592 10 13625 https://digitalcollections.lakeheadu.ca/files/original/8154f0af057b119dd8208a23f584961e.pdf f4397eb22cabad582e48970a6ebf2063 PDF Text Text 69th ANNUAL MEETING Eau Claire, Wisconsin — April 24-25, 2023 INSTITUTE ON LAKE SUPERIOR GEOLOGY Part 1 — Program and Abstracts �Thank you to our sponsors! A SPECIAL THANK YOU TO OUR INDIVIDUAL CONTRIBUTORS: FREDERICK CAMPBELL, VAL CHANDLER, JIM DEGRAFF, THOMAS ERICKSON, TOM FITZ, DAVE GOOD, PAULA LEIER-ENGELHARDT, ALLAN MACTAVISH, BOB MAHIN, GORDON MEDARIS JR., JIM MILLER, STEVEN PINTA, TOD ROUSH, AND GERRY WHITE i �Proceedings of the 69th ILSG Annual Meeting – Part 1 69th ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY April 24-25th Eau Claire, Wisconsin HOSTED BY Rob Lodge, Esther Stewart, Carsyn Ames Co-Chairs University of Wisconsin- Eau Claire and Wisconsin Geological and Natural History Survey Proceedings - Volume 69 Part 1 – Program and Abstracts Compiled and edited by Carsyn Ames Cover Photos. Left— Photograph showing a group of men, women and children traveling through a forest north of Chippewa Falls, Wisconsin in a horse-drawn carriage, Chippewa Co., 1916. Center— Cross-bedding in basal Cambrian sandstone Eau Claire Co., 1919. Right — Outcrops of rhyolite schist along the north fork of the Eau Claire River, Eau Claire Co. 1919. ii �Proceedings of the 69th ILSG Annual Meeting – Part 1 69th INSTITUTE ON LAKE SUPERIOR GEOLOGY VOLUME 69 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD T RIP GUIDEBOOK Trip 1: PRECAMBRIAN GEOLOGY OF THE CHIPPEWA RIVER VALLEY Trip 2: WISCONSIN’S PALEOZOIC STRATIGRAPHY AND TOUR OF CRYSTAL CAVE Trip 3: PRECAMBRIAN GEOLOGY OF THE EAU CLAIRE RIVER VALLEY Trip 4: QUATERNARY GEOLOGY AND GEOMORPHOLOGY OF THE EAU CLAIRE REGION Reference to material in Part 1 should follow the example below: 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.), Institute on Lake Superior Geology Proceedings, 69th Annual Meeting, Eau Claire, Wisconsin, Part 1 - Abstracts and Proceedings. v.69, part 1, p.3738. Published by the 69th 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 iii �Proceedings of the 69th ILSG Annual Meeting – Part 1 ISSN 1042-9964 Part 1: Program and Abstracts Table of Contents Institutes on Lake Superior Geology, 1955-2023 v Sam Goldich and the Goldich Medal vii Goldich Medal Guidelines ix Goldich Medalists and Goldich Medal Committee xi Citation for Goldich Medal Award to Peter Hollings xii Honoring the Pioneers of Lake Superior Geology xii Nomination for Thomas Benton Brooks, Pioneer of Lake Superior Geology xv Memoriams for Stephen Allard, Steven Hauck and Manfred Kehlenbeck xx Eisenbrey Student Travel Awards xxv Joe Mancuso Student Research Awards xxvi Doug Duskin Student Paper Awards and Award Committee xxvii Board of Directors and Session Chairs xxviii Field Trip Leaders and Guidebook Authors xxix Report of the 68th Annual Meeting xxx Technical Program xxxiv Poster Presentations xl Banquet Presentation xliii Abstracts 1-99 iv �Proceedings of the 69th ILSG Annual Meeting – Part 1 Institutes on Lake Superior Geology, 1955-2023 # 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 v �Proceedings of the 69th ILSG Annual Meeting – Part 1 # 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 vi 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 �Proceedings of the 69th 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 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. 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 69th ILSG Annual Meeting – Part 1 INSTITUTE ON LAKE SUPERIOR GEOLOGY GOLDICH MEDAL viii �Proceedings of the 69th 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. ix �Proceedings of the 69th ILSG Annual Meeting – Part 1 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 �Proceedings of the 69th ILSG Annual Meeting – Part 1 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 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 2023 GOLDICH MEDAL RECIPIENT Peter Hollings Goldich Medal Committee Serving through the meeting year shown in parentheses. Steve Kissin (2018-2023*) Lakehead University (Committee Chair) Dorothy Campbell (2019-2024*) Ontario Geological Survey Dean Peterson (2022-2025) Big Rock Exploration *Terms of the committee members were extended 2 years because of the cancelation of the 2020 meeting and the logistical difficulties of voting during the 2021 virtual meeting. xi �Proceedings of the 69th ILSG Annual Meeting – Part 1 Citation for the Goldich Medal Recipient to Peter Hollings ILSG Members, it is our privilege to present the citation for this year’s recipient of the prestigious Goldich Medal to Dr. Peter Hollings. Pete received his Bachelor of Science with Honours in Geology from the Royal Holloway and Bedford New College, University of London in 1992. He continued as a postgraduate research assistant at Royal Holloway and Bedford New College until 1994 when he enrolled as a Ph.D. student at the University of Saskatchewan. He earned his Ph.D. in 1998 and his doctoral dissertation was titled “Geochemistry of the Uchi subprovince.” He had a one-year postdoctoral fellowship at Saskatchewan, followed by a two-year NSERC postdoctoral fellowship at the University of Tasmania. Pete joined the faculty at Lakehead University in 2001 as an Assistant Professor and in 2009 was promoted to full Professor, a title he continues to hold. Since 2013 Pete has been Director of the Centre of Excellence for Sustainable Mining and Exploration (CESME) at Lakehead University. He has served as Chair of the Department of Geology and as interim Dean of the Faculty of Science and Environmental Studies at Lakehead. Pete has been recognized for his research through several awards. In 2004 he and his coauthors were awarded the Julian Boldy Award by the Mineral Deposits Division of the Geological Association of Canada for an outstanding paper. In 2008 he was awarded the William Harvey Gross Medal by the Mineral Deposits Division of the Geological Association of Canada for significant contributions to the field of economic geology by a geoscientist under the age of 40. He was part of the team recognized by an award in 2012 and in 2014 by AMIRA International. In 2015 he was named the NSERC Distinguished Researcher for Lakehead University and in 2016 he was named the Lakehead University Research Chair in the NSERC/CHIR category. He received the Howard Street Robinson Medal from the Geological Association of Canada in 2017. In 2021, a paper on which he was co-author was awarded the 2020 Cameron-Hall Copper Medal for the most outstanding scientific publication in the journal Geochemistry: Exploration, Environment, Analysis (GEEA). Pete was awarded the NOHFC Industrial Research Chair in Mineral Exploration for a term from 2020 to 2025. Pete has an impressive professional record of publications and presentations. As of 2022, he has been first author or co-author of 145 refereed journal articles, 13 book chapters, 234 reports, 136 papers in refereed conference proceedings, and 87 abstracts in conference xii �Proceedings of the 69th ILSG Annual Meeting – Part 1 proceedings. While this is an impressive list of accomplishments, it is Pete’s ongoing contributions to our understanding of Lake Superior geology and to the Institute on Lake Superior Geology that make him a worthy recipient of the Goldich Medal. Pete has extensively conducted research on the geology of the Lake Superior region and the broader Superior Province. He has focused on both the Midcontinent Rift System (MRS) and Archean greenstone belts and their mineral resources. More than 30 of his published papers in refereed journals are on Lake Superior geology as well as about half of both his 30 first-authored conference proceedings and 29 first-authored refereed abstracts. He has contributed to more than 60 technical reports on Lake Superior geology. Of his 27 invited presentations, half have dealt with Lake Superior geology. Pete has a significant number of publications and presentations relevant to the discovery and utilization of natural resources in the Lake Superior region. Some of his numerous economic geology publications and presentations on topics outside of the Lake Superior region are also applicable to our regional geology. An area of emphasis in Peter’s research is the application of geochemistry and petrology to explore for ore deposits, including NiCu-PGE deposits (e.g., Lac des Iles Mine and the Thunder Bay North igneous complex). His other areas of interest include igneous geochemistry of the MRS, Archean greenstone belts and granites, the tectonic setting of komatiites, and Archean gold deposits. As the Director of CESME, he provides leadership in promoting the discovery of and environmentally responsible exploration for natural resources. Pete has also made contributions to understanding of the natural history and environment of the Lake Superior region as demonstrated by numerous publications focused on the timing and evolution of local rocks and mineral deposits. Pete’s research is firmly rooted in field work and uses geochemical and other data to test existing ideas and concepts and to develop new ones. He has successfully used local and regional geochemical data to provide evidence and/or implications for broader geological questions, such as atmospheric oxygen in the Precambrian, continental growth and lithospheric recycling, the Superior Province cratonic keel, and the earliest phases of Midcontinent Rift development. In addition to data-driven new ideas and concepts, Pete’s research efforts have resulted in development of new analytical approaches that can be applied to the Lake Superior region and beyond. As a Professor at Lakehead University, Pete is actively involved the education of geoscientists through classroom teaching and thesis supervision. He is committed to training and mentoring as evidenced by the large role students play in his research. He has supervised and co-supervised 37 honours undergraduate research projects and 32 Masters graduate student theses. Most of this student-focused research has involved Lake Superior geology. His former students now have senior positions with government and industry, and some have gone on to complete PhDs. Moreover, he supports and encourages students to attend and present their research at ILSG. xiii �Proceedings of the 69th ILSG Annual Meeting – Part 1 ILSG plays a significant role in Pete’s professional activities. He has authored/co-authored (many with his students) 75 ILSG abstracts (nearly 4 per year), six ILSG field trip guidebooks, and ILSG Special Publication #1, Field trip guidebook for the Slate Islands, Ontario. At his very first ILSG meeting in 2002, Pete co-authored an abstract and served on the Student Paper Awards Committee. Pete has Chaired or Co-Chaired four in-person annual meetings (Nipigon, 2005; International Falls, 2010; Thunder Bay, 2012; Terrace Bay, 2019) and the virtual meeting in 2021. He has served as the Secretary of the ILSG from 2003 to the present. As Secretary, he is responsible for email communications with the members of ILSG. As a member of the Board, he attends and chairs the annual Board meeting. In ILSG Board meetings he always considers and defends the best interests of Institute. Pete is the ILSG webmaster and played a key role in the current design of the ILSG website which he updates and maintains. Through his efforts, Lakehead University is the digital archive to all of the past ILSG proceedings and field trip guidebooks and provides open access of this content worldwide. A testament to the quality and accessibility of these documents was ILSG’s receiving the 2016 Outstanding Geologic Field Trip Guidebook Series Award by the Geoscience Information Society (GSIS), which Pete accepted on behalf of the Institute. The stature of ILSG in the regional, national, and international geological communities has been elevated because of the increased presence of ILSG on the worldwide web, in large part because of Pete’s efforts. Over the years, we have all witnessed Pete in action. He is collegial, easy to approach and gets along well with others, whether they be students, colleagues, or industry geoscientists. He is both a good listener and a good speaker. And he is open-minded. He has high personal standards and expects them to be reflected in the work of his students and research colleagues. Pete is truly enthusiastic about the geology of the Lake Superior region and about ILSG. Pete has made and continues to make substantial contributions to the field of geology and to the Institute on Lake Superior Geology. Pete has more than met the qualifications that are engraved on the Goldich Medal itself: “For outstanding contributions to the geology of the Lake Superior region.” We congratulate the 2023 Goldich Medalist, Peter Hollings. Citation by: Theodore J. Bornhorst, Goldich Medalist 2008 Mark C. Smyk, Goldich Medalist 2005 xiv �Proceedings of the 69th ILSG Annual Meeting – Part 1 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) 2018-20 not presented 2021 Newton Horace Winchell (1839-1914) 2022 Thomas Leslie Tanton (1890-1971) 2023 Thomas Benton Brooks (1836-1900) xv �Proceedings of the 69th ILSG Annual Meeting – Part 1 2023 Nomination for Thomas Benton Brooks Pioneer of Lake Superior Geology “During many years Major (T.B.) Brooks was the chief authority in the region on matters pertaining to geology, the ores and the mines of the iron region of Lake Superior.”1 Shortly after the U.S. Civil War Major Thomas Benton Brooks moved to the Marquette Iron Range. There over the course of less than a decade, he became the premier geologist, prospector, mining and civil engineer, and mining company executive of the region. During these formative years of the iron ore industry, when the Lake Superior region was providing about one-quarter of the iron ore used in the U.S., he was employed by the Iron Cliffs Company, the predecessor of the ClevelandCliffs Company, the Michigan and Wisconsin Geological Surveys, and served as a consultant to iron ore exploration and mining companies of the region. His contributions had a significant role in mapping the Precambrian geology and iron ranges of Michigan and Wisconsin and a lasting impact on the iron ore industry of the region. As stated by Prof. C.R. Van Hise, Brooks’ successor as the premier geologist of the Lake Superior region2: “Notwithstanding the immense advantage which it has been to have Brooks’ work as a foundation, it has taken many years of labor fairly to complete the structural story to which Brooks contributed important chapters. Only those who have labored in the Lake Superior region and who understand its peculiar difficulties can give Brooks credit for the remarkable work he did. His geological work is my ideal of what should be done in a new region of complex geology.” Thomas B. Brooks was born on June 15, 1836 in Monroe, NY, near the New Jersey border, and died nearby on November 22, 1900. In 1852 at the age of 16, he joined a surveying crew of the Erie Railroad and rapidly advanced from woodsman to instrument man. In 1853 he was employed with the New York Topographic and Geological Survey and then entered the Engineering Department of Union College of Schenectady, NY in 1856, graduating in 1858 in civil engineering. He remained at Union College as an instructor for a year and then took part in topographical surveys in New York, New Jersey, Pennsylvania and the U.S. Gulf Coast. In 1 Quoted from an article by Chas. A. Lawton in the Daily Mining Journal, November 29, 1900 entitled The Late Major Thomas Benton Brooks: Biographical Sketch of a Man Whose Name is Intimately Associated With the Early Development of Michigan’s Iron Mines. The Mining Journal, the predominant daily newspaper of Marquette, Michigan and the Northern Peninsula of Michigan, was founded in 1841. 2 As quoted by Bailey Willis of the U.S. Geological Survey in an obituary for Major Brooks in the Proceedings of the American Association for the Advancement of Science, New Series, Volume 13, No. 325(March 22, 1901), 460-462. xvi �Proceedings of the 69th ILSG Annual Meeting – Part 1 1860 he attended a series of lectures on geology given by Prof. J.P. Lesley former state geologist of Pennsylvania and Professor of Geology at the University of Pennsylvania. This was his only formal education in geology. He volunteered for the Union Army in 1861 and organized an engineering company that had a distinguished record during numerous Civil War campaigns. He retired from the Union Army in 1864 as a brevet colonel after being wounded in the battle of Denly’s Bluff, but referred to himself after the war as Major Brooks. In 1865 after leaving the Union Army he accepted a position with the Geological Survey of New Jersey where he conducted magnetic surveys with a dip needle to locate iron ores and was put in charge of mines and furnaces. Shortly thereafter, he was induced to take charge of the mines of the Iron Cliffs Company in the Marquette Iron Range as vice-president and general manager. He moved to Negaunee, Michigan, where his practical knowledge of geology and engineering, leadership skills, originality, keen powers of observation and deduction, and intense work ethic served him, the company, and the Lake Superior region well. This is where his extensive geological studies began and where he developed the instruments and methodology to exploit the iron ores of the Lake Superior region. He brought the dip needle to the Lake Superior region and was among or possibly was the very first, to use it in iron ore exploration and geologic mapping in the region. He also pioneered the dial (Sun) compass, which he modified for geologic use from the surveying solar compass developed by W.A. Burt. In 1869 he resigned from the Iron Cliffs Company and was given the responsibility of mapping and reporting on the Marquette Iron Range and was placed in charge of the Economic State Geological Survey of the district by the Michigan Geological Survey, essentially becoming the State Geologist of the Northern Peninsula. He received no salary for this position, but he was allowed to receive private funds from numerous iron ore companies and mines. Unfortunately, his intense work schedule took a toll on his health that caused him to leave Marquette with his family in the winter of 1872-73 for London, England and eventually Dresden, Germany, where he hoped to regain his health, but failed to do so. During this period he prepared reports on his iron range geologic studies for publication by the Michigan and Wisconsin Geological Surveys (Brooks, 1873 and 1880), articles on the geology of the region and magnetic surveying instruments and their use published in various journals including the American Journal of Science and Arts (Brooks and Pumpelly, 1872; Brooks, 1875), and co-authored the book “Iron Ores of Missouri and Michigan” (Pumpelly, Brooks, and Schmidt, 1876). During his years involved with the geology and ores of the Lake Superior region Major Brooks made numerous advances in the geological knowledge of the region that have served as a foundation for future studies and developed methods and instruments that proved useful for exploiting the ores of the region for many years. The following are a list of his major lasting accomplishments: • • He with the assistance of R. Pumpelly and R.D. Irving developed the dial (Sun) compass for geologic studies based on the principal of Burt’s surveying solar compass which together with the dip needle that he brought from the Geological Survey of New Jersey were used in the Lake Superior region for nearly a century to locate and outline iron-rich rocks and ores. His publications on these instruments led to their extensive worldwide use. He established procedures for conducting magnetic surveys for geological purposes in the Lake Superior region and methods of interpreting the observations of the surveys based on empirical studies. xvii �Proceedings of the 69th ILSG Annual Meeting – Part 1 • • • • • • • He was the first to describe the magnetic characteristics of the minerals and rocks of the Lake Superior region. He (Brooks, 1872a) recognized that magnetic anomalies observed in the area of non-magnetic Paleozoic (then Silurian) sedimentary rocks of the eastern part of the Northern Peninsula of Michigan were likely derived from the basement Precambrian rocks that crop out to the west. Accordingly, these anomalies could be used to trace the basement rocks and their structure beneath the sedimentary rocks. Furthermore, he realized that anomaly characteristics could be used to determine the depth to magnetic sources and thus, the thickness of the sedimentary rocks. In a similar manner he understood that perhaps the depth of Lake Superior could be determined from analysis of the lake magnetic anomalies. He founded the first assay facility for iron ores in the Lake Superior region in the city of Marquette which facilitated iron ore mining in the region. He conducted one of the first geological surveys of the Marquette, Menominee, Crystal Falls, and Gogebic Iron Ranges. He was the first to understand that the Marquette Iron Range occurs within a 75-km long syncline extending to the west from near Marquette, Michigan (Allen and Martin, 1922). He recognized the stratigraphic position of the copper-bearing rocks of the Northern Peninsula of Michigan and suggested the name Keweenawian (note his spelling) for the age of these rocks in American Journal of Science and Arts articles of 1872 and 1875. Subsequently, the term Keweenawan has been used for these rocks. He had an important role in developing safe, efficient methods of mining iron ores of the Lake Superior region (Brooks, 1972b). He was intensely interested in the education of his children and supported the studies of his son, Alfred Hulse Brooks, a famed geologist of the U.S. Geological Survey, Alaska Branch, who is honored by naming of the Brooks Range of Alaska after him. These are all significant contributions that have had a profound role in understanding of the geology of the Lake Superior region and the exploitation of its ores. They have largely gone unrecognized for the past century and a half, but they clearly distinguish Major Thomas Benton Brooks as a Pioneer of Lake Superior geology. References Allen, R.C., and Martin, H.M., 1922. A brief history of the Geological and Botanical Survey of Michigan. Michigan History Magazine, Volume VI, No. 44: 675-750. Lawton, C.A., 1900. The Late Major Thomas Benton Brooks: Biographical Sketch of a Man Whose Name is Intimately Associated with the Early Development of Michigan’s Iron Mines. The Daily Mining Journal, November 29, 1900. Pumpelly, Raphael, Brooks, T.B., and Schmidt, A., 1876. Iron Ores of Missouri and Michigan. G.P. Putnam’s Sons, New York: 624. Willis, B., 1901. Thomas Benton Brooks. Proceedings of the American Association for the Advancement of Science, Science, New Series, Volume 13, No. 325: 460-462. William J. Hinze, Purdue University xviii �Proceedings of the 69th ILSG Annual Meeting – Part 1 APPENDIX: PUBLICATIONS OF T.B. BROOKS Brooks, T.B., 1872a. On the use of the magnetic needle in mineral explorations on Lake Superior. Van Nostrand’s Eclectic Engineering Magazine (1869-1879), August 1, 1872; Volume 7, No. 44, American Periodicals: 161. Brooks, T.B., 1872b. An analysis of the cost and description of the methods of mining employed in the Marquette Iron Region, Lake Superior, Michigan. Transactions of the American Society of Civil Engineers, Volume XXXIV: 18. Brooks, T.B., and Pumpelly, R., 1872. On the age of the copper-bearing rocks of Lake Superior. American Journal of Science and Arts, Third Series, Volume III, No. XVIII: 428-432. Brooks, T.B., 1873. Geology of Marquette Iron Range, Geology of the Menominee Iron Range, and Geology of the Gogebic and Montreal Iron Ranges. Michigan Geological Survey, Volume 1, Chapters IV, V, VI, VII, and VIII, Part 1, Iron-Bearing Rocks: 117-243. Brooks, T.B., 1875. On the youngest Huronian rocks south of Lake Superior and the age of the copperbearing series. American Journal of Science and Arts, Third Series, Volume III, No. XI: 206211. Brooks, T.B., 1880. Geology of the Menominee Region. In Chamberlin, T.C. (ed.), Geology of Wisconsin, Volume 3, Part 7, Chapters 1, 2, and 3: 430-552. xix �Proceedings of the 69th ILSG Annual Meeting – Part 1 In Memoriam Stephen Allard Stephen Thomas Allard, 59, of Winona, MN, passed away on Friday, September 16, 2022. He was born May 2, 1963, in Manchester, New Hampshire and graduated from Manchester Central High School before going on to receive both his bachelor’s and master’s degrees from the University of New Hampshire, and his doctorate from the University of Wyoming. In 2002, Stephen moved to Winona, MN to begin his career as a professor at Winona State University. After serving for 19 years as a faculty member in the Department of Geoscience, Stephen retired from the university in December of 2021. During his tenure at WSU, Stephen served on several committees and taught 13 different courses drawing on his expertise in hard rock and structural geology. Stephen was dedicated to teaching and mentoring students through field-based research, leading courses and field trips throughout the United States, notably the many summers spent in the Black Hills of South Dakota. (modified from Hartford Courant newspaper) xx �Proceedings of the 69th ILSG Annual Meeting – Part 1 In Memoriam Steven A. Hauck This Fall the Institute on Lake Superior Geology lost a dedicated geologist and friend, Steve Hauck, who was a regular attendee of ILSG (since at least 1984) and worked on countless projects in the Lake Superior Region while employed at the Natural Resources Research Institute (NRRI) in Duluth, MN. During that time, Steve was a mentor to numerous geologists in the region throughout their early and continuing careers. Steve Hauck had just recently moved from Duluth to Euclid, OH where he passed away on October 6, 2022 at the age of 73. Steve as born on May 16, 1949, in Rochester, NY, where he graduated from Gates-Chili High School prior to attending Albion College where he earned a BS in geology. He enlisted in the US Army where he was trained as a Chinese translator and married fellow Albion student and the love of his life Barbara Horsley to whom he was married for 50 years. Steve loved to talk about geology on car trips and impressed his future father-in-law with his knowledge and enthusiasm. Steve later earned a MS degree in geology from the University of North Carolina before embarking on a geology career that eventually led him around the globe. He was first employed by Union Carbide in Grand Junction, CO, where he was responsible for exploration for uranium in the 4-corners region. While at Union Carbide he was also responsible for developing a world-wide exploration program in search of IOCG deposits (as they were later called) and visited many similar deposits including Olympic Dam, Pilot Knob and Pea Ridge in Missouri, and Kiruna iron deposits in Sweden to name a few. Steve’s first ILSG talk (1984) pertained to the distinguishing characteristics of these types of deposits and was titled “Comparison of Middle Proterozoic Iron Oxide Rich Ore Deposits, Mid-Continent, USA, South Australia, Sweden, and the Peoples Republic of China.” Steve was then hired as the second employee of the Minerals Division of the NRRI in 1984 as Research Director and Manager where he worked for over 30 years. He was initially responsible for building and equipping the division, focusing on economic geology, and initially hired graduate students from the University of Minnesota Duluth (UMD). During his tenure at the NRRI, Steve hired well over 30 UMD students (both undergraduate and graduate students) as well as many geologists in their early career years. Projects that he and his coworkers researched ranged from clay deposits in SW MN, to Cu-Ni and Fe-Ti deposits in the Duluth Complex, to the Biwabik Iron Formation, to geochemistry on a wide range of rocks spanning from the Archean to the Cretaceous. He worked closely with fellow geologists at the Minnesota Geological Survey, Minnesota Department of Natural Resources Lands and Minerals, and the U.S. Geological Survey, and collaborated with many geologists across the U.S. and overseas in academia and industry. xxi �Proceedings of the 69th ILSG Annual Meeting – Part 1 Steve was a Co-chair of the ILSG meeting for its 50th Anniversary in Duluth in 2004 and served on the Board of Directors for three years. Overall, Steve participated in three ILSG talks (one as primary author) and ten posters (four as primary author). Steve loved to talk about rocks and encouraged his co-workers to give talks and poster presentations at many of the ILSG meetings. Steve was an avid birder, cultivator of native plants, and shutterbug. He was predeceased by his youngest son, Davis, and his parents Arthur and Jean (Doron). He is survived by wife Barbara, son Steven (Danette), sisters Carlin Eagan (Daniel), Sandra Doron, and Mary McGuire (Mark), and two grandchildren Levi and Abigail. (modifed from Duluth Tribune newspaper) xxii �Proceedings of the 69th ILSG Annual Meeting – Part 1 In Memoriam Manfred Kehlenbeck Manfred was born in Bremen, Germany in 1937 to parents Emma and Theodor. This is where he spent his childhood, amidst the horrors of World War II, like so many of Europe's children. At age 14, Manfred immigrated with his parents to the U.S., landing in New York in July of 1952 and settling with relatives in Long Island until they could become established. Here he completed his high school education, then attended Hofstra University for his undergraduate degree. It was there that he was introduced to the science of geology, which became his lifelong interest and focus of his future education and career. It was also on Long Island that he met Elenore, who would eventually become his wife of 53 years. Manfred went on to Syracuse University in upstate New York to attain his M.Sc. in Geology and gain field experience in the beautiful Adirondack Mountains. And then, moving even further north, he attended Queen's University in Kingston, Ontario where he achieved his Ph.D. Since he has always planned to teach, he then accepted a position at the young Lakehead University in Thunder Bay, Ontario. Here he soon became fascinated with the Precambrian geology of the area and greatly enjoyed his teaching duties. He was a born teacher, winning Teacher of the Year awards both at Lakehead and in the Province. He served five terms as a Geology Department Chair, guiding the department into its M.Sc. program. His years at Lakehead were productive and happy ones. Upon his retirement, Manfred was able to expand on other interests and travel widely. In addition to trips in Canada, the U.S. and Germany, there were four “special” ones – professionally to Russia and China, and then the most fascinating ones, to the Arctic and Antarctic. His other areas of interest and hobbies were in watercolour and pen and ink drawings of local scenes, especially forests, lakes, rocks, and old buildings of NW Ontario xxiii �Proceedings of the 69th ILSG Annual Meeting – Part 1 and many views of Old Fort William. Many of his works hang in Thunder Bay homes. He became an avid gardener, curler and opera lover, and spent many hours volunteering for various causes. It was a happy and fulfilling retirement for Manfred and Elenore until his last illness and unexpected passing in the early morning hours of July 7, 2022 when he drew his last breath at the Thunder Bay Regional Health Sciences Centre after emergency surgery. Our thanks to the I.C.U. staff and especially to Katie and Michaela who were so kind and thoughtful during those last terrible hours, and to N.P. Crystal Kaukinen for the many years of care she had provided. Thanks also to all who have been so kind with phone calls, cards, offers to help, food and rides. Special thanks to Barb Morriss for always checking in, to Sam and Georgina Spivak for all the rides, and to Vince and Frieda DeSa who have been here for me everyday with their help and support – without them, I don't know how I would have survived this devastating time. Manfred was a good, kind, generous man, and loving and devoted husband. He is sorely missed. Auf Wiedersehen mein lieber Manfred. Published by The Thunder Bay Chronicle Journal on Aug. 13, 2022. xxiv �Proceedings of the 69th ILSG Annual Meeting – Part 1 Eisenbrey Student Travel Awards The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name “Eisenbrey” was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. “Ned” Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The number of awards and value are determined by the annual Chair in consultation with the Secretary and Treasurer. Recipients will be announced at the annual banquet. The following general criteria will be considered by the annual Chair, who is responsible for the selection: 1) The applicants must have active resident (undergraduate or graduate) student status at the time of the annual meeting of the Institute, certified by the department head. 2) Students who are the senior author on either an oral or poster paper will be given favored consideration. 3) It is desirable for two or more students to jointly request travel assistance. 4) In general, priority will be given to those in the Institute region who are farthest away from the meeting location. 5) Each travel award request shall be made in writing to the annual Chair, and should explain need, student and author status, and other significant details. Successful applicants will receive their awards during the meeting. xxv �Proceedings of the 69th ILSG Annual Meeting – Part 1 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 2022, the ILSG Board of Directors selected two students to be granted research funding of $1000.00 each from the Joe Mancuso Student Research Fund. The awardees were: Itai Bojdak-Yates Lawrence University Department of Geosciences TOPIC: Detrital zircon provenance study of Paleozoic sandstones from Wisconsin Lillian Glodowski University of Wisconsin- Eau Claire TOPIC: Petrogenesis of the Lynne Zn-CuPb Deposit, Oneida Co., Wisconsin Evan Weber University of Wisconsin- Eau Claire TOPIC: U/Pb Geochronology and Zircon Trace Element Geochemistry of the Pembine-Wausau Terrane of the Proterozoic Penokean Orogen, Wisconsin xxvi �Proceedings of the 69th ILSG Annual Meeting – Part 1 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. xxvii �Proceedings of the 69th ILSG Annual Meeting – Part 1 Board of Directors Board appointment continues through the close of the meeting year shown in parentheses, or until a successor is selected. The terms of Board members were extended 2 years because of cancellation of the 2020 meeting, and the difficulties of virtual voting by the membership during the 2021 meeting. Mike Easton, Chair (2022-2025) — Ontario Geological Survey Mark Smyk (2019-2024*) — Lakehead University Esther Stewart (2018-2023*) – Wisconsin Geological & Natural History Survey Peter Hollings — Secretary (2019-2024*) — Lakehead University Mark A. Jirsa — Treasurer (2022-2025) — Minnesota Geological Survey xxviii �Proceedings of the 69th ILSG Annual Meeting – Part 1 Field Trip Leaders and Guidebook Authors Field trips have been the mainstay of the ILSG since its inception 69 years ago. We want to give a special thanks to the field trip leaders and guidebook authors who volunteered their time and talent in carrying that tradition forward. 1) Precambrian geology of the Chippewa River Valley Rob Lodge- UW- Eau Claire Bob Hopper- UW- Eau Claire 2) Wisconsin’s Paleozoic stratigraphy and tour of Crystal Cave Carsyn Ames- Wisconsin Geological and Natural History Survey Esther Stewart- Wisconsin Geological and Natural History Survey William Batten- Wisconsin Geological and Natural History Survey Eric Stewart- Wisconsin Geological and Natural History Survey Ian Orland- Wisconsin Geological and Natural History Survey 3) Precambrian geology of the Eau Claire River Valley Rob Lodge- UW- Eau Claire Evan Weber- UW- Eau Claire (student) 4) Quaternary geology and geomorphology of the Eau Claire Region Doug Faulkner- UW- Eau Claire Elmo Rawling- Wisconsin Geological and Natural History Survey xxix �Proceedings of the 69th ILSG Annual Meeting – Part 1 REPORT OF THE 68th ANNUAL MEETING OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY The Ontario Geological Survey (OGS), with support from the Geological Survey of Canada (GSC), hosted the 68th Annual Institute on Lake Superior Geology on May 07 – 12, 2023 in the “Cavern” at Science North in Sudbury, Ontario. The meeting consisted of two days of technical sessions with pre- and post-technical 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: the Centre for Excellence and Sustainable Mineral Exploration in Thunder Bay, Gel Exploration Limited, the Northwestern Ontario Prospectors Association, Vale Canada, and the Ontario Geological Survey. We also thank the Individual Contributors to the Student Travel Scholarship fund: Mary Kay Arthur, Mike Beauregard, Ben Berger, Terry Boerboom, Jim DeGraff, Michael and Monica Easton, Dick Heglund, Joanna Hodge, Bob Mahin, Jim Miller, Dean Peterson, Mark and Laurie Severson, Al MacTavish and Graham Wilson. The 2022 meeting was the first in-person meeting held since the 2019 Terrace Bay meeting. An ILSG meeting questionnaire, which ran from January 20 to February 20, 2022, was key to shaping the format and venue of the meeting during a period of rapidly changing COVIDrelated regulations, with most responses favouring an in-person meeting. For technical reasons, a hybrid meeting was not possible. Total meeting registration was 80, including 12 students. This registration is about 80% of the attendance of the last two Sudbury area meetings (Sudbury 1997; Sault Ste. Marie 2006), and was a great turnout given the COVID-related travel restrictions still in place at the time of the meeting. Attendance from the United States was excellent, with attendance from the Sudbury area lower than expected, for unknown reasons. Despite the somewhat lower attendance, the technical program was nevertheless excellent, with a strong focus on Midcontinent Rift geology and mineralization in the Lake Superior region. In addition, four presentations focused specifically on Sudbury area geology. There was also time in the schedule for several impromptu presentations on a variety of topics on Wednesday afternoon prior to the announcement of the student awards. Proceedings Volume 68 was published in two parts. Part 1 – Program and Abstracts, compiled and edited by Michael Easton (OGS), contains 28 published abstracts for 21 oral and 8 poster presentations (one poster did not have an abstract). Students presented 5 oral and 5 poster presentations. Part 2 – Field Trip Guidebooks, also was compiled and edited by Michael Easton. It contains descriptions of three pre-meeting and two post-meeting field trips. Hard copies of the Abstract Volume and Field Trip Guidebooks for trip participants were printed by Johanne Roux and Carlo Castrechino (OGS) after it proved impossible to find a commercial printer who could produce the volumes in time for the meeting. Both volumes are available for download from the Institute on Lake Superior Geology website. Monica Easton is thanked for assisting in preparing the digital versions of both volumes. The 68th ILSG marked only the second time in the Institute’s long history that its annual meeting was held in Sudbury, the last time being in 1997. Since the discovery of distal ejecta xxx �Proceedings of the 69th ILSG Annual Meeting – Part 1 from the Sudbury impact in the western Lake Superior area in 2005, many members of the Institute had suggested that the time was right for another Sudbury meeting. The meeting location enabled organizers to offer five field trips that showcased a variety of Proterozoic rocks in the Sudbury area itself, as well as along the north shore of Lake Huron. Three field trips focussed on the geology and mineralization related to the Sudbury Structure, and the organizers wish to thank the local exploration companies that graciously provided information and access to their properties. Parts of the other two of the field trips had been offered at previous ILSG annual meetings (e.g., Sudbury 1997; Sault Ste. Marie 2006), but both greatly benefitted from the new mapping, research, discoveries and interpretations that had taken place in the intervening years. COVID-related shortages of rental vehicles and/or drivers led to pre-meeting trips being held over several days, which unexpectedly, provided more opportunities for attendees to take in several field trips if they wanted. All the field trips, and the meeting itself, were blessed with sunny weather and a minimum number of pesky insects. Total field trip participation was 96 (excluding leaders and volunteer drivers). A list of field trips is provided below (numbers correspond to trip numbers in the Guidebook volume): Pre-meeting field trips (and leaders) on Saturday, May 07; Sunday, May 8, and Monday, May 9. 5) A cross-section through the Huronian Supergroup at Elliot Lake, Ontario (Michael Easton, Ontario Geological Survey) (May 7) 2) Geology of the Grenville Front in the Sudbury area (Michael Easton, Ontario Geological Survey) (May 8) 1) Traverse across the Sudbury Impact Structure (Wouter Bleeker, Geological Survey of Canada, and Sandra Kamo, University of Toronto; Michael Lesher and Henning Seibel, Laurentian University) (Two-day trip, May 8 and May 9) Post-meeting field trips (and leaders) on Thursday, May12 3) Magmatism and brecciation in the Footwall Rocks of the southwestern Sudbury Structure (Caroline Gordon, Ontario Geological Survey; Carol-Anne Généreux, Laurentian University and Terrane Geoscience; and Brad Clarke, SPC Nickel Corporation) 4) An overview of the geology of the Sudbury Structure (Shirley Péloquin, Ontario Geological Survey) Many registrants attended the welcoming reception on Monday evening, which included an IMAX theatre presentation on “Dinosaurs of Antarctica”. Furthermore, the vast majority of registrants and invited guests attended the annual ILSG banquet on Tuesday night. Although a Homer Award overview presentation was given, no “recipients” were identified during the 2022 annual meeting, or in the previous 3 years! As always, a highlight of the post-banquet activities was presentation of the 2022 Goldich Medal. This year’s very deserving recipient was Terry Boerboom. The Goldich Medal citation was presented by Mark Jirsa, his colleague for many years. Mark described Terry’s contributions to the ILSG and to the greater understanding of Minnesota’s geology over several decades during his time as a student and his 35 years with the Minnesota Geological Survey. Terry is indeed a worthy recipient of this prestigious award. The 68th ILSG saw a return to the usual post-banquet guest speaker tradition. Andy Parmenter xxxi �Proceedings of the 69th ILSG Annual Meeting – Part 1 of the Canadian Nuclear Waste Management Organization (NWMO) travelled from Toronto to give an overview of NWMOs Geoscience site characterization of the Revell batholith in the Ignace area of northwestern Ontario. His talk provided detailed insights into the 3-D character of a Neoarchean granodioritic to granitic intrusion, based on detailed mapping and geophysical, seismic, and geochemical studies, as well as from multiple 1 km-long research cores obtained from the batholith. In 2022, the student paper committee had its usual difficult job of selecting the best among five excellent oral presentations and five poster presentations for the Doug Duskin Student Paper Awards. The committee awarded four prizes, with the best talk award going to Rebecca Price for her talk on “Mineralogy and Petrology of the Good Hope Carbonatite Complex, Marathon, ON” and the best poster award going to Khalid Yahia for his poster on “Geochemical and isotopic composition of Midcontinent Rift-related intrusions of the Thunder Bay North Igneous Complex, northwestern Ontario, Canada”. Runner-up prizes went to Audray Hinkenmeyer for her talk on “Characterizing Late Wisconsinan Rainy Lobe till from the Hudson Bay Lowlands to SW Minnesota: Insights on provenance and ice sheet behavior during Late Wisconsin glaciation” and to Katherine Langfield for her poster on “Slip Kinematics of the Hancock Fault in the Midcontinent Rift System, Keweenaw Peninsula, Michigan”. Eisenbrey Student Travel Grants were given to three students: Connor Caglitoti (Lakehead University), Katherine Langfield (Michigan Technical University), and Miles Harbury (University of Wisconsin, Milwaukee). The Institute’s Board of Directors met on Tuesday, May 10, 2022, and a brief overview of the meeting notes is provided below: 1. Accepted report of the Chairs for the 67th ILSG, Virtual Meeting; as published on the ILSG web site, and minutes of last Board meeting in May 2021. 2. Received, discussed, and accepted 2021-2022 ILSG Financial Summary. 3. Received, discussed, and accepted 2021-2022 report of the Secretary (Hollings). 4. Approved Michael Easton as on-going ILSG Board member 5. Discussed and approved renewal of Mark Jirsa as Institute Treasurer (end of term 2022). This was later approved by a vote of the membership. 6. Discussed and approved replacing Dan England as the “member from industry” on the Goldich Committee (end of term 2022) with Dean Peterson. 7. Approved Eau Claire as the site for the 69th annual ILSG meeting. The meeting will be hosted by Robert Lodge and Esther Stewart. 8. Reviewed and approved the guidelines for the Honouring the Pioneers of Lake Superior Geology with the charge that the document will be reviewed as needed. 9. Future meeting locations were discussed. Ted Bornhorst offered Houghton in 2024, Peter Hinz has offered Kenora as a future site and Mark Jirsa is keen to host the Mountain Iron meeting that was cancelled in 2020 because of the pandemic. In a subsequent discussion, Bernie Saini-Eidukat expressed a willingness to organize a meeting in St. Cloud. 10. The cost of insurance was discussed and it was agreed that the Board of Directors insurance and field trip insurance should be maintained for future meetings and that the costs would be included in the cost of each meeting. The fact that the Institute meets in both the US and Canada is an added complication. xxxii �Proceedings of the 69th ILSG Annual Meeting – Part 1 11. Jirsa advised the board of the donation of polar bear carvings from Mike Beauregard, and it was agreed that a silent auction would be held during the meeting with funds going to support student travel. Dan England later donated two samples with visible gold and, combined, these items raised $395 for the Eisenbrey fund 12. Bornhorst advised that there are a small number of hard copies missing from the MTU archives and that he will work to fill these. It was agreed that the ILSG would make a donation of $1 per member (minimum $100) each year to the library as a “thank you” for their efforts 13. The 68th 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 who were pressed into action as editors, field trip leaders and drivers. Patty Cobin and Ted Bornhorst (A.E. Seaman Mineral Museum, Michigan Technological University) handled the premeeting registration. Ted also supplied the poster boards. Thanks go also to the staff at Science North who helped the meeting run smoothly as well as Bryston’s on the Park in Copper Cliff who provided a first-class banquet dinner, as well as lunches and snacks during the technical sessions. Michael Easton (OGS) and Wouter Bleeker (GSC) Co-Chairs, 68th Institute on Lake Superior Geology xxxiii �Proceedings of the 69th ILSG Annual Meeting – Part 1 TECHNICAL PROGRAM xxxiv �Proceedings of the 69th ILSG Annual Meeting – Part 1 TECHNICAL PROGRAM SUNDAY APRIL 23, 2023 All field trips begin and end at The Lismore Hotel 8:00 am - 5:00 pm PRE-MEETING FIELD TRIPS 1) Precambrian geology of the Chippewa River Valley Rob Lodge and Bob Hooper – UW- Eau Claire 2) Wisconsin’s Paleozoic stratigraphy and tour of Crystal Cave Carsyn Ames – Wisconsin Geological and Natural History Survey 4:00 pm - 10:00 pm Registration (Wilson Hall Lobby) 7:00 pm - 10:00 pm Welcoming Reception (Wilson Hall A/F) MONDAY APRIL 24, 2023 7:30 am – 11:30 am Registration (Wilson Hall Lobby) 8:00 OPENING REMARKS (Wilson B) Rob Lodge and Carsyn Ames, Co-Chairs, 2023 ILSG TECHNICAL SESSION I Session Chair: James DeGraff- Michigan Technological University * 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 abstract, if different than the first author. 8:10 William J. Hinze, and +William Cannon 2023 Pioneer of Lake Superior Geology: Thomas Benton Brooks 8:30 Erika Vye and William Rose Geoheritage as an educational tool to explore relationships with land and water in the Keweenaw 8:50 William Rose New work developing Keweenaw geoheritage awareness 9:10 Matt Carter and Donald Elsenheimer Workshop Outcomes and Updates for the Minnesota Department of Natural Resource’s Drill Core Library 9:30 Dean Peterson On the Importance of Geologic Maps for Mineral Exploration 9:50 9:50 END OF TECHNICAL SESSION I COFFEE BREAK xxxv �Proceedings of the 69th ILSG Annual Meeting – Part 1 TECHNICAL SESSION II Session Chair: Ben Drenth- USGS and Amy Radakovich Block- Minnesota Geological Survey 10:00 Dana Peterson, Paul Bedrosian, and Carol Finn Subsurface characterization of the Duluth Complex and related intrusions from 3D modeling of gravity and magnetotelluric data 10:20 Paul Bedrosian, Tien Grauch, Laurel Woodruff, William Cannon, Benjamin Drenth, Esther Stewart, Dana Peterson, and James Jones Interpreted geophysical cross-sections through the Lake Superior region: Investigating three billion years of geologic history in sixteen lines of data 10:40 Tien Grauch, Sam Heller, Esther Stewart, and Laurel Woodruff Exploring the geology of the Midcontinent Rift under western Lake Superior using a preliminary velocity model of seismic line GLIMPCE C 11:00 Jennifer Smith, Victoria Tschirhart, Loughlin Tuck, Randy Enkin, and David Roy-Guay Exploring the application of full tensor magnetic gradiometry to better define conduit type NiCu-PGE targets 11:20 END OF TECHNICAL SESSION II 11:20-1:00 LUNCH BREAK and LSG BOARD OF DIRECTORS MEETING - lunches not provided to conference attendees- 11:20-1:00 Student Career Panel- (L.E. Phillips Memorial Public Library- 400 Eau Claire St. in the Riverview Room (Room 306)) TECHNICAL SESSION III Session Chair: Marcia Bjørnerud- Lawrence University 1:10 Wouter Bleeker, Jennifer Smith, Michael Hamilton, Sandra Kamo, Pete Hollings, Michael Easton, and Robert Cundari The Midcontinent Rift System: Neither triple junction nor failed rift? 1:30 Matthew Brzozowski, +Pete Hollings, Jing-jing Zhu, and Robert Creaser Contributions of diverse mantle sources during the early stages of Midcontinent Rift formation — Implications for a passive rifting model 1:50 *Daniel 2:10 *Katherine Langfield, 2:30 END OF TECHNICAL SESSION III Lizzadro-McPherson, James DeGraff and Ian Gannon Structural analysis and slip kinematics of the Keweenaw fault system between Bête Grise Bay and Gratiot Lake, Keweenaw County, Michigan James DeGraff, and Nolan Gamet Slip Kinematics of the Keweenaw and Hancock Faults within the Midcontinent Rift System, Upper Peninsula of Michigan xxxvi �Proceedings of the 69th ILSG Annual Meeting – Part 1 2:30 COFFEE BREAK TECHNICAL SESSION IV Session Chair: Pete Hollings- Lakehead University and 2023 Goldich Medalist 2:50 *Tianna 3:10 *Sam Ghantous, 3:30 *Blaize Briggs and Mary Louise Hill Quetico-Wabigoon Subprovince Boundary in the Superior Province north of Thunder Bay, Ontario, Canada 3:50 Margaret Upton, Phillip Larson, Allan MacTavish, and Peter Hinz Summary of the 2022 ILSG Field Trip to Iceland 4:10 END OF TECHNICAL SESSION IV 4:10 POSTER VIEWING - AUTHORS WILL BE PRESENT AT THEIR POSTERS 6:00 RECEPTION AND CASH BAR (Wilson Hall A/F) 7:00 Groeneveld, Peter Hollings, Wyatt Bain, and Lionnel Djon Petrography, geochemistry, and mineralization of the Archean Titan (Roaring River) intrusion, Northwestern Ontario Noah Phillips, Alex Lusk, Julie Newman, and Shaocheng Ji Are serpentine fault mirrors an indicator of seismic slip? A microstructural analysis ANNUAL BANQUET AND AWARDS (Wilson Hall A/F) SPEAKER: Curt Meine- Adjunct Professor at UW- Madison and Senior Fellow with the Aldo Leopold Foundation and Center for Humans and Nature IMAGINING “CONSERVATION GEOLOGY”: LESSONS FROM THE DRIFTLESS AREA TUESDAY APRIL 25, 2023 8:00 INTRODUCTORY REMARKS AND UPDATES (Wilson Hall B) Rob Lodge and Carsyn Ames, Co-Chairs, 2023 ILSG TECHNICAL SESSION V Session Chair: Allan MacTavish- Consulting Geologist and 2021 Goldich Medalist 8:10 *Justin Jonsson, Pete Hollings, Matthew Brzozowski, Wyatt Bain, and Lionnel Djon Petrogenesis of the mineralized horizons in the Offset and Creek zones, Lac des Iles Complex, N. Ontario 8:30 Pete Hollings, Jacob Hanley, Mark Smyk, Larry Heaman, and Brian Cousens Copper-rich melt inclusions from the St. Ignace Island Complex: Implications for magma mixing and mineralization xxxvii �Proceedings of the 69th ILSG Annual Meeting – Part 1 8:50 Alex Steiner, Dean Peterson, and Gabriel Sweet Magma Recharge and the distribution of Copper and Nickel in the Keweenaw Large Igneous Province 9:10 David Good Identifying regional exploration domains for Ni-Cu-PGE deposit types in the Midcontinent Rift 9:30 Julia Steenburg and Anthony Runkel Record of an Ancient Meteorite Impact Buried Beneath the Twin Cities, MN 9:50 COFFEE BREAK 10:10 Benjamin Drenth, Amy Radakovich Block, George Hudak, Kate Souders, and Stacy Saari Geophysical architecture of the Neoarchean Mentor anorthosite intrusive complex, northwestern Minnesota 10:30 Paul Weiblen The Use of Electric Pulse Disaggregation Technology to Recover Nickel Metal from Nickel Sulfide Ore Deposits 10:50 END OF TECHNICAL SESSION V 11:00 ADDITIONAL POSTER VIEWING – AUTHORS ARE ENCOURAGED TO BE AT THEIR POSTERS (Wilson C & D) 11:30-12:30 LUNCH BREAK - lunches not provided to conference attendees- TECHNICAL SESSION VI Session Chair: Laurel Woodruff- USGS and 2014 Goldich Medalist 12:40 *Margaret Upton, Howard Mooers, and Philip Larson Alteration Geochemistry Characterization and 3D Modeling of the Back Forty Volcanogenic Massive Sulfide (VMS) Deposit Stephenson, Upper Peninsula of Michigan, USA 1:00 Robert Lodge Re-evaluating the tectonics and metallogeny of terranes in the Paleoproterozoic Penokean Orogen, Wisconsin 1:20 William Cannon and Benjamin Drenth Eastward transition from banded iron-formation to ferruginous clastic rocks across the central Upper Peninsula of Michigan 1:40 Jamey Jones, William Cannon, Benjamin Drenth, and Paul O’Sullivan Provenance patterns and tectonic styles of ca. 2.3–1.8 Ga metasedimentary strata in northern Michigan based on regional mapping and detrital zircon U-Pb geochronology xxxviii �Proceedings of the 69th ILSG Annual Meeting – Part 1 2:00 *Audray Hinkemeyer, Howard Moores, and Phillip Larson Determining Provenance of Rainy Lobe Till using Geochemistry and Detrital Zircon Geochronology. 2:20 COFFEE BREAK 2:40 Gordon Medaris Jr. and Steven Driese Secular Changes in the Magnitude of Terrestrial Weathering 2:40 END OF TECHNICAL SESSION VI TECHNICAL SESSION VII Session Chair: Carsyn Ames- Wisconsin Geological and Natural History Survey 3:00 Caroline Rose Tips from a GIS Specialist: Moving maps to GeMS, and a utility for georeferencing quadrangles 3:20 Matthew Rehwald, Carsyn Ames, Sarah Bremmer, William Fitzpatrick, Eric Stewart, Bill Batten, and Stephen Mauel Mobile geologic mapping at the Wisconsin Geological and Natural History Survey 3:40 Roger Schulz Outcrop Scale Mapping Utilizing High-Accuracy GNSS with MnDOT’s Virtual Reference Station (VRS) Network: Minnesota Examples 4:00 Stephen Mauel, Eric Stewart, Matthew Rehwald, Esther Stewart, Carsyn Ames, Sarah Bremmer, and William Fitzpatrick 3D geologic mapping at the Wisconsin Geological and Natural History Survey 4:20 END OF TECHNICAL SESSION VII 4:20 BEST STUDENT PAPER AWARDS STUDENT TRAVEL AWARDS CLOSING REMARKS 4:40 END OF TECHNICAL SESSIONS WEDNESDAY APRIL 26, 2023 8:00am – 5:00pm POST-MEETING FIELD TRIPS Field trips begin and end at The Lismore Hotel 3) Precambrian Geology of the Eau Claire River Valley Rob Lodge and Evan Weber– UW- Eau Claire 4) Quaternary Geology and Geomorphology of the Eau Claire Region Doug Faulkner – UW- Eau Claire J. Elmo Rawling III– Wisconsin Geological and Natural History Survey xxxix �Proceedings of the 69th ILSG Annual Meeting – Part 1 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. *Zsuzsanna P. Allerton, Anita Hall, Françoise Roger, and Christian Teyssier Geochronology and Geochemical Analysis of the Giants Range Batholith in Northern Minnesota Carsyn Ames The Wisconsin Geological and Natural History Survey’s (WGNHS) 2020 and 2021 National Geological and Geophysical Data Preservation Program (NGGDPP) Projects *Ryan Barkley, Noah Phillips, and Pete Hollings The geologic setting, structural controls, and geochemical signature of the Eagle River Au deposit in Northwestern Ontario Marcia Bjørnerud, Buchholz, T., Falster, A.U, And Simmons, W.B. Deformation, metamorphism, fluid flow and pegmatite emplacement history of the post-1630 Ma Waterloo Quartzite of southern Wisconsin Amy Radakovich Block, Kate Souders, Benjamin Drenth, George Hudak, Stacy Saari, and Aaron Hirsch New geologic mapping in the Superior Province of northwestern Minnesota, USA: Pennington and Red Lake Counties *Itai Bojdak-Yates, Marcia Bjørnerud, David Malone, and Esther Stewart A revised provenance model for the Elk Mound Group in south-central Wisconsin based on detrital zircon analysis James DeGraff and William Rose Digital Image Capture and Database Compilation of Historic Mining Data from the Keweenaw Copper District, Michigan: A Progress Update Benjamin Drenth and William Cannon Geophysical mapping of the Great Lakes Tectonic Zone and surrounding Precambrian geology in the central Upper Peninsula, Michigan William Fitzpatrick and Eric Stewart Multiple overlapping features spatially associated with lead-zinc-copper mineralization in the Highland quadrangles, southwest Wisconsin, USA *Lillian Glodowski and Robert Lodge Characterizing volcanic host stratigraphy and syn-volcanic intrusions at the Lynne Zn-Pb-Cu deposit, Oneida Co., Wisconsin xl �Proceedings of the 69th ILSG Annual Meeting – Part 1 *Kaine Johnson and Robert Lodge Hydrothermal Alteration Facies of the Eisenbrey Zn-Cu Deposit, Rusk County, Wisconsin *Matthew Leahy and Robert Lodge Petrology and Geochemistry of the Paleoproterozoic Eau Claire Volcanic Complex, Eau Claire, WI *Francisca Nuñez-Ferreira, Lucas Zoet, and J. Elmo Rawling III Morphometry and formation process of eskers developed under the Chippewa Lobe of the Laurentide Ice Sheet *Jordan Peterzon, Noah Phillips, Peter Hollings and Lionnel Djon Fault zone architecture in mafic protoliths at the Lac des Iles mine, northwestern Ontario Caroline Rose, J. Elmo Rawling III, Eric Carson, John Attig, David Mickelson, William Mode, Mark Johnson, and Kent Syverson Quaternary Geology of Wisconsin at a scale of 1:500,000 (in review) Allison Severson, Eric Nowariak, and Phillip Larson Geology and geochemistry of the basal North Shore Volcanic Group and Midcontinent Rift Intrusive Supersuite, Cook County, MN, USA Eric Stewart, William Fitzpatrick, and Carsyn Ames Relay zones in weakly folded and faulted Paleozoic strata and their role localizing Mississippi Valley-type mineralization, southwest Wisconsin, USA *Madeline Taylor and Marcia Bjørnerud Deciphering the metamorphic and deformational history of the Hardwood Gneiss, Felch District, Michigan: Anomalously high-pressure rocks in the heart of the Penokean orogen *Evan Weber, Robert Lodge, and Jeffrey Marsh U/Pb geochronology and zircon petrochronology of Paleoproterozoic magmas from the Marshfield terrane Penokean Orogen, Wisconsin xli �Proceedings of the 69th ILSG Annual Meeting – Part 1 BANQUET PRESENTATION IMAGINING “CONSERVATION GEOLOGY”: LESSONS FROM THE DRIFTLESS AREA Curt Meine Adjunct Professor at UW- Madison and Senior Fellow with the Aldo Leopold Foundation and Center for Humans and Nature The field of conservation biology emerged in the 1980s when scientists became increasingly alarmed about the loss of biodiversity, and decided that they had a responsibility to put their science to work to address the issue. This required not only new interdisciplinary research, but new ways to put knowledge to work in our human and natural communities. Can we imagine a field of conservation geology that similarly seeks to integrate geological knowledge with history and culture, and addresses our concerns for our landscapes and for future generations? The Driftless Area provides ample examples and opportunities to explore that question. xlii �Proceedings of the 69th ILSG Annual Meeting – Part 1 ABSTRACTS 1 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Geochronology and Geochemical Analysis of the Giants Range Batholith in Northern Minnesota ALLERTON, Zsuzsanna1, HALL, Anita1, ROGER, Françoise2, and TEYSSIER, Christian1 1 Earth and Environmental Science Department, University of Minnesota, 150 Tate Hall, 116 Church St. SE, Minneapolis, MN 55455 2 Géosciences Montpellier, Université de Montpellier-Campus Triolet, c.c. 60 Place Eugéne Batallion 34090, Montpellier, Cedex 05, France The Giants Range Batholith (GRB) is a ~ 2.7-billion-year-old (2.7 Ga) granitic unit in northern Minnesota striking SW-NE from east of Ely to Grand Rapids (Figure 1). It is located N-NW of the 1.8 Ga Mesabi Iron Range and the 1.1 Ga Duluth Igneous Complex (DC). During emplacement, the GRB was situated at the southern edge of the Superior Craton, the Archean core of the North American Continent. At its eastern end the GRB is in contact with the Mesoproterozoic DC (1.1 Ga), which is the intrusive segment of the Mid-continent Rift System, and to the west the GRB flanks the Lower Member of the Ely Greenstone Formation. This project has two main goals: (1) better understanding the origin of the GRB; and (2) using the GRB to track the thermal and hydrothermal history of the rocks from the contact with the DC outward. The project included the compilation of existing data, such as geochronology and geochemistry, that have been collected to date on the GRB, based Figure 1. Simplified geologic map of Minnesota's arrowhead on Allison (1925), Griffin & Morey (1969), region showing the Giants Range Batholith in blue. Prior Viswanathan (1971), Boerboom & Zartman studies have been done in the area circled in red. The white dashed box shows the current and proposed area of this (1993), Boerboom (1994), and Southwick project. Modified from Jirsa, M.A., Miller jr., J.D., & Morey, (1994). The Minnesota Geological Survey G.B. (2008). (Jirsa, 2016) acquired U-Pb zircon age dates on selected samples. Only Boerboom & Zartman (1993) and Boerboom (1994) have completed trace element analysis. Their eight samples are from the central section of the GRB (red circle in Figure 1) and were collected along the northern margin. The samples from the GRB main body have not been analyzed for trace elements, and geochronological data are scarce. Our sampling campaign so far has concentrated on the northeastern part of the GRB (white dashed box in Figure 1) and builds on the work of Boerboom & Zartman (1993) and Boerboom (1994). Thin sections were cut and used in transmitted-light petrography to determine mineralogical composition, analyze textures, and identify accessory minerals for radiometric dating. Radiometric dating involved Laser-Ablation Inductively Coupled Plasma Mass-Spectroscopy (LA-CPMS) performed at the University of Clermont-Ferrand, France. We obtained zircon and titanite age dates for samples located within 1000 meters from the DC contact. The zircon grain separates from one sample produced a concordant age date of ~ 2690 ± 10 Ma. In-situ zircon analysis of another sample displays some discordia (Pb loss) that may be associated with hydrothermal alteration related to DC emplacement. The mounted titanite grains and in-situ analysis yielded ages (approx. 2450-2500 2 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Ma) that are consistently younger than zircons from the same samples. Current and future work include further sample collection in the study area (white dashed box in Figure 1) to obtain additional U-Pb dates on titanite and zircon to the thermal and hydrothermal history of GRB at the contact with the DC. The GRB samples also contain abundant apatite grains, some primary and some recrystallized, that will be dated using the U-Pb method to provide new data on the thermal and hydrothermal history of the GRB near the DC contact. Additionally, we will pursue acquiring bulk composition and trace element data in order to better understand the source of magma and the likely tectonic setting in which the GRB was emplaced. References Allison, I.S., 1925. The Giants Range Batholith of Minnesota. The Journal of Geology, 33(5): 488-508. https://doi.org/10.1086/623215. Boerboom, T.J. and Zartman. R.E., 1993. Geology, Geochemistry, and Geochronology of the Central Giants Range Batholith, Northeastern Minnesota. Canadian Journal of Earth Sciences, 30(12): 25102522. https://doi.org/10.1139/e93-217. Boerboom, T., 1994. Short Contributions to the Geology of Minnesota: Alkalic Plutons of Northeastern Minnesota; Report of Investigations 43. Minnesota Geological Survey, ISSN 0076-9177. Frost, B.R. and Frost, C.D., 2008. A geochemical classification for feldspathic igneous rocks. Journal of Petrology 49.11. Griffin, W.L. and Morey, G.B., 1969. Geology of the Isaac Lake Quadrangle, St. Louis County, Minnesota. Published in Cooperation with Minnesota Department of Iron Range Resources and Rehabilitation. Minnesota Geological Survey 5 P-8 Special Publication Series. University of Minnesota. Southwick, D.L., 1994. Short Contributions to the Geology of Minnesota: Assorted Geochronologic Studies of Precambrian Terranes in Minnesota: A Potpourri of Timely Information. Report of Investigations 43. Minnesota Geological Survey, ISSN 0076-9177. 3 �Proceedings of the 69th ILSG Annual Meeting – Part 1 The Wisconsin Geological and Natural History Survey’s (WGNHS)’s 2020 and 2021 National Geological and Geophysical Data Preservation Program (NGGDPP) Projects AMES, Carsyn1, GOTTSCHALK, Brad1, ROSE, Caroline1, SIBLEY, Dave1 1 Wisconsin Geological and Natural History Survey, University of Wisconsin- Madison, 3817 Mineral Point Rd. Madison, WI 53705 The Wisconsin Geological and Natural History Survey (WGNHS) received grants from the United States Geological Survey (USGS)’s National Geologic and Geophysical Data Preservation Program (NGGDPP) for FY2020 and FY2021. This program promotes the preservation and public accessibility of geoscience collections and data. Projects completed during the 2020 grant were 1) to preserve 130 boxes of hand samples, and 2) to convert 10 WGNHS maps to the standard Geologic Map Schema (GeMS) format. The majority of hand samples for this project came from a donation by Gene LaBerge (UW-Oshkosh) who worked extensively in and around Marathon County, and whose work resulted in a Marathon County bedrock map (LaBerge and Myers, 1983) published by WGNHS. The collection includes more than 1500 specimens from 583 separate outcrops. Successfully preserving these samples is of importance as Marathon Co. continues to urbanize and many of the outcrops these samples represent are being demolished due to land development. The 10 maps converted to GeMS format during the project include Pleistocene maps from northwestern Wisconsin and bedrock maps from southern and northeastern Wisconsin. Converting legacy maps to GeMS format is important because the digital use of WGNHS maps allows for wider and broader use by both internal and external stakeholders. Additionally, a survey of WGNHS external partners showed that a majority prefer digital versions of maps and data. Projects for the 2021 grant included 1) expanding the WGNHS data viewer’s capacity to deliver photos of bedrock cores, 2) digitizing borehole data from the Mineral Development Atlas (MDA)- a joint project between the USGS, United States Bureau of Mines (USBM), and state surveys of Wisconsin, Iowa, and Illinois- that gathered information related to metallic mineral exploration and mining in the lead-zinc district, and 3) photograph, log, and permanently archive seven cores from the Lynne Deposit, a volcangenic massive sulfide deposit in Oneida County. WGNHS’s data viewer, created in 2018, saw its capacity expanded to include photos of cores in their collection. The 2021 project used almost 300 donated Wisconsin Department of Transportation (WisDOT) cores to test pilot this new ability and results are available here: https://data.wgnhs.wisc.edu/data-viewer/. An additional part of this project included the correlation of 3300 scanned logs to the boreholes and updating location data for logs and cores. The MDA portion of the 2021 project focused efforts on mine workings in Lafayette Co., WI. Staff at the Survey geolocated more than 17,000 boreholes and corrected polygons for surface workings such as quarries, prospecting sites, and lead diggings. Lastly, WGNHS permanently archived seven cores from the Lynne Deposit, Oneida Co., WI in 2021. These cores were transferred to WGNHS’s samples repository from the University of Wisconsin- Eau Claire where they had been stored temporarily for student study. The cores (totaling approximately 2500 ft) were then logged and photographed by UW-Eau Claire students. These photos were also added to the WGNHS data viewer. References LaBerge, G., and Myers, P., 1983. Precambrian Geology of Marathon County, Wisconsin. Wisconsin Geological and Natural History Survey IC45: 1-88. https://wgnhs.wisc.edu/catalog/publication/000295/resource/ic45. 