River Cosby subdomain, quartzite quartzite only had Archean zircons Mercer anorthosite Southern subdomain, 1222±2 Ma in Prevec (1992) St. Charles Southern subdomain, anorthosite 1206±36 Ma in Prevec (1993) tonalite, Northern subdomain, granodiorite range from 7 sample sites tonalite, Northern subdomain, age of metamorphism, granodiorite range from 6 sample sites U/Pb TIMS zircon, lower intercept Rousell et al. (2012) Abbreviations: GF, Grenville Front; GFTZ, Grenville Front tectonic zone; LA-ICP–MS, laser ablation inductively coupled plasma mass spectrometry; SHRIMP, sensitive high-resolution ion microprobe; TIMS, thermal ionization mass spectrometry. 113 �Proceedings of the 68th ILSG Annual Meeting - Part 2 At the time of formation, the Grenville Front was probably equivalent to the Main Boundary thrust of the current Himalayan orogen, marking the boundary between lowlands to the north and high-standing mountains to the south. afield, perhaps contributing to the fill of late Mesoproterozoic basins in the western and northern parts of North America (e.g., Hoffman and Grotzinger 1993; Rainbird et al. 1997). In the field, the Grenville Front has generally been placed at the southeastern limit of recognizable Southern Province rocks (Lumbers 1975; Davidson 1997, 1998). Locally, however, there are complications that have led to many debates concerning the identity of the Grenville Front and its distinction from other faults that intersect, merge with, or are parallel to it (see discussion in Davidson (1995, 1997, 1998)). In the west, the Grenville Front can be traced from Georgian Bay across the Killarney Magmatic Belt until Coniston where it intersects the Murray and the Creighton faults. Along most of its length in Ontario, the Grenville Front is characterized by a major, intense, moderately southeast-dipping mylonite zone a few metres to tens of metres thick. The mylonitic rocks have a dip-parallel lineation and kinematic indicators show a northwestward thrust sense. Rocks in the immediate foreland adjacent to front-parallel faults, which are generally steeper than the front mylonite zone, show cataclastic deformation. Gneissic and protomylonitic rocks southeast of the front show penetrative ductile deformation with the same northwest-directed thrust sense as the front mylonites. Thus, the Grenville Front marks a transition from brittle to ductile deformation toward the southeast. The timing of isotopic closure in a variety of mineral systems adjacent to the Grenville Front in the Sudbury area was examined by Corfu and Easton (2000). Zircon closed at 995 to 987 Ma, similar to ages reported all along the Grenville Front from Killarney to Labrador by Krogh (1994); this age represents some of the youngest Grenvillian activity in Ontario. Titanite and monazite from the same localities record only slightly younger ages between 989 to 977 Ma, consistent with the slightly lower closure temperatures for these minerals. Rutile and apatite ages from the same samples record ages of 973 to 971 Ma and 959-932 Ma, respectively, consistent with slow cooling along the Grenville Front after last movement at circa 995 Ma. In many places, front-parallel mylonite zones occur within the gneisses southeast of the front, and both these and the front mylonitic rocks exhibit local, superimposed cataclasis. This demonstrates the changing nature of deformation, from ductile to brittle, during the time taken for the orogen to rise and, with accompanying exhumation, to cool. With respect to the time taken for uplift, it is noteworthy that nowhere along the length of the Grenville Front is there any evidence that the foreland was ever the site of a basinal depression that received detritus from an elevated Grenvillian mountain belt (it is probably significant that sediments of suitable age that are part of the Midcontinent Rift fill have Archean or Penokean and not Grenvillian provenance). This can be explained through a combination of factors such as the length of time taken for exhumation, and the effect of crustal thickening within the orogen that may have allowed the foreland to remain isostatically buoyant (e.g., Jamieson and Beaumont 1989). Lack of a foreland basin would have allowed only ephemeral deposition of coarse detritus adjacent to the orogen, and finer detritus to bypass the foreland and to be spread widely farther As mentioned, the Murray fault (Figure 1, 4, 5) is a major west-trending lineament, which locally separates weakly metamorphosed Huronian Supergroup rocks to the north from strongly metamorphosed and deformed, Mesoproterozoicgranite-bearing, Huronian Supergroup rocks to the south. Metamorphic contrast across the Murray fault is most pronounced in proximity to plutonic rocks (e.g., the Eden Lake and Cutler plutons). Northeast of Coniston (Figure 4, 5), the Grenville and Murray faults are thought to merge into the Wanapitei Fault (Davidson 1997). In central Street 114 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 4. A) The juncture between the Grenville Front mylonite zone and the Murray–Wanapitei fault south of Coniston. Stop 2-5 is the same location as Stop 1 in this guide, Stop 2-3 is the same as Stop 3. Abbreviations: GFMZ, Grenville Front mylonite zone; L, lake. B) Regional relationship between the Grenville Front, the Murray fault, and faults extending westward from the Ottawa-Bonnechere rift system. Dashed lines are Neoproterozoic Grenville swarm dikes, inverted triangles in Lake Nipissing are alkalic complexes associated with Neoproterozoic rifting. Both figures from Davidson (1995). 115 �Proceedings of the 68th ILSG Annual Meeting - Part 2 intrusive rocks of both the East Bull Lake and the Nipissing intrusive suites, as well as several types of migmatitic gneisses, likely of Neoarchean age (Easton 2000). The eastern segment, between River Valley and the Ottawa River, includes rocks mainly derived from the adjacent Superior Province. The field trip area straddles the boundary between the central and eastern segments. The GFTZ can be envisaged broadly as an anastomosing network of higher-strain rocks, moderately inclined and shallowing southeastward, surrounding lower-strain pods and lenses that are presumably elongate parallel to the prevalent southeast-plunging stretching lineation. The same style of structure can be seen in many places at outcrop scale and, if one considers porphyroclasts in mylonite, also at microscopic scale. Its southeast margin is ill-defined; as the front-parallel layered structure becomes shallower to the southeast; it also becomes progressively warped (buckled) about gentle, predominantly southeast-plunging axes so that the structural grain expressed at the surface changes from northeast to southeast. Map-scale enveloping surfaces, however, maintain a generally northeast trend. Figure 5. Fault interpretations in the Coniston area. A) after Lumbers (1975); B) modified after Dressler (1984). Stop 6 on the figure is located just southwest of Stop 3 in this guidebook. Figure from Davidson (1997). Township, the Grenville Front (Wanapitei Fault) is displaced to the north along the younger, northtrending, Upper Wanapitei River fault by at least 850 m of sinistral and west-side-up movement (Easton et al. 1996; Easton and Murphy 2002). In the Street Township area east of Sudbury, a rapid southeastward increase in metamorphism is present, with garnet-staurolite-kyanite developed within 800 m of the front and sillimanite-potassium feldspar rocks within 2 km. (Easton and Murphy 2002; Easton et al. 1999). Metamorphism and progressive disruption of the Sudbury swarm dikes occurs much closer to the front than to the southwest, and successive orthopyroxeneclinopyroxene-garnet coronas are fully developed in non-deformed cores of dismantled lenses within a kilometre of the front. Northwest of the front, shaly interbeds in the Mississagi Formation (predominantly cross-bedded feldspathic arenite) and shales of the underlying Pecors Formation are low-grade phyllites (muscovite-chlorite-albitequartz) 800 m from the Grenville Front. Nipissing gabbro contains the assemblage epidote-actinolitechlorite-albite ± quartz. The Grenville Front tectonic zone (GFTZ) The Grenville Front tectonic zone (GFTZ) is a region up to 30 km across lying between the Grenville Front and the Central Gneiss Belt of the Grenville Province (Figure 1). Easton (1992) divided the Grenville Front tectonic zone in Ontario into 3 lithologic segments. The western segment between Killarney and Wahnapitae comprises rocks equivalent in age, geophysical signature, and rock type to the adjacent Killarney Magmatic Belt. The central segment stretches from Wahnapitae to River Valley and contains mafic 116 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Despite its proximity to Sudbury, no rocks that can be related to the Sudbury impact event (e.g., metamorphosed Sudbury breccia; Offset dikes, etc.) have been reported from the GFTZ. the Sudbury dike swarm. There is no relative age constraint on the leucogranite pegmatites, although they are metamorphically recrystallized. East of the Wanapitei River The GFTZ zone near Wanup In contrast, east of the river, several large roundish or ovoid plutonic bodies are present, likely related to the Killarney Magmatic Belt, including the Wanapitei Complex (6 x 2 km in size) and the Cleland stock (3 x 3 km in size). East Bull Lake intrusive suite rocks former larger, more continuous, and better-preserved bodies, including the Red Deer Lake intrusion (11 km long, up to 1 km wide) and a body present along the southeast margin of the Wanapitei Complex. Host gneisses are dominantly quartzofeldspathic and contain. garnet-in both melanosomes and leucosomes (Photos F, G, H, I), and are similar to many of the gneisses present in Street Township only a few kilometres to the north-northeast. Also present are deformed, lenticular granitoid bodies (“metaarkoses” of Lumbers 1975), which in map pattern appear to define broad folds (see maps of Lumbers 1975; Dressler 1984). Similarly, the kyanitebearing paragneiss units west of Wahnapitae (Grant et al. 1962; Pearson 1959; Easton and James 1997) are more continuous, and better preserved, than possible correlative schistose rocks to the west of the Wanapitei River (Stop 13). Calc-silicate gneisses and impure marbles have not been reported from the area east of the river. Sudbury swarm dikes are pod-like, with minimal strike lengths. Late granite pegmatites seem to be less abundant in the area east of the river. Finally, the boundary with the Nepewassi domain is approximately 12-15 km from the Grenville Front. A key observation from the 2021 mapping program by the author is that the Grenville Front tectonic zone in the Sudbury area displays different structural styles whether one is west, or east, of the Wanapitei River. Most of the stops on the field trip are in the area west of the Wanapitei River, primarily for logistical reasons. West of the Wanapitei River West of the river, lithological units consist mainly of highly-strained gneisses, typically migmatitic (Photo A, B), that form thin, near continuous belts interlayered with migmatitic amphibolite, amphibolite and garnet amphibolite. Possible metasedimentary units, including schists containing aluminosilicate minerals, calc-silicate gneisses, and minor impure marble (Photo C), form thin, lenticular, discontinuous units that occur locally within the package of highly-strained gneissic and amphibolitic units. Rocks of the East Bull intrusive suite are locally present, but form thin, discontinuous units. Sudbury swarm dikes are large, with strike lengths of 50 to 100 m. In addition, the boundary with the Nepewassi domain is only 8 km from the Grenville Front. Pegmatite dikes are common west of the Wanapitei River and are predominantly granitic. Some are deformed and concordant or nearconcordant, with gneissosity, whereas others are highly discordant. Large, late, niobium-yttriumfluorine (NYF), variably-zoned, discordant, granitic pegmatite dikes (e.g., Stop 6a, 6b) are common in the area west of the river, and many have been quarried in the past, mainly for feldspar and/or mica (Vos et al. 1981). Narrow (0.5 to 3.0 m wide), garnet-baring, fine-grained leucogranite pegmatites are abundant in the GFTZ near the boundary with Nepewassi domain (Photo D, E) .and were not observed by the author east of the river. The late NYF pegmatites are younger than Structure along the Wanapitei River The feature causing the observed lithological differences across the Wanapitei River has a linear, north-northwest trend and appears as a weak linear magnetic feature in the low-resolution magnetic data available for the area (Figure 6, upper). The course of the Wanapitei River coincides with this structure from Coniston to Estaire, which obscures direct examination of the rocks immediately 117 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Photo C. Rocks west of the Wanapitei River. Impure dolomitic marble and calc-silicate layers in southerly of two calc-silicate and marble bands on the west side of Highway 69/400, northside of Old Wanup Road overpass. Scale card is 9 cm long (UTM 510571E 5140422N). Photo A. Rocks west of the Wanapitei River. Folded granitic leucosome in migmatitic, garnetbearing, gneissic diorite at north end of the roadcut. West side of Highway 69/400, 1.8 km from the Grenville Front (UTM 509315E 5141309N). Photo D. Rocks west of the Wanapitei River. Most of rock face is relatively massive, leucosome-poor migmatitic, garnet-bearing, gneissic granodiorite, however there is a zone that is much more leucosome-rich in the lower, centre part of the photo. West side of Highway 69/400 (UTM 5111057E 5137014N). Photo B. Rocks west of the Wanapitei River. Most of rock face is relatively massive, leucosome-poor migmatitic, garnet-bearing, gneissic granodiorite, however there is a zone that is much more leucosome-rich in the lower, centre part of the photo. West side of Highway 69/400, 1.8 km from the Grenville Front (UTM 509366E 5141236N). Photo E. Close-up of white, garnet-bearing, near concordant white pegmatite dike shown in Photo D. Scale card is 9 cm long. 118 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Photo F. Rocks east of the Wanapitei River. Closeup view of layered felsic metatexite gneiss at this station. Outcrop is on the west side of St. Cloud Road. Scale card is 9 cm long (UTM 515775E 5139250N). Photo H. Rocks east of the Wanapitei River. Layered garnet-bearing felsic metatexite gneiss at this station. Outcrop is on the west side of St. Cloud Road. Scintillometer for scale, instrument is 23 cm long, back of scintillometer is 10 cm square (UTM 515544E 5138949N). Photo G. Rocks east of the Wanapitei River. Layered felsic metatexite gneiss at this station. Outcrop is on the west side of St. Cloud Road. Hammer for scale, handle is 33 cm long (UTM 515544E 5138949N). Photo I. Rocks east of the Wanapitei River. Layered felsic metatexite gneiss at this station. Outcrop is on the west side of St. Cloud Road. Hammer for scale, handle is 33 cm long. 119 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Wanapitei River are consistent with higher temperatures. Regardless, more work needs to be done on rocks from both sides of the northnorthwest-trending structure to understand its origin and tectonic history. Granite Pegmatites in the GFTZ Ercit (1999) demonstrated that most granitic pegmatites in the northern Grenville Province, regardless of age, are classified as niobiumyttrium-fluorine (NYF) type pegmatites with no parental granite, indicating an anatectic origin and post-kinematic emplacement. Lumbers (1975) recognized two types of granitic pegmatite intrusions in the field trip area. Gneissic, deformed, granitic pegmatite intrusions composed mainly of quartz and potassium feldspar with minor mica and amphibole that occur as discontinuous dikes and sills. Zircon from one of these dikes in Cleland Township, east of the Wanapitei River, yielded an age between 1600 and 1700 Ma (Krogh and Davis 1970; Lumbers 1975), suggesting an affiliation with the Killarney magmatic suite. Figure 6. Upper. First vertical derivative of the residual magnetic intensity of the Wanup area showing the location of Wanup, the Grenville Front, and the north-northwest structure along the Wanapitei River. Northwest linear magnetic highs west of the Grenville Front are Sudbury diabase dikes. Lower. Bouguer gravity field. More common are late, post-metamorphic granitic pegmatite intrusions (circa 1000 Ma) that are distinctly zoned, commonly with quartz-rich cores (Stop 6b). Some are locally radioactive because of the presence of allanite (Stop 6a). Many have been quarried in the past, mainly for feldspar and/or mica (Vos et al. 1981). adjacent to the structure. The feature is also apparent in the Bouguer gravity data, separating a broad gravity high on the east side of the river from an area of lower density rocks to the west (Figure 6, lower). Which is interesting, as many of the mafic and granulite facies rocks west of the river have specific gravity values of 3.0 and 3.2 g/cm3. Acknowledgements Field work related to this guidebook was conducted in September to October 2021, with Julie Chartrand of the Ontario Geological Survey providing excellent field assistance. Dr. Manuel Duguet of the Ontario Geological Survey provided a technical review of the manuscript prior to publication. The north-northwest-trending structure is located where there is a flexure in the trace of the GFTZ, from north-northeast from Killarney to Wanup, to northeast from Wanup eastward. A preliminary explanation for the lithological and structural differences across the northnorthwest-tending structure would be that different structural levels are exposed on either side, with a likely deeper, and possibly hotter level on the west compared to the east. The greater degree of migmatization, and the presence of granulite facies rocks in the Nepewassi domain west of the 120 �Proceedings of the 68th ILSG Annual Meeting - Part 2 FIELD TRIP DETAILS we are on the same limb of the Coniston syncline that we will be on at Stop 3 (see Figure 4a). Geological Maps Cross the road and walk slightly west to the roadcut on the north side. The roadcut and rounded outcrops north of the road consist of Nipissing metagabbro that is massive but metamorphosed, containing secondary green amphibole, epidote and chlorite. The metagabbro also has low magnetic susceptibility. This metamorphic assemblage is typical of Nipissing gabbro throughout the Southern Province and thus does not appear to be a function of proximity to the Grenville Front. Geological compilation maps covering all or parts of the area of the field trip include Ames et al. (2005), Dressler (1984), Lumbers (1975) and Card and Lumbers (1973). ROAD LOG Note: Caution should be taken when parking vehicles on the shoulder of the highway and when examining outcrops located along Highway 17 and on other roads along the field trip route. All UTM co-ordinates are given in NAD 83 datum, zone 17. Cross the road again back to the south side. Here we see the northeast contact of the Sudbury diabase dike. Blocks containing its chilled contact with Mississagi sandstone can be found at the roadside. Near this contact the diabase is fine grained and contains xenocrysts of plagioclase which in turn include earlier-crystallized olivine crystals — a common feature at the margins of Sudbury dikes (Bethune 1997; Bethune and Davidson 1997). There is absolutely no evidence in thin section of any metamorphic reaction between these two minerals at this location. Note that this dike is unmetamorphosed and has high magnetic susceptibility. The Sudbury dikes are chemical distinct compared to other dike swarms in the region, and are characterized by TiO2 >2.5 wt.%, >700 ppm barium and >300 ppm Zr (Ketchum and Davidson 2000). This distinctive chemistry allows for these dikes to be recognized south of the Grenville Front, where they serve as important markers of deformation and metamorphic history (Figure 3). Leave from Science North at the junction of Paris Street and Ramsey Lake Road in Sudbury. Head south on Paris toward highway 69. 0.0 km — Junction of Highway 69 and the southeast and southwest bypass. Turn right onto the eastbound ramp and proceed east on Highway 17 after merging on to the Highway. 6.3 km — pull over on the right shoulder by the 1552.0 kilometre sign. Walk ahead (east) onto the roadcut on the south (right) side of the road. This locality is Stop 2-3 in Davidson (1995), Stop 1 in Davidson (1997); Stop 1-1 in Davidson et al. (2002). Stop 1. Mississagi Formation sandstone, Nipissing gabbro sill and a Sudbury swarm olivine-diabase dike UTM co-ordinates 508085E, 5144868N This stop lies within the Southern Province 900 m northwest of the Murray fault and approximately 3 km west-southwest of Stop 3. It is only two kilometres from the Grenville Front (see Figure 4a). Here a southeast-trending, vertical, 65m-thick olivine diabase dike of the 1235 millionyear-old Sudbury dike swarm cuts across the contact between Mississagi sandstone (south side of the road) and Nipissing metagabbro (north side). In the same dike that we see here, but farther to the south on the southeast side of the Murray fault, plagioclase xenocrysts become clouded with fine epidote, and rims of fine actinolite appear between olivine and plagioclase grains. Where Sudbury dikes have been identified in the immediate hanging wall of the Grenville Front, olivine in plagioclase xenocrysts has reaction coronas of pale orthopyroxene with outer rims of pargasite-spinel symplectite, and Ti-Fe oxide grains are surrounded Crossbedding in the sandstone indicates that the steeply dipping beds face north-northwest. In fact, 121 �Proceedings of the 68th ILSG Annual Meeting - Part 2 by Ti-biotite and garnet symplectite. Near the Grenville Front to the southwest, Sudbury dikes cut across and are chilled against a pre-existing mylonitic fabric that is developed in granitoid rocks as young as 1470 Ma (Bethune 1997; Davidson and Ketchum 1993), pointing to the existence of some kind of pre-Grenvillian tectonic front roughly coincident with the Grenville Front sensu stricto. In this regard it is pertinent that, in the hanging wall within a few kilometres of the front, metamorphic monazite in pelitic gneiss (Dudas et al. 1994) and zircon from pegmatitic leucosomes (Krogh 1994) record an age of circa 1445 Ma. produced small (cm-size) poorly defined cones (Spray et al. 2007). Return to vehicles and continue east on the bypass. 11.2 km — Junction of the bypass and Highway 17, take the right merge lane onto Highway 17 and head east toward Coniston. 14.1km — Junction to Coniston (traffic light), continue east on Highway 17. 16.8 km — Junction with Highway 17 and the Coniston Hydro Dam Road just past the overpass over the railway tracks. Turn right onto Coniston Hydro Dam Road. Between Highway 17 and the parking area, the road crosses feldspathic sandstone beds of the Mississagi Formation. Return to vehicles and continue east on the bypass. 9.0 km — pull over on the right shoulder by just after the curve on the way up the hill. Examine outcrops on the east (right) side of the road. 18.4 km — Park in pullout area opposite the gates to the Coniston Hydro Dam. We will walk to the first series of outcrops which are on the north side of the railway tracks (UTM 513570E, 5146857N). This is Stop 2-5 of Davidson (1995), Stop 7 of Davidson (1997), Stops C-1 and C-2 of Easton et al. (1999), and Stop 1-2 of Davidson et al. (2002). Stop 2. Shattercones in Mississagi Formation sandstone UTM co-ordinates 509418E, 5147073N Sudbury is famous for its shatter cones, which are well exposed in units of the Huronian Supergroup, especially the Mississagi Formation. The roadcut exposes numerous large shatter cones developed in quartz arenite of the Mississagi Formation. This outcrop is best visited in late afternoon, where the evening sun provides excellent lighting. Note that the shatter cones are not isolated individuals. The whole outcrop is full of shatter cones, something that is not revealed on a polished glaciated surface. Stop 3a. Mississagi Formation sandstone, Southern Province side of the Grenville Front Two major faults of the Murray fault system in the Southern Province, the Creighton fault and the Murray fault itself (Card 1978), converge eastward and meet just north of Alice Lake, 2 km west of here (see Figure 4, 5). East-northeast of this juncture, a narrow valley in line with the Murray fault marks the Grenville Front. North of this valley are well-preserved Huronian Supergroup sandstones (Mississagi Formation) and Nipissing gabbro (not observed at the stop) at low metamorphic grade, and south of it, high-grade migmatitic gneisses of the Grenville Province. The covered interval between the two conceals the Wanapitei fault, and is as little as 25 m wide in places between Alice Lake and the village of Wahnapitae, 6 km to the northeast. There is no field evidence to suggest that the Murray and Wanapitei faults are not one and the same, contrary to The shatter cone collar around the Sudbury structure forms a near continuous ring extending up to 20 km distant from the contact between the footwall rocks and the Sudbury Igneous Complex. Prior to regional deformation and folding, most shatter cones pointed upward, as their impact source origin was from above. Despite statements to the contrary, no volcanic blast has ever formed a shatter cone collar, and nuclear blasts have at best 122 �Proceedings of the 68th ILSG Annual Meeting - Part 2 published maps of this area (Lumbers 1975; Dressler 1984); the two interpretations are illustrated in Figure 5. by Corfu and Easton (2000) indicates that the metamorphism is indeed Grenvillian and culminated at circa 995 Ma. Mississagi sandstone at this stop displays obvious primary sedimentological features — crossbedding is well preserved and indicates that the beds face the same way as they dip, namely steeply to the northwest. Cleavage in the silty interbeds dips steeply southward and is axial planar to a major, southwest-plunging syncline whose surface trace lies about 2 km to the northwest (see Figure 4a). Metamorphic grade is low: cleavage in the silty interbeds is given by aligned sericite; thin sections show the presence of minor greenish biotite, indicating that the grade is probably no higher than middle greenschist facies. To the east, the trace of the Wanapitei fault lies in the valley along which a power line runs; to the west it passes just south of the slag heaps that can be seen in the distance. Return to vehicles. Retrace route back to Highway 17 and continue east on 17. 20.2 km — Highway 17 and Coniston Hydro Dam Road, turn right and continue east on Highway 17. 22.7 km — Bridge over the Wanapitei River in Wahnapitae village. The Wanapitei fault which, as at Stop 3, marks the Grenville Front in this area, lies in the river valley. Bare hills on the north (left) side of the river are underlain by well-bedded Mississagi sandstone that faces northwest, away from the front, similar to what we observed at Stop 3. The large roadcut to the right (south), just past the bridge and the variety store exposes kyanite-bearing metasedimentary gneiss and mafic gneiss (garnet-bearing amphibolite) derived from gabbro; both of which are cut by coarse-grained pegmatite, itself deformed. Walk back to the outcrop by the road on the opposite side of the valley and railway line. 27.2 km — Junction Highway 17 and 537, continue east on Highway 17. Stop 3b. Gneisses on the Grenville Province side of the Grenville Front 31.5 — Junction Highway 17 and Sunset Road. UTM co-ordinates 5135633E 5146768N 32.6 — Pull off onto gravel area on the south side of the Highway. This locality is Stop 1, Day 3 in Easton, James and Jobin-Bevans (2010). The outcrop on the south side of the railway crossing is composed of migmatitic quartzofeldspathic, mafic and minor pelitic gneiss, and includes narrow mylonite zones that diverge southwestward from the Murray-Wanapitei fault line. The pelitic gneiss contains kyanite and sillimanite, and amphibolite contains garnet, attesting to middle to upper amphibolite facies. Stop 4 (Optional). Shear-Zone Hosted Orthopyroxene Hornblendite Body UTM co-ordinates 525451E 5152056N Examine the outcrop and large blasted boulders present on the west side of the pullout. They belong to an orthopyroxene hornblendite body of the East Bull Lake intrusive suite that is present within a high-strain zone that extends subparallel to the highway. Examples of these highly strained felsic gneisses can be examined in outcrops at the base of the hill east of the pullout. The top of the ridge south of the road and above the stop consists of layered leucogabbronorite of the East Bull Lake intrusive suite. Thus, although proximal to rocks of the suite here, the orthopyroxenite body is not This outcrop exposes a highly deformed mix of granitoid and hornblende gneiss, some with garnet, cut by mylonite zones whose rotated feldspar porphyroclasts indicate south-side-up displacement. These rocks clearly represent an entirely different crustal level to that exposed just 70 m to the north, which implies several kilometres of vertical displacement along the Wanapitei fault, provided that the metamorphism in these rocks is younger than that in the Mississagi Formation. In Street Township to the northwest, geochronology 123 �Proceedings of the 68th ILSG Annual Meeting - Part 2 directly in contact with other rocks of the suite, although that relationship has been observed elsewhere in Street and Awrey townships (Easton and Murphy 2002). Note the large, equant, orthopyroxene crystals, and the fine-grained amphibole matrix. Mineral chemistry indicates that the amphiboles are magnesium hornblende, tremolite and cummingtonite (Buckley et al. 1997; Easton and Murphy 2002). The amphiboles occur as individual grains and complexly exsolved and intergrown grains. Olivine grains from a body located closer to the Grenville Front are Fo73. Orthopyroxene compositions lie in the bronzite field and are magnesium-rich (En75-80). (Easton and Murphy 2002). closely and attempt to assess possible protoliths. Are these metasedimentary or metavolcanic rocks or are they highly deformed orthogneisses? Once at the northeast end of the outcrop, the protolith of some of these gneisses will become readily apparent, due to an area of lower-strain present in a macroscopic fold nose (Photo 2). This locality illustrates the perils of trying to identify protolith in many gneissic terranes, especially in areas of poor, incomplete, and lichencovered exposures. Remember, we are only a kilometre south of the Grenville Front at this locality, and only at upper amphibolite conditions. What cannot be ascertained at this stop is whether these gneisses reflect a rather simple metamorphic history, for example an Archean high-grade metamorphic event followed by reworking during the Grenville, or multiple metamorphic episodes throughout the Archean, the Paleoproterozoic and the Mesoproterozoic. Turn around an retrace route westward on Highway 17 32.7 km — Junction Highway 17 and Sunset Road. Turn right (north) onto Sunset Road and proceed for 400 m. Pull over and park on the left shoulder of the road at the entrance to MTO gravel pit 402002 (phone 705-4775478). Walk to the well exposed outcrops in the central part of the pit. This stop was used by Davidson (1997) but was not included in the guidebook descriptions. Return to vehicles and retrace route back to Highway 17. 34.4 km — Turn right and head west on Highway 17 toward Sudbury. 43.3 km — Junction Highway 17 and 537 in Wahnapitae just east of the bridge. Turn left onto 537 and head south. Stop 5. Gneisses in the Grenville Province 1100 metres south of the Grenville Front 48.8 km — Metamorphosed Sudbury swarm diabase dike is exposed on the west side of the roadcuts which consist mainly of Grenville gneisses. UTM co-ordinates of gate 524338E 5152230N A variety of predominantly migmatitic gneisses are exposed in the well-exposed outcrops in the floor and northwestern wall of this pit. We are approximately 1 km south of the Grenville Front at this stop The grey, garnet-rich, gneisses may have been metasedimentary rocks. In contrast, a variety of mafic rocks, some garnet-bearing, some not, are interlayered with the grey gneisses. Some of these mafic rocks are large rafts in the gneisses (Photo 1), whereas others are thinner, boudinaged and aligned, and may represented dismembered dikes. Garnet is abundant and occurs in both the melanosome and leucosomes of the gneisses. 60.5 km — Junction in Wanup, continue west (straight) toward Highway 400-69. Proceed to the most northeastern exposed outcrop. While doing so, examine the gneisses 124 �Proceedings of the 68th ILSG Annual Meeting - Part 2 69 and is located just to the south of the part of Dill Township that was mapped in detail by Kwak (1968) and Davidson (1997, 1998). This road cut is illustrative of what rocks look like in the Grenville Front tectonic zone only 7 km south of the Grenville Front. The key takeaway is that is near impossible to easily relate the rocks that we see here to any of the rock units that we see on the north side of the front in either the Southern or the Superior provinces. In contrast to Stop 5, which appears to be dominated by an abundance of metasedimentary gneisses, the large roadcuts to the west and the east are more representative of much of the Grenville Front tectonic zone in the Sudbury area, where mafic gneisses predominant, albeit with slivers and layers of rocks that may have originally been metasedimentary. In examining the mafic gneisses in the two roadcuts, pay attention to features such as degree and style of leucosome formation; the presence or absence of garnet, and the abundance of garnet in some of the mafic gneisses, which far exceeds what would normally be generated in a mafic rock during a single-stage metamorphic event. We will examine the west roadcut first. Photo 1. Grey migmatite with mafic pods at Stop 5. Stop 6A. West Roadcut. From west to east the roadcut consists of: Photo 2. Fold in grey migmatite at Stop 5 (524200E 5152280N). In the nose of the fold, even though the rock is still deformed and metamorphosed, it is clear here that the protolith of the rock was a matrix-supported conglomerate of unknown stratigraphic affinity.  Approximately 100 m of interlayered grey to locally rusty, thin layered paragneiss and weakly layered, variably migmatitic mafic gneiss and deformed granitoid layers  13 m wide outcrop gap  Approximately 25 m of variably migmatitic, texturally varied mafic gneiss. This unit hosts a thin sub-horizontal pegmatite. 61.8 km — Pull over and park in the pullout area on the right side of the road. This stop will examine the two large roadcuts to the west and east of the parking area.  Approximately 55 m of gneissic gabbro, with distinct east and west contacts. Along the western contact, relict large plagioclase crystals occur in the groundmass as dark equant to lath shaped crystals and as isolated crystals up to 20 mm long (Photo 4c). Energy dispersive X-ray analysis indicates that the large crystals are andesine, with a composition of An46. Plagioclase xenocrysts are common occurrence in Sudbury swarm dikes in the Sudbury area (Stop 1; Davidson 1997, p.16). The presence of large plagioclase crystals only along the western contact Stop 6. Gneisses in the Grenville Province 6 kilometres southeast of the Grenville Front Pullout 512055E 5137085N The pullout splits a near continuous road cut, approximately 800 m long, and up to 15 m high, into eastern and western halves. The road cut was created during the process of four-laning Highway 125 �Proceedings of the 68th ILSG Annual Meeting - Part 2 may simply reflect better preservation at that locality, as the gneissic gabbro at the contact has moderate magnetic susceptibility (1.1 to 2.3 x 10-3 SI units) compared with the rest of the body (<0.75 x 10-3 SI units). The fact that the gneissic gabbro is likey a metamorphosed Sudbury swarm gabbroic dike is confirmed by geochemistry, as these samples have the high TiO2 (>2.5 wt.%), barium (>700 ppm) barium and Zr (>300 ppm) contents typical of the Sudbury swarm (analyses 1-4, Table 4). The gneissic gabbro hosts a near-vertical feldsparrich pegmatite dike (Photo 3). The eastern part of the pegmatite dike contains bluish apatite crystals (Photo 4a) and allanite (Photo 4b). Scintillometer readings from the eastern part of the dike range from 50 to 98 ppm U and 130 to 208 ppm Th (Easton, unpublished data). The age of the pegmatite is not known, but is younger than circa 1240 Ma, the age of the host Sudbury dike. Photo 4. A) Blue apatite in “trains” along albite crystal boundaries in granitic pegmatite. B) Single allanite crystal in granitic pegmatite. C) Andesine xenocrysts in metamorphosed mafic dike adjacent to pegmatite. Photos from Péloquin et al. (2020). Photo 3. Near-vertical pegmatite dike cutting gneissic gabbro of the Sudbury dike swarm at Stop 6a. Dike is unevenly zoned, with the right (west) side dominated by potassium feldspar, and the left (east) side being more radiogenic and containing more quartz, allanite and apatite. 126 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Table 4. Summary of geochemical and mineral chemistry data from mafic rocks at Stop 6. Work was performed at the OGS Geoscience Laboratory. Co-ordinates in NAD83, Zone 17. Analysis 4 from Easton (2003), analysis 6 from Peck et al. (1995). Analysis Number Sample Number Easting (m) Northing (m) Rock Name SiO2 (wt %) TiO2 Al2O3 Fe2O3total MnO MgO CaO Na2O K2O P2O5 CO2 S LOI Total Ba (ppm) Rb Sr V Pb Th U Nb Y Zr Ni Cr Cu Au (ppb) Pd Pt amphibole pyroxene biotite plagioclase oxide sulphide 1 2 3 4 5 19RME-2001 19RME-2003 19RME-2005 99RME-0337 19RME-2003 511908 5137095 gneissic gabbro, cgr 46.37 2.87 17.01 15.51 0.193 5.56 7.26 3.19 1.40 0.639 <0.023 0.135 0.65 100.74 769 30 389 206 5.9 3.7 2.2 15.1 36.4 229 90 83 84 0.7 0.4 0.3 n/a n/a n/a n/a n/a n/a 511949 5137093 gneissic gabbro, east contact 46.28 3.18 15.88 16.41 0.208 5.23 7.45 3.39 1.38 0.710 0.038 0.211 0.42 100.63 805 31 373 257 7.3 4.8 <1.6 16.1 41.4 259 67 97 96 0.7 0.6 0.4 H none 10% An27-31 ilmenite pyrite 511898 5137080 gneissic gabbro, west contact 46.98 3.01 15.61 16.08 0.201 5.35 7.63 3.22 1.38 0.666 0.042 0.113 0.41 100.63 780 27 390 240 5.6 4.0 <1.6 15.4 38.6 241 80 90 66 1.o 0.6 0.4 H none 10% An27-31 ilmenite pyrrhotite 6 7 90DCP-226 19RME-2004 556111 5109966 Sudbury dike 511951 5137095 leucogabbroic gneiss 402140 5143082 gabbro. East Bull Lake 512003 5137095 migmatitic mafic gneiss 45.53 2.96 16.61 16.92 0.200 5.95 7.76 3.49 1.25 0.59 <0.03 0.08 <0.05 99.73 700 1 363 233 6 2.5 0.7 16 39 243 93 45 58 25 <8 <5 n/a n/a n/a n/a n/a n/a 44.61 1.38 18.11 13.26 0.125 7.63 11.03 2.36 1.02 0.038 0.116 0.249 0.57 100.16 204 31 360 439 3.5 <1.5 <1.6 2.2 18.8 42 12 35 70 1.5 <0.14 0.1 MH none trace An52-65 ilmenite pyrrhotite 48.82 1.55 17.70 15.13 0.20 2.54 8.74 2.76 1.32 0.11 0.09 n/a 1.50 100.37 399 59 263 261 nr nr nr nr 17 78 20 nr 282 nr nr nr n/a n/a n/a n/a n/a n/a 51.10 0.42 13.98 8.61 0.160 10.29 12.38 1.44 0.57 0.034 0.159 0.044 1.00 100.06 64 42 96 212 1.9 <1.5 <1.6 0.7 10.7 29 164 531 65 10.2 24.7 30.6 FH diopside trace An74-77 none pyrite Notes: Major element oxides are in weight %; trace element data are in parts per million, except for Au, Pd, Pt which are in parts per billion. Plagioclase composition of xenocrysts in sample 19RME-2005 is An46-55. Abbreviations: cgr, coarse-grained; FH, ferrohornblende; H, hastingsite; LOI = loss-on-ignition; MH, magnesiohastingsite; n/a = not applicable; nr, not reported. Amphibole classification of Leake et al. (1997). 127 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Table 4 — continued. Analysis Number Sample Number Easting (m) Northing (m) Rock Name 8 9 10 11 12 19RME-2011 19RME-2008 19RME-2014 19RME-2009 19RME-2010 512277 5137074 garnet amphibolite 41.91 0.99 19.86 16.90 0.154 7.30 11.31 0.95 0.52 0.024 0.312 0.306 0.58 512313 5137069 garnet amphibolite 41.72 1.00 18.98 18.00 0.165 8.16 9.92 0.79 0.59 0.023 0.133 0.239 1.10 512290 5137073 ultramafic dike 512312 5137069 paragneiss SiO2 (wt %) TiO2 Al2O3 Fe2O3total MnO MgO CaO Na2O K2O P2O5 CO2 S LOI 511971 5137100 gneissic granodiorite 73.31 0.27 13.98 2.61 0.022 0.81 1.87 3.15 3.70 0.024 0.035 0.011 0.41 46.39 0.56 10.52 10.99 0.166 17.60 9.77 1.33 0.51 0.105 0.239 0.050 1.61 90.32 0.03 5.78 0.47 0.09 0.86 0.39 0.87 0.82 0.012 <0.023 0.019 0.46 Ba (ppm) Rb Sr V Pb Th U Nb Y Zr Ni Cr Cu Au (ppb) Pd Pt TREE 1861 126.6 267 15 39.3 34.7 4.1 7.4 5.2 136 6 25 <9 0.6 0.16 0.19 160.91 131 14.6 551 774 <1.7 0.28 0.16 <0.7 3.9 11 40 58 167 2.3 0.23 0.28 18.73 106 13.6 454 797 <1.7 0.19 0.07 <0.7 4.3 8 31 37 263 2.6 0.15 0.22 17.21 162 8.1 217 230 <1.7 1.11 0.38 1.6 11.7 45 593 1891 30 1.1 4.94 4.36 47.78 1683 13.7 96 <3 <1.7 0.63 0.34 <0.7 4.8 104 5 23 21 <0.6 <0.14 <0.06 76.26 Notes: Major element oxides are in weight %; trace element data are in parts per million, except for Au, Pd, Pt which are in parts per billion. Abbreviations: LOI = loss-on-ignition; n/a = not applicable; nr, not reported; TREE = total rare earth elements. 128 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Table 5. Summary of geochemical data from felsic and mafic intrusive rocks at Stops 7 and 8. Work was performed at the OGS Geoscience Laboratory. Co-ordinates in NAD83, Zone 17. Analysis Number Sample Number Easting (m) Northing (m) Rock Name SiO2 (wt %) TiO2 Al2O3 Fe2O3total MnO MgO CaO Na2O K2O P2O5 CO2 S LOI Ba (ppm) Rb Sr V Pb Th U Nb Y Zr Ni Cr Cu Au (ppb) Pd Pt TREE 1 2 3 4 21RME-021 Stop 7 514042 5129295 gneissic megacrystic granodiorite 21RME-022 Stop 7 5129235 5129037 gneissic megacrystic granodiorite 21RME-024 >0.75 >0.75 0.71 >0.75 >0.75 0.15 0.14 0.08 0.16 0.14 0.20 0.059 0.14 0.167 0.054 0.33 0.190 0.017 0.39 0.430 0.103 0.66 0.196 0.137 0.82 2265 60.0 370 28 17 4.1 <1.3 15.7 32.0 743 5 12 25 1996 56.8 384 46 15 4.3 <1.3 24.4 31.7 541 6 11 17 <2700 95.5 363 33 18 4.7 <1.3 16.3 20.3 544 3 <7 13 666 44.4 434 218 12 3.4 <1.3 15.1 47.2 46 37 22 46 324 22.5 327 185 8 <1.9 <1.3 5.3 31.7 85 85 129 46 >129 >129 >99 >120 >53 514123 5129524 gneissic megacrystic granodiorite 5 21RME-020 21RME-023 Stop 7 Stop 8 514046 514151 5129293 5129037 fine-grained fine-grained gneissic gneissic diorite gabbro Notes: Major element oxides are in weight %; trace element data are in parts per million, except for Au, Pd, Pt which are in parts per billion. Abbreviations: LOI = loss-on-ignition; n/a = not applicable; nr, not reported; TREE = total rare earth elements. 129 �Proceedings of the 68th ILSG Annual Meeting - Part 2  Approximately 16 m of non-migmatitic, gneissic leucogabbro (Photo 5). In thin section, the gneissic leucogabbro has a texture characterized by 120° grain boundaries and consists predominantly of brown amphibole and andesine. Brown amphibole is common under granulite facies conditions (Froese 1973). This is consistent with metamorphic conditions in this part of Dill Township identified by Kwak (1968). Geochemical data suggests the leucogabbro may be part of the East Bull Lake intrusive suite (emplaced circa 2475 Ma; compare analyses 5 and 6, Table 4).  Approximately 10 m of migmatitic mafic gneiss.  4 m of migmatitic granodiorite gneiss (Photo 6), similar to the host rock to samples C96-2 and C96-1 in Corfu and Easton (2001). This granodiorite gneiss has elevated U and Th contents (analysis 8, Table 4) typical of early Paleoproterozoic felsic rocks in the Sudbury area such as the Creighton granite and the Copper Cliff rhyolite (both circa 2460 Ma).  10 m wide outcrop gap  Approximately 35 m of migmatitic mafic gneiss, locally with deformed granitoid veins and layers present. Photo 5. Non-migmatitic, leucogabbroic gneiss at Stop 6a. This rock is cut by the gneissic gabbro of the Sudbury dike swam, and thus is older than 1240 Ma. Scale card is 10 cm long. Stop 6B. Roadcut. From west to east the roadcut consists of:  Approximately 125 m of interlayered grey to locally rusty, thin layered paragneiss, complexly folded, and weakly layered, variably migmatitic mafic gneiss and deformed granitoid layers.  Approximately 45 m of migmatitic mafic gneiss, locally with deformed granitoid veins and layers present. This gneiss hosts a 30 m long, discordant zoned non-radiogenic pegmatite vein.  Approximately 200 m of garnet amphibolite (25-50% garnet) (analysis 9, 10, Table 4) (Photo 7). At one point, it is cut by a near-vertical ultramafic dike (analysis 11, Table 4). A thin (1 m thick), near vertical quartzose gneiss band is likely of metasedimentary origin given its high silica content (analysis 12, Table 4).  Approximately 100 m of interlayered grey to locally rusty, thin layered paragneiss and weakly layered, variably migmatitic mafic gneiss and deformed granitoid layers. Photo 6. Gneissic granodiorite to monzogranite at Stop 6a, possibly related to the Creighton granite. Scale card is 10 cm long. 130 �Proceedings of the 68th ILSG Annual Meeting - Part 2 All of the units are steeply-dipping (70-85°) to the southeast. The age of the paragneiss and mafic gneiss is not well constrained, although Easton and Murphy (2002) and Easton and James (1997) observed that migmatitic mafic gneiss in the Street Township area to the northeast were Archean, whereas non-migmatitic mafic gneisses and amphibolite units were Proterozoic. If the migmatitic granodiorite gneiss in the west roadcut is indeed correlative with similar granitoid rocks in the Street Township and the Sudbury area that are circa 2460 Ma, this would suggest that the migmatitic mafic gneisses in the roadcut are Archean. Photo 7. Close-up of garnetite that constitutes much of the roadcut at Stop 6b. Scale card is 1 cm long. Analyses 9 and 10, Table 4, are from this unit. The garnet-rich mafic gneisses (25-50% garnet) present in the eastern third of the roadcut (Photo 6) are of potential economic interest. Garnet-rich mafic gneisses occur as discontinuous lenses in the Grenville Front tectonic zone from Sudbury to River Valley and have been mined locally in Street Township as a source of garnet (Easton and Murphy 2002). Kwak (1968) shows the presence of other outcrops of garnet-rich gneiss to the northeast of the roadcut, suggesting that they may be more abundant in Dill Township than previously suspected. Easton and Murphy (2002) and Easton (1996, 2003) suggested that these garnet-rich rocks may represent metamorphosed hydrothermal altered rocks similar to those present in volcanogenic massive sulphide (VMS) systems. 63.1 km — Optional Stop – Large roadcuts dominated by grey orthogneiss occur on both sides of the highway (UTM co-ordinates north side 511060E 5136470N, south side 511010E 5136460N), with an approximately 15 m wide Sudbury swarm dike similar to that observed at Stop 6A present in the north roadcut (UTM 511032E 5136448N). 63.3 km — Junction 537 and Estaire Road, turn south onto Estaire Road. 63.6 km — Cross from Grenville Front tectonic zone into Nepewassi domain. 72.3 km — Junction Estaire Road and Nelson Road, turn right onto Nelson Road. 73.7 km — Pull over and stop by roadcut on north side of road. Return to vehicles and continue west on Highway 537. 131 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Stop 7 – Estaire pluton, granulite, Nepewassi domain UTM co-ordinates514045E 5129295N At this location, we are approximately 15 km south of the Grenville Front, and approximately 6.5 km south of the northern boundary of Nepewassi domain. This roadcut consists predominantly of green (Photo 8a) and pink (Photo 8b), garnetbearing, potassium feldspar megacrystic granodioritic gneiss. The gneiss incorporates irregular pods of fine- to medium-grained gneissic diorite (Photo 8c), which could be a co-magmatic phase (analysis 4, Table 5). The green coloration is a typical effect of granulite-facies metamorphism. In thin section, orthopyroxene occurs in the gneissic diorite, along with clinopyroxene- garnet [Alm58Adr2Grs18Prp19Sps4]-hastingsite-biotite (4.5 wt. % TiO2) and andesine. Opaque minerals are pyrrhotite and pyrite. Orthopyroxene is locally altered to calcite and is the only mafic phase that shows alteration. The presence of orthopyroxene indicates that this rock reached temperatures of at least 800°C (Pattison et al. 2003). The overall mineralogy of the green and pink granodioritic gneiss is similar, but there are a few key differences. In the green rock, garnet [Alm66Adr3Grs21Prp7Sps4] is abundant, and occurs primarily along the margins of potassium feldspar megacrysts and large plagioclase laths. Amphibole is hastingsite, plagioclase is oligoclase, potassium feldspar contains 1.3-1.9 wt. % BaO, and biotite contains (3-5 wt. % TiO2). Both magnetite and ilmenite are present, with minor amounts of Al in magnetite and Mn in ilmenite; no sulphide minerals were observed. Fluorapatite and 50-200 micron-size zircon are also present. In contrast, in the pink rock, garnet [Alm67Adr2Grs20Prp10Sps2] is sparser, and plagioclase phenocrysts have andesine cores and oligoclase rims. Opaque minerals are ilmenite (stoichiometric) and minor pyrrhotite and pyrite, and allanite are also present. Neither sulphide mineral nor allanite are in the green rock. Photo 8. A) Fresh surface of garnet-bearing potassium megacrystic gneissic granodiorite at Stop 7 showing green coloration suggestive of granulite facies metamorphism. B) Fresh surface of garnet-bearing potassium megacrystic gneissic granodiorite at Stop 7 showing pink coloration. C) Fine-grained mafic gneissic diorite raft hosted in potassium megacrystic gneissic granodiorite. Scale card in all images is 10 cm long. 132 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Stop 9 – Quartzite, Nepewassi domain The presence of potassium megacrystic granodiorite and fine-grained dioritic rocks is typical of rocks of the Killarney intrusive suite (circa 1740 Ma), which are common in the eastern part of Nepewassi domain (cf. Easton 2014). Preliminary geochemical data for granodiorite samples of the Estaire pluton are characterized by high Ba (>2000 ppm) and high Zr (>500 ppm) contents (analyses 1-3, Table 5.) These geochemical characteristics resemble those of the West Bay pluton in Nepewassi domain, located south of Verner (Van de Kerckhove and Easton 2016). A sample of this unit, from the roadcut at the road junction 100 m from the stop was collected for geochronology, but results were not available at the time of the trip. UTM co-ordinates 513333E 5134250N This roadcut consists of medium layered quartzite, compositionally a quartz arenite, with thin, boudinaged mafic layers (dikes?) (Photo 9). The stop is near the southern end of a 2.2 km long, up to 300 m wide, belt of quartzite, part of which was quarried for silica flux between 1910 and 1924, first by the Canadian Copper Company and later by the International Nickel Company of Canada (MDI 41I07SW00002). Quartzite units occur throughout the Nepewassi domain, most notably in the French River area but also as thin slivers in southeastern Nepewassi domain (Easton 2014; Van de Kerckhove 2016). A sample from this roadcut was collected for geochronology, but results were not available at the time of the trip. Return to vehicles and continue to road junction. 73.8 km — Junction, Nelson, McVittie, Secord roads, turn left onto McVittie Road. 74.1 km — Pull over and stop, roadcuts are present on both sides of the road. We are interested in the north end of the roadcut on the west side of the road. Stop 8 – Estaire pluton, intrusion breccia, amphibolite, Nepewassi domain UTM co-ordinates 514150E 5129080N. From north to south, the roadcut exposes a spectacular intrusion breccia with rafts of finegrained mafic material (gabbro to diorite) hosted by medium-grained granodiorite. The centre part of the outcrop is medium-grained gneissic granodiorite to monzogranite. The south end of the outcrop is a fine- to medium-grained gneissic gabbro to diorite (analysis 5, Table 5). As at the last stop, well-developed intrusion breccias and intercalation of mafic and felsic magmatic phases are typical of the Killarney intrusive suite. Photo 9. Quartzite at stop 9. Note thin amphibolite layer above the end of the hammer handle (which is 40 cm long). Krogh (1989) reported a predominantly Archean population (≥2650 Ma) with a metamorphic age of circa 1060 Ma. Quartzites in southeastern Nepewassi domain studied by Van de Kerckhove (2016) also had predominantly Archean populations (≥2650 Ma, but generally <2700 Ma). Van de Kerckhove (2016) suggested that the populations in the quartzites were suggestive that they may be correlative with the Lorrain or Bar River Formations of the Huronian Supergroup, however, there is nothing unique in the zircon 74.4 km — Turn around and return to Nelson Road, right onto Nelson Road. 75.8 km — Nelson Road and Estaire Road, turn left and head north. 81.6 km — Pull over and stop, examine roadcut on east (right) side of road. 133 �Proceedings of the 68th ILSG Annual Meeting - Part 2 populations of any of the Nepewassi domain quartzites that is diagnostic that they are part of the Huronian Supergroup versus representing another stratigraphic package. Davidson (1997, p.29-32) has an extensive discussion regarding whether or not Huronian Supergroup rocks are present in the Grenville Front tectonic zone. Biotite from near this Stop show evidence for excess argon, yielding ages between 950 to 1020 Ma (Fairbairn, Hurley and Pinson 1960; Hanson and Gast 1967), which are similar to zircon ages from the GFTZ in the Sudbury area (Krogh 1994; Corfu and Easton 2000). Return to vehicles and continue north on Estaire Road. 82.5 km — Enter Wanup pluton 83.3 km — Pull over on right shoulder, outcrop opposite on west side of road. Stop 10 – Wanup pluton, flattened megacrystic granodiorite, Nepewassi domain UTM co-ordinates 512357E, 5135635N This roadcut exposes rocks of another megacrystic granite pluton in Nepewassi domain, the Wanup pluton. We are at the north end of the pluton at this stop, and overall, the Wanup pluton exhibits a more intense gneissic fabric than does the Estaire pluton that we saw at Stops 7 and 8. There are 2 rock types present in the roadcut, a matrix-rich, gneissic megacrystic diorite to granodiorite (Photo 10a, 10b) and a matrix-poor, megacrystic granodiorite to monzogranite (Photo 10c). As was the case for the Estaire pluton, it is likely that the Wanup pluton may be a Killarney intrusive suite body. Photo 10. A) weathered surface of potassium feldspar megacrystic gneiss granodiorite of the Wanup pluton from an outcrop 200m to the southeast of Stop 10. B) fresh surface of matrixpoor potassium feldspar megacrystic gneiss granodiorite of the Wanup pluton from Stop 10. C) fresh surface of matrix-rich potassium feldspar megacrystic gneiss granodiorite of the Wanup pluton from Stop 10. Scale card in all images is 10 cm long. Return to vehicles, continue north on Estaire Road. 84.7 km — Pull over on right shoulder, outcrop on east side. 134 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Stop 11 – Nepewassi terrane boundary? UTM co-ordinates 511070E 5136160N This 100 m long, up to 3 m high, roadcut exposes a variety of tectonites related to the boundary between the Nepewassi domain to the south and the Grenville Front tectonic zone to the north. The south end of the roadcut consists of irregularly layered migmatitic paragneiss with boudinaged mafic layers (Photo 11). Continuing northward, is a transition into a zone of flattened grey granodioritic gneisses and amphibolite (Photo 12). Present within this zone are extremely deformed, porphyroclastic granitic pegmatite dikes that are parallel to the near-vertical gneissic fabric present throughout the roadcut (Photo 13). Photo 12. Flattened grey granodioritic gneiss (left) and flattened amphibolite (right) at Stop 11. Hammer handle is 33cm long. Near the north end of the roadcut, large potassium feldspar porphyroclasts are present locally in the tectonites (Photo 14). Isoclinal folds are also present in the tectonites (Photo 15). Return to vehicles, continue north on Estaire Road. Photo 13. Flattened, migmatitic amphibolite (left) and porphyroclastic granitic pegmatite dike (right) at Stop 11. Hammer handle is 33cm long. Photo 11. Irregularly layered migmatitic paragneiss with boudinaged mafic layers at the south end of Stop 11. Hammer handle is 33cm long. Photo 14. large potassium feldspar porphyroclast at Stop 11. Scale card is 10 cm long. 135 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Return to vehicles and continue north on Estaire Road. 87.7 km — Pull over on right shoulder (UTM 510020E, 5137675N), outcrop on right (east) side. Optional – mafic pod (migmatitic amphibolite) with straight gneiss on north side. 88.5 km — Pull over on right shoulder, outcrop on the right (east) side of Estaire Road. Stop 4.5 of Davidson, Carmichael and Pattison (1990), Stop 2-10 of Davidson (1995). Stop 13 – Garnet amphibolite and schists UTM co-ordinates 509410E 5139350N This approximately 80 m long roadcut consists of several different phases. The southern end of the roadcut consists of weakly migmatitic, garnet amphibolite (analysis 8, Table 6), which is in sharp contact with the pelitic and semi-pelitic schists that constitute the bulk of the outcrop. There are at least 3 schist units in the roadcut, from south to north these are. Photo 15. Folding of leucosome layer in grey, dioritic gneiss, north end of Stop 11. Scale card is 10 cm long.  Biotite-garnet schist with boudinaged quartz layers (quartz veins?) 85.0 km — Junction with 537, continue north on Estaire Road.  Muscovite-garnet schist with andesine porphyroblasts and intercalated with thin layers of semi-pelite to psammite (analysis 5, Table 6). Garnet [Alm75Adr0Grs7Prp14Sps5]. 86.6 km — Pull over on right, outcrop on right (east) side. Stop 12 – Migmatitic amphibolite  Muscovite-staurolite-garnet schist (analysis 6, Table 6) with andesine porphyroblasts. Staurolite is black on the weathered surface and is best seen on slabbed surfaces. Garnet [Alm76Adr1Grs6Prp9Sps5]. UTM co-ordinates 510020E 5137675N This roadcut is representative of much of the fine- to medium-grained amphibolite units with 1% to 5% thin, stringy feldspathic leucosome present in the GFTZ west of the Wanapitei River. This rock was described as “common amphibolite” by Kwak (1969). The affinity of these amphibolite units has not been firmly established. Both Lumbers (1975) and Dressler (1984) considered them as possible equivalents of the Nipissing intrusive suite, however, the presence of leucosome suggests that they might be older. Geochemical data (unpublished) obtained by the author indicates that the amphibolite bodies are more primitive than typical Nipissing intrusive suite rocks. The schist unit can be traced almost continuously over approximately 3.5 km and is near its widest extent at this stop. Only 900 m to the north, on Highway 69 (Photo 16), the unit is only about 15 m thick. Davidson, Carmichael and Pattison (1990) report that at this stop, in thin section, kyanite forms single crystals in contact with all the other minerals (garnet-biotite-muscovite-plagioclasequartz) and that it is commonly grown across large muscovite flakes. Sillimanite occurs as bundles of needles, generally associated with, or replacing, biotite, particularly at the edges of garnet 136 �Proceedings of the 68th ILSG Annual Meeting - Part 2 porphyroblasts. Kwak (1971) reported kyanite and potassium feldspar together as well. Figure 7 presents thermobarometric curves which indicate equilibration at ~8 kbar and 710°C. This is ~1 kbar and 40°C higher than the result obtained from a sample collected only 1.2 km to the north at the golf course, suggesting a thermal gradient of ~50°C/km (Davidson et al. 1990). Figure 7. TWEEQ plot showing 22 thermobarometric curves (5 independent) for pelitic schist with the main-stage assemblage garnet-kyanite-sillimanite-muscovite-biotiteplagioclase-quartz-ilmenite-rutile from a sample collected on the hill above Stop 13, as reported by Davidson, Carmichael and Pattison (1990). Photo 16. Muscovite-potassium feldspar schist on the east side of Highway 69, 900 m north of Stop 13 (analysis 7, Table 6). Schist is bounded to the south by garnet amphibolite (not in the photo) and to the north by irregularly layered grey, migmatitic, tectonite of intermediate composition (lower left of the photo). Rare earth element (REE) patterns for these rocks are inconclusive. The schist samples from this Stop have patterns (Figure 8) consistent with the post-Archean Australian shale composite (Taylor and McLennan 1985), but also with the pattern found in FIII rhyolites (Lesher et al. 1978) associated with volcanogenic massive sulphide (VMS) systems. It is tempting to think that these schists might be metamorphosed equivalents of the Huronian Supergroup, mostly likely the McKim Formation which has the bulk-rock major element composition capable of forming the observed mineral assemblages, most notable a highaluminum content (compare analyses 1-4, Table 6). Although the major element geochemistry supports this possibility, trace element data (analyses 5-7, Table 6) are less conclusive, as none of the samples analyzed have Ni/Co or Cr/Zn ratios typical of post-Archean fine-grained sedimentary rocks, such as the McKim Formation (between 1 and 2.5 and 1 and 1.6, respectively, Tang, Chen and Rudnick 2016). In fact, the schist sample from the site shown in Photo 16 (analysis 7, Table 6), is similar to that of the Nepewassi domain quartzite sample from Stop 9 rather than the McKim Formation. If these schists are part of the McKim Formation, this correlation is only possible if the McKim Formation has undergone extreme tectonic thinning (from approximately 1,000 m thick immediately north of the Grenville Front to approximately 100 m or less here. In addition, the adjacent Huronian Supergroup units, such as the Mississagi Formation, which are sandstonedominated, are nowhere to be seen. It could be that some of the neighbouring mafic host rocks, such as the garnet amphibolite at this stop, be metamorphosed Huronian Supergroup volcanic 137 �Proceedings of the 68th ILSG Annual Meeting - Part 2 rocks. Alternatively, as discussed at Stop 6b, the schists could represent metamorphosed hydrothermally altered rocks, similar to those present in VMS systems. Return to vehicles and continue north on Estaire Road. 88.5 km — Junction Estaire Road, Gladu Road, and Bentley Avenue. Park vehicles and walk to older roadcuts on the south side of Gladu Road. Stop 14 (Optional). Mylonitic rocks near the Grenville Front UTM co-ordinates 508520E 5141335N We are approximately 800 m south of the Grenville Front at Stop 14. The two old roadcuts south of Gladu Road consist of thin-layered, compositionally varied, straight gneisses, with some near vertical, gneissic fabric parallel, porphyroclastic granite pegmatite dikes. As we saw at Stop 5, protolith of these gneisses is not easily determined. Figure 8. Rare earth elements for selected samples normalized to the post-Archean Australian shale composite (Taylor and McLennan 1985). Schist samples from Stop 13 (open and filled triangles, analyses 5, 6, Table 6) and a McKim Formation sample from the Southern Province (filled diamonds) are straight lines centred around 1, suggesting that they could be metasedimentary rocks of the McKim Formation. Similarly, a sample of quartzite from Stop 9 (open diamonds, analysis 4, Table 6) parallels the composite, but at 1/10 the REE content, likely due to the abundance of quartz. Another schist sample (photo 16, open squares, analysis 7, Table 6) and the felsic rock from Stop 6B (open triangles, analysis 8, Table 4) have unusual concave heavy rare earth patterns, suggesting that they may have been affected by alteration. Return to vehicles and continue north on Estaire Road. 91.5 km — Optional. follow Bentley Avenue to turnaround at end for a view across the Grenville Front. Retrace route to Estaire Road, take Highway 69 back into town to Science North. End of road log. 138 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Table 6. Summary of geochemical from rocks at Stop 13 as well as average samples of the McKim Formation. Analysis 1 from Card, Innes and Debicki (1977, Table 8, p.40); analyses 2, 3, from Kwak (1968, 1971), with samples 3 from near Stop 13; analyses 4-8 from Easton (unpublished data, 2022). Analysis Number Sample Number 1 2 Average of Average of 8 Samples, 14 samples McKim Fm 3 4 5 6 7 8 232C 21RME102 Stop 9 513333 5134250 quartzite 21RME029 Stop 13 509443 5139333 schist 21RME030 Stop 13 509401 5139388 schist 21RME063 21RME027 Stop 13 509443 5139333 garnet amphibolite 0.0 >0.75 >0.75 0.12 >0.75 0.00 0.09 0.05 0.00 0.21 0.074 0.035 0.90 0.09 0.174 2.98 0.10 0.105 2.93 0.11 <0.03 1.93 0.06 0.131 0.79 95 47.7 39 5.6 11 3 <1.9 <1.3 0.8 4.8 82 9 37 3 2.97* 5.97* 1802 197.0 57.7 31 283 16 13.8 4.4 15.8 45.9 258 92 184 257 0.42 0.26 1968 196.8 68.7 30 261 15 11.2 3.2 13.8 34.5 200 81 150 166 0.49 0.22 1288 18.8 50.0 14 24 5 <1.9 <1.3 1.9 3.0 54 9 17 2 4.55* 1.21 123 17.4 116 16 268 9 <1.9 <1.3 3.5 21.9 60 127 221 106 n/a n/a 32 202 206 27 <27 Easting (m) Northing (m) Rock Name semi-pelite schist schist SiO2 (wt %) TiO2 Al2O3 Fe2O3total MnO MgO CaO Na2O K2O P2O5 CO2 S LOI/H2O+ 57.90 0.83 21.94 8.10 0.06 2.93 1.01 1.29 2.77 0.12 0.11 nr 3.20 59.20 1.11 20.75 8.54 0.08 2.90 1.34 1.49 3.25 0.15 nr nr 1.56 60.95 1.23 19.90 5.48 0.05 2.60 1.49 2.27 3.74 0.10 nr nr 1.94 Ba (ppm) Rb Sr Ga V Pb Th U Nb Y Zr Ni Cr Cu Ni/Co Cr/Zn Au (ppb) Pd Pt TREE 509866 5140422 schist Notes: Major element oxides are in weight %; trace element data are in parts per million, except for Au, Pd, Pt which are in parts per billion. * indicates Ni/Co or Cr/Zn with Archean ratios. Abbreviations: LOI = loss-on-ignition; n/a = not applicable; nr, not reported; TREE, total rare earth element. 139 �Proceedings of the 68th ILSG Annual Meeting - Part 2 References ——— 1992. Circa 1.75 Ga ages for plutonic rocks from the Southern Province and adjacent Grenville Province: what is the expression of the Penokean orogeny?: Discussion; in Radiogenic Age and Isotopic Studies: Report 6; Geological Survey of Canada Paper 92-2, p.227-228. Aldis, C.H. 2016. Geochronology and geochemistry of Mesoproterozoic granitoids in the Nepewassi domain, Grenville Orogen; unpublished BSc thesis, University of Waterloo, Waterloo, Ontario, 82p. Ames, D.E., Davidson, A., Buckle, J.L. and Card, K.D. 2005. Geology, Sudbury bedrock compilation, Ontario; Geological Survey of Canada, Open File 4570, scale 1:50 000. Card, K.D. and Lumbers, S.B. 1975. Sudbury–Cobalt; Ontario Geological Survey, Map 2361, scale 1:253 440. 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U-Pb evidence for polymetamorphic history of Huronian rocks underlying the Grenville Front Tectonic Zone east of Sudbury, Ontario; Chemical Geology, v.172, p.149171. Davey, S., Bleeker, W., Kamo, S[.L.], Davis, D.[W], Easton, M.[R.] and Sutcliffe, R.H. 2019. Ni-Cu-PGE potential of the Nipissing sills as part of the ca. 2.2 Ga Ungava large igneous province; in Targeted Geoscience Initiative: 2018 report of activities; Card, K.D. 1978. Geology of the Sudbury-Manitoulin area, districts of Sudbury and Manitoulin; Ontario Geological Survey, Report 166, 238p. 140 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Geological Survey of Canada, Open File 8549, p.403-419. zircon: Application to crystallization of the Sudbury impact melt sheet; Geology, v.36, p.383-386. Davidson, A. 1995. Tectonic history of the Grenville Province, Ontario, Precambrian’95 Field Trip Guidebook A5; Geological Survey of Canada, Open File 3142, 149p. Debicki, R.L. 1990. Stratigraphy, paleoenvironment and economic potential of the Huronian Supergroup in the southern Cobalt embayment; Ontario Geological Survey, Miscellaneous Paper 148, 154p. ——— 1997. New information on the Grenville Front near Sudbury, 43rd Annual Institute on Lake Superior Geology, Proceedings, v.43, pt.3, 38p. Dickin, A.P. 1998a. Nd isotope mapping of a cryptic continental suture, Grenville Province of Ontario; Precambrian Research, v.91, p.433-44. ——— 1998. Questions of correlation across the Grenville Front east of Sudbury, Ontario, in Current Research, 1998-C, Geological Survey of Canada, p.145–154. ——— 1998b. Pb isotope mapping of differentially uplifted Archean basement: a case study from the Grenville Province, Ontario; Precambrian Research, v.91, p.445-454. Davidson, A. and Ketchum, J.W.F. 1993. Grenville Front studies in the Sudbury region, Ontario; in Current Research Part C, Geological Survey of Canada, Paper 93-1C, p.271-278. Dickin, A.P. and McNutt, R.H. 1989. Nd model age mapping of the southeast margin of the Archean foreland in the Grenville Province of Ontario; Geology, v.17, p.299-302. Davidson, A., and van Breemen, O. 1994. U-Pb ages of granites near the Grenville Front, Ontario, in Radiogenic age and isotopic studies: report 8, Geological Survey of Canada, Current Research 1994-F, p.107–114. Dressler, B.O. 1984. Sudbury geological compilation; Ontario Geological Survey, Map 2491, scale 1:50 000. Dressler, B.O., Gupta, V.K. and Muir, T.L. 1991. The Sudbury Structure; in Geology of Ontario, Chapter 15, Ontario Geological Survey, Special Volume 4, pt.1, p.593-625. Davidson, A., Easton, R.M., Corriveau, L. and Martignole, J. 2002. Transect of the southwestern Grenville Province; Geological Association of Canada, Saskatoon’02, Fieldtrip B6 Guidebook, 114p. Dudas, F.O., Davidson, A. and Bethune, K.M. 1994. Age of the Sudbury diabase dykes and their metamorphism in the Grenville Province, Ontario; in Radiogenic Age and Isotopic Studies: Report 8; Geological Survey of Canada, Paper 94-F, p.97-106. Davidson, A., Carmichael, D.M. and Pattison, D.R.M. 1990. Metamorphism and Geodynamics of the southwestern Grenville Province, Ontario; IGCP Project 235-304, Field Trip #1 Guidebook, 123p. Easton, R.M. 1992. The Grenville Province; in Geology of Ontario, Chapter 19, Ontario Geological Survey, Special Volume 4, pt.2, p.713-904. Davidson, A., Culshaw, N.G. and Nadeau, L. 1982. A tectono-metamorphic framework for part of the Grenville Province, Parry Sound region, Ontario; in Current Research, Part A, Geological Survey of Canada, Paper 82-1A, p.175-190. ——— 1996. Geology of garnetiferous gneisses in Street Township, District of Sudbury; in Summary of Field Work and Other Activities, 1996, Ontario Geological Survey, Miscellaneous Paper 166, p.7073. Davidson, A., van Breemen, O. and Sullivan, R.W. 1992. Circa 1.75 Ga ages for plutonic rocks from the Southern Province and adjacent Grenville Province: what is the expression of the Penokean orogeny?; in Radiogenic Age and Isotopic Studies: Report 6; Geological Survey of Canada Paper 92-2, p. 107118. ——— 1998. New observations related to the mineral potential of the Southern Province and the Grenville Front tectonic zone east of Sudbury; Ontario Geological Survey, Open File Report 5976, 28p. ——— 2000. Variation in crustal level and large-scale tectonic controls on rare-metal and platinum-group element mineralization in the Southern and Grenville provinces; in Summary of Fieldwork and Davis, D.W. 2008. Sub-million-year age resolution of Precambrian igneous events by thermal extraction– thermal ionization mass spectrometer Pb dating of 141 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Ercit, T.S. 1999. North versus south: NYF pegmatites in the Grenville Province of the Canadian Shield; The Canadian Mineralogist, v.37, p.818-819. Other Activities 2000, Ontario Geological Survey, Open File Report 6032, p.28-1 to 28-16. ——— 2003. Geology and mineral potential of the Paleoproterozoic River Valley Intrusion and related rocks, Grenville Province; Ontario Geological Survey, Open File Report 6123, 171p. Fairbairn, H.W., Hurley, P.M. and Pinson, W.H. 1960. Mineral and rock ages at Sudbury-Blind River, Ontario; Geological Association of Canada, Proceedings, v.12, p.41-66. ——— 2014. Geology and mineral potential of the Nepewassi domain, Central Gneiss Belt, Grenville Province; in Summary of Field Work and Other Activities, 2014; Ontario Geological Survey, Open File Report 6300, p. 16-1 to 16-12. Fedo, C.M., Young, G.M., Nesbitt, H.W., and Hanchar, J.M. 1997. 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Péloquin, S., Berger, B., Easton, R.M. and Kennedy, C. 2020. Dill pegmatite; in Report of Activities 2019, Resident Geologist Program, Kirkland Lake Regional Resident Geologist Report: Kirkland Lake and Sudbury Districts; Ontario Geological Survey, Open File Report 6367, p.55-63. Robertson, J.A., Card, K.D. and Frarey, M.J. 1969. The federal-provincial committee on Huronian Stratigraphy, Progress Report; Ontario Geological Survey, Miscellaneous Paper 31, 26p. Rousell, D.H., Petrus, J.A., Easton, R.M., Tinkham, D.K. and Napoli, M.G. 2012. The tectonometamorphic, magmatic and mineralization history of the Wanapitei Complex, Grenville Front tectonic zone, Ontario; in 58th Institute on Lake Superior Geology, Proceedings, v.58, pt.1, p.75-76. Prevec, S.A. 1992. U-Pb age constraints on Early Proterozoic mafic magmatism from the southern Superior and western Grenville provinces, Ontario; in Radiogenic Age and Isotopic Studies, Report 6, Geological Survey of Canada, Paper 92-2, p.97-106 Sawyer, E.W. 2008. Atlas of migmatites; The Canadian Mineralogist, Special Publication 9, NRC Research Press, Ottawa, 371p. ——— 1993. An isotopic, geochemical and petrographic investigation of the genesis of early Proterozoic mafic intrusions and associated volcanics near Sudbury, Ontario; unpublished PhD. thesis, University of Alberta, Edmonton, Alberta, 223p. Schandl, E.S., Gorton, M.P. and Davis, D.W. 1994. Albitization at 1700+/-2 Ma in the SudburyWanapitei Lake area, Ontario; Canadian Journal of Earth Sciences, v.31, p.597-607. ——— 2004. Basement tracing using Mid-Proterozoic anorthosites straddling a palaeoterrane boundary, Ontario, Canada; Precambrian Research, v.129, p.164-184. Smith, M.D. 2002. The timing and petrogenesis of the Creighton pluton, Ontario: an example of felsic magmatism associated with Matachewan igneous events; unpublished MSc thesis, University of Alberta, Edmonton, Alberta, 123p. Pye, E.G., Naldrett, A.J., and Giblin, P.E, eds. 1984. Geology and Ore Deposits of the Sudbury Structure; Ontario Geological Survey Special Volume 1, 603p. Spray, J., Federowich, J.S. and Kontak, D.J. 2007. An introduction to the geology of the Sudbury impact structure; Intra-Meeting Field Trip, NUNA 2007 Meeting, Sudbury, Ontario, 38p. Raharimahefa, T., Lafrance, B. and Tinkman, D.K. 2014. New structural, metamorphic, and U-Pb geochronological constraints on the Blezardian Orogeny and the Yavapai Orogeny in the Southern Province, Sudbury, Canada; Canadian Journal of Earth Sciences, v.51, p.750-774. Streckeisen, A. 1976. To each plutonic rock its proper name; Earth-Science Reviews, v.12, p. 1-33. Sullivan, R.W. and Davidson, A. 1993. 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Abbreviations of names for rock-forming minerals; American Mineralogist, v.95, p.185-187. 146 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Field Trip 3 – Magmatism and Brecciation in the Footwall Rocks of the Southwestern Sudbury Structure Caroline Gordon Ontario Geological Survey, Sudbury, Ontario P3E 6B5 Carol-Anne Généreux Mineral Exploration Research Centre, Harquail School of Earth Sciences, Laurentian University, Sudbury, Ontario P3E 2C6, Terrane Geoscience Inc., Canada Brad Clarke SPC Nickel Corp., Sudbury, Ontario P3E 5P5 Introduction 2019) and the Ramsey–Algoma granitoid complex (Card 1979), which includes the Cartier (2642 Ma: Meldrum et al. 1997) and Birch Lake (2651 Ma: Kamo 2006; Easton and Heaman 2008; Gordon, et al. 2018a) batholiths (Figure 2). The southern boundary of the Superior Province is overlain unconformably by the supracrustal rocks of the Huronian Supergroup. The Huronian Supergroup was deposited in a continental rift and on a continental platform between 2450 and 2219 million-years ago (Krogh et al. 1984; Bennett et al. 1991) and has been interpreted to represent a partial Wilson cycle (Young 1983; Hoffman 1989; Bennett al. 1991; Young et al. 2001). This one-day field trip presents geological highlights from the Ontario Geological Survey (OGS) Southwest Sudbury Structure bedrock mapping project. This project is part of a collaborative program with the OGS, the Mineral Exploration Research Centre at the Harquail School of Earth Sciences, Laurentian University, and the private sector. Regional Geology The Sudbury Igneous Complex (SIC) is interpreted to represent a melt sheet produced by the impact of a meteorite at 1850 Ma (Dietz 1964; Krogh et al. 1984; Davis 2008). The SIC is part of the Southern Province of the Canadian Shield and is located north of the Grenville Front astride the southern contact of the Archean Superior Province (Figure 1). The term Sudbury Structure, which is used throughout this field guide, refers to the SIC, the Sudbury Basin that contains rocks of the Whitewater Group, and the outer zone of brecciated footwall rocks. The Sudbury Structure is geographically subdivided into north, south and east ranges (Figure 2). The Sudbury area has been intruded by numerous dikes, sills and plutons of various ages (Figure 2). Known intrusions and intrusive suites include, in chronological order, the 1) Joe Lake gabbro (2660 Ma: Bleeker et al. 2015); 2) Matachewan dike swarm (2480-2460 Ma: Heaman 1997; Bleeker et al. 2012); 3) Drury Township, Falconbridge and Frood intrusions of the East Bull Lake Intrusive Suite (2480 Ma: Krogh et al. 1984; James et al. 2002; Keays and Lightfoot 2020); 4) Creighton and Murray plutons (2460 Ma: Bleeker et al. 2015); 5) Nipissing Intrusive Suite (2219 to 2210 Ma: Davey et al. 2019; Noble and Lightfoot 1992; Corfu and Andrews 1986: Bleeker et al. 2015); 6) Trap dike swarm (1750 Ma: Bleeker et al. 2015); 7) Sudbury dike swarm (1238 Ma: Krogh et al. 1987), and; 8) Grenville dike swarm (590 Ma: Kamo et al. 1995). In the Sudbury area, Archean rocks of the Superior Province are part of the Abitibi Subprovince and consist of the Levack Gneiss Complex (2711 to 2647 Ma: Krogh et al. 1984; Wodicka and Card 1995), the Benny greenstone belt (2680-2700 Ma: Ontario Geological Survey 147 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 1. Sketch map showing the regional setting of the Sudbury Igneous Complex and the Huronian Supergroup (modified from Young et al. 2001). The Sudbury area has been affected by several episodes of deformation and metamorphism. Regional metamorphism is thought to have reached mid-greenschist to lower-amphibolite facies and generally increases to the south (Card 1978; Card et al. 1984; Fox 1971). Country rocks adjacent to the SIC were thermally metamorphosed but have since been overprinted by regional metamorphism (Dressler et al. 1991; Jørgensen et al. 2019; Généreux et al. 2021). Ductile deformation of the Southern Province has historical been interpreted to have started prior to or concurrent with emplacement of the Nipissing Intrusive Suite during the Blezardian Orogeny (2415-2219 Ma; Raharimahefa et al. 2014; Stockwell 1982), and continued during the Penokean Orogeny (18901830 Ma; Dressler 1984a; Bennett et al. 1991), the Yavapai and Mazatzal orogenies (1770-1600 Ma; Bailey et al. 2004; Raharimahefa et al. 2014; Papapavlou et al. 2017), and the Grenville Orogeny (1120-980 Ma; Carr et al. 2000). Recent studies have attributed most of the deformation in the Sudbury area to the Yavapai–Mazatzal orogenies (Bailey et al. 2004; Raharimahefa et al. 2014; Papapavlou et al. 2017). The lower age limit of ductile deformation is constrained by the age of the undeformed Sudbury dike swarm (1238±4 Ma; Krogh et al. 1987). Three types of ore environments are associated with the SIC: 1) contact deposits, which are hosted in depressions at the base of the SIC; 2) offset deposits, which occur in quartz diorite dikes that extend for several kilometres from the SIC into the footwall rocks, and; 3) footwall deposits, which are found in the brecciated footwall rocks directly underlying the SIC (cf. Lightfoot 2017). It is generally accepted that nickel-copper-platinum group element (Ni-Cu-PGE) deposits in Sudbury are primarily magmatic and formed by differentiation of a sulphide melt during crystallization of the SIC (Keays and Crocket 1970 148 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 2: Geological map of the Sudbury area (modified from Ames and Farrow 2007). The location of Drury and Denison townships are shown in the lower left of the figure. 149 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Table 1. Mineral deposits in Drury and Denison townships. Table includes names for mines and prospects. Occurrences and discretionary occurrences are not included here. Reference numbers in table correspond with mineral occurrence symbol on Figure 3. Naldrett et al. 1994; Naldrett 1999; Ames and Farrow 2007), with some involvement of hydrothermal fluids (Farrow et al. 1994; Jago et al. 1994; Morrison et al. 1994; Molnár and Watkinson 2001; Péntek et al. 2008). Many deposits in the South Range of the SIC were modified during post 1850-Ma tectonic events (Lightfoot 2017). Mining and Exploration History The southwestern portion of the Sudbury Structure has been an important mining and exploration area since the discovery of Ni-Cu-PGE mineralization related to the SIC. The first deposit discovered in the area was the Worthington deposit in 1884 (cf. Lightfoot 2017). Since then, the southwestern Sudbury Structure has supported numerous past-producing mines and currently hosts one active mine (Totten Mine), advanced prospects (Victoria Project) and undeveloped mineral occurrences (Figure 3; Table 1). The majority of mineral deposits are associated with SIC-related rocks and occur as contact deposits or as offset deposits within the Worthington and Vermilion offset dikes. Commodities include NiCu-PGE, gold (Au), cobalt (Co), silver (Ag), iron (Fe), tin (Sn), selenium (Se), tellurium (Te) and arsenic (As) (Table 1). Several contact and offset deposits were modified by post-SIC deformation and ore is now hosted within shear zones (i.e. Vermilion and Chicago mines). In addition to SIC-related mineralization, known prospects, occurrences and discretionary occurrences include: 1) Ni-Cu±PGE mineralization hosted by the Nipissing gabbro, volcanic and sedimentary rocks of the Huronian Supergroup, and within shear zones; 2) Au-Cu bearing quartz veins; and, 3) uranium-thorium (U-Th) mineralization in pyritic quartz-pebble-rich conglomeratic arenites in the lower Matinenda Formation of the Huronian Supergroup (Figure 3). Quartz veins and quartzites in the area have also been quarried for silica. Ref No. Name Commodity* (Primary/Secondary) 1 Totten Mine Ni, Cu / PGE, Co, Au, Ag 2 Totten #1 Ni, Cu / PGE, Co, Au, Ag 3 Worthington Mine Ni, Cu, PGE / Co, Au, Ag 4 Worthington #2 Ni, Cu, PGE / Co, Au, Ag 5 Howland Pit Ni, Cu / PGE, Co 6 Robinson Mine Ni, Cu / PGE, Co 7 Aer Mine (Rosen and Gersdorffite mines) Ni, Cu / PGE, Co 8 Victoria Mine Ni, Cu / PGE, Au 9 Vermilion Mine Ni, Cu / PGE, Au, Ag, Co, Fe, Sn, Se, Te, As 10 Crean Hill Mine Ni, Cu, PGE / Au, Co, Fe, Ag, Se, Te 11 Lockerby Mine Ni, Cu, Co / PGE, Au, Ag 12 Ellen Pit Ni, Cu / Au, Ag, PGE, Fe, Co, Se, Te 13 Chicago Mine Ni, Cu / PGE, Au, Ag, Co, Fe, Se, Te 14 Sultana Nickel Mine Ni, Cu 15 Delta Occurrence Ni, Cu / PGE, Au 16 McIntyre Mine Ni, Cu 17 Victoria Project Ni, Cu, PGE 18 Alanaen and Maki West (Kerr Addison Prospect) U, Th, Cu *Primary and secondary commodities as listed in the Ontario Mineral Inventory Database (Ontario Geological Survey 2022). 150 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 3. Geological map of Drury and Denison townships (after Gordon et al. 2018a; Gordon and Généreux 2017; Gordon 2018). Mineral occurrences are from Ontario Geological Survey (2022) and Gordon et al. (2018a). Universal Transverse Mercator coordinates are in North American Datum 1983, Zone 17. CCF = Cameron Creek Fault, VLF = Vermilion Lake Fault, FLF = Flack Lake Fault, CAF = Chicago Fault, CF = Creighton Fault, VF = Victoria Fault, CHF = Crean Hill Fault, MF = Murray Fault, WRTH-W = Worthington Offset dike, western limb, WRTH–E = Worthington Offset dike, eastern limb, VO = Vermilion Offset dike, CVDZ = Creighton-Victoria deformation zone. 151 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Geological Overview of the Southwestern Sudbury Structure In the southwestern Sudbury Structure, the magmatic breccia along the base of the SIC (Figure 3) has been subdivided into 4 types based on matrix composition and clast abundance (cf. Gordon et al. 2018a). Geological mapping in the southwest Sudbury Structure was completed between 2015 and 2018 and focussed on Drury and Denison townships (Figure 3) located approximately 50 km west of the City of Greater Sudbury (Figure 2). 1. Clast-rich Sublayer Norite (>30% clasts): heterolithic breccia with abundant gabbroic and lesser amounts of granitoid clasts in a noritic to leuconoritic matrix. For sake of clarity, the authors have omitted the prefix “meta” for the rock names in this field guide (e.g. gabbro versus metagabbro). 2. Clast-poor Sublayer Norite (<30% clasts): heterolithic breccia with a noritic matrix. Sudbury Igneous Complex (SIC) 3. Sublayer Granite Breccia: Heterolithic breccia (>35% clasts) with abundant granitoid clasts and few mafic clasts in a pink-weathering matrix. The SIC is elliptical in shape and approximately 30 km x 60 km in size (Figure 2). It is made-up of three main components: 1) Main Mass, which is a differentiated igneous body; 2) Contact Sublayer, a basal magmatic breccia; and, 3) Offset dikes, quartz diorite dikes emplaced in the footwall of the SIC (Giblin 1984; Dressler et al. 1991; Ames et al. 1997, 1998, 2002). All three components are exposed in the southwest Sudbury Structure. 4. Heterolithic breccia with gabbroic and anorthositic gabbro clasts in a variably textured, leucogabbroic matrix. Clast-rich and clast-poor Sublayer Norite occurs at the SIC–Archean granitoid and SIC–Huronian Supergroup contacts. The Sublayer Granite Breccia unit, which likely represents a variation of the classic Sublayer Granite Breccia, occurs exclusively at the SIC–Archean granitoid contact. Breccia type 4 occurs exclusively at the SIC–Drury Township intrusion contact and, although it has been tentatively classified as sublayer, it may represent Footwall Breccia. Main Mass The SIC, as exposed in the southwestern Sudbury Structure, exhibits the complete Main Mass stratigraphic sequence (Figure 3). From top to bottom, it consists of: 1. Granophyre: leucocratic monzogranite and upper plagioclase-rich phase of granodiorite. Offset Dikes 2. Transition zone quartz gabbro: melanocratic to mesocratic quartz gabbro and monzogabbro with cumulus magnetite and apatite. Offset dikes of the SIC, also known as Offset Sublayer, are radial, concentric, and discontinuous segmented bodies that were emplaced into the footwall of the SIC (cf. Lightfoot 2017). Offset dikes consist of two phases: quartz diorite (QD) and a sulphide-enriched inclusion-bearing quartz diorite (IQD) (Grant and Bite 1984). 3. South Range norite: leucocratic to mesocratic quartz monzogabbro and norite. Contact Sublayer The main mass of the SIC overlies an extensive zone of magmatic breccia known as the Contact Sublayer. The Sublayer is heterolithic in matrix composition and clast type, variably gossanous and laterally discontinuous. It is generally classified into two groups based on the composition of its matrix: Sublayer Norite consists of breccia with a noritic matrix, whereas Sublayer Granite Breccia contains a granitic matrix (cf. Lightfoot 2017). The Worthington Offset is a branching radial offset dike that trends southwest from the SIC contact (Figure 3). The proximal part, occupying a possible embayment in the Victoria Mine area north of Ethel Lake, occurs as numerous faulted segments (Grant and Bite 1984). Southward from the SIC, the offset dike narrows and then broadens before bifurcating into eastern and western limbs. 152 �Proceedings of the 68th ILSG Annual Meeting - Part 2 The eastern limb tapers gradually over a distance of 1500 m at the surface, trends southeast, and broadens at depth (Grant and Bite 1984). The western limb extends to the southwest for at least 15 km, with a thickness between 30 and 100 m. The dike truncates volcanic and sedimentary rocks of the Huronian Supergroup as well as Nipissing gabbro and is crosscut by the Creighton Fault (Figure 3). The Worthington Offset contains both the inclusion-poor QD and mineralized IQD phases. The contact between phases is sharp and inclusions of QD are present within IQD. formed by cataclasis (Lafrance and Kamber 2010; O'Callaghan et al. 2016; Rousell et al. 2003) and/or frictional melting of the target rocks during cratering (Dressler 1984a; Lafrance and Kamber 2010; O'Callaghan et al. 2016; Rousell et al. 2003; Thompson and Spray 1994). Within the southwestern Sudbury Structure, Sudbury Breccia occurs as fine-grained, dark green to greenish-yellow veins, dikes and irregular shaped bodies of various thicknesses and orientations. Thicker breccia veins, typically several metres in width, are usually clast-rich and form corridors along major lithologic contacts and structures. Two major heterolithic Sudbury Breccia belts were identified in Drury Township: 1) at the contact between the Drury Township intrusion and the Archean granitoid, and 2) following the folded contacts between Huronian sedimentary rocks and Nipissing gabbro in south-central Drury Township (Figure 3). These breccia belts range from a few metres to several hundred metres wide. In the belts, breccia matrix typically makes-up more than 40% of the outcrops. The Vermilion Offset dike crops out east of Ethel Lake, south of Crean Hill (Figure 3). The Vermilion Offset occurs within a 200 m long, northwest-striking zone of discontinuous QD and IQD pods at the contact of Sudbury Breccia and sedimentary and volcanic rocks of the Stobie Formation (Grant and Bite 1984; Szentpeteri et al. 2003). Breccias in the Footwall Rocks Two types of breccia occur within the footwall of the SIC: Footwall Breccia and Sudbury Breccia. Footwall rocks of the SIC Footwall Breccia Archean Ramsey-Algoma granitoid complex Footwall Breccia is a parautochthonous breccia that occurs in discontinuous lenses and sheets between the Contact Sublayer and underlying footwall rocks. It consists mainly of brecciated and partially melted footwall rocks, and has an igneous to granoblastic matrix (Lakomy 1990; McCormick et al. 2002). Within the southwestern Sudbury Structure, Footwall Breccia has been identified at the Crean Hill Mine area (Figure 3) (Généreux et al. 2021), where it consists of breccia dikes and pods of various compositions hosted in volcanic rocks of the Stobie Formation. Monzogranite, granite and granodiorite belonging to the Archean Ramsey–Algoma granitoid complex are the oldest rocks in the southwestern Sudbury Structure (Figure 3). The contact between the Archean basement and rocks of the Huronian Supergroup is known to be unconformable (Stockwell 1964; Card 1990) but is highly sheared in the Sudbury area. The transition between the Cartier and Birch Lake batholiths of the Ramsey-Algoma granitoid complex is reportedly within Drury Township, but the exact location of the contact has not been determined (Tolman 1929; Meldrum et al. 1997). U/Pb zircon geochronology on a granitoid sample collected in western Drury Township yielded a minimum age of 2645±1 Ma (Gordon et al. 2018a), which suggests that the granitoid rocks in Drury Township are older than the Cartier batholith and more similar in age to the Birch Lake batholith. Sudbury Breccia All rock types in the Sudbury area that are older than circa 1850 Ma contain various amounts of impact breccia, locally called Sudbury Breccia. It is a parautochthonous breccia with an aphanitic to microcrystalline matrix, and occurs as discontinuous veins or tabular bodies within the footwall rocks. Sudbury Breccia is thought to have 153 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Elliot Lake Group The Elliot Lake Group is the lowermost unit in the Huronian Supergroup and the only unit to contain volcanic rocks. The basal volcanic rocks are interpreted as fissure eruptions related to deeppenetrating crustal faults during rifting (Card 1978). In the Sudbury area, the volcanic rocks are up to 3 km thick and are subdivided into the Elsie Mountain, Stobie and Copper Cliff Formations (Card 1978) (Figure 4). The felsic volcanic rocks of the Copper Cliff Formation are not exposed in Drury and Denison townships and will not be discussed further in this guide. Paleoproterozoic Huronian Supergroup The Huronian Supergroup extends from the east shore of Lake Superior across the north shore of Lake Huron, and northeastward across the Cobalt Embayment to the Noranda area in northwestern Quebec (Bennett et al. 1991) (Figure 1). It consists of a southward thickening, up to 12 km thick, package of sedimentary and volcanic rocks that are subdivided, from oldest to youngest, into the Elliot Lake, Hough Lake, Quirke Lake and Cobalt groups (Bennett et al. 1991; Robertson et al. 1969). The minimum age of the Huronian Supergroup is constrained by the age of the intruded Nipissing Intrusive Suite (2210-2219 Ma: Davey et al. 2019; Corfu and Andrews 1986; Noble and Lightfoot 1992; Bleeker et al. 2015). Its maximum depositional age is constrained by the age of the felsic volcanic rocks of the Copper Cliff Formation (2452-2460 Ma: Krogh et al. 1984; Ketchum et al. 2013; Bleeker et al. 2015). The Huronian Supergroup volcanic rocks in Drury and Denison townships exhibit significant lateral variation from east to west and will be discussed as 3 separate segments: western, central and eastern. In the western segment, the majority of rocks previously identified as volcanic have been reclassified as mylonites of the CreightonVictoria deformation zone (Figure 3) (Gordon et al. 2015; Simard et al. 2016; Généreux et al. 2016; Généreux et al. 2017; Gordon et al. 2018a). In the central segment, which is east of the CreightonVictoria deformation zone, volcanic rocks of the Elsie Mountain Formation are dominated by massive and pillowed basaltic flows that are locally amygdaloidal and porphyritic, with minor amounts of intercalated arenite and siltstone. South of the Elsie Mountain Formation, bimodal volcanic rocks of the Stobie Formation are intercalated with arenite of the Matinenda Formation (Figure 3). In this area, the Stobie Formation consists predominantly of massive and pillowed basaltic to rhyolitic flows with interbedded arenite, siltstone, and minor amounts of pyroclastic rocks. The central and eastern segments are separated by an unnamed northwest-trending fault in Denison Township (Figure 3). The eastern segment is dominated by the bimodal Stobie Formation, which is bound to the north and east by the SIC and Creighton pluton, respectively. The Elsie Mountain Formation is largely absent in the eastern segment, except for a thin sliver of basalt exposed adjacent to the Creighton pluton. The oldest and lowermost Elliot Lake Group consists of an intercalated sequence of sandstone, conglomerate, siltstone, mudstone, and local volcanic rocks (Card 1978). With the exception of the carbonate-bearing Serpent Formation (Quirke Lake Group), the overlying Hough Lake, Quirke Lake and Cobalt groups each contain cyclical repeating sequences of lower conglomeratic units, middle siltstone and mudstone units, and upper sandstone units (Roscoe 1969). Rocks of the lower Huronian Supergroup, specifically the Elliot Lake and Hough Lake groups, are represented in the southwestern Sudbury Structure. In the Sudbury area, most of the Huronian Supergroup strata are subvertical and are approximately west-northwest- to east-trending. Reversals of facing direction define the synclines, anticlines and thrust faults in the area. Despite folding and faulting within formations, the overall younging direction of the stratigraphy is southward. Unit thicknesses stated herein are apparent thicknesses. 154 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 4. Generalized stratigraphy and facies relationships of the Elliot Lake Group in the Southwest Sudbury Structure (modified from Card 1978). The Matinenda Formation, which hosts the uranium-rich pyritic quartz pebble conglomerates in the Elliot Lake area (Figure 1), is interpreted to have been deposited in a braided fluvial environment (Fralick and Miall 1989). In the southwestern Sudbury Structure, the Matinenda Formation is up to 1-km thick and thins eastward where it eventually disappears from the Stobie– McKim Formations contact but crops out as discontinuous layers within the Stobie Formation (Figure 3). The Matinenda Formation consists of subfeldspathic arenite and quartz arenite with quartz-pebble conglomeratic beds. The quartzpebble conglomeratic beds locally contain pyrite and elevated concentrations of U and Th. The arenites, which constitute the bulk of the formation, are massive to crudely bedded, locally displaying graded beds and cross-bedding. The McKim Formation, which is interpreted to represent a marine transgression that gradually drowned the Matinenda fluvial plain (Fralick and Maill 1989), is one of the most aerially extensive Huronian Supergroup units in the southwestern Sudbury Structure. The preserved sequence was significantly thickened (up to 1.5 km) by folding (Figure 3). The McKim Formation consists of interbedded mudstone, siltstone and minor sandstone. The amount of interbedded sandstone increases significantly eastward through Denison Township. The sandy and silty turbidites are 155 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Proterozoic Intrusive Rocks typically thickly laminated to thinly bedded and commonly display cross-bedding, graded beds, ripples and scour marks. Staurolite, chloritoid and rutile porphyroblasts are present, predominantly in the muddier beds. Drury Township Intrusion, Matachewan Dike Swarm and Huronian Synvolcanic Intrusions The Drury Township intrusion, Matachewan dike swarm, and volcanic rocks of the Huronian Supergroup were all emplaced during continental rifting associated with the Matachewan Igneous Event (Heaman 1997). The Drury Township intrusion and Matachewan dike swarm are also tentatively genetically linked to eruption of the basal Huronian Supergroup mafic volcanic rocks (Fahrig 1987; Vogel et al. 1998). Hough Lake Group The Hough Lake Group contains sedimentary rocks of the Ramsay Lake, Pecors and Mississagi Formations. It is the lowest of the 3 groups that display the cyclical repetition of conglomerate— siltstone-mudstone—–sandstone. Each cycle is interpreted to represent a sequence of glaciogenic —marine—fluvial and/or shallow marine deposition (Roscoe 1969; Robertson 1976; Fralick and Miall 1989). The Drury Township intrusion is interpreted as one of several leucogabbro-anorthosite sills of the East Bull Lake Intrusive Suite (Prevec 1993; Prevec and Baadsgaard 2005). Intrusions of the East Bull Lake Suite were emplaced between ~2491 and 2475 Ma and occur in a discontinuous east-northeast-trending belt along the Archean– Proterozoic contact between Elliot Lake and the Ottawa River (Krogh et al. 1984; James et al. 2002; Bleeker et al. 2012; Bleeker et al. 2015). The Drury Township intrusion is up to 2 km thick and is exposed at the Ramsey–Algoma granitoid complex – Huronian Supergroup – SIC contact in Drury Township (Figure 3). The intrusion is composed of medium- to coarse-grained, locally pegmatitic, vari-textured anorthositic gabbro with a marginal gabbroic phase. The Ramsay Lake Formation is up to 300 m thick and consists of 2 conglomerate units: the “beige” and “grey” members, and, an upper sandstone unit, the “sandy” member (Gordon et al. 2018a). The “beige” member is a bilithic conglomeratic subfeldspathic arenite with a beige, quartz-rich matrix and clasts of granite and quartz. The “grey” member is consistent with the classic description of the Ramsay Lake Formation (cf. Young 1991; Bennett et al. 1991). It is a heterolithic conglomeratic wacke or sandstone with a grey, quartz-rich matrix. The “sandy” member is composed of crudely bedded, subfeldspathic arenite, wacke and quartz arenite. The Pecors Formation is 100 to 300 m thick and exposed between the Ramsay Lake and Mississagi Formations (Figure 3). The formation consists of siltstone and mudstone. The siltstone and mudstone are thickly laminated and locally exhibit cross-beds and load structures. The Matachewan dike swarm is an extensive radial dike swarm consisting of north- and northwest-trending mafic dikes, which intruded granitoids of the Superior Province and crop out over 300,000 km2 in Ontario and southwestern Quebec (Halls and Bates 1990). The Matachewan dike swarm was emplaced in 2 main pulses. The first, earlier pulse at circa 2480 Ma is believed to have been coincident with emplacement of the East Bull Lake Intrusive Suite (Krogh et al. 1984; James et al. 2002; Bleeker et al. 2012; Bleeker et al. 2015). The second, and “main pulse” of the Matachewan dike swarm. occurred at circa 2460 Ma (Heaman 1997; Bleeker et al. 2012; Bleeker et al. 2015). In the southwestern Sudbury Structure, northwest- and northeast-trending Matachewan The Mississagi Formation is at least 2 km thick, including thickening by folding (Figure 3). Sandstones of the Mississagi Formation consist of well-sorted, fine- to medium-grained subfieldspathic arenite and quartz arenite. They are thinly to thickly bedded, locally contain beds of siltstone, and commonly display cross-beds. 156 �Proceedings of the 68th ILSG Annual Meeting - Part 2 dikes crosscut the Drury Township intrusion and the Archean Ramsey-Algoma granitoid complex (Figure 3). These mafic dikes are fine- to mediumgrained and locally plagioclase-phyric. The northeast trend for the Matachewan dike swarm in the Sudbury area is atypical; it is possible that their trend represents a small-scale, concentric dike swarm (Gordon et al. 2018a). emplaced between 2219 to 2210 Ma (Davey et al. 2019; Corfu and Andrews 1986; Noble and Lightfoot 1992; Bleeker et al. 2015) as part of the Ungava large igneous province during continental rifting (Ernst and Bleeker 2010; Davey et al. 2019). In the southwestern Sudbury Structure, numerous Nipissing sills intruded the Huronian Supergroup and adjacent Archean basement rocks (Figure 3). Individual sills can reach up to 400 m thick and extend over 2 km in length. The larger Nipissing sills typically occur near or at the contact between the Ramsay Lake and McKim Formations. These larger sills are crudely differentiated, with gabbro and melagabbro phases. Pegmatoidal and/or anorthositic pods occur within the thicker portions of the sills. Smaller, narrower sills, on the other hand, are undifferentiated. Mafic sills that are geochemically similar to the Elsie Mountain and Stobie formations intrude the volcanic rocks of the Elsie Mountain and Stobie Formations as well as sedimentary rocks of the Matinenda and McKim Formations (Figure 3) (Gordon et al. 2018a; Gordon 2021, 2022). These mafic sills likely represent a combination of synvolcanic intrusions and intercalated mafic flows. Locally, peperite textures are preserved where the mafic sills intruded the McKim Formation, suggesting that Huronian Supergroup volcanism continued (or resumed) during deposition of the McKim Formation (Gordon et al. 2018a; Gordon 2021). Most of these mafic sills are 50-100 m wide, but a few larger intrusions are up to 200 m wide and 2 km long (Figure 3). All are roughly east-trending and fine- to medium-grained. Trap Dike Swarm In the Sudbury area, east- to east-northeasttrending mafic dikes, interpreted to belong to the Trap dike swarm (circa 1750 Ma; cf. Bleeker et al. 2015), crosscut the Worthington Offset dike, Sudbury Breccia, folded Nipissing sills and Huronian Supergroup stratigraphy (Figure 3). These dikes are typically less than 50 in width and consist of undeformed quartz diabase. Field relationships suggest that these mafic dikes postdate the SIC event and they are tentatively linked to a post-Penokean rifting event (cf. Bleeker et al. 2015). Creighton Pluton The Creighton pluton is a 2455-2460 Ma subvolcanic sill that was emplaced into the Huronian Supergroup mafic volcanic rocks during rifting (Bleeker et al. 2015). It is interpreted as the high-level magma chamber to rhyolites of the Copper Cliff Formation (Bleeker et al. 2015). The western extent of the Creighton pluton crops out in the northeastern corner of Denison Township (Figure 3). It intruded the mafic volcanic rocks of the Stobie and Elsie Mountain Formations and is truncated to the north by the SIC. The pluton consists of leucocratic granite and porphyritic quartz monzonite. Inclusions of porphyritic quartz monzonite are locally found in the granite phase. Dikes of Unknown Affinity East-northeast-trending mafic dikes of similar appearance to the Trap dikes, but which are geochemically distinct, crosscut the Archean basement, folded Nipissing sills and Huronian Supergroup strata (Gordon et al. 2018a). The mafic dikes are fine- to medium-grained, locally plagioclase-phyric and are variably foliated. In the southern central portion of Drury Township, northwest-trending felsic dikes crosscut folded Nipissing sills. The felsic dikes are up to 5 m wide, quartz-rich, massive and contain inclusions of Nipissing gabbro (Gordon et al. 2018a). Nipissing Intrusive Suite The voluminous Nipissing Intrusive Suite is exposed for over 400 km from the Ontario–Quebec border to Sault Ste. Marie. The intrusive suite was 157 �Proceedings of the 68th ILSG Annual Meeting - Part 2 These mafic and felsic dikes have not been assigned to any known intrusive suite and likely represent one or more magmatic events that occurred after regional folding. They are not included on Figure 3; readers are referred to Gordon et al. (2018a) for more information. McKim Formation are juxtaposed against rocks of the Mississagi, Pecors and Ramsay Lake Formations. Northwest-trending faults produced significant offsets of the Huronian Supergroup strata. In Denison Township, rocks of the Stobie and McKim Formations are juxtaposed against rocks of the Elsie Mountain Formation along a northwesttrending fault (Figure 3). This configuration of Huronian Supergroup strata is truncated to the north by the SIC and crosscut by the CreightonVictoria deformation zone. Reactivation of the northwest-trending faults is indicated by sheared sublayer along the fault. These structures may represent southward extension of the similarly oriented Onaping Fault system which, like the Murray Fault, is thought to have originated as extensional faults during deposition of the Huronian Supergroup and reactivated during subsequent orogenic events (cf. Card 1978; Zolnai et al. 1984). Sudbury Dike Swarm The Mesoproterozoic Sudbury dike swarm (circa 1238 Ma: Krogh et al. 1987) consists of northwest-trending olivine diabase dikes that crosscut the Superior and Southern provinces, and their deformed and metamorphosed equivalents occur within the northwestern Grenville Province (Ketchum and Davison 2000). The Sudbury dike swarm extends ~300 km west and northwest from the Sudbury area. Different tectonic settings for their emplacement have been proposed, which include continental rifting (Shellnut and MacRae 2012), a back arc setting (Ernst and Bleeker 2010), or mantle-plume upwelling (Easton et al. 2021). In the southwestern Sudbury Structure, undeformed, northwest-trending dikes of the Sudbury dike swarm crosscut the Ramsey-Algoma granitoid complex, the Drury Township intrusion, rocks of the Huronian Supergroup and the SIC (Figure 3). The Sudbury dikes consist of undeformed olivine diabase, are fine- to mediumgrained and locally plagioclase-phyric. Pre-impact Foliation An early, southeast-trending foliation is locally preserved within basaltic rocks of the Elsie Mountain Formation adjacent to the Creighton Pluton contact. This foliation is locally crosscut by Sudbury Breccia, which also contains randomly oriented clasts of the foliated basalt and granitoid indicating that deformation started before the Sudbury impact event (Gordon 2018). Structure The southwestern Sudbury Structure displays multiple generations of structural fabrics and major structures, which are described below in chronological order. Bedding Parallel Thrust Faults Bedding-parallel thrust faults are recognized on the basis of stratigraphic repetition and/or absence of specific Huronian Supergroup formations. The thrust faults are folded along with Huronian Supergroup strata (Gordon et al. 2018a; Généreux et al. 2018) and, thus, predate regional folding. In Drury Township, a bedding-parallel foliation is associated with these thrust faults and is locally preserved in the matrix of Sudbury Breccia, indicating that these faults formed after the impact event (Généreux et al. 2018). Murray Fault and Northwest-Trending Faults The Murray Fault is a major east- to eastnortheast-trending structural feature in the Southern Province (Figure 1). It is thought to have originated as an extensional fault during Huronian sedimentation and has been periodically reactivated during subsequent tectonic events (Card and Hutchinson 1972; Card 1978; Zolnai et al. 1984). The Murray Fault truncates the Huronian Supergroup strata in the southeastern area of Denison Township (Figure 3), where rocks of the 158 �Proceedings of the 68th ILSG Annual Meeting - Part 2 The CVDZ is characterized by a strong westnorthwest- to east-trending subvertical foliation, and a steeply plunging eastward-trending stretching lineation. Shear sense indicators correspond to dextral (horizontal) shearing in the west, and south-over-north dextral (oblique) shearing to the east (Généreux et al. 2016; Généreux, et al. 2017). The development of the CVDZ is interpreted as coeval with regional folding (Généreux et al. 2018). Regional Folds, Foliation and Lineation The present structural configuration of the Huronian Supergroup strata in Drury Township is strongly controlled by kilometre-scale, northeasttrending isoclinal folds. Adjacent to the Superior– Southern provinces contact, smaller scale folds are west-northwest to east-trending, following the orientation of the Superior–Southern provinces contact. Throughout Drury Township, Nipissing sills are folded along with the Huronian Supergroup strata or follow their axial trace. Folding decreases in intensity eastward, into Denison Township, and appears to be restricted to the McKim Formation (Figure 3). Northeast-trending Faults This fault group includes the Cameron Creek, Fairbank Lake, Chicago and Vermilion Lake faults, as well as unnamed faults of similar orientation (Figure 3). These northeast-trending brittle-ductile faults produce significant offset of SIC-related rocks, folded Huronian Supergroup strata and Nipissing sills. A moderate to strong east- to east-northeasttrending regional foliation is ubiquitous within the Huronian Supergroup stratigraphy and Sudbury Breccia. The regional foliation overprints Sudbury Breccia, and locally crenulates the beddingparallel foliation associated with the early thrust faults (Généreux et al. 2018). It is axial planar to folds on outcrop and contains a southeast- to southwest-plunging stretching lineation that parallels fold axes. Both fabrics also rotate along with fold orientations adjacent to the SouthernSuperior provinces contact (Généreux et al. 2018). Minor Structures Shattercones, which are impact-related conical fractures with distinctive cone or fan-shaped features, are found within arenites and conglomerates of the Mississagi and Ramsey Lake Formations, respectively. The regional foliation and mylonitic fabric are locally overprinted by north-northwest-trending crenulation cleavage and kink bands. Most kink bands observed are S-shaped and locally form conjugate sets of centimetre-scale box-folds (cf. Généreux et al. 2016; Gordon et al. 2018a). Local, brittle, northwest-trending faults also crosscut all rock types, including Sudbury dikes. Creighton-Victoria Deformation Zone The Creighton-Victoria deformation zone (CVDZ) consists of an east-southeast-trending, 200 to 400-m wide mylonite zone that occurs along the contact between the Archean Superior Province and Paleoproterozoic Southern Province across Drury Township (Figure 3) (Gordon et al. 2015; Simard et al. 2016; Généreux et al. 2016; Généreux et al. 2017; Gordon et al. 2018a). Eastward into Denison Township, the CVDZ extends into a 1.5 km wide deformation corridor bound by the Creighton and Victoria faults (Généreux et al. 2017). The corridor consists of discrete, 5 to 20-m wide, shear zones that follow internal contacts within weakly to moderately foliated Huronian volcanic and sedimentary rocks. 159 �Proceedings of the 68th ILSG Annual Meeting - Part 2 FIELD TRIP DETAILS The Trap dike is massive, grey in colour, homogeneous and fine-grained. It is at least 20-m wide. Along the contact with the Trap dike, the Nipissing gabbro is strongly amphibolitized. Both the Nipissing sill and the Trap dike were metamorphosed to greenschist facies. Plagioclase is partially sausseritized and pyroxene is completely replaced by amphibole and, locally, biotite. Geological Maps Geological compilation maps covering all or parts of the area of the field trip include Ames et al. (2005), Dressler (1984b) and Card and Lumbers (1973). A detailed geology map is available for Drury Township (Gordon et al. 2018b). ROAD LOG Approximately 50 m south of the Trap dike, the Nipissing sill is intruded by olivine diabase of the Sudbury dike swarm (Figure 6). This Sudbury dike can be traced for over 8 km and truncates the folded Huronian Supergroup stratigraphy and the Worthington Offset dike (Figure 3). The contact between the Sudbury dike and Nipissing sill is sharp, strikes northwest and dips steeply northward. The Sudbury dike is at least 25 m wide and has a distinctive brown weathered surface. It is magnetic, massive, fine- to medium-grained and plagioclase-phyric. It is also relatively unaltered, its primary mineralogy consisting of plagioclase, clinopyroxene, orthopyroxene and olivine. The Sudbury dike is truncated by late, northwesttrending brittle faults adjacent to the southern contact with the Nipissing gabbro and within the central area of the exposure (Figure 6). Note: Caution should be taken when parking vehicles on the shoulder of the roads and when examining outcrops along any road along the field trip route. All UTM co-ordinates are given in NAD 83 datum, zone 17. Figure 3 shows the location of the field trip stops. The mileages in the road log represent the distance from one stop to another. 