243 10 13229 https://digitalcollections.lakeheadu.ca/files/original/a3135b02ae42629dc9f91152dd8734c3.pdf ee8b7508494e14facf4b73ce8c98ac45 PDF Text Text 65th Annual Meeting Terrace Bay, Ontario - May 8-9, 2019 Institute on Lake Superior Geology Part 1 – Program and Abstracts �Thank you to our sponsors! Individual contributors to student travel scholarship: Al MacTavish, Mary Kay Arthur, L. Gordon Medaris, Jr., Nick Swanson-Hysell �65th Annual Meeting Institute on Lake Superior Geology May 8-9, 2019 Terrace Bay, Ontario HOSTED BY: Mark Smyk and Pete Hollings Co-Chairs Ontario Geological Survey and Lakehead University Proceedings - Volume 65 Part 1 – Program and Abstracts Compiled and edited by Mark Puumala Cover Photos: Left - Little Pic River Breccia zone, Coldwell Complex, Middle - Toe of pahoehoe flow, Slate Islands. Right - Glacially polished syenite, Coldwell Complex. ��65th Institute on Lake Superior Geology Volume 65 consists of: Part 1: Program and Abstracts Part 2: Field Trip Guidebook Trip 1: The Slate Islands Trip 2: Midcontinent Rift-Related Carbonatites and Diatremes Trip 3: Geology of the Western Schreiber-Hemlo Greenstone Belt Trip 4: Geology of the Nipigon Area Trip 5: A stratigraphic transect across the Northern flank of the Midcontinent Rift near Rossport Trip 6: Geology of the Coldwell alkaline complex Trip 7: Building and ornamental stone sites of the Marathon Area, Ontario Trip 8: Geology of the past-producing Winston Lake Cu-Zn Mine Reference to material in Part 1 should follow the example below: 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, 5-6. Published by the 65th 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 ��Part 1: Program and Abstracts Table of Contents Institutes on Lake Superior Geology, 1955-2019 Sam Goldich and the Goldich Medal iii v Goldich Medal Guidelines vii Goldich Medalists and Goldich Medal Committee ix Citation for Goldich Medal Award to Mark Severson x Honoring the Pioneers of Lake Superior Geology xii Memoriam to Gene L. LaBerge xiii Eisenbrey Student Travel Awards xv Joe Mancuso Student Research Awards xvi Doug Duskin Student Paper Awards and Award Committee xvii Board of Directors and Session Chairs xviii Field Trip Leaders and Guidebook Authors xix Report of the 64th Annual Meeting xx Technical Program xxiii Poster Presentations xxx Abstracts 1-103 ii �Institutes on Lake Superior Geology, 1955-2019 # 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 iii �# 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 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 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 56 2010 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 63 2017 Wawa, Ontario 64 2018 Iron Mountain, Michigan 65 2019 Terrace Bay, Ontario iv 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 �Sam Goldich and the Goldich Medal Sam Goldich received an A.B. from the University of Minnesota in 1929, a M.A. from Syracuse University in 1930, and a Ph.D. from the University of Minnesota in 1936. During World War II Sam worked for the U.S. Geological Survey in mineral exploration. In 1948, Sam returned to the University of Minnesota, and became Professor and Director of the Rock Analysis Laboratory the following year. He rejoined the U.S. Geological Survey in 1959 and was appointed as the first Branch Chief of the Branch of Isotope Geology. Sam returned to academia in 1964 when he went to Pennsylvania State University. He left PSU in 1965 and moved to the State University of New York at Stony Brook, where he stayed for 3 years. Restless yet again, he moved to Northern Illinois University in 1968 where he was a professor until his retirement in 1977. Sam’s final move was to Denver where he became an emeritus at the Colorado School of Mines. Sam died in 2000, less than a month before his 92nd birthday. In the late 1970’s, Geological Society of America Special Paper 182, which included seminal geochronological studies by Sam Goldich and coworkers on the Archean rocks of the Minnesota River Valley, was nearing completion. At this time various ILSG regulars began discussing the possibility of recognizing Sam for his pioneering work on the resolution of age relationships and thus the geology of Precambrian rocks in the Lake Superior region. Three members, R.W. Ojakangas, J.O. Kalliokoski and G.B. Morey, presented the idea to the ILSG Board of Directors in 1978. The Board approved the creation of an award, provided funding could be obtained. It was suggested that collecting one or two dollars at registration for a dedicated account would provide resources for striking the medal. A general request was made to the ILSG membership for donations and Sam himself offered a challenge grant to match the contributions. In total $4,000 was collected and thus began the work of creating the Goldich Medal. The initial Goldich Award was presented to Sam by G.B. Morey in 1979 and consisted of a large paper proclamation. For the actual medal, G.B. Morey consulted with the foundry on production details, while Dick Ojakangas and Jorma Kalliokoski worked on the design of the award, suggesting that it be given for “outstanding contributions to the geology of the Lake Superior region.” Simultaneously, a committee of J.O. Kalliokosi, W.F. Cannon, M.M Kehlenbeck, G.B. Morey, and G. Mursky developed the Award Guidelines that were approved by the ILSG Board. By 1981 all the elements of the Goldich Award had come together, and the second recipient, Carl E. Dutton, Jr., received the Goldich Medal for 50 years of significant contributions to the understanding of the geology of the Lake Superior region. Since the beginning, the Awards Committee has consisted of individuals representing industry, government and academia, with each member of the Committee serving for three years. The medal is now awarded every year at the annual ILSG meeting. Reference: Morey, G.B. and Hanson, G.N. (editors). 1980. Selected studies of Archean gneisses and Lower Proterozoic rocks, southern Canadian Shield. Geological Society of America, Special Paper 182, 175 p. Prepared by various Goldich Medal Awardees, 2007 v �INSTITUTE ON LAKE SUPERIOR GEOLOGY GOLDICH MEDAL vi �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. vii �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. viii �Goldich Medalists 1979 Samuel S. Goldich 1998 Zell Peterman 2016 Mark A. Jirsa 1980 not awarded 1999 Tsu-Ming Han 2017 Philip Fralick 1981 Carl E. Dutton, Jr 2000 John C. Green 2018 Val W. Chandler 1982 Ralph W. Marsden 2001 John S. Klasner 1983 Burton Boyum 2002 Ernest K. Lehmann 1984 Richard W. Ojakangas 2003 Klaus J. Schulz 1985 Paul K. Sims 2004 Paul Weiblen 1986 G.B. Morey 2005 Mark Smyk 1987 Henry H. Halls 2006 Michael G. Mudrey 1988 Walter S. White 2007 Joseph Mancuso 1989 Jorma Kalliokoski 2008 Theodore J. Bornhorst 1990 Kenneth C. Card 2009 L. Gordon Medaris, Jr 1991 William Hinze 2010 William D. Addison & 1992 William F. Cannon Gregory R. Brumpton 1993 Donald W. Davis 2011 Dean M. Rossell 1994 Cedric Iverson 2012 James D. Miller 1995 Gene La Berge 2013 Tom Waggoner 1996 David L. Southwick 2014 Laurel Woodruff 1997 Ronald P. Sage 2015 Rodney J. Ikola 2019 GOLDICH MEDAL RECIPIENT Mark Severson Goldich Medal Committee Serving through the meeting year shown in parentheses. Klaus Schultz (2016-2019) U. S. Geological Survey Dan England (2017-2020) Eveleth Fee Office Steve Kissin (2018-2021) Lakehead University ix �Citation for the Goldich Medal Recipient to Mark Severson ILSG Members, Goldich Medal recipients and guests, it is my honor to present the citation for this year’s recipient of the Goldich Medal, Mark J. Severson. Mark J. Severson has made significant contributions to understanding a vast number or topics associated with the geology of the Lake Superior region during his 30+ year career. Few can say they have contributed to the ILSG as a student, as an industry geologist, as an academic, as a thesis advisor, and as a teacher. In fact, Mark’s contributions to understanding Lake Superior Geology fill at least 15 pages of a Google Scholar search! He exemplifies the essence of an “Institute on Lake Superior Geology” geologist, possessing both exceptional field skills and extraordinary lab skills which have enabled him to conduct comprehensive, high quality scientific research. Mark also possesses the rare skills that allow him to communicate complicated geological features, models, and stories to professionals, students and the public in a way that teaches them (and even more importantly, gets them excited about) the amazing geology of the Lake Superior region and the countless geological wonders in their backyards. Mark’s research in the Lake Superior region began in the mid-1970s after obtaining his Bachelor of Sciences degree in Geology at Western Illinois University. In 1978, he was awarded his Master’s Degree in Geology at the University of Minnesota Duluth, studying the “Petrology and Sedimentation of Early Precambrian Graywackes in the Eastern Vermilion District, Northeastern Minnesota” under the advisement of (at that time) future Goldich Medal awardee Dr. Richard Ojakangas. After stints as an exploration geologist searching for base metals, gold and uranium with US Steel and Santa Fe Pacific Mining, Mark started a distinguished 25-year long career with the Economic Geology Group at the Natural Resources Research Institute (NRRI) at the University of Minnesota Duluth (UMD). While at the NRRI, Mark established himself as one of the leading economic geologists in the Lake Superior region, producing nearly 40 NRRI technical reports, eight geologic maps, and numerous peer-reviewed journal and public poster presentations. The geologic topics covered in this work are diverse, and include: • • • Performing a wide variety of research associated with the igneous stratigraphy, coppernickel-platinum group element and titanium mineralization in the Duluth Complex (which included logging of over 1 million feet of Duluth Complex drill core and the production of 10 NRRI Technical Reports, 1 NRRI geologic map, and 11 peer reviewed journal publications): Completing substantial research evaluating sedimentary environments, mineralization, and the stratigraphy of the Biwabik Iron Formation, culminating in NRRI Technical Report NRRI/TR-2009/09, where he established the “Rosetta Stone” for interpreting the stratigraphy of the Biwabik Iron Formation; Writing numerous technical reports describing SEDEX-type mineralization in Carleton County and the Cuyuna District of eastern and east-central Minnesota, respectively; x �• • • • • • Producing an NRRI technical report describing the history of gold exploration in Minnesota; Completing an NRRI technical report explaining metallic exploration, mining, and processing permits in Minnesota; Co-authoring a significant NRRI technical report which describes rare earth element (REE) mineral potential across Minnesota; Developing technical reports describing clay deposits in the Minnesota River Valley; Producing the most detailed heat flow maps available for the State of Minnesota; and Co-authoring a federally-funded report describing possibilities for the development of pumped-hydro energy storage systems in legacy iron-mining landscapes in northeastern Minnesota; During his time at the NRRI, Mark contributed to the education of undergraduate and graduate students, as well as teachers through his efforts as an Adjunct Professor in the Department of Geology at the University of Minnesota Duluth, as an instructor for the Precambrian Research Center geology field camp, and via the Minnesota Minerals Education Workshop. Throughout his career, Mark collaborated on numerous projects with the Minnesota Geological Survey (MGS). This included co-authoring three Open File Reports (maps and reports) about Duluth Complex mineralization, as well as a significant Report of Investigation which describes the geology and mineral potential of the Duluth Complex and related rocks. It is important to note that these MGS publications were co-authored with Goldich Medal awardees John Green, Jim Miller, Mark Jirsa, and Val Chandler. Since 2013, Mark has worked (and is now “semi-retired”) as a Senior Geologist for Teck American, where he continued to define Cu-Ni resources at the Mesaba Deposit in NE Minnesota. Despite his “semi-retired” status, Mark continues to make significant contributions to understanding Lake Superior geology in his role as Vice President for the Mesabi Range Geological Society. Mark’s contributions to the ILSG since 1989 include authoring or co-authoring 22 abstracts and seven field trip guidebooks, serving as a session chair, and serving on student paper committees over the course of at least 20 ILSG meetings since 1989. It is worth noting that Mark served as the co-chair with Steve Hauck for the 50th Annual ILSG meeting that took place in Duluth in 2004. As well, Mark has undoubtedly increased the knowledge of those attending the many ILSG field trips that he participated in over the past 29 years. All of us who have known and worked with Mark know of his passion for the geology. His significant contributions to understand and teach about the spectacular and diverse geology of the Lake Superior region, as well as his significant contributions to the Institute on Lake Superior Geology have all been accomplished with the highest level of professionalism and distinction. Please join me in congratulating Mark J. Severson as the 2019 recipient of the Goldich Medal from the Institute on Lake Superior Geology. Submitted by George J. Hudak Director, Minerals-Metallurgy-Mining Initiative Natural Resources Research Institute, UMD xi �Honoring the Pioneers of Lake Superior Geology (Adopted by the Board of Directors, 2016) Preamble At the suggestion of Gene LaBerge, the 2016 executive board agreed to implement a program to recognize historic pioneers in the understanding of geology in the Lake Superior region. Beginning with the 2017 annual meeting, nominations will be accepted from the membership for geologists whose work was conducted primarily before inception of the institute in 1955. Biographical sketches of those pioneers will be presented at future annual meetings so that all might appreciate the value of their contributions. Selection of nominees will be decided in part by the organizing committee of each year's annual meeting, in consultation with the Board, to ensure equitable geographic representation in the selection process. Award Guidelines 1) Nominations from the membership will be submitted via the Institute web site and forwarded to the Chair of the next Annual Meeting. The nominations will be no more than half a page in length and will summarise the contribution of the nominee. 2) The Organising Committee will select one or two individuals to be highlighted at the next Annual meeting and submit those names to the Board for approval. 3) The nominator will be requested to prepare a brief presentation to be given during the next annual meeting with a summary to be included in the Proceedings volume. 4) Unsuccessful nominations will be kept by the Secretary for two years and forwarded to the next meeting Chair; these nominations may be resubmitted at a later date. The Board will review this award every five years. Pioneers of Lake Superior Geology 2017 Douglass Houghton (1809-1845) 2018-19 not presented xii �In Memoriam Gene L. LaBerge This winter the Institute on Lake Superior Geology, its members, and countless others lost a dedicated geologist, and outstanding teacher, mentor, colleague, and friend-Gene LaBerge. Gene’s contributions to the geology of the Lake Superior region and the study of iron-formation have been many and impactful. He will be greatly missed, not only for his scientific contributions, but also for his good humor, wise council, and generous nature. Gene was a product of the Northwoods, born and raised in Ladysmith, Wisconsin, the eventual home of the Flambeau copper-gold mine in the mid-1990s. After serving a stint in the U.S, Marine Corp during the Korean War, he went on to study geology, obtaining his B.S., M.S., and Ph.D. degrees all at the University of Wisconsin-Madison. While a graduate student in 1957, he was hired by Ralph Marsden, head of exploration for U.S. Steel, to explore for iron-formation in northern Michigan. This, along with field trips around the region led by Ralph and Stan Tyler of UW-Madison, sparked his interest in iron-formation and the geology of the Lake Superior region. While he was working to finish his Ph.D. research, his advisor, Stan Tyler, suddenly died. Fortunately, Ralph Marsden was able to step in, allowing Gene to finish and receive his Ph.D. His dissertation was on the origin of magnetite in iron-formation. After graduate school, he continued his study of iron-formation accepting a post-doctoral fellowship in Adelaide, Australia where his new bride, Sally, had received a Fulbright Scholarship. During this post-doc, he also spent several months in South Africa. After a year in Australia, Gene accepted a second postdoctoral fellowship, this time with the Geological Survey of Canada in Ottawa. Some of the samples of iron-formation he collected for his studies he subsequently used to build a beautiful fireplace in his house in Omro, Wisconsin. In 1965, Gene joined the faculty at UW-Oshkosh as the third member of the then expanding Geology Department. It would remain his home for all his career. Early on, he and Sally would spend weekends driving the back roads of the Lake Superior region looking for outcrops and planning field trips. Gene’s passion for geology was clearly expressed through the many (>100) overnight field trips he led during his 33-year teaching career. While still a graduate student, he had helped conduct a pebble survey in northern Wisconsin for U.S. Steel, identifying thousands and thousands of pebbles in gravel pits across the region. This experience led him to devise one of his classic (or infamous) student exams-the pebble test- where students had to identify the rock type and mineralogy of small pebbles using only a hand lens. Gene often said that “a rock or mineral was much easier to identify if you had seen it before”. To reinforce that, students in his mineralogy and lithology classes learned to identify not only the most common minerals and rocks but also many less common ones, particularly those important in exploration for certain types of mineral deposits. His classes were always rigorous, comprehensive, and taught with infectious enthusiasm. Gene retired from teaching in 1998. Over his teaching career, Gene xiii �received all the teaching and research awards offered by UW-Oshkosh, the only faculty member to have done so. In the late 1960s, on the advice of Carl Dutton, Gene began mapping the geology around Wausau in Marathon County, Wisconsin for the Wisconsin Geological and Natural History Survey (WGNHS). The project eventually grew to include mapping all of Marathon County in collaboration with Paul Myers of UW-Eau Claire. In 1983, Gene began working part-time for the U.S. Geological Survey (USGS), an association he would maintain both formally and informally for the rest of his career. Much of the work he did for the USGS was done collaboratively with his good friend and colleague John Klasner of Western Illinois University. During his work for the WGNHS and USGS, Gene probably walked over more of Wisconsin and northern Michigan than anyone ever has. Along with authoring many technical journal articles, book chapters, and WGNHS and USGS publications, he used his in-depth knowledge of the geology of the Lake Superior region to write a book for non-specialists, Geology of the Lake Superior Region, first published in 1994. Gene was an active and long-time member of the Institute on Lake Superior Geology (ILSG), giving his first presentation at the1958 meeting in Duluth, only the fourth meeting of the Institute. He went on to give presentations at many more ILSG meetings, served as Chair for two meetings (1969 and 1984), and led field trips for several meetings. Gene received the Goldich Award in 1995 for his significant contributions to the geology of the Lake Superior region and the ILSG. Gene also had other interests and pursuits. He was an avid mineral collector, building a worldclass collection whose centerpiece was gem-quality tourmalines from locations around the world. The collection was effectively displayed in his house in cases he designed and build himself. The collection was eventually sold (a hard decision resulting from having to move and downsize), but not before he had his three daughters select their favorite specimens. In mid-life, when some go out and buy a new sports car or take up sky diving, Gene took up playing the guitar, a talent he found useful for all those nights sitting around a campfire on field trips. In 1999, he published a book with the help of his daughter Michelle, Travels with Sophie, chronicling the experiences of his mother, Louise, while she served as the first supervising teacher of Rusk County, Wisconsin in 1917-1918. Sophie was the name of the Model T Ford his mother used to travel between more than 100 one-room schools in the county at the time. Gene also recently completed a book with George Robinson, curator of the Seaman Mineral Museum at Michigan Technological University, on the minerals in iron ores, Minerals in the Iron Ores of the Lake Superior Region. Gene led an active and productive life that touched and influenced many people through his research, teaching, writing, and public out-reach. He is survived by his wife, Sally, and daughters Michelle, Rene, and Laura and their families. Gene’s passing leaves a hole in the fabric of the lives of all who knew him and called him a friend. Klaus J. Schulz xiv �Eisenbrey Student Travel Awards The 1986 Board of Directors established the ILSG Student Travel Awards to support student participation at the annual meeting of the Institute. The name “Eisenbrey” was added to the award in 1998 to honor Edward H. Eisenbrey (1926-1985) and utilize substantial contributions made to the 1996 Institute meeting in his name. “Ned” Eisenbrey is credited with discovery of significant volcanogenic massive sulfide deposits in Wisconsin, but his scope was much broader—he has been described as having unique talents as an ore finder, geologist, and teacher. These awards are intended to help defray some of the direct travel costs of attending Institute meetings, and include a waiver of registration fees, but exclude expenses for meals, lodging, and field trip registration. The 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. xv �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 2018, the ILSG Board of Directors selected two students to be granted research funding of $750.00 each from the Joe Mancuso Student Research Fund. The awardees were: Jacqueline L. Drazan University of Minnesota-Duluth, MSc, Department of Earth and Environmental Sciences, draza004@d.umn.edu TOPIC: Morphological and Geochemical Comparison between Archean Marine Peperites (Fivemile Lake, MN) and Pleistocene Freshwater Peperites (Sveifluhals, Iceland) Thomas Bodden Michigan Technological University, MSc, Department of Geological and Mining Engineering and Sciences, tjbodden@mtu.edu TOPIC: Stable isotopic composition of calcite precipitated with native copper and other minerals of the Keweenaw Peninsula, Michigan xvi �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. 2019 Student Paper Awards Committee Katarina Bjorkman – Bjorkman Prospecting George Hudak – Natural Resources Resarch Institute–UMD David Good – Western University xvii �Board of Directors Board appointment continues through the close of the meeting year shown in parentheses, or until a successor is selected Esther Stewart (2018-2021) – Wisconsin Geological & Natural History Survey Anthony Pace (2017-2020) – Ontario Geological Survey Christian Schardt (2016-2019) – University of Minnesota Duluth Pete Hollings - Secretary (2016-2019) – Lakehead University Mark Jirsa – Treasurer (2017-2020) – Minnesota Geological Survey Session Chairs Ben Drenth- United States Geological Survey Dan England – Eveleth Fee Office Mary Louse Hill - Lakehead University Amy Radakovich- Minnesota Geological Survey Nicholas Swanson-Hysell – University of California, Berkeley Laurel Woodruff – United States Geological Survey Michael Zieg – Slippery Rock University Shannon Zurevinski – Lakehead University xviii �Field Trip Leaders and Guidebook Authors Field trips have been the mainstay of the ILSG since its inception 65 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) The Slate Islands Pete Hollings – Lakehead University Mark Smyk – Ontario Geological Survey Bill Addison and Philip Fralick – Lakehead University 2) Midcontinent Rift-related carbonatites and diatremes Shannon Zurevinski – Lakehead University Dorothy Campbell and Mark Puumala – Ontario Geological Survey 3) Geology of the western Schreiber-Hemlo greenstone belt Seamus Magnus – Ontario Geological Survey 4) Geology of the Nipigon area Philip Fralick – Lakehead University Robert Cundari – Ontario Geological Survey 5) A stratigraphic transect across the northern flank of the Midcontinent Rift near Rossport Pete Hollings and Philip Fralick – Lakehead University 6) Geology of the Coldwell alkaline complex Allan MacTavish – Panoramic PGMs (Canada) Limited Mark Smyk – Ontario Geological Survey David Good – Western University John McBride – Stillwater Canada Inc. 7) Building and ornamental stone sites of the Marathon area, Ontario Peter Hinz – Ministry of Energy, Northern Development and Mines 8) Geology of the past-producing Winston Lake Cu-Zn Mine Robert Lodge – University of Wisconsin-Eau Claire Mark Smyk and Mark Puumala – Ontario Geological Survey xix �REPORT OF THE 64th ANNUAL MEETING OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY IRON MOUNTAIN, MICHIGAN The U.S. Geological Survey with assistance from the Wisconsin Geological and Natural History Survey hosted the 64th Annual Institute on Lake Superior Geology on May 15 – 18, 2018 at the Pine Mountain Resort in Iron Mountain, Michigan. The meeting consisted of two days of technical sessions with pre- and post-technical session field trips. Laurel Woodruff (USGS), Bill Cannon (USGS), and Esther Stewart (WGNHS) were co-chairs for the 2018 meeting. Tom Mroz and Tom Waggoner helped with pre-meeting logistics. Darlene Comfort and Ted Bornhorst (A.E. Seaman Mineral Museum, Michigan Technological University) handled all pre-meeting registration and printing needs. Ted also supplied the poster boards and helped with many aspects of the meeting. Mary Kay Arthur and Dave Wilhelm (Geological Society of Minnesota) provided valuable logistical assistance on-site at Pine Mountain during the technical sessions. Connie Dicken (USGS) was the media czar for the technical sessions, keeping all presentations on track with fewer glitches than normal. Generous contributions to the ILSG general fund and in support of 2018 student travel scholarships came from Lundin Mining, the Geological Society of Minnesota, Ron Seavoy, Mary Kay Arthur, L. Gordon Medaris, Jr., and Steven Baumann. Total meeting registration was 189 (37 students), an excellent turn-out. Proceedings Volume 64 was published in two parts: Part 1 – Program and Abstracts, edited by Esther Stewart, contains 62 published abstracts for 34 oral and 28 poster presentations; Part 2 – Field Trip Guidebooks, compiled by Bill Cannon, contains descriptions of four field trips, two pre-meeting and two post-meeting. The 64th ILSG marked the second time in its long history that the annual meeting was held in Iron Mountain. The prior meeting was in 2003. Field trips visited two areas new to the ILSG and two trips that provided new stops in areas of prior trips. On Tuesday, May 15, Bill Cannon, Klaus Schulz, Robert Ayuso, and Tom Mroz led a field trip of 46 people to examine the stratigraphy, structure and economic geology of regional Precambrian rocks in the Felch District, Central Dickinson County, Michigan. Also, on Tuesday, Tom Waggoner was the leader of 37 people for a trip that looked at the Paleoproterozoic Hemlock Formation. Most stops of these two trips were to locations and geology new to ILSG attendees. On Friday, May 18 Tom Mroz and Bill Cannon shepherded a large crowd of 56 somewhat wandering individuals on a field trip that looked at the geology of the Menominee Iron Range. This trip had little overlap with a trip of a similar title that was given in 2003 as it included a visit to the Archean Carney Lake Gneiss, newly recognized as containing zircons with cores as old as 3.8 Ga, and an underground tour of the Iron Mountain Iron Mine. Another Friday trip led by Klaus Schulz, with a contribution from Marcia Bjørnerud, examined the granitoid rocks of the PembineWausau terrane in northern Wisconsin. In addition to examining granite, the 30 people on that trip had an additional task of keeping Klaus out of jail when he was caught looking at an outcrop along a railroad line. One hundred and fifteen participants attended the annual ILSG banquet on Wednesday night, cheerfully bringing chairs from the technical session room into the banquet room. After an excellent dessert, everyone moved their chairs back to the technical session room. The 2018 Homer award was given to Pete Hollings for his confidence in the appropriate vehicles one needs to travel around Iceland. Al McTavish, fresh off the success of last year’s Iceland trip, despite the Land xx �Rovers, gave a short promotional presentation for a proposed 2019 trip to another volcanic island, Hawaii. As always, a highlight of the post-banquet activities was presentation of the 2018 Goldich Medal. This year’s very deserving recipient was Val Chandler of the Minnesota Geological Survey (MGS). Val’s wife and three adult children were all able to attend the banquet and award ceremony. The Goldich was presented to Val by David Southwick, Director Emeritus of the MGS and his colleague for many years. Dave’s citation described Val’s many professional contributions to the geophysical mapping and interpretation of Minnesota’s mostly hidden geology. Val’s long history with the ILSG started with a field trip when he was fresh out of Purdue, which serendipitously led to his distinguished career with the MGS. This year’s banquet speaker was Nancy Langston, a professor in the Department of Social Sciences at Michigan Technological University. Dr. Langston (or Nancy, as we all called her) gave a presentation that drew on her recent book titled Sustaining Lake Superior: An extraordinary lake in a changing world. The presentation described past, present, and future environmental challenges to Lake Superior, such as logging, Reserve Mining taconite disposal, and climate change. We all were encouraged by Nancy’s final optimism that with responsible stewardship, the largest freshwater lake in the world will endure. In 2018, the student paper committee had its usual difficult job of selecting the best among 7 excellent oral presentations and 16 excellent poster presentations for the Doug Duskin Student Paper Awards. This year’s committee included Robert Cundari (Ontario Geological Survey), Esther Stewart (WGHNS), performing double duty along with her co-chair responsibilities, and Latisha Brengman (University of Minnesota – Duluth). In the end, there was a three-way tie for first place. Poster awards ($300 each) were awarded to Samuel Hone (Slippery Rock University) for his poster titled: Olivine crystal size distribution in the Black Sturgeon Sill, Nipigon, Ontario, and William Fitzpatrick (University of Wisconsin- Eau Claire) for his poster titled: Mineral chemistries of the Tower Mountain Intrusive Complex Au-deposit, Ontario. Kira Arnold (Lakehead University) was recognized for her oral presentation titled: Geology and geochemistry of the Terrace Bay Batholith, N. Ontario ($400). Eisenbrey Student Travel Grants were given to 19 students: Daniel Wilkes, Emily Gorner, Kira Arnold, Vittoria Smith, and Simon Dolega – all from Lakehead University; Schuyler Borages, Erica Craddock, Ryan Leonard, Walter Johnson-Geis, Lily Atkinson, and Juliana Olsen-Valdez, all from Lawrence University; Jacqueline Drazan, Margaret Upton, and Matthew Matko, from the University of Minnesota-Duluth; Victoria Stinson, University of Saskatoon; Dustin Liikane, University of Toronto, Katharine Rose and Kevin Rupp, both from Western Michigan University, and Joseph Rasmussen, University of Wisconsin-Platteville. The Institute’s Board of Directors met on May 16, 2018 and a brief overview of the meeting is provided below: 1. Accepted the Report of the Chair for the 63rd ILSG from Ted Bornhorst and minutes of the last Board meeting from ILSG secretary, Pete Hollings. 2. Accepted the 2017-2018 ILSG Financial Summary from ILSG treasurer, Mark Jirsa. 3. Approved one co-chair from the 64th annual meeting, Esther Stewart, as the on-going board member. xxi �4. Nominated Steve Kissin from Lakehead University to replace Shannon Zurevinski on the Goldich Committee. 5. Approved Terrace Bay, Ontario as the location for the 2019 ILSG annual meeting with cochairs Pete Hollings and Mark Smyk. The 64th ILSG meeting was a great success and we wish to thank all the people who contributed to that success. The staff of Pine Mountain was professional and responsive to the needs of a large group – plenty of excellent donuts. The weather was perfect, not too hot, not too cold, not rainy, not buggy. The field trips this year had many participants, and thanks are due to field trip leaders, intrepid bus drivers, those who drove support vehicles on field trips and handled each trip’s logistics, as well as everyone else who stepped up when needed. As always, everyone who attended the 64th ILSG was willing to help as necessary and to adapt to any situation that developed. The meeting this year was well attended, and we are heartened by the excellent student participation and attendance, a trend we hope continues. Your co-chairs are very pleased with the final outcomes of the 64th ILSG. Organizing a meeting and compiling the two Proceeding volumes requires a significant time commitment from the cochairs and others, and we thank our respective organizations for their recognition of the importance of the ILSG. We also thank the ILSG community and members who make the experiences of the co-chairs almost fun, especially once the meeting is over, and we encourage others to take on the task. Laurel Woodruff, Bill Cannon, and Esther Stewart Co-Chairs, 64th Institute on Lake Superior Geology xxii �TECHNICAL PROGRAM TUESDAY MAY 7, 2019 All field trips begin and end at the Terrace Bay Cultural Centre 8:00 am - 5:00 pm PRE-MEETING FIELD TRIPS 1) The Slate Islands Pete Hollings – Lakehead University 2) Midcontinent Rift-related carbonatites and diatremes Shannon Zurevinski – Lakehead University 3) Geology of the western Schreiber-Hemlo greenstone belt Seamus Magnus – Ontario Geological Survey 4) Geology of the Nipigon area Philip Fralick – Lakehead University 4:00 pm - 10:00 pm Registration (Terrace Bay Cultural Centre) 7:00 pm - 10:00 pm Welcoming Reception and Poster Session (Terrace Bay Cultural Centre) xxiii �WEDNESDAY MAY 8, 2019 8:00 am – 11:30 am Registration (Terrace Bay Cultural Centre) 8:30 OPENING REMARKS (Terrace Bay Cultural Centre) Pete Hollings and Mark Smyk, Co-Chairs, 2019 ILSG TECHNICAL SESSION I Session Chairs: Shannon Zurevinski – Lakehead University Michael Zieg – Slippery Rock 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. 8:40 Brigitte Gelinas, +Pete Hollings and Richard Friedman Geology and geochemistry of the Laird Lake property and associated gold mineralization, Red Lake greenstone belt, Ontario 9:00 *Munira Afroz, Philip Fralick, Brian Killingsworth, Martin Homann, Pierre Sansjofre and Stefan Lalonde Sulfur, Carbon, and Oxygen Isotope Geochemistry of ~2.93 Ga Mesoarchean Chemical Sedimentary rocks in the Red Lake Area, Ontario 9:20 *Brittany Ramsay, Philip Fralick, Paul Bielski, Martin Homann, Pierre Sansjofre and Stefan Lalonde Mesoarchean chemical sedimentary rocks of northwestern Ontario: Implications for hydrosphere composition in deep time 9:40 Brad Gottschalk and Caroline Rose Recent efforts to curate and provide access to the historical documents of the E.K. Lehmann and Associates Exploration Company 10:00 COFFEE BREAK 10:20 William F. Cannon, Klaus J. Schulz and Benjamin J. Drenth The Dickinson Group in the Central Upper Peninsula of Michigan: Part 1- Age and tectonic setting based on new geophysical, geochronological, and geochemical data xxiv �10:40 Benjamin J. Drenth, William F. Cannon and Klaus J. Schulz The Dickinson Group in the central Upper Peninsula of Michigan: Part 2Geophysical expression and a preliminary interpretation of its eastward extent under Paleozoic cover 11:00 Ryan Clark, David Peate, Alison Kusick, Kenny Horkley and Raymond Anderson Reexamining the Osborne core for new insights into the age and petrology of the Northeast Iowa Intrusive Complex (NEIIC) 11:20 Wouter Bleeker, Michael Hamilton, Sandra Kamo, Dustin Liikane, Jennifer Smith, Pete Hollings, Robert Cundari, Michael Easton and Don Davis High-resolution dating of the magmatic plumbing system of the Midcontinent Rift System—Insights into rift evolution and mineralization processes 11:40 End of Technical Session I 11:40 LUNCH BREAK – BUFFET PROVIDED ILSG BOARD OF DIRECTORS MEETING TECHNICAL SESSION II Session Chairs: Dan England – Eveleth Fee Office Laurel Woodruff – United States Geological Survey 1:00 Mark Puumala Using graphitic sedimentary rock geochemistry as an indicator of gold potential in the Shebandowan greenstone belt, northwestern Ontario 1:20 *Chanelle Boucher and Pete Hollings Geology and geochemistry of ultramafic rocks in the Lake of the Woods area 1:40 *Kira Arnold, Pete Hollings, Seamus Magnus, Shannon Zurevinski and Robert Creaser Geology and geochemistry of the Terrace Bay Batholith, N. Ontario 2:00 David Holder, Francois Robert and John Hay Geological characteristics and structural controls of Au mineralisation at the enigmatic Hemlo deposit 2:20 COFFEE BREAK xxv �2:40 Paul A. Bedrosian Multi-scale AEM and MT mapping of the Precambrian in Upper Michigan, northern Wisconsin, and eastern Minnesota 3:00 John McBride, David Good, D. Hollis and N. Arndt Pilot study: Using ambient noise passive seismic surveys for Ni-Cu-PGE mineral exploration at the Marathon PGM-Cu deposit, Marathon, Ontario 3:20 Dave Good, Pete Hollings and Andrew Jedemann Recognizing MCR magmas generated by partial melting in the SCLM: Lessons from mafic magmas in the Coldwell Complex 3:40 Ross Sherlock and Kate Rubingh Geologic architecture and precious metal mineralization in the southern Abitibi; new insights from the Larder Lake area 4:00 POSTER VIEWING - AUTHORS WILL BE PRESENT AT THEIR POSTERS 5:00 END OF TECHNICAL SESSION II 6:00 RECEPTION AND CASH BAR (Terrace Bay Cultural Centre) 7:00 ANNUAL BANQUET (Terrace Bay Cultural Centre) • Announcement of 66th Annual Meeting Location • 2019 Goldich Award Presentation to Mark Severson xxvi �THURSDAY MAY 9, 2019 8:30 INTRODUCTORY REMARKS AND UPDATES (Terrace Bay Cultural Centre) Pete Hollings and Mark Smyk, Co-Chairs, 2019 ILSG TECHNICAL SESSION III Session Chairs: Mary Louise Hill – Lakehead University Nicholas Swanson-Hysell – University of California-Berkeley 8:40 Wouter Bleeker, +Sandra Kamo, Michael Hamilton and K. Chamberlain New age data and insights into the ca. 1887-1870 Ma Circum-Superior Belt, with startling implications for the Lake Superior area geology 9:00 Robert Michael Easton What do detrital zircon studies of the Huronian Supergroup tell us? an analysis of all published data 9:20 *Sophie Kurucz, Philip Fralick, Stefan Lalonde and Martin Homann Paleoproterozoic snowball earth? Sedimentology and geochemistry of a Huronian glacial cycle 9:40 L.G. Medaris Jr., D.H. Malone, G.C. Hill, B.S. Singer, B.R. Jicha, A. Van Lankvelt, M.L. Williams and P.W. Reiners The Wolf River Orogeny: Geon 14 magmatism, sedimentation, and deformation in the southern Lake Superior region 10:00 COFFEE BREAK 10:20 Jim Miller The importance of “tablesetting” intrusions in creating economic Ni-Cu-PGE deposits in the Midcontinent Rift 10:40 Robert Nowak, Espree Essig and Robert Mahin Geochemical vectoring towards a serpentinized peridotite chonolith, Eagle East NiCu-Co-PGE deposit, Upper Peninsula, Michigan 11:00 Jennifer Smith, Wouter Bleeker, Mike Hamilton, and Duane Petts An investigation into the distribution of chalcophile elements and timing of mineralization within the Crystal Lake intrusion: A U-Pb geochronology and LAICP-MS study 11:20 Jack Gibbons, Tamara Diedrich and Thomas Quigley Petrography of several cobalt-enriched samples from the Atikokan River Intrusions, Atikokan, Ontario xxvii �11:40 End of Technical Session III 11:40 LUNCH BREAK – BUFFET PROVIDED TECHNICAL SESSION IV Session Chairs: Amy Radakovich – Minnesota Geological Survey Ben Drenth – United States Geological Survey 1:00 Thomas W. Buchholz, Alexander U. Falster and Wm. B. Simmons Updated mineralogy of a roadside pegmatite in the Stettin Complex, Wausau Syenite Complex, Marathon County, Wisconsin 1:20 *Paul Bielski and Philip Fralick LA-ICP-MS micro-sampling of iron formation: what it can tell us 1:40 Tamara Diedrich and Stephen Day Neutralization of proton acidity with sequestration of atmospheric CO2 during experimental weathering of intrusive rocks from the Midcontinent Rift System 2:00 Carson G. Prichard, +James J. Student, Jory L. Jonas, Nicole M. Watson and Kevin M. Pangle Catchment geology correlation with fish otolith microchemistry across disparate glacial till depths in the Lake Michigan basin 2:20 COFFEE BREAK 2:40 J.M. DeGraff, C.W. Tyrell, G.E. Hubbell and B.T. Carter Keweenaw Fault system along Bête Grise Bay, Michigan: geometry, kinematics, and tectonic significance 3:00 Esther K. Stewart, V.J.S. Grauch, Laurel G. Woodruff and Samuel Heller Seismic stratigraphy of the 1.1 Ga Midcontinent Rift beneath western Lake Superior shows evidence of differing subsidence histories for syn-magmatic sub-basins 3:20 V.J.S Grauch, Esther K. Stewart, Laurel G. Woodruff and Samuel Heller Evaluating Alternate Geophysical Models along the Isle Royale-Superior Shoal Aeromagnetic Anomaly, Central Lake Superior 3:40 Nicholas L. Swanson-Hysell Insights into Midcontinent Rift development resulting from a strengthened chronostratigraphic framework xxviii �4:20 BEST STUDENT PAPER AWARDS STUDENT TRAVEL AWARDS CLOSING REMARKS 4:40 END OF TECHNICAL SESSIONS FRIDAY MAY 10, 2019 8:00am – 5:00pm POST-MEETING FIELD TRIPS Field trips 5 to 8 begin and end at the Terrace Bay Cultural Centre 5) A stratigraphic transect across the northern flank of the Midcontinent Rift near Rossport Pete Hollings – Lakehead University 6) Geology of the Coldwell alkaline complex Allan MacTavish – Panoramic PGMs (Canada) Limited David Good – Western University 7) Building and ornamental stone sites of the Marathon Area, Ontario Peter Hinz – Ministry of Energy, Northern Development and Mines 8) Geology of the past-producing Winston Lake Cu-Zn Mine Mark Puumala and Mark Smyk – Ontario Geological Survey xxix �POSTER PRESENTATIONS *Thomas J. Bodden, Theodore J. Bornhorst, Florence Begue and Chad Deering Stable isotopic composition of calcite precipitated with native copper and other minerals of the Keweenaw Peninsula, Michigan Terrence J. Boerboom Recognition of probable distal ejecta from the 1850 Ma Sudbury meteorite impact event along the southern edge of the Animikie basin in Minnesota J.M. DeGraff and I.S. DeGraff Southwest Margin of the Midcontinent Rift System in Eastern Lake Superior: Review and Preliminary Interpretation *Jacqueline L. Drazan, George Hudak and Howard Mooers Morphology, mineralogy, texture, and genesis of peperite, Fivemile Lake, Vermilion District, Minnesota: Comparison with Pleistocene peperite, Iceland Benjamin J. Drenth, William F. Cannon and Klaus J. Schulz High-resolution aeromagnetic survey, central Upper Peninsula, Michigan Don Elsenheimer, Cari Deyell-Wurst and Lionel C. Fonteneau Hyperspectral Imaging of Bedrock Core from the Minnesota DNR Drill Core Library: A New Tool for Archival Preservation and Mineral Exploration V.J.S. Grauch and K.J. Schulz Superior Shoal revisited: Evidence for Keweenawan basalts with reversed- and normalpolarity remanent magnetization and early magma chemistry, central Lake Superior Linnea L. Johnson, David H. Malone and John P. Craddock Detrital Zircon Geochronology of Keweenaw Interflow Sediments within the North Shore Volcanic Group, Minnesota, U.S.A. Seamus Magnus Precambrian Geology of the Western Schreiber–Hemlo Greenstone Belt Amy Radakovich, Val Chandler and Mark Jirsa Wawa, undercover: Bedrock geologic and bedrock topographic mapping in north-central Minnesota Laura Ratcliffe Precambrian Geology of the Eastern Shebandowan Greenstone Belt - Insights into Stratigraphy and Structural History xxx �Christian Schardt and Mady David High-technology metals in ore-forming environments and their signature in volcanic-hosted sulfide mineralization in northern Minnesota and Wisconsin K.J. Schulz, W.F. Cannon, L.G. Woodruff and R.A. Ayuso Geochemistry of Archean Gneisses in Dickinson County, Northern Michigan Clarence Surette and Jill Taylor-Hollings Towards understanding geoarchaeological contexts in Northwestern Ontario: The newly formed lithic material comparative collection at Lakehead University Nicholas L. Swanson-Hysell, Sonia M. Tikoo and L.M. Fairchild New paleomagnetic constraints on the formation of the Slate Islands impact structure Nicholas L. Swanson-Hysell, Sarah P. Slotznick and L.M. Fairchild An oxygenated Paleolake Nonesuch and primary detrital hematite in the Freda river system Shiwei Wang, Pete Hollings and Ben Kuzmich Petrological and geochemical characteristics of the granitic rocks from the Dog Lake Granite Chain: Implications for the genesis of Quetico Basin Laurel G. Woodruff, Suzanne W. Nicholson, Connie L. Dicken and Klaus J. Schulz Mineral deposits of the Midcontinent Rift System - A new space/time classification *Jackie Wrage, Adrian Fiege, Brian Konecke, Adam Simon, Philipp Ruprecht and Harald Behrens Sulfur mobility in arc magma systems: Implications for porphyry ore deposits Michael J. Zieg Multiscale Layering in the Black Sturgeon Sill, Nipigon, Ontario * 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. xxxi �ABSTRACTS 1 �Sulfur, Carbon, and Oxygen Isotope Geochemistry of ~2.93 Ga Mesoarchean Chemical Sedimentary rocks in the Red Lake Area, Ontario AFROZ, Munira1, FRALICK, Philip1, KILLINGSWORTH, Bryan2,3, HOMANN, Martin3 SANSJOFRE, Pierre3 and LALONDE, Stefan3 1 Department of Geology, Lakehead University, 955 Oliver Road Thunder Bay, ON P7B 5E1 Canada. Institut de Physique du Globe de Paris, 1 Rue Jussieu, Paris, France. 3European Institute for Marine Studies, CNRS-UMR6538 Laboratoire Géosciences Océan, Technopôle Brest-Iroise, Plouzané, France. 2 Isotope geochemistry provides important insight into ancient marine carbon and sulfur sources and their role in evolving biologic activity. This research studied ~2.93Ga Mesoarchean chemical sedimentary rocks and carbonaceous slate directly underlying the Red Lake carbonate platform to explore these interactions through the analysis of sulfur, carbon and oxygen isotopes. Core samples of sulfidic iron formation, black slate, and carbonate rocks from 11 drill holes through the carbonate platform were analyzed using mass spectrometry. Multiple isotopes of sulfur (i.e. δ32S, δ33S, δ34S, and δ36S) were measured from sulfidic iron formation samples, while δ13C was examined from carbonaceous slate and δ13C and δ18O from inorganic carbonates. δ34S (‰ VCDT) -15 -10 -5 0 5 10 15 EBL-27 PB-32 PB-33 PB-34 PB-35 Figure 1: δ34S plot of samples from different drill-holes. Figure 2: δ34S vs. ∆33S plot of Red Lake samples with additional literature data (After Johnston, 2011) The analysis showed that sulfur in pyrite was derived from multiple sources as evident from the δ34S values in Fig. 1. Near zero values of δ34S indicate sulfur leached from primary sources due to high-temperature hydrothermal fluids (Thode et al., 1961), whereas δ34S values of >5‰ indicate that some of the sulfur was derived from Archean seawater (Ono et al., 2003). Finally, the lower negative values are indicative of bacterial sulfate reduction in sediments (Seal, 2006). In addition, the δ34S vs. ∆33S plot (Fig. 2) reveals that mass-independent fractionation of sulfur (diagonal array of samples) as well as microbial processing of sulfur (horizontal trend of samples) was active in the Mesoarchean sulfidic iron formation (Ono et al., 2003). The organic δ13C isotope plot (Fig. 3) has lighter δ13C values (~ -30‰) near the bottom of the stratigraphy while heavier δ13C values (~ -17‰) are exhibited towards the carbonate platform. This trend, especially less fractionated values of C, indicates that purple sulfur bacteria might be present in the shallow water carbonate platform along with cyanobacteria as these bacteria fractionate carbon isotopes differently (Posth et al., 2017). Furthermore, the dolostone samples have lighter δ18O isotope values (Fig. 4) which suggests dolomitization was not confined to the marine 2 �environment, instead, it was influenced by fresh water that produces lighter isotopic signatures (Wright and Tucker, 1990). Figure 3: δ13C plot with stratigraphy Figure 4: Mg/Ca vs. δ18O plot of carbonates (After Jaffrés et al., 2007) Based on the results, it is concluded that the source of sulfur was varied in the sediments below the Red Lake carbonate platform and was fractionated by both mass-dependent and massindependent processes. The δ13C trend of organic carbon hints that different bacterial communities were living on the carbonate platform. The δ18O signature indicates that dolostones were precipitated from a mixed water environment. References Jaffrés, J. B. D., Shields, G. A., & Wallmann, K. (2007). The oxygen isotope evolution of seawater: A critical review of a long-standing controversy and an improved geological water cycle model for the past 3.4 billion years. Earth-Science Reviews, 83(1–2), 83–122. Johnston, D. T. (2011). Multiple sulfur isotopes and the evolution of Earth’s surface sulfur cycle. Earth-Science Reviews, 106(1–2), 161–183. Ono, S., Eigenbrode, J. L., Pavlov, A. A., Kharecha, P., Rumble, D., Kasting, J. F., & Freeman, K. H. (2003). New insights into Archean sulfur cycle from mass-independent sulfur isotope records from the Hamersley Basin, Australia. Earth and Planetary Science Letters, 213(1), 15–30. Posth, N. R., Bristow, L. A., Cox, R. P., Habicht, K. S., Danza, F., Tonolla, M., Canfield, D. E. (2017). Carbon isotope fractionation by anoxygenic phototrophic bacteria in euxinic Lake Cadagno. Geobiology, 15(6), 798–816. Seal, R. R. (2006). Sulfur Isotope Geochemistry of Sulfide Minerals. Reviews in Mineralogy and Geochemistry, 61(1), 633–677. Thode, H. G., Monster, J., & Dunford, H. B. (1961). Sulphur isotope geochemistry. Geochimica et Cosmochimica Acta, 25(3), 159–174. Wright, V. P., & Tucker, M. E. (1990). Carbonate sedimentology. Blackwell scientific publications. 3 �Geology and Geochemistry of the Terrace Bay Batholith, N. Ontario ARNOLD, Kira1, HOLLINGS, Pete1, MAGNUS, Seamus2, ZUREVINSKI, Shannon1, CREASER, Robert3 1 Department of Geology, Lakehead University, Thunder Bay, Ontario P7B5E1 Ontario Geological Survey, Ministry of Energy, Northern Development and Mines, Earth Resources and Geoscience Mapping Section, 933 Ramsey Lake Road, Sudbury, ON, P3E 6B5, Canada 3 Department of Earth & Atmospheric Sciences, University of Alberta, 126 ESB Edmonton, Alberta, T6G2R3, Canada 2 The Terrace Bay Batholith is a 25 km long oval shaped granitoid intrusion located in the western portion of the Schreiber-Hemlo greenstone belt, part of the larger Wawa-Abitibi terrane (Fig. 1). The pluton, emplaced at 2689±1.1 Ma (Kamo 2016) intrudes circa 2720 Ma metavolcanic rocks, and a nearby pluton of equivalent age intrudes circa 2698-2693 Ma clastic metasedimentary rocks (Kamo 2016; Davis and Sutcliffe 2017). Younger plutonism in the region occurred between 2673 and 2667 Ma (Kamo 2016; Kamo and Hamilton 2017). The purpose of this study was to classify the Terrace Bay Batholith petrographically and geochemically in order to investigate the petrogenesis and tectonic setting in which the pluton formed, and to characterize the gold and base metal mineralization associated with the intrusion. Detailed mapping showed that the pluton can be separated into three mineralogically distinct lithologies (Fig. 1): granodiorite (typically composed of medium to coarse quartz and feldspar phenocrysts in a groundmass of fine-grained amphibole, biotite, disseminated magnetite, and sulphide minerals), monzogranite (composed of medium-grained quartz and feldspar with increased amounts of potassium feldspar and amphibole relative to the granodiorite), and diorite (composed of medium-grained amphibole and plagioclase with little to no quartz or potassium feldspar present). Two types of hydrothermal alteration are present: chlorite-epidote alteration and a pervasive hematite alteration. The faults and shears in the pluton likely acted as pathways for the hydrothermal fluids. Geochemically, the pluton is a homogenous calc-alkaline pluton, with minimal variation between lithologies. The pluton exhibits trace element signatures that are characteristic of suprasubduction zone magmas, including: fractionated heavy rare earth elements, negative high field strength element anomalies, enrichment of Th over light rare earth elements and enrichment of light rare earth elements. The fractionated heavy rare earth elements and the Th-Nb-La systematics are consistent with formation in a subduction zone at depths where garnet is stable. The Sr/Y and La/Yb signatures support formation within the garnet stability field and suggest small amounts of slab-derived melt were incorporated into the mantle wedge. The εNd values ranging from +2.16 to +2.49 suggest that the pluton underwent minimal crustal contamination during melting and emplacement. The emplacement of the pluton was determined to be through multiple injections derived from a single source. Prolonged fractional crystallization may have resulted in the formation of subtle mineralogical variation but no geochemical differences. Molybdenum mineralization in the pluton is spatially associated with gold mineralization, which suggests it was deposited during the same hydrothermal event. Gold and molybdenum mineralization is generally disseminated throughout the pluton at low concentrations, with higher concentrations of the metals hosted in sulphide mineralized quartz veins. Rhenium-Osmium isotopes from samples of molybdenum from these sulphide-mineralized quartz veins yielded an age of 2671 ±12 Ma, as well as postdating the emplacement of the pluton. Candela (1991) suggests that in plutons emplaced at greater depths, aqueous phases will remain dispersed 4 �throughout the magma, resulting in disseminated mineralization such as that in the Terrace Bay pluton. Figure 1. Simplified bedrock geology map of the Terrace Bay batholith and surrounding greenstone belt in Priske, Strey and Syine townships. Modified from Arnold et al. (2017). References Arnold, K.A., Hollings, P., Magnus, S.J. 2017. Geology and mineral potential of the Terrace Bay pluton, western Schneider-Hemlo greenstone belt; in Summary of Fieldwork and Other Activities, 2017, Ontario Geological Survey, Open File Report 6333, p.12-1 to 12. Candela, P. A. 1991. Physics of aqueous phase evolution in plutonic environments. American Mineralogist, p. 76. Davis, D.W. and Sutcliffe, C.N. 2017. U-Pb geochronology by LA-ICPMS in samples from northern Ontario; internal report prepared for the Ontario Geological Survey, Jack Satterly Geochronology Laboratory, University of Toronto, Toronto, Ontario, 131p. Kamo, S.L. 2016. Part A: Report on U-Pb ID-TIMS geochronology for the Ontario Geological Survey: bedrock mapping projects, Ontario, Year 1: 2015-2016; internal report prepared for the Ontario Geological Survey, Jack Satterly Geochronology Laboratory, University of Toronto, Toronto, Ontario, 48p. Kamo, S.L. and Hamilton, M.A. 2017. Part A: Report on U-Pb ID-TIMS geochronology for the Ontario Geological Survey: bedrock mapping projects, Ontario, Year 2: 2016-2017; internal report prepared for the Ontario Geological Survey, Jack Satterly Geochronology Laboratory, University of Toronto, Toronto, Ontario, 72p. Kamo, S.L. 2018. Part A: Report on U-Pb ID-TIMS geochronology for the Ontario Geological Survey: bedrock mapping projects, Ontario, Year 3: 2017-2018; internal report prepared for the Ontario Geological Survey, Jack Satterly Geochronology Laboratory, University of Toronto, Toronto, Ontario, 44p. 5 �Multi-scale AEM and MT mapping of the Precambrian in Upper Michigan, Northern Wisconsin, and Eastern Minnesota BEDROSIAN, Paul A. U.S. Geological Survey, MS 964, Federal Center, Denver, CO, 80225 The U.S. Geological Survey is conducting airborne electromagnetic (AEM) and magnetotelluric (MT) surveys over parts of Minnesota, Upper Michigan, and Wisconsin to map Precambrian geology and inform mineral resource assessments in this complex region. A total of 2,700 line-km of AEM data were collected along 16 regional transects over an area of 75,000 km2. These transects range from 100 to 300 km in length and cross numerous structural boundaries and more than 2 billion years of geology. An additional 100+ MT stations have been collected along some of these transects to refine regional resistivity models (Bedrosian, 2016) based on broadly-spaced EarthScope MT stations (Fig. 1). Pronounced contrasts in electrical resistivity exist between conductive sedimentary/metasedimentary rocks and resistive volcanic/intrusive rocks. Archean rocks of the Superior and Minnesota River Valley provinces are imaged as monolithic resistors (Figure 2), whereas strong conductors are linked to metamorphic graphite and metallic sulfides within Paleoproterozoic (PP) rocks of the Penokean orogen (most notably the Michigamme Formation). These conductors are evident in regional-scale resistivity models (Fig. 1b) and extend well into the lower-crust beneath the Penokean orogen (Bedrosian, 2016). Their upper-crustal geometry is being refined by ongoing MT investigations, while in the near-surface, AEM models image narrow (100s of meters wide) sub-vertical conductors (Fig. 2) extending for tens of kms along mapped or inferred faults and shear zones. Laboratory measurements on core samples confirm the presence of graphite in several of these conductive zones. Additional conductors mapped by the AEM data are preferentially located along the periphery of Archean gneiss domes in the region, suggesting that either the oldest PP units are anomalously conductive, and/or that locallyenhanced metamorphic grade is required to form conductive minerals. MT and AEM models of conductive PP rocks further constrain and refine structural details, such as a southern dip on the Niagara fault and a northward extension of PP rocks beneath younger rocks as far north as the Keweenaw fault. Within rocks of the Mesoproterozoic Midcontinent Rift System (MRS), the primary electrical contrast is between resistive volcanic and intrusive rift rocks and the conductive sedimentary successions of the Oronto Group, the Bayfield Group, and the Jacobsville Sandstone. East of the Keweenaw Peninsula, a thick succession of the latter exhibits a similar resistivity signature to that of the Freda Formation on the other side of the peninsula. Relative to lab measurements on these rocks, conductivity in the AEM models for both Freda and Jacobsville is elevated, possibly indicating elevated salinity in the pore waters of these Precambrian aquifers. Together with well control, the AEM models refine structure in several locations, including recognition of the Bayfield Group as more spatially limited than previously recognized and models that suggest the concealed edge of the MRS crosses the central U.P. along a linear magnetic boundary (near 46°15´N, 86°30´W). Imaged younger features include paleochannels cut into Precambrian sediments beneath Lake Superior (Fig. 2), an eastward-thickening wedge of Paleozoic cover in the Eastern survey area (the Northwestern edge of the Michigan basin), and a veneer of glacial sediments, the variable thickness of which can be mapped along each of the AEM profiles. The latter presents modeling 6 �challenges, currently under investigation, due to strong induced-polarization effects in clay-rich glacial tills. References Bedrosian, P.A. 2016. Making it and breaking it in the Midwest: Continental assembly and rifting from modeling of EarthScope magnetotelluric data, Precambrian Res., 278, 337-361, doi: 10.1016/j.precamres.2016.03.009. Figure 1: (a) AEM flight lines (black) and MT stations (white and green circles) atop magnetic anomaly map. Magnetic highs (red) are primarily due to thick volcanic successions (V), iron formations (IF) and intrusive complexes (IC). (b) Regional electrical resistivity model at 5 km depth. Conductors (red) correlate with PP metasedimentary rocks (shaded). White lines denote regional faults; yellow line indicates profile shown in Figure 2. Figure 2: Interpreted resistivity cross-section derived from AEM data. Profile location highlighted in Figure 1. Vertical exaggeration 10:1. 7 �LA-ICP-MS Micro-Sampling of Iron Formation: What it Can Tell Us BIELSKI, Paul and FRALICK, Philip Department of Geology, Lakehead University, Thunder Bay, ON, Canada The occurrence of iron formation during the Archean is well documented, however the mechanisms of their genesis are poorly understood within shallow waters and even less so within the deep-ocean. At the same time our understanding of Archean deep-ocean chemistry is also limited and poorly constrained. To address these issues, a new method for analysis of sulphide facies iron formation geochemistry is being conducted. This method involves a geochemical analysis of deep-ocean iron formation facies at a sub-lamination scale with attention to possible indicators of deposition rate and changes in water chemistry due to mixing of ambient seawater with a hydrothermal plume. Thus, changes in water chemistry during individual cycles of deposition can be measured. This method is conducted using Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS) alongside Scanning X-Ray Fluorescence (XRF). To test this new application of small scale geochemical analysis, an investigation of the Morley Occurrence was conducted. The Morley Occurrence is a deep-water Neoarchean (~2.7 Ga) sulphide-facies iron formation sitting upon intermediate flows and pyroclastic rocks and overlain by mafic flows and minor turbidites (Fralick et al., 1989). The occurrence itself is about 3 km south-east of Schrieber, Ontario. What makes this site interesting for this application is that oxides replace pyrite at the top of some thin colloform laminations (Fig. 1). These colloform structures are composed of sub-millimeter to millimeter thick pyrite laminations with increasing chert, carbon, and detrital minerals toward their tops. Applying LA-ICP-MS to these pyrite laminations at a sub-laminae level has provided information on geochemical changes of the depositional waters (Fig. 2) in addition to being proof of concept for this application to be used on other facies of iron formation. Laser ablation data (Fig. 2) shows a decrease in Ti upwards through pyrite laminations while Zr increases before resetting at each new lamination. Comparison with data on other hydrothermally sourced metals, such as Ni, Mn, Zn, and Pb, indicates that Ti is of hydrothermal origin while Zr is unrelated to venting fluids. This agrees with the pattern generated when plotting the series of laser ablation shots against Ti and Zr. The source of Zr could be thought to be from detrital origin, however the Zr concentrations are quite low for detrital sediments (Fig. 2). In addition, Zr has a positive relation to Y, Hf, U, and Th (high-field strength elements) along with samples having variable Zr/Hf ratios comfortably below and occasionally significantly above both chondrite and continental values which points to possible preferential scavenging of Hf from seawater by non-detrital sediment leading to fractionation between the two (Bau and Alexander, 2009). This explanation agrees with the data: a resetting Zr value at the beginning of each new pyrite lamination which increases with an assumed decrease in deposition rate with the general rate of deposition based off of chert and detrital sediments increasing upwards. 8 �The LA-ICP-MS data from the Morley Occurrence indicates that hydrothermal influence decreased upwards through each laminae, while seawater influence increased upward. A decreasing deposition rate upward through a laminae resulted in increased scavenging time for elements such as Zr and possibly increased concentration of rainout detritus containing Zr. Figure 1. Left: A photomicrograph of colloform laminations. Right: An example of LA-ICP-MS shots through colloform laminations (Yellow scale bar is 500 um). Figure 2. Plots of LA-ICP-MS Ti and Zr data taken through a set of colloform laminations. Square points represent where each of the three new pyrite laminations begin. References Bau, M. and Alexander, B.W., 2009. Distribution of high field strength elements (Y, Zr, REE, Hf, Ta, Th, U) in adjacent magnetite and chert bands and in reference standards FeR-3 and FeR-4 from the Temagami iron-formation, Canada, and the redox level of the Neoarchean ocean. Precambrian Research, 174(3-4), pp.337-346 Fralick, P.W., Barrett, T.J., Jarvis, K.E., Jarvis, I., Schnieders, B.R. and Vande Kemp, R., 1989. Sulfidefacies iron formation at the Archean Morley occurrence, northwestern Ontario; contrasts with oceanic hydrothermal deposits. The Canadian Mineralogist, 27(4), pp.601-616. 9 �High-resolution dating of the magmatic plumbing system of the Midcontinent Rift System—Insights into rift evolution and mineralization processes BLEEKER, Wouter1, HAMILTON, Michael2, KAMO, Sandra2, LIIKANE, Dustin2,3, SMITH, Jennifer1, HOLLINGS, Pete4, CUNDARI, Robert5, EASTON, Michael6, and DAVIS, Don2 1 Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8 2 Jack Satterly Geochronology Laboratory, U. of Toronto, 22 Russell St., Toronto, Ontario M5S 3B1 3 Dept. of Earth Sciences, University of Toronto, U. of Toronto, 22 Russell St., Toronto, Ontario M5S 3B1 4 Department of Geology, Lakehead University, 955 Oliver Rd, Thunder Bay, Ontario P7B 5E1 5 Ontario Geological Survey, 435 James Street South, Thunder Bay, Ontario P7E 6S7 Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5 Emails: wouter.bleeker@canada.ca; mahamilton@es.utoronto.ca; dustin.liikane@mail.utoronto.ca 6 North America’s Midcontinent Rift System is one of the best preserved and most accessible Proterozoic failed intra-cratonic rifts in the world, and therefore a pre-eminent natural laboratory for understanding the evolution of complex rift systems in cratonic settings, what generates them, what makes them fail, and the myriad of processes associated with their magmatic, sedimentary, and structural evolution. A key dataset that is fundamental to any deeper understanding of this rift system, including its endowment of mineralization, consists of precise and accurate ages on all the different components that make up this rift system. Already, there is a rich literature on dating (mostly U-Pb) of the rift system (see Bleeker, 2018, for a recent summary). Much recent progress has focused on improving the age resolution of volcanic rocks that fill the rift, in conjunction with detailed paleomagnetic investigations, to resolve in more detail the rapidly evolving apparent polar wander path (e.g., Fairchild et al., 2017). Nevertheless, many key components of the rift system, including a wide variety of intrusions that are part of the complex plumbing system of the rift, remain undated or have ages that require refinement, or have dates that are clearly puzzling outliers in the evolving temporal framework of U-Pb ages. Some of the published ages were obtained on limited amounts of small baddeleyite crystals and suffer from associated complications (Pb loss and variable discordance, elevated common Pb and associated corrections, ambiguity in choice of regression line and upper intercept, subtly different systematics between baddeleyite and zircon, etc.). In some cases, there exists doubt on the exact provenance or sample location of dated samples. Our aim is to revisit many, if not all, of the intrusions, and particularly those associated with mineralization, to improve and complete the U-Pb age framework, ideally at ~1 Myr resolution; and to link the intrusive record to the better resolved volcanic record as well as the overall tectono-magmatic evolution of the rift. Initially we have focused on some of the “outliers” such as the Inspiration Sill (Nipigon area, with a published age of 1159±33 Ma), or the iconic Logan Sills overlooking Thunder Bay (Fig. 1), with an age interpretation of 1114.7±1.1 Ma based on limited and discordant baddeleyites (Heaman et al., 2007). These problems can be tackled by searching for more optimum samples in the field, and applying ever improving U-Pb analytical techniques (lower blanks, new and better calibrated spike solutions, chemical abrasion of zircons, etc.). Searching for zircon-bearing samples is the key for ultra-high precision ages. 10 �Figure 1: Above: the iconic Logan Sills (s.s.) overlooking the Kaministiquia River and the city of Thunder Bay. Two sills are visible, having intruded the mudstones and thinly bedded turbiditic sedimentary rocks of the ca. 1.85 Ga Rove Formation, Animikie Basin; an upper main sill capping the mesas, and a thin lower sill forming a minor ledge in the trees. Right: our optimum sample of evolved, late-stage, varitextured and in part pegmatitic gabbro from near the top of the main upper sill, at Mount McKay. Figure 2: Left: Cross-cutting relationship of younger NNE-trending Pigeon River dyke cutting across, and chilled against, older coarse-grained and sparsely porphyritic diabase of one of the main Cloud River dykes. New age data are available for both the Pigeon River and Cloud River dykes. Together with searching for more optimum samples in the field, or in drill core, a key aspect of our study also involves resolving cross-cutting relationships in the field, where they exist, to help guide overall interpretation (Fig. 2). Already we have new and more robust age data on ~10 key units, including previously undated mineralized intrusions, which will be discussed at the meeting. Among those are: the Inspiration Sill, the main Logan Sill (Fig. 1), Pigeon River dykes, Cloud River dyke, Sunday Lake and Current Lake intrusions, Crystal Lake intrusion, Mount. Mollie dyke, Bovine Igneous Complex, and several others. Acknowledgements: we thank numerous industry partners and colleagues at the USGS for their keen interest in this study, their scientific input, and their generous cooperation. References Bleeker, W., Liikane, D.A., Smith, J., Hamilton, M., Kamo, S.L., Cundari, R., Easton, M., and Hollings, P., 2018, Controls on the localization and timing of mineralized intrusions in intra-continental rift systems, with a specific focus on the ca. 1.1 Ga Mid-continent Rift system. Geological Survey of Canada, Open File 8373, p. 15–27. https://doi.org/10.4095/306594. 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, vol. 9, p. 117–133. Heaman, L.M., Easton, R.M., Hart, T.R., Hollings, P., Mac-Donald, C.A., and Smyk, M., 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario. Canadian Journal of Earth Sciences, vol. 44, p. 1055–1086. 11 �New age data and insights into the ca. 1887-1870 Ma Circum-Superior Belt, with startling implications for the Lake Superior area geology BLEEKER, W.1, KAMO, S.2, HAMILTON, M.2, and CHAMBERLAIN, K.3 1 Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, wouter.bleeker@canada.ca Jack Satterly Geochronology Laboratory, U. of Toronto, 22 Russell St., Toronto, Ontario M5S 3B1 3 Department of Geology and Geophysics, University of Wyoming, Laramie, USA 2 Introduction: The ca. 1887-1870 Ma Circum-Superior Belt (Baragar and Scoates, 1981) has long been recognized as one of Canada’s major metallogenic belts, principally because of its world-class Ni-Cu-PGE deposits at Thompson and Raglan, as well as a number of other significant prospects elsewhere along the belt (e.g., Labrador Trough). Discontinuous outcrop along the margin of the Superior craton, remoteness, and the great extent of the belt has hampered detailed correlations between different segments. Although first-order correlations were suspected, and included volcanic rock in the Lake Superior area, U-Pb geochronology has only recently advanced to a point where we can demonstrate that peak mafic-ultramafic magmatism was coeval between localities such as Thompson and Raglan, with large volumes of Mg-rich ultramafic rocks being emplaced at 1882 Ma, both as flows, sills, and feeder dykes. Similar age mafic-ultramafic magmatism is now known from around the Superior craton, from northern Quebec to Minnesota, while ca. 1882 Ma dyke swarms intrude far into the cratonic hinterland. Carbonatites and kimberlites are coeval, within age uncertainty, with the maficultramafic magmatism or precede peak magmatism by a few million years, an age pattern also seen in other large igneous provinces (e.g., Bushveld Complex and slightly earlier carbonatites, Phalaborwa). A model that best explains the present observations is that of a mantle plume impinging on the base of thick cratonic lithosphere of supercraton Superia (Bleeker, 2003), with hot, low-viscosity, plume mantle then flowing laterally into multiple thin spots and incipient rifts, localized along the present margins of the Superior cratonic fragment, where large-scale and nearly synchronous decompression melting ensued at 1882±1 Ma (Fig. 1; see Bleeker and Kamo, 2018; and references therein). Figure 1: Interpreted geodynamic setting of the Circum-Superior Belt: plume ascent, interaction with cratonic lithosphere, and continental breakup. a) Ascending mantle plume impinging on thick lithosphere of the ancestral Superior craton, i.e. supercraton Superia. b) Flattening and rapid lateral flow of hot, buoyant plume mantle to thin spots, leading to nearly synchronous large-volume mafic-ultramafic magmatism (after Bleeker and Kamo, 2018). 12 �Continenal breakup: The emerging picture, with nearly synchronous mafic-ultramafic magmatism around what are now the margins of the Superior craton, clearly indicates a geological setting of continental breakup rather than that of accreting arcs at 1882 Ma. It is thus important to think about the Superior craton, and the geology of the Lake Superior area, in the context of progressive continental breakup. There is growing evidence that the Kaapvaal craton of South Africa, and thus also supercraton Vaalbara, was attached to the southwestern corner of the Superior craton (Bleeker et al., 2016; see also Gumsley et al., 2017), as part of an ancient terrane (also comprising much of Wyoming craton and Karelia) that collided with the growing Superia landmass at ca. 2650 Ma, and remained there until the ca. 1880 Ma breakup event introduced above. Indeed there are ca. 1880 Ma dykes in the eastern Kaapvaal craton. Furthermore, the proposed reconstruction is paleomagnetically viable. On final breakup, the Minnesota River Valley terrane, a piece of the ancient crust of the eastern Kaapvaal craton, was left stranded as an exotic terrane on the southern breakup margin of the Superior craton (Bleeker et al., 2016). Predictions: The startling conclusion must be that Kaapvaal craton, indeed entire Vaalbara, Wyoming and Karelia, were contiguous with the southern Superior craton from ca. 2650 Ma until progressive breakup from ca. 2000 Ma to 1880 Ma, within the context of supercraton Superia. Indeed dykes of exact Bushveld age (2056 Ma) have been identified in the Western Superior craton (Bleeker et al., 2016), and 2167 Ma Biscotasing dykes have been identified in the eastern Kaapvaal craton (with matching trend!), two independent and exact age matches of short-lived mafic magmatic events that demonstrate, without any doubt, a “nearest neighbour” relationship (Bleeker and Ernst, 2006) of these cratonic fragments over this time interval. This leads to another startling prediction: a potential plume track that starts with the LIP-scale Marathon magmatic event in the eastern Lake Superior area, at ca. 2130 Ma, can be traced to the southwest, with pulsed 2125-2100 Ma mafic magmatism along the southern margin of the Superior craton; it then was responsible for the Fort Frances giant mafic dyke swarm at ca. 2070 Ma; it can then be traced into the easternmost Kaapvaal craton where it is fist manifested by the 2060 Ma Phalaborwa and related carbonatites; and, finally, further west, it eroded the lithosphere and produced Earth’s largest layered mafic intrusion at 2056 Ma, the Bushveld Complex. In conclusion: two of the best-known Archean cratons in the world, Superior and Kaapvaal, shared a common history from ca. 2650 Ma terrane collision until ca. 1880 Ma breakup. On breakup, a piece of ancient Kaapvaal crust, Minnesota River Valley terrane, was left behind. Kaapvaal and Superior were joined and contiguous all through the lead-up to the Bushveld Complex, and the magmatism that culminated with the Bushveld Complex started with the ca. 2130 Ma Marathon event, tracing a continuous plume track from the southern Superior craton into the Kaapvaal craton. References Baragar, W.R.A. and Scoates, R.F.J., 1981. In: Developments in Precambrian Geology, v. 4: p. 297–330. Bleeker, W., 2003. Lithos, v. 71(2): p. 99-134; Bleeker, W., Chamberlain, K.R., Kamo, S.L., Hamilton, M., Kilian, T.M. and Buchan, K.L., 2016. 35th IGC, Cape Town, South Africa, Paper Number 5222. Bleeker, W. and Ernst, R., 2006. In: Dyke Swarms—Time Markers of Crustal Evolution. Balkema, Rotterdam, p. 326. Bleeker, W. and Kamo, S.L., 2018. In: GSC Open File 8373, p. 5–14, https://doi.org/10.4095/306592. Gumsley, A.P., Chamberlain, K.R., Bleeker, W., Söderlund, U., de Kock, M.O., Larsson, E.R. and Bekker, A., 2017. PNAS, v. 114(8): p. 1811-1816. 13 �Stable isotopic composition of calcite precipitated with native copper and other minerals of the Keweenaw Peninsula, Michigan BODDEN, Thomas J.1, BORNHORST, Theodore J.2, BÉGUÉ, Florence3, and DEERING, Chad1 1 Department of Geological and Mining Engineering and Sciences, Michigan Tech, Houghton, MI 49931 2 A. E. Seaman Mineral Museum, Michigan Tech, Houghton, MI 49931 3 Institute of Earth Sciences, University of Lausanne, Lausanne, Switzerland Hydrothermal native copper deposits are hosted by Midcontinent Rift-filling volcanic and sedimentary rocks in Michigan’s Keweenaw Peninsula. Butler and Burbank’s (1929) classic U.S.G.S. Professional Paper documented the district-wide paragenesis of hydrothermal mineral precipitation and the relative age of minerals with respect to the precipitation of native copper, the principal ore mineral. Puschner (2002) subdivided hydrothermal mineral paragenesis into three Stages: Stage 1 (pre-native copper), Stage 2 (syn-native copper), and Stage 3 (post-native copper). Since Stage 1 and 2 minerals represent a single continuous episode of mineral deposition, they will be combined for data presentation below. Stage 3 is a distinct later episode (veins that cross-cut native copper deposits) and thus, will be considered separately. Stages 1-2 formed at a significantly higher temperature than Stage 3 based on mineral equilibria, stable isotope pairs, chlorite geothermometry, and fluid inclusions (Puschner, 2002; Livnat, 1983). In addition to temporal (paragenetic) variation of mineral precipitation within the native copper district, both within the district and the broader Keweenaw Peninsula, there is spatial variation in the suite of minerals in a particular locality (Stoiber and Davidson, 1959). The spatial mineral variation corresponds to a regular variation in the temperature of precipitation of minerals from the hydrothermal fluids. This research is an extension of Bornhorst and Woodruff (1997) who proposed fluidmixing was an important mechanism facilitating native copper precipitation on the basis of the variability of stable isotope data derived from Stage 1-2 calcite from the Kearsarge deposit; the largest basalt-hosted deposit in the district. Calcite is good to study the evolution of the hydrothermal fluids as it precipitates in all three stages and with native copper in Stage 2. The purpose of this study was to test the hypothesis of fluid-mixing proposed by Bornhorst and Woodruff (1997) using a geographically broader data set as well as using secondary ion mass spectrometry (SIMS) to obtain in-situ stable isotope values for calcite. We have compiled 159 published oxygen-carbon stable isotope pairs for calcite from Livnat (1983; 88 pairs), Puschner (2002; 31 pairs), and Bornhorst and Woodruff (1997; 40 pairs) determined by traditional bulk mineral analysis. We have added an additional 101 pairs determined by SIMS on selected spots from three samples. Each of the pairs have been grouped according to paragenetic Stage when possible based on sample description, geographic location, and textural observation from cathodoluminescence imaging; those not able to be grouped are not included in the discussion below. Puschner’s (2002) data was only obtained for Stage 1-2 calcite, as was the case for Bornhorst and Woodruff (1997) with the exception of one Stage 3 calcite. The new SIMS data are from: Stage 1-2 calcite from the Quincy deposit, Stage 2 calcite and Stage 3 calcite from the Kearsarge deposit. The variability of the SIMS spot data from only 14 �three samples is similar to the entire range of the 159 bulk samples as a result of averaging by bulk sampling. The oxygen and carbon isotopic composition of the hydrothermal fluids in equilibrium with the calcite has been calculated considering both temperature variation among paragenetic stages and differences in geographic location. For Stage 1-2 calcite a temperature of 250°C +/50°C was used and for Stage 3 calcite a temperature of 125°C +/- 25°C (Puschner, 2002; Livnat, 1983). The variation of Stage 1 and 2 water in equilibrium with calcite is widely scattered between δ18OH2O of about +22 to +4 ‰ and δ13CCO2 of about +3 to -8 ‰ (using midpoint temperatures). The total variability in Stage 1-2 water stable isotopic compositions can only partly be explained by considering paragenetic, spatial, and local temperature variation. Thus, the larger data set compiled for this study, representing Stage 1-2 water in equilibrium with calcite, is consistent with the observations of Bornhorst and Woodruff (1997). To explain the oxygen isotopic data in his limited data set, Puschner (2002) proposed that ore fluids were derived through metamorphism and mixed with a meteoric water at shallow depths during formation of the native copper deposits. The range in δ18OH2O and δ13CCO2 from our data supports this conclusion. Stage 3 ranges in δ18OH2O from about +8 to -2 ‰ and in δ13CCO2 from about 0 to -10 ‰ using midpoint temperatures. In δ18OH2O and δ13CCO2 space Stage 3 calcite is generally different than Stage 1-2, but overlaps with Stage 1-2 at the higher values of δ18OH2O. Stage 3 calcite isotopic composition can only partly be explained by local temperature variation. The range of δ18OH2O for Stage 3 calcite overlaps the expected range of values for meteoric and metamorphic waters and is consistent with a tentative interpretation of fluid-mixing. Further interpretations are in progress. This study was partially supported by an ILSG Student Research Grant. References Bornhorst, T.J., and Woodruff, L.G., 1997, Native Copper Precipitation by Fluid-Mixing, Keweenaw Peninsula, Michigan: Institute on Lake Superior Geology Proceedings, 43rd Annual Meeting, v. 43, part 1, p. 9-10. Butler, B.S., and Burbank, W.S., 1929, The copper deposits of Michigan: U.S. Geological Survey Professional Paper 144, 238 p. Livnat, A., 1983, Metamorphism and copper mineralization of the Portage Lake Lava Series, northern Michigan: Ph.D. Dissertation, University of Michigan, Ann Arbor, 292p. Puschner, U.R., 2002, Very low-grade metamorphism in the Portage Lake Volcanics on the Keweenaw Peninsula, Michigan, USA: Ph.D. Dissertation, University of Basel, Basel, Switzerland, 82p. and appendices Stoiber, R.E., and Davidson, E.S., 1959, Amygdule mineral zoning in the Portage Lake Lava Series, Michigan copper district: Econ. Geol., v. 54, p. 1250-1277, 1444-1460. 15 �Recognition of probable distal ejecta from the 1850 Ma Sudbury meteorite impact event along the southern edge of the Animikie basin in Minnesota BOERBOOM, Terrence J., Minnesota Geological Survey Petrographic examination of a drill core (LM-13-4) obtained in 2013 by Minerals Processing Corporation, located at the southeastern margin of the Animikie basin (Fig. 1), has revealed the presence of an approximately 5 m thick interval with features that can be attributed to distal Sudbury ejecta. These include sphere-in-sphere structures, vesiculated devitrified glass, zoned accretionary lapilli, anatase, and possible (albeit questionable) rare and poorly preserved decorated PDF lamellae in some quartz grains. These features are similar to those described from other locations, including Michigan (Cannon and others, 2010), and Ontario and Minnesota (Addison and others, 2005), among others. This location is approximately 950 km from Sudbury. This core is from a belt that has historically been mapped as part of the southward-adjacent Mille Lacs Group which is thought to predate deposition in the main Animikie basin. However, a more recent reinterpretation (Boerboom, 2009) places this belt in the lower part of the Animikie basin, as part of the Thomson Formation, an interpretation supported by the presence of this ejecta layer. The vertical drill core intersects bedrock at 137’/42m depth (beneath glacial drift) and ends at 437’/133m depth. The ejecta horizon occurs in the Figure 1. Cartoon map of the 291-309’ interval. Bedding is upright and (most likely) dips north an average of Animikie basin showing 60 degrees. Major folds appear to be lacking in this core, despite the presence location of drill core LM13-4. of a weak, nearly vertical cleavage. The approximately 5.5m thick interval attributed to the ejecta horizon lies within a thick, low-grade turbidite sequence. The core above the ejecta horizon is gray and orange-ochre banded ‘ferruginous slate’, likely a more weathered and oxidized version of the gray carbonaceous argillite and graywacke below the ejecta horizon which contains numerous thin beds of brownish carbonate beds and pyrite. The ejecta horizon can be divided into three distinct portions (Fig. 2) – a lower lapilli-rich layer (20cm, not all shown in Fig. 2), a middle fragmental and brecciated layer (10cm), and an upper sandy layer that Figure 2. Lower portion of the eject interval. Top of core is to the left. Bottom 6 inches/13cm is not shown. Drill core is 3.5 cm in width. continues upward for several meters and appears to grade into the overlying turbidites. The lower portion contains abundant dark gray vertically flattened, weakly and concentrically zoned accretionary lapilli that increase in size and abundance upward, in a matrix of small pale green and dark gray, angular shard – like clasts. The middle layer contains angular chert clasts at the base and larger elongate pale green shard-like clasts at the top. The upper layer is composed of a grayish-green sandy graywacke with 1-3mm lapilli in the lower part that are weakly concentrated along bedding planes. This interval may represent an influx of debris ultimately derived from the Sudbury impact crater, possibly a submarine debris flow slumped downslope from its original depositional source, within an otherwise unbroken turbidite sequence. Despite thorough replacement by secondary phyllosilicate minerals, there are many well-preserved features (Fig. 3) that compare to those elsewhere attributed to ejecta fallout. To date positive identification of shocked quartz has been unsuccessful. However, some grains bear parallel arrays of linear bubble trains which may represent decorated quartz planar deformation features (Fig. 3c). Nonetheless there are many other features that can be attributed to ejecta fallout, and the location along the southern margin of the Animikie basin is where it logically would be expected. 16 �Huber and others (2014) describe spherules (their term) from drill cores near Coleraine that contain microcrystalline rutile and anatase in the outer rims. They state that because the transition from rutile to anatase at low pressures is in the 500-600 C range, and because the rocks at Coleraine were only subject to low T metamorphism, that anatase formed in the spherulitic melt droplets as they cooled. Thin sections from core LM13-4 contain abundant anatase as confirmed by SEM and by optical properties. However, the anatase does not occur as fine granular masses, but rather as prismatic crystals up to 0.8mm in length concentrated in microscopically dark-opaque zones interpreted to be deformed devitrified glass shards. The anatase laths are commonly rimmed by zones that are not opaque, implying they may have formed by some secondary mechanism such as diagenesis, hydrothermal alteration or lowgrade metamorphism. In contrast, ilmenite is the dominant Ti-phase within the accretionary lapilli. The significance of this is currently unknown. Ongoing petrographic work will attempt to more positively identify shocked quartz, and SEM and XRD work will be conducted to better characterize the secondary mineralogical assemblages. If further analytical and petrographic data conclude the material is ejecta-bearing, it will be the first such occurrence along the southern Animikie basin in Minnesota. A B C D E Figure 3. A. Sphere-in-sphere structures interpreted as melt droplets, with chloritic cores and sericitic rims. B. Clast of devitrified vesicular glass with internal spherical structures. C. Straight bubble trains in quartz – possible relict deformation lamellae? D. Zoned accretionary lapilli most visible at thin edge of thin section. E. Reflected light image of showing ilmenite (Ilm) in accretionary lapilli, and anatase (An) in dark semiopaque zones (in transmitted light) that are interpreted as possible deformed pumice-like fragments. References Boerboom, T.J., 2009, Bedrock geologic map of Carlton County, Minnesota; Minnesota Geological Survey County Atlas Series C-19, Plate 2; scale 1:100,000. Huber, M.S., McDonald, I., and Koeberl, C., 2014, Petrography and geochemistry of ejecta from the Sudbury impact event: Meteoritics and Planetary Science 49, No. 10, P. 1749-1768 Cannon, W.F., Schulz, K.J., Horton, J.W., Jr., and Kring, D.A., 2010: The Sudbury impact layer in the Paleoproterozoic iron ranges of northern Michigan, USA, GSA Bulletin v. 122; no. 1/2; p. 50–75. Addison, W.D., Brumpton, G. R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W., and Hammond, A.L., 2005: Discovery of distal ejecta from the 1850 Ma Sudbury impact event, Geology; March 2005; v. 33; no. 3; p. 193–196. 17 �Geology and Geochemistry of Ultramafic rocks in the Lake of the Woods Area BOUCHER, Chanelle and HOLLINGS, Pete Department of Geology, Lakehead University, Thunder Bay, Ontario P7B5E1 The Archean komatiites of the Lake of the Woods greenstone belt in Kenora, Ontario formed on the western extension of the Superior Province southern margin and have not been studied using modern methods. Although Archean plate tectonic processes have been the subject of decades of research, the nature of these processes remains the subject of considerable debate. Recent work has investigated the link between komatiites and Archean subduction zones. Komatiites are widespread in Archean terranes and together with spatially associated tholeiitic basalts form an important part of many Late Archean greenstone belts, therefore a better understanding of Archean geodynamic processes and comparison to modern day processes is required. The Lake of the Woods greenstone belt (LWBG) is located in the Western Wabigoon Terrane which is composed dominantly by mafic volcanic rocks with large tonalite-granodiorite plutons. The LWGB is situated along the northwestern margin of the Western Wabigoon Terrane, bounded to the north by the Winnipeg River and English River terranes and to the south by the Quetico terrane (Ayer and Davis, 1997). It consists of a northeast trending metavolcanic plutonic belt that extends for 900km and is about 150km wide (Ayer and Davis, 1997). The LWGB is divided into three supracrustal assemblages: a lowermost mafic volcanic Lower Keewatin assemblage; a compositionally diverse, predominantly volcanic middle Upper Keewatin assemblage; and a predominantly sedimentary uppermost Electrum assemblage (Ayer and Davis, 1997). The Upper Keewatin assemblage consists of 1) mafic to felsic metavolcanic rocks of calc-alkalic affinity, 2) ultramafic to mafic metavolcanic rocks of komatiitic to tholeiitic affinity, and 3) turbiditic metasedimentary rocks (Ayer and Davis, 1997). Detailed mapping in the Upper Keewatin Assemblage identified komatiites on the southern margin of the Long Bay Group. The komatiites are typically metamorphosed to upper greenschist facies and include a variety of schists that do not show any preserved primary textures or mineralogy. Polyhedrally jointed flow tops were observed in rare locations. Mineral assemblages include dominantly anthophyllite-tremolite-chlorite (Fig. 1A) and serpentinetremolite-chlorite (Fig. 1B) schists, as well as lesser talc-tremolite-chlorite schists. These units are moderately to intensely foliated with chlorite and lesser amphibole defining the foliation and also include randomly oriented bladed amphibole grains that typically have tremolite cores and anthophyllite rims. The amphiboles show a chemical transition from core to rim with a loss in Ca as anthophyllite appears. Accessory phases include chromite, magnetite, ilmenite and apatite. Ultramafic rocks are very fine-grained, and mineralogy has been described using a compilation of petrography, XRD (x-ray diffraction) and SEM (scanning electron microscope) analysis. Whole-rock geochemical analyses were conducted on 110 samples collected during field work in 2017 and 2018. The Upper Keewatin Assemblage is composed of dominantly mafic to intermediate volcanic rocks that are typically of tholeiitic affinity with rare calc-alkalic units. A total of 41 samples were determined to be ultramafic. The komatiite units are Al-undepleted rocks that display primitive mantle normalized patterns, as well as major and trace element concentrations consistent with melts derived from outside the garnet stability zone. They can be 18 �subdivided into three suites with primitive mantle patterns that display strong Th and Nb depletions with flat HREEs (heavy rare earth elements), weak Th and Nb depletions with flat HREEs and enriched Th with moderate Nb depletions and flat HREEs. Neodymium isotope analyses, in conjunction with trace element geochemistry, suggests that some units have been weakly to moderately contaminated. Mafic tholeiitic units have low- and high-Ti varieties, in which most units are dark grey to black amphibolites and rare chlorite-tremolite schists. The geochemistry of the mafic units shows similar contamination trends to the ultramafic units. Figure 1. Field photographs of grey to dark green foliated ultramafic metavolcanic rocks. A (LOW17CB19 15U 368614 5467778): Pervasive chlorite creates deep green color and moderate foliation. B (LOW17CB92 15U 368861 5465424): Talc alteration with red staining along strong foliation planes. References Ayer, J.A., and Davis, D.W. 1997. Late Archean evolution of differing convergent margin assemblages in the Wabigoon Subprovince: Geochemical and geochronological evidence from the Lake of the Woods greenstone belt, Superior Province, northwestern Ontario; Precambrian Research, 81:155-178. 19 �Updated mineralogy of a roadside pegmatite in the Stettin Complex, Wausau Syenite Complex, Marathon County, Wisconsin. BUCHHOLZ, Thomas W.1, FALSTER, Alexander. U. 2, and SIMMONS, Wm. B. 2 1 1140 12th Street North, Wisconsin Rapids, Wisconsin 54494; 2Maine Mineral and Gem Museum, PO Box 500, 99 Main Street, Bethel, Maine 04217. In 2017 we presented an initial report regarding a recently re-exposed pegmatite along 120 Avenue in the SW 1/4 NW 1/4 of Sec. 22, T.29, R. 6E near the western margin of the intrusion. It has become apparent that this aplite/pegmatite is the same roadside pegmatite described in Weidman (1907), having been obscured by slumped soil, rock and vegetation for the intervening 110 years. 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 primarily composed of amphibole, pyroxene, tabular and nepheline syenites, and syenite aplite. th In 2017 we reported on the occurrence of albite, arfvedsonite, aegirine, microcline, pyrochlore, monazite-(Ce), bastnäsite-(Ce), cerianite, xenotime-(Y), zinnwaldite, zircon, goethite/hematite replacements after siderite, pyrite, a TiO2 phase, columbite-(Fe), bismuthinite, astrophyllite and fluorite. Several other species were included in the poster presentation but not described in the abstract, hence these are included in the below descriptions. Minor additional xenotime has been identified as sheaves of pale blue crystals in albiterich aplite, associated with aegirine, while further analysis of pyrochlore indicates they are largely fluorcalciopyrochlore, although due to strong, ubiquitous zoning three or more pyrochlore species may be present in various zones in one crystal. Graphite is not uncommon as thin, black crystals in late quartz pods in and near pegmatitic portions of the dike but is easily missed, and several yellow-brown grains of thorbastnäsite have been found in microcline in core zone material. Euxenite-(Y) forms rare small, brown, elongated crystals in pegmatite, and probable thorite and grayite are sparse. Careful visual examination of samples has revealed the first Be-bearing minerals in the Stettin complex in albite-rich aplite, located close to the transition to pegmatite. Phenakite is found as patches of clear, colorless phenakite poikilitically including albite crystals and as isolated grains in pegmatite, and true to its name (from Greek phenas for “deceiver) is difficult to distinguish from similar quartz without the use of optical methods. Bertrandite was found as very pale blue platy crystals in patches in albite near phenakite, and feathery, pale yellow bavenite near bertrandite. Heavy mineral separates have revealed a suite of unusual inconspicuous phases, some present in very small amounts. These include sparse grains of galena and sphalerite, native bismuth with small amounts of a Ca-Bi phase (perhaps kettnerite or beyerite) one small grain of akanthite in native bismuth, and an Ag-Bi-S phase, also in bismuth. The Ag-Bi-S phase remains unidentified as the stoichiometry does not match benjaminite, dantopaite, matildite or pavonite (the known Ag-Bi-S minerals), and paucity of material precludes further investigation. 20 �Cassiterite is not uncommon in some samples, but is difficult to visually distinguish from abundant zircon. Worldwide, cassiterite is very rare from alkalic complexes, as a review of applicable literature and Mindat listings revealed very few occurrences worldwide. Apparently, Sn is not commonly enriched in alkalic environments, although several additional occurrences of cassiterite have been noted in Stettin Complex pegmatites. The occurrence of graphite in and near the core zone suggests a reducing environment during crystallization of those portions of the dike, while the late crystallization of cerianite (requiring oxidation of Ce3+ to Ce4+), replacement of siderite by goethite and hematite, and common partial replacements of aegirine and arfvedsonite by Fe-oxide phases suggests a late transition to an oxidizing environment. References Van Wyck, N. (1994) The Wolf River A-type magmatic event in Wisconsin: U/Pb and Sm/Nd constraints on timing and petrogenesis. Institute on Lake Superior Geology, 40th Annual Meeting, Part 1, Program and Abstracts, p. 81-82. Weidman, Samuel (1907). The Geology of North Central Wisconsin. Wisconsin Geological and Natural History Survey Bulletin No. XVI, Scientific Series No. 4, 697 pp. 21 �The Dickinson Group in the Central Upper Peninsula of Michigan: Part 1- Age and tectonic setting based on new geophysical, geochronological, and geochemical data CANNON, W.F.1, SCHULZ, K.J.1 and DRENTH, Benjamin J.2 1 U.S. Geological Survey, Reston, VA 20192 U.S. Geological Survey, Denver, CO 80225 2 A unique sequence of metasedimentary and metavolcanic rocks is exposed in a ~100 km2 area of central Dickinson County, Michigan. James (1958) divided these rocks into three formations that comprise the Dickinson Group. James et al. (1961) provided additional details of structure and stratigraphy. Our recent geophysical, geochemical, and geochronological studies shed new light on this group and suggest substantial changes to previous interpretations. The basal East Branch Arkose (arkose, conglomerate, and minor basalt flows) grades upward to the Solberg Schist (finer grained-clastics with probable metavolcanic interbeds and a medial banded iron-formation, the Skunk Creek Member). The uppermost formation, the Six Mile Lake Amphibolite (massive to banded hornblende-plagioclase rock), is presumed to be mafic metavolcanics. James et al. (1961) concluded that the Six Mile Lake Amphibolite grades southward into Archean gneiss, so ascribed an Archean age to the entire Dickinson Group. The exposed Dickinson Group lies in a vertical, south-facing monocline about 5 km wide. James et al. (1961) considered this the approximate stratigraphic thickness of the group because they found no indication of internal folding or faulting across that distance. The lack of internal folding is especially well documented for the East Branch Arkose where abundant cross beds all indicate south-facing strata. In the Solberg Schist the Skunk Creek Member can be traced by its strong aeromagnetic anomaly (as much as 2000 nT) as a single horizon for 50 km without indication of structural repetition. This apparent structural simplicity is belied by a ubiquitous penetrative foliation that is steeply dipping and essentially beddingparallel in the East Branch and Solberg as shown by oriented micas, stretched quartz grains, and, in the East Branch Arkose, flattened and elongated pebbles. In the Six Mile Lake Amphibolite oriented hornblende grains define a gently-plunging lineation (fold axes?). Development of these penetrative structures appears to have been synchronous with metamorphism that peaked at about 1.83 Ga (Holm et al., 2007) and thus records deformation during the Penokean orogeny. The exposed Dickinson Group may be the north limb of a large syncline whose southern limb is truncated by a fault against the Archean rocks to the south. East of the exposed area our new aeromagnetic data indicates that the belt of Dickinson Group rocks widens and is more structurally complex (Drenth et al., 2019). The East Branch Arkose contains a significant population of detrital zircons with 2.1 Ga ages (Craddock, et al., 2013) and is clearly Paleoproterozoic rather than Archean. The most abundant clast type in the East Branch conglomerates is orthoquartzite likely derived from the older 2.2-2.3 Ga Sturgeon Quartzite. An age of 2.1 Ga has been determined for the “porphyritic red granite”(prg) (Ayuso et al., 2018), which is surrounded by Dickinson Group strata. The prg likely was an important source of detritus, including zircons, for the East Branch Arkose. Correlation of the Dickinson Group with other Paleoproterozoic sequences of the region is not fully resolved. It is clearly younger than the 2.2-2.3 Ga Chocolay Group and its metamorphism at 1.83 Ga provides an upper age limit. Within current age constraints it could be equivalent to parts of the Menominee and/or Baraga Groups. But, another possibility is that the Dickinson Group is a vestige of a unique sequence deposited during the long hiatus between about 2.1 Ga (prg) and 1.9 Ga (Menominee Group). and provides a record of the final separation of the Superior and Wyoming cratons. The Six Mile Lake amphibolite and a metadiabase sill in the Solberg Schist have distinctive trace element chemistry consistent with a mantle plume source. Within the Lake Superior region that composition is known only in mafic dikes north of Lake Superior (Schulz et al., 2018) that were intruded between 2126 and 2067 Ma and mark a long-lived mantle plume event during separation of the two cratons (Halls et al., 2008). The coarse, locally derived fluvial sediments of the East Branch Arkose are consistent with extensional uplift during which much of the Chocolay Group was stripped from it basement and erosion unroofed 2.1 Ga granite plutons. The ensuing transition of fine-grained clastic sediments and banded iron-formation of the 22 �Solberg Schist marks the transition to marine sedimentation culminating in plume-related mafic volcanism (Six Mile Lake Amphibolite) marking the final continental separation. Geologic map of the Dickinson Group and surrounding units (modified from James et al., 1961). 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, Proceedings of 64th Annual meeting, Part 1: Program and Abstracts. p. 7-8. 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. Drenth, Benjamin J., Cannon, W.F., and Schulz, K.J., 2019, The Dickinson Group in the Central Upper Peninsula of Michigan: Part 2- Geophysical expression and a preliminary interpretation of its eastward extent under Paleozoic cover, Institute on Lake Superior Geology, Proceedings of 65 th Annual meeting, Part 1: Program and Abstracts. Halls, H.C., Davis, D.W., Stott, G.M., Ernst, R.E., and Hamilton, M.A., 2008, The Paleoproterozoic Marathon large igneous province: New evidence for a 2.1 Ga long-lived mantle plume event along the southern Superior Province, {Precambrian Research, v. 162, p. 327-353. Holm, D.K., Schneider, D.A., Rose, S., Mancuso, C., McKenzie, M., Foland, K.A., and Hodges, K.V., 2007, Proterozoic metamorphism and cooling in the southern Lake Superior region, North American and its bearing on crustal evolution, Precambrian Research, v. 157, p. 106-126. James, H.L., 1958, Stratigraphy of pre-Keweenawan rocks in parts Northern Michigan, U.S. Geological Survey Professional Paper 314-C, 24 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. Schulz. K, J., Cannon, W.F., and Woodruff, L.G., 2018, Geochemistry of mafic rocks in Dickinson County, Michigan: Evidence for 2.1 Ga rifting: Institute on Lake Superior Geology, Proceedings of 64 th Annual meeting, Part 1: Program and Abstracts. p. 93-94. 