4 �Proceedings of the 69th ILSG Annual Meeting – Part 1 The geologic setting, structural controls, and geochemical signature of the Eagle River Au deposit in Northwestern Ontario BARKLEY, Ryan1, PHILLIPS, Noah1, HOLLINGS, Pete1 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada The Eagle River orogenic gold deposit is hosted in the Mishibishu greenstone belt of the western Superior craton, approximately 50 km west of Wawa, Ontario. The deposit, an active underground mine, has been in continuous production since 1995 and produced 1.485 (Moz) of Au through to the end of 2021 (SRK, 2022). The average grade is 9.7 g/pt and Au is primarily hosted in highly strained, milky white to grey quartz veins that dip to the north and strike east to west. The shear zones are hosted in an elliptical quartz diorite pluton, extending into iron rich mafic volcanic rocks. The Mishibishu greenstone belt is dominated by granitic plutons, mafic to felsic volcanics, and lesser amounts of metasedimentary packages. U-Pb zircon dates in the belt range from 2.6 to 2.8 Ga, indicating a Neoarchean environment (Keller, 1989). To understand the geological setting, structural controls, and the geochemical signature of the Eagle River deposit, we completed detailed structural field mapping, petrography, and whole rock geochemistry analysis of the rocks in and around the deposit. A total of 41 whole rock geochemistry samples were collected from the area north of the mine. Two suites were identified; suite one consists of calc-alkaline basalt, andesite, dacite, rhyolite, diorite, tonalite and granite. This suite is characterized by enriched La/Smn ratios of 2.06 to 6.83 and negative Nb anomalies (Nb/Nb* of 0.09 to 0.43), consistent with magmas formed in a supra-subduction environment. Suite two consists of tholeiitic basalt, andesitic-basalt and gabbro. This suite is characterized by flatter trace element patterns with La/ Smn ratios of 0.83 to 1.48 and minor Nb anomalies (Nb/Nb*of 0.40 to 0.85), consistent with primitive arc tholeiites (Fig. 1). Figure 1. Primitive mantle normalised diagrams for the calc-alkaline (blue) vs tholeiitic (red) rocks of the study area. Normalising values from Sun and McDonough (1989). Shear zones for this study appear to be ductile. The white to grey, boudinaged quartz veins are highly strained and flattened, indicating a ductile environment. Gold accumulates in areas of high strain. Quartz veins exhibit chessboard extinction patterns and lobate grain boundaries indicating that the veins have recrystallized through grain boundary migration dynamic recrystallization (Fig. 2; Stipp et al, 2002). Deformation therefore occurred at high temperatures in a low stress environment. 5 �Proceedings of the 69th ILSG Annual Meeting – Part 1 0.5 cm Figure 2. Recrystallization of quartz veins via grain boundary migration. References Keller, J., 1989. The evolution of the Mishibishu greenstone belt, near Wawa, Ontario. Electronic Theses and Dissertations. Stipp, M., Holger & Heilbronner, R., & Schmid, S., 2002. Dynamic recrystallization of quartz: Correlation between natural and experimental conditions. Geological Society London Special Publications. 200: 171-190. 10.1144/GSL.SP.2001.200.01.1. Sun, S.S., and McDonough, W.F., 1989. Chemical and Isotopic Systematics of Oceanic Basalts: Implications for Mantle Composition and Processes. In: Saunders, A.D., Norry, M.J., Eds., Magmatism in the Ocean Basins, Geological Society, London, Special Publications, 42: 313-345. S.R.K Consulting, 2022. 43-101 Eagle River Mine, Ontario, Canada, Wesdome Gold: 262. 6 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Interpreted geophysical cross-sections through the Lake Superior region: Investigating three billion years of geologic history in sixteen lines of data BEDROSIAN, Paul A.1, GRAUCH, V.J.S.1, WOODRUFF, Laurel G.2, CANNON, William F.3, DRENTH, Benjamin J. 1, STEWART, Esther K. 4, PETERSON, Dana E.1 and JONES, James V.5 1 U.S. Geological Survey, Building 20, MS 964, Denver Federal Center, Denver, CO 80225 U.S. Geological Survey, 2280 Woodale Drive, Mounds View, MN, 55112 3 U.S. Geological Survey, 12201 Sunrise Valley Dr., MS 954, Reston, VA 20192 4 Wisconsin Geological & Natural History Survey, 3817 Mineral Point Road, Madison, WI 53705 5 U.S. Geological Survey, 4210 University Drive, Anchorage, AK 990508 2 The northern midcontinent is a window into an Archean-Proterozoic continent, and the Mesoproterozoic Midcontinent Rift System (MRS) that nearly tore it apart. This complex tectonic collage has been largely unmodified during the last billion years yet is poorly exposed except in the Lake Superior region. The area is rich in mineral resources, including native and sedimentary copper deposits, iron formations, volcanogenic massive sulfide deposits, and nickel-copper-platinum-group element sulfide mineralization. In 2016, 2,710 line-km of airborne electromagnetic (AEM) and magnetic data were collected along sixteen regional transects spanning parts of three states and more than three billion years of geologic time (Bedrosian, 2019). The transects range from 100 to 300 km in length and cross parts of the Wisconsin Magmatic Terrane, the Penokean fold and thrust belt, the MRS, and the Archean Superior Province (Figure 1). Data modeling was challenging due to poor control on system height and the prevalence of induced polarization effects (Bedrosian et al., 2018). The final electrical resistivity models derived from the AEM data have been translated into interpreted geophysical cross-sections though an iterative, consensus building approach. A team was assembled with varied expertise in the geology, geophysics, and mineral resources of the MRS, Penokean, and Archean assemblages within the region. Over a period of two years, a series of interpretation sessions worked line-by-line through the transects, culminating in a workshop to synthesize and finalize interpretations. Geologic maps, potential-field data, and drill hole logs were examined alongside the AEM resistivity models and incorporated into the resulting interpretations. Constraints from seismic reflection and refraction studies, magnetotelluric models, geochronology, and detrital zircon studies were also considered where available. The resulting annotated geophysical cross-sections are a resource to be drawn and built upon for geologic and tectonic investigations. Some aspects these sections touch upon include: • Internal structure of the Animikie basin and the basal contact of the Duluth Complex • Geometry and deformation of MRS-flanking sedimentary basins • Structure of the MRS Ashland syncline • Geometry and extent of post-magmatic MRS clastics (Oronto and Bayfield Groups) • Geometry, internal variability and provenance of the Jacobsville Sandstone • Geometry and extent of Archean, Penokean, and MRS faults • Extent and dismemberment of Penokean-deformed metasedimentary units • Iron formations and Penokean structures along the early Proterozoic gneiss dome corridor • Patterns of reverse polarity dikes • Phanerozoic cover and underlying structure (e.g., eastern arm of the MRS) • Distribution, thickness, and variability in glacial cover 7 �Proceedings of the 69th ILSG Annual Meeting – Part 1 New insights and refinements are many but include (a) a restricted areal extent for Bayfield Group clastic rocks, (b) multiple distinct subunits within the Jacobsville sandstone, (c) a close stratigraphic relation between Penokean iron formations and conductive sulfide-rich metasediments, and (d) complex deformation and alteration of the main bowl Animikie basin. Figure 1. Location of AEM and magnetic profiles (magenta). Background geology is from a USGS MRS GIS compilation from published sources of the region. References Bedrosian, P.A., 2018. Geologic mapping and tectonic structure of the U.S. midcontinent via reconnaissance AEM, 7th Intl. Wksp on Airborne Electromagnetics, Kolding, Denmark: 4. Bedrosian, P., 2019. Multi-scale AEM and MT mapping of the Precambrian in Upper Michigan, Northern Wisconsin, and Eastern Minnesota, in Puumala, M., (ed.), Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Part 1 - Abstracts and Proceedings. v.65, Part 1: 67. 8 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Deformation, metamorphism, fluid flow and pegmatite emplacement history of the post-1630 Ma Waterloo Quartzite of southern Wisconsin BJØRNERUD, M.1, BUCHHOLZ, T. 2, FALSTER, A. U.3, and SIMMONS, W. B.3 1 Geosciences Department, Lawrence University, Appleton Wisconsin 54911 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494 3 Maine Mineral & Gem Museum, PO Box 500, 99 Main Street, Bethel, Maine 04217 2 The Waterloo Quartzite, one of the upper Paleoproterozoic ‘Baraboo Interval’ quartzites of the southern Great Lakes region (Medaris et al., 2003), experienced a more complex structural history and higher-grade metamorphism (amphibolite facies) than any of the other quartzite units in this group. It is also distinctive in being intruded by bodies of granitic pegmatite. Natural outcrops of the Waterloo quartzite are limited, but a major quarry near the town of Waterloo (43.210 N, 88.450 W) provides three-dimensional exposures and access to a large volume of fragmented rock. This study is based on observations and samples taken at the quarry over several years as it was deepened and enlarged by blasting. The youngest detrital zircons in the Waterloo Quartzite date to 1634 Ma, younger than the 1710 Ma maximum depositional age of the Baraboo Quartzite, and an indication that sediment transport directions changed from southward to northward (modern coordinates) between the times of deposition of the Baraboo and Waterloo units (Schwartz et al., 2018). The protolith of the Waterloo quartzite was primarily pure quartz sandstone but also included pelites and quartz pebble conglomerates with clasts of jasper (Stewart, 2021). The earliest deformational feature in the Waterloo quartzite is a penetrative foliation (S1) defined by aligned grains of sub-mm muscovite in the pelitic layers; this muscovite has yielded an 40 Ar/39Ar age of 1452 +/- 7 Ma and has been interpreted as evidence of a pervasive fluid flow event that introduced potassium into the supermature sediments, in which K was originally absent (Medaris et al., 2003). In the quarry, the S1 foliation is nearly parallel to bedding; both surfaces strike toward the northeast (045° to 055°) and dip moderately (35°-55°) southeast, suggesting that the rocks lie on the SE limb of a tight NW-verging anticline. In places, mm- to cm-scale quartz veins with Ti-rich hematite masses on their margins lie parallel to the foliation and have fibers oriented perpendicular to the foliation. This points to another episode of fluid infiltration under a stress regime distinct from the one that formed the foliation. These early quartz veins are commonly folded and/or boudinaged. The S1 foliation is overprinted by porphyroblasts of andalusite, typically about 0.5 cm in size. Most of these have been altered to muscovite and/or kaolinite, indicating another episode of fluid infiltration. The kaolinite occurs mainly on the margins of the andalusite crystals, giving them a zoned appearance. In many specimens, the retrograded andalusites have a rusty red color that may be related to the presence of hematite in the kaolinitic rims (Geiger et al., 1982). The next structural feature to develop in these rocks are kink-like crenulations in pelitic horizons, seen abundantly in blocks in the quarry waste piles. At two sites where this crenulation cleavage (S2) was observed in place, it strikes N to NNW and dips steeply east. The geometry of the crenulations is strongly influenced by the presence of the andalusite porphyroblasts/ pseudomorphs, many of which have small, asymmetric pressure shadows of quartz and muscovite that seem to be related to the development of the crenulations. Sometime after the formation of the crenulation cleavage, the quartzite was intruded by Kfeldspar-dominated pegmatite dikes ranging in width from 1 cm to 3 m. Pegmatites have been known from the NW portion of the Waterloo Quarry for some years and have been discussed by 9 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Buchholz et al. (2016). The pegmatite dikes have sharp boundaries and granitic textures, with Kfeldspar, quartz and muscovite crystals of equal and uniform size. Blasting of a phyllitic horizon in the NE part of the quarry has recently exposed additional thin (< 5 cm wide) pegmatite dikes, with mmscale chilled margins at contacts with the host rock. Heavy mineral separates from these thin dikes contain fluorapatite, monazite-(Ce), ilmenite, columbite-(Mn), tantalite-(Mn), evidence of significant enrichment of Ta/Nb and Mn/Fe. In addition to the pegmatite dikes, coarse-grained pegmatite-like patches occur in the necks of boudinaged quartz veins and quartzose layers enclosed by phyllites, primarily in the NE corner of the quarry. Unlike the clearly igneous dikes, these patches have irregular boundaries with the host rock and their crystal size is variable. In thin section, K-feldspar and quartz in these patches show a micrographic texture. Muscovite in pelitic layers surrounding the boudins is coarser than in the rest of the rock, and rusty andalusite pseudomorphs are smeared and flattened in the vicinity of the boudins, suggesting that they had already been altered and softened by the time the boudins formed. Although they occur in the same area of the quarry, the pegmatite-like boudin patches do not seem to be physically connected to the pegmatite dikes. The patches are presumably older since they formed during the process of boudinage, while the dikes apparently postdate deformation. The pegmatite-like material in the boudin necks could either be hydro- thermal or produced by in situ melting related to influx of fluids or perhaps a local drop in pressure (mean stress) during boudinage. Examination of heavy mineral separates from the pegmatite-like bodies associated with boudins revealed fluorapatite and monazite-(Ce). One specimen of the boudin material contains small beryl crystals in a pocket-like void. This may be similar to beryl occurrences in regionally metamorphosed rocks in Austria (Franz et al, 1986), believed to have formed between 500- 550⁰C -- slightly higher than maximum metamorphic temperature estimates of 500⁰C for the Waterloo rocks (Medaris et al., 2003). Small crystals of greenish to light brown dravitic tourmaline are also present locally; analysis shows that these are Li-bearing, as are nearby muscovites. Additional mineral phases include chloritoid, spessartine garnet, fluorapatite, and gahnite, all pointing to the introduction of fluids with a rich mix of ions. Although the Waterloo quarry lies only 20 km in the across-strike direction from the south limb of the Baraboo syncline, it is not easy to correlate either the chronology or the orientations of structures at Waterloo with those in the more famous Baraboo Quartzite. Our observations from the Waterloo quarry suggest that the 1470-1450 Ma “Baraboo Orogeny” (Medaris et al., 2021) was a complex, multistage tectonic event whose details have not yet been fully documented. References Buchholz, T.W., Falster, A.U. & Simmons, W.B., 2016. Second Foord Pegmatite Symposium: 22-23. Franz, G., Grundman, G., & Ackermand, D, 1986. Tschermaks Min. Pet. Mitteilungen, 15: 167-192. Geiger, C., Guidotti, C. & Petro, 1982. Geoscience Wisconsin 6: 21-40. Medaris, L.G. & others, 2003. Journal of Geology, 111, doi:10.1086/373967 Medaris, L.G. & others, 2021. Geoscience Frontiers, 12, doi: 10.1016/j.gsf.2021.101174 Schwartz, J.J., Stewart, E.K. and Medaris, L.G., Jr., 2018. ILSG Proceedings, 64: 93–94. Stewart, E.K., 2021. Wisconsin Geological & Natural History Survey Map 508. Stewart. E.K., Brengman, L. & Stewart, E.D., 2021. Journal of Geology, 129, doi:10.1086/713687. 10 �Proceedings of the 69th ILSG Annual Meeting – Part 1 The Midcontinent Rift System: Neither triple junction nor failed rift? BLEEKER, Wouter1, SMITH, Jennifer1, HAMILTON, Michael2, KAMO, Sandra2, HOLLINGS, Pete3, EASTON, Michael4, and CUNDARI, Robert5 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 3 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 4 Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, ON P3E 6B5 5 Ontario Geological Survey, 435 James Street South, Thunder Bay, ON P7E 6S7 2 The Midcontinent Rift System has often been described in terms of i) a failed intracontinental rift system; with ii) a basic ‘triple junction’ architecture, the three arms of the triple junction being represented by the SW arm of Lake Superior, the SE arm of Lake Superior, and a less developed rift structure reaching up into the Lake Nipigon area. Here we challenge both views. Although it is certainly true that on a local scale, i.e. the North American midcontinent, the rift system failed and inverted, it is likely that on a more global scale the system did not fail but led to ocean opening at ca. 1103 Ma, i.e. the waning phase of the “Early Magmatic Stage”3 of the Midcontinent Rift. This led to a global reorganization of plate stresses. The majority of robust structural indicators suggest that this early stage rifting, initiated at ca. 1110 Ma and waning towards ca. 1103 Ma, was oriented on an NW-SE axis or trend (present orientation), from Lake Nipigon to the SE arm of Lake Superior and beyond. The significant gradient in rifting and lithospheric stretching, from Lake Nipigon (minor rifting followed by sagging) to the SE rift arm (major rifting), requires that the rotation pole for this early phase of rifting was situated to the northwest, somewhere in northwest Ontario. At larger distances from this rotation pole, up to 90° of arc away(?), to the southeast (present orientation), lithospheric spreading may have reached ~1000 km and thus likely led to ocean opening. This early phase of rifting with its NW-SE axis came to a close with a marked hiatus of ~4-5 Myr (the “Magmatic Hiatus”), represented in most sections by a distinct unconformity of conglomerates and more shallow dipping basalt flows on top of older, more steeply dipping basalt flows. When rifting resumed, after this significant hiatus, it opened up the SW arm of the Midcontinent Rift organized on a SW-NE trending rift system. This phase was accompanied by the “Main Magmatic Stage” and was initiated at 1099-1098 Ma, the emplacement age of the Duluth Complex (e.g., Paces and Miller, 1993). The marked gradient in rifting and lithospheric stretching on this SWNE rift system, with major crustal stretching in the central part of Lake Superior, and less stretching farther to the southwest, indicates that the rotation pole for this younger phase of rifting was situated well to the southwest, perhaps in Texas or on the future western margin of Laurentia. This SW-NE rift system shows marked jogs, and may have stepped over to the south, through the eastern arm of Lake Superior, and continued to the northeast in the area now obscured by final accretion and collision of the Grenville orogen at ca. 1 Ga. As for the first phase of rifting, we observe locally (in North America) only one proximal end (relative to the rotation poles) of the larger rift systems—systems that may well have been global in scale. Clearly the later SW-NE rift system is distinct from the earlier NW- SE rift system, with completely different rotation poles, and rift axes that are essentially perpendicular to each other. Relevant to the early NW- SE rift phase, the extent of the rifting and the location of its rotation pole, are occurrences of carbonatite complexes in northwest Ontario, and large diabase sills at the base 3 We use here the magmatic stage terminology of Miller and Nicholson (2013) but with modified age boundaries. 11 �Proceedings of the 69th ILSG Annual Meeting – Part 1 of the Athabasca Basin (the Moore Lakes sills), the latter with an age exactly equivalent to those of the Nipigon diabase sills (see Bleeker et al., 2020 and references therein). At a larger scale, there are 1108 Ma magmatic provinces on several other continents (e.g., the Umkondo sills in South Africa; Hanson et al., 2004). And relevant to the younger SW-NE rift phase is the major diabase sill magmatism of the SW USA Diabase Province at 1095-1085 Ma (e.g., Bright et al., 2014; Heaman and Grotzinger, 1992). Clearly, we need to zoom out to develop a broader understanding of the Midcontinent Rift System and see it in a more global context. The GSC-funded project to refine our knowledge of this major rift system started with an attempt to better define the ages (both precision and accuracy) of some of the major events and many of the mineralized intrusions. We currently have ~30 U-Pb samples in various stages of progress and some early results were reported in Bleeker et al. (2020) and Smith et al. (2020). Several others will be discussed as part of this presentation. Our initial focus was to resolve many of the problematic age ‘outliers’, the majority of which were based on extrapolations from sparse and discordant data. Most of these outliers are now gone. Based on our current data and review of the published literature, major age divisions may be summarized as follows: Initiation: ca. 1111-1110 Ma, as best defined by the large Echo Lake subvolcanic layered intrusion (a robust zircon age, reported in Cannon and Nicholson, 2001). Early Magmatic Stage: 1110-1103 Ma, with emplacement of regional diabase sill complexes (ca. 1108-1106 Ma) following early rift intrusions and regional plateau basalt building (1110-1107 Ma). The Tamarack intrusion, still organized on a NNW-SSE dyke-like system, is ca. 1104 Ma. Hiatus: 1103-1099 Ma, in many places marked by an angular unconformity. Main Magmatic Stage: 1099-1092 Ma, initiated with emplacement of the Duluth Complex and later characterized by the very extensive flood basalts of the Portage Lake Volcanic Group. Late Magmatic Stage: 1092-1084 Ma, waning volcanism and intercalated rift-fill sediments. Sagging and Rift-Fill Stage: 1084 to ca. 1060 Ma, final rift fill sedimentation, Oronto Group. References Bleeker, W. et al., 2020. The Midcontinent Rift and its mineral systems: Overview and temporal constraints of Ni-Cu-PGE mineralized intrusions. GSC Open File 8722: 7–35. DOI: org/10.4095/326880. Bright, R.M. et al., 2014. U-Pb geochronology of 1.1 Ga diabase in the southwestern United States: Testing models for the origin of a post-Grenville large igneous province. Lithosphere, 6:135–156. Cannon, W.F. and Nicholson, S.W., 2001. Geology map of the Keweenaw Peninsula and adjacent area. U.S. Geological Survey, Geological Investigations Series, Map I-2696, scale 1:100 000. Heaman, L.M. and Grotzinger, J.P., 1992. 1.08 Ga diabase sills in the Pahrump Group, California: Implications for development of the Cordilleran miogeocline. Geology, 20: 637–640. Miller, J.D. and Nicholson, S.W., 2013. Geology and mineral deposits of the 1.1 Ga Midcontinent Rift in the Lake Superior region – An overview. Precambrian Research Center Guidebook 13-1:1–50. Paces, J.B. and Miller, J.D., 1993. Precise U‐Pb ages of Duluth complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Ga midcontinent rift system. Journal of Geophysical Research: Solid Earth, 98: 13 997–14 013. Smith, J.W. et al., 2020. Timing and controls on Ni-Cu-PGE mineralization within the Crystal Lake Intrusion, 1.1 Ga Midcontinent Rift. GSC Open File 8722: 37–63. 12 �Proceedings of the 69th ILSG Annual Meeting – Part 1 New geologic mapping in the Superior Province of northwestern Minnesota, USA: Pennington and Red Lake Counties BLOCK, Amy Radakovich1, SOUDERS, A. Kate2, DRENTH, Benjamin J.3, HUDAK, George J.4, SAARI, Stacy M. 5, HIRSCH, Aaron C.1 1 Minnesota Geological Survey, 2609 Territorial Road, St. Paul, MN 55114 U.S. Geological Survey, PO Box 25046, MS 963, Denver Federal Center, Denver, CO 80225 3 U.S. Geological Survey, PO Box 25046, MS 973, Denver Federal Center, Denver, CO 80225 4 Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN 55811 5 Minnesota Department of Natural Resources, 1525 3rd Ave E, Hibbing, MN 55746 2 The Earth Mapping Resources Initiative (Earth MRI) is a partnership between the USGS and state geological surveys/science agencies that funds data collection and geologic mapping in order to better characterize areas of potential critical mineral resources. Earth MRI recently funded a highresolution airborne geophysical survey (Allen Langhans and Drenth, 2023; Fig. 1, blue box) and acquisition of new geochronologic (Souders, A.K., in review), petrologic, and geochemical data in a part of the Superior Province in northwestern Minnesota that is prospective for numerous criticalmineral-producing systems. Previous geologic mapping of the area (Jirsa et al., 1999; Jirsa et al., 2011) was limited by an absence of outcrop, limited drill hole data, and only one geochronologic age. Newly acquired data support ongoing bedrock mapping across a large area (Fig. 1, red box); This map highlights the geology of Pennington and Red Lake Counties (Fig. 1, orange box). The map area comprises three conterminous subprovinces of the Archean Superior Province which are situated in unusually close proximity to one another; in the map area, the Quetico metasedimentary province pinches to as little as 5 km of thickness in map view where it separates the Wabigoon and Wawa volcanoplutonic subprovinces on either side. Ages from a biotite tonalite in the Snake River batholith (ca. 2758 Ma), a diorite in the Grygla pluton (ca. 2771 Ma), and a biotite-hornblende tonalite in the Red Lake Falls pluton (ca. 2701 Ma) (Souders, in review) define multiple Neoarchean episodes of intermediate to felsic intrusive activity within the Wabigoon subprovince. Interpretation of the improved aeromagnetic data suggests a revised, more southerly position of the Wabigoon-Quetico subprovince boundary, as well as modifications of numerous other geologic contacts across the map area. Finally, new geochronologic ages confirm an Archean (ca. 2737 Ma) age for the Mentor Anorthosite Intrusive Complex (MAIC) (Souders, in review), and new geophysical interpretations reveal that the MAIC is as much as twice as large and much more structurally complex than previously thought (Drenth et al., this volume). Both findings regarding the MAIC are consistent with what is known of other Archean anorthosites in the Superior Province (Sotirou & Polat, 2020; Polat et al., 2018). Work in the larger Earth MRI mapping area is ongoing. Additional geochronologic data will shed light on the depositional history and timing of mineralization of volcanic strata in both the Wawa and Wabigoon subprovinces. Geochemical analyses will supplement petrographic observations and help refine tectonic provenance of all rock units. A new geologic map of the entire area (Fig. 1, red box) will be completed, and a comprehensive mineral potential model will better assess the potential for critical minerals in the area. 13 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Pennington Red Lake Figure 1. Generalized subprovince map of the Superior Province in northwest Minnesota, USA, showing the location of both the recent geophysical survey (blue outline), ongoing new mapping (red outline) for the EarthMRI project, and the map area for this poster (orange outline). 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. Drenth, et al., this volume. Jirsa, M.A., Chandler, V.W., and Runkel, A.C., 1999. M-092 Bedrock geologic map of northwestern Minnesota. Minnesota Geological Survey. Retrieved from the University of Minnesota Digital Conservancy, https://hdl.handle.net/11299/973. Jirsa, M.A., Boerboom, T.J., Chandler, V.W., Mossler, J.H., Runkel, A.C., and Setterholm, D.R., 2011. Geologic map of Minnesota, bedrock geology: Minnesota Geological Survey State Map S-21, scale 1:500,000. Polat, A., Longstaffe, F.J., and Frei, R., 2018. An overview of anorthosite-bearing layered intrusions in the Archaean craton of southern West Greenland and the Superior Province of Canada: implications for Archaean tectonics and the origin of megacrystic plagioclase: GEODINAMICA ACTA, v. VOL. 30, NO. 1: 84–99, https://doi.org/10.1080/09853111.2018.1427408. Sotiriou, P., and Polat, A. 2020. Comparisons between Tethyan anorthosite‐bearing ophiolites and Archean anorthosite‐bearing layered intrusions: implications for Archean geodynamic processes: Tectonics, v. 39, 35. 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 �Proceedings of the 69th ILSG Annual Meeting – Part 1 A revised provenance model for the Elk Mound Group in south-central Wisconsin based on detrital zircon analysis BOJDAK-YATES, Itai S.1, BJØRNERUD, Marcia1, MALONE, David H.2, and STEWART, Esther K.3 1 Department of Geosciences, Lawrence University, Appleton, WI, 54911, United States Department of Geography, Geology, and the Environment, Campus Box 4400, Illinois State University, Normal, IL, 61790, United States 3 Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd, Madison, WI, 53705, United States 2 The Late Cambrian Elk Mound Group consists of three sandstone formations deposited in a shallow tropical sea: the Mount Simon, Eau Claire, and Wonewoc formations, in ascending order. The formations underlie much of the upper Midwestern United States in vast, thin sheets, which thicken toward the Illinois Basin further south. These formations have long fascinated geologists due to their extraordinary physical and chemical maturity, but they have often eluded explanation thanks to those same qualities. Recent studies have employed detrital zircon (DZ) U-Pb analysis to constrain the sources of the sand, and workers have begun to build regional provenance models that describe the origins of the sand and the routes it took to arrive at its present location. Our study builds upon these models with new samples from the Mount Simon Sandstone, a quartz arenite deposited in terrestrial and shoreface environments (Dott et al. 1986). We analyzed samples from outcrops of nonmarine deposits high on the Wisconsin Arch near Wisconsin Dells, WI, as well as a drill core taken 26 miles east of the Dells (the Triemstra core, near Belle Fountain, WI). We place these samples in the context of previous DZ work in this area, especially a study by Konstantinou et al. (2014). The formations of the Elk Mound Group are poorly defined in central Wisconsin, and the samples reveal a transition from Mesoproterozoic source provinces towards Late Archean source provinces as one moves up section and to the west (Figure 1). This transition is understood to represent a shift from sediments derived from the more local Wolf River Batholith (ca. 1470 Ma) and Penokean orogenies (ca. 1830 Ma) to more distal sediments derived from the Superior Province (ca. 2650 Ma). However, other sedimentary basins such as the Animikie and Huronian basins and the Midcontinent Rift may have contributed sediments as well. The physical maturity of the sand grains supports a recycled origin, as multiple cycles of weathering and erosion would have been necessary to produce such rounded grains (Dott, 2003). Sedimentological details of the sandstone reveal additional information about shifts in provenance. A pair of samples from the Wisconsin Dells area (upper Chapel Gorge and lower Mirror Lake) show relatively high proportions of Penokean-age sediments. The sedimentology of the outcrops sampled records a transition from a dune environment to a braided river environment, and these rivers may have brought sediment from the Penokees. Additionally, the proportional increase in Archean-age sediments correlates with a rise in sea level as one rises through the Elk Mound Group. This correlation suggests that local sediment sources were drowned by sea level transgressions, while the distal Superior Province remained high enough to continue eroding and contribute sediment to a shallow sea already rich in Archean-age sand. Paleocurrent indicators derived from optical borehole image logs from wells across central Wisconsin add to the regional provenance picture with evidence of predominant currents flowing toward the west and southwest, giving some indication of the more immediate source and final transport of these sediments. 15 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Figure 1. DZ data from six samples gathered in central Wisconsin, organized in ascending order through the section and from east to west. (The Triemstra sample is the oldest and furthest east; the Wonewoc sample is the youngest and furthest west.) The Chapel Gorge samples came from the east bank of the Wisconsin River about 1.5 miles north of Wisconsin Dells. The Mirror Lake samples came from the northwest shore of Mirror Lake about 3.5 miles south of Wisconsin Dells. The Wonewoc sample was collected near Wonewoc, WI, about 21.5 miles west of Wisconsin Dells, and was analyzed by Konstantinou et al. (2014). References Dott Jr., R.H., Byers, C.W., Fielder, G.W., Stenzel, S.R., and Winfree, K.E., 1986. Aeolian to marine transition in Cambro-Ordovician cratonic sheet sandstones of the northern Mississippi Valley, USA. Sedimentology, 33: 345-367. Dott Jr., R.H., 2003. The Importance of Eolian Abrasion in Supermature Quartz Sandstones and the Paradox of Weathering on Vegetation-Free Landscapes. The Journal of Geology, 111(4): 387-405. Konstantinou, A., Wirth, K.R., Vervoort, J.D., Malone, D.H., Davidson, C., and Craddock, J.P., 2014. Provenance of Quartz Arenites of the Early Paleozoic Midcontinent Region, USA. The Journal of Geology, 122: 201-216. Lovell, T.R., and Bowen, B.B., 2013. Fluctuations in Sedimentary Provenance of the Upper Cambrian Mount Simon Sandstone, Illinois Basin, United States. The Journal of Geology, 121: 129-154. 16 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Quetico-Wabigoon Subprovince Boundary in the Superior Province north of Thunder Bay, Ontario, Canada BRIGGS, Blaize1, and HILL, Mary Louise1 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada The boundary zone between the Quetico and Wabigoon subprovinces is a complex zone of deformation and metamorphism that historically has been described as a fault, change in metamorphic grade and/or change in lithology. This boundary zone is exposed along Highway 527 within a roughly 23km stretch of highway. At the south end of this zone the DeCourcey Lake outcrop is a strongly foliated, mylonitic gneiss containing quartz, feldspar, garnet, sillimanite, muscovite, and biotite with pegmatites and boudinaged quartz veins that is interpreted to be part of the Quetico subprovince. The north end of the zone is marked by weakly foliated Max Lake conglomerate that displays primary sedimentary textures and is interpreted to be part of the Wabigoon subprovince. Cataclasis was discovered 9.8km north of the DeCourcey Lake outcrop and marks a sharp change from the high-grade amphibolite to granulite facies Quetico lithologies south of the cataclasite to subgreenschist to greenschist facies Wabigoon lithologies to the north. This cataclasite is characteristic of brittle deformation and evidence for a fault that has not been reported in previous studies. The fault is mapped parallel to the foliation of the cataclasite (Fig. 1). This cataclasite is interpreted to be a boundary fault marking the abrupt transition between the Quetico and Wabigoon subprovinces along Highway 527. Figure 1. Map of study area along Highway 527 showing sampled outcrops and new subprovince boundary. Thirteen outcrops along Highway 527 were mapped and sampled for microstructural analysis. 17 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Thin sections created from these samples were used to identify deformation microstructures in quartz and feldspar. Feldspar deformation microstructures are particularly useful and can be used as a proxy for temperature since feldspar needs higher temperatures than quartz to deform internally. Identifying deformation regimes for feldspar is important as most of the rocks within the study area are dominantly composed of quartz and feldspar. Metamorphic Grade Low Temperature 400 ℃ Textures/Deformation Structures -Patchy undulose extinction - Fracturing and cataclasis -Angular grains -Grain size faults with bent cleavage plane/twins Low-Medium 400-500 ℃ Medium 450-600 ℃ High 600 ℃ -Internal fracturing (minor dislocation glide) -Bulging recrystallization (BLG) -Tapered deformation twins -Bent twins -Undulose extinction -Deformation & kink bands -Core & mantle texture -Fine grain recrystallization/uniform grain size -Micro-kinking -Less abundant deformation twins -Sub-grain rotation (SGR) -Bulging recrystallization (BLG) -Core and mantle texture -Myrmekite along foliation planes Table 1. Feldspar deformation structures and corresponding temperatures/metamorphic grade based on descriptions from Passchier and Trouw (2005). References Passchier, C.W. and Trouw, R.A.J., 2005. Microtectonics, Second Edition. Springer. Berlin, New York: 366. 18 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Contributions of diverse mantle sources during the early stages of Midcontinent Rift formation — Implications for a passive rifting model BRZOZOWSKI, Matthew1,2, HOLLINGS, Pete1, ZHU, Jing-jing3, CREASER, Robert4 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada British Columbia Geological Survey, 1810 Blanshard Street, Victoria, BC V8T 4J1 Canada 3 State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, 99 Lincheng West Road, Guiyang, Guizhou Province 550081, PR China 4 Earth & Atmospheric Sciences, University of Alberta, 116 Street & 85 Avenue, Edmonton, AB T6G 2R3, Canada 2 It is generally accepted that the Midcontinent Rift System (MRS) and associated magmatism originated as a result of the impingement and melting of the Keweenaw Plume beneath the crust ca. 1.1 Ga (Hutchinson et al. 1990). This interpretation is based largely on Sm–Nd and Re–Os isotope data, and the need for a heat source to explain the large volumes of magma generated (Cannon 1992; Nicholson et al. 1997; Shirey 1997). This view has recently been challenged, however, given the long duration of magmatism associated with the MRS (Hollings and Heggie 2014) and paleomagnetic evidence that is indicative of rapid plate motion during the formation of the MRS (Swanson-Hysell et al. 2014). Alongside these ambiguities are uncertainties in the nature of the sources that fed the MRS with magma (e.g., plume vs. subcontinental lithospheric mantle)? Clarifying these ambiguities has remained challenging given that many of the earliest magmas in the MRS were variably contaminated by crustal material (e.g., the Nipigon sills), masking potential contributions from distinct mantle sources. Development of a robust genetic model for the early history of the MRS and the critical mineral resources associated with this magmatism requires a firm understanding of these contributions. To address this, we integrated new Os isotope data of Initiation (>1,109 Ma), Early (1,109–1,104 Ma), and Hiatus (1,104–1,098 Ma) stage rocks with variations in their bulk-rock trace-element and Nd isotope geochemistry (Brzozowski et al. 2023). Early MRS rock suites are characterized by highly variable γOsi values of -10 to 3857, with Early Stage melts exhibiting the greatest variability (-10 to 3857) and Initiation Stage melts exhibiting the smallest variability (4 to 50). Given that the γOsi values do not correlate with La/Sm, Gd/Yb, and MgO, this variability could not be due to variable degrees of partial melting, retention of garnet in the mantle, or fractional crystallization, respectively. Several of the rock suites of interest exhibit elevated Th/Nb–Th/La and radiogenic εNdi–Sri values that are indicative of crustal contamination and/or contributions from a subcontinental lithospheric mantle (SCLM) source. Based on numerical modeling, the radiogenic εNdi and γOsi values recorded by the mafic–ultramafic intrusions and sills are indicative of their crystallization from hybrid melts (enriched SCLM-derived melt > plume-derived melt) that assimilated <10% crustal material during emplacement (Fig. 1). In contrast, the melts that fed the diabase sills and subaerial lavas likely originated from depleted portions of the Keweenaw Plume based on their variably negative to positive γOsi values, and were contaminated during emplacement (Fig. 1). Although contamination can explain the range of εNdi values exhibited by the rock suites, it cannot independently account for the range of γOsi values because i) this would require unrealistically high degrees of contamination and ii) not all of the rock suites were contaminated (cf. Wolfcamp Basalt). Rather, it is likely that fractionation of sulfide liquid and/or Os-bearing platinum-group minerals also contributed to this variability. Together, these results indicate that i) not all of the rock suites in the MRS crystallized from plume-derived melts, ii) melt contributions from the SCLM were greatest during the early stages of rift formation, and iii) the MRS likely initiated passively, with plume impingement being a coincidence that provided the energy and material necessary for voluminous 19 �Proceedings of the 69th ILSG Annual Meeting – Part 1 magmatism. Figure 1. Variation in γOsi and εNdi in hybrid magmas generated by mixing of melts from various mantle reservoirs. The numbers along the mixing curves are the mixing percents. The numbers in rounded boxes are the εNdi values of the contaminant. References Brzozowski M.J., Hollings P., Zhu J-J., Creaser R.A., 2023. Osmium isotopes record a complex magmatic history during the early stages of formation of the North American Midcontinent Rift — Implications for rift initiation. Lithos: 436–437:106966. Cannon W.F. 1992. The Midcontinent rift in the Lake Superior region with emphasis on its geodynamic evolution. Tectonophysics 213: 41–48. Hollings P., Heggie G. 2014. Rethinking the Midcontinent Rift–puncturing the ‘Plume Paradigm’. In: 60th Institute on Lake Superior Geology. Hibbing, Minnesota, pp 57–58. Hutchinson D.R., White R.S., Cannon W.F., Schulz K.J., 1990. Keweenaw hot spot: Geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent Rift System. J Geophys Res 95: 10869. Nicholson S.W., Schulz K.J., Shirey S.B., 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 34: 504–520. Shirey S.B. 1997. Re-Os isotopic compositions of Midcontinent rift system picrites: implications for plume – lithosphere interaction and enriched mantle sources. Can J Earth Sci 34: 489–503. Swanson-Hysell N.L., Burgess S.D., Maloof A.C., Bowring S.A., 2014. Magmatic activity and plate motion during the latent stage of Midcontinent Rift development. Geology 42: 475–478. 20 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Eastward transition from banded iron-formation to ferruginous clastic rocks across the central Upper Peninsula of Michigan CANNON, W. F.1, DRENTH, Benjamin J.2 1 U.S. Geological Survey, MS 954, Reston, VA 20192; 2U.S. Geological Survey, Denver, CO 80225 The classic Paleoproterozoic iron-formations of the Lake Superior iron ranges are predominantly banded cherty chemical sedimentary rocks characterized by centimeter-scale interbedding of chert and various iron minerals. New observations from legacy iron exploration drill cores that sampled Precambrian rocks below Paleozoic sediments to the east of the exposed iron ranges in the Upper Peninsula of Michigan show that highly ferruginous fine-grained clastic sedimentary rocks are predominant in that area, and that true cherty iron-formation is a subordinate component of the ferruginous sedimentary section. Most of our information is derived from a collection of cores from proprietary exploration holes held by Cleveland-Cliffs Iron Company, who has allowed us to examine, sample, and describe the rock units. Those holes were drilled to test five large-amplitude magnetic anomalies (Figure 1). Cores from four additional anomalies that are publicly available at the Michigan Geologic Sample Repository were also studied. Figure 1. Reduced to pole aeromagnetic anomaly map showing anomalies sourced in sub-Paleozoic Precambrian basement, names assigned to each magnetic anomaly, and drill holes used in this study. Crosshatched pattern is the area of Paleozoic cover. The ferruginous clastic rocks examined in this study are generally laminated at centimeter- to millimeter-scale and range from fine-grained quartzite to siltstone. Most are even-bedded. Laminae alternate between quartzo-feldspathic and ferruginous; some of the latter are nearly 100% iron minerals. Average iron mineral content of individual short core segments is as much as 50% by visual estimates. All are metamorphosed to varying degrees, but unambiguous relict clastic textures are preserved widely. The combination of textures and mineral content leaves no doubt that these are 21 �Proceedings of the 69th ILSG Annual Meeting – Part 1 clastic rocks that accumulated very anomalous concentrations of iron. Figure 2. A. Whole thin section of thinly interlaminated fine sandstone and siltstone from the LaBranch deposit. Light layers are quartzo-feldspathic fine sandstone. Darkest layers are nearly all magnetite. B. Crossed polars view of quartz, microcline, biotite, and magnetite in fine sandstone from the Gladstone deposit. C. Same view as B in reflected light showing numerous magnetite grains. Bright partial rims on some grains are martite. Figure 3. Schematic section of approximately 100 kilometers showing the transition from Vulcan Ironformation in the west, as exposed on the Menominee Range (Bayley et al., 1966) and Felch Trough (James et al., 1961), to ferruginous clastic-dominated sedimentary rocks in areas covered by Paleozoic sediments in the east. We interpret these ferruginous clastic rocks as the lateral equivalent of the Menominee Group, which includes the major banded iron-formations of the Menominee and other iron-ranges of the western Upper Peninsula of Michigan. They record a gradation from the purely chemical and clasticstarved true banded iron-formations to the west, to a more shoreward facies where fine clastic sedimentation predominated and overwhelmed slow precipitation of chert beds. Intermittent periods of diminished clastic input allowed sporadic deposition of layers of cherty banded iron-formation, some of which are granular, indicating deposition in shallow water. These relationships show that the lateral disappearance of true banded iron-formations resulted from suppression of chemical chert precipitation by the input of fine-grained clastic sediments. However, intense iron deposition persisted into this more proximal fine-clastic-dominated facies resulting in abundant ferruginous clastic rocks. References 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: 96. 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. 22 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Workshop Outcomes and Updates for the Minnesota Department of Natural Resource’s Drill Core Library CARTER, Matt J.1 and ELSENHEIMER, Donald2 1 Minnesota Department of Natural Resources, 1525 3rd Ave E, Hibbing, MN 55746 2 Minnesota Department of Natural Resources, 500 Lafayette Rd, Saint Paul, MN 55155 The Minnesota Department of Natural Resources (DNR) provides public access to more than one million meters of drill core from over 9,000 locations across the state at its Hibbing Drill Core Library (DCL). This archive opened in 1967 and has been an invaluable resource for bedrock mapping, mineral exploration, and research, including numerous ILSG presentations. In November 2022, the DNR convened a workshop to gather stakeholder input on DCL policies and procedures (Carter et al., 2023). A need to update these policies and procedures was identified by DNR staff after conducting a 2022 inventory of DCL holdings and determining its current storage capacity, an assessment of projected core submissions, participation in a National Geological and Geophysical Data Preservation Program (NGGDPP) data management workshop, and a review of the policies and procedures of the United States Geological Survey (USGS) and peer repositories. Workshop participants were affiliated with the mining/mineral exploration industry, government agencies, academic institutions, and consulting firms. Feedback was gathered through exercises and participant surveys that focused on the mission of the DCL, prioritization of storage for various materials, sampling and related policies, and desirable enhancements to DCL databases. DNR staff used input from the workshop and reviewed the mission statements from the USGS, the DNR, and peer repositories to create a mission statement for the DCL. DCL curational decisions on what to add or retain in its collection have not previously been constrained by storage capacity. Given anticipated core submissions, participants were encouraged to consider submission and retention priorities, even with a planned addition of a fourth DCL storage building. It was recommended that prioritization should be given to materials that are costlier to replace, are more difficult to access (present and future) as well as complete (i.e., non-skeletonized) diamond drill hole cores that have economic and/or geologic significance. Suggestions were made in favor of retaining pulp and reject samples derived from bedrock core, while acknowledging the potential for the materials to degrade over time. It was suggested that unless surficial materials (e.g., outcrop, glacial sediments) have historical significance or were from areas with restricted access then they should be given a low storage priority. Participants encouraged the DNR to consider strategies that might optimize storage capacity or lower retention costs, such as standardized containers for unconsolidated materials or off-site storage of lower priority samples within the collection. Established DCL policies for facility visits, sampling protocols, and derivative thin sections and dataset submissions are comparable to peer repositories. While reviewing these policies, workshop participants expressed concerns about missing or oversampled intervals and suggested improving communication about the allowable sample size based on the proposed analyses. It 23 �Proceedings of the 69th ILSG Annual Meeting – Part 1 was generally accepted that samples, unused materials, and thin sections should be returned within a year. Yet, it was recognized that multi-year projects may need accommodations, that regular communication and updates must be provided by visitors who want to retain materials for over a year, and that consequences need to be established and enforced for those that do not follow policies. In general, the DCL could improve the communication of its policies to ensure visitors are better able to follow them. Participants also offered ideas on enhancing online access to DCL holdings and associated datasets. These included improving the accuracy of some drill hole collars as well as the link between historical and other publicly available data to drill holes. Digital images of cores boxes were also desirable and the DNR is conducting a pilot program to evaluate digital image collection. The importance of the DCL and the value it offers to researchers, the local mining and mineral exploration community, and the citizens of Minnesota was emphasized by workshop participants. DNR staff are currently using workshop feedback and relevant policies and procedures at peer repositories to make preliminary curational decisions that support the DCL’s mission on topics such as storage prioritization, development of operational policies, enhancements to associated databases, and future decision-making. Discussions and input on preliminary policy ideas at venues such as ILSG will help craft a published update to DCL policies and procedures. References Carter, M.J., Elsenheimer, D. and Arends, H., 2023. Minnesota Minerals Coordinating Committee Drill Core Library Workshop. Minnesota Department of Natural Resources, Lands and Minerals Division, OFR 411: 57. 24 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Digital Image Capture and Database Compilation of Historic Mining Data from the Keweenaw Copper District, Michigan: A Progress Update DeGRAFF, James1 and ROSE, William1 1 Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931 U.S.A. The Michigan copper rush starting at Copper Harbor in 1843 (Fig. 1) led to 150 years of mining that produced ~7.5 x 106 MT of copper (Bornhorst, T.J. and Barron, R.J., 2011), 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. Many companies invested significantly in trenching, coring, and mining operations that generated an enormous body of geologic information. USGS efforts in the 1940s and 1950s to map bedrock geology and to assess mineral resources have compiled much of this information as bedrock geology maps with supporting cross sections and reports. Though available online in various formats, these map products are the tip of an iceberg of original detailed source data that is not easily accessed. Significant exploratory drilling that postdates map publication has not been utilized for later geologic investigations because of the same difficulty of access. Paper records and microfiche that decay with time are stored at various locations, which further complicates their use. Many groups could benefit from improved access to this vast amount of information. Therefore, we began a ‘skunk-works’ project to identify and gather information into a digital image repository, to extract it into tabular databases, and to explore how to make it available to scientists, industry, land-use planners, and the general public. Figure 1. Michigan’s copper mining district with generalized bedrock geology. Figure modified from (Bornhorst, T.J. and Barron, R.J., 2011). Numbered field trip stops generally define the extent of copper mining and exploration between 1843 and present. Limited mining also occurred on Isle Royale just off the map to the 25 �Proceedings of the 69th ILSG Annual Meeting – Part 1 north. The initial phase of the project was to identify sources, access data, establish procedures, and demonstrate feasibility. We started with drill holes, trenches, and mine openings posted on USGS geology maps of the Keweenaw Peninsula. Features were symbolized in Google Earth from georegistered maps, assigned unique codes, and recorded with their data in tables having a common layout (Stage 1). Derivative tables contain data unique to a class, such as azimuth and inclination of drill holes found on core logs (Stage 2). Data captured up to this stage are useful for positioning and orienting features on maps and in subsurface models. Stage 3 captures geologic data as a function of location in a feature, e.g., distance along a drill hole. Such information, available from core descriptions at the Keweenaw National Historical Park (Keweenaw National Historical Park, 2016), the USGS/Denver Archives (White, W.S., 1985), old reports and plates, often requires careful transcription to extract it from image records. Other potential sources of such mining data include early reports of the Michigan Geological Survey, university archives, and private collections. Besides preserving and making these data available to others in an easy-to-access format, we hope to build subsurface models that can benefit research, mineral exploration, and land-use planning (Fig. 2). Figure 2. Possible uses of the database once it is further developed. Acknowledgements: We thank Ted Bornhorst (MTU), Jeremy Mason (KNHP), Bill Cannon (USGS), and Jenny Stevens (USGS) for making us aware of and facilitating access to the two archives that currently are being digitally captured and tabulated. This work is possible because of the foresight of many late geologists who gathered and preserved the original paper records. References 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 Mid-continent of North America: Geological Society of America Field Guide 24: 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. 26 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Geophysical mapping of the Great Lakes Tectonic Zone and surrounding Precambrian geology in the central Upper Peninsula, Michigan DRENTH, Benjamin J.1, CANNON, William F.2 1 2 U.S. Geological Survey, PO Box 25046, MS 973, Denver Federal Center, Denver, CO 80225 U.S. Geological Survey, 12201 Sunrise Valley Dr., MS 954, Reston, VA 20192 The Great Lakes Tectonic Zone (GLTZ) forms the boundary between the Wawa-Abitibi subprovince (north side) and Minnesota River Valley subprovince (south side) within the Archean Superior Province. The GLTZ is concealed for all of its 1100 km length, except south of Marquette in the central Upper Peninsula of Michigan (Sims, 1991; Sims and Day, 1993). Near KI Sawyer, it is exposed as a NW-striking, 2.3 km wide mylonite zone along a strike length of about 11 km, with a mylonitic foliation that dips steeply to the SW (Sims, 1993). The location extent of the GLTZ is unknown to the east where it is concealed beneath Paleozoic sedimentary rocks. We use legacy aeromagnetic data (Daniels et al., 2009) in combination with modern aeromagnetic data (Drenth and Brown, 2020) and ground gravity data to geophysically characterize the GLTZ and map its eastward extent under cover and map additional nearby covered Precambrian tectonic elements. Discontinuous NW-striking aeromagnetic gradients observed over the mylonite zone are interpreted to be produced by structurally juxtaposed rocks with varying magnetizations, and such relations are observed locally in outcrops. Mapping of similar gradients across the region shows that they are widely distributed, but have highest concentration within 3 km of the center of the GLTZ. Gravity data show a steep regional gradient along the GLTZ trend, which is likely produced by the juxtaposition of a dense greenstone belt on the north against lower-density gneisses and granites on the south. Using the distribution of aeromagnetic gradients, broader aeromagnetic patterns, and the regional gravity gradient, the GLTZ is interpreted to extend about 55 km under cover to the east, where it changes to an E-W strike and possibly NE strike (Fig. 1). Interpretations are less detailed and less certain east of the area covered by high-quality aeromagnetic data. Interpreted Paleoproterozoic features have similar strike as the GLTZ. This includes an undated dike swarm and an elongated trough of variably magnetic and dense Paleoproterozoic strata that extends from the Gwinn district southeast under Paleozoic cover. The trough is truncated on its southeastern margin by an interpreted extension of the Norway Lake fault. The GLTZ is terminated on the east by broad aeromagnetic and gravity highs produced by rocks of the buried eastern arm of the 1.1 Ga Midcontinent Rift. The intersection of the rift and the GLTZ is the location of a change in the strike of the rift from crudely N-S north of the GLTZ to NW south of the GLTZ. References Daniels, D.L., Kucks, R.P., Hill, P.L., and Snyder, S. L., 2009. Michigan magnetic and gravity maps and data: a website for the distribution of data: U.S. Geological Survey Data Series 411: http://pubs.usgs.gov/ds/ds411. Drenth, B.J., and Brown, P.J., 2020. Airborne magnetic survey, Iron Mountain-Chatham region, central Upper Peninsula, Michigan, 2018: U.S. Geological Survey data release, https://doi.org/10.5066/P91EF3CI. 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. Sims, P.K., 1993. Structure map of Archean rocks, Palmer and Sands 7.5-minute quadrangles, Michigan, showing Great Lakes tectonic zone: U.S. Geological Survey Miscellaneous Investigations Map I-2355, 1:24,000 scale. Sims, P.K., and Day, W.C., 1993. Great Lakes tectonic zone -- revisited: U.S. Geological Survey Bulletin 1904- 27 �Proceedings of the 69th ILSG Annual Meeting – Part 1 S, 11 p. Figure 1. Preliminary interpretations. 28 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Geophysical architecture of the Neoarchean Mentor anorthosite intrusive complex, northwestern Minnesota DRENTH, Benjamin J.1, BLOCK, Amy Radakovich2, HUDAK, George J.3, SOUDERS, A. Kate4, SAARI, Stacy5 1 U.S. Geological Survey, PO Box 25046, MS 973, Denver Federal Center, Denver, CO 80225 Minnesota Geological Survey, 2609 Territorial Road, St. Paul, MN 55114 3 Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, MN 55811 4 U.S. Geological Survey, PO Box 25046, MS 963, Denver Federal Center, Denver, CO 80225 5 Minnesota Department of Natural Resources, 1525 3rd Ave E, Hibbing, MN 55746 2 The ca. 2737 Ma (Souders, 2023) Mentor anorthosite intrusive complex (MAIC) lies near the northern margin of the Wawa subprovince of the Archean Superior Province, in an area of northwestern Minnesota where the Wawa, Quetico, and Wabigoon subprovinces are juxtaposed in close proximity (Fig. 1). The rocks of interest are entirely concealed by 10s to >100 m of unconsolidated Quaternary sediments and localized Cretaceous strata and saprolite. The MAIC comprises a large volume of megacrystic anorthosite, with a lesser volume of oxide-rich gabbros. The gabbros are known, from a single borehole intersection at ~70 m depth, to be enriched in vanadium (see http://minarchive.dnr.state.mn.us), and have further potential for chromium and titanium mineralization. New interpretations are based on data from an Earth Mapping Resources Initiative (MRI)-sponsored aeromagnetic survey flown in 2021 and pre-existing ground gravity data, constrained by approximately ten boreholes in the area. The anorthosite is weakly magnetized and dense, with a mean measured density of 2940 kg/m3, producing a 10-60 mGal gravity high. Pervasive epidote alteration is a suggested explanation for the high density of the anorthosite (the density of unaltered anorthite is 2730 kg/m3). The oxide-rich gabbros are strongly magnetized, producing aeromagnetic anomalies as large as 6000 nT, making them readily mappable across the complex. New geophysical interpretations (Fig. 1) suggest that the MAIC is significantly broader in extent than previously interpreted (Jirsa et al., 1999) and can be traced along strike for approximately double its originally interpreted length. The MAIC covers an area of about 640 km2 along a strike length of about 85 km, and forward modeling suggests a depth extent as great as 7 km. The MAIC is here interpreted to be the largest known anorthosite complex in the Superior Province, as measured by preserved extent in map view (cf. Sotiriou and Polat, 2020). The MAIC is observed in drill core to intrude a package of basalt flows at its northwest boundary and is itself intruded by multiple low-density felsic plutons that produce 10-20 mGal, 4-20 km wide gravity lows. The large felsic pluton along the southeastern margin of the MAIC is dated at 2702 ± 6.5 Ma (Souders, 2023), and is here called the Fertile pluton after the nearby town. This tectonomagmatic setting is consistent with other anorthosite complexes of the Superior Province, that commonly intrude packages of mafic volcanic flows and are themselves commonly intruded by felsic plutons (e.g., Polat et al., 2018). Disrupted trends and patterns of geophysical anomalies indicate that the MAIC was variably deformed, likely via both faulting and folding, in a complex fashion. 29 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Figure 1. Preliminary geophysical interpretations of geology surrounding of the Mentor anorthosite intrusive complex and surrounding area. Inset shows location of study area. References Jirsa, M. A., Chandler, V. W., and Runkel, A. C., 1999. M-092 Bedrock geologic map of northwestern Minnesota. Minnesota Geological Survey. Retrieved from the University of Minnesota Digital Conservancy, https://hdl.handle.net/11299/973. Polat, A., Longstaffe, F. J., and Frei, R., 2018. An overview of anorthosite-bearing layered intrusions in the Archaean craton of southern West Greenland and the Superior Province of Canada: implications for Archaean tectonics and the origin of megacrystic plagioclase: GEODINAMICA ACTA, v. 30, 1:84–99. https://doi.org/10.1080/09853111.2018.1427408. Sotiriou, P., and Polat, A. 2020. Comparisons between Tethyan anorthosite‐bearing ophiolites and Archean anorthosite‐bearing layered intrusions: implications for Archean geodynamic processes: Tectonics, v. 39: 35. https://doi.org/10.1029/2020TC006096. Souders A.K., 2023. 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. 30 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Multiple overlapping features spatially associated with lead-zinc-copper mineralization in the Highland quadrangles, southwest Wisconsin, USA FITZPATRICK1, William, and STEWART1, Eric 1 Wisconsin Geological and Natural History Survey, University of Wisconsin-Madison Division of Extension, 3817 Mineral Point Road, Madison, WI, 53705 Several features of the Paleozoic bedrock units of the Upper Mississippi Valley (UMV) leadzinc district have been spatially correlated with sulfide mineralization in the Sinnipee Group including folds and faults (e.g. Heyl et al., 1959) and paleovalleys in the base St. Peter unconformity surface (e.g. Mai and Dott, 1985). The significance of these features in creating fluid pathways with sufficient flow to explain the temperature anomalies associated with ore deposition has been justified by the modeling of Arnold et al. (1996). New detailed 1:24,000 scale mapping of two quadrangles in the Highland area created a detailed structural and stratigraphic framework for this area at the northernmost margin of the UMV district, with the results providing a case study allowing the precise geometry of factors such as fold zones and paleovalleys relative to lead zinc mineralization to be revealed. Numerous E-W and N-S trending fold zones with amplitudes of 20-60 ft were identified during mapping of the Highland quadrangles (Fig. 1). Lead-zinc mineralization as defined by the digitized mineral development atlas (MDA) mine maps (Pepp et al., 2019) is clustered on the margins of the synclines, most commonly found on gently sloping ramps below the crest of adjacent structural highs. The largest deposits in the Highland area are spatially associated with pit zones where the base Platteville drops for an additional 40-80 ft below the trough of the synclines over restricted elliptical areas. These pit zones are the site of the steepest folding observed in the mapped area, and may have been important for compromising the integrity of the overlying Maquoketa formation, providing a fluid pathway for migrating brines through this regionally important aquitard (Arnold et al., 1996). Numerous paleovalleys filled with St. Peter formation were identified during mapping (Fig. 2), with the largest in the southeast and southwest corners of the quadrangles mapped continuing down to the Jordan formation with the Prairie du Chien group entirely removed. By removing the Prairie du Chien group, these paleovalleys provide connectivity between the thick, lower Cambrian sandstone aquifer and the upper St. Peter aquifer, allowing large volumes of migrating brines to migrate upward in section towards the favorable ore host units in the Sinnipee Group (Arnold et al., 1996). In the Highland district, the likely flow paths from these paleovalleys to the places where the Maquoketa aquitard was compromised at the pit zones directly correspond to areas with known lead-zinc mineralization. References Arnold, B.W., Bahr, J.M., and Fantucci, R., 1996. Paleohydrology of the upper Mississippi valley zinc-lead district: Society of Economic Geologists Special Publication, no. 4: 378-389. https://doi.org/10.5382/SP.04.28. Heyl, A.V., Jr., Agnew, A.F., Lyons, E.J., Behre, C.H., Jr., and Flint, A.E., 1959. The geology of the Upper Mississippi Valley zinc-lead district: U.S. Geological Survey Professional Paper 309: 310 p., 24 pls., https://doi.org/10.3133/pp309. Mai, H., and Dott, R.H., Jr., 1985. A subsurface study of the St. Peter sandstone in southern and eastern Wisconsin: Wisconsin Geological and Natural History Survey Information Circular 47: 35 p., 2 pls., https://wgnhs.wisc.edu/catalog/publication/000297. Pepp, K., Siemering, G., and Ventura, S., 2019. Digital atlas of historic mining activity in southwestern Wisconsin, 40 p., https://learningstore.extension.wisc.edu/products/digital-atlas-of-historic-miningfeatures-and-potential-impacts-in-southwestern-wisconsin. 