38.5 km (30 minutes) – Starting at the Willet Green Miller Centre in Sudbury, head west toward Ramsey Lake Road. Turn left onto Ramsey Lake Road and continue west for 1.9 km. Use the left 2 lanes to turn left (south) on Paris Street and continue for 4 km. Use the right lane to take the Highway 17W ramp to Sault Ste. Marie. After 800 m, continue straight to merge onto Hwy 17. Drive west on Hwy 17 for 30 km. Turn right (north) onto Fairbank Lake/Totten Mine Road and continue for 1.1 km. Stop 1 will be on the right (east) side of the road. 6.4 km (~10 minutes) – Head north on Fairbank Lake Road toward Bay Street and drive for 1.4 km. Turn right (north) onto Crean Hill Road and continue for 2.1 km. Turn right at the fork to stay on Crean Hill Road and continue for 2.9 km. Stop 2 is past the gates and will be on the right (east) side of the road. Stop 1 Nipissing sill, Trap and Sudbury dikes UTM coordinates 0471368E 5136502N Exposed on the east side of Fairbank Lake/Totten Mine Road is a large outcrop of Nipissing gabbro intruded by mafic dikes of the Trap and Sudbury dike swarms (Figure 3). The Nipissing gabbro is part of a northwest-trending sill that is up to 300-m wide and 7-km long (Figure 3). The Nipissing gabbro is green-grey, massive and medium-grained. Along its northern margin, it is intruded by a Trap dike (Figure 5). The contact between dikes is sharp, strikes east-northeast and dips steeply southward. Stop 2 Crean Hill - Footwall Breccia UTM coordinates 0473051E 5141707N Just beyond the gates and on both sides of the access road are stripped outcrops that expose variably mineralized Crean Hill Footwall Breccia. The exposure is within 200 m of the verticallydipping SIC contact. The open pit of the pastproducing Crean Hill Mine is located just beyond the fence at the northern edge of the outcrops (Figure 7). 160 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 5. Nipissing gabbro crosscut by a northeast-trending Trap dike (Stop 1). Figure 6. Sudbury dike in contact with Nipissing gabbro and crosscut by northwest-trending brittle faults (Stop 1). 161 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 7. Partial view of the Crean Hill outcrops with the Crean Hill open pit in the background (Stop 2). Crean Hill mine, currently owned by Vale Canada Limited, operated sporadically from 1909 to 2000, producing 744,747 tons grading 2.14% Ni and 2.9% Cu to the end of 1916 (Card 1968). The main orebody consisted of sulphide breccia hosted in brecciated Huronian Supergroup volcanic rocks along the SIC contact (Coleman 1913; Knight 1917; Card 1968), with disseminated low-sulphide PGE mineralization occurring in the underlying brecciated footwall rocks. Monolithic breccias occur as irregular veins in dacite (felsic breccia) and basalt (mafic and quartzofeldspathic breccias). Partial melt patches make up 5–20 vol.% of the host rocks, and consist of quartz and plagioclase, with hornblende porphyroblasts. They terminate en biseau and cut across mafic breccia veins. Heterolithic dioritic breccias occur as pods (bilithic breccia), dikes (breccia dikes), and anastomosing veins (mixed breccia) within basalt (Figure 9). They formed as melts with contactparallel flow textures, which are defined by elongate wispy clasts that wrap around basalt clasts. Partial melt textures are not observed within the dioritic breccias, but narrow (<1 cm) quartzplagioclase leucosomes occur along their contact with the host basalt. The Crean Hill stripped outcrops expose brecciated basalt with minor dacite and quartzofeldspathic arenite of the Huronian Supergroup, which are crosscut by dikes and pods of Footwall Breccia (Figure 8). Four compositions of breccia are found in the outcrop: monolithic felsic, mafic, and quartzofeldspathic breccias, and heterolithic dioritic breccias (Généreux et al. 2021). 162 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 8. Geological map of the Crean Hill outcrops (from Généreux et al. 2021). UTM coordinates are in NAD 83, Zone 17 (Stop 2). 163 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Mineralized mixed breccia appears gossanous on outcrop, containing <5 vol.% of disseminated pyrrhotite, pentlandite, chalcopyrite, gersdorffite and precious metal minerals (PMM), which are commonly associated with epidote-quartz(-calcite) alteration patches. Shear zones appear to be the main control on the distribution of PGE (Gibson et al. 2010), with strongly foliated mixed breccia generally enriched in Au, Pt and Pd. 1.6 km (~3 minutes) – Head southwest on Crean Hill Road toward Fairbank Lake Road and drive for 1.4 km. Turn left (southeast) on the gravel road. Continue east on the gravel road for 200 m, keep left at the fork. Stop 3 is on the north side of the road. Figure 9. Mixed breccia unit crosscut by a heterolithic dioritic breccia dike (from Généreux et al. 2021) (Stop 2). Stop 3 Vermilion Mine – Vermilion Offset Dike, Sudbury Breccia and Stobie Formation A detailed study of the Crean Hill breccias by Généreux et al. (2021) showed that they have significantly lower SiO2 and higher TiO2 than the SIC, suggesting that the breccias likely did not form by injection of SIC melt into the fractured target rocks. Modeling of partial melt compositions during contact metamorphism showed that the dioritic breccia matrices are too mafic to have formed by anatexis during contact metamorphism. Instead, their composition mirrors that of their host rocks, thus they are best interpreted as locallyderived shock melts that formed during shock compression and which were trapped in the basement rocks during cooling of the SIC (Généreux et al. 2021). The breccias were subsequently modified by contact metamorphism (T ≥ 750°C) during cooling of the melt sheet, and by later syn-tectonic regional metamorphism at upper greenschist to amphibolite conditions. UTM coordinates 0472251 E 5140280 N Francis L. Sperry discovered sperrylite (PtAs2) at the Vermilion mine (Wells 1889), which is also the type locality for arsenohauchecornite (Ni18Bi3AsS16), michenerite (PdBiTe) and violarite (FeNi2S4). The mine operated from 1887 to 1916 and produced over 4000 tonnes of ore with exceptionally high grade of 6.64% Ni and 6.89% Cu, including 180 tonnes at 20-25% Cu-Ni, 125 g/t Ag, 125 g/t Pd, 46.9 g/t Pt and 10.3 g/t Au (Holloway et al. 1917). The stripped outcrop exposes the Vermilion Offset quartz diorite (Figure 10 and 11). This offset dike is not connected to the Main Mass of the SIC (Grant and Bite 1984), but instead forms a lens at the contact between Sudbury Breccia and mafic volcanic rocks of the Stobie Formation (Figure 10). At the northernmost part of the outcrop are slightly deformed basaltic volcanic rocks of the Stobie Formation, where bedded lapilli tuff is interlayered with vesicular basalt, pillow breccia and possible flow-top breccia (Figure 12). Volcanic textures such as hyaloclastite, amygdules, and lapilli are generally well-preserved but are locally overprinted by coarse acicular amphiboles. Low-sulphide PGE mineralization is hosted in anastomosing veins of ‘mixed’ dioritic breccia, which contain a medium-grained dioritic matrix intermingled with a fine-grained basaltic matrix. This mixed breccia is locally crosscut by irregularly shaped dioritic breccia dikes (Figure 9) and displays a strong mottled texture that is further complicated by the presence of leucosomes. 164 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 10. Geology of the Vermilion Offset (modified from Grant and Bite 1984). Detailed geology of the Vermilion Mine surface outcrop modified from Lightfoot et al. (1997) (Stop 3). 165 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 11. Overview of the stripped outcrop at the Vermilion mine (Stop 3). The central portion of the main outcrop exposes the Vermilion quartz diorite, which is generally medium-grained and contains up to 20% of small (<5 cm), partially digested, felsic and mafic clasts. At the top of the main outcrop is a 10-m wide section of finer-grained and foliated inclusionbearing quartz diorite (IQD), which contains 2040% of dacite, basalt and amphibolite clasts that are up to 10 m in size. IQD is in sheared contact with basaltic rocks to the north and Sudbury Breccia to the South. Figure 12. Basaltic pillow breccia with wellpreserved hyaloclastite (Stop 3). 166 �Proceedings of the 68th ILSG Annual Meeting - Part 2 1.5 km. Park at marked trail. Walk north on trail for 150 m. Trail leads to outcrops exposed along the powerline. Stop 4 Creighton Fault and Stobie Formation UTM coordinates 474656E 5140835N Along the powerline is an excellent exposure of ductile deformation observed within the deformation corridor bound by the Creighton and Victoria Faults. The Creighton Fault can be traced across most of Denison Township as a prominent, relatively continuous, topographic low with periodic outcrops. Along the powerline, a 20-m wide flow-banded dacite is in sheared contact with a relatively massive, locally amygdaloidal, basaltic flow. Both the intermediate and mafic volcanic rocks are part of the Stobie Formation. Figure 13. Matrix-supported Sudbury Breccia with elongated clasts (Stop 3). Sudbury Breccia is best exposed south and southwest of the main quartz diorite outcrop, closest to the parking area. It occurs as variably foliated, matrix-supported, heterolithic breccia (Figure 13) within brecciated mafic volcanic rocks. Ductile deformation is expressed in the intermediate unit as a strong east-northeasttrending foliation that is consistently oriented counter-clockwise to flow banding, suggesting apparent dextral shearing (Figure 14). An intersection lineation between the flow banding and the foliation steeply plunges to the southeast. The same kinematic indicators are observed in other shear zones in the area, including the Creighton-Victoria mylonite zone farther west (Généreux et al. 2017). The Vermilion ore occurred in shear-hosted sulphide veins and irregular lenses ranging from a few centimetres to 40 cm in diameter. The shear zones are found mainly in Sudbury Breccia and strike parallel to the quartz diorite–Sudbury Breccia contact (Szentpéteri et al. 2003). Disseminated sulphides and platinum-group minerals (PGM) also occur in quartz diorite and foliated Sudbury Breccia, and are associated with irregularly distributed epidote-albite-chlorite alteration patches that range from a few centimetres to several metres in size. The close spatial association of PGM with alteration minerals, their finely disseminated nature, the presence of sulphides-PGM in secondary hydrothermal veins, and the occurrence of sulphide veins within shear zones all suggest a complex multistage magmatic-hydrothermal and metamorphic-hydrothermal origin of the sulphidePGM assemblages (Szentpéteri et al. 2003). 4 km (15 minutes) – Return to Crean Hill Road (~200 m). Turn left (southwest) toward Fairbank Lake Road for 1.3 km. Turn right (northwest) on Fairbank East Road and continue north for approximately 1 km. Turn right (east) on the gravel road. Drive east for Figure 14. Strong pervasive foliation (S) in felsic volcanic rock consistently oriented counterclockwise to flow banding (S0), suggesting apparent dextral shearing (from Généreux et al. 2017) (Stop 4). 167 �Proceedings of the 68th ILSG Annual Meeting - Part 2 13 km (25 minutes) – Walk south 150 m along trail to return to the gravel road. Travel west for 1.5 km. Turn left (south) on Fairbank East Road and continue for 1.1 km. Continue south onto Crean Hill Road for 2.1 km. Turn right (west) onto Fairbank Lake Road. Fairbank Lake Road turns into Spanish River Road after 7 km, continue along Spanish River Road for another 4 km. Where Spanish River Road turns southward, continue straight (west) onto High Falls Road for 200 m. Stop 5 is at the bend along High Falls Road. The outcrops are exposed north of the road. Use the small gravel road adjacent to the outcrop for parking. Stop 5 Synvolcanic mafic sill and McKim Formation UTM coordinates 0460806E 5135978N On the north side of Spanish River Road, a large outcrop contains turbidites of the McKim Formation and a synvolcanic mafic sill (Figure 3). The McKim Formation consists of thickly laminated mudstone with siltstone layers, and contains porphyroblasts of chloritoid and staurolite (Figure 15A). Bedding is subvertical and trends northeast. There is a strongly developed beddingparallel foliation overprinted by regional eastnortheast-trending foliation (Figure 15B). Figure 15. A) Laminated mudstones of the McKim Formation with chloritoid and staurolite porphyroblasts. B) Bedding parallel foliation in thickly laminated mudstones of the McKim Formation overprinted by regional east-northeasttrending foliation, vertical face (Stop 5). North of the McKim Formation a 50-60 m wide mafic sill is exposed. The sill trends parallel to bedding and exhibits distinctly rounded and lobate, mafic enclaves that are enclosed in a felsic, micarich and garnet-bearing matrix (Figure 16). This texture has been interpreted as a peperite, formed as a result of a mafic sill intruding what were originally unconsolidated wet sediments of the McKim Formation. The mafic enclaves are geochemically similar to that of the mafic volcanic rocks of the Elsie Mountain Formation (Gordon et al. 2018; Gordon 2021, 2022). Figure 16. Peperite with fine-grained, amoeboid mafic lobes encompassed by a felsic matrix (Stop 5). 168 �Proceedings of the 68th ILSG Annual Meeting - Part 2 10 km (15 minutes) – Drive east on High Falls Road and continue straight onto Spanish River Road for 4 km, turn left (north) on Fairbank Lake Road, drive north for 5.5 km. Stop 6 is on the left (west) side of Fairbank Lake Road. Stop 6 Drury Township Intrusion and Matachewan dike swarm UTM coordinates 0464241E 5142157N On the west side of Fairbank Lake Road, outcrops of the Drury Township intrusion are exposed. The Drury Township intrusion consists of anorthositic gabbro that is medium- to coarsegrained, locally pegmatitic and vari-textured (Figure 17A). Mineralogy is characterized by greenschist grade assemblages. Plagioclase is almost entirely saussuritized and pyroxenes are replaced by amphibole and chlorite. On the east side of the road, the anorthositic gabbro varies from undeformed to mylonitized (Figure 17B). Where deformed, shear zones and foliation trend northeast and are parallel to the adjacent Chicago Fault (Figure 3). The anorthositic gabbro is also crosscut by a narrow, plagioclase-phyric mafic dike of the Matachewan dike swarm. The Matachewan dike is fine-grained, massive and metamorphosed to greenschist facies. Figure 17. A) Coarse-grained to pegmatitic anorthositic gabbro of the Drury Township intrusion. B) Mylonitized anorthositic gabbro of the Drury Township intrusion (Stop 6). 8 km (10 minutes) – Drive south on Fairbank Lake Road for 5.5 km. Turn left (east) to stay on Fairbank Lake Road and continue for 1.8 km. Turn left (north) on Kidd Copper Mine Road and drive north for 700 m. Stop 7a is on the left (west) side of Kidd Copper Mine Road. The Worthington Offset dike has been mined periodically since 1885 and is host to Sudbury’s most recently developed deposit, Totten Mine, with grades of 1.42% Ni, 1.9% Cu and 4.8 g/t PGM (Lightfoot 2017; Lightfoot and Farrow 2002). Three past producing mines can be found on the Aer-Kidd property (Figure 18). The first is the Howland Pit, which has been filled in and partially reclaimed. The second is the Robinson Mine (Stop 7b), which includes a shaft cap, a large, fenced hole, and small adit (Figure 19). The third pastproducing site is the Rosen and Gersdorffite mines (also known as Aer Mine), which are located northeast of the old mill site and contains a shaft cap. Stop 7 Aer Kidd Property - Worthington Offset Dike The beginning of Kidd Copper Mine Road is Vale Canada Limited property and is gated. The property changes ownership to SPC Nickel Corp. at the crest of the hill just before the Howland pit. The Worthington Offset dike crops out on the west side of the road and is intermittently exposed within the Aer-Kidd Property from the Howland pit to Perch Lake (Figure 18). 169 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 18. Geological map of the Aer-Kidd Property along the Worthington Offset dike. Past-producing mines are located within the dike where amphibole- and inclusion-bearing quartz diorite (AIQD) pods are observed. Stop 7a and 7b are located between the Howland Pit and Robinson Mine. UTM coordinates are in NAD 83, Zone 17 (Stop 7). 170 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 19. Geological map of the stripped outcrop between the Howland Pit and the Robinson Mine sites (Stop 7a). The Stop includes siltstones of the McKim Formation, as well as the quartz-diorite (QD) and inclusion-bearing quartz-diorite (IQD) phases of the Worthington Offset dike. UTM coordinates are in NAD 83, Zone 17. 171 �Proceedings of the 68th ILSG Annual Meeting - Part 2 There are many stripped and forested outcrops of the Worthington Offset dike over the length of the property. The two best surface exposures are: 1) the large, stripped outcrop on the left side of the road between the Robinson Mine and Howland Pit (Stop 7a) (Figure18), and 2) the Robinson Mine site (Stop 7b) (Figure 18, 19). foliated siltstone of the McKim Formation. QD is found farther up the hill, but it is much thinner than that at Stop 7a. Further north is the sharp contact between the QD and IQD. The mineralized matrix of the IQD at this site has a slightly higher grade than at Stop 7a. Massive sulphides are found further north, just below the fence. Stop 7a: Worthington Offset Dike and McKim Formation UTM coordinates 0466620E 5137925N On the west side of Kid Copper Mine Road, past the Howland Pit and core farm, is a large, stripped outcrop sloping north. This outcrop shows the phase separation of the dike well and is representative of the weakly mineralized to unmineralized portions of the Worthington Offset. The outcrop contains a small section of McKim Formation siltstone (near the road) in sharp contact with Worthington quartz diorite (QD) (Figure 19). The siltstone is thickly laminated and exhibits a moderate east-northeast-trending foliation. QD is medium-grained, massive, with local veins and jointing, and contains several rounded sedimentary enclaves adjacent to the contact with the siltstone (Figure 20A). Farther north, the QD phase is crosscut by inclusion-bearing quartz diorite (IQD). The IQD phase is heterolithic and contains inclusions of QD, amphibolite, basalt and siltstone that are enclosed in a massive, medium-grained quartz diorite matrix (Figure 20B). Most inclusions are subrounded and range from a centimeter to submeter in diameter. Sulphide blebs are visible throughout the matrix of the IQD. Figure 20. A) Sharp and linear contact between the Worthington Offset dike and the McKim Formation. Note the rounded sedimentary enclaves in QD near the contact. B) Heterolithic, massive IQD with sub-rounded inclusions derived from local host rocks. Note the pervasive sulphide burns throughout the quartz diorite matrix (Stop 7a). Stop 7b: Mineralization in the Worthington Offset Dike UTM coordinates 0466765E 5138015N 200 m (1 minute) – Continue northeast along Kidd Copper Mine Road for 200 m. Stop 7b is on the left (north) side of the road. Northward up the hill, toward the fenced hole and behind the shaft cap, is the Robinson Mine outcrop. The hill exposes a large portion of the Worthington Offset dike, but the best exposure is the stripped outcrop located south of the fence adjacent to the Robinson pit (Figure 21). The southern edge of the outcrop consists of weakly 172 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 21. Geological map of the stripped outcrop found on the Robinson Mine site at Stop 7b. The Stop includes siltstones of the McKim Formation, and quartz diorite (QD), inclusion-bearing quartz diorite (IQD) and amphibole- and inclusion-bearing quart diorite (AIQD) phases of the Worthington Offset dike. Massive sulphide mineralization is hosted within the AIQD phase present along the northern edge of the exposure. A sub-unit of IQD is found at this location, and locally is called amphibolite-bearing IQD (AIQD). The contact between IQD and AIQD is transitional, which is why the latter is considered a sub-unit of IQD rather than a distinct phase of the dike. AIQD contains almost exclusively large, rounded, amphibolite inclusions, with massive sulphides wrapping around them (Figure 22A). The amphibolite inclusions are dark green, massive, coarse-grained, and can range from a few centimetres to several metres in diameter. These inclusions are thought to have been derived from the nearby Nipissing sills. Similar inclusions have been reported at the Totten Mine, where they are referred to as “Sudbury Gabbros” (Lightfoot 2017). 173 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Massive to semi-massive sulphide mineralization is associated with AIQD and occurs solely within the quartz diorite matrix. The amphibolite inclusions themselves are not mineralized, thus they are quite dilutive to the mineralization of this sub-unit. This style of mineralization is representative of the mineralization observed in the Worthington Offset dike. Chalcopyrite, pyrrhotite, and pentlandite are the dominant sulphide minerals (Figure 22B), and gersdorffite and niccolite have been identified locally in drill core. Sulphide mineralization is not a necessary feature of AIQD, but the latter is always present where mineralization occurs. The distribution of mineralized AIQD is complex and varied. On this property, 4 modeled vertical shoots are known to contain significant sulphide mineralization (Howland, Robinson, Rosen, and Perch Lake). Elsewhere along the Worthington Offset, mineralization is structurally controlled, occurring in bends, folds, and boudins along the dike. A narrow diabase dike is also found at this stop. The dike crosscuts the Worthington Offset dike and has been tentatively assigned to the Trap dike swarm. It is massive, fine-grained, weakly magnetic and can be followed for several metres. Diabase dikes are commonly observed in drill core. Two large olivine diabase dikes of the Sudbury dike swarm also cut through the property. Shear zones are observed throughout the Aer Kidd property, including at this outcrop, where an east-northeast-trending shear zone displaced the offset dike by less than a metre. Such localized displacement is observed along several other shear zones observed on surface and in drill core, and has been reported on many historical mine maps. The orientation of the shear zone is similar to other easttrending shear zones in Denison and Drury townships, including the CVDZ (Figure 3), and likely formed during the same deformation event. Figure 22. A) Mineralized outcrop on the surface exposure of the Robinson Mine, showing AIQD with amphibolite inclusions. The QD matrix between the inclusions hosts massive to semimassive sulphide mineralization. B) Mineralized AIQD in drill core. Two massive sulphide stringers are hosted within quartz diorite matrix and wrap around rounded amphibolite inclusions. Note the smaller amphibolite inclusions the sulphide stringer vein. Dominant sulphide minerals are pyrrhotite and pentlandite, with lesser chalcopyrite (Stop 7b). A narrow diabase dike is also found at this stop. The dike crosscuts the Worthington Offset dike and has been tentatively assigned to the Trap dike swarm. It is massive, fine-grained, weakly magnetic and can be followed for several metres. Diabase dikes are commonly observed in drill core. Two large olivine diabase dikes of the Sudbury dike swarm also cut through the property. 174 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Acknowledgments References The authors would like to thank R-L. Simard for her collaboration and contributions to the Southwest Sudbury Structure bedrock mapping project. R.M. Easton is thanked for his ongoing guidance and assistance on this project, and M. Duguet, S. Evers, J.E. Chartrand, S.J. McIlraith, and P. Gervais for all the helpful discussions, as well as assistance with databases, software logistics, sample preparation and drafting. B. Lafrance and D.K. Tinkham from Laurentian University are thanked for their ongoing advice and helpful discussions in building the geological interpretation. Special thanks are extended to Vale Canada Limited, Lonmin PLC, Wallbridge Mining Company Limited, SPC Nickel Corp., KGHM International, Sudbury Integrated Nickel Operations (a Glencore Company) and private landowners for providing access to their properties, sharing information and knowledge, and for all additional support and assistance that has been provided. Last but not least, this project would not have been possible without our hard-working field assistants, J. Ménard, J. Enright, L. Lebeau, S. Robillard, C. Stone, E. Campbell, G. Broughton, B. Clarke, J. Creppin, M. Jacques and J. AndersonButcher. Ames, D.E. and Farrow, C.E.G. 2007. Metallogeny of the Sudbury mining camp, Ontario; in Mineral Deposits of Canada: A Synthesis of Major DepositTypes, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods, Geological Association of Canada, Mineral Deposits Division, p.329-350. Ames, D.E., Bleeker, W., Heather, K.B. and Wodicka, N. 1997. Timmins to Sudbury transect: new insights into the regional geology and setting of mineral deposits; Geological Association of Canada– Mineral Association of Canada, Joint Annual Meeting, Ottawa, Field Trip Guidebook B6, 133p. Ames, D.E., Davidson, A., Buckle, J.L. and Card, K.D. 2005. Geology, Sudbury bedrock compilation, Ontario; Geological Survey of Canada, Open File 4570, scale 1:50 000. Ames, D.E., Golightly, J.P., Lightfoot, P.C. and Gibson, H.L. 2002. Vitric compositions of the Onaping Formation and their relationship to the Sudbury Igneous Complex, Sudbury Structure; Economic Geology, v.97, p.1541-1562. 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The Birch Lake Batholith, Ontario; American Journal of Science, series 5, v.17, no.101, p.403-424. 180 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Field Trip 4 – Overview of the Sudbury Structure Sudbury District RGP Office Resident Geologist Program, Ontario Geological Survey, Sudbury, Ontario P3E 6B5 Introduction Geology of the Sudbury Area – Brief Overview This field trip is based on the “Sudbury Structure Field Trips” that have been given for over 20 years by the Sudbury District Office of the Ontario Geological Survey, Resident Geologist Program. There are, therefore, numerous contributors, not all of which have been recorded over the years. The Rousell and Brown (2009) Field Guide is also heavily leaned upon. Formation of the Sudbury Structure It is generally accepted that the Sudbury Structure and its associated abundant metal endowment were the result of a collision of a large meteorite (or comet; e.g. Petrus et al. 2015) with the Earth approximately 1850 million-years-ago (Krogh et al. 1984; Davis 2008). The impact happened near the contact of the Archean Superior and the Paleoproterozoic Southern provinces. Greater detail on the Sudbury Structure is presented in the introductory section of Field Trip 1 at this meeting “A Traverse Across the Sudbury Impact Structure” (Bleeker et al. 2022). Furthermore, Lightfoot (2017) gives a very indepth treatment of the Sudbury Igneous Complex and its associated nickel deposits. The entire meteorite, as well as a tremendous mass of earth’s crust, was volatilized on impact. A complex crater (Figure 1) developed and was filled with a ‘melt sheet’ of molten crustal material that segregated and crystallized to become the Sudbury Igneous Complex (SIC); which was then overlain by fragmental “fall-back” material and younger, more typical, sedimentary rocks that constitute the Whitewater Group. This one-day field trip will provide an overview of the Sudbury Structure, one of the most prolific nickel camps in the world and the remnant of one of the world’s largest impact craters. The sites visited provide a cross-section of the Sudbury Structure, including the footwall rocks, the Sudbury Igneous Complex and the crater-fill sedimentary rocks of the Whitewater Group. At the time of the impact the Penokean Orogeny (1870-1835 Ma) was underway. It is posited that the impact occurred in the foreland marine basin at the leading edge of the orogen. The Penokean Orogeny continued after the collision and resulted in modification to the impact crater. The subsequent nearly 2 billion years of tectonic and erosional history also changed the shape and size of the Sudbury Structure, resulting in its current configuration. In the footwall rocks, the evidence for the impact origin of the structure include the Sudbury Breccia (pseudotachylite) and shatter cones. The nickel deposits formed at the base of the Sudbury Igneous Complex, and in the associated quartz-diorite “Offset” dikes. The first of the crater-fill units, the Sandcherry Member of the Onaping Formation, is a fallback breccia from the impact. The subsequent crater-fill units, the Onwatin and Chelmsford Formations, will also be visited. Geology of the Sudbury Structure After the impact, the rocks and structure generated by the impact were subjected to varying degrees and orientations of deformation. The Sudbury Structure, therefore, has been divided into three distinct “ranges” in order to distinguish sections with different footwall rock affinities and The Stops are not in stratigraphic order, but in the order that provides the best safety for the participants (e.g., avoidance of crossing major highways whenever possible). 181 �Proceedings of the 68th ILSG Annual Meeting - Part 2 different degrees of structural over-print. Figure 2 outlines the approximate location of the “North Range”, “South Range” and “East Range”. The East Range is distinguished primarily on its distinct and complicated structural overprint. SIC Footwall Rock Types Sudbury Breccia (Stops 3 and 8) Sudbury Breccia resulted from the shattering of the rocks. that were at or near the surface of the Earth. due to the shock from the impact of the meteorite. This network of in-situ breccias is relatively abundant in zones that occur as far as 15 km from the Sudbury Structure contact, whereas more isolated occurrences have been identified up to 80 km from the exposed footwall contact. Breccia contacts are typically sharp and have a range of configurations from straight and regular through complex, anastomosing, riverine patterns. Sudbury Breccia dikes preferentially conform to pre-Sudbury Event structures and lithological contacts. Breccia Zones range from millimetres to hundreds of metres wide and may be traced for tens of kilometres (e.g., the South Range Breccia Belt). A stratigraphic section (Figure 3A) summarizes the rocks and features that developed as a result of the impact. These features are found in the country “Footwall” rocks as well as in the SIC and the Whitewater Group. A second cross-section (Figure 3B) summarizes the various ore forming environments within the entire Sudbury Structure. In the vicinity of the Sudbury Structure, the Superior Province rocks comprise the Levack Gneiss Complex; a suite of 2711 to 2642 millionyear old metamorphosed and intimately intercallated supracrustal and felsic and mafic intrusive rocks (Krogh et al. 1984; Wodicka and Card 1995). The gneisses are cut by the metagranitoid intrusive rocks of the Cartier batholith (2642 Ma; Meldrum et al. 1997). The Southern Province rocks are characterized by shallow marine and terrestrial metavolcanic and metasedimentary rocks of the Huronian Supergroup (2460 to 2300 Ma; see Trip 5, this volume; Easton and Bennett 2022) and associated mafic-ultramafic intrusions associated with the East Bull Lake intrusive suite (circa 2480 Ma; James et al. 2002) and the Nipissing gabbro suite (circa 2217 Ma; Davey et al. 2019). The character of the distinct Superior versus Southern province rocks is reflected in the Sudbury Structure rocks in proximity with either subprovince (i.e. the nature of Sudbury Breccia in the North Range (Superior Province) is distinct from that of the South Range (Southern Province)). Sudbury Breccia is typically a clast-supported breccia with a pseudotachylitic (glassy) to finely comminuted matrix that has been variably recrystallized. In proximity with the SIC, the matrix displays partial melt in pods and encased lithic fragments. Clasts are generally locally derived, with some exotic material, and are commonly equant with sub-rounded to sub-angular shapes. Economic mineralization can occur in proximity to, and associated, with Sudbury Breccia. The Frood–Stobie and Broken Hammer mines are examples of deposits hosted in, or associated, with Sudbury Breccia. Footwall Breccia (Stop 5: Access requires permission from City of Greater Sudbury) Footwall Breccia is an economically significant unit that hosts deposits such as at the Levack Mine (Figure 4). This style of mineralization is most prevalent in the North Range. Footwall Breccia is not uniformly distributed around the Sudbury Structure contact environment and is absent at the contact in many places. Unmineralized Footwall Breccia is present at Stop 5. Outside the Sudbury Structure proper (in what is referred to as the “Footwall” environment), rock types that resulted from the Sudbury Event include the Sudbury Breccia, Footwall Breccia and Offset Dikes (or Quartz Diorite - QD). Rocks within the Sudbury Structure include the Sudbury Igneous Complex (SIC) and the overlying Whitewater Group supracrustal rocks. 182 �Proceedings of the 68th ILSG Annual Meeting - Part 2 This enigmatic rock type occurs as “Megabreccia” and discontinuous sheets at the SIC contact, as well as part of some Offset Dikes (i.e. Whistle and Foy), and as intrusions into Felsic Norite. It is thickest in “embayments” (interpreted slump features along the over-steepened crater wall) where they provided suitable sites for sulphide accumulation (Figure 4A). The SIC, from bottom to top (or from Footwall contact to the base of the Whitewater Group), includes the Contact Sublayer/Offset Sublayer and the Main Mass (Norite, Quartz Gabbro and Granophyre) units. Contacts between the units are gradational and subject to interpretation. The character of the units in different regions within the Sudbury Structure can be distinct. Footwall Breccia is a heterolithic breccia with angular to sub-rounded fragments of varied sizes. Fragments are mostly locally derived and may include gabbro, diabase, mafic gneiss, and Huronian Supergroup sandstones – dependent upon the rock types of the footwall environment. Fragments of Sudbury Breccia have been identified in this unit, constraining the relative age of these two breccia types. The matrix is what makes this such an enigmatic lithology; it is crystalline rather than fragmental in nature. The matrix is subigneous at the SIC contact and has a metamorphic character with increased distance from the SIC. High temperatures at the SIC contact melted the breccia matrix resulting in the sub-igneous texture (Lakomy 1990). With increased distance from the contact, the temperature gradient resulted in a transition from an igneous-textured breccia matrix to a metamorphic-textured breccia matrix (Fedorowich et al. 2009). Contact Sublayer/Offset Sublayer (Stops 1, 5 and 9) Sublayer rock types are inclusion-bearing, igneous-textured, gabbronoritic or quartz dioritic bodies of varied compositions. The two main Sublayer environments are contact and offset dike. Both environments have potential to host economic Ni-Cu±PGE (Platinum Group Element) mineralization. The Discovery Outcrop (Stop 1) is an example of mineralized Contact Sublayer, and the recently re-opened Totten Mine is an example of economic mineralization in the Offset Sublayer environment. In the contact environment, rocks of the Sublayer are discontinuous lenses or sheets at the contact between the “Main Mass” of the SIC and the Footwall rocks. The presence and thickness of the Contact Sublayer appears to be controlled by the three-dimensional topography of the Footwall contact, with the Sublayer preferentially occurring in troughs (embayments and terraces) at the contact. The Contact Sublayer is a varied mixture of igneous silicate matrix, inclusions of silicate rock material, and magmatic Cu-Ni-Fe sulphides. Fragments include • footwall country rocks that can be identified and correlated with those directly observed in the Footwall, • xenoliths related to the SIC (similar to the basal Norite unit) and • exotic inclusions (anorthosite to dunite) with no known affiliation or source. Sudbury Igneous Complex (SIC) The Sudbury Igneous Complex (SIC) represents the crystalline rocks generated from the impact melt sheet. The current theory suggests that the melt sheet was created on the floor of the evolving impact crater within moments of the impact of the meteorite. This dynamic fluid evolved with the crater. As the crater stabilized the melt sheet differentiated and eventually solidified to form the SIC. The fall-back material (the Onaping Formation of the Whitewater Group) was originally ejected from the crater as it was excavating itself, but then was deposited on top of the melt sheet and was likely partially consumed and incorporated into it. In the Offset environment (Stop 9), the Sublayer is commonly referred to as Quartz Diorite or QD. This type of Sublayer occurs in thin, dike-like intrusions into the Footwall that occur in three morphologies 183 �Proceedings of the 68th ILSG Annual Meeting - Part 2 • • • predominate in this unit and intact crystalline plagioclase laths and prisms are present. radial offsets that extend out nearly orthogonally from the SIC, concentric offsets that are oriented subparallel to but outboard of the SIC, and discontinuous or breccia-hosted offsets. Whitewater Group (Stops 2, 6 and 7) The Whitewater Group encompasses an accumulation of nearly 3 km of supracrustal clastic and chemical “sedimentary” basin-fill rocks. The lowermost Onaping Formation is an atypical clastic rock of contentious origin; however, it is overlain by more recognisably sedimentary units that include the Vermilion, Onwatin and Chelmsford Formations. The Whitewater Group has been recognized only within the confines of the Sudbury Structure. However, it is likely that related clastic sediments (most particularly of the Onaping Formation) were deposited well outside the structure, but were either • not lithified and were dispersed and/or redeposited in other forms or • were eroded away or • have not yet been recognized or • a combination of the above scenarios. Most of the offset dikes are composite intrusions consisting of a central core of inclusion-bearing (commonly sulphide-rich) quartz diorite flanked by relatively inclusion- and sulphide-poor quartz diorite. Main Mass (Stop 4) The Main Mass of the SIC is a gradational series of igneous rocks that can be interpreted as having crystallized from a single differentiating magma (Pattison 2009; Lightfoot et al. 1997). At the base of the Main Mass, the Norite unit is a massive, medium- to coarse-grained, cumulate textured, two-pyroxene gabbronorite with varied amounts of quartz (up to 15%) and numerous accessory minerals. In the South Range, the unit is black as a result of ilmenite growth in plagioclase grains. In the North Range, the plagioclase grains do not typically contain ilmenite in their crystal structure and the rock colour is generally grey. “Mafic Norite” can locally be distinguished at the base of the Main Mass as modally more orthopyroxenerich and quartz-poor than the rest of the Norite, and geochemically by a sharp increase of magnesium. Onaping Formation (Stop 6) Originally described as a tuff, this contentious formation is a thick (1.4 km) accumulation of heterolithic breccias with igneous textures. The base of the formation is intruded by Granophyre of the underlying SIC and the top grades up into the carbonate and mudstone rocks of the overlying Vermilion and Onwatin Formations. The intermediate unit of the SIC Main Mass is a greenish-grey, two-pyroxene Quartz Gabbro. Contacts between the Quartz Gabbro and the flanking Norite and Granophyre are gradational. Quartz contents range from 15-60%. Passing stratigraphically up through the Quartz Gabbro there is an increase in the abundance of granophyric-textured quartzofeldspathic material. At Stop 4 this can be observed as an increase in the pink component of the rock relative to the green component. Recent detailed work by Ames et al. (2009) has led to the interpretation that the Onaping Formation formed in a dynamic and changing environment with an internal stratigraphy that captures the early evolution of the impact crater. “The formation represents a succession of glassrich breccias and coeval hypabyssal intrusions that have been hydrothermally altered to a variable degree.” (Ames et al. 2009) The most compelling evidence that distinguishes the rocks of the Onaping Formation from the volcaniclastic rocks that they texturally resemble is the presence of The upper and most felsic of the SIC Main Mass units is the Granophyre (formerly called micropegmatite). Granophyric intergrowth of quartz-plagioclase-potassium feldspar modally 184 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 1. Complex crater development after a meteorite impact (from Stoffler et al. 1980 in Taylor 1982). Figure 2. Sudbury Structure showing the surrounding geologic provinces and the 3 “ranges” of the SIC. Note the Grenville Province only formed in the area after the impact event (from Rousell and Card 2009). 185 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 3. Schematic sections illustrating the A) stratigraphic and B) ore deposit environments associated with the Sudbury Structure. Note the difference in scale and the absence of the uppermost stratigraphy in B. SIC = Sudbury Igneous Complex, OF = Onaping Formation. Figure from Ames et al. (2008). 186 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 4. A) An idealized representation of the embayment structures and their relationship to slump terraces of the Sudbury Structure. B) A section through the Levack No. 4 ore body (after Morrison 1984). Onwatin Formation (Stop 7) The Onwatin Formation consists of massive to laminated, carbonaceous and sulphidic argillite and siltstone with minor greywacke (Ames et al. 2009). This formation conformably overlies the Vermilion Formation. Its thickness has been interpreted as between 600 m and 1410 m. The formation is interpreted to have developed in a restricted, anoxic basin. shock metamorphosed quartz in the lithic fragments (French 1967; Peredery 1972). This is further supported by the presence of shock induced diamonds, and fullerenes (Becker et al. 1994; Masaitis et al. 1999), shatter cones in lithic fragments (Peredery 1972) and an iridium anomaly (Mungall et al. 2004). At Stop 6, we encounter the chaotic fragmental rocks of the Sandcherry Member of the Onaping Formation. “Bombs” with lithic cores and welldeveloped, banded glass rims are distinctive features of these outcrops. Chelmsford Formation (Stop 2) The extensively exposed Chelmsford Formation is the uppermost preserved unit of the Whitewater Group and occupies the elliptical core of the Sudbury Structure. This unit comprises turbiditic greywackes with a minor argillic component (Ames et al. 2009). The contact between the Chelmsford Formation and the underlying Onwatin Formation is gradational. Well-defined sedimentary structures including channels, flute casts, ripples, convolute laminations and iron-carbonate concretions can be observed in outcrops of the Chelmsford Formation. Vermilion Formation (no stop on this trip) The Vermilion Formation is a carbonaceous argillite unit that was subjected to significant syndepositional hydrothermal alteration. This alteration resulted in economic accumulations of zinc, lead, copper and silver mineralization (Errington and Vermilion mines). This unit, traceable around the Sudbury Structure in drillcore, is only exposed at surface in the southwest part of the basin in the vicinity of the pastproducing Errington Mine. The average thickness of the unit is 13.5 m (Stoness 1994). 187 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 5. MAP OF FIELD TRIP STOPS (geology from Ames et al. 2005) . Red stars indicate active mines A Copper Cliff North Mine B Creighton Mine C Totten Mine D McReedy West Mine E Fraser Mine F Coleman (Lower Coleman) Mine G Nickel Rim South Mine N Garson Mine 188 �Proceedings of the 68th ILSG Annual Meeting - Part 2 FIELD TRIP DETAILS Geological Maps Geological compilation maps covering all or parts of the area of the field trip include Ames et al. (2005), Dressler (1984b), and Card and Lumbers (1977). ROAD LOG Note: Caution should be taken when parking vehicles on the shoulder of roads and highways and when examining outcrops located along major roads along the field trip route. UTM co-ordinates are given in NAD 83 datum, zone 17; latitude and longitude are also provided in decimal degrees). 10.1 km — There are several routes to get to the Discovery Site from Science North. The “easiest” is: Turn left (west) out of Science North onto Ramsey Lake Road. Turn right (north) onto Paris Street (650m). Turn left onto Elm Street (2.75km) and continue on Elm Street/Regional Road 35 (6.7km) to turn off for Discovery Site on right (NE). Trail to site. Stop 1. Sudbury Discovery Site: A) Gossaneous outcrop of Sublayer along strike from original Discovery Outcrop. B) Close-up of stringer sulphides. Ruler marked in inches on the top, centimeters on the bottom. Stop 1 – Discovery Site UTM co-ordinates 495917E, 5151952N latitude-longitude 46.5211243N, 81.0532319W Protected site: NO HAMMERS Caution: Outcrop is adjacent to the active transcontinental line of the Canadian Pacific Railway (CPR). This discovery led to one of Canada’s most active staking-rushes. In just over 100 years the Sudbury Structure has generated tremendous wealth and produced huge masses of critically important raw materials for Ontario’s and the world’s manufacturing. Nickel-copper mineralization was first identified in the Sudbury area by Alexander Murray of the Geological Survey of Canada in 1856. However, it wasn’t until 1883 that the economic significance of the area was appreciated by the public. At this time construction of the Canadian Pacific Railway exposed rich coppernickel mineralization at a site close to the spot commemorated here. In the 1970s the actual discovery outcrop was mined and is now occupied by the water-filled Murray Pit, a few hundred metres from here. At this site, in line with the original discovery outcrop, gabbro-peridotite inclusion-bearing Contact Sublayer is exposed. Mineralization, though perhaps not as rich, is similar to that found in 1883. The Clarabelle No. 2 open-pit and the head frame of the inactive Murray Mine are visible from the Stop. 189 �Proceedings of the 68th ILSG Annual Meeting - Part 2 16.1 km — Northeast on Regional Road 35. Turnoff on right (north). Outcrop 90m west of parking area on north side of highway. Caution: Turn-off is a driveway. Stop 2 – Chelmsford Formation, Whitewater Group UTM co-ordinates 481688E, 5157337N latitude-longitude 46.5693493N, 81.2389532W Well-defined Bouma sequences can be identified in these outcrops. These turbidite beds display excellent sedimentary structures indicating tops, including fining upward sequences, crossbedding, flame structures and rip-ups. Distinctive, large, elongate “concretions” overprint bedding and can be traced within beds over many metres along the outcrop face. A well-developed, steeply dipping cleavage associated with the post-Sudbury event South Range Shear Zone (SRSZ) cuts bedding and appears to stretch the concretions into upright oval cross-sections. 23.5 km — Highway 144. Turn-off on right (NE). Outcrop 50m southeast of parking area on northeast side of the highway. Caution: Outcrops adjacent to Highway 144. Stop 3 – Levack Gneiss Complex with Matachewan dike and Pseudotachylite (Sudbury Breccia) Stop 2. A) Bedded Chelmsford Formation with flame structures. B) ‘Concretions’ (outlined in dashed red line) in Chelmsford Formation. UTM co-ordinates 464642E, 5164239N latitude-longitude 46.6307803N, 81.4619083W This location is part of what is considered the North Range footwall. It is outside the Sudbury Structure. Compositionally heterogeneous, complex gneiss of the Archean Levack gneiss complex is cut by pegmatitic dikes associated with the Archean Cartier Granite and a Paleoproterozoic Matachewan diabase dike. All these rocks are cut by narrow, impact-generated Sudbury Breccia veinlets. These outcrops display a complexity that is inherent in the footwall of the Sudbury Structure. Stop 3. Sudbury Breccia hosted in Levack gneiss. Ruler marked in inches on the top, centimeters on the bottom. 190 �Proceedings of the 68th ILSG Annual Meeting - Part 2 4.8 km — Return south on Highway 144 (4.7km). Turn left on onto Regional Road 8 (Onaping/Levack). Turn-off 160m from intersection on right. Outcrop 30m south of parking area; trail to outcrop. Stop 4 – Quartz Gabbro and Granophyre UTM co-ordinates 468456E, 5162737N latitude-longitude 46.6174539N, 81.4119820W This outcrop shows the gradational contact between the Quartz Gabbro and the Granophyre units of the Sudbury Igneous Complex. Rousell and Brown (2009) describe the granophyre on the southern part of the outcrop as being “approximately three parts micrographic intergrowth (potassium -feldspar and quartz) to one part tabular, plagioclase phenocrysts”. The contact between the granophyre and the quartz-gabbro to the north is described as “a gradual change in the micrographic intergrowth to plagioclase ratio. The contact is arbitrarily placed where the modal plagioclase exceeds that of intergrowth”. 1.9 km — North on Regional Road 8 (450m). Turn left on onto Onaping Drive (1.3km). Turn right onto unnamed road to gate for rehabilitated Onaping Landfill (150m). Outcrop is 250m north of parking area; walk on road to outcrop. Stop 4. A) Sudbury Igneous Complex, quartz gabbro with pink felspars. B) micropegmatite (granophyre) dike. of partial melt pods. Differences are best seen from a distance. Stop 5 – SIC Contact: Levack gneiss, Sublayer and Footwall Breccia (access requires permission from the City of Greater Sudbury) The outcrop on the west exemplifies barren Contact Sublayer. The exposure at the Discovery Site would look like this if it were not strongly mineralized. This rare, nearly barren outcrop of Sublayer shows fragments of norite > ultramafic >> felsic gneiss in an igneous crystalline matrix. UTM co-ordinates 467459E, 5163351N latitude-longitude 46.5892202N, 81.4298083W This stop is at the west end of the highly productive Levack–Onaping embayment structure that hosts the currently operating Fraser, Coleman, and McCreedy West Cu-Ni-PGE mines (Figure 5). Two large outcrop faces expose Contact Sublayer rocks (western exposure) and Footwall Breccia rocks (eastern exposure). The differences between them are subtle and are primarily based on the fragment composition, and the presence or absence The outcrop on the east has been interpreted as unmineralized to weakly mineralized Footwall Breccia. Here, unlike the Sublayer outcrop, the fragments are all Levack gneiss. The Footwall Breccia is locally cut by zones of partial melting characterized by acicular amphibole crystals (that may originally have been pyroxenes) in a pink matrix. 191 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Stop 6 – Onaping Formation, Whitewater Group UTM co-ordinates 470724E, 5159607N latitude-longitude 46.583893N, 81.3821632W Please watch for highway traffic at this site! Caution: Stop requires walking across Highway 144. Traffic can be very heavy and fast-moving. This is the A.Y. Jackson Lookout, a scenic spot to enjoy High Falls. The outcrops to be examined are on the far side of Highway 144 and with the curves in the road and the speed of the traffic it is a dangerous crossing. Stop 5. A) Levack Gneiss Complex. B) Sublayer (dark norite breccia). Stop 6. Breccia of the Onaping Formation, Sandcherry Member. A) General aspect. B) Fragment within a streamlined glass rim. 8.0 km — Return along unnamed road to Onaping Drive (150m). Turn left. Turn right (south) onto Regional Road 8 (1.3km). Turn left (south) on onto Highway 144 (500m). Turn left into A.Y. Jackson Lookout (6.2km). Outcrop is along west side of highway; 230m north of turn-off; 160m from parking area. The Onaping Formation is the lowermost formation of the Whitewater Group. These rocks belong to the Sandcherry Member, a chaotic fragmental basin fill created from the fall-back of ejecta from the impact. Originally described as volcanic in origin, a few features identified in this 192 �Proceedings of the 68th ILSG Annual Meeting - Part 2 unit clearly distinguishes these rocks from the tuffs they closely resemble. The most compelling evidence is geochemical or microscopic: presence of an iridium anomaly (Mungall et al. 2004), presence of “impact diamonds” (Masaitis et al. 1999), and fragments containing shock metamorphosed quartz (French 1967; Peredery 1972). The planar deformation features in the quartz have only ever been identified in rocks affected by an impact and rocks affected by an atomic blast. Look for lithic cored “bombs” and fragments with glassy rims. These rocks do look very much like volcaniclastic material. Stop 7. Onwatin Formation slate. Bedding is near horizontal (white dashed line); cleavage is vertical. 31.4 km — East on Highway 144 to Highway 144S (Lively/Highway 17; 11.7km). Turn right to stay on Highway 144S. Turn left onto Regional Road 24 (Lively; 13.6km). Turn right onto Anderson Drive (5.9km). Turn left into Tom Davies Community Centre (150m). NOTE: the entrance to the community centre is after the building (one-way). Outcrop is on the east side of the building next to the highway. 5.4 km — Turn right (south) out of A.Y. Jackson Lookout onto Highway 144. Continue 5.4 km to stop on right (south). Park on the highway shoulder. Stop 7 – Onwatin Formation, Whitewater Group UTM co-ordinates 475298E, 5159222N latitude-longitude 46.5861084N, 81.3224356W The Onwatin Formation is approximately 600 m thick, comprising carbonaceous and pyritic argillite and minor wacke. The formation is thought to have been deposited in a deep restricted basin with stagnant and anoxic bottom waters. Estimates of the range of total carbon content of the Onwatin Formation vary, and the carbonaceous material may have originated as floating algal mats (Rousell 1984; Arengi 1977). Coleman (1905) estimated a range of 6.8 to 10% carbon in the Onwatin slate, whereas Arengi (1977) calculated 0.26 to 4.05% free carbon in the Onwatin Formation. Arengi (1977) concluded that the carbon occurs as elongated segmented structures, either as individuals or in clots, which resemble modern and fossil algal and fungal filaments. Stop 8 – Pseudotachylite: Sudbury Breccia UTM co-ordinates 488791E, 5141810N latitude-longitude 46.4297697N, 81.1458923W The glaciated outcrop shows fragment-rich pseudotachylite that is part of the South Range Breccia Belt. Fragments consist of lower Huronian Supergroup metasedimentary and metavolcanic rocks, which form the host rocks to the pseudotachylite dike. More specifically, the fragments consist of mafic metavolcanic rocks of the Elsie Mountain Formation, metarhyolite of the Copper Cliff Formation, and metapelite and metaquartzite of the McKim Formation. Fragments may show reaction rims and incipient marginal fragmentation. This suggests that the matrix was a melt. 193 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Stop 9 – Copper Cliff Offset: Copper Cliff No. 1 Mine site UTM co-ordinates 494819E, 5146946N latitude-longitude 46.476050N, 81.0674912W Stop 8. Polymictic Sudbury Pseudotachylite. (GPS is 15 x 6.5 cm). The Copper Cliff Offset is one of the Sudbury Offset Dikes (also referred to as Quartz Diorite Dikes or QD) that host much of the economic copper-nickel-PGM mineralization in the Sudbury Camp. The offset dikes are part of the noritic sublayer, and the Copper Cliff Offset merges with the Main Mass norite in a funnel-shaped embayment to the north. This segment of the offset is about 8 km in length. At this site, the dike lies along the contact between the supracrustal rocks of the Elsie Mountain and Stobie Formations to the east and the Creighton Granite to the west (Cochrane 1984). The dike is cored by the subunit “IQD” or Inclusion Quartz Diorite. This subunit is the host of economic mineralization in the offset environment. Breccia– 10.0 km — Turn right onto Main Street from Anderson Drive (130m). Turn left onto Old Highway 17 (Regional Road 55 to Sudbury; 1.3km). Turn left onto Power Street (7.2km). Turn left onto Godfrey Drive (950m). Parking for stop on right (400m). Stop 85m east along powerline. The Copper Cliff No. 1 Mine was the first underground mine in Sudbury. Production began in 1886. The Copper Cliff North Mine continues production on the same Offset. Of other historical interest, the park on the other side of Godfrey Street was a roast bed. The area has been re-greened. 9.5 km — Return south on Godfrey Drive and turn left onto Balsam Street (130m). Turn left onto Regional Road 55 (1.3km). Continue on Regional Road 55, keeping left at cloverleaf for Big Nickel Drive (Regional Road 34). Regional Road 55 becomes Lorne Street after Big Nickel Drive cloverleaf. Turn left onto Martindale Road (3.6km). Left turn to remain on Martindale (450m). Slight left onto Walford Road at Regent Street intersection (1.4km). Turn left onto Paris Street (750m). Turn right onto Ramsey Lake Road (700m), Stop on right (1.5km). Outcrop begins 50m back (SW) from parking spot along footpath. Stop 9. Copper Cliff No. 1 Mine site. 194 �Proceedings of the 68th ILSG Annual Meeting - Part 2 workers, however, have noted that at other locations around the SIC, shatter cone orientations are in opposing directions (Dressler 1984a). The shatter cones, together with planar features in quartz, are believed to be evidence of a meteorite impact origin for the Sudbury Structure (e.g., Dietz 1964, 1972; French 1972). End of road log. References Ames, D.E., Davidson, A., Buckle, J. and Card, K., 2005. Geology, Sudbury bedrock compilation, Ontario; Geological Survey of Canada, Open file 4570, scale 1:50 000. Stop 10. Shattercones. Pen is 13 cm long. Stop 10– Shatter Cones Ames, D.E., Davidson, A. and Wodicka, N., 2008. Geology of the giant Sudbury polymetallic mining camp, Ontario, Canada; Economic. Geology, v.103, p.1057-1077. UTM co-ordinates 501535E, 5146083N latitude-longitude 46.4683165N, 80.9800069W Protected site: NO HAMMERS This outcrop has some of the best preserved and abundant shatter cones found in the Sudbury area. The host rock here is quartzite of the Mississagi Formation. The shatter cones appear as conical striated features whose surfaces are often micaceous and shiny. They range in length from a few centimetres to about a metre. Large cones may have numerous small cones along their flanks. Cones are exposed only where they control the outcrop surface. On other surfaces intersecting crescent-shaped fractures give the characteristic shattered appearance to the rock. They are best seen here when obliquely illuminated by the late afternoon sun. Ames, D.E., Stoness, J.A. and Rousell, D.H. 2009. Whitewater Group; in A Field Guide to the Geology of Sudbury, Ontario; Ontario Geological Survey, Open File Report 6243, p.37-44. Arengi, J.T. 1977. Sedimentary evolution of the Sudbury Basin; unpublished MSc thesis, University of Toronto, Toronto, Ontario, 141p. Becker, L., Bada, J.L., Winans, R.E. Hunt, J.E., Bunch, T.E. and French, B.M. 1994. Fullerenes in the 1.85billion-year-old Sudbury Impact Structure; Science, v.265 p.642-645 (Erratum v.265 p.1644) Bleeker, W. Kamo, S.L., Henning, S. and Lesher, M. 2022. A traverse across the Sudbury Impact Structure; in 68th Institute on Lake Superior Geology, Proceedings, v.68, pt.2, Guidebook, Field Trip 1. The distinctive fractures, termed shatter cones, form by passage of shock waves through rock, and are found at many astroblemes and “cryptoexplosion” structures. They have also been reported from localities with no known explosive associations. Geological mapping has shown that the SIC is surrounded by a belt of rocks more than 16 km wide containing shatter cones. In some locations, if the rocks are returned to their hypothetical orientation during the Sudbury Event, the apices of the shatter cones appear to point inward toward the basin (French 1972). Other Card, K.D. and Lumbers, S.B. 1977. Sudbury-Cobalt; Ontario Geological Survey, Map 2361, scale 1:253 440. Cochrane, L.B. 1984. Ore Deposits of the Copper Cliff Offset; in The Geology and Ore Deposits of the Sudbury Structure, Ontario Geological Survey, Special Volume 1, p 97-136. Coleman, A.P. 1905. The Sudbury Nickel Region; Report of the Ontario Bureau of Mines, v.14, pt.3, 183p. 195 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Canadian Shield; Economic Geology, v.97, p.15771606. Davey, S., Bleeker, W., Kamo, S.L., Davis, D.W, Easton, M.R. and Sutcliffe, R.H. 2019. Ni-Cu-PGE potential of the Nipissing sills as part of the ca. 2.2 Ga Ungava large igneous province; in Targeted Geoscience Initiative: 2018 report of activities; Geological Survey of Canada, Open File 8549, p.403-419. Krogh, T.E., Davis, D.W. and Corfu, F. 1984. Precise U-Pb Zircon and Baddleyite ages for the Sudbury areal in The geology and ore deposits of the Sudbury Structure, Ontario Geological Survey Special Volume 1; p. 431-446. Davis, D.W. 2008. Sub-million-year age resolution of Precambrian igneous events by thermal extraction– thermal ionization mass spectrometer Pb dating of zircon: Application to crystallization of the Sudbury impact melt sheet; Geology, v.36, p.383-386. Lakomy, R. 1990. Implications for cratering mechanics from a study of the Footwall Breccia of the Sudbury impact structure, Canada; Meteorics, v.25, p 95-207. Lightfoot, P.C. 2017. Nickel sulfide ores and impact melts: Origin of the Sudbury Igneous Complex; Elsevier Inc., 662p. Dietz, R.S. 1964. Sudbury Structure as an astrobleme; Journal of Geology, v.72, p.412-434. Lightfoot, P.C., Doherty, W., Farrell, K., Keays, R.R., Moore, M. and Pekeski, D. 1997. Geochemistry of the Main Mass, Sublayer, Offsets, and Inclusions from the Sudbury Igneous Complex, Ontario; Ontario Geological Survey, Open File Report 5959, 231p. ——— 1972. Sudbury Astrobleme, splash emplaced sublayer and possible cosmogenic ores; in Geological Association of Canada, Special Paper 10, p.754-756. Dressler, B.O. 1984a. The effects of the Sudbury Event and the Intrusion of the Sudbury Igneous Complex on the Footwall Rocks of the Sudbury Structure; in The Geology and Ore Deposits of the Sudbury Structure, Ontario Geological Survey, Special Volume 1, p 97-136. Masaitis, V.L.; Shafranovsky, G.I.; Grieve, R.A.F.; Langenhorst, F.; Peredery, W.V.; Therriault, A. M.; Balmasov, E.L.; Fedorova, I.G.; Dressler, B.O. and Sharpton, V.L. 1999. Impact diamonds in the suevitic breccias of the Black Member of the Onaping Formation, Sudbury Structure, Ontario, Canada; in Large meteorite impacts and planetary evolution; II, Geological Society of America, Special Paper 339, p. 317-320. ——— 1984b. Sudbury geological compilation; Ontario Geological Survey, Map 2491, scale 1:50 000. Easton, R.M. and Bennett, G. 2022. A cross-section through the Huronian Supergroup at Elliot Lake, Ontario; in 68th Institute on Lake Superior Geology, Proceedings, v.68, pt.2, Guidebook, Field Trip 5, 57p. Meldrum, A., Abdel-Rahman, A.F., Martin, R.F. and Wodicka, N. 1997. The nature, age and petrogenesis of the Cartier Batholith, northern flank of the Sudbury Structure, Ontario; Canada; Precambrian Research, v.82, p.265–285. Fedorowich, J.S., Golightly, J.P. and Rousell, D.H. 2009. Breccias in the Footwall; in A Field Guide to the Geology of Sudbury, Ontario; Ontario Geological Survey, Open File Report 6243, p.45-55. Morrison, G.G. 1984. Morphological Features of the Sudbury Structure in Relation to an Impact Origin; in The Geology and Ore Deposits of the Sudbury Structure, Ontario Geological Survey, Special Volume 1; p. 513-520. French, B.M. 1967. Sudbury structure, Ontario: some petrographic evidence for origin by meteorite impact; Science, v.156, p.1094–1098. Mungall, J.E.; Ames, D.E. and Hanley, J.J. 2004. Geochemical evidence from the Sudbury Structure for crustal redistribution by large bolide impacts; Nature, v.429, p.546-548. ——— 1972. Shock-metamorphic features in the Sudbury Structure, Ontario: a review; in Geological Association of Canada, Special Paper 10, p.19-28. Pattison, E.F. 2009. Sudbury Igneous Complex; in A Field Guide to the Geology of Sudbury, Ontario; Ontario Geological Survey, Open File Report 6243, p.56-74. James, R.S., Easton, R.M., Peck, D.C. and Hrominchuk, J.L. 2002. The East Bull Lake intrusive suite: remnants of a ~2.48 Ga large igneous and metallogenic province in the Sudbury area of the 196 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Stöffler D., Gault D.E. and Reimold W.U. 1980. Cratering experiments in non-cohesive and weakly cohesive sand: Excavation mode and ejecta characteristics (abstract); in Papers Presented to the Conference on Multi-ring Basins: Formation and Evolution Lunar and Planetary Institute; p.89-91. Peredery, W.V. 1972. Chemistry of fluidal gases and melt bodies in the Onaping Formation, in New Developments in Sudbury Geology, Geological Association of Canada, Special Paper 10, p.49-59. Petrus, J.A., Ames, D.A. and Kamber, B.S. 2015. On the track of the elusive Sudbury impact: geochemical evidence for a chondrite or comet bolide; Terra Nova, v.27, p.9-20. Stoness, J.A. 1994. The stratigraphy, geochemistry and depositional environment of the Paleoproterozoic Vermilion and Onwatin formations, and their relationship to the Zn-Cu-Pb massive sulphide deposits in the Sudbury Basin; unpublished MSc thesis, Laurentian University, Sudbury, Ontario, 205p. Rousell, D.H. 1984. Onwatin and Chelmsford formations; in The Geology and Ore Deposits of the Sudbury Structure, Ontario Geological Survey, Special Volume 1, p.211-218. Rousell, D.H. and Brown, G.H., editors. 2009. A Field Guide to the Geology of Sudbury, Ontario; Ontario Geological Survey, Open File Report 6243, 200p. Taylor, S.R. 1982. Planetary Science: A Lunar Perspective. Lunar and Planetary Institute. 508p. Wodicka, N. and Card, K.D. 1995. Late Archean history of the Levack gneiss complex, southern Superior Province, Sudbury, Ontario: New evidence from UPb geochronology; in Precambrian ’95, Program with Abstracts, p.191. Rousell, D.H. and Card, K.D. 2009. Geological Setting; in A Field Guide to the Geology of Sudbury, Ontario; Ontario Geological Survey, Open File Report 6243, p.1-6. 197 �Proceedings of the 68th ILSG Annual Meeting - Part 2 198 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Field Trip 5 – An Overview of the Huronian Supergroup in the Elliot Lake area R.M. Easton Earth Resources and Geoscience Mapping Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5 with contributions by G. Bennett Retired, formerly Resident Geologist, Sault Ste. Marie, Ontario Geological Survey Introduction The Huronian Supergroup is one of the Earth’s most studied sequences of rocks. Since the turn of the century the results of hundreds of studies of Huronian rocks have been published in scientific journals and government publications. These studies have led geoscientists, to present evidence for the Earth’s earliest glacial periods, the development of free oxygen in the atmosphere of the early Earth, the deposition of paleoplacer deposits of uranium, and evidence for plate tectonic activity during the Paleoproterozoic. Much of the evidence is based on rock exposures which will be visited during this field trip. The field trip uses road accessible outcrops. All of the road stops can be accessed using a 2-wheel drive vehicle. Unless otherwise stated, all UTM coordinates are in Zone 17, datum NAD 83, which is essentially equivalent to NAD WGS84. Safety Many of the field trip stops are located on highways that are especially busy during the summer season. Care should always be exercised when parking, exiting vehicles, and crossing the roads. Use of safety vests and/or bright clothing is recommended, in order to improve your visibility to motorists. Proterozoic rocks of the Canadian Shield in the Sudbury to Elliot Lake area are assigned to either the Paleoproterozoic Southern Province or the Mesoproterozoic Grenville Province (cf. Card et al. 1972; Wynne-Edwards 1972). The Southern Province in Ontario comprises Paleoproterozoic metasedimentary and metavolcanic rocks of the Huronian Supergroup and gabbroic intrusions of the Nipissing gabbro suite. Also included in the Southern Province are the Sudbury Igneous Complex (SIC), the Whitewater Group; plutonic and minor volcanic rocks of the Killarney Magmatic Belt; and rocks of the Sudbury mafic dike swarm (see Figure 2; Bennett et al. 1991). Table 1 summarizes the major geological events affecting the Superior, Southern and Grenville provinces in the Sault Ste. Marie to Sudbury area. Most of the trip routes are on Crown land or public roadways, but access is on or near private property in some cases. As in all such situations, please respect the property rights of others, so as to maintain good relationships, so that future access for geologists is not adversely affected. Purpose The transect through the Huronian Supergroup at Elliot Lake used by this field trip provides an opportunity to examine nearly all of the major units of the supergroup in a single day. These first pages are intended to give participants new to the Huronian Supergroup of Ontario a summary of what we think we know of these ancient rocks. Much of the material is borrowed from prior ILSG guidebooks (Bennett 2006; Bennett et al. 1997), but with updating and the inclusion of additional stops by R.M. Easton. The Huronian Supergroup (Robertson et al. 1969a; see Figure 3) is a sequence of variably metamorphosed Paleoproterozoic sedimentary and minor volcanic rocks that lie unconformably upon 199 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Archean rocks of the Superior Province. The Huronian rocks extend eastward from Lake Superior, along the north shore of Lake Huron to Sudbury, and then northward to the Noranda area of Quebec; a distance of approximately 450 km (Figures 1, 2). dominated phase at 2724.9±1.4 Ma (Optional Stop C) and a younger, calc-alkaline phase at 2686.5±1.1 Ma (Easton 2010, 2013a). Volcanism had ceased by 2674.8±0.8 Ma, the emplacement age of a granodiorite intrusion into the greenstone belt (Stop 1). The Huronian Supergroup attains its greatest thickness of 12,000 metres southeast of Sudbury (Debicki 1990). The sequence thins northward due to the wedging out of basal units, the thinning of the siliciclastic units, and erosion within the sequence (Roscoe 1969; Frarey and Roscoe 1970). The ages of both the greenstone belts and the Ramsey-Algoma granitoid complex are predominantly in the range of 2695 to 2650 Ma, in contrast to ages from the Abitibi greenstone belt which are typically in the range of 2740 to 2690 Ma. This distinction is important, as it allows for discrimination between potential source regions for the Huronian Supergroup strata, as described in detail in the section on “What Detrital Zircon studies tell us about the Source Region for the Huronian Supergroup”. The circa 2310 Ma U/Pb age from zircons in tuffaceous layers in the Bar River Formation (Hill et al. 2018; Rasmussen et al. 2013) places an upper age limit on the age of deposition of the bulk of Huronian Supergroup, with all sedimentation being completed well before emplacement of the Nipissing gabbroic intrusions at circa 2217 Ma (Davey et al. 2019; Corfu and Andrews 1986; Noble and Lightfoot 1992). The age of the rhyolites of the Copper Cliff Formation (circa 2450 Ma; Bleeker et al. 2015; Ketchum et al. 2013) is probably close to the start of initial deposition of the Huronian Supergroup. Huronian Magmatism Introduction Four distinct, more-or-less coeval (2480 to 2460 Ma), post-Kenoran igneous rock sequences are associated with the Huronian Supergroup:  Mafic dikes in the basement rocks but which do not cut the Huronian Supergroup (Matachewan and Hearst dike swarms). The Archean Basement  Igneous complexes, typically layered, of gabbro, gabbronorite and anorthosite (East Bull Lake intrusive suite). The basement to the Huronian Supergroup consists predominantly of rocks of the Ramsey– Algoma granitoid complex (Card 1979), which includes several large felsic batholiths, including the Cartier (2642 Ma: Meldrum et al. 1997) and Birch Lake (2651 Ma: Kamo 2006) granite batholiths. The batholiths were emplaced into a slightly older, granodiorite and quartz diorite intrusive and gneissic complex, which have ages of 2700 to 2675 Ma (Easton 2013a; Prevec 1993; Ontario Geological Survey, unpublished data).  Mafic to felsic volcanic flows (Elliot Lake Group).  Felsic plutons in the Southern and Grenville Provinces in the Sudbury area (2475 to 2460 Ma). Basement Dikes The granitoid rocks of the Ramsey-Algoma granitoid complex are intruded by mafic dikes of the Matachewan–Hearst swarm, which was likely emplaced in 2 main pulses, the first, earlier pulse at circa 2480 Ma is believed to have been coincident with emplacement of the East Bull Lake intrusive suite of layered intrusions (Krogh, et al. 1984; James et al. 2002a; Bleeker et al. 2015; Heaman 1997). The second and “main pulse” of the Matachewan–Hearst dike swarm occurred at circa 2460 Ma (Bleeker et al. 2015). These intrusive phases are younger than the few greenstone belts present in the Sudbury to Elliot Lake area; the largest of which is the Whiskey Lake greenstone belt, located south and southeast of Elliot Lake. Limited geochronology from the Whiskey Lake greenstone belt indicates 2 main periods of volcanism, an earlier, tholeiitic mafic 200 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 1. Sketch map showing the regional setting of the Sudbury Igneous Complex and the Huronian Supergroup (modified from Young et al. 2001). Layered Gabbro, Gabbronorite and Anorthosite Complexes et al. 2002a, 2002b; Easton et al. 2010). The three largest bodies contain platinum-group element mineralization near their basal contacts (cf. Easton et al. 2010). At the base of the Huronian Supergroup in the Elliot Lake, Agnew Lake and Sudbury areas are several layered gabbro to anorthosite intrusions referred to as the East Bull Lake intrusive suite (Peck et al. 1995; James et al. 2002a, 2002b). These bodies have ages of circa 2475 Ma (Clough and Hamilton 2017; Krogh et al. 1984) and appear to be slightly older than the rocks of the Elliot Lake Group. U/Pb chemical-abraded thermal-ionizationmass-spectrometry (CA-TIMS) zircon ages from the River Valley, Agnew and East Bull Lake intrusions cluster at 2475 Ma (Clough and Hamilton 2017; Easton et al. 2010). Although similar in age to the older phase of the Matachewan dike swarm, there are numerous occurrences of Matachewan dikes cutting intrusive rocks of the East Bull Lake intrusive suite (cf. Easton 2003, 2009; Easton et al. 2010) indicating that emplacement of all these mafic rocks was coeval. All the East Bull Lake intrusive suite bodies found to date have been emplaced into the Archean basement at, or just below, the Archean–Huronian Supergroup boundary. Intrusions of the East Bull Lake intrusive suite are characterized by the presence of anorthositic phases, and locally by a well-developed, primary rhythmic layering of alternating anorthositic and gabbroic layers (cf. James et al. 2002a, 2002b; Easton et al. 2010). Major intrusions of the East Bull Lake intrusive suite occur at Agnew Lake and East Bull Lake, with the largest body, the River Valley intrusion being found in the Grenville Province adjacent to the Grenville Front (cf. James 201 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 2. A time-rock chart for the southeast Lake Superior region (from Bennett 2006). 202 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 3. A stratigraphic column for the Elliot Lake fieldtrip transect (from Bennett 2006). 203 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Table 1. Timing of major geological events and summary of age constraints on the main rock units present in the Sudbury to Elliot Lake area. Event and/or Map Unit Age Constraint (Ma) Comment and/or Source Grenville dike swarm 586±4 Pegmatite vein emplacement 989±2 1000 to 990 Corfu and Easton (2000) 1040 to 1030 Carr et al. (2000) Age of peak metamorphism in the hangingwall of the Grenville Front tectonic zone Age of peak Grenvillian metamorphism in the Central Gneiss Belt Sudbury mafic dike swarm Killarney magmatic belt second-stage magmatism, coincident with magmatism in the Eastern Granite Rhyolite Province and in the Central Gneiss Belt Regional albitization metasomatic event Killarney magmatic belt volcanism and high-level plutonism Northwest-trending regional faults Penokean orogeny (folding and metamorphism of Huronian Supergroup rocks) Impact event and formation of Sudbury breccia Penokean arc formation and magmatism Thrust faulting F2 folding F1 folding Emplacement of Nipissing gabbro sills Huronian Supergroup sedimentation 1238±4 1471±3 Kamo, Krogh and Kumarapeli (1995) Corfu and Easton (2000) emplaced in or along northwest-trending faults in the Southern Province, deformed and metamorphosed in the Grenville Province. Krogh et al. (1987). van Breemen and Davidson (1988) U/Pb monazite, Schandl, Gorton and Davis (1994); fluid focussed along northwest faults 1740, 1747±3, 1749±12 van Breemen and Davidson (1988); Sullivan and Davidson (1993); Davidson and van Breemen (1994) Pre-1700, post-1850 Faults cut Sudbury Structure 1775±10 Peak deformation. Zi et al. (2022) ~1835 Peak metamorphism. Holm et al. (2001) 1701±4 1850±1 1890-1860, 1845-1830 post-F2 pre-regional faulting post-2200, pre-1700, pre 1850? pre-2200 2217±4 >2220 but <2460 Huronian Supergroup felsic volcanism and related plutonic rocks, including the Matachewan dike swarm ~2477 to 2375 (2450±25, 2460±20, 2477±9, 2415±5 Emplacement of East Bull Lake intrusive suite rocks 2475±2 Emplacement of orthopyroxene hornblendite bodies (East Bull Lake suite) 2468±5 Emplacement of alkali feldspar granite and megacrystic granodiorite near River Valley High-grade Archean metamorphism and migmatization Emplacement ages of Archean units in the Sudbury area 2660 to 2665 2647±4 2711±7 to 2642±1 204 Krogh, Davis and Corfu (1984); Davis (2008) Zi et al. (2022) Sudbury breccia localized along these faults, suggesting they are pre-Sudbury Structure Pre-regional faulting, Nipissing sills axial planar to folds Nipissing sills folded of intruded along folds Davey et al. (2019); Corfu and Andrews (1986); Noble and Lightfoot (1992) Youngest detrital grains in Bar River Fm are 2306 Ma (Hill, Davis and Cochran 2018) Krogh, Davis and Corfu (1984), Heaman (1997); Corfu and Easton (2000), Krogh, Kamo and Bohor (1996), Smith (2002); Bleeker et al. (2015) Heaman (geochronologist, University of Alberta, personal communication, 1999); Clough and Hamilton (2017) Corfu and Easton (2000) Bodies intrude Crerar and Pardo gneiss, Easton (2003) Krogh, Davis and Corfu (1984); Wodicka and Card (1995); Ames et al. (2005) Krogh, Davis and Corfu (1984); Wodicka and Card (1995); Chen, Krogh and Lumbers (1995); Meldrum et al. (1997); Ames et al. (2005) �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 4. The cyclicity of Huronian Supergroup rocks (from Bennett 2006). Figure 5. Paleocurrent directions in the Matinenda and the Mississagi Formations (from Bennett 2006). 205 �Proceedings of the 68th ILSG Annual Meeting - Part 2 The Huronian Supergroup Marie area and between 110-300 m thick in the Thessalon area. It consists of 2 distinctive rock types: an upper, well-sorted, grey sandstone and a clast-supported polymictic conglomerate (Bennett et al. 1991). (Figures 3, 6, 7). The Livingstone Creek Formation has not been recognized east of the Quirke Lake Syncline (Bennett 2006). Introduction The Huronian Supergroup is subdivided into 4 groups (Robertson et al. 1969a, 1969b), which in ascending stratigraphic order are: the Elliot Lake Group, Hough Lake Group, Quirke Lake Group and Cobalt Group (Figure 3). Formations of the 3 upper groups, with the exception of the Serpent Formation of the Quirke Lake Group, show regional stratigraphic continuity, and display a remarkable cyclicity of lithological units (Figure 4). Each cycle begins with matrix-supported conglomerate (diamictite), followed by mudstone, siltstone and/or limestone, and capped by a thick sequence of crossbedded, coarse sandstone (Bennett et al. 1991). Paleocurrent studies (cf. Long 1976, 1978; McDowell 1957) have shown flow to the south to southeast, with southeast being the predominant direction (Figure 5). In most areas, clast-supported, polymictic conglomerate is predominant in the lower sections of the Livingstone Creek Formation. Cobble- to boulder-sized clasts, generally of grey granitic rocks and minor mafic plutonic and metamorphic rocks, are set in a sparse matrix of grey coarse arkose or arkosic grit. Bennett (2006) notes that he had not observed clasts of Huronian Supergroup volcanic rocks in these conglomerates. Locally, thin units of crossbedded, grey arkose are interbedded with the conglomerate (Frarey 1977, Bennett et al. 1991). The granitic mega-clasts of the conglomerate member are predominately pale grey in contrast with the predominantly reddish hues of the underlying Archean basement rocks. Some of the granitic megaclasts in the predominately grey conglomerate near the south end of Pine Ridge Road near Thessalon show the distinct texture of the typical Archean, massive, pink, potassium feldspar-megacrystic granite – but with only a hint of the pink color in the phenocrysts. The grey colour of the Livingstone Creek Formation conglomerates appears to be due to the reduction of ferric iron in the feldspars of the granitic clasts, and not a result of differing provenance as some have suggested. This conclusion is supported by Bennett’s (2006) observation that granitic rocks in a “paleosol zone” a few metres to a few tens of metres below the base of the Livingstone Creek Formation commonly are grey as well. Conglomerate units (e.g., Ramsey Lake, Bruce and Gowganda formations) in each of the cycles have been interpreted as being glaciogenic in origin, likely deposited in a marine environment adjacent to an ice shelf. The siltstone and sandstone units are interpreted to represent deposition during warmer intraglacial or post-glacial periods in either fluvial or marine environments (cf. Junnila and Young 1995; Fralick and Miall 1989). The Elliot Lake Group The Elliot Lake Group differs from the overlying Huronian groups in that:  its internal stratigraphy is generally discontinuous and less extensive.  it does not have the diamictite-mudstone-sandstone sequence of the overlying groups.  it contains the only important uranium deposits and the only volcanic rocks of the Huronian Supergroup. The grey, sandstone member of the Livingstone Creek Formation can be distinguished from most Huronian Supergroup sandstones by its uniform grain size (fine- to medium-sand). In addition, mudstone and pebbly units are lacking in the sandstone member of the Livingstone Creek  st formations have disconformable surfaces. The Livingstone Creek Formation Conglomerates and sandstones of the Livingstone Creek Formation (Frarey 1967, 1977) form the lowermost Huronian Supergroup unit The formation is at least 400 m thick in the Sault Ste. 206 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Formation, and carbonate occurs along the foreset beds of the well-developed trough crossbeds. The Huronian Supergroup Volcanic Rocks of the Sault Ste Marie-Elliot Lake area In addition to the well-known exposures in the Thessalon and Sault Ste Marie areas, other possible occurrences of the Livingstone Creek Formation include: Frarey (1967) named the Huronian volcanic rocks in the Thessalon and Sault Ste. Marie areas that overlie the Livingstone Creek Formation as the “Thessalon Formation” (Figures 3, 6).  grey sandstone and conglomerate near Crazy Lake in Nicholas Township (Bennett 1978; Bottrill 1971). Bennett (2006), based on his examination of all known exposures of Huronian Supergroup volcanic rocks as well as all available drill-core and drill-hole logs reporting volcanic rocks in the Elliot Lake–Sault Ste Marie area, concluded that there is no credible evidence for more than one period of Huronian Supergroup volcanism in the Elliot Lake–Sault Ste. Marie area and that all the Huronian Supergroup volcanic rocks west of the nose of the Quirke Lake Syncline are stratigraphically correlative with the Thessalon Formation (Figure 6) (Bennett 1978, 2006; Bennett et al. 1991).  a basal grey sandstone unit directly underlying the Matinenda Formation in Haughton Township (Bennett et al. 1991).  an area of clast-supported, grey granite-cobble conglomerate near Samried Lake (Jackson 2001). The clast size, local source and low stratigraphic position of the Livingstone Creek Formation conglomerates are consistent with deposition as an alluvial fan(s). The uniform, fine- to mediumgrained sand of the trough cross-bedded sandstone member suggests a different, although likely related, more distal depositional environment than that of the conglomerate. The sandstone member may represent deposition by median streams flowing in a fault-bounded valley with walls of Archean rocks partly covered by alluvial fans (Bennett et al. 1991). The well-sorted nature of the sandstone suggests an aeolian component or even aeolian deposition as proposed by Meyer (1983). Unfortunately, none of the many attempts to obtain an absolute age determination from rocks of the Thessalon Formation have been successful. Nonetheless, there is no reason to think that the age of the Thessalon Formation differs significantly from that of the Copper Cliff Formation (circa 2460 Ma). The maximum thickness of the Thessalon Formation in the Sault Ste. Marie area is approximately 650 to 820 m (Frarey 1977). Diamond drilling has indicated at least 670 m of Thessalon Formation volcanic rock under Lake Huron south of the town of Thessalon, and the formation may be up to 1080 m thick north of Bass Lake in Aberdeen Township (Bennett 2006). The Huronian Supergroup Volcanic Rocks – Overview As noted by Easton (2013a), the transition between dominantly subaerial and dominantly submarine deposition of metavolcanic rocks of the Huronian Supergroup occurs in the Elliot Lake area, and this change in depositional environment may have been significant with respect to sedimentary depositional environments in the Elliot Lake area itself. Thus, it may be no coincidence that Huronian Supergroup mafic metavolcanic rocks are found in proximity to all the past-producing uranium mines and current prospects in the Elliot Lake area. In the Sault Ste. Marie, Thessalon and Aberdeen Lake areas, the Thessalon Formation can be subdivided into an upper tholeiitic basalt unit and a lower complex or “mixed member” (Bennett et al. 1991), which includes fractionated rocks, including basaltic andesite, tholeiitic andesite, mugearite, hawaiite and rhyolite. Magnesium-rich basalts with some of the chemical characteristics of komatiites are present in the Dollyberry Lake, Pecors Lake and Thessalon 207 �Proceedings of the 68th ILSG Annual Meeting - Part 2 areas. The lower flow sequences show much lower concentrations of Ni, Cr, and contain higher amounts of Ti and P than do the upper flows (cf. Ketchum et al. 2013). In the Dollyberry Lake area, the upper basalts appear to be missing, possibly due to erosion (Bennett 2006). In most areas the metamorphic grade of the Thessalon Formation flows is lower greenschist facies, although the presence of albite and primary pyroxene, along with elevated sodium contents, indicates sub-greenschist facies at the northern end of the Duncan volcanic belt near Sault Ste. Marie (Bennett 2006). A comprehensive analysis of the geochemistry of the Thessalon Formation volcanic rocks between Sault Ste. Marie and Thessalon, concluded that the lavas are divisible into 7 distinct units based on mapping, petrography and major and trace element geochemistry (Figure 8) (Tomlinson 1996; Ketchum et al. 2013). The 7 units were grouped into 2 “lava series”. The upper lava series (unit 6) is equivalent to the upper tholeiitic basalt sequence of Bennett et al. (1991). The lower lava series (units 1-5, of Tomlinson 1996 and Ketchum et al. 2013) consists mainly of basaltic andesite with subordinate, local rhyolite, mugearite, andesite and high magnesium basalt flows; and corresponds to the “diverse member”of Bennett et al. (1991). Amygdules of epidote, chlorite, calcite, quartz and stilpnomelane in complex zonal arrangements are common. Flattened chlorite-filled amygdules a centimetre or less across are a distinctive feature of most mafic flows of the Thessalon Formation. Pillow structures are rare but are present in most areas. Scoriaceous flow-tops and crosscutting breccias are commonly filled with a fine-grained mixture of quartz and grey to red secondary albite (Bennett 2006). With regard to the geochemistry and tectonic setting of the Thessalon Formation, Tomlinson (1996) stated “that the source of the lavas was metasomatized upper mantle rather than a deep mantle or plume component. Structural subsidence patterns in the Archean basement (Zolnai et al. 1984) are thought to be responsible for lithospheric stretching, in-turn causing mantle upwelling, episodic partial melting and volcanism.” Bennett et al. (1991) proposed that the upper, basaltic flows of the Thessalon Formation (upper lava series) probably represent part of a continental flood basalt sequence, whereas the diverse member (lower lava series) appears to have erupted from central vents. The volcanic rocks of the Quirke Lake Syncline display lithological and geochemical similarities to the lower lava series of the Thessalon Formation west of the Quirke Lake Syncline (Bennett 2006). Syndepositional features present where sedimentary rocks of the Livingstone Creek Formation infill fractures in the underlying Archean basement indicate that initially volcanism was a consequence of rifting. In active rifts, a single uplift and melting event occurs as a plume impacts the lithosphere, but in passive rifts uplift and melting are episodic. In addition, the presence of multiple erosional surfaces in the Elliot Lake Group indicate that many episodes of uplift occurred (Bennett 2006). Therefore, the Huronian rifting event can best be characterized as a typical passive rifting event (Tomlinson 1996). This is consistent with Jolly’s (1987) conclusion that the Thessalon Formation is a continental flood basalt sequence related to continental rifting. The upper, tholeiitic basalt flows of the Thessalon Formation (upper lava series of Tomlinson 1996; Ketchum et al. 2013) are almost uniformly greenish-grey fine- to medium-grained tholeiitic metabasalt. The essential minerals are albite, actinolite, chlorite, clinozoisite, epidote and Fe-Ti oxide. Primary clinopyroxene is present in only a few samples of basalt from the Sault Ste. Marie area. The andesitic rocks of the lower lava series are typically darker and contain stilpnomelane and biotite with green pleochroism (Fe+3 rich?) and albite and actinolite. Quartz is a minor component of the basaltic and andesitic types (Bennett 2006). 208 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 7. Stratigraphic Relationships in the Elliot Lake Group (from Bennett 2006). Figure 8. Internal stratigraphy of the Thessalon Formation, Elliot Lake Group, in the Sault Ste. Marie to Elliot Lake area (from Bennett 2006, modified from Tomlinson 1996)). 209 �Proceedings of the 68th ILSG Annual Meeting - Part 2 higher Ni and lower V contents compared to EMF1. On a Nb/Yb versus Th/Yb diagram (Pearce element diagram), the EMF-2 mafic rocks define a distinct trend separate to that of the EMF-1 and Stobie Formation mafic volcanic rocks. Comparing data from Gordon (2021) with that of Ketchum et al. (2013) on volcanic rocks in the Thessalon area (~200 km to the west), the mafic rocks of EMF-1 and the Stobie Formation are comparable to a Thessalon upper basalt-basaltic andesite unit. The rocks of EMF-2 closely resemble a stratigraphically lower basalt-andesite Thessalon unit. Based on the geochemical similarity with the Thessalon volcanic rocks, similar magmatic processes were likely responsible for the generation of the mafic volcanic rocks of the Thessalon, Elsie Mountain and Stobie Formations. Huronian Supergroup volcanic rocks of the Sudbury Area The volcanic rocks of the Sudbury area differ in terms of internal stratigraphy, overall thickness, and depositional environment from the Huronian Supergoup volcanic rocks in the Sault Ste. Marie– Elliot Lake area. The Huronian Supergroup volcanic sequence in the Sudbury area has been subdivided into the predominately mafic Elsie Mountain (1000 m thick) and Stobie Formations (1500 m thick), and the felsic Copper Cliff Formation (760 m thick). The depositional age of the Copper Cliff Formation is circa 2460 Ma (Beeker et al. 2015; Ketchum et al. 2013; Krogh et al. 1984), and it is likely that the Creighton Granite was coeval with the Copper Cliff Formation (Beeker et al. 2015). The volcanic rocks in the Sudbury area show evidence of submarine eruption from fault-controlled vents along the edge of a depositional basin into which arkosic sandstones were transported from the Archean granitic terrain to the north, with turbidites being deposited from the basin margins (Card 1978a). Sedimentary rocks associated with the Thessalon Formation Some early reports referred to the presence of quartz-pebble conglomerate in the Livingstone Creek Formation, however, this could not be confirmed by Bennett (2006). At many locations, however, a thin unit (< 1 m) of radioactive, pyritic, quartz-pebble conglomerate overlain by a few metres of coarse arkose sand was found to lie upon the Archean basement, or directly atop the Livingstone Creek Formation, where the latter is present. In Duncan Township in the Sault Ste. Marie–Thessalon area, this quartz-pebble conglomerate-arkose sequence occurs in the lower flows of the Thessalon Formation. (Hay 1963; Bennett et al. 1978; Meyer 1983) (Figures 6, 7). Based on recent mapping in the Sudbury area, a preliminary geochemical characterization of the mafic volcanic rocks of the Elsie Mountain and Stobie Formations has been presented (Gordon (2021). Mafic volcanic rocks of the Elsie Mountain Formation are divided into EMF-1 and EMF-2, which are geochemically distinct from each other. Mafic rocks of EMF-1, along with those in the Stobie Formation, are high-Fe tholeiitic basalts. EMF-1 samples in the Elsie Mountain Formation represent the basal lavas, but they are not the most primitive lavas as they have lower MgO, Ni and Cr and higher SiO2 compared mafic lavas in the overlying Stobie Formation. Mafic rocks of EMF1 and the Stobie Formation exhibit similar primitive mantle-normalized trace element profiles, characterized by LREE enrichment relative to HREE and negative Nb-Ta-Ti anomalies. EMF-2 mafic volcanic rocks are tholeiitic andesites with distinct primitive mantlenormalized REE profiles characterized by strongly depleted HREE. EMF-2 mafic rocks also have Bennett et al. (1991) proposed that the conglomerate-arkose units indicate the presence of a disconformity between the volcanic rocks of the Thessalon Formation and the Livingstone Creek Formation. The wide distribution of these units (Figure 6) suggests that they reflect an early erosional period of regional extent. The resistant nature of the mineral assemblage in the conglomerate (assuming an oxygen deficient atmosphere) points to a period of extreme weathering. Some of this quartz-rich regolith may still be visible as a quartz breccia atop the granitic basement west of Highway 639 (Optional Stop G). 210 �Proceedings of the 68th ILSG Annual Meeting - Part 2 (1989) suggested that the Matinenda Formation was deposited from shallow braided streams flowing down a south dipping paleoslope which underwent tilting to the southeast during deposition. Kimberly et al. (1980) reported that the uraniferous conglomerates contained almost no magnetite-ilmenite and had very high K/Na ratios. These are also features of the paleosols beneath the Huronian Supergroup and suggest that the sediment of the Matinenda Formation was formed by the intense weathering of a granitic source terrain, as proposed by Roscoe (1969). The Matinenda Formation, Elliot Lake Group The Matinenda Formation of the Elliot Lake Group is a sequence of arenites and intercalated quartz-pebble conglomerates which host the once strategically important uranium deposits of the Elliot Lake camp where it lies on Huronian Supergroup volcanic rocks and/or the Archean basement (Roscoe 1969; Robertson 1968, 1976). In the Thessalon, Sault Ste. Marie, and Sudbury areas, it consists predominantly of fine-to mediumgrained, subarkose to subwacke, and is probably less than 50 m thick (Bennett 1978). In Haughton Township, the Matinenda Formation lies upon grey sandstones equated with the Livingstone Creek Formation (Bennett 2006) (Figure 7). Two southeast trending ore zones were recognized since the early days of uranium mining in the Elliot Lake camp. The Nordic zone, east of the City of Elliot Lake is about 1.6 km (1 mi) wide and 5.6 km (3.6 mi) long. The Quirke Zone, in the Quirke Lake area, is about 3.2 km (2mi) wide and 9 km (6 mi) long. Basement paleotopography is thought to have had a determining influence on the position and orientation of the zones. Ore grade (approximately 0.1 % U3O8 (850 ppm U)) conglomerate occurs as persistent lenses with individual units up to 4.5 m thick. The uraniferous quartz-pebble conglomerates are commonly well developed at the base of the Matinenda Formation but also occur in the arkose up to 45 m above the base (Roscoe 1969, Robertson 1968). Total mine production from 1955 to 1990 was 160 million tonnes of ore averaging 896 g/t U3O8 for a total uranium metal production of 164,000 tonnes. In the Sudbury area, clastic units correlated with the Matinenda Formation thin rapidly eastward and are intercalated with the mainly metavolcanic rocks of the Stobie Formation and mudstones of the McKim Formation (Card 1978a). The most abundant rock type of the Matinenda Formation in the Elliot Lake area is generally described as medium- to coarse-grained subarkose, arkose and grit consisting of poorly-sorted quartz and feldspar grains set in a matrix of sericite and comminuted rock and mineral and the fragments. The ratio of potassium feldspar to plagioclase feldspar is about 8:1. Minor constituents are pyrite, calcite, chlorite, zircon and rarely, leucoxenecoated iron oxide and monazite. Varied amounts of sericite give the sandstones a green, apple green or greenish-yellow colouration. Well-sorted, quartzpebble conglomerate beds, with well-rounded pebbles and cobbles of quartz and chert, and pebbly subarkose units, are scattered throughout the coarse subarkose of the Matinenda Formation, but are more common near the base, in what has been termed the “floater-reef zone” (Robertson 1968; Pienaar 1963) (Stop 3). The quartz-pebble conglomerate consists mainly of well-rounded, pale- to dark-grey, quartz and chert pebbles in a matrix of pyrite, quartz and/or feldspar grit and sericite. Minable units contain about 15% pyrite. Radioactive minerals include uraninite, brannerite. and uranothorite (Roscoe 1969). Monazite and zircon are common heavy minerals. The sedimentological and mineralogical features of the uranium-bearing zones of the Elliot Lake camp are generally believed to support a modified paleoplacer origin of the ores as outlined by Roscoe (1969). Advocates of this hypothesis propose that prior to the accumulation of significant free oxygen Trough crossbedding, scour, and fill structures are common in the subarkose units (Robertson 1968; Roscoe 1969). Paleocurrent studies have established a northwest source area for the sediment of the Matinenda Formation (McDowell 1957; Long 1978; Figure 5). Fralick and Miall 211 �Proceedings of the 68th ILSG Annual Meeting - Part 2 in the Earth’s atmosphere, southeastward flowing streams carried quartz, pyrite and uraniferous minerals released by the extensive weathering of the Ramsey-Algoma granitoid terrain and deposited them in southeast-trending units constrained by the basement topography (Bennett 2006). McKim Formation. North of the Murray Fault the McKim Formation rarely exceeds a few hundred metres in thickness, whereas south it is at least 2400 metres thick (Debicki 1990). Card (1978) suggested that the change from laminated siltstone in the west to more wacke in the east indicated a change from more distal to proximal facies, in turn suggesting more tectonic activity and possibly a source for the McKim Formation sediments from the east. Fralick and Miall (1989) concluded that the McKim Formation in the Elliot Lake area represented a marine transgression that gradually drowned the Matinenda Formation fluvial plain. The McKim Formation The McKim Formation is the uppermost formation of the Elliot Lake Group. In diamond drill core, the contact between the Matinenda Formation and the McKim Formation is interfingering over 1 to 3 metres, consisting of clean sandstone of the Matinenda Formation and mudstone and wacke of the McKim Formation. Aweres Formation In the Sault Ste. Marie area, the Aweres Formation, a 1700 m thick sequence of conglomerate and sandstone (McConnell 1927), unconformably overlies mafic volcanic rocks of the Thessalon Formation. The internal stratigraphy and rock types of the Aweres Formation are consistent with deposition as an alluvial fan (Bennett 2006). Robertson (1968) gives a thickness of 0 to 100 m for the McKim Formation on the south limb of the Quirke Lake syncline. It is missing on the north limb. The McKim Formation is thickest in the Sudbury area, where it is up to 2400 m thick. Card et al. (1977) recognized 3 facies within the McKim Formation: The base of the formation consists almost entirely of mafic volcanic clasts whereas higher levels show a progressive increase in granitic clasts. The uppermost rocks of the Aweres Formation south of Aweres Lake are mainly arkose with thin pebble conglomerate beds. The lithological variation with stratigraphic height indicates the continual erosion of an uplifted, faultbounded, plateau of Huronian Supergoup volcanic rocks (Bennett 2006).  the “quartz sandstone” facies is equivalent to the Matinenda Formation, and represents thin interbeds of meta-quartz arenite and minor metaconglomerate in the 2 other main facies of the McKim Formation.  the “greywacke” facies of interbedded metawacke, metasiltstone and thin bedded mudstone and siltstone. Ripple marks, cross-laminations, graded beds and Bouma cycles are common. Bedding varies from a few centimetres to 50 cm thick.  the “laminated argillite facies” of thin-bedded mudstone and siltstone, with occasional beds of finegrained wacke. Bedding is commonly less than 1 cm, and rarely exceeds 10 cm. The distinct lithology of the Aweres Formation prevents its direct correlation with other Huronian Supergroup rocks. The upper surface is partly faultbounded. but is unconformably overlain by the Gowganda Formation on Highway 556. It is possible that the Aweres Formation is an erosional remnant of a more extensive alluvial fan system that extended in a more-or-less northeast direction beyond the present northern limit of the Hough Lake and Quirke Lake Groups. The Mississagi Formation may represent a distal depositional environment compared to that of the Aweres Formation (Bennett 2006). Where more highly metamorphosed, rocks of the laminated argillite facies, are best described as metapelites. The metapelites are characterized by high Al2O3 contents (20-25 weight %, Easton 2006b; Card et al. 1977), and moderate Fe/Mg ratios (~2), which is probably why metamorphic porphyroblast development is generally restricted to the laminated argillite facies. The Murray Fault appears to have exerted an important influence on the deposition of the 212 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Hough Lake Group Stratigraphic Relationships within the Elliot Lake Group, Sault Ste Marie-Elliot Lake area Introduction The stratigraphic relationship between the Matinenda, Thessalon, and Livingstone Creek Formations is revealed on a rock face near the northern boundary of Haughton Township about 30 km (18 miles) north of the town of Thessalon (Bennett 2006). Here pyritic quartz-pebble conglomerate of the Matinenda Formation directly overlies an apple-green paleosol on grey, finegrained sandstone correlated with the Livingstone Creek Formation (Bennett 2006). The Hough Lake Group (Robertson et al. 1969a; Roscoe 1969) is lowest of the 3 groups of the Huronian Supergroup that display the cyclic deposition of diamictite; mudstone-siltstone and/or carbonate; and arenite. Each cycle is generally thought to represent a sequence of glaciogenic– marine–fluvial and/or shallow marine deposition (Figure 4). Ramsay Lake Formation The Ramsay Lake Formation is the lowermost unit of the Hough Lake Group and is the oldest of 3 such conglomerate units that define the base of Hough Lake, Quirke Lake and Cobalt Groups (Roscoe 1969; Pienaar 1963) (Figure 3, 4). About 600 m northwest of the aforementioned occurrence, arkose and quartz-pebble conglomerate of the Matinenda Formation disconformably overlie a steeply dipping, eaststriking, mafic dike; the upper few metres of which is a sericite-leucoxene paleosol. The dike cuts grey sandstone and apple-green paleosol of the Livingstone Creek Formation (Bennett 2006). The Ramsay Lake Formation is a widespread, but relatively thin unit. In the Elliot Lake area, the Ramsay Lake Formation ranges from zero to just over 30 m thick (based on diamond drill logs from the Sault Ste. Marie District Geologist’s Office). The Ramsay Lake Formation is 70 to 170 m thick in the Sudbury-Manitoulin area (Card 1978a). Less than 2 km south of the above location Chandler (1976) identified a fault-bounded block of Thessalon Formation volcanic rocks with a minimum thickness of approximately 500 m. The mafic dike referred to above was a feeder for Thessalon flows, since the Thessalon Formation is the only known igneous activity at this stratigraphic level (Bennett 2006). Matrix-supported polymictic conglomerate (diamictite) is the most abundant rock type in the formation, especially near the base. Cobbles in the lowermost few metres usually reflect the underlying rock type (Robertson 1968; Parviainen 1973). Locally, minor amounts of mudstone, wacke and arenite are present. Subround to well-rounded pebbles and cobbles of grey granitic rocks and angular to rounded clasts of dark green to black volcanic rocks generally form less than 30 volume percent of the diamictite. The dark matrix consists of quartz, feldspar, chlorite, muscovite-sericiteillite and pyrite (Parvianen 1973). The above observations show that there was a period of volcanic activity, and a period of erosion, separating the Matinenda and the Livingstone Creek Formations. Since paleosols occur upon Huronian Supergroup flows in the Elliot Lake area, the sub-Matinenda unconformity seen in Haughton Township likely extends east to the Quirke Lake Syncline. In addition, the outcrop pattern of the volcanic rocks on geological maps also suggests that the volcanic rocks are erosional remnants preserved in basement depressions (Bennett 2006). The Thessalon Formation may have once extended beyond the limit suggested from its present outcrop distribution (Figure 6), especially if it were a continental flood basalt sequence. Although some writers have argued for a debris flow origin, most writers now accept the Ramsay Lake Formation as having a significant glaciogenic component (cf. Roscoe 1969; Robertson 1976). Fralick and Maill (1989) identified an ice-proximal association of pebbly sandstone and diamictite; subaqueous gravity flows and ice rainout deposits; and ice-proximal, fluvial outwash deposits. 213 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Mississagi Formation; but Long (1978) argued that the abundance of mud-grade matrix in the immature arenites, the predominance of unimodal paleocurrent directions, and the lack of quartz arenites argued against a marine environment for the Mississagi Formation. Long (1978) concluded that the Mississagi Formation was deposited from braided streams with low to intermediate sinuosity and high width to depth ratios. Pecors Formation The Ramsay Lake Formation is conformably overlain by a sequence of generally dark, bedded and laminated wacke, mudstone, siltstone and sandstone (Roscoe 1969). The Pecors Formation is 30 m thick at Quirke Lake (Robertson 1968) but is as much as 900 m thick south of the Murray Fault in the Sudbury area (Card 1978a). It was not identified in the area between Thessalon and Sault Ste. Marie (Frarey 1977). Ripple marks, graded bedding, cross-laminations parallel laminations, ball and pillow structures, clastic dikes and slumpage features have been reported in the formation. The basal part of the formation is commonly laminated, resembling varves, and in places has dropstones (Robertson 1968; Parvianen 1973). Partial Bouma sequences are common (Card 1978a; Robertson 1976). The Pecors Formation is the result of transgressive units formed in deep water by turbidity currents (Card 1978a). The presence of dropstones is evidence of a cold paleoclimate and provides supporting evidence for the glaciogenic origin of the underlying Ramsay Lake Formation. Beds are commonly about a metre thick but can range from a few centimetres to over 4 m thick. Trough cross-stratification and ripple crossstratification are common sedimentary structures (Long 1978). Cross-stratified beds may show grain-size gradation (McDowell 1957). Long (1978) measured over 2500 cross-stratified units in the Mississagi Formation (Figure 5) and recognized 2 major stream systems: a stream system flowing southeast to east from the Sault Ste. Marie area, which joined a stream system flowing southwest from the Cobalt Plain, thereby forming a southward flowing system southwest of the Sudbury area. These observations suggest that the area now occupied by the Sudbury Igneous Complex was elevated during the time of Mississagi Formation deposition (Long 1978). Mississagi Formation The Mississagi Formation is a thick sequence of predominantly grey, arenitic rocks extending most of the length of the Huronian Supergroup outcrop belt. In the Quirke Lake syncline, the Mississagi Formation is 344 to 704 m thick. South of the Murray Fault the formation is notably thicker, being more than 3000 m thick in the Sudbury area (Card 1978a; Long 1978). Quirke Lake Group Bruce Formation The Bruce Formation extends from the Garden River Indian Reserve near Sault Ste. Marie to about 70 km northeast of Sudbury. It consists mainly of matrix-supported and minor clast-supported conglomerate. Pebbly wacke, arkose, wacke and siltstone are locally present. By far the most dominant rock type in the Mississagi Formation is moderately well-sorted, medium- to coarse-grained subarkose and arkose. Small to medium quartz and/or chert pebble conglomerate is a minor component of the formation; but is more common in the western and northeastern parts of the Huronian belt. Finegrained pyrite along forsets commonly results in rusty staining of outcrops. Greenish, sericitic units form relatively thin planar-bedded units between crossbedded sandstones. Palonen (1973) provided evidence supporting a marine origin for the The Bruce Formation is from 79 to 12 m thick in the Elliot Lake area and is 26 to37 m thick under the main part of the Quirke Lake Syncline (Robertson 1968). Pebble- to boulder-sized, angular to subrounded clasts generally consist of pale-grey granitic rocks, Archean supracrustal rocks and fine-grained mafic clasts. The upper parts of the formation may contain up to 5% carbonate (Robertson 1968). 