23 �Reexamining the Osborne core for new insights into the age and petrology of the Northeast Iowa Intrusive Complex (NEIIC) CLARK, Ryan1, PEATE, David2, KUSICK, Alison2, HORKLEY, Kenny2, and ANDERSON, Raymond2 1 Iowa Geological Survey, University of Iowa, Iowa City, IA 52242 USA 2 Department of Earth and Environmental Sciences, University of Iowa, Iowa City, IA 52242 USA The Keweenawan Midcontinent Rift System (MRS) has been the focus of decades of research for its enigmatic geologic history and its wealth of economic minerals. The latter has been concentrated in the Lake Superior region where the MRS is exposed at or near the land surface. Copper-nickel sulfide and platinum group element deposits have been identified along the north shore in Ontario (Coldwell Complex) and along the western shore in Minnesota (Duluth Complex). These magmatic deposits are related to the MRS and are geophysically distinct, with high amplitude magnetic anomalies and associated gravity highs (Drenth et al., 2015 and Drenth & Brown, 2016). Since 2012, the U.S. Geological Survey (USGS) has conducted two major high-resolution geophysical surveys in northeastern Iowa and southeastern Minnesota in an attempt to better understand the nature of the Precambrian basement geology concealed beneath at least 1,000 feet (300 m) of Paleozoic sedimentary rocks. The surveys, both magnetic and gravity, have succeeded in refining the area previously identified as the Northeast Iowa Plutonic Complex (Anderson, 2006), now called the Northeast Iowa Intrusive Complex (NEIIC). The NEIIC has an aerial extent of over 6,000 mi2 (15,500 km), including several large ring/horseshoe shaped anomalies and associated linear features. Some of these features have been characterized using geophysical techniques, yet with a limited number of boreholes that reach the NEIIC, accurate lithologic and geochronologic data has remained elusive. One iron exploration core, the Osborne core, drilled in 1963 intersected a dike extending northeastward from the main part of the NEIIC and encountered more than 700 feet (213 m) of ultramafic olivine-plagioclase cumulate. The Iowa Geological Survey (IGS) and the University of Iowa Department of Earth and Environmental Sciences are reexamining the Osborne core to identify and characterize datable minerals. A systematic survey of compositional variations was done using a handheld portable XRay Fluorescence (pXRF) analyzer, with replicate analyses made on individual cores pieces, at an average sampling interval of 2 m along the core. These data show there are two distinct zones within the core that have elevated zirconium (Fig. 1), together with high K, P and Rb, indicative of trapped residual liquid. X-ray element mapping and backscatter images have identified baddeleyite and zirconolite minerals in these zones (Fig. 2). Samples have been selected and are being processed for geochronologic analyses. Obtaining a reliable age date from the Osborne core could provide a missing piece to the NEIIC puzzle and help answer the question of whether it is in fact related to the MRS and other economic mineral deposits in the Lake Superior region. 24 �Osborne Core pXRF Results Zr (ppm) 0 200 400 600 800 1000 1800 1900 2000 Depth (ft) 2100 2200 2300 2400 2500 2600 Figure 1: PXRF results for zirconium through the Precambrian sequence encountered in the Osborne core. Figure 2: Backscatter image of a zirconolite crystal from the Osborne core at 2,416' depth. References Anderson, R.R. 2006. Geology of the Precambrian surface of Iowa and surrounding area. Iowa Geological Survey, Open File Map OFM-06-7. Drenth, B.J., Anderson, R.R., Schulz, K.J., Feinberg, J.M., Chandler, V.W., and Cannon, W.F. 2015. What lies beneath: geophysical mapping of a concealed Precambrian intrusive complex along the IowaMinnesota border. Canadian Journal of Earth Science, v. 52, p. 1-15. Drenth, B.J., and Brown, P.J. 2016. Airborne magnetic total-field survey, Manchester region, Iowa, USA. U.S. Geological Survey data release, https://doi.org/10.5066/F7416V52. 25 �Keweenaw Fault System along Bête Grise Bay, Michigan: Geometry, Kinematics, and Tectonic Significance DEGRAFF, J.M. 1, TYRELL, C.W.1, HUBBELL, G.E. 1, and CARTER, B.T. 2 1 Michigan Technological University, Houghton, MI 49931 Structural Geology Consultant (now at Repsol) Houston, TX 77027 2 The Keweenaw Fault (KF) extends along the southern margin of the Midcontinent Rift System from northwest Wisconsin to near Keweenaw Point in Michigan. Reverse movement on the fault has thrust Portage Lake Volcanics (PLV, 1.1 Ga) over younger, mostly flat-lying Jacobsville Sandstone (JS) (Fig. 1), imparting a regional northerly tilt to PLV strata (1). The KF near Keweenaw Point is of interest because 1950s USGS maps (2-3) show five coastal areas with juxtaposed PLV and JS strata connected by an anomalously sinuous fault trace (Fig. 2a). Based on geophysical data, some have proposed that the KF continues offshore beyond Keweenaw Point in an arc curving over 90° to a southeasterly direction (4-5). These geometries seem incompatible with a simple thrust system. Furthermore, a lack of reported slip indicators prevents defining the ratio of dip to strike slip and estimating principal stress directions responsible for fault motion. New mapping of the KF system along Bête Grise Bay reveals that the oddly sinuous fault trace of the 1950s oversimplifies important geologic relationships in the area (Fig. 2a-b). Three to perhaps four of seven PLV-JS contacts previously mapped as faulted instead have an unconformity between PLV lava flows and basal JS strata. Unconformable contacts to the west show fractured, locally saprolitic PLV basalt below moderately dipping JS strata of alternating muddy siltstone, lithic to quartzose sandstone, and pebble conglomerate with angular basalt fragments in a muddy matrix. An unconformable contact to the east shows slightly deformed JS strata overlying steeply dipping, intensely faulted and brecciated PLV strata, indicating major slip on this KF segment before local JS deposition. At other shoreline locations, deformed JS strata truncated on fault contacts with PLV lavas provide evidence of a second period of slip on the KF system after some or all JS deposition. Recognition of unconformable PLV-JS contacts, combined with mapping both onshore and offshore, breaks the sinuous single fault trace of the 1950s into at least six segments generally striking ESE and forming a left-stepping, en echelon pattern. Well exposed fault surfaces near the shoreline have provided many opportunities to measure orientation of slip indicators and to infer slip sense. Analysis of such measurements at 36 sites indicates that the last period of activity on this part of the KF system was dominated by strike slip, with a 2:1 ratio of dextral strike slip to reverse dip slip (N side up). Geologic relationships across major fault segments are consistent with their north sides sliding to the right and upward relative to opposing sides. Inversion of fault-slip data further confirms a mostly strike-slip regime and indicates a maximum shortening direction of N80°W during the last period of fault motion. South of the Bare Hill rhyolite, a major ENE-trending fault appears to link two ESE-trending, en echelon fault segments (Fig. 2b). The linking fault follows the core of a tight upright anticline in PLV strata with an interlimb angle of 30° or less. PLV strata on the SE flank of the anticline dip steeply to moderately SE (counter-regional) for at least 3.5 km along the shore. Poles to bedding on both flanks of the anticline define a fold axis plunging 21° at N82°E. The tightly folded nature of the faulted anticline in relatively rigid strata implies that the fold formed during dextral strike slip on the linking fault or was modified afterward by such shearing. The trace of the KF system changes direction from NNE near Houghton to ESE at Bête Grise Bay (> 70⁰), which mimics the change in strike of PLV layers over the same distance (Fig. 1). Large crustalscale faults often curve and split into segments near their terminations. The new mapping results thus imply that the KF system terminates near the end of the peninsula in a series of fault splays, possibly transferring slip to other faults farther southeast. Based on these results and regional information, we suggest that slip on the KF system changes from mostly reverse dip-slip along its NNE-trending portion 26 �near Houghton to mostly dextral strike-slip near the tip of the Keweenaw Peninsula, and that total slip magnitude decreases over this same distance. Acknowledgements: We appreciate primary funding by the USGS EDMAP program, additional funding by the Keweenaw Community Forest Company, field and GIS support from D. Lizzadro-McPherson, and discussions with USGS geologists W. Cannon, K. Schulz, and L. Woodruff. Figure 1 (left): Keweenaw Peninsula where Portage Lake Volcanics are thrust over Jacobsville Sandstone. Black rectangle near tip of the peninsula marks area of Figure 2. (adapted from 1). Figure 2 (below): Study area along the Keweenaw Fault east from Bête Grise Bay. Main units: PLV mafic = greens; PLV felsic = reds; JS = pink-A / yellow-B. A) USGS maps from 1950s (2-3). B) New map highlighting fault pattern and PLV-JS unconformity. References 1. 2. 3. 4. 5. Cannon, W.F. and Nicholson, S.W., 2001, Geologic Map of the Keweenaw Peninsula and Adjacent Area, Michigan: United States Geological Survey, Map I-2696, Scale = 1:100,000. Cornwall, H. R., 1954, Bedrock Geology of the Lake Medora Quadrangle, Michigan: U.S. Geological Survey, Washington, D.C., Geologic Quadrangle Map GQ-52, scale 1:24,000. Cornwall, H.R., 1955, Bedrock Geology of the Fort Wilkins Quadrangle, Michigan: U.S. Geological Survey, Washington, D.C., Geologic Quadrangle Map GQ-74, scale 1:24,000. 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, p. 305-332. Hinze, W.J., Allen, D.J., Braile, L.W., and Mariano, J., 1997, The Midcontinent Rift System: a major Proterozoic continental rift: in Ojakangas, R.W., Dickas, A.B., and Green, J.C. (eds.), Middle Proterozoic to Cambrian Rifting, Central North America: Boulder, Colorado, Geological Society of America Special Paper 312, p. 7-35. 27 �Southwest Margin of the Midcontinent Rift System in Eastern Lake Superior: Review and Preliminary Interpretation DEGRAFF, J.M.1 and DEGRAFF, I.S.2 1 Michigan Technological University, Houghton, MI 49931 Geologic Consultant, Houston, TX 77042 2 The relatively well-defined southwest branch of the Midcontinent Rift System (MRS) transitions to the less well-defined southeast branch near Keweenaw Point and the postulated Thiel Fault zone (1-3; Fig. 1). The southwest branch has abundant outcrops of rift-related rocks around Lake Superior and has been extended farther southwest beneath Paleozoic strata with geophysics and widely spaced deep drill holes. In contrast, rift-related rocks of the southeast branch mostly lie beneath lakes Superior and Michigan, post-rift Jacobsville Sandstone (JS), or Phanerozoic strata of the Michigan Basin. Current understanding of the southeast branch of the MRS mostly comes from geophysical data and rare deep boreholes that penetrate Precambrian basement. To better define the transition between the two branches of the MRS, we initiated research on the offshore geology between the Keweenaw Peninsula and the south shore of Lake Superior east of Marquette, Michigan (Fig. 2). Data to be obtained and used include new shoreline and underwater outcrop descriptions and existing seismic and potential field data. This poster provides some initial perceptions and thoughts in the context of prior investigations. Outcrops of rift-related rocks in the area are mainly along the Keweenaw Peninsula and Manitou Island, but probably occur at Stannard Rock shoal and perhaps at other shallow areas along an arc from Keweenaw Point southeastward to Munising (Fig. 2). Information about Stannard Rock is limited to sketchy early reports and one rock sample described as “quartzless porphyry” or rhyolite (4-5). If Stannard Rock shoal proves to have rhyolitic rocks, this outcome together with the rhyolitic flows at the bottom of the Amoco St. Amour 1-29R borehole (6) could indicate a significant area of felsic volcanism along the southwest margin of the MRS in eastern Lake Superior. Other relevant outcrops in the area are Jacobsville strata that rim Keweenaw Bay and extend eastward along the south shore of Lake Superior to Sault Ste. Marie. Along the shore near Munising, aerial imagery available through Google Earth shows JS strata on the rift margin generally striking NS and dipping eastward toward the rift axis defined by geophysical data. Qualitative review of available geophysical data provides additional insight into the nature of the rift margin north of Munising and structural trends in pre-rift basement between the two MRS branches. Five seismic reflection lines in eastern Lake Superior define: (1) an uplifted rift flank to the southwest with sub-horizontal strata, and (2) a rift margin-slope with strata dipping moderately northeast toward the rift basin. Some lines show evidence of a component of reverse faulting along the rift margin (7), but others may be interpreted as having only a flexure without obvious faulting. The NNW-trending rift margin has two jogs, a southern one near Munising and a northern one near Stannard Rock, implying that rift-margin faults are not continuous along the entire margin. The orientation of such faults is more than 90° off trend of the Keweenaw Fault, and so their slip direction must differ from that of the Keweenaw Fault. Therefore, faults along this rift margin are best regarded as distinct from the Keweenaw Fault and should have different names (e.g., Munising, Au Train). Broad trends in potential field data are consistent with rift margin trends interpreted on seismic data, including the two jogs. In addition, aeromagnetic data define several circular to arcuate anomalies up to 16 km in diameter that cluster on the rift flank near the jogs in the rift margin (Fig. 2). Each cluster of circular anomalies appears to lie along ENE-trending zones that 28 �may represent crustal-scale fracture zones. It is possible that these anomalies are caused by eruptive centers that sourced volcanic rocks in their immediate surroundings. Further work is required to test these ideas and to improve understanding of this less studied sector of the MRS. Acknowledgements: We appreciate the helpful and encouraging comments by Bill Hinze (Purdue University) during a review of this abstract. Figure 1 (left): Major rock units and faults in the Lake Superior area; KF-Keweenaw Fault, DF-Douglas Fault, IRF-Isle Royale Fault, TF-Thiel Fault (1). Inset map shows extent of Midcontinent Rift System (MRS) from Lake Superior southwest to Kansas (K) and southeast to Detroit (D). Black rectangle is area of Figure 2. Figure 2 (below): Main structural elements between the Keweenaw Peninsula and Munising. H-Houghton, Ma-Marquette, MuMunising, MI-Manitou Island, SR-Stannard Rock; KF-Keweenaw Fault, TF-Thiel Fault, F-unnamed rift-margin faults. Dark red “faults” inferred from geophysical data. Purple and blue features interpreted from aeromagnetic data and explained in poster. References 1. Miller, Jr., J.D., 2007, The Midcontinent Rift in the Lake Superior region: a 1.1 Ga Large Igneous Province: IAVCEI Large Igneous Provinces Commission, p. 1-18. 2. Hinze, W.J., Allen, D.J., Braile, L.W., and Mariano, J., 1997, The Midcontinent Rift System: a major Proterozoic continental rift: in Ojakangas, R.W., Dickas, A.B., and Green, J.C. (eds.), Middle Proterozoic to Cambrian Rifting, Central North America: Boulder, Colorado, Geological Society of America Special Paper 312, p. 7-35. 3. 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, p. 305-332. 4. Irving, R.D., 1883, The copper-bearing rocks of Lake Superior: U.S.G.S. Mono., v. 5, p. 360-361. 5. 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. 6. Ojakangas, R.W. and Dickas, A.B., 2002, The 1.1-Ga Midcontinent Rift System, central North America: sedimentology of two deep boreholes, Lake Superior region, Sediment. Geol., v. 147(1-2), pp. 13-36. 7. 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. 29 �Neutralization of proton acidity with sequestration of atmospheric CO2 during experimental weathering of intrusive rocks from the Midcontinent Rift System DIEDRICH1, Tamara, DAY2, Stephen 1 MineraLogic LLC, 306 W. Superior St., Alworth Building, Suite 408, Duluth, MN 55802 USA 2 SRK Consulting (Canada) Inc., 1066 West Hastings St., Vancouver, BC, V6E 3XS Canada Intrusive rocks associated with the Mesaba Deposit1, contained predominantly in the Bathtub Intrusion of the Duluth Complex2, have been the subject of a comprehensive geochemical characterization program, initiated in 2010, to inform plans for managing water and waste rock on any potential future mining project. This program includes multiple experimental components to characterize subaerial weathering reactions, rates, and products; including, but not limited to, laboratory testwork using an ASTM standard method under the “humidity cell test” configuration, laboratory testing on columns of rock, and a field-based, larger scale barrel test program. Duluth Complex rocks tend to contain abundant olivine and/or plagioclase, both of which are relatively reactive acid-neutralizing and, potentially, carbonate-forming silicate minerals. Experimental weathering outcomes confirm the effectiveness of silicate dissolution in neutralizing proton acidity through three distinct mechanisms: 1) consumption of protons as reactants in silicate mineral dissolution reactions; 2) reaction with dissolved alkalinity formed during dissolution of silicate minerals in the presence of atmospheric CO2; and, 3) as reactants during dissolution of secondary carbonate minerals, which were precipitated as weathering products of primary silicate phases. Furthermore, as suggested by the latter two of the above numerated mechanisms, silicate weathering reactions in the presence of atmospheric CO2, represents a well-established net sink for atmospheric CO2 in the form of carbonate mineral weathering products. Weathering reactions for relatively reactive silicate minerals that are abundant in Duluth Complex rock include those shown below for An50 and olivine, respectively: Na0.5Ca0.5Al1.5Si2.5O8(s) + 1.5 H+ + 6.5 H2O ↔ 0.5 Ca2+ + 0.5 Na+ + 1.5 Al(OH)3 + 2.5 H4SiO2 (Mg,Fe)2SiO4(s) + 4 H+ ↔ 2 (Mg2+, Fe2+) + H4SiO4 Subsequent oxidation and hydrolysis of iron from the olivine breakdown reaction releases hydrogen through the following reaction: Fe2+ + 1/4O2 + 5/2H2O ↔ Fe(OH)3 + 2H+ Every cationic charge unit3 added to solution corresponds to a proton being removed. 1 The Mesaba Deposit is a magmatic copper-nickel-PGM deposit described by >800,000 feet of diamond drilling that is owned by Teck American Inc. a wholly owned subsidiary of Teck Resources Limited. 2 Severson, M J, 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. 3 Charge unit concentration is equal to molar concentration times charge. Release of Fe2+ during dissolution consumes protons, which are re-released upon oxidation and hydrolysis of iron. Therefore, release of iron is overall proton-neutral and not included. 30 �The relationship between molar concentrations of cations and sulfate in weathering test leachate is a robust indicator of leachate pH across all experimental configurations. Figure 1 shows data from 3,053 individual leachate samples from over 40 different tests. The y-axis represents the relative rates of proton consumption and production during weathering, as indicated by the “charge unit balance” (defined in the figure) of the leachate sample. When the composition of the leachate indicates that protons are being consumed by silicate dissolution faster than they are being produced during sulfide mineral oxidation, the leachate pH is higher than the blank; i.e., there is a net decrease in proton concentration. Conversely, pH of the leachate becomes acidic when the composition of the leachate indicates that protons are being released faster than they are consumed. The clear relationship between rates of proton production and consumption, and drainage pH is an indication of the effectiveness of silicate mineral dissolution in neutralization of proton acidity in the weathering tests. Figure 1. Leachate data from weathering tests (n=3053) showing relationship between charge unit balance and pH. “Charge unit balance”, defined as the molar ratio of cationic charge unit concentration to sulfate charge unit concentration (oxidation of one mol of sulfur in pyrrhotite to sulfate releases two protons). When charge unit balance is equal to one (shown as dashed line), the rate of consumption and production of protons during weathering is equal. Dotted line shows lowest pH observed in leachate from blank tests. In addition to sulfide mineral oxidation, dissolution of atmospheric CO2 into rainwater can provide protons for silicate dissolution, through equilibria between dissolved CO2 and carbonic acid (H2CO3), and the subsequent dissociation of carbonic acid to bicarbonate alkalinity and protons. Therefore, in the presence of CO2, consumption of protons during silicate dissolution would continue to drive this reaction toward the reaction products, resulting in accumulation of bicarbonate alkalinity in associated waters. Under select conditions, this carbonate builds up and eventually reacts with the calcium and magnesium released during silicate dissolution to precipitate secondary carbonate minerals. While secondary carbonate minerals have not, yet, been directly detected as experimental products, leachate chemistry suggests that, as the ratio of the rock to water increases for different experimental configurations, the leachate becomes more concentrated in calcium, magnesium, and bicarbonate alkalinity, until, eventually, calcium/magnesium ratios decrease, as calcium carbonate is presumably being preferentially precipitated out of solution. 31 �Morphology, mineralogy, texture, and genesis of peperite, Fivemile Lake, Vermilion District, Minnesota: Comparison with Pleistocene peperite, Iceland. DRAZAN, Jacqueline L.1, HUDAK, George2, MOOERS, Howard1 1 Department of Earth and Environmental Sciences, University of Minnesota - Duluth, 1114 Kirby Dr., 229 Heller Hall, Duluth, MN, 55812; 2Natural Resources Research Institute, 5013 Miller Trunk Hwy, Hermantown, MN, 55811. Peperites are defined as a “rock formed essentially in situ by disintegration of magma intruding and mingling with unconsolidated or poorly consolidated, typically wet sediments” (White et al., 2000, p. 65). Pillowed dikes and associated peperite is well exposed at Fivemile Lake in the Vermilion District of northeastern Minnesota (Hudak et al., 2002; Hudak et al., 2003; Hudak et al., 2004). The rocks are Neoarchean in age (~2.7 billion years, Peterson et al., 2001), and contain well-preserved and well-studied volcanic facies (e.g. Hudak et al., 2002, 2003, 2004). The sequence at Fivemile Lake has been interpreted as recording a series of northeasttrending mafic dikes which have intruded wet volcaniclastic sediments to produce peperite deposits at different levels within the seafloor in a relatively shallow (<1500 m) submarine volcanic system (Hudak et al., 2004). In the current study, outcrops were mapped at a scale of 1:39 with field work focused on extremely detailed mapping to evaluate peperite deposit morphology, mineralogy, and textures. The igneous component is pillowed to massive, dominated by amygdules, and grades into the host sediment (Fig. 1, right). Outside the margins of the pillowed dikes, both globular and blocky peperite comprising isolated igneous clasts floating in the host volcaniclastic sedimentary rock are present. The volcaniclastic sedimentary rocks are moderately- to highly-vesiculated, and locally contain 1cm wide highly vesiculated zones that are parallel- to sub-parallel to igneous component fragments within the peperite deposits. Although textures are well-preserved both at the outcrop and thin-section scales at Fivemile Lake, the original minerals/glass have been postdepositionally altered to quartz, epidote, and carbonate minerals. As an analog for the Archean Fivemile Lake peperite, Pleistocene peperites from three locations in Iceland were described, sampled, and analyzed. Locations include three sites in móberg, two near Sveifluháls, Iceland, and a site at Reynisfyara Beach near Vik, Iceland. Kagy (2011) identified peperites along pillowed dyke margins in móberg near Sveifluháls, Reykjanes Peninsula, Iceland. Both blocky and fluidal types were described along with anomalous igneous clasts in a host rock of hydrothermally altered lapilli tuff (palagonite formation). The rocks are relatively unaltered and contain abundant glass with some alteration to palagonite. Host sediment is highly vesiculated and glassy, with broken, jagged hyaloclastite fragments making up the matrix (Fig. 1, left). Some fragments have phenocrysts, while other fragments are separated by a junky, opaque matrix, likely a result of surficial weathering at the outcrop. Evaluation of the peperite at Fivemile Lake with comparison to Pleistocene peperites aids in identification of primary peperite textures and morphologies. Documentation and mapping of peperites is useful in determining and understanding magma-water interactions and hydrovolcanic processes like magma explosions in wet sediment. Formation of peperites at Fivemile Lake is spatially associated with synvolcanic faults and occurred near the paleo- 32 �seafloor, which is a prospective geologic setting for volcanogenic massive sulfide deposits (Gibson et al., 1999; Rosa et al., 2016). Figure 1: Field images on left from Iceland (Site 2L; Kagy, 2011) indicate pillowed dyke with peperite next to host sediment. On right, field image shows pillow lava intruding and budding off into host sediment on Peperite Point. References 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. 13-48. Hudak, G. J., Newkirk, T. T., Odette, J., and Hauck, S., 2002. Comparative Geology, Stratigraphy, and Lithogeochemistry of the Fivemile Lake, Quartz Hill, and Skeleton Lake VMS Occurrences, Vermilion District, NE Minnesota: Natural Resources Research Institute Technical Report NRRI/TR-2002/03, 390 p. Hudak, G. J., Newkirk, T. T., Odette, J., and Hauck, S., 2003. Comparative Geology, Stratigraphy, and Lithogeochemistry of the Fivemile Lake, Quartz Hill, and Skeleton Lake VMS Occurrences, Vermilion District, NE Minnesota: Natural Resources Research Institute Report of Investigation NRRI/RI-2003/18, 390 p. Hudak, G. J., Newkirk, T. T., Drexler, H., Odette, J. D., and Hocker, S. M., 2004. Neoarchean Peperites in the Vicinity of Fivemile Lake, Vermilion District, NE Minnesota: Institute on Lake Superior Geology, V. 50, Part 1- Proceedings and Abstracts, p. 84-85Kagy, H.M. 2011. Interaction Of Basaltic Dikes And Wet Lapilli Tuff At Glaciovolcanic Centers: A Case Study Of Sveifluháls, Iceland As A Terrestrial Analog For Dike-cryosphere Interaction On Mars, Master’s thesis. University of Pittsburgh, Department of Geology and Planetary Science. Mercurio, E. C. 2011. Processes, Products and Depositional Environments of Ice-Confined Basaltic Fissure Eruptions: A Case Study of the Sveifluháls Volcanic Complex, SW Iceland, Ph.D. dissertation, University of Pittsburgh, Department of Geology and Planetary Science. Peterson, D.M., Gallup, C., Jirsa, M.A., and Davis, D.W. 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. Rosa, C.J.P., McPhie, J., Relvas, J.M.R.S. 2016. Distinguishing peperite from other sediment-matrix igneous breccias: Lessons from the Iberian Pyrite Belt. Journal of Volcanology and Geothermal Research: 315, p. 28-39. White, J.D.L., McPhie. J., Skilling, L. 2000. Peperite: a useful genetic term. Bulletin of Volcanology: 2, p. 65-66. 33 �The Dickinson Group in the Central Upper Peninsula of Michigan: Part 2 - Geophysical expression and a preliminary interpretation of its eastward extent under Paleozoic cover DRENTH, Benjamin J.1, CANNON, William F.2, and SCHULZ, Klaus J.2 1 U.S. Geological Survey, PO Box 25046, MS 964, Denver Federal Center, Denver, CO, 80225 2 U.S. Geological Survey, 12201 Sunrise Valley Dr., MS 954, Reston, VA, 20192 The Dickinson Group crops out in central Dickinson County, Michigan, and includes three formations (described in detail by James et al., 1961) that may contain a unique metasedimentary and volcanic record of the final breakup of the Superior and Wyoming cratons (Cannon et al., this volume). The basal East Branch Arkose is made up of arkose, conglomerate, and basalt flows. The overlying Solberg Schist consists of finer clastic rocks, metavolcanics rocks, and an iron-formation, the Skunk Creek Member. The uppermost member, the Six Mile Lake Amphibolite, consists of mafic metavolcanic rocks. The contact between the East Branch Arkose and Solberg Schist is gradational. The contact between the Solberg Schist and Six Mile Lake Amphibolite is not exposed, but was interpreted to be conformable (James et al., 1961). Where exposed west of the edge of Paleozoic cover, the Dickinson Group forms a nearly vertical, south-facing monocline extending more than 20 km with consistent east-west strike (Fig. 1). The Dickinson Group was originally interpreted as Archean, based on an apparent gradational contact between the Six Mile Lake Amphibolite and Archean granite to the south (James et al. 1961). However, various lines of evidence establish an apparent age range of ~2.1 to 1.83 Ga for the entire Dickinson Group (Holm et al., 2007; Craddock et al., 2013; Ayuso et al., 2018; Schulz et al., 2018; Cannon et al., 2018b; Cannon et al., this volume). Cleary, the nature of the Six Mile Lake Amphibolite-Archean contact (queried on Fig. 1) is critical to the interpretation of the Dickinson Group and merits further study. Parts of the Dickinson Group have geophysically distinctive features compared to surrounding Precambrian rocks. The high-density Six Mile Lake Amphibolite is the dominant source of an east-west elongated, ~13 mGal gravity high that extends 10s of km over both the area of exposure and Paleozoic cover to the east (Drenth et al., 2018). The ~2.1 Ga (Ayuso et al., 2018) “porphyritic red granite” (prg, Fig. 1), a probable source of detritus for sedimentary parts of the Dickinson Group, produces a ~4 mGal gravity low and a zone of mostly quiet aeromagnetic anomalies. Geophysical data show that it is a larger body than shown by previous mapping. Numerous narrow, strike-parallel elongated aeromagnetic highs lie over all units of the Dickinson Group, including the following examples. Aeromagnetic highs with amplitudes up to 600 nT lie over the East Branch Arkose, interpreted to reflect interbedded basalt flows (James et al., 1961). The Skunk Creek Member iron-formation of the Solberg Schist produces an aeromagnetic high with a maximum amplitude of 2000 nT, distinguishing it from other anomaly sources in the area. Other aeromagnetic highs with amplitudes <500 nT do not have confirmed sources, but have been generally ascribed to diabase dikes, gabbroic intrusions, and other magnetic layers within the Dickinson Group (James et al. 1961). A preliminary interpretation of the eastward subcrop extension (under Paleozoic cover) of the Dickinson Group (Fig. 1) is based on 3D inverse gravity modeling of the geometry of the Six Mile Lake Amphibolite, tracing the distinctive aeromagnetic signature of the Skunk Creek Member, and following the strikes of other aeromagnetic anomalies. The volume of the Six Mile Lake Amphibolite is interpreted to increase dramatically to the east of where it is exposed, and the Skunk Creek Member is interpreted to be complexly folded east of the Paleozoic contact, in 34 �contrast to the monoclinal structure to the west. At least two, and perhaps three folds are indicated by aeromagnetic patterns. Collectively, these interpretive observations may be best reconciled by a model that involves complexly faulted and folded Solberg Schist and Six Mile Lake Amphibolite, including a possible thrust sheet (Fig. 1). The broader tectonic significance of this model hinges on the true nature of the Six Mile Lake Amphibolite-Archean contact. Figure 1: Preliminary interpretation of the full extent of the Dickinson Group, modified from James et al. (1961), Craddock et al. (2013), Cannon et al. (2018a,b), and Cannon et al. (this volume). References Ayuso, R. A., Schulz, K. J., Cannon, W. F., Woodruff, L. G., Vasquez, 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, p. 7-8. Cannon, W. F., Schulte, R., and Bickerstaff, D., 2018a, Exposed Precambrian bedrock in part of Dickinson County, Michigan, and Marinette and Florence Counties, Wisconsin: U.S. Geological Survey data release: https://www.sciencebase.gov/catalog/item/59a5b942e4b075bb795913e1. Cannon, W. F., Schulz, K. J., Ayuso, R. A., and Mroz, T. H., 2018b, Field Trip 1: Archean and Paleoproterozoic geology of the Felch District, Central Dickinson County, Michigan, in Cannon, W. F., ed., Institute on Lake Superior Geology 64th Annual Meeting Proceedings Volume 2: Field Trip Guidebooks, p. 1-38. Cannon, W.F., Schulz, K.J., Drenth, B.J., this volume, The Dickinson Group of Dickinson County, Michigan: Part 1- age and tectonic setting based on new geophysical, geochemical, and geochronologic data: Institute on Lake Superior Geology, Proceedings of 65 th Annual Meeting, Part 1: Program and Abstracts. 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. Drenth, B.J., Woodruff, L.G., Schulz, K.J., Cannon, W.F., and Ayuso, R.A., 2018, On the source(s) of the FelchArnold gravity anomaly, Upper Peninsula, Michigan: Institute on Lake Superior Geology, Proceedings of 64 th Annual Meeting, Part 1: Program and Abstracts, p. 27-28. Holm, D. K., et al. (2007). "Reinterpretation of Paleoproterozoic accretionary boundaries of the north-central United States based on a new aeromagnetic-geologic compilation." Precambrian Research, v. 157, p. 71-79. 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. Schulz, K.J., Cannon, W.F., and Woodruff, L.G., 2018, Geochemistry of mafic rocks in Dickinson County, Michigan: evidence for ~2.1 Ga rifting: Institute on Lake Superior Geology, Proceedings of 64 th Annual Meeting, Part 1: Program and Abstracts, p. 93-94. 35 �High-resolution aeromagnetic survey, central Upper Peninsula, Michigan DRENTH, Benjamin J.1, CANNON, William F.2, and SCHULZ, Klaus J.2 1 U.S. Geological Survey, PO Box 25046, MS 964, Denver Federal Center, Denver, CO, 80225 2 U.S. Geological Survey, 12201 Sunrise Valley Dr., MS 954, Reston, VA, 20192 We present a new aeromagnetic dataset from a high-resolution (150 m line spacing, 80 m nominal terrain clearance) regional fixed-wing survey (~37,000 line km) flown over portions of the central Upper Peninsula of Michigan in 2018. The survey footprint includes areas with Precambrian bedrock between Marquette and Iron Mountain and extends eastward over a large area with weakly magnetized Paleozoic sedimentary cover (Fig. 1), which will allow interpretation of Precambrian subcrop. Archean rocks of the gneiss terrane south of the Great Lakes Tectonic Zone (GLTZ), a Neoarchean suture, are generally weakly magnetized. A swarm of north-northeast trending magnetic dikes are imaged cutting the gneiss terrane between the Bush Lake fault and the GLTZ. These dikes are not detected north of the GLTZ, indicating the swarm predates the suture. Magnetic highs north of the GLTZ lie over exposures of the Archean greenstone-granite terrane and trend subparallel to the GLTZ trend. Metasedimentary rocks of the Paleoproterozoic Chocolay and Baraga Groups are generally weakly magnetized. Iron formations within the Menominee Group (i.e., the Vulcan Iron-formation) produce very large amplitude positive anomalies. Anomaly amplitudes in the Felch and Calumet troughs reach ~15,000 nT. Several other very large amplitude anomalies (up to ~35,000 nT) lie over the Paleozoic sedimentary cover to the east and are produced by very strongly magnetized iron formations in the Precambrian subcrop that have been drilled by the private sector (Waggoner, 2007). The Dickinson Group, once thought to be Archean (James et al. 1961) but now considered to be at least partly Paleoproterozoic (e.g., Cannon et al., 2018), is characterized by numerous east-west elongated, narrow magnetic highs. Some of these highs have been interpreted to reflect mafic volcanic rocks and an iron formation, but the sources of others are not explicitly known (James et al., 1961). Multiple generations of likely Proterozoic dikes are expressed in the aeromagnetic data. Numerous reversely polarized dikes interpreted to be Keweenawan (i.e., related to the ~1.1 Ga Midcontinent Rift System) trend east-northeast. Normally polarized dikes that are also likely Keweenawan trend west-northwest. A swarm of northwest-trending dikes of unknown age trends subparallel to the GLTZ. References 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: http://pubs.usgs.gov/of/1999/of99-547/. Cannon, W. F., Schulz, K. J., Ayuso, R. A., and Mroz, T. H., 2018, Field Trip 1: Archean and Paleoproterozoic geology of the Felch District, Central Dickinson County, Michigan, in Cannon, W. F., ed., Institute on Lake Superior Geology 64th Annual Meeting Proceedings Volume 2: Field Trip Guidebooks, p. 1-38. 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. Waggoner, T. D., 2007, Definition of the Proterozoic terrain under the Paleozoic -- central U.P., Michigan: Institute on Lake Superior Geology 53rd Annual Meeting, p. 85-86. 36 �Figure 1: Simplified bedrock geology of the aeromagnetic survey region, modified from Cannon and Ottke (1999) and Cannon et al. (2018). 37 �What do detrital zircon studies of the Huronian Supergroup tell us? an analysis of all published data EASTON, Robert Michael1 1 Adjunct Professor, Department of Earth Sciences, Carleton University, Ottawa, Ontario Since the publication of the first detrital zircon analyses from the Huronian Supergroup in 2006 (Rainbird and Davis 2006), detrital zircon work has been completed on more than 25 samples of the supergroup, from almost every unit (except for the Pecors, Espanola and Bruce formations) (Craddock et al. 2013; Davis et al. 2018; Easton and Heaman 2008, 2011; Hill et al. 2018; Kenny et al. 2018; Long et al. 2011; Ménard 2017; Petrus et al. 2016; Rasmussen et al. 2013). Most of this work occurred in the area between Sudbury and Sault Ste. Marie, all north of the Murray fault, with only 2 samples studied so far from the Cobalt basin northwest of Sudbury. These data are summarized in Table 1, with age ranges and averages based on grains that are < 5% discordant, a lower cutoff than used in most studies. Key observations are: • Zircons between circa 2450 and 2490 Ma, likely derived from either Huronian Supergroup volcanic rocks and/or related mafic and felsic intrusions, so far have been reported only from the Matinenda or the Mississagi formations, generally from sample sites near the base of the supergroup. • Samples from the lower Huronian Sgp (Elliot Lk and Hough Lk groups) are dominated by Geon 26 detritus, consistent with provenance dominated by local sources characteristic of the RamsayAlgoma granitoid complex. Where detailed stratigraphic sampling has occurred, the lowermost units have unimodal populations, becoming more diverse with increasing stratigraphic height (e.g., Easton and Heaman 2011). The only exceptions are the 2 samples from the Cobalt basin, which are dominated by Geon 27 populations, consistent with more >2.7Ga basement in that area. • Above the Mississagi Formation, Geon 27 populations are dominant, but Geon 28, 29 and Geon 30 grains are also commonplace. This may reflect a change in sedimentation style, and/or increased erosion of the hinterland resulting in a wider range of source material becoming available. • The uppermost Huronian Sgp units have ages of circa 2310 Ma (Hill et al. 2018; Rasmussen et al. 2013), meaning deposition of the entire supergroup took place between circa 2460 to 2310 Ma. • Persistent throughout the sequence are occasional Geon 25 grains, typically with ages of 2550-2590; these grains become somewhat more abundant in the upper two groups. These grains have no known local source, and as suggested by Bleeker (pers. comm. 2019). may have a source region to the south, such as the Kaapvall craton, that was subsequently rifted away from North America. • Currently it is not possible to determine if the detrital zircon populations differ between glaciogenic (e.g., Ramsay Lake, Gowganda) units and the non-glaciogenic sandstone units. • Grains >3.0 Ga occur sporadically throughout the supergroup, mainly in the Matinenda and Mississagi formations, and could be sourced locally from Michigan (see Ayuso et al. 2017). More difficult to explain is the population of 29 ancient grains, 3.0-3.6 Ga, in the Gowganda Formation sample from Cobalt. Is this sourced locally in the Cobalt area, or have these grains been transported from sources currently exposed on the northeast shore of Hudson’s Bay? It is unclear if the sampled unit is glaciogenic or not, as the sampled rock type was not specified by Kenny et al. (2017). References Ayuso, R.A., Schulz, K.J., Cannon, W.F., Woodruff, L.G., Vazquez, J.A. and Jackson, J. 2017. Evidence for the presence of Eoarchean crust in northern Michigan; in 63rd Institute on Lake Superior Geology Annual Meeting, Wawa, ON, Proceedings v.63, pt.1, .9-10. 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, 623-644. Davis, D.W., Ménard, J. and Sutcliffe, C.N. 2018. U-Pb geochronology by LA-ICP-MS in samples from northern Ontario; internal report prepared for the Ontario Geological Survey, Jack Satterly Geochronology Laboratory, University of Toronto, Toronto, Ontario, 94p. Easton, R.M. and Heaman, L.M. 2008. Detrital zircon geochronology of Huronian Supergroup sandstones located within the Vernon structure, north of Espanola, Ontario; 54th Institute on Lake Superior Geology, Proceedings, v.54, pt.1, 21-22. 38 �Table 1. Summary of data for all Huronian Supergroup samples based on grains ≤ 5% discordant, in most studies many more grains were analyzed. For samples with significant discordance, the lower numbers shown are for grains ≤ 10% discordant. Also indicated are grains per Geon. All samples are sandstones unless otherwise noted. Samples from he Cobalt Basin are in italics. Abbreviations: cong, conglomerate; EL, Elliot Lake area; MCB, main conglomerate bed; S, Sudbury area; TH, Thessalon area. Formation Bar River mudstone Bar River EL Gordon Lake EL Gordon Lake EL Lorrain EL Gowganda Number n=16 n=62 n=57 n=30 n=172 Range (Ma) 2279-2745 2523-3074 2284-2840 3 sites 2684-2890 2520-3614 Serpent EL Serpent EL-S n=46 n=10 n=19 n=63 n=22 n=130 n=117 n=72 n=65 n=25 n=37 n=36 n=210 n=39 n=27 n=30 n=47 n=36 n=5 n=15 n=28 2549-3576 2531-3317 2531-3317 2443-3617 2591-2832 2388-3286 2414-2978 2544-2949 2656-2887 2526-2719 2607-2821 2533-2752 2366-2906 2505-3774 2451-2714 2650-2742 2620-2897 2617-2776 2634-2651 2621-2684 2546-2838 Main Peak (Ma) 2344 2706 (27>>26) 2317, 2702 (26≈27) 2308, 2308, 2311 2713 (27>26) 2705, 2857, 2965, 3076, 3316 (27>26) 2719 (27>>26) 2688 (5%) 2688 (10%) 2466, 2692 (26>27) 2663 (26>>27) 2477, 2697 (26≈27) 2490, 2560, 2689 2683 (26>>27) 2697 (26>27) 2659 (26>>27) 2677 (26>>27) 2670 (26>>27) 2459, 2703, 2771 2557, 2661 (26>>27) 2457, 2671 (26>>27) 2680 (26>>>27) 2664 (26>>>27) 2649 (26>>>27) 2641 (5%) 2643 (10%) 2641 (26>>>27) n=37 2507-2890 2698 (26≈27) Mississagi EL Mississagi EL-S Mississagi (upper) S Mississagi S Mississagi S Ramsay Lake EL-S Ramsay Lake S Ramsay Lake S cong McKim S Mississagi cong Matinenda S Matinenda EL-S Matinenda EL Matinenda (upper) EL Matinenda EL Matinenda above MCB EL Matinenda below MCB EL Livingstone Creek TH 24 2 4 2 3 25 5 5 3 26 1 18 20 27 2 30 22 28 29 >3.0 2 1 4 3 2 6 34 18 51 6 38 18 29 22 4 8 13 1 57 39 22 28 3 8 7 122 5 4 5 3 3 11 1 3 1 7 9 3 9 4 8 24 18 57 47 44 35 19 27 26 78 22 20 25 47 33 5 15 24 1 17 16 1 1 2 5 2 2 10 4 3 1 3 3 11 3 4 1 9 11 2 2 2 7 2 4 1 1 1 1 3 Easton, R.M. and Heaman, L.M. 2011. Detrital zircon geochronology of Matinenda Formation sandstones (Huronian Supergroup) at Elliot Lake, Ontario: Implications for uranium mineralization; 57th Institute on Lake Superior Geology, Proceedings, v.57, pt.1, 31-32. Hill, C.M., Davis, D.W. and Corcoran, P.L. 2018. New U-Pb geochronology evidence for 2.3 Ga detrital zircon grains in the youngest Huronian Supergroup formations, Canada; Precambrian Research, v.314, 428-433. Kenny, C.G., Petrus, J.A., Whitehouse, M.J., Daly, J.S., and Kamber, B.S. 2017. Hf isotope evidence for effective melt homogenisation at the Sudbury impact crater, Ontario, Canada; Geochimica et Cosmochimica Acta, v.215, 317-336. Long, D.G.F., Ulrich, T. and Kamber, B.S. 2011. Laterally extensive modified placer gold deposits in the Paleoproterozoic Mississagi Formation, Clement and Pardo Townships, Ontario; Canadian Journal of Earth Sciences, v.48, 779-792. Ménard. J.A. 2017. Sedimentary provenance of the Elliot Lake and Hough Lake groups, Huronian Supergroup, Sudbury area; in Summary of Field Work and Other Activities, 2017; Ontario Geological Survey, Open File Report 6333, 17-1 to 17-7. Petrus, J.A., Kenny, G.G., Ayer, J.A., Lightfoot, P.C. and Kamber, B.S. 2016. Uranium-lead zircon systematics in the Sudbury impact crater-fill: implications for target lithologies and crater evolution; Journal of the Geological Society; v.173, 59-75. Rainbird, R.H. and Davis, W.J. 2006. Detrital zircon geochronology of the western Huronian Basin; in 52nd Institute on Lake Superior Geology Annual Meeting, Sault Ste. Marie, ON, Proceedings v.52, pt.1, 55-56. Rasmussen, B., Bekker, A. and Fletcher, I.R. 2013. Correlation of Paleoproterozoic glaciations based on U–Pb zircon ages for tuff beds in the Transvaal and Huronian Supergroups; Earth and Planetary Science Letters, v.382, 173-180. 39 �Hyperspectral Imaging of Bedrock Core from the Minnesota DNR Drill Core Library: A New Tool for Archival Preservation and Mineral Exploration ELSENHEIMER, Don1, DEYELL-WURST, Cari2, and FONTENEAU, Lionel C.3 1 Minnesota Department of Natural Resources, 500 Lafayette Rd, St. Paul, MN 55155 USA Corescan Pty Ltd, 22033 Boul Gouin Ouest, Montreal, QC, CANADA 3 Corescan Pty Ltd, 1/127 Grandstand Road, Ascot WA 6104, AUSTRALIA 2 The Minnesota Department of Natural Resources (DNR) hired Corescan Inc. to scan 4900m of bedrock core from the DNR Drill Core Library (DCL) using Corescan’s hyperspectral core imaging system (Martini et al, 2017). The technique integrates both Visible Near InfraRed (VNIR) and Shortwave Infrared (SWIR) reflectance spectroscopy with high-resolution photography (50 µm) and 3-d laser profiling (200 µm) to identify minerals, estimate mineral abundances and create textural maps at 500 µm resolution. Hyperspectral imaging is a non-destructive analytical technique that supports the archival preservation of limited core material. Project results support DNR land management decisions on state mineral rights and promote mineral exploration and development. This project for the first time will provide public access to hyperspectral imaging data archived within the Coreshed® Virtual Core Library. DNR anticipates public release of project data and public access to Coreshed by summer, 2019. The DNR selected project core from thirty-two (32) drill holes located in five areas in Northern and Central Minnesota with distinct mineral deposits and/or high mineral potential. Initial project results are from an Archean Wabigoon Subprovince greenstone terrane near International Falls (Seine Group) and Biwabik Iron Formation core from the Mesabi Range. The Seine Group of greenschist-facies, metasedimentary and metavolcanic rocks sits at the contact between the Wabigoon and Quetico Subprovinces of the Archean Superior Province (Jirsa et al., 2014). Gold exploration in the region included an active period of drilling in the late 1980’s. Frey (2012) re-logged and re-sampled several of the DCL-archived Seine Group cores, and identified alteration patterns and features favorable for gold mineralization, including greater abundances of porphyoblastic and vein tourmaline. Hyperspectral imaging of twelve archived DCL cores from the area extends Frey’s tourmaline observations to drill cores that (due to active exploration) were not available at the time of his study. There is a positive correlation between gold concentrations and hyperspectral mineral identification of under-recognized tourmaline. Variations in the 2350nm feature position (Bierwirth, 2008) suggest tourmaline compositions within the dravite-schorl series (Figure 1). Complete or near complete transects of the Biwabik Iron Formation (BIF) were imaged in six Mesabi Range drill cores (LWD99-1, LWD99-2, MDDP-2, -5, -7, and -8). Hyperspectral imaging of core from LWD99-2 is able to differentiate microplaty hematite banding from more martite-rich bands. Two chlorite types are also recognized within this same core based on absorption features; an Mg-Fe intermediate composition that occurs in the Virginia Formation and its contact with the underlying Upper Slaty Unit, and a more iron-rich chamosite found in the Lower Cherty Unit and its contact with the underlying Pokegama Quartzite. Average albedo in the visible spectral range (448-740nm) highlights variation within the heavily sampled contact between the BIF and overlying Virginia Formation, where Addison et al. (2005) identified an ~25 to ~58cm thick ejecta layer associated with the 1850Ma Sudbury impact event. White mica is recognized based on absorption features within an ~ 2.6m interval of LWD99-2 core at the transition from BIF to Virginia Formation. Within this occurrence interval, a much smaller ~ 38cm interval with ammonium-rich white mica (feature around 2010nm, Canet et al. (2015)) is recognized in a thin layer of cherty carbonate. The discovery of relatively rare ammonium-rich white mica in association with an identified ejecta layer, if confirmed, would be significant. 40 �References Addison W.D., Brumpton G.R., Vallini D.A., McNaughton N.J., Davis D.W., Kissin S.A., Fralick P.W., and Hammond A.L. (2005) Discovery of distal ejecta from the 1850 Ma Sudbury impact event. Geology 33:193-196. Bierwirth, P.N. (2008) Laboratory and imaging spectroscopy of tourmaline - a tool for mineral exploration. 14th Australasian Remote Sensing and Photogrammetry Conference, Darwin. Canet C., Hernández-Cruz B., Jiménez-Franco A., Pi T., Peláez B., Villanueva-Estrada R.E., Alfonso P., González-Partida E., Salinas S. (2015) Combining ammonium mapping and short-wave infrared (SWIR) reflectance spectroscopy to constrain a model of hydrothermal alteration for the Acoculco geothermal zone, Eastern Mexico. Geothermics 53:154-65. Frey B.A. (2012) International Falls Drill Core Descriptions and Chemistry, Koochiching County, Minnesota. Project 378 Open-File Report, Minnesota Department of Natural Resources, Division of Lands and Minerals, 39p. Jirsa M.A., Boerboom T.J., and Chandler V.W. (2014) M-197 Bedrock Geology of the International Falls and LittleFork 30’x60’ Quadrangles, northern Minnesota. Minnesota Geological Survey, Retrieved from the University of Minnesota Digital Conservancy, http://hdl.handle.net/11299/166157. Martini B.A., Harris A.C., Carey R., Goodey N., Honey F., and Tufilli N. (2017) Automated Hyperspectral Core Imaging – A Revolutionary New Tool for Exploration, Mining and Research. in “Proceedings of Exploration 17: Sixth Decennial International Conference on Mineral Exploration” edited by V. Tschirhart V. and M.D. Thomas, p. 911-922. Figure 1: Hyperspectral imaging of tourmaline within an 8cm-long section of quarter-core from DDH TC35-1. This section is within a larger 4 foot (1.22m) core interval that assayed at 4020ppb Au. Variations in the 2350nm feature position (Bierwirth, 2008) suggest compositions within the dravite-schorl series. 41 �Geology and Geochemistry of the Laird Lake Property and Associated Gold Mineralization, Red Lake Greenstone Belt, Ontario GÉLINAS, Brigitte, HOLLINGS, Pete1, FRIEDMAN, Richard2 1 Department of Geology, Lakehead University, Thunder Bay, Ontario P7B5E1 Pacific Centre for Isotopic and Geochemical Research, University of British Columbia 2 The Red Lake greenstone belt (RLGB) is one of world’s best endowed gold districts and like many other gold-rich regions, the individual deposits are closely associated with regional contacts, in part unconformable (Robert et al., 2005). A regional break in Red Lake separates the Mesoarchean and Neoarchean assemblages and hosts 94% of all gold (production, reserves, and resources; Dubé et al., 2003) in the greenstone belt, yet, the relationship between the two Archean packages is still disputed in terms of tectonic history (Stott, 1996; Stott and Corfu, 1991; Hollings and Kerrich, 2000; Roger et al., 2000; Sanborn-Barrie et al., 2001; 2004; Hollings and Kerrich, 2006). The Laird Lake property encompasses the regional break between the Balmer (2.99 to 2.96 Ga) and the Confederation (2.74 to 2.73 Ga) assemblages on the south-western end of the Red Lake greenstone belt, Northwestern Ontario. Multiple gold occurrences on the Laird Lake property generally occur within 200 m of the regional break and could represent the continuation of a similar gold system as seen at the Madsen Mine. The purpose of this study was to determine the tectonic setting in which the assemblages formed, and to characterize the controls on and nature of the gold mineralization associated with the tectonic contact between the Balmer and Confederation assemblages. Only 10 km east of the study area is the past-producing Madsen Mine, which lies on the north side of the regional break between the Balmer and Confederation assemblages. The ore is locally defined by the Austin and McVeigh ore zone, which displays a characteristic mineral banding (Dubé et al., 2000). Detailed mapping of the Laird Lake area highlighted major differences between the two assemblages (Gélinas, 2018). The Balmer assemblage is typically composed of fine-grained, aphyric, locally pillowed mafic volcanic rocks, ultramafic intrusive and volcanic rocks with flow-breccia textures and local spinifex-bearing clasts, and banded-iron formations. In contrast, the Confederation assemblage consists of porphyritic (feldspar) or poikiloblastic (amphibole) mafic volcanic rocks intercalated with intermediate to felsic volcanic rocks that include crystal lapilli tuffs, crystal tuffs and tuffs. Syn-volcanic and syn- to post-D2 intrusions commonly cross-cut the volcanic packages. A regional foliation (~Etrending) is present throughout the volcanic rocks and increases in intensity at the tectonic contact between the two assemblages where a deformation zone no thicker than 100 m is present within the Balmer assemblage. Whole-rock geochemical analyses were undertaken on 161 samples from the Laird Lake area. The Balmer assemblage is composed of tholeiitic mafic volcanic rocks with minor Al-undepleted komatiites, whereas the Confederation assemblage is composed of transitional mafic and calc-alkalic intermediate to felsic volcanic rocks, which display FI, FII, and FIIIb rhyolite trends. Neodymium isotope analyses, in conjunction with trace element geochemistry, suggests that parts of the Balmer assemblage were weakly contaminated by an older intermediate basement. The data suggests both arc and back arc volcanism within the Confederation assemblage, with the arc rocks showing stronger a crustal component than the back-arc rocks. U-Pb geochronology of volcanic and intrusive Confederation units yielded ages of 2741 ± 19 Ma (FI quartz-feldspar porphyritic crystal tuff) and 2737.68 ± 0.79 Ma (diorite). The geochemistry and age of the tuff correlates within error to the Heyson sequence of the Confederation, whereas the diorite is likely a syn-volcanic intrusion. 42 �The Balmer assemblage is interpreted to represent an oceanic plateau formed by plume magmatism on the margins of the North Caribou Terrane whereas the Confederation assemblage was likely built in an oceanic arc setting where both arc and back arc volcanism were occuring simultaneously. The presence of xenocrystic zircons within the 2741 Ma quartz-feldspar porphyritic crystal tuff suggest that melts within the main arc incorporated xenocrystic zircons during ascent through a thin Mesoarchean crustal fragment. Juxtaposition of the Confederation assemblage onto the Mesoarchean assemblages likely occurred between 2739-2733 Ma. Gold mineralization at the Laird Lake property is controlled by a D2 deformation zone within the Balmer assemblage at the tectonic contact between the Balmer and Confederation assemblages. The mineralization is commonly found associated with a mineral banded parallel to the main D2 fabric, accompanied by disseminated arsenopyrite, pyrrhotite, pyrite ± chalcopyrite, similar to the features observed at the nearby Madsen Mine. The Laird Lake property likely represents the continuation of the same mineralized structure found at both the Madsen and Starrat-Olsen mines and was later displaced as far as 10 km west by the dextral Laird Lake fault post-2704 Ma. References Dubé B, Balmer W, Sanborn-Barrie M, Skulski T, Parker J (2000). A preliminary report on amphibolite-facies, disseminated-replacement-style mineralization at the Madsen gold mine, Red Lake, Ontario. Geological Survey of Canada, Current Research 2000-C17, 14 p. Dubé B, Williamson K., and Malo, M., 2003. Gold mineralization from the Red Lake mine trend: Example from the Cochenour-Willans mine area, Red Lake, Ontario, with new key information from the Red Lake Mine and potential analogy with the Timmins camp. Geological Survey of Canada Current Research 2003-C21, 15 p. Gélinas, B., 2018. Geology and Geochemistry of the Laird Lake Property and Associated Gold Mineralization, Red Lake Greenstone Belt, Northwestern Ontario. Unpublished MSc thesis, Lakehead University, 360 p. Hollings P., and Kerrich R., 2000. An Archean arc basalt – Nb-enriched basalt – adakite association: The 2.7 Ga Confederation assemblage of the Birch-Uchi greenstone belt, Superior Province. Contributions to Mineralogy and Petrology, vol. 139, p. 208-226. Hollings P., and Kerrich R., 2006. Light rare earth element depleted to enriched basaltic flows from 2.8 to 2.7 Ga greenstone belts of the Uchi Subprovince, Ontario, Canada. Chemical Geology, vol. 227, p. 133-153. Robert F, Poulsen HK, Cassidy KF, Hodgson CJ (2005) Gold Metallogeny of the Superior and Yilgarn Cratons. Economic Geology 100th Anniversary volume p. 1001-1033. Rogers N., McNicoll V., van Staal C.R., and Tomlinson K.Y., 2000. Lithogeochemical studies in the UchiConfederation greenstone belt, northwestern Ontario: implications for Archean tectonics; Geological Survey of Canada, Current Research 2000-C16, 11 p. Sanborn-Barrie M, Skulski T, Parker J (2001) Three hundred million years of tectonic history recorded by the Red Lake greenstone belt, Ontario. Geological Survey of Canada, Open File 4594, 30 p. Sanborn-Barrie M, Rogers N, Skulski T, Parker J, McNicoll V, Devaney J (2004) Geology and tectonostratigraphic assemblages, east Uchi Subprovince, Red Lake and Birch–Uchi belts, Ontario. Geological Survey of Canada, Open File 4256; Ontario Geological Survey, Preliminary Map P.3460, scale 1:250 000. Stott G. M., 1996. The geology and tectonic history of the central Uchi Subprovince; Ontario Geological Survey, Open File Report 5952, 178 p. Stott, G. M., and Corfu, F., 1991. Uchi subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p. 145-238. 43 �Petrography of several Co-enriched samples from the Atikokan River Intrusions, Atikokan, Ontario GIBBONS1, Jack, DIEDRICH1, Tamara, QUIGLEY, Thomas2 1 MineraLogic LLC, 306 W. Superior St., Alworth Building, Suite 408, Duluth, MN 55802 USA 2 Great Lakes Exploration Inc., Menominee, MI 49858 USA The Atikokan River Intrusions (ARIs) consist of five or more sulfide and oxide-rich mafic intrusive bodies that have been emplaced along a 28-km section of the Quetico Fault Zone (QFZ) east of Atikokan, ON. Sulfide and oxide mineralization within these intrusions was historically explored for iron ore, and locally developed into at least one small-scale, open pit and underground mine in the late 19th century. The intrusions and associated mineralization are also variably enriched in copper, nickel, and cobalt. Great Lakes Exploration, Inc. (GLE) currently controls an approximately 12-km long stretch of the ARIs, which includes a significant portion of the mineralized intrusions. GLE is currently evaluating the potential for these intrusions to contain cobalt, copper, and nickel at concentrations and in mineral phases that are economically recoverable. Optical petrography (reflected and transmitted light) observations, and bulk geochemical data, from seven ARI hand samples, constrain the nature of Co-mineralization and provide information on textural relationships between minerals, as described here. Petrographic characterization indicates that sulfide mineral assemblages includes pyrrhotite, pyrite, and chalcopyrite. The presence of trace sphalerite was previously identified in other samples by QEMSCAN (conducted by XPS Consulting and Testwork Services). Pyrite occurs as 100- to 200-µm sized grains, while pyrrhotite and chalcopyrite occur as smaller 20- to 50-µm sized grains that compose larger aggregates that partially encompass pyrite grains. None of the observed sulfides display complex intergrowth or exsolution textures in the samples evaluated. Magnetite occurs with most sulfide assemblages, contains both chalcopyrite and pyrrhotite inclusions, appears to be roughly positively correlated with pyrrhotite abundance, and can locally replace pyrite. The abundance (15 to 20 volume percent) and textural relationship (e.g., contains sulfide inclusions and crosscuts/replaces igneous phenocrysts) suggest that at least a portion of the observed magnetite is secondary. Though no stoichiometric cobalt phase was definitively identified via optical petrography, cobalt assay results correlate well with pyrite abundance, consistent with the presence of cobaltiferous pyrite. Textural relationships suggest that the observed sulfide assemblage evolved from an early pyrite- to a late pyrrhotite-dominant assemblage, with chalcopyrite present in both early and late assemblages but likely increased in abundance in the latter assemblage. Examples of sulfide and oxide mineral occurrences are provided in Figure 1A-B. Observations on silicate mineralogy help to establish potential peak metamorphic conditions, understand origin and composition of mineralizing hydrothermal fluids, and provide guidance in constraining timing of mineralization. Coarse-grained chlorite intergrowths with pyrrhotite appear to suggest that a portion of the observed mineralization possibly occurred during metamorphism. The lack of primary igneous minerals, in most samples, suggests that secondary alteration was intense, at least locally, and that peak metamorphic conditions reached upper 44 �greenschist facies; historic reports (MacTavish, 1999) of rarely preserved garnet suggest that metamorphic conditions could have reached lower-most amphibolite facies at other locations within the ARIs. Examples of alteration products and textures are shown in Figure 2A-B. A B Figure 1. Examples of typical ARI sulfide assemblages. Both images taken in plane-polarized, reflected light. A) Coarsegrained pyrite and magnetite locally supported by a pyrrhotite-rich matrix. Trace chalcopyrite occurs between pyrite grains. Magnetite locally replaced several pyrite grains near the center portion of image. Pyrrhotite displays a slender reaction rind. B) . Typical pyrite, pyrrhotite, and chalcopyrite assemblage. Several of the pyrite grains contain a distinct pitted core (partially outlined by dashed line) surrounded by a broad growth zone lacking inclusions, which might possibly indicate pyrite growth occurred in two stages. The scale of the image is the same as Fig. 1A. A B Figure 2. Mineral textures that help to constrain the timing and origin of observed sulfide assemblage. Both images taken in plane-polarized, reflected light. Images have been edited to highlight contrast between mineral phases. A) Large igneous orthoclase phenocryst replaced by magnetite. Trace amounts of pyrite and chalcopyrite occur within the magnetite. Pyrrhotite is absent from the sample. B) Coarse-grained, secondary chlorite is intimately intergrown with pyrrhotite in lower left portion of image. Alteration rind on pyrrhotite is very well developed in this sample. Pyrite locally replaced by magnetite. Chalcopyrite and pyrrhotite exhibit strong spatial association that is typical of this sulfide assemblage. Reference MacTavish, A.D. 1999. The mafic-ultramafic intrusions of the Atikokan-Quetico area, northwestern Ontario; Ontario Geological Survey, Open File Report 5997, 127p. 45 �Recognizing MCR magmas generated by partial melting in the SCLM: Lessons from mafic magmas in the Coldwell Complex GOOD, Dave1, HOLLINGS, Pete2 and JEDEMANN, Andrew2 1Department of Earth Sciences, Western University, London, ON N6A 5B7 Canada 2Department of Geology, Lakehead University, Thunder Bay, ON P7B 5E1 Canada We present new interpretations of a comprehensive data set for basalt and intrusive mafic rocks from the Midcontinent Rift. These data display well-defined trends for trace element abundances that demonstrate variable degrees of partial melting in a plume-like source and subsequent fractional crystallization. For instance, diagrams that compare highly incompatible elements Zr and La, La and Yb, or Nb and Th in MCR rocks show the majority of data plot in a field that spans compositions from E-MORB to OIB along approximately linear trends with constant inter-element ratios. Data that deviates from these MCR trends are explained by interaction of the magma with continental crust during ascent, consistent with elevated Th contents or Rb-Sr and Sm-Nd isotope values that confirm contamination. However, there are cases where evidence such as relative Th or major element abundances contradict isotopic evidence that may or may not agree with crustal contamination. Indeed, multiple isotope systems (Pb-Pb and Nd-Sm) are sometimes in disagreement with respect to the degree of contamination. Another mechanism that might explain such irregularities is partial melting of a metasomatized SCLM source (Furman and Graham, 1999; Sgualdo et al., 2015). It has been well established that initial SCLM isotope values can be overprinted by metasomatism, and therefore isotope systematics, in particular Rb-Sr and Sm-Nd are ineffective for distinguishing between crustal contaminated plume magmas and SCLM-derived magmas. Establishing a set of geochemical criteria that could be used to distinguish between mafic rocks in the MCR that were generated by partial melting in the metasomatized SCLM from plume magmas that were contaminated in the crust is the subject of this presentation. In wide-ranging studies of mantle xenoliths from Africa, New Zealand and Europe, secondary minerals found in veins include phlogopite, clinopyroxene and pargasitic amphibole (Frezzotti et al., 2010; Scott et al., 2014). The trace element signatures of each phase exhibit distinguishing features that, since they are among the first minerals to disappear during a partial melting event, will impart distinctive trace element signatures to the resulting magma composition. For instance, the different compatibilities of Rb, Ba and Sr in amphibole compared to phlogopite, or Nb and Th in amphibole compared to clinopyroxene will result in decoupling of LILE abundances due to the relative proportions of each mineral in the source rock. As these minerals contain very high concentrations of incompatible elements relative to the depleted protolith SCLM rock, a relatively small amount (<1-2%) of each mineral will have a very large impact on the resultant magma composition enabling recognition of trace element signature. Magmas generated from Areas of SCLM that have been less impacted by metasomatism, and thus might have a very low proportion of secondary minerals, will have a depleted HFSE signature marked by sub-chondritic Zr/Y, Zr/Hf and very low La/Yb values, and possibly anomalous Sr and Ba. A key example of volcanic rocks that show contradictory isotopic and geochemical evidence for crustal contamination is Mamainse Point Volcanic Group 5b (Shirey et al., 1994), in 46 �which the εNd values of -3.5 and -6.3 indicate significant crustal contamination, but Pb-Pb data imply a maximum of 2% crustal material. The combination of sub-chondritic Zr/Y and Zr/Hf, low La/Yb, very low La, Th, and TiO2 abundances, and corresponding positive Ba and Sr anomalies is strong evidence for derivation from a weakly metasomatized but initially depleted SCLM source. Examples of MCR magmatism from the Nipigon embayment that exhibit SCLMlike signatures are presented to test the usefulness of key features identified in mafic rocks of the Coldwell Complex that distinguish them as originating from the SCLM. The geochemical characteristics of the Nipigon intrusions are examined as test cases to establish whether or not they were derived from the SCLM. References Beccaluva et al., 2001, J. Pet. 42, 173-187. Bodinier, J.L., Menzies, A.M., et al., 2004, J. Pet. 45, 299-320. Frezzotti, M.L., Ferrando, S. et al., 2010, Geochim. Cosmochim. Acta 74, 3023-3039. Furman, T., and Graham, D. 1999, Lithos 48, 237-262. Good D.J. and Lightfoot P.C., CJES, in press. Hollings, P., Hart, T., Richardson, A., MacDonald, C.A., 2007, CJES 44, 1087-1110. Scott, J.M., Hodgkinson, A. Palin, J.M., et al. 2014, Contrib Mineralogy Petrol., 167: 963. Sgualdo, P., Aviado, K., Beccaluva, L., et al., 2015, Tectonophysics, v. 650, p. 3-17. Shirey, S.B., Klewin K.W., Berg, J.H. and Carlson R.W., 1994, Geochim. Cosmochim. Acta, 58, 44754490. Lightfoot, P.C., Sage, R.P., Doherty, W., Naldrett, A.J. and Sutcliffe, R.H. 1999. OGS OFR 5998, 57p. 47 �Recent Efforts to Curate and Provide Access to the Historical Documents of the E.K. Lehmann and Associates Exploration Company GOTTSCHALK, Brad, and ROSE, Caroline Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, Wisconsin, 53705 In October of 2015, the Wisconsin Geological and Natural History Survey (WGNHS) received a large donation of documents from Kate Lehmann, the daughter of renowned exploration geologist Ernest K. Lehmann. While Lehmann worked primarily in Minnesota, his company, E.K. Lehmann and Associates, also worked in northern Wisconsin from the late seventies into the mid-nineties and, during that time, donated a large amount of core from their drilling projects to WGNHS. After Ernest Lehmann passed away, his family donated the records related to his work in Wisconsin to WGNHS, and the paperwork related to his work in Minnesota to the Minnesota DNR. With financial assistance from the Lehmann family, the Minnesota DNR scanned all of the documents donated to them and added them to their Drill Core Library and Mineral Exploration Collections’ interactive map to provide online access. Staff at WGNHS have provided something similar to our users, but with a more narrowly focused scope. The donation to WGNHS was quite large—31 record boxes of reports and other documents, and two cardboard cases of rolled maps and figures. As WGNHS possesses limited resources, we knew we would have to find a way to focus our curation efforts. Consulting with Tom Evans, an emeritus Survey staff member and an authority on mining in Wisconsin, we decided to concentrate on documents that directly related to rock core in our possession. From December of 2015 to February of 2017, Tom Evans and Brad Gottschalk, the WGNHS archivist, searched for documents that provided data for drillholes. Once these were identified, they matched the Lehmann drillholes to records in our geological database, Geobase. 351 individual drillholes in 67 exploration targets were identified in the Lehmann documents. Of these 351 holes, we had physical core from 289. Some of these Lehmann targets are still considered areas of interest for mineral development. The targets most extensively explored were Bend in Taylor County, Ritchie Creek in Price County, and Horse Shoe in Lincoln County. The documentation for drillholes in these and other targets include location maps, geological maps, logs, geological and geophysical cross-sections, chemical analyses of samples and assay reports. In 2017, we received a grant from the USGS’s National Geological and Geophysical Data Preservation Program (NGGDPP) to scan the Lehmann papers and put them online using an interactive map application. Selecting the documents for scanning was a complicated task. In the paperwork were monthly reports for many of the targets, as well as memos and final reports. The documentation for the drillholes was frequently duplicated in multiple monthly reports as well as in the final report. There were multiple cross-sections for the more widely explored targets, and, especially for the Bend target, which showed promise as a gold deposit, there was a great deal of assay data. Gottschalk and two student employees weeded out duplicates and scanned each unique document. In the end, some drillholes represented in the Lehmann papers were not included in the web application due to poor or incomplete data. After excluding these, we compiled data for 331 drillholes in 65 targets contained in 1153 individual documents. Of the 331 holes represented in the project, we have physical core samples from 288. 48 �As the scanning portion of the project neared completion, Caroline Rose, GIS specialist, began to construct the ArcGIS application that would provide online access to the documents. Rose used ArcGIS Online’s Storymaps templates and Web App Builder to present the Lehmann collection in two web maps: one organized by drillhole and one organized by exploration target. The first map features point locations and details of individual drillholes and links to all related documents. Document details can be followed to show all drillholes related to the document. Documents can be opened in PDF format from the map popup or from a table in the interface. The second web map shows exploration targets, which are collections of drillholes, as circular symbols sized according to the number of related documents. It is immediately apparent that three of the targets are related to more than fifty documents (Bend, Ritchie Creek, and Horse Shoe). Several other targets are related to more than ten documents. The targets are linked to their related documents. Again, documents in PDF format can be opened from the map or the table. Figure 1: The interactive map showing the exploration targets represented in the Lehmann papers. Rose configured the data using ArcGIS Pro to establish many-to-many relationships between the datasets, as one drillhole could be related to many documents, and one document could be related to many drillholes. She then used the ArcGIS Online WebApp Builder to create the interface, and a Storymaps template to create the tabbed layout. At the end of the project, metadata records for the 1153 documents scanned and put online were uploaded to the USGS National Digital Catalog. References Minnesota DNR, 2016, Lehmann Family fund collection of Mineral Exploration Documents (including the Polaris Joint Venture (https://www.dnr.state.mn.us/lands_minerals/polaris/index.html) 49 �Superior Shoal Revisited: Evidence for Keweenawan Basalts with Reversed- and Normalpolarity Remanent Magnetization and Early Magma Chemistry, Central Lake Superior GRAUCH, V.J.S. 1 and SCHULZ, K.J. 2 1 U.S. Geological Survey, MS 964, Federal Center, Denver, CO, 80225 U.S. Geological Survey, MS 954, National Center, Reston, VA, 20192 2 Superior Shoal is an easterly trending, ~20-km-long bathymetric bedrock high below the water’s surface near the center of Lake Superior. Being the only accessible bedrock within a radius of about 70 km, the Shoal can provide evidence critical to understanding the structure of the 1.1 Ga Midcontinent Rift in central Lake Superior, yet debates remain about its geology. Located at the intersection of two geophysically interpreted faults, the bathymetric high is composed of a series of ridges of Keweenawan basalts on the south and a broader ridge of sandstone on the north (Manson and Halls, 1991). Previous studies of Superior Shoal give conflicting results on the age of the basalts based on the polarity of remanent magnetization. Magnetic polarities are commonly used to recognize early (>1100 Ma) Keweenawan lavas (reversed-polarity) from younger (<1100 Ma) lavas (normal-polarity), while acknowledging a separate normal-polarity event between ca. 1101-1103 Ma (Swanson-Hysell et al., 2019). Manson and Halls (1991) concluded from paleomagnetic measurements that the basalts have normal-polarity remanence, whereas Teskey et al. (1991) concluded from analysis of aeromagnetic data that the basalts have reversed-polarity remanence. To resolve the apparent disagreement regarding magnetic polarity, we (1) reviewed the paleomagnetic results from the Manson and Halls study, (2) expanded on the aeromagnetic analysis of the Teskey et al. study, and (3) analyzed basalt samples collected during the paleomagnetic study of Manson and Halls to determine if they are chemically affiliated with typical early or late rift lavas (Nicholson et al., 1997). Review of Paleomagnetic Study of Manson and Halls A review of the methods, analyses, estimated errors, and results of the Manson and Halls (1991) study from their three basalt sites at Superior Shoal give confidence in their results. They found primary normal-polarity components, although orientations are somewhat dissimilar to those expected for typical normal-polarity Keweenawan basalts. They attributed the dissimilar directions to tectonic tilts that are nonuniform, but generally have northerly dip. Expansion of Aeromagnetic Analysis by Teskey et al. Teskey et al. (1991) analyzed the negative aeromagnetic anomaly at Superior Shoal using the principle that magnetic rocks forming rugged bathymetry should produce aeromagnetic anomalies that correspond to bathymetric shapes. In comparing the bathymetry of Superior Shoal to aeromagnetic anomalies along profiles, Teskey et al. noted an inverse correlation between bathymetric and aeromagnetic highs and lows, suggesting a reversed-polarity remanence. Expanding on this approach, a three-dimensional model of bathymetry was assigned magnetizations typical of normal versus reversed polarity for Keweenawan basalts. Comparisons of the magnetic fields computed from these models to the observed aeromagnetic anomaly show a good correspondence with the reversed-polarity model, supporting the conclusion that the bulk of the rock volume at Superior Shoal possesses very strong, reversed-polarity remanence. 50 �Chemical Analysis of Paleomagnetic Samples Recently, 10 samples from basalt sites 1 and 2 of Manson and Halls (1991) were analyzed for major and trace elements. The Superior Shoal basalt samples have similar geochemical characteristics with a limited range in MgO = 5.4 to 8.1 wt.%, TiO2 = 1.6 to 2.4 wt.%, and La/Yb = 6.5 to 7.2. They are most similar in composition to Siemens Creek Type II basalts and are comparable to the Central suite of the Osler Group (Fig. 1), both of which are composed of early, reversed-polarity lavas that are mostly older than ca. 1105 Ma (Nicholson et al., 1997; Swanson-Hysell et al., 2019). The results of the basalt analyses combined with the paleomagnetic results suggest that basalts with early magma chemistry but with normal-polarity remanence are present at Superior Shoal. Reconciliation of the Results A more detailed analysis of flight-line aeromagnetic data allows that basalts of both polarities likely exist at Superior Shoal. Low-amplitude positive anomalies are superposed on the broader, high-amplitude negative anomalies, suggesting that a large volume of reversedpolarity early lavas underlie normal-polarity lavas of smaller volume (and/or lower magnetization). The apparent conflict of normal-polarity, early magma chemistry may be due to (1) magma typical of early rift magmatism that continued erupting into one of the later normalpolarity times, or (2) a previously unrecorded normal polarity event that occurred sometime between 1105 Ma and 1103 Ma. Further study at Superior Shoal appears warranted. References Lightfoot, P.C., Sutcliffe, R.H., and Doherty, William, 1991, Crustal contamination identified in Keweenawan Osler Grop tholeiites, Ontario: A trace element perspective: Journal of Geology, v. 99, p. 739–760. Manson, M.L., and Halls, H.C., 1991, An investigation of Superior Shoal, central Lake Superior, with a manned submersible: Canadian Journal of Earth Sciences, v. 28, p. 145–150. 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. 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, 29 January 2019, https://doi.org/10.1130/B31944.1 Teskey, D.J., Thomas, M.D., Gibb, R.A., Dods, S.D., Kucks, R.P., Chandler, V.W., Fadaie, K., and Phillips, J.D., 1991, High resolution aeromagnetic survey of Lake Superior: Eos, v. 72, no. 8, p. 81, 85–86. 51 �Evaluating Alternate Geophysical Models along the Isle Royale-Superior Shoal Aeromagnetic Anomaly, Central Lake Superior GRAUCH, V.J.S. 1, STEWART, Esther Kingsbury 2, WOODRUFF, Laurel G. 3, and HELLER, Samuel 4 1 U.S. Geological Survey, MS 964, Federal Center, Denver, CO, 80225 Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd., Madison, WI 53705 3 U.S. Geological Survey, 2280 Woodale Drive, St. Paul, MN 55112 4 U.S. Geological Survey, MS 939, Federal Center, Denver, CO, 80225 2 As much as 3 km of Midcontinent rift basalts exposed on the NE-elongate island of Isle Royale (IR) in central Lake Superior dip SE and are commonly regarded as part of the upthrown block of a post-rift reverse fault just off the northern IR shore. A prominent, narrow, aeromagnetic high-low pair (IR-SS anomaly) emanates from the NE tip of IR, curving toward the SE to a strong negative anomaly at Superior Shoal (SS), a bathymetric high near the center of the lake (Fig. 1). The IR-SS anomaly is commonly interpreted as an extension of the IR reverse fault, involving younger (normal magnetic polarity) and possibly older (reversed magnetic polarity) rift basalts. Broad, linear to curvi-linear gravity highs parallel the IR-SS anomaly to the south (Fig. 1). A seismic-reflection line (GLIMPCE A) crosses the IR-SS anomaly at a complicated area of multiple linear magnetic anomalies (Fig. 1). The seismic section shows a 12-km-wide disrupted zone that extends vertically below the complicated area and divides packages of subhorizontal reflections that cannot be connected across the zone. Previous geophysical models of the IR-SS anomaly satisfy some of the data sets, but none integrate all of them satisfactorily. For example, a vertical reverse fault with ~2.5 km of throw has been interpreted and modeled from the seismic-reflection and gravity data (Thomas and Teskey, 1994), but does not account for the shallow basalts observed near SS. Conversely, a magnetic model along flight-line L11260 east of GLIMPCE A (Fig. 1) fits the sharp IR-SS anomaly using a <5-km-wide zone of igneous rock extending from the lake bottom to ~4 km depth (Teskey and Thomas, 1994), but does not fit the broader gravity anomaly. To develop models of the IR-SS anomaly that better integrate all the data and are constrained by new information that early lavas are preserved at SS (Grauch and Schulz, this volume), we tested a number of 2D gravity and magnetic models considering 3 conceptual models: (1) reverse fault with steeply dipping, south-facing basalt layers; (2) localized intrusions, such as volcanic feeder zones or dikes; and (3) remnants of an early-lava plateau extending northward from the IR-SS anomaly. We were unable to construct fully integrated models using the steeply dipping reverse-fault concept. Instead, models with moderately southward-dipping (<45°) basalt layers worked for profiles IRKP and IRKP2 (Fig. 1). The early-lava plateau remnant concept works well for profile L11070 across SS (Fig. 1). This model depicts strongly magnetic, reversed-polarity layers north of the IR-SS anomaly that abruptly terminate at the south side of the shoal (the linear positive anomaly there is the expression of this termination). Geologically, this model suggests that early lavas rest at shallow levels (~1 km depth) north of the IR-SS anomaly, possibly overlying a pre-rift sedimentary basin that has been pervasively intruded by a younger, rift-related mafic igneous complex. This is consistent with preliminary reinterpretations of GLIMPCE A, which suggest that a ~10-km thick, pre-rift sedimentary basin exists north of the IR-SS anomaly. Models that work best for profiles 8ext, 25ext, and 43ext include moderately dipping, south-facing basalts combined with concepts of both early-lava plateau remnants and localized intrusions. 52 �The 2D model testing suggests that (1) the IR-SS aeromagnetic anomaly is likely the product of multiple geologic causes; (2) a steeply dipping reverse fault model is the least favored; and (3) models involving localized intrusions and shallow, early-lava and/or pre-rift rocks at or on the north side of the IR-SS anomaly need to be considered further. 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. Grauch, V.J.S. and Schulz, K.J., 2019. Superior Shoal Revisited: Evidence for early Keweenawan lavas with both reversed and normal-polarity remanent magnetization, central Lake Superior: Institute on Lake Superior Geology 65, Part 1 – Program and Abstracts Teskey, D.J. and Thomas, M.D., 1994. Three-dimensional magnetic modelling of the Midcontinent Rift beneath central Lake Superior: Canadian Journal of Earth Sciences, v. 31, p. 675–681. Thomas, M.D. and Teskey, D.J., 1994. An interpretation of gravity anomalies over the Midcontinent Rift, Lake Superior, constrained by GLIMPCE seismic and aeromagnetic data: Canadian Journal of Earth Sciences, v. 31, p. 682–697. Fig. 1. Aeromagnetic, gravity, and geology maps for the study area showing locations of seismicreflection lines and 2D model profiles. The IR-SS aeromagnetic anomaly is traced on all maps by the dashed yellow line. IR – Isle Royale; SS – Superior Shoal. Gravity and aeromagnetic compilations from Anderson and Grauch (2018). Geology generalized by E. Anderson from a USGS GIS compilation by C. Dicken (accessed January, 2015). 53 �Geological characteristics and structural controls of Au mineralisation at the enigmatic Hemlo deposit. HOLDER, David1, ROBERT, Francois1 and HAY, Jonathan1 1 Barrick Gold Corporation, Hemlo Operations, Marathon, Ontario, Canada. email:david.holder@barrick.com Hemlo is one of Canada’s largest and most well-known mines, producing ~23 Moz Au since discovery in 1981. The deposit is located in the Hemlo-Schreiber greenstone belt within the Wawa subprovince of the Superior craton. The Wawa sub-province, with ~40 Moz Au endowment (past production + reserves + resources) represents the western continuation of the highly auriferous southern Abitibi subprovince (~281 Moz Au). The sub-provinces are separated by the Kapuskasing structural zone, which is thought to have facilitated uplift and erosion of the Wawa block, exposing deep, high-grade metamorphic rocks ranging westward from granulite to amphibolite (e.g. Thompson 2006). Hemlo represents a rather unique deposit which is effectively isolated within the HemloSchreiber greenstone belt. Located along the Hemlo shear zone, Hemlo is hosted within amphibolite grade tectonites of volcanic (predominately volcanoclastics and hypabyssal intrusion) and sedimentary origin (e.g. Muir 1997). The mineralisation is characterised by an unusual metal assemblage with significant enrichments of Mo-As-Sb-Hg-Tl-V-Ba, associated with K-metasomatism and pervasive feldspathisation (Poulsen et al., in press). The unusual characteristics of Hemlo mean it has been the focus of many scientific studies over the past ~35 years. However there is still no consensus regarding the deposit genesis and its origins remain enigmatic. This is in part due to the effects of high-grade metamorphism and intense deformation, which have modified the original character of mineralization and geometry of the ore body. Historically, mining of the deposit has been carried out as 3 distinct operations; David Bell, Golden Giant (Main) and Williams (B- and C-zones) which has further hampered understanding of the system. Since unification of the mine by Barrick, a concerted effort has been made to determine the geological controls of mineralisation, focused primarily on the western-most C-zone, the main area of current operations. The deposit can be split into two distinct zones (Fig. 1); [1] the Williams B-zone and eastern extensions; Golden Giant Main zone and David Bell (referred to as B-zone herein) and [2] the Williams C-zone. The B-zone, which accounts for most of the gold, is a moderate to steeply NE-dipping tabular ore body developed on the contact of a series of felsic volcanic rocks known as the Moose Lake Volcanic complex (MLVC) and a heterolithic volcanoclastic unit locally referred to as the “fragmental” unit (Poulsen et al., submitted). The B-zone represents the “classic” Hemlo ore, characterised by textually destructive K-feldspar alteration (microcline) with abundant pyrite, molybdenite, barite, and a variety of As- and Hg-bearing sulfides and sulfosalts. The grade-thickness distribution on a longitudinal section across the deposit (Fig. 1) highlights the overall NW-plunge of the mineralisation in this zone, with a main shoot plunging ~30o and a number of steeper internal shoots plunging ~60o. The geologic controls of these plunges are poorly understood at present and are the focus of on-going study. 54 �Grade-Thickness Williams C-zone Figure 1: Interpolant gram.meter long sections (looking north) of the B-zone-David Bell (east) and Williams C-zone 100-series (west). The 300-series and B-zone footwall lodes not shown. Black-dashed lines highlight two apparent plunges to the mineralised system [60o-NW and 30o-NW]. Interpolant based on 0.5 g/t indicator grade shell. Williams B- / Golden Giant Main -zone David Bell The C-zone, located in the west part of the deposit comprises two sub-parallel Wstriking moderately (~60o) plunging shoots (Fig. 1) known as the 100- and 300- series lodes. The 100-series mineralisation is developed within a tight NW plunging fold closure of the “fragmental” unit, whereas the 300-series is situated within the MLVC. The mineralisation is characterised by pervasive, textually destructive K-feldspar alteration with Figure 2. Photograph of [A] molybdenite and pyrite disseminations and folded and transposed Kstringers (Fig. 2a), cross-cut by high-grade feldspar alteration and [B] rerecrystallized quartz veins and quartz-pyrite crystallised early quartz vein A with abundant Au visible. replacement zones (Fig. 2b). It is evident from underground exposure and drill-core that the bulk of mineralisation pre-dates metamorphism and deformation: feldspar and quartz-pyrite alteration zones are folded and transposed by the penetrative S2 foliation, which also transposes molybdenite and pyrite stringers (Fig. 2a). The early quartz veins and quartz-pyrite replacements display diffuse lobate contacts typical of recrystallised quartz, with sulfides and visible gold also transposed into the foliation planes (Fig. 2b). The current geometry of the C-zone mineralisation was evidently controlled by the development of F2 folds. The overall moderate to steep NW-plunge of the mineralisation corresponds with plunge measurements of F2 parasitic fold hinges and D2 stretching lineations (e.g. Muir 2003). A late, post-D2 mineralisation event is evident from a number of late crack-seal ribbon veins, oblique to and cross-cutting the S2 fabric, and cutting the earlier quartz-pyrite mineralisation. These distinct and superimposed styles of mineralization indicate a complex and multistage history of the Hemlo deposit, a characteristic common to many giant gold deposits. References Muir, T.L., 1997. Precambrian geology, Hemlo gold deposit area; Ontario Geological Survey, Report 289:1-219 Muir, T.L., 2003. Structural evolution of the Hemlo greenstone belt in the vicinity of the world-class Hemlo gold deposit; Canadian Journal of Earth Science. 40:395-430. Poulsen, H.K., Robert, F. & Barber, R., (submitted) Hemlo Gold System, Superior Province, Canada, Society of Economic Geologists Special Publication on Gold Deposits. Thompson, P.H. 2006. A new metamorphic framework for the Hemlo greenstone belt: Implications for deformation, plutonism, alteration and gold mineralization; Ontario Geological Survey, Open File Report 6190:1-80. 55 �Detrital Zircon Geochronology of Keweenaw Interflow Sediments within the North Shore Volcanic Group, Minnesota, U.S.A. JOHNSON, Linnea L.1, MALONE, David, H.1, CRADDOCK, John, P.2 1 Geography-Geology, Illinois State University, Normal, Illinois 61790 Geology, Macalester College, 1600 Grand Avenue, Saint Paul, Minnesota 55105 2 During the early stages of the Mesoproterozoic Midcontinent Rift, the North Shore Volcanic Group was deposited around 1100 Ma. This group of volcanic rocks, composed of rhyolite, basalt, and andesitic basalt, are interlaid with detrital sediments whose source zircon ages do not coincide with the age of the rift system. These interflow sediments vary in composition, comprised of quartz arenite, lithic arenite, conglomerate, and conglomeratic sandstone. Collection of samples took place at two locations along the north shore of Lake Superior in Minnesota, USA. Samples were collected from ~10 m thick conglomeratic sandstone at Caribou Creek , a ~1 m thick overturned lithic arenite entrained in a xenolith of the Beaver Bay Complex at milepost 61 on Highway 61, and cross bedded sandstones at Leif Ericson Park in Duluth. Zircon analysis using LA-ICPMS at the University of Arizona Laserchron Center, determine the provenance of both these sandstones. Milepost 61 sample set (n=102) contains zircons with a maximum deposition age of 1081 Ma in addition to zircon ages ranging from 1073.0-1879.9 Ma. Using an age probability plot, four peak ages are identified to be 1116, 1440, 1688, 1778 Ma. The Caribou Creek sample set (n=61) contains zircon ages ranging from 1051.2-3184.1 Ma, with three peak ages of 1109, 1377, and 1730 Ma. A total of 101 zircons were analyzed for the Leif Erickson sample. Zircons from this sample ranged in age from 1074-2707 Ma and has a maximum depositional age of 1081 Ma. Age peaks for this sample are 1111, 1446, 1690 and 1778 Ma. Prior notions that interflow sediments were sourced only from within the rift system cannot be entirely true. New data we collected suggests that some of the interflow sediment was derived from an external source outside of the Midcontinent Rift basin. Zircon ages coincide with Archean terranes to the south, and may also include the Midcontinent Granite-Rhyolite, Mazatzal and Yavapai provinces. Fluxes in high lands from reactivation of faults bounding these provinces may have uplifted these potential source areas. References Craddock, J.P., Konstantinou, A., Vervoort, J.D., Wirth, K.R., Davidson, C., Finley-Blasi, L., Juda, N.A., and Walker, E., 2013, Detrital zircon provenance of the Proterozoic Midcontinent Rift, Lake Superior region, USA: Journal of Geology, v. 121, p. 57-73. Davis, D.W., and Green, J.C. 1997, Geochronology of the North American Midcontinent Rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of Earth Sciences, v. 34, p. 476-488. 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, v. 9, p.117-133. Gehrels, G. and Pecha, M., 2014, Detrital zircon U-Pb geochronology and Hf isotope geochemistry of Paleozoic and Triassic passive margin strata of western North America: Geosphere, v. 10, p. 49-65. Gehrels, G.E., Valencia, V., Pullen, A., 2006, Detrital zircon geochronology by Laser-Ablation Multicollector ICPMS at the Arizona LaserChron Center, in Loszewski, T., and Huff, W., eds., Geochronology: Emerging Opportunities, Paleontology Society Short Course: Paleontology Society Papers, v. 11, 10 p. Gehrels, G.E., Valencia, V., Ruiz, J., 2008, Enhanced precision, accuracy, efficiency, and spatial resolution of U-Pb ages by laser ablation–multicollector–inductively coupled plasma–mass spectrometry: Geochemistry, Geophysics, Geosystems, v. 9, Q03017 Jirsa, M.A. 1984. Interflow sedimentary rocks in the Keweenawan North Shore Volcanic Group, northeastern Minnesota. Minn. Geol. Surv. Rep. Invest. 30, 20 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, p. 1–12. Whitmeyer, S.J., and Karlstrom, K E., 2007, Tectonic model for the Proterozoic growth of North America: Geosphere, v. 3, p. 220-259. 56 �Figure1: A regional tectonic map with the Midcontinent Rift and major geologic contacts (Craddock et al., 2013). Milepost 61 and Caribou Creek sample locations are marked with red stars. The location for the KP samples (Craddock et al. 2013) are marked with blue stars. Figure2: Stratigraphic column of the North Shore Volcanic Group strata in the Keweenaw Supergroup, showing the interflow sediments and sample localities (Craddock et al., 2013). Red indicates samples from the interflow sediments. Gray indicates samples from the Beaver Bay Complex and North Shore Volcanics. Figure3: Stacked probability density plot comparing Caribou Creek zircon ages to mile post 61 zircon ages. Age peaks in Ma. Figure 4: Cumulative probability plot comparison of current samples at Caribou Creek and Mile Post 61, with additional interflow sediment data set KP10 and KP 16 (acquired from Craddock et al., 2013). Figure 5: Stacked age probability plots comparing interflow sediment with overlying sandstone units (acquired from Craddock et al., 2013) data set. Age probability plots are categorized with orogeny events. 57 �Paleoproterozoic Snowball Earth? Sedimentology and Geochemistry of a Huronian Glacial Cycle KURUCZ, Sophie1, FRALICK, Philip1, LALONDE, Stefan2, HOMANN, Martin2 1 Department of Geology, Lakehead University, Thunder Bay, ON, skurucz@lakeheadu.ca 2 European Institute for Marine Studies, CNRS-UMR6538 Laboratoire Géosciences Océan, Brest, France The Paleoproterozoic Huronian Supergroup is a ~12km thick sequence of mostly sedimentary rocks that outcrops along the southern margin of the Superior craton and contains evidence for three complete glacial cycles within its stratigraphy. The second glacial event, represented in the Bruce Formation of the Quirke Lake Group is unique because of its overlying cap carbonate, the Espanola Formation, which is the only appreciable carbonate unit within the Huronian Supergroup. A cap carbonate overlying the glacial deposits of the Bruce Formation suggests that the Quirke Lake Group may record evidence for extreme climatic perturbations on the same scale as the later Neoproterozoic glacial cycles, where cap carbonates are ubiquitous overlying glacial deposits. The Neoproterozoic glaciations have been the source of much speculation regarding the cause of the formation of cap carbonates and the possibility of their representing the resulting effects of global ice cover during periods known as ‘Snowball Earth’ events (eg. Kirschvink, 1992). Thus, the presence of a cap carbonate overlying only the second of three glacial deposits in the Huronian Supergroup suggests that the conditions that led to its deposition were unique within the Paleoproterozoic and perhaps akin to those that prevailed during the Neoproterozoic glaciations. To assess the extent of the similarities between the Espanola Formation and the Neoproterozoic cap carbonates, the sedimentology, geochemistry, and isotopic composition of the Bruce glacial event was studied in its entirety. Some of the most interesting and useful results were uncovered through systematic sampling of drill hole E150-2 (Figure 1). Firstly, the presence of a hitherto unmentioned laminated dropstone facies occurs in the uppermost Bruce Formation. This unit is unique because it records evidence of both carbonate precipitation and glacial activity at the same time; a feature that is not recorded elsewhere in the Quirke Lake Group sedimentology. In this facies, 1-10cm thick carbonate-rich laminae occur in a clastpoor diamictite unit with dropstones occasionally punctuating the laminae. The laminated dropstone facies is also exceptional for its extremely negative δ13Ccarb values of ~-10‰, which is on the same order of magnitude as the Shuram-Wonoka anomaly, the most extreme anomaly recorded from the Neoproterozoic cap carbonates (Halverson et al., 2005). Even more perplexing, are the unique REE patterns associated with this extreme δ13Ccarb anomaly. This unit is characterised by REE patterns with consistent negative Eu anomalies, flat light (L) REE and highly variable heavy (H) REE that range from negatively to positively sloped. These patterns stand in stark contrast to REE patterns of samples from the overlying interlaminated carbonate and siltstone facies of the Espanola Formation. Carbonates from the overlying Espanola Formation have patterns with consistently depleted LREE and moderately enriched middle (M) REE, while HREE have a relatively flat pattern that transitions to a positive slope moving up stratigraphy. The relative depletion of LREE in these units that was not present in the underlying laminated dropstone facies indicates a stronger seawater signature, which may reflect a decrease in the influence of meltwater on the geochemical composition. Systematic sampling of the middle and upper Espanola Formation stratigraphy also produced a trend of upwards increasing δ13Ccarb values. Over approximately 110m of stratigraphy the δ13Ccarb values increase from ~4.5‰ to -2‰. This is another feature that has been noted from some Neoproterozoic cap carbonates and has been interpreted to be related to a marine regressive sequence (eg. Giddings and Wallace, 2009). 58 �Thus, the similarity between the Espanola Formation δ13Ccarb values and those of some Neoproterozoic cap carbonates supports the hypothesis that the Espanola Formation may have been formed under similar conditions as its Neoproterozoic counterparts. Figure 1: A ~75m section of stratigraphy sampled from drill hole E150-2 of the contact between the Bruce Formation and Espanola Formation. Red samples (lower REE plot) are from the laminated dropstone facies in the upper Bruce Formation. They have extreme negative δ13C values of approximately -10‰ and consistent negative Eu anomalies. The δ18O values do not show as anomalously low values but are noticeably lower than values further up stratigraphy and fall in the range of -21‰ to -20‰. The purple samples (upper REE plot) are from the interlaminated carbonate and siltstone facies of the lower Espanola Formation. These samples show a rapid trend upwards in δ13C values from ~-4.5‰ to -2‰ and they have REE patterns with consistent LREE depletion and moderate MREE enrichment. References Giddings, J.A., Wallace, M.W., 2009. Sedimentology and C-isotope geochemistry of the “Sturtian” cap carbonate, South Australia. Sediment. Geol. 216, 1–14. Halverson, G.P., Hoffman, P.F., Schrag, D.P., Maloof, A.C., Rice, A.H., 2005. Towards Neoproterozoic composite carbon-isotope record. Geol. Soc. Am. Bull. 117, 1181–1207. Kirschvink, J.L., 1992. Late Proterozoic low-latitude global glaciation - The Snowball Earth. In: Schopf, J.W., Klein, C. (Eds.), The Proterozoic Biosphere. Cambridge University Press, Cambridge, 51– 52. 59 �Precambrian Geology of the Western Schreiber–Hemlo Greenstone Belt MAGNUS, Seamus Ontario Geological Survey, 933 Ramsey Lake Road Sudbury, ON, P3E 6B5 Canada The Schreiber–Hemlo greenstone belt is located within the Wawa–Abitibi terrane of the Superior Province. The greenstone belt includes Neoarchean supracrustal and intrusive rocks that have been crosscut and unconformably overlain by Paleoproterozoic and Mesoproterozoic intrusive and supracrustal rocks of the Southern Province. Bedrock mapping in this area by the Ontario Geological Survey from 2015 to 2018 focussed on the Archean rocks of the western part of the Schreiber–Hemlo greenstone belt, with an emphasis on applying modern geochemical and geochronological techniques. The supracrustal rocks in the western Schreiber–Hemlo greenstone belt are arranged in an upright stratigraphy consisting of four distinct depositional packages, with chemical and clastic metasedimentary rocks along disconformable contacts (Figure 1). The oldest rocks in the greenstone belt are felsic and mafic metavolcanic rocks of Package A, deposited circa 2720 Ma (Davis and Sutcliffe 2017) in a volcanic arc environment. These are overlain by Package B, which is composed mainly of mafic metavolcanic rocks deposited in a “back-arc” volcanic environment. In the western part of the project area, Package B is overlain by Package C, which is composed mainly of mafic metavolcanic rocks deposited in an “oceanic plateau” volcanic environment. In the eastern part of the project area, Package B is overlain by Package D, which is composed of turbiditic wacke and mudstone deposited between 2696 and 2690 Ma (Fralick, Purdon and Davis 2006; Davis and Sutcliffe 2017). The chronostratigraphic relationship between packages C and D is unknown, as contacts between these packages have not been observed. The oldest felsic plutons that crosscut the supracrustal rocks are the circa 2690 Ma Terrace Bay and Steel River plutons (Kamo 2016). Regional ductile deformation likely started at this time, however, whether it began before or after emplacement of the plutons is uncertain. The circa 2667 Ma Santoy Lake pluton shows little evidence for ductile deformation along its margins, which suggests that regional ductile deformation ceased at approximately this time (Kamo 2016). Northwest ductile and brittle-ductile shear zones crosscut and displace all of the Archean rocks. Dikes of the Paleoproterozoic Matachewan, Biscotasing and Marathon dike swarms crosscut Archean rocks in the project area, and outliers of the base of the Paleoproterozoic Gunflint Formation unconformably overlie the Archean rocks at the west end of the project area, southwest of Schreiber. The Coldwell Alkalic Intrusive Complex intrudes the Archean rocks at the east end of the Schreiber–Hemlo greenstone belt. Alkalic diabase dikes crosscut the Archean rocks and the intrusive rocks of the Coldwell Alkalic Intrusive Complex and are believed to be related to volcanism during rifting associated with formation of the Keweenawan Midcontinent Rift. The Archean rocks host a variety of base metal and precious metal occurrences which have been the subject of exploration and limited mining activities for over a century. The circa 2720 Ma felsic metavolcanic rocks are correlative with rocks in the nearby Winston Lake and Manitouwadge areas that host past-producing Zn-Cu mines (Davis, Schandl and Wasteneys 1994; Zaleski, van Breemen and Peterson 1999). Gold mineralization is hosted in sheared and altered metavolcanic rocks and in veined and altered granitoid rocks. Proterozoic rocks in the north shore of Lake Superior region have potential to host magmatic sulphide and oxide mineralization including a variety of transitional metals and rare earth elements. 60 �Figure 1: Simplified geological map of the western Schreiber–Hemlo greenstone belt, highlighting the major Archean rock types, some of the stratigraphic younging indicators observed during this study, all of the U-Pb zircon geochronological data in the area, and the inferred fold axial traces. An inset figure outlines the inferred depositional packages A, B, C and D. Note that Proterozoic diabase dikes, which are abundant in the map area, are not shown for clarity. Abbreviations: DHR = Dead Horse Road, HWY 17 = Trans-Canada Highway 17, LLR = Long Lake Road. See references for ages. All UTM co-ordinates provided using NAD83 in Zone 16. References Davis, D.W., Schandl, E.S. and Wasteneys, H.A. 1994. U-Pb dating of minerals in alteration halos of Superior Province massive sulphide deposits: Syngenesis versus metamorphism; Contributions to Mineralogy and Petrology, v.115, p.427-437. Davis, D.W. and Sutcliffe, C.N. 2017. U-Pb geochronology by LA-ICPMS in samples from northern Ontario, internal report for the Ontario Geological Survey; Jack Satterly Geochronology Laboratory, University of Toronto, Toronto, Ontario, 131p. Fralick, P., Purdon, R.H. and Davis, D.W. 2006. Neo-Archean trans-subprovince sediment transport in southwestern Superior Province: sedimentological, geochemical and geochronological evidence; Canadian Journal of Earth Sciences, v.43, p.1055-1070. Kamo, S.L. 2016. Part A: Report on U-Pb ID-TIMS geochronology for the Ontario Geological Survey: Bedrock Mapping Projects, Ontario, Year 1: 2015-2016, internal report prepared for the Ontario Geological Survey; Jack Satterly Geochronology Laboratory, University of Toronto, Toronto, Ontario, 48p. Zaleski, E., van Breemen, O. and Peterson, V.L. 1999. Geological evolution of the Manitouwadge greenstone belt and Wawa-Quetico subprovince boundary, Superior Province, Ontario, constrained by U-Pb zircon dates of supracrustal and plutonic rocks; Canadian Journal of Earth Sciences, v.36, p.945-966. 