31 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Figure 1. 10ft structure-contour map for the base of the Platteville formation with interpreted fold axes marked by red lines with black outlines with arrows denoting synclines and anticlines. Green polygons mark surface diggings and blue polygons mark underground mine workings from the MDA data digitized by Pepp et al., 2019. Figure 2. Cross section running E-W through the southern part of the Highland quadrangles. Large black arrows mark likely flow paths for mineralizing fluids ascending from St. Peter paleovalleys (Oa) to pit zones which locally breach Maquoketa aquitard. Are serpentine fault mirrors an indicator of seismic slip? A microstructural analysis 32 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Are serpentine fault mirrors an indicator of seismic slip? A microstructural analysis GHANTOUS, Sam1, PHILLIPS, Noah 1, LUSK, Alex 2, NEWMAN, Julie 3, & JI, Shaocheng 4 1 Department of Geology, Lakehead University, Thunder Bay, ON, Canada Department of Geology & Geophysics, Texas A&M University, College Station, TX, USA 3 United States Geological Survey, Denver, CO, USA 4 Department of Civil, Geological and Mining Engineering, École Polytechnique, Montréal, QC, Canada 2 Fault mirrors are smooth, sheened surfaces along a fault plane. An array of microstructures may produce a fault mirror which each have respective formation mechanisms and associated slip velocities. Fault mirrors in certain compositions may be an indicator of ancient earthquakes with seismic slip velocities, but not all fault mirrors are associated with seismic slip. We study the microstructures of two serpentine mirror surfaces, which have not yet been described in the literature, to determine their formation mechanisms and to assess whether they serve as indicators of paleo-seismic slip. One sample is a medium green mirror surface from a late normal fault cutting dunites from the Twin Sisters complex, Washington State, USA. The second mirror surface is pale green and cuts a serpentinite from the Thetford Mines ophiolite in Quebec, Canada. Both fault mirrors have slickenlines on their surfaces indicating that they formed during slip. The mirror surface from the Twin Sisters complex consists of a ~2 micron thick, potentially amorphous, low asperity serpentine layer which may have formed during seismic slip. The mirror surface from the Thetford Mines ophiolite consists of a ~0.5 centimeter-thick layer which is composed of radiating serpentine microcrystallites which are ~ 1 micrometer in length and 10’s to 100’s of nanometers in width. These serpentine microcrystallites are interpreted to have crystallized from a serpentine gel phase during slip. While we hypothesize that these samples are both indicative of seismic slip, similar structures may form if serpentine gels crystallize during aseismic creep. Serpentine fault mirrors may represent paleo-seismic slip, but a microstructural examination of the mirror surface is required to establish a seismic origin. Figure 1. SEM photomicrographs of radiating serpentine microcrystallites from the Thetford Mines ophiolite fault mirror. 33 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Characterizing volcanic host stratigraphy and syn-volcanic intrusions at the Lynne Zn-Pb-Cu deposit, Oneida Co., Wisconsin GLODOWSKI, Lillian N. 1, LODGE, Robert W.D. 1 1 Department of Geology, University of Wisconsin-Eau Claire, 101 Roosevelt Avenue, Eau Claire, WI 54701 The Lynne Zn-Pb-Cu deposit in Oneida County, Wisconsin is one of several volcanogenic massive sulfide (VMS) deposits located within the understudied Paleoproterozoic (1.8-1.9 Ga) Penokean Volcanic Belt (PVB). The PVB formed as the Marshfield and Pembine-Wausau terranes collided and accreted onto the Superior Craton during the Penokean orogeny (Schulz and Cannon, 2007). VMS deposition in Wisconsin has been interpreted to be associated with continental back-arc rifting in a submarine environment. However, little data is available on the deposit-level at Lynne and other deposits in the PVB to test this model. Volcanic and tectonic variability in VMS forming environments and the effect of basement inheritance on metallogeny are important for district-scale exploration. This study constrains the volcanic and tectonic setting at the Lynne deposit via trace element systematics and aims to improve regional metallogenic models in the PVB. Historically, the Lynne deposit was subdivided by Adams (1996) based upon their relative stratigraphic position to the ore horizon into upper and lower “Rhyolite”, “Dacite”, and “Volcaniclastic (VCS)” with mineralized zones occupying the lower VCS unit (Figure 1A). This study relogged seven drill holes from the Lynne deposit and sampled for petrographic and geochemical analyses. The new geochemical data presented in this study reveals there are no petrochemical differences between the upper and lower host strata (Figure 1B). There were also no petrochemical differences observed between the volcanic host rocks and the intruding footwall granodiorite. Therefore, the rocks in this study have been subdivided based simply upon composition and petrography. The volcanic rocks which host the Lynne deposit are comprised primarily of medium to dark grey felsic to intermediate lapilli and crystal tuff. The sedimentary rocks at the Lynne deposit are observed to be very fine-grain, dark grey siltstones with thin parallel laminations and are assumed to be volcanically derived. The Lynne deposit is intruded by a pluton of medium-grained granodiorite which disrupts the lower massive sulfide lenses. The granodiorite appears in a variety of colors ranging from pink and orange to grey and white. Smaller mafic and felsic dikes also crosscut the Lynne deposit. The mafic dikes are dark grey to green with a fine-grain mafic matrix and feldspar phenocrysts. Felsic dikes are commonly light to medium grey with a fine-grain felsic matrix. The geochemical data indicates that VMS deposition at the Lynne deposit occurred in a bimodal-felsic petrochemical assemblage consistent with a continental setting. The shared FII-type lithogeochemistry of the felsic volcanic rocks, granodiorite pluton, and felsic dikes suggests these rocks formed under similar extensional, shallow crustal conditions and originated from the same magmatic system. Combined with the lack of a metamorphic aureole around the pluton, the intruding footwall granodiorite is likely the syn-volcanic intrusion which eventually intruded its own volcanic pile (Galley et al., 2003). Improved geochemical and petrographic data on the Lynne deposit will allow for more accurate and improved models which can be compared to other deposits throughout the PVB and around the world. 34 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Figure 1. A) Geologic cross section of the Lynne deposit highlighting host stratigraphy, bore hole traces, approximate sample locations, and mineralized zones. Modified from Kennedy (1997). B) Rock type classification diagram of the Lynne. Diagram from Pearce (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: 161179. Galley, A.G., 2003. Composite synvolcanic intrusions associated with Precambrian VMS-related hydrothermal systems: Mineralium Deposita, v. 38: 443–473. Kennedy, L.P., 1997. Summary geologic and geotechnical report for the Lynne project Oneida County, Wisconsin, U.S.A., Unpublished report of Noranda Minerals Wisconsin Corp.: 26. Pearce, J.A., 1996. A users guide to basalt discrimination diagrams, Trace Element Geochemistry of Volcanic Rocks: Applications for Massive Sulphide Exploration. Geological Association of Canada, Short Course Notes 12: 79-133. Schulz, K.J. and Cannon, W.F., 2007. The Penokean orogeny in the Lake Superior region. Precambrian Research, 157: 4-25. 35 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Identifying regional exploration domains for Ni-Cu-PGE deposit types in the Midcontinent Rift GOOD, David1 1 Department of Earth Sciences, Western University, London, ON N6A 5B7 Canada A new classification strategy for Midcontinent Rift basalts and associated gabbro and ultramafic rocks is proposed, the main objective being to identify magmatic suites associated with known Ni-Cu-PGE occurrences and their spatial distribution across the rift. The study is based on the idea that units with similar incompatible trace element signatures formed under similar conditions in a similar mantle source region and had been subjected to similar contamination or fractionation processes. Elements used in this study are REE, Th, Nb, and Zr. The approach taken is to identify point cloud clusters (magmatic suites) on contoured point density plots for REE represented by ‘lambda’ parameters which emphasize slope and curvature of REE patterns. The resultant groups are checked in Gd/Yb vs. Th/Nb and Gd/Yb vs. La/Sm diagrams which identify influence by crustal contamination or clinopyroxene fractionation, respectively. Melts produced in a metasomatised mantle source are a special case and are distinguished from contaminated melts in a Zr-Th-La diagram. The data set comprises a total of 1815 samples, 343 of which are basalt, from 70 mafic units. Data are carefully screened for discrepancies and extreme outliers removed. Results indicate a total of eight distinct magmatic suites (Groups 1 to 8). The groups are not listed in stratigraphic order because many units appear simultaneously, and a few are active for long time periods during the MCR event. Highlights of the study with respect to Ni-Cu-PGE mineralized intrusions include: a) Group 1 includes the Current, Seagull and Thunder Intrusions and the Lower Suite basalts of the Osler Volcanic Group; b) Group 2 is the most voluminous and includes the Duluth, Tamarack and Crystal Lake deposits, the Pigeon, Cloud and Arrow intrusions, and basalts of the Greenstone Flows, Upper Suite at Black Bay (OVG) and Upper Groups A and B at Mamainse Point; c) The Eagle deposit is intermediate between Groups 2 and 5 but overlaps the field for all flows in Lower Mamainse Point Group A; d) Group 7 includes the Two Duck Lake (Marathon deposit), Abitibi Dykes and metabasalt unit 3a; e) Group 8 includes the Geordie Lake deposit, Wolfcamp basalt, Copper Island dykes and a few of the Pukaskwa dyke swarm; and f) Groups 3 and 4 are not, as yet, associated with mineralized intrusions and includes the Nipigon sills and basalts of the Centre and Upper Suites of OVG. A map of the Midcontinent Rift showing regional domains for each Group is presented, highlighting the extent of igneous rock domains for each of the known Ni-Cu-PGE deposit types, and their locations relative to the central axis of the MCR. 36 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Exploring the geology of the Midcontinent Rift under western Lake Superior using a preliminary velocity model of seismic line GLIMPCE C GRAUCH, V.J.S.1, HELLER, Sam J.2, STEWART, Esther K.3, and WOODRUFF, Laurel G.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 Wisconsin Geological & Natural History Survey, 3817 Mineral Point Road, Madison, WI 53705 4 U.S. Geological Survey, 2280 Woodale Drive, Mounds View, MN, 55112 2 Seismic-reflection data were collected in the 1980s as part of the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE) to investigate the 1.1 Ga Midcontinent Rift System (MRS). GLIMPCE Line C crosses western Lake Superior from north to south shores (Fig. 1 inset). Many previous workers have interpreted the MRS in Line C as an asymmetric central graben filled with 10–20 km of subaerial basalt flows, overlain by 7-10 km of sedimentary section, and underlain by magmatic underplating. The central graben was interpreted to have formed from extensional normal faults, later reactivated as high-angle reverse faults. The northern part of Line C crosses over a prominent gravity low called the Grand Marais Ridge (GMR; Fig. 1 inset), previously interpreted as an Archean granitic basement high. Line C interpretations are commonly shown on a section plotted against two-way travel time along with a crudely estimated depth scale. We are undertaking a more rigorous approach by developing a detailed velocity model for time to depth conversion. The modeling for Line C is guided by velocities resulting from a pre-existing seismic refraction study, intervals defined by seismic horizons, and correlation with velocity models from neighboring seismic-reflection lines. Velocities are verified using common-reflection point gathers from pre-stack depth migration. Several salient points about the MRS can be gleaned from the preliminary velocity model alone (Fig. 1). The north and south sides of the model are dissimilar, reflecting the disparate geology of the north and south shores. On the south side, we identify an outline reminiscent of a bird (Fig. 1) that helps focus discussion without implying any geologic significance. Aided by a land-based seismic line near the southeast end of Line C, we can tentatively identify the geologic units under the lake within the bird outline (Fig. 1) and interpret a sag basin rather than a graben. The basin contains inferred Porcupine Mountains Volcanics (PM; 6.1 km/s), Portage Lake Volcanics (PLV; 5.9 and 6.5 km/s), with older, possibly reversed magnetic polarity, volcanic units at the base (6.9 km/s). A thick gabbroic sill (6.8 km/s) is inferred within the PLV section. We interpret the truncated PLV (5.9 km/s) and PM (6.1 km/s) intervals at the bird’s head to represent an eroded cliff face of the tilted northern limb of the sag basin. Sheet-like mafic intrusions (7.1 km/s) arise from the lower crust/upper mantle (7.2 km/s) and diverge upwards, following the geometry of the central sag basin. The interpretation that these 7.1 km/s units represent discontinuous or only partially evident magmatic feeder zones is based on their high velocities and sheet-like forms, which in part are constrained by neighboring industry seismic sections. The sedimentary section above the sag basin includes the Oronto Group (3.4, 4.7, 5.2, and 5.6 km/s) and likely Bayfield Group (3.0 km/s). An angular unconformity between Oronto Group (5.6 km/s) and underlying PM (6.1 km/s) at the bird’s head indicates the north limb of the sag basin was tilted prior to deposition. In contrast, the units on the south limb appear conformable. Using aeromagnetic patterns that lead from the north shore into the lake, we tentatively identify a highly reflective (not shown) 4.7 km/s interval as rhyolites of the upper northeast sequence of the North Shore Volcanic Group (NSVG). This unit is interpreted to be angularly unconformable with overlying sedimentary rocks of the same velocity (4.7 km/s). The 5.6 km/s and 6.5 km/s intervals 37 �Proceedings of the 69th ILSG Annual Meeting – Part 1 beneath the interpreted rhyolites are likely older NSVG volcanic rocks that form a carapace over the GMR. The 6.1 km/s velocity of the GMR corroborates its interpretation as a granitic basement high. The model indicates that a 5.6 km/s unit (NSVG?) dives below the bird outline to depths below 15 km. Whether this unit is connected to deeper parts of the sag basin or separated by faulting is obscured by the 7.1 km/s sheet-like intrusions. The sedimentary section on the north side of the model tilts to the south, unconformably overlies volcanic rocks (4.7 and 5.2 km/s) and is truncated by the overlying 3.0 km/s interval (Bayfield Group or equivalent). The sedimentary package on the north side collectively has lower velocities and is thinner than the sedimentary package on the south side. It is unclear if the northern section is correlative with the Oronto Group or represents less consolidated, younger rocks, possibly eroded from the NSVG or basalts at the bird’s head. Identification of velocity intervals and their relations at and under the bird’s head are key to understanding the tectonomagmatic picture but remain somewhat obscure. Suffice to say for Line C that magmatism and syn-magmatic subsidence played a greater role in the origins of the MRS than previously realized. Moreover, unconformable relations within the sedimentary package may be evidence of multiple post-magmatic tectonic events. Figure 1. Preliminary velocity model for GLIMPCE Line C showing velocity intervals in km/s. Inset map shows Line C in relation to the Grand Marais Ridge and neighboring seismic lines in Lake Superior. The white dashed line outlines a bird-like pattern to guide discussion. Velocities near the bird’s head are interfingered only to provide a smooth transition for the depth migration; lines are drawn to better represent the form of the depthconverted seismic horizons, which are not shown for simplicity. Vertical exaggeration=2. 38 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Petrography, geochemistry, and mineralization of the Archean Titan (Roaring River) intrusion, Northwestern Ontario GROENEVELD, Tianna1, HOLLINGS, Peter1, BAIN, Wyatt1, DJON, Lionnel2 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 Archean Titan intrusion, formerly known as the Roaring River mafic intrusion, is one of several mafic-ultramafic complexes in northwestern Ontario that are currently the focus of ongoing PGE exploration. The Titan intrusion is located ~145 km North of Thunder Bay, Ontario, in the Winnipeg River terrane of the western Superior Province and is part of the Roaring River Complex (Figure 1). The Titan intrusion was identified as an underexplored area during a lake sediment survey in 2000 (Ontario Geological Survey). In the following five-year period, there were several periods of prospecting and soil surveys carried out in the area as well as one diamond drilling project, which aimed to determine the extent of the Titan intrusion within the Roaring River Complex and assess the potential for economic Ni-Cu-PGE mineralization. This early exploration revealed petrologic and geochemical similarities between the Titan intrusion and the mineralized mafic-ultramafic rocks in the Lac des Iles (LDI) Complex, which lies ~60 km to the south of the Titan intrusion (Figure 1). The LDI Complex is the largest of a series of mafic and ultramafic intrusions known as the LDI suite, all within the Marmion terrane, and hosts the world-class LDI palladium mine. An unpublished U-Pb age for zircons from the Titan intrusion yielded an age of 2690 ± 3.2 Ma, broadly coeval with the LDI Complex, dated at 2689 ± 1.0 Ma (Heaman and Easton, 2006). Outcrop across Titan is sparse, due to the presence of pervasive glacial till and Proterozoic diabase sills. Samples were collected in the summer of 2021 and analyzed for whole rock and PGE geochemistry, sulphur and Sm-Nd isotope analysis, and detailed petrographic characterization. The intrusion consists of a mix of lithologies, ranging from pyroxenites to gabbros to leucogabbros. The lithologies are distributed throughout the intrusion and suggest a simple magma body, where one pulse of magma underwent fractional crystallization within a closed system. Sulphide mineralization is generally confined to pyrite and chalcopyrite, though inclusions of pyrrhotite were observed occasionally. Sulphide mineralization is typically fine-grained and disseminated, though larger blebs do occur, usually of either pyrrhotite or chalcopyrite. The pyrite is considered to be a hydrothermal phase, likely formed from secondary precipitation while the larger blebs of pyrrhotite are considered to be a primary magmatic phase. The Titan intrusion is characterized by enriched LREE’s and fractionated HREE’s, with negative Nb, Zr, Hf, and Ti anomalies (Figure 2). Titan samples have a range of (La/Sm)N from 0.7 to 3.8, a range of (Gd/Yb)N from 2.3 to 7.4, and a range of Nb/Nb* values from 0.02 to 0.47. The geochemistry behavior is consistent with formation in a supra subduction zone setting, which fits with the regional setting of the Winnipeg River and Marmion terranes during this time period (~2.74-2.69 Ga). Only small amounts of crustal material appears to have been incorporated, based on εNd values of 0.70 to 1.82, compared to an estimated depleted mantle at 2.7 Ga which would have a εNd value of +3. Titan appears to be a simple intrusion when compared to intrusions of similar size in the LDI suite and many of the similarities between Titan and the LDI suite appear to occur from regional characteristics of the area in this time period (~2.74-2.69 Ga). 39 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Figure 1. (left) A regional geology map of the western Superior Province highlighting the approximate locations of the Titan intrusion, the Lac des Iles Complex, and the city of Thunder Bay, modified from Stott et al., 2010. Figure 2. (below) Primitive mantle normalized spider plot, showing representative values for oceanic island basalts (OIB), continental arc, oceanic arc, and the span of values for Titan. Concentrations normalized to primitive mantle from Sun and McDonough (1989) OIB from Sun and McDonough (1989), continental and oceanic arcs from Kelemen et al. (2014). References Heaman, L.M. and Easton, R.M., 2006. Preliminary U/Pb geochronology results: Lake Nipigon Geoscience Initiative. Ontario Geological Survey, Miscellaneous Release-Data 191. Kelemen, P.B., Hanghøj, K., Greene, A.R., 2014. One View of the Geochemistry of Subduction-Related Magmatic Arc, with an Emphasis on Primitive Andesite and Lower Crust. Treatise on Geochemistry, vol. 4: 749-806. Ontario Geological Survey., 2000. Garden-Obonga Lake Area Lake Sediment Survey: Gold and PGE Data; Open File Reports 6028: 76. Stott, G.M., Corkery, M.T., Percival, J.A., Simard, M., 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: 20-1 to 20-10. 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, vol. 42: 313-345. 40 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Determining Provenance of Rainy Lobe Till using Geochemistry and Detrital Zircon Geochronology. HINKEMEYER, Audray M.1, MOOERS, Howard D.1, and LARSON, Phillip C.2, O’SULLIVAN, Paul B.3 1 Department of Earth and Environmental Sciences, University of Minnesota Duluth, Duluth, MN 55812 Vesterheim Geoscience, PLC, Hibbing, MN 3 GeoSep Services, 1521 Pine Cone Road, Moscow, Idaho 83843 2 Till of the Late Wisconsin Rainy lobe (RL), which emanated from the Labradoran sector of the Laurentide ice sheet, is exposed at the surface from SW Minnesota to the extreme NE part of the State. The RL advanced to its maximum limit in southwestern Minnesota well prior to the Last Glacial Maximum (ca. 27-30 ka BP) and retreated into Ontario by 17.9 ka BP. This till exhibits dramatic spatial and temporal changes in provenance from the Hewitt till of SW Minnesota to the Independence till in the NE. While texture, fabric, and physical properties are similar, lithologic changes include a decrease in carbonate and greywacke of the Omarolluk Fm. with an increase in mafic rocks of the Duluth Complex as the ice retreated. The observed change in lithology reflects changes in the mean transport length (MTL) of the till. The MTL is the average distance of transport defined by the indicator lithology abundance. The Hewitt till has a mean transport length of > 1000 km, whereas the Brainerd and Independence tills have mean transport lengths of approximately 400 and 100 km, respectively (Berthold, 2015). Two models have been proposed to explain the lithological differences (particularly carbonate) in RL tills. Goldstein (1989) postulated that the downglacier increase in carbonate in the Hewitt till was the result of progressive incorporation, by regelation or deformation, of older underlying till that was rich in carbonate. However, Goldstein also postulated an accretionary origin for the Wadena drumlins, which would imply continuous deposition rather than erosion. This subglacial erosional vs. depositional paradox remains unresolved. Larson (2008) concluded that the changes in sedimentology and landforms record systematic changes in provenance related to changing basal boundary conditions in the interior of the LIS. As the RL advanced early in the last glacial cycle, a continuous till sheet composed of sediment from Hudson Bay and the Hudson Bay lowlands (HBL) extended to SW MN. As the ice approached its maximum limit, much of this till sheet was then eroded exposing Canadian Shield bedrock along the central portion of the flow path (Fig. 1). Early in this phase of glaciation, the sediments reflect long-distance transport from Hudson Bay, and later phases reflect increased proportions of felsic shield lithologies and Duluth Complex rocks. These two models of Rainy lobe till sedimentology are evaluated using mixing models, till matrix geochemistry, and detrital zircon geochronology. The tills underlying the Hewitt till are typically finer textured and contain significant concentrations of Cretaceous age carbonates and shales. Therefore, a multicomponent mixing model is developed to examine sedimentological variability by incorporation of older, underlying tills (e.g. Goldstein, 1989). To evaluate the model of Larson (2008), which implies long vs. short transport distances, twenty-eight samples collected along a transect from SW to NE Minnesota, and six samples collected from the HBL, were processed and sent for geochemical analysis. Fifteen of these samples were processed and analyses for detrital zircon geochronology using laserablation, ICPMS. 41 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Results of a 48-element analytical suite along with latitude, longitude, and depth were run through a principal component. The first 3 factors were retained for analysis. Factors 1 and 3 distinguished mafic vs felsic igneous rock geochemical signatures and carbonate content, respectively. Factor 1, felsic vs. mafic lithologies, can be used as a proxy for MTL and shows locally vs distally derived lithologies. Factor 3 distinguishes tills based on carbonate content. Core SLL (Independence till) plots positively on factor 1 indicating a short MTL. Core CSS (Brainerd till) represents an intermediate MTL, while cores UMRB and TG (Hewitt till) SW of the Wadena drumlin field have the longest MTL. In addition, the samples with the longest MTL plot in high carbonate space, positive on Factor 3. Detrital zircon age populations represented on probability density plots show that the shortest MTL Figure 1. Factor 1 (MTL) vs. factor 3 (carbonate content). samples have the highest signature of local 1.1 Ga MidContinent Rift zircons. A Kolmogorov-Smirnoff (K-S) test statistically compares age populations and determines if they are statistically different. Results from the K-S test reveal that HBL ages are statistically similar to samples from central Minnesota (core CSS). The mixing model, indicates that the Hewitt till is not a mixture low-carbonate RL till and older underlying tills. Geochemistry, and detrital zircon analyses support the model of Larson (2008). Early deposits of the RL in SW Minnesota are geochemically similar to the high-carbonate HBL samples, indicating a distal provenance. This similarity is also observed in the detrital zircon results from the K-S test. Subsequently younger deposits lose the HBL signature and start to incorporate more felsic craton and eventually mafic signatures of the Mid-Continent rift system. References Berthold, A.J., 2015. Surface Boulder Concentrations of the Late Wisconsinan Rainy Lobe, Minnesota, USA. M.S. Thesis, University of Minnesota Duluth: 48. Goldstein, B.S., 1985. Stratigraphy, sedimentology, and late-Quaternary history of the Wadena drumlin region, central Minnesota: Minneapolis, University of Minnesota, Ph.D. dissertation: 216. Larson, P.C., 2008. Quantification of Glacial Sediment Erosion, Entrainment and Transport Processes and Their Implications for the Dynamic History of the Laurentide Ice Sheet. Ph.D. Dissertation, University of Minnesota: 76. 42 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Copper-rich melt inclusions from the St. Ignace Island Complex: Implications for magma mixing and mineralization HOLLINGS, Pete1, HANLEY, Jacob2, SMYK, Mark1,3, HEAMAN, Larry4, and COUSENS, Brian5 1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1 Canada 2 Department of Geology, Saint Mary’s University, 923 Robie Street, Halifax, NS, B3L 2Y5 Canada 3 Ontario Geological Survey, Ministry of Mines, Suite B002, 435 James St. South, Thunder Bay, ON P7E 6S7 Canada 4 Department of Earth & Atmospheric Sciences, University of Alberta, 126 Earth Sciences Building, Edmonton, AB, T6G 2E3, Canada 5 Ottawa-Carleton Geoscience Centre, Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa. Ontario, K1S 5B6, Canada The St. Ignace Island Complex (SIC) comprises volcanic and intrusive rocks that were emplaced the upper portions of Midcontinent Rift-related, ca.1008 Ma Osler Group volcanic rocks (Davis and Sutcliffe 1985; Fig. 1). The St. Ignace Island complex is a ~26 km2 stock with a core of quartz-feldspar-phyric rhyolites and dacites and an outer ring of anorthosite and gabbro (Sutcliffe and Smith 1988; Giguere 1975). The petrology and geochemistry of the SIC has been described by Smyk et al. (2006) and Hollings et al. (2023). The pink to grey, felsic rocks at the center of the complex are quartz-phyric, with rare pyroxene and feldspar phenocrysts. Textures at a variety of scales show evidence of the mingling and mixing of partially crystallized mafic and felsic liquids in SIC rocks. Mafic and felsic liquids may be incipiently mixed, resulting in partially disaggregated mafic enclaves hosted in a felsic matrix. With progressive mixing, the felsic volcanic domains in the rock become darker and phenocrysts of quartz and alkali feldspar appear embedded in the mafic Figure 1. (A) Map of upper Great Lakes. (B) Regional domains. In the most intensely mixed geology of the St. Ignace Island complex. Age data samples, small, mafic crystalline clots are (black stars) from Davis and Sutcliffe (1985) and Davis dispersed throughout a felsic matrix, and as and Green (1997). (C) Geological map of St. Ignace rare mafic enclaves, consisting of only a thin Island, modified after Giguere (1975). rind of mafic rock surrounding coarsegrained plagioclase phenocrysts. Well-preserved silicate melt inclusions (MI), many completely glassy, were observed in quartz, clinopyroxene and some plagioclase phenocrysts from the felsic and mafic rocks of the SIC, representing some of the oldest unrecrystallized silicate melt inclusions recognised to date. Melt inclusions from quartz from the felsic rocks are broadly rhyolitic in composition whereas those from 43 �Proceedings of the 69th ILSG Annual Meeting – Part 1 plagioclase in the mafic rocks range from basalt to basaltic andesite. The melt compositions are interpreted to represent the end-member liquids in the system with direct evidence of mixing of the two. Concentrations of Cu and Ag (in both mafic and felsic MI), and Mo (in felsic MI), are up to an order of magnitude higher in both the mafic and felsic MI than in continental crust and the host bulk rock concentrations. We propose that the melt inclusions have preserved pre-eruptive metal tenors that were subsequently modified by sulfide saturation, degassing, or post-solidus hydrothermal alteration. The elevated Cu and Ag contents are similar to those noted in arc-related and extremely oxidized early Midcontinent Rift-related rocks and may account for the world-class volcano-sedimentary-hosted Cu(Ag) deposits within the Rift as well as the presence of small, porphyry-style deposits. References Davis, D.W., and Green, J.C., 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution; Canadian Journal of Earth Sciences, v.34: 476488. 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: 1572-1579. Giguere, J.F., 1975. Geology of St. Ignace Island and adjacent islands, District of Thunder Bay; Ontario Division of Mines, Geological Report 118: 35. Hollings, P., Hanley, J., Smyk, M., Heaman, L., and Cousens, B., 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, in review. Smyk, M., Hollings, P., and Heaman, L., 2006. Preliminary investigations of the petrology, geochemistry and geochronology of the St. Ignace complex, Midcontinent Rift, Northern Lake Superior, Ontario. In Wilson, A.C. (ed.), Proceedings and Abstracts, Institute on Lake Superior Geology 52nd Annual Meeting, Proceedings Volume 52, Part 1 – Program and Abstracts, 61-62. Sutcliffe, R.H., and Smith, A.R., 1988. Geology of the St. Ignace Island volcanic-plutonic complex; Summary of Field Work and Other Activities, Ontario Geological Survey, Miscellaneous Paper 141: 368-371. 44 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Hydrothermal Alteration Facies of the Eisenbrey Zn-Cu Deposit, Rusk County, Wisconsin JOHNSON, Kaine, P. 1, and LODGE, Robert W.D. 1 1 Department of Geology and Environmental Sciences, University of Wisconsin-Eau Claire, Eau Claire, WI This study focuses on the hydrothermal alteration zones surrounding the volcanogenic massive sulfide (VMS) Eisenbrey Zn-Cu deposit in Rusk County, northwestern Wisconsin. The Eisenbrey deposit is hosted within the Paleoproterozoic Pembine-Wausau terrane and is a part of the Penokean volcanic belt, along with many other VMS deposits including the Crandon, Lynne, and Flambeau deposits. The goal of this research is to develop a petrographic and geochemical categorization of alteration types and complete a geochemical mass balance to produce specific alteration trends. Data collected on the hydrothermal alteration at the Eisenbrey deposit is being compared with other Wisconsin VMS deposits to produce a better depositional framework for VMS mineralization. The Penokean Orogen is the culmination of various accretionary events and volcanism. The Penokean Orogen began around 1.88 Ga along the southern margin of the Superior Craton. The collision and subsequent accretion of the Pembine-Wausau terrane resulted in subduction moving to the south and began back arc basin development. Most VMS deposits within the Penokean volcanic belt formed within this back arc extensional environment (Shultz and Cannon 2007). Arc magmatism continued until roughly 1.85 Ga. when an Archean crustal fragment, known as the Marshfield terrane, accreted to the Pembine-Wausau terrane & Superior Craton. VMS systems are characterized by volcanic-sedimentary hosted massive sulfide deposits that form at or near sea floor. Formation is associated with convection of metal rich hydrothermal fluids rising through the crust and mobilizing elements. These deposits are commonly poly-metallic with common mineralization of Zn-Cu-Pb-Ag-Au rich sulfides. Hydrothermal alteration in VMS environments results in mobilization of major elements during modification of primary minerals. The style of alteration varies based on the volcanic setting and fluid chemistry, but commonly are noted by gains in MgO, Fe2O3, K2O, and/or SiO2 and losses in Na2O and CaO (Galley et al., 2007). The Eisenbrey deposit (Figure 1) is relatively poorly understood. Regional metamorphism at the Eisenbrey deposit is lower amphibolite facies and has completely recrystallized the alteration zone at the deposit. Eisenbrey deposit is the only known VMS occurrences associated with Algoma-type iron formation and formed within the “Main Arc Sequence” (DeMatties, 2022). Therefore, improving our understanding of the Eisenbrey hydrothermal system can aid in identifying new exploration criteria in non-typical VMS environments for the Penokean Orogen. Samples of the hydrothermal alteration zone at the Eisenbrey deposit were analyzed across twelve drill holes from both the structural hanging wall and footwall to the ore horizon. These samples were initially divided into alteration mineral assemblages based on petrography. Alteration types include chlorite-cordierite-anthophyllite, quartz-anthophyllite-biotite, quartz-white mica, quartz-biotite. These alteration types were then characterized using major and trace element geochemistry and mass balance calculations. The alteration at Eisenbrey has notable gains in Fe2O3 and MnO; with losses in SiO2, MgO, and Na2O. This contrasts alteration at Flambeau, which has gains in K2O and SiO2 (Lodge et al., 2022). 45 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Figure 1. Representative cross-section of the Eisenbrey deposit with representative photomicrographs of common alteration types (right). I. shows Quartz-Anthophyllite alteration (T-22), II. shows Chlorite-Cordierite alteration (T-40), III. Shows Quartz-White Mica alteration (T-22) 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. Galley, A.G., Hannington, M.D., and Jonasson, I.R., 2007. Volcanogenic massive sulphide deposits, in Goodfellow, W.D., ed., Mineral Deposits of Canada: A Synthesis of Major Deposit-Types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods: GAC-MAC, Special Publication No. 5: 141-161. Lodge, R.W.D., Lemke, T.C., Blotz, K.E., 2022. Using Ore Petrography and Geochemical Mass Balance to Constrain the Hydrothermal Environment at the Paleoproterozoic Flambeau Cu-Zn-Au Deposit, Wisconsin, USA. Society of Economic Geology, Society of Economic Geologist Annual Meeting Proceedings, Denver, CO, paper P2.15. Schulz, K.J., and Cannon, W.F., 2007. The Penokean orogeny in the Lake Superior region: Precambrian Research, v. 157: 4–25. 46 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Provenance patterns and tectonic styles of ca. 2.3–1.8 Ga metasedimentary strata in northern Michigan based on regional mapping and detrital zircon U-Pb geochronology JONES, Jamey1, CANNON, William F.2, DRENTH, Benjamin J.3, and O’SULLIVAN, Paul4 1 U.S. Geological Survey, Alaska Science Center, Anchorage, AK U.S. Geological Survey, Geology Energy Minerals Science Center, Reston, VA 3 U.S. Geological Survey, Geology, Geophysics, and Geochemistry Science Center, Denver, CO 4 GeoSep Services LLC, Moscow, ID 2 Detrital zircon U-Pb data from ca. 2.3–1.8 Ga metasedimentary successions in northern Michigan are used to test regional stratigraphic correlations and yield key insights into provenance and tectonic styles along the southern Superior craton. Circa 2.3–2.2 Ga Chocolay Group turbiditic strata and quartzite record initial rifting and basin formation along the southern Superior margin. Unimodal ca. 2.7–2.6 Ga age populations were derived from abundant Archean batholiths in the surrounding region. Distinctive ca. 2.3 Ga populations are rare but present in some samples, but the source(s) of these grains is not well understood. Chocolay Group detrital zircon data are very similar to upper Huronian Supergroup strata to the east and with other global ca. 2.3–2.2 Ga glaciogenic successions. The ca. 2.1 Ga Dickinson Group contains bimodal ca. 2.9 and 2.7 Ga age populations in the East Branch Arkose and Solberg Schist that are distinctive in the region and suggest a mixture of recycled 2.3 Ga Chocolay Group quartzite and more diverse regional Archean basement sources. Minor ca. 2.1 Ga grains indicate derivation from nearby plutonic sources or eroded volcanic equivalents of the same age, consistent with magmatism, regional uplift, and final rifting of the southern Superior craton ca. 2.1. After a ca. 100 Ma hiatus, the Ajibik and Siamo Formations of the ca. 1.90–1.85 Menominee Group have unimodal ca. 2.7–2.6 Ga age populations that suggest continued derivation from ca. 2.7– 2.6 Ga batholiths and (or) recycling of older underlying strata. The Goodrich Formation of the basal Baraga Group (ca. 1.85–1.83 Ga) shows similar patterns. A provenance shift to prominent ca. 1.85 Ga populations occurs in turbiditic strata of the Michigamme Formation (upper Baraga Group), indicating arrival of the outboard Wisconsin magmatic terrane to the south. Michigamme strata record basin evolution between the southern Superior Province and the exotic terrane as it approached and collided during the ca. 1.87–1.83 Ga Penokean orogeny, but the relative role of Penokean versus younger ca. 1.78–1.76 Ga tectonism in regional folding and metamorphism remains uncertain. Additional mapping and geochronology focused on Michigamme strata will better constrain regional depositional ages, facies relationships, and tectono-metamorphic patterns. 47 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Petrogenesis of the mineralized horizons in the Offset and Creek zones, Lac des Iles Complex, N. Ontario JONSSON, Justin1, HOLLINGS, Peter1, BRZOZOWSKI, Matthew1, BAIN, Wyatt1, DJON, Lionnel2 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 48 �Proceedings of the 69th ILSG Annual Meeting – Part 1 (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 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: 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: 435. Seal, R.R., 2006. Sulfur isotope geochemistry of sulfide minerals. Reviews in Mineralogy and Geochemistry, vol. 61: 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: 15-1 to 15-25. 49 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Slip Kinematics of the Keweenaw and Hancock Faults within the Midcontinent Rift System, Upper Peninsula of Michigan LANGFIELD, Katherine1, DeGRAFF, James1, GAMET, Nolan1 1 Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, MI, USA The Keweenaw fault is a major compressional structure along the center of the Keweenaw Peninsula and positioned near the southern edge of the Midcontinent Rift System (MRS). The smaller Hancock fault connects with the hanging wall of the Keweenaw fault and, together, the two faults define a thrust slice. The MRS formed ~1.1 billion years ago when a major extensional event split a significant portion of the ancient North American continent across the Upper Midwest. The rifting produced large volumes of basaltic lava, roughly ending with the Portage Lake Volcanics that have an exposed thickness of 3-5 km along the Keweenaw Peninsula (1). A common interpretation of the Keweenaw fault is that it originally formed as a normal fault during MRS extension and then inverted to become a reverse fault during a post-rift compressional event, most likely the Grenville Orogeny (2,3). Another interpretation is that the Keweenaw and Hancock faults are parts of a detached fault system that was initiated during the Grenville Orogeny (4). Until a few years ago, ideas about these and similar faults in the region considered only dip slip with an either normal or reverse sense of motion. Recent bedrock mapping and measurements of faultslip lineations, however, have revealed a significant component of right-lateral strike-slip on the Keweenaw fault system near its northeastern end which is about twice the magnitude of north-side-up reverse slip (5, 6). To clarify the slip kinematics of this region we utilized bedrock mapping and fault slip measurements between Hancock and Mohawk, MI to clarify the geometry and slip kinematics of the NE-trending Keweenaw and Hancock faults and to relate their characteristics here to what is observed along the more easterly trending portion of the fault system previously studied (Fig. 1). Rose diagrams of slickenlines rakes found along the Hancock and Keweenaw Faults show that both faults have roughly equal dip-slip versus strike-slip components (Fig. 2). This bimodal distribution of rake data differs from previous EDMAP projects, possibly due to the overall curvature of the Keweenaw Peninsula. The strike-slip to dip-slip component ratio was 2:1 (Mueller, 2021). The resulting map from this project indicates that the Keweenaw Fault isn’t a single fault trace, but instead connected fault segments (Fig. 3) The updated map and cross-section from this project proposes a new model for the Keweenaw Fault system kinematics. Acknowledgements This project was funded by the U.S Geological Survey’s EDMAP program under Award No. G21AC10681. This funding was matched by the Department of Geological and Mining Engineering and Sciences of Michigan Technological University, as well as sponsorship by the Michigan Geological Survey. Funding was also provided by the ILSG Student Research Fund for work done in the Quincy Mine, as well as an award by the Michigan Space Grant Consortium. Thanks goes to Tom Wright for access to the Quincy Mine. Additionally, we thank Ian Gannon, Breeanne Heusdens, Jack Hawes, Braxton Murphy, and Dillon Breen for fieldwork assistance. 50 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Figure 1. Map showing geology of the Keweenaw Peninsula. The boxes show the areas for the previous and current EDMAP project. (Cannon and Nicholson, 2001). Figure 2. Rake histograms showing the distribution of low and high angle rake on the Keweenaw fault (A) and Hancock fault (B). Arrows indicate mean rake of each dataset. Figure 3. Updated bedrock geologic map and legend of study area. References Cannon, W.F., and Nicholson, S.W., 2001. Geologic Map of the Keweenaw Peninsula and Adjacent Area, Michigan, U.S. Geological Survey, 1:100000 scale. Cannon, W.F., 1994. Closing of the Midcontinent rift ‒ A far-field effect of Grenvillian compression: Geology, v. 22: 155-158. 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: 127-136. 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, https://doi.org/10.1130/B36186.1. Tyrrell, C.W., 2019. Keweenaw Fault Geometry and Slip Kinematics – Bête Grise Bay, Keweenaw Peninsula, Michigan [M.S. thesis]: Houghton, Michigan, Michigan Technological University: 30. Mueller, S.A., 2021. Structural Analysis and Interpretation of Deformation Along the Keweenaw Fault System West of Lake Gratiot, Keweenaw County, Michigan, Open Access Master’s Thesis, Michigan Technological University 51 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Petrology and Geochemistry of the Paleoproterozoic Eau Claire Volcanic Complex, Eau Claire, WI LEAHY, Matthew D.1, LODGE, Robert W.D.1 Department of Geology and Environmental Sciences, University of Wisconsin-Eau Claire, Eau Claire, WI 54701 USA 1 The 1.8 Ga Eau Claire Volcanic Complex (ECVC) is located in the northwestern portion of Wisconsin primarily exposed in the Eau Claire River valley. The complex is part of the Marshfield terrane of the Penokean Orogen which developed along the southern margin of the Superior craton (Schulz & Cannon, 2007). Following the accretion of a juvenile ocean island arc, now known as the Pembine-Wausau terrane (PWT), with the southern margin of the Superior craton, opposing subduction zones closed the ocean between the accreted PWT and MT resulting in coeval magmatism on both terranes prior to collision around 1850 Ma. The origin of the MT is uncertain but is believed to be a small Archean craton that is either a rifted fragment of the Superior Province (Zi et al., 2021) or Wyoming Province (Malone et al., 2019). The suture between these terranes is the Eau Pleine Shear Zone. Paleoproterozoic subduction-related volcanism began to develop along MT’s northern margin, resulting in arc volcanism and back-arc spreading with associated calc-alkaline felsic magmas (DeMatties, 2022). This volcanism continued until the terrane collided with the subduction trench, resulting in a major compressional event along the Superior craton (Sims et al., 1989; Shultz and Cannon, 2007). This comprehensive interpretation of the tectonic setting fits well with the eastern portion of the MT where rocks are more abundantly exposed. However, the lack of outcrop exposure due to extensive Cambrian sedimentary strata has restricted research and mineral exploration in western parts of the orogen (DeMatties, 2022). This includes the ECVC, which is based on geophysical data, and has high potential for supergene-enriched VMS-style mineralization (DeMatties, 2022). The main objective of this study is to map and sample volcanic, metamorphic, and intrusive packages of the ECVC exposed along the North Fork of the Eau Claire River (Figure 1A) and Chippewa River for whole-rock geochemistry and petrographic analysis. Trace element geochemical data can be used to determine magmatic and tectonic settings of these rocks and improve regional tectonic models for the ECVC and MT. Twenty-four samples were analyzed for major elements via XRF and trace elements via ICPMS. Rock classifications were given in the field, reevaluated during petrographic analysis, and grouped into suites based on geochemistry. The majority of the suites were separated into four main categories: felsic gneiss (Figure 1B), mafic gneiss (Figure 1C), amphibolite (Figure 1D), and granitoid (Figure 1E). Each suite was diagnosed with a tectonic signature using multiple trace element diagrams. Th/Yb versus Nb/Yb displayed geochemical characteristics of deep crustal recycling for the majority of the samples, related to the active subduction that occurred during the advancement of the MT. The only suite that differs from this trend is the amphibolite group, which has a lower Th-Yb-Nb concentration, insinuating magma-crustal interactions with the protolith basalt. A tectonic classification tertiary diagram using La-Y-Nb solidified the theory that calc-alkaline arc magmatism dominated the MT region, while the amphibolite suite trends towards a more tholeiitic arc composition. This interpretation is backed by a magmatic affinity diagram as well using Th-Yb-Zr-Y percents. 52 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Figure 1. (A) Regional map of the Eau Claire River with the North Fork and relative location in Wisconsin, (B) Poorly exposed bedrock of a felsic gneiss, (C) Isoclinal folded trondhjemite at Hamilton Falls trending eastwest, (D) Elongated pipe vesicles on an amphibolite outcrop near Knights Pool, (E) Intrusive contact between pegmatite and amphibolite. 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, 141, 104489. https://doi.org/10.1016/j.oregeorev.2021.104489 Malone, S.J., Nicholson, K.N., and Dowling, C.B., 2019, Preliminary geochemistry on the Marshfield Terrane, west-central Wisconsin: Geological Society of America Abstracts with Programs, doi: 10.1130/abs/2018am-322316. Schulz, K.J., and Cannon, W.F., 2007, The Penokean orogeny in the Lake Superior region: Precambrian Research, v. 157, p. 4–25, doi: 10.1016/j.precamres.2007.02.022. Sims, P.K., Schmus, W.R., Schulz, K.J., and Peterman, Z.E., 1989, Tectono-stratigraphic evolution of the early Proterozoic Wisconsin magmatic terranes of the Penokean orogen: Canadian Journal of Earth Sciences, v. 26, p. 2145–2158, doi: 10.1139/e89-180. Zi, J.-W., and al., et, 2021, Refining the Paleoproterozoic tectonothermal history of the Penokean orogen: New U-Pb age constraints from the pembine-wausau terrane, Wisconsin, USA: doi: 10.1130/gsab.s.14700069. 53 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Structural analysis and slip kinematics of the Keweenaw fault system between Bête Grise Bay and Gratiot Lake, Keweenaw County, Michigan LIZZADRO-McPHERSON, Daniel1, DeGRAFF, James1, and GANNON, Ian2 1 2 Department of Geological and Mining Engineering Sciences, Michigan Technological University, 630 Dow Environmental Sciences, 1400 Townsend Drive, Houghton, MI 49931 USA The Keweenaw fault is perhaps the most important geologic structure on the Keweenaw Peninsula, with an estimated 7-11 km (1) of reverse slip juxtaposing Cu-bearing volcanic strata of the ~1.1 Ga Portage Lake Volcanics above ~1.0 Ga Jacobsville Sandstone. The fault has been interpreted as a riftbounding normal fault later inverted by compressional pulses of the Grenville Orogeny (2) and, more recently, as part of a detached thrust fault system unrelated to an earlier normal fault (1). The fault is shown on published maps as a nearly continuous fault trace whose sinuosity implies multiple fault segments and complex slip dynamics. Recent mapping has revealed that the Keweenaw fault at its most northeastern exposure on land is better characterized as a network of interconnected, left-stepping fault segments with easterly strike and exhibiting a 2:1 ratio of dextral strike slip to reverse slip (3). This project focused on the eastern half of a 2019-2020 EDMAP project (Fig.1) to map the Keweenaw fault system between Bête Grise Bay and Gratiot Lake. New mapping combined with structural and fault-slip analyses produced a revised bedrock geology map (Fig. 2) and a 3D-model (Fig. 3) that better constrain the geometry of the fault system, revealing folds and fault-bounded blocks in the main fault’s footwall. Analyses of fault slip data indicates a strike-to-dip slip ratio of 1.7:1 and a local shortening direction of 083°-263°. Slip along faults is a function of their strike relative to the shortening direction. Eastward transport of faultbounded blocks relative to the distal footwall was facilitated by mostly strike slip on longer EWtrending faults and reverse slip on shorter NEtrending faults, coupled with layer-parallel detachments along weak layer boundaries. The fault network defines a complex multistranded Figure 1. Bedrock geology of the Keweenaw transpressional system with overall dextral strike Peninsula (4), showing the 2017-2018 (grey box) slip and north-side-up reverse slip. Footwall folds and 2019-2020 (green box) EDMAP study areas. in Jacobsville strata adjacent to mostly strike-slip faults are considered to be cogenetic drag folds that formed during the Rigolet phase of the Grenville orogeny. These findings are consistent with recent mapping projects adjacent to the study area and investigations that relate far-field compressive pulses of the Grenville Orogeny to deformation of Keweenawan strata. Acknowledgements Funding provided by the USGS EDMAP program (Award No. G19AC00140) with a matching contribution from the Department of Geological and Mining Engineering and Sciences, Michigan Technological University and additional support from the Keweenaw Community Forest Company. Sponsored by the Michigan Geological Survey. 54 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Figure 2. Keweenaw fault system between Lac La Belle and Gratiot Lake. Deer Lake fault block is in the Keweenaw fault’s footwall between the lakes. Cross-sections shown by thin black lines labeled A - F. Figure 3. Cross-section B-B' showing modeled hanging-wall and footwall structural and stratal relationships across the Deer Lake fault block. References 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: 449–466. Cannon, W.F., Green, A.G., Hutchinson, D.R. et al., 1989. The North American Midcontinent Rift beneath Lake Superior from GLIMPCE seismic reflection profiling. Tectonics, v.8:305-332. Tyrrell, C.W., 2019. Keweenaw Fault Geometry and Slip Kinematics – Bête Grise Bay, Keweenaw Peninsula, Michigan: Michigan Technological University, M.S. thesis: 30. Cannon, W.F. and Nicholson, S.W., 2001. Geologic Map of the Keweenaw Peninsula and Adjacent Area, Michigan. U.S. Geological Survey, Map I-2696, Scale 1:100,000. 55 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Re-evaluating the tectonics and metallogeny of terranes in the Paleoproterozoic Penokean Orogen, Wisconsin LODGE, Robert W.D.1 1 Department of Geology & Environmental Science, University of Wisconsin-Eau Claire, Eau Claire, WI 54701 USA The tectonic model for the development of the Penokean orogen was synthesized in a classic paper by Schutz and Cannon (2007) that compiled decades of mapping, sedimentology, U/Pb geochronology, and geophysical surveys. The orogen started at ca. 1880 Ma with the accretion of Pembine-Wausau terrane, an oceanic arc complex, onto the margin of the Superior Province. A subduction flip after accretion resulted in overprinting continental arc volcanism and rifting (Figure 1A) until the collision a collision of an Archean crustal block, known as the Marshfield terrane, at ca. 1850 Ma. Several undeformed intrusions, interpreted as post-tectonic intrusions, constrain the end of the Penokean orogen at ca. 1835 Ma (Figure 1B). Perhaps the most important event during the orogen was the formation of the ~150 million tonnes of volcanogenic massive sulfide (VMS) deposits in the Pembine-Wausau terrane at ca. 1875 Ma (Sims et al, 1989; Quigley, 2016). This event was widespread across multiple VMS deposits. This presents a clear episode of submarine rifting and was assigned to a period of continental back-arc tectonism by Shultz and Cannon (2007). This is supported by the presence of inherited Archean zircons at the Lynne and Back Forty VMS deposits (Quigley, 2016) indicating the presence of Archean crust during the formation of Pembine-Wausau magmas. However, new U/Pb data has documented a second VMS forming event at ca. 1835 Ma at the Back Forty (Quigley, 2016) and Eisenbrey (Weber and Lodge, 2022) VMS deposits. Recognition of this extensional event has led to an alternate tectonic model wherein back-arc extension reactivated multiple times during ridge subduction (Zi et al., 2021). One of the principal issues that needs to be resolved with the classic Penokean tectonic model is the regional setting of Penokean VMS mineralization. VMS deposits formed in continental settings have different petrochemical associations than those formed in oceanic settings. New lithogeohemical data from mafic and felsic rocks at several VMS deposits (Flambeau, Eisenbrey, Lynne, Wolf River) suggest that most of the deposits hosted in rocks that are consistent with oceanic settings, while some suggest a continental setting. This suggests that the continental setting for the VMS mineralization does not apply to all deposits and that the extent of Archean basement needs to be better defined. Zircon petrochronology provides a mechanism to better resolve the nature of continental basement and its influence on metallogeny by providing a link between age of magmatism and tectonic setting and/or crustal inheritance. Once again, some deposits within the Pembine-Wausau terrane provide evidence for Archean basement, while others do not. However, in the process of discovering new ages, we also discovered that VMS forming environments continued until ca. 1835 Ma in a juvenile, oceanic setting. It was also discovered that some of the rocks from the Eau Claire volcanic complex of the Archean Marshfield terrane were mantle-derived, oceanic magmas that were ~1875 Ma with no evidence for Archean inheritance and seems eerily similar magmas from the Pembine-Wausau terrane. While Penokean magmas are known to intrude Archean rocks in the Black River Falls region of Wisconsin (Weber and Lodge, 2022), they clearly show Archean inheritance. As the hunt for domestic critical minerals makes its way to Wisconsin, the Penokean terranes and their metallogenic setting needs to be re-evaluated. 56 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Figure 2. Illustration of tectonic models proposed by Shultz and Cannon (2007) and various new petrochemical or zircon petrochronology datasets that highlight some inconsistencies in the model. REFERENCES Quigley, A., 2016. Setting of the volcanogenic massive sulfide deposits in the Penokean Volcanic belt, Great Lakes region, USA: Unpublished M.S. thesis, Colorado School of Mines: 95. Schulz, K.J., and Cannon, W.F., 2007. The Penokean orogeny in the Lake Superior region: Precambrian Research, v. 157: 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: 2145-2158. Weber, E.M., and Lodge, R.W.D., 2022. New U/Pb Geochronology from the Proterozoic Penokean Orogen, Wisconsin: Implications for VMS Metallogeny. Society of Economic Geology, Society of Economic Geologist Annual Meeting Proceedings, Denver, CO, paper P5.10. 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 Pembine-Wausau terrane, Wisconsin, USA: Geological Society of America Bulletin, v. 134: 776-790. 57 �Proceedings of the 69th ILSG Annual Meeting – Part 1 3D geologic mapping at the Wisconsin Geological and Natural History Survey MAUEL, Stephen1, STEWART, Eric1, REHWALD, Matthew1, STEWART, Esther K. 1, AMES, Carsyn1, BREMMER, Sarah1, and FITZPATRICK, William1 1 Wisconsin Geological and Natural History Survey, University of Wisconsin-Madison Division of Extension, 3817 Mineral Point Road, Madison, WI, 53705 The Wisconsin Geological and Natural History Survey has constructed a preliminary 14-county 3-D geologic data model across southern Wisconsin. The model was constructed primarily from well construction reports (WCRs) that have been refined for several WGNHS projects, as well as data from the Mineral Development Atlas, borehole geophysics, and data from previous mapping performed at various scales. Well Construction Reports (WCRs) from digital and analog sources were assembled in a GIS geodatabase. The land surface elevation for each well was extracted from a DEM, and the elevation was then used to “hang” each well’s downhole lithology. By displaying and exaggerating the data in 3D, the different lithologies were carefully selected and assigned to geologic formations. Prior to interpolation, statistical outliers were identified, inspected, and edited when appropriate. The elevation for each formation contact was used to interpolate a raster. The resultant raster was inspected to identify obvious outliers, and after the outliers were edited or removed, a “final” raster of each contact was generated. The formation contact rasters can be intersected with a bedrock elevation raster to produce a geologic map. New data can be added to the model when available, and a new updated map can be generated. The products derived from this type of 3D geologic modelling are useful to the public in many applications. Harmful minerals or metals dissolved in groundwater are a realistic concern in Wisconsin, and determining the geologic formation in which a well terminates can help to avoid or resolve water quality issues. 3D geologic modeling can help to inform decision making about land use and land practices, land conservation, zoning and planning, identification of natural hazards, and the construction & engineering of wells, roads, railways, and buildings. 58 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Secular Changes in the Magnitude of Terrestrial Weathering MEDARIS, L. Gordon Jr.1, and DRIESE, Steven G.2 1 2 Department of Geoscience, University of Wisconsin–Madison, Madison, WI 53706 Department of Geosciences, Baylor University, Waco, TX 76798 In a recent investigation of paleosols in the Lake Superior region, the magnitudes of weathering in six Proterozoic paleosols were found to be less than those in four Phanerozoic paleosols and four modern soils (Medaris et al., 2022). However, in view of this relatively small database, the apparent age distinction in the magnitudes of weathering might be spurious, and thus we have expanded the database to test the veracity of secular changes in the magnitude of terrestrial weathering. Twenty-one first-cycle paleosols in igneous and metaigneous rocks with well-characterized and relatively homogenous protolith compositions were selected for comparison. These paleosols occur world-wide, vary in age from 100 Ma to 2960 Ma, and have protolith compositions ranging from gabbro to granite. This expanded database confirms that the magnitude of weathering in Phanerozoic paleosols and modern soils is greater than that in Precambrian paleosols. Potassium metasomatism is a common phenomenon in paleosols (Rye and Holland, 1998), and among the 17 Cambrian and Precambrian paleosols investigated here, 14 experienced potassium metasomatism, which is recorded by the presence of neoblastic muscovite, illite, or microcline. The effect of such K-metasomatism is illustrated in a plot of Al2O3-(CaO*+Na2O)-K2O, where compositional trends for modern soils and unmetasomatized paleosols are oriented subparallel to the AC*N join (Fig. 1A), and those for K-metasomatized paleosols are rotated towards the K apex (Fig. 1B). (A) (B) Figure 1. Protolith compositions and paleosol trends in the system, Al2O3-(CaO*+Na2O)-K2O. A: Modern soils and paleosols without K-metasomatism; B. Paleosols with K-metasomatism. In K-metasomatized paleosols, the amount of K2O removed by weathering is unknown, but may be estimated by comparison to an average for the depth variations of K2O and Na2O in modern soils, for which: (% change K2O) / (% change Na2O) = – 1.40z3 + 0.95z2 – 0.31z + 0.75 where z is normalized depth. Following this approach, the removal of K2O is estimated to be 47 ± 4% for the combined Cambrian and Precambrian paleosols and observed to be 54 ± 21% for the Cretaceous paleosols and 47 ± 21% for modern soils (Fig. 2A). Interestingly, no correlation exists between the percentage of K2O removed and age (or protolith composition; not shown). In contrast, the total 59 �Proceedings of the 69th ILSG Annual Meeting – Part 1 addition of K2O to the weathered profiles, expressed in terms of Depth-Normalized Mass Flux, progressively increases with decreasing age, i.e. 0.74 ± 0.29 at 2960 Ma, 0.96 ± 0.18 at 2450 Ma, 1.04 ± 0.52 at 1600-2200 Ma, and 1.62 ± 0.36 at 500 Ma (Fig. 2B). (A) (B) Figure 2. A: Percentages of K2O removed from soils and paleosols; B: Total K2O added to paleosols, expressed as Depth-Normalized Mass Flux (DNMF). The percentage removal by weathering for the sum of SiO2, CaO, Na2O, and K2O(est or meas) progressively increases from Archean (17.3±1.5%) to Proterozoic (21.0±3.7%) to Cambrian (25.1±3.1%) to Cretaceous (37.1±10.8%) paleosols. In comparison, the percentage of mass removed from five modern soils is 36.0±3.7%, which lies within the values for the Cretaceous paleosols. We suggest that the greater magnitude of weathering in Phanerozoic soils compared to Proterozoic ones is due to higher concentrations of organic acids during Phanerozoic soil formation, which resulted from the emergence of sparse cryptophytes in biological soil crusts in Cambrian time and subsequent greening of the continents with vascular plants from Devonian time to the present. Figure 3. Percentages of the total mass of SiO2, CaO, Na2O, and K2O removed from modern soils and paleosols. References Medaris et al., 2022. Journal of Geology, v. 130, in press. Rye & Holland, 1998. American Journal of Science, v. 298: 621-672. 