214 �Proceedings of the 68th ILSG Annual Meeting - Part 2 The Bruce Formation is generally interpreted as a tillite with minor beds and lenses of glacially derived sandstone. Dropstones have been observed in laminated units (Robertson 1968). Serpent Formation The Serpent Formation occurs throughout much of the Huronian Supergroup outcrop belt; however, it was locally removed by erosion during a period of tectonic activity preceding deposition of the Gowganda Formation of the Cobalt Group. Thickness estimates range from 150 to 1500 m (Bennett et al. 1991). According to Robertson (1968), nowhere in the Blind River–Elliot Lake area is there evidence that the total thickness of the Serpent Formation has been preserved. Casshyap (1969) concluded that the formation was deposited from terrestrial wet-base glaciers. Sims et al. (1981), however, proposed that the Bruce Formation represents an accumulation of debris flows released by normal faulting, a sudden increase in paleoslope, and a sudden increase in water depth. This is consistent with observations made in Porter and Vernon townships showing considerable down-cutting, ranging from 5 to 30 m, of the Bruce Formation into the underlying Mississagi Formation (Easton 2005). The Serpent Formation is mainly fine- to medium-grained, quartz arenite and arkose. Conglomeratic units have been noted, especially near its base. Carbonate is a significant component near the base of the formation in the Elliot Lake area (Robertson 1968). Planar and festoon crossbedding, rip-up clasts, fine-laminations, and mud cracks have been reported. Long (1976) proposed that the Serpent Formation was deposited in a distal braided stream environment with calcareous units representing a sabkha environment. Young (1982) noted that the presence of very large crossbeds and a bimodal size distribution suggest aeolian processes may have been active, at least locally. Espanola Formation The Espanola Formation is the only widespread carbonate unit of the Huronian Supergroup. It is a present from Sault Ste. Marie to the Maple Mountain area, approximately 70 km northeast of Sudbury. Its widespread distribution and distinctive lithology make it the most useful stratigraphic marker unit in the Huronian Supergroup. In the Elliot Lake area, the Espanola Formation can be subdivided into 3 members: a lower limestone member, a middle siltstone- arenite member and an upper dolomite member (Robertson 1968). The latter generally contains 3% to 4% total iron which gives it a distinct brownish hue on weathered surfaces. Contacts between members tend to be gradational. All 3 members are thinly bedded to laminated. The threefold subdivision is less well developed south of the Murray Fault (Young 1982). Cobalt Group Gowganda Formation The Gowganda Formation is a complex sequence of conglomerates, sandstones, siltstones and mudstones, and is the lowermost formation of the Cobalt Group. Its thickness ranges from 1070 m in the Sault Ste. Marie area; to 970 to 1150 m around Whitefish Falls on the north shore of Lake Huron; and from 950 to 2700 m near Sudbury. Near Dunlop Lake, in the Elliot Lake area, the Gowganda Formation is about 600 m thick. Intraformational breccias, mud cracks, ripplemarks, flame structures and ball-and-pillow structures are common sedimentary features. Hofmann et al. (1980) described stromatolites in the Espanola Formation on Quirke Lake. All these features suggest deposition in quiet shallow waters with carbonate deposition being interrupted by influx of fine-grained sediment. Matrix-supported conglomerates are common, especially in the lower parts of the formation. However, these are commonly intercalated with clast-supported conglomerates and sandstone units. Laminated mudstones and siltstone are especially prominent in the upper parts of the Gowganda Formation. Many occurrences of ice-rafted 215 �Proceedings of the 68th ILSG Annual Meeting - Part 2 dropstones have been reported in laminated mudstone-siltstone units. Individual units are generally relatively thin and discontinuous making subdivision of the Gowganda Formation difficult except in well-exposed areas. northern Ontario. It is overwhelmingly an arenite sequence, with local siltstone units present in lower parts of the formation. It is up to 2500 m thick near Sault Ste. Marie and in the LaCloche Syncline, southwest of Sudbury. It is up to 2300 m thick in the Cobalt Basin. Most granitic clasts in Gowganda Formation conglomerates have a distinctly pinkish or reddish hue, in comparison to the grey, granitic clasts in the matrix-supported conglomerates of the stratigraphically lower Ramsay Lake and Bruce Formations. Pink- and red-hued sandstones also first make their appearance in the formation. Roscoe (1969) pointed out the appearance of red coloration (i.e. ferric iron) just above the basal units of the Gowganda Formation, and argued that it represents the appearance of free oxygen in the Earth’s atmosphere, and a change from the previously reducing atmospheric conditions that allowed the accumulation of easily oxidized minerals such as pyrite and uraninite. Roscoe (1969) did, however, emphasize that glaciation is only one of several processes likely responsible for the deposition of the Gowganda Formation. In general, the lower part of the Lorrain Formation is dominated by pink, arkosic sandstone; the middle by hematite-rich subarkose and quartzarenite; and the upper part by pale grey to white mature, quartz-arenite. A distinctive jasper-pebble conglomerate found in the Sault Ste. Marie area is a popular decorative stone, known locally as “pudding stone”. Previously these jasper clasts were thought to be derived from banded iron formations from the Abitibi greenstone belt, however, Bleeker (2018) observed that the jasper clasts of the puddingstone are often angular (i.e. more or less proximal), that they suddenly become a dominant clast type (again suggesting proximal); that there are few if any real banded iron formation clasts; and that the jasper clasts are extremely fine-grained and delicately textured and they do not contain magnetite, unlike banded iron formation samples from the Abitibi greenstone belt which are noticeably more recrystallized. Thus, Bleeker (2018) concluded that the jasper clasts of the Lorrain Formation puddingstone are not of Archean derivation, but represent penecontemporaneous reworking of otherwise poorly-preserved Huronian jasper deposits, possibly associated with a minor volcanic or hydrothermal centre that has not yet been identified. Given that the occurrence of puddingstone is strongly concentrated in the area around Bruce Mines, the source jasper beds were likely local deposits restricted to that part of the Huronian basin, possibly the fine-grained siliceous siltstone and associated layers that have been referred to in some of the early papers on the Huronian as “Bruce Mines Jasper” (Collins 1925). The depositional environment of the diamictites in the Gowganda Formation have been the subject of discussion since Coleman (1905) proposed a glacial origin for these matrix-supported conglomerates. Many subsequent writers including Ovenshine (1965), Casshyap (1969), Lindsay (1971) and Young and Nesbitt (1985) also have supported a glacial, glacial-marine, or glaciolacustrine, origin for the Gowganda Formation diamictites. Card (1968) concluded that, although glaciation may have supplied coarse detritus to the basin initially, debris flows and turbidity currents, related to vertical tectonic movement, may better explain the thickness variations, rock associations and distribution of units in the Gowganda Formation in the Sudbury– Manitoulin area. Lorrain Formation The presence of aluminous minerals is a characteristic feature of the uppermost quartzarenites of the Lorrain Formation. Diaspore and kaolinite are common in the Sault Ste. Marie area The Lorrain Formation is generally wellexposed throughout most of the Huronian Supergroup outcrop belt, where it commonly forms the background to some of the most scenic views in 216 �Proceedings of the 68th ILSG Annual Meeting - Part 2 and north of Elliot Lake (Wood 1973) whereas kyanite, andalusite and kaolinite occur as metamorphosed equivalents in the LaCloche Lake– Killarney area (Card 1978a). Young (1973) and Wood (1973) interpreted the presence of diaspore and kaolinite as the result of post-depositional, insitu, alteration of feldspar under hot and humid climatic conditions. Formation. He also described hematite ooliths and the abundance of grains in the 0.02 to 0.05 mm range, a relatively uncommon grain size in sedimentary rocks. Since this size is found in loess deposits, Wood (1973) proposed that the quartz silt of the Gordon Lake Formation was formed by glacial action, and then carried by the wind and deposited in a tidal flat environment. The presence of abundant detrital hematite in the Lorrain Formation and the occurrence of monazitebearing quartz-pebble conglomerate north of Elliot Lake, have been interpreted by Frarey and Roscoe (1970) as indicating an oxidizing environment. Bar River Formation The Bar River Formation is the uppermost formation of the Huronian Supergroup. It is characterized by quartz-arenite with minor ferruginous arenite and siltstone. It is approximately 300 m thick in the Flack Lake area, north of Elliot Lake. Wright and Rust (1985) concluded that the Bar River Formation was deposited in a tidal environment. Planar and trough crossbedding are common, as are ripple marks and other primary depositional structures. There is no consensus as to the depositional environment of the Lorrain Formation. Most of the sedimentary structures present can be found in either shallow marine or fluvial environments. Wood (1973), Young (1973) and Frarey (1977) favored a fluviatile setting, whereas Pettijohn (1970) supported a marine setting. Card (1976) proposed that the Lorrain Formation resulted from near-shore coastal shelf deposition during episodic marine transgression and regression. Nipissing Intrusions Sills, and minor dikes and cone sheets, of gabbro, diabase and granophyre, commonly referred in the older literature (pre-1995) as “Nipissing diabase”, are the most widespread igneous rocks associated with the Huronian Supergroup. Nipissing intrusions are widely and evenly distributed throughout the Huronian Supergroup outcrop belt but, and with few exceptions, are not recognized in the Archean Ramsey-Algoma granitoid terrane. Individual intrusions may be up to several hundred metres thick and extend over a strike-length area of 10s of kilometres. There is no current consensus on the tectonic setting for emplacement of the Nipissing intrusions. Gordon Lake Formation The Gordon Lake Formation displays a gradational contact with the underlying Lorrain Formation. It is composed predominantly of variegated mudstone, siltstone, chert and minor fine-grained sandstone. The Gordon Lake Formation in the Flack Lake area is subdivided into a lower member of reddish arenite, siltstone, and chert with anhydrite and gypsum nodules; a middle member of green siltstone and mudstone; and an upper member of reddish mudstone, siltstone and chert (Robertson 1986). Sedimentary features include small-scale crossbeds, ripple marks and desiccation cracks. Olivine-bearing hypersthene gabbro, gabbro, feldspathic pyroxenite, two-pyroxene quartz gabbro, hornblende gabbro, granophyric gabbro and granophyre have been identified in Nipissing intrusions. Many Nipissing sills are characterized by chilled margins 50 cm to 5 m wide, overlain by 10-20 m of quartz gabbro, then 100-500 m of hypersthene-poor gabbro-norite and vari-textured diabase (Lightfoot and Naldrett 1996). Some features of the Gordon Lake Formation are unique in the Huronian Supergroup. Wood (1973) noted the abundance of feldspar in marked contrast to rocks of the immediately underlying Lorrain Baddeleyite and/or zircon from Nipissing gabbro sills in the Gowganda area, the Sudbury area, the 217 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Agnew Lake area and north of Thessalon, have all given ages between 2214 and 2219 Ma (Davey et al. 2019; Noble and Lightfoot 1992, Corfu and Andrews 1986; Easton, unpublished data 2021). Buchan and Card (1985) report that paleomagnetic data suggests at least 2 periods of Nipissing intrusive activity. If so, then the 2 paleomagnetic poles formed in a short timespan of circa 5 million years.  colour variations  destruction of primary rock textures accompanied by the development of soil textures  destruction of primary minerals with formation of clay minerals or metamorphic equivalents  dikes of material from overlying sediment washed down into desiccation cracks in the soil  rip-up clasts of overlying sediments Well-preserved paleosols below the Matinenda Formation in the Elliot Lake area have been described by many workers (Roscoe 1969; Pienaar 1963; Robertson 1968; Frarey and Roscoe 1970; Gay and Grandstaff 1980; Kimberly et al. 1984; G-Farrow and Mossman 1988; Prasad and Roscoe 1991; Sutton and Maynard 1992, 1993; Easton 2013b). Lightfoot and Naldrett (1996) discuss the geochemical characteristics of the Nipissing magmas and the potential for platinum group metal deposits. They concluded that parental magmas of remarkably uniform composition underwent in-situ contamination and differentiation in the intrusions. In addition to nickel-copper-PGE mineralization (Jobin-Bevans 2014, 2016; Jobin-Bevans et al. 1998), a spatial association between Nipissing intrusions and 5-element vein-type mineralization has long been recognized, especially in the Cobalt area (cf. Fyon et al. 1992). Bennett et al. (1991) proposed that there are 3 disconformities or unconformities in the Elliot Lake Group, which all have the potential for paleosol development (Figure 7). These are in descending stratigraphic order the sub-Matinenda disconformity, the sub-Thessalon Formation disconformity, and the sub-Livingstone Creek Formation unconformity. Huronian Paleosols and Evidence for Oxygen Accumulation in the Huronian Atmosphere Paleosol Evidence The sub-Livingstone Creek Formation unconformity is the lowest unconformity and is the only entirely sub-Huronian Supergroup unconformity (Figure 3, 7). This unconformity is exposed in the Thessalon area, where the upper few metres of the Archean granitic rocks can be seen to progress from angular, slightly rotated blocks, separated by grey grit and fine-grained sandstone, upward, to more rounded boulders with a higher proportion of finer clastic material (Collins 1925). This zone may be termed a “paleo-regolith”, since there is little or no obvious development of the yellow, sericitic paleosol commonly found in the younger, sub-Matinenda paleosols. It has long been recognized that the study of paleosols (ancient soil profiles) beneath the Huronian Supergroup could provide information pertaining to the development the Earth’s atmosphere and climate during the Proterozoic. Since iron is much less soluble in the ferric state than when in the ferrous state, the behavior of iron in paleosols should provide some indication of the oxygen partial pressure of the environment. Many of the best descriptions of Precambrian paleosols have been from those associated with the Huronian Supergroup unconformity (Gall 1992). Grandstaff et al. (1986) identified 8 features of paleosols; most of which have been described in paleosols beneath the Huronian Supergroup. These features are: Prasad and Roscoe (1996) described 2 paleosols in the same diamond drill core from the Denison Mine at Elliot Lake. One was found above Huronian Supergoup volcanic rocks and another, less well-developed paleosol, was found upon Archean tonalite below a short section of quartzpebble conglomerate and grit below the 9 m thick  stratiform  relatively thin (<20 m)  transitional lower boundary-sharp upper boundary 218 �Proceedings of the 68th ILSG Annual Meeting - Part 2 volcanic unit (Prasad, personal communication, 1997 in Bennett 2006). Gay and Grandstaff (1980) concluded that the upward increase in iron content indicated the presence of free oxygen in early Huronian atmosphere, although at approximately 1% of the present level. They also suggested that the loss of iron in most Huronian paleosols could be due to local reducing environments. Some writers have concluded that the increase in potash (as sericite) in Huronian paleosols is due largely to diagenetic and metamorphic processes that may mask the environmental and hydrologic conditions operative during paleosol development (cf. Gay and Grandstaff 1980; G-Farrow and Mossman 1988). The best developed, and most studied, Huronian paleosols have been found directly below the Matinenda Formation. On mafic rocks, the subMatinenda paleosols can generally be recognized by the presence of an upper, distinctly apple-green to yellowish, sericitic zone which grades downward, over a few centimetres to several metres, to a black, fine-grained, chlorite-rich eluvial zone up to several metres thick. Abundant pseudomorphs of titanium oxide after ilmenite are a feature of paleosols on mafic igneous rocks. Ripup clasts of sericitic paleosol are commonly found in the lower few metres of the overlying Matinenda Formation. Prasad and Roscoe (1996) report significant amounts of carbonate and pyrite in subMatinenda paleosols in the Elliot Lake area. The mineralogy and geochemistry of subLorrain Formation paleosols described by Rainbird et al. (1990) and Sutton and Maynard (1992, 1993) commonly show an enrichment of Fe+3 relative to Fe+2 without a significant loss of total iron. Hematite is a common mineral in the upper parts of sub-Lorrain paleosols, in contrast to the presence of pyrite in sub-Matinenda paleosols. In this respect, the sub-Lorrain paleosols resemble many postGeon 23 paleosols and are consistent with weathering in an oxidizing atmosphere (Prasad and Roscoe 1996; Rainbird et al. 1990). The uppermost sections of sub-Matinenda Formation paleosols developed on Archean granitic rocks is generally an apple-green to yellowish rock composed mainly of quartz and sericite (Robertson 1968; Gay and Grandstaff 1980; Sutton and Maynard 1992), Where the texture of the protolith is well preserved, but the original mineralogy is replaced, the paleosol may be termed a saprolith (Rainbird et al, 1990). The chlorite-rich eluvial zone of paleosols on granitic rocks is generally lacking or relatively thin. Other Evidence Since pyrite and uraninite are unstable under oxidizing conditions, the abundance of detrital pyrite and uraninite in the paleoplacer uranium ore zones in the Matinenda Formation provide evidence for an oxygen deficient atmosphere during weathering, transport and deposition of early Huronian Supergroup sediments. Sub-Matinenda paleosols commonly show the pronounced loss of sodium typical of most paleosols. Calcium and magnesium are also depleted, but there is generally a large increase in potassium content (Gay and Grandstaff, 1980). In most cases, iron and manganese are depleted in the upper parts of the paleosol. This is held to provide evidence of weathering in a reducing environment. Gay and Grandstaff (1980), however, noted an upward increase in total iron in paleosol from the Pronto Mine area. Easton (2013b) locally reported chemical compositions approaching that of a bauxite developed over a mafic substrate in diamond drill core from Elliot Lake. In contrast to the common red beds of more modern clastic sequences (post-Geon 23), sandstones and most granitic clasts below the Cobalt Group are almost all drab coloured despite the abundance of red and pink granitic rocks in the source area (Roscoe 1969, 1973). Frarey and Roscoe (1970) proposed that the drab colour of lower Huronian Supergroup clastic rocks is due to the lack of free oxygen in the atmosphere during their deposition. 219 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Red-hued, hematite-bearing rocks, which Roscoe (1969) proposed mark the presence of an oxidizing atmosphere, make an appearance with the Gowganda Formation of the Cobalt Group, and are important in parts of the Lorrain and Gordon Lake Formations. fault, it is generally interpreted as an inverted growth fault; i.e. an early listric normal fault active during sedimentation; which during later compression became a thrust or reverse fault (Card 1978a; Jackson 2001; Zolnai et al. 1984). The rocks of the Huronian Supergroup have been subjected to several deformational events (Table 1). This is particularly evident south of the Murray Fault. In the Whitefish Falls area, Young and Nesbitt (1985) concluded that some large-scale folding was related to syn-depositional and/or postdepositional deformation of unconsolidated sediment. Early syndepositional deformation is indicated also the unconformity beneath the Gowganda Formation; and the presence of ragged, slumped contacts and large slump blocks along major faults (Card 1978a; Young 1983). Not all workers, however, accept the above explanation for the preservation of uraninite and pyrite, and the observed change in colour with stratigraphic position. For example, Ohmoto (1996) has stated “the loss of total iron in paleosols of all ages is not due to a reducing atmosphere but to the reductive dissolution of ferric hydroxides under an oxic atmosphere”. Regional Tectonic Patterns and Metamorphism Major structures in the Huronian Supergroup outcrop belt follow 2 trends: 1) west-northwest trending folds and faults in the Sault Ste. MarieElliot Lake area; and 2) east to northeast striking folds and faults in the Sudbury-Manitoulin area south of the Murray fault. These 2 orientations are associated with differing fold styles, metamorphic grade and metamorphic fabric. Convincing evidence of at least one important pre-Nipissing (circa 2217 Ma) deformational event, historical assigned to the apocryphal Blezardian orogeny (Stockwell 1982), comes from the observation that Nipissing bodies in the Sudbury-Whitefish Falls area transect axial surfaces of major folds (Card 1978a). Such relationships are not observed north of the Murray Fault (Jackson 2001; Robertson 1964). North of the Murray Fault, Nipissing sills tend to occupy structures parallel to the axial plane of the Chiblow anticline, suggesting pre-Nipissing. folding. Easton (2006a), in the Porter-Vernon area north of Espanola and north of the Murray Fault, noted that at least 2 periods of folding are present, roughly orthogonal to one another — the resulting interference forms a dome-and-basin pattern (Figure 10). F1 folds Nipissing gabbro intrusions in the lowermost part of the stratigraphy (in the Hough Lake and Quirke Lake Groups), whereas Nipissing gabbro appears to be emplaced along fractures related to F2 axial planes (all groups). This suggests either multiple periods of gabbro emplacement, or more likely, that gabbro emplacement occurred syn-folding. In either case, folding cannot be significantly younger than the emplacement age of the Nipissing intrusions. In the Sault Ste Marie–Elliot Lake area, fault and fold structures generally trend west-northwest to northwest. Folds are generally upright, and open, with gentle, variably-plunging hinges. There is only weak development of minor tectonic structures; metamorphic grade is subgreenschist (Figure 9, Card 1978b). The major structural features of the Elliot Lake area include a gently south-dipping homocline south of the Flack Lake fault, the open fold of the Quirke Lake Syncline, and the Chiblow Anticline to the south of the Quirke Lake Syncline. In the Elliot Lake area, neither Jackson (2001) nor Easton (2009, 2013a) found any evidence of a detachment at, or near, the basement-cover interface. Notable changes occur across the northeasttrending faults of the Murray Fault system, the most significant structural feature of the Huronian Supergroup outcrop belt. Because many formations show a significant increase in thickness south of the 220 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 9. Metamorphism of the Huronian Supergroup. Figure from Card (1978b). Jackson (2001) also noted evidence of preNipissing faults north of the Murray Fault zone. Easton (2006a), again in the Porter-Vernon area, made the same observation, and recognized at least 5 major fault sets, 4 of which are post-folding.  The earliest faults are north-trending and juxtapose Archean granitic basement against Huronian Supergroup strata. These faults appear to have been fluid conduits, as indicated by the presence of large quartz vein systems and microbrecciation in the Archean basement, and hydrothermal annealing of quartz in sedimentary rocks adjacent to the faults. obscured by subsequent vertical movement, and the fact that these faults are the loci for the development of extensive zones of Sudbury breccia. The localization of Sudbury breccia along this fault set suggests that it may have developed at circa 1850 Ma due to the Sudbury impact.  Finally, significant vertical displacement, occurs along a major set of closely spaced northwesttrending faults. Some of these faults are the loci of Sudbury swarm diabase dikes (circa 1240 Ma), which are undeformed and unmetamorphosed, suggesting that this fault set formed between 1850 and 1240 Ma.  East-northeast faults also juxtapose Huronian Supergroup strata against basement rocks, but are post- F1 folding, with both vertical and lateral movement. They may be associated with a set of north to northeast, dominantly normal faults, which may have an older thrust component.  Most significant in terms of map pattern, at least in the southern part of the Porter-Vernon area nearest the Murray fault system, are east to east-northeast normal faults across which major changes in stratigraphic level occur. There may be a thrust component to these faults, but if so, it has been Following emplacement of the Nipissing intrusions, but prior to the emplacement of the Sudbury Igneous Complex (1850 Ma), there was further deformation and regional metamorphism. Rb/Sr isotopic studies of Huronian Supergroup metasedimentary rocks indicate that metamorphic resetting occurred at 1900-1850 Ma (Fairbairn et al. 1969). This age range is correlative with the Penokean Orogeny of Michigan and Minnesota (Sims et al. 1981), which has a peak metamorphic age of circa 1835 Ma (Holm et al. 2001). 221 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 10. Simplified geological map of the northeast shore of Agnew Lake, showing the distribution of fold styles within Porter and southern Vernon townships. The contact between the Mississagi and Bruce formations has been highlighted to illustrate the fold pattern, and units stratigraphically above the Bruce Formation are shown by a pattern. Between the Cameron Creek and Midport faults, the area is dominated by a dome and basin geometry, indicating the presence of 2 fold generations, with approximately perpendicular axial planes. North of the Midport fault, the early, north-oriented fold style (F1) dominates. Figure from Easton (2005). 222 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Recently, increasing evidence suggests, that even in its type area, the Penokean orogeny is not as significant a regional event as had been previously thought. In fact, there is increasing evidence that the Yavapai orogeny (Geon 17) may be responsible for much of the deformation previously attributed to the Penokean (cf. Zi et al. 2022; Holm et al., 2018; Schulz and Bjornerud 2018; Raharimahefa et al. 2014). Clearly additional work on the timing and extent of deformation affecting the Huronian Supergroup is needed in Ontario, especially away from Sudbury where there has been considerable resetting of isotopic systems by the Sudbury impact event. it have deformed Huronian Supergroup sedimentary rocks that were not yet deposited. Jackson (2001) also proposed that the “inverted growth-fault” model, as applied by Zolnai et al. (1984) to structural-stratigraphic relationships in the Huronian Supergroup outcrop belt may, in some cases, be interpreted as thrust faults with flats following depositional boundaries, and ramps that cut up through the stratigraphic section. Given the data available, neither model could be rejected for major northwest-trending faults in the Sault Ste. Marie area (Jackson 2001). Jackson (1994) points out that the curvature of the Flack Lake fault is in the opposite direction to that expected if it is a thrust fault, as proposed by Zolnai et al. (1984). After the emplacement of the Sudbury Igneous Complex (SIC) at 1850 Ma (Davis 2008), and the deposition of the Whitewater Group, there is evidence of further deformation and low-grade metamorphism of the Huronian Supergroup, followed by intrusion of granite plutons at circa 1740 Ma and circa 1450 Ma, predominately in the Killarney area and in what is now the Grenville Province in the Sudbury area. The intensity of postSIC deformation and retrograde meta-morphism increase south of the Murray Fault, especially in the area between the SIC and the Grenville Front. The Murray Fault system separates moderately deformed, low grade metamorphic rocks to the north from multi-deformed, higher-grade rocks of the Sudbury–Manitoulin area to the south. The Sudbury–Manitoulin area is characterized by open to sub-isoclinal, flattened buckle folds with upright to northward overturned axial surfaces. Elongate domes and basin are formed by reversals in plunge. Penetrative axial place cleavage and steeply plunging rodding and/or mineral lineations are well developed. More than one age of major and minor structures can be discerned south of the Murray Fault (Jackson 2001). A study of magnetic fabrics, strain patterns, and microstructures in granitoid rocks of the Creighton and Murray granites and their Huronian Supergroup host rocks (Riller 1996) lent credence to the concept of a pre-2220 Ma “Blezardian orogeny” (Stockwell 1982). Riller (1996) concluded that major folding and amphibolite facies regional metamorphism in the Sudbury area was coeval with the emplacement of the Creighton the Murray granites, which at the time yielded discordant upper intercept ages of 2333+33/-22 Ma (Frarey et al. 1982) and 2388+20/-13 Ma (Krogh et al. 1984). Subsequent work on the Creighton granite by Bleeker et al. (2015), Kenny et al. (2017) and Smith (2002), and on the Murray granite by Krogh et al. (1996), have yielded ages of 2460±20 Ma and 2477±9 Ma, respectively, suggesting that the Blezardian orogeny was not widespread, nor could Metamorphism south of the Murray Fault ranges from lower greenschist to lower amphibolite facies (Figure 9). Rocks of higher metamorphic grade occur in 2 zones or nodes, one along the Murray Fault system itself and another northwest of the Grenville Front. Both zones coincide with major anticlinoria, although in detail, metamorphic isograds transect fold axes (Jackson 2001). Highergrade metamorphic nodes do not coincide with the few granitic intrusions that intrude the Huronian Supergroup rocks south of the Murray Fault. The inferred 1900 to 1850 Ma age of metamorphism is much younger than the age of the Creighton and Murray granites (circa 2460 Ma) yet older than the circa 1740 Ma and 1450 Ma Cutler and Chief Lake granites. 223 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Jackson (2001) considered the origin of the high-grade staurolite-biotite assemblages of the McKim Formation in the hanging wall of the Murray Fault as one of the most enigmatic aspects of the tectonic history of the Southern Province. He concluded that geobarometry indicates a relatively low- pressure metamorphism (2-3 kbar, bathozone 2 of Carmichael 1978) at high temperature (Figure 11). These conditions differ significantly from the 6-7 kbar pressures (bathozone 5 of Carmichael 1978) estimated for the Penokean metamorphism in Minnesota as determined by Holm and Selverstone (1990). Jackson (2001) concluded that the high-temperature metamorphism was at or below pressure corresponding to the thickness of the Huronian Supergroup rock column, thereby precluding crustal thickening as the origin of the metamorphism. Jackson (2001) concluded that a high heat flow regime, such as that developed in areas of crustal extension and related mantle upwelling, was the cause. Such a model is compatible with Card’s (1964) view that the highgrade metamorphism may be the result of rapid, focused heat flow. Figure 11. Pressure-temperature (P–T) grid showing the location of major mineral assemblages in the system KFMASH, after Spear (1993). Bathozones from Carmichael (1978). Also indicated are possible P– T paths for different parts of the Southern Province. Abbreviations: and = andalusite, as = aluminosilicate, bt = biotite, chl = chlorite, cld = chloritoid; crd = cordierite, grt = garnet, kfs = potassium feldspar, ky = kyanite, ms = muscovite, prl = pyrophyllite, qtz = quartz, sil = sillimanite, st = staurolite; B1 = Baldwin Township initial conditions, B2 = Baldwin Township peak conditions, B3 = Baldwin Township retrograde path; DK = diaspore to kyanite path. Figure from Easton (2006b). 224 �Proceedings of the 68th ILSG Annual Meeting - Part 2 In contrast, at the Great Bend of the Spanish River, interpreted as a high-grade metamorphic node (Card 1978b), and which has been the subject of detailed metamorphic studies (Card 1964; Fox 1971), the co-existing assemblage staurolitechloritoid in metapelitic rocks of the McKim Formation, along with the local presence of andalusite, places the area in bathozone 3 of Carmichael (1978) (Figure 11). Furthermore, work on McKim Formation metapelites in Baldwin Township that contain relict kyanite, north of the Murray Fault near Espanola, Easton (2006b) concluded that minimum metamorphic conditions corresponded to bathozone 4 (>4 kbar, ≥500°C; Carmichael 1978). Upon reaching peak temperatures, the metapelites cooled quickly to lower grade, likely in a fluid-rich environment, which retrogressed most minerals except for kyanite (Figure 11). Minerals such as chloritoid, andalusite and staurolite, were especially susceptible to retrograde alteration, as they would have already started to break down during the period of increasing temperature (Easton 2006b). the observations in Baldwin Township and the proposed tectonic thickening model (Easton 2006b), presents a considerably more complex metamorphic history for the south-central Southern Province than has been previously envisaged (e.g., Card 1978b; Bennett et al. 1991; Bennett 2006). If nothing else, it emphasizes the need for additional, modern, metamorphic studies of the Huronian Supergroup throughout the Sault Ste. Marie to Sudbury area. What detrital zircon studies tell us about the source region for the Huronian Supergroup The publication of the first detrital zircon analyses from the Huronian Supergroup (Rainbird and Davis 2006) took place at the same time as this field trip was last run back in May 2006. Since then, detrital zircon work has been completed on more than 25 samples of the Huronian Supergroup (Table 2), and from almost every unit (except for the Pecors, Espanola and Bruce formations) (Craddock et al. 2013; Davis et al. 2018; Easton and Heaman 2008, 2011; Hill et al. 2018; Kenny et al. 2017; Long et al. 2011; Ménard 2017, 2019; Petrus et al. 2016; Rasmussen et al. 2013). Most of this work occurred in the area between Sudbury and Sault Ste. Marie, all north of the Murray fault, with only 2 samples studied so far from the Cobalt basin northwest of Sudbury. These data are summarized in Table 2, with age ranges and averages based on grains that are < 5% discordant, a lower cutoff than used in many studies. Key observations are: Easton (2006b) argued that tectonic thickening is the most common explanation used to account for the transition from andalusite to kyanite and provides an explanation for the syn-kinematic character of the metamorphic porphyroblasts. It also can account for the differences between Baldwin Township and the Great Bend area. The model of Easton (2006b) explains other metamorphic mineralogical anomalies in the Southern Province. For example, at low temperatures, but similar pressures, the reaction of diaspore to kyanite occurs, which would account for the presence of reported occurrences of diaspore and kyanite (Card 1978a, 1978b; Church 1967; Chandler et al. 1969). It accounts for the folding of metamorphic isograds, as reported by Jackson (2001) in the May Township area. It also provides an alternate explanation for the highgrade metamorphic nodes in the Southern Province other than the presence of focussed heat and fluid zones proposed by Card (1978b). The resulting tectonic history of the Southern Province, based on  Zircons between 2450 and 2490 Ma, likely derived from either Huronian Supergroup volcanic rocks and/or related mafic and felsic intrusions, so far have been reported only from the Matinenda or the Mississagi Formations, generally from sample sites near the base of the formations.  Samples from the lower Huronian Supergroup (Elliot Lake and Hough Lake Groups) are dominated by Geon 26 detritus (see Table 2), consistent with provenance dominated by local sources characteristic of the Ramsay-Algoma granitoid complex. Where detailed stratigraphic sampling has occurred, the lowermost units have unimodal populations, 225 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Paleoproterozoic rocks of the Thessalon Formation and the East Bull Lake intrusive suite have ΕNdT values ranging from 2.58 to -2.28 (Easton 2012; Prevec 1993), suggesting derivation from a primary magma originating from a depleted mantle source, which locally was affected by minor amounts of crustal contamination. becoming more diverse with increasing stratigraphic height (e.g., Easton and Heaman 2011). The only exceptions are the 2 samples from the Cobalt basin, which are dominated by Geon 27 populations, consistent with more Geon 27 basement in that area.  Above the Mississagi Formation, Geon 27 populations are dominant, but Geon 28, 29 and Geon 30 grains are also commonplace (see Table 2). This may reflect a change in sedimentation style, and/or increased erosion of the hinterland resulting in a wider range of source material becoming available. In contrast, the Matinenda Formation sandstone samples have negative ΕNdT, ranging from -0.52 to -9.21 (Easton 2012). Samples with the highest negative ΕNdT were also enriched in Th, most likely due to the presence of monazite. The magnitude of the negative ΕNdT values in the Matinenda Formation indicates a negligible contribution from the volcanic and intrusive rocks of the Whiskey Lake greenstone belt, all of which have positive ΕNdT. The Matinenda Formation data can be explained if the sandstones contain a significant component derived from a suite of radiogenic granites located 30 to 40 km north and northwest of Elliot Lake which have ΕNdT of -6.19 (Easton 2012). These radiogenic granites typically contain 8 to 33 ppm U and 30 to 50 ppm Th (Easton 2010), thus they are also a potential source of uranium. In contrast, uranium contents of other Archean felsic intrusions in the Elliot Lake area are 1 to 4 ppm (Easton 2010). It may be no coincidence that the most negative of the Matinenda Formation samples was collected only a few metres below the mineralized Main Conglomerate Bed.  The uppermost Huronian Supergroup units have ages of circa 2310 Ma (Hill et al. 2018; Rasmussen et al. 2013), meaning deposition of the entire supergroup occurred between 2460 to 2310 Ma.  Persistent throughout the sequence are occasional Geon 25 grains, typically with ages of 2550-2590; these grains become somewhat more abundant in the upper 2 groups. These grains have no known local source, and as suggested by Bleeker (personal communication, 2019). may have a source region to the south, such as the Kaapvall craton, that was subsequently rifted away from North America.  Currently it is not possible to determine if the detrital zircon populations differ between glaciogenic (e.g., Ramsay Lake) and non-glaciogenic sandstone units. In the Elliot Lake area, the Ramsay Lake Formation has a zircon population consisting only of Geon 26 and Geon 27 grains, similar to the population present in the underlying Matinenda Formation (Easton and Heaman 2011; Ménard 2019).  Grains >3000 Ma occur sporadically throughout the Huronian Supergroup, mainly in the Matinenda and Mississagi Formations, and could be sourced locally from Michigan (see Ayuso et al. 2017). More difficult to explain is the population of 29 ancient grains, 3000-3600 Ma, in the Gowganda Formation sample from Cobalt. Is this sourced locally in the Cobalt area, or have these grains been transported from sources currently exposed on the northeast shore of Hudson’s Bay? It is unclear if the sampled unit is glaciogenic or not, as the sampled rock type was not specified by Kenny et al. (2017). In summary, the neodymium data, and the detrital zircon and geochemical data reported by Easton and Heaman (2011), are all consistent with a local source region that included radiogenic granites found north and northwest of Elliot Lake. The absence of similar radiogenic granites north of the Huronian Supergroup between Elliot Lake and Sault Ste. Marie may explain why the Matinenda Formation west of Elliot Lake contains no significant uranium occurrences. Nd isotope data reported by Easton (2012) supports the conclusions based on the detrital zircon studies. Archean felsic volcanic and granodiorite samples from the Whiskey Lake greenstone belt, have positive ΕNdT values close to the depleted mantle evolution curve. 226 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Table 2. Summary of data for all Huronian Supergroup samples based on grains ≤ 5% discordant, in most studies many more grains were analyzed. For samples with significant discordance, the lower numbers shown are for grains ≤ 10% discordant. Also indicated are grains per Geon. All samples are sandstones unless otherwise noted. Samples from the Cobalt Basin are in italics. Abbreviations: cong, conglomerate; EL, Elliot Lake area; MCB, main conglomerate bed; S, Sudbury area; TH, Thessalon area. Table updated from Easton (2019). Number Range (Ma) Bar River mudstone Bar River EL Gordon Lake EL Gordon Lake EL Lorrain EL Gowganda Formation n=16 n=62 n=57 n=30 n=172 2279-2745 2523-3074 2284-2840 3 sites 2684-2890 2520-3614 Serpent EL Serpent EL-S Mississagi EL Mississagi EL Mississagi EL-S Mississagi (upper) S Mississagi S Mississagi S Ramsay Lake EL Ramsay Lake EL-S Ramsay Lake S Ramsay Lake S cong McKim S Mississagi cong Matinenda S n=46 n=10 n=19 n=125 n=63 n=22 n=130 n=117 n=72 n=84 n=65 n=25 n=37 n=36 n=210 n=39 2549-3576 2531-3317 2531-3317 2420-3499 2443-3617 2591-2832 2388-3286 2414-2978 2544-2949 2658-2781 2656-2887 2526-2719 2607-2821 2533-2752 2366-2906 2505-3774 Matinenda EL-S n=27 2451-2714 Matinenda EL Matinenda (upper) EL Matinenda EL Matinenda above MCB EL Matinenda below MCB EL Livingstone Creek TH n=30 n=47 2650-2742 2620-2897 2344 2706 (27>>26) 2317, 2702 (26≈27) 2308, 2308, 2311 2713 (27>26) 2705, 2857, 2965, 3076, 3316 (27>26) 2719 (27>>26) 2688 (5%) 2688 (10%) 2450, 2679 (26>27) 2466, 2692 (26>27) 2663 (26>>27) 2477, 2697 (26≈27) 2490, 2560, 2689 2683 (26>>27) 2678 (26>>27) 2697 (26>27) 2659 (26>>27) 2677 (26>>27) 2670 (26>>27) 2459, 2703, 2771 2557, 2661 (26>>27) 2457, 2671 (26>>27) 2680 (26>>>27) 2664 (26>>>27) Main Peak (Ma) n=36 n=5 n=15 n=28 2617-2776 2634-2651 2621-2684 2546-2838 2649 (26>>>27) 2641 (5%) 2643 (10%) 2641 (26>>>27) n=37 2507-2890 2698 (26≈27) 227 24 25 26 27 28 29 >3.0 1 18 20 2 30 22 2 1 4 3 2 5 5 3 2 6 34 18 51 6 38 18 29 9 4 8 45 24 18 57 47 44 54 35 19 27 26 78 22 22 4 8 34 13 1 57 39 22 30 28 3 8 7 122 5 11 1 3 1 1 7 13 9 20 4 25 47 5 3 3 3 33 5 15 24 1 17 16 19 4 2 3 1 1 2 3 5 2 2 10 4 3 1 3 3 11 3 10 4 1 9 11 2 2 7 2 2 4 1 1 1 1 3 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Tectonic Models for the Development of the Huronian Basin More recently, Roscoe and Card (1992), noting the close stratigraphic correlation between the Paleoproterozoic sequences of the Wyoming craton and the Huronian Supergroup, proposed that the Superior and Wyoming cratons are rifted portions of what was once a single continental land mass. They suggest the direction of the Matachewan-Hearst dike swarm (2475-2460 Ma) indicates an east-west tensional regime, which resulted in, a Huronian basin elongated in a northsouth direction. On this larger craton, sediment was deposited in a southward-deepening intracratonic basin. Roscoe and Card (1992) proposed that it was during the Nipissing igneous event (2217 Ma) that successful rifting of the Superior Province took place with the eventual drifting of part of the missing Superior Province to its present location as Wyoming craton. They attribute pre-Nipissing folding to the Blezardian orogeny of Stockwell (1982) and the later, more important, deformation to be coeval with the Penokean orogeny of Michigan, Wisconsin and Minnesota (Roscoe and Card 1992). Various tectonic models have been proposed for the early development and later deformation of the Huronian basin. Many reconstructions are essentially modifications of the model put forth by Dietz and Holden (1966), which stated that the Huronian Supergroup represents a rift and passive margin sequence that was compressed, partly tectonically buried, and metamorphosed during a collision with the Superior craton and another mass which overrode its southern edge. Zolnai et al. (1984) and Bennett et al. (1991) accepted the essential aspects of the Dietz and Holden (1966) model. The model proposes that rifting and continental break-up was coeval with Huronian Supergroup volcanism (2475-2450 Ma) and that the much later regional deformation occurred coincident with the Penokean Orogeny (1860-1835 Ma). This model does not attempt to account for the multiple deformation events affecting Huronian Supergroup rocks or the origin of the Nipissing intrusions. Jackson (2001) supported the model of Roscoe and Card (1992) since the high heat flow, which he considers necessary to give the observed features of the high-grade metamorphic rocks, would be a necessary effect of mantle upwelling during continental break-up. He also interpreted some of the early high-strain deformation as being consistent with a Nipissing-age break-up of the Superior craton. Young (1982) proposed that the Huronian Supergroup was deposited in an aulocogen, an easterly-trending fault-bounded trough, which opened toward an ocean in the area now occupied by the Grenville Province. Sims et al. (1981) concluded that the Huronian Supergroup, the Marquette Range Supergroup, and Animikie Group rocks were deposited as intra-continental, fault-controlled basins along a major, Neoarchean structure, the Great Lakes Tectonic Zone. 228 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Acknowledgements The senior author wishes to thank Mike Hailstone, former Sault Ste. Marie Resident Geologist (after Gerry Bennett) for introducing him to this trip back in 2008. This was followed by a mapping program in the Elliot Lake to Pecors Lake area between 2009 and 2011. Figure 12. Index map for included geological maps and some areas mentioned in the text. Figure modified from Bennett (2006). 229 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figure 13. Legend for Figure 14. Figure modified from Bennett (2006) 230 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Figures 14. Upper. Geological map of the Quirke Lake syncline with stop locations. Lower. Geological map of the Flack Like area with stop locations. Both modified from Bennett (2006). 231 �Proceedings of the 68th ILSG Annual Meeting - Part 2 FIELD TRIP DETAILS 0.0 km. Bridge across Depot Lake (approximately 15.4 km from visitor centre). Enter into the Whiskey Lake greenstone belt (Archean). Set odometer to zero. Geological Maps Geological compilation maps covering all or parts of the area of the field trip include Giblin and Leahy (1979); Johns, McIlraith and Muir (2003); and Easton (2013a). The southern part of the field trip area, including optional Stops A-C and Stop 1, as well as the Whiskey Lake greenstone belt and the eastern part of the Quirke Lake syncline, are depicted on Easton (2013a). 1.0 km. Approximately a kilometre north of the bridge, pull over on the right shoulder on the passing lane up the hill. Examine exposures on the large rock face on the right side of the road. Highway 108. This is a new stop. Optional STOP A: Archean metasedimentary rocks of the Whiskey Lake greenstone belt ROAD LOG UTM co-ordinates 381165E 5133640N Note: Caution should be taken when parking vehicles on the shoulder of the highway and when examining outcrops located along Highways108 and 639 and on other roads along the field trip route. All UTM co-ordinates are given in NAD 83 datum, zone 17, which is essentially equivalent to NAD WGS84. Here we see thin- to medium-bedded turbidites of the Archean Whiskey Lake greenstone belt. Partial Bouma sequences are present in the thicker turbidite beds. The turbidites likely have a high felsic volcaniclastic component, as a sample from this outcrop for detrital zircon study yielded no zircons or titanite (Photo1). Note: This guidebook describes a total of 25 stops (17 stops and 8 optional stops). If one is starting in Elliot Lake, it is possible to do all of the stops in one day. If coming from Sudbury, then some stops will need to be omitted (mainly the optional stops). All 16 stops from Bennett (2006) are included in the road log, along with an additional 9 stops added by the senior author. Leave from Science North at the junction of Paris Street and Ramsey Lake Road in Sudbury. Head to Highway 17 and proceed west toward Sault. Ste. Marie. At the junction of Highway 17 and 108, turn right onto Highway 108 and head north to Elliot Lake (29 km (18 mi) from, Highway 17). Set odometer to zero at this point. Photo 1. Thick volcaniclastic bed in turbidites at Optional Stop A. Hammer handle is 33 cm long. From the junction with Highway 17 to the bridge at Depot Lake, the highway passes through the Ramsey-Algoma granitoid complex. The rocks along the highway have not been studied in detail. The granitoid rocks include xenoliths of mafic rock and are cut by numerous dikes of the Matachewan dike swarm (circa 2460 Ma), which are mediumgrained, medium-green, and locally plagioclase porphyritic. Also present are fine-grained, flinty, mafic dikes that may represent feeders to Huronian Supergroup volcanic rocks. Although not observed in this outcrop, as one heads along the highway and up stratigraphy, the turbidites pass into thinly bedded mudstones and then into a magnetite facies iron formation. These metasedimentary rocks lie atop the older (circa 2740 Ma) of the 2 volcanic sequences that comprise the Whiskey Lake greenstone belt (Easton 2013a). Return to vehicles, continue on Highway 108 232 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Archean Whiskey Lake greenstone belt (Photo 2). A U/Pb zircon sample from this outcrop yielded a CA-TIMS age of 2724.9±1.4 Ma (Hamilton in Easton 2010) indicating that these rocks are among the oldest in the greenstone belt. 2.3 km (1.44 mi). Turnoff to Elliot Lake airport on the left. Continue straight. 3.6 km (2.25 mi). Pull over on the shoulder of the road. Examine long roadcut on the right side of the highway. This is a new stop. The northwestern outcrop consists of extremely flattened tuff-breccia (Photo 2). The higher strain in this outcrop may reflect its location in the contract strain aureole of the large granodiorite body that we will examine at Stop 1. Return to vehicles, continue straight (westward) on Highway 108. Optional STOP B: Stone Ridge intrusion UTM co-ordinates 379693E 5135505N The roadcut exposes part of an east-trending metagabbro intrusion, termed the Stone Ridge intrusion. The intrusion is 700 to 1000 m wide, with a minimum strike length of 15 km. It lies 1 to 2 km south of, and roughly parallels, the Archean– Proterozoic unconformity (Easton 2009, 2013a). As seen in this roadcut, large parts of the intrusion contain preserved primary mineralogy. The predominant rock type is a weakly metamorphosed, grey to light grey weathering, medium-grained, leuconorite to leucogabbronorite. Where recrystallized, orthopyroxene is altered to amphibole, and the rock takes on a greener colour. Texturally, the body is remarkably uniform, but coarse-grained to pegmatitic patches of gabbro occur along the northern margin of the intrusion. Matachewan dikes (circa 2460 Ma), which were observed to intrude the body (Easton 2009), would preclude the Stone Ridge Intrusion being part of the Nipissing intrusive suite, which was not emplaced until circa 2217 Ma, suggesting that the Stone Ridge intrusion is more likely part of the East Bull Lake intrusive suite. 6.1km (3.81 mi). Turnoff to the left takes you to the Discovery Site lookout. Several large boulders representative of the uraniumbearing “Main Conglomerate Bed” are present in the parking area of the lookout. Looking northeast from the highway and/or the lookout, you can see a large ridge of greenish sandstone of the Matinenda Formation. The first mine in the Elliot Lake Camp, the Buckles Mine, was located at the base of this ridge opposite the turnoff. 6.8 km (4.25 mi). Pull over and park near the middle of a large roadcut on the north side of the highway This is a new stop. STOP 1. Thessalon Formation feeder dike and Archean granodiorite UTM co-ordinates 377092E 5136767N The roadcut contains a 15 m wide, near-vertical mafic dike which is vesicular (Photo 3). The vesicular nature of the dike is best observed on the roadcut on the south side of the Highway (Photo 4). The mafic dike has the composition of a tholeiitic andesite on a Jensen discrimination diagram (54.9 wt.% SiO2, 2.38 wt.% K2O, magnesium number of 36 (data in Easton (2013b)) and is a high-K basaltic andesite in the IUGS total-alkali silica classification. Return to vehicles, continue northward of Highway 108 to the junction with Nordic Road and the golf course. 5.3 km (3.31 mi.) Junction with Nordic Road. Park safely and examine outcrops on the northeast and northwest sides of the intersection. This is a new stop. Optional STOP C: Archean metavolcanic rocks of the Whiskey Lake greenstone belt UTM co-ordinates 378483E 5136436N geochronology site; 378486E 5136443 flattened tuff The northeastern outcrop consists of reversely graded felsic tuffs and felsic tuff-breccias of the 233 �Proceedings of the 68th ILSG Annual Meeting - Part 2 Photo 3. Roadcut on north side of Highway 108 at Stop 1. The dark, 10 m wide, vertical, steep-walled mafic dike is intruded Archean granodiorite, which has a U/Pb zircon age of 2674.8±0.8 Ma (Easton 2013a). This dike likely feed Thessalon Formation flows. Photo 4 Close-up of mafic dike rock on the south side of Highway 108 at Stop 1 with irregular, white vesicles in the dike. Pen is 13.5 cm long. The medium-grained granodiorite that forms the bulk of the roadcut is typical of the younger intrusive bodies within the Ramsey-Algoma granitoid belt. A sample collected from the roadcut on the south side of the road yielded a CA-TIMS age of 2674.8±0.8 Ma (Easton 2013a). The youngest age reported so far from metavolcanics of the Whiskey Lake is 2685.5±1.1 Ma (Easton 2013a). Photo 2. Felsic volcanic rocks at Optional Stop C. Upper. Lapilli tuff in the northeastern outcrop which was sampled for geochronology. Knife is 9 cm long. Middle. Moderately flattened tuff breccia from the northwestern outcrop. Lower. Strongly flattened tuff breccia from the northwestern outcrop. Hammer handle is 33 cm long. Return to vehicles, continue straight (westward) on Highway 108. 234 �Proceedings of the 68th ILSG Annual Meeting - Part 2 7.1 km (4.44 mi). Pull over and park on the shoulder of the highway. Cross over to the former route of the highway and walk west toward the intersection of the new and the old highways. This is a new stop. Optional STOP D: Paleoweathering of Archean granodiorite UTM co-ordinates 376567E 5136603N Proceeding west along the outcrops present on the north side of the former highway, you can observe the progression from grey granodiorite to reddishweathering granodiorite to greenish granodiorite, with the change in coloration reflecting increasing paleoweathering of the granodiorite (Photo 5). Key elemental changes between the 3 phases are summarized in Table 3, and include increasing iron, manganese, magnesium, potassium and aluminum contents, with decreasing silica content. Table 3. Element changes related to paleoweathering in granodiorite at Optional Stop D. Data from Easton (2013b). Major elements are in weight percent. Abbreviations. LOI, loss on ignition; CIA, chemical index of alteration. Element Photo 5. Paleoweathering in Archean granodiorite at Optional Stop D. Upper. Grey, unweathered granodiorite. Middle. red-green weathering granodiorite. Lower. Green weathering granodiorite. See Table 3 for chemistry on each type. Pen is 9 cm long. Photo 5 Grey, not weathered upper Red weathered middle Green weathered lower SiO2 69.1 60.1 52.7 Al2O3 15.7 19.4 19.6 Fe2O3total 3.2 5.0 9.9 MnO 0.02 0.06 0.12 MgO 1.9 2.7 5.5 CaO 1.0 0.5 0.7 Na2O 6.8 7.9 6.9 K2O 1.0 2.5 0.8 LOI 1.3 2.0 3.5 CIA 89 87 70 Th (ppm) 7 12 13 TiO2, P2O5, U, Zr nearly constant in all 3 samples Return to vehicles, continue straight (west) on Highway 108. Continue straight on the highway past Hillside Drive South. 235 �Proceedings of the 68th ILSG Annual Meeting - Part 2 9.2 km (5.75 mi). Roadcut to the right of the highway just west of the junction with Esten Drive north consists of coarse sandstone and pebblestone of the Matinenda Formation similar to what we will see at Stop 3. The pink colouration in this roadcut is related to its proximity to a fault located approximately along the highway route. UTM co-ordinates 370462E, 5137991N The low outcrops on north side of Spine Road are grey, buff and dark-grey sandstone and radioactive, pyritic, quartz-pebble conglomerate of the Matinenda Formation. Pebble units, located near the top of the outcrop, are rusty-weathering, about 20-30 cm thick, and dip about 10 degrees to the north (Photo 6). Pebbles in this outcrop are 1-2 cm across and are generally much smaller than the typical pebbles in the ore zones of the Elliot Lake mines. Scintillometer readings from the pebble beds, collected by R.M. Easton between 2009 and 2022, range from 15,000 to 18,000 counts per second, with 600 to 800 ppm U and 600 to 800 ppm Th, with the U content being typical of the ore grades from the Elliot Lake camp. Continue straight on the highway and past the first few stoplights to Hillside Drive North. Turn left (west) onto Hillside Drive North. If going directly to Stop 2, once on Hillside Drive North, continue west for approximately 1 km to Spine Road. Turn right (west) onto Spine Road and drive past the hospital to Lawrence Avenue at the far west end of Spine Road (~2.1 km). Park in the turn-around at the end of Spine Road for Stop 2. This is Stop 15 of Bennett et al. (1997) and Stop 1.2 of Bennett (2006). If going to Optional Stop E, continue west on Hillside Drive North for about 400 to Spruce Avenue. Turn right on Spruce Avenue and continue approximately 100 m to Valley Crescent, turn right onto Valley Crescent and follow it for approximately 350 m to Balsam Place. Turn right on to Balsam Place and stop at the end of the cull-de-sac. This is Stop 1.1 in Bennett (2006). Optional STOP E: McKim Formation and Nipissing Diabase Photo 6. Matinenda Formation at Stop 2. Hammer head is resting on radioactive, rusty-weathering pebble beds that are the focus of this stop. UTM co-ordinates 372751E, 5138695N Outcrops on the east side of the cul-de-sac are mudstone and grey sandstone of the Mckim Formation. Note the deflection of the axial plane cleavage in the mudstone units. The movement of adjacent beds inferred from the deflection of the cleavage indicates the south limb of a syncline. A gabbroic dike is separated from the sedimentary rocks by a zone of sheared and fractured rocks. The McKim Formation is missing on the north limb of the syncline. Ruzicka and LeCheminant, (1984) reported the radioactive conglomerate contains “rare-earthelement-bearing uranothorite (?), large zircons, a Ti-U-Si-Fe phase (brannerite?), chalcopyrite and chromite. The distribution of radioactive minerals in the conglomerate displays layering thus indicating a detrital origin of these grains”. STOP 2. Radioactive quartz-pebble conglomerate, Matinenda Formation 236 �Proceedings of the 68th ILSG Annual Meeting - Part 2 STOP 3. Coarse sandstone, floater reef zone, Matinenda Formation, Elliot Lake Group The dark-grey areas in the radioactive beds are due to the presence of minor amounts radioactive carbon generally known in the Elliot Lake area as “thucolite” also referred to as a hydrocarbon kerogen. Ruzicka and LeCheminant, (1984) noted that several generations of carbon occur in the conglomerates of the Matinenda Formation. The earliest generation occurs as layers concordant with the bedding or as a component of the matrix and appears to have been deposited in areas of quiescent sedimentation during the last phase of an upward-fining sedimentary cycle. UTM co-ordinates 373385E 5137830N Created in 2014, this rock face exposes greenish coarse sandstone and pebblestone of the Matinenda Formation. Compared to Stop 2, we are now only 25 m higher in the stratigraphy above the “Main Conglomerate Bed” in what is locally referred to as the “Floater Reef Zone”. The name is derived from the fact that many of the pebblestone beds are variably radioactive (typically 2,000 to 5,000 counts per second). Scintillometer data collected by the senior author from this Stop give 20 to 25 ppm U and 130 to 150 ppm Th. Later generations of thucolite are probably remobilized phases of the first generation. The carbonaceous matter in the Elliot Lake ores is comparable in occurrence and composition to hydrocarbon in the Witwatersrand gold reefs; interestingly Ruzicka and LeCheminant (1984) report elevated gold content (1000-2000 ppb) in the carbonaceous matter of the Elliot Lake ore beds. The radioactive carbon at this site is reported to be auriferous, although the gold content is not available. At the north-end of the rock face is a 5 m wide medium-grained mafic dike cutting the Matinenda Formation. The affinity of the dike is unknown, and no geochemical data are available for this dike. It cannot be a Matachewan dike as it cuts the Matinenda Formation, and it does not have the scintillometer characteristics of a Nipissing gabbro (potassium is too low). It could be related to a suite of east-trending dikes in the Elliot Lake area, such as the one at Stop 14. In 1955 Rio Algom Mines Limited completed a diamond drill hole about 30 m south of this location. The drill log shows that the radioactive beds exposed here are about 35 metres above the Archean basement rocks. This drilling indicated that there are no ore-grade units in this area. Return to vehicles. Exit the south end of the mall parking lot, turn left onto Hillside Drive South, continue east on Hillside to the traffic lights (Highway 108), approximately 300 m. Grab samples collected by G. Bennett in 1982 and reported in Bennett (2006) returned up to 0.80 lbs U3O8/ton and 0.78 lbs ThO2/ton (340 ppm U, 340 ppm Th). A continuous chip sample returned 0.31 lbs U3O8/ton and 0.53 lbs ThO2/ton (130 ppm U, 232 ppm Th). Turn left onto Highway 108 and head north. Reset odometer to zero Retrace route on Spine Road to Hillside Drive. At the traffic light, turn right onto Ontario Drive heading south. Vacant lot on the right is the site of the former Algo Mall, which had a catastrophic collapse on June 23, 2012. Continue for approximately 450 m on Ontario Avenue and where the road bends, continue straight into the retail mall parking lot. Park and examine the large rock face on the west side of the mall parking lot. This is a new stop. 2.1 km (1.31 mi), Westview Park on Elliot Lake on the left. 1.1 km (0.69 mi). Miners Memorial Park and Horne Lake on the right. Cliff on the east side of Horne Lake consists of Mississagi Formation sandstone (Photo 7). 2.4 km (1.5 mi). Stanleigh Road on the right, 237 �Proceedings of the 68th ILSG Annual Meeting - Part 2 (R.M. Easton, unpublished data). The broad age range of zircons in this sample contrasts greatly with the limited age range (2658-2781 Ma) present in a conglomerate of the underlying Ramsay Lake Formation, which was sampled by Ménard and Easton in 2017, 13 km to the east-southeast of the field trip stop (Ménard 2019). The population in the Ramsay Lake Formation sample is almost identical to the 4 Matinenda Formation samples studied by Easton and Heaman (2011), only a few hundred metres to the south of the conglomerate. Photo 7. View from the Miners Memorial across Horne Lake showing cliff of Mississagi Formation sandstone. 3.5 km (2.19 mi). Pull-over where snowmobileATV trail intersects Highway 108. Cross Highway to outcrop on the west side. This is Stop 2.3 of Young (1991), Stop 16 of Bennett et al. (1997) and Stop 1.3 of Bennett (2006). STOP 4. Mississagi Formation, Hough Lake Group UTM co-ordinates 371717E, 5140426N One-metre-thick beds of grey sandstone of the Mississagi Formation on the west side of the highway display the rusty staining on the face of the outcrop reflecting the minor pyrite content along the foreset beds of trough cross-beds (Photo 8). The paleocurrent direction (from the west) can be best observed on the upper surface of the outcrop. — Please exercise caution when walking on smooth, wet rock surfaces — The grey colour of these sandstones and the presence of apparent detrital pyrite are held by most geoscientists to indicate the very low partial pressure of free oxygen of the atmosphere during the deposition of the Mississagi Formation. Photo 8. Crossbedding in Mississagi Formation sandstone at Stop 4. Return to vehicles, continue north on Highway 108 for 1.4 km. 4.1 km (2.56 mi). Pull over on the right shoulder of the passing lane part way up the hill. This is Stop 2.4 of Young (1991); Stop 17 of Bennett et al. (1997) and Stop 1.4 of Bennett (2006). STOP 5. Nipissing gabbro, altered Mississagi Formation, Bruce Formation UTM co-ordinates 371652E, 5140948N J.A. Ménard and R.M. Easton collected a sample for detrital zircon geochronology from this stop in 2017 (Photo 8). The zircon population ranged in age from 2420 to 3499 Ma, with notable peaks at 2450 Ma (13 grains) and 2679 Ma (45 grains), and with 10 Geon 28 grains and 13 grains ≥3000 Ma (Ménard 2019). The abundance of circa 2450 Ma grains compared to other Mississagi Formation samples (see Table 2) is not unexpected given the abundance of Elliot Lake Group metavolcanic rocks to the north of this site near Dunlop Lake We will start by examining the low outcrops on the west side of Highway 108. Please exercise caution when crossing the highway. Low, rounded outcrops at the south end are Mississagi Formation sandstones similar to those we saw at Stop 4, but slightly pink in colour, likely due to the formation of albite by hydrothermal fluids from the adjacent Nipissing gabbro intrusion (east roadcut). After an outcrop gap, conglomerate 238 �Proceedings of the 68th ILSG Annual Meeting - Part 2 of the Bruce Formation is well-exposed. In particular note a clast with a gneissic fabric, as well as numerous, subrounded to subangular, white, granitoid clasts typical of the formation (Photo 9). The abundant matrix is dark-grey to black. Large, glassy, quartz grains are abundant on fresh surfaces of the conglomerate, another feature typical of the formation. The glassy, black, appearance results from the dark matrix behind the clear quartz (Bennett 2006). has been interpreted as the result of erosion by high-pressure, waterborne sediment, presumably by the melting of an adjacent Pleistocene ice sheet (Bennett 2006). North of gabbro sill, the upper portion of the Mississagi Formation is exposed along the east side of the highway and is also pinkish. A few tens of metres northward, the Mississagi Formation is overlain by diamictite of the Bruce Formation (UTM 371668E, 5141245N). The dispersed megaclasts in the Bruce Formation are predominantly grey granitic rocks with smaller mafic clasts, predominantly Thessalon Formation volcanic rocks. At this locale, there is no evidence of a significant disconformity at the base of the Bruce Formation. Return to vehicles and proceed approximately 300 m to near the top of the hill and park on the right shoulder. 4.4 km (2.75 mi). Roadcuts are present on both sides of the highway. This is Stop 18 of Bennett et al. (1997) and Stop 1.5 of Bennett (2006). STOP 6. Espanola Formation, Quirke Lake Group and Nipissing gabbro sills The base of the Espanola Formation is a green, laminated unit about a metre or so thick. Laminated silty limestones and minor thin, chert beds of the limestone member of the Espanola Formation (Photo 10), overlie the green unit. At this location, the proximity of Nipissing gabbro sills (Photo 11) has led to the development of calc-silicate minerals including: grossular garnet, diopside, idocrase (vesuvianite), and wollastonite typical of a skarn (Robertson 1968; Bennett 2006). Wollastonite (identified by X-ray diffraction) is found just below the north-dipping gabbro sill near the north end of the exposure, where it occurs as sub-parallel groups of pale grey to white prismatic crystals about 1 mm wide and up to a cm long. The pink coating on joint surfaces is apophyllite (KFCa4[Si8O20]8H20) an uncommon mineral (identified by X-ray diffraction), sometimes found in amygdules in basalts, but which is also Photo 9. Ramsay Lake Formation conglomerate at Stop 4. Note a gneissic clast just below the rusty spot at the centre of the photo. Knife is 9 cm long. A sill-like body of Nipissing gabbro is exposed at the south end of the roadcut on the east side of the highway. Rhythmic, compositional layering is visible on the vertical face of the roadcut. Additional evidence of hydrothermal activity along the intrusion contact is seen by dark-green to black chlorite deposited along fractures in the gabbro (Bennett 2006). Near the north end of the Nipissing outcrop face, note the relatively planar, striated, surface is truncated by more irregular surface that 239 �Proceedings of the 68th ILSG Annual Meeting - Part 2 associated with calc-silicates. Young (1991) states that the small scale thrust faults and folds in the limestone on the west side of the highway are probably the result of slumping during early tectonic activity. The upper, ferruginous dolostone-bearing member of the Espanola Formation and the overlying Serpent Formation are not present at this location but are well represented on the north limb of the syncline. Young (1991) and Bennett (2006) both suggested that the missing Serpent Formation and ferruginous dolostone member were removed during a period of pre-Gowganda Formation erosion, which are visible in roadcuts along the next stretch of the highway. Alternatively, a fault could be present in the linear valley at the end of the outcrop, which has truncated stratigraphy. Photo 11. Nipissing sill (dark) in contact with Espanola Formation marbles (white) at Stop 5. Note discoloration of the marbles near the sill contact. View west across Highway 108. 4.5-5.0 km (2.81-3.13 mi). Diamictite and minor sandstone of the Gowganda Formation are exposed in a near-continuous roadcut along Highway 108. In these exposures, megaclasts of pink granite, grey granite and granitic gneiss and mafic rocks are widely distributed in a dark green matrix. A typical example can be seen at 4.8 km. Most geologists now consider at least some of the diamictites in the Gowganda Formation to be tillites, although a debris-flow origin, either glaciogenic or as submarine debris flows, is a more reasonable interpretation at specific localities. Roscoe (1969) places the appearance of free oxygen in the atmosphere (“oxyatmoversion”) as coinciding with the appearance of the reddish hue of hematite just above the base of the Gowganda Formation. Return to vehicles and continue north on Highway 108. 6.5 km (4.06 mi). The stop is at a large roadcut at the top of a hill, near a communication tower. Park at the south end of the roadcut on the east side of the highway. This is Stop 2.5 of Young (1991), Stop 19 of Bennett et al. (1997) and Stop 1.6 of Bennett (2006). Photo 10. Espanola Formation at Stop 5. Note white recessive weathering limestone beds and thinly laminated, darker, calc-silicate and mudstone beds. Hammer handle is 33 cm long. This is an impressive exposure though a stratified sequence of diamictites, clastsupported conglomerates and sandstones of the Gowganda Formation. 240 �Proceedings of the 68th ILSG Annual Meeting - Part 2 STOP 7. Stratified Gowganda Formation, Cobalt Group UTM co-ordinates 371841E, 5143180N At the south end of the roadcut on the west side of the highway, massive diamictite is exposed. It is overlain by about 50 cm of laminated mudstone with dropstones, in turn overlain by a thick succession of lenticular beds of coarse pink and pink-grey arkosic sandstone interbedded with distinct beds of diamictite (Photo 12), pebbly sandstone and clast-supported polymictic conglomerate (Photo 13). Some conglomerate units display normal and reverse grading suggestive of debris flows. Some sandstones contain large clasts. Some clasts in the diamictite show striations, suggestive of a glacial origin. Clasts in the conglomerate are mainly wellrounded fragments, but some rip-up clasts of sandstone are also present. The rocks displayed here may be interpreted as debris and mass flows, possibly formed in an ice-proximal setting by resedimentation of glacial debris at a retreating glacial margin (Young 1991; Bennett 2006). Photo 12. Gowganda Formation at Stop 7 showing pink-grey arkosic sandstone (lower) overlain by matrix- to clast-supported conglomerate. Hammer handle is 33 cm long. Note the predominance of red and pink granitic clasts, in marked contrast to the pale grey clasts of the Bruce Formation seen earlier at Stop 5. There is also a significant proportion of black pebble to cobble-sized clasts. The mineral assemblage and metamorphic grade of a few mafic clasts examined by G. Bennett many years ago indicated that the clasts were probably from Thessalon Formation basaltic flows (Bennett 2006). Return to vehicles and continue north on Highway 108. 7.4 km (4.63 mi). Pink sandstone and diamictite of the Gowganda Formation. 9.4 km (5.88 mi). Diamictite with large boulder, Gowganda Formation. Photo 13. Gowganda Formation matrix-supported conglomerate (lower) overlain by clast-supported conglomerate at Stop 7. Hammer handle is 33 cm long. 12.0 km (7.5 mi). Stanrock Road. Reset odometer. Turn east onto Stanrock Road. 241 �Proceedings of the 68th ILSG Annual Meeting - Part 2 2.0 km (1.25 mi). park on the south side (right) of the road opposite a roadcut on the north side. This is Stop 2.6 of Young (1991) and Stop 1.7 of Bennett (2006). STOP 8. Laminated varvite? Gowganda Formation, Cobalt Group UTM co-ordinates 374911E, 5147007N Laminated siltstone/mudstone of the Gowganda Formation on the north side of the road. This unit has been interpreted as being similar to the varves found as deposits in Pleistocene glacial lakes. Continue east on Stanrock Road. 9.5 km (5.94 mi). Turn right onto Popeye Lake Road. 9.8 km (6.13 mi). Park and examine outcrops on the west side of the road. STOP 9. Gowganda—Serpent Formation disconformity UTM co-ordinates 381485E, 5144900N The Serpent Formation consists of fine, wellsorted sandstone and siltstone, and is typically light grey. The exposure at this Stop shows typical sandstones of the formation (Photo 14), just below the disconformity with the overlying Gowganda Formation. The contact can be seen partway up the hill above the road level exposures of the Serpent Formation. The contact is sharp but irregular, and the Gowganda Formation consists of polymictic, matrix-supported, conglomerate Photo 14. Serpent Formation. Upper. Crossbedding in sandstone at Stop 9. Lower. Indistinct, medium bedding in fine sandstone at Stop 9. Hammer handle is 33 cm long. Optional STOP F. Gowganda—Serpent Formation disconformity UTM co-ordinates 375102E, 5150499N The Serpent Formation is not present in the south limb of the Quirke Lake Syncline and in the Blind River—Sault Ste. Marie area where it was probably removed during a period of preGowganda Formation erosion. At this location, on the south side of the road, well-sorted sandstone of the Serpent Formation is overlain by polymictic conglomerate of the Gowganda Formation. The contact is sharp but irregular. Evidence of a subGowganda Formation disconformity at this location is based on the presence of pebble and cobbles of the Serpent Formation near the base of the overlying Gowganda Formation. Return to vehicles, retrace route to Highway 108. Reset odometer to zero at the junction. Turn right onto Highway 108 and continue north. 3 km (1.9 mi). Denison Mine Road - Turn east. Reset odometer to 0. 1.5 km (0.95 mi). This is Stop 2.7 of Young (1991), Stop 20 of Bennett et al. (1997) and Stop 1.8 of Bennett (2006). Return to Highway 108. Reset odometer at Highway 108 and Denison Mine Road. 242 �Proceedings of the 68th ILSG Annual Meeting - Part 2 0.5 km (0.3 mi). Disseminated carbonate in sandstone of the Serpent Formation on the east side of Highway 108. The detrital zircon sample of the Serpent Formation reported by Rainbird and Davis (2006) and Craddock et al. (2013) came from this roadcut. The population ranged from 2549 to 3576 Ma, with the dominant population at Geon 27, but with 11 Geon 28 grains (see Table 2). 1.1 km (0.68 mi). Road to Quirke Lake and the former Panel Mine. Reset odometer to zero. Turn east onto Panel Mine Road. 0.6 km (0.38 mi) and 1.5 km (0.9 mi). Two outcrop areas are exposed on the west side of the road. The southern of the 2 areas is currently betterexposed. The second area to the north is Stop 2.8 of Young (1991), Stop 21 of Bennett et al. (1997) and Stop 1.9 of Bennett (2006). Photo 15. Dropstone in laminated dolostone of the Espanola Formation at Stop 10. Pen is 13 cm long. STOP 11. Ramsay Lake Formation overlain by Pecors Formation STOP 10. Upper member of the Espanola Formation, Quirke Lake Group UTM co-ordinates 377379E, 5152019N Diamictites of the Ramsay Lake Formation contain cobbles of grey granitic rocks, mafic clasts of Huronian Supergroup metavolcanic rocks and Archean felsic metavolcanic clasts in an abundant dark-grey to black sandy matrix. The Ramsay Lake Formation is overlain by dark laminated siltstone and mudstone of the Pecors Formation (Photo 16). The latter contains a few dropstones (Photo 17). Note: the Matinenda Formation of the Elliot Lake Group, expected between the basement and the Ramsay Lake Formation, is truncated by the Ramsay Lake Formation in this area. The Matinenda Formation does occur in the mine workings down-dip from this location. UTM co-ordinates area 1, 374350E, 5151383N area 2, 375258E, 51511331N Both areas exposure ferruginous dolomite and siltstone of the upper member of the Espanola Formation. At the first stop, ripple marks are visible on some bedding surfaces. The upper member is the uppermost of the 3 members of the Espanola Formation recognized in the Elliot Lake area (Robertson 1968). It is characterized by intercalated siltstone and reddishbrown weathering, ferruginous dolostone beds containing 3-4% FeO. Intraformational breccia, ripple marks, small-scale crossbedding, and a variety of soft sediment features are present, but only faintly visible on the south outcrop (Photo 15). Near the east end of the second outcrop a grey clastic dike crosses stratification at a high angle. If you continue east to the end of the Panel Mine Road, you enter the rehabilitated area of the former Panel Mine. There is little evidence of the uranium mine and mill complex that was on this site until 1993. Return to vehicles and continue east on Panel Mine Road. 4.1 km (2.5 mi). This is Stop 2.9 of Young (1991), Stop 22 of Bennett et al. (1997) and Stop 1.10 of Bennett (2006). 243 �Proceedings of the 68th ILSG Annual Meeting - Part 2 1.9 km (1.19 mi). Near the top of the hill a fresh roadcut on the west side of Highway 639 exposes greenish, coarse-grained sandstone of the Matinenda Formation (UTM 372210E 5152225N. We are near the top of the floaterreef zone, so slightly higher stratigraphically than at Stop 3. 2.5 km (1.6 mi). This is Stop 23 of Bennett et al. (1997) and Stop 1.11 of Bennett (2006). We will not visit this stop due to logistical and accessibility reasons. Optional STOP G. Huronian Supergroup volcanic rocks of the Thessalon Formation Photo 16. Laminated mudstone of the Pecors Formation, Stop 11. Hammer handle is 33 cm long. UTM co-ordinates 371508E, 5152302N A gated road leads west from Highway 639 to one of the Quirke Mine tailings dams. Park near the gate and walk a short distance along a rough road from the gate to the base of the tailings dam. Note the very dark green to black, flattened, chlorite amygdules characteristic of the Huronian Supergroup mafic volcanic rocks between Sault Ste. Marie and Elliot Lake. Cross the stream and proceed northward a short distance along a rough road to the crest to the low hill. The Huronian Supergroup volcanic rock at this location (UTM 371428E, 5152342N) include hawaiite and mugearite (Bennett 2006). The eastward-trending, south dipping unconformity between the Archean granitic basement rocks and Huronian Supergroup volcanic rocks is visible near the crest of the hill. There appears to be no paleosol development at this location. Near the west end of the outcrop, a thin, quartz-pebble conglomerate or breccia unit, consisting mainly of angular, quartz-clasts, overlies the granitic rocks at the base of the volcanic unit. Scattered, isolated, mainly cobblesized clasts of quartz are also present along the unconformity. Photo 17. Laminated mudstone of the Pecors Formation containing a dropstone at Stop 11. Knife is 9 cm long. Return to vehicles, retrace route back to Highway 108. Reset odometer to zero at highway. Turn right and continue north on Highway 108. Tailings dam of the Quirke Mine is visible west of the Highway. 0.8 km (0.5 mi). Highway 108 ends and Highway 639 begins. 1.0 km (0.6 mi). Diamictite of the Bruce Formation is exposed on the west side of the highway. 1.5 km (0.9 mi). Outcrops of Mississagi Formation are exposed along Highway 639. Note the yellowish colour characteristic of the Mississagi Formation where it lies directly on the Archean granitic basement (Robertson 1968). Some visitors to this site have proposed that the contact between the Archean and Huronian Supergroup volcanic rocks is not an unconformity, but a fault contact. During a visit to the Stanleigh Mine in 1990, however, G. Bennett observed identical scattered, quartz pebbles along the contact of the Huronian Supergroup volcanic rocks 244 �Proceedings of the 68th ILSG Annual Meeting - Part 2 where they overlie Archean mafic volcanic rocks in a haulage drift on the south limb of the Quirke Lake Syncline (Bennett 2006). Bennett (2006) proposed that these quartz cobbles are lag deposits left behind when the finer sediment was washed off the surface. A few kilometres west of this stop, occurrences of this conglomerate unit contain more rounded quartz grains, and locally are overlain by a thin arkosic sandstone. Return to vehicles and continue north on Highway 639. 2.6 to 10 km (1.63-6.25 mi). Highway 639 passes through Archean granitoid rocks cut by Matachewan and other diabase dikes before entering into dominantly mafic volcanic rocks of the Ompa Lake greenstone belt which has similar ages to the Whisky Lake greenstone belt, namely circa 2685 Ma (see summary in Easton 2010). Photo 18. Pillow structures in Archean mafic metavolcanic rocks at Optional Stop H. Scale card is 10 cm long. Photo from Bennett (2006, p.40). 9.3 km (5.81 mi). This is Stop 24 of Bennett et al. (1997) and Stop 1.12 of Bennett (2006). 11.3 km (7.06 mi). Provincial Park. Optional STOP H. Pillowed Archean metavolcanic rocks 11.6 km (7.25 mi). Large roadcut on the east side of the road. Park on the shoulder. This is an added stop. UTM co-ordinates 368530E, 515773N Entrance to Mississagi STOP 12. Bar River Formation, Cobalt Group Archean mafic metavolcanic rocks with welldeveloped pillow structures are exposed, on a north-sloping outcrop, on the east side of the highway. The pillows are deformed, however, facing directions can easily be determined. Small amygdules are concentrated near the upper surface of many pillows. Lichen growth since 2006 has rendered this stop less spectacular than as indicated in Photo 18. UTM coordinates 367555E, 5159855N Thin to medium bedded, pale grey sandstone of the Bar River Formation with herringbone crossbedding. Return to vehicles and continue north on Highway 639. 12.3 km (7.69 mi). Jim Christ Lake to the northeast (formerly Christman Lake). Park on the right shoulder beside a low ridge of partially blasted roadcuts on the north side of the highway. This is Stop 25 of Bennett et al. (1997) and Stop 1.13 of Bennett (2006). This once spectacular roadcut has suffered from needless road construction damage in recent years. Return to vehicles and continue north on Highway 639. 10.2 km (6.38 mi). Flack Lake fault occupies a valley near this point. 10.7 km (6.69 mi). Outcrops of hematite-stained sandstone of the Bar River Formation. 245 �Proceedings of the 68th ILSG Annual Meeting - Part 2 STOP 13. Bar River Formation, Cobalt Group STOP 15. Gordon Lake Formation, Cobalt Group UTM co-ordinates 366742E, 5160652N UTM co-ordinates 363062E, 5163194N Sandstones and mudstones of the Bar River Formation at this stop display ripple marks, mud cracks and sinuous structures, which have been described as possible worm casts. Comparison with desiccation structures in the Gordon Lake Formation led Young (1969) to suggest that these features are the result of the transportation of consolidated desiccation fracture fillings. Siltstones and sandstones of the Gordon Lake Formation display ripple marks, desiccation cracks, cross bedding and a late cleavage. Note: the presence of pyrite in contrast to hematitic nature of the Gordon Lake Formation near the top of the formation. Return to vehicles and continue north on Highway 639. This outcrop area was sampled by Rainbird and Davis (2006) and Craddock et al. (2013) for detrital zircon geochronology. Population range was 25233074 Ma, with peaks at 2531, 2705 and 2726 Ma. 19.6 km (12.25 mi). Park on the right shoulder roughly midway in a lengthy roadcut on a south-facing hill which has almost near continuous exposures of Nipissing gabbro. Examine outcrops on the east side of the road. Return to vehicles and continue north on Highway 639. 16.1 km (10.01 mi). This is Stop 26 of Bennett et al. (1997) and Stop 1.14 of Bennett (2006). STOP 14. Red beds of the Gordon Lake Formation, Cobalt Group UTM co-ordinates 364395E, 5162758N Laminated, maroon buff or green siltstone and mudstone, and minor chert, represent the upper part of the Gordon Lake Formation. Desiccation cracks and ripple marks are present, as are reduction spots in the maroon beds. It was near this stop that Hill et al. (2018) collected a green siltstone sample which had a limited zircon population (27 grains), but with the 4 youngest grains giving an age of 2302±19 Ma, and with another cluster of 5 grains at 2364±16 Ma. These ages, as well as those of Rasmussen et al. (2016), suggest deposition occurred at circa 2300 Ma. Other populations were 6 grains at 2525±15 Ma, with older grains ranging from 2674 to 3158, but dominated by Geon 27 grains (Hill et al. 2018). Photo 19. Mafic dike at Stop 16 showing sparse, small, plagioclase phenocrysts. Hammer handle is 33 cm long. STOP 16. Nipissing gabbro and post-Nipissing dike UTM co-ordinates 361850E, 5164825N The medium-grained, slightly greenish gabbro is typical of the Nipissing intrusions in the Elliot Lake area. At the stop, the gabbro is cut by a 3 m wide, near-vertical, sharp-walled fine-grained mafic dike that is east-trending. Small plagioclase phenocrysts occur throughout the dike (Photo 19). Return to vehicles and continue north. 17.4 km (10.88 mi). Park on the shoulder of the road and examine outcrops on the east side of the road. This is Stop 27 of Bennett et al. (1997) and Stop 1.15 of Bennett (2006). 246 �Proceedings of the 68th ILSG Annual Meeting - Part 2 STOP 17. Lorrain Formation, Cobalt Group Similar dikes have been observed elsewhere in the Elliot Lake area (Easton 2013a, 2013b), but have yet to be assigned to any specific dike swarm. It is possible they could be related to the circa 1750 Ma Trap dike swarm, or the 2125-2105 Ma Marathon dike swarm. The senior author attempted to obtain an age on the dike, but no suitable phases were recovered for geochronology. Hunt and Roddick (1987) reported a K-Ar whole rock age of 1325 Ma for this dike. UTM co-ordinates 361771E, 5166967N White to pale pink quartz arenite of the upper Lorrain Formation is exposed on the east side of the highway. The detrital zircon sample reported by Rainbird and Davis (2006) and Craddock et al. (2013) came from this Stop. Here, Geon 27 zircons are twice as abundant as Geon 26 zircons, and there are also several Geon 28 zircons present (see Table 2). Return to vehicles and continue north on Highway 639. 23.5 km (15.69). Junction Highway 639 and 546 at Little White River Road. End of Field Trip. Retrace route back to Elliot Lake and return to Sudbury. 21.8 km (13.63 mi). This is Stop 28 of Bennett et al. (1997) and Stop 1.16 of Bennett (2006). End of road log. 247 �Proceedings of the 68th ILSG Annual Meeting - Part 2 References Buchan, K.L. and Card, K.D. 1985. Preliminary comparison of petrographic and paleomagnetic characteristics of Nipissing diabase intrusions in northern Ontario; in Current Research, Part A, Geological Survey of Canada, Paper 85-1A, p.131140. Ames, D.E., Davidson, A., Buckle, J.L. and Card, K.D. 2005. Geology, Sudbury bedrock compilation, Ontario; Geological Survey of Canada, Open File 4570, scale 1:50 000. Ayuso, R.A., Schulz, K.J., Cannon, W.F., Woodruff, L.G., Vazquez, J.A. and Jackson, J. 2017. Evidence for the presence of Eoarchean crust in northern Michigan; in 63rd Institute on Lake Superior Geology, Proceedings, v.63, pt.1, p.9-10. Card, K.D. 1964. Metamorphism in the Agnew Lake area, District of Sudbury, Ontario, Canada; Geological Society of America, Bulletin, v.75, p.1011-1030. ——— 1976. 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Petrology of the Bruce and Gowganda formations and its bearing on Huronian sedimentation in the Espanola-Willisville area, Ontario, Canada; Paeleogeography, Paleoclimatology and Paleoecology, v.6, p.5-36. Chandler, F.W. 1973. Geology of McMahon and Morin townships, District of Algoma, Ontario Division of Mines, Geoscience Report 112, 77p. Davey, S., Bleeker, W., Kamo, S[.L.], Davis, D.[W], Easton, M.[R.] and Sutcliffe, R.H. 2019. Ni-Cu-PGE potential of the Nipissing sills as part of the ca. 2.2 Ga Ungava large igneous province; in Targeted Geoscience Initiative: 2018 report of activities; Geological Survey of Canada, Open File 8549, p.403-419. ——— 1976. Geology of the Saunders Lake area, District of Algoma, Ontario Division of Mines, Geoscience Report 155, 46p. Chandler, F.W., Young, G.M. and Wood, J. 1969. Diaspore in early Proterozoic quartzite (Lorrain Formation) of Ontario; Canadian Journal of Earth Sciences, v.6, p.337-340. Davidson, A., and van Breemen, O. 1994. U-Pb ages of granites near the Grenville Front, Ontario, in Radiogenic age and isotopic studies: report 8, Geological Survey of Canada, Current Research 1994-F, p.107–114. Chen, Y.D., Krogh, T.E. and Lumbers, S.B. 1995. Neoarchean trondhjemitic and tonalitic orthogneiss identified within the northern Grenville Province in Ontario by precise U-Pb dating and petrologic studies; Precambrian Research, v.72, p.263-281. Davis, D.W. 2008. Sub-million-year age resolution of Precambrian igneous events by thermal extraction– thermal ionization mass spectrometer Pb dating of zircon: Application to crystallization of the Sudbury impact melt sheet; Geology, v.36, p.383-386. Church, W.R. 1967. The occurrence of kyanite, andalusite and kaolinite in lower Proterozoic (Huronian) rocks of Ontario; Geological Association of Canada–Mineralogical Association of Canada, Annual Meeting, Kingston, Ontario, Program with Abstracts, p.14. Davis, D.W., Ménard, J. and Sutcliffe, C.N. 2018. U-Pb geochronology by LA-ICP-MS in samples from northern Ontario; internal report prepared for the Ontario Geological Survey, Jack Satterly Geochronology Laboratory, University of Toronto, Toronto, Ontario, 94p. Coleman, A.P. 1905. The Lower Huronian ice age; Journal of Geology, v.16, no. 2, p.149-158. Debicki, R.L. 1990. Stratigraphy, paleoenvironment and economic potential of the Huronian Supergroup in the southern Cobalt embayment; Ontario Geological Survey, Miscellaneous Paper 148, 154p. Collins, W.H. 1925. North shore of Lake Huron; Geological Survey of Canada, Memoir 143, 160p. Clough, C.E. and Hamilton, M.A. 2017. Matachewan LIP revisited: a revised, high-resolution U-Pb age for the East Bull Lake intrusion and associated units; Geological Association of Canada—Mineralogical Association of Canada, Abstracts, v.40, p.65. Dietz, R. S. and Holden, J.C, 1966. Miogeosynclines in space and time; Journal of Geology, v.74, p.566-583. Easton, R.M. 2003. 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The Grenville Province; in Variations in Tectonic Styles in Canada, Geological Association of Canada, Special Paper 11, p.263-334. 255 �� Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Proceedings, 2022 Description An account of the resource Institute on Lake Superior Geology. Sudbury, Ontario. May 10-11, 2022. Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology. Date A point or period of time associated with an event in the lifecycle of the resource 2022 Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/7d55375f5ea82d59f6506545db132114.pdf 1c223630c51274ed83ad28800080031c PDF Text Text International Ni-Cu Symposium August 6-8th 2024 Lakehead University, Thunder Bay, Canada �i International Ni-Cu Symposium August 6-8th 2024 Lakehead University, Thunder Bay, Canada Meeting Chair - Pete Hollings Organising committee - Matt Brzozowski, Robert Cundari, David Good, Peter Hinz, Al MacTavish, Jim Miller, Dean Rossel, Mark Smyk Reference to material in this volume should follow the example below: Authors, 2024, Abstract title, 2024 International Ni-Cu Symposium Abstracts Volume, Thunder Bay, August 6-8th 2024, p. xx-xx. �Thank you to our sponsors See you next time! �iii Table of Contents One parental magma for them all: Unveiling the crystallization of the Raptor Zone, Tamarack Intrusive Deposit, Minnesota .................................................................................................................. 1 Augustin, C.T.1*, Mungall, J.1 .............................................................................................................. 1 A Paradigm Shift: The Evolution of Nickel-bearing Ultramafic Emplacement Models............................ 3 Aubut A. .............................................................................................................................................. 3 The LDI Intrusive Suite: Geology, tectonic setting, magmatic evolution, and possible controls of sulphide mineralization ........................................................................................................................... 5 Bain, W.B.1,2, Tolley, J. 1, Djon, L.M.3, Hamilton, M.A.4, and Hollings, P.1 ............................................ 5 Mineral geochemistry and textural relations of Ni sulfides and Co arsenides ores from the atypical Avebury nickel deposit, western Tasmania, Australia ............................................................................ 7 Barillas-Diaz, J.L.1, Cooke D.R.1, Zhang, L.1, Wei Hong1, Cajal, Y.1, Denholm, J.2 and Chisnall, T.2 ....... 7 Whole Rock Geochemistry and Down Hole Vectoring as an Exploration Strategy in the Coldwell Complex .................................................................................................................................................. 8 Boucher, C.1, Pitts, MJ. 1, Good, D.J.2, and Laxer, M.2 .......................................................................... 8 What does magmatic sulfide liquid hide? ............................................................................................... 9 Cherdantseva, M.V. 1, Anenburg M.2, Mavrogenes J.E.2 and Fiorentini M.L.1 ..................................... 9 Characterization of Sulfides in Gorgona Island Komatiites: Insights into Cretaceous Mantle Plume Melting and Magmatic Processes ......................................................................................................... 11 Camilo Conde1, Mateo Espinel1, Ana Elena Concha2 ......................................................................... 11 Magmatic and hydrothermal evolution of the PGE-Cu-Ni Current Lake deposit .................................. 12 Corredor Bravo, A. P.1, Hollings, P.1, Brzozowski, M.1, and Heggie, G.2 ............................................. 12 Sulfide percolation and drainback process in magmatic conduit system in the Huangshan-Jingerquan mineralization belt ................................................................................................................................ 14 Deng, Y.-F.1, Xie-Yan Song2 and Feng Yuan1 ...................................................................................... 14 Weathering of non-ore grade rock from Duluth Complex deposits: Outcomes from comprehensive pre-mining geochemical characterization............................................................................................. 16 Diedrich, T.R.1 and Theriault S.2......................................................................................................... 16 Application of FactSage to Model the Compositional Variability of the Ni-Cu-PGE Mineralization at the Main Zone of the Tamarack Intrusive Complex .............................................................................. 18 El Ghawi, A.K.1 and Mungall, J.E.1 ..................................................................................................... 18 Petrophysics Applied to Magmatic Sulfide Deposits: The Physical Properties - Mineralogy Link ......... 20 Enkin, R.J.1 ......................................................................................................................................... 20 Regional changes in plume-generated stress linked to MCR (Keweenawan LIP) chonolith emplacement ........................................................................................................................................ 23 Ernst, R.E.1, El Bilali, H.1, Buchan, K.L.2 and Jowitt, S.M.3 .................................................................. 23 �iv A proposed cryptic common thread among Ni-Cu-PGE-(Au-Te) systems spanning the boundary between Laurasia and Gondwana......................................................................................................... 25 Fiorentini, M.1*, Holwell, D.2, Blanks, D.3, Cherdantseva, M.1, Denyszyn, S.4, Ince, M.1, Vymazalova, A.3, and Piña Garcia, R.5 .................................................................................................................... 25 How exploration geologists can and should use “soft NSRs” to represent assays of Ni-Cu-PGE mineralization ....................................................................................................................................... 27 Goldie, R.J. ......................................................................................................................................... 27 Characterizing the Early (Plume) and Main (Rifting) Stages in the evolution of the Midcontinent Rift 28 Good, D.J. .......................................................................................................................................... 28 Lithospheric structure controls for large magmatic Ni-Cu discoveries ................................................. 30 Hayward, N.1,2 ................................................................................................................................... 30 Deep orogenic magmatic Ni and Cu sulfide systems in the Curaçá Valley, Brazil ................................. 32 Holwell, D.A.1, Thompson, J.2 , Blanks, D.E.1, Oliver, E.1, Porto, P.3, Oliviera, E.3., Tomazoni, F.4, Lima, A. 4, D’Altro R.,4., Graia, P.4 and Sant’Ana, T.4. .................................................................................. 32 Spatial distribution, lithological associations, and geochemical signatures of Ring of Fire Intrusive Suite within the McFaulds Lake Greenstone Belt in the Superior Province: Implications for the Ni-CuPGE, Cr, and Fe-Ti-V Metal Endowment of the Region ......................................................................... 33 Houlé, M.G.1,2, Sappin, A.-A.1, Lesher, C.M.2, Metsaranta, R.T.3, Rayner, N.4, and McNicoll, V.4....... 33 Spatial distribution of mafic and ultramafic units in the Canadian north: Implications for critical minerals (Ni, Cu, Co, PGE) potential ...................................................................................................... 35 Houlé, M.G.1,2, Bédard, M.-P.1, Lesher, C.M.2, and Sappin, A.-A.1 ..................................................... 35 Copper and komatiitic magmatism – source of copper in the Sakatti Cu-Ni-PGE deposit in northern Finland................................................................................................................................................... 37 Höytiä, H.1,2, Peltonen P.1, Halkoaho, T.3, Makkonen, H.V.3,5 and Virtanen, V.J.1,5 ........................... 37 The Koperberg Suite of the Okiep Copper District - an overlooked target for magmatic nickel sulphides in a convergent margin system ............................................................................................. 39 Hunt J.P.1, van Schalkwyk L.1, Smart E.1 and Benhura C.1.................................................................. 39 A multi-methodological approach: Combining textural observations and geochronology to study the J-M Reef Package and its Hanging Wall, Stillwater Complex, Montana ................................................ 41 Jenkins, M.C.1*, Corson, S.2, Geraghty2, E., Kamo S.L. 3, Lowers, H.4, and Mungall, J.E.5 ................... 41 Nickel-copper-platinum group elements potential of mafic and ultramafic intrusions in northwestern Ontario .................................................................................................................................................. 44 Jonsson, J.1, Malegus, P.1, Churchley, S.1, Price, R.1 ........................................................................... 44 Petrogenesis of the mineralized horizons in the Offset and Creek zones, Lac des Iles Complex, N. Ontario .................................................................................................................................................. 46 Jonsson, J.1, Hollings, P.1, Brzozowski, M.1, Bain, W.1, Djon, L.2 ........................................................ 46 Quantum full tensor magnetic gradiometry to better define conduit type Ni-Cu-PGE targets ............ 48 Kaski, K.1, Smith, J.1, Tschirhart, Victoria1, Heggie, G., Enkin, R.1 ...................................................... 48 �v Exploration-Based Classification Scheme for Magmatic Ni-Cu-(PGE) Systems ..................................... 50 Lesher C.M.1 and Houlé M.G.2,1 ......................................................................................................... 50 Thermodynamic constraints on the generation of cubanite-rich magmatic sulfides ........................... 52 Maghdour-Mashhour, R.1, Mungall, J.1 ............................................................................................. 52 Constraining the Sunday Lake mineralization: A Ni-Cu-PGE deposit .................................................... 54 Mexia, K.1, Hollings, P. 1 ..................................................................................................................... 54 Primitive arc magmatism and the development of magmatic Ni-Cu-PGE mineralization in Alaskantype ultramafic-mafic intrusions ........................................................................................................... 56 Milidragovic, D.1,2, Nixon, G.T.3, Spence, D.W.2, Nott, J.A.2, Goan, I.R.2, Scoates, J.S.2 ...................... 56 Stratigraphy of the Grasset Ultramafic Complex and its Ni-Cu-(PGE) mineralization, Abitibi Greenstone Belt, Superior Province, Canada. ....................................................................................... 58 Milier, K.1, Houlé M.G.2 and Saumur B.M.1 ....................................................................................... 58 Geochemistry of Camp Lake Block Lac des Iles Palladium Mine, N. Ontario, Canada .......................... 60 Njipmo Ngoko, B.1, Hollings, P.1, Djon, L.2 and Hamilton, M.3 ........................................................... 60 Hf-Nd-Pb isotopic evidence for variable impact devolatilization in the Sudbury Igneous Complex and its relevance for Ni-Cu-(PGE) sulfide ore formation.............................................................................. 62 Peters, D.1, Lesher C.M.1 and Pattison E.1.......................................................................................... 62 Deformation in mafic protoliths: Impacts from late faults on Ni-Cu-PGE mineralization at Lac des Iles Mine, Canada ........................................................................................................................................ 64 Peterzon, J.1, Phillips, N.1, Hollings, P.2, and Djon, M.L.2 ................................................................... 64 Formation of euhedral silicate megacrysts within magmatic massive sulfides .................................... 66 Raisch, D.1, Staude S.1, Fernandez, V. 2 and Markl G.1 ....................................................................... 66 Applying Magnetic Vector Inversion (MVI) on Aeromagnetic Data in the Thunder Bay Region of the Mid-Continent Rift ................................................................................................................................ 68 Riahi, S.1, Mungall J.E.1, Ernst, R.E1 ................................................................................................... 68 Potential links between the Midcontinent Rift (MCR) related Baraga-Marquette dyke swarm and early MCR related magmatic Ni-Cu sulfide deposits in Michigan, USA. ................................................ 70 Rossell, D.M.1*, Strandlie, J.2.............................................................................................................. 70 Texture and composition of Fe-Ti oxides of the Neoarchean Big Mac mafic intrusion and its implication for Fe-Ti-V-(P) mineralization in the McFaulds Lake greenstone belt, Superior Province, Canada .................................................................................................................................................. 72 Sappin, A.-A.1, Houlé, M.G.1,2*, Metsaranta, R.T. 3, and Lesher, C.M.2............................................... 72 Complexly zoned pyroxenes at Kevitsa record magma mixing and survive alteration ......................... 74 Schoneveld, L.1, Luolavirta, K.2,3, Barnes, SJ1 , Hu, S.1 , Verrall, M.1 and Le Vaillant, M.1 ................... 74 New indicator mineral signatures for nickel sulfide exploration .......................................................... 76 Schoneveld, L.E.1, Williams M.1, Salama, W.1, Spaggiari, C. V.1, Barnes, S. J. 1, Le Vaillant, M. 1, Siegel, C. 1, Hu, S. 1, Birchall, R. 1, Baumgartner, R. 1, Shelton, T. 1, Verrall, M. 1, and Walmsley, J. 1 . 76 �vi Apatite as an indicator for volatile involvement in the genesis of the Marathon Cu-PGE deposit, northwestern Ontario ........................................................................................................................... 78 Shahabi Far, M.1, Good, D.2 and Samson, I3 ...................................................................................... 78 Geochemical and Petrologic Investigation of the Eagle’s Nest Intrusion, McFaulds Lake Greenstone Belt, Ontario, Canada ............................................................................................................................ 81 Sheshnev, V.1, Hollings, P.1, Phillips N.J.1, Weston, R.J.2, Deller, M.2 and Campbell, D.2 .................... 81 Reconstitution of the Merensky Reef footwall during chamber replenishment .................................. 83 Smith, W.D.1,2, Henry, H.3, Maier, W.D.4, Muir, D.D.4, Heinonen, J.S.5,6, Andersen, J.Ø7................... 83 Future research areas to aid in exploration for Ni sulfides ................................................................... 85 Sproule, R.A.1 ..................................................................................................................................... 85 Exploring the footwall: Sulfide Mineralization in the footwall Granite of the Maturi Deposit, Minnesota. ............................................................................................................................................ 86 Steiner, R. A.1 ..................................................................................................................................... 86 The Anatomy of a Cu-Ni-Co-PGE Mineralized Mafic Magmatic System: The South Kawishiwi Intrusion of the Duluth Complex, Northeastern Minnesota ................................................................................ 90 Sweet, G.S.1 and Peterson, D.M.2 ...................................................................................................... 90 Multi-thermochronological records of cooling, denudation and preservation of ancient ultrabasic magmatic ore deposits: An example from the Neoproterozoic Jinchuan giant magmatic Cu-Ni sulfide deposit .................................................................................................................................................. 94 Ni Tao1,2*, Jiangang Jiao1, Jun Duan1, Haiqing Yan1, Ruohong Jiao3, Hanjie Wen1 ........................... 94 Compositional variability in olivine: New data on the occurrences of Ni and Co as guides to mineral prospectivity ......................................................................................................................................... 95 Thakurta, J.1, Wagner, Z.1, Nagurney, A.2, and Schaef, H.T. 2 ............................................................ 95 The effects of diagenetic and metamorphic processes on the sulphur liberation from the Virginia Formation black shale during magmatic assimilation by the Duluth Complex, Minnesota, USA ......... 97 Virtanen V.J.1,2, Heinonen J.S.2,3, Märki L. 4, Galvez M.E. 5 and Molnár F.6......................................... 97 Mantle-to-crust scale chemical fractionation and sulphide saturation of the Paleoproterozoic komatiites of the Central Lapland Greenstone Belt, Finland – implications for geochemical exploration ............................................................................................................................................ 99 Virtanen V.J.1,2, Höytiä H.M.A.2, Iacono-Marziano G.1, Yang S.3, Moilanen M.3 and Törmänen T.4 . 99 Ni-Cu-PGE prospectivity of the Mackenzie Large Igneous Province ................................................... 102 Williamson, M.-C.1, Rainbird, R.H.1, O’Driscoll, B.2 and Scoates, J.S.3.............................................. 102 Siluro-Devonian Mafic-Ultramafic Intrusions in New Brunswick, Northern Appalachians, and their Associated Nickel-Copper-Cobalt Sulphide Deposits: A preliminary review ....................................... 103 Yousefi, F.1, Lentz D.R.1, Walker J.A.2 ,Thorne K.G.3, and Karbalaeiramezanali A. 3 ......................... 103 Geochemistry of Archean komatiitic greenstone terranes of the Wyoming Province: implications for geodynamic setting and mineralization .............................................................................................. 105 Zieman, L.J.1*, Poletti, J.E.1, and Jenkins, M.C.1 ............................................................................... 105 �1 One parental magma for them all: Unveiling the crystallization of the Raptor Zone, Tamarack Intrusive Deposit, Minnesota Augustin, C.T.1*, Mungall, J.1 1 * Mineral Deposits Laboratory, Earth Sciences Department, Carleton University, Ottawa. claudiaaugustin@cmail.carleton.ca ___________________________________________________________________________ The Tamarack Intrusive Complex (TIC) is one of the mafic complexes intruded in the context of the Midcontinent Rift (MCR) system in the Midwestern United States. The Tamarack Intrusive Complex is located ca. 80 km west of Duluth, Minnesota, and it is intruded within the Paleoproterozoic (~1.85 Ga) slates and greywackes of the Thomson Formation of the Animikie Group [1,2]. It was emplaced in the Early Stage of the MCR, with a baddeleyite U-Pb age of 1105.6 ± 1.2 Ma [1] and zircon Concordia age of 1103.81±0.92 [3]. The TIC is characterized by an aeromagnetic anomaly with a broader, rounded region at the south leading into a narrower, elongated extension towards the north, extending approximately 13 km northwest-southeast and varying from hundreds of m to ca. 4 km in width [1]. Its morphology contains distinct shaped intrusive bodies, such as the ovoid-shaped Bowl Intrusion in the south and a dike-like area in the north, which includes the Raptor zone [1,2; figure 1]. Figure 1 Schematic local geological map and cross-section of the Raptor zone. The rocks of the Raptor zone usually show a consistent vertical sequence, except when in proximity to lateral contacts, where drill cores show a more complex variation in texture and mineralogy. Usually, the sequence consists of a basal portion of fine-grained olivine cumulate rock; therefore, this unit will be called Basal Raptor Zone Unit (BRZ), keeping the name consistent with what has been used for previous TIC studies. The most abundant primary minerals in the BRZ unit are olivine, clinoand ortho-pyroxene, and plagioclase (figure 2a). The olivine size ranges from 170 µm to 3.3 mm, but most grains are <0.5 mm. The coarser grains of olivine are more prevalent in the upper section and gradually diminish downwards. Commonly, the coarser olivine grains display plane-oriented dendritic exsolution of chromium-spinel and clinopyroxene along a consistent orientation. Above this unit is a thick, coarse-grained olivine cumulate called CGORaptor unit (figure 2b). The mineral proportions of the CGORaptor are variable along the stratigraphy; the intercumulus/cumulus ratios phases increase to the center, i.e., the cumulus phase decreases towards the upper and lower contacts. These two rock units are characterized by similar primary mineralogy and classified as feldspathic lherzolite, with the most notable difference being a variation in olivine grain size and a slight increase in earlier chromium-spinel. The subtle grain-size distinction makes it difficult to identify their gradual contact visually. The upper portion of CGORaptor shows intercalation of olivine cumulates with �2 pockets/domains of a varitextured gabbro. The gabbro that is intercalated with CGO and the contact with it is mostly diffused, marked only by the disappearance of olivine cumulate. Figure 2: EDS phase maps showing textural differences between the BRZ (a) and CGO(b), with minor large olivines in a finer matrix in the BRZ compared with the more uniform CGO. To address the composition and evolution of the melt parental to the CGORaptor rocks of the TIC, we have modeled crystallization using the alphaMELTS thermodynamic software [4-5]. The starting composition used was derived from the chilled margin of the Raptor zone. The cooling of the liquid under isobaric conditions and fO2 at the fayalite–magnetite–quartz (FMQ) solid oxygen buffer produced a similar sequence of crystallization, modal proportions of solids to the observed bulk-rock and mineral compositions of all major constituents of the rocks of the Raptor Zone. This method successfully mirrored the crystallization order, the relative amounts of solid phases, and the chemical composition of the primary cumulus minerals. Our results show a crystallization sequence beginning with olivine (Fo87), followed by clinopyroxene, chromium-rich spinel, orthopyroxene, and plagioclase. Specifically, at 1170 °C, the simultaneous formation of olivine and clinopyroxene, adjusted in proportion, reflects the varied compositions within the unit. Moreover, the liquid remaining at this temperature aligns with the mineralogy and composition observed in the gabbro unit. Using the same composition and parameters but slightly increasing the fO2 levels to NNO, the model predicts that spinel forms earlier, leading to similar BRZ composition and mineralogy. This change explains the prevalent spinel and the observed exsolution textures between cr-spinel and clinopyroxene in the coarse-grained olivine—features typically linked to variations in cooling rates and oxygen fugacity. Our thermodynamic analysis shows that the three main rock types in the Raptor Zone can originate from a single magma source, with only minor adjustments needed to explain their variations. The categorization into BRZ and CGO units appears to be based on slight differences in oxidation states and crystal sizes rather than suggesting they are from two separate magmatic intrusions. The findings suggest these units might represent different stages of the same magmatic event. References: [1] Goldner B (2011) Min University Thesis [2] Taranovic V et al. Lithos 212-215 (6-31) [3] Bleeker W et al. (2020) Geol Survey of Canada, Open File 8722, p. 7–35 [4] Asimow P D (1998) Am. Mineral. 83 (1127-1132) [5] Smith P M and Asimow P D (2005) Geochem. Geophys. Geosyst. 