61 �Pilot study: Using ambient noise passive seismic surveys for Ni-Cu-PGE mineral exploration at the Marathon PGM-Cu deposit, Marathon, Ontario MCBRIDE, J.1, GOOD, D.2, HOLLIS D.3, and AARNDT, N.3 1 Stillwater Canada Inc. 90 Peninsula Rd. Marathon, ON P0T 2E0, Canada 2 Department of Earth Sciences, University of Western Ontario, London, ON N5A 5B7, Canada 3 Sisprobe, 38240 Maylan, France Active seismic surveys are a powerful geophysical tool for exploring to significant depth, and are commonly used in the oil and gas industry. However, because of the high cost and environmental impact associated with conducting a seismic survey, this method is rarely used for mineral exploration. Nevertheless, with the increased difficulty of finding economic mineral deposits, exploration companies continue to look deeper and there is a growing need to develop cheaper methods with less environmental impact to do so. Passive seismic methods currently being tested by SISPROBE Inc. at the Marathon deposit have the advantage of being a low impact and low-cost method for examining velocity contrast in geologic units to depths below surface approaching 1 km. Passive seismic methods use ambient noise generated from the natural environment. At Marathon, the dominant noise source is wave action in Lake Superior with a minor contribution from waves in the North Atlantic Ocean. Additional noise is generated by traffic on the nearby highway and railway. The use of autonomous seismic data recorders allows for flexibility when designing sensor arrays, which is necessary in remote or environmentally sensitive areas that include challenging topography. The Coldwell Complex is approximately 25 km in diameter and is composed of three centers of predominantly alkaline magmatism that intruded the Archean greenstone terrane (Mitchell and Platt, 1977) along the northern margin of the Midcontinent rift between 1108 and 1094 Ma (Heaman et al., 2007). Centre I is composed of augite syenite, quartz syenite and the Eastern Gabbro Suite. The Eastern Gabbro Suite outcrops along the eastern and northern margin of the complex and is composed of numerous gabbroic to ultramafic intrusions of the Layered and Marathon Series that cut a 1 km thick pile of metabasalt (Good et al., 2015; and Good and Lightfoot, 2019). Mineralization at the Marathon PGM-Cu deposit is hosted by Two Duck Lake gabbro and ultramafic rocks of the Marathon Series. The Marathon PGM-Cu deposit is an ideal site to test the passive seismic technique because of the extensive geological database and the distinct petrophysical property contrast exhibited by the various syenites and gabbros of the complex, and the underlying Archean metavolcanic rocks of intermediate composition. A preliminary noise survey was completed in 2017 to test ambient source signal-to-noise ratio. It was determined that wave action from Lake Superior generates sufficient ambient noise to proceed to a production scale survey. In 2018, a production scale survey was completed with 90 sensors deployed at 300 m spacing in an array that is elongated parallel to wave propagation in order to maximize signal pairs. 62 �The geophones used were GSX-1 single channel units, which collected data in the vertical direction. They recorded data every 4 ms for a total of 26 days (Hollis, 2018). The density and P-wave velocities for representative samples of each lithologic unit at the deposit were measured at Western University. These measurements were used to constrain interpretations of lithological boundaries determined from the 3D velocity inversion model for the survey data. Augite syenite (Vp of 5500 m/s and Rho 2650 km/m3) overlies the Two Duck Lake gabbro (Vp 6200 m/s and Rho 3100 kg/m3) while the Archean metavolcanic footwall (Vp 5000 m/s and Rho 2800 kg/m3) lies below the gabbro. The ultramafic (Vp 6800 m/s and Rho 3500 kg/m3) units that host the mineralization occur as lenses and pods that are distinguishable from the gabbro units. The geological boundary between the Two Duck Lake gabbro and the Archean metavolcanic footwall was successfully resolved by the survey. The survey also identified a high-velocity anomaly down dip from the Marathon PGM-Cu deposit at a depth of 600 m. The anomaly has a velocity value that is representative of an ultramafic unit. To validate the velocity anomaly, a 6 km gravity line was completed over the area which confirmed a high-density body at depth. By combining both passive seismic and gravity methods along with the structural association of the anomaly along feeder conduits, the anomaly is interpreted to be an accumulation of dense minerals such as magnetite, apatite, olivine and sulfide in a conduit setting. The passive seismic technique therefore identified an exploration target at depth where previous electromagnetic and magnetic surveys had not. Passive seismic geophysics is an excellent technique for the mineral exploration industry as it brings considerable depth penetration with the advantages of 3D seismic imaging, at low cost while being sensitive to the environment. References Good D.J., Epstein R., McLean, K., Linnen R., and Samson, I., 2015. Evolution of the Main Zone at the Marathon Cu-PGE Sulphide Deposit, Midcontinent Rift, Canada: Spatial Relationships in a Magma Conduit Setting. Economic Geology, v. 110, pp. 983-1008. Good D.J. and Lightfoot P.C., in press, Significance of Metasomatized Lithospheric Mantle in the Formation of Early Basalts and Cu-PGE Sulfide Mineralization in the Coldwell Complex, Midcontinent Rift, Canada, Canadian Journal of Earth Sciences, 2019. Heaman, L., Easton, M., Hart, T., Hollings, P., McDonald, C., and Smyk, M., 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario: Canadian Journal of Earth Sciences v. 44, pp. 1055-1086 Hollis D., 2018, Marathon Passive Seismic Project, internal report, Sisprobe, 24 Allee des Vulpains, 38240 Meylan, France. Mitchell, R., and Platt, R., 1977. Field guide to the aspects of the geology of the Coldwell alkaline complex: Institute on Lake Superior Geology, Technical Report 63 �The Wolf River Orogeny: Geon 14 Magmatism, Sedimentation, and Deformation in the Southern Lake Superior Region MEDARIS, L. G. Jr.1, MALONE, D. H.2, HILL, G. C.2, SINGER, B. S.1, JICHA, B. R.1, VAN LANKVELT, A.3, WILLIAMS, M. L.3, and REINERS, P. W.4 1 Department of Geoscience, University of Wisconsin–Madison, Madison, WI 53706 Department of Geography, Geology, and the Environment, Illinois State University, Normal, IL 61790 3 Department of Geosciences, University of Massachusetts–Amherst, Amherst, MA 01003 4 Department of Geosciences, University of Arizona, Tucson, AZ 85721 2 The Proterozoic Wolf River Batholith (WRB), which is the most prominent Precambrian geological feature in northeastern Wisconsin, was first described in 1975 by Van Schmus et al. and initially interpreted to represent an episode of anorogenic igneous activity by analogy with the classic Proterozoic rapakivi granites in Finland (Anderson & Cullers, 1978). Subsequently, it was recognized that the WRB is the local expression of a transcontinental belt of Geon 14 granites that were again interpreted to be anorogenic (Anderson, 1983). More recent investigations reveal that emplacement of these transcontinental Geon 14 granites along the eastern and southern margins of Laurentia was associated with an orogenic event involving continental arc magmatism, sedimentation, and deformation (Whitmeyer & Karlstrom, 2007; Daniel et al., 2013), certain aspects of which are now recognized as being related to the Wolf River event in Wisconsin. Magmatism The WRB underlies a minimum area of 1.45 x 104 km2 and consists predominantly of alkaline biotite granite and biotite–hornblende adamellite and subordinate quartz syenite, monzonite, and anorthosite (Anderson & Cullers, 1978). U–Pb zircon ages for the different plutons range from 1468 ± 4 to 1484 ± 2 Ma, with the main part of the batholith yielding an average crystallization age of 1476 ± 2 Ma (DeWayne & Van Schmus, 2007). To the west of the WRB in Marathon county, the Wausau syenite and Nine Mile granite plutons yield older crystallization ages of 1522 and 1506 Ma, respectively. Oxygen, Sm–Nd, and Lu–Hf isotopic data indicate that the WRB was derived from partial melting of the late Paleoproterozoic crust in the region (Anderson & Morrison, 2005; DeWayne & Van Schmus, 2007; Goodge & Vervoort, 2006). Sedimentation The Baldwin conglomerate occurs at the northeastern margin of the WRB, where it lies unconformably on the Geon 18 Macauley gneiss and Waupee metavolcanic and metasedimentary rocks and is intruded by the 1470 Ma Hager porphyry. The Baldwin conglomerate is polymict and chemically immature, containing clasts of the underlying lithologies set in a medium–grained arkosic matrix. A relative probability plot for detrital zircons in the Baldwin conglomerate displays a prominent Geon 14 (Wolf River) peak, subordinate Geon 16 (Mazatzal), Geon 17 (Yavapai), and Geon 18 (Penokean) peaks, and a single detrital zircon at 2690 Ma (Algoman) (Fig. 1). The maximum age of deposition (MAD) calculated from the youngest statistically homogenous population (MSWD ≤ 1.0) is 1458 ± 10 Ma. These results demonstrate that deposition of the Baldwin conglomerate was synchronous with crystallization of the WRB. Figure 1. Relative probability plot for detrital Deformation and recrystallization Evidence zircons in the Baldwin conglomerate for Geon 14 deformation associated with the WRB is best revealed by metasedimentary rocks of the post– Mazatzal Baraboo Interval. In the Baraboo Range, muscovite parallel to slatey cleavage in four samples 64 �of Seeley Slate yields 40Ar/39Ar cooling ages of 1473, 1483, 1493, and 1496 Ma (all with ± 3Ma), and muscovite decorating crenulation cleavage in Waterloo metapelite yields 1465 ± 7 Ma. In addition, cooling ages of 1472 ± 3, 1480 ± 11, and 1469 ± 11 Ma have been obtained for muscovite in breccia in the Baraboo Quartzite, in hydrothermal veins at the base of the quartzite, and in metamorphosed paleosol beneath the quartzite. Figure 3. U/Th–He ages for hematite in Baraboo metapelite Figure 2. Th map and U-Pb ages for monazite in Seeley Slate Monazite occurs as a detrital mineral in the Seeley Slate, and some grains exhibit new monazite rims that extend parallel to cleavage (Fig. 2). Electron probe microanalysis and dating of monazite were done using the UMass Ultrachron probe. Detrital monazite cores yield Penokean and Archean ages; rims yield a date of 1502 ± 30 Ma, comparable to the age of the WRB. In the Baraboo Quartzite, folded metapelite layers consisting largely of pyrophyllite contain tiny grains (50–100 m in diameter) of recrystallized hematite. Such hematite yields a mean U/Th–He age of 1507 ± 153 Ma (Fig. 3), which is consistent with the ages obtained for muscovite and monazite by other geochronologic methods. Note that the Baraboo Interval sedimentary rocks containing evidence for Geon 14 folding and recrystallization, e.g. the Baraboo and Waterloo quartzites, are located within the trans-continental belt of Geon 14 granites, whereas those located outside the transcontinental belt, e.g. the Sioux and Barron quartzites, are neither folded nor recrystallized. Despite the massive character of different Wolf River plutons and “anorogenic” appearance of the batholith itself, it is now clear that emplacement of the WRB was accompanied by Geon 14 sedimentation and deformation and can be viewed as an orogenic event. The Wolf River orogeny provides a link between the Pinwarian orogeny to the northeast and the Picuris orogeny to the southwest, thus completing the transcontinental extent of Geon 14 orogenesis in North America. References Anderson, 1983, GSA Memoir 161, 133–154; Anderson & Cullers, 1978, Precam. Res. 7, 287–324. Anderson & Morrison, 2005, Lithos 80, 45–60; Daniel et al., 2013, GSA Bull. 125, 1423–1441. DeWayne & Van Schmus, 2007, Precam. Res. 157, 215–234. Goodge & Vervoort, 2006, Earth Planet. Sci. Lett. 243, 711–731. Whitmeyer & Karlstrom, 2007, Geosphere 3, 220-259; Van Schmus et al., 1975, GSA Bull. 86, 907–914. 65 �The Importance of “Tablesetting” Intrusions in Creating Economic Ni-Cu-PGE Deposits in the Midcontinent Rift MILLER, Jim University of Minnesota Duluth (emeritus) and JDM GeoConsulting, Shuniah, ON (mille066@umn.edu) Some of the most promising targets for economic Ni- Cu-PGE sulfide deposits in the Lake Superior region are associated with small-scale ultramafic-mafic intrusions emplaced during early stages of the 1.1Ga Midcontinent Rift. While many of these intrusions share well documented attributes – small size, sub-horizontal conduit geometries, high grades and tenors of Ni-Cu-PGE sulfide ore, ultramafic host rock – one common attribute that is not so well known is the association of these mineralized intrusions with precursor intrusions. I refer to these earlier intrusions as “tablesetting” intrusions (TSI) as their emplacement appears to have played a major role in producing the well mineralized intrusions that followed. Before discussing what role TSI plays, the basic structural, lithologic and geochemical attributes of the TSI associated with four well-studied MCR ultramafic intrusions will be described. I am familiar with these intrusions through the MS thesis research of my UMD graduate students – Eagle (Mulcahy, 2018), Tamarack (Goldner, 2011), BIC (Foley, 2011), and Current Lake (Chaffee, 2015) - and through many years of discussions with exploration geologists such as Dean Rossell (Rio Tinto), Bob Mahin (Eagle/Lundin), Al MacTavish (MagmaMetal/Panoramic), and Geoff Heggie (Magma Metals/Panoramic). The discovery of the Eagle deposit in 2002 by Dean Rossell and his Rio Tinto/Kennecott crew in the Baraga Basin area north of Marquette, Michigan set off a flurry of exploration activity in the Lake Superior region that continues to this day. Eagle is the only MCR-related Ni-Cu-PGE deposit that has progress to active mining, which began in 2014, soon after the property was acquired by Lundin Mining. In 2015, continued exploration in the area revealed additional economic mineralization in the subhorizontal conduit of the nearby Eagle East intrusion. With total minelife of the Eagle and Eagle East deposits projected to end in 2023, the company is aggressively exploring for additional deposits in the area. One of the main vectoring tools being employed is to seek out pyroxenite dikes. As observed at both Eagle and Eagle East, weakly mineralized pyroxenite to melagabbro (PYX unit) occurs at the margins of the main peridotite body that hosts the bulk of the Ni-Cu-PGE mineralization. Weakly mineralized pyroxenite also occurs as xenoliths in well-mineralized peridotite - the HTBX and IBRX units (Mulcahy, 2018). It is hoped that tracing the occurrences of the tablesetting PYX rock type will lead to discovery of another mineralized peridotite body in the vicinity of Eagle/Eagle East. Concurrent with their exploration in Upper Michigan, RioTinto/Kennecott was seeking an Eagle-like occurrence in the Paleoproterozoic Animikie Basin in east-central Minnesota. This led to the discovery of significant Ni-Cu-PGE mineralization in 2008 in the Tamarack intrusion/deposit. The main zone of massive to semi-massive sulfide mineralization occurs in the “tail” section of the tadpole-shaped subhorizontal intrusion where two distinct peridodite bodies come into contact – 1) a deeper CGO unit, which is characterized by coarse cumulus olivine and significant intercumulus clinopyroxene and plagioclase, and 2) an overlying FGO unit, which is characterized by finer grained cumulus olivine and only a minor intercumulus component. Whereas Goldner (2011) concluded from petrographic and geochemical attributes that the CGO unit is the precursor intrusion, the local Rio Tinto/Kennecott crew has interpreted the FGO is the earlier intrusion. In either case, the mineralization is clearly focused where the two intrusive components come into contact. The Thunder Bay North PGE-Cu-Ni deposit associated with the Current Lake Intrusion was discovered by Magma Metals in 2006 near the occurrence of glacial boulders of well mineralized peridotite on the shoreline of Current Lake. Like Tamarack and Eagle, the mineralization is hosted by peridotite, and like Tamarack, it has a subhorizontal tadpole shape (chonolithic). However, it is intrusive 66 �into Archean granitoids and metasedimentary rocks rather than Paleoproterzoic black shales. Another significant difference is that the precursor rock is a strongly contaminated, commonly xenolith-rich lithology that Magma Metals termed the Hybrid Unit (with red and gray varieties). Petrographic studies by Chaffee (2015) determined this rock is a weakly mineralized quartz gabbro that is variably discolored by hematitic staining. Geochemical modelling also showed that the parental magma to the hybrid unit is a contaminated equivalent to the peridotitic magma that followed emplacement of the hybrid. The hybrid intrusions were emplaced in an orthogonal pattern of subhorizontal and subvertical dikes. The main mineralized peridotite tended to be emplaced at the intersections of the vertical and horizontal hybrid dikes to form chonolith-shaped bodies. The Bovine Igneous Complex (BIC) is a funnel-shaped intrusion that, like Eagle, was emplaced into the Paleoproterozoic Baraga Basin of Upper Michigan. Because it is one of the few ultramafic intrusions with surface exposures, exploration activity on BIC by Dean Rossell and his Rio Tinto/Kennecott crew began in the mid-90’s, though significant mineralization was not discovered until 2006. Detailed petrographic and geochemical studies by Foley (2011) on two drill core profiling the igneous stratigraphy of BIC showed it to be composed of two well differentiated intrusive cycles. The lower (early) sequence grades from an Ol cumulate upward to a Cpx+Ol cumulate. This is overlain by a cumulus reversal back to an Ol cumulate that grades upward to a Cpx+Ol cumulate, then an Ox+Cpx±Ol cumulate, and is capped by a Pl+Cpx+Ox cumulate. Both differentiated sequences show smooth cryptic layering of Mg/Fe ratio in olivine and augite, and Ca/Na ratio in plagioclase. Ni-Cu-PGE enriched sulfide mineralization occurs intermittently through the lower ultramafic sequence and at the basal contact between of the upper differentiated sequence. Although economic grades of mineralization appear to be lacking in these sequences, the possibility of finding more concentrated and higher tenor sulfide in the as yet undiscovered conduit to BIC seems high. The main take-aways from the observation made of these four mineralized ultramafic intrusions in regard to the role of the precursor “tablesetting” intrusions (TSI) are: 1) TSI are important for establishing the plumbing system for subsequent intrusions. 2) TSI serve to pre-heat and begin devolatilization of sulfide-bearing country rock, but because of rapid heat loss to cold country rock, they tend to generate little sulfide of low tenor. 3) Subsequent intrusions of hot ultramafic magmas into a pre-heated and structurally compromised country rock created by the TSI are able to cool slowly and create cumulate lithologies. Larger, more closed intrusions may become well differentiated (BIC), whereas in narrow chonolithic intrusions that are perhaps open to surface, large volumes of ultramafic magma can pass through resulting in the crystallization of uniformly primitive cumulates (Eagle, Tamarack, Current Lake). 4) In open chonolithic systems, the dynamic passage of large volumes of metal-rich ultramafic magma that bore through and inflate the precursor TSI, can upgrade the tenor of any early-formed sulfide in the TSI and any additional sulfide devolatilized by the new heat pulse. 5) Wherever the intrusive plumbing network creates subhorizontal sheets, channels, or tube-shaped (chonolith) conduits, this allows for gravitational concentration of enriched sulfide liquid. UMD MS Theses Chaffee, M., 2015, Petrographic and Geochemical Study of the Hybrid Rock Unit Associated with the Current Lake Intrusive Complex. Foley, D., 2011, Petrology and Cu-Ni-PGE Mineralization of the Bovine Igneous Complex, Baraga County, Northern Michigan. Goldner, B., 2011, Petrology and Cu-Ni-PGE Mineralization of the Tamarack Intrusion, Aitkin and Carlton Counties, Minnesota. Mulcahy, C., 2018, Emplacement and Crystallization Histories of Cu-Ni-PGE Sulfide-mineralized Peridotites in the Eagle and Eagle East Intrusions. 67 �Geochemical Vectoring Towards a Serpentinized Peridotite Chonolith, Eagle East Ni-Cu-Co-PGE Deposit, Upper Peninsula, Michigan NOWAK, Robert1, ESSIG, Espree1, MAHIN, Robert1 1 Eagle Mine Exploration, 200 Echelon Drive, Negaunee, MI 49866 Serpentinization is a low temperature (≤500°C), surficial to hypabyssal metasomatic process in which pyroxene [(Ca,Mg,Fe)2Si2O6] and olivine [(Mg,Fe)2SiO4] react with H2O +/CO2 to form hydrous silicates (serpentine) +/- hydroxides (brucite) +/- carbonates (dolomite, magnesite, calcite), and +/- Fe-oxides (magnetite) (Huang et al., 2017; Kelemen and Matter, 2008). These chemical reactions can result in the transfer of Mg2+, Ca2+, and Si4+, the oxidation of Fe2+ (Kelemen and Matter, 2008), and, under very reducing conditions, the formation of nickel alloys (awaruite (Ni3Fe; Preiner et al., 2018; Lawley, 2018). Pervasive serpentinization of peridotite can incorporate up to 13-15% H2O by weight and result in an estimated volume increase up to 40% (Schroeder et al., 2002; Shervais et al., 2005). This process can significantly alter the properties of ultramafic to mafic rocks, resulting in decreased density, seismic velocity (Miller and Christensen, 1997), and rheological strength (Escartin et al., 2001), in addition to an increase in magnetic susceptibility (Toft et al., 1990). The Eagle and Eagle East magmatic Ni-Cu-Co-PGE deposits, which formed during the Midcontinent Rift (MCR), are hosted within intensely serpentinized peridotite chonoliths. The aim of this study was to investigate whether a geochemical signature could be detected from drill core analyses outside the main serpentinized peridotite chonoliths and potentially utilized as an exploration vector toward mineralized peridotite. A pyroxenite sheet dike, which extends along strike and is crosscut by the Eagle East ore-hosting chonolith, was the focus of this study. Over fifty samples, collected from drill core intercepts of the pyroxenite sheet dike, were analyzed for major, minor, and trace elements (using ICP-MS and XRF methods) and utilized to generate 3-D models (using Leapfrog software). Pyroxenite intercepts ~400 meters away from any secondary intrusion (i.e. peridotite chonolith or gabbroic stock) were used as a baseline comparison to pyroxenite intercepts above and below the Eagle East peridotite chonolith. Pyroxenite samples above the Eagle East peridotite conduit have relatively enriched (on the order of 1 to 5 wt %) SiO2 and MgO values, and relatively depleted CaO contents (on the order of 1.5 to 2 wt%) relative to baseline pyroxenites. The enrichment and depletion trends become most pronounced ~50 meters above the flat-lying Eagle East conduit. Pyroxenite samples below the Eagle East chonolith contain relatively enriched SiO2, MgO, and CaO values, except for pronounced depletions within ~50 meters of the chonolith contact. Enrichment of nickel (on the order of 0.2 to 1 wt%) in the lower pyroxenites can extend up to 500 meters away from the lower chonolith keel contact (Fig. 1). The overall pattern of depletion proximal to the Eagle East chonolith is interpreted as resulting from near contact related serpentinization of the pyroxenites. The overall pattern of enrichment distal to intense serpentinization is interpreted as redistribution of these elements outside the zone of intense serpentization into less-altered pyroxenites. In summary, serpentinization is an important component to consider when modelling and interpreting the major and base metal element content of ultramafic and mafic rocks which can potentially host Ni-Cu-Co-PGE mineralization. The occurrence of proximal depletion, coupled with distal enrichment of MgO, CaO, and SiO2 may provide exploration criteria that could be 68 �used to vector towards serpentinized peridotite. The redistribution of Ni from serpentinized olivine, presumably into the mineral awaruite, displayed the most widespread detection halo (up to 500 meters outside the Eagle East system). The implications of this study could aid in determining the nickel prospectivity of a magmatic system and improve estimations on the potential size of a serpentinized system based on the scale of the geochemical halo observed. Figure 1: Long-section, looking southeast, showing nickel content (ppm) of pyroxenite samples projected onto the modelled pyroxenite plane. The modelled serpentinized Eagle East chonolith surface with massive- (red) and semi-massive sulfide ore-bodies (yellow) is also shown. References Escartin, J., Hirth, G. and Evans, B., 2001. Strength of slightly serpentinized peridotites: Implications for the tectonics of oceanic lithosphere. Geology, 29, 1023-1026. Huang, R., Lin, C., Sun, W., Ding, X., Zhan, W., Zhu, J., 2017. The production of iron oxide during peridotite serpentinization: Influence of pyroxene. Geoscience Frontiers, 8, 1311-1321. Kelemen, P.B., and Matter, J., 2008. In situ carbonation of peridotite for CO 2 storage. PNAS, 105, 17295-17300. Lawley, C., 2019. Gold and PGE mobility during serpentinization. PDAC technical session in: advances in mineral systems modelling of Ni-Cu-PGE and gold, v. 2019 Preiner, M., Xavier, J.C., Sousa, F.L., Zimorski, V., Neubeck, A., Lang, S.Q., Greenwell, H.C., Kleinermanns, K., Harun,T., McCollom,T.M., Holm, N.G., and Martin, W.F., 2018. Serpentinization: Connecting Geochemistry, Ancient Metabolism and Industrial Hydrogenation. Life, 41, 1-22. Schroeder, T., John, B. and Frost, B.R., 2002. Geologic implications of seawater circulation through peridotite exposed at slow-spreading mid-ocean ridges. Geology, 30, 367-370. Shervais, J.W., Kolesar, P. and Andreasen, K., 2005. A field and chemical study of serpentinization-Stonyford, California: Chemical flux and mass balance. International Geology Review, 47, 1-23. Toft, P.B., Arkani-Hamed, J. and Haggerty, S.E., 1990. The effects of serpentinization on density and magnetic susceptibility: a petrophysical model. Physics of the Earth and Planetary Interiors, 65, 137-157. 69 �Catchment Geology Correlation with Fish Otolith Microchemistry Across Disparate Glacial Till Depths in the Lake Michigan Basin PRICHARD, Carson G1., STUDENT, James J2., JONAS, Jory L3., WATSON, Nicole M1., and PANGLE Kevin L1. 1 Central Michigan University, Department of Biology, Mount Pleasant, Michigan, 48859 USA Central Michigan University, College of Science and Engineering, Center for Elemental and Isotopic Analysis, Mount Pleasant, Michigan, 48858 USA 3 Michigan Department of Natural Resources, Charlevoix Fisheries Research Station, Charlevoix, Michigan, 49720 USA 2 Fish otoliths are calcium carbonate boney-like structures found in fish ears that grow concentrically, and as such they preserve a chemical record of select environmental changes during life. This study used laser ablation inductively coupled plasma mass spectrometry (LAICP-MS) to record variations in signal intensities of magnesium (25Mg), calcium (43Ca), manganese (55Mn), copper (65Cu), zinc (66Zn), strontium (88Sr), barium (137Ba), and lead (208Pb) isotopes in steelhead (Oncorhynchus mykiss) otoliths. These signals were then converted to trace element concentrations (in ppm) along transects that represent a timespan when each fish resided in a particular catchment. A portion of the otolith data used in this study was previously used to build models that discriminate Wild-and Hatchery-Origin steelhead across the Lake Michigan Basin (Watson et al., 2018). The current study incorporated results from 538 WildOrigin steelhead otoliths that were collected in 2014 and 2015 (Prichard et al., 2019). A general introduction to otolith microchemistry applications, trace element uptake in otoliths, Michigan steelhead, and Michigan Geology and otolith microchemistry will be presented. Glacial deposits obfuscate much of the Michigan bedrock influence on stream chemistry, and this in turn influences the utility of Sr isotope systematics in lower peninsula Michigan streams as compared to bedrock dominated fluvial systems. A primary application of otolith microchemistry is distinguishing natal origins of individual fish within a mixed-stock fishery. Stocks must be distinguishable according to stockspecific microchemistry patterns, with accurate stock assignment contingent upon microchemistry assessment of all sources contributing to the mixed-stock fishery. However, otolith microchemistry signatures of individual fish, upon which classification models are built, likely represent only a portion of the variability that exists for the stocks corresponding to each natal source. To statistically infer expected otolith microchemistry patterns among unsampled catchment areas proximal to sampled areas, we tested the hypothesis that variation in catchment geology among 35 stream sites across the Lake Michigan basin is correlated with the variation in otolith microchemistry signatures of age-0 steelhead collected at those sites. Matrices of Mahalanobis distances between all pairs of individual fish were calculated for each of the following: (1) assignment scores from discriminant function analysis of the variation among sites based on otolith microchemistry, and (2) the geology (bedrock age, bedrock lithology, and 70 �surficial geology) underlying the catchments upstream of each of the sites where fish were sampled. Based on Mantel tests, these matrices were found to be significantly correlated, indicating that age-0 steelhead that exhibit greater differences in otolith microchemistry signatures tended to come from sites exhibiting greater differences in catchment geology. Surficial geology alone was more correlated with otolith microchemistry than bedrock age, bedrock lithology, or any combinations of the three geological datasets. The significant relationship between geology and otolith microchemistry, although weak, supports tenuous hydrologic and geologic bases for delineating natal source geographic boundaries. References Prichard, C.G., Student, J. J., Jonas, J. L., Watson, N. M., and Pangle, K. L., (2019) Geologic variability underlying stream catchment areas correlates with fish otolith microchemistry across disparate glacial till depths, Fisheries Research, submitted Dec., 2018 and is currently under revision. Watson, N. M., Prichard, C.G., Jonas, J. L., Student, J. J., and Pangle, K. L., (2018) Otolith ChemistryBased Discrimination of Wild- and Hatchery-Origin Steelhead across the Lake Michigan Basin, North American Journal of Fisheries Management, ISSN: 0275-5947 DOI: 10.1002nafm.10178. 71 �Using graphitic sedimentary rock geochemistry as an indicator of gold potential in the Shebandowan greenstone belt, northwestern Ontario PUUMALA, Mark Ontario Geological Survey, Ministry of Energy, Northern Development and Mines, Resident Geologist Program, Suite B002, 435 James Street South, Thunder Bay, Ontario, P7E 6S7 Graphitic sedimentary rocks are a common feature of Archean greenstone belts. Due to their high carbon content, these rocks tend to be more metalliferous than non-carbonaceous sedimentary rocks. They also act as strong reducing agents to hydrothermal fluids and can sequester metals and other elements from those fluids (Barrie 2004). Springer (1985) noted that graphitic argillites in the Abitibi greenstone belt often contain anomalous concentrations of gold (up to 0.5 ppm Au), and that much higher concentrations (up to 15 ppm Au) can be found in graphitic argillites that show evidence of hydrothermal alteration (e.g., quartz veining and carbonate alteration). Given their relative abundance in Archean greenstone belts and their response to gold-bearing hydrothermal fluids, the geochemistry of graphitic sedimentary rocks should provide information to assist in the search for mesothermal gold deposits. Detailed geochemical studies completed in the Abitibi greenstone belt by Barrie (2004) demonstrated that graphitic argillite proximal to the Owl Creek, Hoyle Pond, Holloway and HoltMcDermott mines typically contains elevated concentrations of gold (Au), arsenic (As), antimony (Sb) and mercury (Hg). Based on the results of this work, Barrie (2004) developed a method of calculating a hydrothermal alteration index that is based on concentrations of these elements and is normalized to graphitic and carbonaceous (non-carbonate) carbon (C*) and sulphur (S). Normalization of the data accounts for the likelihood that the degree of metal sequestration from hydrothermal fluids will be proportional to the graphitic/carbonaceous carbon and sulphur contents of the rock. The alteration index equation is as follows: log (Au x Hg x As x Sb)/(C* x S); where concentrations of Au and Hg are in parts per billion, concentrations of As and Sb are in parts per million and concentrations of C* and S are in weight %. Alteration index (AI) values of >6.5 were deemed to be very significant and indicative of sample collection within 1 km of ore, while AI values <5.5 were considered insignificant. During the 2017 and 2018 field seasons, staff of the Thunder Bay Resident Geologist Office collected 72 samples of graphitic sedimentary rock from various locations in the Shebandowan greenstone belt west of the City of Thunder Bay. The program included the collection of outcrop samples and drill core samples. Drill core was obtained from the Ontario Geological Survey’s Thunder Bay and Conmee Township core repositories. The purpose of this sampling was to test the applicability of the graphitic argillite gold alteration index method of Barrie (2004) as an exploration targeting tool in the Shebandowan greenstone belt, and to establish a geochemical database for graphitic sedimentary rocks. Samples were analysed by the Ontario Geological Survey Geoscience Labs in Sudbury for the same comprehensive suite of major, minor and trace elements that were included in the Abitibi greenstone belt studies of Barrie (2004). This paper will focus on the work that was completed in 2017 (48 samples) near known gold and base metal occurrences, as results are still pending from the 2018 sampling program. As shown on Figure 1, Alteration index (AI) values exceeding 6.5 were obtained from samples collected near three known gold prospects located in the Shabaqua area (West Zone, Bylund and South Zone). No highly significant AI values were obtained from samples collected proximal to volcanogenic massive sulphide (VMS) or ultramafic rock-hosted Ni-Cu occurrences located further to the south in Conmee, Adrian, Sackville and Aldina townships. 72 �The highest AI value was obtained from an outcrop grab sample collected at the West Zone. Two more West Zone samples (1 outcrop and 1 drill core) also displayed elevated AI values. Gold at the West Zone is hosted in 2 brecciated, silicified and sulphide mineralized chert horizons. These horizons are both approximately 3 m wide and have assayed up to 6.87 g/t Au over 3.05 m. Anomalous AI values of 7.31 and 6.43 were obtained from two drill core samples collected proximal to the Bylund gold prospect. Gold mineralization on the Bylund property occurs in a 125 m wide zone of carbonate-altered rocks and stockwork quartz-carbonate veins. The anomalous AI value near the South Zone gold occurrence was obtained from a surface grab sample collected from a historic exploration trench. There are no known surface gold showings in proximity to this sample location. However, it is located approximately 20 m from the collar of the diamond drill hole that intersected the South Zone gold mineralization. The results of this study are consistent with the findings of Barrie (2004) and demonstrate that graphitic mudstone geochemistry can be used as a gold exploration targeting tool in the Shebandowan greenstone belt. The Bylund-West Zone-South Zone corridor in the Dawson Road Lots area has been identified as a high priority gold exploration target. Figure 1. Map illustrating gold alteration index (AI) values for graphitic sedimentary rock samples collected in the vicinity of the South Zone, West Zone and Bylund gold showings near Shabaqua, Ontario. AI values of >6.5 suggest that the sample site may be located within 1 km of a significant gold mineralized structure. Map grid is provided in UTM NAD83, Zone 16 co-ordinates. References Barrie, C.T. 2004. Geochemistry of exhalates and graphitic argillites near VMS and gold deposits, an Ontario Mineral Exploration Technologies (OMET) project; C.T. Barrie and Associates Ltd., Ottawa, ON, 126p. Springer, J. 1985. Carbon in Archean rocks of the Abitibi belt (Ontario-Quebec) and its relation to gold distribution; Canadian Journal of Earth Sciences, v.22, p.1945-1951. 73 �Wawa, undercover: Bedrock geologic and bedrock topographic mapping in north-central Minnesota RADAKOVICH, Amy1, CHANDLER, Val1, and JIRSA, Mark1 1 Minnesota Geological Survey, 2609 Territorial Road West, St. Paul, MN 55114 Recently published bedrock geology and bedrock topography maps for four counties in north-central Minnesota (Chandler and Radakovich, 2018; Jirsa and Chandler, 2016; Radakovich and Chandler, 2016a, b, c; Radakovich and Chandler 2018a, b, c) serve as a case study for mapping Precambrian geology in areas of almost complete cover by glaciogenic sediment. Production of the maps therefore relied heavily on data from geophysical investigation methods; successes and challenges are discussed herein. Some exploration drill core and minimal outcrop data locally guided bedrock geology mapping; however, aeromagnetic and gravity data proved to be the most useful tools for deciphering bedrock composition in most of the four county area. Basement geology (Fig. 1A) consists chiefly of Archean metavolcanic, metasedimentary, and metaplutonic rocks of the Wawa subprovince, intruded by a suite of northwest-trending Paleoproterozoic mafic dikes. Geophysical modeling refined the characterization of the Archean Leech Lake Structural Discontinuity and several buried Archean iron formations across the study area. Younger sedimentary rocks of the Paleoproterozoic Animikie Group overlie the basement bedrock in several separate, formerly-continuous basins in eastern parts of the study area. Sparse drilling data indicate poorly consolidated sedimentary strata overlying bedrock of all ages, particularly in and along topographic lows in the Precambrian surface. Pollen analysis verified a Cretaceous age for these poorly consolidated sedimentary rocks. Bedrock topographic surfaces (Fig. 1B) were hand-contoured based on limited bedrock elevation data from rare bedrock outcrops, exploration drill hole records, and drilling records of the small percentage of wells that reached the bedrock surface. As a result of the paucity of direct bedrock elevation information, a significant amount of data from passive seismic and conventional seismic soundings allowed bedrock elevation to be inferred over a large portion of the four-county area. Depth to bedrock calculations indicate that several hundred feet to as much as over 1000 feet of Quaternary glacial sediment covers the bedrock surface across most of the region. In some locations, bedrock composition and structure appear to have played a role in the development of paleo drainages on the bedrock surface; in others, drainage seems to have been less affected by apparent bedrock composition. One of the most difficult obstacles to depicting the bedrock topography was recognition of Cretaceous bedrock between the Precambrian weathering surface and the bottom of the Quaternary sediment. This poorly consolidated material is generally transparent to geophysical methods and rarely recognized by well drillers. 74 �A B Figure 1. A) Bedrock geologic map of Wadena, Becker, Hubbard, and Cass (WaBeHuCa) Counties in Minnesota. Includes Archean, Paleoproterozoic, and Cretaceous strata. A full legend for bedrock units can be obtained in the referenced publications and will be discussed during the talk. B) Bedrock topographic map of WaBeHuCa counties. Sun illumination angle 315°, Sun elevation 45°. 5x vertical exaggeration. References Chandler, V.W., and Radakovich, A.L., 2018, Bedrock Geology, pl. 2 of Lusardi, B.A., project manager, Geologic atlas of Hubbard County, Minnesota: Minnesota Geological Survey County Atlas C-41, pt. A, 6 pls., scale 1:100,000. Jirsa, M.A., and Chandler, V.W., 2017, Bedrock Geology, pl. 2 of Bauer, E.J., project manager, Geologic atlas of Becker County, Minnesota: Minnesota Geological Survey County Atlas C-42, pt. A, 6 pls., scale 1:100,000. Radakovich, A.L., and Chandler, V.W., 2016a, Bedrock topography and depth to bedrock, pl. 5 of Lusardi, B.A., project manager, Geologic atlas of Wadena County, Minnesota: Minnesota Geological Survey County Atlas C-40, pt. A, 5 pls., scale 1:200,000. ------ 2016b. Bedrock Geology, pl. 2 of Lusardi, B.A., project manager, Geologic atlas of Wadena County, Minnesota: Minnesota Geological Survey County Atlas C-40, pt. A, 5 pls., scale 1:100,000. ------ 2016c, Bedrock topography and depth to bedrock, pl. 6 of Bauer, E.J., project manager, Geologic atlas of Becker County, Minnesota: Minnesota Geological Survey County Atlas C-42, pt. A, 6 pls., scale 1:200,000. ------ 2018a, Bedrock Topography and Depth to Bedrock, pl. 6 of Lusardi, B.A., project manager, Geologic atlas of Hubbard County, Minnesota: Minnesota Geological Survey County Atlas C-41, pt. A, 6 pls., scale 1:200,000. ------ 2018b, Bedrock Topography and Depth to Bedrock, pl. 6 of Lusardi, B.A., project manager, Geologic atlas of Cass County, Minnesota: Minnesota Geological Survey County Atlas C-43, pt. A, 6 pls., scale 1:200,000. ------ 2018c, Bedrock Geology, pl. 2 of Lusardi, B.A., project manager, Geologic atlas of Cass County, Minnesota: Minnesota Geological Survey County Atlas C-43, pt. A, 6 pls., scale 1:200,000. 75 �Mesoarchean Chemical Sedimentary Rocks of Northwestern Ontario: Implications for Hydrosphere Composition in Deep Time 1RAMSAY, Brittany, 1FRALICK, Philip, 1BIELSKI, Paul, 2HOMANN, Martin, SANSJOFRE, Pierre2, and LALONDE, Stefan2 1 Department of Geology, Lakehead University, Thunder Bay, Canada, bjramsay@lakeheadu.ca European Institute for Marine Studies, CNRS-UMR6538 Laboratoire Géosciences Océan, Brest, France 2 The 2.88 Ga carbonate sediments on Woman Lake, within the Uchi Subprovince of the Superior Province, preserve a chemical record of the Archean hydrosphere. Chemical sediments act as proxies for ancient waters by incorporating rare earth elements (REE) and isotopic signatures into their crystal lattice as they precipitate, thereby documenting the chemical composition of the waters from which they formed (Webb et al., 2009). Comparing the sedimentologic characteristics of Archean units to modern analogues enable us to determine the depositional environments of the past. Detailed stratigraphic columns linked with geochemical and isotopic data permit a more complete understanding of the depositional environment and evolutionary processes occurring at this early time in Earth’s history. At the base of the carbonate platform, lying atop rhyolitic Archean basement, is a massive carbonate grainstone unit, interbedded with minor crinkly-stratiform silicified microbial mats. A sharp contact separates them from a thrombolite unit (TB), composed of discontinuous and clotted laminations of dark, organic rich carbonate and white carbonate cement. Up section within the TB unit colloform stromatolites develop. This unit transitions into a stromatolitic unit that is comprised of 12-15cm thick carbonate grainstone (CG) alternating with 5-8cm thick pustular stromatolites (PS) for ~5m. Geochemically interesting trends reminiscent of both Archean and modern oxygenbearing signatures are evident in shale-normalized REE spectra (Fig. 1). REE concentrations were determined from weak acetic acid leaches by ICP MS and laser ablation-ICP-MS at the European Institute for Marine Studies (IUEM). The CG’s display distinct negative Ce anomalies and slightly positive Eu anomalies while the PS show weaker negative Ce anomalies and no Eu anomaly (Fig. 1b). The TB unit displays consistent spectra with negligible Ce anomalies and positive Eu anomalies (Fig. 1a). Laser ablation-ICP-MS was used to obtain REE spectra exclusively from the TB’s white calcite cements (Fig. 1c), which show a more pronounced Ce anomaly. Stable C and O isotopes were also analyzed at IUEM. The PS have slightly lower δ 13C values compared to the CG and TB (Fig. 2), however all samples fall within a relatively restricted range (-1.19 to 1.22‰). Positive Eu and negative Ce anomalies are generally accepted as robust indicators of the influence of hydrothermal fluids and the presence of free oxygen, respectively (Derry and Jacobson, 1992). They are unique among REE in that they have two possible valence states (Eu2+,3+, and Ce3+,4+). In high temperature hydrothermal fluids, Eu3+ is reduced to Eu2+, which renders it more soluble than its trivalent neighbors, a process that enriched Archean seawater with Eu and imparted a positive Eu anomaly on precipitating carbonates. Negative Ce anomalies result from the oxidation of Ce3+ to Ce4+ and the subsequent removal of the less soluble Ce4+ from solution. Once oxidized, Ce readily adsorbs onto particulate matter, removing it from solution and permitting Ce depleted chemical precipitation. Stable carbon isotopes in marine carbonate rocks are widely used as proxies for carbon cycling through time and are commonly employed to track organic carbon burial, which 76 �preferentially sequesters 12C and leads to 13C enrichment in residual dissolved inorganic carbon (Schidlowski, 2001). Throughout most of earth’s history δ13C varies only slightly from 0‰ (Veiser, 2001), and Woman Lake carbonates are no exception. The PS are slightly more negative compared to the CG and TB, likely due to the PS being more abundant in organic matter. The geochemical anomalies and isotopic signatures present at Woman Lake seem to indicate that the carbonates precipitated from two different fluid sources. The positive Eu anomaly suggests a hydrothermal source which is characteristic of Archean seawater, while the Ce anomaly suggests fluids interacted with free oxygen. Stratigraphically, TB’s precipitated first and contain a positive Eu anomaly (Fig. 1a), suggesting they precipitated within seawater. The overlying Stromatolitic Unit (PS and CG) contain negative Ce anomalies (Fig. 1b), which imply that they were deposited nearshore, where the PS could grow and interact with oxygen-bearing freshwater. The TB cements contain a more pronounced Ce anomaly compared to the whole rock composition (Fig.1c). It is possible that the cements inherited the Ce anomaly from the overlying pustular stromatolites and grainstones. If they were subaerially exposed to rain, they may have partially dissolved and percolated through CG, reprecipitating with negative Ce anomalies in spaces within the TB producing the clotted fenestral appearance. It is also possible that the pustular stromatolites produced locally oxic conditions while the thrombolites were not as capable. References Derry, L.A., Jacobsen, S.B., (1990) The chemical evolution of Precambrian seawater: Evidence from REEs in banded iron formations. Geochim. Cosmochim. Acta 54, 29652977. Schidlowski, M. 2001. Carbon isotopes as biogeochemical recorders of life over 3.8 Ga of Earth history: Evolution of a concept. Precambrian Res., v. 106, p. 117–134. Veizer, J., 2003. Isotopic evolutions of seawater on geological time scales: sedimentological perspective, in Lentz, D.R. ed., Geochemistry of Sediments and Sedimentary Rocks: Evolutionary Considerations to Mineral DepositForming Environments: Geological Association of Canada, GeoText 4, p. 53-68. Webb, G., Nothdurft, L., Kamber, B., Kloprogge, T., and Zhao, J. 2009. Rare earth element geochemistry of scleractinian coral skeleton during meteoric diagenesis: a sequence through neomorphism of aragonite to calcite. Sedimentology, vol. 56, p. 14331463. 77 �Precambrian Geology of the Eastern Shebandowan Greenstone Belt - Insights into Stratigraphy and Structural History RATCLIFFE, Laura M.1 1 Earth Resources and Geoscience Mapping Section, Ontario Geological Survey, Sudbury, Ontario P3E 6B5 The Shebandowan greenstone belt (SGB), located in the Wawa-Abitibi terrane of the Superior Province, extends 150 km west from Thunder Bay to Quetico Provincial Park and has an arcuate shape. It is bordered by the Quetico Subprovince to the north and wraps around the Northern Light–Perching Gull Lakes batholithic complex to the south. Paleoproterozoic sedimentary rocks overlie the SGB’s southeastern extent. This presentation reports on a multiyear project by the Ontario Geological Survey focused on updating the bedrock geology of the eastern part of the SGB, and new insights into the stratigraphy and structural history of the eastern SGB are explored. This work builds on previous work by Shegelski (1980), Williams et al. (1991), Berger (1993) and Corfu and Stott (1998). The Shebandowan greenstone belt contains a succession of supracrustal rocks and their syn-eruptive intrusive equivalents. Geochronological data and previous geological studies have defined 3 main supracrustal assemblages: the Greenwater assemblage (circa 2720 Ma), the Kashabowie assemblage (circa 2695 Ma), which is not currently recognized in the eastern SGB, and the Shebandowan assemblage (circa 2690 to 2680 Ma) (Corfu and Stott 1998). Based on the interpretation of Corfu and Stott (1998), the SGB has undergone 2 main stages of deformation (D1 and D2). D1 deformation occurred at approximately 2695 Ma, and is thought to have tectonically imbricated rocks of the Greenwater and Kashabowie assemblages across the SGB, this event is poorly understood in the eastern SGB. D2 deformation is constrained between 2685 and 2680 Ma and thought to record oblique northwest-directed compression. Regionally, the emplacement of sanukitoid plutons between 2685 and 2680 Ma is thought to have occurred during the waning of D2. In the eastern part of the SGB the Greenwater assemblage (circa 2720 Ma) comprises predominately massive, aphyric mafic volcanic flows with minor aphyric, pillowed and locally variolitic mafic flows. Among the mafic volcanic flows are thin layers of felsic and ultramafic volcanic rocks 100 to 750 m thick, as well and thin 100 m layers of terrigenous-clastic sedimentary rocks. Syn-eruptive intrusive mafic and ultramafic rocks occur throughout the Greenwater assemblage as sills and dikes. In the eastern part of the SGB the Shebandowan assemblage (circa 2690 Ma) is comprised of predominately intermediate volcaniclastic to epiclastic, heterolithic, amphibole- and plagioclase-phyric tuff, lapilli tuff, tuff breccia and course tuff breccia and/or terrigenous-clastic wacke, siltstone and conglomerate. The conglomerate commonly contains sedimentary fragments. The contact between the Greenwater and Shebandowan assemblages regionally has been interpreted to be an unconformity (Corfu and Stott 1998), however it is not been clearly demonstrated in outcrop. Mapping as part of this project has identified a distinct lithostratigraphic unit separating rocks from the Greenwater and Shebandowan assemblages. The 1 to 1.5 km thick (in plan view) “boundary zone” comprises intermediate tuffs and flows and/or wacke to siltstone, interlayered with chemical sedimentary rocks and lenses of conglomerate (containing chemical sedimentary rocks ± mafic, ± ultramafic, ± felsic volcanic fragments). The “boundary zone” may be interpreted as the inferred lower stratigraphic unit of the Shebandowan assemblage or as the rocks deposited during a transitional period between the Greenwater and Shebandowan assemblages. Work is ongoing to evaluate the geologic context of this distinctive unit and its significance with respect to the stratigraphy of the SGB. Some new constraints on the timing of deformation in the eastern SGB are provided by local outcrop observations and targeted geochronological analyses. An outcrop where structural relationships are well 78 �exposed consists of a wacke deposited after 2694±3 Ma (Davis, Ménard, and Sutcliffe 2018), that is assigned to the Shebandowan assemblage intruded by a set of tonalite dikes emplaced at 2682 ± 2 Ma (Davis, Ménard, and Sutcliffe 2018). Bedding and a layer parallel foliation in the wacke are folded and the folding is cross-cut by the tonalite dikes (Photo 1A). The tonalite dikes are also folded and boudinaged (Photo 1B), and finally a weakly penetrative cleavage overprints the previously described features. These observations indicate there were multiple phases of deformation affecting the Shebandowan assemblage rocks and that deformation continued past the emplacement of the dikes circa 2680 Ma. Corfu and Stott (1998) considered the final phase of deformation in the SGB to be between 2685 Ma and 2680 Ma and that tectonic activity was quiescent after 2680 Ma in contrast to the adjacent Quetico Subprovince, and other greenstone belts farther east in the Wawa Subprovince, where deformation is recorded after 2680 Ma. However, the previously described outcrop observations show that there were multiple phases of deformation in the SGB after circa 2680 Ma. Thus, tectonic activity in the eastern SGB continued for longer than previously interpreted. Photo 1: Photographs from a well exposed outcrop showing structural relationships. Two geochronology samples were analyzed from this location and ages are displayed (Davis, Ménard, and Sutcliffe 2018) (290782E 5363887N). Photo A) Folded primary layering and layer parallel schistosity (S0 and S1) cross cut by a tonalite dike. The dike is outlined by the black dashed line. The primary layering and layer parallel schistosity is indicated by the white dashed line. The folding event preceding dike emplacement is annotated in in white and black (F1). Photo B) Tonalite dike emplaced in a thinly bedded wacke and siltstone is folded and boudinaged. The dike is outlined by the black dashed line. The primary layering and layer parallel schistosity (S 0 and S1) is indicated by the white dashed line. The folding event following dike emplacement is annotated in white and black (F2). Compass is 22 cm long including sighting arm. The UTM co-ordinates are provided using NAD83 in Zone 16. References Berger, B.R. 1993. Geology of Adrian and Marks townships; Ontario Geological Survey, Open File Report 5862, 90p. 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. Davis, D.W., Ménard, J. and Sutcliffe, C.N. 2018. U-Pb geochronology of samples from northern Ontario, Part B: LA-ICP-MS; internal report prepared for the Ontario Geological Survey, Jack Satterly Geochronology Laboratory, University of Toronto, Toronto, Ontario, 94p. Shegelski, R.J. 1980. Archean cratonization, emergence and red bed development, Lake Shebandowan area, Canada; Precambrian Research, v.12, p.331-347. 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. 79 �High-technology metals in ore-forming environments and their signature in volcanichosted sulfide mineralization in northern Minnesota and Wisconsin. SCHARDT, Christian and DAVID, Mady Department of Earth and Environmental Sciences, University of Minnesota-Duluth, 1114 Kirby Dr. Duluth, MN 55812 While the use of high-technology metals (HTMs), such as In, Ge, Ga, and Tl, is increasing in essential industrial applications and renewable energy technologies, our understanding of the sourcing and accumulation of these elements is insufficient. This applies to both their general distribution in various geological environments, their sourcing by ore-forming processes, and their deposition in selected ore deposits from which they are being mined. Typical concentrations for these metals are very low in most rock types (0.1 - 2 ppm; e.g., Terashima, 2001) and may reach concentrations > 1 % in certain ore deposit types (Murao et al., 2008; Kampunzu et al., 2009) by substituting for common metals (Zn, Cu, Sn) in familiar ore mineral such as sphalerite, chalcopyrite, or stannite (Johan, 1988, Pavlova et al, 2015). The formation of ore deposits showing elevated values of In, Ge, Ga, and Tl (volcanichosted massive sulfides, granitic tin deposits, MVT deposits) are relatively well understood but it is unclear why these metals do not accumulate in other ore-forming environments, i.e. SEDEX, SSC, or porphyry deposits, known to contain elevated concentrations of other HMTs. Little research has been conducted into the general thermodynamic behavior or potential enrichment mechanisms and there is no organized database of the concentration of these metals in various geological settings or their sourcing in ore-forming systems. Previous work (Schardt and David, 2018) initiated data collection and analysis of these metals in common rock types and an interpretation of potential sources. In this study, the database has been expanded to include most common ore-forming environments to better understand commonalities and differences with regards to selected HMTs and evaluate volcanic-hosted massive sulfide signatures from northern Minnesota (Vermilion district) and Wisconsin (Penokian Volcanic Belt). Figure 1 plots all available data (whole rock, mineral, ore material, alteration) for the most common ore deposit types and compares them to available data from the Vermilion district as well as known volcanic-hosted massive sulfide deposits in the Penokean Volcanic Belt of Wisconsin. Except for Tl, both environments show low average concentrations compared to similar formation environments (V). Data would suggest that either a) hydrothermal processes unrelated to volcanogenic massive sulfide formation (e.g., lower- temperature SEDEX, epithermal, gen. hydrothermal, SSC, MVT) are more efficient at sourcing and concentration HTMs, or b) the difference in host/source rock has a significant influence on the ability of the ore-forming system to source and accumulate HTMs. Most rock types have very similar HTM concentrations, except for Ga, which is significantly more abundant in volcanic rocks (~ 1 ppm vs. ~ 20 ppm). Higher fluid temperatures, such as those found in granitic ore systems (G in figure 1) do not exhibit any specific HTM enrichment but their whole-rock data (stippled line in figure 1) indicate that In may be more mobile under these conditions. This interpretation is speculative as available data are scattered and these elements are not routinely analyzed. While no systematic work has been conducted to assess the behavior of these metals a robust database is now available to study their distribution in hydrothermal systems and apply results to exploration efforts in Minnesota and Wisconsin. 80 �Figure 1. Plot of HTM concentrations as a function of ore-forming environment. Vertical bars represent minimum, average, and maximum values for each ore deposit type (see text). Solid lines denote trend in average concentrations (all data) while stippled line shows whole-rock concentrations only. L – low temperature (SSC, MVT); V – volcanogenic (all types of VHMS); H – hydrothermal (SEDEX, epithermal, hydrothermal), G – graniterelated (skarn, tin, porphyry). References Johan, Z, 1988, Indium and Germanium in the Structure of Sphalerite: an Example of Coupled Substitution with Copper. Mineralogy and Petrology, v. 39, p.211 – 229 Kampunzu, A.B., Cailteux J.L.H., Kamona, A.F., Intiomale, M.M., and Melcher, F., 2009, Sediment-hosted Zn–Pb– Cu deposits in the Central African Copperbelt, Ore Geology Reviews, v. 35, p. 263-297 Murao, S., Deb, M., and Furuno, M., 2008, Mineralogial evolution of indium in high grade tin-polymetallic hydrothermal veins - A comparative study from Tosham, Haryana state, India and Goka, Naegi district, Japan, Ore Geology Reviews, v. 33, p. 490-504 Pavlova, G.G., Palessky, S.V., Borisenko, A.S., Vladimirov, A.G., Seifert, T., and Phane, L.A. (2015) Indium in cassiterite and ores of tin deposits. Ore Geology Reviews, v. 66, p. 99–113 Schardt, C., and David, M., 2018, High-technology metal behavior in ore-forming environments and its implication for the Vermilion District, northern Minnesota, Proceedings of the Institute on Lake Superior Geology, v. 64, p. 91-92 Terashima, S. (2001) Determination of Indium and Tellurium in Fifty Nine Geological Reference Materials by Solvent Extraction and Graphite Furnace Atomic Absorption Spectrometry. Geostandards Newsletter, v. 25, p. 127 - 132 81 �Geochemistry of Archean Gneisses in Dickinson County, Northern Michigan SCHULZ, K.J.1, CANNON, W.F.1, WOODRUFF, L.G.2, AND AYUSO, R.A.1 1 U.S. Geological Survey, 954 National Center, Reston, VA 20192, 2 U.S. Geological Survey, Mounds View, MN 55112 A terrane composed largely of Meso- to Paleoarchean gneisses and granitic rocks occurs along the southern margin of the Neoarchean Superior Craton in the Lake Superior region. These rocks are best documented from exposures in the Minnesota River Valley (MRV) in southwestern Minnesota, but they also occur in basement uplifts in northern Michigan including the Watersmeet Dome in the MareniscoWatersmeet area, the Carney Lake Gneiss north of the Menominee iron range, and the Southern Complex south of the Marquette Trough. Recent studies in the MRV have shown a range in ages primarily between ~2.6 Ga to ~3.5 Ga, representing both primary intrusive events and metamorphic/tectonic overprints (Bickford et al., 2007 and references therein). Similarly, dating of the gneisses in the Watersmeet Dome (Miska et al., 2018) and Carney Lake Gneiss (Ayuso et.al, 2018) have a range of ages from ~1.8 Ga to ~3.6 Ga, but also several spot analyses of zircon cores and xenocrysts that date at ~3.8 Ga. Thus, the northern Michigan gneisses show evidence of an Eeoarchean component and effects of the Penokean orogeny neither of which are seen in the MRV. Here we report on the geochemistry of Archean gneisses from Dickinson County in northern Michigan including the Carney Lake Gneiss. The Archean rocks in Dickinson County are described in James et al., (1961) and Bayley et al., (1966). As is typical of Archean gneiss terranes, the rocks consist mostly of variably banded and deformed tonalite-trondhjemite-granodiorite (TTG) gneisses and granites. Geochemistry Major elements Major element geochemistry of the gneisses in Dickinson County span the compositional range from tonalite to granite. Using the granite classification of Frost et al., (2001), the gneisses are magnesian and mostly calcic, although some of the more felsic gneisses are alkali calcic. A notable feature of the gneisses is that they are weakly to strongly peraluminous and corundum normative (Fig, 1A). The Na 2O content ranges from 3 to 5 wt.%, and the K2O content ranges from ~1 to 5 wt.% (medium- to high-K range); Na2O/ K2O ratios are mostly <2. There is a positive correlation between Na2O and Al2O3 contents, and a negative correlation between K2O and Al2O3 contents. Trace elements Unlike many Archean TTG suites which are typically characterized by Sr contents >400 ppm, the Sr contents of the Dickinson County gneisses are variable but <400 ppm. Rubidium/Sr ratios are variable, ranging from <0.1 to ~1, and show a negative correlation with Al2O3. Samples exhibit significant variation in chondrite normalized REE patterns, both in terms of pattern steepness and size of the Eu anomaly (Fig. 1B, C). The (La/Yb) N for samples from the Carney Lake Gneiss varies from 29 to 183 with no to moderately negative Eu anomalies; the more felsic samples tend to have higher light REE abundances and larger negative Eu anomalies (Fig. 1B). In contrast, two biotite gneiss samples from near Felch have much flatter patterns ((La/Yb) N of 14 and 24) and moderate to large negative Eu anomalies (Fig. 1C). Two samples of Norway Lake Gneiss from north of Felch have intermediate sloped REE patterns ((La/Yb)N of 44 and 58), no Eu anomaly for the tonalite sample, and moderate negative anomaly for the more granitic sample (Fig. 1C). All samples have negative Nb and Ta anomalies on primitive mantle normalized trace element plots. Comparison with Archean TTG suites Archean TTG suites are commonly silica-rich (SiO2 >64 wt.%, but commonly ≥70 wt.%), have high Na2O (>3.0 wt.%) and Na2O/K2O (>2), and low ferromagnesian element contents (Moyen and Martin, 2012). They trend from metaluminous to slightly peraluminous (A/CNK ~1; normative corundum <1%), with A/CNK increasing in more granitic compositions. Two subgroups are recognized: most 82 �Archean TTG suites have high Al2O3 (>15 wt.% at 70 wt.% SiO2) with high Sr contents (>400 ppm) and strongly fractionated REE patterns ((La/Yb)N up to 150); the second subgroup has lower Al2O3 (<15 wt. %) as well as lower Sr and less fractionated REE patterns. The Archean gneisses and granitic rocks in Dickinson County have the geochemical characteristics of typical TTG suites with respect to Al2O3 and strongly fractionated REE patterns. However, most samples are more potassic and less sodic than typical TTG and have lower Sr contents (<400 ppm). In addition, the Dickinson County samples are all peraluminous. Discussion Most models for the genesis of typical TTG involve partial melting of garnet amphibolite or eclogite (Moyen and Martin, 2012). However, the geochemical characteristics of the Archean gneisses in Dickinson County suggest a more complex petrogenesis involving melting of preexisting evolved crustal sources. This is supported by the presence of ~3.8 Ga xenocrystic zircons in some samples (Ayuso et al., 2018). It has been proposed that the Archean gneiss terrane in the Lake Superior region is a remnant of the Wyoming Craton, which was rifted from the Superior Craton in the Paleoproterozoic. In this regard, it may be significant that the quartzofeldspathic gneisses and granitoids in the Wyoming Craton have similar geochemical characteristics to the Archean gneisses in Dickinson County including relatively high K2O, low Sr, variably steep REE patterns, and are also mostly peraluminous (Frost et al., 2006). 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, Proceedings of 64th Annual meeting, Part 1: Program and Abstracts, p. 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, 96 p. Bickford, M.E., Wooden, J.L., Bauer, R.L., and Schmitz, M.D., 2007, Paleoarchean gneisses in the Minnesota River Valley and northern Michigan, USA, in Van Kranendonk, M.J., Smithies, R.H., and Bennett, V.C., eds., Earth’s Oldest Rocks, Developments in Precambrian Geology, v. 15, p. 731–750. Frost, B.R., Collins, C.G., Arculus, R.J., Ellis, D.J., and Frost, C.D., 2001, A geochemical classification of granitic rocks: Journal of Petrology, v. 42, p. 2033–2048. Frost, C.D., Frost, B.R., Kirkwood, Robert, and Chamberlain, K.R., 2006, The tonalite-trondhjemite-granodiorite (TTG) to granodiorite-granite (GG) transition in the late Archean plutonic rocks of the central Wyoming Province: Canadian Journal of Earth Sciences, v. 43, p. 1419–1444. 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. Miska, M.A., Mueller, P.A., and Bermudez, Katherine, 2018, Paleoarchean crust of the Minnesota-Michigan corridor: Evidence from the Watersmeet Dome, northern Michigan: Geological Society of America Abstracts with Programs, v. 50, no. 6, doi: 10.1130/abs/2018AM-318140. Moyen, Jean-Francois, and Martin, Hervé, 2012, Forty years of TTG research: Lithos, v. 148, p. 312–336. 83 �Geologic Architecture and Precious Metal Mineralization in the Southern Abitibi; New Insights from the Larder Lake Area SHERLOCK, Ross, RUBINGH, Kate and the Metal Earth research team Mineral Exploration Research Center, Harquail School of Earth Sciences, Laurentian University, Sudbury Ontario Metal Earth is one of the largest mineral exploration research project ever undertaken and is a fully funded $104M / 7 year research project focused on the processes responsible for differential metal endowment and ore localization during the Archean. A major focus of Metal Earth is to use geological and geophysical data to define crust to mantle scale differences across ancestral fault systems and volcanic centres that have variable metal endowment. As part of the Metal Earth project, research has focused on a ~40 km long north south geologic transect that is centered over the Cadillac-Larder Lake break and extends northward into the Ben Nevis volcanic complex and to the south over the Lincoln Nipissing shear zone. The Cadillac-Larder Lake break is a regionally extensive crustal break and hosts a number of gold deposits including the Kerr Addison mine which historically produced over 11Moz of gold. The Ben Nevis volcanic complex (2696.6 ± 1.3 Ma), part of the Blake River group (2701 ± 3 – 2698.5 ± 2Ma), is correlative to the Noranda VMS camp but lacks significant metal endowment. The Lincoln Nipissing shear zone is similar to the Cadillac-Larder Lake break, in that it juxtaposes different geologic domains and is marked by ultramafic volcanic rocks, clastic sedimentary rocks and Timiskaming aged small volume intrusive rocks and associated gold prospects. At both the Cadillac-Larder Lake break and the Lincoln Nipissing shear zone, ultramafic rocks of the Larder Lake group (ca. 2710-2704 Ma) (Piché in Quebec) are unconformably overlain by clastic rocks of the Timiskaming (2677-2670 Ma) or Hearst assemblage (<ca. 2700 Ma). This suggests that the original geologic relationship was stratigraphic in nature and subsequently overprinted by deformation and alteration associated with the gold deposits, in contrast to the previous interpretations that only considered a structural emplacement. Recent geological and geophysical surveys from the Metal Earth research project indicate that the Cadillac-Larder Lake break is well-resolved using seismic methods to depths of over 30 km and has a corresponding MT conductivity anomaly. In contrast, the Lincoln Nipissing shear zone, although sharing similar characteristics to the Cadillac-Larder Lake break, is poorly resolved by seismic and MT methods, perhaps correlating with the relative lack of metal endowment along the shear zone. This is MERC-Metal Earth publication number MERC-ME2018-177. 84 �An investigation into the distribution of chalcophile elements and timing of mineralization within the Crystal Lake intrusion: A U-Pb geochronology and LA-ICP-MS study SMITH, Jennifer1, BLEEKER, Wouter1, HAMILTON, Mike2 and PETTS, Duane1 1 Geological Survey of Canada, 601 Booth Street, Ottawa, Canada; email:jennifer.smith6@canada.ca Jack Satterly Geochronology Laboratory, Dept. of Earth Sciences, University of Toronto, 22 Russell St., Toronto, Canada 2 A detailed geochemical and isotopic study is underway on the 1099.1 ± 1.2 Ma (Heaman et al. 2007) Crystal Lake intrusion, which has previously been compared to the proximal Duluth Complex (Thomas 2015). The aim of this study is to gain further insights into the controls on ore genesis within the ‘mainrift’ intrusions (Miller and Nicholson 2013). The Crystal Lake intrusion, located 47 km southwest of Thunder Bay, Ontario, Canada, outcrops as a prominent Y-shaped body within the Paleoproterozoic Animikie basin, intruding sulfur-bearing shale, argillite and greywacke of the Rove Formation. Although a number of dating studies have been undertaken on the MCR (see Heaman et al. 2007, for a relatively recent compilation), many of the intrusions either lack the precision that is now possible with routine chemical abrasion U-Pb geochronology or are yet to be dated. We are currently undertaking a comprehensive geochronology study throughout the MCR, this includes detailed dating of the Crystal Lake intrusion. In addition to refining Heaman’s et al. (2007) baddeleyite age of 1099.1 ± 1.2 Ma we aim to constrain the relationship of the northern and southern limbs of the intrusion. Furthermore, we plan to untangle the timing of the Crystal Lake intrusion relative to other MCR intrusive events including the NE-trending Pigeon River dykes, the NW-trending Cloud River dykes and the sulfide-bearing Mount Mollie intrusion developed to the east. Ni-Cu-PGE sulfide mineralization is developed within the northern and southern limbs of the Crystal Lake intrusion in association with vari-textured gabbros and irregular Cr-spinel-bearing horizons. The association of sulfides and metal enrichment with pegmatitic/taxitic units is also observed within other Ni-Cu-PGE deposits such as the ca. 1108 Ma (Heaman and Machado 1992) Coldwell Complex, Merensky Reef, Norilsk and Voisey’s Bay. Sulfide mineralization is largely disseminated, with massive sulfides (<50 cm in thickness) developed locally within the northern limb. The disseminated ores are variable in texture with globular (capped and uncapped), blebby and interstitial sulfides identified. Silicate-capped sulfide globules have been recognized in other Ni-Cu sulfide deposits (e.g. Norilsk, Insizwa Complex; Barnes et al. 2017; Le Vaillant et al. 2017) and are interpreted as being the remnants of former segregation vesicles that attached to an immiscible sulfide melt (Mungall et al. 2015). Within the Crystal Lake intrusion, the morphology of the caps, which are comprised of amphiboles, clays, chlorite and calcite, is variable. Convex silicate caps, identical to those modelled by Mungall et al. (2015), are present along with very irregular silicate attachments. The implications of degassing, which appears to be a common process within the Ni-Cu ore systems, for sulfide transportation and deposition is yet to be constrained. Furthermore, the cause (e.g. contamination, pressure changes) and timing of degassing relative to crystallization is not well understood. A detailed elemental deportment study is currently in progress, focused on characterizing the distribution and mineralogy of platinum-group minerals (PGMs). Element mapping of sulfides by LAICP-MS has been used to further investigate the control on the distribution of the chalcophile elements during sulfide fractionation. Preliminary observations indicate that Pd resides in solid solution within pentlandite (1 – 150 ppm) and as small As-Bi and Sb-bearing PGMs. Within the massive sulfides Pdbearing minerals show a strong association with nickel arsenides resulting in lower concentrations of Pd (~1 ppm) in the pentlandite than typical of other sulfide assemblages (10–150 ppm). Platinum is not compatible in any of the sulfide phases, instead occurring as discrete As and Sb-bearing PGMs. The PGMs are found either enclosed or attached to sulfides or within secondary silicates around the altered margins of the sulfides. It is yet to be established whether the crystallization of Cr-spinel and/or lowtemperature alteration of the sulfides has had any control on the mineralogy and distribution of PGEs. 85 �Element mapping of the sulfides by LA-ICPMS has revealed some interesting structural and or/mineralogical controls in the distribution of chalcophile elements. Although not observed throughout the primary sulfide assemblage, some unaltered sulfides are characterized by a strong microfabric (Fig. 1). This fabric is defined by several elements including As, Mo, Bi, Pb, Pd and Re which appear to be preferentially concentrated along thin, parallel linear features within the pyrrhotite-pentlanditechalcopyrite assemblage. The molybdenum map also shows thicker banding and elevated concentrations within pyrrhotite (Fig. 1). Interestingly this fabric is not confined to a particular sulfide phase. This is best shown by Figure 1. LA-ICP-MS element maps of primary sulfide As, Mo and Re which clearly cut across the assemblage grain boundaries of pyrrhotite, pentlandite and chalcopyrite, which suggests that this fabric was developed subsequent to crystallization of all three phases. For other elements such as Pd and Pb, the fabric is restricted to the pyrrhotite and pentlandite (Fig. 1). Silicate infilled fractures appear to cut the fabric as shown in the As map. Further work is in progress to determine the controls on selected element mobility (i.e. low temperature alteration or deformation) and to gain an understanding at what scale this remobilization is occurring. If various elements are remobilized over long distances, then it could have implications for vectoring of Ni-Cu ore systems. Element mapping by LA-ICP-MS is an extremely powerful tool, providing unparalleled detail at the micro scale. This technique provides insight into the behavior and mobility of chalcophile elements during sulfide fractionation, low temperature alteration and/or deformation and may provide a link to larger element haloes associated with some Ni-Cu-PGE deposits. References Barnes, S.J., Mungall, J., Le Vaillant, M., Godel, B., Lesher, M., Holwell, D., Lightfoot, P., Krivolutskaya, N., and Wei, B., (2017) Sulphide-silicate textures in magmatic Ni-Cu-PGE sulphide ore deposits: Disseminated and net-textured ores. American Mineralogist 102:473–506. Heaman, L.M., and Machado, N., (1992) Timing and origin of midcontinent rift alkaline magmatism, North America: evidence from the Coldwell Complex. Contributions to Mineralogy and Petrology 110:289–303. Heaman, L.M., Easton, R.M., Hart, T.R., Hollings, P., MacDonald, C.A., Smyk, M., (2007) Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario. Canadian Journal of Earth Sciences 44:1055–1086. Le Vaillant, M., Barnes, S.J., Mungall, J.E., Mungall, E.L., (2017) Role of degassing of the Noril’sk nickel deposits in the Permian–Triassic mass extinction event. Proceedings of the National Academy of Sciences 114:2485–2490. Miller, J.D., Nicholson, S.W., (2013) Geology and mineral deposits of the 1.1 Ga Midcontinent Rift in the Lake Superior region – An overview; in Field Guide to the Cu-Ni-PGE Deposits of the Lake Superior Region (ed) JD Miller. Precambrian Research Center Guidebook 13-1:1–50. Mungall, J.E., Brenan, J.M., Godel, B., Barnes, S.J., Gaillard, F., (2015) Transport of metals and sulphur in magmas by flotation of sulphide melt on vapour bubbles. Nature Geoscience 8:216–219. 86 �Seismic stratigraphy of the 1.1 Ga Midcontinent Rift beneath western Lake Superior shows evidence of differing subsidence histories for syn-magmatic sub-basins STEWART, Esther K.1, GRAUCH, V.J.S.2, WOODRUFF, Laurel G.3, and HELLER, Samuel4 1 Wisconsin Geological & Natural History Survey, 3817 Mineral Point Road, Madison, WI 53705 U.S. Geological Survey, MS 964, Federal Center, Denver, CO 80225 3 U.S. Geological Survey, 2280 Woodale Drive, St. Paul, MN 55112 4 U.S. Geological Survey, MS 939, Federal Center, Denver, CO 80225 2 The nature of the Midcontinent Rift where it is hidden beneath Lake Superior can only be understood from geophysical interpretation and inferences from geologic concepts onshore. Many decades of collecting geophysical data and improving the geologic framework for the Lake Superior region have led to evolving paradigms on its structure and geologic history. We are taking a new look at the existing geophysical data considering recent age dating and geologic mapping as part of an ongoing effort to characterize the 3D geometry of the rift through time. In particular, we are constructing a seismic stratigraphic framework from reflection data collected in the 1980s to characterize the basalt-filled sub-basins present beneath the western lake. Integration with gravity and aeromagnetic data and correlation of seismic stratigraphy to onshore geology provides insight into the geometry of the sub-basins and the timing and rates of subsidence and concurrent magma accumulation. We interpret 3 reprocessed and 7 geolocated images of industry seismic sections and public seismic data (GLIMPCE Line C) using a 3D visualization software platform. Seismic sections were converted from time to depth using existing seismic refraction models as guides. Dips and thicknesses of onshore geology projected onto nearby seismic profiles tie seismic stratigraphy to mapped geologic units. Aeromagnetic modeling helps distinguish igneous rocks with strong, reversed- versus normal-polarity remanent magnetization, which constrains the cooling ages of syn-rift rocks to before or after about 1100 Ma, respectively. Seismic facies and reflection geometry image volcanic flows, sills, intrusions, and pre-rift crust. We have identified three seismic stratigraphic units. The lowest unit has clinoform reflection geometry and may represent pre-rift sediments intruded by sills. The middle and upper units have synform reflection geometry, but aeromagnetic modeling and correlations to onshore geology suggest the middle unit represents older (>1100 Ma) basalts with reversed-polarity and the upper unit represents younger (<1100 Ma) basalts with normal magnetic polarity. As pointed out by previous workers (e.g., Allen et al., 1997) we do not observe large normal faults bounding sub-basins. Except for a possible growth fault of limited extent at depth on GLIMPCE Line C (Fig. 1), we observe sub-basins that sag and thicken toward their centers, implying that syn-magmatic subsidence may have been the primary control on basin development in western Lake Superior. The sub-basins are flanked by two previously recognized seismic highs that are associated with gravity lows: Grand Marais Ridge (GMR) south of Grand 87 �Marais, MN, and White’s Ridge (WR), centered on the Bayfield Peninsula (Fig. 1; Allen et al. 1997). An isopach map of the seismic stratigraphic unit interpreted as normal-polarity basalts shows differences in thickness and age of basin fill within sub-basins surrounding GMR (Fig. 1). We correlate most of the basin fill in a western, bowl-shaped basin to the Beaver Bay Complex and younger North Shore Volcanics that were deposited or emplaced on the present-day northern lake shore of Minnesota between ca 1096 – 1094 Ma. In contrast, only 4-9 km of normal-polarity basalt infills an adjacent, elongate basin that wraps around the southeast and eastern sides of GMR. We correlate this material with the ca 1094 – 1091 Ma Portage Lake Volcanics on Michigan’s Keweenaw Peninsula. The difference in basin geometry and age indicates each basin developed independently. The western basin subsided and infilled at some 5mm per year. After the main subsidence and infilling of the western basin waned, the adjacent, shallower basin began to subside at some 3 to 1.3mm per year. Reflections interpreted as reversed- and normalpolarity basalts in the shallower basin truncate against the southern side of GMR. Truncation of these once laterally continuous basalt layers was likely caused in part by the basins’ different subsidence histories and relative uplift of GMR. A mechanism for the syn-magmatic basin subsidence and dramatic difference in subsidence rates is unclear. Fig 1: Isopach map of normal-polarity basalt. Onshore geology highlights the distribution of reversed- and normalpolarity basalt and overlying sedimentary units. Purple lines locate seismic profiles, with line GLIMPCE C labeled. Reference Allen, D.A., Hinze, W.J., Dickas, A.B., and Mudrey, M.G., Jr., 1997, Integrated geophysical modeling of the North American Midcontinent Rift System: new interpretations for western Lake Superior, northwestern Wisconsin, and eastern Minnesota, in Ojakangas, R.W., Dickas, A.B., and Green, J.C., eds., Middle Proterozoic to Cambrian Rifting, Central North America: Geological Society of America Special Paper 312, p. 47-72. 88 �Towards understanding geoarchaeological contexts in Northwestern Ontario: The newly formed lithic material comparative collection at Lakehead University SURETTE, Clarence, and TAYLOR-HOLLINGS, Jill Department of Anthropology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1; clsurett@lakeheadu.ca, jstaylo1@lakeheadu.ca Stone tools were created by flintknapping, which is the process of utilizing percussion and pressure flaking techniques on fine-grained siliceous raw materials that were carefully selected by ancient people. Excellent preservation of lithics in the boreal forest of Northwestern Ontario provides some of the best evidence in the archaeological record for reconstructing these past human activities. Professional archaeologists have been finding and interpreting stone tools from the earliest known Palaeo or Early Period (ca. 9,000-7,000 years before present) sites in the Thunder Bay region beginning in the 1950s (Dawson, 1984; MacNeish, 1952). However, little is still known about the variation and sources of them in Northwestern Ontario, despite lithics being the most commonly found artifact class. For site descriptions, archaeologists must try to identify lithic artifacts to the best of their abilities, both describing the material and then attempting to find the source from which people had obtained it. Although geologists are not typically as concerned about the macroscopic nuances of each flakeable material (e.g., chert rather than brown banded Hudson Bay Lowland chert), archaeologists note variability in order to understand ancient miners’ choices whether for better flaking, durability of formed edges, or sometimes even esthetic appeal. These descriptions often reflect superficial macroscopic observations and a misunderstanding of regional geological characterizations even in terms of major rock types (i.e., sedimentary, igneous, or metamorphic). During the cataloguing process, many archaeologically recovered lithic types are erroneously categorized, leaving many artifacts to fall into the category of “unknown material type”. Perhaps one of the biggest challenges in Northwestern Ontario is the limited availability of comparative collections to try and identify lithic raw materials. To counteract this issue, Surette, colleagues, and students in the Department of Anthropology at Lakehead University began collecting geoarchaeological samples of knappable materials in 2011, and at primary sources where possible. Secondary and tertiary source examples were also collected, even though their contexts are more complicated to understand from a geoarchaeological perspective. Geological maps were examined and geoarcheological contexts were considered (ancient hydrology, stratigraphy, etc.) to determine where these silica rich materials might be located and which would have been accessible at different times. In addition, we have started sharing and trading for samples with archaeologists and geologists from other institutes in Canada and the U.S.A. to build a representative library. To date, there are nearly 5,000 samples from both countries, of which 1,300 are from various locations in Ontario. One of the better understood hosts of raw materials is the Gunflint Formation near Thunder Bay, which has been the recent focus of much sampling for the comparative collection (e.g., Vickruck, 2018). 89 �Unfortunately, archaeologists in Canada rarely describe knappable rocks and minerals by attributes, in detail, or using correct geological terminology, even for quarry sites where there is typically one or limited sources found. This factor is problematic because it is then difficult for other researchers to determine if their samples are made of the same material - perhaps found at a different site due to trading or people obtaining quarry samples and utilizing them elsewhere. In Northwestern Ontario, there are also few basic descriptions of flakeable materials used by early Indigenous populations (Hamilton, 1981 for Lac Seul, Taylor-Hollings, 2017 regarding the Bloodvein River, and Vickruck, 2015 for the Thunder Bay region). Therefore, we aim to change that in our discipline through the study of samples in the Lakehead University lithic collection and make this information available to other researchers (eventually online). The Lakehead University lithic material comparative collection in the Department of Anthropology will provide archaeologists and geoscientists with opportunities to examine the minutia of knappable rocks and minerals, with emphasis on the Gunflint Formation but also from many other regions. This new database provides raw material examples that can be studied in many different ways, either at a large scale or microscopic studies of individual rocks or minerals. Due to having a large collection, we now know that different sources can produce similar flakeable rocks, which emphasizes the need to chemically test them to clarify provenance. The next steps are to catalogue and properly describe these samples in the collection. We also plan to develop methods for analyzing these materials with non-destructive techniques, which may be also applied to artifact characterization. Combined, this will help us address both the problem of not knowing about sources in Northwestern Ontario and illustrating these materials properly through basic lithological descriptions and in some cases, further nondestructive geoarchaeological analytical techniques. Ultimately, that will help us address the selection processes of ancient Indigenous people in the area. References Dawson, K.C.A. 1984. A history of archaeology in Northern Ontario to 1983 with bibliographic contributions. Ontario Archaeology 42:27-92. Hamilton, S. 1981. The archaeology of Wenesaga Rapids. Archaeology Research Report 17, Archaeology and Heritage Planning Branch, Ontario Ministry of Culture and Recreation, Toronto. MacNeish, R. 1952. A possible early site in the Thunder Bay district, Ontario. National Museum of Canada, Bulletin No. 126, pp. 23-47. Department of Northern Affairs and National Resources, Ottawa. Taylor-Hollings, J. 2017. “People lived there a long time ago”: Archaeology, ethnohistory, and traditional use of the Miskweyaabiziibee (Bloodvein River), northwestern Ontario. Unpublished PhD dissertation, Department of Anthropology, University of Alberta, Edmonton. Vickruck, C. 2018. Investigating the qualities of raw lithic material and the selection pressures of lithic materials from the Gunflint Formation, in Ontario Canada. Master of Environmental Studies: Northern Environments and Cultures, Lakehead University, Thunder Bay. 90 �Insights into Midcontinent Rift development resulting from a strengthened chronostratigraphic framework SWANSON-HYSELL, Nicholas L. Department of Earth and Planetary Science, University of California, Berkeley Correlation of volcanostratigraphic sequences across the Midcontinent Rift has been a long time focus of research efforts with major advances made on the basis of lithostratigraphy (e.g. Green, 1982), magnetostratigraphy (e.g. Books, 1972), chemostratigraphy (e.g. Nicholson et al., 1997), and U-Pb geochronology-based chronostratigraphy (e.g. Davis and Green, 1997). As the result of an effort to obtain high-resolution U-Pb geochronological constraints on paleomagnetic poles from the Midcontinent Rift and constrain rates of rapid plate motion, we have published 14 new chemical abrasion–isotope dilution– thermal ionization mass spectrometry (CA-ID-TIMS) 206Pb/238U dates (Swanson-Hysell et al., 2015; Fairchild et al., 2017; Swanson-Hysell et al., 2019). Single zircon analyses of chemically-abraded grains improves the geochronology of previously dated units in addition to resulting in high-precision dates developed for previously undated units. These dates can be used to construct a chronostratigraphic framework for Midcontinent Rift volcanic and sedimentary succession shown in Figure 1 that is further informed by magnetostratigraphic data and paleomagnetic pole position. Figure 1: Chronostratigraphic correlation of Midcontinent Rift volcanic sequences across the Lake Superior Region of North America, informed by new U-Pb dates. The numbered circles correspond to CA-ID-TIMS 206Pb/238U dates. The analytical uncertainty, which can be used when comparing these dates to one another, is less than the time represented by the height of the circles. Extrapolated eruption rates, paleomagnetic data (both polarity and pole position) and 207Pb/206Pb dates (not shown) inform the chronostratigraphic interpretation, but the chronostratigraphy is most robust in the proximity of the 206Pb/238U dates. Figure from Swanson-Hysell et al., 2019. 91 �To aid in the discussion of geomagnetic polarity zones, I propose naming them following the guidelines of the International Commission on Stratigraphy resulting in the Alona Bay reversed-polarity zone, the Flour Bay normal-polarity zone, the Flour Bay reversed-polarity zone and the Portage Lake normalpolarity zone with current evidence suggesting that the Portage Lake normal-polarity zone was particularly long-lived (Driscoll and Evans, 2016). The geomagnetic polarity timescale and the major inclination change recorded as the result of paleolatitude change throughout of the time period of active Midcontinent Rift magmatism continue to present opportunities to constrain the chronostratigraphy of volcanic, intrusive and sedimentary rocks of the rift. Future high-precision 206Pb/238U dates on intrusive units coupled with paleomagnetic data may offer further opportunities related to the polarity record in the earliest history of rift development. The mafic nature of the earliest Midcontinent Rift lavas (including picrites), and the associated lack of zircon, continues to present challenges to efforts to robustly constrain the chronostratigraphy of the early magmatic stage of rift development. Age constraints on the angular unconformities within the Osler Volcanic Group, between the North Shore Volcanic Group & Schroeder-Lutsen Basalts and at the base of the Oronto Group in northern Wisconsin are insightful as relates to the timescale of active rifting and the transition to the thermal subsidence stage of rift development. Post-rift unconformities can be particularly useful for constraining the timing of the end of rifting as they juxtapose underlying syn-rift strata with post-rift strata. The Brownstone Falls unconformity at which Oronto Group sedimentary rocks overlie progressively lower stratigraphic levels of the Porcupine Volcanics, Portage Lake Volcanics, and Kallander Creek Volcanics (Fig. 1) is well-explained as a post-rift unconformity. The volcanics underlying the unconformity can be interpreted as syn-rift strata with the overlying Oronto Group having been deposited during widespread thermal subsidence. The syn-rift strata in this interpretation include the Portage Lake Volcanics which could constrain the post-rift phase to postdate 1091.59 ± 0.27/0.52/1.3 Ma (Swanson-Hysell et al., 2019). The youngest well-dated magmatic product from the Midcontinent Rift is the Davieaux Island rhyolite of the Michipicoten Island Formation (1083.52 ± 0.23/0.35/1.2 Ma; Fairchild et al., 2017) although the Bear Lake volcanics are likely younger still (Kulakov et al., 2018). When combined with paleomagnetic data, these chronostratigraphic constraints provided evidence for very rapid motion of Laurentia leading up to the collisional orogenesis associated with the Ottawan phase of the Grenvillian orogeny. References Books, K., 1972, Paleomagnetism of some Lake Superior Keweenawan rocks: U.S. Geological Survey Professional Paper 760, 42 p. Davis, D., and Green, J., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Sciences, v. 34, p. 476–488, https://doi.org/10.1139/e17039 Driscoll, P.E., and Evans, D.A.D., 2016, Frequency of Proterozoic geomagnetic superchrons: Earth and Plan etary Science Letters, v. 437, p. 9–14, https://doi.org /10.1016/j.epsl.2015.12.035. 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, v. 9, p. 117–133, https://doi.org/10.1130/L580.1. Green, J.C., 1982, Geology of Keweenawan extrusive rocks in GSA Memoir v. 156 Geology and Tectonics of the Lake Superior Basin https://doi.org/10.1130/MEM156-p47 Kulakov, E., Bornhorst, T. J., Deering, C., and Moore, J. B. 2018. The youngest magmatic activity of the Midcontinent Rift at Bear Lake, Keweenaw Peninsula, Michigan: ILSG Program and Abstracts, v. 64. Nicholson, S., Shirey, S., Schultz, K., and Green, J., 1997, Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: Implications for multiple mantle sources during rift development: Canadian Journal of Earth Sciences, v. 34, p. 504–520, https://doi.org/10.1139 /e17041. Swanson-Hysell, N.L., Burgess, S.D., Maloof, A.C., and Bowring, S.A., 2014, Magmatic activity and plate motion during the latent stage of Midcontinent Rift development: Geology, v. 42, p. 475–478, https://doi.org/10.1130/G35271.1. 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, https://doi.org/10.1130/B31944.1. 92 �An oxygenated Paleolake Nonesuch and primary detrital hematite in the Freda river system SWANSON-HYSELL, Nicholas L., SLOTZNICK, Sarah P. and FAIRCHILD, Luke M. Department of Earth and Planetary Science, University of California, Berkeley In addition to preserving an extended interval of volcanism, sediment deposited within the thermal subsidence phase (Cannon and Hinze, 1992) of the Midcontinent Rift provide an exceptional record of late Mesoproterozoic terrestrial environments. The Oronto Group, deposited during this thermal subsidence phase commences with the Copper Harbor Conglomerate ca. 1086 Ma, which represents terrestrially-deposited alluvial fan and fluvial sediments (Elmore, 1984). The Nonesuch Formation overlies the Copper Harbor Conglomerate and is interpreted as a lacustrine facies association (Stewart and Mauk, 2017) that is exposed along a >250-km-long belt in northern Michigan and Wisconsin. The sediments of the Nonesuch Formation indicate that the lake in northern Wisconsin and Michigan (referred to here as Paleolake Nonesuch) was large and persistent. Lacustrine sedimentation continued until after the transition into the overlying Freda Formation — a transition typical based on color rather than lithofacies. The overlying Freda Formation is dominantly composed of channelized sandstone and overbank siltstone deposits that were deposited within a terrestrial fluvial environment (Ojakangas et al., 2001). Five drill cores from northern Wisconsin were used by Stewart and Mauk (2017) to develop a sequence stratigraphic framework for the Nonesuch Formation. In work published in Slotznick et al. (2018) we focused on two of these cores (DO-8 and WC-9) and have now extended our analyses to outcrop sections as well as cores from the Presque Isle syncline. In this research, we have used experimentally determined estimates of magnetization and coercivity on samples that span sections of the Nonesuch Formation. These rock magnetic data can be used to interpret three distinct magnetic facies (Slotznick et al., 2018). These three facies and their juxtaposition can be explained as the result of an oxycline in Paleolake Nonesuch. The detrital input to the lake is preserved in facies 2 and included magnetite and hematite due to weathering and oxidation of the source igneous material during transport. Magnetic facies 1 is associated with sediments in the deepest part of the lake with a magnetic mineral assemblage that shows that delivered iron oxides underwent reductive dissolution through microbial metabolic processes. Much of this iron and iron within sheet silicates reacted with sulfide to form pyrite, but sulfide availability was restricted to pore waters and not sufficient to sulfidize all of the available reactive iron. These data indicate that the sediments in the deepest part of the lack were anoxic, possibly with anoxia extending into the water column. In contrast, intermediate oxygen levels in waters throughout much of the lake allowed for the preservation of detrital magnetite and hematite in facies 2. In the shallow waters of the lake recorded in facies 3, oxic conditions prevailed and most of the detrital magnetite, as well as iron in other phases, was oxidized to hematite. In Slotznick et al. (2019), we interpreted this vertical sequence of facies to reflect a stacking of laterally distributed environments such that the transition from the deepest-water low-iron-oxide facies into the intermediate-water magnetite-rich facies and the shallower-water hematite-rich facies is the result of an oxycline within the ancient lake. The depth dependence of the oxycline is similar to that found in modern eutrophic lakes wherein the aerobic respiration of descending organic matter leads to a decrease in dissolved oxygen with depth. Overall, these data indicate that this ca. 1.1 billion-year-old lake was more deeply oxygenated than has previously been interpreted (e.g. Cumming et al., 2013) providing a hospitable environment for the diverse biota that was present in the lake that included early eukaryotes (Wellman and Strother, 2015). This framework is strengthened by new data from a section along Potato River Falls that was near a paleo-highlands. In this section, the magnetic mineralogy is dominated by Facies 2 and 3 through repeated fluvial-lacustrine cycles without the anoxia seen in the deeper part of the lake. 93 �The interpretation of a low-latitude paleolatitude of Laurentia at the time of Nonesuch and Freda deposition is reliant on magnetizations held by hematite (Henry et al., 1977). While detrital hematite in sediment can lead to a primary depositional remanent magnetization, alteration of minerals through interaction with oxygen can lead to the post-depositional formation of hematite. We have used the exceptionally-preserved fluvial sediments of the Freda Formation to gain insight into the timing of hematite remanence acquisition and its magnetic properties. This deposit contains siltstone intraclasts that were eroded from a coexisting lithofacies and redeposited within channel sandstone. Thermal demagnetization, petrography and rock magnetic experiments on these clasts reveal two generations of hematite. One population of hematite demagnetized at the highest unblocking temperatures and records paleomagnetic directions that rotated along with the clasts. This component is a primary detrital remanent magnetization. The other component is removed at lower unblocking temperatures and has a consistent direction throughout the intraclasts. This component is held by finer-grained hematite that grew and acquired a chemical remanent magnetization following deposition resulting in a population that includes superparamagnetic nanoparticles in addition to remanence-carrying grains. This primary magnetization can be successfully isolated from co-occurring authigenic hematite through high-resolution thermal demagnetization. These data lend credence to existing paleomagnetic data from the Freda Formation as well as future efforts to develop such data at higher resolution. References Cannon WF, Hinze WJ (1992) Speculations on the origin of the North American midcontinent rift. Tectonophysics 213:49–55 Cumming VM, Poulton SW, Rooney AD, Selby D (2013) Anoxia in the terrestrial environment during the late Mesoproterozoic. Geology 41:583–586. Elmore RD (1984) The Copper Harbor Conglomerate: A late Precambrian fining- upward alluvial fan sequence in northern Michigan. Geol Soc Am Bull 95:610–617. Fairchild, L. M., Swanson-Hysell, N. L., Ramezani, J., Sprain, C. J., & Bowring, S. A. (2017). The end of Midcontinent Rift magmatism and the paleogeography of Laurentia. Lithosphere, 9(1), 117-133. doi: 10.1130/L580.1 Henry, S., Mauk, F., & Van der Voo, R. (1977). Paleomagnetism of the upper Ke- weenawan sediments: Nonesuch Shale and Freda Sandstone. Canadian Journal of Earth Science, 14, 1128-1138. doi: 10.1139/e77-103 Ojakangas RW, Morey GB, Green JC (2001) The Mesoproterozoic midcontinent rift system, Lake Superior region, USA. Sediment Geol 141-142:421–442. Slotznick, S., Swanson-Hysell, N.L., and Sperling, E. (2018), An oxygenated Mesoproterozoic lake revealed through magnetic mineralogy, PNAS, doi:10.1073/pnas.1813493115 Stewart EK, Mauk JL (2017) Sedimentology, sequence-stratigraphy, and geochemical variations in the Mesoproterozoic Nonesuch formation, Northern Wisconsin, USA. Precambrian Res 294:111–132. Swanson-Hysell, N.L., Ramenzani, J., Fairchild, L.M. and Rose, I. (2019), Failed rifting and fast drifting: Midcontinent Rift development, Laurentia’s rapid motion and the driver of Grenvillian orogenesis, Geological Society of America Bulletin, doi:10.1130/B31944.1. Wellman CH, Strother PK (2015) The terrestrial biota prior to the origin of land plants (embryophytes): A review of the evidence. Palaeontology 58:601–627. 94 �New paleomagnetic constraints on the formation of the Slate Islands impact structure SWANSON-HYSELL, Nicholas L., TIKOO, Sonia M. and FAIRCHILD, L.M. Department of Earth and Planetary Science, University of California, Berkeley Pioneering paleomagnetic study by Halls (1975, 1979) was central to establishing an impact origin for the Slate Islands Impact structure. We have conducted further paleomagnetic study of both the injectite lithic breccia dikes throughout the structures as well as the target rocks which have an impactrelated overprint. These data enable further conclusions about the timescale of cratering processes and the origin of the impact-related overprint discovered by Halls (1975, 1979). In our work, we previously used lithic breccia dikes that were injected into the target rock during crater excavation to constrain the rate of crater modification within the central uplift of the Slate Islands Impact structure (Fairchild et al., 2019). We studied both the matrix as well as the clasts within the breccia dikes throughout the impact structure paleomagnetically. These data revealed uniform and linear paleomagnetic directions both in the matrix and in the clasts that are best interpreted as being due to frictionally heating above the magnetite Curie temperature (580 °C) during dike emplacement and subsequently cooling in situ through magnetic blocking temperatures. The work of Halls (1979) used data from the matrix of the breccia dikes to argue that the dikes acquired a thermal remanent magnetization during cratering and our data provide strong support for this hypothesis. The tight clustering of paleomagnetic directions from these breccia dikes indicates that the dikes cooled and locked in their magnetic remanence during a time interval in which the impact structure had reached a stable state with no major ongoing structural rotations. Applying a conductive cooling model to the thinnest sampled breccia dike demonstrates that magnetic remanence was being acquired six minutes after emplacement which indicates a stable crater structure at that time. This constraint of a stable crater structure only minutes after impact that results from breccia dike paleomagnetism is a rare case in which a geological process can be resolved on such a short time scale. We have also pursued paleomagnetic study paired with hydrocode modeling of the Slate Islands impact structure host rocks. The goal of this study is to determine the relative contribution of the competing nature of shock and thermal effects on the magnetization of moderately to highly shocked rocks within the crater. Target rocks within the central peaks of complex craters are often exposed to pressures >10 GPa (such as the case in the Slate Islands; Dressler et al., 1998) and thus experience shock heating to >200 °C (Stewart et al., 2007) as well as baked contact heating to higher temperatures associated with the emplacement of impact breccias (Fairchild et al., 2016). Under such conditions, magnetization acquired during the passage of a shock wave (shock remanent magnetization; SRM) would be largely overprinted by a higher intensity thermal overprint during post-impact cooling (Tikoo et al., 2015). In our thermal demagnetization experiments, the impact-related magnetization component was predominantly removed from samples by laboratory unblocking temperatures of ~250-350°C consistent with heating to such temperatures. These data, combined with results from paleointensity experiments, support an interpretation that the Slate Islands overprint is primarily thermal in origin. This result is consistent with paleomagnetic studies of several other terrestrial impact craters, which report thermallyinduced magnetizations in target rocks (Elbra et al., 2007; Jackson and Van der Voo, 1986). The potential for thermal and viscous remagnetization, as well as the acquisition of chemical remanence from hydrothermal activity (Pesonen et al., 1999; Quesnel et al., 2013), render it unlikely that SRM will be preserved in rocks as the dominant impact-related magnetization in the majority of settings. 