60 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Morphometry and formation process of eskers developed under the Chippewa Lobe of the Laurentide Ice Sheet NUÑEZ-FERREIRA, Francisca1, ZOET, Lucas1, and RAWLING III, J Elmo 2 1 2 Department of Geoscience, University of Wisconsin-Madison, Madison, WI, 53705 Wisconsin Geological and Natural History Survey, University of Wisconsin‐Madison, Madison, WI, 53705 Eskers are an important indicator of paleo subglacial hydrologic conditions and a good alternative to direct glaciological observations because they are one of the few landforms that record those processes. Esker morphology and sedimentology is useful to gain insight into how sediment transport relates to subglacial hydrology along channels, which in consequence provides understanding on ice dynamics. However, large discrepancies in the formation mechanisms of eskers still exist and there are even fewer attempts to investigate the influence of soft bed conditions on this process. To address this, we analyzed the morphometry and distribution of eskers formed under the Chippewa Lobe of the Laurentide Ice Sheet (Figure 1). This includes mapping the sinuosity and spatial distribution with 2m resolution LiDAR, comparing these to sediment thickness derived from a water well data base, and examining the sediment sequence of one large esker exposed to sand and gravel extraction (~20 m tall) (Figure 2). The LiDAR analysis revealed a direct relation between sinuosity and length of eskers formed in soft bed conditions, with a mean of 1.07 that is very similar to eskers formed under hard bed conditions. Eskers spacing over the soft bed of the Chippewa Lobe appear closer than over hard beds in Canada (e.g Storrar et al, 2014). The spacing of eskers decrease when the ice margin retreats, meaning that melt rates increase (Boulton et al, 2009; Hewitt, 2011). Moreover, the relation between the distribution of eskers and till thickness indicates that eskers formed preferentially over thin layers of sediment, specifically near 18 meters for the Chippewa Lobe. The results from the grain size distribution of the large esker showed that the critical shear stress changed nonmonotonically throughout the formation of the esker. As such, we can assume that the water velocity or depth of the channel likely changed sporadically with time while the esker formed. Figure 1. Distribution of eskers formed under the Chippewa Lobe during the Last Ice Age. 61 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Figure 2. Location of the selected esker for sediment analysis. The yellow start shows the location of a pit where the samples were extracted for the analysis. References Boulton, G.S., Hagdorn, M., Maillot, P.B., & Zatsepin, S., 2009. Drainage beneath ice sheets: groundwater–channel coupling, and the origin of esker systems from former ice sheets. Quaternary Science Reviews, 28(7-8): 621-638. Hewitt, I.J., 2011. Modelling distributed and channelized subglacial drainage: the spacing of channels. Journal of Glaciology, 57(202): 302-314. Storrar, R.D., Stokes, C.R., & Evans, D.J., 2014. Morphometry and pattern of a large sample (> 20,000) of Canadian eskers and implications for subglacial drainage beneath ice sheets. Quaternary Science Reviews, 105: 1-25. 62 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Subsurface characterization of the Duluth Complex and related intrusions from 3D modeling of gravity and magnetotelluric data PETERSON, Dana E. 1, BEDROSIAN, Paul A. 1 and FINN, Carol A. 1 1 U.S. Geological Survey, MS 973, Federal Center, Denver, CO 80225 The Mesoproterozoic Duluth Complex and related intrusions in northeastern Minnesota make up the second largest exposed mafic intrusive complex in the world, second only to the Bushveld Complex in Africa. It 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. Given the complex geology of the area, 3D modeling is necessary to provide a complete picture of the variable densities and geometries of intrusive suites throughout the Duluth Complex as well as their extent at depth. In this study, we use Bouguer gravity data collected over the past ~60 years and magnetotelluric data collected in 2019 to create new 3D models of density, resistivity, and subsurface structure of the region constrained by geologic data. We use the results of these models to calculate the total volume of the Beaver Bay Complex, Duluth Complex, and onshore North Shore Volcanic Group, and estimate preliminary intrusion and emplacement rates using age estimates from Swanson-Hysell et al. (2021). We model both thickness and density of intrusive and volcanic rocks in the region using Oasis GMSYS-3D. The igneous layer in our starting model is 11 km thick with a constant density of 2,941 kg/m3. Other surfaces in the model include topography, near surface glacial deposits, a high-density lower crustal layer, and the base of the crust. We start our inversion by allowing the basal surface of the igneous units to vary and then invert for density within the igneous layer, within a range of 2,630-3,180 kg/m3. Our gravity modeling indicates that intrusive and volcanic rocks reach a maximum thickness ~23 km, or half the crustal column, with densities ranging from ~2,730-3,030 kg/m3 and a mean density of 2,940 kg/m3. The thickest, highest density areas of the model are beneath the Beaver Bay Complex and other mapped diabase intrusions. We interpret the two thickest areas in our gravity model as feeder zones for the Beaver Bay intrusive complex and possibly also for the Duluth Complex, in-line with interpretations arising from previous gravity studies in the area (Allen, 1994; Miller et al., 2002). Preliminary volume estimates from 3D gravity modeling indicate the present-day Duluth Complex, Beaver Bay Complex, and onshore volcanic rocks constitute ~92,100 km3 of igneous material. We calculate the volume of separate mapped units by extending the mapped geologic boundaries at the surface to depth within our 3D model. Three major geologic groups each comprise ~30% of this total volume: 1) the North Shore Volcanic Group, 2) diabase units of the Beaver Bay Complex and intrusions to the northeast and southwest of it, and 3) the Duluth Complex Layered and Anorthositic series. The older Early gabbro series and Felsic series of the Duluth Complex make up the remaining ~10% volume. 206Pb/238U zircon ages for the Anorthositic and Layered series from Swanson-Hysell et al. (2021) indicate that rocks of these units were emplaced contemporaneously over a period of 500,000 ± 260,000 years, suggesting an emplacement rate of ~0.06 km3/year, assuming a constant rate on magma input. Using recently acquired magnetotelluric data, we invert for resistivity in the study area using ModEM (Kelbert et al., 2014). Our magnetotelluric model highlights an arcuate low resistivity anomaly at depths of ~9-20 km, westwardly adjacent to the high-density and high resistivity feeder zones (Figure 1). This anomaly may represent a plane of weakness along which magma intruded to form the Beaver Bay Complex and the Duluth Complex. Low resistivities in this case would be attributed to sulfide or graphitic mineralization that developed along the contact between intruding magma and country rock. These resistivities are also similar to values observed in the Paleoproterozoic metasedimentary rocks of the Animikie Basin, located to the southwest of the Duluth Complex. The 63 �Proceedings of the 69th ILSG Annual Meeting – Part 1 spatial and temporal relationship between the Animikie Basin and Duluth Complex raises a tantalizing hypothesis that the basin structure may have preferentially localized magma intrusion. If this was the case, entrainment of conductive metasedimentary rocks of the Animikie Group along a pre-existing fault could also explain the arcuate low resistivity anomaly observed adjacent to the highly resistive feeder zones. Figure 1. Depth slice through the 3D magnetotelluric resistivity model at ~14 km depth. Black dashed line is the surface extent of the Duluth Complex and related intrusive and volcanic rocks. White lines are 5 km contours of Duluth Complex thickness extracted from our gravity model, starting from 10 km. Cyan line is the outline of Lake Superior. Acknowledgements Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. government. 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. Kelbert, A., Meqbel, N., Egbert, G.D. and Tandon, K., 2014. ModEM: A modular system for inversion of electromagnetic geophysical data. Computers & Geosciences, 66:40-53. Miller, J.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. Retrieved from the University of Minnesota Digital Conservancy, https://hdl.handle.net/11299/58804. Swanson-Hysell, N.L., Hoaglund, S.A., Crowley, J.L., Schmitz, M.D., Zhang, Y., & Miller Jr, J.D., 2021. Rapid emplacement of massive Duluth Complex intrusions within the North American Midcontinent rift. Geology, 49(2): 185-189. https://doi.org/1.1130/G47873.1. 64 �Proceedings of the 69th ILSG Annual Meeting – Part 1 On the Importance of Geologic Maps for Mineral Exploration PETERSON, Dean1 1 Big Rock Exploration, 2505 West Superior Street, Duluth, MN 55806 The basis for most types of geologic investigations is fundamentally rooted in geologists’ observations and interpretations made of landscapes, exposed rocks, and surficial materials in their natural habitat: “in the field”. Coherent geologic maps, which may take many years to create, represent assembled collections of observations in context of space and geologic time, requiring teams of geologists who are usually employed by federal or state/province geological surveys. The outcomes of these concerted efforts in the field are published geologic maps at various scales. It is these works of publicly funded geologic mapping that form the foundation upon which mineral exploration programs and mineral resource developments are built (Figure 1). These early endeavors are key components in national goals to define domestic resources of critical minerals. In decades past, many university geology students in the USA (including the author) were employed as summer interns assisting geological survey geologists in the bedrock geologic mapping of 1:24,000 scale quadrangles. This type of early professional experience can have profound implications for the careers of these students. Student knowledge gained includes the understanding of what it takes to systematically map bedrock exposures and structural zones, categorize the various rock types into lithologic map units, write out detailed descriptions of these map units, generate geologic cross sections and correlation diagrams, and putting all of these components together into a map that the geologic survey will subsequently publish. In today’s mineral industry, geologic maps are largely digital compilations of publicly available regional/district scale GIS data (downloaded and/or digitized from geological survey websites) merged/overlain with detailed industry geologic mapping of prospects and/or project areas. For the most part, the mineral industry quickly compiles digital data into geologic databases and is seemingly always searching for new ways to quickly capture data in the field digitally. The ease with which the mineral industry can generate digital geologic map products today can be good, bad, or ugly. The state of such geologic map outcomes by industry entities rests largely on the knowledge and experience of the company geologists. The US Geological Survey’s (USGS) Earth Mapping Resources Initiative (Earth MRI) program is a partnership of the USGS, the Association of American State Geologists (AASG) and other governmental, Tribal, and private-sector entities to update the nation’s surface and subsurface mapping to improve our knowledge of the geologic framework in the United States and to identify areas that may have the potential to contain undiscovered critical mineral resources. In November 2021, the US government passed the Infrastructure Investment and Jobs Act, one outcome of which is an investment of $320 million into Earth MRI to develop a better understanding of sustainable mineral production and mine waste options. An industry appeal to Earth MRI programs is to reinvigorate the education of future professional geologists by employing hundreds of geology student interns in upcoming geologic mapping projects. 65 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Figure 1. The mineral development trapezoid. 66 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Fault zone architecture in mafic protoliths at the Lac des Iles mine, northwestern Ontario PETERZON, Jordan1, PHILLIPS, Noah1, HOLLINGS, Peter1, DJON, Lionnel2 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 Faults are important geologic structures that host earthquakes and serve as permeable pathways through the upper crust. From an economic perspective, faults may transport and trap mineralized fluids. In turn, trapped mineralization may be offset or remobilized by later faulting. Fault zones are complex structures that produce an array of fault rock fabrics and architectures. Fault zone architecture typically consists of three components: 1) a fault core where most of the slip has been accommodated, 2) a damage zone bounding the fault core where fracture density increases with proximity to the fault core, and 3) an undeformed and less altered protolith. (Faulkner et al., 2010). Permeability is significantly 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. Faults may therefore 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). Fault zone architecture has been well studied in felsic to intermediate protoliths but studies on mafic protoliths are lacking. Here, we examine late faults within the mafic Lac des Iles complex to characterize fault zone architecture in mafic protoliths. The Lac des Iles complex is a series of mafic-ultramafic intrusive bodies occurring within the Marmion terrane of the Superior Province. The complex has been dated at 2689 ± 1.0 Ma and was emplaced into a ~3.01 – ~2.68 Ga granite-greenstone terrane (Djon et al., 2018). The Lac des Iles mine, owned and operated by Impala Canada, is a working Pt-Pd mine which is classified as a structurally controlled magmatic sulfide deposit. Extensive Ni-Cu-PGE mineralization has been offset by two late reverse faults in the high-grade zones (>4 g/t Pd): the Camp Lake fault and the Offset fault. A depletion in Pt-Pd mineralization is observed surrounding the late Camp Lake fault which extends ~180m into the hanging wall and ~145m into the footwall. Five drill holes that cross the late faults were logged and sampled in detail, with a fracture density counting program conducted systematically in the hanging wall and footwall. Fracture density increases as a power law function with proximity to the fault core and correlates with alteration. Tonalite has a higher fracture density and fracture density decay rate than gabbronorites near the fault. Fracture density and hematite/epidote alteration are more intense in the damage zone when faults cut through tonalite than when faults cut through gabbro. Fault cores in tonalite display a range of textures, from chlorite-rich gouges to fault breccias with calcitic matrix, while fault cores in gabbro only display chlorite-rich gouges. In this study, felsic protoliths have a higher fracture density than mafic protoliths indicating that fluid flow was 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. 67 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Figure 1. Simplified regional map of the Lac des Iles intrusive complex (Djon et al., 2018). Figure 2. (A): Fracture density data from a single drill hole displaying an increase in fractures with proximity to faulting. (B): Underground exposure of the Camp Lake Fault at Lac des Iles. (C): Schematic of a typical fault zone architecture with corresponding cartoons of typical fracture density and permeability across the fault (Faulkner et al., 2010). 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, mechanics and fluid flow properties of fault zones. Journal of Structural Geology, 32 (11): 1557-1575. 68 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Mobile geologic mapping at the Wisconsin Geological and Natural History Survey REHWALD, Matthew1, AMES, Carsyn1, BREMMER, Sarah1, FITZPATRICK, William1, STEWART, Eric1, BATTEN, William1 and MAUEL, Stephen1 1 Wisconsin Geological and Natural History Survey, University of Wisconsin-Madison Division of Extension, 3817 Mineral Point Road, Madison, WI, 53705 The Wisconsin Geological and Natural History Survey (WGNHS) currently collects and analyzes data using a number of mobile applications for different purposes. Field data collection has become a tool for collection of new data and the verification of existing data. It has allowed the survey to create an automated pipeline to capture photos, notes, as well as record location information into one central location for a respective project. We had 4 objectives to implement while incorporating mobile applications. The application had to be 1) easy to use, 2) efficient, 3) easy to update, and 4) capable of displaying many datasets in the field. At the WGNHS we utilize mobile field applications for the collection of new data and the verification of existing map data. A mobile field application has the advantage of making many different data sets available to the user in the field within the flexible scale of a mobile GIS application. The incorporation of other mobile applications (FieldMove Clino) for data collection can increase efficiencies and are a vital to aid interpretations. Mobile field applications allow for field reconnaissance from almost anywhere. When considering a large project with a lot of data, increasing efficiency in field mapping techniques without compromising quality is important. Automating much of the data collection and data transfer eliminates the need for individuals to spend time cataloging digital pictures, copying field notes, and uploading field data. It’s a great advantage to be able to easily update or add additional map layers and data, and to see the data already collected. A visual display of data collection progress is useful in time management and project planning. The ease of which an application can be updated consumes time and affects project budget. Ease of use is also important, Accessibility and technical expertise should not be barriers to data collection. The ease of use of a mapping application has positive impacts the project participants, the project budget, and the project output. 69 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Figure 1. Diagram of the flow of geologic data from the source to and from the mobile application. Managing the data allows for customization of the functionality and the display of the data. 70 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Quaternary Geology of Wisconsin at a scale of 1:500,000 (in review) ROSE, Caroline1, RAWLING III, J. Elmo1, CARSON, Eric C.1, ATTIG, John W.1, MICKELSON, David M.1, MODE, William N.2, JOHNSON, Mark D.3, and SYVERSON, Kent M.4 1 Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, WI 53705 University of Wisconsin–Oshkosh Department of Geology, 645 Dempsey Trail, University of Wisconsin– Oshkosh, Oshkosh, WI 54901 3 Department of Earth Sciences, University of Gothenburg, Gothenburg, Sweden 4 University of Wisconsin–Eau Claire Dept. of Geology, 145 Phillips Hall, University of Wisconsin-Eau Claire, Eau Claire, WI 54702 2 In 2023 the Wisconsin Geological and Natural History Survey staff expect to publish a new statewide compilation map of Quaternary geology at a scale of 1:500,000. A preliminary version is presented here by the principal cartographer. Pre-existing statewide coverages of the surficial geology are limited to Chamberlin’s 1881 map of Quaternary formations and Hadley and Pelham’s 1976 map of glacial deposits at 1:500,000, which differentiates only six map units. No modern compilation of the surficial geology of the state at a scale of 1:500,000 or larger has been completed before. Figure 1. Statewide Quaternary geologic mapping in Wisconsin: Left: Chamberlin’s 1881 “Quaternary Formations of Wisconsin”. Center: Hadley and Pelham’s 1976 “Glacial Deposits of Wisconsin”. Right: Draft polygons of 1:500,000 scale surficial geologic map being compiled by WGNHS geologists. This effort began in 2019 due to a one-time funding opportunity from the US Geological Survey’s National Cooperative Geologic Mapping Program. Authors compiled previous mapping at 1:100,000 scale for 44 of Wisconsin’s 72 counties, along with partial mapping at the 1:100,000 scale and/or mapping at the 1:250,000 scale for 13 additional counties. Some areas had no prior mapping available at detailed scales. New map units have been developed for the 1:500,000 scale and are divided into glacial and nonglacial sediment that is characterized by lithology and subdivided by geomorphology. Glacial deposits are mapped at the formation level following the WGNHS Lexicon of Pleistocene Stratigraphic Units. We use color hue to differentiate among the various glacial formations by source areas with green groupings derived from the Superior basin, blue groupings from the 71 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Michigan basin. We assign the darkest colors to the strings of moraines and hummocky till marking the extent of glacial lobes of the most recent Wisconsin Glaciation. Nonglacial Quaternary units are generally shown in warm colors, including modern alluvium, colluvium, lake deposits, and meltwater stream deposits, with small pockets of terraces which are highlighted along major river valleys. The Driftless Area in southwestern Wisconsin shows the dendritic patterns of eroding colluvium along branching alluvial tributaries with windblown silt on the uplands. Some large deposits of organic sediment and areas of exposed or thinly covered bedrock are included at this scale. This map layout is being produced entirely in ArcGIS Pro, which is a relatively new layout process for our office. We are organizing the GIS data according to the USGS standard Geologic Map Schema (“GeMS”), and we make use of this data structure to draw the unit description text in the Explanation of Map Units (legend) directly from a table in the geodatabase using a dynamic text element. This saves us from the extra work of synchronizing the layout text with the database text. Although ArcGIS Pro does not natively offer an easy solution for geologic map legends, we have been able to find a series of work-arounds to achieve the desired legend layout. References Chamberlin, T.C., 1881. General map of the Quaternary formations of Wisconsin, plate 2 of Atlas of the Geological Survey of Wisconsin: [Madison, Wisc.], Wisconsin Geological Survey, scale approximately 1:960,000. Hadley, D.W., and Pelham, J.H., 1976. Glacial deposits of Wisconsin—Sand and gravel resource potential: Wisconsin Geological and Natural History Survey Map M061: 19 p., 1 pl., scale 1:500,000, https://wgnhs.wisc.edu/catalog/publication/000385 [Previously Map 10.]. Acomb, L., Attig, J.W., Baker, R.W., Brownell, J., Clayton, Lee, Fricke, C., Frolking, T.A., Frye, J.C., Hemstad, C., Jacobs, P.M., Johnson, M.D., Knox, J.C., Leigh, D.S., Mason, J.A., McCartney, M.C., Mickelson, D.M., Mode, W.N., Muldoon, M.A., Need, E.A., Schneider, A.F., Simpkins, W.W., Socha, B.J., Syverson, K.M., Willman, H.B., 2011. Lexicon of Pleistocene Stratigraphic Units of Wisconsin: Wisconsin Geological and Natural History Survey Technical Report 001: 180. 72 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Tips from a GIS Specialist: Moving maps to GeMS, and a utility for georeferencing quadrangles ROSE, Caroline1 1 Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, WI 53705 USA The USGS has recently been requiring that geologic mapping deliverables use their new standard database format, called the Geologic Map Schema, or “GeMS.” The Wisconsin Geological and Natural History Survey (WGNHS) began converting geologic maps into the GeMS format four years ago. I will offer a brief overview of GeMS and will use the GIS data for the Geology of LaCrosse County map (available for download on our website) to demonstrate how GeMS captures the components of a geologic map in geodatabase format. We have created several documents to facilitate the process of migrating maps into GeMS, and we have made them available in this Github repository: https://github.com/wgnhs/gems. My advice to anyone beginning this process is to first consult our “Workflow Overview” document for a high-level summary of the steps. When completing the GeMS-specified attributes, the “Quick-Reference Sheets” are a convenient arrangement of the GeMS documentation, with each layer or table printed on a separate reference sheet, to put focus on one layer or table at a time. To help verify that a GeMS database is complete, the “GeMS Fields Checklist” is designed to help in confirming completion of GeMS attributes. Two of our documents address the process of authoring metadata for a GeMS geodatabase. The document titled “Metadata For GeMS Maps - Step by Step in ArcCatalog” is a guide to starting FGDC metadata in ArcCatalog before using the USGS-provided metadata script. The “Metadata Summary for GeMS Fields” is a reference to show where GeMS attributes appear in the FGDC metadata, as produced by the metadata script. All of these documents are housed on our github page, along with other resources such as python scripts and slides from various presentations. We are making it a priority to share these with other GeMS users; we hope these resources are useful to other organizations working through the process of converting maps into GeMS. I will also briefly summarize how we have involved the GeMS format in our map layouts in ArcGIS Pro by drawing from the Description of Map Units table to automatically lay out the legend using Dynamic Text elements. In the second half of this talk, I will give an overview of a semi-automated utility for georeferencing maps, especially USGS quadrangles. The software is called QuadG+ and was developed by USGS and University of Wisconsin collaborators to build the Historical Topographic Map Collection. It is available for free download at https://geography.wisc.edu/quad-g/ and has proven useful to Wisconsin survey staff for georeferencing maps with field notes. 73 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Figure 1. The free software QuadG+ automatically detects the corners and other control marks in a scan of a USGS quadrangle References Burt, James E., Jeremy White, Gregory Allord, Kenneth Then, A-Xing Zhu, 2022. Quad-G+: Automated Georeferencing of Scanned Map Images User Manual Version 2.13 December 2022. University of Wisconsin – Madison. Accessed March 27, 2023. https://uwmadison.app.box.com/s/tkccw1j5u3ensn2e10hrl1eiek78z9r6/file/1125666147300. 74 �Proceedings of the 69th ILSG Annual Meeting – Part 1 New work developing Keweenaw geoheritage awareness ROSE, William1 1 Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931 U.S.A. Telling Keweenaw Geostories in ~ten minutes. Old stories of Keweenaw geohistory have been made into web-based illustrated summaries meant to fill awareness of geoheritage from literature sources. About 8-15 minutes long with ~20 illustrations, these stories tell about the Ontonagon Boulder, Douglass Houghton, Louis Agassiz, Jane Schoolcraft and Hiawatha, Pasties and Keweenaw Miners, Big Annie and the 1913 Strike, Discovery of the Keweenaw Fault, the Green Rock at Copper Harbor, Ben Franklin and Lake Superior, and the Discovery of the C&H Conglomerate. The stories may be viewed online (https:// vimeo.com/showcase/9801619). They show how local history is guided by geology. They are intended to supplement local and statewide awareness and pride. Bringing the Boulder Home to the UP. The Ontonagon Boulder was a legendary float of native copper 75 �Proceedings of the 69th ILSG Annual Meeting – Part 1 which was on the west branch of the Ontonagon River until 1847 (https://vimeo.com/showcase/9801619/ video/785968264). The word of mouth of this unusual precious rock led to widespread interest, but it was difficult to move. Dispute over the ownership of the Boulder was spirited, and eventually it ended up in Washington DC at the Smithsonian Mineral Science Museum. The Boulder is considered a sacred object by Ojibway (Erik Redix, 2017, American Indian Quarterly, 41 (3)). Repatriation of the boulder to the UP was applied for, but rejected by the Smithsonian in 2000. UP residents and tourists have no access to this iconic legend. Currently (for decades) the boulder resides out of public view. We propose a loan of the boulder to allow it to visit museums such as Cranbrook, Univ of Michigan and the AE Seaman Mineralogical Museum, partner of the Keweenaw National Historic Park Building a Statue of a feminist labor leader. Anna Klobuchar Clemenc was a feminist labor leader in Calumet during the miners’ strike of 1913 (https://vimeo.com/showcase/9801619/video/748833299). She had fame for her leadership of labor parades when she wrapped herself in the American Flag to inhibit the violent confrontations. Tall and homely, “Big Annie” used her personnage advantageously and allied with the Western Federation of Miners. She worked with Mother Jones and with the women’s vote efforts in Washington. She was the first member of the Michigan Women's Hall of Fame. The Michigan legislature has officially named June 17 as “Big Annie Day”. A bronze life-sized statue of her is planned for permanent display in Red Jacket, outside of the Calumet Opera House and one block away from the Italian Hall. For more info: https://www.facebook.com/profile.php?id=100090193837168 76 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Outcrop Scale Mapping Utilizing High-Accuracy GNSS with MnDOT’s Virtual Reference Station (VRS) Network: Minnesota Examples SCHULZ, Roger1 1 Big Rock Exploration Geologic mapping has long been utilized to visualize the underlying geology of a region. An important tool used in geologic mapping are those that resolve the mapper’s locations at a given time. The tools used to locate a mapper have advanced greatly since the time of pace and compass, chains, and grids. With that advancement comes ever more accurate location data. One of the most common modern mapping tools utilizes satellite networks to send a signal from which location data is calculated on a consumer grade handheld GPS unit. While handheld GPS’s are useful in mapping moderate to small scale (e.g., 1:5000 or 1:24,000), the accuracy limitations of these units are not capable of resolving outcrop-size maps (e.g., <1:250 scale). Given the limited outcrop in places like the Lake Superior region, it is necessary to extract all possible data from a given outcrop, lending greater importance to small-scale maps. Attaining a level of location accuracy needed for such outcrop scale mapping requires additional real-time corrections of satellite data. The Global Navigation Satellite System (GNSS) encompasses three major satellite networks operated by the USA (GPS), Russia (GLONASS), and the EU (Galileo). When utilized within the GNSS framework, it is possible to have reliable satellite coverage anywhere in the world, a requirement for accurate location data. GNSS functions via one-way communication of radio waves from the satellites to a receiver that calculates distance from the satellite to the receiver. Distance calculations based on the speed of the signal (c) and the time differential (Δt) between the signal being sent then picked up by the receiver (D = c • Δt). To triangulate the position of the observer, this calculation must be solved by multiple satellites. This results in positional data that is generally accurate to 10m in the horizontal, at best. The reason for the inaccuracy is that the atmosphere interferes with the speed of the signal resulting in a delay. It is possible to achieve more accurate data by correcting for this differential delay using established ground-based networks. Differential correction using Virtual Reference Station (VRS) utilizes base stations at control monuments that continually collect positional data generating an average position that can be used to determine the degree of atmospheric delay. When used in a network of base stations, the average atmospheric delay for an area can be determined. The regional delay, or differential, can be communicated to a handheld unit over an internet connection, thereby eliminating the effect of atmospheric delay. Positions can then be determined to centimeter-scale accuracy, a requirement of mapping outcrop scale features. MNDOT has implemented a statewide Virtual Reference Station (VRS) network with over 140 base stations over control monuments whose purpose is to correct for the atmospheric delay and generate high-accuracy GNSS datasets.2 This network is free to use for anyone. Figure 1 below is a case study from South Pass, Wyoming where a trench was mapped at 1:250 using a Trimble Geo 7x. The trench this study area contained auriferous quartz veins and barren quartz veins anastomose along a pair of sheared faults separated by several meters and are connected ladder veinlets. Without the decimeter-scale accuracy of the corrected positional data, it would not have been possible to accurately locate the geology, geochemical samples, or structural data within the trench and the adjacent outcrops. Such an approach could be extremely useful in visualizing complex intrusive outcrops in the Duluth Complex, tracing of the contacts of lava flows and interflow sediments along the shore of Lake Superior, and veins and stockworks within Archean rocks. 77 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Figure 1. Trench Mapping and Sampling for Relevant Gold Corp at the Golden Buffalo Project - South Pass, Wyoming. by Big Rock Exploration LLC References GNSS Timing and Atmospheric Interferences: How GNSS Is Solving These Problems. Global GPS Systems, 24 Jan. 2023, https://globalgpssystems.com/gnss/gnss-timing-and-atmospheric-interferences-how-gnss-issolving-these-problems/. Land Management. MnCORS Network - Land Management - MnDOT, https://www.dot.state.mn.us/surveying/cors/index.html. Understanding RTK VRS Networks. Global GPS Systems, 24 Jan. 2023, https://globalgpssystems.com/gnss/understanding-rtk-vrs-networks/. 78 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Geology and geochemistry of the basal North Shore Volcanic Group and Midcontinent Rift Intrusive Supersuite, Cook County, MN, USA SEVERSON, Allison R.1, NOWARIAK, Eric S.1, LARSON, Phillip C.2 1 Minnesota Geological Survey, Department of Earth and Environmental Sciences, University of Minnesota-Twin Cities, MN, USA 2 Vesterheim Geoscience PLC, Duluth, MN, USA Northeastern Minnesota preserves complex relationships between Mesoproterozoic volcanic flows and comagmatic gabbroic to granophyric intrusive rocks associated with the ca. 1.1 Ga Midcontinent Rift System (MRS), as well as Paleoproterozoic metasedimentary rocks. Over the last two years, bedrock mapping of nine 1:24K quadrangles in northeastern-most Minnesota (Fig. 1) has elucidated some of these relationships between the Rove Formation, Logan sills, Puckwunge sandstone, reversely polarized North Shore Volcanic Group (NSVG), and gabbroic and granophyric rocks of the Midcontinent Rift Intrusive Supersuite. Results described herein are based on field and thin section observations, and associated geochemistry, which will be compiled and published as part of the Minnesota Geological Survey’s County Geologic Atlas Series. Volcanic rocks lie conformably on top of the Puckwunge sandstone in the eastern map area (Fig. 1). In the western part of the map area, the Crocodile Lake Gabbro (CLG) is in contact with the Paleoproterozoic Rove Formation to the north, with the Rove being highly deformed, metamorphosed, and partially melted proximal to the contact. South of the CLG, is the coeval Cucumber Lake Granophyre (CLGp), which is in contact with the Grand Portage Lavas (GPL), Esther Lake Lavas (EL), and Hovland Lavas (HL) of the NSVG to the south. The NSVG youngs from north to south, and transitions from mafic to more felsic from north to south which is most evident in the transition from the GPL to the overlying EL (Fig. 1). Geochemically, this sequence evolves along a strong tholeiitic trend (Fig. 2). Lithologic and geochemical patterns suggest the >1108 Ma GPL, EL, and HL were likely sourced from a long-lived, evolving magma. The basal GPL amygdaloidal basalt preserves 5 - 75 cm long pillows with somewhat enigmatic siliceous, carbonate, and glassy selvages that also preserve hyaloclastic and perlitic textures. These flows are geochemically primitive and contain abundant altered olivine, pyroxene, and oxide phenocrysts. The pillowed basal unit grades into thick, massive to ophitic basaltic and basaltic andesite flows of the EL. The transition from the GPL is also marked by a change in trace element geochemistry from an enriched mantle to a more depleted mantle signature. The base of the HL consists of a package of strongly glomeroporphyritic, amygdaloidal andesites and basaltic andesites transitioning to porphyritic rhyolite and icelandite. Porphyritic basaltic to andesitic lavas in the westernmost map area also preserve pillow structures, but these flows vary in thickness and extent, suggesting aqueous subbasins within the HL volcanic basin. Intercalated throughout the HL are abundant dikes and sills of ultraphyric diabase containing 15-60% of >5 mm plagioclase phenocrysts within a basaltic, locally ophitic very fine-grained groundmass. These intrusives are interpreted to be hypabyssal and locally cut across volcanic stratigraphy. Though these dikes and sills are endemic to the area, temporal relationships between these intrusives, the surrounding volcanics, and the Brule-Hovland Gabbro are unknown. The ca. 1107 Ma CLG and the CLGp comprise some of the earliest known rocks within the intrusive Duluth Complex. Basal gabbroic cumulates of the CLG grade into dioritic-monzonitic rocks of the Crocodile Lake “Mixed Zone”, below the contact with the overlying CLGp. This Mixed Zone is typified by complex dikes and plagioclase cumulate rocks, rich in micrographic interstitial felsic mesostasis. Abundant quench textures and pegmatitic zones, as well as distinct geochemical patterns 79 �Proceedings of the 69th ILSG Annual Meeting – Part 1 suggest the Mixed Zone is a “cap” to the CLG rather than a gradual transition to the CLGp. REE patterns and Eu anomalies within these coeval intrusives suggest liquid immiscibility between mafic and felsic components of the source magma may have played a significant role in their genesis (Fig 3.). Other intrusive gabbroic rocks include the texturally varied Brule-Hovland Gabbro, which cross-cuts the HL. Figure 1. Regional geologic map of northeastern Cook County, MN. Ongoing partially USGS-funded STATEMAP projects outlined with bold lines. Generalized geology is from MGS miscellaneous map series M-119. Figure 2. AFM diagram of volcanic rocks. Figure 3. Chondrite-normalized REE diagram of Crocodile Lake and Cucumber Lake intrusives, based on Sun and McDonough, 1989. 80 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Exploring the application of full tensor magnetic gradiometry to better define conduit type NiCu-PGE targets SMITH, Jennifer1, TSCHIRHART, Victoria1, TUCK, Loughlin2, ENKIN, Randy1, and ROYGUAY, David3 1 Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8 Defence Research and Development Canada, Ottawa 3 SBQuantum,Sherbrooke, QC, J1H 1Z1 2 Magmatic Ni-Cu-PGE sulfide deposits are often associated with small conduit- or chonolithtype intrusions. These deposit types are notoriously challenging exploration targets owing to: 1) their small size, 2) lack of alteration halo or distal footprint, 3) complex and variable morphology, and 4) unpredictable depositional sites of sulfides (Barnes 2023). Furthermore, mafic rocks commonly retain significant remanent magnetization which, if not detected, can result in inaccurate modelling and targeting of these deposits. With a significant increase in the global production of Ni forecasted for the transition to a low-CO2 future, these deposit types will likely become an increasingly important source of Ni, both in Canada and globally. With fewer new discoveries being made, despite increased exploration expenditure, new methods and knowledge are needed to facilitate successful exploration at the regional and deposit scales and to ultimately secure a stable Ni supply. Historically, exploration has traditionally relied on geophysics (gravity, magnetics, electromagnetics), to identify potential mafic and/or ultramafic host intrusions, with airborne magnetic surveying dominating due to its low cost, and ability to survey vast areas rapidly and systemically. Although there is incredible value in Total Magnetic Intensity (TMI) data there are numerous limitations to this approach (e.g. non-uniqueness, scalar measurements, can’t distinguish remanence from induced field). The full tensor magnetic gradiometry (FTMG) technique, which measures the full magnetic gradient tensor at each measurement point, overcomes many of the limitations of TMI data. Advantages of FTMG include: (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 provide a more complete picture of the subsurface magnetic properties and improved discrimination between magnetic sources. This leads to improved imaging of complex structures, more accurate models of the subsurface, and improved understanding of geological processes. While quantum FTMG is in use by industry, practicalities relating to the system hamper its widespread deployment. Currently, existing quantum FTMG relies on SQUID technology for large scale airborne surveying. The application of SQUID technology has shown great benefits due to the enhanced sensitivity and fidelity of the system. However, these systems typically weigh ~270 kg and require extremely low sensor temperatures, making them impractical for ground and uncrewed aerial vehicle (UAV) surveying. These limitations have warranted the development of a complimentary ground and UAV quantum FTMG system such as the diamond-based quantum magnetometer in development by SBQuantum. This rugged and compact system leverages quantum properties of nitrogen vacancy (NV) centres in a diamond to provide highly accurate, quantum-based FTMG measurements. The Geological Survey of Canada (GSC) is in the early stages of establishing a new collaborative partnership with Defence Research and Development Canada (DRDC), SBQuantum, and numerous other industry and academic partners. The aim of this partnership and wider project is to derisk quantum magnetic gradiometer use across Canada with the purpose of facilitating widespread adoption by the Canadian exploration industry, academia and the military. This will be achieved 81 �Proceedings of the 69th ILSG Annual Meeting – Part 1 through the field testing and validation of the ruggedized quantum FTMG system developed by SBQuantum. As part of this project, SBQ’s quantum magnetic gradiometer will be deployed on several Canadian critical mineral systems, allowing comparison with traditional airborne and/or ground total magnetic field systems and non-quantum FTMG systems. As part of this, a detailed study will be undertaken on the Ni-Cu-PGE bearing Escape Lake intrusion in northern Ontario, which presents as a complicated magnetic signal that is strongly affected by remanent magnetization and associated with the 1.1. Ga Midcontinent Rift. With conventional total field geophysical methods unable to address the challenging features which are often characteristic of small, conduit-type magmatic sulfide deposits, this case study will explore the use and application of quantum FTMG in the context of improving targeting of conduit type Ni-Cu-PGE deposits. This study will be the first to generate publicly accessible quantum FTMG data over critical mineral deposits in Canada and will act to improve exploration capacity by validating tools useful for critical metal deposits whose complex geophysical expressions are not easily resolved by traditional geophysical techniques. The increased accuracy of these quantum technologies, which map the magnetic field at an enhanced scale, provide the ability to resolve the complexity of these deposits. Providing enhanced tools to facilitate exploration and delineate deposits better will aid with the identification of new Canadian deposits of critical metals needed for the lower carbon and digitized economy supply chain. This will aid Canada’s Critical Minerals Strategy set forth in the 2022 Federal Budget. References Barnes, S.J., 2023. Lithogeochemistry in exploration for intrusion-hosted magmatic Ni–Cu–Co deposits. Geochemistry: Exploration, Environment, Analysis, 23(1): geochem2022-025. 82 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Record of an Ancient Meteorite Impact Buried Beneath the Twin Cities, MN STEENBERG, Julia R. 1, and RUNKEL, Anthony C. 1 1 Minnesota Geological Survey, 2606 W. Territorial Rd., St. Paul, MN 55114 USA An impact crater is proposed in the southeast part of the Twin Cities metropolitan area, 11 miles (18 km) south of St. Paul within an area with significant residential and industrial development. The crater lies within a predictable package of Paleozoic sedimentary rocks in the Twin Cities structural basin where near its center includes 14 formations with a total thickness of about 1,200 feet (365 meters) (Mossler, 2008; Mossler, 2013). Paleozoic formations are characterized by widespread layers of sandstone, shale, and carbonate deposited in shallow seas during the Cambrian and Ordovician Periods (500 to 450 Ma). They are underlain by Mesoproterozoic (1,100 Ma) sedimentary and volcanic rocks of the Keweenawan Supergroup associated with the Midcontinent Rift. Paleozoic rocks in this area have limited exposure along the Mississippi and Minnesota River bluffs, roadcuts, and rock quarries, but elsewhere are buried beneath a variable thickness of Quaternary glacial sediments. Without extensive exposures, a variety of subsurface datasets are used for bedrock mapping including core, drill cuttings, geophysical logs, passive seismic stations, and driller’s descriptions from water well records. While mapping the bedrock geology of Dakota County, an area of discordance with the surrounding Paleozoic stratigraphy was observed in geologic cuttings samples, and corroborated with additional cuttings, geophysical logs and water wells driller’s records. Drill samples reveal as much as 575 ft (175 m) of anomalous sandstone, siltstone and shale with some intervals containing abundant cloudy and fractured quartz sand grains. The samples are from an area entirely buried by several hundred feet of glacial deposits within a deep buried channel carved into the surrounding bedrock layers adjacent to the Mississippi River near the city of Inver Grove Heights. Beneath the anomalous sequence of strata and in additional samples near the site, local Cambrian and Mesoproterozoic stratigraphic layers are recognized but are out of the usual stratigraphic order and in places entirely overturned. Microscopic investigation has resulted in the detection of shocked metamorphic features including planar deformation features (PDFs) in the fractured quartz grains, confirming the impact origin of this structure (Fig. 1). As such, this area is referred to as the Pine Bend Impact Structure (PBIS) (Steenberg, in prep). Based on the available geologic data near the site and current models of crater formation from similarly sized structures in layered sedimentary target rocks we interpret this feature to be a complex crater, approximately 4 km wide with an apparent central uplift and possible terraced rims (Grieve, 1991). The total disturbed area may be as large as 9 square miles (23 square kilometers). Based on published crater- to- meteor size ratios, the size of the meteor is estimated to be several hundred meters in diameter (Grieve and Pilkington, 1996). Due to its location, within a buried bedrock valley, the upper sequence of this structure has been removed by erosion, making it difficult to precisely date the impact. It may be as old as Late Cambrian (~490 Ma), having occurred during or after deposition of the Jordan Sandstone based on the age of the overturned strata and the apparent lack of carbonate from the overlying Prairie du Chien Group in the samples. We have also collected a pebble with PDFs from strata approximating the Jordan-Prairie du Chien contact in an outcrop about 10 kilometers from the crater. If this pebble is ejecta from the PBIS, it also supports a latest Cambrian or very Early Ordovician age of impact. This would make the PBIS older than known craters in surrounding states which are Ordovician and younger (French et al., 2004; French et al., 2018). The dynamic nature of our planet has left us with a small sample size of terrestrial impact structures, nearly 200 confirmed impact structures are currently recognized on Earth (Gottwald et al., 2020). Although Minnesota has known impact debris from the Sudbury Impact Structure, this would 83 �Proceedings of the 69th ILSG Annual Meeting – Part 1 be Minnesota’s first documented crater, giving us a rare opportunity to better understand the important geological and biological effects of meteorite impact events on Earth. Figure 1. Photomicrographs of mounted quartz sandstone rock chips from a cuttings sample, sample depth is 525 feet. A- Two sets of planar features and feather features. B- One set of decorated planar deformation features. Photos by L. Ferriere, Natural History Museum, Vienna, Austria. References French, B.M., Cordua, W., and Plescia, J.B., 2004. The Rock Elm meteorite impact structure, Wisconsin: Geology and shock-metamorphic effects in quartz. GSA Bulletin, 116: 200–218. French, B.M., McKay, R.M., Liu, H.P., Briggs, D.E.G., and Witzke, B.J., 2018. The Decorah structure, northeastern Iowa: Geology and evidence for formation by meteorite impact. GSA Bulletin, 130: 2062– 2086. Gottwald, M., Kenkmann, T., and Reimold, W.U., 2020. Terrestrial impact structure. In: TheTan-DEM-X Atlas, Part 1 and 2, Friedrich Pfeil, Munich, Germany. Verlag Dr. Grieve, R.A.F., 1991. Terrestrial impact: the record in the rocks. Meteoritics, 26: 175–194. Grieve, R.A.F., and Pilkington, M., 1996. The signature of terrestrial impacts. AGSO Journal of Australian Geology and Geophysics, 16: 399-420. Mossler, J.H., 2008. Paleozoic stratigraphic nomenclature for Minnesota. Minnesota Geological Survey Report of Investigations RI-65: 76, 1 pl. Mossler, J.H., 2013. Bedrock geology of the Twin Cities ten-county metropolitan area, Minnesota. Minnesota Geological Survey Miscellaneous Map M-194: scale 1:125,000. Steenberg, J.R., in prep. Bedrock geology, pl. 2 of Steenberg, J.R., project manager, Geologic atlas of Dakota County, Minnesota. Minnesota Geological Survey County Atlas C-57: 6 pls., scale 1:100,000. 84 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Magma Recharge and the distribution of Copper and Nickel in the Keweenaw Large Igneous Province STEINER, Alex1, PETERSON, Dean1, SWEET, Gabriel1 1 Big Rock Exploration, 2505 W. Superior Street, Duluth, MN 55806. The Keweenaw large igneous province (LIP) was formed over a protracted period of magmatism that emplaced Cu-Ni-PGE bearing mafic to ultramafic intrusions along the arcuate MidContinent Rift system, thus creating one of the largest critical mineral resources in North America. The magmatic activity associated with the Keweenaw LIP has been divided into a series of tectonomagmatic stages extending from at least 1115 Ma to 1080 Ma. Of the six stages of formation, significant orthomagmatic sulfide deposits were formed during Stage 1 (plume impact stage; 11151110 Ma), Stage 2 (early stage; 1110-1105 Ma), and the Stage 4 (the main stage; 1101-1094 Ma). Stage 1 and 2 intrusions in the Minnesota and Michigan are Ni-rich while those of stage 4 in the Duluth Complex are considerably more copper-rich. Here we discuss a potential mechanism of copper enrichment via magma recharge-evacuation-fractional crystallization (REAFC) where the parameters of differentiation are based upon a conceptual model for the formation of continental LIPs. It has been recognized that continental LIPs form in a series of phases that reflect the conditions of magma generation and differentiation prior to the eruption and eventual formation of continental flood basalts (Jerram and Widdowson, 2005). Early phases of LIP formation are dominated by more primitive lavas, that pass through a magma plumbing system that is immature and inefficient at differentiating magmas (Steiner et al., 2021). However, the magmatic plumbing system of the most voluminous eruptive phase is mature and capable of differentiating magmas to a considerable degree. The key difference between these two periods is the amount of magma recharge, which has a profound impact on the geochemical composition of the resultant magmas where compatible elements become buffered and incompatible elements become enriched (Lee, Lee and Wu, 2014). The relative Cu-enrichment of mineralized Stage 4 intrusions compared to earlier Ni-rich Stage 1 and 2 intrusions may be explained by several mechanisms. Mechanisms such as sulfide upgrading and high-R factors have been recognized as important contributors to Cu-rich mineralization (Peterson and Boerst, 2013). However, recent chemo-stratigraphic examinations of Keweenaw LIP lavas from the Keweenaw Peninsula have demonstrated that REAFC processes are controlling erupted lava compositions during the eruption of Stage 4 lavas (Davis et al., 2021). To test the effect of REAFC on the proportions of Ni and Cu that may be available to form an orthomagmatic sulfide deposit, REAFC geochemical modelling utilizing the equations of Lee et al. (2014) were performed on a generalized basaltic composition (MgO = 10%, Ni = 250 ppm, Cu = 116 ppm (Prinz, 1967)). Figure 1 demonstrates the liquid line of descent for Cu, Ni, and MgO during REAFC differentiation and pure fractional crystallization. During pure fractional crystallization, MgO and Ni behave compatibly, gradually decreasing in concentration with continued differentiation while Cu gradually increases. However, during REAFC differentiation, both Ni and MgO become buffered while Cu becomes decoupled, increasing in concentration while Ni and MgO remain the constant. The consequence of this decoupling is that Cu can become considerably enriched relative to Ni, thereby producing a magma that contains greater than anticipated Cu concentrations compared to pure fractional crystallization. When this Cu-enriched magma reaches sulfur saturation, the subsequent sulfide magma would have a greater abundance of Cu to scavenge, resulting in the Cu-rich sulfide deposits observed in in the Duluth Complex. 85 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Figure 1. REAFC calculations (Lee, Lee and Wu, 2014) for a generic basalt. Model parameters are recharge/evacuation = 0.43, assimilation = 0.07, fractional crystallization = 0.5. Crystallizing phases were olivine (25%), plagioclase (65%), and clinopyroxene (15%). References Davis, W.R. et al., 2021. Geochemical, petrographic, and stratigraphic analyses of the Portage Lake Volcanics of the Keweenawan CFBP: implications for the evolution of main stage volcanism in continental flood basalt provinces, Geological Society, London, Special Publications: SP518-2020–221. doi:10.1144/SP518-2020-221. Jerram, D.A. and Widdowson, M., 2005. The anatomy of Continental Flood Basalt Provinces: geological constraints on the processes and products of flood volcanism, Lithos, 79(3): 385–405. doi:https://doi.org/10.1016/j.lithos.2004.09.009. Lee, C.-T.A., Lee, T.C. and Wu, C.-T., 2014. Modeling the compositional evolution of recharging, evacuating, and fractionating (REFC) magma chambers: Implications for differentiation of arc magmas, Geochimica et Cosmochimica Acta, 143: 8–22. doi:10.1016/j.gca.2013.08.009. Peterson, D. and Boerst, K., 2013. Twin Metals Minnesota’s Maturi Deposit, in 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 RegionOctober 6-13, 2013, Field Trip Guidebook: 45–57. Prinz, M., 1967. Geochemistry of basaltic rocks: trace elements. In’, Basalts, 1: 271–333. Steiner, R.A. et al., 2021. Initial Cenozoic Magmatic Activity in East Africa: New Geochemical Constraints on Magma Distribution within the Eocene Continental Flood Basalt Province, Geological Society, London, Special Publications: SP518-2020–262. doi:10.1144/SP518-2020-262. 86 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Relay zones in weakly folded and faulted Paleozoic strata and their role localizing Mississippi Valley-type mineralization, southwest Wisconsin, USA STEWART, Eric1, FITZPATRICK, William1, and AMES, Carsyn1 1 Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, WI, 53705 Folds and faults have long been known to play a role in localizing Mississippi Valley-type zinclead mineralization in the historic Upper Mississippi Valley base metal district (UMVD) of southwestern Wisconsin. However, a simple correlation between mineralization and map-scale structures is overly simplistic since it does not explain why mineralization often occurs only along isolated portions of folds and faults. New 1:24,000 scale geologic mapping as part of the United States Geological Survey Earth Mapping Resources Initiative (EarthMRI) in the Stitzer region of the northern UMVD was initiated to improve understanding of the relationship between folds, faults, and mineralization. The Mineral Point anticline is the dominant structure in the Stitzer area (Figure 1). It is an asymmetric, northwest-trending gentle fold with a maximum amplitude of around 180 feet. The fold deforms platform Cambrian and Ordovician siliciclastic and carbonate strata, and contains several doubly plunging segments. Structural highs along the fold (Figure 1) correspond to aeromagnetic anomalies (Daniels and Snyder, 2002). Deformation bands in sandstone are common along the more steeply dipping northeast limb of the fold. The asymmetry of the fold and the correspondence of structural highs to aeromagnetic anomalies suggest the Mineral Point anticline is a forced fold, forming from thrust reactivation of a buried Precambrian fault. At depth near the Precambrian basement, the segments of the Mineral Point anticline probably transition into fault segments. Simple 2D kinematic modeling suggests contraction is highest near the base of the overlying folded section. If deformation bands accommodate some of the contraction in the basal siliciclastic sequence, then significant numbers of deformation bands are probably present low in the Paleozoic section. Mineralization and historic mining are heavily concentrated where two segments of the Mineral Point anticline overlap, and a third smaller anticline terminates (Figure 1). The area between the overlapping segments of the Mineral Point anticline is interpreted to represent the area above a relay zone between thrust segments. As mineralizing brines approached the Mineral Point anticline from the south, flow was probably altered due to the abundance of impermeable deformation bands. Flow conduits developed in the relay zone between fault-fold segments, focusing the brines upward and concentrating mineralization. References Carlson, J., 1961. Geology of the Montfort and Linden Quadrangles, Wisconsin, in Geology of parts of the Upper Mississippi Valley zinc-lead district. U.S. Geological Survey Bulletin 1123–B: 95– 138, 2 pls. Daniels, D. and Snyder, S., 2002. Wisconsin aeromagnetic and gravity maps and data; a web site for distribution of data. U.S. Geological Survey Open-File Report 2002-493. Taylor, A., 1964. Geology of the Rewey and Mifflin quadrangles, Wisconsin, in Geology of parts of the Upper Mississippi Valley zinc-lead district. U.S. Geological Survey Bulletin 1123–F: 279–360, 2 pls. West, W., 1971. Geologic map of the Ellenboro quadrangle, Grant County, Wisconsin. U.S. Geological Survey Geologic Quadrangle Series 959: 1 pl. 87 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Figure 1. Simplified structure contour map of the base of the Ordovician Platteville Formation. Mines are concentrated in the SE portion of the map near the junction of three anticline-syncline pairs. Additional data sources include the Mineral Development Atlas, Carlson (1961), West (1971), and Taylor (1964). 88 �Proceedings of the 69th ILSG Annual Meeting – Part 1 Deciphering the metamorphic and deformational history of the Hardwood Gneiss, Felch District, Michigan: Anomalously high-pressure rocks in the heart of the Penokean orogen TAYLOR, Madeline1 and BJØRNERUD, Marcia1 1 Geosciences Department, Lawrence University, Appleton Wisconsin 54911 The Neoarchean Hardwood Gneiss is a mafic granulite with minor metapelites, exposed over an area of about 6 km2 between the towns of Foster City and Hardwood, MI, 8 km southeast of the eastern end of the Paleoproterozoic “Felch Trough” (James, 1961). The area lies at the heart of the ca. 1.85 Ga Penokean orogen and within the superimposed Yavapai-age (1.75 Ga) ‘gneiss dome corridor’ (Drenth et al., 2021). In contrast to the primarily felsic gneisses of the region, which contain inherited zircons with ages of 3.8-3.5 Ga, the Hardwood Gneiss is mostly mafic and yields no zircons older than 2.7 Ga (Ayuso et al., 2018). Zircons from the Hardwood also record a period of growth between 2.2 and 1.9 Ga, which does not correspond to any known thermal events in the region (Cannon et al., 2018). Most notably, the Hardwood experienced much higher-pressure metamorphism than any other rocks in the region. Using a variety of geo- thermometers and -barometers, Peterson & Geiger (1990) concluded that the mafic rocks underwent two distinct metamorphic events, the first, ‘M1’, at 8.2-11.6 kbar and ca. 770°C, and a another, ‘M2’, at 6-10 kbar and 610-740°C, while the pelites experienced only the second. Maximum pressure estimates for the nearby Peavy metamorphic node, in contrast, are < 5 kbar (Attoh and Klasner, 1989). It is difficult to explain how the Hardwood complex, with its distinctive geochronologic and metamorphic signatures, came to be incorporated into the Penokean orogen. This study presents detailed field, petrographic and microstructural observations that may help constrain the origin and history of the Hardwood Gneiss. Peterson & Geiger (1990) identified three compositional units in the Hardwood: metabasite, amphibolite, and metapelite. The metapelite, a garnet-biotite schist, is clearly a distinct unit, exposed in the western end of the outcrop area, but our work suggests that the amphibolite and metabasite are both part of a heterogeneous igneous complex that included anorthositic, gabbroic and noritic horizons – perhaps a Neoarchean layered mafic intrusion. If this complex was of mantle origin, it could explain the absence of older Archean zircons. In addition to their compositional variety, the metamafic rocks display a wide range of metamorphic and deformational textures. In outcrop, they have a strong, apparently mylonitic, foliation that dips mainly NE but is somewhat variable in orientation and may be folded. In thin section, microstructures show that a combination of brittle and plastic deformation mechanisms contributed to the intense fabric. In plagioclase-rich horizons, the feldspars tend to be the largest crystals, apparently surviving as porphyroclasts. These show both cataclastic fracturing and highly distorted twins, a combination usually interpreted to indicate that deformation took place at mid-crustal depths and temperatures of ca. 500°, the brittle-plastic transition for feldspars. These intensely deformed rocks show evidence of only limited, and heterogeneous, recrystallization, either dynamic or static. This suggests that deformation was brief and that the rocks cooled quickly after deformation ceased. Garnet-bearing horizons within the mafic complex display especially remarkable textures. Clusters and trains of garnets, apparently broken -- and in some cases, shattered – are engulfed in a very fine-grained (<0.01mm) feldspathic matrix. The unusual shapes of some of the garnet fragments – including crescents and splinters – may indicate seismic fragmentation (Hawemann et al., 2019). The largest garnet fragments tend to have inclusion-free cores and ‘spongy’ poikilitic rims, while smaller fragments are commonly poikilitic throughout, with a notable 89 �concentration of opaque inclusions. In some cases, the edges of the small garnet fragments are so diffuse that they cannot be seen in plane light. Peterson & Geiger (1990) interpreted the poikilitic rims and small inclusion-rich garnets as records of a second metamorphic event, but we speculate that these are resorption features rather than overgrowths. ‘Spongy’ or ‘amoeboid’ poikilitic rims are known to form around granulite-facies garnets during the introduction of external fluids (Baxter et al., 2017), or when garnets are engulfed in pseudotachylyte melts (Austrheim et al., 1996). Because they form under disequilibrium conditions, such resorption rims are unlikely to yield reliable P-T results, and this could account for the large range of P-T conditions Peterson & Geiger (1990) suggested for their ‘M2’ metamorphic event. Given the shattered nature of the Hardwood garnets, we tentatively speculate that the very fine-grained material in which they occur could represent coseismic fault rock – either (devitrified) pseudotachylyte or/and ultracataclasite flushed with seismically-pumped fluids. In this interpretation, the Hardwood complex would have experienced only one high-P/T metamorphic event, followed by mylonitization, cataclasis and seismic faulting. If the quasi- brittle deformation of the feldspars – which seems to be part of the same event that shattered the garnets -- can be interpreted as occurring at ca. 500°, the deformation would have had to happen well after the granulite-facies event. However, feldspar plasticity can be suppressed in very dry rocks (e.g. Bjørnerud & Austrheim, 2004), so it is also possible that the seismic event(s) occurred under high-temperature conditions and possibly soon after the granulite facies metamorphism that formed the inclusion-free garnets. Whether any of these events occurred during the Penokean orogeny remains unclear. One possible constraint on the timing of the main foliation-forming event comes from the occurrence of an unfoliated mafic within a feldspathic layer in the Hardwood Gneiss on the south bank of the East Branch of the Sturgeon River. If this sill could be dated or linked geochemically with known mafic magmatic units in the region, this would establish the youngest possible deformation age for the Hardwood Gneiss. The Hardwood pelites are classic garnet-biotite schists, with asymmetric quartz-vein boudins and garnet ‘tails’ that suggest normal-sense shear along the NE-dipping foliation. Low- angle normal faulting would be the most efficient way to juxtapose deep crustal rocks like the Hardwood Gneiss against the shallower units that surround it. But the pelites, which represent supracrustal material and record amphibolite rather granulite-facies conditions, lie on the western edge of the Hardwood outcrop area, so top-to-the-east normal slip does not help explain how the high-pressure mafic units were brought up from depth. The area between the Felch Trough and the Niagara Fault is among the most structurally complex of parts of the Penokean/Yavapai orogen, with many anastomosing faults of different generations (Drenth et al., 2021). The orientations of structures within the Hardwood complex have almost certainly been altered since their formation by later faulting and tilting. For now, the Hardwood Gneiss remains a micro- terrane of unknown provenance within the Penokean orogen. References Attoh, K. & Klasner, J., 1989. Tectonics 8: 911-933. Austrheim, H., Erambert, M., & Boundy, T., 1996. Earth & Planetary Science Letters 139: 223-238. Ayuso, R., et al., 2018. Institute on Lake Superior Geology Proceedings 64: 7-8. Baxter, E., Caddick, M., & Dragovic, B., 2017. Rev. Min. & Geochem. 83, 469–533. doi: 10.2138/rmg.2017.83.15 Bjørnerud, M. & Austrheim, H., 2004. Geology 32: 765-768. Cannon, W.F., Schulz, K., Ayuso, R. & Mroz, T., 2018. ILSG Field Trip Guidebook 64: 1-38. Drenth, B., Cannon, W.F., Schulz, K., & Ayuso, R., 2021. Precam. Res. 369. doi: 10.1016/j.precamres.2021.106205 Hawemann, F., et al., 2019. Solid Earth 10: 1635-1649. doi: 10.5194/se-10-1635-2019 James, H., Clark, L., Lamey, C., & Pettijohn, F., 1961. USGS Professional Paper 310. Peterson, J. & Geiger, C., 1990. Journal of Geology 98: 273-281. 