6(1-8) �3 A Paradigm Shift: The Evolution of Nickel-bearing Ultramafic Emplacement Models Aubut A. M Sibley Basin Group Ltd., PO Box 304, Nipigon, Ontario. sibley.basin.group@gmail.com ___________________________________________________________________________ An important class of nickel deposit are those hosted by stratabound dunite-peridotite bodies. This class includes the Kambalda district of Australia, Pechenga in the Kola Peninsula of western Russia, Kabanga in south-central Africa, the Shaw Dome area of northern Ontario, Raglan in northern Quebec and Thompson in northern Manitoba. All have been, or currently are attributed to the intrusion of ultramafic sills [e.g. 2,8,9]. Key evidence in support of this model is that the ultramafic bodies typically exhibit at least some differentiation and are sub-concordant to the host sediments. This tendency to default to an intrusion model now includes the Tamarack deposit in Minnesota [11] even though another model, one that incorporates extrusion, may be just as valid. Despite the prevalence of the intrusion model there are many nickel deposits hosted by ultramafic bodies that display clear evidence of being the product of extrusive flows, often exhibiting the same key features used to invoke an intrusive origin [e.g. 1,3,4,7]. Major komatiite hosted nickel deposits share some common features: 1) the nickel mineralisation is hosted by ultramafic rocks; 2) the sulphides are at the stratigraphic base of the host ultramafics; 3) the ultramafic rocks are hosted by, or in contact with, sulphidic and carbonaceous argillaceous rocks; 4) the ultramafic bodies are stratabound and generally conformable to the host lithology; and 5) they are hosted within extensional basins usually with a significant sedimentary component with Kambalda being the one exception. As Maier et al. [3] point out, the reason magmatic feeder systems rather than large intrusions are important hosts to economic nickel deposits is because of flow dynamics. Rice and Moore [11] have studied flow dynamics and concluded that open-channel flows were turbulent, and that this turbulence was required to expose the sulphides present to enough magma to generate the tenors observed. This turbulence explains how sedimentary sulphides can be integrated and assimilated by ultramafic magma and result in significant nickel tenors, nickel in 100% sulphide [4,5]. Turbulent flow is difficult, if not impossible, to explain by a simple intrusive mechanism. In addition, to get the size of deposit observed there needs to be significant volumes of ultramafic magma. The one environment that does allow turbulent flow to take place, and have the volumes required, is with high volume surface flows with gravity settling of the magmatic and assimilated sedimentary sulphides, along with significant magma mixing to get the observed partitioning of the silicate nickel into the sulphides. But there is a density “problem” in that ultramafic magmas are typically denser than the host rocks, especially when they are sedimentary. This paradox is typically glossed over or totally ignored. For example, see Hubbert et al. [5]. Ultramafic magma is not buoyant as the contrast is negative. So, how were these high-density liquids able to ascend through the crust? When rocks melt, they become about 10% less dense. In the case of ultramafic rocks, they have an average density of about 3.1 grams per cubic centimetre (g/cc) depending on the proportion of olivine present which has a density of 3.27–4.27 g/cc. Hubbert et al. [5] assumed a value of 2.8 g/cc. The average crust has a density of 2.7 g/cc or less and thus buoyancy could not have taken place. To move upward from the mantle through the crust there must have been a mechanism other than buoyancy. �4 An alternative mechanism proposed in the literature is “overpressure” defined by Walwer et al. [12] as “the difference between the pressure inside the magma and the local pressure acting orthogonal to the magma body wall.” Melting of the mantle creates magma plumes that move upward due to buoyancy to the Mantle-Crust boundary where the magma collects and then moves laterally thus creating extensional forces in the overlying crust. This accumulating magma would be constrained by the overlying lithostatic load and in doing so would build up overpressure. Eventually the crust would thin enough such that vertical fractures would form allowing the trapped magma to escape, not through buoyancy but due to the built-up overpressure exceeding the lithostatic load. At surface the hot, dense ultramafic magma would then flow over, and into, deep water sediments where the magma would mechanically and thermally erode and assimilate sulphide rich sediments. This mechanism would explain the correlation with rift basins, as well as how a dense magma can penetrate a less dense substrate and produce the type of volumes required to attain high R values, while also generating the turbulent flow needed to assure incorporation, and assimilation of sulphide with resultant nickel partitioning required to get the high tenors typical of most sulphide deposits found associated with extensional basins. An extrusive model is more compatible with these commonalities and issues. It explains why the host ultramafic bodies are stratabound. It provides a better mechanism for incorporating sedimentary sulphide. It provides more opportunity for high R values creating high tenors. And it presents a tectonic environment, rifted basins, that can be easily targeted. Currently nickel is an under explored commodity primarily because, using the intrusion model, limited opportunities are available. The flow model on the other hand is more robust as it does a better job of explaining things like the high volumes of magma needed and the fluid dynamics required to ensure thorough mixing of the denser sulphides with the magma to attain the tenors present in these deposits. In addition, being tied to a specific tectonic event, rifting, it is not fixed in time or place as much as the intrusive model is. While intrusive environments do exist where these conditions are met, they are always in primary magma conduits. References: [1] Arndt NT (1975) Unpub Ph.D. Thesis, U of T. [2] Bleeker W (1990) Unpub PhD Thesis, UNB. [3] Hill RET et al. (1995) Lithos 34: 159-188. [4] Hubbert HE and Sparks RSJ (1985) J of Petro 26-3: 694-725. [5] Hubbert HE et al. (1984) Nature 309:19-22. [6] Maier WD et al. (2001) Cana Mine 39:547-556. [7] Marston RJ et al. (1981) Econ Geol 76:1330-1363. [8] Melezhik VA et al. (1994) Tran Inst Min Meta B 103:B129-B145. [9] Naldrett AJ (1981) Econ Geol 75th Anni Volu :628-685. [10] Rice A and Moore JM (2001) Cana Mine 39:491-503. [11] Taranovic V et al. (2018) Econ Geol 113-5:1161-1179. [12] Walwer D et al. (2021) Phys of the Earth and Plan Inte 312, �5 The LDI Intrusive Suite: Geology, tectonic setting, magmatic evolution, and possible controls of sulphide mineralization Bain, W.B.1,2, Tolley, J. 1, Djon, L.M.3, Hamilton, M.A.4, and Hollings, P.1 1 Department of Geology, Lakehead University, Thunder Bay, ON P7B 5E1, Canada British Columbian Geological Survey, Victoria, BC V8T 4J1, Canada 3 Impala Canada, Thunder Bay, ON P7B 6T9, Canada 4 Department of Earth Sciences, University of Toronto, Toronto, ON M5S 3B1, Canada 2 ___________________________________________________________________________ The Archean Lac des Iles (LDI) complex hosts a world-class platinum group element (PGE) deposit. This mafic-ultramafic intrusive complex is situated near the suture between the Wabigoon and Quetico subprovices and is spatially associated with a suite of satellite intrusions: the Tib Lake, Legris Lake, Wakinoo Lake, Demars Lake, Dog River, and Buck Lake intrusions- known collectively as the LDI intrusive suite (Fig 1 a). Textural, petrographic and geochemical similarities between the LDI Mine Block intrusion and the LDI intrusive suite suggest a genetic association and potentially a comparable degree of PGE mineralization. Here, we present an overview of the geology of the LDI intrusive suite and provide new U-Pb age dates, Sm-Nd isotopes, and parental melt modelling. Zircon U-Pb ages for the Buck Lake (2698.1 ± 1.6 Ma), Wakinoo Lake (2696.6 ± 0.8 Ma), Demars Lake (2694.1 ± 1.5 Ma), Legris Lake (2690.6 ± 0.8 Ma), Dog River (2689.9 ± 0.7 Ma), and Tib Lake (2685.9 ± 1.6 Ma) intrusions show a spatial trend of younging to the north and demonstrate a temporal association with the Lac des Iles Mine Block intrusion (2689.0±1.0 Ma; [1]; Fig 1 b). Whole rock εNdT values from the Wakinoo Lake, Tib Lake, Legris Lake, and Lac des Iles intrusions overlap and similarly display a trend of increasingly negative values with decreasing age (Fig 1 c). These patterns likely reflect the initial assimilation of Wabigoon tonalite country rock early in the magmatic evolution of the LDI intrusive suite and progressively more assimilation of Quetico metasedimentary rocks in later stages. Model parental magma compositions for the LDI intrusive suite produce similar trace element profiles with highly fractionated REE content, moderately negative Ta-Nb and Zr-Hf anomalies, and strong enrichment in the large ion lithophile elements. This pattern is consistent with an arc setting and might indicate a common source reservoir of parental melt. The observed Sm-Nd isotopic signature of the LDI intrusive suite supports this interpretation and suggests that host rock assimilation was a main control of the magmatic differentiation of individual intrusions. However, magma mixing may also have occurred during the formation of the Tib Lake and North LDI intrusions, as indicated by the more primitive compositions of individual cyclic units [2]. Magmatic sulphides from the Legris Lake intrusion have δ34S values that overlap the mantle range but trend toward the composition of Wabigoon tonalite [3]. This suggests that external S addition drove sulphide saturation during its formation. However, a comparison of whole rock S/Se and Cu/Pd ratios of mineralized lithologies suggests sulphide melt retention during emplacement was a key control on the scale of sulphide mineralization in the Legris Lake intrusion and other intrusions of the LDI intrusive suite. �6 Fig 1. a. Regional geologic map showing locations of Thunder Bay, the Lac des Iles mine (in red), and the Lac des Iles intrusive suite (in blue). b. U-Pb ages for individual intrusions in the Lac des Iles intrusive suite. c. Whole-rock εNdT values for the LDI intrusive suite and host rock lithologies. North LDI, South LDI and Shelby Lake diorite data from Brügmann et al. [4] References: [1] Stone D (2010) Ontario Geological Survey, Open File Report 5422:1–130 [2] Djon LM et al. (2017) Can Min 55:349-374 [3] Bain WM et al. (2023) Min Deps doi:10.1007/s00126-023-01183-x [4] Brügmann MJ et al. (1997) Precambrian Res 81:223-239 �7 Mineral geochemistry and textural relations of Ni sulfides and Co arsenides ores from the atypical Avebury nickel deposit, western Tasmania, Australia Barillas-Diaz, J.L.1, Cooke D.R.1, Zhang, L.1, Wei Hong1, Cajal, Y.1, Denholm, J.2 and Chisnall, T.2 1 Centre for Ore Deposit and Earth Sciences (CODES), University of Tasmania, Private Bag 79, Hobart, TAS 7001, Australia, joseluis.barillasdiaz@utas.edu.au 2 Avebury Nickel Mine, Trial Harbour Road Zeehan TAS 7469, Australia ___________________________________________________________________________ The unusual Avebury metasomatic nickel sulfide deposit in western Tasmania was discovered in 1998 and is the best-known case of an economic hydrothermal-remobilized Ni deposit [1]. The nickel sulfide ores are hosted in the Middle Cambrian serpentinized peridotites of the allochthonous maficultramafic ophiolite complex, while cobalt arsenides within the Neoproterozoic Crimson Creek volcanoclastic sequence. The Avebury Ni deposit lies in the halo of the strongly fractionated, reduced Devonian Sn-mineralized ~360 Ma Heemskirk granite [2]. Apatite U-Pb ages from 374 ± 14 Ma to 347 ± 15 Ma from mineralized serpentinite and Crimson Creek skarn imply that hydrothermal remobilization of Ni-Co occurred at Avebury due to hydrothermal fluids derived from Devonian Heemskirk granite. The compositional and mineralogical transformations associated with chemical reactions triggered by the response of hydrothermal fluids from the granite resulted in a magnesianskarn including brucite + diopside + hedenbergite + augite and tremolite-actinolite in the ultramafic rocks and pyroxene + garnet + axinite-(Mg) ± ludwigite and tourmaline in the volcanoclastic rocks of Crimson Creek. The dominant nickel sulfide mineral at Avebury is pentlandite, which is associated with pyrrhotite and minor chalcopyrite. Pentlandite is hosted in olivine + clinopyroxene cumulates, which have been serpentinized in most cases where pentlandite occurs mainly as relatively coarse-grained sulfide blebs with pyrrhotite. Pentlandite also occurs in relatively fine-grained shattered disseminations within actinolite. The coarse-grain pentlandite is fractured and encapsulated by magnetite, and Niarsenides have partly replaced pentlandite grains. Pentlandite has altered slightly along grain edges to violarite and pyrite. Chalcopyrite may occur as exsolution intergrowths in millerite and pentlandite. The high-resolution XRF scanning analysis from core rock and whole rock assay from mineralized serpentinite samples show positive Ni/Ti and Ni/Cr ratios and discriminated between two nickel mineralization zones. The Ni vs MgO diagram shows that nickel mineralization is hosted primarily in MgO-rich and pyroxene-rich serpentinites. In contrast, the low-MgO and Cr-rich serpentinite negatively correlate with Ni. However, the serpentinite FeO-rich positively correlates with pentlandite rich in cobalt. Although some serpentinite horizons have strong metasomatism, all the serpentinized ultramafics have >16% magnetite and are depleted in Al2O3, TiO2, Sr, Y and Zr. The whole rock assay results indicate a negative correlation of Cu and Zn with Ni. Mineral characterization using an automated energy dispersive X-ray spectroscopy mineral mapping (AMICS) shows nickel sulfides and cobalt arsenides do not coexist in the same mineral assemblage. Cobaltite, alloclasite and minor glaucodot are the two main arsenides of cobalt restricted to the magnesianskarn of prehnite + augite and hedenbergite in Crimson Creek. The laser ablation analyses (LA-ICPMS) in pentlandite minerals from the Avebury deposit do not show strong correlations with other elements. However, a small group of pentlandite shows incipient correlations between Au, Ag and Co. Analysis in pentlandite and pyrrhotite shows some crystals with Pt values between 2.5 to 4.0 ppb. Cobaltite shows a slight trend in which the cobalt content decreases as the Ni content increases. On the other hand, the pyrite crystals show a strong correlation between Au, Co, Cu and Ni. The correlation between nickel and cobalt in pentlandite is modest in the Avebury deposit compared to Trial Harbour pentlandite, which shows strong correlations between these two elements. The paragenesis relationships, mineral textures, and compositional trends exhibited by Ni-Co ores at the Avebury deposit provide evidence of a multi-stage depositional history. References: [1] Keays R and Jowitt S (2013) Ore Geology Reviews 52: 4–17 [2] Hong W eta al. (2017) Gondwana Research 46: 124–140 �8 Whole Rock Geochemistry and Down Hole Vectoring as an Exploration Strategy in the Coldwell Complex Boucher, C.1, Pitts, MJ. 1, Good, D.J.2, and Laxer, M.2 1 2 Generation Mining, Marathon, ON, Canada. cboucher@genpgm.com Department of Earth Sciences, Western University, London, ON N6A 5B7, Canada ___________________________________________________________________________ The Eastern Gabbro-Basalt Suite of the Coldwell Complex has been widely explored for decades by various operators, resulting in the discovery of numerous exploration Prospects and Deposits. Although numerous economic and academic studies have been completed on the flagship Marathon Deposit, Sally deposit, and Boyer and Four Dams occurrences, little work has been done to advance understanding of relationships between trace-element geochemistry and mineralization at the Complex-sized scale. For instance, earlier work has described stratigraphic and trace-element relationships between metabasalt and the mineralized Two Duck Lake intrusions, and between mineralized and unmineralized phases of the host gabbro. In this presentation we examine these relationships at a larger scale and test for their usefulness as an exploration vector tool in the Coldwell Complex. A second objective of this presentation is to examine the 3D spatial relationships between Cu/Pd and Cu/S and the associated mineralization style, footwall topography and faulting at the Marathon deposit. This study takes advantage of the dynamic conduit model that it is used to explain many features of Cu-PGE mineralization in the Marathon Series rocks. For instance, the spatial distribution of mineralization relative to topographic lineaments is explained by magma transport along early fault zones that were reactivated late in the history of the complex to create the lineaments. This study also takes advantage of significant changes or inflection points in the trends for Cu/Pd, Cu/S, Pd/Au, and Cu/Ni values between the three dominant mineralization styles in the Two Duck Lake gabbro: Footwall Zone, Main Zone, and W-Horizon. Large deflections in the downhole trends of these ratios, particularly Cu/Pd, act as a proxy for identification of individual pulses of magma (or stacking of intrusions). Although contacts between pulses are difficult to recognize in thick packages of gabbro, they can be identified by sharp changes in Cu, Pd and S content or, more importantly, by inflection points in metal ratio proxy trends (Cu/Pd or Cu/S). Here we present results of our study for these factors at the deposit scale and propose key features that might be useful for recognizing settings in the conduit model from down hole assay data. The implementation of geochemistry and downhole vectoring will continue to advance and provide insight into refined geological modelling. Future work on in-depth classification of units will include Layered Series rocks and proximity to major structures, differentiation of TDL Gabbro based on mineralogy and texture, origin of the two varieties of oxide-melatroctolite pods and relationship to underlying conduits and identifying key indicators to aid in lithological classification based on basic assay package. �9 What does magmatic sulfide liquid hide? Cherdantseva, M.V. 1, Anenburg M.2, Mavrogenes J.E.2 and Fiorentini M.L.1 1 Centre for Exploration Targeting, School of Earth Sciences, University of Western Australia, Australia, maria.cherdantseva@uwa.edu.au 2 Research School of Earth Sciences, Australian National University, Canberra, Australia ___________________________________________________________________________ In natural examples, magmatic sulfides hosted in mafic-ultamafic intrusions, regardless of textural variability (massive, globular, net-textured, disseminated), are almost ubiquitously found in spatial association with alkali-, lithophile- and volatile-rich minerals, such as phlogopite, ilmenite, chlorite, amphibole, calcite, etc. These minerals display diverse textures, either surrounding sulfide margins or found inside sulfides as euhedral crystals as well as irregular, rounded or vermicular inclusions. The presence of the listed minerals in association with sulfides has been previously attributed to secondary processes, late circulation of fluids or highly differentiated melts [1, 2, 3]. However, existing models fail to provide a satisfactory explanation on why these alkali-, lithophile- and volatilerich minerals so often occur in direct contact with sulfides or as inclusions in them. Here, we argue that the common spatial association of alkali-, lithophile- and volatile-rich minerals with magmatic sulfides could be explained by the partial dissolution of lithophile and volatile elements in sulfide liquid at high temperature and pressure and their subsequent release upon cooling of the system. Indeed, several experimental studies show that at high temperatures and additional various conditions (e.g., oxygen fugacity, melt composition), regular magmatic sulfide liquid has the capacity to dissolve a wide range of lithophile elements (such as Al, Mg, Mn, Ti, Ca, K, etc. [4, 5, 6]), halogens (Cl, Br, F, I [6, 7]) and water [8]. However, there has never been a clear connection made between formation of alkali-, lithophile- and volatile-rich minerals in close spatial association with sulfides and the potential chalcophile behaviour of some lithophile elements and halogens dissolved in sulfide liquids under some specific conditions. We put forward the idea that a genetic link between these elements and sulfide liquid could not only explain the formation of volatile-rich halos around sulfides but also elucidate the cryptic link between magmatic and hydrothermal mineralising processes as explained below. Our new experiments were conducted to investigate the potential of magmatic sulfide liquids to dissolve K, Na and chloride in magmatic conditions (1200-850 °C, 5 kbar, ΔFMQ = –1.5). All experiments were run using piston cylinder apparatus at the National Australian University. The experiments were run in 3.5 mm Pt capsules lined with graphite to prevent sulfides from coming into contact with the metal capsule. The Pt capsule was welded and enclosed within 5/8-inch MgO-PyrexNaCl assembly (Fig. A1a). Temperature measurement was carried out with a B-type Pt-Rh thermocouple. We investigated the fate of these elements as the system crystallizes, both in isolation and in equilibrium with silicate melts. The experiments where sulfide liquid was mixed with K, Na and Cl without presence of silicate melt had layered set-up to monitor the melting and mixing process between sulfide phases, alkalis and Cl. Three runs with the same set up and and starting composition were heated up to 1100 °C (at 5 kbar) and then cooled down and quenched at different temperatures (1100 °C, 850 °C and 300 °C). The result of the experiments show that at high temperature the initial layering is not retained and sulfide liquid homogenizes, dissolving ~3 wt% of K, 0.3 wt% of Na and 0.03 wt% of Cl. During quenching, sulfide liquid forms elongated skeletal crystals of mss and interstital residual mixed sulfide matrix. Medium temperature experiment consisted of rounded grains of Ni-rich monosulfide solid solution (mss) in a Cu-rich fine-grained matrix interpreted as quenched liquid. The mss contains negligible concentrations of alkali elements and Cl (< 0.03 wt% of Na, <0.03 wt% K and < 0.003 wt% Cl), whereas the Cu-rich sulfide matrix contains 2.7 wt% of K, 0.6 wt% of Na, and 0.6 wt% of Cl. Slowly cooled to 300 °C experiment contain �10 alkali- and Cl-free pyrrhotite, pentlandite, chalcopyrite and alkali-rich sulfides such as murunskite (K2(Cu,Fe)4S4) and djerfisherite (K6(Fe,Cu,Ni)25S26Cl). The second experiments were designed to examine the behavior of sulfides in equilibrium with silicate melt. The high temperature experiment was quenched after heating to 1250 °C (at 5 kbar), resulting in the formation of sulfide globules comprising elongate skeletal crystals of alkali-free mss intergrown with sulfide matrix of mixed Fe-Ni-Cu composition containing up to 2 wt% Na and 1.3 wt% K, along with 0.1 wt% chloride. Another experiment was slowly cooled from 1250 °C to 300 °C (at 5 kbar) and crystallized to an alkali-rich silicate matrix composed of chromian spinel, nepheline, apatite, Na–K–Ca-carbonate, clinopyroxene and sulfide globules. The sulfide blebs differentiated to pyrrhotite, pentlandite, chalcopyrite and bornite with K, Na or Cl concentrations below detection limit. Results of our experiment show that sulfide liquid can dissolve a substantial amount of alkalis and Cl at high pressures and temperature at geologically relevant redox conditions. Incorporation of these elements into the melt network of magmatic sulfide liquid can affect its physical properties. Thus, the presence of alkalis and Cl dissolved in sulfides could play a crucial role in reducing the melting point of mantle sulfides, akin to the effect of other fluxes on silicate assemblages [9]. Consequently, the presence of alkalis, Cl and water may enhance sulfide melting in localized mantle domains, where molten metal-rich sulfides can be extracted and incorporated into ascending magmas without the requirement of anomalously high heat triggers, widening the spectrum of geodynamic scenarios where fertile melts can be generated on a global scale [10]. Our slowly cooled experiments indicate that alkalis and Cl become immiscible with sulfide liquid during cooling and crystallization. Indeed, magmatic sulfides have never been documented to contain any impurities of lithophile elements or halogens. The only known K and Cl-rich sulfides, such as djerfisherite and murunskite, are very rare and form only in extremely alkali-rich conditions [11]. As a result of immiscibility, it is proposed that sulfide liquid “sweats out” the alkalis and chloride during magma crystallization. This process erases any direct evidence of the former presence of alkalis and Cl in the sulfide itself. Instead, it leaves behind a subtle association of alkali silicates surrounding them, including phlogopite, amphibole, scapolite, and Cl-apatite. However, this process of direct exsolution of Cl, K, Na and water [8] contributes into the metal butget of overlying hydrothermal systems. Magmatic hydrothermal fluids enriched in chloride and alkalis may be important carriers of Cu, Au, and PGEs [e.g., 12] which tend to form aqueous chloride complexes. The exsolution of chalcophile metals, alkalis, and Cl as well as their partitioning into magmatic-hydrothermal fluids supports previous models that link mineralized deep magmatic systems to overlying hydrothermal systems [13]. In summary, alkalis and chlorine play a pivotal role in enhancing metal extraction from the mantle by reducing the melting point of sulfides and lowering their density. During crystallization, these elements exsolve from sulfide liquids into adjacent silicates and late fluid phases, thus increasing the mineralizing potential of magmatic-derived hydrothermal fluids. References: [1] Kanitpanyacharoen W and Boudreau AE (2013) Miner Depos 48(2):193–210 [2] Yuan Q et al. (2023) Lithos 438-439:107014 [3] Ballhaus C and Stumpfl E (1986) Contrib Mineral Petrol 94(2):193-204 [4] Kiseeva E and Wood B (2015) Earth Planet Sci Lett 424:290-294 [5] Wood B and Kiseeva E (2015) Am Mineral 100:2371-2379 [6] Steenstra E et al. (2020) Geochim Cosmochim Ac 273:275-290 [7] Mungal J and Brenan J (2003) Can Min 41(1):207-220 [8] Wykes J and Mavrogenes J (2005) Econ Geol 100:157-164 [9] Sakamaki T (2017) Chem Geol 475:135-139 [10] Holwell DA et al. (2019) Nat Commun 10(1):1–10 [11] Osadchii VO et al. (2018) Contrib to Mineral Petrol 173 (5):1–9 [12] Sullivan N et al. (2022) Geochim Cosmochim Ac 316:230-252 [13] Heinrich C and Connolly J (2022) Geol 50(10):1101-1105 �11 Characterization of Sulfides in Gorgona Island Komatiites: Insights into Cretaceous Mantle Plume Melting and Magmatic Processes Camilo Conde1, Mateo Espinel1, Ana Elena Concha2 1 University of Geneva, 2 Universidad Nacional de Colombia ___________________________________________________________________________ The demand for copper, aluminum, nickel, zinc, and lead is ever increasing. Advances in new models and technology are helping the exploration industry to discover new resources of these important minerals and meet the requirements of the global population. This theme will include all aspects of exploration of these metals, from genesis and mineral processing to the circular economy. Komatiites from Gorgona Island, Colombia, are unique as the only Phanerozoic spinifex-textured ultramafic lavas and the only Cretaceous-age occurrences globally reported (dated at approximately 90 million years old (Kerr et al., 1997)). These rocks have been central to discussions about high temperature melting in mantle plumes, with recent studies developing into the melting event's details, source materials, and melting depths. This study is the first focus on sulfides within Gorgona komatiites, showing the presence of interstitial sulfides, typically larger than 20 microns. Through detailed petrography, SEM imaging, and QUEMSCAN analysis, the research aims to identify and characterize these sulfides, identifying their composition and relating them with magmatic processes. Key sulfides identified include chalcopyrite, pyrite, pentlandite and pyrrhotite positioning Gorgona as a significant new site for magmatic sulfides studies. For the sulfide characterization, the electron microprobe analyzer (EPMA), provide precise compositional data crucial for understanding the magmatic evolution. This is particularly important as it helps determine the timing of sulfur saturation, which in turn reveals whether nickel or copper with PGE becomes more prevalent. Understanding these processes is vital for developing nickelcopper-PGE models and gaining insights into mantle-core conditions, underscoring the geological significance of the Gorgona komatiites. �12 Magmatic and hydrothermal evolution of the PGE-Cu-Ni Current Lake deposit Corredor Bravo, A. P.1, Hollings, P.1, Brzozowski, M.1, and Heggie, G.2 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada. acorredo@lakeheadu.ca 2 Clean Air Metals, 1004 Alloy Drive, Thunder Bay, ON P7B 6A5 Canada. gheggie@cleanairmetals.ca ___________________________________________________________________________ The Mesoproterozoic (1,106.6 ± 1.6 Ma [1]) Current intrusion forms part of the PGE-CuNi mineralized Thunder Bay North Intrusive Complex. The Current intrusion consists of a northwest-trending conduit-type body (wehrlite, lherzolite, olivine gabbronorite ± troctolite) associated with the earliest stages of the Midcontinent Rift System (MRS; [2]) that intruded Archean rocks of the Quetico Basin and is associated to the Quetico Faults System that cross the boundaries between the Quetico basin and the Wabigoon terrane in the Superior Province [3]. To date the intrusion hosts four mineralized zones (Fig. 1); the Current and Bridge Zone in the northwest, the Beaver-Cloud Zone in the middle, and the 437-Southeast Anomaly (SEA) Zone is in the southeast [4]. Geochemical analysis of the intrusion reveal a Figure 1. Schematic model of the Current intrusion well-defined primitive mantle-normalized and the Quetico country rock. Illustration compiled pattern resembling ocean island basalt, in Leapfrog using data provided by Clean Air Metals characterized by LREE enrichment and small Inc. positive anomalies in Nb, La, and Ce relative to Th, suggesting no, or minimal, crustal contamination. The La/Smn values in samples from the Current intrusion range from 1.8 to 2.6, consistent with previous studies and suggesting the originated from an enriched mantle plume. The enriched composition of the magma in the intrusion aligns with other mineralized and unmineralized intrusions related to the MRS, including the Escape, Seagull, Lone Island intrusions, and the Nipigon Sills [5,6,7,8]. The intrusion has slightly lower Sri (0.7021 to 0.7043) and εNd (-1.18 to -4.02) than the typical values of the mantle source at 1100 Ma as well as the Nipigon Sills, Seagull intrusion, and Coubran volcanics [5,6,9]. Given the absence of geochemical anomalies that indicate assimilation of the Archean crust, an enriched SCLM is suggested to have interacted with the parental magma to generate the slightly negative εNd values. Stable isotope analysis suggest that the rocks of the intrusion underwent interactions with magmatic fluids (δ2H from −40 to −80‰, δ18O from 5.5 to 7.0‰; [10,11]), meteoric fluids (δ2H <-80‰, δ18O <5.5‰; [12]), and crustal derived fluids (δ18O >7‰; Figure 2; [13,14]). �13 The assessment of alteration intensity and micro-textural features in the intrusion identified three distinct domains, each showing varying secondary mineral assemblages. Domain A consists of antigorite, actinolitetremolite, clinochlore, epidote, sericite, pyrite, millerite, secondary pyrrhotite, chamosite and secondary magnetite. Domain B consists mainly of lizardite-chrysotile and an increase in the modal abundances Figure 2. δ18O and δ2H values of bulk rock in the four of clinochlore, epidote, sericite, mineralized zones of the Current intrusion (Current, Bridge, pyrite, millerite, and secondary Beaver-Cloud, and 437-SEA) and the surrounding country rock magnetite relative to Domain A. of the Quetico basin. Domain C is composed of talc and carbonate minerals that have replaced the secondary minerals of Domains A and B. Domains A and B were formed by fluids with H2O content derived from meteoric and magmatic sources. Domain A indicates high-temperature alteration processes, with the presence of antigorite suggesting temperatures exceeding 300°C [15]. In contrast, Domain B formed from fluids at lower temperatures (<300 °C; [16]), primarily due to the presence of lizardite-chrysotile. Domain C is associated with later crustal fluids with CO2 contents below 50°C [16]. The alteration processes that have modified the Current intrusion involved the mobilization and incorporation of major elements such as Na2O, Fe2O3, K2O, and CaO in the replacement of primary silicates by secondary silicates, as well as a reduction in mineral volume during the replacement of primary sulfides by secondary sulfides and oxides. References: [1] Bleeker W et al. (2020) Geological Survey of Canada 8722: 7-35 [2] Woodruff L et al. (2020) Ore Geology Reviews 126: 103716 [3] Williams H (1991) Ontario Geological Survey 833-403 [4] Kuntz G et al. (2022) Princeton University 171-204 [5] Heggie G (2005) Lakehead University 365 [6] Hollings P et al. (2007b) Canadian Journal of Earth Sciences 44(8): 1111-1129 [7] Caglioti C (2023) Lakehead University 242 [8] Yahia K (2023) Lakehead University 148 [9] Cundari R (2012) Lakehead University 154 [10] Loewen M et al. (2019) Earth and Planetary Science Letters 508: 62-73 [11] Taylor H (1968) Contributions to Mineralogy and Petrology19(1): 1-71 [12] Ripley E and Al-Jassar T (1987) Economic Geology 82(1): 87-107 [13] Li H (1991) Mcmaster University 138 [14] Ripley E et al. (1993) Economic geology 88(3): 679-696 [15] Evans B (2004) International Geology Review 46(6): 479-506 [16] Barnes I et al. (1973) Economic Geology 68(3): 388-398 �14 Sulfide percolation and drainback process in magmatic conduit system in the Huangshan-Jingerquan mineralization belt Deng, Y.-F.1, Xie-Yan Song2 and Feng Yuan1 1 Ore Deposit and Exploration Center (ODEC), Hefei University of Technology, Hefei 230009, Anhui, P. R. China, dengyufeng@hfut.edu.cn 2 State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, 46th Guanshui Road, Guiyang 550002, P. R. China ___________________________________________________________________________ Magma conduit systems consist of a series of flow-through dykes and sills (Barnes et al., 2016). When sulfides segregated at depth are carried by ascending mafic magmas, they would settle out in magma feeders or chambers at shallower depths as the flow velocities decreased. The differentiated sulfide rich melts in the upper magma chamber could drain back into the feeder dykes to form massive sulfide veins. The Huangshan-Jingerquan Ni-Cu metallogenic belt is located at the southern margin of the Central Asian Orogenic Belt. The total Ni metal reserve of the deposits is about a million tonnes. This makes it the largest orogenic Ni-Cu metallogenic belt worldwide (Deng et al., 2022). The Huangshandong, Huangshan, Tulaergen deposits are the biggest magmatic Ni-Cu deposits in this area, the morphology of the sulfide-bearing mafic-ultramafic complex and occurrence of the Ni-Cu sulfide orebodies in the deposits are obviously different. The Huangshandong complex is rhombus-shaped, ~3.5 km long with a maximum width of 1.2 km. The complex was emplaced in the Gandun Formation carbonaceous slate and meta-sandstone intercalated with limestone. The Huangshandong deposit contains 90 million metric tonnes (Mt) of sulfide ores at average grades of 0.40 wt% Ni (Song et al., 2021). Several ore horizons comprised of disseminated and net-textured sulfides are located at the base of the lherzolite within the complex. A series of concave lenticular orebodies within the gabbronorite occur at the western end of the complex. The tadpole-shaped Huangshan complex is 3.8 km long and up to 0.8 km wide. The base of the complex dips to the west to a depth of ~1000 m and becomes shallower to the east. It was emplaced into the sulfur-barren meta-sandstone and limestone of the Gandun Formation. There is an up to 50m thick thermal metamorphic aureole containing cordierite and epidote around the Huangshan complex. The Huangshan deposit contains 80.4 Mt of sulfide ores with average grades of 0.54 wt% Ni (Zhou et al., 2004). The main stratiform sulfide orebody comprised of disseminated and net-textured sulfides occurs at the base of the lherzolite, which is underlain by gabbronorite. The small Tulaergen dyke consists of lherzolite, websterite and gabbro, and was emplaced in the Wutongwozi Formation meta-tuff and meta-sandstone. The Tulaergen deposit contains ~37 Mt of sulfide ores with average grades of 0.45 wt% Ni (Mao et al., 2008). Variably sized lenticular Ni-Cu sulfide orebodies comprised of disseminated and net-textured sulfides are situated in the central part of the lherzolite . The Ni grade is higher in the upper part of the orebodies than in the lower part. A Fe-rich massive ore vein occurs within the disseminated ores and a Cu-rich massive ore body extends from the ultramafic dyke to the wall-rock (Zhao et al., 2019). The Ni-Cu sulfide deposits along the Huangshan-Jingerquan belt were formed in different locations at different depths in independent conduit systems. The migration and deposition processes of the sulfide liquids in these conduit systems are analogous to the model proposed by Barnes et al. (2016). We propose that some of the sulfides were deposited where the magma pathways changed direction and formed the Tulaergen sulfide-mineralized dyke in the Wutongwozi Formation at relatively deep levels (Deng et al., 2021). The negative correlations between IPGE and Pd/Ir of the Tulaergen massive ore veins suggest a differentiation between IPGE and PPGE triggered by fractional crystallization of the sulfide melt (Zhao et al., 2019). The massive ore veins embedded within the disseminated ores are likely the result of drain back of differentiated sulfide liquids along fractures within the �15 disseminated orebody. Whereas, other sulfide-rich liquids were carried upward into shallow magma chambers. There, the reduction in flow velocity caused the precipitation of sulfide that formed the stratiform or lenticular orebodies in the large magma chambers at relatively shallow depths, such as the Huangshan and Huangshandong complexes hosted in the Gandun Formation. References: [1] Deng Y-F et al (2022) Economic Geology 117: 1867-1879 [2] Song X-Y et al (2021) Lithos390-391 doi:10.1016/j.lithos.2021.106114 [3] Zhao Y (2019) Geochimica et Cosmochimica Acta 249:42-58 [4] Barnes S (2016) Ore Geology Reviews 76:296-316 �16 Weathering of non-ore grade rock from Duluth Complex deposits: Outcomes from comprehensive pre-mining geochemical characterization Diedrich, T.R.1 and Theriault S.2 1 2 MineraLogic LLC, 306 W Superior St., Suite 920, Duluth, MN USA 55802, tdiedrich@mnlogic.com MineraLogic LLC, St. Paul, MN, USA ___________________________________________________________________________ The Duluth Complex, a large, predominantly mafic, intrusive complex in northeastern Minnesota, USA associated with the 1.1 Ga Mid-Continent Rift System, hosts several magmatic copper-nickelcobalt and platinum group element (Cu-Ni-Co ± PGE) deposits. These deposits are generally located along the northwestern boundary of the complex, and in proximity to the Paleoproterozoic-aged metasediments of the Animikie Basin. NewRange Copper Nickel LLC (“NewRange”) is currently assessing and/or engaged in development of the Mesaba and NorthMet deposits within the Duluth Complex. Complementing these efforts, NewRange has conducted an extensive and comprehensive program to characterize the environmental geochemistry of non-ore grade rock, ore, tailings, and unconsolidated surficial materials associated with the deposits. This program includes standard mine waste characterization methods, e.g., ASTM humidity cell tests (HCT); custom designed tests to provide information at different scales of evaluation; multi-faceted mineral characterization components; and field weathering tests. The results of the test program both provide a robust basis for identifying waste rock and water management strategies which would be protective of the environment during mining, and elucidate aspects of the fundamental weathering behavior of gabbroic composition rock. Non-ore grade rocks and tailings from these deposits contain minor amounts of the iron sulfide mineral pyrrhotite, which, during weathering in the presence of oxygen, releases proton acidity through the reaction: 2FeS(po) + 2.75O2 + 2.5H2O → 2Fe2+ + 2SO42- + 5H+ (1) If the reaction continues to proceed in the presence of adequate oxygen, the iron will oxidize and, under circum-neutral pH conditions, precipitate as iron oxides, hydroxides, or oxyhydroxides, generalized as the following: Fe2+ + ¼ O2 + H+ → Fe3+ + ½ H2O (2) Fe3+ + 3H2O → Fe(OH)3(s) + 3H+ (3) While rocks from the deposits do not contain appreciable carbonate minerals to neutralize this proton acidity, they do contain abundant plagioclase and olivine—both of which can neutralize the proton acidity produced during the above reactions during weathering. The environmental geochemical characterization program indicates that there are at least three distinct, but related, neutralization mechanisms active in non-ore rock and tailings from the Duluth Complex. The first neutralization mechanism is the consumption of protons as reactants in silicate mineral dissolution reactions. Common weathering reactions for relatively reactive silicate minerals that are abundant in the complex include the following: Plagioclase (anorthite) dissolution CaAl2Si2O8(s) + 2H+ + H2O → Ca2+ + Al2Si2O5(OH)4(s) (4) Olivine (forsterite) dissolution Mg2SiO4(s) + 4 H+ → 2Mg2+ + H4SiO4 (5) As shown from reactions (4) and (5), every cationic charge unit (for example, 2 cationic charge units for every mol Mg2+ and Ca2+) produced corresponds to a proton being consumed as a reactant. Furthermore, in the presence of atmospheric CO2, dissolution of CO2 into rainwater results in reactions driving towards equilibria between carbonic acid, proton acidity, and bicarbonate alkalinity: �17 H2CO3 → H+ + HCO3- (6) Weathering of silicate minerals in the presence of carbonic acid under neutral pH conditions tends to move reaction (6) toward the reaction products, resulting in accumulation of bicarbonate alkalinity in associated waters. Reaction with the accumulated alkalinity represents a second potential neutralization mechanism. Finally, under select hydrologic conditions (low water to rock ratios), bicarbonate produced in reaction (6) could build up and eventually react with the calcium and magnesium released during reactions (4) and (5) to precipitate carbonate minerals in situ. This reaction leads to the third neutralization mechanism, dissolution of secondary carbonate minerals, and, further, provides a means of capturing and transforming atmospheric CO2 into stable solid phases in the rock. Outcomes from the environmental characterization program support the long-term effectiveness of these three mechanisms in neutralizing acidity from low sulfur rock. A subset of tests have been running for approximately 19 years, and, thus provide direct observational evidence at the multidecadal scale (Fig. 1). Furthermore, geochemical trends from these tests indicate that neutralization reactions will persist at least as long as the sulfide oxidation potential exists. Figure 1. 10th percentile (“P10”) of pH values observed over long-term kinetic testing as a function of initial sulfur content. Each circle represents one HCT. Test durations vary, with the longest tests running for approximately 19 years. The potential to generate drainage with pH less than the blank is dependent on initial sulphur content, with all samples starting with less than approximately 0.2% sulphur maintaining a neutral pH throughout testing. �18 Application of FactSage to Model the Compositional Variability of the Ni-CuPGE Mineralization at the Main Zone of the Tamarack Intrusive Complex El Ghawi, A.K.1 and Mungall, J.E.1 1 Carleton University; Mineral Deposits. Lab Herzberg Laboratories 1125 Colonel By Drive, Ottawa, Ontario, Canada; Karimelghawi@cmail.carleton.ca ___________________________________________________________________________ The Tamarack Intrusive Complex (1105.6 ± 1.2 Ma) is located in NE Minnesota and was emplaced during the early magmatic stage of the Midcontinental Rift System (MRS) [1]. The TIC is composed of a Dike intrusion in the north where the Ni-Cu-PGE mineralization is hosted, and a less explored Bowl intrusion in the south, (Fig. 1). The Dike area of the complex can be divided into many zones which are, from north to south, the Raptor Zone, the Main Zone, and the 164 Zone (Fig.1). Sulfide mineralization in these zones occur as disseminated (1-8 wt.% S), semi-massive (8-25 wt.% S), and massive sulfides (> 25 wt.% S), composed dominantly of pyrrhotite, pentlandite, and chalcopyrite. Massive sulfide bodies in the Main Zone are mostly hosted in the country rocks between the FineGrained Olivine (FGO) and Coarse-Grained Olivine (CGO) Intrusions (Fig.1). Some thin massive sulfide veins also occur in the Main Zone, crosscutting the CGO intrusion. Figure 1: a) Outline of the Tamarack Intrusive Complex. b) Cross section through Main Zone, looking north. Modified after [2]. To understand the compositional variability of the sulfide mineralization at the Main Zone of the TIC, as well as the evolution of the sulfide and silicate magma, chalcophile element compositions (Ni, Cu, Pt, Pd) of sulfide-mineralized rocks have been reported, and a thermodynamic model was developed using the thermodynamic software FactSage 8.3. The FactSage software package uses the ChemSage Gibbs energy minimization routine to minimize the total Gibbs energy of a system with a given set of constraints, and with the availability of the thermodynamic database for the system of interest [3]. These databases have been developed from the optimization of data from the literature, and from new experimental results [3]. The silicate magma composition that is equilibrated with the sulfide liquid in the TIC has been inferred using FactSage. An isenthalpic assimilation-fractional crystallization model has been �19 followed starting with the composition of the Mamainse Point Formation, Volcanic Group 2, that is associated with the same stage that the TIC was emplaced in [4]. The contaminant that was used in this model is the Virginia Formation shale. An R-factor model was then implemented to assess the effects of varied silicate to sulfide mass ratios on the composition of the sulfides at the Main Zone of the TIC [5]. The Rfactor curve passes through the disseminated sulfides, most of which occur between R = 700 and R = 1500 (Fig. 2). The semi massive sulfides are depleted in Pt and Pd compared with the disseminated sulfides. The massive sulfides that mainly occur in the country rocks are Pt and Pd poor and Ni rich, suggesting that these sulfides might be dominated by accumulated monosulfide solid solution (MSS), and there might have been a net loss of fractionated sulfide liquid from the Main Zone of the TIC (Fig.2). The sulfide melt composition calculated at an R factor equal to 900 was then inputted into FactSage and an equilibrium crystallization run was then performed. Trends of MSS and sulfide liquid were generated (Fig. 2). The sulfide melt composition at R = 900 coexists with the early crystallizing MSS at the sulfide liquidus temperature of 1038 °C. With cooling and crystallization of MSS, the sulfide liquid becomes more enriched in Pt, Pd, and Cu. Most semi massive sulfide compositions can be represented as mixtures of MSS and liquid. The extreme enrichment in Pt and Pd shown by sulfide veins cannot be explained solely in terms of MSS fractionation and will be the subject of future study. Figure 2: Variation of Ni, Pt, and Pd versus Cu in the disseminated, semi massive, massive sulfides, and sulfide veins from the Main Zone of the TIC. Concentrations are represented in 100% sulfide. The orange circles along the black curves represent sulfide compositions at different R factors. Solid and liquid compositions during equilibrium crystallization of a sulfide liquid formed at R= 900 are represented by horizontal lines and crosses, respectively. Tie-lines are represented in green dashed lines connecting the coexisting liquid and the early crystallizing solids at 1038 °C and at 817 °C. References: [1] Goldner B (2011) MSc Thesis: 155 [2] Talon Metals (2022) Technical Report [3] Bale C et al. (2009) Calphad 33(2): 295-311 [4] Lightfoot P (1999) OGS 5998: 91 [5] Campbell IH and Naldrett AJ (1979) Econ Geol 74: 1503-1506 �20 Petrophysics Applied to Magmatic Sulfide Deposits: The Physical Properties Mineralogy Link Enkin, R.J.1 1 Geological Survey of Canada, POB 6000, Sidney, BC V8L 4B2, CANADA, randy.enkin@nrcan-rncan.gc.ca ___________________________________________________________________________ Modern mineral exploration demands interpretation formed by the integration of two principal activities: geological mapping and geophysical survey collection. The linking element is the physical properties of rocks, which must be measured, compiled, and analysed. The current emphasis on critical minerals is motivating us to look deeper into previously explored regions to understand the geological settings that are conducive to discovering economic critical mineral systems. Figure 1, Conceptual framework describing the behaviour of various physical properties commonly measured by the mining industry. [1] Physical properties are directly controlled by the bulk composition, the mineralogy, and the texture of rocks [2]. Gravity and magnetic surveys reflect density and magnetic properties, which can mostly be described by the relative amounts of three principal components of mineral families: the light minerals: quartz+feldspar+calcite, the dark minerals: ferromagnesian silicates, and magnetite. Ore minerals and porosity add and subtract density. Importantly, igneous rocks formed in the upper crust usually have a ~10:1 ratio of ferromagnesian silicates to magnetite concentration, and most subsequent geological processes lead to magnetite loss. Electric resistivity and chargeability are controlled by permeability and ore minerals which effectively form networks of wires and capacitors, as revealed by equivalent circuit analysis of spectral impedance measurements. �21 Figure 2, Henkel Plot, Density vs Log(Magnetic Susceptibility), of rocks in the Canadian Rock Physical Property Database. [3] Figure 3, Igneous rocks formed in the upper-crust fall on the Magnetite Trend (FM/M~10), whereas most other geological processes are magnetite destructive. [2] �22 Ultramafic environments, which commonly host Ni-Cu deposits, have a distinctive set of petrophysical properties, which bears directly on their geophysical signatures [4]. Originating from deep, reduced levels, unaltered ultramafics are typically dense and paramagnetic. On hydration and serpentinization, rocks become extremely low density, and iron is rejected from ferromagnesian silicates to form high concentrations of magnetite. These rocks are extremely magnetic and usually display high Koenigsberger ratios, meaning that magnetic remanence dominates aeromagnetic surveys. Carbonation transforms rocks to dense, paramagnetic bodies. Examples from British Columbia and Ontario will illustrate these exotic trends and processes. Figure 4, Henkel plot of ultramafic rocks in the Canadian Cordillera, displaying physical property changes with degree of serpentinization. [4] Through understanding the physical properties - mineralogy link, geophysical interpretation leads to delineation of geological processes and better exploration strategies. References: [1] Dentith, et al. (2020), Geophysical Prospecting, 68: 178-199 doi.org/10.1111/1365-2478.12882 [2] Enkin RJ, et al. (2020), Geochemistry, Geophysics, Geosystems, 21: doi.org/10.1029/2019GC008818 [3] Enkin RJ (2018), Geological Survey of Canada Open File 8460, doi.org/10.4095/313389 [4] Cutts JA, et al. (2021), Geochemistry, Geophysics, Geosystems, 22: doi.org/10.1029/2021GC009989 �23 Regional changes in plume-generated stress linked to MCR (Keweenawan LIP) chonolith emplacement Ernst, R.E.1, El Bilali, H.1, Buchan, K.L.2 and Jowitt, S.M.3 1 Department of Earth Sciences, Carleton University, Ottawa K1S 5B6, Richard.Ernst@Carleton.ca. 273 Fifth Ave., Ottawa K1S 2N4, Canada 3 Nevada Bureau of Mines and Geology, University of Nevada Reno, 1664 N. Virginia Street, Reno 89503, Nevada, USA 2 ___________________________________________________________________________ Introduction: Changes in regional stresses contribute to the formation of many types of ore deposits. Here, we consider the role of plume-generated stresses in metallogeny, and the role of giant dyke swarms of LIPs in monitoring those stresses. We begin with our just-published analysis of the Siberian Traps LIP, its giant dyke swarms and its Norilsk-Talnakh ores [1], and then we consider the Mid-Continent Rift / Keweenawan LIP event as a second example. Norilsk-Talnakh ores of the Siberian Traps LIP: Plume-generated 90° stress change recorded by the transition from radiating to circumferential dolerite dyke swarms of the Siberian Traps LIP may be linked to emplacement of Norilsk-Talnakh ore deposits. [2] showed that the timing of Norilsk-Talnakh Ni-Cu-PGE mineralization in the Siberian Traps LIP is associated with a 90° change in stress, which they attributed to changes in plate stresses. However, as detailed in [1], we propose that this 90° stress change associated with Norilsk-Talnakh mineralization could instead be due to changing plume dynamics as monitored by the transition from the LIP’s giant radiating dolerite dyke swarm to its circumferential swarm (Fig. 1). As noted in [1], the 90° transition from a regional radiating swarm to a circumferential swarm involves a decrease in the radial sigma 1 stress followed by an increase in a hoop-like sigma 1 stress. This implies an intervening period in which the stress is isotropic, a period that we associated with emplacement of the Norilsk-Talnakh mineralization. It is possible that this stress drop led to release of volatiles and allowed ascent and/or lateral emplacement of gas-buoyed magmatic sulphides (e.g. [3-5]). Figure 1: LEFT: Distribution of dyke swarms and volcanic feeder zones associated with the Siberian Traps LIP; modified after [6]. A generalized version of the overall radiating system of dykes and feeder zones is superimposed in orange, and a generalized version of the circumferential dykes is in light purple. Dyke sets: E = Ebekhaya; KO = Kochikha; M = Maimecha. N = Norilsk feeder zones to volcanic flows, which correlate with major fault zones, including the prominent Norilsk-Kharaelakh fault (KF). RIGHT: Timing of volcanic assemblages in the Norilsk region (younging upward), compared with the stress orientations after [2] and with the matching dyke swarm pattern from [1]. Mid-Continent Rift System (Keweenawan LIP): We consider this as a possible example of plume related stress change linked to chonolith mineralization. This major (~1112-1090 Ma) LIP event in the �24 Great Lakes region of North America is associated with an arcuate zone of rifting and a significant number of mineralized intrusions (“chonoliths” and ‘tube-like conduits” in [7]; and “conduit type intrusions” in [8]. [8] noted two main stages in this LIP: the ~1112-1105 Ma Plateau stage, and the ~1100-1092 Ma Rift stage, followed by Late Rift and Post-Rift stages. The numerous chonoliths (conduit type intrusions) were mostly emplaced during the Plateau stage. Figure 2. The 1112–1090 Ma Keweenawan LIP of the Mid-continent Rift of North America. Key elements include volcanics, sills, a circumferential dyke swarm, and exposed and buried intrusive complexes. Also shown are the older ca. 1140 Ma Abitibi dyke swarm and coeval lamprophyre dykes, which may represent a radiating dyke system, and may be related to 1150 Ma Corson diabase intrusions [9] centred just west of the figure. Rift-parallel circumferential Keweenawan dykes from west to east: CC = Carlton County, PR = Pigeon River, CI = Copper Island, P = Pukaskwa, M = Mamainse Point. BM = Baraga-Marquette dykes. Keweenawan sills: LS = Logan, NS = Nipigon sills. Intrusive complexes: DIC = Duluth, CIC = Coldwell, NEIIC = northeastern Iowa. Ca. 1140 Ma radiating dykes: A = Abitibi, ED = Eye-Dashwa, L = lamprophyre dykes. Interpreted mid-crustal intrusive complexes are shown schematically as brown circles. The Goodman Swell has been interpreted as locating the centre of an underlying mantle plume. More details in [10]. [10] described a giant circumferential dyke swarm for the Keweenawan LIP / Midcontinent Rift (Fig. 2), analogous to a Venusian corona. The ages of Pigeon River dykes [7], which we interpret as a portion of the circumferential swarm, indicate emplacement during the Rifting stage, perhaps in association with spreading of the plume head. In our interpretation, plume head arrival and initial domal uplift may have occurred 30 my earlier at 1140-1150 Ma, associated with emplacement of the 1141 Ma radiating Abitibi swarm (Fig. 2; [11]. We speculate that the radiating stress regime at 1140 Ma associated with plume generated uplift persisted until the Plateau stage before transitioning to the circumferential stress regime associated with the Rifting stage. The chonoliths/conduit type intrusions, such as Tamarack, BIC, Eagle and Current Lake [7-8], were mostly emplaced during the Plateau stage, i.e. during our proposed transition from radiating to circumferential stresses. This is a similar timing to our interpretation for the Norilsk-Talnakh ores of the Siberian Traps LIP (Fig. 1; [1]. References: [1] Ernst R et al. (2024) Econ Geol 119: 243–249 [2] Begg et al. (2018). Ch 1, in Mondal S and Griffin W (ed.) Processes and ore deposits of ultramafic-mafic magmas through space and time: Elsevier, p. 1–46. [3] Lesher (2019) Can J Earth Sci 56: 756-773 [4] Yao Z-s and Mungall J (2022) E Sci Rev 227: 103964 [5] Barnes S et al. (2023) Geology 51 (11): 1027-1032 [6] Buchan K and Ernst R (2019), In: Srivastava R et al (eds.) Dyke swarms of the world – a modern perspective: Springer, p. 1–44, [7] Bleeker W et al. (2020). In Bleeker W and Houlé M (ed.). Geol Surv Canada Open File 8722. [8] Woodruff L et al (2020) Ore Geol Rev 126: 103716 [9] McCormick K et al (2018) Can J Earth Sci 55: 111-117 [10] Buchan K and Ernst R (2021) Gondwan Res. 100: 25–43 [11] Ernst R et al. (2018). Earth Planet Sci Lett 502: 244-252 �25 A proposed cryptic common thread among Ni-Cu-PGE-(Au-Te) systems spanning the boundary between Laurasia and Gondwana Fiorentini, M.1*, Holwell, D.2, Blanks, D.3, Cherdantseva, M.1, Denyszyn, S.4, Ince, M.1, Vymazalova, A.3, and Piña Garcia, R.5 1 Centre for Exploration Targeting, Australian Research Council Industrial Transformation Training Centre in Critical Resources for the Future, School of Earth Sciences, University of Western Australia, Australia marco.fiorentini@uwa.edu.au 2 Centre for Sustainable Resource Extraction,School of Geography, Geology and Environment, University of Leicester, United Kingdom 3 BHP Metals Exploration, United Kingdom 4 Department of Earth Sciences, Memorial University of Newfoundland, Canada 5 Dpto. Mineralogía y Petrología, Universidad Complutense Madrid, Spain ___________________________________________________________________________ The long-lived geodynamic evolution of the boundary between Laurasia and Gondwana may have created the ideal conditions for the genesis of a trans-continental Ni-Cu-PGE-(Au-Te) mineralised belt in Europe. This working hypothesis stems from the recent understanding that orogenic processes play a fundamental role in the triggering of chemical and physical processes for the transport of metals from the metasomatised mantle through to various crustal levels. An insight into the polyphased genetic evolution of magmatic sulfide mineral systems is provided by a series of mineralised occurrences located in the Bohemian Massif, Czech Republic. Here, a series of hydrated gabbros contain magmatic sulfides ranging in texture from disseminated to matrix and blebby. These alkaline intrusions with a markedly sodic nature host magmatic sulfide mineralisation revealing a mantle-like signature, with in-situ ∂34S values ranging from -2.4 to +1.8‰. New TIMS UPb data pinpoint emplacement and crystallisation of these mineralised magmas at 363.9 ± 0.6 Ma, with Sm-Nd model ages pointing to involvement of a metasomatised Mesoproterozoic lithospheric mantle in a post-orogenic geodynamic framework. Mineralised intrusions in the Bohemian Massif are strongly analogous to a series of Permo-Triassic (290-250 Ma) hydrated and carbonated ultramafic alkaline pipes containing Ni-Cu-PGE-(Te-Au) mineralisation emplaced in the lower continental crust in the Ivrea Zone, Italy. Despite the significant age difference, mineralisation in the Bohemian Massif and Ivrea Zone is similar in terms of their geochemical and isotopic characteristics, pointing to similar ore forming processes and mantle sources having operated in a syn- to post-Variscan Orogen setting. A subsequent mineralising event is recorded in the Ivrea Zone at ~200 Ma, most likely associated with the Central Atlantic Magmatic Province (CAMP). It is argued that this event reactivated and focussed lower-crustal carbonate- and metal-rich sulfide mineralisation associated with the Permo-Triassic pipes into the ~200 Ma mineralised intrusion known as La Balma Monte Capio. Mineralised systems in the Bohemian Massif and Ivrea Zone are markedly different in size, geometry and overall metal endowment from the larger and better-known Aguablanca system in southern Spain. However, they all share distinctive geochemical and isotopic characteristics pointing to a common DNA: their association with the complex and multi-phase activation of the margin between Laurasia and Gondwana across the Variscan metallogenic belt from the Devonian to the Triassic. �26 The nature and localisation of the magmatic sulfide mineral systems along this belt indicate that enhanced potential for ore formation at lithospheric margins may be due not only to favourable architecture, but also to localised enhanced metal and volatile fertility. This hypothesis may explain why ore deposits along the margins of lithospheric blocks are not distributed homogeneously along their entire extension but generally form clusters. As mineral exploration is essentially a search space reduction exercise, this new understanding may prove to be important in predictive exploration targeting for new mineralised camps in Europe and elsewhere globally, as it provides a way to prioritise segments with enhanced fertility along extensive lithospheric block margins. �27 How exploration geologists can and should use “soft NSRs” to represent assays of Ni-Cu-PGE mineralization Goldie, R.J. Independent Analyst and Director, 54 Peach Willow Way, Toronto, Ontario, Canada M2J 2B6 Raymondgoldie@outlook.com __________________________________________________________________________ A Net Smelter Return (NSR) is the net revenue generated by a block of mineralization, less off-site costs (Goldie and Tredger [1]). Three procedures for computation of the NSRs of Ni-Cu-PGE sulphide mineralization are in common use: values calculated by accountants; mine-specific estimates prepared by mine operators, and “soft estimates” (Goldie [2]). Soft estimates are useful in representing assays of samples taken during exploration for Ni-Cu-PGE deposits. Their computation is based on statistical analyses of the grades and metallurgical properties of ores at operating Ni-Cu-PGE mines, and the smelting and refining fees paid by those mines. There are three reasons why exploration geologists should express assays of samples as soft estimates of NSRs: (i) representing assays as single numbers facilitates their graphical representation, such as on contour maps; (ii) the computation of soft estimates may reveal that, as is common in mineralization that is rich in PGE, the mineralization contains substances or has mineralogical issues that could lead to a smelter penalizing or even rejecting a potential mine’s products (Goldie [3]); (iii) representation of assays as single numbers not only facilitates their comprehension by the readers of company press releases, it may also reduce the chances that investors apply invalid rules-of-thumb to those assays, resulting in expensive misunderstandings. References: [1] Goldie R and Tredger P (1991) Geosci Canada 18:159-171 [2] Goldie R (2023) Min Economics https://doi.org/10.1007/s13563-023-00400-3 [3] Goldie R (2022) Aust Inst Mining & Metal, Int Mining Geol Conf: 222-235 �28 Characterizing the Early (Plume) and Main (Rifting) Stages in the evolution of the Midcontinent Rift Good, D.J. Department of Earth Sciences, Western University, London, ON N6A 5B7, Canada, dgood3@uwo.ca ___________________________________________________________________________ The mid-Proterozoic Midcontinent Rift (Keweenawan Large Igneous Province) contains the most diverse assemblage of basalt rock types for any LIP on earth. In this study, six of the eight main basalt types in the rift are compared to the global distributions of ocean plateau, ocean island basalts and continental large igneous province basalts using a combination of two sophisticated classification strategies based on high precision incompatible trace element data (after O’Neill, 2016 and Pearce et al., 2021). The two basaltic sequences that are not described here occur in the northeast quadrant of the Midcontinent Rift and were shown by Good et al. (2021) to have been derived from a metasomatically modified mantle source. Thus, they are not suitable candidates for interpretation using the classification strategies as applied here. Basalt data for the Midcontinent Rift were compiled by the author from detailed studies of trace element geochemistry at numerous sites around Lake Superior by several researchers during the past 30 years. Data for oceanic basalts were compiled by O’Neill (2016) as part of his impressive study to highlight the usefulness of calculated coefficients to characterize REE diagram patterns (λ0, λ1 and λ2). Data for continental Large Igneous Provinces were compiled by Pearce et al. (2021) to show the usefulness of geochemical proxy diagrams to define which of the various petrological mechanisms operated during their formation (the LIP Print Approach). Figure 1: Discrimination boundaries for basalts sourced from different Mantle Regions plotted on the O’Neill diagram (left hand side). See text for discussion. Group 52 corresponds to basalt that shows characteristics of both plume and upper mantle source. Taken together, these comparisons show that Midcontinent Rift data in groups 2, 3 and 4 are like ocean plateau basalts and groups 1 and 5 are like ocean island basalts. That is, data are in excellent agreement with the hypothesis that basalt in group 2 was derived by partial melting in the Upper Mantle whereas groups 5 and 1 were derived by partial melting in the Mantle Plume, but at depths below the pyrope garnet and majorite garnet stability boundaries, respectively. This and other evidence suggest Groups 3 and 4 were derived by partial melting in a subduction modified depleted mantle source. Based on these inferred origins for the various basalt units, the Midcontinent Rift exhibits spatial and temporal zonation. Spatially, the mantle plume was centred beneath the west �29 central portion of what is now Lake Superior. Temporally, the effects of mantle plume volcanism occurred throughout the Early Stage of the Midcontinent Rift but had vanished before the end of the Hiatus Stage. During the subsequent Main Stage of magmatism, mafic rocks were derived primarily from the Upper Mantle, presumably by decompression melting as the crust thinned during extension. Figure 2: Midcontinent Rift basalt of groups 1 to 6 plotted in the LIP print diagrams of Pearce et al. (2021). See text for discussion. Figure 3: Model for basaltic melt source regions of the Midcontinent Rift Event: (a) During the Early Stage, most melts are generated in the mantle plume with lesser amounts generated in the overlying mantle and/or subduction modified lithospheric or asthenospheric mantle; (b) During the Main stage, most of the melts are generated by decompression melting in the upper mantle as the crust thins during extension. References: [1] O’Neill, H.St.C, (2016) Journal of Petrology, Vol. 57, No. 8, 1463–1508 [2] Pearce, J.A. et al., (2021) Lithos 392–393 (2021) [3] Good, D.J. et al. (2021) Journal of Petrology, 2021-07, Vol.62 (7) �30 Lithospheric structure controls for large magmatic Ni-Cu discoveries Hayward, N.1,2 1 Centre for Exploration Targeting, University of Western Australia, Crawley, WA 6009, Australia. NHayward@protonmail.com 2 PredictOre Pty Ltd, 1/40 Victory Terrace, East Perth, WA 6009, Australia ___________________________________________________________________________ To sustain the clean energy transition, society needs to increase the reserve base of green and critical mineral ore deposits containing metals such as copper (Cu), lithium (Li), nickel (Ni), cobalt (Co), rare earth elements (REE) and platinum group elements (PGE). Discovery of large new polymetallic Ni-Cu (±PGE, Co) sulfide deposits can help meet this need, but their discovery rates have declined over the last 25 years, and they present very difficult greenfield exploration targets because of their rare occurrence, very small footprints, large range in formation depths, concealment among extensive magmatic provinces, and increasing challenges for exploration land access. The fact that most recent Ni sulfide discoveries were found in magmatic provinces that had no previously known Ni-sulfide resources favours a first-mover Ni exploration strategy. The minerals industry needs improved mineral system models that more accurately predict the location of new districts (camps) and large deposits in remote and covered terrains with low data quality and availability. This study [1] demonstrates that low-cost three-dimensional lithospheric structure targeting has the power to significantly improve the accuracy and precision of targeting large magmatic Ni discoveries. It also addresses a disconnect between conceptual academic models for magmatic Ni-Cu (also Cu-Au) systems, which largely omit lithospheric structural controls on magma flux and intrusion emplacement, and the practice of explorers to empirically target proximity to lithospheric-scale fault zones for mineralised intrusions. This disconnect is exacerbated by a lack of quantitative analyses of the spatial accuracy, precision and causality of lithospheric structures that are inferred to be control ore deposition, which this study also addresses. The 1st-order (subprovince-scale) lithospheric structure control on magmatic Ni-Cu ore distribution is widely accepted to be along the sutured edges of paleo-cratonic blocks with preserved Archean subcontinental lithospheric mantle [2]. However, 2nd- to 3rd-order controls on emplacement of district-scale mineralised intrusion clusters and individual deposits along craton edges remain poorly understood. Two alternative models previously proposed are: (i) emplacement of dyke-like intrusions in dilational jogs along strike-slip faults [3], and (ii) emplacement of intrusion clusters near intersections of transverse translithospheric faults (TLFs) [4,5]. These models invoke predominantly vertical magma transport along fault conduits with subjacent sulphide saturation. Other models invoke long-distance lateral magma transport through interconnected sill and dyke complexes and potential for distal sulphide saturation [6,7] which, if correct, would greatly increase the permissive search space. New structural interpretations and quantitative analyses were completed globally for 72 Ni deposits with >50kt Ni (equivalent) metal. This extensive sample population covers a range of magmatic Ni deposit settings from intracratonic to pericratonic and arc-related, and from Mesoarchean to Cenozoic. Six detailed case studies addressing the lithospheric structure architecture controls on giant Ni deposits will be presented for Voisey’s Bay, Noril’sk-Talnakh, Kabanga, Jinchuan, West Musgrave, and the Cape Smith Belt. Less detailed examples will also be shown from the Midcontinent Rift, southern Africa, China, and western Australia. From quantitative analysis of the 72 regional structural case studies, the 1st-order control for all large magmatic Ni-Cu deposits is observed to be ≤30 km from paleocraton edge-parallel translithospheric faults, and specifically in their hangingwall where inclined. This relationship holds for all magmatic NiCu deposit settings. Furthermore, large intracontinental Ni deposits are also located ≤30 km from 2ndorder transverse translithospheric faults that intersect paleocraton edges (Fig. 1). However, for pericratonic and Archaean greenstone komatiite settings, proximity of Ni deposits to transverse �31 translithospheric fault intersections is not widely recognised or preserved. In one exception, clusters of komatiitic Ni deposits in the Agnew-Wiluna greenstone belt are observed to have a semi-regular spatial periodicity along strike with a mean spacing of ~22 km, and this is controlled by the intersection of local cryptic transverse rift faults [8]. Prioritising target proximity to certain translithospheric fault intersections can significantly reduce subprovince-scale search areas (~104-105 km2) to a few prospective districts (~102 km2). The largest deposits are found closest to (but rarely within) the most prominent translithospheric faults. At smaller scales, a few deposits are localised along small-scale dilational jogs in wrench faults, but this control is relatively rare. At deposit scale, controls on emplacement of mineralised channel-like flows and pipe-like intrusions (chonoliths) are typically more stratigraphic than structural, where overpressured, high temperature magmas self-generate pathways. Productive stratigraphic horizons are dominated by rheologically weak and highly fusible metasedimentary or gneissic units. A model (Fig. 2) is proposed where the root zones of translithospheric fault intersections initially channel fertile mantle melts into the deep crust. Ascent of buoyant overpressured magmas is then dispersed up to a few 10s km lateral to inclined master fault conduits through complex dyke-sill-dyke networks in steeper hangingwall fault splays, their damage zones, and rheologically weak contacts. The extreme magma flux required to form large Ni sulfide deposits results from positive magmadeformation feedbacks and bottom-up self-organisation. Targeting translithospheric fault intersections therefore requires a more systematic bottom-up and hierarchal approach to structural mapping, where the roots of cryptic lithospheric faults are defined, and structures are rated by scale, dip, and geodynamic behaviour. Fig. 1: Deposit size class versus distance to both edge-parallel and transverse TLFs. Fig. 2: Concept section showing dispersal of ascending mafic-ultramafic melts through dyke-sill networks with high magma flux in hangingwall of paleocraton edge translithospheric fault zone. References: [1] Hayward N (2024) Submitted to Econ Geol [2] Begg G et al (2010) Econ Geol 105: 1057-1070 [3] Lightfoot P and Evans-Lamswood D (2015) Ore Geol Rev 64: 354-386 [4] Myers J et al (2008) Can J Earth Sci 45: 909-934 [5] Begg et al (2018) Processes and Ore Deposits of Ultramafic-Mafic Magmas through Space and Time, Elsevier: 1-46 [6] Lesher C (2019) Can J Earth Sci 56: 756-773 [7] Ernst R et al (2019) J Volcanol Geotherm 384: 75-84 [8] Perring C (2016) Econ Geol 111: 1159-1185 �32 Deep orogenic magmatic Ni and Cu sulfide systems in the Curaçá Valley, Brazil Holwell, D.A.1, Thompson, J.2 , Blanks, D.E.1, Oliver, E.1, Porto, P.3, Oliviera, E.3., Tomazoni, F.4, Lima, A. 4, D’Altro R.,4., Graia, P.4 and Sant’Ana, T.4. 1 Centre for Sustainable Resource Extraction, University of Leicester, UK PetraScience Consultants, Vancouver, Canada 3 Ero Copper, Vancouver, Canada 4 Ero Caraiba, Brazil 2 ___________________________________________________________________________ The magmatic sulfide ores of the Curaçá Valley, Brazil, form an unusual subgroup of intrusion-related sulfide deposits. They are Cu-rich in general, with some Ni-dominant deposits on a district scale. They are located in small, hydrous mafic-ultramafic intrusions emplaced into the lower-mid crust at around peak metamorphic conditions. The metallogeny of the majority of known Curaçá Valley deposits are dominated Cu-sulfide deposits with abundant bornite, chalcopyrite with magnetite and hydrous silicates; phlogopite being abundant to semi massive in places. They have high Cu/Ni and Au/PGE ratios and have abundant telluride minerals. In addition, recently discovered Ni-rich deposits contain pyrrhotite, with pentlandite loops, some Co-rich pyrite, very minor chalcopyrite that is associated with phlogopite. Both deposit types are very low in IPGE (Os, Ir, Ru) and Rh. The Cu-Au-Te signature of the Curaçá Cu deposits, with abundant hydrous phases, particularly phlogopite, is consistent with an alkaline mafic genetic model, as these metallogenic characteristics have been identified in many of intrusions worldwide and usually represent post-subduction magmatic systems [1,2]. There are (at least at present) many more Cu occurrences identified in the Valley than Ni ones, and if the district is taken as a whole, then the overall metallogenic signature is still Cu-Au-Te dominant with some Ni and PGE. However, further discoveries of Ni would change this overall mass balance. An alternative, or possibly additional process that may have occurred is large scale sulfide liquid fractionation, where Ni-rich mss separates from Cu-rich sulfide liquid that crystallises at a lower temperature to Cu-rich iss. The general Cu-Au-Te(+Pd) signature of the Cu ores from the Curaçá Valley are entirely consistent with an iss signature, but it would imply sulfide liquid fractionation within the magmatic plumbing system on a district scale of km to tens of km. Whilst this may seem extreme, the process is clearly scalable from the mm to cm scale seen in many sulfide blebs and patches up to deposit scale such as the Cu-rich veins at Sudbury. Textural differences are striking, with the Ni ores having sulfides as disseminations, interstitial patches and net textured and massive sulfides representative of sulfide coexisting with silicate minerals. The Cu ores in stark contrast commonly show textures indicative of migrating Cu sulfide liquid, intruding as veins and breccia fills along with net-textures and insterstitial sulfides. The importance of phlogopite and other volatile-rich mineral phases with the Cu sulfide would also be consistent with a fractionated, volatile-rich sulfide liquid migrating over a wide range of distances. It is possible that the Curaçá Valley (and the O’okiep district in South Africa), represent deep magmatic sulfide systems at the roots of orogenic belts, formed from hydrous, metasomatized mantle sources, and where sulfide liquid fraction on a km-scale can produce both Ni- and Cu-rich deposits across a district. Regardless of the preferred individual or combined model, there is clearly potential for further discoveries in this complex setting References: [1] Holwell DA (2019) Nat Com 3511 [2] Blanks DE (2020) Nat Com 4342 �33 Spatial distribution, lithological associations, and geochemical signatures of Ring of Fire Intrusive Suite within the McFaulds Lake Greenstone Belt in the Superior Province: Implications for the Ni-Cu-PGE, Cr, and Fe-Ti-V Metal Endowment of the Region Houlé, M.G.1,2, Sappin, A.-A.1, Lesher, C.M.2, Metsaranta, R.T.3, Rayner, N.4, and McNicoll, V.4 1 Geological Survey of Canada, Lands and Minerals Sector, Natural Resources Canada, 490 Couronne Street, Québec City, QC G1K 9A9 Canada michel.houle@nrcan-rncan.gc.ca 2 Mineral Exploration Research Centre, Harquail School of Earth Sciences, Goodman School of Mines, Laurentian University, 935 Ramsey Lake Road, Sudbury, ON P3E 2C6 Canada 3 Earth Resources and Geoscience Mapping Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, ON P3E 6B5 Canada 4 Geological Survey of Canada, Lands and Minerals Sector, Natural Resources Canada, 601 Booth Street, Ottawa, ON K1A 0E9 Canada ___________________________________________________________________________ The McFaulds Lake greenstone belt (MLGB) is an arcuate (>200km long) belt within the Superior Province in northern Ontario that records episodic volcanism and sedimentation spanning from ca. 2.83 to 2.70 Ga and has been subdivided into several tectonostratigraphic assemblages [1]. One of the dominant geological features of the Mesoarchean to Neoarchean MLGB is the semi-continuous trend of mafic to ultramafic intrusions belonging to the Ring of Fire intrusive suite (RoFIS) [2], which hosts world-class Cr deposits, a major Ni-Cu-(PGE) deposit, and potentially significant Fe-Ti-V-(P) prospects. Intrusive bodies of the RoFIS occur within almost all volcanic-dominated supracrustal rock assemblages. The RoFIS has been subdivided into two subsuites based on their spatial distribution, lithological associations, geochemical signatures, and mineralization styles: the Ekwan River (ERSS) and Koper Lake (KLSS) subsuites [3, 4]. Although the mafic to ultramafic intrusive bodies of these subsuites have similar emplacement/crystallization ages (KLSS = 2732.9 to 2735.5 Ma vs. ERSS = 2732.6 to 2734.1 Ma), they are significantly different in many respects: 1) the KLSS is spatially much more restricted than the ERSS; 2) the KLSS is composed of dunite, peridotite, chromitite, pyroxenite, and gabbro, whereas the ERSS is composed of abundant gabbro and ferrogabbro with lesser anorthosite and rare pyroxenite and does not contain any olivine-rich ultramafic rocks; 3) the KLSS typically hosts Cr and Ni-Cu-(PGE) mineralization (e.g., mainly within the Esker intrusive Complex), whereas the ERSS typically hosts Fe-Ti-V-(P) mineralization (e.g., Big Mac and Thunderbird intrusions); and 4) the KLSS (higher MgO, Ni and Cr) and ERSS (higher FeOT, Ti and V) have clear differences in their geochemical trends indicating a distinct geochemical evolution (Fig. 1). Furthermore, ERSS ferrogabbro locally intrudes KLSS units, however, the opposite relationship is also observed at one locality. The magmatic evolution is still being debated, but the above observations suggest temporally overlapping but discrete ultramafic-dominated (KLSS) and mafic-dominated (ERSS) intrusions with complex contact relationships, rather than a single, large, tectonically dismembered layered ultramafic-mafic intrusion, as previously suggested [2]. A newly recognized intrusive body in the area contains olivine-rich ultramafic rocks and chromitite seams, like other members of KLSS, but both are enriched in Fe relative to rocks of the KLSS. This highlights the presence of several types of oxide-rich mineralization within the RoFIS. These include high Cr and low Fe chromitite seams typically associated with most of the Esker intrusive complex, intermediate Cr and Fe chromitite seams sporadically associated with parts of the Esker intrusive complex, and high Fe and low Cr magnetitite seams typically associated with EKSS’s intrusive bodies. Regardless of their origin, the exceptional metal endowments, and the wide diversity of mineral deposit types within the mafic and ultramafic rocks of the RoFIS, including Cr, Ni-Cu-(PGE), and Fe-TiV-(P) mineralization, of the McFaulds Lake greenstone belt highlight the likelihood of discovering additional mineral resources elsewhere within the Superior Province and other frontier areas throughout the Canadian Shield. �34 Figure 1: Binary plots of major and trace elements (anhydrous and normalized to 100%) of the mafic to ultramafic intrusions within the Koper Lake and Ekwan River subsuites of the Ring of Fire intrusive suite. A) FeOT versus MgO. B) Ni versus MgO. C) Ti versus Cr. D) Cr/V versus MgO. Data are from [5, 6, and Houlé, unpublished data]. References: [1] Metsaranta RT and Houlé MG (2020) Open File Rep 6359:360p. [2] Mungall JE et al. (2011) Proc GAC-MAC-SEG-SGA Ann Meeting Ottawa 2011:148 [3] Houlé MG et al. (2018) Open File Rep 8589:441-448 [4] Houlé MG et al. (2020) Open File Rep 8722:141-163 [5] Kuzmich B et al. (2015) Open File Rep 7856:115-123 [6] Metsaranta RT (2017) Ont Geol Surv Misc Rel Data 347 �35 Spatial distribution of mafic and ultramafic units in the Canadian north: Implications for critical minerals (Ni, Cu, Co, PGE) potential Houlé, M.G.1,2, Bédard, M.-P.1, Lesher, C.M.2, and Sappin, A.-A.1 1 Geological Survey of Canada, Lands and Minerals Sector, Natural Resources Canada, 490 Couronne Street, Québec City, QC G1K 9A9 Canada; michel.houle@nrcan-rncan.gc.ca 2 Mineral Exploration Research Centre, Harquail School of Earth Sciences, Goodman School of Mines, Laurentian University, 935 Ramsey Lake Road, Sudbury, ON P3E 2C6 Canada ___________________________________________________________________________ The transition to low-carbon economy that is taking place in Canada and elsewhere around the world is driving renewed interest in critical minerals, especially in battery minerals, like Ni and Co. Canada is one of the world's leading magmatic sulfide Ni producers, as attested by the presence of at least 5 world-class Ni mining districts (e.g., Sudbury-ON, Thompson-MB, Raglan/Expo-QC, Voisey’s Bay-NL, and Lynn Lake-MB). These Ni-Cu-Co-(PGE) deposits are associated mainly with magmatic maficultramafic mineral systems. Canada contains a very large number of mafic and ultramafic units across the country, but their total abundance is unknown and of these, only a handful are partially to well characterized. As example, a recent global compilation has reported only 52 layered intrusions in Canada [1]. Thus, an extensive compilation of mafic and ultramafic unit area is currently underway by the Geological Survey of Canada (GSC) to aid in identifying historic and future mineral resources (Fig. 1). Figure 1. Distribution of mafic and ultramafic units within northern Canada. Grey dashed line represents the approximate boundary of GEM-GeoNorth area (north of ~54° N latitude). Geological provinces are from [2]. NiCu-Co-(PGE) deposits: 1 = Canalask/Wellgreen, 2 = Turnagain, 3 = Muskox, 4 = Dinty, 5 = Axis/Currie/Rea, 6 = Nickel King, 7 = West Bear, 8 = Lynn Lake, 9 = Ferguson Lake, 10 = Rankin Inlet, 11 = Raglan Nickel Belt – Raglan and Expo horizons, 12 = Hope Advance sector, 13 = Chrysler-Erickson sector, 14 = Redcliff sector, 15 = Blue Lake sector, and 16 = Voisey’s Bay. The first step in this compilation is a large-scale spatial inventory of mafic and ultramafic units. To date, over fifteen thousand units have been catalogued north of ~54° N latitude (within the GEMGeoNorth area), based on geological maps available at scales ranging from large scale (1:500,000 to �36 1:63,360) to more detailed scale (1:5,000 or less), in the vicinity of known and historic Ni-Cu-(PGE) deposits, and where areas of interest have been identified due to the preponderance of maficultramafic units or nickel showings. Within the GEM-GeoNorth area, the largest proportions of mafic and ultramafic bodies are related to three major Proterozoic Large Igneous Provinces (LIPs) worldwide: the Franklin LIP (~0.72 Ga), the Mackenzie LIP (~1.27 Ga), and the Circum-Superior LIP (~1.88 Ga), which exhibit quite variable metal endowments [3]. Thus far, no deposits have been found in the Franklin LIP, only small Ni-Cu-(PGE) and Cr deposits have been identified in the Mackenzie LIP (e.g., Muskox), whereas world-class mining districts occur within the Circum-Superior LIP (e.g., Raglan, Thompson). Because of the size of the Muskox intrusion (over 120 km long), its worldwide recognition, and the historical work done by the GSC in 1960s [4], this prospective unit will receive a special attention within the framework of this compilation. In the Canadian context, magmatic Ni-Cu-Co-(PGE) deposits with variable abundances of sulfides/alloys and metal ratios have formed throughout geological time (Mesoarchean to Cenozoic), from a wide range of parental magmas (komatiitic to quartz dioritic), in a wide range of tectonic settings (extensional to convergent), so none of these attributes are particularly critical exploration variables. Almost all the historic and current Canadian production comes from large mining districts (e.g., Sudbury, Thompson, Voisey’s Bay, Raglan, and Lynn Lake), all of which still have significant large brownfield potential. However, several other regions have excellent greenfields potential, as evidenced by the presence of many historic and recently discovered Ni-Cu-Co-(PGE) deposits. The preliminary results of the GSC compilation indicate, for example, that more than 50 Ni-Cu-Co-(PGE) deposits occur north of ~54° N latitude, including Triassic flood basalt-related subvolcanic intrusions (e.g., Wellgreen, Canalask) and Jurassic plutonic zoned/composite complexes (e.g., Turnagain) within the Cordillera Province; Neoarchean norite- and gabbro-related intrusions (e.g., Nickel King, Ferguson Lake), Paleoproterozoic komatiite-related (e.g., Rankin Inlet) and gabbro-related (e.g., Lynn Lake) intrusions within the Western Churchill; Paleoproterozoic volcanic (e.g., Raglan) and subvolcanic (e.g., Expo Ungava) komatiitic basalt-related lava channels and channelized dikes within the Central Churchill; Paleoproterozoic volcanic-subvolcanic picritic to komatiitic basalt-related intrusions, differentiated ultramafic to mafic sills, and glomeroporphyritic gabbroic sills within the Eastern Churchill; and Mesoproterozoic plutonic troctolitic (e.g., Voisey’s Bay) intrusions within the Nain Province. The degree of preservation of these deposits ranges from essentially unmetamorphosed and undeformed (e.g., Voisey’s Bay) through low-grade metamorphosed with very localized deformation (e.g., Raglan) to medium- and high-grade metamorphosed with widespread deformation (e.g., Ferguson Lake, Thompson). Overall, the ubiquitous distribution of ultramafic and mafic units highlighted by this compilation indicates that there is not only significant potential for the discovery of additional Ni-Cu-Co-(PGE) mineralization in traditional and established mining camps, but also has tremendous potential for the discovery of new Ni-Cu-Co-(PGE) and Cr-PGE deposits in under-explored regions of Canada. References: [1] Smith WD and Maier WD (2021) Earth Sci Rev 220:1-36 [2] Wheeler JO et al (1996) GSC A Map Series 1860A [3] Ernst RE (2014) Larg Ign Prov; Camb Univ Press: 667 [4] Scoates JS and Scoates RFJ (2024) Lithos 474-475: 1-40 �37 Copper and komatiitic magmatism – source of copper in the Sakatti Cu-NiPGE deposit in northern Finland Höytiä, H.1,2, Peltonen P.1, Halkoaho, T.3, Makkonen, H.V.3,5 and Virtanen, V.J.1,5 1 Department of Geosciences and Geography, P.O. Box 64, FI-00014 University of Helsinki, Finland Anglo American plc (AA Sakatti Mining Oy), Tuohiaavantie 2, FI-99600 Sodankylä, Finland 3 Geological Survey of Finland, P.O. Box 1237, FI-70211 Kuopio, Finland 4 Geological Survey of Finland, Vuorimiehentie 2K, FI-02150 Espoo 5 Suomen Malmitutkimus Oy, Kuopio, Finland 6 Institut des Sciences de la Terre d’Orléans, UMR 7327, CNRS/Université d’Orléans/BRGM, Orléans, France 2 ___________________________________________________________________________ Copper is an important commodity in most of the magmatic Ni-Cu-platinum group element (PGE) sulfide deposits. Several nickel camps and deposits, e.g. Noril’sk (Russia), Sudbury and Raglan (Canada), and Jinchuan (China), host individual mineralizations and mineralization types that are more enriched in Cu compared to Ni. Host rocks of these Cu-enriched Ni-deposits vary from mafic (derived from tholeiitic parental magmas) to ultramafic (derived from ferropicritic or komatiitic basaltic parental magmas) and they bear evidence of variable, but generally high silicate/sulfide mass ratios (R factor) from c. 100 to > 1000 during their formation [1.Important Cu-enrichment mechanisms also include mantle source with low Ni/Cu, fractional crystallization of segregated sulfide phase, assimilation of Cu from external source, and post-magmatic modification of sulfides by fluids. Sakatti is a Cu-Ni-PGE deposit in the Paleoproterozoic c. 2.5-1.8 Ga Central Greenstone Belt (CLGB) in northern Finland with total reported resources of 44.4 Mt @ 1.9% Cu, 0.96 % Ni, 0.05% Co, 0.64 g/t Pt, 0.49 g/t Pd and 0.33 g/t Au [2]. The deposit was discovered by Anglo American Plc in 2009 and can be sub-divided into six distinct ore types: 1) Ni-rich massive ore, 2) Cu-rich massive ore, 3) Ni-Cu interstitial ore in gabbronorites, 4) Cu-rich disseminated ore, 5) Cu-PGE-rich stockwork vein ore, and 6) Py-rich massive ore. The mineral assemblage consists of chalcopyrite, pyrrhotite, pentlandite, pyrite and Ni-Pt-Pd tellurides of the melonite-merenskyite-moncheite series. The sulfide phase shows evident fractionation from Ni-rich monosulfide solid solution (mss) to Cu-rich intermediate sulfide solid solution (iss) [3, 4]. Bulk of the sulfides in Sakatti show narrow range of δ34S, between +2 and +4 ‰, indicating non-magmatic source of sulfur for much of the deposit. The Sakatti sulfide deposit is underlain by argillaceous sediments with thick anhydrite-gypsum intervals, some of which, are in direct contact with the cumulates and show prominent magma-country interaction. The sulfide ores in Sakatti are hosted by chonolith-style magma conduit composed of ortho-, mesoand adcumulates, pegmatoidal gabbronorites and fine-grained komatiitic rocks. These are derived from a komatiitic parental magma in equilibrium with Fo92-93 olivine (c. 19–21 wt. % MgO). Olivine in the Sakatti deposit contains relatively high Ni contents (2500–3500 ppm), which can be due orthopyroxene fractionation in the lower crust en route to surface [5]. Typical mineral assemblage contains olivine + chromite ± orthopyroxene ± clinopyroxene ± plagioclase. All host rocks show one to two orders of magnitude enrichment in LREE compared to that of chondrite. The age of the ultramafic magmatism is constrained to c. 2054 Ma [6], which corresponds to a global Ni-Cu-PGE mineralizing event with coeval ages in e.g. Bushveld (South Africa), Mirabela (Brazil) and Elanskii (Ukraine) complexes, related to the final break-up of the supercontinent Kenorland. With R factor modelling it is not possible to achieve the observed low Ni/Cu ratio at Sakatti. The same is true also with the N factor (zone refining) or with the multistage upgrading modelling. Therefore, four other processes that could account for the anomalously high Cu-content and low Ni/Cu of Sakatti are discussed: 1) Magma generation from Cu-enriched metasomatized mantle source 2) removal of Ni-rich mss at depth, 3) Assimilation of copper from country rocks, and 4) postmagmatic upgrade of the Cu grades. �38 [1] Cu-enriched mantle source is commonly attributed to metasomatized mantle. Uncontaminated CLGB komatiites have MREE-enriched hump-shaped patterns, reflecting limited marks of metasomatized source at the time of their separation [7]. Mantle source alone contributing the copper contents in Sakatti is doubtful, as the degrees of partial melting for parental melts are high (c. 15-25 %) [5, 7]. [2] Brownscombe et al. [3] proposed that the primary mss was segregated at earlier stage and the Cu-rich portion of it was re-assimilated and injected into the current host cumulates by later magmas that did not equilibrate with the sulfides, possibly due to a kinetically controlled process, similar to that proposed for varying metal tenors in the Raglan deposits [8]. However, the most primitive olivine cumulates also host the most primitive mss, indicating that host magma took part to the sulfide segregation to some degree. R factors for Sakatti are generally low (50–100) and the modelled Ni/Cu values are generally much higher than the ones observed, therefore indicating that there must be additional processes contributing to the varying Ni/Cu ratios. However, an alternating option could arise from computational simulations, where Ni/Cu ratios between 1.9 and 0.4 ratios can be produced for sulfides during closed fractional crystallization scenario depending on the initial sulfur content of the parental magma [5]. [3] Magma-sulfate interaction textures, positive δ34S, elevated Fe3+ contents in chromite [9] and similarity in REE-patterns between cumulates and sulfate rocks indicate that Sakatti host rocks have assimilated their sulfate-bearing country rocks during ascent and/or in-situ. However, most of the seemingly unaltered sulfate sediments bear very low Cu contents, and besides, regionally potential assimilants have Cu contents typically below 150 ppm [10, 11]. Yet copper collection during assimilation could be facilitated by oxidized magma, coexisting magmatic fluid(s) [12] and formation of xenomelts [13], which would form as a response to assimilation of carbonatesulfate sediments. [4] Re-Os [14], U-Pb [6], Pb-Pb, and Cu isotope results [15] point towards later remobilization of the Cu-rich portions of the ore. However, no obvious alteration patterns resulting from late hydrothermal fluids are found in the deposit. Age constraint for post-magmatic modification spans from c. 1.9 to 1.8 Ga [6, 14], which include ages of the numerous Au and IOCG (Iron-Oxide -Copper-Gold) deposits within the CLGB [16], suggesting mobility of copper during this period. Massive sulfide ores, however, pose a strong chemical buffer, which means they are not easily extensively affected by fluid activity. The discussed processes are not mutually exclusive and could have contributed to the high Cu budget. The available data indicates that processes 2) and 4) were the dominant controls of Cu. [1] Burrows D and Lesher M (2012) Econ Geol 16:515–552 [2] Anglo American Ore Reserves and Mineral Resources Report (2022) [3] Brownscombe W et al. (2015) Min Dep of Finland:211–252 [4] Fröhlich F et al. (2021) Can Min 59:1485–1510 [5] Virtanen V et al. (in review) [6] Höytiä et al. (in review) [7] Hanski E and Kamenetsky V (2013) Chem Geol 343:25–37 [8] Li Y and Mungall J (2022) Econ Geol 117:1131–1148 [9] Silventoinen S (2020) M.Sc. thesis Uni Helsinki, 95 p. [10] Haverinen J (2020). M.Sc. thesis, Uni Helsinki, 82 p [11] Köykkä J et al. (2019) Precamb Res 331:105364 [12] Iacono-Marziano G et al. (2017) Ore Geol Rev 90:399–413 [13] Lesher C (2017) Ore Geol Rev 90:465–484 [14] Moilanen M et al. (2021) Ore Geol Rev 132:104044 [15] Höytiä H et al. (2023) 14th Int Pt Symposium Abs Vol:235–236 [16] Niiranen T (2005) PhD thesis synopsis D6, Uni Helsinki, 27 p. �39 The Koperberg Suite of the Okiep Copper District - an overlooked target for magmatic nickel sulphides in a convergent margin system Hunt J.P.1, van Schalkwyk L.1, Smart E.1 and Benhura C.1 1 Orion Minerals, 16 North Road, Dunkeld West, Randburg 2196, South Africa, johnpaul.hunt@orionminerals.com.au ___________________________________________________________________________ The Okiep Copper District (OCD) is the oldest formal mining district in South Africa dating back to 1852, having produced 2.2 Mt of Cu from 32 mines and 70% of this total having been mined from just 5 mines. It is located in the Bushmanland Subprovince of the Namaqua Sector of the Namaqua-Natal Metamorphic Province (NNMP) which is younger than but broadly contemporaneous with the Grenville-Kibaran orogenies associated with the amalgamation of the Rodinia supercontinent (Figure 1). Steep northwards subduction occurred to the south of the NNMP. Roll-back of the subducting slab causing dextral trans-tensional extension in the continental back-arc environment, where the Bushmanland Subprovince is presently located. Metamorphic grade, in general, increases from amphibolite facies in the north to upper granulite facies in the south. Namaquan orogenesis occurred in two episiodes: the Okiepian Episode (1180-1210 Ma) involving crustal shortening and the intrusion of large volumes of granitic sheets (now granite gneiss); and the Klondikean Episode (10201040 Ma) involving mafic underplating, ultra-high-temperature metamorphism, granitic sheets, dextral transtension, constrictional fabrics, and crustal thinning [1] and importantly the intrusion of the Koperberg Suite. The Koperberg Suite is by volume predominantly anorthositic with associated jotunite, biotite diorite, leuconorite, norite, hypersthenite, and glimmerite intruded as discrete magmatic events. It intruded as ENE and ESE oriented, irregular and discontinuous dykes, sills and plugs into an overwhelmingly granulite-facies granite-gneiss terrane, which were commonly focused within kinked anticlines Figure 2. Distribution of ore deposits and mining districts in the various Subprovinces and Terranes of the Namaqua Sector of the Namaqua-Natal Metamorphic Province. The Okiep Copper District is located in the �40 northern portion of the Bushmanland Supbprovince, with the Kliprand Nickel District located approximately 150km to the southeast [2]. known as ‘steep structures’. The quartzites and metapelites of the Khurisberg Subgroup have historically been a potentially lithological control with the majority of known mineralised intrusions occurring stratigraphically above this horizon. It has long been established that the sequence of intrusion is from felsic to mafic: anorthosite was the earliest intruded magma, followed by ferrodiorites, then norites, and ultimately orthopyroxenites (hypersthenites) and magnetitites. The majority of mineralisation is associated with the increasingly more mafic lithotypes, the majority being hosted by magnetitite, orthopyroxenite and norite, then ferrodiorite, and only a small proportion of mineralisation being hosted by anorthosite. The Koperberg Suite ores are grouped based on the main sulphide assemblage [3], namely the: 1. Carolusberg-type ore: the most abundant type characterised by a bn-mgt (± cp) assemblage 2. Narrap-type ore: characterised by a typical iss assemblage (cp + po ± pn), 3. Hoit-type ore: an intermediate assemblage characterised by a bn-cp It had long been held that the overwhelmingly abundant bn-mgt assemblage within the OCD was a consequence of post-magmatic oxidation of a primary sulphide assemblage as represented by the Narrap type, however, recent trace element and isotopic studies suggest this not to be the case [3]. Oxidation of the magma liquid and the corresponding immiscible sulphide liquid occurs with progressive crystallisation and fractionation of Fe2+-rich phases and post-magmatic oxidation of the sulphide is not supported by textural and geochemical observations. The Hondekloof Ni-Cu deposit is located approximately 150km SE of the OCD in the Kliprand Nickel District (KND). This gabbronorite-hosted basal massive sulphide mineralisation is part of a larger suite of intrusives including anorthosite, norite, quartz norite, diorite, glimmerite, and an earlier extensively developed charnoenderbite. The mineralisation assemblage of magnetic pyrrhotite with minor exsolved cobaltian pentlandite, chalcopyrite as well as pyrite is typical of orthomagmatic Ni-Cu-Co bearing sulphide bodies derived from a typical mss assemblage [4]. On the basis of petrological and petrochemical similarities, the gabbronorite host is correlated with a pre-Koperberg Suite “two pyroxene granulite” of the OCD, effectively having an identical gabbronoritic mineralogy and chemistry. This mafic unit was historically regarded as being unmineralized and therefore avoided. A two-stage model was proposed [4] which is simplified as follows: Stage 1. an early nickeliferous mss sulphide liquid was extracted from the magma chamber associated with preto syn-tectonic gabbronorites. Stage 2. renewed tectonism and compression of the magma chamber resulted in the extraction of first an anorthositic suite, followed by increasingly more mafic assemblages and ultimately the most hypermelanic phases and the low-S, high-mgt, cupriferous residual iss sulphide liquid from the base of the magma chamber. The exploration implications for the OCD is that the historical exploration and exploitation has concentrated on bn-mgt rich ores, traced on surface and followed down to depth, or efficiently mapped by magnetic geophysical surveys. The distribution of “two-pyroxene granulites” has been mapped but entirely disregarded until now. A number of known deposits have elevated Ni concentrations, such as Okiep East and Narap Mine, and it is noted that these are in proximity to increased occurrences of two-pyroxene granulites. Modern transient electromagnetic (TEM) surveys have only recently been completed and map a number of discrete anomalies both in proximity to Koperberg Suite intrusives and distinct from them. At two localities, Ezelsfontein East and Nous, both located within the OCD, drilling confirmed the presence of massive and disseminated Ni-Cu sulphide, establishing proof of concept and opening up the OCD to new aspects in its exploration potential. References: [1] Dewey J et al. (2006) Precam Res 150(3-4), 173–182 [2] Rozendaal A et al. (2017) SAJG 120(1), 153–186 [3] Marima E (2022) Unpubl. MSc Univ. Rhodes 120p [4] Hamman J N et al. (1996) SAJG 99(2), 153-16 �41 A multi-methodological approach: Combining textural observations and geochronology to study the J-M Reef Package and its Hanging Wall, Stillwater Complex, Montana Jenkins, M.C.1*, Corson, S.2, Geraghty2, E., Kamo S.L. 3, Lowers, H.4, and Mungall, J.E.5 1 U.S. Geological Survey, Geology, Minerals, Energy, and Geophysics Science Center, Spokane, Washington, USA *mcjenkins@usgs.gov 2 Sibanye-Stillwater, Columbus, Montana, USA 3 Jack Satterly Geochronology Laboratory, University of Toronto, Toronto, Ontario, Canada 4 U.S. Geological Survey, Geology, Geophysics, and Geochemistry Science Center, Denver, Colorado, USA 5 Department of Earth Sciences, Carleton University, Ottawa, Ontario, Canada ___________________________________________________________________________ The J-M Reef is a world-class platinum-group element (PGE) deposit hosted in the 2.709 Ga Stillwater Complex in Montana, USA [1, 2]. The J-M Reef is the accumulation of PGE-enriched sulfide minerals located in the Anorthosite subzone I (ASZI) of the Troctolite-Anorthosite zone I in the Lower Banded series of the complex (Fig. 1A). Anorthosite subzone I is comprised of anorthosites, troctolites, peridotites, and norite adcumulates and heteradcumulates. The cumulates that host economic J-M Reef sulfide mineralization are generally coarse-grained to pegmatoidal in texture and may be any of the rock types found in ASZI. These coarse-grained rocks are called the Reef Package (Fig. 1B). The top of the Reef Package is marked by a textural discontinuity between the coarse-grained cumulates and relatively fine-grained cumulates in the hanging wall. The surface that marks the top of the Reef Package is informally called the hanging wall contact and economic PGE mineralization is not found above this contact [3]. The sulfide mineralization that makes up the J-M Reef may not always be present; therefore, tracing the reef location during mine development can be challenging [1]. The hanging wall contact can always be identified in drill core and underground workings even where the J-M Reef is not present making this contact an important marker horizon during mining. �42 Figure 3. 1A) Stratigraphic section showing the series and zone nomenclature for the Stillwater Complex [4]. 1B) Stratigraphic section showing the subzones of Troctolite-Anorthosite zone I [3, 5, 6]. The general location of the hanging wall contact (HWC) is shown as a dashed line. 1C) Preliminary U-Pb zircon ages (yellow) and published zircon ages of the JM Reef from Wall et al. (2018; blue) [2]. Zircon mean ages are shown as points and error bars correspond to 2σ. Electron backscattered diffraction was used to investigate the microtextural change at the hanging wall contact from four intersections. In general, the results show that rocks in the hanging wall are characterized by finer crystal sizes and a well-developed B-type fabric typical of cumulates from layered mafic intrusions (Fig. 2) [7]. In contrast, the rocks that host the J-M Reef are found to be coarse-grained and do not have a strong rock fabric indicating that they likely crystallized under conditions where crystal settling, compaction, or magmatic flow did not impact the orientations of the crystals. Instead, the Reef Package may have crystallized in situ where crystals grew to impingement without a preferred orientation. These findings do not resolve the origin of the hanging wall contact as it could plausibly represent either a resumption of normal layered mafic intrusion petrogenetic processes like crystal settling and/or compaction or it could represent a pre-existing cumulate layer that acted as an aquitard to the magma that formed the Reef Package. Figure 4. Bivariate plots showing rock fabrics from the hanging wall (HW) and Reef Package (RP) based on the foliation number (F#) vs the lineation number (L#) defined as the ratios of the maximum eigen value divided by the intermediate eigen value for the crystallographic axes. The F# is equal to e1/e2 for the (010) plane and the L# is equal to the e1/e2 for the [100] direction. Stillwater cumulates from the Picket Pin (PP) area are shown as solid black triangles. The shaded fields show where data from other layered mafic intrusions (LMIs), fast spreading centers (FSC), and slow spreading centers (SSC) plot on the diagram [3, 7]. To test the hypothesis that the hanging wall contact represented a cumulate layer that existed prior to the emplacement of the magma that formed the Reef Package, high-precision chemical abrasionisotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) zircon U-Pb dating was used to determine the age of rocks below, above, and within the Reef Package (Fig. 1B). The mean ages of zircons below the Reef Package are approximately the same as those in the Reef Package. In contrast, mean ages from zircons in the hanging wall are older than the Reef Package—including one substantially older sample (SW48904-150-153) from Norite subzone (Fig. 1B). These results support the hypothesis that the hanging wall contact represents the base of a pre-existing cumulate layer that caused the magma that formed the J-M Reef Package to pool at the level of the Reef Package. The zircon ages are consistent with out-of-sequence CA-ID-TIMs zircon ages that have been reported from Stillwater [2] and the Bushveld [8, 9] complexes. The age results do not place firm constraints on the origin of the J-M Reef deposit as either the hydromagmatic model [10] or orthomagmatic �43 model [11] could plausibly form the reef with or without the presence of an overlying igneous aquitard layer. References: [1] Jenkins et al. (2020) Econ Geol 115: 1799-1826 [2] Wall et al. (2018) J Petrol 59: 153-190 [3] Jenkins et al. (2022) J Petrol 63: egac053 [4] Todd et al. (1982) Econ Geol 77: 1454-1480 [5] Turner et al. (1985) Mont Bur Min Geol 92: 210-230 [6] Corson et al. (2002) 9th Plat Symp 101-102 [7] Cheadle and Gee (2017) Elem 13: 409-414 [8] Mungall et al. (2016) N Comm 7: 13385 [9] Scoates et al. (2021) J Petrol 62: egaa107 [10] Boudreau (1999) J Petrol 40: 755-772 [11] Jenkins et al. (2021) Precambr Res 367: 106457 Note: Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. �44 Nickel-copper-platinum group elements potential of mafic and ultramafic intrusions in northwestern Ontario Jonsson, J.1, Malegus, P.1, Churchley, S.1, Price, R.1 1 Resident Geologist Program, Ontario Geological Survey, Ministry of Mines, Suite B002, 435 James Street South, Thunder Bay, ON P7E 6S7 Canada ___________________________________________________________________________ Globally, magmatic sulphide deposits host significant resources of nickel, copper, cobalt and platinum group elements (PGE). These deposits occur as concentrations of sulphide minerals hosted within mafic to ultramafic intrusive rocks and are widespread across Ontario, occurring in every Precambrian geologic terrane. Ontario is home to 10 operating mines in magmatic sulphide deposits: 9 within the Paleoproterozoic Sudbury Igneous Complex and one within the Neoarchean Lac des Iles Complex. In 1999, Operation Treasure Hunt was initiated by the Ontario Government to stimulate mineral exploration by acquiring new airborne geophysical data, surficial and bedrock geochemical data, and development of new methods. In 2003, following completion of the Operation Treasure Hunt project, the Ontario Geological Survey published a report that assessed 109 mafic to ultramafic intrusions across Ontario [2]. The purpose of this part of Operation Treasure Hunt was to characterize and publish data for intrusions known to be prospective for PGE-dominated magmatic sulphide mineralization. Many of the intrusions studied during Operation Treasure Hunt were host to significant known mineralization, including current and past-producing mines, and several of these intrusions are the focus of ongoing mineral exploration. Despite the work by Vaillancourt et al. [2], there are hundreds of mafic to ultramafic intrusions in Ontario that have not been systematically assessed for magmatic sulphide mineralization potential. Many of these intrusions have favourable characteristics for potentially containing magmatic sulphide deposits, including geophysical anomalies (e.g., magnetic, conductivity), overburden geochemical anomalies and known sulphide mineralization. In 2023, the Resident Geologist Program of the Ontario Geological Survey initiated a project to systematically characterize geochemistry of a subset of mafic-ultramafic intrusions in northwestern Ontario that largely have not been subject to significant historical evaluation by academic researchers, government surveys, or mineral exploration companies. Evaluating the geochemistry of mafic to ultramafic intrusions can provide insight into the magma history, tectonic setting and potential for economic metal endowment. Factors that may influence metal endowment, that can be determined from the examination of geochemical data, include determining magma source characteristics, the timing of sulphur saturation and the degree of interaction of the magma(s) with their country rocks. Careful evaluation of physical characteristics and whole-rock geochemistry can inform future mineral exploration and/or the development of models for the emplacement of mafic to ultramafic intrusions and any hosted mineralization. Initial sample collection and analytical work took place during 2023. Areas of interest are shown in Figure 1, and include the Red Lake, Onaman–Tashota, and Heaven Lake greenstone belts. In this display, we provide examples of preliminary results and interpretations from areas targeted in the first year of field work, including the Trout Bay intrusion (Red Lake greenstone belt), Westwood intrusion (northeast of the Lumby Lake greenstone belt), and the Big Ghee Lake intrusion (south of the Shebandowan greenstone belt). �45 Figure 1. Simplified bedrock geology map of a portion of northwestern Ontario, showing project target areas: Red Lake greenstone belt (outlined in blue); Heaven Lake greenstone belt (outlined in black); and Onaman–Tashota greenstone belt (outlined in white). Regional geology modified from Ontario Geological Survey [1]. References [1] Ontario Geological Survey 2011. 1:250 000 scale bedrock geology of Ontario; Ontario Geological Survey, Miscellaneous Release—Data 126 – Revision 1. [2] Vaillancourt, C., Sproule, R.A., MacDonald, C.A. and Lesher, C.M. 2003. Investigation of maficultramafic intrusions in Ontario and implications for platinum group element mineralization: Operation Treasure Hunt; Ontario Geological Survey, Open File Report 6102, 335p. �46 Petrogenesis of the mineralized horizons in the Offset and Creek zones, Lac des Iles Complex, N. Ontario Jonsson, J.1, Hollings, P.1, Brzozowski, M.1, Bain, W.1, Djon, L.2 1 2 Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Impala Canada, 69 Yonge Street, Suite 700 Toronto, ON M5E 1K3 Canada ___________________________________________________________________________ The Lac des Iles Complex is a Neoarchean (2.69 Ga; D.W. Davis cited in Stone et al., 2003) polyphase mafic-ultramafic complex located in the Marmion terrane of the Superior Province, 85 km north of Thunder Bay, Ontario, Canada. The intrusive complex can be subdivided into two discrete subcomplexes: the ultramafic-dominated North Lac des Iles Complex and the mafic-dominated South Lac des Iles Complex (SLDIC). The SLDIC has been subdivided into four intrusive series, termed the gabbronorite, breccia, norite, and diorite series (Decharte et al., 2018). To date, economic Pd-rich mineralization has been discovered in both the breccia and norite series, and occurs proximal to the contacts between the breccia and gabbronorite series and between the breccia and norite series. The objectives of this study are to i) evaluate the mechanisms of formation of the mineralized horizons near the contact between the breccia and norite domains in the Offset and Creek zones of the SLDIC, ii) evaluate the role that crustal contamination played in this process, and iii) assess the tectonic setting in which the SLDIC formed. The breccia and norite series are both composed of varitextured, brecciated, and equigranular leucocratic-melanocratic norites and gabbronorites, and their altered equivalents. The breccia series contains a greater proportion of brecciated and varitextured rocks, while the norite series contains a greater proportion of equigranular rocks. All pre-alteration lithologies are essentially plagioclaseorthopyroxene cumulates with varyingly minor quantities of interstitial clinopyroxene, biotite, magnetite, chalcopyrite, pentlandite, and pyrrhotite. Variable degrees of hydrothermal alteration are indicated by the presence of tremolite-actinolite and talc (after pyroxenes), chlorite and sericite (after plagioclase), and pyrite (after pyrrhotite). Although the breccia and norite series are mineralogically similar, the breccia series is generally more leucocratic (i.e., higher plagioclase/pyroxene ratio) than the norite series. Neodymium isotopic evidence indicates that the Offset and Creek Zone magmas were crustally contaminated. ɛNd values of 19 analyzed samples range from +0.38 to -3.47 (median = -2.13), which is consistently more negative than the ɛNd value of +2.24 expected in an uncontaminated mantlederived magma that crystallized at 2.69 Ga. The crustal contaminant that imparted the negative ɛNd values is unlikely to be the tonalitic gneiss that hosts the SLDIC, as the ɛNd value of one reported tonalitic gneiss sample is -1.77 (Brugmann et al., 1997). The lack of correlation between ɛNd and geochemical or spatial variations suggests that variable crustal contamination was not the cause of the geochemical variability observed within the Offset and Creek Zones. Samples from both the breccia and norite series have similar trace-element chemistry, including enriched LILE/LREE patterns, flat HREE patterns, and pronounced negative Nb anomalies. Although these characteristics can be caused by assimilation of crustal material, it is more likely that they are the result of formation of the parental magma in a magmatic arc. Evidence for this interpretation includes low Nb/Yb ratios, high Ba/Th ratios, low Th content, and the lack of correlation between geochemical variability and Nd isotopic variability. Evidence from S isotopes of sulfide minerals and whole-rock geochemistry suggests that the addition of crustal S was not necessary in the formation of the Pd-rich mineralization within the Offset and Creek zones. δ34S values of 54 crystals from 17 samples range from -0.37‰ to +3.28‰ VCDT (median = +1.11‰), with values from 52 of 54 crystals falling in the expected range of mantle-derived sulfur (0 ± 2‰; Seal, 2006). Based on the association of low Cu/Pd ratios with high Pd values, Offset and Creek zone ores formed at high R factors, which were likely high enough to cause the PGE �47 enrichment without incorporation of crustal sulfur. The higher degree of Pd enrichment in the Offset Zone compared to the Creek Zone was likely due to a greater amount of sulfide liquid in the Offset Zone that also underwent higher R factors; the distribution of sulfide liquid and magma flow may have been influenced by primary structural constraints on the geometry of the intrusion. No evidence was found for significant low-temperature remobilization of chalcophile elements, including the PGEs. The compositional variability observed within the breccia and norite domains suggests that both domains formed via multiple pulses of compositionally similar magma. The proximity of mineralization to the interpreted feeder conduits suggests that the distribution of mineralization is largely the result of PGMs/Pd-rich pentlandite crystallizing as the magma transitioned from the feeder structure outwards into the periphery of the intrusive complex. This process may have repeated several times as successive magma pulses infiltrated the partially crystallized intrusive complex, resulting in the redistribution of ores in brecciated zones. References: Brugmann, G.E., Reischmann, T., Naldrett, A.J., and Sutcliffe, S.H., 1997. Roots of an Archean volcanic arc complex: the Lac des Iles area in Ontario, Canada. Precambrian Research, vol. 81, p. 223-239. Decharte, D., Hofton, T., Marrs, G., Olson, S., Peck, D., Perusse, C., Roney, C., Taylor, S., Thibodeau, D., and Young, B., 2018. Feasibility study for Lac des Iles mine incorporating underground mining of the Roby Zone. North American Palladium, NI 43-101 Technical Report, 435p. Seal, R.R., 2006. Sulfur isotope geochemistry of sulfide minerals. Reviews in Mineralogy and Geochemistry, vol. 61, p. 633-677. Stone, D., Lavigne, M.J., Schnieders, B., Scott, J., and Wagner, D., 2003. Regional geology of the Lac des Iles area, in Summary of Field Work and Other Activities 2003. Ontario Geological Survey, Open File Report 6120, p. 15-1 to 15-25. �48 Quantum full tensor magnetic gradiometry to better define conduit type NiCu-PGE targets Kaski, K.1, Smith, J.1, Tschirhart, Victoria1, Heggie, G., Enkin, R.1 1 Natural Resources Canada, Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8 __________________________________________________________________________ Magmatic Ni-Cu-PGE sulfide deposits are frequently associated with small conduit-type intrusions. These deposit types are challenging exploration targets due to their limited size, absence of distinct alteration halo or distant footprint, complex and variable morphology, and unpredictable depositional sites of sulfides [1]. Additionally, mafic rocks often retain significant remanent magnetization, which, if overlooked, can lead to inaccurate modelling and targeting of these deposits. The dwindling number of new Ni discoveries over the last decade highlights the necessity for the development and implementation of novel methods to facilitate improved detection and targeting of these deposit types at the regional to deposit scales. Traditional airborne Total Magnetic Intensity (TMI) data is the most used and cost-effective surveying method for identifying and delineating intrusions which can host nickel deposits. Although there is incredible value in TMI data there are challenges with data interpretation including issues of nonuniqueness, scalar measurements, and the inability of TMI to differentiate remanence from the induced field. The full tensor magnetic gradiometry (FTMG) technique, which measures the full magnetic gradient tensor at each measurement point, overcomes many of these limitations and offers numerous advantages including: (a) superior resolution of near-field sources, (b) enhanced detectability at low-magnetic latitudes, (c) automatic removal of the regional field and diurnal variations, and (d) additional target information from a single flight line. FTMG can therefore provide improved discrimination of magnetic sources and a more complete picture of the subsurface magnetic properties. Commercialized quantum FTMG sensors currently use Superconducting Quantum Interference Device (SQUID) technology and due to their size and strict temperature requirements are most appropriate for large-scale airborne surveys. With SQUID sensors being unsuitable for ground and uncrewed aerial vehicle (UAV) surveys a new generation of compact, rugged diamond-based quantum magnetometers are in development and offer an alternative FTMG technology for ground and UAV surveying. Although quantum FTMG offers significant advantages in sensitivity and the opportunity for improved targeting of ore deposits, its widespread adoption by the mining industry has been hindered, in part, by a lack of capabilities and expertise in the areas of data handling and interpretation. As part of a larger collaborative research project, the Geological Survey of Canda (GSC) with Defense Research and Development Canada, aim to de-risk quantum magnetic gradiometer use across Canada through the field testing and validation of quantum FTMG systems and comparing them with traditional total magnetic field systems and non-quantum FTMG systems. As part of this project, the GSC is undertaking a comprehensive study on the Ni-Cu-PGE bearing Escape and Current Intrusions of the Thunder Bay North Intrusive Complex which present as complicated magnetic signals that are strongly affected by remanent magnetization. Here we present preliminary results from the processing of TMI data (Fig. 1) provided by Clean Air Metals Inc. and compare this with newly acquired SQUID FTMG data. Unconstrained (Fig. 2) and constrained magnetic susceptibility inversions derived from both datasets are presented to examine the 3D geometry and extent of the Ni-Cu-PGE mineralized mafic-ultramafic intrusions. Magnetization vector inversions (MVI) are also presented and offer additional insights into the extent and strength of remanent magnetization developed in association with these intrusions. Physical rock properties �49 of the intrusions are used to further validate the MVI models and gain insights into the processes controlling the localization of remanent magnetization. This study marks the first instance of generating publicly accessible quantum FTMG data covering critical mineral deposits in Canada. Ultimately, the aim is to enhance exploration capabilities by validating tools applicable to critical metal deposits, whose intricate geophysical characteristics pose challenges for conventional geophysical techniques. Figure 5. Residual magnetic intensity of the Escape and Current Intrusions of the Thunder Bay North Intrusive Complex. Figure 6. Unconstrained inversion results representing highest modelled magnetic susceptibility contrasts in the Escape and Current Intrusions of the Thunder Bay North Intrusive Complex. References: [1] Barnes, S.J., 2023. Lithogeochemistry in exploration for intrusion-hosted magmatic Ni–Cu–Co deposits. Geochemistry: Exploration, Environment, Analysis, 23(1), pp.geochem2022-025. �50 Exploration-Based Classification Scheme for Magmatic Ni-Cu-(PGE) Systems Lesher C.M.1 and Houlé M.G.2,1 1 Mineral Exploration Research Centre, Harquail School of Earth Sciences, Goodman School of Mines, Laurentian University, Sudbury, ON P3E 2C6, Canada, mlesher@laurentian.ca 2 Geological Survey of Canada, Lands and Minerals Sector, Natural Resources Canada, 490 Couronne Street, Québec City, QC G1K 9A9, Canada ___________________________________________________________________________ Magmatic Ni-Cu-Co-(PGE) deposits have typically been classified on the basis of age, magma type, and tectonic setting [e.g., 1] or cumulus mineralogy [2], but they formed throughout geological time (Mesoarchean to Cenozoic) from a wide range of parental magmas (komatiitic to quartz dioritic) with different cumulus mineralogy in a wide range of tectonic settings (extensional to convergent), so none of these attributes are particularly useful exploration variables. A more useful classification is based on the nature of the host units: 1) impact melt sheets, 2) differentiated layered maficultramafic intrusions, 3) channelized mafic-ultramafic lavas/sills/dikes, 4) differentiated/zoned maficultramafic pipes/plugs/stocks, and 5) orogenic peridotites, each of which is fundamentally different: Group Group 1 