95 �References Collins, G. S. (2014), Numerical simulations of impact crater formation with dilatancy, J. Geophys. Res. Planets, 119, 26002619, doi:10.1002/2014JE004708. Dressler, B. O., V. L. Sharpton, and B. C. Schuraytz (1998), Shock metamorphism and shock barometry at a complex impact structure: Slate Islands, Canada, Contrib. Min. Petrol., 130, 275-287, doi:10.1007/s004100050365. Elbra, T., A. Kontny, L. J. Pesonen, N. Schleifer, and C. Schell (2007), Petrophysical and paleomagnetic data of drill cores from the Bosumtwi impact structure, Ghana, Meteorit. Planet Sci., 42, 829-838. Fairchild, L. M., N. Swanson-Hysell, and S. M. Tikoo (2016), A matter of minutes: Breccia dike paleomagnetism provides evidence for rapid crater modification, Geology, 44, 723-726, doi:10.1130/G37927.1 Halls, H. C. (1975), Shock-induced remanent magnetisation in late Precambrian rocks from Lake Superior, Nature, 255, 692-695, doi:10.1038/255692a0. Halls, H. C. (1979), The Slate Islands meteorite impact site: a study of shock remanent magnetization., Geophys. J. Roy. Astr. S., 59, 553-591, doi:10.1111/j.1365-246X.1979.tb02573.x. Jackson, M., and R. Van der Voo (1986), A paleomagnetic estimate of the age and thermal history of the Kentland, Indiana cryptoexplosion structure, J. Geol., 94, 713-723. Pesonen, L. J., S. Elo, M. Lehtinen, T. Jokinen, R. Puranen, and L. Kivekas (1999), Lake Karikkoselka impact structure, central Finland: New geophysical and petrographic results., Geol. S. Am. S., 339, 131-147. Stewart, S. T., A. Seifter, G. B. Kennedy, M. R. Furlanetto, and A. W. Obst (2007), Measurements of emission temperatures from shocked basalt: hot spots in meteorites, Proc. 38th Lunar and Planetary Science Conference, 2413. Tikoo, S. M., J. Gattacceca, N. L. Swanson-Hysell, B. P. Weiss, C. Suavet, and C. Cournede (2015), Preservation and detectability of shock-induced magnetization, J. Geophys. Res., doi:10.1002/2015JE004840. Quesnel, Y., J. Gattacceca, G. R. Osinski, and P. Rochette (2013), Origin of the central magnetic anomaly at the Haughton impact structure, Canada, Earth Planet. Sci. Lett., 367, 116-122. 96 �Petrological and geochemical characteristics of the granitic rocks from the Dog Lake Granite Chain: Implications for the genesis of Quetico Basin WANG, Shiwei1,2, HOLLINGS, Pete2 and KUZMICH, Ben2 1 School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China 2 Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada, P7B 5E1. The Archean Quetico Basin is a metasedimentary terrane of the Superior Province between the Wawa-Abitibi Terrane to the south and the Western Wabigoon, Winnipeg River, and the Marmion terranes to the north. The majority of plutonic rocks within the Quectio Basin are granitoids (Williams, 1991), and are an important tool to investigate the evolution and genesis of the Quetico Basin and the nature of Archean tectonic processes (Sawyer et al., 2002). Percival (1989) studied the geology of granitic intrusions in the western Quetico basin and identified three main types: (i) an older, rare, white foliated hornblende-biotite tonalite, (ii) a pink, magnetic, medium- to coarse-grained, mostly massive, biotite leucogranite (e.g., the Lac La Croix Batholith), and (iii) a white-grey, medium-grained to pegmatitic, muscovite leucogranite (e.g., the Sturgeon Lake Batholith). He proposed that the muscovite leucogranites were S-type granites with a metasedimentary source, whereas the pink biotite granite had lower SiO2, and higher Na2O, K2O, and Sr than the S-type granite and could have been derived from either an S-type or I-type source (Percival, 1989). The Dog Lake Granite Chain, consisting of six ovoid magnetite-bearing intrusions (Shabaqua, Silver Falls, Trout Lake, Barnum Lake, White Lily and Penasen Lake), is characterized by a linear trend that parallels the tectonic boundary between the Wawa-Abitibi terrane to the south, and the Quetico Basin to the north. The intrusive rocks in the Dog Lake Granite Chain can be divided into three units: monzodiorite, microcline-phyric monzonite/quartz monzonite, and granite. The majority of monzodiorite and monzonites are metaluminous, whereas the granites are peraluminous. Whole rock data show that the Al2O3 and LREE (La, Ce, Pr and Nd) contents of the monzodiorite show a positive correlation with SiO2, whereas the monzonite and granite units show a negative correlation. Similarly, the K2O content of the monzodiorite unit correlates positively with SiO2 content, whereas the monzonite and granite units show no strong correlation. The HREE (Er, Tm, Yb and Lu) contents of the monzodiorite and monzonite units decrease with SiO2 content, whereas in the granites they increase. In combination this suggests that the three units were likely derived from different magmatic sources. Mantle-like whole rock ɛNd (+1.30 to +1.76) and zircon ɛHf (+0.34 to +7.27) values, and arclike geochemical signatures characterized by the enrichment of large ion lithophile elements (LILE) and low HSFE, suggesting that the monzodiorite unit was likely generated by partial melting of the mantle wedge above a subduction zone. The monzonite units show I-type granite 97 �signatures with the positive whole rock ɛNd (+0.81 to +1.23) and slightly enriched zircon ɛHf (0.21 to +3.81) values, consistent with them having formed from re-melting of metavolcanic rocks. The S-type granites exhibit positive Ce anomalies, negative Eu anomalies and a ranges of whole rock ɛNd (-1.75 to +0.43) and zircon ɛHf (-2.04 to +3.37) values, suggesting a mixed source comprised of arc-like orogenic sediments and minor metavolcanic rocks. Therefore, we conclude that the addition of voluminous melt generated by partial melting of arc-like orogenic sediments likely caused the transition from I-type to S-type magmas as the magmatic system evolved. References Percival, J.A., and Williams, H.R. 1989. The Late Archean Quetico accretionary complex, Superior Province, Canada. Geology 17(1), 23-25. Sawyer, D., 2002. "Discovering plate boundaries: A classroom exercise designed to allow students to discover the properties of tectonic plates and their boundaries." Rice University. http://plateboundary.rice.edu/intro.html Accessed 17 December. Williams, H.R., 1991. Quetico Subprovince. In: Thurston, P.C., Williams, H.R., Sutcliffe, R.H., and Stott, G.M. (Eds.), Geology of Ontario; Ontario Geological Survey Special Vol. 4. 98 �Mineral deposits of the Midcontinent Rift System - A new space/time classification WOODRUFF, Laurel G.1, NICHOLSON, Suzanne W.2, DICKEN, Connie L.2, and SCHULZ, Klaus J.2 1 U.S. Geological Survey, 2280 Woodale Drive, St. Paul, MN 55112 U.S. Geological Survey, MS 954, 12201 Sunrise Valley Drive, Reston, VA 20192 2 The Midcontinent Rift System (MRS) hosts a diverse suite of magmatic and hydrothermal mineral deposits, largely known from rift rocks exposed at or near the surface in the Lake Superior region (Nicholson et al., 1992). Most of these deposits, which are significant past, present, and likely future providers of critical minerals, can be placed into a new space/time metallogenic framework (Fig. 1). This framework was developed using 552 mineral deposits compiled from the U.S. Geological Survey Mineral Resources Data System (MRDSa) and the Ontario Ministry of Energy, Northern Development and Mines Mineral Deposit Inventory (MDIb). Deposits were classified by deposit type, host rock age and type, and estimated mineralization age. The deposits were then put into a tectonic evolutionary framework for the MRS, which showed that many deposits formed in unique spatial and temporal stages of rift evolution. The distribution of 106 zircon/baddeleyite age dates, also compiled in this study, reflects three main magmatic MRS stages: 1) an early Plateau Stage from ~1113 to ~1105 Ma, characterized by widespread subaerial volcanism (e.g., magnetically reversed North Shore Volcanic Group, Osler Volcanics, Siemans Creek Volcanics) and related intrusive activity (e.g., Coldwell Complex, Early Gabbroic/ Felsic Series of the Duluth Complex); 2) a Rift Stage (~1102 to ~1091 Ma), characterized by eruption of thick sections of subaerial flood basalts largely confined to central, sagging basins (e.g., magnetically normal North Shore Volcanic Group, Portage Lake Volcanics) accompanied by voluminous intrusive events (e.g., Anorthosite/Troctolite Series of the Duluth Complex, Beaver Bay Complex, and Mellen Complex); and 3) a Late-Rift Stage (~1090 to ~1083 Ma) with diminished sporadic, mainly andesitic/felsic volcanism (e.g., Lakeshore Traps, Michipicoten Island Volcanics). A Post-Rift Stage is dominated by sediment deposition from the margins of the rift as subsidence within the central basins continued because of thermal collapse. This created thick sections of sedimentary rock (Copper Harbor Conglomerate, Nonesuch Formation, and Freda Sandstone that comprise the Oronto Group) that overlie stacked basalt flows within rift basins. The time frame for Oronto Group deposition and its relationship to clastic sediments of the Bayfield Group and Jacobsville Sandstone, the youngest rocks assigned to MRS history, are poorly constrained. The last event that put a close to the MRS was a Compressional Stage (~1060 and ~1040 Ma) that created reverse faults along some margins of the rift and carried older rocks over younger. Mineralization in the MRS evolved within this broad tectonic context, beginning with intrusive magmatic deposits that formed contemporaneously with intrusion of MgO-rich mafic/ultramafic magmas during the Plateau Stage. Deposit types in this stage include: 1) small conduit-type sulfide deposits (e.g., the Ni-Cu-PGE Eagle, Tamarack, and Thunder Bay North deposits, and the Cu-PGM Marathon deposit); 2) layered Ti-Fe-(V) deposits in the Duluth Complex Early Gabbro Series; and 3) Nb-U(±Th±REE) in alkaline intrusions in Ontario. Magmatic mineral deposits related to the MRS Rift Stage include large contact-type disseminated Cu-Ni-PGE sulfide deposits and small but potentially economic Ti-Fe(-V) oxide ultramafic deposits along the basal section of the Duluth Complex. With diminished volcanism and increased sedimentation during the Late/Post-Rift Stages, hydrothermal fluids became an 99 �increasingly important component of mineralization. Deposits that formed during this time frame are thought to include: 1) chalcocite in basalt (e.g., 543S); 2) copper sulfide veins (e.g., Coppercorp); and 3) Cu-Mo breccia pipes in the Mamainse Point area, which all may have a hybrid magmatic/hydrothermal origin. Additional hydrothermal deposits within this time period are: 1) a complex group of metal-bearing veins in Ontario (e.g., Ag-bearing veins of the Mainland and Island Groups, unconformity Pb-Zn-Ba(±U) veins); and 2) stratiform Cu deposits in the Nonesuch Formation (e.g., White Pine and Copperwood). The final, major MRS mineralizing event occurred during the Compressional Stage and created the world-famous Keweenawan native Cu(±Ag) deposits, contained within the Rift Stage Portage Lake Volcanics. This space/time classification of MRS mineral deposits is outlined in a USGS Story Mapc. Figure 1. Simplified geologic map of the Lake Superior region, showing pre-MRS and MRS-related rocks, distinguished by stage, type, and magnetic polarity. Mineral deposits compiled from USGS and OGS databases have been classified by deposit type and put into a time-space evolutionary model for the MRS. Geology and mineral deposits taken from the USGS MRS geodatabase. 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. a https://mrdata.usgs.gov/mrds/ https://www.mndm.gov.on.ca/en/mines-and-minerals/applications/ogsearth/mineral-deposits-mdi c https://wim.usgs.gov/geonarrative/MRS_mineral_deposits/ b 100 �Sulfur mobility in arc magma systems: Implications for porphyry ore deposits WRAGE, Jackie1, FIEGE, Adrian1, KONECKE, Brian1, SIMON, Adam1, RUPRECHT, Philipp2, BEHRENS, Harald3 1 Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan, USA 2 Department of Geological Sciences and Engineering, University of Nevada, Reno, Nevada, USA 3 Institute of Mineralogy, Leibniz University Hannover, Callinstrasse 3, 30167 Hannover, Germany Porphyry ore deposits supply two-thirds of the world’s Cu and nearly all of its Mo, as well as significant amounts of Au, Ag, and critical elements such as Re, Se and Te. These deposits form as a result of arc-related volcanism, when partial melts generated by dehydration melting of the subducting basaltic ocean crust percolate upwards through the mantle wedge and accumulate at the base of the crust in a process called underplating. As these mafic magmas fractionate, felsic melts segregate and ascend to the middle and upper crust where they form magma chambers that are thought to be the source of ore fluids in porphyry systems. However, a major problem with sourcing porphyry fluids from intermediate to felsic magmas is that mass balance calculations indicate that such silicic magmas cannot supply all of the S in porphyry ore deposits. The most plausible explanation for the excess S in porphyry deposits is underplating of middle to upper crustal silicic magma chambers by decompressing, volatile-saturated mafic magma that delivers volatiles such as S, H2O, and Cl, and possibly metals into the overlying felsic magma. This study explores the effects of underplating on volatile exchange between a mafic recharge magma and felsic host magma by simulating an underplating scenario. Diffusion-couple experiments were performed wherein a cylinder of mafic magma (basaltic andesite) was juxtaposed beneath a cylinder of felsic magma (dacite) and run under a range of pressuretemperature-composition-redox conditions relevant for upper crustal arc magma (porphyry) systems. The most intriguing finding is the development of a redox gradient of ~1.8 log units fO2 at the mafic-felsic interface of the most oxidizing (FMQ+4) experiments, where the mafic melt is oxidized, and the felsic melt is reduced. Sulfur x-ray absorption near-edge structure (S-XANES) analyses also indicate complex S-speciation near the mafic-felsic interface in the most reducing (FMQ+1) experiment. Such a gradient affects the speciation of redox-sensitive elements such as S and moderates mass transfer from mafic to felsic melt, as well as affecting the metalscavenging potential of an exsolved magmatic-hydrothermal volatile phase. Studying the effects of underplating in arc systems is paramount to understanding the source(s) and mechanisms responsible for the titration of volatiles and metals into ore forming environments and could help reconcile the excess sulfur problem in volcanic systems. 101 �Multiscale Layering in the Black Sturgeon Sill, Nipigon, Ontario ZIEG, Michael J. Department of Geography, Geology, and the Environment, Slippery Rock University, Slippery Rock, PA 16057 The basic petrology of the Nipigon sills was established by Richard Sutcliffe thirty years ago: “The diabase occurs primarily as … 150 to 200 m thick sills with a textural stratigraphy indicating that the sills represent single cooling units. Compositional variation in the sills indicates that they crystallized from several magma pulses.” (Sutcliffe, 1987) “Fractionation in the sills is attributed to flowage differentiation and movement of residual liquids … toward the top of the sections.” (Sutcliffe, 1989). The current study has confirmed and amplified these interpretations of sill-scale processes. A continuous drill core through the Black Sturgeon sill (Zieg & Wallrich, 2018), a 250 m Nipigon group (Hollings et al., 2007; 2010) mafic intrusion, has yielded far more detailed stratigraphic variations than were previously available. With this newly-available data, Sutcliffe’s original model can be shown to apply at multiple scales. Specifically, although textural evidence for “non-unit” a cooling history remains elusive, smaller-scale magma batches can be recognized within the larger-scale magma pulses. Additionally, there is compositional evidence for upward movement of evolved liquids within any sub-section of the sill. These patterns have been traced down to a scale on the order of tens of centimeters, with individual textural and mineralogical discontinuities visible on the thin section scale. The relationship between classical igneous layering and the observed compositional and textural variations is unclear. Much of the data from the Black Sturgeon sill suggests that olivine (and plagioclase) concentrations in this system reflect the injection of antecryst-laden magma. A similar type of analysis on a more traditionally layered intrusion might shed light on the extent to which the processes that controlled layer formation in the BSS also contributed to layer formation in other systems. References Hollings, P., Hart, T., Richardson, A. & MacDonald, C. A. (2007). Geochemistry of the Mesoproterozoic intrusive rocks of the Nipigon Embayment, northwestern Ontario: Evaluating the earliest phases of rift development. Canadian Journal of Earth Sciences, v. 44, p. 1087–1110. Hollings, P., Smyk, M., Heaman, L. M. & Halls, H. (2010). The geochemistry, geochronology and paleomagnetism of dikes and sills associated with the Mesoproterozoic Midcontinent Rift near Thunder Bay, Ontario, Canada. Precambrian Research, v. 183, p. 553–571. Sutcliffe, R. H. (1987). Petrology of Middle Proterozoic diabases and picrites from Lake Nipigon, Canada. Contributions to Mineralogy and Petrology, v. 96, p. 201–211. Sutcliffe, R. H. (1989). Mineral variation in Proterozoic diabase sills and dykes at Lake Nipigon, Ontario. The Canadian Mineralogist, v. 27, p. 67–79. Zieg, M.J, & Wallrich, B.M. (2018). Emplacement and differentiation of the Black Sturgeon Sill, Nipigon, Ontario: A principal component analysis. Journal of Petrology, v. 59, p. 2385–2412. 102 �Figure 1. Compositional profiles. Higher Ni concentrations coincide with olivine-dominated portions of the system, regardless of scale. Distinct compositional layering can be recognized at all three scales. Figure 2. Textural profiles. Mean plagioclase length is typically lower in parts of the system that have accumulated olivine. This pattern can be traced down to olivine accumulations less than a meter thick. Although textural variations can be recognized at all three scales, there is little clear evidence for discrete cooling units. Rather, textural variations apparently reflect different magma batches (crystal cargo). 103 � https://digitalcollections.lakeheadu.ca/files/original/86325f55313fe9ffc025e80c4ac4582c.pdf b1fa39b1d9f97f9eb95b50db71f9d1b8 PDF Text Text 65th Annual Meeting Terrace Bay, Ontario - May 8-9, 2019 Institute on Lake Superior Geology Part 2 – Field Trip Guidebook �Thank you to our sponsors! Individual contributors to student travel scholarship: Al MacTavish, Mary Kay Arthur, L. Gordon Medaris, Jr., Nick Swanson-Hysell �65th Annual Meeting Institute on Lake Superior Geology May 8-9, 2019 Terrace Bay, Ontario HOSTED BY: Mark Smyk and Pete Hollings Co-Chairs Ontario Geological Survey and Lakehead University Proceedings - Volume 65 Part 2 – Field Trip Guidebook Compiled and edited by Al MacTavish and Pete Hollings Cover Photos: Left - Pillowed Archean metabasalt, Schreiber Beach, Middle - Layered Eastern Border Gabbro, Coldwell Complex. Right - Foliated Archean metavolcanic rocks, Slate Islands. ��65th Institute on Lake Superior Geology Volume 65 consists of: Part 1: Program and Abstracts Part 2: Field Trip Guidebook Trip 1: The Slate Islands Trip 2: Midcontinent Rift-Related Carbonatites and Diatremes Trip 3: Geology of the Western Schreiber-Hemlo Greenstone Belt Trip 4: Geology of the Nipigon Area Trip 5: A stratigraphic transect across the Northern flank of the Midcontinent Rift near Rossport Trip 6: Geology of the Coldwell alkaline complex Trip 7: Building and ornamental stone sites of the Marathon Area, Ontario Trip 8: Geology of the past-producing Winston Lake Cu-Zn Mine Reference to material in Part 2 should follow the example below: Magnus, S., 2019. Geology of the Western Schreiber-Hemlo Greenstone Belt. In; MacTavish, A. and Hollings, P. (Eds.), Institute on Lake Superior Geology Proceedings, 65th Annual Meeting, Terrace Bay, Ontario, Part 2 - Field trip guidebook, v.65, part 2, 3-31. Published by the 65th 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 65th ILSG Annual Meeting - Part 2 Table of Contents 65th Annual Meeting..........................................................................................................A Introduction, safety considerations and acknowledgements................................................1 Field trip 1 - The Slate Islands.............................................................................................2 Field trip 2 - Midcontinent Rift-Related Carbonatites and Diatremes.................................3 Field trip 3 - Geology of the Western Schreiber-Hemlo Greenstone Belt.........................14 Field trip 4 - Geology of the Nipigon Area........................................................................43 Field trip 5 - A stratigraphic transect across the Northern flank of the Midcontinent Rift near Rossport.............................................................................................................60 Field trip 6 - Geology of the Coldwell alkaline complex..................................................75 Field trip 7 - Building and ornamental stone sites of the Marathon Area, Ontario..........105 Field trip 8 - Geology of the past-producing Winston Lake Cu-Zn Mine.......................113 -i- �Proceedings of the 65th ILSG Annual Meeting - Part 2 Introduction, safety considerations and acknowledgements Pete Hollings Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada and Mark Smyk Resident Geologist Program, Ontario Geological Survey, Ministry of Energy, Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada This volume is intended to serve not only as a guide for 65th ILSG field trip participants but also as a reference for those planning to revisit these areas at a later date. Consequently we have included UTM coordinates in the NAD 83 datum for stops, as well as instructions on how to reach them. As some of the stops are on private and staked land, please be sure to obtain the land owners’ permission before entering their land. Contact the staff of the Resident Geologist Program in Thunder Bay for current ownership information. We are once again offering field trips onto Lake Superior. This creates a number of unique safety issues. Please exercise caution when getting in and out of the boats as the outcrops are often extremely slippery. Personal flotation devices must be worn in the boats at all times. If you are planning to revisit these sites please be very careful, as Lake Superior is dangerous; waves can often be many metres high and even in midsummer fog can appear very quickly. either major highways or busy logging roads. Please take care when crossing or parking along these roads. We would like to thank all the other authors who contributed to this field guide, all those who provided comments and/or assisted with the running of the field trips themselves (Shannon Zurevinski, Rob Cundari, Phil Fralick, Dorothy Campbell, Mark Puumala, Robert Lodge, Al MacTavish, Dave Good, John McBride, Peter Hinz, Seamus Magnus, Bill Skrepichuk). We appreciate the assistance and cooperation of the exploration and mining companies in providing us access and information concerning their properties, particularly Superior Lake Resources Ltd. (Winston Lake Mine), Plato Gold Corp. (Good Hope Niobium), Rudy Wahl (Madonna), Jerry Blakely (Shack Lake), Red Rock Indian Band (Ruby Lake), Alex Pleson (Stenlund), John Ternowesky (Dead Horse North) , Stillwater-Sibanye (Marathon) and Ontario Parks (Slate Islands and Neys). The other field trips will be visiting stops along Figure 1. Map showing the location of the eight field trips offered in 2019. -1- �Proceedings of the 65th ILSG Annual Meeting - Part 2 Field trip 1 - The Slate Islands Pete Hollings Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Mark Smyk Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Bill Addison and Phil Fralick Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada The field guide for the Slate Islands has previously been published as ILSG Special Publication #1 and is available on the ILSG website - www.lakesuperiorgeology.org InstItute on Lake superIor GeoLoGy Special publication #1 FIeLd trIp GuIdebook For the sLate IsLands, ontarIo pete hoLLInGs, Mark sMyk, bILL addIson & phIL FraLIck -2- �Proceedings of the 65th ILSG Annual Meeting - Part 2 Field trip 2 - Midcontinent Rift-Related Carbonatites and Diatremes Shannon Zurevinski Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Dorothy Campbell and Mark Puumala Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Introduction The Midcontinent Rift System (MCR) of North America is one of the largest known aborted rifts, and extends ~2200km from Kansas, north to the Lake Superior area. There are several MCR-related fault systems in Lake Superior and the surrounding terrain, as evidenced by seismic reflections, gravity and various magnetic anomalies. Some of the most fascinating geology of the Terrace Bay-Marathon area of Northwestern Ontario is where Archean rocks of the Superior Province are intruded by diatremes, alkalic rocks, carbonatites and ultramafic lamprophyres, which are mostly related to the MCR and the Trans-Superior Tectonic Zone (TSTZ). Many of these intrusions are located along the strike of the Big Bay-Ashburton Fault (which is the proposed northern extension of the Thiel Fault), representing the most northerly component of the TSTZ (Sage, 1982). The Coldwell Alkalic Complex, Killala Lake Alkalic Complex, Chipman, Prairie Lake and Good Hope Carbonatite occurrences, the Dead Horse Creek Diatreme, and various ultramafic lamprophyres lie along the extrapolated arm of the fault system (Fig. 1). We would like to acknowledge the earlier work of Ron Sage and David H. Watkinson, who guided an extensive field trip into the Alkalic rocks of the MCR during the 41st Annual Meeting of the Institute in 1995. This field trip will serve as an update to the prospecting and exploration completed over the last 25 years and will visit: 1) the North Dead Horse property; 2) the diamondiferous Madonna dyke; 3) the Prairie Lake Carbonatite; and 4) the Good Hope Carbonatite (Fig. 2). We also acknowledge Rudy Wahl for his hard work and boots-on-the-ground prospecting that has led to the discoveries of both the Madonna dyke and Good Hope Carbonatite. A special thanks to Seamus Magnus for providing assistance with the descriptions of the North Dead Horse subcomplex trenches. Figure 1. A regional map of the area north of Lake Superior between Terrace Bay and Marathon, showing the alkaline complexes, diatremes, carbonatites and ultramafic lamprophyres of the area. (modified from Smyk et al., 1993). Acknowledgements We are grateful to Rudy Wahl (Madonna dyke and Good Hope Carbonatite), John Ternowsky (North Dead Horse Property) and Nuinsco Resources (Prairie Lake Carbonatite) for permissions to access the properties for this field trip. Stop descriptions Stop 1: The North Dead Horse Creek Diatreme -3- UTM Coordinates 523959E 5409978N (parking lot) �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 2. General geology with field trip stops (geology from the Ontario Geological Survey). Introduction The Dead Horse Creek Diatreme is hosted by the metasedimentary and metavolcanic rocks of the Schreiber-White River greenstone belt, within the Wawa Subprovince (Sage, 1982). It is located approximately 1km from the western margin of the Coldwell Alkalic Complex (Fig. 2). Sage (1982) describes the complex as a broad spectrum of heterolithic breccias that have undergone varying degrees of alteration and are variably radioactive, and subsequently divided the complex into five subcomplexes (North, South, East, West, and Central; Fig. 3). Past exploration programs by Gulf Minerals Canada (1977) focused on the uranium mineralization of the West Dead Horse and the North Dead Horse subcomplexes (Fig. 3). Subsequent exploration on the property by Highwood Resources Ltd. (1985) focused on exploration for beryllium, yttrium, and cerium; Unocal Canada’s (1987) main interest was exploring the potential for yttrium; while Canadian International Mining (2011) focused on exploration for the Rare Earth Elements (REEs). The mineralized zone of the West Dead Horse subcomplex has been described as “diverse, exotic, hydrothermally altered, and rare metal mineralized” (Sage, 1982). Unpublished U-Pb geochronology of zircons from the Figure 3. The Dead Horse Creek Diatreme geology map. Shown here are the 5 subcomplexes. Also note the Gulf minerals drilling program (investigated for U). After Sage (1982) -4- �Proceedings of the 65th ILSG Annual Meeting - Part 2 Dead Horse Creek West subcomplex, were found to be 1128.7 ± 6 Ma (Sage, 1995). The West Dead Horse subcomplex (400m x 1600m elongate in a N-NE direction) has been the focus of extensive geochemical and mineralogical studies, and is described as an occurrence of heterolithic and carbonate-rich breccias and veins (Smyk et al., 1993; Potter and Mitchell, 2005). Smyk et al. (1993) reported Heavy Rare Earth Element (HREE) enrichment from the mineralized zone of the West Dead Horse occurrence to be up to 1004ppm ΣHREE. Potter and Mitchell (2005) summarized the REE mineralogy of the occurrence, including a phenakite-bearing quartz vein, Ca-zirconosilicate, zircon, thorite, uraninite, apatite, xenotime-Y, monazite-Ce and rutile. Smyk et al. (1993) proposed that the volcaniclastic breccia formed during the early stages of the MCR event and incorporated clasts of Archean metasedimentary and granitic rocks. They proposed that after the emplacement of the volcaniclastic breccia, U-Be-Zr mineralization occurred along fault structures after being introduced via A-type granitic fluids, and was followed by alkaline metasomatism. The mineralized zone is not related to the igneous activity that produced the volcaniclastic breccia, rather the porous breccia allowed for the deposition of the U-Be-Zr mineralization. Potter and Mitchell (2005) provide a genetic model for the complex based on the exotic mineral suite present (including the accessory REE mineralogy), and concluded that upon emplacement of the nearby Coldwell Alkalic complex (1108 ±1 Ma; Heaman & Machado, 1992), the Dead Horse volcaniclastic breccia was subjected to thermal metamorphism and post-Pleistocene supergene alteration. Nb-Ti-V-Cr-bearing alkaline fluids were introduced into the same fault system and reacted with the initial mineralization, creating the suite of exotic minerals which was further diversified by supergene alteration (Potter and Mitchell, 2005). The remaining question: What is the source of these Nb-Ti-V-Cr-bearing fluids? While the Coldwell Alkalic Complex does contain the A-type granitic rocks that would produce these fluids, the timing of the emplacement of both the Coldwell Alkalic Complex and the Dead Horse Creek diatreme would need further geochronological studies to constrain this relationship. Canadian International Minerals Inc. (CIM) recently conducted an extensive exploration program on the North Dead Horse property, including trenching, assaying, and geophysical surveys. The showing is arranged in a cross, and the E-W trending stripping is ~250m, while the N-S stripping is ~320m in length (Fig. 4). This recently exposed trenching provides an Figure 4. The North Dead Horse Creek trench map (modified from Magnus 2019). -5- �Proceedings of the 65th ILSG Annual Meeting - Part 2 excellent opportunity to observe the breccia and its various components. Stop 1a: North Dead Horse: Trachytic Diabase Dyke UTM Coordinates 524072E 5410174N Occurring along the trail from the parking area is a trachytic diabase dyke. This is described as a mafic dyke containing feldspar phenocrysts with a magnetic matrix (Fig. 5). These dykes are widespread throughout the area and also occur within the Coldwell Alkalic Complex. Figure 6. Photograph of the grey breccia from the North Dead Horse trench, cut by an alkalic dyke. Figure 5. (a - top) Photograph of the Trachytic diabase dyke from Stop #1. (b - bottom) Close-up photograph of the feldspar phenocrysts of the Trachytic diabase dyke. Stop 1b: North Dead Horse: East-West Trench Figure 7. Photograph of the dark grey breccia from the North Dead Horse trench, showing large clasts of banded metasediments, and hematized and zoned fragments, as well as chert fragments. Grey and red heterolithic breccias are found at the North Dead Horse trenches, cut by various mafic and intermediate dykes (Fig. 4). (Figs. 6 and 7). CIM describes the matrix of the grey breccias as carbonate-rich. The clasts are generally composed of amphibolite, wacke, and granitoid rocks (Fig. 6). Some wacke clasts are several meters in length. Brecciation of the various clasts appears to be found along bedding and schistosity planes. The light grey breccia is host to relatively unaltered clasts that show their primary and structural fabrics The red breccia is dominant in the western part of the trench and is highly altered (Fig. 8). The clasts are UTM Coordinates 524210E 5410389N -6- �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 8. Photographs of the red breccia from the North Dead Horse trench, showing altered hematized metasediments, and both granitic and mafic metavolcanic clasts. composed of granitic and mafic metavolcanics, altered metasediment, and lesser chert. The metasedimentary clasts are hematized and heavily altered at the rims, with the degree of alteration reduced towards the core of the clasts. Some clasts are sulphide-rich. The matrix is magnetic and there is carbonate and chlorite alteration present. Rare earth mineralization on the property appears to be associated with areas containing the red breccia unit (Fig. 4). Minerals identified by Potter and Mitchell (2005) on the property include albite, potassium feldspar, quartz, calcite, apatite, phenakite, aegirinejervisite, aegirine-natalyite, allanite, barite, barylite, coffinite, Ca-Mn-silicate, magnetite, monazite-(Ce), niobian vanadium rutile, pyrite, thorite, thoro-gummite, thortveitite, uraninite, vanadium crichtonite, xenotime(Y), ankerite-dolomite and zircon (Quist, 2011). (S. Magnus, personal communication). The east end of the E-W trench is host to a recessively weathered dyke with an unknown, possibly carbonatitic composition. Stop 2: The Madonna Dyke UTM Coordinates 530377E 5427200N Introduction Located approximately 30kms northwest of Marathon, the Madonna Dyke was discovered by Rudy Wahl in 2007 (Fig. 9). The Madonna Dyke is North-South trench The north-south trench contains mostly grey breccias. The grey breccia has large, relatively unaltered metasediment, amphibolite, and granitic clasts (as described above). Intermediate alkalic dykes, mafic dykes, and late fractures crosscut all breccias. The mafic dykes are similar to the dyke from Stop 1a (and occur throughout the area and within the Coldwell Alkalic Complex). The intermediate alkalic dykes are fine-grained and grey to pink in colour. The intermediate dykes are less common than the mafic dykes. Preliminary studies have shown the intermediate alkalic dykes to have up to 2000ppm ΣREEs, and it is reported that the altered and mineralized zones of North Dead Horse are coincident with these intermediate REE-enriched alkalic dykes Figure 9. Photo of the Madonna dyke (photo courtesy of R. Wahl). -7- �Proceedings of the 65th ILSG Annual Meeting - Part 2 approximately 1 to 2m in width, outcrops for ~50m, strikes 009°, and dips 65° towards the west (Wahl website). Sampling recovered 66 diamonds from a 1205.80 kg sample, which includes white, green, yellow, brown and grey diamonds (http://users.renegadeisp. com/~rwahl/Kimberlite%20Targets%20Available%20 for%20Option.htm). In 2018, R. Wahl completed drilling 82m to the southwest of the Madonna Dyke and intersected the same diamondiferous dyke over a width of 2.78m. The dyke is underlain by Neoarchean biotite granite gneiss and hornblende granite gneiss, as well as post tectonic quartz monzonites (Coates, 1970). The dyke intrudes near lineament intersections that could represent zones of crustal weakness that may have acted as a pathway for the ascent of mantlederived magmas (Kozlowski, 2016). Classification The Madonna dyke been classified as a diamondiferous alnöite ultramafic lamprophyre (Kozlowski, 2016). It is described as a mafic hypabyssal rock with medium- to fine-grained rounded phenocrysts and a fine-grained dark-green to black groundmass (Fig. 10). The dyke has an orange-brown rind on its weathered surface. The phenocrysts (1 to 10mm in diameter) are estimated to make up to 50% of the modal abundance and are identified as pseudomorphs after olivine, pyroxene, and oxides rimmed by carbonate and lesser late-stage calcic amphiboles (Kozlowski, 2016). The groundmass consists of calcite (after melilite), Figure 10. Photo of the Madonna dyke showing rounded phenocrysts set in a dark green groundmass. Also note the orange-brown rind on the weathered surface (photo courtesy of R. Wahl). phlogopite, magnetite, apatite and some alteration products (Kozlowski, 2016). Mineralogy of the Madonna Dyke Kozlowski (2016) summarizes the mineral chemistry of the Madonna dyke used to provide proper classification of the dyke. Pseudomorphed olivine occurs as microphenocrysts, phenocrysts, and rare macrocrysts replaced by serpentine, magnetite, and calcite. A few fresh olivine macrocrysts show mantle compositions ranging from Fo91 to Fo92 (Kozlowski, 2016). Clinopyroxenes are aluminous diopside with Al2O3 ranging from 3.11 to 14.47 wt.%. Groundmass micas have kinoshitalite–phlogopite compositions, with up to 4 wt.% BaO and 20.9 wt.% Al2O3. Spinelgroup mineral compositions follow Magnetic Trend #2 – the Titanomagnetite Trend, where spinels range in composition from aluminous magnesian chromite to titanian magnesian chromite to titanian chromite to members of the ulvöspinel-magnetite series (Fig. 11; Kozlowski, 2016). Spinel-group minerals occur as red chromium spinel phenocrysts to macrocrysts with magnesium-rich cores and iron-rich rims, often associated with olivine phenocrysts and macrocrysts. They also occur as fine-grained opaque groundmass titanomagnetites with altered cores, and as reaction products forming a necklace texture around olivine. Atoll spinel is present. Although the Madonna dyke shows some textural and petrogenetic features of kimberlites, the mineralogy, including the presence of calcite after melilite and amphibole, are analogous with an ultramafic lamprophyre of alnöitic affinity Figure 11. Reduced spinel prism with compositions of Madonna dyke spinels (blue = core; red = rim) showing a magmatic trend 2 – the titanomagnetite trend (green arrow) with a hiatus in the middle (classification after Mitchell, 1986). -8- �Proceedings of the 65th ILSG Annual Meeting - Part 2 Table 1 Summary of petrographic features of the Madonna Dyke compared to kimberlite and UML (ultramafic lamprophyre). After Mitchell (1995b) and Birkett et al. (2004). Olivine (macrocrysts) Kimberlites common rare (phenocrysts) common common common Mica common phlogopite common phlogopite not observed (groundmass) common, phlogopite kinoshitalite common Al-biotite common, phlogopite kinoshitalite Spinels abundant, Mg-chromite to Mg ulvöspinel common, Mg-chromite to Ti-magnetite common, Mg-chromite to Ti-magnetite (atoll) very common present present (necklace) present present present Perovskite common, Sr- and REEpoor common, Sr- and REEpoor not observed Diopside absent common, Al- and Tirich common, Al- rich Apatite common, Sr- and REEpoor common, Sr- and REEpoor common, Sr- and REEpoor (skeletal) rare rare common Calcite abundant common common Melilite absent common common Amphibole absent present present (phenocrysts) (Kozlowski, 2016). Table 1 provides a summary of the features of the Madonna dyke compared to kimberlite rocks and ultramafic lamprophyres. Stop 3: The Prairie Lake Carbonatite Complex UTM Coordinates 520150E 5431100N Introduction The Prairie Lake Carbonatite Complex is located ~26 km from the shores of Lake Superior (Figs. 1 & 2). It covers a surface area of ~8.8 km2 and generally consists of foidolitic and carbonatitic rocks (Fig. 12; Sage, 1987). It is generally a small arcuate intrusion emplaced in Archean gneisses along the TSTZ (Sage, 1987). The complex has been actively explored since 1968 for U and Nb, and is considered ‘multicommodity’ with its potential residual apatite deposits. There is significant modal heterogeneity of the rocks at the Prairie Lake complex. Wu et al. (2017) summarize the variation of the rock types present: 1) calcite carbonatite; 2) biotite pyroxenite; 3) the ijolitic series rocks; 4) potassic syenites; 5) heterogeneous UML rare Madonna Dyke carbonatites; and 6) rare dolomitic carbonatite (Fig. 12). The niobium mineralization in the Prairie Lake Carbonatite complex includes Na- and Ca- pyrochlore, latrappite, loparite, U-pyrochlore, Ce-pyrochlore, Pbpyrochlore, marianoite, and wohlerite (Mitchell, 2015). The pyrochlore has a wide range of compositions and are complexly zoned and resorbed. The Nb mineralization is distributed mainly between perovskite, pyrochlore, and Nb-Zirconolite and tends to be REE-poor with in situ alteration (Mitchell, 2015). Zurevinski and Mitchell (2015) describe the only known worldwide occurrence of orbicular ijolite from within the Prairie Lake Complex (Fig. 13). This occurrence had been previously noted and described by Sage (1987, 1995). The orbicules occur in an ijolite matrix, and the mineralogy of the orbicules is similar to that of their host ijolite (nephelene, diopside, calcite, apatite, andradite-melanite garnet, titanite, etc.; Fig. 13; Zurevinski and Mitchell, 2015). Detailed mineralogy and petrology have shown that the orbicular ijolite represents an interaction of a partially crystallized quenched ijolitic melt, in contact with a second pulse -9- �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 12. A map of the Prairie Lake Carbonatite complex. From Sage (1987). of consanguineous ijolite magma. Immersion in the latter resulted in a sub-solidus diffusion and annealing recrystallization (i.e., magma mixing; Zurevinski and Mitchell, 2015). Figure 13. Orbicular ijolite from the Prairie Lake Carbonatite Complex (from Zurevinski and Mitchell, 2015). Recent detailed geochronology has been completed on the Prairie Lake complex by Wu et al. (2016). In summary, U-Pb with baddeleyite from the carbonatite gave emplacement ages of 1157 ± 2.3 Ma and 1158 ± 3.8 Ma; baddeleyite from the ijolite-series rocks gave 1163 ± 3.6 Ma and U-Pb apatite from the carbonatite gave an emplacement age at ~1160 Ma. Therefore, the findings of Wu et al. (2016) reveal that all units were synchronously emplaced at ~1160 Ma. Furthermore, Sr-Nd-Hf tracer isotopic studies showed that the Prairie Lake carbonatites, ijolites, and syenite rocks had identical isotopic composition, therefore, the silicate and carbonatite rocks are co-genetic and thereby related by fractional crystallization processes (Wu et al., 2016). This data has reinforced the previous conclusions that Prairie Lake is the earliest manifestation of midcontinent rift magmatism, and is not genetically related to the nearby Coldwell or Killala Alkalic complexes (Rukhlov and Bell, 2010). - 10 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Stop 3: Prairie Lake Calciocarbonatite (Sövite) (West) The area is noted by the prominent hill surrounded by low-lying marshy land. Generally, there is very sparse outcropping of extensively weathered calciocarbonatite. As you walk up the hill, you will view cobbles and boulders of calciocarbonatite and siliciocarbonatite rocks. (Figs. 14 and 15). The deep cuts on the right side of the trail will show deeply weathered carbonatite-rich soils, and in some areas, relict primary banding may be observed. The calcite carbonatite mineralogy varies throughout the complex, but generally contains calcite, apatite, olivine, phlogopite, pyrite, and magnetite. The modal layers and bands that are sometimes observed at Prairie Lake are represented by various oxides (commonly magnetite), interlayered between calcitedominant layers. Stop 4: The Good Hope Carbonatite UTM Coordinates 519363 E 5431721 N (Parking area) Introduction The Good Hope Carbonatite is located approximately 28km North of Hwy 17 and was discovered in 2015 by Rudy Wahl (Fig. 2). It occurs within the magnetic “low” on the Northwestern flank of the Prairie Lake Carbonatite, in low-lying marshy land. The Nb property has undergone mapping, geophysical surveys, trenching, and drilling since its discovery. Plato Gold Corp. has an option agreement with Rudy Wahl and other claim holders on the property. Figure 14. Banded sövite (calciocarbonatite) from the Prairie Lake Carbonatite Complex (photo courtesy of M. Smyk). The Good Hope property is host to carbonatite, ijolite, and alkali granite. Alkali granite is fine- to medium-grained and characterized by the abundance of orange and red potassium feldspar and quartz (Selway, 2017). At surface, the medium-grained carbonatite has a reddish-brown rind and appears in some cases to be brecciated (Fig. 16). The brecciated material shows angular to subround fragments with buff-white carbonate stringers present (Puumala et al., 2015; Fig. 16). Investigations from the drill core samples have Figure 15. Weathered carbonatite from the Prairie Lake Carbonatite Complex (photo courtesy of M. Smyk). Figure 16. Brecciated carbonatite from the Good Hope Carbonatite. - 11 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 led to the classification of the Good Hope carbonatitic rocks. Three types of carbonatite have been identified on the property: calciocarbonatite, ferrocarbonatite, and siliciocarbonatite (Cleaver, 2017). Alkali granitic breccia with carbonatite veins has been identified in the drill core (Selway, 2017). The veins correlate with the higher-grade mineralization (>1.0 wt. % Nb2O5) (Selway, 2017). Niobium mineralization is primarily concentrated in pyrochlore, which are characterized by low UO3 and are ThO2-free (https://www.platogold. com/projects/good-hope-niobium-project/).A pyrochlore-group mineral that has no ThO2 and low UO3 content is important as these radionuclides (Th, U) end up in the slag during processing and can be quite problematic. Other minerals found occurring within the carbonatite rocks include calcite, ferrodolomite, siderite, apatite, ferrocolumbite, mica, and pyrochloregroup minerals (Cleaver, 2017). Cleaver (2017) divided the carbonatites into two paragenetic varieties, pyrochlore-rich and pyrochlorepoor. Mineralogical and petrological evidence from the pyrochlore-rich carbonatites show early crystallized cumulates of apatite and Na-Ca pyrochlore minerals, while the pyrochlore-poor carbonatites appear to represent a later stage of crystallization (Cleaver, 2017). Cleaver (2017) concluded that the mineralogy of the Good Hope occurrence is different to that of the carbonatites occurring in the western and southern margins of the Prairie Lake carbonatite. The differences in mineralogy, coupled with a different magnetic signature, carbonatite texture, weathering profile, and distinct topography, indicate that the Good Hope occurrence is perhaps not directly related to the Prairie Lake carbonatite, however, the genetic relationship remains unknown (Cleaver, 2017). At this stop we will visit the areas that have been the focus of the exploration program over the last few years. Recent trenching has uncovered various outcrops of the ijolites, carbonatites and alkali granites present on the property. References Birkett, T.C., McCandless, T.E., and Hood, C.T. 2004. Petrology of the Renard igneous bodies; host rocks for diamond in the northern Otish Mountains region, Quebec. Lithos 76 (1): 475-490. Cleaver, A. 2017. Mineralogy and petrology of the Good Hope carbonatite occurrence, Marathon, Ontario. Unpublished HBSc. thesis, Lakehead University. Figure 17. Abundant pyrochlore in carbonatite at 102m, sample #1219065 (from Selway, 2017). Figure 18. Apatite mineralization in a carbonatite vein, as shown with a UV light system. Photo courtesy of R. Wahl. Coates, M.E. 1970. Geology of the Killala-Vein Lakes area, District of Thunder Bay, Ontario Department of Mines, Geology Report 81, 35p. Heaman, L.M. and Machado, N. 1992. Timing and origin of midcontinent rift alkaline magmatism, North America: evidence from the Coldwell Alkaline Complex. Contributions to Mineralogy and Petrology 110:289-303. Kozlowski, A. 2016. The mineralogy and petrology of the diamondiferous Madonna Dyke, Marathon, ON; unpublished HBSc. thesis, Lakehead University, Thunder Bay, ON, 72p. Mitchell, R.H. 1986. Kimberlites: Mineralogy, Geochemistry, and Petrology: New York, Plenum Press, 442 p. Mitchell, R.H. 1995. The role of petrography and lithogeochemistry in exploration for diamondiferous rocks. Journal of Geochemical Exploration, 53: 339350. Mitchell, R.H. 2015. Primary and secondary niobium mineral deposits associated with carbonatites. Ore Geology Reviews 64:626-641. Puumala, M.A., Campbell, D.A., Tims, A., Debicki, R.L., Pettigrew, T.K., and Brunelle, M.R. 2015. Report of Activities 2014. Resident Geologist Program, Thunder Bay South Regional Resident Geologist Report: Thunder Bay South District; Ontario Geological Survey, Open File Report 6303, 75p. Puumala, M.A., Campbell, D.A., Tuomi, R.D., Pettigrew, T.K. and Hinz, S.L.K. 2018. Report of Activities - 12 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 2017, Resident Geologist Program, Thunder Bay South Regional Resident Geologist Report: Thunder Bay South District; Ontario Geological Survey, Open File Report 6338, 101p. Sage, R.P. and Watkinson, R.H. 1995. Alkalic rocks of the Midcontinent rift, Institute of Lake Superior Geology, 41st Annual Meeting, Proceedings Vol. 41, Part 2a, Marathon, Ontario. Potter, E.G. and Mitchell, R.H. 2005. Mineralogy of the Dead Horse creek volcaniclastic breccia complex, Northwestern Ontario, Canada. Contributions to Mineralogy and Petrology, 150: 212-229. Selway, J. 2017. Assessment report for geological mapping program, Good Hope Niobium Property, Marathon, ON, Canada. Plato Gold Corp. 116p. Quist, B. 2011. Dead Horse Creek Rare Earth property, Walsh and Grain Townships, Thunder Bay Mining Division; Thunder Bay District, Assessment Report, AFRO 2.52396, 174p. Rukhlov, A.S. and Bell, K. 2010. Geochronology of carbonatites from the Canadian and Baltic Shields, and the Canadian Cordillera: clues to mantle evolution. Mineralogy and Petrology 98: 11-54. Sage, R.P. 1982. Mineralization in diatreme structures north of Lake Superior, Ontario Geological Survey Study, vol. 27, Ontario Ministry of Natural Resources, Toronto, p79. Sage, R.P. 1987. Geology of Carbonatite - Alkalic Rock Complexes in Ontario: Prairie Lake Carbonatite Complex, District of Thunder Bay; Ontario Geological Survey, Study 46, 9Ip Smyk, M.C., Taylor, R.P., Jones, P.C., and Kingston, D.M. 1993. Geology and geochemistry of the West Dead Horse Creek rare metal occurrence, Northwestern Ontario. Exploration and Mining Geology, 2:245251. Wahl, R. Wahl’s prospecting renegadeisp.com/~rwahl/ website. http://users. Wu, F.Y., Mitchell, R.H., Li, Q-L., Zhang, C., and Yang, Y-H. 2017. Emplacement age and isotopic composition of the Prairie Lake Carbonatite complex, Northwestern Ontario, Canada. Geological Magazine 154(2): 217236. Zurevinski, S.E. and Mitchell, R.H. 2015. Petrogenesis of orbicular ijolites from the Prairie Lake complex, Marathon, Ontario: Textural evidence from rare processes of carbonatitic magmatism. Lithos 239: 234-244. - 13 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Field trip 3 - Geology of the Western Schreiber-Hemlo Greenstone Belt Seamus Magnus Ontario Geological Survey, 933 Ramsey Lake Road Sudbury, ON, P3E 6B5 Canada Preface This field trip guidebook was prepared for a 1-day pre-meeting field trip held in conjunction with the Institute on Lake Superior Geology (ILSG) Annual Meeting hosted in Terrace Bay, Ontario from May 7 to 10, 2019. This geological guidebook was written to showcase the preliminary results of four years of bedrock mapping conducted by the author for the Ontario Geological Survey from 2015 to 2018 in the Schreiber–Hemlo greenstone belt (Magnus and Walker, 2015; Magnus and Arnold, 2016; Arnold et al., 2017; Magnus, 2017a,b; Magnus and Hastie, 2018). The coincidence of this meeting being held in Terrace Bay (within the mapping area) as the project was wrapping up provided the perfect opportunity to showcase these preliminary results to a broad audience. The ILSG meeting has never been hosted in Terrace Bay, however the meeting was hosted in the nearby towns of Nipigon in 2005 and Marathon in 1995. A field guide, “Geology of the Schreiber Greenstone Assemblage and its Gold and Base Metal Mineralization” was prepared for the 1995 meeting in Marathon (Smyk and Schnieders 1995); two of the five stops from that field guide are revisited in this field guide, but with updated information. The tectonically diverse geological history of the Lake Superior region has made it a playground for geologists of every discipline. The north shore of Lake Superior has over a century of mining and exploration history, including precious metals, base metals rare earth metals, and unique industrial minerals such as the colourful marbles at Ruby Lake. The location of Paleoindian sites along the north shore of Lake Superior, which was likely settled during glacial retreat at about 10,000 years before present, tend to be located in close proximity to the cherty rocks of the Gunflint formation, a source for tooling material (Norris, 2012). In fact there is evidence to show that ancient peoples south of the lake were mining, using and trading native copper as early as 4,000 years before present (Pleger, 2000). Furthermore, the Ojibway story of Nanabush and Waub-Ameek (the Giant Beaver) describes the glacial history of the Great Lakes (Snake et al., 1991), albeit it in a mythological way. Indeed, the geology of the Lake Superior area has been of interest to humans for a long time, and the author is thankful for the opportunity to learn a little more about the geological history of the area, and even more thankful to be able to share this knowledge. Safety Some of the field trip stops are located on the Trans Canada Highway 17 which is busy year-round and especially during the summer months. This highway is the major transportation route between western and eastern Canada, and as such much of the traffic along this highway includes transport trucks and logging trucks which have great momentum, especially when fully loaded. The terrane along the north shore of Lake Superior is rugged, thus the highway in this area has many hills and blind curves and the road is mostly restricted to two lanes with narrow shoulders. To maximize the safety of the field trip participants and that of the drivers on the highway, and to minimize the effect that our presence has on the flow of traffic, the author has selected field trip stops that provide ample parking space away from the shoulders of the highway and have suitable sight-lines with the traffic. The area along the north shore of Lake Superior is prone to inclement weather conditions, with dense fog possible at any time of year, causing additional risk for drivers and pedestrians; please use extreme caution during foggy periods. Care should always be exercised when parking, exiting vehicles and crossing the roads. Use of safety vests and/or bright clothing is recommended to improve your visibility to motorists. Stop 3 involves some driving along a railway This field trip guide is also available as Ontario Geological Survey, Open File Report 6357, and can be downloaded from: http://www.geologyontario.mndm.gov.on.ca/mndmaccess/mndm_dir.asp?type=pub&id=OFR6357 - 14 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 maintenance path underlain by sand, gravel and rough ground. Two-wheel drive vehicles are capable of driving on this path, but vehicles with higher clearance and all-wheel or four-wheel-drive are preferred. This stop also involves a short hike away from the parking spot, across a railroad track and up and down a steeplygraded slope. Participants should be aware of the potential for railroad traffic and “slips, trips and falls” hazards. It is recommended that anyone following this field guide individually should bring first aid supplies, food and water. Cell phone service coverage at Stop 3 is not 100% for all providers, especially in areas with more rugged terrain; participants should ensure that their cell phones have adequate connection to their networks before driving down the railroad access path, and again before hiking to the outcrop. Most of the trip routes and sites are on Crown land or public roadways, but access is on or near private property for some routes. As in all such situations, please respect the property rights of others to maintain good relationships with the landowners so that future access for geologists is not adversely affected. Terminology A number of terms used in this report are outlined below. named according to Jensen (1976). Regional Geology The bedrock along the north shore of Lake Superior hosts rocks spanning roughly 1.9 billion years of Earth’s history, from the beginning of the Mesoarchean era to the end of the Mesoproterozoic era, and include a diverse range of rocks formed in a variety of tectonic settings. Neoarchean Geological Setting The Superior Province is an Archean Craton that forms part of the North American continental shield. Rocks of the Superior Province, which range in age from circa 3.4 Ga to 2.6 Ga, are arranged in greenstone belts and plutonic domains. The Superior Province has been subdivided into terranes in which the rocks share similar lithological, geochemical, age and isotopic characteristics and structural and metamorphic histories (Stott et al., 2010). The relationship between these terranes during the early stages of their formation is unclear, however the histories of their evolution converge at circa 2700 Ma, when the terranes were amalgamated to form the Superior Craton (Stott et al., 2010). All whole rock chemical analyses that appear in this report were done at the Geoscience Laboratories, Ontario Geological Survey, Ministry of Northern Development and Mines, Sudbury. All chondrite- and primitive mantle-normalized data or diagrams referred to or shown in this report use the normalizing values of Sun and McDonough (1989). Three major terranes are present near the north shore of Lake Superior; the Wawa–Abitibi Terrane to the south, the Wabigoon Terrane to the north, and the Quetico Terrane between them (Fig. 1). The Wawa– Abitibi granite-greenstone terrane contains Neoarchean volcanic rocks erupted through juvenile oceanic crust and is interpreted to represent an oceanic arc depositional environment (Williams, 1989). The Wabigoon granitegreenstone terrane contains Neoarchean volcanic rocks erupted through and deposited upon Mesoarchean crust, is interpreted to represent a continental arc depositional environment, and is considered to have been a “protocontinent” (Williams 1989). The Quetico terrane is composed mainly of turbiditic siliciclastic rocks with sparse slivers of oceanic crust and is interpreted to represent an accretionary wedge deposited offshore of the Wabigoon “proto-continent” (Williams, 1989; Fralick et al., 2006). A preliminary compilation of geochronological data for these terranes (Fig. 2) helps to visualize the timing of events. Rock type names based on major element analyses are based on the Total Alkalis versus Silica diagram (TAS; LeMaitre, 1989), except for more ultramafic rocks such as basaltic komatiites, which have been Sedimentary rocks in the Manitouwadge greenstone belt and in the western Schreiber–Hemlo greenstone belt, both along the northern margin of the WawaAbitibi Terrane (see Fig. 1), contain detrital zircon For the sake of simplicity, the name “Wabigoon Terrane” is used in figures and in the text to refer to the collective granite and greenstone domains between the Quetico and English River metasedimentary terranes. As used in this report, “Wabigoon Terrane” includes several subdivisions included in Stott et al. (2010). Terminology for clastic sedimentary rocks, such as wacke and mudstone, follows Pettijohn (1975). Terminology for volcaniclastic rocks, such as tuffaceous conglomerates, follows Schmid (1981). - 15 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 1. Regional map of the north shore of Lake Superior, displaying Archean and Proterozoic geology. White stars indicate local past-producing and currently producing mines. Abbreviations: O-TGB = Onaman–Tashota Greenstone Belt, MGB = Manitouwadge Greenstone Belt, ESHGB = Eastern Schreiber–Hemlo Greenstone Belt, WSHGB = Western Schreiber–Hemlo Greenstone Belt, HWY 17 = Trans-Canada Highway 17. Geology from Ontario Geological Survey 2011; Terrane and domain boundaries from Stott et al. (2010). populations that are correlative with those in the Quetico Terrane and in the Beardmore–Geraldton greenstone belt (Zaleski et al., 1999; Fralick et al., 2006; Tóth, 2018; Tóth et al., 2015; Fig. 2). This suggests that during deposition of the sedimentary sequences, the Wabigoon, Quetico and Wawa–Abitibi terranes were a contiguous depositional environment. In this interpreted environment, detrital material from both the ongoing Wabigoon continental arc volcanism and from erosion of Mesoarchean crust of the Wabigoon “proto-continent” was deposited into a fore-arc accretionary wedge. As the Wawa–Abitibi oceanic arc approached the proto-continent, sediments from the continent began to fill the basin between them, eventually spilling over onto the still-active WawaAbitibi volcanic arc (Fralick et al., 2006). The end of supracrustal rock formation in the northern Lake Superior region is marked at circa 2690 Ma by crosscutting felsic plutons (Figs. 1, 2); plutonism in the region was accompanied by regional deformation and metamorphism from circa 2690 to circa 2670 Ma (Fig. 2). The three terranes were deformed synchronously during three main events; 1) early thrusting during collision of the terranes (D1), 2) upright folding during continued compression (D2), and 3) late transpressional shearing (D3; Williams, 1989). These deformational events likely represent a succession of different styles of deformation during a single protracted event; not three distinct events (Williams, 1989). The structural histories for the Shebandowan (Corfu and Stott, 1998) and Manitouwadge greenstone belts (Zaleski et al., 1999) and the eastern part of the Schreiber–Hemlo greenstone belt (Muir, 2003) are similar to Williams’ (1989) broad interpretation for the region, however the timing and development of deformation is slightly different for each greenstone belt and within each terrane. These differences are likely caused by uncertainties in the geochronological data, inconsistencies in interpretations of all of the geological and related data, and the diachronous nature of regional deformation itself (Corfu and Stott, 1998). - 16 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 2. A preliminary compilation of geochronological data for the Wabigoon, Quetico and Wawa–Abitibi Terranes. Error bars have not been illustrated, for the sake of visual simplicity. Abbreviations: Beard.–Gerald. = Beardmore–Geraldton, Win. L. = Winston Lake, Sch.–Hem. = Schreiber–Hemlo. Numbers correspond to sources for geochronology data: 1 = Anglin et al. (1988), 2 = Blackburn et al. (1985), 3 = Corfu (2000), 4 = Corfu and Muir (1989), 5 = Corfu and Stott (1986), 6 = Corfu and Stott (1998), 7 = Davis (1996), 8 = Davis and Sutcliffe (2017), 9 = Davis, Beakhouse and Jackson (1998), 10 = Davis et al. 1985, 11 = Davis, Pezzutto and Ojakangas (1990), 12 = Davis et al. (1994), 13 = Fage (2011), 14 = Fralick and Davis (1999), 15 = Fralick et al. (2006), 16 = Hart et al. (2002), 17 = Kamo (2015), 18 = Kamo (2016), 19 = Kamo and Hamilton (2017), 20 = Tóth and Lafrance (2018) and references therein, and 21 = Zaleski et al. (1999). - 17 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Paleoproterozoic Geological Setting Several Paleoproterozoic mafic dike swarms are present in the area north of Lake Superior, including dikes from the Matachewan (2480-2445 Ma; Heaman, 1997; Bleeker et al., 2012), Biscotasing (2175-2166 Ma; Buchan et al., 1993; Davis and Stott, 2003; Halls and Davis, 2004; Hamilton and Stott, 2008) and Marathon (2122-2100 Ma; Halls et al., 2008) dike swarms (Fig. 3). Sedimentary rocks of the Animikie Group unconformably overly the Superior Craton and the Paleoproterozoic dike swarms (Fig. 1). The base of the Animikie Group is defined by a thin, locally developed Kakabeka Conglomerate, which hosts carbonaceous microfossils (Gunflintia and Huronosporia) preserved in cherty stromatolites, interpreted to have formed in near-shore and shallow water environments (e.g., Wacey et al., 2013). These rocks are overlain by iron formation, carbonate rocks and siliciclastic rocks of the Gunflint Formation, which is interpreted to have been deposited during multiple marine transgressions in an extensional basin between the Penokean volcanic arc and the Superior Craton prior to their collision at circa 1.86 Ga (Fralick et al., 2002) or, alternatively, in a foreland basin north of the Penokean fold-thrust belt (Ojakangas et al., 2001). The Gunflint Formation is overlain by fine-grained argillites and slates of the Rove Formation, which contain a mixture of Archean zircons and circa 1.83-1.77 Ga zircons (Heaman and Easton, 2006) and are interpreted to have been deposited in a deep marine setting between the assembled Laurentian Craton and the circa 1.8-1.7 Ga Yavapai volcanic arc (Whitmeyer and Karlstrom, 2007). The boundary between the Gunflint Formation and the Rove Formation, and their lithostratigraphic equivalents in the USA, is marked by an unusual rock unit thought to represent distal ejecta from the circa 1.85 Ga Sudbury impact (Addison et al., 2005; Cannon et al., 2010). Mesoproterozoic Geological Setting The circa 1.4 Ga Sibley Group, a sequence of sediments deposited in alluvial-fluvial, lacustrine and eolian settings unconformably overlies the Paleoproterozoic Animikie Group (Rogala et al., 2005). The circa 1.1 Ga Keweenawan Midcontinent Rift event caused widespread magmatic activity in the Lake Figure 3. Simplified geological map of the Schreiber–Marathon area highlighting the Proterozoic formations in the area. All UTM coordinates provided using NAD83 in Zone 16. - 18 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Superior area. Pre-rift intrusive rocks are preserved north of Lake Superior, including the circa 1157-1164 Ma Prairie Lake carbonatite-ijolite complex (Rukhlov and Bell, 2010; Wu et al., 2017). Early-rift rocks north of Lake Superior include 1120-1110 Ma mafic to ultramafic intrusions such as the Thunder Bay North intrusive complex, Kitto and Seagull intrusions and the Logan diabase sills (Bleeker et al., 2018 and references therein). Most of the preserved rift-related rocks were emplaced between 1109 and 1093 Ma and include both intrusive and supracrustal rocks, including the Osler Volcanic Group, the Nipigon and Inspiration diabase sills and the circa 1108 Ma Coldwell alkalic intrusive complex (Bleeker et al., 2018 and Liikane et al., 2018, and references therein). Younger dike rocks include the circa 1099-1095 Ma Pigeon River and Cloud river dike swarms (Liikane et al., 2018). The supracrustal rocks include several packages of mafic and felsic volcanic rocks and sedimentary rocks, which are overlain by late-rift volcanic and sedimentary rocks as young as circa 1083 Ma (Miller and Nicholson, 2013 and references therein). These supracrustal rocks crop out primarily south of Lake Superior in Minnesota, Wisconsin and Michigan, and occur sporadically along the northern and eastern shores of Lake Superior. In the Terrace Bay area, mafic volcanic rocks of the <1108 Ma Osler Group unconformably overlie the circa 1400 Ma Sibley Group (Davis and Sutcliffe, 1985; Heaman and Easton, 2006), and two groups of volcanic rocks of unknown age unconformably overlie the circa 1108 Ma Coldwell Alkalic Intrusive Complex (Heaman and Machado, 1987, 1992) the Coubran Lake and Wolfcamp Lake volcanic rocks (Fig. 3; Cundari, 2012; Davis, 2016; Davis et al., 2017). Archean Geology of the Western Schreiber-Hemlo Greenstone Belt The western Schreiber–Hemlo greenstone belt is a roughly 50km long belt of supracrustal and intrusive rocks bounded on its north and west sides by Archean granitoid plutonic rocks. It extends southward under Lake Superior and is separated from the eastern Schreiber–Hemlo greenstone belt by the Mesoproterozoic Coldwell Alkalic Intrusive Complex (Fig. 1). The greenstone belt is apparently connected to a greenstone belt in the Winston Lake–Big Duck Lake area to the north by a north-trending sliver of greenstone, but the relationship between the belt and the Archean volcanic rocks on the Slate Islands, 10km to the south, is unknown (Fig. 1). Stratigraphy of the Schreiber–Hemlo Greenstone Belt Based on stratigraphic way-up indicators such as flow contacts, pillow cusps and graded bedding, the supracrustal rocks of the western Schreiber–Hemlo greenstone belt are arranged in upright, generally open folds that are locally intensified to tight and isoclinal folds proximal to pluton boundaries and in shear zones (Figs. 4 and 5). The supracrustal rocks have been subdivided into several stratigraphic packages based on common volcanic and sedimentary facies as well as geochemical characteristics and geochronological constraints (Fig. 4, inset). At the time of the ILSG field trip, geochemical and geochronological data for the rocks north and west of the Terrace Bay pluton is pending, thus the stratigraphic arrangement presented herein is tentative. East of the Terrace Bay pluton, three packages of supracrustal rocks are present: Package A, dominated by felsic metavolcaniclastic rocks; Package B, dominated by mafic metavolcanic rocks and Package D, a sequence of turbiditic wackes equivalent to the McKellar Harbour formation of Fralick et al. (2006); Package C is not present (Magnus and Walker, 2015; Magnus and Arnold, 2016; Magnus, 2017a,b). North and west of the Terrace Bay pluton, Packages A and B are present, however Package B is disconformably overlain by Package C, another sequence of distinct mafic metavolcanic rocks, and package D is not present (Magnus and Hastie, 2018). The highly strained and structurally complex area between the Terrace Bay and Santoy Lake plutons appears to mark the boundary between these two stratigraphic sections. Package A Package A is composed mainly of felsic volcaniclastic rocks, including tuffs, crystal tuffs, tuffaceous conglomerates and minor coherent flows. The crystal tuffs contain plagioclase phenocrysts and, in many cases, contain blue quartz phenocrysts. The tuffaceous conglomerates are generally clastsupported and contain pebble to cobble-sized clasts of coherent felsic rocks with similar plagioclase and blue quartz phenocrysts in a felsic tuffaceous matrix (Magnus, 2017b). This package contains minor mafic to intermediate massive to pillowed flows, including some massive flows with a high concentration of quartz and/or calcite-filled amygdules. Chert and sulphide- - 19 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 4. Simplified geological map of the western Schreiber–Hemlo greenstone belt, highlighting the major Archean rock types, some of the stratigraphic younging indicators observed during this study, all of the U-Pb zircon geochronological data in the area, and the inferred fold axial traces. An inset figure outlines the inferred depositional packages A, B, C and D. All UTM coordinates provided using NAD83 in Zone 16. Figure 5. Map of the Western Schreiber–Hemlo greenstone belt outlining domains with distinct structural and metamorphic characteristics. Abbreviations: JMMHSZ = Jackfish-Middleton-McKellar Harbour Shear Zone, HWY 17 = Trans-Canada Highway 17. All UTM coordinates provided using NAD83 in Zone 16. - 20 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 bearing chemical metasedimentary rocks are present in this package, most commonly near and along the contact between this package and Package B. An incredibly well-preserved package of felsic, intermediate and mafic metavolcanic rocks southwest of the town of Schreiber is stratigraphically correlative with Package A (i.e. below Package B, from younging directions), however the volcanic facies are different than those present in the majority of Package A as described in the previous paragraph (Magnus and Hastie, 2018). The most notable difference is the absence of the blue quartz phenocrysts present throughout the remainder of Package A. There are also fewer tuffs and crystal tuffs; the felsic rocks present are predominantly massive, plagioclase porphyritic flows and breccias with angular clasts of similar plagioclase porphyritic material. Several intermediate, plagioclase and amphibole porphyritic massive to brecciated flows are also present, as well as massive to pillowed mafic flows up to 200m thick. Interflow formations in this sequence include chert and magnetite-bearing chemical metasedimentary rocks and tuffaceous wackes and conglomerates. Package A contains the oldest known rocks in the western Schreiber–Hemlo greenstone belt; three samples from the top of the package have ages of circa 2720 Ma, which is correlative with similar felsic volcanism in the Winston Lake area, in the Manitouwadge greenstone belt and with the Greenwater assemblage in the Shebandowan greenstone belt (Davis et al., 1994; Davis and Sutcliffe, 2017). Volcanic rocks of this age have not yet been identified in the eastern Schreiber–Hemlo Greenstone Belt, however there are numerous felsic volcanic formations on that portion of the Schreiber–Hemlo belt that do not have any age information. Older phases of both the Pukaskwa and Black Pic batholiths from the eastern Schreiber–Hemlo belt, however, have yielded similar circa 2720 Ma ages (Corfu and Muir, 1989; Beakhouse and Davis, 2005). The mafic to intermediate rocks in this package contain trace element concentrations consistent with both arc volcanic and oceanic plateau volcanic settings. A-B Disconformity In the Schreiber area, a substantial sequence of interbedded graphitic argillite, chert, sulphide facies iron formation and felsic tuffaceous breccias represents a disconformity between packages A and B. The Elwood and Morley base metal sulphide occurrences occur along this horizon. Above this disconformity, there are several massive to pillowed flows which are overlain by a chemical metasedimentary sequence that includes graphitic argillite, sulphide facies iron formation and marble. So far two exposures of this horizon have been observed. In one, the marble is composed mainly of calcite with minor silicate and sulphide components; the other is a breccia, with mafic volcanic, argillite and sulphide clasts supported by a matrix of calcite. The significance of these marbles is unknown and requires further study. Is the carbonate derived from a primary sedimentary source? Could the breccia have been formed in a karstlike environment? Or was the carbonate introduced during later hydrothermal alteration? Elsewhere in the belt, similar chert and sulphide facies iron formation are concentrated along the A-B disconformity, including one occurrence of marble south of the Foxtrap Lake pluton. However, the occurrence of these rocks is more sporadic, which may be a consequence of their location in areas that are more highly strained and metamorphosed than the well-preserved occurrence in the Schreiber area. A unique feature of the A-B disconformity, which crops out along Highway 17 in Schreiber, is a sequence of interbedded turbiditic wacke and siltstone with basaltic andesitic composition that was deposited either synchronously or directly on top of the chemical metasedimentary rocks. The mafic composition of these rocks indicates they were derived primarily from a mafic volcanic source. No zircons have been found in these rocks, which precludes detrital geochronology, but a whole rock Sm-Nd isotopic study may help identify possible sources for this mafic sediment. Package B Package B is dominated by massive to pillowed mafic flows with two distinct geochemical populations based on trace elements that are consistent with both oceanic plateau volcanism and back-arc basin volcanism, respectively. Flows with trace elements consistent with an oceanic plateau volcanic setting are typical green to grey-green massive to pillowed flows, with lath-shaped plagioclase microphenocrysts visible in thin section and small vesicles concentrated around the edges of the pillows. Some massive flows with this chemistry also contain abundant calcite-filled amygdules. Flows with trace elements consistent with a back-arc volcanic setting have distinct variolitic - 21 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 textures as well as irregular-shaped cavities that likely served as conduits for volatile fluids and gases rather than typical vesicles, which are not present in these rocks (Magnus, 2017b). marked by a sequence of chert, graphitic argillite and sulphide facies iron formation that is continuous along the entire contact. One occurrence of a coherent felsic flow, with a perlitic texture and possible flow banding, has been observed near the top of this package near the Steel River (Magnus, 2017b). Package D, also known as the McKellar Harbour Formation (Fralick et al., 2006), represents the youngest known supracrustal rocks in the western Schreiber– Hemlo greenstone belt. This package is composed of a sequence of interbedded turbiditic wacke, sandstone and mudstone with normal graded bedding and sharp bedding contacts. B-C Disconformity North and west of the Terrace Bay pluton, the top of Package B is marked by a horizon of felsic volcaniclastic rocks including tuffs, tuffaceous wackes and tuffaceous conglomerates, locally interbedded with chert and sulphide facies iron formation. This horizon is located between the Terrace Bay pluton and the Lunch Lake pluton (an area known locally as the Empress Structure) and wraps around the southeastern edge of the Terrace Bay pluton (Fig. 4, inset). Package C Package C is dominated by massive to pillowed mafic to intermediate flows with trace element concentrations that are predominantly consistent with oceanic plateau volcanism, including a more thorium-enriched variety of that chemical signature and some calc-alkalic arc volcanism. The rocks in this package commonly contain medium grained equant plagioclase phenocrysts, which are uncommon in the other metavolcanic packages. This package lacks the variolitic back-arc volcanic rocks that are a distinctive feature of Package B. Apart from the felsic volcaniclastic rocks that mark the lower contact between this package and Package B, several other isolated lenses of felsic volcaniclastic material have been observed in this package. A sequence of tuffaceous metasedimentary rocks along the western edge of the Santoy Lake pluton may represent the top of Package C. This sequence includes tuffaceous wackes with abundant plagioclase phenocrysts which locally display graded bedding interbedded with tuffaceous conglomerates. The tuffaceous conglomerates are clast-supported and polymictic, with a variety of felsic and mafic metavolcanic rocks, including clasts of mafic rocks with equant plagioclase phenocrysts like those in the mafic rocks of Package C (Magnus, 2017b). B-D Disconformity East of the Terrace Bay pluton, packages B and D are in disconformable contact. This disconformity is Package D – McKellar Harbour Formation The youngest detrital zircon at the base of the package is 2696±3 Ma, which marks the maximum age of deposition for the package, and the youngest detrital zircon from the top of the package is 2693±4 Ma, which suggests that the basin had a source for young zircons during deposition (Fralick et al., 2006) 2006). The sedimentary rocks are crosscut by the 2689.6±2 Ma Steel River pluton (Kamo and Hamilton, 2017), which places a minimum age of deposition for the package, suggesting deposition occurred between 2690 and 2696 Ma. There is volcanism recorded during the 26962689 Ma interval in both the eastern Schreiber–Hemlo greenstone belt (Corfu and Muir, 1989; Davis and Lin, 2003) and in the nearby Shebandowan greenstone belt which could have provided young detrital material during deposition of the package. Older detrital zircons are present in this package, including several Mesoarchean zircons at circa 2900 Ma (Fralick et al., 2006) and a single concordant Paleoarchean grain at 3423±10 Ma (Davis and Sutcliffe, 2017). This implicates a continental component for the source of the sediments, which is interpreted to have been the Wabigoon proto-continent (Fralick et al., 2006) and/or the Minnesota River Valley terrane. Mafic to Intermediate Intrusive Rocks A series of sill-like gabbroic rocks that intrude Package B are parallel to stratigraphy and have chemistry similar to the nearby variolitic mafic metavolcanic rocks. These have been interpreted as syn-volcanic intrusions and may in some cases represent medium to coarse grained massive flows. In several of these bodies, rocks with basaltic chemistry display spinifex-like textures composed of abundant elongate amphibole crystals; invariably these rocks are associated with massive rocks of basaltic komatiitic chemistry, which may represent cumulate phases within an ultramafic flow or sill. - 22 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Mafic intrusive rocks that occur locally along the contact between packages B and D east of the Terrace Bay pluton, as well as near the contact between packages B and C northwest of the Terrace Bay pluton, typically have elongate, “plumose” amphibole crystals with interstitial plagioclase feldspar. These intrusions have chemistry similar to arc basalts, which helps distinguish them from the rocks with the spinifex-like texture described above. The Lunch Lake and Longworth Lake plutons are both composed mainly of diorite and have generally equigranular hypidiomorphic textures. The Lunch Lake pluton also includes porphyritic diorite with distinct blue quartz phenocrysts, which have not been observed in any other mafic or intermediate intrusive rock in the belt, but which are common in the felsic volcanic and volcaniclastic rocks. Felsic Intrusive Rocks Felsic intrusive rocks that surround and crosscut the greenstone belt were emplaced over a period of at least 20 million years (Figs. 1, 2). The Terrace Bay, Steel River and Syenite Lake plutons have ages between 2690-2680 Ma (Kamo, 2016; Kamo and Hamilton, 2017, 2018). These plutons are generally oblate and irregular in shape and have well-developed foliations along their contacts that penetrate up to 500m into the surrounding rocks. The Terrace Bay and Steel River plutons are both composed mainly of grey, equigranular to quartz and/or alkali feldspar porphyritic granodiorite with minor dioritic components. The Syenite Lake pluton is composed mainly of pink alkali feldspar porphyritic quartz monzonite and quartz monzodiorite. The Foxtrap Lake and Little Pic River plutons, as well as the small pluton between them, have ages between 2680-2670 Ma (Kamo and Hamilton, 2017, 2018). These plutons are round in plan view, have well-developed foliations along their margins, which continue up to 1 km into the surrounding rocks. These plutons are composed mainly of grey, equigranular to alkali-feldspar porphyritic granodiorite. The Santoy Lake pluton, with a zircon age of 2667±4 Ma (Kamo, 2016), is round to irregular in shape and has a weakly developed foliation along its contacts. This pluton is composed of pink quartz monzonite to monzonite, with local alkali feldspar porphyritic varieties, and contains a distinctly low abundance of mafic minerals. The Crossman Lake batholith (age unknown) is elongate and has a very well-developed foliation along its southern contact that penetrates up to 3km into the supracrustal rocks to the south. The intrusion is composed of equigranular white to grey trondhjemite, tonalite and granodiorite. A small pluton south of the town of Schreiber (age unknown) is irregular in shape and does not have foliations developed along its contacts. This intrusion is mostly composed of grey to pink quartz porphyritic granite with more intermediate varieties towards its southern contact. Quartz and/or feldspar porphyritic felsic dikes (age unknown) are abundant around Schreiber and northwest of the Terrace Bay pluton but are uncommon elsewhere in the greenstone belt. The “Whitesand Lake Batholith” (age unknown) is heterogeneous and appears to include more than one distinct intrusion; further mapping is required to better delineate the granitoid rocks of this batholith. Archean Structural Geology The degree of metamorphism and the nature of structural fabrics varies throughout the western Schreiber–Hemlo greenstone belt. Distinct domains containing common metamorphic mineral assemblages and structural fabrics are illustrated in Figure 5. Few crosscutting relationships have been observed between these different fabrics, thus, the structural features displayed on the map have been separated into two structural events; the early penetrative ductile fabrics, and the later more discrete brittle-ductile fabrics (Fig. 5). Observations of stratigraphic younging indicators, bedding-cleavage relationships and fold closures as well as stratigraphic correlation using geochronology have provided evidence for the upright folded Archean stratigraphy in the Schreiber–Hemlo greenstone belt (Fig. 4). The intensity of deformation and metamorphism tends to increase towards the granitoid plutons (Fig. 5). The supracrustal rocks in a 1 to 1.5 km wide zone along the northern margin of the greenstone belt have amphibolite facies mineral assemblages and display strong penetrative foliations which define tight to isoclinal fold axial planes that are parallel to the margin. South of this zone, the rocks are generally less deformed, have greenschist to amphibolite facies mineral assemblages and have east to northeast trending foliations and fold axial traces. These fold - 23 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 axial traces indicate that the rocks were deformed under northwest-directed compression during regional ductile deformation (Fig. 4). Discrete, outcrop-scale shear zones within the “Open Folding” domains (Fig. 5) are also east to northeast striking and have kinematic indicators that indicate reverse, south-side up vertical displacement with a dextral horizontal component, which is consistent with northwestdirected compression. are generally parallel with the regional ductile fabric. A thin zone of high strain up to 200 m wide is present along the margins of the circa 2690 Ma Terrace Bay pluton (Kamo and Hamilton, 2018). Kinematic indicators along the strained margins of the pluton indicate reverse, south-side up vertical displacement with a dextral horizontal component. In plan view, this pluton and the 2682.3 ± 1.1 Ma Syenite Lake pluton (Kamo and Hamilton, 2018), which are both hosted within the greenstone belt, strike northeast and resemble large-scale dextral sigma clasts. This suggests that the bulk of horizontal displacement in the greenstone belt during regional ductile deformation was dextral. The Terrace Bay pluton is the oldest known pluton in this part of the Schreiber–Hemlo greenstone belt; there has been no evidence to determine whether regional ductile deformation had commenced prior to emplacement of this pluton. Several occurrences of polymictic, clast-supported conglomerate with pebble to cobble-sized clasts and interspersed lenses of stromatolitic chert unconformably overly the Archean basement along the shore of Lake Superior southwest of Schreiber. This conglomerate represents the base of the Gunflint Formation, and hosts the famous microfossils Gunflintia and Huronosporia (e.g., Wacey et al., 2013). The 2674.1 ± 1.3 Ma Foxtrap Lake pluton (Kamo and Hamilton, 2018) truncates several east-trending fold axial traces, which are overprinted by fold axial traces that are parallel to the pluton margins. This indicates that regional ductile deformation had commenced prior to 2674 and continued after emplacement of the pluton. The 2667 ± 4 Ma Santoy Lake pluton (Kamo, 2016) truncates several east-trending fold axial traces and has very thin zones of strain along its margins. This suggests that the pluton was emplaced during the later stages of regional ductile deformation. There are two highly strained zones to note in the greenstone belt (Fig. 5). 1) The Jackfish–Middleton– McKellar Harbour shear zone is a 1-2 km wide zone of greenschist-facies rocks which are isoclinally folded and heavily sheared along lithological contacts. 2) The Empress Structure, located in a narrow band between the Terrace Bay and Lunch Lake plutons, hosts amphibolite facies rocks which are isoclinally folded and sheared with a moderate penetrative foliation throughout the zone. Throughout the greenstone belt, other thinner, unnamed ductile shear zones occur that Conjugate northwest-striking and north to northeast striking brittle-ductile shear zones and faults crosscut the greenstone belt and offset lithological contacts and the ductile fabrics. Proterozoic Geology Gunflint Formation Diabase Dikes A multitude of diabase dikes are present throughout the Schreiber-Terrace Bay area. Where possible, these dikes have been assigned to dike swarms that have been previously recognized in the area based on their orientation and chemistry. Dikes from three Paleoproterozoic dikes swarms have been recognized, including the circa 2460 Ma Matachewan Swarm, the circa 2170 Ma Biscotasing Swarm and the circa 2120 Ma Marathon Swarm (Bleeker et al., 2012; Halls and Davis, 2004; Halls et al., 2008, respectively). Three northeast-striking dikes with chemistry similar to dikes of the circa 1096 Ma Pigeon River Swarm (Liikane et al., 2018) have been observed. Previously the Pigeon River Swarm has only been recognized in the Thunder Bay area, however the dikes in the Schreiber–Hemlo greenstone belt are approximately along strike from those dikes (180 km). These dikes do not present a distinctive geophysical signature in the aeromagnetic dataset, thus tracing their extent is difficult. A series of east to northeast striking subalkalic dikes is present east of Terrace Bay which have not yet been correlated with other regional events. The author believes these to be related to the circa 1.1 Ga Keweenawan Midcontinent Rift. The most abundant dikes in the area are a series of west to northwest striking alkalic, olivine tholeiitic diabase dikes that are similar in chemistry to, and - 24 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 appear to point directly to, the Wolfcamp Lake volcanic rocks that unconformably overly the Coldwell Alkalic Intrusive Complex. If this interpretation is correct, then it is likely that the alkalic dikes are related to an episode of Keweenawan volcanism younger than 1108 Ma (the age of the Coldwell Complex which they crosscut). as massive sulphide in irregularly-shaped veins, and in sulphide-bearing quartz veins. Where these structures crosscut felsic metavolcanic rocks, the rocks are typically altered from grey to beige, and feldspar phenocrysts are altered to a distinct bright green colour (Magnus and Hastie, 2018). Mineral Potential Proterozoic rocks near the north shore of Lake Superior host a variety of commodities, including nickel, copper and platinum group elements associated with mafic to ultramafic intrusions; other transitional metals, such as niobium, tantalum and titanium and rare earth elements associated with carbonatitic (e.g., the Prairie Lake carbonatite-ijolite complex) and alkalic rocks (e.g., the Coldwell Alkalic Intrusive Complex); and diamond, associated with lamprophyric and kimberlitic dikes. The Western Schreiber–Hemlo greenstone belt has a long history of mineral exploration and has potential for a variety of styles of base and precious metal mineralization. Base metal sulphide and precious metal occurrences are associated with supracrustal rocks throughout the greenstone belt. Sulphide mineralized rocks occur near and along the upper contact of the circa 2720 Ma metavolcanic rocks of Package A. The host rocks are sulphide facies iron formation and chert, interbedded with felsic volcaniclastic rocks and garnetiferous mafic metavolcanic rocks. These rocks are lithologically similar and chronologically correlative with circa 2720 Ma metavolcanic rocks in the Winston Lake and Manitouwadge areas, which both host past-producing Zn-Cu-Ag base metal mines (see Fig. 1; Davis et al., 1994; Zaleski et al., 1999). Gold and base metal sulphide occurrences are associated with highly strained supracrustal rocks, including the JMMHSZ, the Empress Structure, strained rocks surrounding plutons and other discrete shear zones throughout the greenstone belt. Mineralization in these shear zones typically occurs in quartz and carbonate veins in the shear zones and in silicate and carbonate altered haloes adjacent to the veins. The Hemlo gold deposit, located in the eastern part of the Schreiber–Hemlo greenstone belt, is hosted in highly strained and altered supracrustal rocks (Fig. 1; e.g. Muir, 2003). Gold occurrences, with minor silver, molybdenum and copper mineralization, are associated with the Terrace Bay pluton and other granitoid rocks in the map area. Mineralization occurs in sulphide-bearing quartz veins and in altered granitoid rock adjacent to the veins. The veins are typically straight, with sharp contacts, and occur in parallel sets and in “stockwork” arrangements (Arnold et al., 2017; Marmont, 1984). Zinc, silver and lead mineralization (with minor copper and gold) is associated with north to northeast striking faults near Schreiber. Mineralization occurs Road Log Note: Caution should be taken when parking vehicles on the shoulder of the highway and when examining outcrops located along Highway 17. All UTM coordinates are provided in NAD83, Zone 16. Figures 3 and 4 show the location of the field trip stops. The primary focus of this trip is on the Archean rocks of the Schreiber–Hemlo Greenstone Belt, however because of the abundance of Proterozoic diabase dikes in the area, some of the stops will also feature Proterozoic rocks. Note the mileages in the road log are not cumulative, rather each number is the distance from one stop to another. 41.7 km - Starting in Terrace Bay, drive east along Highway 17 for roughly 42km (25 minutes). About 1.5km (1 minute) before the parking spot, you will pass under two power transmission lines and Ripple Lake on the southeast side of the road; begin to slow down at this point, to make sure vehicles behind you have time to react, as you will be pulling off the road shortly. As you approach the parking spot, you will drive downhill across the McKellar Creek bridge. The parking spot is on the right (south) side of the road at the east end of this bridge just past the guardrail. Reset odometer to zero as subsequent mileages to stops are based on starting here. If you pass the stop, turn at the junction between Highway 17 and Dead Horse Road and find a suitable location to turn around, and retrace your route back to the stop. - 25 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 40.3 km - If starting in Marathon, drive west along Highway 17 for 40.3 km (29 minutes). After crossing the Little Pic River Bridge, then passing Dead Horse Road, Stop 1 will be on the south (left) side of the highway. Beware of oncoming traffic prior to turning into the parking area; east-bound vehicles will be driving speedily downhill at this location. Stop 1. Sheared rocks, diabase and lamprophyre UTM coordinates 521355E 5407099N There are outcrops on the north and south sides of the highway at this location. The highway marks a contact between metasedimentary rocks to the north and mafic metavolcanic and intrusive rocks to the south. The metasedimentary rocks north of the highway are wacke and siltstone with local sulphide mineralization. Box folds are observed throughout these rocks, which indicate that the rocks have been subjected to beddingparallel shearing. Several biotite porphyritic ultramafic lamprophyre dikes, up to 8cm wide, crosscut the metasedimentary rocks, which exhibit iron carbonate alteration haloes adjacent to the dikes. A single 240m long, near vertically-faced outcrop is present south of the highway. In this large outcrop, the mafic rocks display varied degrees of strain. In more highly strained areas, fractures and quartz veins tend to be parallel to the strong foliation, and where the outcrop surface is parallel to the foliation plane, geometric shapes appear in the outcrop. In less strained areas, fractures are more irregular, and parallel sets of quartz veins dip shallowly to the south, nearly perpendicular to the foliation in the outcrop. These areas are most easily observed by viewing the entire outcrop from the north side of the highway. All primary textures have been obliterated in the highly strained areas, where a strong steeply dipping foliation hosts folded and boudinaged quartz veins and box folds and local sulphide mineralization. Quartz vein boudins with both sigma and delta asymmetries indicate the latest displacement was vertical; reverse motion with the northern side moving up towards the south. Two lineations are present in the outcrop; one shallowly plunging lineation which is interpreted to be a crenulation lineation, and a steeply-plunging lineation which is interpreted to be a stretching lineation, both formed during the reverse shearing event. The trend and plunge of these lineations vary throughout the outcrop as the foliations that host them waver. The two areas of more competent rock are composed of massive medium-grained equigranular aphyric mafic rock, which may represent either massive metavolcanic flows or intrusive rocks. In the western zone, a dike of granodioritic rock crosscuts the mafic rock. Near the west end of the outcrop, two adjacent alkalic diabase dikes crosscut the Archean rocks, striking west and dipping to the north. Together, these dikes are about 10m wide. These dikes are generally equigranular with fine-grained chilled margins and fractures orthogonal to the contacts. A 5cm wide feldspar porphyritic diabase dike is present at the very west end of the outcrop. Several north-striking biotite porphyritic ultramafic lamprophyre dikes are present near the middle and at the east end of the outcrop, similar to those on the north side of the highway. The mineralogy and texture of these dikes, including their associated iron carbonate alteration, are typical of lamprophyre dikes in this area. Return to vehicles and turn left to drive west on Highway 17. 12.9 km - Drive west on Highway 17 for 12.9 km (8 minutes). Two minutes before the stop, you will pass Black Fox Lake on the right (north) side. The parking location for Stop 2 will be on the left (south) side of the highway in a turn-around location, on the east side of the Steel River Bridge (Fig. 6). If you miss the stop, there is a suitable turn-around location on the west side of the bridge. Stop 2. Turbiditic Wacke UTM coordinates 508700E 5402577N There are outcrops on both the north and south sides of the highway at this location. The outcrop on the north side of the highway is shorter in length, and has cleaner outcrop surfaces, so it will be the focus of this stop. Stop 2 is the same locality as Stop 1 in Smyk and Schnieders (1995). This outcrop is composed of southward-younging normally graded wacke interbedded with mudstone (Photo 1). On the south side of the road, and in outcrops along Santoy Bay and on Lawson Island, the younging direction of graded beds switches from south to north repetitively within tens of metres. These tight younging reversals are the main evidence for isoclinal folding in the area (Fig. 6). - 26 - A pervasive schistosity throughout the outcrop, �Proceedings of the 65th ILSG Annual Meeting - Part 2 sheared during folding. Return to the vehicles and turn left to drive west on Highway 17. Photo 1. This photograph displays normally graded wacke interbedded with mudstone. Arrows point to enigmatic features which may be related to primary loading structures, folding or both. The outcrop surface is horizontal and a scale card is pointing north. axial planar to local isoclinal folding, is at a low angle to bedding. Along this schistosity, the bedding planes display a strange pattern in which the more fissile mudstone layers form tabular projections into the sandstone, whereas the sandstone layers form more lobate projections into the mudstone. This may be interpreted as a primary loading structure that has been 4.6 km - Drive west on Highway 17 for 4.6 km (3 minutes). You will be driving uphill on a moderate to steep grade, and any westbound transport trucks on this stretch of road will be driving below the speed limit with their 4-way flashers on. Do not pass these trucks; stay well behind them to ensure they do not decide to pull over and stop. After cresting the hill, the next turn will be about 1 minute away. Look for a “road intersection” sign and prepare to turn left (south). 2.2 km - Drive down this gravel road south towards Lake Superior, always keeping to the left at any junction. Eventually you will turn east and pass a gravel pit and a railway crossing. Slow down considerably and continue onward. ~2.0 km - The road turns from gravelly to sandy and narrows to a single lane. Continue driving eastward with caution. There are several branches of this trail that quickly lead to dead ends; if you reach a dead end, turn around and try another path. If at any time you feel unsafe or are unsure whether your vehicle is Figure 6. Simplified geological map of the Santoy Bay area, which includes stops 2 and 3. Several rocks of interest near Stop 3 are also indicated but will not be visited during this field trip. All UTM coordinates provided using NAD83 in Zone 16. - 27 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 capable of driving in this terrain, turn back and skip ahead to Stop 4. The parking spot for Stop 3 is located at UTM 505400E, 5402555N, in a location where the path widens and there is a clearing through the trees on the south side of the path. 250-300 m - Unpack your hiking gear, including food, water and first aid equipment. Walk south along a footpath towards the railway; there is good line-ofsight along the rails at this location to see any oncoming trains. For the 2019 ILSG trip, there will be a flagged trail through the bush towards the lakeshore outcrop (Stop 3). For future users of this guidebook, you will have to navigate directly southward to the lake. Safety Note: the hill between the railway and Lake Superior is steeply sloping and built out of cobbles. This is a health and safety risk to those with reduced mobility. If at any time you feel unsafe to continue, turn back and skip ahead to Stop 4. Stop 3. Variolitic Mafic Flows UTM coordinates 505366E 5402266N This lakeshore outcrop, which has been kept lichen free by winter ice in Lake Superior is the best exposure of variolitic mafic flows in the western Schreiber– Hemlo Greenstone Belt (Fig. 6). Exposure here is good enough to trace flow contacts and to observe various macrotextures present in at least four consecutive flows (Fig. 7). Pillows are generally 3m across, with single rinds up to 3 cm thick, and selvages filled with glassy material and hydrothermal minerals like quartz, calcite and epidote. These pillows lack vesicles or amygdules, but some pillows situated at or near the top of flow sequences contain elongate, discontinuous, quartz- and carbonate-filled cavities, which the author interprets to represent large, formerly gas-filled cavities. The most conspicuous feature of these pillows is the variolitic texture (Fig. 7). Inward from the chilled margins, the pillows are dark green and very finegrained, with only a few small varioles (up to 2 mm). Varioles become larger (up to 8 mm) and more abundant toward the core of the pillow, where they appear to have amalgamated to produce a more massive, leucocratic pillow core (Fig. 7). The interiors of the varioles are concentrically zoned with bands of calc-silicate minerals. The distribution of varioles in the pillows is not always perfectly concentric; their distribution seems to be more erratic at the tops of the pillows. Very few pillows display multiple concentric variolitic and non-variolitic bands. In pillows that contain both the gas cavities and varioles, the gas cavities are always located in the dark green, non-variolitic upper portions of the pillows. The massive flows, which may be traced in this Figure 7. A) Simplified bedrock geology map of the shoreline outcrop at Stop 3, and B) illustration of the macrotextures observed in outcrop along A–A’. All UTM coordinates provided using NAD83 in Zone 16. - 28 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 outcrop for up to 50 m in length, have widths that vary in proportion to their lengths (i.e., thinner flows are less laterally extensive). The internal structure of the flows is similar to that observed in the pillows, with non-variolitic, dark green material inside the rinds that grades into massive variolitic cores. The transition from rind to variolitic core at the base of the flows occurs over approximately 5 cm, whereas, at the top of the flows, the varioles have coalesced into lobes and pods that appear pillow-like, without the necessary rinds (i.e., pseudopillows). Randomly oriented, elongate crystals of amphibole are present in the massive, medium-grained core of the thickest flow in this outcrop. These flows and all other variolitic mafic flows in the greenstone belt have trace element contents consistent with mafic rocks erupted in “back-arc basin” volcanic environments. This is the defining characteristic of Depositional Package B, which is dominated by rocks of this chemistry intercalated with mafic rocks of “oceanic plateau” volcanic affinity. Nearby outcrops of a perlitic felsic flow (accessible along a footpath) and outcrops of spinifex-textured mafic to ultramafic rocks (accessible along Highway 17 or along the railway) are indicated in Figure 6. These rocks will not be visited during this field trip. Please use caution, especially along the highway and the railway, if you decide to visit these rocks. Return to vehicles by the same path you took to the outcrop. Turn the vehicles around and drive west along the path. ~2.0 km - Drive back along the sandy path the way you entered, until you reach the gravel pit. 2.2 km - Keep to the right and drive north until the gravel road intersects with Highway 17. 