90 �Alteration Geochemistry Characterization and 3D Modeling of the Back Forty Volcanogenic Massive Sulfide (VMS) Deposit Stephenson, Upper Peninsula of Michigan, USA UPTON, Margaret1, MOOERS, Howard1, LARSON, Phillip2 1 Department of Earth and Environmental Sciences, University of Minnesota - Duluth, 1114 Kirby Drive, 102 Heller Hall, Duluth, MN 55812 2 Cleveland-Cliffs Hibbing Taconite Company. Hibbing, MN 55746 The Gold Resources Back Forty zinc-and-gold-rich polymetallic volcanogenic massive sulfide (VMS) deposit is located near Stephenson in the Upper Peninsula of Michigan. In general, VMS deposits are created in submarine environments when heated seawater circulates through oceanic crust and precipitates base and precious metals at or near the seafloor due to both cooling and neutralization of the ore fluid. In the process, host rock mineralogy and geochemistry are modified by both downwelling and upwelling hydrothermal fluids, which produces distinct alteration mineral assemblages and metasomatic changes within the host rock (Shanks and Thurston, 2012; Galley et al., 2007). Alteration mineral assemblages and their spatial distribution can be used to unravel the geochemical evolution of the system and help locate mineralization. The relationship between host rock and alteration mineralogy is not well understood or documented at the Back Forty Deposit but essential for understanding its genesis. This study 1) identifies the alteration mineral assemblage present at the Back Forty Deposit using lithogeochemistry results; 2) calculates the elemental gains and losses associated with hydrothermal alteration; 3) develops a working method for immediate qualitative alteration values from core logging; and 4) creates a model of the alteration zonation in coordination with the existing stratigraphy and mineralization. Core from nine drill holes (~ 2,950 meters), were logged to observationally identify alteration mineral assemblages, intensity, and their textural characteristics. The deposit, hosted in felsic pyroclastic rocks, shows mostly sericite alteration, which was used to establish an alteration intensity scale of 1-4 (1: weak, 5: intense). Major alteration mineral assemblages observed were sericite ± silica ± chlorite. Sericite alteration is pervasive throughout the deposit (2.5-3.5) with silica alteration intensity ranging from 1-2 and a few areas of silica flooding (3.5-4.5). Weak to moderate (1.5-2.5) chlorite alteration occurred throughout the deposit within the host rhyolite crystal tuff units as spotty chlorite coarse-grained agglomerations. Lithogeochemistry (1,300 count) was evaluated using the alteration box plot (Large et al., 2001) and the isocon mass balance method (Grant, 1986) (fig. 1), which are essential in quantitative assessment of chemical changes associated with alteration mineral assemblages and their spatial distribution, and the identification of hydrothermal fluid pathways and mineralization vectors within the deposit. In addition to using isocon results, alteration box plot results were modeled based on sericite, chlorite, and total alteration. The production of cross sections based upon this numerical modeling identify the alteration mineral zonation and its relative extent; this model is evaluated to determine the relationship between massive sulfide mineralization and alteration intensity. From these results, downhole core logging of alteration assigned numeric values (“quick log”) is evaluated as a method to make fast-paced exploration decisions while awaiting longer lead-time lithogeochemical results. By leveraging the process and combination of core logging for alteration mineralogy and intensity paired with geochemical analysis, it may be possible to determine the origin direction of hydrothermal fluid flow associated with mineral deposition and aid in future exploration efforts to locate additional mineralization on the Back Forty Deposit property. 91 �Results from this study show the sericite alteration is most significantly related to Zn and Cu mineralization, whereas the chlorite alteration is most associated with Ag, Au, and Pb. Distinctly depleted species associated with sericite alteration include Ba, Sr, Na2O, Rb; with chlorite commonly depleted in Br, Ba, Sr, Na2O. Least v. Intense Sericite Alteration 50 45 45 Be Au Ge Zn Ga U Cu 40 35 More Altered Cs Dy Ce La Ag Ni Cr 20 Tl Hf Sn Sc TiO2 10 MgO As Yb Tl Mo Zr Pr Pr Hg Cd U Ir 15 Lu Sc 10 Hf V TiO2 Te Ba Tb MnO Br Eu Ga Cs Co Nd K2O Be In 20 Er Ho Y Cr2O3 Nb Ge y = 0.996x R² = 0.996 Na2O CaO 10 15 20 25 30 5 NdYb Sm Br Rb Gd Dy Tm Eu Bi Ba AL2O3 K2O 35 40 45 50 Cr Ta Sr y = 1.067x R² = 0.999 Na2O CaO BaO Re 0 Least Altered Th MgO P2O5 0 Lu SiO2 La Ce Ir Ho Sr 5 Ni Pb In Cd 0 25 BaO Tb V Co Bi Gd Er Hg Fe2O3 5 Rb Pb Y MnO Sb Cu Fe2O3 Sm P2O5 15 Tm Cr2O3 Ta Se W 30 W Zr 25 Zn 35 SiO2 Nb 0 40 Th 30 Least v. Intense Chlorite Altered 50 5 10 15 20 25 30 35 40 45 50 Least Altered Figure 1. ISOCON plot of selected elements used to compare elemental gains and losses between least and most altered samples. Isocon line of best fit is defined by relative immobile. Species above the isocon line are enriched; below are depleted (Grant, 1986). References Aquila Resources (now Gold Resources), data current as of April 2021. Galley, A., Hannington, M., Jonasson, I., 2007. Volcanogenic Massive Sulphide Deposits. Geological Survey of Canada, Special Publication 5: 141-161. Grant, J. A., 1986. The Isocon Diagram: A Simple Solution to Gresens' Equation for Metasomatic Alteration. Economic Geology, v. 81: 1976-1982. Large, R. R., Gemmell, B.J., Paulick, H., 2001. The Alteration Box Plot: A Simple Approach to Understanding the Relationship between Alteration Mineralogy & Lithogeochemistry Associated with Volcanic-Hosted Massive Sulfide Deposits. Economic Geology, v. 96: 957-971. Shanks, W.C.P., Thurston, R., 2012. Volcanogenic Massive Sulfide Occurrence Models. USGS Scientific Investigations Report 2010–5070–C: 363. 92 �Summary of the 2022 ILSG Field Trip to Iceland UPTON, Margaret1, LARSON, Phillip2, MACTAVISH, Allan3, HINZ, Peter4 1 Department of Earth and Environmental Sciences, University of Minnesota - Duluth, 1114 Kirby Drive, 102 Heller Hall, Duluth, MN 55812 2 Cleveland-Cliffs Hibbing Taconite Company. Hibbing, MN 55746 3 AGC GeoConsulting, 777 Red River Road, Thunder Bay, ON P7B IJ9 4 Retired, Ontario Ministry of Energy, Northern Development and Mines, Thunder Bay, ON During Summer of 2022 (July 26-August 9), a group of 16 people set out to tour the diverse and awe-inspiring geology of Iceland, led by ILSG representative geologists Phil Larson, Peter Hinz, and Allan MacTavish. The 15 day trip held many surprises for all involved, including the worst stretch of weather Phil has experienced in Iceland (of 11 visits!) as well as a once-in-a-lifetime experience to see a volcanic eruption. In addition to the trip leaders, the group of 16 people included 3.5 professional geologists, 1.5 graduate students, one retiree, one Goldich Medal laureate, and 9 members of the Minnesota Geological Society. Stops throughout the trip focused on a wide range of topics: • Volcanism, both historical and the 2021 Geldingadalir eruption; Figure 1. Photo credit: Tom Hart • Icelandic cuisine, lore, and the historical and cultural evolution of the nation; • Environmental geochemistry of subsurface and near-surface processes; • Volcanic flows, igneous petrology for mineralogy and volcanic textures; • Geothermal energy and its uses; • Hydrology and hydrologic events related to glaciers and volcanics; • Geomorphology as it relates to ecology, volcanics, and glaciers • Wind, water, and glacial erosional features; glacial nomenclature By special arrangement, the trip was scheduled to overlap with the onset of the 2022 Meradalir eruption (fig.1). An advance party made a midnight scouting foray to the vent site before a Force 13 gale descended on the island. 93 �This presentation summarizes the highlights from the trip (fig.2). Featured locations include the Fagradalsfjall eruptions on the Reykjanes Peninsula, the Vestmannaeyjar Islands, climbing atop and viewing the Laki fissure from above, trekking to the highlands to view Askja and its pumice fields, the Jökulsárgljúfur canyon and scablands, free roadside hákarl stands, being lowered into a dormant magma chamber, a sampling of geothermal pools, plus the many epic waterfalls along the way! Figure 2. Generalized map of the trip route. 94 �GEOHERITAGE AS AN EDUCATIONAL TOOL TO EXPLORE RELATIONSHIPS WITH LAND AND WATER IN THE KEWEENAW VYE, Erika1 and ROSE, William2 1 Great Lakes Research Center, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931 Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931 2 Geoheritage is an 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). Geoheritage strongly emphasizes the importance of the varied personal values people have for geologic features and the wide-ranging relationships we have with landscapes. As such, geoheritage is an effective geoscience communication tool affording place-based learning experiences that nurture our sense of place, deepen our Earth science literacy, and inspire stewardship and protection of our place. This presentation explores geoheritage education and outreach initiatives in Michigan’s Keweenaw Peninsula for both formal and informal learning communities. The Keweenaw Peninsula sits at the heart of the Midcontinent Rift and is renowned for the world’s largest accessible native copper deposit and Lake Superior, the largest freshwater lake on Earth. These geologic processes and features have fostered varied human relationships with the landscape, including the oldest metal workings in the Western Hemisphere and the European immigration wave of 18401910 triggered by the Copper Boom. This intersection of deep time, industrial, and cultural heritage has been the focus of teacher professional learning institutes and student internship experiences that explore the compelling geoheritage of our place. These programs: a) focus on complex environmental issues rooted in Earth systems processes of importance within the community, b) emphasize strong community partnerships that bring together varied values and perspectives of our place; c) explore other ways of knowing about the dynamic and interconnected geologic and human stories that serve as the foundation of the landscape’s past, present, and future through equitable knowledge exchange, and d) elevate Earth science literacy for educators and students by connecting the underpinning geology to current environmental issues with wide-ranging impacts in our communities today such as cultural identity, subsistence uses, recreation, and sense of place. The geologic formations of the Midcontinent Rift are beautifully exposed in the Keweenaw for researchers, teachers, students, and geotourists. As the Keweenaw shifts from an extractive industrial economic past, geoheritage initiatives support a future based on education, conservation, and sustainable tourism. Current initiatives in our community include a) the development of geotourism opportunities - Keweenaw Geotours, b) strong partnerships with local conservation groups to maintain access to world-class geosites that provide outstanding Earth science learning opportunities, and c) exploration of recreational opportunities including the concept of a shoreline hiking trail following the high water mark of Lake Superior. Geoheritage education and outreach opportunities help foster a culture of stewardship, increase Earth science literacy, and provide opportunities to share our varied relationships with land and water. 95 �Figure 1. Teachers and students explore the geoheritage of the Keweenaw by land and water. References 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. Geological Society of America 2017. GSA Position Statement: Geoheritage. Retrieved from: https://www.geosociety.org/documents/gsa/positions/pos20_Geoheritage.pdf. National Park Service and American Geosciences Institute 2015. America’s Geologic Heritage: An Invitation to Leadership. NPS 999/129325. National Park Service, Denver, Colorado. Reynard, E. and Brilha, J., 2017. Geoheritage: Assessment, protection, and management. Elsevier, ISBN: 9780128095317. 96 �U/Pb geochronology and zircon petrochronology of Paleoproterozoic magmas from the Marshfield terrane Penokean Orogen, Wisconsin WEBER, Evan1, LODGE, Robert W.D.1, MARSH, Jeffrey2 1 Department of Geology and Environmental Science, University of Wisconsin-Eau Claire, Phillips Hall Eau Claire, WI 54701 2 Department of Earth Sciences, Laurentian University, 933 Ramsey Lake Rd, Sudbury, ON P3E 6H5, Canada This study presents U-Pb, Hf-Lu, and trace isotopic element data from zircons obtained from volcanic and intrusive rocks from the Paleproterozoic Penokean magmas within the Marshfield terrane in northern Wisconsin. The Penokean Orogen hosts both the Proterozoic Pembine-Wausau and the Archean Marshfield terranes. The Eau Pleine Shear Zone is interpreted as the paleosuture zone between these two terranes (Sims et al., 1989). Both terranes host volcanic and intrusive rocks that were formed during the Penokean orogen. The Pembine-Wausau terrane is a juvenile arc system that was developed through subduction during the Penokean orogen that accreted against the Superior craton. The volcanic rocks in this terrane are tholeiitic and calcalkaline in nature (Schulz and Cannon, 2007). The Marshfield terrane is thought to be an accreted fragment of an Archean craton that collided with the Pembine-Wausau terrane and the Superior craton (Klier, 2019). The Marshfield terrane is mainly comprised of gneisses that underlie Early Proterozoic volcanic rocks (Sims et al., 1989), but due to poor exposure of these rocks this terrane is poorly understood. This study aims to provide a better understanding of the volcanic terranes in the region to improve regional models of the southern portion of the Penokean orogen. Samples were collected from Big Falls and other locations within the Eau Claire volcanic complex, as well as from granites and gneisses exposed in Black River Falls. Zircons from these samples were then analyzed at Laurentian University (Sudbury, Ontario, Canada) via Split-Stream Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LASS-ICP-MS) to obtain U-Pb, HfLu, and trace isotopic element data. Results reveal complex age relationships and basement architectures. The Big Falls gneiss, part of the Eau Claire volcanic complex lying south of the Eau Pleine Shear Zone (Fig. 1), resulted in an interpreted U-Pb age of 1874.7±2.1 Ma (Fig. 1) which is consistent with VMS-forming events in the Pembine-Wausau terrane. Zircon trace element geochemistry from the Eau Claire volcanic complex indicate rocks formed from a hydrated but reduced melt. This melt may have occurred in a back-arc setting where decompression occurred in a metasomatized mantle, which is characteristic of back-arc signatures. Hf-Lu isotopic data from the Eau Claire volcanic complex show the rocks here lack an Archean inheritance. The data from the Eau Claire volcanic complex was compared to a granite intrusion in Black River falls and both the Eisenbrey and Lynne VMS deposits in the Pembine-Wausau terrane. Based on Hf-Lu data, the Black River Falls granite showed inheritance of basement, which is expected based on field relationships with Archean rocks from the Marshfield terrane. The Eisenbrey and Lynne deposit have juvenile signatures which is characteristic of an oceanic arc system. According to trace isotopic element data, the VMS deposits also formed from a more oxidized and hydrated melt which is a similar geodynamic setting seen in the Eau Claire volcanic complex. Since we would expect basement inheritance in the Eau Claire volcanic complex, these results question what is known about the Marshfield terrane and its relationship to the Penokean. 97 �Figure 1. Geologic map of Eau Claire and Chippewa Falls area highlighting the location of Big Falls alongside a concordia diagram plotting the age of Big Falls at 1874.7±2.1 Ma. Cathodoluminescence imaging of individual zircons are also shown with their corresponding ages. 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. Klier, J.J., 2019. The Marshfield Terrane: Redefinition of Origin Through Zircon Geochronology and Geochemistry [MSc thesis]: Ball State University: 115. Schulz, K.J. and Cannon, W.F., 2007. The Penokean orogeny in the Lake Superior region. Precambrian Research 157: 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 Science, v. 26: 2145-2158. 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 Pembine-Wausau terrane, Wisconsin, USA: GSA Bulletin, v. 134: 776–790. 98 �The Use of Electric Pulse Disaggregation Technology to Recover Nickel Metal from Nickel Sulfide Ore Deposits WEIBLEN, Paul1 1 Minnesota Geological Survey (Retired), 2609 West Territorial Road, St. Paul, MN 55114 All metals, except for the noble metals like gold, occur in nature as metal sulfides. The chemical process “Plat Sol”1 can be used to recover nickel metal from nickel sulfide ores. The demand for nickel metal has increased dramatically due to the need for nickel metal for electric vehicle batteries. Elon Musk, always ahead of the curve, has signed an agreement with Talon Metals to be the sole recipient of any nickel metal they produce. Similarly, the Biden Administration is encouraging a transition from fossil Fuel vehicles to electric vehicles.2 However, a particle size of less than a millimeter is required for the feed to the Plat Sol process. Electric Pulse Disaggregation Technology3 provides much more efficient and less expensive method than conventional crushing and grinding for reducing the particle size of ore samples. Figure 1 provides details on the disaggregater. Inside the 3D printed gray cap on the left below is a stainless steel hemisphere with a pointed electrode projecting upward. On the right, is a black 3D printed cap with an electrode like the one above. When the two caps are screwed together, a sphere is formed. The electrodes are separated ~ 5 mm forming a spark gap. The two hemispheres are filled with water and inch-sized sample fragments. When the 50KV power supply is turned on the discharge across the spark gap vaporizes the water, which in turn separates different minerals along their grain boundaries. Examples of “zapped” Talon Metals nickel sulfide ore will be shown. Figure 1. Image of the disaggregator set up. References Google “Plat Sol” https://www.whitehouse.gov/briefing-room/statements-releases/2021/08/05/fact-sheet-president-bidenannounces-steps-to-drive-american-leadership-forward-on-clean-cars-and-trucks/ https://www.researchgate.net/project/Electric-pulse-disaggregation-and-hydroseparation-for-mineral-processing *** Abstract Withdrawn*** 99 � https://digitalcollections.lakeheadu.ca/files/original/0b0e110830002a4043b2d65b71c5ca51.pdf 2cbbb59065186547c55e7817dcd81e3e PDF Text Text 69th ANNUAL MEETING Eau Claire, Wisconsin — April 24-25, 2023 INSTITUTE ON LAKE SUPERIOR GEOLOGY Part 2 — Field Trip Guidebooks �Thank you to our sponsors! A SPECIAL THANK YOU TO OUR INDIVIDUAL CONTRIBUTORS: FREDERICK CAMPBELL, VAL CHANDLER, JIM DEGRAFF, THOMAS ERICKSON, TOM FITZ, DAVE GOOD, PAULA LEIER-ENGELHARDT, ALLAN MACTAVISH, BOB MAHIN, GORDON MEDARIS JR., JIM MILLER, STEVEN PINTA, TOD ROUSH, AND GERRY WHITE i �Proceedings of the 69th ILSG Annual Meeting – Part 2 69th ANNUAL MEETING INSTITUTE ON LAKE SUPERIOR GEOLOGY April 24-25th Eau Claire, Wisconsin HOSTED BY Rob Lodge, Esther Stewart, Carsyn Ames Co-Chairs University of Wisconsin- Eau Claire and Wisconsin Geological and Natural History Survey Proceedings - Volume 69 Part 2 – Field Trip Guidebooks Compiled and edited by Rob Lodge Cover Photos. Upper — Photograph of E.O. Ulrich taking notes in the field describing the Cambrian Mount Simon Formation in the Chippewa Falls region in 1913. Lower — Photograph of geologists E.F. Bean and E.C. Edwards fording the Eau Claire River at Morrison’s Ford in 1919. iii �Proceedings of the 69th ILSG Annual Meeting – Part 2 69th INSTITUTE ON LAKE SUPERIOR GEOLOGY VOLUME 69 CONSISTS OF: PART 1: PROGRAM AND ABSTRACTS PART 2: FIELD T RIP GUIDEBOOK Trip 1: PRECAMBRIAN GEOLOGY OF THE CHIPPEWA RIVER VALLEY Trip 2: WISCONSIN’S PALEOZOIC STRATIGRAPHY AND TOUR OF CRYSTAL CAVE Trip 3: PRECAMBRIAN GEOLOGY OF THE EAU CLAIRE RIVER VALLEY Trip 4: QUATERNARY GEOLOGY AND GEOMORPHOLOGY OF THE EAU CLAIRE REGION Reference to material in Part 2 should follow the example below: Lodge and Hooper, 2023. Precambrian geology of the Chippewa River Valley: A transect through the western Marshfield Terrane. in Lodge, R.W.D. (Ed.), Institute on Lake Superior Geology Proceedings, 69th Annual Meeting, Eau Claire, Wisconsin, Part 2 – Field Trip Guidebooks. v.69, part 2, p.1-26. Published by the 69th 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 iv �Proceedings of the 69th ILSG Annual Meeting – Part 2 Part 2: Field Trip Guidebooks Table of Contents Page Field Trip 1: Precambrian geology of the Chippewa River valley: A transect through the western Marshfield Terrane Field Trip 2: Wisconsin’s Paleozoic stratigraphy and tour of Crystal Cave Field Trip 3: Precambrian Geology of the Eau Claire River Valley: Re-discovering the Eau Claire Volcanic Complex Field Trip 4: Quaternary Geology and Geomorphology of the Eau Claire Region v 1 27 48 71 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Field Trip 1 – Precambrian geology of the Chippewa River Valley: A transect through the western Marshfield Terrane Robert W.D. Lodge and Robert L. Hooper Department of Geology & Environmental Science, University of Wisconsin-Eau Claire, Eau Claire, Wisconsin 54701 of the 18.2 Mt Back Forty VMS deposit in Michigan, easing of the Wisconsin sulfide mining moratorium in 2017, and a recent national push for securing critical mineral resources. However, this has also highlighted the lack of modern datasets, notably lithogeochemistry, on these deposits that could be used to further our knowledge of the mineral-forming systems in the VMS belt. The Pembine-Wausau terrane has received most of the historic and recent attention since it hosts approximately 150 million tonnes of known VMS mineralization. However, little attention has been given to the Penokean volcanic deposits that overprinted the Marshfield Terrane that are presented in this guidebook. DeMatties (2022) recognized the gap in knowledge for these Penokean volcanic deposits within the Marshfield Terrane, also called the Eau Claire Volcanic Complex, and highlighted their exploration potential. Introduction The erosional outliers of Precambrian bedrock in the Chippewa River Valley represent the southernmost extent of the Canadian Shield before it is completely covered by Paleozoic sedimentary strata. The rocks exposed here are part of the Paleoproterozoic Penokean Orogeny, a collisional orogen that resulted from the accretion of the Pembine-Wausau and Marshfield terranes onto the (present-day) southern margin of the Superior Province. This region was last visited by members of the Institute of Lake Superior Geology in 1980 when a field trip through the region was conducted by Paul Myers (Myers et al., 1980). Since this time, there has been ‘new’ U/Pb data collected by the USGS (Sims et al. 1989) and others (Van Wyke et al, 1997; Klier, 2019), regional syntheses of the Penokean volcanogenic massive sulfide (VMS) mineralization (DeMatties 1989; 1994; 2018; 2022), maps published by government surveys (Mudrey et al, 1987; Brown 1988), and orogenwide tectonic model (Shultz and Cannon, 2007) that is being revisited based on new U/Pb data (Zi et al., 2021). After forty years of advancing our knowledge of the Penokean Orogen, it is worth touring again. The portion of the Marshfield terrane that is visited in this guidebook is well known, but grossly understudied and much of its regional context is unknown. Students from the University of Wisconsin-Eau Claire have been visiting many of the locations in this guidebook for decades to learn how to map and describe rocks in the field, measure structures and interpret geologic histories, and learn the basic mechanics of field work. Faculty, students, and alumni from Eau Claire consider these outcrops classic. This guidebook will (re-)introduce these rocks and provide an updated view on their context to the Marshfield terrane and Penokean Orogen. Ongoing research in this region hopes to expand the lithogeochemistry and zircon petrochronology database to better delineate the The Penokean Orogen is perhaps best known for hosting numerous VMS deposits. The passing of the “Prove-it-first” law, or sulfide mining moratorium, in 1997 effectively shut down mineral exploration and mining in Wisconsin. One of the most complete descriptions of several deposits was published by the Institute of Lake Superior Geology (LeBarge, 1996). More recently, the mineral exploration industry has been reinvigorated because of the 2002 discovery 1 �Proceedings of the 69th ILSG Annual Meeting - Part 2 geodynamic evolution and crustal architecture of this region. Determining the presence or absence of Archean basement throughout the Marshfield terrane will help refine terrane boundaries and improve our understanding of the metallogeny of the region to assist in future mineral exploration efforts. in a suprasubduction zone setting and are now structurally juxtaposed along the southern edge of the Archean Superior Province during the earliest phases of forming the Columbia, or Nuna, supercontinent (LaBerge and Myers, 1984; Sims et al., 1989; Schulz and Cannon, 2007). The orogen is host to at least 150 million metric tonnes (Mt) of VMS and associated mineralization (DeMatties, 1994, 2018) but remains one of the more poorly understood and underexplored mineral districts in North America. Regional Geology The Paleoproterozoic Penokean Orogen (ca. 1.8 Ga) in the Lake Superior region (Figure 1) is a classic Precambrian orogenic belt comprised of dominantly sub-marine volcanic rocks and associated plutons. The Penokean rocks formed The Penokean Orogen has been divided into the Interior and Exterior domains. The Exterior Figure 1 – Geologic map of the major tectonic assemblages and major structures of the Penokean Orogen. Notable and important abbreviations that are important for this guidebook are EPSZ, Eau Pleine shear zone; NFZ, Niagara fault zone. Figure from Shultz & Cannon (2007). 2 �Proceedings of the 69th ILSG Annual Meeting - Part 2 domains are sutured to the Superior Craton by the Niagara fault zone (Figure 1). The Exterior domain consists of passive margin, rift, and forearc basin sediments and Archean crustal blocks from the Superior Province that were folded and faulted in the foreland part of the orogen. The Interior Domain consists of two accreted terranes, the Pembine-Wausau and Marshfield terranes, that are sutured by the Eau Pleine Shear zone (Figure 1). The PembineWausau terrane is a composite accreted oceanic arc overprinted by continental-margin magmatism and hosts numerous VMS deposits and occurrences (DeMatties, 1994; Shultz & Cannon, 2007) (Figure 2). The Marshfield terrane is composed of Archean crustal fragments of unknown origin that were overprinted by Penokean magmas during the orogen (Figure 2) and is described in more detail in the section to follow. Shultz and Cannon (2007) synthesized tectonic events during the Penokean Orogeny based on a detailed compilation of lithologic, structural, sedimentological, isotopic, and geochronological datasets. This classic model proposed that an oceanic arc, now referred to as the PembineWausau terrane, collided with the southern Superior Province around 1880 Ma. Following a subduction flip from south-directed to northdirected subduction, continental arc magmatism and back arc extension followed until about 1850 Ma when convergence with an Archean crustal block, known as the Marshfield Terrane accreted to the southern edge of the Wausau- Pembine Terrane along the Eau Pleine Shear Zone (ESPZ). Sedimentation related to this convergence in a foreland basin setting continued until about 1835 Ma. The end of the Penokean orogen was constrained by a series of undeformed posttectonic plutons dated at 1830 Ma which stitched the terranes. Figure 2 – Schematic tectonic evolution of the Penokean Orogen provided by Shultz and Cannon (2007) based on geophysical, sedimentological, and geochronological complications. contradictory data came when Quigley (2016) obtained a high-precision U/Pb zircon age of 1832.98 ± 0.52 Ma from a rhyolite at the Back Forty deposit via CA-ID-TIMS. The other analyzed VMS deposits across the PembineWausau terrane by Quigley (2016) provided consistent U/Pb zircon ages ca. 1875 Ma and supported the Shultz and Cannon (2007) tectonic model. Additional U/Pb zircon ages from volcanic units (Beecher Formation) and plutonic rocks (Dunbar Gneiss, Newingham Tonalite) in the eastern part of the orogen by Zi et al. (2021) However, this classic tectonic model for the evolution of the Penokean orogen has recently been re-evaluated in light of new U/Pb data obtained throughout the orogen. The first 3 �Proceedings of the 69th ILSG Annual Meeting - Part 2 supported the younger extensional tectonic event proposed by Quigley (2016). These new ages resulted in a revised Penokean tectonic model where long-lived northward subduction along a continental margin with repeated extensional and contractional regimes in response to retreat and advance of the subducting oceanic plate (Figure 3). Weber and Lodge (2022) obtained a U/Pb age of 1831.4 ± 2.0 Ma on the dacite unit hosting the Eisenbrey deposit in the western part of the orogen, suggesting that this second VMS forming event was widespread. A summary of the geochronology is presented in Figure 4. part of the Marshfield terrane and lie immediately south of the Eau Pleine Shear Zone. Current tectonic models suggest that the Marshfield Terrane represents an Archean microcontinent of uncertain origins (Sims et al., 1989; Schulz and Cannon, 2007; Zi et al., 2021). Some of the earliest work on the terrane by Sims et al. (1989) noted only eight Archean U/Pb ages from isolated outcrops along the Wisconsin, Black, and Chippewa Rivers, many of which were compiled from unpublished sources. One of those was the gneiss exposed at Jim Falls (Stop 3 in this guidebook) which was dated at 2522 ± 22 Ma. Current tectonic reconstructions usually have Paleoproterozoic volcanic rocks in the Marshfield terrane being deposited on Archean basement at about 1870–1860 Ma. The Paleoproterozoic volcanic sequence is referred to as the Eau Claire Volcanic Complex by DeMatties (2018; 2022) and are preserved only as erosional remnants. The Eau Claire Volcanic Complex consists principally of an interlayered sequence of felsic to mafic volcanic rocks, dacite porphyry, and a variety of clastic and chemical sedimentary rocks (Sims et al., 1989). Some conglomerates contain granitic gneissic clasts that were interpreted to be Archean (Myers et al. 1980), but no definitive ages were determined on the clasts. Throughout the Marshfield terrane there are various Paleoproterozoic intrusions of gabbro, diorite, and tonalite. These have U/Pb ages of 1835-1865 Ma (Sims et al., 1989; Van Wyck and Johnson, 1997; Weber and Lodge, 2022). Otherwise, our knowledge of the Archean basement of the Marshfield terrane and associated Paleoproterozoic volcanic rocks remains as sparse as the outcrop exposures. Figure 3 - Schematic illustration of the revised tectonic model of the Penokean Orogen. Figure is from Zi et al. (2021). Abbreviations: NF—Niagara fault zone; EPSZ—Eau Pleine shear zone.Marshfield Terrane The study of the Marshfield terrane remained stagnant until new U/Pb and Lu-Hf isotopic data from zircons was published as a masters thesis (Kleir, 2019). The new isotopic data in the Marshfield Terrane collected from the Chippewa and Yellow River valleys will be presented at various stops on this field trip. In our opinion, one of the most significant results was that the “Archean” rocks from the Jim Falls region of Marshfield Terrane This guidebook visits field sites from the northwestern exposures of rocks interpreted to be 4 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 4 - Time-space plot for the tectonic components of the Penokean Orogen. Plot is from Zi et al. (2021). See citation for references on data sources. Sims et al. (1989) is a metasedimentary rock that has a significant proportion of Paleoproterozoic zircons (Kleir, 2019). While the data clearly indicates the presence of Archean rocks in the sedimentary source region, the sedimentary provenance does not require that Archean rocks represent the basement architecture in the northern part of the Marshfield Terrane. U/Pb ages from the northern part of the Marshfield Terrane collected in the Chippewa and Yellow River areas are interpreted as Paleoproterozoic in age (~1.83-1.88 Ga) and Hf isotopies indicate a juvenile source (without Archean contributions). This finding raises questions about the extent of the Archean basement in the Marshfield Terrane and consequently, the basement architecture in 5 �Proceedings of the 69th ILSG Annual Meeting - Part 2 the region. Preliminary geochemistry from Klier (2019) and our ongoing studies in the region show interesting trends that will help distinguish petrogenetic processes. Figure 5 highlights the REE trace element characteristics of these deposits and their application to each stop in subsequent sections below. thermometry determined temperatures between 719-769°C (Hannack and Radwany, 2018). A rutile U/Pb age of 1835 Ma from Sims et al. (1989) in the Eau Claire Volcanic Complex (Big Falls – Fieldtrip 3 in this volume) may indicate the timing of metamorphism since new zircon U/Pb age from the same region provided a crystallization age of ~ 1875 Ma (Weber and Lodge, 2022). Regional metamorphism in this region is at lower to upper amphibolite facies. Hornblendeplagioclase thermo-barometry from gneisses in the Chippewa River valley indicate metamorphism occurred at temperatures between 606-646°C and pressures between 5.74-6.64 Kbar (Hafften and Radwany, 2018). A sample of amphibolitic gneiss from the Eau Claire Volcanic Complex using the edenite-richterite Field Trip Stops The overall objective to this guidebook is to tour the Precambrian exposures of the Marshfield terrane along a southwest-northeast transect as exposed in the Chippewa River Valley. Starting within the city of Chippewa Falls, the trip will work its way to the northwest along the river and presumably get closer to the terrane boundary at the Eau Pleine Shear Zone. Stops 1-4 and 6 are all within the Marshfield Terrane, whereas Stop 5 is considered the southernmost exposure of the Pembine-Wausau Terrane. Fieldtrip stops are summarized in Figure 6. New data have us questioning what we know about the Marshfield Terrane. For example: Where exactly is the northern boundary of the Marshfield terrane in the Eau Claire region, and how much of the Marshfield Terrane, as currently defined, has an Archean basement architecture? Most of the locations in this guidebook are at the downstream side of hydro-electric dams. These areas are prone to sudden flooding and the upmost caution and careful planning should be used prior to visiting these locations. In addition, rocks here are uneven and slippery especially when wet. To access larger sections of outcrops, low water conditions or ladders (temporary bridges) may be required. In addition, all locations in this region may contain poisonous plants (e.g. nettle, poison ivy) and black-legged ticks that can transmit diseases. While this is unlikely to be a concern in early spring during the 2023 ILSG conference, future users of this manual should plan appropriately. Figure 5 - Trace element diagrams from the rocks in the Chippewa River valley region. Data from Cornell is preliminary data from ongoing projects whereas the remainder is from Klier (2019). (A) Classification diagram from Pearce (1996) showing protolith compositions. (B) Primitive mantle-normalized rare earth element diagram using normalizing values from Sun and McDonough (1989). 6 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 6 - Regional geology of the Chippewa Falls and Eau Claire region showing fieldtrip stops and approximate location of the Eau Pleine Shear zone. Rocks to the south of the Shear Zone are interpreted to be part of the Marshfield Terrane, whereas rocks to the north are part of the Pembine-Wausau terrane. Figure compiled from Mudrey et al. (1987) and Brown (1988). 7 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Stop #1 – Nonconformity at Irvine Park Claire and Chippewa Falls region are some of the southernmost exposures of the crystalline basement in the Lake Superior region before it disappears beneath the undeformed Paleozoic sedimentary strata. This is one of the many exposures of the “Great Unconformity” that is present throughout this region. Details of this unconformity in this region is described in detail in the most recent ILSG guidebook presented in Eau Claire (Chan et al. 1991) and is summarized below. Lat: 44.9542° Long: -91.3972° Precambrian Unconformity The Precambrian- Cambrian boundary is represented by a highly variable surface in the mid-continent area. In west-central Wisconsin, the Precambrian surface forms an extensive planation surface with a regional SW dip of less than 1 degree. Archean iron formations in the Black River Falls region and Proterozoic quartzites throughout the state, most famously the Baraboo Syncline, form isolated monadnocks on the peneplain. The peneplain was mantled by a layer of paleosols as much as several hundred feet thick. In some areas, Cambrian rocks directly rest upon barren, moderately weathered Precambrian rocks. Considering the low paleolatitude of the continent during the Cambrian, deep weathering of the Precambrian surface must have occurred before the Upper Cambrian deposition. The Precambrian basement, however, shows variable degrees of weathering and the weathering is complicated by potassium metasomatism overprinting associated with Silurian and Devonian K-rich basinal brines (Lui, 1997; Lui et al., 2003). Potassium metasomatism along the unconformity is responsible for the development of illite, interlayered I/S and authigenic Kfeldspar in both saprolites and in the Cambrian rocks in the Chippewa River Valley. Where the Precambrian is mafic (gabbros, amphibolites and gneisses) the potassium metasomatism commonly produces a distinctive bright bluegreen clay (celadonite) seen at Stop 2 on this field trip and at Big Falls (Fieldtrip 3 in this guidebook). These potassic brines have been The outcrop described at this stop is located along the east bank of Duncan Creek within Irvine Park in Chippewa Falls. Upon entering the park, drive north on Irvine Park Drive past the zoo and bison enclosure until the inter-section with Bear Den Road. There is ample parking in this area near the intersection that crosses Duncan Creek to the east and find the small foot trail that leads northward to the outcrop. Potential hazards include poisonous plants and ticks, but they are unlikely to be a problem in early spring. There is also uneven and potentially wet ground. The purpose of this location is to highlight the conditions that are impeding the study of the Precambrian bedrock in the region. The Eau 8 �Proceedings of the 69th ILSG Annual Meeting - Part 2 implicated in the formation of MVT deposits in the Tri-state region (Aleinkoff et al., 1993). At this location (and numerous others) where the Cambrian formations are in contact with Precambrian plutonic rocks, the basement is heavily spheroidal weathered (Photo 1, Figure 7) and is generally deeply altered to kaolinite saprolite and then metasomatically altered to illite I/S and authigenic Kspar. Mt. Simon Formation sandstone and conglomerate fill the wedges among the spheroids. In some areas such as Little Falls and Big Falls in Eau Claire County (Fieldtrip 3, this volume) or Rock Dam in Jackson County, Cambrian sandstones rest upon Proterozoic amphibolite and meta-rhyolites that are only partially altered. Mount Simon Formation is a coarse-grained, medium to thick-bedded quartz pebble conglomerate. The topographic relief on the Precambrian surface was probably only a few meters as sedimentary channels are typically less than 1 meter deep. The presence of trace fossils (rusophycus and Climactichnites, or trilobite burrows/tracks; Photo 2) and planar and bipolar cross-bedding (Photo 3) suggest a littoral or shallow marine tidal flat environment of deposition for the lower part of the formation. The upper part of the Mt. Simon Formation contains feldspathic quartz arenite with smallscale ripple bedding, brachiopod fragments (lingula sp.), and worm trails (planolites) The Photo 2 - Climactichnites fossil from the lower Mt. Simon Formation collected along the Chippewa River near downtown Eau Claire. Climachtinites trace fossils are typical of tidal flats in the Cambrian. Field of view is ~1m across. Photo 1 – Irvine Park outcrop photograph showing nonconformity between Cambrian Mt. Simon Formation (above) and Paleoproterozoic trondhjemite (below). Photo courtesy of Scott Clark (UW-Eau Claire Cambrian Mount Simon Formation The sediment above the unconformity consists largely of Upper Cambrian Mt. Simon Formation deposited on the mid-continent region of North America during the Dreisbachian transgression. The Mount Simon Formation is a fine to coarsegrained, moderately to well sorted, quartz arenite with a local basal conglomerate. The Mount Simon Formation varies in thickness 40 to 180 meters in the Upper Mississippi Valley. Locally, in the Chippewa Valley area, the lower part of the Photo 3 - Cross-bedded conglomerate and sandstone of the Cambrian Mount Simon Formation in the Irvine Park area, Chippewa Falls. Photo courtesy of Scott Clark (UW-Eau Claire). 9 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 7 – Conceptual field sketch of the unconformity at Irvine Park. Figure from Chan et al. (1991). Stop 2 – Penokean and Mid-Continent Rift Intrusions at Lake Wissota Dam grain size distribution shows a generally fining upward sequence. Precambrian Intrusion Lat: 44.9429° Long: -91.3425° The Paleoproterozoic biotite tonalite (trondhjemite) showing spheroidal and saprolitic weathering at this location is interpreted to be similar to the larger trondhjemite intrusion that underlies much of the Chippewa River valley. The trondhjemite can be more easily observed below the Chippewa Falls hydroelectric dam in downtown Chippewa Falls and below the Wissota hydroelectric dam (Stop 2 in this guidebook). The biotite trondhjemite at Chippewa Falls Hydro was dated by Van Schmus (1980) at 1,840 ± 15 Ma. Saprolites like the one exposed here are characteristic of areas of prolonged tropical to subtropical weathering on a granitoid bedrock surface of low relief. The saprolite at this outcrop contains high clay content and angular quartz and feldspar. The alteration intensity increases approaching the Cambrian Mt Simon Formation. This outcrop is located on the downstream side the dam on Lake Wissota. Drive to the end of 74th Avenue in Chippewa Falls and park in the Chippewa Rod and Gun Club & Marina. From here, you can walk southward along the access road to the dam (about 1 km). There are several places to cross the small steam to access the largest part of the outcrop. The largest potential hazard at this location is the stream crossing and uneven, wet walking area. A ladder or other temporary structure might be required to assist in crossing the stream if water levels are high. This outcrop highlights some of the magmatic history in this region. The majority of the exposure here is a Paleoproterozoic biotite tonalite (trondhjemite) that has local pods and dikes or medium gray biotite tonalite and alkali feldspar granite pegmatite. The outcrop is intruded by at least three gabbroic dykes associated with the mid-continent rift. The largest 10 �Proceedings of the 69th ILSG Annual Meeting - Part 2 of which is clearly visible in arial view (Figure 8). The entire area is covered by thin outwash gravels and silts that varies with seasonal flooding events. Some of the tonalite near the Chippewa River displays the same spheroidal weathering seen at Irvine Park so this location is just below the Great Unconformity and displays some of the same associated potassic alteration along faults and joints seen in Irvine Park (Stop 1). The potassic alteration is responsible for most of the pink color seen in outcrop. Figure 8 - (top) Generalized geology of the Wissota Dam region. Figure modified from Myers et al. (1980). (bottom) Aerial view of the outcrops with the mid-continent rift highlighted. Image obtained from Google Earth. tonalite for rocks with higher mafic concentrations. The oldest, abundant rock at Wissota Dam is a weakly foliated hornblende, biotite trondhjemite composed of oligoclase (50%), quartz (30%), microcline (5%), biotite (10%), and 5% hornblende with common accessory euhedral to subhedral titanite (Photo 4). Weak foliation strikes Nl5°W and dips steeply east. This is intruded by small dykes and masses of medium-grained, medium-grey hornblendebiotite tonalite (± epidote) that locally contains Granitoid Intrusive Suite Most of the Paleoproterozoic intrusive igneous rocks at Wissota are quartz diorites or tonalites with various proportions of hornblende and biotite. For clarity, and to be consistent with the terminology used by previous geologists working in the Chippewa River Valley, on this field trip we refer to the lightest colored tonalites (color index 15 or less) as trondhjemite and reserve 11 �Proceedings of the 69th ILSG Annual Meeting - Part 2 trondhjemite is an FI-type felsic rock with strongly depleted HREE (Figure 5) representing deep crustal melting (Hart et al. 2004). The trondhjemite is cut by east-northeast-trending pegmatite veins and pods and quartz ± pyrite) and epidote veinlets. Potassic alteration especially along any fractured surfaces the trondhjemites produces a pink color in outcrop (Photo 5). Minor cataclastic fault zones cut the granitoid intrusions with leftlateral displacement. A thin, branching discordant sheet of foliated biotite trondhjemite approximately 1-3 meters wide and trends N55°W. Drag folded foliation in the enclosing rocks indicates left-lateral displacement. Photo 4 – Photographs of main lighter colored tonalite phase at Wissota Dam. (A) Photomicrographs in plane-polarized light showing feldspar grains are lightly weathered with opaque rims around titanite. In cross-polarized light, quartz grains show moderate undulatory extinction. Photo from Klier (2019). B) Field photograph of biotite tonalite (trondhjemite) showing medium-grained, equigranular texture. Feldspars weather pink in color and mafic phases tend to be recessively weathered. lenticular xenoliths of banded amphibolite. The tonalite pods show no grain size diminution and sometimes have irregular shapes suggesting that some tonalites may be enclaves of earlier phases of the trondhjemite. In other cases, the tonalites are clearly dykes crosscutting the trondhjemite. The tonalite dikes tend to be unaltered with vitreous dark-brown biotite (~25%) and lack foliation. All minerals in the foliated tronhjemite show internal fracturing and dislocation, and contain quartz grains with undulatory extinction, display grain boundary migration and dynamic quartz recrystallization (Klier, 2019). The Photo 5 – Photographs of pegmatite and associated alteration at Wissota Dam. (A) Thin quartz-epidote veining and potassic alteration surrounding conjugate fracture sets adjacent to pegmatite. (B) Coarse grained texture of the pegmatite. Feldspar crystals can be as large as 5-7 cm in size. 12 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Slickenside fault surfaces elsewhere in this outcrop have similar strike and dip with the slickensides plunging 5°NW. of the largest dyke and the mineral chemistry was examined using SEM-EDS and optical petrography. The main dyke has an aphanitic chilled margin a few cm wide along both the north and south sides and the grain size consistently coarsens toward the middle of the dike into a medium grained olivine gabbro (Photo 7A). A prominent set of joints perpendicular to the cooling surface along the dike walls are interpreted as columnar joints and these are especially prominent on the south side below the power lines. More pronounced columnar joints are also seen in one of the smaller (2m wide) dikes along the northwest side of the area next to the Chippewa River. No internal contacts are apparent at the outcrop scale suggesting that this large dike represents a single cooling unit of magma intruded into the upper crust. West of the power lines the dike is cut by two faults, one left lateral strike slip fault with a few meters of displacement and a low angle reverse fault with well-developed chlorite slickensides and extensive alteration including chlorite and hematite, and calcite filled tension fractures. Mid-Continent Rift Dykes Three diabase dykes related to mid-continent rift extension intrude the granitoids (Photo 6) exposed below the Wissota Dam spillway and the largest dike is an ENE trending (~N65E) olivine tholeiite that averages 47m in width. The large dyke has a notable sharp and chilled margin. Unpublished data from the dyke at this location and others along the Chippewa River indicate a tholeiitic composition that shows slightly more Mg-enrichment trends on AFM diagrams in comparison to other parts of the dyke swarm in the region. Ongoing student-faculty research at the University of Wisconsin-Eau Claire is examining the composition of the large dyke at Wissota Dam. Samples were collected as a cross section The chilled margins consist of very finegrained plagioclase with variable compositions ranging from An35 to An63, in a devitrified-glass matrix crowded with submicron Fe-Ti oxides and sparse sub-calcic augite (Average cpx = [Mg.68Fe.60Ca.55Al.09Ti.02Mn.01] [Si1.91Al.09O6]). Within two meters of the north side of the diabase the dike contains single crystals of labradorite up to 10 cm across apparently sourced from a deeper magma chamber and transported (floated?) upwards during intrusion of the dike. Locally the chilled margin is altered to chlorite and very fine grained bright blue-green celadonite (Photo 7D), alkali-feldspar and dark red biotite. Five samples collected from the central 35 m of the dike all consists of an olivine gabbro with a well-developed ophitic texture (Photo 7B). The mineralogy from the center includes both titaniferous augite (1-2wt% TiO2) and titanaugite (>2wt% TiO2) oikocrysts with pink and lavender Photo 6 – Photographs showing sharp, chilled margin of mid-continent rift gabbro with Paleoproterozoic trondhjemite intrusion. 13 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Photo 7 – Photographs of mid-continent rift gabbroic dyke at Wissota Dam. (A) Outcrop photo showing fractured and weathered surface of dyke. Weathered surface shows medium-grained texture. (B) photomicrograph in crosspolarized light (40x) showing ophitic texture. (C) Photomicrograph in plane-polarized light (40x) showing aggregate of euhedral to subhedral olivine (ol) crystals (Fo 40-45) in plagioclase and cpx matrix where cpx as pinkish purple pleochroism typical of the titanaugite composition. (D) Photomicrograph in plane-polarized light (100x) showing greenish blue celadonite (cel) replacing biotite (bt) in the transitition zone between the chilled margin and dikes central olivine gabbro. pleochroism, laths of normally zoned plagioclase with labradorite cores (An55-65) and thin rims of andesine (An30-35) and unusually large aggregates of euhedral to subhedral olivine containing over 50 individual olivine crystals (Photo 7C). The augite and biotite show little variation across the dyke but the olivine becomes progressively more Fe-rich towards the south with an average of Fo43 in the north to Fo35 near the southern contact. The opaque minerals are primarily ilmenite with titaniferous magnetite lamellae often rimmed with a highly titaniferous reddish orange biotite. In the transition zone between the chilled margin and the center 30 m of the dyke much of the biotite is replaced (altered) with celadonite K(Mg,Fe2+)(Al,Fe3+)[Si4O10](OH)2 with a brilliant blue-green color in plane polarized light (Photo 7D). Unusual olivine aggregates (glomerocrysts?) occur throughout the central 35m of the dyke and often consist of more than 50 crystals (Photo 8). In some of the olivine aggregates the minerals show at least some crystallographic alignment. Olivine within individual aggregates have a very narrow range of chemistry. In one aggregate 16 grains were analyzed and the average composition was Fo47.5±0.2(2σ); this standard deviation is about the 14 �Proceedings of the 69th ILSG Annual Meeting - Part 2 magma with limited chemical variation and could be produced by turbulent flow (synneusis) agglomeration or by a high degree of undercooling and ripening of dendritic olivine. It seems very likely that this dike was an active conduit for magmas reaching the surface to produce MCR lava flows even though Chippewa Falls is almost 200 km south of the main MCR rift axis. Geochemical results which are pending should help further constrain the system. Stop 3 – Gneisses and Pegmatites at Jim Falls Photo 8 – Photomicrograph in cross-polarized light of olivine aggregate (glomerocrysts) showing consistent orientation of olivine crystals in the cluster. Gray crystals all have an optic axis almost perpendicular to the section. Magnification 100X. Lat: 45.0549° Long: -91.2734° same size as the analytical error for EDS analysis on olivine. Olivine aggregates have been described from several basaltic conduits where they have been attributed to differential crystal movement during turbulent flow in an active conduit such as at Kilauea (Helz, 1987) or as xenocrysts extracted from a deforming cumulate. However, there is no reference to aggregates with such a large number of crystals. The texture and chemistry of the aggregates at Wissota come closest to matching olivine aggregates collected from lava flows at La Reunion (Welsch et al., 2013) which they ascribe to rapid dendritic crystal growth and ripening under a high degree of undercooling (-ΔT > 60°C) from low viscosity basalts. Petrographic Interpretation: The dyke is sourced from a lower-level fractionated magma chamber of enriched basalt (E-MORB or alkali olivine parent) as evidenced by the olivine composition (~Fo40), plagioclase (An60) and titanaugite/ilmentite modal mineralogy. Plagioclase zoning from An60 cores to An30 rims suggests at least limited reaction with wall rocks. As a fractionated magma it seems likely that the ascending magma contained phenocrysts of both olivine and plagioclase that were kinetically fractionated by turbulent flow resulting in a chilled margin largely devoid of phenocrysts. The olivine aggregates form in equilibrium with This outcrop is located within the spillway of the hydroelectric dam near the community of Jim Falls. About 500 m north from the intersection of 15 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Highway 178 and County Highway Y (the main road into the community of Jim Falls), there will be an old, abandoned bridge that that used to be the access bridge to the community. There is ample parking in front of this bridge. On the south side of the old bridge is a small foot path that leads down to the outcrops along the river. These outcrops are smoothly polished from the flooding at the dam. They are uneven and quite slippery when wet. If water levels are high, there are also outcrops immediately downstream of the spillway to the north. This location has intensely folded amphibolite and biotite quartzofeldspathic gneiss that is intruded by granitoid intrusions and associated pegmatites (Figure 9). Intense shearing and metamorphism results in little preserved primary texture within the gneisses and amphibolites. On the east bank of the river is a gabbroic dyke associated with the mid-continent rift. Figure 10 - Tera-Wasserburg concordia diagram of biotite quartzofeldspathic gneiss from Jim Falls. A wide spread of ages is suggestive of a detrital origin. Figure from Klier (2019). sedimentary rocks. Trace element geochemistry from Klier (2019) was inconclusive in determining volcanic protolith because of low Ti abundance. Preliminary petrography from student projects at the University of Wisconsin-Eau Claire and Kleir (2019) seem to suggest that amphibolites are rather rare. Other geochemical results from ongoing research at the University of Wisconsin-Eau Claire are pending. The outcrops at this stop are one of the original “Archean” exposures of the Marshfield terrane that was described in in Sims et al. (1989), but new data in the region casts doubt on that original interpretation. The gneisses at this location were assigned a U/Pb age of 2522 ± 22 by Sims et al. (1989). Little description of the data was provided in that original reference and the date itself was referenced as unpublished data from personal communication. Klier (2019) resampled the gneiss from the region and analyzed zircons using LA-ICPMS. The resulting data clearly shows a large spread of ages and a significant portion of those are Paleoproterozic in age. There are clearly older sources of detritus for these meta-sedimentary rocks, some as old as 2841 Ma. However, the dominant source of detritus was Paleoproterozoic (Figure 10). Based on this new data, the Jim Falls region is not obviously an Archean crustal fragment. A biotite quartzofeldspathic gneiss was sampled by Klier (2019) for U/Pb geochronology. Based on recent field work, this rock appears to be the dominate lithology that exists in the immediate region around and under the bridge. Kleir (2019) describes the rock containing classic mylonitic textures and is comprised of quartz (60%), alkali feldspar (25%), biotite (15%), and trace zircon (Photo 9A). There are prominent bands of porphryoblastic quartz and cryptocrystalline biotite. Biotite is also present rarely as larger “destroyed” grains. Quartz has undulatory extinction and has undergone grain boundary migration recrystallization. Some feldspar grains display domino-type fragmented porphyroclastic textures. Portions of feldspar grains have diminished to sericite. Weakly Amphibolites and Gneisses Myers et al. (1980) interpreted the amphibolites and gneisses in this region to be derived from mafic volcanic rocks and associated 16 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 9 – Precambrian geologic map of Chippewa River near Jim Falls. Figure digitized from Myers et al. (1980). 17 �Proceedings of the 69th ILSG Annual Meeting - Part 2 isoclinally folded amphibolite occur in the granitic rocks. Granitoids and Pegmatites Granitic rocks range in composition from trondhjemite to alkali feldspar granite. Pegmatite dike intrusion occurred at several stages of "granite" intrusion. Mineralogy includes alkali feldspar (65%) quartz (32%), plagioclase (3%), and trace zircon.. The grains are subhedral to anhedral with intergrowths and granophyric textures occasionally present. Quartz grains appear stretched and strained and have undergone either subgrain rotation recrystallization or grain boundary migration recrystallization (Klier, 2019). The older granitic rocks are foliated and locally mylonitized. Shearing and boudinage of pegmatite stringers transposed them into oblique concordance with lamination in the enclosing rocks. A rough correlation can be made between relative age and concordance of veinlets. thinly laminated amphibolite was intruded by granite so that lenticular slices of the amphibolite were dragged en echelon away from the wall (Photo 10). The coarse granite pegmatite intruded under stress contains en echelon fractures filled with very coarse quartz. Photo 9 – (A) Photomicrograph in plane-polarized light of biotite quartzofeldspathic gneiss showing sericite-altered feldspar crystals and pronounced dynamic recrystallization of matrix. Foliation defined by elongation of grains and alignment of biotoite. Photo from Klier (2019). (B) Outcrop photo showing gneiss intruded by boudinaged granitic dykes. Gneissic layering is very fine and difficult to see in this photo. chlortizied biotite bands define foliation (Photo 9A). Garnetiferous hornblende gneiss and schist are folded with high-amplitude isoclinal folds with a persistent ENE strike. Small (F2) folds plunge gently east-northeast. These are folded F1 isoclinal folds, and a few hinges can be found in the outcrop. Some of the granitic pegmatites appear to be folded as well or are slightly boudinaged (Photo 9B), suggesting that pegmatites intruded prior to F2 or where exploiting layering within the folded gneisses and amphibolites during emplacement. Xenoliths of Photo 10 – Typical intrusive relationship between pegmatite surrounding amphibolites and gneisses. Small pegmatite veinlets and en echelon fracturing along margins is common resulting in lens-shaped gneissic fragments. 18 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Stop #4 – Amphibolites and Gneisses at Cornell Dam outcrops almost continuously for 4 km down the river. The amphibolite could also be classed as a gneissic, mafic hornblende tonalite or hornblende gneiss. This is also one of the few areas that this trip visits that you can potentially see primary depositional features! Immediately below the dam, the rock is a fine-grained amphibolite with elongate bulbous inclusions that appear to stretched pillows (Photo 11A) that contain local irregular to lens-shaped quartz-epidote nodules (Photo 11B). Lat: 45.1625° Long: -91.1596° The amphibolite is composed of subhedral to anhedral, lensoidal hornblende clusters (54%) with coarse, lensoidal porphyroclasts of twinned plagioclase (28%) and fine-grained quartz. Banding in the amphibolite Is cut by lenticular segments of granite and quartz veinlets. Garnets This outcrop is located within the spillway of the hydroelectric dam near the community of Cornell. About 500 m southwest from the bridge into Cornell on Highway 178 is the Wisconsin Department of Natural Resources Ranger Station where there is ample parking. Just south of the Ranger Station is a small road (called Pine Point Road) that leads toward the water. There are foot trails and gated roads (accessible by foot) that lead toward the outcrops at the dam and by the river. These outcrops are uneven but are generally dry and easily traversed under normal river conditions. If water levels are high, outcrops can also be visited on the shoreline above the dam near the Municipal Works buildings in Cornell. Photo 11 – Flow-like features in the amphibolites at Cornell Dam. (A). Streched pillow-like structures with cm-scale darkened pillow margins. (B) Irregular quartz-epidote nodules that are common in submarine or hydrothermally-altered submarine flows. Myers et al (1980) described this location as a laminated (foliated) garnet amphibolite that 19 �Proceedings of the 69th ILSG Annual Meeting - Part 2 in the amphibolite tend to be moderately poikioblastic with minor rotational features. The distribution of garnet clusters shows little relation to banding. Trace element chemistry of these rocks show clear tholeiitic trends and flat REE patterns on normalized diagrams (Figure 5). Further downstream from the dam, the rock becomes notably lighter in color and there appears to be a lower percentage of amphiboles. These rocks share similar trace element patterns (Figure 5) and are interpreted to be genetically related. The reason for the change in texture may be due to increase structural modification and gneissic banding development. The outcrops are also intruded by mafic dykes that clearly cross-cut the dominant foliation (Photo 12). These dykes trend N40°W and are approximately 30-50 cm in apparent thickness with no obvious chill margin. Since these dykes cross-cut the structural fabric, they are assumed to be related to the mid-continent rift. However, no petrography or chemistry has been done to confirm this hypothesis. of the river, there is a small vehicle parking area and footpath that leads to the dam on 260th Avenue about 100 m west of the intersection with County Highway M. This trail will take you to the dam and carefully navigate to the north bank of the river downstream of the dam. To access the south bank of the river, drive to the end of Irvine Avenue before it turns into a private driveway. There are numerous small foot paths that will lead to the south bank of the river. Photo 12 – Gabbroic dyke intruding through amphibolites at Cornell Dam. Stop 5 – Amphibolites and Deformed Diorite at Holcombe Dam The outcrops at this location are considered part of the Pembine-Wausau terrane, or is it? While the location of the Eau Pleine Shear Zone becomes problematic in this region, Sims et al. (1989) consider the Jump River Shear Zone the northern boundary of the Marshfield Terrane. Magnetic lineaments mark this shear zone and extend it close to these outcrops. Depending on Lat: 45.2251° Long: -91.1289° As time permits, each side of the river at Holcombe Dam has different rock types to examine, but are vastly different approaches to see them. To visit the outcrops on the north bank 20 �Proceedings of the 69th ILSG Annual Meeting - Part 2 the map, the shear zone lies just north or just south of the outcrops at this location (e.g. Mudrey et al., 1987). The rocks at this stop are a gneissic quartz diorite intrusion and an amphibolite schist (Figure 11). So, Marshfield or Pembine-Wausau terrane? The newest geochronology from the region is inconclusive. The rock exposed on the southern bank of the river below the dam is a foliated amphibolebiotite schist (Photo 13). Klier (2019) describes this rock as banded at the microscopic scale. Quartz grains are well banded, fairly subhedral to anhedral and feature undulatory extinction. Their boundaries are somewhat irregular and indicative of bulging recrystallization. Amphibole grains are hornblende to tremolite. Biotite appears as brown to light green grains and typically feature Photo 13 – Photomicrograph in plane-polarized light of amphibole-biotite schist with trace amounts of epidote in a quartzofeldspathic matrix. Figure from Klier (2019). Figure 11 – Precambrian geologic map of the Holcombe Dam region. Unit Abbreviations: qd: quartz-diorite, bgn: banded gneiss, ams: amphibolite schist. Figure from Myers et al. (1980). 