 Impact melt sheets Group 2 
 Differentiated layered maficultramafic intrusions Group 3 
 Channelized mafic-ultramafic flows/sills/dikes Subgroup A Exogenetic (external S ± metals) B Endogenetic (internal metals ± S) A Layered differentiated intrusions B Composite differentiated intrusions C Weakly layered differentiated intrusions A Flows B Sills Setting Impact structure Primarily large igneous province Primarily large igneous province Group 5 Orogenic peridotites Convergent B Zoned noncomposite C Unzoned composite D Unzoned non-composite A Ophiolite complexes B Peridotite massifs Bushveld SA, Great Dyke ZI, Muskox NU, Stillwater MT Duluth MN, Montcalm ON Americano do Brasil BR, Bird River MB, Kotalahti FI D Chonoliths A Zoned composite Sudbury ON Morokweng SA C Dikes Group 4 
 Differentiated/zone d mafic-ultramafic pipes/plugs/stocks Examples Oceanic crust/ mantle Alexo ON, Kambalda WA, Perseverance WA, Raglan QC Dumont QC, Jinchuan CH, Mt Keith AU, Namew Lake SK, Norilsk RU, Pechenga RU, Thompson MB Eagle MI, Eagle’s Nest ON, ExpoMéquillon QC, Hongquiling CH, Huangshan CH, Limae CH, Voisey’s Bay NL, Qingkuangshan CH Kalatongke CH, Limoeiro BR, Mirabella BR, Nebo-Babel WA, Nkomati (Uitkomst) SA, Savannah WA, Tamarack MN Duke Island AK, Giant Mascot BC, Mordor AU, Xiarihamu CH Jingbulake CH, Lynn Lake “EL” MB, Gordon Lake ON, Aguablanca SP, Lynn Lake “A” MB, Turnagain BC Lynn Lake “FLGC” MB, HituraVammala FI Acoje PH, Baptiste (Decar) BC, Potosí CU, Oman, Shetland UK, Troodos CY Ivrea-Verbano IT Group 1 impact melt sheets thus far include only one example with economic Ni-Cu-PGE mineralization, the 1850 Ma, 260 km-diameter Sudbury (ON) structure [see e.g., 3]. The 146 Ma, 80 km-diameter Morokweng (SA) structure contains subeconomic Fe-Ni-Co sulfide nodules and veins that appear to be derived in part from the impactor [e.g., 4]. No other impact structures with Ni-Cu- �51 PGE mineralization have been identified [e.g., 5], most likely because they were too small to generate enough impact melt and/or lacked the abundant economic (e.g., Shakespeare) to subeconomic (e.g., Nipissing and East Bull Lake Intrusive Suites) Cu-Ni-PGE mineralization in the target rocks at Sudbury. Group 2 differentiated layered intrusions commonly host sub- to uneconomic reef-type PGE-(Cu)(Ni) mineralization (e.g., Centre Hill ON, Romeo II QC), but sometimes contain economic reef-type PGE-(Cu)-(Ni) mineralization (e.g., Bushveld Merensky and UG-2 reefs, Stillwater J-M reef, Great Dyke MSZ) and where they do contain Ni-Cu-(PGE) mineralization it is normally low-grade (e.g., Duluth Complex, Muskox). Because they are A) periodically replenished and well-differentiated magma chambers (e.g., Bushveld), B) composite differentiated intrusions (e.g., Duluth), or C), weakly layered differentiated intrusions they are only rarely/locally dynamic enough to generate high-grade Ni-Cu(PGE) mineralization. Group 3 channelized mafic-ultramafic flows/sills/dikes include some of the world’s largest, highestgrade Ni-Cu-(PGE) deposits/camps (e.g., Raglan, Thompson, Kambalda, Jinchuan, Norilsk-Talnakh) and many small high-grade deposits (e.g., Eagle, Tamarack, Eagle’s Nest). They are typically enriched in olivine or Opx, poorly to weakly differentiated, and interpreted to have formed at high magma fluxes, enhancing thermomechanical erosion of S-bearing country rocks and upgrading of metal contents in sulfide xenomelts. In low-grade deposits, Ni-Co-IPGE in olivine can be redistributed into sulfides during serpentinization (e.g., Dumont, Mt Keith). Group 4 differentiated/zoned mafic-ultramafic pipes/plugs/stocks have typically been subdivided based on their cumulus mineralogy into: Opx-poor (e.g., Uralian-Alaskan type), Opx-rich (e.g., Giant Mascot-type), Gabbroic, and Noritic [e.g., 2], but those characteristics also apply to many deposits in Group 3. Most are zoned and/or multiphase, representing relatively low magma fluxes. They can contain economic mineralization (e.g., Aguablanca, Giant Mascot, Lynn Lake, Xiarihamu), but typically have low tonnages, grades, and tenors. Group 5 ophiolites and peridotite massifs (AKA orogenic peridotites) often contain subeconomic to economic abundances of Cr ± PGE mineralization, and typically only contain currently economic abundances of Ni after being lateritized [6]. However, the sparse amounts of Ni-Cu-(PGE) may be “upgraded” by liberation of Ni-Co-IPGE during serpentinization of olivine under fO2 conditions that favour stabilization of Ni sulfides and/or Ni ± Pt ± Ir-Os alloys (e.g., Decar). Each group exhibits variations in form, degree of olivine/Opx accumulation, and degree of differentiation, sometimes hampering classification into Groups 2, 3, and 4. They also exhibit variations in original (and current) orientations, compositions, and degrees of zoning/differentiation/ layering/brecciation. They also formed from a wide range of magma types, some derived from depleted peridotitic mantle (undepleted in PGE relative to Ni-Cu-Co) and some derived from fertilized pyroxenitic mantle (depleted in PGE relative to Ni-Cu-Co). The single most important element to generating high-grade and high-tonnage deposits appears to be high magma flux, but lower-grade and lower tonnage deposits can form at lower magma fluxes. References: [1] Naldrett AJ (2004) Springer: 728 pp. [2] Nixon GT et al. (2015) Geol Surv Canada OF7856: 17-34 [3] Lightfoot PC (2016) Elsevier: 680 pp. [4] Hart RJ et al. (2002) EPSL 198: 49-62 [5] James S et al. (2022) Energy Geosci 3: 136-146 [6] Golightly JP (2010) SEG Spec Publ 15: 451–485 �52 Thermodynamic constraints on the generation of cubanite-rich magmatic sulfides Maghdour-Mashhour, R.1, Mungall, J.1 1 Department of Earth Sciences, Carleton University, 2115 Herzberg Laboratories, Ottawa, Ontario K1S 5B6, Canada ___________________________________________________________________________ Nickel (Ni) and Copper (Cu) are paramount for advancing sustainability and enhancing human wellbeing, serving as indispensable elements in modern technology and pivotal components in green energy solutions. We launched a study of Ni-Cu ore deposits from the Keweenawan Large Igneous Province (LIP) to unravel their intricate geochemical and thermodynamic conditions, crucial for understanding their genesis and optimizing ore extraction methods, thereby bolstering industrial efficiency and sustainability. The Keweenawan LIP, emplaced within the ca. 1.1 Ga Mid-Continent Rift (MCR), comprises by maficultramafic intrusions and flood basalts extending across Lake Superior in Ontario and Minnesota [1]. The MCR preserves a broad array of magmatic sulfide deposits in a relatively unmetamorphosed state, offering a unique opportunity for detailed study and understanding of primary processes that are commonly obscured by later metamorphism. MCR deposits exhibit variable concentrations of cubanite (CuFe2S3) alongside the more prevalent chalcopyrite (CuFeS2). Cubanite content ranges widely from less than 1% to as high as 80% of the Cu sulfide mode [2], posing a major metallurgical challenge. The presence of cubanite prolongs flotation circuit processing times, necessitating a delicate balance between efficiency and optimization to separate Cu sulfides from tails effectively [3]. The occurrence of cubanite and chalcopyrite cannot be inferred from Cu-Ni-S assay and must be observed petrographically. Our primary aim is an innovative approach to mitigate cubanite prevalence within the circuit by precisely identifying cubanite-rich geometallurgical zones exclusively through assay databases, thereby circumventing the need for costly petrography and SEM analyses. The first essential step is to comprehend the thermodynamic controls imposed by intensive parameters, including oxygen and sulfur fugacity (fO2 and fS2), which contribute to the stability of cubanite in a system where silicate melt, and sulfide melt are in equilibrium. Subsequently, we explore the required parental magma chemical composition and intensive variables necessary at elevated temperatures to ensure the stability of cubanite as the system cools down to lower temperatures. To address these questions, we utilized FactSage 8.3 to model the evolution of a cubanite-favorable anhydrous magmatic closed system initially comprising ~15 wt% sulfide liquid and ~85% silicate melt at the liquidus temperature. Re-equilibration of the model system to lower temperatures allowed us to determine the conditions required at the liquidus that would result in the development of cubanite-rich sulfide assemblages upon cooling to near ambient temperatures. Our investigation yielded novel findings that cubanite stability is achieved at log fS2 of -14, log fO2 of -37, and a temperature of 270 degrees Celsius. These conditions correspond to a low-temperature ambient state, akin to a parental magma composition with log fS2 of -0.7 and log fO2 of -7.2 at the liquidus temperature indicating a condition slightly more reduced than the Quartz-Fayalite-Magnetite (QFM) buffer (ΔQFM -1). �53 We have also uncovered a diverse array of model cubanite-bearing low-temperature assemblages, including various combinations of pentlandite, pyrrhotite, chalcopyrite, talnakhite, and mooihoekite. Whereas the abundances of pentlandite, pyrrhotite, and chalcopyrite display a wide spectrum of sensitivity to fO2 and fS2, our findings reveal five distinct assemblages—incorporating chalcopyrite, talnakhite, and mooihoekite—that showcase high sensitivity to even two decimal points of shifts in fO2 and fS2. As fO2 decreases and fS2 increases, these assemblages undergo transitioning from chalcopyrite to talnakhite and ultimately to mooihoekite. It is noteworthy that cubanite exhibits stability even in hydrous systems, albeit under extremely reduced conditions. For some cubanite-bearing assemblages, such as those with mooihoekite, cubanite stability necessitates an exceptionally reduced environment, with ΔQFM reaching as low as -3.3 and log fS2 dropping to -2.5. As our study progresses, our next phase entails conducting quantitative and qualitative mineral classification through petrography and SEM X-ray mapping of representative samples sourced from Ni-Cu deposits spanning distinct intrusions across the Mid-Continent Rift (MCR). Our aim is to compare sulfide paragenesis within cubanite-rich domains across the MCR with thermodynamically generated model compositions and assemblages provided by FactSage. Additionally, we will incorporate geochemical insights to establish a link between bulk rock assay data and the presence of cubanite in the Ni-Cu deposits. This approach will enable us to delineate geometallurgical domains potentially requiring modified beneficiation circuits. References: [1] Taranovic et al. (2015) Can Min 24(2): 347 [2] Ripley and Alawi (1986) Lithos, 212: 16-31 [3] Muzinda et al. (2018) Min Eng, 125: 34-41 �54 Constraining the Sunday Lake mineralization: A Ni-Cu-PGE deposit Mexia, K.1, Hollings, P. 1 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, On P7B 1J4, Canada kmexiad@lakeheadu.ca ___________________________________________________________________________ The Sunday Lake Intrusion (SLI) is located 25 km north of Thunder Bay, Ontario, and hosts Ni-Cu-PGE mineralization. It has been dated at 1109.0±1.3 [1], and as such is related to the plateau stage of the ~1115 to 1106 Ma Midcontinent Rift System (MRS; [2], [3]). The SLI is a tabular shaped intrusion emplaced in Archean rocks of the Quetico Basin that becomes more tube-like to the northwest where it is hosted by Archean granitoids. It is emplaced along the Crock Fault, which is interpreted to be a splay of the main Quetico Fault [3]. It varies from 350 meters to 1000 meters in thickness. The intrusion consists of mafic-ultramafic layers divided into three series: the Upper Gabbro Series, the Lower Gabbro Series, and the Ultramafic Series (Fig 1.) [3]. Reef-style sulphide mineralization (2-10 vol.%) is present in the lower zones of the intrusion, consisting of disseminated to blebby chalcopyrite-pyrrhotite-pyrite-cubanite in an olivine melagabbro (Fig. 2). The Ultramafic series mineralization shows a laterally extensive 20 meters thicks layer with enrichment in Cu-Pt, Pd and Au at levels of 3-10 g/t Pt+Pd+Au [3]. The main objective of this project is to characterize the paragenetic sequence of the Sunday Lake Intrusion and to study the effects of crustal contamination on mineralization. This project utilizes two representative drill holes from which a total of 71 samples were collected. A total of thirty polished thin sections were generated for petrographic studies. Rocks were classified based on relative proportions of olivine, clinopyroxene, and plagioclase with modal rock names such as melagabbro, olivine melagabbro, and wehrlites. Downhole diagrams of trace and major elements vary within the layered intrusion, but both plume-like compositions (Fig. 3A), and evidence for contamination by host rocks (Fig. 3B). Variation in composition suggest other geological processes such as episodes of melt re-injection, contamination, assimilation, and fractional crystallization. These processes likely lead to the generation of sulphides and further precipitation. Sixteen samples have been sent for Sm-Nd and Rb-Sr isotope studies to assess the paragenetic history of the Sunday Lake Intrusion mineralization. cm Figure 2. Photograph of sample SL23KM41 showing an olivine melagabbro with disseminated and blebby sulphides. �55 A B Figure 3. Primitive mantle normalized REE spider diagram of two samples. A: Sample showing a plume-like trend. B: Sample suggesting an interaction with the host rock. Normalising values from [5]. References: [1] Bleeker, W., et al. "The Midcontinent Rift and its mineral systems: Overview and temporal constraints of Ni-Cu-PGE mineralized intrusions." Targeted Geoscience Initiative 5 (2020): 7-35. [2] Heaman, L. M., Easton, R. M., Hart, T. R., MacDonald, C. A., Hollings, P., & Smyk, M. (2007). Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario. Canadian Journal of Earth Sciences, 44(8), 1055-1086. [3] Flank, S. (2017). The Petrography, Geochemistry and Stratigraphy of the Sunday Lake Intrusion, Jacques Township, Ontario. School of graduate studies. [4] Woodruff, L. G., Schulz, K. J., Nicholson, S. W., & Dicken, C. L. (2020). Mineral deposits of the Mesoproterozoic Midcontinent Rift system in the Lake Superior region–a space and time classification. Ore Geology Reviews, 126, 103716. [5] Sun, S. S., & McDonough, W. F. (1989). Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. Geological Society, London, Special Publications, 42(1), 313-345. [6] Miller, J.D. (2020). Report on the Petrography, Geochemistry, and Lithostratigraphy of DDH SL10026 from the Southern Sunday Lake Intrusion. JDM GeoConsulting. �56 Primitive arc magmatism and the development of magmatic Ni-Cu-PGE mineralization in Alaskan-type ultramafic-mafic intrusions Milidragovic, D.1,2, Nixon, G.T.3, Spence, D.W.2, Nott, J.A.2, Goan, I.R.2, Scoates, J.S.2 1 Geological Survey of Canada-Pacific; 1500-605 Robson St., Vancouver, BC, V6B 5J3; dejan.milidragovic@nrcan-rncan.gc.ca 2Pacific Centre for Isotopic and Geochemical Research; Department of Earth, Ocean and Atmospheric Sciences; University of British Columbia 3British Columbia Geological Survey ___________________________________________________________________________ Zoned ultramafic-mafic plutonic rocks in convergent margin settings represent trans-crustal magmatic feeders [1,2] to coeval, and better studied, arc volcanoes. Arc lavas, which are on average basaltic to andesitic, represent differentiated and largely degassed magmatic products [3,4] and only rarely provide a clear glimpse into the earliest stages of arc magma evolution [5,6,7]. The study of lower- to mid-crustal arc cumulates, which include high-temperature liquidus lithologies, is complimentary and necessary to establish a holistic understanding of arc magmatism and mantlecrust metal transfer. Ultramafic-mafic convergent margin intrusions are typically composed of rocks comprised of variable proportions of olivine ±Cr-spinel, clinopyroxene, amphibole, and magnetite. Plagioclase is volumetrically minor and appears relatively late in the crystallization sequence, consistent with high parental magma water contents. The absence of orthopyroxene distinguishes the predominantly abundant class of “Alaskan-type” intrusions (e.g., Tulameen, Polaris, Turnagain), which are the focus of this presentation, from orthopyroxene-rich “Giant Mascot–type” intrusions [8]. Alaskan-type intrusions have long been recognized for their platinum group element (PGE) potential, hosted principally within micrometer-size platinum group metal (PGM) inclusions (e.g., laurite, isoferroplatinum, tetraferroplatinum) in thin chromite-rich horizons and massive schlieren occurring in dunite. Alaskan-type intrusions may also host significant magmatic Ni-Cu-PGE sulfide mineralization in dunite and wehrlite (e.g., Turnagain [9]) and notable palladium-subgroup PGE (PPGE) concentrations may occur in association with Cu-rich sulfides (e.g., chalcopyrite ±bornite) in more evolved clinopyroxene- and hornblende-rich rock types [10,11]. The mineralization style and potential in Alaskan-type intrusions is a reflection of the interplay between: 1) degree of country rock assimilation during emplacement and differentiation, and 2) the oxidation state of the primary, mantle-derived melts. Evolution of oxidized arc magmas [12] through assimilation of either S-rich or relatively reduced country rock favours early sulfide saturation and formation of magmatic Ni-Cu-PGE sulfides in hightemperature dunite and wehrlite. At Turnagain, assimilation of country rocks is indicated by the isotopic composition of sulfides, which show non-uniform d34S values (+4.2 to -12.3 ‰ [13,14]) that are largely intermediate between those of the depleted mantle (-1.28 [15]) and surrounding phyllite (-11.6 to -20.1 [13,14]). Magmatic chalcopyrite from the Polaris Alaskan-type intrusion has uniform near-chondritic sulfur isotope compositions (d34S =-0.19 +0.48/-0.32‰) that are markedly lighter than those of the country rocks (δ34S = +7.4 +1.3/-1.7), indicating that the evolution of primitive mantle-derived magma(s) occurred without appreciable country rock assimilation [16]. The differentiation of primitive arc magma without contamination from country rocks favours crystallization of PGM in association with chromite-bearing dunite and immiscibility of Cu-PPGE-Au-rich sulfide from the more differentiated clinopyroxene, magnetite ±hornblende-saturated magmas. In principle, the nature of PGM (i.e., Ptenriched vs. IPGE-enriched) and the onset of sulfide immiscibility in systems not affected by country rock assimilation are governed by the oxidation state of the primary magma, and by extension, the oxidation state of the sub-arc mantle wedge. The predominance of Pt-alloys, such as those observed at the Tulameen intrusion, indicates moderately oxidized parental magmas (log f(O2) <FMQ+2), �57 where Pt is likely to be near saturation [17]. In contrast, the absence of Pt-alloys and predominance of Ir-Ru-Os alloys and laurite (e.g., Polaris) indicates strongly oxidized parental magmas (log f(O2) ≥FMQ+2) [11]. In the absence of country rock assimilation, sulfide immiscibility may be attained through reduction in the oxidation state of the magma, most likely triggered by magnetite fractionation [18]. The oxidation of the FeS component in the melt to form magnetite (e.g., 6 FeS melt + 4O2 = 2 Fe3O4 magnetite + 3S2 [19,20]) is consistent with the Cu-rich character of the earliest formed immiscible magmatic sulfides at both Tulameen and Polaris [10,11]. The diverse magmatic Ni-Cu-PGE mineralization styles of Alaskan-type intrusions reflect the complexity of arc magmatism. Key controlling factors include: 1) first-order differences in the oxidation state of the sub-arc mantle that may relate to the composition and nature of the subducted oceanic crust [16,21], and 2) the composition and volume of crust that is assimilated during magma ascent and emplacement. References: [1] Cashman K V et al. (2017) Science 355: 9 [2] Spence D W et al. (2024) Lithos 474-475: 107578 [3] Müntener, O and Ulmer P (2018) Am J Sci 318: 64-89 [4] Ding S et al. (2023) Geochem Geophys Geosys 24: e2022GC010552 [5] Russell J K and Snyder L D (1997) Can Min 35, 521-541 [6] Milidragovic D et al. (2016) Earth Planet Sci Lett 454: 65-77 [7] Till C B (2017) Am Min 102: 931-947 [8] Nixon G T et al. (2015) GSC Open File 7856: 17-34 [9] Mudd G and Jowitt S (2014) Econ Geol 109: 1813-1841 [10] Nixon G T et al. (2020) GSC Open File 8722: 197-218 [11] Milidragovic D et al. (2021) Can Min 59: 1627-1660 [12] Cottrell E et al. (2022) Geophys Monogr 266, 33-61 [13] Scheel J E (2007) UBC MSc thesis, 201 p [14] Jackson-Brown S (2017) UBC MSc thesis, 272 p [15] Labidi J et al. (2013) Nature 501: 208-211 [16] Milidragovic D et al. (2023) Earth Planet Sci Lett 620: 118337 [17] Borisov A and Palme H (2000) Am Mineral 85: 1665-1673 [18] Jenner F E et al. (2010) J Petrol 51: 2445-2464 [19] Wohlgemuth-Ueberwasser C C et al. (2013) Min Dep 48: 115-127 [20] Lesher C M (2017) Ore Geol Rev 90: 465-484 [21] Canil D and Fellows S A (2017) Earth Planet Sci Lett 470: 73-86 �58 Stratigraphy of the Grasset Ultramafic Complex and its Ni-Cu-(PGE) mineralization, Abitibi Greenstone Belt, Superior Province, Canada. Milier, K.1, Houlé M.G.2 and Saumur B.M.1 1 Université du Québec à Montréal (UQAM), Département des sciences de la Terre et de l’Atmosphère, 201 avenue du Président Kennedy, Montréal, QC H2X3Y7, Canada. 2 Geological Survey of Canada, Lands and Minerals Sector, Natural Resources Canada, 490 Couronne Street, Québec City, QC G1K 9A9 Canada ___________________________________________________________________________ In the Abitibi Greenstone Belt (AGB), komatiitic rocks are prospective for Ni-Cu-PGE mineralization. Most of these occur within the Kidd-Munro and Tisdale assemblage located in the southern parts of the AGB [1]. The Grasset Ultramafic Complex (GUC) of the northern AGB is a notable exception, as it hosts one of the largest Type I komatiitic Ni-(Cu)-(PGE) deposits in the entire Abitibi. [1]. Located in the Harricana-Turgeon area, the GUC is an 8 kilometre long ultramafic corridor (Fig. 1A) within the volcano-sedimentary Manthet Group interpreted as part of the Deloro assemblage (2734-2724 Ma). The country rocks mostly consist of felsic to mafic volcanic rocks with gabbroic sills and graphitic mudstones. The GUC occurs within felsic volcanic and graphitic sediments that may contain semimassive to massive sedimentary sulfides intervals. However, it can crosscut the local stratigraphy. The GUC consists of thick ultramafic cumulate bodies (Fig. 1B, C) and komatiitic lava flows within the GUC central area (Fig. 1C). Both host Ni-(Cu-PGE) mineralization, such as that observed in the GUC central area and in the southern end of the GUC. The latter hosts the Grasset deposit. Figure 7 A) Simplified geological map of the GUC area [2]. B) Geological map of the Grasset area [3]. C) Geological map of the GUC central area [4]. The Grasset deposit consists of a peridotitic body (Fig. 1A) dipping to the southwest, cut by the Sunday Lake fault to the southeast (Fig. 1A, B), and dominated by olivine meso- to orthocumulate with lesser intervals of olivine adcumulate. The ultramafic rocks have undergone a significant degree of talc-serpentine-carbonate alteration, and primary mineral assemblages have been completely obliterated. The ultramafic body does not exhibit much lithological variation, especially in its central portions where it occurs as a homogenous olivine cumulate unit. Toward the northwest, the ultramafic splits into two bodies interleaved with felsic volcanics (Fig. 1A). The lower and upper contacts within the country rocks are sharp and gradually shifts from pyroxenite (Fig. 2B) to peridotite. Locally, relicts of “olivine hopper crystal” crescumulates (Fig. 2A) occurs within the cumulate body. Three Ni-Cu-(PGE) mineralized horizons (H1, H2, H3) occurs at different levels of the Grasset ultramafic body. H1 occurs along the basal contact between the ultramafic and the footwall rocks (Fig. 2B) and consists of disseminated to net-textured and semi-massive to massive sulfides. H2 �59 is very sparse and cannot be confidently defined as a clear mineralized horizon. H3, the main horizon, occurs in the upper part of the Grasset ultramafic unit. Its thickness can be up to 55 m, consisting of several intervals from disseminated, to heavy disseminated and net-textured sulfides (Fig. 2C) with rare massive sulfide intervals. Sulfide assemblages of H3 and H1 differ. H3 is largely composed of pyrrhotite (Po) ≈ pentlandite (Pn) >> chalcopyrite (Cpy), With pyrite (Py) occasionally replacing Po. In contrast, H1 exhibits a more common magmatic sulfide paragenesis of Po >> Pn >> Cpy. However, when normalized to 100% sulfide, H3 average grade is 15.1% Ni, 1.4% Cu, 0.31% Co and 12.1 ppm Pt+Pd, whereas H1 tenors are lower showing an average grade of 7.6% Ni, 1.0% Cu, 0.15% Co and 5 ppm Pt+Pd. Despite these tenor variations, H1 and H3 show similar Ni/Cu (8-11) and Pd/Pt ratios (1.8-2.0). Figure 2: A) Relict of hopper crystal in an olivine crescumulate. B) H1 disseminated sulfides within the pyroxenite in contact with the hornfelsed footwall felsic tuff (Right). C) H3 net-textured sulfides. D) Komatiitic flow top breccia. E) Disseminated sulfides within the olivine cumulate of a komatiitic flow. F) Olivine mesocumulate of the poorly differentiated cumulate, note the presence of elongated olivine. The GUC central area is composed of a series of komatiitic flows and thick cumulate ultramafic bodies dipping to the west. These komatiitic flows occur between the felsic volcanics and graphitic sediments (Fig. 1C). The flows consist of several flow top breccias (Fig. 2D) underlain by olivine ortho- to mesocumulates (Fig. 2E) that progressively decrease in thickness toward the stratigraphic top. The earliest flows, at the base of the sequence, appear to contain the bulk of the Ni-(Cu)-(PGE) mineralization in this area. This mineralization occurs at the bottom of the olivine cumulate with disseminated (Fig. 2E) to net-textured and massive sulfides. The thick ultramafic cumulates (Fig. 2F) are poorly differentiated bodies, composed of olivine ortho- to mesocumulate. These ultramafic bodies do not show clear field evidence of intrusive relationships, but they occur at varying local stratigraphic levels. They exhibit sparse disseminated sulfides, but rarely massive sulfides at the basal contact. In conclusion, the GUC is a komatiitic sequence consisting of extrusive komatiitic flows and thick olivine cumulate bodies. The system could thus host both Type I and II komatiite-associated mineralization. The GUC could represent a volcanic-subvolcanic komatiitic succession where extrusive facies are more likely to be found in the GUC central area. The extrusive or intrusive origin of Grasset remains unclear at this stage. However, the occurrence of crescumulate and several Ni-(Cu)-PGE horizons suggests the existence of several ultramafic subunits within the Grasset unit. The Grasset deposit highlights the potential for new Ni discoveries hosted in the Deloro assemblage and for similar discoveries in underexplored area such as the northern parts of the AGB. References: [1] Houlé MG et al. (2017). Rev in Econ Geol 19: 103-132 [2] Archer Exploration (2023). Corporate presentation [3] Tucker MJ et al (2019). Proc 15th SGA Biennial Meeting 2: 497-500 [4] Balmoral Ressources Ltd (2020). Roundup �60 Geochemistry of Camp Lake Block Lac des Iles Palladium Mine, N. Ontario, Canada Njipmo Ngoko, B.1, Hollings, P.1, Djon, L.2 and Hamilton, M.3 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada. bnjipmo@lakeheadu.ca 2 impalacanada, 69 Yonge Street, Suite 700, Toronto ON, Canada M5E 1K3 Canada. lionneldjon@gmail.com 3 Jack Satterly Geochronology Laboratory, University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1, Canada The Archean Lac des Iles suite is located just north of the Wabigoon-Quetico boundary [1], approximately 90 kilometers north of Thunder Bay in Northwestern Ontario. This suite of intrusions includes discrete mafic and ultramafic complexes associated with sanukitoids, which were emplaced along deep-seated regional faults [2]. Among these, only the Lac des Iles Complex hosts economically significant palladium deposits, specifically at the Lac des Iles mine. The complex is divided into two parts: North Lac des Iles and South Lac des Iles. The North Lac des Iles mainly comprises ultramafic rocks such as websterite, clinopyroxenite, wherlite, lherzolite, dunite, and peridotite [3]. In contrast, South Lac des Iles is primarily composed of mafic rocks such as gabbro, gabbronorite, norites, and melanorite [4] and is the main host of the Roby, Offset, and Camp Lake zones. This study focuses on the Camp Lake zone, the deepest part of the palladium deposit, recently highlighted by exploration drilling. The aim is to characterize the petrological, geochronological, and geochemical attributes of the Camp Lake zone and compare these with those of the Roby and Offset zones. Four main petrographic subtypes have been identified within the Camp Lake zone: leucogabbronorites, mesogabbronorites, melagabbronorites, and norite. The rock textures are generally equigranular or varitextured. Petrographic studies show these rocks mainly consist of a mixture of pyroxenes and plagioclase. The pyroxenes predominantly comprise orthopyroxene with minor clinopyroxene, which are partially to completely replaced by amphiboles (cummingtonite, actinolite, and tremolite). The plagioclase is weakly to moderately altered and generally retains its original habit. The Camp Lake rocks exhibit magmatic sulfide contents ranging from 0.5% to 3%, dominated by pyrrhotite, pentlandite, and chalcopyrite, with minor pyrite. Sulfide minerals often occur as blebs or disseminated grains intergrown with silicate minerals. A new zircon U-Pb age was acquired for the mineralized Camp Lake rocks, yielding an emplacement age of 2690.56 ± 0.80 Ma [5], closely similar to that of the Roby and Offset deposits [6]. Geochemical analysis of the Camp Lake Zone rocks shows enrichment in LREE (La/Smn ranging from 1.29 to 7.75, with a median of 3.30), unfractionated HREE (Gd/Ybn ranging from 0.56 to 1.49, with a median of 0.88), and a negative Nb anomaly. These values are similar to those of the Roby and Offset zones and are consistent with a subduction zone setting [7]. Also, similar to the Roby-Offset deposits, PGE values in Camp Lake range between 1.0 g/t and 3.0 g/t, with variations in the rocks increasing with Cu and Ni content. However, Camp Lake is distinguished by higher proportions of pyrrhotite compared to chalcopyrite and lower Pd/Pt and Cu/Pd ratios than the other zones. Data show that the Camp Lake zone exhibits lower δ34S values, ranging from (-1.1‰ to +0.3‰), while the Roby and Offset zones show wider variations ranging from (-0.37 to +3.28‰) [8]. This observation suggests that the sulfur in the Camp Lake zone is of mantle origin and that the sulfide was less affected by hydrothermal processes, leading to more limited sulfide alteration. References: [1]. Lavigne, M.J., & Michaud, M.J. (2001). The Lac des Iles Palladium Deposit, Ontario, Canada. Economic Geology. Volume 10, pages 1-17. [2]. Impala Canada. (2017). Technical Report on the Lac des Iles Palladium Mine. Impala Canada. �61 [3]. Djon, L., Smith, M., Johnson, R., & Brown, T. (2017). Canadian Journal of Earth Sciences, 54, 12341250. [4]. Gomwe, T. (2008). Geology and Mineralization of the Lac des Iles Complex. In: Platinum-Group Elements in Magmatic Ore Deposits. Springer, pp. 123-145. [5]. Hamilton, M.A., 2024. Report on U-Pb CA-ID-TIMS geochronology of diorite and gabbro samples from Lac des Iles – related intrusions at Wakinoo, Buck Lake, Demars Lake, and Dog River, NW Ontario. Unpublished report prepared for Prof. P. Hollings, Department of Geology, Lakehead University, Ontario. 14p. [6]. Peck, D., Houle, M.G., & Smith, M.P. (2016). Economic Geology, 111, 833-858. [7]. Peck, D., Houle, M. G., et Smith, M. P. (2016) Geology, Petrology, and Controls on PGE Mineralization of the Southern Roby and Twilight Zones, Lac des Iles Mine, Canada, p. 43 [8]. Jonsson, J. (2023). Petrogenesis of mineralized horizons in the Offset and Creek zones, Lac des Iles Complex, N. Ontario, pages 146-168. �62 Hf-Nd-Pb isotopic evidence for variable impact devolatilization in the Sudbury Igneous Complex and its relevance for Ni-Cu-(PGE) sulfide ore formation Peters, D.1, Lesher C.M.1 and Pattison E.1 1 Laurentian University, Sudbury, ON P3E 2C6, Canada, dpeters@laurentian.ca ___________________________________________________________________________ The Sudbury Igneous Complex (SIC), generally believed to be the remnant of a large, 1850 Ma bolide impact, hosts one of the world’s largest magmatic Ni-Cu-(PGE) sulfide mining camps. It consists of i) the Main Mass, the crystallization product of the impact melt sheet), ii) underlying discontinuous lenses of variably mineralized magmatic and anatectic breccias, iii) radial and concentric, variable mineralized quartz dioritic offset dikes, and iv) overlying fallback/suevitic breccias. The ultimate source for all metals and sulfur is the immediate target rocks melted during the impact event, but the timing and mechanisms of ore formation are still being debated. Most current models assume that all metals and sulfur completely dissolved in the impact melt sheet and subsequently exsolved and sank toward the bottom, where they accumulated in local embayments or troughs, either by convective currents [1, 2] and/or gravity-driven density flows [3]. However, this process is slow and difficult to reconcile with the observed heterogeneities in the Pb>S>Os isotopic compositions of the sulfide ores around the SIC [4, 5, 6] and would require an initially heterogeneous impact melt sheet from which the sulfide ores subsequently exsolved. An alternative model is that significant amounts of Pb [7] and S [8], as well as Zn-Cd-Rb-Cs [9] and other volatile elements were volatilized during the impact event, followed by localized thermomechanical erosion of S ± metal-bearing footwall rocks by the superheated impact melt sheet [3, 10], forming local sulfide xenomelts, which then accumulated in local embayments and troughs [3]. Impact devolatilization would have left volatile elements such as Pb and S more susceptible to postimpact modifications by thermomechanical erosion, whereas more refractory elements such as Hf or Nd [11] would have been largely preserved during impact, making them less susceptible to postimpact modifications. Characterising the Hf-Nd-Pb isotopic composition of the Main Mass (the crystallized impact melt sheet) therefore presents an excellent opportunity to better understand i) the characteristics of the initial impact melt sheet, ii) post-impact contamination processes, and iii) formation of the sulfide ores associated with the SIC. Preliminary results of Hf isotope analysis on zircons by LA-MC-ICP-MS from four Main Mass transects across the North Range of the SIC show a narrow range in Hf isotope compositions (εHf1850Ma between -8 and -12, Figure 1A), similar to previously published data for the South Range of the SIC [12]. Similarly, literature data for whole-rock Nd isotope compositions across the North Range [13, 14] also show a narrow range (εNd1850Ma between -7 and -9, Figure 1B), which suggests effective vertical and lateral homogenization of the initial impact melt across the North Range prior to crystallization. Lead isotope compositions on the other hand, while being relatively homogeneous throughout the Granophyre, Quartz Gabbro and Felsic Norite (Δ207Pb/204Pb between 300 and 450), become more variable towards the base of the Main Mass, especially within the Mafic Norite (Δ207Pb/204Pb between 100 and 400, Figure 1C) [7, 15]. The greater Pb isotopic variability in the Mafic Norite can be attributed to the greater susceptibility of Pb to post-impact contamination by thermomechanical erosion, which would have been most significant at the base of the melt sheet. The decoupling of the more variable Pb isotopes from the more homogenous Hf and Nd isotopic compositions within the Mafic Norite therefore provides strong evidence for impact devolatilization of Pb>S>>Os>Nd>Hf. Although a contribution from the impact melt sheet cannot be entirely excluded, the current Hf-Nd-Pb isotopic evidence from the Main Mass favours a model in which the sulfide ores dominantly formed at the base by local thermomechanical erosion of S-bearing footwall rocks. Additional analyses of Nd and Pb isotopic compositions of the Main Mass across the North Range are in progress to confirm the results. �63 Figure 1: Stratigraphic variations in Hf, Nd, and Pb isotopic compositions throughout the North Range Main Mass of the Sudbury Igneous Complex. Individual analyses are shown in grey, unit averages (±1σ) in the colour of the respective lithology. Black lines and shaded blue squares show the overall average (±1σ) for the North Range Main Mass. A. εHf1850Ma variations throughout the North Range Main Mass. B. εNd1850Ma variations throughout the North Range Main Mass. C. Δ207Pb/204Pb variations throughout the North Range Main Mass. Hf data are from this study, Nd data are from [13, 14], Pb data are from [7, 15]. For calculation of Δ207Pb/204Pb see [7]. GRAN – Granophyre, QGAB – Quartz Gabbro, FSNR – Felsic Norite, MFNR – Mafic Norite References: [1] Lightfoot P et al. (2001) Econ Geol 96: 1855-1875 [2] Zieg M and Marsh B (2005) GSA Bulletin 117: 1427-1450 [3] Wang Y et al. (2022) Econ Geol 117: 1-28 [4] Darling J et al. (2012) GCA 99: 1-17 [5] Ripley E et al. (2015) Econ Geol 110: 1125-1135 [6] Morgan J et al. (2002) GCA 66: 273-290 [7] McNamara G et al. (2017) Econ Geol 112: 569-590 [8] Lesher C (2019) GAC-MAC 42: 130-131 [9] Kamber B and Shoenberg R (2020) EPSL 544: 116356 [10] Prevec S and Cawthorn R (2002) JGR 107: B8 2176 [11] Lodders K (2003) Astrophysics Journal 591: 1220-1247 [12] Kenny G. et al. (2017) GCA 215: 317-336 [13] Faggart B et al. (1985) Science 230: 436-439 [14] Dickin A et al. (1996) GCA 60: 1605-1613 [15] Dickin A et al. (1999) GSA Special Paper 339: 361-371 �64 Deformation in mafic protoliths: Impacts from late faults on Ni-Cu-PGE mineralization at Lac des Iles Mine, Canada Peterzon, J.1, Phillips, N.1, Hollings, P.2, and Djon, M.L.2 1 2 Lakehead University, 955 Oliver Road, Thunder Bay ON. P7B 5E1, Canada; jpeterzo@lakeheadu.ca Impala Canada, 69 Yonge Street, Suite 700 Toronto ON. M5E 1K3, Canada __________________________________________________________________________ Fault zones are complex structures that serve as permeable pathways through the upper crust; however, the impact of host lithology on damage zone development remains poorly understood. The development of fault cores and damage zones is typically controlled by the strength and composition of the protolith, conditions of deformation, and fluid chemistry [1], this is particularly true for faults hosted in mafic lithologies where damage zones control hydration in mafic crust. Permeability is significantly enhanced in damage zones due to the high density of fractures and is diminished in fault cores when a clay rich gouge is present. Faults therefore may act as conduits or barriers for fluid flow depending on the proportion of fault core to damage zone [2]. Trapped mineralization may be offset or remobilized by later faulting. This study investigates the deformation and alteration geochemistry footprint of late faults within the mafic-ultramafic intrusions at the Lac des Iles mine (Figure 1). The 2,689 +/- 1.0 Ma Lac des Iles Complex (LDIC) [3] is a series of intrusive bodies hosted within the ~3.01 – 2.68 Ga granitegreenstone Marmion terrane of the Superior Province, Canada. Ni-Cu-PGE mineralization has been offset, and depleted in areas surrounding the fault zone, including the damage zone and fault core, by the reverse Offset Fault and hypothesized reverse Camp Lake Fault. Palladium depletion is hypothesized to be from fluid flow through the fault damage zones. Fracture densities from the hanging wall of each fault were measured to determine the damage zone and fault core width in both gabbronorites and tonalites (Figure 2). Tonalites have a higher fracture density than the gabbronorites, suggesting fluid flow would be more effective in felsic protoliths, which in turn may contribute to metal remobilization, implying that host rock lithology has a strong control over fault zone structure, mineralization, and alteration assemblages. Metal contents display depletions in areas surrounding faults, and show a strong correlation with fracture density measurements. It is likely that a frictionally weak, chlorite rich fault core likely impeded the development of a more fracture dense damage zone in the gabbronorites, as opposed to a silica-rich brecciated fault core in the tonalites. Deformation conditions of the Camp Lake and Offset Fault zones were studied through scanning electron microscopy (SEM) and electron microprobe analyses. Preliminary results from this support our hypothesis of a silicified fault core in tonalites (Figure 3) and a chlorite-rich fault core in gabbronorites and reveal three generations of chlorite growth: prefaulting at ~350°C, syn-faulting at ~150 – 200°C, and post-faulting at ~150°C [4] (Figure 4). We aim to highlight the importance of fluid-rock interactions in the development of fault core and damage zone structures in mafic protoliths, and their associated impact on Ni-Cu-PGE mineralization. References: [1] Caine et al. (1996) Geology, 24 (11): 1025-1028 [2] Faulkner et al. (2010) Journal of Structural Geology, 32 (11): 1557-1575 [3] Djon et al. (2018) Economic Geology, 113 (3): 741-767 [4] Wiewóra and Weiss (1990) Clay Minerals, 25: 83-92 �65 �66 Formation of euhedral silicate megacrysts within magmatic massive sulfides Raisch, D.1, Staude S.1, Fernandez, V. 2 and Markl G.1 1 Department of Geosciences, University of Tübingen, Schnarrenbergstrasse 94-96, D-72076 Tübingen, Germany Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom Corresponding author: Dominic.raisch@uni-tuebingen.de 2 _________________________________________________________________________ In the magmatic massive sulfide ore from Nova-Bollinger (Western Australia), large (up to 10 cm) silicate crystals, completely enclosed in massive sulfides, are common where sulfides infiltrate older silicate rocks. This texture could provide a new insight into the infiltration and the role of the magmatic sulfides in the nucleation and growth processes of these crystals. At Nova-Bollinger, the megacrysts consist of pyroxene, garnet and plagioclase (Fig. 1) and are typically observed in association with emulsion-textured sulfides at the sulfide infiltration front from the orebody into the silicate rocks. The infiltrated country rock itself consists of amphibolite- to granulite-facies metamorphosed mafic granulite [2] with an assemblage of plagioclase, pyroxene, amphibole ± garnet. Infiltration of hot sulfide melt caused parts of the country rock to incongruently melt producing both tonalitic melt and peritectic orthopyroxene and garnet. While the peritectic silicates formed margins at the contact between the sulfides and the country rock, the newly formed immiscible buoyant silicate melt formed an upward counterflow through the descending, denser sulfide melt, resulting in the formation of an emulsion [1, 3]. The assemblage of the country rock may contain the same minerals as the megacrysts of the emulsion texture, but they are clearly distinguishable both optically and chemically. Garnet, for example, is only occasionally present in the immediate country rock depicting a mostly poikilitic morphology with rarely any euhedral crystals larger than 800 µm, in contrast to the up to 6 cm euhedral and sometimes even skeletal garnet of the emulsion texture. In addition, the garnet and pyroxene megacrysts of the emulsion texture show distinct negative Eu-anomalies (Eu/Eu*= 0,17 for both minerals) with a strong depletion in light REE (Fig. 2) and in some cases display round multisulfide inclusions, as visible by computed tomography scans. Both characteristics are missing in the country rock counterparts as well as in the gabbroic host silicate melt. These observations argue for a magmatic origin of to the megacrysts via crystallisation from the silicate melt portion of the emulsion texture. The large grain size may be the result of the constant movement of the emulsion (to keep it stabilized [REF]), where the constant bumping of silicate melt droplets onto the growing crystals provides enough material to garnet, pyroxene or plagioclase to allow them grow to megacrysts within this emulsion. Once the movement of the melts decreases, the immiscible melts can separate, leaving the megacrysts behind in massive sulfides. While plagioclase coexists with garnet and pyroxene, pyroxene and garnet never coexist as megacrysts, which may be due to a temperature effect. This is based on the observation that pyroxene is mostly associated with mono-sulfide solid solution, which records temperatures up to 1100°C [4], whereas garnet is associated with intermediate sulfide solid solution, which starts to crystallise at temperatures around 880°C [4]. Besides other magmatic Ni-Cu sulfide deposits (i.e., Kambalda, Western Australia [1]), partly skeletal megacrysts are also found associated with emulsion textures of anatectic sedimentary exhalative deposits in massive sulfides (e.g. cordierite, pyroxene, and feldspar from the granulite-facies Silberberg deposit in Germany, [5]). �67 Figure 8 Plagioclase megacrysts in massive sulfides from Nova-Bollinger. Figure 9 Primitive mantle normalized [6] REE-pattern of orthopyroxene from Nova-Bollinger. References: [1] Staude S et al. (2017) Ore Geol Rev 90:446-464 [2] Clark C et al. (2014) Precambrian Res 204:1-21 [3] Barnes S et al. (2018) Ore Geol Rev 101:629-651 [4] Craig JR & Kullerud G (1969) Soc Eco Geo Monogr 4:344-358 [5] Staude et al. (2023) Miner Deposita 58:987-1003 [6] Lyubetskaya T & Korenaga J (2007) Solid Earth 112 �68 Applying Magnetic Vector Inversion (MVI) on Aeromagnetic Data in the Thunder Bay Region of the Mid-Continent Rift Riahi, S.1, Mungall J.E.1, Ernst, R.E1 1 Department of Earth Sciences, Carleton University, Ottawa, Ontario, Canada, Shokouhriahinajafaba@cunet.carleton.ca The research presented here applies the Magnetic Vector Inversion (MVI) technique to aeromagnetic datasets of the region surrounding Thunder Bay. The intrusions related to the Mid-Continent Rift contain several deposits containing high-grade mineralization zones that are abundant in platinum (Pt), palladium (Pd), copper (Cu), and nickel (Ni). Given the pivotal role of geophysical data in mineral exploration and the proven efficacy of magnetic data in delineating mineralized zones, our aim is to deepen the understanding of the geological attributes and the potential for mineralization in the Thunder Bay region deposit by applying MVI. Aeromagnetic Data Acquisition: The aeromagnetic data was used in this pilot study obtained from the USGS website [1], representing compilations of previously published survey data from various geological surveys and organizations. These Figure 1. TMI and analytic signal (AS) of the area including the Current Lake and Escape Lake areas. compilations, produced using industry-standard techniques, were analytically continued to a surface drape of 150 m and 300 m above ground and gridded to 250 m and 500 m cell size, respectively. They offer consistent datasets suitable for onshore geology mapping and magnetic modeling extending across the lake shore [1]. Total Magnetic intensity (TMI) data of the study area and the analytic signal (AS), with the magnetic units are shown in Figure 1. Magnetic Vector Inversion (MVI): Magnetization vector inversion (MVI) is employed to replicate the distribution of magnetization vectors within subsurface blocks [2-4]. This technique involves calculating the overall distribution of magnetization vectors from the components within each underground block. MVI enables the simultaneous analysis of complex geological scenarios, such as the overlay of multiple sources with �69 diverse remanent magnetization directions, and facilitates the complete retrieval of magnetization vector data [5-8]. All modeling and comparisons in the examples presented herein were conducted using the Geosoft VOXI Earth Modeling system. The aeromagnetic dataset was inverted to generate 3D voxel MVI susceptibility models employing the Geosoft VOXI Earth Modeling system (Fig. 2). Strong magnetic anisotropy is evident in the southwest corner of the region. Future efforts will focus on highresolution exploration data sets over recognized chonoliths including Tamarack and Current Lake to seek distinctive magnetic vector characteristics of these small but valuable intrusions. Fig 2. 3D MVI VOXEL model and MVI vectors, the above color bar gives the susceptibility in SI. The axes are in meters. The lower color bar gives the normalized amplitude in SI. References: [1] Anderson, E.D., and Grauch, V.J.S. (2018), Updated aeromagnetic and gravity anomaly compilations and elevation-bathymetry models over Lake Superior: U.S. Geological Survey data release, https://doi.org/10.5066/F7F18X8S. [2] Wang, M.Y., Di, Q.Y., Xu, K., Wang, R. (2004), Magnetization vector inversion equations and forward and inversed 2-D model study, Chinese Journal of Geophysics, 47, 601–609. [3] Lelievre, P.G. & Oldenburg, D.W. (2009), A 3D total magnetization inversion applicable when significant, complicated remanence is present, Geophysics, 74, L21–L30. [4] Ellis, R.G., de Wet, B., Macleod, I.N., (2012), Inversion of magnetic data for remanent and induced sources, in ASEG Extended Abstracts, pp. 1–4. [5] Kubota, R., Uchiyama, A. (2005), Three-dimensional magnetization vector inversion of a seamount, Earth, Planets and Space, 57, 691–699. [6] MacLeod, I.N., Ellis, R.G. (2016), Quantitative magnetization vector inversion, in ASEG Extended Abstracts, pp. 1–6. [7] Liu, S., Hu, X., Zhang, H., Geng, M. & Zuo, B. (2017), 3D magnetization vector inversion of magnetic data: improving and comparing methods, Pure and Applied Geophysics, 174, 4421–4444. [8] Ghalehnoee, M.H., Ansari, A. (2022), Compact magnetization vector inversion, Geophysical Journal International, 228, 1–16. �70 Potential links between the Midcontinent Rift (MCR) related BaragaMarquette dyke swarm and early MCR related magmatic Ni-Cu sulfide deposits in Michigan, USA. Rossell, D.M.1*, Strandlie, J.2 1Talon Metals, Tamarack, MN, USA 2 Eagle Mines, Marquette, MI, USA *rossell@talonmetals.com ___________________________________________________________________________ The~1100Ma Midcontinent Rift (MCR) system can be traced across the central United States and Canada as a ~2000km long gravity high, but the only surface exposures of the volcanics, intrusions and sediments that make up the MCR are in the Lake Superior region. Despite the large extent of the MCR, historic MCR related mineral production has been almost exclusively from the portion of the MCR in Michigan. The MCR related mineral deposits shown in Figure 1, range from the famous Keweenaw volcanic hosted Native Cu deposits and the large “White Pine type” sediment hosted chalcocite deposits to the Eagle magmatic Ni-Cu sulfide mine, the only currently producing Ni mine in the USA. In contrast to many Large Igneous Provinces which are relatively short-lived events of a few million years or less, the main period of MCR related magmatism spans ~20my [1]. The USGS [1] subdivides MCR volcanism into two main phases, an Early Plateau Stage (~1112-1105Ma) which largely occurred during a period of reversed magnetic polarity and later Rift stages (~1102-1090Ma) which occurred during a period of normal magnetic polarity . Figure 10 Geology map of the Western portion of the Upper Peninsula of Michigan, USA showing the distribution of dykes of the Baraga Dyke swarm and the various types of mineral deposits and prospects associated with the MCR (modified from Michigan Geologic Survey state geology map). The Baraga dyke swarm is located on the south side of the MCR in the western portion of the Upper Peninsula of Michigan , USA (fig. 1). The dyke swarm is comprised of more than 100 mafic-ultramafic �71 dykes wide enough (+10m) to be visible in proprietary high resolution airborne magnetic data sets (the dykes shown in figure 1), and likely hundreds more, to thin to be discernible from airborne data, but frequently intersected in drilling in the area. The dykes can be divided into three types based on geochronology, magnetic polarity, orientation and chemistry that are referred to in figure 1 as the “metal depleted”, “Cr Rich”, and “Reversely Polarized” dykes The oldest known dykes within the dyke swarm are the “Metal Depleted ”dykes, which are only recognized as a pair of east-west trending dykes on the north and south side of the Eagle Ni-Cu mine. These two gabbroic dykes have very different trace element chemistry from all the other dykes in the Baraga-Marquette swarm (most notably having below detection limits PGE contents) and are the only dated dykes (1120Ma+/4my [2]). The youngest known dykes are the “reversely polarized” set of gabbroic dykes that have distinctive ophitic to sub-ophitic textures, generally East-West orientations, high TiO2 contents , mantle like Cu/Zr ratios and the highest Pd contents of any of the dyke sets. Although, all attempts to date these dykes have been unsuccessful, they cross-cut both the East Eagle intrusion dated at 1107.3+/-3.7ma [3] and the BIC intrusions dated at 1106.2+/- 1.3Ma [4]. Despite the cross cutting relationships, Paleomagnetic data suggests they might be similar in age to the Eagle intrusions [5]. The third type of dykes making up the Baraga-Marquette dyke swarm are a NW-SE trending set of dykes that range from centimetres to >70m in width. Although they have a wide range of MgO contents, the sampled dykes all have much higher Cr contents(>500ppm) than the other two types of dykes. The are often amygdaloidal, Cr Rich dykes typically do not have visible sulfides, but do resemble the amygdaloidal pyroxenite margins of the well mineralized olivine cumulates that host mineralization in the Eagle and Eagle East deposits. The Ni-Cu-PGE mineralized, pipe like conduits at Eagle, Eagle East and BIC also align closely with Cr Rich dykes, suggesting a potential temporal and genetic relationship (feeder dykes). The pronounced 30-40 degree change in orientation between the likely similar aged, reversely polarized dykes and Cr Rich dykes might indicate a change in the orientation of the regional stress fields associated with the emplacement of the mineralized intrusions. References: [1] Woodruff, L et al. (2020) Ore Geol. Rev. 126 [2] Dunlop, M (2013) Indiana Univ. MSc thesis (93p.) [3] Ding X et al. (2010) Geochem. Geophys. Geosyst. v.11(3) [4] Bleeker W et al. (2020) personal communication [5] Foucher M (2018) Michigan Tech. Univ. PhD dissertation (173p.) �72 Texture and composition of Fe-Ti oxides of the Neoarchean Big Mac mafic intrusion and its implication for Fe-Ti-V-(P) mineralization in the McFaulds Lake greenstone belt, Superior Province, Canada Sappin, A.-A.1, Houlé, M.G.1,2*, Metsaranta, R.T. 3, and Lesher, C.M.2 1 Geological Survey of Canada, Lands and Minerals Sector, Natural Resources Canada, 490 Couronne Street, Québec City, QC G1K 9A9 Canada anne-aurelie.sappin@nrcan-rncan.gc.ca 2 Mineral Exploration Research Centre, Harquail School of Earth Sciences, Goodman School of Mines, Laurentian University, 935 Ramsey Lake Road, Sudbury, ON P3E 2C6 Canada 3 Earth Resources and Geoscience Mapping Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, ON P3E 6B5 Canada * Presenter _________________________________________________________________________ The McFaulds Lake greenstone belt (MLGB), also known as the “Ring of Fire” area, is a region with great potential for orthomagmatic Cr-platinum-group element (PGE), Ni-Cu-(PGE), and Fe-Ti-V-(P) mineralization, as attested by the discovery of the world-class Black Thor – Big Daddy – Black Horse – Black Creek – Blackbird Cr-(PGE) system, the Eagle’s Nest Ni-Cu-(PGE) deposit, and the Thunderbird, Butler West, Butler East, and Big Mac Fe-Ti-V-(P) prospects. Most of the mafic-ultramafic intrusions hosting orthomagmatic mineralization in the area belong to the ca. 2736˗2732 Ma Ring of Fire intrusive suite (RoFIS) (e.g., [1], [2]). This suite includes mafic and ultramafic-dominated intrusions associated with Cr-(PGE) and Ni-Cu-(PGE) mineralization (Koper Lake subsuite) and mafic-dominated intrusions associated with Fe-Ti-V-(P) mineralization (Ekwan River subsuite) [2]. The latter are the most abundant, but also more widespread geographically. The Big Mac intrusion is the largest intrusion belonging to the Ekwan River subsuite. It forms a broadly layered, subconcordant sill, and comprises various flavors of gabbro (± Fe-Ti oxides), minor anorthosite, and rare pyroxenite. These lithologies exhibit partially preserved cumulate textures composed mostly of plagioclase and clinopyroxene (almost completely altered to amphibole) with local magnetite and ilmenite, apatite, and Fe-Ni-Cu sulfides. Fe-Ti oxide mineralization in the Big Mac intrusion occurs as massive (> 80% Fe-Ti oxides) to semi-massive (40 to 80% Fe-Ti oxides) magnetiteilmenite layers, net-textured (20 to 35% Fe-Ti oxides) to patchy net-textured (10 to 25% Fe-Ti oxides), and locally as millimeter- to a few centimeter-thick stringers (Fig. 1). Massive to semi-massive Fe-Ti oxide layers are mainly located in the northern part of the intrusion, whereas patchy to net-textured oxides are more widespread throughout. All lithologies typically contain at least several percent disseminated Fe-Ti oxides (< 10%). Based on whole-rock geochemical data, the best mineralized interval (9.5 m thick) has an average composition of 68 wt.% FeOt, 17 wt.% TiO2, and 0.48 wt.% V2O5. The Big Mac sill also contains disseminated pyrrhotite, pentlandite, pyrite, and chalcopyrite (< 10% sulfides) throughout the intrusion, and millimeter-thick stringers of chalcopyrite, pyrite, and pyrrhotite. In the northern part of the intrusion, the semi-massive to massive magnetite layers contain patchy net-textured pyrrhotite, pentlandite, pyrite, and chalcopyrite (10 to 20% sulfides; Fig. 1E-F) with up to 1.6% Ni100 (Ni at 100% sulfides) and 1.8% Cu100. Fe-Ti oxides are well preserved in the Big Mac intrusion and their chemical composition can be used to characterize the internal stratigraphy, to determine which parts are more prospective for V and P mineralization, and to estimate the conditions for the genesis of the Fe-Ti oxide layers. The Big Mac intrusion appears to have crystallized from high-Fe parental magmas that were injected from a feeder conduit located in the northernmost part of the intrusion. Based on the presence of more primitive magnetite and ilmenite compositions in the northern part of the intrusion and more evolved signatures in the southern part, the rocks in the northern part likely represent more conduitproximal facies that are more prospective for Fe-Ti-V mineralization, whereas the rocks in the southern part likely represent more distal facies that are more prospective for Fe-Ti-P mineralization. The trace element contents of magnetite also suggest that the crystallization of the Fe-Ti oxide layers in the Big Mac intrusion occurs under relatively oxidized conditions (fO2 > FMQ + 1). The Big Mac �73 magnetite displays many characteristics (e.g., texture, chemical composition) in common with magnetite in other mafic-dominated intrusions of the Ekwan River subsuite (e.g., Thunderbird, Butler West, Butler East). This attests to the Fe-Ti-V-(P) potential of the large ferrogabbroic magmatic event that affected the MLGB at ca. 2735˗2732 Ma [3] and formed the Ekwan River subsuite. Figure 1: (A) Simplified and schematic graphic log of drill core BM09-04 located in the northern part of the Big Mac intrusion. (B-G) Photomicrographs of polished thin sections in plane-polarized transmitted (B-D) and reflected (E-G) light showing the different oxide textural facies in the Big Mac intrusion. (B) Disseminated, anhedral grain of magnetite in mesocratic gabbro. (C) Disseminated, rounded grain of magnetite in clinopyroxenite. (D) Net-textured magnetite in melanocratic gabbro. (E-F) Semi-massive magnetite with patchy net-textured pyrrhotite, pentlandite, and chalcopyrite. Anhedral magnetite contains ilmenite exsolutions as anhedral grains and lamellae. (G) Massive magnetite with ilmenite exsolutions as anhedral crystals and thick lamellae. Abbreviations: Amp = amphibole, Cpx = clinopyroxene, Cpy = chalcopyrite, Grt = garnet, Ilm = ilmenite, Mag = magnetite, Pl = plagioclase, Pn = pentlandite, Po = pyrrhotite. References: [1] Houlé M.G. et al. (2015) Geological Survey of Canada, Open File 7856, pp. 35–48. [2] Houlé M.G. et al. (2019) Geological Survey of Canada, Open File 8549, pp. 441–448. [3] Houlé M.G. et al. (2020) Geological Survey of Canada, Open File 8722, p. 141–163. �74 Complexly zoned pyroxenes at Kevitsa record magma mixing and survive alteration Schoneveld, L.1, Luolavirta, K.2,3, Barnes, SJ1 , Hu, S.1 , Verrall, M.1 and Le Vaillant, M.1 1 CSIRO Mineral Resources, Perth, 6151, Australia Geopool Oy, Teknobulevardi 3−5, 01530 Vantaa, Finland. kirsi.luolavirta@geopool.fi 3 Oulu Mining School, Faculty of Technology P.O. Box 3000, FI-90014 University of Oulu, Finland 2 ___________________________________________________________________________ Magmatic Ni-Cu-(Platinum Group Element—PGE) sulfide deposits are generally linked to dynamic systems and conduit-type emplacements of mafic-ultramafic magmas. Schoneveld et al. [1] demonstrated a common feature of variable titanium (Ti) and chromium (Cr) zoning patterns in cumulus pyroxenes in various mineralized intrusions (e.g. Noril’sk-Talnakh, Nova-Bollinger, Jinchuan) and attributed these features to reflect a high-flux magmatic environment with wall rock assimilation and related fluctuating cooling rates where pyroxenes crystallized. On the contrary, according to the authors, barren intrusions were characterized by simple normally zoned pyroxenes. Pyroxene zoning was therefore suggested to serve as a potential prospectivity indicator for magmatic Ni-Cu-PGE sulfide deposits. However, on many occasions, the primary mineralogy of the ore hosts has been subjected to variable degrees of hydrothermal alteration, potentially hindering the usability of the pyroxene zoning approach in exploration. This dilemma is being tackled by mapping pyroxene zoning patterns of samples recording variable degrees of amphibole alteration. Additionally, pyroxene has been shown to record magma histories in volcanic settings [2] and also has the potential to record important magmatic histories in these ore deposits. In this research, microbeam X-ray fluorescence (XRF) mapping techniques were applied to the mineralized Kevitsa intrusion, in northern Finland to study pyroxene zoning patterns. Synchrotronbased µXRF chemical imaging using multidetector Maia arrays has proved especially effective [3], allowing entire thin sections to be imaged at micrometer-scale resolution in a matter of hours (Australian Synchrotron, operated by ANSTO). This allows many grains with varying crystal orientations to be analyzed and detailed visualization of chemical zoning. The mafic-ultramafic Kevitsa intrusion (2.06 Ga) is hosted by a volcano-sedimentary sequence in the Central Lapland greenstone belt. A disseminated Ni-Cu-(PGE, Au, Co) sulfide ore deposit occurs within the central parts of ultramafic olivine-pyroxene cumulates. The deposit has been mined since late 2011 and is currently operated by Boliden. The sample set comprises 29 thin sections collected from various parts of the intrusion representing mineralized and non-mineralized domains within the intrusion. Most of the samples are clinopyroxene-olivine mesocumulates with variable modes of olivine, augite, and oikocrystic or transitional cumulus to poikilitic orthopyroxene (bronzite/enstatite). These textures are characteristic throughout the ultramafic part of the Kevitsa intrusion. The samples have also been exposed to variable degrees of hydrothermal alteration and many clinopyroxene grains have begun the transformation to amphibole. Very complex pyroxene zoning patterns are observed throughout the Kevitsa intrusion (Figure 1). Hence, the Kevitsa intrusion provides yet another example of a sulfide ore-bearing variant of a maficultramafic intrusive body with diagnostic complex zoning patterns of pyroxene minerals. The observed styles and magnitudes of clinopyroxene zonation in Kevitsa, however, are unusual when compared to other ore-bearing intrusive bodies [1]. A common feature for clinopyroxe grains is highly Cr-poor cores, followed by strong oscillatory patterns in the mantles, often ending in a rim of very low Cr and high Ti values. Similarly, the clinopyroxene in the most nickel-rich ore zones shows enriched nickel rims. These patterns are best explained by open magma chamber processes, consistent with Luolavirta et al. [4]. The nickel enrichment and chemical oscillations recorded in the pyroxene crystal structure suggest an influx of new, Ni-rich melt into the partially solidified crystal mush at Kevitsa. The clinopyroxene zoning patterns are not reflected in the oikocrystic �75 orthopyroxene that generally records smooth normal zoning. This indicates post-cumulus growth of orthopyroxene (cf. slow nucleation as cumulus mineral). Cr-rich Cr-poor Figure 1. Examples of end-member zoning styles in the Kevitsa pyroxenes with traverses across the grains showing the Cr and Ti content that causes each distinct zoning type. A) normal zoning from trapped liquid reactions B) sector zoning with B1 and B2 showing different sectioning effects of this zoning type C) abrupt zoning D) oscillatory zoning E) crater-zoned clinopyroxene with the content of Cr and Ti of the traverse shown in F). G) crater zoning schematic H) Moat zoned clinopyroxene grain I) traverse of Cr and Ti content across the grain J) moat zoning schematic. The examination of the preservation of the zoning patterns with alteration reveals that Cr zonation is visible through the early stages of amphibole alteration, with preservation being enabled by the presence of Cr-rich epitaxial amphibole. However, the remnant zoning is lost as the amphibole alteration progresses. It is worth noting that the complex zoning patterns are observed in almost every sample, regardless of the location relative to the ore-bearing domain of the intrusion (some are located up to a few hundred meters away from the deposit). Hence, to enhance the methodology as an exploration tool, further research is needed to outline the distal extent of this fingerprint away from the ore within mineralized intrusions of reasonable size. References: [1] Schoneveld et al. (2020) Zoned Pyroxenes as Prospectivity Indicators for Magmatic Ni-Cu Sulfide Mineralization. Front. Earth Sci. 8:256. [2] Ubide et al. (2019) Sector-zoned clinopyroxene as a recorder of magma history, eruption triggers, and ascent rates: Geochim. Cosmochim. Acta. 251:265-283. [3] Barnes et al. (2020) Imaging trace-element zoning in pyroxenes using synchrotron XRF mapping with the Maia detector array: Benefit of low-incident energy. Am. Min. 105:136–140 [4] Luolavirta et al. (2018) In-situ strontium and sulfur isotope investigation of the Ni-Cu-(PGE) sulfide orebearing Kevitsa intrusion, northern Finland. Min. Dep. 53:1019–1038 �76 New indicator mineral signatures for nickel sulfide exploration Schoneveld, L.E.1, Williams M.1, Salama, W.1, Spaggiari, C. V.1, Barnes, S. J. 1, Le Vaillant, M. 1, Siegel, C. 1, Hu, S. 1, Birchall, R. 1, Baumgartner, R. 1, Shelton, T. 1, Verrall, M. 1, and Walmsley, J. 1 1 Mineral Resources, CSIRO, Western Australia Corresponding Author: Louise.Schoneveld@csiro.au ___________________________________________________________________________ Discovery of new ore deposits is becoming more difficult as we explore beneath deep cover. Commonly, exploration programs start from geophysical targeting and move straight into drilling, which is expensive and has a low sampling density. Nickel sulfide deposits specifically have little to no hydrothermal footprints and usually have small sulfide targets, therefore, this sampling practice risks missing potential key sulfide intercepts and abandoning fertile ground. Exploring using indicator minerals can give additional information before drilling has commenced to identified prospective areas and can continue to be used in early drilling programs to allow focus on more prospective intrusions. In this study, we develop key chemical signatures within minerals that indicate Ni prospectivity and prove the effectiveness of mineral indicators for use in exploration. Australia hosts one third of the world’s nickel (Ni) deposits and most are located in Western Australia therefore this area was the focus of our study. Comprising 11 detailed case studies from Western Australia and one from South Australia, paired with existing global mineral chemistry data from CSIRO databases, the aim of each case study was to understand the mineral deposit or exploration camp in detail, to provide context for the indicator mineral signatures that were measured. We analysed both komatiitic systems as well as intrusionhosted systems, sampling from both known mineralised and apparently barren examples. Further, we sampled the regolith and cover above these deposits to determine as to whether indicator minerals can survive weathering and transport processes. We analysed spinel minerals (chromite-magnetite), olivine, pyroxene, apatite, ilmenite, and plagioclase for their trace elements using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). This large and robust dataset is ideal for machine learning applications. We used random forest models to distinguish the key trace element contents of each mineral that signifies mineralisation and the confidence of each prediction. Spinel was the largest dataset in this study, with over 7,000 LA-ICP-MS analyses. This large dataset allowed for confident (77%) predictions of mineralised vs non-mineralised occurrences using the machine learning models. The key elements underpinning these predictions were Co, Ga, V, Ni, and Cr. Using the trace element data, it may also be possible to predict the volume of sulfide associated with an individual spinel grain. This has implications for vectoring toward larger and more economic deposits. Analysis of the cover and regolith showed that chromite is not significantly affected by weathering. A study of the Black Swan nickel mine in Western Australia shows that the trace element contents in spinel are consistent across the talc-altered, serpentine-altered, and fresh examples of the komatiite. This suggests that the spinel family would be a robust resistate indicator mineral for Ni exploration. Olivine is this database's next largest mineral collection, with over 1,400 LA-ICP-MS analyses. Using the machine learning models, the trace elements in olivine can be used to accurately (95%) predict �77 that the host intrusion was mineralised; however, the unmineralised category has poorer recall (60%), which suggests a greater likelihood of false positive predictions. Pyroxene can be examined for trace element (Cr-Ti) variation within grains to understand if the intrusion has the potential to be a conduit. Although not a direct indicator of sulfide presence, it can indicate the potential for high-R factors and, therefore, a metal-rich sulfide (if sulfide saturation has occurred). Minerals such as olivine, pyroxene, and plagioclase do not survive weathering and are not considered resistant indicator minerals. However, they can still be analysed in fresh rock to assess as to whether the subject intrusion has potential to host Ni-sulfide orebodies. The other minerals (apatite, ilmenite, and plagioclase) have less than 1,000 LA-ICP-MS analyses for each phase in this database. Although they show promise in being robust indicator minerals, a larger training dataset should be accumulated before their use in exploration. Ilmenite specifically was found to be the most common mineral in heavy mineral concentrates and is easily separated with a magnet (figure 1). The trace elements in ilmenite show confident predictions for prospectivity, however, the database needs to be expanded to develop ilmenite as an additional resistant indicator mineral. In this project, we have developed analysis and data-handling workflows, and machine-learning models for Ni-sulfide exploration. Although these models were primarily developed using Western Australian case studies, these exploration tools are applicable globally. Figure 11: Magnetic and heavy liquid separation from the same stream sediment sample, A) magnetically separated; B) heavy liquid separation. The heavy liquid separation was carried out on the remaining fraction after magnetic separation. �78 Apatite as an indicator for volatile involvement in the genesis of the Marathon Cu-PGE deposit, northwestern Ontario Shahabi Far, M.1, Good, D.2 and Samson, I3 1 Department of Earth Sciences, Carleton University, Ottawa, ON (maryam.shahabifar@carleton.ca) Department of Earth Sciences, Western University, London, ON 3 Department of Earth and Environmental Sciences, University of Windsor, ON 2 ___________________________________________________________________________ The Marathon Cu-PGE deposit of the Mesoproterozoic (1106 ± 1 Ma) Coldwell alkaline complex contains three types of mineralization with different textural, mineralogical, and geochemical characteristics: Footwall Zone, Main Zone, and W-Horizon. The relative roles of volatiles in metal enrichment in this deposit remain a point of debate. In this study, the significance of hydrothermal fluids in directly precipitating ore minerals or causing their later modification using the texture and composition of apatite is investigated. The textural relationships of apatite with other minerals indicate two types of apatite generation: early apatite and late apatite. Early apatite crystals are homogeneous with no textural or chemical zoning. Late apatite crystals exhibit diverse zoning patterns including oscillatory zoning, patchy zoning, and replacement textures (Fig. 1). The zoning in apatite is associated with Si and rare earth elements (REE) changes. Late apatite grains reveal replacement zones along crystal rims as well as around cracks containing monazite and/or allanite inclusions; this feature will be referred to as replacement apatite in this study (Fig. 1). The earlier apatite grains that show replacement zones are referred to as late metasomatized apatite. The overall decrease in Cl/F ratios of the late apatite from the Footwall to the W Horizon (Fig. 2) can be explained by magma degassing similar to the suggested model for the Bushveld and Stillwater complexes [1][2][3]. Primary fluid and monazite inclusions in the replacement rims of the metasomatized late apatite associated with hydrous minerals can be interpreted to have resulted from the interaction of volatiles with the late-stage gabbroic melts. Experimental studies indicate that monazite and other REE-minerals can be formed as a result of fluid-induced coupled dissolutionreprecipitation processes [4] via fluorapatite interaction with H2O, 40/60 CO2/H2O, and KCl brine [5][6]. Given that the metasomatized late apatite has an overall higher Cl/F ratio compared to the other apatite grains (Fig. 2), the fluid must have been Cl-rich. The metasomatized late apatite and their replacement rims with monazite inclusions are usually associated with residual hydrous melt aggregates and are more abundant in W Horizon. This indicates that late-stage hydrous melts and associated exsolved fluids are more abundant in the W Horizon than in the other two zones. The ubiquitous presence of hydrothermal alteration around the residual hydrous melt aggregates certainly indicates that a hydrous fluid exsolved from the late-stage melts. The presence of hydrothermal carbonate and epidote in the late assemblages as well as the presence of carbonate as an alteration of apatite in the replacement rims indicates that fluid also must have contained CO2 or other carbonic species. Given that sulfide minerals in the W Horizon mostly occur in association with biotite and hornblende as either interstitial coarse crystals or interstitial phase in the residual hydrous melt aggregates, the Cl- carbonic-enriched volatiles exsolved from late-stage magma must have been played a critical role for PGE-enrichment in the W Horizon. Allanite as either inclusions, filling voids or cracks, or along the rims of late metasomatized apatite or independent grains are much coarser grains compared to monazite (Fig. 1) suggesting that the early nucleated monazite must have interacted with later possibly more NaCl or CaCl2-rich fluid reacted with the surrounding silicate rocks to form allanite [5][6][7][8]. This is consistent with elevated Cl contents of alteration products (amphibole with up to 3.9 wt% Cl) associated with metasomatized �79 late apatite with higher Cl content and suggests that the late-stage hydrothermal fluid was Clenriched. The occurrence of allanite in the Footwall Zone and Main Zone but rare occurrence in the W Horizon indicates that the late-stage fluid infiltration must have been less dominant in the W Horizon. This is consistent with relatively fewer secondary hydrous minerals in the W Horizon. High metal contents of the replacement rims of apatite in the Footwall Zone and their association with chalcopyrite indicate that metals and S were mobilized by these volatiles. Much of the chalcopyrite in the Main Zone has replaced pyrrhotite and is intergrown with hydrous silicate minerals, which also suggests that Cu was introduced into the system, presumably by volatiles. This observation can be explained by a process in which volatiles fluxed through the Footwall Zone and transported Cu to the Main Zone. Replacement of pyrrhotite by chalcopyrite in the Main Zone and associated Cu metasomatism must have occurred after pyrrhotite crystallization in the Main Zone suggesting Cu remobilized with later-stage hydrothermal fluid. Chalcopyrite inclusions occurrence within voids in the replacement zones of apatite as well as along the cracks within apatite where allanite occurs, could suggests that this fluid could be the Cl-rich hydrothermal fluid that is responsible for the allanite formation. The sources of these late-stage volatiles are not constrained yet, although one possibility could be the devolatilization of the Archean country rocks. a b c Aln d Metasomatized late apatite Replacement rim e Mnz Aln f Ap Aln Fig. 1: Back-scattered electron images (BSE) showing diverse zoning and textures in the late apatite: a) oscillatory zoning with Si and REE changes between zones, b) patchy zoning of late apatite from W Horizon showing difference is carbon concentration between the zones, c) allanite filling the cracks and voids within apatite, d) metasomatized late apatite showing replacement zones around the rims and along cracks, e) zoomed-in image from red box on image c showing monazite inclusions within the replacement zone, f) allanite as overgrowth rim of apatite. Aln: alanite, Ap: apatite, Mnz: monazite. �80 2.0 2.0 Metasomatized Late apatite Metasomatized Late apatite Replacement rim Replacement rim Late apatite Late apatite apatite Early Early apatite 1.5 Cl/F Cl/F 1.5 1.0 1.0 0.5 0.5 0.0 Footwall Zone 0.0 Main Zone W Horizon Fig. 2: Box-whisker plot comparing Cl/F W values of different apatite generations and textures from different part Footwall Zone Main Zone Horizon of the Marathon deposit. The lower, middle, and upper lines in each box represent 25%, median and, and 75% of the data, respectively. The lower whisker represents the 10th percentile and the upper whisker represents the 90th percentile. Circles show outliers. References: [1] Boudreau A and McCallum I (1989) Contrib Mineral Petrol 102:138-153 [2] Boudreau A et al. (1995) Contrib Mineal Petrol 122:289-300 [3] Willmore C et al. (2000) J Petrol 41:1517-1539 [4] Pan Y and Fleet M (2002) Rev in Mineral Geochem 48:13-49 [5] Harlov D and Förster (2003) Amer Miner 88:1209-1229 [6] Spear F (2010) Chem Geol 279:55-62 [7] Budzyń B et al. (2011) Amer Miner 96:1547-1567 [8] Jonsson E et al. (2016) Amer Miner 101:1769-1782 �81 Geochemical and Petrologic Investigation of the Eagle’s Nest Intrusion, McFaulds Lake Greenstone Belt, Ontario, Canada Sheshnev, V.1, Hollings, P.1, Phillips N.J.1, Weston, R.J.2, Deller, M.2 and Campbell, D.2 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1. vsheshne@lakeheadu.ca 2 Wyloo Metals, 1-1127 Premier Way, Thunder Bay, Ontario, P7B 0A3. ___________________________________________________________________________ The Eagle’s Nest orthomagmatic Ni-Cu-(PGE) deposit is situated in the northern portion of the Superior Province within the McFaulds Lake greenstone belt, approximately 500km northeast of Thunder Bay, Ontario. The deposit contains 11.1 million tonnes of proven and probable reserves grading 1.68% Ni, 0.87% Cu, 0.89g/t Pt, 3.09g/t Pd and 0.18g/t Au [1]. The Eagle’s Nest intrusion is associated with the mafic-ultramafic magmatism of the Ring of Fire intrusive suite between 2736 and 2732 Ma and is part of the ultramafic-dominated Koper Lake subsuite [2,3]. The Eagle’s Nest intrusion was emplaced along a sub-horizontal conduit, forming a blade-shaped dike [4]. Mineralization is consistent with gravitational sulfide segregation at the basal, northwestern contact of the intrusion. A post emplacement, regional deformation event, rotated the intrusion into its present day, subvertical orientation, with a width of ~500m, thickness of ~150m and vertical extent >1600m. The mineralized ore body of the Eagle’s Nest intrusion consists of a zoned pyrrhotite – pentlandite – chalcopyrite assemblage with massive sulfide mineralization at the northwestern contact gradationally becoming, net-textured and disseminated to the southeast [5]. Mungall et al. [6] estimated the parental magma to be a low-Mg komatiitic magma with ~22% MgO and ~12% FeOT. More recently, Zuccarelli et al. [5] reported the most magnesian olivine within the mineralized portion of the intrusion is Fo86, which is consistent with a picritic parental magma composition. Contradictions among the estimated parental magma composition and the most magnesian olivine found within the intrusion, require further constraints on the composition of the melt that formed the mineralized system. Geochemical, petrographic, mineral chemistry, and radiogenic isotope techniques, are being used to characterize the unmineralized portions of the Eagle’s Nest intrusion, to characterize the associated chilled dikes in the vicinity of the intrusion, and to constrain the parental magma characteristics that formed the Eagle’s Nest deposit. This will allow for a more holistic approach to determining the primary melt composition. One-hundred and thirty-six samples were collected from drill core. Samples comprise five tonalitic wall-rock samples, 44 mafic-ultramafic chilled dike samples, and 87 intrusion samples. Intrusion samples comprise of mafic-ultramafic lithologies that include peridotite (Fig. 1), gabbro, and units identified as chilled margins of the main intrusion. One-hundred and twenty-one samples were analysed using Inductively Coupled Plasma Atomic Emission Spectroscopy and Inductively Coupled Mass Spectroscopy for major oxides and trace elements. A total of 30 polished thin section were prepared comprising seven peridotite, eight contact, and 15 offshoot dike samples. A total of 20 samples were selected for analysis of Sm-Nd isotopes. Three different approaches are used to evaluate the parental magma composition that formed the Eagle’s Nest intrusion. The first two approaches will examine chilled margins preserved along the length of the intrusion and within the magmatic breccia matrix situated within the hanging-wall of the chonolith. The third approach will examine the chemical composition of olivine grains preserved within the ultramafic lithologies of the intrusion. To further constrain the contamination history and identify primitive melt compositions, Sm-Nd isotope data will also be examined. �82 Figure 1. Photomicrograph of a peridotite sample depicting poikilitic textured orthopyroxene with preserved fresh olivine within the oikocryst surrounded by cumulus serpentinized olivine (XPL: crosspolarized light). References: [1] Burgess et al. (2012) Micon Int Ltd: 197 [2] Metsaranta et al. (2015) Geol Surv of Can Opn File Rep 7856: 61-73 [3] Houlé et al. (2020) Geol Surv of Can Opn File Rep 8722: 141-163 [4] Barnes S.J. and Mungall J.E. (2018) Econ Geol 113: 789-798 [5] Zuccarelli et al. (2022) Econ Geol 117(8): 1731-1759 [6] Mungall et al. (2010) Soc of Econ Geol Sp Pub 15: 539-557 �83 Reconstitution of the Merensky Reef footwall during chamber replenishment Smith, W.D.1,2, Henry, H.3, Maier, W.D.4, Muir, D.D.4, Heinonen, J.S.5,6, Andersen, J.Ø7 1 Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada CSIRO Mineral Resources, 26 Dick Perry Avenue, Kensington, Perth, WA 6151, Australia 3 Géosciences Environment Toulouse, Université de Toulouse III Paul Sabatier, 14 Avenue E. Belin, 31400 Toulouse, France 4 School of Earth & Environmental Sciences, Cardiff University, United Kingdom, CF10 3AT 5 Department of Geosciences and Geography, University of Helsinki, P.O. Box 64, 00014, Helsinki, Finland 6 Geology and Mineralogy, Åbo Akademi University, Akademigatan 1, 20500 Åbo, Finland 7 Camborne School of Mines, University of Exeter, Penryn, United Kingdom, TR10 9EZ 2 __________________________________________________________________________ The Merensky Reef of the Bushveld Complex was discovered in 1924 by Hans Merensky, whilst following up on platinum-group mineral concentrates that Andries Lombaard had panned from a stream in the eastern Bushveld Complex [1]. This discovery was to be significant, and the aptly named Merensky Reef was the focus of intense scientific research for the ensuing 100 years, providing insight into the formation of reef-style platinum-group element occurrences in layered mafic-ultramafic intrusions. However, many aspects of the petrogenesis of such reef-style occurrences remain debated despite a century of investigations. The layered mafic-ultramafic rocks of the 2.056 Ga Bushveld Complex are together known as the Rustenburg Layered Suite, which itself has been divided into five stratigraphic units, including the Marginal, Lower, Critical, Main, and Upper Zones [2]. The Merensky Reef occurs in the Upper Critical Zone, which predominantly consists of interlayered norite, anorthosite, chromitite, and orthopyroxenite [3]. Several researchers have proposed that the Merensky Reef marks a regional unconformity that formed when preexisting semicrystalline cumulates (i.e., resident cumulates) interacted with relatively primitive melt that replenished the overlying melt column [4,5]. This replenishment event is believed to have thermally- and (or) chemically-eroded the resident cumulates, leading to the development of the Merensky Reef stratigraphy and its world-class platinum-group element mineralization. This study represents a detailed investigation of the Merensky Reef footwall at the Rustenburg Platinum Mine in the western lobe of the Bushveld Complex. At this location, the Merensky Reef is a single layer of coarse-grained orthopyroxenite that is bracketed by mm-scale chromitite seams. These units are underlain by a cm-scale anorthosite that in-turn is underlain by leuconorite. We have employed electron probe microanalysis and electron back-scatter diffraction to characterize changes in the footwall rocks with proximity to the reef and thermodynamic simulations using Magma Chamber Simulator to constrain the effect chamber replenishment may have on different resident cumulates. The leuconorite hosts normally zoned orthopyroxene crystals with poikilitic overgrowths and cumulus plagioclase crystals that define a non-random fabric consistent with gravitational settling in a quiescent melt. The anorthosite consists of variably zoned cumulus plagioclase crystals that are traversed by sub-vertical domains of sulfides, pyroxenes, and accessory phases. These plagioclase crystals record a non-random fabric that strengthens with proximity to the reef, and it is proposed to have formed by gravitational settling followed by the removal of phases in the plagioclase interstices. The contact between the leuconorite and anorthosite is marked by features that are consistent with trapped liquid shift, such as a relatively increased abundance of intercumulus phases and relatively low orthopyroxene molar Mg/(Mg+Fe)] values. Very fine-grained chromite crystals are concentrated at the margins of orthopyroxene crystals in the leuconorite, but practically vanish in the overlying anorthosite where they occur only sparsely in the sub-vertical domains. The lower chromitite shares a knife-sharp contact with the underlying anorthosite. The lower chromitite comprises both amoeboidal and blocky chromite crystals [6], that display no spatial preference (i.e., host grain, stratigraphic location) nor any statistically significant chemical differences. The key difference �84 between the two chromite forms is that amoeboidal crystals host greater degrees of internal misorientation as well as abundant polymineralic inclusions. Thermodynamic simulations show that anorthosite residues, amongst other lithologies, may form as replenishing melts react with noritic cumulates. The initial modelled footwall melts assimilated by the replenishing melt are relatively volatile-rich and become Cr-bearing once resident cumulus orthopyroxene is consumed [7]. It is proposed that chamber replenishment triggered the reconstitution of resident noritic cumulates to anorthosite residues (Fig. 1A-B). The replenishing melt was likely saturated in chromite and sulfide melt, whereby skeletal chromite precipitated close to the melt-cumulate interface. The porosity generated in the footwall facilitated the downward percolation of sulfide melt that in turn helped to displace trapped silicate melts upward to the level of the proto-reef (Fig. 1C-D). The initially relatively volatile-rich footwall melts triggered dissolutionreprecipitation of skeletal to amoeboidal chromite, and the chromitite grew as auxiliary Cr3+ and Al3+ was liberated from the footwall. Figure 1. Petrogenetic model for replenishment-driven footwall reconstitution at the Rustenburg Platinum Mine. A. Deposition of leuconoritic (orthopyroxene = opx + plagioclase = pl) cumulates by gravitational settling of silicates in a quiescent melt. B. Basal influx of relatively primitive melt that entrains blocky chromite (cr) and sulfide (sul) melt. Skeletal chromite crystals form by supercooling close to the base of the replenishing melt and reconstitution of resident leuconoritic cumulates begins. C. Footwall melts are initially volatile-bearing and Cr-undersaturated (light blue arrows), triggering dissolution-reprecipitation of skeletal chromitites to form amoeboidal chromites. D. The footwall melts become Cr-saturated (green arrows) as orthopyroxene and accessory chromite are consumed. This leads to further chromite precipitation and the formation of the lower chromitite. These footwall melts are displaced upwards by down-going sulfide melts, which may also instigate coarsening of plagioclase and orthopyroxene oikocrysts. Black arrows to the side of diagrams denote the lithology. References: [1] Cawthorn RG (1999) S. Afr. J. Geol. 102(3):178-183 [2] Cawthorn RG (2015) In:Layered Intrusions pp. 517-587 [3] Cameron EN (1982) Econ Geol 77:1307-1327 [4] Viring RG and Cowell MW (1999) S. Afr. J. Geol. 102:192-208 [5] Roberts MD et al. (2007) Min Dep 79:169-186 [6] Vukmanovic Z et al. (2013) Contrib Min Pet 165:1031-1050 [7] Scoon RN and Costin G (2018) J. Pet. 59(8):1551-1578 �85 Future research areas to aid in exploration for Ni sulfides Sproule, R.A.1 1 Rio Tinto Exploration, Salt Lake City, UT, USA ___________________________________________________________________________ Discovery rates for magmatic nickel sulphide deposits have declined over the last thirty years and particularly over the last ten years. We are not discovering a sufficient number of high-quality low carbon footprint nickel sulphide deposits in a timely manner to meet society’s needs. Exploration is moderately successful at the deposit scale in a fertile intrusion and after initial discovery of sulfides. This is largely determined by the effectiveness of detection of conductive sulphides by EM technologies in massive-dominated NiS deposits, or the generally large footprint (e.g., magnetics, gravity, surface geochemistry) of large disseminated NiS deposits amenable to open pit mining. However, exploration struggles to identify new fertile lithospheric regions, new favourable terranes and potential camps. We also lack fundamental detailed understandings on the relationship and timing of nickel sulfide deposits to tectonic cycles, and the processes that form, enrich and accumulate sulfides. This is particularly true when we consider the full range of prospective parental magma compositions and host rock lithologies over the complete range of crustal levels. Moreover, both research and exploration activity have also largely focussed on magmatic Ni systems to the relative detriment of other types of important NiS deposits including sediment-hosted (e.g., Enterprise, Zambia) and hydrothermal types (e.g., Jaguar, Brazil). At present, our knowledge can be improved by developing: (1) an improved understanding of fertile lithospheric regions; (2) other geological environments conducive to forming Tier 1 NiS deposits; (3) detailed 3D nickel sulphide ore deposit models and footprints (geology, geophysics, geochemistry and mineralogy) for mineralized systems from a range of parental magma compositions, crustal depths and a range of tectonic settings. �86 Exploring the footwall: Sulfide Mineralization in the footwall Granite of the Maturi Deposit, Minnesota. Steiner, R. A.1 1 Big Rock Exploration, 2505 W Superior Street, Duluth MN 55806. alex@bigrockexploration.com ___________________________________________________________________________ The 1.1 Ga Keweenawan large igneous province generated voluminous magmatism resulting in the eruption of extensive flood basalts and the emplacement of sub-volcanic intrusions now exposed along the flanks of Lake Superior [1]. In northeastern Minnesota, two intrusive sequences of the Layered Series, the Partridge River Intrusion (PRI) and South Kawishiwi Intrusion (SKI), are known to host significant Cu-Ni-PGE sulfide mineralization [1]. The Maturi Cu-Ni-PGE deposit is located in the northern part of the SKI where the footwall is composed of granitic rocks of the Giants Range Batholith (GRB). The majority of Cu-Ni-PGE-enriched sulfides are disseminated throughout a 50-150m-thick basal mineralized zone (BMZ), though locally occur as massive to semi-massive sulfide occurrences along the basal contact (Figure 1). The mineralized rocks of the BMZ were emplaced in a series of three crystal-laden troctolitic pulses or stages that are divided on the basis of sulfide metal tenor, whole rock composition, and textural variations detailed in Peterson [2] (Figure 1). The first pulse, Stage 1, is sulfide poor and begins to delaminate the overlying anorthosite rocks from the footwall. Stage 2 contains abundant country rock xenoliths and more sulfide droplets that are carried within the crystal slurry and those sulfides are higher Cu, Ni, and PGE tenors than the prior Stage 1. Stage 3 is yet more enriched in metals, with the highest metal tenors found there and is also the most mafic pulse, often containing melatroctolite or sub-dunite horizons. Stages 2 and 3 are broadly emplaced above prior pulses, but locally erode down into the previous pulse in areas of channelized magma flow and may erode down to the granite below. Enigmatically, the underlying granite commonly hosts magmatic sulfide mineralization. That mineralization may occur as massive Ni-rich sulfide at the intrusion contact or extend as deep as 100 meters below the basal contact as Cu-rich sulfides (Figure 1). Sulfur isotope data show that the sulfide in the mineralized granite originated from the same source as that in the overlying troctolite [3, 4]. Here we present a mechanism by which melting and density-driven displacement drives magmatic Cu-Ni-PGE sulfide mineralization into the footwall granite of the Maturi deposit. Three of the drill cores were selected from the Maturi deposit that represent all three stages in contact with the underlying footwall granite [2]. Core logging and subsequent petrographic observations show that the granite reached pyroxene hornfels grade metamorphism and underwent partial melting due to thermal input from the overlying intrusion (Figure 2). Abundant leucosomes and sieve textured feldspars with trapped silicate melt record pervasive melting in the GRB. Leucosome patches and feldspar sieves have been observed to contain massive to semi-massive sulfide suggesting a relationship between location of partial melts and sulfide liquid, perhaps physical displacement of the former by the latter (Figure 3). Mass-balance equations using the isocon method of Grant [5] were used to explore the geochemical parameters to provide insight into the relationship of partial melts and sulfide liquid. When elements that partition into pyroxene (Cr, Mg, Mn) are treated as restite (not removed or added to the original �87 lithology) it becomes clear that an exchange of sulfide for partial melt is occurring (Figure 4). Elements that would partition into the silicate liquid during melting (REE, LIL, K, Ba) become depleted relative to the restite while components of the sulfide (S, Ni, Cu) become enriched. Samples of the footwall with the strongest sulfide mineralization show the strongest depletion of partial melt elements and the strongest enrichment of sulfide liquid components. The face that sulfide liquid and partial melts occupy the same textural space within the rock (e.g., leucosome patches between restite phases and sieve texture in plagioclase) and the geochemical signature showing the removal of partial melt components and addition of sulfide liquid components leads to the conclusion that mineralization in the footwall of the Maturi deposit is caused by the displacement of partial melt for a denser sulfide liquid. Such a process should not only result in mineralization of the footwall but also contamination of the overlying intrusion by partial melts. White [6] identified geochemical markers for contamination of the overlying BMZ by the footwall rocks, which became more intense in proximity to the footwall contact. This study finds abundant as networks and pods of partial melts throughout the GRB. Therefore, it is reasonable to assume that the amount of liquid displacement that can occur is limited by the amount of sulfide liquid available to penetrate the footwall. While there is large reservoir of sulfide present as the disseminated sulfides in the intrusions, that amount of that sulfide that may interact with the footwall interface is unclear. However, contamination of the silicate magma in the vicinity of the footwall rocks would reduce the sulfur carrying capacity in a magma that is already sulfur saturated thus providing an additional sulfide liquid reservoir to displace partial melts in the GRB. The formation of such a reservoir is evidenced by Ni-rich massive sulfide occurrences at the footwall contact intercepted during drilling. It is notable that the majority of the massive sulfide occurrences are found where the footwall is in contact with Stage 3; this being the latest mineralizing pulse would therefore introduce the greatest heat budget to the footwall rocks (Figure 1). It is below these locations that partial melting and footwall mineralization is most intense. By understanding both the emplacement sequence and mechanism of mineralized intrusions it is possible to constrain the focusing of heat into the country rock. Such constraints provide insight into targeting basal accumulations of sulfide within intrusions as well as unconventional mineralization hosted within the country rocks. Figure 1 – cartoon cross-section of the basal mineralized zone at Maturi highlighting areas on footwall mineralization below stages 2 and 3. �88 Figure 2 – partial melt pocket or leucosome surrounding remnant feldspar grains with orthopyroxene found in the melt (left). Melt pockets inside of feldspar grain resulting in sieve texture. Figure 3 – net-textured partial melt + pyroxene surrounding remnant feldspar and pyroxene. Arrow indicates sulfide that surrounds pyroxene in the same manner as partial melts elsewhere in the section. �89 Figure 4 – example isocon plot where the isocon is a best-fit line for MgO, MnO, and Cr2O3. The green field indicates components that are enriched relative to the isocon while the red field indicates depletion. References: [1] Miller, J.D. Jr. et al (2002) Minnesota Geological Survey Report of Investigations 58 [2] Peterson D.M. (2012), Duluth Metals ltd Presentation to Twin Metals Minnesota LLC [3] Ripley, E. M. and Alawi, J. A. (1986) Canadian Mineralogist 24:347-368 [4] Molnar, F. et al., (2009) Geological Society of America Abstract [5] Grant, J. A. (1986) Economic Geology 81:1976-1982 [6] White, C. R. (2010) MS Thesis University of Minnesota Duluth �90 The Anatomy of a Cu-Ni-Co-PGE Mineralized Mafic Magmatic System: The South Kawishiwi Intrusion of the Duluth Complex, Northeastern Minnesota Sweet, G.S.1 and Peterson, D.M.2 1,2 Big Rock Exploration, 2505 West Superior Street, Duluth MN, 55806, gabe@bigrockexploration.com ___________________________________________________________________________ In 1977, the Minnesota Department of Natural Resources published the first district-scale gradetonnage estimate [1] of Cu-Ni and TiO2 along the western margin of the Duluth Complex. These estimates, which utilized 324 of the 903 holes drilled through 1976 (285,902 meters), included 4.4 billion tons at 0.66% Cu and 0.2% Ni as well as 220 million tons at >10% TiO2 and brought to light the potential world-class scale of the Duluth Complex mafic magmatic system. Since the 1977 gradetonnage estimate, approximately 1,993 new exploration holes totaling over 802,360 meters have been drilled in the Duluth Complex area