7.3 km - Turn left and drive west on Highway 17 for 7.3km (4 minutes). Along the way, you will get an excellent view over Jackfish Lake and the terraced hills to the north. Those hills are the location of the historic Empress gold mine, which produced 112 oz Au at 0.10 oz/ton from 1896-1897. After passing the lake and beginning to drive uphill, prepare to slow down for Stop 4. The parking spot for Spot 4 is a turn-around spot on the left (south side) of the highway, on the west end of a large granite and diabase outcrop. Stop 4. Granite, sheared mafic rocks and diabase dikes UTM coodinates 503233E 5411222N There are outcrops on the north and south sides of the highway at this location. On the south side of the highway, grey to pink granodiorite of the Terrace Bay pluton is crosscut by a 50 m wide plagioclase porphyritic diabase dike. This dike trends generally northward and is aligned with a north-northeast trending geophysical anomaly consistent with dikes of the Biscotasing dike swarm. There are smaller plagioclase porphyritic dikes with chill margins that crosscut the large dike. On the north side of the road, there are two outcrops composed of a series of southward dipping panels of granite, mica schist, mafic intrusive rocks and massive felsic rocks. The bottom of the western outcrop is massive grey to pink granodiorite of the Terrace Bay pluton, the top of the outcrop is a dike or sill of weakly foliated massive, fine grained aphyric felsic rock and a panel of mica schist lies between them. The mica schist is composed of biotite and chlorite with abundant quartz and calcite veins. The dominant foliation in the mica schist dips more steeply to the south than the contacts between the schist and the felsic rocks between which it is sandwiched. This looks like a C-S structural fabric; the contacts between units represent the “C” plane, and the strong foliation in the schistose rock represents the “S” plane. Quartz veins in the schist are boudinaged; asymmetric boudins (mostly sigma clasts) and the orientation of the C-S fabric both indicate south-side up reverse displacement northward. Box folds in this schist post-date the sigmoidal quartz boudins. A biotite porphyritic ultramafic lamprophyre dike crosscuts the rocks at the west end of the outcrop. The eastern outcrop is a massive sheared mafic rock composed mainly of amphibole and biotite with minor quartz, feldspar and carbonate minerals, cut by a small granitoid dike at the west end of the outcrop. Southward dipping shear zones crosscut this rock with C-S fabrics similar to those observed in the western outcrop, indicating the same south-side up reverse displacement. The trace element composition of this mafic rock is similar to that of the granitoid rocks in the Terrace Bay pluton, with higher concentrations of transitional elements such as iron, magnesium, chromium, vanadium, nickel and copper. This rock is interpreted to represent mafic country rock that was - 29 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 altered during emplacement of the Terrace Bay pluton. Return to the vehicles and turn left to drive west on Highway 17. 17.4 km - Drive west on Highway 17 for 17.4km (12 minutes). You will pass through the town of Terrace Bay. On the west side of town, just west of the Aguasabon River, turn left at the intersection between the highway and Aguasabon Gorge Road. Note the outcrops of grey granodiorite, locally altered to pink granodiorite, along the highway towards Terrace Bay. Most of the rocks in the Terrace Bay pluton look like this. 750 m - Drive to the end of the road. There will be a large parking area with outhouses and picnic tables. There is a boardwalk with railings at the south end of this parking lot that leads towards a vista with a view of the Aguasabon Falls and Lake Superior. Safety Note: Although there are small foot-paths off of the main boardwalk, do not hop over the railing to walk on these paths. Falling into the gorge would lead to death. Stop 5. Aguasabon Falls Gorge; structures and alteration UTM coordinates 490833E 5403233N This vista overlooks granitoid rocks of the Terrace Bay pluton (Photo 2). Granodiorite in the pluton is normally grey; locally, the granodiorite is altered pink, caused by hydrothermal alteration around regionalscale shear-zones and faults. North, northwest and northeast-striking faults, which correlate with similar shear zones that crosscut the supracrustal rocks of the greenstone belt, control the vertical cliff faces in the Aguasabon River Gorge. Return to vehicles, drive back along Aguasabon Gorge Road and turn left to drive west on Highway 17. 8.1 km - Drive west on Highway 17 for 8.1 km (5 minutes). Along the way you will get an excellent view of Lake Superior (Terrace Bay). One minute before the next turn, you will pass an intersection between the highway and Worthington Bay Road. You will then cross a train bridge; turn right onto Hays Lake Road 200m north of the bridge (Fig. 8). Note the large outcrop of sulphide-bearing chert and graphitic argillite at the beginning of Hays Lake Road. Drive for 800 metres along Hays Lake Road; there Photo 2. View of the Aguasabon Falls Gorge, with Lake Superior and the Slate Islands in the background. Photo taken from a vista at the end of Aguasabon Gorge Road. will be a clearing in the trees on the north side of the road. Pull your vehicle safely to the shoulder of the road and park. Stop 6. Harkness Hays and Gold Range UTM coordinates 483711E 5404933N North of the road, there is a northeast-trending ridge of outcrops that have been the subject of gold exploration for more than a century, with the earliest staking recorded in 1917. Over the following several decades, numerous adits and shafts were used to sample the bedrock in this ridge, which hosts the HarknessHays property to the west and the Gold Range property to the east. The Harkness-Hays property produced 200 oz Au at 2.58 oz/t during intermittent mining activities between 1920 and 1936; the Gold Range property produced 36.35 oz Au at 0.91 oz/t during intermittent mining activities from 1921 to 1941 (Schnieders et al., - 30 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 8. Simplified geological map of the Schreiber area, which includes field trip stops 6, 7 and 8. The mafic metasedimentary rocks and the Morley occurrences, which are correlative with the rocks at Stop 7, are indicated, as well as nearby occurrences of marble associated with other chemical metasedimentary rocks. A box outlines the area covered in Figure 9. Abbreviations: Ag = silver, Au = gold, Cu = copper, Fe = iron, g = graphite, grt = garnet, Mo = molybdenum, Pb = lead, py = pyrite, S = sulphur, Zn = zinc. All UTM coordinates provided using NAD83 in Zone 16. 1996). The focus for this stop will be on the HarknessHays property, which is the most easily accessible (and has the better historic gold grade). These outcrops are composed mainly of massive to pillowed mafic metavolcanic rocks, crosscut by quartz feldspar porphyritic felsic dikes and biotite porphyritic lamprophyre dikes. These rocks are located in a moderately strained zone along the northwest edge of the Terrace Bay pluton and contain amphibolite facies mineral assemblages (Figs. 5 and 8). Quartz veins in the northeast-striking foliation, parallel to the contact with the pluton, host gold-bearing sulphides and occurrences of native gold. Native gold at this site is found most commonly in white, vuggy quartz veins. Much of the bedrock has been blasted, and quartz vein bearing rocks are dispersed throughout the resultant pile of rocks. It is recommended that visitors search through this pile of rock, rather than scale the pile to access the steep, cliffy outcrops. Return to vehicles, turn the vehicles around and drive back towards Highway 17. 5.3 km - Turn right and drive westward on Highway 17 for 5.3 km (4 minutes). Along the way you will pass through the town of Schreiber. After passing the Villa Blanca Inn, on the west side of town you will begin driving uphill. The parking spot for Stop 7 will be on the right (north) side of the road at the top of this hill; prepare to stop. Note as you drive through Schreiber, on the right (northeast) side of the road are several outcrops of turbiditic mafic-derived metasedimentary rocks. - 31 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Stop 7. Elwood Occurrence UTM coordinates 479511E 5407266N There are outcrops on the north and south sides of the highway at this location (Fig. 8). On both sides of the highway, the outcrops are an upright north-dipping sequence of interbedded chert and graphitic argillite with minor beds of felsic tuffaceous conglomerate. Sulphide mineralization is disseminated throughout the outcrop and occurs in calcite-sulphide veins and as conformable lenses of massive sulphide. The sulphide minerals are dominantly pyrrhotite and pyrite with minor chalcopyrite. On the north side of the highway, a massive mafic flow marks the top of this sequence and forms an erosion-resistant cap on the outcrop. At the base of this flow, the rock contains abundant siliceous xenoliths of chert ripped up from the underlying cherty units, as well as abundant quartz and calcite amygdules, likely caused by the release of volatile fluids from the underlying sediments as the mafic flow was deposited. One 15cm wide dike of similar composition crosscuts the underlying rocks and is interpreted to be a feeder dike to the flow. The sequence of sedimentary rocks is roughly 80 m thick and represents a significant disconformity between the arc volcanic rocks of Package A and the back-arc basin volcanic rocks of Package B (Fig. 4). These rocks have a strong electromagnetic signature which is traceable along strike; eastward, the anomaly coincides with the mafic metasedimentary rocks along the highway in Schreiber and with chemical metasedimentary rocks at the Morley occurrence southeast of town (Fig. 8). Several other chemical sedimentary rocks occur east of town, including two occurrences of marble interbedded with argillite and sulphide facies iron formation and the outcrop of sulphide-bearing chert and argillite observed earlier at the intersection of Highway 17 and Hays Lake Road (Fig. 8). Whether these nearby rocks represent separate sequences or are part of the same depositional sequence but separated by cryptic folding requires further, more detailed mapping. The strata in this area are arranged in open, upright folds, which is apparent in this outcrop. However, the more fissile argillite-rich zones have developed C-S fabrics, kink folds and kinematic indicators like sigma and delta clasts that all indicate a significant amount of dextral shearing has affected these rocks. Because the only place locally that this deformation has been observed is in these argillitic rocks, the timing and cause of this shearing is unknown. Return to vehicles and turn left to drive east on Highway 17. 1.2 km - Drive 1.2 km into the town of Schreiber and take the third right onto Winnipeg Street. This is the street immediately east of the Golden Rail chip truck. 600 m - Drive to the end of Winnipeg Street, where you will see a railroad museum, and turn right onto Scotia Street. 70 m - Drive 1 block west on Scotia Street, then turn left on Subway Street. 210 m - Driving south on Subway Street, you will pass beneath the railway. Take the first right onto Isbester Drive. 2.3 km - Drive south to the end of Isbester Drive. There is a parking lot at the end of the road, and an outhouse and a gazebo down near the beach. Walk down the footpath to the beach, then turn right (west) and walk towards the first outcrop. This is Stop 8. Stop 8. Schreiber Beach Outcrops UTM coordinates 478600E 5404600N The outcrops at Schreiber Beach (Photo 3) are the best exposure of a conformable sequence of Archean metavolcanic and metasedimentary rocks in the Schreiber–Hemlo greenstone belt and perhaps throughout Ontario (Fig. 9). From the easternmost outcrop to the Schreiber Channel Provincial Nature Reserve, where exposures of Proterozoic Photo 3. View of Schreiber Beach and the rocky shoreline westward, taken from a vista along the Casque-Isles Trail. - 32 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 9. Simplified geological map of the shoreline west of Schreiber Beach on Lake Superior. Metavolcanic flows and metasedimentary sequences are labelled I-XII and i-iii respectively. Proterozoic rocks of interest are labelled. Note that the Casque-Isles trail is well constrained for two km west of the Schreiber Beach, but the author was not able to get better a resolution trace of the trail further west. All UTM co-ordinates provided using NAD83 in Zone 16. rocks interrupt the Archean exposure, there are three kilometers (as the crow flies) of near-continuous rocky shoreline, with less than 5% of the shoreline covered by cobble beaches. Winter ice scraping against the rocky shoreline keeps the rocks up to several metres inland lichen-free, displaying beautiful mineral, volcanic and sedimentary textures. The rocks are not pervasively deformed; only crosscut by discrete fractures with minimal displacement and up to metrewide shear zones. There are only a few altered areas in which the primary textures of the rocks are preserved. The upright stratigraphy strikes northwest and dips shallowly to the northeast, such that the straight, eastwest trending shoreline provides for us a perfect crosssection; walking west along the shoreline guides us down through stratigraphy. The lead author only had the opportunity to map the shoreline between Schreiber Beach and Twin Harbours, and so that stretch will be the focus of this description. From compiled data, the Archean rock between Twin Harbours and the granite-greenstone contact to the west is composed mainly of mafic metavolcanic rock. The world-famous Schreiber Channel Stromatolite outcrop and associated Gunflint conglomeratic rocks occurs along this stretch of shoreline, and a Keweenawan diabase sill occupies the Archean-Proterozoic unconformity. Eastward from Twin Harbours, higher in the stratigraphy, are five consecutive mafic metavolcanic flows up to 200 m in apparent thickness (roughly 175 m in true thickness, with an estimated 30 degree dip; Fig. 9). These flows are massive at the base, with medium to coarse-grained equigranular textures that could easily be mistaken for mafic intrusive rocks. Thin beds of sulphide-bearing chert are present along the flow contacts. Flow II is crosscut by an alkalic diabase dike, flow III is crosscut by two north-trending, carbonate altered mafic dikes up to 25 m wide, and flow V is crosscut by a series of north-trending dikes that display unique Liesegang textures. A 50 m thick sequence (i) of tuffaceous conglomerates with pebble to cobble-sized mafic and felsic volcanic clasts lies atop flow V (Fig. 9). Minor graded beds of sandy to gravelly material are present within these conglomerates. These rocks are crosscut by alkalic diabase dikes, and unconformably overlain by an outlier of the Gunflint Formation basal conglomerate, including cherty stromatolite domes similar to those observed at the Schreiber Channel Provincial Nature Reserve. The conglomerate is massive, polymictic and clast-supported, with dominantly pebble to cobblesized clasts. The conglomerates are interpreted to have been deposited in a shallow water environment akin to the cobble beaches present along the shores of Lake Superior today. - 33 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Eastward from metasedimentary sequence i are two consecutive massive to pillowed mafic flows similar to flows I-V. Another outlier of the Gunflint basal conglomerates and stromatolites, crosscut by an alkalic diabase dike, unconformably overlies pillows near the top of flow VI (Fig. 9). A thin sequence of tuffaceous wacke (ii) marks the top of flow VII. This is overlain by an intermediate volcanic flow (VIII) with plagioclase and amphibole phenocrysts. This flow is generally massive, with an agglomerate of similar composition at its top. This agglomerate grades into the polymictic tuffaceous conglomerate (iii) similar to the conglomerates in sequence i. Another massive intermediate flow (IX) with plagioclase and amphibole phenocrysts overlies the conglomerates of sequence iii roughly 100 m wide, with agglomerate at its top similar to that of flow VIII. This agglomerate is overlain by a 3 m sequence of normally graded tuffaceous wacke, which is then overlain by a massive to brecciated felsic flow with feldspar phenocrysts (Flow X). Two consecutive massive to pillowed mafic flows (XI and XII) overlie Flow X with a 1 m wide sequence of banded chert and magnetite between them. Outcrop exposure ends just above the base of Flow XII, which is then covered by the sands of Schreiber Beach. This eastern-most outcrop will be the subject of our last stop on the field trip (Fig. 9). To access the shoreline to the west, one could either walk along the rocky shoreline, which is quite rugged and slippery when wet, or follow the CasqueIsles Trail, which intermittently jogs down towards the shoreline (Fig. 9). There is a stream with steep-sided banks roughly halfway between Schreiber Beach and Twin Harbours, at the contact between flows IV and V. At the mouth of this stream, there are cobbles and boulders along the shoreline that may be crossed if there are no waves on Lake Superior and the outflow from the stream is minimal. A small wooden footbridge, wide enough for one person, crosses this stream about 500 metres to the north along the Casque-Isles Trail. Neither of these choices are particularly safe, so exercise extreme caution whichever path you choose to follow. Note that aside from the Schreiber Channel stromatolites, most of the rocks observed between here and Twin Harbours are massive to pillowed mafic flows (Fig. 9). To reach the Schreiber Channel stromatolites, it is recommended to start at the west end of the trail in Rossport or to approach the location by boat. End of road log. Acknowledgments The author would like to thank the field crews from the summers of 2015 (Joseph Walker, Andrea Nywening, Matthew Hanewich and Lauren Madronich), 2016 (Kira Arnold, Mallory Metcalf, Lucas Wolfe and Haley Aldred), 2017(Kira Arnold, Joshua Nguyen, Maddison Hodder and Gabrielle Klemt) and 2018 (Evelyn Moorhouse, Cassandra Powell, Mateo DoradoTroughton, Jessica Verschoor, Shadman Islam and Rachel Bourassa) for their hard work and perseverance through the particularly rough terrain. The author would like to thank Evan Hastie for co-leading the 2018 field season. The author would also like to thank the Richards family of Terrace Bay, who hosted the crew at their Jackfish Lake cottages on Highway 17 during the 2015-2018 field seasons, with special thanks to local prospector Wayne Richards, for all of his logistical aid and for sharing his abundance of local mineral exploration knowledge. Thanks to the people of Pic River and Pic Mobert First Nations communities for their gracious blessing and for allowing us to work on their traditional lands. The author would also like to thank local prospector Rudy Wahl, Mike Koziol of Alto Ventures Ltd. And Troy Gill of Sanatana Resources for tours of their properties and allowing us access to their properties over the last several field seasons. Thanks also to Mark Smyk, Dorothy Campbell and Mark Puumala of the Resident Geologist Program Thunder Bay office for their help during this project. Thanks to Michael Easton and Riku Metsaranta for their careful edits, and to Laura Ratcliffe for help with the figures. References Addison W.D., Brumpton, G.R., Vallini, D.A., McNaughton, N.J., Davis, D.W., Kissin, S.A., Fralick, P.W. and Hammond, A.L. 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact event; Geology, v.33, p.193-196. Anglin, C.D., Franklin, J.M., Loveridge, W.D., Hunt, P.A. and Osterberg, S.A. 1988. 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Geological studies of the Wabigoon, Quetico and Abitibi–Wawa subprovinces, Superior Province of Ontario, with emphasis on the structural development of the Beardmore–Geraldton belt; Ontario Geological Survey, Open File Report 5724, 189p. Wu, F-Y, Mitchell, R.H., Li, Q-L, Zhang, C. and Yang, Y-H. 2017. Emplacement age and isotopic composition of the Prairie Lake carbonatite complex, northwestern Ontario, Canada; Geological Magazine, v.154, no.2, p.217-236. Zaleski, E., van Breemen, O. and Peterson, V.L. 1999. Geological evolution of the Manitouwadge greenstone belt and Wawa-Quetico subprovince boundary, Superior Province, Ontario, constrained by U-Pb zircon dates of supracrustal and plutonic rocks; Canadian Journal of Earth Sciences, v.36, p.945-966. - 38 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Appendix 1. Geochemical data for typical samples of the major rock types in the western Schreiber–Hemlo greenstone belt. All analyses were performed at the OGS Geoscience Laboratories, Sudbury. Complete data are available in Magnus (2017) (Tuuri and Walsh townships), Magnus (2019) (Syine township) and in upcoming Miscellaneous Release—Data reports for Priske and Strey townships. Sample Number Related Stop 17SJM013C 17SJM013C 17SJM013C 15SJM068A 15SJM068A Stop 2 Stop 2 Stop 2 Rock Name mafic volcanic rock Package B back arc volcanic 505364 5402267 base of flow mafic volcanic rock Package B back arc volcanic 505364 5402267 top of flow S&S Stop 2A mafic volcanic rock Package B back arc volcanic 506861 5402734 base of flow basalt mafic volcanic rock Package B back arc volcanic 505364 5402267 middle of flow basalt back arc basin 48.67 back arc basin 50.07 basaltic andesite back arc basin 52.7 S&S Stop 2A mafic volcanic rock Package B back arc volcanic 506861 5402734 "spinifex" texture basalt back arc basin 49.56 0.9 1.02 1.05 0.66 Al2O3 12.32 14.05 14.21 Cr2O3 0.078 0.044 0.054 Formation Volcanic Setting Easting (m) Northing (m) Notes TAS rock name Tectonic Setting SiO2 (wt %) TiO2 Fe2O3 total 16SJM157 A near Stop 2 17SJM082C 15SJM204B none melanogab bro back arc basin 46.17 picrobasalt back arc basin 36.97 mafic volcanic rock Package C plateau volcanic 501620 5412183 thoriumenriched basalt S&S Stop 2B mafic volcanic rock Package B plateau volcanic 506190 5403315 trachytic texture basalt continental arc 47.89 continental arc 45.45 0.34 0.22 2.18 1.93 12.08 6.2 4.43 15.36 14.76 0.08 0.31 0.58 0.008 0.02 basalt Package B back arc volcanic 506123 5402406 n/a 12.99 12.44 11.28 12.07 11.66 10.62 14.57 13.13 MnO 0.194 0.199 0.213 0.173 0.137 0.175 0.242 0.339 MgO 11.08 8.11 6.01 11.06 24.23 26.37 2.54 3.08 CaO 10.036 8.439 9.481 9.384 5.055 6.66 12.959 8.382 0.93 1.02 2.37 1.93 0.02 <0.02 0.42 2.9 Na2O K2 O 0.23 2.59 0.27 0.08 0.01 0.03 1.19 0.73 P2O5 0.099 0.109 0.118 0.047 0.027 0.018 0.356 0.372 LOI Total Mg Number 3.41 2.99 2.98 2.99 6.02 14.12 1.73 9.1 100.95 101.11 100.75 100.12 100.18 100.19 99.47 100.2 0.82 0.78 0.74 0.83 0.92 0.93 0.49 0.56 Th (ppm) 0.382 0.424 0.436 0.212 0.097 0.062 1.151 0.765 Nb 3.098 3.47 3.706 1.29 0.597 0.478 8.719 9.274 Ta 0.205 0.23 0.236 0.071 0.033 0.024 0.562 0.528 Ti 5267 5972 6139 4028 1952 1326 12625 11341 Zr 73 80 85 42 22 14 172 142 1.08 1.07 1.07 1.05 1.08 0.98 2.53 3.65 La/LuCI Total REE 44.06 49.17 52.90 25.42 10.89 8.57 112.25 108.66 Notes: “Formation” refers to the depositional package, pluton or dike swarm that the sample is related to; “Volcanic Setting” refers to the volcanic environment inferred using different geochemical and geological parameters; “Tectonic Setting” refers to the inferred tectonic setting of mafic rocks, based on Cabanis and Lecolle (1989); “TAS Rock Name” refers to the rock name based on the Total Alkalis versus Silica diagram from Le Maitre (1989); Major element oxides are in weight %; trace element data are in parts per million; Mg number = atomic Mg/Mg + Fe, where Fe = total Fe expressed as ferrous iron; this value is dimensionless La/LuCI includes elements normalized using values from Sun & McDonough (1995); this value is dimensionless Abbreviations: LOI = loss-on-ignition; n/a = not applicable; REE = rare earth elements; S&S = Smyk and Schnieders (1995); TAS = Total Alkalis versus Silica; - 39 24- �Proceedings of the 65th ILSG Annual Meeting - Part 2 Appendix 1, continued Sample Number Related Stop Rock Name Formation Volcanic Setting Easting (m) Northing (m) Notes TAS Rock Name Tectonic Setting SiO2 (wt %) 17SJM016B 17SJM016A 16SJM213A 16WM043A Stop 8 mafic volcanic rock Package A Stop 8 mafic volcanic rock Package A none lapilli tuff none crystal tuff none tuffaceous conglomerate none tuff like Stop 2 wacke Package A Package A arc volcanic arc volcanic arc volcanic B-C Disconformity arc volcanic Package D arc volcanic B-C Disconformity arc volcanic 478600 5404609 pillow core, Flow XI 478600 5404609 base of flow, Flow XII 512241 5409187 498541 5402882 clasts up to 5cm andesite dacite dacite 499867 5412118 with interbedded chert dacite 513955 5411351 n/a basaltic andesite transitional arc 514731 5408709 quartz and feldspar phenocrysts rhyolite calc-alkaline calc-alkaline calc-alkaline calc-alkaline calc-alkaline calc-alkaline 58.89 70.26 76.1 69.22 64.6 63.96 54.7 17SJM129C 17SJM168B 16WM027A n/a dacite TiO2 1.32 0.74 0.47 0.1 0.56 0.49 0.57 Al2O3 15.92 15.16 14.66 12.05 13.14 12.35 15.92 Cr2O3 0.027 0.022 <0.002 0.01 0.009 0.024 0.03 Fe2O3total 11.85 6.44 3.75 1.35 3.61 5.66 5.99 MnO 0.11 0.084 0.052 0.047 0.034 0.17 0.094 MgO 3.31 4.53 1 0.11 4.04 3.55 3.14 CaO 2.651 5.437 3.518 0.502 2.99 6.886 3.511 Na2O 2.38 3.63 4.27 0.45 2.82 2.19 4.03 K2 O 1.84 0.51 1.35 8.99 0.61 1.09 1.79 P2O5 0.12 0.174 0.113 0.017 0.145 0.108 0.179 LOI 5.62 5.33 1.28 0.85 2.23 2.35 1.04 Total 99.86 100.98 100.76 100.64 99.42 99.5 100.28 0.60 0.79 0.59 0.31 0.86 0.77 0.74 Th (ppm) 0.559 2.941 3.728 5.249 1.816 1.764 9.035 Nb 3.969 6.999 6.471 9.436 4.74 3.572 7.056 Ta 0.251 0.468 0.603 0.918 0.335 0.275 0.471 Ti 6481 5823 2781 563 3209 2842 3462 Mg Number Zr La/LuCI 86 175 187 167 133 86 158 3.29 13.81 6.25 5.12 6.33 9.55 19.71 Total REE 61.83 163.67 101.45 124.07 80.13 58.96 181.12 Notes: “Formation” refers to the depositional package, pluton or dike swarm that the sample is related to; “Volcanic Setting” refers to the volcanic environment inferred using different geochemical and geological parameters; “Tectonic Setting” refers to the inferred tectonic setting of mafic rocks, based on Cabanis and Lecolle (1989); “TAS Rock Name” refers to the rock name based on the Total Alkalis versus Silica diagram from Le Maitre (1989); Major element oxides are in weight %; trace element data are in parts per million; Mg number = atomic Mg/Mg + Fe, where Fe = total Fe expressed as ferrous iron; this value is dimensionless La/LuCI includes elements normalized using values from Sun & McDonough (1995); this value is dimensionless Abbreviations: LOI = loss-on-ignition; n/a = not applicable; REE = rare earth elements; S&S = Smyk and Schnieders (1995); TAS = Total Alkalis versus Silica; - 4025 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Appendix 1, continued Sample Number Related Stop Rock Name 17SJM147A 17SJM038A 15JW094A 17KA040A 15JW006A 16SJM171A 17SJM200A 17SJM033B Stops 4 and 5 granodiorite none granodiorite none granite none monzogranit e none tonalite none quartz syenite none porphyritic dike Terrace Bay pluton Foxtrap Lake pluton Santoy Lake pluton Steel River pluton Syenite Lake pluton porphyritic dikes 506412 5410351 dacite Crossman Lake batholith 488795 5423075 dacite 522826 5412786 rhyolite 507404 5412210 rhyolite 511259 5405636 dacite 487180 5415617 trachy-dacite 482667 5405457 dacite 64.15 69.74 71.6 70.18 none quartz monzodiori te Little Pic River pluton 524180 5413445 trachyandesite 61.77 64.34 67.92 67.67 TiO2 0.44 0.36 0.18 0.28 0.58 0.53 0.25 0.25 Al2O3 15.19 15.19 16.04 14.99 17.36 14.05 15.82 14.87 Cr2O3 0.017 0.006 <0.002 0.004 <0.002 0.02 0.006 0.022 Formation Easting (m) Northing (m) TAS rocktype SiO2 (wt %) Fe2O3 total 4.04 3.56 1.37 2 5.1 6.1 2.21 2.73 MnO 0.078 0.062 0.024 0.025 0.086 0.082 0.04 0.042 MgO 3.03 0.98 0.5 0.55 2.17 2.91 1.03 2.49 CaO 3.89 3.282 1.122 1.129 4.113 2.713 2.067 2.937 Na2O 4.57 4.09 6.31 5.23 4.67 2.84 4.95 5.58 K2 O 2.45 1.52 2.95 4.66 2.86 2.42 4.17 0.89 P2O5 0.256 0.13 0.085 0.127 0.292 0.138 0.136 0.099 LOI 1.46 1.19 0.81 0.81 0.72 3.81 Total 99.7 100.14 101.14 100.08 99.84 100.04 99.3 101.02 Mg Number 0.80 0.60 0.67 0.60 0.70 0.72 0.72 0.83 Th (ppm) 9.668 3.212 5.215 32.841 6.723 7.388 13.741 1.795 Nb 5.497 7.076 3.251 11.59 6.843 5.554 7.084 2.326 Ta 0.378 0.726 0.194 0.683 0.385 0.42 0.612 0.193 Ti 2551 2046 1106 1661 3396 3278 1477 1438 Zr 190 160 94 287 191 144 174 87 24.83 5.45 19.80 52.14 23.09 17.57 31.85 17.62 La/LuCI 3.39 Total REE 244.97 77.55 84.17 270.77 230.87 137.44 185.46 66.37 Notes: “Formation” refers to the depositional package, pluton or dike swarm that the sample is related to; “TAS Rock Name” refers to the rock name based on the Total Alkalis versus Silica diagram from Le Maitre (1989); Major element oxides are in weight %; trace element data are in parts per million; Mg number = atomic Mg/Mg + Fe, where Fe = total Fe expressed as ferrous iron; this value is dimensionless La/LuCI includes elements normalized using values from Sun & McDonough (1995); this value is dimensionless Abbreviations: LOI = loss-on-ignition; n/a = not applicable; REE = rare earth elements; S&S = Smyk and Schnieders (1995); TAS = Total Alkalis versus Silica; - 26 41 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Appendix 1, continued Sample Number Related Stop Rock Name Formation Easting (m) Northing (m) Notes TAS rocktype Tectonic Setting SiO2 (wt %) 16SJM178D 15SJM019B 15JW072E 16SJM119C 15SJM068B 17SJM006B 16SJM139A Stop 1 none Stops 1 and 4 Stop 4 S&S Stop 2A none none alkalic alkalic lamprophyre subalkalic subalkalic subalkalic subalkalic diabase diabase diabase diabase diabase diabase rift-parallel rift-parallel lamprophyre Biscotasing Marathon Matachewan Pigeon River alkalic dikes alkalic dikes dikes 515772 523556 512213 517970 506861 498906 506570 5408307 5407664 5405453 5406078 5402734 5412471 5407803 n/a trachytic n/a n/a n/a n/a n/a texture basalt trachy-dacite foidite basalt basalt basalt basalt continental continental intercontinental continental calc-alkaline calc-alkaline continental arc arc rift arc arc 46.11 60.78 29.55 49.74 47.65 50.08 48.02 TiO2 1.19 0.49 4.35 1.22 0.74 1.45 1.93 Al2O3 14.54 15.66 3.97 14.77 13.35 13.68 16.04 Cr2O3 0.02 <0.002 0.1 0.02 0.11 0.02 0.02 Fe2O3total 12.77 8.63 15.76 14.04 10.82 14.92 13.86 MnO 0.217 0.234 0.269 0.206 0.171 0.19 0.193 MgO 5.51 0.34 15.94 6.4 10.47 6.9 6.04 CaO 10.656 1.814 13.069 10.428 11.079 8.774 9.742 Na2O 3.22 6.41 0.1 2.27 1.71 2.81 2.75 K2 O 1.53 4.82 2.35 0.4 0.31 0.39 0.48 P2O5 1.099 0.077 0.876 0.118 0.117 0.098 0.214 LOI 2.89 0.82 12.61 0.7 3.33 1.5 0.54 Total 99.92 100.08 99.04 100.32 99.87 100.83 99.86 0.70 0.18 0.85 0.71 0.84 0.72 0.70 Mg Number Th (ppm) 9.282 56.416 8.508 1.138 0.496 3.17 1.669 Nb 63.339 >277 124.706 4.426 2.866 6.18 11.382 Ta 2.739 14.976 7.901 0.28 0.129 0.435 0.774 Ti 7279 2967 >25000 6976 4623 3630 11126 Zr 180 1041 375 80 62 148 160 26.63 19.55 53.01 2.16 4.20 11.19 3.50 La/LuCI Total REE 468.09 923.53 418.37 56.87 62.82 144.04 103.50 Notes: “Formation” refers to the depositional package, pluton or dike swarm that the sample is related to; “Tectonic Setting” refers to the inferred tectonic setting of mafic rocks, based on Cabanis and Lecolle (1989); “TAS Rock Name” refers to the rock name based on the Total Alkalis versus Silica diagram from Le Maitre (1989); Major element oxides are in weight %; trace element data are in parts per million; Mg number = atomic Mg/Mg + Fe, where Fe = total Fe expressed as ferrous iron; this value is dimensionless La/LuCI includes elements normalized using values from Sun & McDonough (1995); this value is dimensionless Abbreviations: LOI = loss-on-ignition; n/a = not applicable; REE = rare earth elements; S&S = Smyk and Schnieders (1995); TAS = Total Alkalis versus Silica; - 4227 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Field trip 4 - Geology of the Nipigon Area Philip Fralick Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada and Robert Cundari Resident Geologist Program, Ontario Geological Survey, Ministry of Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Introduction The Nipigon area, west of Terrace Bay, hosts Neoarchean, amphibolite-facies metamorphic and intrusive rocks of the Quetico Subprovince, as well as overlying Mesoproterozoic Sibley Group sedimentary rocks and Midcontinent Rift-related mafic intrusive rocks. This trip buids upon previous ILSG trips, namely Fralick et al. (2000) and Smyk and Kissin (2005). A lot of this guide is taken from them. The field trip begins on Highway 17 just north of Lake Helen focusing on metamorphosed clastic sedimentary rocks, their high-grade metamorphic equivalents and derived granitic rocks of the Quetico Subprovince (Stops 1-6; Fig. 1). The trip continues east of Nipigon highlighting Proterozoic sedimentary rocks of the Sibley Group and intrusive mafic rocks related to the Midcontinent Rift (Stops 7-10; Fig. 1). Many stops, especially Stop 1 through 6, are on road cuts along a narrow section of highway. Please use extreme caution when viewing road side outcrops. Regional Geology - Quetico Subprovince The Quetico Subprovince of the Superior Province is situated between the Wabigoon and Wawa volcanoplutonic subprovinces that bound the Quetico on its northern and southern margins, respectively. This east- Figure 1: Geology of the Nipigon area field trip stop locations. - 43 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 trending subprovince has a fairly consistent width of 70 km and is composed predominantly of metasedimentary rocks and their migmatitic and anatectic derivatives (Fig. 2; Williams, 1991). In general, the boundaries of the Quetico, whether or not they may be primary and/or tectonic, have been mapped as steeply dipping surfaces across which there is commonly a distinct contrast in lithology. However, LITHOPROBE deep seismic has shown that the Quetico and Beardmore-Geraldton volcanic and sedimentary belts of the Wabigoon Subprovince are thrust over the volcanic rocks of the Onaman-Tashota terrane of the Wabigoon Subprovince at an angle of 12 degrees (Geller, 2012). This agrees well with prior interpretations of the Quetico as an accretionary prism (Devaney and Williams, 1989) and the Beardmore-Geraldton area as its associated forearc basin (Barrett and Fralick, 1989; Fralick et al., 1992) that were thrust northward onto the Wabigoon arc during Wawa arc collision. An overview of the lithologic, metamorphic, structural and tectonic characteristics of the Quetico Subprovince has most recently been provided by Easton (2000): “The intensity of metamorphism varies within the subprovince, such that rocks marginal to the subprovince tend to be at lower grade than in the interior. The lowest metamorphic grade is found along the northern boundary with the Wabigoon subprovince (Pirie and Mackasey, 1978). Locally, subgreenschist- to greenschistfacies rocks occur along the southern boundary (Borradaile, 1982), but typically, there is a rapid rise in metamorphic grade north of the Wawa subprovince, especially north of Manitouwadge, where a belt of metasedimentary granulites occurs within the Quetico subprovince close to, and parallel with, the northern margin of the Wawa Subprovince (Coates, 1968; Williams and Breaks, 1989, 1990; Pan et al., 1994). As a result, grade distribution is asymmetrical, with the maximum in temperature and pressure occurring south of the central Quetico, locally coincident with the southern margin.” In contrast, Seemayer (1992) also described an asymmetric metamorphic grade distribution that had metamorphic grade increasing from south to north across the Quetico, southwest of Lake Nipigon. The southern margin was characterized by greenschist-facies rocks, the central portions were at amphibolite facies and the northern margin displayed an abrupt decrease in grade Figure 2. Generalized regional geology of the Quetico Subprovince (after Williams, 1991). - 44 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 adjacent to the Wabigoon Subprovince to the north. The Quetico Fault, which is normally situated at the northern margin of the Quetico in this western area, lies well within the Quetico southwest of Lake Nipigon (Seemayer, 1992). Seemayer (1992) determined temperatures based on garnet-biotite thermometry ranging from 526°C at the southern margin of the Quetico Subprovince, increasing asymmetrically to a maximum of 714°C, then falling sharply at the northern margin to 517°C. Pressure calculated at near peak temperature was 5 ± 1.5 kbar. Easton (2000) described the regional metamorphic conditions and metamorphic history: “P–T conditions increase from west to east, for example, 500ºC and 2.5 kbar at the Minnesota border west of Thunder Bay (Percival, 1989), to 700–780ºC and 5.4–6.1 kbar adjacent to the Kapuskasing structural zone (Percival and McGrath, 1986; Percival, 1989). Typical conditions in the central region are on the order of 620ºC and 3.3 kbar (Percival, 1989). Granulites north of Manitouwadge yield 680–770ºC and 4.4–6.4 kbar (Pan et al., 1994; Percival, 1989). The regional variation in P–T can be ascribed to a relatively shallow level of erosion in the west (<10 km) and a deeper level in the east (>12 km) (Percival et al., 1985). Rocks located east of the Kapuskasing structural zone are believed to be generally at upper-amphibolite-facies conditions (Williams, 1991).” facies being structurally controlled within thrustbounded panels (Williams, 1991).” “In contrast, the main phase of regional metamorphism (M2), which produced the observed map-pattern (Fig. 2), occurred late syntectonically (Sawyer, 1983; Williams, 1991). The general sequence of isograds, based on the appearance of diagnostic assemblages in pelites, is Chl–Ms–Bt, Grt+And+Sil, Grt+Crd+Sil, in situ granitic leucosome, and Opx (Pirie and Mackasey,1978; Percival and Stern, 1984). The common occurrence of Grt–And in metapelites in the western Quetico subprovince is diagnostic of bathozone 2 (<3.4 kbar; Carmichael, 1978), whereas the presence of Sil–St in the eastern Quetico is diagnostic of bathozones 3 and 4 (3.4– 5.5 kbar).” “As noted by Williams (1991), tectonic thickening of the sedimentary pile and intrusion of minor I-type granitic rocks occurred prior to the thermal acme. Most of the large pre- to syntectonic granitic bodies are peraluminous and have sedimentary sources but display little evidence of thermal contact metamorphism; one exception is the South Beatty Lake pluton in the northern Quetico subprovince (Pirie & Mackasey, 1978). Steeply dipping thermal gradients, local increases in temperature around large plutons, and the general association of the highest-grade rocks with abundant generation of leucosome, indicate that the source of heat was a combination of burial, upward magmatic transport, and tectonism.” “Evidence for an earlier, medium-pressure, low-temperature, pre-tectonic or early syntectonic metamorphism comes from four areas within the subprovince. In the Atikokan region, and in northern Minnesota, both at the northern margin of the Quetico subprovince, an early M1 metamorphic peak between D1 and D2 produced Ky–St–Bt assemblages (Ayres, 1978; Tabor et al., 1989). Kyanite inclusions in plagioclase within Grt–Sil–Bt–Pl–Qtz schist near Raith, north of Thunder Bay, have been reported by Percival et al. (1985). Kehlenbeck (1976) also presented textural evidence for a polymetamorphic history along the northern margin of the Quetico subprovince north of Thunder Bay. Again, along the northern margin of the subprovince, in the Beardmore–Geraldton area (Williams, 1989), amphibolite-facies conditions were attained prior to D2 deformation, with the distribution of “In the northern Quetico, M1 metamorphism is estimated to have occurred between 2698 Ma, the maximum age of sedimentation (Davis et al., 1990) and 2688+4 Ma, the age of emplacement of the late syntectonic Blalock pluton (ibid). In the southern Quetico, M1 occurred after 2690 Ma, the maximum age of sedimentation (Zaleski et al., 1999). The timing of M2 metamorphism is less well constrained and may have been protracted. In the Manitouwadge area, (ibid) constrained regional D2 deformation to 2680– 2677 Ma and suggested that migmatization in both the northern Wawa and southern Quetico subprovinces occurred after 2679 Ma, broadly coincident with D3 deformation. This inference is consistent with observations elsewhere in - 45 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 the Quetico that contact aureoles around late plutons, dated at 2671±2 to 2665±2 Ma, as well as late granitic pegmatites dated at 2653±4 Ma, overprint regional metamorphic fabrics (Percival and Sullivan, 1988; Percival, 1989). North of Manitouwadge, Pan et al. (1998) reported a U–Pb zircon age of 2666±1 Ma from a granitic pegmatite concordant with respect to the D3 fabric, and suggested that the regional amphibolite-facies metamorphism occurred between 2671 and 2665 Ma, consistent with the ages cited above. The timing of peak granulite-facies metamorphism north of Manitouwadge appears to be some 15 Ma younger, on the basis of U–Pb zircon ages of 2650± and 2651±3 Ma from a mafic granulite and a tonalitic leucosome, respectively (ibid). Zaleski & van Breemen (1997) reported that titanite ages young with increasing metamorphic grade, ranging from ~2686 Ma in the southern Manitouwadge greenstone belt to ~2640 Ma in the southern Quetico, suggesting that the thermal effects of regional metamorphism may have lasted over ~30 million years, from 2677 to 2640 Ma, in the higher-grade parts of the Wawa and Quetico subprovinces. On the basis of their regional geological and geochronological studies, Zaleski et al. (1999) concluded that “M2 metamorphism occurred after the tectonic juxtaposition of the Quetico and Wawa subprovinces.” (14 to 15 km deep in the crust; Easton, 2000; Percival, 1989). The nomenclature of Mehnert (1968) has been used in describing migmatites in this field guide. Regional Geology - Sibley Group The Mesoproterozoic, 1.4 Ga, Sibley Group crops out in a ~175 km wide by 400 km long ovoid under Lake Superior extending north to the south and west of Lake Nipigon with a minimum thickness of 950 m inferred from drill core (Rogala et al., 2007). The group is a predominantly flat-lying red-bed sequence which can be broken up into five lithological units; Pass Lake Formation, Rossport Formation, Kama Hill Formation, Outan Island Formation and Nipigon Bay Formation (Fig. 3). The following is mainly based on research conducted and published on by Becky Rogala and Riku Metsaranta: This field trip will cover the southern half of the Quetico Subprovince, from south of Beardmore to Nipigon (Fig. 1). As mentioned above, there is an asymmetric distribution in metamorphic grade, with a gradual progression from greenschist-facies, clastic metasedimentary rocks near the northern contact with the Wabigoon Subprovince; to lower amphibolite-facies schists and gneisses; through to upper amphibolitefacies migmatites and derived granitic rocks near the southern contact with the Wawa Subprovince near Nipigon. Thermal and pressure maximum occurs south of the center of the Quetico. The metamorphic character is of high-temperature/low-pressure (Abukuma-type) metamorphism, associated with the abundant intrusion of granitoid rocks and the regional distribution of migmatites derived from the metasedimentary rocks (Kamineni et al., 1988; Percival and McGrath, 1986; Percival, 1989; Williams, 1989). Peak metamorphic assemblages in the field trip area suggest conditions >650ºC and 5 kbar, corresponding to bathozones 4 to 5 The Pass Lake Formation consists of two members; the basal Loon Lake Member conglomerates and the overlying Fork Bay Member sandstones. The basal conglomerates are typically only a few metres thick with a maximum 15 m thickness in topographic lows within the basement rock. Conglomerates of the Loon Lake Member begin the large-scale, thinning-upward succession of the lower portion of the group and are dominantly composed of associated basement material (Rogala et al., 2007). They were deposited in channels eroded into the basement below braided streams in a semi-arid environment as highlighted by dolocrete horizons in floodplain sediment. Lower energy streams deposited trough cross-stratified sandstone. In places these materials were reworked into cobble-pebble beach deposits as a lacustrine system to the south expanded northward. With transgression fining- and thinning-upwards successions of sheet sandstone were deposited offshore from river mouths, while in areas with higher sediment supply deltaic forced regression occurred. With time the location of the lacustrine system in an area of internal drainage resulted in saline conditions developing. Alternating dolomitic red and light grey thin layers of the Channel Island member, Rossport Formation, attest to cyclic changes in organic productivity leading to more organic material in the bottom sediment reducing the oxidized iron. Higher hematitic clay contents in the red layers may be the - 46 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 3. Lithostratigraphy, depositional environments and climate during Sibley Group Sedimentation (from Rogala et al., 2007). result of sediment water influx leading to more rapid deposition and therefore less organic sediment. Tectonic instability brought this phase of sedimentation to an end by flooding the lacustrine system with sheet sandstone probably derived from the south because of north down-tilting of the terrain (Rogala et al., 2007). Strandline stromatolitic dolomite of the Middlebrun Bay Member was laid down as the lake shrunk. Subaerial exposure caused the development of karst topography, terra rosa and other types of soil horizons on the dolomite. Intrabasin mass flows composed of clasts from underlying Sibley lithologies commonly developed during this time period. With the end of widespread lacustrine conditions the water table remained close to the surface resulting in the precipitation of gypsum and carbonate in extensive mudflats of the Fire Hill Member, Rossport Formation. Saline ponds with gypsiferous stromatolites and teepee structures developed in lower areas. Higher mudflats were too dry for evaporates to form. This is the end of continuous sedimentation in the lower Sibley Group. A time gap of unknown extent separates the underlying playa system from overlying deltaic - 47 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 deposits of the Kama Hill and Outen Island Formations (Rogala et al., 2007). The Kama Hill Formation represents a ~50 m thick wave-rippled and hummocky cross-stratified sandstone and mudstone package which abruptly overlies the upper Fire Hill Member of the Rossport Formation (Rogala et al., 2007). Paleocurrent indicators suggest material comprising the Kama Hill Formation was derived from the south to southeast with the unit being thickest in the central portion and thinning toward the east (Cheadle, 1986). The Kama Hill Formation is dominated by horizontally laminated siltstone and fine-grained sandstones with interbeds of mudstone and ripple-laminated, fine-grained sandstone (Rogala et al., 2007). It represents prodelta and distal bar deposits. The Outan Island Formation consists of two members; the lower Lyon Member including coarsening- and thickening-upward sandstone successions overlain by siltstone and ripple laminated sandstone and the Hele Member consisting of a fining-and thinningupward sandstone succession overlain by mud-cracked siltstone (Rogala et al., 2007). The Lyon Member represents coarsening- and thickening-upwards, sandy distributary mouth bars overlain by the Hele Member fluvial system with extensive floodplains deposited in a non-arid climate (Fralick and Zaniewski, 2012; Ielpi et al., 2018). The coarsening upwards delta lobes are on the same scale as the modern Mississippi giving the impression that this was a substantive drainage system. A slight angular unconformity exists at the top of the Outan Island Formation with the overlying Nipigon Bay Formation. Four hundred and fifty meters of Nipigon Bay sandstones make up half of the Sibley Group. They were deposited sometime between 1.4Ga and 1.1 Ga. This Formation represents large Aeolian dunes developed in an arid setting. Franklin et al. (1980) originally inferred the Sibley Basin to be a result of subsidence caused from the ~1.1 Ga Midcontinent Rift system. This was called into question with age constraints on the Sibley Group, based on U-Pb geochronology of detrital zircons and stratigraphy, less than 1440 Ma for the entire Sibley Group (Rogala et al., 2007). The basal conglomerate of the Osler Group, erosively overlying the Nipigon Bay Formation, gives a lower age constraint of 1109 Ma (Davis and Sutcliffe, 1985) for the Nipigon Bay, whereas the other formations have a tighter lower limit of 1339±33 Ma derived from diagenetic Sr geochronology (Franklin et al., 1980). Cheadle (1986) noted intercalation of English Bay Complex rhyolites with Sibley sandstones, which together with geochronology, debunks the Sibley Group as being of MCR affinity and lends the notion that the succession was in fact 250-350 m.y. older than the MCR. The current model holds that the Sibley Group was deposited in a half graben-controlled basin (Rogala et al., 2007) inferred to be a product of large-scale thermal subsidence following the ~1550 Ma thermal plume event which produced the English Bay Complex (Hollings et al., 2004). Regional Geology - Midcontinent Rift Mesoproterozoic intrusive, volcanic and minor sedimentary rocks associated with the MCR collectively constitute the Keweenawan Supergroup. On the northern margin of the MCR, Keweenawan rocks include a variety of intrusive rocks and Osler Group volcanic rocks, which represent some of the earliest magmatism in the MCR. Ages range from ca. 1140 Ma (Heaman et al., 2007) to ages younger than the magnetic polarity reversal that occurred between 1105 and 1102 Ma (Davis and Green, 1997). The majority of mafic and ultramafic rocks in the Lake Nipigon and northern Lake Superior areas, including the Nipigon and Logan sills, appear to have been emplaced in a short, magnetically reversed, interval between ca. 1115 and 1100 Ma (Heaman et al., 2007). Emplacement of alkalic intrusions, such as the 1108 Ma Coldwell Complex (Heaman and Machado, 1992), and filling of much of the submerged part of the rift in Lake Superior, also occurred in this period. This was followed by a period of magnetically normal, waning mafic and felsic magmatism, between 1096 and 1085 Ma, that is preserved mainly along the Lake Superior shore by units such as the Crystal Lake (1099±1 Ma), Moss Lake (1095±2 Ma) and Blake (1095±2 Ma) gabbros, and a Pigeon River dyke near Arrow River (1093±3 Ma; Heaman et al., 2007). Hypabyssal Mafic Rocks Diabase sills, extending from the vicinity of Thunder Bay to east of Lake Nipigon, represent the northern remnants of the Midcontinent Rift, and have previously been referred to as the Logan sills (Stockwell et al., 1972). However, a geochemical difference has been noted between the sills to the north and south of the City of Thunder Bay (Hart, 2003; Hart et al., 2005). Hollings et al. (2007a) proposed that the - 48 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 term Logan Igneous Suite, which would fall within the Midcontinent Rift Intrusive Supersuite (Miller et al., 2002), should be applied to all the diabase sills in the area north of Lake Superior, with subdivision into the informal terms, Nipigon sills for the sills north of Thunder Bay, and Logan sills to the south. Nipigon Sills Nipigon sills are commonly massive, medium- to coarse-grained, olivine-tholeiitic diabase/gabbros (Sutcliffe, 1989; Hart and MacDonald, 2007). Nipigon sills are dominantly present throughout the Lake Nipigon area but have also been recognized in the Thunder Bay area (Hollings et al., 2007b). Nipigon sills are characterized by a massive, subophitic to ophitic, plagioclase and clinopyroxene texture with trace to 3% olivine and 1-2% modal magnetite (Hart et al., 2005). Nipigon sills display a reverse magnetic polarity and generally form thick, columnar jointed sheets. Sills commonly intrude Sibley group sedimentary rocks but also can be found in contact with Archean rocks of the Quetico subprovince and the Marmion and Winnipeg River terranes. Sills often intrude earlier emplaced ultramafic units of the Nipigon Embayment as well as the 1129.0 ± 2.3 Ma Pillar lake Volcanic rocks and the 1546.5 ± 3.9 Ma English Bay Complex (Heaman et al., 2007) providing evidence for their emplacement during the second main phase of magmatism (Hart and MacDonald, 2007). The shallow dipping Nipigon diabase sills are estimated to cover an area in excess of 20,000 km2 (Sutcliffe 1991) ranging in thickness from <5 m to >180 m (Hart and Macdonald, 2007). Stops Stop 1 – Glacier Lake Batholith Leucogranite UTM coordinates 0409911E 5445897N Large road cuts on both sides of Highway 11 provide excellent exposures of the Glacier Lake Batholith (GLB). At this location, it consists of white, massive to locally foliated, muscovite < biotite, medium- to coarsegrained granite. Localized pods of tourmaline-biotitemuscovite- potassium feldspar pegmatite occur within the granite. These pods may reach 1 m in diameter and locally contain tourmaline-quartz intergrowths. Numerous, curvate, fibrolite-muscovite+tourmaline veins up to 1 cm thick also occur in the host granite. Purple fluorite is exposed on a fractured outcrop face on the west side of the highway (Fig. 4). Field Trip Road Log Stop Locality Terrace Bay to Nipigon Lake Helen stops Intersection of Hwy.’s 11 &17 in the Town of Nipigon Take Hwy 11 north 1 Glacier Lake Batholith 2 Biotite leucogranite 3 Migmatite - roadside rest area Pull-off area 4 Pegmatitic granite 5 Pegmatitic granite 6 Pegmatites in migmatite Nipigon Stops Intersection of Hwy.’s 11 &17 in the Town of Nipigon Take Hwy 17 east 7 Stendlund Barite-Amethyst 8 Ruby Lake O1 Polygonal Diabase O2 Migmatites 9 Kama Hill 10 Unconformity at Gurney O3 Sunrise-Sunset Fluorite - 49 - km 105 km 0 17.1 10.8 7.4 5.9 5.2 4.85 4.6 0 6.8 (4.1 km into site) 18.4 21.4 39.9 �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 4. Fluorite on fracture surface in sericite- and fibrolitebearing Leucogranite (Stop 1). Stop 2 – Glacier Lake Batholith Leucogranite/ Migmatite UTM coordinates 0407163E 5440408N This is another example of the southern margin of the Glacier Lake Batholith where it is in contact with metasedimentary migmatite (Fig. 5). White, biotite-muscovite leucogranite contains rare, bluegreen apatite. Foliation is developed in feldspathic segregations and biotitic seams. Leucogranite dykes and migmatite locally exhibit a lit-par-lit structure. Tight to isoclinal folds have developed in the quartzbiotite-feldspar schist (Fig. 6). All rocks display boudinage and folding. Folded leucosome suggests an early and protracted deformation history (Fig. 7). Metasedimentary migmatite enclaves are common in the white, S-type pegmatitic granites along the highway. Note that less evolved S-type granite may contain biotite as the only mica. Narrow diabase Figure 5. Contact between folded metasedimentary migmatite and Leucogranite (Stop 2). Figure 6. Folded metasedimentary migmatite (Stop 2). dykes cut the country rocks on the western side of the highway. Figure 7. Folded metasedimentary schist, leucosome development (Stop 2). - 50 - incipient �Proceedings of the 65th ILSG Annual Meeting - Part 2 Stop 3 – Migmatite UTM coordinates 0406986E 5437331N (Rest Area; Coffee) Migmatites are exposed on the shore and islands of Lake Helen and on a large, glacially streamlined, whaleback outcrop behind the highway rest stop/ pulloff area. There is quite a bit of variation in the relative amounts of leucosome and restite (Fig. 8). Boudinaged and pytgmatically folded leucosome pods and veins occur in a quartzo-feldspathic matrix with narrow, biotitic restite septa. Figure 9. Pegmatitic K-spar - quartz – biotite granite (Stop 4). 10). The megacrystic, foliated granite may represent an earlier, xenolith-bearing phase that was intruded by the massive granite while still relatively warm and plastic. Figure 8. Lit-par-lit migmatites with large metasedimentary inclusions (schollen) (Stop 3 area). Stop 4 – Pegmatitic Granite UTM coordinates - 0407272E 5435208N This outcrop shows a transition in magmatism from S-type (i.e. Glacier Lake Batholith) to I-type granitoids. Pink, coarse-grained, biotite granite here consists of pink, coarse-grained, perthitic K-feldspar, quartz, coarse-grained biotite, and brown, altered plagioclase (up to 4 cm long). These weakly peraluminous, pegmatitic, biotite granites are relatively primitive and are younger than the white, two mica leucogranites. Such rocks are probably of I-type origin and typically are metasedimentary enclave free. Bulk rock levels of rare-elements are very low: 128 ppm Rb, 2.63 ppm Cs, 1.8 ppm Nb, 0.39 ppm Ta (F. Breaks, OGS, personal communication, 2004). A K-feldspar-megacrystic, pink biotite granite (Fig. 9) with a shallowly eastdipping foliation and metasedimentary xenoliths is intruded by a more massive granite at this locality. The intrusive contact is embayed and scalloped, suggestive of co-mingling magmatic textures (Fig. Figure 10. Scalloped contact (arrow) between foliated biotite granite (top) and pegmatitic granite (bottom) (Stop 4). Stop 5 – Pegmatitic Granite UTM coordinates - 0407458E 5434906N A large, bare, whaleback outcrop on the east side of the highway consists of coarse-grained to pegmatitic, massive, homogeneous pink granite (i.e. the younger granitic unit at Stop 4; Fig. 11). Crystals or interstitial patches of quartz, biotite and locally sericitized K-feldspar average 2 to 3 cm in size (Fig. 12). Individual feldspar megacrysts may attain lengths of over 60 cm. Stop 6 – Pegmatite Dykes in Migmatite UTM coordinates 0407587E 5434690N Approximately 200 m south of the massive pink granite, white pegmatite dykes intrude fine-grained - 51 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 11. “Whaleback” outcrop of coarse-grained to pegmatitic granite; note large (>60 cm) feldspar megacryst (circled) (Stop 5). metasedimentary schist. This dark, fine-grained, feldspathic, biotite schist displays a well-developed foliation and minor folds, indicative of a long, protracted ductile deformation history, perhaps coeval with dyke emplacement. Note the sub-horizontal mineral lineation. Flattening of feldspars, producing small-scale, augen structures, is also indicative of hightemperature (>500º C) deformation. The boudinaged and annealed, sericite- and biotite-bearing dykes range from a few centimetres (lit-par-lit structured) to several metres in thickness. They are mineralogically and texturally interesting, containing both cordierite and quartz-tourmaline intergrowths with alkali (locally sericitized) feldspar. Garnet is conspicuous by its absence. Breaks et al. (2003) described potassic, biotite pegmatite that grades into a medium-grained, biotite granite near this stop (UTM: Easting 408339; Northing 5433016). The pegmatite contains coarse, euhedral Figure 13. Quartz-tourmaline intergrowth in pegmatite dyke (Stop 6). Figure 12. Coarse-grained to pegmatitic granite (Stop 5). K-spar; quartz; prismatic, medium- to coarse-grained, black tourmaline (schorl-dravite; Fig. 13); and fine grained, green and blue fluor-apatite (0.3 to 0.9 weight % MnO). A biotite-rich, metasomatic contact occurs between the biotite granite and diabase. Graphic, coarse-grained cordierite (<2 cm) and tourmaline occur in the granite near a diabase contact. Lunch Stop – Nipigon Marina UTM coordinates 0408076E 5429202N (Rest Area) Stop 7 – Stenlund Amethyst- Barite Occurrence UTM coordinates 0413918E 5429779N The property is underlain by flat-lying Sibley Group sedimentary rocks of the Lower Rossport Formation. Brick-red to orange muddy dolomite are interbedded with buff-coloured units. Small beige reduction spots occur in the red, hematite-rich dolomites. In contrast to this early reduction of hematite by flecks of organic matter late alteration (reduction of the ferric iron) occurs along the joints. Veins are exposed in two locations, 60 m and 170 m north of Highway 17. At the occurrence nearer the highway, 0.5 to 6.0 cm wide quartzbarite+/-amethyst veins occupy a parallel fracture set apparently controlled by a dominant set at 070°-085° SE. Vein breccias up to 20 cm wide and larger cavities and vugs appear to have developed preferentially in a sandier unit which locally overlies the red dolostone. Brecciated rock fragments are grey and green in some cases, possibly due to the vein alteration. Small, parallel quartz-barite filled fractures and veinlets, also striking 070°, occur 110 m north of the occurrence. Pieces of barite, baritic vein and breccia float are abundant in the vicinity. However, their source was not found. The - 52 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 flat relief and lack of outcrop necessitates extensive overburden stripping. Colorless, smoky and amethystine quartz and rare citrine occur with white and pink, bladed to massive barite (Fig. 14). Drusy quartz is commonly associated with barite seams. Lavender to deep purple amethyst crystals with pyramidal terminations are 0.2 to 15 mm across at their bases. Hematite has been reduced to an unspecified mineral. Figure 15. Extracted blocks of multicoloured and banded Ruby Lake marble (Stop 8). quarry (Hinz et al., 1994). Calcite, dolomite, epidote and opaque minerals were noted in thin section by Hinz et al. (1994) from the Nipigon River quarry. A number of chemical analyses for Nipigon River marble (wt.%) were also provided (Table 1). Figure 14. Drusy white quartz and amethyst with barite (Stop 7). Stop 8 – Ruby Lake Marble Quarry UTM coordinates 0414973E 5426071N Variegated, multicoloured, banded marble has been quarried here for landscaping stone (Fig. 15). Approximately 175 tonnes of marble were quarried and shipped in 1998 (D. MacAlpine, personal communication, 1999). The dimensions of the largest, transported block was 1.8 by 0.75 by 0.50 m (1.8 tonnes; ibid). Approximately 398 tonnes of marble were quarried and shipped in 1999 (D. MacAlpine, pers. comm., 2000). This marble consists of contact metamorphosed, Mesoproterozoic, Rossport Formation dolostone and other, calcareous sedimentary rocks in the contact metamorphic aureole of Midcontinent Rift-related Nipigon diabase sills. It has previously been termed Nipigon River marble and was quarried from 1883 to ca. 1910 at a site on the eastern side of the Nipigon River, approximately 6 km west of the Ruby Lake Shallow exploration trenches on the side of the road leading to the top of Ruby Mountain (i.e. top of the upper sill) have exposed copper-mineralized marble. Fine-grained, disseminated blebs of native copper (0.1 to 1.0 mm) occur along calcite-coated, hairline fractures parallel to bedding planes. Mineralized fractures are most easily recognized where secondary (supergene) malachite has formed. The top of the adjacent (middle?) sill is exposed farther along the road (optional stop 2). Similar, copper-mineralized, calcareous units have been noted near a diabase sill contact at Hughes Point, 2.5 km to the south-southwest by Schnieders et al. (1996). At this location, 2 to 5 cm wide calcite veins contain disseminated covellite (after chalcocite?) and malachite. The orientation of the veins are roughly parallel to joints developed in the adjacent sill. Grab samples have returned up to 3.065% Cu, nil Au and nil Ag (ibid). Franklin (1970) described copper-mineralized stromatolitic units near Disraeli Lake. A variety of copper minerals, including digenite, cuprite, covellite, Table 1. Chemical analyses from the Nipigon River Marble. From Hinz et al. (1994). Sample 89MCK-09 89MCK-10 SiO2 35.21 29.19 TiO2 0.20 0.10 Al2O3 8.20 5.20 Fe2O3 2.04 2.04 FeO 1.53 0.00 - 53 - MnO 0.05 0.03 MgO 23.56 27.63 CaO 26.74 35.32 Na2O 0.00 0.00 K2O 2.38 0.41 P2O5 0.09 0.07 �Proceedings of the 65th ILSG Annual Meeting - Part 2 chalcopyrite, native copper and malachite, were identified as open-space fillings in the vuggy host rock (ibid) suggested that the copper was introduced epigenetically from a gabbro to peridotite plug that intruded these Sibley Group rocks. The close spatial association of copper mineralized rocks with diabase sill contacts in the vicinity of Ruby Lake supports this theory. Alternatively these deposits are very similar to those in the Zambian copper belt where organic matter in the stromatolitic mats reduced copper travelling in groundwater forming large syngenetic ores. The marble is stromatolitic, though this is difficult to ascertain in most places due to the lack of pinnacles. The stromatolites are best seen on the tops of beds where they protrude, causing the contact to become wavy. Hints of the presence of stromatolites are visible, giving the impression that a large amount of the horizontal layering is stromatolitic S-mat (smooth mat that has a crinkly appearance). Non-crinkly, commonly lighter coloured layers interlaminated with the S-mat are storm layers of dolomitic silt washed in by wave activity. This sequence was deposited in a strandline proximal position in the playa, as denoted by the presence of teepee structures in this horizon at other locations and lacustrine deposits below the stromatolites and sub-aerial deposits above. This infers that lake size had stabilized during this interval, eliminating the large-scale fluctuations in shoreline positioning. Optional Stop 1 – Polygonal Jointed Diabase Sill UTM coordinates 0414746E 5426684 N This optional stop is accessed via a steep, rough trail ~500 m northwest from the turnoff to the Ruby Lake Marble quarry to the top of Ruby Mountain. Exposed here is an excellent exposure of the chilled, upper margin of a Nipigon diabase sill. The diabase is massive, homogeneous, fine- to medium-grained and locally feldspar-phyric. The large pavement outcrop displays polygonal cooling joints (seen in plan view) also referred to as “tortoise-shell” texture (Fig. 16). Figure 16. Polygonal jointed (“Tortoise-shell” texture) diabase (Optional Stop 1). disrupted (Fig. 17). There is a relatively high proportion of quartzo-feldspathic neosome matrix to the xenoliths, suggesting high(er) degrees of partial melting. Patches and dykelets of coarse-grained, biotite-quartz-feldspar neosome contain garnet, cordierite and large (>10 cm), euhedral green apatite crystals. Stop 9 – Kama Hill UTM coordinates 0425272E 5428131N Diabase-capped Kama Hill provides an excellent roadside exposure of the Rossport Formation (Fig. 18). The sequence is dominated by interlaminated shaley dolostone and dolomitic red shale with layer thicknesses ranging from millimeters to ten centimeters. There is some cyclicity in layer thickness variation up through the sequence, but it is not a strong trend. Some carbonate-dominated layers contain Optional Stop 2 –Migmatites UTM coordinates 0425006E 5429614N This optional stop displays schollen (raft)-structured migmatites in which folded and schlieric, mafic, paleosome xenoliths have been highly deformed and Figure 17. Schollen structure in migmatite (Optional Stop 2). - 54 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 18. View looking northwest at Kama Hill. Lower outcrop is Rossport Formation. Upper cliff is a Nipigon diabase sill (Stop 9). coarse- to medium-grained sand grains. Desiccation cracks are present, but not common. The central areas of some thicker more dolomitic (lighter-coloured) layers contain molds where gypsum crystals have weathered out. The alternation between red, more clay-rich layers, and gray, more dolomitic layers, represents climatic fluctuations over large time periods, as secular paleomagnetic trends measured in core of this facies indicates that the average meter of sediment took thousands of years to be deposited (Rogala, 2003). This compares well with modern playa systems and is reasonable as most of the sediment is dolomite formed from precipitation of HCO3-, Ca+2 and Mg+2 from solution in inflowing water. The lake was not totally drying up so there was always a standing body of saline water for the new fresh water to mix with. This lowering of salinity should have caused dissolution prior to evaporation over time causing more precipitation. See if you can see evidence of this. Ca ratio in the lake water and may have been the result of the precipitation of gypsum in the central lake. However, this is unlikely as higher salinities are expected in the shallower, marginal areas than the lake center, and thus, gypsum precipitation should be initiated in the shallows, producing shale-gypsumdolomite triplets. The lack of these means that gypsum precipitation was not necessary to increase the Mg/Ca ratio. The ambient ratio in the lake water itself must have been high enough for the precipitation of dolomite During rainy periods, water influx into the lake brought and deposited hematite-rich clays. In dry periods, evaporation from the internal drainage system resulted in lake contraction, hypersalinity and the precipitation of dolomite. This requires a high Mg/ Towards the top of the alternating layers (cyclic facies) thick sandstone sheets start appearing. These are the harbingers of tectonic rotation of the basin causing the basin to switch from northern drainage to southern drainage. Tension is recorded in sandstone Synsedimentary deformation manifests itself as small to large slump folds and brecciation of units in places. The sequence is intruded by diabase dykes which bake immediately adjacent sediments. Some evidence has been put forward that the diabase intruded watery sediment, but we have not seen clear indications of this. Structural controls on the emplacement of local sills were suggested by Antonellini and Cambray (1992). - 55 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 layers developing vertical joints along which they rotate as they extend. The sandstones also represent the dying phases of the lake. Overlying this succession is a chert-carbonate unit (Middlebrun Bay Member; Cheadle, 1986), which locally hosts stromatolites and hydrocarbons. It represents a shoreline microbialite similar to those present in some modern lakes. It is mostly composed of S-mat (crinkly layers) with extensive brecciation and silicification. The silicification is early, probably driven by rotting organic matter creating acidic conditions that favours carbonate replacement by silica. The brecciation is also quite early likely resulting from both dewatering of underlying muds having upward progress impeded by the impermeable microbialites and rainwater causing weakening of the layer through dissolution. A freshwater altered zone can be seen at the top of the dolostone. The siltstones and fine-grained sandstones immediately above the dolostone also contain stromatolites; one of the few places siliciclastic stromatolites can be seen in the rock record. As the lake shrank sub-aerial mudflats of the upper Rossport Formation were deposited on top of the dolostone. Pieces accessible in a pile of excavated talus display ripple marks, mudcracks and rare raindrop impressions. In many areas these siltstones and mudstones contain abundant gypsum, but it is only present in core as it readily weathers out of outcrops. Stop 10 – Unconformity at Gurney UTM coordinates 0440046 E 5419045 N The basal unconformity between the Sibley Group and underlying granitoids is exposed at this location (Fig. 19). A channel is visible, eroded into basement, and containing matrix-supported conglomerates overlain by cross-stratified sandstones. It represents episodes of subaerial debris flow activity interspersed with normal flash flood runoff. Archean granitic rocks are altered at the unconformity and likely represent a pre-Sibley weathered regolith. This weathered paleosol, noted by Gill (1926) and Moorhouse (1960), was locally described by Scott (1987). Friable, blotchy, red and green granite hosts quartz-carbonate veins between exfoliation blocks. Feldspars have been hematitized and/or destroyed; ferromagnesian minerals have been chloritized (ibid). Limited sampling of drill core from the Black Sturgeon Lake area cited by Scott (1987) suggests that this Figure 19. Unconformity between weathered Archean granite and Pass Lake Formation debris flows and sandstones, Highway 17 at Gurney (Stop 10). alteration may typically involve marked increases in Fe2O3, MgO, H2O) and decreases in Na2O, CaO and perhaps K2O. Paleomagnetic data suggest that this weathering was equatorial (G. Borradaille, unpublished data, 1999). As noted by Franklin (1978), Scott (1987) and Tanton (1948), a number of uranium occurrences are associated with altered Archean granitoids and overlying sedimentary rocks within the Sibley basin, prompting comparisons with the Athabasca basin in Saskatchewan. Favourable local parameters for supergene uranium deposits include: (i) uranium-enriched basement rocks (quartz monzonites, pegmatites); (ii) onlaps of basal Sibley sandstones on Archean paleotopographic “highs”; and (iii) Keweenawan(?) faults that extend to the basement (ibid). Optional Stop 3 – Sunrise-Midday Veins UTM coordinates 0454557E 5415513N The Sunrise vein is 18 m wide in the Highway 17 roadcut. It strikes approximately northeast and dips vertically with sharp contacts. Detailed mapping has not been successful in tracing this fluorite-bearing zone along strike. To the northeast, it is covered by alder swamp and glacial till. To the southwest, there is no outcrop along the strike of the vein. The Midday vein is about 5.8 m wide, strikes 74° and dips 68° northwest. It is located to the west of the Sunrise vein in the same roadcut. It extends into a swampy area to the west and pinches out 61 m east of the road-cut. It has a possible maximum strike length of 305 m. Both the Sunrise and Midday veins contain barite and fluorite, but very little amethyst. Brecciation in the - 56 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Midday vein is not as apparent as in the Sunset vein, although epidotization is very strong, giving the zones a greenish-yellow colour on fresh surfaces. The fluorite and barite occur as narrow veinlets along irregular fractures within these zones. Assay values are erratic due to the irregular nature of the mineralization. One section across the Sunrise vein assays up to 23.11% CaF2 over 3 m. References Antonellini, M.A. and Cambray, F.W. 1992. Relations between sill intrusions and bedding parallel extensional shear zones in the Mid-continent rift system of the Lake Superior region; Tectonophysics, v.212, p.33 1-349 Ayres, L.D. 1978. Metamorphism in the Superior Province of northwestern Ontario and its relationship to crustal development; in Metamorphism in the Canadian Shield, Geological Survey of Canada, Paper 78-10, p.25-36. Barrett, T.J. and Fralick, P.W., 1989. Turbidites and iron formation, Beardmore-Geraldton Ontario: application of a combined ramp/fan model to Archean clastic and chemical sedimentation. Sedimentology, v. 36, p. 221-234. Borradaile, G.J. 1982. Comparison of Archean structural styles in two belts of the Canadian Superior Province; Precambrian Research 19, p.179-189. Breaks, F.W., Selway, J.B. and Tindle, A.G., 2003. Fertile peraluminous granites and related rare-element mineralization in pegmatites, Superior Province, northwest and northeast Ontario: Operation Treasure Hunt; Ontario Geological Survey, Open File Report 6099, 179p. Carmichael, D.M., 1978. Metamorphic bathozones and bathograds: a measure of the depth of postmetamorphic uplift and erosion on the regional scale; American Journal of Science, 278, p.769-797. Cheadle, B.A.1986. Alluvial-playa sedimentation in the lower Keweenawan Sibley Group, Thunder Bay District, Ontario; Canadian Journal of Earth Sciences, v.23, p.527-542. Coates, M.E., 1968. Stevens–Kagiano Lake area; Ontario Department of Mines, Geological Report 68, p. Davis, D.W. and Sutcliffe, R.M., 1985. U-Pb ages from the Nipigon plate and northern Lake Superior; Geological Society of America Bulletin, v.96, p.1572-1579. Davis, D.W. and Green, J.C., 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of Earth Sciences, 34: 476-488. Davis, D.W., Pezzutto, F., & Ojakangas, R.W., 1990. The age and provenance of metasedimentary rocks in the Quetico Subprovince, Ontario, from single zircon analyses: implications for Archean sedimentation and tectonics in the Superior Province; Earth and Planetary Science Letters, v.99, p.195-205. Devaney, J.R. and Williams, H.R., 1989. Evolution of an Archean subprovince boundary: a sedimentological and structural study of part of the Wabigoon-Quetico boundary in northern Ontario. Canadian Journal of Earth Sciences, v. 26, p. 1013-1026. Fralick, P. W., Wu, J., and Williams, H.R., 1992. Trench and slope basin deposits in an Archean metasedimentary belt, Superior Province, Canada. Canadian Journal of Earth Sciences, v. 29, p. 2551-2557. Fralick, P., Smyk, M. and Mailman, M. 2000. Geology and stratigraphy of the Mesoproterozoic Sibley Group; Institute on Lake Superior Geology, 46th Annual Meeting, Thunder Bay, Ontario, Proceedings Volume 46, part 2, Field Trip Guidebook, p.1-41. Fralick, P.W. and Zaniewski, K., 2012. Sedimentology of a wet pre-vegetation floodplain assemblage. Sedimentology, v. 59, p. 1030-1049. Hart, T.R. 2003. Keweenawan mafic and ultramafic intrusive rocks of the Lake Nipigon and Crystal Lake areas, northwestern Ontario; 49th Institute on Lake Superior Geology, Proceedings volume 49, Part 1, Programs and abstracts, p.21-22. Hart, T.R., MacDonald, C.A., Hollings, P., and Richardson, A., 2005. Proterozoic intrusive rocks of the Nipigon Embayment and Midcontinent Rift. In, T.O. Tormanen and T.T Alapieti, 10th International platinum Symposium Extended Abstracts, Geology Survey of Finland, 365-368. Hart, T.R. and MacDonald, C.A., 2007. Proterozoic and Archean Geology of the Nipigon Embayment: implications for emplacement of the Mesoproterozoic Nipigon diabase sills and mafic to ultramafic intrusions. Canadian Journal of Earth Sciences 44: 1021-1040. Heaman, L.M., Easton, M., Hart, T.R., Hollings, P., Macdonald, C.A., and Smyk, M., 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario. Canadian Journal of earth Sciences 44: 1055-1086. Heaman, L.M. and Machado, N., 1992. Timing and origin of midcontinent rift alkaline magmatism, North America: evidence from the Coldwell Complex; Contributions to Mineralogy and Petrology, v. 110, p. 289-303. Hollings, P.N., Fralick, P.W., and Kissin S.A., 2004. Geochemistry and geodynamic implications of the Mesoproterozoic English Bay granite-rhyolite complex, northwestern Ontario. Canadian Journal of - 57 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 boundary in the De Courcey – Smiley Lakes area, northwestern Ontario; Canadian Journal of Earth Sciences, v.13, p.737-748. Earth Sciences, v. 44, p. 389-412. Hollings, P., Hart, T., Richardson, A., and MacDonald, C.A., 2007a. Geochemistry of the mid-Proterozoic intrusive rocks of the Nipigon Embayement, northwestern Ontario. Canadian Journal of Earth Sciences 44: 1087-1110. Hollings, P.N., Smyk, M.C., and Hart, T., 2007b. Geochemistry of Midcontinent Rift-related mafic dikes and sills near Thunder Bay: New insights into geographic distribution and the geochemical affinities of Nipigon and Logan sills and Pigeon River and other dikes. 53rd Institute on Lake Superior Geology, Annual Meeting, Proceedings volume 53, Part 1. Lutsen, Minnesota, May 2007, pp. 40–41. Ielpi, A., Fralick, P.W., Ventra, D., Ghinassi, M., Lebeau, L., Marconato, A., Meek, R., and Rainbird, R.H., 2018. Fluvial floodplains prior to greening of the continents: Stratigraphic record, geodynamic setting, and modern analogs. Sedimentary Geology, v. 372, p. 140-172. Mehnert, K.R., 1968. Migmatites and the origin of granitic rocks. Elsevier. 405p. Miller, J.D., Smyk, M.C., Severson, M.J., Lavigne, M.J., and Middleton, R.S. 2002. PGE occurrences in mafic intrusions around western Lake Superior, USA and Canada; 9th International Platinum Symposium, Field Trip Guidebook, 135p. Moorhouse, W.W. 1960. The Gunflint iron range in the vicinity of Port Arthur; Ontario Department of Mines, Annual Report, v.69, pt.7, p.1-40. Pan, Y., Fleet, M.E., and Williams, H.R., 1994. Granulite facies metamorphism in the Quetico subprovince, north of Manitouwadge, Ontario; Canadian Journal of Earth Sciences, v.31, p.1427-1439. Easton, R.M. 2000. Metamorphism of the Canadian Shield, Ontario, Canada I: The Superior Province; The Canadian Mineralogist, v.38, p.287-317. Pan, Y., Fleet, M.E., and Heaman, L.M., 1998. Thermotectonic evolution of an Archean accretionary complex: U–Pb geochronological constraints on granulites from the Quetico subprovince, Ontario, Canada; Precambrian Research, v.92, p.117-128. Franklin, J.M. 1970. Metallogeny of the Proterozoic rocks of the Thunder Bay District, Ontario; unpublished Ph.D. thesis, The University of Western Ontario, London, 304p Percival, J.A and Stern, R.A., 1984. Geological synthesis in the western Superior Province, Ontario; in Current Research, Part A, Geological Survey of Canada, Paper 84-1A, p.397-407. Franklin, J.M. 1978. Uranium mineralization in the Nipigon area, Thunder Bay District, Ontario; in Current Research, Part A, Geological Survey of Canada, Paper 78-lA, p.275-282. Percival, J.A and Sullivan, R.W., 1988. Age constraints on the evolution of the Quetico belt, Superior Province, Ontario; in Radiogenic Age and Isotopic Studies: Report 2, Geological Survey of Canada, Paper 88-2, p.97-108. Franklin, J.M., McIlwaine, W., Poulsen, K., and Wanless, R. 1980 Stratigraphy and depositional setting of the Sibley Group, Thunder Bay District, Ontario, Canada. Canadian Journal of Earth Sciences, v. 17, p. 633-650. Geller, P., 2012. A review of the western Superior Lithoprobe Line 3. Unpublished Honours Thesis, Lakehead University, 45 pp. Gill, J.E. 1926. Gunflint iron-bearing formation, Ontario; Geological Survey of Canada, Summary Report, 27C, p.28c-88c. Hinz, P., Landry, R.M., and Gerow, M.C. 1994. Dimension stone occurrences and deposits in northwestern Ontario; Ontario Geological Survey, Open File Report 5890, 191p. Kamineni, D.C., Stone, D., and Johnston, P.J. 1988. Metamorphism of Quetico sedimentary rocks near Atikokan, Ontario; in Program with abstracts, Geological Association of Canada-Mineralogical Association of Canada-Canadian Society of Petroleum Geologists Annual Meeting, v.13, p.A63. Kehlenbeck, M.M., 1976. Nature of the Quetico–Wabigoon Percival, J.A., 1989. A regional perspective of the Quetico metasedimentary belt, Superior Province, Canada; Canadian Journal of Earth Sciences, v.26, p.677693. Percival, J.A. and McGrath, P.H., 1986. Deep crustal structure and tectonic history of the northern Kapuskasing uplift of Ontario: an integrated petrological-geophysical study; Tectonics, v.5, p.553-572. Percival, J.A. and McGrath, P.H., 1986. Deep crustal structure and tectonic history of the northern Kapuskasing uplift of Ontario: an integrated petrological-geophysical study; Tectonics, v.5, p.553-572. Percival, J.A., Stern, R.A., and Digel, M.R., 1985. Regional geological synthesis of western Superior Province, Ontario; in Current Research, Part A, Geological Survey of Canada, Paper 85-1A, p.385-397. Pirie, J. and Mackasey, W.O., 1978. Preliminary examination of regional metamorphism in parts of the Quetico metasedimentary belt, Superior Province, Ontario; Geological Survey of Canada, Paper 78-10, p.37-48. Rogala, B., 2003. The Sibley Group: A lithostratigraphic, - 58 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 geochemical, and paleomagnetic study. Unpublished MSc thesis, Lakehead University, Thunder Bay, 254 pp. Rogala, B., Fralick, P.W., Heaman, L.M., and Metsaranta, R., 2007. Lithostratigraphy and chemostratigraphy of the Mesoproterozoic Sibley Group, northwestern Ontario, Canada. Canadian Journal of Earth Science 44, 1131-1149. Sawyer, E.W., 1983. The structural history of a part of the Archean Quetico metasedimentary belt, Superior Province, Canada; Precambrian Research, v.22, p.271-294. Schnieders, B.R., Smyk, M.C., Speed, A.A., and McKay, D.B. 1996. Mineral occurrences in the NipigonMarathon area, Volumes 1 and 2, Ontario Geological Survey, Open File Reports 5951, 912 p. Scott, J.F. 1987. Uranium occurrences of the Thunder BayNipigon-Marathon area; Ontario Geological Survey, Open File Report 5634, l58p. Seemayer, B.E., 1992. Variations in metamorphic grade in metapelites in transects across the Quetico Subprovince north of Thunder Bay, Ontario; unpublished M.Sc. thesis, Lakehead University, Thunder Bay, 163p. Smyk, M.C. and Kissin, S.A. 2005. Geology and rare element pegmatites of the Quetico Subprovince near Nipigon, 51st Institute on Lake Superior Geology, Proceedings volume 51, pt.2d,14p. Stockwell, C.H., McGlynn, J.C., Emslie, R F., Sanford, B.V., Norris, A.W., Donaldson, J.A., Fahrig, W.F., and Currie K L. 1972. Geology of the Canadian Shield, in Geology and Economic Minerals of Canada, edited by R.J.W. Douglas, Geological Survey of Canada, Economic Geology Report 1, 838 p. Sutcliffe, R.H. 1989. Mineral variation in Proterozoic diabase sills and dykes at Lake Nipigon, Ontario; Canadian Mineralogist, v.27, p.67-79 Sutcliffe, R.H., 1991. Proterozoic geology of the Lake Superior area. In: Geology of Ontario. Edited by P.C. Thurston, H.R. Williams, R.H. Sutcliffe, and G.M. Stott. Ontario Geological Survey, Special Vol. 4, Part 1, pp. 405–484. Tabor, J.R., Hudleston, P.J., and Mcloughlin, J., 1989. Metamorphism of the Quetico supracrustals north of the Vermillion granitic complex, northern Minnesota; Geological Association of Canada – Mineralogical Association of Canada, Program with Abstracts, v.14, p.A38. Tanton, T.L. 1948. Radioactive nodules in sediments of the Sibley series, Nipigon, Ontario; Transactions of the Royal Society of Canada, v. XLII, series III, section IV, p.69-75. Williams, H.R., 1989. Geological studies in the Wabigoon, Quetico, and Abitibi–Wawa subprovinces, Superior Province of Ontario, with emphasis on the structural development of the Beardmore–Geraldton belt; Ontario Geological Survey, Open File Report 5724p. Williams, H.R., 1991. Quetico subprovince; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, v.1, p.383-403. Williams, H.R. and Breaks, F.W., 1989. Geological studies in the Manitouwadge–Hornepayne area; Summary of Field Work and Other Activities 1989, Ontario Geological Survey, Miscellaneous Paper 146, p.7991. Williams, H.R. and Breaks, F.W., 1990. Geological studies in the Manitouwadge–Hornepayne area; Summary of Field Work and Other Activities 1990, Ontario Geological Survey, Miscellaneous Paper 151, p.4751. Zaleski, E. and van Breemen, O., 1997. Age constraints on plutonism, metamorphism and deformation across the Wawa–Quetico subprovince boundary near the Manitouwadge greenstone belt, northeastern Ontario; Institute on Lake Superior Geology, Program with Abstracts, v.43, p.67-68. Zaleski, E., van Breemen, O., and Peterson, V.L., 1999. Geological evolution of the Manitouwadge greenstone belt and Wawa–Quetico subprovince boundary, Superior Province, Ontario, constrained by U–Pb zircon dates of supracrustal and plutonic rocks; Canadian Journal of Earth Sciences, v.36, p.945-966. - 59 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Field trip 5 - A stratigraphic transect across the Northern flank of the Midcontinent Rift near Rossport Pete Hollings and Philip Fralick Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada Safety As this trip will be taking place on Lake Superior it will be weather dependent and could be cancelled or curtailed at very short notice. Please exercise caution when getting in and out of the boats as the outcrops are often extremely slippery. Life jackets must be worn in the boats at all times. It will probably be very cold out on the lake so please dress warmly. Introduction This field guide has been updated from Hollings and Fralick (2005) that was published as part of the 51st ILSG meeting in Nipigon, Ontario. We have updated the regional geology to include some more recent work and modified some stop descriptions to reflect changing lake levels. Regional geology Archean granites, outcropping along the shoreline near Rossport, are unconformably overlain by strata of the Gunflint Formation (Fig. 1). These sediments were deposited on a south facing shelf at approximately 1878 Ma (Fralick et al., 2002). The Formation consists of a Lower Member composed of basal stromatolitic bioherms overlain by ankeritic, interclastic grainstones. A regressive, karstified surface caps the northern portion of this assemblage (Fralick and Barrett, 1995) and is succeeded by the Upper Member. It begins with a repetition of the underlying lithologies to which, higher in the succession, are added carbonaceous shales, tuffs and rarely mafic volcanic rocks. These chemical and fine-grained siliciclastic sediments record parasequence development on a storm-dominated shelf (Pufahl and Fralick, 2004) forming the relatively stable portion of a back-arc basin (Kissin and Fralick, 1994; Hemming et al., 1995) prior to compressive northward thrusting of the arc at approximately 1860 to 1835 Ma. As the compression of the Penokean Orogeny waned the sea again transgressed over the area depositing Rove Formation black, carbonaceous shales gradually transitioning upward into turbidites (Morey, 1967). The turbitite fan fed off of a delta prograding to the SSE, with sediment sourced from the rising TransHudson Mountains (Maric and Fralick, 2005). This depositional cycle occurred at 1832 Ma (Kissin et al., 2003; Addison et al., 2005). The lower portion of the Gunflint Formation in the Rossport area is poorly exposed. Lithologies present in the limited outcrop of the Lower Gunflint are similar to those in the succession comprising the thin, basal Kakabeka Conglomerate and overlying interclastic grainstones present in exposures to the west near Thunder Bay. Good exposure of the Upper Gunflint exists on Quarry Island and consists of possible basaltic flow rocks with associated stromatolites, overlain by a succession of medium- to coarse-grained, graded, sandstone beds. The geochemistry of the sandstones is similar to Archean rocks to the north indicating probable derivation from this source. Black shales, lithically correlative with the Rove Formation, outcrop on an island approximately 5 km to the west. The shales do not overlie the turbidite succession on Quarry Island where arenites of the Sibley Group disconformably rest on an erosion surface at the top of the Gunflint sandstones. The basal unit of the Sibley Group is the Pass Lake Formation. It is composed of the conglomeratic Loon Lake Member and the overlying sandstones of the Fork Bay Member (Cheadle, 1986). Where the basal conglomerates are present they either represent: 1) large channel fills cutting down into sandstones to shales with abundant caliche zones, or; 2) more laterally continuous conglomerates interbedded and overlain by parallel laminated, medium-grained sandstones. The former represent channel gravels in braided fluvial systems and the latter coarse-grained strandline deposits. The overlying Fork Bay sandstones likewise - 60 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 A B Figure 1b Osler Group 1108 Ma Nipigon diabase Stop 8 Sibley Group Lake Superior Animikie Group Figure 1c 1105 Ma Lake Superior 200 km C 15 km Rossport N 48°30’ Archean rocks 48°30’ Stop 7 Keweenawan intrusive rocks Quarry Island Stop 6 Simpson Island Stop 5 Stop 4 Stop 3 Vein Island Osler Group volcanics Channel Island Keweenawan sediments Sibley Group sediments Gunflint Formation Archean basement Stop 2 Stop 1 Wilson Island Copper Island 87°30’ 87°45’ 48°45’ 1 km 48°45’ Figure. 1. Map showing the location of the field trip area. B) Regional geology map showing the extent of the Osler Volcanic Group. Age data from Davis and Sutcliffe (1985) and Davis and Green (1997). Modified after Sutcliffe (1986). C) Geological map of the Osler Volcanic Group showing sample locations. Modified after Giguerre (1975). record both braided fluvial deposition and subaqueous, strand proximal sand-sheet development. In addition to upward thinning and fining successions developed during transgressive systems tract formation, other sandstone assemblages thicken and coarsen upwards representing progradational, delta lobe outbuilding. The delta prograded into a lacustrine setting that isotopes (C, O, S and Sr) indicate became more saline with time (Metsaranta, 2006). This is consistent with Cheadle’s (1986) findings and those of Rogala et al. (2007) based on regional paleogeography. The increasing salinity of the water resulted in dolomite, minor gypsum and rarer barite and celestite precipitation mixed with mud deposition. A red and green banding developed in this assemblage due to periodic anoxia of the bottom sediments. The final desiccation of the lacustrine basin is recorded by the development of strandline microbial bioherms (stromatolites) which are overlain by either a terra rosa (red, wind-delivered soil) or subareal, intraformational, mass-flow conglomerates. This is succeeded upwards by mudstones with abundant gypsum nodules representing mudflats formed in an arid climatic setting where hypersaline groundwaters precipitated gypsum. Together all these fine-grained sediments comprise the Rossport Formation. It is overlain by the Kama Hill Formation; a coarsening upwards deltaic succession recording flooding of the basin and development of a more humid climate. The age of the Pass Lake and Rossport Formations can be bracketed between laser ablation MS youngest ages on detrital zircons of 1600 Ma and a Rb-Sr isochron of 1339 Ma (Franklin, 1978). The youngest detrital zircons in the deltaic succession of the Outan Island Formation is 1443±31 Ma. The Sibley Group is very well exposed along the shorelines of the islands off Rossport. The basal disconformity can be seen about two thirds of the way up the cliff face on the western side of Quarry Island where it overlies graded sandstone beds of the Gunflint Formation. Blocks of medium-grained sandstone were extracted from the cliff face on the island for use in the construction of buildings in Thunder Bay. These - 61 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 V N Portage Lake IV Volcanics Upper Suite 1100 1105 R R 1110 Portage lake Volcanics IV Kallander Creek Volcanics Siemens Creek Volcanics Mamainse Point Michipicoten Island Michipicoten Island Formation Group 7 V (Group 8) III Group 6 IV IV Osler Group Age (Ma) 1095 Copper Harbour Conglmerate Isle Royale Copper Harbour Conglomerate 1090 The oldest rift-related rocks on which U-Pb age determinations have been performed lie along the northwestern portion of the rift. These include, from NE to SW, the alkaline intrusive rocks of the Coldwell Complex (1108±1 Ma, Heaman and Machado, 1992), the lower Osler Group volcanic rocks (1108+4/-2 to 1105±2 Ma, Davis and Sutcliffe, 1985; Davis and Green, 1997), the Logan Sills (1109+4/-2 Ma, Davis and Sutcliffe, 1985), the lower portion of the North Shore Volcanics (1108±2 Ma, Davis and Green, 1997), the Isle Royale Black Bay Peninsula Lake Nipigon Upper Michigan NW Wisconsin 1085 Formation, underlies the same disconformity. This highlights the fact that approximately 600 m of erosive downcutting occurred in the Rossport area before the basal Osler was deposited. II I Bessemer Quartzite Lower Suite I Simpson Isl Cgl Nipigon Sills Schroeder Basalts V Beaver Bay Complex Mostly basalt units Duluth Complex Great Conglomerate and Group 5 Central Suite III NE Minnesota SW limb Groups 3,4 Group 2 Group 1 III II I IV IV North Shore Volcanic Group sandstones are medium- and large-scale planar crossstratified and may represent a sandflat composed of transverse bars in a braided stream or small barchan, eolian dunes. Rare pebbles indicate the former may be the case but this evidence is not conclusive. Channel and Copper Islands contain excellent exposures of the lacustrine rocks with outbuilding of channel systems along the paleolake margins. One of the best outcrops of the strandline stromatolitic carbonates occurs on Channel Island and will be visited during this field trip. On Copper Island the Rossport Formation is overlain disconformably by pebbly, fluvial conglomerates of the basal Osler. Thirty kilometers to the west the uppermost unit of the Sibley, the Nipigon Bay Ely’s Peak Basalts I, II, III Nopeming sandstone Archean Basement Figure 2. Schematic correlations of volcanic rocks of the Midcontinent Rift based on the stratigraphic position of distinctive basalt sequences, magnetic polarity and absolute age where possible. Modified after Nicholson et al. (1997). Dashed lines in Upper Michigan section separate lower and upper members of Kallander Creek and Siemens Creek volcanics. Left hand column shows magnetic polarity. Roman numerals I-IV refer to five distinctive laterally extensive basalt compositions identified on the south shore of western Lake Superior. Where equivalent basalt compositions occur in other stratigraphic successions, the appropriate Roman numeral is noted (see Nicholson et al., 1997 for data sources). Shaded regions represent intervals in which contacts are covered or obscured by plutonic rocks. - 62 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Swamper Lake Gabbro and Nathan’s Series intrusive rocks (1107 Ma, Paces and Miller, 1993) and the lower portion of the Powder Mill Group (1107±2 Ma, Davis and Green, 1997). These older units outcrop on the rift flanks where erosion has removed the younger rift sequence or, in the case of the Coldwell and Logan Sills, are intruded into older rocks immediately north of the rift. More recently a number of older sill complexes have also been identified in the vicinity of the Nipigon Embayment (Heaman et al., 2007). The Osler Volcanic Group comprise a ~3km thick sequence (Cannon et al., 1989) lying unconformably above Sibley Group metasedimentary rocks (Fig. 2). The volcanic sequence is overlain and intruded by the St. Ignace Island Volcanic-Plutonic Complex an intercalated sequence of basaltic rocks and rhyolitic flows (Sutcliffe and Smith, 1988). Detailed descriptions of the Osler Group have been provided by McIlwaine and Wallace (1976), Lightfoot et al. (1991) and Keays and Lightfoot (2015). Generally the mafic flows of the Osler Group consist of massive to amygdaloidal flows, with locally developed ropey tops and pahoehoe textures (Sutcliffe and Smith, 1988). The flows range in thickness from 5cm to 30m (Lightfoot et al., 1991) with a regional dip of ~6-15° S (Giguerre, 1975; Lightfoot et al. 1991; Hollings et al., 2007). The majority of the exposed section is magnetically reversed with only the upper 100m displaying a normal polarity (Halls, 1974). Recent work by Swanson-Hysell et al., (2014) has shown that there is a progressive change in the paleomagnetic sequence of the Osler volcanic rocks that is consistent with a ca. 25° of latitudinal motion of Laurentia. The contact between the two units is marked by the presence of the Puff Island conglomerate and a discordance between the basalt flows above and below the contact. This has been interpreted as representing a significant break in the eruption history. A felsic porphyry near the base of the Osler Group has yielded an age of 1107.5+4/-2 Ma (Davis and Sutcliffe, 1985) whereas zircons from the Agate Point rhyolite towards the top of the reversely magnetized sequence have yielded an age of 1105±2 Ma (Fig. 1b; Davis and Green, 1997). Within the Osler Group interflow sediments are typically thin and of limited extent. Field descriptions of the sedimentary successions appear in Giguere (1975) and McIlwaine and Wallace (1976). They show that there are two main zones of sedimentary rocks within the Osler Group. One occurs near the base of the volcanic pile. The other is present approximately 2700 meters higher in the succession marking the paleomagnetic reversal. Lightfoot et al. (1991) in a study of the Osler Volcanic Group exposed along the shores of the Black Bay Peninsula to the west of the field trip location proposed that the major and trace element geochemical data could be used to subdivide the flows into an Upper, Central and Lower Suite although the boundaries between the suites were not clear cut. While the geochemical compositions of the Central (750900m) and Upper suites (1900-3000m) overlap their Lower Suite (0-750m) is distinguished by elevated Mg numbers (0.55-0.7 versus 0.3-0.6), lower Al2O3 (8-12 wt% versus 13-17wt%), lower Th/Nb ratios (0.090.70 versus 0.3-0.6) but higher Gd/Ybn ratios (3.5-4.5 versus 1.6-2.6). Nicholson et al. (1997) concluded that there were five geochemically distinct flood-basalt compositions within the Mid-continent rift that are common to most sections and appear in approximately the same stratigraphic order (Fig. 2) They recognized a lower suite in the Siemens Creek Volcanics (Basalt Type 1; Fig. 2), which they suggest is analogous to the Lower Suite of Lightfoot et al. (1991). In the United States this unit is less than 100m thick whereas the Lower Suite of Lightfoot et al. (1991) is ~750m thick. They report a narrow range of εNd(1100) values for the Siemens Creek Volcanics of -0.7 to +0.7. Nicholson et al. (1997) further suggest that there is a broadly recognizable suite of basalts above this (Basalt type II) which includes the upper Siemens Creek Volcanics and in the upper part of the Grand Portage lavas (Fig. 2). The suite is characterized by slight negative Nb anomalies and a range of εNd(1100) values of -1.4 to -6.9. They suggest that this may be analogous to the most primitive members of the Central Suite of Lightfoot et al. (1991). The volcanic flows of the Osler Group on Wilson Island are all basalts or basaltic andesites (SiO2 = 4756 wt%; MgO = 5-16 wt%; Hollings et al., 2007). The basalts are characterized by LREE enrichment (La/Smn = 1.5-3.9) in conjunction with moderately fractionated HREE (Gd/Ybn = 1.5-3.7) and slight positive to moderately negative Nb anomalies (Nb/ Nb* = 0.56-1.13; Hollings et al., 2007; Fig. 3). Major and trace element data show trends of increasing SiO2 and decreasing MgO and display strong positive correlations between La/Smn, Th/La and Th/Nb with height (Fig. 4). This correlation is most pronounced - 63 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 in conjunction with LREE enrichment and strongly fractionated HREE are comparable to modern OIB, albeit at lower absolute abundances (Fig. 3). When compared to other flood basalt sequences the more primitive basalts from this study closely resemble basalts from the Parana-Etendeka flood basalt sequence (Fig. 4; Gibson et al., 2000). The εNd data from the most primitive members of the Osler Group is consistent with an enriched mantle plume rather than a contaminated depleted mantle source, given the lack of trace element evidence for contamination in these samples. Depleted mantle at 1100 Ma would have had a positive εNd perhaps as high as +6 whereas an enriched plume source would have εNd ~0 (Nicholson and Shirey, 1990; Shirey et al., 1994). Up sequence the basalts are characterized by higher SiO2, Th and La/Smn abundances in conjunction with increasingly negative Nb and Ti anomalies and εNd(1106) values of -4 to -5. This is consistent with contamination of these basalts by an older lithospheric component characterized by pronounced LREE enrichment, high Th abundances but generally unfractionated HREE (Hollings et al., 2007). 100 10 Hawaiian OIB Deccan Traps CFB Parana-Etendeka CFB 1 Th Nb La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Al V Sc Rock/Primitive Mantle 100 10 Type 1 Lower Suite 1 100 Th Nb La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Al V Sc 10 Type 2 The sedimentary successions near the base of the Osler Group constitute the Simpson Island Formation Figure 3. Comparison of primitive mantle normalized and have recently been described in detail by Hollings plots from the Osler Group with A) Phanerozoic OIB and et al. (2007). They are composed of a Lower Member Continental Flood Basalts (CFB) and B & C) the Lower and dominated by trough cross-stratified, medium-grained Central suites of Lightfoot et al. (1991). From Hollings et al. sandstones directly overlying basement and an Upper (2005). Member with a greater variety of siliclastic units. The above 400m with samples from the base of the Lower Member sits on an irregular, erosional surface stratigraphy displaying more or less constant values of cut into the underlying quartz arenites of the Nipigon these ratios (Fig. 4). Measured 143Nd/144Nd ratios for Bay Formation, Sibley Group. A massive pebble-cobble the seven Osler basalts analysed range from 0.511857- conglomerate overlies the unconformable surface 0.512286 with εNd(t=1106Ma) of +0.3 to -5.3 (Hollings et and is in turn overlain by decameter-scale layers of al., 2007). The high incompatible element abundances, coarse-grained and pebbly sandstone (Fig. 5, Section 1 900 800 Central Suite Th Nb La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Al V Sc SiO2 MgO Fe2O3 Th La/Smn Gd/Ybn Th/La Th/Nb εNd 700 600 500 400 300 200 100 0 40 50 60 5 10 15 10 15 1 2 3 4 1 2 3 4 1 2 3 4 0.05 0.10 0.15 0.20 0.10 0.15 0.20 -6 -4 -2 0 Figure 4. Geochemical stratigraphy of the Osler Group on Vein and Wilson Islands. The stratigraphic position of the samples has been calculated assuming a dip of 10° parallel to the section. From Hollings et al. (2005). - 64 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 V V V V V V V V V 6 No Longitudinal 24 Complex V V V V V Major 12 V V V V 6 V V V V V V 18 V 3 0 Section 5. V V No O/C V V V V V Nipigon Bay V V V Ripples Pebbles Hummocky Cross-Strat. V V V V V No O/C ? m. m V V V V V Complex No O/C ? m. Sandy Sheetfloods 6 Sheetfloods and Debris flows 3 V V V V V V V V V V Conglomerate Pebbly V. Coarse Medium V. Fine Siltstone Shale Sst. V V V V V V 7 15 V V V 8 m. No O/C 12 21 18 15 12 Bar V 0 Massflow 3 m. No O/C V V V V V V V V V V V V V 5 6 V V V V V V 15 V V V V V V V 10 km Older Units Section Locations Section 7. V V V V V V V V V V Sand Channels with Small Gravel Longitudinal Bars Stacked Channels 9 Sheetflood Sands with Channels 6 Small Stacked Channels Distal 6 V 4 2 1 3 VV V Sedimentary Rocks Igneous Rocks 1 27 Sandy Channels to Distributary Mouth Bar 9 V m Gravel Channels 26 V V V V V Osler Group 24 29 23 V V Section 6. 