21 �Proceedings of the 69th ILSG Annual Meeting - Part 2 shear bands with cryptocrystalline biotite crosscutting crystals. Klier (2019) obtained a U/Pb zircon age via LA-ICPMS of 1858 ± 1.0 Ma (Figure 12). This age does not definitively put the rocks in this region in Marshfield or PembineWausau terrane. There does not appear to be any Archean inherited zircons, as one might expect if Penokean magmas were overprinting an Archean crustal block. Photo 14 – Outcrop of quartz diorite on north bank of Chippewa River at Holcombe Dam. quartz, with minor amounts of biotite, muscovite and epidote. The quartz diorite contains two types of inclusions: hornblende rich ultramafic inclusions and spotted mafic inclusions. Ultramafic inclusions occur along the northwest portion of the exposed quartz diorite, generally less than 0.5 meters in length, although one is at least 2 meters long. Ultramafic inclusions are composed of 7585% hornblende and 11—13% biotite with a small amount of plagioclase (meta pyroxenites?). Chlorite occurs as an alteration product of biotite and less commonly of hornblende and can compose more than 20% of the rock. Figure 12 – Tera-Wasserburg concordia diagram of amphibolitic schist on the south bank of the Chippewa River near Holcombe dam. Figure from Klier (2019). Stop 6 – Tonalites and quartz diorites at Cadott Bridge On the north bank of the river, the outcrop is predominately a synkinematic quartz diorite (Photo 14; Figure 13). The quartz diorite is a medium-grained, dark to medium grey rock with rusty weathering surfaces. It is faintly foliated and has white discontinuous bands and lenticles which are more quartz rich than the rest of the rock. Quartz diorite is composed of plagioclase (32-51%), quartz (11-31%) and mafic minerals (12-33%). Mafic minerals range from entirely hornblende to entirely biotite. The quartz diorite is cut by medium-grained granite pods with migmatitic contacts and by finer-grained dykes with sharp contacts. The granite is a pink, faintly foliated rock which locally contains porphyritic microcline grains reaching 1 cm in size. Granitic rocks consist of plagioclase, microcline and Lat: 45.9535° Long: -91.1508° This outcrop shows is easily accessible under the Main Street bridge in Cadott. Just north of the bridge near the Main Street-Yellow Street intersection there is a parking area on the north bank of the river. From this parking area, there are footpaths that lead to the waters edge. The predominant rock type here is foliated biotite quartz diorite to biotite tonalite (Figure 14) composed of plagioclase (An25-35, 30-55%), quartz (10-40%), hornblende (0-25%) and biotite (0-15%) (Myers et al. 1980). Mafic minerals are partly replaced by chlorite (of several varieties), epidote, and sericite. Magnetite (1-5%) is a by- 22 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 13 – Detailed outcrop map of the quartz diorite gneiss on the north bank of the Chippewa River at Holcombe Dam. Figure from Myers et al. (1980). Figure 14 – Precambrian geologic map of the region downstream of Cornell Dam. Figure modified from Myers et al. (1980). 23 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Photo 15 – Mylonitized biotite tonalite at the Cadott Bridge on the Yellow River. Photo 16 – Photograph of elongated xenoliths (?) of chlorite-rich metavolcanic rocks in biotite tonalite at Cadott Bridge on the Yellow River. product in the chloritization of hornblende. The tonalites have been mylonitized (Photo 15) and locally recrystallized and contain lenticular xenoliths of chlorite and epidote-rich metavolcanic(?) rock (Photo 16). The older cataclastic foliation is axial-planar to isoclinally folded pegmatite, aplite, and quartz layers. These rocks are cut by a pervasive N65-75°W trending foliation and mylonitic shear zones. Acknowledgements Despite decades of regular visits from groups from the University of Wisconsin-Eau Claire, the most extensive detailed maps and rock descriptions were provided by Paul Myers and collaborators in the 1980 ILSG guidebook (Myers et al, 1980). There is some recent research activity in the region, but between new but pending analyses and future ambitions, the descriptions and maps provided in that ILSG guidebook are the most detailed and accurate for the region. A lot of the geologic descriptions have been updated and figures have been digitized while adding new data and insights where available. West of the Yellow River bridge, foliated biotite tonalite encloses angular xenoliths of hornblende tonalite or amphibolite containing strongly deformed aplite and pegmatite stringers. Isoclinally folded quartz, aplite, and pegmatite veinlets exist as angular xenoliths in a lighter biotite tonalite. 24 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Hafften, D., and Radwany, M., 2018, Geothermobarometry of a Precambrian amphibolite from Cornell WI: Proceedings of the Institute on Lake Superior Geology 64th Annual Meeting, Iron Moutain, Michigan, p. 45-46. In addition, the authors of this guidebook would like to thank the countless undergraduate and graduate students that have worked on these outcrops and have continued to inspire new work in the region. Specific acknowledgement is deserving to Matt Leahy and his efforts in digitizing figures and compiling geochemistry for the guidebook. Hannack, G., and Radwany, M., 2018, HornblendePlagioclase thermometry of the Eau Claire River Complex, western Wisconsin: Proceedings of the Institute on Lake Superior Geology 64th Annual Meeting, Iron Mountain, Michigan, p. 47-48. References Aleinikoff, J.N., Walter, M., Kunk, M.J., and Hearn, P.P., Jr., 1993, Do ages of authigenic K-feldspar date the formation of Mississippi Valley–type PbZn deposits, central and southeastern United States? Pb isotope evidence: Geology, v. 21, p. 73– 76. Hart, T. R., Gibson, H. L., and Lesher, C. M., 2004, Trace element geochemistry and petrogenesis of felsic volcanic rocks associated with volcanogenic massive Cu-Zn-Pb sulfide deposits: Economic Geology, v. 99, p. 1003-1013. Helz, R. T. 1987, Diverse olivine types in the lava of the 1959 eruption of Kilauea Volcano and their bearing on eruption dynamics. USGS Professional Paper 1350, p. 691-722. Brown, B. A., 1988, Bedrock geology of Wisconsin, west-central sheet, Wisconsin Geological and Natural History Survey Map 87–11b. Chan, L. S., Myers, P. E., and Hay, R. L., 1991, Features and significance of the PrecambrianCambrian contact in western Wisconsin. Institute of Lake Superior Geology 37th Annual Meeting, Eau Claire, Wisconsin, Field Trip Guidebook 2, 17 p. Klier, J. J., 2019, The Marshfield Terrane: Redefinition of origin through zircon geochronology and geochemistry: Unpub. M.S. thesis, Ball State University, 115 p. LaBerge, G. L., 1996, Volcanogenic massive sulfide deposits of northern Wisconsin: A commemorative volume, Proceedings of the 42nd Annual Meeting of the Institute on Lake Superior Geology, Cable, Wisconsin. DeMatties, T. A., 1989, A proposed geologic framework for massive sulfide deposits in the Wisconsin Penokean volcanic belt: Economic Geology, v. 84, p. 946-952. LaBerge, G. L., and Myers, P. E., 1984, Two early Proterozoic successions in central Wisconsin and their tectonic significance: Geological Society of America Bulletin, v. 95, p. 246-253. DeMatties, T. A., 1994, Early Proterozoic volcanogenic massive sulfide deposits in Wisconsin: An overview: Economic Geology, v. 89, p. 1122-1151. Lui, J., 1997, K-Metasomatism in Uppermost Precambrian Rocks in West-Central, Wisconsin and Southeastern, Missouri. Unpub. PhD thesis, University of Illinois. 227p. 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. Lui, J. Hay, R. L., Deino, A. and Kyser, T. K., 2003, Age and origin of authigenic K-feldspar in uppermost Precambrian rocks in the North American Midcontinent. Geological Society of America Bulletin, v. 115, p. 422-433. 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, article 104489. 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. 25 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Mudrey, M. G., LaBerge, G. L., Myers, P. E., and Cordua, W. S., 1987, Bedrock geology of Wisconsin, northwest sheet, Wisconsin Geological and Natural History Survey Map 88-7. Quigley, A., 2016, Setting of the volcanogenic massive sulfide deposits in the Penokean Volcanic belt, Great Lakes region, USA: Unpub. M.S. thesis, Colorado School of Mines, 95 p. 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. 21452158. Van Schmus, W. R., 1980, Chronology of igneous rocks associated with the Penokean orogeny in Wisconsin: Geological Society of America Special Paper, v. 182, p. 159-168. Van Wyck, N., and Johnson, C. M., 1997, Common Lead, Sm-Nd, and U-Pb constraints on petrogenesis, crustal architecture, and tectonic setting of the Penokean orogeny (Paleoproterozoic) in Wisconsin: Geological Society of America Bulletin, v. 109, p. 799-808. Weber, E. M., and Lodge, R. W. D., 2022, New U/Pb Geochronology from the Proterozoic Penokean Orogen, Wisconsin: Implications for VMS Metallogeny: Society of Economic Geologists Annual Meeting, Denver, CO, paper P5.10. Welsch, B., Faure, F., Famin, V., Barronet, A., Bachelery, P., 2013, Dendritic Crystallization: A Single Process for all of the Textures of Olivine in Basalts?, Journal of Petrology, v.543, p. 539-574. 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: Geological Society of America Bulletin, v. 134, p. 776-790. 26 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Field Trip 2 – Wisconsin’s Paleozoic stratigraphy and tour of Crystal Cave Carsyn Ames, Esther Stewart, William “Bill” Batten, Eric Stewart, Ian Orland Wisconsin Geological and Natural History Survey, University of Wisconsin- Madison, 3817 Mineral Point Rd. Madison, WI 53705 Introduction The Cambrian-Ordovician strata exposed in western Wisconsin were deposited during the major Sauk and Tippecanoe marine transgressions onto the interior of the Laurentian continent (Sloss, 1963). These rocks compose the regional aquifer system, host disseminated sulfide mineralization that contribute to groundwater contamination, and are locally mined as proppant for fracking in the oil and gas industry. Additionally, variable hardness of these units in part controls the formation of ledges and hillslopes in the fluvially-dissected Driftless Area of southwestern Wisconsin. During this field trip, we will focus on Cambrian and lower Ordovician strata of the Sauk sequence (Figures 1 and 2). We start our day touring Crystal Cave, a cave system developed along joints within the Ordovician Prairie du Chien Group dolostone. For the rest of the day, we will visit outcrop exposures of the Cambrian Jordan Formation, Tunnel City Group, and Wonewoc Formation sandstones, and if time permits- the Eau Claire and Mount Simon Formations. We hope this field trip will provide an opportunity to discuss similarities and differences between units deposited on the western side of the Wisconsin Arch and those deposited on the eastern side, where field trip authors have focused much of their work. Additionally, we welcome and encourage discussion between participants that have knowledge of or experience working with these stratigraphic units. Figure 1. Correlation of map units showing relative ages of Cambrian-Ordovician units. COpg: Parfreys Glen Formation, Ce: Elk Mound Group, Ctl: Lone Rock Formation of the Tunnel City Group, Ctm: Mazomanie Formation of the Tunnel City Group, Ctc: Tunnel City Group, Ct: Trempealeau Group, Opc: Prairie du Chien Group including the Oneota and Shakopee Formations, Oa: Ancell Group, including the St. Peter and Glenwood Formations, Osp: Sinnipee Group, including the Platteville, Decorah, and Galena Formations. From Stewart (in revision). Ages from Gradstein et al. (2020). Cambrian-Ordovician strata in the southern Lake Superior Region were deposited on an essentially flat continental shelf in a shallow epeiric sea well within the Laurentian continent (Figure 3, Runkel et al., 2012, 2020). These strata overlie Precambrian bedrock of variable ages across the Great Unconformity, a surface characterized by locally significant topographic relief and weathering and exposed in outcrops around the Eau Claire area. The regional paleogeography that controlled sediment source to sink was defined by several structural highs, including the Transcontinental Arch, Wisconsin Dome, and Wisconsin Arch, and several basins, including the Hollandale Embayment, Illinois Basin, and Michigan Basin (Figures 3 and 4, A very brief geologic history of the Cambrian-Ordovician strata in Western Wisconsin Regional depositional model and setting 27 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 2. Generalized stratigraphic column of Wisconsin. From: Bedrock Stratigraphic Units in Wisconsin Bedrock Stratigraphic Units in Wisconsin [small] - WGNHS. 28 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 3. From Runkel 2020, Figure 4. This figure illustrates a depositional model developed for southeastern Minnesota. The Cambrian- Ordovician strata in Wisconsin are thought to have been deposited in a similar fashion. Runkel et al., 1998). The field stops we will visit in western Wisconsin lie west of the Wisconsin Arch and straddle the eastern edge of the Transcontinental Arch and the southwest flank of the Wisconsin Dome. These structural highs were periodically subaerially exposed and eroded during deposition of Paleozoic units. dominated Great American Carbonate Bank (Figures 3 and 5; Runkel et al., 2012). Sandy Cambrian sediments of the Mt. Simon, Wonewoc, and Jordan Formations were deposited in shoreface, aeolian, wave-, and tideinfluenced settings within the inner detrital belt. Mixed, fine-grained sandstone, siltstone, shale, and carbonate of the Eau Claire Formation, Trempealeau and Tunnel City Groups were winnowed and trapped within a transitional, relatively deeper water moat that separated the inner detrital belt from the Great American Carbonate Bank (Runkel et al., 2012). Dolomite of the Prairie du Chien Group was deposited in relatively shallow water, subtidal to peritidal settings on this carbonate bank. Interfingering sandstone, shale, and carbonate record marine transgressions and regressions that caused reciprocal expansion and contraction of the facies belts. During sea level rise, the carbonate bank advanced landward as siliciclastic-dominated nearshore environments were drowned. Cambrian-Ordovician siliciclastic and carbonate units were deposited in a nearshore, sandstone-dominated inner detrital belt that passed offshore into a relatively deeper water moat, which in turn transitioned into a carbonate- Figure 4. from Runkel and others 1998, regional map showing locations of the Wisconsin Dome, Wisconsin Arch, Transcontinental Arch and Hollandale Embayment. Other depositional basins are shown on map (Michigan and Illinois Basins), as well as regional extent of Paleozoic units. 29 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 5. from Runkel et. al., 2012 showing the depositional environments that produce interfingering of different Cambrian-Ordovician siliciclastic and carbonate units across the Midwest. Conversely, during sea level fall, sandy nearshore facies of the inner detrital belt expanded seaward, limiting carbonate deposition. Paleozoic sedimentary rocks were gently folded and faulted in the Paleozoic, probably related to far-field effects of continental margin orogenic events. Structures in Wisconsin rarely exceed 200 feet in structural relief. Recent mapping in Wisconsin and Minnesota suggests folds and faults are probably related to reactivation of much older Precambrian structures (Figure 8). Deformation probably occurred in at least two pulses: once during the Ordovician (Mossler, 2006; Steenberg and Retzler, 2016; Stewart E.K., 2021) and at least once later in the Paleozoic (Heyl and others, 1959; Carlson, 1961). The importance of these folds and faults for groundwater studies is a topic of active interest. In northern Illinois, sandstones in the core of the Sandwich Fault zone have an order of magnitude reduction in horizontal hydraulic conductivity compared to the surrounding rocks (Hadley and others, 2020). In eastern Wisconsin, the Beaver Dam anticline is associated with a statistically significant increase in detection of dissolved arsenic in groundwater wells (Stewart E.D. and others, 2021). The Wisconsin Arch and its influence on Cambrian-Ordovician strata Strata deposited in areas east (for example, Dodge, Fond du Lac, and Jefferson Counties, Wisconsin) and west (for example, the outcrops we will visit today) of the Wisconsin Arch (Figure 4) were deposited in different sub-basins and tapped different local sediment source areas. In addition, the Dodge, Fond du Lac, and Jefferson County map areas were situated in more proximal locations on the Wisconsin Arch relative to today’s field stop locations. Therefore, the eastern sections include more pronounced exposure surfaces, condensed, or eroded sections, and typically include thinner and less abundant fine-grained intervals. Figure 6 shows a generalized stratigraphic column for Jefferson County (east of Wisconsin Arch), with accompanying pXRF elemental data. Figure 7 shows a stratigraphic column from Trempealeau County in western Wisconsin, south of this trip’s field stops, and west of the Wisconsin Arch. Regional and county scale mapping The most recent regional map for West-Central Wisconsin was published in 1988 by Bruce Structural observations on the CambrianOrdovician strata in Wisconsin 30 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 6. From Stewart (in revision), Bedrock geology of Jefferson County. Jefferson County is east of the Wisconsin Arch. 31 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 7. Core log, Gamma Ray log, and pXRF logs from the Arcadia core, Trempealeau County. Modified slightly from Zambito et al. (2018). Trempealeau County is west of the Wisconsin Arch. Brown (WGNHS). This map (Figure 9) includes all of the Paleozoic units we will see today, as well as the older Proterozoic and Archean rocks that make up the bedrock to the east of Eau Claire. Many of the stops for this field trip were found using the Cambrian contacts from this map. Field Trip Stops Stop 1: Crystal Cave, Spring Valley, WI (Contributed by Ian Orland, WGNHS) UTM location 4964692.71N) for stop (559180.94E, and includes walking, ducking, and climbing 7 stories. Please exercise caution while inside the cave as surfaces may be uneven. Below is a brief We will be touring the cave with staff from Crystal Cave. The tour is moderately strenuous 32 �Proceedings of the 69th ILSG Annual Meeting - Part 2 synopsis of a recent collaboration between WGNHS and UW-Madison’s Geoscience Department on speleothems from southern Wisconsin: Caves are fascinating natural features, and can preserve geologic records of past environments. Relatively recent advances in the methods and precision of geochemical analyses have established cave formations (speleothems) as important scientific tools for understanding climate changes of the last 500,000 years. A number of groups have studied the geochemistry of speleothems in the Lake Superior region. In Wisconsin, much of this work has happened at Cave of the Mounds in Blue Mounds, WI, just outside of the terminal moraine of the Laurentide Ice Sheet and some 20 miles southwest of Madison. While that cave is not the destination for this field trip, this section is intended to highlight the types of information we can learn from caves like Crystal Cave. Both caves are privately-owned show caves that were opened for tours in the late 1930s/early 1940s. Crystal Cave is situated in Prairie du Chien Group dolomites of the Early Ordovician (~475 Ma), while Cave of the Mounds is in Sinnipee Group dolomites of the Middle Ordovician (~465 Ma). The formation ages of passages in each cave are poorly constrained. Stalagmites and stalactites from Cave of the Mounds, however, have recorded environmental signals for >250,000 years. Cave of the Mounds: permafrost record Researchers from UW-Madison collected the first seven stalagmite samples in 2015 for modern U-Th geochronological analysis at UM-Twin Cities. Initial results prompted further sampling and analyses; Batchelor et al. (2019) reports 141 U-Th dates from 19 cave carbonate (speleothem) samples ranging from 250–2 ka. The temporal distribution of these ages revealed hiatuses of stalagmite growth in the cave during both of the last glacial maxima, demonstrating the presence and duration of permafrost (Figure 10). Notably, Figure 8. Cross-section from Dodge County, eastern flank of the Wisconsin Arch, south-central Wisconsin. Note offset of Precambrian basement and Cambrian Elk Mound Group (Ce) through Ordovician Prairie du Chien Group (Opc) and subtle folding of younger units. From Stewart E.K. (2021). 33 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 9. Bedrock geologic of west-central Wisconsin from Brown, 1988. 34 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 10 (*from Batchelor et al., 2019). Speleothem U‐Th ages at Cave of the Mounds (COM) in context with regional and global paleoclimate records. (a) MIS boundaries (odd numbers=interglacial periods, even numbers=glacial periods). (b) Stacked records of δ18O (‰) from benthic marine foraminifera annotated with MIS substage names (Lisiecki & Raymo, 2005). (c) Summer insolation (21 June at 43°N). (d) Atmospheric CO2 (ppm) and (e) CH4 (ppb) concentrations. (f) U‐Th ages from COM speleothems with associated 2σ uncertainties (this study) and statistically significant growth hiatuses (gray and red vertical bars). (g) Paleo‐permafrost reconstructions based on geomorphic features in Wisconsin, including ice wedge casts and polygons (Clayton et al., 2001). (h) U‐Th dates of speleothems from caves in the Midwestern United States in order of decreasing latitude. References provided in the main text. MIS = Marine Isotope Stage changes in the δ18O signal during a time period when warm periods are recorded in polar ice cores and stronger monsoons are recorded in tropical stalagmites (Figure 11). the 18 ky duration of the growth hiatus at MIS 2 was much longer than the hiatus that overlaps MIS 6 (5 ky), consistent with more extensive continuous permafrost in the region during the last glacial period. A combination of microscopic imaging and analysis showed that the δ18O changes each happened in ~10 years, and comparison to a climate model demonstrated that the δ18O changes likely happened as a result of >10°C warming above the cave. These results speak to how quickly and dramatically those polar Cave of the Mounds: Decadal warming events during the last glacial period Earlier this year, Batchelor et al. (2023) published a record of the oxygen isotope ratios (δ18O) of calcite from a Cave of the Mounds stalagmite that grew during the last glacial period. Their interpretation focused on a number of rapid 35 �Proceedings of the 69th ILSG Annual Meeting - Part 2 warming events were propagated across the Laurentide Ice Sheet, which is important for better understanding the dynamics of rapid climate change. As you enjoy the tour of Crystal Cave, consider what geologic stories might be captured in its Figure 11 (*from Batchelor et al., 2023). Stalagmite CM-5 δ18O record in comparison to other regional δ18O records of the last-glacial period. a, Cave of the Mounds (COM; this study) δ 18O record (black line), with associated U-Th ages (black dots/2SD error). Note the error of our age model ranged from 520 to 2800 years and was on average 730 years. b, A stalagmite δ18O record from Buckeye Creek Cave, WV (red line) showing relatively lowmagnitude δ18O changes during the last glacial period. c, A compilation of Chinese speleothem δ 18O records (orange line), showing high-magnitude δ18O changes, which reflects the sensitivity of the East Asian monsoon system to high-latitude warmings (DO events) during the last glacial period. *Note the scale of the y-axis in panels A-C are the same to allow for one-to-one comparison. d, The North Greenland Ice Sheet Project (NGRIP) δ18O record (blue line), showing the timing of abrupt warming DO Events (labeled #s). 36 �Proceedings of the 69th ILSG Annual Meeting - Part 2 speleothems. If you have ideas or questions, feel free to reach out to Ian Orland (orland@wisc.edu)! Stop 2: Prairie Du Chien Group- Kraemer Quarry Entrance Outcrop- 850th Ave between Lincoln Rd and 870th Ave intersections. UTM location 4966004.74N) for stop (564719.58E, We do not have permission to enter the quarryDo not enter the quarry. There is an outcrop of the Prairie Du Chien Group just outside of the quarry gate that continues down the hill from the quarry entrance. This outcrop appears to be a very sandy portion of the Prairie Du Chien Group, possibly representing the lower most Stockton Hill Member of the Oneota Formation, or an interfingering of the Jordan Sandstone within the basal Prairie Du Chien Group. Figure 12. Massive beds, of sandy, carbonate cemented Prairie Du Chien Group. Just to the right of the quarry entrance are massive, 1-2m thick beds (Figure 12). To the left, and down the hill, the massive beds continue and just below them thinly bedded, lighter color units begin to appear (Figure 13). The portion of the outcrop that continues down the hill also contains what may be the Prairie Du Chien Gp./Jordan Fm. contact in the ditch just below the road grade between the outcrop and the road (Figure 14). While not recognized in the formal bedrock stratigraphic column for Wisconsin, the thinly bedded, lighter color units may also represent the Coon Valley Member of the Oneota Formation, often recognized and mapped in Minnesota (Steenberg, J.R., and Retzler, A.J., 2016). We will depart on 850th Ave. by continuing down the slope. To the left near the toe of the slope, there is a valley floor with a barn and small pasture. Figure 13. Massive beds of Prairie Du Chien GroupStockton Hill Member? Possibly atop thinly bedded interfingerings of Jordan sandstone. Looking across the valley floor, there is an outcrop of Jordan sandstone just across the creek (Figure 15). 37 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 15. View of valley floor, at toe of slope driving down 850th Ave., looking across pasture towards Jordan outcrop just across creek. 18). This outcrop (Figure 17) is likely close to the base of the Figure 14. Arrow pointing to possible Prairie Du Chien Gp/Jordan Fm. contact in ditch just below road grade. Jordan/St. Lawrence contact (labeled in Figure 16), and the floodplain that the Eau Galle River runs through most likely represents the top of the St. Lawrence Formation. Stop 3: Jordan Formation- Cth B and 770th (Spring Lake, Wisconsin) UTM location 4962594.39N) for stop The Jordan Formation of the Trempealeau Group has been highly studied in both Wisconsin and Minnesota (Mudrey, M.G. Jr. ed, 1997 and references therein). The distinction between and regional application of the quartzose and feldspathic sandstones in this formation have also been debated (Runkel 1994 and Byers and Dott, 1995). Overall, the Jordan Formation represents a coarsening upward sequence that is conformable (562605.40E, Lithofacies of the Jordan Formation are described in Runkel, 1994 and are as follows: 1) very fine-grained hummocky cross-stratified and burrowed sandstone, 2) fine-grained, trough cross-stratified and burrowed sandstone, 3) medium- to coarse-grained, large-scale crossstratified sandstone and 4) thinly interbedded sandstone, mudstone and shale. They note that lithofacies 4 may only be relevant to certain areas in Minnesota (Figure 18). Authors are open to discussion as to where this particular outcrop falls in Runkel’s 1994 classification schema (Figure Figure 16. View of the outcrop across Cth B with approximate contacts labeled. 38 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Iron staining Figure 17. A. View of outcrop. Note the cross bedding and iron staining above hammer. B. Possible iron concretions? PDC Jordan Silcrete at its basal contact with the St. Lawrence, and unconformable with the Prairie Du Chien Group contact at its top. The Coon Valley member of the Jordan is not formally recognized in the stratigraphic column of Wisconsin (WGNHS Figure 19. Photo of core from Jefferson County, Wisconsin (east of Wisconsin Arch) modified from Kusick, 2022 M.S. Thesis. This interval of core shows the contact between the Jordan Formation and the Prairie Du Chien Group. 2011), though it has been noted above the Jordan Fm. in southern parts of the state. A recent M.S. dissertation (Kusick, 2022) discussed, in detail, both the stratigraphy and depositional environments of the CambrianOrdovician units east of the Wisconsin Arch. Kusick (2022) describes the Jordan sandstone as being comprised of only 2 facies of cross stratified sandstone and shaly sandstone, and as being deposited in an upper to lower shoreface environment. These authors would also like to note that locally, the Jordan Formation east of the Wisconsin Arch includes silcrete and clay, and hosts disseminated sulfides (Figure 19). Stop 3a: Rock Elm Impact Structure- Rock Elm, WI- lunch at Nugget Lake County Park UTM location 4948450.76N) Figure 18. Figure 2 from Runkel, 1994 illustrating the different lithofacies of the Jordan Sandstone in Minnesota. 39 for stop (561573.58E, �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 20. Core photos showing jumbled and deformed Cambrian strata from the southern edge of the Rock Elm central uplift - from WGNHS archives. No set stop, informational only as we’ll be eating lunch at a park within the crater. detected in detrital zircon grains and is interpreted to be caused by the impact (Cavosie et al., 2015). The field trip will go through the Rock Elm impact structure (Figure 20), located in Pierce County around 35 miles WSW of Eau Claire (Figure 21). The Rock Elm impact structure is the largest deformation event recorded in the Paleozoic section of western Wisconsin. The structure contains a 6.5 km diameter ring boundary fault and a central uplift 1 km across (Cordua, 1985). Where control exists, the ring boundary fault is thought to have accommodated 45 meters of down-in-the-center displacement (French and others, 2004). Much of the interior of the ring boundary fault is filled with the relatively flat-lying Rock Elm shale and the overlying Washington Road sandstone, which have a combined thickness of approximately 48 meters. These units are unique to the area, and do not exist outside of the ring fault. These units are described based on numerous outcrops, many given in Cordua (1987) and Cunningham and others (2011). The central uplift contains outcrops of tilted Mt. Simon Formation (Figure 20), which suggests 250 to 300 meters of uplift within the core zone relative to rocks outside the impact structure (French et al., 2004). Reidite, a high pressure polymorph of zircon, has been Stop 4: Skolithos burrows in Tunnel City Group-330th Ave. between HWY 25 and Cth Y (Private Property!!!) UTM location 4961335.76N) for stop (586184.90E, We will park on a private drive and walk east along the road to this outcrop. This stop in the Tunnel City Group is an excellent example of Skolithos burrows (Figure 22) which are common in the Tunnel City Group. This outcrop is likely the Tomah Member of the Lone Rock Formation. Excellent examples of cross-stratification can be seen at this outcrop as well. 40 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 21. Map plate from the 2007 Wisconsin Geological and Natural History Survey Open File Report on the Rock Elm impact structure (Cordua and Evans, 2007). 41 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 22. Tunnel City Group outcrop with excellent examples of Skolithos burrows and possibly multiple types of cross-stratification. “Tunnel City Group (Cambrian) The Tunnel City Group is comprised of the Lone Rock and Mazomanie Formations. Similar to neighboring La Crosse County (Evans, 2003), the Mazomanie Formation was not recognized in Trempealeau County. Stop 5: St. Lawrence Formation/Tunnel City Group road cut- Cth C and Cth Y UTM location 4958918.89N) for stop (587368.83E, Lone Rock Formation. The Lone Rock Formation (Figure 23) and its members are identifiable in the map area. The members, from oldest to youngest, are Birkmose, Tomah, and Reno; these are not differentiated at the map scale. The Birkmose Member is a dolomitecemented, coarse-grained, glauconitic sandstone to sandy dolostone with flat-pebble conglomerates; the Tomah Member is a tan to white-colored, medium-grained, glauconitic quartz sandstone; and the Reno Member is a glauconitic medium- to coarse-grained quartz sandstone with flat-pebble conglomerates. Palaeophycus and Skolithos are common, as is hummocky cross-stratification and crossstratification bounded by horizontal bedding surfaces. The contact with the overlying St. Lawrence Formation is sharp and unconformable. The road cut is just west of the intersection of Cth C and Cth Y. We will park and walk to this outcrop. Cth C is a fairly busy road, please exercise caution when you decide to cross. Recent mapping in Trempealeau County, southeast of stops 5 and 6, has produced interesting work on both the geological relationships of the Cambrian- Ordovician rocks, and the quality of groundwater in the west-central part of the state. Zambito and others, 2018 published the following unit description for the Tunnel City Group: 42 �Proceedings of the 69th ILSG Annual Meeting - Part 2 phyllosilicate mineral. Another interesting part of this road cut is the bench approximately 35ft up. This bench feature likely represents the unconformable contact Zambito and others, 2018 alluded to with the overlying St. Lawrence Formation. The Mazomanie Formation is generally not observed in this part of the state and is more prevalent in the southern parts of the state where it interfingers the Lone Rock Formation (Mudrey, M.G. Jr. ed, 1997 and references therein). St. Lawrence Fm. above bench Reno Member Tomah Member A 2019 study by Zambito and others investigated the relationship between groundwater quality and the geochemistry of the Tunnel City-Wonewoc units in western Wisconsin. This study notes that sulfide bearing minerals are disseminated between the two units in west-central Wisconsin, and they call for more work to better understand the geochemical effects of oxidation of sulfide minerals during groundwater pumping in this part of the state. Our next stop will be at an outcrop of Wonewoc sandstone, and we will pass other outcrops of this unit on our drive. Figure 23. Road cut showing contacts between the St. Lawrence Fm., and Reno and Tomah Mbrs. of the Lone Rock Fm. This is the view from the north side of Cth C. The Lone Rock Formation is commonly exposed in shale pits, along roads leading to ridgetops, and at the top of sand mine high walls where the Wonewoc Formation is extracted and the Birkmose Member forms the caprock. The formation is approximately 150 feet thick in the map area. Elemental data for part of the Lone Rock Formation is shown in plate 2 [figure 7]. These data show the formation’s lithologic variability, in particular the distinct upper carbonate-cemented and lower sandstone dominated intervals in the Birkmose; the lower interval consists of reworked quartz grains from the underlying Wonewoc with interspersed, rare glauconite grains and phosphatic brachiopods.” These authors find this to be an excellent, and representative description of the unit for the westcentral region. – Zambito and others (2018) Overall, the Tunnel City Group both east and west of the Wisconsin Arch are quite similar. As examined at this stop, west of the Arch, the Tunnel City Gp. East of the Arch is also a quartz sandstone with glauconite and trace amounts of shale. Stop 6: Wonewoc road cut- Cth Y UTM location 4959793.83N) Figure 24 shows a small part of the westernmost portion of the outcrop. The very dark, greenish-black bed just below the more resistant dolomitic bed is rich in the mineral glauconite, which is an iron potassium for stop (593272.27E, The Wonewoc Formation is a fine to coarse sandstone unit with medium to thick beds, highangle trough cross-stratification and some 43 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 24. A. View of westernmost point of the outcrop. B. close up of the phosphatic rich, friable sandstone of the Tomah Member. orange, iron rich bed on the south side of Cth Y that doesn’t seem to appear in the north face of the outcrop. feldspar (Mudrey, M.G. Jr. ed, 1997). Brachiopod fragments, Skolithos burrows (which we observed at stop 4), and Climactichnites are somewhat common in the fossiliferous Ironton Member of this formation. The upward contact (Figure 25) with the Tunnel City Group is gradational and fines upward; the basal contact with the Eau Claire Formation has been debated as to whether it is gradational or not (Mudrey, M.G. Jr. ed, 1997 and Ostrom 1978). Note the In the 1990s, to better characterize aquifer and confining units, the Minnesota Geological Survey began focusing on the hydrostratigraphic characteristics of geologic units (1998 ILSG field guide). Hydrostratigraphic subdivisions include: 1) fine clastic; 2) coarse clastic; 3) carbonate; 4) clastic/carbonate mix. While this approach has not been implemented as part of bedrock mapping in Wisconsin, its importance has been recognized by Wisconsin hydrogeologists in lithologically complex units such as the Eau Claire Formation (Bradbury and Runkel, 2011). The authors are open to questions, and discussion of this method as it may pertain to future groundwater study needs across the Midwest. Additional stops if time permits: Devil’s Punchbowl – Eau Claire Formation: UTM location 4966909.29E) Figure 25. View of the Wonewoc outcrop on Cth Y. 44 for stop (582783.00N, �Proceedings of the 69th ILSG Annual Meeting - Part 2 Park in the parking lot, walk east towards the stairs, and take them down the trail into the Punchbowl. Acknowledgements A special thanks to Eric Stewart and Ian Orland for contributing content to this guide. A very special thanks to Bill Batten for helping scout field locations and always knowing where to find the best contacts. Additionally, this guide would not have been possible with consulting Dave LePain’s mapping notes and field guides from Pierce and St. Croix counties and the work of others who have previously published work on these Paleozoic units. This is a classic stop for field trips in this part of the state. Devil’s Punchbowl is managed by the Landmark Conservancy- please do not use rock hammers on the outcrops and be good stewards of the landscape. This outcrop shows the relationship between the Eau Claire Formation and the Wonewoc Formation. Expect to see a fine-grained sandstone with swaley cross beds in the Eau Claire Formation, and mediumto coarse-grained, cross-stratified sandstone in the Wonewoc Formation at this location (Mudrey, M.G. Jr. ed, 1997). References Batchelor, C. J., Marcott, S. A., Orland, I. J., He, F., and Edwards, R. L., 2023. Decadal warming events extended into central North America during the last glacial period. Nature Geoscience 16: pages 257261, Hwy 37/Hendricks Ave and Silver Springs Dr.- Mt. Simon Formation: UTM location 4958712.01E) for stop (615774.61N, Batchelor C. J., Orland I. J., Marcott S. A., Slaughter R., Edwards R. L., Zhang P., and Li X., 2019. Distinct permafrost conditions across the last two glacial periods in mid-latitude North America. Geophysical Research Letters 46: pages 1331813326, This is a typical Mt. Simon Formation exposure (Figure 26), coarse- to mediumgrained, cross-bedded, iron stained, sandstone, interbedded with shale and fine grained sandstone (Mudrey, M.G. Jr. ed, 1997). Bradbury, K. R., & Runkel, A. C., 2011. Recent advances in the hydrostratigraphy of Paleozoic bedrock in the Midwestern United States. GSA Today, v. 21, pages 10-12. Byers C.W. and Dott R.H. Jr., 1995 Sedimentology and depositional sequences of the Jordan Formation (Upper Cambrian), Northern Mississippi Valley, Journal of Sedimentology, v. B65, no.3, pages 289-305. Cavosie, A. J., Erickson, T. M., & Timms, N. E., 2015. Nanoscale records of ancient shock deformation: Reidite (ZrSiO4) in sandstone at the Ordovician Rock Elm impact crater. Geology, 43(4), pages 315-318. Cordua, W. S. 1985. Rock Elm structure, Pierce county, Wisconsin: a possible cryptoexplosion structure. Geology, 13(5), pages 372-374. Figure 26. View of the Mt. Simon Formation outcrop. This location is heavily iron stained and exhibits excellent examples of sedimentary structures like trough cross stratification and channel forms. Cordua, W. S., 1987. The Rock Elm Disturbance, Pierce County Wisconsin, in Balaban, N. (ed.), Field trip guidebook for the Upper Mississippi 45 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Valley, Minnesota, Iowa and Wisconsin, prepared for the 21st annual meeting of the Geological Society of America North-central section, Minnesota Geological Survey Guidebook Series #15, pages 123-152. Central Section, Geological Society of America, May1-2, 114 pages. Ostrom, M.E.,1978. Stratigraphic relations of Lower Paleozoic rocks of Wisconsin, Wisconsin Geological and Natural History Survey Field Trip Guidebook 3, pages 3-22 Cordua W.S. and Evans T.J., 2007. Geology of the Rock Elm Complex, Pierce County, Wisconsin, Wisconsin Geological and Natural History Survey Open File Report WOFR2007-02, Map, 1 plate. Runkel A.C., 1994. Deposition of the uppermost Cambrian (Croixian) Jordan Sandstone, and the nature of the Cambriand-Ordovician boundary in the Upper Mississippi Valley, Geological Society of America Bulletin, vol. 43: pages 60-71 Cunningham, J., Dolliver, H., and Cordua, W., 2011. Flaming meteors, dark caves and raging water: geological curiosities of western Wisconsin, in Miller, J.D, Hudack, G., Wittkop, C., and McLaughlin, P.I. (eds.), Archean to Anthropocene: Field Guides to the Geology of the Mid-continent of North America, Geological Society of America Guidebook Field guide 24, pages 411-424. Runkel A.C. McKay, R.M., and Palmer, A.R., 1998. High-resolution sequence stratigraphy of lower Paleozoic sheet sandstones in central North America: The role of special conditions of cratonic interiors in development of stratal architecture. GSA Bulletin, v.110 no.2., pages 188-210. doi:10.1130/B26117.1 French, B. M., Cordua, W. S., & Plescia, J. B., 2004. The Rock Elm meteorite impact structure, Wisconsin: Geology and shock-metamorphic effects in quartz. Geological Society of America Bulletin, 116(1-2), 200-218. Runkel, Anthony C., Robert M. McKay, Clinton A. Cowan, James F. Miller, and John F. Taylor, 2012, The Sauk megasequence in the cratonic interior of North America: Interplay between a fully developed inner detrital belt and the central great American carbonate bank, in J. R. Derby, R. D. Fritz, S. A. Longacre, W. A. Morgan, and C. A. Sternbach, eds., The great American carbonate bank: The geology and economic resources of the Cambrian – Ordovician Sauk megasequence of Laurentia: AAPG Memoir 98, p. 1001 – 1011. Carlson, J.E., 1961. Geology of the Montfort and Linden Quadrangles, Wisconsin, in Geology of parts of the Upper Mississippi Valley zinc-lead district: U.S. Geological Survey Bulletin 1123– B, pages 95–138, 2 pls., scale 1:24,000, Gradstein, F.M., Ogg, J.G., Schmitz, M.D. and Ogg, G.M. eds., 2020. Geologic time scale 2020. Elsevier. Runkel, A.C., 2020. Minnesota at a Glance Paleozoic History of Southeastern Minnesota-Ancient Tropical Seas. Minnesota Geological Survey. Retrieved from the University of Minnesota Digital Conservancy, Heyl, A.V., Jr., Agnew, A.F., Lyons, E.J., Behre, C.H., Jr., and Flint, A.E., 1959, The geology of the Upper Mississippi Valley zinc-lead district: U.S. Geological Survey Professional Paper 309, 310 pages., 24 pls. Sloss, L.L., 1963. Sequences in the cratonic interior of North America. Geological Society of America Bulletin, 74(2), pages 93-114. Kusick, A. R., 2022. Stratigraphy, Sedimentology, and Deformational Significance of Cambrian and Early Ordovician Strata Along the Southeast Wisconsin Arch (M.S. dissertation, The University of Wisconsin-Milwaukee). Steenberg, J.R., and Retzler, A.J., 2016. Bedrock geology, plate 2 of Geologic atlas of Washington County: Minnesota Geological Survey County Atlas Series C–39, Part A, scale 1:100,000, Mossler, J.H., 2006, Bedrock Geology of the Prescott quadrangle, Washington and Dakota counties, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M–167, scale 1:24,000, Stewart E.D., Stewart E.K., Bradbury, K.R., Fitzpatrick, W.A., 2021. Correlating Bedrock Folds to Higher Rates of Arsenic Detection in Groundwater, Southeast Wisconsin, USA, Groundwater, v59, no.6, pages 829-838. Mudrey, M.G. Jr. ed, 1997. Guide to field trips in Wisconsin and Adjacent areas of Minnesota. Prepared for the 31st Annual meeting of the North- 46 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Stewart, E.K., 2021. Bedrock geology of Dodge County, Wisconsin: Wisconsin Geological and Natural History Survey Map Series M–508, scale 1:100,000, Stewart (in revision). Bedrock Geologic map of Jefferson County, Wisconsin: WGNHS Map Series, 1 plate, 1:100,000-scale. Wisconsin Geological and Natural History Survey [WGNHS], 2011, Bedrock stratigraphic units in Wisconsin: Wisconsin Geological and Natural History Survey Educational Series 51, 2 p. Zambito J.J IV, Mauel, S. W., Haas, L.D., Batten, W.G., Chase, Streiff, C.M., P.M., Niemisto, E.M., Heyrman, E.J., 2018. Preliminary Bedrock Geology of Southern Trempealeau County, Wisconsin, Wisconsin Geological and Natural History Survey Open File Report WOFR2018-01, 2 plates scale 1:100,000, 27 pages Zambito J.J IV, Haas, L.D., Parsen, M.J., McLaughlin, P.I., 2019. Geochemistry and mineralogy of the Wonewoc-Tunnel City contact interval strata in western Wisconsin, Wisconsin Geological and Natural History Survey Open File Report WOFR2019-01: 28. 47 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Field Trip 3 – Precambrian Geology of the Eau Claire River Valley: Re-discovering the Eau Claire Volcanic Complex Robert W.D. Lodge, Evan M. Weber, Robert L. Hooper Department of Geology & Environmental Science, University of Wisconsin-Eau Claire, Eau Claire, Wisconsin 54701 of the “prove-it-first” law, or sulfide mining moratorium, in 1997 effectively shut down mineral exploration and mining activities in the region. More recently, the mineral exploration industry has been reinvigorated because of the 2002 discovery of the 18.2 Mt Back Forty deposit in Michigan, easing of the sulfide mining moratorium in 2017, and a recent national push for securing domestic critical mineral resources. However, this has also highlighted the lack of modern datasets on Wisconsin’s mineral deposits that could be used to further our knowledge of the mineral-forming systems in the belt. The Pembine-Wausau Terrane has received most of the historic and recent attention since it hosts approximately 150 million tonnes of known VMS mineralization. However, little attention has been given to the Penokean volcanic deposits that overprinted the Marshfield Terrane that are presented in this guidebook. These volcanic deposits host a VMS prospect (Butler Prospect) and therefore the geodynamic setting of these volcanic rocks clearly are favorable for submarine hydrothermal activity. DeMatties (2022) recognized the gap in knowledge for these Penokean volcanic deposits, known as the Eau Claire Volcanic Complex, within the Marshfield Terrane and their exploration potential. It is a little embarrassing how little we know about the Eau Claire region considering the mineral wealth of the rest of the orogen. Current research at the University of Wisconsin-Eau Claire is aimed at the addressing this issue. Introduction The erosional outliers of Precambrian bedrock in the Eau Claire River valley represent the southernmost extent of the Canadian Shield before it is completely covered by Paleozoic sedimentary strata. The rocks exposed here are part of the Paleoproterozoic Penokean Orogeny, a collisional orogen that resulted from the accretion of the Pembine-Wausau and Marshfield terranes onto the southern margin of the Superior Province. This region was last visited by members of the Institute of Lake Superior Geology in 1980 when a field trip through the region was conducted by Paul Myers and colleagues (Myers et al., 1980) when it was called the “Chippewa Amphibolite Complex”. Since then, the “Eau Claire River Complex” was defined and described in detail by Cummings (1984). There has been ‘new’ U/Pb data collected by the USGS (Sims et al. 1989) and others (Van Wyck et al, 1997; Klier, 2019; Weber and Lodge, 2022), regional syntheses of the Penokean volcanogenic massive sulfide (VMS) mineralization (DeMatties 1989; 1994; 2018; 2022), maps published by government surveys (Brown, 1988), and orogen-wide tectonic model (Shultz and Cannon, 2007) that is being revisited based on new U/Pb data (Zi et al., 2021). The rocks that will be visited on this trip are a critical part of evaluating the tectonic models for the Penokean Orogen and have not been examined using modern analytical techniques. The Penokean Orogen is perhaps best known for hosting numerous VMS deposits. In fact, one of the most complete descriptions of several deposits was published by the Institute of Lake Superior Geology (LeBarge, 1996). The passing The portion of the Eau Claire Volcanic Complex that is visited in this guidebook is not well exposed and its regional context is poorly constrained. Students from the University of 48 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Wisconsin-Eau Claire have been visiting Big Falls and Little Falls locations in this guidebook for decades to learn how to map and describe rocks in the field, measure structures and interpret geologic histories, and learn the basic mechanics of field work. Faculty, students, and alumni from Eau Claire consider these outcrops classic. This guidebook will (re-)introduce these rocks and present some of the ongoing research with the Eau Claire Volcanic Complex. The outcrops visited in this guidebook are accessible by foot, but many others were accessed by kayaking in the Eau Claire River. Ongoing research in this region hopes to expand the lithogeochemistry and zircon petrochronology database to better delineate the geodynamic evolution and crustal architecture of this region. Determining the presence or absence of Archean basement throughout the Marshfield terrane will help refine terrane boundaries and improve our understanding of the metallogeny of the region to assist in future mineral exploration efforts. Regional Geology The Paleoproterozoic Penokean Orogen (ca. 1.8 Ga) in the Lake Superior region (Figure 1) is a classic Precambrian orogenic belt comprised of Figure 1: Geologic map of the major tectonic assemblages and major structures of the Penokean Orogen. Notable abbreviations that are important for this guidebook are EPSZ, Eau Pleine shear zone; NFZ, Niagara fault zone. Figure from Shultz & Cannon (2007). 49 �Proceedings of the 69th ILSG Annual Meeting - Part 2 dominantly submarine volcanic rocks formed in a suprasubduction zone setting that are now structurally juxtaposed along the southern edge of the Archean Superior Province during the earliest phases of forming the Columbia, or Nuna, supercontinent (LaBerge and Myers, 1984; Sims et al., 1989; Schulz and Cannon, 2007). The orogen is host to at least 150 million metric tonnes (Mt) of VMS and associated mineralization (DeMatties, 1994, 2018) but remains one of the more poorly understood and underexplored mineral districts in North America. The Penokean Orogen has been divided into the Interior and Exterior domains. These domains are sutured by the Niagara Fault Zone (Figure 1). The Exterior domain consists of passive margin, rift, and forearc basin sediments and Archean crustal blocks from the Superior Province that were deformed in the folded and faulted foreland part of the orogen. The Interior Domain consists of two accreted terranes, the Pembine-Wausau and Marshfield terranes. These terranes are sutured by the Eau Pleine Shear Zone (Figure 1). The PembineWausau Terrane is a composite accreted oceanic arc overprinted by continental-margin magmatism and hosts numerous VMS deposits and occurrences (DeMatties, 1994; Shultz & Cannon, 2007) (Figure 2). The Marshfield Terrane is composed of Archean crustal fragments of unknown origin that was overprinted by Penokean-aged magmas during the Penokean orogen (Figure 2) and is described in more detail in the sections to follow. Figure 2 - Schematic tectonic evolution of the Penokean Orogen provided by Shultz and Cannon (2007) based on geophysical, sedimentological, and geochronological compilations. continental arc volcanism and back arc extension developed until about 1850 Ma until the collision with the Marshfield terrane began. During this ocean closure, a double subduction zone with concurrent northward and southward subduction resulted in arc magmatism on both the PembineWausau and Marshfield terranes. Sedimentation related to this convergence in a foreland basin setting continued until about 1835 Ma. The end of the orogen was constrained by undeformed post-tectonic plutons dated at 1830 Ma that stich shear zones. Shultz and Cannon (2007) synthesized the tectonic events that formed the Penokean Orogen (summarized in Figure 2) based on a detailed compilation of lithologic, structural, sedimentological, and geochronological datasets. This classic model proposed that an oceanic arc, now the Pembine-Wausau Terrane, collided with the southern margin of the Superior Province around 1880 Ma. Following a subduction flip from south-directed to north-directed subduction, 50 �Proceedings of the 69th ILSG Annual Meeting - Part 2 However, this classic tectonic model for the evolution of the Penokean Orogen has recently been re-evaluated considering new U/Pb data. The first contradictory data came when Quigley (2016) obtained a high-precision U/Pb zircon age of 1832.98 ± 0.52 Ma from a rhyolite at the Back Forty deposit via CA-ID-TIMS. This younger age was in stark contrast to the other VMS deposits that yielded U/Pb zircon ages of ca. 1875 Ma. Additional U/Pb zircon ages reported by Zi et al. (2021) from volcanic units (Beecher Formation) and plutonic rocks (Dunbar Gneiss, Newingham Tonalite) in the eastern part of the orogen supported the younger extensional tectonic event proposed by Quigley (2016). These new ages resulted in a revised Penokean tectonic model where long-lived northward subduction along a continental margin with repeated extensional and contractional regimes in response to retreat and advance of the subducting oceanic plate (Figure 3). Weber and Lodge (2022) obtained a U/Pb age of 1831.4 ± 2.0 Ma on the dacite unit hosting the Eisenbrey deposit in the western part of the orogen, suggesting that this second VMS forming event was widespread. A summary of the geochronology is presented in Figure 4. Figure 3 - Schematic illustration of the revised tectonic model of the Penokean Orogen. Figure is from Zi et al. (2021). Abbreviations: NF—Niagara fault zone; EPSZ—Eau Pleine shear zone. Marshfield Terrane This guidebook visits the only Penokean volcanic complex south of the Eau Pleine Shear Zone and is interpreted to part of the Marshfield Terrane. The Marshfield Terrane represents an Archean microcontinent of uncertain origins (Sims et al., 1989; Schulz and Cannon, 2007; Zi et al., 2021). Some of the earliest works on the terrane by Sims et al. (1989) noted eight Archean U/Pb ages from isolated outcrops along the Wisconsin, Black, and Chippewa Rivers; many of which were compiled from unpublished sources. Paleoproterozoic volcanic rocks in the Marshfield terrane were deposited about 1835-1865 Ma (Sims et al., 1989; Van Wyck, 1995; Klier, 2019; Weber and Lodge, 2022). These supracrustal rocks were referred to as the Eau Claire River Complex by Cummings (1984) or the Eau Claire Volcanic Complex by DeMatties (2018; 2022). They consist of an interlayered sequence of felsic to mafic volcanic rocks, dacite porphyry, and a variety of clastic and chemical sedimentary rocks (Sims et al., 1989). Some conglomerates contain granitic gneissic clasts that were interpreted to be Archean (Myers et al. 1980), but no definitive ages were determined on the clasts. Otherwise, our knowledge of the Archean Marshfield terrane and associated Paleoproterozoic volcanic rocks remains as sparse as the outcrop exposures. Eau Claire Volcanic Complex The Eau Claire Volcanic Complex is poorly documented and understudied mainly due to its inaccessible outcrops in remote parts of the Eau Claire River valley. Myers et al. (1980) described 51 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 4 - Time-space plot for the tectonic components of the Penokean Orogen. Plot is from Zi et al. (2021). See citation for references on data sources. supracrustal amphibolites, metarhyolites and metasediments in the Eau Claire River valley and classified them as part of the Chippewa Amphibolite Complex. This informal classification of the high metamorphic grade rocks in the Eau Claire-Chippewa River area was eventually grouped with the Marshfield Terrane by Sims et al. (1989) and Shultz and Cannon (2007). The first time that that the Eau Claire “Complex” was officially referred to was by Cummings (1984) when discussing the petrology and geochemistry of the gneisses in the Big FallsLittle Falls area (Stops 1 and 2 in this guidebook). After that, research in the Eau Claire Volcanic Complex essentially ceased. Sims et al. (1989) reported a U/Pb rutile age from Big Falls of ca. 1835 Ma. In fact, the words “Eau Claire” are not used in the Shultz and Cannon (2007) regional synthesis. DeMatties (2018; 2022) refers to the Eau Claire Complex when discussing the volcanic complexes in the Penokean, but largely cites the work of Myers et al. (1980). 52 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Preliminary Hf-isotope data and zircon trace elements reveal that the rocks analyzed in the Eau Claire Volcanic Complex are juvenile, mantlederived melts with no inheritance from older sources (See Weber et al. 2023, Part 1 of this volume). This suggests that these volcanic rocks are not forming on Archean basement, as one would expect if the Eau Claire Volcanic Complex was emplaced onto the Marshfield Terrane. Additionally, magnetic lineaments on aeromagnetic maps for the region appear to crosscut the interpreted position of the Eau Pleine Shear Zone and the overall fabric as outlined by magnetics appears constant (Figure 5). Ongoing research in the region seeks to better define the relationship of the Eau Claire Volcanic Complex to the Marshfield Terrane and the architecture of the basement in this area. Field Trip Stops The overall objective of this guidebook is to tour the accessible parts of the Eau Claire Volcanic Complex as exposed in the Eau Claire River valley and surrounding tributaries. The guidebook can be divided into two main regions: The Big Falls-Little Falls and North Fork regions. The Big Falls-Little Falls region represents the classic “Eau Claire Complex” originally described by Cummings (1984). The North Fork region is much more remote and rarely visited by geologists. In fact, it is not obvious that anyone has studied these rocks since they were first reported by Myers et al. (1980). These more remote parts of the complex are currently being studied (see Leahy and Lodge, Part 1 of this volume) to determine their regional context. Some of that data will be presented herein. The goal of this work is to determine if the Eau Claire Volcanic Complex is a volcanic center built upon Archean crust (continental arc) or is juvenile Figure 5 - Total field aeromagnetic map of the Eau Claire River region showing the location of Eau Pleine Shear Zone and field trip regions (Big Falls, North Fork). White dashed lines highlight a couple of magnetic lineaments that extend through the suturing shear zone. Magnetic maps from Daniels and Snyder (2002). 53 �Proceedings of the 69th ILSG Annual Meeting - Part 2 (oceanic arc) as this is important implications for the regional metallogeny and mineral systems. Most of the locations in this guidebook are on riverside outcrops. These areas are prone to sudden flooding and the upmost caution and careful planning should be used prior to visiting these locations. In addition, rocks here are uneven and potentially slippery. To access larger sections of outcrops, low water conditions may be required. In addition, all locations in this region may contain poisonous plants (e.g. nettle, poison ivy) and black-legged ticks that can transmit diseases. While this is unlikely to be a concern in early spring during the 2023 ILSG conference, future users of this manual should plan appropriately. Figure 6 - Generalized Precambrian geologic map of the Big Falls-Little Falls area of the Eau Claire River. Figure modified from Cummings (1984). Big Falls Region The banded amphibolite, gneisses, and intrusions in the Big Falls region of the Eau Claire River are some of most studied Precambrian exposures in this region and are visited multiple times a year by introductory and upper division geology classes at the University of WisconsinEau Claire. It is in this region that the term “Eau Claire River Complex” was first introduced by Cummings (1984) and this terminology has since been adopted by others (e.g. DeMatties, 2018) to describe the volcanic rocks present in the Marshfield Terrane. Figure 7 - Metamorphic conditions from geothermobarometic studies at Big Falls indicated by the yellow star. Data is from unpublished student project at the University of Wisconsin-Eau Claire. The region consists of mostly amphibolitic and felspathic gneisses that are intruded and brecciated by tonalite (Figure 6). Regional metamorphism in this region is at lower to upper amphibolite facies. A sample of amphibolitic gneiss from the Eau Claire Volcanic Complex using the edenite-richterite thermometry determined temperatures between 719-769 °C (Hannack and Radwany, 2018). Unpublished data from University of Wisconsin-Eau Claire class projects using garnet-biotite thermobarometry estimate peak metamorphic conditions at 765 °C and 11.5 kbars (Figure 7). A rutile U/Pb age of 1835 Ma from Sims et al. (1989) in the Eau Claire Volcanic Complex may indicate the timing of metamorphism. New research in this region provides our first glimpse into the trace element characteristics of these rocks (Figure 8). Rocks from both Big Falls and Little Falls have mafic protoliths with EMORB to oceanic arc like abundances of Th, Nb, and Yb (Pearce, 2008). On normalized trace element diagrams, samples have elevated LREE, low Th/La ratios, negative Nb and Ti anomalies. These trace element characteristics features are common in back-arc environments. Additionally, feldspathic units sampled have extremely 54 �Proceedings of the 69th ILSG Annual Meeting - Part 2 depleted trace element signatures and positive Eu anomalies, suggesting that they may be fractionated crystal cumulates. This broadly supports the interpretation of Cummings (1984) that the protolith of the Big Falls gneisses are a layered mafic intrusion. Stop 1: Amphibolite Gneisses and the “Great Unconformity” at Big Falls County Park Lat: 44.8215° Long: -91.2953° Figure 8 – Preliminary trace element geochemistry from the Big Falls-Little Falls area of the Eau Claire Volcanic Complex. Top: Trace element classification diagram from Pearce (1996) modified from Winchester and Floyd (1977). Middle: Mantle source discrimination diagram from Pearce (2008). Bottom: Primitive mantle-normalized trace element diagram using values from Sun and McDonough (1989). This location is accessible from the north entrance to Big Falls County Park off Eau Claire County Highway Q. There is a parking lot at this entrance with plenty of parking for park visitors. Follow the paved foot path eastward toward the river. Once on the riverbank, walk northward for 55 �Proceedings of the 69th ILSG Annual Meeting - Part 2 about 50 m to reach the outcrops at the falls. Note that outcrops on the south bank of the falls will have to be accessed via the south entrance to the park on County Highway K. During very low water conditions, it is possible to hop across the outcrops to access the south bank. This region has some steep-edged rock cliffs adjacent to the river and there are springs that keep some areas wet and slippery. Please watch your step. This stop highlights the geology along the north side of Big Falls County Park where the rocks are exposed along the Eau Claire River. In addition to the Precambrian rocks, this location also has a great exposure of the “Great Unconformity” with overlying Cambrian Mount Simon Formation. The Eau Claire River flows along the nonconformity between the Cambrian and Precambrian rocks, where the river has eroded the overlying Cambrian units away exposing the Precambrian basement rocks. At this stop, we highlight four locations that highlight different units seen here at Big Falls County Park (Figure 9). Photo 1: Photographs of the banded amphibolitic gneiss at Big Falls County Park. (A) Banded amphibole gneiss at Big Falls. B: Photomicrograph in plane-polarized light (25x) of large garnet porphyroblasts in quartzofeldspathic and hornblende matrix. Location 1: The Banded Amphibole Gneiss The banded amphibole gneiss (Photo 1A) is best described as a fine-grained banded gneiss with alternating hornblende-rich and plagioclaserich layers. The hornblende-rich layers range anywhere from less than 1 cm to ~15 cm and contain about 85% hornblende and 15% granulated plagioclase with sparse idioblastic garnet. The plagioclase-rich layers are consistently thicker and contain approximately 15% hornblende. The garnets though scarce occur as coarse grained porphyroblasts in both layers (Photo 1B) although these garnets often show retrograde alteration back to hornblende. The garnets are typically poikioblastic (Photo 2) with quartz, plagioclase and occasionally biotite inclusions. The hornblende occurs as euhedral to subhedral grains and the plagioclase as very-fine grained, dynamically recrystallized, matrix. The granulated plagioclase is typically labradorite to bytownite but anorthite (An92) occurs as cores in some of the idioblastic hornblende to create an Photo 2: Photomicrograph in plane polarized light of poikioblastic garnet in a hornblende and granulated plagioclase matrix (50X magnification). unusual bi-modal plagioclase population (Photo 3). Multiple shear zones and isoclinal folds are present throughout the outcrop, providing evidence for multiple deformation periods. Partially annealed shear zones can be traced across almost the entire unit that truncate and offset banding. There are also asymmetric 56 �Proceedings of the 69th ILSG Annual Meeting - Part 2 amphibole schist is enriched in MgO and FeO and depleted in CaO and Al2O3 in comparison to the banded amphibole gneiss (Table 1). Both major element (Table 1) and trace elements (Figure 8) characterize this as a MORB-like composition. Isoclinal folds of the foliation and the presence of ductile shear zones indicate deformation throughout this unit. The contact between the banded amphibole gneiss and the amphibole schist is buried by slumping blocks of the Cambrian Mount Simon Formation and vegetation. Photo 3: Photomicrograph in partially crossed polars of idioblastic hornblende with anorthite cores (An 92) in a matrix of granulated plagioclase (An72) in the banded amphibole gneiss at Big Falls (Magnification is 50x). amphibolite inclusions kinematics. that show variable This unit was sampled for a recent zircon petrochronology study and produced significant results that question what is currently understood about the southern portion of the Penokean Orogen. This unit yields a U/Pb age of 1874.7 ± 2.1 Ma which temporally correlates with other VMS-forming events across the PembineWausau terrane. Zircon trace element geochemistry of the sample indicate the sample formed in a hydrated but reduced melt in a backarc setting where decompression was occurring in a metasomatized mantle (Weber and Lodge, 2022). Hf-Lu data from the banded amphibolite gneiss indicates a lack of older basement inheritance. The zircon petrochronology from these rocks contradicts the interpretation that Eau Claire Volcanic Complex was emplaced into the Archean Marshfield Terrane. These results have motivated additional research in the Eau Claire Volcanic Complex. Photo 4: Outcrop photo of the feldspathic gneiss at Big Falls. Location 3: Transition Gneiss and Feldspathic Gneiss The amphibole schist gradually grades into the transition gneiss for a few meters as the unit contains fewer amphibole-rich layers and the plagioclase rich layers become more prominent (Photo 4). The feldspathic gneiss is primarily made of plagioclase, with 10-20% hornblende and lesser amounts of chlorite, epidote, and localized sulfidation with some pyrite mineralization. Location 2 Amphibole Schist Despite strong metamorphic recrystallization and structural fabric overprinting, there are some primary igneous textures that are preserved (Cummings, 1984). The compositional banding and layering throughout all units appear to be The further west along the Eau Claire River, the amphibole schist is exposed. This unit is best described as a dark green to black, fine grained thinly banded amphibole schist. Hornblende is the primary amphibole with lesser amounts of plagioclase. Based on whole rock XRF data, the 57 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 9: Geologic map showing outcrop locations at the Big Falls stop. Modified from Cummings (1984). Table 1. Whole rock major element geochemistry (via XRF) from Big Falls for banded amphibole gneiss (location 1), amphibolite (location 2), and altered rocks at the Precambrian-Cambrian contact (location 3). Average MORB composition (Winters, 2010) is included for comparison. Unit Unaltered Banded Gneiss 1 Banded Gneiss 2 Banded Gneiss 3 Altered (Depth) surface .5m 1.0m 1.5m Unaltered Amphibolite 1 Amphibolite 2 Average MORB (Winter 2010) Altered (Depth) surface .25m .5m .65m .75m 1.0m SiO2 TiO2 Al2O3 Fe2O3T CaO MgO MnO Na2O K2O P2O5 Totals 47.10 48.43 46.63 0.17 0.21 0.23 30.43 30.56 30.24 2.14 3.35 2.47 15.18 14.65 14.51 0.72 1.38 0.88 0.03 0.04 0.03 2.21 2.97 2.85 0.20 0.25 0.18 0.05 0.04 0.03 98.23 101.88 98.05 52.06 50.26 52.55 52.92 0.44 0.74 0.34 0.32 16.66 15.28 17.36 18.23 6.72 11.56 7.28 5.79 0.88 0.63 1.06 0.80 5.21 6.18 5.17 4.91 0.04 0.05 0.03 0.02 0.11 0.05 0.05 0.09 9.28 9.14 9.04 9.37 0.03 0.28 0.03 0.03 91.43 94.17 92.91 92.48 51.10 52.66 50.50 1.34 1.71 1.56 15.50 13.25 15.30 13.73 12.12 11.50 9.07 8.05 11.50 5.79 6.57 7.47 0.19 0.20 n/a 3.11 3.56 2.62 0.29 1.46 0.16 0.23 0.17 0.13 100.35 99.75 100.74 61.93 62.79 61.48 62.62 60.07 58.69 0.94 0.99 0.84 0.88 0.95 0.87 17.36 17.18 17.04 16.26 16.84 16.31 4.98 6.10 5.04 5.42 5.60 7.30 0.63 0.65 0.64 0.64 0.64 0.66 2.70 3.05 3.02 2.68 2.89 3.24 0.02 0.04 0.02 0.04 0.04 0.07 0.32 0.20 0.21 0.23 0.07 0.08 7.10 7.22 6.97 7.15 7.45 7.18 0.19 0.19 0.18 0.18 0.21 0.19 96.17 98.41 95.44 96.10 94.76 94.59 58 �Proceedings of the 69th ILSG Annual Meeting - Part 2 primary. Anorthositic autoliths are incorporated in a fine-banded, more mafic matrix near the transitional gneiss. The banding in the autolith is discordant to banding in the matrix and appear to be concentrated in bands but are not associated with boudinage fabrics. These observations were critical in interpreting the protolith of this region. Location 4: The Great Unconformity and the basal portion of the Mt. Simon Formation The large hillside on the north bank of the river and above the Precambrian outcrops is the Cambrian Mount Simon Formation. The base of the Mt. Simon Formation is a mix of coarsegrained quartz arenites and quartz pebble conglomerates. Photo 5: Nonconformity between the Cambrian Mount Simon Formation and the amphibolites at Big Falls County Park. The Great Unconformity creates a nonconformable contact between the Precambrian units at the previous three locations and the base of the Mt. Simon Formation. At Big Falls, a thin blue-green celadonite clay layer (Figure 10) has formed along the nonconformity as a result of Kmetasomatism from basinal brines. The Kmetasomatism at the unconformity is recognized throughout the midcontinent region and is related to MVT lead zinc deposits in the Tri-state region (See field trip 1 in this volume). Locally the celadonite acts as a fluid barrier for springs that flow along the unconformity. In other places, the contact appears to be relatively sharp with little alteration (Photo 5). Figure 10: A-CN-K diagram showing chemical change due to weathering and the alternative path of Kmetasomatism. The celadonite at Big Falls is not a weathering profile and requires adding substantial potassium and to produce the celadonite and authigenic K-spar seen along the unconformity (see Table 1 for chemistry). Stop 2 –Tonalite Breccia and Gneisses at Little Falls. Lat: 44.8103° Long: -91.2825° bridge at this location for the Eau Claire River flood stage measurement. Just north of the bridge there is a small parking area on the west side of the road. There are several small foot paths that lead down to the river’s edge. The outcrops are mostly exposed immediately around and under the bridge. The quality of exposure here changes all the time as flooding conditions sometimes This location is just on the north side of the County Highway K bridge that crosses the Eau Claire River and is 300 m north of the exit to the south entrance of Big Falls County Park. Google Maps calls this place the East Eau Claire Canoe Landing, but the USGS refers to this location as Little Falls (so does the faculty and students at the University of Wisconsin-Eau Claire) and uses the 59 �Proceedings of the 69th ILSG Annual Meeting - Part 2 buries parts of the outcrop with sand and downed vegetation. To access the south bank of the river, a short bush traverse (100-300 m) will be required from the southside of the bridge along the riverbank. If water level and time permit, this stop has four locations of interest that highlight the intrusive history in the region. The outcrops in this area expose an inclusion-rich intrusive contact between a foliated tonalite and lensoidal amphibolite and are cut by younger pegmatitic and mid-continent rift diabase dykes (Figure 11). This location is used to teach students at the University of Wisconsin-Eau Claire about interpreting relative geologic time and observing contact relationships. The absolute ages of these rocks are unknown as recent attempts to isolate zircons from the tonalite were unsuccessful. Photo 6: Gneissic tonalite breccia highlighting some of the banded gneiss xenoliths. Some of the smaller xenoliths here are elongated. significant assimilation of amphibolite. Biotite commonly produces a crude foliation that may have formed from hornblende during a later deformation. Location 1: Gneissic Tonalite Breccia It is clear the tonalite is metamorphosed and is an important part of determining the nature of the Eau Claire Volcanic Complex. However, efforts to constrain the timing of this intrusive event have yielded conflicting results. Van Schmus (1980) yielded a U/Pb age of 1842 ± 10 Ma utilizing zircon fractions (i.e. not modern single crystal methods). Sims et al. (1989) reported a U/Pb age of 1856 ± 5 Ma from a xenolith at Little Falls. Assuming that the amphibolite xenoliths at Little Falls are from the same amphibolite unit at Big Falls that was dated at 1875 Ma, then there is a clear conflict. The tonalite was sampled for that recent petrochronologic study (Weber and Lodge, 2022) but yielded very few zircons. Resolving the timing of emplacement and tectonic setting of this intrusion will help better understand the geodynamic setting of the Eau Claire Volcanic Complex. The gneissic tonalite breccia is the most prominent unit at Little Falls (Photo 6). Roughly 90% of xenoliths in the breccia are characterized as banded gneiss to banded amphibolite containing 50-80% hornblende and 20-40% plagioclase with lesser amounts of biotite. These xenoliths range in size anywhere from less than 1cm to greater than 20 cm, and they are hosted in a biotite tonalite intrusion which destroys the older banded gneiss. The banded gneiss xenoliths are also elongated and contain folds. Ultramafic xenoliths are scarce but also present. These xenoliths contain over 90% hornblende with lesser amounts of epidote-clinozoisite and plagioclase. Occurring in localized clusters, the fragmented ultramafic xenoliths indicate these were most likely part of a larger block but separated during the tonalite intrusion event (Myers et al., 1980). The tonalite is composed of 35-40% plagioclase (An50-55), about 30% hornblende, 25-30% quartz, 5-10% biotite, and accessory epidote. Myers et al. (1980) interpreted the fabric in the rock as flow-lamination, however it is parallel to regional magnetic lineaments suggesting it may be a structural fabric. Large variation in mafic mineral abundance indicates Location 2: Diabase Along the north side of the river and east of the bridge, lies one of many mid-Proterozoic diabase dykes associated with the mid-continent rift in the 60 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 11: Geologic map of the Little Falls area showing the locations of interest at this stop. Figure modified from Myers et al (1980). Eau Claire region. Like many other diabase dike outcrops in the area, this diabase exhibits both clean columnar jointing and well-defined chilled margins. This diabase is only a few meters in size and disappears beneath the surrounding overburden (Figure 11). The diabase has a medium-grained, equigranular texture and does not have any foliation or recrystallization textures and is clearly post-metamorphism. throughout the Eau Claire volcanic complex along the Eau Claire River where the Precambrian rocks are exposed. Their macro- and microscopic characteristics indicate they are Location 3: Pegmatite Dike On the west bank of the river lies a 2 m wide pegmatite dyke (Photo 7). Outlier boulders of this pegmatite can be found on the eastern bank of the river. The alkali-feldspar crystals in this outcrop reach sizes greater than 30 cm. Various sizes of quartz veins also crosscut this unit. This granite pegmatite dike is one of a handful seen Photo 7: 18m-wide pegmatite in the Eau Claire River downstream from Little Falls. 