by a number of companies and the State of Minnesota. The physical formation processes of sulfide-bearing mafic intrusions remains one of the most important concepts for geologists engaged in exploring mafic magmatic systems for ore deposits. It is critically important to understand that the delivery of sulfide-bearing and potentially crystal-laden magmas into a growing intrusion is an iterative process confined to the spatial geometry of the system. The delivered magma will change with time (intrusion rate, crystallinity, xenolith content, sulfide content & tenor) and early batches of crystallizing magma are commonly cut and eroded by subsequent magmas (with their own unique intrusion rate, crystallinity, sulfide content & tenor). This work describes a new synthesis of decades of detailed mapping (>30,000 outcrops mapped), exploration and definition drilling (787,908 meters of core in 1899 holes), geochemistry (101,882 drill core and 8,267 surface sample analyses), geophysical surveying, and modeling by the authors and others in the South Kawishiwi Intrusion (SKI) and its Nickel Lake Macrodike (NLM) feeder dike. The outcomes of this new synthesis can perhaps be used as a proxy from which geologists can explore other mafic magmatic systems across the globe. The SKI is a shallow dipping (~24º east-southeast) sill-like troctolitic intrusion exposed in a 10- x 32kilometer arcuate band along the northwestern margin of the Duluth Complex. It extends from the edge of the Mesaba deposit (which is within the adjacent Partridge River and Bathtub intrusions) on the southwest, to the Spruce Road deposit on the northeast (Fig. 1). The SKI initially intruded between a hangingwall of the Duluth Complex Anorthositic Series rocks and a footwall composed of Paleoproterozoic sedimentary rocks, i.e., the Virginia Formation (VF) and Biwabik Iron Formation (BIF) in the southwest, and exclusively granitoid rocks of the Archean Giants Range Batholith in the northeast. The local presence of xenoliths of the BIF and VF as inclusions within the northern SKI and the NLM are interpreted as far-traveled country-rock blocks and not, as Severson et al. [2] interpreted, Paleoproterozoic sedimentary units assimilated in-situ from the immediate footwall during emplacement of the SKI. The basal stratigraphic section of the SKI was first described in great detail by Severson [3] and culminated with the SKI igneous stratigraphy being subdivided into 17 different units. In 2008, geologists from Duluth Metals Limited came to the realization that the contact-type mineralization at the Maturi deposit formed from initial basaltic composition SKI magmas that intruded as sulfidebearing, crystal-laden (plagioclase & olivine), magmatic slurries. Based on this interpretation, the company reinterpreted the sulfide-bearing basal zone of the SKI at the Maturi deposit into the Basal Mineralized Zone, or BMZ. This new interpretation was based on the geometry of the system (silllike sub-horizontal intrusion) and the inherent crystallinity of the SKI magmas. The channelized flow of these phenocryst-rich magmas led to crystal sorting and melting of the footwall granitic rocks to create the heterogeneous lithologies and textures of the BMZ. Years of detailed geological mapping, integrated with geological logging of all available drill holes, and a comprehensive assembly and interpretation of all geochemical data has led to a simplified overall igneous stratigraphy of the �91 intrusion. This stratigraphy has been subdivided into five basic units, including the Upper SKI, the SKI Break, the Middle SKI, the Main AGT, and the BMZ (Figure 2). Figure 1. Bedrock geologic map of the South Kawishiwi Intrusion and surrounding terranes. Yellow outlines define the approximate boundaries of compliant NI 43-101 resource estimates of the labeled Cu-Ni-Co-PGE deposits. In 2012, and after much additional drilling, the geology of the Maturi deposit BMZ was reevaluated once again by the geologic staff of Duluth Metals Limited, Twin Metals Minnesota, and geologists from the consulting firm AMEC. The reanalysis utilized a significant volume of new, high-quality geochemical and geological data to complete an updated mineral resource classification by AMEC. Mineralization in both the BMZ and footwall at the Maturi deposit area were reclassified based on patterns in the physical distribution of mineralization as projected on down-hole plots. Sulfide mineralization at Maturi is characterized by several distinct patterns, including A) very low grade, �92 fine-grained intervals showing low variability (Stage 1) that probably represent initial chilled magmas, B) moderate Cu-Ni and low PGE grade, xenolith-bearing (BIF, VF, basalt & anorthosite), mineralized zones showing low variability (Stage 2), and C) clean, higher grade, (Cu-Ni and PGE), xenolith-poor mineralized troctolite zones with higher variability and commonly bounded by low grade selvages (Stage 3). Significantly, most of the contacts between different mineralized intervals are typically quite abrupt. A single hole might contain one or several distinct mineralized intervals within the BMZ, including higher grade intervals with the highestgrade intervals occurring at the top, middle, or bottom of the section. Based on these criteria, four intrusive subunits, characterized by common grade profiles, were defined in the BMZ. In addition, two distinct suites of mineralization were identified in the footwall rocks, including NiCo enriched semi-massive to massive sulfide zones and disseminated Cu-PGE enriched zones deep in the footwall granitoids. All the newly classified zones of the BMZ at the Maturi Deposit are shown stratigraphically in Figure 3 and diagrammatically in Figure 4. Figure 2. Simplified igneous stratigraphy of the SKI. Figure 3. Revised igneous stratigraphy of the BMZ and adjacent rocks within the Maturi deposit. The classifications derived from this exercise were validated by multivariate statistical analysis of geochemical data, including principal component analysis and factor analysis. This investigation revealed distinct geochemical fingerprints of mineralization within the BMZ as well as several possible subdivisions of the BMZ based on both the physical distribution patterns of mineralization and the geochemistry of the host rocks. The Maturi subunits defined and validated were determined to occur in a consistent stratigraphic order and are correlative across the deposit. The current lithostratigraphic model for Maturi effectively discriminates between higher- and lowergrade SKI mineralization and provides a realistic geological model for mineralization throughout the intrusion. The new data allowed correlation of units from hole-to-hole and section-to-section �93 resulting in a very robust geologic model upon which Twin Metals Minnesota is building preliminary mine plans. Figure 4. Detailed idealized view of the BMZ intrusive stages at the Maturi Cu-Ni-PGE deposit. A fundamental aspect of the ever-developing ore deposit model of the SKI is an understanding of the initial conditions of the magmatic system – its crystallinity, sulfur capacity, geochemistry, and geometry – and how the sulfur saturated SKI magma lived, worked, and died. Such understanding includes the realization that the magma was a crystal-liquid (silicate and sulfide liquids) slurry and the identification of magma channel ways and sub-channels and their associated thermal anomalies. In addition, the SKI magmas locally melted the footwall granitoid rocks, and the addition of SiO2 into the sulfide-bearing troctolitic melts of the SKI induced additional sulfide immiscibility, stripping the melts of dissolved Ni and Co and forming high-grade massive sulfide ores locally at the basal contact and within the highly metamorphosed footwall Archean granitoids. In the end, hard work and intellectual geologic thought has been used to identify and understand one of the world’s largest resources of Cu-Ni-PGEs (Table 1). Table 1. Grade-Tonnage tabulation for deposits of the SKI. References [1] Listerud W and Meineke D (1977) MNDNR Report 93: 1-74 [2] Severson M et al. (2002) MGS RI 58: 164-200 [3] Severson, M (1994) NRRI TR 93/94: 1-210 �94 Multi-thermochronological records of cooling, denudation and preservation of ancient ultrabasic magmatic ore deposits: An example from the Neoproterozoic Jinchuan giant magmatic Cu-Ni sulfide deposit Ni Tao1,2*, Jiangang Jiao1, Jun Duan1, Haiqing Yan1, Ruohong Jiao3, Hanjie Wen1 1 Department of Geology, Northwest University, Xi’an, China, ni.tao@chd.edu.cn School of Earth Science and Resources, Chang'an University, Xi’an, China 3 School of Earth and Ocean Sciences, University of Victoria, Victoria, Canada 2 ___________________________________________________________________________ The post-mineralization denudation history and preservation of ore deposits have significant scientific and practical implications for ore deposit preservation condition, ore-forming potential evaluation, and deep ore prospecting. Ancient Cu-Ni sulfide ore deposits are characterized by complex magmatic evolution and a long-term geological history. How to quantify their denudation degree and emplacement depth is currently the focus and challenge of ore deposit preservation research. This study strategically chooses the Jinchuan giant magmatic Cu-Ni sulfide deposit as an example, with the Neoproterozoic ore-bearing plagioclase lherzolite as the main target, combined with its Paleoproterozoic metamorphic country rocks and early Paleozoic diorite veins for comparison. Multi-thermochronological analyses applied include apatite and zircon (U-Th)/He dating, apatite fission-track analysis, plagioclase and hornblende 40Ar/39Ar dating. The aims are to trace the thermal history of the ore-bearing intrusion, calculate its denudation thickness by integrating regional geological records, set up inversion models for verifying the calculated denudation thickness as well as determining emplacement depth of the ore-bearing intrusion. On this basis, by judging the relationship between the denudation thickness and the emplacement depth of the ore-bearing intrusion, this study clarifies the preservation degree of Jinchuan Cu-Ni sulfide deposit. The results may provide a new thermochronological paradigm for studying the preservation conditions and evaluating deep ore exploration potential of (ancient) ultrabasic Cu-Ni sulfide magmatic ore deposits. �95 Compositional variability in olivine: New data on the occurrences of Ni and Co as guides to mineral prospectivity Thakurta, J.1, Wagner, Z.1, Nagurney, A.2, and Schaef, H.T. 2 1 Natural Resources Research Institute, University of Minnesota, 5013 Miller Trunk Highway, Hermantown, MN 55811, USA 2 Pacific Northwest National Laboratory, 902 Battelle Boulevard, P.O. Box 999, Richland, WA 99352 USA ___________________________________________________________________________ Concentrations of trace constituents in olivine have been measured from a wide variety of maficultramafic intrusive and volcanic igneous rocks in different tectonic settings in North America. Samples include rocks from different locations of the 1.1 Ga old Midcontinent Rift System (MRS), such as the layered Duluth gabbroic Complex in Minnesota, and the peridotitic intrusions at Eagle in Michigan and Tamarack in Minnesota. The Cretaceous to Jurassic Dunite-peridotite rocks from Red Mountain Ultramafic Complex at the Kenai Chrome mine, the Eklutna ultramafic rocks, and the Alaskan-type ultramafic complex at Duke Island in Alaska represent small intrusive bodies in convergent tectonic settings. Alkali basalts with olivine phenocrysts from the Springville volcanic suite in Arizona constitute Pleistocene volcanic rocks. While the content of Ni is inversely correlated with the presence of sulfide minerals in the assemblages, a larger and more significant variation has been observed with respect to the origin, modes of occurrence and tectonic settings of the rocks in this study. Considerable variations are observed in different intrusions of the Duluth Complex in terms of the nature of the host rock: whether olivine gabbro or troctolite. Ni in the olivine gabbro ranges between 1800 and 2000 ppm while in the representative troctolite units it ranges between 700 to 900 ppm. Very high contents of Ni in olivine, ranging from 2000 to 2700 ppm are seen in small peridotitic intrusive bodies at the MRS such as feldspathic peridotite in Eagle, the Bowl and Fine Grained Olivine (FGO) intrusions of Tamarack. The dunite-peridotite at Kenai and Eklutna show comparable high values but values in the olivine clinopyroxenite unit of the Alaskan-type Complex at Duke Island are less than 800 ppm. A substantial range in Ni-content of olivine from 500 to almost 2500 ppm is observed in the olivine basalt at the Springville Volcanic Suite where individual phenocrysts of olivine show growth rims of changing Ni-content from core to rim (Figure 1). The Co-content of olivine in the olivine gabbro and troctolite units of the Duluth complex range from 300 to 400 ppm and 400 to 600 ppm respectively. Samples from Eagle, Tamarack as well as Duke Island cluster between 300 and 400 ppm. However, the dunite-peridotite at Kenai and Eklutna show values less than 250 ppm. From the new dataset and data available from previous studies [1, 2 and 3] it is evident, that with other factors being similar, Ni shows a positive correlation with the MgO-content while a negative correlation with Co is evident from the new data. Starting with the composition of magma from source rocks, changing fO2 conditions and H2O-content, leading to factors such as liquid evolution by fractional crystallization, assimilation, and re-equilibration of magma with preexisting Ni- and Co-rich rocks, a continuous spectrum of changing concentrations of trace metals in olivine can be envisioned from the available dataset. Such trace metal concentrations in olivine are important not only as indicators of Ni-rich sulfide mineralized zones in the associated rocks, but also as tools to evaluate the possibility of extraction of such critical metals from the ongoing development of new methods of metal-extraction from nonconventional sources such as olivine. �96 Figure 1: Concentrically zoned olivine phenocrysts in an olivine basalt from the Springville Volcanic Field in Arizona. Ni-Co concentrations change along the zones. References: [1] Barnes, J.B. (2023) Am Min 108:1-17 [2] Li, C. and Ripley, E.M. (2010) Chem Geo 275: 99-104 [3] Marek, L., Arevalo, R.D., Puchtel, I.S., Fiorentini, M.L. and Nisbet, E.G. (2019) Am Min 104: 1143-1155 �97 The effects of diagenetic and metamorphic processes on the sulphur liberation from the Virginia Formation black shale during magmatic assimilation by the Duluth Complex, Minnesota, USA Virtanen V.J.1,2, Heinonen J.S.2,3, Märki L. 4, Galvez M.E. 5 and Molnár F.6 1 Institute des Sciences de la Terre d’Orléans (ISTO), CNRS-Université d’Orléans-BRGM, France Department of Geosciences and Geography, University of Helsinki, Helsinki, Finland 3 Geology and Mineralogy, Faculty of Science and Engineering, Åbo Akademi University, Turku, Finland 4 METAS, Federal Institute of Metrology, Bern, Switzerland 5 Department of Earth Sciences, ETH Zürich, Zürich, Switzerland 6 Department of Mineralogy, Institute of Geography and Earth Sciences, Eötvös Loránd University, Budapest, Hungary 2 ___________________________________________________________________________ The Duluth Complex, Minnesota, USA, contains large low-grade disseminated Cu-Ni(-PGE) sulphide resources hosted in troctolites and smaller massive sulphide lenses hosted in norites [1]. Several lines of evidence, including sulphur isotopes, suggest that both deposit types formed by assimilation of sulphur from the Virginia Formation black shale. In the Virginia Formation, sulphur is mainly hosted in micron-scale disseminated pyrite with the exception of the peculiar carbon and sulphur-rich Bedded Pyrrhotite Unit that is characterized by pyrrhotite laminae with mm-scale thickness [1,2]. The Bedded Pyrrhotite Unit has been identified as an important source of sulphur especially to the norite-hosted massive sulphide occurrences [1,2]. However, the processes that caused the carbon and sulphur enrichment in the Bedded Pyrrhotite Unit have not been studied in detail. We used optical and scanning electron microscopy as well as Raman spectroscopy to characterize the normal Virginia Formation black shale and the Bedded Pyrrhotite Unit with emphasis on the carbonaceous materials (CM) and sulphides. Regionally metamorphosed and contactmetamorphosed samples were studied from both units. Whole-rock chemical data was acquired to measure H2O, Corg, and S in the samples. In the normal Virginia Formation, CM is present as uniformly dispersed submicron-scale flakes as typical for buried organic material (Fig. 1a). Raman spectroscopy revealed several defect bands (D1 to D4, see Fig. 1) meaning that the CM is structurally aromatic but turbostratic (i.e., aromatic sheets as in graphite but not in highly organized stacked-sheet structure). Structural ordering of residual CM is a suitable geothermometer as it changes systematically with temperature and it is not subject to retrograde resetting [3,4]. The CM geothermometer of Lahfid et al. [4] indicates that the Virginia Formation reached peak temperature of 300–340 ± 50 °C during regional metamorphism prior to the formation of the Duluth Complex. Figure 12. Reflected-light microphotographs of a) the normal black shale and b) the Bedded Pyrrhotite Unit in the regionally metamorphosed Virginia Formation. Typical Raman spectra of the carbonaceous materials (CM) with structure-related bands (G, D1 to D4) indicated as well as the whole-rock Corg, sulphur (S), and H2O �98 contents are shown. The arrow indicates that CM in b) represents remnants of accumulated oil. Abbreviations: Ab = albite, Ccp = chalcopyrite, Chl = chlorite, Ms = muscovite, Po = pyrrhotite, Py = pyrite, Qz = Quartz. The regionally metamorphosed Bedded Pyrrhotite Unit contains microscale fracture zones enriched in CM and sulphur (Fig. 1b). These zones are characterized by irregularly shaped quartz and sulphide grains that are rotated relative to the bedding (Fig. 1b). Carbonaceous material is found as pore space fillings (Fig. 1b) and as grain coatings suggesting that it represents oil residuals. Raman spectroscopy confirms that the CM in the Bedded Pyrrhotite Unit is structurally different from the CM in the normal black shale (Fig. 1b). Due to the migratory origin of the CM, we cannot reliably apply the geothermometer to the Bedded Pyrrhotite Unit. We suggest that the pore space, which facilitated oil infiltration, formed in the microfracture zones due to dissolution of soluble precursor sedimentary clasts, which are now replaced by quartz and sulphides (Fig. 1b). Pyrrhotite precipitation in diagenetic conditions is kinetically limited, hence the original sulphide in the Bedded Pyrrhotite Unit was probably pyrite (or some typical metastable diagenetic sulphide like greigite). We suggest that the original sulphide was converted to pyrrhotite during low-temperature hydropyrolysis of the CM during regional metamorphism. Whole-rock chemical data shows that the pyrite-bearing normal black shale experienced loss of H2O, Corg, and sulphur due to muscovite and chlorite breakdown as well as pyrite conversion to pyrrhotite caused by the Duluth Complex. The contact-metamorphosed Bedded Pyrrhotite Unit experienced the same metamorphic conditions but shows no systematic depletion of volatiles. In fact, the contact-metamorphosed Bedded Pyrrhotite Unit is the most Corg and sulphur rich part of the Virginia Formation. We suggest that sulphur was conserved through contact metamorphism because of the stability of pyrrhotite during devolatilization as shown in previous experiments [5]. This means that extensive partial melting of the Bedded Pyrrhotite Unit was required to liberate sulphur to the Duluth Complex magma. Consequently, the sulphide occurrences in association with Bedded Pyrrhotite Unit xenoliths are generally in the norites, which show more signs of assimilation Unit compared to the troctolites [1,2]. We also observed that prograde cordierite in the contactmetamorphosed Bedded Pyrrhotite Unit (Fig. 2a) is consistently replaced by biotite and muscovite at the vicinity of the pyrrhotite laminae (Fig. 2b). This indicates retrograde hydration event introduced H2O and possibly Corg and sulphur to the contact-metamorphosed normal black shale. Our findings highlight some key diagenetic and regional metamorphic processes that are important for magmatic ore genesis as they affect the CM and sulphur budget in black shales as well as the reactions that liberate sulphur upon magmatic assimilation. Figure 13. Back-scattered electron images showing a) the prograde mineral assemblage and b) the retrograde mineral assemblage of the contact-metamorphosed Bedded Pyrrhotite Unit. In a) prograde cordierite (crd) is surrounded by K-feldspar (Kfs), whereas in b) small anhedral cordierite is surrounded by retrograde phlogopite (Phl). Abbreviations: Gr = graphite, Pl = plagioclase, Po = pyrrhotite, Qz = quartz. References: [1] Thériault R and Barnes S-J (1998) Can Min 36:869-886 [2] Samalens N et al. (2017) Ore Geol Rev 81:173-187 [3] Beyssac O et al. (2002) J Metamorphic Geol 20:859-871 [4] Lahfid A et al. (2010) Terra Nova 22:354-360 [5] Virtanen V et al. (2021) Nat Commun 12:1-12 �99 Mantle-to-crust scale chemical fractionation and sulphide saturation of the Paleoproterozoic komatiites of the Central Lapland Greenstone Belt, Finland – implications for geochemical exploration Virtanen V.J.1,2, Höytiä H.M.A.2, Iacono-Marziano G.1, Yang S.3, Moilanen M.3 and Törmänen T.4 1 Institut des Sciences de la Terre d’Orléans, UMR 7327, CNRS/Université d’Orléans/BRGM, Orléans, France Department of Geosciences and Geography, University of Helsinki, Helsinki, Finland 3 Oulu Mining School, University of Oulu, Oulu, Finland 4 Geological Survey of Finland, Rovaniemi, Finland 2 ___________________________________________________________________________ In the Central Lapland Greenstone Belt (CLGB) komatiites are present along a >250 km long SE-NW zone across the northern Finland (Fig. 1). The CLGB komatiites formed at ca. 2.05 Ga mostly as underwater eruptions on a sedimentary basin, which is known to have contained abundant sulphurous black shales and evaporites [1]. This association with sulphurous sedimentary rocks makes the CLGB komatiites promising targets for Cu-Ni-PGE sulphide deposits. Indeed, these sedimentary rocks supplied sulphur to the Kevitsa and Sakatti Cu-Ni(-PGE) sulphide deposits (Fig. 1), which formed during the same magmatic event as the CLGB komatiites [1,2]. To understand the petrogenesis of the CLGB komatiites from their mantle source to their crustal sink, we conducted computational thermodynamic simulations to constrain the chemical fractionation and sulphide saturation state without the effects of assimilation. These simulations guide identification of chemical anomalies related to assimilation and sulphide saturation in the CLGB komatiites and related intrusive rocks. Figure 14. Geological map showing the distribution of the Central Lapland Greenstone Belt komatiites. We defined the parental melt of the CLGB komatiites using a chilled margin of a komatiitic dyke from Kevitsa, which represents quenched olivine-saturated melt [3]. We added olivine to the chilled margin composition to reversely fractionate it to be in equilibrium with the most primitive olivine (Fo92) in Sakatti [1]. Using this method, we constrained major element oxides, Ni, Cu, and rare earth elements (REE) for the komatiitic (MgO = 20.6 wt.%) parental melt. Assuming adiabatic propagation through the lithosphere, the parental melt should be compositionally identical to the primary mantle melt and allows constraining the mantle melting conditions. We used REEBOX PRO [4] to define Ti and REE contents as well as temperature of the adiabatically melting mantle source. Several mantle sources and mantle potential temperatures were tested. Consistent with the previous studies related to the mantle source of the CLGB komatiites [3,5,6], we found that pyrolite mantle-source with depleted MORB -type REE contents is suitable. The best fit of Ti, REE, and temperature was reached with the mantle potential temperature of 1575 °C and with degree of melting at 15–20 %. The �100 mantle potential temperature determines that melting starts at ca. 5 GPa and the required degree of melting is reached at ca. 3 GPa (equivalent to ca. 100 km depth). Major element oxide composition of the parental melt (assumed here as identical to the primary mantle melt as noted above) is well compatible with literature data from mantle melting experiments with pyrolite mantle source [7]. We calculated the sulphur content at sulphide saturation (SCSS) for the primary mantle melt using the parental melt composition (major element oxides, Ni, and Cu) and the final pressuretemperature conditions in the mantle using the parameterization of Smythe et al. [8]. This constrains the maximum sulphur content of the primary mantle melt to 1172 ppm. With the typical range of sulphur content for a depleted mantle source of 150–200 ppm [9] and with the degree of mantle melting at 15–20%, the initial sulphur content of the CLGB komatiites is estimated to be 750–1172 ppm. To examine chemical fractionation of the CLGB komatiites in crustal conditions (25 MPa), we conducted closed-system fractional crystallization simulations using Magma Chamber Simulator [10]. For SCSS, we used the same parameterization [7] as with the mantle melting simulations. Using new and literature data [1,2,3,5,6,11,12,13,14], we compiled a comprehensive whole-rock (n = 299–403 depending on the element) and olivine (n = 917) chemistry database for the CLGB komatiites and spatiotemporally related rocks (from Kevitsa and Sakatti) to evaluate the simulation results. We find that closed-system fractional crystallization produces a good fit to the reference data for major element oxides and Ni (Fig. 2a). Importantly, simulated Ni contents in olivine are in good agreement with natural data (Fig. 2a) and could be used to identify Ni-depleted olivine to indicate those CLGB komatiites that experienced early sulphide saturation. Sulphur and Cu show highly incoherent behaviour in the reference data set and were likely affected by sulphide accumulation, degassing, and post-magmatic alteration. However, the simulation results are compatible with literature data for S (Fig. 2b) and Cu from chromite-hosted melt inclusions from the CLGB komatiites [6], which show relatively coherent behaviour compared to the whole-rock data. Depending on the initial S content (750–1172 ppm, see above), our SCSS simulations show that both Ni-rich (Ni/Cu = 1.9) and Cu-rich (Ni/Cu = 0.4) sulphide melt could have formed from the CLGB komatiite melt upon closed-system fractional crystallization (Fig. 2b). Moreover, the simulations indicate that the S content of CLGB komatiite melt was constantly close to SCSS starting from the liquidus (Fig. 2b). Accordingly, assimilation of sulphur-bearing country rocks has the potential to form relatively large sulphide accumulations within this region. Figure 15. Closed-system fractional crystallization simulation results shown on a) MgO (wt.%) vs Ni (ppm) and b) MgO (wt.%) vs. S (ppm) diagrams. The data clouds in a) represent whole-rock and olivine data from the Central Lapland Greenstone Belt (CLGB) komatiites and related rocks (Kevitsa and Sakatti). Sulphur contents in b) are shown only for chromite-hosted melt inclusions from the CLGB komatiites. �101 References: [1] Brownscombe W et al. (2015) Min Dep of Finland 211-252 [2] Luolavirta K et al. (2018) Lithos 296-299:37-53 [3] Puchtel I et al. (2020) Chem Geol 554:1-23 [4] Brown E and Lesher C (2016) Geochem Geophys Geosystems 17:3929-3968 [5] Hanski E et al. (2001) J Pet 42:855-876 [6] Hanski E and Kamenetsky V (2013) Chem Geol 343:25-37 [7] Walter M (1998) J Pet 39:29-60 [8] Smythe D et al. (2017) Am Min 102:795-803 [9] Lorand J-P and Luquet A (2016) Rev Mineral Geochem 81:441-488 [10] Bohrson W et al. (2014) J Pet 55:1685-1717 [11] Luolavirta K et al. (2018) Bull Geol Soc Finland 90:5-32 [12] Patten C et al. (2023) Min Dep 58:461-488 [13] Saverikko M (1985) Bull Geol Soc Finland 57:55-87 [14] Törmänen T et al. (2016) Min Dep 51:411-430 �102 Ni-Cu-PGE prospectivity of the Mackenzie Large Igneous Province Williamson, M.-C.1, Rainbird, R.H.1, O’Driscoll, B.2 and Scoates, J.S.3 1 Geological Survey of Canada, 601 Booth St, Ottawa, ON, K1A 0E8 Canada Email: marie-claude.williamson@nrcan-rncan.gc.ca 2 University of Ottawa, Marion Hall, Ottawa, ON, K1N 6N5 Canada 3 PCIGR, University of British Columbia, 2020-2207 Main Mall, Vancouver, BC, V6T 1Z4 Canada ___________________________________________________________________________ Large igneous provinces (LIPs) are high volume, intraplate magmatic events that consist of flood basalts, gabbro sills and dykes +/- layered intrusions. Most LIPs are emplaced over a time span of ~50 My or less [1], and there is strong evidence that the flood basalt volcanism occurs over even shorter time intervals (<1-2 My). The 1.27 Ga Mackenzie LIP includes flood basalts and feeder dykes of the Coppermine River Group (CRG), the Muskox intrusion and the Mackenzie dyke swarm. Previous studies of the Mackenzie LIP have focused on each of these three elements of the magmatic architecture, which resulted in many geological maps, datasets and samples archived at the GSC’s Earth Materials Facility [2, 3, 4]. We propose to revisit previous work [5] and fill knowledge gaps [6] to produce a regional synthesis of the Mackenzie LIP that specifically highlights Ni-Cu-PGE prospectivity. Knowledge about the Ni-Cu-PGE prospectivity of the Mackenzie LIP is largely based on previous mapping and laboratory studies of the Muskox intrusion and its putative feeder dyke [7, 8]. In contrast, the prospectivity of CRG flood basalts and feeder dykes is unknown. In this presentation, we summarize the methodology and anticipated results of a new GSC project on the Ni-Cu-PGE prospectivity of the Mackenzie LIP. We will adopt a multidisciplinary approach and a different research lens, one that specifically investigates the contact zone(s) and structures between the CRG and the Muskox intrusion. Our objectives are to: (1) fill knowledge gaps on the CRG feeder dykes and marginal rocks of the Muskox intrusion and evaluate the prospectivity of contact zones between intrusions and country rocks; (2) identify channelized lava flows, sills and dykes using remote predictive mapping; and (3) publish a synthesis that will focus specifically on Ni-Cu-PGE prospectivity. Detailed remote predictive mapping of feeder dykes will further our understanding of ore genesis in channelized lava flows, sills, and dykes [9]. Additionally, mineralogical and geochemical studies of picritic lava flows will establish mantle melting temperatures, thus providing constraints on the timing and composition of magma fluxes during the lifetime of the LIP. Another important aspect of studying the picrites is to establish genetic links with the Muskox feeder dyke. Finally, our aim is to reconstruct the timing and duration of magmatism in the Mackenzie LIP and establish links to potential mineralization using high-precision geochronology of the Mackenzie dykes and of the CRG lava flows. The results will increase our knowledge base of Mackenzie LIP architecture, and of the NiCu-PGE prospectivity of the CRG flood basalts and feeder dykes, and of the marginal rocks of the Muskox intrusion. References: [1] Ernst R E and Bleeker W (2010) Can J Earth Sci 47, 695-739 [2] Mackie R A et al. (2009) Precambrian Res 172: 46-66 [3] Skulski T et al. (2018) GSC Open File 8522, 37 p. [4] Williamson M-C et al. (2023) 14th Int Pt Symp: 160-163 [5] Ernst R E et al. (2010) GSC Open File 6016, 14 p. [6] Scoates J S and Scoates R F J (2024) Lithos 474-475: 107560 [7] Hulbert L (2005) GSC Open File 4881 (CD-ROM) [8] Day J M D et al. (2013) Lithos 182-183: 242-258 [9] Lesher M (2019) Can J Earth Sci 56: 756-773 �103 Siluro-Devonian Mafic-Ultramafic Intrusions in New Brunswick, Northern Appalachians, and their Associated Nickel-Copper-Cobalt Sulphide Deposits: A preliminary review Yousefi, F.1, Lentz D.R.1, Walker J.A.2 ,Thorne K.G.3, and Karbalaeiramezanali A. 3 1 1: Department of Earth Sciences, University of New Brunswick, Fredericton, NB E3B5A3 Canada fazilat.yousefi@unb.ca 2 Geological Surveys Branch, Department of Natural Resources and Energy Development, Bathurst, New Brunswick, E2A 7B8 Canada 3 Geological Surveys Branch, Department of Natural Resources and Energy Development, Fredericton, New Brunswick, E3B 5H1Canada ___________________________________________________________________________ In the Appalachian-Caledonian region, several mafic-ultramafic intrusions host notable Ni-Cu-Co sulphide mineralization, as well as platinum-group elements. Notable examples in New Brunswick (NB) [1] include St. Stephen, Goodwin Lake, Mechanic Settlement, and Portage Brook intrusions. With the exception of Mechanic Settlement (Proterozoic), these occurrences are Silurian-Early Devonian, and formed during the terminal stages of the Acadian Orogeny [2]. Powderhorn Lake and Portage Ni-Cu occurrences represent examples associated with mafic and ultramafic intrusions in Newfoundland (NF). The Moxie, Katahdin, Union, Alexander, Moosehorn Plutonic Suite, and Pocomoonshine Gabbro-Diorite in Maine (USA) are examples of hosting Ni-Cu sulphide mineralization [3, 4]. The location of Devonian mafic-ultramafic intrusions linked to Ni-Cu, Co, and PGE sulphide mineralization in Maine, NB, and NF are shown below on a map, showing the tectonic zones of the Canadian Appalachians (Fig. 1). This preliminary study explores occurrences of Ni-Cu sulphide mineralization, cobalt, platinum-group elements, and their mafic-ultramafic intrusions in NB. The compositions of these mafic-ultramafic intrusions include gabbro, gabbronorite, olivine gabbro, olivine gabbronorite, anorthosite, peridotite, and troctolite. The sulphide mineral assemblages in these mafic-ultramafic rocks are dominated by pyrrhotite, pentlandite, and chalcopyrite. The assimilation of sulphide-bearing Cambro-Ordovician metasedimentary rocks typical of the Gander zone, and the local attainment of sulphide-silicate equilibrium are key factors in the formation of immiscible sulphide melts. For instance, in southern NB, the Siluro-Devonian St. Stephen Intrusion has an extremely low mass ratio of silicate magma to sulphide melt indicating a preferential assimilation of sulphide-rich portions of the Cambro-Ordovician Cookson Formation within the host St. Croix terrane. The scattered coarse sulphide blebs within the host intrusion indicates either solidification of the rock shortly after the formation of immiscible sulphide droplets or a high yield strength of the magma that prevented sulphide blebs from efficiently settling – differentially segregating [1]. The mafic-ultramafic intrusions in New Brunswick have low silica contents (38.2 to 51.28 wt.%) and FeOt/MgO ratios (<5), displaying calc-alkaline to tholeiitic features. Variations in Al2O3, Fe2O3t, MgO, and CaO in most samples can be explained by the fractional crystallization - accumulation of olivine, both pyroxenes, and plagioclase. Preliminary lithogeochemistry indicates a wide variation in Cr (up to 1300), with Ni (up to 1100 ppm), Cu (up to 635 ppm), and Co (up to 150 ppm) content outside of the mineralized zones. Earlier separation of sulphides seems to be the reason for the typically low concentrations of chalcophile and platinumgroup elements in these basic intrusive rocks. There is an enrichment of light rare earth elements relative to heavy rare earth elements in these mafic-ultramafic intrusions. The host intrusions are characterized by enrichment of large-ion lithophile elements (e.g., Rb, Ba, Sr) and are depleted in high-field strength elements (e.g., Nb, Ta, Zr, Hf, Ti), with much lower Ta/La (0.04) than primitive mantle (0.06; [5]). These unique characteristics may be attributed to the involvement of continental crust, which generally lacks Ta and Nb. The elevated Th/Nb(averaging 0.25) and La/Hf (averaging 8.6) support an island arc basalt affinity for these intrusions. Referring to an example (Moxie Pluton) in Maine Appalachian Orogeny [6], the emplacement of mafic-ultramafic intrusions occurred due to crustal fracturing in the late stages of the Acadian Orogeny, leading to a local tensional regime that generated a bimodal (mafic & felsic) igneous suite. According to the high positive ɛNd values �104 presented [7], it is inferred that the magmas responsible for forming these mafic-ultramafic intrusions originated by decompression of a modified mantle. Fig. 1: Distribution of Devonian mafic-ultramafic intrusions associated with Ni-Cu sulphide, cobalt, and platinum group element (PGE) mineralization in Maine (USA), New Brunswick, and Newfoundland, situated within the Canadian Appalachians (modified from [8]). References: [1] Paktunc A.D (1989) Econ Geol 84: 817-840 [2] Ruitenberg A (1968) NB Dept. Nat. Resources Rept. Inv 7: 47 p [3] McLaughlin K.J et al. (2003) Atl. Geol 39: 123-146 [4] Slack J.F et al. (2022) Atl. Geol 58: 155-191 [5] Ye X.T (2015) J Asian Earth Sci 113: 75-89 [6] Thompson J.F.H (1984) Am J Sci 284: 462-483 [7] Whalen et al. (1996) Can J Earth Sci 33: 140-155 [8] Hibbard J and Karabinos P (2013) Geosci. Canada 40: 303-317 �105 Geochemistry of Archean komatiitic greenstone terranes of the Wyoming Province: implications for geodynamic setting and mineralization Zieman, L.J.1*, Poletti, J.E.1, and Jenkins, M.C.1 1 U.S. Geological Survey, Geology, Minerals, Energy, and Geophysics Science Center, Spokane, Washington, USA *lzieman@usgs.gov ___________________________________________________________________________ Archean komatiites are important host rocks of some Ni-Cu sulfide deposits [1] and are hypothesized to be the parental melt of several Archean layered mafic intrusions that host world-class platinumgroup element (PGE) deposits [e.g., 2, 3]. The Archean Wyoming Province in the western United States contains two greenstone terranes that include komatiitic metavolcanic rocks: South Pass in the southern Wind River Range and Bradley Peak in the Seminoe Mountains, Wyoming. These Archean greenstone terranes have primarily been explored for Au, Cu, Fe, and Zn [4, 5]. However, the age, geodynamic setting, and sulfide mineralization potential of the spatially associated maficultramafic metavolcanic rocks are poorly understood. Here, new major and trace element geochemistry as well as detrital zircon geochronology constrain the volcanic environment and the emplacement ages of these komatiitic metavolcanic units. Metavolcanic units from the Bradley Peak region preserve primary igneous textures, including parallel and random spinifex (Fig. 1A), whereas igneous textures are overprinted by schistose textures in the South Pass metavolcanic rocks. Like most global komatiites, mafic-ultramafic rocks from both terranes have been metamorphosed up to amphibolite facies and contain tremolite, actinolite, serpentinite, chlorite, talc, and/or epidote. This work focuses on elements that are resistant to alteration [e.g., Mg, Al, Ti, and rare earth elements (REE); 6]. The metavolcanic rocks in both Bradley Peak and South Pass greenstone belts contain basaltic to ultramafic komatiites, as well as high-Mg and high-Fe tholeiitic basalts based on the Al-Mg-(Fe+Ti) classification scheme of [7] (Fig. 1B). The subset of komatiitic samples (n = 20) have MgO contents predominantly ranging from 10 to 23 wt. %. These low MgO contents (< 30 wt. %) suggest low degrees of partial melting or high degrees of crustal contamination relative to komatiites associated with major Ni deposits [e.g., 8]. Like most Archean komatiites [e.g., 6], komatiites from both greenstone terranes are predominately Al-undepleted (i.e., Munro-type) based on their chondritic Gd/Yb and Al2O3/TiO2 ratios (Fig. 1C). The absence of heavy REE enrichments indicates the komatiitic magmas were generated at mantle depths shallower than the garnet stability field (< 300 km). The South Pass komatiites are highly enriched in light REE relative to a primitive komatiite melt, whereas the Bradley Peak komatiites are not enriched in light REE. These trends suggest that the South Pass komatiites have experienced higher percentages of crustal assimilation than the Bradley Peak komatiites. This interpretation supports previous studies that proposed the South Pass ultramafic rocks intruded continental shelf sedimentary rocks at the southern margin of the Wyoming craton, whereas the Bradley Peak ultramafic rocks were deposited in a sediment-starved ocean basin within a rift [9, 10]. Because komatiites lack minerals suitable for geochronology, emplacement ages of the ultramafic units were better constrained using detrital zircon U-Pb geochronology for metasedimentary rocks interbedded with the metavolcanic rocks in each greenstone terrane (Fig. 1D). Significant age populations were determined to identify the youngest age peak, which corresponds with the maximum depositional age (MDA), in addition to the weighted mean age for each sample [11]. In the Bradley Peak region, the weighted mean age for a metagraywacke from the Seminoe Formation, which overlies the ultramafic rocks, constrains the Bradley Peak ultramafic rocks to be older than 2721 ± 15 Ma. In the South Pass region, a metagraywacke from the unit overlying the komatiites (Miners Delight Formation) has a weighted mean age of 2673 ± 16 Ma, which agrees with published data and the previously accepted age for this greenstone terrane of 2.67 Ga [12]. Two pelitic schist samples interbedded with the komatiite units record MDA ranges ca. 3007-3049 Ma. This MDA range constrains komatiite units to younger than 3.01 Ga, but permits the komatiite units to be older than the previously assumed age of 2.67 Ga. �106 Figure 1. A) Sub-parallel spinifex texture preserved in the Bradley Peak metavolcanic rocks. B) Al-Mg-(Fe+Ti) cation classification plot after [7]. Hypothetical Stillwater parental melt (orange star) is from [2]. C) Gd/Yb vs. Al2O3/TiO2 for the subset of komatiitic rocks from (B) in comparison to global komatiites after [12]. Inset: TiO2 vs. Al2O3 illustrating Al-depleted (Al2O3/TiO2 ≈ 20) and Al-undepleted (Al2O3/TiO2 ≈ 10) trends. D) Detrital zircon age data. Vertical scales in probability density plots, calculated after [12], are reduced to 25%. A crystallization age is given for igneous sample 23BP25 (a). Weighted mean age (b) is given for samples with one significant age peak. The MDA (c) is given for samples with more than one significant age population. These komatiites do not satisfy several criteria typically thought to be important for Ni-Cu ore genesis [e.g., 1]— they were generated from relatively low degree partial melting and, in the case of the Bradley Peak greenstone, lack geochemical signatures of significant crustal assimilation, which is widely accepted to be a source of sulfur for ore genesis [1]. Contrarily, they are Al-undepleted and erupted at cratonic margins, characteristic of komatiites that have been associated with major Ni deposits [8]. Furthermore, the geochronological data do not rule out that either greenstone terrane was erupted synchronously with the emplacement of the 2.7 Ga Stillwater Complex in the Archean Wyoming Province, which is thought to have an Al-undepleted komatiitic parental melt (see Fig. 1B and 1C). Future work is needed to test if eruption of the komatiites is related to the emplacement of this layered intrusion or other magmatic systems in the Wyoming Province. References: [1] Barnes S J et al. (2016) Ore Geol Rev 76:296-316 [2] Jenkins M C et al. (2021) Precambr Res 367:106457 [3] Eales H and Costin G (2012) Econ Geol 107:445-465 [4] Hausel D (1991) WY State Geo Survey 44:1-129 [5] Hausel D (1994) WY State Geo Survey 50:1-24 [6] Barnes S J et al. (2004) Mineral Petrol 82:259-293 [7] Jensen (1976) Ontario Geo Survey 66 [8] Mole D et al. (2014) Proc Natl Acad Sci 111:10083-10088 [9] Grace et al. (2006) Can J Earth Sci 43:1445-1466 [10] Frost C et al. (2006) Can J Earth Sci 43:1533-1555 [11] Gehrels G (2009) Excel Age Pick Program [12] Arndt N and Lesher C (2004) Cambridge U Press [13] Saylor J and Sundell K (2016) Geosphere 12:203-22 Note: Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. � https://digitalcollections.lakeheadu.ca/files/original/d7158bd5cc5c05c60eb8187bcbef6867.pdf db1969b8ed755f1d201e2bac65fca824 PDF Text Text International Ni-Cu Symposium August 6-8th 2024 Lakehead University, Thunder Bay, Canada �i International Ni-Cu Symposium August 6-8th 2024 Lakehead University, Thunder Bay, Canada Meeting Chair - Pete Hollings Organising committee - Matt Brzozowski, Robert Cundari, David Good, Peter Hinz, Al MacTavish, Jim Miller, Dean Rossel, Mark Smyk Reference to material in this volume should follow the example below: Authors, 2024, Abstract title, 2024 International Ni-Cu Symposium Abstracts Volume, Thunder Bay, August 6-8th 2024, p. xx-xx. �Thank you to our sponsors See you next time! �iii Table of Contents One parental magma for them all: Unveiling the crystallization of the Raptor Zone, Tamarack Intrusive Deposit, Minnesota .................................................................................................................. 1 Augustin, C.T.1*, Mungall, J.1 .............................................................................................................. 1 A Paradigm Shift: The Evolution of Nickel-bearing Ultramafic Emplacement Models............................ 3 Aubut A. .............................................................................................................................................. 3 The LDI Intrusive Suite: Geology, tectonic setting, magmatic evolution, and possible controls of sulphide mineralization ........................................................................................................................... 5 Bain, W.B.1,2, Tolley, J. 1, Djon, L.M.3, Hamilton, M.A.4, and Hollings, P.1 ............................................ 5 Mineral geochemistry and textural relations of Ni sulfides and Co arsenides ores from the atypical Avebury nickel deposit, western Tasmania, Australia ............................................................................ 7 Barillas-Diaz, J.L.1, Cooke D.R.1, Zhang, L.1, Wei Hong1, Cajal, Y.1, Denholm, J.2 and Chisnall, T.2 ....... 7 Whole Rock Geochemistry and Down Hole Vectoring as an Exploration Strategy in the Coldwell Complex .................................................................................................................................................. 8 Boucher, C.1, Pitts, MJ. 1, Good, D.J.2, and Laxer, M.2 .......................................................................... 8 What does magmatic sulfide liquid hide? ............................................................................................... 9 Cherdantseva, M.V. 1, Anenburg M.2, Mavrogenes J.E.2 and Fiorentini M.L.1 ..................................... 9 Characterization of Sulfides in Gorgona Island Komatiites: Insights into Cretaceous Mantle Plume Melting and Magmatic Processes ......................................................................................................... 11 Camilo Conde1, Mateo Espinel1, Ana Elena Concha2 ......................................................................... 11 Magmatic and hydrothermal evolution of the PGE-Cu-Ni Current Lake deposit .................................. 12 Corredor Bravo, A. P.1, Hollings, P.1, Brzozowski, M.1, and Heggie, G.2 ............................................. 12 Sulfide percolation and drainback process in magmatic conduit system in the Huangshan-Jingerquan mineralization belt ................................................................................................................................ 14 Deng, Y.-F.1, Xie-Yan Song2 and Feng Yuan1 ...................................................................................... 14 Weathering of non-ore grade rock from Duluth Complex deposits: Outcomes from comprehensive pre-mining geochemical characterization............................................................................................. 16 Diedrich, T.R.1 and Theriault S.2......................................................................................................... 16 Application of FactSage to Model the Compositional Variability of the Ni-Cu-PGE Mineralization at the Main Zone of the Tamarack Intrusive Complex .............................................................................. 18 El Ghawi, A.K.1 and Mungall, J.E.1 ..................................................................................................... 18 Petrophysics Applied to Magmatic Sulfide Deposits: The Physical Properties - Mineralogy Link ......... 20 Enkin, R.J.1 ......................................................................................................................................... 20 Regional changes in plume-generated stress linked to MCR (Keweenawan LIP) chonolith emplacement ........................................................................................................................................ 23 Ernst, R.E.1, El Bilali, H.1, Buchan, K.L.2 and Jowitt, S.M.3 .................................................................. 23 �iv A proposed cryptic common thread among Ni-Cu-PGE-(Au-Te) systems spanning the boundary between Laurasia and Gondwana......................................................................................................... 25 Fiorentini, M.1*, Holwell, D.2, Blanks, D.3, Cherdantseva, M.1, Denyszyn, S.4, Ince, M.1, Vymazalova, A.3, and Piña Garcia, R.5 .................................................................................................................... 25 How exploration geologists can and should use “soft NSRs” to represent assays of Ni-Cu-PGE mineralization ....................................................................................................................................... 27 Goldie, R.J. ......................................................................................................................................... 27 Characterizing the Early (Plume) and Main (Rifting) Stages in the evolution of the Midcontinent Rift 28 Good, D.J. .......................................................................................................................................... 28 Lithospheric structure controls for large magmatic Ni-Cu discoveries ................................................. 30 Hayward, N.1,2 ................................................................................................................................... 30 Deep orogenic magmatic Ni and Cu sulfide systems in the Curaçá Valley, Brazil ................................. 32 Holwell, D.A.1, Thompson, J.2 , Blanks, D.E.1, Oliver, E.1, Porto, P.3, Oliviera, E.3., Tomazoni, F.4, Lima, A. 4, D’Altro R.,4., Graia, P.4 and Sant’Ana, T.4. .................................................................................. 32 Spatial distribution, lithological associations, and geochemical signatures of Ring of Fire Intrusive Suite within the McFaulds Lake Greenstone Belt in the Superior Province: Implications for the Ni-CuPGE, Cr, and Fe-Ti-V Metal Endowment of the Region ......................................................................... 33 Houlé, M.G.1,2, Sappin, A.-A.1, Lesher, C.M.2, Metsaranta, R.T.3, Rayner, N.4, and McNicoll, V.4....... 33 Spatial distribution of mafic and ultramafic units in the Canadian north: Implications for critical minerals (Ni, Cu, Co, PGE) potential ...................................................................................................... 35 Houlé, M.G.1,2, Bédard, M.-P.1, Lesher, C.M.2, and Sappin, A.-A.1 ..................................................... 35 Copper and komatiitic magmatism – source of copper in the Sakatti Cu-Ni-PGE deposit in northern Finland................................................................................................................................................... 37 Höytiä, H.1,2, Peltonen P.1, Halkoaho, T.3, Makkonen, H.V.3,5 and Virtanen, V.J.1,5 ........................... 37 The Koperberg Suite of the Okiep Copper District - an overlooked target for magmatic nickel sulphides in a convergent margin system ............................................................................................. 39 Hunt J.P.1, van Schalkwyk L.1, Smart E.1 and Benhura C.1.................................................................. 39 A multi-methodological approach: Combining textural observations and geochronology to study the J-M Reef Package and its Hanging Wall, Stillwater Complex, Montana ................................................ 41 Jenkins, M.C.1*, Corson, S.2, Geraghty2, E., Kamo S.L. 3, Lowers, H.4, and Mungall, J.E.5 ................... 41 Nickel-copper-platinum group elements potential of mafic and ultramafic intrusions in northwestern Ontario .................................................................................................................................................. 44 Jonsson, J.1, Malegus, P.1, Churchley, S.1, Price, R.1 ........................................................................... 44 Petrogenesis of the mineralized horizons in the Offset and Creek zones, Lac des Iles Complex, N. Ontario .................................................................................................................................................. 46 Jonsson, J.1, Hollings, P.1, Brzozowski, M.1, Bain, W.1, Djon, L.2 ........................................................ 46 Quantum full tensor magnetic gradiometry to better define conduit type Ni-Cu-PGE targets ............ 48 Kaski, K.1, Smith, J.1, Tschirhart, Victoria1, Heggie, G., Enkin, R.1 ...................................................... 48 �v Exploration-Based Classification Scheme for Magmatic Ni-Cu-(PGE) Systems ..................................... 50 Lesher C.M.1 and Houlé M.G.2,1 ......................................................................................................... 50 Thermodynamic constraints on the generation of cubanite-rich magmatic sulfides ........................... 52 Maghdour-Mashhour, R.1, Mungall, J.1 ............................................................................................. 52 Constraining the Sunday Lake mineralization: A Ni-Cu-PGE deposit .................................................... 54 Mexia, K.1, Hollings, P. 1 ..................................................................................................................... 54 Primitive arc magmatism and the development of magmatic Ni-Cu-PGE mineralization in Alaskantype ultramafic-mafic intrusions ........................................................................................................... 56 Milidragovic, D.1,2, Nixon, G.T.3, Spence, D.W.2, Nott, J.A.2, Goan, I.R.2, Scoates, J.S.2 ...................... 56 Stratigraphy of the Grasset Ultramafic Complex and its Ni-Cu-(PGE) mineralization, Abitibi Greenstone Belt, Superior Province, Canada. ....................................................................................... 58 Milier, K.1, Houlé M.G.2 and Saumur B.M.1 ....................................................................................... 58 Geochemistry of Camp Lake Block Lac des Iles Palladium Mine, N. Ontario, Canada .......................... 60 Njipmo Ngoko, B.1, Hollings, P.1, Djon, L.2 and Hamilton, M.3 ........................................................... 60 Hf-Nd-Pb isotopic evidence for variable impact devolatilization in the Sudbury Igneous Complex and its relevance for Ni-Cu-(PGE) sulfide ore formation.............................................................................. 62 Peters, D.1, Lesher C.M.1 and Pattison E.1.......................................................................................... 62 Deformation in mafic protoliths: Impacts from late faults on Ni-Cu-PGE mineralization at Lac des Iles Mine, Canada ........................................................................................................................................ 64 Peterzon, J.1, Phillips, N.1, Hollings, P.2, and Djon, M.L.2 ................................................................... 64 Formation of euhedral silicate megacrysts within magmatic massive sulfides .................................... 66 Raisch, D.1, Staude S.1, Fernandez, V. 2 and Markl G.1 ....................................................................... 66 Applying Magnetic Vector Inversion (MVI) on Aeromagnetic Data in the Thunder Bay Region of the Mid-Continent Rift ................................................................................................................................ 68 Riahi, S.1, Mungall J.E.1, Ernst, R.E1 ................................................................................................... 68 Potential links between the Midcontinent Rift (MCR) related Baraga-Marquette dyke swarm and early MCR related magmatic Ni-Cu sulfide deposits in Michigan, USA. ................................................ 70 Rossell, D.M.1*, Strandlie, J.2.............................................................................................................. 70 Texture and composition of Fe-Ti oxides of the Neoarchean Big Mac mafic intrusion and its implication for Fe-Ti-V-(P) mineralization in the McFaulds Lake greenstone belt, Superior Province, Canada .................................................................................................................................................. 72 Sappin, A.-A.1, Houlé, M.G.1,2*, Metsaranta, R.T. 3, and Lesher, C.M.2............................................... 72 Complexly zoned pyroxenes at Kevitsa record magma mixing and survive alteration ......................... 74 Schoneveld, L.1, Luolavirta, K.2,3, Barnes, SJ1 , Hu, S.1 , Verrall, M.1 and Le Vaillant, M.1 ................... 74 New indicator mineral signatures for nickel sulfide exploration .......................................................... 76 Schoneveld, L.E.1, Williams M.1, Salama, W.1, Spaggiari, C. V.1, Barnes, S. J. 1, Le Vaillant, M. 1, Siegel, C. 1, Hu, S. 1, Birchall, R. 1, Baumgartner, R. 1, Shelton, T. 1, Verrall, M. 1, and Walmsley, J. 1 . 76 �vi Apatite as an indicator for volatile involvement in the genesis of the Marathon Cu-PGE deposit, northwestern Ontario ........................................................................................................................... 78 Shahabi Far, M.1, Good, D.2 and Samson, I3 ...................................................................................... 78 Geochemical and Petrologic Investigation of the Eagle’s Nest Intrusion, McFaulds Lake Greenstone Belt, Ontario, Canada ............................................................................................................................ 81 Sheshnev, V.1, Hollings, P.1, Phillips N.J.1, Weston, R.J.2, Deller, M.2 and Campbell, D.2 .................... 81 Reconstitution of the Merensky Reef footwall during chamber replenishment .................................. 83 Smith, W.D.1,2, Henry, H.3, Maier, W.D.4, Muir, D.D.4, Heinonen, J.S.5,6, Andersen, J.Ø7................... 83 Future research areas to aid in exploration for Ni sulfides ................................................................... 85 Sproule, R.A.1 ..................................................................................................................................... 85 Exploring the footwall: Sulfide Mineralization in the footwall Granite of the Maturi Deposit, Minnesota. ............................................................................................................................................ 86 Steiner, R. A.1 ..................................................................................................................................... 86 The Anatomy of a Cu-Ni-Co-PGE Mineralized Mafic Magmatic System: The South Kawishiwi Intrusion of the Duluth Complex, Northeastern Minnesota ................................................................................ 90 Sweet, G.S.1 and Peterson, D.M.2 ...................................................................................................... 90 Multi-thermochronological records of cooling, denudation and preservation of ancient ultrabasic magmatic ore deposits: An example from the Neoproterozoic Jinchuan giant magmatic Cu-Ni sulfide deposit .................................................................................................................................................. 94 Ni Tao1,2*, Jiangang Jiao1, Jun Duan1, Haiqing Yan1, Ruohong Jiao3, Hanjie Wen1 ........................... 94 Compositional variability in olivine: New data on the occurrences of Ni and Co as guides to mineral prospectivity ......................................................................................................................................... 95 Thakurta, J.1, Wagner, Z.1, Nagurney, A.2, and Schaef, H.T. 2 ............................................................ 95 The effects of diagenetic and metamorphic processes on the sulphur liberation from the Virginia Formation black shale during magmatic assimilation by the Duluth Complex, Minnesota, USA ......... 97 Virtanen V.J.1,2, Heinonen J.S.2,3, Märki L. 4, Galvez M.E. 5 and Molnár F.6......................................... 97 Mantle-to-crust scale chemical fractionation and sulphide saturation of the Paleoproterozoic komatiites of the Central Lapland Greenstone Belt, Finland – implications for geochemical exploration ............................................................................................................................................ 99 Virtanen V.J.1,2, Höytiä H.M.A.2, Iacono-Marziano G.1, Yang S.3, Moilanen M.3 and Törmänen T.4 . 99 Ni-Cu-PGE prospectivity of the Mackenzie Large Igneous Province ................................................... 102 Williamson, M.-C.1, Rainbird, R.H.1, O’Driscoll, B.2 and Scoates, J.S.3.............................................. 102 Siluro-Devonian Mafic-Ultramafic Intrusions in New Brunswick, Northern Appalachians, and their Associated Nickel-Copper-Cobalt Sulphide Deposits: A preliminary review ....................................... 103 Yousefi, F.1, Lentz D.R.1, Walker J.A.2 ,Thorne K.G.3, and Karbalaeiramezanali A. 3 ......................... 103 Geochemistry of Archean komatiitic greenstone terranes of the Wyoming Province: implications for geodynamic setting and mineralization .............................................................................................. 105 Zieman, L.J.1*, Poletti, J.E.1, and Jenkins, M.C.1 ............................................................................... 105 �1 One parental magma for them all: Unveiling the crystallization of the Raptor Zone, Tamarack Intrusive Deposit, Minnesota Augustin, C.T.1*, Mungall, J.1 1 * Mineral Deposits Laboratory, Earth Sciences Department, Carleton University, Ottawa. claudiaaugustin@cmail.carleton.ca ___________________________________________________________________________ The Tamarack Intrusive Complex (TIC) is one of the mafic complexes intruded in the context of the Midcontinent Rift (MCR) system in the Midwestern United States. The Tamarack Intrusive Complex is located ca. 80 km west of Duluth, Minnesota, and it is intruded within the Paleoproterozoic (~1.85 Ga) slates and greywackes of the Thomson Formation of the Animikie Group [1,2]. It was emplaced in the Early Stage of the MCR, with a baddeleyite U-Pb age of 1105.6 ± 1.2 Ma [1] and zircon Concordia age of 1103.81±0.92 [3]. The TIC is characterized by an aeromagnetic anomaly with a broader, rounded region at the south leading into a narrower, elongated extension towards the north, extending approximately 13 km northwest-southeast and varying from hundreds of m to ca. 4 km in width [1]. Its morphology contains distinct shaped intrusive bodies, such as the ovoid-shaped Bowl Intrusion in the south and a dike-like area in the north, which includes the Raptor zone [1,2; figure 1]. Figure 1 Schematic local geological map and cross-section of the Raptor zone. The rocks of the Raptor zone usually show a consistent vertical sequence, except when in proximity to lateral contacts, where drill cores show a more complex variation in texture and mineralogy. Usually, the sequence consists of a basal portion of fine-grained olivine cumulate rock; therefore, this unit will be called Basal Raptor Zone Unit (BRZ), keeping the name consistent with what has been used for previous TIC studies. The most abundant primary minerals in the BRZ unit are olivine, clinoand ortho-pyroxene, and plagioclase (figure 2a). The olivine size ranges from 170 µm to 3.3 mm, but most grains are <0.5 mm. The coarser grains of olivine are more prevalent in the upper section and gradually diminish downwards. Commonly, the coarser olivine grains display plane-oriented dendritic exsolution of chromium-spinel and clinopyroxene along a consistent orientation. Above this unit is a thick, coarse-grained olivine cumulate called CGORaptor unit (figure 2b). The mineral proportions of the CGORaptor are variable along the stratigraphy; the intercumulus/cumulus ratios phases increase to the center, i.e., the cumulus phase decreases towards the upper and lower contacts. These two rock units are characterized by similar primary mineralogy and classified as feldspathic lherzolite, with the most notable difference being a variation in olivine grain size and a slight increase in earlier chromium-spinel. The subtle grain-size distinction makes it difficult to identify their gradual contact visually. The upper portion of CGORaptor shows intercalation of olivine cumulates with �2 pockets/domains of a varitextured gabbro. The gabbro that is intercalated with CGO and the contact with it is mostly diffused, marked only by the disappearance of olivine cumulate. Figure 2: EDS phase maps showing textural differences between the BRZ (a) and CGO(b), with minor large olivines in a finer matrix in the BRZ compared with the more uniform CGO. To address the composition and evolution of the melt parental to the CGORaptor rocks of the TIC, we have modeled crystallization using the alphaMELTS thermodynamic software [4-5]. The starting composition used was derived from the chilled margin of the Raptor zone. The cooling of the liquid under isobaric conditions and fO2 at the fayalite–magnetite–quartz (FMQ) solid oxygen buffer produced a similar sequence of crystallization, modal proportions of solids to the observed bulk-rock and mineral compositions of all major constituents of the rocks of the Raptor Zone. This method successfully mirrored the crystallization order, the relative amounts of solid phases, and the chemical composition of the primary cumulus minerals. Our results show a crystallization sequence beginning with olivine (Fo87), followed by clinopyroxene, chromium-rich spinel, orthopyroxene, and plagioclase. Specifically, at 1170 °C, the simultaneous formation of olivine and clinopyroxene, adjusted in proportion, reflects the varied compositions within the unit. Moreover, the liquid remaining at this temperature aligns with the mineralogy and composition observed in the gabbro unit. Using the same composition and parameters but slightly increasing the fO2 levels to NNO, the model predicts that spinel forms earlier, leading to similar BRZ composition and mineralogy. This change explains the prevalent spinel and the observed exsolution textures between cr-spinel and clinopyroxene in the coarse-grained olivine—features typically linked to variations in cooling rates and oxygen fugacity. Our thermodynamic analysis shows that the three main rock types in the Raptor Zone can originate from a single magma source, with only minor adjustments needed to explain their variations. The categorization into BRZ and CGO units appears to be based on slight differences in oxidation states and crystal sizes rather than suggesting they are from two separate magmatic intrusions. The findings suggest these units might represent different stages of the same magmatic event. References: [1] Goldner B (2011) Min University Thesis [2] Taranovic V et al. Lithos 212-215 (6-31) [3] Bleeker W et al. (2020) Geol Survey of Canada, Open File 8722, p. 7–35 [4] Asimow P D (1998) Am. Mineral. 83 (1127-1132) [5] Smith P M and Asimow P D (2005) Geochem. Geophys. Geosyst. 6(1-8) �3 A Paradigm Shift: The Evolution of Nickel-bearing Ultramafic Emplacement Models Aubut A. M Sibley Basin Group Ltd., PO Box 304, Nipigon, Ontario. sibley.basin.group@gmail.com ___________________________________________________________________________ An important class of nickel deposit are those hosted by stratabound dunite-peridotite bodies. This class includes the Kambalda district of Australia, Pechenga in the Kola Peninsula of western Russia, Kabanga in south-central Africa, the Shaw Dome area of northern Ontario, Raglan in northern Quebec and Thompson in northern Manitoba. All have been, or currently are attributed to the intrusion of ultramafic sills [e.g. 2,8,9]. Key evidence in support of this model is that the ultramafic bodies typically exhibit at least some differentiation and are sub-concordant to the host sediments. This tendency to default to an intrusion model now includes the Tamarack deposit in Minnesota [11] even though another model, one that incorporates extrusion, may be just as valid. Despite the prevalence of the intrusion model there are many nickel deposits hosted by ultramafic bodies that display clear evidence of being the product of extrusive flows, often exhibiting the same key features used to invoke an intrusive origin [e.g. 1,3,4,7]. Major komatiite hosted nickel deposits share some common features: 1) the nickel mineralisation is hosted by ultramafic rocks; 2) the sulphides are at the stratigraphic base of the host ultramafics; 3) the ultramafic rocks are hosted by, or in contact with, sulphidic and carbonaceous argillaceous rocks; 4) the ultramafic bodies are stratabound and generally conformable to the host lithology; and 5) they are hosted within extensional basins usually with a significant sedimentary component with Kambalda being the one exception. As Maier et al. [3] point out, the reason magmatic feeder systems rather than large intrusions are important hosts to economic nickel deposits is because of flow dynamics. Rice and Moore [11] have studied flow dynamics and concluded that open-channel flows were turbulent, and that this turbulence was required to expose the sulphides present to enough magma to generate the tenors observed. This turbulence explains how sedimentary sulphides can be integrated and assimilated by ultramafic magma and result in significant nickel tenors, nickel in 100% sulphide [4,5]. Turbulent flow is difficult, if not impossible, to explain by a simple intrusive mechanism. In addition, to get the size of deposit observed there needs to be significant volumes of ultramafic magma. The one environment that does allow turbulent flow to take place, and have the volumes required, is with high volume surface flows with gravity settling of the magmatic and assimilated sedimentary sulphides, along with significant magma mixing to get the observed partitioning of the silicate nickel into the sulphides. But there is a density “problem” in that ultramafic magmas are typically denser than the host rocks, especially when they are sedimentary. This paradox is typically glossed over or totally ignored. For example, see Hubbert et al. [5]. Ultramafic magma is not buoyant as the contrast is negative. So, how were these high-density liquids able to ascend through the crust? When rocks melt, they become about 10% less dense. In the case of ultramafic rocks, they have an average density of about 3.1 grams per cubic centimetre (g/cc) depending on the proportion of olivine present which has a density of 3.27–4.27 g/cc. Hubbert et al. [5] assumed a value of 2.8 g/cc. The average crust has a density of 2.7 g/cc or less and thus buoyancy could not have taken place. To move upward from the mantle through the crust there must have been a mechanism other than buoyancy. �4 An alternative mechanism proposed in the literature is “overpressure” defined by Walwer et al. [12] as “the difference between the pressure inside the magma and the local pressure acting orthogonal to the magma body wall.” Melting of the mantle creates magma plumes that move upward due to buoyancy to the Mantle-Crust boundary where the magma collects and then moves laterally thus creating extensional forces in the overlying crust. This accumulating magma would be constrained by the overlying lithostatic load and in doing so would build up overpressure. Eventually the crust would thin enough such that vertical fractures would form allowing the trapped magma to escape, not through buoyancy but due to the built-up overpressure exceeding the lithostatic load. At surface the hot, dense ultramafic magma would then flow over, and into, deep water sediments where the magma would mechanically and thermally erode and assimilate sulphide rich sediments. This mechanism would explain the correlation with rift basins, as well as how a dense magma can penetrate a less dense substrate and produce the type of volumes required to attain high R values, while also generating the turbulent flow needed to assure incorporation, and assimilation of sulphide with resultant nickel partitioning required to get the high tenors typical of most sulphide deposits found associated with extensional basins. An extrusive model is more compatible with these commonalities and issues. It explains why the host ultramafic bodies are stratabound. It provides a better mechanism for incorporating sedimentary sulphide. It provides more opportunity for high R values creating high tenors. And it presents a tectonic environment, rifted basins, that can be easily targeted. Currently nickel is an under explored commodity primarily because, using the intrusion model, limited opportunities are available. The flow model on the other hand is more robust as it does a better job of explaining things like the high volumes of magma needed and the fluid dynamics required to ensure thorough mixing of the denser sulphides with the magma to attain the tenors present in these deposits. In addition, being tied to a specific tectonic event, rifting, it is not fixed in time or place as much as the intrusive model is. While intrusive environments do exist where these conditions are met, they are always in primary magma conduits. References: [1] Arndt NT (1975) Unpub Ph.D. Thesis, U of T. [2] Bleeker W (1990) Unpub PhD Thesis, UNB. [3] Hill RET et al. (1995) Lithos 34: 159-188. [4] Hubbert HE and Sparks RSJ (1985) J of Petro 26-3: 694-725. [5] Hubbert HE et al. (1984) Nature 309:19-22. [6] Maier WD et al. (2001) Cana Mine 39:547-556. [7] Marston RJ et al. (1981) Econ Geol 76:1330-1363. [8] Melezhik VA et al. (1994) Tran Inst Min Meta B 103:B129-B145. [9] Naldrett AJ (1981) Econ Geol 75th Anni Volu :628-685. [10] Rice A and Moore JM (2001) Cana Mine 39:491-503. [11] Taranovic V et al. (2018) Econ Geol 113-5:1161-1179. [12] Walwer D et al. (2021) Phys of the Earth and Plan Inte 312, �5 The LDI Intrusive Suite: Geology, tectonic setting, magmatic evolution, and possible controls of sulphide mineralization Bain, W.B.1,2, Tolley, J. 1, Djon, L.M.3, Hamilton, M.A.4, and Hollings, P.1 1 Department of Geology, Lakehead University, Thunder Bay, ON P7B 5E1, Canada British Columbian Geological Survey, Victoria, BC V8T 4J1, Canada 3 Impala Canada, Thunder Bay, ON P7B 6T9, Canada 4 Department of Earth Sciences, University of Toronto, Toronto, ON M5S 3B1, Canada 2 ___________________________________________________________________________ The Archean Lac des Iles (LDI) complex hosts a world-class platinum group element (PGE) deposit. This mafic-ultramafic intrusive complex is situated near the suture between the Wabigoon and Quetico subprovices and is spatially associated with a suite of satellite intrusions: the Tib Lake, Legris Lake, Wakinoo Lake, Demars Lake, Dog River, and Buck Lake intrusions- known collectively as the LDI intrusive suite (Fig 1 a). Textural, petrographic and geochemical similarities between the LDI Mine Block intrusion and the LDI intrusive suite suggest a genetic association and potentially a comparable degree of PGE mineralization. Here, we present an overview of the geology of the LDI intrusive suite and provide new U-Pb age dates, Sm-Nd isotopes, and parental melt modelling. Zircon U-Pb ages for the Buck Lake (2698.1 ± 1.6 Ma), Wakinoo Lake (2696.6 ± 0.8 Ma), Demars Lake (2694.1 ± 1.5 Ma), Legris Lake (2690.6 ± 0.8 Ma), Dog River (2689.9 ± 0.7 Ma), and Tib Lake (2685.9 ± 1.6 Ma) intrusions show a spatial trend of younging to the north and demonstrate a temporal association with the Lac des Iles Mine Block intrusion (2689.0±1.0 Ma; [1]; Fig 1 b). Whole rock εNdT values from the Wakinoo Lake, Tib Lake, Legris Lake, and Lac des Iles intrusions overlap and similarly display a trend of increasingly negative values with decreasing age (Fig 1 c). These patterns likely reflect the initial assimilation of Wabigoon tonalite country rock early in the magmatic evolution of the LDI intrusive suite and progressively more assimilation of Quetico metasedimentary rocks in later stages. Model parental magma compositions for the LDI intrusive suite produce similar trace element profiles with highly fractionated REE content, moderately negative Ta-Nb and Zr-Hf anomalies, and strong enrichment in the large ion lithophile elements. This pattern is consistent with an arc setting and might indicate a common source reservoir of parental melt. The observed Sm-Nd isotopic signature of the LDI intrusive suite supports this interpretation and suggests that host rock assimilation was a main control of the magmatic differentiation of individual intrusions. However, magma mixing may also have occurred during the formation of the Tib Lake and North LDI intrusions, as indicated by the more primitive compositions of individual cyclic units [2]. Magmatic sulphides from the Legris Lake intrusion have δ34S values that overlap the mantle range but trend toward the composition of Wabigoon tonalite [3]. This suggests that external S addition drove sulphide saturation during its formation. However, a comparison of whole rock S/Se and Cu/Pd ratios of mineralized lithologies suggests sulphide melt retention during emplacement was a key control on the scale of sulphide mineralization in the Legris Lake intrusion and other intrusions of the LDI intrusive suite. �6 Fig 1. a. Regional geologic map showing locations of Thunder Bay, the Lac des Iles mine (in red), and the Lac des Iles intrusive suite (in blue). b. U-Pb ages for individual intrusions in the Lac des Iles intrusive suite. c. Whole-rock εNdT values for the LDI intrusive suite and host rock lithologies. North LDI, South LDI and Shelby Lake diorite data from Brügmann et al. [4] References: [1] Stone D (2010) Ontario Geological Survey, Open File Report 5422:1–130 [2] Djon LM et al. (2017) Can Min 55:349-374 [3] Bain WM et al. (2023) Min Deps doi:10.1007/s00126-023-01183-x [4] Brügmann MJ et al. (1997) Precambrian Res 81:223-239 �7 Mineral geochemistry and textural relations of Ni sulfides and Co arsenides ores from the atypical Avebury nickel deposit, western Tasmania, Australia Barillas-Diaz, J.L.1, Cooke D.R.1, Zhang, L.1, Wei Hong1, Cajal, Y.1, Denholm, J.2 and Chisnall, T.2 1 Centre for Ore Deposit and Earth Sciences (CODES), University of Tasmania, Private Bag 79, Hobart, TAS 7001, Australia, joseluis.barillasdiaz@utas.edu.au 2 Avebury Nickel Mine, Trial Harbour Road Zeehan TAS 7469, Australia ___________________________________________________________________________ The unusual Avebury metasomatic nickel sulfide deposit in western Tasmania was discovered in 1998 and is the best-known case of an economic hydrothermal-remobilized Ni deposit [1]. The nickel sulfide ores are hosted in the Middle Cambrian serpentinized peridotites of the allochthonous maficultramafic ophiolite complex, while cobalt arsenides within the Neoproterozoic Crimson Creek volcanoclastic sequence. The Avebury Ni deposit lies in the halo of the strongly fractionated, reduced Devonian Sn-mineralized ~360 Ma Heemskirk granite [2]. Apatite U-Pb ages from 374 ± 14 Ma to 347 ± 15 Ma from mineralized serpentinite and Crimson Creek skarn imply that hydrothermal remobilization of Ni-Co occurred at Avebury due to hydrothermal fluids derived from Devonian Heemskirk granite. The compositional and mineralogical transformations associated with chemical reactions triggered by the response of hydrothermal fluids from the granite resulted in a magnesianskarn including brucite + diopside + hedenbergite + augite and tremolite-actinolite in the ultramafic rocks and pyroxene + garnet + axinite-(Mg) ± ludwigite and tourmaline in the volcanoclastic rocks of Crimson Creek. The dominant nickel sulfide mineral at Avebury is pentlandite, which is associated with pyrrhotite and minor chalcopyrite. Pentlandite is hosted in olivine + clinopyroxene cumulates, which have been serpentinized in most cases where pentlandite occurs mainly as relatively coarse-grained sulfide blebs with pyrrhotite. Pentlandite also occurs in relatively fine-grained shattered disseminations within actinolite. The coarse-grain pentlandite is fractured and encapsulated by magnetite, and Niarsenides have partly replaced pentlandite grains. Pentlandite has altered slightly along grain edges to violarite and pyrite. Chalcopyrite may occur as exsolution intergrowths in millerite and pentlandite. The high-resolution XRF scanning analysis from core rock and whole rock assay from mineralized serpentinite samples show positive Ni/Ti and Ni/Cr ratios and discriminated between two nickel mineralization zones. The Ni vs MgO diagram shows that nickel mineralization is hosted primarily in MgO-rich and pyroxene-rich serpentinites. In contrast, the low-MgO and Cr-rich serpentinite negatively correlate with Ni. However, the serpentinite FeO-rich positively correlates with pentlandite rich in cobalt. Although some serpentinite horizons have strong metasomatism, all the serpentinized ultramafics have >16% magnetite and are depleted in Al2O3, TiO2, Sr, Y and Zr. The whole rock assay results indicate a negative correlation of Cu and Zn with Ni. Mineral characterization using an automated energy dispersive X-ray spectroscopy mineral mapping (AMICS) shows nickel sulfides and cobalt arsenides do not coexist in the same mineral assemblage. Cobaltite, alloclasite and minor glaucodot are the two main arsenides of cobalt restricted to the magnesianskarn of prehnite + augite and hedenbergite in Crimson Creek. The laser ablation analyses (LA-ICPMS) in pentlandite minerals from the Avebury deposit do not show strong correlations with other elements. However, a small group of pentlandite shows incipient correlations between Au, Ag and Co. Analysis in pentlandite and pyrrhotite shows some crystals with Pt values between 2.5 to 4.0 ppb. Cobaltite shows a slight trend in which the cobalt content decreases as the Ni content increases. On the other hand, the pyrite crystals show a strong correlation between Au, Co, Cu and Ni. The correlation between nickel and cobalt in pentlandite is modest in the Avebury deposit compared to Trial Harbour pentlandite, which shows strong correlations between these two elements. The paragenesis relationships, mineral textures, and compositional trends exhibited by Ni-Co ores at the Avebury deposit provide evidence of a multi-stage depositional history. References: [1] Keays R and Jowitt S (2013) Ore Geology Reviews 52: 4–17 [2] Hong W eta al. (2017) Gondwana Research 46: 124–140 �8 Whole Rock Geochemistry and Down Hole Vectoring as an Exploration Strategy in the Coldwell Complex Boucher, C.1, Pitts, MJ. 1, Good, D.J.2, and Laxer, M.2 1 2 Generation Mining, Marathon, ON, Canada. cboucher@genpgm.com Department of Earth Sciences, Western University, London, ON N6A 5B7, Canada ___________________________________________________________________________ The Eastern Gabbro-Basalt Suite of the Coldwell Complex has been widely explored for decades by various operators, resulting in the discovery of numerous exploration Prospects and Deposits. Although numerous economic and academic studies have been completed on the flagship Marathon Deposit, Sally deposit, and Boyer and Four Dams occurrences, little work has been done to advance understanding of relationships between trace-element geochemistry and mineralization at the Complex-sized scale. For instance, earlier work has described stratigraphic and trace-element relationships between metabasalt and the mineralized Two Duck Lake intrusions, and between mineralized and unmineralized phases of the host gabbro. In this presentation we examine these relationships at a larger scale and test for their usefulness as an exploration vector tool in the Coldwell Complex. A second objective of this presentation is to examine the 3D spatial relationships between Cu/Pd and Cu/S and the associated mineralization style, footwall topography and faulting at the Marathon deposit. This study takes advantage of the dynamic conduit model that it is used to explain many features of Cu-PGE mineralization in the Marathon Series rocks. For instance, the spatial distribution of mineralization relative to topographic lineaments is explained by magma transport along early fault zones that were reactivated late in the history of the complex to create the lineaments. This study also takes advantage of significant changes or inflection points in the trends for Cu/Pd, Cu/S, Pd/Au, and Cu/Ni values between the three dominant mineralization styles in the Two Duck Lake gabbro: Footwall Zone, Main Zone, and W-Horizon. Large deflections in the downhole trends of these ratios, particularly Cu/Pd, act as a proxy for identification of individual pulses of magma (or stacking of intrusions). Although contacts between pulses are difficult to recognize in thick packages of gabbro, they can be identified by sharp changes in Cu, Pd and S content or, more importantly, by inflection points in metal ratio proxy trends (Cu/Pd or Cu/S). Here we present results of our study for these factors at the deposit scale and propose key features that might be useful for recognizing settings in the conduit model from down hole assay data. The implementation of geochemistry and downhole vectoring will continue to advance and provide insight into refined geological modelling. Future work on in-depth classification of units will include Layered Series rocks and proximity to major structures, differentiation of TDL Gabbro based on mineralogy and texture, origin of the two varieties of oxide-melatroctolite pods and relationship to underlying conduits and identifying key indicators to aid in lithological classification based on basic assay package. �9 What does magmatic sulfide liquid hide? Cherdantseva, M.V. 1, Anenburg M.2, Mavrogenes J.E.2 and Fiorentini M.L.1 1 Centre for Exploration Targeting, School of Earth Sciences, University of Western Australia, Australia, maria.cherdantseva@uwa.edu.au 2 Research School of Earth Sciences, Australian National University, Canberra, Australia ___________________________________________________________________________ In natural examples, magmatic sulfides hosted in mafic-ultamafic intrusions, regardless of textural variability (massive, globular, net-textured, disseminated), are almost ubiquitously found in spatial association with alkali-, lithophile- and volatile-rich minerals, such as phlogopite, ilmenite, chlorite, amphibole, calcite, etc. These minerals display diverse textures, either surrounding sulfide margins or found inside sulfides as euhedral crystals as well as irregular, rounded or vermicular inclusions. The presence of the listed minerals in association with sulfides has been previously attributed to secondary processes, late circulation of fluids or highly differentiated melts [1, 2, 3]. However, existing models fail to provide a satisfactory explanation on why these alkali-, lithophile- and volatilerich minerals so often occur in direct contact with sulfides or as inclusions in them. Here, we argue that the common spatial association of alkali-, lithophile- and volatile-rich minerals with magmatic sulfides could be explained by the partial dissolution of lithophile and volatile elements in sulfide liquid at high temperature and pressure and their subsequent release upon cooling of the system. Indeed, several experimental studies show that at high temperatures and additional various conditions (e.g., oxygen fugacity, melt composition), regular magmatic sulfide liquid has the capacity to dissolve a wide range of lithophile elements (such as Al, Mg, Mn, Ti, Ca, K, etc. [4, 5, 6]), halogens (Cl, Br, F, I [6, 7]) and water [8]. However, there has never been a clear connection made between formation of alkali-, lithophile- and volatile-rich minerals in close spatial association with sulfides and the potential chalcophile behaviour of some lithophile elements and halogens dissolved in sulfide liquids under some specific conditions. We put forward the idea that a genetic link between these elements and sulfide liquid could not only explain the formation of volatile-rich halos around sulfides but also elucidate the cryptic link between magmatic and hydrothermal mineralising processes as explained below. Our new experiments were conducted to investigate the potential of magmatic sulfide liquids to dissolve K, Na and chloride in magmatic conditions (1200-850 °C, 5 kbar, ΔFMQ = –1.5). All experiments were run using piston cylinder apparatus at the National Australian University. The experiments were run in 3.5 mm Pt capsules lined with graphite to prevent sulfides from coming into contact with the metal capsule. The Pt capsule was welded and enclosed within 5/8-inch MgO-PyrexNaCl assembly (Fig. A1a). Temperature measurement was carried out with a B-type Pt-Rh thermocouple. We investigated the fate of these elements as the system crystallizes, both in isolation and in equilibrium with silicate melts. The experiments where sulfide liquid was mixed with K, Na and Cl without presence of silicate melt had layered set-up to monitor the melting and mixing process between sulfide phases, alkalis and Cl. Three runs with the same set up and and starting composition were heated up to 1100 °C (at 5 kbar) and then cooled down and quenched at different temperatures (1100 °C, 850 °C and 300 °C). The result of the experiments show that at high temperature the initial layering is not retained and sulfide liquid homogenizes, dissolving ~3 wt% of K, 0.3 wt% of Na and 0.03 wt% of Cl. During quenching, sulfide liquid forms elongated skeletal crystals of mss and interstital residual mixed sulfide matrix. Medium temperature experiment consisted of rounded grains of Ni-rich monosulfide solid solution (mss) in a Cu-rich fine-grained matrix interpreted as quenched liquid. The mss contains negligible concentrations of alkali elements and Cl (< 0.03 wt% of Na, <0.03 wt% K and < 0.003 wt% Cl), whereas the Cu-rich sulfide matrix contains 2.7 wt% of K, 0.6 wt% of Na, and 0.6 wt% of Cl. Slowly cooled to 300 °C experiment contain �10 alkali- and Cl-free pyrrhotite, pentlandite, chalcopyrite and alkali-rich sulfides such as murunskite (K2(Cu,Fe)4S4) and djerfisherite (K6(Fe,Cu,Ni)25S26Cl). The second experiments were designed to examine the behavior of sulfides in equilibrium with silicate melt. The high temperature experiment was quenched after heating to 1250 °C (at 5 kbar), resulting in the formation of sulfide globules comprising elongate skeletal crystals of alkali-free mss intergrown with sulfide matrix of mixed Fe-Ni-Cu composition containing up to 2 wt% Na and 1.3 wt% K, along with 0.1 wt% chloride. Another experiment was slowly cooled from 1250 °C to 300 °C (at 5 kbar) and crystallized to an alkali-rich silicate matrix composed of chromian spinel, nepheline, apatite, Na–K–Ca-carbonate, clinopyroxene and sulfide globules. The sulfide blebs differentiated to pyrrhotite, pentlandite, chalcopyrite and bornite with K, Na or Cl concentrations below detection limit. Results of our experiment show that sulfide liquid can dissolve a substantial amount of alkalis and Cl at high pressures and temperature at geologically relevant redox conditions. Incorporation of these elements into the melt network of magmatic sulfide liquid can affect its physical properties. Thus, the presence of alkalis and Cl dissolved in sulfides could play a crucial role in reducing the melting point of mantle sulfides, akin to the effect of other fluxes on silicate assemblages [9]. Consequently, the presence of alkalis, Cl and water may enhance sulfide melting in localized mantle domains, where molten metal-rich sulfides can be extracted and incorporated into ascending magmas without the requirement of anomalously high heat triggers, widening the spectrum of geodynamic scenarios where fertile melts can be generated on a global scale [10]. Our slowly cooled experiments indicate that alkalis and Cl become immiscible with sulfide liquid during cooling and crystallization. Indeed, magmatic sulfides have never been documented to contain any impurities of lithophile elements or halogens. The only known K and Cl-rich sulfides, such as djerfisherite and murunskite, are very rare and form only in extremely alkali-rich conditions [11]. As a result of immiscibility, it is proposed that sulfide liquid “sweats out” the alkalis and chloride during magma crystallization. This process erases any direct evidence of the former presence of alkalis and Cl in the sulfide itself. Instead, it leaves behind a subtle association of alkali silicates surrounding them, including phlogopite, amphibole, scapolite, and Cl-apatite. However, this process of direct exsolution of Cl, K, Na and water [8] contributes into the metal butget of overlying hydrothermal systems. Magmatic hydrothermal fluids enriched in chloride and alkalis may be important carriers of Cu, Au, and PGEs [e.g., 12] which tend to form aqueous chloride complexes. The exsolution of chalcophile metals, alkalis, and Cl as well as their partitioning into magmatic-hydrothermal fluids supports previous models that link mineralized deep magmatic systems to overlying hydrothermal systems [13]. In summary, alkalis and chlorine play a pivotal role in enhancing metal extraction from the mantle by reducing the melting point of sulfides and lowering their density. During crystallization, these elements exsolve from sulfide liquids into adjacent silicates and late fluid phases, thus increasing the mineralizing potential of magmatic-derived hydrothermal fluids. References: [1] Kanitpanyacharoen W and Boudreau AE (2013) Miner Depos 48(2):193–210 [2] Yuan Q et al. (2023) Lithos 438-439:107014 [3] Ballhaus C and Stumpfl E (1986) Contrib Mineral Petrol 94(2):193-204 [4] Kiseeva E and Wood B (2015) Earth Planet Sci Lett 424:290-294 [5] Wood B and Kiseeva E (2015) Am Mineral 100:2371-2379 [6] Steenstra E et al. (2020) Geochim Cosmochim Ac 273:275-290 [7] Mungal J and Brenan J (2003) Can Min 41(1):207-220 [8] Wykes J and Mavrogenes J (2005) Econ Geol 100:157-164 [9] Sakamaki T (2017) Chem Geol 475:135-139 [10] Holwell DA et al. (2019) Nat Commun 10(1):1–10 [11] Osadchii VO et al. (2018) Contrib to Mineral Petrol 173 (5):1–9 [12] Sullivan N et al. (2022) Geochim Cosmochim Ac 316:230-252 [13] Heinrich C and Connolly J (2022) Geol 50(10):1101-1105 �11 Characterization of Sulfides in Gorgona Island Komatiites: Insights into Cretaceous Mantle Plume Melting and Magmatic Processes Camilo Conde1, Mateo Espinel1, Ana Elena Concha2 1 University of Geneva, 2 Universidad Nacional de Colombia ___________________________________________________________________________ The demand for copper, aluminum, nickel, zinc, and lead is ever increasing. Advances in new models and technology are helping the exploration industry to discover new resources of these important minerals and meet the requirements of the global population. This theme will include all aspects of exploration of these metals, from genesis and mineral processing to the circular economy. Komatiites from Gorgona Island, Colombia, are unique as the only Phanerozoic spinifex-textured ultramafic lavas and the only Cretaceous-age occurrences globally reported (dated at approximately 90 million years old (Kerr et al., 1997)). These rocks have been central to discussions about high temperature melting in mantle plumes, with recent studies developing into the melting event's details, source materials, and melting depths. This study is the first focus on sulfides within Gorgona komatiites, showing the presence of interstitial sulfides, typically larger than 20 microns. Through detailed petrography, SEM imaging, and QUEMSCAN analysis, the research aims to identify and characterize these sulfides, identifying their composition and relating them with magmatic processes. Key sulfides identified include chalcopyrite, pyrite, pentlandite and pyrrhotite positioning Gorgona as a significant new site for magmatic sulfides studies. For the sulfide characterization, the electron microprobe analyzer (EPMA), provide precise compositional data crucial for understanding the magmatic evolution. This is particularly important as it helps determine the timing of sulfur saturation, which in turn reveals whether nickel or copper with PGE becomes more prevalent. Understanding these processes is vital for developing nickelcopper-PGE models and gaining insights into mantle-core conditions, underscoring the geological significance of the Gorgona komatiites. �12 Magmatic and hydrothermal evolution of the PGE-Cu-Ni Current Lake deposit Corredor Bravo, A. P.1, Hollings, P.1, Brzozowski, M.1, and Heggie, G.2 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada. acorredo@lakeheadu.ca 2 Clean Air Metals, 1004 Alloy Drive, Thunder Bay, ON P7B 6A5 Canada. gheggie@cleanairmetals.ca ___________________________________________________________________________ The Mesoproterozoic (1,106.6 ± 1.6 Ma [1]) Current intrusion forms part of the PGE-CuNi mineralized Thunder Bay North Intrusive Complex. The Current intrusion consists of a northwest-trending conduit-type body (wehrlite, lherzolite, olivine gabbronorite ± troctolite) associated with the earliest stages of the Midcontinent Rift System (MRS; [2]) that intruded Archean rocks of the Quetico Basin and is associated to the Quetico Faults System that cross the boundaries between the Quetico basin and the Wabigoon terrane in the Superior Province [3]. To date the intrusion hosts four mineralized zones (Fig. 1); the Current and Bridge Zone in the northwest, the Beaver-Cloud Zone in the middle, and the 437-Southeast Anomaly (SEA) Zone is in the southeast [4]. Geochemical analysis of the intrusion reveal a Figure 1. Schematic model of the Current intrusion well-defined primitive mantle-normalized and the Quetico country rock. Illustration compiled pattern resembling ocean island basalt, in Leapfrog using data provided by Clean Air Metals characterized by LREE enrichment and small Inc. positive anomalies in Nb, La, and Ce relative to Th, suggesting no, or minimal, crustal contamination. The La/Smn values in samples from the Current intrusion range from 1.8 to 2.6, consistent with previous studies and suggesting the originated from an enriched mantle plume. The enriched composition of the magma in the intrusion aligns with other mineralized and unmineralized intrusions related to the MRS, including the Escape, Seagull, Lone Island intrusions, and the Nipigon Sills [5,6,7,8]. The intrusion has slightly lower Sri (0.7021 to 0.7043) and εNd (-1.18 to -4.02) than the typical values of the mantle source at 1100 Ma as well as the Nipigon Sills, Seagull intrusion, and Coubran volcanics [5,6,9]. Given the absence of geochemical anomalies that indicate assimilation of the Archean crust, an enriched SCLM is suggested to have interacted with the parental magma to generate the slightly negative εNd values. Stable isotope analysis suggest that the rocks of the intrusion underwent interactions with magmatic fluids (δ2H from −40 to −80‰, δ18O from 5.5 to 7.0‰; [10,11]), meteoric fluids (δ2H <-80‰, δ18O <5.5‰; [12]), and crustal derived fluids (δ18O >7‰; Figure 2; [13,14]). �13 The assessment of alteration intensity and micro-textural features in the intrusion identified three distinct domains, each showing varying secondary mineral assemblages. Domain A consists of antigorite, actinolitetremolite, clinochlore, epidote, sericite, pyrite, millerite, secondary pyrrhotite, chamosite and secondary magnetite. Domain B consists mainly of lizardite-chrysotile and an increase in the modal abundances Figure 2. δ18O and δ2H values of bulk rock in the four of clinochlore, epidote, sericite, mineralized zones of the Current intrusion (Current, Bridge, pyrite, millerite, and secondary Beaver-Cloud, and 437-SEA) and the surrounding country rock magnetite relative to Domain A. of the Quetico basin. Domain C is composed of talc and carbonate minerals that have replaced the secondary minerals of Domains A and B. Domains A and B were formed by fluids with H2O content derived from meteoric and magmatic sources. Domain A indicates high-temperature alteration processes, with the presence of antigorite suggesting temperatures exceeding 300°C [15]. In contrast, Domain B formed from fluids at lower temperatures (<300 °C; [16]), primarily due to the presence of lizardite-chrysotile. Domain C is associated with later crustal fluids with CO2 contents below 50°C [16]. The alteration processes that have modified the Current intrusion involved the mobilization and incorporation of major elements such as Na2O, Fe2O3, K2O, and CaO in the replacement of primary silicates by secondary silicates, as well as a reduction in mineral volume during the replacement of primary sulfides by secondary sulfides and oxides. References: [1] Bleeker W et al. (2020) Geological Survey of Canada 8722: 7-35 [2] Woodruff L et al. (2020) Ore Geology Reviews 126: 103716 [3] Williams H (1991) Ontario Geological Survey 833-403 [4] Kuntz G et al. (2022) Princeton University 171-204 [5] Heggie G (2005) Lakehead University 365 [6] Hollings P et al. (2007b) Canadian Journal of Earth Sciences 44(8): 1111-1129 [7] Caglioti C (2023) Lakehead University 242 [8] Yahia K (2023) Lakehead University 148 [9] Cundari R (2012) Lakehead University 154 [10] Loewen M et al. (2019) Earth and Planetary Science Letters 508: 62-73 [11] Taylor H (1968) Contributions to Mineralogy and Petrology19(1): 1-71 [12] Ripley E and Al-Jassar T (1987) Economic Geology 82(1): 87-107 [13] Li H (1991) Mcmaster University 138 [14] Ripley E et al. (1993) Economic geology 88(3): 679-696 [15] Evans B (2004) International Geology Review 46(6): 479-506 [16] Barnes I et al. (1973) Economic Geology 68(3): 388-398 �14 Sulfide percolation and drainback process in magmatic conduit system in the Huangshan-Jingerquan mineralization belt Deng, Y.-F.1, Xie-Yan Song2 and Feng Yuan1 1 Ore Deposit and Exploration Center (ODEC), Hefei University of Technology, Hefei 230009, Anhui, P. R. China, dengyufeng@hfut.edu.cn 2 State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, 46th Guanshui Road, Guiyang 550002, P. R. China ___________________________________________________________________________ Magma conduit systems consist of a series of flow-through dykes and sills (Barnes et al., 2016). When sulfides segregated at depth are carried by ascending mafic magmas, they would settle out in magma feeders or chambers at shallower depths as the flow velocities decreased. The differentiated sulfide rich melts in the upper magma chamber could drain back into the feeder dykes to form massive sulfide veins. The Huangshan-Jingerquan Ni-Cu metallogenic belt is located at the southern margin of the Central Asian Orogenic Belt. The total Ni metal reserve of the deposits is about a million tonnes. This makes it the largest orogenic Ni-Cu metallogenic belt worldwide (Deng et al., 2022). The Huangshandong, Huangshan, Tulaergen deposits are the biggest magmatic Ni-Cu deposits in this area, the morphology of the sulfide-bearing mafic-ultramafic complex and occurrence of the Ni-Cu sulfide orebodies in the deposits are obviously different. The Huangshandong complex is rhombus-shaped, ~3.5 km long with a maximum width of 1.2 km. The complex was emplaced in the Gandun Formation carbonaceous slate and meta-sandstone intercalated with limestone. The Huangshandong deposit contains 90 million metric tonnes (Mt) of sulfide ores at average grades of 0.40 wt% Ni (Song et al., 2021). Several ore horizons comprised of disseminated and net-textured sulfides are located at the base of the lherzolite within the complex. A series of concave lenticular orebodies within the gabbronorite occur at the western end of the complex. The tadpole-shaped Huangshan complex is 3.8 km long and up to 0.8 km wide. The base of the complex dips to the west to a depth of ~1000 m and becomes shallower to the east. It was emplaced into the sulfur-barren meta-sandstone and limestone of the Gandun Formation. There is an up to 50m thick thermal metamorphic aureole containing cordierite and epidote around the Huangshan complex. The Huangshan deposit contains 80.4 Mt of sulfide ores with average grades of 0.54 wt% Ni (Zhou et al., 2004). The main stratiform sulfide orebody comprised of disseminated and net-textured sulfides occurs at the base of the lherzolite, which is underlain by gabbronorite. The small Tulaergen dyke consists of lherzolite, websterite and gabbro, and was emplaced in the Wutongwozi Formation meta-tuff and meta-sandstone. The Tulaergen deposit contains ~37 Mt of sulfide ores with average grades of 0.45 wt% Ni (Mao et al., 2008). Variably sized lenticular Ni-Cu sulfide orebodies comprised of disseminated and net-textured sulfides are situated in the central part of the lherzolite . The Ni grade is higher in the upper part of the orebodies than in the lower part. A Fe-rich massive ore vein occurs within the disseminated ores and a Cu-rich massive ore body extends from the ultramafic dyke to the wall-rock (Zhao et al., 2019). The Ni-Cu sulfide deposits along the Huangshan-Jingerquan belt were formed in different locations at different depths in independent conduit systems. The migration and deposition processes of the sulfide liquids in these conduit systems are analogous to the model proposed by Barnes et al. (2016). We propose that some of the sulfides were deposited where the magma pathways changed direction and formed the Tulaergen sulfide-mineralized dyke in the Wutongwozi Formation at relatively deep levels (Deng et al., 2021). The negative correlations between IPGE and Pd/Ir of the Tulaergen massive ore veins suggest a differentiation between IPGE and PPGE triggered by fractional crystallization of the sulfide melt (Zhao et al., 2019). The massive ore veins embedded within the disseminated ores are likely the result of drain back of differentiated sulfide liquids along fractures within the �15 disseminated orebody. Whereas, other sulfide-rich liquids were carried upward into shallow magma chambers. There, the reduction in flow velocity caused the precipitation of sulfide that formed the stratiform or lenticular orebodies in the large magma chambers at relatively shallow depths, such as the Huangshan and Huangshandong complexes hosted in the Gandun Formation. References: [1] Deng Y-F et al (2022) Economic Geology 117: 1867-1879 [2] Song X-Y et al (2021) Lithos390-391 doi:10.1016/j.lithos.2021.106114 [3] Zhao Y (2019) Geochimica et Cosmochimica Acta 249:42-58 [4] Barnes S (2016) Ore Geology Reviews 76:296-316 �16 Weathering of non-ore grade rock from Duluth Complex deposits: Outcomes from comprehensive pre-mining geochemical characterization Diedrich, T.R.1 and Theriault S.2 1 2 MineraLogic LLC, 306 W Superior St., Suite 920, Duluth, MN USA 55802, tdiedrich@mnlogic.com MineraLogic LLC, St. Paul, MN, USA ___________________________________________________________________________ The Duluth Complex, a large, predominantly mafic, intrusive complex in northeastern Minnesota, USA associated with the 1.1 Ga Mid-Continent Rift System, hosts several magmatic copper-nickelcobalt and platinum group element (Cu-Ni-Co ± PGE) deposits. These deposits are generally located along the northwestern boundary of the complex, and in proximity to the Paleoproterozoic-aged metasediments of the Animikie Basin. NewRange Copper Nickel LLC (“NewRange”) is currently assessing and/or engaged in development of the Mesaba and NorthMet deposits within the Duluth Complex. Complementing these efforts, NewRange has conducted an extensive and comprehensive program to characterize the environmental geochemistry of non-ore grade rock, ore, tailings, and unconsolidated surficial materials associated with the deposits. This program includes standard mine waste characterization methods, e.g., ASTM humidity cell tests (HCT); custom designed tests to provide information at different scales of evaluation; multi-faceted mineral characterization components; and field weathering tests. The results of the test program both provide a robust basis for identifying waste rock and water management strategies which would be protective of the environment during mining, and elucidate aspects of the fundamental weathering behavior of gabbroic composition rock. Non-ore grade rocks and tailings from these deposits contain minor amounts of the iron sulfide mineral pyrrhotite, which, during weathering in the presence of oxygen, releases proton acidity through the reaction: 2FeS(po) + 2.75O2 + 2.5H2O → 2Fe2+ + 2SO42- + 5H+ (1) If the reaction continues to proceed in the presence of adequate oxygen, the iron will oxidize and, under circum-neutral pH conditions, precipitate as iron oxides, hydroxides, or oxyhydroxides, generalized as the following: Fe2+ + ¼ O2 + H+ → Fe3+ + ½ H2O (2) Fe3+ + 3H2O → Fe(OH)3(s) + 3H+ (3) While rocks from the deposits do not contain appreciable carbonate minerals to neutralize this proton acidity, they do contain abundant plagioclase and olivine—both of which can neutralize the proton acidity produced during the above reactions during weathering. The environmental geochemical characterization program indicates that there are at least three distinct, but related, neutralization mechanisms active in non-ore rock and tailings from the Duluth Complex. The first neutralization mechanism is the consumption of protons as reactants in silicate mineral dissolution reactions. Common weathering reactions for relatively reactive silicate minerals that are abundant in the complex include the following: Plagioclase (anorthite) dissolution CaAl2Si2O8(s) + 2H+ + H2O → Ca2+ + Al2Si2O5(OH)4(s) (4) Olivine (forsterite) dissolution Mg2SiO4(s) + 4 H+ → 2Mg2+ + H4SiO4 (5) As shown from reactions (4) and (5), every cationic charge unit (for example, 2 cationic charge units for every mol Mg2+ and Ca2+) produced corresponds to a proton being consumed as a reactant. Furthermore, in the presence of atmospheric CO2, dissolution of CO2 into rainwater results in reactions driving towards equilibria between carbonic acid, proton acidity, and bicarbonate alkalinity: �17 H2CO3 → H+ + HCO3- (6) Weathering of silicate minerals in the presence of carbonic acid under neutral pH conditions tends to move reaction (6) toward the reaction products, resulting in accumulation of bicarbonate alkalinity in associated waters. Reaction with the accumulated alkalinity represents a second potential neutralization mechanism. Finally, under select hydrologic conditions (low water to rock ratios), bicarbonate produced in reaction (6) could build up and eventually react with the calcium and magnesium released during reactions (4) and (5) to precipitate carbonate minerals in situ. This reaction leads to the third neutralization mechanism, dissolution of secondary carbonate minerals, and, further, provides a means of capturing and transforming atmospheric CO2 into stable solid phases in the rock. Outcomes from the environmental characterization program support the long-term effectiveness of these three mechanisms in neutralizing acidity from low sulfur rock. A subset of tests have been running for approximately 19 years, and, thus provide direct observational evidence at the multidecadal scale (Fig. 1). Furthermore, geochemical trends from these tests indicate that neutralization reactions will persist at least as long as the sulfide oxidation potential exists. Figure 1. 10th percentile (“P10”) of pH values observed over long-term kinetic testing as a function of initial sulfur content. Each circle represents one HCT. Test durations vary, with the longest tests running for approximately 19 years. The potential to generate drainage with pH less than the blank is dependent on initial sulphur content, with all samples starting with less than approximately 0.2% sulphur maintaining a neutral pH throughout testing. �18 Application of FactSage to Model the Compositional Variability of the Ni-CuPGE Mineralization at the Main Zone of the Tamarack Intrusive Complex El Ghawi, A.K.1 and Mungall, J.E.1 1 Carleton University; Mineral Deposits. Lab Herzberg Laboratories 1125 Colonel By Drive, Ottawa, Ontario, Canada; Karimelghawi@cmail.carleton.ca ___________________________________________________________________________ The Tamarack Intrusive Complex (1105.6 ± 1.2 Ma) is located in NE Minnesota and was emplaced during the early magmatic stage of the Midcontinental Rift System (MRS) [1]. The TIC is composed of a Dike intrusion in the north where the Ni-Cu-PGE mineralization is hosted, and a less explored Bowl intrusion in the south, (Fig. 1). The Dike area of the complex can be divided into many zones which are, from north to south, the Raptor Zone, the Main Zone, and the 164 Zone (Fig.1). Sulfide mineralization in these zones occur as disseminated (1-8 wt.% S), semi-massive (8-25 wt.% S), and massive sulfides (> 25 wt.