32 Longitudinal No O/C Trough Cross-Strat. Small Irregular Lenses Paleocurrent Direction V V V V V V V V V V V V V V V V V V V V V V V V V V Bar 0 0 Complex Rhyolite V V V V =318o 15 9 Longitudinal Parallel Lamination V V V V Bar Tail Sand Sheet Longitudinal Bar Nipigon Bay V Sandy Channel 3 Bar Sandy Channel V Section 4. m Basalt INTERFLOW SEDIMENTS m V Channel O/C =268o V 9 3 0 V l e g e n d 18 No 6 V V Sst. V 9 V Major Sandy Channel 0 Bar V m V 3 O/C 15 Section 1. V V No O/C V n = 38 V V 21 o Section 3. m V = 265 27 Section 2. Conglomerate Pebbly V. Coarse Medium V. Fine Siltstone Shale N m V SIMPSON ISLAND FORMATION ( Basal Sediments ) 3 0 Massflow Sheetflood Sands with Channels Figure 5. Sections of sedimentary rocks in the Osler Group. Sections 1 through 4 are the basal sedimentary succession of the Lower Member, Simpson Island Formation, at different locations (see inset map). Sections 5 and 6 are of the Upper Member, Simpson Island Formation, interlayered with basal basalt flows. Section 7 is the sedimentary assemblage near the top of the Osler Group on Puff Island. From Hollings et al. (2005). - 65 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 1). Sandstones are parallel laminated, commonly have cross-stratified tops, more rarely contain pebbly transverse ribs and chute and pool-like structures. The central portion of the succession is composed of trough cross-stratified, medium-grained sandstone organized into a stacked assemblage of lenses. Pebble stringers and pebbly sandstones commonly occur on the deeper portions of curving set boundaries (Fig. 5, Section 2). Massive pebble-cobble conglomerates sharply overly the sandstone succession (Fig. 5, Sections 2 and 4). The conglomerates contain trough cross-stratified, mediumgrained sandstone lenses; decameter- to meter-scale wedges of planar cross-stratified sandstone and are interbedded with assemblages of trough cross-stratified sandstones up to one meter thick. Another assemblage of trough cross-stratified sandstone, similar to the one in the central portion of the succession, caps the basal sedimentary assemblage (Fig. 5, Sections 3 and 4). Clast lithologies in the pebble-cobble population are dominated by quartz, chert, various types of volcanic rocks, red siltstone, metamorphosed granite and at the east end of the outcrop belt, on Copper Island, a higher proportion of unmetamorphosed red granite. Current indicators show flow was to the west, averaging 265°. A sedimentary assemblage also occurs near the upper limit of outcrop of the Osler Formation, at the top of the magnetically reversed interval (Fig. 5, Section 7). These interflow sediments are located on Puff Island and overly a felsic porphyry with a U-Pb age of 1105 Ma (Davis and Green, 1997). They contain: sharp sided assemblages of laterally continuous, pebbly, coarsegrained sandstone beds with caliche horizons which are scoured into by small lenses of conglomerate; large scours filled with trough cross-sets over a meter thick; stacked assemblages of irregular lenses filled by trough cross-stratified, coarse-grained, pebbly sandstone; and, poorly sorted, disorganized, massive boulder-cobble conglomerate. Clasts are all volcanic, ranging from quartz-feldspar porphyries to mafic compositions. Paleocurrents on large-scale sedimentary structures consistently show flow to the southeast. The Simpson Island Formation is composed of a laterally continuous sedimentary succession up to 25 meters thick and discontinuous sedimentary units up to 30 meters thick interlayered with the basal basalt flows. The lowest sedimentary beds fill channelways cut into the underlying sandstones of the Nipigon Bay Formation. The channel fills and overlying sedimentary assemblage represent a braided stream system, similar to the South Saskatchawan model (Miall, 1978), where dunes composed of coarse-grained sand migrated down the channels and gravelly longitudinal bars with chute channels and bar edge sand wedges form the higher relief areas (Fig. 5). The fluvial interpretation is consistent with Tanton (1931) and McIlwaine and Wallace (1976). Clast lithologies indicate debris was mainly derived from erosion of local lithologies. Stops The trip will depart from the public dock at Rossport and will undertake a traverse through the stratigraphy of the Midcontinent Rift, starting with the youngest rocks of the Osler Volcanic Group, proceeding through the Sibley and Gunflint Formations and finishing with a look at the granites of the Archean basement (Fig. 1). In order to make the most of the calmer weather typical of early mornings we will first travel for approximately 30 minutes to the most southerly outcrop on Wilson Island. Stop 1 – Osler volcanics, Wilson Island UTM coordinates 0462794E 5402095N Flows of the Osler Group on Wilson Island are typically >1m thick, frequently amygdaloidal towards the top and bottom of the flow with a massive core and rarely displayed a pahoehoe texture on the flow surface. The basalts are characterized by clinopyroxene and Figure 6. Well-developed vesicle column in basaltic flows, Stop 1 on Wilson Island. - 66 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 8. Pahoehoe texture at Stop 2, Wilson Island. Figure 7. Approximately 2m thick mafic flow cut by sediment filled cooling crack. Stop 1, Wilson Island. plagioclase phenocrysts in a groundmass of plagioclase, augite and Fe-Ti oxides. Rarely, basalts from the base of the sequence contained pseudomorphed olivine phenocrysts. The basalts have all been subjected to low-grade metamorphism ranging from zeolite to prehnite-pumpellyite facies (McIlwaine and Wallace, 1976). At this stop, approximately 500m above the basal conglomerates, are exposed a sequence of thin rubbly basalt flows ~50cm thick with rare massive flows ~2-4m thick and thin interflow sediments. These thick flows host well-developed vesicle columns (Fig. 6). Geochemically basalts at this outcrop are similar to the Central Suite of Lightfoot et al. (1991). Sedimentary units are predominantly quartz sandstones with thin shale partings. These are best interpreted as sands washing into small hollows on the surface of the flow units with a mud drape settling out towards the top of the layer. In places units that appear to have been deposited on the surface of basalt flows connect into sub-vertical cooling cracks in the flows. Sediment filled cooling cracks can be up to 2m deep (Fig. 7). Stop 2 – Osler Volcanics, North end of Wilson Island UTM coordinates 0461810E 5403438N Exposed at this outcrop, are the lower flows of the Osler Volcanic Group ~300m above the conglomerates of the Upper Simpson Island Formation. The basaltic Figure 9. Toe lobe in pahoehoe basalt flow at Stop 2, Wilson Island. flows are generally massive, ranging in thickness from 1-3m. Flow tops vary from rubbly to well-developed pahoehoe textures (Fig. 8). The basalts are vesicular and amygdaloidal and in places the vesicles are elongated giving them almost a pipe-like appearance. Geochemically basalts at this outcrop are similar to the Central Suite of Lightfoot et al. (1991). In some well-developed flow lobes are preserved (Fig. 9). At the north end of the outcrop the flows are cut by a 2-3m wide mafic dyke. This dyke is geochemically distinct from the flows but comparable to the older diabase intrusions in the vicinity of Lake Nipigon. Stop 3 – Upper Simpson Island Formation, Daylight Point, Wilson Island - 67 - UTM coordinates 0461450E 5404650N �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 10. Fine-grained red sandstones with thin shale partings forming the base of the deltaic deposits at Stop 3. separates this assemblage from overlying mediumto large-scale, trough cross-stratified, coarse-grained sandstones to conglomerates (Fig. 12). Paleocurrent indicators show flow to the west, though with a higher variance than other sections. Clast lithologies are probably locally derived from both Archean and Proterozoic sources. The fine-grained sandstone near the base of the section represents a wave modified deltafront (i.e., a distributary mouth bar of a small delta). The presence of small, dish-shaped scours suggests a shallow water environment with no large channels. The upper part of the sequence represents badly organized river deposits with gravelly, longitudinal bar forms and channels filled with sand. The planar cross sets at the base of the cliff were formed by transverse bars, while the trough cross beds represent migrating dunes. Figure 11. Oscilation ripples with overlying hummocky, medium-grained sandstone showing the effects of wave reworking on the sediments forming the delta front at Stop 3. Figure 12. Cross-stratified sandstone and massive conglomerate forming the upper portion of the prograding deltaic succession present in the Upper Member of the Simpson Island Formation present at Stop 3. These sediments represent a longitudinal bar-channel complex of a braided stream. A sedimentary assemblage of the Upper Member occurs on Wilson Island, overlying approximately 50 meters of basal basalt. This coarsening upwards succession has oscillation rippled, very fine-grained sandstones at its base (Fig. 5, Section 6). These coarsen upwards by the addition of increasing amounts of medium-grained, parallel laminated to hummocky cross-stratified to oscillation rippled, decimeter-scale sandstone beds (Figs. 10, 11). A covered interval Figure 13. Interlayered red siltstones and dolostones (lower unit underlying the more massive strandline carbonate with overlying mass-flow deposits) were deposited in a saline lake away either temporally or spatially from areas of coarse sand influx. The colour banding reflects the position of the redox boundary as the sediments accumulated. The grey layers commonly have slightly higher dolomite contents possibly reflecting higher organic productivity leading to more photosynthetically mitigated carbonate precipitation (higher dolomite content) and heavier organic loading to the sediment (redox boundary moving upward to at or above sediment water interface). Stop 4, Mary Ann Bay. - 68 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 14. Stromatolitic layering (smooth mat with small pinnacles) with interbedded coarse silt to very fine-grained sand storm layers (white). This is typical of strandline to sabkha environments and especially open water ponds on the sabkha. Some of the storm layers were remobilized in the form of clastic dykes and sills. Stop 4 – Sibley Group, Mary Ann Bay, Channel Island UTM coordinates 0462769E 5405999N A succession of grey dolomites interbedded with red siltstones outcrop on the shoreline here (Fig. 13). These are interpreted to be part of the cyclic facies (Channel Island Member) of the Rossport Formation as exposed at Kama Hill. The dolomite and minor gypsum indicate a hypersaline environment interpreted to be lacustrine because of the multiple deltas and sand sheets building in from a variety of directions. The cyclic facies is overlain by stromatolites and interbedded thin, sandy, carbonate storm layers (Fig. 14) of the Middlebrun Bay Member. This meter thick assemblage is similar to recent sabkha deposits on the south shore of the Persian Gulf, and in particular the open water ponds on this sabkha where sandy storm layers are well preserved. The upper few centimeters of the stromatolitic unit is altered to a grey-green layer that represents a weathered horizon interpreted to have formed as the sequence became subaerial and the stromatolites weathered in situ. This weathered zone is traceable throughout the basin with the strandline deposits below it commonly containing well developed tepee structures. The carbonates are overlain by a massflow unit with intraformational clasts of red siltstone, sandstone and dolostone up to boulder size. Although the contact between the carbonates and the mass flow Figure 15. Odd shaped structures of probable stromatolitic origin. within the Gunflint Formation. Stop 5, Quarry Island units is locally obscured by the intrusion of a sill, the transition is interpreted to represent a minor time gap based on the weathered zone which expands to thick terra rosa (soil) deposits at other locations. Stop 5 – Gunflint Formation, Quarry Island UTM coordinates 0462371E 5406786N A succession of sandstones and mafic volcanic rocks outcrop on the south shore of Quarry Island. On the northeastern end of the outcrop area a gabbro, probably related to the Midcontinental Rift, is exposed. Next to this is a small outcrop of stromatolites with a box-like appearance (Fig. 15). The rectangular to square outline of the mounds contrasts with the round to oval appearance of classic stromatolites, though their organic origin is exemplified by the high angle layering, which, when projected into the area now eroded, can be seen to form mounded structures. Areas between the stromatolites are infilled with coarser siliciclastic sandstones and cherty clasts. The next outcrop of Gunflint volcanic rocks is problematic. Mafic volcanic flow rocks occur interbedded with Upper Gunflint lithologies southwest of Thunder Bay. These are also associated with stromatolites that developed on the firm substrate of the flow tops. Thus, the igneous rocks in the Gunflint assemblage on Quarry Island could be correlative to the other flow rocks, but - 69 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 16. Well graded sandstone layers. Though these are probably turbidites they were not deposited in excessively deep water and may, in fact, represent distal tempestites. Stratigraphic position is problematic as these beds may belong to either the Upper Gunflint or Rove Formations. Stop 5, Quarry Island. it is difficult to conclusively show that these rocks are extrusive. Possible flow banding is present, as are areas of finer and coarser material in individual units. The igneous rocks are overlain by medium- to coarsegrained sandstones with bed thicknesses averaging approximately 30 cm. The sandstones are dark in appearance giving the impression they were derived Figure 17. Unusual markings on the bedding planes of the graded sandstones. Stop 5, Quarry Island. Figure 18. A second example of unusual marking observed at Stop 6. The origin of these markings is unclear. Stop 5, Quarry Island from mafic detritus, but their geochemistry indicates an intermediate source similar in composition to the Archean crust to the north. The layers are excellently graded (Fig. 16) with the only sedimentary structure being sporadically developed parallel lamination. Beds such as these are commonly thought of as typical turbidites and the deposits ascribed to reasonably deep water. However, it must be remembered that graded bedding simply means a decelerating flow deposited the bed, which can occur in any water depth. These beds may be tempestites, ie. beds formed by storm events, in this case in water deeper than storm wave base but certainly not anything approaching abyssal depths. Or they may have formed from inter- or overflow sediment-water plumes off river mouths, though lack of current reworking of bed tops makes this unlikely. Alternatively they may represent prodelta deposits formed by slumping of the delta front. All of these environments are relatively shallow which agrees with the presence of stromatolites not far stratigraphically below the graded beds. Another interesting point concerning these clastic units is that although such sandstones are common in the upper Rove Formation they are not present in the Gunflint at any other location. Thus, their stratigraphic position is debatable. The third unusual attribute is the presence of difficult to interpret structures on some bedding planes. Series of enechelon small crack-fill like features cut across bedding planes (Fig. 17). In addition a jellyfishlike impression was found on a bedding plane (Fig. 18). This feature had five-fold symmetry, similar to echinoderms, but the age of the rocks and its presence in sandstone leads to the distinct possibility that it was manufactured. - 70 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 19. Silicified alteration envelopes adjacent to quartz veins in basal Gunflint carbonates, near Rossport. Photo courtesy of Mark Smyk. Stop 6 – Pass Lake Formation, Quarry Island Figure 21. Black chert with gossan within the basal conglomerate of the Gunflint Formation at Gut Point. Photo courtesy of Mark Smyk. capable of creating such an organization of lithofacies. UTM coordinates 0461720E 5406798N - this stop The presence of the pebble is significant as only freak will be time dependant. wind-storms, such as tornadoes, can move material of This outcrop consists of a cliff-face in sandstones, this weight, and these do not form sand dunes. So, it is which were quarried, and the blocks produced used in more likely that these deposits are subaqueous but this the construction of buildings in Thunder Bay (Fralick rests only on a slim piece of evidence. et al., 2000). Here we see the basal sediments of the Sibley Group, the Pass Lake Formation. The Pass Stop 7 – Basal conglomerate of the Gunflint, Gut Lake forms a diverse group of basal coarse clastic Point deposits representing environments ranging from UTM coordinates 0461610E 5408587N braided fluvial through to subaqueous sand sheets. The unconformity and basal Gunflint are exposed The medium-grained sandstones present in this cliff are organized into a series of large-scale planar cross- at Gut Point as a thin, discontinuous veneer along the stratified sets with normal to low dip angles. Sorting is lakeshore on top of Archean basement (Fig. 19). The fairly good and only one pebble has been found in the basement is a medium-grained, equigranular granite, succession. Assemblages such as this pose a dilemma which has been altered (sausseritized/chloritized) in formulating an interpretation of their depositional beneath the basal Gunflint. The basal conglomerate is environment. Both aeolian sand dunes and sandflats up to 30 cm thick and occupies depressions in the paleocomposed of transverse bars in braided rivers are erosion surface in the basement. The conglomerate is matrix-supported, with subangular to rounded pebbles of white, sugary quartz, lesser cherty and lithic fragments and minor jasper in a medium-grained, sandy matrix (Fig. 20). A black, pyritiferous chert breccia is marked by a conspicuous gossan (Fig. 21). Sulphide mineralization may be related to a persistent, parallel fracture set at 115°. Fractures may host quartz-calcitebarite veins ranging from < 1 to 20 cm wide as well as vein breccia. A conjugate fracture set at right angles to the first is locally developed. Silicification adjacent to the veins has preserved a thin (1 to 5 cm) veneer of Figure 20. Matrix supported basal conglomerate of the Gunflint from being eroded (especially the carbonate Gunflint Formation containing rounded pebbles of quartz, units). A 2m wide diabase dyke strikes at 115° through chert and lithic fragments. Photo courtesy of Mark Smyk. the outcrop. - 71 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 km in size and intrudes the Schreiber greenstone belt; no radiometric date has been generated. Feldspar phenocrysts are typically 3-4 cm across (Fig. 23), subhedral to euhedral and in places appear to display localized alignment suggestive of flow banding. Acknowledgements We would like to thank Mark Smyk and John Scott for their help and advice in the preparation of this field guide. In particular Mark Smyk provided text and photographs for Stop 7. References Addison, W.D., Brumpton, G.R., Vallini, D.A., Davis, D.W., Kissin, S., Fralick, P.W., McNaughton, N.J., and Hammond, A., 2005. Discovery of distal ejecta from the 1850 Ma Sudbury impact event. Geology, 33, 193-196. Cannon, W., Green, A., Hutchinson, D., Lee, M., Milkereit, B., Behrendt, J., Halls, H., Green, J., Dickas, A., Morey, G., Sutcliffe, R., and Spencer, C., 1989. The North American Midcontinent Rift beneath lake Superior from GLIMPCE seismic reflection profiling. Tectonics, 8, 305-332. Figure 22. Porphyritic Archean granite, Selim Point. Carter, M.W. 1988. Geology of the Schreiber-Terrace Bay area, District of Thunder Bay; Ontario Geological Survey, Open File Report 5692, 287p. Cheadle, B.A., 1986. alluvial-playa sedimentation in the lower Keweenawan Sibley Group, Thunder Bay District, Ontario. Canadian Journal of Earth Sciences, v. 23, p. 527-542. Figure 23. Feldspar phenocrysts in Archean porphyritic granite at Selim Point. Stop 8 – Archean basement, Selim Point UTM coordinates 0469219E 5409146N From the dock in Rossport return to Highway 17 and head east for ~5 km. Turn right on to Lakeshore Drive just west of Whitesand Provincial Park. Follow the dirt road to a parking spot opposite a small tombola (Fig. 22). The porpyritic granite exposed here is Archean in age and part of the Wawa Subprovince. The area was mapped by Carter (1988) who described the rocks as porphyritic pink, hornblende + biotite alkali feldspar granite, a phase of the Whitesand Lake Batholith. The porphyritic “facies” is surrounded by massive pink and grey phases of alkali feldspar granite that is not exposed at this locality. The batholith is about 8 x 16 Davis, D.W., and Green, J.C., 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of Earth Sciences, 34, 476-488. Davis, D.W. and Sutcliffe, R., H., 1985. U-Pb ages from the Nipigon plate and northern Lake Superior. Geological Society of America Bulletin, 96, 1572-1579.Davis, D.W., and Green, J.C., 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution. Canadian Journal of Earth Sciences, 34, 476-488. Fralick, P.W. and Barrett, T.J., 1995. Depositional controls on iron formation associations in Canada. In, ed by A.G. Plint, Sedimentary Facies Analysis, Special Publication of the International Association of Sedimentologists, v. 22, p. 137-156. Fralick, P.W., Kissin, S.A. and Davis , D.W., 2002. The age of the Gunflint Formation, Ontario, Canada: single zircon U-Pb age determinations from reworked - 72 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 volcanic ash. Canadian Journal of Earth Sciences, v. 39, p. 1085-1091. Fralick, P.W., Smyk, M. and Mailman, M., 2000. Geology and stratigraphy of the Mesoproterozoic Sibley Group. In, ed. by P. Fralick, Field trip Guide Books, Forty-Sixth Annual Meeting, Institute of Lake Superior Geology. p. 7-42. Franklin, J.M., 1978. The Sibley Group, Ontario. In, Rubidium-strontium isochron age studies, Report 2, Geological Survey of Canada, Paper 77-14, p. 31-34. Giguere, J.F., 1975. Geology of St. Ignace Island and adjacent islands, District of Thunder Bay. Ontario Ministry of natural Resources, Geological Report 118, 35p. Halls, H.C., 1974. A paleomagnetic reversal in the Osler Volcanic Group, Northern Lake Superior. Canadian Journal of Earth Sciences, 11, 1200-1207/ Heaman, L.M., and Machado, N., 1992. Timing and origin of the Midcontinent Rift alkaline magmatism, North America: evidence from the Coldwell Complex. Contributions to Mineralogy and Petrology, 110, 289-303. Hemming, S.R., McLennan, S.M. and Hanson, G.N., 1995. Geochemical and Nd/Pb isotopic evinence for the provinance of the early Proterozoic verginia Formation, Minnisota. Implications for the tectonic setting of the Animikie Basin. Journal of Geology, v. 103, p. 147-168. G.R., 2003. New zircon ages from the Gunflint and Rove Formations, northwestern Ontario. Proceedings Institute of lake Superior Geology, Lightfoot, P., Sutcliffe, R., and Doherty, W., 1991. Crustal contamination identified in Keweenawan Osler Group tholeiites, Ontario: A trace element perspective. Journal of Geology, 99, 739-760. McIlwaine, W.H., and Wallace, H., 1976. Geology of the Black Bay Peninsula Area, District of Thunder Bay, Accompanied by Map 2304, scale 1 inch to 1 mile. Ontario Division of Mines, GR133, 54p. Maric, M. and Fralick, P.W., 2005. Sedimentology of the Rove and Virginia formations and their tectonic significance. Institute of Lake Superior Geology, v. 51, p. 41-42. Metsaranta, R.T., 2006. Sedimentology and geochemistry of the Mesoproterozoic Pass Lake and Rosport Formations. Sibley Group. Unpublished MSc. Thesis, Lakehead University, 217 pp. Morey, G.B., 1967. 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(Ed.), Institute on Lake Superior Geology Proceedings, 51st Annual Meeting, Nipigon, Ontario, Part 2 - Field trip guidebook, v.51, part 2, 57-70. Paces, J.B., and Miller, J.D, Jr., 1993. Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota; geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagnetic processes associated with the 1.1 Ga Midcontinent Rift System. Journal of Geophysical Research, B, Solid Earth and Planets, vol.98, no.8, pp.13,997-14,013. Hollings, P., Fralick, P. and Cousens, B., 2007. Geochemistry and sedimentology of the Osler Formation: Evaluating rifting in the Proterozoic. Canadian Journal of Earth Sciences, 44, 389-412. Keays, R. and Lightfoot, P., 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. Kissin, S.A. and Fralick, P.W., 1994. Early Proterozoic volcanics of the Animikie Group, Ontario and Michigan, and their tectonic significance. Proceedings Institute of Lake Superior Geology, v. 40, p. 18-19. Kissin, S.A., Vallina, D.A., Addison,W,D. and Brumpton, Pufahl, P.K. and Fralick, P.W., 2004. Depositional controls on paleoproterozoic shallow-water iron formation accumulation, Gogebic Range, Wisconsin, U.S.A. Sedimentology, v. 54, p. 791-808. Rogala, B., Fralick, P.W., Heaman, L.M. and Metsaranta, R., 2007. Lithostratigraphy and chemostratigraphy of the Mesoproterozoic Sibley Group, northwestern Ontario, Canada. Canadian Journal of Earth Sciences, v. 44, p. 1131-1149. Shirey, S., Lewin, K., Berg, J., and Carlson, R., 1994. Temporal changes in the sources of flood basalts: Isotopic and trace element evidence from the 1100 Ma old Keweenawan Mamainse Point Formation, - 73 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Ontario, Canada. Geochimica et Cosmochimica Acta, 58, 4475-4490. Sutcliffe, R. H., 1986. The petrology, mineral chemistry and tectonics of Proterozoic rift-related igneous rocks at Lake Nipigon, Ontario. Unpublished Ph.D. thesis, University of Western Ontario, London, 325p. Sutcliffe, R.H., and Smith, A.R., 1988. Project number 87-17. Geology of the St. Ignace Island volcanicplutonic complex. Summary of Fieldwork and Other Activities 1988. Ontario Geological Survey Miscellaneous Paper 141, 368-371. Swanson-Hysell, N. L., Vaughan, A. A., Mustain, M. R. and Asp, K. E., 2014. Confirmation of progressive plate motion during the Midcontinent Rift’s early magmatic stage from the Osler Volcanic Group, Ontario, Canada. Geochem. Geophys. Geosyst., 15, 2039–2047, doi:10.1002/2013GC005180 Tanton, T.L., 1931. Fort William and Port Arthur, and Thunder Cape map areas, Thunder Bay District, Ontario. Geological survey of Canada Memoir 167, 222p. - 74 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Field trip 6 - Geology of the Coldwell alkaline complex Allan MacTavish Panoramic PGMs (Canada) Limited, Thunder Bay, ON, Canada Mark Smyk Resident Geologist Program, Ontario Geological Survey, Ministry of Energy, Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada David Good Earth Sciences Dept., Western University, London, Ontario, Canada and John McBride Stillwater Canada Inc., Marathon, Ontario, Canada The Coldwell Alkaline Complex Trip will consist of two parts comprising 1) a west to east transect through southern part of the complex and 2) a visit to the Marathon Cu-PGE Deposit. This guide is modified from the 2017 ILSG Field Guide. Part 1: Transect Through the Coldwell Alkaline Complex Allan MacTavish and Mark Smyk A variety of Mesoproterozoic, Midcontinent Riftrelated alkalic and carbonatitic rocks occur within several intrusive complexes on or near the northern shore of Lake Superior (Figs. 1 and 2). They include the Coldwell and Killala Lake alkaline complexes, the Prairie Lake and Chipman Lake carbonatites, and numerous diatremes and related dikes in the vicinity of Dead Horse Creek (Sage, 1982, 1985, 1987; Fig. 2). These complexes are spatially localized and structurally controlled by the Trans-Superior Tectonic Zone (TSTZ), a north-northeast-trending structure that extends for over 600km and includes the Thiel Fault in Lake Superior (Klasner et al., 1982). Alkaline magmatism related to Midcontinent rifting occurred along the TSTZ from approximately 1.2 to 1.0 Ga (Table 1). It has been postulated that the TSTZ may represent Figure 1. Midcontinent Rift geology and the locations of mafic/ultramafic intrusions (After Miller et al., 1995). - 75 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 alkaline complexes are both thought by some to have formed as the result of ring fracturing and caldera collapse. The abundance of observed xenolithic blocks and roof pendants suggests that these complexes are presently exposed at relatively high structural levels. Figure 2. Regional geology (Sage, 1991) in the vicinity of the Trans-Superior Tectonic Zone (TSTZ), extension of the Thiel Fault (B). Key to numbering: 30 – Chipman Lake fenites / carbonatite dikes; 31 – Killala Lake alkaline complex; 32 – Prairie Lake Carbonatite; 33 – Coldwell alkaline complex; 36 – Slate Islands; 47 – Dead Horse Creek diatremes; 48 – McKellar Creek diatreme; 49 – Gold Range Diatreme; 50 – Neys Diatreme; A – Michipicoten Fault; C – Killala Lake Deformation Zone. part of a failed arm of a Keweenawan-age triple junction (Weiblen, 1982; Mitchell and Platt, 1982b) or the intersection of a late fracture system with the rift (Mitchell et al., 1983). Local alkalic and carbonatite complexes have been emplaced at inflections in the trends of major structural zones, or at sites of crossfaulting (Sage, 1991). The Coldwell and Killala Lake Similar ages for numerous mafic intrusions in the Nipigon Embayment (cf. Heaman et al., 2007) and the alkalic rocks of the Coldwell Complex (1108 ± 1 Ma; Heaman and Machado, 1992) indicate the contemporaneous production of tholeiitic and alkalic magmas during Midcontinent rifting. The oldest magnetization, found in the gabbros and augite syenites on the eastern side of the complex, records a concordant pole position with reversed polarity at about 1109 ± 5 Ma on the Keweenawan segment of the Precambrian apparent polar wander path (Lewchuk and Symons, 1990). The localization of the alkalic magmatism offaxis, dominantly northeast of the central rift, prompted Heaman and Machado (1992) to suggest that this may have been a region of maximum lithospheric extension during rifting. U/Pb data (Heaman and Machado, 1992) demonstrate that most rock units in the Coldwell Complex were emplaced within a relatively short time span (<3 million years) ca. 1108 Ma, and support the contention that the complex experienced relatively rapid cooling from initial emplacement temperatures to at least ~500º C. Strontium-, neodymium- and lead-isotopic compositions of selected minerals from different phases of the complex (Heaman and Machado, 1992) display considerable scatter, suggesting that their magmas had different isotopic compositions. The initial strontium- and neodymium-isotopic compositions of clinopyroxene and plagioclase from one of the earliest gabbroic phases are identical to data derived from primitive olivine tholeiites from the Midcontinent Rift and indicate that the majority of magmas, both Table 1 MCR-related Alkaline Magmatism Occurring Along the TSTZ Lithologic Unit/Complex Coldwell Alkaline Complex Be-Zr Zone crosscutting Dead Horse Creek diatreme Prairie Lake Carbonatite Lamprophyre Dyke, McKellar Harbour Gabbro (biotite), Killala Lake Complex Syenite, Killala Lake Complex ( -2.49% discordant; 1.82% discordant) 1 Age(s) (Method) 1108 ± 1 Ma (U/Pb) 1112.7 ± 4 Ma (U/Pb) 1128.7 ± 6 Ma (U/Pb) 1130 ± 10Ma (Rb/Sr) 1145 + 15/10 Ma (U/Pb) ~1160 Ma (U/Pb) 1185 ± 90 Ma (K/Ar) 1 2 1050 ± 35 Ma (Rb/Sr) 2 - 76 - Reference Heaman and Michado (1987) Krogh and Wilkinson (M. Smyk pers. Comm., 1995) Pollock (1987) Queen et al. (1996) Wu et al. (2016) Coats (1970) Bell and Blenkinsop (1980) �Proceedings of the 65th ILSG Annual Meeting - Part 2 tholeiitic and alkaline, have a uniform, nearly chondritic isotopic composition (ibid). Samarium- neodymium data, supported by oxygen-isotopic and whole-rock geochemical data, indicate that crustal contamination played a small, varied role in the generation of the Coldwell magmas (Bohay, 1997). In addition to small, variable amounts of assimilation of upper and lower crust, the parental plume magmas also interacted with the lithospheric upper mantle to a small degree (ibid). Local alkalic and carbonatitic intrusive rocks host a variety of characteristic base, precious, titaniferous, phosphate, and rare metal occurrences (cf. Smyk and Sage, 1995). They include the following: 1. Magmatic Cu-Ni-PGE (± Au, Ag) in gabbros of the Killala Lake and Coldwell complexes; 2. Magmatic Ti-V±apatite deposits in the Eastern Border Gabbros of the Coldwell Complex; 3. Magmatic U, Nb (+ wollastonite, apatite) in the Prairie Lake carbonatite (Sage, 1987); 4. Late-stage magmatic Nb-Y-F-family rare earth elements in syenite pegmatites (Alexander, 2007); 5. A Be-Zr-U-Th-Y mineralized zone crosscutting the Dead Horse Creek diatreme (Smyk et al., 1993; Potter, 2004); and 6. Pb-Zn-Ag-mineralized quartz-carbonate veins (Kissin and McCuaig, 1988). The Coldwell Alkaline Complex (Fig. 3) covers an area of ~580km2, making it one of the largest alkalic complexes in the world and the largest in North America. It was emplaced during the early stages of the Midcontinent rift system, which includes: early large and small mafic to ultramafic intrusions (i.e. Seagull Lake Complex, Thunder Bay North Intrusive Complex); Keweenawan flood basalts, the Duluth Complex, the Nipigon and Logan sills, and a variety of non-diabase mafic to ultramafic dyke-rocks. The Coldwell Complex was mapped by Kerr (1910a, 1910b), Puskas (1967), and Walker et al. (1993b, 1993c), and comprises three, superimposed ring subcomplexes or magmatic centers (Mitchell and Platt, 1978) that young progressively (Centers 1 to 3) to the southwest (Fig. 4). Walker et al. (1993) and Barrie et al. (2002) dispute the series of ring dykes or sheeted cones interpretation and suggest that the complex is a composite lopolith or sill. The intrusive centres can be generally described as follows: Center 1: Generally silica-saturated rocks with oversaturated residue; chiefly consisting of the Eastern and Western border gabbros (the oldest rocks within the complex) and later iron-rich augite syenite and syenite-syenodiorite (Mitchell and Platt, 1978, 1982; Mulja, 1989); Center 2: Generally silica-undersaturated alkalic rocks with oversaturated residue; consisting of locally nepheline- and hastingsite-bearing miaskitic nepheline syenite, and numerous volumetrically minor alkaline lamprophyre and analcime tinguaite dykes (Mitchell and Platt, 1978, 1982; Laderoute, 1989; and Mulja, 1989); and Center 3: Silica-oversaturated alkalic rocks with oversaturated residue; consisting of magnesiohornblende syenites, quartz syenites, and minor granites (Mitchell and Platt, 1994; LukosiusSanders, 1988). The mineralogy of the main lithologic units is listed in Table 2. The superimposition of the three intrusive centres and a complex, protracted magmatic history has produced a myriad of hybrid rocks, igneous breccias, and ambiguous crosscutting relationships. The wide variety of lamprophyric and other dyke rocks occurring within the complex (as described by Mitchell and Platt, 1994) include (in order of emplacement): Figure 3. Generalized geology of the Coldwell Alkaline Complex (after Walker et al., 1993) with field trip stops. - 77 - 1. Mafic ocellar lamprophyre (camptonitic variety) 2. Quartz-bearing, mafic lamprophyres (camptonitic variety) �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 4. Coldwell Alkaline Complex magmatic centres: CI (Centre 1), CII (Centre 2), and CIII (Centre 3). Generalized geology after Mitchell and Platt (1994). 3. Sannaite-type lamprophyres 4. Monchiquitic-type lamprophyres 5. Feldspar glomeroporphyritic and alkali basalt dikes 6. Analcime tinguaite (heronite) Abundant large rafts and/or roof pendants of mafic volcanic rocks are mapped throughout the Coldwell Complex and in places exhibit horizontal extensional cooling cracks on a ten to hundreds of metres scale that are thought consistent by some workers with subhorizontal bedding. For the most part the roof pendants may be the lowermost portions of the Keweenawan flood basalt sequences suggesting that the complex is barely unroofed and is exposed at a very shallow crustal level (Mitchell and Platt, 1994; Sage and Watkinson, 1995; Barrie et al., 2002). It is also highly probably that some, or most of the mafic rafts observed within the complex that could not be roof pendants are detached portions of chilled complex roof or wall rocks (finegrained gabbros). The mafic intrusive rocks occurring within Centers Table 2. Mineralized Zones Associated with the Mafic Intrusive Rocks of Centres 1 and 2. Intrusion (Centre) Eastern Gabbro (1) Western Gabbro (1) Two Duck Lake (1) Malpas Lake (1) Geordie Lake (2) Alkalic gabbro (2) Lithologic Units Layered gabbro cumulates (olivine gabbro, gabbro, troctolite, anorthositic (leuco-) gabbro); Fe-Ti oxide ± apatite cumulates Massive and layered series gabbro; olivine-bearing Gabbronorite, olivine gabbronorite, olivine-bearing gabbro, leucogabbro Hornblende gabbro to monzodiorite; olivine ferrogabbro to ferrodiorite; olivine gabbro to diorite Amphibole-bearing olivine gabbro Troctolite, olivine gabbro Biotite gabbro Biotite- and olivine-gabbro - 78 - Reference(s) Shaw (1994, 1997); Lum (1973); Barrie et al. (2002) Penczak (1992); Wilkinson (1983) Shaw (1994, 1997) Dahl et al. (1987) Shaw (1994, 1997) Mulja (1989); MacTavish et al. (1987); Good, pers. com. (2019) Mitchell and Platt (1982b) Walker et al. (1993a) �Proceedings of the 65th ILSG Annual Meeting - Part 2 1 and 2 are tabulated, with their associated mineralized zones (Table 2). Magmatic, gabbro-hosted Cu-Ni-PGE deposits in the Coldwell Complex have been the focus of much exploration and research for the past 60 years. Mineralized zones occur within the border gabbro at the eastern (Marathon deposit; Skipper Lake Zone) and western (Middleton occurrences) margins of the complex, and within its interior at Geordie Lake. The Geordie Lake mineralized zones, hosted by a younger (?) gabbro, are enriched in tellurium and silver and have higher Pd:Pt ratios (~19) (Mulja and Mitchell, 1991) than the border gabbro-hosted deposits (~4; Smyk, 2001). Geochemical variations in mineralized zones in the Coldwell Complex are shown in Figure 5. A table of selected Coldwell Complex deposits and mineralized zones is shown below (Table 3). Marked similarities exist between the mineralization style, geochemistry, and host rocks of Coldwell Complex-, Duluth Complex-, and the Crystal Lake gabbro-hosted deposits near Thunder Bay. Similarities include mineral textures, abundance and compositions, crystallization paths for the host gabbros, silicatesulphide associations, trace-element trends and Figure 5. Discrimination plot for PGE-mineralized samples for Coldwell and other Midcontinent Rift-related intrusions. Data from Good (1992), MacTavish (unpublished data, Resident Geologist’s Files, Thunder Bay), Watkinson et al. (1983), Wilkinson (1983), and unpublished data, Resident Geologist’s Files, Thunder Bay. Duluth Complex composite data from Hauck et al. (1997). chalcophile element fractionation trends (Good and Crockett, 1994a). Research by Watkinson and Ohnenstetter (1992) and Good and Crockett (1994a, 1994b) produced debate between the relative importance of magmatic and hydrothermal processes in local copper-nickel-PGE Table 3. Selected Coldwell Alkaline Complex Deposits and Mineralized Zones Mineralized Zone Marathon Geordie Lake Grade / Significant Assays Ore Mineralogy Reference(s) Measured and Indicated InPit Resources: 114.8 Mt @ 0.775 g/t Pd, 0.228 g/t Pt, 0.083 g/t Au, 0.241% Cu, 1.567 g/t Ag; Proven and Probable In-Pit Reserves: 91.447 Mt @ 0.832 g/t Pd, 0.237 g/t Pt, 0.085 g/t Au, 0.247% Cu, 1.440 g/t Ag (January 2010) Measured and Indicated Resources (above $13.00/t cut-off): 32.42 Mt @ 0.61 g/t Pd, 0.04 g/t Pt, 0.05 g/t Au, 0.37% Cu, 2.93 g/t Ag Chalcopyrite ≤ pyrrhotite >> pentlandite > cubanite ≤ pyrite; sphalerite, hollingworthite, atokitezvyaginstevite, sperrylite, Bikotulskite, michenerite, merenskyite, monceite, stibiopalladinite, paolovite, merteite II, palladoarsenide, unnamed (Pd As ), nickeline, majakite, argentian gold Chalcopyrite, bornite, pyrite, millerite, siegenite, pentlandite, galena, chalcocite, melonite, hessite, unnamed (Ag Te ), altaite, kotulskite, merenskyite, michenerite, sopcheite, Pdbismuthotelluride, paolovite, Pdarsenide, guanglinite, Pdantimonide, sperrylite, electrum, Pd1.6As1.5Ni, AgSb Chalcopyrite, pyrrhotite, pentlandite, sphalerite, pyrite Chalcopyrite, bornite, pentlandite, cobaltite, galena, chalcocite; telargpalite, polarite, kotulskite, taimyrite, merteite, zvyagintsevite, plumbopalladinite, majakite, tetraferroplatinum n/a Marathon PGM Corporation Ohnenstetter et al. (1991); Watkinson and Ohnenstetter (1992); Good and Crocket (1994a, 1994b) 5 2 3 4 Middleton Skipper Lake average grade of 1.05 g/t Pd+Pt+Au over 12 m Area 41 0.48 g/t Pt+Pd+Au over 202 m, incl. 1.23 g/t Pt+Pd+Au over 61 m - 79 - 2 news release, Marathon PGM Corporation, May 04, 2010 Mulja (1989); Mulja and Mitchell (1990, 1991) Penczak (1992) MacTavish (2000) Benton Resources Corp. �Proceedings of the 65th ILSG Annual Meeting - Part 2 mineralization processes. Watkinson and Ohnenstetter (1992) presented field, petrographic and mineralchemical data that support the interaction of magmatic sulphide mineral assemblages with a chlorine-rich mixture of magmatic (deuteric) fluid and volatile species generated by the breakdown of assimilated xenoliths at low temperatures. However, Good and Crockett (1994a, 1994b) contended that element migration took place over only very short distances and that the original, bulk sulphides were not enriched in copper and PGE by later fluids. The information within this field trip guide was taken from a variety of sources, including guidebooks from previous field trips to the Coldwell Complex: Puskas (1970); Loubat (1972); Mitchell and Platt (1977, 1982a, 1994); Smyk and Sage (1995), Smyk (2001), Smyk (2010), and unpublished field observations and mapping completed by A. MacTavish (1992). All UTM co-ordinates listed are NAD83 Zone 16 with locations shown on Figure 6.   Stop descriptions Stop C1: Natrolite-Bearing Syenite and Massive FeTi-oxides UTM coordinates 525528E 5405511N 29.4 to 29.9 km west of the Highway 626 and Highway 17 junction Description: This exposure displays natrolitebearing, pegmatitic syenite (Photo 1). Reddish orange natrolite (an acicular or prismatic zeolite mineral replacing nepheline) patches up to 15cm in diameter, crystals of perthitic feldspar up to 30cm in length, and crystals or black amphibole up to 25cm in length comprise the bulk of this syenite (Photo 2). Mitchell and Platt (1994) reported accessory pleochroic clinopyroxene, zircon, titanite, and biotite. Natrolite has locally been ascribed to the hydrothermal alteration of primary nepheline and has also been referred to as “hydronepheline” by local workers. The syenite is intruded by a camptonite lamprophyre dike (Mitchell and Platt, 1994) and also hosts large, medium-grained gabbro xenoliths (Photo 3), up to 1m in thickness and sometimes up to 5m in length (west-side of the highway), that exhibit 1 to 2cm wide, dark reaction rims adjacent to the enclosing syenite. To the east, the pegmatitic syenite gives way to finer grained nepheline syenite in which chalky-weathering nepheline may be Photo 1. Pink, natrolite-bearing, pegmatitic augite syenite. Photo credit D. Campbell. Photo 2. Pegmatitic syenite containing reddish orange patches of natrolite, light pinkish perthitic feldspar, and black amphibole. Photo credit A. MacTavish. Photo 3. Large gabbro xenolith located on the west side of the highway. Please note that the xenolith has been crosscut by fine-grained syenite veins and that the syenite below the xenolith is varitextured to pegmatitic in texture, whereas the syenite above is medium- to coarse-grained. Photo credit A. MacTavish. - 80 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 6a. Northern portion of Neys Shoreline geological map starting at Prisoner’s Cove, Neys Provincial Park with Field Stop locations. - 81 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 6b. Central portion of Neys Shoreline geological map, Neys Provincial Park with Field Stop locations. - 82 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 6c. Southern portion of Neys Shoreline geological map, Neys Provincial Park with Field Stop locations. - 83 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 observed. Rare natrolite grains are also present. Farther east, a variety of equigranular and pegmatitic syenites are exposed. Near the eastern end of the outcrop (UTM 525625E, 5404825N), a large xenolith of gabbro-hosted, massive titaniferous magnetite has been exposed. Minor clinopyroxene, plagioclase and apatite occur within the massive oxide unit. Analyses completed in 1951 and reported by Hinz and Landry (1994) indicated total iron and titanium values ranging between 33 and 45%, and 4.5 and 13.5%, respectively; phosphorus contents ranged up to 0.371%. Photo 4. Syenite outcrop on the south side of the highway containing blocks of mafic xenoliths often occurring in elongated, semi-continuous rafts. Photo credit A. MacTavish. Photo 5. Elongated, jig-saw-fit xenolithic block exhibiting both angular and lobate/cuspate (amoeboid) margins. Please note the lighter-coloured, pinkish-grey elongated xenolithic block with apparently sharp margins located below the dark grey block. This lower block appears to be coarser-grained and may possibly be different in composition. Photo credit A. MacTavish. Stop C2: Little Pic River Breccia Zone UTM coordinates 527478E 5405531N 27.3 km west of the Highway 626 and Highway 17 junction These road cuts, particularly along the south side of the highway, expose spectacular intrusive breccias within the youngest rocks of the complex, along the east side of the fault zone that the Little Pic River occupies. The breccias often occur as semi-continuous, fragmented, elongate rafts (western end of southern rock cut, Photo 4) that consist of angular to rounded blocks of fine- to medium-grained, equigranular, mafic (gabbroic?) rocks within a groundmass of pink, mediumgrained, quartz syenite. In some cases blocks can exhibit both angular and lobate to cuspate (amoeboid) margins (see Photo 5). The mafic rocks comprising the blocks were interpreted as oligoclase-bearing basalt by Mitchell and Platt (1982a). Subsequent discussion and study has led to the suggestion of perhaps 2 texturally discernable types of basic xenoliths, those with: (1) sharp, angular margins, and (2) those with lobate to cuspate margins. In this model, the angular xenoliths represent synplutonic basalts which are now preserved elsewhere as megaxenoliths in younger intrusions. The cuspate-margined xenoliths may represent the effects of mixing between two contemporaneous gabbroic/ basaltic and syenite magmas (i.e., magma mixing or co-mingling). Cuspate, possible chilled margins with quench-textured clinopyroxene, plagioclase and skeletal olivine have been noted in similar xenoliths to the south on the Coldwell Peninsula by G. Shore (personal communication with M. Smyk, 1995) and suggest the quenching of the basic magma against the cooler, syenitic magma. These are reasonable hypotheses and there are definitely at least two types and textures of xenoliths; however, they do not completely explain the presence of blocks exhibiting both margin types as observed by the senior author of this guide and shown in Photo 5. Texturally there also seems to be three different types of xenoliths: the most abundant are dark grey to black, very finegrained xenoliths (Photo 4); medium-grained, greyish pink xenoliths with somewhat less distinct, but still relatively sharp margins (Photo 5), and several unusual zones where there is are subvertical zones of rounded, dark grey, amphibole-phyric xenoliths within a pinkish, mafic groundmass. Are these some sort of breccia dykes or just hybridized zones of xenoliths (Photo 6; what do you think?)? Although isolated xenoliths are - 84 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 granites and have been interpreted to be the result of fractional crystallization of mantle-derived, basaltic magma (Lukosius-Sanders, 1988; Mitchell et al., 1993). Stop C3: Prisoners Cove, Neys Provincial Park (Sample Collecting Prohibited!) UTM coordinates 527984E 5402537N The descriptions (unpublished mapping/field descriptions, MacTavish, 1992) herein are for 14 substops along the shoreline south of Prisoner’s Cove; however, due to time constraints only the northernmost stops will be visited. 23.5km west of the Hwy 626 and Hwy 17 junction; 2.8km south of Hwy 17 to the park headquarters and then south along the shoreline trail Photo 6. Rounded dark grey, amphibole-phyric xenoliths/ inclusions within a fine- to medium-grained, pinkish mafic groundmass (breccia dyke?). Photo credit M. Puumala. common, there are many areas within these outcrops where incipient or in-situ brecciation characterized by syenite dykes and “jig-saw puzzle/jig-saw fit” breccias are observed, where brecciated fragments can be fitted back together. Miarolitic cavities, up to several centimetres in width, contain euhedral quartz, feldspar, and calcite crystals. The breccia zone persists to the east, towards the scenic lookout located 800m to the east. The south side of the highway is underlain by oligoclase gabbro and quartz syenite, while various, xenolithic-bearing syenitic rocks are exposed on the north side. These pyroxene- and amphibole-(ferro-edenite) bearing syenites contain xenoliths of alkali gabbro, alkali diorite and other, equigranular to porphyritic syenites. Near the lookout turnoff, gray, nepheline-bearing syenite intrudes the mafic rocks and contains orange natrolite. Sannaite and ocellar, camptonitic lamprophyre dikes have been reported near this site by Mitchell and Platt (1994) who proposed the following order of local emplacement: Mg-hornblende syenite → contaminated Fe-edenite syenite → Fe-edenite syenite → quartz syenite (earliest → latest) Lukosius-Sanders (1988) classified the local rocks as miaskitic, metaluminous syenites enriched in U, Th, REE and Zr. These syenites have affinities to A-type General Description: The wave-washed, glacially polished outcrops along the shoreline of Lake Superior at Prisoner Cove and south for over a kilometre along the western side of the Coldwell Peninsula exhibit a variety of lithologic, textural, and crosscutting features that characterize much of the Center 2 magmatism in the Coldwell Complex. In its simplest sense, this composite stop displays the contact between alkalic biotite gabbro and amphibole-nepheline syenite, but the enigmatic effects of assimilation and hybridization have severely complicated and obscured many of the primary features. In all cases within the nepheline syenitic rocks exposed along the shoreline at this stop the nepheline has been completely altered to the zeolite mineral natrolite which weathers to orange-coloured pits. Medium- to coarse-grained, olivine- and enclavebearing, biotite gabbro comprises much of the eastern portion of the outcrops. Gabbro xenoliths occur within the syenite and within hybrid phases along their mutual contact, which trends roughly north-south, parallel to the shoreline. The outcrops often exhibit a pitted surface resulting from the preferential weathering of mafic enclaves consisting of biotite-olivine gabbro to biotite-clinopyroxene gabbro or leucogabbro (Walker et al., 1992) within a more syenitic groundmass. The syenitic groundmass consists of fine- to coarse-grained nepheline (altered to natrolite) syenite with minor acicular amphibole and poikilitic biotite. Mitchell and Platt (1994) have identified the amphibole as hastingsite. - 85 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Distinct to diffuse layering, a nebulous to locally distinct igneous foliation, and localized soft-sediment style magmatic deformation exists within the amphibole-nepheline syenite. Identifiable, undisturbed layering may be oriented parallel to the apparent syenite/gabbro contact and dips from very steeply to vertically in the north to shallow to moderate to the east (where measurable) in the south. Observed soft-sediment deformation features consist of flame structures, fluid-escape features, slump folds, and isolated well-layered syenite blocks surrounded by obvious fluid escape textures. Much past discussion has focused on whether the observed structures have resulted simply from igneous process, syn- or postintrusion shearing, or a combination of these processes. The present author’s strongly favour igneous processes since the observed fracturing is very localized, is late and brittle, and does not appear to have affected the foliation or layering within the surrounding rocks in any observable way. It is highly probable that the crystallizing magma chamber was often shocked by MCR tectonic activity. These earthquakes then caused the slumping of unstable crystallizing layers along chamber walls; allowed the isolation of broken, but relatively intact layered blocks; and allowed trapped deuteric fluids formed during the fractionation process to escape upwards through the broken layers. Upon close examination the fracturing presently observed in outcrop obviously took place after the chamber was completely crystallized and was able to deform in a brittle manner. Sub-Stop C3a (527980E, 5402571N): This area, located on the point to the north and west of the old Photo 7. Wispy, relatively mafic in appearance, hybrid amphibole-nepheline syenite exhibiting diffuse discontinuous layers. Photo credit A. MacTavish. flat-bottomed boats, mainly consists of foliated, wispy, hybridized amphibole-nepheline syenite with diffuse discontinuous “layers” (Photo 7). The core of this outcrop is flanked to the northeast by a heterogeneous zone containing large numbers of rounded to angular, variably assimilated (metasomatized?) and disaggregated inclusions/xenoliths of biotite gabbro. Reaction rims around these inclusions are readily visible. Also the inclusion-rich zone, as a whole, seems to be enclosed within a diffuse reaction zone when compared to the hybrid syenites adjacent to the west. The western margin of the exposure is a medium- to coarse-grained hybridized syenite with numerous very coarse-grained to pegmatitic inclusions of amphibolenepheline syenite. At the northwestern tip of the outcrop is an elongate, diffuse zone of apparently nonhybridized, non-foliated syenite (possibly the original parent syenite?). Sub-Stop C3b (527950E, 5402535N): This stop, located 30m west-southwest of the old boats near the shoreline, consists of a 4 to 5m wide, west-northweststriking, brittle fracture zone hosting a 70 to 100cm thick, dark greenish-grey, ocellar lamprophyre dyke at its northern margin near the water’s edge. The ocellae present within the dyke are composed of reddish, recessive-weathering carbonate (±zeolites?) which are elongated parallel to dyke margins (elongated by flow?). The lamprophyre dyke is also enveloped by a brick-red alteration halo that is not completely within the fracture zone and also extends into the unfractured hybrid syenites to the north for up to 5m. This red halo could be due to either hematization or K-alteration. Similar, subparallel fracture zones can also be observed about 10m and 23m to the south. Sub-Stop C3c (527966E, 5402475N): This stop is located ~50m east-southeast of Sub-stop C3b, and consists of a zone of large blocks (?) of coarse-grained, natrolite-bearing, biotite gabbro to biotite melagabbro that are surrounded by fine- to medium-grained amphibole-nepheline syenite containing diffuse gabbro xenolith ghosts. It is distinctly possible that this is not a zone of xenoliths/inclusions at all, but the exposed upper contact of an underlying biotite gabbro that is part of the biotite gabbro body located about 40m to the southeast (see Sub-Stop C3e, below) where the syenite is observed to overly the gabbro. These blocks (?) are cross-cut by narrow horizontal and subvertical syenite veins and dykes (Photo 8). - 86 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Photo 8. Large biotite gabbro xenolith/inclusion (?) crosscut by veins and dykes of amphibole-nepheline syenite. Photo credit A. MacTavish. Photo 9. Biotite gabbro xenoliths/inclusions separating from and original larger block and beginning to assimilate (?) into the surrounding syenite melt through a process of metasomatism and disaggregation. Photo credit A. MacTavish. Photo 10. Disaggregating biotite gabbro xenoliths exhibiting subparallel reaction haloes. Photo credit M. Puumala. Sub-Stop C3d (528004E, 5402440N): This location (45m southeast of Sub-stop C3c) consists of an irregular zone of gabbro xenoliths/inclusions, surrounded by fine- to medium-grained, weakly foliated syenite (Photos 9 and 10). The xenoliths are in the process of being broken down in stages from originally angular, cohesive blocks to diffuse groupings of amoeboid to wispy mafic remnants within a mafic mineral-rich, hybrid syenite. This process is probably not assimilation in the strictest sense, but is more likely a process of chemical (rather than thermal) invasion through metasomatism that over time breaks down the xenoliths and then eventually disaggregates the mineral constituents of the blocks to the point where they are then assimilated into the syenite melt. The hybridized (?) syenite surrounding the xenoliths exhibit aligned amphibole grains that may indicate flow (?) around and between fragments. There are also places where there are noticeable (up to 15cm thick) halos surrounding zones of xenoliths that consist of aligned amhibole grains that are somewhat separated into diffuse bands. Sub-Stop C3e (528014E, 5402433N): Located only 12m southeast of Sub-Stop 3d and consists of coarsegrained, knobby-weathering, biotite gabbro that has been cross-cut by numerous hair thin to 5cm thick, very fine- to fine-grained syenite stringers and veins and the occasional, larger, fine-grained to pegmatitic syenite vein (pegmatite is in centre of these veins; Photo 11). There are numerous leucocratic clots (oikocrysts?) of plagioclase (Photo 12) throughout. Sub-Stop C3f (527982E, 5402324N): This substop (~115m west-southwest of Sub-stop C3e) consists of an irregular, variably assimilated zone of mafic Photo 11. Biotite gabbro crosscut by fine-grained to pegmatitic syenite dyke (centre) and thinner syenite veins (centre left). Photo credit A. MacTavish. - 87 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 magmatic layering dips shallowly to the west and west-southwest at between 20 and 26° and there is a possible weak alignment of K-feldspar laths parallel to layering. The bases of the undulating modal layers are defined by a sharp increase in amphibole content. The best defined layering is near the lake with layering becoming increasingly more diffuse, disrupted, folded (slumping?), and contorted to the east until it becomes unrecognizable. (gabbroic?) xenoliths of highly variable size ranges. Many blocks are in the last stages of assimilation where the original xenoliths are now merely ghosts infilled with isolated mafic remnants and considerable numbers of hornblende grains. Sub-Stop C3h (527971E, 5402265N): At this location (~27m south of Sub-stop C3g) are two, subparallel, aphanitic to fine-grained, ocellar lamprophyre dykes (Photo 13) occupying a narrow southeaststriking fracture zone. The dykes dip to the northeast between 54° and subvertical. The ocellae (immiscible liquid droplets) are usually centralized within the dykes away from the strongly chilled dyke margins and are infilled with several minerals including applegreen and greyish minerals (zeolites?), and possibly white calcite. Sub-Stop C3g (527978E, 5402292N): This location (~30m south of Sub-stop C3f) consists of locally welldeveloped modal layering within medium- to locally coarse-grained amphibole-nepheline syenite. The Sub-Stop C3i (527987E, 5402198N): At this location (~70m south of Sub-stop C3h) is a zone of leopard mottles in moderately mafic, often grain-sizelayered (?) amphibole nepheline syenite. Photo 12. Leucocratic clots of plagioclase (oikocrysts) within biotite gabbro. Photo credit A. MacTavish. Sub-Stop C3j (527971E, 5402265N): This location (~80m south of Sub-stop C3i) is, for lack of a better name, a “Layer Breccia Zone” where there has been strong disruption, localized rotation, and folding of original magmatic syenite layers (Fig. 7). Finergrained syenite containing acicular amphibole grains has flowed around the layer blocks and alignment of those amphibole grains mirrors flow directions. The zone is surrounded by a highly disturbed hybrid mixtite with few measurable features. Thinner blocks consist of a series of thin modal layers of highly variable textures. The thicker layers are usually the coarsest, are sometimes size-graded, and contain glomerocrysts of K-feldspar (with include amphibole and natrolite after nepheline) up to 1.5cm in diameter surrounded by acicular amphibole grains and recessive-weathering altered nepheline. Photo 13. Ocellar lamprophyre dyke in narrow fracture zone. Photo credit A. MacTavish. Sub-Stop C3k (528000E, 5402039N): Located 77m south of Sub-stop C3j. This sub-stop comprises a well-layered block of amphibole-nepheline syenite (~6.5m by 3.5m in size) that is surrounded by a highly distorted zone of fine- to very-coarse-grained (varitextured) syenitic material that appears to have flowed around the block (Photo 14 and Fig. 8). This - 88 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Photo 14. A well-layered block of amphibole-nepheline syenite that is surrounded by a highly distorted zone of fineto very-coarse-grained (varitextured) syenitic material that appears to have flowed around the block. Photo credit A. MacTavish. Also refer to Figure 9. Figure 7. Hand drawn detailed map of Layer Breccia Zone ‘A’. Mapping by A. MacTavish (1992). Abbreviation key: vcgr = very coarse-grained; f-cgr = fine- to coarse-grained; m-vcgr = medium-verycoarse-grained; int = intermediate; LS = Leopard spots (mottles). isolated, Spectacular Block ‘B’, consists of a sequence of three thick layers where amphibole and K-feldspar are aligned subparallel to layer bases. The base of each layer is undulatory on the scale of a single very coarse feldspar crystal. Figure 8. Hand drawn detailed map of Layer Breccia Zone ‘A’. Mapping by A. MacTavish (1992). Abbreviation key: fgr = fine-grained; mgr = medium-grained; cgr = coarse-grained; vcgr = very coarse-grained; f-cgr = fine- to coarse-grained; m-vcgr = medium-very coarse-grained; int = intermediate. - 89 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Sub-Stop C3l (528004E, 5402016N): A further 23m south of Sub-stop C3k is a zone characterized by well-developed syenite layering (Photo 15), some possible magmatic channel scours, some localized soft-sediment-style deformation, and a few zones of intense, localized layer disruption. Most of the layers within the southern part of the zone are quite flat lying (18°W dip). Photo 16. Crosscutting, vein-like body possibly resulting from the movement of volatile-rich magmatic fluids. Photo credit A. MacTavish. Photo 15. Well-developed, relatively flat-lying, magmatic layering within amphibole-nepheline syenite. Photo credit A. MacTavish. Sub-Stop C3m (528011E, 5402006N): This substop is located 12m southeast of Sub-stop C3l and is directly adjacent to it. It consists of a zone of disturbed and distorted syenite layering similar to that observed north of the isolated block observed at Sub-stop C3k. Contorted and convoluted layering is common and folding is observed locally with the disturbance increasing in intensity to the south. Most noticeable in this area is a deformed, crosscutting, texturally variable, vein-like body (Photo 16) composed of mobile “flowbanded” material. The margins of this “Vein C” (Fig. 9) are often irregular, possibly due to volatile fluid seepage (?) and it is often cored by coarse-grained to pegmatitic veinlets and pods. It is possible that this structure has erupted from the nose of a slump fold. Sub-Stop C3n (528020E, 5401977N): This final sub-stop is located 30m east-southeast of Sub-stop C3m and consists of a large slump-fold (Fig. 10) composed of medium- to very coarse-grained, modallyand normally grain-size graded syenite layers (Photo 17). This was interpreted as slump folding due to the presence of at least three axial planar directions present within three separate folds all in close proximity to each other. Unfortunately since mapping was completed in Figure 9. Hand drawn detailed map of Vein ‘C’. Mapping by A. MacTavish (1992). Abbreviation key: fgr = fine-grained; mgr = medium-grained; cgr = coarse-grained; vcgr = very coarse-grained; f-mgr = fine to medium-grained; c-vcgr = coarse to very coarse-grained; LM = Leopard mottles; peg = pegmatitic; int = intermediate 1992 this exposure has become partially obscured by the growth of lichen. - 90 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 tholeiitic lineage, contemporaneous with the Coldwell Complex. Fresh, metasomatized and hornfelsed, andesine-oligoclase basalt flows are estimated to attain a thickness of 5 km (Mitchell and Platt, 1994; Nicol, 1990). Assimilation and brecciation of the flows by subsequent gabbroic to syenitic magmatism has resulted in the widespread development of basaltic xenoliths ranging from 1m to over 1 km in size, comprising a roof pendant in the central part of the complex (Walker et al., 1992). Walker et al. (1992) subdivided these basaltic rocks into three main units: Figure 10. Hand drawn detailed map of the Area ‘D’ slump fold. Mapping by A. MacTavish (1992). Abbreviation key: fgr = fine-grained; cgr = coarse-grained; f-mgr = fine to medium-grained; f-cgr = fine to coarse-grained; int = intermediate 1. Aphanitic to fine-grained, massive, locally amygdaloidal (?) / ocellar basalt; 2. Medium-grained, diabasic (ophitic) basalt; and 3. Aphanitic to medium-grained, feldspar-phyric, diabasic (ophitic) basalt. At Wolf Camp Lake, aphanitic basalts contain round to amoeboid, epidote- and quartz-filled structures up to 2cm in diameter that have been interpreted as amygdules (Photo 18). Well-defined, amygdule-bearing zones dip 8° to the southwest in this vicinity (Walker et al., 1992). The basaltic roof pendant is locally underlain and enveloped by feldspar-phyric amphibole syenite and Fe-rich augite syenite. Photo 17. Lichen-obscured slump fold within layered amphibole-nepheline syenite. Photo credit A. MacTavish. Stop C4: Hornfelsed Basaltic Roof Pendants, Wolf Camp Lake UTM coordinates 541775E 5404189N 8.6 km west of the Highway 626 and Highway 17 junction Description: Hornfelsed basaltic rocks overlying the complex were recognized early in its mapping by Tuominen (1967) and Puskas (1970) and likely represent a volcanic edifice that has been subsequently eroded (Sage 1986). Mitchell and Platt (1994) and Nicol (1990) have considered these basalts to have a Photo 18. Amygdules within the basaltic roof pendant located near Wolf Camp Lake. Photo credit D. Campbell. Stop C5: Layered Fe-rich Augite Syenite (Alternate stop if time allows) UTM coordinates 544782E 5398443N 680 m west along the shoreline of Lake Superior from the end of the James River industrial road along the waterfront in Marathon; OR 150m south of Carden Cove road, 0.3 km past CPR tracks (park at 544864E, 5398750N) - 91 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Description: Broad expanses of glacially polished and wave-washed, massive Fe-rich augite syenite occur all along this part of the Lake Superior shoreline near Marathon. Fresh surfaces vary from dark green-brown to black, despite a buff to white weathered surface. Small dimension stone quarries were developed and produced in this area during the 1930’s. Much of the stone was shipped to larger centres in the American mid-west and Toronto. Fe-rich augite syenite (formerly referred to as ferroaugite syenite) comprises a large portion of the exposure in the eastern half of the Coldwell Complex. It appears to be a sheet-like intrusion that dips approximately 15° toward the center of the complex, sandwiched between the underlying Eastern Border Gabbro and an overlying, recrystallized amphibolequartz syenite; it also intrudes the basaltic roof pendants (Walker et al., 1992; 1993a). Crystallization of the syenite inwards from its upper and lower contacts produced mineralogical and compositional variations across it (Walker et al. 1993a). Constituent minerals include iridescent, lathlike, cryptoperthitic feldspar (up to 30% interstitial), and variable amounts of fayalite, amphibole, aenigmatite, and rare quartz. Coarse-grained to pegmatitic portions of the syenite host a variety of REE-bearing fluoro-carbonates, quartz, chalcedony, and molybdenite. Iridescent feldspar, known locally as “spectrolite”, was recently (2010) commercially extracted on a very small-scale from pegmatite at Shack Lake near Marathon. Although this unit is typically massive, rhythmic to chaotic layering is locally developed and where observed commonly dips shallowly towards the centre of the complex. At this site, layering strikes at 070° and dips 60° north. The layering is unusual in that it is defined by an intercumulus mineral (augite) rather that by cumulus phases (feldspar). at ~45°. This thickly layered sequence is underlain by massive gabbro near the contact with the Archean country rocks. The macrorhythmic layering is laterally discontinuous, pinching out over distances of 5 to 10m and contacts are sharp and conformable (Shaw, 1994, 1997). Rhythmic layering is modal and has been related to variation in the respective proportions of plagioclase (An60-35), augite (Fo67-43), minor orthopyroxene (En55-66), and Fe-Ti-oxides by Lum (1973). Modal plagioclase varies from approximately 60 to 80% in the leucocratic layers and 20 to 35% in the meso- to melanocratic layers (Shaw, 1994). A second band of layered gabbro, separated from the first by massive gabbro, is exposed on top of the long rock cut (Photo 19). Here, the macrorhythmic layering (Photo 20) produces relatively thin (1 to 5cm) to medium thick (5 to 100cm) layers that can be traced for over 35m along Photo 19. Macrorhythmic layering within the Eastern Border Gabbro. Photo credit M. Smyk. Stop C6: Layered Eastern (Border) Gabbro UTM coordinates 549199E 5398010N 1.7 km east of the Highway 626 and Highway 17 junction Description: Layering in the Eastern Border Gabbro shows distinct variations in style, is usually parallel to the eastern contact of the gabbro, and dips 20° to 60° toward the center of the complex (Shaw, 1994, 1997). At this stop, layering strikes approximately north and dips west towards the rest of the complex Photo 20. Macrorhythmic modal layering within the Eastern Border Gabbro. Photo credit A. MacTavish. - 92 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 strike. Layer contacts are sharp, locally scalloped and conformable. Trough cross-bedding has been noted on vertical faces by Shaw (1994). This stop is also close to the contact between the Eastern Border Gabbro and the Fe-rich augite syenite to the west. Pegmatitic syenite dykes intrude the gabbro at this locality and contain miarolitic cavities. McLaughlin (1990) has reported the presence of a variety of REE-bearing fluorocarbonates (bastnaesite, parisite, synchisite), Nb-bearing phases, and zircon in pegmatitic syenite with quartz, feldspar, and sodic amphibole. Stop C7 (Alternate Stop): Eastern Contact of the Coldwell Complex UTM coordinates 549656E 5396238N 3.3 to 3.6km east of the Highway 626 and Highway 17 junction Description: A number of highway rock cuts and outcrops expose the eastern contact of the Coldwell Complex with the enclosing Archean greenstone belt country rocks. Center 1 gabbros, the oldest rocks of the complex, form a ring dyke that forms the eastern and northern margins of the complex where it is in contact with Archean supracrustal and granitoid rocks. The reverse magnetization of these gabbros (Lilley, 1964) produces prominent magnetic “lows” on aeromagnetic maps. The most recent and comprehensive study of the Eastern Border Gabbro was conducted by Shaw (1994, 1997) who noted that more than 90% of the unit consists of layered gabbro. At this location, varitextured, unlayered Eastern Border Gabbro is in contact with, and contains numerous xenoliths of, Archean metasedimentary rocks. This has produced hybrid and contaminated phases and rheomorphic breccia. Crosscutting Center 1 syenite dikes are commonly pegmatitic. Amethystine quartz, calcite, and molybdenite occur in vugs within this chaotic contact zone. Disseminated iron- and copper-sulphides occur in biotite-rich, varitextured gabbro (Dunlop Occurrence), which has experienced sporadic exploration since the discovery of copper in the early 1950’s. It was last drilled in 1992 by Noranda Inc. with the best assay intervals grading 0.35% Cu/6.0m and 0.42% Cu/4.0m, respectively (Resident Geologist’s Files, Thunder Bay). A grab sample of rusty-weathering, moderately magnetic, fine- to medium-grained gabbro with coarse biotite and blebby chalcopyrite graded 5090ppm Cu, 494ppm Ni, 241ppm Zn, 8ppb Pd, 2ppb Pt and 22ppb Au (ibid). Overgrown pits are located just inside the tree line, west of the highway (UTM 549575E, 5396290N). Shaw (1994; 1997), Walker et al. (1993a, 1993b, 1993c), Currie (1980), and Tucker (1995) have documented a number of occurrences of rheomorphic breccia associated with the Eastern Border Gabbro along its intrusive, basal contact with the Archean supracrustal country rocks. Breccia units are characterized by chaotic flow fabrics that surround flow-oriented clasts situated in a medium-grained, granitic matrix. This unit has been somewhat enigmatic, having been alternatively described by earlier workers as conglomerate and ignimbrite (Resident Geologist’s Files, Thunder Bay). Similar exposures of this map unit also occur along the western contact of the complex, north of Middleton (cf. Wilkinson, 1983). Locally, pods of breccia vary from 20 to 75m in width and are up to 250m long. The breccia exposed along Highway 17 at this site contains mainly hornfelsed Archean clastic metasedimentary and metavolcanic rocks and massive vein quartz. In the vicinity of Two Duck Lake, the breccia contains fine-grained gabbro clasts (Tucker, 1995). The breccia varies from clast- to matrix-supported; the matrix consists of equigranular quartz, feldspar, and minor biotite, clino- and orthopyroxene, and opaque minerals; and tourmaline and prehnite overgrowths have been noted (Tucker, 1995). Rounded to angular clasts range in size from 0.5 to over 100cm and locally have developed 1 to 2cm wide, chlorite-rich reaction rims that are thickest where they are matrix-supported (Shaw, 1994). Magnetite and quartz¬feldspar-tourmaline veins cut both matrix and clasts. Quartzo-feldspathic rinds and crosscutting veinlets have been interpreted to be the result of partial melting of the felsic material during assimilation. The close association between rheomorphic breccia and the Eastern Border Gabbro suggests that the intrusion of the gabbro led to the brecciation and partial melting of the country rocks (Shaw, 1994, 1997; Tucker, 1995). - 93 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 PART 2: Marathon Cu-PGM Deposit David Good and John McBride Introduction to the Marathon Deposit The Cu-PGM sulphide mineralization of the Marathon deposit is hosted by the Two Duck Lake Gabbro, the latest mafic intrusive event and consequently the most continuous gabbroic body within the Eastern Gabbro Suite at the Marathon deposit. The Eastern Gabbro Suite, located around the eastern and northern margin of the Coldwell, was composed initially of a thick sequence of tholeiitic basalt that was subsequently intruded by a much larger volume of leucocratic to ultramafic intrusions that caused contact metamorphism of the basalt to pyroxene-hornfels grade (Good et al., 2015). All of these units are represented at the Marathon deposit (Fig. 1). The topography of the Coldwell is characterized by deep valleys and steep cliffs that form strong surface lineaments. Two lineaments at the Marathon deposit correspond to north dipping normal faults (north side down) with displacement of approximately 50 metres. Two Duck Lake Intrusion The Two Duck Lake intrusion is irregular in shape and elongated north-south (Fig. 2). The dip at the east contact is variable from nearly flat (at the south end) Figure 1. Geology of the Marathon deposit (after Good et al., 2015) highlighting location of field trip stops. Stops are marked with red dot and labelled as stop 1a, etc. Note two normal faults that correspond to strong surface lineaments (dashed lines) to vertical and locally overturned where the footwall overhangs the intrusion. The intrusion is composed of coarse-grained to pegmatitic olivine gabbro and troctolite. Modal layering is rare. The TDL gabbro was interpreted to have formed by intrusion of a nearly homogeneous plagioclase crystal mush by Good and Crocket (1994). But recent work suggests the intrusion formed by accumulation of several pulses of magma in a conduit setting (Good, Figure 2. 3d isometric view of the Two Duck Lake intrusion (from Good et al., 2015). Three coloured portions indicate blocks that were offset by normal faults with north side down by up to 60 metres. Note that numerous intrusions of mineralized Mt-Ol-Cpx-Ap rock (yellow) occur in the vicinity of major feeder zones, but those above the 6300 feeder but are not shown. - 94 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 2010; Ruthart, 2012; Good et al., 2015; and Shahabi Far, 2017). Multiple feeder channels were inferred by Good et al. (2015) to occur in the vicinity of several coincident features, including: deep V- or U-shaped channels in the footwall contact; topographic lineaments; very thick mineralized intervals; and irregular-shaped intrusions of olivine-magnetite-clinopyroxene-apatite. Age relationships Evidence suggests that all units in the Coldwell were emplaced within a short time interval between about 1108 Ma and 1105 Ma (Heaman and Machado, 1992; Good et al. in preparation). Age relationships, based on cross-cutting contacts and U-Pb age dating for the various mafic units are summarized in Figure 3. The metabasalt is interpreted to correlate with Mamainse Point Volcanic Group 1 (Fig. 4) which was emplaced at approximately 1108 Ma (Keays and Lightfoot, 2015). Two Duck Lake gabbro and associated breccia (Fig. 5) and occurs within several thick and continuous shallow-dipping lenses that parallel the footwall contact. The disseminated sulphides are concentrated in troughs along the footwall contact that approximately follow topographic lineaments (Fig. 6). The lenses are referred to as the Footwall, Main, and Hangingwall zones and the W Horizon. Sulfides in the Footwall, Main, and Hanging-wall zones consist predominantly of chalcopyrite and pyrrhotite with minor amounts of cubanite, bornite, pentlandite, cobaltite, and pyrite. Sulfides occur interstitial to primary silicates and also in association with hydrous silicates such as amphibole, chlorite, and minor serpentine (Watkinson and Ohnenstetter, 1992; Samson et al., 2008). Chalcopyrite occurs as separate grains or as rims on pyrrhotite grains. Some chalcopyrite is intergrown with highly calcic plagioclase (An70–An80) in replacement zones at the margins of plagioclase crystals (Good and Crocket, 1994; Shahabifar, 2016). The metabasalt was subsequently intruded by the following units, listed in order from oldest to youngest, layered troctolite sill of the Marathon Series, gabbroic anorthosite and olivine gabbro of the Layered Series, Two Duck Lake gabbro and various ultramafic units composed of magnetite +/- olivine +/- apatite +/clinopyroxene of the Marathon Series, Malpa Lake intrusion, and syenite. The W horizon is characterised by extreme PGE enrichment relative to Cu with several 2m thick drill hole intersections having 20 to 70 ppm Pd and Cu/Pd as low as 3 (e.g., Fig. 7, top and bottom photos). The best intersection contains 34 ppm Pd and 9.6 ppm Pt over 10 m. Mass balance considerations, assuming initial magma contained 10 ppb Pd, would require a magma column on the order of 34 km to generate the 34 ppm Pd in this interval. Disseminated sulfide mineralization is hosted by the The W Horizon is commonly difficult to identify in drill core because it typically contains only trace sulfides, but if sulfides are present, they consist of Mineralization Figure 3. Relative timing of mafic metavolcanic and intrusive events (age dates after Good et. al, in preparation) in the Eastern Gabbro Suite of the Coldwell Alkaline Complex. - 95 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 4. Correlation diagram showing range of ages for the Coldwell units compared to volcanic and intrusive units in the Midcontinent Rift (after Keays and Lightfoot, 2015). Figure 5. Stratigraphic section through the Main zone and overlying troctolite sill. Note the saw tooth pattern for Cu, Pd and Cu/Pd indicating individual pulses of sulphide-bearing crystal slurry. Unit 2d, breccia of metabasalt blocks and Two Duck Lake gabbro; unit 3bd, coarse grained ophitic and pegmatitic Two Duck Lake gabbro; unit 4a, breccia of footwall blocks and Two Duck Lake gabbro (from Good et al., 2015). - 96 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 6. Three versions of top view for the Marathon deposit showing 3d topography (green surface) and contoured footwall surface models. Note the troughs and ridges (left hand image) correspond to surface lineaments. Note the higher-grade assays for Cu (>0.5%) and Pd (>3 ppm) are aligned within zones that parallel troughs within the 3d footwall surface model. Figure 7. PGE rich samples from the W Horizon contain fresh clinopyroxene, olivine and plagioclase. Top photo sample with 107 ppm Pd+Pt+Au and 203 ppm Cu. Bottom photo sample with 70 ppm Pd+Pt+Au and 0.86 % Cu. chalcopyrite and bornite with minor pyrrhotite and trace amounts of pentlandite, cobaltite, and pyrite (Ruthart, 2012). Deposit Model Exploration strategies in the Coldwell are based on the conduit model and a schematic magmatic plumbing system such as that envisioned by Barnes et al. (2016) - 97 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 (Fig. 8). Evidence for a magma conduit setting at the Marathon deposit were described by Good et al. (2015), and include: consistent with rotation of sill from west dipping in the north to sub horizontal or north dipping in the south. • association with metavolcanic rocks • fault control (mineralization parallel to topographic lineaments) • brecciation and assimilation • accumulation in trough setting • flow through PGE upgrading • tube shaped intrusion • gravity driven back flow (Mt-Ol-Cpx-Ap cumulates) • high heat flux (wide zone of pyroxenehornfels grade metavolcanic rocks) Figure 9. Map at south end of Marathon deposit showing location of stops 1 and 2. Figure 8. Step 3 of schematic illustration for magmatic plumbing system (after Barnes et al., 2016) Field Trip Stops Stop 1a: South end of Troctolite Sill (Figs. 10b) UTM coordinates 549995E 5403560N Trench exposure with coarse-grained mottled augite troctolite shows large fresh oikocrysts of olivine (brown), clinopyroxene (black) and magnetite (black) and subhedral plagioclase (white). The layered troctolite sill is an important marker horizon because it occurs just above the top of the Main Mineralized zone and is an indicator of the relative fault offset that occurred along E-W–trending normal faults at 5404500 and 5404900 North (Fig. 1). The sill dips moderately west at the north end, but flattens out in the south to sub-horizontal (Fig. 9b). Note layering is approximately east west at Stop 1a, Figure 10. 3d image (iso view) of geology at south end of Marathon deposit showing location of stops 1 and 2 on trenched outcrops (black polygons): (a) shows orientation of footwall surface troughs approximately perpendicular to contact, and the red centre line of the W horizon at surface on the splat trench; (b) top (light blue) and bottom surfaces (dark blue) of the troctolite sill. Gap in the troctolite sill surfaces represents location where W Horizon and TDL gabbro cuts the through the sill; (c) surface model of W Horizon (yellow). - 98 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Stop 1b: Southeast corner of Marathon deposit (Figs. 10 and 11) UTM coordinates 550090E 5403395N Trench outcrop exposure of Two Duck Lake gabbro within a shallow dipping bowl-shaped depression in the footwall (Fig. 10a) includes W Horizon and Main zone type mineralization. Figure 11. Plan map of trench at the southeastcorner of the Marathon deposit including assay table for samples 44 to 56 located in channel immediately north of the historic trench. Red circle marks location of historic trench (ca. mid 1960’s) with high copper mineralization. The unit was not assayed for Pd until 2005. The channel sample located just north of the red circle returned assays of 3.37 ppm Pd+Pt+Au, and 0.35% Cu over 18.6 m. East-west layering in TDL gabbro is visible just south of trench. Outcrop shows textural evidence for cross cutting intrusions of subophitic olivine gabbro. - 99 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Stop 2a: Splat trench and the W Horizon (Fig.12) UTM coordinates 549900E 5403804N Stops 2a and 2b are located on the east and northwest branches of the splat trench, respectively, and highlight two locations along the W horizon as it drapes over top of a north plunging ridge in the footwall. Figure 12. Plan map of the stripped outcrop on the east arm of the splat trench highlights the distinct coarse-grained to pegmatitic subophitic olivine gabbro of the Two Duck Lake intrusion. Stratigraphic top of the section is to the northeast (compare to Fig. 9a and 9c). Note the large xenolith/breccia of metavolcanic rock along the east edge of the outcrop. Codes: 3a, medium-grained (1-5 mm) Two Duck Lake gabbro; 3b, coarse-grained (5mm to 1cm); 3d, pegmatitic gabbro; 3f, magnetite and clinopyroxene rich gabbro; 4, breccia; 2a, metavolcanic rock. Mineralization consists of disseminated cpy, bn and minor po. Assay table for samples 9 to 24 within the saw-cut channel on the outcrop have an average grade of 2.64 g/t Pd+Pt+Au and 0.1% Cu over 25.1 m. - 100 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Stop 2b: Splat trench and the W Horizon (Fig. 13) UTM coordinates 549810E 5403825N Stop 2b: Malachite zone - Splat trench Figure 13. Plan map of the northwest outcrop on the splat trench highlights the distinct coarse-grained to pegmatitic subophitic olivine gabbro of the Two Duck Lake intrusion. Stratigraphic top of the section is to the north (compare to Fig. 9a and 9c). Note the large xenolith/breccia zone of metavolcanic rock along the east edge of the outcrop. Codes: 3a, mediumgrained (1-5 mm) Two Duck Lake gabbro; 3b, coarse-grained (5mm to 1cm); 3d, pegmatitic gabbro; 3f, magnetite and clinopyroxene rich gabbro; 4, breccia; 2a, metavolcanic rock. Mineralization consists of disseminated cpy, bn and minor po. Assay table for samples 84 to 93 within the saw-cut channel on the outcrop have an average grade of 2.13 g/t Pd+Pt+Au and 0.36% Cu over 17 m. References in ferroan olivine gabbros of the Coldwell Complex, Ontario; in The Geology, Geochemistry, Mineralogy and Mineral Beneficiation of Platinum-Group Elements. Edited by L.J. Cabri; Canadian Institute of Mining and Metallurgy and Petroleum, Special Volume 54, p.321-337. Alexander, M. 2007. The mineralogy of NYF pegmatites from the Coldwell Alkaline Complex, northwestern Ontario; unpublished MSc thesis, Lakehead University, Thunder Bay, Ontario. Barnes, S.J., Cruden, A.R., Arndt N., Saumur B.M. 2016. The mineral system approach applied to magmatic Ni–Cu–PGE sulphide deposits, Ore Geology Reviews 76, 296-316. Barrie, C. Tucker, MacTavish, A.D., Walford, P.C., Chataway, R., and Middaugh, R., 2002. Contacttype and magnetite reef-type Pd-Cu mineralization Bell, K. and Blenkinsop, J. 1980. Grant 42: Ages and initial 87Sr-86Sr ratios from alkaline complexes of Ontario; in Geoscience Research Grant Program, Summary of Research, 1974-1980, Ontario Geological Survey, Miscellaneous Paper 93, p.16-23. Bohay, T.J. 1997. The Coldwell alkaline complex, Ontario: Magmatic affinity as determined by an isotopic - 101 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 and geochemical study; unpublished MSc thesis, McMaster University, Hamilton, Ontario, 135p. and deposits in northwestern Ontario; Ontario Geological Survey, Open File Report 5889, 145p. Coates, M.E. 1970. Geology of the Killala–Vein lakes area, Ontario; Ontario Department of Mines, Geological Report 81, 35p. 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, Econ Geol, 110, 1235-1267. Good, D.J. 1992. Genesis of copper-precious metal sulfide deposits in the Port Coldwell alkalic complex, Ontario; unpublished PhD thesis, McMaster University, Hamilton, Ontario, 203p. Good, D.J. 1993. Genesis of copper-precious metal sulfide deposits in the Port Coldwell Alkalic Complex, Ontario Geoscience Research Grant Program, Grant No. 341, Ontario Geological Survey, Open File Report 5839, 23. Good, D.J. and Crockett, J.H. 1994a. Genesis of the Marathon Cu-platinum-group element deposit, Port Coldwell alkaline complex, Ontario: A Midcontinent Rift-related magmatic sulfide deposit; Economic Geology, v.89, p.131-149. Good, D.J. and Crockett, J.H., 1994b. Origin of albite pods in the Geordie Lake gabbro, Port Coldwell alkaline complex, northwestern Ontario: Evidence for latestage hydrothermal Cu-Pd mineralization; The Canadian Mineralogist, v.32, p.681-701. 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, Econ Geol v.110, p.983-1008. Hauck, S.A., Severson, M.J, Zanko, L., Barnes, S.-J., Morton, P., Alminas, H., 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; Geological Society of America, Special Paper 312, p.137-185. Heaman, L.M., Easton, R.M., Hart, T.R., Hollings, P., MacDonald, C.A. and Smyk, M. 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon Region, Ontario; Canadian Journal of Earth Sciences, v.44, no.8, p.1055-1086. Heaman, L.M. and Machado, N. 1987. Isotope geochemistry of the Coldwell alkaline complex: 1. U-Pb studies on accessory minerals; Geological Association of Canada–Mineralogical Association of Canada, Joint Annual Meeting, Saskatoon, Saskatchewan, Program with abstracts, p.54. Heaman, L.M. and Machado, N 1992. Timing and origin of the Midcontinent Rift alkaline magmatism, North America: Evidence from the Coldwell complex; Contributions to Mineralogy and Petrology, v.110, p.289-303. Hinz, P. and Landry, R. 1994. Industrial mineral occurrences Kerr, H.L. 1910a. Geological map of part of the north shore of Lake Superior, District of Thunder Bay; Ontario Bureau of Mines, Annual Report Map 19B, scale 1:63 360. Kerr, H.L. 1910b. Nepheline syenites of Port Coldwell; Ontario Bureau of Mines, Annual Report, v.19, p.194-232. Kissin, S.A. and McCuaig, T.C. 1988. The genesis of silver vein deposits in the Thunder Bay area, northwestern Ontario: Geoscience Research Grant Program, Summary of Research, 1987-1988; Ontario Geological Survey, Miscellaneous Paper 140, p.146156. Klasner, J.S., Cannon, W.F. and Van Schmus, E.R. 1982. The Pre-Keweenawan tectonic history of the southern Canadian Shield and its influence on the formation of the Midcontinent Rift; in Geology and Tectonics of the Lake Superior Basin, Geological Society of America, Memoir 156, p.27-46. Lewchuk, M.T. and Symons, D.T.A. 1990. Paleomagnetism of the late Precambrian Coldwell complex, Ontario, Canada; Tectonophysics, v.184, p.73-86. Laderoute, D.G. 1987. The petrology, geochemistry, and petrogenesis of alkaline dyke rocks from the Coldwell Alkaline complex; unpublished M.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 89p. Tectonophysics, v.184, p.73-86. Lilley, F.E.M. 1964. An analysis of the magnetic features of the Port Coldwell intrusion; unpublished BSc thesis, University of Western Ontario, London, Ontario, 89p. Lukosius-Sanders, J. 1988. Petrology of the syenites from Center III of the Coldwell alkaline complex, northwestern Ontario; unpublished MSc thesis, Lakehead University, Thunder Bay, Ontario, 141p. Lum, H.K. 1973. Petrology of the eastern gabbro and associated sulphide mineralization of the Coldwell alkaline complex, Ontario; unpublished BSc thesis, Carleton University, Ottawa, Ontario, 68p. MacTavish, A. 2000. A new style of PGE mineralization within the Coldwell alkaline complex, northwestern Ontario; Ontario Exploration and Geoscience Symposium, Toronto, December 11-12, 2000, Speaker Abstracts, p.3. MacTavish, A., Lukosius-Sanders, J. and Jowett, R. 1987. Geological report of the Joa Option (Geordie Lake - 102 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 property), St. Joe Canada Inc.; unpublished report, Resident Geologist’s Files, Thunder Bay, 7p. MacTavish, A Smyk, M., Good, D., and McBride, J., 2017. Transect Through the Coldwell Alkaline Complex In; MacTavish, A. and Hollings, P. (Eds.), Institute on Lake Superior Geology Proceedings, 63rd Annual Meeting, Wawa, Ontario, Part 2 - Field trip guidebook, v.63, part 2, 1-44. McLaughlin, R.M. 1990. Accessory rare metal mineralization in the Coldwell alkaline complex, northwest Ontario; unpublished MSc thesis, Lakehead University, Thunder Bay, Ontario, 123p. Miller, J.D., Jr., Nicholson, S.W., and Cannon, W.F. 1995. The Midcontinent rift in the Lake Superior region, in Miller, J.D., Jr., ed., Field trip guidebook for the geology of ore deposits of the Midcontinent rift in the Lake Superior region; Minnesota Geological Survey Guidebook Series, no. 20, p.1-22. Mitchell, R.H. and Platt, G. R. 1978. Mafic mineralogy of ferroaugite syenite from the Coldwell alkaline complex, Ontario, Canada; Journal of Petrology, v.19, p.627-651. Mitchell, R.H. and Platt, G. R. 1982a. The Coldwell alkaline complex; in Field Trip Guidebook, Proterozoic geology of the northern Lake Superior area, Geological Association of Canada–Mineralogical Association of Canada, Joint Annual Meeting, Winnipeg, Manitoba, p.42-61. Mitchell, R.H. and Platt, G. R. 1982b. Mineralogy and petrology of nepheline syenites from the Coldwell alkaline complex, Ontario, Canada; Journal of Petrology, v.23, p.186-214. Mitchell, R.H. and Platt, G. R. 1994. Aspects of the geology of the Coldwell alkaline complex: Field trip A2, Geological Association of Canada–Mineralogical Association of Canada, Joint Annual Meeting, Waterloo, Ontario, 36p. Mitchell, R.H., Platt, G.R., Lukosius-Sanders, J., ArtistDowney, M. and Moogk-Pickard, S. 1993. Petrology of syenites from Center III of the Coldwell alkaline complex, northwestern Ontario, Canada; Canadian Journal of Earth Sciences, v.30, p.145-158. Mitchell, R.H., Platt, R.G. and Cheadle, S.P. 1983. A gravity study of the Coldwell complex, northwestern Ontario, and its petrological significance; Canadian Journal of Earth Sciences, v.20, p.1631-1638. Mulja, T. 1989. Petrology, geochemistry, sulphide- and platinum-group element mineralization of the Geordie Lake intrusion; unpublished MSc thesis, Lakehead University, Thunder Bay, Ontario, 234p. Mulja, T. and Mitchell, R.H. 1990. Platinum-group minerals and tellurides from the Geordie Lake intrusion, Coldwell complex, northwestern Ontario; Canadian Mineralogist, v.28, p.489-501. Mulja, T. and Mitchell, R.H. 1991. The Geordie Lake intrusion, Coldwell Complex, Ontario: Palladiumand tellurium-rich disseminated sulfide occurrence derived from an evolved tholeiitic magma; Economic Geology, v.86, p.1050-1069. Nicol, D.N. 1990. Assimilation of basic xenoliths with Center 3 syenites of the Coldwell Complex, Ontario; unpublished MSc thesis, Lakehead University, Thunder Bay, Ontario, 59p. Ohnenstetter, D., Watkinson, D.H. and Dahl, R. 1991. Zoned hollingworthite from the Two Duck Lake intrusion, Coldwell complex, Ontario; American Mineralogist, v.76, p.1694-1700. Penczak, R.S. 1992. Petrology and mineral chemistry of the Middleton copper occurrence of the Western gabbro, Coldwell alkaline complex, Ontario; unpublished BSc thesis, University of Waterloo, Ontario. Pollock, S.J. 1987. The isotopic geochemistry of the Prairie Lake carbonatite complex; unpublished MSc thesis, Carleton University, Ottawa, Ontario, 71p. Potter, E.G. 2004. The rare and exotic mineralogy of the western subcomplex of the Dead Horse Creek diatreme, northwestern Ontario; unpublished MSc thesis, Lakehead University, Thunder Bay, Ontario. Puskas, F.W. 1967. Port Coldwell area; Ontario Department of Mines, Preliminary Map P.114, scale 1:31 680. Puskas, F.W. 1970. The Port Coldwell alkali complex; in Proceedings, 16th Institute on Lake Superior Geology, Thunder Bay, Ontario, p.87-100. Queen, M., Heaman, L.M., Hanes, J.A., Archibald, D.A. and Farrar, E. 1996. 