61 �Proceedings of the 69th ILSG Annual Meeting - Part 2 clearly younger than the Penokean deformation and metamorphism and could be related to Mazatzal or Yavapai orogenic events to the south like the Wolf River Batholith in northcentral Wisconsin. The mineralogy of the pegmatites is very complicated with many accessory carbonate, phosphate and oxide phases enriched in Nb, Y, F U, Th and REE. Zircon in the pegmatites indicate extreme fractionation (Figure 12). The zircons also show considerable xenotime (Y,P) substitution and considerable solid solutions with both coffinite (USiO4) and thorite (ThSiO4). The pegmatites, despite their pink color, are primarily composed of plagioclase with an overprint of potassic alteration. Most of the alkali-feldspar occurs along cleavages, crystal boundaries and fractures indicating it is a late phase in pegmatite formation. The plagioclase in the pegmatites is primarily albite but ranges from An0 to An30. The euhedral and inclusion free garnets (Photo 8) have a limited range of chemistry close to 50% almandine and 50% spessartine which is similar to magmatic garnet compositions in other garnet-quartz-albite pegmatites (Muller et al., 2018). Figure 12. Zr/Hf in pegmatites from the Eau Claire River Complex. The Eau Claire River pegmatites have many characteristics of pegmatites in the Nb/Y/F (NYF) family of rare element pegmatites and NYF pegmatites are always associated with metaluminous to alkaline (or peralkaline) granites (Cerny and Ercit, 2005) Location 4: Amphibolitic Gneiss The xenoliths within the tonalite are assumed to be derived from the nearby outcrops of the amphibolitic gneisses. Unlike the planar banding at Big Falls, the amphibolitic gneisses here is more lensoidal with cm- to dm-scale lens-shaped, hornblende-rich pods surrounded by more plagioclase-rich “matrix” and quartz-veining. Photo 8. Top: Photomicrograph (25X in plane polarized light) of garnet cluster in quartz and albite from Little Falls pegmatite dike on the west side of the river. Bottom: Almandine/spessartine garnet clusters at the same location at Little Falls are magmatic in origin. 62 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Detailed work has not been completed on these outcrops but are assumed to be petrogenetically related to the amphibolites at Big Falls. Future research will examine these exposures more closely. the trace element geochemistry and zircon petrochronology of the rocks in this region to make better links with the rest of the Eau Claire Volcanic Complex. Much of that data is still pending or preliminary, so results will be forthcoming soon. The goal for the field trip in this region is to show as many of the rocks as possible, regardless of how much we know about them. North and South Fork Region This is the part of the trip where information on these rocks is sparse and new data is only just becoming available. To our knowledge, the rocks in the North and South Fork areas of the Eau Claire River have not been studied in any detail since Myers et al. (1980). Much of the area is remote and sparsely developed and very few outcrops are easily accessible. Field work in the 2022 summer relied on one-way, day-long kayak trips along different segments of the North Fork. Aside from the occasional powerline, field work on these stretches of the Eau Claire River felt wild and remote. This field work aimed to characterize The region consists of amphibolites, feldspathic gneisses, and foliated granitoids and are cross-cut by younger, undeformed granitic pegmatites (Figure 13). A metarhyolite from this region yielded an age of 1858 ± 5 Ma (Sims et al, 1989) and was one of the key samples that linked Penokean volcanic processes to the Marshfield terrane. Myers et al. (1980) interpreted mappable contacts between intrusions and foliated Figure 13: Geologic map of the North and South Fork of the Eau Claire River in eastern Eau Claire County and western Clark County. Figure modified from Myers et al. (1980). 63 �Proceedings of the 69th ILSG Annual Meeting - Part 2 supracrustal rocks in this area to be sheared and nearly vertical and that they enclose lensoidal fault slices which have been juxtaposed mainly by strike-slip displacement. Outcrops of amphibolite in the southern part of the map (Figure 13) near the confluence of the North and South Forks of the Eau Claire River were interpreted to be part of the “Chippewa Amphibolite Complex” (Myers et al., 1980) which is broadly supported by regional aeromagnetic maps (Figure 5). Myers et al. (1980) interprets the volcanic rocks in this region to unconformably on amphibolites, but geochronologic data is lacking to make any absolute local or regional correlations. Regional metamorphic grade is estimated to be upper greenschist to lower amphibolite based on the presence of garnet, epidote, muscovite, and hornblende. Preliminary geochemical results from the North Fork region begin to reveal the setting of these volcanic and intrusive rocks. Volcanic protoliths are bimodal (Figure 14) with tholeiitic mafic rocks with oceanic affinities (Figure 15) and FI- to FII-type felsic rocks arc-like affinities (Figure 16). More work needs to be done before we can concretely interpret the setting of this region of the Eau Claire Volcanic Complex. Figure 15: Trace element geochemical characteristics of the mafic rocks from the North Fork region of the Eau Claire River. Top: Magmatic affinity diagram for sub-alkaline basalts from Ross & Bedard (2009). Middle: Mantle source discrimination diagram from Pearce (2008). Bottom, Primitive mantle-normalized trace element diagram using values from Sun and McDonough (1989). Figure 14: Trace element classification diagram from Pearce (1996), modified from Winchester & Floyd (1977). 64 �Proceedings of the 69th ILSG Annual Meeting - Part 2 These settings are not typical of continental settings, which continues to question the relationship between the Eau Claire Volcanic Complex and Marshfield Terrane. Stop 3 – Amphibolite and Intrusions at Knights Pool Lat: 44.7482° Long: -90.9669° Figure 16 – Trace element geochemical characteristics of felsic rocks from the North Fork region of the Eau Claire River. Top: Nb/Y discrimination diagram for granites from Pearce (1984). Middle: F-type felsic discrimination diagram from Hart et al. (2004). Bottom: Primitive mantle-normalized diagram using values from Sun & McDonough (1989). The directions to get to this stop are a little more elaborate since it is in a more remote location. From the community of Augusta, take State Highway 27 north for 4.4 miles to County Road GG. Turn east on to County Road GG and drive 4.7 miles to the intersection of Channey 65 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Road just after the bridge over the Eau Claire River. Turn east on Channey Road (note that this is an unpaved road) and drive for 4.6 miles until the road crosses the North Fork of the Eau Claire River. This location is called Knights Pool and is labelled by signage. The accessible outcrops in this region are immediately beneath the bridge and are accessible by foot trails. There are larger outcrops north of the bridge that are accessible by a small, 150 m bush traverse along a sparsely used trail. At both locations, rocks are immediately adjacent to a shallow but fast-moving river and caution should be used. Complex exposed in this section of the river upand downstream of this location. However, this is the only easily accessible section by foot. At Knights Pool, there are mostly strongly foliated and deformed amphibolites that are intruded by a biotite granodiorite (Figure 17). Amphibolite: The amphibolite at Knights Pool is characterized by a fine to very fine grained mafic lineated amphibolite with stretched quartzfilled amygdules, relict pillow structures, and wispy textures suggesting a mafic flow (Photo 9). Thin sections of the amphibolite clearly show the lineations present in the amphibolite here (Photo 10). Several stages of deformation occurred starting with the amphibolite being isoclinally folded, then intruded by aplite veins, and intruded by large granodiorite body (Myers et al., 1980). The shearing in of the granodiorite body created a mylonite gneiss along the contact with the amphibolite. Knights Pool is located at the bridge on Channey Road as it crosses the North Fork of the Eau Claire River along the southern edge of the North Fork Eau Claire River State Natural Area. There is more of the Eau Claire Volcanic Photo 9: Outcrop of the amphibolite showing both the isoclinal folds and strained amygdules present in the unit. Trace element geochemistry of the amphibolites at Knights Pool are notably LREEdepleted with strong negative Nb anomalies (Figure 15). This suggests it was derived from strongly depleted but metasomatized mantle. This type of environment, presumably a mature backarc, rarely exists in a continental setting. The granitoids in the map area have classic enriched LREE and Th with depleted Nb and HREE signatures suggesting they are related to a Figure 17: Geologic map of the Knights Pool area. Map is modified from Myers et al. (1980). 66 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Photo 10: Photomicrographs in plane-polarized light of the amphibolite at Knights Pool amphibolite under thin section at 25x magnification (A) and 100x magnification (B). Hornblende forms a clear lineation. Photo 11: Photomicrographs of the biotite granodiorite at Knights Pool in (A) plane-polarized light, and (B) cross-polarized light. Both images are 25x magnification. the village of Rock Dam. Turn northward on Butler Road and use the parking lot on Hay Creek Lake just south of the bridge over Hay Creek. The outcrop of phyllite are under the bridge and near the Rock Dam spillway. The nonconformity and metarhyolite can be better accessed from within the Rock Dam Campground near campsite 90. different tectonic event when the crust was thicker, and garnet was stable to deplete HREE. Granodiorite Intrusion: The medium to coarse grained biotite granodiorite intruded the amphibolite creating a mylonitic fabric along the contact. The intrusion, shearing along the contact, caused the folding of the aplite veins seen in the amphibolite. Quartz is strongly recrystallized and biotite concentrations define a weak foliation. Thin section photos highlight how the quartz is being recrystallized, as well as show the alignment of biotite aggregates (Photo 11). This location reveals another nonconformity between the Precambrian and Cambrian Mount Simon Formation. The metarhyolites and phyllites in this area are strongly mylonitized and primary structures are difficult to interpret. The metarhyolite at this location described by Myers et al. (1980) is the only reference to a rhyolitic unit in the Eau Claire Volcanic Complex. Since Sims et al. (1989) dated a metarhyolite at 1858 ± 5 Ma in the Eau Claire River and cited Myers et al. (1980), it presumably came from this location. If that is the case, then the rocks at Rock Dam are of regional significance because it is one of Stop 4 – Metarhyolite and Phyllite at Rock Dam Lat: 44.7338° Long: -90.8469° From the previous stop, continue eastward on Channey Road until it ends at County Highway H. Turn southward and drive 1.5 miles to Rock Dam Road. Turn eastward and take this road into 67 �Proceedings of the 69th ILSG Annual Meeting - Part 2 estimated mineral percentages and matrix of this rock is composed of a very fine-grained alkalifeldspar (57%), quartz (35%), muscovite (3%), magnetite (2%), and biotite (1%). The quartz eyes along with the absence of feldspar porphyroclasts suggests that the quartz either originated as phenocrysts or clasts in a tuff (Myers et al. (1980). Foliation trends east-west and is near vertical. Phyllite: Closer to the base of Rock Dam lies a muscovite-rich phyllite composed of alkalifeldspar, quartz, and muscovite (Photo 12). This outcrop lacks both the quartz eyes and biotite possibly indicating a separate protolith than the metarhyolite (Myers et al. 1980). Photo 12: Outcrop photo of strongly foliated phyllite at Rock Dam near spillway. Cambrian strata can be seen in background on opposite bank of river. Mt. Simon Formation: The basal part of the Mt. Simon Formation, a conglomerate layer containing pebbles of vein quartz and rhyolite, is exposed at this location (Photo 13). The contact between the Mt. Simon and the Precambrian metarhyolite shows about 5 m of relief. Many of the locations exposing the Great Unconformity in the Eau Claire region show deep weathering of the underlying Precambrian rocks, however there is not much weathering of the Precambrian metarhyolite here. the very few dated Penokean supracrustal rocks within the Marshfield terrane. Mylonitized Metarhyolite: Myers et al. (1980) admittingly conceded that determining the protolith of this outcrop can be challenging considering the similarities between sheared porphyritic rhyolites and leucogranites. The mylonite here is pale pink metamorphosed porphyritic rhyolite containing quartz eyes that can be described as phenocrysts or clasts. These quartz eyes are roughly 1-2.5 mm in size and under thin section show a subrectangular to lenticular shape (Myers et al. (1980). The 68 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Photo 13: Basal pebble conglomerate in Cambrian strata overlying metarhyolite at Rock Dam. Stop 5 – Metavolcanic Rocks at Mead Lake Lat: 44.7885° Long: -90.7742° From the previous stop, continue northward on Butler Road for 0.8 miles to the intersection with Willard Road. Drive eastward on Willard Road for 2.0 miles and turn north onto County Road M. Drive northward on County Road M for 1.6 miles and turn eastward onto Rocky Run Road. Drive 1.2 miles on Rocky Run Road and turn northward on Bruce Mountain Road that will turn into South Lake Road. South Mead Lake Park will be 1.4 miles down this road. Park there, and the outcrops are on the riverbank west of the Mead Lake Dam spillway. The bedrock exposed at this location is primarily a foliated, fine-grained chloritic metavolcanic rock (Photo 14). There has been no known study of this rock, and our data is still pending. Nonetheless, it is apparent that the metamorphic grade seems to be decreasing in this part of the Eau Claire Volcanic Complex. This is in stark contrast to the rocks in the Chippewa River valley (Fieldtrip 1, this volume) and Big Falls region (Stops 1-2). Future work in the region will utilize every outcrop, even small ones like this location, to better describe and define the tectonics and metallogeny of the Eau Claire Volcanic Complex. Photo 14: Outcrop photo of metavolcanic rocks at Mead Lake Dam. 69 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Wisconsin: An overview: Economic Geology, v. 89, p. 1122-1151. Acknowledgements Despite decades of regular visits from groups from the University of Wisconsin-Eau Claire, the most extensive detailed maps and rock descriptions were provided by Paul Myers and collaborators in the 1980 ILSG guidebook. Outside of Big Falls County Park, many of those locations have not been visited since then. A lot of the geologic descriptions from those lesserknown areas have been updated from Myers et al. (1980) and figures have been digitized while adding new data and insights where available. 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, article 104489. In addition, the authors of this guidebook would like to thank the countless undergraduate and graduate students that have worked on these outcrops and have continued to inspire new work in the region. Specific acknowledgement is deserving to Matt Leahy and his undergraduate research project in the North Fork region in providing some insight into that part of the complex. Hannack, G., and Radwany, M., 2018, HornblendePlagioclase thermometry of the Eau Claire River Complex, western Wisconsin: Proceedings of the Institute on Lake Superior Geology 64th Annual Meeting, Iron Mountain, Michigan, p. 47-48. Hart, T. R., Gibson, H. L., and Lesher, C. M., 2004, Trace element geochemistry and petrogenesis of felsic volcanic rocks associated with volcanogenic massive Cu-Zn-Pb sulfide deposits: Economic Geology, v. 99, p. 1003-1013. References Klier, J. J., 2019, The Marshfield Terrane: Refedinition of origin through zircon geochronology and geochemistry: Unpub. M.S. thesis, Ball State University, 115 p. Brown, B. A., 1988, Bedrock geology of Wisconsin, west-central sheet, Wisconsin Geological and Natural History Survey Map 87–11b. 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. 75-86. LaBerge, G. L., 1996, Volcanogenic massive sulfide deposits of northern Wisconsin: A commemorative volume, Proceedings of the 42nd Annual Meeting of the Institute on Lake Superior Geology, Cable, Wisconsin. Cerny, P. and Ercit, T. S., 2005, The classification of granitic pegmatites revisited. Canadian Mineralogist, v.43, p. 2005-2026. LaBerge, G. L., and Myers, P. E., 1984, Two early Proterozoic successions in central Wisconsin and their tectonic significance: Geological Society of America Bulletin, v. 95, p. 246-253. Daniels, D. L., and Snyder, S. L., 2002, Wisconsin aeromagnetic and gravity maps and data, U.S. Geological Survey Open-File Report 02-493. 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. DeMatties, T. A., 1989, A proposed geologic framework for massive sulfide deposits in the Wisconsin Penokean volcanic belt: Economic Geology, v. 84, p. 946-952. Muller, A., Spratt, J., Thomas, R., Williamson, B.J., and Seltmann, R., 2018, Canadian Mineralogist, v. 56, p. 657-687. DeMatties, T. A., 1994, Early Proterozoic volcanogenic massive sulfide deposits in 70 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Pearce, J. A., 1996, A users guide to basalt discrimination diagrams: Trace Element Geochemistry of Volcanic Rocks: Applications for Massive Sulphide Exploration. Geological Association of Canada, Short Course Notes, v. 12, p. 79-133. pre- and early Proterozoic rocks in Wisconsin: Unpub. Ph.D. thesis, University of Wisconsin Madison, 295 p. Van Wyck, N., and Johnson, C. M., 1997, Common Lead, Sm-Nd, and U-Pb constraints on petrogenesis, crustal architecture, and tectonic setting of the Penokean orogeny (Paleoproterozoic) in Wisconsin: Geological Society of America Bulletin, v. 109, p. 799-808. Pearce, J. A., 2008, Geochemical fingerprinting of oceanic basalts with applications to ophiolite classification and the search for Archean oceanic crust: Lithos, v. 100, p. 1-4. Weber, E. M., and Lodge, R. W. D., 2022, New U/Pb Geochronology from the Proterozoic Penokean Orogen, Wisconsin: Implications for VMS Metallogeny: Society of Economic Geologists Annual Meeting, Denver, CO, paper P5.10. Pearce, J. A., Harris, N. B. W., and Tindle, A. G., 1984, Trace element discrimination diagrams for the tectonic interpretation of granitic rocks: Journal of Petrology, v. 25, p. 956-983. Quigley, A., 2016, Setting of the volcanogenic massive sulfide deposits in the Penokean Volcanic belt, Great Lakes region, USA: Unpub. M.S. thesis, Colorado School of Mines, 95 p. Winchester, J. A., and Floyd, P. A., 1977, Geochemical discrimination of different magma series and their differentiation products using immobile elements: Chemical Geology, v. 20, p. 325-343. Ross, P.-S., and Bédard, J. H., 2009, Magmatic affinity of modern and ancient subalkaline volcanic rocks determined from trace-element discriminant diagrams.: Canadian Journal of Earth Sciences, v. 46, p. 823-839. Winter, J. D., 2010, An Introduction to Igneous and Metamorphic Petrology, Prentice Hall, 697p. 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: Geological Society of America Bulletin, v. 134, p. 776-790. 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. 21452158. Sun, S., and McDonough, W. F., 1989, Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes, in Saunders, A. D., and Norry, M. J., eds., Magmatism in the Ocean Basins, Geological Society Special Publication, v. 42, p. 313-345. Van Schmus, W. R., 1980, Chronology of igneous rocks associated with the Penokean orogeny in Wisconsin: Geological Society of America Special Paper, v. 182, p. 159-168. Van Wyck, N., 1995, Oxygen and carbon isotopic constraints on the development of eclogites, Holsnpy, Norway, and, Major and trace element, common Pb, Sm-Nd, and zircon geochronology constraints on petrogenesis and tectonic setting of 71 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Field Trip 4 – Quaternary Geology and Geomorphology of the Eau Claire Region Douglas J. Faulkner Department of Geography and Anthropology, University of Wisconsin-Eau Claire, Eau Claire, WI 54701 J. Elmo Rawling, III Wisconsin Geological and Natural History Survey, Madison, WI 53705 Phillip H. Larson Earth Science Programs, EARTH Systems Laboratory, Minnesota State University Mankato, Mankato, MN 56001 72 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Introduction Eau Claire lies close to the outermost edge of the former Chippewa Lobe of the Laurentide Ice Sheet as it existed during late Wisconsinan time (MIS-2) (Fig. 1). To the south are older glacial deposits and then the Driftless Area, which apparently was never glaciated. This all-day field trip will concentrate on three aspects of the region’s landscape development from the late Wisconsinan to the late Holocene: glacial, fluvial and aeolian. Glacial Landscapes Northern Wisconsin was glaciated multiple times in the Quaternary. The oldest glacial deposits were derived from the northwest and were likely deposited prior to 780,000 ka. These include the Pierce and Marathon Formations (Rawling et al., in review; Syverson et al., 2011). These deposits are poorly preserved where they occur at the surface (Rawling et al., in review) and although their occurrence is documented in the subsurface (Attig 1985 and 1993; Woodruff et al., 2004), their regional distribution is poorly documented. During the most recent glaciations, ice flowed from the northeast through the Superior Basin until it was thick enough to spill over the regional bedrock divide (Attig and Rawling, 2018). Ice formed during an earlier glaciation deposited glacial and meltwater sediment of the River Falls Formation (Syverson, 2007). River Falls tills and outwash are preserved on uplands in the Eau Claire area, and landforms associated with this advance have been eroded and are not preserved. The best preservation of landforms is associated with the late Wisconsinan ice (ca. 25–11.5 yr B.P.), which formed the Chippewa Lobe that reached its maximum extent at the Chippewa Moraine. This ice was subject to stagnation whenever the ice profile in the Superior Basin lowered, resulting in an ice margin landscape consisting of broad (10s of kilometers) zones of stagnant ice features such as disintegration ridges, ice-walled lakes, and kettles. Figure 1. Top: Map of Wisconsin showing areas covered by lobes of the southern Laurentide Ice Sheet during the late Wisconsinan (MIS-2) Glaciation; inset map shows distribution of ice in the Great Lakes region. The red circle shows the location of Eau Claire near the southern edge of Chippewa Lobe. Bottom: Schematic illustration showing how moraines form over time. Supraglacial sediment accumulates at a stable ice margin (time one). As glacier retreats, buried ice is preserved under supraglacial sediment and minor moraines form if margin temporarily stabilizes (time two). The distribution of moraines after ice has melted (time three; modified from Attig and Rawling, 2018). 73 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Fluvial Landscapes The process of knickzone migration and incision up the LCR was episodic and unexpectedly prolonged. The episodic history of knickzone migration and incision is clearly indicated by the number and spatial distribution of terraces found in the LCR valley below the Wissota. Instead of two terrace levels resulting from the two episodes of abrupt base-level fall, there are as many as seven (Fig. 2). Each of these levels represents a period when the river was migrating laterally and forming a floodplain, followed by an episode of renewed incision that left the floodplain as a terrace. The prolonged history of knickzone migration and incision along the LCR is revealed by the optically stimulated luminescence (OSL) ages of terrace alluvium from several sites in the LCR valley (Fig. 3). These OSL ages indicate that knickzone migration took thousands of years longer than studies of modern alluvial streams affected by minor base-level falls suggest it should’ve taken (Begin 1986, 1988; Begin et al., 1981) The Chippewa River is the second largest stream in Wisconsin, draining a watershed of approximately 25,000 km2 to the upper Mississippi River (UMR). During the Late Wisconsinan, it was the primary stream draining meltwater from the Chippewa Lobe. Overloaded with glacigenic sediment, the lower Chippewa River (or LCR, which refers to the river beyond the Chippewa Moraine) filled its bedrock valley with sandy outwash to depths exceeding 50 meters. Then, sometime between 18-16 ka, the UMR incised 15 m, and at ~13.4 ka, it incised an additional 40 m (Knox 2007; Loope 2012; Gran et al. 2013). Each of these incision episodes abruptly lowered the base level of the LCR, creating knickzones that migrated up the LCR and its tributaries. The incision resulting from knickzone migration created the Wissota terrace, a prominent landform in the LCR valley that marks the maximum height of LCR aggradation during the Late Wisconsinan (Andrews 1965). Figure 2. Terraces of the LCR valley. The names of terraces below the Wissota terrace are based on their height above the modern floodplain and distance below the Wissota. From lowest to highest these are T-1, T-2, T-3, T-4, T-5, and T-6. One terrace that does not fit into the T1-T6 schema is found in the relatively narrow bedrock valley downstream from the Eau Galle-Chippewa River confluence. Named the Maxville Terrace, this terrace slopes from the level of the Wissota at its upvalley end to a level equivalent to T-4 in the UMR valley. (Figure from Faulkner et al. (2016).) 74 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 3. A proposed model of the evolution of the longitudinal profile of the LCR in response to UMR incision labeled with OSL ages of terrace alluvium. The profiles were constructed by connecting scattered terrace remnants, except for the Wissota and Maxville terraces, which are relatively continuous features. (Modified from Faulkner et al. (2016).) Autogenic variations in the amount of sediment supplied to the river likely explain why the migration of knickzones and incision up the LCR was episodic and prolonged. Incision resulting from knickzone migration would’ve created a relatively deep narrow channel with steep, unstable banks. Bank collapse, promoted by lateral stream erosion, would have greatly increased the supply of sediment to the stream. With more sediment to transport, the stream would no longer have had excess power, causing knickpoint migration and incision to slow or cease altogether. This, in turn, would’ve allowed lateral stream migration and floodplain formation to occur. Over time, lateral erosion and bank failure would’ve caused the banks to move away from the stream and become less steep, leading to a reduction in the amount of sediment supplied from them to the LCR. With a declining sediment supply, the stream would’ve once again had excess power, resulting in renewed knickzone migration and incision, at least until the process repeated itself farther upstream. The supply of sediment to the LCR from its tributaries was also subject to autogenic variations. LCR incision resulting would have lowered base level for its tributary streams, creating knickzones that then migrated up them. The subsequent tributary incision would have caused a dramatic increase in the supply of sediment to the LCR. Sediment supplied from tributaries would have remained high until their incised channel banks began to stabilize. But until that happened, high amounts of tributary sediment would have affected knickpoint migration and incision on the LCR, slowing it down and possibly causing it to stop. It is likely that autogenic variations in sediment from the largest tributaries, the Red Cedar River and the Eau Claire River, had the biggest impact on the LCR. Aeolian Landscapes There is abundant evidence throughout the Eau Claire region that wind has been a significant geomorphic agent during the late Quaternary. The most widespread evidence of aeolian activity is provided by deposits of loess, which were mainly sourced from the outwash plains of meltwater streams, including that of the Chippewa River (Schaetzl et al., 2014; Schaetzl et al., 2018; Fig. 4). Loess deposition during the Late Wisconsinan in the Eau Claire region began no later than 24 ka and continued until as recently as 10 ka (Schaetzl et al., 2014). Over this interval, the dominant processes of loess transport and deposition 75 �Proceedings of the 69th ILSG Annual Meeting - Part 2 2014). Later, strong northwest winds entrained sands from outwash and weathered sandstone. As they traveled over existing loess, saltating sands remobilized it and kept it in suspension until topographic barriers blocked further sand movement, which allowed loess to accumulate in their lee. Today, large swaths of the region are loess-free, with the thickest loess found on the southeast sides of prominent sandstone inselbergs and ridges (Schaetzl et al., 2018). A variety of sandy aeolian landforms, such as parabolic dunes, sand sheets, sand ramps, and sand stringers, also attest to the geomorphic significance of wind in the Eau Claire region. While these landforms are generally subtle and apparent only on LiDAR-derived DEMs, they are widespread (Fig. 5). They also have a generally consistent orientation, which indicates that they were primarily formed by west-northwesterly winds. In addition, a dozen OSL ages from different landforms reveal that sandy aeolian landforms in the region were being deposited between 13 and 9 ka (Schaetzl et al., 2018; Millett, 2019; Mataitis, 2020; Shandonay et al., 2022). Figure 4: Extent and thickness of loess within in western Wisconsin, as derived from Natural Resources Conservation Service county soil surveys (from Schaetzl et al., 2018.) apparently changed (Schaetzl et al., 2018). Existing evidence indicates that, early on, loess was primarily deflated from the outwash plains of the Chippewa River and its meltwater tributaries and deposited downwind of them (Schaetzl et al., Figure 5. Parabolic dunes (Millett, 2019) and sand stringers (Schaetzl et al., 2018; Mataitis, 2020) identified from lidar-derived DEMs, aerial photographs, and soil survey data in and near the LCR valley. 76 �Proceedings of the 69th ILSG Annual Meeting - Part 2 buried and melted. The dominant landforms in the area are ice-walled lake plains (Figs. 6 and 7; Clayton et al., 2001), such as the one the Obey Center is built upon. These form when lakes develop in the stagnant ice landscape preserved in permafrost conditions. It is likely that permafrost conditions were in northern Wisconsin until ~13.5 ka (Attig and Rawling, 2018; Batchelor, 2019). They are composed of laminated finegrained sediment that is typically fine sand and silt. These landscapes contain organic material further south that have aided in interpreting the timing of the Lake Michigan Lobe (Curry et al., 2018); however, organic material is typically not preserved in northern Wisconsin. Field Trip Stops UTM coordinates are in zone 15, WGS84 datum Stop 1: Copper Falls Glacial Deposits UTM coordinates 623464E, 5008658N Till of the Copper Falls Formation is reddish brown, sandy (~30 – 80% sand; Syverson, 2007; Syverson et al, 2011), and sourced from the northeast. Copper Falls till is distinguished from older River Falls till primarily by the landscape they underlie. Copper Falls sediment is found in relatively unmodified landscapes formed during the late Wisconsinan. Glacial landforms (moraines, eskers, drumlins, ice-walled lake plains…) are well persevered. River Falls sediment is associated with a highly eroded landscapes and likely formed before the late Wisconsinan Glaciation. This roadcut exposes typical unsorted glacial deposits of the Copper Falls Formation deposited as stagnant ice melted. Stop 2. David R. Obey Ice Age Interpretive Center UTM coordinates 624514E, 5009009N The interpretive center is located in the Chippewa Moraine State Recreation Area and within the Chippewa Moraine. The moraine formed when late Wisconsin ice was at its maximum position (Syverson, 2007). The landscape here is generally described as hummocky, and formed as stagnant ice was Figure 6. Schematic illustration showing the formation of ice walled lake plains (from Clayton et al., 2001). (A) Supraglacial sediment forms at the surface of stagnant ice and lakes occupies low areas where sorted sediment accumulates. (B) Hummocky topography with ice-walled lake plains remain after the ice has melted. Figure 7. Lidar-derived DEM showing the hummocky topography of the Chippewa Moraine near Stops 1 and 2. Ice walled lake plains are abundant within the moraine, which contrasts with the flat landscape formed by meltwater streams (outwash). 77 �Proceedings of the 69th ILSG Annual Meeting - Part 2 suggest that it happened during the late Holocene (Fig. 3). Stop 3. Colluvium Exposure UTM coordinates 625123E, 5001761 N The falls consist of a series of small knickpoints formed in early Proterozoic bedrock consisting of banded amphibolites with granitic intrusions (Myers and Maercklein, 1978). The angular form of the knickpoints, along with angular boulders of the same lithology scattered along the channel bed, suggest that the river is incising here primarily by mean of hydraulic plucking. Hydraulic plucking occurs when flows are deep and fast, leading to a zone of flow separation and low pressure on the downstream face of knickpoints. If the bedrock of a knickpoint is sufficiently jointed and weathered, the resulting drag force will pull blocks away from the knickpoint face. Polished rock surfaces with rare grooves and potholes indicate that abrasion by bedload sediment is also playing a role here in channel incision, although its effects appear secondary to that of plucking. The landscape beyond the LIS margin was greatly affected by periglacial processes. One of the most profound effects was the stripping of hillslopes by the mass-wasting process of solifluction (Clayton et al., 2001). Evidence of solifluction is provided by relict deposits of colluvium (colluvial aprons) that mantle bedrock slopes in areas of former permafrost. Here we see a prime example of such a colluvial deposit, which consists of an unsorted mixture of sandstone clasts (pebble to boulder in size) supported in a matrix of silty sand. The sand and the sandstone clasts were likely derived from the underlying bedrock (sandstone of the Cambrian Eau Claire Formation) by intense freeze-thaw weathering during the Late Wisconsinan. The silty material probably is loess that winds deflated and transported to the sight from nearby outwash plains. While the Chippewa has clearly incised into the Jim Falls bedrock, incision overall has been minimal. The lack of incision is likely due, in part, to the weathering and erosion-resistant nature of the bedrock. An additional factor is the relatively short amount of time that the river has been incising at this location. Given the model of long-profile evolution in Figure 3, incision didn’t start here until sometime after 4.7 ka. Stop 4. Jim Falls UTM coordinates 635875E, 4990538 N) The Chippewa at Jim Falls is an example of a superimposed river (Fig. 8). Here, the Chippewa River incised through a cover of outwash and till and encountered a topographic high in the buried a bedrock landscape. It did not, however, incise to its present level in one episode of downcutting, as evidenced by two terraces that are apparent at and near this site. The highest terrace grades to the Wissota terrace, the maximum level of aggradation of the lower Chippewa River. This indicates that the river continued to flow at this level after the glacial margin had retreated to the north of this site. Remnants of a terrace approximately 3 meters below the Wissota (best seen downstream from the east end of the pedestrian bridge), which is cut into till, indicate a period of channel stability before the river incised further to the buried bedrock. When deeper incision occurred is unknown, although OSL ages of terrace alluvium farther down valley The site today clearly is highly modified by a dam, appropriately named the Jim Falls Dam. The original Jim Falls Dam was built in 1923 to utilize the hydraulic head provided by the falls to generate electricity. It was designed so that the falls were bypassed and left dry except during high flows, when excess water was released through a spillway that was located at the falls upstream end. The dam was redeveloped in 1988 and now has the highest generation capacity of any hydropower dam in Wisconsin (~60 MW). This redevelopment included moving the main spillway from the head of the falls to a location adjacent to a new main powerhouse. It also included constructing a smaller spillway and 78 �Proceedings of the 69th ILSG Annual Meeting - Part 2 auxiliary powerhouse where the main spillway had been. This was done so that a minimum flow of 240 cfs could be released down the bypassed reach year-round, except for the period April 1- May 31, when flow through the reach is increased to 850 cfs to enhance fish spawning habitat. Figure 8. Lidar-derived DEM of Jim Falls and surrounding area. Stop 5. Wildenberg Quarry Evidence for permafrost during the Late Wisconsinan is widespread in Wisconsin, with a hypothesized permafrost interval in central Wisconsin from ca. 33 to 15 ka (Batchelor et al., 2019) and as late as 13.5 ka in northern Wisconsin (Attig and Rawling, 2018). This interval is, however, poorly constrained in the Eau Claire region due to a lack of 14C datable materials in features diagnostic of permafrost. OSL dating now makes it possible to date proxy geomorphic features, like ice-wedge pseudomorphs, to help constrain this (Schaetzl et al., 2021). UTM coordinates 616648 E, 4970372 N Note: This is private property. No access is allowed without owner’s permission. Glacial sediment of the River Falls Formation includes till that is lithologically like the Copper Falls Formation and melt-water stream sediment (Syverson, 2007; Severson et al., 2011). These can be distinguished from the Copper Falls formation because they occur at the top of highly eroded landscapes and the soils in them are more developed. There are no primary glacial landforms associated with the River Falls Formation, likely due to intense modification by periglacial processes in permafrost conditions during the late Wisconsinan. The ice-wedge pseudomorphs in this quarry are sand wedges (Fig. 9). Sand wedges such as these form in periglacial settings when thermal contraction of frozen ground in winter forms 79 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 9. Sand wedges exposed in the headwall of the Wildenberg quarry in July 2018. (Photograph by Randy Schaetzl.) cracks in the soil. If this happens in a cold, dry, wind-swept environment with sand available for transport, sand will blow into and fill the cracks. Over time, repeated cracking and filling will form vertical structures that generally taper with depth. OSL dating of sand wedges in this quarry and in another quarry located 60 km to the south indicate that thermal contraction cracks existed and were filling with sand from no later than 19.3 ka until 14.7 ka. Schaetzl et al. (2021) interpret these ages as documenting when permafrost in the region most likely ended. Interestingly, the OSL ages from this quarry (15.1 and 14.7 ka) are younger than those from the quarry 60 km to the south (19.3, 19.1, and 18.3 ka). These may represent a time-transgressive spatial relationship in that permafrost possibly degraded earlier at the more southerly location and remained longer at the more northerly one, although this is purely speculative given the large errors on the OSL ages (1.4 to 2.2 ka at 1). That said, the larger and more complex morphologies of the sand-wedges found at this quarry do suggest the possibility of more intense sand-wedge development due to more prolonged permafrost conditions. City incorporated it into its bicycle-pedestrian trail system. In addition to being historically significant, the High Bridge (Fig. 10) affords an excellent view of many of the terrace levels found in the LCR valley. The High Bridge itself is at the level of T6. To the east, trees and houses can be seen at the top of the Wissota terrace scarp, which is 6-7 meters above T-6. The Wissota can also be seen to the west where the pedestrian-bicycle trail cuts into its scarp. Looking downstream, lower terraces are difficult to discern from this vantage point, although the residential and business districts located near the river provide clues. These built-up areas are all above the 100-year flood level. That is, they all are on terraces. In Eau Claire, there is little active floodplain. This suggests that incision below the lowest terrace level occurred here recently (within the last 2.3 ka according to the model of long-profile evolution in Fig. 3). Upstream from the High Bridge is the Dells Dam. This dam, which was built in 1924 for the purpose of generating electricity, is situated at an unusually good site for a dam on the LCR – a bedrock gorge. This gorge was formed when the river incised into a cover of glacial outwash that buried a low ridge of Cambrian sandstone (Mt. Simon Formation) connecting Mt. Simon (the conical bedrock hill located approximately 500 m northeast of the dam) to the bedrock uplands located west-northwest of it. In other words, the river here did not incise into its pre-glacial bedrock valley, which is located east of Mt. Stop 6. High Bridge UTM coordinates 617832E, 4964561N This stop is on the so-called High Bridge in the city of Eau Claire. Standing 26 m above the Chippewa River, the High Bridge was built in 1880 as a railroad bridge and was an innovative bridge for its time. It was abandoned in 1992, and the City acquired ownership in 2007. In 2015, the 80 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Simon and the High Bridge. This is an example of an epigenetic gorge (Ouimet et al., 2008), and the river likely carved it sometime after 7.4 ka (see Fig. 3). Incision is actually still occurring here, as evidenced by a 1.5-m bedrock knickpoint located 180 m downstream of the bridge. Figure 10. Lidar-derived DEM showing the stream terraces found in Eau Claire in the vicinity of the High Bridge. upvalley and on the valley’s other side. The last episode of incision, below T-1, occurred within the last 2.3 ka based on an OSL date of T-1 fill from a site also located 6.5 km upvalley (Fig. 3). In the downstream direction, the valley below the Wissota is predominantly floodplain with only rare lower terrace remnants. In addition, the river’s planform switches from a single-thread meandering shape to a multi-thread anabranching one that extends downvalley for a distance of 8.5 km. At that point, it returns to a single-channel meandering form. It is uncertain why this anabranching reach exists, although its similarity to the sedimentation zones of wandering gravel bed streams in British Columbia described by Desloges and Church (1987) suggests a cause. Given its location immediately downstream of Stop 7. Sand Hill Cemetery UTM coordinates 599549E, 4958254N This stop is at the edge of the Wissota terrace tread and top of the Wissota terrace scarp. To the north, a braided channel is apparent in the subtle rolling topography of the Wissota tread, providing evidence of the Chippewa’s glacial past (Fig. 11). To the south, the Wissota scarp descends nearly 30 m to the lowest terrace in the valley (T-1). Looking upvalley, this terrace can be identified by the farmland situated on it. Lowlying land that is wooded is either floodplain or paleochannels cutting across the T-1 surface. Incision here below the Wissota level occurred between 10 and 9 ka, based on four OSL dates of Wissota fill obtained from sites located 6.5 km 81 �Proceedings of the 69th ILSG Annual Meeting - Part 2 the reach that was recently incised below T-1, it could be the result of sedimentation resulting from that incision event (Fig. 12). A pronounced convexity in the long profile of the modern river along the anabranching reach supports this hypothesis (Faulkner et al., 2016). Figure 11. Lidar-derived DEM showing the fluvial and aeolian landforms in the vicinity of the Sand Hill Cemetery (Stop 7), which are discussed in the text. Figure 12. Cartoon showing the setting of the anabranching reach (and hypothesized sedimentation zone) downstream of the reach that is incised below the T-1 terrace (adapted from Adams et al., 2016). 82 �Proceedings of the 69th ILSG Annual Meeting - Part 2 A short distance (~200 m) west of the Sand Hill Cemetery, an elongate wooded hill—informally named the Steffes-Zanoni site—can be seen rising above the low-relief landscape of the Wissota terrace. (This hill can also be seen in Fig. 11.) Its summit is approximately 8 m higher than the surrounding Wissota terrace surface, and its elongate form runs parallels to the Wissota terrace scarp. The landform is composed of sand that is mineralogically indistinguishable from the terrace sediments beneath it, although it is relatively finer-grained and more well-sorted, indicating that it is aeolian sand (Millett, 2019). It is also morphologically complex, consisting of multiple parabolic forms coalesced together with smaller parabolic forms on top of them. These forms--smaller parabolic dunes superimposed on larger older ones—indicate repeated aeolian activity at this site. The OSL ages of samples obtained from depths of 1.7 m and 2.5 m near the hill’s northwest end suggest a depositional age of the landform’s upper part to be ~0.5 ka. Larson et al. (2008) identified a dune similar to this one in the city of Eau Claire and called it a cliff-top parabolic dune (because of its form and its proximal relationship to the Wissota terrace scarp). Since then, many cliff-top parabolic dunes have been noted in the Eau Claire region (note the large number of parabolic dunes situated along the Wissota scarp in Fig. 5). Why these dunes exist will be discussed at our final stop. with parabolic forms oriented perpendicular to them—are hypothesized to have had a similar genetic origin (Larson et al., 2008; Millett, 2019). In their proposed model, a river cuts into the base of a high fill-terrace scarp. This creates an unstable cutbank and promotes mass wasting that removes vegetation from the scarp face. Wind that then blows against such a scarp gets compressed, which causes its velocity to go up. The increase in velocity enhances the wind’s ability to entrain exposed sandy sediment and transport it up the scarp face. When this happens, the sandy sediment ultimately settles out at the top of the scarp as wind velocity is reduced there. This leaves behind “cliff-top dunes.” (Fig. 14). Given this model of dune genesis, one should expect cliff-top dunes to have different orientations and depositional ages compared to other parabolic dunes not in cliff-top positions. This is indeed the case. Non-cliff-top dunes generally have a northwest-southeast orientation and depositional ages older than 10 ka. Cliff-top dunes display a variety of orientations (perpendicular to their scarps) and are generally much younger. OSL ages from a cliff-top dune in the city of Eau Claire indicate a period of aeolian deposition at ca. 6.0 ka, while two from the Steffes-Zanoni site (Stop 7) indicate that deposition in the uppermost dune sediments occurred at ca. 0.5 ka. At the Kiwanis site, eight OSL ages point to two depositional episodes: at ca. 0.9 ka and 0.5 ka. Stop 8. Town of Union Conservancy The model of Larson et al. (2008) of cliff-top dune genesis suggests that these should be forming wherever the Chippewa River is eroding laterally into Wissota terrace fill. This, however, is not the case; today, all cliff-top dunes in the LCR valley are stabilized by vegetation and no longer moving. Thus, the genesis of these landforms is doubtless more complex than the model of Larson et al. (2008) suggests, with climatic variability likely playing a key role in the process of aeolian sedimentation and dune formation at cliff-top locations (Millett, 2019). For example, during humid climate intervals UTM coordinates 607504E, 4959523N Note: This site involves walking on unpaved trails for an approximate distance of one mile. There are some short sections of the trail that are moderately steep. Parabolic dunes in cliff-top positions are especially prominent at this location, which is informally called the Kiwanis Site (Fig. 13). Like those seen at the previous stop, these dunes are situated atop the Wissota terrace scarp. These dunes and others that are similarly situated in the LCR valley—above high Wissota terrace scarps 83 �Proceedings of the 69th ILSG Annual Meeting - Part 2 (such as at the present), high cutbanks carved by the river into Wissota fill are colonized readily by vegetation, which inhibits the entrainment and transport of sand up terrace scarps. In contrast, during arid intervals, vegetation cover is greatly reduced, especially on steep well-drained terrace scarps, allowing wind to entrain and transport sand up them. OSL ages from the Kiwanis Site and the Steffes-Zanoni Site support the significance of climate variability in the formation of cliff-top dune. At Kiwanis, these ages indicate two pulses of aeolian deposition – the first at ca. 0.9 ka and a second at ca. 0.5 ka, with the latter coinciding with ages from the Steffes-Zanoni Site. If correct, these pulses happened during the Medieval Climatic Anomaly when well documented dry periods affected the mid-continent of North America (reviewed in Millett, 2019). forms enclosed by a subtle linear ridge appear to be anthropogenic features. It is likely, based onthe morphology and distribution of these features, that their creation was tied to the genesis of the prominent aeolian dunes found nearby and, potentially, culturally linked to a wellestablished late Woodland period of mound building in the upper Mississippi River valley – a period which would have coincided with the formation of the Kiwanis and Steffes-Zanoni dunes. Based on the configuration of these features, it is hypothesized that this site represents the “Thunderer,” an effigy of a bird-like deity, or sky being. The spotted Thunderer, with “spots” represented by the location of the mounds within the linear structure, is associated with the West and the bringer of storms. If true, Native peoples may have watched the dunes form during a period of more aridity during episodes of higher winds/storms, resulting in this site being spiritually significant at that time and now an important site of cultural heritage (R. Schirmer, personal communication). Lastly, closer examination of the Kiwanis Site (Fig. 13) reveals landforms in close proximity to the dunes that do not look to be of natural origin. Several hemispherical (or conical) mound-like Figure 13. The Kiwanis Site. Cliff-top parabolic dunes are situated directly above the Chippewa River on top of the Wissota terrace scarp. Also note the hemispherical mounds and linear ridge of hypothesized anthropogenic origin. 84 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Figure 14. Model of cliff-top dune genesis induced by lateral river erosion (adapted from Larson et al., 2008). Begin, Z.B. 1986. Determination of “diffusion” erosion coefficients for some tributaries of Oaklimiter Creek, North-Central Mississippi, in Hadley, R.F., ed., Drainage Basin Sediment Delivery. IAHS Publication, p. 447–462. References Adams, H.R., Vincent, A.M., and Faulkner, D.J. 2016. Patterns of downstream fining on the lower Chippewa River, Wisconsin: Abstracts, Annual Meeting - American Association of Geographers, San Francisco, CA Begin, Z.B. 1988. Application of a diffusion-erosion model to alluvial channels which degrade due to base-level lowering: Earth Surface Processes and Landforms, v. 13, p. 487–500. Andrews, G.W. 1965. Late Quaternary geologic history of the Lower Chippewa Valley, Wisconsin. Geological Society of America Bulletin: v. 76, p. 113–124. Begin, Z.B., Meyer, D.F., and Schumm, S.A. 1981. Development of longitudinal profiles of alluvial channels in response to base-level lowering: Earth Surface Processes and Landforms, v. 6, p. 49–68. Attig, J.W. 1985. Pleistocene Geology of Vilas County, Wisconsin: Wisconsin Geological and Natural History Survey, Information Circular 51, 32 p. Clayton, L., Attig, J.W., and Mickelson, D.M. 2001. Effects of late Pleistocene permafrost on the landscape of Wisconsin: Boreas, v. 30, p. 173–188. Attig, J.W. 1993. Pleistocene Geology of Taylor County, Wisconsin: Wisconsin Geological and natural History Survey, Bulletin 90, 25 p. Curry, B.B., Lowell, T.V., Wang, H., and Anderson, A.C. 2018. Revised time-distance diagram for the Lake Michigan Lobe, Michigan Subepisode, Wisconsin Episode, Illinois, USA, in Kehew, A., and Curry, B.B., eds., Quaternary Glaciation of the Great Lakes Region: Process, Landforms, Sediments, and Chronology: Geological Society of America Special Paper 530, p. 1–12. Attig, J.W., and Rawling, J.E., III. 2018. Influence of persistent buried ice on late glacial landscape development in part of Wisconsin’s Northern Highlands, in Kehew, A., and Curry, B.B., eds., Quaternary Glaciation of the Great Lakes Region: Process, Landforms, Sediments, and Chronology: Geological Society of America Special Paper 530, p. 1–12. Desloges, J.R., and Church, M. 1987. Channel and floodplain facies in a wandering gravel-bed river, in Ethridge, F.G., Flores, R.M., Harvey, M.D., Weaver, J.N., eds., Recent Developments in Fluvial Sedimentology: Special Publication. Society of Economic Paleontologists and Mineralogists, p. 99–109. Batchelor, C.J., Orland, I.J., Marcott, S.A., Slaughter, R., Edwards, R.L., Zhang, P., Li, X., and Cheng, H. 2019. Distinct permafrost conditions across the last two glacial periods in midlatitude North America: Geophysical Research Letters, v. 46, no. 22, p. 13318-13326. 85 �Proceedings of the 69th ILSG Annual Meeting - Part 2 Faulkner, D.J., Larson, P.H., Jol, H.M., Running, G.L., Loope, H.M., and Goble, R.J. 2016. Autogenic incision and terrace formation resulting from abrupt late-glacial base-level fall, lower Chippewa River, Wisconsin, USA: Geomorphology, v. 266, p. 75–95. Schaetzl, R.J., Forman, S.L., and Attig, J.W. 2014. Optical ages on loess derived from outwash surfaces constrain the advance of the Laurentide Ice Sheet out of the Lake Superior Basin, USA: Quaternary Research, v. 81, p. 318–329. Schaetzl, R.J., Larson, P.H., Faulkner, D.J., Running, G.L., Jol, H.M., and Rittenour, T.M. 2018. Eolian sand and loess deposits indicate west-northwest paleowinds during the Late Pleistocene in western Wisconsin, USA: Quaternary Research, v. 89, p. 769–785. Gran, K.B., Finnegan, N., Johnson, A.L., Belmont, P., Wittkop, C., and Rittenour, T. 2013. Landscape evolution, valley excavation, and terrace development following abrupt postglacial baselevel fall: Geological Society of America Bulletin, v. 125, p. 1851–1864. Schaetzl, R.J., Running, G.L., Larson, P., Rittenour, T., Yansa, C., and Faulkner, D. 2022. Luminescence dating of sand wedges constrains the Late Wisconsin (MIS 2) permafrost interval in the upper Midwest, USA: Boreas, v. 51, p. 385– 401. Knox, J.C., 2007. The Mississippi River System, in Gupta, A., ed., Large Rivers: Geomorphology and Management. John Wiley & Sons, Chichester, England ; Hoboken, NJ, p. 145–182. Larson, P.H. McDonald, J., Baker, A., Dryer, W.P., Running, G.L., Faulkner, D.J. and Jol, H.M. 2008. Geomorphology of cliff-top parabolic dunes within the lower Chippewa River valley, upper Putnam Park, Eau Claire, Wisconsin: Abstracts, Annual Meeting - Association of American Geographers, Boston, MA. Schirmer, R. 2023. Personal communication regarding archeology in the upper Mississippi valley and the Kiwanis Site. 3/8/2023. Shandonay, K.L., Bowen, M.W., Larson, P.H., Running, G.L., Rittenour, T., and Mataitis, R. 2022. Morphology and stratigraphy of aeolian sand stringers in southeast Minnesota and western Wisconsin, USA: Earth Surface Processes and Landforms, v. 47, p. 2863–2876. Loope, H.M., Mason, J.A., Knox, J.C., Goble, R.J., Hanson, P.R., Young, A.R., and Curry, B.B. 2012. Late Wisconsinan aggradation and incision history of the upper Mississippi River, USA: Abstracts with Programs - Geological Society of America, v. 44, p. 455. Syverson, K.M. 2007. Pleistocene Geology of Chippewa County, Wisconsin: Wisconsin Geological and natural History Survey, Bulletin 103, 53 p. Mataitis, R. 2020. “Geomorphology and chronology of sand stringer deposition beyond the ice margin: Southeastern Minnesota and western Wisconsin, USA.” M.S. Thesis. Minnesota State University, Mankato. Syverson, K.M., Clayton, L., Attig, J.W., and Mickelson, D.M. 2011. Lexicon of Pleistocene Stratigraphic Units of Wisconsin: Wisconsin Geological and Natural History Survey Technical Report 1, 180 p. Millett, J. 2019. “Cliff-top dunes in the lower Chippewa River valley of west-central Wisconsin.” M.S. Thesis. Minnesota State University, Mankato. Woodruff, L.G., Attig, J.W., and Cannon, W.F. 2004. Geochemistry of glacial sediments in the area of the Bend massive sulfide deposit, north-central Wisconsin: Journal of Geochemical Exploration, v. 82, p. 97–109. Myers, P.E., and Maercklein, D.A. 1978. Amphibolites and Granites at Jim Falls: Wisconsin Geology of Wisconsin – Outcrop Descriptions, Geological and Natural History Survey, 7 p. Rawling III, J.E., Carson, E.C., Attig, J.W., Mickelson, D.M., Mode, W.N., Johnson, M.D., Syverson, KM. (in preparation). The Quaternary Geology of Wisconsin. Wisconsin Geological and Natural History Survey. Map Sale 1:500,000. 86 � 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, 2023 Description An account of the resource Institute on Lake Superior Geology. Eau Claire, Wisconsin. April 24-25, 2023. 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 2023-04-24 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/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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Nicholson, S.W., Shirey, S.B., Schulz, K.J., and Green, J.C., 1997, Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development: Canadian Journal of Earth Sciences, v. 34, p. 504-520. Ohr, M., 1993, Geochronology of diagenesis and low-grade metamorphism in pelites: Ph.D. dissertation, The University of Michigan, Ann Arbor, MI.,161 p. Ojakangas, R.W., Morey, G.B., Green, J.C., 2001. The Mesoproterozoic midcontinent rift system, Lake Superior region, USA. Sedimentary Geology, v. 141, p. 421–442. Paces, J.B., 1988, Magmatic processes, evolution and mantle source characteristics contributing to the petrogenesis of Midcontinent rift basalts: Portage Lake Volcanics, Keweenaw Peninsula, Michigan: Ph.D. Dissertation, Michigan Technological University, Houghton, 413p. 