% S), composed dominantly of pyrrhotite, pentlandite, and chalcopyrite. Massive sulfide bodies in the Main Zone are mostly hosted in the country rocks between the FineGrained Olivine (FGO) and Coarse-Grained Olivine (CGO) Intrusions (Fig.1). Some thin massive sulfide veins also occur in the Main Zone, crosscutting the CGO intrusion. Figure 1: a) Outline of the Tamarack Intrusive Complex. b) Cross section through Main Zone, looking north. Modified after [2]. To understand the compositional variability of the sulfide mineralization at the Main Zone of the TIC, as well as the evolution of the sulfide and silicate magma, chalcophile element compositions (Ni, Cu, Pt, Pd) of sulfide-mineralized rocks have been reported, and a thermodynamic model was developed using the thermodynamic software FactSage 8.3. The FactSage software package uses the ChemSage Gibbs energy minimization routine to minimize the total Gibbs energy of a system with a given set of constraints, and with the availability of the thermodynamic database for the system of interest [3]. These databases have been developed from the optimization of data from the literature, and from new experimental results [3]. The silicate magma composition that is equilibrated with the sulfide liquid in the TIC has been inferred using FactSage. An isenthalpic assimilation-fractional crystallization model has been �19 followed starting with the composition of the Mamainse Point Formation, Volcanic Group 2, that is associated with the same stage that the TIC was emplaced in [4]. The contaminant that was used in this model is the Virginia Formation shale. An R-factor model was then implemented to assess the effects of varied silicate to sulfide mass ratios on the composition of the sulfides at the Main Zone of the TIC [5]. The Rfactor curve passes through the disseminated sulfides, most of which occur between R = 700 and R = 1500 (Fig. 2). The semi massive sulfides are depleted in Pt and Pd compared with the disseminated sulfides. The massive sulfides that mainly occur in the country rocks are Pt and Pd poor and Ni rich, suggesting that these sulfides might be dominated by accumulated monosulfide solid solution (MSS), and there might have been a net loss of fractionated sulfide liquid from the Main Zone of the TIC (Fig.2). The sulfide melt composition calculated at an R factor equal to 900 was then inputted into FactSage and an equilibrium crystallization run was then performed. Trends of MSS and sulfide liquid were generated (Fig. 2). The sulfide melt composition at R = 900 coexists with the early crystallizing MSS at the sulfide liquidus temperature of 1038 °C. With cooling and crystallization of MSS, the sulfide liquid becomes more enriched in Pt, Pd, and Cu. Most semi massive sulfide compositions can be represented as mixtures of MSS and liquid. The extreme enrichment in Pt and Pd shown by sulfide veins cannot be explained solely in terms of MSS fractionation and will be the subject of future study. Figure 2: Variation of Ni, Pt, and Pd versus Cu in the disseminated, semi massive, massive sulfides, and sulfide veins from the Main Zone of the TIC. Concentrations are represented in 100% sulfide. The orange circles along the black curves represent sulfide compositions at different R factors. Solid and liquid compositions during equilibrium crystallization of a sulfide liquid formed at R= 900 are represented by horizontal lines and crosses, respectively. Tie-lines are represented in green dashed lines connecting the coexisting liquid and the early crystallizing solids at 1038 °C and at 817 °C. References: [1] Goldner B (2011) MSc Thesis: 155 [2] Talon Metals (2022) Technical Report [3] Bale C et al. (2009) Calphad 33(2): 295-311 [4] Lightfoot P (1999) OGS 5998: 91 [5] Campbell IH and Naldrett AJ (1979) Econ Geol 74: 1503-1506 �20 Petrophysics Applied to Magmatic Sulfide Deposits: The Physical Properties Mineralogy Link Enkin, R.J.1 1 Geological Survey of Canada, POB 6000, Sidney, BC V8L 4B2, CANADA, randy.enkin@nrcan-rncan.gc.ca ___________________________________________________________________________ Modern mineral exploration demands interpretation formed by the integration of two principal activities: geological mapping and geophysical survey collection. The linking element is the physical properties of rocks, which must be measured, compiled, and analysed. The current emphasis on critical minerals is motivating us to look deeper into previously explored regions to understand the geological settings that are conducive to discovering economic critical mineral systems. Figure 1, Conceptual framework describing the behaviour of various physical properties commonly measured by the mining industry. [1] Physical properties are directly controlled by the bulk composition, the mineralogy, and the texture of rocks [2]. Gravity and magnetic surveys reflect density and magnetic properties, which can mostly be described by the relative amounts of three principal components of mineral families: the light minerals: quartz+feldspar+calcite, the dark minerals: ferromagnesian silicates, and magnetite. Ore minerals and porosity add and subtract density. Importantly, igneous rocks formed in the upper crust usually have a ~10:1 ratio of ferromagnesian silicates to magnetite concentration, and most subsequent geological processes lead to magnetite loss. Electric resistivity and chargeability are controlled by permeability and ore minerals which effectively form networks of wires and capacitors, as revealed by equivalent circuit analysis of spectral impedance measurements. �21 Figure 2, Henkel Plot, Density vs Log(Magnetic Susceptibility), of rocks in the Canadian Rock Physical Property Database. [3] Figure 3, Igneous rocks formed in the upper-crust fall on the Magnetite Trend (FM/M~10), whereas most other geological processes are magnetite destructive. [2] �22 Ultramafic environments, which commonly host Ni-Cu deposits, have a distinctive set of petrophysical properties, which bears directly on their geophysical signatures [4]. Originating from deep, reduced levels, unaltered ultramafics are typically dense and paramagnetic. On hydration and serpentinization, rocks become extremely low density, and iron is rejected from ferromagnesian silicates to form high concentrations of magnetite. These rocks are extremely magnetic and usually display high Koenigsberger ratios, meaning that magnetic remanence dominates aeromagnetic surveys. Carbonation transforms rocks to dense, paramagnetic bodies. Examples from British Columbia and Ontario will illustrate these exotic trends and processes. Figure 4, Henkel plot of ultramafic rocks in the Canadian Cordillera, displaying physical property changes with degree of serpentinization. [4] Through understanding the physical properties - mineralogy link, geophysical interpretation leads to delineation of geological processes and better exploration strategies. References: [1] Dentith, et al. (2020), Geophysical Prospecting, 68: 178-199 doi.org/10.1111/1365-2478.12882 [2] Enkin RJ, et al. (2020), Geochemistry, Geophysics, Geosystems, 21: doi.org/10.1029/2019GC008818 [3] Enkin RJ (2018), Geological Survey of Canada Open File 8460, doi.org/10.4095/313389 [4] Cutts JA, et al. (2021), Geochemistry, Geophysics, Geosystems, 22: doi.org/10.1029/2021GC009989 �23 Regional changes in plume-generated stress linked to MCR (Keweenawan LIP) chonolith emplacement Ernst, R.E.1, El Bilali, H.1, Buchan, K.L.2 and Jowitt, S.M.3 1 Department of Earth Sciences, Carleton University, Ottawa K1S 5B6, Richard.Ernst@Carleton.ca. 273 Fifth Ave., Ottawa K1S 2N4, Canada 3 Nevada Bureau of Mines and Geology, University of Nevada Reno, 1664 N. Virginia Street, Reno 89503, Nevada, USA 2 ___________________________________________________________________________ Introduction: Changes in regional stresses contribute to the formation of many types of ore deposits. Here, we consider the role of plume-generated stresses in metallogeny, and the role of giant dyke swarms of LIPs in monitoring those stresses. We begin with our just-published analysis of the Siberian Traps LIP, its giant dyke swarms and its Norilsk-Talnakh ores [1], and then we consider the Mid-Continent Rift / Keweenawan LIP event as a second example. Norilsk-Talnakh ores of the Siberian Traps LIP: Plume-generated 90° stress change recorded by the transition from radiating to circumferential dolerite dyke swarms of the Siberian Traps LIP may be linked to emplacement of Norilsk-Talnakh ore deposits. [2] showed that the timing of Norilsk-Talnakh Ni-Cu-PGE mineralization in the Siberian Traps LIP is associated with a 90° change in stress, which they attributed to changes in plate stresses. However, as detailed in [1], we propose that this 90° stress change associated with Norilsk-Talnakh mineralization could instead be due to changing plume dynamics as monitored by the transition from the LIP’s giant radiating dolerite dyke swarm to its circumferential swarm (Fig. 1). As noted in [1], the 90° transition from a regional radiating swarm to a circumferential swarm involves a decrease in the radial sigma 1 stress followed by an increase in a hoop-like sigma 1 stress. This implies an intervening period in which the stress is isotropic, a period that we associated with emplacement of the Norilsk-Talnakh mineralization. It is possible that this stress drop led to release of volatiles and allowed ascent and/or lateral emplacement of gas-buoyed magmatic sulphides (e.g. [3-5]). Figure 1: LEFT: Distribution of dyke swarms and volcanic feeder zones associated with the Siberian Traps LIP; modified after [6]. A generalized version of the overall radiating system of dykes and feeder zones is superimposed in orange, and a generalized version of the circumferential dykes is in light purple. Dyke sets: E = Ebekhaya; KO = Kochikha; M = Maimecha. N = Norilsk feeder zones to volcanic flows, which correlate with major fault zones, including the prominent Norilsk-Kharaelakh fault (KF). RIGHT: Timing of volcanic assemblages in the Norilsk region (younging upward), compared with the stress orientations after [2] and with the matching dyke swarm pattern from [1]. Mid-Continent Rift System (Keweenawan LIP): We consider this as a possible example of plume related stress change linked to chonolith mineralization. This major (~1112-1090 Ma) LIP event in the �24 Great Lakes region of North America is associated with an arcuate zone of rifting and a significant number of mineralized intrusions (“chonoliths” and ‘tube-like conduits” in [7]; and “conduit type intrusions” in [8]. [8] noted two main stages in this LIP: the ~1112-1105 Ma Plateau stage, and the ~1100-1092 Ma Rift stage, followed by Late Rift and Post-Rift stages. The numerous chonoliths (conduit type intrusions) were mostly emplaced during the Plateau stage. Figure 2. The 1112–1090 Ma Keweenawan LIP of the Mid-continent Rift of North America. Key elements include volcanics, sills, a circumferential dyke swarm, and exposed and buried intrusive complexes. Also shown are the older ca. 1140 Ma Abitibi dyke swarm and coeval lamprophyre dykes, which may represent a radiating dyke system, and may be related to 1150 Ma Corson diabase intrusions [9] centred just west of the figure. Rift-parallel circumferential Keweenawan dykes from west to east: CC = Carlton County, PR = Pigeon River, CI = Copper Island, P = Pukaskwa, M = Mamainse Point. BM = Baraga-Marquette dykes. Keweenawan sills: LS = Logan, NS = Nipigon sills. Intrusive complexes: DIC = Duluth, CIC = Coldwell, NEIIC = northeastern Iowa. Ca. 1140 Ma radiating dykes: A = Abitibi, ED = Eye-Dashwa, L = lamprophyre dykes. Interpreted mid-crustal intrusive complexes are shown schematically as brown circles. The Goodman Swell has been interpreted as locating the centre of an underlying mantle plume. More details in [10]. [10] described a giant circumferential dyke swarm for the Keweenawan LIP / Midcontinent Rift (Fig. 2), analogous to a Venusian corona. The ages of Pigeon River dykes [7], which we interpret as a portion of the circumferential swarm, indicate emplacement during the Rifting stage, perhaps in association with spreading of the plume head. In our interpretation, plume head arrival and initial domal uplift may have occurred 30 my earlier at 1140-1150 Ma, associated with emplacement of the 1141 Ma radiating Abitibi swarm (Fig. 2; [11]. We speculate that the radiating stress regime at 1140 Ma associated with plume generated uplift persisted until the Plateau stage before transitioning to the circumferential stress regime associated with the Rifting stage. The chonoliths/conduit type intrusions, such as Tamarack, BIC, Eagle and Current Lake [7-8], were mostly emplaced during the Plateau stage, i.e. during our proposed transition from radiating to circumferential stresses. This is a similar timing to our interpretation for the Norilsk-Talnakh ores of the Siberian Traps LIP (Fig. 1; [1]. References: [1] Ernst R et al. (2024) Econ Geol 119: 243–249 [2] Begg et al. (2018). Ch 1, in Mondal S and Griffin W (ed.) Processes and ore deposits of ultramafic-mafic magmas through space and time: Elsevier, p. 1–46. [3] Lesher (2019) Can J Earth Sci 56: 756-773 [4] Yao Z-s and Mungall J (2022) E Sci Rev 227: 103964 [5] Barnes S et al. (2023) Geology 51 (11): 1027-1032 [6] Buchan K and Ernst R (2019), In: Srivastava R et al (eds.) Dyke swarms of the world – a modern perspective: Springer, p. 1–44, [7] Bleeker W et al. (2020). In Bleeker W and Houlé M (ed.). Geol Surv Canada Open File 8722. [8] Woodruff L et al (2020) Ore Geol Rev 126: 103716 [9] McCormick K et al (2018) Can J Earth Sci 55: 111-117 [10] Buchan K and Ernst R (2021) Gondwan Res. 100: 25–43 [11] Ernst R et al. (2018). Earth Planet Sci Lett 502: 244-252 �25 A proposed cryptic common thread among Ni-Cu-PGE-(Au-Te) systems spanning the boundary between Laurasia and Gondwana Fiorentini, M.1*, Holwell, D.2, Blanks, D.3, Cherdantseva, M.1, Denyszyn, S.4, Ince, M.1, Vymazalova, A.3, and Piña Garcia, R.5 1 Centre for Exploration Targeting, Australian Research Council Industrial Transformation Training Centre in Critical Resources for the Future, School of Earth Sciences, University of Western Australia, Australia marco.fiorentini@uwa.edu.au 2 Centre for Sustainable Resource Extraction,School of Geography, Geology and Environment, University of Leicester, United Kingdom 3 BHP Metals Exploration, United Kingdom 4 Department of Earth Sciences, Memorial University of Newfoundland, Canada 5 Dpto. Mineralogía y Petrología, Universidad Complutense Madrid, Spain ___________________________________________________________________________ The long-lived geodynamic evolution of the boundary between Laurasia and Gondwana may have created the ideal conditions for the genesis of a trans-continental Ni-Cu-PGE-(Au-Te) mineralised belt in Europe. This working hypothesis stems from the recent understanding that orogenic processes play a fundamental role in the triggering of chemical and physical processes for the transport of metals from the metasomatised mantle through to various crustal levels. An insight into the polyphased genetic evolution of magmatic sulfide mineral systems is provided by a series of mineralised occurrences located in the Bohemian Massif, Czech Republic. Here, a series of hydrated gabbros contain magmatic sulfides ranging in texture from disseminated to matrix and blebby. These alkaline intrusions with a markedly sodic nature host magmatic sulfide mineralisation revealing a mantle-like signature, with in-situ ∂34S values ranging from -2.4 to +1.8‰. New TIMS UPb data pinpoint emplacement and crystallisation of these mineralised magmas at 363.9 ± 0.6 Ma, with Sm-Nd model ages pointing to involvement of a metasomatised Mesoproterozoic lithospheric mantle in a post-orogenic geodynamic framework. Mineralised intrusions in the Bohemian Massif are strongly analogous to a series of Permo-Triassic (290-250 Ma) hydrated and carbonated ultramafic alkaline pipes containing Ni-Cu-PGE-(Te-Au) mineralisation emplaced in the lower continental crust in the Ivrea Zone, Italy. Despite the significant age difference, mineralisation in the Bohemian Massif and Ivrea Zone is similar in terms of their geochemical and isotopic characteristics, pointing to similar ore forming processes and mantle sources having operated in a syn- to post-Variscan Orogen setting. A subsequent mineralising event is recorded in the Ivrea Zone at ~200 Ma, most likely associated with the Central Atlantic Magmatic Province (CAMP). It is argued that this event reactivated and focussed lower-crustal carbonate- and metal-rich sulfide mineralisation associated with the Permo-Triassic pipes into the ~200 Ma mineralised intrusion known as La Balma Monte Capio. Mineralised systems in the Bohemian Massif and Ivrea Zone are markedly different in size, geometry and overall metal endowment from the larger and better-known Aguablanca system in southern Spain. However, they all share distinctive geochemical and isotopic characteristics pointing to a common DNA: their association with the complex and multi-phase activation of the margin between Laurasia and Gondwana across the Variscan metallogenic belt from the Devonian to the Triassic. �26 The nature and localisation of the magmatic sulfide mineral systems along this belt indicate that enhanced potential for ore formation at lithospheric margins may be due not only to favourable architecture, but also to localised enhanced metal and volatile fertility. This hypothesis may explain why ore deposits along the margins of lithospheric blocks are not distributed homogeneously along their entire extension but generally form clusters. As mineral exploration is essentially a search space reduction exercise, this new understanding may prove to be important in predictive exploration targeting for new mineralised camps in Europe and elsewhere globally, as it provides a way to prioritise segments with enhanced fertility along extensive lithospheric block margins. �27 How exploration geologists can and should use “soft NSRs” to represent assays of Ni-Cu-PGE mineralization Goldie, R.J. Independent Analyst and Director, 54 Peach Willow Way, Toronto, Ontario, Canada M2J 2B6 Raymondgoldie@outlook.com __________________________________________________________________________ A Net Smelter Return (NSR) is the net revenue generated by a block of mineralization, less off-site costs (Goldie and Tredger [1]). Three procedures for computation of the NSRs of Ni-Cu-PGE sulphide mineralization are in common use: values calculated by accountants; mine-specific estimates prepared by mine operators, and “soft estimates” (Goldie [2]). Soft estimates are useful in representing assays of samples taken during exploration for Ni-Cu-PGE deposits. Their computation is based on statistical analyses of the grades and metallurgical properties of ores at operating Ni-Cu-PGE mines, and the smelting and refining fees paid by those mines. There are three reasons why exploration geologists should express assays of samples as soft estimates of NSRs: (i) representing assays as single numbers facilitates their graphical representation, such as on contour maps; (ii) the computation of soft estimates may reveal that, as is common in mineralization that is rich in PGE, the mineralization contains substances or has mineralogical issues that could lead to a smelter penalizing or even rejecting a potential mine’s products (Goldie [3]); (iii) representation of assays as single numbers not only facilitates their comprehension by the readers of company press releases, it may also reduce the chances that investors apply invalid rules-of-thumb to those assays, resulting in expensive misunderstandings. References: [1] Goldie R and Tredger P (1991) Geosci Canada 18:159-171 [2] Goldie R (2023) Min Economics https://doi.org/10.1007/s13563-023-00400-3 [3] Goldie R (2022) Aust Inst Mining & Metal, Int Mining Geol Conf: 222-235 �28 Characterizing the Early (Plume) and Main (Rifting) Stages in the evolution of the Midcontinent Rift Good, D.J. Department of Earth Sciences, Western University, London, ON N6A 5B7, Canada, dgood3@uwo.ca ___________________________________________________________________________ The mid-Proterozoic Midcontinent Rift (Keweenawan Large Igneous Province) contains the most diverse assemblage of basalt rock types for any LIP on earth. In this study, six of the eight main basalt types in the rift are compared to the global distributions of ocean plateau, ocean island basalts and continental large igneous province basalts using a combination of two sophisticated classification strategies based on high precision incompatible trace element data (after O’Neill, 2016 and Pearce et al., 2021). The two basaltic sequences that are not described here occur in the northeast quadrant of the Midcontinent Rift and were shown by Good et al. (2021) to have been derived from a metasomatically modified mantle source. Thus, they are not suitable candidates for interpretation using the classification strategies as applied here. Basalt data for the Midcontinent Rift were compiled by the author from detailed studies of trace element geochemistry at numerous sites around Lake Superior by several researchers during the past 30 years. Data for oceanic basalts were compiled by O’Neill (2016) as part of his impressive study to highlight the usefulness of calculated coefficients to characterize REE diagram patterns (λ0, λ1 and λ2). Data for continental Large Igneous Provinces were compiled by Pearce et al. (2021) to show the usefulness of geochemical proxy diagrams to define which of the various petrological mechanisms operated during their formation (the LIP Print Approach). Figure 1: Discrimination boundaries for basalts sourced from different Mantle Regions plotted on the O’Neill diagram (left hand side). See text for discussion. Group 52 corresponds to basalt that shows characteristics of both plume and upper mantle source. Taken together, these comparisons show that Midcontinent Rift data in groups 2, 3 and 4 are like ocean plateau basalts and groups 1 and 5 are like ocean island basalts. That is, data are in excellent agreement with the hypothesis that basalt in group 2 was derived by partial melting in the Upper Mantle whereas groups 5 and 1 were derived by partial melting in the Mantle Plume, but at depths below the pyrope garnet and majorite garnet stability boundaries, respectively. This and other evidence suggest Groups 3 and 4 were derived by partial melting in a subduction modified depleted mantle source. Based on these inferred origins for the various basalt units, the Midcontinent Rift exhibits spatial and temporal zonation. Spatially, the mantle plume was centred beneath the west �29 central portion of what is now Lake Superior. Temporally, the effects of mantle plume volcanism occurred throughout the Early Stage of the Midcontinent Rift but had vanished before the end of the Hiatus Stage. During the subsequent Main Stage of magmatism, mafic rocks were derived primarily from the Upper Mantle, presumably by decompression melting as the crust thinned during extension. Figure 2: Midcontinent Rift basalt of groups 1 to 6 plotted in the LIP print diagrams of Pearce et al. (2021). See text for discussion. Figure 3: Model for basaltic melt source regions of the Midcontinent Rift Event: (a) During the Early Stage, most melts are generated in the mantle plume with lesser amounts generated in the overlying mantle and/or subduction modified lithospheric or asthenospheric mantle; (b) During the Main stage, most of the melts are generated by decompression melting in the upper mantle as the crust thins during extension. References: [1] O’Neill, H.St.C, (2016) Journal of Petrology, Vol. 57, No. 8, 1463–1508 [2] Pearce, J.A. et al., (2021) Lithos 392–393 (2021) [3] Good, D.J. et al. (2021) Journal of Petrology, 2021-07, Vol.62 (7) �30 Lithospheric structure controls for large magmatic Ni-Cu discoveries Hayward, N.1,2 1 Centre for Exploration Targeting, University of Western Australia, Crawley, WA 6009, Australia. NHayward@protonmail.com 2 PredictOre Pty Ltd, 1/40 Victory Terrace, East Perth, WA 6009, Australia ___________________________________________________________________________ To sustain the clean energy transition, society needs to increase the reserve base of green and critical mineral ore deposits containing metals such as copper (Cu), lithium (Li), nickel (Ni), cobalt (Co), rare earth elements (REE) and platinum group elements (PGE). Discovery of large new polymetallic Ni-Cu (±PGE, Co) sulfide deposits can help meet this need, but their discovery rates have declined over the last 25 years, and they present very difficult greenfield exploration targets because of their rare occurrence, very small footprints, large range in formation depths, concealment among extensive magmatic provinces, and increasing challenges for exploration land access. The fact that most recent Ni sulfide discoveries were found in magmatic provinces that had no previously known Ni-sulfide resources favours a first-mover Ni exploration strategy. The minerals industry needs improved mineral system models that more accurately predict the location of new districts (camps) and large deposits in remote and covered terrains with low data quality and availability. This study [1] demonstrates that low-cost three-dimensional lithospheric structure targeting has the power to significantly improve the accuracy and precision of targeting large magmatic Ni discoveries. It also addresses a disconnect between conceptual academic models for magmatic Ni-Cu (also Cu-Au) systems, which largely omit lithospheric structural controls on magma flux and intrusion emplacement, and the practice of explorers to empirically target proximity to lithospheric-scale fault zones for mineralised intrusions. This disconnect is exacerbated by a lack of quantitative analyses of the spatial accuracy, precision and causality of lithospheric structures that are inferred to be control ore deposition, which this study also addresses. The 1st-order (subprovince-scale) lithospheric structure control on magmatic Ni-Cu ore distribution is widely accepted to be along the sutured edges of paleo-cratonic blocks with preserved Archean subcontinental lithospheric mantle [2]. However, 2nd- to 3rd-order controls on emplacement of district-scale mineralised intrusion clusters and individual deposits along craton edges remain poorly understood. Two alternative models previously proposed are: (i) emplacement of dyke-like intrusions in dilational jogs along strike-slip faults [3], and (ii) emplacement of intrusion clusters near intersections of transverse translithospheric faults (TLFs) [4,5]. These models invoke predominantly vertical magma transport along fault conduits with subjacent sulphide saturation. Other models invoke long-distance lateral magma transport through interconnected sill and dyke complexes and potential for distal sulphide saturation [6,7] which, if correct, would greatly increase the permissive search space. New structural interpretations and quantitative analyses were completed globally for 72 Ni deposits with >50kt Ni (equivalent) metal. This extensive sample population covers a range of magmatic Ni deposit settings from intracratonic to pericratonic and arc-related, and from Mesoarchean to Cenozoic. Six detailed case studies addressing the lithospheric structure architecture controls on giant Ni deposits will be presented for Voisey’s Bay, Noril’sk-Talnakh, Kabanga, Jinchuan, West Musgrave, and the Cape Smith Belt. Less detailed examples will also be shown from the Midcontinent Rift, southern Africa, China, and western Australia. From quantitative analysis of the 72 regional structural case studies, the 1st-order control for all large magmatic Ni-Cu deposits is observed to be ≤30 km from paleocraton edge-parallel translithospheric faults, and specifically in their hangingwall where inclined. This relationship holds for all magmatic NiCu deposit settings. Furthermore, large intracontinental Ni deposits are also located ≤30 km from 2ndorder transverse translithospheric faults that intersect paleocraton edges (Fig. 1). However, for pericratonic and Archaean greenstone komatiite settings, proximity of Ni deposits to transverse �31 translithospheric fault intersections is not widely recognised or preserved. In one exception, clusters of komatiitic Ni deposits in the Agnew-Wiluna greenstone belt are observed to have a semi-regular spatial periodicity along strike with a mean spacing of ~22 km, and this is controlled by the intersection of local cryptic transverse rift faults [8]. Prioritising target proximity to certain translithospheric fault intersections can significantly reduce subprovince-scale search areas (~104-105 km2) to a few prospective districts (~102 km2). The largest deposits are found closest to (but rarely within) the most prominent translithospheric faults. At smaller scales, a few deposits are localised along small-scale dilational jogs in wrench faults, but this control is relatively rare. At deposit scale, controls on emplacement of mineralised channel-like flows and pipe-like intrusions (chonoliths) are typically more stratigraphic than structural, where overpressured, high temperature magmas self-generate pathways. Productive stratigraphic horizons are dominated by rheologically weak and highly fusible metasedimentary or gneissic units. A model (Fig. 2) is proposed where the root zones of translithospheric fault intersections initially channel fertile mantle melts into the deep crust. Ascent of buoyant overpressured magmas is then dispersed up to a few 10s km lateral to inclined master fault conduits through complex dyke-sill-dyke networks in steeper hangingwall fault splays, their damage zones, and rheologically weak contacts. The extreme magma flux required to form large Ni sulfide deposits results from positive magmadeformation feedbacks and bottom-up self-organisation. Targeting translithospheric fault intersections therefore requires a more systematic bottom-up and hierarchal approach to structural mapping, where the roots of cryptic lithospheric faults are defined, and structures are rated by scale, dip, and geodynamic behaviour. Fig. 1: Deposit size class versus distance to both edge-parallel and transverse TLFs. Fig. 2: Concept section showing dispersal of ascending mafic-ultramafic melts through dyke-sill networks with high magma flux in hangingwall of paleocraton edge translithospheric fault zone. References: [1] Hayward N (2024) Submitted to Econ Geol [2] Begg G et al (2010) Econ Geol 105: 1057-1070 [3] Lightfoot P and Evans-Lamswood D (2015) Ore Geol Rev 64: 354-386 [4] Myers J et al (2008) Can J Earth Sci 45: 909-934 [5] Begg et al (2018) Processes and Ore Deposits of Ultramafic-Mafic Magmas through Space and Time, Elsevier: 1-46 [6] Lesher C (2019) Can J Earth Sci 56: 756-773 [7] Ernst R et al (2019) J Volcanol Geotherm 384: 75-84 [8] Perring C (2016) Econ Geol 111: 1159-1185 �32 Deep orogenic magmatic Ni and Cu sulfide systems in the Curaçá Valley, Brazil Holwell, D.A.1, Thompson, J.2 , Blanks, D.E.1, Oliver, E.1, Porto, P.3, Oliviera, E.3., Tomazoni, F.4, Lima, A. 4, D’Altro R.,4., Graia, P.4 and Sant’Ana, T.4. 1 Centre for Sustainable Resource Extraction, University of Leicester, UK PetraScience Consultants, Vancouver, Canada 3 Ero Copper, Vancouver, Canada 4 Ero Caraiba, Brazil 2 ___________________________________________________________________________ The magmatic sulfide ores of the Curaçá Valley, Brazil, form an unusual subgroup of intrusion-related sulfide deposits. They are Cu-rich in general, with some Ni-dominant deposits on a district scale. They are located in small, hydrous mafic-ultramafic intrusions emplaced into the lower-mid crust at around peak metamorphic conditions. The metallogeny of the majority of known Curaçá Valley deposits are dominated Cu-sulfide deposits with abundant bornite, chalcopyrite with magnetite and hydrous silicates; phlogopite being abundant to semi massive in places. They have high Cu/Ni and Au/PGE ratios and have abundant telluride minerals. In addition, recently discovered Ni-rich deposits contain pyrrhotite, with pentlandite loops, some Co-rich pyrite, very minor chalcopyrite that is associated with phlogopite. Both deposit types are very low in IPGE (Os, Ir, Ru) and Rh. The Cu-Au-Te signature of the Curaçá Cu deposits, with abundant hydrous phases, particularly phlogopite, is consistent with an alkaline mafic genetic model, as these metallogenic characteristics have been identified in many of intrusions worldwide and usually represent post-subduction magmatic systems [1,2]. There are (at least at present) many more Cu occurrences identified in the Valley than Ni ones, and if the district is taken as a whole, then the overall metallogenic signature is still Cu-Au-Te dominant with some Ni and PGE. However, further discoveries of Ni would change this overall mass balance. An alternative, or possibly additional process that may have occurred is large scale sulfide liquid fractionation, where Ni-rich mss separates from Cu-rich sulfide liquid that crystallises at a lower temperature to Cu-rich iss. The general Cu-Au-Te(+Pd) signature of the Cu ores from the Curaçá Valley are entirely consistent with an iss signature, but it would imply sulfide liquid fractionation within the magmatic plumbing system on a district scale of km to tens of km. Whilst this may seem extreme, the process is clearly scalable from the mm to cm scale seen in many sulfide blebs and patches up to deposit scale such as the Cu-rich veins at Sudbury. Textural differences are striking, with the Ni ores having sulfides as disseminations, interstitial patches and net textured and massive sulfides representative of sulfide coexisting with silicate minerals. The Cu ores in stark contrast commonly show textures indicative of migrating Cu sulfide liquid, intruding as veins and breccia fills along with net-textures and insterstitial sulfides. The importance of phlogopite and other volatile-rich mineral phases with the Cu sulfide would also be consistent with a fractionated, volatile-rich sulfide liquid migrating over a wide range of distances. It is possible that the Curaçá Valley (and the O’okiep district in South Africa), represent deep magmatic sulfide systems at the roots of orogenic belts, formed from hydrous, metasomatized mantle sources, and where sulfide liquid fraction on a km-scale can produce both Ni- and Cu-rich deposits across a district. Regardless of the preferred individual or combined model, there is clearly potential for further discoveries in this complex setting References: [1] Holwell DA (2019) Nat Com 3511 [2] Blanks DE (2020) Nat Com 4342 �33 Spatial distribution, lithological associations, and geochemical signatures of Ring of Fire Intrusive Suite within the McFaulds Lake Greenstone Belt in the Superior Province: Implications for the Ni-Cu-PGE, Cr, and Fe-Ti-V Metal Endowment of the Region Houlé, M.G.1,2, Sappin, A.-A.1, Lesher, C.M.2, Metsaranta, R.T.3, Rayner, N.4, and McNicoll, V.4 1 Geological Survey of Canada, Lands and Minerals Sector, Natural Resources Canada, 490 Couronne Street, Québec City, QC G1K 9A9 Canada michel.houle@nrcan-rncan.gc.ca 2 Mineral Exploration Research Centre, Harquail School of Earth Sciences, Goodman School of Mines, Laurentian University, 935 Ramsey Lake Road, Sudbury, ON P3E 2C6 Canada 3 Earth Resources and Geoscience Mapping Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, ON P3E 6B5 Canada 4 Geological Survey of Canada, Lands and Minerals Sector, Natural Resources Canada, 601 Booth Street, Ottawa, ON K1A 0E9 Canada ___________________________________________________________________________ The McFaulds Lake greenstone belt (MLGB) is an arcuate (>200km long) belt within the Superior Province in northern Ontario that records episodic volcanism and sedimentation spanning from ca. 2.83 to 2.70 Ga and has been subdivided into several tectonostratigraphic assemblages [1]. One of the dominant geological features of the Mesoarchean to Neoarchean MLGB is the semi-continuous trend of mafic to ultramafic intrusions belonging to the Ring of Fire intrusive suite (RoFIS) [2], which hosts world-class Cr deposits, a major Ni-Cu-(PGE) deposit, and potentially significant Fe-Ti-V-(P) prospects. Intrusive bodies of the RoFIS occur within almost all volcanic-dominated supracrustal rock assemblages. The RoFIS has been subdivided into two subsuites based on their spatial distribution, lithological associations, geochemical signatures, and mineralization styles: the Ekwan River (ERSS) and Koper Lake (KLSS) subsuites [3, 4]. Although the mafic to ultramafic intrusive bodies of these subsuites have similar emplacement/crystallization ages (KLSS = 2732.9 to 2735.5 Ma vs. ERSS = 2732.6 to 2734.1 Ma), they are significantly different in many respects: 1) the KLSS is spatially much more restricted than the ERSS; 2) the KLSS is composed of dunite, peridotite, chromitite, pyroxenite, and gabbro, whereas the ERSS is composed of abundant gabbro and ferrogabbro with lesser anorthosite and rare pyroxenite and does not contain any olivine-rich ultramafic rocks; 3) the KLSS typically hosts Cr and Ni-Cu-(PGE) mineralization (e.g., mainly within the Esker intrusive Complex), whereas the ERSS typically hosts Fe-Ti-V-(P) mineralization (e.g., Big Mac and Thunderbird intrusions); and 4) the KLSS (higher MgO, Ni and Cr) and ERSS (higher FeOT, Ti and V) have clear differences in their geochemical trends indicating a distinct geochemical evolution (Fig. 1). Furthermore, ERSS ferrogabbro locally intrudes KLSS units, however, the opposite relationship is also observed at one locality. The magmatic evolution is still being debated, but the above observations suggest temporally overlapping but discrete ultramafic-dominated (KLSS) and mafic-dominated (ERSS) intrusions with complex contact relationships, rather than a single, large, tectonically dismembered layered ultramafic-mafic intrusion, as previously suggested [2]. A newly recognized intrusive body in the area contains olivine-rich ultramafic rocks and chromitite seams, like other members of KLSS, but both are enriched in Fe relative to rocks of the KLSS. This highlights the presence of several types of oxide-rich mineralization within the RoFIS. These include high Cr and low Fe chromitite seams typically associated with most of the Esker intrusive complex, intermediate Cr and Fe chromitite seams sporadically associated with parts of the Esker intrusive complex, and high Fe and low Cr magnetitite seams typically associated with EKSS’s intrusive bodies. Regardless of their origin, the exceptional metal endowments, and the wide diversity of mineral deposit types within the mafic and ultramafic rocks of the RoFIS, including Cr, Ni-Cu-(PGE), and Fe-TiV-(P) mineralization, of the McFaulds Lake greenstone belt highlight the likelihood of discovering additional mineral resources elsewhere within the Superior Province and other frontier areas throughout the Canadian Shield. �34 Figure 1: Binary plots of major and trace elements (anhydrous and normalized to 100%) of the mafic to ultramafic intrusions within the Koper Lake and Ekwan River subsuites of the Ring of Fire intrusive suite. A) FeOT versus MgO. B) Ni versus MgO. C) Ti versus Cr. D) Cr/V versus MgO. Data are from [5, 6, and Houlé, unpublished data]. References: [1] Metsaranta RT and Houlé MG (2020) Open File Rep 6359:360p. [2] Mungall JE et al. (2011) Proc GAC-MAC-SEG-SGA Ann Meeting Ottawa 2011:148 [3] Houlé MG et al. (2018) Open File Rep 8589:441-448 [4] Houlé MG et al. (2020) Open File Rep 8722:141-163 [5] Kuzmich B et al. (2015) Open File Rep 7856:115-123 [6] Metsaranta RT (2017) Ont Geol Surv Misc Rel Data 347 �35 Spatial distribution of mafic and ultramafic units in the Canadian north: Implications for critical minerals (Ni, Cu, Co, PGE) potential Houlé, M.G.1,2, Bédard, M.-P.1, Lesher, C.M.2, and Sappin, A.-A.1 1 Geological Survey of Canada, Lands and Minerals Sector, Natural Resources Canada, 490 Couronne Street, Québec City, QC G1K 9A9 Canada; michel.houle@nrcan-rncan.gc.ca 2 Mineral Exploration Research Centre, Harquail School of Earth Sciences, Goodman School of Mines, Laurentian University, 935 Ramsey Lake Road, Sudbury, ON P3E 2C6 Canada ___________________________________________________________________________ The transition to low-carbon economy that is taking place in Canada and elsewhere around the world is driving renewed interest in critical minerals, especially in battery minerals, like Ni and Co. Canada is one of the world's leading magmatic sulfide Ni producers, as attested by the presence of at least 5 world-class Ni mining districts (e.g., Sudbury-ON, Thompson-MB, Raglan/Expo-QC, Voisey’s Bay-NL, and Lynn Lake-MB). These Ni-Cu-Co-(PGE) deposits are associated mainly with magmatic maficultramafic mineral systems. Canada contains a very large number of mafic and ultramafic units across the country, but their total abundance is unknown and of these, only a handful are partially to well characterized. As example, a recent global compilation has reported only 52 layered intrusions in Canada [1]. Thus, an extensive compilation of mafic and ultramafic unit area is currently underway by the Geological Survey of Canada (GSC) to aid in identifying historic and future mineral resources (Fig. 1). Figure 1. Distribution of mafic and ultramafic units within northern Canada. Grey dashed line represents the approximate boundary of GEM-GeoNorth area (north of ~54° N latitude). Geological provinces are from [2]. NiCu-Co-(PGE) deposits: 1 = Canalask/Wellgreen, 2 = Turnagain, 3 = Muskox, 4 = Dinty, 5 = Axis/Currie/Rea, 6 = Nickel King, 7 = West Bear, 8 = Lynn Lake, 9 = Ferguson Lake, 10 = Rankin Inlet, 11 = Raglan Nickel Belt – Raglan and Expo horizons, 12 = Hope Advance sector, 13 = Chrysler-Erickson sector, 14 = Redcliff sector, 15 = Blue Lake sector, and 16 = Voisey’s Bay. The first step in this compilation is a large-scale spatial inventory of mafic and ultramafic units. To date, over fifteen thousand units have been catalogued north of ~54° N latitude (within the GEMGeoNorth area), based on geological maps available at scales ranging from large scale (1:500,000 to �36 1:63,360) to more detailed scale (1:5,000 or less), in the vicinity of known and historic Ni-Cu-(PGE) deposits, and where areas of interest have been identified due to the preponderance of maficultramafic units or nickel showings. Within the GEM-GeoNorth area, the largest proportions of mafic and ultramafic bodies are related to three major Proterozoic Large Igneous Provinces (LIPs) worldwide: the Franklin LIP (~0.72 Ga), the Mackenzie LIP (~1.27 Ga), and the Circum-Superior LIP (~1.88 Ga), which exhibit quite variable metal endowments [3]. Thus far, no deposits have been found in the Franklin LIP, only small Ni-Cu-(PGE) and Cr deposits have been identified in the Mackenzie LIP (e.g., Muskox), whereas world-class mining districts occur within the Circum-Superior LIP (e.g., Raglan, Thompson). Because of the size of the Muskox intrusion (over 120 km long), its worldwide recognition, and the historical work done by the GSC in 1960s [4], this prospective unit will receive a special attention within the framework of this compilation. In the Canadian context, magmatic Ni-Cu-Co-(PGE) deposits with variable abundances of sulfides/alloys and metal ratios have formed throughout geological time (Mesoarchean to Cenozoic), from a wide range of parental magmas (komatiitic to quartz dioritic), in a wide range of tectonic settings (extensional to convergent), so none of these attributes are particularly critical exploration variables. Almost all the historic and current Canadian production comes from large mining districts (e.g., Sudbury, Thompson, Voisey’s Bay, Raglan, and Lynn Lake), all of which still have significant large brownfield potential. However, several other regions have excellent greenfields potential, as evidenced by the presence of many historic and recently discovered Ni-Cu-Co-(PGE) deposits. The preliminary results of the GSC compilation indicate, for example, that more than 50 Ni-Cu-Co-(PGE) deposits occur north of ~54° N latitude, including Triassic flood basalt-related subvolcanic intrusions (e.g., Wellgreen, Canalask) and Jurassic plutonic zoned/composite complexes (e.g., Turnagain) within the Cordillera Province; Neoarchean norite- and gabbro-related intrusions (e.g., Nickel King, Ferguson Lake), Paleoproterozoic komatiite-related (e.g., Rankin Inlet) and gabbro-related (e.g., Lynn Lake) intrusions within the Western Churchill; Paleoproterozoic volcanic (e.g., Raglan) and subvolcanic (e.g., Expo Ungava) komatiitic basalt-related lava channels and channelized dikes within the Central Churchill; Paleoproterozoic volcanic-subvolcanic picritic to komatiitic basalt-related intrusions, differentiated ultramafic to mafic sills, and glomeroporphyritic gabbroic sills within the Eastern Churchill; and Mesoproterozoic plutonic troctolitic (e.g., Voisey’s Bay) intrusions within the Nain Province. The degree of preservation of these deposits ranges from essentially unmetamorphosed and undeformed (e.g., Voisey’s Bay) through low-grade metamorphosed with very localized deformation (e.g., Raglan) to medium- and high-grade metamorphosed with widespread deformation (e.g., Ferguson Lake, Thompson). Overall, the ubiquitous distribution of ultramafic and mafic units highlighted by this compilation indicates that there is not only significant potential for the discovery of additional Ni-Cu-Co-(PGE) mineralization in traditional and established mining camps, but also has tremendous potential for the discovery of new Ni-Cu-Co-(PGE) and Cr-PGE deposits in under-explored regions of Canada. References: [1] Smith WD and Maier WD (2021) Earth Sci Rev 220:1-36 [2] Wheeler JO et al (1996) GSC A Map Series 1860A [3] Ernst RE (2014) Larg Ign Prov; Camb Univ Press: 667 [4] Scoates JS and Scoates RFJ (2024) Lithos 474-475: 1-40 �37 Copper and komatiitic magmatism – source of copper in the Sakatti Cu-NiPGE deposit in northern Finland Höytiä, H.1,2, Peltonen P.1, Halkoaho, T.3, Makkonen, H.V.3,5 and Virtanen, V.J.1,5 1 Department of Geosciences and Geography, P.O. Box 64, FI-00014 University of Helsinki, Finland Anglo American plc (AA Sakatti Mining Oy), Tuohiaavantie 2, FI-99600 Sodankylä, Finland 3 Geological Survey of Finland, P.O. Box 1237, FI-70211 Kuopio, Finland 4 Geological Survey of Finland, Vuorimiehentie 2K, FI-02150 Espoo 5 Suomen Malmitutkimus Oy, Kuopio, Finland 6 Institut des Sciences de la Terre d’Orléans, UMR 7327, CNRS/Université d’Orléans/BRGM, Orléans, France 2 ___________________________________________________________________________ Copper is an important commodity in most of the magmatic Ni-Cu-platinum group element (PGE) sulfide deposits. Several nickel camps and deposits, e.g. Noril’sk (Russia), Sudbury and Raglan (Canada), and Jinchuan (China), host individual mineralizations and mineralization types that are more enriched in Cu compared to Ni. Host rocks of these Cu-enriched Ni-deposits vary from mafic (derived from tholeiitic parental magmas) to ultramafic (derived from ferropicritic or komatiitic basaltic parental magmas) and they bear evidence of variable, but generally high silicate/sulfide mass ratios (R factor) from c. 100 to > 1000 during their formation [1.Important Cu-enrichment mechanisms also include mantle source with low Ni/Cu, fractional crystallization of segregated sulfide phase, assimilation of Cu from external source, and post-magmatic modification of sulfides by fluids. Sakatti is a Cu-Ni-PGE deposit in the Paleoproterozoic c. 2.5-1.8 Ga Central Greenstone Belt (CLGB) in northern Finland with total reported resources of 44.4 Mt @ 1.9% Cu, 0.96 % Ni, 0.05% Co, 0.64 g/t Pt, 0.49 g/t Pd and 0.33 g/t Au [2]. The deposit was discovered by Anglo American Plc in 2009 and can be sub-divided into six distinct ore types: 1) Ni-rich massive ore, 2) Cu-rich massive ore, 3) Ni-Cu interstitial ore in gabbronorites, 4) Cu-rich disseminated ore, 5) Cu-PGE-rich stockwork vein ore, and 6) Py-rich massive ore. The mineral assemblage consists of chalcopyrite, pyrrhotite, pentlandite, pyrite and Ni-Pt-Pd tellurides of the melonite-merenskyite-moncheite series. The sulfide phase shows evident fractionation from Ni-rich monosulfide solid solution (mss) to Cu-rich intermediate sulfide solid solution (iss) [3, 4]. Bulk of the sulfides in Sakatti show narrow range of δ34S, between +2 and +4 ‰, indicating non-magmatic source of sulfur for much of the deposit. The Sakatti sulfide deposit is underlain by argillaceous sediments with thick anhydrite-gypsum intervals, some of which, are in direct contact with the cumulates and show prominent magma-country interaction. The sulfide ores in Sakatti are hosted by chonolith-style magma conduit composed of ortho-, mesoand adcumulates, pegmatoidal gabbronorites and fine-grained komatiitic rocks. These are derived from a komatiitic parental magma in equilibrium with Fo92-93 olivine (c. 19–21 wt. % MgO). Olivine in the Sakatti deposit contains relatively high Ni contents (2500–3500 ppm), which can be due orthopyroxene fractionation in the lower crust en route to surface [5]. Typical mineral assemblage contains olivine + chromite ± orthopyroxene ± clinopyroxene ± plagioclase. All host rocks show one to two orders of magnitude enrichment in LREE compared to that of chondrite. The age of the ultramafic magmatism is constrained to c. 2054 Ma [6], which corresponds to a global Ni-Cu-PGE mineralizing event with coeval ages in e.g. Bushveld (South Africa), Mirabela (Brazil) and Elanskii (Ukraine) complexes, related to the final break-up of the supercontinent Kenorland. With R factor modelling it is not possible to achieve the observed low Ni/Cu ratio at Sakatti. The same is true also with the N factor (zone refining) or with the multistage upgrading modelling. Therefore, four other processes that could account for the anomalously high Cu-content and low Ni/Cu of Sakatti are discussed: 1) Magma generation from Cu-enriched metasomatized mantle source 2) removal of Ni-rich mss at depth, 3) Assimilation of copper from country rocks, and 4) postmagmatic upgrade of the Cu grades. �38 [1] Cu-enriched mantle source is commonly attributed to metasomatized mantle. Uncontaminated CLGB komatiites have MREE-enriched hump-shaped patterns, reflecting limited marks of metasomatized source at the time of their separation [7]. Mantle source alone contributing the copper contents in Sakatti is doubtful, as the degrees of partial melting for parental melts are high (c. 15-25 %) [5, 7]. [2] Brownscombe et al. [3] proposed that the primary mss was segregated at earlier stage and the Cu-rich portion of it was re-assimilated and injected into the current host cumulates by later magmas that did not equilibrate with the sulfides, possibly due to a kinetically controlled process, similar to that proposed for varying metal tenors in the Raglan deposits [8]. However, the most primitive olivine cumulates also host the most primitive mss, indicating that host magma took part to the sulfide segregation to some degree. R factors for Sakatti are generally low (50–100) and the modelled Ni/Cu values are generally much higher than the ones observed, therefore indicating that there must be additional processes contributing to the varying Ni/Cu ratios. However, an alternating option could arise from computational simulations, where Ni/Cu ratios between 1.9 and 0.4 ratios can be produced for sulfides during closed fractional crystallization scenario depending on the initial sulfur content of the parental magma [5]. [3] Magma-sulfate interaction textures, positive δ34S, elevated Fe3+ contents in chromite [9] and similarity in REE-patterns between cumulates and sulfate rocks indicate that Sakatti host rocks have assimilated their sulfate-bearing country rocks during ascent and/or in-situ. However, most of the seemingly unaltered sulfate sediments bear very low Cu contents, and besides, regionally potential assimilants have Cu contents typically below 150 ppm [10, 11]. Yet copper collection during assimilation could be facilitated by oxidized magma, coexisting magmatic fluid(s) [12] and formation of xenomelts [13], which would form as a response to assimilation of carbonatesulfate sediments. [4] Re-Os [14], U-Pb [6], Pb-Pb, and Cu isotope results [15] point towards later remobilization of the Cu-rich portions of the ore. However, no obvious alteration patterns resulting from late hydrothermal fluids are found in the deposit. Age constraint for post-magmatic modification spans from c. 1.9 to 1.8 Ga [6, 14], which include ages of the numerous Au and IOCG (Iron-Oxide -Copper-Gold) deposits within the CLGB [16], suggesting mobility of copper during this period. Massive sulfide ores, however, pose a strong chemical buffer, which means they are not easily extensively affected by fluid activity. The discussed processes are not mutually exclusive and could have contributed to the high Cu budget. The available data indicates that processes 2) and 4) were the dominant controls of Cu. [1] Burrows D and Lesher M (2012) Econ Geol 16:515–552 [2] Anglo American Ore Reserves and Mineral Resources Report (2022) [3] Brownscombe W et al. (2015) Min Dep of Finland:211–252 [4] Fröhlich F et al. (2021) Can Min 59:1485–1510 [5] Virtanen V et al. (in review) [6] Höytiä et al. (in review) [7] Hanski E and Kamenetsky V (2013) Chem Geol 343:25–37 [8] Li Y and Mungall J (2022) Econ Geol 117:1131–1148 [9] Silventoinen S (2020) M.Sc. thesis Uni Helsinki, 95 p. [10] Haverinen J (2020). M.Sc. thesis, Uni Helsinki, 82 p [11] Köykkä J et al. (2019) Precamb Res 331:105364 [12] Iacono-Marziano G et al. (2017) Ore Geol Rev 90:399–413 [13] Lesher C (2017) Ore Geol Rev 90:465–484 [14] Moilanen M et al. (2021) Ore Geol Rev 132:104044 [15] Höytiä H et al. (2023) 14th Int Pt Symposium Abs Vol:235–236 [16] Niiranen T (2005) PhD thesis synopsis D6, Uni Helsinki, 27 p. �39 The Koperberg Suite of the Okiep Copper District - an overlooked target for magmatic nickel sulphides in a convergent margin system Hunt J.P.1, van Schalkwyk L.1, Smart E.1 and Benhura C.1 1 Orion Minerals, 16 North Road, Dunkeld West, Randburg 2196, South Africa, johnpaul.hunt@orionminerals.com.au ___________________________________________________________________________ The Okiep Copper District (OCD) is the oldest formal mining district in South Africa dating back to 1852, having produced 2.2 Mt of Cu from 32 mines and 70% of this total having been mined from just 5 mines. It is located in the Bushmanland Subprovince of the Namaqua Sector of the Namaqua-Natal Metamorphic Province (NNMP) which is younger than but broadly contemporaneous with the Grenville-Kibaran orogenies associated with the amalgamation of the Rodinia supercontinent (Figure 1). Steep northwards subduction occurred to the south of the NNMP. Roll-back of the subducting slab causing dextral trans-tensional extension in the continental back-arc environment, where the Bushmanland Subprovince is presently located. Metamorphic grade, in general, increases from amphibolite facies in the north to upper granulite facies in the south. Namaquan orogenesis occurred in two episiodes: the Okiepian Episode (1180-1210 Ma) involving crustal shortening and the intrusion of large volumes of granitic sheets (now granite gneiss); and the Klondikean Episode (10201040 Ma) involving mafic underplating, ultra-high-temperature metamorphism, granitic sheets, dextral transtension, constrictional fabrics, and crustal thinning [1] and importantly the intrusion of the Koperberg Suite. The Koperberg Suite is by volume predominantly anorthositic with associated jotunite, biotite diorite, leuconorite, norite, hypersthenite, and glimmerite intruded as discrete magmatic events. It intruded as ENE and ESE oriented, irregular and discontinuous dykes, sills and plugs into an overwhelmingly granulite-facies granite-gneiss terrane, which were commonly focused within kinked anticlines Figure 2. Distribution of ore deposits and mining districts in the various Subprovinces and Terranes of the Namaqua Sector of the Namaqua-Natal Metamorphic Province. The Okiep Copper District is located in the �40 northern portion of the Bushmanland Supbprovince, with the Kliprand Nickel District located approximately 150km to the southeast [2]. known as ‘steep structures’. The quartzites and metapelites of the Khurisberg Subgroup have historically been a potentially lithological control with the majority of known mineralised intrusions occurring stratigraphically above this horizon. It has long been established that the sequence of intrusion is from felsic to mafic: anorthosite was the earliest intruded magma, followed by ferrodiorites, then norites, and ultimately orthopyroxenites (hypersthenites) and magnetitites. The majority of mineralisation is associated with the increasingly more mafic lithotypes, the majority being hosted by magnetitite, orthopyroxenite and norite, then ferrodiorite, and only a small proportion of mineralisation being hosted by anorthosite. The Koperberg Suite ores are grouped based on the main sulphide assemblage [3], namely the: 1. Carolusberg-type ore: the most abundant type characterised by a bn-mgt (± cp) assemblage 2. Narrap-type ore: characterised by a typical iss assemblage (cp + po ± pn), 3. Hoit-type ore: an intermediate assemblage characterised by a bn-cp It had long been held that the overwhelmingly abundant bn-mgt assemblage within the OCD was a consequence of post-magmatic oxidation of a primary sulphide assemblage as represented by the Narrap type, however, recent trace element and isotopic studies suggest this not to be the case [3]. Oxidation of the magma liquid and the corresponding immiscible sulphide liquid occurs with progressive crystallisation and fractionation of Fe2+-rich phases and post-magmatic oxidation of the sulphide is not supported by textural and geochemical observations. The Hondekloof Ni-Cu deposit is located approximately 150km SE of the OCD in the Kliprand Nickel District (KND). This gabbronorite-hosted basal massive sulphide mineralisation is part of a larger suite of intrusives including anorthosite, norite, quartz norite, diorite, glimmerite, and an earlier extensively developed charnoenderbite. The mineralisation assemblage of magnetic pyrrhotite with minor exsolved cobaltian pentlandite, chalcopyrite as well as pyrite is typical of orthomagmatic Ni-Cu-Co bearing sulphide bodies derived from a typical mss assemblage [4]. On the basis of petrological and petrochemical similarities, the gabbronorite host is correlated with a pre-Koperberg Suite “two pyroxene granulite” of the OCD, effectively having an identical gabbronoritic mineralogy and chemistry. This mafic unit was historically regarded as being unmineralized and therefore avoided. A two-stage model was proposed [4] which is simplified as follows: Stage 1. an early nickeliferous mss sulphide liquid was extracted from the magma chamber associated with preto syn-tectonic gabbronorites. Stage 2. renewed tectonism and compression of the magma chamber resulted in the extraction of first an anorthositic suite, followed by increasingly more mafic assemblages and ultimately the most hypermelanic phases and the low-S, high-mgt, cupriferous residual iss sulphide liquid from the base of the magma chamber. The exploration implications for the OCD is that the historical exploration and exploitation has concentrated on bn-mgt rich ores, traced on surface and followed down to depth, or efficiently mapped by magnetic geophysical surveys. The distribution of “two-pyroxene granulites” has been mapped but entirely disregarded until now. A number of known deposits have elevated Ni concentrations, such as Okiep East and Narap Mine, and it is noted that these are in proximity to increased occurrences of two-pyroxene granulites. Modern transient electromagnetic (TEM) surveys have only recently been completed and map a number of discrete anomalies both in proximity to Koperberg Suite intrusives and distinct from them. At two localities, Ezelsfontein East and Nous, both located within the OCD, drilling confirmed the presence of massive and disseminated Ni-Cu sulphide, establishing proof of concept and opening up the OCD to new aspects in its exploration potential. References: [1] Dewey J et al. (2006) Precam Res 150(3-4), 173–182 [2] Rozendaal A et al. (2017) SAJG 120(1), 153–186 [3] Marima E (2022) Unpubl. MSc Univ. Rhodes 120p [4] Hamman J N et al. (1996) SAJG 99(2), 153-16 �41 A multi-methodological approach: Combining textural observations and geochronology to study the J-M Reef Package and its Hanging Wall, Stillwater Complex, Montana Jenkins, M.C.1*, Corson, S.2, Geraghty2, E., Kamo S.L. 3, Lowers, H.4, and Mungall, J.E.5 1 U.S. Geological Survey, Geology, Minerals, Energy, and Geophysics Science Center, Spokane, Washington, USA *mcjenkins@usgs.gov 2 Sibanye-Stillwater, Columbus, Montana, USA 3 Jack Satterly Geochronology Laboratory, University of Toronto, Toronto, Ontario, Canada 4 U.S. Geological Survey, Geology, Geophysics, and Geochemistry Science Center, Denver, Colorado, USA 5 Department of Earth Sciences, Carleton University, Ottawa, Ontario, Canada ___________________________________________________________________________ The J-M Reef is a world-class platinum-group element (PGE) deposit hosted in the 2.709 Ga Stillwater Complex in Montana, USA [1, 2]. The J-M Reef is the accumulation of PGE-enriched sulfide minerals located in the Anorthosite subzone I (ASZI) of the Troctolite-Anorthosite zone I in the Lower Banded series of the complex (Fig. 1A). Anorthosite subzone I is comprised of anorthosites, troctolites, peridotites, and norite adcumulates and heteradcumulates. The cumulates that host economic J-M Reef sulfide mineralization are generally coarse-grained to pegmatoidal in texture and may be any of the rock types found in ASZI. These coarse-grained rocks are called the Reef Package (Fig. 1B). The top of the Reef Package is marked by a textural discontinuity between the coarse-grained cumulates and relatively fine-grained cumulates in the hanging wall. The surface that marks the top of the Reef Package is informally called the hanging wall contact and economic PGE mineralization is not found above this contact [3]. The sulfide mineralization that makes up the J-M Reef may not always be present; therefore, tracing the reef location during mine development can be challenging [1]. The hanging wall contact can always be identified in drill core and underground workings even where the J-M Reef is not present making this contact an important marker horizon during mining. �42 Figure 3. 1A) Stratigraphic section showing the series and zone nomenclature for the Stillwater Complex [4]. 1B) Stratigraphic section showing the subzones of Troctolite-Anorthosite zone I [3, 5, 6]. The general location of the hanging wall contact (HWC) is shown as a dashed line. 1C) Preliminary U-Pb zircon ages (yellow) and published zircon ages of the JM Reef from Wall et al. (2018; blue) [2]. Zircon mean ages are shown as points and error bars correspond to 2σ. Electron backscattered diffraction was used to investigate the microtextural change at the hanging wall contact from four intersections. In general, the results show that rocks in the hanging wall are characterized by finer crystal sizes and a well-developed B-type fabric typical of cumulates from layered mafic intrusions (Fig. 2) [7]. In contrast, the rocks that host the J-M Reef are found to be coarse-grained and do not have a strong rock fabric indicating that they likely crystallized under conditions where crystal settling, compaction, or magmatic flow did not impact the orientations of the crystals. Instead, the Reef Package may have crystallized in situ where crystals grew to impingement without a preferred orientation. These findings do not resolve the origin of the hanging wall contact as it could plausibly represent either a resumption of normal layered mafic intrusion petrogenetic processes like crystal settling and/or compaction or it could represent a pre-existing cumulate layer that acted as an aquitard to the magma that formed the Reef Package. Figure 4. Bivariate plots showing rock fabrics from the hanging wall (HW) and Reef Package (RP) based on the foliation number (F#) vs the lineation number (L#) defined as the ratios of the maximum eigen value divided by the intermediate eigen value for the crystallographic axes. The F# is equal to e1/e2 for the (010) plane and the L# is equal to the e1/e2 for the [100] direction. Stillwater cumulates from the Picket Pin (PP) area are shown as solid black triangles. The shaded fields show where data from other layered mafic intrusions (LMIs), fast spreading centers (FSC), and slow spreading centers (SSC) plot on the diagram [3, 7]. To test the hypothesis that the hanging wall contact represented a cumulate layer that existed prior to the emplacement of the magma that formed the Reef Package, high-precision chemical abrasionisotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) zircon U-Pb dating was used to determine the age of rocks below, above, and within the Reef Package (Fig. 1B). The mean ages of zircons below the Reef Package are approximately the same as those in the Reef Package. In contrast, mean ages from zircons in the hanging wall are older than the Reef Package—including one substantially older sample (SW48904-150-153) from Norite subzone (Fig. 1B). These results support the hypothesis that the hanging wall contact represents the base of a pre-existing cumulate layer that caused the magma that formed the J-M Reef Package to pool at the level of the Reef Package. The zircon ages are consistent with out-of-sequence CA-ID-TIMs zircon ages that have been reported from Stillwater [2] and the Bushveld [8, 9] complexes. The age results do not place firm constraints on the origin of the J-M Reef deposit as either the hydromagmatic model [10] or orthomagmatic �43 model [11] could plausibly form the reef with or without the presence of an overlying igneous aquitard layer. References: [1] Jenkins et al. (2020) Econ Geol 115: 1799-1826 [2] Wall et al. (2018) J Petrol 59: 153-190 [3] Jenkins et al. (2022) J Petrol 63: egac053 [4] Todd et al. (1982) Econ Geol 77: 1454-1480 [5] Turner et al. (1985) Mont Bur Min Geol 92: 210-230 [6] Corson et al. (2002) 9th Plat Symp 101-102 [7] Cheadle and Gee (2017) Elem 13: 409-414 [8] Mungall et al. (2016) N Comm 7: 13385 [9] Scoates et al. (2021) J Petrol 62: egaa107 [10] Boudreau (1999) J Petrol 40: 755-772 [11] Jenkins et al. (2021) Precambr Res 367: 106457 Note: Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. �44 Nickel-copper-platinum group elements potential of mafic and ultramafic intrusions in northwestern Ontario Jonsson, J.1, Malegus, P.1, Churchley, S.1, Price, R.1 1 Resident Geologist Program, Ontario Geological Survey, Ministry of Mines, Suite B002, 435 James Street South, Thunder Bay, ON P7E 6S7 Canada ___________________________________________________________________________ Globally, magmatic sulphide deposits host significant resources of nickel, copper, cobalt and platinum group elements (PGE). These deposits occur as concentrations of sulphide minerals hosted within mafic to ultramafic intrusive rocks and are widespread across Ontario, occurring in every Precambrian geologic terrane. Ontario is home to 10 operating mines in magmatic sulphide deposits: 9 within the Paleoproterozoic Sudbury Igneous Complex and one within the Neoarchean Lac des Iles Complex. In 1999, Operation Treasure Hunt was initiated by the Ontario Government to stimulate mineral exploration by acquiring new airborne geophysical data, surficial and bedrock geochemical data, and development of new methods. In 2003, following completion of the Operation Treasure Hunt project, the Ontario Geological Survey published a report that assessed 109 mafic to ultramafic intrusions across Ontario [2]. The purpose of this part of Operation Treasure Hunt was to characterize and publish data for intrusions known to be prospective for PGE-dominated magmatic sulphide mineralization. Many of the intrusions studied during Operation Treasure Hunt were host to significant known mineralization, including current and past-producing mines, and several of these intrusions are the focus of ongoing mineral exploration. Despite the work by Vaillancourt et al. [2], there are hundreds of mafic to ultramafic intrusions in Ontario that have not been systematically assessed for magmatic sulphide mineralization potential. Many of these intrusions have favourable characteristics for potentially containing magmatic sulphide deposits, including geophysical anomalies (e.g., magnetic, conductivity), overburden geochemical anomalies and known sulphide mineralization. In 2023, the Resident Geologist Program of the Ontario Geological Survey initiated a project to systematically characterize geochemistry of a subset of mafic-ultramafic intrusions in northwestern Ontario that largely have not been subject to significant historical evaluation by academic researchers, government surveys, or mineral exploration companies. Evaluating the geochemistry of mafic to ultramafic intrusions can provide insight into the magma history, tectonic setting and potential for economic metal endowment. Factors that may influence metal endowment, that can be determined from the examination of geochemical data, include determining magma source characteristics, the timing of sulphur saturation and the degree of interaction of the magma(s) with their country rocks. Careful evaluation of physical characteristics and whole-rock geochemistry can inform future mineral exploration and/or the development of models for the emplacement of mafic to ultramafic intrusions and any hosted mineralization. Initial sample collection and analytical work took place during 2023. Areas of interest are shown in Figure 1, and include the Red Lake, Onaman–Tashota, and Heaven Lake greenstone belts. In this display, we provide examples of preliminary results and interpretations from areas targeted in the first year of field work, including the Trout Bay intrusion (Red Lake greenstone belt), Westwood intrusion (northeast of the Lumby Lake greenstone belt), and the Big Ghee Lake intrusion (south of the Shebandowan greenstone belt). �45 Figure 1. Simplified bedrock geology map of a portion of northwestern Ontario, showing project target areas: Red Lake greenstone belt (outlined in blue); Heaven Lake greenstone belt (outlined in black); and Onaman–Tashota greenstone belt (outlined in white). Regional geology modified from Ontario Geological Survey [1]. References [1] Ontario Geological Survey 2011. 1:250 000 scale bedrock geology of Ontario; Ontario Geological Survey, Miscellaneous Release—Data 126 – Revision 1. [2] Vaillancourt, C., Sproule, R.A., MacDonald, C.A. and Lesher, C.M. 2003. Investigation of maficultramafic intrusions in Ontario and implications for platinum group element mineralization: Operation Treasure Hunt; Ontario Geological Survey, Open File Report 6102, 335p. �46 Petrogenesis of the mineralized horizons in the Offset and Creek zones, Lac des Iles Complex, N. Ontario Jonsson, J.1, Hollings, P.1, Brzozowski, M.1, Bain, W.1, Djon, L.2 1 2 Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Impala Canada, 69 Yonge Street, Suite 700 Toronto, ON M5E 1K3 Canada ___________________________________________________________________________ The Lac des Iles Complex is a Neoarchean (2.69 Ga; D.W. Davis cited in Stone et al., 2003) polyphase mafic-ultramafic complex located in the Marmion terrane of the Superior Province, 85 km north of Thunder Bay, Ontario, Canada. The intrusive complex can be subdivided into two discrete subcomplexes: the ultramafic-dominated North Lac des Iles Complex and the mafic-dominated South Lac des Iles Complex (SLDIC). The SLDIC has been subdivided into four intrusive series, termed the gabbronorite, breccia, norite, and diorite series (Decharte et al., 2018). To date, economic Pd-rich mineralization has been discovered in both the breccia and norite series, and occurs proximal to the contacts between the breccia and gabbronorite series and between the breccia and norite series. The objectives of this study are to i) evaluate the mechanisms of formation of the mineralized horizons near the contact between the breccia and norite domains in the Offset and Creek zones of the SLDIC, ii) evaluate the role that crustal contamination played in this process, and iii) assess the tectonic setting in which the SLDIC formed. The breccia and norite series are both composed of varitextured, brecciated, and equigranular leucocratic-melanocratic norites and gabbronorites, and their altered equivalents. The breccia series contains a greater proportion of brecciated and varitextured rocks, while the norite series contains a greater proportion of equigranular rocks. All pre-alteration lithologies are essentially plagioclaseorthopyroxene cumulates with varyingly minor quantities of interstitial clinopyroxene, biotite, magnetite, chalcopyrite, pentlandite, and pyrrhotite. Variable degrees of hydrothermal alteration are indicated by the presence of tremolite-actinolite and talc (after pyroxenes), chlorite and sericite (after plagioclase), and pyrite (after pyrrhotite). Although the breccia and norite series are mineralogically similar, the breccia series is generally more leucocratic (i.e., higher plagioclase/pyroxene ratio) than the norite series. Neodymium isotopic evidence indicates that the Offset and Creek Zone magmas were crustally contaminated. ɛNd values of 19 analyzed samples range from +0.38 to -3.47 (median = -2.13), which is consistently more negative than the ɛNd value of +2.24 expected in an uncontaminated mantlederived magma that crystallized at 2.69 Ga. The crustal contaminant that imparted the negative ɛNd values is unlikely to be the tonalitic gneiss that hosts the SLDIC, as the ɛNd value of one reported tonalitic gneiss sample is -1.77 (Brugmann et al., 1997). The lack of correlation between ɛNd and geochemical or spatial variations suggests that variable crustal contamination was not the cause of the geochemical variability observed within the Offset and Creek Zones. Samples from both the breccia and norite series have similar trace-element chemistry, including enriched LILE/LREE patterns, flat HREE patterns, and pronounced negative Nb anomalies. Although these characteristics can be caused by assimilation of crustal material, it is more likely that they are the result of formation of the parental magma in a magmatic arc. Evidence for this interpretation includes low Nb/Yb ratios, high Ba/Th ratios, low Th content, and the lack of correlation between geochemical variability and Nd isotopic variability. Evidence from S isotopes of sulfide minerals and whole-rock geochemistry suggests that the addition of crustal S was not necessary in the formation of the Pd-rich mineralization within the Offset and Creek zones. δ34S values of 54 crystals from 17 samples range from -0.37‰ to +3.28‰ VCDT (median = +1.11‰), with values from 52 of 54 crystals falling in the expected range of mantle-derived sulfur (0 ± 2‰; Seal, 2006). Based on the association of low Cu/Pd ratios with high Pd values, Offset and Creek zone ores formed at high R factors, which were likely high enough to cause the PGE �47 enrichment without incorporation of crustal sulfur. The higher degree of Pd enrichment in the Offset Zone compared to the Creek Zone was likely due to a greater amount of sulfide liquid in the Offset Zone that also underwent higher R factors; the distribution of sulfide liquid and magma flow may have been influenced by primary structural constraints on the geometry of the intrusion. No evidence was found for significant low-temperature remobilization of chalcophile elements, including the PGEs. The compositional variability observed within the breccia and norite domains suggests that both domains formed via multiple pulses of compositionally similar magma. The proximity of mineralization to the interpreted feeder conduits suggests that the distribution of mineralization is largely the result of PGMs/Pd-rich pentlandite crystallizing as the magma transitioned from the feeder structure outwards into the periphery of the intrusive complex. This process may have repeated several times as successive magma pulses infiltrated the partially crystallized intrusive complex, resulting in the redistribution of ores in brecciated zones. References: Brugmann, G.E., Reischmann, T., Naldrett, A.J., and Sutcliffe, S.H., 1997. Roots of an Archean volcanic arc complex: the Lac des Iles area in Ontario, Canada. Precambrian Research, vol. 81, p. 223-239. Decharte, D., Hofton, T., Marrs, G., Olson, S., Peck, D., Perusse, C., Roney, C., Taylor, S., Thibodeau, D., and Young, B., 2018. Feasibility study for Lac des Iles mine incorporating underground mining of the Roby Zone. North American Palladium, NI 43-101 Technical Report, 435p. Seal, R.R., 2006. Sulfur isotope geochemistry of sulfide minerals. Reviews in Mineralogy and Geochemistry, vol. 61, p. 633-677. Stone, D., Lavigne, M.J., Schnieders, B., Scott, J., and Wagner, D., 2003. Regional geology of the Lac des Iles area, in Summary of Field Work and Other Activities 2003. Ontario Geological Survey, Open File Report 6120, p. 15-1 to 15-25. �48 Quantum full tensor magnetic gradiometry to better define conduit type NiCu-PGE targets Kaski, K.1, Smith, J.1, Tschirhart, Victoria1, Heggie, G., Enkin, R.1 1 Natural Resources Canada, Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8 __________________________________________________________________________ Magmatic Ni-Cu-PGE sulfide deposits are frequently associated with small conduit-type intrusions. These deposit types are challenging exploration targets due to their limited size, absence of distinct alteration halo or distant footprint, complex and variable morphology, and unpredictable depositional sites of sulfides [1]. Additionally, mafic rocks often retain significant remanent magnetization, which, if overlooked, can lead to inaccurate modelling and targeting of these deposits. The dwindling number of new Ni discoveries over the last decade highlights the necessity for the development and implementation of novel methods to facilitate improved detection and targeting of these deposit types at the regional to deposit scales. Traditional airborne Total Magnetic Intensity (TMI) data is the most used and cost-effective surveying method for identifying and delineating intrusions which can host nickel deposits. Although there is incredible value in TMI data there are challenges with data interpretation including issues of nonuniqueness, scalar measurements, and the inability of TMI to differentiate remanence from the induced field. The full tensor magnetic gradiometry (FTMG) technique, which measures the full magnetic gradient tensor at each measurement point, overcomes many of these limitations and offers numerous advantages including: (a) superior resolution of near-field sources, (b) enhanced detectability at low-magnetic latitudes, (c) automatic removal of the regional field and diurnal variations, and (d) additional target information from a single flight line. FTMG can therefore provide improved discrimination of magnetic sources and a more complete picture of the subsurface magnetic properties. Commercialized quantum FTMG sensors currently use Superconducting Quantum Interference Device (SQUID) technology and due to their size and strict temperature requirements are most appropriate for large-scale airborne surveys. With SQUID sensors being unsuitable for ground and uncrewed aerial vehicle (UAV) surveys a new generation of compact, rugged diamond-based quantum magnetometers are in development and offer an alternative FTMG technology for ground and UAV surveying. Although quantum FTMG offers significant advantages in sensitivity and the opportunity for improved targeting of ore deposits, its widespread adoption by the mining industry has been hindered, in part, by a lack of capabilities and expertise in the areas of data handling and interpretation. As part of a larger collaborative research project, the Geological Survey of Canda (GSC) with Defense Research and Development Canada, aim to de-risk quantum magnetic gradiometer use across Canada through the field testing and validation of quantum FTMG systems and comparing them with traditional total magnetic field systems and non-quantum FTMG systems. As part of this project, the GSC is undertaking a comprehensive study on the Ni-Cu-PGE bearing Escape and Current Intrusions of the Thunder Bay North Intrusive Complex which present as complicated magnetic signals that are strongly affected by remanent magnetization. Here we present preliminary results from the processing of TMI data (Fig. 1) provided by Clean Air Metals Inc. and compare this with newly acquired SQUID FTMG data. Unconstrained (Fig. 2) and constrained magnetic susceptibility inversions derived from both datasets are presented to examine the 3D geometry and extent of the Ni-Cu-PGE mineralized mafic-ultramafic intrusions. Magnetization vector inversions (MVI) are also presented and offer additional insights into the extent and strength of remanent magnetization developed in association with these intrusions. Physical rock properties �49 of the intrusions are used to further validate the MVI models and gain insights into the processes controlling the localization of remanent magnetization. This study marks the first instance of generating publicly accessible quantum FTMG data covering critical mineral deposits in Canada. Ultimately, the aim is to enhance exploration capabilities by validating tools applicable to critical metal deposits, whose intricate geophysical characteristics pose challenges for conventional geophysical techniques. Figure 5. Residual magnetic intensity of the Escape and Current Intrusions of the Thunder Bay North Intrusive Complex. Figure 6. Unconstrained inversion results representing highest modelled magnetic susceptibility contrasts in the Escape and Current Intrusions of the Thunder Bay North Intrusive Complex. References: [1] Barnes, S.J., 2023. Lithogeochemistry in exploration for intrusion-hosted magmatic Ni–Cu–Co deposits. Geochemistry: Exploration, Environment, Analysis, 23(1), pp.geochem2022-025. �50 Exploration-Based Classification Scheme for Magmatic Ni-Cu-(PGE) Systems Lesher C.M.1 and Houlé M.G.2,1 1 Mineral Exploration Research Centre, Harquail School of Earth Sciences, Goodman School of Mines, Laurentian University, Sudbury, ON P3E 2C6, Canada, mlesher@laurentian.ca 2 Geological Survey of Canada, Lands and Minerals Sector, Natural Resources Canada, 490 Couronne Street, Québec City, QC G1K 9A9, Canada ___________________________________________________________________________ Magmatic Ni-Cu-Co-(PGE) deposits have typically been classified on the basis of age, magma type, and tectonic setting [e.g., 1] or cumulus mineralogy [2], but they formed throughout geological time (Mesoarchean to Cenozoic) from a wide range of parental magmas (komatiitic to quartz dioritic) with different cumulus mineralogy in a wide range of tectonic settings (extensional to convergent), so none of these attributes are particularly useful exploration variables. A more useful classification is based on the nature of the host units: 1) impact melt sheets, 2) differentiated layered maficultramafic intrusions, 3) channelized mafic-ultramafic lavas/sills/dikes, 4) differentiated/zoned maficultramafic pipes/plugs/stocks, and 5) orogenic peridotites, each of which is fundamentally different: Group Group 1 
 Impact melt sheets Group 2 
 Differentiated layered maficultramafic intrusions Group 3 
 Channelized mafic-ultramafic flows/sills/dikes Subgroup A Exogenetic (external S ± metals) B Endogenetic (internal metals ± S) A Layered differentiated intrusions B Composite differentiated intrusions C Weakly layered differentiated intrusions A Flows B Sills Setting Impact structure Primarily large igneous province Primarily large igneous province Group 5 Orogenic peridotites Convergent B Zoned noncomposite C Unzoned composite D Unzoned non-composite A Ophiolite complexes B Peridotite massifs Bushveld SA, Great Dyke ZI, Muskox NU, Stillwater MT Duluth MN, Montcalm ON Americano do Brasil BR, Bird River MB, Kotalahti FI D Chonoliths A Zoned composite Sudbury ON Morokweng SA C Dikes Group 4 
 Differentiated/zone d mafic-ultramafic pipes/plugs/stocks Examples Oceanic crust/ mantle Alexo ON, Kambalda WA, Perseverance WA, Raglan QC Dumont QC, Jinchuan CH, Mt Keith AU, Namew Lake SK, Norilsk RU, Pechenga RU, Thompson MB Eagle MI, Eagle’s Nest ON, ExpoMéquillon QC, Hongquiling CH, Huangshan CH, Limae CH, Voisey’s Bay NL, Qingkuangshan CH Kalatongke CH, Limoeiro BR, Mirabella BR, Nebo-Babel WA, Nkomati (Uitkomst) SA, Savannah WA, Tamarack MN Duke Island AK, Giant Mascot BC, Mordor AU, Xiarihamu CH Jingbulake CH, Lynn Lake “EL” MB, Gordon Lake ON, Aguablanca SP, Lynn Lake “A” MB, Turnagain BC Lynn Lake “FLGC” MB, HituraVammala FI Acoje PH, Baptiste (Decar) BC, Potosí CU, Oman, Shetland UK, Troodos CY Ivrea-Verbano IT Group 1 impact melt sheets thus far include only one example with economic Ni-Cu-PGE mineralization, the 1850 Ma, 260 km-diameter Sudbury (ON) structure [see e.g., 3]. The 146 Ma, 80 km-diameter Morokweng (SA) structure contains subeconomic Fe-Ni-Co sulfide nodules and veins that appear to be derived in part from the impactor [e.g., 4]. No other impact structures with Ni-Cu- �51 PGE mineralization have been identified [e.g., 5], most likely because they were too small to generate enough impact melt and/or lacked the abundant economic (e.g., Shakespeare) to subeconomic (e.g., Nipissing and East Bull Lake Intrusive Suites) Cu-Ni-PGE mineralization in the target rocks at Sudbury. Group 2 differentiated layered intrusions commonly host sub- to uneconomic reef-type PGE-(Cu)(Ni) mineralization (e.g., Centre Hill ON, Romeo II QC), but sometimes contain economic reef-type PGE-(Cu)-(Ni) mineralization (e.g., Bushveld Merensky and UG-2 reefs, Stillwater J-M reef, Great Dyke MSZ) and where they do contain Ni-Cu-(PGE) mineralization it is normally low-grade (e.g., Duluth Complex, Muskox). Because they are A) periodically replenished and well-differentiated magma chambers (e.g., Bushveld), B) composite differentiated intrusions (e.g., Duluth), or C), weakly layered differentiated intrusions they are only rarely/locally dynamic enough to generate high-grade Ni-Cu(PGE) mineralization. Group 3 channelized mafic-ultramafic flows/sills/dikes include some of the world’s largest, highestgrade Ni-Cu-(PGE) deposits/camps (e.g., Raglan, Thompson, Kambalda, Jinchuan, Norilsk-Talnakh) and many small high-grade deposits (e.g., Eagle, Tamarack, Eagle’s Nest). They are typically enriched in olivine or Opx, poorly to weakly differentiated, and interpreted to have formed at high magma fluxes, enhancing thermomechanical erosion of S-bearing country rocks and upgrading of metal contents in sulfide xenomelts. In low-grade deposits, Ni-Co-IPGE in olivine can be redistributed into sulfides during serpentinization (e.g., Dumont, Mt Keith). Group 4 differentiated/zoned mafic-ultramafic pipes/plugs/stocks have typically been subdivided based on their cumulus mineralogy into: Opx-poor (e.g., Uralian-Alaskan type), Opx-rich (e.g., Giant Mascot-type), Gabbroic, and Noritic [e.g., 2], but those characteristics also apply to many deposits in Group 3. Most are zoned and/or multiphase, representing relatively low magma fluxes. They can contain economic mineralization (e.g., Aguablanca, Giant Mascot, Lynn Lake, Xiarihamu), but typically have low tonnages, grades, and tenors. Group 5 ophiolites and peridotite massifs (AKA orogenic peridotites) often contain subeconomic to economic abundances of Cr ± PGE mineralization, and typically only contain currently economic abundances of Ni after being lateritized [6]. However, the sparse amounts of Ni-Cu-(PGE) may be “upgraded” by liberation of Ni-Co-IPGE during serpentinization of olivine under fO2 conditions that favour stabilization of Ni sulfides and/or Ni ± Pt ± Ir-Os alloys (e.g., Decar). Each group exhibits variations in form, degree of olivine/Opx accumulation, and degree of differentiation, sometimes hampering classification into Groups 2, 3, and 4. They also exhibit variations in original (and current) orientations, compositions, and degrees of zoning/differentiation/ layering/brecciation. They also formed from a wide range of magma types, some derived from depleted peridotitic mantle (undepleted in PGE relative to Ni-Cu-Co) and some derived from fertilized pyroxenitic mantle (depleted in PGE relative to Ni-Cu-Co). The single most important element to generating high-grade and high-tonnage deposits appears to be high magma flux, but lower-grade and lower tonnage deposits can form at lower magma fluxes. References: [1] Naldrett AJ (2004) Springer: 728 pp. [2] Nixon GT et al. (2015) Geol Surv Canada OF7856: 17-34 [3] Lightfoot PC (2016) Elsevier: 680 pp. [4] Hart RJ et al. (2002) EPSL 198: 49-62 [5] James S et al. (2022) Energy Geosci 3: 136-146 [6] Golightly JP (2010) SEG Spec Publ 15: 451–485 �52 Thermodynamic constraints on the generation of cubanite-rich magmatic sulfides Maghdour-Mashhour, R.1, Mungall, J.1 1 Department of Earth Sciences, Carleton University, 2115 Herzberg Laboratories, Ottawa, Ontario K1S 5B6, Canada ___________________________________________________________________________ Nickel (Ni) and Copper (Cu) are paramount for advancing sustainability and enhancing human wellbeing, serving as indispensable elements in modern technology and pivotal components in green energy solutions. We launched a study of Ni-Cu ore deposits from the Keweenawan Large Igneous Province (LIP) to unravel their intricate geochemical and thermodynamic conditions, crucial for understanding their genesis and optimizing ore extraction methods, thereby bolstering industrial efficiency and sustainability. The Keweenawan LIP, emplaced within the ca. 1.1 Ga Mid-Continent Rift (MCR), comprises by maficultramafic intrusions and flood basalts extending across Lake Superior in Ontario and Minnesota [1]. The MCR preserves a broad array of magmatic sulfide deposits in a relatively unmetamorphosed state, offering a unique opportunity for detailed study and understanding of primary processes that are commonly obscured by later metamorphism. MCR deposits exhibit variable concentrations of cubanite (CuFe2S3) alongside the more prevalent chalcopyrite (CuFeS2). Cubanite content ranges widely from less than 1% to as high as 80% of the Cu sulfide mode [2], posing a major metallurgical challenge. The presence of cubanite prolongs flotation circuit processing times, necessitating a delicate balance between efficiency and optimization to separate Cu sulfides from tails effectively [3]. The occurrence of cubanite and chalcopyrite cannot be inferred from Cu-Ni-S assay and must be observed petrographically. Our primary aim is an innovative approach to mitigate cubanite prevalence within the circuit by precisely identifying cubanite-rich geometallurgical zones exclusively through assay databases, thereby circumventing the need for costly petrography and SEM analyses. The first essential step is to comprehend the thermodynamic controls imposed by intensive parameters, including oxygen and sulfur fugacity (fO2 and fS2), which contribute to the stability of cubanite in a system where silicate melt, and sulfide melt are in equilibrium. Subsequently, we explore the required parental magma chemical composition and intensive variables necessary at elevated temperatures to ensure the stability of cubanite as the system cools down to lower temperatures. To address these questions, we utilized FactSage 8.3 to model the evolution of a cubanite-favorable anhydrous magmatic closed system initially comprising ~15 wt% sulfide liquid and ~85% silicate melt at the liquidus temperature. Re-equilibration of the model system to lower temperatures allowed us to determine the conditions required at the liquidus that would result in the development of cubanite-rich sulfide assemblages upon cooling to near ambient temperatures. Our investigation yielded novel findings that cubanite stability is achieved at log fS2 of -14, log fO2 of -37, and a temperature of 270 degrees Celsius. These conditions correspond to a low-temperature ambient state, akin to a parental magma composition with log fS2 of -0.7 and log fO2 of -7.2 at the liquidus temperature indicating a condition slightly more reduced than the Quartz-Fayalite-Magnetite (QFM) buffer (ΔQFM -1). �53 We have also uncovered a diverse array of model cubanite-bearing low-temperature assemblages, including various combinations of pentlandite, pyrrhotite, chalcopyrite, talnakhite, and mooihoekite. Whereas the abundances of pentlandite, pyrrhotite, and chalcopyrite display a wide spectrum of sensitivity to fO2 and fS2, our findings reveal five distinct assemblages—incorporating chalcopyrite, talnakhite, and mooihoekite—that showcase high sensitivity to even two decimal points of shifts in fO2 and fS2. As fO2 decreases and fS2 increases, these assemblages undergo transitioning from chalcopyrite to talnakhite and ultimately to mooihoekite. It is noteworthy that cubanite exhibits stability even in hydrous systems, albeit under extremely reduced conditions. For some cubanite-bearing assemblages, such as those with mooihoekite, cubanite stability necessitates an exceptionally reduced environment, with ΔQFM reaching as low as -3.3 and log fS2 dropping to -2.5. As our study progresses, our next phase entails conducting quantitative and qualitative mineral classification through petrography and SEM X-ray mapping of representative samples sourced from Ni-Cu deposits spanning distinct intrusions across the Mid-Continent Rift (MCR). Our aim is to compare sulfide paragenesis within cubanite-rich domains across the MCR with thermodynamically generated model compositions and assemblages provided by FactSage. Additionally, we will incorporate geochemical insights to establish a link between bulk rock assay data and the presence of cubanite in the Ni-Cu deposits. This approach will enable us to delineate geometallurgical domains potentially requiring modified beneficiation circuits. References: [1] Taranovic et al. (2015) Can Min 24(2): 347 [2] Ripley and Alawi (1986) Lithos, 212: 16-31 [3] Muzinda et al. (2018) Min Eng, 125: 34-41 �54 Constraining the Sunday Lake mineralization: A Ni-Cu-PGE deposit Mexia, K.1, Hollings, P. 1 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, On P7B 1J4, Canada kmexiad@lakeheadu.ca ___________________________________________________________________________ The Sunday Lake Intrusion (SLI) is located 25 km north of Thunder Bay, Ontario, and hosts Ni-Cu-PGE mineralization. It has been dated at 1109.0±1.3 [1], and as such is related to the plateau stage of the ~1115 to 1106 Ma Midcontinent Rift System (MRS; [2], [3]). The SLI is a tabular shaped intrusion emplaced in Archean rocks of the Quetico Basin that becomes more tube-like to the northwest where it is hosted by Archean granitoids. It is emplaced along the Crock Fault, which is interpreted to be a splay of the main Quetico Fault [3]. It varies from 350 meters to 1000 meters in thickness. The intrusion consists of mafic-ultramafic layers divided into three series: the Upper Gabbro Series, the Lower Gabbro Series, and the Ultramafic Series (Fig 1.) [3]. Reef-style sulphide mineralization (2-10 vol.%) is present in the lower zones of the intrusion, consisting of disseminated to blebby chalcopyrite-pyrrhotite-pyrite-cubanite in an olivine melagabbro (Fig. 2). The Ultramafic series mineralization shows a laterally extensive 20 meters thicks layer with enrichment in Cu-Pt, Pd and Au at levels of 3-10 g/t Pt+Pd+Au [3]. The main objective of this project is to characterize the paragenetic sequence of the Sunday Lake Intrusion and to study the effects of crustal contamination on mineralization. This project utilizes two representative drill holes from which a total of 71 samples were collected. A total of thirty polished thin sections were generated for petrographic studies. Rocks were classified based on relative proportions of olivine, clinopyroxene, and plagioclase with modal rock names such as melagabbro, olivine melagabbro, and wehrlites. Downhole diagrams of trace and major elements vary within the layered intrusion, but both plume-like compositions (Fig. 3A), and evidence for contamination by host rocks (Fig. 3B). Variation in composition suggest other geological processes such as episodes of melt re-injection, contamination, assimilation, and fractional crystallization. These processes likely lead to the generation of sulphides and further precipitation. Sixteen samples have been sent for Sm-Nd and Rb-Sr isotope studies to assess the paragenetic history of the Sunday Lake Intrusion mineralization. cm Figure 2. Photograph of sample SL23KM41 showing an olivine melagabbro with disseminated and blebby sulphides. �55 A B Figure 3. Primitive mantle normalized REE spider diagram of two samples. A: Sample showing a plume-like trend. B: Sample suggesting an interaction with the host rock. Normalising values from [5]. References: [1] Bleeker, W., et al. "The Midcontinent Rift and its mineral systems: Overview and temporal constraints of Ni-Cu-PGE mineralized intrusions." Targeted Geoscience Initiative 5 (2020): 7-35. [2] Heaman, L. M., Easton, R. M., Hart, T. R., MacDonald, C. A., Hollings, P., & Smyk, M. (2007). Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario. Canadian Journal of Earth Sciences, 44(8), 1055-1086. [3] Flank, S. (2017). The Petrography, Geochemistry and Stratigraphy of the Sunday Lake Intrusion, Jacques Township, Ontario. School of graduate studies. [4] Woodruff, L. G., Schulz, K. J., Nicholson, S. W., & Dicken, C. L. (2020). Mineral deposits of the Mesoproterozoic Midcontinent Rift system in the Lake Superior region–a space and time classification. Ore Geology Reviews, 126, 103716. [5] Sun, S. S., & McDonough, W. F. (1989). Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. Geological Society, London, Special Publications, 42(1), 313-345. [6] Miller, J.D. (2020). Report on the Petrography, Geochemistry, and Lithostratigraphy of DDH SL10026 from the Southern Sunday Lake Intrusion. JDM GeoConsulting. �56 Primitive arc magmatism and the development of magmatic Ni-Cu-PGE mineralization in Alaskan-type ultramafic-mafic intrusions Milidragovic, D.1,2, Nixon, G.T.3, Spence, D.W.2, Nott, J.A.2, Goan, I.R.2, Scoates, J.S.2 1 Geological Survey of Canada-Pacific; 1500-605 Robson St., Vancouver, BC, V6B 5J3; dejan.milidragovic@nrcan-rncan.gc.ca 2Pacific Centre for Isotopic and Geochemical Research; Department of Earth, Ocean and Atmospheric Sciences; University of British Columbia 3British Columbia Geological Survey ___________________________________________________________________________ Zoned ultramafic-mafic plutonic rocks in convergent margin settings represent trans-crustal magmatic feeders [1,2] to coeval, and better studied, arc volcanoes. Arc lavas, which are on average basaltic to andesitic, represent differentiated and largely degassed magmatic products [3,4] and only rarely provide a clear glimpse into the earliest stages of arc magma evolution [5,6,7]. The study of lower- to mid-crustal arc cumulates, which include high-temperature liquidus lithologies, is complimentary and necessary to establish a holistic understanding of arc magmatism and mantlecrust metal transfer. Ultramafic-mafic convergent margin intrusions are typically composed of rocks comprised of variable proportions of olivine ±Cr-spinel, clinopyroxene, amphibole, and magnetite. Plagioclase is volumetrically minor and appears relatively late in the crystallization sequence, consistent with high parental magma water contents. The absence of orthopyroxene distinguishes the predominantly abundant class of “Alaskan-type” intrusions (e.g., Tulameen, Polaris, Turnagain), which are the focus of this presentation, from orthopyroxene-rich “Giant Mascot–type” intrusions [8]. Alaskan-type intrusions have long been recognized for their platinum group element (PGE) potential, hosted principally within micrometer-size platinum group metal (PGM) inclusions (e.g., laurite, isoferroplatinum, tetraferroplatinum) in thin chromite-rich horizons and massive schlieren occurring in dunite. Alaskan-type intrusions may also host significant magmatic Ni-Cu-PGE sulfide mineralization in dunite and wehrlite (e.g., Turnagain [9]) and notable palladium-subgroup PGE (PPGE) concentrations may occur in association with Cu-rich sulfides (e.g., chalcopyrite ±bornite) in more evolved clinopyroxene- and hornblende-rich rock types [10,11]. The mineralization style and potential in Alaskan-type intrusions is a reflection of the interplay between: 1) degree of country rock assimilation during emplacement and differentiation, and 2) the oxidation state of the primary, mantle-derived melts. Evolution of oxidized arc magmas [12] through assimilation of either S-rich or relatively reduced country rock favours early sulfide saturation and formation of magmatic Ni-Cu-PGE sulfides in hightemperature dunite and wehrlite. At Turnagain, assimilation of country rocks is indicated by the isotopic composition of sulfides, which show non-uniform d34S values (+4.2 to -12.3 ‰ [13,14]) that are largely intermediate between those of the depleted mantle (-1.28 [15]) and surrounding phyllite (-11.6 to -20.1 [13,14]). Magmatic chalcopyrite from the Polaris Alaskan-type intrusion has uniform near-chondritic sulfur isotope compositions (d34S =-0.19 +0.48/-0.32‰) that are markedly lighter than those of the country rocks (δ34S = +7.4 +1.3/-1.7), indicating that the evolution of primitive mantle-derived magma(s) occurred without appreciable country rock assimilation [16]. The differentiation of primitive arc magma without contamination from country rocks favours crystallization of PGM in association with chromite-bearing dunite and immiscibility of Cu-PPGE-Au-rich sulfide from the more differentiated clinopyroxene, magnetite ±hornblende-saturated magmas. In principle, the nature of PGM (i.e., Ptenriched vs. IPGE-enriched) and the onset of sulfide immiscibility in systems not affected by country rock assimilation are governed by the oxidation state of the primary magma, and by extension, the oxidation state of the sub-arc mantle wedge. The predominance of Pt-alloys, such as those observed at the Tulameen intrusion, indicates moderately oxidized parental magmas (log f(O2) <FMQ+2), �57 where Pt is likely to be near saturation [17]. In contrast, the absence of Pt-alloys and predominance of Ir-Ru-Os alloys and laurite (e.g., Polaris) indicates strongly oxidized parental magmas (log f(O2) ≥FMQ+2) [11]. In the absence of country rock assimilation, sulfide immiscibility may be attained through reduction in the oxidation state of the magma, most likely triggered by magnetite fractionation [18]. The oxidation of the FeS component in the melt to form magnetite (e.g., 6 FeS melt + 4O2 = 2 Fe3O4 magnetite + 3S2 [19,20]) is consistent with the Cu-rich character of the earliest formed immiscible magmatic sulfides at both Tulameen and Polaris [10,11]. The diverse magmatic Ni-Cu-PGE mineralization styles of Alaskan-type intrusions reflect the complexity of arc magmatism. Key controlling factors include: 1) first-order differences in the oxidation state of the sub-arc mantle that may relate to the composition and nature of the subducted oceanic crust [16,21], and 2) the composition and volume of crust that is assimilated during magma ascent and emplacement. References: [1] Cashman K V et al. (2017) Science 355: 9 [2] Spence D W et al. (2024) Lithos 474-475: 107578 [3] Müntener, O and Ulmer P (2018) Am J Sci 318: 64-89 [4] Ding S et al. (2023) Geochem Geophys Geosys 24: e2022GC010552 [5] Russell J K and Snyder L D (1997) Can Min 35, 521-541 [6] Milidragovic D et al. (2016) Earth Planet Sci Lett 454: 65-77 [7] Till C B (2017) Am Min 102: 931-947 [8] Nixon G T et al. (2015) GSC Open File 7856: 17-34 [9] Mudd G and Jowitt S (2014) Econ Geol 109: 1813-1841 [10] Nixon G T et al. (2020) GSC Open File 8722: 197-218 [11] Milidragovic D et al. (2021) Can Min 59: 1627-1660 [12] Cottrell E et al. (2022) Geophys Monogr 266, 33-61 [13] Scheel J E (2007) UBC MSc thesis, 201 p [14] Jackson-Brown S (2017) UBC MSc thesis, 272 p [15] Labidi J et al. (2013) Nature 501: 208-211 [16] Milidragovic D et al. (2023) Earth Planet Sci Lett 620: 118337 [17] Borisov A and Palme H (2000) Am Mineral 85: 1665-1673 [18] Jenner F E et al. (2010) J Petrol 51: 2445-2464 [19] Wohlgemuth-Ueberwasser C C et al. (2013) Min Dep 48: 115-127 [20] Lesher C M (2017) Ore Geol Rev 90: 465-484 [21] Canil D and Fellows S A (2017) Earth Planet Sci Lett 470: 73-86 �58 Stratigraphy of the Grasset Ultramafic Complex and its Ni-Cu-(PGE) mineralization, Abitibi Greenstone Belt, Superior Province, Canada. Milier, K.1, Houlé M.G.2 and Saumur B.M.1 1 Université du Québec à Montréal (UQAM), Département des sciences de la Terre et de l’Atmosphère, 201 avenue du Président Kennedy, Montréal, QC H2X3Y7, Canada. 2 Geological Survey of Canada, Lands and Minerals Sector, Natural Resources Canada, 490 Couronne Street, Québec City, QC G1K 9A9 Canada ___________________________________________________________________________ In the Abitibi Greenstone Belt (AGB), komatiitic rocks are prospective for Ni-Cu-PGE mineralization. Most of these occur within the Kidd-Munro and Tisdale assemblage located in the southern parts of the AGB [1]. The Grasset Ultramafic Complex (GUC) of the northern AGB is a notable exception, as it hosts one of the largest Type I komatiitic Ni-(Cu)-(PGE) deposits in the entire Abitibi. [1]. Located in the Harricana-Turgeon area, the GUC is an 8 kilometre long ultramafic corridor (Fig. 1A) within the volcano-sedimentary Manthet Group interpreted as part of the Deloro assemblage (2734-2724 Ma). The country rocks mostly consist of felsic to mafic volcanic rocks with gabbroic sills and graphitic mudstones. The GUC occurs within felsic volcanic and graphitic sediments that may contain semimassive to massive sedimentary sulfides intervals. However, it can crosscut the local stratigraphy. The GUC consists of thick ultramafic cumulate bodies (Fig. 1B, C) and komatiitic lava flows within the GUC central area (Fig. 1C). Both host Ni-(Cu-PGE) mineralization, such as that observed in the GUC central area and in the southern end of the GUC. The latter hosts the Grasset deposit. Figure 7 A) Simplified geological map of the GUC area [2]. B) Geological map of the Grasset area [3]. C) Geological map of the GUC central area [4]. The Grasset deposit consists of a peridotitic body (Fig. 1A) dipping to the southwest, cut by the Sunday Lake fault to the southeast (Fig. 1A, B), and dominated by olivine meso- to orthocumulate with lesser intervals of olivine adcumulate. The ultramafic rocks have undergone a significant degree of talc-serpentine-carbonate alteration, and primary mineral assemblages have been completely obliterated. The ultramafic body does not exhibit much lithological variation, especially in its central portions where it occurs as a homogenous olivine cumulate unit. Toward the northwest, the ultramafic splits into two bodies interleaved with felsic volcanics (Fig. 1A). The lower and upper contacts within the country rocks are sharp and gradually shifts from pyroxenite (Fig. 2B) to peridotite. Locally, relicts of “olivine hopper crystal” crescumulates (Fig. 2A) occurs within the cumulate body. Three Ni-Cu-(PGE) mineralized horizons (H1, H2, H3) occurs at different levels of the Grasset ultramafic body. H1 occurs along the basal contact between the ultramafic and the footwall rocks (Fig. 2B) and consists of disseminated to net-textured and semi-massive to massive sulfides. H2 �59 is very sparse and cannot be confidently defined as a clear mineralized horizon. H3, the main horizon, occurs in the upper part of the Grasset ultramafic unit. Its thickness can be up to 55 m, consisting of several intervals from disseminated, to heavy disseminated and net-textured sulfides (Fig. 2C) with rare massive sulfide intervals. Sulfide assemblages of H3 and H1 differ. H3 is largely composed of pyrrhotite (Po) ≈ pentlandite (Pn) >> chalcopyrite (Cpy), With pyrite (Py) occasionally replacing Po. In contrast, H1 exhibits a more common magmatic sulfide paragenesis of Po >> Pn >> Cpy. However, when normalized to 100% sulfide, H3 average grade is 15.1% Ni, 1.4% Cu, 0.31% Co and 12.1 ppm Pt+Pd, whereas H1 tenors are lower showing an average grade of 7.6% Ni, 1.0% Cu, 0.15% Co and 5 ppm Pt+Pd. Despite these tenor variations, H1 and H3 show similar Ni/Cu (8-11) and Pd/Pt ratios (1.8-2.0). Figure 2: A) Relict of hopper crystal in an olivine crescumulate. B) H1 disseminated sulfides within the pyroxenite in contact with the hornfelsed footwall felsic tuff (Right). C) H3 net-textured sulfides. D) Komatiitic flow top breccia. E) Disseminated sulfides within the olivine cumulate of a komatiitic flow. F) Olivine mesocumulate of the poorly differentiated cumulate, note the presence of elongated olivine. The GUC central area is composed of a series of komatiitic flows and thick cumulate ultramafic bodies dipping to the west. These komatiitic flows occur between the felsic volcanics and graphitic sediments (Fig. 1C). The flows consist of several flow top breccias (Fig. 2D) underlain by olivine ortho- to mesocumulates (Fig. 2E) that progressively decrease in thickness toward the stratigraphic top. The earliest flows, at the base of the sequence, appear to contain the bulk of the Ni-(Cu)-(PGE) mineralization in this area. This mineralization occurs at the bottom of the olivine cumulate with disseminated (Fig. 2E) to net-textured and massive sulfides. The thick ultramafic cumulates (Fig. 2F) are poorly differentiated bodies, composed of olivine ortho- to mesocumulate. These ultramafic bodies do not show clear field evidence of intrusive relationships, but they occur at varying local stratigraphic levels. They exhibit sparse disseminated sulfides, but rarely massive sulfides at the basal contact. In conclusion, the GUC is a komatiitic sequence consisting of extrusive komatiitic flows and thick olivine cumulate bodies. The system could thus host both Type I and II komatiite-associated mineralization. The GUC could represent a volcanic-subvolcanic komatiitic succession where extrusive facies are more likely to be found in the GUC central area. The extrusive or intrusive origin of Grasset remains unclear at this stage. However, the occurrence of crescumulate and several Ni-(Cu)-PGE horizons suggests the existence of several ultramafic subunits within the Grasset unit. The Grasset deposit highlights the potential for new Ni discoveries hosted in the Deloro assemblage and for similar discoveries in underexplored area such as the northern parts of the AGB. References: [1] Houlé MG et al. (2017). Rev in Econ Geol 19: 103-132 [2] Archer Exploration (2023). Corporate presentation [3] Tucker MJ et al (2019). Proc 15th SGA Biennial Meeting 2: 497-500 [4] Balmoral Ressources Ltd (2020). Roundup �60 Geochemistry of Camp Lake Block Lac des Iles Palladium Mine, N. Ontario, Canada Njipmo Ngoko, B.1, Hollings, P.1, Djon, L.2 and Hamilton, M.3 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada. bnjipmo@lakeheadu.ca 2 impalacanada, 69 Yonge Street, Suite 700, Toronto ON, Canada M5E 1K3 Canada. lionneldjon@gmail.com 3 Jack Satterly Geochronology Laboratory, University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1, Canada The Archean Lac des Iles suite is located just north of the Wabigoon-Quetico boundary [1], approximately 90 kilometers north of Thunder Bay in Northwestern Ontario. This suite of intrusions includes discrete mafic and ultramafic complexes associated with sanukitoids, which were emplaced along deep-seated regional faults [2]. Among these, only the Lac des Iles Complex hosts economically significant palladium deposits, specifically at the Lac des Iles mine. The complex is divided into two parts: North Lac des Iles and South Lac des Iles. The North Lac des Iles mainly comprises ultramafic rocks such as websterite, clinopyroxenite, wherlite, lherzolite, dunite, and peridotite [3]. In contrast, South Lac des Iles is primarily composed of mafic rocks such as gabbro, gabbronorite, norites, and melanorite [4] and is the main host of the Roby, Offset, and Camp Lake zones. This study focuses on the Camp Lake zone, the deepest part of the palladium deposit, recently highlighted by exploration drilling. The aim is to characterize the petrological, geochronological, and geochemical attributes of the Camp Lake zone and compare these with those of the Roby and Offset zones. Four main petrographic subtypes have been identified within the Camp Lake zone: leucogabbronorites, mesogabbronorites, melagabbronorites, and norite. The rock textures are generally equigranular or varitextured. Petrographic studies show these rocks mainly consist of a mixture of pyroxenes and plagioclase. The pyroxenes predominantly comprise orthopyroxene with minor clinopyroxene, which are partially to completely replaced by amphiboles (cummingtonite, actinolite, and tremolite). The plagioclase is weakly to moderately altered and generally retains its original habit. The Camp Lake rocks exhibit magmatic sulfide contents ranging from 0.5% to 3%, dominated by pyrrhotite, pentlandite, and chalcopyrite, with minor pyrite. Sulfide minerals often occur as blebs or disseminated grains intergrown with silicate minerals. A new zircon U-Pb age was acquired for the mineralized Camp Lake rocks, yielding an emplacement age of 2690.56 ± 0.80 Ma [5], closely similar to that of the Roby and Offset deposits [6]. Geochemical analysis of the Camp Lake Zone rocks shows enrichment in LREE (La/Smn ranging from 1.29 to 7.75, with a median of 3.30), unfractionated HREE (Gd/Ybn ranging from 0.56 to 1.49, with a median of 0.88), and a negative Nb anomaly. These values are similar to those of the Roby and Offset zones and are consistent with a subduction zone setting [7]. Also, similar to the Roby-Offset deposits, PGE values in Camp Lake range between 1.0 g/t and 3.0 g/t, with variations in the rocks increasing with Cu and Ni content. However, Camp Lake is distinguished by higher proportions of pyrrhotite compared to chalcopyrite and lower Pd/Pt and Cu/Pd ratios than the other zones. Data show that the Camp Lake zone exhibits lower δ34S values, ranging from (-1.1‰ to +0.3‰), while the Roby and Offset zones show wider variations ranging from (-0.37 to +3.28‰) [8]. This observation suggests that the sulfur in the Camp Lake zone is of mantle origin and that the sulfide was less affected by hydrothermal processes, leading to more limited sulfide alteration. References: [1]. Lavigne, M.J., & Michaud, M.J. (2001). The Lac des Iles Palladium Deposit, Ontario, Canada. Economic Geology. Volume 10, pages 1-17. [2]. Impala Canada. (2017). Technical Report on the Lac des Iles Palladium Mine. Impala Canada. �61 [3]. Djon, L., Smith, M., Johnson, R., & Brown, T. (2017). Canadian Journal of Earth Sciences, 54, 12341250. [4]. Gomwe, T. (2008). Geology and Mineralization of the Lac des Iles Complex. In: Platinum-Group Elements in Magmatic Ore Deposits. Springer, pp. 123-145. [5]. Hamilton, M.A., 2024. Report on U-Pb CA-ID-TIMS geochronology of diorite and gabbro samples from Lac des Iles – related intrusions at Wakinoo, Buck Lake, Demars Lake, and Dog River, NW Ontario. Unpublished report prepared for Prof. P. Hollings, Department of Geology, Lakehead University, Ontario. 