40Ar/39Ar phlogopite and U-Pb perovskite dating of lamprophyre dykes from the eastern Lake Superior region: Evidence for a 1.14 Ga magmatic precursor to Midcontinent Rift volcanism; Canadian Journal of Earth Sciences, v.33, p.958-965. Ruthart R. 2012. Characterization of high-PGE, low-sulphur mineralization at the Marathon PGE-Cu deposit, Ontario: M.Sc. thesis, Waterloo, ON, University of Waterloo, 145 p. Sage, R.P. 1982. Mineralization in diatreme structures north of Lake Superior; Ontario Geological Survey, Study 27, 79p. Sage, R.P. 1985. Geology of carbonatite-alkaline rock complexes in Ontario: Chipman Lake area; Ontario Geological Survey, Study 44, 40p. Sage, R.P. 1986. Alkalic rock complexes – carbonatites of northern Ontario and their economic potential; unpublished PhD thesis, Carleton University, Ottawa, Ontario, 335p. Sage, R.P. 1987. Geology of carbonatite-alkaline rock complexes in Ontario: Prairie Lake carbonatite - 103 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 complex, District of Thunder Bay; Ontario Geological Survey, Study 46, 91p. Tuominen, H.V. 1967. Port Coldwell area; Ontario Department of Mines, Map P.114, scale 1:15 840. Sage, R.P. 1991. Alkaline rock, carbonatite and kimberlite complexes of Ontario, Superior Province; in Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p. 683-709. Walker, E.C., Sutcliffe, R.H., Shaw, C.S.J., Shore, G.T. and Penczak, R.S. 1992. Geology of the Port Coldwell alkaline complex; in Summary of Field Work, 1992, Ontario Geological Survey, Miscellaneous Paper 160, p.108-119. Sage, R.P. and Watkinson, D.H. 1995. Alkalic rocks of the Midcontinent rift; Institute on Lake Superior Geology, Marathon, ON, Proceedings Volume 41:2A, 79p. Samson, I.M., Fryer, B.J., and Gagnon, J.E. 2008. The Marathon Cu-PGE deposit, Ontario: Insights from sulphide chemistry and textures, in Goldschmidt conference, p. 820. Shaw, C.S.J. 1994. Petrogenesis of the eastern gabbro, Coldwell alkaline complex, Ontario; unpublished PhD thesis, University of Western Ontario, London, Ontario, 292p. Shahabi Far, M.. 2016. The magmatic and volatile evolution of gabbros hosting the Marathon PGE-Cu deposit: evolution of a conduit system, PhD thesis, University of Windsor, Ontario. Shaw, C.S.J.1997. The petrology of the layered gabbro intrusion, eastern gabbro, Coldwell alkaline complex, northwestern Ontario, Canada: Evidence for multiple phases of intrusion in a ring dyke; Lithos, v.40. p.243-259. Smyk, M.C., Taylor, R.P., Jones, P.C. and Kingston, D.M. 1993. Geology and geochemistry of the West Dead Horse Creek rare-metal occurrence, northwestern Ontario; Exploration and Mining Geology, v.2, no.3, p.245-251. Smyk, M.C. and Sage, R.P. 1995. Geology and mineralization of intrusive complexes of the Marathon, Ontario area; in Field Trip Guidebook for the Geology and Ore Deposits of the Midcontinent Rift in the Lake Superior region, International Geological Correlation Program, Project 336, Field Conference and Symposium, Duluth, Minnesota, August 19 to September 1, 1995, p.182-193. Tucker, C. 1995. Origin of breccia associated with the Eastern Gabbro, Coldwell alkaline complex, northwestern Ontario; unpublished BSc thesis, University of Western Ontario, London, 57p. Walker, E.C., Sutcliffe, R.H., Shaw, C.S.J., Shore, G.T. and Penczak, R.S 1993a. Precambrian geology of the Coldwell Alkaline Complex; Ontario Geological Survey, Open File Report 5868, 30p. Walker, E.C., Sutcliffe, R.H., Shaw, C.S.J., Shore, G.T. and Penczak, R.S 1993b. Precambrian geology, Port Coldwell complex, west half; Ontario Geological Survey, Preliminary Map P.3232, scale 1:20 000. Walker, E.C., Sutcliffe, R.H., Shaw, C.S.J., Shore, G.T. and Penczak, R.S 1993c. Precambrian geology, Port Coldwell complex, east half; Ontario Geological Survey, Preliminary Map P.3233, scale 1:20 000. Watkinson, D.H., Whittaker, P.J. and Jones, P.L. 1983. Platinum group elements in the eastern gabbro, Coldwell complex, northwestern Ontario; Ontario Geological Survey, Miscellaneous Paper 113, p.183191. Watkinson, D.H. and Ohnenstetter, D. 1992. Hydrothermal origin of platinum-group mineralization in the Two Duck Lake intrusion, Coldwell Complex, Northwestern Ontario: Canadian Mineralogist, v. 30, p. 121-136. Weiblen, P.W. 1982. Keweenawan intrusive rocks; Geological Society of America Memoir 156, p.57-82. Wilkinson, S.J. 1983. Geology and sulphide mineralization of the marginal phases of the Coldwell complex, northwestern Ontario; unpublished MSc thesis, Carleton University, Ottawa, Ontario, 129p. Wu, F.Y., Mitchell, R.H., Li, Q-L., Zhang, C., and Yang, Y-H. 2017. Emplacement age and isotopic composition of the Prairie Lake Carbonatite complex, Northwestern Ontario, Canada. Geological Magazine 154(2): 217236. - 104 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Field trip 7 - Building and ornamental stone sites of the Marathon Area, Ontario Peter Hinz Mineral Exploration & Development Section, Ontario Ministry of Energy, Northern Development and Mines, 435 James Street South, Suite B002 Thunder Bay, Ontario, P7E 6S7, Canada Foreward This tour will examine two past-producing “granite” quarries, two “granite” exploration sites in the Marathon area, one ornamental stone occurrence and, time permitting, the possible source of the recently popular “yooperlite” cobbles found in the Upper Peninsula of Michigan. Introduction (taken from Hinz et al., 1994) The dimension and monument stone industry in northwestern Ontario has a long history and is linked to the development and prosperity of the region. One of the earliest commercial operations was located on Vert Island in Nipigon Bay of Lake Superior. The Mesoproterozoic Sibley Group yielded an attractive red sandstone which was extracted by the Chicago Verte Island Sandstone Company. The stone was used locally for the construction of the Canadian National Railway and shipped to Chicago, Winnipeg, southern Ontario, and other U.S. cities for construction uses. Development of some of the earliest quarries in the Marathon and Nipigon areas was directly related to the construction of the Canadian Pacific Railway in the late 1880’s. Syenites of the Coldwell Alkaline Complex in the vicinity of Marathon and sandstones south of Nipigon were used in the construction of railway trestles to span the Black, Pic, Little Pic, Steel, and Nipigon rivers. Today these trestles show very little wear and are a testament to the long-standing durability of the stones. Although markets for dimension stone decreased in the early 1900’s, production continued at the Simpson Island sandstone quarry (1900 to 1910) and at the Bannerman and Horne quarry (1912 to 1915) near Ignace. The next period of quarry development took place during the late 1920’s to early 1930’s. Five small scale quarries operated northwest of Marathon along the Canadian Pacific Railway. Black and brown granites were extracted and shipped to customers in Toronto, Buffalo, Chicago, and Detroit. In 1932, the last of these quarries closed due to the loss of market. opened a quarry approximately 12 km southwest of the town of Vermilion Bay. This highly popular pink granite began production in 1954 and continued sporadically under various names until 1991 when the quarry, now named Granite Quarriers (GQI) Inc., closed. In 1981, Nelson Granite Limited of Sussex, New Brunswick began production of an identical granite from a quarry immediately south of the highway from the Granite Quarriers Inc. site. This quarry has operated year-round since that time and is still in production. Currently, 2019, Nelson Granite Limited is the only stone producer operating in northwestern Ontario. Nelson Granite produces a range of colours including pink, yellow, green, brown, and white granite from four quarries located north of Kenora and west of Vermilion Bay. Northwestern Ontario stone is shipped around the world for a range of uses including: building stone for interior and exterior uses; monumental stone; and landscape uses including pavers, benches, and accent pieces. Detailed descriptions of the historic quarries, their operations, geology and geotechnical test results are provided in Hinz et al. (1994). Descriptions of current producers are available in the Kenora portion of the Report of Activities 2018 (Paterson et al., 2019). Geologic setting Puumala (2018) provides the following general geological description of the Coldwell Alkaline Complex. “The geology of this area is dominated by rocks of the Coldwell Alkaline Complex. The Coldwell Complex was emplaced into Neoarchean supracrustal rocks of the Wawa Subprovince of the Superior Province during the Mesoproterozoic Midcontinent Rift event at ca. 1108 +/- 1 Ma (Heaman and Machado, 1992). The complex approximately bisects the Schreiber-Hemlo greenstone belt and is located at the north end of the Thiel fault, a zone of faulting which separates grabens with different subsidence history in the rift (Cannon et al., 1989). In 1948, the Vermilion Pink Granite Company - 105 - The Coldwell Complex was mapped by Kerr (1910a, �Proceedings of the 65th ILSG Annual Meeting - Part 2 1910b), Puskas (1967), and Walker et al. (1993), and comprises three, superimposed ring sub-complexes or magmatic centers (Mitchell and Platt, 1978) that young progressively (Centers 1 to 3) to the southwest. The rocks of Center 1 are silica-saturated and include the Eastern and Western border gabbros (the oldest rocks within the complex) and later iron-rich augite syenite and syenite-syenodiorite (Mitchell and Platt, 1978, 1982; Mulja, 1989). Center 2 includes silica-undersaturated alkalic rocks with oversaturated residue. Rock types include nepheline- and hastingsite-bearing miaskitic nepheline syenite, and numerous volumetrically minor alkaline lamprophyre and analcime tinguaite dykes (Mitchell and Platt, 1978, 1982; Laderoute, 1987; and Mulja, 1989). Center 3 includes silica-oversaturated alkalic rocks with oversaturated residue consisting of magnesio-hornblende syenites, quartz syenites, and minor granites (Mitchell and Platt, 1994; LukosiusSanders, 1988). The map area covers much of the Eastern border gabbro, which hosts numerous occurrences of Magmatic Cu-Ni-PGE (± Au, Ag) and Ti-V±apatite mineralization (MacTavish and Smyk, 2017). The Marathon Cu-PGE deposit is the most notable example of the deposits hosted in the Eastern border gabbro. Another gabbroic intrusion within the interior of the Coldwell Complex hosts the Geordie Lake Cu-PGE deposit (MacTavish and Smyk, 2017). Centre 1 augite and amphibole syenite has previously been quarried as dimension stone in Marathon, just to the south of the Coldwell Complex map area (Hinz et al., 1994), and these rocks continue to see periodic exploration interest. Late-stage syenite pegmatites that host occurrences of Nb-Y-F-rare earth elements also occur in the area (Alexander, 2007).” The current field trip stops will feature rocks of all intrusive centres as shown in Figure 1: 1. Center 1: Stops 1, 2, 3, and 4; 2. Center 2: Stops 5 and 6; 3. Center 3: Stop 7. MacTavish and Smyk (2017), Walker et. al. (1993) and Hinz et. al. (1994) provide descriptions of the lithologies which will be visited. From MacTavish and Smyk (2017), “Fe-rich augite syenite (formerly referred to as ferroaugite syenite) comprises a large portion of the exposure in the eastern half of the Coldwell Complex. It appears Figure 1. Coldwell Complex maps showing field trip stop locations within the three intrusive centres. to be a sheet-like intrusion that dips approximately 15° toward the center of the complex, sandwiched between the underlying Eastern Border Gabbro and an overlying, recrystallized amphibole-quartz syenite; it also intrudes the basaltic roof pendants (Walker et al., 1992, 1993a). Crystallization of the syenite inwards from its upper and lower contacts produced mineralogical and compositional variations across it (Walker et al.; 1993a). Constituent minerals include iridescent, lathlike, cryptoperthitic feldspar (up to 30% interstitial), and variable amounts of fayalite, amphibole, aenigmatite, and rare quartz. Coarsegrained to pegmatitic portions of the syenite host a variety of REE-bearing fluoro-carbonates, quartz, chalcedony, and molybdenite. Iridescent feldspar, known locally as “spectrolite”, was recently (2010) commercially extracted on a very small-scale from pegmatite at Shack Lake near Marathon. Feldsparporphyritic amphibole syenite contains two textural variants, a feldspar porphyritic amphibole syenite with an aphanitic to medium-grained groundmass and interstitial amphibole; and a later intrusion of mediumgrained amphibole syenite with columnar feldspar and interstitial amphibole.” Walker et al. (1993) stated that “the amphibolenepheline syenite (Unit 13) is white to red, mesocratic to leucocratic, medium-grained with variable proportions of feldspar, nepheline, amphibole, biotite, apatite, and zeolites. Locally the nepheline syenite is well-layered - 106 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 with melanocratic olivine-nepheline syenite grading into mesocratic syenite. Spectacular orbicular layering occurs on the south shore of Pic Island. An intergranular texture resulting from intergrown feldspar, amphibole, and nepheline is typical of the unit. Near lineaments and lithological contacts the amphibole-nepheline syenite becomes red. Texturally different varieties of amphibole-nepheline syenite occur near the contacts and include mesocratic nepheline-amphibole syenite with near-equant euhedral amphibole prisms and mesocratic amphibole-nepheline syenite with interstitial amphibole and euhedral columnar feldspar. The amphibole-natrolite-nepheline syenite (Unit 14) is an extremely variable rock unit that intrudes the roof pendant mafic volcanics, gabbro, iron-rich augite syenite, amphibole syenite, and amphibolenepheline syenite. The main rock type within this unit is a gray to pink, mesocratic, amphibole-natrolitenepheline syenite with variable amounts of natrolite, lath feldspar and acicular amphibole. The textural complexities of the amphibole-natrolite-nepheline syenite is considered to be a product of assimilation and mixing of a variety of rock compositions in a solid, semi-molten or molten state and synplutonic intrusion of the alkaline gabbro.” From Hinz et al. (1994), “Amphibole-natrolitenepheline syenite: contains primarily anhedral, “turbid” crystals of perthitic feldspar. The turbid areas are caused by the presence of numerous vacuoles. The reddish colour of the stone may be due to ironstaining of the vacuoles and fractures within the feldspar crystals. Anhedral pyroxene (augite), biotite, amphibole (hornblende), and opaque minerals occur together.” Field Trip Stops Field trip stops are illustrated in Figure 2. Stop 1: Peninsula Quarry (1927-1932) UTM coordinates 544191E 5399826N In the 1880’s, prior to commercial production, several small quarries were operated supplying stone for the construction of river abutments and railway trestles in support of the construction of the Canadian National Railway. In 1927, “commercial operations were initiated by Peninsula Granite Quarries Ltd. on 17 claims located on the east shore of Carden Cove” (Hinz et al., 1994). Figure 2. Geology of the Coldwell Alkaline Complex with field trip stop locations. - 107 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Peninsula Granite Quarries Ltd. operated four quarries at various points along Carden Cove, north of the town of Marathon. Most of the historic infrastructure related to the operations is lost however remains of the original derrick, grout shovel, and steam engine can still be seen on-site (Figs. 3 and 4). sheet below to a depth of 10ft did not reach another sheeting plane. The rift is roughly parallel to the sheeting planes.” Stop 2: Coldspring Quarry (1931-1938?) UTM coordinates 545332E 5398469N From Hinz et al. (1994): “The ground covering the black granite was purchased by the Cold Spring Granite Company from the Peninsula Granite Quarries Ltd. During the year, a new quarry was opened with a new derrick, drilling equipment and power plant. Twelve men were employed to quarry blocks that weighted up to 35 tons. In 1931, twenty car loads of black granite were sent to Cold Spring, Minnesota for fabrication (Thomson, 1932; The Northern Miner, 1931). In the late 1930s the quarry operations were abandoned due to the lack of market.” The claims are underlain by Fe-rich augite syenite as described above in Walker et al. (1992, 1993a) and MacTavish and Smyk (2017). The stone is mediumto coarse-grained, dark brown to black in fresh-cut surfaces. Two sets of jointing are documented, the most prominent is 335° with a dip of 50° south and a second is 005° with a vertical dip. Sheeting (horizontal) fractures range from 0.45m to 2.4m and dip 8-10° west (Thomson, 1932). Figure 3. Remains of the derrick at the Peninsula Quarry. Stop 3: Shack Lake Spectrolite (1963-present) UTM coordinates 546698E 5399605N Figure 4. Remains of the steam winch at the Peninsula Quarry. The Peninsula Quarry site is underlain by iron-rich augite syenite and amphibole syenite as described above in Walker et al. (1992, 1993a) and MacTavish and Smyk (2017).” Thomson (1931) described the jointing, “Two sets of joints are seen in the quarry. The most prominent strikes almost due north and varies in dip from vertical to 70°W. The cross-jointing strikes east-west and is nearly vertical. At the quarry the north-south joints are 70ft apart and run parallel for at least 500ft. Rectangular blocks of a size limited only by plant capacity can be quarried. The sheets lie horizontally and exhibit an even and well-defined floor. The first sheet quarried had a maximum thickness of 14ft. Drilling in the next From Hinz et al. (1994): “The Shack Lake occurrence was first staked in 1963 by C.S. Downey, sporadic exploration work including diamond drilling and blasting was conducted on the site since then. For a time in the early 1990’s the property owners of the time, Jon and Audrey Ferguson considered opening a “pickyour-own” operation similar to the amethyst operations in the Thunder Bay area, this never came to fruition. At the time the Town of Marathon adopted spectrolite as the “town mineral”. Currently the property is held by G. Blakely who has conducted additional diamond drilling and blasting. Spectrolite is a variation of labradorite, with a deeper and wider range of colours (full spectrum) hence its name. Spectrolite was first identified in Finland. The spectrolite occurs within the syenite as two phases: large crystals up to 10cm across in pegmatite dikes cross-cutting the syenite; and - 108 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 smaller crystals within the contact zone between the pegmatite and medium-grained host syenite. In both cases the spectrolite displays bluish to yellow-gold schillerescence. Ribbe (1983) states that‘Schiller’ may be used to refer to diffuse, often silvery reflections from mutually oriented, platy inclusions, especially common in labradorite parallel to (010) (Rayleigh, 1923). Portions of ferro-augite syenite (commercially known as “black granite”) in the eastern part of the Coldwell Alkalic Complex near Marathon are coarse grained to pegmatitic. Large feldspar crystals may display yellow-orange to blue schillerescence and have been locally termed “spectrolite”. Two properties near Shack Lake, 2 km northwest of Marathon, have undergone limited exploration in the past but are being re-examined by owners Don Wilkinson, and Jon and Audrey Ferguson, respectively. The main potential usage of the “spectrolite”-rich syenite is as decorative or ornamental stone for use in cabochons, bookends and perhaps tiles.” From Schnieders et al. (1991): “Pegmatitic zones are commonly deeply weathered and are not amenable to large block quarrying. Hand picking and sorting can be undertaken on a small scale. Prospectors should investigate any coarse-grained to pegmatitic sections of syenite or dikes for feldspars that display this characteristic schiller effect. In deeply weathered outcrops, the feldspars commonly remain intact and retain their schiller colours. Stripping and blasting may be required to obtain “fresher”, unfractured material. X-ray diffraction analysis of the “spectrolite” shows the presence of plagioclase and minor K-feldspar (antiperthite). Examination of the mineral in oils shows that it is oligoclase. The schiller effects may be brought about by diffraction that occurs at the boundary of the exsolution lamellae (H. DeSouza, Ontario Geological Survey, personal communication, 1990).” The stone is a generally coarse-grained black to olivebrown with some greenish sections. Compositionally the stone is an iron-rich augite syenite as described above in Walker et al. (1992, 1993a) and MacTavish and Smyk (2017). Hinz et al. (1994) reports testing done by Cold Spring Granite (Canada) Ltd. yielded the following physical properties: Bulk specific gravity: 2.738 Percent absorption (48 hours): 0.560 Compressive strength: 20,130 (psi) dry, 18,420 (psi) wet Modulus of Rupture: 1,420 (psi) dry, 1,530 (psi) wet Testing completed by “Twin City Testing Corp., St. Paul, Minnesota (Assessment Files, Thunder Bay). Stop 5: Yooperlite Source Location UTM coordinates 536816E 5404785N In 2018 the US media was a-buzz over the discovery of a new “mineral” with the unofficial name of “yooperlite”. In a 2018 paper, Laughlin et al. (2018) reported that yooperlite is a fluorescent sodalitebearing syenite which occurs in the Upper Peninsula of Michigan as beach pebbles and cobbles. The authors indicate that it is probable the bedrock source is likely the Coldwell Alkaline Complex in Ontario. Centre 2 of the Coldwell Complex hosts amphibolenatrolite-nepheline syenite (Unit 14) within which, the fluorescent mineral hackmanite, a sulphur-bearing variety of sodalite, has been identified. It can be postulated that the yooperlite pebbles and cobbles found along the Lake Superior shoreline of Michigan originated from this unit and were glacially transported to their current location and subsequently wave-washed and tumbled. From Sage & Watkinson (1995). “Stop 20: Biotite gabbro intruded by various types of nepheline syenite. This stop is at a broad curve in Highway 17 and one should be very CAREFUL OF VEHICLES. Stop 4: Marathon Black Occurrence (1990-1994) UTM coordinates 543576E 5402637N The Marathon Black occurrence was staked by D. Petrunka in the early 1990’s when interest in building stone was high and the Cold Spring Granite (Canada) Ltd. was active in the area. Mr. Petrunka was able to secure funding to remove test blocks from the site, however further development did not occur. Starting at approximately 18.8km outcrops on the east side of the highway of grey to buff pyroxeneamphibole syenite contain orange fluorescing hackmanite, a variety of sodalite. This mineral can only readily be seen with a UV lamp. The syenite contains numerous mafic xenoliths up to 25 to 30cm in maximum length. The xenoliths are subangular to - 109 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 subrounded and the mafic minerals within the syenite tend to occur in clots. The larger xenoliths may have feldspar phenocrysts or porphyroblasts up to 1.0cm. The phenocrysts or porphyroblasts display a seriate size distribution and comprise up to 5 % of the rock. At the centre of the curve at 19.2km an alkalic biotite gabbro is exposed on the north side of the highway. The medium- to coarse-grained gabbro is extensively cut by medium- to coarse-grained syenite and some of the nepheline has been altered to reddish orange “hydronephelinite”. There may be two ages of alkalic gabbro with the older coarser grained gabbro displaying dark selvages up to 7 or 8cm wide and an irregular shape suggesting that it behaved in a more or less ductile manner. The alkalic gabbros have a clotty mafic mineral assemblage. The surface of the coarser grained biotite gabbro is pitted from the weathering of mafic clots. Dikes of nepheline syenite pegmatite occur on the north side of the highway at the inflection in the curve. At this site, two ages of nepheline syenite pegmatite of essentially identical composition cut each other. The trends of these dikes are approximately 340° dipping 60° south and 090° dipping 50° north. The dike trending 090° cuts the dike trending 340° and both are on the order of 20 to 30 cm in width. The dikes have been sketched by Puskas (1970). Both dikes are zoned from an amphibole-rich margin to a feldspar-natrolite (hydronephelinite)-rich core. The central parts of these dikes are commonly relatively rich in reddish orange “hydronephelinite”. West from the two dikes toward the hackmanite-bearing outcrop, coarse-grained alkalic biotite gabbro is intruded by medium- to coarsegrained pyroxene, amphibole syenite with traces of nepheline. Both of these rock types are in turn cut by coarse-grained nepheline syenite dikes. Brecciation is so intensive that the outcrop is an igneous breccia. West toward the hackmanite-bearing outcrop the syenite is pink to grey, fine- to medium-grained, inequigranular seriate with clotty assemblages of mafic minerals. On the south side of the highway coarse-grained alkalic gabbro appears to be cut by medium-grained alkalic gabbro which is in turn intruded by mottled pink to grey, inequigranular seriate amphibole syenite. There is a slight coarsening in texture next to the medium-grained gabbro and a dike of similar material projects from the contact with the medium-grained gabbro through both phases of alkalic gabbro” Stop 6: Marathon Red Occurrence (1960-1994) UTM coordinates 532030E 5402401N The first attempts to quarry red syenite occurred in the late 1880’s near Port Coldwell. From the 1960’s to mid-1980’s exposures of red granite east of Neys Lunch along the Trans-Canada Highway were staked several times, however no further work was recorded. D. Petrunka staked the claims in 1985 and optioned the ground to Cold Spring Granite (Canada) Ltd. Coldspring removed blocks from the highway exposures to evaluate the colour and market suitability. Diamond drilling conducted by Coldspring determined that the red colour of the syenite was limited to top 9.1m of the unit below which it changed to pink. In 1990 Cold Spring terminated the option agreement. The stone is an amphibole-natrolite-nepheline syenite described above by Walker et al. (1993). From Hinz et al. (1994): “In thin section, the stone is composed of primarily anhedral, “turbid” crystals of perthitic feldspar. The turbid areas are caused by the presence of numerous vacuoles. The reddish colour of the stone may be due to iron-staining of the vacuoles and fractures within the feldspar crystals. Anhedral pyroxene (augite), biotite, amphibole (hornblende), and opaque minerals occur together.” The stone is red-brown on fresh surface and orangered on the weathered surface, medium- to coarsegrained with some randomly distributed mafic knots. Hinz et al. (1994) reports samples sent to the Geoscience Laboratories in Sudbury yielded the following physical properties: Bulk specific gravity: 2.75 Percent absorption (2 hours): 0.22, (48 hours): 0.32 Compressive strength: 22,049 (psi) Modulus of Rupture: 1,394 (psi) Stop 7: Little Pic River Quarry (circa. 1884) UTM coordinates 527367E 5405024N This optional stop will be dependant on timing and weather. The construction of the Canadian Pacific Railway (C.P.R.) in the mid 1880’s required good stone quality for piers and abutments at river crossings (Fig. 5). Prospecting parties advanced along proposed rights-ofway ahead of construction in order to identify locations - 110 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 5. CPR construction across the Little Pic River, circa. 1884. Photo compliments of the Thunder Bay Historical Society. of quarriable stone. This quarry, previously unknown to Ministry staff, was one such location where the stone was quarried and land transported to the bridge construction site. Fig. 6 shows a work crew at the Little Pic River Quarry during the construction of the C.P.R., circa. 1884. References Alexander, M. 2007. The mineralogy of NYF pegmatites from the Coldwell Alkaline Complex, northwestern Ontario; unpublished MSc thesis, Lakehead University, Thunder Bay, Ontario. Cannon, W.F., Green, A.G., Hutchinson, D.R., Lee, M., Milkereit, B., Behrendt, J.C., Halls, H.C., Green, J.C., Dikas, A.B., Morey, G.B., Sutcliffe, R. and Spencer, C. 1989. The North Midcontinent Rift beneath Lake Superior from GLIMPCE seismic reflection profiling; Tectonics, v.8, p.305-332. Heaman, L.M. and Machado, N. 1987. Isotope geochemistry of the Coldwell alkaline complex: 1. U-Pb studies on accessory minerals; Geological Association of Canada–Mineralogical Association of Canada, Joint Annual Meeting, Saskatoon, Saskatchewan, Program with abstracts, p.54. Hinz, P., Landry, R.M. and Gerow, M.C. 1994. Dimension stone occurrences and deposits in northwestern Ontario; Ontario Geological Survey, Open File Report 5890, 191.p. Kerr, H.L. 1910a. Geological map of part of the north shore of Lake Superior, District of Thunder Bay; Ontario Bureau of Mines, Annual Report Map 19B, scale 1:63 360. Kerr, H.L. 1910b. Nepheline syenites of Port Coldwell; Ontario Bureau of Mines, Annual Report, v.19, p.194-232. Laderoute, D.G. 1987. The petrology, geochemistry, and petrogenesis of alkaline dyke rocks from the Figure 6. Little Pic River Quarry, circa. 1884. Photo compliments of the Thunder Bay Historical Society. Coldwell Alkaline complex; unpublished M.Sc. Thesis, Lakehead University, Thunder Bay, Ontario, 89p. Tectonophysics, v.184, p.73-86. Laughlin, R., Carlson, S.M., Olds, T.A. and Miller 2018. A New Find of Fluorescent Sodalite From Michigan’s Upper Peninsula. Mineral News, Vol. 34, No. 5, May, 2018. Lukosius-Sanders, J. 1988. Petrology of the syenites from Center III of the Coldwell alkaline complex, northwestern Ontario; unpublished MSc thesis, Lakehead University, Thunder Bay, Ontario, 141p. MacTavish. A. and Smyk, M.C. 2017. Archean and Proterozoic geology of the Marathon-Hemlo area, 63rd Institute on Lake Superior Geology Proceedings, v. 63, Part 1, Field Trip Guidebook, p. 1-31. Mitchell, R.H. and Platt, G. R. 1978. Mafic mineralogy of ferroaugite syenite from the Coldwell alkaline complex, Ontario, Canada; Journal of Petrology, v.19, p.627-651. Mitchell, R.H. and Platt, G. R. 1982a. The Coldwell alkaline complex; in Field Trip Guidebook, Proterozoic geology of the northern Lake Superior area, Geological Association of Canada–Mineralogical Association of Canada, Joint Annual Meeting, Winnipeg, Manitoba, p.42-61. Mitchell, R.H. and Platt, G. R. 1994. Aspects of the geology of the Coldwell alkaline complex: Field trip A2, Geological Association of Canada–Mineralogical Association of Canada, Joint Annual Meeting, Waterloo, Ontario, 36p. Mulja, T. 1989. Petrology, geochemistry, sulphide- and platinum-group element mineralization of the Geordie Lake intrusion; unpublished MSc thesis, Lakehead University, Thunder Bay, Ontario, 234p. Paterson, W.P.E., Lichtblau, A.F., Ravnaas, C., Lewis, S.O., Tuomi, R.D., Fudge, S.P., Pettigrew, T.K. and Wiebe, K. 2019. Report of Activities 2018, Resident Geologist Program, Red Lake Regional Resident - 111 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Geologist Report: Red Lake and Kenora Districts; Ontario Geological Survey, Open File Report 6351, 127p Puskas, F.P. 1967. Geology of the Port Coldwell Area, District of Thunder Bay; Ontario Department of Mines , Open File Report 5014, 92p. Puskas, F.W. 1970. The Port Coldwell alkali complex; in Proceedings, 16th Institute on Lake Superior Geology, Thunder Bay, Ontario, p.87-100. Puumala, M.A. 2018. Geological description of the Coldwell Alkaline Complex. Personal correspondence. Unpublished Report, p.1. Rayleigh, Lord. 1923. Studies of Iridescent Colour, and the Structure Producing It. III; The Colours of Labrador Feldspar. Proceedings of the Royal Society (Londaon) 103A, p.34-45. Ribbe, P.H. (1983) Chemistry, structure and nomenclature of feldspars. in: Feldspar Mineralogy. (P.H. Ribbe, editor). Reviews in Mineralogy 2. Mineralogical Society of America, Washington, D.C. p. 1-19. Sage, R.P. and Watkinson, D.H. A. 1995. Alkalic Rocks of the Midcontinent Rift, 41st Institute on Lake Superior Geology Proceedings, v. 41, Part 2a, Field Trip Guidebook, 79p. Schnieders, B.R., Smyk, M.C. and Hinz, P. 1991. SchreiberHemlo Resident Geologist’s District; in Report of Activities 1990, Resident Geologists, Ontario Geological Survey, Miscellaneous Paper 152, p.141171. Thomson, J.E. 1931. Geology of the Heron Bay Area, District of Thunder Bay; Ontario Department of Mines, v.XL, pt .2, p.21-39. Accompanied by map 40d. Thomson, J.E. 1933. Geology of the Heron Bay Area, Thunder Bay District, Ontario; Ontario Department of Mines, Annual Report 1932, v.XLI, pt.6,,p.34-37. Walker, E.C., Sutcliffe, R.H. , Shaw, C.S.J. , Shore, G.T. and Penczak, R.S. 1992. Geology of the Coldwell Alkaline Complex; in Summary of Field Work and Other Activitie s 1992, Ontario Geological Survey, Miscellaneous Paper 160, p. 108-119. Walker, E.C., Sutcliffe, R.H., Shaw, C.S.J., Shore, G.T., and Penczak, R.S. 1993. Precambrian geology of the Coldwell Alkalic Complex; Ontario Geological Survey, Open File Report 5868, 30p. - 112 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Field trip 8 - Geology of the past-producing Winston Lake Cu-Zn Mine Robert W.D. Lodge Department of Geology, University of Wisconsin-Eau Claire, WI 54702-4004 Mark Smyk and Mark Puumala Resident Geologist Program, Ontario Geological Survey, Ministry of Energy, Northern Development and Mines, Thunder Bay, Ontario, P7E 6S7, Canada Introduction The Winston Lake greenstone belt is best known for hosting economic volcanogenic massive sulphide (VMS) deposits totalling ~ 6 million tons of Zn-CuPb ore (Ontario Geological Survey, 2011). The belt is located along the northern margin of the Wawa Subprovince of the Wawa-Abitibi terrane and is about 20 km north of Schreiber, Ontario (Pye, 1964; Severin et al., 1991). The Winston Lake greenstone belt is tectono-stratigraphically equivalent to ca. 2720 Ma greenstone belts along the northern margin of the Wawa Subprovince, such as the Shebandowan (Corfu and Stott, 1998), Manitouwadge (Zaleski et al., 1999), and Vermilion (Peterson et al., 2001) greenstone belts (Fig. 1). Regional metamorphic grade in the belt is lower amphibolite facies (Williams et al., 1991). et al., 2008; Lodge et al., 2015). There are also several field trips and published guidebooks (Severin et al., 1991; Smyk and Schnieders, 1995; Lodge, 2012). The previous research in these areas has been extremely valuable throughout the planning of this field trip and preparation of the guidebook. Note that, in the descriptions of some of the units, primary igneous names are used rather than their metamorphic names (e.g., gabbro versus amphibolite). The geochemical and isotopic data presented in this guidebook are available for download through the Ontario Geological Survey (Lodge and Chartrand, 2013). The Winston Lake Greenstone Belt (Fig. 2) is a small belt located directly north of, and almost connected to the Schreiber-Hemlo greenstone belt (Williams et al., 1991); however, the contact relationship of these belts is poorly constrained (Carter, 1982b, a). Unlike the many other greenstone belts in the region, the Winston Lake greenstone belt has not been mapped at a regional scale since the 1960’s (Pye, 1964). The belt is bound to the north by the Quetico Subprovince, to the west by the Winston Lake batholith, and to the south by the Crossman Lake Batholith (Severin et al., 1991). Rocks in the western part of the belt that host the past-producing Winston Lake Mine were initially interpreted as metasedimentary rocks because of the presence of aluminosilicate minerals (Pye, 1964). They were later interpreted to be hydrothermally altered felsic and mafic volcanic assemblages. The Winston Lake greenstone belt and its VMShosting strata have received a considerable amount of research at a property scale (e.g., Osterberg, 1993; Schandl et al., 1995), a belt scale (Lodge et al., 2014), and a subprovince scale (e.g., Polat et al., 1999; Kerrich Figure 1. Geochronology of northern most greenstone belts in the Western Wawa Subprovince. Most of the belts have experienced most of their formation ca. 2720 Ma. Figure from Lodge et al. (2014) and was compiled from numerous geochronological studies (Corfu and Stott, 1998; Zaleski et al., 1999; Peterson et al., 2001; Lodge et al., 2013, 2014). - 113 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 2: Geology of the Winston Lake greenstone belt. Figure compiled by Lodge et al. (2015) from various published and unpublished maps (Pye, 1964; Ritcey, 1992; Osterberg, 1993). Regional Geology The Winston Lake belt has been informally subdivided into two main lithotectonic assemblages: the Winston Lake Assemblage (Fig. 3A) and the Big Duck Lake Assemblage (a thick mafic unit composing most of the belt in Fig. 3B; Severin et al., 1991; Polat et al., 1999; Lodge et al., 2014). The Winston Lake Assemblage is host to the VMS deposits and is composed of calc-alkaline, bimodal volcanic and siliciclastic rocks (Gorton and Schandl, 1995). The Big Duck Lake Assemblage consists of Mg- to Fe-tholeiitic basalts, quartz-feldspar porphyry dykes and sills, and their brecciated equivalents (Ritcey, 1992; Polat et al., 1999). It has been assumed that the Big Duck Lake Assemblage conformably overlies the Winston Lake Assemblage and that the contact was intruded by a thick differentiated gabbro (Osterberg, 1993). This field trip will not examine the Big Duck Lake Assemblage and therefore it will not be discussed further. If interested, please consult the references cited above (in particular: Ritcey, 1992). Prior to the research of Lodge et al. (2014), only one U-Pb age of 2723 ± 3 Ma was obtained from a felsic volcanic rock associated with the Winston Lake orebody (Davis et al., 1994). More recent geochronological data indicate that the entire Winston Lake Assemblage and the Zenith gabbro are all ca. 2720 Ma (Lodge et al., 2014). The structural history of the belt is also poorly constrained and two main structural events are Figure 3: Geology of the Winston Lake greenstone belt. The belt is subdivided into the VMS-hosting Winston Lake Assemblage (A) and Big Duck Lake Assemblage (B). Figure compiled by Lodge et al. (2014). - 114 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 interpreted: D1 manifested as tilting of stratigraphy and a foliation development (north-northwest striking foliation in the Winston Lake Assemblage; weststriking foliation in the Big Duck Lake Assemblage), and D¬2 represented by minor folds and faulting that offset contacts at the map scale (Osterberg, 1993). The VMS-hosting Winston Lake Assemblage is dominated by felsic volcanic and siliciclastic rocks. Despite the high degree of metamorphism and relatively high degree of deformation, many primary volcanic features are preserved in the volcanic rocks. Reliable younging directions obtained from pillowed flows and cross-bedding in volcaniclastic rocks suggest an eastward-younging stratigraphy. The oldest supracrustal strata in this part of the belt are felsic volcaniclastic and siliciclastic rocks. These are conformably overlain by a quartz-feldspar porphyry flow that is associated with the Pick Lake VMS deposit. Altered mafic flows (named Ladder and Middle mafic units) are interlayered with the Camp and Main felsic units that host the Winston Lake VMS deposit. The feldspar-phyric felsic volcanic rocks were then presumably overlain by mafic flows of the Big Duck Assemblage, which was followed by the emplacement of a thick, synvolcanic differentiated gabbro at that contact; the gabbro sill hosts the Zn-rich Zenith orebody. The general stratigraphy of the Winston Lake Assemblage is illustrated in Figure 4. History of Exploration Mining and Mineral Much of the mining and exploration history for the Winston Lake greenstone belt is published internally within the companies that have explored and mined these deposits. Most of these are not externally available. However, the Mineral Deposit Inventory published by the Ontario Geological Survey (2019) has summarized current and historical information available for these deposits. Much of the historical information provided in this section is summarized from this database. The size and grade of the deposits in the Winston Lake area are summarized in Table 1. Massive Zn-mineralization, in what is now interpreted to be a synvolcanic gabbroic sill, was first discovered in 1879 by prospectors and became the Zenith Mine. A total of 1065 tons of ore, averaging approximately 45% Zn, was shipped to a smelter between 1891 and 1899. Between 1899 and 1901, 2700 tons of sphalerite-rich ore was mined and concentrated by Grand Calumet Mining Company Limited (Ontario Geological Survey, 2019) Very little exploration was undertaken in the area until the grounds were claimed by Zenmac Metal Mines Ltd. in 1952. In the late Figure 4. Schematic cross-section looking north-northwest through the strata hosting the VMS orebodies of the Winston Lake area. Figure modified from unpublished Inmet Mining Corp. figures based on stratigraphic terminology from Lodge et al. (2014). - 115 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Table 1. Summary of mining activity in the Winston Lake area (Resident Geologist’s Files, Thunder Bay South District, Thunder Bay). Mine Winston Lake Years of Production 1988-1999 Ore Milled (tonnes) 3 268 698 Zenith Zenmac 1891–1901 1966–1970 3416 164 200 1960’s, almost 165,000 t at a grade of 16.5% Zn were mined from the Zenmac deposit. After the Zenith Mine closed, the property was stagnant until 1978 when Corporation Falconbridge Copper (CFC) completed reconnaissance geological mapping and lithogeochemical sampling in the region. The “metasediments” (Bartley, 1940; Pye, 1964) were re-interpreted as metamorphosed felsic volcanics. This was followed by more detailed property mapping, lithogeochemistry, and geophysical surveys, which defined the alteration zone that is in the immediate footwall to the gabbro. Areas of Na2O depletion and FeO, MgO, and Zn enrichment were outlined in the calc-alkaline volcanic rocks. CFC geologists also realized that the presence of massive sphalerite in the gabbro was unusual. The presence of VMS-like lithogeochemical signatures in the footwall to the gabbro led to the interpretation that the Zenith orebody Commodities and Grade 1.04% Cu, 14.56% Zn, 32.32 g/t Ag, 1.41 g/t Au 45% Zn 16.5% Zn was likely a large xenolith from a larger VMS orebody hosted at the top of the calc-alkaline felsic volcanic strata below the gabbro. With this newly recognized VMS potential, diamond drilling began in 1981 and targeted the felsic volcanic rocks at the base of the gabbro. In 1982, after only drilling five holes, CFC intersected 2.1 metres of massive sulphides containing 1.1% Cu, 19.1% Zn, 22.2 g/t Ag and 0.73 g/t Au. Mining began in 1988 and continued until the mine was officially closed in 1998. Later research by Osterberg (1993) and Lodge et al. (2014) outlines the lateral extent of alteration in the Winston Lake camp (Fig. 5). The surface expression of the Pick Lake orebody, the Anderson occurrence, was first reported by local prospectors in 1952. There was some shallow diamond drilling completed in the area but nothing materialized. CFC picked up the claims following the discovery of the Winston Lake deposit in 1982. In 1984, diamond Figure 5. Lateral variations in the stratigraphic thicknesses in the Winston Lake assemblage. Distance between sections is not to scale. Strata are hung from the bottom of the Ladder mafic unit. Figure from Lodge et al. (2014). For location of figure and legend, refer to Figure 3. - 116 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 drilling testing the down-dip extension of the Anderson occurrence discovered the Pick Lake orebody. In 1993, Metall Mining (formally Minnova, and operators of the mine at the time) began a 2200 metre drift to mine the Pick Lake deposit through the mine workings at Winston Lake. Doiron et al. (1997) proposed that the dyke-like nature of the deposit, combined with durchbewegung ore textures and sulphide injection structures, suggested that the Pick Lake deposit was a remobilized massive sulphide ore body. The Pick Lake Mine was abandoned when the Winston Lake Mine shut down in 1998. After the shut-down of the mine limited exploration was carried out on the property. There were several mapping and geophysical projects carried out on select parts of Superior Lake Resources’ current property between 2000-2011 by various exploration companies, but no further resources were outlined. In 2017-18, the mineral properties overlying the Pick Lake and Winston Lake deposits were acquired by Superior Lake Resources Limited. Superior Lake have completed new drilling programs that updated the resources at the Pick Lake and Winston Lake deposits (Table 2) and ground geophysical surveys that defined new targets for future exploration (Fig. 6). Field Trip Stops All coordinates are reported in NAD 83, Zone 16 The purpose of this portion of the field trip is to introduce the geological setting of strata hosting the VMS ore bodies at Winston Lake. As most of the lithofacies of the greenstone belt are difficult to access, this part of the trip will focus on camp-scale features and what role they played in the discovery of the orebodies. The stops will focus on the immediate footwall strata to the Winston Lake deposit and will highlight some of the geochemical and geochronologic data obtained from recent studies (Lodge et al., 2014). The field stops are illustrated in Figure 7. Figure 6. Recent ground EM geophysical results in the vicinity of the Winston Lake and Pick Lake ore bodies. Figure obtained from Superior Lake Resources website (www.superiorlake.com.au). Table 2. Updated resources at the Pick Lake and Winston Lake deposits (www.superiorlake.com.au). Resource Category Indicated Inferred Total // Weighted Average Tonnage (Mt) 2.07 0.28 2.35 Zn (%) 18.0 16.2 17.7 (News Release, Superior Lake Resources Limited, March 7, 2019) - 117 - Cu (%) 0.9 1.0 0.9 Au (g/t) 0.38 0.31 0.38 Ag (g/t) 34 37 34 �Proceedings of the 65th ILSG Annual Meeting - Part 2 leucocratic gabbro to pyroxenite (this is more apparent at Stop 2). The Zenith orebody appears to be associated with the transition from gabbro to pyroxenite. The Zenith orebody is essentially mined out but there are a few metre-scale slivers of massive sphalerite remaining. The ores are strictly Zn-rich and contain only minor amounts of pyrite, pyrrhotite, and chalcopyrite. Like most zinc-rich ores, they commonly are coated by white chalky zinc oxide minerals. The grain size of the sphalerite is generally coarse, most likely recrystallized during regional metamorphism. Stop 2 – Differentiated Gabbro UTM Coordinates 472337E 5425082N Figure 7. Geology of the Winston Lake mine area highlighting the location of field trip stops. Figure modified from Lodge (2012) based on data presented by Lodge et al. (2014). For location of figure and legend, refer to Figure 3. Stop 1 – Zenith Mine Open Pit UTM Coordinates 473182E 5424996N This stop is inside of the main Winston Lake Mine gate. It is a short drive along mine property roads. From where we park, walk along the base of the cliff toward the lake. WARNING: The walk into the main pit area is on a narrow and steep-sided path. Walking to the exposures of sulphides in a large group is discouraged. This stop represents the remnants of the original discovery in the Winston Lake area. The gabbro (now amphibolite) in this area is massive with some low-angle shears cutting up the outcrop. The gabbro is differentiated and consist of phases ranging from This stop is on the main road about 100 metres back from the Winston Lake mine gate along the side of the road. The different phases of the gabbro intrusion are well-exposed on either side of the road. This outcrop is very large and is dissected by a stream. We will not be crossing the stream. This stop highlights the complexity and multiple phases of the gabbro. On either side of the road, near the gate to the Winston Lake mine site, are perfectly exposed road cuts and stripped exposures of the gabbro that hosts the Zenith orebody. At this stop, we are less than 100 metres from the base of the intrusion. Particularly on the south side of the road toward the creek, the gabbro has a layered appearance with layers of pyroxenite (now mostly hornblende) and more plagioclase-rich leucocratic layers. In other areas, there are pegmatitic patches with large centimetre-scale plagioclase crystals. The compositional layering in this intrusion ranges from centimetre-scale to metre-scale Photo 1. (Left) Zenith mine open pit. (Right) Sphalerite-rich xenolith in gabbro. - 118 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 At this stop, the contact between the gabbro and the underlying volcanic rocks is exposed along road cuts leading toward the mine property. Immediately below the contact with the gabbro is a layered siliceous tuff unit that is known as the Winston Lake Interval. About 450 metres down dip of this unit is the Winston Lake main orebody. Photo 2. Differentiated gabbro showing melanocratic and leucocratic layers. layers. Most of these variations are not mappable. A leucocratic pegmatitic phase of the gabbro was sampled for U-Pb geochronology to determine the age of the gabbro. Zircons separated from the gabbro were low-U, typical of gabbroic zircons, indicating that they were magmatic in origin rather than xenocrystic. These grains were analyzed using TIMS analysis at the University of Toronto, yielding a U-Pb age of 2719.2 ± 4.0 Ma. This age indicates that the gabbro that intruded and entrained the Winston Lake VMS body is synvolcanic, and the same age (or slightly younger) as the host rocks. The finely laminated layers range in thickness from a few millimetres to 1-2 centimetres and range in composition from felsic tuff, chert, and lesser mafic tuff. This unit is laterally extensive (Fig. 3) and continues almost the entire length of the Winston Lake assemblage. Trace element and REE patterns suggest that this ash layer has an FII to FIII type felsic composition (Hart et al., 2004). There is little variation in this volcaniclastic unit, both compositionally and texturally, although it is locally interlayered with mafic flows. Stop 4 – Altered Felsic Volcanic Rocks in Footwall UTM Coordinates 472123E 5424987N Continue south(west) along the road for about 150 metres to the next stop. This stop is near the trail entrance to stops W5 to W7. There is about 100 metres of roadcut outcrop of variably altered felsic volcanic rocks here to examine. This stop is along the main road about 100 metres west from Stop 2. It is a large roadcut outcrop of the contact between the gabbro and the underlying felsic volcanic rocks. This stop, and nearby outcrops typify the alteration facies (assemblages) found within the uppermost Main and Camp felsic units. These altered units are laterally extensive, but relatively thin (Figs. 3 & 4) and it is not known how many flows are represented within this unit. Unaltered equivalents of this rock are usually massive, coherent units that are quartz- and plagioclase-phyric, Photo 3. Bedded felsic volcaniclastic unit known as the Winston Lake Horizon. Photo 4. Altered felsic volcanic rock now a quartzmuscovite-sillimanite-biotite schist. Stop 3 – Winston Lake Horizon UTM Coordinates 472200E 5425057N - 119 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 and locally contain flow banding. Minor felsic tuffs and tuff breccias have also been described in this unit elsewhere in the camp (Osterberg, 1993). This unit has a U-Pb age of 2723 ± 3 Ma (Davis et al., 1994). Altered versions of these quartz- and feldspar-phyric flows contain variable amounts of biotite, muscovite, sillimanite, and cordierite. Phenocrysts may be locally preserved, but are more difficult to see. Lesser altered equivalents contain quartz-muscovite-biotite-feldspar assemblages. With increasing degree of alteration, the rocks contain cordierite, sillimanite knots, garnet and anthophyllite. Major element geochemistry shows extensive Na2O depletion and local Fe-Mg enrichment. Trace element and REE patterns indicate that this unit is a FIII-type felsic volcanic (Fig. 8). the main road and we will be returning on the same trail and we will look at the lithofacies passed over on the return trip. WARNING: This is an ATV trail and there are many wet places and irregular surfaces. Please walk with caution and try and stay dry. This outcrop of the Ladder Mafic Unit contains some of the most spectacular features that will be observed Stop W5 – “Ladder” Mafic Flow UTM Coordinates 471800E 5425200N From the last stop, enter the trail that leads westward from the main road. The next stop is approximately 500 metres in along the trail. This is the furthest stop from Photo 5. (Top) Coarse grained orthoamphibole. (Bottom) Orthoamphibole-garnet-biotite schist. Figure 8. Geochemical discrimination plots for the felsic units in the Winston Lake greenstone belt. One notable geochemical distinguisher for the different felsic units in the Winston Lake Assemblage is the differences in Zr/Ti. Figure from Lodge et al. (2014). Fields in plots A and B are from Lesher et al. (1986) and Hart et al. (2004). - 120 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 during this field trip. In addition to the coarse-grained mineral assemblages associated with metamorphosed hydrothermal alteration, the overall lack of significant deformation in this area has preserved the volcanic textures in the rock (Fig. 9). Younging directions are still determinable in this unit despite strong alteration and they indicate an eastward younging of the strata. As with most units in the Winston Lake assemblage, this unit is laterally extensive, but relatively thin. Unaltered equivalents of the Ladder Flow contain plagioclase phenocrysts. At the eastern limit of this outcrop are spectacular exposures of an orthoamphibole-cordierite assemblage within the altered pillowed mafic flows. The pillows are metre-scale and their selvages are resistive weathering. There appears to be very little compositional difference between the pillow interior and the selvages, with the exception of slightly more biotite. Stratigraphically below the pillows is a 10 metre thick massive basalt flow. This massive flow is very homogeneous and the only variation are gradational changes in the size of the orthoamphibole crystals. These crystals can be up to 10 centimetres in size and are usually randomly oriented. The matrix is mostly medium grained cordierite and minor biotite in this massive lithofacies. Near the lower contact with the altered felsic volcanic rocks, the Ladder Flow contains abundant clots, sheets, and veins of porphyroblastic garnet. In addition to garnet-orthoamphibole-cordierite, there are also local concentrations of biotite and chlorite. This alteration style is interesting, but difficult to interpret. In some places it appears as if the garnet clots represent altered clasts in a breccia. In other locations, they may be altered pillow margins or may be deformed veins. Regardless, the sharp transition to massive orthoamphibole-cordierite altered flow into a more chaotic orthoamphibole-garnet-cordierite zone may indicate the transition from a massive to breccia facies of this unit. It may also represent a different chemical gradient within the alteration zone. Geochemically, the garnet-bearing rocks are still mafic in composition. The geochemical characteristics from the Ladder Mafic Unit, as well as other mafic units throughout the greenstone belt, are summarized in Figure 10. All of the mafic flows in the footwall to the Winston Lake mine have similar geochemical characteristics. Most noteworthy is that they are all calc-alkalic to transitional in their magmatic affinities and have pronounced negative Nb anomalies. In the hangingwall strata (i.e. Big Duck Lake Assemblage), the flows are tholeiitic and have flat rare earth and trace element patterns on normalized element plots (Lodge et al., 2014). Stop 6 – Trail Showing UTM Coordinates 471867E 5425051N From the previous stop, return eastward back toward the main road for approximately 150 metres. This stop is a flat, rusty outcrop that we walked over to get to the Figure 9. Outcrop sketch of the contact between the altered mafic “Ladder flow” and the underlying altered felsic volcanics in the Winston Lake area. Sketch only incorporates areas that were cleaned and there are additional outcrops in the area. - 121 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Figure 10. Various geochemical discrimination plots for mafic rocks from the Winston Lake greenstone belt. Basalts from the Ladder and Middle units tend to be have more calc-alkalic to transitional magmatic affinities. Sills and flows from the Big Duck Lake assemblge are mainly tholeiititc. Figure from Lodge et al. (2014). Fields in A from Shervais (1982). Fields in B from Ross & Bedard (2009). Fields in C from Piercey et al. (2002). Fields in D modified from Polat (2009). Ladder Flow. This stop represents one of the many smaller mineralized intervals in the Winston Lake camp. The mineralization in the “Trail Showing” is located near the contact between the “Ladder Flow” and underlying felsic volcanic rock and it is hosted within a siliceous, bedded volcaniclastic unit up to 15 centimetres thick. It contains over 6000 ppm Cu and Zn-bearing metamorphic minerals such as gahnite are also present. The bedded volcaniclastic unit is altered to a quartz-cordierite-biotite-garnet mineral assemblage containing variable amounts of orthoamphibole and sillimanite. The layering in the rock appears to be relict primary volcanic texture. This unit is distinct and is present at the contact between the Ladder mafic flow and the underlying massive, altered quartz-feldspar-phyric Main felsic flow, which the trail passes over from there to the main road. The alteration assemblages in the massive flow are the same as those observed at Stop 4. Stop 7 – Contact of Ladder Flow and Altered Felsic Volcanic Rocks UTM Coordinates 471918E 5424992N Continuing back toward the main road along the trail, this stop is approximately 100 metres from the previous stop. This is the last stop on the trail before returning to the main road. Photo 6. Felsic volcaniclastic rock hosting disseminated sulphides. This stop shows the contact between the Ladder mafic flow and the overlying altered Main felsic volcanic rocks, which constitute the immediate footwall strata to the Winston Lake deposit. Cleaning of this otherwise black and featureless exposure resulted in a near-perfect exposure of an altered basaltic flow top breccia that is intermingled with the overlying felsic tuff. This is one of a few places where these flow features are exposed at surface. The variety of flow facies exposed within - 122 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 rocks that dip eastward underneath this unit (Fig. 4). We will not see the host rocks to the Pick Lake deposit on this trip because they are not easily accessible. The surface expression of the Pick Deposit is known as the Anderson Occurrence and is 850 m west of this location. Photo 7. Upper contact of altered mafic flow (Ladder Flow) and altered felsic volcanic. the Ladder mafic flow indicates that the contact at this stop is not a peperite caused by a sill mingling with overlying unconsolidated tuff. Rather, it is more likely that the felsic tuff settled into on top of the basaltic flow top breccia. The flow here is altered to an orthoamphibole-garnet assemblage with retrograde chlorite and biotite. The garnet porphyroblasts are evenly distributed and occur in the felsic volcanic above the contact. This suggests some chemical exchange between the lithofacies at the contact during metamorphism, as previously suggested by Gorton and Schandl (1995). The overlying felsic tuff is altered to a quartz-muscovite-sillimanite-biotite mineral assemblage. Stop 8 – Pick Lake Vent Raise Area UTM Coordinates 471579E 5424177N About 150 metres south of the trail entrance on the main road is the gate to the Pick Lake deposit. The Pick Lake vent raise, and the next stop are about 1.1 kilometres from the gate. There are plenty of outcrops around the former vent raise to examine that show a variety of alteration facies. WARNING: Although the shaft has a concrete cap and is safe to walk on, please do not walk directly on old mine workings. This stop is in the middle of the thickest part of the quartz-feldspar-phyric felsic flow that forms the Main felsic unit of the Winston Lake assemblage and the hanging wall to the Pick Lake orebody. The Pick Lake deposit is about 300 metres directly below the mine workings near this stop. The orebody is not associated with the rocks exposed here, rather is associated with felsic volcaniclastic and siliciclastic Based on this location alone, it is not clear whether this part of the Main Felsic Unit is definitively an intrusion or extrusion (e.g. Osterberg, 1993). There appears to be very little textural variation in the unit throughout the area. The unit is massive and the only variations are in the degree of alteration and mineral assemblages. The alteration is pervasive and mineralogical changes are gradational and do not appear to represent primary compositional layering. If it is an extrusive unit, it is a very massive flow with only a thin brecciated carapace (seen at Stop W9). Stratigraphically above the Ladder Mafic Unit, the Main Felsic Unit appears to exhibit phenocryst sizes in unaltered parts of this unit that are much larger (3-4 millimetres) compared to the Camp Felsic Unit in the footwall to the Winston Lake orebody. A sample of this unit was submitted for U-Pb dating by TIMS analysis at the University of Toronto. The sample yielded a homogeneous population of zircon that produced an age of 2721 ± 1 Ma. This confirms that the unit is the same age as the host rocks to the Winston Lake orebody. The Main felsic unit has a notably higher Zr/Ti ratio that the Camp felsic unit. All the felsic magmas in this camp are FIII type felsic magmas. Photo 8. Quartz-muscovite-biotite-garnet schist near the Pick Lake mine shaft. - 123 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Stop 9 – Pick Lake Felsic Breccias UTM Coordinates 471998E 5424667N This stop is a roadside outcrop on the Pick Lake road approximately 100 metres away from the gate. This is the last stop inside the Pick Lake gated road. At this stop, the monolithic breccia phase of the felsic quartz-feldspar-phyric flow from the Main Felsic Unit is well exposed on the roadside. It is not certain if this breccia is a volcanic or structural texture. Given that the main part of this body is thick, massive and homogeneous, and that it has been confirmed to be the same age of the surrounding volcanic rocks, it is possible that this may represent the synvolcanic intrusion that was the feeder for the overlying felsic flows and volcaniclastic units above the Ladder mafic unit. It is common for hypabyssal synvolcanic intrusions to be brecciated at their margins. Alternatively, it could represent the breccia margin of a flow. Opinions are encouraged! The fragments are lenticular and stretched defining a pronounced stretching lineation. They contain quartz and plagioclase phenocrysts that compose up to 15% of the fragment. The anastomosing matrix is composed of quartz-biotite-muscovite mineral assemblages and composes up to 25% of the rock. There is no obvious layering in the rock but there is some variation in the abundance and size of the clasts that may represent crude bedding. Stop 10 – Tuffaceous Metasedimentary Rocks UTM Coordinates 472699E 5423145N This stop is 1.8 kilometres southward on the main road from the Pick Lake gate and is adjacent to the power lines. There are several roadcut outcrops that are worth checking out. This is the final stop of the field trip. The tuffaceous metasedimentary rocks are along strike and south from the orebodies, and do not contain mineral assemblages indicative of significant hydrothermal alteration. They were classified as “intermediate volcaniclastics” by Osterberg (1993). There are many sedimentary structures in this unit, such as cross-bedding, that suggesting it is a reworked volcaniclastic deposit. The composition of the rock suggests that the provenance is mostly mafic with only a minor felsic component. In this stop, mineral assemblages range from biotitequartz-garnet to biotite-quartz-hornblende and even Photo 10. Low-angle cross-bedded mafic-intermediate tuffaceous metasedimentary rock. local concentrations of lapilli-sized clasts of mafic composition. These compositional variations are on the metre- to outcrop-scale. A sample of this unit within the finer-grained, cross-bedded part of the exposure was sent for detrital zircon analysis at the LA-ICP-MS lab at Laurentian University. The results show a single peak centered around 2720 Ma, suggesting that the source of detritus was local and from 2720 Ma volcanic units (Lodge et al., 2014). The composite, mafic-felsic composition of these rocks suggests that they are not primary volcanic deposits. They may represent distal, reworked facies of slump fans deposited during rifting and sourced from bimodal volcanic units within the Winston Lake assemblage. Photo 9. Matrix-supported quartz-phyric felsic volcanic breccia. - 124 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Acknowledgements The guidebook represents a revised version of a field trip guidebook published by the Ontario Geological Survey (OFR 6282). Much of the text and images has been re-used from this publication. A complete citation for that publication is below: Lodge, RWD (2012). Winston Lake and Manitouwadge revisited: Modern views of two volcanogenic massive sulphide (VMS)-endowed greenstone belts: A field trip guidebook. Ontario Geological Survey, Open File Report 6282, 37 p. In addition, the authors would like to thank management of Superior Lake Resources for allowing members of the Institute on Lake Superior Geology to access their mineral exploration property. References Bartley, M.W. 1940. Geology of the Big Duck-Aguasabon Lakes area. Ontario Geological Survey, Map 49k. Carter, M.W. 1982a. Precambrian geology of the Terrace Bay area, northeast sheet, Thunder Bay District. Ontario Geological Survey, Preliminary Map 2557. Carter, M.W. 1982b. Precambrian geology of the Terrace Bay area, northwest sheet, Thunder Bay District. Ontario Geological Survey, Preliminary Map 2556. Corfu, F. and Stott, G.M. 1998. Shebandowan greenstone belt, western Superior Province: U-Pb ages, tectonic implications, and correlations. Geological Society of America Bulletin 110, p. 1467-1484. Davis, D.W., Schandl, E.S., and Wasteneys, H.A. 1994. U-Pb dating of minerals in alteration halos of Superior Province massive sulfide deposits; syngenesis versus metamorphism. Contributions to Mineralogy and Petrology 115, p. 427-437. Doiron, D., Siddiqui, M., and Smyk, M.C. 1997. Preliminary investigations of the Pick Lake deposit, Winston Lake Mine, Ontario: A remobilized massive sulphide orebody; 43rd Institute on Lake Superior Geology, Program with Abstracts, Sudbury, Ontario, p.17-18. Gorton, M.P. and Schandl, E.S. 1995. An unusual sink for rare earth elements: the rhyolite-basalt contact of the Archean Winston Lake volcanogenic massive sulphide deposit, Superior Province, Canada. Economic Geology 90, p. 2065-2072. 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 99, p. 1003-1013. Kerrich, R., Polat, A., and Xie, Q. 2008. Geochemical systematics of a 2.7 Ga Kinojevis Group (Abitibi), and Manitouwadge and Winston Lake (Wawa) Ferich basalt-rhyolite associations: Backarc rift oceanic crust? Lithos 101, p. 1-23. Lesher, C.M., Goodwin, A.M., Campbell, I.H., and Gorton, M.P. 1986. Trace-element geochemistry of oreassociated and barren, felsic metavolcanic rocks in the Superior Province, Canada. Canadian Journal of Earth Sciences 23, p. 222-237. Lodge, R.W.D. 2012. Winston Lake and Manitouwadge revisited: Modern views of two volcanogenic massive sulphide (VMS)-endowed greenstone belts. A field trip guidebook., Ontario Geological Survey Open File Report 6282, p. 37. Lodge, R.W.D. and Chartrand, J.E. 2013. Establishing regional geodynamic settings and the metallogeny of volcanogenic massive sulphide mineralization of greenstone belt assemblages (circa 2720 Ma) of the Wawa Subprovince via geochemical comparisons, Ontario Geological Survey, Miscellaneous Release Data 306. Lodge, R.W.D., Gibson, H.L., Stott, G.M., Franklin, J.M., and Hamilton, M.A. 2014. Geodynamic reconstruction of the Winston Lake greenstone belt and VMS deposits: New trace element geochemistry and U-Pb geochronology. Economic Geology 109, p. 1291-1313. Lodge, R.W.D., Gibson, H.L., Stott, G.M., Franklin, J.M., and Hudak, G.J. 2015. Geodyamic 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 52, p. 196-214. Lodge, R.W.D., Gibson, H.L., Stott, G.M., Hudak, G.J., and Jirsa, M. 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 235, p. 264-277. Ontario Geological Survey 2019. Mineral Deposit Inventory; Ontario Geological Survey, Mineral Deposit Inventory (April 2019 update), online database.Osterberg, S.A. 1993. Stratigraphy, physical volcanology, and hydrothermal alteration of the footwall rocks to the Winston Lake massive sulfide deposits, northwestern Ontario. University of Minnesota at Minneapolis, 351 p.. Peterson, D., Gallup, C., Jirsa, M., and Davis, D.W. 2001. Correlation of the Archean assemblages across the U.S.-Canadian border: Phase I geochronology, 47th Annual Meeting, Institute on Lake Superior Geology, Proceedings Volume 47, Part 1 - Program and Abstracts, p. 77-78. - 125 - �Proceedings of the 65th ILSG Annual Meeting - Part 2 Piercey, S.J., Mortensen, J.K., Murphy, D.C., Paradis, S., and Creaser, R.A. 2002. Geochemistry and tectonic significance of alkalic mafic magmatism in the Yukon-Tanana terrane, Finlayson Lake region, Yukon. Canadian Journal of Earth Sciences 39, 17291744. Polat, A. 2009. The geochemistry of Neoarchean (ca. 2700 Ma) tholeiitic basalts, transitional to alkaline basalts, and gabbros, Wawa Subprovince, Canada: Implications for petrogenetic and geodynamic processes. Precambrian Research 168, p. 83-105. Polat, A., Kerrich, R., and Wyman, D.A. 1999. Geochemical diversity in oceanic komatiites and basalts from the late Archean Wawa greenstone belts, Superior Province, Canada: trace element and Nd isotope evidence for a heterogeneous mantle. Precambrian Research 94, p. 139-173. Pye, E.G. 1964. Mineral deposits of the Big Duck Lake area, district of Thunder Bay, Ontario Department of Mines Geological Report 27, 58 p.. Ritcey, D.J. 1992. Geology and mineralization in the vicinity of Big Duck Lake, Ontario. Unpublished MSc. thesis, University of Ottawa, 235 p.. 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 46, p. 823-839. Schandl, E.S., Gorton, M.P., and Wasteneys, H.A. 1995. Rare earth element geochemistry of the metamorphosed volcanogenic massive sulfide deposits of the Manitouwadge mining camp, Superior Province, Canada; a potential exploration tool? Economic Geology and the Bulletin of the Society of Economic Geologists 90, p. 1217-1236. Severin, P.W.A., Balint, F., and Sim, R. 1991. Geological setting of the Winston Lake massive sulphide deposit, Mineral Deposits in the Western Superior Province, Ontario, Geological Survey of Canada Open File 2164, p. 58-73. Shervais, J.W. 1982. Ti-V plots and the petrogenesis of modern and ophiolitic lavas. Earth and Planetary Science Letters 59, p. 101-118. Smyk, M.C. and Schnieders, B.R. 1995. Geology of the Schreiber greenstone assemblage and its gold and base metal mineralization; 41st Institute on Lake Superior Geology, Proceedings volume 41, pt.2c, Marathon, Ontario, 77 p.Williams, H.R., Stott, G.M., Heather, K.B., Muir, T.L., Sage, R.P., 1991. Wawa Subprovince, in: Thurston, P.C., Williams, H.R., Sutcliffe, R.H., Stott, G.M. (Eds.), Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 1, p. 485-541. Zaleski, E., van Breemen, O., and Peterson, V.L. 1999. Geological evolution of the Manitouwadge greenstone belt and Wawa-Quetico subprovince boundary, Superior Province, Ontario, constrained by U-Pb zircon dates of supracrustal and plutonic rocks. Canadian Journal of Earth Sciences 36, 945-966. - 126 - � 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, 2019 Description An account of the resource Institute on Lake Superior Geology. Terrace Bay, Ontario. May 8-9, 2019. 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 2019 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/56fcc75435efc57f9f999bad9bcf01b1.JPG 9c43255db5112e429d18e7f38157ebea 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 Cairine Budner fonds 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 Nahjus Athletic Club Young boys exercise drill Subject The topic of the resource Sports Description An account of the resource Black and white photograph of the Nahjus Young boy's exercise drill on stage. The boys are dressed in Nahjus uniforms for the time Date A point or period of time associated with an event in the lifecycle of the resource 1926 - 1927 Contributor An entity responsible for making contributions to the resource Donor S. Anttila Rights Information about rights held in and over the resource Public domain Format The file format, physical medium, or dimensions of the resource Photograph Type The nature or genre of the resource Still image Identifier An unambiguous reference to the resource within a given context PHIII-0012c (lxii) scan#BUD-0035 1926 1927 exercise drill gymnasts Nahjus Athletic Club stage Young boys https://digitalcollections.lakeheadu.ca/files/original/88e4ca0cb9d8c30674f915adad33010c.pdf 00a00568506a2bb2ddd39848707548a8 PDF Text Text www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Isle Royale: Keweenaw Rift Geology Figure 1: Native copper in a vein on Washington Island, Isle Royale (photo by Justin Olson). This occurrence of copper was found all over the Keweenaw and Isle Royale, but humans dug them out and made pits and small mines to extract the precious metal. It was traded across the North American continent by Native Americans. Later Europeans re-excavated the indigenous pits and eventually developed major mining activity. This mining of copper was an economic pay-off of a geologic event that brought deep-seated heavy elements to earth’s surface more than one billion years ago. The wilderness preservation of Isle Royale may explain why such occurrences happen there but not on the Keweenaw, except in underwater places like Great Sand Bay. Physical Volcanology of Large Lava Flows Middle Proterozoic Continental Tholeiitic Flood Basalts of the 1.1 Ga Keweenaw Rift (Rodinia). Field trip Institute of Lake Superior Geology, May 25-30, 2013 Bill Rose, Justin Olson Michigan Technological University 1 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Table of Contents Topic Page No. Purpose and Philosophy Introduction (Basic References) Broad Background (Some geo background with web links) Specific Background pages: Basalt (the mother liquor of the planets) Paleomagnetism (great tool to see geologic history) Geochemistry (esoteric? geochemistry) Basalt types (field hand specimen petrology) Physical features of large lava flows Columnar Joints (entablature, colonade) Mafic Volcaniclastic Deposits (pyroclastic rocks of the rift) Isle Royale Lava Stratigraphy (nomenclature of flows) Ophitic texture (understanding an unusual igneous texture) Pegmatite (in situ differentiation of thick lava flows--also pegmatoid, dolerite) Amygdaloid (lava flow tops with bubble holes filled with colored minerals) Copper (Why native copper here?) Conglomerate (alluvial fan and fluvial sediments) LIDAR (new 2 m resolution LIDAR topography data) Specific field areas we will visit: Washington Harbor (Windigo, Grace Island) NWCoast (Hugginin Cove, Wendigo) McCargoe Cove (Minong Mine) Amygdaloid Island (Amygdaloid channel, Belle Isle, MVD) Blake Point (Upper and lower ophite, pegmatite) Passage Island Snug Harbor Scoville Point (entablature jointing) Lookout Louise (Monument Rock) Red Rock Point Raspberry Island (Segregation cylinders, vesicle cylinders, pegmatites) Tookers and Davidson Mott Island (Conglomerate) Lighthouse (Amygdaloidal flow top minerals) Ojibway What to take Home (why Isle Royale is geologically unique--what it is known for) Acknowledgements Bibliography (where this information comes from) Latitude-Longitude locations 2 3 4 6 11 14 16 19 21 28 29 32 33 35 38 40 42 43 45 48 50 51 53 56 56 57 60 62 65 70 71 74 76 77 78 79 85 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Purpose and Philosophy This field guide aims to give anyone interested in geology and Isle Royale an interpretation of things that can be seen outside in this unique National Park. We try to avoid jargon, in spite of some of the words above on this page. Each part of the earth’s surface offers part of the evidence of past events for us to interpret. On Isle Royale we see rocks which reflect earth about 1.1 Billion years ago, and we can interpret what this rock record means. These interpretations are speculative and they evolve constantly, reflecting new observations. This field guide is an update of a guide from 1994. One very important source is a geologic map done by N. King Huber of the US Geological Survey. On this geologic map of Isle Royale, this geologic map (Figure 2) the western part is mostly tan, and the eastern part is mostly green. The colors reflect glacial outwash gravels and moraines that mostly bury the bedrock in the west, while those materials are absent in the east. Because of our interest in the rift lavas, this trip focuses on the Eastern part of Isle Royale, which has only minimal glacial cover, although we do pass through Washington Harbor and part of the western portion. Isle Royale has remarkably few visitors, especially considering that it is a national park and is, for many people, within a couple of days travel. This lack of tourism can be explained partially by the park's island location and by the fact that a trip to Isle Royale seems to require a deeper commitment, of both time and money, than other vacations might require. But the very people whom you would most expect to want to visit Isle Royale don't go. When you compare the popularity of various national parks, the public's avoidance of Isle Royale is obvious, perhaps even more obvious to me because of my position. As a professor at Michigan Technological University for more than 40 years, I have had direct contact with hundreds of ecologically-minded students, many of them geology majors, who are committed to the outdoors and to field experiences. However, very few of these students go to the park, even though they live for years in Houghton, MI, which is the home of the Ranger III, one of the principal transporters of visitors to and from Isle Royale. Likewise, many of the geologists I have known have visited all of the geological sites around Lake Superior and the other Great Lakes, but only a few of them have been to Isle Royale. This is a remarkable contradiction, something I'm at a loss to explain. It seems to attest to America's addiction to the automobile; maybe people just can't stomach the thought of being separated from their car for a few days! At any rate, I hope that this guide and its website (http://www.geo.mtu.edu/~raman/SilverI/ IRKeweenawRift) will encourage more geologists, as well as other people, to visit the park. Besides the fact that Isle Royale has outstanding geological sites, a trip there can be made at moderate expense, and the park offers comfortable facilities and logistics that most geologists would find agreeable. I recommend taking a week to visit and using kayak, canoe or motor boat (bring along or rent from the park concession) to allow access to the many wave-washed outcrops. ...Bill Rose April 2013 3 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Introduction Before you go to Isle Royale on the field trip, you may wish to read some geological sources. Which ones you read could depend on your interests. One source for all who are interested in the geology is Huber (1975): USGS Bulletin 1309 (http://pubs.usgs.gov/bul/1309/report.pdf). This booklet covers much of Isle Royale geology and is well illustrated. A more academic version of Huber’s geology is USGS Prof Paper 754-C-also downloadable for free (http://pubs.usgs.gov/pp/0754c/report.pdf). There is also a report on the glacial geology, more useful in Western Isle Royale (http://pubs.usgs.gov/pp/0754a/ report.pdf). The geologic map (Figure 2) is downloadable also and can be used in GIS format with Google Earth or other base maps. For the Keweenaw Peninsula, GIS data on the geology and mineral deposits is available from Cannon et al., USGS OFR 99-149, 1999 (http://pubs.usgs.gov/of/1999/ of99-149/). The age information on the Keweenawan rocks is one of the most vital pieces of data. Those interested in age should consult Davis & Paces, 1990, and Nicholson et al., 1997. The petrology and geochemistry of the Volcanic Rocks of the Portage Lake Volcanics is thoroughly explored by Paces, 1988. Figure 3: Schematic cross section of Isle Royale, showing tilted lava and conglomerate layers. The sections which follow are specifically designed to provide background information on various geologic topics. Figure 2 (next page): Geologic map of Isle Royale National Park (Huber, 1973). (http://www.nature.nps.gov/geology/inventory/publications/map_graphics/isro_map_graphic.pdf) 4 �40 15 pu 8 pu pu 16 45 : :: 26 2018 % 12 psp 15 18 12 5 37 6 : : pu pu pu 5 : Qal pm 8 :: 5 : 5 18 5 10 Qal 17 15 13 5 pu 19 pu Ì : 20 pu 55 45 pth 15 pei » »»»» 15 7 : 6 18 : pu 13 18 pu Qal 20 45 4 5 : 10 15 50 15 : Ì Qal : pg 10 »» »» 15 10 5 15 20 10 10 50 psp Qal 35 »»» 15 45 15 10 15 45 pu 40 20 10 Qal Qal 15 10 40 15 Ì 18 pu 12 15 20 17 15 18 10 5 National Park Service U.S. Department of the Interior pei 17 pu Ì 10 ¹ 10 15 16 13 pu ¹ pu : : 15 10 May 2008 ¹ pu 22 20 20 :: : :: : : : 20 ¹ Qal Ì 40 pu 30 30 ¹ 10 21 ¹ 15 17 ¹ 16 17 ¹ pei 18 18 ¹ o ¹ ¹ o oo oo ooo ¹¹ ¹ ¹ ¹¹ o oo o o o oo o o o ooo ¹ ¹ o o o o oo oo oo o o o ¹ ¹ ¹ ¹ ¹ ¹ o o oo ¹ ¹ %% % %%% o %%% ¹ % % % %%% %%%% %% 10 Miles 15 Kilometers ¹ 35 Qal 15 15 ¹ psp 15 ¹¹ php pu psp 18 ¹ Ì 20 10 pu Qal 10 :: pu ¹¹ 9 13 7 17 ¹ Qal 14 pu pu 11 » ¹ 20 Qal 23 15 ¹ pu pm 22 16 ¹ pu pu pth 9 8 : : :: : : : :: : :: : : : : : :: :: : :: : : : : : : : : : :: : : : 13 pu ¹ 18 pu 8 12 ¹ pg psp 12 12 ¹ 28 pu 14 10 ¹ 17 Qal 9 10 ¹ 5 ¹ cb 10 ¹ 8 ¹ » 6 pei Qal : 8 5 7.5 o ¹ o 10 ¹ 9 28 ¹ 18 14 9 26 o 18 psp 13 9 26 o php 14 pu 8 27 3.75 oo o¹ o pg pu 12 ¹ 15 Qts 27 ¹ pu 16 9 ¹ ¹ Qal Qal 15 9 25 o 17 11 18 ¹ o ¹ ¹¹ pth pu 20 12 ¹ ¹¹ ¹ ¹ ¹ ¹ ¹ ¹ 13 15 20 ¹ ¹o o o oo ¹ ¹ ¹ oo ¹o o o oo o oo o o o o o ¹ oooo ¹¹ oo ¹ ¹ ¹ ¹ ¹ ¹ ¹ ¹ ¹ ¹ ¹o oo¹ o ¹¹o ¹¹ oo o ¹ ¹ ¹ ¹ o o pu Qal 14 14 14 20 o oo o o 17 o 16 20 o oo oo o o ¹ oo o o o 20 o 18 Qal 17 17 17 cp o 28 Qal 17 19 18 19 o Qal Qts 17 19 15 o o oo ¹ o o ¹ ¹ ¹ oo ¹¹ ¹ ¹ o¹ ¹ ¹ ¹ 5 pu 8 15 19 18 21 o o o 17 17 18 o 22 15 pm cb 18 19 ooo 25 php Qts 16 10 cu Qal o o o Qal : 10 14 11 18 0 2.5 : pu 13 10 16 18 ¹¹¹ ¹ ¹ oo oo o Qal 11 Qal 18 18 11 12 Qal 17 18 % 18 pg 19 17 13 o o oo o 14 11 14 ¹ : : : Qg pu : 13 o strike and dip of lava flows prospect pit pu Qal Qal % 12 o downthrown side of fault strike and dip of beds Geologic Attitude and Observation Localities : mine shaft Mine Feature Localiteis » diamond drill hole Ì # Paleoshorelines beach crest, known or certain wave-cut cliff, known or certain wave-cut cliff, approximate unknown offset/displacement, concealed unknown offset/displacemen, approximate unknown offset/displacement, know or certain kettle, known or certain drumlin, known or certain direction of glacial movement, known or certain Glacial Line Features Faults 12 base of pegmatitic zone in Greenstone Flow (pg) Linear Geologic Units Qal pg psp 15 pu 13 cu 10 12 cb % pu 15 15 % ¹¹ ¹ ¹ ¹ %%%%% % 9 12 11 o ¹ % %%%% %%% % %% %%%% %% cb 0 ¹ Isle Royale National Park Michigan 11 pwi : : Geologic Map of Isle Royale National Park known or certain Geologic Contacts approximate concealed shoreline shoreline, approximate Qal - alluvium Qts - talus, slopewash, and glacial drift Qg - glacial till cu - Copper Harbor Conglomerate, undivided cc - Copper Harbor Conglomerate, chiefly conglomerate 22 # 9 10 : cb - Copper Harbor Conglomerate, boulder and cobble conglomerate cp - Copper Harbor Conglomerate, pebble conglomerate cs - Copper Harbor Conglomerate, chiefly sandstone pu - Portage Lake Volcanics, lava flows, undivided php 18 pth pu psc - Portage Lake Volcanics, sandstone and conglomerate pp - Portage Lake Volcanics, pyroclastic rocks psp - Portage Lake Volcanics, Scoville Point Flow pei - Portage Lake Volcanics, Edwards Island Flow pmp - Portage Lake Volcanics, Middle Point Flow 19 pu : # # # : 12 ¬ 5 pli - Portage Lake Volcanics, Long Island Flow # # 12 : pth - Portage Lake Volcanics, Tobin Harbor Flow 17 # # # Qal : pwi - Portage Lake Volcanics, Washington Island Flow # ## pm : pg pwi : 10 : pg - Portage Lake Volcanics, Greenstone Flow 20 #15 # #15 # 10 11 : : :: : : pgi - Portage Lake Volcanics, Grace Island Flow 18 pu 9 9 : : pm - Portage Lake Volcanics, Minong Flow 20 12 pu pu cu 10 cu 9 : : : ph - Portage Lake Volcanics, Huginnin Flow 15 Qts 8 9 : : php - Portage Lake Volcanics, Hill Point Flow 14 %%% % 15 17 Qal : pai - Portage Lake Volcanics, Amygdaloid Island Flow water 18 %%%% ¹ 13 10 : : : % % % % % % % % % % % % % % % % % % % % % % gradational o ¹ : Produced by Geologic Resources Division : Geologic Units 13 10 : : oo o oo o o www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Broad Background Any individual place on Earth exhibits only tiny windows of Earth history. In the Keweenaw and Isle Royale, we can see into events that range from about 1.2 billion years ago until perhaps about 0.9 billion (Davis and Paces, 1990), and we can also see the deposits of the glacial periods of the last few million years. To see the record of other times we must travel to where we can see rocks of those ages are at the surface. This is the very best place to see the exposed rocks of the midcontinent rift (Figure 4). This rift extended from at least Kansas to Detroit, but it is exposed only near Lake Superior. At the time of rifting there were huge differences in the configuration of the continents and a huge supercontinent, Rodinia, was assembled, a hodgepodge of pieces of what is now North America, Antarctica, Europe and South America. And it was beginning to break up. In the Keweenaw we get a remarkable opportunity to look at rocks produced during the rifting period of Rodinia, which preceded the orogens shown in green in Figure 5. The orogens mark the areas where continental blocks approached each other at about 1.1 by ago. The orogeny in eastern North America, which eventually ended the Keweenaw Rifting episode, produced an orogen known as the Grenville Front. (Cannon, 1994). Figure 4 Map of the Mesoproterozoic Midcontinent Rift System, showing insets A: the extent of the rift as currently known and B: The main copper districts. from Bornhorst and Barron, 2012. 6 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 5 Two schematic maps of the Rodinia supercontinent showing how pieces of various modern continents are thought to have been assembled more than 1 billion years ago. Sources: John Goodge (left) and KE Karlstrom et al., 1999 (right). Rodinia’s assembly acted like a great blanket for a large area of Earth’s surface, preventing heat loss and creating an opportunity for heat to build up underneath. A great hot spot formed under the blanket. The continent began to split with very hot dike swarms. When the splitting opened the rift, magma was erupted in huge amounts—a supereruption. The ancient Earth contained more radioactive heat producers so the potential for big eruptions was greater. We still think that most of Earth’s heat comes from radioactivity, and we still expect Large Igneous Provinces (LIPs) to develop when and where mantle hot spots occur. But perhaps LIPs are getting smaller as time passes and natural radioactivity declines. Figure 6: Schematic view of the mantle plume head which developed over a hotspot, and which is thought to have led to the midcontinent rift, the great ponded flood basalt lavas of the Keweenaw and Isle Royale. High heat flow focussed on the Lake Superior region led to continental splitting and spreading, forming a rift basin (shown in red) which curved around the current Keweenaw Peninsula. From K Schulz, pers comm., USGS. 7 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Heat flow on Earth is declining with time as natural radioactivity continues to be spent. Convection of Earth’s core and mantle do not produce steady heat transfer from earth’s core to the surface. Since volcanism is driven by higher than average heat flow, volcanism comes and goes as heat flow changes in time and place. Overall, heat declines, but in any time or place, it can vary markedly in both directions. Super-eruptions result from very high heat flow conditions. Figure 7: Map of Supereruptions of the past 2 million years on Earth. Note correlation with the ring of fire. From Geological Society of London. The Midcontinent rift was driven by high heat flow, and it certainly represents a type of supereruption or Large Igneous Province (LIP). To explain the distributions of LIPs in time and place, volcanologists refer to plates, hotspots and/or mantle plumes, much of which which are far from our direct access. These plates, hotspots and plumes come and go, plates move over hotspots and/or plumes, and time/space series patterns are not clearly defined or predictable. This requires volcanologists to consider the deep thermal origin of volcanism, which is fundamental geophysics of the deep Earth and especially the mantle and core. We lack explanations to explain why deep Earth heat transfer leads to massive volcanism at rare intervals and in widely scattered surficial locations. The surface manifestations may be huge volumes of volcanic rocks. The environmental consequences must be large, but are mostly uncertain. From the recent record of LIPs, a relationship of the timing of LIPs with extinctions of living species is advanced. Volcanologists agree that super-eruptions lie in Earth’s future, but the time and place is uncertain. 8 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 8: Map of some LIPs on Earth, plotted in red with yellow hotspot locations and at right, their ages and with extinction evidence, based on the number of animal families represented. Interruptions in the trend toward diversity occur at extinctions. From Bresson, 2011. Heat flow bottom line: The Keweenaw Rift record shows how the earth has highly irregular deep-seated convective events that help shape the planet. They come and go in time and space. Once the hot spot of the whole world, the Keweenaw now has heat flow that is far below average. Figure 9: Stratigraphic units of the Keweenawan from Bornhorst and Barron, 2012. On Isle Royale the upper part of the Portage Lake Volcanics and the Copper Harbor Conglomerate are found. Figure 9 shows this mid-Proterozoic Keweenawan Supergroup, which contains all the formations of the rift. These consist of lavas from the deep earth and redbed sediments, shed off of the top of Rodinia into the gaping rift. On Isle Royale we find only the Portage Lake Volcanics and the Copper Harbor Conglomerate, while on the Keweenaw we have all the formations. The Lavas of the Portage Lake Lava Series are the result of a continental rift, very much like the currently active Red Sea. Existence of a rift is a way to explain how such huge volumes of lava could have been erupted. It also helps explain the syncline shown in Figure 11. A great crack across North America formed, stretching from Kansas to the UP and then on to Detroit. Figures 4 and 6 show the western limb of a feature called the “mid-continent gravity high,” a linear feature that extends from Kansas to Lake Superior where it coincides with the Lake Superior Syncline. This feature is mostly completely invisible, but was detected by geophysicists working with gravity meters, who showed that the gravity attraction of 9 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift earth to the instrument is measurably higher, indicating dense rock underneath. Figure 6 shows a buried dense rock region colored red. The dense rock could be the dense black lava flows we have in the Keweenaw, and their gravity shows that the rift was hundreds of miles long. Drill holes have penetrated the lavas in Kansas and Iowa, so we know that lavas are there—it is not just gravity detection. A second geophysical anomaly, this one even more deeply buried, has been discovered extending from Lake Superior southward to near Toledo Ohio (Figure 4 or 6). This adds to the definition of the hypothesized Keweenaw rift, which is sometimes described as a continental scale fissure, which resembles what happened in the Atlantic to separate Europe from North America. The rift breaks through all the older rock units (Figure 10). Figure 10: Map of Minnesota, Wisconsin, Iowa and Upper Michigan, showing the rift rocks in grey, over the proposed geologic terrane map of Precambrian basement rocks in the northern U.S. continental interior. WRB: Wolf River batholith. Underlying gray-toned base map is the newly compiled regional aeromagnetic anomaly map “Craton margin domain” represents sedimentary and volcanic rocks deposited during the interval 2.3–1.77 Ga; stippled pattern represents area affected by Penokean deformation; cross-hatched pattern represents area termed ‘gneiss dome corridor’ which was affected by Yavapai-interval deformation (Schneider et al., 2004). GIPB: Green Island plutonic belt; BS: Baraboo syncline. Figure and caption from Holm et al., Pre-C Res., 2007. Figure 11: The Synclinal nature of the layers on the Keweenaw and Isle Royale, visualized by NK Huber, USGS. 10 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift The idea of a syncline comes from observed features in geology. In the Keweenaw we cannot see the whole syncline--far from it! We just see the rocks dipping toward the north at Copper Harbor and those dipping to the south on Isle Royale. In between is how geologists earn their money! Figure 12 shows a confirmation of the synclinal nature of the rift rocks, based on seismic geophysics. Implications of this hypothesis: 1. Layers of rock extended from the Keweenaw to Isle Royale, apparently filling a basin. 2. Something caused the basin to subside. 3. The basin has influenced the formation of Lake Superior. 4. The basin may continue beyond the Lake. 5. Its importance could extend much farther than explaining the tilting. Figure 12: Profile across eastern Lake Superior, confirming the geometry of the rift with seismic geophysics (Modified from Behrendt et al. (1988). Basalt Isle Royale is mainly underlain by basaltic lava, the result of hundreds of successive eruptions from the Rift. Mostly this basalt made its way to the surface rapidly, but some was held in magma chambers and evolved before erupting. Basalt is the most common composition of lava rocks that cool from magma, liquid rock that rises from the deep Earth at volcanoes. Today basalt is forming at many active rifts, including Iceland, the East African Rift Valley, the Red Sea and the Rio Grande Valley of New Mexico and Colorado. 11 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Basalt is the result of partial melting of meteoritic material, so it forms on other terrestrial planets as well as Earth, making it the “mother liquor” of volcanoes on terrestrial planets. It is found all over Earth, but especially under the oceans and in other areas where Earth’s crust is thin. It formed in the Isle Royale-Keweenaw region because of the Midcontinent Rift. Most of Earth’s surface is basalt lava, but basalt makes up only a small fraction of continents. Keweenaw lavas are mainly basaltic: continental flood basalts with isotopic signatures close to bulk composition of Earth (Paces, 1988). Within the sequence of flows there are several cycles of evolution in subcrustal magma chambers. Overall the lavas become slightly more primitive with time. The ages are well established from U-Pb dating of zircons. Most of the great outpouring of rift lavas occurred in about 2 million years. Figure 13: U-Pb dates on zircons from pegmatite zones of the Portage Lake Volcanics, Keweenaw Peninsula (Paces and Miller, 1993). Lane (1911) first recognized and described the mirror-image geological and lithological similarity of the PLV and the CHC on both sides of the Syncline (Figure 14), and further suggested that the great lava flow of the Keweenaw Peninsula (Greenstone Flow, Figure 13) and the large flow of Isle Royale are the same. Huber (1973a) strongly supports Lane's correlations. Longo (1984), after extensive field mapping and sampling at Isle Royale and the Keweenaw, gives field observations and geochemical data that also strongly confirms the correlation of the Greenstone flow. Figure 14: Sketch map of Lane, 1911, which suggest the correlations of layers between the Keweenaw and Isle Royale. 12 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 15: Summary of ages and correlations of Keweenawan age rocks around Lake Superior (from K Schulz, pers comm, USGS, modified from Nicholson et al., 1997). This correlation means that the Greenstone flow is one of the earth's largest lava flows; according to Longo (1984), it has an aggregate volume of 1650 km3 (396 mi3), comparable to the Roza flow of the Columbia River Flood basalts, which is estimated to be 1300 km3 (312 mi3) by Swanson et al. (1975). The areal extent of the Roza, 40,000 km2 (15,450 mi2), is much larger than the Greenstone flow, 5000 km2 (1930 mi2), a comparison which results from the ponding of the Greenstone within the rift basin. Thus, the solidification of the Greenstone flow is a kind of magma ocean experiment, the likes of which is rare on this planet. Table 1 at left is from Self et al., 1998. 13 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Paleomagnetism The conceptual model for Earth’s magnetic field is that of a dipole (i.e., bar magnet) positioned at Earth’s center and aligned with the rotational axis of the Earth. This allows us to predict the direction of the magnetic field at any location on Earth’s surface using the fundamental equations of a dipole field. This equation gives a direct relation between magnetic inclination and geographic latitude at the point of observation. The geomagnetic field irregularly reverses (i.e. a magnetic compass which points north will now point south and vice versa) and these reversals are symmetrical (i.e. the normal and reversed field directions are exactly anti-parallel). The above is the fundamental assumption used to reconstruct continents to their past positions using the ancient magnetic field recorded in rocks (fossil magnetism). The record of the strength and direction of Earth’s magnetic field (paleomagnetism, or fossil magnetism) is an important source of our knowledge about Earth’s evolution throughout the entire geological history. This record is preserved by many rocks from the time of their formation. The paleomagnetic data have played an instrumental role in deciphering the history of our planet including a decisive evidence for continental drift and global plate tectonics. The data have also been crucial for better understanding the problems of regional and local tectonics, geodynamics, and thermal history of our planet. The ~1.1 billion-year-old North American Midcontinent Rift paleomagnetism has been intensively studied since early 1960s (for example, see a review in Halls and Pesonen, 1982). The rifting began during an interval of reversed polarity of geomagnetic field. The reversely magnetized (“reversed”) lavas (the Siemens Creek Formation of Powder Mill Group, the lowermost part of North Shore Volcanics, Osler Volcanics, and the lower part of Mamainse Point Formation) are found in many locations around Lake Superior (see figure 15). This early stage magmatism occurred from 1108 to approximately 1105 million years ago. The period of active magmatism was followed by a quiescence period when a geomagnetic field reversal took place. Magmatism renewed by 1102 Ma (Ojakangas et al., 2001) during the normal polarity interval. During this interval, a sequence of Portage Lake lava flows erupted within a two to three million year interval around 1095 million years ago. These rocks represent the main stage of the rift-related magmatism. All younger sedimentary and igneous suites exposed on the Keweenaw peninsula (the Copper Harbor conglomerate, LST, etc) have normal polarity magnetization. However, the geomagnetic field reversal mentioned above is characterized by an asymmetry, manifested in natural magnetization recorded by Keweenawan rocks that crop out around the Lake Superior (e.g., Palmer, 1970; Halls and Pesonen, 1982; Pesonen and Halls, 1983; Schmidt and Williams, 2003). Most but not all of the reversely magnetized lava flows and dikes of this age consistently have characteristic 14 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 16: Equal area projection of the western hemisphere showing the Logan Loop on the polar wandering curve. Letters are keyed to the table below. From Robertson and Fahrig, 1971. 15 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift directions of magnetization that are about 20 to 40 degrees steeper in inclination than their normally magnetized (“normal”) equivalents, while declinations show the expected 180 degree relationship. The paleomagnetic pole positions derived from these normally and reversely magnetized rocks define a noticeable amount of apparent polar wander that forms the western arm of the so-called “Logan Loop” (Robertson & Fahrig, 1971). The two most favored hypotheses for this reversal asymmetry are either apparent polar wander during Keweenawan times (Davis and Green, 1997; Schmidt and Williams, 2003) or the presence of a persistent non-dipole field causing the geomagnetic field to depart from a geocentric axial dipole geometry (Pesonen and Nevanlinna, 1981; Halls and Pesonen, 1982; Nevanlinna and Pesonen, 1983; Pesonen and Halls, 1983). The recent study of this problem (Swanson-Hysell et al., 2009) on lavas from Mamainse Point shows that the geomagnetic reversal asymmetry observed in rocks of Keweenawan age is an artifact of the rapid motion of North America during this time. The other study by Kern et al, (2012) on rocks of the alkaline Coldwell Complex (Ontario, Canada) also suggests no asymmetry in geomagnetic reversal during Keweenawan time. Basalt Geochemistry and field types of basalt on Isle Royale Paces (1988) conducted detailed study of the composition of the lavas of the PLV, studying a complete section on the Keweenaw Peninsula. He provided a description of the texture and thickness (see Basalt Types); chemical composition (Table 2); mineral chemistry (Figure 2); and petrography (Table 3). The lavas resemble other younger examples of continental flood basalts (see also LIPS sources) with their main composition being olivine tholeiite that contains high MgO and Ni, but also have enrichment of highly incompatible elements. There are only minor amounts of more evolved (have more complicated history) magmas and overall the magmas become more primitive (less complicated history) with time. Isotopically (Nd and Sr) the lavas are very close to bulk earth values. Paces (1988) describes the rocks: PLV lava flows display a relatively limited number of textures based on the relationships between dominant mineralogical constituents. These components originally included groundmass plagioclase, olivine, clinopyroxene, iron-titanium oxide, volcanic glass or mesostasis, occasional phenocrysts or microphenocrysts of plagioclase, and sometimes olivine. Textures that developed within the coarsest portion of different lava flows range from fine-grained intergranular through subophitic and ophitic. This same range in textures can be observed in individual, thick lava flows which grade from intergranular chilled flow margins to a coarsely ophitic flow interior. True quench textures (Lofgren, 1971) including skeletal, dendritic or spherulitic olivine and pyroxene, have not been observed in PLV basalts. PLV lava flows do not preserve evidence of an extensive pre-eruptive crystallization history. Chilled margins are generally aphanite. Occasionally, lavas contain minor amounts (usually less than 1%) of small euhedral phenocrysts of plagioclase (often with melt inclusion-rich cores) and sometimes olivine. When present, both of these phases commonly exhibit glomeroporphyritic tendencies. Neither the plagioclase nor olivine phenocrysts show obvious evidence of 16 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Table 2 17 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Table 3 disequilibrium with the liquid. Except for rounded plagioclase cores, both olivine and plagioclase phenocrysts are in apparent textural equilibrium with the liquid. Slightly porphyritic lavas frequently exhibit serrate textures. The dominant textural element in all lavas is the framework of groundmass plagioclase laths. This framework is a randomly-oriented, felt-like structure of interlocking euhedral to subhedral laths. Rarely, the partial alignment of laths forms crude trachytic fabric, indicating movement of magma after at least partial crystallization. The second most prominent textural element is defined by clinopyroxene crystals and their relationships to the plagioclase lath framework. In all cases, clinopyroxene has clearly crystallized later than olivine and plagioclase. Clinopyroxene crystals exhibit 18 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift intergranular to ophitic textures depending both on the size of the clinopyroxene crystals as well as the size of the plagioclase laths. Melaphyric flows and chilled flow margins contain small, blocky clinopyroxene crystals intergranular to the plagioclase framework. In many (but not all) thicker flows, clinopyroxene grains begin to enclose subophitically, and eventually ophitically, plagioclase and olivine crystals as the massive flow interior is approached. The boundary between subophitic and ophitic textures is gradational and is exceeded when a significant number of plagioclase laths are completely enclosed by the surrounding clinopyroxene oikocrysts. Absolute size of the oikocryst is not definitive: a large clinopyroxene grain may only subophitically enclose large groundmass plagioclase laths, however the same sized grain may ophitically enclose plagioclase laths of smaller dimensions. Thus, over half, 60-70% (volume basis), of most PLV lavaflows are typically composed of a plagioclase lath framework with loosely packed clinopyroxene oikocrysts. The remaining interstitial space within the plagioclase framework and between oikocrysts is filled with variable proportions of intergranular olivine, iron-titanium oxides, and intersertal volcanic "glass. " Evidence of gas exsolution is preserved in some flow interiors as vesicular cavities of ellipsoidal to highly irregular shapes. Diktytaxitic textures, however, are not apparent. Vesicles are particularly well preserved in thinner flows which quenched rapidly; however, they are observable in some thicker flow interiors as well. --Paces 1988 We conclude that the lavas of the Portage Lake Volcanics are typical of basaltic LIPs on earth and also chemically resemble the basalts of the moon and Mars. In the field, we can see some textural variety of basalts. Basalt is mainly made of two minerals: Plagioclase feldspar and pyroxene. Basalt has several textural varieties such as glassy, massive, porphyritic, vesicular, scoriaceous. Porphyrite or porphyritic basalt (see photos above) is characterized by obvious crystals, usually of plagioclase, which is often white or tan in color. These crystals are typically 19 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift interpreted as phases that formed before eruption, where magma was being stored (in a “magma chamber”).On Isle Royale, there are five main examples of porphyritic basalt flows: The Scoville Point (psp), Hill Point (php) Tobin Harbor (pth) Grace Island (pgi) and Huginnin (ph). “Trap,” melaphyre, or massive basalt typically has no conspicuous crystals, and in its interior regions has a uniform grey or grey brown color (see photos above). On Isle Royale there are four large “Trap” flows: Edwards Island (pei) Long Island (pli), Minong (pm) and Amygdaloid Island (pai). Ophite or Ophitic basalt (see photos above) exhibits a sometimes subtle, knobby texture with equidimensional pyroxenes usually between 0.5 and about 3 cm. On Isle Royale there are 3 main ophitic flows: Washington Island (pwi), Greenstone (pg) and Hill Point (php) . 20 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift These textural types of basalt reflect environment of deposition in part. Thicker flows which cooled more slowly are more likely to be ophitic, as figure 17 shows. Figure 17: Histogram plots of numbers of flows within the Portage Lake Volcanics which had melaphryic (Trap), and Ophitic textures. Subophitic textures are intermediary between ophitic and melaphyric. From Paces, 1988. Two conclusions emerge from Paces’ work: (1) the lavas are compositionally similar throughout the section and generally are high magnesium, olivine tholeiites; and (2) the flows range from less than 10 m (33 ft.) to more than 100 m (330 ft.) thick, and the thicker ones are more likely to have ophitic textures. Physical features of lava flows A summary statement from a review paper about basalt flows (Self et al., 1998): The most common rock type at the surface of the Earth, and on the other terrestrial planets, is basalt. Basaltic lavas come in two forms: aa and pahoehoe (from the Hawaiian ‘a’ā and pāhoehoe). Pahoehoe flows have often been thought of as small, slow-moving, inconsequential lavas. It is thus not surprising that the processes involved in the emplacement of large, fastmoving, channelized aa flows have received greater attention (see Kilburn & Luongo 1993, Crisp & Baloga 1994, Pinkerton & Wilson 1994, and references therein). However, as in the fable of the tortoise and the hare, it is the slow but unrelenting pahoehoe lava flows that 21 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift ultimately grow larger and longer than the spectacular but short-lived channelized rivers of lava that produce aa flows. In terms of both areal coverage and total volume, pahoehoe flows dominate basaltic lavas in the subaerial and submarine environments on Earth. The most abundant type of lava, submarine pillows, is closely related to pahoehoe in their style of emplacement (e.g. Macdonald 1953, Williams & McBirney 1979). A compilation of the rather sparse information on intermediate length (50–100 km) and long (>100 km) lava flows on the Earth (Table 1) shows that pahoehoe is far more common in these larger flows. Several large extraterrestrial flows also seem to be pahoehoe (e.g. Theilig & Greeley 1986, Bruno et al 1992, Campbell & Campbell 1992). The emplacement of pahoehoe flows is therefore a fundamental process in crustal formation on the Earth and the other terrestrial planetary bodies. Isle Royale and Keweenaw lava flows exhibit pahoehoe features, and do not show pillows or other subaqueous physical aspects. Therefore, here we use descriptive material from volcanological literature that describe pahoehoe flood basalts (Hon et al. 1994; Goff, 1996, Self et al., 1998 and Thordarson & Self, 2012). A generalized cross section of an “inflated” pahoehoe flood basalt is shown in Figure 18. Figure 18: Idealized cartoon of the cross section through an inflated pahoehoe lobe. The lobe is divided into three sections on the basis of vesicle structures, jointing, and crystal texture. The upper crust makes up 40–60% of the lobe and the lower crust is 20–100 cm thick, irrespective of the total lobe thickness. Upper crust: Vesicular, often with discrete horizontal vesicular zones (VZs) that form during active inflation. Bubble size increases with depth. Prismatic or irregular jointing, sometimes equivalent to the entablature in thick lava flows. Petrographic texture ranges from hypohyaline to hypocrystalline (90–10% glass). Core: Very few vesicles. Porosity is dominated by diktytaxitic voids. Vesicles are mostly in the silicic residuum, which forms vesicle cylinders (VCs) and vesicle sheets (VSs). Holocrystalline (<10% glass). Lower crust: Nearly as vesicular as the upper crust, few joints, and 50– 90% glass. from Self et al., 1998. 22 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Development of the layers shown in figure 18 is likely the result of a sequence of inflation and deflation events as observed at Kilauea and Mauna Loa and depicted and explained by Hon et al., in Figure 18 and below: Inflated pahoehoe sheet flows have a distinctive horizontal upper surface, which can be several hundred meters across, and are bounded by steep monoclinal uplifts. The inflated sheet flows we studied ranged from 1 to 5 m in thickness, but initially propagated as thin sheets of fluid pahoehoe lava, generally 20-30 cm thick. Individual lobes originated at outbreaks from the inflated front of a prior sheet-flow lobe and initially moved rapidly away from their source. Velocities slowed greatly within hours due to radial spreading and to depletion of lava stored within the source flow. As the outward flow velocity decreases, cooling promotes rapid crustal growth. At first, the crust behaves plastically as pahoehoe toes form. After the crust attains a thickness of 2-5 cm, it behaves more rigidly and develops enough strength to retain incoming lava, thus increasing the hydrostatic head at the flow front. The increased hydrostatic pressure is distributed evenly through the liquid lava core of the flow, resulting in uniform uplift of the entire sheet-flow lobe. Initial uplift rates are rapid (flows thicken to 1 m in 1-2 hours), but rates decline sharply as crustal thickness increases, and as outbreaks occur from the margins of the inflating lobe. One flow reached a final thickness of nearly 4 m after 350 hr. Inflation data define powerlaw curves, whereas crustal cooling follows square root of time relationships; the combination of data can be used to construct simple models of inflated sheet flows. As the flow advances, preferred pathways develop in the older portions of the liquid-cored flow; these pathways can evolve into lava tube systems within a few weeks. Formation of lava tubes results in highly efficient delivery of lava at velocities of several kilometers per hour to a flow front that may be moving 1-2 orders of magnitude slower. If advance of the sheet flow is terminated, the tube remains filled with lava that crystallizes in situ rather than draining to form the cave-like lava tubes commonly associated with pahoehoe flows. Inflated sheet flows from Kilauea and Mauna Loa are morphologically similar to some thick Icelandic and submarine sheet flows, suggesting a similar mechanism of emplacement. The planar, sheet-like geometry of flood-basalt flows may also result from inflation of sequentially emplaced flow lobes rather than nearly instantaneous emplacement as literal floods of lava. 23 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 19: A figure schematically describing the development of Hawaiian pahoehoe lavas with inflation and deflation (from Hon et al., 1994). 24 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift An inflated view of lava flow sections is probably appropriate for Isle Royale, given the ponded constraints of the rift valley and the thickness of flows observed in the Portage Lake Volcanics. Figure 20 shows a sequential interpretative development of layers in Portage Lake lava flows, based on numerous examples of flows exposed in cross section. This view has similarities with Figure 18, and also is analogous with solidification in sills, based on work by Bruce Marsh (Figure 21; Marsh et al., 1991; Mangan & Marsh, 1992), and also shows how liquid can be squeezed out of mush below forming a cylindrical feature moving up and then trapped in a horizontal layer. Figure 20: Cross section cartoons of Keweenawan lava flows at various stages of solidification, from Paces (1988). A is an early stage when crust has formed on the top and bottom of the flow, While B and C show later stages in solidification as liquid (darkest color) is progressively restricted to the interior, away from the cooling margins where magma is becoming a crystal mush and eventually a solid, and segregations develop from mushy regions. Figure 21: Cross section of a sill, solidifying from its top and bottom and building crystal mush layers from its cooling surfaces both above and below a liquid layer near the sill’s center. Temperature and crystal size vary with height as shown and segregations form and can rise from the lower part of the flow, but get trapped in tabular zones above the center. 25 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Vesicular zones in the PLV tend to be mineralized by zeolite and prehnite-pumpellyite facies minerals, which is an overprint over strictly physical volcanological features. Figure 21 shows a typical pattern of vesicular zones within these lavas and Figure 23 shows some typical segregation cylinders. Goff (1996) has made an extensive study of vesicle cylinders which we suggest are equivalent to segregation cylinders, and develop above the lower solidification front of the lava flow. Figure 22: Cross section of an idealized lava flow within the Portage Lake Volcanics, showing four kinds of regions where gas filled vesicles typically later become mineralized by hydrothermal fluids. Pipe Vesicles (see Figure 23) develop at the base of the flow, perhaps the result of boiling of trapped meteoric water from the soil below the flow. Segregation cylinders or vesicle cylinders (Figure 24) develop above the solidifying lower contact zone, and rise to the flow center or beyond, creating vertical vesicle rich features. At the flow top, vesicles develop as the lava crust thickens and solidifies, with vesicles being more numerous and smaller at the top and less numerous and larger across the first meter or so of flow thickness. A pegmatite zone is found occasionally above the flow midpoint, marked by tabular zones, with thin flows marked by vesicular layers (called vesicle sheets) and thicker flows being termed doleritic, with larger and more conspicuous plagioclase laths. 26 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 23: Pipe vesicles, filled with Calcite and laumontite, seen at the base of a 5m thick PLV lava flow from near Eagle Harbor on the Keweenaw Peninsula. Figure 24: Two examples of mineralized vesicle cylinders or segregation cylinders, from the Keweenaw (left) and Isle Royale (right). These features are generally found below the flow midpoint, and have variable vertical extension. Some conclusions about the lavas of the Keweenaw Rift: 1. The overall physical characteristics resemble other examples from much younger flood basalts and other basaltic volcanoes. 