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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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. 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"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/749c5c6ee43eb071b1c6eea81168ff01.pdf 7cea60901a11884b9ca5c94d5c9eda94 PDF Text Text Institute on Lake Superior Geology Special Publication #1 Field Trip Guidebook for the Slate Islands, Ontario Pete Hollings, Mark Smyk, Bill Addison & Phil Fralick �Institute on Lake Superior Geology Special Publication #1 Field Trip Guidebook for the Slate Islands, Ontario Pete Hollings Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Mark Smyk Resident Geologist’s Office, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Bill Addison R.R. 2, Kakabeka Falls, Ontario, P0T 1W0, Canada Phil Fralick Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Cover Photos: Top - Shatter cone clast in impact breccia; Middle - Interflow sandstone unit in Paleoproterozoic basalts; Bottom - West coast of Patterson Island. �Institute on Lake Superior Geology Special Publication #1 Field Trip Guidebook for the Slate Islands, Ontario Reference to material in this volume should follow the example below: Hollings, P., Smyk, M., Addison, B. and Fralick, P., 2006. Field trip guidebook for the Slate Islands. Institute on Lake Superior Geology, Special Publication 1, p. 21. Published by the Institute on Lake Superior Geology and distributed by the ILSG Secretary: Pete Hollings - ILSG Secretary Department of Geology Lakehead University 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada Email: peter.hollings@lakeheadu.ca ILSG website: www.lakesuperiorgeology.org ISSN 1042-9964 �ILSG Special Publication #1 - The Slate Islands Table of Contents Introduction..........................................................................................................................1 Safety Considerations..........................................................................................................1 Acknowledgements..............................................................................................................1 Regional Geology................................................................................................................2 One Archipelago, Two Possible Origins..............................................................................5 The Case for an Extraterrestrial Impact Origin for the Slate Islands Structure.........5 The Case for a Cryptoexplosion Origin for the Slate Islands Structure...................10 The Debate...............................................................................................................11 Economic Geology.............................................................................................................12 Stops...................................................................................................................................13 Stop A – “Honeymoon Bay” near Cove Island........................................................13 Stop B – Sunday Harbour.........................................................................................14 Stop C – Horace Cove..............................................................................................15 Stop D – Western shore of Patterson Island.............................................................16 Stop E – McGreevy Harbour....................................................................................18 References..........................................................................................................................20 -i- �ILSG Special Publication #1 - The Slate Islands Introduction Safety Considerations This volume is intended to serve not only as a guide for participants during the August 2006 field trip to the Slate Islands, but also as a reference for those planning to revisit the area at a later date. Consequently we have included UTM coordinates (NAD 83 datum) for stops. The Slate Islands are covered by southern boreal forest with some shoreline arctic-alpine disjunct flora and is protected as a Natural Environment Provincial Park with no visitor facilities. Rock collecting and sampling is prohibited throughout the entire archipelago unless a permit is first obtained from the Ministry of Natural Resources: Slate Islands Provincial Park Ministry of Natural Resources P.O. Box 970 Nipigon, ON P0T 2J0 A field trip to the Slate Islands creates a number of unique safety issues. Please exercise caution when getting in and out of the boats, as the outcrops are often sharp and extremely slippery. Personal flotation devices should be worn in the boats at all times. If you are planning to revisit these sites please be very careful. Lake Superior is a cold, dangerous lake; waves can often be metres high and even in mid-summer fog can appear very quickly. A GPS system, compass and maps should be utilized. We strongly encourage you to charter a large boat from the mainland rather than trying to make the trip to the islands yourself. Phone: (807) 825-3403 This is the first time a publication has been produced for a field trip that is not directly associated with an ILSG Annual Meeting. However, the location of the Slate Islands dictates that field trips to the islands are best made later in the summer when weather and lake conditions are more conducive to travel. Acknowledgements We would like to thank all those who provided comments on this guide and assisted with the running of the field trips, particularly Doug Caldwell and John Scott. Woodland Caribou on the Slate Islands -1- �ILSG Special Publication #1 - The Slate Islands Regional Geology The Slate Islands comprise a 7 km-wide archipelago of 17 islands located in northern Lake Superior approximately 12 km southeast of Terrace Bay (Fig. 1). The geology of the islands has been mapped by Coleman (1901), Parsons (1918) and by Sage (1975, 1991). The islands comprise both Archean and Proterozoic rocks. The Archean rocks are part of the Schreiber-Hemlo greenstone belt (Wawa Subprovince). Paleoproterozoic sequences include the Gunflint and Rove Formations of the Animikie Group. Mesoproterozoic Keweenawan basalts are interpreted to be an extension of the Osler Group of the Midcontinent Rift (Sage, 1991). Sage (1978, 1991) mapped greenschist facies Archean metavolcanic rocks and subvolcanic intrusive rocks ranging in composition from calc-alkaline dacite to tholeiitic basalt. The Archean supracrustal rocks consist of coarse felsic pyroclastic units, felsic to mafic tuffs, feldspar-phyric flows and amygdaloidal, pillowed and variolitic mafic flows with thin interbeds of argillite and siltstone (Fig. 2; Sage, 1991). On the basis of pillow facing directions Sage proposed that an antlicinal structure crosses the centre of Mortimer Island. The pillowed flows are most common on Mortimer and Delaute islands (Fig. 2); pillows are typically bun- to mattress-shaped and up to 2m across (Sage, 1991). In places on Mortimer Island the massive and pillowed basalts grade into flow breccias. Volcanic and intrusive rocks of more felsic compositions are found on Patterson, Dupuis, Spar and Leadman islands (Fig. 2) and have been interpreted by Sage (1991) to be highly sheared, amygdaloidal and porphyritic carbonatised sequences. Archean metasedimentary rocks are relatively rare and are predominantly volcaniclastic as they appear to interfinger with the volcanic flows (Sage, 1991). The Archean mafic volcanic rocks from the Slate Islands are can be subdivided into two distinct geochemical suites. One suite is characterized by flat primitive mantle-normalized patterns typical of tholeiitic rocks found in modern oceanic plateaus whereas the second, more abundant, suite is characterized by weakly LREE-enriched patterns with minor negative Nb anomalies, characteristic of rocks formed in an island arc setting (Fig. 3; P. Hollings, unpublished data). Similar assemblages have been reported in the Schreiber-Hemlo greenstone belt (Polat et al., 1998). Figure 1. Map showing the location of the Slate Islands. -2- �Figure 2. Geological map of the Slate Islands. Modified after Sage (1991). -3- 1 km Patterson Island Mortimer Island D Horace Cove C Edmonds Island McColl Island 87°00’ Sunday Harbour B McGreevy E Harbour Bowes Island 87°00’ A1 Dupuis Island Delaute Island A2 Cove Island Field trip stop 84°40’ Mafic volcanic rocks Felsic volcanic rocks Metasedimentary rocks Mafic intrusive rocks Felsic intrusive rocks Archean Animikie group Osler group Diabase dikes Breccias Post-Archean N ILSG Special Publication #1 - The Slate Islands 84°40’ �ILSG Special Publication #1 - The Slate Islands Figure 3. Primitive mantle normalised diagram showing representative samples of the two geochemical suites recognised amongst the Archean mafic volcanic rocks of the Slate Islands. ST33 = arc-type, ST38 = plateau-type. Normalising values from Sun and McDonough (1989). On the western shore of Patterson Island, Sage (1991) reported an ~20m thickness of iron formation of the Gunflint Formation lying unconformably on the Archean basement and below Mesoproterozoic Keweenawan basalts (Fig. 2). The lowermost three metres of the sequence consists of interbedded jaspilitic chert, hematite and carbonate overlain by a sequence of hematitic chert. The argillites are generally massive and only locally display well-developed bedding. Recent re-examination of outcrops on eastern Mortimer Island and Delaute Island mapped as Paleoproterozoic Rove Formation clastic sedimentary rocks, has resulted in them being reinterpreted as Archean metasedimentary rocks analogous to the McKellar Harbour turbidite sequence on the mainland and this is now reflected on Figure 2. Keweenawan basalts unconformably overlie the Gunflint rocks (Fig. 2) and form an ~120m thick flow sequence that dips ~80° at its base and diminishing to ~25° towards its top (Sage, 1991). This implies some degree of block rotation. Twenty-two individual flows can be recognized within the upper portion of the sequence. Interflow contacts are typically sharp and often marked by thin interflow sedimentary units (Sage, 1991). The basalts are typically vesicular and amygdaloidal, and in places show poorly developed ropy flow tops. The feldspar and pyroxene-phyric basalt flows are incipiently to completely altered, to sericite, carbonate and calcite (P. Hollings, unpublished data). However, even the relatively unaltered samples are significantly more altered than Osler basalts in the vicinity of Rossport (Hollings et al., 2006). Red, medium-grained, well-sorted, arkosic sandstone interflow units consist of sub-rounded to sub-angular grains of predominantly quartz, plagioclase, Kfeldspar, volcanic rock fragments and amphibole. The feldspars, especially the K-feldspar, are commonly intensely weathered (seriticised). Most grains have very fine-grained hematitic coatings. The sandstone is relatively matrix-poor with an earlier phase of radiating chalcedony and quartz fans to drusy cement overgrown by a later stage of blocky carbonate, void-filling cements. Halls (1974) proposed that the paleomagnetic signature of the basalts was comparable to the lower portions of the Osler volcanic group in northwestern Lake Superior. The aforementioned rocks are also intruded by a number of Keweenawan dikes and breccia bodies which commonly occupy and obscure lithologic contacts (Sage, 1991). Hinze et al. (1966), on the basis of aeromagnetic data, interpreted the presence of two major faults, which intersected to the south of the Slate Islands (Fig. 4). Sage (1991) has proposed that the onshore extension of the Figure 4. Faults of eastern Lake Superior with inferred directions of movement from Sage (1991). Modified from Hinze et al. (1966). -4- �ILSG Special Publication #1 - The Slate Islands northeast-trending Big Bay-Ashburton Bay Fault may be related to northeast-trending structures associated with the Mesoproterozoic Midcontinent Rift-related alkalic carbonatitic complexes at Deadhorse Creek and Prairie Lake. Sage (1991) reported that, in the vicinity of Patterson Island, breccia dikes cut and enclose blocks of lamprophyre with carbonatite affinity that have yielded a K-Ar age of ~300 Ma. Recently this unit has been dated using the U-Pb method, yielding a Keweenawan age of ~1100 Ma (L. Heaman, University of Alberta, personal communication, 1994, referenced in Dressler et al., 1999) suggesting the young K-Ar ages are likely the result of resetting. The breccias have been used to both argue for and against a meteor impact theory for the Slate Islands and are discussed further below. One Archipelago, Two Possible Origins In addition to the complex bedrock geology, the islands have also been the focus of interest and debate because they are considered by some to represent the “best-preserved, medium-sized, meteor impact structure on Earth” (V. Sharpton, Lunar and Planetary Institute, NASA, pers. comm. 1995). However, this theory is not universally accepted and Sage (1991, 1999) has proposed an endogenous cryptoexplosion process for formation of the islands. The Case for an Extraterrestrial Impact Origin for the Slate Islands Structure Before discussing the evidence for an impact, it is worth outlining the basic dynamics of a hypervelocity extraterrestrial impact. The continuous process that occurs during an impact is more readily understood if it is dealt with in stages (summarized by French, 1998, and outlined in particular for the Slate Islands by Dressler et al., 1998). 1) Contact/Compression Phase. As the impactor hits, hypervelocity shock waves are generated in both the impactor and the target rocks which forces the bedrock downwards and outwards, instantaneously vapourizing the impactor and the target rock near the point of impact, while further away the target melts as the shock pressures attenuate. 2) Decompression/Excavation Phase. The shock wave is immediately followed by a rarefaction or tensional wave, decompressing the remaining rock and allowing it to relax, opening fractures large and small, driving material downward, outward and upward, excavating an extremely short-lived, steep-sided, unstable transient crater. 3) Central Peak Formation. If the impactor and consequent forces are large enough, the unloading of deep bedrock by the removal of overlying rock plus the decompression following the shock wave, results in material in the bottom of the crater rebounding upward into a central peak within the transient crater (as when a drop of water hits the calm surface of a pond). 4) Transient Crater Collapse and Formation of the Final Crater. As the transient crater reaches its maximum size, the fractured and faulted oversteepened walls begin collapsing into the crater in a rush, meeting and mixing with the likewise collapsing central uplift, before settling into an approximation of the final crater form. The entire process from first contact by the impactor to this stage has not lasted much more than 5-10 minutes in most craters, perhaps 15 minutes in the very largest craters a couple of hundred of kilometres in diameter. 5) Long Term Adjustment. Then begins a long process of adjustment, lasting decades to many millennia, depending on many things, but primarily crater size. During this final stage, loose debris continues settling, aided by tremors as stresses are released, hydrothermal activity begins in medium-sized to large craters, cooling continues, and finally, consolidation and lithification of breccias takes place. A number of authors have proposed that the Slate Islands have preserved the site of a meteor impact (Halls, 1975, 1976; Robertson and Grieve, 1976; Halls and Grieve, 1976; Halls and Stesky, 1978; Dressler et al., 1995, 1998, 1999). The islands themselves have been identified as the central uplift of a mediumsized impact structure, which, bathymetry suggests, is surrounded by a submerged annular trough ringed by a ridge 30 to 32 km in diameter (Halls and Grieve, 1976; Dressler et al., 1995), representing the suggested final crater diameter. A crater this size implies an ~1.5 km diameter impactor with an arrival velocity of ~ 15 km/s. This circular feature was also transected and confirmed by the Great Lakes International Multidisciplinary Program of Crustal Evolution (GLIMPCE) seismic reflection line (Fig. 5; Mariano and Hinze, 1994). According to Dressler et al. (1998) almost all the rocks of the archipelago are somewhat brecciated and -5- �ILSG Special Publication #1 - The Slate Islands Figure 5. Reprocessed northern part of GLIMPCE Line A (courtesy of B. Milkereit, Geological Survey of Canada, 1994, published in Dressler et al., 1999). Left vertical axis is seconds of two-way time, right vertical axis is approximate depth in kiometres and horizontal axis shows shot points. X is the westward projection of the approximate centre of the central uplift. The distance from the centre of the central uplift (approximate geographic centre of the archipelago) to R is 15-16 km. R lies approximately where the rim of the structure is placed based on bathymetry. The strong reflections at 0.5 s may represent arenites of the Jacobsville Formation and not multiple reflections of the lake bottom which is at relatively shallow depth in the area investigated here. AB: Keweenawan basalt; BC: Jacobsville Formation. From Dressler et al. (1999). they propose that the bedrock can be considered a megabreccia, although the detailed mapping of Sage (1991) showed good structural coherence across the islands. For Sage (1999) this structural coherence between the blocks and with rocks on the mainland argues for an endogenous origin for the breccias on the islands. All local rocks have been intruded by a network of anastamosing breccia bodies ranging in colour from brick-red to greenish grey. The breccias consist of sharply angular to sub-rounded fragments up to four metres across derived from local Precambrian rocks. The breccias have been ascribed to both endogenous intrusive activity (e.g., Sage, 1991) and meteor impact (e.g., Sharpton et al., 1996) and have been used to argue both for and against the impact theory on the Slate Islands. The age of the Slate Islands structures and breccia bodies is poorly constrained (Table 1). Grieve et al. (1995) proposed an age of <350 Ma based on similarities in the erosional level between the Slate Islands and the ~350 Ma Charlevoix structure in Quebec. Sharpton et al. (1996) have proposed an age of 500-800 Ma based on the presence of clasts of the Mesoproterozoic Jacobsville sandstone (southern shore of Lake Superior, Michigan) and absence of any Devonian or Ordovician carbonates. However, the Slate Islands sandstone clasts are similar to sandstone interflow units found within the Osler basalts on the western shore of Patterson Island and, thus, may not be Jacobsville sandstone. More recent Ar-Ar age determinations on impact-generated pseuodotachylites have yielded spectra consistent with an age of ~450 Ma (Fig. 6; Sharpton et al., 1997; Dressler et al., 1999). Features that have been used in support of an impact event include dikes of clastic-matrix breccia Figure 6. 40Ar-39Ar release spectra. Samples 95SL103 and 95SL13:3e: dark gray, inclusion bearing “impact melts” (Keweenawan basalt). Sample 94B1D2, inclusion-poor pseudotachylite. From Dressler et al. (1999). -6- �ILSG Special Publication #1 - The Slate Islands Table 1. Stratigraphic age constraints on the Slate Islands impact. The age of the Jacobsville is ~1100 Ma, however these clasts may be interflow sandstones from within the Osler-like volcanic flows (see text). From Dressler et al. (1999). (Halls and Grieve, 1976; Sage, 1991), which include; pseudotachylites, polymictic allogenic breccias and monomictic autoclastic breccias (Sharpton et al., 1996) concentrated on the eastern shore of Patterson Island as well as Mortimer, Dupuis and Delaute islands. Shatter cones occur throughout the islands but are most obvious in the Keweenawan basalts. They are interpreted to have formed from the passage of a high-pressure shock wave (Dietz, 1964). They are characterized by a surface decorated with linear ridges and grooves (horsetail striations) that radiate from the apex of the cone. On the Slate Islands most shatter cones range from 2cm to ~30cm long; those in the Keweenawan rocks are 10 to 30 cm long (Sharpton et al., 1996). In addition Sharpton et al. (1996) reported a number of “mega cones” at least 10m long (and possibly up to 20m) in McGreevy Harbour (Fig. 2). As with the breccias the origin of the cones themselves remains controversial (Sharpton et al., 1996). Dressler et al. (1999) has suggested that the shatter cones formed during the compressional phase of the impact (Fig. 7) and indicate a minimum shock pressure in the target rocks of 3 GPa. Sage (1991) observed that shatter cones were most extensive close to breccia outcrops and used this to argue that the explosive emplacement of diatreme dikes was responsible for their formation. However, more detailed work (Sharpton et al., 1996) indicated that the shatter cones are ubiquitous on the islands. Dressler et al. (1995, 1999) have reinterpreted Figure 7. Formation of the Slate Islands impact structure. A) preimpact target; B) contact and compression; C) excavation; D) central uplift; E) central uplift collapse and modification; F) final structure; G) present structure, black areas indicate impact melt overlain by allogenic breccias (assumed, not shown in DF). a, Proterozoic and younger supracrustal rocks: Deformed Archean greenstone assemblage (assumed in annular trough); b, Mafic metavolcanics, minor metasediments and intrusive rocks; c, Intermediate and felsic metavolcanics, minor metasediments and intrusive rocks. From Dressler et al. (1999). -7- �ILSG Special Publication #1 - The Slate Islands Sage’s diatremes as impact breccia bodies (e.g., Bunte breccia, suevite). Microscopic planar deformation features (PDFs) in quartz and feldspar have been observed in rocks from the Slate Islands by Halls and Grieve (1976), Sage (1991) and Dressler et al. (1994). These planar lamellae are shock induced micro-melt zones <2-3 μm wide along crystallographic axes. They first appear in quartz at pressures of ~8 GPa along the {0001} and {1011} axes and at ~10 GPa they begin to appear along the {1013} axis (French, 1998). PDFs are considered diagnostic of the extremely intense shock waves produced during hypervelocity impacts. However, a single set of PDFs can easily be confused with Bohm lamellae and other planar features, and thus, two or more criss-crossing sets of PDFs along different measured crystallographic axes are the preferred diagnostic features. Crisscrossing sets of PDFs are seen both in the Slate Islands host rocks and in breccia components. In a detailed study of these features Dressler et al. (1998) showed a zone of maximum shock intensity on Patterson Island (Fig. 8), suggesting the location of the impact was slightly west of the center of Patterson Island. types cross-cut other breccias, plus features such as PDFs, allowed them to hypothesize when various features formed during the impact process. It is within this context that Dressler and Sharpton (1997) place their interpretations of the breccias (Figs. 7, 8 & 9; Table 2). The breccias identified by the authors include: A detailed study of the breccias on the Slate Islands has been undertaken by Dressler and Sharpton (1997) who have estimated that breccias make up ~15 to 25% of the Islands’ rocks. Interpretation of which breccia • Pseudotachylites which are thought to have formed as a result of brittle-or brittle-ductile seismic faulting and instantaneous melting due to the passage of the hypersonic shock wave during the compressional phase of the impact event. Pseudotachylites are relatively rare in the archipelago and occur as small veins and dikes. The early formation of these pseudotachylites is supported by the presence of clasts of pseudotachylite in the breccias. • Polymictic clastic matrix breccias are the most abundant breccia type on the islands but are more common on Patterson Island than on the outlying islands. The breccias contain a wide variety of clasts from all host lithologies, that are angular to sub-rounded, and range in size from <1mm to several metres. These are interpreted to have formed when decompression allowed opening of fractures within the crater walls and floor (Dressler et al., 1999) excavating the crater to a depth of ~1.5 km in approximately 1 minute. Figure 8. Sketch map of Slate Islands impact structure, located in northern Lake Superior. Dashed lines show concentric trends of coast lines and structural elements indicating crater center on western side of Patterson Island (approximate location is shown by cross). Previous estimates of crater center, based on shatter cone orientations (Stesky and Halls, 1983) or shock isobars deduced from planar deformation features in quartz (Grieve and Robertson, 1976), are shown as filled circles. Shatter-coned outcrops are shown as small unfilled circles. Filled diamond shows location of >10m shatter cone. Map is adapted from Sharpton et al. (1996). Shaded field is the area of highest shock values from Dressler et al. (1998). -8- �ILSG Special Publication #1 - The Slate Islands Table 2. Slate Islands impact breccias. From Dressler and Sharpton (1997). Figure 9. Section across the Slate Islands complex impact structure showing distribution of breccias investigated. Minor polymictic clastic matrix breccias are also present further away from the centre of the structure than shown here. Profile is based on bathymetric information from around the archipelago and on topographic maps of the islands. From Dressler and Sharpton (1997). -9- �ILSG Special Publication #1 - The Slate Islands The presence of Proterozoic clasts in breccias dominated by Archean material has been used to argue for downward movement and mixing of clasts over distances possibly as much as 5 km (Dressler and Sharpton, 1997). • Allogenic breccia deposits containing altered glass fragments (suevites) or with no glass fragments (Bunte breccia) are present on Dupuis and Patterson islands and have been used to argue for a shallower erosion level for the archipelago (Sharpton and Dressler, 1996). The Bunte breccias are interpreted to have formed either as fall-back deposits in the crater or as ground-surge deposits. The breccias contain mainly Proterozoic clasts, supporting their origin as fall-back deposits (Dressler et al., 1999). Dressler et al. (1999) have also reported the presence of suevite breccias and use the absence of aerodynamically shaped glass fragments to argue that they are also fall-back breccias. • Monomictic, autochthonous breccias are found on Mortimer Island and a number of the small outlying islands. The breccias comprise angular, densely packed fragments typically up to 20 cm in size within a matrix of similar clastic rock powder. These are interpreted to have formed late in the impact process during the crater modification phase (Fig. 7) as huge blocks of rock slumping off the transient crater walls ground together during their slide into the crater over several minutes (Dressler et al., 1999). The breccias are often autoclastic with transitional borders with their host rocks (Dressler and Sharpton, 1997). Halls and Grieve (1976) and Grieve and Robertson (1976) proposed that the Slate Islands represented uplifted basement that preserved breccias injected during impact into the crater subfloor (~0.5 to 1.5 km below the central peak). However, Sharpton et al. (1996) have suggested that the allogenic and autoclastic breccias indicate that the present exposure surface is only a few hundred metres below the original ground surface. This issue is not without controversy and is discussed further in Halls (1997), Grieve and Robertson (1997) and Sharpton and Dressler (1997). cone orientations and shock barometry (Stesky and Halls, 1983; Grieve and Roberston, 1976) suggested that it was closer to the center of Patterson Island (Fig. 8). However, given that both locations are within 1.5 km of each other and given that the impacting body was estimated to have a diameter of ~1.5 km, none of the proposed locations should be considered definitive (Sharpton et al., 1997). The Case for a Cryptoexplosion Origin for the Slate Islands Structure The title of Sage’s (1999) paper clearly stated his case: “The Slate Islands: A Uniquely Sited Cryptoexplosion Structure”. He noted that the Slate Islands are situated on or near the Proterozoic-Archean boundary and at the intersection of two major inferred faults, the Big Bay-Ashburton Bay Fault (or accommodation zone) and the Michipicoten Fault. He also noted that the Slate Islands lie on the flank of the Midcontinent Rift where crustal thickness reaches 50 km or more, and on a topographic ridge, extending southwest to Superior Shoals and northeast to the mainland, which bisects this thick crust. Perhaps most importantly to Sage, the Slate Islands are close to the Port Coldwell Alkalic Rock Complex, the Kilalla Lake Alkalic Rock Complex, Prairie Lake Carbonatite, Deadhorse Creek Diatremes and McKellar Creek Diatremes (Sage, 1999), all of Keweenawan age. Sage (1991) argued that “the possibility of a meteorite impact at this precise location – on a ridge traversing the Lake Superior Basin, on the nose of an Archean fold structure, at the precise location of the Proterozoic-Archean contact, at the precise location of two intersecting regional faults, and at the precise location of highly volatile alkalic magmatism – is too incredible to accept (Sage, 1991, p.56)”. Sage (1978) presented a number of geological observations favouring a non-impact origin many of which were elaborated upon in Sage (1991). These included: Current interpretations based on topographic and structural trends place the crater center in the westcentral part of Patterson Island (Fig. 8; Sharpton et al., 1996) whereas earlier interpretations based on shatter - 10 - • Clast size sorting in the diatremes from finegrained at the margins to coarse-grained at the center is typical of laminar flow (Sage, 1978) and more likely to occur in a diatreme than by downward intrusion. • Orientation of shatter cones was not consistent with a central impact structure. • Contact metamorphic effects between the breccia and alkalic diabase indicates that hydrothermal �ILSG Special Publication #1 - The Slate Islands activity accompanied breccia emplacement. • The presence or absence of an igneous matrix in breccia dikes on the islands and the mainland does not preclude an igneous origin. The Debate The debate – cryptoexplosion vs impact – over the origin of the Slate Islands structure largely mirrors (in a more genteel way) the vigorous debate which began when Dietz (1964) proposed that the Sudbury Structure was due to an impact. When Alvarez et al. (1980) proposed that the K/T extinction was caused by an impact, the debate became a nasty scientific controversy. Now, some 26 years later, the debates over Sudbury and the Chicxulub-K/T extinction crater are resolved in favour of an impact origin for both. The debate is summarized by Powell (1998) in an interesting, very readable popular book, “Night Comes to the Cretaceous”. To reiterate Sage’s statement, “the possibility of a meteorite impact at this precise location – on a ridge traversing the Lake Superior Basin, on the nose of an Archean fold structure, at the precise location of the Proterozoic-Archean contact, at the precise location of two intersecting regional faults, and at the precise location of highly volatile alkalic magmatism – is too incredible to accept” (Sage, 1991, p.56). Halls (1979) counters that the absence of complex overlapping shatter cone sets argues against the multiple emplacement events proposed by Sage (1978). On a larger scale Halls (1978) argued that the regional faults proposed by Sage (1978) are only inferred from geophysical data and the magnetic anomalies may also delineate the unfaulted margin of the Keweenawan basin. Halls (1979) also provided alternative explanations for the apparent coincidences suggested by Sage, observing that the lower and upper Precambrian contact predates the shock event and cannot be used to argue either for or against. The Slate Islands debate centres mainly on the interpretation of three sets of features: the breccias, shatter cones, and planar deformation features (PDFs). Sage (1991) has proposed that the breccias originated from the forcible emplacement of volatilerich magmas formed at depths > 35 km which have risen to a shallower level and exsolved a gas phase. The higher volatile contents of clasts and matrix have been argued to support this model. However, arguing against an origin at depths of ~35 km is the absence of deep-seated or magmatic material in the breccias (Robertson and Grieve, 1979). Another breccia problem is explaining how dikes containing upper level Paleoproterozoic fragments were emplaced into lower level Archean rock. Halls and Grieve (1976) were the first to suggest a downward injection of breccias into fractures (during the crater modification stage following the passage of the initial shock wave) as a result of an impact event (Robertson and Grieve, 1979). However, Sage (1991) has countered that the presence of stratigraphically high level clasts at depth could also be explained by collapsing fluid columns after the emplacement of diatremes. Today, the various breccia types (pseudotachylites, polymictic, allogenic, and monomictic autochthonous), their relationships within each other, and their locations within the Slate Islands structure seem to be best explained by their production during various phases of the impact process (Sharpton and Dressler, 1997; Dressler and Sharpton, 1997; Dressler et al., 1999). Sage (1991) has also advocated that the forceful emplacement of diatremes formed at depths > 35 km could account for the shock features – shatter cones and PDFs – preserved on the islands. Sage (1999) provides a number of examples of other occurrences of planar deformation lamellae and shock textures that may have been produced by kimberlite emplacement, however, he acknowledges that these features have also been interpreted as having been formed by impact events. Robertson and Grieve (1979) observed that shatter cone formation is a function of lithology as well as shock pressure and the fissile Archean metavolcanics would display more poorly developed cones than the structurally isotropic Keweenawan flows. The distribution of microscopic shock effects has been recorded by Grieve and Robertson (1976) who showed that the intensity of these features increase in a consistent fashion from the coast inward to the proposed impact centre (see Fig. 8). Roberston and Grieve (1979) also argued that diatreme emplacement is normally considered to be a process of “drilling and venting by gas streaming” rather than by violent explosions and that the pressures induced by this process are unlikely to exceed 1.5 GPa, whereas pressures of 2-6 GPa are required to generate shatter cones (French, 1998). Halls (1979) took issue with Sage’s measurements of shatter cones suggesting that he did not use the correct measurement procedure and did not properly correct - 11 - �ILSG Special Publication #1 - The Slate Islands his data and that stated that there is no convincing spatial correlation between shatter cones and breccia dikes, and furthermore, the breccias contain shatterconed clasts (Halls and Grieve, 1976). sampled by Resident Geologist staff in 1995 near Cove Island returned 604 ppm Zn, <100 ppm Cu and 157 ppm Pb (Resident Geologist’s Files, Thunder Bay South District, Thunder Bay). PDFs seem beyond debate, so long as they are really PDFs (and not Böhm lamellae), based upon measured widths and spacing of lines and particularly based upon their alignment along measured crystallographic axes. French (1998) has summarized a large body of laboratory experimental evidence and field evidence on PDFs, as do Dressler et al. (1998). Dressler et al. (1998) measured a large number of PDF orientations in Slate Islands quartz crystals. They found PDFs aligned along many different axes, notably the {1013} and {1012} axes, indicative of shock pressures as high as 18 GPa, pressures equivalent to those many hundreds of kilometers depth within Earth. Such observations cannot be explained by diatremes. A little discussed problem for the impact hypothesis is the apparent absence of evidence for an impact on the mainland north shore of Lake Superior, only 15 km from the proposed impact centre, which supposedly produced a crater ~15-16 km in radius. Thus, on both side of the debate, problems still have to be resolved, but the majority of the evidence described from the Slate Islands Structure is best explained by an impact. Economic Geology A synopsis of the mineral exploration history and mineralization is provided by Sage (1991). Two styles of mineralization in the Archean metavolcanic rocks have garnered the most exploration interest: 1) lode gold; and 2) volcanogenic massive sulphide copperzinc. Gold is associated with quartz-carbonate veins in deformed and altered (Fe-carbonate, sericite, chlorite, tourmaline) rocks. More than 20 occurrences of visible gold in float boulders of quartz vein material have been recorded on the islands (Resident Geologist’s Files, Thunder Bay South District, Thunder Bay). Visible gold has also been noted in-situ near Horace Cove (aka St. Mary’s Bay). Massive sulphides occur in felsic metavolcanic rocks or as fragments in pyroclastic rocks. Massive pyrite sampled by Sage (1991) returned 0.11% Cu and 0.28% Zn. A sulphide-facies banded iron formation - 12 - �ILSG Special Publication #1 - The Slate Islands Stops are cut by a number of grey, heterolithic breccia dikes The field trip stops are labeled with letters rather than numbered in sequence as access to all stops, particularly on the outer shores of the islands, is weather-dependent. Many of the stops require some wading in order to reach all the outcrops so a change of footwear is recommended. Please take care when getting in and out of boats as outcrops are usually extremely slippery. with 0.5 to 5 cm, angular to sub-rounded clasts (Figs. 11 & 12). Narrow (<1 cm) dikelets may extend into the wall rock from the parent breccia dike. Stop A – “Honeymoon Bay” near Cove Island UTM coordinates – 0502004E 5386377N This small bay near Cove Island (Fig. 2) provides exposures of strongly sheared Archean felsic metavolcanic rocks at its northeast end (Stop A1). These metavolcanic rocks are phyllitic, displaying a pronounced west-trending, steeply dipping foliation with minor folds and kink bands. They are intruded by a 1 m wide, Paleoproterozoic diabase dike that zigzags across the outcrop (Fig. 10). A small (50 cm) wide breccia dike crosscuts the metavolcanic rocks. Metavolcanic rocks along the western shore of the bay Figure 12. Heterolithic breccia dyke at Stop A1. From this point groups will be shuttled out to a small island to the east of Honeymoon Bay (Stop A2; Fig. 2; UTM coordinates 0502426E 5386437N). This island consists of heterolithic breccia with clasts over 2 m across (Fig. 13). From the top of the island it is possible to look down upon a series of anastamosing dark grey breccia dikes under the water in the bay (Fig. Figure 10. Diabase dyke intruding Archean felsic metavolcanic rocks at Stop A1. Figure 11. Breccia dyke intruding Archean felsic metavolcanic rocks at Stop A1. Dykelets marked by arrows. Figure 13. Typical breccia exposed on the breccia island at Stop A2. - 13 - �ILSG Special Publication #1 - The Slate Islands from larger, parent dikes. Stop B – Sunday Harbour UTM coordinates – 0500801E 5386425N Figure 14. Anastamosing breccia dykes at Stop A2. 14) and across to a second island where an ~2 m wide, recessively weathered clast of reddish metavolcanic rock is clearly visible. You will need to get your feet wet to fully appreciate this outcrop. The majority of clasts typically range between <1 to 10 cm in size. They are derived from Archean metavolcanic rocks, Mesoproterozoic diabase and a variety of nondescript, fine-grained, variably altered rocks of indeterminate origin. Ragged, injected bodies of breccia may extend The beach consists of reworked glaciofluvial sediments characterized by a variety of locally derived and exotic rounded cobbles and boulders. Most of these are felsic plutonic and mafic metavolcanic rocks of the Schreiber-Hemlo greenstone belt. The exotic clasts are best exemplified by what have been termed “omars” (Prest, 1990), glacial erratics of massive, dark siliceous greywacke that contain light-toned (generally buff-weathering) calcareous concretions which are typically subspherical and weather recessively (Fig. 15). Omars, which commonly occur in and on eskers and outwash, but which also may be found in till and lacustrine deposits, are inferred to have been derived from the Omarolluk Formation of the Belcher Group in southeastern Hudson Bay (Prest et al., 2000). Most of the erratics were dispersed northwestward and westward across the Hudson Bay Paleozoic Basin by Labrador Sector ice, followed by westward and southwestward movement of ice across the Paleozoic and Archean terrain of northern Ontario, northern Manitoba and the upper Midwestern United States. A series of breccias are exposed on the eastern shore of Sunday Harbour. Dressler et al. (1999) reported the presence of two allogenic breccias at this outcrop: a Bunte Breccia is reported from the southern portion of the outcrop and a suevite breccia to the north. The southern end of the outcrop is a heterolithic grey breccia with clasts up to 50cm wide. It also contains clasts with well-developed shatter cones (Fig. 16). The grey breccia contains conspicuous reddish metavolcanic clasts and rare mafic to ultramafic clasts up to 50 cm across. At this location, the breccia is quite friable and easily dislodged from outcrop faces. Narrow diabase dikes with quartz-filled tension gashes intrude the metavolcanic rocks and are exposed just offshore in shallow water. Foliation orientations in the metavolcanic rocks are variable, suggesting either large-scale folding or rotation of large blocks of country rocks. Reddish alteration zones appear as dike-like bodies, cutting the grey breccia in places. Figure 15. Omars from the beach at Sunday Harbour (Stop B). - 14 - �ILSG Special Publication #1 - The Slate Islands chalcopyrite and hematite are noted. Grab sampling of vein material by Resident Geologist’s Program staff returned up to 3.95 ounces Au per ton and 0.2 ounce Ag per ton (Resident Geologist’s Files, Thunder Bay South District). The style of mineralization, alteration and deformation resembles that at Heron Bay, on the mainland shore of Lake Superior, approximately 50 km east of this location. The following synopsis of gold exploration at Horace Cove (aka St. Mary’s Bay) was modified from that of Sage (1991). Parsons (1918) concluded that the gold showing on the northwest corner of Horace Cove was the most promising of the known gold occurrences. From 1960 to 1963 Kimberly-Clark Pulp and Paper Company Limited conducted a mineral exploration program of the islands to test two gold showings. The main gold showing (St. Mary’s Bay occurrence) is on the northwestern corner of Horace Cove and the second occurrence (Cosen’s Showing) lies 240 m to the northeast. Figure 16. Shatter cone clast in breccia at Sunday Harbour (Stop B). Stop C – Horace Cove UTM coordinates – 0497386E 5387206N Shoreline outcrops at this location expose pervasively Fe-carbonatized and sericitized, schistose Archean metavolcanic rocks (Fig. 17). A strong, westtrending and steeply dipping foliation has resulted in the development of fissile, phyllitic rocks that also contain quartz, chlorite, green mica (chromian muscovite) and pyrite. Hydrothermal alteration and deformation preclude definitive recognition of the protolith. Sage (1991) has noted schistose basaltic to andesitic rocks, as well as dacitic to rhyolitic flows in the vicinity. Quartz- and feldspar-phyric units and sections containing quartz blebs (amygdules?) are also noted. Thin section analysis of this quartz-sericite schist by Nichols (1963) revealed a fine-grained groundmass of quartz blebs and scaly intergrowths of sericite that hosts siderite euhedra, altered albite and prochlorite and quartz amygdules. Along the shoreline, quartz-carbonate veins, with which most of the gold is associated, occupy a 050°trending fracture set. They range up to approximately 8 cm in width and are locally folded. Visible gold, pyrite, In 1960 Kimberly-Clark contracted an aeromagnetic and electromagnetic survey of the island. In 1961 and 1962 trenching, bulldozing, stripping, sampling and geologic mapping was done by the company over both the St. Mary’s Bay zone and Cosen’s showing. At St. Mary’s Bay bulldozer stripping to depths of 1.6 to 2.0 m exposed an area of approximately 18,900 m2; at Cosen’s showing 240 m to the north, approximately 5350 m2 of similar stripping was completed (G.E. Parsons, consulting geologist, personal communication, 1976). In 1963, Kimberly-Clark formed the Slate Island Mining Company Limited (The Northern Miner, September 19, 1963). Kimberly-Clark held a 50% interest, Junior Frood Mines Limited 25%, Upper Canada Mines Limited 12.5% and Cadamet Mines Limited 12.5% in Figure 17. Intensely sheared Archean metavolcanic rocks and folded quartz-carbonate vein at Horace Cove (Stop C). - 15 - �ILSG Special Publication #1 - The Slate Islands this new company (Financial Post Survey of Mines, 1964, p.169). In 1963, this company completed 20 diamond drill holes, totalling an estimated 1974 m on the St. Mary’s Bay zone (G.L. Puttock, personal communication, 1974). This work disclosed variable, but locally very high-grade, gold mineralization in quartz veins of short strike length and over narrow widths of 2 to 10 cm. Mineral exploration of the islands ceased with the termination of the efforts of KimberlyClark. In April 1973, finding the islands of no further use to them, Kimberly-Clark transferred its rights to the islands back to the Crown. Subsequently, in September 1973 the islands were removed from staking. The gold-bearing, quartz-carbonate veins of St. Mary’s Bay zone and Cosen’s showing display a strong southwesterly strike. Since the host rocks of the veins are folded into a northwest-trending sequence, these veins are approximately normal to stratigraphy as was observed in several places along the eastern shore of Patterson Island. Some evidence for shear folding of the quartz veins is indicated at the St. Mary’s Bay zone by the irregular “sawtooth” pattern of some of the veins. Nichols (1963) suggested that gold-bearing quartz veins on Patterson Island occurred in the nose of a fold and occupied shear and tension fractures. Based on samples and descriptions by G.E. Parsons (consulting geologist for Kimberly-Clark Pulp and Paper Company Limited, personal communication, 1974), gold locally occurs in three ways. These are: (1) in association with pyrite within the quartzcarbonate veins; (2) as flakes and thin sheets along the flanks of the quartz-carbonate veins; and (3) as thin sheets or flakes along schistosity planes of the rocks enclosing the quartz-carbonate veins. Sampling by Sage (1991) of various quartz veins returned nil to insignificant gold values except for the St. Mary’s Bay zone, where assays of 0.5 ounce Au per ton over widths of 2 to 3 cm were obtained. The quartz veins vary from tabular, lensoid, clearly defined veins to irregular anastomosing structures with no clearly discernible attitude. An average of 98 clearly defined veins gave an average width of 10.7 cm and a length of 5.5 m (Sage, 1991). Reddish-brown, coarse-grained carbonate is an ubiquitous, accessory to dominant mineral and pyrite is common to abundant. Rarely, black needle-like crystals of tourmaline were noted. A contoured stereonet plot of 167 quartz vein attitudes indicated a rather broad spread of attitudes with one and possibly two maxima. The strongest maximum defines a 070°-trending vein set dipping approximately 60° southeast. The second maximum defines a 035°trending, vertically dipping vein set. The intersection of these two trends would define a lineation striking 210°, plunging about 14° southwest. Brummer (1962) delineated an area of sericite schist and shearing extending for 300 m north-south and 570 m east-west at the northern end of Horace Cove. Pyroclastic rocks, diorites and porphyritic metavolcanic rocks were also noted. Three steeply dipping to vertical vein sets were identified: a major set at 035°; and minor sets at 063° and 050° to 060°. The 40 veins that had been discovered at that point ranged in strike length between 16 and 60 m and in width from 0.5 to 20 cm, averaging 5 cm. Brummer (1962) noted that approximately 80% of the gold occurred as this films along the outer vein margins. The altered wall rock was not sampled for assay; Nichols (1963) noted an absence of gold in wallrock. Stop D – Western shore of Patterson Island UTM coordinates – ca. 495965E 5387400N This location on the western shore of Patterson Island (Fig. 2) is a microcosm of Slate Islands geology, in that a variety of rock types and geologic features are exposed. The southernmost outcrops (UTM coordinates - 495965E 5387324N) are sheared Archean metavolcanic rocks which are unconformably overlain by hematite-jasper banded iron formation and ferruginous shales of the Paleoproterozoic Gunflint Formation (Fig. 18). Figure 18. Unconformable contact between Keweenawan basalts (left) and Animikie Gunflint Formation (right). - 16 - �ILSG Special Publication #1 - The Slate Islands The base of the Gunflint outcrop consists, very approximately, of 20m of interlayered grainstone and slaty iron formation. Diabase and red breccia dikes have obscured the lower contact of the Gunflint Formation. The grainstone layers are 1 to 30 cm thick and are composed of intraclasts of chert and jasper. Clast sizes range up to that of small pebbles but are dominated by medium-to coarse-grained sand. The rock has a pervasive quartz cement. The slaty iron formation layers form bundles less than one to several centimetrers thick. Individual layers are millimeter to sub-millimeter in thickness. They are composed of magnetite mixed with what is probably siliciclastic clay and silt. This unit is overlain by approximately 7 m of just the slaty iron formation. This denotes a rapid change from shallower, storm-dominated, bottom to deeper, more quiescent conditions, a trend similar to the Gogebic iron formation successions described from Wisconsin (Pufahl and Fralick, 2004). The exposed upper part of the Gunflint section dips approximately 20° to 30° to the north. The upper contact of the Gunflint with overlying Mesoproterozoic (Keweenawan) flood basalts is also obscured by diabase and breccia bodies. Small, delicate shatter cones (< 5 cm long) are developed in the argillaceous portions of the sedimentary rocks (UTM coordinates - 495964E 5387369N; Fig. 19). Figure 20. Amygdaloidal Keweenawan basalt. Stop D on Patterson Island. approximately 1m thick interflow sandstone unit can be accessed by wading across the small bay. Mediumgrained, interflow red sandstones form successions up to a couple of metres thick. Bed thicknesses vary from a few centimeters to approximately 1 m. The sandstones are massive; sedimentary structures, aside from upper flow regime parallel laminations, are not A series of north-striking basalt flows ranging from 1 to 2 m thick outcrop along the shoreline. Sage (1991) has noted 22 separate flows in this section. The basalts are vesicular and amygdaloidal and dip approximately 20° to 60° to the west. Pipe amygdules occur near flow bases; coalescing amygdules may form flow-parallel lenses and bands (Fig. 20). Ropy flow tops, characteristic of pahoehoe lava, are locally preserved (Fig. 21). An Figure 21. Pahoehoe texture developed on basalt flow tops at Stop D on Patterson Island. Figure 19. Shatter cones in the Gunflint Formation at Stop D on Patterson Island. well preserved. This may be the result of fluid escape, especially during heating by overlying basalt flows. Trough-like structures in the top of one bed overlain by basalt flows may represent gouge marks where blocks of solidified lava caught up in the overriding basalt flow has been dragged through the unlithified sand in a manner analogous to glacial striae (Figs. 22 & 23). The channels are oriented in an east-west direction, perpendicular to the strike of the flows. The interflow sandstone is thicker than interflow sedimentary rocks in Osler basalts on Wilson Island (Hollings and Fralick, 2005). In addition to the thick interflow unit, thin layers of baked interflow mudstone can be seen within and - 17 - �ILSG Special Publication #1 - The Slate Islands Figure 24. Shattercones in Keweenawan diabase, Stop D Patterson Island. Figure 22. Keel marks left in the upper surface of an interflow sandstone unit at Stop D, Patterson Island. Flow direction is parallel to black arrows. between the basalt flows. Shatter cones are particularly well-developed in the basalt flows and in the boulders and blocks that litter the beach (Fig. 24). In places the shatter cones exceed 20 cm in length. Stop E – McGreevy Harbour UTM coordinates – 500825E 5390752 N Dressler et al. (1999) have interpreted the structures preserved in the Archean felsic volcanic rocks at this site as large shatter cones, the largest being ~10m high (Fig. 25). To the west of the larger shatter cone a partial cone may be preserved that would imply a total length on the order of 20m. It is difficult to disembark at this site and equally hard to clamber on the steep talus cascading into the water. The scale of these large features is better appreciated from 15 to 20 m offshore. Figure 23. Close-up of keel marks left in the upper surface of an interflow sandstone unit at Stop D, Patterson Island. - 18 - �ILSG Special Publication #1 - The Slate Islands Figure 25. Large shatter cone visible in the cliff side in McGreevy Harbour (Stop E). - 19 - �ILSG Special Publication #1 - The Slate Islands References Alvarez, L.W., Alvarez< W., Asaro, F., andMichel, H.V. 1980. Extraterrestrial cause for the CretaceousTertiary extinction. Science, 269, 1095-1108. Brummer, J.J. 1962. Gold-bearing veins, St. Mary’s Bay, Patterson Island, Lake Superior, Ontario; unpublished report, Resident Geologist’s Files, Thunder Bay South District, 5p. Cannon, W.F., Green, A.G., Hutchinson, D.R., Lee, M., Milkereit, B., Behrendt, J.C., Halls, H.C., Green, J.C., Dickas, A.B., Morey, G.B., Sutcliffe, R. and Spencer, C., 1989. The North American Midcontinent Rift Beneath Lake Superior from GLIMPCE Seismic Reflection Profiling; Tectonics, v. 8, n. 2, p. 305332. Grieve, R. A. F., Rupert, J., Smith, J., and Therriault, A., 1995, The record of terrestrial impact cratering: GSA Today, v. 5, p. 189, 194–196. Halls, H.C., 1974. A Keweenawan sequence from the Slate Islands, Northern Lake Superior. Institute on Lake Superior Geology, 20th Annual Meeting, Sault Ste. Marie, Ontario, p.14. Halls, H.C., 1975. Shock-induced remanent magnetization in Late Precambrian rocks from Lake Superior. Nature, 255, 692-695. Halls, H.C., 1976. The Slate Islands: The Central Uplift of a meteorite impact crater. Institute on Lake Superior Geology, 22nd Annual Meeting, St Paul, Minnesota, p.27 Card, K.D., Sanford, B., and Davidson, A., 1994. Bedrock geology, Lake Superior, Ontario, USA. Natural Resources Canada, Map NL-16/17-G. Halls, H.C., 1979. Diatremes and shock features in Precambrian rocks of the Slate Islands, northeastern Lake Superior: Discussion. Geological Society of America Bulletin, 90, 1084-1086. Coleman, A.P., 1901. The Slate Islands in Iron Ranges of Northwestern Ontario, Ontario Bureau of Mines, v. 11, 552-555. Halls, H.C., 1997. New constraints on the Slate Islands impact structure: Comments and Reply. Geology, 25, 666. Dietz, R. S., 1964, Sudbury structure as an astrobleme: Journal of Geology, v. 72, p. 412–434. Halls, H. C., and Grieve, R. A. F., 1976, The Slate Islands: A probable complex meteorite impact structure in Lake Superior: Canadian Journal of Earth Sciences, v. 13, p. 1301–1309. Dressler, B.O. and Sharpton, V.L., 1997. Breccia formation at a complex impact crater: Slate Islands, Lake Superior, Ontario, Canada. Tectonophysics, 275, 285-311. Dressler, B.O., Sharpton, V.L., Schuraytz, B. and Scott, J., 1994. Bunte breccia, impact melt and suevite at teh SLate Islands impact structure, Ontario. Ontario Geological Survey, Miscellaneous Paper 163, 59-61. Dressler, B.O., Sharpton, V.L., Schnieders, B. and Scott, J., 1995. New observations at the Slate Islands impact structure, Lake Superior. Ontario Geological Survey, Miscellaneous Publication 164: 53-61. Halls, H. C., and Stesky, R.M., 1978. Paleomagnetic and shatter cone measurements from the Slate Islands, Northern Lake Superior. Canadian Geophysical Union, 5th Annual Meeting, London, Ontario, p.34. Hinze, W.J., O’Hara, W.N., Trow, J.W. and Secor, G.B., 1966. Aeromagnetic studies of eastern Lake Superior, in Steinhart, J.S., ed., The earth beneath the continents. American Geophysical Union, Geophysical Monograph, 10, 95-110. Dressler, B.O., Sharpton, V.L. and Schuraytz, B.C., 1998. Shock metamorphism and shock barometry at a complex impact structure: Slate Islands, Canada. Contributions to Mineralogy and Petrology, 130, 275-287. Hollings, P., and Fralick, P., 2005. A stratigraphic transect across the northern flank of the Midcontinent Rift near Rossport. In; Hollings, P. (Ed.), Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Part 2 - Field trip guidebook, v.51, part 2, 57-70. Dressler, B.O., Sharpton, V.L. and Copeland, P., 1999. Slate Islands, Lake Superior, Canada: A mid-size, complex impact structure. Geological Society of America, Special Paper 339, 109-124. Hollings, P., Fralick, P. and Cousens, B., 2006. Geochemistry and sedimentology of the Osler Formation: Evaluating rifting in the Proterozoic. Canadian Journal of Earth Sciences, in press. French, B.M., 1998. Traces of catastrophe. Lunar and Planetary Institute, Contrib. 954, 120 pp. Mariano, J., and Hinze, W. J., 1994, Structural interpretation of the Midcontinental rift in eastern Lake Superior from seismic reflection and potential-field studies: Canadian Journal of Earth Sciences, v. 31, p. 619– 628. Grieve, R. A. F., and Robertson, P. B., 1976, Variations in shock deformation at the Slate Islands impact structure, Lake Superior, Canada: Contributions to Mineralogy and Petrology, v. 58, p. 37–49. Grieve, R. A. F., and Robertson, P. B., 1997. New constraints on the Slate Islands impact structure: Comments and Reply. Geology, 25, 666-667. Nichols, L.C. 1963. Gold occurrences on the Slate Islands of Lake Superior; unpublished B.Sc. thesis, Queen’s University, Kingston, Ontario, 61p. Norris, A.W. and Sanford, B.V., 1969. Paleozoic and - 20 - �ILSG Special Publication #1 - The Slate Islands Mesozoic geology of the Hudson Bay lowlands. In Hood, P.J., ed., Earth science symposium on the Hudson Bay. Geological Survey of Canada, Paper 68-53, 169-205. Parsons, A.L., 1918. Slate Islands, Lake Superior. Ontario Bureau of Mines, Annual Report, 27, 155-167. Polat, A., Kerrich, R., and Wyman, D., 1998. The late Archean Schreiber-Hemlo and White River-Dayohessarah greenstone belts, Superior Provnce: Collages of oceanic plateaus, oceanic arcs, and subductionaccretion complexes. Tectonophysics, 289, 295-326. Powell, J.L.,. 1998. Night comes to the Cretaceous. Harcourt Brace and Company, New York, 250 pp. Prest, V.K. 1990. Laurentide ice-flow patterns: A historical review, and implications of the dispersal of Belcher Island erratics; Géographie physique et Quaternaire, v.44, p.113-136. Sharpton, V., Dressler, B., Herrick, R., Schnieders, B., Scott, J., 1996. New constraints on the Slate Islands impact structure, Ontrio Canada. Geology, 24, 851-854. Sharpton, V.L., Copeland, P., Dressler, B.O. and Spell, T.L., 1997. New age constraints on the Slate Islands impact structure, Lake Superior, Canada. Lunar and Planetary Science Conference XXVIII, 1287-1288. Stesky, R. M., and Halls, H. C., 1983, Structural analysis of shatter cones from the Slate Islands, northern Lake Superior: Canadian Journal of Earth Sciences, v. 20, p. 1–18. Sun S. S., and McDonough., W.F., 1989. Chemical and isotopic systematics of oceanic basalts: Implications for mantle composition and processes. In Magmatism in the ocean basins. Edited by A.D. Saunders, and M.J. Norry. Geological Society Special Publication. 42: 313-345. Prest, V.K., Donaldson, J.A. and Mooers, H.D. 2000. The Omar story: The role of Omars in assessing glacial history of west-central North America; Géographie physique et Quaternaire, v.54, no.3, p.257-270. Pufahl, P., and Fralick, P., 2004. Depositional controls on Paleoproterozoic iron formation accumulation, Gogebic Range, Lake Superior Region, USA. Sedimentology, 51, 791-808. Roberston, P.B. and Grieve, R.A.E., 1976. Comparison of the distribution of shock metamorphism at Charlevoix, P.Q., and Slate Islands, Ontario. Geological Association of Canada – Mineralogical Association of Canada, Annual Meeting, Program with Abstracts, v. 1, p.42. Roberston, P.B. and Grieve, R.A.E., 1979. Diatremes and shock features in Precambrian rocks of the Slate Islands, northeastern Lake Superior: Discussion. Geological Society of America Bulletin, 90, 10871088. Sage, R.P., 1975. Slate Islands. Ontario Division of Mines, Ministry of Natural Resources, Map P. 997, scale 1” to 0.25 miles. Sage, R.P., 1978. Diatremes and shock features in Precambrian rocks of the Slate Islands, northeastern Lake Superior. Geological Society of America Bulletin, 89, 1529-1540. Sage, R. P., 1991, Precambrian geology, Slate Islands: Ontario Geological Survey Report 264, 111 p. Sage, R.P., 1999. The Slate Islands: A uniquely sited cryptoexplosion structure. In Summary of Field work and other activities 1999, Ontario Geological Survey, Open File Report 6000, 28-1 to 28-13. Sharpton, V. and Dressler, B., 1997. New constraints on the Slate Islands impact structure: Comments and Reply. Geology, 25, 668-669. - 21 - � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Institute on Lake Superior Geology: Special Publication #1: Field Trip Guidebook for the Slate Islands, Ontario Description An account of the resource Institute on Lake Superior Geology: Special Publication #1: Field Trip Guidebook for the Slate Islands, Ontario Creator An entity primarily responsible for making the resource Institute on Lake Superior Geology Date A point or period of time associated with an event in the lifecycle of the resource 2006 Contributor An entity responsible for making contributions to the resource Pete Hollings Mark Smyk Bill Addison Phil Fralick Format The file format, physical medium, or dimensions of the resource PDF Language A language of the resource English https://digitalcollections.lakeheadu.ca/files/original/fcd597fd04341c5410f6bd4bf5205c36.jpg 5d7f5305c197c3463addab12e3a296c6 https://digitalcollections.lakeheadu.ca/files/original/6431947cdb03ed6c8ffe67b147211b9e.jpg 6885962d98d090f3bbb5f9710181c5ed 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 Ken Armson fonds Subject The topic of the resource Forestry Description An account of the resource A collection of thousands of photographic slides depicting Ken Armson's work in the field of forestry, 1952-1995. Creator An entity primarily responsible for making the resource Ken Armson Date A point or period of time associated with an event in the lifecycle of the resource 1952-1995 Rights Information about rights held in and over the resource These images have been digitized and shared on this site with permission. Most are still under copyright. 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/. Subject The topic of the resource Forest Products Industry Creator An entity primarily responsible for making the resource Ken Armson Date A point or period of time associated with an event in the lifecycle of the resource 1957-06-15 Format The file format, physical medium, or dimensions of the resource JPG Type The nature or genre of the resource Still image Identifier An unambiguous reference to the resource within a given context Armson-Soils-15 Title A name given to the resource Intense Ae and Bh Developed in Old Heathland Now Planted With Spruce Slide Courtesy of J.A. (Steve) Brodie Description An account of the resource Intense Ae and Bh developed in old heathland now planted with spruce slide courtesy of J.A. (Steve) Brodie. Spodosol, P, C, F, D. Coverage The spatial or temporal topic of the resource, the spatial applicability of the resource, or the jurisdiction under which the resource is relevant Czechoslovakia https://digitalcollections.lakeheadu.ca/files/original/60fa851635e79658876744e7a101db0a.jpg 302a545fbf920cee5c0e3ed1a87c48dc 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 Lakehead University Collection Description An account of the resource Photographs from Lakehead University's history: people, events, and campus. 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 Inter provincial Pipeline Company Bursaries Subject The topic of the resource University Life People Description An account of the resource Inter provincial Pipeline Company Bursaries $250 each LU W @ Dr. A. Booth to Robert Breeze, Taina Hordy, Victor Kapanen, Peter Linklater, Frank Pianka, Jawed Shaikh, Peter Siczkar, Brian Somerville, Bruce Staus, Barbara Tamminen A faculty member presenting an award to a student. A group with students and other faculty stand nearby and observe. Creator An entity primarily responsible for making the resource Lakehead University Awards Type The nature or genre of the resource Still Image Identifier An unambiguous reference to the resource within a given context UG6-B-II-78 Awards Barbara Tamminen Brian Somerville Bruce Staus Dr. A. 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Creator An entity primarily responsible for making the resource Northwestern Ontario Development Association Format The file format, physical medium, or dimensions of the resource JPG Type The nature or genre of the resource Still image Identifier An unambiguous reference to the resource within a given context NODA_M-72-2 Coverage The spatial or temporal topic of the resource, the spatial applicability of the resource, or the jurisdiction under which the resource is relevant Canada - Ontario - Thunder Bay Canada - Ontario - Port Arthur https://digitalcollections.lakeheadu.ca/files/original/43f40a3c7b69b815f31d2a12c24603ff.jpg 6849f9b305dcb134e209f4dc7832924d 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 Intercity Shopping Plaza Subject The topic of the resource Business and Industry Communities in Northwestern Ontario Description An account of the resource Intercity Shopping Plaza, aerial photo, Port Arthur, September 1957. Trees along Memorial Avenue are also visible. 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Title A name given to the resource Interior Elevator Company Photograph Subject The topic of the resource Business and Industry Description An account of the resource A black and white photograph taken of the first elevator company owned by N.M Paterson. Description reads “The first prairie elevator company that N.M Patterson owned. This photo, taken around 1918, shows farmers lining up to empty grain wagons outside the Paterson elevator” Rights Information about rights held in and over the resource Public Domain Format The file format, physical medium, or dimensions of the resource JPG Language A language of the resource English Type The nature or genre of the resource Still Image Identifier An unambiguous reference to the resource within a given context General Archives-280e Date A point or period of time associated with an event in the lifecycle of the resource 1918