14p. [6]. Peck, D., Houle, M.G., & Smith, M.P. (2016). Economic Geology, 111, 833-858. [7]. Peck, D., Houle, M. G., et Smith, M. P. (2016) Geology, Petrology, and Controls on PGE Mineralization of the Southern Roby and Twilight Zones, Lac des Iles Mine, Canada, p. 43 [8]. Jonsson, J. (2023). Petrogenesis of mineralized horizons in the Offset and Creek zones, Lac des Iles Complex, N. Ontario, pages 146-168. �62 Hf-Nd-Pb isotopic evidence for variable impact devolatilization in the Sudbury Igneous Complex and its relevance for Ni-Cu-(PGE) sulfide ore formation Peters, D.1, Lesher C.M.1 and Pattison E.1 1 Laurentian University, Sudbury, ON P3E 2C6, Canada, dpeters@laurentian.ca ___________________________________________________________________________ The Sudbury Igneous Complex (SIC), generally believed to be the remnant of a large, 1850 Ma bolide impact, hosts one of the world’s largest magmatic Ni-Cu-(PGE) sulfide mining camps. It consists of i) the Main Mass, the crystallization product of the impact melt sheet), ii) underlying discontinuous lenses of variably mineralized magmatic and anatectic breccias, iii) radial and concentric, variable mineralized quartz dioritic offset dikes, and iv) overlying fallback/suevitic breccias. The ultimate source for all metals and sulfur is the immediate target rocks melted during the impact event, but the timing and mechanisms of ore formation are still being debated. Most current models assume that all metals and sulfur completely dissolved in the impact melt sheet and subsequently exsolved and sank toward the bottom, where they accumulated in local embayments or troughs, either by convective currents [1, 2] and/or gravity-driven density flows [3]. However, this process is slow and difficult to reconcile with the observed heterogeneities in the Pb>S>Os isotopic compositions of the sulfide ores around the SIC [4, 5, 6] and would require an initially heterogeneous impact melt sheet from which the sulfide ores subsequently exsolved. An alternative model is that significant amounts of Pb [7] and S [8], as well as Zn-Cd-Rb-Cs [9] and other volatile elements were volatilized during the impact event, followed by localized thermomechanical erosion of S ± metal-bearing footwall rocks by the superheated impact melt sheet [3, 10], forming local sulfide xenomelts, which then accumulated in local embayments and troughs [3]. Impact devolatilization would have left volatile elements such as Pb and S more susceptible to postimpact modifications by thermomechanical erosion, whereas more refractory elements such as Hf or Nd [11] would have been largely preserved during impact, making them less susceptible to postimpact modifications. Characterising the Hf-Nd-Pb isotopic composition of the Main Mass (the crystallized impact melt sheet) therefore presents an excellent opportunity to better understand i) the characteristics of the initial impact melt sheet, ii) post-impact contamination processes, and iii) formation of the sulfide ores associated with the SIC. Preliminary results of Hf isotope analysis on zircons by LA-MC-ICP-MS from four Main Mass transects across the North Range of the SIC show a narrow range in Hf isotope compositions (εHf1850Ma between -8 and -12, Figure 1A), similar to previously published data for the South Range of the SIC [12]. Similarly, literature data for whole-rock Nd isotope compositions across the North Range [13, 14] also show a narrow range (εNd1850Ma between -7 and -9, Figure 1B), which suggests effective vertical and lateral homogenization of the initial impact melt across the North Range prior to crystallization. Lead isotope compositions on the other hand, while being relatively homogeneous throughout the Granophyre, Quartz Gabbro and Felsic Norite (Δ207Pb/204Pb between 300 and 450), become more variable towards the base of the Main Mass, especially within the Mafic Norite (Δ207Pb/204Pb between 100 and 400, Figure 1C) [7, 15]. The greater Pb isotopic variability in the Mafic Norite can be attributed to the greater susceptibility of Pb to post-impact contamination by thermomechanical erosion, which would have been most significant at the base of the melt sheet. The decoupling of the more variable Pb isotopes from the more homogenous Hf and Nd isotopic compositions within the Mafic Norite therefore provides strong evidence for impact devolatilization of Pb>S>>Os>Nd>Hf. Although a contribution from the impact melt sheet cannot be entirely excluded, the current Hf-Nd-Pb isotopic evidence from the Main Mass favours a model in which the sulfide ores dominantly formed at the base by local thermomechanical erosion of S-bearing footwall rocks. Additional analyses of Nd and Pb isotopic compositions of the Main Mass across the North Range are in progress to confirm the results. �63 Figure 1: Stratigraphic variations in Hf, Nd, and Pb isotopic compositions throughout the North Range Main Mass of the Sudbury Igneous Complex. Individual analyses are shown in grey, unit averages (±1σ) in the colour of the respective lithology. Black lines and shaded blue squares show the overall average (±1σ) for the North Range Main Mass. A. εHf1850Ma variations throughout the North Range Main Mass. B. εNd1850Ma variations throughout the North Range Main Mass. C. Δ207Pb/204Pb variations throughout the North Range Main Mass. Hf data are from this study, Nd data are from [13, 14], Pb data are from [7, 15]. For calculation of Δ207Pb/204Pb see [7]. GRAN – Granophyre, QGAB – Quartz Gabbro, FSNR – Felsic Norite, MFNR – Mafic Norite References: [1] Lightfoot P et al. (2001) Econ Geol 96: 1855-1875 [2] Zieg M and Marsh B (2005) GSA Bulletin 117: 1427-1450 [3] Wang Y et al. (2022) Econ Geol 117: 1-28 [4] Darling J et al. (2012) GCA 99: 1-17 [5] Ripley E et al. (2015) Econ Geol 110: 1125-1135 [6] Morgan J et al. (2002) GCA 66: 273-290 [7] McNamara G et al. (2017) Econ Geol 112: 569-590 [8] Lesher C (2019) GAC-MAC 42: 130-131 [9] Kamber B and Shoenberg R (2020) EPSL 544: 116356 [10] Prevec S and Cawthorn R (2002) JGR 107: B8 2176 [11] Lodders K (2003) Astrophysics Journal 591: 1220-1247 [12] Kenny G. et al. (2017) GCA 215: 317-336 [13] Faggart B et al. (1985) Science 230: 436-439 [14] Dickin A et al. (1996) GCA 60: 1605-1613 [15] Dickin A et al. (1999) GSA Special Paper 339: 361-371 �64 Deformation in mafic protoliths: Impacts from late faults on Ni-Cu-PGE mineralization at Lac des Iles Mine, Canada Peterzon, J.1, Phillips, N.1, Hollings, P.2, and Djon, M.L.2 1 2 Lakehead University, 955 Oliver Road, Thunder Bay ON. P7B 5E1, Canada; jpeterzo@lakeheadu.ca Impala Canada, 69 Yonge Street, Suite 700 Toronto ON. M5E 1K3, Canada __________________________________________________________________________ Fault zones are complex structures that serve as permeable pathways through the upper crust; however, the impact of host lithology on damage zone development remains poorly understood. The development of fault cores and damage zones is typically controlled by the strength and composition of the protolith, conditions of deformation, and fluid chemistry [1], this is particularly true for faults hosted in mafic lithologies where damage zones control hydration in mafic crust. Permeability is significantly enhanced in damage zones due to the high density of fractures and is diminished in fault cores when a clay rich gouge is present. Faults therefore may act as conduits or barriers for fluid flow depending on the proportion of fault core to damage zone [2]. Trapped mineralization may be offset or remobilized by later faulting. This study investigates the deformation and alteration geochemistry footprint of late faults within the mafic-ultramafic intrusions at the Lac des Iles mine (Figure 1). The 2,689 +/- 1.0 Ma Lac des Iles Complex (LDIC) [3] is a series of intrusive bodies hosted within the ~3.01 – 2.68 Ga granitegreenstone Marmion terrane of the Superior Province, Canada. Ni-Cu-PGE mineralization has been offset, and depleted in areas surrounding the fault zone, including the damage zone and fault core, by the reverse Offset Fault and hypothesized reverse Camp Lake Fault. Palladium depletion is hypothesized to be from fluid flow through the fault damage zones. Fracture densities from the hanging wall of each fault were measured to determine the damage zone and fault core width in both gabbronorites and tonalites (Figure 2). Tonalites have a higher fracture density than the gabbronorites, suggesting fluid flow would be more effective in felsic protoliths, which in turn may contribute to metal remobilization, implying that host rock lithology has a strong control over fault zone structure, mineralization, and alteration assemblages. Metal contents display depletions in areas surrounding faults, and show a strong correlation with fracture density measurements. It is likely that a frictionally weak, chlorite rich fault core likely impeded the development of a more fracture dense damage zone in the gabbronorites, as opposed to a silica-rich brecciated fault core in the tonalites. Deformation conditions of the Camp Lake and Offset Fault zones were studied through scanning electron microscopy (SEM) and electron microprobe analyses. Preliminary results from this support our hypothesis of a silicified fault core in tonalites (Figure 3) and a chlorite-rich fault core in gabbronorites and reveal three generations of chlorite growth: prefaulting at ~350°C, syn-faulting at ~150 – 200°C, and post-faulting at ~150°C [4] (Figure 4). We aim to highlight the importance of fluid-rock interactions in the development of fault core and damage zone structures in mafic protoliths, and their associated impact on Ni-Cu-PGE mineralization. References: [1] Caine et al. (1996) Geology, 24 (11): 1025-1028 [2] Faulkner et al. (2010) Journal of Structural Geology, 32 (11): 1557-1575 [3] Djon et al. (2018) Economic Geology, 113 (3): 741-767 [4] Wiewóra and Weiss (1990) Clay Minerals, 25: 83-92 �65 �66 Formation of euhedral silicate megacrysts within magmatic massive sulfides Raisch, D.1, Staude S.1, Fernandez, V. 2 and Markl G.1 1 Department of Geosciences, University of Tübingen, Schnarrenbergstrasse 94-96, D-72076 Tübingen, Germany Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom Corresponding author: Dominic.raisch@uni-tuebingen.de 2 _________________________________________________________________________ In the magmatic massive sulfide ore from Nova-Bollinger (Western Australia), large (up to 10 cm) silicate crystals, completely enclosed in massive sulfides, are common where sulfides infiltrate older silicate rocks. This texture could provide a new insight into the infiltration and the role of the magmatic sulfides in the nucleation and growth processes of these crystals. At Nova-Bollinger, the megacrysts consist of pyroxene, garnet and plagioclase (Fig. 1) and are typically observed in association with emulsion-textured sulfides at the sulfide infiltration front from the orebody into the silicate rocks. The infiltrated country rock itself consists of amphibolite- to granulite-facies metamorphosed mafic granulite [2] with an assemblage of plagioclase, pyroxene, amphibole ± garnet. Infiltration of hot sulfide melt caused parts of the country rock to incongruently melt producing both tonalitic melt and peritectic orthopyroxene and garnet. While the peritectic silicates formed margins at the contact between the sulfides and the country rock, the newly formed immiscible buoyant silicate melt formed an upward counterflow through the descending, denser sulfide melt, resulting in the formation of an emulsion [1, 3]. The assemblage of the country rock may contain the same minerals as the megacrysts of the emulsion texture, but they are clearly distinguishable both optically and chemically. Garnet, for example, is only occasionally present in the immediate country rock depicting a mostly poikilitic morphology with rarely any euhedral crystals larger than 800 µm, in contrast to the up to 6 cm euhedral and sometimes even skeletal garnet of the emulsion texture. In addition, the garnet and pyroxene megacrysts of the emulsion texture show distinct negative Eu-anomalies (Eu/Eu*= 0,17 for both minerals) with a strong depletion in light REE (Fig. 2) and in some cases display round multisulfide inclusions, as visible by computed tomography scans. Both characteristics are missing in the country rock counterparts as well as in the gabbroic host silicate melt. These observations argue for a magmatic origin of to the megacrysts via crystallisation from the silicate melt portion of the emulsion texture. The large grain size may be the result of the constant movement of the emulsion (to keep it stabilized [REF]), where the constant bumping of silicate melt droplets onto the growing crystals provides enough material to garnet, pyroxene or plagioclase to allow them grow to megacrysts within this emulsion. Once the movement of the melts decreases, the immiscible melts can separate, leaving the megacrysts behind in massive sulfides. While plagioclase coexists with garnet and pyroxene, pyroxene and garnet never coexist as megacrysts, which may be due to a temperature effect. This is based on the observation that pyroxene is mostly associated with mono-sulfide solid solution, which records temperatures up to 1100°C [4], whereas garnet is associated with intermediate sulfide solid solution, which starts to crystallise at temperatures around 880°C [4]. Besides other magmatic Ni-Cu sulfide deposits (i.e., Kambalda, Western Australia [1]), partly skeletal megacrysts are also found associated with emulsion textures of anatectic sedimentary exhalative deposits in massive sulfides (e.g. cordierite, pyroxene, and feldspar from the granulite-facies Silberberg deposit in Germany, [5]). �67 Figure 8 Plagioclase megacrysts in massive sulfides from Nova-Bollinger. Figure 9 Primitive mantle normalized [6] REE-pattern of orthopyroxene from Nova-Bollinger. References: [1] Staude S et al. (2017) Ore Geol Rev 90:446-464 [2] Clark C et al. (2014) Precambrian Res 204:1-21 [3] Barnes S et al. (2018) Ore Geol Rev 101:629-651 [4] Craig JR & Kullerud G (1969) Soc Eco Geo Monogr 4:344-358 [5] Staude et al. (2023) Miner Deposita 58:987-1003 [6] Lyubetskaya T & Korenaga J (2007) Solid Earth 112 �68 Applying Magnetic Vector Inversion (MVI) on Aeromagnetic Data in the Thunder Bay Region of the Mid-Continent Rift Riahi, S.1, Mungall J.E.1, Ernst, R.E1 1 Department of Earth Sciences, Carleton University, Ottawa, Ontario, Canada, Shokouhriahinajafaba@cunet.carleton.ca The research presented here applies the Magnetic Vector Inversion (MVI) technique to aeromagnetic datasets of the region surrounding Thunder Bay. The intrusions related to the Mid-Continent Rift contain several deposits containing high-grade mineralization zones that are abundant in platinum (Pt), palladium (Pd), copper (Cu), and nickel (Ni). Given the pivotal role of geophysical data in mineral exploration and the proven efficacy of magnetic data in delineating mineralized zones, our aim is to deepen the understanding of the geological attributes and the potential for mineralization in the Thunder Bay region deposit by applying MVI. Aeromagnetic Data Acquisition: The aeromagnetic data was used in this pilot study obtained from the USGS website [1], representing compilations of previously published survey data from various geological surveys and organizations. These Figure 1. TMI and analytic signal (AS) of the area including the Current Lake and Escape Lake areas. compilations, produced using industry-standard techniques, were analytically continued to a surface drape of 150 m and 300 m above ground and gridded to 250 m and 500 m cell size, respectively. They offer consistent datasets suitable for onshore geology mapping and magnetic modeling extending across the lake shore [1]. Total Magnetic intensity (TMI) data of the study area and the analytic signal (AS), with the magnetic units are shown in Figure 1. Magnetic Vector Inversion (MVI): Magnetization vector inversion (MVI) is employed to replicate the distribution of magnetization vectors within subsurface blocks [2-4]. This technique involves calculating the overall distribution of magnetization vectors from the components within each underground block. MVI enables the simultaneous analysis of complex geological scenarios, such as the overlay of multiple sources with �69 diverse remanent magnetization directions, and facilitates the complete retrieval of magnetization vector data [5-8]. All modeling and comparisons in the examples presented herein were conducted using the Geosoft VOXI Earth Modeling system. The aeromagnetic dataset was inverted to generate 3D voxel MVI susceptibility models employing the Geosoft VOXI Earth Modeling system (Fig. 2). Strong magnetic anisotropy is evident in the southwest corner of the region. Future efforts will focus on highresolution exploration data sets over recognized chonoliths including Tamarack and Current Lake to seek distinctive magnetic vector characteristics of these small but valuable intrusions. Fig 2. 3D MVI VOXEL model and MVI vectors, the above color bar gives the susceptibility in SI. The axes are in meters. The lower color bar gives the normalized amplitude in SI. References: [1] Anderson, E.D., and Grauch, V.J.S. (2018), Updated aeromagnetic and gravity anomaly compilations and elevation-bathymetry models over Lake Superior: U.S. Geological Survey data release, https://doi.org/10.5066/F7F18X8S. [2] Wang, M.Y., Di, Q.Y., Xu, K., Wang, R. (2004), Magnetization vector inversion equations and forward and inversed 2-D model study, Chinese Journal of Geophysics, 47, 601–609. [3] Lelievre, P.G. & Oldenburg, D.W. (2009), A 3D total magnetization inversion applicable when significant, complicated remanence is present, Geophysics, 74, L21–L30. [4] Ellis, R.G., de Wet, B., Macleod, I.N., (2012), Inversion of magnetic data for remanent and induced sources, in ASEG Extended Abstracts, pp. 1–4. [5] Kubota, R., Uchiyama, A. (2005), Three-dimensional magnetization vector inversion of a seamount, Earth, Planets and Space, 57, 691–699. [6] MacLeod, I.N., Ellis, R.G. (2016), Quantitative magnetization vector inversion, in ASEG Extended Abstracts, pp. 1–6. [7] Liu, S., Hu, X., Zhang, H., Geng, M. & Zuo, B. (2017), 3D magnetization vector inversion of magnetic data: improving and comparing methods, Pure and Applied Geophysics, 174, 4421–4444. [8] Ghalehnoee, M.H., Ansari, A. (2022), Compact magnetization vector inversion, Geophysical Journal International, 228, 1–16. �70 Potential links between the Midcontinent Rift (MCR) related BaragaMarquette dyke swarm and early MCR related magmatic Ni-Cu sulfide deposits in Michigan, USA. Rossell, D.M.1*, Strandlie, J.2 1Talon Metals, Tamarack, MN, USA 2 Eagle Mines, Marquette, MI, USA *rossell@talonmetals.com ___________________________________________________________________________ The~1100Ma Midcontinent Rift (MCR) system can be traced across the central United States and Canada as a ~2000km long gravity high, but the only surface exposures of the volcanics, intrusions and sediments that make up the MCR are in the Lake Superior region. Despite the large extent of the MCR, historic MCR related mineral production has been almost exclusively from the portion of the MCR in Michigan. The MCR related mineral deposits shown in Figure 1, range from the famous Keweenaw volcanic hosted Native Cu deposits and the large “White Pine type” sediment hosted chalcocite deposits to the Eagle magmatic Ni-Cu sulfide mine, the only currently producing Ni mine in the USA. In contrast to many Large Igneous Provinces which are relatively short-lived events of a few million years or less, the main period of MCR related magmatism spans ~20my [1]. The USGS [1] subdivides MCR volcanism into two main phases, an Early Plateau Stage (~1112-1105Ma) which largely occurred during a period of reversed magnetic polarity and later Rift stages (~1102-1090Ma) which occurred during a period of normal magnetic polarity . Figure 10 Geology map of the Western portion of the Upper Peninsula of Michigan, USA showing the distribution of dykes of the Baraga Dyke swarm and the various types of mineral deposits and prospects associated with the MCR (modified from Michigan Geologic Survey state geology map). The Baraga dyke swarm is located on the south side of the MCR in the western portion of the Upper Peninsula of Michigan , USA (fig. 1). The dyke swarm is comprised of more than 100 mafic-ultramafic �71 dykes wide enough (+10m) to be visible in proprietary high resolution airborne magnetic data sets (the dykes shown in figure 1), and likely hundreds more, to thin to be discernible from airborne data, but frequently intersected in drilling in the area. The dykes can be divided into three types based on geochronology, magnetic polarity, orientation and chemistry that are referred to in figure 1 as the “metal depleted”, “Cr Rich”, and “Reversely Polarized” dykes The oldest known dykes within the dyke swarm are the “Metal Depleted ”dykes, which are only recognized as a pair of east-west trending dykes on the north and south side of the Eagle Ni-Cu mine. These two gabbroic dykes have very different trace element chemistry from all the other dykes in the Baraga-Marquette swarm (most notably having below detection limits PGE contents) and are the only dated dykes (1120Ma+/4my [2]). The youngest known dykes are the “reversely polarized” set of gabbroic dykes that have distinctive ophitic to sub-ophitic textures, generally East-West orientations, high TiO2 contents , mantle like Cu/Zr ratios and the highest Pd contents of any of the dyke sets. Although, all attempts to date these dykes have been unsuccessful, they cross-cut both the East Eagle intrusion dated at 1107.3+/-3.7ma [3] and the BIC intrusions dated at 1106.2+/- 1.3Ma [4]. Despite the cross cutting relationships, Paleomagnetic data suggests they might be similar in age to the Eagle intrusions [5]. The third type of dykes making up the Baraga-Marquette dyke swarm are a NW-SE trending set of dykes that range from centimetres to >70m in width. Although they have a wide range of MgO contents, the sampled dykes all have much higher Cr contents(>500ppm) than the other two types of dykes. The are often amygdaloidal, Cr Rich dykes typically do not have visible sulfides, but do resemble the amygdaloidal pyroxenite margins of the well mineralized olivine cumulates that host mineralization in the Eagle and Eagle East deposits. The Ni-Cu-PGE mineralized, pipe like conduits at Eagle, Eagle East and BIC also align closely with Cr Rich dykes, suggesting a potential temporal and genetic relationship (feeder dykes). The pronounced 30-40 degree change in orientation between the likely similar aged, reversely polarized dykes and Cr Rich dykes might indicate a change in the orientation of the regional stress fields associated with the emplacement of the mineralized intrusions. References: [1] Woodruff, L et al. (2020) Ore Geol. Rev. 126 [2] Dunlop, M (2013) Indiana Univ. MSc thesis (93p.) [3] Ding X et al. (2010) Geochem. Geophys. Geosyst. v.11(3) [4] Bleeker W et al. (2020) personal communication [5] Foucher M (2018) Michigan Tech. Univ. PhD dissertation (173p.) �72 Texture and composition of Fe-Ti oxides of the Neoarchean Big Mac mafic intrusion and its implication for Fe-Ti-V-(P) mineralization in the McFaulds Lake greenstone belt, Superior Province, Canada Sappin, A.-A.1, Houlé, M.G.1,2*, Metsaranta, R.T. 3, and Lesher, C.M.2 1 Geological Survey of Canada, Lands and Minerals Sector, Natural Resources Canada, 490 Couronne Street, Québec City, QC G1K 9A9 Canada anne-aurelie.sappin@nrcan-rncan.gc.ca 2 Mineral Exploration Research Centre, Harquail School of Earth Sciences, Goodman School of Mines, Laurentian University, 935 Ramsey Lake Road, Sudbury, ON P3E 2C6 Canada 3 Earth Resources and Geoscience Mapping Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, ON P3E 6B5 Canada * Presenter _________________________________________________________________________ The McFaulds Lake greenstone belt (MLGB), also known as the “Ring of Fire” area, is a region with great potential for orthomagmatic Cr-platinum-group element (PGE), Ni-Cu-(PGE), and Fe-Ti-V-(P) mineralization, as attested by the discovery of the world-class Black Thor – Big Daddy – Black Horse – Black Creek – Blackbird Cr-(PGE) system, the Eagle’s Nest Ni-Cu-(PGE) deposit, and the Thunderbird, Butler West, Butler East, and Big Mac Fe-Ti-V-(P) prospects. Most of the mafic-ultramafic intrusions hosting orthomagmatic mineralization in the area belong to the ca. 2736˗2732 Ma Ring of Fire intrusive suite (RoFIS) (e.g., [1], [2]). This suite includes mafic and ultramafic-dominated intrusions associated with Cr-(PGE) and Ni-Cu-(PGE) mineralization (Koper Lake subsuite) and mafic-dominated intrusions associated with Fe-Ti-V-(P) mineralization (Ekwan River subsuite) [2]. The latter are the most abundant, but also more widespread geographically. The Big Mac intrusion is the largest intrusion belonging to the Ekwan River subsuite. It forms a broadly layered, subconcordant sill, and comprises various flavors of gabbro (± Fe-Ti oxides), minor anorthosite, and rare pyroxenite. These lithologies exhibit partially preserved cumulate textures composed mostly of plagioclase and clinopyroxene (almost completely altered to amphibole) with local magnetite and ilmenite, apatite, and Fe-Ni-Cu sulfides. Fe-Ti oxide mineralization in the Big Mac intrusion occurs as massive (> 80% Fe-Ti oxides) to semi-massive (40 to 80% Fe-Ti oxides) magnetiteilmenite layers, net-textured (20 to 35% Fe-Ti oxides) to patchy net-textured (10 to 25% Fe-Ti oxides), and locally as millimeter- to a few centimeter-thick stringers (Fig. 1). Massive to semi-massive Fe-Ti oxide layers are mainly located in the northern part of the intrusion, whereas patchy to net-textured oxides are more widespread throughout. All lithologies typically contain at least several percent disseminated Fe-Ti oxides (< 10%). Based on whole-rock geochemical data, the best mineralized interval (9.5 m thick) has an average composition of 68 wt.% FeOt, 17 wt.% TiO2, and 0.48 wt.% V2O5. The Big Mac sill also contains disseminated pyrrhotite, pentlandite, pyrite, and chalcopyrite (< 10% sulfides) throughout the intrusion, and millimeter-thick stringers of chalcopyrite, pyrite, and pyrrhotite. In the northern part of the intrusion, the semi-massive to massive magnetite layers contain patchy net-textured pyrrhotite, pentlandite, pyrite, and chalcopyrite (10 to 20% sulfides; Fig. 1E-F) with up to 1.6% Ni100 (Ni at 100% sulfides) and 1.8% Cu100. Fe-Ti oxides are well preserved in the Big Mac intrusion and their chemical composition can be used to characterize the internal stratigraphy, to determine which parts are more prospective for V and P mineralization, and to estimate the conditions for the genesis of the Fe-Ti oxide layers. The Big Mac intrusion appears to have crystallized from high-Fe parental magmas that were injected from a feeder conduit located in the northernmost part of the intrusion. Based on the presence of more primitive magnetite and ilmenite compositions in the northern part of the intrusion and more evolved signatures in the southern part, the rocks in the northern part likely represent more conduitproximal facies that are more prospective for Fe-Ti-V mineralization, whereas the rocks in the southern part likely represent more distal facies that are more prospective for Fe-Ti-P mineralization. The trace element contents of magnetite also suggest that the crystallization of the Fe-Ti oxide layers in the Big Mac intrusion occurs under relatively oxidized conditions (fO2 > FMQ + 1). The Big Mac �73 magnetite displays many characteristics (e.g., texture, chemical composition) in common with magnetite in other mafic-dominated intrusions of the Ekwan River subsuite (e.g., Thunderbird, Butler West, Butler East). This attests to the Fe-Ti-V-(P) potential of the large ferrogabbroic magmatic event that affected the MLGB at ca. 2735˗2732 Ma [3] and formed the Ekwan River subsuite. Figure 1: (A) Simplified and schematic graphic log of drill core BM09-04 located in the northern part of the Big Mac intrusion. (B-G) Photomicrographs of polished thin sections in plane-polarized transmitted (B-D) and reflected (E-G) light showing the different oxide textural facies in the Big Mac intrusion. (B) Disseminated, anhedral grain of magnetite in mesocratic gabbro. (C) Disseminated, rounded grain of magnetite in clinopyroxenite. (D) Net-textured magnetite in melanocratic gabbro. (E-F) Semi-massive magnetite with patchy net-textured pyrrhotite, pentlandite, and chalcopyrite. Anhedral magnetite contains ilmenite exsolutions as anhedral grains and lamellae. (G) Massive magnetite with ilmenite exsolutions as anhedral crystals and thick lamellae. Abbreviations: Amp = amphibole, Cpx = clinopyroxene, Cpy = chalcopyrite, Grt = garnet, Ilm = ilmenite, Mag = magnetite, Pl = plagioclase, Pn = pentlandite, Po = pyrrhotite. References: [1] Houlé M.G. et al. (2015) Geological Survey of Canada, Open File 7856, pp. 35–48. [2] Houlé M.G. et al. (2019) Geological Survey of Canada, Open File 8549, pp. 441–448. [3] Houlé M.G. et al. (2020) Geological Survey of Canada, Open File 8722, p. 141–163. �74 Complexly zoned pyroxenes at Kevitsa record magma mixing and survive alteration Schoneveld, L.1, Luolavirta, K.2,3, Barnes, SJ1 , Hu, S.1 , Verrall, M.1 and Le Vaillant, M.1 1 CSIRO Mineral Resources, Perth, 6151, Australia Geopool Oy, Teknobulevardi 3−5, 01530 Vantaa, Finland. kirsi.luolavirta@geopool.fi 3 Oulu Mining School, Faculty of Technology P.O. Box 3000, FI-90014 University of Oulu, Finland 2 ___________________________________________________________________________ Magmatic Ni-Cu-(Platinum Group Element—PGE) sulfide deposits are generally linked to dynamic systems and conduit-type emplacements of mafic-ultramafic magmas. Schoneveld et al. [1] demonstrated a common feature of variable titanium (Ti) and chromium (Cr) zoning patterns in cumulus pyroxenes in various mineralized intrusions (e.g. Noril’sk-Talnakh, Nova-Bollinger, Jinchuan) and attributed these features to reflect a high-flux magmatic environment with wall rock assimilation and related fluctuating cooling rates where pyroxenes crystallized. On the contrary, according to the authors, barren intrusions were characterized by simple normally zoned pyroxenes. Pyroxene zoning was therefore suggested to serve as a potential prospectivity indicator for magmatic Ni-Cu-PGE sulfide deposits. However, on many occasions, the primary mineralogy of the ore hosts has been subjected to variable degrees of hydrothermal alteration, potentially hindering the usability of the pyroxene zoning approach in exploration. This dilemma is being tackled by mapping pyroxene zoning patterns of samples recording variable degrees of amphibole alteration. Additionally, pyroxene has been shown to record magma histories in volcanic settings [2] and also has the potential to record important magmatic histories in these ore deposits. In this research, microbeam X-ray fluorescence (XRF) mapping techniques were applied to the mineralized Kevitsa intrusion, in northern Finland to study pyroxene zoning patterns. Synchrotronbased µXRF chemical imaging using multidetector Maia arrays has proved especially effective [3], allowing entire thin sections to be imaged at micrometer-scale resolution in a matter of hours (Australian Synchrotron, operated by ANSTO). This allows many grains with varying crystal orientations to be analyzed and detailed visualization of chemical zoning. The mafic-ultramafic Kevitsa intrusion (2.06 Ga) is hosted by a volcano-sedimentary sequence in the Central Lapland greenstone belt. A disseminated Ni-Cu-(PGE, Au, Co) sulfide ore deposit occurs within the central parts of ultramafic olivine-pyroxene cumulates. The deposit has been mined since late 2011 and is currently operated by Boliden. The sample set comprises 29 thin sections collected from various parts of the intrusion representing mineralized and non-mineralized domains within the intrusion. Most of the samples are clinopyroxene-olivine mesocumulates with variable modes of olivine, augite, and oikocrystic or transitional cumulus to poikilitic orthopyroxene (bronzite/enstatite). These textures are characteristic throughout the ultramafic part of the Kevitsa intrusion. The samples have also been exposed to variable degrees of hydrothermal alteration and many clinopyroxene grains have begun the transformation to amphibole. Very complex pyroxene zoning patterns are observed throughout the Kevitsa intrusion (Figure 1). Hence, the Kevitsa intrusion provides yet another example of a sulfide ore-bearing variant of a maficultramafic intrusive body with diagnostic complex zoning patterns of pyroxene minerals. The observed styles and magnitudes of clinopyroxene zonation in Kevitsa, however, are unusual when compared to other ore-bearing intrusive bodies [1]. A common feature for clinopyroxe grains is highly Cr-poor cores, followed by strong oscillatory patterns in the mantles, often ending in a rim of very low Cr and high Ti values. Similarly, the clinopyroxene in the most nickel-rich ore zones shows enriched nickel rims. These patterns are best explained by open magma chamber processes, consistent with Luolavirta et al. [4]. The nickel enrichment and chemical oscillations recorded in the pyroxene crystal structure suggest an influx of new, Ni-rich melt into the partially solidified crystal mush at Kevitsa. The clinopyroxene zoning patterns are not reflected in the oikocrystic �75 orthopyroxene that generally records smooth normal zoning. This indicates post-cumulus growth of orthopyroxene (cf. slow nucleation as cumulus mineral). Cr-rich Cr-poor Figure 1. Examples of end-member zoning styles in the Kevitsa pyroxenes with traverses across the grains showing the Cr and Ti content that causes each distinct zoning type. A) normal zoning from trapped liquid reactions B) sector zoning with B1 and B2 showing different sectioning effects of this zoning type C) abrupt zoning D) oscillatory zoning E) crater-zoned clinopyroxene with the content of Cr and Ti of the traverse shown in F). G) crater zoning schematic H) Moat zoned clinopyroxene grain I) traverse of Cr and Ti content across the grain J) moat zoning schematic. The examination of the preservation of the zoning patterns with alteration reveals that Cr zonation is visible through the early stages of amphibole alteration, with preservation being enabled by the presence of Cr-rich epitaxial amphibole. However, the remnant zoning is lost as the amphibole alteration progresses. It is worth noting that the complex zoning patterns are observed in almost every sample, regardless of the location relative to the ore-bearing domain of the intrusion (some are located up to a few hundred meters away from the deposit). Hence, to enhance the methodology as an exploration tool, further research is needed to outline the distal extent of this fingerprint away from the ore within mineralized intrusions of reasonable size. References: [1] Schoneveld et al. (2020) Zoned Pyroxenes as Prospectivity Indicators for Magmatic Ni-Cu Sulfide Mineralization. Front. Earth Sci. 8:256. [2] Ubide et al. (2019) Sector-zoned clinopyroxene as a recorder of magma history, eruption triggers, and ascent rates: Geochim. Cosmochim. Acta. 251:265-283. [3] Barnes et al. (2020) Imaging trace-element zoning in pyroxenes using synchrotron XRF mapping with the Maia detector array: Benefit of low-incident energy. Am. Min. 105:136–140 [4] Luolavirta et al. (2018) In-situ strontium and sulfur isotope investigation of the Ni-Cu-(PGE) sulfide orebearing Kevitsa intrusion, northern Finland. Min. Dep. 53:1019–1038 �76 New indicator mineral signatures for nickel sulfide exploration Schoneveld, L.E.1, Williams M.1, Salama, W.1, Spaggiari, C. V.1, Barnes, S. J. 1, Le Vaillant, M. 1, Siegel, C. 1, Hu, S. 1, Birchall, R. 1, Baumgartner, R. 1, Shelton, T. 1, Verrall, M. 1, and Walmsley, J. 1 1 Mineral Resources, CSIRO, Western Australia Corresponding Author: Louise.Schoneveld@csiro.au ___________________________________________________________________________ Discovery of new ore deposits is becoming more difficult as we explore beneath deep cover. Commonly, exploration programs start from geophysical targeting and move straight into drilling, which is expensive and has a low sampling density. Nickel sulfide deposits specifically have little to no hydrothermal footprints and usually have small sulfide targets, therefore, this sampling practice risks missing potential key sulfide intercepts and abandoning fertile ground. Exploring using indicator minerals can give additional information before drilling has commenced to identified prospective areas and can continue to be used in early drilling programs to allow focus on more prospective intrusions. In this study, we develop key chemical signatures within minerals that indicate Ni prospectivity and prove the effectiveness of mineral indicators for use in exploration. Australia hosts one third of the world’s nickel (Ni) deposits and most are located in Western Australia therefore this area was the focus of our study. Comprising 11 detailed case studies from Western Australia and one from South Australia, paired with existing global mineral chemistry data from CSIRO databases, the aim of each case study was to understand the mineral deposit or exploration camp in detail, to provide context for the indicator mineral signatures that were measured. We analysed both komatiitic systems as well as intrusionhosted systems, sampling from both known mineralised and apparently barren examples. Further, we sampled the regolith and cover above these deposits to determine as to whether indicator minerals can survive weathering and transport processes. We analysed spinel minerals (chromite-magnetite), olivine, pyroxene, apatite, ilmenite, and plagioclase for their trace elements using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). This large and robust dataset is ideal for machine learning applications. We used random forest models to distinguish the key trace element contents of each mineral that signifies mineralisation and the confidence of each prediction. Spinel was the largest dataset in this study, with over 7,000 LA-ICP-MS analyses. This large dataset allowed for confident (77%) predictions of mineralised vs non-mineralised occurrences using the machine learning models. The key elements underpinning these predictions were Co, Ga, V, Ni, and Cr. Using the trace element data, it may also be possible to predict the volume of sulfide associated with an individual spinel grain. This has implications for vectoring toward larger and more economic deposits. Analysis of the cover and regolith showed that chromite is not significantly affected by weathering. A study of the Black Swan nickel mine in Western Australia shows that the trace element contents in spinel are consistent across the talc-altered, serpentine-altered, and fresh examples of the komatiite. This suggests that the spinel family would be a robust resistate indicator mineral for Ni exploration. Olivine is this database's next largest mineral collection, with over 1,400 LA-ICP-MS analyses. Using the machine learning models, the trace elements in olivine can be used to accurately (95%) predict �77 that the host intrusion was mineralised; however, the unmineralised category has poorer recall (60%), which suggests a greater likelihood of false positive predictions. Pyroxene can be examined for trace element (Cr-Ti) variation within grains to understand if the intrusion has the potential to be a conduit. Although not a direct indicator of sulfide presence, it can indicate the potential for high-R factors and, therefore, a metal-rich sulfide (if sulfide saturation has occurred). Minerals such as olivine, pyroxene, and plagioclase do not survive weathering and are not considered resistant indicator minerals. However, they can still be analysed in fresh rock to assess as to whether the subject intrusion has potential to host Ni-sulfide orebodies. The other minerals (apatite, ilmenite, and plagioclase) have less than 1,000 LA-ICP-MS analyses for each phase in this database. Although they show promise in being robust indicator minerals, a larger training dataset should be accumulated before their use in exploration. Ilmenite specifically was found to be the most common mineral in heavy mineral concentrates and is easily separated with a magnet (figure 1). The trace elements in ilmenite show confident predictions for prospectivity, however, the database needs to be expanded to develop ilmenite as an additional resistant indicator mineral. In this project, we have developed analysis and data-handling workflows, and machine-learning models for Ni-sulfide exploration. Although these models were primarily developed using Western Australian case studies, these exploration tools are applicable globally. Figure 11: Magnetic and heavy liquid separation from the same stream sediment sample, A) magnetically separated; B) heavy liquid separation. The heavy liquid separation was carried out on the remaining fraction after magnetic separation. �78 Apatite as an indicator for volatile involvement in the genesis of the Marathon Cu-PGE deposit, northwestern Ontario Shahabi Far, M.1, Good, D.2 and Samson, I3 1 Department of Earth Sciences, Carleton University, Ottawa, ON (maryam.shahabifar@carleton.ca) Department of Earth Sciences, Western University, London, ON 3 Department of Earth and Environmental Sciences, University of Windsor, ON 2 ___________________________________________________________________________ The Marathon Cu-PGE deposit of the Mesoproterozoic (1106 ± 1 Ma) Coldwell alkaline complex contains three types of mineralization with different textural, mineralogical, and geochemical characteristics: Footwall Zone, Main Zone, and W-Horizon. The relative roles of volatiles in metal enrichment in this deposit remain a point of debate. In this study, the significance of hydrothermal fluids in directly precipitating ore minerals or causing their later modification using the texture and composition of apatite is investigated. The textural relationships of apatite with other minerals indicate two types of apatite generation: early apatite and late apatite. Early apatite crystals are homogeneous with no textural or chemical zoning. Late apatite crystals exhibit diverse zoning patterns including oscillatory zoning, patchy zoning, and replacement textures (Fig. 1). The zoning in apatite is associated with Si and rare earth elements (REE) changes. Late apatite grains reveal replacement zones along crystal rims as well as around cracks containing monazite and/or allanite inclusions; this feature will be referred to as replacement apatite in this study (Fig. 1). The earlier apatite grains that show replacement zones are referred to as late metasomatized apatite. The overall decrease in Cl/F ratios of the late apatite from the Footwall to the W Horizon (Fig. 2) can be explained by magma degassing similar to the suggested model for the Bushveld and Stillwater complexes [1][2][3]. Primary fluid and monazite inclusions in the replacement rims of the metasomatized late apatite associated with hydrous minerals can be interpreted to have resulted from the interaction of volatiles with the late-stage gabbroic melts. Experimental studies indicate that monazite and other REE-minerals can be formed as a result of fluid-induced coupled dissolutionreprecipitation processes [4] via fluorapatite interaction with H2O, 40/60 CO2/H2O, and KCl brine [5][6]. Given that the metasomatized late apatite has an overall higher Cl/F ratio compared to the other apatite grains (Fig. 2), the fluid must have been Cl-rich. The metasomatized late apatite and their replacement rims with monazite inclusions are usually associated with residual hydrous melt aggregates and are more abundant in W Horizon. This indicates that late-stage hydrous melts and associated exsolved fluids are more abundant in the W Horizon than in the other two zones. The ubiquitous presence of hydrothermal alteration around the residual hydrous melt aggregates certainly indicates that a hydrous fluid exsolved from the late-stage melts. The presence of hydrothermal carbonate and epidote in the late assemblages as well as the presence of carbonate as an alteration of apatite in the replacement rims indicates that fluid also must have contained CO2 or other carbonic species. Given that sulfide minerals in the W Horizon mostly occur in association with biotite and hornblende as either interstitial coarse crystals or interstitial phase in the residual hydrous melt aggregates, the Cl- carbonic-enriched volatiles exsolved from late-stage magma must have been played a critical role for PGE-enrichment in the W Horizon. Allanite as either inclusions, filling voids or cracks, or along the rims of late metasomatized apatite or independent grains are much coarser grains compared to monazite (Fig. 1) suggesting that the early nucleated monazite must have interacted with later possibly more NaCl or CaCl2-rich fluid reacted with the surrounding silicate rocks to form allanite [5][6][7][8]. This is consistent with elevated Cl contents of alteration products (amphibole with up to 3.9 wt% Cl) associated with metasomatized �79 late apatite with higher Cl content and suggests that the late-stage hydrothermal fluid was Clenriched. The occurrence of allanite in the Footwall Zone and Main Zone but rare occurrence in the W Horizon indicates that the late-stage fluid infiltration must have been less dominant in the W Horizon. This is consistent with relatively fewer secondary hydrous minerals in the W Horizon. High metal contents of the replacement rims of apatite in the Footwall Zone and their association with chalcopyrite indicate that metals and S were mobilized by these volatiles. Much of the chalcopyrite in the Main Zone has replaced pyrrhotite and is intergrown with hydrous silicate minerals, which also suggests that Cu was introduced into the system, presumably by volatiles. This observation can be explained by a process in which volatiles fluxed through the Footwall Zone and transported Cu to the Main Zone. Replacement of pyrrhotite by chalcopyrite in the Main Zone and associated Cu metasomatism must have occurred after pyrrhotite crystallization in the Main Zone suggesting Cu remobilized with later-stage hydrothermal fluid. Chalcopyrite inclusions occurrence within voids in the replacement zones of apatite as well as along the cracks within apatite where allanite occurs, could suggests that this fluid could be the Cl-rich hydrothermal fluid that is responsible for the allanite formation. The sources of these late-stage volatiles are not constrained yet, although one possibility could be the devolatilization of the Archean country rocks. a b c Aln d Metasomatized late apatite Replacement rim e Mnz Aln f Ap Aln Fig. 1: Back-scattered electron images (BSE) showing diverse zoning and textures in the late apatite: a) oscillatory zoning with Si and REE changes between zones, b) patchy zoning of late apatite from W Horizon showing difference is carbon concentration between the zones, c) allanite filling the cracks and voids within apatite, d) metasomatized late apatite showing replacement zones around the rims and along cracks, e) zoomed-in image from red box on image c showing monazite inclusions within the replacement zone, f) allanite as overgrowth rim of apatite. Aln: alanite, Ap: apatite, Mnz: monazite. �80 2.0 2.0 Metasomatized Late apatite Metasomatized Late apatite Replacement rim Replacement rim Late apatite Late apatite apatite Early Early apatite 1.5 Cl/F Cl/F 1.5 1.0 1.0 0.5 0.5 0.0 Footwall Zone 0.0 Main Zone W Horizon Fig. 2: Box-whisker plot comparing Cl/F W values of different apatite generations and textures from different part Footwall Zone Main Zone Horizon of the Marathon deposit. The lower, middle, and upper lines in each box represent 25%, median and, and 75% of the data, respectively. The lower whisker represents the 10th percentile and the upper whisker represents the 90th percentile. Circles show outliers. References: [1] Boudreau A and McCallum I (1989) Contrib Mineral Petrol 102:138-153 [2] Boudreau A et al. (1995) Contrib Mineal Petrol 122:289-300 [3] Willmore C et al. (2000) J Petrol 41:1517-1539 [4] Pan Y and Fleet M (2002) Rev in Mineral Geochem 48:13-49 [5] Harlov D and Förster (2003) Amer Miner 88:1209-1229 [6] Spear F (2010) Chem Geol 279:55-62 [7] Budzyń B et al. (2011) Amer Miner 96:1547-1567 [8] Jonsson E et al. (2016) Amer Miner 101:1769-1782 �81 Geochemical and Petrologic Investigation of the Eagle’s Nest Intrusion, McFaulds Lake Greenstone Belt, Ontario, Canada Sheshnev, V.1, Hollings, P.1, Phillips N.J.1, Weston, R.J.2, Deller, M.2 and Campbell, D.2 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1. vsheshne@lakeheadu.ca 2 Wyloo Metals, 1-1127 Premier Way, Thunder Bay, Ontario, P7B 0A3. ___________________________________________________________________________ The Eagle’s Nest orthomagmatic Ni-Cu-(PGE) deposit is situated in the northern portion of the Superior Province within the McFaulds Lake greenstone belt, approximately 500km northeast of Thunder Bay, Ontario. The deposit contains 11.1 million tonnes of proven and probable reserves grading 1.68% Ni, 0.87% Cu, 0.89g/t Pt, 3.09g/t Pd and 0.18g/t Au [1]. The Eagle’s Nest intrusion is associated with the mafic-ultramafic magmatism of the Ring of Fire intrusive suite between 2736 and 2732 Ma and is part of the ultramafic-dominated Koper Lake subsuite [2,3]. The Eagle’s Nest intrusion was emplaced along a sub-horizontal conduit, forming a blade-shaped dike [4]. Mineralization is consistent with gravitational sulfide segregation at the basal, northwestern contact of the intrusion. A post emplacement, regional deformation event, rotated the intrusion into its present day, subvertical orientation, with a width of ~500m, thickness of ~150m and vertical extent >1600m. The mineralized ore body of the Eagle’s Nest intrusion consists of a zoned pyrrhotite – pentlandite – chalcopyrite assemblage with massive sulfide mineralization at the northwestern contact gradationally becoming, net-textured and disseminated to the southeast [5]. Mungall et al. [6] estimated the parental magma to be a low-Mg komatiitic magma with ~22% MgO and ~12% FeOT. More recently, Zuccarelli et al. [5] reported the most magnesian olivine within the mineralized portion of the intrusion is Fo86, which is consistent with a picritic parental magma composition. Contradictions among the estimated parental magma composition and the most magnesian olivine found within the intrusion, require further constraints on the composition of the melt that formed the mineralized system. Geochemical, petrographic, mineral chemistry, and radiogenic isotope techniques, are being used to characterize the unmineralized portions of the Eagle’s Nest intrusion, to characterize the associated chilled dikes in the vicinity of the intrusion, and to constrain the parental magma characteristics that formed the Eagle’s Nest deposit. This will allow for a more holistic approach to determining the primary melt composition. One-hundred and thirty-six samples were collected from drill core. Samples comprise five tonalitic wall-rock samples, 44 mafic-ultramafic chilled dike samples, and 87 intrusion samples. Intrusion samples comprise of mafic-ultramafic lithologies that include peridotite (Fig. 1), gabbro, and units identified as chilled margins of the main intrusion. One-hundred and twenty-one samples were analysed using Inductively Coupled Plasma Atomic Emission Spectroscopy and Inductively Coupled Mass Spectroscopy for major oxides and trace elements. A total of 30 polished thin section were prepared comprising seven peridotite, eight contact, and 15 offshoot dike samples. A total of 20 samples were selected for analysis of Sm-Nd isotopes. Three different approaches are used to evaluate the parental magma composition that formed the Eagle’s Nest intrusion. The first two approaches will examine chilled margins preserved along the length of the intrusion and within the magmatic breccia matrix situated within the hanging-wall of the chonolith. The third approach will examine the chemical composition of olivine grains preserved within the ultramafic lithologies of the intrusion. To further constrain the contamination history and identify primitive melt compositions, Sm-Nd isotope data will also be examined. �82 Figure 1. Photomicrograph of a peridotite sample depicting poikilitic textured orthopyroxene with preserved fresh olivine within the oikocryst surrounded by cumulus serpentinized olivine (XPL: crosspolarized light). References: [1] Burgess et al. (2012) Micon Int Ltd: 197 [2] Metsaranta et al. (2015) Geol Surv of Can Opn File Rep 7856: 61-73 [3] Houlé et al. (2020) Geol Surv of Can Opn File Rep 8722: 141-163 [4] Barnes S.J. and Mungall J.E. (2018) Econ Geol 113: 789-798 [5] Zuccarelli et al. (2022) Econ Geol 117(8): 1731-1759 [6] Mungall et al. (2010) Soc of Econ Geol Sp Pub 15: 539-557 �83 Reconstitution of the Merensky Reef footwall during chamber replenishment Smith, W.D.1,2, Henry, H.3, Maier, W.D.4, Muir, D.D.4, Heinonen, J.S.5,6, Andersen, J.Ø7 1 Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada CSIRO Mineral Resources, 26 Dick Perry Avenue, Kensington, Perth, WA 6151, Australia 3 Géosciences Environment Toulouse, Université de Toulouse III Paul Sabatier, 14 Avenue E. Belin, 31400 Toulouse, France 4 School of Earth & Environmental Sciences, Cardiff University, United Kingdom, CF10 3AT 5 Department of Geosciences and Geography, University of Helsinki, P.O. Box 64, 00014, Helsinki, Finland 6 Geology and Mineralogy, Åbo Akademi University, Akademigatan 1, 20500 Åbo, Finland 7 Camborne School of Mines, University of Exeter, Penryn, United Kingdom, TR10 9EZ 2 __________________________________________________________________________ The Merensky Reef of the Bushveld Complex was discovered in 1924 by Hans Merensky, whilst following up on platinum-group mineral concentrates that Andries Lombaard had panned from a stream in the eastern Bushveld Complex [1]. This discovery was to be significant, and the aptly named Merensky Reef was the focus of intense scientific research for the ensuing 100 years, providing insight into the formation of reef-style platinum-group element occurrences in layered mafic-ultramafic intrusions. However, many aspects of the petrogenesis of such reef-style occurrences remain debated despite a century of investigations. The layered mafic-ultramafic rocks of the 2.056 Ga Bushveld Complex are together known as the Rustenburg Layered Suite, which itself has been divided into five stratigraphic units, including the Marginal, Lower, Critical, Main, and Upper Zones [2]. The Merensky Reef occurs in the Upper Critical Zone, which predominantly consists of interlayered norite, anorthosite, chromitite, and orthopyroxenite [3]. Several researchers have proposed that the Merensky Reef marks a regional unconformity that formed when preexisting semicrystalline cumulates (i.e., resident cumulates) interacted with relatively primitive melt that replenished the overlying melt column [4,5]. This replenishment event is believed to have thermally- and (or) chemically-eroded the resident cumulates, leading to the development of the Merensky Reef stratigraphy and its world-class platinum-group element mineralization. This study represents a detailed investigation of the Merensky Reef footwall at the Rustenburg Platinum Mine in the western lobe of the Bushveld Complex. At this location, the Merensky Reef is a single layer of coarse-grained orthopyroxenite that is bracketed by mm-scale chromitite seams. These units are underlain by a cm-scale anorthosite that in-turn is underlain by leuconorite. We have employed electron probe microanalysis and electron back-scatter diffraction to characterize changes in the footwall rocks with proximity to the reef and thermodynamic simulations using Magma Chamber Simulator to constrain the effect chamber replenishment may have on different resident cumulates. The leuconorite hosts normally zoned orthopyroxene crystals with poikilitic overgrowths and cumulus plagioclase crystals that define a non-random fabric consistent with gravitational settling in a quiescent melt. The anorthosite consists of variably zoned cumulus plagioclase crystals that are traversed by sub-vertical domains of sulfides, pyroxenes, and accessory phases. These plagioclase crystals record a non-random fabric that strengthens with proximity to the reef, and it is proposed to have formed by gravitational settling followed by the removal of phases in the plagioclase interstices. The contact between the leuconorite and anorthosite is marked by features that are consistent with trapped liquid shift, such as a relatively increased abundance of intercumulus phases and relatively low orthopyroxene molar Mg/(Mg+Fe)] values. Very fine-grained chromite crystals are concentrated at the margins of orthopyroxene crystals in the leuconorite, but practically vanish in the overlying anorthosite where they occur only sparsely in the sub-vertical domains. The lower chromitite shares a knife-sharp contact with the underlying anorthosite. The lower chromitite comprises both amoeboidal and blocky chromite crystals [6], that display no spatial preference (i.e., host grain, stratigraphic location) nor any statistically significant chemical differences. The key difference �84 between the two chromite forms is that amoeboidal crystals host greater degrees of internal misorientation as well as abundant polymineralic inclusions. Thermodynamic simulations show that anorthosite residues, amongst other lithologies, may form as replenishing melts react with noritic cumulates. The initial modelled footwall melts assimilated by the replenishing melt are relatively volatile-rich and become Cr-bearing once resident cumulus orthopyroxene is consumed [7]. It is proposed that chamber replenishment triggered the reconstitution of resident noritic cumulates to anorthosite residues (Fig. 1A-B). The replenishing melt was likely saturated in chromite and sulfide melt, whereby skeletal chromite precipitated close to the melt-cumulate interface. The porosity generated in the footwall facilitated the downward percolation of sulfide melt that in turn helped to displace trapped silicate melts upward to the level of the proto-reef (Fig. 1C-D). The initially relatively volatile-rich footwall melts triggered dissolutionreprecipitation of skeletal to amoeboidal chromite, and the chromitite grew as auxiliary Cr3+ and Al3+ was liberated from the footwall. Figure 1. Petrogenetic model for replenishment-driven footwall reconstitution at the Rustenburg Platinum Mine. A. Deposition of leuconoritic (orthopyroxene = opx + plagioclase = pl) cumulates by gravitational settling of silicates in a quiescent melt. B. Basal influx of relatively primitive melt that entrains blocky chromite (cr) and sulfide (sul) melt. Skeletal chromite crystals form by supercooling close to the base of the replenishing melt and reconstitution of resident leuconoritic cumulates begins. C. Footwall melts are initially volatile-bearing and Cr-undersaturated (light blue arrows), triggering dissolution-reprecipitation of skeletal chromitites to form amoeboidal chromites. D. The footwall melts become Cr-saturated (green arrows) as orthopyroxene and accessory chromite are consumed. This leads to further chromite precipitation and the formation of the lower chromitite. These footwall melts are displaced upwards by down-going sulfide melts, which may also instigate coarsening of plagioclase and orthopyroxene oikocrysts. Black arrows to the side of diagrams denote the lithology. References: [1] Cawthorn RG (1999) S. Afr. J. Geol. 102(3):178-183 [2] Cawthorn RG (2015) In:Layered Intrusions pp. 517-587 [3] Cameron EN (1982) Econ Geol 77:1307-1327 [4] Viring RG and Cowell MW (1999) S. Afr. J. Geol. 102:192-208 [5] Roberts MD et al. (2007) Min Dep 79:169-186 [6] Vukmanovic Z et al. (2013) Contrib Min Pet 165:1031-1050 [7] Scoon RN and Costin G (2018) J. Pet. 59(8):1551-1578 �85 Future research areas to aid in exploration for Ni sulfides Sproule, R.A.1 1 Rio Tinto Exploration, Salt Lake City, UT, USA ___________________________________________________________________________ Discovery rates for magmatic nickel sulphide deposits have declined over the last thirty years and particularly over the last ten years. We are not discovering a sufficient number of high-quality low carbon footprint nickel sulphide deposits in a timely manner to meet society’s needs. Exploration is moderately successful at the deposit scale in a fertile intrusion and after initial discovery of sulfides. This is largely determined by the effectiveness of detection of conductive sulphides by EM technologies in massive-dominated NiS deposits, or the generally large footprint (e.g., magnetics, gravity, surface geochemistry) of large disseminated NiS deposits amenable to open pit mining. However, exploration struggles to identify new fertile lithospheric regions, new favourable terranes and potential camps. We also lack fundamental detailed understandings on the relationship and timing of nickel sulfide deposits to tectonic cycles, and the processes that form, enrich and accumulate sulfides. This is particularly true when we consider the full range of prospective parental magma compositions and host rock lithologies over the complete range of crustal levels. Moreover, both research and exploration activity have also largely focussed on magmatic Ni systems to the relative detriment of other types of important NiS deposits including sediment-hosted (e.g., Enterprise, Zambia) and hydrothermal types (e.g., Jaguar, Brazil). At present, our knowledge can be improved by developing: (1) an improved understanding of fertile lithospheric regions; (2) other geological environments conducive to forming Tier 1 NiS deposits; (3) detailed 3D nickel sulphide ore deposit models and footprints (geology, geophysics, geochemistry and mineralogy) for mineralized systems from a range of parental magma compositions, crustal depths and a range of tectonic settings. �86 Exploring the footwall: Sulfide Mineralization in the footwall Granite of the Maturi Deposit, Minnesota. Steiner, R. A.1 1 Big Rock Exploration, 2505 W Superior Street, Duluth MN 55806. alex@bigrockexploration.com ___________________________________________________________________________ The 1.1 Ga Keweenawan large igneous province generated voluminous magmatism resulting in the eruption of extensive flood basalts and the emplacement of sub-volcanic intrusions now exposed along the flanks of Lake Superior [1]. In northeastern Minnesota, two intrusive sequences of the Layered Series, the Partridge River Intrusion (PRI) and South Kawishiwi Intrusion (SKI), are known to host significant Cu-Ni-PGE sulfide mineralization [1]. The Maturi Cu-Ni-PGE deposit is located in the northern part of the SKI where the footwall is composed of granitic rocks of the Giants Range Batholith (GRB). The majority of Cu-Ni-PGE-enriched sulfides are disseminated throughout a 50-150m-thick basal mineralized zone (BMZ), though locally occur as massive to semi-massive sulfide occurrences along the basal contact (Figure 1). The mineralized rocks of the BMZ were emplaced in a series of three crystal-laden troctolitic pulses or stages that are divided on the basis of sulfide metal tenor, whole rock composition, and textural variations detailed in Peterson [2] (Figure 1). The first pulse, Stage 1, is sulfide poor and begins to delaminate the overlying anorthosite rocks from the footwall. Stage 2 contains abundant country rock xenoliths and more sulfide droplets that are carried within the crystal slurry and those sulfides are higher Cu, Ni, and PGE tenors than the prior Stage 1. Stage 3 is yet more enriched in metals, with the highest metal tenors found there and is also the most mafic pulse, often containing melatroctolite or sub-dunite horizons. Stages 2 and 3 are broadly emplaced above prior pulses, but locally erode down into the previous pulse in areas of channelized magma flow and may erode down to the granite below. Enigmatically, the underlying granite commonly hosts magmatic sulfide mineralization. That mineralization may occur as massive Ni-rich sulfide at the intrusion contact or extend as deep as 100 meters below the basal contact as Cu-rich sulfides (Figure 1). Sulfur isotope data show that the sulfide in the mineralized granite originated from the same source as that in the overlying troctolite [3, 4]. Here we present a mechanism by which melting and density-driven displacement drives magmatic Cu-Ni-PGE sulfide mineralization into the footwall granite of the Maturi deposit. Three of the drill cores were selected from the Maturi deposit that represent all three stages in contact with the underlying footwall granite [2]. Core logging and subsequent petrographic observations show that the granite reached pyroxene hornfels grade metamorphism and underwent partial melting due to thermal input from the overlying intrusion (Figure 2). Abundant leucosomes and sieve textured feldspars with trapped silicate melt record pervasive melting in the GRB. Leucosome patches and feldspar sieves have been observed to contain massive to semi-massive sulfide suggesting a relationship between location of partial melts and sulfide liquid, perhaps physical displacement of the former by the latter (Figure 3). Mass-balance equations using the isocon method of Grant [5] were used to explore the geochemical parameters to provide insight into the relationship of partial melts and sulfide liquid. When elements that partition into pyroxene (Cr, Mg, Mn) are treated as restite (not removed or added to the original �87 lithology) it becomes clear that an exchange of sulfide for partial melt is occurring (Figure 4). Elements that would partition into the silicate liquid during melting (REE, LIL, K, Ba) become depleted relative to the restite while components of the sulfide (S, Ni, Cu) become enriched. Samples of the footwall with the strongest sulfide mineralization show the strongest depletion of partial melt elements and the strongest enrichment of sulfide liquid components. The face that sulfide liquid and partial melts occupy the same textural space within the rock (e.g., leucosome patches between restite phases and sieve texture in plagioclase) and the geochemical signature showing the removal of partial melt components and addition of sulfide liquid components leads to the conclusion that mineralization in the footwall of the Maturi deposit is caused by the displacement of partial melt for a denser sulfide liquid. Such a process should not only result in mineralization of the footwall but also contamination of the overlying intrusion by partial melts. White [6] identified geochemical markers for contamination of the overlying BMZ by the footwall rocks, which became more intense in proximity to the footwall contact. This study finds abundant as networks and pods of partial melts throughout the GRB. Therefore, it is reasonable to assume that the amount of liquid displacement that can occur is limited by the amount of sulfide liquid available to penetrate the footwall. While there is large reservoir of sulfide present as the disseminated sulfides in the intrusions, that amount of that sulfide that may interact with the footwall interface is unclear. However, contamination of the silicate magma in the vicinity of the footwall rocks would reduce the sulfur carrying capacity in a magma that is already sulfur saturated thus providing an additional sulfide liquid reservoir to displace partial melts in the GRB. The formation of such a reservoir is evidenced by Ni-rich massive sulfide occurrences at the footwall contact intercepted during drilling. It is notable that the majority of the massive sulfide occurrences are found where the footwall is in contact with Stage 3; this being the latest mineralizing pulse would therefore introduce the greatest heat budget to the footwall rocks (Figure 1). It is below these locations that partial melting and footwall mineralization is most intense. By understanding both the emplacement sequence and mechanism of mineralized intrusions it is possible to constrain the focusing of heat into the country rock. Such constraints provide insight into targeting basal accumulations of sulfide within intrusions as well as unconventional mineralization hosted within the country rocks. Figure 1 – cartoon cross-section of the basal mineralized zone at Maturi highlighting areas on footwall mineralization below stages 2 and 3. �88 Figure 2 – partial melt pocket or leucosome surrounding remnant feldspar grains with orthopyroxene found in the melt (left). Melt pockets inside of feldspar grain resulting in sieve texture. Figure 3 – net-textured partial melt + pyroxene surrounding remnant feldspar and pyroxene. Arrow indicates sulfide that surrounds pyroxene in the same manner as partial melts elsewhere in the section. �89 Figure 4 – example isocon plot where the isocon is a best-fit line for MgO, MnO, and Cr2O3. The green field indicates components that are enriched relative to the isocon while the red field indicates depletion. References: [1] Miller, J.D. Jr. et al (2002) Minnesota Geological Survey Report of Investigations 58 [2] Peterson D.M. (2012), Duluth Metals ltd Presentation to Twin Metals Minnesota LLC [3] Ripley, E. M. and Alawi, J. A. (1986) Canadian Mineralogist 24:347-368 [4] Molnar, F. et al., (2009) Geological Society of America Abstract [5] Grant, J. A. (1986) Economic Geology 81:1976-1982 [6] White, C. R. (2010) MS Thesis University of Minnesota Duluth �90 The Anatomy of a Cu-Ni-Co-PGE Mineralized Mafic Magmatic System: The South Kawishiwi Intrusion of the Duluth Complex, Northeastern Minnesota Sweet, G.S.1 and Peterson, D.M.2 1,2 Big Rock Exploration, 2505 West Superior Street, Duluth MN, 55806, gabe@bigrockexploration.com ___________________________________________________________________________ In 1977, the Minnesota Department of Natural Resources published the first district-scale gradetonnage estimate [1] of Cu-Ni and TiO2 along the western margin of the Duluth Complex. These estimates, which utilized 324 of the 903 holes drilled through 1976 (285,902 meters), included 4.4 billion tons at 0.66% Cu and 0.2% Ni as well as 220 million tons at >10% TiO2 and brought to light the potential world-class scale of the Duluth Complex mafic magmatic system. Since the 1977 gradetonnage estimate, approximately 1,993 new exploration holes totaling over 802,360 meters have been drilled in the Duluth Complex area by a number of companies and the State of Minnesota. The physical formation processes of sulfide-bearing mafic intrusions remains one of the most important concepts for geologists engaged in exploring mafic magmatic systems for ore deposits. It is critically important to understand that the delivery of sulfide-bearing and potentially crystal-laden magmas into a growing intrusion is an iterative process confined to the spatial geometry of the system. The delivered magma will change with time (intrusion rate, crystallinity, xenolith content, sulfide content & tenor) and early batches of crystallizing magma are commonly cut and eroded by subsequent magmas (with their own unique intrusion rate, crystallinity, sulfide content & tenor). This work describes a new synthesis of decades of detailed mapping (>30,000 outcrops mapped), exploration and definition drilling (787,908 meters of core in 1899 holes), geochemistry (101,882 drill core and 8,267 surface sample analyses), geophysical surveying, and modeling by the authors and others in the South Kawishiwi Intrusion (SKI) and its Nickel Lake Macrodike (NLM) feeder dike. The outcomes of this new synthesis can perhaps be used as a proxy from which geologists can explore other mafic magmatic systems across the globe. The SKI is a shallow dipping (~24º east-southeast) sill-like troctolitic intrusion exposed in a 10- x 32kilometer arcuate band along the northwestern margin of the Duluth Complex. It extends from the edge of the Mesaba deposit (which is within the adjacent Partridge River and Bathtub intrusions) on the southwest, to the Spruce Road deposit on the northeast (Fig. 1). The SKI initially intruded between a hangingwall of the Duluth Complex Anorthositic Series rocks and a footwall composed of Paleoproterozoic sedimentary rocks, i.e., the Virginia Formation (VF) and Biwabik Iron Formation (BIF) in the southwest, and exclusively granitoid rocks of the Archean Giants Range Batholith in the northeast. The local presence of xenoliths of the BIF and VF as inclusions within the northern SKI and the NLM are interpreted as far-traveled country-rock blocks and not, as Severson et al. [2] interpreted, Paleoproterozoic sedimentary units assimilated in-situ from the immediate footwall during emplacement of the SKI. The basal stratigraphic section of the SKI was first described in great detail by Severson [3] and culminated with the SKI igneous stratigraphy being subdivided into 17 different units. In 2008, geologists from Duluth Metals Limited came to the realization that the contact-type mineralization at the Maturi deposit formed from initial basaltic composition SKI magmas that intruded as sulfidebearing, crystal-laden (plagioclase & olivine), magmatic slurries. Based on this interpretation, the company reinterpreted the sulfide-bearing basal zone of the SKI at the Maturi deposit into the Basal Mineralized Zone, or BMZ. This new interpretation was based on the geometry of the system (silllike sub-horizontal intrusion) and the inherent crystallinity of the SKI magmas. The channelized flow of these phenocryst-rich magmas led to crystal sorting and melting of the footwall granitic rocks to create the heterogeneous lithologies and textures of the BMZ. Years of detailed geological mapping, integrated with geological logging of all available drill holes, and a comprehensive assembly and interpretation of all geochemical data has led to a simplified overall igneous stratigraphy of the �91 intrusion. This stratigraphy has been subdivided into five basic units, including the Upper SKI, the SKI Break, the Middle SKI, the Main AGT, and the BMZ (Figure 2). Figure 1. Bedrock geologic map of the South Kawishiwi Intrusion and surrounding terranes. Yellow outlines define the approximate boundaries of compliant NI 43-101 resource estimates of the labeled Cu-Ni-Co-PGE deposits. In 2012, and after much additional drilling, the geology of the Maturi deposit BMZ was reevaluated once again by the geologic staff of Duluth Metals Limited, Twin Metals Minnesota, and geologists from the consulting firm AMEC. The reanalysis utilized a significant volume of new, high-quality geochemical and geological data to complete an updated mineral resource classification by AMEC. Mineralization in both the BMZ and footwall at the Maturi deposit area were reclassified based on patterns in the physical distribution of mineralization as projected on down-hole plots. Sulfide mineralization at Maturi is characterized by several distinct patterns, including A) very low grade, �92 fine-grained intervals showing low variability (Stage 1) that probably represent initial chilled magmas, B) moderate Cu-Ni and low PGE grade, xenolith-bearing (BIF, VF, basalt & anorthosite), mineralized zones showing low variability (Stage 2), and C) clean, higher grade, (Cu-Ni and PGE), xenolith-poor mineralized troctolite zones with higher variability and commonly bounded by low grade selvages (Stage 3). Significantly, most of the contacts between different mineralized intervals are typically quite abrupt. A single hole might contain one or several distinct mineralized intervals within the BMZ, including higher grade intervals with the highestgrade intervals occurring at the top, middle, or bottom of the section. Based on these criteria, four intrusive subunits, characterized by common grade profiles, were defined in the BMZ. In addition, two distinct suites of mineralization were identified in the footwall rocks, including NiCo enriched semi-massive to massive sulfide zones and disseminated Cu-PGE enriched zones deep in the footwall granitoids. All the newly classified zones of the BMZ at the Maturi Deposit are shown stratigraphically in Figure 3 and diagrammatically in Figure 4. Figure 2. Simplified igneous stratigraphy of the SKI. Figure 3. Revised igneous stratigraphy of the BMZ and adjacent rocks within the Maturi deposit. The classifications derived from this exercise were validated by multivariate statistical analysis of geochemical data, including principal component analysis and factor analysis. This investigation revealed distinct geochemical fingerprints of mineralization within the BMZ as well as several possible subdivisions of the BMZ based on both the physical distribution patterns of mineralization and the geochemistry of the host rocks. The Maturi subunits defined and validated were determined to occur in a consistent stratigraphic order and are correlative across the deposit. The current lithostratigraphic model for Maturi effectively discriminates between higher- and lowergrade SKI mineralization and provides a realistic geological model for mineralization throughout the intrusion. The new data allowed correlation of units from hole-to-hole and section-to-section �93 resulting in a very robust geologic model upon which Twin Metals Minnesota is building preliminary mine plans. Figure 4. Detailed idealized view of the BMZ intrusive stages at the Maturi Cu-Ni-PGE deposit. A fundamental aspect of the ever-developing ore deposit model of the SKI is an understanding of the initial conditions of the magmatic system – its crystallinity, sulfur capacity, geochemistry, and geometry – and how the sulfur saturated SKI magma lived, worked, and died. Such understanding includes the realization that the magma was a crystal-liquid (silicate and sulfide liquids) slurry and the identification of magma channel ways and sub-channels and their associated thermal anomalies. In addition, the SKI magmas locally melted the footwall granitoid rocks, and the addition of SiO2 into the sulfide-bearing troctolitic melts of the SKI induced additional sulfide immiscibility, stripping the melts of dissolved Ni and Co and forming high-grade massive sulfide ores locally at the basal contact and within the highly metamorphosed footwall Archean granitoids. In the end, hard work and intellectual geologic thought has been used to identify and understand one of the world’s largest resources of Cu-Ni-PGEs (Table 1). Table 1. Grade-Tonnage tabulation for deposits of the SKI. References [1] Listerud W and Meineke D (1977) MNDNR Report 93: 1-74 [2] Severson M et al. (2002) MGS RI 58: 164-200 [3] Severson, M (1994) NRRI TR 93/94: 1-210 �94 Multi-thermochronological records of cooling, denudation and preservation of ancient ultrabasic magmatic ore deposits: An example from the Neoproterozoic Jinchuan giant magmatic Cu-Ni sulfide deposit Ni Tao1,2*, Jiangang Jiao1, Jun Duan1, Haiqing Yan1, Ruohong Jiao3, Hanjie Wen1 1 Department of Geology, Northwest University, Xi’an, China, ni.tao@chd.edu.cn School of Earth Science and Resources, Chang'an University, Xi’an, China 3 School of Earth and Ocean Sciences, University of Victoria, Victoria, Canada 2 ___________________________________________________________________________ The post-mineralization denudation history and preservation of ore deposits have significant scientific and practical implications for ore deposit preservation condition, ore-forming potential evaluation, and deep ore prospecting. Ancient Cu-Ni sulfide ore deposits are characterized by complex magmatic evolution and a long-term geological history. How to quantify their denudation degree and emplacement depth is currently the focus and challenge of ore deposit preservation research. This study strategically chooses the Jinchuan giant magmatic Cu-Ni sulfide deposit as an example, with the Neoproterozoic ore-bearing plagioclase lherzolite as the main target, combined with its Paleoproterozoic metamorphic country rocks and early Paleozoic diorite veins for comparison. Multi-thermochronological analyses applied include apatite and zircon (U-Th)/He dating, apatite fission-track analysis, plagioclase and hornblende 40Ar/39Ar dating. The aims are to trace the thermal history of the ore-bearing intrusion, calculate its denudation thickness by integrating regional geological records, set up inversion models for verifying the calculated denudation thickness as well as determining emplacement depth of the ore-bearing intrusion. On this basis, by judging the relationship between the denudation thickness and the emplacement depth of the ore-bearing intrusion, this study clarifies the preservation degree of Jinchuan Cu-Ni sulfide deposit. The results may provide a new thermochronological paradigm for studying the preservation conditions and evaluating deep ore exploration potential of (ancient) ultrabasic Cu-Ni sulfide magmatic ore deposits. �95 Compositional variability in olivine: New data on the occurrences of Ni and Co as guides to mineral prospectivity Thakurta, J.1, Wagner, Z.1, Nagurney, A.2, and Schaef, H.T. 2 1 Natural Resources Research Institute, University of Minnesota, 5013 Miller Trunk Highway, Hermantown, MN 55811, USA 2 Pacific Northwest National Laboratory, 902 Battelle Boulevard, P.O. Box 999, Richland, WA 99352 USA ___________________________________________________________________________ Concentrations of trace constituents in olivine have been measured from a wide variety of maficultramafic intrusive and volcanic igneous rocks in different tectonic settings in North America. Samples include rocks from different locations of the 1.1 Ga old Midcontinent Rift System (MRS), such as the layered Duluth gabbroic Complex in Minnesota, and the peridotitic intrusions at Eagle in Michigan and Tamarack in Minnesota. The Cretaceous to Jurassic Dunite-peridotite rocks from Red Mountain Ultramafic Complex at the Kenai Chrome mine, the Eklutna ultramafic rocks, and the Alaskan-type ultramafic complex at Duke Island in Alaska represent small intrusive bodies in convergent tectonic settings. Alkali basalts with olivine phenocrysts from the Springville volcanic suite in Arizona constitute Pleistocene volcanic rocks. While the content of Ni is inversely correlated with the presence of sulfide minerals in the assemblages, a larger and more significant variation has been observed with respect to the origin, modes of occurrence and tectonic settings of the rocks in this study. Considerable variations are observed in different intrusions of the Duluth Complex in terms of the nature of the host rock: whether olivine gabbro or troctolite. Ni in the olivine gabbro ranges between 1800 and 2000 ppm while in the representative troctolite units it ranges between 700 to 900 ppm. Very high contents of Ni in olivine, ranging from 2000 to 2700 ppm are seen in small peridotitic intrusive bodies at the MRS such as feldspathic peridotite in Eagle, the Bowl and Fine Grained Olivine (FGO) intrusions of Tamarack. The dunite-peridotite at Kenai and Eklutna show comparable high values but values in the olivine clinopyroxenite unit of the Alaskan-type Complex at Duke Island are less than 800 ppm. A substantial range in Ni-content of olivine from 500 to almost 2500 ppm is observed in the olivine basalt at the Springville Volcanic Suite where individual phenocrysts of olivine show growth rims of changing Ni-content from core to rim (Figure 1). The Co-content of olivine in the olivine gabbro and troctolite units of the Duluth complex range from 300 to 400 ppm and 400 to 600 ppm respectively. Samples from Eagle, Tamarack as well as Duke Island cluster between 300 and 400 ppm. However, the dunite-peridotite at Kenai and Eklutna show values less than 250 ppm. From the new dataset and data available from previous studies [1, 2 and 3] it is evident, that with other factors being similar, Ni shows a positive correlation with the MgO-content while a negative correlation with Co is evident from the new data. Starting with the composition of magma from source rocks, changing fO2 conditions and H2O-content, leading to factors such as liquid evolution by fractional crystallization, assimilation, and re-equilibration of magma with preexisting Ni- and Co-rich rocks, a continuous spectrum of changing concentrations of trace metals in olivine can be envisioned from the available dataset. Such trace metal concentrations in olivine are important not only as indicators of Ni-rich sulfide mineralized zones in the associated rocks, but also as tools to evaluate the possibility of extraction of such critical metals from the ongoing development of new methods of metal-extraction from nonconventional sources such as olivine. �96 Figure 1: Concentrically zoned olivine phenocrysts in an olivine basalt from the Springville Volcanic Field in Arizona. Ni-Co concentrations change along the zones. References: [1] Barnes, J.B. (2023) Am Min 108:1-17 [2] Li, C. and Ripley, E.M. (2010) Chem Geo 275: 99-104 [3] Marek, L., Arevalo, R.D., Puchtel, I.S., Fiorentini, M.L. and Nisbet, E.G. (2019) Am Min 104: 1143-1155 �97 The effects of diagenetic and metamorphic processes on the sulphur liberation from the Virginia Formation black shale during magmatic assimilation by the Duluth Complex, Minnesota, USA Virtanen V.J.1,2, Heinonen J.S.2,3, Märki L. 4, Galvez M.E. 5 and Molnár F.6 1 Institute des Sciences de la Terre d’Orléans (ISTO), CNRS-Université d’Orléans-BRGM, France Department of Geosciences and Geography, University of Helsinki, Helsinki, Finland 3 Geology and Mineralogy, Faculty of Science and Engineering, Åbo Akademi University, Turku, Finland 4 METAS, Federal Institute of Metrology, Bern, Switzerland 5 Department of Earth Sciences, ETH Zürich, Zürich, Switzerland 6 Department of Mineralogy, Institute of Geography and Earth Sciences, Eötvös Loránd University, Budapest, Hungary 2 ___________________________________________________________________________ The Duluth Complex, Minnesota, USA, contains large low-grade disseminated Cu-Ni(-PGE) sulphide resources hosted in troctolites and smaller massive sulphide lenses hosted in norites [1]. Several lines of evidence, including sulphur isotopes, suggest that both deposit types formed by assimilation of sulphur from the Virginia Formation black shale. In the Virginia Formation, sulphur is mainly hosted in micron-scale disseminated pyrite with the exception of the peculiar carbon and sulphur-rich Bedded Pyrrhotite Unit that is characterized by pyrrhotite laminae with mm-scale thickness [1,2]. The Bedded Pyrrhotite Unit has been identified as an important source of sulphur especially to the norite-hosted massive sulphide occurrences [1,2]. However, the processes that caused the carbon and sulphur enrichment in the Bedded Pyrrhotite Unit have not been studied in detail. We used optical and scanning electron microscopy as well as Raman spectroscopy to characterize the normal Virginia Formation black shale and the Bedded Pyrrhotite Unit with emphasis on the carbonaceous materials (CM) and sulphides. Regionally metamorphosed and contactmetamorphosed samples were studied from both units. Whole-rock chemical data was acquired to measure H2O, Corg, and S in the samples. In the normal Virginia Formation, CM is present as uniformly dispersed submicron-scale flakes as typical for buried organic material (Fig. 1a). Raman spectroscopy revealed several defect bands (D1 to D4, see Fig. 1) meaning that the CM is structurally aromatic but turbostratic (i.e., aromatic sheets as in graphite but not in highly organized stacked-sheet structure). Structural ordering of residual CM is a suitable geothermometer as it changes systematically with temperature and it is not subject to retrograde resetting [3,4]. The CM geothermometer of Lahfid et al. [4] indicates that the Virginia Formation reached peak temperature of 300–340 ± 50 °C during regional metamorphism prior to the formation of the Duluth Complex. Figure 12. Reflected-light microphotographs of a) the normal black shale and b) the Bedded Pyrrhotite Unit in the regionally metamorphosed Virginia Formation. Typical Raman spectra of the carbonaceous materials (CM) with structure-related bands (G, D1 to D4) indicated as well as the whole-rock Corg, sulphur (S), and H2O �98 contents are shown. The arrow indicates that CM in b) represents remnants of accumulated oil. Abbreviations: Ab = albite, Ccp = chalcopyrite, Chl = chlorite, Ms = muscovite, Po = pyrrhotite, Py = pyrite, Qz = Quartz. The regionally metamorphosed Bedded Pyrrhotite Unit contains microscale fracture zones enriched in CM and sulphur (Fig. 1b). These zones are characterized by irregularly shaped quartz and sulphide grains that are rotated relative to the bedding (Fig. 1b). Carbonaceous material is found as pore space fillings (Fig. 1b) and as grain coatings suggesting that it represents oil residuals. Raman spectroscopy confirms that the CM in the Bedded Pyrrhotite Unit is structurally different from the CM in the normal black shale (Fig. 1b). Due to the migratory origin of the CM, we cannot reliably apply the geothermometer to the Bedded Pyrrhotite Unit. We suggest that the pore space, which facilitated oil infiltration, formed in the microfracture zones due to dissolution of soluble precursor sedimentary clasts, which are now replaced by quartz and sulphides (Fig. 1b). Pyrrhotite precipitation in diagenetic conditions is kinetically limited, hence the original sulphide in the Bedded Pyrrhotite Unit was probably pyrite (or some typical metastable diagenetic sulphide like greigite). We suggest that the original sulphide was converted to pyrrhotite during low-temperature hydropyrolysis of the CM during regional metamorphism. Whole-rock chemical data shows that the pyrite-bearing normal black shale experienced loss of H2O, Corg, and sulphur due to muscovite and chlorite breakdown as well as pyrite conversion to pyrrhotite caused by the Duluth Complex. The contact-metamorphosed Bedded Pyrrhotite Unit experienced the same metamorphic conditions but shows no systematic depletion of volatiles. In fact, the contact-metamorphosed Bedded Pyrrhotite Unit is the most Corg and sulphur rich part of the Virginia Formation. We suggest that sulphur was conserved through contact metamorphism because of the stability of pyrrhotite during devolatilization as shown in previous experiments [5]. This means that extensive partial melting of the Bedded Pyrrhotite Unit was required to liberate sulphur to the Duluth Complex magma. Consequently, the sulphide occurrences in association with Bedded Pyrrhotite Unit xenoliths are generally in the norites, which show more signs of assimilation Unit compared to the troctolites [1,2]. We also observed that prograde cordierite in the contactmetamorphosed Bedded Pyrrhotite Unit (Fig. 2a) is consistently replaced by biotite and muscovite at the vicinity of the pyrrhotite laminae (Fig. 2b). This indicates retrograde hydration event introduced H2O and possibly Corg and sulphur to the contact-metamorphosed normal black shale. Our findings highlight some key diagenetic and regional metamorphic processes that are important for magmatic ore genesis as they affect the CM and sulphur budget in black shales as well as the reactions that liberate sulphur upon magmatic assimilation. Figure 13. Back-scattered electron images showing a) the prograde mineral assemblage and b) the retrograde mineral assemblage of the contact-metamorphosed Bedded Pyrrhotite Unit. In a) prograde cordierite (crd) is surrounded by K-feldspar (Kfs), whereas in b) small anhedral cordierite is surrounded by retrograde phlogopite (Phl). Abbreviations: Gr = graphite, Pl = plagioclase, Po = pyrrhotite, Qz = quartz. References: [1] Thériault R and Barnes S-J (1998) Can Min 36:869-886 [2] Samalens N et al. (2017) Ore Geol Rev 81:173-187 [3] Beyssac O et al. (2002) J Metamorphic Geol 20:859-871 [4] Lahfid A et al. (2010) Terra Nova 22:354-360 [5] Virtanen V et al. (2021) Nat Commun 12:1-12 �99 Mantle-to-crust scale chemical fractionation and sulphide saturation of the Paleoproterozoic komatiites of the Central Lapland Greenstone Belt, Finland – implications for geochemical exploration Virtanen V.J.1,2, Höytiä H.M.A.2, Iacono-Marziano G.1, Yang S.3, Moilanen M.3 and Törmänen T.4 1 Institut des Sciences de la Terre d’Orléans, UMR 7327, CNRS/Université d’Orléans/BRGM, Orléans, France Department of Geosciences and Geography, University of Helsinki, Helsinki, Finland 3 Oulu Mining School, University of Oulu, Oulu, Finland 4 Geological Survey of Finland, Rovaniemi, Finland 2 ___________________________________________________________________________ In the Central Lapland Greenstone Belt (CLGB) komatiites are present along a >250 km long SE-NW zone across the northern Finland (Fig. 1). The CLGB komatiites formed at ca. 2.05 Ga mostly as underwater eruptions on a sedimentary basin, which is known to have contained abundant sulphurous black shales and evaporites [1]. This association with sulphurous sedimentary rocks makes the CLGB komatiites promising targets for Cu-Ni-PGE sulphide deposits. Indeed, these sedimentary rocks supplied sulphur to the Kevitsa and Sakatti Cu-Ni(-PGE) sulphide deposits (Fig. 1), which formed during the same magmatic event as the CLGB komatiites [1,2]. To understand the petrogenesis of the CLGB komatiites from their mantle source to their crustal sink, we conducted computational thermodynamic simulations to constrain the chemical fractionation and sulphide saturation state without the effects of assimilation. These simulations guide identification of chemical anomalies related to assimilation and sulphide saturation in the CLGB komatiites and related intrusive rocks. Figure 14. Geological map showing the distribution of the Central Lapland Greenstone Belt komatiites. We defined the parental melt of the CLGB komatiites using a chilled margin of a komatiitic dyke from Kevitsa, which represents quenched olivine-saturated melt [3]. We added olivine to the chilled margin composition to reversely fractionate it to be in equilibrium with the most primitive olivine (Fo92) in Sakatti [1]. Using this method, we constrained major element oxides, Ni, Cu, and rare earth elements (REE) for the komatiitic (MgO = 20.6 wt.%) parental melt. Assuming adiabatic propagation through the lithosphere, the parental melt should be compositionally identical to the primary mantle melt and allows constraining the mantle melting conditions. We used REEBOX PRO [4] to define Ti and REE contents as well as temperature of the adiabatically melting mantle source. Several mantle sources and mantle potential temperatures were tested. Consistent with the previous studies related to the mantle source of the CLGB komatiites [3,5,6], we found that pyrolite mantle-source with depleted MORB -type REE contents is suitable. The best fit of Ti, REE, and temperature was reached with the mantle potential temperature of 1575 °C and with degree of melting at 15–20 %. The �100 mantle potential temperature determines that melting starts at ca. 5 GPa and the required degree of melting is reached at ca. 3 GPa (equivalent to ca. 100 km depth). Major element oxide composition of the parental melt (assumed here as identical to the primary mantle melt as noted above) is well compatible with literature data from mantle melting experiments with pyrolite mantle source [7]. We calculated the sulphur content at sulphide saturation (SCSS) for the primary mantle melt using the parental melt composition (major element oxides, Ni, and Cu) and the final pressuretemperature conditions in the mantle using the parameterization of Smythe et al. [8]. This constrains the maximum sulphur content of the primary mantle melt to 1172 ppm. With the typical range of sulphur content for a depleted mantle source of 150–200 ppm [9] and with the degree of mantle melting at 15–20%, the initial sulphur content of the CLGB komatiites is estimated to be 750–1172 ppm. To examine chemical fractionation of the CLGB komatiites in crustal conditions (25 MPa), we conducted closed-system fractional crystallization simulations using Magma Chamber Simulator [10]. For SCSS, we used the same parameterization [7] as with the mantle melting simulations. Using new and literature data [1,2,3,5,6,11,12,13,14], we compiled a comprehensive whole-rock (n = 299–403 depending on the element) and olivine (n = 917) chemistry database for the CLGB komatiites and spatiotemporally related rocks (from Kevitsa and Sakatti) to evaluate the simulation results. We find that closed-system fractional crystallization produces a good fit to the reference data for major element oxides and Ni (Fig. 2a). Importantly, simulated Ni contents in olivine are in good agreement with natural data (Fig. 2a) and could be used to identify Ni-depleted olivine to indicate those CLGB komatiites that experienced early sulphide saturation. Sulphur and Cu show highly incoherent behaviour in the reference data set and were likely affected by sulphide accumulation, degassing, and post-magmatic alteration. However, the simulation results are compatible with literature data for S (Fig. 2b) and Cu from chromite-hosted melt inclusions from the CLGB komatiites [6], which show relatively coherent behaviour compared to the whole-rock data. Depending on the initial S content (750–1172 ppm, see above), our SCSS simulations show that both Ni-rich (Ni/Cu = 1.9) and Cu-rich (Ni/Cu = 0.4) sulphide melt could have formed from the CLGB komatiite melt upon closed-system fractional crystallization (Fig. 2b). Moreover, the simulations indicate that the S content of CLGB komatiite melt was constantly close to SCSS starting from the liquidus (Fig. 2b). Accordingly, assimilation of sulphur-bearing country rocks has the potential to form relatively large sulphide accumulations within this region. Figure 15. Closed-system fractional crystallization simulation results shown on a) MgO (wt.%) vs Ni (ppm) and b) MgO (wt.%) vs. S (ppm) diagrams. The data clouds in a) represent whole-rock and olivine data from the Central Lapland Greenstone Belt (CLGB) komatiites and related rocks (Kevitsa and Sakatti). Sulphur contents in b) are shown only for chromite-hosted melt inclusions from the CLGB komatiites. �101 References: [1] Brownscombe W et al. (2015) Min Dep of Finland 211-252 [2] Luolavirta K et al. (2018) Lithos 296-299:37-53 [3] Puchtel I et al. (2020) Chem Geol 554:1-23 [4] Brown E and Lesher C (2016) Geochem Geophys Geosystems 17:3929-3968 [5] Hanski E et al. (2001) J Pet 42:855-876 [6] Hanski E and Kamenetsky V (2013) Chem Geol 343:25-37 [7] Walter M (1998) J Pet 39:29-60 [8] Smythe D et al. (2017) Am Min 102:795-803 [9] Lorand J-P and Luquet A (2016) Rev Mineral Geochem 81:441-488 [10] Bohrson W et al. (2014) J Pet 55:1685-1717 [11] Luolavirta K et al. (2018) Bull Geol Soc Finland 90:5-32 [12] Patten C et al. (2023) Min Dep 58:461-488 [13] Saverikko M (1985) Bull Geol Soc Finland 57:55-87 [14] Törmänen T et al. (2016) Min Dep 51:411-430 �102 Ni-Cu-PGE prospectivity of the Mackenzie Large Igneous Province Williamson, M.-C.1, Rainbird, R.H.1, O’Driscoll, B.2 and Scoates, J.S.3 1 Geological Survey of Canada, 601 Booth St, Ottawa, ON, K1A 0E8 Canada Email: marie-claude.williamson@nrcan-rncan.gc.ca 2 University of Ottawa, Marion Hall, Ottawa, ON, K1N 6N5 Canada 3 PCIGR, University of British Columbia, 2020-2207 Main Mall, Vancouver, BC, V6T 1Z4 Canada ___________________________________________________________________________ Large igneous provinces (LIPs) are high volume, intraplate magmatic events that consist of flood basalts, gabbro sills and dykes +/- layered intrusions. Most LIPs are emplaced over a time span of ~50 My or less [1], and there is strong evidence that the flood basalt volcanism occurs over even shorter time intervals (<1-2 My). The 1.27 Ga Mackenzie LIP includes flood basalts and feeder dykes of the Coppermine River Group (CRG), the Muskox intrusion and the Mackenzie dyke swarm. Previous studies of the Mackenzie LIP have focused on each of these three elements of the magmatic architecture, which resulted in many geological maps, datasets and samples archived at the GSC’s Earth Materials Facility [2, 3, 4]. We propose to revisit previous work [5] and fill knowledge gaps [6] to produce a regional synthesis of the Mackenzie LIP that specifically highlights Ni-Cu-PGE prospectivity. Knowledge about the Ni-Cu-PGE prospectivity of the Mackenzie LIP is largely based on previous mapping and laboratory studies of the Muskox intrusion and its putative feeder dyke [7, 8]. In contrast, the prospectivity of CRG flood basalts and feeder dykes is unknown. In this presentation, we summarize the methodology and anticipated results of a new GSC project on the Ni-Cu-PGE prospectivity of the Mackenzie LIP. We will adopt a multidisciplinary approach and a different research lens, one that specifically investigates the contact zone(s) and structures between the CRG and the Muskox intrusion. Our objectives are to: (1) fill knowledge gaps on the CRG feeder dykes and marginal rocks of the Muskox intrusion and evaluate the prospectivity of contact zones between intrusions and country rocks; (2) identify channelized lava flows, sills and dykes using remote predictive mapping; and (3) publish a synthesis that will focus specifically on Ni-Cu-PGE prospectivity. Detailed remote predictive mapping of feeder dykes will further our understanding of ore genesis in channelized lava flows, sills, and dykes [9]. Additionally, mineralogical and geochemical studies of picritic lava flows will establish mantle melting temperatures, thus providing constraints on the timing and composition of magma fluxes during the lifetime of the LIP. Another important aspect of studying the picrites is to establish genetic links with the Muskox feeder dyke. Finally, our aim is to reconstruct the timing and duration of magmatism in the Mackenzie LIP and establish links to potential mineralization using high-precision geochronology of the Mackenzie dykes and of the CRG lava flows. The results will increase our knowledge base of Mackenzie LIP architecture, and of the NiCu-PGE prospectivity of the CRG flood basalts and feeder dykes, and of the marginal rocks of the Muskox intrusion. References: [1] Ernst R E and Bleeker W (2010) Can J Earth Sci 47, 695-739 [2] Mackie R A et al. (2009) Precambrian Res 172: 46-66 [3] Skulski T et al. (2018) GSC Open File 8522, 37 p. [4] Williamson M-C et al. (2023) 14th Int Pt Symp: 160-163 [5] Ernst R E et al. (2010) GSC Open File 6016, 14 p. [6] Scoates J S and Scoates R F J (2024) Lithos 474-475: 107560 [7] Hulbert L (2005) GSC Open File 4881 (CD-ROM) [8] Day J M D et al. (2013) Lithos 182-183: 242-258 [9] Lesher M (2019) Can J Earth Sci 56: 756-773 �103 Siluro-Devonian Mafic-Ultramafic Intrusions in New Brunswick, Northern Appalachians, and their Associated Nickel-Copper-Cobalt Sulphide Deposits: A preliminary review Yousefi, F.1, Lentz D.R.1, Walker J.A.2 ,Thorne K.G.3, and Karbalaeiramezanali A. 3 1 1: Department of Earth Sciences, University of New Brunswick, Fredericton, NB E3B5A3 Canada fazilat.yousefi@unb.ca 2 Geological Surveys Branch, Department of Natural Resources and Energy Development, Bathurst, New Brunswick, E2A 7B8 Canada 3 Geological Surveys Branch, Department of Natural Resources and Energy Development, Fredericton, New Brunswick, E3B 5H1Canada ___________________________________________________________________________ In the Appalachian-Caledonian region, several mafic-ultramafic intrusions host notable Ni-Cu-Co sulphide mineralization, as well as platinum-group elements. Notable examples in New Brunswick (NB) [1] include St. Stephen, Goodwin Lake, Mechanic Settlement, and Portage Brook intrusions. With the exception of Mechanic Settlement (Proterozoic), these occurrences are Silurian-Early Devonian, and formed during the terminal stages of the Acadian Orogeny [2]. Powderhorn Lake and Portage Ni-Cu occurrences represent examples associated with mafic and ultramafic intrusions in Newfoundland (NF). The Moxie, Katahdin, Union, Alexander, Moosehorn Plutonic Suite, and Pocomoonshine Gabbro-Diorite in Maine (USA) are examples of hosting Ni-Cu sulphide mineralization [3, 4]. The location of Devonian mafic-ultramafic intrusions linked to Ni-Cu, Co, and PGE sulphide mineralization in Maine, NB, and NF are shown below on a map, showing the tectonic zones of the Canadian Appalachians (Fig. 1). This preliminary study explores occurrences of Ni-Cu sulphide mineralization, cobalt, platinum-group elements, and their mafic-ultramafic intrusions in NB. The compositions of these mafic-ultramafic intrusions include gabbro, gabbronorite, olivine gabbro, olivine gabbronorite, anorthosite, peridotite, and troctolite. The sulphide mineral assemblages in these mafic-ultramafic rocks are dominated by pyrrhotite, pentlandite, and chalcopyrite. The assimilation of sulphide-bearing Cambro-Ordovician metasedimentary rocks typical of the Gander zone, and the local attainment of sulphide-silicate equilibrium are key factors in the formation of immiscible sulphide melts. For instance, in southern NB, the Siluro-Devonian St. Stephen Intrusion has an extremely low mass ratio of silicate magma to sulphide melt indicating a preferential assimilation of sulphide-rich portions of the Cambro-Ordovician Cookson Formation within the host St. Croix terrane. The scattered coarse sulphide blebs within the host intrusion indicates either solidification of the rock shortly after the formation of immiscible sulphide droplets or a high yield strength of the magma that prevented sulphide blebs from efficiently settling – differentially segregating [1]. The mafic-ultramafic intrusions in New Brunswick have low silica contents (38.2 to 51.28 wt.%) and FeOt/MgO ratios (<5), displaying calc-alkaline to tholeiitic features. Variations in Al2O3, Fe2O3t, MgO, and CaO in most samples can be explained by the fractional crystallization - accumulation of olivine, both pyroxenes, and plagioclase. Preliminary lithogeochemistry indicates a wide variation in Cr (up to 1300), with Ni (up to 1100 ppm), Cu (up to 635 ppm), and Co (up to 150 ppm) content outside of the mineralized zones. Earlier separation of sulphides seems to be the reason for the typically low concentrations of chalcophile and platinumgroup elements in these basic intrusive rocks. There is an enrichment of light rare earth elements relative to heavy rare earth elements in these mafic-ultramafic intrusions. The host intrusions are characterized by enrichment of large-ion lithophile elements (e.g., Rb, Ba, Sr) and are depleted in high-field strength elements (e.g., Nb, Ta, Zr, Hf, Ti), with much lower Ta/La (0.04) than primitive mantle (0.06; [5]). These unique characteristics may be attributed to the involvement of continental crust, which generally lacks Ta and Nb. The elevated Th/Nb(averaging 0.25) and La/Hf (averaging 8.6) support an island arc basalt affinity for these intrusions. Referring to an example (Moxie Pluton) in Maine Appalachian Orogeny [6], the emplacement of mafic-ultramafic intrusions occurred due to crustal fracturing in the late stages of the Acadian Orogeny, leading to a local tensional regime that generated a bimodal (mafic & felsic) igneous suite. According to the high positive ɛNd values �104 presented [7], it is inferred that the magmas responsible for forming these mafic-ultramafic intrusions originated by decompression of a modified mantle. Fig. 1: Distribution of Devonian mafic-ultramafic intrusions associated with Ni-Cu sulphide, cobalt, and platinum group element (PGE) mineralization in Maine (USA), New Brunswick, and Newfoundland, situated within the Canadian Appalachians (modified from [8]). References: [1] Paktunc A.D (1989) Econ Geol 84: 817-840 [2] Ruitenberg A (1968) NB Dept. Nat. Resources Rept. Inv 7: 47 p [3] McLaughlin K.J et al. (2003) Atl. Geol 39: 123-146 [4] Slack J.F et al. (2022) Atl. Geol 58: 155-191 [5] Ye X.T (2015) J Asian Earth Sci 113: 75-89 [6] Thompson J.F.H (1984) Am J Sci 284: 462-483 [7] Whalen et al. (1996) Can J Earth Sci 33: 140-155 [8] Hibbard J and Karabinos P (2013) Geosci. Canada 40: 303-317 �105 Geochemistry of Archean komatiitic greenstone terranes of the Wyoming Province: implications for geodynamic setting and mineralization Zieman, L.J.1*, Poletti, J.E.1, and Jenkins, M.C.1 1 U.S. Geological Survey, Geology, Minerals, Energy, and Geophysics Science Center, Spokane, Washington, USA *lzieman@usgs.gov ___________________________________________________________________________ Archean komatiites are important host rocks of some Ni-Cu sulfide deposits [1] and are hypothesized to be the parental melt of several Archean layered mafic intrusions that host world-class platinumgroup element (PGE) deposits [e.g., 2, 3]. The Archean Wyoming Province in the western United States contains two greenstone terranes that include komatiitic metavolcanic rocks: South Pass in the southern Wind River Range and Bradley Peak in the Seminoe Mountains, Wyoming. These Archean greenstone terranes have primarily been explored for Au, Cu, Fe, and Zn [4, 5]. However, the age, geodynamic setting, and sulfide mineralization potential of the spatially associated maficultramafic metavolcanic rocks are poorly understood. Here, new major and trace element geochemistry as well as detrital zircon geochronology constrain the volcanic environment and the emplacement ages of these komatiitic metavolcanic units. Metavolcanic units from the Bradley Peak region preserve primary igneous textures, including parallel and random spinifex (Fig. 1A), whereas igneous textures are overprinted by schistose textures in the South Pass metavolcanic rocks. Like most global komatiites, mafic-ultramafic rocks from both terranes have been metamorphosed up to amphibolite facies and contain tremolite, actinolite, serpentinite, chlorite, talc, and/or epidote. This work focuses on elements that are resistant to alteration [e.g., Mg, Al, Ti, and rare earth elements (REE); 6]. The metavolcanic rocks in both Bradley Peak and South Pass greenstone belts contain basaltic to ultramafic komatiites, as well as high-Mg and high-Fe tholeiitic basalts based on the Al-Mg-(Fe+Ti) classification scheme of [7] (Fig. 1B). The subset of komatiitic samples (n = 20) have MgO contents predominantly ranging from 10 to 23 wt. %. These low MgO contents (< 30 wt. %) suggest low degrees of partial melting or high degrees of crustal contamination relative to komatiites associated with major Ni deposits [e.g., 8]. Like most Archean komatiites [e.g., 6], komatiites from both greenstone terranes are predominately Al-undepleted (i.e., Munro-type) based on their chondritic Gd/Yb and Al2O3/TiO2 ratios (Fig. 1C). The absence of heavy REE enrichments indicates the komatiitic magmas were generated at mantle depths shallower than the garnet stability field (< 300 km). The South Pass komatiites are highly enriched in light REE relative to a primitive komatiite melt, whereas the Bradley Peak komatiites are not enriched in light REE. These trends suggest that the South Pass komatiites have experienced higher percentages of crustal assimilation than the Bradley Peak komatiites. This interpretation supports previous studies that proposed the South Pass ultramafic rocks intruded continental shelf sedimentary rocks at the southern margin of the Wyoming craton, whereas the Bradley Peak ultramafic rocks were deposited in a sediment-starved ocean basin within a rift [9, 10]. Because komatiites lack minerals suitable for geochronology, emplacement ages of the ultramafic units were better constrained using detrital zircon U-Pb geochronology for metasedimentary rocks interbedded with the metavolcanic rocks in each greenstone terrane (Fig. 1D). Significant age populations were determined to identify the youngest age peak, which corresponds with the maximum depositional age (MDA), in addition to the weighted mean age for each sample [11]. In the Bradley Peak region, the weighted mean age for a metagraywacke from the Seminoe Formation, which overlies the ultramafic rocks, constrains the Bradley Peak ultramafic rocks to be older than 2721 ± 15 Ma. In the South Pass region, a metagraywacke from the unit overlying the komatiites (Miners Delight Formation) has a weighted mean age of 2673 ± 16 Ma, which agrees with published data and the previously accepted age for this greenstone terrane of 2.67 Ga [12]. Two pelitic schist samples interbedded with the komatiite units record MDA ranges ca. 3007-3049 Ma. This MDA range constrains komatiite units to younger than 3.01 Ga, but permits the komatiite units to be older than the previously assumed age of 2.67 Ga. �106 Figure 1. A) Sub-parallel spinifex texture preserved in the Bradley Peak metavolcanic rocks. B) Al-Mg-(Fe+Ti) cation classification plot after [7]. Hypothetical Stillwater parental melt (orange star) is from [2]. C) Gd/Yb vs. Al2O3/TiO2 for the subset of komatiitic rocks from (B) in comparison to global komatiites after [12]. Inset: TiO2 vs. Al2O3 illustrating Al-depleted (Al2O3/TiO2 ≈ 20) and Al-undepleted (Al2O3/TiO2 ≈ 10) trends. D) Detrital zircon age data. Vertical scales in probability density plots, calculated after [12], are reduced to 25%. A crystallization age is given for igneous sample 23BP25 (a). Weighted mean age (b) is given for samples with one significant age peak. The MDA (c) is given for samples with more than one significant age population. These komatiites do not satisfy several criteria typically thought to be important for Ni-Cu ore genesis [e.g., 1]— they were generated from relatively low degree partial melting and, in the case of the Bradley Peak greenstone, lack geochemical signatures of significant crustal assimilation, which is widely accepted to be a source of sulfur for ore genesis [1]. Contrarily, they are Al-undepleted and erupted at cratonic margins, characteristic of komatiites that have been associated with major Ni deposits [8]. Furthermore, the geochronological data do not rule out that either greenstone terrane was erupted synchronously with the emplacement of the 2.7 Ga Stillwater Complex in the Archean Wyoming Province, which is thought to have an Al-undepleted komatiitic parental melt (see Fig. 1B and 1C). Future work is needed to test if eruption of the komatiites is related to the emplacement of this layered intrusion or other magmatic systems in the Wyoming Province. References: [1] Barnes S J et al. (2016) Ore Geol Rev 76:296-316 [2] Jenkins M C et al. (2021) Precambr Res 367:106457 [3] Eales H and Costin G (2012) Econ Geol 107:445-465 [4] Hausel D (1991) WY State Geo Survey 44:1-129 [5] Hausel D (1994) WY State Geo Survey 50:1-24 [6] Barnes S J et al. (2004) Mineral Petrol 82:259-293 [7] Jensen (1976) Ontario Geo Survey 66 [8] Mole D et al. (2014) Proc Natl Acad Sci 111:10083-10088 [9] Grace et al. (2006) Can J Earth Sci 43:1445-1466 [10] Frost C et al. (2006) Can J Earth Sci 43:1533-1555 [11] Gehrels G (2009) Excel Age Pick Program [12] Arndt N and Lesher C (2004) Cambridge U Press [13] Saylor J and Sundell K (2016) Geosphere 12:203-22 Note: Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource International Ni-Cu Symposium. August 6-8th 2024, Lakehead University, Thunder Bay, Canada. Abstracts. Description An account of the resource International Ni-Cu Symposium. August 6-8th 2024, Lakehead University, Thunder Bay, Canada. Abstracts. Creator An entity primarily responsible for making the resource International Ni-Cu Symposium Date A point or period of time associated with an event in the lifecycle of the resource 2024-08 Contributor An entity responsible for making contributions to the resource Pete Hollings Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English Type The nature or genre of the resource Text https://digitalcollections.lakeheadu.ca/files/original/74e827a63cf38b24a3118cf8b12eb371.pdf b48a2c449c6d06e50789cf2709e4786d PDF Text Text )mpertal '9rhrr Dnuglµers nf tt,e �mpire �-�.!).. �.. 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