2. The PLV are subaerial, inflated pahoehoe flows which are ponded and do not deflate after eruption. 3. The volumes of PLV flows are as large as any known in other flood basalts. 4. Because their thicknesses are in excess of hundreds of feet, PLV flows show more pronounced in situ differentiation than other examples. 27 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Columnar Joints Like mudcracks, columnar joints form from volume contraction. In the mudcracks, volume decreases with drying, while in lava flows or volcanic tuffs it is cooling that drives the contraction. Lava flows display a variety of columns (Figure 25), often with a stratigraphic pattern. Colonnade is a coarser, more regular pattern often found at the base of the flow. Entablature is more irregular, and often found near the top. Sometimes there is a sandwich colonnadeentablature-colonnade structure like Figure 25 (Long & Wood, 1986). The Portage Lake Volcanics show columnar joints in many places. They also exhibit difference scales and styles of jointing. Figure 25: Schematic diagram of columnar jointing pattern in the Columbia River flood basalt near Bend, Oregon (left), compared with an actual photograph of one good example of a lava cross section. Individual sections never match perfectly because of environmental variables. The recognition of the role of water infiltration in the formation of certain kinds of entablature jointing (see above) in the Columbia River Flood basalts by Long & Wood, 1986 was an especially important insight (see Iceland examples especially), as was the detailed work on column formation by DeGraff and Aydin (1993) and DeGraff et al. (1989). On Isle Royale, colonnade style jointing can be seen in many places, although it is less perfectly developed than many worldwide examples. Entablature jointing is also prominent at Isle Royale, especially in the Edwards Island flow (pei) and the top of the Greenstone Flow (pg). To demonstrate the variability of columnar joints in lavas and tuffs, the field trip website explores a large collection of columnar joint photographs (http://www.geo.mtu.edu/~raman/SilverI/ IRKeweenawRift/Columnar_Joints/Columnar_Joints.html). 28 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 26: Colonnade style jointing on Isle Royale. Left photo shows Monument Rock, an exhumed (sea stack) column several meters in diameter. Right photo shows rude 5m diameter columns in the Greenstone Flow (pg). For more, see also (http://www.geo.mtu.edu/~raman/ SilverI/IRKeweenawRift/IR_Column_examples/IR_Column_examples.html) Figure 27: Entablature style jointing in the Edwards Island Flow, Scoville Point, Isle Royale. Scale of these joints is 7-12 cm. Mafic Volcaniclastic Deposits Kilauea and Iceland mainly produce lava flows like those on Isle Royale, but near their vents we find compositionally similar pyroclastic rocks of a variety of types. These pyroclastic rocks, called mafic volcaniclastic deposits (MVD) are also a minor part of the rock record at Isle Royale. We note that such rocks are well known at most continental flood basalt provinces (see Ross et al. 2005). Mechanisms for generation of these deposits include magmatic and phreatomagmatic processes. On both Isle Royale and the Keweenaw such deposits are noted in a few stratigraphic horizons. 29 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift In a review paper, Ross et al., 2005, have summarized worldwide occurrences of MVD: Flood volcanic provinces are assumed generally to consist exclusively of thick lavas and shallow intrusive rocks (mostly sills), with any pyroclastic rocks limited to silicic compositions. However, mafic volcaniclastic deposits (MVDs) exist in many provinces, and the eruptions that formed such deposits are potentially meaningful in terms of potential atmospheric impacts and links with mass extinctions. The province where MVDs are the most voluminous—the Siberian Traps—is also the one temporally associated with the greatest Phanerozoic mass extinction. A lot remains to be learned about these deposits and eruptions before a convincing genetic link can be established, but as a first step, this contribution reviews in some detail the current knowledge on MVDs for the provinces in which they are better known, i.e., the North Atlantic Igneous Province (including Greenland, the Faeroe Islands, the British Isles, and tephra layers in the North Sea basin and vicinity), the Ontong Java plateau, the Ferrar, and the Karoo. We also provide a brief overview of what is known about MVDs in other provinces such as the Columbia River Basalts, the Afro-Arabian province, the Deccan Traps, the Siberian Traps, the Emeishan, and an Archean example from Australia. The thickest accumulations of MVDs occur in flood basalt provinces where they underlie the lava pile (Faeroes: >1 km, Ferrar province: >400 m, Siberian Traps: 700 m). In the Faeroes case, the great thickness of MVDs can be attributed to accumulation in a local sedimentary basin, but in the Ferrar and Siberian provinces the deposits are widespread (>3x105 km2 for the latter). On the Ontong Java plateau over 300 m of MVDs occur in one drill hole without any overlying lavas. Where the volcaniclastic deposits are sandwiched between lavas, their thickness is much less. In most of the cases reviewed, primary MVDs are predominantly of phreatomagmatic origin, as indicated by the clast assemblage generally consisting of basaltic clasts of variable vesicularity (dominantly non- to poorly-vesicular) mixed with abundant country rock debris. The accidental lithic components often include loose quartz particles derived from poorly consolidated sandstones in underlying sedimentary basins (East Greenland, Ferrar, Karoo). These underlying sediments or sedimentary rocks were not only a source for debris but also aquifers that supplied water to fuel phreatomagmatic activity. In the Parana´–Etendeka, by contrast, the climate was apparently very dry when the lavas were emplaced (aeolian sand dunes) and no MVDs are reported. Volcanic vents filled with mafic volcaniclastic material, a few tens of metres to about 5 km across, are documented in several provinces (Deccan, North Atlantic, Ferrar, Karoo); they are thought to have been excavated in relatively soft country rocks (rarely in flood lavas) by phreatomagmatic activity in a manner analogous to diatreme formation. 30 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift On Isle Royale at least three occurrences of MVDs were noted by NK Huber: 1. A breccia found above the Amygdaloid Island Flow (pai) (Figure 28), 2. a Tuff-breccia unit at the top of the Minong Flow (pm) and 3. A Tuff-breccia above the Greenstone Flow (pg). On the field trip we plan to visit the Amygdaloid Island occurrence. We will also see sedimentary units on Mott Island which resemble MVD. Figure 28: Breccia occurring above the Amygdaloid Island Flow, collected from the south shore near the E end of the island (from Huber, 1973). Lava Stratigraphy on Isle Royale. Huber (1973) named eleven distinctive lava flows (Figure 30) from the sequence of lava units on Isle Royale, using their field characteristics (see above). These units can be traced across the island generally paralleling the elongation of the whole island. These named flows are generally the thickest and most resistant to erosion so they make topographic highs and project as islands at the margins of the main island, accounting for the smaller units of the archipelago. This layered stratigraphy is quite regular (Figures 29, 30, 31). Figure 29: Cliff section of Icelandic lavas, showing a sequence of parallel layers with variable thicknesses. We do not generally have vertical sequences like this at Isle Royale, but the layers must have very similar geometry. Photo from along the south coast of Iceland near Hof, 2008. 31 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 30: Named lava flows of Isle Royale (from Huber, 1973). Map symbols in red. psp pei pmp pli pth pwi pp pg pgi pm ph php pp pai 32 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 31: Longitudinal Stratigraphic section showing variations in thickness of the Portage Lake Volcanics and Copper Harbor Conglomerate on Isle Royale. From Huber, 1973. Ophitic Texture and Ophite significance Keweenaw rift rocks include a somewhat rare textural variety of basalt called ophite or ophitic basalt. Ophitic texture is defined inconsistently, but it is an important variety of basalt texture where pyroxene (or occasionally olivine) forms larger crystals and typically contains numerous crystals of plagioclase (Figure 32). Pyroxenes may vary from < 1 to 10 cm and may include as many as hundreds of plagioclases. In the field the pyroxenes are often 1-2 cm in diameter and give the rock a distinctive aspect. There may be a brownish or orange region surrounding the pyroxenes which may represent a glassy remnant of magma melt. Overall the ophite is thought to represent a solidified remnant of a dendritic crystal mush. 33 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Crystal size and form in volcanic rocks is known to be influenced by the rates of cooling in the immediate vicinity of the growing crystal. Slow cooling in a pluton leads to large, Figure 32: Ophitic cobbles (left) and a wet surface of an ophitic lava flow (right) are common on Isle Royale and the Keweenaw, and quite rare elsewhere in the world. The ovoid features in both photos are clinopyroxenes, while the orange or reddish material surrounding the pyroxene is typically a glassy mesostasis which is now altered to chlorite or corrensite. equidimensional crystals, while very rapid cooling can lead to no crystals at all (glass or obsidian). Intermediate cooling rates can lead to unusual shapes of crystals (spherulites, “bow ties”, spinifex, and ophitic) as crystals nucleate or grow at accelerated rates as crystallization, which requires more time than allowed by the environmental cooling of the lava, cannot keep pace and exhibits disequilibrium (Lofgren, 1980). The rate of heat loss (undercooling or supercooling) during the solidification is thus thought to cause ophitic texture, where pyroxene is growing rapidly and plagioclase is forming many more nuclei. Because ophites may completely crystallize and can be coarse-grained, especially with respect to pyroxene, some are termed gabbro rather than basalt. At first geologists looking at ophitic lava flows in the Keweenaw wondered whether they were sills. There is a tendency for ophitic textures to be found in large basaltic intrusive rock bodies such as sills, suggesting that overall they reflect relatively slow solidification. Overall ophitic texture is ubiquitous and could be a hallmark of the Keweenaw Rift lavas. Paces (1988; see Figure 17) found that the average thickness of ophitic Keweenaw flows was 33 m (range 11-140m), while subophitic ones were 12 m (range 4-45 m), and traps (melaphyres) about 5 m thick (range 2-60m). We note that the overall average thickness of Keweenawan flows is about 10-11m, much greater than what we see at modern volcanoes like Kilauea (average flow about 0.5 m thick). The differences are likely the result of ponding within the rift valley, where volcanism filled the rift basin rather than running off a slope away from the vent, as happens at Kilauea. So ophitic texture is a hallmark of slow cooling that is apparently related to ponding of the lavas. 34 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Pegmatites or pegmatoids in lava flows In Keweenawan flows pegmatite (or pegmatoid) layers are conspicuous (Fig 33), especially in the thicker flows. They appear to be analogous to vesicle sheets that are found in most flood basalts, but they may result from more evolution during the longer solidification times. Figure 33: This collection of beach cobbles shows obvious texture of Keweenawan lava pegmatite—note conspicuous plagioclase laths. These layers have vesicular texture and are typically mineralized with zeolite facies minerals. The thickest lava flows in the Keweenawan Portage Lake Volcanics contain horizons called “pegmatites,” “pegmatoids,” or “dolerites.” The following description of these features is from Longo (1984): Lacroix (1928, 1929) coins the term ''pegmatitoide" to describe the coarse-grained zones considered to represent the final stages of differentiation in basaltic lavas of France. The lavas of Michigan's Copper Country show similar differentiates for which Lane (1893) applies the term "doleritic." Cornwall (1951) adopts the textural term "pegmatite" from the usage of Butler and Burbank (1929). He changed the confusing "doleritic" term to "pegmatitic facies, " and subsequently described such units in the Greenstone flow, Big Trap, and several other large flows within the PLV on the Keweenaw Peninsula. For the present study, the term "pegmatoid zone" from Lindsley et al. (1971) is adopted to encompass the portion of the Greenstone flow with numerous en echelon, lens-shaped pegmatoids, associated granophyric phases, and subophitic layers. Texturally, pegmatoids are coarse grained when compared to ophitic zones. Coarse plagioclase laths dominate with interstitial, subhedral clinopyroxene and abundant interstitial to somewhat poikilitic magnetite and ilmenite. Consequently, the pegmatoids are strongly magnetic compared to ophitic units. This suggests that a higher titaniferous magnetite/ ilmenite ratio for magmatoids than for ophites. Visual inspection generally reveals a greater overall opaque (oxide) concentration in the pegmatoids. Subophitic layers are often found hosting the en echelon pegmatoids. These layers, like pegmatoids, are strongly magnetic and very coarse grained stratiform features, but contain less abundant, smaller sized pyroxene. The 35 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift contacts between pegmatoids and subophitic units are usually sharp, although instances of gradational contacts have been observed. Subophitic layers grade into the ophites and seem to occupy the greatest volume of the pegmatoid zone. They have been observed to pinch out within pegmatoid units and may not be continuous planar features throughout the flow. Perhaps pegmatoid units are not only lens-shaped but also flattened amoeboid-like features interfingering with subophitic layers. The frequency of pegmatoids and subophitic layers increases proportionally with increasing flow thicknesses. Both vary in thickness and shape and typically occur in the upper half of a lava flow. Pegmatoids have also been observed as auto intrusions, such as in the entablature on Isle Royale and the upper ophite on the Keweenaw Peninsula. The stratiform pegmatoids are usually found armoring the tops of cliffs formed of the lower ophite. The extension of weak vertical joint patterns into the pegmatoid (forming crude large columns) suggests that pegmatoids may be part of the colonnade. In most cases pegmatoid zones separate a basal colonnade from an upper colonnade. Pegmatoids are not unique to thick flows of the PLV. Lindsley et at. (1971) assert that three of the thicker flows from the Picture Gorge Basalt contained pegmatoid lenses. Santin (1969) discusses the presence of pegmatoids in horizontal basalts of the Lanzarote and Fuerteventura Islands in the Canarian Archipelago. --Longo 1984 Pegmatites are found to be especially well developed in thicker flows such as the Greenstone (pg), which can be more than 1200 ft thick. Pegmatite layers up to 30 ft thick are found above the flow’s midpoint at a stratigraphic layer analogous to the vesicle sheets near the top of the core of idealized pahoehoe flows as described by Self et al., 1998 (see Flow Structure section, above). Cornwall (1951) shows a Greenstone flow section from the Keweenaw in Figure 34. Upper Ophite pg Lower Ophite Figure 34: Columnar section (left) and cross section (above) of the Greenstone flow (pg) as exposed in overlapping diamond drillholes from Delaware, Michigan (Keweenaw Peninsula). Pegmatite is shown as black layers and occurs in the upper part of an unusually thick (1300 ft) lava flow. Granophyre was not found in the cores but is projected based on field data (from Cornwall, 1951). 36 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift The texture of pegmatite in thick flows is coarser and the plagioclase laths may be as large as several cm (fig 35). Figure 35: Polished surface of pegmatite boulder from Passage Island, Isle Royale, showing plagioclase laths of several cm. In thinner flows pegmatite layers are thin (often a few cm) and resemble vesicle sheets (see figure 36). Figure 36: Thin pegmatite or vesicle sheet from 6 m thick lava flow of Lake Shore Traps, Silver Island, Keweenaw Peninsula. Pegmatite layers or vesicle sheets in thinner flows are texturally similar to segregation cylinders and lie stratigraphically above them (Figure 37). 37 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 37: Tabular pegmatite layer within horizontally fractured section of 20-30 m thick flow on Raspberry Island. This 6 cm thick layer is about 5 m stratigraphically above segregation cylinders. Amygdaloidal Minerals in Portage Lake Volcanics on Isle Royale To find minerals at Isle Royale or in the Keweenaw, you should walk the coastlines, especially those that are well wave-washed. The waves expose the minerals and pebbles of various minerals, can be found on adjacent beaches. Using a canoe or small boat, and watershoes and taking plenty of time, walk the shore and watch for veins and amygdaloids. Observe the interiors of basalt flows where vesicle cylinders, pegmatites, joints and veins may expose these distinctive minerals (Figure 22). Individual minerals are sometimes difficult to identity, even for experts, but certain groups of minerals can be distinguished very easily (see Table below). The colors of amygdaloidal minerals are highly variable and distinctive (Figure 38). 38 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 38: A selection of beach pebbles showing various colors of amygdaloidal minerals. See also photos of specific minerals (http://www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift/ Amygdaloid/Pages/Amygdaloid_2.html) For another test, you can use your finger nail. The phyllosilicates, chlorite, corrensite, and saponite are all of green color and very soft minerals. You can easily scratch them with your finger nail. The other green minerals as pumpellyite or prehnite are much harder and you will not be able to scratch them with your fingernail. In fact, in pebbles along the shore they stand out, since they are not as easily eroded as the surrounding rock. The pink unusual color of prehnite of Isle Royale often is the result of very tiny inclusions of native copper which makes it similar to the zeolite thomsonite.The zeolite family is in general difficult to identify, but the zeolite, laumontite, can easily be recognized. It is of white or pink color and if you touch it with your finger nail it will split up into small fibers. What’s next? After mastering the mineral identifications in the boulders, students can also look at amygdular minerals to study the order that minerals were deposited in those vesicles, what mineralogists call paragenesis. 39 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Native Copper and the mining The Midcontinent Rift is the most important and notable location on Earth for native copper. This is truly a cosmic oddity, because copper in nature is typically found as a sulfide. Indeed, Goldschmidt classified copper with a group of elements called “chalcophile”. So why does copper occur in the Midcontinent Rift as native copper (Fig 39)? This is a major puzzle. Figure 39: NATIVE COPPER VEIN ON WASHINGTON ISLAND, ISLE ROYALE NATIONAL PARK. THIS VIEW MAY BE LIKE WHAT NATIVE AMERICANS FOUND WHEN THEY FIRST VISITED THE COPPER COUNTRY. SUCH OCCURRENCES ARE NOT COMMON ANYMORE —THEY WERE DUG OUT OF THE WAVE-WASHED SHORELINES. Could sulfur have been purged from the magma source region or from its magma chambers? This idea is suggested by the early ultramafic dikes which apparently represent the beginning of Midcontinent Rift and which contain apparently immiscible sulfide bodies containing Ni, Cu and rare earth elements (Ding et al., 2012). These dikes could represent magmas derived from mantle material that was melted more completely than when the mantle produces basalt. And this magma may have exsolved sulfide liquid before it was intruded into dikes. Loss of sulfur from the source region or a magma chamber may result in a sulfur-depleted environment favoring native copper? This is a speculation! Another explanation of sulfur loss is that loss of sulfur through degassing of magma from magma oceans would be facilitated by the ponding and long solidification times. Awareness of sulfur emissions from eruptions is heightened by recent studies of eruptions and climate. Could extensive degassing during Keweenawan rifting play a role in eventually forming native copper ore deposits? Speculation! Keweenawan native copper deposits seem to be associated with widespread hydrothermallyinduced zeolite and prehnite-pumpellyite facies metamorphism (Stoiber & Davidson, 1959; Jolly, 1974) which mineralized the permeable lava flow tops and sediment layers of the Portage Lake Volcanics, apparently about 30 ma after the rift volcanism, during the period of Grenvilleinduced deformation of the rift syncline (Bornhorst & Barron, 2012; Nicholson et al., 40 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift 1997 ,Cannon, 1994; Bornhorst et al., 1988) when there was faulting of the rift which enhanced fluid flow within the syncline. There is a rich lore about indigenous ancient copper mining in the Lake Superior region. Most of it is highly speculative and is unsupported, but it is fervently believed. The abundant archeaological copper relicts (Figure 40) leave no doubt that copper was mined at Isle Royale thousands of years ago and traded across North America and beyond. These early mines found native copper in veins at the surface. They left behind pits and dumps. Figure 40: Archeological Copper relicts of midcontinent rift native copper from the Michigan Tech Archives. These materials and open pits left behind show that ancient people mined copper in the Keweenaw and on Isle Royale. Mining by Europeans started in the 1800s on both the Keweenaw and Isle Royale. The Isle Royale mines were all marginal efforts and did not last more than a few years. 41 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Copper Harbor Conglomerate The Copper Harbor Conglomerate occurs on the SW sector of Isle Royale and has been studied by N K Huber, USGS OFR 754-B. Huber gives the following introductory comments: The Copper Harbor Conglomerate, in its type area on the Keweenaw Peninsula of Michigan, was named and defined so as to include a thick sequence of sedimentary rocks, previously separated (in ascending order) into the Great, Middle, and Outer Conglomerates, with intervening lava flows, the Lake Shore Traps (Lane and Seaman, 1907, p. 690-691; Lane, 1911, p. 37-40). On the Keweenaw Peninsula, the Copper Harbor Conglomerate conformably overlies the Portage Lake Volcanics (middle Keweenawan), and locally the two formations interfinger. The Portage Lake Volcanics consists primarily of lava flows; minor sedimentary rocks, similar to those within the Copper Harbor Conglomerate, are intercalated between flows (hereafter referred to as interflow sedimentary rocks). The transition between the two formations reflects a gradual cessation of volcanic activity and the growing dominance of a sedimentary regime. The Copper Harbor Conglomerate is overlain by the Nonesuch Shale and Freda Sandstone (upper Keweenawan, Fig 41). Figure 41: Local Stratigraphic units. Approximately four-fifths of Isle Royale is underlain by volcanic flows and minor clastic rocks of the Portage Lake Volcanics, which dip 10°-20° to the southeast in the vicinity of their contact with the overlying Copper Harbor Conglomerate (Huber, 1973b, Wolff & Huber, 1973). The Copper Harbor Conglomerate underlies the remaining one-fifth of Isle Royale and is confined to the southwestern part of the archipelago; it dips 5°-28° to the southeast. The contact between the Copper Harbor Conglomerate and the Portage Lake Volcanics appears to be conformable; the top of the Copper Harbor Conglomerate, however, is not exposed. If the Nonesuch Shale and other formations that overlie the Copper Harbor Conglomerate on the Keweenaw Peninsula are present in the Isle Royale area, they lie beneath Lake Superior to the southeast. Consisting of fluvial subaerial sandstones, siltstones and conglomerates, The CHC shows transport directions that generally spill into the rift valley (see Fig 42). Huber gives many details of the CHC on Isle Royale in his OFR. 42 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 42: Plot of observed and interpreted paleocurrents seen in the Copper Harbor Conglomerate, Isle Royale (Wolff & Huber, 1973). On the Keweenaw Peninsula the Copper Harbor Conglomerate is partly made up of alluvial fans (Elmore, 1984). On Isle Royale the sandy and silty units are more abundant and cobble sizes are generally smaller. LIDAR Topographic Surveys of Isle Royale LIDAR (LIght Detection and Ranging or Laser Imaging Detection and Ranging) survey of all of Isle Royale, with a nominal resolution of about 2 m is a new resource for understanding landscapes. The data we show here came from Seth De Pasqual, at Isle Royale National Park. It reveals a striking topography which shows the dipping lava beds, and the prominent large lava flows, like the the region NE of Windigo. Differential erosion of lava flows occurs when soft material, like what is found in the amygdaloidal flow tops and along faults is preferentially removed and makes a topographic low, while the massive flow interiors resist erosion and become topographic highs. Glacial deposits mask the lava layers in part, especially southward in the image, where the flows are mostly covered, but protrude through glacial cover. The glacial materials are softer, but they also reveal wonderful geological information. Drumlins are asymmetrical glacial features (Figure 43) which reveal the direction of glacial movement. Figure 44 shows an area near Lily Lake, which depicts conspicuous drumlins south of the lake. The pattern shows the direction of movement (from east to west) clearly, and the degree of elongation is also indicative of the rate of movement. 43 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 43: Schematic diagram of a drumlin, showing how its shape may be related to the direction of ice movement (www.geography-site.co.uk). psp Figure 44: LIDAR topography image of Lily Lake region, Isle Royale National Park, showing multiple drumlins. LIDAR is advantageous over conventional DEM (digital elevation models) for glacial features, but the good resolution of LIDAR also clarifies structural information on the lava flows. The second LIDAR image (Figure 45) shows dramatic bending of the lava flow layering that is remarkably regular in most places on Isle Royale. The bending likely reflects deformation related to faulting associated with McCargoe Cove. The LIDAR offers an opportunity to do interpretation, which will reveal details of the rift formation and its subsequent deformation. Figure 45: (next page) LIDAR topographic image of Pickerel Cove area, Isle Royale. The layered lava flow sequences of Isle Royale stand out clearly as resistant flow interiors resist erosion and stand up to higher levels. Faults which offset the flow layers are also detected as eroded topographic lows. Here there is apparent bending of the lava flows. 44 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Specific Field areas we will visit: Washington Harbor and Windigo The field trip starts at Grand Portage, Minnesota, where we will take the Voyageur II east to Washington Harbor and Windigo about 35 km (22 mi) offshore. Between the Minnesota shoreline and Isle Royale, the strike of Keweenawan rocks, known as the North Shore Volcanics in Minnesota (1109-1100 Ma), changes from E-W to about N 55° E, where the PLV formation (1096-1094 Ma; Figure 13) at Isle Royale begins. This discontinuity could be partially related to the Isle Royale Fault (IRF), which the Voyageur crosses between Grand Portage and Isle Royale. This is a thrust fault which bounds the north flank of the rift, apparently associated with the inversion of the Midcontinent Rift. The IRF was detected in the GLIMPCE (Great Lakes International Multidisciplinary Program on Crustal Evolution) seismic profile (Figure 12) collected on a NS line E of the Keweenaw Peninsula, far from Isle Royale, but along the north flank of the rift zone. It is thought to extend W to at least the SW end of Isle Royale, where Isle Royale is mantled with a much thicker portion of glacial cover and the glacial features are much more prominent (see pp. 20-21 and 41-54 in Huber 1983). 45 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift The bedrock geology of the Washington and Grace Harbor areas (Figure 46) includes four large flows that continue all the way to the other end of the island. The Greenstone Flow (pg) crosses the center of Washington Island and outcrops in several places SSE of Windigo. The Tobin Harbor flow (pth) outcrops at South Rock, SW of Washington Island. The Minong Flow (pm) outcrops S of McGinty Cove, and the Scoville Point Flow (psp) outcrops near Middle Point on the S side of Grace Harbor. The thickest flows in this area are the Washington Island Flow (pwi) and the Grace Island Flow (pgi). Both of these flows occur only locally, from the end of Washington Island to a point between Windigo and Sugar Mountain, a distance of about 14.5 km (9 rni) along strike. The lava flows here dip at 15-20° SE, an attitude that is similar for younger flows on Isle Royale. Vertical N-S trending fractures, with little offset, cut across the bedrock strata near Washington Harbor (Figure 46, 47, 48). Huber (1983) interprets these as structures related to the warping of the Lake Superior Syncline. South of Grace Harbor, the bedrock of the island is buried by till. Washington Harbor ! ! Grace Harbor Figure 46: Portion of figure 2 (Geologic map of Isle Royale, Huber, 1973) showing the area along the west end of Isle Royale. Most of the map indicates glacial deposits, shown in tan, which cover much of the lavas and conglomerates. The prominent locations where bedrock penetrates the glacial deposits are shown in bright colors. Most of eastern Isle Royale has little glacial cover. 46 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 47: Oblique Google Earth view of Washington Island, looking E, showing N-S faults and tilted lava flows. Note: the blue triangle indicates direction and angle of dip (right at about 20 deg.) Figure 48: Oblique Google Earth view of Washington Harbor, looking E. The Windigo area was the site of the last serious mining on Isle Royale, from 1890 to 1892. After failure and closure of mines farther E, the Wendigo Copper Company (renamed from the Isle Royale Land Corporation) founded a mining venture on 8000 acres of land at Washington Harbor, under the leadership of Jacob Houghton, brother of Douglass Houghton. The town site was named Ghyllbank and was located near the present site known as Windigo. The mine site, about 2 km (1.25 mi) inland to the NE, was named Wendigo. People built roads all around the W end of Isle Royale, and 135 people lived at the mine site. The company did diamond drill exploration, as well as extensive trenching. In 1892, the miners gave up and left. When mining stopped, the company tried to sell land to tourists and resort owners (Rakestraw 1965). 47 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift The N Side of Isle Royale: The Hill Point Flow After Windigo, we travel along a straight section of the coast following the Hill Point Flow (php). Muted by glacial deposits, the layered strata of lava flows shows in the geomorphology. Note the cross cutting faults (dotted lines in Figure 49), which are conspicuous in this area. These faults may have formed during deformation of the rift during its subsidence and during the Grenville Orogeny. The faults may have enhanced fluid flow, zeolite facies metamorphism, and copper mineralization. The faulted Windigo area is one place where some mining occurred. Figure 49: Oblique Google Earth View, looking S from Hugginin Cove, directly across the stratigraphy, with flows dipping away from the view. With the flows dipping SE, moving toward the N side of the island takes us further into the PLV section, until we reach the horizon of the Hill Point Flow (php). This is an ophitic flow, forming imposing cliffs along the shore from Hugginin Cove all the way to Todd Harbor, a distance of about 24 km (15 mi). This flow also makes up the majority of shoreline from Pickerel Cove all the way to Hill Point itself, at the W end of Five Finger Bay, about 64 km (40 mi) from Windigo. The tilted strata along the shore make the shoreline steep, and the prevailing winds from the NNW can make conditions treacherous for small boats. The Hill Point Flow is a coarse-grained, ophitic unit with augite oikocrysts of 2 cm (0.8 in) or more. The vertical fractures superimposed across the dipping strata are noticeable throughout the entire flow. From the west area of the flow to the east area, the fractures gradually begin to change from N-S to more N-E trending. According to Longo (1984), the Hill Point Flow may correlate with a large flow on the Keweenaw Peninsula, the Scales Creek Ophite, which extends all along the Keweenaw Peninsula for more than 160 km (100 mi) of strike length, and right through Houghton, which is about 110 km (68 mi) SSE of Hugginin Cove. 48 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift LIDAR survey (Figure 50), from Seth De Pasqual, at Isle Royale National Park, reveals a striking topography which shows the dipping lava beds, and the prominent large lava flows, like the Hill Point Flow (php) and the Minong flow (pm) in this image of the region NE of Windigo. Differential erosion of lava flows occurs when soft material, like what is found in the amygdaloidal flow tops and along faults is preferentially softer and evolves to a topographic low, while the massive flow interiors resist erosion and become topographic highs. The prominent NS faulting of the lava layers is obvious, as are less extensively altered NE trending faults. Glacial deposits partially mask the lava layers, especially southward in the image, where the Grace Island (pgi) and Greenstone Flows (pg) are mostly covered, but protrude through glacial cover. Trails are plotted in yellow. This LIDAR data is advantageous for structural geology study because of its sensitivity to faults. It also reveals details of glacial (drumlins, outwash, kames, etc.) and postglacial features (shorelines, mine pits, dumps and roads). php pm php php pm pm pgi pg pgi pg Figure 50: LIDAR topographic image of comparable area to Figure 48, showing how LIDAR is advantageous for structural studies. (Seth De Pasqual, NPS). Trails in yellow. McCargoe Cove 49 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift At the midpoint of the island is McCargoe Cove, which is a linear, 3.2 km (2 mi) long inlet (Figure 51) that follows a large fracture zone, trending N 30º E to a campground site located along an ancient Native American portage route and near a mine, the Minong Mine. Native Americans left hundreds of ancient pits as relics of mining over centuries at this site, and in 1874 three companies were formed in Detroit to exploit the potential here. They built a dock and a warehouse, and started to build a railroad. Some large masses of copper were successfully mined, and the community here grew for several years in spite of difficult winter conditions. But mining did not last beyond 1885 (Rakestraw 1965). Figure 51: Oblique Google Earth View of McCargoe Cove, looking SW. Lava layers are dipping to the left with steeper dips below and shallower ones above. LIDAR survey (nominal resolution of about 2 m; Figure 52), from Seth De Pasqual, at Isle Royale National Park, reveals a striking topography which shows the dipping lava beds, and the prominent large lava flows, like the the Minong Flow (pm) in this image of the region W of the McCargoe Campground. 50 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift pm pm Figure 52: LIDAR topography of the area west of the McCargoe Campground, showing the pits and dumps of the Minong Mine. These features cannot be resolved in DEM-based topo maps with lesser resolution. As in previous examples, increased erosion of lava flows in the amygdaloidal flow tops and along faults makes topographic lows, and flow layers and prominent NE trending faulting is obvious. Here the mine pits and dumps associated with the Minong Mine are also easily resolved, which shows how LIDAR can map topographic features that are difficult to resolve through vegetative cover. Copper mineralization in the area above a thick lava flow is common, perhaps due to the effect of channeling fluid, as the flow interiors are relatively impermeable and act as a hydrologic dam. The Amygdaloid Channel From McCargoe Cove, we will continue to the NE, passing through the Amygdaloid Channel (Figure 53). Amygdaloid Island is composed of the oldest lavas of the PLV on Isle Royale and is supported by a large flow, the Amygdaloid Island flow (pai), which is a fine-grained basalt (termed "trap"). At the W end of Amygdaloid Island is the National Park Service (NPS) ranger station near Kjaringa Kjeft. Crystal Cove, 3.2 km (2 mi) E of the station, was, beginning in 1906, a private residence and fishery. 51 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 53: Oblique Google Earth View of the Amygdaloid Channel, looking NE, with Amygdaloid Island to the left and beds dipping to the right at increasingly shallow angles. As we travel through the Amygdaloid Channel, drowned ridge and valley topography of Isle Royale will become very visible, with more resistant lava flows holding up linear islands. Amygdaloid Island is the site of mafic volcaniclastic deposits (pp on Amygdaloid Island in Fig 53). It also has a sea arch (left) which is located almost directly opposite the keyhole. (Fig 54). Shipwrecks are numerous on the many "reefs" found all around the NE end of Isle Royale. Opposite Crystal Cove on the south side of Amygdaloid Island is Belle Isle, a beautiful campground accessible only by boat and canoe, located on the site of a resort that operated in the 1920s, when it served the grand lake steamers of that period. Figure 54: Sea Arch on Amygdaloid Island. 52 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Blake Point—a key locality As we round the tip of Isle Royale to Blake Point (Figure 55), we are moving up in the stratigraphic sequence. We will first cross the Hill Point Flow (php) at Hill Point, then the Minong Flow (pm) near Locke Point, and finally the Greenstone Flow (pg) at the Palisades. The Greenstone Flow is perhaps Earth's largest lava flow. Blake Pt Locke Pt Hill Pt Figure 55: Blake Point segment of Geologic Map of Isle Royale (Huber, 1973). At right, photos of columns at the Palisades on the anti-dip slope just west of Blake Point. The following are comments by Longo (1984): Similarities in the stratigraphic sequence of Isle Royale and the Keweenaw Peninsula of Michigan were recognized by numerous workers prior to 1851. The first thorough study of both areas, conducted by Lane (1893, 1911), resulted in the correlations of specific rock units. One unit in particular, due to its persistence as a prominent ridge on both Isle Royale and the Keweenaw Peninsula, became Lane's most convincing evidence for a correlation across this section of the Lake Superior syncline. 53 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Lane (1893) states, "The backbone ridge thus agrees in every way with the great corresponding ridge on the Keweenaw Point." Outcrop and drill core data by Lane (1893) reveal this unit as a single immense, differentiated lava flow. Lane (1893) refers to the flow as "the Greenstone, the 'backbone' and biggest ophite of all, with the bed at its base we correlate as the Allouez Conglomerate. " The Greenstone's great thickness and differentiated nature led some workers to consider it as an intrusive sill (Seaman and Seaman 1944; Van Hise and Leith 1911). However, convincing data have proven this unit to be a lava flow (Lane 1893,1911; Butler and Burbank 1929; Broderick 1935, 1946; Cornwall 1951), and henceforth known as the Greenstone flow. Huber (1973a) confirms the similarities of the Greenstone flow on Isle Royale and the Keweenaw Peninsula, and he supported the correlation. --Longo 1984 The shoreline around Blake Point offers the best view of the Greenstone Flow, better than any other sites at Isle Royale or the Keweenaw Peninsula (Figure 56). On the way to the campground in Merrit Lane, the starting point of our Blake Point walk, we will pass the NW side of Edwards Island, which has good exposures of entablature columnar joints in the Edwards Island flow (pei). The boat will let us off at the Merrit Lane Campground for our walk to Blake Point. We will follow the shoreline from Merrit Lane around the point, remaining close to the wave-washed rocks, yet trying to keep our feet dry. Most of the walk is on the upper ophite unit of the Greenstone flow. (The entablature part of the Greenstone and its flow top is underneath Merrit Lane, and we will see parts of this from the boat later). The upper ophite exhibits a poorly-developed columnar structure all along the walk, with the columns perpendicular to the bedding. The size of the oikocrysts increases from top to bottom. After rounding the corner, we will cut through the bushes to descend a cliff that marks the lower anti-dip face of the upper ophite. At the base of this cliff, we will see wave-washed exposures of the pegmatoid, here about 23 m (75 ft.) thick. The contact here appears to be quite sharp, although Huber (1973a) says it is frequently gradational. The pegmatoid underlies the low shoreline and also the area under the light tower. A section of the Greenstone flow is exposed on Passage Island, a 2 km (1.2 mi) long island that can be seen about 4 km (2.5 mi) offshore from Blake Point. Around the corner from the tower and vertically down about 4 m (13 ft.) is the contact with the lower ophite (which is too difficult for us to reach safely). Longo (1984) describes the contact as a gradation over about 1 m of thickness. From here, we will return by the same route to Merrit Lane. Weather permitting, we will travel around the point in the boat to examine the lower ophite cliffs along the Palisades. The columns exposed on the anti-dip slope are up to several meters across. The base of the Greenstone flow is not exposed here. Figure 56 (next page): Oblique Google Earth View of Blake Point, looking SW, with beds dipping about 25 degrees to the SE. Most of the large land mass is underlain by the Greenstone flow, and its three distinct layers can be seen and outlined here better than anywhere else. Below is a photo from just offshore at Blake Point, at the water level. 54 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 57: LIDAR survey of Blake Point area. Most of the land imaged here is the Greenstone Flow and this image is remarkable in showing indications of layering, and also the different character of layers. On the south side of the main land body, facing Merrit Lane, are eroded remnants of the Upper Ophite layer, with its columnar jointing and dipslope aspect. On the North side there is a steep slope (Palisades) where the Lower Ophite is exposed on an antidip slope. Between these two layers lies the pegmatite of the Greenstone, which is 75 ft thick and which appears to be eroding in an irregular, wavy pattern. It is remarkable that the LIDAR shows information about these three layers. This information is valuable because we typically do not have very good exposures. Seth DePasqual, IRNP. peg peg upper ophite peg upper ophite peg upper ophite 55 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Passage Island and Gull Rocks Farther to the NE, off of Blake Point, Passage Island, 3.5 miles from Blake Point, and Gull Rocks, 8.7 miles away (Figure 58), are both built of rift lava, including the Greenstone Flow, which is found at the E end of Passage I. These are the most easterly subaerial exposures of the PLV near Isle Royale. Figure 58: Oblique Google Earth View of Passage I and Gull Rocks (same scale, but offset). To the right is a piece of the Isle Royale Geologic Map (Huber, 1973). Snug Harbor At this wonderful location in Rock Harbor, the National Park Service has chosen to concentrate its Isle Royale services and concessions for visitors. The Lodge and Visitor Center is where the field trippers will sleep, catch their boat rides and have evening discussions. The location coincides with two of Huber’s named lava flows: the Scoville Point (psp) porphyrite and the Edwards Island Trap (pei) (Figure 59). This location allows boat access to both Tobin and Rock Harbor, as well as foot trails to Scoville Point, Mount Franklin, and Daisy Farm, and is a safe harbor. 56 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 59: Oblique Google Earth View of Snug Harbor, Looking NE. Scoville Point For part of this day's trip we will walk on the rocky dip slope of the Scoville Point flow (psp), facing Rock Harbor along the shore (Figure 60). Huber (1973) describes the basalt of this flow as containing "fine, equant, millimeter sized, plagioclase crystals distributed uniformly through a fine grained matrix." He says the thickness is 30-60 m (l00-200 ft.). There are not many features that can be seen in outcrop, but the flow is very resistant to erosion and buttresses the shoreline. We will take the Stoll Trail (white line in Fig. 60), which goes along the shore of Rock Harbor. Along here, we will see 5000-year-old Nipissing shorelines and glacially grooved outcrops of the Scoville Point flow. Outwash cover here is meager, but kettle lakes and morainal zones occur. On the upper map in Fig. 60, GPS markers identify the points of interest/inquiry. Also, we will be able to see the ophitic flows above and below the Scoville Point flow along the way. About 0.8 km (0.5 mi) from the Lodge lie ancient mine pits, attributed to Native Americans who occupied this area from about 5000 yrs BP during the period of the Nipissing stage. The mining was apparently informal and quite limited in any one place, but there are more than 1000 such pits all over Isle Royale according to Rakestraw (1965). 57 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 60: Oblique Google Earth Views of Stoll Trail to Scoville Point looking N and SW. Trail is a white line, and marked points are GPS marked locations. As we near Scoville Point, the Scoville Point flow (psp) dominates the shoreline and has steep smooth exposures. At the point itself, we will look at the excellent exposures of the Scoville Point flow, the ophitic flows below it, and the Edwards Island flow (pei), which underlies the companion point located just to the NW of Scoville Point. 58 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift There is a good exposure of cellular amygdaloid in one of the ophitic flows, and the Edwards Island Flow shows well developed entablature jointing (Figures 61, 62). Figure 61: Shoreline exposure of Edwards Island flow near Dashler Cabin, Scoville Point (entablature joints in pei). This view shows a vertical cross section of the joints on the antidip slope. Figure 62: Columnar joints in the Edwards Island Trap (pei) at the Dashler Cabin near Scoville Point. This view is perpendicular to the joints and shows their polygonal forms. The scale of the polygons is about 7-10 cm. While looking at the columnar joints in the Edwards Island flow (pei) at Scoville Point near the Dashler Cabin (Figs 61, 62), we should discuss whether this jointing pattern is indeed entablature jointing in the sense of Long & Wood (1986), and whether we should infer that the Edwards Island flow was indeed cooled in part by being flooded by surface water. (see also section on columnar jointing above) 59 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift We will return to the lodge via the Tobin Harbor Trail, which is easier to hike. It stays near the shore of Tobin Harbor, mostly atop the Edwards Island flow. Just NE of the Rock Harbor Lodge on the return trail is the site of the Smithwick Mine remains; this mine was discovered in 1843 and actually operated in 1847 and 1848. The work done here mostly consisted of exploratory shafts and excavations, and it is unclear whether much ore was found (Rakestraw, 1965). Lookout Louise and Monument Rock From Mirror Lake to Lookout Louise, we will hike about 1.6 km (1 mi) long and 85 m (280ft.) up (Figure 63). We will begin on the Tobin Harbor flow, but after passing the Lake we will walk Figure 63: Oblique Google Earth View, looking SW at Lookout Louise. Trails plotted in white. on the Greenstone Flow, following a dip slope up to Lookout Louise. At about the halfway point, the trail passes Monument Rock (Figure 64), an individual column from the colonnade of the upper ophite that is exposed as an erosional remnant. 60 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 64: Woodcut from Ackerman Lithographers, New York, showing Monument Rock in the 1840s. This old view is advantageous because the modern forest blocks an overall view like this one. Huber (1983, see especially pp 47-55) suggests that Monument Rock was formed by wave cut shoreline processes along a former "raised" shoreline, which he associates with glacial Lake Minong, about 10 Ka. From Lookout Louise we will look over the steep, anti-dip slope of the lower ophite and see Five Finger Bay, Duncan Narrows, and Amygdaloid Island. LIDAR topographic survey (Figure 65) came from Seth De Pasqual, at Isle Royale National Park. It reveals a striking topography which shows the dipping lava beds. Prominent large lava flows, like the Greenstone flow (pg) are obvious features in this image. Differential erosion of lava flows occurs when soft material, such as that found in the amygdaloidal flow tops and along faults, is preferentially removed and makes a topographic low, while the massive flow interiors resist erosion and become topographic highs. In this image we can also see the different layers of the Greenstone flow, including the Upper Ophite, the Pegmatite, and the Lower Ophite. 61 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift old shorelines lower ophite pegmatite upper ophite pg old shorelines Monument Rock upper ophite pth pth Figure 65: Shaded LIDAR topographic map of area near Lookout Louise, Isle Royale. This image shows what are thought to be ancient lake shorelines, which demonstrate that Monument Rock, far from the shore today, was once close to the lake shore and could have had a sea stack aspect. The image also shows layering textures in the Greenstone Flow which could reflect the Upper and Lower Ophite and the Pegmatite. Image from Seth De Pasqual, IRNP. Trails are shown in yellow. Post glacial shorelines can be seen in parts of this image also, and including in the vicinity of Monument Rock, itself far from the current shoreline. This arrangement suggests that the freestanding form of Monument Rock is consistent with its formation as a “sea stack”, and a remnant of the upper ophite of the Greenstone Flow, which is mostly eroded from this place. This interpretation was first suggested by N.K. Huber. Red Rock Point and Porter Island At Red Rock Point (Figure 66), we will pass excellent examples of entablature jointing of the upper part of the Greenstone flow. The basalt of the entablature is melaphyre (“trap”), very fine grained. The curvi-columnar nature of a few of the columns resembles some of the Columbia River basalt descriptions (Figure 25). Long and Wood (1986) suggest that entablature jointing 62 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift results when extensive floods that are created from disrupted drainages cause dramatic quenches of solidifying flood basalts. Figure 66: Oblique Google Earth image of eastern parts of Tobin Harbor, looking SW. Around the corner of Red Rock Point is a feature that Longo (1984) describes as follows: A large autointrusive dike was found intruding (N 20° W, 65° E) the columnar-jointed melaphyre at Red Rock Point. Despite an apparent lack of aplites, the dike is texturally similar to the stratiform pegmatoid. It is composed of randomly oriented, euhedral plagioclase laths with interstitial, subhedral augite and pigeonite (no poikilitic textures occur). The plagioclase laths are immense by comparison to the microlites of a typical ophitic unit. Three characteristic features of the dike are: (1) the abundant plagioclase phenocrysts (up to 1 cm (0.4 in)), (2) a blue-green hue from plagioclase altered to chlorite in the dike, and (3) alignment of plagioclase laths parallel to the dike contact, forming an igneous lamination. Amygdules are more abundant along the dike contact also. The process of autointrusion is similar to the mechanisms of pegmatoid formation, except that after the residual liquid is pressed out of the hosting crystal mesh, the differentiated magma is squeezed up into the vertical tensional fractures. --Longo 1984 63 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 67: Sag-Flowout structure, from McKee and Stradling (1970). Longo (1984) interprets the auto intrusion to be related to a sag flowout structure (Figure 67), described by McKee and Stradling (1970) as: a large structure that develops as the crust of a partly solidified flow founders and causes the upward escape of the flow's fluid interior (see figure, above). Below the water level at Red Rock Point is an occurrence of coarse grained granophyric rock, which can be found in beach cobbles and boulders and may occur within the Greenstone flow itself. The origin of granophyres in sills are not well understood, and Figure 68 from Marsh et al., 1991 shows some of his ideas, including existing silicic material which was carried in during intrusion. Figure 68: Some possible positions of granophyre within sheet-like intrusions. the left two panels show residual fluids forming lenses. The panel at right shows an accumulation of granophyre at the upper contact which may have existed upon emplacement (Marsh et al., 1991). We will also pass Porter's Island, which includes exposures of a fragmental rock that Huber (1973a) interprets as pyroclastic (pp). The same unit can be found on the Tobin Harbor shoreline opposite Newman Island. However, according to Longo (1984), these exposures may represent the fragmental top of the greenstone flow. The breccia unit, which is about 1-5 m (3.3-16.4 ft.) thick, contains rounded and semirounded fragments of the Greenstone flow set in a finer matrix that has amoeboid-shaped, agate amygdules. Longo did an extensive petrographic study but could not find any evidence of shards or pumice. He did, however, find bow-tie spherulitic plagioclases in the matrix, which suggests an undercooled texture for the basaltic material there. This unit occurs at the top of the Greenstone flow along about 15 km (9.3 mi) of strike length (approximately to Mt. Ojibway), according to Huber's map. Similar units are found at the top of the Greenstone flow on the Keweenaw Peninsula (Longo 1984). 64 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 69: Oblique Google Earth Image of Raspberry Island, looking N. Raspberry Island At Raspberry Island (Fig 69), about 0.5 km (0.3 mi) SE of Rock Harbor Lodge, we spend the day looking at a remarkable set of exposures nearby that provide an impression of some of the solidification features of an ophitic flow (approximately 20-30 m thick). At least since 2000, and in increasing amounts, low lake levels have made these exposures more numerous and accessible. One of many small islands along the S side of Rock Harbor, Raspberry Island is three ophitic flows of the undivided PLV (pu) dipping 15° SE. The uppermost of these flows is extensively exposed on a wave-washed dip slope. This shoreline receives strong storm waves and, fortunately has wave-washed exposures about 1 km (0.6 mi) long. They expose the flow interior, with the top of the flow eroded away and the base buried. A loop trail goes around the W half of the island, marked by informative signs about the unique ecosystem of this island, which features frequent fog and damp, moss-rich swamps. Among the unusual plants is the pitcher plant (Sarracenia puerperia), which is an insectivorous plant that flourishes in the swamp along the loop trail. 65 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift First, we will visit the W end of the island, where the regional attitude of the lava flows is seen in the view along strike toward Smithwick Island across the Smithwick Channel (Fig 70). Dip Slope Smithwick I 3 flows dip SE Anti-dip Slope Figure 70: Photo of Smithwick Island taken from Raspberry Island, showing a gently dipping sequence of three lava flows with obvious dip and anti-dip slopes. The dip of 20-25 degrees to the SE is typical of Isle Royale. The point on Raspberry Island facing the Channel is underlain by the oldest of the three flows on the island. We will walk on a dip slope that shows some of the jointing pattern we will also observe on the SE sides of Davidson and Smithwick Islands. Next, we will head to the SE corner of the island to observe some poorly-developed columns in the uppermost Raspberry Island flow, before looking at vesicle and segregation cylinders, and vesicle sheets or pegmatites. On the wave-washed SE shore are two zones of exposures of vesicle cylinders. Paces (1988) describes vesicle cylinders (Goff 1996) in the PLV: Vesicle pipes are elongated, tube-like structures, 10-30 cm (4-12 in) in diameter and 0.5-2 m (1.6-6.6 ft.) in length, containing somewhat coarser and more prismatic crystals compared to the adjacent groundmass. They are oriented vertically and occur predominantly in the bottom half of the flow. The origins and dynamic behavior of vesicle cylinders are poorly understood; however they appear to represent an accumulation of exsolved magmatic gas bubbles which migrate upwards through the magma during the period when the cooling magma behaves as a Bingham plastic (i.e., possesses a finite yield strength, Walker 1987). --Paces 1988 66 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 71: Segregation cylinders standing up as resistant to wave washing, forming small mounds separated by a few feet. The flow is tilted about 20 degrees to the left in this view. Figure 72: Vesicle cylinder or segregation cylinder from Raspberry Island, showing its cylindrical shape in 3 dimensions. Here at Raspberry Island, exposures of vesicle cylinders (Figures 71, 72) show a fairly regular spacing, 1-3 m (3-10 ft) apart, and a marked variety of textures; some were evidently preserved almost as voids, while others are filled with material that closely resembles vesicular pegmatoid. An interesting aspect of the exposures here is the relationship between the ophitic textures of the flow and the vesicle cylinders: The grain size of oikocrysts seems to be diminished by the proximity to the vesicle cylinder. Vesicle cylinders (Goff, 1996) are found mainly in only two areas along this shoreline. This may reflect their restricted occurrence in a thin part (less than a few meters thick) of this flow. Based on limited field examination, this thin part seems to be in the lower part of the flow. The comparisons between this occurrence and written descriptions, by Paces (1988) of the PLV on the Keweenaw, by Marsh et al. (1991) of solidification in sheet-like basaltic bodies, and those from Hon et al. (1994) and Self et al. (1998), are illuminating. Also featured conspicuously along the E shore of Raspberry Island are slickenside surfaces. A study of the fault slickenfibers allowed Witthuhn-Rolf (1997) to use geometrical and statistical methods to define the kinematics of the closing of the rift (Figure 73). In Witthuhn-Rolf's study, 67 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift a) Figure 73: Equal area rose diagrams of the trend of slickensides on reverse faults on island along Rock Harbor, Isle Royale National Park (Witthuhn-Rolf, 1997). Mon EAST AND WEST CARIBOU INNER lULL, OUTER HILL AND DAVIDSON Equal Area +:..... = RASPBERRY AND EDWARDS STOKLEY BAY, TOOKER, SHAW AND SMITHWICK ISLANDS Figure 32: Left: Equal area rose diagrams of the trend of slickensides on (a) normal and (b) reverse faults on Isle Royale (Witthuhn 1993). Notice the similar trends that define the resolved shear stress on the faults. Right: Rose diagrams ofthe trends of slickensides on reverse faults measured on islands along the SE shoreline of Isle Royale (Witthuhn 1993). Figure 74: Epidote-coated slickenside surfaces along faults exposed in Raspberry Island lava flows. Raspberry and Edwards Islands offered one of the largest populations of measurements. The measurements revealed two consistent stress fields, for each limb of the syncline, that would satisfy the conditions envisioned for the opening and closing of the Midcontinent Rift. Most of the faults on Isle Royale, including both normal and reverse faults, trend NE. This suggests that the reverse faults represent reactivated normal faults. The orientation of reverse faults at Isle Royale differs significantly from the predominately N-S trending structures measured in the PLV on the Keweenaw Peninsula. 68 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 75: 4 cm thick vesicle sheet or pegmatoid layer within horizontally fractured section of basaltic lava flow at Raspberry Island. About two-thirds of the way along the shore of Raspberry Island, the exposures that occur are stratigraphically higher in the flow. Here the flow has a laminar structure that consists of fractures that are parallel to the bedding and spaced about 0.5-3 cm (0.2-1.2 in) apart. Within this part of the flow, vesicle cylinders are not seen, but small pegmatoid lenses (vesicle sheets: Figure 75) occur. Paces (1988) describes them: Pegmatoid horizons are similar to vesicle cylinders in that they consist of gas-rich, coarsely crystalline, granophyric material. However, they occur as discontinuous lenses and layers, typically 10 cm (4 in) to several meters thick, and are usually located between the flow top and most massive portion of the flow interior. Pegmatoids are best developed in thicker flows that have cooled slowly enough to allow in situ differentiation (Cornwall 1951; Lindsley et al. 1971). This material represents the last remaining volatile-rich liquid, which is injected into fractures oriented sub-parallel to the upperflow surface. Both vesicle cylinders and pegmatoid layers contain significant void space in the form of vesicles and gas pockets and contribute to the permeability of the lava flows. --Paces 1988 The origin of the pegmatoids is likely related to the process by which the vesicle cylinders were formed. However, for the pegmatoid origin, the rise of material in channels is limited by the 69 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift thermal gradient and by the associated solidification that happens above the zone of pegmatoids, so the material is blocked and accumulates in lensoid layers (Figure 75). It is possible that Keweenawan flows preserve the inflated nature of ponded flood basalts well because runout of inflated flows such as can occur on sloping volcanoes is prevented by the riftfilling geometry. Tookers and Davidson Islands Figure 76: Oblique Google Earth Image of Tookers I looking N. One of many small islands strung out along the south side of Rock Harbor, Tookers (Fig 76) has some nice exposures of lava flow tops on its south side. Flow tops are amygdaloidal and less resistant to weathering. Flow interiors are massive and featureless, except they nearly always have at least poorly-developed columns. Figure 77: Exposure of contact between two lava flows, showing a black massive, relatively fresh upper flow, in contact with a reddish altered amygdaloidal flow top. Photo from 7 Mile Point, Keweenaw Peninsula. Flow Top 70 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Figure 78: Wave-washed lava surface on SE corner of Davidson Island showing polygonal jointing pattern with 2-4 m diameter polygons. Such patterns may be seen on many ophitic flows on Isle Royale. Figure 79: Oblique Google Earth Image of Davidson I looking W. On Davidson Island (Fig 79) is the Boreal Research Center, a residence for researchers at Isle Royale. We will walk around this small island, visiting another exposure of the epiclastic sedimentary rocks and an exposure of a columnar-jointed, ophitic flow on the SE corner of the island (Figure 78). The wave-washed shoreline has exposed a surface perpendicular to the columns, which are 2-3 m (6-10 ft.) across. Large columns seem to be a regular feature of ophitic flows at Isle Royale. Mott Island We will stop at Mott Island (Figures 79, 80) to visit one of the best exposures of sedimentary units within the PLV, found at the SW end of the island, facing East Caribou Island near the Park headquarters complex. There are seven such units mapped by Huber (1973) in the Chippewa 71 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Harbor area. Most of them are remarkably constant in thickness and lithology throughout their lateral extents, which are 65 km (40 mi) or more. Figure 80 : Detail of Isle Royale Geologic Map (Huber, 1973) which shows part of Eastern Isle Royale including Mott Island. The brown colored unit is the interflow sediment we will visit. Paces (1988) reports the following about interflow sediments in the PLV: Occasionally, lava flows are separated by intervening sheets and lenses of terrigenous clastic sediment. Twenty two major interflow sedimentary horizons occur scattered throughout the PLV section and are described by Butler and Burbank (1929), White (1952), and Merk and Jirsa (1982). Interflow sedimentary beds vary in thickness from less than 1 cm (0.4 in) thick fine-grained siltstones filling fractures between flow top fragments to coarse boulder conglomerates over 100 m (330 ft.) thick locally. Typically, interflow sediments are poorly sorted, lithologically immature conglomerates and sandstones derived from a nearby volcanic source of some relief and deposited in an alluvial fan-type environment (Merk and Jirsa 1982). 72 �rough and ly toward gh, finally ce to form usands of Lake VolKeweenaw represent is volcanic e sedimened with the Lake Vole Copper nd other above the ported by basin from gins. This of streams, at the lavas f the basin, mes, reverover large t of a basin filled (fig. flows were oward the as filling by nwarping. was intered downslopes that bris to be y, with the c activity, mitted the er Harbor nger Kem a thick e the vol- enawan or osed along www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift A. Lava erupts near the center'Of the basin and spreads laterally toward the margins to form a sequence of. lava flows. Y 8. The basin subsides, and during a lull in volcanic activity gravels are swept into the basin and'spread out over the uppermost lava flow. C. Volcanic activity resumes, and the cycle starts over again. FLOOD BASALTS AND SEDIMENTS Figure 81 : Cross section of rift valley showing theaccumulation process of interbedding. 43) showing of lava from (Fig. fissure vents in the Center of the rift, sometimes by this sandstone, together with similar alternating with infilling sediments from sandstone exposed in the southwestern outside the rift (Huber, 1973). part of the basin (fig. 39). The gross synclinal form of the Keweenawan basin resulted from subsidence coincident with filling of the basin rather than later folding by squeezing. However, Keweenawan strata near the margins of the basin, as73 on the Keweenaw Peninsula and Isle Royale, were subsequently steepened Transportation was generally from the SE to NW*, or from basin margins towards the center of the subsiding graben (White 1952). Although the interflow sediments are volumetrically insignificant within the PLV (3% of the total lithologic volume) (Merk and Jirsa 1982; White 1971), they form distinct and relatively continuous stratigraphic marker horizons within an otherwise monotonous volcanic pile. The occurrence of occasional interflow sediments implies that rates of lava flow extrusion, sedimentation, and/or tectonic subsidence were not constant during the formation of the PLV. White (1960) shows that a subsidence-depositional equilibrium was established so that both lava flows and sediments were deposited on near-horizontal surfaces. Most lava flows were deposited directly on top of the underlying lava flow top indicating a more-or-less constant and relatively short repose period between eruptions. The infrequent presence of sedimentary beds between lava flows may indicate occasional hiatuses in magma extrusion, which allowed or alluvial fans to transgress out towards the center of the basin. Conversely, interflow sedimentary horizons may mark brief periods of increased depositional rates possibly related to episodic normal faulting and basin subsidence. --Paces 1988 *this quote refers to the Keweenaw, where Paces worked--on Isle Royale directions are reversed. �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Edison Fishery and the Lighthouse The Fishery (Figure 82) itself is a restored camp that is occupied each summer by a retired Lake Superior fisherman and his family; this man is employed by the Park to interpret what life was like here during the heyday of Isle Royale fishing camps, from before the establishment of the Park in 1936 until the sea lamprey invasion of the 1950s. Figure 82 Oblique Google Earth View of Edison Fishery and the Lighthouse looking SW. The lavas that underlie the site of the fishery and the lighthouse are a sequence of 45-50 ophitic flows, which occur between the Scoville Point flow (psp) and the overlying CHC. As we walk around the point we will see several flow tops exposed, good examples of cellular amygdaloids. This is an excellent place to find (but not to collect!) Isle Royale greenstone, a nodular, compact form of pumpellyite that is prized as a semi-precious gemstone (Huber 1983, see pp. 58-9). The geological purpose of stopping here is to look at the flow sections along the wave-washed shoreline, following it from this point to Tonkin Bay. We can also look at the amygdule mineral suite, which can be found on the pebble beaches. The amygdules of Isle Royale's flows contain a variety of secondary minerals, listed alphabetically (by Huber) as barite, calcite, chlorite, copper, datolite, epidote, laumontite, natrolite, prehnite, pumpellyite (chlorastrolite or “greenstone”), 74 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift quartz (agate), and thomsonite (see section on Amygdaloid). The prehnite is unusual in that it contains disseminated native copper inclusions and has a pink color, which has caused some to confuse it with thomsonite (Huber 1969). Overall, the assemblage is zeolite facies and prehnitepumpellyite facies, representing a slightly lower grade than much of the Keweenaw Peninsula area. This lower mineralization temperature may partially explain the lower abundances of native copper on Isle Royale than those found on the Keweenaw Peninsula. This metamorphic event reflects a large hydrothermal (hot, geothermal brine which was pumped through the porous flow tops and conglomerates of the Portage Lake Volcanics for years after the volcanism ended (Jolly, 1972). Figure 83: Oblique Google Earth View of Mt Franklin and Ojibway tower, looking SW. The view looks directly along the strike of the lava flows, which are dipping gently to the east. 75 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Franklin and Ojibway The Mount Franklin Trail begins 0.3 km (0.2 mi) W of Three Mile Campground (Figure 83). The trail immediately climbs a ridge supported by the Scoville Point Flow (psp), then levels off and descends. We will cross a boardwalk over a swamp and arrive at a valley where there is a junction with the Tobin Harbor Trail, 0.8 km (0.5 mi) from Three Mile Campground. We will continue on the Mount Franklin Trail, straight ahead, crossing the Tobin Creek swamp and then climbing a ridge underlain by the Tobin Harbor flow (pth). From here we will descend to cross another swamp and then begin the 300 ft. ascent of the Greenstone ridge. The entire swamp and ascent is underlain by the great Greenstone Flow (pg). At the top of the ridge there is a junction with the Greenstone Ridge Trail, which we will take left to go about 0.5 km (0.3 mi) to Mount Franklin, elevation 330 m (l080 ft.). Here there is a good view of the N side of the island, including Five Finger Bay, Lane Cove, and Amygdaloid Island, as well as of the Canadian Shoreline, including the Logan Sills and the Sleeping Giant. The Greenstone Flow is indeed the backbone of the island, forming the most prominent ridge all along; only at Blake Point, however, is a reasonably complete section through the flow exposed. The contact between the pegmatoid and the lower ophite units of the Greenstone is mainly located near the crest of the Greenstone ridge. The lower ophite underlies the N slope, which is a steep, anti-dip slope, and the pegmatoid armors the gentler dip slope to the S. Figure 84: Oblique Google Earth view of Ojibway Tower and Daisy Farm, looking E. 76 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Following the same trail, we will descend sharply to a wooded area and level off for about 0.4 km (0.25 mi) before climbing again. We will then reach the ridge crest and follow it for another 3.2 km (2 mi), with occasional outstanding views, to the Mount Ojibway tower. This structure was built in 1962 and was used initially as a fire tower. Now it is used for monitoring acid rain, along with other environmental monitoring. We can climb the tower stairs for full views of the surroundings, both to the N and S. From the tower we will descend to the Daisy Farm Campground via the Mount Ojibway Trail. (Figure 84). We will go down from the ridge to the first level spot and then begin to rise over a smaller ridge. The beginning of this small ridge is the approximate location of the top of the Greenstone flow; the ridge top and the dip slope to the S is underlain by the Tobin Harbor flow (pth). At the base of this ridge we will cross a swamp fed by Tobin Creek. Then we will ascend Ransom Hill, which has the Long Island Flow (pli) on its anti-dip (N) slope and the Edwards Island Flow (pei) on its dip slope (S) side, where there is some entablature jointing. From Ransom Hill, the trail descends to Daisy Farm Campground. Daisy Farm is located on the site of an old mining community, called Ransom, which was founded in 1847 with the clearing of land and the construction of a smelter. The mining prospects dimmed quickly, however, and the mining activity ended only two years later in 1849. Then, in 1866, all the buildings burned down. In later years, the place was the site of a sawmill, a garden that supplied vegetables to Rock Harbor Lodge, and a Civilian Conservation Corps (CCC) camp, which was a foundation for youth employment, developed by Roosevelt during the depression (Rakestraw 1965). What to take home After a several day journey, what are the earth sciences messages that stick with you? What are the globally significant issues that stand out? What is uniquely interesting about the place and time that is recorded in rocks here? What big ideas emerge from this geology? 1. Rodinia, a Proterozoic supercontinent, blanketed Earth’s mantle, and the higher heat flow of 1.1 billion years ago triggered huge volumes of hot magmatism from the mantle, first giving rise to ultramafic dike swarms, then basalts in huge quantities.  These dikes split the great supercontinent, but a nearby continental collision (Grenville) was apparently what prevented the formation of an ocean basin. 2. Large Scale Flood basalts occurred for a brief period, lasting only a few million years in the Keweenaw and Isle Royale.  These eruption rates, much higher than average, apparently were driven by a mantle plume. They are similar to other continental flood basalts and mafic large igneous provinces (LIPs) in these respects. There are volcanic, plutonic, and sedimentary elements to the mantle plume and rifting (see map, below). 3. Ponding of magma happened in a great crack—the midcontinent rift basin, locally called the Keweenaw Rift. Because lava solidifies by heat loss from the lower surface where it is contact with the cold ground and the upper surface where it is in contact with the air, thick lava flows cool much more slowly than thin ones, because the massive flow interiors, far 77 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift from the top and bottom of the flow, are shielded from heat loss. The Keweenaw Rift has flows as thick as 1200 ft, thicker than those found in other mafic LIPs. 4. The in situ differentiation within the largest lava flows may have occurred because of the existence of a large ponded magma body (not unlike a magma ocean) within the rift valley for perhaps up to a millennium. This results in pegmatite or dolerite horizons within the large flows, features that are not common in younger flood basalts. Vesicle cylinders and segregation cylinders are also conspicuous features of these ponded flows which occur in the lower parts of the flows, reflecting compaction of a dendritic mush consisting of ophitic crystals. 5. The hotspot (mantle plume head), along with the rifting it caused, created a big, elongate hole in the continent, that was partially filled with basalt and redbed sediments. This hole has persisted until now and it is this hole that coincides very closely to the position of Lake Superior. 6. An unexplained unique aspect of this rift situation is native copper mineralization. Though other rifts have all of the other mineral deposit types of the 1.1 Ga Lake Superior area, none has native copper. We are puzzled by this cosmic geochemical oddity. What happened to the sulfur usually found with chalcophile elements? 7. Fossils are difficult to find in Keweenawan rocks, generally, but cyanobacteria are conspicuous. Stromatolites within the rift basin here are associated with an oxidized ocean and an atmosphere that was holding at least some free oxygen. Following the redbeds of the rift were the multiple Snowball Earth events. Acknowledgements The opportunity to write a detailed guide to Isle Royale and to lead a field trip comes from the cooperation of many people. Lori Witting did the planning and financing issues for the trip. Mark Klawiter planned the food and field logistics. Bob Barron helped with numerous details. I would like to thank Liz Valencia and Greg Bickings of Isle Royale National Park for permitting and helping plan this field trip. Steve Roblee was an eager boat pilot. King Huber provided us with a complete set of his many publications about Isle Royale and also with lots of cheerful encouragement. Jim Paces, Tony Longo, and Rick Wunderman provided me with a lot of insight on the volcanic geology of Isle Royale. Kate Witthuhn-Rolf supplied some unpublished data. Discussions with Bruce Marsh and Angus Hellawell about solidification helped me to understand a little better what may have been going on inside Isle Royale's lava flows. Seth De Pasqual at IRNP provided LIDAR maps for the guide and explanations of them. Evgeniy Kulakov worked on the paleomagnetic information for us. George Robinson helped find some great mineral specimens to illustrate the zeolite facies 78 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift amygdaloids, and John Jaszsak helped with photographing them. Many researchers provided material for me to learn about and communicate about Isle Royale. There are so many crucial words and illustrations that are needed, and I tried to use as many as I could. Here are some of the names: Ted Bornhorst, Bill Cannon, Henry Cornwall, Jim DeGraff, Doug Elmore, Fraser Goff, John Green, Ken Hon, Wayne Jolly, Susanne Nicholson, Dick Ojakangas, Lauri Pesonen, Anthony Philpotts, Suzanne Schmidt, Steve Self, Dick Stoiber, George Walker, Walter White. Others are in the Bibliography. Ken VanDellen helped with editing the text and clarifying the English. References Cited • Basalt Volcanism Study Project, 1981, Basaltic volcanism on the terrestrial planets, Pergamon Press, Inc., New York, 1286 pp. • Bondre, N.R., R.A. Duraiswami, G. Dole (2004) Morphology and emplacement of flows from the Deccan Volcanic Province Bulletin of Volcanology, 66 , pp. 29–45 • Behrendt, J.C., A.G. Green, W.F. Cannon, D.R. Hutchinson, M. Lee, B. Milkereit, W.F. Agena, C. Spencer (1988) Crustal structure of the Midcontinent Rift System: results from the GLIMPCE deep seismic reflection profiles Geology, 16 , pp. 81–85. • 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: Geological Society of America Special Paper 312, p. 127–136. • Bornhorst, T.J., and Brandt, D., 2009, Michigan’s earliest geology: The Precambrian, in Schaetzl, R., Darden, J., and Brandt, D., eds., Michigan Geography and Geology: New York, Pearson Custom Publishing, p. 24–39. • 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: New York, Pearson Custom Publishing, p. 150–173. • 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 and the Bulletin of the Society of Economic Geologists, v. 83, p. 619–625. • Bornhorst, TJ and R Barron, 2011, Copper deposits of the western Upper Peninsula of Michigan, Geol Soc Amer Field Guide 24: 83-99. • Brannon, J.C. 1984, Geochemistry of successive lava flows of the Keweenawan North Shore Volcanic Group, Ph.D. dissertation, Washington University, St. Louis, MO, 312 pp. • Bresson, David, 2011 Large Igneous Provinces and mass extinctions. Scientific American, September 16, 2011 (http://blogs.scientificamerican.com/history-of-geology/2011/09/16/largeigneous-provinces-and-mass-extinctions/) 79 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift • Broderick, T.M., 1935, Differentiation in lavas of the Michigan Keweenawan, Geol. Soc. Am. Bull., v. 46, pp. 503-58. • Broderick, T.M., Hohl, C.D., and Eidemiller, H.N., 1946, Recent contributions to the geology of the Michigan copper district, Econ. Geol., v. 41, pp. 675-725. • Brown, A.C., 2006, Genesis of native copper lodes in the Keweenaw district, northern Michigan: A hybrid evolved meteoric and metamorphogenicmodel: Economic Geology and the Bulletin of the Society of Economic Geologists, v. 101, p. 1437–1444. • Butler, B.S. and Burbank, W.S., 1929, The copper deposits of Michigan, USGS. Prof Pap., No. 144, 238 pp. • Cannon, W.F., and Phillips, B.A.M., 2007, Geologic and cultural history of the Grand Portage National Monument [field trip 2]: Institute on Lake Superior Geology, Annual Meeting, Lutsen, MN, Part 2 – Field Trip Guidebook, v. 53, pages 24-52. • Cannon, W.F., 1994, Closing of the Midcontinent rift: a far-field effect of Grenvillian compression, Geology, v. 22, pp. 155-8. • Cannon, W.E et al., 1989, The North American Midcontinent rift beneath Lake Superior from GLIMPCE seismic reflection profiling, Tectonics, v. 8, pp. 30532. • 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, 12, p. 728–744, doi:10.1029/93TC00204. • Carmichael, I.S.E., Turner, EJ., and Verhoogen, J., 1974, Igneous Petrology, McGraw-Hill, New York. • Clark, J.A, Hendriks, M., Timmermans, T.J., Struck, C., and Hilverda, K.J., 1994, Glacial isostatic deformation of the Great Lakes region, Geol. Soc. Am. Bull., v. 106, pp. 19-31. • Cornwall, H.R., 1951, Differentiation in lavas of the Keweenawan series and the origin of the copper deposits of Michigan, Geol. Soc. Am. Bull., v. 62, pp. 159-202. • Crisp, J., and S. Baloga, 1994, The influence of crystallization and entrainment of cooler material on the emplacement of basaltic aa lava flows, J. Geophys. Res., 99: 11819-11831. • Davis, D.W. and Green, J.C., 1997. Geochronology of the North American Midcontinent Rift in western Lake Superior and implication for its geodynamic evolution. Canadian Journal of Earth Sciences, 35, 476-488. • Davis, D.W., and Paces, J.B., 1990, Time resolution of geologic events on the Keweenaw Peninsula and implications for development of the Midcontinent Rift system: Earth and Planetary Science Letters, v. 97, p. 54–64, doi:10.1016/0012-821X(90)90098-I. • DeGraff, J.M. and Aydin, A, 1993, Effect of thermal regime on growth increment and spacing of contraction joints in basaltic lava, J. Geoph. Res., v. 98, pp. 6411-30. • DeGraff, J.M., Long, P.E., and Aydin, A, 1989, Use of joint-growth directions and rock textures to infer thermal regimes during solidification of basaltic lava flows, J. Volc. and Geotherm. Res., v. 38, pp. 309-24. • Ding, X., Ripley, E.M., Shirey, S.B., and Li, C., 2012, Os, Nd, O, and S isotopic constraints on country rock contamination in the conduit-related Eagle Cu-Ni-(PGE) deposit, 80 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift • • • • • • • • • • • Midcontinent Rift System, Upper Michigan: Geochimica et Cosmochimica Acta, v. 89, p. 10-30. Elmore, R.D., 1984, The Copper Harbor Conglomerate: A late Precambrian fining-upward alluvial fan sequence in northern Michigan: Geological Society of America Bulletin, v. 95, p. 610–617, doi:10.1130/0016 -7606(1984)95<610:TCHCAL>2.0.CO;2. Elmore, R.D., Milavec, G.J., Imbus, S.W., and Engel, M.H., 1989, The Precambrian Nonesuch Formation of the North American Mid-Continent Rift, sedimentology and organic geochemical aspects of lacustrine deposition: Precambrian Research, v. 43, p. 191–213, doi: 10.1016/0301-9268(89)90056-9. Foster, J.W. and Whitney, J.D., 1851, Report on the geology of the Lake Superior land district, Washington, DC, AB. Hamilton, 400 pp. Goff, F.E., 1977, Vesicle cylinders in vapor-differentiated basalt flows, Ph.D. dissertation, University of California, Santa Cruz, CA, 83 pp. Goff, F. E.. 1996, Vesicle cylinders in vapor-differentiated basalt flows, J Volcanol Geoth Res 71: 167-185. Goodge, J. W., Vervoort, J. D., Fanning, C. M., Brecke, D. M. Farmer, G. L., Williams, I. S., Myrow, P. M., DePaolo, D. J., 2008, A Positive Test of East Antarctica-Laurentia Juxtaposition Within the Rodinia Supercontinent: Science, v. 321, p. 235-240. [PDF] Grand Portage National Monument, 1986, Grand Portage, National Park Service, U.S. Department of the Interior, GPO: 1986--491-414/20056. Green, J.C., 1982, Geology of Keweenawan extrusive rocks, Geo. Soc. Am. Mem., v. 156: 47-55. Green, J.C., 1989, Physical volcanology of mid-proterozoic plateau lavas: the Keweenawan North Shore Volcanic Group, Minnesota, Geol. Soc. Am. Bull., v. 101, pp. 486-500. Halls, H.C. and Pesonen, L.J., 1982. Paleomagnetism of Keweenawan rocks, Geological Society of America, Memoir, 156, 173-201. Hellawell, A, Sarazin, J.R., and Steube, R.S., 1993, Channel convection in partly solidified systems, Phil. Trans. R. Soc. Land., v. 345, pp. 507-44. • Holm, D.K., D.A. Schneider, S. Rose, C. Mancuso, M. McKenzie, K.A. Foland, K.V. Hodges 2007 Proterozoic metamorphism and cooling in the southern Lake Superior region, North America and its bearing on crustal evolution, Precambrian Research 157:106–126. • Hon, K. , J. Kauahikaua, R. Denlinger, K. Mackay Emplacement and inflation of pahoehoe sheet flows; observations and measurements of active lava flows on Kilauea Volcano, Hawaii Geological Society of America Bulletin, 106 (3) (1994), pp. 351–370 • Huber, N.K., 1969, Pink copper-bearing prehnite from Isle Royale National Park, Michigan, USGS. Prof Pap., No. 650-D, pp. D63-8. • Huber, N.K., 1973, Geologic map of Isle Royale National Park, Keweenaw County, Michigan, USGS. Map, No. 1-796. • Huber, N.K., 1973a, The Portage Lake Volcanics (middle Keweenawan) on Isle Royale, Michigan, U.S.G.S. Prof Pap., No. 754C, 32 pp. 81 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift • Huber, N.K, 1973b, Glacial and postglacial geologic history of Isle Royale National Park, Michigan, U.S.G.S. Prof Pap., No. 754-A, 15 pp. • Huber, N.K., 1983, The Geologic Story of Isle Royale National Park, U.S.G.S. Bull., No. 1309, 66 pp. • Jerram, D.A., M. Widdowson, 2005 The anatomy of continental flood basalt provinces: geological constraints on the processes and products of flood basalt volcanismLithos, 79 (3– 4) , pp. 385–405 • Hjartarson A. 1988. The great Thjorsa lava: Earth's largest Holocene lava flow. Natturufraedingurinn 58:1–16 • Jolly, W.T., 1974, Behavior of Cu, Zn, and Ni during prehnite-pumpellyite rank metamorphism of the Keweenawan basalts, northern Michigan: Economic Geology and the Bulletin of the Society of Economic Geologists,69, p. 1118–1125. • Karlstrom, K.E., Harlan, S.S., Williams, M.L., McLelland, J., Geissman, J.W., and Ahaell, K., 1999. Refining Rodinia: Geologic Evidence for the Australia Western U.S. (AUSWUS) connection for Proterozoic Supercontinent Reconstructions. GSA Today, v.9, n. 10, October, 1999 • Katterhorn, SA & CJ Schaefer, 2008, Thermal–mechanical modeling of cooling history and fracture development in inflationary basalt lava flows, Journal of Volcanology and Geothermal Research 170 (2008) 181–197 • Kern, A.N., Kulakov, E.V., Smirnov, A.V., Diehl, J.F., and K. Chamberlain, 2012. Paleomagnetism of the Coldwell Complex (Ontario, Canada): New Data and New Insights. American Geophysical Union Fall meeting, abstract GP21A-1131. • Keszthelyi, L., S. Self Some physical requirements for the emplacement of long basaltic lava flows Journal of Geophysical Research, 103 (B11) (1998), pp. 27,447–27,464 • Kilburn C. R. J. and G. Luongo 1993. Active Lavas - Monitoring and Modelling . UCL Press, London, 384pp. • Klewin, K.W., and Shirey, S.B., 1992, The igneous petrology and magmatic evolution of the Midcontinent Rift system: Tectonophysics, v. 213, p. 33-40. • Lacroix, A, 1928, Les Pegmatitoides des Roches Volcaniques a Facies Basaltiques, Ac. Sci. Paris Comptes Rendus, v. 187, pp. 321-6. • Lacroix, A, 1929, Les Pegmatitoides des Roches Volcaniques a Facies Basaltiques: A Propos de Celles du Wei-Tchang, Bull. Geol. Soc. China, v. 8, pp. 45-9. • Lane, AC., 1893, Geological report on Isle Royale, Michigan, Geol. Surv. ofMichigan, v. 6., pp. 1-265. • Lane, A.C., 1911, The Keweenaw series of Michigan, Michigan Geol. and Biol. Surv. Pub. 6, Geol. Ser. 4, v. 2, 983 pp. • Lane, A. C., and Seaman, A. E., 1907, Notes on the geological section of Michigan, Part 1. The pre-Ordovician: Journal of Geology, v. 15, p. 680-695. • Lindsley, D.H., Smith, D., and Haggerty, S.E., 1971, Petrography and mineral chemistry of a differentiated flow of Picture Gorge Basalt near Spray, Oregon, Carnegie Inst. of Washington, Yearbook 69, pp. 264-85 82 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift • Lofgren. G.E., 1980, Experimental studies on the dynamic crystallization of silicate melts, Physics of Magmatic Processes (ed. RB. Hargraves), Princeton Univ. Press, Princeton, NJ, pp. 487-551. • Long, P.E. and Wood, BJ., 1986, Structures, textures and cooling histories of Columbia River basalt flows, Geol. Soc. Am. Bull., v. 97, pp. 1144-55. • 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. • Mangan, M. and B. D. Marsh. 1992. Solidification front fractionation in phenocryst -free sheet-like magma bodies. J. Geology, v. 100, p. 605-620. • Marsh, B.D., Gunnarsson, B., Congdon, R., and Carmody, R., 1991, Hawaiian basalt and Icelandic rhyolite: indicators of differentiation and partial melting, Geologische Rundschau, 80/2, pp. 481510. • McKee, B. and Stradling, D., 1970, The sag flowout: a newly described volcanic structure, Geol. Soc. Am. Bull., v. 81, pp. 2035-44. • Merk, G.P. and Jirsa, M.A., 1982, Provenance and tectonic significance of the Keweenawan interflow sedimentary rocks, Geol. Soc. Am. Mem., v. 156, pp. 97-105. • Nevanlinna, H., and Pesonen L.J., 1983. Late Precambrian Keweenawan asymmetric polarities as analyzed by axial offset dipole geomagnetic models, Journal of Geophysical Research, 88, 645–658. • 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. • 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. 10,851-10,868. • Ojakangas, R.W., Morey, G.B. and Green J.C., 2001. 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Volc., 56, 108. • Rakestraw, L., 1965, Historic mining on Isle Royale, reprinted in Borealis Isle Royale, Natural History Association, Houghton, MI. • Robertson, J.M., 1975, Geology and mineralogy of some copper sulfide deposits near Mount Bohemia, Keweenaw County, Michigan: Economic Geology and the Bulletin of the Society of Economic Geologists, v. 70,1202–1224. • Robertson, W.A. and Fahrig, W.F., 1971. The great Logan paleomagnetic loop-the polar wandering path from the Canadian Shield rocks during the Neohelikian Era. Canadian Journal of Earth Sciences, 8, 1355-1372. • Rogan W, S Blake and I Smith 1996 In situ chemical fractionation in thin basaltic lava flows: examples from the Auckland volcanic field, New Zealand, and a general physical model JVGR 74: 89-99 • Ross PS, L Ukstins Peate, MK McClintock, YG Yu, IP Skilling, JDL White and BF Houghton, 2005, Mafic volcaniclastic deposits in flood basalt provinces: A review, Journal of Volcanology and Geothermal Research 145 (2005) 281–314 • Santin, S.F., 1969, Pegmatitoides in the horizontal basalts of the Lanzarote and Fuerteventura Islands, Series I, Bull. Volc., v. 33, pp. 989-1007. • Schneider, D., Holm, D. K., Boyle, C. O., Hamilton, M., and Jercinovic, M., 2004, Paleoproterozoic development of a gneiss dome corridor in the southern Lake Superior region, U.S.A.: In Whitney, Tessyier, and Siddoway (eds) Gneiss domes in orogeny: Geological Society of America Special Paper 380, p.339-357. • Schmidt, S.T. and Robinson, D. 1997. 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. Geol. Soc. Am. Bull. 109, 683-697. • Schmidt, P. W., and Williams G.E., 2003. Reversal asymmetry in Mesoproterozoic overprinting of the 1.88-Ga Gunflint Formation, Ontario, Canada: non-dipole effects or apparent polar wander?, Tectonophysics, 377,7–32, 2003. • Seaman, A.E. and Seaman, W.A., 1944, Geological column Lake Superior Region in general: Michigan Geol. Surv. Div., Progress Report No. 10. • Self, S., L. Keszthelyi, T. Thordarson The importance of pahoehoe Annual Review of Earth and Planetary Sciences, 26 (1998), pp. 81–110 • Stoiber, R.E., and Davidson, E.S., 1959, Amygdule mineral zoning in the Portage Lake Lava Series, Michigan copper district: Economic Geology, 54, p. 1250–1277, p. 1444–1460. 84 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift • Swanson, D.A., Wright, T.L., and Helz, RT., 1975, Linear vent systems (and estimated rates of magma production and eruption) for the Yakima basalt of the Columbia plateau, Am. J. Sci., v. 275, pp. 877-905. • Thordarson, T., and S. Self (1998), The Roza Member, Columbia River Basalt Group: A gigantic pahoehoe lava flow field formed by endogenous processes?, J. Geophys. Res., 103(B11), 27411–27445, doi:10.1029/98JB01355. • Tomkeieff, S.I., 1940, The basalt lavas of Giants Causeway district of Northem Ireland: Bull. Volcanol., v. 6, pp. 90-143. • Van Hise, C.R. and Leith, C.K., 1911, The geology of the Lake Superior region, U.S.G.S. Mon., No. 52, 381 pp. • Trubitsin, M. Kaban and M. Rothacher: "Mechanical and thermal effects of floating continents on the global mantle convection", PHYSICS OF THE EARTH AND PLANETARY INTERIORS (Vol. 171, S. 313-322). • Walker, G.P.L., 1987, Pipe vesicles in Hawaiian basaltic lavas: their origin and potential paleoslope indicators, Geol., v. 14, pp. 84-7. • White, W.S., 1952, Imbrication and initial dip in a Keweenawan conglomerate bed, J. Sed. Pet., v. 22, pp. 189-99. • White, W.S., 1960, The Keweenawan lavas of Lake Superior: an example of flood basalts, Am. J. Sci., v. 258-A, pp. 367-74. • White, W.S., 1971, Geologic setting of the Michigan copper district, In Guidebook for Field Conference. Michigan Copper District, Sept. 30 - Oct. 2, (ed. W.S. White), Soc. Econ. Geol., Michigan Technological University, Houghton, MI, pp. 3-17. • Whitmeyer S. J. and K E. Karlstrom 2007 Tectonic model for the Proterozoic growth of North America Geosphere 2007;3;220-259 • Witthuhn-Rolf, K.M., 1997, A structural analysis of the Midcontinent rift in Michigan and Minnesota, Geol Soc Amer Special Paper 312: 97-113. • Wolff, RG. and Huber, N.K., 1973, The Copper Harbor Conglomerate (middle Keweenawan) on Isle Royale, Michigan, and its regional implications, U.S.G.S. Prof Pap., No. 754-B, 15 pp. • Worster, M.G. and Huppert, H.E., 1993, The crystallization of lava lakes, J. Geoph. Res., v. 98, pp. 15,891901. Lat-Long Locations of this field trip You have noticed there is no road log for this trip. This is because there are no roads! Download all the locations from the web here: www.geo.mtu.edu/~raman/IsleRoyalekmz.zip These files will be readily ingested by Google Earth software or GPS software and provide precise locations for all the sites described here. A table of the Latitude and Longitude of all these sites is listed here so it can be used to manually transfer this information if needed. These values may be entered manually into GPS or Google Earth. 85 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Name Longitude Latitude 3 Mile CG -88.52960447 48.12410194 Amygdaloid Island Ranger St -88.65598008 48.13570657 Arch Amgd I -88.62558448 48.14889669 Belle Isle CG -88.58562501 48.15234621 Big Cols Davidson -88.51062061 48.1249503 Big Cols Rasp I -88.47841662 48.14023711 Big Cols Rasp -88.48350224 48.13787684 Blake Pt -88.42229232 48.19082848 Caribou Arch -88.56108894 48.09705948 Caribou CG -88.57214509 48.09498673 Cop Harb Cong RC -89.23205406 47.85168084 Crystal Cove -88.58980015 48.15869417 Daisy Farm CG -88.59552193 48.09214022 Davidson I -88.51535972 48.12257809 Duncan Bay CG -88.52185527 48.150598 Edison Fishery -88.58317221 48.08946992 Edwards Is -88.43527441 48.17172245 Gull Rocks East -88.26162826 48.26236504 Hill Pt -88.52528802 48.1655558 Johnson Is -88.58571927 48.14731944 Keyhole -88.61806043 48.14501207 L Louise -88.47250078 48.16924628 Lane Cove CG -88.5570814 48.14486573 Lighthouse -88.57937109 48.08979679 86 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Name Longitude Latitude Little Todd CG -88.92697185 48.02005966 Locke Pt -88.45901399 48.18450616 McCargoe CG -88.7082605 48.08740121 Merrit Lane CG -88.42972709 48.18442853 Minong Mine -88.72005096 48.08347491 Moose Skulls -88.59063351 48.08709128 Mott I Dock -88.54739095 48.10720599 Mott Sediment -88.55002491 48.10429157 Ollies Rocks -88.71228558 48.11692273 Ophite php Wash Hbr -89.17944981 47.93491098 Ophite pwi Wash Hbr -89.23071872 47.87582894 Passage Island Dock -88.35571791 48.23122681 Passage Light -88.36567255 48.22354584 Pickerel Cove -88.65241933 48.12402173 Pine Mountain -88.72816055 48.08439633 Porphyrite pgi Wash Hbr -89.21618244 47.88214454 Porphyrite pmp Wash Hbr -89.21962318 47.8702359 Porphyrite ph Wash Hbr -89.18438864 47.93089216 Porter I -88.44598813 48.17423934 Rasp I Dock -88.47534021 48.14220455 Rasp Seg Cyls -88.47477863 48.1405457 Raspberry Pegs -88.46879695 48.14351368 Red Rock Pt -88.45413695 48.17139189 -89.313 47.867 Rock of Ages Light 87 �www.geo.mtu.edu/~raman/SilverI/IRKeweenawRift Name Longitude Latitude Scoville Pt -88.44940521 48.16322165 Snug Harbor -88.4852324 48.14576228 South Rock -89.27218772 47.86125303 Susie Islands -89.5736758 47.96604403 Suzy's Cave -88.51477842 48.13207674 Todd Harbor CG -88.8219923 48.05083223 Tookers I -88.50329307 48.12941722 Trap pm Wash Hbr -89.2212717 47.90837977 Trap2 pm Wash Hbr -89.14971047 47.93373916 Voyaguer II Dock -89.65254479 47.96263767 Wendigo Mine -89.15127391 47.93227937 Wilson I -88.83672187 48.05654853 Windigo Dock -89.15820212 47.91194955 88 � 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 Isle Royale: Keweenaw Rift Geology. Field Trip, 2013. Description An account of the resource Isle Royale: Keweenaw Rift Geology Physical Volcanology of Large Lava Flows Field Trip, Institute on Lake Superior Geology May 25-30, 2013 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 2013 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/ec689b4f34e01892a88bce6b510af3ba.jpg 78ca391c21961a09271883bf4ba56d12 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 Cairine Budner fonds 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 Nahjus Athletic Club Junior Girl's drill team Subject The topic of the resource Sports Description An account of the resource Black and white photograph of Nahjus Athletic Club Junior Girl's Drill Team on stage. Girls are dressed in Nahjus uniforms of the time. A forest scene is a backdrop on the stage Date A point or period of time associated with an event in the lifecycle of the resource 1950 Rights Information about rights held in and over the resource Public domain Format The file format, physical medium, or dimensions of the resource Photograph Type The nature or genre of the resource Still image Identifier An unambiguous reference to the resource within a given context PHIII-0012c (lx) scan# BUD-0036 1950 drill team gymnastics gymnasts junior girl's team Nahjus Athletic Club https://digitalcollections.lakeheadu.ca/files/original/dbc7fdb89b11d34f8eaa66aa282739da.jpg 60b46ac92b66f8a1006d8b94c709eebe 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 Cairine Budner fonds 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 Nahjus Athletic Club Young Girls pyramid Subject The topic of the resource Sports Description An account of the resource Black and white photograph of Young Girls pyramid at Nahjus Park on stage. Girls are dressed in Nahjus uniforms for the time Date A point or period of time associated with an event in the lifecycle of the resource 1944-45 Contributor An entity responsible for making contributions to the resource Donor C. Budner Rights Information about rights held in and over the resource Public domain Format The file format, physical medium, or dimensions of the resource Photograph Type The nature or genre of the resource Still image Identifier An unambiguous reference to the resource within a given context PHIII-0012c (lviii) BUD-0038 1944 1945 gymnastics gymnasts Nahjus Athletic Club Nahjus Park pyramides Young Girls https://digitalcollections.lakeheadu.ca/files/original/037f46f622e669b4833f22d6d9974017.jpg fe6f1529ad6ea51ca5b8bf0e76b82a0d 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 Cairine Budner fonds 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 Nahjus drill Subject The topic of the resource Sports Description An account of the resource Black and white photograph of men and women performing a Nahjus drill - drama presentation on a stage. A trellis with flowers is the background Contributor An entity responsible for making contributions to the resource Donor C. Budner Rights Information about rights held in and over the resource Public domain Format The file format, physical medium, or dimensions of the resource Photograph Type The nature or genre of the resource Still image Identifier An unambiguous reference to the resource within a given context PHIII-0012c (lviii) scan# BUD-0039 drama presentation gymnastics Nahjus drill https://digitalcollections.lakeheadu.ca/files/original/72f4eaef4c27d2927b02865330b29105.jpg 8a3be0cf82888c5841b96d23cf8f3d2a 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 Cairine Budner fonds 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 Nahjus Athletic Club Young Girls drill Subject The topic of the resource Sports Description An account of the resource Black and white photograph of Young Girls drill on stage. The girls are dressed in Nahjus uniforms for the time Date A point or period of time associated with an event in the lifecycle of the resource 1926-27 Contributor An entity responsible for making contributions to the resource Donor S. Anttila Rights Information about rights held in and over the resource Public domain Format The file format, physical medium, or dimensions of the resource Photograph Type The nature or genre of the resource Still image Identifier An unambiguous reference to the resource within a given context PHIII-0012c (lvii) scan# BUD-040 1926 1927 drill gymnastics gymnasts Nahjus Athletic Club stage uniforms Young Girls https://digitalcollections.lakeheadu.ca/files/original/0ed5dede2ffb373303fe592d3b21b7be.jpg 0262cb433ceb4874a306abc0e7d178a6 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 Cairine Budner fonds 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 Nahjus Athletic Club Married Women gymnastics group Subject The topic of the resource Sports Description An account of the resource Black and white photograph of Nahjus Married Women gymnastic group performing Rhythmic Wand drill at Nahjus Park. Pianist is Bertha Beck and coach is Cairine Beck. Women are dressed in the Nahjus uniform for the time Date A point or period of time associated with an event in the lifecycle of the resource 1950 Contributor An entity responsible for making contributions to the resource Donor C. Budner Rights Information about rights held in and over the resource Public domain Format The file format, physical medium, or dimensions of the resource Photograph Type The nature or genre of the resource Still image Identifier An unambiguous reference to the resource within a given context PHIII-0012c (lvi) scan# BUD-0041 1950 gymnastics Married Women Nahjus Athletic Club Nahjus Park rhythmic wand drill https://digitalcollections.lakeheadu.ca/files/original/4438b487b186d69857ed04075d5b0ba2.jpg a329db5d841ccd76f5df6a985389ef61 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 Cairine Budner fonds 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 Nahjus Athletic Club Married Women Gym Group Subject The topic of the resource Sports Description An account of the resource Black and white photograph of Nahjus Married Women Gymnastics group preparing for Wand drill at Nahjus Park. L to R 1. Cairine (Beck Budner) coach, 6. Hunni Amadeo. Women are dressed in uniforms for the time Date A point or period of time associated with an event in the lifecycle of the resource 1950 Contributor An entity responsible for making contributions to the resource Donor C. Budner Rights Information about rights held in and over the resource Public domain Format The file format, physical medium, or dimensions of the resource Photograph Type The nature or genre of the resource Still image Identifier An unambiguous reference to the resource within a given context PHIII-0012c (lv) scan# BUD-0042 1950 Cairine Budner gymnastics Hunni Amadeo Married Women Nahjus Athletic Club Nahjus Park Wand drill https://digitalcollections.lakeheadu.ca/files/original/8e4a7b911b6deaabb310c58cb8831846.jpg daca83dd47e3cf54d259ec6f8d40914f 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 Cairine Budner fonds 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 Nahjus Athletic Club Junior Girls Drill team Subject The topic of the resource Sports Description An account of the resource Black and white photograph of Nahjus Junior Girls Drill team on stage. Girls are dressed in Nahjus uniforms of the time. A forest scene is a backdrop on the stage Date A point or period of time associated with an event in the lifecycle of the resource 1950 Rights Information about rights held in and over the resource Public domain Format The file format, physical medium, or dimensions of the resource Photograph Type The nature or genre of the resource Still image Identifier An unambiguous reference to the resource within a given context PHIII-0012c (lii) scan #BUD-0043 1950 Girls Drill team gymnastics Nahjus Athletic Club stage